DUAL-MODULATION-FREQUENCY PHOTOPYROELECTRIC TECHNIQUE - APPLICATION TO THERMOPHYSICAL INVESTIGATION OF (NH4)HSO4 AND (ND4)(2)TECL6 CRYSTALSAT LOW-TEMPERATURE

The photopyroelectric (PPE) method for thermo-optic materials investig ation uses a pyroelectric sensor to measure the photothermal heating o f the sample. We report on a self-calibrated PPE technique consisting of simultaneous excitation of the PPE cell at two frequencies. The cor responding signals are retrieved from the multiplexed pyroelectric res ponse with two lock-in amplifiers. Their ratio depends only on the sen sor's specific heat and the sample's thermal effusivity, as temperatur e-dependent quantities. This scheme has the advantage that it cancels out possible fluctuations of excitation intensity and of the pyroelect ric coefficient. Experimental results with (NH4)HSO4 and (ND4)(2)TeCl6 crystals show that this feature is particularly important when studyi ng structural phase transitions at low temperature (20 to 300 K). Fina lly it is found that below 240 K epoxy resin is a better coupling medi um than silicon grease. (C) 1997 Society of Photo-Optical Instrumentat ion Engineers.