Characterization of Materials by Thermoanalytical Techniques
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MRS BULLETIN/JULY1988
TG-DTA and even simultaneous TGDTA-EGA as the volatile products are swept by a carrier gas into some gas analyzing de vice. The common approach is to use linear heating rates. The actual rate selected depends not only on the capabilities of the instrument fumace-sensor combination but also on the process under investigation and the trade-off between the accuracy required and the time available. The use of several heating rates will frequently provide additional information such as activation energy for well-defined and non-overlapping reactions,2 while the use of different sample sizes, atmospheres, and flow rates may reveal the presence of thermal or mass transport limitations.34 Mass transport is important for readily reversible reactions, e.g., the decomposition of carbonates or hydrates where the
reaction temperature has a pronounced dependence on the partial pressure of the product gas. This article will consider the thermal methods listed in Table I sequentially, with a very brief discussion of the generalized apparatus and a few examples of how the method has been applied to evaluate materials and processes. After the specific references for this article are listed several general purpose monographs and chapters which describe many more applications and delve into the equipment and principles in greater detail. Thermogravimetry (TG) and Thermomagnetometry (TM) These two techniques are described together because they basically involve the same instrument, a thermobalance, except that TM imposes a magnetic field gradient on the sample to measure magnetic effects. The technique consists of suspending the sample from a mass sensor, which can vary from a piezoelectric sensor to a quartz spring but is more commonly the modern electrobalance. The sample is enclosed in a Chamber whose temperature is measured and programmed and in which the atmosphere can be controlled. The recent interest in high Tc superconductors presents a prime example of the application of TG techniques. The oxygen content, x, in Ba^C^O* is critical to its electrical properties. One of the methods used most offen to determine x is reduction of the sample H2 in accordance with Eq. 1. 2Ba2YCu3Or + (2x - 7)H2 -+ 4BaO + Y 2 0 3 + 6Cu + (2x - 7)H20 (1)
Table 1. Common Thermoanalytical Techniques Method
Common Abbrevlatlon
Property Measured
Thermogravimetry Differential Thermal Analysis Differential Scanning Calorimetry Evolved Gas Analysis
TG(TGA)
Mass AT between sample & reference
Thermodilatometry Thermomechanical Analysis or Dynamic Thermomechanometry Thermomagnetometry
TD TMA
Heat adsorbed or evolved by sample Nature & amount of evolved species Dimension Deformation/nonoscillatory load
DMA
Deformatipn/oscillatory load
TM
Relative magnetic susceptibility
DTA DSC EGA
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Characterization of Materials by Thermoanalytical Techniques
The reaction is accomplished by flowing the H2-containing a t m o s p h e r e through the thermobalance while heati n g thie s a m p l e to a t e m p e r a t u r e approaching 1000
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