Lecture 6 Dta & Dsc01

8/14/2019 Lecture 6 Dta & Dsc01

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Lecture 6 DTA & DSC 01 1

DIFFERENTIAL THERMAL ANALYSIS (DTA)

AND

DIFFERENTIAL SCANNING CALORIMETRY (DSC)

The Differential Thermal Analysis (DTA) and Differential Scanning

Calorimetry (DSC) have always used to study the same phenomena.

Differential Thermal Analysis (DTA) is a thermal technique where the

sample temperature is compared with the temperature of a reference

(an inert material) and the temperature difference is recorded as a

function of the sample, reference or furnace temperature while the

sample is being heated or cooled at a constant rate.

The temperature change is detected by a differential method as shown

in Figure 6.1.


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Figure 6.1 The basic concept of DTA system


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The DTA method is very useful for the characterization of materials that willundergo physical or chemical changes by the heat effect without the mass

change.

The temperature change of the sample is due to either an endothermic orexothermic reaction or enthalpy change.

The related processes include: a phase change

fusion

melting crystal structural inversion

destruction of the crystal lattice

boiling

sublimation

evaporation

dehydration reaction

other reactions.

The phase change, dehydration, reduction and some of the decompositionprocesses produce endothermic effect, while crystallization, oxidation and

some of the decomposition reactions give exothermic effects.


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Comparison between the thermal method and the differential thermal

analysis are shown in Figure 6.2

The Normal Thermal Method:

In curves (a) and (b) the

temperature is measured as a

function of the furnace

temperature or time, while the

sample is continuously heated at a

constant rate of temperature

increase.

No enthalpy change is indicated by

the curve (a), but an endothermic

and exothermic enthalpy change is

indicated by the curves in (b).

Subsequently no deviation in curve

(a), but there are deviation in

curves (b): the change in Ti is dueto exothermic or endothermic

change.

These changes subside at Tf and

the sample temperature returns to

that of the furnace temperature.

The Differential Thermal Method:

In curve (c) the difference between the sample

temperature and that of the reference (TsTr) ismeasured as a function of the system

temperature, T.

At Ti, the curve deviates from horizontal and

forms an upward peak (exothermic) or

downward peak (endothermic).

The final reaction temperature Tf, does not occur

at the top of the peak, but lies on the higher

temperature side of the relevant peak. Its exactposition depends on the instrumental system

used.

Therefore, in a differential thermal method, a

very small temperature change can be detected

easily and the peak area is proportional to the

enthalpy change (H) and the sample mass.
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DTA curve is a plot of the temperature difference (T= Ts - Tr) versus the temperature

(T). Endothermic process is shown as a downward peak, while exothermic process

is shown as an upward peak (Figure 6.3)

Figure 6.3

Exothermic

Endothermic

Four types of transitions

detected by DTA method:

I second order transition,

where a change in the

horizontal line is detected

(e.g. glass transition);

II a sharp endothermic curve

due to the fusion or melting

process;

III a broad endothermic curve

due to a decomposition or

dissociation process;

IV an exothermic curve due to

the crystalline phase

change.


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Two approaches in the temperature sensing system of DTA (Figure 6.4):

Temperature Sensor in DTA

1. Classic DTA: the thermocouple is immersed into the sample and the reference materials

2. Heat flux DTA: the thermocouple is placed outside the sample and the reference

materials

Figure 6.4


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Differential Scanning Calorimetry (DSC)

DSC is a technique in which the difference in the heat flow (power) to the sample (container) and the

reference (container) is detected versus time or temperature while the sample is continuously heated

in a certain atmosphere

i) Power Compensated DSC: the sample and the reference are heated separately by

using different heaters and the temperature difference between the sample and the

reference is maintained zero while the electrical power needed to maintain that both

temperatures are the same (P= d(Q/dt) is measured (Figure 6.5)

Two Types of DSC:

Figure 6.5

The heat absorbed by the sample per unit time is

compensated accurately by the difference in the

electrical power (P) supplied to the heaters

P = dq / dt = . (CSCR)

The peak area is, therefore, directlyproportional to the value ofH. In the power

compensated DSC the calibration constant

needed to convert the peak area into joule is

the electricalconversion factor.


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The Effect of High Temperature

Most DSC systems are designed to measure heat capacity and H attemperatures of less than 700 oC. At higher temperatures, radiation (which

increases with the temperature rise) becomes more significant.

Most DTA systems work on the principle of heat conductivity at lowertemperatures where the heat radiation is low. At higher temperatures the heat

transition from its source to the sample takes place at faster rate and if the

thermocouple is placed below the sample holder, the system becomes less

sensitive.

When a sample undergoes a reaction, there is a temperature gradient in the

sample. In order to obtain a meaningful calorimetric data, the sample size should

be minimum to reduce the heat gradient in the sample. However, if the sample

consists of different materials it is usually difficult to obtain a sufficiently small butrepresentative sample.

The Effect of the Sample Size

Power Compensated DSC


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ii) Heat Flux DSC: the sample and the reference are heated using the sameheat source and the temperature difference T is measured (Figure 6.6). The

signal is converted into the power difference Pand this value is plotted versus

temperature or time

Figure 6.6 Heat flux DSC (Source: Haines F 3.4)

The DSC curve is a plot of P versus

temperature (or time). Since an endothermic

process involves absorption of more heat by the

sample than the reference, the process is

indicated by the upward peak.

In a heat flux DSC system thethermocouple is placed under the

sample and the reference containers

(Figure 6.8 (a)-(e)). The base line

variation is due to a change in the heat

capacity of the sample and it is not

influenced by other properties of the

sample. The DSC system can be usedfor the determination of heat capacity

of the material. However, since the

base line variation is influenced by the

physical properties of the sample

container material, the instrument

needs to be calibrated using a

standard material.


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The calibration constant Kis a thermal factor (varies with temperature) that is

used to convert the peak area into joule.

The main components of the DTA/DSC system is shown in Figure 6.7.

The area under the peak is directly proportional to the heat of reaction:

H = KTdt = K. (peak area)

THE INSTRUMENTATION SYSTEM OF DSC/DTA

The main components ot the DTA/DSC system:

The DTA/DSC detector and amplifier

The furnace and temperature sensor

Computer and temperature programmer

Recorder, plotter or any data collection device
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The DTA/DSC Detector

In the DCS/DTA method, a thermocouple is used to detect the temperature change while the sample

being heated under controlled conditions.

For a low temperature process, a copperconstantan or cromel-alumel thermocouple may be used,

while heating at high temperature or under corrosive atmosphere the Pt-Pt/13%Rh thermocouple isused.

The choice of thermocouple that can be used for a DSC/DTA system is shown in Figure 6.8.

A DTA/DSC experiment normally uses between 10 20 mg of sample.

A sample container may also be specially designed for use in high pressure thermal experiment. For

example in thepressure DSC(PDSC) technique, the sample container can should be able to stand

up to 70 atm pressure. Hence, reactions that produce gas at high temperature can be studied.

For example in the stability study of accelerated oxidative experiment by using a PDSC to a cooking

oil under high oxygen pressure or under catalytic reduction of organic compound by using hydrogen.

The Sample Container

Various materials may be used to make the sample plate and crucible, but most of the sample

container for use at lower temperatures (less than the melting point of Al: 660o

C), are made fromaluminum.

Platinum or ceramic may be used as the materials for the sample holder when the experiments

involve heating at higher temperatures. However, the thermal conductivity difference must be taken

into account in making a choice of the sample holder to be used.
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Reference Materials

DTA and DSC are defined as the differential techniques because the behaviour

of the sample is compared with the behaviour of a reference.

All thermal properties that could be involved in the thermal analysis experiment

must be taken into account.

For example, emissivity of the sample might change when the sample changed

phase, or react or change in colour.

Among the commonly used materials for the reference are calcined alumina

Al2O3 or carborundum, SiC.

Sometimes the sample is diluted with the reference material.

In this case, the sample does not react with the reference material

Dilution of the sample with the reference material will normally improves the

base line and the the shape of the DTA peak.


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Figure 6.9: The normal DTA or DSC curve for a melting material

During the melting process,

completion of the reaction is

indicated by the tip of the

peak.

Curve A indicates the curve

that is obtained experimentallyreturns gradually to the

baseline (curve A)

Theoretically, the curve (peak)

should return immediately to

the baseline (curve B)

Active area


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Figure 6.10 A change in the temperature detected by the thermocouple

before and after the thermal reaction which is due to a change in the

thermal conductivity of the sample material (Source: Haines F 3.7)


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The Second Type Theory: The Relationsh ip BetweenTand The Sample Temperature

This theory takes into account various

relationship between the temperaturedifference T and the sample

temperature at certain times in order to

obtain the activation energy and the

order of the reaction.

The theory, however, does not take into

account the effect of heat transfer

There exists a relationship between the

activearea of the DTA peak thermogramand the quantity of heat involved.

The active area of the DTA/DSC curve

evolved from the thermal reaction process,

while the other areas are produced as the

T returns to the baseline after thereaction has completed (point E in Figure

6.9).

The shape of the curve (curve B) is

exponential due to the influence of natural

heating or cooling process and it depends

on the arrangement of the sample holder.

Therefore, the plot of ln(T) vs time

beyond the point E of the curve A will belinear.

The active area is the area that beginsfrom the point where the curve starts to

the point E of the curve.

The relationship between the active area

and the reaction heat is shown by the

following equation:

Heat = A(active area)

whereA = slope of the plot ln(T) vs time

The theory assumes that the physical

properties of the sample and the reference

do not change during the thermal

experiment

Therefore, the curve region after the point

E is influenced by the instrumental system

used.


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For melting, crystal change and zero order reactions:

The active region of the DTA curve (related to the reaction) ends near the tip of the

DTA peak

A portion of the DTA curve exponentially returns to the baseline which is influenced by

the instrumental parameters

Therefore, the end of the reaction and theA factor can be precisely determined.

For reactions other than melting, the Tposition before the curve returns to thebaseline when the reaction has been completed will depend on the type of the

reaction.

However, if the reaction is controlled by a different type mechanism, the reaction ends up

while the Tgets near to the baseline.

Under such conditions, determination of the constantA would be difficult.

The DTA curve that ends up at high temperature might contain a mixture of reaction

mechanisms and the final portion of the curve is exponential, hence, differ from one device

to another.


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Since the tip of the DTA peak might

not really represent the reaction

process difficulty could arise if two

close consecutive reactions

produce two overlapping peaks orcompletely combined peak.

i) due to the structural inversion of silica at

573 oC

ii) due to the crystal transition of potassium

sulphate at 583 oC.

The ability to separate such

overlapping peaks indicate the

resolution of the instrument.

For example: a mixture of silica and potassium

sulphate show two peaks:

Most DTA theory assume that:

a) during the thermal reaction, the physical properties of the material does not change

and it does not depend on temperature,

a) the baseline is the same before and after the reaction.

However, most DTA/DSC curves do not have the same baseline before and after

the reaction because physical changes always occur during the thermal reaction.


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The baseline change of the DTA/DSC

curve will affect:

calculation of the area under the DTA

peak

the magnitude of error in the areacalculation which depends on the peak

size: the smaller the peak the larger the

error

the actual determination of the baseline

If the baseline is changed, estimation of the

peak area may be carried out by first drawing a

straight line across the peak from where the

peak leaves the baseline to the point where the

peak meets the baseline again (Figure 6.11(a)) The result is that the measured peak area

tends to become larger

Two baselines approach may also be used as

shown in Figure 6.11(b).

Figure 6.11 Estimation of thearea under the DTA/DSC curve by

constructing a baseline before andafter the reaction as shown by the

dotted lines

(a)

(b)


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CALIBRATION

The instrument temperature must be properly calibrated in order to obtain accurate data.

For the DSC system, sensitivity of the calorimetry need to be calibrated.

Some of the reference materials recommended by the ICTAC (International Confederation for

Thermal Analysis and Calorimetry) for the calibration of DTA/DSC system is given in Table 6.1.

The DSC thermogram of the melting process of pure indium metal (99.999%) is

shown in Figure 6.12.

The melting point 156.6 oC is obtained by determining the intersection point Te.

The area marked under the peak, AS in the Figure is used for the determination of

calorimetry sensitivity constant, K:

K = HS . mS /AS

Where HS = the fusion enthalpy of indium (28.71 J/g)

mS = mass of the indium sample (g)

AS = peak area (cm2)

K = calorimetric sensitivity (J/cm2)
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Figure 6.12 DSC peak for indium melting process which shows the determination ofintersection point Te (6 mg, 10 K/min, nitrogen) (Source: Haines: F 3.11)

Calibration for a wide operational temperature

range using various materials allows

determination of relationship between K and

temperature.

Example: pure indium sample 6.68 mg gives a

peak with area of 21.94 cm2. Calculate the value

ofKat 156 oC.

Answer: K = (28.70 x 6.68 x 10-3) / 21.94

= 8.74 x 10-3 J / cm2

Note !

The DTA or DSC curve depends on the conditions of the sample

and the instrument:

Heating Rate Sample (chemical properties, purity, origin) Sample mass (volume, packing, distribution, dissociation) Crucible (material, geometry) Atmosphere (gas, static, flowing)

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Lecture 6 Dta & Dsc01