DIRECTIONAL SOLIDIFICATION OF BI/MNBI COMPOSITIONS

Abstract: The invention provides a method for the directional solidification of Bi/MnBi compositions to produce magnetic single domain size MnBi particles, in aligned array, having a particle size of less than 1 micron and having an Mn concentration of up to 50 atomic percent, which comprises directionally solidifying a composition containing Mn and Bi, said composition containing from 0.1 to 50% by atomic fraction Mn, at a solidifying rate of 0.5 cm/h and at a thermal gradient, in the liquid at the liquid-solid interface, of from 10°C/cm to 1000°C/Cm.

Description: DIRECTIONAL SOLIDIFICATION OF Bi/}hBi COMPOSITIONS This invention relates to the improved permanent magnet properties of MnBi rods, in the equilibrium phase (LTP) grown by directional solidification. Moreover, this invention relates to AMBi rods which contain a high coercivity or IIC phase which provides exceptionally high permanent magnet properties at cryogenic temperature and can serve as a magnetic switch. The aforesaid high coercivity or HC phase is found to coexist with the LTP phase. The novelly produced BiftinBi rods of this invention serve as permanent magnet material for temperatures in the range of -5O0C to +l500C and at cryogenic temperature. These permanent magnet properties are comparable to presently used sintered rare earth-cobalt permanent magnets and can be mass produced at reduced costs. The permanent magnet (ferromagnetic) character of MnBi has long been established by a number of investigators. The MnBi intermetallic, which has a hexagonal NiAs crystal structure, possesses one of the highest magnetocrystalline anisotropiss gl = 9.1 x 106 erg/cm3, K2= 2.6 x 106 erg/cm3) and magnetostrictions ( = -800 x 10 ) of any material other than the rare earthcobalt alloys. In order to optimize the permanent magnet figures of merit for this system, i.e. remanent induction, intrinsic coercivity and energy product: (1) the magnetic moment of each magnetic region or domain should be aligned with every other region such that each moment lies along the easy axis of magnetization and (2) the average domain size should be reduced below a critical size at which a singledomain structure is of lower energy (and hence more probable to exist) than a multi-domain structure and below which magnetization reversal by coherent rotation is favoured over domain wall motion, reversed domain nucleation or incoherent curling processes. However, the permanent magnet figures of merit may also be a sensitive function of the shape of the magnetic domain (shape anisotropy), the elastic deformation of a domain under application of an external magnetic field (magnetostriction), local demagnetizing fields at surface irregularities or inclusions of the domains (strain), local elastic strains near dislocations and other lattice defects (magnetostriction, anti-ferromagnetic coupling, domain wall pinning), a local Bi-rich region produced by preferential oxidation of Mn (local reduction in magnetocrystalline anisotropy) or the coexistence of a second magnetic phase which could be ferromagnetic (magnetic coupling) or paramagnetic (ultra fine WInBi particles). Previous attempts to produce permanent magnets using MnBi have involved hot-pressing of powder in a very high external magnetic field which presumably aligns the eas axis of magnetization of different particles. The coercive field for MnBi powder was found to be extremely sensitive to average particle size, varying from intrinsic coercivity mRc of 600 Oe for 100 pm diameter particles to 12000 Oe for particles less than 3 ,um in size. Unfortunatel, due to the shape and deformation of the powder crystallites, permanent magnet properties achieved with MnBi powder are: remanent induction (Br) = 4300 gauss intrinsic coercivity (mac) = 3400 Oe maximum static energy product (B11) max 4.3 Mg-Oe at room temperature as compared with the theoretical maximum of: Br - 8200 gauss H = 35000 Oe (BH) max = 17 MG-Oe We have used a different approach involving plane-front directional solidification to achieve particles of MnBi whose size is an order of magnitude less ((0.5 than those obtained previously by grinding and crushing. Accordingly, this invention involves a method for the directional solidification of Bi/MnBi compositions to produce magnetic single domain size MnBi particles, in aligned array, having a particle size of less than 1 micron and having an Mn concentration of up to 50 atomic percent which comprises directionally solidifying a composition containing Mn and Bi, said composition containing from 0.1 to 50% by atomic fraction Mn, at a solidifying rate of 0.5 cm/h to 50 cm/h and at a thermal gradient, in the liquid at the liquid-solid interface, of from lO0C/cm to 10000C/cm. In one embodiment of the above process, a magnetic single domain size MnBi particle composite is obtained which contains about 0.72% by weight Mn (eutectic) which employs a solidifying rate of 30 cm/h at a thermal gradient of 1000C/cm. In another embodiment of the above process, a magnetic single domain size MnBi particle composite is obtained which contains about 0.72 to 20.8% by weight Mn (Mn rich off-eutectic) which employs a solidifying rate of 0.5 cm/h to 50 cm/h and a thermal gradient of from 100 to 10000C/cm. In another embodiment, the resulting MnBi composite is post-heat treated in a temperature range of from 2000C to 2600C for a period of from 1 to 100 hr. under an inert atmosphere. In yet another embodiment, this invention is directed to the Mn-Bi composite having improved permanent magnetic properties comprising single domain size MnBi particles, in aligned array, having a particle size of less than 1 micron and an Mn concentration of up to 50% by atomic fraction, said composite having one ferromagnetic MnBi phase and the post heat treated composites thereof. The inventive method disclosed herein involves the directional solidification at various growth velocities (0.5 cm/hr to 100 cm/hr) and improved thermal gradients (10 C/cm to 1000 C/cm) of eutectic and off-eutectic Bi/mNBi resulting in MnBi rods characterised by near theoretical permanent magnet properties after post-heat treatment. SAtIl'LE PRE1'StATION AND PROCESSING Samples were prepared using high purity Mn (99.9)and Bi (99.999) which were melted together in an evacuated quartz crucible above 446 C, the temperature at which the stoichiomet ic MnBi compound forms. The smarting proport- iOHS were Mn-rich. Portions of these starting boules were then remelted in a specially designed evacuated quartz crucible and directionally solidified at various growth velocities and thermal gradients using the Bridgman- Stockbarger technique. Control1d directional solidification of Mn-Bi produces an aligned array of chevrontype cross-section shaped MnBi rods whose average 1 effective rod diameter follows a (growth velocity) 2 law in thermal gradients varying from lO0C/cm to 10000C/cm. MAGNETIC PROPERTIES Magnetization, measured parallel to the growth direction for MlBi and parallel as well as perpendicular for Bi, as a function of applied field (-230 to 230 kG) at temperatures of 4.2, 77, 295 and 538 K was used to characterize the magnetic properties. At room temperature, there is ample experimental evidence for the existence of two ferromagnetic MnBi phases. One phase, the low-temperature (LTF) pr equilibrium MnBi phase has an anisotropy energy which is a monotonically decreasing function below 500 K with an expected intrinsic coercivity of 30 kOe for Stoner-Wohlfarth single domain, coherent rotation particles at room temperature. The other so-called quenched-high-temperature-phase (QHTP), which is stable above 613 K, can be represented as Mnl 08Bi where it is assumed that 15 of the Mn atoms of the MnBi compound occupy octahedral, interstitial positions in its NiAs structure. Recently, we have observed the existence of a third magnetic phase and termed it the High Coercive Strength or UC phase. We find that the HC phase is paramagnetic at room temperature and orders ferromagnetically between 295 K and 77 K with an intrinsic coercivity of 120 kOe at 77 K, and with a remanent induction of about 2800 gauss. The magnitude of the intrinsic coercivity and shape of the demagnetization curve for the HC phase at 77 K is suggestive of Stoner-Wohlfarth single domain, coherent rotation particles. The amount of HC phase is a function of growth velocity and thermal gradient. We find more of the HC phase and correspondingly less of the LTP for larger growth velocities and thermal gradients. The room temperature intrinsic coercivity for samples processed only by directional solidification (which we call the as-grown state) appears to be a weakly dependent function of the average dispersed MnBi particle size except for very hgih growth velocities where the samples become almost paramagnetic in character, i.e., where at least 50 of the MnBi particles consist of the HC phase. We have seen dramatic improvment at room temperature not only in saturation magnetization and remanent induction but also in intrinsic coercivity and energy product after a 24 hour heat-treatment anneal, since the HC phase undergoes conversion to the LTP phase as a function of the heat treatment anneal. Our results suggest that the coexistence of the HC phase with the LTP phase limits the room temperature intrinsic coercivity of the LTP phase for effective MnBi particle diameters 1) > 2.0 Mm and enhance it for tD 7 3.5 Mm This behaviour is similar to that observed in certain rare earth-cobalt materials in which the introduction of a region of reduced magnetocrystalline anisotropy (magnetic defect) can either enhance (pinning) or decrease (nucleation of reverse domains) the intrinsic coercivity. We have achieved permanent magnet properties for those MnBi rods containing the LTP phase which are 598% the theoretical maximum. Over the temperature range from -500C to +1500 C, these permanent magnet figures of merit are competitive with those of the rare earth-cobalt family currently produced by a sintering process. In addition, we are able to produce the.HC phase, though unstable at elevated temperatures, which is stable below 100 K with magnetic properties which makes it a superior permanent magnet material for use in cryogenic environments. We propose producing rods of (LTS)1 x(EC)x(OxNl) MnBi rods by directional solidification, varying the amount (volume fraction) of MnBi by either (a) varying the initial Mn composition and growing off-eutectic or (b) hot-rolling or extruding the ds product or (c) extracting these rods from their Bi-matrix by a suitable solvent (by which the Bi may be recovered and recycled) and forming various forms or shapes whose permanent magnet figures of merit can be optimized for use at room and elevated temperatures (-500C to + 1500C) for motor applications (for example) or for use at cryogenic temperatures (L100 K) cr as a magnetic switch (paramagnetic at T?290 K and ferrimagnetic for To100 K). This choice of magnetic properties can be achieved by a suitable heat-treatment anneal of the as-grown MnBi rods (i.e. a suitable x in (LTP)l-x(HC) ). Also, since the cost in materials (50% Mn + 50 % Bi) in making 1 gram of MnBi is at least one order of magnitude lifer than the cost in materials (33% Sm + 6/9 Co) of making 1 gram of SmCo5 and in view of the opinion that the overriding cost in mass production is ultimately materials cost limited, the alternative of LTP MnBi rod magnetis of equivalent (to sintered SmCo or (misch-metal) Co5) permanent magnet performance 5 is attractive. The choice of a binder material can be a polymer material for HC rod, permanent magnets at cryogenic temperatures, LTP rod permanent magnets (-500C to +1500C) or for the magnetic switch (LTP)1-x(HC)X (for intermediate O < x < l) or a compaction of LTP rods for room to elevated temperature regimes. ExMiPLE Employing a Bridgman-Stockbarger type directional solidifdcation apparatus whose hot zone is maintained at 4600 C, whose cold zone is maintained at 250 C, thus giving rise to a thermal gradient in the liquid melt at the solid-liquid interface of about 1000 C/cm. The melt is directionally solidified at a growth velocity (furnace speed) of about 30 cm/h. blorphology amd Magnetic Properties The above solidification conditions for the eutectic composition (.72 mn) results in a compositionally uniform product whose morphology is characterised by an aligned ensemble of MnBi rods whose long axis lies along the solidification direction (direction of heat flow) with an average MnBi rod diameter of 0.5 micron. The magnetic properties in the as grolfn state (no post-solidification heat treatment) are characterised by a non-equilibrium magnetic phase (high coervicity or HC). This phase is paramagnetic at room temperature and orders ferrimagnetically at 240 K in the ordered state, i.e., below 24O0K, the saturation magnetization is 220 emu/cm3 and the resistance to demagnetization or intrinsic coercivity is 120 kOe. For those samples which are heat treated between 200 and 2600C for periods of between 1 and 100 hours the non-equilibrium HC phase transforms to the equilibrium ferromagnetic LTP Mn-Bi phase or a fully transformed sample, the saturization magnetization is 660 ema/em3 and the intrinsic coercivity is 35 kOe at room temperature; These values are the theoretically predicted maximum for equilibrium MnBi.

Claims: CLAIMS 1. A method for the directional solidification of Bi/MnBi compositions to produce magnetic single domain size MnBi particles, in aligned array, having a particle size of less than 1 micron and having an Mn concentration of up to 50 atomic percent, which comprises directionally solidifying a composition containing Mn and Bi, said composition containing from 0.1 to 50 by atomic fraction Mn, at a solidifying rate of 0.5 cm/h to 50 cm/h and at a thermal gradient, in the liquid at the liquid-solid interface, of from lO0C/cm- to 1O000C/cm. 2. The method of claim 1 for the production of a magnetic single domain size MnBi particle composite which contains from 0.72% by weight Mn (eutectic), wherein said solidifying rate is 30 cm/h and said thermal gradient is 1000 C/cm. 3. The method of claim 1 for the production of a magnetic single domain size MnBi particle composite which contains from 0.72 to 20.8 by weight Mn (Mn rich off eutectic), wherein said solidifying rate is 0.5 cm/h to 50cm/h and said thermal gradient is from 1000C/cm to 10000C/cm. 4. The method of claim 1 for the production of a magnetic single domain size MnBi particle composite which contains from 0.11 to 0.72 by weight Mn (Bi rich off eutectic), wherein said solidifying rate of 0.5 cm/h to 50 cm/h and said thermal gradient is from 100 cm to 2000C/cm. 5. The method of claim 1 wherein the resulting MnBi composite is post-heat treated in a temperature range of from.200 to 2600C for a period of from 1 to 100 hr. underaninert atmosphere such as argon gas. 6. The method of claim 5 wherein said post-heat treatment is effected to a vacuum furnace at a temperature of irom 200 to 260 C for a period of 1 to 100 hr. and at a pressure of less than 10-5 Torr. or in an inert atmosphere. 7. The method of claim 5 wherein said post-heat treatment is effected by hot rolling or extruding ata temperature in the range of 200-260 C. 8. An Mn-Bi composite having improved permanent magnet properties comprising single domain size MnBi particles in aligned array,having a particle size of less than 1 micron and an BIn concentration of up to 50% by atomic fraction, said composite having one ferromagnetic and one ferrimagnetic MnBi phase. 9. The post-heat treated composite of claim 8, wherein said post-heat treatment comprises heating of said composite at a temperature of from 200 to 2600C for a period of from 1 to 100 hr. under a pressure of at least 10 5 Torr. or in an inert atmosphere.

Inventors: LARSON, DAVID J., PIRICH, RONALD G.
Applicants: GRUMMAN AEROSPACE CORPORATION
Application year: 1984
Main classification: C30B21
IPC codes: C22C22, C22C1, B22D27, C22C12, H01F1, C30B21, C22F1