Abstract:
A process for mechanically strengthening a permanent magnet includes providing nanofibers or nanotubes, providing a ferromagnetic metal, defining a mixture by mixing the ferromagnetic metal with the nanofibers or nanotubes and sintering the mixture.

Description:
FIELD OF THE INVENTION 
     The present disclosure generally relates to rare earth permanent magnets. More particularly, the present disclosure relates to a process for mechanically strengthening rare earth permanent magnets by sintering the magnet raw materials with carbon and boron nanofibers or nanotubes. 
     BACKGROUND OF THE INVENTION 
     Rare-earth iron permanent magnets combine the magnetization of iron or cobalt with the anisotropy of a light rare-earth metal in intermetallic compounds which exhibit nearly ideal hysteresis. The rare-earth iron magnets may be indispensable components in a vast range of electronic and electromechanical devices. The mechanical strength of rare-earth permanent magnets may be limited by the sintered nature of these materials. In order to use these magnets in rotating machinery applications such as motors, generators and flywheel bearings, for example and without limitation, the magnets may require structural strengthening or reinforcement. 
     Permanent magnets used in rotating machinery may utilize metal or composite overwrap to increase their design limit for high speed rotation. However, the performance limits of permanent magnets with overwrap may still be limited. Moreover, the physical overwrap of permanent magnets may increase the cost and complexity of the system which utilizes the magnet and may also hinder or limit inspection of the magnet, reduce emission of heat from the system and limit application of the system to medium speeds of the magnet. 
     Therefore, a process for intrinsically strengthening rare earth permanent magnets on a molecular scale by sintering the magnet raw materials with carbon and boron nanofibers or nanotubes is needed. 
     SUMMARY 
     The present disclosure is generally directed to a process for mechanically strengthening a permanent magnet. Generally, the process may include providing nanofibers or nanotubes; providing a ferromagnetic metal; defining a mixture by mixing the ferromagnetic metal with the nanofibers or nanotubes; and sintering the mixture. 
     In some embodiments, the process may include providing nanofibers or nanotubes; providing a ferromagnetic metal and a rare earth metal; defining a mixture by mixing the ferromagnetic metal and the rare earth metal with the nanofibers or nanotubes; pressing and shaping the mixture; and sintering the mixture. 
     In some embodiments, the process may include processing carbon and boron nitride nanofibers or nanotubes by reacting amorphous carbon and boron powder with ammonia in the presence of iron catalyst particles; providing iron oxide and a rare earth metal; defining a mixture by mixing the iron oxide and the rare earth metal with the nanofibers or nanotubes; pressing and shaping the mixture; and sintering the mixture. 
     In some embodiments, the process may include processing carbon and boron nitride nanofibers or nanotubes by reacting amorphous carbon and boron powder with ammonia in the presence of iron catalyst particles at a temperature of from about 1100 degrees C. to about 1400 degrees C. for a time period of from about 20 minutes to about 2 hours; providing oxides of iron, neodymium and boron; defining a mixture by mixing the oxides of iron, neodymium and boron with the nanofibers or nanotubes; pressing and shaping the mixture; and sintering the mixture at a temperature of between about 1,000 degrees and about 3,000 degrees C. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a flow diagram of an illustrative embodiment of a process for mechanically strengthening rare earth permanent magnets. 
         FIG. 2  is a flow diagram of an exemplary process for processing carbon and boron nanofibers or nanotubes in implementation of the process for mechanically strengthening rare earth permanent magnets. 
         FIG. 3  is a flow diagram of an exemplary process for sintering the carbon and boron nanofibers or nanotubes with the magnet raw materials in fabrication of the rare earth permanent magnets. 
         FIG. 4  is a flow diagram of an alternative illustrative embodiment of a process for mechanically strengthening rare earth permanent magnets. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to practice the disclosure and are not intended to limit the scope of the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     The present disclosure is directed to a process for mechanically strengthening rare earth permanent magnets in which carbon and boron nitride (BN) nanofibers or nanotubes or nanotubes (hereinafter nanofibers or nanotubes) are used to mechanically strengthen the sintered permanent magnet material. An iron-based catalyst may be used in processing of the nanofibers or nanotubes. The chemistry of the carbon-based and boron-based nanofibers or nanotubes which is derived from the iron-based catalyst may be compatible with permanent magnet raw materials. In some embodiments, oxides of iron, neodymium and boron) may be mixed together, pressed and sintered to fabricate an NdFeB permanent magnet. Additionally, boron nitride nanofibers or nanotubes are chemically compatible with the permanent magnet stoichiometry. 
     Due to the sintering process which is used for their fabrication, permanent magnets may be structurally weak. On the other hand, carbon and boron nitride nanofibers or nanotubes are among the strongest known materials. The size of the carbon and boron nitride nanofibers or nanotubes is such that the nanofibers or nanotubes will significantly strengthen the matrix of the permanent magnet without interfering with the uniformity or strength of the permanent magnet domains which are required to produce magnets with very high magnetic field capability. 
     Referring initially to  FIG. 1 , a flow diagram  100  of an illustrative embodiment of a process for mechanically strengthening rare earth permanent magnets is shown. In block  102 , carbon and boron nitride nanofibers or nanotubes are processed with an iron-based catalyst. In block  104 , in some embodiments of the process, oxides of iron, neodymium and boron may be selected. In some embodiments, cobalt and other materials such as nickel and/or rare earth metals may be used. In some embodiments, an oxide of a rare earth metal may be used. In block  106 , the selected metal oxides are mixed with the iron-based carbon and boron nanofibers or nanotubes. In block  108 , the mixture of iron-based carbon and boron nanofibers or nanotubes and the selected oxides are pressed, shaped and sintered to form rare earth permanent magnets. 
     Referring next to  FIG. 2 , a flow diagram  200  of an exemplary process for processing carbon and boron nitride nanofibers or nanotubes (block  102  in  FIG. 1 ) in implementation of the process for mechanically strengthening rare earth permanent magnets is shown. In block  202 , iron catalyst particles may be provided. In block  204 , the iron catalyst particles may be applied to high-temperature substrates such as alumina, silicon carbide, platinum and/or molybdenum, for example and without limitation. In block  206 , amorphous carbon and boron powder may be reacted with a small quantity of ammonia in a flowing atmosphere of nitrogen in the presence of the iron catalyst particles. The iron catalyst particles may be added to the reaction mixture in the range of up to several weight percent. Prior to reaction of the carbon and boron powder with the ammonia in block  206 , the iron catalyst particles may be briefly milled with a hydrocarbon solvent and ceramic grinding medium. Reaction temperatures may range from about 1100 degrees C. to about 1400 degrees C. for from about 20 minutes to about 2 hours. In block  208 , the carbon and boron nanofibers or nanotubes may be formed as a result of the reaction in block  206 . 
     Referring next to  FIG. 3 , a flow diagram  300  of an exemplary process for sintering the carbon and boron nanofibers or nanotubes with the magnet raw materials (block  108  in  FIG. 1 ) in fabrication of the rare earth permanent magnets is shown. In block  302 , the mixture of selected oxides and nanofibers or nanotubes is shaped into a block of desired form. In block  304 , the block is subjected to sintering temperatures. In some embodiments, the block may be subjected to sintering at a temperature of between about 1,000 degrees and about 3,000 degrees C., preferably between about 1,170 degrees and about 1,270 degrees C. In block  306 , in some embodiments the block may next be subjected to aging temperatures of between about 800 degrees and about 900 degrees C. In block  308 , the block may be cooled to room temperature. The rare earth permanent magnet block may be fabricated in the form of the part which is to be utilized in the motor, generator, flywheel, bearing or other dynamic application of the magnet. 
     Referring next to  FIG. 4 , a flow diagram  400  of an alternative illustrative embodiment of a process for mechanically strengthening rare earth permanent magnets is shown. In block  402 , carbon and boron nitride nanofibers or nanotubes may be processed with an iron-based catalyst. This step may be carried out according to the process which was heretofore described with respect to  FIG. 2 . In block  404 , oxides of iron, neodymium and boron may be selected. In block  406 , the nanofibers or nanotubes may be dry-mixed in a selected stoichiometry with the oxides selected in block  404 . In block  408 , the mixture of nanofibers or nanotubes may be pressed and sintered with the selected oxides to form the rare earth permanent magnets. This step may be carried out according to the process which was heretofore described with respect to  FIG. 3 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.