A method for improving magnetic entropy change of as-prepared alloys by heat treatment

Abstract

The application discloses a method for improving the magnetic entropy change of a prepared alloy through heat treatment, and comprises the following steps: (1) ultrasonic cleaning, ethanol washing and natural drying of nickel particles, manganese particles and gallium particles; (2) repeatedly melting the mixed raw material particles obtained by mixing the nickel particles, the manganese particles and the gallium particles to obtain an alloy ingot; (3) casting the alloy ingot into an alloy rod by using a copper mold suction casting method; (4) polishing, ultrasonic cleaning, ethanol washing and drying of the alloy rod; and (5) performing phased heat treatment on the treated alloy rod in a vacuum environment by using a mode of gradually reducing heat treatment temperature and gradually increasing heat treatment time, and cooling the alloy rod to room temperature in the furnace after the phased heat treatment is completed. The application can solve the technical problems that the magnetic entropy change of the existing nickel-manganese-gallium alloy cannot be effectively improved under high magnetic field intensity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of preparation of large magnetic entropy change alloy materials. Specifically, it is a method for improving the magnetic entropy change of a prepared alloy through heat treatment. BACKGROUND

[0002] The typical chemical composition of the nickel-manganese-gallium alloy is Ni x Mn y Ga z (50≤x≤56, 22≤y≤30, 22≤z≤30), which can exhibit both martensitic phase transition and magnetic phase transition within the temperature range of -80℃ to 80℃. The alloy has both ferromagnetism and ferroelasticity, and exhibits significant magnetic heat effect during the magnetic structure phase transition. Therefore, it is used as a solid-state refrigeration working medium to replace traditional gas compression technology, and has the characteristics of high efficiency and environmental protection. However, since the nickel-manganese-gallium alloy belongs to a first-order magnetic structure phase transition material, it has the defects of large hysteresis and wide transition temperature range, which limits its application in actual use, as it often requires a higher driving magnetic field. The existing technology often optimizes the composition ratio of nickel-manganese-gallium, or modifies it by doping other metal elements, or improves its preparation process to optimize its magnetic refrigeration performance.

[0003] Chinese patent document CN1584082A discloses a large magnetic entropy change compound and a preparation method thereof. By controlling the proportion of the three metal elements of nickel, manganese and gallium and optimizing the process, the finally prepared nickel-manganese-gallium compound has a magnetic entropy change of 0-16 J / kgK at a magnetic field of 0-2T. However, the large magnetic entropy change compound has the following defects: the preparation of the nickel-manganese-gallium alloy needs to be carried out at a relatively high temperature, and the proportion of each element needs to be strictly controlled during the preparation process. If the large magnetic entropy is realized by controlling the proportion of metal elements, the transition temperature may not be ideal, which will affect the overall magnetic performance. In addition, although the nickel-manganese-gallium alloy prepared by the patent has a large magnetic entropy change at a magnetic field of 0-2T, due to the limitation of the crystal structure of the nickel-manganese-gallium alloy material, the magnetic entropy change usually reaches a peak at a lower magnetic field (such as 1.6T), and the magnetic entropy change cannot be effectively improved by further increasing the magnetic field strength (such as a magnetic field strength of 7T). SUMMARY

[0004] Therefore, the technical problem to be solved by the present application is to provide a method for improving the magnetic entropy change of a prepared alloy through heat treatment, so as to solve the technical problems that the existing preparation process of the nickel-manganese-gallium alloy needs to strictly control the proportion of each element to realize a large magnetic field change, the process is difficult, and the magnetic entropy change of the prepared nickel-manganese-gallium alloy cannot be effectively improved at a high magnetic field strength.

[0005] To solve the above technical problems, the present application provides the following technical solutions:

[0006] A method for improving the magnetic entropy change of an as-prepared alloy by heat treatment comprises the following steps:

[0007] Step (1), ultrasonically cleaning the high-purity nickel particles, high-purity manganese particles and high-purity gallium particles, washing them with anhydrous ethanol and naturally drying them after the ultrasonic cleaning is completed; ultrasonic cleaning followed by washing with anhydrous ethanol can completely remove stubborn contaminants on the surfaces of the high-purity nickel particles, high-purity manganese particles and high-purity gallium particles, ensuring that the particle surfaces are cleaner, avoiding oxidation, and ensuring that the quality and performance of the materials in the subsequent alloy processing process will not be affected; natural drying can ensure that the metal particles can achieve the required purity and performance in subsequent processing;

[0008] Step (2), placing the mixed raw material particles obtained by mixing the treated high-purity nickel particles, high-purity manganese particles and high-purity gallium particles into an arc melting furnace, and repeatedly melting them under the protection of an inert gas to obtain an alloy ingot;

[0009] Step (3), transferring the alloy ingot into a crucible with a copper mold suction casting, and casting the alloy rod by the copper mold suction casting method;

[0010] Step (4), grinding the alloy rod and then ultrasonically cleaning it, washing it with anhydrous ethanol and letting it dry naturally;

[0011] Step (5), subjecting the treated alloy rod to stage-by-stage heat treatment in a vacuum environment, and cooling the alloy rod to room temperature in the furnace after the stage-by-stage heat treatment, thereby obtaining a magnetic entropy change alloy;

[0012] The staged heat treatment method is: the heat treatment is divided into at least four stages in the manner of "gradually decreasing the heat treatment temperature and gradually increasing the heat treatment time", and the heat treatment temperature of the first stage is 770~790℃, the heat treatment temperature of the last stage is 500~550℃, and the total heat treatment time is 33~41h.

[0013] The nickel-manganese-gallium alloy rod is heat-treated by the staged heat treatment method of the present invention of "gradually decreasing the heat treatment temperature and gradually increasing the heat treatment time", and the initial treatment temperature, the final treatment temperature and the total heat treatment time are controlled within the above-mentioned range. This is beneficial to optimizing the microstructure of the alloy, regulating the magnetic phase transition, enhancing the magnetic interaction, improving the thermal stability and improving the reaction kinetics, and can effectively enhance the magnetic entropy change of the prepared alloy under high magnetic field strength.

[0014] In the above-mentioned method for improving the magnetic entropy change of the prepared alloy by heat treatment, in step (1), the particle size of the high-purity nickel particles is Φ3*3 mm, and the mass fraction of nickel in the high-purity nickel particles is greater than or equal to 99.995wt%; the particle size of the high-purity manganese particles is 1~10mm, and the mass fraction of manganese in the high-purity manganese particles is greater than or equal to 99.9wt%; the particle size of the high-purity gallium particles is 2~5 mm, and the mass fraction of gallium in the high-purity gallium particles is greater than or equal to 99.999wt%; compared with bulk high-purity nickel, high-purity manganese and high-purity gallium, the present invention uses granular metal raw materials and controls the particle size range of each metal raw material within the above-mentioned range, which can significantly improve the phase change rate of each alloy element during smelting, improve the uniformity of the alloy, and create favorable conditions for subsequently obtaining a nickel-manganese-gallium alloy with a higher magnetic entropy change under high magnetic field strength.

[0015] In the above-mentioned method for enhancing the magnetic entropy change of the as-prepared alloy by heat treatment, in step (2), the molar ratio of the high-purity nickel particles, the high-purity manganese particles and the high-purity gallium particles is (50~53): (22~26): (22~26).

[0016] In the above-mentioned method for improving the magnetic entropy change of the as-prepared alloy by heat treatment, in step (1), the particle size of the high-purity nickel particles is Φ3*3 mm, and the mass fraction of nickel in the high-purity nickel particles is equal to 99.995wt%; the particle size of the high-purity manganese particles is 1~10 mm, and the mass fraction of manganese in the high-purity manganese particles is equal to 99.9wt%; the particle size of the high-purity gallium particles is 2~5 mm, and the mass fraction of gallium in the high-purity gallium particles is equal to 99.999wt%.

[0017] In the above-mentioned method for improving the magnetic entropy change of the prepared alloy by heat treatment, in step (2), the mixed raw material particles are placed in the first crucible of a magnetron tungsten pole vacuum arc melting furnace, and the titanium block is placed in the second crucible; the magnetron tungsten pole vacuum arc melting furnace is evacuated and filled with high-purity argon as a protective gas; the titanium block in the second crucible is first melted, and then the mixed raw material particles in the first crucible are melted, and after melting, they are cast to obtain an intermediate ingot; the intermediate ingot is turned over and repeatedly melted and cast to obtain an alloy ingot; and electromagnetic stirring is turned on during the melting process.

[0018] In the above method of improving the magnetic entropy change of the prepared alloy by heat treatment, in step (2), the arc melting furnace is evacuated to 10 -4Pa, the purity of high-purity argon is greater than or equal to 99.99%; the temperature of the mixed raw material particles is 1450~1500℃, the casting temperature is 1300~1350℃, and the cooling rate after casting is 10~50℃ / s; the number of repeated melting and casting is 4~5 times; the electromagnetic stirring rate during the melting process is 300~600rpm; if the number of repeated melting and casting is greater than 5 times, the risk of oxidation pollution will increase, and problems such as grain coarsening and composition deviation will occur, thereby reducing the performance of the alloy; but if the number of repeated melting and casting is less than 4 times, the alloy is difficult to completely dissolve, more inclusions remain, and there are also problems such as poor surface quality and incomplete phase transformation, which affect the uniformity and performance of the alloy. The present invention controls the melting temperature, melting stirring rate, casting temperature, and cooling rate after casting in a reasonable range during the repeated melting process, and controls the number of repeated melting to fully dissolve the alloy components, reduce the residual inclusions, improve the uniformity and performance of the alloy ingot, and provide conditions for subsequent copper mold suction casting to obtain an alloy bar with ideal performance.

[0019] In the above-mentioned method for improving the magnetic entropy change of the prepared alloy by heat treatment, in step (3), during the copper mold suction casting process to obtain the alloy rod, the melting temperature of the alloy ingot is 1450~1500℃, the casting temperature is 1300~1350℃, the suction casting negative pressure is 100~500Pa, and the cooling rate is 10~50℃ / s; the diameter of the alloy rod is 8~10mm. Copper mold suction casting helps to optimize the casting quality of the alloy, improve the uniformity of the obtained alloy rod, and optimize its mechanical properties; if the repeatedly melted alloy ingot is not subjected to copper mold suction casting, but is directly polished and then subjected to subsequent modulation treatment, the subsequent heat treatment effect will be poor due to the existence of inherent defects, coarse grains, uneven composition and other problems in the alloy ingot. Compared with other conventional methods, the copper mold suction casting method under the above-mentioned process parameters of the present invention is used to cast alloy ingots into rods, which can effectively avoid alloy oxidation, and can effectively eliminate the inherent defects in the alloy ingots, refine the grain size, and improve the uniformity of the composition, which is beneficial to improving the treatment effect of subsequent staged heat treatment.

[0020] In the above-mentioned method for improving the magnetic entropy change of the prepared alloy by heat treatment, in step (4), the sandpaper used for grinding is 2000 mesh, and the surface roughness is polished to less than or equal to 0.025 μm, so as to achieve a smooth and reflective mirror effect. After cutting, the metal surface of the alloy bar may produce oxides, burrs, residual processing fluids or other contaminants; after grinding by the above-mentioned grinding method, these surface impurities can be removed, providing a clean surface for subsequent heat treatment; if the heat treatment is directly performed without grinding, it may lead to uneven heat treatment effect, oxide layer and contaminants affecting the heat treatment effect, poor surface quality and other problems.

[0021] In the above method for improving the magnetic entropy change of the prepared alloy by heat treatment, in step (5), the treated alloy rod is first placed in a porcelain boat, both ends of the porcelain boat are sealed with titanium blocks, and then the sealed porcelain boat is placed in a quartz glass tube and sealed with a vacuum rotary tube sealing device; the sealed quartz glass tube is placed in a resistance furnace for staged heat treatment; the vacuum degree in the quartz glass after sealing is 10 -3 Pa.

[0022] In the above-mentioned method of improving the magnetic entropy change of the prepared alloy by heat treatment, in step (5), the staged heat treatment method is as follows: first, the temperature is increased to 770~790℃ at a heating rate of 5~10℃ / min and kept at this temperature for 1~2h (under this treatment condition, the necessary phase change and homogenization of the alloy rod can be ensured; if the heating rate of the heat treatment in this stage is too fast, the alloy components may be precipitated or uneven, and if the heating rate is too slow, the phase change process of the alloy may be affected; if the temperature is too high, the alloy elements may be volatilized or lost, and if the temperature is too low, the alloy composition may be uneven; if the holding time is too long, some components may be precipitated, and if the holding time is too short, the alloy may not fully react); then, the temperature is reduced to 740~755℃ at a cooling rate of 5~10℃ / min and kept at this temperature for 1~2h (this stage can further promote the phase change of the alloy and the diffusion of elements, ensuring that the structure of the alloy is more uniform; at this stage, if the temperature is too high, the magnetic response of the material may be reduced; if the temperature is too low, the composition may be uneven; the holding time Too long may increase the risk of oxidation; too short holding time will lead to incomplete reaction); then cool down to 630~650℃ at a cooling rate of 5~10℃ / min and hold for 10~12h (this stage can fully diffuse the components in the alloy, optimize the phase structure, and lay the foundation for grain refinement and magnetic property improvement; if the temperature is too high at this stage, it will affect the stability of the alloy composition; if the temperature is too low, it will cause uneven structure; if the holding time is too long, it will affect the final structure of the alloy; if the holding time is too short, it will cause unstable alloy performance); finally, cool down to 500~550℃ at a cooling rate of 5~10℃ / min and hold for 20~24h (the final heat treatment stage can ensure complete diffusion of elements in the alloy, optimize the microstructure, and enable it to maintain stable performance in long-term use; if the temperature is too high at this stage, it will affect the quality of the alloy; if the temperature is too low, it will affect the alloy performance; if the holding time is too long, it will affect the final structure of the alloy; if the holding time is too short, it will affect the thermal stability of the alloy). After the holding period, cool to room temperature with the furnace. The present invention divides the heat treatment into four stages by adopting the method of "gradually decreasing the heat treatment temperature and gradually increasing the heat treatment time", and controls the heat treatment temperature and heat treatment time of each stage within a specific range. It can effectively improve the lattice structure of the nickel-manganese-gallium alloy and significantly improve its magnetic entropy change performance under a higher magnetic field intensity (7T).

[0023] In the above-mentioned method for improving the magnetic entropy change of the prepared alloy by heat treatment, in step (1), the particle size of the high-purity nickel particles is Φ3*3mm, and the mass fraction of nickel in the high-purity nickel particles is equal to 99.995wt%; the particle size of the high-purity manganese particles is 1~10mm, and the mass fraction of manganese in the high-purity manganese particles is equal to 99.9wt%; the particle size of the high-purity gallium particles is 2~5mm, and the mass fraction of gallium in the high-purity gallium particles is equal to 99.999wt%;

[0024] In step (2), the molar ratio of high-purity nickel particles, high-purity manganese particles and high-purity gallium particles is 2:1:1. Under this ratio, the process conditions of the present invention can be used to prepare a material with the chemical formula Ni 52 Mn 26 Ga 22 Magnetic entropy change alloy;

[0025] Put the mixed raw material particles into the first crucible of the magnetic tungsten vacuum arc melting furnace, and put the titanium block into the second crucible; evacuate the magnetic tungsten vacuum arc melting furnace to 10 -4 Pa was followed by the addition of high-purity argon gas with a purity greater than or equal to 99.99% as a protective gas; the titanium block in the second crucible was first smelted, and then the mixed raw material particles in the first crucible were smelted at a smelting temperature of 1450°C. After smelting, the mixture was cast at 1300°C, and then cooled to room temperature at a cooling rate of 50°C / s to obtain an intermediate ingot; the intermediate ingot was flipped over and repeatedly smelted and cast five times to obtain an alloy ingot; electromagnetic stirring was turned on during the smelting process, and the electromagnetic stirring rate was 500 rpm;

[0026] In step (3), during the copper mold suction casting process to obtain the alloy rod, the melting temperature of the alloy ingot is 1500°C, the casting temperature is 1350°C, the suction casting negative pressure is 500 Pa, and the cooling rate is 10°C / s; the diameter of the alloy rod is 8 mm;

[0027] In step (4), the sandpaper used for polishing is 2000 mesh, and the surface roughness is polished to be less than or equal to 0.025 μm;

[0028] In step (5), the treated alloy rod is first placed in a porcelain boat, both ends of the porcelain boat are sealed with titanium blocks, and then the sealed porcelain boat is placed in a quartz glass tube and sealed with a vacuum rotary tube sealing device; the sealed quartz glass tube is placed in a resistance furnace for staged heat treatment; the vacuum degree in the quartz glass after sealing is 10 -3 Pa;

[0029] The staged heat treatment method is as follows: first, heating to 775 DEG C at a heating rate of 7 DEG C / min, holding for 1.5 h, then, cooling to 750 DEG C at a cooling rate of 7 DEG C / min, holding for 2 h, then, cooling to 650 DEG C at a cooling rate of 7 DEG C / min, holding for 10 h, finally, cooling to 500 DEG C at a cooling rate of 7 DEG C / min, holding for 20 h, and then cooling to room temperature in the furnace.

[0030] The technical scheme of the present application achieves the following beneficial technical effects:

[0031] The method for improving the magnetic entropy change of a prepared alloy by heat treatment of the present application can improve the magnetic entropy change of the prepared Ni 50-53 Mn 22-26 Ga 22-26 Heusle alloy under high magnetic field intensity. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The structure characterization graph of the Ni 52 Mn 26 Ga 22 Heusle alloy prepared by the embodiment of the present application;

[0033] Figure 2 The magnetic heat characterization graph of the Ni 52 Mn 26 Ga 22 Heusle alloy prepared by the embodiment of the present application (after heat treatment, the magnetic field direction is "⊥");

[0034] Figure 3 The magnetic heat characterization graph of the Ni 52 Mn 26 Ga 22 Heusle alloy prepared by the embodiment of the present application (preparation state, the magnetic field direction is "⊥");

[0035] Figure 4 The magnetic heat characterization graph of the Ni 52 Mn 26 Ga 22 Heusle alloy prepared by the embodiment of the present application (after heat treatment, the magnetic field direction is "∥");

[0036] Figure 5 The magnetic heat characterization graph of the Ni 52 Mn26 Ga 22 Magnetic heat characterization map of Heusler alloy (as-prepared state, magnetic field direction is "||"). DETAILED DESCRIPTION

[0037] The method for improving the magnetic entropy change of the as-prepared alloy by heat treatment in this embodiment includes the following steps:

[0038] Step (1), high-purity nickel particles, high-purity manganese particles and high-purity gallium particles are placed in an ultrasonic cleaner for ultrasonic cleaning. After ultrasonic cleaning, the particles are washed with anhydrous ethanol and naturally dried. The mass fraction of nickel in the high-purity nickel particles is 99.995wt%, the mass fraction of manganese in the high-purity manganese particles is 99.9wt%, and the mass fraction of gallium in the high-purity gallium particles is 99.999wt%. The particle size of the high-purity nickel particles is Φ3*3 mm, the particle size of the high-purity manganese particles is 1-10 mm, and the particle size of the high-purity gallium particles is 2-5 mm.

[0039] Step (2), the mixed raw material particles obtained by mixing the high-purity nickel particles 35.5217 g, the high-purity manganese particles 16.9579 g and the high-purity gallium particles 17.8528 g treated in step (1) are placed in the first crucible of a magnetic tungsten electrode vacuum arc melting furnace, and a titanium block is placed in the second crucible. The magnetic tungsten electrode vacuum arc melting furnace is vacuumed to 10 -4 Pa by a mechanical pump and then high-purity argon (purity 99.999%) is filled as a protective gas. First, the titanium block in the second crucible is melted to absorb the residual oxygen in the melting furnace, and then the mixed raw material particles in the first crucible are melted. After melting, the intermediate ingot is obtained by casting. The intermediate ingot is flipped and repeatedly melted and cast to obtain an alloy ingot. The electromagnetic stirring is turned on during the melting process. The temperature for melting the mixed raw material particles is 1450℃, the casting temperature is 1300℃, and the cooling rate after casting is 50℃ / s. The number of repeated melting and casting is 5 times. The electromagnetic stirring rate during the melting process is 500 rpm.

[0040] Step (3), the alloy ingot is transferred to the crucible with copper mold suction casting by a turning spoon, and the alloy rod is obtained by copper mold suction casting. During the process of copper mold suction casting to obtain the alloy rod, the melting temperature of the alloy ingot is 1500℃, the casting temperature is 1350℃, the suction casting negative pressure is 500 Pa, and the cooling rate is 10℃ / s. The diameter of the alloy rod is 8 mm. The alloy rod is an as-prepared Ni 52 Mn 26 Ga 22 Heusler alloy.

[0041] Step (4), cutting the alloy rod with a diameter of 8 mm to obtain Φ3×5 mm alloy rods, polishing with 2000 grit sandpaper until the surface roughness is less than or equal to 0.025 μm, and then placing it in an ultrasonic cleaner for ultrasonic cleaning. After the ultrasonic cleaning is completed, washing with anhydrous ethanol and drying naturally;

[0042] Step (5), put the treated alloy rod into a high temperature resistant hollow ceramic tube (porcelain boat), plug both ends with titanium blocks, then put the porcelain boat with the alloy rod into a high temperature resistant quartz glass tube, and use a vacuum rotary tube sealing device to seal the quartz tube (10 -3 Pa); the sealed quartz glass tube is placed in a resistance furnace for staged heat treatment. After the staged heat treatment is completed, it is cooled to room temperature with the furnace to obtain heat-treated Ni 52 Mn 26 Ga 22 Heusle alloy.

[0043] The staged heat treatment method is: first, heat up to 775℃ at a heating rate of 7℃ / min and keep warm for 1.5h, then cool down to 750℃ at a cooling rate of 7℃ / min and keep warm for 2h, then cool down to 650℃ at a cooling rate of 7℃ / min and keep warm for 10h, and finally cool down to 500℃ at a cooling rate of 7℃ / min and keep warm for 20h. After the insulation is completed, cool to room temperature with the furnace.

[0044] The Ni prepared in this example 52 Mn 26 Ga 22 The structure of the Heusle alloy was characterized, and the prepared Ni 52 Mn 26 Ga 22 Heusle alloy and heat-treated Ni prepared in step (5) 52 Mn 26 Ga 22 Heusle alloys were subjected to magnetocaloric characterization.

[0045] Figure 1 Ni prepared in the embodiment of the present invention 52 Mn 26 Ga 22 Structural characterization of Heusle alloy, from Figure 1 It can be seen that the sample is austenite and martensite coexisting at room temperature.

[0046] Figure 2 and Figure 3 Heat treated Ni 52 Mn 26 Ga 22Heusle alloy and as-prepared Ni 52 Mn 26 Ga 22 Magnetocaloric characterization results of Heusle alloy in the magnetic field direction of "⊥". From Figure 2 and Figure 3 It can be seen from and that the amplitude of the magnetic entropy change of the alloy is greatly enhanced after heat treatment, indicating that the heat treatment process can optimize the microstructure of the alloy and improve its magnetic properties.

[0047] Figure 4 and Figure 5 are heat-treated Ni 52 Mn 26 Ga 22 Heusle alloy and as-prepared Ni 52 Mn 26 Ga 22 Magnetocaloric characterization results of Heusle alloy in the magnetic field direction of "∥". From Figure 4 and Figure 5 It can be seen from and that the amplitude of the magnetic entropy change of the alloy is greatly enhanced after heat treatment, indicating that the heat treatment process can optimize the microstructure of the alloy and improve its magnetic properties.

[0048] Table 1

[0049]

[0050] The Ni 52 Mn 26 Ga 22 Heusle alloy prepared by the present application is suitable for magnetic refrigeration, intelligent materials and other fields. Compared with traditional materials, this alloy has a higher magnetic entropy change under high magnetic field strength, not only improving the refrigeration efficiency, but also increasing the environmental friendliness, adjustability and thermal stability, and therefore has strong competitiveness.

[0051] Obviously, the above examples are only examples for clearly illustrating, but not limitation to the embodiments. For those skilled in the art, other different forms of changes or variations can be made on the basis of the above description. Here, it is not necessary and impossible to enumerate all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the patent application claims.

Claims

1. A method for increasing the magnetic entropy change of an as-prepared alloy by heat treatment, characterized in that: The steps include: Step (1), ultrasonically cleaning the high-purity nickel particles, high-purity manganese particles and high-purity gallium particles, washing them with anhydrous ethanol and drying them naturally; Step (2), mixing the treated high-purity nickel particles, high-purity manganese particles and high-purity gallium particles to obtain mixed raw material particles, placing them into an arc melting furnace and repeatedly melting them to obtain an alloy ingot; Step (3), casting the alloy ingot by copper mold suction casting to obtain alloy rod; Step (4), grinding, ultrasonic cleaning, washing with anhydrous ethanol and air-drying the alloy rod in sequence; Step (5), subjecting the treated alloy rod to stage-by-stage heat treatment in a vacuum environment, and obtaining a magnetic entropy change alloy after the stage-by-stage heat treatment is completed; In step (2), the molar ratio of the high-purity nickel particles, the high-purity manganese particles, and the high-purity gallium particles is (50-53): (22-26): (22-26); In step (3), during the copper mold suction casting process to obtain the alloy rod, the melting temperature of the alloy ingot is 1450-1500°C, the casting temperature is 1300-1350°C, the suction casting negative pressure is 100-500 Pa, and the cooling rate is 10-50°C / s; the diameter of the alloy rod is 8-10 mm; The staged heat treatment method is: first, heat up to 770~790℃ at a heating rate of 5~10℃ / min and keep warm for 1~1.5h, then cool down to 740~755℃ at a cooling rate of 5~10℃ / min and keep warm for 2~3h, then cool down to 630~650℃ at a cooling rate of 5~10℃ / min and keep warm for 10~12h, finally cool down to 500~550℃ at a cooling rate of 5~10℃ / min and keep warm for 20~24h. After the insulation is completed, cool to room temperature with the furnace.

2. The method for improving the magnetic entropy change of as-prepared alloy by heat treatment according to claim 1, characterized in that: In step (1), the particle size of the high-purity nickel particles is Φ3*3 mm, and the mass fraction of nickel in the high-purity nickel particles is greater than or equal to 99.995wt%; the particle size of the high-purity manganese particles is 1~10 mm, and the mass fraction of manganese in the high-purity manganese particles is greater than or equal to 99.9wt%; the particle size of the high-purity gallium particles is 2~5 mm, and the mass fraction of gallium in the high-purity gallium particles is greater than or equal to 99.999wt%.

3. The method for improving the magnetic entropy change of as-prepared alloy by heat treatment according to claim 1, characterized in that: In step (2), the mixed raw material particles are placed in the first crucible of a magnetron tungsten vacuum arc melting furnace, and the titanium block is placed in the second crucible; the magnetron tungsten vacuum arc melting furnace is evacuated and filled with high-purity argon as a protective gas; the titanium block in the second crucible is first melted, and then the mixed raw material particles in the first crucible are melted, and after melting, they are cast to obtain an intermediate ingot; the intermediate ingot is turned over and repeatedly melted and cast to obtain an alloy ingot; and electromagnetic stirring is turned on during the melting process.

4. The method for improving the magnetic entropy change of as-prepared alloy by heat treatment according to claim 3, characterized in that: In step (2), the arc melting furnace is evacuated to 10 -4 Pa, the purity of high-purity argon is greater than or equal to 99.99%; the melting temperature of the mixed raw material particles is 1450~1500℃, the casting temperature is 1300~1350℃, and the cooling rate after casting is 10~50℃ / s; the number of repeated melting and casting is 4~5 times; the electromagnetic stirring rate during the melting process is 300~600rpm.

5. The method for improving the magnetic entropy change of as-prepared alloy by heat treatment according to claim 1, characterized in that: In step (4), the sandpaper used for polishing is 2000 mesh, and the surface is polished to a surface roughness of less than or equal to 0.025 μm.

6. The method for improving the magnetic entropy change of as-prepared alloy by heat treatment according to claim 1, characterized in that: In step (5), the treated alloy rod is first placed in a porcelain boat, both ends of the porcelain boat are sealed with titanium blocks, and then the sealed porcelain boat is placed in a quartz glass tube and sealed with a vacuum rotary tube sealing device; the sealed quartz glass tube is placed in a resistance furnace for staged heat treatment; the vacuum degree in the quartz glass after sealing is 10 -3 Pa.

7. The method for improving the magnetic entropy change of as-prepared alloy by heat treatment according to claim 1, characterized in that: In step (1), the particle size of the high-purity nickel particles is Φ3*3mm, and the mass fraction of nickel in the high-purity nickel particles is equal to 99.995wt%; the particle size of the high-purity manganese particles is 1~10mm, and the mass fraction of manganese in the high-purity manganese particles is equal to 99.9wt%; the particle size of the high-purity gallium particles is 2~5mm, and the mass fraction of gallium in the high-purity gallium particles is equal to 99.999wt%; in step (2), the molar ratio of the high-purity nickel particles, the high-purity manganese particles and the high-purity gallium particles is 2:1:1; In step (2), the mixed raw material particles are placed in the first crucible of the magnetic tungsten vacuum arc melting furnace, and the titanium block is placed in the second crucible; the magnetic tungsten vacuum arc melting furnace is evacuated to 10 -4 Pa was followed by the addition of high-purity argon gas with a purity greater than or equal to 99.99% as a protective gas; the titanium block in the second crucible was first smelted, and then the mixed raw material particles in the first crucible were smelted at a smelting temperature of 1450°C. After smelting, the mixture was cast at 1300°C, and then cooled to room temperature at a cooling rate of 50°C / s to obtain an intermediate ingot; the intermediate ingot was flipped over and repeatedly smelted and cast five times to obtain an alloy ingot; electromagnetic stirring was turned on during the smelting process, and the electromagnetic stirring rate was 500 rpm; In step (3), during the copper mold suction casting process to obtain the alloy rod, the melting temperature of the alloy ingot is 1500°C, the casting temperature is 1350°C, the suction casting negative pressure is 500 Pa, and the cooling rate is 10°C / s; the diameter of the alloy rod is 8 mm; In step (4), the sandpaper used for polishing is 2000 mesh, and the surface roughness is polished to be less than or equal to 0.025 μm; In step (5), the treated alloy rod is first placed in a porcelain boat, both ends of the porcelain boat are sealed with titanium blocks, and then the sealed porcelain boat is placed in a quartz glass tube and sealed with a vacuum rotary tube sealing device; the sealed quartz glass tube is placed in a resistance furnace for staged heat treatment; the vacuum degree in the quartz glass after sealing is 10 -3 Pa; The staged heat treatment method is: first, heat up to 775℃ at a heating rate of 7℃ / min and keep warm for 1.5h, then cool down to 750℃ at a cooling rate of 7℃ / min and keep warm for 2h, then cool down to 650℃ at a cooling rate of 7℃ / min and keep warm for 10h, and finally cool down to 500℃ at a cooling rate of 7℃ / min and keep warm for 20h. After the insulation is completed, cool to room temperature with the furnace.

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