METHOD AND APPARATUS FOR THERMOACOUSTIC ANALYSIS (TAA)
Mirchandani, Vinod Ramchand
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A prototype of an apparatus for Thermoacoustic Analysis (TAA) was designed and implemented which allows thermal analysis of solid materials by a non-destructive technique using ultrasonic waves. Longitudinal ultrasonic waves are generated by a high voltage RF pulse excitation of an X-cut quartz transducer at its resonant frequency of 4 MHz. The waves are coupled to and from the material by two identical buffer glass rods while the material temperature is ramped accurately by a PID temperature controller. The chanqes in the transit time of the acoustic waves and their relative attenuation, are monitored with sample temperature. Transit time measurements have been carried out by noting the time difference between the reflected and the transmitted waves. Relative attenuation measurements are done by taking the peak-to-peak voltage of the received signal at various sample temperatures with respect to the peak-to-peak voltage of the received signal at room temperature. The apparatus can be used at room tempezature as well as from room temperature to 250°C at a maximum heating rate of about 10°C/min and with a resolution of around 10 ns in transit time measurement. The accuracy of the transit time measurement is thus around 10 ns or better and that of the peak-to-peak received signal voltage is around 1 mV. The apparatus was applied at room temperature to determine the mechanical properties of amorphous polymers and thermally cycled composites. A correlation between normalised velocity, relative attenuation and flexural strength with number of thermaI cycles is obtained for three types of composites. Furthermore, the usefulness of the apparatus has been demonstrated by carrying out ultrasonic measurements as a function of temperature at different heating rates for polycarbonat.e (PC). An excellent correlation between TAA, viscosity, and the conventional thermal analysis techniques viz. Differential Scanning Calorimetry (DSC), Thermomicrohardness (T�H) for polycarbonate (PC) has been found. The TAA technique developed herein has been also applied to Poly(methyl methacrylate) (PMMA), Polyphenylenesulphide (PPS) and a near-stoichiometric amorphous arsenic triselenide alloy.