WO2008088774A2

WO2008088774A2 - Improved process for making boron intride

Info

Publication number: WO2008088774A2
Application number: PCT/US2008/000454
Authority: WO
Inventors: Jennifer Klug; Dawn Krencisz; Anand Murugaiah; Chandrashekar Raman
Original assignee: Momentive Performance Materials Inc.
Priority date: 2007-01-12
Filing date: 2008-01-11
Publication date: 2008-07-24
Also published as: WO2008088774A3

Abstract

A process for making boron nitride with improved processing yield, controlled crystal size and controlled agglomerate hardness. To control the agglomerate hardness, carbon may be added to the crude BN as a dopant before the firing step in an amount ranging from 4 to 20 wt. %. For yield improvement, the starting mixture of an oxygen-containing boron compound and a nitrogen-containing source may be reduced in size such that at least one of a) at least 15 wt% of the individual raw materials or the blended raw materials are less than 20 microns in diameter and b) the average particle size is between 1 and 60 microns (μm).

Description

IMPROVED PROCESS FOR MAKING BORON NITRIDE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to, and the benefit of the priority date of, U.S. Provisional Patent Application No. 60/884,643, filed on January 12,

2007.

BACKGROUND

[0002] The present embodiments relate to a process for making boron nitride with improved processing yield, controlled crystal size and controlled agglomerate hardness.

[0003] Boron nitride ("BN") comes in a variety of crystalline structures and has a variety of uses from polishing agents to lubricants. Hexagonal boron nitride ("hBN") is a very desirable form as a white composition having hexagonal layer structure similar to graphite in platelet morphology. BN has found uses in many applications such as thermal conductivity applications, electrical insulation applications, corrosion resistance applications, lubrication applications, plastic additives, electronic materials, non-oxidizing ceramics sintering filler powder, makeup materials, medical additives, etc. Additionally, boron nitride can be molded and used in composite materials or as a raw material for cubic boron nitride.

[0004] BN can be manufactured in a high temperature reaction between inorganic raw materials, e.g., reacting boric acid / boric oxide with melamine / urea to make BN. In the prior art, the reaction step typically has a low mass yield and the resulting BN is often rather impure because of unreacted boric oxide. The boric oxide impurities further decrease yield in any secondary firing or crystal growth step. The low yield of BN is a significant reason for the high cost of manufacture. [0005] Hexagonal BN (hBN) crystals have a high intrinsic thermal conductivity and it is desirable to grow large crystals. However growing large crystals is difficult since high thermal energy and the presence of a liquid phase such as boric oxide are required. Ramping the temperature directly to high temperatures during the crystal growth step will result in rapid vaporization of the boric oxide, resulting in reduced grain growth potential. A number of methods have been used in the prior art to get around this conflict between using high temperature and sustaining the presence of boric oxide such as: using grain growth aids (other than boric oxide); long soak times at one final soak temperature (>1500°C); using pressure to maintain favorable grain growth conditions (above 1 bar pressure); re-firing the hBN; pre-compacting the hBN to provide favorable kinetics and material transport for grain growth; and combinations of the above. However, these processes are prone to increase the cost, particularly with the complex process steps.

[0006] In earlier applications, grain-growth agents such as oxides, nitrides, borates or carbonates of alkali or alkaline earth metals have been used. However, with the use of the grain-growth agent, a wash step is required after the crystal growth step to obtain high purity BN, which significantly increases manufacturing costs. Further, even trace impurities of alkali or alkaline earth metals left behind after a wash step are undesirable for some applications. In another embodiment of the prior art, boric oxide is used as a grain-growth agent at atmospheric pressure, but only moderate crystal growth can be achieved. This is because the high vapor pressure of boric oxide leads to rapid evaporation and depletion of boric oxide.

[0007] There also exist a number of references in the prior art where carbon is added during the BN manufacturing process to either suppress crystal growth (which is desirable for certain applications with particle size limitations) or to improve the purity of the final product by reacting with the oxygen impurities. Japanese Publication Nos. 4016502, 4083706, 62056308, and US Patent No. 4,784,978 disclose processes wherein carbon is added to the initial raw materials, typically a boron containing compound such as boric acid and a nitrogen containing compound such as melamine or urea. Japanese Publication No. 10203806 also discloses adding a carbon source at the raw material stage but further specifies that the particle size of the carbon source should not be more than 1/5* the size of the boric acid. In Japanese Publication No. 61256905, 5-15 wt. % carbon is added with respect to the entire amount of impurities in a crude hexagonal boron nitride, for making final powder which is more than 98 wt.% pure and wherein the particle diameter is less than 0.5 microns. [0008] Recently BN is increasingly being used as a filler in thermoplastic and thermoset resins to enable the fabrication of thermally conducting plastic shapes and parts. There are limitations in incorporating the BN powder into these matrices. Boron nitride agglomerates with high tap density have better flowability and demonstrate lower viscosity in resins than small crystals/platelets; however, they will cause the plastics to have lower part strength. Single crystal BN platelets will have less of an impact on part strength, but cause processing issues and the viscosity of the filled polymer increases significantly. Over and above these technical challenges, the high cost of manufacture of BN limits its use as a filler in resins which are typically significantly cheaper than boron nitride. Hence there is a need for an improved process for making boron nitride for improved processing yield, controlled crystal size and controlled agglomerate hardness, which facilitate the use of BN in these applications.

SUMMARY OF THE INVENTION

[0009] In one aspect, there is provided a process for producing a boron nitride compound by reacting a mixture comprising an oxygen-containing boron compound with a nitrogen-containing source; wherein the mixture is reduced in size such that at least one of a) at least 15 wt% of the blended material is less than 20 microns in diameter and b) the average particle size is between 1 and 60 microns (μm) prior to the reaction of the oxygen-containing boron compound with the nitrogen-containing source at a temperature of at least 900⁰C

[0010] In a second aspect, there is provided a process for producing a boron nitride compound by reacting a mixture comprising an oxygen-containing boron compound with a nitrogen-containing source wherein the mixture may or may not be subject to particle size reduction before reaction at a temperature of at least 900⁰C

[0011] In another aspect, there is provided a process for producing a hexagonal boron nitride compound, wherein a carbonaceous compound is added to a crude boron nitride as a dopant in an amount of 4.5 to 20 wt. % carbon prior to heat treating of the crude boron nitride in a nitrogenous atmosphere at a temperature of at least 1600⁰C. The crude boron nitride preferably contains between 5 and 25 wt% oxygen as an impurity. The addition of the carbon dopant in this step helps control the agglomerate hardness.

[0012] In another aspect, there is provided a hexagonal boron nitride compound having improved hardness, wherein said boron nitride is produced by adding a carbonaceous compound during formation of said boron nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a chart showing the relationship of crush strength of boron nitride as a function of carbon loading.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The present embodiments relate to processes to make boron nitride using various processing steps and/or components to modify or control the properties of the finished product. As used herein, the terms "size", "D50", "average particle size", and "particle size" refer to the normalized mean diameter of the particles. As used herein, the term "calcining" refers to a heating step to react the raw materials to form crude boron nitride. "Crude boron nitride" is defined as a boron nitride with an amorphous or turbostratic microstructure. This is in contrast to hBN, which has a primarily crystalline structure. Crude boron nitride also typically has various impurities, such as oxygen or oxides. As used herein, "% Theoretical yield" is calculated as equaling: actual mass yield x (1- mass fraction B₂O3 (assuming all oxygen present as boric oxide))/(theoretical mass yield of raw materials with 100% conversion to BN). [0015] In the present processes to produce boron nitride, a boron source and a nitrogen source are used as starting materials, reacting to form a compound in which a boron atom and a nitrogen atom coexist. [0016] In one embodiment of the invention, the boron source is an oxygen containing boron compound. In this instance, the oxygen-containing boron compound may be selected from the group of boric acid, boron oxide, boric oxide providing substances such as boron trioxide, diboron dioxide, tetraboron trioxide or tetraboron pentoxide, and borate ores such as colemanite, ulexite, pandermite, danburite, datolite, and mixtures thereof. In another embodiment, the oxygen-containing boron compound comprises 50 wt. % boric acid and 50 wt. % ulexite. In yet another embodiment of the invention, boric acid is used as the oxygen-containing boron compound.

[0017] In one embodiment, the nitrogen-containing compound may include organic primary, secondary and/or tertiary amines such as diphenylamine, dicyandiamide, ethylene amine, hexamethylene amine, melamine, urea, and mixtures thereof. In one embodiment, melamine is used as the nitrogen- containing compound. In another embodiment, urea is used, either alone or with melamine, as the nitrogen-containing source.

[0018] In one embodiment, the starting materials comprise from about 45-

60 wt. % boric acid, and 40 to 55 wt. % melamine. In another embodiment, the starting materials comprise about 52.5 wt. % boric acid, and about 47.5 wt. % melamine. In yet a third embodiment, the starting material comprises about 60 wt% boric acid and about 40 wt% melamine.

[0019] In other embodiments, the starting materials comprise 35-60 wt% urea and 40-65 wt% boric acid. In another embodiment, the starting material comprises 45-65 wt% boric acid, 15-50% melamine, and 5-40% urea. [0020] Process Steps: The process for making BN of the invention may be carried out as a batch process, or as a continuous process, and may include the following process steps:

[0021] Size Reduction/ blending. In certain embodiments, melamine and boric acid or boric oxide are reacted to form BN at a temperature above 600⁰C. The reaction between melamine and boric oxide is essentially a solid phase reaction where the reaction is expected to occur at the interface of melamine and boric oxide particles. At these high temperatures, the sublimation and degradation of melamine leads to melamine mass loss and reactivity loss, resulting in incomplete reaction and boric oxide impurities. Raw materials having a large particle size will have a smaller fraction of the particles available for reaction (due to a lower surface area to mass ratio) while the rest of the particle is likely to be lost due to sublimation or will have reduced reaction rates. [0022] Thus, in one embodiment, a size reduction step is employed to achieve significant size reduction of the raw material particles prior to reaction (calcining) of the raw materials to form BN₁ thus facilitating easier and more complete reaction between the raw materials during the subsequent calcining reaction step. The size reduction step results in a mixture wherein the individual raw material boron-containing compound particles and/or nitrogen-containing compound particles have average particle sizes of 1 to 60 microns (μm) or the mixture is reduced in size such that at least 15 wt% of the raw material particles are less than 20 microns in size. As noted above, "particle size" refers to the normalized mean diameter of the particles. It should be noted that a raw material may satisfy one of these requirements, but not the other, or it may satisfy both of these requirements.

[0023] In other embodiments, the raw material particles may have particle sizes in the range of from 20-40 microns, in other embodiments from 30-45 microns, from 30-60 microns. In other embodiments, at least 20 wt. % of the raw material particles are less than 20 microns in size, and in other embodiments at least 30% are less than 20 microns in size.

[0024] As noted above, the size reduction step can be performed on either or both of the boron containing compound and nitrogen containing compound raw materials. Likewise, the size reduction step can be conducted before, during or after a step of blending the nitrogen containing compound and the boron containing compound. If performed during or after the blending step, then the size reduction will obviously be performed on particles of both the raw materials. If performed during the blending step, a high shear blender or some other high shear mixing apparatus is used to both blend and crush the material concurrently. If done before or after blending of the raw materials, the size reduction step can be done in a mill or other apparatus suitable for crushing or otherwise reducing the size of the particles.

[0025] By subjecting the raw material particles to a size reduction step, improvements in yield of from about 10% up to 50% or more can be achieved, compared to product made with commercially available materials which do not undergo a size reduction process. Such commercially available materials include melamine with an average particle size of about 90 microns or greater and boric acid or boric oxide with an average particle size of about 250-350 microns. The use of such commercially available raw materials without a size reduction step will typically result in a blend with an estimated average particle size of 150-175 microns when blended in ratios described herein.

[0026] Agglomerates containing particles of the boron and nitrogen sources may form during the blending process due to heat applied externally or generated during a high shear blending process. The material temperature may be as high as 275⁰C. These agglomerates may reach sizes up to a few millimeters. However, as long as size reduction of the particles occurs as described above before the agglomeration, the same benefits with regard to more complete reaction of the raw materials will be observed. [0027] In one embodiment, melamine (with an average starting particle size of about 90 μm) and boric acid (with an average starting particle size of about 300 μm) are simultaneously size reduced and blended in a high shear blender, producing a final blend with an average particle size for the blended materials of about 40 μm. As discussed in the subsequent sections, this blend of raw materials, when subjected to a subsequent calcining reaction to form boron nitride, gives improved yields, as evidenced by low boric oxide impurities. [0028] In another embodiment for intensive/high shear blending, a mill or other apparatus for reducing particle size (such as an attritor mill) is used to blend the raw materials (either before or after blending) to a final average particle size of about 5 μm prior to reaction, also leading to high yield in the reaction step. In one such process, the raw materials are blended in a plough or paddle blender for at least 15 minutes with added intensifying choppers.

[0029] In another embodiment, the individual raw materials are reduced in particle size before mixing them and then subsequently mixed to produce a blend with an average particle size of about 45 μm or less. In such an instance when size reduction is conducted prior to blending, the blending does not need to be performed under high shear, and can be conducted in a low shear blender, for instance. The same improved yield results, as described herein, are expected if the initial particle size of the raw materials meet the same criteria without the need for a size reduction step.

[0030] In one exemplary process, a mixture of 50% boric acid and 50% melamine is simultaneously size reduced and blended in a production scale vertical high shear blender with high intensifier speeds to impart shear that breaks the particles down and reduces their size while also blending the raw materials together. The high heat generated during this process results in the agglomeration of these fine particles. The agglomerated raw material optionally may then be further processed as described below.

[0031] Carbon Addition: In another embodiment, a carbon containing material is added to the raw materials prior to calcining, such as during the raw material blending process. This addition of carbon can be done with or without size reduction of the raw material, to further carbothermically reduce the boric oxide to improve reaction yields and decrease oxygen content after calcining, as described in more detail below. The oxygen content of the resultant BN will be lower in such an embodiment. For example, when 1 wt. % carbon is added to the blend, the resultant oxygen was less than 8 wt%. However, the BN produced may have a higher carbon content, in one embodiment around 3 wt%. The carbon containing material may comprise one of the materials described herein below.

[0032] In yet another embodiment, water may be added to the blending process to improve interaction of boric acid and melamine to aid in re- agglomerating the particles. The amount of water added may vary from about 2 to 25 weight percent. In one embodiment where 17 wt. % water is added to the blend after 30 minutes, the resultant oxygen content in the BN after calcining at 1100⁰C in ammonia is 6 wt% with a carbon content of 0.5 wt%. [0033] In another exemplary process a mixture comprising boric acid, melamine, and urea (which may be commercially available in the form of pills or pellets) may be blended ( such as in a lab scale plough blender with intensifying choppers) to impart high shear and particle size reduction,. The blends may then be pressed into pills with a tableting machine (as described in more detail below), and reacted as described below.

[0034] Optional pre-heatinα / drying step: In one embodiment of the invention and after the size reduction/blending step, the starting material may be dried at temperatures of about 100 to 300⁰C from 0.5 to 15 hours to drive off any moisture in the reactants and create porosity between the raw materials, forming aggregates of materials in the form of nuggets, chunks, or pellets. [0035] Optional Crushing of the Precursors: In one embodiment, after the size reduction/mixing step or after the drying step, the starting raw materials may be crushed or broken into small pieces using conventional equipment such as roller mills, cross beater mills, rolling discs and the like.

[0036] Optional Densification ("Pilling") Step: In one embodiment, after the size reduction/blending step, and the optional drying step, the mixed precursors may be crushed and densified using a process known in the art such as tableting, briquetting, extruding, pilling, and compacting, among others. In this step, the crushed mixture is densified into pellets weighing from 0.1g to 200 g each. In one embodiment, the pellets have an average weight of about 10 g or less. In a second embodiment, the crushed mixture is densified into pellets with an average weight of about 2 g or less.

[0037] In one embodiment, the densification/pelletizing steps are carried out in one extruding step, wherein the raw materials including optional dopants are fed in a twin screw extruder or similar equipment with a binder, such as polyvinyl alcohol; polyoxyethylene-based nonionic surfactants; polycarboxylic acid salts such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid; polyoxazolines such as poly(2-ethyl-2-oxazoline); stearic acid; N₁N'- ethylenebisstearamide; sorbitan compounds such as sorbitan monostearate; and the like. The material is then subsequently dried and pelletized upon exit from the extruder.

[0038] The exit pellets can be fed in a continuous process directly into the reaction vessel for the next step, or in yet another embodiment, processed through a furnace for additional drying prior to being fed into the reaction vessel, wherein boron nitride is formed.

[0039] Calcination: Next, in the calcination or reaction step, the blended powder or densified material is fired in a nitrogenous atmosphere in a reaction chamber, wherein the chamber is heated to an elevated temperature of from about 700 ⁰C to 1600⁰C to react the raw materials to form crude boron nitride. In one embodiment, the calcining is carried out at 1000-1400 ⁰C, wherein the process temperature is held for about 0.1 to 30 hours, wherein the nitrogenous purge is maintained at a rate sufficient to sustain a non-oxidizing environment. [0040] In one exemplary process, raw materials are maintained in ammonia while being fired at a temperature from about 1000 to 1600⁰C for 0.25 hours to 12 hours. In a second exemplary process, the raw materials are fired at about 1200⁰C for about 4 hours. In certain embodiments, the nitrogenous atmosphere may be ammonia or a mixture of ammonia and an inert gas. [0041] The steps described above can be carried out as a batch process whereby the powder or loose pellets are introduced into a reaction chamber for firing.- Alternately, the steps may be carried out as part of a continuous process, wherein the material is continuously fed into a reaction vessel. In one example of a continuous process, the reaction vessel is passed through the furnace assembly by a force feed mechanism wherein as each vessel container is introduced into the furnace assembly, each previous vessel container is moved one container length through the furnace. In another example of a continuous process, the sample in the form of pellets or powder is introduced into a rotary calciner or reaction chamber under a nitrogenous atmosphere on one end, and is collected on the other end as crude BN.

[0042] Optional Crushing of the Crude BN: In one embodiment, after the calcination process the crude BN is crushed or broken into small pieces that can be later densified using known processes such as tableting, briquetting, extruding, pilling, and compacting, among others. The crushing can be done using conventional equipment such as roller mills, cross beater mills, rolling discs and the like. In one embodiment, the crushed materials are broken into pieces weighing between 10 mg to 10 g each. In yet another embodiment, the materials are broken into pieces weighing about 0.2 g each.

[0043] Optional Carbon Doping to Control Agglomerate Hardness: In one embodiment, after the calcination process and the optional crushing step, but prior to a final heat treatment step (as described below), carbon is added to the crude BN as a dopant in an amount ranging from 4.5 to 20 wt. % carbon as a percentage of the total crude BN weight including impurities. In one embodiment, the carbon dopant is added in an amount ranging from 4.5 to 8 wt.% carbon. In another embodiment, carbon is added in an amount of 5 to 10 wt. % carbon.

[0044] The addition of carbon increases the agglomerate hardness and thereby improves the processability and thermal conductivity of the final BN product in thermal applications, amongst other applications. The carbon source for use as a dopant can be in a carbonaceous solid or liquid form, including but not limited to cornstarch, carbon black, soot, graphite powder, sugar, agar, melamine, corn syrup, pitch, and molasses. In one embodiment, the carbon dopant is carbon black having a surface area ranging from 7-12 m²/g, with 99.9% of particles going through a 325-mesh sieve. In one embodiment, 5 wt % carbon is blended with crude BN powder of 75% purity. In another embodiment, 10 wt% pitch is blended with crude BN powder of 85% purity.

[0045] Particle hardness may be measured by various methods, including the following: The BN is roll crushed and then screened in a vibratory screener. A 200-mesh screen and a 325-mesh screen are used. The material that falls through the 200-mesh screen and stays upon the 325-mesh screen are then measured using a Microtrac laser particle analyzer. A comparison is made between the D50 with 20 seconds of internal ultrasonication at 25 watts and 40 seconds of ultrasonication. A percentage breakdown is calculated by subtracting the difference of the D50s and dividing by the D50 at 20 seconds. According to this method, BN with 1.2% carbon had an average breakdown of 10% while an otherwise identically produced BN with 8.2% carbon had an average breakdown of 6%. Similarly, contrasting 20 seconds of ultrasonication with 80 seconds of ultrasonication, the material with 1.2% carbon had an average breakdown of 22% while the material with 8.2% carbon had an average breakdown of 17%. [0046] Final Heat Treatment: The crude BN is subjected to a final heat treatment step to remove impurities and convert the crude BN to crystalline (hexagonal) BN. This final heat treatment is typically conducted at a temperature in the range of from about 1600⁰C to about 2100⁰C, for a period of time generally from about 4 hrs to 40 hrs.

[0047] Intermediate temperature hold during Heat Treatment Step: During the final heat treatment step, the crude BN may optionally be heated to an intermediate temperature of 1600⁰C to 1900⁰C and held for 0.16 hrs to 12 hrs, and then heated to the final heat treatment temperature of 1850⁰C to 2100⁰C anywhere from 0.16 hrs up to 72 hrs. This intermediate hold ensures sustained presence of boric oxide, which could otherwise vaporize more quickly during a single high temperature hold. This acts as a grain growth/refinement aid and allows the BN to crystallize and grow to larger sizes as opposed to a single final temperature hold. Alternatively, similar results to the intermediate hold can be obtained by sufficiently slowing the heating/ramp rate to one final high temperature hold to provide adequate time at the above intermediate temperatures.

[0048] In one embodiment, the crude BN in the form of powder is heated to 1650⁰C for 4 hrs, and then heated to a final temperature of 2050⁰C for 12 hrs in a nitrogenous atmosphere in a batch furnace. In another embodiment, the crude BN in the form of Vz diameter pellets was heated in a batch furnace under nitrogen to an intermediate temperature of 1800⁰C and held for 2 hrs, and then heated to 2000⁰C for 6 hrs. In another embodiment, the crude BN in the form of V-i pellets was heated in a continuous pusher furnace such that the crude BN was subjected to an intermediate temperature of 1700⁰C - 1725⁰C for 45 minutes and then held at 1950⁰C for 5 hrs. In yet another embodiment, the crude BN was heated to one final temperature of 1975°C with a ramp rate of 100°C/hr up to 1600⁰C, and then was slowed to 10°C/hr 1975°C. It was then held at 1975°C for 30 hrs. In another embodiment, the above was repeated to a final temperature of 2000⁰C for up to 72 hrs.

Examples

[0049] The following examples were made using compositions of melamine and boric acid as described in paragraph [0018] [0050] Example 1 : Boric acid and melamine was blended in a v-blender for 30 minutes with an intensifier bar. The blended material was milled in an attritor mill for 30 minutes at 60 rpm. The blended material was pressed into compacts using a cold hydraulic press. The compacted material made with and without milling were reacted in a tube furnace for 30 minutes at 1450⁰C in a nitrogen atmosphere.

[0051] Example 2: A mixture of boric acid and melamine was blended in a vertical high shear blender at a low speed (900 rpm) for 60 minutes reaching a maximum temperature of 80° C (2a). In a separate experiment (2b), the same composition of boric acid and melamine was blended in the same mixer for 30 minutes at high speed (1800 rpm) and 30 minutes at low speed (900 rpm), achieving a maximum temperature of 135°C. After blending, each material was compacted in a cold press. Both materials were reacted at 1450° C in a nitrogen atmosphere in a tube furnace. Table 1 summarizes results from both Examples 1 and 2 as compared to a control sample using boric acid and melamine that was not milled or otherwise subject to particle size reduction Table 1

[0052] Example 3: Boric acid and melamine were blended in a production scale vertical high shear blender with a high speed setting for the first 30 minutes. Due to high energy imparted during the blending process, the temperature reached up to 135⁰C accompanied by evolution of water vapor from the conversion of boric acid to boric oxide phases. The blender speed was then reduced to a slow speed setting to maintain the same blend temperature for an additional 30 minutes. The high energy of the blending process causes the boric acid and melamine particles to break down and re-agglomerate. The resulting blend was pressed into briquettes and fired in a rotary calciner at 1100⁰C for 30 minutes. The resulting particle size distribution and calcined oxygen is listed below:

[0053] Example 4: In another set of examples, mixtures containing 50 -

55% boric acid and 45 - 50% melamine were blended in a lab scale vertical high shear mixer with different speed settings and times, resulting in different applied shear, temperature and particle size of the blends. These blends were pressed into compacts and were calcined in a lab scale calciner at 1450⁰C for 30 mins under flowing nitrogen. Higher shear and longer blend times generally resulted in lower particle size distributions and calcined oxygen values accompanied by higher yields except for 1500 rpm and 60 minute blend, which resulted in agglomeration of the particles giving a larger particle size distribution as listed in the table below. As demonstrated, a high yield was achieved with this material despite the large particle size distribution since they were first broken down into a distribution with at least 15% of the particles smaller than 20 microns.

[0054] Example 5: In yet another example, mixtures comprising boric acid, melamine, urea and carbon were blended in a reciprocating-ball-mill for 30 seconds. The blends were pressed into pills with a cold press, and were fired in lab scale calciner at 1450⁰C for 30 minutes in nitrogen. The results are shown in the table below. The addition of carbon resulted in reduced oxygen, although somewhat higher carbon content.

[0055] Example 6: Mixtures containing 50 - 55% boric acid and 45 - 50% melamine were blended in a (medium shear) lab scale plough blender. Different shear settings were applied by varying the time the intensifying chopper was on during the total blend time, resulting in different particle size of the blends. These blends were pressed into compacts and were calcined in a lab scale calciner at 1450⁰C for 30 minutes under flowing nitrogen. Higher shear applied by running the choppers for longer times during blending generally resulted in lower Particle Size Distribution (PSDs) and calcined oxygen values accompanied with higher yields as listed in the table below. The chopper time in the table is given as a percent of the total blending time.

[0056] Example 7: Various blends were made with boric acid and melamine in the ratios described in paragraph [0018]. The boric acid and melamine were either unmilled or milled separately in an attritor mill before blending in a v-blender for 30 minutes. The results are shown below.

[0057] Example 8. In this example, the starting material comprises crude

BN with oxygen content of 18 weight % made using the method described in Example 1. Three batches are made with 0 wt %, 2 wt % or 4 wt % carbon black plus 2% corn starch (which has 1.2 wt% C) for total carbon contents of 1.2, 3.2, and 5.2 wt%. Each blend is pressed into compacts. These compacts are nitrided in a production scale furnace under nitrogen for 6 hours at 195O⁰C. The compacts are crushed in a finger crusher, and then roll crushed through a 3-high roll crusher.

[0058] Oxygen is measured with a LECO TC-436 AR Oxygen/Nitrogen

Determinator. Carbon is measured with a LECO HF-400/IR-412 instrument. Surface area is measured with a NOVA 2000 BET instrument. Tap density is measured by weighing out a known mass of powder into a graduated cylinder. The cylinder is then tapped on a Dual Autotap machine for 3000 taps. Tap density is defined as the initial mass divided by the final. Particle size is measured with a Microtrac X100 laser light scattering instrument using a particle refractive index of 1.74 and a fluid refractive index of 1.33. [0059] The BN is used as a filler in silicone oil as the polymer matrix (Dow

200 - 100 cst viscosity) at a filler concentration ranging from 35 to 55 wt % BN in silicone fluid. The silicone + BN mixture is mixed via a lab scale FlackTek speed mixer at approximately 3500 rpm for 20 seconds. Viscosity is measured twice in silicone oil (Dow 200 - 100 cst viscosity) using AR-2000 TA Rheometer, and recorded at 1/s shear rate. Crush strength is measured using an lnstron Compression Test with a crosshead speed of 0.2 in/min with a 100-pound load cell.

[0060] For thermal conductivity measurements, the BN is used in pads made with Sylgard 184 (100% silicone, available from DOW Corning) Silicone Resin and curing agent Sylgard 184 as the polymer matrix. The Sylgard fluids are first mixed in speed mixer for 20 seconds at 3500 RPM, then followed by addition of BN fillers, and then mixed for 20 seconds at 3500 RPM. The mixtures are placed in a 3"x5"rectangular mold and pressed at 125°C for 30 minutes to form pads of 0.5 to 1.5 mm in thickness. Bulk thermal conductivity is measured via a Mathis ™ Hot Disk Thermal Constant Analyzer. Through plane thermal conductivity is measured via a Netzsch LFA 447 Laser Flash Analyzer. The results are shown in the table below.

[0061] Example 9: Amorphous BN with oxygen content of 15 wt % was blended with 2 wt % cornstarch, 1.5 wt % water, and varying levels of carbon black. Blends were made with 0 wt %, 2 wt %, 4.5 wt %, 7 wt %, or 9.5 wt % carbon black. Each blend was compacted into disks one inch in diameter and 0.25 inches in thickness. These disks were nitrided in a production scale furnace for 6 hours at 195O⁰C. The disks were turned down with a lathe and the top and bottom were ground flat to final dimensions. The samples were measured using ASTM standard D 3967-95a, Standard Test Method for Splitting Tensile Strength of Intact Rock Core Specimens. The results are shown below.

[0062] Example 10: Amorphous BN with oxygen content of 18 wt % is blended with 1.5 wt. % water plus additives according to the chart below. This blend is compacted into briquettes the size of small almonds, approximately 1 inch long by 0.5 inches in diameter. These briquettes are nitrided in a production scale furnace under nitrogen for 6 hours at 195O⁰C. The briquettes were crushed using the lnstron Compression Test with a crosshead speed of 0.2 in/min with a 100-pound load cell. The crush strength results are illustrated in the table below. Figure 1 is a chart illustrating crush strength as a function of carbon loading.

[0063] Example 12: In this example, the starting material comprises crude

BN mixed with carbon and having oxygen and carbon content as shown in the following table. This blend is pressed into briquettes. These briquettes are nitrided in a production scale furnace for 6 hours at 195O⁰C. The compacts are crushed in a finger crusher, and then roll crushed through a 3-high roll crusher. The resulting BN properties and crush strengths are listed in the table.

[0064] The crushed materials were then screened through a 200 mesh then a 325 mesh screens. The portion that was -200 mesh/+325 mesh was then tested for particle breakdown. Particle breakdown is defined as the percent change in the particle size (D50) after ultrasonication.

[0065] Example 13: Crude BN in the form of powder, containing 10 wt% oxygen impurity, was heated at a rate of 100°C/hr in a batch furnace in nitrogen atmosphere to 1750⁰C for 4 hrs, and then further heated to a final temperature of 2050⁰C for 2 hrs. The final product was crushed in a reciprocating ball mill for 2 minutes, and the particle size was measured. The average particle size was measured to be 10 microns. This compared with a average particle size of 6 - 7 microns if the product was fired to a final temperature of 2050⁰C for 6 hrs. [0066] Example 14: In this example, crude BN with 15% oxygen impurity was blended with 2 wt% corn starch and pressed into pellets of 10 gms each, and was fired in a graphite tube furnace under flowing nitrogen to 1850⁰C at a ramp rate of 250°C/hr and held for 3 hrs. It was then heated further to 2050⁰C for 3 hrs at a ramp rate of 250°C/hr and then cooled. The pellets were hand crushed using a mortar and pestle, and then further crushed using a reciprocating ball mill for 2 mins. The average particle size measured was 11 microns. [0067] Example 15: Crude BN was blended with 2wt% corn starch and was pressed into briquettes in a compacting machine and was fired in a pusher type graphite furnace under nitrogen such a way that the crude BN briquettes were subjected to a intermediate hold of 1750⁰C to 1800⁰C for 2 hrs and then was heated to 1950⁰C for 4 hrs. The resulting refined BN was crushed with a pulverizing mill, and then with a hammer mill. The resulting particle size measured was in the order of 9 to 12 microns.

[0068] Example 16: Crude BN with 14% oxygen impurity was compacted in a cold press without any binder and then was fired in a batch furnace under nitrogen to 1800⁰C for 8 hrs and then was heated to 1975°C for 30 hrs. The average particle size of the resulting BN was the order of 10 - 12 microns. [0069] While the subject novel concept has been described with reference to the foregoing embodiments and considerable emphasis has been placed herein on the structures and structural interrelationships between the component parts of the embodiments disclosed, it will be appreciated that other embodiments can be made and that many changes can be made in the embodiments illustrated and described without departing from the principles of the subject novel concept. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. Accordingly, it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the present novel concept and not as a limitation. As such, it is intended that the subject novel concept be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims and any equivalents thereof.

Claims

What is claimed is:

1. A process for producing boron nitride, said process comprising: blending an oxygen-containing boron compound with a nitrogen- containing source to form a raw material blend; wherein said blend comprises particles having at least one of a) a particle size distribution such that at least 15 wt% of said particles have a particle size of less than 20 microns, and b) an average particle size of 1 to 60 microns; and reacting the oxygen-containing boron compound with the nitrogen- containing source at a temperature of at least 900⁰C to form boron nitride.

2. The process of claim 1 , wherein at least one of the boron compound and the nitrogen-containing source is subjected to a particle size reduction process.

3. The process of claim 2, wherein the particle size reduction is done in a high shear blending or high intensity blending process.

4. The process of claim 3, wherein the high shear blending or high intensity blending is done using attritor milling, ball milling, hammer milling, jet milling, disk milling, high intensity mixing, plough/paddle blending with intensifying choppers and combinations thereof.

5. The process of claim 2 wherein the particle size reduction is performed during said blending.

6. The process of claim 2 wherein the particle size reduction is performed subsequent to blending said boron compound and said nitrogen-containing source.

7. The process of claim 2, wherein the particle size reduction process is performed prior to blending said boron compound and said nitrogen-containing source.

8. The process of claim 2, wherein both of said boron compound and said nitrogen-containing source are subjected to a particle size reduction process.

9. The process of claim 1 , further comprising the step of heating treating said BN at a temperature in the range of from about 1600⁰C to about 2100⁰C after said reaction step.

10. The process of claim 1 , wherein said reaction step is carried out in a nitrogenous atmosphere comprising an inert gas and ammonia between 900 and 145O⁰C for a period of at least 10 minutes. (Should 9 and 10 be switched or does it matter?)

11. The process of claim 1 , wherein the boron nitride yield of said process is increased by at least 10% over the yield of a similar process wherein the particle size of [the blend] falls within either a) or b).

12. The process of claim 1 , further comprising the step of adding a carbon containing source to said raw material blend.

13. A process for producing a hexagonal boron nitride, said process comprising: blending an oxygen-containing boron compound with a nitrogen- containing source; reacting said boron compound with said nitrogen containing source to form boron nitride; adding a carbon containing source to said boron nitride to form a carbon doped boron nitride, wherein said carbon containing source is added in an amount of from about 4.5 to 20 weight % to said boron nitride; and heat treating said carbon doped boron nitride in a nitrogenous atmosphere to form hexagonal boron nitride. Do we need to add the carbon concentration in this claim?

14. A process according to claim 13, wherein said heat treating step is conducted such that the boron nitride is heated to an intermediate temperature of 1600⁰C to 1900⁰C and held for 0.16 hrs to 12 hrs, and then heated to a temperature in the range from 1850⁰C to 2100⁰C for 0.16 hrs to 72 hrs.

15. The process of claim 13, wherein the hexagonal boron nitride formed has total oxygen concentration of not more than 0.5 wt%.

16. The process of claim 13, wherein at least one of said boron compound and nitrogen-containing source comprise particles having at least one of a) a particle size distribution such that at least 15 wt% of said particles have a particle size of less than 20 microns, and b) an average particle size of 1 to 60 microns.

17. The process of claim 13, wherein said boron nitride formed after said reacting step comprises a crude boron nitride having an oxygen content of from 5 to 25.

18. The process of claim 13, wherein the carbon containing source is selected from the group consisting of carbon black, graphite powder, cornstarch, agar, melamine, corn syrup, molasses, pitch, asphalt and mixtures thereof.

19. The process of claim 13, wherein the carbon doped boron nitride mixture is compacted prior to heat treating.

20. A hexagonal boron nitride compound having improved hardness, wherein said boron nitride is produced by adding a carbonaceous compound during formation of said boron nitride.