Thermal analysis.

Thermal analysis is a technique where the physical properties of materials can be studied as they change with temperature. Several methods were used in this work but they are distinguished from one another by the property which is measured.

 Differential thermal analysis (DTA): temperature difference

 Differential scanning calorimetry (DSC): heat difference

 Thermogravimetric analysis (TGA): mass

Simultaneous thermal analysis (STA) normally refers to the simultaneous application of thermogravimetry and differential scanning calorimetry to one and the same sample in a single instrument. The test conditions are entirely the same for the TGA and DSC signals (e.g same atmosphere, gas flow rate, vapour pressure of the sample, heating rate, etc). The

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essence of all these techniques is that the sample's response is recorded as a function of temperature and time.

Temperature can be controlled in a predetermined way, either by continuous increase or decrease in temperature at a constant rate or by carrying out a series of determinations at different temperatures with isothermal measurements. In addition to that it can control its environment with different atmospheres, e.g. measurements can be carried out in air or under an inert gas (e.g. nitrogen or helium). Moreover, measurements can be carried out under reducing or oxidizing atmospheres. This technique can be used to study phase transitions of the materials.

2.8.1 Differential Thermal Analysis

DTA is a thermal analysis techniques used to determine the thermal properties and phase changes of a material. It involves heating or cooling a test sample against an inert reference under the same conditions while recording any temperature difference between the sample and reference. Thermal conductivities and the heat capacities of the test and reference samples are not the same. 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 obtained relative to the reference sample. The baseline of the DTA curve should display discontinuities at the transition temperatures and the slope of the curve at any point will depend on the microstructure formation at that particular temperature.

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2.8.2 Differential scanning calorimetry

Differential scanning calorimetry is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and the reference are maintained at equal temperature throughout the experiment. Temperature program for a DSC analysis is designed that the sample holder temperature increases linearly as a function of time. The reference sample should have a well defined heat capacity over the range of temperatures to be scanned.

The principal of this technique is that when a sample undergoes a physical transformation (e.g. phase transitions) more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether less or more heat must flow to the sample depends on whether it is an exothermic or endothermic process. When the sample undergoes exothermic processes such as crystallization less heat is required to raise the sample temperature. By observing the difference in heat flow between the sample and reference, DSC can measure the amount of heat absorbed or released by the sample during such transitions. Also it is used to observe more subtle phase changes (e.g. glass transitions).

2.8.3 Thermogravimetric analysis

Similar to the DTA and DSC, TGA also is a thermo analytical technique. TGA performs on samples that determine changes in weight with temperature. It involves heating a sample in an inert or different atmosphere and then measuring the change in weight. The weight change over particular temperature ranges offers indication of the thermal stability and the composition of the sample. It also provides information on estimation of moisture and

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volatiles, oxidative stability, assessment of composition, and decomposition kinetics. TGA uses heat to force reactions and physical changes in materials and also it provides quantitative measurement of mass change in materials associated with transition and thermal degradation.

Simultaneous TGA-DTA/DSC measures both heat flow and weight changes (TGA) in a material as a function of temperature or time in a controlled atmosphere. Simultaneous measurement of these two material properties simplifies the interpretation of the results. Characteristic thermogravimetric curves are given for specific materials and chemical compounds due to unique sequence from physicochemical reactions occurring over specific temperature ranges and heating rates. These unique characteristics are related to the molecular structure of the sample.

In this work thermogravetric analysis was carried out on NETZSCH TG 209 instrument to evaluate the oxygen content (weight loss) at different temperatures and to compare with neutron diffraction results. The initial weight of the sample was about 20 mg. The effect of buoyancy was corrected using pre-empty crucible runs under corresponding gas atmosphere and flow rates. The samples were heated up to 900 oC in the TG furnace at a heating rate of 5 oC min-1 in air.

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Figure 2.6: Image of the NETZSCH TG 209 instrument used for thermogravimetric analysis

In order to understand any phase transition behaviour, simultaneous TGA/DSC was carried out using identical test conditions. Sample's response is recorded as a function of temperature.