WO2009051888A1

WO2009051888A1 - Doped ultrafine metal carbide particles

Info

Publication number: WO2009051888A1
Application number: PCT/US2008/073436
Authority: WO
Inventors: Noel R. Vanier; Cheng-Hung Hung
Original assignee: Ppg Industries Ohio, Inc.
Priority date: 2007-10-17
Filing date: 2008-08-18
Publication date: 2009-04-23

Abstract

Ultrafine metal carbide particles with dopants are disclosed. The ultrafine metal carbide particles may comprise boron carbide doped with Al, Ti, W, Zr, Mg, N, Fe, Na, Ca, Si, Y, La, Hf, Ta, Mo, Ni, Co, V, Nb, Ce, Mn, Li, Nd and the like. The dopant material is uniformly distributed in the ultrafine metal carbide particles on a submicron scale, which results in improved sintering and mechanical properties.

Description

DOPED ULTRAFINE METAL CARBIDE PARTICLES

GOVERNMENT CONTRACT

[0001] This invention was made with United States government support under Contract Number W911NF-05 -9-0001 awarded by DARPA. The United States government has certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application is a continuation-in-part of U.S. Application Serial No. 11/468,424 filed August 30, 2006, which is incorporated herein by reference.

FIELD OF THE INVENTION

[0003] The present invention relates to metal carbide particles, and more particularly relates to the production of ultrafine metal carbide particles with dopants.

BACKGROUND INFORMATION

[0004] Boron carbide particles having particle sizes of greater than 0.2 micron have been produced by solid phase synthesis using B₂O₃ and carbon as starting reactant materials and subsequent milling. Such particles may be sintered to form various products such as armor panels and abrasion resistant nozzles. Sintering aids may be added to such boron carbide particles by milling in order to obtain a mixture that is homogeneous on a macro scale. However, these mixtures are not uniform on a microscale, and such non-uniformities may adversely affect sintering of the particles and cause defects in the sintered bodies that degrade mechanical properties.

SUMMARY OF THE INVENTION

[0005] In certain respects, the present invention is directed to a method for making ultrafine doped metal carbide particles comprising: introducing a metal- containing precursor, a carbon-containing precursor and at least one dopant into a plasma; heating the precursors and the at least one dopant by the plasma to form the ultrafine doped metal carbide particles from the precursors; and collecting the ultrafine doped metal carbide particles.

[0006] In other respects, the present invention is directed to ultrafine doped metal carbide particles comprising: ultrafine metal carbide particles having an average size of less than 100 nm; and dopant material substantially uniformly distributed in the ultrafine metal carbide particles.

[0007] In further respects, the present invention is directed to a sintered material comprising ultrafine metal carbide particles sintered in the presence of dopant material substantially uniformly distributed in the ultrafine metal carbide particles.

BRIEF DESCRIPTION QF THE DRAWINGS

[0008] Fig. 1 is a flowchart depicting the steps of certain methods of the present invention.

[0009] Fig. 2 is a partially schematic sectional view of an apparatus for producing ultrafine metal carbide particles including a feed line for a mixture of metal-containing precursor, carbon-containing precursor and dopant in accordance with certain embodiments of the present invention.

[0010] Fig. 3 is a partially schematic sectional view of an apparatus for producing ultrafine metal carbide particles including a feed line for a mixture of metal-containing precursor, carbon-containing precursor and dopant in accordance with certain embodiments of the present invention.

[0011] Fig. 4 is a micrograph of ultrafine boron carbide particles doped with Al in accordance with an embodiment of the present invention.

[0012] Figs. 5 and 6 are electron energy loss spectroscopy (EELS) images of the ultrafine aluminum doped boron carbide particles shown in Fig. 4, with Fig. 5 mapping elemental boron and Fig. 6 mapping elemental aluminum.

DETAILED DESCRIPTION

[0013] For purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating

- 9 - examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0014] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0015] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of "1 to 10" is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0016] In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of "or" means "and/or" unless specifically stated otherwise, even though "and/or" may be explicitly used in certain instances.

[0017] Certain embodiments of the present invention are directed to methods for making ultrafine doped metal carbide particles, as well as the ultrafine doped metal carbide particles and consolidated products produced by such methods. Examples of ultrafine metal carbides that may be produced include boron carbides such as B₄C, B₁₃C₂, B₈C, B₁₀C, B₂₅C. Other ultrafine doped metal carbides that may be produced in accordance with the present invention include tungsten carbide, titanium carbide, silicon carbide, aluminum carbide, iron carbide, zirconium carbide, magnesium aluminum carbide, hafnium carbide and the like. [0018] As used herein, the term "ultrafine metal carbide particles" refers to metal carbide particles having a B.E.T. specific surface area of at least 5 square meters per gram, such as 20 to 200 square meters per gram, or, in some cases, 30 to 100 square meters per gram. As used herein, the term "B.E.T. specific surface area" refers to a specific surface area determined by nitrogen adsorption according to the ASTMD 3663- 78 standard based on the Brunauer-Emmett- Teller method described in the periodical "The Journal of the American Chemical Society", 60, 309 (1938).

[0019] In certain embodiments, the ultrafine metal carbide particles made in accordance with the present invention have a calculated equivalent spherical diameter of no more than 200 nanometers, such as no more than 100 nanometers, or, in certain embodiments, 5 to 50 nanometers. As will be understood by those skilled in the art, a calculated equivalent spherical diameter can be determined from the B.E.T. specific surface area according to the following equation:

Diameter (nanometers) = 6000 / [BET(m²/g) _* p (grams/cm³)]

[0020] In certain embodiments, the ultrafine metal carbide particles have an average particle size of no more than 100 nanometers, in some cases, no more than 50 nanometers or, in yet other cases, no more than 30 or 40 nanometers. As used herein, the term "average particle size" refers to a particle size as determined by visually examining a micrograph of a transmission electron microscopy ("TEM") image, measuring the diameter of the particles in the image, and calculating the average particle size of the measured particles based on magnification of the TEM image. One of ordinary skill in the art will understand how to prepare such a TEM image and determine the average particle size based on the magnification. The size of a particle refers to the smallest diameter sphere that will completely enclose the individual particle.

[0021] In accordance with certain embodiments of the invention, dopant precursor materials are provided along with metal and carbon precursors as the starting materials for the production of the ultrafine metal carbide particles. Dopant materials that may be incorporated in the ultrafine metal carbide particles include Al, Ti, W, Zr, Mg, N, Fe, Na, Ca, Si, Y, La, Hf, Ta, Mo, Ni, Co, V, Nb, Ce, Mn, Li, Nd and the like. The dopant is typically present in an amount up to about 10 atomic percent, for example, from about 0.01 to about 2 or 5 atomic percent. [0022] As the ultrafine metal carbide particles are formed, the dopant is incorporated in each of the ultrafine metal carbide particles and/or on the surface of the ultrafine metal carbide particles. The dopant is therefore uniformly distributed on a submicron or nano scale, which provides uniform dispersion of the dopant when the ultrafine particles are subsequently sintered or otherwise consolidated. Materials sintered from the ultrafine doped metal carbide particles are substantially devoid of non- uniformities and defects that normally would result from doping, thus resulting in significantly improved mechanical properties.

[0023] As used herein, the term "substantially uniformly distributed in the ultrafine metal carbide particles", when referring to the dopant material, means that the dopant is incorporated within the ultrafine metal carbide particles and/or on the surfaces of the ultrafine metal carbide particles such that the dopant is evenly distributed with the powder on a submicron scale. Standard transmission electron microscopy (TEM) techniques may be used to determine such uniform dopant distributions. When such ultrafine metal carbide particles are subsequently sintered or otherwise consolidated, the resultant products comprise ultrafine dopant materials uniformly distributed throughout the consolidated body. Such dopant materials are typically less than 100 nm in size, for example, less than 50 nm.

[0024] Fig. 1 is a flow diagram depicting certain embodiments of the methods of the present invention. A metal-containing precursor, carbon-containing precursor and dopant are provided as feed materials. In the embodiment shown in Fig. 1, the precursors and dopant are provided from three separate sources. However, the feed materials may be provided from a single source or from multiple sources.

[0025] In one embodiment, the metal-containing and carbon-containing precursors may be provided in liquid form. The term "liquid precursor" means a precursor material that is liquid at room temperature. In accordance with certain embodiments in which boron carbide powders are produced, suitable liquid boron- containing precursors include borate esters and other compounds containing boron- oxygen bonds. For example, the liquid boron-containing precursor may comprise trimethylboroxine, trimethylborate and/or triethylborate. The carbon-containing precursor may be in liquid form and may comprise aliphatic carbon atoms and/or aromatic carbon atoms. For example, the liquid carbon-containing precursor may comprise acetone, iso-octane and/or toluene. In certain embodiments, the liquid carbon- containing precursor may comprise an organic liquid with a relatively high C:H atomic ratio, e.g., greater than 1:3 or greater than 1:2. Furthermore, such liquid hydrocarbon precursors may also have a relatively high C:O atomic ratio, e.g., greater than 2:1 or greater than 3:1.

[0026] In certain embodiments, a single liquid may be provided as the feed material. For example, boron-containing compounds such as B₂O₃ or borax particles may be suspended or dissolved in an organic liquid such as methanol, glycerol, ethylene glycol or dimethyl carbonate. Thus, the liquid boron-containing precursor and liquid carbon-containing precursor may comprise hydrocarbon solvents in which particulate boron-containing precursors are at least partially suspended or dissolved. As another example, polypropylene powder may be suspended in trimethylborate liquid.

[0027] In accordance with certain embodiments, the ratio of boron-containing precursor to carbon-containing precursor is controlled in order to control the composition of the resultant boron carbide and/or in order to control the formation of excess boron or excess carbon in the ultrafine boron carbide particles. For example, if an excess amount of boron-containing precursor is used, excess boron may form on or in the ultrafine boron carbide particles, which may react with oxygen or air to form oxide compounds. As a further example, an excess amount of carbon-containing precursor in the starting feed material may cause the formation of graphite on or in the resultant boron carbide particles.

[0028] In certain embodiments, the metal and boron precursors and the dopant(s) may be provided in solid particulate form. For example, ultrafine boron carbide particles may be produced from B₂O₃ as the boron source, carbon black or a polymer such as polypropylene as the carbon source, and a metal, metal oxide, metal carbonate, metal salt, solid organometallic or metal hydroxide as the dopant source. Alternatively, the carbon source may be a liquid as described above, or a gas such as methane or natural gas.

[0029] As shown in Fig. 1 , in accordance with certain methods of the present invention, the metal-containing precursor, carbon-containing precursor and dopant are contacted with a carrier. The carrier may be a gas that acts to suspend or atomize the precursors in the gas, thereby producing a gas-stream in which the precursors are entrained. Suitable carrier gases include, but are not limited to, argon, helium, hydrogen, or a combination thereof.

[0030] Next, in accordance with certain embodiments of the present invention, the precursors and dopant(s) are heated by a plasma system, e.g., as the entrained precursors flow into a plasma chamber, yielding a gaseous stream of the precursors and/or their vaporized or thermal decomposition products and/or their reaction products. In certain embodiments, the precursors are heated to a temperature ranging from 1,500° to 20,000°C, such as 1,700° to 8,000°C.

[0031] In certain methods of the present invention, after the gaseous stream is produced, it is contacted with one or more quench streams that are injected into the plasma chamber through at least one quench stream injection port. For example, the quench streams are injected at flow rates and injection angles that result in impingement of the quench streams with each other within the gaseous stream. The material used in the quench streams is not limited, so long as it adequately cools the gaseous stream to facilitate the formation or control the particle size of the ultrafine doped metal carbide particles. Materials suitable for use in the quench streams include, but are not limited to, inert gases such as argon, helium, nitrogen, hydrogen gas, ammonia, mono, di and polybasic alcohols, hydrocarbons, amines and/or carboxylic acids.

[0032] In certain embodiments, the particular flow rates and injection angles of the various quench streams may vary, so long as they impinge with each other within the gaseous stream to result in the rapid cooling of the gaseous stream. For example, the quench streams may primarily cool the gaseous stream through dilution, rather than adiabatic expansion, thereby causing a rapid quenching of the gaseous stream, before, during and/or after the formation of the ultrafine metal carbide particles prior to passing the particles into and through a converging member, such as a converging-diverging nozzle, as described below.

[0033] In certain embodiments of the invention, after contacting the gaseous product stream with the quench streams to cause production of ultrafine doped metal carbide particles, the ultrafine particles may be passed through a converging member, wherein the plasma system is designed to minimize the fouling thereof. In certain embodiments, the converging member also comprises a diverging section, e.g., a converging-diverging (De Laval) nozzle. In these embodiments, while the converging- diverging nozzle may act to cool the product stream to some degree, the quench streams perform much of the cooling so that a substantial amount of the ultrafine doped metal carbide particles are formed upstream of the nozzle. In these embodiments, the converging-diverging nozzle may primarily act as a choke position that permits operation of the reactor at higher pressures, thereby increasing the residence time of the materials therein. The combination of quench stream dilution cooling with a converging-diverging nozzle appears to provide a commercially viable method of producing ultrafine particles using a plasma system, since, for example, in certain embodiments the feed materials can be used effectively without the necessity of heating the feed materials to a gaseous state before injection into the plasma. Alternatively, liquid feed materials may be vaporized prior to introduction to the plasma system.

[0034] As is seen in Fig. 1, in certain embodiments of the methods of the present invention, after the ultrafine doped metal carbide particles exit the plasma system, they are collected. Any suitable means may be used to separate the ultrafine particles from the gas flow, such as, for example, a bag filter, cyclone separator or deposition on a substrate.

[0035] Fig. 2 is a partially schematic sectional diagram of an apparatus for producing ultrafine doped metal carbide particles in accordance with certain embodiments of the present invention. A plasma chamber 20 is provided that includes a feed inlet 50 which, in the embodiment shown in Fig. 2, is used to introduce a mixture of the metal-containing precursor, carbon-containing precursor and dopant into the plasma chamber 20. In another embodiment, the feed inlet 50 may be replaced with one or more separate inlets (not shown) for the metal-containing precursor, carbon-containing precursor and/or dopant(s). Also provided is at least one carrier gas feed inlet 14, through which a carrier gas flows in the direction of arrow 30 into the plasma chamber 20. The carrier gas may act to suspend or atomize the precursors in the gas, thereby producing a gas-stream with the entrained precursors which flows towards plasma 29. Numerals 23 and 25 designate cooling inlet and outlet respectively, which may be present for a double-walled plasma chamber 20. In these embodiments, coolant flow is indicated by arrows 32 and 34.

[0036] In the embodiment depicted by Fig. 2, a plasma torch 21 is provided. The torch 21 may thermally decompose or vaporize the metal-containing precursor, carbon- containing precursor and dopant(s) within or near the plasma 29 as the stream is delivered through the inlet of the plasma chamber 20, thereby producing a gaseous stream. As is seen in Fig. 2, the precursors are, in certain embodiments, injected downstream of the location where the arc attaches to the annular anode 13 of the plasma generator or torch.

[0037] A plasma is a high temperature luminous gas which is at least partially (1 to 100%) ionized. A plasma is made up of gas atoms, gas ions, and electrons. A thermal plasma can be created by passing a gas through an electric arc. The electric arc will rapidly heat the gas by resistive and radiative heating to very high temperatures within microseconds of passing through the arc. The plasma is often luminous at temperatures above 9,000 K.

[0038] A plasma can be produced with any of a variety of gases. This can give excellent control over any chemical reactions taking place in the plasma as the gas may be inert, such as argon, helium, or neon, reductive, such as hydrogen, methane, ammonia, and carbon monoxide, or oxidative, such as oxygen, nitrogen, and carbon dioxide. Inert or reductive gas mixtures may be used to produce ultrafine doped metal carbide particles in accordance with the present invention. In Fig. 2, the plasma gas feed inlet is depicted at 31.

[0039] As the gaseous product stream exits the plasma 29 it proceeds towards the outlet of the plasma chamber 20. An additional reactant, as described earlier, can optionally be injected into the reaction chamber prior to the injection of the quench streams. A supply inlet for the additional reactant is shown in Fig. 2 at 33.

[0040] As is seen in Fig. 2, in certain embodiments of the present invention, the gaseous stream is contacted with a plurality of quench streams which enter the plasma chamber 20 in the direction of arrows 41 through a plurality of quench stream injection ports 40 located along the circumference of the plasma chamber 20. As previously indicated, the particular flow rate and injection angle of the quench streams is not limited so long as they result in impingement of the quench streams 41 with each other within the gaseous stream, in some cases at or near the center of the gaseous stream, to result in the rapid cooling of the gaseous stream to control the particle size of the ultrafine doped metal carbide particles. This may result in a quenching of the gaseous stream through dilution.

[0041] In certain methods of the present invention, contacting the gaseous stream with the quench streams may result in the formation and/or control of the particle size of the ultrafine particles, which are then passed into and through a converging member. As used herein, the term "converging member" refers to a device that restricts passage of a flow therethrough, thereby controlling the residence time of the flow in the plasma chamber due to pressure differential upstream and downstream of the converging member.

[0042] In certain embodiments, the converging member comprises a converging- diverging (De Laval) nozzle, such as that depicted in Fig. 2, which is positioned within the outlet of the plasma chamber 20. The converging or upstream section of the nozzle, i.e., the converging member, restricts gas passage and controls the residence time of the materials within the plasma chamber 20. It is believed that the contraction that occurs in the cross sectional size of the stream as it passes through the converging portion of nozzle 22 changes the motion of at least some of the flow from random directions, including rotational and vibrational motions, to a straight line motion parallel to the plasma chamber axis. In certain embodiments, the dimensions of the plasma chamber 20 and the material flow are selected to achieve sonic velocity within the restricted nozzle throat.

[0043] As the confined stream of flow enters the diverging or downstream portion of the nozzle 22, it is subjected to an ultra fast decrease in pressure as a result of a gradual increase in volume along the conical walls of the nozzle exit. By proper selection of nozzle dimensions, the plasma chamber 20 can be operated at atmospheric pressure, or slightly less than atmospheric pressure, or, in some cases, at a pressurized condition, to achieve the desired residence time, while the chamber 26 downstream of the nozzle 22 is maintained at a vacuum pressure by operation of a vacuum producing device, such as a vacuum pump 60. Following passage through nozzle 22, the ultrafine doped metal carbide particles may then enter a cool down chamber 26.

[0044] As is apparent from Fig. 2, in certain embodiments of the present invention, the ultrafine doped metal carbide particles may flow from cool down chamber 26 to a collection station 27 via a cooling section 45, which may comprise, for example, a jacketed cooling tube. In certain embodiments, the collection station 27 comprises a bag filter or other collection means. A downstream scrubber 28 may be used if desired to condense and collect material within the flow prior to the flow entering vacuum pump 60.

[0045] In certain embodiments, the residence times for materials within the plasma chamber 20 are on the order of milliseconds. When the metal-containing and carbon-containing precursors are provided in liquid form, they may be injected under pressure (such as from 1 to 300 psi) through a small orifice to achieve sufficient velocity to penetrate and mix with the plasma. In addition, in many cases the injected liquid stream is injected normal (90° angle) to the flow of the plasma gases. In some cases, positive or negative deviations from the 90° angle by as much as 30° may be desired.

[0046] Fig. 3 is a partially schematic diagram of an apparatus for producing ultrafine particles in accordance with certain embodiments of the present invention. A plasma chamber 120 is provided that includes a precursor feed inlet 150. Also provided is at least one carrier gas feed inlet 114, through which a carrier gas flows in the direction of arrow 130 into the plasma chamber 120. As previously indicated, the carrier gas acts to suspend the precursor in the gas, thereby producing a gas-stream suspension of the precursor which flows towards plasma 129. Numerals 123 and 125 designate cooling inlet and outlet respectively, which may be present for a double- walled plasma chamber 120. In these embodiments, coolant flow is indicated by arrows 132 and 134.

[0047] In the embodiment depicted by Fig. 3, a plasma torch 121 is provided. Torch 121 thermally decomposes the incoming gas-stream suspension of precursors within the resulting plasma 129 as the stream is delivered through the inlet of the plasma chamber 120, thereby producing a gaseous product stream. As is seen in Fig. 3, the precursors are, in certain embodiments, injected downstream of the location where the arc attaches to the annular anode 113 of the plasma generator or torch. [0048] In Fig. 3, the plasma gas feed inlet is depicted at 131. As the gaseous product stream exits the plasma 129 it proceeds towards the outlet of the plasma chamber 120. As is apparent, a reactant, as described earlier, can be injected into the reaction chamber prior to the injection of the quench streams. A supply inlet for the reactant is shown in Fig. 3 at 133.

[0049] As is seen in Fig. 3, in certain embodiments of the present invention, the gaseous product stream is contacted with a plurality of quench streams which enter the plasma chamber 120 in the direction of arrows 141 through a plurality of quench stream injection ports 140 located along the circumference of the plasma chamber 120. As previously indicated, the particular flow rate and injection angle of the quench streams is not limited so long as they result in impingement of the quench streams 141 with each other within the gaseous product stream, in some cases at or near the center of the gaseous product stream, to result in the rapid cooling of the gaseous product stream to produce ultrafine particles. This results in a quenching of the gaseous product stream through dilution to form ultrafine particles.

[0050] In certain embodiments of the present invention, such as is depicted in Fig. 3, one or more sheath streams are injected into the plasma chamber upstream of the converging member. As used herein, the term "sheath stream" refers to a stream of gas that is injected prior to the converging member and which is injected at flow rate(s) and injection angle(s) that result in a barrier separating the gaseous product stream from the plasma chamber walls, including the converging portion of the converging member. The material used in the sheath stream(s) is not limited, so long as the stream(s) act as a barrier between the gaseous product stream and the converging portion of the converging member, as illustrated by the prevention, to at least a significant degree, of material sticking to the interior surface of the plasma chamber walls, including the converging member. For example, materials suitable for use in the sheath stream(s) include, but are not limited to, those materials described earlier with respect to the quench streams. A supply inlet for the sheath stream is shown in Fig. 3 at 170 and the direction of flow is indicated by numeral 171.

[0051] By proper selection of the converging member dimensions, the plasma chamber 120 can be operated at atmospheric pressure, or slightly less than atmospheric pressure, or, in some cases, at a pressurized condition, to achieve the desired residence time, while the chamber 126 downstream of the converging member 122 is maintained at a vacuum pressure by operation of a vacuum producing device, such as a vacuum pump 60. Following production of the ultrafine particles, they may then enter a cool down chamber 26.

[0052] As is apparent from Fig. 3, in certain embodiments of the present invention, the ultrafine particles may flow from cool down chamber 126 to a collection station 127 via a cooling section 145, which may comprise, for example, a jacketed cooling tube. In certain embodiments, the collection station 127 comprises a bag filter or other collection means. A downstream scrubber 128 may be used if desired to condense and collect material within the flow prior to the flow entering vacuum pump 160.

[0053] The precursors may be injected under pressure (such as greater than 1 to 100 atmospheres) through a small orifice to achieve sufficient velocity to penetrate and mix with the plasma. In addition, in many cases the injected stream of precursors is injected normal (90° angle) to the flow of the plasma gases. In some cases, positive or negative deviations from the 90° angle by as much as 30° may be desired.

[0054] The high temperature of the plasma may rapidly decompose and/or vaporize the precursors. There can be a substantial difference in temperature gradients and gaseous flow patterns along the length of the plasma chamber. It is believed that, at the plasma arc inlet, flow is turbulent and there is a high temperature gradient from temperatures of about 20,000 K at the axis of the chamber to about 375 K at the chamber walls. At the nozzle throat, it is believed, the flow is laminar and there is a very low temperature gradient across its restricted open area.

[0055] The plasma chamber is often constructed of water cooled stainless steel, nickel, titanium, copper, aluminum, or other suitable materials. The plasma chamber can also be constructed of ceramic materials to withstand a vigorous chemical and thermal environment.

[0056] The plasma chamber walls may be internally heated by a combination of radiation, convection and conduction. In certain embodiments, cooling of the plasma chamber walls prevents unwanted melting and/or corrosion at their surfaces. The system used to control such cooling should maintain the walls at as high a temperature as can be permitted by the selected wall material, which often is inert to the materials within the plasma chamber at the expected wall temperatures. This is true also with regard to the nozzle walls, which may be subjected to heat by convection and conduction.

[0057] The length of the plasma chamber is often determined experimentally by first using an elongated tube within which the user can locate the target threshold temperature. The plasma chamber can then be designed long enough so that the materials have sufficient residence time at the high temperature to reach an equilibrium state and complete the formation of the desired end products.

[0058] The inside diameter of the plasma chamber may be determined by the fluid properties of the plasma and moving gaseous stream. It should be sufficiently great to permit necessary gaseous flow, but not so large that recirculating eddys or stagnant zones are formed along the walls of the chamber. Such detrimental flow patterns can cool the gases prematurely and precipitate unwanted products. In many cases, the inside diameter of the plasma chamber is more than 100% of the plasma diameter at the inlet end of the plasma chamber.

[0059] In certain embodiments, the converging section of the nozzle has a high aspect ratio change in diameter that maintains smooth transitions to a first steep angle (such as > 45°) and then to lesser angles (such as < 45°) leading into the nozzle throat. The purpose of the nozzle throat is often to compress the gases and achieve sonic velocities in the flow. The velocities achieved in the nozzle throat and in the downstream diverging section of the nozzle are controlled by the pressure differential between the plasma chamber and the section downstream of the diverging section of the nozzle. Negative pressure can be applied downstream or positive pressure applied upstream for this purpose. A converging-diverging nozzle of the type suitable for use in the present invention is described in United States Patent No. RE37,853 at col. 9, line 65 to col. 11, line 32, the cited portion of which being incorporated by reference herein.

[0060] The following examples are intended to illustrate certain embodiments of the present invention, and are not intended to limit the scope of the invention. Example 1

[0061] Undoped ultrafine boron carbide particles were produced using a DC thermal plasma reactor system similar to that shown in Fig. 2. The main reactor system included a DC plasma torch (Model SG-100 Plasma Spray Gun commercially available from Praxair Technology, Inc., Danbury, Connecticut) operated with 80 standard liters per minute of argon carrier gas and 24 kilowatts of power delivered to the torch. A liquid precursor feed composition comprising the materials and amounts listed in Table 1 was prepared and fed to the reactor at a rate of 7 grams per minute through a gas assisted liquid nebulizer located about 0.5 inch down stream of the plasma torch outlet. At the nebulizer, 15 standard liters per minute of argon were delivered to assist in atomization of the liquid precursors. Following a 10 inch long reactor section, a plurality of quench stream injection ports were provided that included 6 1/8 inch diameter nozzles located 60° apart radially. A 7 millimeter diameter converging-diverging nozzle was provided 4 inches downstream of the quench stream injection port. Quench argon gas was injected through the quench stream injection ports at a rate of 145 standard liters per minute. The produced undoped boron carbide particles were purified using methanol to remove boron oxide and/or boric acid.

Table 1

Commercially available from Alfa Aesar, Ward Hill, Massachusetts.

[0062] The measured B.E.T. specific surface area of the produced material was 30 square meters per gram using a Gemini model 2360 analyzer (available from Micromeritics Instrument Corp., Norcross, Georgia), and the calculated equivalent spherical diameter was 79 nanometers.

Example 2

[0063] Ultrafine boron carbide particles from nitrogen-containing liquid precursors were prepared using the apparatus and conditions identified in Example 1 , with the feed materials and amounts listed in Table 2. Table 2

[0064] The produced boron carbide particles were purified using methanol to remove boron oxide and/or boric acid. The measured B.E.T. specific surface area of the produced material was 33 square meters per gram using a Gemini model 2360 analyzer (available from Micromeritics Instrument Corp., Norcross, Georgia), and the calculated equivalent spherical diameter was 72 nanometers.

Example 3

[0065] Ultrafine aluminum doped boron carbide particles from liquid precursors were prepared using the apparatus and conditions identified in Example 1 and the feed materials and amounts listed in Table 3.

Table 3

Commercially available from Alfa Aesar, Ward Hill, Massachusetts.

[0066] The produced particles had a theoretical composition of 5 atomic percent aluminum and the balance boron carbide. The produced aluminum doped boron carbide particles were purified using methanol to remove boron oxide and/or boric acid. The measured B.E.T. specific surface area of the produced material was 83 square meters per gram using a Gemini model 2360 analyzer (available from Micromeritics Instrument Corp., Norcross, Georgia), and the calculated equivalent spherical diameter was 29 nanometers. Fig. 4 is a micrograph of a transmission electron microscopy (TEM) image of a representative portion of the particles. The micrograph was prepared by weighing out 0.2 to 0.4 grams of the particles and adding those particles to methanol present in an amount sufficient to yield an adequate particle density on a TEM grid. The mixture was placed in a sonicater for 20 minutes and then dispersed onto a 3 millimeter TEM grid coated with a uniform carbon film using a disposable pipette. After allowing the methanol to evaporate, the grid was loaded into a specimen holder which was then inserted into a TEM instrument. Elements of boron and aluminum were identified and mapped using electron energy loss spectroscopy (EELS) of the TEM image of Fig. 4. Fig. 5 and Fig. 6 are EELS images for elemental boron and elemental aluminum, respectively. The uniform distribution of aluminum with respect to the boron carbide particles is as shown in Fig. 6.

[0067] The aluminum doped boron carbide particles were then sintered by uniaxially pressing at 300 MPa, followed by heating of the pressed pellet in a helium atmosphere at a rate of 10⁰C per minute to a temperature of 2,300⁰C, holding for 100 minutes, then cooling to room temperature.

Example 4

[0068] Ultrafine multielement doped boron carbide particles from liquid precursors were prepared using the apparatus and conditions identified in Example 1 and the feed materials and amounts listed in Table 4.

Table 4

Commercially available from Alfa Aesar, Ward Hill, Massachusetts.

[0069] The produced particles had a theoretical composition of 0.6 atomic percent iron, 0.3 atomic percent tungsten, 0.35 atomic percent silicon, 0.1 atomic percent titanium, 0.06 atomic percent aluminum, 0.06 atomic percent calcium, 0.02 atomic percent sodium, and the balance boron carbide. The produced doped boron carbide particles were purified using methanol to remove boron oxide and/or boric acid. The measured B.E.T. specific surface area of the produced material was 30 square meters per gram using a Gemini model 2360 analyzer (available from Micromeritics Instrument Corp., Norcross, Georgia), and the calculated equivalent spherical diameter was 79 nanometers.

[0070] Additional doped ultrafine boron carbide particle samples were produced as shown in Table 5. The percentages listed are theoretical atomic percentages.

Table 5

[0071] It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the foregoing description. Such modifications are to be considered as included within the following claims unless the claims, by their language, expressly state otherwise. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims

WE CLAIM:

1. A method for making ultrafine doped metal carbide particles comprising: introducing a metal-containing precursor, a carbon-containing precursor and at least one dopant into a plasma; heating the precursors and the at least one dopant by the plasma to form the ultrafine doped metal carbide particles from the precursors; and collecting the ultrafine doped metal carbide particles.

2. The method of Claim 1, wherein the metal carbide comprises boron carbide, tungsten carbide, titanium carbide, silicon carbide, aluminum carbide, iron carbide, and zirconium carbide.

3. The method of Claim 1, wherein the metal carbide comprises boron carbide.

4. The method of Claim 1 , wherein the ultrafine doped metal carbide particles comprise B₄C, B₁₃C₂, B₈C, B₁₀C and/or B₂₅C.

5. The method of Claim 1, wherein the ultrafine doped metal carbide particles comprise B₄C.

6. The method of Claim 1, wherein the dopant comprises Al, Ti, W, Zr, Mg, N, Fe, Na, Ca, Si, Y, La, Hf, Ta, Mo, Ni, Co, V, Nb, Ce, Mn, Li and/or Nd.

7. The method of Claim 1, wherein the dopant comprises Al, Ti, W, Zr and/or Mg.

8. The method of Claim 1 , wherein the dopant is present in the ultrafine metal carbide particles in an amount up to about 10 atomic percent.

9. The method of Claim 1, wherein the dopant is present in the ultrafine metal carbide particles in an amount of from about 0.01 to about 2 atomic percent.

10. The method of Claim 1, wherein the dopant is substantially uniformly distributed in the ultrafine metal carbide particles.

11. The method of Claim 1 , wherein the ultrafine doped metal carbide particles have an average particle size of less than 100 nm.

12. The method of Claim 12, wherein the dopant material in the ultrafine doped metal carbide particles has an average size of less than 100 nm.

13. The method of Claim 1, wherein the metal-containing precursor is introduced in liquid form.

14. The method of Claim 1, wherein the carbon-containing precursor is introduced in liquid form.

15. The method of Claim 1, wherein the dopant is introduced in liquid form.

16. The method of Claim 1, wherein the metal-containing precursor is introduced in solid form.

17. The method of Claim 1, wherein the carbon-containing precursor is introduced in solid form.

18. The method of Claim 1, wherein the dopant is introduced in solid form.

19. The method of Claim 1, further comprising mixing the metal- containing precursor, carbon-containing precursor and at least one dopant before the introduction into the plasma.

20. The method of Claim 1, further comprising contacting the metal- containing precursor, carbon-containing precursor and at least one dopant with a carrier gas before the introduction into the plasma.

21. An ultrafine metal carbide powder produced by the method of Claim 1.

22. Ultrafine doped metal carbide particles comprising: ultrafine metal carbide particles having an average size of less than 100 nm; and dopant material substantially uniformly distributed in the ultrafine metal carbide particles.

23. The ultrafine doped metal carbide particles of Claim 22, wherein the metal carbide comprises boron carbide and the dopant comprises up to about 10 atomic percent Al, Ti, W, Zr, Mg, N, Fe, Na, Ca, Si, Y, La, Hf, Ta, Mo, Ni, Co, V, Nb, Ce, Mn, Li and/or Nd.

24. A sintered material comprising ultrafine metal carbide particles sintered in the presence of dopant material substantially uniformly distributed in the ultrafine metal carbide particles.

25. The sintered material of claim 24, wherein the metal carbide comprises boron carbide and the dopant comprises up to about 10 atomic percent Al, Ti, W, Zr, Mg, N, Fe, Na, Ca, Si, Y, La, Hf, Ta, Mo, Ni, Co, V, Nb, Ce, Mn, Li and/or Nd.