Thermal Analysis SSK 4242

Lecture 4 4-Nov-04 1

Thermal AnalysisSSK 4242

Factors Affecting the TGA Thermograms:

1.Instrumental factors

1.Sample factors



1. The Effects of Instrumental (Balance) Factors on the TGA Thermogram:

a) The Effects of Heating Rate

For an endothermic reaction, the Ti and Tf depends on the heating rate;

The initial temperature (Ti) becomes higher when the heating rate is higher compared to the initial temperature when the heating rate is lower;

The final temperature (Tf) becomes higher when the heating rate is higher compared to the final temperature when the heating rate is lower;

The temperature range (Tf - Ti) is also larger when the heating rate is higher compared to the temperature range when the heating rate is lower;



An example of the effect of heating rate on the TGA thermogram of Ferum(II) Carbonate is shown in Figure 3.17;

At any temperature range, the decomposition level becomes higher when the heating rate is lower compared to that when the heating rate is higher;

For an exothermic reaction, the sample temperature becomes higher than the furnace temperature and the temperature difference between that of the sample and that of the furnace also becomes bigger;

When there is a chain of reactions, the heating rate may become the major factor that will influence whether or not resolution/separation of the reactions will be achieved.

a) The Effects of Heating Rate (Cont’d)



b) The Effects of Furnace Atmosphere on the Thermogram

The effects of atmosphere on the thermogravimetry curve depends on the following:

1. Type of reactions, 2. Conditions of the decomposition products,3. Type of the atmosphere used in the experiment.

Three types of reaction that can be studied whether they are reversible or non reversible are:

• If an inert gas is used as the atmosphere, its function is to remove the decomposition products of reactions (1) and (2) and to avoid reaction (3).

• If the atmosphere used contains the same gas as the released gaseous products, it will affect the reversible reaction (1) but does not give any effect on the irreversible reactions (2) and (3).

1) As(1) Bs(2) + Cg

2) As(1) Bs(2) + Cg

3) As(1) + Bg(1) Cs(2) + Dg(2)



Examples of the effect of the atmosphere on the reaction thermograms for reversible and irreversible reactions are given in Figure 3.18.

In this example, a 500 mg sample of hydrated calcium oxalate was heated at a rate of 300 oC/hr in the atmosphere of nitrogen and oxygen, respectively.

b) The Effects of Furnace Atmosphere on the Thermogram (Cont’d)

(I) CaC2O4 · H2O(s) CaC2O4(s) + H2O(g)

(II) CaC2O4(s) CaCO3(s) + CO(g)

(III) CaCO3(s) CaO(s) + CO2(s)

Reaction (I) is a dehydration process. Neither N2 nor O2 used as the atmosphere will affect the reaction, hence their role is to remove the liberated water molecules

Reaction (II) proceeds under the N2 atmosphere. When the atmosphere was changed to oxygen, the O2 reacts exothermally with the released CO(g) as a secondary reaction which increases the temperature of the unreacted solid sample, which in turn increases the rate of decomposition. Therefore, under the oxygen atmosphere decomposition of calcium oxalate takes place at a faster rate and completes at a lower temperature than that under the N2 atmosphere.

Reaction (III), theoretically, is not affected by the use of N2 or O2 as the atmosphere. However, Figure 3.18 shows that a small change occurred at the step (III) of the thermogram. This is due to the difference in physical properties (in terms of particle size, surface area, and crystal defects, etc.) of CaCO3 formed in step (II) when the atmosphere was changed from N2 to O2.



The Effect of Atmospheric Pressure

In the case of Reaction (III) (reversible reaction), an increase of atmospheric pressure of the CO2 in the furnace atmosphere will increase the initial temperature, Ti.

Remember Le Chatelier Principle !?

The initial decomposition temperature changes from 400 oC at lower partial pressure to 900 oC at higher partial pressure (760 Torr).

See Figure 3.19.

(III) CaCO3(s) CaO(s) + CO2(s)



Figure 3.22 shows the TGA curves of CuSO4·5H2O under high and low atmospheric pressure and correction for buoyancy.

The Effect of Atmospheric Vacuum Pressure

Under certain conditions: vacuum condition and the sample has a certain mass and thickness and using certain sample holder, the sample appeared to have increased in mass (buoyancy effect) during thermal decomposition.

This is observed during dehydration of CaC2O4.H2O (see Figure 3.21)

Decomposition of CaCO3 takes place faster in the atmosphere with greater thermal conductivity, which indicates that the rate of sample decomposition is influenced by the speed of heat flow in the thermal system. Figure 3.20 shows that the thermal conductivity of helium, nitrogen and argon are in the order of He > N2 > He. Therefore, the decomposition rate of CaCO3 in the respective atmospheric gas will follow the same order.

The Effect Of Atmospheric Gas Thermal Conductivity



The Effect of Sample Holder

Figure 3.23 The effect of sample holder geometry on the TGA curve of C2O4.H2O under a dynamic CO2 atmosphere. Dotted line: quartz plate, solid line: porcelain crucible.

Figure 3.24 The TGA curves of CuSO4.5H2O using a crucible and a thin aluminum plate. Curve (1) (500 mg) obtained using a crucible, and curve (2) (200 mg) obtained by using a thin plate. Heating rate 10 oC/min.

Figure 3.25 The effect of heat sink on the TGA curve for lead carbonate (a) aluminum block, (b) aluminum plate. Heating rate 450 oC/hr.

Figure 3.26 The effect of sample packing and the sample holder geometry on the removal of water from the CaC2O4 H2O sample.



2) The Effects of Sample Properties on TGA Thermograms

a) The Effect of Sample Mass

The mass of the sample may influence the TGA curve in three ways:

1. The extent of exothermic or endothermic reaction that has caused the sample temperature to depart from a linear change. The larger the sample mass, the bigger the departure from linearity.

2. The degree of gaseous products absorption through the pores around the solid sample particles. Under a static atmosphere, the surrounding atmosphere will soon be influenced by the whole sample.

3. The existence of a large thermal gradient in the sample, especially when the sample has a low thermal conductivity.



In order to detect the presence of an intermediate, smaller sample may be used as shown in the Figure 3.27.

The presence of an intermediate CuSO4.3H2O appeared more evidently when the sample mass was 0.426 mg compared to that when a larger sample mass (18.00 mg) was used.

The Effects of Sample Properties on TGA

Thermograms (Cont’d)

a) The Effect of Sample Mass (Cont’d)

A better resolution for the detection of an intermediate reaction is obtained by using smaller sample mass



Other examples of the effects of sample mass on the TGA curve:


Thermograms (Cont’d) a) The Effect of Sample Mass (Cont’d)

Figure 3.28 shows the effect of sample mass on the TGA curve of CaC2O4. H2O in a static atmosphere and a heating rate of 300 oC/hr. Sample mass: (a) 126 mg, (b) 250 mg, and (c) 500 mg.



Other examples of the effects of sample mass on the TGA curve:


Thermograms (Cont’d) a) The Effect of Sample Mass (Cont’d)

Figure 3.29 The effect of sample mass on the thermal decomposition of calcite.



b) The Effect of Particle Size on the TGA Thermogram

The Effects of Sample Properties

on TGA Thermograms (Cont’d)

Figure 3.30 shows the effect of different particle sizes on the TGA thermogram of dehydration of whewellite (CuC2O4.H2O) under vacuum. Solid line: 150 mesh; dotted line: single crystal.

The difference in particle size causes differences in the rate of absorption of the gaseous products, which in turn changes the shape of the thermogram.

Decrepitation of large particles may cause the thermogram to show sudden loss of the sample mass.

The small and homogeneous particle size enables the changes to be closer to the equilibrium conditions and more complete decomposition process.

Sample with large particle size has a lower surface area to mass ratio that normally decomposes at higher temperature than that of the sample with smaller particle size.



Sources of Errors in TGA Analysis

The various sources of errors in TGA analysis are interrelated to each other:

The sample holder air buoyancy Convection current and turbulent of heat in the furnace Random fluctuation of the balance and recording mechanism The effect of furnace induction The effect of electrostatics towards the balance mechanism The environment of the balance Condensation of materials on the balance fulcrum Temperature measurement and calibration Weighing calibration of the recording balance The flow of recording chart paper Reaction between the sample and sample holder material Temperature fluctuation The effect of momentum shift in vacuum conditions



The Effect of Air Buoyancy

The gas phase density decreases with temperature increase At 300 oC the reduction of gas phase density (and hence the

increase in buoyancy) effect towards the sample increases to about one and half times higher than that when the temperature is 25 oC.

In air atmosphere, this may cause a mass variation of 0.6 mg/cm3.

The changes of gas (air) density (and hence the buoyancy) (mg/cm3) versus temperature at two different pressure is shown in Figure 3.31.

Gas

den

sity

, m

g/cm

3

Figure 3.31.



Buoyancy affects not only the balance system and the sample holder, but also on the mass changes and the volume of the sample during the decomposition processes.

Figure 3.32 shows the effect of buoyancy and the correction done on the TGA curve of a large CaCO3 sample in the CO2 atmosphere.

Curve (I): no correction was done on the effect of buoyancy

Curve (II): correction has been done on the effect of buoyancy to the crucible and the crucible support

Curve (III): correction done includes the effect of buoyancy towards the sample.

The Effect o

f Buoyancy

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Thermal Analysis SSK 4242