基于光热技术的氧化钇透明陶瓷缺陷特性研究

Defects Characteristics of Y_(2)O_(3) Transparent Ceramics Based on Photothermal Technique

针对氧化钇透明陶瓷在制备过程中产生的气孔、杂质等缺陷难以实现定量化无损检测、评估的问题,本研究团队提出了基于光热热透镜效应的扫描成像测量方法,建立了氧化钇透明陶瓷表面和体内不同区域的光热吸收信号与气孔、杂质等缺陷之间的关联。根据吸收幅值的统计分布特征,采用以E+3σ(E和σ分别为吸收的平均值和标准差)为阈值对扫描图像进行分割的方法来表征缺陷的分布。与光学显微镜观测等方法相比,该方法具有灵敏度高、可准确表征缺陷的吸收等优点,可以为缺陷对材料吸收特性和热学特性的影响提供评判依据,进一步为氧化钇透明陶瓷制备工艺的改进提供重要支撑。

Objective Yttrium oxide(Y_(2)O_(3))transparent ceramics have the advantages of high melting point,good chemical stability,wide optical transparency(230 nm-8.0μm),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 Y_(2)O_(3) transparent ceramics,which causes the ceramics cannot achieve complete density.The pores and impurities significantly reduce the optical transmittance of Y_(2)O_(3) 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 Y_(2)O_(3) 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 absorptioncharacteristics 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 apowerful 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 Y_(2)O_(3) 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 Y_(2)O_(3) 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 Y_(2)O_(3) 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).Y_(2)O_(3) 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+3σas 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.