Patent Application
20120282130
The present invention relates to rare earth element-containing permanent magnet materials and to a method of making the materials by carbothermic reduction of a rare earth element-containing oxide including at least one of neodymium and praseodymium in a manner to form a rare earth element-containing intermediate alloy material as a master alloy for reacting with suitable non-rare earth metal alloying elements and boron and/or carbon to make a permanent magnet material at lower cost with improved properties.
A world-wide market ($4.3 billion) for the Nd2Fe14B-based permanent magnets is now well established, but the costs of these magnets has risen quite rapidly because the price of neodymium has risen by a factor of five times in the past three years due to Chinese export controls and pricing. Thus a new and more economical process for their preparation is very attractive, and if it were environmentally friendly this would be a plus.
Currently, the Nd2Fe14B alloy is prepared by melting the three alloy constituents in the appropriate amounts. The costliest Nd (neodymium) constituent is prepared from the oxide, Nd2O3, by converting it to NdF3 and then reducing the fluoride electrolytically in a fused LiF bath. This process is quite costly since several steps are required and each involves a high consumption of electrical power. Furthermore, fluorine gas is a by-product which presents a serious environmental problem. This combined with the large amount of energy used in processing accounts for a rapid increase in the price of Nd metal in the past several years.
Nd metal also can be prepared by the metallothermic reduction of NdCl3 or NdF3 using calcium metal as the reductant followed by a separate casting step to remove excess calcium. In these processes, CaCl2 or CaF2 slag is produced and must be adequately and safely returned to the environment. The chloride process also presents a second problem as a result of residual chlorine being incorporated in the Nd metal and into Nd2Fe14B permanent magnet material made using the Nd metal. The presence of the chlorine ion in Nd2Fe14B renders the product susceptible to corrosion and oxidation such that the product must be protected by a coating from the ambient environment to prevent degradation of the permanent magnet.
The present invention provides in an embodiment a carbothermic reduction method wherein a rare earth element-containing oxide including at least one of neodymium (Nd) and praseodymium (Pr), and optionally one or more other rare earth elements (including one or more of La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y which are alloying agents to modify the magnetic properties of the permanent magnet material) is reduced in the presence of carbon and a source comprising a reactant element selected from the group consisting of silicon (Si), germanium (Ge), tin (Sn), lead (Pb), arsenic (As), antimony (Sb) and bismuth (Bi) to form a rare earth element-containing intermediate alloy material. This intermediate alloy is useful as a master alloy for making a permanent magnet material. The source of the reactant element can comprise elemental silicon, germanium, tin, lead, arsenic, antimony, and/or bismuth, or the oxides thereof, or other compounds thereof, that can participate in the carbothermic reduction reaction to form the intermediate material.
Moreover, the carbothermic reduction method of the invention can provide for an efficiency of greater than 90% and is also environmentally friendly since no slag is formed during preparation, and the only by-product is carbon monoxide gas, which it utilized as a starting material for preparing organic compounds, or as a component of producer gas (also known as water gas) for cogeneration of heat or electricity.
The present invention provides in another embodiment a method wherein the rare earth element-containing intermediate alloy material (as a master alloy) is reacted with one or more suitable non-rare earth metal alloying elements and boron and/or carbon to make a permanent magnet material. The permanent magnet material can include, but is not limited to, Nd2Fe14B+Si material, Pr2Fe14B+Si material, (Nd/Pr)2Fe14B+Si material wherein the materials exhibit useful magnetic remnant magnetization and coercivity properties comparable to those of commercial Nd2Fe14B permanent magnets and improved corrosion and oxidation resistance.
In an illustrative embodiment of the invention, the permanent magnet material can be represented by RxTMyB1-zCz+E where R is includes at least one of Nd and Pr and optionally one or more rare earth elements selected from the group consisting of La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y; TM is selected from the group consisting of Fe, Co, V, Nb, Ti, Zr, Al, and Ga; B and C are boron and carbon respectively; and where E is a reactant element selected from the group consisting of silicon, germanium, tin, lead, arsenic, antimony and bismuth. The value of x can range from 1.5 to 2.5, the value of y can range from 12 to 16, and the value of z can range from 0 to 0.5. The ratio of the aggregate amount (e.g. aggregate atomic %) of RxTMyB1-z Cz to the amount (atomic %) of E preferably is 2 or greater. The permanent magnet materials may be processed by any suitable means to achieve the microstructure required for optimal magnetic properties such as the coercivity, remanence, energy product and magnetic ordering temperature. One process involves making particulates comprising the permanent magnet material and bonding the particulates using a binder to form a bonded permanent magnet.
The present invention is advantageous in that the rare earth element-containing intermediate alloy is used as a master alloy to make a permanent magnet material in a single step process wherein the above-described intermediate alloy is reacted with one or more non-rare earth metals (e.g. Fe) and boron and/or carbon. Still further, the reactant element, such as silicon, can be present in the permanent magnet material in an amount effective to improve corrosion and oxidation resistance in ambient environments as compared to the same material without the reactant element.
Other advantages of the present invention will become more readily apparent from the following detailed description taken in conjunction with the following drawings.
a and 5b show the transmission electron microscopic (TEM) micrograph of Nd2Fe14B melt spun ribbon described in Example 5 at two different magnifications and
The present invention provides in an embodiment a carbothermic reduction method for reducing a rare earth element-containing oxide including at least one of neodymium (Nd) and praseodymium (Pr) at temperatures below 1800 degrees C. The rare earth element-containing oxide is reduced in the presence of carbon (reducing agent) and a source comprising a reactant element selected from the group consisting of silicon (Si), germanium (Ge), tin (Sn), lead (Pb), arsenic (As), antimony (Sb) and bismuth (Bi) in order to form a rare earth element-containing intermediate alloy material that comprises at least one of Nd and Pr, and optionally other rare earth elements selected from the group consisting of La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y (as alloying agents to modify the magnetic properties of the permanent magnet material) and the reactant element (Si, Ge, Sn, Pb, As, Sb and/or Bi). For purposes of illustration but not limitation, when silicon (Si) comprises the reactant element, the intermediate alloy material can comprise an alloy of Nd and/or Pr and Si, such as a binary NdSix or PrSix alloy or a ternary (NdPr)Six alloy where the value of x depends upon the carbothermic reduction reaction conditions. The alloy can be substantially stoichiometric such as Nd5Si3, Nd5Si4, Pr5Si3 or Pr5Si4, which thus comprise intermetallic compounds, or it can be non-stoichiometric such as for example, Nd5Si3.62 and others as well. The rare earth element-containing intermediate alloy provides a master alloy for making a permanent magnet material as will be described below.
The carbothermic reduction process is a solid state, diffusion controlled process and intimate contact between the carbon reducing agent and the oxide particles and source of reactant element is employed for the reduction to reach completion. The optimum particle size of the rare earth-containing oxide, carbon, and source of the reactant element and the best conditions for milling and blending the mixture thereof can be determined empirically to this end. The Examples below illustrate certain exemplary parameters for carrying out the carbothermic reduction reaction.
For purposes of illustration and not limitation, the rare earth element-containing oxide can comprise suitable oxide particulates that include Nd oxide, Pr oxide, mixtures thereof, or mixed Nd/Pr oxides and optionally other rare earth oxides or mixtures thereof when the other rare earth elements noted above are to be optionally present in the intermediate alloy. For example, Nd2O3, and/or Pr6O11 (or an oxide in the range from Pr2O3 to PrO2), and the mixed oxide Nd2O3 and PrOx (where 1.50≦x≦2.00) particulates thereof can be used in practice of the invention and are available as commercial grade, high purity oxide particles (purity of 99.9%) in a size range of 40 to 200 μm from Santoku America Company.
The carbon used as the reducing agent in the carbothermic reduction reaction can be of any suitable type, such as including but not limited to, Shawinigan (acetylene black) type available from Chevron Chemical Co. that is 100% compressed, 325 mesh, and contains less than 0.05% ash and can be used as-received. The Examples described below used such carbon in the “as-received” condition. Other types of carbon that can be used include, but are not limited to acetylene black type.
The source of the reactant element can be selected from elemental silicon, elemental germanium, elemental tin, elemental lead, elemental arsenic, elemental antimony and/or elemental bismuth as well as alloys thereof one with another and/or with other elements, one or more oxides thereof, or other compounds thereof that can participate in the carbothermic reduction reaction to form the intermediate alloy material enriched in Nd and/or Pr. For purposes of illustration and not limitation, suitable Si is available as commercial grade particles having high purity (e.g. 99.9% purity) in a size range of 100 to 250 μm from Arco Solar Company. SiO2 is available as commercial grade silica particles having high purity (e.g. 99.9% purity) in a size range of 40 to 50 μm from Alfa-Aesar Company.
In an embodiment of the invention, a particulate mixture of the rare earth element-containing oxide, carbon reducing agent, and the source of reactant element is prepared by milling the particles and blending them together. The mixture is then formed into a paste by adding a binder in a solvent carrier to the mixture. The paste then can be formed into cubes (or other shaped bodies) and air dried to form briquettes, which have good strength and are easily loaded into the tantalum, Al2O3 or other reduction crucible. The dried briquettes can be heated in a tungsten resistance or other type of furnace under vacuum to an appropriate temperature at or above the onset temperature of the carbothermic reduction reaction and for a time to complete the reduction reaction to form the intermediate alloy comprising Nd and/or Pr and possibly other optional rare earth elements, and the reactant element (Si, Ge, Sn, Pb, As, Sb and/or Bi). The reaction can be monitored using a quadrupole gas analyzer to monitor by-product gases such as CO. The particulate mixture preferably is heated to the liquid or molten state after the carbothermic reduction reaction is completed to allow the oxygen and carbon time to react and form CO, thereby reducing the oxygen and carbon content of the intermediate alloy material to a relatively low content such as about 1.5 weight % of O and C or less for purposes of illustration and not limitation.
The intermediate alloy material has a controlled content of Si or other reactant element so that the Si or other reactant element content of the final permanent magnet material made using the intermediate alloy does not degrade magnetic properties and also has a beneficial effect of increasing corrosion and oxidation resistance of the permanent magnet material.
Furthermore, the carbothermic-silicide reduction method of the invention is environmentally friendly since no slag is formed during preparation, and the only by-produce is carbon monoxide gas, which can be absorbed or ignited to carbon dioxide; or utilized as a starting material for preparing organic compounds, or as a component of producer gas (also known as water gas) for cogeneration of heat or electricity. In addition the process is quite efficient, yields as high as 95% have been realized.
Pursuant to another embodiment of the present invention, the rare earth element-containing intermediate alloy is used as a master alloy in making a permanent magnet material. For purposes of illustration and not limitation, the intermediate alloy is reacted with one or more suitable non-rare earth metal alloying elements, and boron and/or carbon to make a permanent magnet material comprising a rare earth element including one or both of Nd and Pr and possibly other optional rare earth alloying additives, the non-rare earth metal, boron and/or carbon, and the reactant element selected from the group consisting of silicon, germanium, tin, lead, arsenic, antimony and bismuth in controlled concentration. The non-rare earth metal preferably comprises Fe yet the invention envisions replacing some or much of the Fe with one or more other non-rare earth metals selected from the group consisting of Co, V, Nb, Ti, Al, and Ga.
For purposes of illustration and not limitation, the present invention can be practiced to make a permanent magnet material that includes, but is not limited to, Nd2Fe14B+Si material, Pr2Fe14B+Si material, (Nd/Pr)2Fe14B+Si material when the non-rare earth metal is Fe and the reactant element is Si. The amount of Si (or other reactant element) is controlled within the range of about 1 to about 10 atomic %, preferably about 2 to about 6 atomic %, or so that the resulting permanent magnet materials exhibit useful magnetic coercivity and magnetic remanence properties comparable to ordinary grade Nd2Fe14B magnet, and improved corrosion and oxidation resistance. That is, the inclusion of Si or other reactant element does not degrade the magnetic properties of the permanent magnet material produced pursuant to the invention and is included in an amount effective to increase its corrosion and oxidation resistance in ambient environments.
In an illustrative embodiment of the invention, the permanent magnet material can be represented by RxTMyB1-zCz+E where R includes at least one of Nd and Pr and optionally other rare earth elements selected from the group consisting of La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y; TM comprises one or more elements selected from the group Fe, Co, V, Nb, Ti, Zr, Al, and Ga; B and C are elemental boron and carbon, respectively; and where E is a reactant element selected from the group consisting of silicon, germanium, tin, lead, arsenic, antimony and bismuth. The value of x can range from 1.5 to 2.5, the value of y can range from 12 to 16, and the value of z can range from 0 to 0.5. The ratio of the aggregate amount (e.g. aggregate atomic %) of RxTMyB1-zCz to the amount (atomic %) of E is 2 or greater. The permanent magnet material can be made by introducing a source of the non-rare earth metal (e.g. Fe) and source of B and/or C to a molten bath of the rare earth-enriched intermediate alloy material. For purposes of illustration and not limitation, for making a Nd2Fe14B+Si material, Pr2Fe14B+Si material, (Nd/Pr)2Fe14B+Si material, appropriate amounts of iron and boron can be introduced into the melted intermediate Nd/PrSix master alloy to make the above permanent magnet materials. Electrolytic iron and commercial grade ferro-boron can be added in the appropriate stoichiometry to form the Nd2Fe14B+Si using a partitioned crucible or added using a vibrating hopper attached to the reduction/casting furnace for purposes of illustration and not limitation.
Particular permanent magnet materials of the invention can be represented (Nd1-xRx)TM14X+E; (Pr1-xRx)TM14X+E; and [(Nd/Pr)1-xRx]TM14X+E where R is optional and selected from the group consisting of La, Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y; where TM is selected from the group consisting of Fe, Co, V, Nb, Ti, Al, and Ga; where X is at least one of B and C; where E is selected from the group consisting of Si, Ge, Sn, Pb, As, Sb and Bi; and x is 0 to 0.6.
The molten permanent magnet material can be melt spun to ribbon, atomized by various techniques to form generally spherical or other shape atomized particles, cast to ingot shape and pulverized to powder particles, and otherwise treated to provide various forms of the material for subsequent use in producing a permanent magnet shape. The permanent magnet materials can be heat treated to optimize their magnetic properties as described below. The invention envisions in a further embodiment making particulates comprising the permanent magnet material by melt spinning, atomization, and pulverizing, heat treating, and bonding the particulates using a binder to form a bonded permanent magnet. The invention also envisions in another further embodiment making particulates comprising the permanent magnet material as described and sintering the particulates to form a sintered permanent magnet.
The following Examples are offered to further illustrate practice of the invention but not limit the scope of the invention.
This example illustrates conduct of the carbothermic reduction process using silica (SiO2) to prepare a Nd5Si3.5 intermediate alloy material.
The respective Nd2O3 and SiO2 particulates are first dried separately at 800 degrees C. in air to remove any adhering moisture, non-oxidized material and/or absorbed gases and screened to the size listed, −212 μ. The mixture was blended for 2 hours in a Turbula commercial blender, mixed with ˜50 cc of acetone containing 3 wt. % polypropylene carbonate (binder), manually formed into ˜1.3 cm cube briquettes, and air dried overnight.
The maximum pressure obtained in the furnace was ˜600 μ@ 1500° C.
The alloy had as-melted a carbon and oxygen content of C=1.69 wt. % and 0=1.31 wt. % and had a 35.6% weight loss. This amount, which exceeded the theoretical value of 32.9% for the removal of C and O, was due to vaporization of SiO and a small amount of Nd or NdO.
This example illustrates conduct of the carbothermic reduction process using elemental silicon (Si) to prepare a Nd5Si3.5 intermediate alloy material.
The Nd2O3 particulates are first dried at 800 degrees C. in air to remove any adhering moisture, non-oxidized material and/or absorbed gases and screened to the size listed, −212 μm. The mixture was blended for 2 hours in Turbula commercial blender, mixed with ˜45 cc of acetone containing 3 wt. % polypropylene carbonate (binder), manually formed into ˜1.3 cm cube briquettes, and air dried overnight.
The maximum pressure obtained in the furnace was ˜600 μm @ 1500° C. plus.
The alloy had as-melted carbon and oxygen contents of C=1.03 wt. % and 0=0.73 wt. % and had a 26.6% weight loss (theoretical 20.3%).
From SEM analysis this alloy had the composition of Nd5Si3.62 and was used to prepare the Nd2Fe14B+Si alloy CEA-1-55 in the next Example.
This example illustrates the conduct of the carbothermic process using elemental silicon (Si) and compacting the prepared briquettes into wafer form. The compacted wafers occupy only one-third the volume of the briquettes and consequently much more material can be processed in the reduction step. A Nd5Si3.52 intermediate alloy was prepared.
The Nd2O3 particulates are first dried at 800° C. in air to remove any adhering moisture, non-oxidized material and/or absorbed gases and screened to −212 μm. The mixture was blended for 2 hours in Turbula commercial blender, mixed with ˜40 cc of acetone containing 3 wt. % polypropylene carbonate (binder), manually formed into ˜1.3 cm cube briquettes, and air dried overnight. These briquettes were compacted into 2.5 cm diameter by ˜0.4 cm thick round wafers using a conventional harden right angle cylinder die and ram. Approximately 1.0×103 kg/cm2 pressure was used to form the wafers. Two or three briquettes were used to prepare each wafer.
The maximum pressure obtained in the furnace was ˜470 μm at 1540° C. plus. The alloy was shiny, had been molten and no reaction was noted when a sample was placed in water for 72 hours indicating very little or no neodymium carbide phase present. The as-prepared alloy had carbon and oxygen contents of C=1.11 wt. % and 0=0.91 wt. %. An 86.6% yield of Nd was obtained and the alloy had a calculated composition of Nd5Si3.52.
This example illustrates conduct of the carbothermic reduction process using elemental silicon (Si) to prepare a Nd5Si3.22 intermediate alloy material using a tantalum reduction crucible with a floating lid to enhance the yield of alloy.
The Nd2O3 particulates are first dried at 800° C. in air to remove any adhering moisture, non-oxidized material and/or absorbed gases and screened to −212 μm. The mixture was blended for 2 hours in Turbula commercial blender and 40 grams were mixed with ˜27 cc of acetone containing 3 wt. % polypropylene carbonate (binder), manually formed into ˜1.3 cm cube briquettes, and air dried overnight. These briquettes were compacted into 1.58 cm diameter by ˜0.4 cm thick round wafers using a conventional harden right angle cylinder die and ram. Approximately 2.1×103 kg/cm2 pressure was used to form the wafers. One or two briquettes were used to prepare each wafer.
The maximum pressure obtained in the furnace was ˜230 μm at 1540° C. plus. Fluctuation in the pressure was observed during the reduction step due to the raising and lowering of the floating lid over the crucible.
The alloy was shiny, had been molten, and a sample did not react in water after 72 hours indicating little or no neodymium carbide present. The yield of neodymium was 94% and the alloy had carbon and oxygen contents of C=1.42 wt. % and O=0.79 wt. %. The alloy had a calculated composition of Nd5Si3.22.
This example illustrates preparation of Nd2Fe14B+Si from the Nd5Si3.62 alloy (CEA-1-55) above as follows:
The above components were arc-melted together under argon on a cold copper hearth. The resultant ingot had a composition of 25.71 wt. % Nd, 69.69 wt. % Fe, 0.96 wt. % B+3.62 wt. % Si. The ingot was then melt spun at 20 m/sec to form ribbon which was sealed in quartz under Ar and annealed for 20 minutes at 800° C. and then quenched in an ice bath (designated as CEA-1-55).
a and 5b shows a transmission electron microscopic (TEM) micrograph of Nd2Fe14B melt spun ribbon similarly melt spun and annealed at 750 degrees C. for 20 minutes, removed from the furnace and quenched in an ice bath at two different magnifications and
The magnetic measurement results after the heat treatment were as follows: remnant magnetization=7.1 kG; coercivity=2.7 kOe; and energy product (BHmax)=6.1 MG-Oe These measured properties are similar to those of the lowest commercial grade Nd2Fe14B permanent magnets.
More generally, the melt spun ribbons can be heat treated at 650 to 850 degrees C. for 10 to 30 minutes to develop optimum magnetic properties. The optimum heating temperature for Nd2Fe14B+Si ribbons is higher than that of the material without Si.
The arc-melted samples containing Si exhibited superior oxidation resistance compared to an arc-melted material without Si. For example, after 64 days at 300 degrees C., the Si-containing material (CEA-1-55) pursuant to the invention is six (6) times more resistant based on weight gain.
This example illustrates preparation of Nd2Fe14B+Si where the content of Si is varied to determine affect on magnetic properties. Samples of Nd2Fe14B+0% Si, 2% Si, 3% Si, 4% Si, and 5% Si where % is weight % were made by alloying the elements in an arc-melting step, and heat treating in a manner similar to that described above.
A better comparison of the permanent magnet properties of the Nd—Fe—B—Si product prepared from the Nd—Si metallic alloy produced by using the carbothermic-silicide method is the energy product calculated from the B—H curves in the second quadrant, see
The rate at which the Nd—Fe—B—Si material is quenched has a pronounced affect on the grain size of the magnetic material in the ribbons.
TiC has been known as a grain refining agent to refine the Nd—Fe—B grain size in the rapidly solidified ribbons. The influence of TiC in the Nd—Fe—B—Si alloys is illustrated in
As seen in
Although the invention has been described in connection with certain illustrative embodiments, those skilled in the art will appreciate that changes and modifications can be made therein within the scope of the invention as set forth in the appended clams.
This application claims benefits and priority of U.S. provisional application Ser. No. 61/280,198 filed Oct. 30, 2009, the disclosure of which is incorporated herein by reference.
This invention was made with government support under Contract No. DE-AC02-07CH11358 awarded by the Department of Energy. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61280198 | Oct 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2010/002847 | Oct 2010 | US |
| Child | 13506427 | US |
