Method for producing improved an anisotropic magent through extrusion

Abstract

A method for producing an improved anisotropic magnet through an extrusion process wherein the container involved in the extrusion process is first coated, on its internal surface, with a material to prevent the bonding of the interior surface to the particle charge to be extruded and, thereby, to prevent cracking due to tensile stresses. In addition, an improved anisotropic extrusion product, with enhanced magnetic properties, can be obtained by holding the temperature of the extrusion product at temperatures between 1000° F. and 1750° F. for between 1 and 10 hours.

Description

FIELD OF THE INVENTION

[0002] This invention relates to a method for producing, by an extrusion process, an improved anisotropic magnet which is crack-free and has enhanced magnetic properties.

BACKGROUND OF THE INVENTION

[0003] It is known to produce a fully dense permanent magnet alloy particle having crystal alignment through an extrusion process whereby a particle charge of a permanent magnet alloy composition is placed in a container and the container is evacuated, sealed and heated to an elevated temperature. The container is then extruded to achieve crystal alignment and to compact the charge to full density to produce the desired fully dense body. U.S. Pat. No. 4,881,984 (Chandhok et al.) describes such a method.

[0004] In practice however, this method has suffered from disadvantages which have precluded use of the resulting magnet bodies. More specifically, the extruded magnet body has been prone to cracking due to tensile stresses which are set up in the magnet body during cooling of the magnet after the extrusion process. During the extrusion process, a magnet body, which typically is comprised of neodymium-iron-cobalt-boron-gallium (NdFeCoBGa) or prasedymium-iron-cobalt-copper-boron-gallium (PrFeCoCuBGa) alloys, bonds with the container involved in the extrusion process—typically a cylindrical steel can. The coefficient of thermal expansion between the container and the magnet charge are at variance such that the aforementioned tensile stresses are created. During cooling, these stresses result in cracking of the finished product which, in turn, has prevented the use of this process to be used commercially. Further, magnets produced through the extrusion processes of the prior art have lacked enhanced magnetic properties of similar magnets produced by other means.

SUMMARY OF THE INVENTION

[0005] It is the purpose of the present invention, therefore, to provide an improved method for producing an anisotropic magnet through extrusion which reduces the tensile stresses that result in cracking of the finished product. An additional object of this invention is to provide an improved method for producing anisotropic magnet bodies wherein the internal surface of the container involved in the extrusion process is lined with a material to prevent the bonding of the container to the magnet charge and reduce resultant tensile stresses which are created during cooling. Several materials may be used for this coating, including inert materials such as fused silica, high temperature refractory cement and boron nitride. These materials may be applied as a paste or spray. In a preferred embodiment, boron nitride provides a smooth surface on the final magnetic body. Other possible coating materials include zirconia, yttria, ceria and other rare earth oxides and their combinations.

[0006] The improved extrusion process described by this invention has direct application to the production of NdFeB anisotropic magnets. Additionally, the improved extrusion process described by this invention may be used to fabricate hexagonal ferrite magnets (i.e., barium, strontium and lanthanum ferrites) and provide high energy products in the range 2-5 MG Oe and, also, samarium—iron nitrides of the composition Sm2Fe17Nx and MnBi magnets.

[0007] It is the further object of this invention to provide an improved method for producing an anisotropic magnet body through an extrusion process wherein the extrusion product is subjected to a post-extrusion thermal treatment to improve and enhance the permanent magnet properties such as remanence, coercivity and energy product. In particular, the post-extrusion thermal treatment consists of holding the extrusion product at temperatures from 1000° F. to 1750° F. for 1 to 10 hours.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] It is known in the prior art to produce anisotropic magnets through an extrusion process whereby a particle charge of the desired permanent magnet alloy is placed in a container, the container is sealed and heated (along with the charge) and extruded to achieve a magnet having crystal alignment and full density. An example of the prior art in this regard is Chandhok et al. Unfortunately, and as discussed above, the finished product resulting from this methodology has yet to be used commercially because of tensile stresses and cracking which result in this extruded magnet body. Such tensile stresses and related cracking occur because of variances between the coefficient of thermal expansion between the container involved in the extrusion process and the magnetic alloy. More specifically, the charge and resulting magnetic body bond with the container during the extrusion process and, upon cooling after the extrusion process, temperature tensile stresses are set up in the magnetic body which result in cracks.

[0009] The above-described tensile stresses, and resulting cracks in the finished magnetic body, can be prevented through use of a coating on the internal surface of the container involved in the extrusion process. More specifically, in accordance with one embodiment of the invention, the container involved in the extrusion process can be coated with a material to prevent the bonding of the interior surface of the container to the charge and resulting magnetic body. Preferably, the coating is comprised of an inert material from the group consisting of fused silica, high temperature refractory cement and boron nitride. These materials may be applied as a paste or spray. In a preferred embodiment of the present invention, boron nitride is used as the coating because it produces a final magnetic body which has a smoother surface. Additionally, the coating of the internal surface of the container involved in the extrusion process can also be comprised of the group consisting of zirconia, yttria, ceria and other rare earth oxides and their combinations.

[0010] This improved extrusion process has, in a preferred embodiment, application to Nd(Fe,Co) (B,Ga) magnets. However, this improved extrusion process is also applicable to fabrication of hexagonal ferrites (i.e., barium, strontium and lanthanum ferrites) and, in particular, hexagonal ferrites that produce high energy products in the range of 2-5 MG Oe and Sm2Fe17Nx and MnBi magnets.

[0011] Additionally, the magnetic properties of an extruded anisotropic magnet can be improved or enhanced through a post-extrusion thermal treatment. More specifically, in accordance with another embodiment of the present invention, the extrusion product, i.e. the resulting magnetic body, can be subjected to a post-extrusion thermal treatment which consists of holding the extrusion product at temperatures from between 1000° F. to 1750° F. for several hours. Through this post-extrusion treatment, enhanced magnetic properties such as remanence, coercivity and energy product are achieved.

Claims

1. A method for producing an improved anisotropic magnet, said method comprising placing a particle charge of a composition from which said magnet is to be produced in a container, evacuating and sealing said container, heating said container and said particle charge and extruding said container and particle charge, wherein said container, prior to introduction of said particle charge, is coated on the internal surface of said container with a material to prevent the bonding of said interior surface of said container to said particle charge and, thereby, to prevent cracking due to tensile stresses.

2. The method of claim 1 wherein said material is an inert material selected from the group consisting of fused silica, high temperature refractory cement and boron nitride.

3. The method of claim 1 wherein said material is an inert material selected from the group consisting of zirconia, yttria, ceria, other rare earth oxides and combinations of zirconia, yttria, ceria and other rare earth oxides.

4. The method of claim 1 wherein said coating is applied as a paste.

5. The method of claim 1 wherein said coating is applied as a spray.

6. The method of claim 1 wherein said method further includes the step of subjecting the post extrusion product to a thermal treatment which consists of holding the temperature of said extrusion product at temperatures between 1000° F. and 1750° F. to obtain enhanced magnetic properties.

7. The method of claim 6 wherein said thermal treatment is applied for between 1 and 10 hours.

8. A method for producing an improved anisotropic magnet with enhanced magnetic properties, said method comprising placing a particle charge of a composition from which said magnet is to be produced in a container, evacuating and sealing said container, heating said container and said particle charge, extruding said container and particle charge, and subjecting the post extrusion product to a thermal treatment which consists of holding the temperature of said extrusion product at temperatures between 1000° F. and 1750° F.

9. The method of claim 8 wherein said thermal treatment is applied for between 1 and 10 hours.