instrumentation concept of thermal analysis

8/14/2019 Instrumentation Concept of Thermal Analysis

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Lecture 2 4-Nov-04 1

Thermal Analysis

SSK 4242

Instrumentation Concept of Thermal Analysis

Ref: Haines 1995

Thermocouple

Furnace

Sample

Detector device

Gas inlet

Gas outlet

Temperature programmer

Detector amplifier anddata collection

Display:

computer/recorder


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Thermal Analysis

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The normal shapes of thermal

analysis curves which shows

the initial temperature (Ti), thefinal temperature (Tf) and peak

temperature (TP)

Thermal Analysis Curves (Thermograms)

S

ignal

Temperature, oC

Ti

Tp

Tf


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Thermal Analysis

SSK 4242

Computer in Thermal Analysis

Computer becomes an important component that plays the

following roles in modern thermo analysis instruments :

as a means to provide operational instructions during

thermal analysis experiment

collection and interpretation, storage and retrieval of

operational and experimental data

user friendly instrument that makes modern thermal

analysis experiments more interesting, convenient, and

yet more precise and accurate.

Simulation of the behaviour of the instrument and the

sample under certain conditions


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Thermal Analysis

SSK 4242

Many analytical methods give results that are

characteristics to the particular samples. For example, IR

produces spectrum of a polymer that is characteristics of

that polymer which is less influenced by the sampling

method, or the instrument used, or the time when the

experiment is carried out.

Thermal analysis, on the other hand, is less specific which

produces result that is dependent of the experimental

conditions. This is due to the dynamic nature of the thermalanalysis process.

Thermal analysis methods are less specific.


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Thermal Analysis

SSK 4242Factors that Influencethe Thermal AnalysisResults

FurnaceFurnace

Gas flow

Heat flow

Sample

holder

Liberated

gas or

volatile

products

The Dynamic of

Thermal Analysis

Process


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Thermal Analysis

SSK 4242

In thermal analysis instrument, the signal

generated by the sensor depends on:

The extent of the reaction and the rate of reaction The extent to which the measured properties have changed

and the rate of the changes The heat transfer by conduction Heat convection The radiation of heat within and around the instrument

Reaction of the sample with the environment

Thermal analysis reports must include statement on:

All the experimental conditions involved in the experiment,

Comparison between sample can only be made on the basis

that all conditions of the experiments are the same,

Any differences must be clearly stated in detail in the reports.

FactorsthatInfluence

theThermalAnaly

sis

Results


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Thermal Analysis

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The signal generated by the thermal sensor depends on:

The rate of reaction and the extent of the reaction How far the measured changes have taken place and the rate of

change Heat transfer by conduction Heat convection Heat radiation within the instrument Reaction of the sample with its surrounding environment

In thermal analysis

The thermal analysis report should mention all the experimental

conditions used in the experiment.

Comparison between samples can only be made if theexperimental conditions for the samples involved are the same.

Any discrepancy must be clearly spell out.

Factors that Influencethe Thermal AnalysisResults


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Thermal Analysis

SSK 4242

e) Trace contamination

This may become catalyst to the decomposition reaction and change

the order of reaction.

Sample FactorFactors that Influencethe Thermal AnalysisResults

a) Sample history:

Information regarding the original

source of the sample and any pre-treatment received by the sample

c) The following

information (if known)

should also be stated in

terms of: Purity Chemical composition Chemical formula

b) Diluent material:

An inert material only should beused as the diluent in thermal

analysis The sample : diluent composition

should be known The diluent may influence the

thermal conductivity of the sample.

d) Particle size:

Particle size may influence the

thermogram, particularly when this

involves surface reaction.


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Thermal Analysis

SSK 4242

a) Sample holder material

Should not react with the sample during the experiment. Aluminium Platinum Silica Alumina

Changing the sample holder may change the rate of heat flowdue to heat conductivity differences of the sample holder

materials.

Sample Holder/Crucible

b) Sample holder geometry

A wide and shallow crucible will facilitate absorption of reactantgas into the sample and diffusion out of the gaseous or volatile

products A narrow and deep crucible will prevent the free movement of

the gasses.


See Sample Holders
http://crucibles%20sample%20holders.ppt/http://crucibles%20sample%20holders.ppt/


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Thermal Analysis

SSK 4242

a) The rate of heating (dT/dt) may be programmed:

At a fix rate of 0 K/min: isothermal condition At a rate of 10 K/min: normal rate (commonly used) At a high rate of 100 K/min: used in simulation of combustion

process of materials.

The Effect of Heating Rate

b) Thermal analysis may also be carried out by cooling.

c) The following process normally proceed to reach a finite value:

Rate of heat transfer, Physical changes, and

Most reactions

Every sample reacts differently when heated or cooled at different

heating/cooling rates.



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Thermal Analysis

SSK 4242

Heat is transferred by the following process: conduction convection radiation

Thermal Lag

where there is a temperature gradient within a body while undergoing heat

treatment due to heat transfer process.the sample temperature is seen to

have higher temperature than its actual temperature.

Correction of thermal lag effects:

Te = Te,0 + . + c Where:Te = uncorrected temperature observed

at a heating rate = a constantTe,0 = corrected temperature at = 0 = heating rate (K/min)c = correction factor, determined by

calibration

Thermal lag becomes

larger when the rate of

heating gets higher

Te = 303.56 + 0.074 . - 0.2= 304.10 K

Example: determination of the

melting point of Gallium



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Thermal Analysis

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Nearly equilibrium condition Where heating rate is very low, less than 1 K/min mostly used.

The Effect of Heating Rate

High heating rate Some portions of the sample melt before the sample begins to

decompose.

Low heating rate

All reactions have taken place before the sample begins to melt. Low heating rate gives better resolution for two adjacent

changes.



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Thermal Analysis

SSK 4242The Effect of Heating Rate

Ti,fast > Ti,slowTf,fast > Tf,slow(Tf - Ti)fast > (Tf - Ti)slow

Heating rate effects the initial

temperature, final temperatureand the temperature range

where the changes occur during

heat treatment.

Rate of heating can either:

Linear, OR Complex with certain pattern

Example: isothermal at 25 oC for 2 min., followed by 50 K/min up to 300 oC;

isothermal at 300 oC for 10 min., and then cooled at a rate of 5 K/min to room

temperature.

Signal

Temperature, oC

Ti

Tp

Tf

See an Example

http://lecture%202%20s%2013%20effect%20of%20heating%20rate.ppt/http://lecture%202%20s%2013%20effect%20of%20heating%20rate.ppt/


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Thermal Analysis

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The Effect of Atmosphere

The gas used to provide atmosphere strongly influences

The heat transfer Chemical process of the sample The reaction products

Although the atmosphere is inert towards the sample, the rate

of heat transfer is influenced by the type of the atmosphere

used

Four types of gasses that are normally used as the

atmosphere in thermal analysis are as follows:

Gas at 1 atm Thermal conductivity(10-2 J/(cmK) at373 K

HeliumNitrogenAirCarbon monoxide

17.773.093.172.23

The table shows that heat transfer by conduction in helium is almost eight

times higher than that in carbon monoxide.


Figure 3.20shows theeffect of temperature

on the thermal

conductivity of helium,

nitrogen and argon

Exercise ?
http://lecture%202%20s%2014%20atmos%20thermal%20conduct%20temp.ppt/http://lecture%202%20s%2014%20exercise%20atmospheric.ppt/http://lecture%202%20s%2014%20exercise%20atmospheric.ppt/http://lecture%202%20s%2014%20atmos%20thermal%20conduct%20temp.ppt/


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Thermal Analysis

SSK 4242The Effect of Atmospheric Partial Pressure

Remember Le Chatelier Principle ?

CaCO3 CaO + CO2

1

rea

cti o

n

fracti o

n

(1

-

)

Temperature

The effect of carbon monoxide partial

pressure on the TG curve of calcium

carbonate AR. 110 mg heated at 100

K/min in

1. 1.0 atm N2,

2. 0.1 atm CO2

,

3. 0.3 atm CO2,

4. 0.5 atm CO2,

5. 0.7 atm CO2,

6. 1.0 atm CO2.

In experiment (2) (5), N2 was

added to maintain the total pressureof 1 atm.

Original Ref.: F.W. Wilburn, J.H. Sharp,

J. Thermal Anal., 1993, 40, 133

FactorsthatInfluence

theThermalAnalysis

Results

Exercise ?
http://lecture%202%20s%2015%20exercise%20partial%20pressure.ppt/http://lecture%202%20s%2015%20exercise%20partial%20pressure.ppt/


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Thermal Analysis

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If the heat treatment is accompanied

by chemical reactions, the thermal

events may change completely.

Example, metal is stable when heated

in nitrogen but will be oxidized when

heated in air.

The Effect of Atmosphere

If the reaction involves an

equilibrium, according to the Le

Chatelier Principle, high

concentration of the reaction

products will force the reaction to

shift backwards.

CaCO3 CaO + CO2

If the sample reaction produces

gas, its own atmosphere will be

formed on its surface that may

change the equilibrium, kinetics and

heat transfer processes.

Calcium Carbonate

Begins to decompose at temperature< 700 oC in the atmosphere that

contains low concentration of CO2 The compound does not decompose

until the temperature reaches > 900oC in the CO2 at a pressure of 1 atm

(See Figure)

The rate of gas flow is important since static atmosphere will not remove the

released gas during the heat treatment and does not remove heat from the

sample.


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Thermal Analysis

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The Effect of Sample Mass

Comparison between thermal

analysis curves can only be made ifthe sample size are the same.

The following will affect thethermal analysis:

Physical properties Sample size Sample packing Sample density Particle size

If the sample size is too small (1 g), thesignal will become very small, individual

crystals may independently react that leads

to the formation of multiple responses/peaks.

On the other hand, if the sample size is too

large, the signal will be large but resolution/separation of adjacent multiple consecutive

reactions will difficult.Very large sample size will only be

needed if the when the sample

consists of several components and

the component of interest

constitutes a small fraction.


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Thermal Analysis

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SCRAM: the five important parametersin thermal analysis

S = Sample

C = Crucible or sample holder

R = Rate of heating

A = Atmosphere

M = Mass of sample


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Thermal Analysis

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Simultaneous Thermal Analysis

A single thermal analysis technique does not give sufficientinformation for scientists to decide conclusively about the

processes taken place during the sample heat treatment. For

example, DTA with an endothermic peaks does not indicate

whether the process is a chemical reaction or a physical change

such as melting, whether a gas has been released or not.

TG thermogram on the same sample will indicate whether or not

there is a mass loss within the temperature range to ensure that

the process is not melting. Conversely, no mass lost indicates

that there is melting process.

The use of combined simultaneous techniques produce more

complete information although this means that the optimum

conditions of a single method will have to be sacrificed.


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Thermal Analysis

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Complementary Methods

Two different techniques are used towards different sample or different

time. For example, the TG method is used for first sample of a polymerand the DTA method is used for the second sample of the same polymer.

Hyphenated techniques, e.g. TG-DTA, or DSC-optometry are simultaneous

techniques in which two methods are used at the same time for the

analysis of a sample. Simultaneous techniques are preferred to avoid

difficulties arise from the use of single techniques

Almost any analytical methods can be used as the complementary for

thermal analysis. For example, released gas can be separated using

chromatographic techniques, dissolved and titrate, measured using

electrochemical or spectroscopic techniques such as IR or atomic

spectroscopy.

Solid reaction products may be studied by surface characterization, study

their catalytic activities and analysed using AAS etc.

All the analytical methods are important in providing full understanding

towards the process taken place during the heat treatment of thermal

analysis.

instrumentation concept of thermal analysis

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