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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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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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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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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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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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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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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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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.
Factors that Influencethe Thermal AnalysisResults
See Sample Holders
http://crucibles%20sample%20holders.ppt/http://crucibles%20sample%20holders.ppt/8/14/2019 Instrumentation Concept of Thermal Analysis
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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.
Factors that Influencethe Thermal AnalysisResults
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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
Factors that Influencethe Thermal AnalysisResults
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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.
Factors that Influencethe Thermal AnalysisResults
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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
Factors that Influencethe Thermal AnalysisResults
http://lecture%202%20s%2013%20effect%20of%20heating%20rate.ppt/http://lecture%202%20s%2013%20effect%20of%20heating%20rate.ppt/8/14/2019 Instrumentation Concept of 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.
Factors that Influencethe Thermal AnalysisResults
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/8/14/2019 Instrumentation Concept of Thermal Analysis
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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/8/14/2019 Instrumentation Concept of 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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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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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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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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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.