A salt column for adiabatic demagnetization refrigeration system and preparation method thereof

Abstract

A salt column for an adiabatic demagnetization refrigeration system comprises: an anisotropic thermal conductive frame, a high-density in-situ grown paramagnetic metal salt adiabatic demagnetization refrigeration medium, and matching flanges and frame accessories; wherein the anisotropic thermal conductive frame comprises a hot end plate (1), a cold end plate (2), a middle column (3), and a thermal conductive array (4). The salt column utilizes an axially configured high thermal conductive frame, and by closely combining with the in-situ grown paramagnetic metal salt, effectively improves the heat conduction efficiency between the paramagnetic metal salt and the working surface, reduces the temperature gradient inside the salt column, and improves the magnetic entropy utilization rate during its thermal cycle. At the same time, the original in-situ growth technology effectively reduces the voids in the salt column while ensuring good thermal contact between the refrigerant salt and the frame, thereby obtaining a high volume magnetic entropy density.

Description

Technical Field

[0001] The invention relates to the technical field of adiabatic demagnetization refrigeration, and in particular to a core salt column for an ultra-low temperature adiabatic demagnetization refrigeration system and a preparation method thereof. Background Art

[0002] Magnetic refrigeration technology can be traced back to the discovery of magnetocaloric effect in 1881. Its core principle is to use the change of magnetic entropy of magnetic materials during magnetization and demagnetization to achieve refrigeration. That is, firstly, the low entropy magnetization state of magnetic materials is achieved through heat exchange with the heat sink, and then the magnetic materials are demagnetized under adiabatic conditions to achieve the purpose of cooling the system.

[0003] The salt column composed mainly of paramagnetic metal salts is the refrigerant of the adiabatic demagnetization refrigeration system and its functional core. It should have the characteristics of large magnetic entropy change, high thermal conductivity, and high magnetic entropy density. However, the adiabatic demagnetization refrigeration medium currently used, especially the refrigerant used in ultra-low temperature environment (mainly inorganic paramagnetic metal salts), has a low thermal conductivity that limits the improvement of the system's refrigeration performance. In addition, the high porosity of the paramagnetic metal salt column is also an important factor leading to its low refrigeration efficiency. Its impact is not only reflected in the negative impact of the voids in the salt column on thermal conductivity, but also in the fact that the reduction in density reduces the magnetic entropy density per unit volume, thereby reducing the overall refrigeration efficiency of the system. Summary of the invention

[0004] The purpose of the present application is to overcome the above existing problems and provide a salt column and a preparation method thereof that can effectively improve the efficiency of an adiabatic demagnetization refrigeration system.

[0005] The high-efficiency salt column of the adiabatic demagnetization refrigeration system comprises an axially arranged anisotropic heat-conducting frame, a high-density in-situ grown paramagnetic metal salt refrigeration medium, and matching flanges, frames and other accessory components.

[0006] The heat-conducting frame includes a cold-end plate, a hot-end plate, a middle column and a high-density heat-conducting array arranged in an axial direction. The cold-end plate and the hot-end plate are the connection structures between the salt column and the heat sink and the refrigeration working surface. The two plates are connected as a whole by the middle column to provide support for the heat-conducting array composed of metal wires. The heat-conducting array is arranged in the axial direction and is composed of a high-density metal wire array, which can effectively improve the overall axial thermal conductivity of the salt column.

[0007] The adiabatic demagnetization refrigeration medium is uniformly and densely filled in the entire heat-conducting frame by an in-situ growth method. The close contact between the medium and the heat-conducting frame realizes efficient heat exchange between the medium and the frame, thereby improving the overall refrigeration efficiency of the system.

[0008] The salt column of the adiabatic demagnetization refrigeration system should also include matching flanges, frames and other auxiliary structures.

[0009] The present invention also includes a method for preparing the salt column:

[0010] (1) A heat-conducting frame structure consisting of a cold-end electrode plate, a hot-end electrode plate, a center column, and an axially arranged high-density heat-conducting array is assembled.

[0011] (2) Prepare a saturated paramagnetic metal salt solution and quantitatively inject it into the heat conduction frame to ensure that the metal salt crystals grow layer by layer from bottom to top in the heat conduction frame at a depth of about 1 mm per layer.

[0012] (3) By changing the solution temperature and using negative pressure to control the solvent evaporation rate, the crystallization conditions are adjusted so that the size of the metal salt crystal grains is comparable to the spacing of the metal wire array of the thermal conductive frame and is in close contact with the thermal conductive frame.

[0013] (4) Repeat the above steps (2) and (3) until the paramagnetic metal salt crystals fill the entire thermal conductive frame.

[0014] The beneficial technical effects of the present invention include:

[0015] The present invention is proposed for the functional core salt column of the adiabatic demagnetization refrigeration system. Based on the basic principle requirements of adiabatic demagnetization refrigeration, its working fluid core must have a high volume magnetic entropy density and at the same time have a high thermal conductivity efficiency. In particular, the thermal conductivity performance between the salt column working fluid and the working surface will seriously affect the overall refrigeration efficiency of the system. However, the existing adiabatic demagnetization refrigeration salt columns are mostly single crystals or blocks pressed from paramagnetic metal salts. The thermal conductivity, as an inherent property of the working fluid material, cannot be further improved. The low density caused by pressing not only further reduces its thermal conductivity, but also reduces the important volume magnetic entropy density, thereby greatly limiting the refrigeration efficiency of the adiabatic demagnetization refrigeration system. In this regard, the present invention provides a new and efficient salt column solution and a preparation method thereof.

[0016] The salt column provided by the present application utilizes an axially configured high thermal conductivity frame, and by closely combining with the paramagnetic metal salt grown in situ, effectively improves the heat conduction efficiency between the paramagnetic metal salt and the working surface, greatly reduces the temperature gradient inside the salt column, and thus effectively improves the utilization rate of magnetic entropy during its thermal cycle. At the same time, the original in-situ growth technology effectively reduces the voids in the salt column while ensuring good thermal contact between the refrigerant salt and the frame, thereby obtaining a high volume magnetic entropy density. By improving thermal conductivity and magnetic entropy density, the salt column provided by the present application can greatly improve the efficiency of the adiabatic demagnetization refrigeration system. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic structural diagram of a salt column and its matching flange, housing and other accessory components for an adiabatic demagnetization refrigeration system provided by an embodiment of the present invention.

[0018] Figure 2 It is a schematic diagram of the three-dimensional structure of the hot end plate or the cold end plate provided in an embodiment of the present invention.

[0019] Figure 3 It is a schematic diagram of the planar structure of a hot end plate or a cold end plate provided in an embodiment of the present invention.

[0020] Figure 4 It is a schematic diagram of the three-dimensional structure of a salt column for an adiabatic demagnetization refrigeration system provided by an embodiment of the present invention.

[0021] Figure 5 It is a schematic diagram of the matching scheme between the end face flange and the electrode plate provided in an embodiment of the present invention.

[0022] in:

[0023] 1-hot end plate; 2-cold end plate; 3-middle column; 4-thermal conductive array; 5-high-density paramagnetic salt; 6-hot end flange; 7-salt column frame shell; 8-cold end flange. DETAILED DESCRIPTION

[0024] To make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. However, those skilled in the art will appreciate that the present invention is not limited to the accompanying drawings and the following embodiments.

[0025] In the description of the invention, it should be noted that the orientation or positional relationship indicated by the terms "length", "width", "upper", "lower", "far", "near", etc. is based on the orientation or positional relationship shown in the drawings, which is only for the convenience of describing the invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and cannot be understood as limiting the specific protection scope of the invention. In addition, the terms "first" and "second" are only used for descriptive purposes to distinguish technical features, and do not have substantial meanings, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features.

[0026] The invention is a salt column developed based on magnetocaloric effect and used in an ultra-low temperature adiabatic demagnetization refrigeration system.

[0027] Figure 1 It is a schematic diagram of the structure of a salt column for an adiabatic demagnetization refrigeration system provided by an embodiment of the present invention.

[0028] like Figure 1 , Figure 4As shown, the salt column for the adiabatic demagnetization refrigeration system includes an anisotropic thermal conductive frame, a high-density in-situ grown paramagnetic metal salt adiabatic demagnetization refrigeration medium, and matching flanges, frames and other auxiliary components. The anisotropic thermal conductive frame includes a hot end plate 1, a cold end plate 2, a middle column 3 and a thermal conductive array 4.

[0029] In one embodiment of the present application, the anisotropic thermally conductive frame is made of high-purity copper with a gold-plated surface. The hot end plate 1 and the cold end plate 2 are the connection structures between the salt column and the heat sink and the refrigeration working surface. The hot end plate 1 and the cold end plate 2 are connected as a whole by the middle column 3 to provide support for the thermal conductive array 4 composed of metal wires. The thermal conductive array 4 is arranged parallel to the middle column 3 and is composed of 1200 copper wires with a diameter of 0.3mm and a purity of 99.99%, ensuring that it still has high thermal conductivity in the ultra-low temperature zone (sub-Kelvin temperature zone), thereby effectively improving the overall axial thermal conductivity of the salt column.

[0030] The heat conducting array 4 is in good thermal contact with the hot end plate 1 and the cold end plate 2 to ensure good thermal conductivity. Figure 2 , Figure 3As shown, in one embodiment of the present invention, the hot end plate 1 and the cold end plate 2 are circular flat plates with a diameter of 40 mm and a thickness of 3 mm, each of which is designed with 1200 circular micropores with a diameter of 0.5 mm. The number of circular micropores is related to the area of ​​the hot end plate 1 and the cold end plate 2. According to the design concept of the present invention, the density of micropores on the hot end plate 1 and the cold end plate 2 is about 0.5 to 2 micropores per square millimeter. The spacing of the micropores is related to the size of the paramagnetic metal salt grains used - the spacing between the two heat conducting wires needs to be smaller than the grain diameter of the paramagnetic metal salt crystals, because the thermal conductivity inside the grains is much greater than the thermal conductivity at the grain boundaries. Such a setting of the heat conducting array is conducive to avoiding the grain boundaries and achieving efficient heat conduction. The diameter of the metal wire (such as copper wire) can be adjusted according to the working conditions. For the embodiment of the present invention, the reasonable range is between 0.1 mm and 0.4 mm. The distance between the centers of each two micropores is 1 mm, and 1200 micropores are evenly distributed, which are used to fix the end points of the metal wires in the heat conducting array 4. The heat-conducting array 4 is composed of the same number of copper wires as the micropores of the hot end plate 1 or the cold end plate 2, and its two ends are connected to the hot end plate 1 and the cold end plate 2 through the micropores. According to one embodiment of the present invention, the above connection method is: the metal wire is passed between the through holes of the two end plates (similar to weaving), and the two ends of the metal wire are wound and fixed on the through holes of the two end plates; a longer metal wire can be interlaced and wound into several heat-conducting arrays; the two ends of the whole metal wire need to be fixed on the cold end plate or the hot end plate by welding. The total volume of the heat-conducting array 4 in this embodiment accounts for about 6.75% of the volume of the entire salt column. In a traditional salt column without a heat-conducting array, the contact area between the adiabatic demagnetization refrigerant and the metal heat-conducting frame is only the sum of the areas of the two end plates, while the refrigerant in the embodiment provided by the present invention is filled in the gap of the heat-conducting array 4. Compared with the existing technology, the contact area between the paramagnetic metal salt adiabatic demagnetization refrigerant and the metal heat-conducting frame in the embodiment of the present invention is increased by about 33 times. When the distance between the hot end plate 1 and the cold end plate 2 is 100 mm, the thermal conductivity of the heat conducting frame at around 5 K can reach 11.7 W / K.

[0031] The hot end flange 6 and the cold end flange 8 have the same structure. Since they are arranged at working surfaces with different temperatures, they are respectively called the hot end flange 6 or the cold end flange 8 (hereinafter collectively referred to as flanges). Both ends of the flange have flat heat conduction planes to achieve good heat conduction at the cross section. In addition, they also have a matching mechanism with the hot end plate or the cold end plate. The matching mechanism can be changed according to different use environments and working conditions, such as Figure 5 It is a structural schematic diagram of the flange-plate assembly solution adopted in the embodiment of the present application.

[0032] In addition, the matching structure with external components can be designed and processed on the flange according to different use environments and working conditions, including but not limited to: threaded holes, clamps, threaded sleeves and other structures.

[0033] The salt column frame shell 7 is a cylindrical thin-walled structure, which is fixed by pressing the hot end flange 6 and the cold end flange 8 from both ends during assembly, wrapping the paramagnetic refrigerant crystals of the salt column and playing a packaging role for the salt column.

[0034] The present invention also includes a method for preparing the salt column:

[0035] (1) The heat-conducting frame structure consisting of the cold-end plate 2, the hot-end plate 1, the center column 3 and the axially arranged high-density heat-conducting array 4 is assembled.

[0036] (2) A saturated paramagnetic metal salt solution is prepared and quantitatively injected into the heat conducting frame. This ensures that the metal salt crystals grow layer by layer from bottom to top in the heat conducting frame at a level of about 1 mm per layer. The metal salt solute in the solution will continuously precipitate into metal salt crystals as the solution solvent evaporates. Preferably, by controlling the solution concentration to be constant (saturated solution) and controlling the amount of the metal salt solution dripped each time, a layer of metal salt crystals with a certain thickness can be precipitated in the heat conducting frame (for example, in this embodiment, metal salt crystals are grown with a thickness of 1 mm per layer). After the upper layer of metal salt crystals is completely precipitated, a saturated metal salt solution of the same concentration and volume is dripped in and its solvent is evaporated until the metal salt crystals are completely precipitated. Repeating this operation can control the deposition of metal salt crystals layer by layer. For the control of the thickness of each layer, the solution concentration is constant, the volume of the solution dripped each time is constant, and the volume of the crystals precipitated after the solvent is completely evaporated is constant. The volume of the crystals deposited each time is calculated by multiplying the layer thickness by the cross-sectional area of ​​the heat conducting frame, and then the volume of the solution dripped each time is reversely calculated, so that the thickness of each layer can be controlled to be 1 mm. Preferably, the paramagnetic metal salt solution is a lithium ytterbium fluoride solution, more preferably, the lithium ytterbium fluoride is LiYb 1-x M x F 4 (0≤x<0.5, M is selected from one of lanthanum (La), cerium (Ce), gadolinium (Gd), and lutetium (Lu)). (3) By changing the solution temperature and assisting with negative pressure, the solvent evaporation rate is controlled, thereby adjusting the crystallization conditions so that the metal salt crystal size is equivalent to the spacing of the metal wire array of the heat-conducting frame and is in close contact with the heat-conducting frame. The adjustment of the above temperature and negative pressure is related to the saturated vapor pressure of the solvent used and the heat resistance of the metal salt solute. When evaporating the solvent to deposit the solute, the solvent evaporation rate can be adjusted by increasing the solution temperature or controlling the negative pressure of the environment. If the solute has a low heat resistance temperature, the adjustable temperature range of the solution is limited. To accelerate the evaporation of the solvent, only a negative pressure environment can be selected. In addition, the solvent evaporation rate is also related to its own saturated vapor pressure. Therefore, the solution temperature, vacuum degree (negative pressure), and solution evaporation rate need to be flexibly adjusted according to different working conditions.

[0037] (4) Repeat the above steps (2) and (3) until the paramagnetic metal salt crystals fill the entire thermal conductive frame.

[0038] The salt column used in the adiabatic demagnetization refrigeration system of the embodiment of the present invention has the following characteristics:

[0039] (1) The refrigerant salt is densely packed. Compared with the 70-80% volume density prepared by the traditional pressing method, the salt column prepared by the layering method can effectively fill the gaps between the grains, increasing the volume density to more than 95%, thereby maximizing the volume magnetic entropy density of the salt column.

[0040] (2) The working fluid salt crystals are tightly connected to the heat-conducting frame. The grains nucleate and grow first on the surface of the heat-conducting metal wire, which reduces the thermal resistance between the grains and the heat-conducting frame and is beneficial to the heat exchange between the working fluid and the heat-conducting frame.

[0041] (3) The working salt crystallizes uniformly. By precisely controlling the precipitation and growth conditions of the crystals in the solution, the working salt grain size can be made uniform and equivalent to the 1 mm spacing between the heat-conducting metal wires, thereby reducing the impact of grain boundary heat transfer on heat exchange.

[0042] The above characteristics can effectively increase the overall axial thermal conductivity of the salt column to 10W / K, which is more than twice that of the commercial GGG single crystal (3.77W / K) of the same appearance.

[0043] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for manufacturing a salt column for an adiabatic demagnetization refrigeration system, characterized in that: The salt column for the adiabatic demagnetization refrigeration system comprises: an anisotropic heat-conducting frame, a high-density in-situ grown paramagnetic metal salt adiabatic demagnetization refrigeration medium, a hot end flange and a cold end flange, and frame accessory components; wherein the anisotropic heat-conducting frame comprises a hot end plate (1), a cold end plate (2), a middle column (3), and a heat-conducting array (4); The manufacturing method comprises: Step 1, assembling a heat-conducting frame structure consisting of a cold-end electrode plate (2), a hot-end electrode plate (1), a center column (3), and an axially arranged high-density heat-conducting array (4); Step 2: prepare a saturated paramagnetic metal salt solution and quantitatively inject it into the heat conduction frame to ensure that the metal salt crystals grow layer by layer from bottom to top in the heat conduction frame at a level of 1 mm per layer; the metal salt solute in the solution is continuously precipitated into metal salt crystals as the solution solvent evaporates; Step 3, by changing the solution temperature and using negative pressure to control the solvent evaporation rate, the crystallization conditions are adjusted so that the size of the metal salt crystal is comparable to the spacing of the metal wire array of the heat-conducting frame, the spacing between the two copper wires is smaller than the grain diameter of the paramagnetic metal salt crystal, and the metal salt crystal is in close contact with the heat-conducting frame; Step 4, repeating the above steps 2 and 3 until the paramagnetic metal salt crystals fill the entire thermal conductive frame; Step 2 includes: preparing a saturated paramagnetic metal salt solution, controlling the amount of the metal salt solution added each time, so that a layer of metal salt crystals with a certain thickness can be precipitated in the heat-conducting frame; after the previous layer of metal salt crystals is completely precipitated, adding a saturated metal salt solution of the same concentration and the same volume and evaporating the solvent until the metal salt crystals are completely precipitated; repeating this operation to control the metal salt crystals to be deposited layer by layer; for the control of the thickness of each layer, the solution concentration is constant, the volume of the solution dripped each time is constant, and the volume of the crystals precipitated after the solvent is completely evaporated is constant; the volume of the crystals deposited each time is calculated by multiplying the layer thickness and the cross-sectional area of ​​the heat-conducting frame, and then the volume of the solution dripped each time is reversely calculated to control the thickness of each layer to be 1 mm; The total volume of the heat conducting array (4) accounts for 6.75% of the volume of the entire salt column used in the adiabatic demagnetization refrigeration system; The hot end plate (1) is a connection structure between the salt column and an external heat sink, and the cold end plate (2) is a connection structure between the salt column and an external refrigeration working surface; the hot end plate (1) and the cold end plate (2) are connected as a whole by a middle column (3) to provide support for a heat conductive array (4) composed of metal wires; the heat conductive array (4) is arranged parallel to the middle column (3); The heat conductive array (4) is in thermal contact with the hot end plate (1) and the cold end plate (2); The heat conductive array (4) is composed of the same number of copper wires as the circular micro-holes on the hot end plate (1) or the cold end plate (2), and its two ends pass through the circular micro-holes to be connected to the hot end plate (1) and the cold end plate (2) respectively.

2. The method for manufacturing a salt column for an adiabatic demagnetization refrigeration system according to claim 1, characterized in that: The anisotropic heat-conducting frame is made of high-purity copper with a gold-plated surface, and the purity of the high-purity copper is above 99.99%.

3. The method for manufacturing a salt column for an adiabatic demagnetization refrigeration system according to claim 1, characterized in that: The thermal conductive array (4) is composed of 1200 copper wires with a diameter of 0.3 mm and a purity of 99.99%.

4. The method for manufacturing a salt column for an adiabatic demagnetization refrigeration system according to claim 1 or 2, characterized in that: The hot end electrode plate (1) and the cold end electrode plate (2) are circular flat plates with a diameter of 40 mm and a thickness of 3 mm, and each is designed with 1200 circular microholes with a diameter of 0.5 mm.

5. The method for manufacturing a salt column for an adiabatic demagnetization refrigeration system according to claim 4, characterized in that: The distance between the centers of every two circular microholes is 1 mm, and 1200 microholes are evenly distributed and used to fix the end points of the metal wires in the heat conduction array (4).

6. The method for manufacturing a salt column for an adiabatic demagnetization refrigeration system according to claim 5, characterized in that: The density of micropores on the hot end plate (1) and the cold end plate (2) is 0.5 to 2 micropores per square millimeter.

7. The method for manufacturing a salt column for an adiabatic demagnetization refrigeration system according to claim 1, characterized in that: The copper wire constituting the heat conducting array (4) is connected to the circular micro-holes on the hot end plate (1) or the cold end plate (2) by: passing the metal wire between the circular micro-holes of the hot end plate (1) and the cold end plate (2), and wrapping the two ends of the metal wire around and fixing them on the circular micro-holes; and fixing the two ends of the whole metal wire on the hot end plate (1) or the cold end plate (2) by welding.

8. The method for manufacturing a salt column for an adiabatic demagnetization refrigeration system according to claim 1, characterized in that: In step three, the adjustment of temperature and negative pressure is related to the saturated vapor pressure of the solvent used and the heat resistance of the metal salt solute. When evaporating the solvent to deposit the solute, the solvent evaporation rate is adjusted by increasing the solution temperature or controlling the environmental negative pressure. If the solute has a low heat resistance temperature, the adjustable temperature range of the solution is limited, and a negative pressure environment is used to accelerate the evaporation of the solvent.

Images