Multi-type barker code modulation laser excitation double-layer material photoacoustic piezoelectric thermal diffusivity measurement method

Abstract

The invention discloses a method for measuring photoacoustic piezoelectric thermal diffusivity of a multi-type barker code modulation laser excitation double-layer material, which comprises the following steps of: modulating laser light intensity by using a barker code signal with an optional carrier wave to form barker code excitation light, and exciting a double-layer sample by the barker code excitation light to generate a photoacoustic signal; when the characteristic parameters of the excitation light of the barker code and the thicknesses of the upper layer and the lower layer of the sample are determined, a photoacoustic signal is detected by a piezoelectric transducer adhered to the rear surface of the sample; the thermal diffusivity corresponding to the maximum value of a correlation peak is the thermal diffusivity of the upper-layer material by performing correlation operation processing on a series of theoretical photoacoustic signals and measured photoacoustic signals corresponding to different thermal diffusivity of the upper-layer material and different elastic property ratios of the upper-layer material and the lower-layer material, and the double-layer sample is turned over for same measurement and processing. The thermal diffusivity of the lower-layer material is obtained. The method provides a lossless, rapid and accurate measurement means for measuring the thermal diffusivity of each layer of material of the double-layer sample.

Description

Technical Field

[0001] The invention relates to the field of solid material thermal property detection, and in particular to a non-destructive, rapid, accurate and quantitative characterization method for photoacoustic piezoelectric thermal diffusivity of double-layer materials excited by multi-type Barker code modulated lasers. Background Art

[0002] Inhomogeneous materials can exhibit different properties in different parts of their interior, such as density, hardness, thermal conductivity, etc. The spatial variation of internal properties gives them unique performance and broad application prospects. As one of the most important inhomogeneous materials, double-layer materials have been widely used in aerospace, automotive industry, electronic equipment and other fields. Thermal properties are basic physical properties that cannot be ignored in the application process of materials. Among them, thermal diffusivity reflects how fast heat spreads in the material. Accurate measurement and calculation of thermal diffusivity can better solve problems such as heat dissipation of heating elements and thermal conduction of insulation materials. Therefore, it is particularly important to achieve non-destructive, accurate and efficient measurement of the thermal diffusivity of double-layer materials.

[0003] Photoacoustic and photothermal technology is a non-destructive measurement technology based on the photoacoustic and photothermal effects of matter. It characterizes the surface, sub-surface and in-body properties of samples by detecting and analyzing the thermal waves and acoustic waves stimulated by modulated lasers in samples. Among them, photoacoustic and piezoelectric technology is one of the important technologies for detecting the thermophysical properties of materials. It combines the photoacoustic effect with the piezoelectric effect and has the advantages of high sensitivity and easy operation. This technology uses a laser beam with periodic light intensity modulated to stimulate the sample to stimulate thermal waves, and acoustic waves are generated inside the sample based on the thermoelastic effect. The acoustic wave signal carrying the thermophysical information of the sample is detected by a piezoelectric transducer, thereby inverting the thermal diffusivity of the sample.

[0004] At present, the measurement range of thermal diffusivity of materials by photoacoustic piezoelectric technology is limited. First, the material thermal diffusivity measurement methods based on photoacoustic piezoelectric technology are mostly based on single-layer models, and the thermal diffusivity of single-layer materials is mostly measured; second, the material thermal diffusivity measurement methods based on photoacoustic piezoelectric technology include frequency domain measurement methods and time domain measurement methods, but the frequency domain measurement method is time-consuming, the system is complex and expensive, and the time domain measurement method using square wave signals or chirp signals to modulate lasers can quickly and effectively measure the thermal diffusivity of materials, but the signal-to-noise ratio is not optimal. Therefore, in order to expand the application scope of photoacoustic technology, in order to address the problem of fast and high-precision measurement of thermal diffusivity of double-layer materials, it is an urgent need to develop photoacoustic piezoelectric technology for fast, non-destructive, high-precision, and quantitative thermal diffusivity measurement of double-layer materials in the field of material thermal property detection and photoacoustic and photothermal fields. Summary of the invention

[0005] The technical problem to be solved by the present invention is: how to overcome the deficiencies of the frequency-domain photoacoustic piezothermal diffusivity measurement method and the existing time-domain photoacoustic piezothermal diffusivity measurement method for measuring the thermal diffusivity of bilayer materials, and propose a new time-domain photoacoustic measurement method for bilayer materials to achieve non-destructive, fast, high signal-to-noise ratio, and accurate measurement of the thermal diffusivity of bilayer materials.

[0006] The technical problem proposed by the present invention is solved as follows: The present invention proposes a multi-type Barker code modulated laser excitation photoacoustic piezothermal diffusivity measurement method for bilayer materials. The system includes a computer 1, a function generator 2, a laser 3, a beam splitter 4, a total reflector 5, a focusing lens 6, a bilayer sample to be measured 7, a piezoelectric transducer 8, a focusing lens 9, a photodetector 10, and a data acquisition card 11. It is characterized in that: The computer 1 controls the function generator 2 to generate a Barker code signal with an optional carrier wave. The Barker code signal modulates the laser 3 to form a laser beam with intensity Barker code modulation. After passing through the beam splitter 4, the Barker code modulated laser beam is divided into a transmitted light beam and a reflected light beam. The transmitted light beam is reflected by the total reflector 5 and focused by the focusing lens 6 to be used as the excitation light to excite the bilayer sample to be measured 7 to generate a photoacoustic signal. This photoacoustic signal is detected by the piezoelectric transducer 8 coupled to the rear surface of the sample, and is related to the thermal diffusivity of the upper layer material in the bilayer sample, the elastic properties and thickness of the upper and lower layers of materials, and the characteristics of the Barker code excitation light. At the same time, the reflected light beam is focused by the focusing lens 9 and received by the photodetector 10 as a real-time monitoring signal of the excitation light. The photoacoustic signal and the real-time monitoring signal of the excitation light are simultaneously collected by the data acquisition card 11 and uploaded to the computer 1. The computer 1, based on the real-time monitoring signal of the excitation light and the frequency-domain bilayer photoacoustic piezoelectric model, obtains a series of theoretical photoacoustic signals corresponding to different thermal diffusivities of the upper layer material and different ratios of elastic parameters of the upper and lower layers of materials when the characteristics of the Barker code excitation light and the thicknesses of the upper and lower layers of materials are known. By performing signal correlation operation processing on this series of theoretical photoacoustic signals and the measured photoacoustic signals collected, the thermal diffusivity corresponding to the theoretical photoacoustic signal with the maximum peak value of the correlation signal is the thermal diffusivity of the upper layer material. By turning the bilayer sample over and performing the same measurement and processing, the thermal diffusivity of the lower layer material can be obtained.

[0007] The Barker code signal with an optional carrier wave generated by the function generator 2 is a low-frequency signal, satisfying the quasi-steady state approximation of elasticity theory f << c / (L + H), where f is the modulation light frequency, c is the sound speed in the sample to be measured, L is the thickness of the upper layer material, and H is the thickness of the lower layer material; and the upper layer material of the bilayer sample to be measured satisfies the thermal thickness condition. Where D is the thermal diffusivity of the upper layer material, that is, the thermal wave only appears in the upper layer, and the lower layer only serves as a sound propagation medium.

[0008] The carrier-selectable Barker code signal is composed of a Barker code sequence of length N, wherein N has seven length values ​​of 2, 3, 4, 5, 7, 11, and 13 bits; the optional carrier signals of the Barker code signal include square wave signals, sine signals, triangle wave signals, and chirp signals. When the sine signal, triangle wave signal, and square wave signal are selected as carrier signals, the symbol width should be a positive integer multiple of the carrier signal period; when the chirp signal is selected as the carrier signal, the symbol width should be a positive integer multiple of the chirp duration.

[0009] The steps for the computer 1 to generate a series of theoretical photoacoustic signals are: Fourier transform the real-time monitoring signal of the excitation light, multiply it with the photoacoustic response function under the frequency domain double-layer photoacoustic piezoelectric model corresponding to different thermal diffusivities and different upper and lower layer elastic property ratios, and then normalize the complex vector so that its two-norm is 1.

[0010] The inversion step of the thermal diffusivity of the double-layer material is to perform correlation operation on a series of theoretical photoacoustic signals generated by the computer 1 and the measured photoacoustic signals, that is, to perform Fourier transform on the measured photoacoustic signals, take the complex conjugate of its spectrum and multiply it with a series of theoretical photoacoustic signals, and perform inverse Fourier transform to obtain a series of time-domain correlation signals, and take the thermal diffusivity corresponding to the theoretical photoacoustic signal with the largest peak value of the correlation signal as the thermal diffusivity of the upper material. The double-layer sample is turned over and the same measurement and processing are performed to obtain the thermal diffusivity of the lower material, and finally the thermal diffusivity of each layer of the double-layer sample is obtained.

[0011] The beneficial effects of the present invention are as follows: the shortcomings of the existing photoacoustic piezoelectric thermal diffusivity measurement method, such as the limited measurement sample range, the slow measurement speed of the frequency domain photoacoustic piezoelectric thermal diffusivity measurement method, and the low measurement accuracy of the existing time domain photoacoustic piezoelectric thermal diffusivity measurement method, are overcome. A Barker code signal with better correlation and optional carrier for modulation of laser and a related demodulation method are adopted, and a multi-type carrier Barker code time domain double-layer photoacoustic piezoelectric theoretical model is established based on the frequency domain double-layer photoacoustic piezoelectric model, thereby realizing non-destructive, fast, accurate, and high signal-to-noise ratio measurement of the thermal diffusivity of each layer of the double-layer sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a schematic diagram of the system of the present invention, wherein 1 is a computer, 2 is a function generator, 3 is a laser, 4 is a transflective mirror, 5 is a total reflective mirror, 6 and 9 are focusing lenses, 7 is a double-layer sample to be tested, 8 is a piezoelectric transducer, 10 is a photoelectric detector, and 11 is a data acquisition card.

[0013] Figure 2 It is an 11-bit Barker code modulation signal with a sinusoidal carrier and a carrier period equal to the code element width.

[0014] Figure 3 This is the theoretical model of Barker code time-domain double-layer photoacoustic piezoelectric.

[0015] Figure 4 Figure 1 is the thermal diffusivity inversion algorithm block diagram and signal-related processing results. Among them, r(t) is the excitation light signal, s(t) is the measured photoacoustic signal, FFT and IFFT are fast Fourier transform and inverse fast Fourier transform, T(D,p) is the frequency domain photoacoustic piezoelectric response function of the double-layer sample, and Z* is the complex conjugate. DETAILED DESCRIPTION

[0016] Combine the following Figures 1 to 4 The present invention specifically describes a method for measuring the photoacoustic piezoelectric thermal diffusivity of a double-layer material excited by a multi-type Barker code modulated laser. However, it should be understood that the accompanying drawings are provided only for a better understanding of the present invention and should not be construed as limiting the present invention. The specific implementation steps are as follows:

[0017] (1) Establishment of experimental system

[0018] a. Figure 1 As shown, the computer 1 controls the function generator 2 to generate a Barker code signal with a selectable carrier. Different carriers (square wave signal, sine signal, triangle wave signal, chirp signal) have different correlation characteristics and measurement performances. Taking an 11-bit sine carrier Barker code signal as an example, Figure 2 As shown, its expression is as follows:

[0019]

[0020] Among them, C n =[+1,+1,+1,-1,-1,-1,+1,-1,-1,+1,-1] is an 11-bit Barker code sequence, T0 is the symbol width, ω0=2π / T0 is the carrier angular frequency, and u(t) is the unit step function.

[0021] b. Barker code signal modulation The laser 3 forms a laser beam with light intensity Barker code modulation, which is divided into a transmitted light beam and a reflected light beam after passing through the reflector 4.

[0022] c. The transmitted light is reflected by the total reflection mirror 5 and focused by the focusing lens 6 as the excitation light to excite the double-layer sample 7 to generate a photoacoustic signal. The photoacoustic signal is detected by the piezoelectric transducer 8 coupled to the rear surface of the sample. The photoacoustic signal is related to the thermal diffusivity of the upper material in the double-layer sample, the elastic properties and thickness of the upper and lower layers, and the Barker code excitation light characteristics (the Barker code time-domain double-layer photoacoustic piezoelectric theoretical model is shown in Figure 3 shown).

[0023] d. The reflected light is focused by the focusing lens 9 and received by the photodetector 10 as a real-time monitoring signal of the excitation light. Its waveform is similar to Figure 2 The Barker code signal is consistent.

[0024] e. The measured photoacoustic signal and the real-time monitoring signal of the excitation light are simultaneously collected by the data acquisition card 11 and uploaded to the computer 1 .

[0025] (2) Signal processing and thermal diffusivity inversion

[0026] a. Figure 4 As shown, computer 1 performs fast Fourier transform on the excitation light signal r(t) and the measurement photoacoustic signal s(t) to obtain their spectra R(ω) and S(ω).

[0027] b. Computer 1 compares the spectrum R(ω) of the excitation light signal with the spectrum R(ω) of the double-layer photoacoustic piezoelectric model (such as Figure 3 The frequency domain photoacoustic piezoelectric response function T(ω, D, p) of the upper material with different upper and lower elastic parameter ratios (as shown) is multiplied to obtain a series of complex vectors, and then the complex vectors are normalized to make their bi-norm 1 to obtain a series of theoretical photoacoustic signals. The frequency domain photoacoustic piezoelectric response function T(ω, D, p) in the double-layer piezoelectric model is expressed as:

[0028]

[0029] in,

[0030]

[0031] A=pL+H, σ 2 =iω / D

[0032] ω=2πf is the angular frequency, D is the thermal diffusivity of the upper material of the double-layer sample, L is the thickness of the upper material, H is the thickness of the lower material, and p is the elastic performance ratio of the upper and lower layers.

[0033] c. Take the complex conjugate of the measured photoacoustic signal spectrum S(ω), multiply it with the series of theoretical photoacoustic signals obtained in step b, and perform inverse Fourier transform to obtain a series of time-domain correlation signals. The thermal diffusivity corresponding to the theoretical photoacoustic signal with the largest peak value of the time-domain correlation signal is the thermal diffusivity of the upper material.

[0034] d. Turn the double-layer sample over and perform the same measurement and processing to obtain the thermal diffusivity of the lower layer material, and finally obtain the thermal diffusivity of each layer of the double-layer sample.

[0035] The present invention proposes a method for measuring the thermal diffusivity of double-layer materials using multi-type Barker code modulated laser excitation photoacoustic piezoelectricity. Based on the frequency domain double-layer photoacoustic piezoelectricity theory, a multi-type carrier Barker code time domain double-layer photoacoustic piezoelectricity theoretical model is established. A Barker code signal with good correlation performance and optional carrier is used to modulate the laser to excite the double-layer sample to generate a photoacoustic signal. The thermal diffusivity is inverted by a correlation demodulation method to obtain the thermal diffusivity of each layer of the double-layer sample. This method provides a non-destructive, fast, accurate, and high signal-to-noise ratio detection method for measuring the thermal diffusivity of double-layer materials.

Claims

1. A method for measuring the photoacoustic piezoelectric thermal diffusivity of double-layer materials using multi-type Barker code modulated laser excitation, characterized in that: A computer (1) controls a function generator (2) to generate a Barker code signal with a selectable carrier. The Barker code signal modulates a laser (3) to form a laser beam with light intensity Barker code modulation. The Barker code modulated laser beam passes through a transflector (4) and is divided into a transmitted light beam and a reflected light beam. The transmitted light is reflected by a total reflector (5) and focused by a focusing lens (6) to serve as an excitation light to excite a double-layer sample (7) to generate a photoacoustic signal. The photoacoustic signal is detected by a piezoelectric transducer (8) coupled to the rear surface of the sample and is related to the thermal diffusivity of the upper layer material in the double-layer sample, the elastic properties and thickness of the upper and lower layers of the material, and the Barker code excitation light characteristics. At the same time, the reflected light is focused by a focusing lens (9) and received by a photodetector (10) as a real-time monitoring signal of the excitation light. The real-time monitoring signal of the excitation light is simultaneously collected by the data acquisition card (11) and uploaded to the computer (1). Based on the real-time monitoring signal of the excitation light and the frequency domain double-layer photoacoustic piezoelectric model, the computer (1) obtains a series of theoretical photoacoustic signals corresponding to different thermal diffusivities of the upper layer material and different elastic property ratios of the upper and lower layers of the material, when the Barker code excitation light characteristics and the thickness of the upper and lower layers of the material are known. The series of theoretical photoacoustic signals are subjected to signal correlation processing with the collected measured photoacoustic signals, and the thermal diffusivity corresponding to the theoretical photoacoustic signal with the largest correlation signal peak value is obtained, which is the thermal diffusivity of the upper layer material. The double-layer sample is turned over and the same measurement and processing are performed to obtain the thermal diffusivity of the lower layer material, and finally the thermal diffusivities of the materials of each layer of the double-layer sample are obtained. The optional Barker code signal of the carrier wave generated by the function generator (2) is a low-frequency signal, satisfying the quasi-steady state approximation of elasticity mechanics f << c / (L + H), where f is the modulation light frequency, c is the sound velocity in the sample to be measured, L is the thickness of the upper layer material, and H is the thickness of the lower layer material; and the upper layer material of the double-layer sample to be measured satisfies the thermal thickness condition Among them, D is the thermal diffusivity of the upper layer material, that is, the thermal wave only appears in the upper layer, and the lower layer only serves as the sound propagation medium. The Barker code signal is composed of a Barker code sequence of length N, and N has 7 length values of 2, 3, 4, 5, 7, 11, and 13 bits. The optional carrier signals of the Barker code signal include several types such as sine signal, square wave signal, triangular wave signal, and chirp signal. When choosing sine signal, square wave signal, or triangular wave signal as the carrier signal, the symbol width should be an integer multiple of the carrier signal period. When choosing chirp signal as the carrier signal, the symbol width should be an integer multiple of the chirp duration. The steps of the computer (1) generating a series of theoretical photoacoustic signals are: Fourier transforming the real-time monitoring signal of the excitation light, multiplying it with the photoacoustic response function of the frequency domain double-layer photoacoustic piezoelectric model corresponding to different thermal diffusivities and different upper and lower layer elastic property ratios, and then normalizing the complex vector so that its two-norm is 1. The inversion step of the thermal diffusivity of the double-layer material is to perform correlation operations on a series of theoretical photoacoustic signals generated by the computer (1) and the measured photoacoustic signals, that is, to perform Fourier transform on the measured photoacoustic signals, take the complex conjugate of the spectrum and multiply it with a series of theoretical photoacoustic signals, and perform inverse Fourier transform to obtain a series of time-domain correlation signals. The thermal diffusivity corresponding to the theoretical photoacoustic signal with the largest peak value of the correlation signal is the thermal diffusivity of the upper material.

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