Differential Thermal Analysis | Clay Minerals | Soil Mineralogy

In this article we will discuss about:- 1. Apparatus of Differential Thermal Analysis 2. Types of Thermal Peak Reactions 3. Applications.

Apparatus of Differential Thermal Analysis:

The DTA apparatus consist of the following:

1. Sample holder, usually ceramic, nickel, or platinum.

2. Furnace with a temperature controller to provide a constant rate of heating.

3. Thermocouples for the measurement of temperature and the difference in temperature between the sample and the reference material.

4. Recorder for the thermocouple output.

The amount of sample required is about 1 g. Although the temperatures at which thermal reactions take place are a function only of the sample, the size and shape of the reaction peaks are affected by the thermal characteristics of the apparatus and the heating rate.

Two thermocouples are used, one placed in the inert material and another in the sample. The other ends of the two thermocouples are connected to a voltmeter.

The reference material must not suffer any change in the state over the temperature range employed to examine the sample. As the temperature increases, any change in the phase by the sample results in a negative or posi­tive temperature signal from the thermocouples.

In a similar manner, any change in state that involves a latent heat of transition causes the temperature of the sample to lag or lead that of the reference material and identify the change of state and the temperature at which it occurred.

Types of Thermal Peak Reactions:

The reactions in the sample during heating are of two types – (a) Endothermic reaction and (b) Exothermic reaction. In endothermic reactions, the sample absorbs heat causing a decrease in ΔT, whereas in exothermic reactions the sample liberates heat causing an increase of ΔT. The analysis of test results consists of comparing the sample curve with those for known materials so that each deflection can be accounted for.

The important thermal reactions that generate peaks on the thermogram are as follows:

1. Dehydration:

Water in a soil may be present in three forms in addition to free pore water – (a) Adsorbed water or water of hydration, which is driven off at 100°C-300°C, (b) Interlayer water such as in halloysite and expanded smectite, and (c) Crystal lattice water in the form of (OH) ions, the removal of which is termed as dehydroxylation.

Dehydroxylation destroys mineral structures. The temperature at which the major amount of crystal lattice water is lost is the most indicative property for the identification of minerals. Dehydration reactions are endothermic and occur in the range of 500°C – 1000°C

2. Crystallization:

New crystals form from amorphous materials or from old crystals destroyed at a lower temper­ature. Crystallization reactions usually are accompanied by an energy loss and thus are exothermic, occurring between 800°C and 1000°C.

3. Phase Changes:

Some crystal structures change from one form to another at a specific temperature and the energy of transformation shows up as a peak on the thermogram. For example, quartz changes from the α to β form reversibly at 573°C. The peak for the quartz phase change is sharp and its amplitude is nearly in direct pro­portion to the amount of quartz present.

The quartz peak is frequently masked within the peak for some other reacting material but may be readily identified by determining the thermogram during cooling of the sample or by letting it cool first and then rerunning it. The other minerals are destroyed during the initial run while the quartz reaction is reversible.

4. Oxidation:

Exothermic oxidation reactions include the combustion of organic matter and the oxidation of Fe²⁺ to Fe+. Organic matter oxidizes in the 250°C-450°C temperature range. Beside quartz, the only common non-clay minerals m soils that give thermal reactions with large peaks are carbonates and free oxides such as gibbsite, brucite, and goethite. The carbonates give very large endothermic peaks between about 800°C and 1000°C, and the oxides have an endothermic peak between about 250°C and 450°C

5. Quantitative Analysis:

Theoretically, the area of the reaction peak is a measure of the amount of mineral present in the sample. For sharp, large amplitude peaks such as the quartz inversion at 573°C and the kaolinite endotherm at 650°C, the amplitude can be used for quantitative analysis. In either case, calibration of the apparatus is necessary, and the overall accuracy is of the order of ± 5%.

Applications of Differential Thermal Analysis:

DTA can be used for a wide variety of applications in addition to the identification of clay minerals such as pharmaceutical and food industries, cement industry, archeology, and environmental studies.