Abstract:
Particles of iron and nickel are added to a flowing plasma stream which does not chemically alter the iron or nickel. The iron and nickel are heated and vaporized in the stream, and then a cryogenic fluid is added to the stream to rapidly cause the formation of nanometer size particles of iron and nickel. The particles are separated from the stream. The particles are preferably formed as single crystals in which the iron and nickel atoms are organized in a tetragonal L1 0  crystal structure which displays magnetic anisotropy. A minor portion of an additive, such as titanium, vanadium, aluminum, boron, carbon, phosphorous, or sulfur, may be added to the plasma stream with the iron and nickel to enhance formation of the desired crystal structure.

Description:
This invention was made with U.S. Government support under Agreement No. DE-AR0000186 awarded by the Department of Energy. The U.S. Government may have certain rights under this invention. 
    
    
     TECHNICAL FIELD 
     This invention pertains to the formation of nanometer size particles of iron-nickel alloys in which the iron and nickel atoms are arranged in the tetragonal L1 0  crystal structure. Mixtures of iron and nickel atoms are formed in their vapor state and the iron-nickel vapor is cooled very rapidly to form nanometer size particles in which the iron and nickel atoms are organized in the tetragonal L1 0  crystal structure. 
     BACKGROUND OF THE INVENTION 
     There is a continuing need for relatively inexpensive, high performance permanent magnet materials. For example, in the automotive vehicle industry there is a particular need for such permanent magnet materials, having relatively high curie temperatures Tc (&gt;300° C.), in traction motors, generators, and other applications. 
     Iron-nickel alloys are believed to offer permanent magnet properties providing they can be formed in the tetragonal L1 0  crystal structure. There is a need to form very small particles of compositions of elemental iron and nickel that may be consolidated into unitary shapes to serve as permanent magnets. Iron (atomic number 26) and nickel (atomic number 28) are similarly-sized transition element atoms. A molten mixture of elemental iron and nickel may be solidified as a face-centered cubic (fcc) crystal structure with the iron and nickel atoms in a disordered arrangement. But the disordered fcc crystal structure of iron and nickel atoms does not provide the magnetic anisotropy that is necessary for permanent magnet properties. There is a need for a method by which iron and nickel atoms may be formed into nanometer size particles of iron-nickel alloys in which the iron and nickel atoms are arranged in layers such that the resulting crystals are not cubic, but tetragonal and in the L1 0 -type AuCu 1 crystal structure to provide magnetic anisotropy. 
     SUMMARY OF THE INVENTION 
     This invention provides a method for forming nanometer size particles of iron and nickel having a L1 0 -type tetragonal crystal structure. When prepared in this crystal structure the iron-nickel composition particles are magnetically anisotropic and have useful permanent magnet properties. 
     In accordance with the invention, solid particles of iron and nickel are introduced into a process medium which is initially a plasma or plasma stream and which quickly heats the particles to form a vapor of iron and nickel atoms. The plasma is suitably formed, as in a DC plasma torch, from a neutral material such as nitrogen that does not chemically react with iron or nickel during their residence in the plasma processing medium. Preferably, the plasma is an element that is not condensable to a liquid at a temperature above 25° C. The plasma is initially at a temperature of many thousand degrees Kelvin, for example, 10,000 Kelvin, and a vapor of a mixture of iron and nickel is quickly formed. A very cold (below about 100K), inert fluid, such as liquid argon, or its vapor, is introduced into the plasma processing medium, containing iron-nickel vapor, to cool the iron-nickel mixture very rapidly to a temperature below 300° C. The vapor mixture of iron and nickel is rapidly transformed into particles of iron and nickel having a particle size smaller than about 250 nanometers. This process is utilized to quickly form and separate particles in which iron and nickel atoms are organized as successive layers of iron atoms and of nickel atoms in the arrangement characteristic of the L1 0 -type tetragonal crystal structure. 
     Preferably, each quenched particle consists of a single crystal of the iron and nickel atoms in the tetragonal L1 0  crystal structure. But, if necessary, particles that are partly amorphous, or have a high density of crystallographic defects such as dislocations may be carefully heat treated in an inert gas atmosphere to complete crystal formation. The heat treatment may be performed in the presence of an applied magnetic field in order to impose a preferential direction for formation of the L1 0  structure. But the particles must not be heated to a temperature (above about 320° C.) at which the crystal structure may convert to a disorganized crystal arrangement of the iron and nickel atoms. The nanometer size particles are collected and available for consolidation into a desired magnet body shape. 
     In accordance with a preferred embodiment of the invention, a flowing plasma stream is generated like that, for example, produced in a DC plasma generator or torch. A steady stream is established in a defined flow path. The plasma stream may have a generally circular cross-section. Solid pieces or particles of iron and nickel are introduced into the plasma stream. Preferably, but not necessarily, iron and nickel particles are introduced separately into the plasma, each at a plurality of locations around the perimeter of the flowing stream. The iron and nickel materials are quickly vaporized and mixed in the flowing plasma stream. 
     When the vapor/plasma process stream has been suitably established, a cryogenic fluid, such as liquid argon or liquid helium, is introduced into the vapor steam in an amount suitable to quench the iron-nickel vapor and form nanometer-size particles of iron and nickel composition. It is intended that the particles be cooled to a temperature below about 300° C. in the quench zone. As the quench fluid is added, the composite flowing stream may be confined and narrowed in cross-section so as to facilitate separation of the iron-nickel particles from the stream, and their recovery. The quenchant may also be separately recovered. 
     Preferably the additions of iron and nickel to the plasma processing stream are managed to produce single crystal particles of FeNi no larger than about 250 nm in size. In general, it is preferred that nickel constitutes about 25 to 67 weight percent of iron and nickel content of the particles. In one embodiment it is preferred that nickel constitutes about 45 to 55 weight percent of the iron and nickel content of the particles, and in another embodiment it is preferred that nickel constitutes about 25 to 39 weight percent of the iron/nickel content. 
     A minor amount of an additive element (A) may be included in the iron and nickel materials introduced into the plasma processing medium. Preferably, A is one or more of the elements selected from the group consisting of titanium, vanadium, aluminum, boron, carbon, phosphorus, and sulfur. The overall iron, nickel, and additive combination is to comprise no more than about fifteen weight percent of A and, preferably, no more than about ten weight percent A. The additive may be used in an amount to stabilize the formation of the iron/nickel combination in its tetragonal L1 0  crystal phase. 
     Accordingly, a method is provided to form a mixture of iron, nickel, and optionally an additive, convert it to a vapor mixture, and rapidly condense nanometer size particles of an organized arrangement of atoms having the tetragonal L1 0  crystal structure. The particles may be consolidated into suitable magnet body shapes by practices such as sintering, hot pressing, hot deformation, spark plasma sintering, or the like. A magnetic field may be applied prior to consolidation to magnetize and align the particles. Alternatively, the particles may be consolidated and the solid body magnetized after consolidation. In either case, complex magnetization patterns (e.g., magnetic poles) may be imposed on the solid compact after consolidation using an appropriate magnetizing fixture. 
     Other objects and advantages of the invention will be apparent from a description of illustrative embodiments of the practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing FIGURE is an enlarged schematic illustration of an organized layered arrangement of iron atoms  10  and nickel atoms  12  in a single cell of a L1 0  tetragonal crystal structure. In this illustration, each layer of atoms of the crystal cell is filled with either iron atoms or nickel atoms. Because of the slightly different sizes of the iron and nickel atoms, the cell is tetragonal. This organized layered arrangement of the iron and nickel atoms provides their L1 0  tetragonal crystals with magnetocrystalline anisotropy. In this illustration, the preferred magnetic direction of the crystal cell is in the vertical direction. The use of additive atoms in the practice of the invention (not illustrated in the drawing FIGURE) serves to enhance or stabilize this basic arrangement of the iron and nickel atoms in the basic L1 0  tetragonal crystal structure. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In one aspect of the present invention, a method is provided to convert particles of iron and nickel, or particles of an alloy of iron and nickel, using vapor phase and quench processing into nanometer size particles of single-crystals of iron and nickel atoms which are organized in a L1 0  tetragonal crystal structure. 
     The method comprises the formation of a plasma volume or stream, created using a composition that does not react chemically with the iron or nickel. Preferably, but not necessarily, the plasma is formed and used as a flowing high temperature stream to which the iron, nickel, and additive elements, if used, are added. The plasma may be formed from a suitable gas that does not chemically alter the iron or nickel. The gas may be, for example, helium, argon or nitrogen. The plasma initially is at a very high temperature of the order of several thousand degrees Kelvin. The plasma is used in the present process to form a high temperature processing medium into which iron and nickel particles are added and vaporized to form a quenchable mixture. As described above in this specification, the vapor mixture is maintained only for a brief period of time and is then quenched to condense the iron, nickel, and any additive atoms as a solid mixture in the form of very small particles. In general, it is preferred to use the plasma in the form of a flowing process stream with a generally round cross-section, or like perimeter, to facilitate the addition of the starting particles at a plurality of locations around the circumference of the plasma stream. 
     Thermal plasmas are often generated in plasma torches when a flowing gas is energized by an electrical discharge, such as a direct current (DC), alternating current (AC), or radio frequency (RF) discharge. A plasma stream in the nature of a DC torch stream is suitable for use as the high temperature processing stream. In a typical DC plasma generator, a gas stream of nitrogen (e.g.) is flowed through a circular tube, along an axial cathode toward an anode ring near the outlet of the tube. A high voltage DC arc discharge is maintained between the downstream end of the axial cathode, near the anode ring. As the nitrogen passes through the DC discharge at a suitable flow rate, it is converted into a highly ionized gas; a plasma. The use of a plasma processing stream is preferred in the practice of this invention because the flowing stream may be quickly and effectively utilized to receive additions of iron, nickel, and additive, to affect their conversion to a mixed vapor, and to accommodate the quenching of the vapor to recover very small, rapidly solidified particles of the permanent magnet material. Accordingly, it is preferred that the stream is established with a generally circular cross-section. Thus, the plasma stream may be enclosed or otherwise formed with a defined periphery, suitable for the addition of the iron, nickel, and any additive solids to be processed. 
     Thus, as soon as the plasma processing stream has been established, it is utilized. Suitable amounts and proportions of iron and nickel particles are injected into the high temperature stream so that they are quickly melted and vaporized. In general it is preferred to utilize the plasma processing stream by introducing the solid materials at several locations around the periphery of the stream and, if necessary, along the flow path of the plasma stream. In a preferred embodiment, iron particles and nickel particles are separately introduced into the plasma stream. When the product is to contain an additive element or elements it may be preferred to pre-form alloys of the iron, nickel, and additive(s). The materials may be added, for example, in predetermined proportions by pushing individual or alloyed particles through feed tubes into the flowing plasma stream. Of course, the rate of addition of the iron and nickel must be in proportion to the capacity of the plasma stream to receive them and immediately melt them to form a vapor of the metal elements to be mixed. Thus, a continuous length-wise portion of the flowing plasma processing stream is utilized to receive and rapidly melt and vaporize the predetermined combinations of iron, nickel, and any additive elements to be prepared as a vapor suitable for quenching. Depending on the predetermined thermal capacity of the plasma process stream, less than a meter or so of its flowing length may be required for this step of the process. 
     When a suitable vaporized mixture of the elements has been formed, the mixed vapor is quenched to recover the added elements in the form of small solid iron-nickel-based particles. By this stage of the process, the initially plasma material may have cooled into a high temperature gas that is carrying the metal vapor. Again, the generally confined perimeter of the flowing process stream may be utilized for the effective addition of a very low temperature (cryogenic) quench fluid into the stream. Preferably, the quench fluid is directed into the process fluid in several radially inwardly-directed streams applied from the circumference or perimeter of the flowing process stream. 
     Liquid argon (initially at about 83 Kelvin) is a preferred quench fluid. Of course, argon has a very narrow liquid temperature range and will soon be converted to a vapor as it encounters the plasma process stream. Liquid helium or liquid nitrogen may also be used as a quench fluid. In order to better utilize the quench fluid and the process stream, it is preferred to add quench fluid from a plurality of locations around the perimeter of the flowing process stream. 
     The addition of the quench fluid increases the mass of the flowing stream as it is cooled. If the flowing process stream has not been physically combined within a tube or the like to preserve its thermal content, the quenched process stream may now be directed into a confining tube or the like. The cross-section of the process stream may initially be allowed to expand and cool. But it is then desired to funnel or narrow the stream in which the solid particles of iron and nickel are being formed. This is to facilitate separation of the precipitated iron-nickel-additive particles from the process stream. It is, of course, desirable to completely recover all metal added to the plasma stream. This may be accomplished by passing the channeled, particle-containing, process stream through a suitable filter or centrifuge. 
     It is also generally desirable to recover the argon or other quench material for reuse. It may also be desirable to recover the working gas used to form the plasma. 
     The practice of the described process is to form generally uniformly-sized particles of (Fe 100−x Ni x ) 100−y A y  composition where the particles are no larger than about 250 nanometers in diameter or largest dimension. A representative sample of the particles may be examined and characterized by X-ray diffraction. 
     Preferably, the particles consist of single crystals of the (Fe 100−x Ni x ) 100−y A y  composition and in the tetragonal L1 0  crystal structure. A schematic illustration of a single crystal cell is presented in the drawing FIGURE. It is seen that alternate layers of the cell consist of iron atoms  10  and nickel atoms  12 . Ideally, this alternate layer arrangement of the iron and nickel atoms, with interspersed additive atoms (if included) would continue throughout the cells of a single crystal particulate material 
     If the quenched particles are not fully crystallized, they may be heat treated in an inert atmosphere at a temperature below about 300° C. for a time determined experimentally, or by experience, to complete the crystallization of the quenched particles. Other methods of inducing complete crystallization in the recovered particles include pressurization under a suitable gas, or application of an applied magnetic field, or combinations of the above, such as heat treatment in the presence of an applied magnetic field. Also mechanical processing of the particles such as rolling, swaging, or ball milling of the particles may be utilized to complete crystallization in the small particles. Combinations of these practices may also be used to induce further crystallization. 
     The process is conducted to obtain the (Fe 100−x Ni x ) 100−y A y  composition in the form of particles having the magnetically anisotropic, tetragonal, L1 0  crystal structure. Preferably, each particle is a single crystal of the desired structure. As stated it is preferred that the nickel content of the iron-nickel mixture be, by weight, 25 to 67 percent of the total of iron and nickel; x=25-67. Within the overall preferred proportions of iron and nickel are two preferred sub-ranges by weight which are found to reflect good combinations of iron and nickel. These weight ranges are reflected by x=45 to 55 weight percent Ni and x=25 to 39 weight percent Ni. 
     When one or more additives (A) are added with the iron and nickel, it is preferred that y be no greater than 15 percent by weight of the total of Fe, Ni, and A. More preferably, it is preferred that y be less than or equal to 10% by weight. It is preferred that an additive, A, is selected to be one or more elements selected from the group consisting of Ti, V, Al, B, C, P, and S. 
     In many permanent magnet applications it will be necessary to consolidate the iron-nickel particles into permanent magnet body shapes for use in electric motors, magnetic actuators, and the like. Such consolidation may be accomplished by any of many suitable methods which do not adversely affect the desired tetragonal L1 0  crystal structure of the particles. A permanent magnet may be formed by magnetizing and magnetically aligning the particles prior to consolidation, or by magnetizing the solid body in its entirely, or in regions, after consolidation is complete. 
     Practices of the invention have been disclosed as specific illustrations which are not intended to limit the proper scope of the invention.