DUAL-MODULATION-FREQUENCY PHOTOPYROELECTRIC TECHNIQUE - APPLICATION TO THERMOPHYSICAL INVESTIGATION OF (NH4)HSO4 AND (ND4)(2)TECL6 CRYSTALSAT LOW-TEMPERATURE
M. Chirtoc et al., DUAL-MODULATION-FREQUENCY PHOTOPYROELECTRIC TECHNIQUE - APPLICATION TO THERMOPHYSICAL INVESTIGATION OF (NH4)HSO4 AND (ND4)(2)TECL6 CRYSTALSAT LOW-TEMPERATURE, Optical engineering, 36(2), 1997, pp. 363-369
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.