Thermal Analysis

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Thermal Analysis

DTA & DSC

Dr. Prafulla Kumar Sahu

M.Pharm., Ph.D.

Raghu College of Pharmacy, Visakhapatnam.

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Thermal analysis

•

• Therma Thermal analysis l analysis includes a grincludes a group of techoup of techniques in whicniques in which specifich specific

physical properties of a material are measured as a function of physical properties of a material are measured as a function of temperature.

temperature.

•

• The techniques The techniques include the meainclude the measurement surement of temperaof temperatures at whtures at whichich

changes may occur, the measurement of the energy absorbed changes may occur, the measurement of the energy absorbed

((endothermic transitionendothermic transition ) ) or evolved (or evolved (exothermic transitionexothermic transition ) ) during aduring a phase transition or a chemical reaction, and the assessment of

phase transition or a chemical reaction, and the assessment of physical changes resulting from changes in temperature.

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Introduction

•

Examples of properties:

–

– Heat-reHeat-related phase changes and lated phase changes and degradations,degradations,

crystallizations, heat capacities, heats of reaction, crystallizations, heat capacities, heats of reaction, glass transitions, curing rates for adhesives, and glass transitions, curing rates for adhesives, and weight changes.

weight changes. •

•

The properties ar

The pr

operties are observed either

e observed either by

by

monitoring temperature or heat flow in and

out of the

out of the sample or by monitoring the sample

sample or by monitoring the sample

weight during the process.

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Classification

•

• Differential thermal analysisDifferential thermal analysis (DTA) is a technique in(DTA) is a technique in

which the temperature difference between the sample which the temperature difference between the sample tested and a reference material is measured while both tested and a reference material is measured while both are subjected to the controlled temperature program. are subjected to the controlled temperature program.

•

• Differential scanning calorimetryDifferential scanning calorimetry (DSC) is a technique(DSC) is a technique

in which the heat flow difference between the sample and in which the heat flow difference between the sample and reference material is monitored while both are subjected reference material is monitored while both are subjected to the controlled temperature program.

to the controlled temperature program.

•

• Thermogravimetric analysisThermogravimetric analysis (TGA) is a technique in(TGA) is a technique in

which the weight of a

which the weight of a sample is monitored during thesample is monitored during the controlled temperature program.

controlled temperature program.

•

• Thermomechanical analysisThermomechanical analysis (TMA)(TMA) •

• Dynamic mechanical analysisDynamic mechanical analysis •

• Enthalpimetric analysisEnthalpimetric analysis •

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T

Trends

rends

•

Various

Various environments

environments

(vacuum, inert, or

controlled gas composition) and

controlled gas composition) and heating rates

heating rates

from 0.1 to 500°C/min are available for

temperatures

ranging from −190 to 1400°C.

•

The anal

The analysis of g

ysis of gas(es) r

as(es) released

eleased by the

by the

specimen as a function of temperature is

possible when thermal analysis equipment is

coupled

with

with Fourier

Fourier

-transform

infrared

detection or with a

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Applications

•

Environmental measurements:

vapor pressure,vapor pressure,

thermal stability, flammability, softening thermal stability, flammability, softening temperatur

temperatures, and boilies, and boiling points.ng points.

•

Compositional analysis:

phase diagrams, freephase diagrams, free

versus bound water, solvent retention, additive versus bound water, solvent retention, additive analysis, mineral characterization, and polymer analysis, mineral characterization, and polymer system analysis.

system analysis.

•

Important area of product reliability:

heat-capacityheat-capacity

data, liquid-crystal transitions, solid fat index, purity, data, liquid-crystal transitions, solid fat index, purity, polymer cures, polymer quality control, glass

polymer cures, polymer quality control, glass transitions, Curie point, and fiber properties. transitions, Curie point, and fiber properties.

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•

Information on

stability

:

modulus changes,

creep studies, expansion coefficients, and

antioxidant evaluation.

•

Dynamic properties of materials:

visco-elastic

measurements, impact resistance, cure

characteristics, elastic modulus, loss modulus,

and shear modulus.

•

Chemical reactions:

heats of transition,

reaction kinetics, catalyst evaluation, metal–

gas reactions, and crystallization phenomena.

Applications

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DTA & DSC

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Principles

•

Both of these methods relate to the

monitoring of the heat absorbed or evolved

during the heating of a sample and a

refer

reference

ence in equivalent

in equivalent environ

environments.

ments.

•

Differential thermal analysis (DTA)

monitors

monitors temperature difference

temperature difference

, while

differential scanning calorimetry (DSC)

measures the

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•

• If an inert sample, such asIf an inert sample, such as aluminaalumina, is heated at , is heated at a constant rate of 10°Ca constant rate of 10°C

min−1, the

min−1, the temperature-temperature-against-time against-time curvecurve is practically ais practically a straight linestraight line..

•

• AA sample that reacts or meltssample that reacts or melts within the temperature range studied willwithin the temperature range studied will

give

give small changes on its small changes on its temperature-timtemperature-time curvee curve. By heating both a. By heating both a reactive sample and an inert reference together at the same rate, these reactive sample and an inert reference together at the same rate, these small differences may be

small differences may be detecteddetected andand amplifiedamplified as a function of as a function of temperature.

temperature.

•

• Example:Example: If 10 mg of metallic indiumIf 10 mg of metallic indium are heated as sample and a are heated as sample and a similarsimilar

amount of alumina as referenc

amount of alumina as reference, both heat at nearly the e, both heat at nearly the same rate untilsame rate until around

around 156°C the indium starts to melt156°C the indium starts to melt. This absorbs energy and the. This absorbs energy and the temperature of the indium rises less fast. This goes on

temperature of the indium rises less fast. This goes on until all the indiumuntil all the indium has melted when the temperatures of the liquid

has melted when the temperatures of the liquid indium and aluminaindium and alumina again rise at the same rate.

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•

T

Two alternative strategies can now be

wo alternative strategies can now be

adopted.

–

Differential thermal analysis or DTA

strategy

–

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DTA strategy

•

• If the temperatures of sample S and reference R areIf the temperatures of sample S and reference R are

measured and the temperature difference recorded:

measured and the temperature difference recorded: DTDTAA strategy

strategy

ΔT = TS − TR ΔT = TS − TR

•

• A downward peak (i.e. a minimum) is recorded. UnderA downward peak (i.e. a minimum) is recorded. Under

carefully controlled instrumental conditions, this may be carefully controlled instrumental conditions, this may be relate

related to the d to the enthalpy change for the thermal event:enthalpy change for the thermal event:

•

• Where, A is the aWhere, A is the area of the rea of the temperature-titemperature-time peak fme peak fromrom

initial (i) to final ( f ) point initial (i) to final ( f ) point ..

•

• This leads to quantitati This leads to quantitative orve or heat-flux differentialheat-flux differential

scanning calorimetry

scanning calorimetry (heat-flux DSC). The negative sign is(heat-flux DSC). The negative sign is requir

required since ed since the enthalpy change on the enthalpy change on melting is positive,melting is positive, but

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Power-compensated DSC

strategy

•

The second

The second strategy is to c

strategy is to contro

ontrol the amou

l the amount of

nt of

heat supplied to sample and reference so that

their temperatures stay as nearly the same as

possible.

•

Using separate heaters for sample and

reference allows measurement of the

difference in power

difference in power ΔP

ΔP to be measured. With

to be measured. With

proper control and calibration, this will give the

enthalpy change of the peak directly:

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•

• A reference material that does not melt A reference material that does not melt in the temperaturein the temperature

range. Its temperature would match the temperature of range. Its temperature would match the temperature of the surroundings (TE) for the entire

the surroundings (TE) for the entire temperature program.temperature program.

•

• Consider plotting the difference (Consider plotting the difference ( ΔT = TS − TR ΔT = TS − TR) between) between

the temperature of the sample (TS)

the temperature of the sample (TS) and the temperature of and the temperature of the reference material (TR) vs. the temperature of the

the reference material (TR) vs. the temperature of the surro

surroundings. Initially, there would undings. Initially, there would be no be no difference,difference, ΔT = ΔT = 0,

0, since the sample and surroundings are heated equally.since the sample and surroundings are heated equally.

•

• However, when the sample melts, TS lags behind TRHowever, when the sample melts, TS lags behind TR

temporarily, making

temporarily, making ΔT ΔT negative.negative.

•

• AfAfter melting is ter melting is complete, the sample catches up complete, the sample catches up such thatsuch that

the two temperatures are again equal,

the two temperatures are again equal, ΔT = 0 ΔT = 0..

•

• A plot of A plot of ΔT ΔT vs. TEvs. TE then results a negative peak in the DTAthen results a negative peak in the DTA

curve when the sample melts. curve when the sample melts.

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Endothermic curve

•

• The negative peak s The negative peak shown is a rhown is a result of anesult of an endothermic processendothermic process

(a process that absorbs heat) such as

(a process that absorbs heat) such as melting.melting.

•

• Other endothermic processOther endothermic processes, other es, other than melting, would than melting, would alsoalso

produce a negative peak. produce a negative peak.

•

• Examples: a chemical reaction or a Examples: a chemical reaction or a decomposition.decomposition. •

• The particular char The particular characteristics of this peak (shacteristics of this peak (shape, width,ape, width,

sharpness, smoothness, etc.) provide clues concerning the sharpness, smoothness, etc.) provide clues concerning the

sample composition and properties that are the object of a DTA. sample composition and properties that are the object of a DTA.

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•

• ExothExothermic processes (processes that evolve ermic processes (processes that evolve heat) mayheat) may

also occur during

also occur during the experiment.the experiment.

•

• This would pr This would produce a suroduce a surge in the samge in the sample temperaturple temperature, ite, it

would produce a positive peak in the

would produce a positive peak in the ΔT ΔT vs. TEvs. TE plot.plot.

•

• ExothExothermic processes include crystallization as ermic processes include crystallization as well aswell as

some chemical

some chemical and decomposition reactions.and decomposition reactions.

Exothermic curve

DTA curve of DTA curve of

the the exothermic exothermic process process

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DTA curve of kaolinite

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Schematic diagram of DTA or DSC Schematic diagram of DTA or DSC

apparatus apparatus

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Instrumentation

•

The temperature, both for the

sample and the

sample and the refer

reference and also

ence and also

the furnace is measured by

thermocouples, or resistance

sensors.

•

Higher sensitivity and greater

stability are obtained if multiple

sensors of inert material are used.

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Factors influencing thermal analysis

•

• The sampleThe sample is generally about 10 mg of powder, fibers oris generally about 10 mg of powder, fibers or

reactants such as monomers for plastic production. These reactants such as monomers for plastic production. These are placed into the

are placed into the cruciblecrucible,, which should be unreactivewhich should be unreactive

and stable over the temperature range used. Platinum, silica, and stable over the temperature range used. Platinum, silica, aluminum, or alumina crucibles are commonly used. The

aluminum, or alumina crucibles are commonly used. The sample and reference pans (either with alumina powder or sample and reference pans (either with alumina powder or sometimes an empty pan) are placed in their holders within sometimes an empty pan) are placed in their holders within the furnace, generally a wire-wound electrical heater

the furnace, generally a wire-wound electrical heater controlled by the computer program.

controlled by the computer program.

•

• The rate of heatingThe rate of heating is user-determined, often about 10 K is user-determined, often about 10 K

min−1, but for the best approach to equilibrium, low heating min−1, but for the best approach to equilibrium, low heating rates are needed, and isothermal experiments may also be rates are needed, and isothermal experiments may also be carried out. High heating rates save time, and can simulate carried out. High heating rates save time, and can simulate situations like burning.

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•

The atmosphere

surrounding the

samples can be controlled. A slow flow of

nitrogen gas will give an almost inert

atmosphere and sweep away harmful

products. Oxygen may be used to study

the oxidative stability of polymers. Carbon

dioxide will react with some oxides to form

carbonates.

•

The mass

of the sample, together with its

volume and packing is important since

these determine the heat transfer and the

diffusion of gases across the sample.

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Applications

•

Unlike other thermal methods, DTA and DSC are

not compound-specific, they are still most

important test methods for a wide

important test methods for a wide variety of

variety of

disciplines and materials.

•

Inorganic materials, salts

Inorganic materials, salts and complex

and complexes :

es :

physical prop

physical properties, chemical changes

erties, chemical changes and

and

qualitative thermal behavior.

•

Minerals and fuel (coal and oil)

•

New materials (e.g. liquid crystals) are

discover

discovered, DSC is frequently used to

ed, DSC is frequently used to test.

test.

•

The greatest use is in the phar

The greatest use is in the pharmaceutical and

maceutical and

polymer industries.

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•

• One special use of DSC for physical changes is theOne special use of DSC for physical changes is the

determination of

determination of purity.purity.

–

– While a pure substance melts sharply, perhaps overWhile a pure substance melts sharply, perhaps over

a few tenths of a degree near its true melting point, a few tenths of a degree near its true melting point, an impure sample may start to melt several

an impure sample may start to melt several

degrees below this temperature, and will give a degrees below this temperature, and will give a broad peak.

broad peak.

•

• Computer analysis of the shape of this peak allows anComputer analysis of the shape of this peak allows an

estimation of purity, but does not provide any estimation of purity, but does not provide any information on the nature of the impurities. information on the nature of the impurities.