Rapid consolidation method for preparing bulk metastable iron-rich materials
Interstitially modified compounds of rare earth element-containing, iron-rich compounds may be synthesized with a ThMn 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.
1. A method of forming a bulk magnet shape by consolidation, the method comprising:
providing particles of a compound expressed by formula R 1+w Fe 12−y M y N z , R is one or more elements selected from the group consisting of Ce, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, w is in the range [−0.1, 0.3], M is one or more elements selected from the group consisting of Mo, Ti, V, Cr, B, Al, Si, P, S, Sc, Ti, V, Co, Ni, Zn, Ga, Ge, Zr, Nb, Hf, Ta, and W; the value of y is in the range [1, 4], N is nitrogen, the value of z is in the range [0.1, 3], and the particles of the compound have a tetragonal crystal structure, corresponding to ThMn 12 tetragonal crystal structure, and permanent magnet properties,
determining a heating temperature, heating period, and consolidation pressure at which a volume of the particles of the compound is 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 heating and compaction of the particles of the compound into a bulk magnet shape comprising both thermogravimetric analysis and differential scanning calorimetry analysis of the particles and analysis of the crystal structure of particles processed by the thermogravimetric and differential scanning calorimetry analyses,
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 bulk magnet shape while retaining the permanent magnet properties of the original particles.
2. The method of claim 1 wherein R is a combination of Ce and Nd and M is molybdenum.
3. The method of claim 1 wherein the value of w is in the range [0.05, 0.15], the value of y is in the range of [1, 2], and the value of z is on the range of [0.5, 1.5].
4. The method of claim 1 wherein the particles of the R 1+w Fe 12−y M y N z compound have maximum dimensions no greater than forty-five micrometers.
5. The method of claim 1 wherein the compound is formed by the reaction of nitrogen gas with particles of a previously formed R 1+w Fe 12−y M y compound without increasing the maximum dimensions of the particles to values greater than forty-five micrometers.
6. The method of claim 1 in which an electron microscopy characterization is used in crystal structure analysis of the particles of a the compound which were subjected to the thermogravimetric and differential scanning calorimetry analyses.
7. The method of claim 1 in which the heating period at the selected heating temperature is no more than ten minutes.