A method for measuring the temperature distribution of a diamond anvil cell in a cryostat

Abstract

The present invention provides a method for measuring the temperature distribution of a diamond anvil cell in a cryostat. By adopting a new method for arranging thermoresistors and thermocouples on the cryostat and the diamond anvil cell, combining experimental temperature measurement with finite element analysis, the measurement of the sample temperature and temperature distribution under low temperature and high pressure can be ultimately realized, and the temperature difference between the sample cavity and the fixture can be accurately obtained. Compared with non-contact temperature measurement methods and traditional contact temperature measurement methods, the present invention is simple and efficient, improving the calibration rate and accuracy. The present invention is easy to implement and has good experimental repeatability; it solves problems such as the small size of the sample cavity, high requirements for the sealing of the cryostat, and difficult arrangement of thermocouples; it makes up for the experimental errors caused by using the temperature on the surface of the cryostat to replace the temperature of the anvil surface of the diamond anvil cell. And according to the corresponding model, it can be extended to the temperature difference between the temperature measurement points on the outer wall of other fixtures and the sample cavity.

Description

Technical Field

[0001] The present invention relates to a method for measuring the temperature of a sample chamber under high pressure in a cryostat, and particularly to a method for measuring the temperature distribution of a diamond anvil cell in a cryostat under extreme conditions. Background Art

[0002] The diamond anvil cell (DAC) is currently the only static pressure device that can reach millions of atmospheres and is a very important research tool in the field of condensed matter. Using a specially made diamond anvil cell, a high pressure of about 500 GPa can be achieved, and this record is continuously broken with the development of technology. In the laboratory, we usually combine high pressure, temperature, magnetic field, and electric field to measure the physical property changes of materials to discover new material properties. Among them, temperature, as an important physical quantity, is of great significance in the research of high pressure, geophysics, solid physics, and high-temperature superconductivity. However, there are still many problems to be solved in the in-situ temperature measurement technology under high pressure based on the diamond anvil cell. The most important problem is how to measure the true temperature of the diamond anvil cell anvil surface during the experiment in real time.

[0003] Due to the small size of the sample chamber and the high requirement for the sealing of the cryostat, it is not easy to arrange thermocouples during the pressurization experiment. Currently, the methods for measuring the temperature of the diamond anvil cell anvil surface under high pressure can be mainly divided into two categories: contact type and non-contact type temperature measurement methods. (1) The contact type temperature measurement method usually fixes several thermal resistors or thin film thermocouples on the outer wall of the diamond or the surface of the cryostat for measurement, and takes the average value of the measurement, regarding the temperature of the outer wall of the diamond or the surface of the cryostat as the temperature of the diamond anvil surface; the advantage is that the temperature value can be directly obtained and the consistency is good; the disadvantage is that even though the heat conduction rate of copper is very fast, due to the time required for heat conduction, the anvil surface temperature lags behind the surface temperature of the cryostat, and the distance difference between the two will still cause a large error in the measured temperature and cannot be exactly the same as the true temperature of the diamond anvil surface; moreover, this method has high requirements for thermal resistors and thermocouples, requires a large number of thermal resistors or thermocouples to be used, and is expensive; in addition, in a high-pressure experiment, there is a possibility of fracture at the edge of the diamond anvil surface. (2) The non-contact type temperature measurement method mainly measures the temperature of the sample through blackbody radiation; the advantage is that there is no limitation, and the temperature of the temperature measurement point can be calculated through indirect measurement, which is very convenient and there is no need to worry about the influence on the experimental device under high pressure; the disadvantage is that the temperature difference of the measurement result is large and the repeatability is poor; the values obtained from two experiments have a large gap, which is unacceptable for experiments with high precision requirements.

[0004] Therefore, providing a method for accurately measuring the temperature of the diamond anvil cell anvil surface in a cryostat is an urgent problem to be solved at present. Summary of the Invention

[0005] To solve the above technical problems, the present invention adopts a contact measurement method, combines the finite element analysis method, and through experiments, analyzes the temperature difference between the diamond anvil surface and the outer wall of the fixture, and gives a calibration curve, so as to obtain an effective anvil surface temperature according to the temperature of the diamond outer wall. Specifically, a method for measuring the temperature distribution of a diamond anvil cell in a cryostat provided by the present invention includes the following steps:

[0006] (1) Place the anvil surfaces of two diamond anvil cells opposite to each other up and down and fix them in the pressure groove of a cryogenic press; fix a thermocouple between the anvil surfaces of the two diamond anvil cells, and make the anvil surfaces of the two diamond anvil cells closely contact the thermocouple by applying pressure, and maintain a certain extrusion so that the thermocouple is in full contact with the anvil surface; the thermocouple is connected to the data acquisition end of a temperature controller, and the temperature controller receives the temperature value measured by the thermocouple;

[0007] (2) Place the cryogenic press in the sample holder of the cryostat and clamp the press with bolts so that the outer wall of the press is in full contact with the inner wall of the sample holder;

[0008] (3) Set at least one fixed reference temperature measurement point and one adjustable test temperature measurement point on the surface of the cryostat sample holder. Each of the reference temperature measurement point and the test temperature measurement point is provided with at least one thermal resistor, and the thermal resistor is connected to the data acquisition end of the temperature controller, and the temperature controller receives the temperature value measured by the thermal resistor at the temperature measurement point;

[0009] (4) Place a heating rod as a heat source in the heating hole of the cryostat. The heating rod is connected to the output end of the temperature controller, and the temperature controller controls the heating rod to increase and decrease the temperature to control the temperature inside the sample holder;

[0010] (5) Construct a finite element model according to the geometric structure and size of the entire cavity of the cryostat, the sample holder, the cryogenic press and the diamond anvil cell;

[0011] (6) Conduct finite element calculations for both the heating and cooling modes: read the temperature values N, M, and P of the corresponding temperature measurement points through the thermal resistors arranged at the reference temperature measurement point and the test temperature measurement point of the cryostat sample holder, and the thermocouple between the anvil surfaces of the diamond anvil cells. Among them, N is the temperature of the reference temperature measurement point, M is the temperature of the test temperature measurement point, and P is the temperature of the anvil surface of the diamond anvil cell;

[0012] (7) Take the change rate of the temperature N of the reference temperature measurement point of the cryostat sample holder as the change rate A of the surface temperature of the cryostat sample holder obtained by the experiment, and input this rate as a thermal boundary condition into the finite element model;

[0013] (8) Change the surface temperature change rate B of the cryostat sample holder in the finite element model, that is, change the slope of the temperature change function curve. Substitute the change rate B as the new thermal boundary condition into the finite element model, and simulate the overall temperature distribution through the finite element model. At this time, three temperature values M′, N′, and P′ are obtained, where M′, N′, and P′ are the temperatures of the experimental temperature measurement points, reference temperature measurement points, and diamond anvil temperatures under the simulated environment, respectively, corresponding to M, N, and P under the experimental conditions in sequence;

[0014] (9) Compare P and P′ as well as A and B. If P≠P′ or A≠B, continue to change the surface temperature change rate B of the cryostat sample holder until the anvil surface temperature of the diamond anvil coincides with the experimentally measured anvil surface temperature, and at the same time satisfy P = P′ and A = B. Then it is considered that the anvil surface temperature P and the surface temperatures N and M at this time are accurate temperatures, that is, the actual temperatures of the diamond anvil anvil surface and the cryostat sample holder surface are obtained; at the same time, the temperature difference between the surface temperature and the anvil surface temperature at this temperature change rate is obtained, △T = N - P, and the overall temperature distribution of the cryostat sample holder is obtained;

[0015] (10) By measuring the actual temperature of the cryostat sample holder surface, the actual temperature of the diamond anvil anvil surface can be inversely deduced.

[0016] The thermocouple mentioned is a T-type thermocouple, and the thermal resistor is a platinum thermal resistor.

[0017] The finite element model mentioned is modeled using Comsol software.

[0018] In step 8, the temperature change curve is set as:

[0019] Heating: T = (α - β) / 15 * t + β

[0020] Cooling: T = α - (α - β) / 37 * t

[0021] Among them, T is the temperature of the heating hole of the cryostat sample holder, with the unit of K, t is the variable, with the unit of min; α is the initial temperature value of the sample holder, and β is the preset experimental temperature end value; only by changing the denominator size can the temperature change rate be changed; by setting different initial temperatures α, final target temperatures β, and different desired temperature change rates, the curve of the simulated experimental environment temperature changing with time can be obtained, and this curve is set as the boundary condition.

[0022] The beneficial effects of the present invention:

[0023] Since the temperature of the sample chamber on the anvil surface of the diamond anvil cell is not affected under high pressure, the present invention only needs to measure under atmospheric pressure conditions to obtain temperature calibration, thus avoiding the problem of edge fracture of the sample chamber on the anvil surface of the diamond anvil cell. In the selection of thermocouples, the present invention can choose T-type thermocouples and platinum resistance thermometers to measure temperature, combine a simpler and more effective method to measure the temperature and provide a method for measuring the temperature field distribution, greatly shortening the time cost of calibration experiments and obtaining a quite good temperature calibration difference. The present invention selects the heat transfer module in Comsol software for heat conduction simulation, and selects transient and steady-state field thermal analysis therein. By restoring the experimental conditions and reasonably modeling, reasonable and accurate experimental results can be obtained. The present invention adopts a new method for arranging resistance thermometers and thermocouples on the cryostat and the diamond anvil cell, combines experimental temperature measurement and finite element analysis, and finally realizes the measurement of the sample temperature and temperature distribution under low temperature and high pressure, and can accurately obtain the temperature difference between the sample chamber and the fixture. Compared with non-contact temperature measurement methods and traditional contact temperature measurement methods, the present invention is simple and efficient, improving the calibration rate and accuracy. The method of the present invention is simple, easy to implement, and has good experimental repeatability; it solves problems such as the small size of the sample chamber, the high requirement for the sealing of the cryostat, and the difficulty in arranging thermocouples; it makes up for the experimental error caused by using the temperature on the surface of the cryostat to replace the temperature on the anvil surface of the diamond anvil cell. And according to the corresponding model, it can be extended to the temperature difference between other temperature measurement points on the outer wall of the fixture and the sample chamber, which has very important significance for experiments on temperature physical quantities related to high pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of the overall temperature measurement process of the present invention;

[0025] Figure 2 is a schematic diagram of the finite element analysis simulation of the temperature measurement of the cryostat of the present invention;

[0026] Figure 3 is a schematic diagram of the distribution map of the temperature measurement points of the present invention;

[0027] Figure 4 is a schematic diagram of the surface temperature and the anvil surface temperature of the present invention;

[0028] Figure 5 is a schematic diagram of the three-point temperature stability judgment of the present invention;

[0029] Figure 6 is a schematic diagram of the temperature difference experiment with different heating and cooling rates of the present invention;

[0030] Figure 7 is a schematic diagram of the temperature difference simulation with different heating and cooling rates of the present invention;

[0031] Figure 8 is a schematic diagram of the temperature difference between the experiment and the simulation of the present invention. Detailed implementation mode

[0032] Refer to Figure 1 、 2 as shown below:

[0033] A method for measuring the temperature distribution of a diamond anvil cell in a cryostat provided by the present invention includes the following steps:

[0034] (1) Place the anvils of two diamond anvil cells facing each other up and down and fix them in the pressure groove of a cryogenic press; select an ultra-thin sheet type T thermocouple as the temperature measuring element for measuring the diamond anvil surface, use cryogenic varnish to stick the thermocouple thin film temperature measuring head on the lower diamond anvil surface, use the upper diamond anvil surface to squeeze the lower anvil surface, and make the two diamond anvil surfaces closely contact the thermocouple by applying pressure, and maintain a certain extrusion so that the thermocouple is in full contact with the anvil surface; the wire diameter of the thermocouple is: 0.08mm * 2 wires, the outer dimension is: 0.1 * 0.2mm, and it is connected to the data acquisition end of the temperature controller. The temperature controller receives the temperature value measured by the thermocouple;

[0035] (2) Place the cryogenic press in the sample clamp of the cryostat and clamp the press with bolts so that the outer wall of the press is in full contact with the inner wall of the sample clamp;

[0036] (3) Set at least one fixed reference temperature measuring point and one adjustable test temperature measuring point on the surface of the cryostat sample clamp. The reference temperature measuring point is set in the middle of the sample clamp surface, close to the heating hole of the cryostat, and the test temperature measuring point is set at the lower part of the sample clamp surface, close to the diamond anvil cell; at least one thermal resistor is provided at the reference temperature measuring point and the test temperature measuring point respectively. Use cryogenic varnish to install the two thermal resistors at the reference temperature measuring point and the test temperature measuring point respectively. The thermal resistor is a platinum thermal resistor and is connected to the data acquisition end of the temperature controller. The temperature controller receives the temperature value measured by the thermal resistor at the temperature measuring point; the temperature controller is a lakeshore 336 temperature controller;

[0037] (4) Place a heating rod as a heat source in the heating hole of the cryostat. The heating rod is connected to the output end of the temperature controller. The temperature controller controls the heating rod to increase and decrease the temperature to control the temperature in the sample clamp; the resistance value of the heating rod is 25Ω and the rated power is 100w;

[0038] (5) Using Comsol software, a finite element model is constructed according to the geometric structures and sizes of the entire cavity of the cryostat, the sample holder, the cryo-press, and the diamond anvil cell. Among them, the finite element model uses three-body modeling directly in Comsol software to highly restore the experimental environment. The diameter of the entire cavity is 76 mm and the thickness is 65 mm. The sample holder has an irregular shape. In this embodiment, its thickness is simulated to be the same as that of the press. The diameter of the press is 47.62 mm, and the height (including the diamond anvil cell) is 27.40 mm. The diamond anvil cell is simulated as a cylinder with an overall height of 2.32 mm, a bottom diameter of 3.60 mm, and a height of 1 mm. The upper anvil surface has a diameter of 1 mm and is overall frustum-shaped.

[0039] (6) Finite element calculations are carried out for the heating and cooling modes respectively: The temperature values N, M, and P of the corresponding temperature measurement points are read through the thermal resistors arranged at the reference temperature measurement point and the test temperature measurement point of the cryostat sample holder, and the thermocouple between the anvil surfaces of the diamond anvil cell. Among them, N is the reference temperature measurement point temperature, M is the test temperature measurement point temperature, and P is the diamond anvil cell anvil surface temperature.

[0040] (7) The change rate of the reference temperature measurement point temperature N of the cryostat sample holder is used as the surface temperature change rate A of the cryostat obtained from the experiment, and this rate is input into the finite element model as a thermal boundary condition.

[0041] (8) Change the surface temperature change rate B of the cryostat sample holder in the finite element model, that is, change the slope of the temperature change function curve. Substitute the change rate B as the new thermal boundary condition into the finite element model, and simulate the overall temperature distribution through the finite element model. At this time, three temperature values M′, N′, and P′ are obtained. Among them, M′, N′, and P′ are the test temperature measurement point temperature, the reference temperature measurement point temperature, and the diamond anvil cell temperature in the simulated environment, corresponding to M, N, and P under the experimental conditions in turn.

[0042] (9) Compare P and P′, and A and B. If P≠P′ or A≠B, then continue to change the surface temperature change rate B of the cryostat sample holder until the diamond anvil cell anvil surface temperature coincides with the experimentally measured anvil surface temperature, and at the same time satisfy P = P′ and A = B. Then it is considered that the anvil surface temperature P and the surface temperatures N and M are accurate temperatures at this time, that is, the actual temperatures of the diamond anvil cell anvil surface and the cryostat sample holder surface are obtained. At the same time, the temperature difference between the surface temperature and the anvil surface temperature at this temperature change rate, △T = N - P, is obtained, and the overall temperature distribution of the cryostat sample holder is obtained.

[0043] (10) By measuring the actual temperature of the cryostat sample holder surface, the actual temperature of the diamond anvil cell anvil surface can be inversely deduced.

[0044] In step 8 of this embodiment, the temperature change curve is set as:

[0045] Heating: T = (α - β) / 15 * t + β

[0046] Cooling: T = α - (α - β) / 37 * t

[0047] Wherein, T is the temperature of the heating hole of the sample clamp of the cryostat, in K, t is a variable, in min; α is the initial temperature value of the sample clamp, which is set to 300 K in this embodiment; β is the preset experimental temperature end value, which is set to the liquid nitrogen temperature of 77 K in this embodiment; the experimental measurement temperature of the present invention is in the range of 77 K of the liquid nitrogen temperature to 300 K of the normal temperature, but is not limited to only this range, and the method of the present invention is also applicable to high-temperature temperature measurement.

[0048] During the cooling process, the initial temperature value of the entire sample clamp is 300 K, and the experimental condition is in vacuum, that is, an adiabatic process.

[0049] Table 1

[0050]

[0051] According to the data in Table 1, it can be seen that the cooling rate is 2.7 K / min. As time goes by, the surface temperature of the sample clamp of the cryostat and the temperature of the anvil surface of the diamond anvil cell gradually decrease.

[0052] Judge whether the dynamic equilibrium condition is reached by whether the difference values at points M, N, and P are stable.

[0053] In this embodiment, such as Figure 3 , 4 , 5, the temperature differences between any two of the three temperatures at points M, N, and P are: the temperature difference between points M and N is b; the temperature difference between points M and P is a; the temperature difference between points P and N is c. Among them, a, b, and c must satisfy that one is equal to the sum of the other two, for example, a = b + c.

[0054] Therefore, when the dynamic equilibrium is reached, a, b, and c must be unchanged and remain in a stable state. This can be seen from Figure 5 In the figure, the right-angled rectangular frame represents the stable state. At this time, it can be seen from the first part of the figure that the temperature curve is linear. The rounded rectangular frame represents the variable temperature state. It can be seen from the first part of the figure that the temperature curve has twists and turns, indicating that the temperature field is unstable, and the measured temperature difference does not correspond to the variable temperature rate. It can be seen from the second part and the third part of the figure that the change trends of a, b, and c are all changed simultaneously, indicating an accompanying relationship between them.

[0055] In LabVIEW, we control the change of the temperature rate and simultaneously observe the changes of a, b, and c. When all three reach a stable state, it means that the temperature field is stable at this time. At this time, as Figure 4As shown, the anvil surface temperature T1 and the non-anvil surface temperature T2 (taking a fixed point, i.e., the reference point N) are read, and the calculated ΔT = T1 - T2 difference corresponds to the temperature change rate at this time.

[0056] This method greatly shortens the measurement of the temperature difference ΔT corresponding to different temperature change rates.

[0057] The temperature field distribution is obtained through the combination of finite element analysis and experiments, as Figure 6 shown in the scatter plot of the experimental temperature change difference versus the temperature change rate, which is a relatively linear scatter distribution.

[0058] Using finite element analysis, a model of the entire sample holder is constructed, boundary conditions are set, and at the same time, according to the temperature measurement principle, the scatter plot of the simulated temperature change difference versus the temperature change rate is obtained as Figure 7 shown.

[0059] Table 2

[0060]

[0061] Table 2 describes the difference between the experimental value and the heating simulation value. As Figure 8 shown, it represents the degree of closeness between the differences.

[0062] Comparing Figure 6 、 Figure 7 , it can be seen that the difference gradually increases, indicating that in the case of high temperature change rates, the experimental error is "amplified". This experimental error is inevitable and it is necessary to reduce the error to obtain high consistency. As Figure 7 After the test rate is 7K / min and onwards, it can be observed that there is an obvious jump in the experiment, which is due to the difference brought by heat sources at two different positions during cooling and heating.

Claims

1. A method for measuring the temperature distribution of a diamond anvil cell in a cryostat, characterized in that: It includes the following steps: (1) Set the anvils of two diamond anvil cells facing each other up and down, and fix them in the groove of a low-temperature press; fix a thermocouple between the anvils of the two diamond anvil cells. By applying pressure, make the anvils of the two diamond anvil cells closely contact the thermocouple, and maintain a certain extrusion so that the thermocouple is in full contact with the anvils; the thermocouple is connected to the data acquisition end of a temperature controller, and the temperature controller receives the temperature value measured by the thermocouple; (2) Place the low-temperature press in the sample clamp of a low-temperature thermostat, and clamp the press with bolts so that the outer wall of the press is in full contact with the inner wall of the sample clamp; (3) Set at least one fixed reference temperature measurement point and one adjustable test temperature measurement point on the surface of the sample clamp of the low-temperature thermostat. Each of the reference temperature measurement point and the test temperature measurement point is provided with at least one thermal resistor, and the thermal resistor is connected to the data acquisition end of the temperature controller. The temperature controller receives the temperature value measured by the thermal resistor at the temperature measurement point; (4) Place a heating rod as a heat source in the heating hole of the low-temperature thermostat. The heating rod is connected to the output end of the temperature controller, and the temperature controller controls the heating rod to raise and lower the temperature to control the temperature in the sample clamp; (5) Construct a finite element model according to the geometric structure and size of the entire cavity of the low-temperature thermostat, the sample clamp, the low-temperature press, and the diamond anvil cells; (6) Conduct finite element calculations for both the heating and cooling modes respectively: read the temperature values N, M, and P of the corresponding temperature measurement points through the thermal resistors arranged at the reference temperature measurement point and the test temperature measurement point of the sample clamp of the low-temperature thermostat, and the thermocouple between the anvils of the diamond anvil cells. Among them, N is the temperature of the reference temperature measurement point, M is the temperature of the test temperature measurement point, and P is the temperature of the anvils of the diamond anvil cells; (7) Take the change rate of the temperature N of the reference temperature measurement point of the sample clamp of the low-temperature thermostat as the change rate A of the surface temperature of the sample clamp obtained from the experiment, and input this temperature change rate as a thermal boundary condition into the finite element model; (8) Change the surface temperature change rate B of the cryostat sample holder in the finite element model, that is, change the slope of the temperature change function curve, substitute the temperature change rate B as the new thermal boundary condition into the finite element model, and simulate the overall temperature distribution through the finite element model. At this time, three temperature values are obtained , where are the experimental temperature measurement point temperature, reference temperature measurement point temperature, and diamond anvil temperature in the simulation environment, corresponding to M, N, and P under the experimental conditions in sequence; (9) Compare P and and A and B. If or , then continue to change the surface temperature change rate B of the cryostat sample holder until the anvil surface temperature of the diamond anvil cell coincides with the experimentally measured anvil surface temperature, while satisfying and A = B. Then it is considered that the anvil surface temperature P and the surface temperatures N and M are accurate temperatures at this time, that is, the actual temperatures of the anvil surface of the diamond anvil cell and the surface of the cryostat sample holder are obtained; at the same time, the temperature difference between the surface temperature and the anvil surface temperature at this temperature change rate, △T = N - P, is obtained, and the temperature distribution of the entire cryostat sample holder is obtained. (10) By measuring the actual temperature on the surface of the sample clamp of the low-temperature thermostat, the actual temperature of the anvils of the diamond anvil cells can be inversely deduced.

2. A method for measuring the temperature distribution of a diamond anvil cell in a cryostat according to claim 1, characterized in that: The thermocouple selected is a T-type thermocouple, and the thermal resistor selected is a platinum thermal resistor.

3. A method for measuring the temperature distribution of a diamond anvil cell in a cryostat according to claim 1, characterized in that: The finite element model is modeled using Comsol software.

4. A method for measuring the temperature distribution of a diamond anvil cell in a cryostat according to claim 1, characterized in that: In step 8, set the temperature change curve as: Temperature rise: ; Cooling: ; where T is the temperature of the heating hole of the sample clamp of the low-temperature thermostat, with the unit of K, t is the variable, with the unit of min; α is the initial temperature value of the sample clamp, and β is the preset experimental temperature end value; changing the temperature change rate only needs to change the denominator size; by setting different initial temperatures α, final target temperatures β, and different temperature change rates, a curve showing the change of the simulated experimental environment temperature over time can be obtained, and this curve is set as the boundary condition.

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