Objective Yttrium oxide (Y2O3) transparent ceramics have the advantages of high melting point, good chemical stability, wide optical transparency (230 nm-8.0 μm), high infrared transmittance, low phonon energy, and high thermal conductivity. They have great application value in high-temperature infrared windows, domes, infrared detectors, luminescent media, lasers, and semiconductor industries. During the preparation process, due to the limitation of the processing technology, tiny pores may be formed on and in the grain boundaries of Y2O3 transparent ceramics, which causes the ceramics cannot achieve complete density. The pores and impurities significantly reduce the optical transmittance of Y2O3 transparent ceramics and produce large absorption, reducing the ceramics’ mechanical and thermal properties, leading to their breakage and failure in extreme environments with high temperatures, high speed, and strong impact. Therefore, it is important to measure and characterize defects, such as pore impurities, in Y2O3 transparent ceramics. Although, methods such as an optical microscopy, scanning electron microscopy, electronic analytical balance, X-ray tomography, and ultrasonic testing can observe the morphology of pores and other defects from the macro or micro levels, they cannot measure the absorption characteristics of defects or detect defects that are not visible in visual imaging but have abnormal absorption. When examining faults in the body, several approaches may cause sample damage. Thermal lens technology based on the photothermal effect is frequently used to identify absorption properties and defects in weakly absorbing solid materials, such as thin films and optical glass. The materials will exhibit thermal deformation on the surface or body when they are excited by a powerful pump light. The thermal lens technology can measure thermal deformation as a result of light absorption. This method has a high detection sensitivity, can precisely evaluate defect absorption properties, and offers a noncontact and nondestructive assessment.
Methods Build surface absorption and in-body absorption measurement devices based on the principle of photothermal and thermal lenses. After the pump laser is modulated by the chopper and focused by the lens, it is incident perpendicular to the surface or the body of the sample after being modulated by the chopper and focused by the lens. The temperature field of the material at the focal spot changes, causing local refractive index variations to produce a “heat slope”. After the beam is extended, the lens focuses the probe light obliquely into the sample surface or body, overlapping the focal position of the pump laser. First, the cerium oxide polishing liquid is used to polish the transparent surface of Y2O3 transparent ceramics on both sides and an X-ray fluorescence spectrometer is used to measure the main components. Thereafter, a scanning electron microscope is used to measure the ceramic surface morphology, an optical profiler is used to measure the surface roughness, and a spectrophotometer is used to measure transmittance. Then, the built photothermal measurement device is used to measure the stability, contrast, and absorbance scan results.
Results and Discussions It is verified that the absorption stability of the photothermal measurement system on the surface and in the body is less than 5% using fused silica glass. Comparing the photothermal measurement results with the same area of Y2O3 transparent ceramics using an optical microscope, the unevenness can reach 80%, indicating that the photothermal measurement system can characterize sample defects. According to the statistical characteristics, the average value plus three times the standard deviation (E+3σ) is used as the segmentation threshold. The images are binarized to indicate the location of the defect (Fig. 6). The scanning measurement of absorption in the different areas of the Y2O3 transparent ceramic surface shows that the absorption of different sample areas have large differences, a high degree of unevenness, and defects such as scratches (Fig. 7). Y2O3 transparent ceramic body absorption measurement results show that there are only small-sized pores and impurities in the body, and the proportion of defects is about 3% (Fig. 8). As a result, using photothermal scanning imaging technology, a link between the absorption signal and ceramic defect may be created, allowing for high-sensitivity detection of the defect and the evaluation of the unevenness of the sample absorption.
Conclusions The experimental results show that the absorption amplitude of surface and internal defects is significantly higher than the intrinsic absorption amplitude of ceramics, and the absorption unevenness caused by surface and internal defects is basically above 50%. The statistical distribution of absorption amplitudes on the surface and in the body shows that the low intrinsic absorption distribution is nearly Gaussian. The image is segmented using E+3σ as the threshold to determine the defect distribution area according to the statistical characteristics. According to the calculation of the binarized image, the defect area accounts for about 3%. Due to the influence of processing, the absorption amplitude and the proportion of defect area on the ceramic surface are higher than the area without processing influence. The experimental results establish the relationship between absorption and defects and realize the precise positioning of surface and internal defects, which is of great significance in improving the ceramic preparation process.
