1. Field of the Invention (Technical Field)
The present invention relates to a process for production of boron nitride powders exhibiting smooth spherical morphology, spheroidal particles with xe2x80x9cbladedxe2x80x9d surface morphology, spheroidal particles with protruding xe2x80x9cwhiskers,xe2x80x9d and fully xe2x80x9cbladedxe2x80x9d particles with platelet morphology, and particles having turbostratic or hexagonal crystal structure. The process utilizes aerosol assisted vapor phase synthesis (AAVS), nitriding organoboron precursors through a boron oxide nitride intermediary composition, to form spherical and modified spherical boron nitride powders. The process can be achieved through use of an aerosol assisted vapor phase reactor system (AAVRS), and it has significant use in preparation of the preferred spheroidal boron nitride powders for use in the microelectronic, polymer, and cosmetic industries as well as in traditional ceramic markets (e.g., aerospace and automotive products).
2. Background Art
Boron nitride (BN) is a well-known, commercially produced refractory non-oxide ceramic material. Boron nitride properties are highly dependent on its crystalline structure. The most common structure for BN is a hexagonal crystal structure. This structure is similar to the carbon structure for graphite, consisting of extended two-dimensional layers of edge-fused six-membered (BN)3 rings. The rings arrange in crystalline form where B atoms in the rings in one layer are above and below N atoms in neighboring layers and vice versa (i.e., the rings are shifted positionally with respect to layers). The intraplanar Bxe2x80x94N bonding in the fused six-membered rings is strongly covalent while the interplanar Bxe2x80x94N bonding is weak, similar to graphite.
Historically, commercial boron nitride articles have been prepared by hot pressing BN powders obtained from classical metallurgical high-temperature synthesis (e.g., boric acid treated with urea at 1000xc2x0 C., xe2x80x9chot-pressed BNxe2x80x9d; or BN obtained by chemical vapor deposition (CVD) growth, xe2x80x9cpyrolytic BNxe2x80x9d). Pyrolytic BN is considered the more typical form in the industry, given the absence of binders and improved crystallinity and grain features. (Unless otherwise indicated, properties of BN described in these background materials are representative of pyrolytic BN.) Under these typical solid state synthesis conditions, BN is typically obtained as a mixture of meso-graphitic and turbostratic modifications that contain varying degrees of disorder of the ideal hexagonal BN structure (h-BN). Fully ordered h-BN is only obtained with careful attention to synthetic detail. (Paine, RT, Narula, CK. Synthetic Routes to Boron Nitride. Chem. Rev. 90: 73-91, 1990.)
Commercial applications for h-BN are well established in several traditional ceramic markets. In particular, the high temperature stability, chemical inertness, lubricity, electrical resistivity and thermal conductivity make BN powders ideal for fabrication of products used in aerospace, automotive and microelectronic products, including large crucibles, heat sinks, mold liners and electrical insulators. Unlike carbon, h-BN is a colorless or (in cases where impurities are present with defect states in the electronic band gap) white material. In its powder form, it can be processed by classical powder forming methods into simple and complex shapes. Since it is soft, hot pressed bodies can be easily machined. In the absence of oxygen and moisture, BN is stable above 2000xc2x0 C.; however, it combusts in oxygen near 900xc2x0 C. The layered, hexagonal crystal structure results in anisotropic physical properties that make this material unique in the overall collection of non-oxide ceramics.
Examples of various known or attempted methods to produce spheroidal BN through hexagonal modification include several high-temperature, metallurgical or chemical vapor deposition (CVD) reactions. (Paine, RT, Narula, CK. Synthetic Routes to Boron Nitride. Chem. Rev. 90: 73-91, 1990.) From the commercial standpoint, h-BN is obtained as a powder most often from multi-step processes that employ boric oxide, sodium borate or boric acid as the boron raw material and urea, melamine and/or ammonia as the nitriding source. These reactions are driven by the thermodynamic stability of BN and the reducing nitridation conditions that remove impurities.
Carbothermal reduction conditions also can be employed to remove oxygen. Commercial powder producers manipulate reaction conditions in order to achieve target powder purity, grain size, sinterability and crystallinity. These features, in turn, influence powder processibility and finished product performance. It is important to note that commercial powders are usually obtained either as agglomerates having irregular morphology or as primary particles with a platelet morphology. The latter is a macroscopic manifestation of the inherent crystal structure of h-BN.
Recently, interest in inorganic ceramic/organic polymer composites containing BN powders for thermal management applications has arisen. It has been suggested in the art that a spherical morphology BN powder would be useful to enhance powder processing of polymers. However, a commercial source of such powders is not available. One known process to obtain small, laboratory-scale samples of spheroidal BN involves reacting trichloroborazine with an aminosilane to form a polymer that dissolves in liquid ammonia (NH3). The resulting solution may be used to form an aerosol containing poly(borazinylamine). The aerosol is then passed through a reaction furnace to produce a boron nitride powder composed of primary particles having spherical morphology. Further nitridation in an NH3 atmosphere at a temperature of 1600xc2x0 C., over a period of time of at least eight hours, gives h-BN particles of overall spheroidal shape with protruding non-uniform blades. This process is not commercially viable since it requires the use of an expensive, commercially unavailable polymer that is made only from an expensive commercially unavailable monomer. (Lindquist, DA et al. Boron Nitride Powders Formed by Aerosol Decomposition of Poly(borazinylamine) Solutions. J. Am. Ceram. Soc. 74 (12) 3126-28, 1991.)
As another example, a second method reacts boron trichloride with ammonia, a combination typically used to make platelet morphology h-BN by CVD. The resulting powders are treated at high temperature in a graphite furnace under vacuum. (The patent suggests formation of spherical primary particles although no evidence of the actual morphology is provided.) This process, if successful, is not commercially attractive due to the expense of the starting material, BCl3, and the formation of a corrosive by-product HCl that tends to leave chloride impurities in powders. (European Patent Office Publication No. 0 396 448.)
A third and potentially more practical process for the formation of spherical morphology h-BN powders utilizes a process where an aerosol is generated from a saturated (0.9M) aqueous solution of boric acid. The aerosol is passed into a heated tubular reactor where it is nitrided by NH3 in a temperature range of between 600xc2x0 C. and 1500xc2x0 C., preferably between 1000xc2x0 C. and 1200xc2x0 C. A powder product, BNxOy, is collected that contains significant amounts of oxygen, typically between 40 wt. % to 55 wt. %. The primary particles have spherical particle diameters in the range 0.1 micron to 5 microns. These powders are subsequently nitrided in a second stage in a temperature range of between 1000xc2x0 C. to 1700xc2x0 C. under a flowing stream of NH3. The oxygen contents of the resulting boron nitride powders are less than 4 wt. % and the particles retain the spherical morphology. (Pruss et al., Aerosol Assisted Vapor Synthesis of Spherical Boron Nitride Powders. Chem. Mater. 12(1), 19-21, 2000; U.S. Pat. No. 6,348,179 to Pruss et al.)
Although the process described by Pruss et al. is practically useful for the production of spherical morphology BN powders, it possesses several drawbacks, including: (a) large amounts of water are injected into the tubular reaction zone in the form of aerosol droplets thereby diluting the NH3 reactant that is required for nitridation of H3BO3 dissolved in the aqueous aerosol droplets; (b) the large amounts of injected water act as a back-reactant with BNxOy aerosol powders; (c) water is also formed as a reaction byproduct in the first stage aerosol nitridation; (d) the BNxOy powders formed in the first-stage nitridation reaction contain large amounts of oxygen; (e) the large amounts of oxygen are difficult to remove in the second-stage nitridation; and (f) there is significant loss of boron as a volatile component during the nitridation process. Due to these drawbacks alternative solventless or non-aqueous solvent-based aerosol chemical systems have been sought in the industry.
Very few readily available, inexpensive boron reagents exist that are soluble in a non-aqueous solvent appropriate for aerosol formation or aerosol pyrolysis. Similarly, there are very few inexpensive, liquid-phase boron reagents that might be employed directly without a solvent to generate an aerosol. However, at least one family of boron reagents does exist that is commercially available in large quantities at relatively low cost and is soluble in non-aqueous solvents: trialkoxyboranes or trialkylborates, (RO)3B (e.g., R=Me(CH3), Et(C2H5), Pr(C3H3), Bu(C4H9)). These are free-flowing liquids at 23xc2x0 C. In addition, there is evidence in the literature that suggests that trialkylborates, (RO)3B, react with the common nitriding reagent ammonia, NH3.
For example, U.S. Pat. No. 2,629,732, discloses that (RO)3B (R=lower mol. wt. alkyl groups preferably CH3) reacts with NH3 in a 1:1 ratio in the gas phase at normal atmospheric pressure and temperature to give adducts, (RO)3B.NH3. Further, other examples in the literature describe a reaction of (CH3O)3B with NH3 that is claimed to form an adduct (MeO)3B.NH3 that sublimes at 45xc2x0 C. and allegedly is stable to at least 375xc2x0 C. (Goubeau et al., Z. Anorg. Allgem. Chem. 266, 161-174, 1951.) Goubeau et al. also describe reactions that employ other reactant ratios which produce complex product mixtures that are not identified. The chemistry is proposed to involve elimination of methanol and dimethyl ether. U.S. Pat. No. 2,824,787 to May et al. claims the formation of BN from pyrolysis of a gas mixture of (MeO)3B and NH3 at a furnace temperature above about 850xc2x0 C. The resulting product is a white powder containing B, N, O, C, and H in varying amounts depending upon reaction conditions. This powder is then heated in NH3 atmosphere to 900xc2x0-1100xc2x0 C. to obtain BN. The ""787 patent does not describe the morphology and crystallinity of the BN. However, it is likely that these processes produce BN with the traditional platelet morphology.
Further, in a series of patents, Bienert et al. describe the formation of boron-nitrogen-hydrogen compounds, BN3xe2x88x92xH6xe2x88x923x, from the reaction of boron halides or basic acid esters with NH3 in a heated gas flow tube held at 200xc2x0 C. or 500xc2x0 C. The resulting compounds are claimed to be useful for making detrition-resistant boron nitride pressed bodies, boron nitride powder and semiconduction components. (Bienert et al., Ger. Offen. No. 1,943,581; Ger. Offen. No. 1,943,582; Ger. Offen. No. 2,004,360; U.S. Pat. No. 3,711,594.) Finally, Murakawa et al. describe the use of (EtO)3B in a hot gas stream of air and methane to form B2O3 and C. A powder compact was subsequently heated at 900xc2x0 C. in N2. It was claimed that h-BN with spherical morphology (ave. diameter, approximately 0.14 micron) formed. (Japanese Patent No. JP60,200,811 to Kokai et al.)
There remains a real need in the art for a process yielding spherical h-BN in high concentration without high oxygen impurities, utilizing commonly available, inexpensive precursors such as trialkylborates.
The present invention is a method for an aerosol assisted vapor phase synthesis (AAVS) process of boron nitride (BN) wherein organoboron precursors are nitrided in one or two heating steps, and wherein a boron oxide nitride intermediary composition is formed after the first heating step and is further nitrided to form resultant spheroidal boron nitride powders including spheroidal particles that are smooth, bladed, have protruding whiskers, and are of turbostratic or hexagonal crystalline structure.
The method of the present invention comprises forming a boron-nitrogen-oxygen-carbon-hydrogen, BNxOyCz, precursor for BN comprising: providing an organoboron precursor, an inert carrier gas, and a nitriding agent; aerosolizing the precursor; introducing the carrier gas into a chamber containing the aerosolized precursor and forming a combined gas stream; sweeping the combined gas stream into a heated furnace; injecting the nitriding agent into the furnace; allowing the nitriding agent and aerosolized precursor to react to form a powder of a boron-nitrogen-oxygen-carbon-hydrogen composition; and collecting the powder.
Further steps in the method may be taken as follows:
In the present invention, the nitriding agent and aerosolized precursor are reacted in a first heating step. The resultant boron-nitrogen-oxygen-carbon-hydrogen composition is heated in a second heating step in the presence of a nitriding agent, inert gas, or vacuum.
The nitriding agent may comprise NH3, N2/H2, N2, alkylamines, hydrazine, cyanamide, dicyanamide, hydroxylamines, or mixtures thereof. The nitriding agent may comprise a liquid, which is aerosolized and is swept into the furnace by a carrier gas.
The organoboron precursor agent may comprise an alkylborate. The alkylborate may comprise a trialkylborate. Further, the trialkylborate may comprise (MeO)3B, (EtO)3B, (PrO)3B, or (BuO)3B. However, the precursor agent may comprise a polyborate. The polyborate may comprise a boroxine. Further, the organoboron precursor may comprises an azeotropic mixture. The azeotropic mixture may comprises an alkylborate and alcohol. The alkylborate may be a trimethylborate and the alcohol may be methanol. The organoboron precursor may be dissolved in simple alcohols, alkanes, or arenes prior to aerosolization, thereby increasing the percentage of carbon in the resulting BNxOyCz powder. Further, the organoboron precursor may be dissolved in liquid ammonia prior to aerosolization.
The aerosolized organoboron precursor and carrier gas, and the nitriding agent are simultaneously swept or injected into the furnace. The flow of the combined gas stream (organoboron precursor and carrier gas) may have a predetermined flow rate. The injection step of the nitriding agent may have a predetermined flow rate.
The step of heating the furnace comprises the step of maintaining a temperature of between approximately 600xc2x0 C. and approximately 1800xc2x0 C. Further, the temperature may be maintained between approximately 1200xc2x0 C. and approximately 1800xc2x0 C.
The boron-nitrogen-oxygen-carbon-hydrogen powder may be collected on a powder collection device, which may be a filter.
Additional steps of grinding the resultant BNxOyCz powder, spreading the powder over an oxide, and melting the powder over the oxide thereby reacting the powder with the oxide and forming a BN thin film layer may additionally be taken. Further, the BNxOyCz powder may be collected on a substrate and then melted, forming a boron nitride thin film layer.
An method of the present invention comprises preparing h-BN by the following steps: providing an organoboron precursor, an inert carrier gas, and a nitriding agent; aerosolizing the precursor; introducing the carrier gas into a chamber containing the aerosolized precursor and forming a combined gas stream; sweeping the combined gas stream into a heated furnace; injecting the nitriding agent into the furnace; allowing the nitriding agent and aerosolized precursor to react to form a powder of a boron-nitrogen-oxygen-carbon-hydrogen composition during a first heating step; heating the boron-nitrogen-oxygen-carbon-hydrogen composition in a second heating step in the presence of the nitriding agent; allowing the nitriding agent and boron-nitrogen-oxygen-carbon-hydrogen composition to react with the nitriding agent in the second heating step to form a boron-nitrogen powder; and collecting the powder.
Further steps in this method may be taken as follows:
The nitriding agent may comprise NH3, N2/H2, N2, alkylamines, hydrazine, cyanamide, dicyanamide, hydroxylamines, or mixtures thereof. The nitriding agent may comprise a liquid, which is aerosolized and is swept into the furnace by a carrier gas.
The organoboron precursor agent may comprise an alkylborate. The alkylborate may comprise a trialkylborate. Further, the trialkylborate may comprise (MeO)3B, (EtO)3B, (PrO)3B, or (BuO)3B. However, the precursor agent may comprise a polyborate. The polyborate may comprise a boroxine. Further, the organoboron precursor may comprise an azeotropic mixture. The azeotropic mixture may comprise an alkylborate and an alcohol. The alkylborate may be trimethylborate and the alcohol may be methanol. The organoboron precursor may be dissolved in simple alcohols, alkanes, or arenes prior to aerosolization, thereby increasing the percentage of carbon in the resulting BNxOyCz powder. When the organoboron precursor is dissolved in alcohols, alkanes, or arenes, the resultant BN compound is microporous or nanoporous. Further, the organoboron precursor may be dissolved in liquid ammonia prior to aerosolization.
The aerosolized organoboron precursor and carrier gas, and the nitriding agent are simultaneously swept or injected into the furnace. The flow of the combined gas stream (organoboron precursor and carrier gas) may have a predetermined flow rate. The injection step of the nitriding agent may have a predetermined flow rate.
The step of heating the furnace comprises a first heating step of maintaining a temperature of between approximately 600xc2x0 C. and approximately 1800xc2x0 C. Further, the temperature may be maintained between approximately 1200xc2x0 C. and approximately 1800xc2x0 C.
The boron-nitrogen-oxygen-carbon-hydrogen powder may be collected on a powder collection device, which may be a filter. The powder may then be placed in a second furnace and subjected to the second heating step in the second furnace. Contrarily, the boron-nitrogen-oxygen-carbon-hydrogen powder may not be collected, but may be vented in a gas stream entrained with the powder into a second furnace before the second heating step.
Both heating steps may be performed in one furnace, which may be a vertical furnace. The second heating step may comprise maintaining the temperature between approximately 600xc2x0 C. and approximately 1800xc2x0 C. Further, the temperature may be maintained between approximately 1200xc2x0 C. and approximately 1800xc2x0 C.
Modified h-BN particles are formed by varying the aerosol conditions or reactor conditions to form turbostratic structures, bladed spherical particles, platelet particles, or particles having crystalline whisker growth.
The formed BN particles may comprise a diameter range between approximately 0.05 xcexcm and approximately 100 xcexcm.
A primary object of the present invention is preparing a BN powder comprising a spherical morphology.
Another object of the present invention is preparing a BN powder comprising a modified spherical morphology.
Yet another object of the present invention is providing a simple, one or two-step synthesis process for preparation of BN.
Another object of the present invention is to provide a process yielding highly pure levels of h-BN, wherein impurities of such a BN product would comprise additional elements or non-spherical BN particles.
A further object of the present invention is to provide a process utilizing simple organoboron precursors.
Yet another object of the present invention is to provide a simple process for utilizing trialkylborates as a precursor to BN.
A primary advantage of the present invention is preparing a BN powder comprising a spherical morphology.
Another advantage of the present invention is preparing a BN powder comprising a modified spherical morphology.
Yet another advantage is that BN may be prepared in a simple one or two-step process.
Another advantage of the present invention is the use of simple, available, inexpensive organoboron compounds as precursor to BN.
A further advantage is the highly purified h-BN product, wherein impurities of such a BN product would comprise additional elements or non-spherical BN particles.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.