A high-temperature environment with a rich Gd core-shell structure R-T-B rare earth permanent magnet and its preparation method

Abstract

This invention provides a core-shell structure R-T-B rare earth permanent magnet with Gd-rich cores for high-temperature environments. In the magnet, the rare earth element R content is 29.0 wt.% to 34.0 wt.%, and R is composed of Gd and R1. The Gd element content is 1.0 wt.% to 20.0 wt.% of the magnet mass. The main phase grains contain main phase grains with a volume ratio of 20 vol.% to 80 vol.% of Gd-rich cores and Gd-poor shells. This invention employs a dual-alloy method to separately melt alloy sheets with high and low Gd contents, then prepares sintered magnets. During high-temperature diffusion treatment, Gd elements in the high-Gd-content main phase diffuse into the R-rich phase at the grain boundaries. Ultimately, the high-Gd-content main phase grains form an anti-shell structure with a Gd-depleted shell and a Gd-rich core. The Gd-depleted shell significantly reduces the degradative effect of Gd on the magnet's coercivity, thus fully utilizing the characteristics of Gd to prepare a magnet with a low temperature coefficient and high coercivity. Grain boundary diffusion treatment of Dy / Tb elements can further improve the magnet's coercivity, preparing R-T-B rare-earth permanent magnets suitable for higher operating temperatures.

Description

Technical Field

[0001] This invention relates to a core-shell structured RTB rare earth permanent magnet with a Gd-rich core for use in high-temperature environments and its preparation method, belonging to the field of rare earth magnets. Background Technology

[0002] Rare-earth permanent magnets (RTB) are widely used in modern industry due to their extremely high energy product, which enables effective miniaturization of equipment. In recent years, with the development of the new energy industry, the market share of RTB magnets in wind power generation and electric vehicles has been increasing year by year. However, because the temperature coefficient of RTB magnets is negative, their magnetic properties deteriorate as temperature increases. Since the operating temperature of a car engine is around 200℃, it is necessary to improve the high-temperature magnetic properties of the magnets to ensure normal motor operation.

[0003] Currently, the high-temperature magnetic properties of RTB magnets are mainly improved by increasing coercivity and reducing the temperature coefficient of magnetic properties (the temperature coefficient itself is a negative value; comparisons of the magnitude of the temperature coefficient in this paper all use its absolute value, the same below). The heavy rare earth element Gd can significantly reduce the temperature coefficient of the magnet, thereby improving its high-temperature magnetic properties. However, Gd can also have adverse effects on magnet performance. On the one hand, Gd is a heavy rare earth element, and in R2T… 14 In the boron phase, Gd and Fe are antiferromagnetically coupled. Adding excessive Gd during the smelting stage reduces the remanence of the magnet. On the other hand, the anisotropy field of the main phase corresponding to Gd is low, and its presence in large quantities in the magnet reduces the coercivity. Therefore, currently, adding Gd to magnets to reduce their temperature coefficient comes at the cost of reduced remanence and coercivity.

[0004] Grain boundary diffusion is an effective method to improve the coercivity of magnets. However, if the diffusion process is not reasonable or the diffusion time is too long, a reverse shell distribution phenomenon of heavy rare earth elements will occur. That is, after the main phase grains of the magnet form a core-shell structure through grain boundary diffusion of heavy rare earth elements, the content of heavy rare earth elements in the core of the main phase grains of the magnet is high, while the content of heavy rare earth elements in the shell is low.

[0005] During grain boundary diffusion, in the initial stage, the high concentration of diffusion sources on the magnet surface diffuses along the R-rich phase at the grain boundaries into the magnet's interior. At this time, because the concentration of heavy rare earth elements (HREEs) in the R-rich phase is higher than that in the main phase grains of the magnet, HREEs diffuse into the main phase grains, forming a shell rich in HREEs on the surface of the main phase grains. However, as the diffusion time increases, the grain boundary diffusion sources on the magnet surface are consumed. The concentration of HREEs in the R-rich phase gradually decreases as diffusion progresses, eventually leading to a situation where the HREE content in the main phase grains of the magnet is higher than that in the R-rich phase at the grain boundaries. At this point, the diffusion direction of HREEs changes from the main phase grains to the R-rich phase, ultimately forming a main phase with an anti-shell structure of HREEs within the magnet. That is, the surface of the main phase grains is a shell with a lower HREE content, while the core of the main phase grains is a core with a higher HREE content. In traditional grain boundary diffusion, elements with high anisotropic fields, such as Dy or Tb, are generally used as grain boundary diffusion sources to improve the coercivity of the magnet by enhancing the anisotropic field on the surface of the main phase grains. However, when an anti-shell structure of Dy or Tb elements appears, the coercivity of the magnet will be significantly reduced.

[0006] In RTB-based rare-earth permanent magnets, the anisotropic field of the Gd-corresponding main phase is lower than that of the Pr or Nd-corresponding main phases. Therefore, to avoid a decrease in the magnet's coercivity, Gd is generally not used as a grain boundary diffusion source during grain boundary diffusion. However, if the process is controlled to promote the formation of a special structure in the magnet, where the core of the main phase grains is rich in Gd and the edges of the main phase grains are poor in Gd, i.e., a Gd-inverse shell structure is formed, the magnet with this structure can, on the one hand, utilize the low temperature coefficient of the Gd main phase to improve the high-temperature magnetic properties of the magnet. On the other hand, since a Gd-depleted shell is formed at the edge of the main phase grain of the magnet, and the coercivity mechanism of NdFeB magnets is nucleation type, if the coercivity at the edge of the main phase of the magnet is low, it will lead to a decrease in the coercivity of the entire main phase grain. Therefore, the Gd-depleted shell at the edge of the main phase grain of the magnet can significantly reduce the deterioration effect of Gd element on the coercivity of the magnet, thereby making full use of the characteristics of Gd to prepare a magnet with a low temperature coefficient and high coercivity. Summary of the Invention

[0007] To address the technical problem that adding Gd elements to reduce the temperature coefficient of RTB rare earth permanent magnets significantly reduces the remanence and coercivity of the magnets, this invention provides a core-shell structure RTB rare earth permanent magnet with a Gd-rich core for high-temperature environments and its preparation method.

[0008] The technical solution adopted in this invention is as follows:

[0009] A core-shell structured RTB rare earth permanent magnet with a Gd-rich core for high-temperature environments, wherein the rare earth element R content in the magnet is 29.0 wt.% to 34.0 wt.%, R is composed of Gd and R1, the Gd content is 1.0 wt.% to 20.0 wt.% of the magnet mass, and the balance of R is R1;

[0010] The magnet contains a main phase R2T 14 B and grain boundaries are rich in R phase. The main phase grains contain 20 vol.% to 80 vol.% (volume ratio) of Gd-rich nucleus main phase grains. The Gd-rich nucleus main phase grains are composed of Gd-rich cores and Gd-poor shells. The difference between the core Gd content H1 (wt.%) and the shell Gd content H2 (wt.%) is δH = H1 - H2. δH satisfies δH = (0.05 to 0.40)R with the total rare earth element content R in the magnet.

[0011] Furthermore, the magnet composition of the high-temperature environment RTB rare-earth permanent magnet with a core-shell structure rich in Gd cores includes:

[0012] R: 29.0wt.%~34.0wt.%,

[0013] B: 0.9 wt.% ~ 1.1 wt.%

[0014] M: 0.1 wt.% ~ 10.0 wt.%

[0015] The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe;

[0016] The R is composed of Gd and R1, with Gd content ranging from 1.0 wt.% to 20.0 wt.% of the magnet mass, and the balance of R being R1; R1 is composed of R3 or a combination of R2 and R3; R2 is at least one of the rare earth elements Dy and Tb, and R3 is at least one of Nd, Pr, Ho, La, and Ce, preferably one or two of Nd and Pr; more preferably, more than 75 wt.% of R3 is Nd.

[0017] When the magnet contains R2, the R2 content is 0.1 wt.% to 2.0 wt.% of the magnet mass, and the Gd-poor shell of the Gd-rich main phase grains of the magnet contains 0.05 wt.% to 0.5 wt.% of R2 element;

[0018] M is at least one of Al, Cu, Ga, Zr, Ti, Nb, Zn, Sn, W, Mo, Hf, Au, and Ag, preferably one or more of Cu, Ga, and Zr;

[0019] The high-temperature environment uses a core-shell structured RTB rare-earth permanent magnet with a Gd-rich core, prepared according to one of the following methods:

[0020] Method (1): When the magnet does not contain R2 element

[0021] High-Gd content alloy raw materials and low-Gd content alloy raw materials were prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning according to the composition ratio. The high-Gd content SC sheets and low-Gd content SC sheets were used to prepare alloy powder. The alloy powder was molded by an orientation magnetic field and isostatically pressed to prepare magnet blanks. After vacuum sintering, high-temperature diffusion treatment was carried out to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment.

[0022] The high-temperature diffusion treatment is carried out at a diffusion temperature of 800-1000℃ and a holding time of 5-25h. After the holding time is completed, the temperature is cooled to below 200℃ and then raised to 400-650℃ and held for 2-10h to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment.

[0023] Method (II): When the magnet contains R2 element:

[0024] High-Gd content alloy raw materials and low-Gd content alloy raw materials, both free of R2 element, are prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning. Alloy powder is obtained from the high-Gd content SC sheets and low-Gd content SC sheets. The alloy powder is molded and isostatically pressed into magnet blanks by an orientation magnetic field. After vacuum sintering, high-temperature diffusion treatment is performed. The resulting matrix magnet is processed into magnetic sheets with a thickness of 0.5-10.0 mm. After surface treatment, a heavy rare earth element diffusion layer with a thickness of 3-100 μm is deposited on the surface of the magnetic sheet. Then, grain boundary diffusion treatment is performed to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich nuclei for high-temperature environment.

[0025] The heavy rare earth element diffusion layer is any one or both of Dy and Tb.

[0026] The heavy rare earth element diffusion layer is generally deposited by vapor deposition, magnetron sputtering or multi-arc ion plating.

[0027] In method (I) or method (II), the diffusion temperature of the high-temperature diffusion treatment is 800-1000℃, the holding time is 5-25h, after the holding time is completed, the temperature is cooled to below 200℃ and then raised to 400-650℃, and held for 2-10h to obtain the magnet substrate.

[0028] In method (ii), the diffusion temperature of the grain boundary diffusion treatment is 800-1000℃, the holding time is 5-25h, after the holding time is completed, the temperature is cooled to below 200℃ and then heated to 400-650℃, and held for 2-10h to obtain the RTB rare earth permanent magnet with a core-shell structure rich in Gd cores for high-temperature environment.

[0029] The difference in Gd content between the high Gd content SC tablets and the low Gd content SC tablets is ≥5.0 wt.%, the Gd content in the high Gd content SC tablets is 5.0 to 34.0 wt.%, and the Gd content in the low Gd content SC tablets is preferably 0.

[0030] Furthermore, the composition of each component in the high Gd content alloy raw material is as follows:

[0031] R3: 0–29 wt.%, R3 is one or more of Nd, Pr, Ho, La, and Ce, preferably more than 75 wt.% of R3 is Nd.

[0032] Gd: 5.0~34.0wt.%

[0033] B: 0.9 wt.% ~ 1.1 wt.%

[0034] M: 0.1 wt.% ~ 10.0 wt.%

[0035] The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe;

[0036] The composition of each component in the low Gd content alloy raw material is as follows:

[0037] R3: 5-34 wt.%, R3 is one or more of Nd, Pr, Ho, La, Ce, preferably more than 75 wt.% of R3 is Nd;

[0038] Gd: 0 to X wt.%, and the Gd content of high Gd alloy raw materials is -X≥5.0wt.%; preferably, the Gd content is 0;

[0039] B: 0.9 wt.% ~ 1.1 wt.%

[0040] M: 0.1 wt.% ~ 10.0 wt.%

[0041] The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe;

[0042] The mass ratio of the high-Gd content alloy raw material to the low-Gd content alloy raw material should ensure that the Gd element content in the final magnet after mixing is 1.0 wt.% to 20.0 wt.%, and in actual production it is generally 1:0.25 to 4.

[0043] Furthermore, the alloy powder is prepared from the high-Gd-content SC sheets and the low-Gd-content SC sheets by mixing the high-Gd-content SC sheets and the low-Gd-content SC sheets, and then preparing the alloy powder by hydrogen crushing and air jet milling; or by separately hydrogen crushing the high-Gd-content SC sheets and the low-Gd-content SC sheets, mixing them, and then preparing the alloy powder by air jet milling; or by separately hydrogen crushing the high-Gd-content SC sheets and the low-Gd-content SC sheets, and then mixing the resulting powders to prepare the alloy powder.

[0044] The vacuum sintering process is as follows: heating to 1020-1110℃ and holding for 3-10 hours.

[0045] In method (ii), the surface treatment process of the magnetic sheet is to remove rust and oil stains from the surface of the magnet by means of sandblasting, pickling and other methods.

[0046] In method (ii), the heavy rare earth element diffusion layer is a pure heavy rare earth element metal, a heavy rare earth element hydride, or an alloy of heavy rare earth elements and other metal elements, wherein the heavy rare earth element is at least one of Dy and Tb.

[0047] In method (ii), a heavy rare earth element diffusion layer with a thickness of 3 to 100 μm is deposited on the surface of the magnetic sheet. Preferably, the heavy rare earth element diffusion layer is deposited on the surface of the magnet perpendicular to the orientation direction, and preferably not on the surface of the magnet not perpendicular to the orientation direction.

[0048] This invention also provides a method for preparing a core-shell structured RTB rare-earth permanent magnet with a Gd-rich core for high-temperature environments, wherein the method comprises one of the following:

[0049] Method (1): When the magnet does not contain R2 element:

[0050] High-Gd content alloy raw materials and low-Gd content alloy raw materials were prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning according to the composition ratio. The high-Gd content SC sheets and low-Gd content SC sheets were used to prepare alloy powder. The alloy powder was molded by an orientation magnetic field and isostatically pressed to prepare magnet blanks. After vacuum sintering, high-temperature diffusion treatment was carried out to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment.

[0051] The high-temperature diffusion treatment is carried out at a diffusion temperature of 800-1000℃ and a holding time of 5-25h. After the holding time is completed, the temperature is cooled to below 200℃ and then raised to 400-650℃ and held for 2-10h to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment.

[0052] Method (II): When the magnet contains R2 element:

[0053] High-Gd content alloy raw materials and low-Gd content alloy raw materials, both of which do not contain R2 element, are prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning. The high-Gd content SC sheets and low-Gd content SC sheets are used to prepare alloy powder. The alloy powder is molded and isostatically pressed into magnet blanks by an orientation magnetic field. After vacuum sintering, high-temperature diffusion treatment is performed. The resulting matrix magnet is processed into magnetic sheets with a thickness of 0.5 to 10.0 mm. After surface treatment, a heavy rare earth element diffusion layer with a thickness of 3 to 100 μm is deposited on the surface of the magnetic sheet. Then, grain boundary diffusion treatment is performed to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich nuclei for high-temperature environment.

[0054] The heavy rare earth element diffusion layer is any one or two of Dy and Tb;

[0055] The composition of each component in the high Gd content alloy raw material is as follows:

[0056] R3: 0–29 wt.%, R3 is one or more of Nd, Pr, Ho, La, and Ce;

[0057] Gd: 5.0~34.0wt.%,

[0058] B: 0.9 wt.% ~ 1.1 wt.%

[0059] M: 0.1 wt.% ~ 10.0 wt.%

[0060] The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe;

[0061] The composition of each component in the low Gd content alloy raw material is as follows:

[0062] R3: 5-34 wt.%, R3 is one or more of Nd, Pr, Ho, La, Ce, preferably more than 75 wt.% of R3 is Nd;

[0063] Gd: 0~X wt.%, and the Gd content of high Gd content alloy raw materials is -X≥5.0wt.%;

[0064] B: 0.9 wt.% ~ 1.1 wt.%

[0065] M: 0.1 wt.% ~ 10.0 wt.%

[0066] The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe;

[0067] The composition of the obtained high-temperature environment RTB rare-earth permanent magnet with a core-shell structure rich in Gd cores is as follows:

[0068] R: 29.0wt.%~34.0wt.%,

[0069] B: 0.9 wt.% ~ 1.1 wt.%

[0070] M: 0.1 wt.% ~ 10.0 wt.%

[0071] The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe;

[0072] The R is composed of Gd and R1, with the Gd element content being 1.0 wt.% to 20.0 wt.% of the magnet mass, and the balance of R being R1; R1 is R3 or is composed of R2 and R3; R3 is at least one of Nd, Pr, Ho, La, and Ce;

[0073] R2 is at least one of the rare earth elements Dy and Tb. When the magnet contains R2, the R2 content is 0.1 wt.% to 2.0 wt.% of the magnet mass. The Gd-rich core main phase grains of the magnet contain 0.05 wt.% to 0.5 wt.% of R2 element in their Gd-poor shells.

[0074] M is at least one of Al, Cu, Ga, Zr, Ti, Nb, Zn, Sn, W, Mo, Hf, Au, and Ag;

[0075] The magnet contains a main phase R2T 14 B and grain boundaries are rich in R phase, and the main phase grains contain Gd-rich core main phase grains with a volume ratio of 20 vol.% to 80 vol.%. The Gd-rich core main phase grains are composed of Gd-rich cores and Gd-poor shells. The difference between the core Gd content H1 (wt.%) and the shell Gd content H2 (wt.%) is δH = H1 - H2. δH satisfies δH = (0.05 to 0.40)R with the total rare earth element content R in the magnet.

[0076] This invention employs a dual-alloy method to prepare core-shell structured rare-earth permanent magnets (RTBs) with Gd-rich cores for high-temperature environments. Specifically, two alloy sheets with high and low Gd contents are melted separately. The two alloy sheets are then mixed and processed into alloy powder via hydrogen crushing and air jet milling. Alternatively, the two alloy sheets are hydrogen-crushed separately, mixed, and then processed into alloy powder via air jet milling. The resulting alloy powder is then oriented, isostatically pressed, and sintered to obtain the sintered magnet. The sintered magnet undergoes a high-temperature diffusion treatment at 800–1000℃ for 5–25 hours. After the high-temperature diffusion treatment, the magnet is cooled to below 200℃ and then heated to 400–650℃, holding at that temperature for 2–10 hours.

[0077] During the high-temperature diffusion process in the magnet, due to the significant Gd concentration difference between the high-Gd-content main phase grains and the R-rich phase at the magnet grain boundaries, Gd diffuses from the main phase grains into the R-rich phase at high temperatures. The Gd diffused into the R-rich phase then continues to diffuse along the molten grain boundary towards areas with lower Gd concentrations, thus diluting the Gd in the R-rich phase near the high-Gd-content main phase grains and further promoting the outward diffusion of Gd from the main phase. Since the diffusion rate of Gd within the main phase grains is much lower than the diffusion rate from the main phase grains to the R-rich phase at the grain boundaries (i.e., the diffusion rate from the grain center to the grain edge is much lower than the diffusion rate from the grain edge to the R-rich phase), the high-Gd-content main phase grains eventually form an anti-shell structure with a Gd-depleted shell and a Gd-rich core.

[0078] The presence of the heavy rare earth element Gd in the main phase grains of a magnet can significantly reduce the temperature coefficient of the magnet, thereby improving its high-temperature magnetic properties. However, on the one hand, Gd is antiferromagnetically coupled with Fe, and adding too much Gd during the smelting stage will reduce the remanence of the magnet. On the other hand, the anisotropy field of the main phase corresponding to Gd is low, and its presence in large quantities in the magnet will reduce the coercivity of the magnet. However, by controlling the formation of an anti-shell structure with a Gd-poor shell and a Gd-rich core in the high-Gd content main phase grains using the method of this invention, on the one hand, the low temperature coefficient of the Gd main phase can be utilized to improve the high-temperature magnetic properties of the magnet. On the other hand, since a Gd-poor shell is formed at the edge of the main phase grains, the degrading effect of Gd on the coercivity of the magnet can be significantly reduced, thus fully utilizing the characteristics of Gd to prepare a magnet with a low temperature coefficient and high coercivity.

[0079] To ensure the diffusion of Gd from the high-Gd-content main phase grains into the R-rich phase at the magnet grain boundaries, a significant Gd concentration gradient between the main phase grains and the R-rich phase at the grain boundaries is necessary. Therefore, this invention employs a dual-alloy process to separately melt high-Gd-content and low-Gd-content alloy sheets and prepare mixed powders. To maintain a large concentration difference of Gd between the high-Gd-content main phase grains and the surrounding R-rich phase at the grain boundaries, thus facilitating the outward diffusion of Gd from the high-Gd-content main phase grains at high temperatures, the Gd content difference between the high-Gd-content SC alloy sheet and the low-Gd-content SC alloy sheet must be ≥5.0 wt.%. In this invention, the Gd content of the high-Gd-content SC sheet is 5.0–34.0 wt.%, and the low-Gd-content SC sheet is preferably Gd-free.

[0080] Furthermore, the rare earth content of the magnet also affects the diffusion of Gd in the high-Gd-content main phase grains. As the rare earth content of the magnet increases, the content of the R-rich phase at the grain boundaries also increases. Gd diffused from the high-Gd-content main phase grains is more easily diluted by the grain boundary phase, thus promoting further outward diffusion of Gd from the high-Gd-content main phase grains. This invention has found that in main phase grains with a Gd-element anti-shell structure, the difference δH between the Gd content H1 (wt.%) in the core of the main phase grain and the Gd content H2 (wt.%) in the shell satisfies δH = (0.05~0.40)R with respect to the rare earth content R of the magnet. That is, as the rare earth content of the magnet increases, the difference in Gd content between the core and shell of the main phase grain with the Gd-element anti-shell structure becomes greater, indicating that more Gd diffuses outward from the shell of the main phase grain.

[0081] Meanwhile, this invention can further improve the coercivity of the magnet and reduce its temperature coefficient through Dy / Tb grain boundary diffusion, thus preparing RTB rare earth permanent magnets suitable for use at higher operating temperatures.

[0082] This invention first prepares a main phase grain with a Gd-rich core reverse shell structure, and then performs grain boundary diffusion of elements such as Dy / Tb. This enriches the Gd-poor shell of the Gd-rich core main phase grain in the magnet with a high concentration of Dy / Tb elements, allowing the Gd-rich core main phase grain to form a Dy / Tb-rich shell with a higher anisotropic field, thereby further improving the coercivity of the magnet. Since heavy rare earth elements have a more negative formation energy when forming the main phase, they are more likely to replace light rare earth elements (Pr, Nd, La, Ce, etc.) to form the main phase. This invention first prepares a magnet with Gd-rich core main phase grains. Because the magnet has a core-shell structure, and the shell has a low content of heavy rare earth elements (Gd) and a high content of light rare earth elements, Dy / Tb is more likely to form a Dy / Tb-rich shell in the Gd-rich core main phase grain during the heavy rare earth element grain boundary diffusion process. If Gd is uniformly distributed within the main phase grains of the magnet, it will hinder the formation of Dy / Tb-rich shells in the main phase grains during the grain boundary diffusion process of heavy rare earth elements, thus reducing the grain boundary diffusion effect. Simultaneously, during the grain boundary diffusion process of heavy rare earth elements, other non-Gd-rich main phase grains will also form grains with Dy / Tb-rich shell structures, further improving the coercivity of the magnet and enabling the fabrication of RTB rare earth permanent magnets suitable for higher operating temperatures.

[0083] The beneficial effects of this invention are as follows: Two alloy sheets with high and low Gd contents are melted separately using a dual-alloy method, and then a mixed powder is prepared. A sintered magnet is obtained through orientation molding, isostatic pressing, and sintering. During high-temperature diffusion treatment, due to the large Gd concentration difference between the high-Gd-content main phase grains and the R-rich phase at the magnet grain boundaries, Gd diffuses from the high-Gd-content main phase grains to the R-rich phase at the magnet grain boundaries at high temperatures. Due to the influence of the rare earth element content and diffusion rate, the high-Gd-content main phase grains of the magnet eventually form an anti-shell structure with a Gd-poor shell and a Gd-rich core. The low temperature coefficient of the Gd main phase can improve the high-temperature magnetic properties of the magnet. Simultaneously, the formation of a Gd-poor shell at the edges of the main phase grains significantly reduces the deteriorating effect of Gd on the magnet's coercivity, thus fully utilizing the characteristics of Gd to prepare a magnet with a low temperature coefficient and high coercivity. Furthermore, through subsequent grain boundary diffusion treatment of Dy / Tb elements, a high concentration of Dy / Tb elements is enriched in the Gd-poor shell of the Gd-rich main phase grains of the magnet; while the non-Gd-rich main phase grains also form a shell with a high concentration of Dy / Tb during the grain boundary diffusion process, further improving the coercivity of the magnet, thereby preparing RTB rare earth permanent magnets suitable for use at higher temperatures. Attached Figure Description

[0084] Figure 1(a), (b), and (c) are SEM micrographs of the final magnets from Experiments No. 2, No. 5, and No. 6, respectively.

[0085] Figure 2 This is a schematic diagram of the microstructure of the Gd-rich nucleus main phase grains in Experiment No. 5 magnet, as well as the energy spectrum point scans of the Gd-poor shell and Gd-rich core of the Gd-rich nucleus main phase grains.

[0086] Figure 3 This is an EPMA surface scan Tb spectrum of magnet No. 9 after Tb diffusion. Detailed Implementation

[0087] The technical solution of the present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0088] When the magnet does not contain the R2 element, the magnet is prepared according to the following method:

[0089] Two types of SC alloy sheets with high Gd content and low Gd content were prepared by mixing raw materials in a certain proportion and then using vacuum induction melting and strip spinning.

[0090] The two types of SC sheets are mixed and then subjected to hydrogen crushing and air jet milling to prepare alloy powder; alternatively, the two types of SC sheets are hydrogen crushed separately and then mixed and subjected to air jet milling to prepare alloy powder; or the two types of SC sheets are hydrogen crushed separately, subjected to air jet milling, and then mixed to prepare alloy powder. The alloy powder is oriented and isostatically pressed to prepare a magnet green blank, and then the magnet is heated to 1020-1110℃ in a vacuum environment and held for 3-10 hours to prepare a sintered magnet.

[0091] The sintered magnet is heated to 800–1000℃ and held for 5–25 hours for high-temperature diffusion treatment. After the holding period, it is cooled to below 200℃ and then heated to 400–650℃ and held for 2–10 hours to obtain the magnet.

[0092] When the magnet contains the element R2, the magnet is prepared according to the following method:

[0093] Two types of SC alloy sheets with high Gd content and low Gd content were prepared by vacuum induction melting and strip spinning after the raw materials were mixed in a certain proportion.

[0094] The two types of SC sheets are mixed and then subjected to hydrogen crushing and air jet milling to prepare alloy powder; alternatively, the two types of SC sheets are hydrogen crushed separately and then mixed and subjected to air jet milling to prepare alloy powder; or the two types of SC sheets are hydrogen crushed separately, subjected to air jet milling, and then mixed to prepare alloy powder. The alloy powder is oriented and isostatically pressed to prepare a magnet green blank, and then the magnet is heated to 1020-1110℃ in a vacuum environment and held for 3-10 hours to prepare a sintered magnet.

[0095] The sintered magnet is heated to 800–1000℃ and held for 5–25 hours for high-temperature pre-diffusion treatment. After the holding period, it is cooled to below 200℃ and then heated to 400–650℃ and held for 2–10 hours to obtain the base magnet.

[0096] The base magnet is machined into magnetic sheets with a thickness of 0.5 to 10.0 mm, and surface treatment is carried out by methods such as sandblasting and pickling to remove rust and oil stains from the magnet surface.

[0097] A heavy rare earth element diffusion layer with a thickness of 3–100 μm is deposited on the surface of the magnetic sheet using methods such as vapor deposition, magnetron sputtering, or multi-arc ion plating. A heavy rare earth diffusion source is covered on the surface of the magnet perpendicular to the orientation direction, while the surface of the magnet not perpendicular to the orientation direction is preferably not covered with a heavy rare earth diffusion source. The heavy rare earth diffusion source is a pure heavy rare earth element metal, a heavy rare earth element hydride, or an alloy of a heavy rare earth element with other metal elements, wherein the heavy rare earth element is at least one of Dy and Tb.

[0098] The magnetic sheet with a surface-deposited diffusion source is subjected to grain boundary diffusion treatment at a diffusion temperature of 800–1000℃ for 5–25 h. After the heat treatment is completed, the temperature is cooled to below 200℃ and then raised to 400–650℃ for 2–10 h to obtain the magnet.

[0099] The final magnet underwent surface treatment using sandblasting to expose a fresh surface. The magnet's room-temperature and high-temperature magnetic properties were tested using a NIM magnetic performance testing system, and its temperature coefficient was calculated. The magnet's microstructure was observed using SEM, and the volume ratio of Gd-rich main phase grains to the main phase grains was statistically analyzed using ImageJ software. The magnet's composition was analyzed using ICP, and the composition of micro-regions was analyzed using EPMA.

[0100] Example 1:

[0101] After the raw materials are mixed in a certain proportion, Nd is prepared by vacuum induction melting and belt spinning. 12 Pr 3. 0B 0.95 Co 0.95 Cu 0.1 Ga 0.15 Gd 17 Zr 0.15 Fe bal High Gd content SC tablets and Nd-containing tablets 24.75 Pr 7.25 B 0.95 Co 0.95 Cu 0.1 Ga 0.15 Zr 0.15 Fe bal Gd-free SC tablets.

[0102] Two types of SC sheets were mixed in different weight ratios and then pulverized by hydrogen crushing and air jet milling. The alloy powder was oriented and isostatically pressed to prepare a magnet green blank. Then, the magnet was heated to 1090℃ in a vacuum environment and held for 6 hours to prepare a sintered magnet.

[0103] The sintered magnet is heated to 880℃ and held for 15 hours for high-temperature diffusion treatment. After the holding period, it is cooled to below 200℃ and then heated to 520℃ and held for 5 hours to obtain the magnet.

[0104] The final magnet underwent surface treatment using sandblasting to expose a fresh surface. The magnet's magnetic properties at room temperature (20℃) and high temperature (150℃) were measured using a NIM magnetic performance testing system, and the temperature coefficient was calculated. The magnet's microstructure was observed using SEM, and the volume ratio of Gd-rich main phase grains to the main phase grains was statistically analyzed using ImageJ software. The magnet's composition was analyzed using ICP, and the composition of micro-regions was analyzed using EPMA.

[0105] The volume ratio of Gd-containing main phase grains in the magnet was controlled by mixing high-Gd content and Gd-free spools at different weight ratios. (In this embodiment, the densities of high-Gd content and Gd-free magnets are close, and the volume ratio of high-Gd content and Gd-free main phase grains can be approximately controlled by controlling the weight ratio of different SC sheets.) The proportion of Gd-rich main phase grains in the main phase grains was analyzed using SEM and ImageJ software. The proportion of high-Gd content spools, the final Gd content of the magnet, the proportion of Gd-containing main phase grains, and the proportion of Gd-rich main phase grains during the mixing of spools in Experiments No.1 to No.7 are shown in Table 1.

[0106] Table 1

[0107] Experiment No. 1 2 3 4 5 6 7 Percentage of high Gd content tablets (wt.%) 0 10.1 20.3 50.2 80.2 90.1 100.0 Gd content in magnet (wt.%) 0 1.7 3.4 8.4 13.5 15.4 17.0 Percentage of high Gd content main phase grains (vol.%) 0 9.3 20.0 50.2 79.9 90.0 100.0 Volume ratio of Gd-rich core main phase (vol.%) 0 9.3 20.0 50.2 79.9 8.1 0

[0108] The absolute values ​​of the magnetic properties of the magnets in Experiments No.1 to No.7 at room temperature and high temperature (150℃), the remanence temperature coefficient α at 150℃, and the coercivity temperature coefficient β are shown in Table 2.

[0109] Table 2

[0110]

[0111] As shown in Table 1, Experiments No. 1 and No. 7 produced magnets from single alloys with and without Gd content, respectively. In Experiment No. 1, when the magnet was entirely made from a single alloy without Gd, the Gd content was zero, and there were no Gd-rich main phase grains in the magnet's main phase. However, when the magnet was entirely made from high-Gd content single alloys (Experiment No. 7), because Gd was present in all the main phase grains, the Gd concentration gradient around the Gd-containing main phase grains was small, making it difficult for Gd to diffuse outward. Therefore, Gd-rich main phase grains could not be formed. It is evident that the proportion of Gd-rich main phase grains in the final magnet does not increase linearly with the increase of the proportion of Gd-containing main phase grains.

[0112] Combined with the SEM image of the magnet in Experiment No. 2 ( Figure 1 (a) and performance data show that when the volume ratio of high Gd content main phase grains is less than 20 vol.%, after high-temperature grain boundary diffusion, all high Gd content main phase grains will transform into Gd-rich nucleus main phase grains. At this time, the proportion of high Gd content main phase grains in the magnet before grain boundary diffusion and the proportion of Gd-rich nucleus main phase grains in the magnet after grain boundary diffusion are the same. However, because the proportion of Gd-rich nucleus main phase grains is relatively small, the improvement effect on the temperature coefficient of the magnet is limited, and the high-temperature magnetic properties of the magnet are poor.

[0113] In Experiments No. 3 to No. 5, when the proportion of high-Gd content main phase grains in the magnet was between 20 vol.% and 80 vol.%, a significant proportion of Gd-free main phase grains and R-rich grains at grain boundaries still existed in the magnet. Therefore, a large concentration gradient of Gd elements existed around the high-Gd content main phase grains, promoting the outward diffusion of Gd elements from the outer side of the high-Gd content main phase grains, thus forming Gd-rich nucleus main phase grains. After high-temperature grain boundary diffusion, all the high-Gd content main phase grains would transform into Gd-rich nucleus main phase grains; that is, the proportion of high-Gd content main phase grains before grain boundary diffusion was the same as the proportion of Gd-rich nucleus main phase grains after grain boundary diffusion. Because of the higher content of Gd-rich nucleus main phase grains in the magnet, the magnetic properties and temperature coefficient of the magnet were significantly improved. Therefore, the high-temperature magnetic properties of the magnet were higher than those in Experiments No. 1 and No. 2.

[0114] In Experiment No. 6, when the proportion of high-Gd content main phase grains in the magnet exceeded 80 vol.%, the proportion of Gd-free main phase grains in the magnet was relatively small, and the Gd concentration gradient around the high-Gd content main phase grains was small. Therefore, the Gd element in the high-Gd content main phase grains was difficult to diffuse outward, ultimately resulting in the proportion of Gd-rich nucleus main phase grains in the magnet being much lower than the proportion of high-Gd content main phase grains in the magnet. Figure 1(c) As can be seen from Experiment No. 6, the proportion of high Gd content main phase grains in the magnet is 90.0 vol.%, while after high-temperature diffusion, the proportion of Gd-rich nucleus main phase grains is only 8.1 vol.%. Although the temperature coefficient of the magnet is also low at this time, the lack of sufficient Gd-rich nucleus main phase grains to optimize the Gd element distribution leads to a deterioration in the magnet's room temperature magnetic properties. The compensation effect of the temperature coefficient cannot make up for the impact of the lower room temperature magnetic properties, so the high-temperature magnetic properties of the magnet will also deteriorate.

[0115] Microstructure of magnet No. 5 in experiment No. 5 was observed using SEM. Figure 1 (b) The magnet composition was analyzed using EPMA point scanning. A schematic diagram of the point scanning of the core-shell composition is shown below. Figure 2 As shown. The results show that the Gd content of the shell and core of the Gd-rich main phase grains in Experiment No. 5 magnet is 8.8 wt.% and 15.3 wt.%, respectively. It can be seen that after grain boundary diffusion, the amount of Gd element diffused outward from the outer side of the high Gd content main phase grains is relatively large, and the high Gd content main phase grains are fully transformed into a structure with a Gd-poor shell and a Gd-rich core.

[0116] This embodiment employs a dual-alloy method to separately melt two alloy sheets: one with high Gd content and the other without Gd, and then prepares a mixed powder. The powder is then oriented, isostatically pressed, and sintered to obtain a sintered magnet. During the high-temperature diffusion treatment of the sintered magnet, due to the significant Gd concentration difference between the high-Gd-content main phase grains and the R-rich phase at the magnet grain boundaries, Gd diffuses from the main phase grains to the R-rich phase at the grain boundaries. Due to the influence of the rare-earth element content and diffusion rate, the high-Gd-content main phase grains eventually form an anti-shell structure with a Gd-poor shell and a Gd-rich core. The low temperature coefficient of the Gd main phase improves the high-temperature magnetic properties of the magnet. Simultaneously, the formation of a Gd-poor shell at the edges of the main phase grains significantly reduces the degradative effect of Gd on the magnet's coercivity, thus fully utilizing the characteristics of Gd to prepare a magnet with both a low temperature coefficient and high coercivity.

[0117] In this invention, the proportion of high Gd content main phase grains in the magnet before diffusion needs to be controlled between 20 vol.% and 80 vol.%. Within this range, the high Gd content main phase grains will fully transform into Gd-rich nucleus main phase grains after high-temperature diffusion. This can significantly improve the temperature coefficient of the magnet and enhance its high-temperature magnetic properties. When the number of high Gd content main phase grains before diffusion at the magnet grain boundaries is too small (less than 20 vol.%), although the high Gd content main phase grains will also transform into Gd-rich nucleus main phase grains after high-temperature diffusion, the improvement in the magnet's temperature coefficient is not significant. When the proportion of high Gd content main phase grains in the magnet before diffusion exceeds 80 vol.%, the excessively high Gd element concentration around the high Gd content main phase grains will inhibit the outward diffusion of Gd elements from the main phase grains, resulting in a reduction in the number of Gd-rich nucleus main phase grains in the magnet, leading to a deterioration in the magnet's magnetic properties at both room temperature and high temperature.

[0118] Example 2:

[0119] After the raw materials are mixed in a certain proportion, Nd is prepared by vacuum induction melting and belt spinning. 12 Pr 3. 0B 0.95 Co 0.95 Cu 0.1 Ga 0.15 Gd 17 Zr 0.15 Fe bal High Gd content SC tablets and Nd-containing tablets 24.75 Pr 7.25 B 0.95 Co 0.95 Cu 0.1 Ga 0.15 Zr 0.15 Fe bal Gd-free SC tablets.

[0120] Two types of SC sheets were mixed in different weight ratios and then pulverized by hydrogen crushing and air jet milling. The alloy powder was oriented and isostatically pressed to prepare a magnet green blank. Then, the magnet was heated to 1090℃ in a vacuum environment and held for 6 hours to prepare a sintered magnet.

[0121] The sintered magnet is heated to 880℃ and held for 15 hours for high-temperature diffusion treatment. After the holding period, it is cooled to below 200℃ and then heated to 520℃ and held for 5 hours to obtain the base magnet.

[0122] The base magnet is machined into a magnetic sheet with a thickness of 2.0 mm, with the thickness direction of the magnet being the orientation direction. Surface treatment is carried out by sandblasting and pickling to remove rust and oil stains from the magnet surface.

[0123] A pure Tb layer with a thickness of 15 μm was deposited on the surface of the magnet perpendicular to the orientation direction using a multi-arc ion plating method, while no Tb layer was deposited on other surfaces.

[0124] The magnetic sheet with a surface-deposited diffusion source is subjected to grain boundary diffusion treatment at a diffusion temperature of 900℃ for 15 hours. After the heat treatment is completed, the temperature is cooled to below 200℃ and then raised to 520℃ for 3 hours to obtain the magnet.

[0125] The final magnet underwent surface treatment using sandblasting to expose a fresh surface. The magnet's room-temperature and high-temperature magnetic properties were tested using a NIM magnetic performance testing system, and its temperature coefficient was calculated. The magnet's microstructure was observed using SEM, and the volume ratio of Gd-rich main phase grains to the main phase grains was statistically analyzed using ImageJ software. The magnet's composition was analyzed using ICP, and the composition of micro-regions was analyzed using EPMA.

[0126] The composition of the base magnet in Experiments No. 8 to No. 10 is the same as that in Experiments No. 2, No. 5, and No. 6, respectively.

[0127] The final magnet contents (Gd, Tb, and proportions of high-Gd-content main phase grains and Gd-rich main phase grains) of Experiments No. 8 to No. 10 are shown in Table 3.

[0128] Table 3

[0129] Experiment No. 8 9 10 Gd content in magnet (wt.%) 1.7 13.5 15.3 Tb content in magnets (wt.%) 0.42 0.42 0.40 Percentage of high Gd content main phase grains (vol.%) 9.3 80.1 90.0 Volume ratio of Gd-rich core main phase (vol.%) 9.3 80.1 8.2

[0130] The absolute values ​​of the magnetic properties of magnets in Experiments No. 8 to No. 10 at room temperature and high temperature (150℃), the temperature coefficient of remanence α at 150℃, and the temperature coefficient of coercivity β are shown in Table 4.

[0131] Table 4

[0132]

[0133] When the volume ratio of high Gd content main phase grains in the magnet is within the range recommended in this invention (Experiment No. 9), after the first high-temperature diffusion, all high Gd content main phase grains are transformed into Gd-rich nucleus main phase grains. Because the Gd content is low in the shell of the Gd-rich nucleus main phase grains, during Tb diffusion at the grain boundaries, Tb atoms more frequently replace Pr and Nd atoms in the shell of the Gd-rich nucleus main phase grains. Therefore, a certain concentration of Tb atoms will accumulate in the Gd-poor shell of the Gd-rich nucleus main phase grains. The Tb distribution spectrum of the magnet after Tb diffusion in Experiment No. 9 is shown in the EPMA surface scan. Figure 3It can be seen that after Tb diffusion, Tb can relatively easily replace Pr and Nd elements in the Gd-poor shell, thus forming a relatively uniform Gd-poor Tb-rich shell. EPMA point scan analysis revealed that after Tb grain boundary diffusion, the Tb content in the Gd-poor shell of the Gd-rich nucleus main phase grains is between 0.05 wt.% and 0.5 wt.%. Simultaneously, non-Gd-rich nucleus main phase grains also form a Tb-rich shell on the outside of the main phase grains after Tb diffusion. Tb element improves the room-temperature coercivity of the magnet by increasing the anisotropic field of the Gd-poor shell in the Gd-rich nucleus main phase grains. Combined with the effect of Gd element in improving the temperature coefficient of the magnet, RTB rare-earth permanent magnets suitable for higher operating temperatures can be prepared.

[0134] In Experiment No. 8, when the proportion of Gd-rich main phase grains was low (<20 vol.%), although the room-temperature remanence and coercivity of the magnet were high after Tb diffusion, the magnetic properties decreased significantly at high temperatures due to the large temperature coefficient. In Experiment No. 10, when the proportion of Gd-rich main phase grains was too high (>80 vol.%), the magnet could not form enough Gd-rich main phase grains during the initial high-temperature diffusion. During Tb grain boundary diffusion, Tb elements needed to replace Gd elements to enter the main phase grains. Since the formation energies of the corresponding main phases of Gd and Tb were similar, it was difficult for Tb to enter the main phase of the magnet. Therefore, the increase in coercivity of the magnet after Tb diffusion was small. Furthermore, the magnet's room-temperature magnetic properties were poor; although the temperature coefficient was low, the high-temperature magnetic properties remained poor.

[0135] In this embodiment, a dual-alloy method is used to separately melt two alloy sheets: one with high Gd content and the other without Gd, and then prepare a mixed powder. After orientation molding, isostatic pressing, and sintering, a sintered magnet is obtained. During the high-temperature diffusion treatment of the sintered magnet, due to the large Gd concentration difference between the high-Gd-content main phase grains and the R-rich phase at the magnet grain boundaries, Gd diffuses from the main phase grains to the R-rich phase at the grain boundaries. Due to the influence of the rare earth element content and diffusion rate, the high-Gd-content main phase grains eventually form an anti-shell structure with a Gd-poor shell and a Gd-rich core. During the grain boundary diffusion process of Dy / Tb heavy rare earth elements, because the Gd content is low at the shell of the Gd-rich core main phase grains, Dy / Tb can replace Pr and Nd elements in the Gd-poor shell and enter the main phase grains. By increasing the anisotropic field of the Gd-poor shell to improve the coercivity of the magnet, and combining this with the effect of Gd element to improve the temperature coefficient of the magnet, the high-temperature magnetism of the magnet can be significantly improved, thus preparing RTB rare earth permanent magnets suitable for use at higher operating temperatures.

[0136] Example 3:

[0137] After the raw materials are mixed in a certain proportion, Nd is prepared by vacuum induction melting and belt spinning. 12 B0.95 Cu 0.1 Ga 0.15 Gd 20 Zr 0.15 Fe bal High Gd content SC tablets and Nd-containing tablets 32+x B 0.95 Cu 0.1 Ga 0.15 Zr 0.15 Fe bal For the Gd-free SC sheet, the values ​​of x in Experiments No. 11 to No. 14 are -2, 0, 2, and 4, respectively.

[0138] Two types of SC sheets, one with high Gd content and the other without Gd, were mixed in a 1:1 weight ratio and then powdered by hydrogen crushing and air jet milling. The alloy powder was then oriented and isostatically pressed to prepare a magnet green blank. The magnet was then heated to 1080℃ in a vacuum environment and held for 6 hours to prepare a sintered magnet.

[0139] The sintered magnet is heated to 880℃ and held for 15 hours for high-temperature diffusion treatment. After the holding period, it is cooled to below 200℃ and then heated to 505℃ and held for 5 hours to obtain the magnet.

[0140] The final magnet underwent surface treatment using sandblasting to expose a fresh surface. SEM was used to observe the magnet's microstructure, and ImageJ software was used to determine the volume ratio of Gd-rich main phase grains to the main phase grains. ICP analysis was used to analyze the magnet's composition, and EPMA analysis was used to analyze the composition of micro-regions within the magnet.

[0141] The total rare earth content, the proportion of high Gd content main phase grains, and the proportion of Gd-rich nucleus main phase grains in the magnets of Experiments No.11 to No.14 are shown in Table 5.

[0142] Table 5

[0143] Experiment No. 11 12 13 14 Total rare earth content R 31.0 32.0 33.0 34.0 Percentage of high Gd content main phase grains (vol.%) 49.5 49.5 49.7 49.8 Volume ratio of Gd-rich core main phase (vol.%) 49.5 49.5 49.7 49.8

[0144] As shown in Table 5, the proportion of high Gd content main phase grains in magnets of different experimental groups with different total rare earth contents is between 49.0 vol.% and 50.0 vol.%. After high-temperature diffusion, all high Gd content main phase grains are transformed into Gd-rich nucleus main phase grains.

[0145] The difference δH between the Gd element content H1 (wt.%) of the Gd-rich core and the Gd element content H2 (wt.%) of the Gd-poor shell in magnets with different total rare earth contents was analyzed by EPMA point scanning. The results are shown in Table 6.

[0146] Table 6

[0147] Experiment No. 11 12 13 14 δH (wt.%) 5.52～6.64 6.45～7.63 7.05～8.72 8.41～9.18

[0148] As shown in Table 6, when the proportion of Gd-rich core main phase grains in the magnet is the same, the difference in Gd element content between the Gd-rich core and the Gd-poor shell of the Gd-rich core main phase grains increases with the increase of the total rare earth content of the magnet. That is, with the increase of the total rare earth content of the magnet, the Gd element in the high Gd content main phase grains is more likely to diffuse outward during the high-temperature diffusion process.

[0149] In this embodiment, a dual-alloy method is used to separately melt two alloy sheets: one with high Gd content and the other without Gd, and then prepare a mixed powder. After orientation molding, isostatic pressing, and sintering, a sintered magnet is obtained. During the high-temperature diffusion treatment of the sintered magnet, due to the large Gd concentration difference between the high-Gd-content main phase grains and the R-rich phase at the magnet grain boundaries, Gd diffuses from the main phase grains to the R-rich phase at the grain boundaries. Due to the influence of the rare earth element content and diffusion rate, the high-Gd-content main phase grains eventually form an anti-shell structure with a Gd-depleted shell and a Gd-rich core. As the total rare earth content of the magnet increases, the Gd concentration in the R-rich phase at the grain boundaries surrounding the high-Gd-content main phase grains decreases, thus making it easier for Gd to diffuse outward from the high-Gd-content main phase grains. Simultaneously, as the total rare earth element content of the magnet increases, the volume ratio of the R-rich phase at the grain boundaries increases, further diluting the Gd element diffused from the high-Gd-content main phase grains. Ultimately, the Gd content difference between the Gd-rich core and the Gd-poor shell of the Gd-rich main phase grains in the magnet increases with the increase of the total rare earth content. In this invention, based on the different total rare earth contents of the magnet and the Gd concentration in the high-Gd-content main phase grains, after high-temperature diffusion, the Gd element difference δH between the Gd-rich core and the Gd-poor shell of the Gd-rich main phase grains satisfies δH = (0.05~0.40)R with respect to the total rare earth content R of the magnet.

[0150] Example 4:

[0151] After the raw materials are mixed in a certain proportion, Nd is prepared by vacuum induction melting and belt spinning. 32- x B 0.95 Cu 0.1 Ga 0.15 Gd x Zr 0.15 Fe bal High Gd content SC tablets and Nd-containing tablets 32 B 0.95 Cu 0.1 Ga 0.15 Zr 0.15 Fe bal For the Gd-free SC sheet, the values ​​of x in Experiments No. 15 to No. 17 are 1, 3, and 5, respectively.

[0152] Two types of SC sheets, high-Gd and Gd-free, were mixed in a 1:1 weight ratio and then powdered by hydrogen crushing and air jet milling. The alloy powder was oriented and isostatically pressed to prepare a magnet green blank. The magnet was then heated to 1080℃ in a vacuum environment and held for 6 hours to prepare a sintered magnet.

[0153] The sintered magnet is heated to 880℃ and held for 15 hours for high-temperature diffusion treatment. After the holding period, it is cooled to below 200℃ and then heated to 505℃ and held for 5 hours to obtain the magnet.

[0154] The final magnet underwent surface treatment using sandblasting to expose a fresh surface. The magnet's magnetic properties at room temperature (20℃) and high temperature (150℃) were measured using a NIM magnetic performance testing system, and the temperature coefficient was calculated. The magnet's microstructure was observed using SEM, and the volume ratio of Gd-rich main phase grains to the main phase grains was statistically analyzed using ImageJ software. The magnet's composition was analyzed using ICP, and the composition of micro-regions was analyzed using EPMA.

[0155] The proportions of high-Gd main phase grains and Gd-rich main phase grains in magnets of Experiments No.15 to No.17 are shown in Table 7.

[0156] Table 7

[0157] Experiment No. 15 16 17 Percentage of high Gd content main phase grains (vol.%) 49.3 49.2 49.5 Volume ratio of Gd-rich core main phase (vol.%) 0.3 1.2 25.3

[0158] As shown in Table 7, the proportion of high-Gd content main phase grains in magnets No. 15 to No. 17 was the same, but the proportion of Gd-rich main phase grains differed after high-temperature diffusion. The formation of Gd-rich main phase grains mainly relies on the large Gd concentration gradient between the high-Gd content main phase grains and the surrounding R-rich phase at the grain boundaries to promote the outward diffusion of Gd elements from the Gd-containing main phase grains. When the Gd content in the high-Gd content main phase grains of the magnet is too low, the driving force for the outward diffusion of Gd elements is small because the concentration difference between the high-Gd content main phase grains and the R-rich phase at the grain boundaries is small. Therefore, the formation process of Gd-rich main phase grains is hindered, ultimately leading to a decrease in the proportion of Gd-rich main phase grains in the high-temperature diffusion magnet.

[0159] The temperature coefficients of the magnets in Experiments No. 15 to No. 17 at 150℃ are shown in Table 8.

[0160] Table 8

[0161] Experiment No. ∣α∣(％ / ℃) ∣β∣(％ / ℃) 15 0.16 0.51 16 0.15 0.50 17 0.1 0.46

[0162] As shown in Table 8, Experiments No. 15 and No. 16 have a larger temperature coefficient at 150℃ due to the lower Gd content and smaller proportion of Gd-rich nucleus-rich main phase grains in the magnets. Therefore, the magnets have poor high-temperature magnetism.

[0163] This invention employs a dual-alloy method to separately melt two alloy sheets: one with high Gd content and the other without Gd, and then prepares a mixed powder. After orientation molding, isostatic pressing, and sintering, a sintered magnet is obtained. During the high-temperature diffusion treatment of the sintered magnet, due to the large Gd concentration difference between the high-Gd-content main phase grains and the R-rich phase at the magnet grain boundaries, Gd diffuses from the main phase grains to the R-rich phase at the grain boundaries. Due to the influence of the rare earth element content and diffusion rate, the Gd-containing main phase grains of the magnet eventually form an anti-shell structure with a Gd-depleted shell and a Gd-rich core. The formation of the Gd-rich core main phase grains mainly relies on the large Gd concentration gradient between the high-Gd-content main phase grains and the surrounding R-rich phase at the grain boundaries to promote the outward diffusion of Gd from the Gd-containing main phase grains. When the Gd content in the high-Gd-content main phase grains of the magnet is too low, the concentration difference between the high-Gd-content main phase grains and the R-rich phase at the grain boundaries is small, resulting in a weak driving force for Gd diffusion. Therefore, the formation process of the Gd-rich nucleus main phase grains is hindered, ultimately leading to a reduction in the proportion of Gd-rich nucleus main phase grains in the high-temperature diffusion magnet. Therefore, in this invention, the Gd content in the high-Gd-content alloy sheet is 5.0–34.0 wt.%.

[0164] Example 5:

[0165] After the raw materials are mixed in a certain proportion, Nd is prepared by vacuum induction melting and belt spinning. 12 B 0.95 Cu 0.1 Ga 0.15 Gd 20 Zr 0.15 Fe bal High Gd content SC tablets and Nd-containing tablets 12+x B 0.95 Cu 0.1 Ga 0.15 Gd 20-x Zr 0.15 Fe bal For the low Gd content SC tablets, the x values ​​in Experiments No.18 to No.21 were 2, 5, 7 and 20, respectively.

[0166] Two types of SC sheets with high Gd content and low Gd content were mixed in a 1:1 weight ratio and then powdered by hydrogen crushing and air jet milling. The alloy powder was oriented and isostatically pressed to prepare a magnet green blank. Then, the magnet was heated to 1080℃ in a vacuum environment and held for 6 hours to prepare a sintered magnet.

[0167] The sintered magnet is heated to 880℃ and held for 15 hours for high-temperature diffusion treatment. After the holding period, it is cooled to below 200℃ and then heated to 505℃ and held for 5 hours to obtain the magnet.

[0168] The final magnet underwent surface treatment using sandblasting to expose a fresh surface. SEM was used to observe the magnet's microstructure, and ImageJ software was used to determine the volume ratio of Gd-rich main phase grains to the main phase grains. ICP analysis was used to analyze the magnet's composition, and EPMA analysis was used to analyze the composition of micro-regions within the magnet.

[0169] The proportions of high Gd content main phase grains and Gd-rich nucleus main phase grains in magnets of Experiments No.18 to No.21 are shown in Table 9.

[0170] Table 9

[0171] Experiment No. 18 19 20 21 Percentage of high Gd content main phase grains (vol.%) 50.1 50.0 49.9 49.5 Volume ratio of Gd-rich core main phase (vol.%) 0.1 50.6 51.2 49.5

[0172] As shown in Table 9, the proportion of high-Gd content main phase grains in magnets from Experiments No. 18 to No. 21 was the same, but the proportion of Gd-rich main phase grains differed after high-temperature diffusion. The formation of Gd-rich main phase grains mainly relies on a large Gd concentration gradient between the high-Gd content main phase grains and the surrounding R-rich grain boundaries, which promotes the outward diffusion of Gd elements from the high-Gd content main phase grains. In Experiment No. 18, when the Gd concentration difference between the high-Gd and low-Gd main phase grains was too small, the driving force for the outward diffusion of Gd elements from the high-Gd main phase grains was weak. Therefore, the formation process of Gd-rich main phase grains was hindered, ultimately leading to a decrease in the proportion of Gd-rich main phase grains in the magnet after high-temperature diffusion. In Experiments No. 19 and No. 20, due to the Gd concentration difference between high-Gd-content main phase grains and low-Gd-content main phase grains being ≥5.0wt.%, the driving force for the outward diffusion of Gd elements in the high-Gd-content main phase grains was greater. After high-temperature diffusion, all the high-Gd-content main phase grains were transformed into Gd-nucleated main phase grains. At the same time, some of the low-Gd-content main phase grains in Experiments No. 19 and No. 20 were also transformed into Gd-nucleated main phase grains.

[0173] The absolute values ​​of the room temperature magnetic properties and 150℃ magnetic properties of the magnets in Experiments No.18 to No.21, the remanence temperature coefficient α at 150℃, and the coercivity temperature coefficient β are shown in Table 10.

[0174] Table 10

[0175]

[0176] In Experiment No. 18, the amount of Gd-rich main phase grains in the magnet was relatively small, resulting in poor magnetic properties at room temperature. Although the magnet also had a low temperature coefficient, the compensation effect of the temperature coefficient could not make up for the lower magnetic properties at room temperature, so the magnetic properties at high temperature also deteriorated. In Experiments No. 19 and No. 20, due to the larger number of Gd-rich main phase grains, the magnetic properties of the magnet were better at both room temperature and high temperature. Furthermore, comparing Experiments No. 19 and No. 21, it can be seen that as the Gd concentration difference between high-Gd and low-Gd main phase grains increases, the magnetic properties of the magnet improve.

[0177] This invention employs a dual-alloy method to separately melt two alloy sheets with high and low Gd contents, then prepares a mixed powder. After orientation molding, isostatic pressing, and sintering, a sintered magnet is obtained. During the high-temperature diffusion treatment of the sintered magnet, due to the large Gd concentration difference between the high-Gd-content main phase grains and the R-rich phase at the magnet grain boundaries, Gd diffuses from the main phase grains to the R-rich phase at the grain boundaries. Due to the influence of the rare earth element content and diffusion rate, the Gd-containing main phase grains eventually form an anti-shell structure with a Gd-poor shell and a Gd-rich core. The formation of the Gd-rich core main phase grains mainly relies on the large Gd concentration gradient between the high-Gd-content main phase grains and the surrounding R-rich phase at the grain boundaries to promote the outward diffusion of Gd from the high-Gd-content main phase grains. When the Gd concentration difference between high-Gd-content and low-Gd-content main phase grains in a magnet is less than 5.0 wt.%, the outward diffusion of Gd from the high-Gd-content main phase grains is suppressed, resulting in a smaller number of Gd-rich nucleus main phase grains and poorer magnetic properties of the magnet. Therefore, in this invention, to ensure the smooth transformation of high-Gd-content main phase grains into Gd-rich nucleus main phase grains, the Gd concentration difference between high-Gd-content and low-Gd-content main phase grains needs to be ≥5.0 wt.%. Furthermore, the greater the Gd concentration difference between high-Gd-content and low-Gd-content main phase grains, the higher the magnetic properties of the magnet. Therefore, in this invention, the low-Gd-content main phase grains are preferably Gd-free.

Claims

1. A R-T-B rare earth permanent magnet having a core-shell structure with Gd-rich cores for use in a high temperature environment, characterized by The magnet composition includes: R: 29.0wt.%~34.0wt.% B: 0.9wt.%~1.1wt.% M: 0.1wt.%~10.0wt.% The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe; The R is composed of Gd and R1, with the Gd element content being 1.0 wt.% to 20.0 wt.% of the magnet mass, and the balance of R being R1; R1 is R3 or is composed of R2 and R3; R3 is at least one of Nd, Pr, Ho, La, and Ce; R2 is at least one of rare earth elements Dy and Tb. When the magnet contains R2, the R2 content is 0.1 wt.% to 2.0 wt.% of the magnet mass, and the Gd-poor shell of the Gd-rich core main phase grain of the magnet contains 0.05 wt.% to 0.5 wt.% of R2 element. M is at least one of Al, Cu, Ga, Zr, Ti, Nb, Zn, Sn, W, Mo, Hf, Au, and Ag; The magnet contains a main phase R2T 14 The magnet contains B and R-rich phases at grain boundaries, and the main phase grains contain Gd-rich core main phase grains with a volume ratio of 20 vol.% to 80 vol.%. The Gd-rich core main phase grains consist of Gd-rich cores and Gd-poor shells. The difference between the core Gd content H1 (wt.%) and the shell Gd content H2 (wt.%) is δH = H1 - H2. δH satisfies δH = (0.05~0.40)R with the total rare earth element content R in the magnet.

2. The R-T-B based rare earth permanent magnet with core-shell structure having Gd-rich core for high temperature environment according to claim 1, characterized in that The high-temperature environment uses a core-shell structured RTB rare-earth permanent magnet with a Gd-rich core, prepared according to one of the following methods: Method (1): When the magnet does not contain R2 element: High-Gd content alloy raw materials and low-Gd content alloy raw materials were prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning according to the composition ratio. The high-Gd content SC sheets and low-Gd content SC sheets were used to prepare alloy powder. The alloy powder was molded by an orientation magnetic field and isostatically pressed to prepare magnet blanks. After vacuum sintering, high-temperature diffusion treatment was carried out to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment. The high-temperature diffusion treatment is carried out at a diffusion temperature of 800~1000℃ and a holding time of 5~25h. After the holding time is completed, the temperature is cooled to below 200℃ and then heated to 400~650℃ and held for 2~10h to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment. Method (2): When the magnet contains R2 element: High-Gd content alloy raw materials and low-Gd content alloy raw materials, both of which do not contain R2 element, are prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning. The high-Gd content SC sheets and low-Gd content SC sheets are used to prepare alloy powder. The alloy powder is molded and isostatically pressed into magnet blanks by an orientation magnetic field. After vacuum sintering, high-temperature diffusion treatment is performed. The resulting matrix magnet is processed into magnetic sheets with a thickness of 0.5~10.0 mm. After surface treatment, a heavy rare earth element diffusion layer with a thickness of 3~100 μm is deposited on the surface of the magnetic sheet. Then, grain boundary diffusion treatment is performed to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich nuclei for high-temperature environment. The heavy rare earth element diffusion layer is any one or both of Dy and Tb.

3. The R-T-B based rare earth permanent magnet with core-shell structure having Gd-rich core for high temperature environment according to claim 2, characterized in that The difference in Gd content between the high-Gd content SC tablets and the low-Gd content SC tablets is ≥5.0 wt.%, and the Gd content in the high-Gd content SC tablets is 5.0~34.0 wt.%.

4. The R-T-B based rare earth permanent magnet with core-shell structure having Gd-rich core for high temperature environment according to claim 2, characterized in that The composition of each component in the high Gd content alloy raw material is as follows: R3: 0~29wt.%, R3 is one or more of Nd, Pr, Ho, La, and Ce; Gd: 5.0~34.0 wt.% B: 0.9wt.%~1.1wt.% M: 0.1wt.%~10.0wt.% The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe; The composition of each component in the low Gd content alloy raw material is as follows: R3: 5~34 wt.%, R3 is one or more of Nd, Pr, Ho, La, and Ce, and more than 75 wt.% of R3 is Nd; Gd: 0~X wt.%, and the Gd content of high Gd alloy raw materials is -X≥5.0wt.%; B: 0.9wt.%~1.1wt.% M: 0.1wt.%~10.0wt.% The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe; The mass ratio of the high-Gd content alloy raw material to the low-Gd content alloy raw material should ensure that the Gd element content in the final magnet after mixing is 1.0 wt.%~20.0 wt.%.

5. The high-temperature environment RTB rare-earth permanent magnet with a core-shell structure and rich in Gd cores as described in claim 3 or 4, characterized in that... The Gd content in the low Gd content SC tablets is 0.

6. The high-temperature environment RTB rare-earth permanent magnet with a core-shell structure rich in Gd cores as described in claim 2, characterized in that... The alloy powder is prepared from the high-Gd-content SC sheets and the low-Gd-content SC sheets by mixing the high-Gd-content SC sheets and the low-Gd-content SC sheets, and then preparing the alloy powder by hydrogen crushing and air jet milling; or by separately hydrogen crushing the high-Gd-content SC sheets and the low-Gd-content SC sheets, mixing them, and then preparing the alloy powder by air jet milling; or by separately hydrogen crushing the high-Gd-content SC sheets and the low-Gd-content SC sheets, and then mixing the resulting powders to prepare the alloy powder.

7. The R-T-B based sintered magnet with core-shell structure having Gd-rich core for high temperature use according to claim 2, wherein 0.05 ≤ x ≤ 0.

15. In method (I) or method (II), the diffusion temperature of the high-temperature diffusion treatment is 800~1000℃, the holding time is 5~25h, after the holding time is completed, the temperature is cooled to below 200℃ and then raised to 400~650℃, and the holding time is 2~10h. In method (ii), the diffusion temperature of the grain boundary diffusion treatment is 800~1000℃, the holding time is 5~25h, after the holding time is completed, the temperature is cooled to below 200℃ and then heated to 400~650℃, and held for 2~10h to obtain the RTB rare earth permanent magnet with a core-shell structure rich in Gd core for high temperature environment.

8. The R-T-B based sintered magnet with core-shell structure having Gd-rich core for high temperature use according to claim 2, wherein 0.05 ≤ x ≤ 0.

15. In method (ii), a heavy rare earth element diffusion layer with a thickness of 3~100μm is deposited on the surface of the magnetic sheet, a heavy rare earth element diffusion layer is deposited on the surface of the magnet perpendicular to the orientation direction, and no heavy rare earth element diffusion layer is deposited on the surface of the magnet not perpendicular to the orientation direction.

9. A method for producing a core-shell structured R-T-B rare earth permanent magnet having a Gd-rich core for use in a high temperature environment, characterized by The method is one of the following: Method (1): When the magnet does not contain R2 element High-Gd content alloy raw materials and low-Gd content alloy raw materials were prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning according to the composition ratio. The high-Gd content SC sheets and low-Gd content SC sheets were used to prepare alloy powder. The alloy powder was molded by an orientation magnetic field and isostatically pressed to prepare magnet blanks. After vacuum sintering, high-temperature diffusion treatment was carried out to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment. The high-temperature diffusion treatment is carried out at a diffusion temperature of 800~1000℃ and a holding time of 5~25h. After the holding time is completed, the temperature is cooled to below 200℃ and then heated to 400~650℃ and held for 2~10h to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich core for high-temperature environment. Method (2): When the magnet contains R2 element: High-Gd content alloy raw materials and low-Gd content alloy raw materials, both of which do not contain R2 element, are prepared into high-Gd content SC sheets and low-Gd content SC sheets respectively by vacuum induction melting and strip spinning. The high-Gd content SC sheets and low-Gd content SC sheets are used to prepare alloy powder. The alloy powder is molded and isostatically pressed into magnet blanks by an orientation magnetic field. After vacuum sintering, high-temperature diffusion treatment is performed. The resulting matrix magnet is processed into magnetic sheets with a thickness of 0.5~10.0 mm. After surface treatment, a heavy rare earth element diffusion layer with a thickness of 3~100 μm is deposited on the surface of the magnetic sheet. Then, grain boundary diffusion treatment is performed to obtain the core-shell structure RTB rare earth permanent magnet with Gd-rich nuclei for high-temperature environment. The heavy rare earth element diffusion layer is any one or two of Dy and Tb; The composition of each component in the high Gd content alloy raw material is as follows: R3: 0~29wt.%, R3 is one or more of Nd, Pr, Ho, La, and Ce; Gd: 5.0~34.0 wt.% B: 0.9wt.%~1.1wt.% M: 0.1wt.%~10.0wt.% The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe; The composition of each component in the low Gd content alloy raw material is as follows: R3: 5~34 wt.%, R3 is one or more of Nd, Pr, Ho, La, and Ce, and more than 75 wt.% of R3 is Nd; Gd: 0~X wt.%, and the Gd content of high Gd alloy raw materials is -X≥5.0wt.%; B: 0.9wt.%~1.1wt.% M: 0.1wt.%~10.0wt.% The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe; The composition of the obtained magnet is: R: 29.0wt.%~34.0wt.% B: 0.9wt.%~1.1wt.% M: 0.1wt.%~10.0wt.% The balance is T and other unavoidable impurities, wherein T is Fe or Fe and Co, and when T contains Co, more than 75.0 wt.% of T is Fe; The R is composed of Gd and R1, with the Gd element content being 1.0 wt.% to 20.0 wt.% of the magnet mass, and the balance of R being R1; R1 is R3 or is composed of R2 and R3; R3 is at least one of Nd, Pr, Ho, La, and Ce; R2 is at least one of rare earth elements Dy and Tb. When the magnet contains R2, the R2 content is 0.1 wt.% to 2.0 wt.% of the magnet mass, and the Gd-poor shell of the Gd-rich core main phase grain of the magnet contains 0.05 wt.% to 0.5 wt.% of R2 element. M is at least one of Al, Cu, Ga, Zr, Ti, Nb, Zn, Sn, W, Mo, Hf, Au, and Ag; The magnet contains a main phase R2T 14 The magnet contains B and R-rich phases at grain boundaries, and the main phase grains contain Gd-rich core main phase grains with a volume ratio of 20 vol.% to 80 vol.%. The Gd-rich core main phase grains consist of Gd-rich cores and Gd-poor shells. The difference between the core Gd content H1 (wt.%) and the shell Gd content H2 (wt.%) is δH = H1 - H2. δH satisfies δH = (0.05~0.40)R with the total rare earth element content R in the magnet.

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