United States Patent (10) Patent No.: US 7,341.702 B2 Pulitz, Jr

USOO7341702B2

(12) United States Patent (10) Patent No.: US 7,341.702 B2 Pulitz, Jr. et al. (45) Date of Patent: Mar. 11, 2008

(54) PROCESS FOR PRODUCING BORON 3,189,412 A 6, 1965 Wood et al...... 423,390 NITRIDE 3.241,918 A 3/1966 Lenihan et al. 3,261,667 A 7, 1966 O’Connor (75) Inventors: Donald William Pulitz, Jr., Parma, OH 3,415,625. A 12, 1968 Bablet al. (US); Laurence Maniccia, Lyndhurst, 4,107,276 A 8, 1978 Schwetz et al. 4,971,779 A * 11/1990 Paine et al...... 423,290

OH (US); Chandrashekar Raman, 5,854.155.w - I A * 12, 1998 K.awasaki et al. 501 (96.4 North Royalton, OH (US); Anand 6,319,602 B1 1 1/2001 Fauzi et al...... 428/366 Murugaiah, North Olmsted, OH (US) 6,348,179 B1* 2/2002 Paine et al...... 423,277 6,824,753 B2 * 1 1/2004 Paine et al...... 423,277 (73) Assignee: Momentive Performance Materials 7,060,237 B1 * 6/2006 Paine et al...... 423,277 Inc., Wilton, CT (US) FOREIGN PATENT DOCUMENTS (*) Notice: Subject to any disclaimer, the term of this CN 1539729. A 10, 2004 patent is extended or adjusted under 35 DE 4108367 C1 8, 1992 U.S.C. 154(b) by 20 days. EP O918O39 5, 1999 EP 1053973 5, 2000 (21) Appl. No.: 11/266,157 GB 874165 8, 1961 GB 874166 8, 1961 (22) Filed: Nov. 3, 2005 GB 12412O6 8, 1961 JP 06-040713 2, 1994 (65) Prior Publication Data US 2006/0140838 A1 Jun. 29, 2006 * c1tedcited bby examiner Primary Examiner Wayne A. Langel Related U.S. Application Data (74) Attorney, Agent, or Firm—Dominick G. Vicari (60) Eyal application No. 60/639,714, filed on Dec. (57) ABSTRACT (51) Int. Cl. A process for producing boron nitride of high purity and COIB 2/064 (2006.01) sh thermal split, "In a oxygen-containing (52) U.S. Cl...... 423A290 Sourceoron incompound the presence is ofreacted a dopant with at a atemperature nitrogen-containing of at least (58) SField oflication Classification file f Searchlet ...... h hist 423A290 1000°C.O for at least one hour, and wherein the dopant forms ee appl1cauon Ille Ior complete searcn n1story. metal borate impurities with a vaporizing temperature that is (56) References Cited lower than the highest processing temperature in the pro CCSS, U.S. PATENT DOCUMENTS 2.922,699 A 1/1960 Lauzau 24 Claims, No Drawings US 7,341,702 B2 1. 2 PROCESS FOR PRODUCING BORON are combined into one single firing step, wherein the metal NITRIDE borate impurities vaporize away forming high purity boron nitride crystals. Lastly, the invention relates to an improved CROSS-REFERENCE TO RELATED process wherein the washing/leaching of the reaction prod APPLICATIONS 5 uct is optional for a boron nitride of high purity. This application claims the benefits of U.S. 60/639,714 SUMMARY OF THE INVENTION filed Dec. 28, 2004, which patent application is fully incor porated herein by reference. The invention relates to a process for producing a poly 10 crystalline hexagonal boron nitride (hBN) compound of high FIELD OF THE INVENTION purity and high thermal conductivity by reacting an oxygen containing boron compound with a nitrogen-containing The invention relates generally to a process for producing Source in the presence of a dopant at a temperature of at least hexagonal boron nitride using a dopant material for a high 1000° C. for at least one hour, wherein the dopant is a purity boron nitride product. 15 metal-containing compound that forms a metal borate with a vaporizing temperature that is lower than the highest BACKGROUND OF THE INVENTION processing temperature. Boron nitride is a thermally stable, highly refractory The invention further relates to a process for producing a material of increasing commercial significance. Typically, 20 hexagonal boron nitride compound in the presence of boron nitride is produced by processes wherein boric acid is lithium nitrate as a dopant. utilized as the boron Source of reaction compositions. Sug Lastly, the invention relates to a process for producing a gested processes for producing boron nitride from boric acid hexagonal boron nitride compound in the presence of are described in U.S. Pat. Nos. 2,922,699; 3,241,918; and lithium nitrate as a dopant, and wherein the processing 3.261,667 as well as in British Pat. Nos. 874,166; 874,165; as temperature ranges from 1000 to 2300° C. and 1,241,206. JP Patent Publication No. 06-040713 dis closes a process for producing boron nitride from coleman DESCRIPTION OF THE INVENTION ite. In these prior art processes for making boron nitride, As used herein, approximating language may be applied borate starting material containing alkali/alkali earth metal 30 to modify any quantitative representation that may vary compounds—particularly Sodium and calcium com without resulting in a change in the basic function to which pounds—when purged with ammonia at temperatures of it is related. Accordingly, a value modified by a term or 1200° C. or higher, form boron nitride plus by-products that terms, such as “about and “substantially.” may not to be require additional washing/treatments steps to recoverboron limited to the precise value specified, in some cases. nitride of high purity. Some of the by-products are various 35 The term “processing temperature' may be used inter forms of calcium borate, which are removed from the boron changeably with the term “process temperature,” which nitride by leaching with hydrochloric acid. Others processes refers to the temperature in the equipment/step in the process use a de-ionized water wash to purify the boron nitride. U.S. for making hBN in the invention. Pat. No. 3,415,625 discloses a continuous or batch process The term “highest processing temperature' or “highest for a boron nitride product of high purity after a washing/ 40 process temperature” refers to the highest process tempera treatment step. ture as measured in the steps and/or equipment used in the U.S. Pat. No. 4,045,186 discloses the use of LiN to react process of making hBN in the invention. with small particle-sized boron nitride for the subsequent As used herein, thermal conductivity of the boron nitride recrystallization of larger-sized crystalline hexagonal boron refers to the thermal conductivity of a sample prepared from nitride from the mixture at elevated temperatures of greater 45 a mixture of 40 vol. 96 hBN (approximately 60 wt.%) in than 1100° C. China Patent Publication No. CN1539729A Sylgard 184 silicone resin and Sylgard 184 curing agent, discloses a process for preparing boron nitride from boron both commercially available from Dow Corning Corp, using trifluoride ether and lithium nitride by solvent heat synthesis a commercially available Hot Disk constant thermal ana method. German Patent Publication No. DE4108367C1 dis lyzer. An illustrative example is a mixture of 10.0 g of BN closes a process to prepare boron nitride with mainly a 50 material in 6.24 g Sylgard 184 silicone resin and 0.62 g of hexagonal structure that is comprised of reacting a suspen Sylgard 184 curing agent. After mixing, the samples are sion of lithium nitride (in anhydrous di(2-6C)alkyl ether) allowed to cure and sample pads are prepared, and the with an excess of trifluoroborane di(1-6C) alkyl etherate thermal conductivity is measured. The Gage R&R data for (pref. dibutyl etherate) with stirring, at 20-230° C. for 2-24 testing thermal conductivity has shown that these results are hours. 55 accurate within +/-0.5 W/mK. In addition to the washing requirement and before the Raw Materials for use in the Process of the Invention. washing step, the prior art processes typically require a Generally in processes to produce boron nitride, a boron two-step approach to make boron nitride crystals. The first Source and a nitrogen Source are used as starting materials, step is a calcination step, wherein the reactants are heated to reacting to form a compound in which a boron atom and a a temperature of up to 1100° C., forming an incompletely 60 nitrogen atom coexist. The present invention relates to a reacted boron nitride in the “turbostratic' form. In the process to produce high purity boron nitride, wherein a second sintering step, the turbostratic boron nitride is heated dopant is mixed with the starting materials of an oxygen to a temperature of 1500 to 2300° C. to control the crystal containing boron compound and an organic nitrogen-con linity and purity of the final boron nitride product. taining compound in a reaction at an elevated temperature. The invention relates to a process for preparing boron 65 The metal borate formed during the reaction from the dopant nitride of high purity and good yield. Additionally, in the metal is vaporized, and therefore not required to be leached process of the invention, the calcination and sintering steps or washed out. US 7,341,702 B2 3 4 In one embodiment of the invention, the oxygen-contain time ranges from 4 hours at 200° C. to 15 hours at 150° C. ing boron compound is selected from the group of boric in a static environment. In yet another embodiment, the acid, boron oxide, boric oxide providing Substances such as starting material is dried at 250° C. for 3 hours. boron trioxide, diboron dioxide, tetraboron trioxide or tet In one embodiment, the reactants and the dopant are raboron pentoxide, and borate ores such as colemanite, contained in a vessel/capsule, and placed into a high tem ulexite, pandermite, danburite, datolite, and mixtures perature furnace (a pusher type furnace or a rotating kiln) up thereof. In one embodiment of the invention, the oxygen to 1000° C. for up to 2 hours. In this preheat step, the containing boron compound comprises 50 wt.% boric acid evolution of the gases is controlled by a counter-flow of and 50 wt.% ulexite. In another embodiment of the inven nitrogen directing the evolved gases toward the furnace tion, boric acid is used as the oxygen-containing boron 10 entrance. compound. Optional Crushing of the Precursors: In one embodiment, In one embodiment of the invention, the nitrogen-con after the mixing/blending step or after the drying step, the taining compound comprises organic primary, secondary starting raw materials are crushed or broken into Small and tertiary amines Such as diphenylamine, dicyandiamide, pieces that can be later densified, using conventional equip ethylene amine, hexamethylene amine, melamine, urea, and 15 ment Such as roller mills, cross beater mills, rolling discs and mixtures thereof. In one embodiment, melamine is used as the like. In one embodiment, the crushed materials are the nitrogen-containing compound. broken into pieces weighing between 10 mg to 10 g each. In The dopant for mixing with the starting materials is yet another embodiment, the materials are broken into selected from compounds that form metal borates with a pieces weighing about 0.2 g each. vaporizing temperature that is lower than the highest pro Optionally after the crushing step and when the boron cessing temperature. Examples of Such metal-containing Source is an alkali or alkali earth metal borate compound compounds include barium compounds such as barium Such as ulexite, the crushed material is mixed with silica. oxide; cesium compounds such as cesium oxide; potassium The calcium in the ulexite reacts with the silica to produce compounds such as potassium hydroxide; strontium com calcium silicate, limiting the formation of 3CaO.B.O. pounds such as strontium oxide; rubidium compounds Such 25 which may otherwise be formed, thus giving a high yield of as rubidium oxide; and lithium compounds Such as lithium BN in the final reaction. In one embodiment, the total oxide, lithium nitrate, lithium acetylacetonate, lithium amount of silica to ulexite is maintained at a molar ratio of cyclopentadienide, dilithium phthalocyanine, dilithium salt, SiO/CaO of less than 0.5. In a second embodiment, the lithium acetate, lithium acetylacetonate, lithium amide, molar ratio is maintained at a rate of less than 1.0. lithium bis(trimethylsilyl)amide, lithium tetrahydridobo 30 Optional Preheating and Densification (“Pilling) Step: In rate, lithium carbonate, lithium dimethylamide, lithium one embodiment after the mixing/blending step, the mixed hydride, lithium hydroxide, lithium metaborate, lithium precursors are dried/crushed and densified using a process molybdate, lithium niobate, lithium perchlorate, lithium known in the art Such as tableting, briquetting, extruding, peroxide, lithium tetraborate, lithium borohydride, lithium pilling, and compacting, among others. In this step, the triethylhydridoborate, and lithium oxalate; and mixtures 35 crushed mixture is densified into pellets weighing from 0.1 thereof. g to 200 g each. In one embodiment, the pellets have an In one embodiment, lithium nitrate is used as the dopant average weight of ~10 g. In a second embodiment, the material for the starting mixture, formingborate compounds crushed mixture is densified into pellets with an average that evaporate at temperatures below the highest processing weight of about 2 g. temperature of 1800° C., making the final washing/leaching 40 In one embodiment, the densification/pelletizing steps are step to remove impurities from the boron nitride reaction carried out in one extruding step, wherein the raw materials product to be an optional step. including the dopant and optional silica are fed in a twin In one embodiment, the starting material comprises from screw extruder or similar equipment with a binder, Such as 30 to 55 wt.% boric acid, 40 to 50 wt.% melamine, and 1 polyvinyl alcohol; polyoxyethylene-based nonionic Surfac to 5 wt.% of lithium containing dopant material. In another 45 tants; polycarboxylic acid salts such as acrylic acid, meth embodiment, the starting material comprises 52.5 wt.% acrylic acid, itaconic acid, boletic acid, and maleic acid; boric acid, 44.5 wt.% melamine, and 3 wt.% lithium nitrate. polyoxazolines Such as poly(2-ethyl-2-oxazoline); Stearic Process Steps: The process for making hBN of the inven acid: N,N'-ethylenebisstearamide; sorbitan compounds such tion may be carried out as a batch process, or as a continuous as Sorbitan monostearate; and the like. The material is then process, including the following process steps. 50 subsequently dried and pelletized upon exit from the Mixing/blending. In the initial step, the starting materials extruder. including the dopant are mixed or otherwise blended The exit pellets can be fed in a continuous process directly together in a dry state in Suitable equipment Such as a into the reaction vessel for the next step, or in yet another blender. embodiment, processed through a furnace of 200° C. for Optional pre-heating/drying step: In one embodiment of 55 additional drying prior to being fed into the reaction vessel, the invention and after the mixing/blending step, the starting wherein the high-purity boron nitride of the invention is material is dried at temperatures of about 100 to 400° C. formed. from 0.5 to 15 hours to drive off any moisture in the Combined Calcination and Heat Treatment Step: In the reactants and create porosity between the raw materials, next step, the pellets are fired in a nitrogenous atmosphere forming aggregates of materials in the form of nuggets, 60 in a reaction chamber, wherein the chamber is heated up chunks, or pellets. from room temperature at a rate of 20 to 1200° C. per hour In one embodiment, the drying is carried out at a tem to at an elevated temperature of 1200 to 2300° C. The perature from 150 to 250° C. The drying operation can be process temperature is then held for about 1 to 30 hours, carried out in air, or in a nitrogen or ammonia atmosphere. wherein the nitrogen purge is maintained at a rate Sufficient The drying time depends on the drying temperature and also 65 to Sustain a non-oxidizing environment. whether the drying step is performed in a static atmosphere, In one embodiment, the pellets are maintained in ammo or with circulating air or gas. In one embodiment, the drying nia while being fired to 1200 to 1600° C. for 2 to 12 hours. US 7,341,702 B2 5 6 In a second embodiment, the pellets are fired at 1400°C. for In another embodiment of the invention, the BN product about 4 hours. In a third embodiment, the pellets are fired to of the invention is further heat-treated or sintered at 1700 to a temperature of 1800° C. from room temperature at a rate 2100° C. in a non-oxidizing gas atmosphere such as nitro of 500° C. per hour. The temperature is then held at 1800° gen, ammonia, or argon. This treatment results in the C. for 5 hours, wherein a nitrogen purge is maintained. progress of crystallization, for a BN product of improved In yet another embodiment, the reaction is carried out at crystallinity and purity and a thermal conductivity of at least a process temperature in excess of 2000°C. to achieve a high 0.3 W/mK. In one embodiment of the invention, the BN reaction rate, using for example, a plasma jet furnace. In yet powder has a thermal conductivity in the range of 3 to 15 another embodiment, the nitrogenous atmosphere is a mix W/mK. ture of ammonia and an inert gas. 10 Applications of the BNPowder Made from the Invention: The steps described above can be carried out as a batch The high purity boron nitride powder of the present inven process whereby the loose pellets are introduced into a tion can be used as a filler for thermal management appli reaction chamber for firing. In another embodiment, the cations, such as in composites, polymers, greases, and fluids. steps are carried out as part of a continuous process, wherein The boron nitride powder can also be used in hot pressing the pellets are continuously fed into a reaction vessel. In one 15 applications, or as a precursor feed stock material in the embodiment of a continuous process, the reaction vessel is conversion of hexagonal boron nitride to cubic boron passed through the furnace assembly by a force feed mecha nitride. In another embodiment, the material is used for nism wherein as each vessel container is introduced into the making a hexagonal boron nitride paste. As used herein, furnace assembly, each previous vessel container is moved paste is a semisolid preparation. This method involves one container length through the furnace. providing a boron nitride slurry and treating the slurry under It should be noted that in the process of the invention, the conditions effective to produce a boron nitride paste includ pellets containing the reactants and dopants are fired in one ing from about 60 wt.% to about 80 wt.% solid hexagonal single step at an elevated processing temperature forming boron nitride. BN crystals, as opposed to the prior art processes wherein a In thermal management applications, e.g., thermal con two-step process is required. 25 ductivity sheets, the boron nitride of the invention is used as The final product of the single combined calcination and a filler in a matrix comprising at least one selected from heat treatment step herein (as opposed to the 2 steps of the among thermoplastic resins such as polyethylene, polypro prior art) is a high crystallinity product with a high purity of pylene, polystyrene, poly-p-Xylene, polyvinyl acetate, poly at least 99% boron nitride. In one embodiment, the BN acrylate, polymethyl methacrylate, polyvinyl chloride, poly product has a purity of at least 99.5% purity. 30 vinylidene chloride, fluorine-base plastic, polyvinyl ether, As used herein, a high crystallinity BN product refers to polyvinyl ketone, polyether, polycarbonate, thermoplastic a product of uniform, smooth feel product having a purity of polyester, polyamide, diene base plastic, polyurethane-base at least 90% boron nitride as well as generally over 99% plastic, silicone and inorganic plastic; and thermosetting purity. The degree of crystallinity relates to the degree of resins such as a phenol resin, furan resin, Xylene/formalde Smoothness of “feel during rubbing, as opposed to a rough, 35 hyde resin, ketone/formaldehyde resin, urea resin, melamine chalky feel for poorly crystalline material (as obtained after resin, aniline resin, alkyd resin, unsaturated polyester resin the 1 step of the 2-step process of the prior art, wherein the and epoxy resin. turbostratic boron nitride is formed in the calcination step of In one embodiment of a thermal management application, up to 1100° C.). the boron nitride of the invention is used as a filler for a Optional Milling Step: Lastly, the reaction product of fired 40 pellets is optionally cooled to temperature before being matrix comprising up to 60 wt.% of a composite selected milled through a jet milling process, into a high purity boron from the group of silicone, epoxy, acrylic, vinyl chloride, nitride product in the form of platelets between 0.1 to 60 polyurethane, and mixtures thereof, for a thermal conduc microns in size. In one embodiment the reaction product is tivity of at least 0.50 W/m-K. milled into powder having an average particle size of 1 to 5 45 In thermal management applications, it is found that the microns. The size is nominal, since each particle is not of dopants help increase the thermal conductivity of the boron Such size, but a conglomerate of loosely bonded ultrafine, nitride product. In embodiments wherein LiNO is used as Submicron crystallites. the dopant or seed material for making the hBN of the In yet another embodiment of the invention, the boron invention, the thermal conductivity of the samples employ nitride powder is crushed and screened to produce agglom 50 ing the BN of the invention is much higher than the thermal erates from 40 mesh to 325 mesh, which materials can be conductivity of samples employing BN made using the used in thermal applications. undoped process of the prior art. It is noted that the reaction product of the invention is EXAMPLES 1-4: Four samples are made containing considerably more pure than the boron nitride of the prior art melamine and boric acid in a weight ratio of 23:27. Example undoped process. As previously indicated, it is Suspected 55 1 contains only melamine and boric acid; example 2 con that metal borate impurities vaporize away at relatively low tains an additional amount of 1.5 wt % lithium nitrate; temperatures, for a high purity boron nitride product. example 3 has 3.0 wt.% lithium nitrate; and example 4 has Other Optional Post Treatment Steps: As indicated, the 5.0 wt.% lithium nitrate. All of samples are dried at 275°C. boron nitride product of the process of the invention is of overnight. The dried material is then pressed into cylindrical high purity Such that acid wash/leaching is not needed. 60 pills and fired in nitrogen at a rate of 500° C. per hour from However, in one embodiment of the invention and as an room temperature to 1800° C. and held for 3 hours. The pills optional step for a boron nitride product of exceptional prepared from the four examples are weighed and milled purity, the product is optionally subject to a washing treat with a ball mill to be analyzed. Samples pads are made and ment. In one embodiment, the final BN product is washed measured for thermal conductivity. with a solvent such as methanol or a 1% aqueous Solution of 65 The results for thermal conductivity (“TC”) and total nitric acid, to produce a fine, white, high purity crystalline yields computed for the examples are given in the table boron nitride powder. below. US 7,341,702 B2 8 9. The process of claim 8, wherein the reaction is carried TABLE out for a period of up to 30 hours. 10. The process of claim 9, wherein the reaction is carried Example 1 Example 2 Example 3 Example 4 out for a period of up to 12 hours. % dopant O.0% 1.5% 3.0% S.0% 11. The process of claim 10, wherein the reaction is TC (W/mK) 1.8 1.5 7.2 7.1 carried out for a period of up to 6 hours. Yield (%) 19.3 2O.O 19.9 18.2 12. The process of claim 1, wherein the dopant comprises at least one compound selected from the group consisting of The boron nitride products prepared from the process of barium compounds, cesium compounds, potassium com the invention show higher thermal conductivity than the 10 pounds, strontium compounds, rubidium compounds, product of the prior art, i.e., having a thermal conductivity lithium nitrate, lithium oxide, lithium acetylacetonate, of at least 1.5 W/mK. Thermal conductivity, in a way, can be lithium cyclopentadienide, dilithium phthalocyanine, dil used as an indication of the purity of the boron nitride (BN) ithium salt, lithium acetate, lithium acetylacetonate, lithium substance. If the BN product contains impurities such as amide, lithium bis(trimethylsilyl)amide, lithium tetrahydri borate compounds or oxygen, the thermal conductivity will 15 doborate, lithium carbonate, lithium dimethylamide, lithium be hindered. hydride, lithium hydroxide, lithium metaborate, lithium This written description uses examples to disclose the molybdate, lithium niobate, lithium perchlorate, lithium invention, including the best mode, and also to enable any peroxide, lithium tetraborate, lithium borohydride, lithium person skilled in the art to make and use the invention. The triethylhydridoborate, lithium oxalate, and mixtures thereof. patentable scope of the invention is defined by the claims, 13. The process of claim 12, wherein the dopant com and may include other examples that occur to those skilled prises at least one compound selected from the group in the art. Such other examples are intended to be within the consisting of barium oxide, barium nitrate, cesium oxide, scope of the claims if they have structural elements that do cesium nitrate, potassium hydroxide, strontium oxide, stron not differ from the literal language of the claims, or if they tium nitrate, rubidium oxide, rubidium nitride, lithium include equivalent structural elements with insubstantial 25 nitrate, lithium oxide, lithium acetylacetonate, lithium differences from the literal languages of the claims. cyclopentadienide, dilithium phthalocyanine, dilithium salt, lithium acetate, lithium acetylacetonate, lithium amide, All citations referred herein are expressly incorporated lithium bis(trimethylsilyl)amide, lithium tetrahydridoborate, herein by reference. lithium carbonate, lithium dimethylamide, lithium hydride, The invention claimed is: 30 lithium hydroxide, lithium metaborate, lithium molybdate, 1. A process for producing hexagonal boron nitride com lithium niobate, lithium perchlorate, lithium peroxide, pound, which comprises: lithium tetraborate, lithium borohydride, lithium triethylby reacting in a reaction Zone an oxygen-containing boron dridoborate, lithium oxalate, and mixtures thereof. compound with a nitrogen-containing Source in the 14. The process of claim 12, wherein the dopant is present presence of one or more compounds used as dopants at 35 in an amount of at least 1 wt.% of the total weight of the a processing temperature of at least 1000°C. for at least oxygen-containing boron compound and the nitrogen-con one hour to form a hexagonal boron nitride compound, taining source. wherein the dopants are metal compounds that form 15. The process of claim 12, wherein the dopant is metal borates with a vaporizing temperature that is selected from the group consisting of barium oxide, cesium lower than the processing temperature, wherein the 40 oxide, potassium hydroxide, strontium oxide, rubidium oxygen-containing boron compound is introduced into oxide, lithium nitrate, and mixtures thereof. the reaction Zone as a Solid. 16. The process of claim 13, wherein the dopant is lithium 2. The process of claim 1, wherein the hexagonal boron nitrate, and wherein the amount of dopant present is at least nitride compound displays a thermal conductivity of at least 1 wt.% of the total weight of the oxygen-containing boron 1.5 W/mk. 45 compound and the nitrogen-containing Source. 17. The process of claim 16, wherein the dopant is lithium 3. The process of claim 1, wherein the hexagonal boron nitrate, and wherein the amount of dopant present is at least nitride compound displays a thermal conductivity of at least 2 wt.% of the total weight of the oxygen-containing boron 3 W/mk. compound and the nitrogen-containing Source. 4. The process of claim 1, wherein the hexagonal boron 50 18. The process of claim 13, wherein the dopant com nitride compound displays a thermal conductivity of at least prises at least one compound selected from the group 5 W/mk. consisting of lithium nitrate, lithium oxide, lithium acety 5. The process of claim 1, wherein the oxygen-containing lacetonate, lithium cyclopentadienide, dilithium phthalocya boron compound reacts with the nitrogen-containing Source nine, dilithium salt, lithium acetate, lithium acetylacetonate, in the presence of the dopant at a processing temperature 55 lithium amide, lithium bis(trimethylsilyl)amide, lithium tet ranging from 1000 to 2300° C. rahydridoborate, lithium carbonate, lithium dimethylamide, 6. The process of claim 5, wherein the processing tem lithium hydride, lithium hydroxide, lithium metaborate, perature is at least 1850° C. and the hexagonal boron nitride lithium molybdate, lithium niobate, lithium perchlorate, compound has a final purity as produced of at least 95%. lithium peroxide, lithium tetraborate, lithium borohydride, 7. The process of claim 6, wherein the processing tem 60 lithium triethylhydridoborate, lithium oxalate, and mixtures perature is at least 2000°C. and the hexagonal boron nitride thereof. compound has a final purity as produced of at least 99%. 19. The process of claim 18, wherein the dopant com 8. The process of claim 5, wherein the oxygen-containing prises at least one compound selected from the group boron compound reacts with the nitrogen-containing Source consisting of lithium nitrate, lithium oxide, lithium carbon in the presence of the dopant at a processing temperature 65 ate, lithium hydride, lithium hydroxide, lithium metaborate, ranging from 1000 to 2300° C., and for a period of up to 72 lithium peroxide, lithium tetraborate, lithium borohydride, hours. and mixtures thereof. US 7,341,702 B2 9 10 20. The process of claim 19, wherein the dopant is lithium 23. The process of claim 22, wherein the processing nitrate. temperature is at least 2000° C. and the product as formed 21. The process of claim 1, wherein said process is a has a purity of at least 99% boron nitride. continuous process. 22. A process for producing hexagonal boron nitride 24. A process of for producing hexagonal boron nitride compound, which process comprising the steps of compound, comprising: a) forming a mixture of an oxygen-containing boron a) forming a mixture of an oxygen-containing boron compound, a nitrogen-containing Source, and a dopant, compound, a nitrogen-containing Source, and a dopant, the dopant being present at an amount of at least 1 wt. the dopant being present at an amount of at least 1 wt. % of the total weight of the oxygen-containing boron 10 % of the total weight of the oxygen-containing boron compound and the nitrogen-containing source, wherein compound and the nitrogen-containing source, wherein the mixture comprises only solids; the mixture comprises only solids: b) heating the mixture at a temperature of at least 200°C. b) Subjecting the mixture to a processing temperature of for at least /2 hour to dry off any moisture in the at least 2000° C. for at least one hour to form a mixture; 15 c) Subjecting the mixture to a processing temperature of at hexagonal boron nitride compound: least 1200° C. for at least one hour to form a hexagonal wherein the dopant 1S a metal compound that forms a boron nitride compound; metal borate with a vaporizing temperature that is wherein the dopant is a metal compound that forms a lower than the processing temperature. metal borate with a vaporizing temperature that is 20 lower than the processing temperature. k . . . .