PREPARATION OF SOLID AGGREGATE BORON NITRIDE CRYSTALS

Abstract: High density boron nitride crystal aggregate is prepared by compacting high density boron nitride powder using an apparatus comprising a first hollow, substantially cylindrical container (11) containing the boron nitride powder (13), a fluid-like material (15), a second hollow, substantially cylindrical, rigid container (17) and high explosive material (18) which produces shock waves.

Description: P EPARATION OF SOLID AGGREGATE BORON NITRIDE CRYSTALS Field of the Invention This invention relates to the production of the solid aggregate crystals of boron nitride by a shock wave process. Background of the Invention Boron nitride is similar to carbon in that both materials have a soft hexagonal form which can be converted under high pressure conditions to two very hard forms: (1) the cubic form with a zincblende crystalline structure, and (2) the hexagonal form with a wurtzite crystalline structure. The soft form and the two hard forms have specific gravity of about 2.28 and 3.49, respectively. The soft form will be referred to as low density boron nitride, and the hard forms as high density boron nitride. The high density cubic form will be designated here as cubic boron nitride, CBN, and the high density hexagonal form will be designated wurtzite boron nitride, wBN. To produce high density boron nitride, one may begin with low density boron nitride and apply static or dynamic high pressures to produce small aggregates, usually no more than 100 microns in diameter. However, many commercial applications of high density boron nitride require much larger aggregates. The subject invention provides method and apparatus to start with small particle sizes of cubic or wurtzite boron nitride or mixtures of both and convert them into polycrystalline aggregates of sizes of one centimeter and greater. In U.S. Patent No. 3,401,019, Cowan and Holtz an disclose a method for producing a shock wave of sufficient intensity to convert carbon to diamond initially, and using a contiguous cooling medium to keep the temperature of the shocked material below 2000°C, and preferably below 1800°C, where the cooling medium also has sufficient thermal conductivity that excessive graphitization after release of the shock wave pressure does not occur. Cowan et al remark that the straightforward shock synthesis technique is incapable of producing satisfactory yields of diamond due to excessive graphitization of the diamond, which is initially hotter than the carbon from which it is converted, upon release of the shock wave pressures. Cowan et al begin with carbon (preferably graphite) compacted to a density of about 75 percent of the theoretical density for diamond, and apparently allow the chosen cooling medium to surround and fill the interstitial regions of the compacted carbon. The admixture is subjected to a shock wave of at least 750- kilobars, and preferably 1000-2000 kilobars. The Cowan et al invention begins with graphite and requires very high shock wave pressures to convert the carbon to diamond form. In U.S. Patent No. 3,568,248, Cowan discloses an end closure or plug for a cylindrical container of material that is to be subjected to a shock wave. The plug comprises a first, substantially cylindrically shaped section in contact with the sample at one end along the cylinder longitudinal axis, and having a shock impedance (the product of shock wave velocity in, and initial density of, the material) equal to the shock impedance of the sample; and a second section, in contact with the first section along the cylinder longitudinal axis at the second end of the first section, having a shock impedance substantially equal to the shock impedance of the first section's material, and arranged to carry off most of the longitudinally-propagating shock wave by spallation of one end of the second section. In one embodiment, the first section has a gradually decreasing porosity as one moves away from the sample toward the second end, and the second section has a gradually increasing porosity as one continues in the same direction. The Cowan invention contemplates that the shock wave will move primarily along the longitudinal axis of the cylinder. United States Patent No. 3,399,254, issued to Dunnington, describes a method of bonding together diamond particles in powder form using shock waves. The sample (powder) is confined in the interior of a hollow flat slab or disc, and one broad, flat face of this container is impacted by an explosively-driven flyer plate or projectile. The shock wave pressures used here are limited to modest values C-?30Q Kilobars) due to sample recovery problems. Balchan and Cowan, in U.S. Patent No. 3,667,911, disclose a method of shock wave treatment of a solid material such as diamond, boron nitride or silicon carbide powder by propagating the shock wave axially along the sample at substantially uniform velocity, where the sample's physical extension in the axial direction is much greater than its physical extension in any direction transverse to the axial direction. The shock wave itself is generated (1) by impacting the sample at one end by an explosively-driven impact plate, or (2) by detonating a high explosive in contact at one end with the sample. The sample may be contained in a container. Ideally, the shock wave is planar, with the defining plane being perpendicular to the axial direction of propagation, and with sufficient shock wave energy that the associated wave pressure is substantially constant throughout the perpendicular plane. The sample's axial length should substantially exceed the length or distance required to achieve shock wave steady state conditions (the "start-up length") which is approximately five times the transverse diameter of the sample; or a solid material, having the same density, shock impedance and transverse diameter as the sample, and having an axial length at least equal to the start-up length, should be provided at an axial position between and in contact with both the sample and the axial position for application of the pressure pulse that produces the shock wave. Details of propagation of a shock in any direction differing substantially from the axial direction (e.g., in a radial direction) are not discussed, and this would probably be inconsistent with the application of the Balchan and Cowan invention. This Patent contains a good mathematical discussion of the generation and propagation of shock waves in a material. A method of aggregating small, hard particles, such as diamond, into larger aggregates by passage of shock waves therethrough is disclosed by Balchan and Cowan in U.S. Patent No. 3,851,027. The sample particles are dispersed interstitially in a carrier matrix having smaller porosity and greater post-shock deformability than the interstitial particles, usually formed as a flat disc or slab, and one broad face of the carrier matrix is impacted by an explosively-driven projectile or driver plate to produce a shock wave that travels through the matrix/interstitial particles combination and bonds many of the hard particles together. It appears that the hard particles are intended to coalesce into a multiplicity of larger size aggregates as the use of the carrier matrix appears to prevent aggregation of all the hard particles into a single mass. The hard particles that are bondable by this technique allegedly include diamond, boron nitride, silicon carbide and silicon nitride, and shock wave pressures of 100 kilobars and up are used for this purpose. Again, the direction of shock wave propagation is axial. Axial propagation of a shock through brittle inorganic aggregates (^<4500 μ diameter) powders such as -alumina, barium ferrite, barium titanate, silicon carbide, boron carbide, magnesium oxide, titanium carbide and bismuth telluride, is disclosed in U.S. Patent No. 3,367,766, issued to Barrington and Bergmann. The shock wave is produced by detonation of an explosive contiguous with one end of a container that confines the powder to be aggregated. Two U.S. patents issued to Corrigan, Nos. 4,201,757 and 4,231,980, disclose the use of an explosively-driven flyer plate to generate shock waves in low density boron nitride to produce the high density wurtzite form of boron nitride. The impact of the flyer plate on one surface of the low density boron nitride is arranged to produce a shock wave in the direction of the C-axis (longitudinal) as usual. The shock compression according to Corrigan allegedly "snaps" the low density boron nitride constituent atoms from loosely packed crystalline form to a close-packed crystalline form of high density and hardness. The technique is apparently orientation-dependent, as the inventor emphasizes that the shock wave must be directed along the C-axis of the soft form of boron nitride. The shock wave pressure range used by Corrigan is 100-500 kilobars. The pressure as noted is applied to the low density form of boron nitride (recrystaliized pyrolytic boron nitride) rather than the high density forms of boron nitride. The Corrigan patent describes a process for producing the wurtzite high density form of boron nitride having an average particle size of 100 microns. None of the patents discussed above discloses and seriously discusses the technique of generation and propagation of radial shock waves within the sample; most of these patents begin with the sample in powder form, not as small aggregates that are to be further aggregated into larger size aggregates; and these techniques do not appear to produce high density aggregates of size one cm or greater. Summary of the Invention One object of the invention is to provide method and apparatus to convert small particle sizes of cubic or wurtzite boron nitride or mixture of both into polycrystalline aggregates of sizes one centimeter or greater. Another object is to provide a shock wave synthesis method for producing solid aggregate crystals of boron nitride, using high density boron nitride as a starting material. Another object is to provide method and apparatus for producing solid aggregate crystals of boron nitride that do not depend upon orientation of the starting materials. Another object is* to provide method and apparatus for producing solid aggregate crystals of boron nitride of diameters large enough for use in grinding, drilling and machining. Another object is to provide method and apparatus for producing solid aggregate crystals of boron nitride or Mohs hardness substantially- 9.5 or more, and Knoop microhardness of substantially 30 gigaNewtons/ or more. Another object is to provide method and apparatus for producing large solid aggregates of high density boron nitride with Knoop hardness substantially greater than can be achieved by static consolidation of high density boron nitride. Other objects of the invention, and advantages thereof, will become clear by reference to the following detailed description and the accompanying drawings. In one embodiment, the method comprises the steps of: Providing high density boron nitride in the form of powder (2-100 microns diameter) inside a first cylindrical container; surrounding the first container on all sides with a predetermined fluid-like material, such as a metal powder of moderate relative density; surrounding the fluid-like material with a hollow, substantially cylindrical, rigid container that is closed at both ends, is in contact with the fluid-like materials it surrounds, and has its interior purged of substantially all air; surrounding the rigid container on the side wall and one end wall with the predetermined amount of high explosive having a detonation velocity D that is greater than the shock wave velocity S of the fluid-like materials, and detonating the high explosive at the end wall of the rigid container. Brief Description of the Drawings Figure 1 illustrates one configuration for the tubes containing boron nitride used in the invention. Figure 2 is a schematic cross-sectional longitudinal view of a first embodiment of the invention. Figure 3 is a schematic cross-sectional longitudinal view of a second embodiment of the invention. Figure 4 is a schematic view of the movement of initial and reflected shock waves produced in the apparatus of the invention. Figure 5 is a graph of development of local pressure with time at a representative point in the fluid-like medium in the first embodiment. Figure 6 is a graph of development of local pressure with time at a representative point in the fluid-like medium in the second embodiment. Figure 7 is a schematic cross-sectional end view of the apparatus used in the first or second embodiment of the invention. Detailed Description The invention provides a method of using explosively-generated shock waves in certain fluid-like materials to produce large solid aggregate crystals of boron nitride, starting from fine powders of high density boron nitride, with reference to Figure 1, the interior of a hollow, rigid, substantially cylindrical container 11 (which may be comprised of copper or some other suitable metal) is filled with high density boron nitride powder 13 and evacuated to a pressure less than -5 -7 10 torr, and preferably to 10 torr or less, and closed and sealed by pinching the tubes at one or both ends of the tube to exclude substantially all gases and provide an airtight container. The boron nitride powder 13 will initially have a density of about 50 percent of the solid, high density boron nitrides. The tubes 11 are then placed in a suitable fluid-like, pressure-transmitting material 15 (which may be a low strength material such as copper or a powder of such material of about 50 percent normal density), which is in turn placed in a second hollow, rigid, substantially cylindrical container 17 ( Figure 2) that is filled with the tubes and fluid-like material. The second container 17 may have a side wall 17s of 0.3-0.7 cm thick steel or other hard metal, and have outer diameter of 6.35cm. The container 17 should, in its preferred embodiment, have two end walls 17e, each comprising a steel outer wall 3-5 cm thick, 17eo and an inner wall 17ei of softer metal such as Al or Mg that is 1.5-3 cm thick. The container is surrounded on the side wall 17s and one end wall 17e with a high explosive 18 that may be 7-12 cm thick. In one embodiment, shown in Figures 2 and 7, the combination of rigid tubes 11 and fluid-l ke material 15 extends to the center line CC of the container 17. In a second embodiment, shown in Figure 3, a central core of the interior of the second container 17 is filled by a solid, substantially cylindrical, metal mandrel 19. The metal mandrel is required when a powder material is used for the fluid-like material. with either the embodiment of Figure 2 or of Figure 3, the high explosive is detonated at or adjacent to one end wall 17e, and the detonation wave sweeps longitudinally along the container perimeter as shown in Figure 4, imploding the container walls toward the container center line CC and producing a radially converging shock wave in the combination of fluid-like material 15 and tubes 11. The high explosive detonation velocity D is chosen to be greater than the shock wave velocity S for the fluid-like material so that the initial shock wave moves in the direction S- as shown in Figure 4, and produces a "reflected" shock wave moving outward in the general direction of S as shown, we have found that use of a fluid-like material 15 in the form of a powdered metal such as copper, or preferably a powder of a higher density material such as tungsten metal or tungsten carbide, allows much larger shock wave pressures to be transmitted to the tubes compared to a solid material. If the embodiment of Figure 2 is employed, the radially converging shock wave S. drills or otherwise produces a central channel in the assembly when it reflects from the center line CC. The reflected shock wave S has a greater amplitude than the incident shock wave S. and moves radially outward, producing for radial positions away from the center line CC a pressure profile such as shown in Figure 5. The first pressure wave S- pre-compresses the boron nitride powder, and the second pressure wave S compresses the powder further to its final density. The boron nitride powder is transformed (permanently) into large aggregate crystals of boron nitride of diameter as large as 1 cm and greater. After the passage of additional time (a few microseconds), one or more release waves R pass through the material and decrease the local pressure to approximately ambient conditions, when a tube 11 is placed on the center line, only a single pressure peak occurs, of extraordinary amplitude, that is of little value for the production of large solid aggregate crystals of boron nitride. If the embodiment of Figure 3 (using a substantially centrally positioned, solid, metal mandrel 19) is employed, the radial shock wave again produces a two-fold rise in pressure as shown in Figure 6. The use of two pressure waves in succession rather than a single higher amplitude wave causes less local thermal heating of the boron nitride aggregate, and the resulting aggregate has a higher density [ — 3.49 gm/cm ) and other superior properties. The mandrel is required to reduce the material axial velocity when a powder is used for the fluid-like material. The high axial velocity material flow is disruptive to the nearby tubes containing the boron nitride powder. The separation in time of the two pressure pulses, denoted by S. and S , is a measure of the radial distance of the pressure measurement point from either the center line of the container 17 (first embodiment) or the mandrel perimeter (second embodiment). Preferably, the two pressure rises sho.wn in Figures 5 and 6 should be separated as much as possible in time so that the pressure is applied to the boron nitride aggregate for as long a time period as possible, we have found that the best radial position for the tubes 11 is approximately midway between the center line (first embodiment) or mandrel perimeter (second embodiment) and the initial inner diameter of the container 17, as illustrated in Figure 7. we have found that large aggregate crystals of boron nitride are best produced if a second pressure wave pulse amplitude of P = 500 - 750 kilobars is attained, with the first amplitude pulse being somewhat lower than the second. The pressure-transmitting material 15 in Figures 2 and 3 may be a metal such as copper, iron, tantalum, tungsten or uranium in powder or other fluid-like form; the use of a powdered form increases the shock wave pressure transmitted through the medium 15. At the same time, a material initially in powdered form undergoes a large relative volume change in response to applied pressure. This results in a control! bly lower shock wave velocity in a powder than in a solid of the same material. The high explosive detonation velocity D should preferably be at least 8 km/sec to best achieve the desired aggregation into large crystals of high density boron nitride. This may be achieved with mixtures such as pentolite (60% PETN/40% TNT) or with single explosives such as PETN or RDX that have detonation velocities as high as 9 km/sec. High explosives with detonation velocities D = 6-8 km/sec will often produce the appropriate shock waves for large crystal aggregation, but detonation velocities D • < 6 km/sec are unsatisfactory here. The material 15 is preferably powdered copper or other suitable powdered metal at approximately half normal density, with this choice of materials, the initial pressure wave amplitude is usually~550 kilobars. with these pressure wave amplitudes, the boron nitride aggregate can be consolidated to 99 percent of its theoretical maximum density. CA. Brookes, in "The Mechanical Properties of Cubic Boron Nitride - A Perspective View", Second International Conference on Hard Materials, Rhodes, Greece (Sept. 23-28, 1984) has compared Knoop indentation hardness of cubic boron nitride with the hardness of two types of diamond, along different crystallographic directions. The Knoop 2 hardness of cubic boron nitride varies from 29 to 43 gigaNewtons/M , depending upon stress plane and direction. Generally, the diamond hardness is 1.4-2.5 times that of the corresponding cubic boron nitride. Using the conventional mineralogical of Mohs scale (range 1-10 with diamond hardness being 10), it would be difficult to distinguish the Mohs hardness of boron nitride and diamond. Although the preferred embodiments of the invention have been shown and described herein, modification and variation in the CBN form the wBN form or mixtures of both may be made without departing from the scope of the invention.

Claims: CLAIMS 1. A method for compaction of powder (2-100 microns) of high density boron nitride into a high density crystal aggregate of size up to 1 cm in the cubic zincblende form, the wurtzite form or mixtures thereof, the method comprising the steps of: providing boron nitride high density powder with bulk density of substantially 50 percent theoretical density inside a first substantially cy1indrical container; surrounding the first container on all sides with a predetermined, fluid-like material having a characteristic shock wave velocity S; surrounding the predetermined material on all sides with a hollow, substantially cylindrical, rigid container that has a side wall and two rigid end walls, and is in contact with the predetermined material, and has its container interior purged of substantially all gases; surrounding the rigid container at one end wall thereof and on its side wall with a predetermined amount of high explosive material having a detonation velocity D, where D is greater than S; and detonating the high explosive material at one of the end walls of the rigid container. -15- 2. The method according to Cl aim 1 , wherein the said flui d-l ike material is drawn from a class consisting of copper, iron, tantalum, tungsten and uranium, and this material is provided in powder form of density substantially 50 percent of theoretical density of the material or 3 a low strength solid metal of density greater than 8 gm/c . 3. Apparatus for compaction of f ne powders of high density boron nitride into a high density crystal aggregate in the zincblende form, the wurtzite form or mixtures thereof, the apparatus comprising: a first hollow, substantially cylindrical container containing the boron nitride powder; a predetermined, fluid-like material having a characteristic snock wave velocity S surrounding and being in contact with the first cylindrical container on all sides; a second hollow, substantially cylindrical, rigid container, which has a longitudinal center line, a side wall and two rigid end walls surrounding and in contact with the fluid-like material on the side wall and end walls of the second container, where the second container has its interior purged of substantially all gases; a predetermined amount of high explosive material surrounding and being in contact with the exteriors of the side wall and one end wall of the second container, where the high explosive has a detonation velocity D greater than the shock wave velocity S; and means to detonate the high explosive at the end wall that is surrounded by the high explosive. -16- 4. Apparatus according to Cl aim 3, wherein said fluid-l ike material is drawn from a class consisting of copper, iron, tantalum, tungsten and uranium in powdered form of density substantially 50 percent of theoretical solid density or in solid form of a low strength material 3 of density greater than 8 g /cm . 5. Apparatus according to Claim 4, wherein said high explosive detonation velocity D satisfies D 8 km/sec. 6. Apparatus according to Claim 4, wherein said hgh explosive material is a mixture of PETN and TNT of ratio by weight substantially 60 percent and 40 percent, respectively. 7. Apparatus according to Claim 4, further including a solid, metal, substantially cylindrical body having a longitudinal center line that is substantially coincident with the longitudinal center line of said second container and is positioned in the interior of said second container. 8. A new material prepared by the method described in Claim 1 having substantially higher hardness than single crystals or aggregates produced by the application of static pressure, and having a density of 3 substanti ally 3.49 gm/cm .

Inventors: CLINE CARL F, HARE ALLAN, WILKINS MARK L, CLINE, CARL, F., HARE, ALLAN, WILKINS, MARK, L.
Applicants: CLINE CARL F, HARE ALLAN, WILKINS MARK L, CLINE, CARL, F., HARE, ALLAN, WILKINS, MARK, L.
Application year: 1985
Main classification: C01B21
IPC codes: B01J3, C04B35