Temperature stable permanent magnet

Abstract: A rare earth element containing permanent magnet which retains its magnetic properties at elevated temperatures by a combination of reducing the temperature coefficient of intrinsic coercivity lower than -0.2%/°C, and increasing the intrinsic coercivity to over 10 kO e .

Description: Field of the Invention The invention relates to a rare earth element containing permanent magnet which retains its magnetic properties at elevated temperature so that it may be used in applications where elevated temperatures are encountered. Permanent magnets containing one or more rare earth elements and a transition element are well known for use in a variety of magnet applications. These include applications where the assembly with which the magnet is used encounters elevated temperature conditions. These applications include electric motors and magnetic bearings operating in high temperature environments. In these high temperature applications, maximum operating temperatures as high as 400 to 750°C are encountered and magnets employed in these applications must retain their magnetic properties at these temperatures. Description of the Prior Art As may be seen from the magnetic properties set forth in Table 1, the Sm 2 TM 17 demonstrates the best temperature performance relative to the other magnet compositions of Table 1, particularly from the standpoint of energy product at elevated temperature. Historically, studies of Sm 2 TM 17 magnets have been categorized into those relating to remanence and energy product, intrinsic coercivity, and temperature compensation by reducing the coefficient of remanence. Characteristically, remanence is increased by the partial substitution of Co with Fe. Further improvements have been made by controlling the alloy composition and processing. A near zero temperature coefficient of remanence was achieved by the partial substitution of Sm with a heavy rare earth element such as Gd or Er. However, the intrinsic coercivity of magnets of this type decrease sharply with increased temperature up to about 200°C. The intrinsic coercivity is dependent upon the microstructure of these magnets and particularly is a fine cell structure consisting of 2:17 phase cells and cell boundaries of a 1:5 phase. The homogeneous precipitations inside the main phase cells pin the domain wall movement and thus enhance coercivity. The precipitation hardened 2:17 magnets are typically Sm(Co, Fe, Cu, Zr) x with x = 7.2-8.5. The 1:5 cell boundaries impede the domain wall motion which has a similar effect to that of the homogeneous wall pinning. The magnets characterized by low intrinsic coercivity generally exhibit homogeneous wall pinning and high intrinsic coercivity magnets show strong inhomogeneities (mixed pinning). Therefore, the cell structure, cell boundaries, and intercell distance are important factors in determining the coercivity of these magnets. The microstructure is controlled by chemistry and heat treatment. A high coercivity 2:17 magnet is preferred for high temperature applications. A permanent magnet alloy consisting of the composition R(Co 1-x-y-z Fe x Cu y Hf z ) A is disclosed in US 4,172,717. In this alloy R is Sm, Ce or a rare earth element, or a combination thereof. x is equal to or is between 0.01 and 0.40, y is equal to or is between 0.02 and 0.25, z is equal to or is between 0.001 and 0.15, and A is equal to or is between 6.5 and 8.3. Such a permanent magnet is characterised in that the amount of Cu substitution required for a desired degree of precipitation hardening may be decreased by the addition of Hf and the amount of Fe substitution may be increased, whereby a permanent magnet having excellent magnetic characteristics may be obtained. In accordance with the invention, a rare earth element containing permanent magnet having a microstructure comprising a Sm 2 CO 17 phase cell structure and Sm 1 Co 5 phase cell boundaries is provided having a Curie temperature of ≥750°C, a temperature coefficient of intrinsic coercivity of between 0 and -0.2%/°C measured over a temperature range of 21°C to 150°C, intrinsic coercivity at room temperature of ≥10 kOe, remanence at room temperature of ≥8 kG, and an energy product at room temperature of ≥15 MGOe, with a maximum operating temperature of ≥300°C. Preferably, the Curie temperature is ≥800°C, the temperature coefficient of intrinsic coercivity is between 0 and -0.15%/°C measured over a temperature range of 21°C to 150°C, intrinsic coercivity at room temperature is ≥15 kOe, the remanence at room temperature is ≥8 kG, and the energy product at room temperature is ≥15 MGOe, with the maximum operating temperature being ≥500°C. More preferably, the temperature coefficient of intrinsic coercivity is between 0 and -0.10%/°C measured over a temperature range of 21°C to 150°C, the intrinsic coercivity at room temperature is ≥20 kOe, the remanence at room temperature is ≥8 kG, and the energy product at room temperature is ≥15 MGOe, with the maximum operating temperature being ≥700°C. The composition of the alloy preferably is Sm(Co 1-x-y-z Fe x Cu y M z ) w , where w is 6 to 8.5, x is 0.10 to 0.30, y is 0.05 to 0.15, z is 0.01 to 0.04. A heavy rare earth element may be substituted for Sm in an amount up to 50 wt%. M is at least one of Zr, Hf, Ti, Mn, Cr, Nb, Mo, and W. Preferably W is 6.5 to 7.5. The invention is also considered to reside in a method of manufacturing a magnet as defined herinabove. It is accordingly an advantage of the present invention to provide a permanent magnet that exhibits near zero irreversible losses of magnetic properties at temperatures of 400 to 750°C. There now follows a description of preferred embodiments of the invention by way of example, with reference being made to: Figure 1 which is a graph showing irreversible losses of conventional magnets and magnets in accordance with the invention as a function of temperature. Although improving the coercivity of 2:17 magnets (up to about 30 kOe) increase the operating temperature, the maximum operating temperature limit is still about 300°C, which is well below typical high-temperature applications where temperature of 400 to 750°C are encountered. To increase the operating temperature range, it is necessary not only to increase coercivity but also to have a negative temperature coefficient of coercivity that approaches a zero value. Hence, it is necessary to produce a negative, near zero temperature coefficient of coercivity along with increasing the intrinsic coercivity to increase the maximum operating temperature (MOT) over 400°C. Hence, in accordance with this invention, the magnets thereof characterized by enhanced temperature stability have a reduced temperature coefficient of coercivity and high intrinsic coercivity. Specific Examples Four Sm 2 TM 17 magnets were produced and tested, with the compositions reported in Table 2. Chemical Compositions by AT.% of Various 2:17 Alloys Alloy %Sm %Co %Fe %Cu %Zr SM:TM A 11.3 59.8 20.5 6.0 2.0 1:7.8 B 11.7 57.0 24.5 4.8 2.0 1:7.6 C 6Sm/6Ce 58.9 18.8 8.8 1.5 1:7.3 D 12.4 60.2 17.7 7.9 1.8 1:7.0 These alloys were melted in a vacuum induction melting furnace and melts were poured into a copper mold, with respect to alloys A, B, and C, or the melt was atomized into fine powder by the use of an inert gas, with alloy D. The alloys cast into the copper mold upon cooling and solidification were crushed to form powders. The crushed powders from alloys A, B, and C, and the atomized powders of alloy D, were further ground to fine powders having a particle size of about 4 to 8 µm by nitrogen gas jet milling. The milled powders were isostatically pressed while being magnetically aligned. The pressed compacts were sintered at temperatures between 1180-1220°C for 1.5 hours followed by homogenization at temperatures of 1170-1190°C for five hours. The sintered magnets were ground and sliced to form 15 mm diameter and 6 mm thick samples for testing. These samples were aged at 800-850°C for 8 to 16 hours followed by slow cooling. The magnetic properties of the aged magnets were measured at room temperature and at 150°C with a hysteresigraph and a high temperature search coil. The irreversible flux loss was estimated by measuring the flux difference with an Helmholtz coil before and after exposing the magnet to elevated temperatures. The magnet samples were held at temperatures up to 250°C for one hour in a convection oven, and held for six hours each at temperatures of 350, 450, 550, and 650°C, respectively, in a vacuum furnace. The permanence coefficient (Bd/Hd) was 1 because UD was 6/15 = 0.4. The Curie temperature was measured by a VSM. The optimum magnetic properties of most alloys were obtained by sintering at 1200°C, 1175°C homogenization, and 830°C aging cycle. The magnetic properties of these magnet samples were measured at room temperature and are reported in Table 3. Magnetic Properties of Various 2:17 Magnets Alloy B r , kG H ci , kOe H c , kOe H k , kOe BH max , MGOe A 10.0 28.5 9.4 11.2 25.2 B 10.9 2.1 1.5 1.5 12.8 C 9.0 0.7 - - 2.7 D 8.3 18.6 7.9 13.2 16.8 ½A+½C 8.7 17.8 6.4 3.5 15.4 ½B+½D 10.2 31.5 9.5 13.8 25.0 This data establishes that the standard magnet A exhibits a coercivity (28.5 kOe) as high as that achieved conventionally. The Fe-rich, low copper containing magnet B exhibited a high remanence and low coercivity. The Ce substituted alloy magnet C, exhibited both a low remanence and extremely low coercivity. The Cu-enriched, 1:7 magnet sample D, exhibited a low remanence, moderately high intrinsic coercivity, and very good loop squareness. Although alloys B and C produce low coercivity, the magnets of these blended alloys exhibited very high coercivities. Since magnets made from alloys B and C exhibited very low coercivities, there were no further tests of these magnets. Magnets made from alloys A and D and from blends of A + C and B + D were measured at 150°C with the same hysteresigraph. The intrinsic coercivity values at room temperature (21°C) and at 150°C, and the calculated temperature coefficient of intrinsic coercivity between 21 and 150°C are listed in Table 4. Coercivities at Room Temperature and 150°C and Temperature Coefficient of H ci (β) Alloy H ci , Room Temp. H ci , 150°C β (21-150°C) kO e kO e %°C -1 A 28.5 18.0 -0.29 D 18.6 15.5 -0.13 ½A+½C 17.8 8.7 -0.39 ½B+½D 31.5 20.8 -0.26 The typical 2:17 magnet A exhibits a typical temperature coefficient of H ci of about -0.30%/°C while magnet D exhibits an improved value of -0.13%/°C. The irreversible losses of the magnets at various temperatures are listed in Table 5. Irreversible Losses (%) of Magnets A and D After Exposure to Elevated Temperatures Temperature (°C) A D 20 0.00 0.00 150 0.00 0.00 250 -0.46 -0.84 350 -2.61 -2.11 450 -12.75 -2.53 550 -34.10 -3.80 650 -60.00 -14.00 The irreversible losses of magnets A and D are plotted in Figure 1. Magnet A starts to increase with respect to irreversible losses at 350°C, and magnet D at about 550°C. This indicates that although both high intrinsic coercivity and low temperature coefficients of intrinsic coercivity are essential for improving temperature stability, the latter is more effective than the former. The MOT is increased by producing a near zero negative temperature coefficient of intrinsic coercivity. This establishes that the magnet should have a temperature coefficient of coercivity with an absolute magnitude of between 0 and -0.15%/°C and intrinsic coercivity greater than 15 kOe for applications at temperatures of 500°C and higher. The Curie temperature of the magnets A and D, measured with a VSM, are listed in Table 6. Curie Temperature of Magnets A and D Alloy T c (°C) A 825 D 840 The Curie temperatures are over 800°C which is much higher than the desired operating temperature of 500°C. Consequently, a magnet having an MOT over 500°C in accordance with the invention is provided by producing a temperature coefficient of intrinsic coercivity of between 0 and -0.15%/°C and increasing the intrinsic coercivity over 15 kOe. A further increase in MOT over 700°C can be achieved by producing a temperature coefficient of coercivity of between 0 and -0.10%/°C and increasing the intrinsic coercivity greater than 20 kOe. The change in the temperature coefficient of intrinsic coercivity to a value nearer to zero (or the improvement in temperature stability) is due to the suppression of thermally activated domain wall motion, which is related to the microstructure of the magnet. Thus, the temperature stable magnet has a fine composite structure of 2:17 phase cell and thick 1:5 boundaries which consists of Sm, Co, Cu-rich phases. The following definitions are used herein: VSM - vibrating sample magentometer B r - remanence (BH) max - energy product H ci - intrinsic coercivity β - temperature coefficient of coercivity MOT - maximum operating temperature T c - Curie temperature Any feature disclosed hereinabove in relation to one embodiment of the invention may be applied to any other embodiment, if necessary following suitable modification.

Claims: A permanent magnet having a microstructure comprising a Sm 2 Co 17 phase cell structure and Sm 1 Co 5 phase cell boundaries, with a Curie temperature of ≥750°C, a temperature coefficient of intrinsic coercivity between 0 and -0.2%/°C measured over a temperature range of 21°C to 150°C, intrinsic coercivity at room temperature of ≥10 kOe, remanence at room temperature of ≥8 kG, and an energy product at room temperature of ≥15 MGOe, with a maximum operating temperature of ≥300°C. The permanent magnet of Claim 1, wherein the Curie temperature is ≥800°C, the temperature coefficient of intrinsic coercivity is between 0 and -0.15%/°C measured over a temperature range of 21°C to 150°C, the intrinsic coercivity at room temperature is ≥15 kOe, the remanence at room temperature is ≥8 kG, and the energy product at room temperature is ≥15 MGOe, with the maximum operating temperature being ≥500°C. The permanent magnet of Claim 2, wherein the temperature coefficient of intrinsic coercivity is between 0 and -0.10%/°C measured over a temperature range of 21°C to 150°C, the intrinsic coercivity at room temperature is ≥20 kOe, the remanence at room temperature is ≥8 kG, and the energy product at room temperature is ≥15 MGOe, with the maximum operating temperature being ≥700°C. The permanent magnet of any preceding claim, consisting essentially of Sm(Co 1-x-y-z Fe x Cu y M z ) w , where w is 6 to 8.5, x is 0.10 to 0.30, y is 0.05 to 0.15, z is 0.01 to 0.04, wherein a heavy rare earth element may be substituted for Sm in an amount up to 50 wt%, M is at least one Zr, Hf, Ti, Mn, Cr, Nb, Mo and W. The permanent magnet alloy of Claim 4, wherein w is 6.5 to 7.5. A rare earth element containing permanent magnet having a microstructure comprising a Sm 2 Co 17 phase cell structure and Sm 1 Co 5 phase cell boundaries, having a Curie temperature of ≥750°C, a temperature coefficient of intrinsic coercivity of between 0 and -0.2%/°C measured over a temperature range of 21°C to 150°C, intrinsic coercivity at room temperature of ≥10 kOe, remanence at room temperature of ≥8 kG, and an energy product at room temperature of ≥ 15 MGOe, with a maximum operating temperature of ≥300°C.

Inventors: KIM ANDREW S, KIM, ANDREW S.
Applicants: CRUMAX MAGNETICS INC, CRUMAX MAGNETICS INC.
Application year: 1996
Main classification: H01F1
IPC codes: H01F1