PROCESS FOR PRODUCING DIAMOND PARTICLES HAVING A SELECTED MORPHOLOGY

Abstract: A process for producing diamond particles having predetermined sizes and shapes. This process involves the formation of silicon carbide particles, or particles convertible to silicon carbide, that are then subjected to a non-decomposible halocarbon gas at temperatures between about 800 and 1200<o> C whereby the silicon carbide is converted through a metathesis step to the corresponding diamond particle. The resultant diamond particles have sizes from a few microns to several hundreds of microns and a range of shapes including needles, fibers, bulky materials as well as porous diamond particles depending upon the shapes of the silicon carbide particles. Preferably, the silicon carbide should be the beta phase form to increase the efficiency of the reaction. Several alternative methods are described for the initial preparation of the silicon carbide particle that is used in the conversion.

Description: -1- D scription Process For Producing Diamond Particles" Having A Selected Morphology Technical Field The field of this invention is generally that relating to the preparation of materials useful in grinding and polishing and the like, and more particularly to a process for producing diamond particles for these uses. Background Art Man-made diamond particles are produced by several processes known in the art. For example, commercial grade diamond particles have been produced at 130 kilobars and 3000° C. In another process, using nickel or iron as a catalyst, such particles are produced at 27 kilobars and 1400° C. The products of these high temperature processes are generally referred to as General Electric-type diamonds because the General Electric Co. utilizes such processes. An additional process for preparing commercial grade diamond particles involves the shock-wave synthesis of the diamond particles under conditions achieving from 0.3 to 1.5 megabars pressure at about 1000 to 2000° C. This is a process generally utilized by E. I. DuPont and Company and therefore such particles are referred to as DuPont-type diamonds. Still another process for the production of commercial grade diamond particles is that disclosed in our Patent No. 4,228,142, issued October 14, 1980. This patent is incorporated herein by reference. When the products of the above-identified processes are viewed using scanning electron microscopy, the particles are found to be rather blocky in shape and some appear to be single crystals. Other parti¬ cles are polycrystalline. The polycrystalline diamond particles exhibit multi-edges and therefore generally are considered to be a preferred type for polishing and grinding. Diamond particles in the polishing range that are produced by these processes are typically in the range of 0.5 to about 85 microns and these are referred to as micron powder. The size range of diamond particles in the grinding range, primarily natural or G.E.-type, is typically from 85 to 850 microns and is often referred to as diamond grit. Various sizing techniques are used to separate the particles so that very uniform particle size fractions are available. The smaller sized precision graded particles are utilized for finish grinding, lapping and polishing, while the larger sizes are used in grinding, sawing or drilling applications, often incorporated in or bonded to cutting tools. All of the above-described known processes produce particles having generally a blocky structure or morphology and therefore perform quite uniformly in their various applications. However, in view of the superior performance of particles with multi- edges for grinding applications, it would appear that it would be desirable to uniformly achieve particles having such structure. Also, particles of other specific shapes (morpho¬ logies) have advantages for various applications. One such shape or morphology is that of fibers as V. r i? used for strengthening materials. Methods are known for the growing of experimental filamentary diamond crystals. Such crystals possess very high strength. These filamentary crystals are grown upon a seed diamond single crystal as a substrate (see, for example, Jl. of Crystal Growth, 2_, p. 380, 1968). Each seed crystal is individually supported, and the filamentary diamond crystal is grown epitaxially in a carbon-containing gas phase. The large scale production of such diamond fibers, is therefore, impractical by these methods. Accordingly, it is one object of the present invention to provide a method for the preparation of diamond particles having a predetermined structure or morphology. It is another object to provide diamond parti¬ cles for use in grinding, polishing and the like applications wherein the surface of the particle is extremely irregular. It is another object of the invention to produce diamond particles having the desired shapes selected from fibers, blocky particles, porous particles, or particles having severely irregular external shapes. Further objects of the invention will become apparent upon consideration of the following descrip¬ tion. Disclosure of the Invention In accordance with the present invention, a beginning structure, which may be organic, is chosen or fabricated having the shape of the intended diamond particle. Thereafter, this structure is converted to be substantially beta silicon carbide having the desired structure. This silicon carbide is then converted to diamond using the process dis¬ closed in our ϋ. S. Patent No. 4,228,142 or an equivalent low temperature, low pressure process. Best Mode for Carrying Out the Invention As discussed in U. S. Patent 4,228,142, diamond particles can be prepared by the reaction of a fluorocarbon with silicon carbide in a temperature range of about 800 to 1200° C. A preferred tempera¬ ture of about 1000° C provides for the optimum production of the diamond particles. Furthermore, the reaction appears to proceed more readily when the silicon carbide is in the beta phase (cubic close packed) rather than the alpha phase (hexagonal) . The presence of iron or nickel as a promoter is also beneficial. Also, since diamonds are stable under certain conditions to 1600° C, the reaction range may be extended with an attendant increase in reaction velocity (see "Physical and Mechanical Properties of Diamond", H. B. Dyer, Proceedings: Ind. Diamond Conf. , P.I, 1967) . Silicon carbide is available from many sources in the industry. It is used, for example, in many high temperature applications. Also, fibers of SiC are used in the strengthening of aluminum. One conventional method for the production of silicon carbide is through the use of rice hulls. Rice hulls are a unique waste product of agriculture. O The rice kernel, along with its hulls, contains a high carbon content as well as a high silica (Si0 2 ) content. When these rice hulls are heated at a temperature of about 1820° C, they are converted to silicon carbide because of this carbon and silicon content. This method of forming silicon carbide from rice hulls is described in U. S. Patent Nos. 3,754,076 and 4,248,844. The resultant product has several forms of morphology including needles, blocky pieces, rough surfaced cylinders and the like. The needles may be separated from the total silicon carbide by a flotation process and then may be used in applications where they provide strengthening to metal matrices, etc. The remaining configurations of the silicon carbide from the needle production are considered waste product but are useful in other applications. Silicon carbide is also conventionally produced using other source ingredients. The silicon portion is derived from such materials as silicon metal, various silicates, silica, silicic acid, silicones, and naturally occurring substances such as diatoma- ceous earth, radiolaria, etc. The carbon sources are typically coke, graphite, charcoal, resins, carbon black, natural carbonaceous materials, etc. Depending upon the conditions of the reactions, SiC particles of various sizes and shapes result and are primarily beta-type. Typical of these other methods are disclosed in U. S. Patent Nos. 4,133,689 and 4,162,167. U. S. Patent No. 3,927,181 describes the preparation of hollow spheres of SiC, and U. S. Patent No. 3,726,737 describes the preparation of "corrugated paper" SiC. The waste product from the rice hull method of preparing silicon carbon needles or fibers was investigated as an inexpensive source of silicon carbide for the production of diamond particles via metathesis using fluorocarbons according to process of U. S. Patent 4,228,142. An x-ray diffraction analysis of the silicon carbide indicated that the ratio of beta phase to alpha phase silicon carbide was approximately 2:1. Further analysis indicated that the material had approximately 25% carbon, 25% alpha phase silicon carbide and 50% beta phase silicon carbide. A 261 gram portion of this material was mixed with 5 weight percent iron powder. This was heated in a stainless steel retort with carbon tetrafluoride flowing through the retort using a porous graphite diffuser. The run was continued for 18 hours at 1000° C. The product powder weight was 173 g. Spectrographic analysis showed, by weight, approximately 5 percent iron, 1 percent fluorine and 350 parts per million silicon. This resultant product was subjected to leaching and density separation to isolate the product diamond powder. A yield of 56 karats (one karat equals 0.2 g) was determined from a 150 g portion of the reacted powder. The yield efficiency of this reaction was calculated to be 17.4% based on the amount of beta-type silicon carbide in the powder and taking into account the free carbon, iron and alpha silicon carbide, and the amount of the εtarting beta silicon carbide in the powder. An x-ray diffraction study of the product showed the existence of diamonds and some graphite possibly entrained within the diamond particles during formation. These diamond particles were black, probably due to this graphite. An unexpected morphology of the diamond particles was noted. When compared with the morphology of the starting silicon carbide, the diamond morphology appeared to be substantially that * ' the silicon carbide; i.e., these diamond parti. s had exact counterparts in the starting SiC material. Thus, where a few needles of silicon carbide were present in the initial starting material, a portion of the diamond particles were also needle-shaped. Other shaped particles of silicon carbide, such as rough surfaced cylinders, become rough cylinders in the resultant diamond powder. Furthermore, what appeared to be relatively hollow silicon carbide particles likewise became hollow diamond particles. The study showed, therefore, that diamond particles of a desired morphology can be produced by selecting the morphology of the starting material; namely, in this case, the silicon carbide. Individual diamond crystals were produced having sizes up to 75 microns diameter, and polycrystaline diamond agglomerates and/or porous shapes up to 600 microns diameter were produced. In addition, rods of 70 to 700 microns in diameter and 300 to 600 microns long were produced as well as "whiskers" 1 micron wide by 30 microns long and needles 10 microns wide and 750 microns - ~vv. ' .' * τ\ long. These sizes, as observed using a scanning electron microscope, are given as illustration and not a limitation of the process. This study demonstrated that a variety of diamond shapes are possible and probably unlimited if the various shapes of silicon carbide can be produced. Thus, instead of a mixture of shapes of silicon carbide as are obtained from the rice hulls, a more uniform shape will result in a higher concen¬ tration of a particular diamond shape if most of the starting silicon carbide has that particular morph¬ ology. Accordingly, diamond needles may be prepared from needle-like silicon carbide which may be used for the strengthening of various composites in the same manner as the silicon carbide has been used in the past. Furthermore, the very porous diamond structures that can be achieved from porous SiC will permit a more complete bonding to any matrix for grinding wheels and the like. Very rough diamond particles can be used in the grinding art, and the very small particles of diamond can be used for polishing applications. This, therefore, makes possible the formation of diamond particles of various sizes and shapes at a relatively low temper¬ ature and at normal pressures. Almost any organic material can be heated in an inert atmosphere, such as argon, to give a carbon residue. Since the shape of the organic material is often retained, it is therefore possible to tailor the carbonized particle shape. Solid or hollow starting materials may be used. The body may be dipped in silica gel, ethyl silicates, silicic acid, methyltrichlorosilane (MTS) liquid, sodium silicate or other silica- or silicon -containing material before or after carbonization which would then react with the carbon body to form silicon carbide. If MTS liquid is used, it rapidly reacts with moisture in the air to form a silicon-containing film or crust. Several of the naturally occurring pollens have a very raspy external shape. Typical of these are pollens of ragweed, sunflower, dandelion, marigold, etc. Numerous of these are illustrated in "The Particle Atlas" Edition Two, Ann Arbor Science Publ., Inc., Ann Arbor, MI (1980). When converted to silicon carbide by the above-cited steps, the resultant silicon carbide can be converted to diamond particles having the same raspy external shape. In addition, there are naturally occurring organic carbonaceous particles having other shapes that may be carbonized, converted to SiC and then to corresponding diamond particles having a morphology like that of the starting material. Typical of such particles are pollens of willow, privet, etc. , having uneven surfaces which permit binding in matrix materials. Others, like corn silk and milk¬ weed fibers are extremely elongated, i.e., needle¬ like. There are larger sized objects, such as cockleburs, sycamore nuts, chestnut burs and the like that may be similarly used. In a similar manner, there are naturally occurring materials that contain predominately silicon or silica (SiO_). These may be mixed with carbon in some form, and then at least partially converted to SiC. Typical of such materials are diatomaceous earth, in its several forms, radiolaria (siliceous skeletons) and infusorial earth. The preparation of silicon carbide from diatomaceous earth and petroleum coke, for example, is described in Chemical Abstracts No. 28948r, 0, p. 120, 1974. Even materials which result in the formation of alpha phase silicon carbide may be utilized. It is known that alpha silicon carbide can be converted to the beta phase by heating in a nitrogen atmosphere of about 3MPa (435 psi) at about 2500°C (see Communi¬ cations of the Am. Cer. Soc. , C-177,1981). This step of producing the beta silicon carbide would enhance the formation of the diamond particle since, as stated above, the beta silicon carbide reacts more readily to form diamonds. The present invention is not limited to the use of silicon carbide made from naturally occuring materials. A specific shape may be constructed by extrusion, stamping, molding and the like of the necessary Si and C ingredients. The conversion then produces the SiC having the desired shape which may then be converted to diamond having that same shape. In some instances it may not be necessary to convert the entire body to silicon carbide before the conversion to the diamond form of carbon. Since methyltrichlorosilane (MTS) decomposes to beta silicon carbide above 1000° C, it will be possible to produce at least a coating of beta silicon carbide by chemical vapor deposition methods upon almost any shaped particles (mandrels) in a fluidized bed. Then using, for example, the carbon tetrafluoride reaction, diamonds of that shape could be produced. This method would permit the preparation of diamond particles of larger size than may be possible using the complete silicon carbide metathesis reaction process. Diamond particles of a size from a few microns up to several hundred microns can be produced using the above described methods. When the starting material, i.e. , silicon carbide, has a wide range of particle sizes and shapes, the product will contain diamond particles having this same wide range of sizes and shapes. If a particular size and/or shape diamond particle is preferred, a sizing and shape sorting step may be added to separate these specific sizes and/or shapes after the diamonds are formed. Alternatively, a sizing and shape selection may be performed on the initial silicon carbide. In this way, the process is made more efficient in that only those sizes and shapes which are desired are present to react with the gas used for the conversion. In either case, a leaching and density separation step may be necessary to retain only the diamond portion of the reaction products. Although the above referenced patent 4,228,142 utilizes a fluorocarbon as the reactant gas, and O more particularly carbon tetrafluoride, other gases are believed to be useful in carrying out the present invention. For example, it may be possible to substitute at least a portion of the fluorine atoms with other halogen atoms to achieve the conversion of silicon carbide to the diamond. Thus, several of the halocarbons may be suitable for use with the present invention. These will include a multiple combination of carbon .with fluorine, chlorine, bromine, iodine .and/or hydrogen. Several such gaseous combinations are more readily available at lower costs than the carbon tetrafluoride. The halocarbon used in the process should not decompose at temperatures up to that used for the conversion of the silicon carbide to diamond unless the decom¬ position yields a halocarbon phase which reacts with silicon carbide to produce the diamond form. Certain forms of solid halocarbons, e.g. Teflon, may also be used when an effective gaseous halocarbon results at the reaction temperature. From the foregoing, it will be understood by those versed in the art that diamond particles of a wide range of sizes and shapes may be produced. The particular shape is controlled by the shape of the silicon carbide body which is subjected to the metathesis conversion to diamond. Accordingly, a particularly shaped diamond particle is produced by intentionally forming a silicon carbide particle of that particular shape. The diamond is formed by subjecting the silicon carbide particle to a halo- SjR O carbon atmosphere at about 1000° C. If desired, suitable diluent gases (helium, argon, nitrogen, hydrogen, etc.) may be used as well as inert filler materials such as excess carbon in some form. The resultant diamond particles are then useful according to their size and shape as well as to the external surface of these shapes. They may be tailored to particularly achieve the desired result in the art of strengthening, grinding, polishing, etc. It will also be recognized by those skilled in the art that the conditions utilized in demonstrating the present invention, as set forth above, are not necessarily the most efficient. For example, conversion in a rotating furnace where tumbling and other mixing occurs will bring about increased contact between the halocarbon and the silicon carbide. This will result in higher yields at reduced reaction times. Also, a fluidized reactor may be used. Further, since the reaction is exothermic, precautions against severe temperature excursions in the material being processed and/or in the furnace may be desirable. In addition, it will be recognized that a material may be moved through series of environments to achieve silicon carbide conversion and ultimate diamond production in a continuous process. Thus, the ingredients may be fed into processing equipment for the automatic production of a diamond product of a selected size and shape. tø- OM '*-- /tf It is, of course, understood that although preferred embodiments of the present invention have been illustrated and described, various modifications thereof will become apparent to those skilled in the art. Accordingly, the scope of the invention should only be defined by the appended claims and the equivalents thereof.

Claims: Claims 1. A process for preparing commercial diamond particles having a selected size and shape, which comprises treating silicon carbide particles having said selected size and shape to a metathesis reaction in the presence of a halocarbon gas at a temperature of about 800° to below about 1600° C. 2. The process of Claim 1 wherein said silicon carbide particles are predominately beta silicon carbide, and said halocarbon gas is carbon tetrafluoride. 3. The process of Claim 1 wherein said silicon carbide is a mixture of a alpha silicon carbide and beta silicon carbide, and further comprises converting said alpha silicon carbide to further beta silicon carbide by heating said alpha silicon carbide in a nitrogen atmosphere of about 1 to about 3 MPa at a temperature of about 1800° C to about 2500° C prior to said metathesis reaction. 4. The process of Claim 1 wherein said silicon carbide particles are produced by heating a organic substance under conditions to form carbonized particles having said selected size and shape and converting at least a portion of said carbonized particles to said particles of silicon carbide in the presence of a silicon-containing substance. OMPI V.'If 5. The process of Claim 1 wherein said silic- carbide particles are produced by heating a silicon- containing particle of said selected size and shape in the presence of a carbon-containing substance under conditions sufficient to produce said silicon carbide particles. 6. The process of Claim 1 wherein said silicon carbide particles are produced by vapor depositing silicon carbide upon a mandrel. 7. The process of Claim 6 further comprising removing said mandrel prior to said metathesis reaction. 8. The process of Claim 7 wherein said mandrel is carbon, and said mandrel is removed by oxidizing said carbon. 9. The process of Claim 1 wherein said silicon carbide particles are produced by mixing an carbona¬ ceous substance with a silicon-containing substance, forming a particle of said selected size and shape, and reacting said carbonaceous substance and said silicon-containing substance in said particle under conditions sufficient to produce said silicon carbide particle. 10. The process of Claim 4 wherein said organic substance is rice hulls. 11. The process of Claim 10 wherein said carbonizing and said converting steps are simultaneous. 12. The process of Claim 4 wherein said carbonized particles are treated with said silicon-containing substance prior to said converting step to produce said particles of silicon carbide. 13. The process of Claim 1 wherein said halocarbon decomposes only at a temperature corresponding to the temperature of said metathesis reaction. 14. The process of Claim 1 wherein said silicon carbide particles have a range of sizes and shapes, and wherein said selected size and shape is separated from said range prior to said metathesis reaction of producing said diamond particles. 15. The process of Claim 1 wherein said silicon carbide particles have a range of sizes and shapes, and wherein said selected size and shape of said diamond particles is separated from all products of said methathesis reaction with said silicon carbide particles. 16. The process of Claim 2 wherein said metathesis reaction is carried out at a temperature of about 1000° C. 17. The process of Claim 1 wherein said diamond particle is elongated and has a diameter of about 1 to about 10 microns, and a length of about 30 to about 750 microns. 18. The process of Claim 1 wherein said diamond particles have a selected size and shape of about 1 to about 600 microns having an extremely rough surface. 19. The process of Claim 1 wherein said selected diamond particles are substantially rods having a diameter of about 70 to 200 microns and a length of about 300 to 600 microns, and wherein said rods are highly porous. 20. The process of Claims 1, 4 or 9 wherein said steps are carried out substantially continuously. 21. The process of Claim 1 wherein said metathesis reaction is carried out in a rotating furnace. 22. The process of Claim 21 further comprising operating said furnace to maintain said diamond particles at a temperature not exceeding about 1600° C. ^3RE OMP {/ Λ , V! -19- 23. Diamond rods having a diameter of about 70 to 200 microns and a length of about 300 to 600 microns as produced according to the process of Claim 1 using silicon carbide rods having substantially a diameter of about 70 to 200 microns and a length of about 300 to 600 microns. 24. Porous diamond particles as produced by the process of Claim 1 wherein said silicon carbide particles are porous.- 25. Diamond particles produced according to the process of Claims 10 or 11. 26. Diamond fibers produced according to the process of Claim 1 wherein said silicon carbide are fibers.

Inventors: CONDON JAMES B, HOLCOMBE JR CRESSIE E, CONDON, JAMES, B., HOLCOMBE, JR., CRESSIE, E.
Applicants: CONDON JAMES B, HOLCOMBE CRESSIE E JUN, CONDON, JAMES, B., HOLCOMBE, JR., CRESSIE, E.
Application year: 1983
Main classification: C01B31
IPC codes: C01B31