Thermal Diffusivity Measurement By Photoacoustic Piezoelectric Technique With Barker Code Modulated Laser Excitation

Thermophysical properties of materials are one of the important properties of materials.Thermal diffusivity is an important characteristic parameter of thermal physical properties of materials,which reflects the speed of heat diffusion in materials.Therefore,it is particularly important to realize nondestructive,rapid,quantitative and accurate measurement of thermal diffusivity of materials.Photoacousto-piezoelectrics is a new technique in the field of nondestructive testing.It is used to measure the thermal diffusivity of materials because of its high sensitivity and specificity.At present,the measurement method of thermal diffuser of materials based on optoacoustic piezoelectric technology needs to be further improved in terms of measurement accuracy and measurement speed.In thesis,the time-domain electro-thermal diffuser of optical acoustic pressure based on Buck code pulse compression technology is carried out to achieve rapid and accurate measurement of thermal diffuser of materials.Specific research contents are as follows:Firstly,the Bakker code theory of time-domain opto-acoustic pressure electrothermal diffuser measurement is established.Based on the single layer and double layer optical acousto-piezoelectricity model in frequency domain,the expression of single layer and double layer optical acousto-piezoelectricity signal in time domain of Barker code laser modulation is derived,and the theory of Barker code optical acoustopiezoelectricity in time domain is established.Based on the signal processing method of Barker code modulation and matching filter demodulation,the inversion method of electro-thermal diffuser of Barker code in time domain is proposed,and the measurement theory of electro-thermal diffuser of Barker code in time domain is established.Secondly,based on the Barker code time-domain opto-acoustic piezoelectricity theoretical model,the simulation research of the parameters affecting the measurement of the electro-thermal diffuser of the Barker code in the time-domain single layer and double layer opto-acoustic pressure is carried out.The influences of laser modulation frequency,thickness of the sample to be measured,thermal diffuser of the sample to be measured and elastic parameters of the sample to be measured on the sensitivity of thermal diffuser measurement in the theory of single-layer and double-layer photoacoustic pressure electrothermal diffuser measurement in the frequency domain were analyzed.Furthermore,in the theory of single-layer and double-layer photoacoustic pressure electrothermal diffuser measurement in the time domain of Barker code,Effects of Barker code sequence,thermal diffusivity of sample to be measured,carrier frequency of Barker code signal,thickness of sample to be measured and elastic parameters of sample to be measured on the sensitivity of thermal diffusivity measurement.The results show that the thermal diffusivity of the sample under test is the most sensitive when the thermal diffusivity length of the sample under test is a certain value of the thickness of the sample,and provides a theoretical basis for the optimization of the experimental system in the future.Finally,based on the Barker code time-domain opto-acoustic pressure electrothermal diffuser measurement theory,the Barker code time-domain opto-acoustic pressure electrothermal diffuser measurement system was established,and the experimental study on the thermal diffuser measurement of single-layer and double-layer materials was carried out.By using thermal diffuser sensitivity analysis,the parameters of Barker code time-domain opto-acoustic pressure electrothermal diffuser measurement experiment system are optimized.The thermal diffuser values of single-layer samples(iron,copper and nickel)and double-layer samples(nickel-iron)are measured respectively.The relative errors of the experimental results are within 3% and 5%,respectively.This study shows that the Barker code time-domain photoacoustic pressure electrothermal diffusivity measurement method can measure the thermal diffusivity of single-layer and double-layer materials quickly,highly sensitive and highly accurate,and provides a new means for the measurement of thermal diffusivity of materials.