Rapid consolidation method for preparing bulk metastable iron-rich materials

Abstract

Interstitially modified compounds of rare earth element-containing, iron-rich compounds may be synthesized with a ThMn.sub.12 tetragonal crystal structure such that the compounds have useful permanent magnet properties. It is difficult to consolidate particles of the compounds into a bulk shape without altering the composition and magnetic properties of the metastable material. A combination of thermal analysis and crystal structure analysis of each compound may be used to establish heating and consolidation parameters for sintering of the particles into useful magnet shapes.

Claims

1. A method of forming a bulk magnet shape by consolidation of particles having permanent magnet properties, the method comprising: providing particles of a compound expressed by the formula (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y, in which compound the value of x is in the range [0, 1], R is an element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y; w is in the range [0.1, 0.3], the element M is one or more of Mo, Ti, V, Cr, B, Al, Si, P, S, Sc, Co, Ni, Zn, Ga, Ge, Zr, Nb, Hf, Ta, and W, and the value of y is in the range [1, 4], the particles of the compound having a tetragonal crystal structure, corresponding to the ThMn.sub.12 tetragonal crystal structure, and permanent magnet properties; determining a heating temperature, heating period, and compaction pressure at which a volume of the particles of the compound may be consolidated under pressure into a bulk magnet shape, having a density no less than 90% of the density of the original particles, without decomposition of the compound or loss of its tetragonal crystal structure or permanent magnet properties, the determination of the heating temperature, heating period, and compaction pressure for the heating and compaction of the particles of a specific compound into a bulk magnetic shape comprising both thermogravimetric analysis and differential scanning calorimetry analysis of the particles and analysis of the crystal structure of the particles processed by the thermogravimetric and differential scanning calorimetry analyses; and confining a volume of the particles in a die for forming the bulk magnet shape and applying the predetermined compaction pressure for consolidation of the particles while passing a pulsing direct current through the confined volume of particles to heat the particles to the predetermined heating temperature and for the predetermined heating time to produce the consolidated bulk magnet shape while retaining the permanent magnet properties of the original particles of the compound.

2. The method of claim 1 wherein the value of x of the (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y compound is in the range [0.6, 1], the value of w is in the range [0.05, 0.15], and the value of y is in the range of [1, 2].

3. The method of claim 1 wherein the particles of the (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y compound have maximum dimensions no greater than forty-five micrometers.

4. The method of claim 1 wherein R is Nd and M is molybdenum in the particles of the (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y compound.

5. The method of claim 4 wherein the value of x of the (Ce.sub.1xNd.sub.x).sub.1+wFe.sub.12yMo.sub.y compound is in the range [0.6, 1], the value of w is in the range [0.05, 0.15], and the value of y is in the range of [1, 2].

6. The method of claim 1 in which the heating period at the selected heating temperature is no more than ten minutes.

7. The method of claim 1 in which the particles of the (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y compound are provided by preparing a molten stoichiometric mixture of the selected proportions of the selected Ce, R, Fe, and M elements of the compound, solidifying the mixture to an ingot, and comminuting the ingot to form particles having maximum dimensions no greater than about forty-five micrometers.

8. The method of claim 1 in which the particles of the (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y compound are provided by preparing a molten stoichiometric mixture of the selected proportions of the selected Ce, R, Fe, and M elements of the compound, solidifying the mixture by melt spinning of the molten mixture to form ribbons or particles of the compound, and comminuting the ribbons or particles to smaller particles having maximum dimensions no greater than about forty-five micrometers.

9. A method of forming a bulk magnet shape by consolidation of particles having permanent magnet properties, the method comprising: providing particles of a compound expressed by the formula (Ce.sub.1xNd.sub.x).sub.1+wFe.sub.12yMo.sub.y, in which compound the value of x is in the range [0, 1], w is in the range [0.1, 0.3], and the value of y is in the range [1, 4], the particles of the compound having a tetragonal crystal structure, corresponding to the ThMn.sub.12 tetragonal crystal structure, and permanent magnet properties; determining a heating temperature, heating period, and compaction pressure at which a volume of the particles of the compound may be consolidated under pressure into a bulk magnet shape, having a density no less than 90% of the density of the original particles, without decomposition of the compound or loss of its tetragonal crystal structure or permanent magnet properties, the determination of the heating temperature, heating period, and compaction pressure for the heating and compaction of the particles of a specific compound into a bulk magnetic shape comprising both thermogravimetric analysis and differential scanning calorimetry analysis of the particles and analysis of the crystal structure of the particles processed by the thermogravimetric and differential scanning calorimetry analyses; and confining a volume of the particles in a die for forming the bulk magnet shape and applying the predetermined compaction pressure for consolidation of the particles while passing a pulsing direct current through the confined volume of particles to heat the particles to the predetermined heating temperature and for the predetermined heating time to produce the consolidated bulk magnet shape while retaining the permanent magnet properties of the original particles of the compound.

10. The method of claim 9 wherein the value of x of the (Ce.sub.1xNd.sub.x).sub.1+wFe.sub.12yMo.sub.y compound is in the range [0.6, 1], the value of w is in the range [0.05, 0.15], and the value of y is in the range of [1, 2].

11. The method of claim 9 wherein the particles of the (Ce.sub.1xNd.sub.x).sub.1+wFe.sub.12yMo.sub.y compound have maximum dimensions no greater than forty-five micrometers.

12. The method of claim 9 in which the heating period at the selected heating temperature is no more than ten minutes.

13. The method of claim 9 in which an electron microscopy characterization is used in the crystal structure analysis of particles of a specific compound which were subjected to the thermogravimetric analysis and the differential scanning calorimetry analysis.

14. The method of claim 9 in which the particles of the (Ce.sub.1xNd.sub.x).sub.1+wFe.sub.12yMo.sub.y compound are provided by preparing a molten stoichiometric mixture of the selected proportions of the Ce, Nd, Fe, and Mo elements of the compound, solidifying the mixture to an ingot, and comminuting the ingot to form particles of the compound having maximum dimensions no greater than about forty-five micrometers.

15. The method of claim 9 in which the particles of the (Ce.sub.1xNd.sub.x).sub.1+wFe.sub.12yMo.sub.y compound are provided by preparing a molten stoichiometric mixture of the selected proportions of the Ce, Nd, Fe, and Mo elements of the compound, solidifying the mixture by melt spinning of the molten mixture to form ribbons or particles of the compound, and comminuting the ribbons or particles to smaller particles of the compound having maximum dimensions no greater than about forty-five micrometers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic, front elevation view of a die with a cylindrical cavity in which interstitially-modified rare earth-iron magnet powder with ThMn.sub.12 type crystal structure is compacted in the round cylindrical cavity of a die between diametrically opposing, upper and lower punches. The die cavity is enclosed so as to provide and maintain the powder in an oxygen-free environment. Means is provided for detecting the temperature of the magnet powder and for passing a pulsed direct current directly through the compacted powder to quickly sinter it into a dense cylinder magnet body.

(2) FIG. 2 is a graph displaying methods of thermal analysis of the compound, (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3, over temperatures (abscissa) in the range from about room temperature to about 800 C., using differential scanning calorimetry (DSC, left ordinate, in arbitrary units) and thermogravimetry analysis (TGA, right ordinate, in arbitrary units). The DSC curves show the heat flow into or out of the specimen during heating. The TGA curve indicates changes in the weight of the sample with increasing temperature. The four boxes, inserted on the face of the graph, are x-ray diffraction patterns, respectively, of the as-nitrided sample after the nitriding treatment but before heating, the sample after heating at 432 C., the sample after heating at 560 C., and the sample after heating at 800 C. The inverted triangle symbol on each of the four x-ray diffraction patterns identify diffraction peaks indicative of the presence of an iron-molybdenum (FeMo) impurity phase resulting from decomposition of the original (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 compound with the required 1-12 phase.

(3) FIG. 3 (a) displays room temperature demagnetization curves for as-nitrided (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 powder prior to consolidation and for bulk magnets made by SPS.

(4) FIG. 3 (b) is the demagnetization curve of the spark plasma sintered bulk (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 magnet SPS-600 measured at 400 K (127 C.).

DESCRIPTION OF PREFERRED EMBODIMENTS

(5) Interstitially modified rare earth-iron magnet powder with ThMn.sub.12 type crystal structure is prepared in the form of (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z in which suitable R elements, M elements, and N elements are described and specified in the Summary section of this specification. Suitable and preferred value ranges for x, w, y, and z are also specified in the Summary section. As stated, in the case of many compounds, the formed powder particles of the (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z compound will not retain their essential 1-12 crystal structure if they are overheated or retained at an elevated temperature too long. A compacted volume of the prepared rare earth-iron magnet powder is consolidated into a densified bulk magnet body using a sintering process in which a pulsed direct electric current (DC) is passed directly through the compressed body of powder as it is held and compacted in a forming die. A suitable spark plasma sintering process may be used to consolidate the powder and retain substantially the same permanent magnet properties produced in the original powder.

(6) In a specific illustrative example, a selected preformed (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y compound powder or a selected preformed (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z powder, either having the 1-12 crystal structure, is loaded in a graphite or metal die and consolidated by a Spark Plasma Sintering (SPS) technique as described herein. Compared to other consolidation methods such as liquid phase sintering or hot pressing, SPS uses the joule heating from high pulsed DC electric current directly passed through the green compact, thereby enabling the rapid sintering of dense samples at reduced temperature. The compound powder is held under pressures of, for example, 60-120 MPa while the holding time at the selected maximum sintering temperature is up to five to ten minutes. For example, the DC current is suitably pulsed at a rate of, e.g., 70 Hertz, with a pulse duration of 12 ms, and a 2 ms pause. Current flow is controlled so as to quickly heat the compacted powder to a predetermined temperature level and no higher. For example, the temperature of the compacted powder may be increased at rates of 50 to 150 Celsius degrees per minute. The rapid sintering rate and reduced sintering temperature make SPS suitable for consolidating the metastable (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y or (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z magnet powder that is susceptible to decomposition when protractedly exposed to elevated temperature.

(7) An example of a SPS type sintering apparatus 10 for sintering the metastable modified rare earth-iron powder is illustrated in FIG. 1. In this illustration, sintering apparatus 10 comprises a round graphite die 12 with a vertical open-ended round cylindrical cavity 14 sized for holding a predetermined volume of the metastable RFe-M or RFe-M-N powder 16. In an illustrative example described below in this specification the composition of the powder was (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3.

(8) The lower end of vertical cavity 14 was closed by the round shaft 20 of lower stainless steel punch 18. Round shaft 20 was sized to fit closely, but movably, in die cavity 14 for applying compaction pressure and, if desired, to conduct DC electrical current to the volume of rare earth-iron compound powder 16. Shaft 20 supported the lower portion of the volume of rare earth-iron powder 16. Punch 18 also has a larger diameter round head 22 for application of pressure (and if desired an electrical current) to the volume of powder 16. Upper stainless steel punch 24 was sized and shaped like lower punch 22. Upper punch 24 comprised round shaft 26 and round head 28 which served functions complementary, but directionally opposing, to punch 22. The cross-hatched rectangle indicates the potential use of a chamber 34, or the like, around the powder volume 16 for isolating it from an oxidizing atmosphere or other atmosphere that could alter the composition and crystal structure of the modified rare earth-iron composition being compacted. Chamber 34 may be evacuated to a suitable level of vacuum or back-filled with a protective, non-oxidizing gas such as, for example, nitrogen or argon.

(9) Means indicated by un-filled arrows 36 is provided to provide a very substantial compacting force (e.g., 60 MPa to 110 MPa) to punches 20, 26. And means 32 is provided to direct a substantial pulsed DC current (indicated by solid lines with a directional arrow leading to punches 18, 24) through the powder volume 16 to directly heat the powder as pressure is applied to the powder by the opposing compacting action of punches 20, 26. Also, a thermocouple 38, or other suitable temperature sensing means, may be placed in the die for timely and continuous sensing of the temperature of the powder 16 as it is being compacted and sintered. Such temperature measurements may be used to manage the amount and duration of pulsed DC current through the powder 16 as it is being consolidated without altering its composition or crystal structure, or appreciably diminishing the magnetic properties of the powder placed in the die. At the completion of the SPS sintering process the current flow is stopped, the punches 20, 26 opened, and a shaped bulk permanent magnet body removed from cavity 14.

(10) As an illustrative example, a powder of the composition, (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5, was prepared, having the 1-12 tetragonal crystal structure. The composition was to be subsequently nitrogenated. It was found that in order to develop hard magnetic properties of the described (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z compounds with 1-12 tetragonal structure, it was necessary to form the compound by a rapid solidification process, specifically by melt spinning.

(11) Melt-spun ribbons of (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5 were prepared by induction melting a stoichiometric mixture of pure elements (Ce, Nd, Fe, and Mo) into a homogeneous liquid volume. The liquid volume was formed in a suitable round bottom container, adapted to permit the controlled or measured withdrawal of a stream of the liquid from the bottom of the container. Then, a fine liquid stream was continually drained downwardly from the container of the liquid onto the circumferential rim of a 10 inch diameter, Cr plated, Cu wheel rotating at a surface wheel speed v.sub.s=17.5 m/s. In such melt spinning operations, the flow rate of the descending molten liquid stream and the speed and mass of the quench wheel stream are coordinated to obtain a suitable rate of solidification of the liquid. The molten liquid volume was thus progressively rapidly quenched upon contact of the liquid stream with the rim of the spinning wheel to produce small, fragmented, solidified ribbons of the starting composition which were collected as they were thrown from the quench surface of the wheel. A relatively small volume of the molten liquid was prepared in this example, and it was not necessary to cool the rotating copper wheel because the volume of liquid was all solidified before the relatively massive copper wheel was appreciably heated above its initial ambient temperature. In processing a substantial volume of the molten rare earth-iron compound, however, it may be necessary to cool the quench wheel to assure suitably rapid solidification of the molten stream to obtain the necessary 1-12 crystal structure.

(12) After cooling to ambient temperature, the collected ribbon particles were ball milled under argon and sieved to a particle size smaller than 45 m prior to nitriding. Nitriding, using pure nitrogen gas, was performed on the powder which had been placed in a Hiden Isochema Intelligent Gravimetric Analyzer (IGA). The nitriding parameters for (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5 were: nitriding pressure P=10 bar, time t=34 h, and temperature T=500 C. The nitrogen absorption is calculated from the weight difference before and after nitriding, assuming all nitrogen atoms go into the 1-12 phase. The nitride compound, (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 was formed. The particle size of the starting compound, (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5, was not appreciably increased by the addition of nitrogen, and the particles (powder) of the nitrided compound were considered ready for compaction.

(13) When the magnetic compound is one with which there is no previous sintering experience, it is preferred (and usually necessary) to conduct thermal evaluation analyses and crystal structure analyses and compositional analyses of sample portions of the powder of a selected (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z composition before spark plasma sinter processing the main portion of the powder in order to determine the temperature limit that will retain the 1-12 crystal structure and the permanent magnet properties in the consolidated bulk magnet body. Examples of such thermal and compositional analyses will be illustrated in the making of bulk magnets of the rapidly solidified and nitride powders of the (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 composition.

(14) In summary, test sample bulk magnets of nominal composition (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 were sintered by a managed spark plasma sintering process in the temperature range of 550-700 C., compaction pressure range of 60-104 MPa, and using either nitrogen or argon as a protective atmosphere. The processing parameters and properties of the sintered compounds are summarized in the Table below in this specification. But, importantly, it was first necessary to predetermine sintering conditions for consolidation of the (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 powder without altering the composition or crystal structure of the compound with the 1-12 crystal structure.

(15) A combination of experimental techniques such as thermal and X-ray diffraction analysis and theoretical calculation based on a metal diffusion model have been used in order to establish the limits of sintering temperature. FIG. 2 displays the differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) results, together with X-ray diffraction patterns at temperatures corresponding to potential thermal events of the (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 powder.

(16) As can be seen from FIG. 2, the DSC cycle 1 curve shows a broad exothermic peak that disappears in the DSC second cycle, but which lacks well defined sharp peaks throughout the heating from about 50 C. to 700 C. In FIG. 2, the arrow labeled Exo marks the direction of exothermic transformation, the magnitude of which is indicated in arbitrary units. From derivatives of the DSC cycle 1 curve, two inflection points were identified near 462 C. and 520 C. The DSC results are consistent with the TGA analysis.

(17) X-ray analysis of post thermal cycling samples at the temperatures identified by TGA revealed no noticeable phase change in samples of the compound after heat treatment at 432 C. But X-ray analyses revealed a slight increase of a FeMo impurity phase at 560 C., and decomposition of the 1-12 phase at 800 C. These findings suggested that the decomposition of (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z is a kinetic process whose rate is determined by the diffusion of dominant metal element Fe.

(18) Starting at the second inflection point of 520 C. identified in the DSC curve (shown in FIG. 2), samples of the as-nitrided, (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3, powder were annealed for 3, 9, 27, and 81 minutes, respectively, and X-ray diffraction patterns of the annealed samples were prepared and analyzed. Then a calculation was made of the annealing temperature T that would make a Fe atom diffuse the same distance in 3 min as it does in 81 min when annealed at 520 C., using the equation:
2(Dt)|.sub.t=81 min,T=520 C.2(Dt)|.sub.t=3 min,T=596 C.
where D=D.sub.0 exp(E.sub.a/kT) is the diffusion coefficient at temperature T, D.sub.0=1.0 mm.sup.2/s, E.sub.a=250 kJ/mol is the activation energy, and t is time. In this way, it can be estimated that annealing at 596 C. for 3 min is equivalent to annealing at 520 C. for 81 min, and annealing at 687 C. for 3 min is equivalent to annealing at 596 C. for 81 min. Samples of (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 powder were repeatedly annealed for 3-81 min at increasing temperature set points estimated by the above method until significant FeMo impurity phase, the byproduct of 1-12 phase decomposition, could be observed in the X-ray diffraction pattern.

(19) In furtherance of the thermal analysis, a series of X-ray diffraction patterns were obtained after annealing for periods of 3 minutes, 9 minutes, 27 minutes and 81 minutes at each of 520 C. (793 K), 596 C. (869 K), and 687 C. (960 K), respectively. Analysis of the respective patterns showed that (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 is stable at 520 C. and that the diffraction pattern after 81 min heating showed no noticeable difference compared to that of the as-nitrided sample. Annealing at 596 C. accelerates the decomposition process as the intensity of the FeMo peak shows a small but discernible increase with increasing annealing time. At 687 C., (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 decomposes at a much faster pace as characteristic peaks associated with the unwanted FeMo phase can be easily observed after only 3 min.

(20) The above-described annealing tests suggested that there exists an opportunity window to sinter (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 up to 687 C. and the bulk magnet may retain reasonable extrinsic magnetic properties if the sample can be sintered in a few minutes. It is for this reason that SPS is chosen to consolidate (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3, as heating and cooling rates of up to 1000 C./min can be achieved in this advanced sintering method.

(21) A series of powder samples were sintered by SPS at temperatures in the range of 500-700 C. and X-ray diffraction patterns of bulk (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 magnets were obtained. It was found that the bulk magnets sintered between 550 and 650 C. maintained a major 1-12 phase, while the one sintered above 675 C. showed significant decomposition into FeMo and Fe based nitrides.

(22) To better assess the phase change during annealing and SPS, Bruker Diffrac Plus Evaluation software was used to analyze the diffraction patterns obtained on the sintered samples and to plot the semi-quantitative phase percentage as functions of holding time and heating temperature. It was concluded that at sinter temperatures below or at 596 C. (869 K), (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 powder exhibits good resistance to decomposition. The 1-12 phase accounts for over 96 wt % in the alloy even after the most severe 81 min annealing. (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 shows much stronger inclination to decompose at 687 C. (960 K). After having been heated for 81 min, over 30 wt % of 1-12 phase has decomposed into impurity phases such as FeMo and Fe nitrides, and 1-12 phase is less than 70 wt % in the alloy.

(23) Sintered magnets deviate from the decomposition trend lines of the powder and show greater propensity to decompose at lower temperature due to (1) the simple Fe diffusion model used for powder samples assumed atmospheric pressure, while the applied ram pressure of 60 MPa could be a contributing factor to induce a higher Fe diffusion rate during the sintering process; (2) the inhomogeneous temperature field in the green compact during the heating stage may accelerate the decomposition process; and (3) the thermal stability test was performed in an Ar protected environment while SPS was carried out in N.sub.2. The more rapid degradation during SPS compared to heating the powder emphasizes the need to minimize time and temperature exposure during consolidation.

(24) Portions of the (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 powder were used in a spark plasma sintering process using a die section and a sintering apparatus like that described in connection with FIG. 1. The pulsed DC current was passed through the compacted powder to rapidly heat the powder to predetermined temperatures of 550 C., 600 C., 650 C., 675 C., and 700 C. In the forming of the bulk magnets of this compound, the typical dwelling time at the selected maximum temperature for the sintering was five minutes. Each densified bulk magnet shape was then removed from its forming die. The pressure applied to the powder was 60 MPa except for a pressure of 104 MPa used in forming a comparative sample at 600 C. The formed bulk magnet pieces were 3 mm in diameter and 1.2 to 1.7 mm in height.

(25) The following Table summarizes the physical and extrinsic magnetic properties of bulk (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 magnets. A sintering temperature of 600 C. or greater is needed to obtain a dense sample with over 90% of theoretical density. However, when the sintering temperature is greater than 675 C., the magnetic properties worsen precipitously. As expected, increasing pressure is helpful to improve density and is a better alternative in place of higher sintering temperature to retain the desired 1-12 phase. In one example (sintering temperature of 675 C.*) it was found that changing the protective inert gas from nitrogen to argon for the sintering resulted in slightly improved coercivity in the bulk magnet.

(26) TABLE-US-00001 TABLE T.sub.sinter P P.sub.rel (BH).sub.max B.sub.r H.sub.ci 4M.sub.19 ( C.) (MPa) (g/cm.sup.3) (%) (MGOe) (kG) (kOe) (kG) Powder NA 8.48 NA 5.32 6.54 3.13 9.13 550 60 6.58 77.6% 5.11 6.59 3.22 8.92 600 60 7.94 93.6% 4.98 6.58 3.34 9.12 600 104 8.05 94.9% 4.97 6.62 3.40 9.21 650 60 7.70 90.8% 4.61 6.60 3.11 9.18 675 60 8.12 95.7% 3.94 7.22 2.07 9.78 675* 60 7.98 94.1% 3.73 6.83 2.41 9.52 700 60 7.66 90.3% 3.29 9.94 0.45 12.21

(27) The values of 4M were obtained at the highest magnetic field of 19 kOe.

(28) FIG. 3 (a) displays room temperature demagnetization curves for as-nitrided (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 powder prior to consolidation and bulk magnets made by SPS. The respective demagnetization curves are for the bulk magnets prepared by SPS as described above and in the Table and sintered at 550, 600, 650, 675, and 700 degrees Celsius. Each of the bulk magnets is believed to be magnetically isotropic. As seen in FIG. 3(a), except for the samples sintered at or above 675 C., SPS samples have identical demagnetization curves as that of the as-nitrided starting powder, indicating SPS is a viable technique to consolidate metastable 1-12 nitrides.

(29) FIG. 3 (b) is the demagnetization curve of the best performing (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 magnet SPS-600 (the third entry in the above Table) at 400 K (127 C.). Using the modified Stoner-Wohlfarth model, we estimated that the uniaxial anisotropy H.sub.a is no less than 3.2 T at 127 C. (400 K). Curie temperature T.sub.c of the bulk magnet is 600 K, the same as that of the as-nitrided powder.

(30) In conclusion, metastable (Ce.sub.0.2Nd.sub.0.8).sub.1.1Fe.sub.10.5Mo.sub.1.5N.sub.1.3 has been successfully consolidated using a rapid sintering technique SPS. The parameters of the sintering process were devised using selected thermal stability tests. In the case of the selected compound, the tests indicated an opportunity window for sintering the nitrides below 687 C. on the time scale of few minutes. It was also found that the actual SPS sintering conditions increased the propensity for decomposition and lowered the upper sintering temperature limit. The described experimental results indicated a sintering temperature between 600-650 C. was suitable for obtaining dense samples with excellent room temperature magnetic properties. At room temperature, the best performing bulk magnet is 95% dense and has H.sub.ci=3.4 kOe, remanence B.sub.r=6.6 kG, magnetization 4M=9.2 kG, and energy product (BH).sub.max=5.0 MGOe. At elevated temperature of 127 C. (400 K), the sample possesses H.sub.ci=1.6 kOe, H.sub.a3.2 T, and 4M=9.2 kG.

(31) In accordance with practices of this invention, a group of (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.y compounds and of (Ce.sub.1xR.sub.x).sub.1+wFe.sub.12yM.sub.yN.sub.z compounds can be formed in the form of powder particles having 1-12 tetragonal crystal structures and permanent magnet properties. But the respective particulate compounds could be metastable and tend to decompose upon standard processes for consolidation of the particles into bulk shapes for magnet applications. Particles of each of the respective compounds may be thermally analyzed to determine suitable sintering conditions for consolidation of the particulate compounds by a suitable spark plasma sintering process into useful magnet shapes.

(32) The effects of heating temperatures, heating times, and consolidation pressures on small particles of the respective compounds may be analyzed using practices such as differential scanning calorimetric analysis (DSC) and thermal gravimetric analysis (TGA). The effects of the heating tests on the test samples may be evaluated, for example, by analysis of the crystal structure of the compounds after heating. X-ray diffraction or other electron microscopy may be used to assess phase changes and changes in crystal structure. Also it is found that the use of diffusion models, especially models directed at the diffusion rate of iron, are useful in arriving at suitable conditions for SPS processing of particles of the respective compounds.

(33) Practices of the invention have been illustrated by the use of specific examples which are not intended to limit the scope of the following claims.