Differential Thermal Analysis

DTA involves heating or cooling a test sample and an inert reference under identical conditions, while recording any temperature difference between the sample and reference. This differential temperature is then plotted against time, or against temperature. Changes in the sample which lead to the absorption or evolution of heat can be detected relative to the inert reference.

Differential temperatures can also arise between two inert samples when their response to the applied heat{treatment is not identical. DTA can therefore be used to study thermal properties and phase changes which do not lead to a change in enthalpy. The baseline of the DTA curve should then exhibit discontinuities at the transition temperatures and the slope of the curve

at any point will depend on the microstructural constitution at that temperature. DTA curve can be used as a ¯nger print for identi¯cation purposes, for example, in the study of clays where the structural similarity of di®erent forms renders di®raction experiments difcult to interpret.

The area under a DTA peak can be to the enthalpy change and is not a®ected by the heat capacity of the sample. DTA may be de¯ned formally as a technique for recording the di®erence in temperature between a substance and a reference material against either time or temperature as the two

specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate. Apparatus The key features of a di®erential thermal analysis kit are as follows (Fig. 1): 1. Sample holder comprising thermocouples, sample containers and a ceramic or metallic

block.

2. Furnace.

3. Temperature programmer.

4. Recording system.

The last three items come in a variety of commercially available forms and are not be discussed in any detail. The essential requirements of the furnace are that it should provide a stable and su±ciently large hot{zone and must be able to respond rapidly to commands from the temperature programmer. A temperature programmer is essential in order to obtain constant heating rates. The recording system must have a low inertia to faithfully reproduce variations in the experimental set{up. Fig. 1: Schematic illustration of a DTA cell. The sample holder assembly consists of a thermocouple each for the sample and reference, surrounded by a block to ensure an even heat distribution. The sample is contained in a small crucible designed with an indentation on the base to ensure a snug ¯t over the thermocouple

bead. The crucible may be made of materials such as Pyrex, silica, nickel or platinum, depending on the temperature and nature of the tests involved. The thermocouples should not be placed in direct contact with the sample to avoid contamination and degradation, although

sensitivity may be compromised.

Thermal Analysis Techniques

Thermal analysis comprises a group of techniques in which a physical property of a substance

is measured as a function of temperature, while the substance is subjected to a controlled

temperature programme. In differential thermal analysis, the temperature diference that develops

between a sample and an inert reference material is measured, when both are subjected

to identical heat{treatments. The related technique of di®erential scanning calorimetry relies

on di®erences in energy required to maintain the sample and reference at an identical

temperature.

Length or volume changes that occur on subjecting materials to heat treatment are detected

in dilatometry; X{ray or neutron di®raction can also be used to measure dimensional changes.

Both thermogravimetry and evolved{gas analysis are techniques which rely on samples which

decompose at elevated temperatures. The former monitors changes in the mass of the specimen

on heating, whereas the latter is based on the gases evolved on heating the sample. Electrical

conductivity measurements can be related to changes in the defect density of materials or to

study phase transitions.

Differential Thermal Analysis (DTA)