Thermal Analysis

Differential Scanning Calorimetry (DSC)

� Principle

� DSC measures the differences in heat flow into a substance and a reference as a function of

sample temperature while both are subjected to a controlled temperature program

� DSC provides access to accurate thermodynamic data as well as information regarding reactivity and

phase transformations

DSC – types

� DSC is the most sophisticated and advanced of the thermal methods.

� There are two principal types:

� power compensated DSC

� heat-flux DSC

Power Compensated DSC-principles

� Temperature difference is maintained zero, i.e., ∆T = 0, by supplying heat into the sample or reference

according to heat emission or absorption

� Electrical power is proportional to heat change in the sample

i.e., P = I2.R

Power Compensated DSC-principles

� Rate of change of power input is plotted against average S & R temperature

� x-axis (abscissa) is temperature and the y-axis (ordinate) is difference in power input (which is

proportional to the heat change i.e., enthalpy)

Instrumentation

Furnace and sample holder

How does it work?

� Small, flat samples are contained in shallow pans, with the aim of making a good thermal contact between

sample, pan and heat flux plate.

� Symmetrical heating of the cell, and therefore S and R, is achieved by constructing the furnace from a

metal of high thermal conductivity – for example, silver

� Sample and reference material are heated by separate heaters in two independent furnaces

� The furnaces are imbedded in a large temperature-controlled heat sink

� Sample holders are above the furnaces

� Pt resistance thermometers are imbedded in the furnaces to monitor the temperatures of sample and

reference continuously

� There is provision for establishing gas flow through the cell, to sweep away volatiles, provide the required

atmosphere, and to assist in heat transfer.

� Control of the furnace, signal acquisition, and data storage and analysis are handled by a computer.

� Two control circuits are used to obtain differential thermograms

� One for average temperature control

� One for ∆T control

� S and R temp. signals are fed into a differential amplifier via a comparator circuit that determines which

is greater

� The amplifier output then adjusts the power input to the two furnaces in such a way that their

temperatures are kept identical

� Throughout the experiment, S and R are isothermal

� A signal proportional to difference in power input to the S and R furnaces is transmitted to the da

acquisition system

� The power differences are plotted as a function of the

Heat flux DSC

How does it work?

� S and R are heated by a single heater

� Differential heat flow into the S and R pans is monitored by chromel disk/constantan thermocouple

� Differential heat flow in to S and R pans is

thermocouple junctions

� Sample temp. is estimated by chromel/alumel junctio

Purge gases

� Typical purge gases are air/N

� He is useful for efficient heat transfer and removal of volatiles.

� Ar is preferred as an inert purge when examining samples that can react with nitrogen.

� The experiment can also be carried

A signal proportional to difference in power input to the S and R furnaces is transmitted to the da

are plotted as a function of the sample temperature and its unit is

single heater

into the S and R pans is monitored by chromel disk/constantan thermocouple

in to S and R pans is directly proportional to the difference in output

Sample temp. is estimated by chromel/alumel junction under the sample disk

Typical purge gases are air/N2

He is useful for efficient heat transfer and removal of volatiles.

Ar is preferred as an inert purge when examining samples that can react with nitrogen.

The experiment can also be carried out under a vacuum or under high pressure.

A signal proportional to difference in power input to the S and R furnaces is transmitted to the data

and its unit is milli Watts (mW)

into the S and R pans is monitored by chromel disk/constantan thermocouple

directly proportional to the difference in output of the two

Ar is preferred as an inert purge when examining samples that can react with nitrogen.

out under a vacuum or under high pressure.

Crucibles

Choice of crucible is critical.

• Thermal properties of crucible.

• Reactive properties with samples.

• Catalytic behaviour with samples.

Aluminum: inexpensive, low temp

Copper: used as catalyst (testing polymers)

Gold: higher temp, expensive

Platinum: still higher temp, expensive.

Alumina (Al2O3): very high temp

Sapphire: crystalline alumina, more chemically resistant than amorphous Al

DSC thermograms

• Thermal properties of crucible.

• Reactive properties with samples.

• Catalytic behaviour with samples.

(testing polymers)

Platinum: still higher temp, expensive.

Sapphire: crystalline alumina, more chemically resistant than amorphous Al2O3.

S – glass transition

Ex – exothermic reaction

En – endothermic reaction

Enthalpy changes

� The DSC curve may show an exothermic or endothermic peak

� The enthalpy changes associated with the events occurring are given by the area under the peaks.

� Peaks may be characterized by:

� Position (i.e., start, end, extrapolated onset and peak temperatures)

� Size (related to the amount of material and energy of the reaction)

� Shape (which can be related to the kinetics of the process)

Some possible processes giving enthalpic peaks

Process Exotherm Endotherm

Solid-solid transition * *

Crystallization *

Melting *

Vaporisation *

Sublimation *

Adsorption *

Desorption *

Desolvation (drying) *

Decomposition * *

Solid-solid reaction * *

Solid-liquid reaction * *

Solid-gas reaction * *

Curing

Polymerization

Catalytic reactions

Measurement of ∆H of thermal transitions

Calibration of DSC

� Temperature calibration is carried out by running standard materials, usually very pure metals with

accurately known melting points.

*

*

Catalytic reactions *

H of thermal transitions

is carried out by running standard materials, usually very pure metals with

accurately known melting points.

is carried out by running standard materials, usually very pure metals with

� Energy calibration may be carried out by using either known heats of fusion for metals, commonly

indium, or known heat capacities.

Effect of heating rate

� Slower heating rates will more accurately depict the onset temperature of transformation

� Two transformations which are very close in temperature range may be more distinctly seen as separate

peaks, whereas they may be mistaken for a singl

Applications of DSC in materials analysis

Polymer characterization

Exo

EndoH

eat flow

(m

W)

Slower heating rates will more accurately depict the onset temperature of transformation

Two transformations which are very close in temperature range may be more distinctly seen as separate

peaks, whereas they may be mistaken for a single transformation under a rapid heating rate

Applications of DSC in materials analysis

Temperature (0 C)

Slower heating rates will more accurately depict the onset temperature of transformation

Two transformations which are very close in temperature range may be more distinctly seen as separate

e transformation under a rapid heating rate

� Here is the DSC curve for a polymeric material such as high density polyethylene (HDPE). We see three

phase transition temperatures: glass transition temperature (Tg), crystallization temperature (Tc), and the

melting temperature (Tm)

Polymer characterization

� Tg may be used to identify polymers

� The amount or effectiveness of a plasticizer may be judged by how much it reduces Tg or affects the

shape of the transition.

� Examination of the transitions in polymer blends gives information as to their compatibility.

� Curing reactions result in an increase in Tg and measurements can be used to monitor the extent of cure.

Polymer characterization

� Thermogram of the blend shows two distinct Tg.

� Therefore, the components of this blend are immiscible in each other

� Tg also varies with chain length for a related group of polymers.

� Additional features occurring in the glass transition region, often a superimposed endothermic peak, are

related to the aging undergone by the material in the glassy state, and can sometimes obscure the

transition, making precise temperature measurement difficult.

exo

en

do

Polymer A

Polymer B

Blend of A+B

Analysis of explosives

� Ammonium perchlorate is an important component of high explosives. The stability of this material is

critical to their safe handling. Mechanism of decomposition was investigated.

Literature values for Ea 37 - 260 kJ/mol with different mechanisms proposed. This

and identified the activation energy as 115 kJ/mol.

Analysis of lubricants

� An important test in the automotive industry is to determine the stability of lubricating oils at elevated

temperatures and pressures. This will impact

brought to a high operating temperature and held there under an oxygen atmosphere.

perchlorate is an important component of high explosives. The stability of this material is

critical to their safe handling. Mechanism of decomposition was investigated.

260 kJ/mol with different mechanisms proposed. This work clarified the mechanism

and identified the activation energy as 115 kJ/mol.

An important test in the automotive industry is to determine the stability of lubricating oils at elevated

temperatures and pressures. This will impact its utility as a lubricant in motors. In this case, the oil is

brought to a high operating temperature and held there under an oxygen atmosphere.

perchlorate is an important component of high explosives. The stability of this material is

critical to their safe handling. Mechanism of decomposition was investigated.

work clarified the mechanism

An important test in the automotive industry is to determine the stability of lubricating oils at elevated

its utility as a lubricant in motors. In this case, the oil is

brought to a high operating temperature and held there under an oxygen atmosphere.

At some point, the oil begins to oxidize and then quickly decomposes exothermically. Note how the synth

has a much longer oxidation induction time (OIT) than does the mineral oil.

Guidelines for interpreting data

It is better to have some idea of what transitions to look for and why. For example:

1. What type of sample is it?

2. What type of transitions can it undergo?

3. What would any changes appear as?

4. What is the temperature range of interest?

5. What data are available from complimentary techniques, e.g. TGA?

6. Has there been any previous analysis?

Then examine the data:

1. Is the event an endotherm or exotherm?

2. Is the event repeatable on a fresh sample or on a reheat?

3. What happens on cooling?

4. Is the event the same in a sealed and unsealed pan?

5. Is the transition sharp or gradual, large or small?

6. Does the event look real? Thermal events are not normally excessively sharp.

At some point, the oil begins to oxidize and then quickly decomposes exothermically. Note how the synth

has a much longer oxidation induction time (OIT) than does the mineral oil.

It is better to have some idea of what transitions to look for and why. For example:

transitions can it undergo?

3. What would any changes appear as?

4. What is the temperature range of interest?

5. What data are available from complimentary techniques, e.g. TGA?

6. Has there been any previous analysis?

the event an endotherm or exotherm?

2. Is the event repeatable on a fresh sample or on a reheat?

4. Is the event the same in a sealed and unsealed pan?

5. Is the transition sharp or gradual, large or small?

ok real? Thermal events are not normally excessively sharp.

At some point, the oil begins to oxidize and then quickly decomposes exothermically. Note how the synthetic oil

Text book

� D. A. Skoog et al., Principles of instrumental analysis, fifth edition, Harcourt Publishers, 2001.

Reference

� R. F. Speyer, Thermal analysis of materials, Marcel Dekker, 1994.