18

THERMAL ANALYSIS

18.1 INTRODUCTION

Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Thermometric methods are distinct analytical tools; several commonly used methods are distinguished from one another by the property which is measured as mentioned in Table 18.1. Data are obtained in the form of continuously recorded curves; these may be regarded as thermal data.

Table 18.1 Common thermal methods and the property measured

18.2 THERMOGRAVIMETRIC ANALYSIS

Thermogravimetric analysis (TGA) is a method of thermal analysis in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate), or as a function of time (with constant temperature and/or constant mass loss). TGA can provide information about physical phenomena, such as second-order phase transitions, including vaporisation, sublimation, absorption, adsorption and desorption. Similarly, TGA can also provide information about chemical phenomena, including chemisorptions, desolvation (dehydration), decomposition and solid-gas reactions (oxidation or reduction).

The technique of TGA is concerned with an analysis of sample weight change curve. The technique involves change in weight of system under examination as the temperature is increased at a predetermined and preferably at a linear rate. Automatic recording thermo balance can give the data manual recording as well as curve of weight change of the sample versus sample temperature. TGA has been widely used in recent years due to the easy availability of automatic, continuously recording sophisticated thermobalances. These are reliable, rugged and very accurate.

TGA is usually two types—dynamic TGA and isothermal or static TGA.

Dynamic TGA

In this type of analysis, the sample undergoes continuous increase in temperature, usually linear with time.

Isothermal TGA

In this type of analysis, the sample is maintained at a constant temperature for a certain period of time, during which any changes in weights are noted.

18.2.1 Principle of TGA

The principle of the technique can be illustrated by the weight loss curve of a hypothetical compound MCO₃.2H₂O as shown in Figure 18.1.

Figure 18.1 Weight loss curve of a hypothetical compound MCO₃.2H₂O

In the curve, at point ‘A’, one water molecule is evolved, and the temperature at A is called “minimum weight loss temperature”. A break is obtained in the curve ‘B’ due to the stoichiometry approaching of MCO₃.2H₂O. Further heating results in the formation of anhydrous MCO₃ weight levels from C to D. The drying temperature of MCO₃ is somewhere between C and D the values of C and D depend upon the heating rate of the furnace. A slower heating rate will shift this temperature to lower values. At point D, the MCO₃ starts decomposition and evolves CO₂ and forms MO; the weight level from E to F is the same due to the thermal stabilities of the original sample. The intermediate compounds and the final product can be ascertained by an examination of the various regions in the curve. The curve is quantitative in that the calculation can be made to determine the stoichiometry of the compound at any given temperature.

Example: Thermogravimetric determination of copper and silver alloys are based on relative stabilities of nitrates are shown in Figure 18.2.

Silver nitrate (AgNO₃) is stable up to 473°C, where it starts decomposing into NO₃ and O₂ and finally metallic silver is left at 608°C.

Cupric nitrate (Cu(NO₃)₂), however, decomposes into CuO in two steps. CuO remains stable up to 950°C.

Figure 18.2 Thermogravimetric curve of copper and silver alloys based on relative stabilities of nitrates

In a thermogravimetric curve, it should be noted that the decomposition temperature is a function of method apparatus and procedure. Moreover, the balance must be calibrated preferably each time it is used by placing a known weight on the pan to give reference mark. This is done so that in the upper right corner, correction must also be applied for the apparent weight change of the empty sample pan to get the actual weight change taking place in the specimen, the apparent weight changes is usually caused by the inter play of a complex combination of various factors, such as contained geometry, radiation effect, the atmosphere in the furnace, air buoyancy, conduction effects in the furnace, etc.

18.2.2 Applications of TGA

TGA is commonly used to determine the selected characteristics of materials that exhibit either mass loss or gain due to decomposition, oxidation or loss of volatiles such as moisture. Common applications of TGA are as follows:

1. Materials characterisation through analysis of characteristic decomposition patterns.
2. Studies of degradation mechanisms and reaction kinetics.
3. Determination of organic content in a sample.
4. Determination of inorganic content in a sample like ash, which may be useful for corroborating predicted material structures or simply used as a chemical analysis. It is an especially useful technique for the study of polymeric materials, including thermoplastics, thermosets, elastomers, composites, plastic films, fibres, coatings and paints.
5. Determination of the composition of complex mixtures.
   1. Determines the purity and thermal stability of analytical reagents, including primary strands.
   2. Determines correction of error in gravimetric analysis
   3. New weighing composition in gravimetric analysis and determination of their thermal stability ranges.
   4. Weighing substances which are unstable at ambient temperatures such as that those which absorb CO₂ and H₂O from air.
   5. The study of properties of materials in relation to the methods useful for their preparation.
   6. For evaluation of various filtration techniques, such as ignition of filler paper and so on.
   7. Discovery of new methods of separation.
   8. Studying the sublimation behaviour of various substances.

18.3 DIFFERENTIAL THERMAL ANALYSIS

Differential thermal analysis (DTA) is a technique in analytical chemistry for identifying and quantitatively analysing the chemical composition of substances by observing the thermal behaviour of sample as it is heated. The technique is based on the fact that as a substance is heated, it undergoes reaction and phase changes that involve absorption or emission of heat. In DTA, the temperature of the test material is measured relative to that of an adjacent inert material. A thermocouple imbedded in the test piece and another in the inert material are connected so that any differential temperatures generated during the heating cycle are graphically recorded as a series of peaks on moving chart. The amount of heat involved and temperature at which these changes take place are characteristics of individual element or compounds. The identification of substance compound, therefore, is accomplished by comparing DTA curves obtained from unknown with those of known elements or compounds. Moreover, the amount of substance present in a composite sample will be related to the area under the peaks in the graph, and this amount can be determined by comparing the area of characteristic peak under identical conditions.

18.3.1 Principle of DTA

The thermal effects associated with the physical and chemical changes described above are measured by a differential method in which the sample temperature is continuously compared against the temperature of the thermally inert reference material. The difference in temperature, called the differential temperature (DT), is recorded as a function of reference material temperature or time, assuring that the furnace temperature rise is linear with time.

In a typical experiment, a furnace is used which contains a sample holder. The latter has two identical and symmetrically located chambers. One set of thermocouple junctions is inserted into the inert material such as aluminium oxide and the other set of thermocouple junction is placed in another chamber containing the sample. Other temperature detecting devices, which also have been employed, are thermistor, resistance and thermometer. The furnace and the sample block temperature are then increased at a linear rate and the temperature difference between the sample and reference material is continuously measured against the reference material temperature.

Figure 18.3 Differential thermal curve

The differential thermal curve is shown in Figure 18.3. At point A, it has been assumed that the sample undergoes some type of endothermic reaction. It is also evident from the sample temperature curve that the sample temperature is no longer linear with respect to time but logs the furnace temperature as a result of absorption heat. The reaction becomes complete at B and the sample temperature increases and after some time, it becomes equal to the furnace temperature again at C. During the actual transition which begins at A, the sample and reference material temperature differ ideally in the case of differential temperature curve. A peak ABC, with maximum at B is thus obtained in the curve. Beyond B, the curve retains to base the ΔT = 0 due to equalisation of sample and reference temperatures. Only an endothermic transition is illustrated here. Hence, each substance will, in general, give a curve whose number, shape and position of various endothermic and exothermic peaks act as a means of qualitative identification of the substance. This technique can also be used to quantitatively evaluate the amount of substance present by making use of the fact that the reaction is proportionally to the amount of reacting substance.

18.4 REVIEW QUESTIONS

18.4.1 Fill in the Blanks

1. The measuring unit in DTA is ___________.

   [Ans. Rate of exchange weight]

2. The condition at which we measure the change of weight in TGA is ___________.

   [Ans. Temperature]

18.4.2 Multiple-choice Questions

1. What is the apparatus used in TGA?
   1. Thermobalance
   2. Calorimeter
   3. DTA Apparatus
   4. All the above

   [Ans. a]

2. What is the parameter used to examine the change in weight in thermometric method?
   1. Temperature
   2. Light
   3. Pressure
   4. None of these

   [Ans. a]

18.4.3 Short Answer Questions

1. Explain any two applications in thermogravimetric analysis.

   Ans.: 1. Determines the purity and thermal stability of analytical chemistry

   2. Determines the composition of complex structure.

2. Write the types of thermogravimetric analysis.

   Ans.: The types of thermogravimetric analysis are as follows:

   1. Dynamic TGA
   2. ISO thermal
3. What is the apparatus used in DTA?

   Ans.: Thermobalance is the apparatus used in DTA.

4. How many types of thermal methods are observed?

   Ans.: There are four methods:

   1. Thermogravimetric analysis
   2. Differential thermal analysis
   3. Derivative thermogravimetric analysis
   4. Thermometric titrations

18.4.4 Descriptive Questions

Q.1 Give a detailed note on thermogravimetric analysis.

Q.2 Explain differential thermal analysis with a detailed sketch.