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The application of photoacoustic microscopy to characterizing coal macerals will be presented. Photoacoustic microscopy can be used as an analytical tool that is responsive to the thermal-elastic properties of individual macerals. In a typical experiment, the crushed-particle coal pellet is mounted on a piezoelectric transducer, and the unit is mounted on the stage of a reflectance microscope. Upon absorbing the chopped light, the temperature of the maceral rises and falls with the same frequency as the modulated light. The resulting temperature variation produces a periodic strain that is detected by the transducer, whose output voltage represents the photoacoustic signal. It is known that the photoacoustic signal is a function of the absorbing maceral's density, specifi heat, and coefficient of linear expansion. The unique ability to probe the thermal-elastic properties of macerals is a principal advantage of photoacoustic microscopy when applied to the study of coal macerals.
A standard reflectance microscope is modified to measure both the reflectance and photoacoustic data from the same macerals. Since reflectance depends on the optical parameters of the maceral, and the photoacoustic signal depends on the thermal-elastic properties of the maceral, the two measurements are complementary. Macerals which differ in density, specific heat, or linear expansion exhibit different photoacoustic responses, even though they may display the same optical properties. In this respect, photoacoustic microscopy offers a potentially valuable way of differentiating between macerals which have identical optical properties but different thermal-elastic properties. Data will be presented showing both the photoacoustic and reflectance measurements from different vitrinite mac rals of the same sample, as well as from samples of different rank.
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