Photoacoustic (PA) spectra for Zn1-x Co x O mixed-crystal powders with various Co concentrations and sintering temperatures were measured. The PA spectra showed three peaks of Co2+ absorption between 560 nm and 660 nm. On the other hand, the PA spectra between 800 nm and 1000 nm were almost flat, and these are expected to be dominated by light scattering effects. The PA spectra were normalized by using the Co2+ absorption peaks in order to compare the sample size and the PA signals intensity, and the sintering temperature dependence of the PA spectra is discussed. The PA signal intensity decreased with the increase of the sintering temperature. The decrease of the PA spectra seems to be caused by small light scattering effects for the large clusters under higher temperature sintering, where the grains and clusters size increase. We could evaluate the grain growth in the sintering processes by PA spectroscopy in a noncontact mode.

Photoacoustic spectra (PA) of Zn 1−x Co x O alloy semiconductors sintered as powdered samples were measured to estimate the optical absorption. Photoacoustic spectroscopy (PAS) is insensitive to the light scattering effects on the samples. CoO molecular percentages in the alloy were changed from 0 to 20 mol%. The samples were sintered at 600 or 950 • C for 24 h in a quartz glass tube. From the PA spectra in the short wavelength region, we plotted (ρhν) 2 vs hν and estimated the band-gap energy (E g ) of the alloy semiconductors, where ρ is the PA signal intensity and hν is the excitation light photon energy. E g of ZnO-CoO alloy decreases as the CoO content increase and reaches to 2.19 eV in the 20 mol% CoO sample. Although the band-gap variation in the lower-CoO-content samples (CoO content below 10 mol%) is large, it becomes small in the high CoO concentration samples. This result concurred with the result of dissolving CoO in ZnO as obtained by X-ray diffraction measurement.

ZnO varistors degraded under various conditions were evaluated by photoacoustic spectroscopy (PAS). The degradation where the grain boundary is damaged by DC bias stress is more than that by AC bias stress. PAS, however, reveals that the interior of the grains of the sample degraded by AC bias stress is much more damaged than that by the DC bias stress. The PA signal intensity at a wavelength of more than 500 nm increases and the dispersion of the spectrum decreases throughout the wide wavelength range considered with the degradation time. In particular, the decrease of the spectral dispersion below 500 nm is caused by the change of the electronic states at the interface, that is, the increase of the recombination center of the space charge. The annealing effect on the degradation of ZnO varistors was also studied. The PA spectrum of the sample annealed in N2 gas corresponds to those of the sample degraded by DC and AC bias stress for a long time. This suggests that the degradation of ZnO varistors is closely related to the release of oxygen from both ZnO grain interior and grain boundaries.

PA spectra with several sintering temperatures on the Bi doped and Pr doped ZnO varistors were measured using the gas microphone method and compared, where the grain structure was observed by the laser microscope. The PA signals in the long wavelength excitaion light decreased as the sintering temperature increased, although the PA signals were large at the short wavelength region on the all samples. The PA signal intensity seems not to depend on the surface area caused by the grain structure change, but it might also relate to the quality of the sintered samples such as the deep levels or defects in the grain. The Pr doped samples need the higher sintering temperature to get the high quality samples than the Bi doped samples.

The noncontact imaging of the buried structures is carried out in the open-air atmosphere by using the photothermal deflection (PTD) method. We applied these techniques to the layered samples. Besides the PTD images for the optically opaque buried structures, the parameters of the materials such as thermal diffusivity can also be calculated from the PTD amplitude and phase signal in the PTD scanning images. When the PTD signals at two different modulation frequencies are used, the thermal diffusivity of the buried structure can be obtained from the PTD signal outside of the sample nondestructively. 78.20.Hp; 42.30.Va; 65.90.+i Recently, the demand for the nondestructive detection method on the optically opaque buried structures has been increasing by the development of complex functional materials. Since the photoacoustic and photothermal phenomena (PPP) were applied to the nondestructive imaging in the early 1980's [1-4], the photoacoustic microscope (PAM) techniques have been used for the observation and the evaluation of both the surface and inner defects [5,6]. The nondestructive evaluations of the grooved metal planes [7] and the simulated pitting corrosions [8] were reported. The thermal parameters can be obtained in the photoacoustic techniques [9]. Thermal diffusivity is obtained from the modulation frequency dependence of photoacoustic signals [10,11]. The thermal properties are analyzed at the plasma-sprayed zirconia coating [12]. PACS:The PPP is used to detect the nonradiative process in the materials. The photothermal deflection (PTD) method is a kind of PPP [13] and it enables us to measure the absorption spectra and thermal properties of the specimen without contact. In the PTD method, chopped monochromatic excitation light is irradiated onto the sample. The change in the refractive index of the atmosphere just above the sample surface is detected by the crossing probe light which is incident on the sample surface at low angle [14]. The excitation beam is scanned through the sample surface by the mechanical stage with a micropositioner, and the photothermal deflection image can be obtained. This method is superior to the other PAM techniques because it does not require samples to be confined in a photoacoustic (PA) cell like the gas-microphone PA spectroscopy, or the sensors to be directly attached to the sample as in the piezoelectrictransducer method. Therefore, the PTD method allows us to study the temperature-dependent photothermal characteristics of the samples directly.In the PTD methods, both amplitude and the phase signals are obtained simultaneously [15], and one can obtain several parameters of the material such as the optical absorption coefficient and the thermal diffusivity. Furthermore, one can get the depth profile of signals by changing the modulation frequency. Although the thermal properties were obtained from the modulation frequency dependence of the PTD signals in the scanned images until now [10,11], we apply the vector model [15] to the scanning imaging methods and calculat...