Photoacoustic investigation of the effect of excess lead oxide on thermal diffusivity of PLZT ceramic

Abstract

The versatility as well as the potential of the photoacoustic (PA) technique as a material characterization method has been established by several workers [1–6]. Apart from providing direct optical absorption spectra, the PA technique can also be used to perform depth profile analysis, thermal characterization as well as investigation of nonradiative relaxation processes [7–10]. In a typical PA experimental arrangement the sample enclosed in an airtight cavity is exposed to an intensity modulated light beam. The resulting periodic heating of the sample is strongly dependent on the interplay of three factors, namely, the optical absorption coefficient at the incident radiation wavelength, lightinto-heat conversion efficiency and the heat diffusion through the sample. The light-into-heat conversion efficiency of each material depends on the nonradiative de-excitation processes taking place within the sample. The dependence of the PA signal on the rate of heat diffused through the sample allows us to perform thermal characterization, especially the determination of thermal diffusivity. Ceramics are considered as the optimum materials to solve a number of scientific and technological problems due to the availability of raw material as well as their resistance to corrosion and irradiation. The unique electro-optic and photo-electric properties of transparent ferroelectric ceramics (TFC) have helped them to acquire a significant fraction of the solid state optoelectronic device market, particularly in high speed light modulators and shutters, thermal and light filters, electrically controlled color filters, alphanumeric displays, block data composers, video projectors and optoelectronic voltmeters [11–16]. The majority of the TFC compositions include materials prepared on the basis of the well-known PZT system with the ABO3 perovskite structure. One such material is lanthanum doped lead zirconate-titanate (PLZT), in which some Pb2+ ions in the A sites are replaced by the higher valence La3+ ions. As a result of the difference in the valency between Pb2+ and La3+, some of the A sites and B sites will be vacant in order to maintain electrical neutrality in the structure. Earlier studies show that the nonstoichiometry of lead oxide as well as the doping of lanthanum introduces a large number of vacancies in PLZT [17, 18]. This letter deals with a study of the effect of excess lead oxide on the thermal diffusivity of PLZT ceramic carried out using the laser induced photoacoustic technique. Since the material preparation was done at 1200 ◦C, some fraction of the lead may escape during the preparation, so the actual lead content may be slightly less than the initial concentration. Samples were prepared with excess lead oxide in different weight percentages and the thermal diffusivity was determined for each sample with different amounts of excess lead oxide. The experimental set up used for the present investigation is similar to the one used by George et al. [19] as shown in Fig. 1. The 488 nm line of an argon ion laser (Liconix 5000 series) was used as the pump beam. The laser beam at a power level of 70 mW was intensity modulated using an electromechanical chopper (Ithaco HMS 230) before it was made to fall on the sample. The sample compartment of the non-resonant PA cell made of stainless steel has a diameter of 8 mm and a depth of 5 mm. The PA signal produced in the cavity was detected using a miniature electret microphone kept in a side chamber coupled to the sample compartment. The output of the microphone was processed using a lock-in-amplifier (Stanford Research Systems SR 510). We measured the PA signal amplitude as a function of chopping frequency. Charpentier et al. have presented a frequency analysis of the PA signal for the determination of thermal diffusivity based on the RosencwaigGersho theory for the PA effect [20, 21]. According to this model, knowing the actual thickness (ls) of the sample and the characteristic frequency ( fc) at which the sample becomes thermally thick, the thermal