Modern Techniques of Materials Characterisation                1   MODERN TECHNIQUES OF MATERIALS CHARACTERISATIONBy :B. R...
Modern Techniques of Materials Characterisation                  2MS 9157 MODERN TECHNIQUES OF MATERIALS CHARACTERISATION ...
Modern Techniques of Materials Characterisation                  3                                        Unit ILaws of re...
Modern Techniques of Materials Characterisation                    4a boundary between two different isotropic media, such...
Modern Techniques of Materials Characterisation                      5angle of refraction θ2 is less than the angle of inc...
Modern Techniques of Materials Characterisation                     6Fig. : The numerical aperture with respect to a point...
Modern Techniques of Materials Characterisation                  7transferred. However, light travels slower through any g...
Modern Techniques of Materials Characterisation                   8ContrastContrast is the difference in visual properties...
Modern Techniques of Materials Characterisation                     9depending on the polarization of the light. This effe...
Modern Techniques of Materials Characterisation                    10PolarizationPolarization (also polarisation) is a pro...
Modern Techniques of Materials Characterisation                  11         Linear                           Circular     ...
Modern Techniques of Materials Characterisation                 12Unpolarized lightMost sources of electromagnetic radiati...
Modern Techniques of Materials Characterisation               13All modern optical microscopes designed for viewing sample...
Modern Techniques of Materials Characterisation                   14Etchants :In industry, etching, also known as chemical...
Modern Techniques of Materials Characterisation                15Optical microscopeThe optical microscope, often referred ...
Modern Techniques of Materials Characterisation                16             Fig. : Basic optical transmission microscope...
Modern Techniques of Materials Characterisation                 17Light sourceMany sources of light can be used. At its si...
Modern Techniques of Materials Characterisation                 18Some microscopes make use of oil immersion lens. These o...
Modern Techniques of Materials Characterisation                  19Operation :                       Fig. : Optical path i...
Modern Techniques of Materials Characterisation                   20In most microscopes, the eyepiece is a compound lens, ...
Modern Techniques of Materials Characterisation                    21ComaAnother type of aberration is coma, which derives...
Modern Techniques of Materials Characterisation                 22Chromatic aberrationChromatic aberration is caused by th...
Modern Techniques of Materials Characterisation                23Other types of aberrationOther kinds of aberration includ...
Modern Techniques of Materials Characterisation                   24element lens system for which all planar wave fronts a...
Modern Techniques of Materials Characterisation                   25                          Fig. : Barrel distortion sim...
Modern Techniques of Materials Characterisation                 26Phase contrast microscopyPhase contrast microscopy is an...
Modern Techniques of Materials Characterisation              27BackgroundThe technique was invented by Frits Zernike in th...
Modern Techniques of Materials Characterisation                  28A practical implementation of phase-contrast illuminati...
Modern Techniques of Materials Characterisation                 29geometric path length). Adding an adjustable offset phas...
Modern Techniques of Materials Characterisation                  301. Unpolarised light enters the microscope and is polar...
Modern Techniques of Materials Characterisation                    31causes a change in phase of one ray relative to the o...
Modern Techniques of Materials Characterisation                    32polycrystalline silicon), and defects in them or cont...
Modern Techniques of Materials Characterisation               33                                         Unit IVPrimary el...
Modern Techniques of Materials Characterisation                   34The binding energy of a particular electron is the ene...
Modern Techniques of Materials Characterisation                35Figure :Generalized illustration of interaction volumes f...
Modern Techniques of Materials Characterisation                36  Differences between TEM and SEM:TEM                    ...
Modern Techniques of Materials Characterisation                37Difference Between AFM and STMAFM vs STMAFM refers to Ato...
Modern Techniques of Materials Characterisation                383. The tip in AFM touches the surface gently touches the ...
Modern Techniques of Materials Characterisation                   39as tip shape and contact force are considered.CONSISTE...
Modern Techniques of Materials Characterisation                   40VISUALIZATION:STM can visualize and even manipulate at...
Modern Techniques of Materials Characterisation                 41http://www.matter.org.uk/tem/depth_of_field.htmDark fiel...
Modern Techniques of Materials Characterisation                    42Fig. : Diagram illustrating the light path through a ...
Modern Techniques of Materials Characterisation                 43Bright field microscopyBright field microscopy is the si...
Modern Techniques of Materials Characterisation                   44  •   The sample has to be stained before viewing. The...
Modern Techniques of Materials Characterisation                  45                          •        Bright field illumin...
Modern Techniques of Materials Characterisation                  46The transmission electron microscope is used to charact...
Modern Techniques of Materials Characterisation                  47and diffracted beams. This allows one to observe planar...
Modern Techniques of Materials Characterisation                  48electron beam interacts readily with the sample, an eff...
Modern Techniques of Materials Characterisation                    49polishing to remove any scratches that may cause cont...
Modern Techniques of Materials Characterisation                    50them will be scattered to particular angles, determin...
Modern Techniques of Materials Characterisation                   51selective diffraction analysis. SAD patterns are a pro...
Modern Techniques of Materials Characterisation                 52                         Fig. : Schematic diagram of an ...
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
Modern Techniques of Materials Characterisation
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Modern Techniques of Materials Characterisation

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Modern Techniques of Materials Characterisation

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Modern Techniques of Materials Characterisation

  1. 1. Modern Techniques of Materials Characterisation 1 MODERN TECHNIQUES OF MATERIALS CHARACTERISATIONBy :B. Ramesh, (Ph.D.),Associate Professor of Mechanical Engineering,St. Joseph’s College of Engineering,Jeppiaar Trust, Chennai-119Ph.D. Research Scholar,College of Engineering, Guindy campus,Anna University,Chennai-25. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  2. 2. Modern Techniques of Materials Characterisation 2MS 9157 MODERN TECHNIQUES OF MATERIALS CHARACTERISATION LTPC 3003AIM:OBJECTIVE: Characterisation of materials is very important for studying the structure ofmaterials and to interpret their propertiesUNIT-I METALLOGRAPHIC TECHNIQUES 8Specimen preparation techniques, components of microscope, Resolution, depth offocus, polarized light, phase contrast, differential interference microscopy, hot stage andquantitative metallographic techniquesUNIT-II X-RAY DIFFRACTION TECHNIQUES 12Crystallography basics, characteristic spectrum, Bragg’s law, Diffraction methods –Laue, rotating crystal and powder methods. Intensity of diffracted beams –structurefactor calculations and other factors. Cameras- Laue, Debye-Scherer cameras, Seeman-Bohlin focusing cameras. Diffractometer – general feature and optics, proportional,scintillating and Geiger counters.UNIT-III APPLICATION OF X-RAY DIFFRACTION 9Determination of crystal structure, lattice parameter, phase diagram and residual stress– quantitative phase estimation, ASTM catalogue of Materials identificationUNIT-IV ELECTRON MICROSCOPY 8Construction and operation of Transmission electron microscope – Selected AreaElectron Diffraction and image formation, specimen preparation techniques.Construction, modes of operation and application of Scanning electron microscope,Energy Dispersive Spectroscopy, Electron probe micro analysis (EPMA), ScanningTunnelling Microscope (STM) and Atomic Force MicroscopeUNIT-V CHEMICAL AND THERMAL ANALYSIS 8Basic principles, practice and applications of X-ray spectrometry, Wave dispersive X- rayspectrometry, Auger spectroscopy, Secondary ion mass spectroscopy – proton inducedX-ray Emission spectroscopy, Differential thermal analysis, differential scanningcalorimetry DSC and thermogravimetric analysis TGA Total: 45TEXTBOOKS:1. Cullity, B. D.,“ Elements of X-ray diffraction”, Addison-Wesley Company Inc., New York, 3rd Edition, 2000.2. Cherepin and Malik, “Experimental Techniques in Physical Metallurgy", Asia Publishing Co. Bombay, 1968.REFERENCE BOOKS:1. Brandon D. G, “Modern Techniques in Metallography”, Von Nostrand Inc NJ, USA, 1986..2. Thomas G., “Transmission electron microscopy of metals”, John Wiley, 1996.3. Weinberg, F., “Tools and Techniques in Physical Metallurgy”, Volume I & II, Marcel and Decker, 1970 Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  3. 3. Modern Techniques of Materials Characterisation 3 Unit ILaws of reflection Fig. : Diagram of specular reflectionIf the reflecting surface is very smooth, the reflection of light that occurs is calledspecular or regular reflection. The laws of reflection are as follows: 1. The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane. 2. The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.Law of refractionIn optics and physics, Snells law (also known as Descartes law, the Snell–Descarteslaw, and the law of refraction) is a formula used to describe the relationship between theangles of incidence and refraction, when referring to light or other waves passing through Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  4. 4. Modern Techniques of Materials Characterisation 4a boundary between two different isotropic media, such as water and glass. The law saysthat the ratio of the sines of the angles of incidence and of refraction is a constant thatdepends on the media. The refractive index can be calculated by rearranging the formulaaccordingly.Named after Dutch mathematician Willebrord Snellius, one of its discoverers, Snells lawstates that the ratio of the sines of the angles of incidence and refraction is equivalent tothe ratio of velocities in the two media, or equivalent to the opposite ratio of the indicesof refraction:with each θ as the angle measured from the normal, v as the velocity of light in therespective medium (SI units are meters per second, or m/s) and n as the refractive index(which is unitless) of the respective medium.Fig. : Refraction of light at the interface between two media of different refractiveindices, with n2 > n1. Since the velocity is lower in the second medium (v2 < v1), the Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  5. 5. Modern Techniques of Materials Characterisation 5angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray in thehigher-index medium is closer to the normal.Numerical apertureIn optics, the numerical aperture (NA) of an optical system is a dimensionless numberthat characterizes the range of angles over which the system can accept or emit light. Theexact definition of the term varies slightly between different areas of optics.In most areas of optics, and especially in microscopy, the numerical aperture of an opticalsystem such as an objective lens is defined bywhere n is the index of refraction of the medium in which the lens is working (1.0 for air,1.33 for pure water, and up to 1.56 for oils), and θ is the half-angle of the maximum coneof light that can enter or exit the lens. In general, this is the angle of the real marginal rayin the system. The angular aperture of the lens is approximately twice this value (withinthe paraxial approximation). The NA is generally measured with respect to a particularobject or image point and will vary as that point is moved. In microscopy, NA generallyrefers to object-space NA unless otherwise noted.In microscopy, NA is important because it indicates the resolving power of a lens. Thesize of the finest detail that can be resolved is proportional to λ/NA, where λ is thewavelength of the light. A lens with a larger numerical aperture will be able to visualizefiner details than a lens with a smaller numerical aperture. Assuming quality (diffractionlimited) optics, lenses with larger numerical apertures collect more light and willgenerally provide a brighter image, but will provide shallower depth of field. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  6. 6. Modern Techniques of Materials Characterisation 6Fig. : The numerical aperture with respect to a point P depends on the half-angle θof the maximum cone of light that can enter or exit the lens.Depth of focusDepth of focus is a lens optics concept that measures the tolerance of placement of theimage plane (the film plane in a camera) in relation to the lens. In a camera, depth offocus indicates the tolerance of the films displacement within the camera, and istherefore sometimes referred to as "lens-to-film tolerance."Depth of focus vs depth of fieldWhile the phrase depth of focus was historically used, and is sometimes still used, tomean depth of field, in modern times it is more often reserved for the image-side depth.Depth of field is the range of distances in object space for which object points are imagedwith acceptable sharpness with a fixed position of the image plane (the plane of the filmor electronic sensor). Depth of focus can have two slightly different meanings. The first isthe distance over which the image plane can be displaced while a single object planeremains in acceptably sharp focus;[1][2] the second is the image-side conjugate of depth offield.[2] With the first meaning, the depth of focus is symmetrical about the image plane;with the second, the depth of focus is greater on the far side of the image plane, though inmost cases the distances are approximately equal.Where depth of field often can be measured in macroscopic units such as meters and feet,depth of focus is typically measured in microscopic units such as fractions of a millimeteror thousandths of an inch.The same factors that determine depth of field also determine depth of focus, but thesefactors can have different effects than they have in depth of field. Both depth of field anddepth of focus increase with smaller apertures. For distant subjects (beyond macro range),depth of focus is relatively insensitive to focal length and subject distance, for a fixed f-number. In the macro region, depth of focus increases with longer focal length or closersubject distance, while depth of field decreases.Refractive indexThe refractive index or index of refraction of a substance is a measure of the speed oflight in that substance. It is expressed as a ratio of the speed of light in vacuum relative tothat in the considered medium.[note 1] The velocity at which light travels in vacuum is aphysical constant, and the fastest speed at which energy or information can be Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  7. 7. Modern Techniques of Materials Characterisation 7transferred. However, light travels slower through any given material, or medium, that isnot vacuum. (See: light in a medium).[1][2][3][4]A simple, mathematical description of refractive index is as follows: n = velocity of light in a vacuum / velocity of light in mediumHence, the refractive index of water is 1.33, meaning that light travels 1.33 times as fastin a vacuum as it does in water.The refractive index, n, of a medium is defined as the ratio of the speed, c, of a wavephenomenon such as light or sound in a reference medium to the phase speed, vp, of thewave in the medium in question:Fig. : Refraction of light at the interface between two media. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  8. 8. Modern Techniques of Materials Characterisation 8ContrastContrast is the difference in visual properties that makes an object (or its representationin an image) distinguishable from other objects and the background. In visual perceptionof the real world, contrast is determined by the difference in the color and brightness ofthe object and other objects within the same field of view. Fig. : Changes in the amount of contrast in a photoBirefringenceBirefringence, or double refraction, is the decomposition of a ray of light (and otherelectromagnetic radiation) into two rays (the ordinary ray and the extraordinary ray)when it passes through certain types of material, such as calcite crystals or boron nitride, Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  9. 9. Modern Techniques of Materials Characterisation 9depending on the polarization of the light. This effect can occur only if the structure ofthe material is anisotropic (directionally dependent). If the material has a single axis ofanisotropy or optical axis (i.e. it is uniaxial), birefringence can be formalized by assigningtwo different refractive indices to the material for different polarizations. Thebirefringence magnitude is then defined bywhere ne and no are the refractive indices for polarizations parallel (extraordinary) andperpendicular (ordinary) to the axis of anisotropy respectively.[1]The reason for birefringence is the fact that in anisotropic media the electric field vector and the dielectric displacement can be nonparallel (namely for the extraordinarypolarisation), although being linearly related.Birefringence can also arise in magnetic, not dielectric, materials, but substantialvariations in magnetic permeability of materials are rare at optical frequencies. Liquidcrystal materials as used in Liquid Crystal Displays (LCDs) are also birefringent.[2]Fig.: Rays passing through a positively birefringent material. The optical axis isperpendicular to the direction of the rays, so the ray polarized perpendicularly tothe optic axis has a greater refractive index than the ray polarized parallel to it. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  10. 10. Modern Techniques of Materials Characterisation 10PolarizationPolarization (also polarisation) is a property of certain types of waves that describes theorientation of their oscillations. Electromagnetic waves, such as light, and gravitationalwaves exhibit polarization; acoustic waves (sound waves) in a gas or liquid do not havepolarization because the direction of vibration and direction of propagation are the same.By convention, the polarization of light is described by specifying the orientation of thewaves electric field at a point in space over one period of the oscillation. When lighttravels in free space, in most cases it propagates as a transverse wave—the polarization isperpendicular to the waves direction of travel. In this case, the electric field may beoriented in a single direction (linear polarization), or it may rotate as the wave travels(circular or elliptical polarization). In the latter cases, the oscillations can rotate eithertowards the right or towards the left in the direction of travel.Polarization stateThe shape traced out in a fixed plane by the electric vector as such a plane wave passesover it (a Lissajous figure) is a description of the polarization state. The followingfigures show some examples of the evolution of the electric field vector (black), withtime(the vertical axes), at a particular point in space, along with its x and y components(red/left and blue/right), and the path traced by the tip of the vector in the plane (purple):The same evolution would occur when looking at the electric field at a particular timewhile evolving the point in space, along the direction opposite to propagation.In the leftmost figure above, the two orthogonal (perpendicular) components are in phase.In this case the ratio of the strengths of the two components is constant, so the directionof the electric vector (the vector sum of these two components) is constant. Since the tipof the vector traces out a single line in the plane, this special case is called linearpolarization. The direction of this line depends on the relative amplitudes of the twocomponents.In the middle figure, the two orthogonal components have exactly the same amplitudeand are exactly ninety degrees out of phase. In this case one component is zero when theother component is at maximum or minimum amplitude. There are two possible phaserelationships that satisfy this requirement: the x component can be ninety degrees aheadof the y component or it can be ninety degrees behind the y component. In this specialcase the electric vector traces out a circle in the plane, so this special case is calledcircular polarization. The direction the field rotates in depends on which of the two phaserelationships exists. These cases are called right-hand circular polarization and left-handcircular polarization, depending on which way the electric vector rotates and the chosenconvention. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  11. 11. Modern Techniques of Materials Characterisation 11 Linear Circular EllipticalAnother case is when the two components are not in phase and either do not have thesame amplitude or are not ninety degrees out of phase, though their phase offset and theiramplitude ratio are constant.[2] This kind of polarization is called elliptical polarizationbecause the electric vector traces out an ellipse in the plane (the polarization ellipse).This is shown in the above figure on the right.The "Cartesian" decomposition of the electric field into x and y components is, of course,arbitrary. Plane waves of any polarization can be described instead by combining any twoorthogonally polarized waves, for instance waves of opposite circular polarization. TheCartesian polarization decomposition is natural when dealing with reflection fromsurfaces, birefringent materials, or synchrotron radiation. The circularly polarized modesare a more useful basis for the study of light propagation in stereoisomers.Though this section discusses polarization for idealized plane waves, all the above is avery accurate description for most practical optical experiments which use TEM modes,including Gaussian optics. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  12. 12. Modern Techniques of Materials Characterisation 12Unpolarized lightMost sources of electromagnetic radiation contain a large number of atoms or moleculesthat emit light. The orientation of the electric fields produced by these emitters may notbe correlated, in which case the light is said to be unpolarized. If there is partialcorrelation between the emitters, the light is partially polarized. If the polarization isconsistent across the spectrum of the source, partially polarized light can be described asa superposition of a completely unpolarized component, and a completely polarized one.One may then describe the light in terms of the degree of polarization, and the parametersof the polarization ellipse.The biological microscope usually is a transmission microscope with light coming frombelow.The metallurgical microscope usually is a reflection microscope with light coming fromabove. Either by a external light source from above or through the lens with beamsplitters.Components of a microscope Fig. : Basic optical transmission microscope elements(1990s) Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  13. 13. Modern Techniques of Materials Characterisation 13All modern optical microscopes designed for viewing samples by transmitted light sharethe same basic components of the light path, listed here in the order the light travelsthrough them: • Light source, a light or a mirror (7) • Diaphragm and condenser lens (8) • Objective (3) • Ocular lens (eyepiece) (1)In addition the vast majority of microscopes have the same structural components: • Objective turret (to hold multiple objective lenses) (2) • Stage (to hold the sample) (9) • Focus wheel to move the stage (4 - coarse adjustment, 5 - fine adjustment)Aberrations :Summary of AberrationsIn an ideal optical system, all rays of light from a point in the object plane wouldconverge to the same point in the image plane, forming a clear image. Theinfluences which cause different rays to converge to different points are calledaberrations. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  14. 14. Modern Techniques of Materials Characterisation 14Etchants :In industry, etching, also known as chemical milling, is the process of using acids, basesor other chemicals to dissolve unwanted materials such as metals, semiconductormaterials or glass. This process has been used on a wide variety of metals with depths ofmetal removal as large as 12mm (0.5 in).Common etchantsFor aluminium • sodium hydroxideFor steels • hydrochloric and nitric acids • ferric chloride for stainless steels • Nital (a mixture of nitric acid and ethanol, methanol, or methylated spirits for mild steels.2% Nital is common etchant for plain carbon steels.For copper • cupric chloride • ferric chloride • ammonium persulfate • ammonia • 25-50 % nitric acid. • hydrochloric acid and hydrogen peroxideFor silica • hydrofluoric acid (HF) is a very efficient etchant for silicon dioxide. It is however very dangerous if it comes into contact with the body.Peroxymonosulfuric acid, also known as persulfuric acid, peroxysulfuric acid, or asCaros acid, is H2SO5, a liquid at room temperature.Ammonium, sodium, and potassium salts of H2SO5 are used in the plastics industry aspolymerization initiators, etchants, desizing agents, soil conditioner, and for decolorizingand deodorizing oils. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  15. 15. Modern Techniques of Materials Characterisation 15Optical microscopeThe optical microscope, often referred to as the "light microscope", is a type ofmicroscope which uses visible light and a system of lenses to magnify images of smallsamples. Optical microscopes are the oldest design of microscope and were designedaround 1600. Basic optical microscopes can be very simple, although there are manycomplex designs which aim to improve resolution and sample contrast. Historicallyoptical microscopes were easy to develop and are popular because they use visible lightso the sample can be directly observed by eye.The image from an optical microscope can be captured by normal light-sensitive camerasto generate a micrograph. Originally images were captured by photographic film butmodern developments in CMOS and later charge-coupled device (CCD) cameras allowthe capture of digital images. Purely Digital microscopes are now available which justuse a CCD camera to examine a sample, and the image is shown directly on a computerscreen without the need for eye-pieces.Alternatives to optical microscopy which do not use visible light include scanningelectron microscopy and transmission electron microscopy.Components Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  16. 16. Modern Techniques of Materials Characterisation 16 Fig. : Basic optical transmission microscope elements(1990s)All modern optical microscopes designed for viewing samples by transmitted light sharethe same basic components of the light path, listed here in the order the light travelsthrough them: • Light source, a light or a mirror (7) • Diaphragm and condenser lens (8) • Objective (3) • Ocular lens (eyepiece) (1)In addition the vast majority of microscopes have the same structural components: • Objective turret (to hold multiple objective lenses) (2) • Stage (to hold the sample) (9) • Focus wheel to move the stage (4 - coarse adjustment, 5 - fine adjustment)These entries are numbered according to the image on the right.Ocular (eyepiece)The ocular, or eyepiece, is a cylinder containing two or more lenses; its function is tobring the image into focus for the eye. The eyepiece is inserted into the top end of thebody tube. Eyepieces are interchangeable and many different eyepieces can be insertedwith different degrees of magnification. Typical magnification values for eyepiecesinclude 2×, 5× and 10×. In some high performance microscopes, the optical configurationof the objective lens and eyepiece are matched to give the best possible opticalperformance. This occurs most commonly with apochromatic objectives.ObjectiveThe objective is a cylinder containing one or more lenses that are typically made of glass;its function is to collect light from the sample. At the lower end of the microscope tubeone or more objective lenses are screwed into a circular nose piece which may be rotatedto select the required objective lens. Typical magnification values of objective lenses are4×, 5×, 10×, 20×, 40×, 50×, 60× and 100×. Some high performance objective lenses mayrequire matched eyepieces to deliver the best optical performance.StageThe stage is a platform below the objective which supports the specimen being viewed.In the center of the stage is a hole through which light passes to illuminate the specimen.The stage usually has arms to hold slides (rectangular glass plates with typicaldimensions of 25 mm by 75 mm, on which the specimen is mounted). Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  17. 17. Modern Techniques of Materials Characterisation 17Light sourceMany sources of light can be used. At its simplest, daylight is directed via a mirror. Mostmicroscopes, however, have their own controllable light source - normally a halogenlamp.CondenserThe condenser is a lens designed to focus light from the illumination source onto thesample. The condenser may also include other features, such as a diaphragm and/orfilters, to manage the quality and intensity of the illumination. For illuminationtechniques like dark field, phase contrast and differential interference contrastmicroscopy additional optical components must be precisely aligned in the light path.FrameThe whole of the optical assembly is traditionally attached to a rigid arm which in turn isattached to a robust U shaped foot to provide the necessary rigidity. The arm angle maybe adjustable to allow the viewing angle to be adjusted.The frame provides a mounting point for various microscope controls. Normally this willinclude controls for focusing, typically a large knurled wheel to adjust coarse focus,together with a smaller knurled wheel to control fine focus. Other features may be lampcontrols and/or controls for adjusting the condenser.Objective lensesOn a typical compound optical microscope there are a selection of lenses available fordifferent applications. Many different objective lenses with different properties andmagnification are available.Typically there will be around three objective lenses: a low power lens for scanning thesample, a medium power lens for normal observation and a high power lens for detailedobservation. The typical magnification of objective lenses depends on the intendedapplication, normal groups of lens magnificaitons may be [4×, 10×, 20×] for lowmagnification work and [10×, 40×, 100×] for high magnification work.Objective lenses with higher magnifications normally have a higher numerical apertureand a shorter depth of field in the resulting image.Oil immersion objective Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  18. 18. Modern Techniques of Materials Characterisation 18Some microscopes make use of oil immersion lens. These objectives must be used withoil (immersion oil) between the objective lens and the sample. The refractive index of theimmersion oil is higher than air and this allows the objective lens to have a largernumerical aperture. The larger numerical aperture allows collection of more light makingdetailed observation of faint details possible.Immersion lenses are designed so that the refractive index of the oil and of the cover slipare closely matched so that the light is transmitted from the specimen to the outer face ofthe objective lens with minimal refraction. An oil immersion lens usually has amagnification of 40 to 100×.MagnificationThe actual power or magnification of a compound optical microscope is the product ofthe powers of the ocular (eyepiece) and the objective lens. The maximum normalmagnifications of the occular and objective are 10× and 100× respectively giving a finalmagnification of 1000×.Magnification and micrographsWhen using a camera to capture a micrograph the effective magnification of the imagemust take into account the size of the image. This is independent of whether it is on aprint from a film negative or displayed digitally on a computer screen.In the case of photographic film cameras the calculation is simple; the final magnificationis the product of: the objective lens magnification, the camera optics magnification andthe enlargement factor of the film print relative to the negative. A typical value of theenlargement factor is around 5× (for the case of 35mm film and a 6×4 inch print).In the case of digital cameras the size of the pixels in the CMOS or CCD detector and thesize of the pixels on the screen have to be known. The enlargement factor from thedetector to the pixels on screen can then be calculated. As with a film camera the finalmagnification is the product of: the objective lens magnification, the camera opticsmagnification and the enlargement factor. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  19. 19. Modern Techniques of Materials Characterisation 19Operation : Fig. : Optical path in a typical microscopeThe optical components of a modern microscope are very complex and for a microscopeto work well, the whole optical path has to be very accurately set up and controlled.Despite this, the basic operating principles of a microscope are quite simple.The objective lens is, at its simplest, a very high powered magnifying glass i.e. a lenswith a very short focal length. This is brought very close to the specimen being examinedso that the light from the specimen comes to a focus about 160 mm inside the microscopetube. This creates an enlarged image of the subject. This image is inverted and can beseen by removing the eyepiece and placing a piece of tracing paper over the end of thetube. By carefully focusing a brightly lit specimen, a highly enlarged image can be seen.It is this real image that is viewed by the eyepiece lens that provides further enlargement. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  20. 20. Modern Techniques of Materials Characterisation 20In most microscopes, the eyepiece is a compound lens, with one component lens near thefront and one near the back of the eyepiece tube. This forms an air-separated couplet. Inmany designs, the virtual image comes to a focus between the two lenses of the eyepiece,the first lens bringing the real image to a focus and the second lens enabling the eye tofocus on the virtual image.In all microscopes the image is intended to be viewed with the eyes focused at infinity(mind that the position of the eye in the above figure is determined by the eyes focus).Headaches and tired eyes after using a microscope are usually signs that the eye is beingforced to focus at a close distance rather than at infinity.The essential principle of the microscope is that an objective lens with very short focallength (often a few mm) is used to form a highly magnified real image of the object.Here, the quantity of interest is linear magnification, and this number is generallyinscribed on the objective lens casing. In practice, today, this magnification is carried outby means of two lenses: the objective lens which creates an image at infinity, and asecond weak tube lens which then forms a real image in its focal plane.[3]Aberrations of lensesLenses do not form perfect images, and there is always some degree of distortion oraberration introduced by the lens which causes the image to be an imperfect replica ofthe object. Careful design of the lens system for a particular application ensures that theaberration is minimized. There are several different types of aberration which can affectimage quality.Spherical aberrationSpherical aberration occurs because spherical surfaces are not the ideal shape with whichto make a lens, but they are by far the simplest shape to which glass can be ground andpolished and so are often used. Spherical aberration causes beams parallel to, but distantfrom, the lens axis to be focused in a slightly different place than beams close to the axis.This manifests itself as a blurring of the image. Lenses in which closer-to-ideal, non-spherical surfaces are used are called aspheric lenses. These were formerly complex tomake and often extremely expensive, but advances in technology have greatly reducedthe manufacturing cost for such lenses. Spherical aberration can be minimised by carefulchoice of the curvature of the surfaces for a particular application: for instance, a plano-convex lens which is used to focus a collimated beam produces a sharper focal spot whenused with the convex side towards the beam source. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  21. 21. Modern Techniques of Materials Characterisation 21ComaAnother type of aberration is coma, which derives its name from the comet-likeappearance of the aberrated image. Coma occurs when an object off the optical axis ofthe lens is imaged, where rays pass through the lens at an angle to the axis θ. Rays whichpass through the centre of the lens of focal length f are focused at a point with distance ftan θ from the axis. Rays passing through the outer margins of the lens are focused atdifferent points, either further from the axis (positive coma) or closer to the axis (negativecoma). In general, a bundle of parallel rays passing through the lens at a fixed distancefrom the centre of the lens are focused to a ring-shaped image in the focal plane, knownas a comatic circle. The sum of all these circles results in a V-shaped or comet-like flare.As with spherical aberration, coma can be minimised (and in some cases eliminated) bychoosing the curvature of the two lens surfaces to match the application. Lenses in whichboth spherical aberration and coma are minimised are called bestform lenses. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  22. 22. Modern Techniques of Materials Characterisation 22Chromatic aberrationChromatic aberration is caused by the dispersion of the lens material—the variation ofits refractive index, n, with the wavelength of light. Since, from the formulae above, f isdependent upon n, it follows that different wavelengths of light will be focused todifferent positions. Chromatic aberration of a lens is seen as fringes of colour around theimage. It can be minimised by using an achromatic doublet (or achromat) in which twomaterials with differing dispersion are bonded together to form a single lens. This reducesthe amount of chromatic aberration over a certain range of wavelengths, though it doesnot produce perfect correction. The use of achromats was an important step in thedevelopment of the optical microscope. An apochromat is a lens or lens system which haseven better correction of chromatic aberration, combined with improved correction ofspherical aberration. Apochromats are much more expensive than achromats.Different lens materials may also be used to minimise chromatic aberration, such asspecialised coatings or lenses made from the crystal fluorite. This naturally occurringsubstance has the highest known Abbe number, indicating that the material has lowdispersion. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  23. 23. Modern Techniques of Materials Characterisation 23Other types of aberrationOther kinds of aberration include field curvature, barrel and pincushion distortion, andastigmatism.Petzval field curvature, named for Joseph Petzval, describes the optical aberration inwhich a flat object normal to the optical axis (or a non-flat object past the hyperfocaldistance) cannot be brought into focus on a flat image plane. Consider an "ideal" single- Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  24. 24. Modern Techniques of Materials Characterisation 24element lens system for which all planar wave fronts are focused to a point a distance ffrom the lens. Placing this lens the distance f from a flat image sensor, image points nearthe optical axis will be in perfect focus, but rays off axis will come into focus before theimage sensor, dropping off by the cosine of the angle they make with the optical axis.This is less of a problem when the imaging surface is spherical, as in the human eye.Most photographic lenses are designed to minimize field curvature, and so effectivelyhave a focal length that increases with ray angle. Fig. : Field curvature: the image plane is not flat.Barrel distortion In "barrel distortion", image magnification decreases with distance from the optical axis. The apparent effect is that of an image which has been mapped around a sphere (or barrel). Fisheye lenses, which take hemispherical views, utilize this type of distortion as a way to map an infinitely wide object plane into a finite image area. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  25. 25. Modern Techniques of Materials Characterisation 25 Fig. : Barrel distortion simulationPincushion distortion In "pincushion distortion", image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the centre of the image are bowed inwards, towards the centre of the image, like a pincushion. A certain amount of pincushion distortion is often found with visual optical instruments, e.g. binoculars, where it serves to eliminate the globe effect. Fig. : Pincushion distortion simulation Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  26. 26. Modern Techniques of Materials Characterisation 26Phase contrast microscopyPhase contrast microscopy is an optical microscopy illumination technique in whichsmall phase shifts in the light passing through a transparent specimen are converted intoamplitude or contrast changes in the image.A phase contrast microscope does not require staining to view the slide. This type ofmicroscope made it possible to study the cell cycle.As light travels through a medium other than vacuum, interaction with this mediumcauses its amplitude and phase to change in a way which depends on properties of themedium. Changes in amplitude give rise to familiar absorption of light, which iswavelength dependent and gives rise to colours. The human eye measures only theenergy of light arriving on the retina, so changes in phase are not easily observed, yetoften these changes in phase carry a large amount of information.The same holds in a typical microscope, i.e., although the phase variations introduced bythe sample are preserved by the instrument (at least in the limit of the perfect imaginginstrument) this information is lost in the process which measures the light. In order tomake phase variations observable, it is necessary to combine the light passing through thesample with a reference so that the resulting interference reveals the phase structure ofthe sample.This was first realized by Frits Zernike during his study of diffraction gratings. Duringthese studies he appreciated both that it is necessary to interfere with a reference beam,and that to maximize the contrast achieved with the technique, it is necessary to introducea phase shift to this reference so that the no-phase-change condition gives rise tocompletely destructive interference.He later realised that the same technique can be applied to optical microscopy. Thenecessary phase shift is introduced by rings etched accurately onto glass plates so thatthey introduce the required phase shift when inserted into the optical path of themicroscope. When in use, this technique allows phase of the light passing through theobject under study to be inferred from the intensity of the image produced by themicroscope. This is the phase-contrast technique.In optical microscopy many objects such as cell parts in protozoans, bacteria and spermtails are essentially fully transparent unless stained. (Staining is a difficult and timeconsuming procedure which sometimes, but not always, destroys or alters the specimen.)The difference in densities and composition within the imaged objects however oftengive rise to changes in the phase of light passing through them, hence they are sometimescalled "phase objects". Using the phase-contrast technique makes these structures visibleand allows their study with the specimen still alive.This phase contrast technique proved to be such an advancement in microscopy thatZernike was awarded the Nobel prize (physics) in 1953. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  27. 27. Modern Techniques of Materials Characterisation 27BackgroundThe technique was invented by Frits Zernike in the 1930s for which he received theNobel prize in physics in 1953. Phase-contrast microscopy is a mode available on mostadvanced light microscopes and is most commonly used to provide contrast oftransparent specimens such as living cells or small organisms.Description1. Condenser annulus2. Object plane3. Phase plate4. Primary image plane Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  28. 28. Modern Techniques of Materials Characterisation 28A practical implementation of phase-contrast illumination consists of a phase ring(located in a conjugated aperture plane somewhere behind the front lens element of theobjective) and a matching annular ring, which is located in the primary aperture plane(location of the condensers aperture).Two selected light rays, which are emitted from one point inside the lamps filament, getfocused by the field lens exactly inside the opening of the condenser annular ring. Sincethis location is precisely in the front focal plane of the condenser, the two light rays arethen refracted in such way that they exit the condenser as parallel rays. Assuming that thetwo rays in question are neither refracted nor diffracted in the specimen plane (location ofmicroscope slide), they enter the objective as parallel rays. Since all parallel rays arefocused in the back focal plane of the objective, the back focal plane is a conjugatedaperture plane to the condensers front focal plane (also location of the condenserannulus). To complete the phase setup, a phase plate is positioned inside the back focalplane in such a way that it lines up nicely with the condenser annulus.Only through correctly centering the two elements can phase contrast illumination beestablished. A phase centering telescope that temporarily replaces one of the oculars isused, first to focus the phase element plane and then center the annular illumination ringwith the corresponding ring of the phase plate.An interesting variant in phase contrast design was once implemented (by the microscopemaker C. Baker, London) in which the conventional annular form of the two elementswas replaced by a cross-shaped transmission slit in the substage and corresponding cross-shaped phase plates in the conjugate plane in the objectives. The advantage claimed herewas that only a single slit aperture was needed for all phase objective magnifications.Recentring and rotational alignment of the cross by means of the telescope wasnevertheless needed for each change in magnification.Differential interference contrast microscopyDifferential interference contrast microscopy (DIC), also known as NomarskiInterference Contrast (NIC) or Nomarski microscopy, is an optical microscopyillumination technique used to enhance the contrast in unstained, transparent samples.DIC works on the principle of interferometry to gain information about the opticaldensity of the sample, to see otherwise invisible features. A relatively complex lightingscheme produces an image with the object appearing black to white on a greybackground. This image is similar to that obtained by phase contrast microscopy butwithout the bright diffraction halo.DIC works by separating a polarised light source into two orthogonally polarizedmutually coherent parts which are spatially displaced (sheared) at the sample plane, andrecombined before observation. The interference of the two parts at recombination issensitive to their optical path difference (i.e. the product of refractive index and Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  29. 29. Modern Techniques of Materials Characterisation 29geometric path length). Adding an adjustable offset phase determining the interference atzero optical path difference in the sample, the contrast is proportional to the path lengthgradient along the shear direction, giving the appearance of a three-dimensional physicalrelief corresponding to the variation of optical density of the sample, emphasising linesand edges though not providing a topographically accurate image.The light pathFig. : The components of the basic differential interference contrast microscopesetup. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  30. 30. Modern Techniques of Materials Characterisation 301. Unpolarised light enters the microscope and is polarised at 45°. Polarised light is required for the technique to work.2. The polarised light enters the first Nomarski-modified Wollaston prism and isseparated into two rays polarised at 90° to each other, the sampling and reference rays. Main article: Wollaston prism Wollaston prisms are a type of prism made of two layers of a crystalline substance, such as quartz, which, due to the variation of refractive index depending on the polarisation of the light, splits the light according to its polarisation. The Nomarski prism causes the two rays to come to a focal point outside the body of the prism, and so allows greater flexibility when setting up the microscope, as the prism can be actively focused.3. The two rays are focused by the condenser for passage through the sample. These tworays are focused so they will pass through two adjacent points in the sample, around0.2 μm apart. The sample is effectively illuminated by two coherent light sources, one with 0° polarisation and the other with 90° polarisation. These two illuminations are, however, not quite aligned, with one lying slightly offset with respect to the other. Fig. : The route of light through a DIC microscope. The two light beams should be parallel between condenser and objective4. The rays travel through adjacent areas of the sample, separated by the shear. Theseparation is normally similar to the resolution of the microscope. They will experiencedifferent optical path lengths where the areas differ in refractive index or thickness. This Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  31. 31. Modern Techniques of Materials Characterisation 31causes a change in phase of one ray relative to the other due to the delay experienced bythe wave in the more optically dense material. The passage of many pairs of rays through pairs of adjacent points in the sample (and their absorbance, refraction and scattering by the sample) means an image of the sample will now be carried by both the 0° and 90° polarised light. These, if looked at individually, would be bright field images of the sample, slightly offset from each other. The light also carries information about the image invisible to the human eye, the phase of the light. This is vital later. The different polarisations prevent interference between these two images at this point.5. The rays travel through the objective lens and are focused for the second Nomarski-modified Wollaston prism.6. The second prism recombines the two rays into one polarised at 135°. Thecombination of the rays leads to interference, brightening or darkening the image at thatpoint according to the optical path difference. This prism overlays the two bright field images and aligns their polarisations so they can interfere. However, the images do not quite line up because of the offset in illumination - this means that instead of interference occurring between 2 rays of light that passed through the same point in the specimen, interference occurs between rays of light that went through adjacent points which therefore have a slightly different phase. Because the difference in phase is due to the difference in optical path length, this recombination of light causes "optical differentiation" of the optical path length, generating the image seen.Advantages and disadvantagesDIC has strong advantages in uses involving live and unstained biological samples, suchas a smear from a tissue culture or individual water borne single-celled organisms. Itsresolution[specify] and clarity in conditions such as this are unrivaled among standard opticalmicroscopy techniques.The main limitation of DIC is its requirement for a transparent sample of fairly similarrefractive index to its surroundings. DIC is unsuitable (in biology) for thick samples, suchas tissue slices, and highly pigmented cells. DIC is also unsuitable for most nonbiological uses because of its dependence on polarisation, which many physical sampleswould affect.One non-biological area where DIC is useful is in the analysis of planar siliconsemiconductor processing. The thin (typically 100-1000 nm) films in silicon processingare often mostly transparent to visible light (e.g., silicon dioxide, silicon nitride and Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  32. 32. Modern Techniques of Materials Characterisation 32polycrystalline silicon), and defects in them or contamination lying on top of thembecome more visible. This also enables the determination of whether a feature is a pit inthe substrate material or a blob of foreign material on top. Etched crystalline features gaina particularly striking appearance under DIC.Image quality, when used under suitable conditions, is outstanding in resolution andalmost entirely free of artifacts unlike phase contrast. However analysis of DIC imagesmust always take into account the orientation of the Wollaston prisms and the apparentlighting direction, as features parallel to this will not be visible. This is, however, easilyovercome by simply rotating the sample and observing changes in the image. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  33. 33. Modern Techniques of Materials Characterisation 33 Unit IVPrimary electron :A primary electron is usually a high energy electron which starts outside the crystal (e.g.in the beam of an electron microscope). It may be elastically scattered or may excitevarious processes in the crystal by being inelastically scattered.Secondary electrons of various types can be emitted from a solid following itsbombardment with primary electrons.Secondary electron :A secondary electron arises as a result of the interaction of a primary electron with aspecimen. In principle the term refers to all electrons emitted from a specimen after it hasbeen bombarded with primary electrons, X-rays or other radiation. In practice the phrasemost commonly refers to low-energy electrons (kinetic energy less than 50eV) emittedfrom the specimen in a scanning electron microscope (SEM).Back scattered electron:A backscattered electron is a high energy primary electron which suffers large angle (>90°) scattering and re-emerges from the entry surface of a specimen. Backscatteredelectrons usually have energies close to that of the primary electron beam. They are ofgreatest interest to SEM users, giving surface sensitive information.Auger electron :An Auger electron has characteristic energy related to the electronic transitions within theatom which have caused it to be emitted. Emission of an Auger electron is an alternativeto the emission of a characteristic X-ray. The energy of an Auger electron, EA, is given byEA = E1 - E2 - E3, whereE1 = energy of atom with inner-shell vacancy,E2 = energy of atom with outer-shell vacancy, andE3 = binding energy of emitted (Auger) electron.Binding energy : Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  34. 34. Modern Techniques of Materials Characterisation 34The binding energy of a particular electron is the energy which would be required toremove it from the atom to an infinite distance.e.g. K (1s) electrons in aluminium = 1559 eVAuger emission example:If a K-shell electron is knocked out of an atom, the resulting inner-shell (K) vacancy canbe filled by an outer-shell (e.g. L2) electron. If the resulting energy difference is lost bethe emission of an L3 electron this will be known as a K-L2, L3 Auger electron.It will have characteristic energyEK-L2,L3 = EK - EL2 - Ebinding, L3Characteristic X-ray :A characteristic X-ray can be emitted from an excited atom when an outer-shell (e.g. L)electron jumps in to fill an inner-shell (e.g. K) vacancy. It has an energy characteristic ofthe atom and can therefore be used for analytical purposes. Its energy is the differencebetween the energies of the atom with an inner-shell vacancy and the same atom with anouter-shell vacancy.The emission of the excess energy when an atom de-excites (decays or relaxes) canalternatively be achieved by the production of an Auger electron.Cathodoluminescence :Cathodoluminescence is the emission of light in response to irradiation by electrons. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  35. 35. Modern Techniques of Materials Characterisation 35Figure :Generalized illustration of interaction volumes for various electron-specimeninteractions. Auger electrons (not shown) emerge from a very thin region of the samplesurface (maximum depth about 50 Å) than do secondary electrons (50-500 Å). Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  36. 36. Modern Techniques of Materials Characterisation 36 Differences between TEM and SEM:TEM SEMElectron beam passes through thin Electron beam scans over surface of sample.sample.Specially prepared thin samples or Sample can be any thickness and is mounted on anparticulate material are supported on aluminum stub.TEM grids.Specimen stage halfway down column. Specimen stage in the chamber at the bottom of the column.Image shown on fluorescent screen. Image shown on TV monitor.Image is a two dimensional projection of Image is of the surface of the sample.the sample.Resolution : 0.2 nm 2 nmMagnification: 500 000 X 200 000 XA TEM (transmission electron A SEM (scanning electron microscope) imagesmicroscope) images using the electrons using the electrons reflected from a specimen. Thethat pass through it. A TEM image takes image from an SEM thus looks somewhat like aa bit more interpretation as we’re not normal photo (we’re used to imaging using theused to seeing images of light that’s light reflected from objects).passed through thingsA TEM is a Transmission Electron A SEM is a Scanning Electron Microscope.Microscope. A very thin specimen, This is the type where you insert a specimencoated in gold, is inserted in the specimen into the scanning chamber and an electron beamchamber of the microscope. An electron scans the surface of the speciman. The electronbeam is then directed through the beam knocks electrons away from the specimenspecimen and produced a negative image and a sensor captures the electrons. Theon a plate coated with a phosphorus captured electrons are then converted bycoating. Photographs are taken of the electronic to a image displayed on a monitor.image by opening a trap door in the plate Pictures of this electronic image can also beand exposing negative film or electronic printed.sensors for a digital image. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  37. 37. Modern Techniques of Materials Characterisation 37Difference Between AFM and STMAFM vs STMAFM refers to Atomic Force Microscope and STM refers to Scanning TunnelingMicroscope. The development of these two microscopes is considered a revolution in theatomic and molecular fields.When talking of AFM, it captures precise images by moving a nanometer sized tip acrossthe surface of the image. The STM captures images using quantum tunneling.Of the two microscopes, the Scanning Tunneling Microscope was the first to bedeveloped.Unlike the STM, the probe makes a direct contact with the surface or calculates theincipient chemical bonding in AFM. The STM images indirectly by calculating thequantum degree tunneling between he probe and sample.Another difference that can be seen is that the tip in AFM touches the surface gentlytouches the surface whereas in STM, the tip is kept at a short distance from the surface.Unlike the STM, the AFM does not measure the tunneling current but only measures thesmall force between the surface and the tip.It has also been seen that the AFM resolution is better than the STM. This is why AFM iswidely used in nano-technology. When talking of the dependence between force anddistance, the AFM is more complex than the STM.When Scanning Tunneling Microscope is normally applicable to conductors, the AtomicForce Microscope is applicable to both conductors and insulators. The AFM suits wellwith liquid and gas environments whereas STM operates only in high vacuum.When compared to STM, the AFM gives a more topographic contrast direct heightmeasurement and better surface features.Summary1. AFM captures precise images by moving a nanometer sized tip across the surface ofthe image. The STM captures images using quantum tunneling.2. The probe makes a direct contact with the surface or calculates the incipient chemicalbonding in AFM. The STM images indirectly by calculating the quantum degreetunneling between he probe and sample. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  38. 38. Modern Techniques of Materials Characterisation 383. The tip in AFM touches the surface gently touches the surface whereas in STM, the tipis kept at a short distance from the surface.4. AFM resolution is better than the STM. This is why AFM is widely used in nano-technology.5. When Scanning Tunneling Microscope is normally applicable to conductors, theAtomic Force Microscope is applicable to both conductors and insulators.6. The AFM suits well with liquid and gas environments whereas STM operates only inhigh vacuum.7. Of the two microscopes, the Scanning Tunneling Microscope was the first to bedeveloped.STM Vs AFMScanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) orscanning force microscopy (SFM) are inventions of Scanning Probe microscopy atechnique that forms images of surfaces using a physical probe that scans the specimen.An image of the surface is obtained by mechanically moving the probe in a raster scan ofthe specimen, line by line, and recording the probe-surface interaction as a function ofposition. STM is a powerful instrument that is used for imaging surfaces at the atomiclevel while AFM is one of the primary tools for imaging, measuring, and manipulatingmatter at the Nano-scale.INVENTED:Scanning Tunneling Microscopy (STM) was invented in 1981 and was developed byGerd Binnig and Heinrich Rohrer.Atomic Force Microscopy (AFM) was invented in 1985 and was also developed by GerdBinnig and Heinrich Rohrer.IMAGE:STM gives two-dimensional image of the atoms.AFM gives three-dimensional surface profile of the Nano-objects.RESOLUTION:STM gives better resolution than AFM because of the exponential dependence of thetunneling current on distance.The force-distance dependence in AFM is much more complex when characteristics such Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  39. 39. Modern Techniques of Materials Characterisation 39as tip shape and contact force are considered.CONSISTED OF:STM uses a sharpened conducting tip.AFM uses a conductive AFM cantilever (typically silicon or silicon nitride with a tipradius of curvature on the order of nanometers) with a sharp tip (probe) at its end that isused to scan the specimen surface.DEPENDED ON:STM relies on electrical current between the tip and the surface.AFM relies on movement due to the electromagnetic forces between atoms.TUNNELING CURRENT:STM record the tunneling current.AFM does not record the tunneling current but the small force between the tip and thesurface.TIP USED:STM uses a sharpened conducting tip (metallic tip).AFM uses a conductive AFM cantilever.INTERACTION:In case of STM Interaction between probe and material surface is monitored is tunnelingcurrent.While in AFM Interaction between probe and material surface is monitored is van derWaals force.PHYSICAL CONTACT:In STM Tip and substrate are in very close proximity but not actually in physical contact.While in AFM Tip and substrate are actually in physical contact.ATTACHMENT OF TIP:Tip is not attached to a tiny leaf spring in case of Scanning tunneling microscopy.In Atomic force microscope Tip is attached to a tiny leaf spring, the cantilever, which hasa low spring constant. Bending of this cantilever is detected, often with the use of a laserbeam, which is reflected from the cantilever.MOUNTED ON:Tip mounts on the scanner when we have scanning tunneling microscope.Sample mounts on the scanner when we have atomic force microscope.TIP SPACE:STMs Tip is kept at a short distance from the surface.While AFMs Tip is not kept at a short distance from the surface but it gently touches thesurface. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  40. 40. Modern Techniques of Materials Characterisation 40VISUALIZATION:STM can visualize and even manipulate atoms.AFM can easily image non-conducting objects i.e., DNA and proteins etc.USED FOR:STM is a powerful instrument that is used for imaging surfaces at the atomic level. STMis being used for the conductance of single molecule.The AFM is one of the primary tools for imaging, measuring, and manipulating matter atthe Nano-scale.ADVANTAGES & DISADVANTAGES:• In STM the two parameters are integrally linked for voltage calculation.• AFM offers the advantage that the writing voltage and tip-to-substrate spacing can becontrolled independently.• AFM gives three-dimensional image while STM only gives two-dimensional image.This is the advantage of AFM over STM.• Resolution of STM is higher than AFM. STM gives true atomic resolution.• An AFM cannot scan images as fast as a STM, requiring several minutes for a typicalscan, while a STM is capable of scanning at near real-time, although at relatively lowquality.Depth of field and depth of focusThe depth of field, Dob is the range of distance along the optical axis in which thespecimen can move without the image appearing to lose sharpness. This obviouslydepends on the resolution of the microscope.The depth of focus, Dim is the extent of the region around the image plane in which theimage will appear to be sharp. This depends on magnification, MT.Both depth of field and depth of focus are strongly dependent on changes in aperture(hence the semiangle n ) and working distance (dob). Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  41. 41. Modern Techniques of Materials Characterisation 41http://www.matter.org.uk/tem/depth_of_field.htmDark field microscopyDark field microscopy (dark ground microscopy) describes microscopy methods, inboth light and electron microscopy, which exclude the unscattered beam from the image.As a result, the field around the specimen (i.e. where there is no specimen to scatter thebeam) is generally dark.Light Microscopy ApplicationsIn optical microscopy, darkfield describes an illumination technique used to enhance thecontrast in unstained samples. It works by illuminating the sample with light that will notbe collected by the objective lens, and thus will not form part of the image. This producesthe classic appearance of a dark, almost black, background with bright objects on it.The lights pathThe steps are illustrated in the figure where an upright microscope is used. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  42. 42. Modern Techniques of Materials Characterisation 42Fig. : Diagram illustrating the light path through a dark field microscope. 1. Light enters the microscope for illumination of the sample. 2. A specially sized disc, the patch stop (see figure) blocks some light from the light source, leaving an outer ring of illumination. 3. The condenser lens focuses the light towards the sample. 4. The light enters the sample. Most is directly transmitted, while some is scattered from the sample. 5. The scattered light enters the objective lens, while the directly transmitted light simply misses the lens and is not collected due to a direct illumination block (see figure). 6. Only the scattered light goes on to produce the image, while the directly transmitted light is omitted.Advantages and disadvantages.Dark field microscopy is a very simple yet effective technique and well suited for usesinvolving live and unstained biological samples, such as a smear from a tissue culture orindividual water-borne single-celled organisms. Considering the simplicity of the setup,the quality of images obtained from this technique is impressive.The main limitation of dark field microscopy is the low light levels seen in the finalimage. This means the sample must be very strongly illuminated, which can causedamage to the sample. Dark field microscopy techniques are almost entirely free ofartifacts, due to the nature of the process. However the interpretation of dark field imagesmust be done with great care as common dark features of bright field microscopy imagesmay be invisible, and vice versa.While the dark field image may first appear to be a negative of the bright field image,different effects are visible in each. In bright field microscopy, features are visible whereeither a shadow is cast on the surface by the incident light, or a part of the surface is lessreflective, possibly by the presence of pits or scratches. Raised features that are toosmooth to cast shadows will not appear in bright field images, but the light that reflectsoff the sides of the feature will be visible in the dark field images. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  43. 43. Modern Techniques of Materials Characterisation 43Bright field microscopyBright field microscopy is the simplest of all the optical microscopy illuminationtechniques. Sample illumination is transmitted (i.e., illuminated from below and observedfrom above) white light and contrast in the sample is caused by absorbance of some ofthe transmitted light in dense areas of the sample. Bright field microscopy is the simplestof a range of techniques used for illumination of samples in light microscopes and itssimplicity makes it a popular technique. The typical appearance of a bright fieldmicroscopy image is a dark sample on a bright background, hence the name.Light pathThe light path of a bright field microscope is extremely simple, no additional componentsare required beyond the normal light microscope setup. The light path therefore consistsof: • Transillumination light source, commonly a halogen lamp in the microscope stand. • Condenser lens which focusses light from the light source onto the sample. • Objective lens which collects light from the sample and magnifies the image. • Oculars and/or a camera to view the sample image.Bright field microscopy may use critical or Köhler illumination to illuminate the sample.PerformanceBright field microscopy typically has low contrast with most biological samples as fewabsorb light to a great extent. Stains are often required to increase contrast whichprevents use on live cells in many situations. Bright field illumination is useful forsamples which have an intrinsic colour, for example chloroplasts in plant cells.Bright field microscopy is a standard light microscopy technique and thereforemagnification is limited by the resolving power possible with the wavelength of visiblelight.SummaryAdvantages • Simplicity of setup with only basic equipment required.Limitations • Very low contrast of most biological samples. • Low apparent optical resolution due to the blur of out of focus material. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  44. 44. Modern Techniques of Materials Characterisation 44 • The sample has to be stained before viewing. Therefore, live cells cannot be viewed.Enhancements • Reducing or increasing the amount of the light source via the iris diaphragm. • Use of an oil immersion objective lens and a special immersion oil placed on a glass cover over the specimen. Immersion oil has the same refraction as glass and improves the resolution of the observed specimen. • Use of sample staining methods for use in microbiology, such as simple stains (Methylene blue, Safranin, Crystal violet) and differential stains (Negative stains, flagellar stains, endospore stains). • Use of a colored (usually blue) or polarizing filter on the light source to highlight features not visible under white light. The use of filters is especially useful with mineral samples. Comparison of transilumination techniques used to generate contrast in a sample of tissue paper. 1.559 μm/pixel. • Dark field illumination, sample contrast comes from light scattered by the sample. Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  45. 45. Modern Techniques of Materials Characterisation 45 • Bright field illumination, sample contrast comes from absorbance of light in the sample.Transmission electron microscopyTransmission electron microscopy (TEM) is a microscopy technique whereby a beamof electrons is transmitted through an ultra thin specimen, interacting with the specimenas it passes through. An image is formed from the interaction of the electrons transmittedthrough the specimen; the image is magnified and focused onto an imaging device, suchas a fluorescent screen, on a layer of photographic film, or to be detected by a sensor suchas a CCD camera.TEMs are capable of imaging at a significantly higher resolution than light microscopes,owing to the small de Broglie wavelength of electrons. This enables the instruments userto examine fine detail—even as small as a single column of atoms, which is tens ofthousands times smaller than the smallest resolvable object in a light microscope. TEMforms a major analysis method in a range of scientific fields, in both physical andbiological sciences. TEMs find application in cancer research, virology, materials scienceas well as pollution and semiconductor research.At smaller magnifications TEM image contrast is due to absorption of electrons in thematerial, due to the thickness and composition of the material. At higher magnificationscomplex wave interactions modulate the intensity of the image, requiring expert analysisof observed images. Alternate modes of use allow for the TEM to observe modulations inchemical identity, crystal orientation, electronic structure and sample induced electronphase shift as well as the regular absorption based imaging.The first TEM was built by Max Knoll and Ernst Ruska in 1931, with this groupdeveloping the first TEM with resolving power greater than that of light in 1933 and thefirst commercial TEM in 1939.Uses: Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  46. 46. Modern Techniques of Materials Characterisation 46The transmission electron microscope is used to characterize the microstructure ofmaterials with very high spatial resolution. Information about the morphology, crystalstructure and defects, crystal phases and composition, and magnetic microstructure canbe obtained by a combination of electron-optical imaging (sub-Ångstrom in the Titan, 2.5Å point resolution in the Tecnai), electron diffraction, and small probe capabilities.Further, the Titan provides significant in situ capabilities, allowing for the investigationof how material structure can evolve due to different environmental factors. The trade-offfor this diverse range of structural information and high resolution is the challenge ofproducing very thin samples for electron transmission.Principles of operation:The transmission electron microscope uses a high energy electron beam transmittedthrough a very thin sample to image and analyze the microstructure of materials withatomic scale resolution. The electrons are focused with electromagnetic lenses and theimage is observed on a fluorescent screen, or recorded on film or digital camera. Theelectrons are accelerated at several hundred kV, giving wavelengths much smaller thanthat of light: 200kV electrons have a wavelength of 0.025Å. However, whereas theresolution of the optical microscope is limited by the wavelength of light, that of theelectron microscope is limited by aberrations inherent in electromagnetic lenses, to about1-2 Å.Because even for very thin samples one is looking through many atoms, one does notusually see individual atoms. Rather the high resolution imaging mode of the microscopeimages the crystal lattice of a material as an interference pattern between the transmitted Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  47. 47. Modern Techniques of Materials Characterisation 47and diffracted beams. This allows one to observe planar and line defects, grainboundaries, interfaces, etc. with atomic scale resolution. The brightfield/darkfieldimaging modes of the microscope, which operate at intermediate magnification,combined with electron diffraction, are also invaluable for giving information about themorphology, crystal phases, and defects in a material. Finally the microscope is equippedwith a special imaging lens allowing for the observation of micromagnetic domainstructures in a field-free environment.The TEM is also capable of forming a focused electron probe, as small as 20 Å, whichcan be positioned on very fine features in the sample for microdiffraction information oranalysis of x-rays for compositional information. The latter is the same signal as that usedfor EMPA and SEM composition analysis (see EMPA facility), where the resolution is onthe order of one micron due to beam spreading in the bulk sample. The spatial resolutionfor this compositional analysis in TEM is much higher, on the order of the probe size,because the sample is so thin. Conversely the signal is much smaller and therefore lessquantitative. The high brightness field-emission gun improves the sensitivity andresolution of x-ray compositional analysis over that available with more traditionalthermionic sources.Restrictions on Samples:Sample preparation for TEM generally requires more time and experience than for mostother characterization techniques. A TEM specimen must be approximately 1000 Å orless in thickness in the area of interest. The entire specimen must fit into a 3mm diametercup and be less than about 100 microns in thickness. A thin, disc shaped sample with ahole in the middle, the edges of the hole being thin enough for TEM viewing, is typical.The initial disk is usually formed by cutting and grinding from bulk or thin film/substratematerial, and the final thinning done by ion milling. Other specimen preparationpossibilities include direct deposition onto a TEM-thin substrate (Si3N4, carbon); directdispersion of powders on such a substrate; grinding and polishing using special devices(t-tool, tripod); chemical etching and electropolishing; lithographic patterning of wallsand pillars for cross-section viewing; and focused ion beam (FIB) sectioning for sitespecific samples.Artifacts are common in TEM samples, due both to the thinning process and to changingthe form of the original material. For example surface oxide films may be introducedduring ion milling and the strain state of a thin film may change if the substrate isremoved. Most artifacts can either be minimized by appropriate preparation techniques orbe systematically identified and separated from real information.Sample preparationSample preparation in TEM can be a complex procedure. TEM specimens are required tobe at most hundreds of nanometers thick, as unlike neutron or X-Ray radiation the Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  48. 48. Modern Techniques of Materials Characterisation 48electron beam interacts readily with the sample, an effect that increases roughly withatomic number squared (z2).[14] High quality samples will have a thickness that iscomparable to the mean free path of the electrons that travel through the samples, whichmay be only a few tens of nanometers. Preparation of TEM specimens is specific to thematerial under analysis and the desired information to obtain from the specimen. As such,many generic techniques have been used for the preparation of the required thin sections.Materials that have dimensions small enough to be electron transparent, such as powdersor nanotubes, can be quickly prepared by the deposition of a dilute sample containing thespecimen onto support grids or films. In the biological sciences in order to withstand theinstrument vacuum and facilitate handling, biological specimens can be fixated usingeither a negative staining material such as uranyl acetate or by plastic embedding.Alternately samples may be held at liquid nitrogen temperatures after embedding invitreous ice.[35] In material science and metallurgy the specimens tend to be naturallyresistant to vacuum, but still must be prepared as a thin foil, or etched so some portion ofthe specimen is thin enough for the beam to penetrate. Constraints on the thickness of thematerial may be limited by the scattering cross-section of the atoms from which thematerial is comprised.Tissue sectioningBy passing samples over a glass or diamond edge, small, thin sections can be readilyobtained using a semi-automated method.[36] This method is used to obtain thin,minimally deformed samples that allow for the observation of tissue samples.Additionally inorganic samples have been studied, such as aluminium, although thisusage is limited owing to the heavy damage induced in the less soft samples. [37] Toprevent charge build-up at the sample surface, tissue samples need to be coated with athin layer of conducting material, such as carbon, where the coating thickness is severalnanometers. This may be achieved via an electric arc deposition process using a sputtercoating device.Sample stainingDetails in light microscope samples can be enhanced by stains that absorb light; similarlyTEM samples of biological tissues can utilize high atomic number stains to enhancecontrast. The stain absorbs electrons or scatters part of the electron beam which otherwiseis projected onto the imaging system. Compounds of heavy metals such as osmium, lead,or uranium may be used prior to TEM observation to selectively deposit electron denseatoms in or on the sample in desired cellular or protein regions, requiring anunderstanding of how heavy metals bind to biological tissues.Mechanical millingMechanical polishing may be used to prepare samples. Polishing needs to be done to ahigh quality, to ensure constant sample thickness across the region of interest. Adiamond, or cubic boron nitride polishing compound may be used in the final stages of Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  49. 49. Modern Techniques of Materials Characterisation 49polishing to remove any scratches that may cause contrast fluctuations due to varyingsample thickness. Even after careful mechanical milling, additional fine methods such asion etching may be required to perform final stage thinning.Chemical etchingCertain samples may be prepared by chemical etching, particularly metallic specimens.These samples are thinned using a chemical etchant, such as an acid, to prepare thesample for TEM observation. Devices to control the thinning process may allow theoperator to control either the voltage or current passing through the specimen, and mayinclude systems to detect when the sample has been thinned to a sufficient level of opticaltransparency.Ion etchingIon etching is a sputtering process that can remove very fine quantities of material. Thisis used to perform a finishing polish of specimens polished by other means. Ion etchinguses an inert gas passed through an electric field to generate a plasma stream that isdirected to the sample surface. Acceleration energies for gases such as argon are typicallya few kilovolts. The sample may be rotated to promote even polishing of the samplesurface. The sputtering rate of such methods is on the order of tens of micrometers perhour, limiting the method to only extremely fine polishing.More recently focussed ion beam methods have been used to prepare samples. FIB is arelatively new technique to prepare thin samples for TEM examination from largerspecimens. Because FIB can be used to micro-machine samples very precisely, it ispossible to mill very thin membranes from a specific area of interest in a sample, such asa semiconductor or metal. Unlike inert gas ion sputtering, FIB makes use of significantlymore energetic gallium ions and may alter the composition or structure of the materialthrough gallium implantation.[38]Selected area (electron) diffraction:Selected area (electron) diffraction (abbreviated as SAD or SAED), is acrystallographic experimental technique that can be performed inside a transmissionelectron microscope (TEM).In a TEM, a thin crystalline specimen is subjected to a parallel beam of high-energyelectrons. As TEM specimens are typically ~100 nm thick, and the electrons typicallyhave an energy of 100-400 kiloelectron volts, the electrons pass through the sampleeasily. In this case, electrons are treated as wave-like, rather than particle-like (see wave-particle duality). Because the wavelength of high-energy electrons is a fraction of ananometer, and the spacings between atoms in a solid is only slightly larger, the atomsact as a diffraction grating to the electrons, which are diffracted. That is, some fraction of Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  50. 50. Modern Techniques of Materials Characterisation 50them will be scattered to particular angles, determined by the crystal structure of thesample, while others continue to pass through the sample without deflection.As a result, the image on the screen of the TEM will be a series of spots—the selectedarea diffraction pattern, SADP, each spot corresponding to a satisfied diffractioncondition of the samples crystal structure. If the sample is tilted, the same crystal willstay under illumination, but different diffraction conditions will be activated, anddifferent diffraction spots will appear or disappear. Fig. : SADP of a single austenite crystal in a piece of steelSAD is referred to as "selected" because the user can easily choose from which part ofthe specimen to obtain the diffraction pattern. Located below the sample holder on theTEM column is a selected area aperture, which can be inserted into the beam path. Thisis a thin strip of metal that will block the beam. It contains several different sized holes,and can be moved by the user. The effect is to block all of the electron beam except forthe small fraction passing through one of the holes; by moving the aperture hole to thesection of the sample the user wishes to examine, this particular area is selected by theaperture, and only this section will contribute to the SADP on the screen. This isimportant, for example, in polycrystalline specimens. If more than one crystal contributesto the SADP, it can be difficult or impossible to analyze. As such, it is useful to select asingle crystal for analysis at a time. It may also be useful to select two crystals at a time,in order to examine the crystallographic orientation between them.As a diffraction technique, SAD can be used to identify crystal structures and examinecrystal defects. It is similar to x-ray diffraction, but unique in that areas as small asseveral hundred nanometers in size can be examined, whereas x-ray diffraction typicallysamples areas several centimeters in size.A diffraction pattern is made under broad, parallel electron illumination. An aperture inthe image plane is used to select the diffracted region of the specimen, giving site- Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  51. 51. Modern Techniques of Materials Characterisation 51selective diffraction analysis. SAD patterns are a projection of the reciprocal lattice, withlattice reflections showing as sharp diffraction spots. By tilting a crystalline sample tolow-index zone axes, SAD patterns can be used to identify crystal structures and measurelattice parameters. SAD is essential for setting up DF imaging conditions. Other uses ofSAD include analysis of: lattice matching; interfaces; twinning and certain crystallinedefects [1].SAD is used primarily in material science and solid state physics, and is one of the mostcommonly used experimental techniques in those fields.Scanning electron microscopeThe scanning electron microscope (SEM) is a type of electron microscope that imagesthe sample surface by scanning it with a high-energy beam of electrons in a raster scanpattern. The electrons interact with the atoms that make up the sample producing signalsthat contain information about the samples surface topography, composition and otherproperties such as electrical conductivity.Scanning process and image formation Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119
  52. 52. Modern Techniques of Materials Characterisation 52 Fig. : Schematic diagram of an SEM.In a typical SEM, an electron beam is thermionically emitted from an electron gun fittedwith a tungsten filament cathode. Tungsten is normally used in thermionic electron gunsbecause it has the highest melting point and lowest vapour pressure of all metals, therebyallowing it to be heated for electron emission, and because of its low cost. Other types ofelectron emitters include lanthanum hexaboride (LaB6) cathodes, which can be used in astandard tungsten filament SEM if the vacuum system is upgraded and field emissionguns (FEG), which may be of the cold-cathode type using tungsten single crystal emittersor the thermally-assisted Schottky type, using emitters of zirconium oxide.The electron beam, which typically has an energy ranging from 0.5 keV to 40 keV, isfocused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter. Thebeam passes through pairs of scanning coils or pairs of deflector plates in the electroncolumn, typically in the final lens, which deflect the beam in the x and y axes so that itscans in a raster fashion over a rectangular area of the sample surface.When the primary electron beam interacts with the sample, the electrons lose energy byrepeated random scattering and absorption within a teardrop-shaped volume of thespecimen known as the interaction volume, which extends from less than 100 nm toaround 5 µm into the surface. The size of the interaction volume depends on theelectrons landing energy, the atomic number of the specimen and the specimens density.The energy exchange between the electron beam and the sample results in the reflectionof high-energy electrons by elastic scattering, emission of secondary electrons byinelastic scattering and the emission of electromagnetic radiation, each of which can be Compiled by : Mr. B. Ramesh, Associate Professor of Mechanical Engineering, St. Joseph’s College of Engineering, Jeppiaar Trust, Chennai-119

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