Chemistry of inorganic compounds – Boron or compound thereof – Oxygen containing
Reexamination Certificate
2002-04-23
2004-11-30
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Boron or compound thereof
Oxygen containing
C423S290000
Reexamination Certificate
active
06824753
ABSTRACT:
BACKGROUND OF THE INVENTION
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 “bladed” surface morphology, spheroidal particles with protruding “whiskers,” and fully “bladed” 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 B—N bonding in the fused six-membered rings is strongly covalent while the interplanar B—N 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 1000° C., “hot-pressed BN”; or BN obtained by chemical vapor deposition (CVD) growth, “pyrolytic BN”). 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 2000° C.; however, it combusts in oxygen near 900° 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 (NH
3
). 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 NH
3
atmosphere at a temperature of 1600° 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, BCl
3
, 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 NH
3
in a temperature range of between 600° C. and 1500° C., preferably between 1000° C. and 1200° C. A powder product, BN
x
O
y
, 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 1000° C. to 1700° C. under a flowing stream of NH
3
. 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
Janik Jerzy F.
Kroenke William J.
Paine Robert T.
Pruss Eugene A.
Wood Gary L.
Fain Katy C.
Langel Wayne A.
Mays & Fain, LLP
Science & Technology Corporation @UNM
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