Q.5. (a) Draw diagram of apparatus and give methodology of DTA. Discuss factors affecting DTA results.
Ans.5. (a) Differential Thermal Analysis
In differential thermal analysis (DTA), a sample and a thermally inert reference material differencebetween the sample and the reference is measured as a function of the temperature applied. Because, during transition, the sample may either absorb or evolve heat, the difference in the temperature between the sample and the standard is equivalent to the temperature of transition and can indicate if the transition is endothermic or exothermic. Usually, ΔT is plotted against the temperature, T, or as a function of time (t).
DTA data are probably the most accurate of all thermal techniques, because the thermocouple is insert into the sample; however, only the temperature of a transition and not the amount of heat can be measured from a DTA curve, as the area under the peak is not proportional to the amount of energy transferred into or out of the sample.
Factors affecting the DTA curve
The DTA curve is affected by a large number of factors than the TG curve. Majority of these factors are common to TG. The various factors affecting the DTA curve are as follows.
(1) Environmental factors.
(2) Instrumental factors.
(3) Sample factors.
1. Environmental factor.
The DTA technique is more sensitive to the gaseous environment around the sample than the TG. Furthermore, the reaction of gaseous atmosphere with the sample may also produce extra peaks in the curve. For example, oxygen in air which causes an oxidation reaction may give rise to an exothermic peak.
In DTA studies two types of gaseous atmosphere are used: (a) a static gaseous atmosphere. (b) a dynamic gaseous atmosphere. Generally, the peak minimum temperature is shifted to lower temperatures in the dynamic gas technique. However, the shapes of the curve remain similar in the two techniques.
For example: effect of the introduction of an atmosphere of the evolved gas on the DTA curve of decomposition of SrCO3. If the DTA curve of this decomposition is recorded in an oxygen atmosphere, the peak due to rhombic – hexagonal transition of SrCO3 and the decomposition peak overlap each other. When CO2 is introduced as a dynamic atmosphere, the transition peak remain at 927ºC but the decomposition peak (SrCO3 = SrO + CO2) gets shifted to much higher temperature.
2. Instrumental factors
(a). Sample holder. The geometry and material used in the fabrication of the sample holder affect the resolution, shape and size of the DTA peaks.
If the sample holder are made from material of high thermal conductivity (e.g. metals), sharp exothermic peaks but relatively flat endothermic peaks are obtained. For better resolution, the size of the holders and amount of the sample should be as small as possible.
(b). Differential temperature sensing device. Generally, heats of transitions are much less as compared to the heat of reactions. Therefore, the differential temperatures in the former case are much smaller and their pre amplification is essential.
If the wires used in temperature sensing devices are much thick, more distortion of the peak heights and the peak temperatures may take place. However, if thinner wires are used, lesser distortion in peak height and peak temperatures may take place
(c). Furnace Characteristics. The type of winding shows a direct effect on the DTA curve. If the winding used in furnace is not uniform, the base line is changed. This type of effect is generally seen in hand-wood furnaces. But this problem is not seen in commercial equipment’s because they employ machine-wound furnaces.
(d). Temperature- programmer controller. One should be quite careful while selecting the temperature-programmer controller because a constant heating rate is required in DTA. If one employs the switching off or on or full power, considerable noise may occur particularly at temperatures above 700ºC. If one has to measure small differential temperature, one should maintain highest accuracy, control and precision in temperature measurement.
(e). Thermal regime.The heating rate has a great influence on the DTA curves. Higher the heating rates, higher the peak temperature and sharper the peaks with greater intensity. Generally, heating rates of 10 to 20º per minute are employed. In some cases, even higher heating rates are preferred. If the sample temperature is used as a reference material, this minimizes the shift in the peak temperature to higher values with faster heating rates.
(f). Recorder.DTA curve is greatly influenced by the type, span, chart-speed and pen-response of a recorder. As the differential signal lies in the range of several tens to hundreds of microvolts, one should be very careful while selecting the recorder span and is some cases it becomes essential to pre amplify the signal. The same choice is also true for the sensitivity of the recorder. If proper sensitivity is not selected, weaker signals would not be recorder whereas the stronger signals might undergo damping. If faster charts speeds are used, DTA peaks get flattened out.
3. Sample characteristics
(i). Physical:In quantitative DTA, one makes the assumption that heat capacity remains constant with the progress of the reaction. But this is changing with the progress of the reaction. In order to maintain the heat capacity nearly constant, the usual practice is to mix a small quantity of the sample with an excess amount of an inert material. But this creates other problems.
The packing density affects the overall thermal conductivity of the sample and hence the heat flow. But it is not possible to get reproducible packing densities and it is never constant.
The effect of particle size is related to the effects of packing density and overall thermal conductivity of the sample. In general, following inferences are drawn from the variation in size of the particle:
1. Particle size alters the peak area. These areas decreased with increasing particle size.
2. Particle size also influences the peak temperatures. Generally with increase in particle size, the peak temperature is shifted to higher values.
3. Particle size also alters the completion temperature. Generally the completion temperature decreases with decreasing particle size.
The weight of the sample also influences the peak intensity and temperatures. Both these increase with increasing weight. But it is important to remember that with smaller amounts of the sample the resolution is greatly increased.
In order to maintain the heat capacity nearly constant during heating, the sample is generally mixed with diluents. Generally, diluent affects the area, temperature and even resolution of the DTA peaks.
(ii).Chemical:The chemical reactivity of the sample, the sample holder, thermocouple material, the ambient gaseous environment and added diluents greatly alter the DTA peaks. Therefore, one should make every effort to select these materials as inert chemically as possible with the sample.
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