阅读说明：本技术 基于介质探测器的薄膜纵向热扩散系数测量系统和方法 (System and method for measuring longitudinal thermal diffusion coefficient of film based on medium detector ) 是由 郑飞虎 陈师杰 张冶文 于 2020-10-30 设计创作，主要内容包括：本发明涉及一种基于介质探测器的薄膜纵向热扩散系数测量系统和方法,用于测量薄膜的纵向热扩散系数,系统包括脉冲光源、分光镜、介质探测器、前置电流放大器、示波器和光电触发装置,介质探测器包括依次设置的接地金属电极、介质薄膜和加压金属电极,接地金属电极保持接地,加压金属电极连接有直流高压电源,加压金属电极还连接前置电流放大器的输入端,前置电流放大器的接地端接地。被测薄膜的一侧设有激光光靶,另一侧用于连接介质探测器的接地金属电极端。与现有技术相比,本发明简单易操作,能够快速完成测量,可用于测量BOPP、PI、PVDF等多种被广泛用于电气绝缘领域、微电子器件领域的介质薄膜和其它金属导电薄层。(The invention relates to a system and a method for measuring a longitudinal thermal diffusion coefficient of a film based on a medium detector, which are used for measuring the longitudinal thermal diffusion coefficient of the film. One side of the film to be measured is provided with a laser light target, and the other side is used for connecting a grounding metal electrode end of the medium detector. Compared with the prior art, the method is simple and easy to operate, can quickly finish measurement, and can be used for measuring various dielectric films and other metal conductive thin layers which are widely used in the fields of electrical insulation and microelectronic devices, such as BOPP, PI, PVDF and the like.)

1. A film longitudinal thermal diffusion coefficient measuring system based on a medium detector is used for measuring the longitudinal thermal diffusion coefficient of a measured film (5), and comprises a pulse light source (1), a spectroscope (2), a front current amplifier (12), an oscilloscope (13) and a photoelectric trigger device (3),

the film longitudinal thermal diffusivity measuring system further comprises a medium detector, the medium detector comprises a grounding metal electrode (7), a medium film (6) and a pressurizing metal electrode (8) which are sequentially arranged, the grounding metal electrode (7) is grounded, the pressurizing metal electrode (8) is connected with a direct-current high-voltage power supply (9), the pressurizing metal electrode (8) is further connected with the input end of the front current amplifier (12), and the grounding end of the front current amplifier (12) is grounded; a laser light target (4) is arranged on one side of the film (5) to be detected, and the other side of the film is directly adhered to the grounding metal electrode (7) of the medium detector through a thermal coupling agent;

the two sides of the dielectric film (6) are respectively provided with the grounding metal electrode (7) and the pressurizing metal electrode (8) after being metallized, and the film to be detected (5) is provided with the opaque laser light target (4);

the spectroscope (2) is used for receiving the pulse light beam of the pulse light source (1) and dividing the pulse light beam into a first light beam and a second light beam, the first light beam irradiates the laser light target (4), and the second light beam irradiates the photoelectric trigger device.

2. A dielectric film based film longitudinal thermal diffusivity measurement system according to claim 1, wherein a blocking capacitor and a protection circuit (11) are further connected in series in a connection line between the pressurizing metal electrode (8) and the pre-current amplifier (12), and the protection circuit (11) is used for protecting the pre-current amplifier (12).

3. The system for measuring the longitudinal thermal diffusivity of a dielectric film based on a film as claimed in claim 2, wherein a switch SW is further connected in series in a connection line between the blocking capacitor and the protection circuit (11), the switch SW comprises a fixed end, a first switching end and a second switching end, the fixed end is connected with the blocking capacitor, the first switching end is connected with the protection circuit (11), and the second switching end is connected with the ground terminal of the ground metal electrode (7).

4. A dielectric film based film longitudinal thermal diffusivity measurement system as claimed in claim 3 further comprising a shielding box (10), wherein said film under test (5), dielectric detector, dc blocking capacitor, protection circuit (11) and switch SW are all disposed within said shielding box (10).

5. The system for measuring the longitudinal thermal diffusivity of a film based on a medium detector as claimed in claim 1, characterized in that a protective resistor is connected in series in a connection line between the pressurizing metal electrode (8) and the direct current high voltage power supply (9).

6. A media detector-based film longitudinal thermal diffusivity measurement system as claimed in claim 1 wherein said photo-triggering device is a photodiode (3).

7. The system of claim 1, wherein the first light beam is transmitted by the beam splitter (2) and the second light beam is reflected by the beam splitter (2).

8. A film longitudinal thermal diffusivity measurement method using a medium probe based film longitudinal thermal diffusivity measurement system as claimed in claim 1 comprising the steps of:

installing a film to be measured with a laser light target on one side in a film longitudinal thermal diffusion coefficient measuring system;

the pulse light source is utilized to perform transient heating on the film to be detected with the laser light target, the film to be detected is transmitted into the medium detector through the film to be detected to form thermal disturbance, a direct-current high-voltage power supply is utilized to apply direct-current voltage to the medium detector, and a displacement current signal generated by the thermal disturbance in the medium detector is acquired through the preposed current amplifier and the oscilloscope;

simulating the temperature distribution in the film to be tested and the medium detector and the displacement current signal generated by the medium detector by adopting a numerical calculation method, carrying out curve fitting on the actual displacement current signal and the simulated displacement current signal to obtain a simulated displacement current signal meeting preset fitting conditions, and taking the film longitudinal thermal diffusion coefficient corresponding to the simulated displacement current signal as a test result of the film longitudinal thermal diffusion coefficient.

9. The method of claim 8, wherein the analog displacement current signal acquisition process comprises the steps of:

according to a preset heat conduction model of the dielectric film, a finite element method is utilized to carry out multi-physical-field coupling to obtain the thermal expansion change of the dielectric film, so that the capacitance change of the dielectric film is calculated, and the magnitude of the simulated displacement current is calculated;

the heat conduction model is constructed according to a heat conduction equation of the medium film, and the expression of the heat conduction equation is as follows:

wherein z is a position variable in the thickness direction, d₁To be measured film thickness, d₂Thickness of the thin film medium detector, D₁For the measured film thermal diffusion coefficient, D₂The thermal diffusion coefficient of the thin film medium detector; t is₁(z, T) is the temperature distribution in the thickness direction in the measured film, T₂(z, t) is the temperature distribution in the thin film medium detector along the thickness direction;

the thermal conductivity boundary conditions of the thermal conductivity equation are:

in the formula, k₁Thermal conductivity of the measured film, k₂The thermal conductivity of the thin film medium detector is shown, delta (t) is a Gaussian distribution function of laser, q is pulse laser energy, and eta is a laser absorption coefficient;

the initial conditions of the heat transfer equation are:

T(z,t)＝T₀，t＝0

in the formula, T₀Is at room temperature;

the computational expression of the heat conduction model is as follows:

in the formula, I' is a heat conduction function, T is temperature, BC is a heat conduction boundary condition, IC is an initial condition, omega is a geometric model domain of the composite film, rho is density, c_TIs the specific heat of the material;

the calculation expression of the analog displacement current is as follows:

in the formula, I is an analog displacement current, C is a capacitance of the medium detector, and U is a voltage applied to both ends of the medium detector.

10. The method according to claim 8, wherein the fitting condition is specifically that a goodness-of-fit of a curve corresponding to the actual displacement current signal and the simulated displacement current signal is calculated, and when the goodness-of-fit is greater than a preset goodness-of-fit threshold, the simulated displacement current signal satisfies the fitting condition;

the calculation expression of the goodness of fit is as follows:

in the formula, R²Is a calculated value of goodness of fit, where y is the data to be fitted,the data mean value is the data mean value to be fitted, Y is the fitting value, n is the total number of data points, and i is the ith data point;

the goodness threshold is not lower than 0.95.

Technical Field

The invention relates to the field of measurement of longitudinal thermal diffusion coefficients of films, in particular to a system and a method for measuring the longitudinal thermal diffusion coefficients of the films based on a medium detector.

Background

The thermal diffusion coefficient of the film is an important thermophysical parameter of a film material, so that the film can be used more reasonably and effectively only by accurately mastering the thermal diffusion coefficient of the film, and the design of an electronic device and the thermal management in the working process are facilitated. When a microelectronic device works, energy loss is usually converted into heat energy to be released, heat accumulation is easy to generate, material fusion can be directly caused by internal temperature rise to cause line fault and even explosion, and the safety and stability of the electronic element work and the service life are directly determined by the heat dissipation capacity of the thin film used as insulation and isolation on the surface of the element.

A method for measuring the thermal diffusivity of a film disclosed in publication No. CN109557129A, comprising: 1. after a layer of metal electrode is arranged between the film to be detected and the auxiliary film, the two films are jointed to form a sample to be detected, and a layer of metal electrode is arranged on each of two sides of the sample to be detected; 2. applying a direct current electric field to metal electrodes on two sides of the film to be tested, and simultaneously vertically striking the metal electrode on one side of the film to be tested by using pulse laser; 3. collecting displacement current generated by pulse laser in a sample to be detected; 4. and transforming the time domain signal of the displacement current to a complex frequency domain to obtain an electric field-frequency relation curve in the sample to be measured, selecting the frequency at the interface on the curve, and calculating by combining the thickness of the thin film.

The method for measuring the thermal diffusion coefficient of the film adopts a double-layer composite film structure, voltage is applied to an interface area of the double-layer film during measurement, and the voltage applying mode is only suitable for measuring the thermal diffusion coefficient of the insulating film. The frequency corresponding to the interface in the electric field-frequency relation curve obtained by the complex frequency domain displacement current is a range, but not a frequency point, so that the corresponding spatial position is difficult to accurately determine by using the frequency range in the scale change relation, and the measurement precision is finally influenced; and the double-layer materials of the film need to be completely the same, otherwise, the relation between the characteristic frequency range and the interface space position cannot be calculated by using a scale transformation method.

Disclosure of Invention

The invention aims to overcome the defects that the film thermal diffusivity measurement is not accurate enough, and double-layer materials are required to be identical or cannot be measured in the prior art CN109557129A, and provides a film longitudinal thermal diffusivity measurement system and a method based on a medium film.

The purpose of the invention can be realized by the following technical scheme:

a film longitudinal thermal diffusion coefficient measuring system based on a medium film is used for measuring the longitudinal thermal diffusion coefficient of a measured film and comprises a pulse light source, a spectroscope, a front current amplifier, an oscilloscope and a photoelectric trigger device.

The film longitudinal thermal diffusivity measuring system further comprises a medium detector, the medium detector comprises a grounding metal electrode, a medium film and a pressurizing metal electrode which are sequentially arranged, the grounding metal electrode is grounded, the pressurizing metal electrode is connected with a direct-current high-voltage power supply, the pressurizing metal electrode is further connected with the input end of the front current amplifier, and the grounding end of the front current amplifier is grounded; and one side of the film to be detected is provided with a laser light target, and the other side of the film to be detected is directly adhered to the grounding metal electrode of the medium detector through a thermal coupling agent.

The medium film is metallized to form two electrodes which are the grounding metal electrode and the pressurizing metal electrode respectively, and the measured film is provided with the opaque laser light target;

the spectroscope is used for receiving the pulse light beam of the pulse light source and dividing the pulse light beam into a first light beam and a second light beam, the first light beam irradiates the laser light target, and the second light beam irradiates the photoelectric trigger device.

Furthermore, a blocking capacitor and a protection circuit are also connected in series in a connecting circuit between the pressurizing metal electrode and the pre-current amplifier, and the protection circuit is used for protecting the pre-current amplifier.

Further, a switch SW is further connected in series in a connecting line between the blocking capacitor and the protection circuit, the switch SW includes a fixed end, a first switching end and a second switching end, the fixed end is connected with the blocking capacitor, the first switching end is connected with the protection circuit, and the second switching end is connected with a grounding end of the grounding metal electrode.

Furthermore, the film longitudinal thermal diffusivity measuring system also comprises a shielding box, and the measured film, the medium detector, the blocking capacitor, the protection circuit and the switch SW are all arranged in the shielding box.

Furthermore, a protective resistor is connected in series in a connecting line of the pressurizing metal electrode and the direct-current high-voltage power supply.

Further, the photoelectric trigger device is a photodiode.

Further, the first light beam is transmitted light of the beam splitter, and the second light beam is reflected light of the beam splitter.

The invention also provides a system and a method for measuring the longitudinal thermal diffusivity of a film based on a medium detector, which are characterized by comprising the following steps:

installing a film to be measured with a laser light target arranged on one side surface in a film longitudinal thermal diffusion coefficient measuring system;

the measured film with the laser light target is subjected to transient heating by the pulse light source, and is transmitted into a medium detector through an interface of the measured film and the medium detector to form thermal disturbance, a direct-current high-voltage power supply applies direct-current voltage to the medium detector, and a displacement current signal generated by the thermal disturbance in the medium detector is acquired by the preposed current amplifier and the oscilloscope;

simulating the temperature distribution in the film to be tested and the medium detector and the displacement current signal generated by the medium detector by adopting a numerical calculation method, carrying out curve fitting on the actual displacement current signal and the simulated displacement current signal to obtain a simulated displacement current signal meeting preset fitting conditions, and taking the film longitudinal thermal diffusion coefficient corresponding to the simulated displacement current signal as a test result of the film longitudinal thermal diffusion coefficient.

Further, the process of obtaining the simulated displacement current signal of the displacement current simulation model comprises the following steps:

according to a preset heat conduction model of the dielectric film, the finite element method is utilized to solve the temperature distribution and the thermal deformation of the dielectric detector and the transient change of the capacitance of the film dielectric detector, and the magnitude of the displacement current of the model is calculated;

the heat conduction model is constructed according to a heat conduction equation of the medium film, and the expression of the heat conduction equation is as follows:

wherein z is a position variable in the thickness direction, d₁To be measured film thickness, d₂Thickness of the thin film medium detector, D₁For the measured film thermal diffusion coefficient, D₂The thermal diffusion coefficient of the thin film medium detector; t is₁(z, T) is the temperature distribution in the thickness direction in the measured film, T₂(z, t) is the temperature distribution in the thin film medium detector along the thickness direction;

the thermal conductivity boundary conditions of the thermal conductivity equation are:

in the formula, k₁Thermal conductivity of the measured film, k₂The thermal conductivity of the thin film medium detector is shown, delta (t) is a Gaussian distribution function of laser, q is pulse laser energy, and eta is a laser absorption coefficient;

the initial conditions of the heat transfer equation are:

T(z,t)＝T₀，t＝0

in the formula, T₀Is at room temperature;

the computational expression of the heat conduction model is as follows:

in the formula, I' is a heat conduction function, T is temperature, BC is a heat conduction boundary condition, IC is an initial condition, omega is a geometric model domain of the composite film, rho is density, c_TIs the specific heat of the material;

the calculation expression of the analog displacement current is as follows:

in the formula, I is an analog displacement current, C is a capacitance of the medium detector, and U is a voltage applied to both ends of the medium detector.

Further, the fitting condition is specifically that a goodness of fit of a curve corresponding to the actual displacement current signal and the simulated displacement current signal is calculated, and when the goodness of fit is greater than a preset goodness threshold, the simulated displacement current signal meets the fitting condition;

the calculation expression of the goodness of fit is as follows:

in the formula, R²Is a calculated value of goodness of fit, where y is the data to be fitted,the data mean value is the data mean value to be fitted, Y is the fitting value, n is the total number of data points, and i is the ith data point;

the goodness threshold is not lower than 0.95.

Compared with the prior art, the invention has the following advantages:

(1) the invention adds a medium detector composed of a grounding metal electrode, a medium film and a pressurizing metal electrode, the medium detector is connected with a film to be measured, and the measuring principle is as follows: the thermal pulse with known parameters generates attenuation and dispersion after passing through the measured film, and the thermal pulse with the known parameters is transmitted to the medium film in the medium detector, and the thermal diffusion coefficient of the measured film can be reversely deduced by analyzing the displacement current characteristics on the medium film;

although the measured films of the comparison file in the application and the background technology are both of a double-layer composite structure, the double-layer films in the comparison file generate displacement current, the application only has electric field distribution in the medium detector film, the displacement current is induced at the metal electrode end, the double-layer films can be different materials, and the defect that a scale conversion method cannot process heterogeneous double-layer materials is avoided.

(2) The invention is a non-contact transient thermal measurement technology, has small damage to a sample and flexible test mode, and can measure the insulating dielectric film with the thickness of submicron.

(3) Compared with most measuring methods, the measuring method for the longitudinal thermal diffusion coefficient of the film obviously shortens the testing period and can quickly obtain the thermal diffusion coefficient of the dielectric film.

(4) The invention is simple and easy to operate, can quickly finish measurement, and can be used for measuring various dielectric films and other metal conductive thin layers which are widely used in the fields of electrical insulation and microelectronic devices, such as BOPP, PI, PVDF and the like.

Drawings

FIG. 1 is a schematic diagram of a system for measuring the longitudinal thermal diffusivity of a dielectric film-based film in accordance with an embodiment of the present invention;

FIG. 2 is a graph showing the fitting comparison effect of the best fit solution of the actual data and the simulated data of the BOPP measured film with the thickness of 4.8 micrometers and the thickness of 6.8 micrometers respectively in the embodiment;

in the figure, 1, a heating pulse light source, 2, a spectroscope, 3, a photodiode, 4, a laser light target, 5, a film to be detected, 6, a dielectric film, 7, a grounding metal electrode, 8, a pressurizing metal electrode, 9, a direct current high-voltage source, 10, a shielding box, 11, a protection circuit, 12, a pre-current amplifier, 13 and an oscilloscope are arranged.

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.

Example 1

The embodiment provides a film longitudinal thermal diffusion coefficient measuring system based on a film medium detector, which is used for measuring the longitudinal thermal diffusion coefficient of a measured film 5, and comprises a pulse light source 1, a spectroscope 2, a front current amplifier 12, an oscilloscope 13, a photoelectric trigger device and a medium detector;

the medium detector comprises a grounding metal electrode 7, a medium film 6 and a pressurizing metal electrode 8 which are sequentially arranged, one side of a measured film 5 is provided with a laser light target 4, the other side of the measured film is used for being connected with the grounding metal electrode 7, the grounding metal electrode 7 is grounded, the pressurizing metal electrode 8 is connected with a direct-current high-voltage power supply 9, the pressurizing metal electrode 8 is also connected with the input end of a front-mounted current amplifier 12, and the grounding end of the front-mounted current amplifier 12 is grounded;

the film medium detector 6 is metallized to form two electrodes, namely the grounding metal electrode 7 and the pressurizing metal electrode 8, and the detected film 5 is provided with the opaque laser light target 4;

the spectroscope 2 is used for receiving the pulse light beam of the pulse light source 1 and dividing the pulse light beam into a first light beam and a second light beam, wherein the first light beam irradiates the laser light target 4, and the second light beam irradiates the photoelectric trigger device.

The laser light target 4 is preferably a carbon black nano thin layer, and can absorb light and heat.

The direct-current high-voltage power supply 9 can input positive high voltage or negative high voltage, when the direct-current high-voltage power supply 9 inputs the positive high voltage, the pressurizing metal electrode 8 in the medium detector is the positive electrode, and the grounding metal electrode 7 is the negative electrode; when a negative high voltage is input by the direct current high voltage power supply 9, the pressurizing metal electrode 8 in the medium detector is a negative electrode, and the grounding metal electrode 7 is a positive electrode.

In a preferred embodiment, a blocking capacitor and a protection circuit 11 are further connected in series to a connection line between the pressurizing metal electrode 8 and the pre-current amplifier 12, and the protection circuit 11 is used for protecting the pre-current amplifier 12.

Further, as a preferred embodiment, a switch SW is further connected in series in a connection line between the blocking capacitor and the protection circuit 11, the switch SW includes a fixed end, a first switching end and a second switching end, the fixed end is connected to the blocking capacitor, the first switching end is connected to the protection circuit 11, and the second switching end is connected to the ground end of the negative metal electrode 7.

Further, as a preferred embodiment, a protective resistor is connected in series to a connection line between the positive electrode metal electrode 8 and the dc high-voltage power supply 9.

Further, as a preferred embodiment, the film longitudinal thermal diffusivity measuring system further comprises a shielding box 10, and the measured film 5, the medium detector, the blocking capacitor, the protection circuit 11, the switch SW and the protection resistor are all arranged in the shielding box 10.

In a preferred embodiment, the optical triggering device is a photodiode 3.

In a preferred embodiment, the first light beam is transmitted light of the beam splitter 2, and the second light beam is reflected light of the beam splitter 2.

The above preferred embodiments are combined together to obtain an optimal embodiment, and the film longitudinal thermal diffusivity measuring system based on the film medium detector in the embodiment comprises a pulse light source 1, a spectroscope 2, a pre-current amplifier 12, an oscilloscope 13, a photoelectric trigger device, a measured film 5, a grounding metal electrode 7, a film medium detector 6, a pressurizing metal electrode 8, a blocking capacitor, a switch SW, a protection circuit 11, a protection resistor, a direct-current high-voltage power supply 9 and a shielding box 10.

The functions of the respective components in the system of the present embodiment are described below.

The pulse light source 1 selects infrared pulse laser, the diameter of a light spot is 1mm, the single pulse energy is 3mJ, the pulse width is 25ns, the wavelength is 1064nm, and the infrared pulse laser is used for heating a sample.

The photoelectric trigger device is completed by adopting a photodiode 3, and the output end of the photodiode 3 is connected with the trigger end of an oscilloscope 13.

The shielding box 10 is used for placing a sample, the film 5 to be tested needs to be tightened and fixed, and the shielding box 10 can be used for placing interference of external signals to current signals. The shielding box 10 is externally provided with a cable with double shielding effect for signal transmission.

And one side of the film 5 to be detected is provided with an optical target which is attached to the surface of the sample to be detected in an evaporation mode, so that the film 5 to be detected can absorb the infrared pulse laser. The thin film to be measured and the medium detector are connected through a trace of thermal coupling agent.

The direct-current high-voltage power supply 9HVDC is externally connected with direct-current voltage to the medium film 6, and the electric field inside the measured film 5 is generally ensured to be 10 kV/mm.

Protective resistor R for limiting current, selection resistor 3X 10⁸Ω。

The blocking capacitor C can isolate the dc high voltage on one hand, and can be used for signal coupling on the other hand, since the thermal pulse response signal of the measured film 5 is a current signal, the impedance of the measured film 5 is much larger than the capacitive reactance of the blocking capacitor C, and 20nF is adopted.

The time for reaching the steady state thermal equilibrium from the application of the pulse laser to the measured film is about 1-3ms, so that the impact frequency of the thermal pulse laser is selected to be 1 Hz.

The switch SW is used for determining the conduction of the signal of the film 5 to be detected, and in the pressurizing process, the switch SW should be ensured to be in a grounding state, and the switch SW is turned on after the voltage is stabilized.

The protection circuit 11 is used to prevent the instantaneous surge current generated when the thin film breaks down from damaging the pre-current amplifier 12. The acquisition of the signal is completed by a pre-current amplifier 12 and an oscilloscope 13. The displacement current is amplified by the pre-current amplifier 12. The displacement current data is collected by the oscilloscope 13, and the oscilloscope 13 selects an average sampling mode for sampling.

The invention also provides a film longitudinal thermal diffusion coefficient measuring method of the film longitudinal thermal diffusion coefficient measuring system based on the film medium detector, which comprises the following steps:

installing a film to be measured with a laser light target on one side in a film longitudinal thermal diffusion coefficient measuring system;

the measured film with the laser light target is subjected to transient heating by the pulse light source, and is transmitted into a medium detector through an interface of the measured film and the medium detector to form thermal disturbance, a direct-current high-voltage power supply applies direct-current voltage to the medium detector, and a displacement current signal generated by the thermal disturbance in the medium detector is acquired by the preposed current amplifier and the oscilloscope;

simulating the temperature distribution in the film to be tested and the medium detector and the displacement current signal generated by the medium detector by adopting a numerical calculation method, carrying out curve fitting on the actual displacement current signal and the simulated displacement current signal to obtain a simulated displacement current signal meeting preset fitting conditions, and taking the film longitudinal thermal diffusion coefficient corresponding to the simulated displacement current signal as a test result of the film longitudinal thermal diffusion coefficient.

The following describes a specific implementation process.

The system of the embodiment is used for testing the thermal conductivity of the biaxially oriented polypropylene film, and comprises the following steps:

a BOPP film (sample to be measured) with the thickness of 6.8 mu m and one BOPP film (medium detector) with the thickness of 5.8 mu m and with carbon black plated on one side and aluminum plated on two sides are adhered together through a thermal coupling agent and are placed into a shielding box to be tightened, so that interference of external signals on measurement signals is prevented.

And a pulse light source is turned on, and beam splitting is carried out through the spectroscope, so that the laser light target receives laser heat radiation with certain energy while the oscilloscope has a trigger signal.

The dc high voltage power supply is turned on to gradually increase the voltage, which keeps the switch SW in an off-ground state.

After the voltage is stabilized, the shading plate is opened to carry out pulse heating on the sample, the laser light target absorbs heat and then propagates in the film to be detected, direct current voltage is applied to the front surface and the rear surface of the medium detector, and the medium detector is equivalent to a capacitor at the moment. The thermal pulse reaches the medium detector after passing through the film to be detected, transient temperature gradient distribution is generated in the medium detector, induced charge on the electrode is changed due to local thermal deformation, displacement current is generated in an external loop, and a front current amplifier is opened to acquire a displacement current signal through an oscilloscope.

For the same material and the tested sample films with different thicknesses, the signal generation time and the peak time generated by the displacement current on the medium film are different. In the experimental process, BOPP films with the thicknesses of 4.8 mu m and 6.8 mu m are used as the tested sample films.

Satisfying the Fourier heat transfer law when laser heats the film material, because the facula diameter is little, film thickness is less than the horizontally size far away, at the heat transfer in-process, can regard as a one-dimensional heat transfer process, the heat-conduction equation is:

wherein z is a position variable in the thickness direction, d₁To be measured film thickness, d₂Is the thickness of a thin film medium detector, in particular to the thickness of a medium thin film in the thin film medium detector, D₁For the measured film thermal diffusion coefficient, D₂The thermal diffusion coefficient of the thin film medium detector; t is₁(z, T) is the temperature distribution in the thickness direction in the measured film, T₂(z, t) is the temperature distribution in the thin film medium detector along the thickness direction;

the heat conduction equation requires setting the heat conduction boundary conditions:

in the formula, k₁For the film to be testedThermal conductivity, k₂The thermal conductivity of the thin film medium detector is shown, delta (t) is a Gaussian distribution function of laser, q is pulse laser energy, and eta is a laser absorption coefficient;

the initial conditions of the mathematical model are:

T(z,t)＝T₀，t＝0

in the formula, T₀Is at room temperature;

and solving the heat conduction function I' of the multilayer composite material structure by adopting a finite element method. By minimizing the general function I', a numerical solution of the temperature distribution of the model can be obtained, with the formula:

the heat conduction model in the one-dimensional thickness direction can be simplified as follows:

in the formula, I' is a heat conduction function, T is temperature, BC is a heat conduction boundary condition, IC is an initial condition, omega is a geometric model domain of the composite film, rho is density, c_TIs the specific heat of the material;

and solving the temperature distribution and the thermal deformation of the medium detector and the transient change of the capacitance of the medium detector by using a finite element method, and calculating the magnitude of the displacement current of the model. The displacement current is calculated by the following formula.

In the formula, I is an analog displacement current, C is a capacitance of the medium detector, and U is a voltage applied to both ends of the medium detector.

When BOPP films with the finite element simulation thicknesses of 4.8 mu m and 6.8 mu m are used as tested sample films, response current of a medium film is used for solving the heat diffusion coefficient in a fitting mode, and a specific determination algorithm comprises the following steps:

(1) the data obtained by the experiment is led into Matlab, the signal obtained by the experiment measurement is obtained after the amplification of the preposed current amplifier, the amplitude of the actual displacement current is obtained by dividing the amplification factor of the preposed current amplifier, and for the signal with relatively high frequency, because the bandwidth of the preposed current amplifier is limited, the signal exceeding the bandwidth frequency of the preposed current amplifier can be attenuated by the amplification factor, and the frequency response calibration is needed.

(2) And setting an initial value of the thermal diffusion coefficient of the measured film in the simulation model, and calculating the simulated response current of the dielectric film under the initial value condition.

(3) The simulated displacement current and the displacement current actually acquired and measured are discrete signals, and are processed by a difference method, so that the abscissa time t of each discrete point is kept consistent, and subsequent calculation is facilitated.

(4) And judging the fitting degree of the experimental curve and the simulated curve according to the numerical value of the goodness of fit, wherein the formula of the goodness of fit is as follows:

where y is the data to be fitted,the mean value of the data to be fitted is obtained, and Y is a fitting value;

(5) under the condition of an initial value, the fitting degree generally cannot reach an optimal solution, parametric scanning is set, the variation step length of the thermal diffusion coefficient is given, and fitting goodness values under the thermal diffusion coefficients of a plurality of given measured films are obtained.

(6) The closer the simulated curve is to the measured curve, the corresponding goodness of fit R²The closer to 1, according to R²Can obtain an optimal solution for the thermal diffusivity. As shown in FIG. 2, the thermal diffusion coefficient of the film sample measured is 0.95 × 10 according to the corresponding fitting comparison effect graph when the fitting degree of the actual data and the simulation data of the BOPP measured film with the thickness of 4.8 micrometers and 6.8 micrometers respectively is the optimal solution^-7m²/s。

The foregoing detailed description of the preferred embodiments of the invention has been presented. It should be understood that numerous modifications and variations could be devised by those skilled in the art in light of the present teachings without departing from the inventive concepts. Therefore, the technical solutions available to those skilled in the art through logic analysis, reasoning and limited experiments based on the prior art according to the concept of the present invention should be within the scope of protection defined by the claims.