Thermoanalytical techniques

Introduction
Thermal Analysis is the term applied to a group of methods and techniques in
which chemical or physical properties of a substance, a mixture of substances or a
reaction mixture are measured as function of temperature or time, while the
substances are subjected to a controlled temperature programme.
THERMOANALYTICAL TECHNIQUES
DIFFERENTIAL
SCANNING
CALORIMETRY
(DSC)
Thermogravimetry
(TG)
Differential
Thermal
Analysis
(DTA)
Thermo-
Mechanical
Analysis(TMA)
2

DIFFERENTIAL SCANNING CALORIMETER
Principle :
 It measures the heat into or out of a sample.
 The difference in the amount of heat required to
increase the temperature of a sample and reference is
measured as a function of temperature.
– The differences in heat flow occur with the occurrence
of two major events:
1) The heat capacity of the sample which increases with
temperature.
2) Transitions occur. 3

Instrumentation Basics:
• Types of DSC Instruments:
1. Power compensated DSC
2. Heat flux DSC
3. Modulated DSC
4. High sensitivity DSC (HS-DSC)
6

• In power compensated DSC, the base of the sample holder unit is in direct
contact with a reservoir of coolant.
• Resistance sensor measures the temperature of the base of the holder and a
resistance heater.
• Phase changes in sample temperature difference is detected
electrical power gets supplied to bring temperature difference below
threshold value.
• Temperatures of both are increased or decreased linearly.
• The power needed to maintain the sample temperature equal to the
reference temperature is measured.
 Advantages over Heat flux DSC:
 Its response time is more rapid than heat flux DSC.
 It is also capable of higher resolution.
8

• In heat flux DSC, the difference in the heat flow into the sample and
reference is measured while the sample temperature is changed at a
constant rate.
• Both sample and reference are heated by a single heating unit.
• Heat flows into both the sample and reference material via an
electrically heated constant thermoelectric disk is given by
where,
f (T, t) is the kinetic response of the sample in J mol ˉˡ
10

3.Modulated DSC Instruments (MDSC) :
• In MDSC, a sinusoidal function is superimposed on the overall temperature
program.
• And produces a micro heating and cooling cycle as the overall temperature
is increased or decreased.
• The overall signal is mathematically deconvoluted into two parts
1. A reversing heat flow signal- Associated with the heat capacity component
of the thermogram .
2. Non- reversing heat flow signal- Associated with the kinetic processes.
 Step transitions appear only in the reversing heat flow signal.
 Exothermic and endothermic events may appear in either or in both
signals.
• It can be useful in resolving polymer glass transitions, which are difficult to
analyze owing to concomitant solvent evaporation.
11

Sample Preparation
• If possible, clean and dry your sample prior to DSC analysis.
• Wear gloves or use forceps when handling your DSC sample.
• DSC samples should be small enough.
• Powder samples or flat solid samples (less than 2 mm tall) work best.
• Sample weight should be between 0.5 and 100 mg.
DSC sample preparation procedures :
• Weigh sample about 0.5 mg with the analytical balance. Record the weight.
• Use forceps to place sample.
• Use forceps to place the aluminium pan lid on top of your sample.
• Use forceps to load the aluminium pan and sample into the sample encapsulating
press .
• Align the sample pan in the encapsulating press, and press down on the handle to
seal the aluminium pan.
• Use the empty pan as a reference sample.
• Need inert gas like nitrogen to avoid oxidation or decomposition.
12

Precaution :
Sample Decomposition
 The DSC is capable of heating samples to 600°C.
 Many materials may decompose during the heating, which can
generate hazardous by-products.
WARNING: If you are using samples that may emit harmful gases,
vent the gases in an appropriate manner. In general, samples
should not be heated above their decomposition temperatures to
prevent the release of hazardous materials.
13

Instrument Operation
• Powering up the instrument,
• Loading your sample,
• Setting your testing conditions,
• Running a scan and collecting
data
• Analyzing your results.
14

Applications
 Quantitative application includes the determination of heat of fusion and the extent
of crystallization for crystalline materials.
 Glass transition temperatures and melting points are useful for qualitative
classification of materials.
 DSC is used to study ‘aging’ and shelf life of pharmaceuticals, as well as other basic
research and development.
 In the Food Industry, DSC has numerous applications to monitor thermal events
discussed earlier such as melting, crystallization, etc, as well as decomposition,
denaturation , dehydration, polymorphism, oxidation, etc.
 Evaluation of metal catalysts by pressure DSC .
 Measurement of heat capacity


2
C
   
 Measurement of thermal conductivity

 

   
1
2
1
T
T
T
T
p
p dT
T
H C dT S
15

DIFFERENTIAL THERMAL ANALYSIS(DTA)
• DTA is a technique in which the difference in temperature
between a substance and a reference material is measured as a
function of temperature while the substance and reference
material are subjected to a controlled temperature program.
16

Fig: Differential thermogram showing types of changes encountered with
polymeric materials.
17

Principle
It is a technique in which the temperature of the substance under
investigation is compared with the temperature of a thermally inert material.
 Initial increase in ΔT is due to the glass transition.
 Tg is the charecteristic temperature at which glassy amorphous polymers
become flexible or rubberlike.
 When heated to the glass transition temperature, the polymer changes
from a glass to a rubber and results in no change in enthalpy.
 The heat capacity of rubber is different from that of glass , which results
in the lowering of the baseline. No peak appears during this transition
because of the zero enthalpy change.
18

INSTRUMENTATION
Sample holder
Sample and reference cells (Al)
Sensors
Pt or chromel/alumel thermocouples one for the sample and one for the
reference joined to differential temperature controller
Furnace
Alumina block containing sample and reference cells
Temperature controller
Controls for temperature program and furnace atmosphere
Recording system 20

Applications
Qualitative and Quantitative Identification of Minerals: Detection
of any minerals in a sample
Polymeric Materials: DTA useful for the characterization of
polymeric materials in the light of identification of thermo physical ,
thermo chemical, thermo mechanical and thermo elastic changes or
transitions.
Measurement of Crystalline: Measurement of the mass fraction of
crystalline material in semi crystalline polymers.
Analysis of Biological Materials: DTA curves are used to date bone
remains or to study archaeological materials.
21

Thermogravimetry (TG) :
• It is the study of weight changes of a specimen as a function of temperature.
• This technique is useful for transformations involving the absorption or evolution of
gases from a specimen consisting of a solid phase.
• Reactant(s)  Product(s) + Gas (a mass loss)
• Gas + Reactant(s)  Product(s) (a mass gain)
• A plot of mass versus temperature permits evaluation of thermal stabilities, rate of
reaction, reaction processes, and sample composition.
• Measurements of changes in sample mass with temperature are made using
thermobalance. The balance should be in a suitably enclosed system so that the
atmosphere can be controlled.
 Processes occurring without change in mass (e.g., the melting of a sample)
obviously cannot be studied by TG.
22

Perkin Elmer TGA7 Thermogravimetric Analyser
23

Instrumentation
1. Recording balance
2. Furnace
3. Furnace programmer or
controller
4. Recording device
24

1)Microbalance:
It is used to record a change in mass of sample/substance.An ideal
microbalance must possess following features:
• It should have accuracy and reproducibility and rapid response.
• It should provide electronic signals to record the change in mass using a
recorder.
• It should have the capacity of auto-weight/mass adjustment.
• It should be mechanically and electrically stable and user friendly.
• The thermobalance has a clamp which is used to hold the microbalance arm.
After the sample has been placed on microbalance, it is left for 10-15 min to
stabilize.
25

2)Sample holder
Available with different size and shape depending upon the weight of sample.
Made up of glass, quartz, aluminium, stainless steel etc.
2 TYPES 1)Shallow pan for holding samples which eliminates gas, vapours or volatile
matter by diffusion during heating.
2)Deep crucible
3)Furnace
4)Furnace temperature programmer
o These are the controller which can provide gradual rise of temperature at a fixed rate.
o This device has a coarse and fine control knobs through which desired temperature
with respect to rate/ time can be obtained.
o This controlling is done by increasing voltage through the heated element by motor
driven variable transformer or by different thermocouples.
27

INTERPRETATION OF TG
• We can consider that the TG curve directly indicates the characteristic
of that compound.
• This is due to the sequence of physicochemical events that occur
under particular conditions over the temperature range
• Ex:1. calcium carbonate
decomposes –single step(800oC–900o C)
forms calcium oxide (a stable solid) & gas CO2
CaCO3 (s) CaO (s) + CO2 (g)
• Ex: 2 ammoniun nitrate
NH4NO3 (g) N2O (g) + 2H20 (g)
(300 Oc) (two volatile prods)
28

• Ex: calcium oxalate monohydrate (CaC2O4.H2O) is heated
in air up to 1000 oC.
• We see a well separated three thermal decompositions
Step 1: hydrate water is lost:
CaC2O4. H2O (s) CaC2O4 (s) + H2O
Step 2 : Anhydrous salt decomposes
CaC2O4 (s) CaCO3 (s) + CO (g)
Step 3: carbonate decomposes.
CaCO3 (s) CaO (s) + CO2 (g)
31

APPLICATIONS OF TG :
 Determining the purity and thermal stability of both primary and
secondary standards.
 Investing the correct drying temperature and the suitability of various
weighing forms for gravimetric analysis.
 Determining the composition of alloys and mixtures.
Eg. For calcium and strontium binary mixture. Here the
decomposition temperature for CaCO3 is 65̊C to 85̊C.
Where for strontium carbonate the decomposition temperature is 95̊C
to 115̊C.
Both carbonates decompose to their oxides with the evolution of CO2.
Hence amount of Ca and strontinum present in the mixture may be
calculated from weight losses due to the evolution of CO2 at the
lower and higher temperature respectively.
32

Thermo-Mechanical Analysis(TMA)
 It provides measurements of penetration, expansion, contraction and extension
of materials as a function of temperature.
 Volume expansion characteristics of samples are measured by placing the
sample in a quartz cylinder fitted with a flat tipped quartz probe in a cylinder
piston arrangement.
 Sample volume changes are translated into linear motion of the piston.
 In this technique, dimensional changes in a sample are the primary
measurement, while the sample is heated, cooled, or studied at a fixed
temperature.
33

Principle:
 A dilatometer is used to determine the linear thermal expansion of a
solid as a function of temperature.
 In this a small load acts on the specimen .
 The measured expansion of the specimen can be used to determine
the coefficient of linear thermal expansion .
 The 1st heating phase yields information about the actual state of the
specimen, including its thermal and mechanical history. When
thermoplastics soften, especially above the glass transition,
orientations and stresses may relax, as a result of which post
crystallization and recrystallization processes may occur.
 To determine the coefficient of expansion as a material
characteristic, the material must undergo reversible changes and so
forth during a 2nd heating phase that has followed controlled
cooling.
34

 The sample sits on a support within the furnace. Resting upon it is a probe to
sense changes in length, which are measured by a sensitive position
transducer, normally a Linear Variable Displacement Transducer (LVDT).
 The probe and support are made from a material such as quartz glass
(vitreous silica), which has a low, reproducible, and accurately known
coefficient of thermal expansion, and also has low thermal conductivity,
which helps to isolate the sensitive transducer from the changing
temperatures in the furnace.
 The quartz probe contains a thermocouple which measures the sample
temperature.
 Any movement of the sample is translated into a movement of the
transformer core and results in an output that is proportional to the
displacement of the probe, and whose sign is indicative of the direction of
movement.
 The temperature range is from liquid nitrogen to 850̊C.
 In this technique (TMA), dimensional changes in a sample are the primary
measurement, while the sample is heated, cooled, or studied at a fixed
temperature.
36

Typical probes:
•Where the specimen is large enough, a
macroprobe with a contact area of approx. 28
mm2 is used.
• For smaller specimens, for example, with an
edge length less than 6 mm, a normal probe
with a contact area of approx. 5 mm2 is
suitable.
•Penetration probes with a small contact
area of just about 0.8 mm2 are suitable for
determining the glass transition temperature.
37

Calibration
• The TMA apparatus must be calibrated in terms of length and
temperature. If the loading force is applied electromagnetically, it
may be calibrated with the aid of known weights.
• A new calibration must be performed every time a probe is changed.
Calibration and actual test measurements must be performed
using the same probe geometry, applied load, heating rate, and
temperature range (starting temperature). The specimen should be
the same size as the calibration specimen.
• The calibration factor, F, is calculated as follows:
F=
 Calibrating the Length
 Calibrating the Temperature
38

Samples
• TMA samples should be of 2-6 mm in diameter and 2-10 mm in
height.
• Samples for thermal expansion and glass transition
measurements should have parallel and flat top and bottom
surfaces.
• NOTE: To avoid making a mess with molten thermoplastics, you
should encapsulate thermoplastic polymer samples in aluminum
foil.
• To avoid reactions between molten metals and the quartz probe
and stage, you should sandwich your metallic sample between
two plates of aluminum oxide.
39

Instrument Operation
• Powering up the instrument,
• Loading your sample,
• Setting your testing conditions,
• Running a scan and collecting data, and Analyzing your results.
40

Precaution:
Sample Decomposition.
 The TMA is capable of heating samples to 1000°C.
 Many materials may decompose during the heating, which can
generate hazardous by products.
WARNING: If you are using samples that may emit harmful
gases, vent the gases in an appropriate manner. In general,
samples should not be heated above their decomposition
temperatures to prevent the release of hazardous materials or
contamination of the TMA
41

APPLICATIONS:
 Thermo mechanical measurements are routinely
used to investigate mechanical stability and
measure thermal expansion coefficients.
 Use of the technique on the small scale as a
micro-analytical tool to identify the distribution of
two materials within a matrix and also on a larger
scale to investigate the firing of a ceramic
material.
42

REFERENCES
1. Mita, T., “Recent Advances in Heat-Resistant Polymers.General Technical Centre
“Publication,No 8,p.117.
2. D. A. Skoog et al., “Principles of instrumental analysis”, fifth edition, Harcourt
Publishers, 2001.
3. Willard.”Instrumental methods of analysis”.seventh edition.pg. 761-776
4. Andrew J.Paasztor.”Handbook of instrumental techniques for analytical
chemistry”.pg.909-927
5. H.Kaur.”Instrumental methods of chemical analysis”.pg.979-991
6. Gurdeep Chatwal.”Thermal methods”.pg.2.701-2.759
7. Blaine,R.L.,Du Pont “Thermal Analysis Technical Report”,No.TA-40 Wilmington,DE,Du
pont,1989
8. Sharma B.K. Goel Publishing House “Instrumental Methods of Analysis” pg. 234-24337

Thermoanalytical techniques

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