Method for improving corrosion resistance of sintered neodymium-iron-boron magnet and application of mixed alloy

Abstract

The invention discloses a method for improving the corrosion resistance of a sintered neodymium-iron-boron magnet and application of mixed alloy. The method comprises the following steps: 1) obtaining an alloy A casting sheet, an alloy B casting sheet and an alloy C casting sheet through vacuum rapid hardening casting sheets; (2) the alloy A casting piece, the alloy B casting piece and the alloy C casting piece are crushed to obtain alloy A coarse powder, alloy B coarse powder and alloy C coarse powder; 3) crushing the alloy A coarse powder, the alloy B coarse powder and the alloy C coarse powder to obtain alloy A fine powder, alloy B fine powder and alloy C fine powder; (4) the alloy A fine powder, the alloy B fine powder and the alloy C fine powder are mixed, and a pressed blank is obtained; (5) sintering the pressed blank and performing secondary heating to obtain a sintered neodymium-iron-boron magnet; and 6) performing a weightlessness experiment on the sintered neodymium-iron-boron The sintered neodymium-iron-boron magnet with the corrosion resistance improved through the method has good corrosion resistance.

Description

Technical Field

[0001] The invention relates to a method for improving the corrosion resistance of a sintered NdFeB magnet and application of a mixed alloy. Background Art

[0002] Sintered NdFeB magnets are widely used in motors, electronics, medical devices and other fields due to their excellent magnetic properties. In recent years, with the increasing scarcity of rare earth resources, the application of high-abundance rare earth elements (such as Ce, La, etc.) in sintered NdFeB magnets has attracted more and more attention. However, compared with traditional rare earth elements such as Pr and Nd, the addition of high-abundance rare earth elements often reduces the corrosion resistance of magnets. The main reasons are as follows: (1) The electrode potential of high-abundance rare earth elements (such as Ce and La) is quite different from that of elements such as Fe and B. They are prone to form micro-batteries in a humid environment, accelerating the electrochemical corrosion of magnets. (2) The addition of high-abundance rare earth elements will change the composition and structure of the grain boundary phase of the magnet, reduce the stability of the grain boundary phase, and make it more susceptible to corrosion by corrosive media. (3) High-abundance rare earth elements (such as Ce) are easily oxidized at high temperatures to form loose and porous oxides, which provide channels for the penetration of corrosive media and accelerate the corrosion of magnets.

[0003] CN104347216A discloses a neodymium iron boron magnetic material with a composite addition of lanthanide elements. The neodymium iron boron magnetic material is composed of the following components in mass percentage: Nd or PrNd: 0.1-32%, B: 0.9-1.5%, Dy: 0-10%, Co: 0.5-10%, Cu: 0.05-0.25%, lanthanide elements (La or Ce): 0.1-15%, trace elements M (one or more of Al, Zr, Nb, Mn, Ga, Ti, Sn): 0.1-4.5%, and the balance is Fe. The preparation method of the magnetic material includes smelting, powdering, molding and sintering. The neodymium iron boron magnetic material contains Co and Ti. The increase of Co and Ti will reduce the remanence of the magnet, affect the magnetic properties of the magnet and increase the cost. In addition, the preparation process of the NdFeB magnetic material makes it difficult to effectively control the composition of the main phase and grain boundary phase, which will lead to the enrichment of more La or Ce elements at the grain boundaries, easily reducing the corrosion resistance of the NdFeB magnetic material.

[0004] CN105427993A discloses a low-weight-loss NdFeB magnet and its preparation method. The low-weight-loss NdFeB magnet is primarily composed of the following components (by mass percentage): Nd: 27-32.5%, B: 1-1.2%, Co: 2-3%, alloying elements M1 (two or more of Al, Cu, Zn, and Ga): 0.15-2.75%, alloying elements M2 (one or more of Mo, Nb, and Zr): 0.05-1.1%, rare earth elements (excluding Nd): 0.7-1.3%, and the balance Fe. The low-weight-loss NdFeB magnet is prepared by the following method: raw materials are mixed according to the components and mass percentages of the NdFeB magnet, smelted, and cast into a spinning strip, followed by hydrogen crushing and airflow grinding to produce a powder, which is then pressed into shape, sintered at high temperature, and surface treated to obtain the low-weight-loss NdFeB magnet. NdFeB magnets contain Zr and Gd. Their production requires Zn plating and phosphating with a zinc-based phosphating solution, followed by an epoxy resin coating. This process is complex and expensive. The addition of Zr and Gd reduces the magnet's remanence, affecting its magnetic properties.

[0005] CN115360008A discloses a sintered NdFeB magnet with high corrosion resistance and high magnetic properties, and its preparation method. The sintered NdFeB magnet includes the element Co, which comprises a main phase and a grain boundary phase. The element Co is distributed in the grain boundary phase and the main phase of the grains. The sintered NdFeB magnet satisfies the following conditions: w1 ≥ w2, where w1 is the content of the element Co distributed in the grain boundary phase of the grains, and w2 is the content of the element Co distributed in the main phase of the grains. The sintered NdFeB magnet contains elements such as Ti and Zr, and the preparation of the sintered NdFeB magnet requires the use of rare earth elements for grain boundary diffusion, resulting in a complex process and high production costs. The addition of Ti and Zr reduces the magnet's remanence, affecting its magnetic properties. Summary of the Invention

[0006] In view of this, an object of the present invention is to provide a method for improving the corrosion resistance of sintered NdFeB magnets. After the corrosion resistance of the sintered NdFeB magnets is improved by this method, the weight loss of the sintered NdFeB magnets is at most 6.5 mg / cm after a 96-hour weight loss test under the conditions of 120°C, 100% relative humidity and 0.2 MPa absolute vapor pressure. 2 , having good corrosion resistance. Another object of the present invention is to provide a use of the mixed alloy. The present invention adopts the following technical solution to achieve the above object.

[0007] In one aspect, the present invention provides a method for improving the corrosion resistance of a sintered NdFeB magnet, comprising the following steps:

[0008] 1) providing raw materials according to the chemical compositions of alloy A, alloy B, and alloy C, respectively; then, subjecting the raw materials of alloy A, alloy B, and alloy C to vacuum rapid solidification casting to obtain alloy A castings, alloy B castings, and alloy C castings;

[0009] The alloy A is composed of the following components by weight percentage: 28-40 wt% of Pr-Nd alloy, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities;

[0010] The alloy B is composed of the following components by weight percentage: 2-15 wt% of Pr-Nd alloy, 16-32 wt% of Ce, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.01-1 wt% of Ga, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities;

[0011] The alloy C is composed of the following components by weight percentage: 30-45 wt% Pr, 6-20 wt% Ni, 2-16 wt% Nb, 0.5-1.5 wt% B, and the balance Fe and unavoidable impurities;

[0012] 2) hydrogen crushing the alloy A flakes, alloy B flakes, and alloy C flakes obtained in step 1) to obtain alloy A coarse powder, alloy B coarse powder, and alloy C coarse powder, respectively;

[0013] 3) The alloy A coarse powder obtained in step 1) is mixed with an antioxidant and a lubricant and then crushed by jet mill to obtain an average particle size D 50 The alloy A fine powder is 0.2 to 12 μm; the alloy B coarse powder is mixed with an antioxidant and a lubricant and then crushed by a jet mill to obtain an average particle size D 50 The alloy B fine powder is 0.2-12 μm, the alloy C coarse powder is mixed with antioxidant and lubricant and then crushed by jet mill to obtain the average particle size D 50 Alloy C fine powder of 2 to 250 nm;

[0014] 4) mixing the Alloy A fine powder, Alloy B fine powder, and Alloy C fine powder obtained in step 3) to obtain a mixed powder, and then orienting and isostatically pressing the mixed powder in a magnetic field to obtain a green compact; wherein the mass ratio of the Alloy A fine powder to the Alloy B fine powder is 20 to 120:100, and the ratio of the mass of the Alloy C fine powder to the sum of the masses of the Alloy A fine powder and the Alloy B fine powder is 0.8 to 2:100;

[0015] 5) sintering the green compact obtained in step 4) at 960-1180° C. under vacuum conditions to obtain a sintered magnet A; heating the sintered magnet A at 880-1080° C. for the first time to obtain a sintered magnet B; and heating the sintered magnet B at 500-800° C. for the second time to obtain a sintered NdFeB magnet;

[0016] 6) The sintered NdFeB magnet obtained in step 5) was cut into samples with a size of 10-15 mm × 10-15 mm × 6-10 mm, and the samples were subjected to a weight loss experiment for 96 hours at 120° C., a relative humidity of 100% and an absolute vapor pressure of 0.2 MPa.

[0017] According to the preparation method of the present invention, preferably, in step 1), the vacuum degree of the vacuum rapid solidification casting sheet is 0.1-10 Pa;

[0018] The thickness of the alloy A casting sheet is 0.2-1.2 mm, the thickness of the alloy B casting sheet is 0.2-1.2 mm, and the thickness of the alloy C casting sheet is 0.2-1.2 mm.

[0019] According to the preparation method of the present invention, preferably, in step 3), the antioxidant is selected from at least one of polyethylene glycol octane, petroleum ether, isozincohol propionate, tributyl borate, and 2,6-di-tert-butyl-p-cresol.

[0020] According to the preparation method of the present invention, preferably, in step 3), for any one of the alloy coarse powders of Alloy A coarse powder, Alloy B coarse powder and Alloy C coarse powder, the mass of the corresponding antioxidant used is 0.2 to 12 wt‰ of the mass of the alloy coarse powder.

[0021] According to the preparation method of the present invention, preferably, the lubricant is selected from at least one of aviation kerosene, isopropyl alcohol, zinc stearate, isooctyl oleate, and triethanolamine borate.

[0022] According to the preparation method of the present invention, preferably, in step 3), for any one of the alloy coarse powders of Alloy A coarse powder, Alloy B coarse powder and Alloy C coarse powder, the mass of the corresponding lubricant used is 0.2 to 12 wt‰ of the mass of the alloy coarse powder.

[0023] According to the preparation method of the present invention, preferably, in step 4), the intensity of the magnetic field is at least 1 T, the oxygen content during orientation molding is 1 to 80 ppm; and the isostatic pressing treatment is cold isostatic pressing treatment.

[0024] According to the preparation method of the present invention, preferably, in step 5), the vacuum degree of the vacuum condition is 0.1×10 -2 ~1×10 -2Pa, the sintering time is 0.5 to 6 hours, and the sintering oxygen content is 0.1 to 50 ppm.

[0025] According to the preparation method of the present invention, preferably, in step 5), the vacuum degree of the first heating is 0.1×10 -3 ~1×10 -3 Pa, the first heating time is 0.5 to 4 hours, and the oxygen content of the first heating is 0.1 to 50 ppm;

[0026] The vacuum degree of the second heating is 0.1×10 -3 ~1×10 -3 Pa, the second heating time is 0.5 to 4 hours, and the oxygen content of the second heating is 0.1 to 50 ppm.

[0027] On the other hand, the present invention also provides a use of a mixed alloy in improving the corrosion resistance of a sintered NdFeB magnet, preferably, the mixed alloy consists of alloy A, alloy B and alloy C;

[0028] The alloy A is composed of the following components by weight percentage: 28-40 wt% of Pr-Nd alloy, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities;

[0029] The alloy B is composed of the following components by weight percentage: 2-15 wt% of Pr-Nd alloy, 16-32 wt% of Ce, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.01-1 wt% of Ga, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities;

[0030] The alloy C is composed of the following components by weight percentage: 30-45 wt% Pr, 6-20 wt% Ni, 2-16 wt% Nb, 0.5-1.5 wt% B, and the balance Fe and unavoidable impurities;

[0031] The corrosion-resistant sintered NdFeB magnets with a size of 10-15mm x 10-15mm x 6-10mm were subjected to a weight loss test for 96 hours at 120°C, 100% relative humidity and 0.2MPa absolute vapor pressure. The weight loss was no more than 6.5mg / cm 2 .

[0032] The present invention significantly improves the corrosion resistance of Ce-containing sintered NdFeB magnets by adding Ni and Nb in a specific ratio and combining this with specific process conditions. The sintered NdFeB magnets, which have been improved in corrosion resistance by the present invention, experienced a weight loss of at most 6.5 mg / cm3 after a 96-hour weight loss test at 120°C, 100% relative humidity, and 0.2 MPa absolute vapor pressure. 2 . DETAILED DESCRIPTION

[0033] The present invention will be further described below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

[0034] The “weightlessness” mentioned in the present invention refers to the mass loss per unit surface area of the magnetic orientation surface caused by corrosion when the magnet is exposed to high temperature and high pressure water vapor environment, and the unit is milligrams per square centimeter (mg / cm 2 ).

[0035] The "vacuum degree" mentioned in the present invention refers to absolute vacuum degree; the smaller the value, the higher the vacuum degree.

[0036] The average particle size D 50 ” represents the equivalent diameter of the largest particle when the cumulative distribution in the particle size distribution curve is 50%.

[0037] <Method for Improving Corrosion Resistance of Sintered NdFeB Magnets>

[0038] The method for improving the corrosion resistance of sintered NdFeB magnets includes vacuum rapid solidification casting, preparing coarse alloy powder, preparing fine alloy powder, compacting, sintering and tempering, and weight loss testing, which is described in detail below.

[0039] Vacuum rapid solidification casting steps

[0040] Raw materials are provided according to the chemical composition of alloy A, alloy B and alloy C respectively. Then, the raw materials of alloy A, alloy B and alloy C are respectively cast by vacuum rapid solidification to obtain alloy A casting sheet, alloy B casting sheet and alloy C casting sheet.

[0041] According to one embodiment of the present invention, alloy A may be composed of the following components, by weight percentage: 28-40 wt% of Pr-Nd alloy, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities.

[0042] In Alloy A of the present invention, the amount of Pr-Nd alloy may be 28-40 wt%, preferably 29-35 wt%, and more preferably 30-32 wt%. The amount of Al may be 0.05-0.8 wt%, preferably 0.1-0.5 wt%, and more preferably 0.15-0.3 wt%. The amount of Cu may be 0.05-0.8 wt%, preferably 0.1-0.6 wt%, and more preferably 0.2-0.5 wt%. The amount of B may be 0.5-1.5 wt%, preferably 0.6-1.3 wt%, and more preferably 0.8-1.2 wt%.

[0043] According to one embodiment of the present invention, alloy B may be composed of the following components, by weight percentage: 2-15 wt% of Pr-Nd alloy, 16-32 wt% of Ce, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.01-1 wt% of Ga, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities.

[0044] In the alloy B of the present invention, the amount of Pr-Nd alloy may be 2-15 wt%, preferably 3-13 wt%, and more preferably 5-12 wt%. The amount of Ce may be 16-32 wt%, preferably 18-30 wt%, and more preferably 20-25 wt%. The amount of Al may be 0.05-0.8 wt%, preferably 0.08-0.6 wt%, and more preferably 0.1-0.5 wt%. The amount of Cu may be 0.05-0.8 wt%, preferably 0.08-0.5 wt%, and more preferably 0.1-0.3 wt%. The amount of Ga may be 0.01-1 wt%, preferably 0.05-0.8 wt%, and more preferably 0.1-0.5 wt%. The amount of B may be 0.5-1.5 wt%, preferably 0.6-1.3 wt%, and more preferably 0.8-1.2 wt%.

[0045] According to one embodiment of the present invention, in the Pr-Nd alloy, the mass ratio of Pr to Nd may be 1:1-8, preferably 1:1.5-7, and more preferably 1:2-6.

[0046] According to one embodiment of the present invention, the alloy C may be composed of the following components by weight: 30-45 wt% Pr, 6-20 wt% Ni, 2-16 wt% Nb, 0.5-1.5 wt% B, and the balance being Fe and unavoidable impurities.

[0047] In the alloy C of the present invention, the amount of Pr can be 30-45 wt%, preferably 32-42 wt%, and more preferably 35-40 wt%. The amount of Ni can be 6-20 wt%, preferably 8-15 wt%, and more preferably 9-12 wt%. The amount of Nb can be 2-16 wt%, preferably 3-15 wt%, and more preferably 4-12 wt%. The amount of B can be 0.5-1.5 wt%, preferably 0.8-1.4 wt%, and more preferably 1-1.3 wt%.

[0048] In the present invention, the Ni and Nb elements act directly on the grain boundary regions of the magnet and are not present in the matrix phase, substantially not affecting the remanence and magnetic energy product of the magnet. Furthermore, the addition of the Ni element increases the corrosion potential of the grain boundary phase, contributing to improved corrosion resistance of the magnet. The Nb element forms a Nb alloy dispersed phase, which can inhibit the expansion of grain boundary corrosion, thereby improving the corrosion resistance of the magnet. However, if the amount of Ni and Nb added in the present invention is too low, the above-mentioned effects cannot be achieved; if the amount added is too high, the cost is increased and the stability of the magnet is not conducive.

[0049] The element ratio of alloy A, alloy B and alloy C of the present invention is limited to the above range and matched with a reasonable mass ratio of alloy A, alloy B and alloy C, which can ensure that the elements of the sintered NdFeB magnet achieve optimal synergy, which is beneficial to reducing the weight loss of the sintered NdFeB magnet under high temperature, high pressure and high humidity conditions and improving the corrosion resistance of the sintered NdFeB magnet.

[0050] According to one embodiment of the present invention, the raw materials of alloy A, alloy B and alloy C can be placed in a furnace respectively, refined under vacuum conditions and protective gas to obtain alloy liquid; the alloy liquid is cast onto a rotating cooling copper roller to obtain alloy flakes.

[0051] According to one embodiment of the present invention, the vacuum degree of the vacuum condition may be 0.1-10 Pa, preferably 0.2-5 Pa, and more preferably 0.5-3 Pa.

[0052] The protective gas involved in the present invention can be selected from at least one of nitrogen and inert gases, and the inert gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe); preferably, the protective gas is at least one of nitrogen, helium, neon, and argon; more preferably, the protective gas is at least one of nitrogen, helium, and argon.

[0053] According to one embodiment of the present invention, the refining temperature may be 1200-1600° C., preferably 1250-1550° C., and more preferably 1300-1500° C. The refining time may be 10-30 min, preferably 12-28 min, and more preferably 13-26 min.

[0054] According to one embodiment of the present invention, the casting temperature may be 1150-1550° C., preferably 1200-1500° C., more preferably 1250-1450° C. The linear speed of the cooling copper roller may be 2-10 m / s, preferably 3-9 m / s, and more preferably 4-8 m / s.

[0055] According to one embodiment of the present invention, the thickness of the alloy A casting sheet may be 0.2 to 1.2 mm, preferably 0.25 to 1.0 mm, and more preferably 0.3 to 0.8 mm. The thickness of the alloy B casting sheet may be 0.1 to 1.2 mm, preferably 0.25 to 1.0 mm, and more preferably 0.3 to 0.8 mm. The thickness of the alloy C casting sheet may be 0.2 to 1.2 mm, preferably 0.25 to 1.0 mm, and more preferably 0.3 to 0.8 mm.

[0056] Reasonable vacuum rapid solidification casting conditions can make the obtained alloy castings more suitable for preparing alloy coarse powder.

[0057] In the present invention, the raw materials of each alloy can be pure elements or intermediate alloys containing the elements, without any particular limitation. The purity of the pure elements or intermediate alloys is at least industrial purity (99.9 wt%).

[0058] Preparation steps of alloy coarse powder

[0059] Alloy A flakes, Alloy B flakes and Alloy C flakes were subjected to hydrogen crushing to obtain Alloy A coarse powder, Alloy B coarse powder and Alloy C coarse powder respectively.

[0060] The hydrogen crushing in the present invention can be achieved using any hydrogen crushing method or hydrogen crushing device known in the art.

[0061] According to one embodiment of the present invention, the hydrogen pressure of hydrogen crushing may be 0.01 to 0.9 MPa; preferably 0.1 to 0.6 MPa; more preferably 0.2 to 0.4 MPa.

[0062] Reasonable hydrogen crushing conditions can make the obtained alloy coarse powder more suitable for preparing alloy fine powder.

[0063] Steps for preparing alloy fine powder

[0064] Alloy A coarse powder, Alloy B coarse powder and Alloy C coarse powder are respectively mixed with an antioxidant and a lubricant and then crushed by air jet mill to obtain Alloy A fine powder, Alloy B fine powder and Alloy C fine powder.

[0065] According to one embodiment of the present invention, the antioxidant can be selected from at least one of polyethylene glycol octane, petroleum ether, isozinc propionate, tributyl borate, and 2,6-di-tert-butyl-p-cresol, preferably at least one of polyethylene glycol octane, petroleum ether, and isozinc propionate, more preferably polyethylene glycol octane or petroleum ether.

[0066] In the present invention, for any one of Alloy A coarse powder, Alloy B coarse powder and Alloy C coarse powder, the mass of the corresponding antioxidant used can be 0.2 to 12 wt‰ of the mass of the alloy coarse powder, preferably 0.5 to 8 wt‰, and more preferably 0.8 to 6 wt‰.

[0067] Reasonable antioxidant ratio is beneficial to prevent the alloy fine powder from being oxidized during mixing and sintering, and reduce the impurity element content of sintered NdFeB magnets.

[0068] According to one embodiment of the present invention, the lubricant can be selected from at least one of aviation kerosene, isopropyl alcohol, zinc stearate, isooctyl oleate, and triethanolamine borate, preferably at least one of aviation kerosene, isopropyl alcohol, and zinc stearate, more preferably isopropyl alcohol or zinc stearate.

[0069] In the present invention, for any of the alloy coarse powders of Alloy A, Alloy B, and Alloy C, the mass of the lubricant used can be 0.2 to 12 wt‰ of the mass of the alloy coarse powder, preferably 0.5 to 8 wt‰, and more preferably 0.8 to 6 wt‰. A reasonable lubricant ratio helps prevent powder particle agglomeration, improves the orientation and magnetic properties of the sintered NdFeB magnet, and helps improve the corrosion resistance of the sintered NdFeB magnet.

[0070] In the present invention, mixing can be achieved in any device known in the art for stirring and mixing, and is not particularly limited herein, for example, a spiral mixer, a vertical mill, and the like.

[0071] According to one embodiment of the present invention, the mixing time may be 0.5 to 5 hours, preferably 1 to 5 hours, and more preferably 1 to 3 hours.

[0072] In the present invention, the jet mill crushing can be achieved by using any type of jet mill known in the art. The jet mill crushing is to use airflow to accelerate the mixed coarse powder and then cause the mixed powder to collide with each other and crush.

[0073] According to one embodiment of the present invention, the airflow for jet milling can be selected from at least one of nitrogen flow, neon flow, and argon flow; preferably at least one of nitrogen flow and argon flow; more preferably nitrogen flow.

[0074] According to one embodiment of the present invention, the air flow pressure of the jet mill can be 0.1-1 MPa; preferably 0.2-0.9 MPa; more preferably 0.3-0.8 MPa.

[0075] According to one embodiment of the present invention, the average particle size D of the alloy A fine powder is 50 The average particle size D of the alloy B fine powder can be 0.2 to 12 μm, preferably 0.5 to 10 μm, and more preferably 2 to 8 μm. 50 The average particle size D of the alloy C fine powder can be 0.2 to 12 μm, preferably 0.5 to 10 μm, and more preferably 2 to 8 μm. 50 The diameter may be 2 to 250 nm, preferably 50 to 230 nm, and more preferably 100 to 220 nm.

[0076] A reasonable particle size range of alloy fine powder is beneficial to improving the fluidity of powder during mixing, improving the lubrication effect during mixing, and improving the uniformity of raw materials during compaction. It is also conducive to achieving the best synergy between the elements, thereby improving the corrosion resistance of sintered NdFeB magnets.

[0077] Green pressing steps

[0078] Alloy A fine powder, Alloy B fine powder and Alloy C fine powder are mixed to obtain a mixed powder, and then the mixed powder is subjected to orientation molding and isostatic pressing in a magnetic field to obtain a green compact.

[0079] According to one embodiment of the present invention, the mass ratio of Alloy A fine powder to Alloy B fine powder may be 20 to 120:100, preferably 20 to 100:100, and more preferably 50 to 100:100. The ratio of the mass of Alloy C fine powder to the sum of the masses of Alloy A fine powder and Alloy B fine powder may be 0.8 to 2:100, preferably 0.8 to 1.5:100, and more preferably 0.9 to 1.5:100.

[0080] In the present invention, mixing can be achieved in any device known in the art for stirring and mixing, and is not particularly limited herein, for example, a spiral mixer, a vertical mill, and the like.

[0081] The present invention divides sintered NdFeB magnets into three groups, which are cast into sheets and powdered separately, and then mixed and pressed into billets. This treatment can better refine the grains, optimize the grain boundary phase and improve the uniformity of the composition, which is beneficial to reducing the irreversible magnetic flux loss and weight loss of the sintered NdFeB magnets under heating, and improving the high-temperature stability and corrosion resistance of the sintered NdFeB magnets.

[0082] According to one embodiment of the present invention, the mixing time may be 0.5 to 5 hours, preferably 1 to 5 hours, and more preferably 1 to 3 hours.

[0083] According to one embodiment of the present invention, during orientation molding, the magnetic field orientation and the pressing direction of the mixed powder are parallel or perpendicular to each other. The intensity of the magnetic field can be at least 1T, preferably 1-3T, and more preferably 1.2-2T.

[0084] In the present invention, in order to prevent the mixed powder from being excessively oxidized, the oxygen content during orientation molding may be 1 to 80 ppm, preferably 5 to 70 ppm, and more preferably 10 to 60 ppm.

[0085] According to one embodiment of the present invention, the isostatic pressing treatment is preferably cold isostatic pressing treatment. The pressure of the cold isostatic pressing can be 100-500 MPa, preferably 120-450 MPa, and more preferably 150-400 MPa.

[0086] Reasonable pressing conditions are beneficial to reducing the weight loss of sintered NdFeB magnets under high temperature, high pressure and high humidity conditions and improving the corrosion resistance of sintered NdFeB magnets.

[0087] Sintering and tempering steps

[0088] The compact is sintered at 960-1180° C. under vacuum conditions to obtain a sintered magnet A. The sintered magnet A is heated at 880-1080° C. for the first time to obtain a sintered magnet B. The sintered magnet B is heated at 500-800° C. for the second time to obtain a sintered NdFeB magnet.

[0089] According to one embodiment of the present invention, the vacuum degree of the vacuum condition can be 0.1×10 -2 ~2×10 -2 Pa, preferably 0.2×10 -2 ~1.8×10 -2 Pa, more preferably 0.5×10 -2 ~1.5×10 -2 Pa.

[0090] According to one embodiment of the present invention, the sintering temperature may be 960-1180° C., preferably 980-1150° C., more preferably 1000-1100° C. The sintering time may be 0.5-6 h, preferably 1-5 h, more preferably 2-4 h.

[0091] According to one embodiment of the present invention, in order to prevent the green compact from being excessively oxidized during sintering, the oxygen content during sintering may be 0.1 to 50 ppm, preferably 0.5 to 25 ppm, and more preferably 1 to 10 ppm.

[0092] According to one embodiment of the present invention, the vacuum degree of the first heating can be 0.1×10 -2 ~2×10 -2 Pa, preferably 0.2×10-2 ~1.8×10 -2 Pa, more preferably 0.5×10 -2 ~1.5×10 -2 Pa.

[0093] According to one embodiment of the present invention, the temperature of the first heating may be 880-1080° C., preferably 890-1050° C., more preferably 900-1000° C. The time of the first heating may be 0.5-4 h, preferably 1-4 h, more preferably 1-3 h.

[0094] According to one embodiment of the present invention, in order to prevent the sintered magnet A from being excessively oxidized during heating, the oxygen content during the first heating may be 0.1 to 50 ppm, preferably 0.5 to 25 ppm, and more preferably 1 to 10 ppm.

[0095] According to one embodiment of the present invention, the vacuum degree of the second heating can be 0.1×10 -2 ~2×10 -2 Pa, preferably 0.2×10 -2 ~1.8×10 -2 Pa, more preferably 0.5×10 -2 ~1.5×10 -2 Pa.

[0096] According to one embodiment of the present invention, the temperature of the second heating may be 500-800° C., preferably 550-750° C., more preferably 600-700° C. The second heating time is 0.5-4 h, preferably 1-4 h, more preferably 1-3 h.

[0097] According to one embodiment of the present invention, in order to prevent the sintered magnet B from being over-oxidized during heating, the oxygen content during the second heating may be 0.1 to 50 ppm, preferably 0.5 to 25 ppm, and more preferably 1 to 10 ppm.

[0098] The sintering and double heating of the present invention can be achieved in any sintering equipment known in the art, such as, but not limited to, a vacuum sintering furnace.

[0099] Controlling the sintering and double heating conditions within the above ranges is beneficial to reducing the weight loss of the sintered NdFeB magnet under high temperature, high pressure and high humidity conditions and improving the corrosion resistance of the sintered NdFeB magnet.

[0100] Weightlessness experiment steps

[0101] The sintered NdFeB magnets were cut into samples with a size of 10-15 mm×10-15 mm×6-10 mm, and the samples were subjected to a weight loss experiment for 96 h at 120° C., a relative humidity of 100%, and an absolute vapor pressure of 0.2 MPa.

[0102] According to one embodiment of the present invention, the size of the cut sintered NdFeB magnet sample may be 10-15mm×10-15mm×6-10mm, preferably 11-15mm×11-15mm×7-10mm, and more preferably 11-13mm×11-13mm×7-9mm.

[0103] According to one embodiment of the present invention, the weight loss of the sample after the experiment can be at most 6.5 mg / cm 2 , preferably at most 6.45 mg / cm 2 , more preferably at most 6.4 mg / cm 2 .

[0104] <Purpose>

[0105] The present invention also provides a use of the mixed alloy in improving the corrosion resistance of sintered NdFeB magnets.

[0106] In the present invention, the mixed alloy consists of alloy A, alloy B and alloy C.

[0107] According to one embodiment of the present invention, alloy A may be composed of the following components, by weight percentage: 28-40 wt% of Pr-Nd alloy, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities.

[0108] In Alloy A of the present invention, the amount of Pr-Nd alloy may be 28-40 wt%, preferably 29-35 wt%, and more preferably 30-32 wt%. The amount of Al may be 0.05-0.8 wt%, preferably 0.1-0.5 wt%, and more preferably 0.15-0.3 wt%. The amount of Cu may be 0.05-0.8 wt%, preferably 0.1-0.6 wt%, and more preferably 0.2-0.5 wt%. The amount of B may be 0.5-1.5 wt%, preferably 0.6-1.3 wt%, and more preferably 0.8-1.2 wt%.

[0109] According to one embodiment of the present invention, alloy B may be composed of the following components, by weight percentage: 2-15 wt% of Pr-Nd alloy, 16-32 wt% of Ce, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.01-1 wt% of Ga, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities.

[0110] In the alloy B of the present invention, the amount of Pr-Nd alloy may be 2-15 wt%, preferably 3-13 wt%, and more preferably 5-12 wt%. The amount of Ce may be 16-32 wt%, preferably 18-30 wt%, and more preferably 20-25 wt%. The amount of Al may be 0.05-0.8 wt%, preferably 0.08-0.6 wt%, and more preferably 0.1-0.5 wt%. The amount of Cu may be 0.05-0.8 wt%, preferably 0.08-0.5 wt%, and more preferably 0.1-0.3 wt%. The amount of Ga may be 0.01-1 wt%, preferably 0.05-0.8 wt%, and more preferably 0.1-0.5 wt%. The amount of B may be 0.5-1.5 wt%, preferably 0.6-1.3 wt%, and more preferably 0.8-1.2 wt%.

[0111] According to one embodiment of the present invention, the alloy C may be composed of the following components by weight: 30-45 wt% Pr, 6-20 wt% Ni, 2-16 wt% Nb, 0.5-1.5 wt% B, and the balance being Fe and unavoidable impurities.

[0112] In the alloy C of the present invention, the amount of Pr can be 30-45 wt%, preferably 32-42 wt%, and more preferably 35-40 wt%. The amount of Ni can be 6-20 wt%, preferably 8-15 wt%, and more preferably 9-12 wt%. The amount of Nb can be 2-16 wt%, preferably 3-15 wt%, and more preferably 4-12 wt%. The amount of B can be 0.5-1.5 wt%, preferably 0.8-1.4 wt%, and more preferably 1-1.3 wt%.

[0113] According to one embodiment of the present invention, the sintered NdFeB magnet after the mixed alloy to improve the corrosion resistance is cut into samples with a size of 10-15 mm × 10-15 mm × 6-10 mm. The sample is subjected to a weight loss test for 96 hours at 120°C, a relative humidity of 100% and an absolute vapor pressure of 0.2 MPa. The weight loss can be at most 6.5 mg / cm 2 , preferably at most 6.45 mg / cm 2 , more preferably at most 6.4 mg / cm 2 .

[0114] <Test method>

[0115] The mass of the sintered NdFeB magnets before and after the experiment was measured using an analytical balance with an accuracy of 0.1 mg at room temperature (25°C), and the weight loss was calculated.

[0116] <Ingredients>

[0117] Unless otherwise specified, the raw materials in the following examples are all commercially available products.

[0118] The high temperature and high pressure steam test device is the EHS-212(M) high pressure accelerated aging test chamber.

[0119] Example 1

[0120] The raw materials are provided according to the chemical compositions of alloy A, alloy B and alloy C as follows:

[0121] Chemical composition of alloy A: Pr-Nd alloy (mass ratio of Pr to Nd is 1:3): 30.5wt%, Al: 0.5wt%, Cu: 0.1wt%, B: 1.0wt%, Fe and unavoidable impurities: 67.9wt%;

[0122] Chemical composition of alloy B: Pr-Nd alloy: 10wt%, Ce: 20.5wt%, Al: 0.1wt%, Cu: 0.1wt%, Ga: 0.5wt%, B: 0.9wt%, Fe and unavoidable impurities: 67.9wt%;

[0123] Chemical composition of alloy C: Pr: 35 wt %, Ni: 10 wt %, Nb: 5 wt %, B: 1.2 wt %, Fe and unavoidable impurities: 48.8 wt %.

[0124] The prepared Alloy A raw materials were placed in a vacuum rapid solidification casting furnace. After evacuation to a vacuum degree of 1 Pa, argon gas was filled in for protection and refined at 1450°C for 15 minutes to obtain liquid Alloy A. The Alloy A liquid was then poured at 1420°C onto a cooling copper roller rotating at a linear speed of 6 m / s to obtain Alloy A sheets with a thickness of 0.3 mm.

[0125] The prepared Alloy B raw materials were placed in a vacuum rapid solidification casting furnace. After evacuation to a vacuum degree of 1 Pa, argon gas was filled in for protection and refined at 1400°C for 15 minutes to obtain liquid Alloy B. The liquid Alloy B was then poured at 1350°C onto a cooling copper roller rotating at a linear speed of 6 m / s to obtain Alloy B sheets with a thickness of 0.3 mm.

[0126] The prepared Alloy C raw materials were placed in a vacuum rapid solidification casting furnace, evacuated to a vacuum degree of 1 Pa, and then filled with argon protection. Refined at 1430°C for 20 minutes to obtain liquid Alloy C. The Alloy C liquid was then poured at 1400°C onto a cooling copper roller rotating at a linear speed of 6m / s to obtain Alloy C sheets with a thickness of 0.3mm.

[0127] The alloy A flakes, alloy B flakes and alloy C flakes were hydrogen crushed at 0.25 MPa to obtain alloy A coarse powder, alloy B coarse powder and alloy C coarse powder, respectively.

[0128] The alloy A coarse powder, polyethylene glycol octane accounting for 1 wt‰ of the mass fraction, and isopropanol accounting for 1 wt‰ of the mass fraction were placed in a spiral mixer and mixed for 1 hour to obtain a mixture A. The mixture A was placed in a jet mill and crushed under nitrogen flow at 0.7 MPa to obtain an average particle size D 50 The alloy A fine powder is 3.5 μm. The alloy B coarse powder, polyethylene glycol octane with a mass fraction of 1 wt‰ and isopropanol with a mass fraction of 1 wt‰ are placed in a spiral mixer and mixed for 1 hour to obtain a mixture B. The mixture B is placed in a jet mill and crushed by nitrogen flow at 0.7 MPa to obtain an average particle size of D 50 The alloy B fine powder is 3.5 μm. The alloy C coarse powder, polyethylene glycol octane with a mass fraction of 1 wt‰ and isopropanol with a mass fraction of 1 wt‰ are placed in a spiral mixer and mixed for 1 hour to obtain a mixture C. The mixture C is placed in a jet mill and crushed by nitrogen flow at 0.7 MPa to obtain an average particle size of D 50 The alloy C fine powder is 200nm.

[0129] Alloy A, Alloy B, and Alloy C fine powders were mixed in a screw mixer at a mass ratio of 50:50:1 for 1.5 hours to obtain a mixed powder. The mixed powder was then subjected to orientation molding in a 2T magnetic field and an oxygen content of 50 ppm. Subsequently, it was cold isostatically pressed at 200 MPa to obtain a green compact.

[0130] At a vacuum degree of 1×10 -2 Pa, the green compact was sintered at 1040 ° C for 3 h to obtain sintered magnet A. -2 Pa and oxygen content of 5ppm, sintered magnet A was heated at 910℃ for 1.5h to obtain sintered magnet B. -2 Pa and oxygen content of 5ppm, the sintered magnet B was heated at 650℃ for 2h to obtain a sintered NdFeB magnet.

[0131] Comparative Example 1

[0132] Except that the alloy A fine powder, alloy B fine powder and alloy C fine powder were mixed in a mass ratio of 50:50:0.5, the rest was the same as in Example 1.

[0133] Comparative Example 2

[0134] The raw materials are provided according to the chemical composition of alloy A and alloy B as follows:

[0135] Chemical composition of alloy A: Pr-Nd alloy: 30.5wt%, Al: 0.5wt%, Cu: 0.1wt%, B: 1.0wt%, Fe and unavoidable impurities: 67.9wt%;

[0136] Chemical composition of alloy B: Pr—Nd alloy: 10 wt %, Ce: 20.5 wt %, Al: 0.1 wt %, Cu: 0.1 wt %, Ga: 0.5 wt %, B: 0.9 wt %, Fe and unavoidable impurities: 67.9 wt %.

[0137] The prepared Alloy A raw material was placed in a vacuum rapid solidification casting furnace. After evacuation to a vacuum degree of 1 Pa, argon gas was filled in for protection and refined at 1450°C for 15 h to obtain liquid Alloy A. The Alloy A liquid was then poured at 1420°C onto a cooling copper roller rotating at a linear speed of 6 m / s to obtain Alloy A sheets with a thickness of 0.3 mm.

[0138] The prepared Alloy B raw materials were placed in a vacuum rapid solidification casting furnace. After evacuation to a vacuum degree of 1 Pa, argon gas was filled in for protection and refined at 1400°C for 15 minutes to obtain liquid Alloy B. The liquid Alloy B was then poured at 1350°C onto a cooling copper roller rotating at a linear speed of 6 m / s to obtain Alloy B sheets with a thickness of 0.3 mm.

[0139] Alloy A and Alloy B were hydrogen crushed at 0.25 MPa to obtain Alloy A and Alloy B coarse powders, respectively.

[0140] The alloy A coarse powder, polyethylene glycol octane accounting for 1 wt‰ of the mass fraction, and isopropanol accounting for 1 wt‰ of the mass fraction were placed in a spiral mixer and mixed for 1 hour to obtain a mixture A. The mixture A was placed in a jet mill and crushed under nitrogen flow at 0.7 MPa to obtain an average particle size D 50 The alloy A fine powder is 3.5 μm. The alloy B coarse powder, polyethylene glycol octane with a mass fraction of 1 wt‰ and isopropanol with a mass fraction of 1 wt‰ are placed in a spiral mixer and mixed for 1 hour to obtain a mixture B. The mixture B is placed in a jet mill and crushed by nitrogen flow at 0.7 MPa to obtain an average particle size of D 50 The alloy B fine powder is 3.5 μm.

[0141] Alloy A and Alloy B fine powders were mixed in a screw mixer at a mass ratio of 50:50 for 1.5 hours to obtain a mixed powder. The mixed powder was then subjected to orientation molding in a 2T magnetic field with an oxygen content of 50 ppm, followed by cold isostatic pressing at 200 MPa to obtain a green compact.

[0142] At a vacuum degree of 1×10 -2 Pa, the green compact was sintered at 1040 ° C for 3 h to obtain sintered magnet A. -2 Pa and oxygen content of 5ppm, sintered magnet A was heated at 910℃ for 1.5h to obtain sintered magnet B. -2 Pa and oxygen content of 5ppm, the sintered magnet B was heated at 650℃ for 2h to obtain a sintered NdFeB magnet.

[0143] Experimental Example 1

[0144] The sintered NdFeB magnets prepared in Example 1, Comparative Example 1, and Comparative Example 2 were tested using the following methods:

[0145] The sintered NdFeB magnets were cut into 12 mm × 12 mm × 8 mm rectangular blocks using a wire-cut electric discharge machine to serve as test samples. The test samples were placed in a high-temperature, high-pressure steam test apparatus. Following GB / T 40792-2021, a weight loss experiment was conducted for 96 hours at 120°C, 100% relative humidity (RH), and an absolute vapor pressure of 0.2 MPa. The weight loss data are shown in Table 1.

[0146] Table 1

[0147] serial number <![CDATA[Weight loss (mg / cm 2 )]]> Example 1 6.4 Comparative Example 1 7.3 Comparative Example 2 20.6

[0148] As can be seen from Table 1, the sintered NdFeB magnets whose corrosion resistance is improved by the method of the present invention have lower weight loss under high temperature, high humidity and high pressure, and have better corrosion resistance.

[0149] The present invention is not limited to the above-mentioned embodiments. Any modification, improvement, or substitution that can be conceived by those skilled in the art without departing from the essential content of the present invention shall fall within the scope of the present invention.

Claims

1. A method for improving the corrosion resistance of sintered NdFeB magnets, comprising the following steps: 1) providing raw materials according to the chemical compositions of alloy A, alloy B, and alloy C, respectively; then, subjecting the raw materials of alloy A, alloy B, and alloy C to vacuum rapid solidification casting to obtain alloy A castings, alloy B castings, and alloy C castings; The alloy A is composed of the following components by weight percentage: 28-40 wt% of Pr-Nd alloy, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities; The alloy B is composed of the following components by weight percentage: 2-15 wt% of Pr-Nd alloy, 16-32 wt% of Ce, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.01-1 wt% of Ga, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities; The alloy C is composed of the following components by weight percentage: 30-45 wt% Pr, 6-20 wt% Ni, 2-16 wt% Nb, 0.5-1.5 wt% B, and the balance Fe and unavoidable impurities; 2) hydrogen crushing the alloy A flakes, alloy B flakes, and alloy C flakes obtained in step 1) to obtain alloy A coarse powder, alloy B coarse powder, and alloy C coarse powder, respectively; 3) The alloy A coarse powder obtained in step 1) is mixed with an antioxidant and a lubricant and then crushed by jet mill to obtain an average particle size D 50 The alloy A fine powder is 0.2 to 12 μm; the alloy B coarse powder is mixed with an antioxidant and a lubricant and then crushed by a jet mill to obtain an average particle size D 50 The alloy B fine powder is 0.2 to 12 μm, and the alloy C coarse powder is mixed with an antioxidant and a lubricant and then crushed by a jet mill to obtain an average particle size of D 50 Alloy C fine powder of 2 to 250 nm; 4) mixing the Alloy A fine powder, Alloy B fine powder, and Alloy C fine powder obtained in step 3) to obtain a mixed powder, and then orienting and isostatically pressing the mixed powder in a magnetic field to obtain a green compact; wherein the mass ratio of the Alloy A fine powder to the Alloy B fine powder is 20 to 120:100, and the ratio of the mass of the Alloy C fine powder to the sum of the masses of the Alloy A fine powder and the Alloy B fine powder is 0.8 to 2:100; 5) sintering the green compact obtained in step 4) at 960-1180° C. under vacuum conditions to obtain a sintered magnet A; heating the sintered magnet A at 880-1080° C. for the first time to obtain a sintered magnet B; and heating the sintered magnet B at 500-800° C. for the second time to obtain a sintered NdFeB magnet; 6) The sintered NdFeB magnet obtained in step 5) was cut into samples with a size of 10-15 mm × 10-15 mm × 6-10 mm, and the samples were subjected to a weight loss experiment for 96 hours at 120° C., a relative humidity of 100% and an absolute vapor pressure of 0.2 MPa.

2. The preparation method according to claim 1, characterized in that In step 1), the vacuum degree of the vacuum rapid solidification casting sheet is 0.1-10 Pa; The thickness of the alloy A casting sheet is 0.2-1.2 mm, the thickness of the alloy B casting sheet is 0.2-1.2 mm, and the thickness of the alloy C casting sheet is 0.2-1.2 mm.

3. The preparation method according to claim 1, characterized in that In step 3), the antioxidant is selected from at least one of polyethylene glycol octane, petroleum ether, isozinc propionate, tributyl borate, and 2,6-di-tert-butyl-p-cresol.

4. The preparation method according to claim 1, characterized in that In step 3), for any one of the alloy coarse powders of Alloy A coarse powder, Alloy B coarse powder and Alloy C coarse powder, the mass of the corresponding antioxidant used is 0.2 to 12 wt‰ of the mass of the alloy coarse powder.

5. The preparation method according to claim 1, characterized in that In step 3), the lubricant is selected from at least one of aviation kerosene, isopropyl alcohol, zinc stearate, isooctyl oleate, and triethanolamine borate.

6. The preparation method according to claim 1, characterized in that In step 3), for any one of the alloy coarse powders of Alloy A coarse powder, Alloy B coarse powder and Alloy C coarse powder, the mass of the corresponding lubricant used is 0.2 to 12 wt‰ of the mass of the alloy coarse powder.

7. The preparation method according to claim 1, characterized in that In step 4), the intensity of the magnetic field is at least 1 T, and the oxygen content during orientation molding is 1 to 80 ppm; and the isostatic pressing treatment is cold isostatic pressing treatment.

8. The preparation method according to claim 1, characterized in that In step 5), the vacuum degree of the vacuum condition is 0.1×10 -2 ~2×10 -2 Pa, the sintering time is 0.5 to 6 hours, and the sintering oxygen content is 0.1 to 50 ppm.

9. The preparation method according to claim 1, characterized in that In step 5), the vacuum degree of the first heating is 0.1×10 -2 ~2×10 -2 Pa, the first heating time is 0.5 to 4 hours, and the oxygen content of the first heating is 0.1 to 50 ppm; The vacuum degree of the second heating is 0.1×10 -2 ~2×10 -2 Pa, the second heating time is 0.5 to 4 hours, and the oxygen content of the second heating is 0.1 to 50 ppm.

10. Use of a mixed alloy in improving the corrosion resistance of sintered NdFeB magnets, characterized in that: The mixed alloy consists of alloy A, alloy B and alloy C; The alloy A is composed of the following components by weight percentage: 28-40 wt% of Pr-Nd alloy, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities; The alloy B is composed of the following components by weight percentage: 2-15 wt% of Pr-Nd alloy, 16-32 wt% of Ce, 0.05-0.8 wt% of Al, 0.05-0.8 wt% of Cu, 0.01-1 wt% of Ga, 0.5-1.5 wt% of B, and the balance being Fe and unavoidable impurities; The alloy C is composed of the following components by weight percentage: 30-45 wt% Pr, 6-20 wt% Ni, 2-16 wt% Nb, 0.5-1.5 wt% B, and the balance Fe and unavoidable impurities; The corrosion-resistant sintered NdFeB magnets with a size of 10-15mm x 10-15mm x 6-10mm were subjected to a weight loss test for 96 hours at 120°C, 100% relative humidity and 0.2MPa absolute vapor pressure. The weight loss was no more than 6.5mg / cm 2 .