Study On Photoacoustic Technique Based On Piezoelectric Detection

Nondestructive Testing(NDT) is an important mean for scientific research, material property measurement, and product quality control. As a new development of NDT, photoacoustic(PA) techniques feature uniqueness, high sensitivity, simplicity and high reliability, thus being among the research hot spots of NDT and widely applied in ingredient analysis and optical, thermal, structural, and elastic property measurements of materials. In this thesis, based on piezoelectric detection, a series of fundamental research on PA techniques including photoacoustic piezoelectric(PAPE) technique, scanning photoacoustic microscopy(SPAM), and photoacoustic spectroscopy(PAS) were carried out to characterize thermal and optical properties of materials. PA techniques based on piezoelectric detection have been further developed, and their applications have been extended.Firstly, the studies of the PAPE technique and its application to the thermal diffusivity measurement were carried out, and the PA characterization of thermal diffusivity was achieved, including:(1) Based on the simplified thermoelastic theoretical model, the conventional frequency domain(FD) PAPE technique was used to determine the thermal diffusivities of materials, and its application range has been specified. By measuring the thermal diffusivities of metals and composite materials, such as copper, aluminum, and YAG, it was found that the conventional FD PAPE technique can effectively characterize the thermal properties of materials with hard elastic properties, while it is not valid for thin and soft materials whose vibration is vulnerable to the attached PZT.(2) A time-domain(TD) PAPE characterization of thermal diffusivity was developed, and the measurement system was accordingly simplified. Based on the Fourier transform and the conventional FD PAPE theory, the TD PAPE model was established and successfully applied to the investigation of the thermal diffusivities of materials such as copper, nickel, and reduced activation ferritic/martensitic(RAFM) steel CLF-1. TD PAPE technique provides a simple, fast, and effective way for thermal diffusivity determination.(3) A modified FD PAPE theory was developed, and measurements of thermal diffusivities of thin and soft materials by the PAPE technique were realized for the first time. Based on the two-layer FD PAPE model, the FD simplified thermoelastic model was revised, and the influence of the PZT on the sample vibration was taken into account. The modified technique was successfully applied to the effective thermal diffusivity determination of thin metal samples, porcine tissues and RAFM steel CLF-1. The modified technique provides an effective thermal diffusivity measurement methodology with higher accuracy than the conventional one.(4) A modified TD PAPE theory was developed, and the measurement accuracy of the TD PAPE technique was improved. Based on the TD PAPE theory, by introducing the modified FD PAPE theory, the modified TD PAPE theory was established and successfully applied to the TD measurement of the thermal diffusivity of nickel, with higher measurement accuracy with respect to the TD PAPE technique.Secondly, the studies of the SPAM technique and its application to the thermal diffusivity imaging were carried out, and the thermal diffusivity distribution imaging(TDDI) of materials characterized by the PA technique was realized, including:(1) The PA imaging system, scanning photoacoustic microscope, was set up. Based on the photoacoustic imaging theory, a SPAM system was set up, and the PA amplitude and phase images of aluminum sample with defective region were obtained, by which the reliability of the system was verified.(2) The TDDI methodology via SPAM-PAPE technique was established, and the PA TDDI was realized for the first time. Based on the SPAM system and the PAPE thermal diffusivity determination theory, the SPAM-PAPE technique was developed and successfully applied to the TDDI of homogeneous materials(copper, steel) and inhomogeneous materials(composed by copper, soldering tin and/or epoxy resin). SPAM-PAPE technique provides an effective way for TDDI, and the application of PA imaging was extended.Finally, the studies of the PAS and its characterization of the optical absorption property of materials were carried out, and the unique optical absorption property characterization of materials by PA technique was realized, including:(1) A non-integrating-sphere type diffuse reflection spectroscopic method was developed, and the analysis of the unique optical absorption properties of Chinese herb medicines was realized. By setting up the Ultraviolet, visible, near-infrared diffuse reflection spectroscopy system and mid-infrared transmission spectroscopic system, the unique optical absorption properties of the Chinese herb medicines such as Rhizoma Coptidis, Cortex Phellodendri, Bulbus Fritillariae Cirrhosae, Caulis Clematidis Armandii etc. were analyzed via their spectra and the corresponding derivative spectra. Results show that the optical spectroscopy can characterize the uniqueness of Chinese herb medicine.(2) A piezoelectric detection based PAS system was set up, and for the first time applied to the quality control of Chinese herb medicines. By designing and processing a high-performance solid PA cell with piezoelectric detector, a piezoelectric PAS system was set up. The study of obtaining the PAS spectra of Chinese herb medicines, such as Rhizoma Coptidis, Cortex Phellodendri, Bulbus Fritillariae Cirrhosae, Caulis Clematidis Armandii and etc. has been carried out. By comparing with the optical spectroscopic measurement results, the PAS results were validated. The experimental results show that PAS can effectively identify various Chinese herb medicines, which can be a complementary method to the optical spectroscopy for quality control of Chinese herb medicines.