1. A method for measuring the thermal diffusivity of a metal material based on photoacoustic signal matched filtering is characterized by comprising the following steps of: the function generator (1) generates a chirp signal and modulates the laser (2) to make the laser emit a laser beam with chirp modulation of light intensity, the laser beam passes through the transflective mirror (3), the total reflective mirror (4) and the focusing lens (5) and then excites the front surface of a metal sample (6) to be detected and generates a photoacoustic signal in the laser beam, and the photoacoustic signal is detected by a piezoelectric transducer (7) coupled to the rear surface of the sample; a small part of light split by the transflective mirror (3) is received by the photoelectric detector (8), so that the real-time monitoring of the time domain characteristics of the excitation light intensity is realized; the data acquisition card (9) acquires the photoacoustic signal and the excitation light intensity time domain signal and transmits the photoacoustic signal and the excitation light intensity time domain signal to the computer (10); the computer (11) generates a series of reference signals according to the excitation light intensity signals monitored in real time and by combining the frequency domain photoacoustic transfer functions corresponding to different thermal diffusivity; and respectively carrying out correlation operation on the reference signals and the measured photoacoustic signals, wherein the reference signal with the highest correlation peak value is the matched filter, and the corresponding thermal diffusivity is the measured value.

2. The photoacoustic signal matched filter-based metal material thermal diffusivity measurement method of claim 1, characterized in that: the initial frequency and the cut-off frequency of the chirp modulation signal generated by the function generator 1 are both low frequencies, namely, the quasi-steady state approximation f < < c/L of the elastomechanics is met, wherein f is the frequency of the generated sound wave, c is the sound velocity in the sample to be detected, and L is the size of the sample; the time-bandwidth product of the chirp signal should be an integer, i.e., the product of the difference between the chirp start frequency and the cutoff frequency and the chirp duration is an integer.

3. The photoacoustic signal matched filter-based metal material thermal diffusivity measurement method of claim 1, characterized in that: the laser (2) is a continuous laser capable of realizing light intensity analog modulation, and the output light intensity and the modulation electric signal have good linearity.

4. The photoacoustic signal matched filter-based metal material thermal diffusivity measurement method of claim 1, characterized in that: the thickness of the piezoelectric transducer (7) should be much smaller than the thickness of the sample so that its effect on the sample vibrations is negligible.

5. The photoacoustic signal matched filter-based metal material thermal diffusivity measurement method of claim 1, characterized in that: the sampling frequency of the data acquisition card (9) should be much higher than the chirp cut-off frequency.

6. The photoacoustic signal matched filter-based metal material thermal diffusivity measurement method of claim 1, characterized in that: the specific algorithm for generating a series of reference signals by the computer (10) is that the Fourier transform is firstly carried out on the excitation light intensity time-domain signal monitored in real time, then the Fourier transform is multiplied by the frequency-domain photoacoustic transfer function corresponding to different thermal diffusivity, and then the complex vector is normalized to enable the two norm to be 1.

7. The photoacoustic signal matched filter-based metal material thermal diffusivity measurement method of claim 1, characterized in that: the specific algorithm for inversion of matched filtering thermal diffusivity is that a series of reference signals in claim 6 are respectively subjected to correlation operation with measured photoacoustic signals, the reference signal with the highest correlation peak value is found, namely the matched filter is found, the corresponding thermal diffusivity is the measured value, and the signal-to-noise ratio is the largest at the moment.

Background

The thermophysical properties of the material can be divided into two types, namely transport properties and thermodynamic properties, wherein the transport properties refer to properties related to energy and momentum transfer processes, and specific parameters comprise heat conductivity coefficient, thermal diffusivity, thermal radiation parameters (emissivity, absorptivity and reflectivity) and the like; the latter refers to properties related to the law of state transition and energy conversion in thermal phenomena, such as specific heat, coefficient of thermal expansion, etc. The thermophysical property parameter is a basis for measuring whether the material can adapt to the working environment of a specific thermal process or not and is a key for basic research, analysis calculation and engineering design of the specific thermal process, so that the thermophysical property parameter of the material can be measured and represented nondestructively, quickly, quantitatively and accurately, and the thermophysical property parameter measuring instrument can not only provide innovative research service in the field of material science, but also provide guarantee for industrial production and quality monitoring.

Since the introduction of photoacoustic photothermal technology in the seventies of the last century, the development of photoacoustic photothermal technology has become one of the important branches in the field of nondestructive testing and evaluation. The photoacoustic photothermal technology is based on the photoacoustic photothermal effect of substances, utilizes dynamically modulated laser to excite diffused waves in a sample, and utilizes optical and acoustic methods to realize detection of the diffused waves, so that the characteristics of the surface, subsurface and body of the sample are estimated. Due to the advantages of no damage, dynamic property, quantification, high sensitivity, strong specificity and the like, the photoacoustic photothermal technology is widely applied to nondestructive quantitative characterization of various materials in the aspects of optics, thermal property, electricity, mechanics, components, structures and the like. Due to the strong correlation between the photoacoustic signal and the thermal property of the sample, the photoacoustic technology can realize the quantitative characterization of the thermophysical property parameters of the material.

The traditional photoacoustic technology mainly adopts a single-frequency excitation and phase-locked demodulation mode on the modulation and demodulation of signals. In this mode, the thermal diffusivity of the material to be measured needs to be scanned from low frequency to high frequency, phase-locked measurement is performed at each single frequency point to obtain amplitude and phase information, and then data of the amplitude frequency and the phase frequency measured by experiments are fitted by using a theoretical model, so that the thermal diffusivity is extracted. This method is very time consuming, typically requires on the order of ten minutes to complete a measurement, and the signal-to-noise ratio is not optimized, so the development of a rapid, non-destructive, and quantitative photoacoustic technique for thermal diffusivity measurement is an urgent need in the field of material thermophysical property detection and photoacoustic photothermal.

Disclosure of Invention

The invention aims to solve the problems that: how to overcome the defects of the existing method for measuring the thermal diffusivity by adopting the photoacoustic technology, and the novel photoacoustic measurement method for the thermal diffusivity is provided, so that the rapid, nondestructive and quantitative measurement of the thermal diffusivity is realized.

The technical problem proposed by the invention is solved as follows: the system comprises a function generator 1, an excitation laser 2, a transflective mirror 3, a total-reflective mirror 4, a focusing lens 5, a metal sample 6 to be detected, a piezoelectric transducer 7, a photoelectric detector 8, a data acquisition card 9 and a computer 10, and is characterized in that: the function generator 1 generates a chirp signal and modulates the laser 2 to make the laser send out a laser beam modulated by light intensity chirp, the laser beam passes through the transflective mirror 3, the total reflective mirror 4 and the focusing lens 5 to excite the front surface of a metal sample 6 to be measured and generate a photoacoustic signal in the metal sample, and the photoacoustic signal is detected by a piezoelectric transducer 7 coupled to the rear surface of the sample; a small part of light split by the transflective mirror 3 is received by the photoelectric detector 8, so that the real-time monitoring of the time domain characteristics of the excitation light intensity is realized; the data acquisition card 9 acquires the photoacoustic signal and the excitation light intensity time domain signal and transmits the photoacoustic signal and the excitation light intensity time domain signal to the computer 10; the computer 10 generates a series of reference signals according to the excitation light intensity signals monitored in real time and by combining the frequency domain photoacoustic transfer functions corresponding to different thermal diffusivity; and respectively carrying out correlation operation on the reference signals and the measured photoacoustic signals, wherein the reference signal with the highest correlation peak value is the matched filter, and the corresponding thermal diffusivity is the measured value.

The initial frequency and the cut-off frequency of the chirp modulation signal generated by the function generator 1 are both low frequencies, namely, the quasi-steady state approximation f < < c/L of the elastomechanics is met, wherein f is the frequency of the generated sound wave, c is the sound velocity in the sample to be detected, and L is the size of the sample; the time-bandwidth product of the chirp signal should be an integer, i.e., the product of the difference between the chirp start frequency and the cutoff frequency and the chirp duration is an integer.

The laser 2 should be a continuous laser capable of realizing analog modulation of light intensity, and the output light intensity and the modulation electric signal should have good linearity.

The thickness of the piezoelectric transducer 7 should be much smaller than the thickness of the sample so that its effect on the sample vibrations is negligible.

The sampling frequency of the data acquisition card 9 should be much higher than the chirp cut-off frequency.

The specific algorithm for generating a series of reference signals by the computer 10 is to perform fourier transform on the excitation light intensity time-domain signal monitored in real time, multiply the signal by the frequency-domain photoacoustic transfer function corresponding to different thermal diffusivity, and normalize the complex vector to make the two-norm 1.

The specific algorithm for inversion of matched filtering thermal diffusivity is that a series of reference signals are respectively subjected to correlation operation with measured photoacoustic signals, the reference signal with the highest correlation peak value is found, namely a matched filter is found, the corresponding thermal diffusivity is a measured value, and the signal-to-noise ratio at the moment is the maximum

The invention has the beneficial effects that: the method overcomes the defects of low measurement speed and the like of the existing photoacoustic thermal diffusivity measurement method, completes signal processing, filtering and quantitative measurement of target parameters in the same step, and realizes nondestructive, quantitative, rapid and economic thermophysical measurement of materials.

Drawings

FIG. 1 is a schematic diagram of the system of the present invention, in which 1 is a function generator, 2 is an excitation laser, 3 is a transflective mirror, 4 is a total-reflection mirror, 5 is a focusing lens, 6 is a metal sample to be measured, 7 is a piezoelectric transducer, 8 is a photodetector, 9 is a data acquisition card, and 10 is a computer.

Fig. 2 shows experimental measurement signals of a certain red copper sample. (a) The time domain signal of the excitation light intensity is monitored in real time, and (b) is the corresponding photoacoustic signal.

FIG. 3 shows signal matched filteringAnd an algorithmic block diagram for quantitative thermal diffusivity measurement. Where f (t) is the time domain signal of the excitation light intensity, s (t) is the photoacoustic signal, FFT and IFFT are the fast Fourier transform and inverse transform, Z^*To take the complex conjugate, T (D) is the frequency domain photoacoustic transfer function, which is a function of the thermal diffusivity, D.

Detailed Description

The following describes a method for measuring thermal diffusivity of a metal material based on photoacoustic signal matched filtering according to the present invention with reference to fig. 1 to 3. It is to be understood, however, that the drawings are provided for a better understanding of the invention and are not to be construed as limiting the invention. The specific implementation steps are as follows:

(1) and (5) establishing an experimental system. The experimental system for measuring the thermal diffusivity of the metal material based on the photoacoustic signal matched filtering, which is shown in fig. 1, is constructed and comprises a function generator 1, an excitation laser 2, a transflective mirror 3, a total reflective mirror 4, a focusing lens 5, a metal sample to be measured 6, a piezoelectric transducer 7, a photoelectric detector 8, a data acquisition card 9 and a computer 10.

a. The excitation laser 2 is selected as a semiconductor laser, so that light intensity analog modulation can be realized, and good linearity is provided between the output light intensity and the modulation electric signal.

b. The function generator 1 is connected with the laser 2, the function generator is guaranteed to be capable of sending out chirp electric signals, and the safety range of the amplitude of the output signals of the function generator is set based on the driving signal data provided by the laser specification.

c. And adjusting the whole light path to enable most of laser energy to pass through the transflective lens 3, the total reflective lens 4 and the focusing lens 5 to form a focusing light spot to excite the sample 6.

d. A piezoelectric transducer 7 is coupled to the back surface of the sample, the thickness of which is much smaller than the thickness of the sample, so that its effect on the sample vibrations is negligible.

e. A small part of light split by the transflective mirror 3 is received by the photoelectric detector 8, so that the real-time monitoring of the time domain characteristics of the excitation light intensity is realized.

f. The data acquisition card 9 acquires the photoacoustic signal and the excitation light intensity time domain signal and transmits the photoacoustic signal and the excitation light intensity time domain signal to the computer 10.

(2) Experimental measurement and signal acquisition. Based on the experimental system, a metal material thermal diffusivity measurement experiment based on photoacoustic signal matched filtering is carried out.

a. For example, the sample to be tested is round red copper, the diameter is 20 mm, and the thickness is 2 mm;

b. the average power of the excitation light used in the experiment was 2 w, the chirp initiation frequency was 20 hz, the cut-off frequency was 120 hz, and the chirp time was 1 sec.

c. The experimental measurement signals of the sample are shown in fig. 2, wherein (a) is the excitation light intensity time domain signal monitored in real time, and (b) is the corresponding photoacoustic signal.

(3) Signal processing and thermal diffusivity measurement. Based on the obtained excitation light intensity time domain signal and the photoacoustic signal, and combining the frequency domain photoacoustic transfer functions corresponding to different thermal diffusivity, a series of reference signals are generated; and respectively carrying out correlation operation on the reference signals and the measured photoacoustic signals, wherein the reference signal with the highest correlation peak value is the matched filter, and the corresponding thermal diffusivity is the measured value. The specific algorithm is shown in fig. 3.

a. Firstly, fast Fourier transform is carried out on the measured time domain signal F (t) of the excitation light intensity to obtain the frequency spectrum F (omega).

b. F (omega) is multiplied point by the frequency domain photoacoustic transfer function T corresponding to different thermal diffusivities, and the transfer function T (omega, D) is given by the following formula

Where L is the sample thickness, ω ═ 2 π f is the angular frequency, and D is the thermal diffusivity. It is readily seen that the transfer function T (ω, D) is a function of thermal diffusivity.

c. And respectively normalizing a series of complex vectors obtained by multiplying F (omega) and T (omega, D) corresponding to different thermal diffusivity to enable the two norms of the complex vectors to be 1, thereby obtaining a series of normalized frequency domain reference signals.

d. The photoacoustic signal s (t) is subjected to fast Fourier transform, the frequency spectrum of the photoacoustic signal s (t) is subjected to complex conjugation, then the photoacoustic signal s (t) is multiplied by the series of reference signals, and the frequency spectrum of the photoacoustic signal s (t) is subjected to inverse Fourier transform, and s (t) and the reference signals are essentially subjected to correlation operation.

f. And (t) comparing the time domain correlation peaks after the correlation operation with the reference signals, wherein the reference signal with the largest peak value is the matched filter, and the corresponding thermal diffusivity is the measured value of the thermal diffusivity of the sample. For the experimental results shown in FIG. 2, the measured value of thermal diffusivity was 108mm²/s。

The invention provides a metal material thermal diffusivity measuring method based on photoacoustic signal matched filtering, which is characterized in that a laser beam modulated by light intensity chirp is used for exciting a metal sample to be measured and generating a photoacoustic signal in the metal sample, and signal processing, filtering and quantitative measurement of target parameters are completed in the same step by searching a matched filter, so that a nondestructive, quantitative, rapid and economic characterization method can be provided for thermophysical detection of materials.