Properties of glass

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Properties of glass

  1. 1. Contents Introduction..................................................................................................................................... 2 OPTICAL PROPERTIES OF GLASS............................................................................... 2 1. Refractive Index~(velocity of light in vacuo, or air)/(velocity of light in medium) ......... 2 Internal Reflection:...................................................................................................................... 3 2. Dispersion* and Abbe-number*........................................................................................... 3 3. Temperature Coefficient of Refractive Index (Δn/ΔT) ...................................................... 3 4. Stress-Optical Coefficient (B)............................................................................................... 4 5. Transmittance........................................................................................................................ 4 a. Internal Transmittance* (τ).................................................................................................. 4 b. Coloration Code* (λ80/λ5) .................................................................................................... 5 Thermal Properties ................................................................................................................ 6 1. Transformation Temperature* (Tg).................................................................................... 6 2. Sag Temperature (Ts) ........................................................................................................... 6 3. Strain Point (T10 14.5 )............................................................................................................... 6 4. Annealing Point (T10 13 ).......................................................................................................... 6 5. Softening Point (T107.6)........................................................................................................... 7 6. Thermal Conductivity (λ) ..................................................................................................... 7 7. Specific Heat (Specific Heat Capacity) (cp) ........................................................................ 7 References ............................................................................................................................... 7
  2. 2. GLASS Introduction A glass is an inorganic non metallic material that does not have a crystalline structure. Such materials are said to be amorphous and are virtually solid liquids cooled at such a rate that crystals have not been able to form.Typical glasses range from the soda-lime silicate glass for soda bottles to the extremely high purity silica glass for optical fibers. Glass is widely used for windows, bottles, glasses for drinking, transfer piping and recepticles for highly corrosive liquids, optical glasses, windows for nuclear applications etc. The main constituent of glass is silicon dioxide (SiO2). The most common form of silica used in glassmaking has always been sand.Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700o C. Adding other chemicals to sand can considerably reduce the temperature of the fusion. The addition of sodium carbonate ( Na2CO3) known as soda ash,in a quantity to produce a fused mixture of 75% Silica (SiO2) and 25% of sodium oxide (Na 2O), will reduce the temperature of fusion to about 800o C. However, a glass of this composition is water soluble and is known as water glass. In order to give the glass stability, other chemicals like Calcium Oxide (CaO) and magnesium oxide (MgO) are needed. The raw materials used for introducing CaO and MgO are their carbonates, limestone (CaCO3) and dolomite (MgCO3), which when subjected to high temperatures give off carbon dioxide leaving the oxides in the glass. OPTICAL PROPERTIES OF GLASS Glasses are among the few solids that transmit visible light • Thin film oxides might, but scattering from grains limit their thickness • Glasses form the basic elements of virtually all optical systems • World-wide telecommunications by optical fibers • Aesthetic appeal of fine glassware- 'crystal' chandeliers • High refractive index/birefringentPbO-based glasses • Color in cathedral windows, art glass, etc. 1. Refractive Index~(velocity of light in vacuo, or air)/(velocity of light in medium) Snell's Law: Note: unit less quantity n (air) = 1.0003 water = 1.33 sapphire = 1.77 diamond = 2.42 f-SiO2 = 1.458 heavy flint = 1.89
  3. 3. Internal Reflection: Critical angle (Brewster's angle) θc below which light is totally reflected: Note: larger n means greater θc, and so more light (from a broader distribution of incident angles) will be internally reflected. High index materials (diamonds, PbO glasses) look 'brilliant' when facets are cut so that internal reflection returns light from large faces that originally collected the light. Note too: internal reflection is important for transmission of light down an optical fiber. 2. Dispersion* and Abbe-number* The main dispersion is expressed by (nF-nc) and (nF'-nc’). The Abbe-number is defined: 3. Temperature Coefficient of Refractive Index (Δn/ΔT) The refractive index of optical glass changes with the temperature. The tem-perature coefficient of the refractive index, (Δn/ ΔT) abs., is measured at 20°C intervals between –40~80°C in a vacuum, using an interference-dilatometer to detect changes in both optical path length and dilation of the specimen. The light source used is a He-Ne gas laser (632.8nm).
  4. 4. For calculation of the temperature coefficient of the relative refractive index (Δn / ΔT) rel. in air at 101.325 kPa, the following equation is given: 4. Stress-Optical Coefficient (B) Ideally, the optical properties of glass are isotropic through fine annealing. Birefringence may be observed, however, when external forces are applied or when residual stresses are present (commonly the result of rapid cooling). The optical path difference δ (nm) associated with birefringence is linearly proportional to both the applied tensile or compressive stress, σ (105 Pa) and the thickness d (cm) of the specimen and is given by the following equation: The proportionality constant, B (10-12 / Pa), in this equation is proper constant of each glass type and referred to as the stress-optical coefficient. Stress-optical coefficients are obtained by measuring the optical path difference caused at the center of a glass disk with He-Ne laser light, when the disk is subject to a compressive load in a diametral direction. 5. Transmittance The transmittance characteristics of optical glasses in this catalog are expressed by two terms. One is "Internal Transmittance" and the other is "Coloration Code". a. Internal Transmittance* (τ) Internal transmittance (τ) refers to transmittance obtained by excluding reflection losses at the entrance and exit surfaces of the glass. Internal transmittance values over the wavelength range
  5. 5. from 280 to 1,550nm are calculated from transmittance measurements on a pair of specimens with different thicknesses. Internal transmittance values obtained for 5mm and 10mm thick glasses are given as τ5mm and τ10mm. The internal transmittance τ for glass with arbitrary thickness d can be obtained from these values by using: where ô0 refers, to the internal transmittance given in the tables for glass with thickness d0 equal to either 5 mm or 10 mm. b. Coloration Code* (λ80/λ5) Optical glasses exhibit almost no light absorption over a wavelength range ex-tending through the visible to the near infra-red. The spectral transmittance characteristics of optical glasses can be simply summarized with the coloration code λ80/λ5. The coloration code is determined in the following way. The internal transmittance of a specimen with thickness 10 ± 0.1mm is measured from 280nm to 700nm. Wavelengths are rounded off to the nearest 10nm and expressed in units of 10nm. λ80 is the wavelength for which the glass exhibits 80% transmittance while λ5 is the wavelength at which the glass exhibits 5% transmittance. For example, a glass with 80% transmittance at 398 nm and 5% transmittance at 362nm has a coloration code 40 / 36 The coloration code is generally applied for transmittance control of optical glasses. Fig.Designation of the Coloration Code in Spectral Transmittance curve.
  6. 6. Thermal Properties 1. Transformation Temperature* (Tg) The glass transformation temperature 'T'g refers to the temperature at which the glass transforms from a lower temperature glassy state to a higher temperature super-cooled liquid state. This behavior is illustrated in Fig. below which shows thermal expansion measured as a function of temperature. A differential thermal dilatometer is used for the measurement as it maintains a uniform temperature distribution within the furnace to ±1°C. As illustrated in the figure, the transformation temperature is determined by the intersection point of the two tangents of the high and low temperature ranges of the thermal expansion curve. 2. Sag Temperature (Ts) In the thermal expansion curve shown in Fig. above, the Sag Temperature (Ts) is defined as temperature at which thermal expansion stops increasing and actually begins to decrease with increasing temperature. This behavior is not due to an intrinsic property of the glass but is rather due to deformation of the glass under the load applied in these measurements. The viscosity of the glass at Ts corresponds to about 1010 to 1011 dPa•s. 3. Strain Point (T10 14.5 ) The strain point, T1014.5, represents a temperature at which internal stresses in a glass are relieved after a few hours. The viscosity of the glass at that temperature corresponds to about 1014.5 dPa•s. 4. Annealing Point (T10 13 ) The annealing point, T1013, represents a temperature at which internal stresses in a glass are relieved after a few minutes. The viscosity of the glass at that temperature corresponds to about 1013 dPa•s.
  7. 7. 5. Softening Point (T107.6) The softening point, T107.6, represents a temperature at which a glass begins to remarkably soften and deform under its own weight. The viscosity of the glass at that temperature corresponds about 107.6 dPa•s. 6. Thermal Conductivity (λ) The thermal conductivity ë is the quotient obtained by dividing the density of heat flow rate by the temperature gradient, that is, the quotient obtained by dividing the heat quantity transferring through a unit area in a unit time, by the temperature difference per unit distance, and expressed in W / (m•K). Note. 1 W / (m•K) = 8.600 0 x 10-1 kcal / (h•m•°C) = 2.388 89x 10-3 cal / (s•cm•°C) 7. Specific Heat (Specific Heat Capacity) (cp) The specific heat, cp, is the quotient obtained by dividing the heat capacity of a substance by the mass, that is, the heat quantity required for increasing the temperature of a substance of unit mass by one unit (1K or 1°C) and expressed in kJ / (kg • K). References 1. Optical Glass, HOYA CORPORATION USA OPTICS DIVISION http://www.hoyaoptics.com/ 2. Shelby Chapter 10, Optical Properties, Cer103 Notes, R.K. Brow 3. http://www.wikipedia.com/opticalglass

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