Photopyroelectric of the Thermal Diffusivity of Recrystallized High Purity Aluminum

Abstract. Thermal diffusivity measurements on high purity (99.99% pure) aluminum samples are reported, using chirped and sinusoidally-modulated laser excitation and photopyroelectric detection of the thermal wave response. The results are found to be dependent on the degree of recrystallization of the specimen: for cold-rolled aluminum, the thermal diffusivity is measured to be c~ = 3.10 x 10 -5 m2/s, which rises to 3.88 x 10 -5 mZ/s for fully recrystallized samples, and then falls to 3.53 “ 10 -5 mZ/s for samples exhibiting the initial stages of grain growth. Introduction The measurement of laser-excited thermal wave phenomena through the use of pyroelectric detectors, known as the photopyroelectric (PPE) technique, has recently been demonstrated to be a simple and sensitive method for the thermal analysis of condensed matter [1-4]. Photothermal techniques, of which photo- pyroelectric methods form a subset, have already been shown to be applicable to nondestructive materials evaluation [5-7] and to the monitoring of industrial processes, measuring such things as doping levels in semiconductors [8-9], and metal oxide homogeneity in magnetic recording media [10-11]. Among the many alternative photothermal detection techniques, however, PPE methods combine high temporal resolution with high sensitivity and low cost [12]; con- sequently pyroelectric materials such as thin-film polyvinylidene fluoride (PVDF) show great potential for use as sensor elements in thermal and spectro- scopic photothermal detection systems. In previous reports the PPE technique was applied to the study of the thermal properties of quartz, aluminum and stainless steel [13-15], as well as to the high-Tc superconductors [16-20]. The present work reports PPE measure- ments for the thermal diffusivity of high purity aluminum in various stages of recrystallization, using a chirped laser excitation technique and a thin PVDF