A new light driven spectrometer for the determination of complex heat capacities and conductivities by combination of effusivity and diffusivity measurements

We describe the construction, performance and first application of a spectr ometer for measurements of frequency-dependent complex thermal diffusivitie s and effusivities. Knowledge of these quantities enables the complex dynam ic specific heat and the thermal conductivity to be calculated. The techniq ue uses the properties of thermal waves initiated at the surface of plateli ke samples by the absorption of light with an oscillating intensity. A regi stration of the resulting temperature oscillations in the absorbance layer at the upper surface of the sample yields the effusivity, an analogous meas urement at the bottom gives the diffusivity. The detection of both amplitud es and phases of the signals leads to complex quantities. Measurements enco mpass a frequency range from 10(-2) to 10(2) Hz. The apparatus was used in a study of the reversible surface melting of poly(ethylene oxide). Both the magnitude and the characteristic time range of this process could be deter mined. (C) 2001 Elsevier Science B.V. All rights reserved.