Measurement of the thermal diffusivity of thin films with bolometers and with pyroelectric temperature sensors

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Abstract
Several methods for the measurement of the thermal diffusivity of thin films are developed. Common to all methods is the generation of a thermal wave via the absorption of intensity-modulated light on one surface of the film; resistive thin film bolometers and pyroelectric sensors are used for the temperature recording. For free-bearing films the thermal diffusivity is determined from front and rear side temperature spectra as a function of the modulation frequency, measured with resistive bolometers. If a pyroelectric material is mounted on a heat sink, the thermal diffusivity is obtained from the spectrum of the pyroelectric current. For very thin films pyroelectric substrates are recommended as temperature sensors. A novel method is also proposed, in which the thermal diffusivity is obtained without the influence of any other material properties and any sensor characteristics. Experimental results are presented with a 25 μm thick PVDF film and with a 1.25 μm thick Polyglutamate Langmuir-Blodgett film on a 9 μm thick PVDF substrate. The thermal diffusivity for the PVDF films is D = 6.2 × 10−8 m2/s and for the Polyglutamate Langmuir-Blodgett film D = 5.5 × 10−8 m2/s at room temperature. The techniques are theoretically discussed and various methods for the analysis of measured data are introduced.