A method and system for measuring the thermal expansion coefficient of a film-like material at ultra-low temperatures based on speckle interferometry

Abstract

The application relates to a method and system for measuring the thermal expansion coefficient of film-shaped materials under ultralow temperature based on speckle interference, and belongs to the technical field of material thermal performance testing. The method solves the problems that the film-shaped materials are not suitable for contact measurement under ultralow temperature due to poor strength, and the uneven surface of the film-shaped materials cannot meet the requirements of general optical methods. Technical points of the application are as follows: after the to-be-tested material is processed, the material is placed in a low-temperature thermostat; when the low-temperature thermostat cools the sample to a target value and is stable, the low-temperature thermostat is stopped; real-time speckle patterns and temperature values are recorded and saved synchronously; after the test is completed, the speckle patterns are processed by using a program to generate an interference pattern; the average fringe spacing in the interference pattern is obtained to directly calculate the surface strain value; finally, a series of strain-temperature values can be obtained, a curve is drawn, and the thermal expansion coefficient of the to-be-tested sample is calculated; and the application is suitable for measuring the thermal expansion coefficient of materials under ultralow temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of material thermal performance testing, and in particular relates to the technical field of material thermal expansion coefficient detection. Background Art

[0002] The coefficient of thermal expansion (CTE) is a key parameter characterizing the thermophysical properties of materials. It is a crucial mechanical and thermal parameter of materials and is crucial for the design of precision structures. For materials subjected to temperature fluctuations, the CTE determines their resistance to thermal shock, the magnitude of thermal stress, and the extent of thermally induced deformation. Accurately measuring a material's CTE is of great practical significance.

[0003] Currently, the commonly used methods for measuring the coefficient of thermal expansion are the pin expansion method and the Michelson interferometer method. The pin expansion method is a non-absolute measurement method that requires further calibration. Furthermore, it is a contact measurement method, and the mechanical stress between the pin and the sample can affect the accuracy of the test results, resulting in relatively low test accuracy. In the Michelson interferometer method, the two beams of light that interfere with each other come from reflections from the sample and the standard, respectively. The inevitable thermal expansion of components such as the standard and the sample stage will affect test accuracy, and the method of reading the number of interference fringes introduces random errors. The test results are not intuitive, and can only measure the average coefficient of thermal expansion within a certain temperature range.

[0004] In the current prior art, for example, the method for testing the thermal expansion coefficient of materials at low temperatures described in the Chinese invention patent "A testing device and testing method for the thermal expansion coefficient of materials at low temperatures" (publication number CN 110146542 A, publication date 2019.08.20) uses a capacitive displacement sensor to measure the thermal expansion coefficient of the material. A vacuum pump is required to maintain the vacuum state of the environment. However, slight or strong vibrations are inevitable during the operation of the vacuum pump, which will have a certain impact on the measurement results.

[0005] At the same time, because most materials have poor strength at low temperatures, they are not suitable for contact measurement methods; and most samples, especially film samples, cannot meet the requirements of general optical methods because of their uneven surfaces. Summary of the Invention

[0006] The present invention proposes a method and system for measuring the thermal expansion coefficient of membrane materials at ultra-low temperatures based on speckle interferometry. This method solves the problem that when using traditional methods to measure the thermal expansion coefficient of membrane materials in ultra-low temperature environments, the membrane materials have poor strength and are not suitable for contact measurement, and their uneven surface cannot meet the requirements of general optical methods. In addition, the cryostat that maintains the low temperature environment has strong vibrations and non-negligible rigid displacement during operation, which solves the problem that many experimental methods used in normal temperature environments cannot be implemented well.

[0007] A method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry, the method comprising:

[0008] Under a vacuum environment, placing the material to be tested in a cryostat, and controlling the cryostat to cool to a minimum temperature so that the material to be tested reaches the minimum temperature and remains stable;

[0009] Two beams of coherent light are incident obliquely on the surface of the material to be tested;

[0010] Controlling the cryostat to stop the temperature control operation so that the temperature of the material to be tested is autonomously increased to room temperature, and collecting speckle images and temperatures on the surface of the material to be tested to obtain multiple sets of data during the process, each set of data including a speckle image and a corresponding temperature;

[0011] The obtained multiple sets of data are processed to obtain the thermal expansion coefficient of the material to be tested.

[0012] Furthermore, the two coherent light beams have the same incident angle, which does not exceed 15°.

[0013] Furthermore, a sample holder is provided inside the low-temperature thermostat.

[0014] Furthermore, the cross section of the sample holder is in the shape of an inverted concave letter.

[0015] Furthermore, a scale line is provided on the top of the sample holder.

[0016] Furthermore, the speckle image and temperature of the material to be measured are recorded by a recording device.

[0017] Furthermore, the recording device includes a camera and a temperature controller, the camera is used to collect speckle images, and the temperature controller is used to collect the surface temperature of the material.

[0018] Furthermore, the lowest temperature of the material to be tested is 35K.

[0019] Furthermore, the speckle image and temperature processing of the surface of the material to be measured is specifically as follows:

[0020] The speckle image is processed using a difference method to obtain an interference pattern, a surface strain value of the material to be measured is obtained according to the interference pattern, and a thermal expansion coefficient of the material is obtained according to the surface strain value.

[0021] The present invention also provides a system for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry, the system comprising:

[0022] Cooling module: including a low-temperature thermostat, used to cool the material to be tested;

[0023] Interference module: used for obliquely injecting two beams of coherent light onto the surface of the material to be tested;

[0024] Data acquisition module: used to collect speckle images and temperature of the surface of the material to be tested;

[0025] Data processing module: used for processing the speckle image and temperature to obtain the thermal expansion coefficient of the material to be tested.

[0026] Beneficial effects of the present invention:

[0027] Since thin film and sheet materials have low rigidity, traditional instruments and methods cannot directly measure them. However, the method described in the present invention realizes a non-contact method for measuring the thermal expansion coefficient of materials at ultra-low temperatures.

[0028] There is an existing technology that can measure the thermal expansion coefficient of materials at low temperatures using strain gauge bonding or similar methods. However, this contact measurement method not only damages the sample, but also poses a problem of how to keep the sample and strain gauge firmly bonded. In addition, a vacuum pump must be used to maintain the vacuum during the entire measurement process, and the vibration of the vacuum pump can lead to inaccurate measurement results.

[0029] The present invention uses speckle interferometry to directly obtain surface strain information of the material being measured, namely the gradient of displacement, without considering the thermal strain of the instrument itself. It is particularly suitable for deformation measurement in low-temperature environments that requires complex instrumentation. Compared with existing image correlation methods, the measurement method described in the present invention does not require the use of a particularly high-pixel camera to achieve higher-precision measurements.

[0030] The present invention uses a low-temperature thermostat to maintain the low temperature of the material to be measured, and by arranging a coherent optical path outside the low-temperature thermostat, it is possible to measure the in-plane deformation and thermal expansion coefficient of the material to be measured.

[0031] The method provided by the present invention is simple to operate, anti-interference, has a wide temperature measurement range, and can measure various samples. For high-cost materials, the present invention can measure them economically and conveniently without consuming materials.

[0032] The method described in the present invention uses an optical method to place the material in a closed system to maintain a low temperature, and collects the temperature and speckle image of the measured sample under the condition that the vacuum equipment, molecular pump and low-temperature thermostat compressor are all turned off. This avoids the problems of material damage during the measurement process due to the reduction in the strength of the measured material and measurement failure and inaccurate results caused by instrument vibration in a low-temperature environment.

[0033] Many materials and devices used in the aerospace field cannot be made into traditional rod shapes. The present invention can measure the thermal expansion coefficient of film-shaped and sheet-shaped materials.

[0034] The present invention realizes a non-contact method for measuring the thermal expansion coefficient of materials at ultra-low temperatures, especially solving the difficulty of traditional instruments and methods in directly measuring thin film materials and sheet materials, and filling the gaps in the test temperature range and materials of traditional thermal expansion coefficient measuring devices.

[0035] The method of the present invention is suitable for detecting the thermal expansion coefficient of a material in an ultra-low temperature environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is an optical path diagram of the speckle interferometry method according to the first embodiment, in which 1 is a laser, 2 is a beam splitter, 3 is a convex lens, 4 is a convex lens, 5 is a reflector, 6 is a reflector, 7 is a transparent optical window of a cryostat, and 8 is a test surface of a sample to be tested;

[0037] Figure 2a This is a photograph of a system for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry as described in embodiment 14;

[0038] Figure 2b This is a schematic diagram of measuring the thermal expansion coefficient of a film material at ultra-low temperature based on the speckle interferometry method according to the fourteenth embodiment;

[0039] Figure 3 A processed shape of the sample to be tested as described in Implementation Method 1 and Implementation Method 11;

[0040] Figure 4 The fixture for holding the material as described in embodiment 1 can be directly installed in a cryostat for testing;

[0041] Figure 5 This is a screenshot of the data post-processing program described in Implementation Example 1 running. The upper left portion shows the image generated by subtraction, and the lower left portion shows a curve chart after column-wise averaging and filtering of the image. The location of the valley value can be read from this, and is marked with a dotted line on the upper left image.

[0042] Figure 6 The measurement results of the copper material described in the eleventh embodiment at 50K to 150K, wherein the ordinate is the strain value, unit: μ, the abscissa is the temperature value, unit: K, the black dotted line is the reference value in the literature, and the red dotted curve is the measurement result of this embodiment;

[0043] Figure 7 The measurement results of the 6061 aluminum alloy material described in the twelfth embodiment at 50K to 150K, wherein the ordinate is the strain value, unit: μ, the abscissa is the temperature value, unit: K, the black dotted line is the measurement result using a commercial instrument, and the red dotted curve is the measurement result of this embodiment;

[0044] Figure 8 A screenshot of the measurement results obtained using the automatic solution function for a certain material described in Embodiment 13 at 50-200 K, in which the vertical axis is the strain value in units of μ and the horizontal axis is the temperature value in units of K, in the program window.

[0045] Figure 9 An algorithm flowchart for automatic solution in data post-processing for Embodiment 1. DETAILED DESCRIPTION

[0046] Embodiment 1:

[0047] Reference Figure 1 , Figure 3 , Figure 8 This embodiment is described.

[0048] A method for measuring the thermal expansion coefficient of a film-shaped material at ultra-low temperature based on speckle interferometry, the method comprising:

[0049] In a vacuum environment, the material to be measured is placed in a cryostat, and the cryostat is controlled to reach the lowest temperature, so that the material to be measured reaches the lowest temperature and remains stable;

[0050] Two coherent lights are obliquely incident on the surface of the material to be measured;

[0051] The cryostat is controlled to stop temperature control operation, so that the material to be measured is self-heated to room temperature, and speckle images and temperatures of the surface of the material to be measured are collected during the process to obtain a plurality of sets of data, each set of data containing a speckle image and a corresponding temperature;

[0052] The obtained plurality of sets of data are processed to obtain the temperature-strain characteristics and the thermal expansion coefficient of the material to be measured.

[0053] Specifically, in actual applications, a vacuum device can be used to realize the vacuum environment, i.e., the cryostat is placed in the vacuum cavity of the vacuum device. The vacuum device can use a molecular pump to realize vacuum control.

[0054] The lowest temperature refers to the lowest temperature that the cryostat can reach, and the material can reach a lowest temperature of about 35 K. During the temperature recovery process, the valve between the cryostat and the molecular pump is closed, so that the internal of the cryostat can still maintain a certain degree of vacuum. In actual experiments, it takes about 1 hour for the material to recover to 200 K.

[0055] The two coherent light beams impinge on the same location on the surface of the material being measured, interfering with each other and producing a random, speckled pattern associated with the surface, known as speckle interferometry. Excessive angles of incidence can prevent the two beams from illuminating the same location on the surface, or the overlap area between the two beams is too small, making measurement impossible. The angle of incidence should not exceed 15 degrees. The two coherent light beams are routed through an optical path outside the cryostat, typically using optical components such as a beam splitter or prism. This is accomplished by ensuring that the two beams have the same angle of incidence.

[0056] Here are some examples to illustrate the above method:

[0057] S1: placing the material to be tested in a cryostat, turning on the vacuum equipment, the molecular pump and the cryostat, using the cryostat to cool the material to be tested to the lowest temperature, and obliquely injecting two coherent beams of light onto the surface of the material to be tested;

[0058] S2: closing the valve of the pipeline connecting the molecular pump and the cryostat, turning off the molecular pump, and after the molecular pump is completely shut down, turning on the recording device, obtaining the speckle image and temperature of the surface of the material to be tested according to the recording device, and then turning off the mechanical pump, the compressor, and the water chiller in sequence;

[0059] S3: When the temperature of the material to be tested rises to room temperature, turning off the recording device to obtain the speckle image and temperature of the sample surface recorded by the recording device;

[0060] S4: Processing the speckle image and temperature of the sample surface to obtain the strain-temperature characteristics and thermal expansion coefficient of the material.

[0061] Specifically:

[0062] The cryostat is a sealed chamber connected to a vacuum pump and a molecular pump via piping, which are used to evacuate the interior of the chamber. Since vacuum pumps can typically only reach pressures of a few Pa, a molecular pump is required to lower the pressure inside the chamber and achieve a higher vacuum. The use of vacuum pumps and molecular pumps is standard for almost all cryostats because, at ultra-low temperatures, the temperature difference between the interior of the device and the surrounding environment is very large. Evacuation helps isolate convective heat transfer, significantly reducing heat leakage.

[0063] like Figure 1As shown, laser light emitted by laser 1 is split into two coherent beams by beamsplitter prism 2. Each beam is then expanded by convex lenses 3 and 4, and then reflected by mirrors 5 and 6. The beams ultimately strike the test surface of the sample at the same angle of incidence, from the left and right sides through the transparent optical window of the cryostat. Interference occurs on the surface, forming a speckle pattern. Temperature changes in the sample cause thermal strain in the material, manifesting as differences in total displacement at different points on the test surface, ultimately causing regular variations in the speckle pattern. By subtracting the two speckle patterns, regular interference fringes can be obtained if the corresponding deformations are small. By calculating the average fringe spacing of the interference fringes, the strain values ​​corresponding to the relative thermal expansion of the two patterns can be obtained. By fixing one of the two patterns as the reference image and continuously varying the other, a series of strain values ​​can be obtained under the same reference. In practice, a computer collects images and temperatures at fixed intervals. This method can be used to obtain strain-temperature data for the sample under test over a narrow temperature range.

[0064] In this embodiment, the material to be tested is first processed into a specific shape. For rigid and easily processed materials, such as aluminum alloy, magnesium alloy, copper, etc., the material to be tested can be prepared into a specific shape. Figure 3 The shape shown in the figure should not be made too smooth during processing, and uniform diffuse reflection should be ensured on the surface to be tested. Materials that are difficult to process, or film materials or materials with higher costs should be processed into rectangular sheets. For film samples, they can be prepared into rectangular sheets with a size not exceeding 2cm x 3cm. For block samples, external threads should be processed on them, and the threads should be screwed into the screw holes at the end of the cold head during the test. If the material is not suitable for processing threads, it should be processed into thin sheets of 1.0cm x 2.5cm or similar sizes, with a thickness not greater than 1mm. For materials that cannot be processed into rough surfaces, as well as transparent materials, the surface needs to be pre-treated before testing to ensure diffuse reflection, such as artificially creating scratches and spraying matte paint, and then the material to be tested is clamped on the sample holder.

[0065] For processing Figure 3 For samples of the shape shown in the figure, just screw the bottom thread into the threaded hole of the cryostat. For samples processed into a rectangular shape, use Figure 4 The sample is clamped with the special clamp shown, and the clamp holding the sample is screwed into the threaded hole of the low temperature thermostat.

[0066] The cryostat is fixed on an optical platform and connected to a vacuum pump and a molecular pump, so as to reduce heat leakage of the cryostat and enable the cryostat to reach the lowest temperature.

[0067] Before turning on the recording device to start recording, the interior of the cryostat is evacuated using a vacuum device and cooled to the lowest temperature the cryostat can reach. Then, the laser is turned on so that the optical path simultaneously directs two coherent beams of light toward the sample surface.

[0068] During the test, it is crucial to ensure that the cryostat is free of any vibrations, as this will affect the camera imaging process and the quality of the speckle pattern. Turning off the vacuum equipment, molecular pump, and compressor of the cryostat mentioned in the previous step is the most basic approach. Furthermore, it is also important to ensure that the piping connected to the cryostat does not come into direct contact with any equipment that may vibrate, and that there are no other obvious sources of vibration near the optical platform.

[0069] This embodiment adopts a method of first lowering the temperature to the lowest temperature that the cryostat can reach, and after it stabilizes, turning off the vacuum equipment, molecular pump and compressor of the cryostat, and then measuring the strain of the sample during the temperature rise process. This avoids the influence of vibration during the operation of the cryostat on the measurement results.

[0070] After shutting down the cryostat's compressor, the temperature of the sample and related components within the cryostat slowly returns to room temperature. The recording device captures the speckle pattern on the sample surface at regular intervals and records the corresponding sample temperature. Once the temperature returns to a higher level, the rate of temperature change slows, and the recording device can be shut down.

[0071] The obtained temperature and speckle image sequences are processed using a post-processing program, which includes an automatic solution algorithm. The flowchart of the automatic solution algorithm is shown in FIG. Figure 8 As shown in the figure, the program uses the subtraction method to generate speckle interferometry patterns between two selected images. (Specifically, when describing "strain," "deformation," or "temperature difference," the terms must be specified for two specific states, which can also be understood as "relative values." As explained below, as long as the relative deformation between the two images is not significant, subtraction can be performed to generate an interference pattern.) When the relative deformation between the two images is not significant, a relatively clear interference fringe pattern can be generated, exhibiting the appearance of vertical stripes. After binarization and Gaussian filtering of the interference fringe image, the grayscale values ​​of the stripes are statistically analyzed using a column-by-column pixel averaging method to obtain one-dimensional data. This one-dimensional data is then low-pass filtered to obtain the peak and valley values ​​of the filtered data. The average strain value of the sample surface is calculated from the fringe spacing calculated from the peak and valley values. The post-processing program also simply implements the function of obtaining the strain-temperature curve of the sample throughout the entire test process. By continuously transforming the two subtraction images, a series of discrete temperature and relative strain values ​​can be obtained. These values ​​can be integrated through a specific method to obtain a complete strain-temperature curve.

[0072] When the deformation scale of the sample surface exceeds the speckle size, the correlation of the image disappears, and interference fringes cannot be obtained at this time. When obtaining the strain-temperature relationship in a large temperature range, the following method needs to be adopted: divide the entire interval into several small intervals to ensure that the vast majority of images in each sub-interval can be subtracted from the same reference image to generate clearly identifiable interference fringes; calculate the strain-temperature relationship in each sub-interval, and finally splice the data of each sub-interval to obtain the strain-temperature relationship in the large temperature range. In the process of automatic calculation, all images are first divided into several sub-intervals, and the solution is performed independently and in parallel for each sub-interval, which greatly improves the solution speed while ensuring the stability of the solution method. The process of the above algorithm is an existing technology that can be mastered by those skilled in the art.

[0073] Implementation method 2:

[0074] This embodiment is a further example of the two coherent light beams described in the method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry described in the first embodiment.

[0075] The two coherent light beams described in this embodiment have the same incident angle, which does not exceed 15°.

[0076] Specifically:

[0077] The two coherent light beams are completely located in a normal temperature environment outside the cryostat, and the two coherent light beams are incident oppositely from two sides of the cryostat.

[0078] Implementation method three:

[0079] This embodiment is a further example of the cryostat described in the method for measuring the thermal expansion coefficient of a film material at an ultra-low temperature based on the speckle interferometry method described in the first embodiment.

[0080] The cryostat described in this embodiment is provided with a sample holder.

[0081] Specifically: the sample seat is used to install the material to be tested.

[0082] Implementation method four:

[0083] This embodiment is a further example of the sample holder described in the method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry described in the third embodiment.

[0084] The cross section of the sample holder in this embodiment is an inverted concave shape.

[0085] Specifically:

[0086] The sample holder is shaped like an inverted "concave" character when viewed from the front, with the side facing the camera. After the sample is placed, a gasket is fixed with bolts on only one side to compress the sample; on the other side, the sample is simply placed freely on the bolts. After clamping, heat conduction only occurs on the left / right side of the sample, which can reduce the temperature gradient of the sample in the vertical direction.

[0087] Implementation method five:

[0088] This embodiment is a further example of the sample holder described in the method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry described in the third embodiment.

[0089] In this embodiment, a scale line is provided on the top of the sample holder.

[0090] Specifically:

[0091] The scale lines are used to measure the camera magnification.

[0092] Specifically:

[0093] The camera magnification D, in pixels / mm, is defined as the number of pixels in the image corresponding to a length of 1 mm on the surface of the sample being measured when the camera is focused.

[0094] Implementation method six:

[0095] This embodiment further illustrates the speckle image and temperature of the material to be measured in the method for measuring the thermal expansion coefficient of a film-like material at ultra-low temperature based on the speckle interferometry method described in the first embodiment.

[0096] In this embodiment, the speckle image and temperature of the material to be measured are recorded by a recording device.

[0097] Implementation method seven:

[0098] This embodiment is a further example of the recording device described in the method for measuring the thermal expansion coefficient of a film material at an ultra-low temperature based on the speckle interferometry method described in the sixth embodiment.

[0099] The recording device described in this embodiment includes a camera and a temperature controller.

[0100] Specifically:

[0101] When using a camera to take pictures, in order to minimize interference from the environment and camera background noise, it should be placed in a dark environment and take pictures using a small aperture and low sensitivity parameters. At this time, the camera's exposure time is relatively long and is extremely susceptible to vibration interference. In addition, the basic GM cycle-based low-temperature thermostat has an internal piston that will cause rigid displacement that can be directly observed in the camera.

[0102] The temperature controller is used in conjunction with the low-temperature thermostat, and the temperature controller establishes communication with the computer.

[0103] The computer includes a computer program, and the functions of the computer program include acquiring images and temperatures, obtaining interference patterns using a difference method, and automatically processing to achieve the function of automatically generating a strain-temperature curve. When the measurement temperature range is large, the computer program divides it into several subintervals of equal length, calculates the strain value for each subinterval in parallel and separately, and finally summarizes the complete strain value and strain-temperature relationship.

[0104] Implementation method eight:

[0105] This embodiment is a further example of the lowest temperature of the material to be measured described in the method for measuring the thermal expansion coefficient of a film-like material at ultra-low temperature based on the speckle interferometry method described in the first embodiment.

[0106] The minimum temperature of the material to be tested in this embodiment is 35K.

[0107] Implementation method nine:

[0108] This embodiment further illustrates the speckle image and temperature processing on the surface of the material to be measured described in the method for measuring the thermal expansion coefficient of a film-like material at ultra-low temperature based on the speckle interferometry method described in the first embodiment.

[0109] The speckle image and temperature processing of the surface of the material to be measured described in this embodiment are specifically as follows:

[0110] The speckle image is processed using a difference method to obtain an interference pattern, a surface strain value of the material to be measured is obtained according to the interference pattern, and the temperature-strain characteristics and thermal expansion coefficient of the material are obtained according to the surface strain value.

[0111] Specifically:

[0112] According to the principle of speckle interferometry, the phase difference between two adjacent dark fringes of the interference fringes obtained by the subtraction method is 2π. Assuming that the interference pattern obtained by subtracting two speckle patterns is obtained, the temperatures corresponding to the two speckle patterns are T1 and T2, unit: K, the average dark fringe spacing or the average bright fringe spacing of the interference pattern is x in the picture, unit: pixel, the incident angle of the two incident light beams in the optical path is θ, and the laser wavelength is λ, unit: nm, then the relative strain of the two speckle patterns can be calculated as:

[0113]

[0114] Where λ is the laser wavelength, D is the camera magnification, x is the average dark fringe spacing or the average bright fringe spacing of the interference pattern, and θ is the incident angle of the two beams.

[0115] Here, the thermal expansion coefficient of the material is assumed to be positive. In fact, for the extremely rare cases where the thermal expansion coefficient is negative or both positive and negative, it can be determined by observing the width and movement direction of the interference fringes at different temperatures. We will not elaborate on this here.

[0116] Implementation method ten:

[0117] This embodiment further illustrates the method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry described in the first embodiment.

[0118] like Figure 1 As shown, laser light emitted by laser 1 is split into two coherent beams by beamsplitter prism 2. Each beam is then expanded by convex lenses 3 and 4, and then reflected by mirrors 5 and 6. The beams ultimately strike the test surface of the sample at the same angle of incidence, from the left and right sides, through the transparent optical window of the cryostat. Interference occurs on the surface, forming a speckle pattern. Temperature changes in the sample cause thermal strain in the material, manifesting as differences in total displacement at different points on the test surface, ultimately causing regular variations in the speckle pattern. By subtracting the two speckle patterns, regular interference fringes can be obtained if the corresponding deformations are small. By calculating the average fringe spacing of the interference fringes, the strain values ​​corresponding to the relative thermal expansion of the two patterns can be obtained. By fixing one of the two patterns as the reference image and continuously varying the other, a series of strain values ​​can be obtained under the same reference. In practice, a computer collects images and temperatures at fixed intervals. This method can be used to obtain strain-temperature data for the sample under test over a narrow temperature range.

[0119] When taking photos with a camera, to minimize interference from the surrounding environment and camera noise, it should be placed in a dark environment, using a small aperture and low sensitivity parameters. At this time, the camera's exposure time is relatively long, making it extremely susceptible to vibration interference. Furthermore, the piston in the basic GM cycle-based cryostat inherently causes rigid displacement that can be directly observed in the camera. To eliminate the effects of these vibrations, the present invention modifies the cryostat's operating procedures. First, the cryostat is cooled to the minimum temperature according to the normal steps, and the vacuum valve of the cryostat is tightened. Then, the molecular pump is turned off. After it is completely shut down, the program is started to start counting, and then the mechanical pump, compressor, and chiller are quickly turned off in sequence.

[0120] In order to obtain the key information of the camera magnification, the concentric circles or scale lines pre-processed on the sample surface can be used, or when using the sample holder proposed in this embodiment, the scale value on it can be directly read.

[0121] The camera magnification D (unit: pixels / mm) mentioned here is defined as the number of pixels in the image corresponding to a 1mm length of the sample surface captured by the camera after focusing. According to the principle of speckle interferometry, the phase difference between two adjacent dark fringes in the interference fringes obtained by subtraction is 2π. Assuming that two speckle patterns are obtained by subtracting the interference pattern, the corresponding temperatures are T1 and T2 (unit: Kelvin), the average dark fringe spacing or the average bright fringe spacing of the interference pattern is x (unit: pixel) in the image, the incident angle of the two beams in the optical path is θ, and the laser wavelength is λ (unit: nm), the relative strain of the two speckle patterns can be calculated as:

[0122]

[0123] Here, the thermal expansion coefficient of the material is assumed to be positive. In fact, for the extremely rare cases where the thermal expansion coefficient is negative or mixed, it can be determined by observing the width and movement direction of the interference fringes at different temperatures. This will not be described in detail here. Through the above operation, the average thermal expansion coefficient of the measured sample in the temperature range of T1 to T2 can also be calculated as:

[0124]

[0125] The thermal expansion coefficient calculated using the above formula is often an average value over a wide temperature range. It is generally used as a preliminary estimate and to determine whether the result has significant deviations. It is not considered the final measurement result. To obtain a more detailed thermal expansion coefficient, the data obtained from the strain-temperature image should be used as the basis.

[0126] Because the cryostat compressor must be turned off during testing, the sample temperature constantly fluctuates, especially at ultra-low temperatures, where temperature fluctuations are more dramatic. Conventional temporal phase shifting methods are unsuitable. Furthermore, due to the limited viewing angle of the cryostat's optical window, the optical path for measuring in-plane displacements is theoretically inconvenient for obtaining phase using spatial carrier methods. Therefore, this embodiment employs a primitive image subtraction method to obtain interference fringes. Without precise phase information, the subtraction method can only provide limited information directly from the interference pattern, one of which is the brightness and darkness information within the interference fringes. When the temperature distribution in the test area is relatively uniform, a series of vertical interference fringes should be generated. Therefore, a program is used to perform column averaging on the interference fringe image, converting it into a one-dimensional column-averaged brightness sequence. Filtering this data produces smoothed data and relatively accurate locations of the maximum and minimum interference fringes.

[0127] In order to make the obtained interference pattern as vertically distributed as possible so as to perform column averaging operation on a single image, according to the determination formula of the interference pattern phase:

[0128]

[0129]

[0130] where δ is the additional phase value due to surface deformation in speckle interferometry, λ is the wavelength of incident light, u x is the displacement in x direction, θ is the incident angle.

[0131] Meanwhile, considering that the bright and dark of the fringes are modulated by δ / 2 in the above formula, the absolute value of should be as small as possible, in other words, when the temperature of the sample surface changes, the deformation of the sample to be measured along the y direction (vertical direction) (i.e. u x in the above formula) should tend to be consistent. By designing the shape of the sample holder, the heat transfer from the sample to the sample holder can be approximated as a one-dimensional case. The measure taken by the present application is to make the sample contact the sample holder only at the leftmost end, as shown in Figure 4 .

[0132] When all the interference fringes are processed by this idea, the data in the whole test interval are examined, and the system error introduced by this method will be reduced to a certain extent.

[0133] In the present embodiment, the experimental data acquisition and data processing are performed by a special program. The data acquisition device can communicate with the temperature controller, and the device acquires image and temperature data at fixed time intervals and stores them in the computer. The interface displays the real-time gray-scale image and the pattern generated by the difference method, so that the experimenter can obtain the approximate data of surface expansion in time and discover experimental problems in time. The data processing program only needs to import the acquired experimental data before processing, and the two images involved in the difference method can be freely selected.

[0134] The program has the function of automatically calculating the strain-temperature curve. After inputting the required calculation range, it is divided into several subintervals. For each subinterval, a suitable common reference image needs to be found, so that more than 50% of the images in the interval can obtain clear and correct fringe position information after the difference is made. In order to speed up the solving speed, the present embodiment uses multi-process parallel computing to solve this part. Under this operation method, the solutions of each subinterval are independent of each other, and parallel solving is feasible and reliable, and does not sacrifice the solving speed of each subinterval.

[0135] Suppose there are 100 pictures in each subinterval. In general, in order to find the above suitable common reference image, about 100-200 times of operation are needed, and the time consumption is about 1 min. Each operation needs to be performed on all images in the subinterval according to the following formula:

[0136]

[0137] I is the brightness distribution of the speckle pattern (intentionally written in distribution form to aid understanding), and α can be used to perform nonlinear transformation on the image brightness information.

[0138] That is, the image brightness (grayscale value) difference needs to be calculated, and then its power α (usually α>2, with this parameter, it can achieve nonlinear adjustment of image contrast). The image obtained in this embodiment is 720x400, so each solution requires a considerable amount of calculation. It is necessary to introduce parallel computing to speed up the solution.

[0139] When all subintervals are calculated, each subprocess returns its calculated strain value sequence to the main program, which then merges and removes erroneous and empty data to finally obtain a complete temperature and strain value sequence, which is then used to draw a graph.

[0140] It is worth noting that due to the long duration of the experiment, unpredictable vibrations will inevitably occur outside the laboratory during the temperature recovery process, which will interfere with the captured images, resulting in less valid data and making it impossible for the automatic solution program to run completely. At this time, manual data extraction is required for solution.

[0141] Implementation method eleven:

[0142] This embodiment further illustrates the method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry described in the first embodiment.

[0143] Low-temperature thermal expansion test experiment of copper material in the temperature range of 50~150K

[0144] According to the measurement method, Figure 3 The copper sample shown in the figure was screwed into the cold end of the cryostat before the experiment, and silicone grease was evenly applied to the contact surface between the sample and the cold end. The surface of the silicon diode temperature probe provided with the cryostat was also wrapped with silicone grease, and the silicon diode was fastened to the sample surface with 5mm wide Kapton tape to ensure good thermal contact between the silicon diode and the sample surface during temperature changes.

[0145] By manually processing the experimental data, the complete strain-temperature relationship in the temperature range of 50-150K was finally obtained. Figure 6 For a clearer comparison, the reference values ​​in the literature have been appropriately shifted.

[0146] Implementation Method Twelve:

[0147] This embodiment further illustrates the method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry described in the first embodiment.

[0148] Aluminum alloy 6061 material 50-150K temperature low temperature thermal expansion test experiment

[0149] The experimental method and the embodiment are consistent, and the experimental data is also manually processed, and the complete 50-150K temperature range strain-temperature relationship is obtained as shown in Figure 7 , and compared with the measurement results of the commercial instrument. It is worth noting that when analyzing the strain-temperature curve of the thermal expansion behavior, attention should be paid to the slope of the curve, rather than the strain value, that is, the point on the curve itself.

[0150] Embodiment thirteen:

[0151] This embodiment is a further illustration of the method for measuring the thermal expansion coefficient of film-shaped materials at ultra-low temperature based on speckle interferometry according to embodiment one.

[0152] Copper material 50-200K temperature range low temperature thermal expansion experiment, using automatic solving program for calculation

[0153] The code written by Python3 carries out automatic strain-temperature solving in the whole temperature range according to the automatic solving algorithm of embodiment one, selects the 300th-6000th frame, and divides it into 114 subintervals at equal intervals according to every 50 pictures. Using a computer with AMD Ryzen7 5800X (8 cores and 16 threads, memory is DDR4 3200MHz under 2x16G dual channel), the program is calculated in parallel by 16 processes, and after each process solves a subinterval, according to the multi-process module built-in Python3, the program will automatically activate the corresponding process of the interval to be solved. Finally, it takes 5min23s to complete the whole solving, and the solving result is shown in Figure 8 .

[0154] Embodiment fourteen:

[0155] Refer to Figure 2a , 2b to illustrate this embodiment.

[0156] The system for measuring the thermal expansion coefficient of film-shaped materials at ultra-low temperature based on speckle interferometry according to this embodiment comprises:

[0157] Cooling module: comprising a low-temperature thermostat for cooling the material to be measured;

[0158] Interference module: for obliquely injecting two coherent lights into the surface of the material to be measured;

[0159] Data acquisition module: for acquiring the speckle image and temperature of the surface of the material to be measured;

[0160] Data processing module: used for processing the speckle image and temperature to obtain the thermal expansion coefficient of the material to be tested.

Claims

1. A method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry, characterized in that: The method comprises: Under a vacuum environment, placing the material to be tested in a cryostat, and controlling the cryostat to cool to a minimum temperature so that the material to be tested reaches the minimum temperature and remains stable; Two beams of coherent light are incident obliquely on the surface of the material to be tested; Controlling the cryostat to stop the temperature control operation so that the temperature of the material to be tested is autonomously increased to room temperature, and collecting speckle images and temperatures on the surface of the material to be tested to obtain multiple sets of data during the process, each set of data including a speckle image and a corresponding temperature; The obtained multiple sets of data are processed to obtain the thermal expansion coefficient of the material to be tested.

2. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 1, characterized in that: The incident angles of the two coherent light beams are the same, and the incident angles do not exceed 15°.

3. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 1, characterized in that: A sample holder is provided inside the low-temperature thermostat.

4. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 3, characterized in that: The cross section of the sample seat is an inverted concave shape.

5. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 3, characterized in that: A scale line is provided on the top of the sample holder.

6. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 1, characterized in that: The speckle image and temperature of the material to be measured are recorded by a recording device.

7. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 6, characterized in that: The recording device includes a camera and a temperature controller, the camera is used to collect speckle images, and the temperature controller is used to collect the surface temperature of the material.

8. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 1, characterized in that: The lowest temperature of the material to be tested is 35K.

9. The method for measuring the thermal expansion coefficient of a film material at ultra-low temperature based on speckle interferometry according to claim 1, characterized in that: The speckle image and temperature processing of the surface of the material to be measured are specifically as follows: The speckle image is processed using a difference method to obtain an interference pattern, a surface strain value of the material to be measured is obtained according to the interference pattern, and a thermal expansion coefficient of the material is obtained according to the surface strain value.

Images