Patent Description:
Titanium diboride powder, among other metal and alloy boride powders, are promising materials for ultrahigh temperature ceramic applications because of the properties they exhibit including excellent hardness, high melting point, wear resistance, good thermal and electrical conductivity, and chemical inertness. Conventional manufacture methods utilize carbothermal reduction, mechanical alloying, sol-gel methods, or high temperature synthesis. Carbothermal reduction is a simple and commonly used method but leads to impurities, unwanted grain size, extensive subsequent processing, and added cost. Titanium diboride (among other metal and alloy borides) is a promising material for engineering applications including abrasive and cutting tools, wear-resistance coating, cemented carbide, cathodes for aluminum electrolysis cells, and crucibles for holding molten metals. Titanium alloy/titanium boride reinforced composites offer high stiffness and strength from reinforcing boride particles and toughness from the titanium alloy matrix. Other transition metal borides are also promising materials for ultrahigh temperature ceramics applications and are gaining attention for new hypersonic vehicle developments.

Given the growing application for metal boride powders, an improved method of production is needed that overcomes the drawbacks of conventional production methods.

A prior art method and assembly for boriding metals using a fluidized bed is disclosed in <CIT>.

In one aspect of the present disclosure, a method for producing a powder in accordance with claim <NUM> is provided.

In another aspect of the present disclosure, an assembly for producing a powder in accordance with claim <NUM> is provided.

The present summary is provided only by way of example, and not limitation. Other aspects of the present disclosure will be appreciated in view of the entirety of the present disclosure, including the entire text, claims and accompanying figures.

While the above-identified figures set forth embodiments of the present invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features, steps and/or components not specifically shown in the drawings.

Titanium diboride powders, among other metal and alloy boride powders, are promising materials for ultrahigh temperature ceramic applications. The present disclosure seeks to overcome drawbacks of conventional manufacture of these powders by combining boriding powders and fluidized bed technology to produce a boriding gas stream that can be delivered to a second fluidized bed reactor for incorporation into the lattice structure of a metal oxide, metal hydroxide, or alloy powders.

<FIG> is a schematic view of dual-fluidized bed reactor assembly <NUM> for producing metal boride and boron-doped powders. Assembly <NUM> includes first fluidized bed reactor <NUM> (hereinafter, "first reactor"), configured to produce a boriding gas stream, and second fluidized bed reactor <NUM> (hereinafter, "second reactor"), fluidly connected to first fluidized bed reactor <NUM> and configured to produce a metal boride or boron-doped powder for use in commercial applications. First and second reactors <NUM>, <NUM> are fluidly connected by conduit <NUM>, which can include a plurality of interconnected conduits (unnumbered) and valves <NUM>, <NUM>, <NUM>, <NUM>, and which can deliver the boriding gas stream to and from a plurality of optional contaminant scrubbers <NUM>, <NUM>, <NUM>, filters <NUM>, <NUM>, and heat exchangers <NUM>, <NUM> to produce an uncontaminated and heated stream of boriding gas suitable for boriding powders of metal oxides, metal hydroxides, and alloys in second reactor <NUM>.

First and second reactors <NUM> and <NUM> are known in the art and can be of substantially identical configuration although operated using different operational parameters and conditions. For simplicity, first and second reactors <NUM> and <NUM> are described congruently, although it should be appreciated that first and second reactors <NUM> and <NUM> can differ and may be individually optimized to accommodate differing operations.

First and second reactors <NUM> and <NUM> include furnace 40a/40b, reactor chamber 42a/42b, fluidizing gas inlet 44a/44b connected to inert gas source 46a/46b, and exhaust gas outlet 48a/48b. Furnace 40a/40b can be a single-zone or multi-zone furnace as illustrated in the embodiments in <FIG>, which include three heating zones, each with temperature controls 50a/50b. Multiple heating zones are not necessary but can provide better temperature control and uniformity of temperature throughout reactor chamber 42a/42b. Furnace 40a can be capable of heating reactor chamber 42a and maintaining a reactor chamber temperature in excess of <NUM>,<NUM> degrees Celsius for up to <NUM> hours. Furnace 40b can generally be operated at a lower temperature but can be configured to heat reactor chamber 42b and maintain a reactor chamber temperature in excess of <NUM> degrees Celsius for up to <NUM> hours.

Reactor chamber 42a/42b is positioned within a chamber of furnace 40a/40b. Reactor chamber 42a/42b can be suspended from a top of furnace 40a/40b by tube 52a/52b, such that reactor chamber walls are spaced apart from furnace walls to allow fluidizing gas to flow along an outer wall of reactor chamber 42a/42b before entering reactor chamber 42a/42b at a bottom end. Reactor chamber 42a/42b has porous plate 54a/54b on a bottom end which retains a powder within reactor chamber 42a/42b while allowing a fluidizing gas to enter reactor chamber 42a/42b.

An inert gas, such as argon or helium, can enter an inner chamber of furnace 40a/40b through gas inlet 44a/44b at the top of first/second reactor <NUM>/<NUM>. The fluidizing gas is heated in the furnace before entering reactor chamber 42a/42b. In alternative embodiments, fluidizing gas can also be heated in a preheater (not shown) upstream of furnace 40a/40b. Powder within reactor chamber 42a/42b can be fluidized by the inert gas, mixed, and heated within reactor chamber 42a/42b. Exhaust gas can exit first and second reactors <NUM> and <NUM> through exhaust gas outlet 48a/48b.

First reactor <NUM> is configured to fluidize and decompose boriding powders contained in reactor chamber 42a. Boriding powders can include commercially available boriding powders, such as EKABOR(R)<NUM>. Preferred boriding powders can include but are not limited to mixtures including <NUM>% boron carbide (B<NUM>C) and <NUM>% sodium carbonate (Na<NUM>CO<NUM>), <NUM>% B<NUM>C and <NUM>% Borax (Na<NUM>B<NUM>O<NUM>), and <NUM>% B<NUM>C. Some boriding powders contain large amounts of diluent such as silicon carbide (SiC) in addition to activators like potassium tetrafluoroborate (KBF<NUM>), sodium tetrafluoroborat (NaBF<NUM>) and ammonium tetrafluoroborate (NH<NUM>BF<NUM>). Diluents can be chosen from either Al<NUM>O<NUM>, SiC, ZrO<NUM>, or varying combinations. The presence of SiC can result in competitive simultaneous siliconizing, while KBF<NUM> can introduce safety issue due to the potential for hydrogen fluoride (HF) formation. For this reason, it may be desirable to avoid such powders. The presence of small amounts of KBF<NUM> can be handled by including scrubbers <NUM>-<NUM> to remove any HF or fluorinated species produced in the production of the boriding gas stream. Scrubbers <NUM>-<NUM> may be left out of assembly <NUM> depending on the composition of the boriding powder. Alternatively, a bypass conduit <NUM> can cause the boriding gas stream to bypass scrubbers <NUM>-<NUM> for boriding powders that do not contain KBF<NUM> or other fluorinated compounds.

Boriding powder can have particle sizes ranging from <NUM> microns to <NUM> millimeters. The fluidizing velocity can be set in accordance with particle size, with larger particles requiring substantially higher fluidizing velocities of the inert gas stream (e.g., a maximum fluidizing velocity to circulate <NUM> micron particles can be <NUM>/s, while a maximum fluidizing velocity to circulate <NUM> millimeter particles can be <NUM>/s). Fluidizing velocity can also be adjusted based on the temperature of reactor chamber 42a, with higher fluidizing velocity generally required at lower temperatures. Fluidizing velocity can be controlled by valve 58a and/or mass flow controller 60a.

The temperature required to decompose boriding powder to produce the boriding gas stream can vary depending on the powder composition. Typically, temperatures between <NUM> and <NUM>,<NUM> degrees Celsius can cause decomposition, however, higher temperatures, including up to <NUM>,<NUM> degrees Celsius, may be necessary for some powder compositions. Temperatures as low as <NUM> degrees Celsius may also be suitable for the decomposition of some boriding powder compositions. The boron source (i.e. B<NUM>C, amorphous boron, etc.) decomposes to form gas streams containing boron trihalides (e.g. BF<NUM>, BCl<NUM>). For instance, when a boriding mixture containing amorphous boron and NH<NUM>Cl is decomposed, the resulting gas stream can contain HCl, BCl<NUM>, BCl<NUM>H, BCl<NUM>, BH<NUM>, BCl, Cl, and BCIH. For complete decomposition, reactor chamber 42a can be held at a decomposition temperature for a period of time ranging from <NUM> to <NUM> hours. The time required for complete decomposition can vary depending on the temperature of reactor chamber 42a, amount of material, particle size, flow conditions, and ramp rate (i.e., time to reach decomposition temperature). A slower ramp rate can reduce the required hold time once the decomposition temperature is reached.

An inline gas chromatograph/mass spectrometer (GC/MS) <NUM> can be located in fluid communication with conduit <NUM> to sample an exhaust gas from reactor chamber 42a. GC/MS <NUM> can be positioned to sample exhaust gas between outlet 48a of first reactor <NUM> and inlet 44b of second reactor <NUM>. As illustrated in <FIG>, GC/MS <NUM> can be disposed upstream of scrubbers <NUM>-<NUM>, although other locations are contemplated. GC/MS <NUM> can be used to detect the presence or absence of the boriding gas stream and provide such information in real time to a controller or operator. Exhaust gas from first reactor <NUM> can be delivered to second reactor <NUM> at any time during the decomposition process, including before the boriding gas stream is produced. However, reaction in second reactor <NUM> will not occur until boriding gas is delivered from first reactor <NUM>. GC/MS <NUM> can be used to determine a start time for reaction in second reactor <NUM> as indicated by the presence of boriding gas in the exhaust gas from reactor <NUM>. In alternative embodiments, exhaust gas from first reactor <NUM> can be vented through bypass line <NUM> to an exhaust vent <NUM> thereby bypassing second reactor <NUM> completely until GC/MS <NUM> detects the presence of the boriding gas stream in conduit <NUM>. Exhaust gas can be directed through a series of scrubbers <NUM>-<NUM>, if necessary, to remove contaminants, before venting to the atmosphere. When the boriding gas stream is detected, bypass conduit <NUM> can be closed and the boriding gas stream can be directed through conduit <NUM> to second reactor <NUM>. Before entering second reactor <NUM>, the boriding gas stream can be directed through scrubbers <NUM>-<NUM> to remove contaminants, as previously discussed, and a series of filters <NUM>, <NUM> to remove any moisture that may be present in the gas stream. Heat exchangers <NUM>, and <NUM> can be used to condense water vapor that might be present in the gas stream and to reheat the boriding gas stream, respectively.

The fluidizing velocity of the boriding gas stream may not be sufficient to fluidize powder in second reactor <NUM>. Supplemental fluidizing velocity can be provided by an inert gas (e.g., argon or helium) from inert gas source 46b. The velocity of inert gas can be controlled by valve 58b and/or mass flow controller 60b. The inert gas stream and boriding gas stream can be mixed in mixer <NUM> prior to delivery to second reactor <NUM>.

Second reactor <NUM> is configured to fluidize powder capable of producing a metal boride when heated in the presence of the boriding gas stream. Suitable powders can include metal oxides and hydroxides of Group IV, V, and VI metals, including but not limited to titanium dioxide (TiO<NUM>), and alloys. While titanium diboride (TiB<NUM>) has been recognized as a promising material for ultrahigh temperature ceramics applications, other transition metal borides are of interest, including but not limited zirconium diboride (ZrB<NUM>), hafnium diboride (HfB<NUM>), niobium diboride (NbB<NUM>), and tantalum diboride (TaB<NUM>). The risk of sintering can be increased with use of metal hydroxides and, therefore, use of metal oxide and alloy powders may be preferred. The boriding gas stream is mixed with the powders in reactor chamber 42b to produce boron-doped powders and/or metal boride. The powders in reactor chamber 42b can be metal oxides (i.e. MO<NUM>) or metal hydroxides (i.e. H<NUM>MO<NUM>), where M=metal (including alloys), O=oxygen, and H=hydrogen, or can be an alloy (e.g., titanium, aluminum, cobalt, copper or nickel base). As boriding species from the boriding gas stream contact the surface of the metal oxide or metal hydroxide powder particles, the surface oxygen and OH species will gradually be removed and replaced by the boriding species, which adsorb on the surface. This surface functionalization results in "borided" surface, or as referred to herein as a "boron-doped powder. " The boriding gas will also diffuse into the surface of alloy powders. As reaction time and temperature increase and the thickness of the surface boride layer increases, the boron species begin to diffuse into the bulk structure of the powder (displacing the bulk oxygen in oxides and hydroxides) to form bulk metal boride powders (referred to interchangeably herein as metal "diborides"). Reactor chamber 42b can be used to produce both boron-doped powders and metal boride.

It will be understood by one of ordinary skill in the art that the degree of doping or boriding will depend on the operational parameters, including but not limited to reactor temperature, fluidizing velocity, particle size, availability of boriding species (i.e., boriding gas concentration), and residence time. The boriding gas stream can be recycled through reactor chamber 42b to optimize the boriding reaction. The boriding gas stream can be recycled from exhaust outlet 48b back to inlet 44b through conduit <NUM>. Second reactor <NUM> can be held at a boriding temperature for a period of time ranging from <NUM> to <NUM> hours to allow for complete reaction of the powder in reactor chamber 42b. Similar to the decomposition of boriding powders, the boriding process time can vary depending on the temperature of reactor chamber 42b, amount of material, particle size, flow conditions, and ramp rate. Particle size of powder in reactor chamber 42b can generally range from <NUM> to <NUM>. A second inline GC/MS <NUM> or suitable sensor can be placed on an outlet conduit from exhaust outlet 48b of second reactor <NUM> to detect the presence of boriding gas stream and/or byproducts of the reaction between the boriding gas stream and metal oxide or metal hydroxide powders (e.g., carbon dioxide or water) to assist in determining if the reaction is complete and whether or not recycle of the exhaust stream should be continued.

First reactor <NUM> continues to produce the boriding gas stream and provide the boriding gas stream to second reactor <NUM> throughout the boriding process. Boriding powder can be added intermittently to reactor chamber 42a of first reactor <NUM> or upon completion of decomposition of available powder in reactor chamber 42a. Second reactor <NUM> can be maintained at a boriding temperature even in the absence of a boriding gas stream under inert gas, such that the boriding process can begin as soon as the boriding gas stream becomes available.

<FIG> is a flow diagram of method <NUM> for producing metal borides or boron-doped powders using assembly <NUM> substantially as described with respect to <FIG>. In step <NUM>, B<NUM>C is decomposed in first reactor <NUM> to produce a boriding gas stream. As disclosed with respect to <FIG>, the decomposition process can take <NUM> to <NUM> hours depending on the amount of material, flow conditions, particle size, temperature of reactor chamber 42a, and ramp rate. In step <NUM>, the boriding gas stream is delivered to second reactor <NUM>. Prior to delivery, the boriding gas stream can be passed through a series of scrubbers and/or filters to remove contaminants and moisture that may be present. If the boriding gas stream does not have a flow velocity sufficient to fluidize powder in second reactor <NUM>, supplemental inert gas can be delivered to obtain a desired fluidizing velocity in reactor chamber 42b. The fluidized powder is mixed with the boriding gas stream and heated in step <NUM> to allow boron to diffuse into the lattice structure of the metal oxides, hydroxides, or alloys to form a metal boride or boron-doped powder. When the boriding process is complete, the delivery of the boriding gas stream is discontinued as first reactor <NUM> is isolated from second reactor <NUM> and shut down. In step <NUM>, second reactor <NUM> is shut down and allowed to cool, reactor chamber 42b is purged with an inert gas, and the metal boride or boron-doped powder is removed.

Conventional metal boride powder manufacturing methods can lead to impurities, unwanted grain size, extensive subsequent processing, and added cost. Dual fluidized bed reactor assembly <NUM> overcomes many of the drawbacks of the conventional processes and can be used to produce high purity metal borides with optimal mechanical, thermal, and electrical properties.

Claim 1:
A method for producing a powder for ultrahigh temperature ceramic applications, the method comprising:
producing a boriding gas stream from a first powder in a first fluidized bed reactor (<NUM>);
delivering the boriding gas stream to a second fluidized bed reactor (<NUM>) through a conduit (<NUM>) fluidly connecting the first and second fluidized bed reactors (<NUM>, <NUM>), the second fluidized bed reactor (<NUM>) containing a second powder;
fluidizing the second powder in the second fluidized bed reactor (<NUM>); and
mixing the second powder with the boriding gas stream such that a metal boride powder or the second powder with a borided surface is formed;
wherein the first powder comprises boron carbide and the second powder is selected from the group consisting of metal oxides, metal hydroxides, and alloys, and the method further comprises:
fluidizing the boron carbide in a first chamber (42a) of the first fluidized bed reactor (<NUM>); and
heating the first chamber (42a) to a temperature ranging from approximately <NUM> to <NUM> degrees Celsius to promote decomposition of the boron carbide and formation of the boriding gas stream.