Abstract:
Means are described for suppressing spontaneous diamond nucleation in the vicinity of diamond seed material located in reaction vessel construction used in the growth of diamond by the process disclosed in U.S. Pat. No. 3,297,407--Wentorf, Jr. 
     In assembly of the reaction vessel a portion of the lower surface of the plug of catalyst-solvent metal is disposed in contact with the diamond seed material. Preferably all of the balance of the lower surface area of the catalyst-solvent plug adjacent the seed material is covered with a disc or layer of a material different from the catalyst-solvent metal employed and selected from a list of specific materials that suppress diamond nucleation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 412,190, filed Nov. 2, 1973, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The synthesis of diamond crystals by high pressure, high temperature processes has become well established commercially. Preferred methods for making diamonds are disclosed and claimed in U.S. Pat. Nos. 2,947,610--Hall et al and 2,947,609--Strong. Apparatus for the conduct of such processes is described and claimed in U.S. Pat. No. 2,941,248--Hall. The Hall et al, Strong and Hall patents are incorporated by reference. 
     Diamond growth in the aforementioned processes occurs by the diffusion of carbon through a thin metallic film of any of a series of specific catalyst-solvent materials. Although such processes are very successfully employed for the commercial production of industrial diamond, the ultimate crystal size of such diamond growth is limited by the fact that the carbon flux across the catalyst film is established by the solubility difference between graphite (the typical starting material) and the diamond being formed. This solubility difference is generally susceptible to significant decrease over any extended period due to a decrease in pressure in the system and/or poisoning effects in the graphite being converted. 
     On the other hand, in the method of growing diamond on a diamond seed crystal disclosed in U.S. Pat. No. 3,297,407--Wentorf, Jr. (incorporated by reference) a difference in temperature between the diamond seed and the source of carbon is relied upon to establish a concentration gradient in carbon for deposition on the seed. Catalyst-solvents disclosed in the aforementioned Hall et al and Strong patents are used in the temperature gradient method as well. The growth of diamond on the seed material is driven by the difference in solubility of diamond in the molten catalyst-solvent metal at the nutrient (source of carbon) and at the seed, between which locations a temperature gradient exists. Most important, this general type of reaction vessel configuration presents a pressure stable system so that pressure can more readily be kept in the diamond stable region. 
     By very carefully adjusting pressure and temperature and utilizing relatively small temperature gradients with extended (relative to growth times for thin film method) growth times larger diamonds can be produced by the method as taught in the Wentorf patent than by the thin-film method. However, attempts to increase crystal sizes to much greater than about 1/20 carat (by increasing the growth times by 5 to 10 times the aforementioned &#34;extended&#34; growth times) has shown that with these longer growth times the strong tendency for spontaneous nucleation of diamond crystals to occur at the underside of the molten catalyst-solvent metal develops into a serious problem, because that diamond nucleation occurring near the diamond seed material competes with the growth from the seed diamond resulting in the development of multiple crystals which collide as they grow and as a consequence produce stress fractures in the colliding crystals. 
     Therefore, in order to enlarge the potential of the Wentorf discovery it is important to overcome the problem of spontaneous nucleation in the vicinity of the diamond seed material. 
     SUMMARY OF THE INVENTION 
     The instant invention provides a solution to this problem of spurious diamond crystal growth thereby improving the capability of the Wentorf invention for producing larger, sounder crystal growth with increased growth periods in a reliable manner. 
     Thus, means are provided for suppressing spontaneous diamond nucleation in the vicinity of the diamond seed material. In the assembly of the reaction vessel employed a disc, or layer, of a material different from the catalyst-solvent and selected from a specific group of materials is disposed over that surface of the plug of catalyst metal, which makes contact with the portion of the diamond seed(s) to serve as a &#34;template&#34; for the new diamond growth. Each diamond seed makes contact with the plug of catalyst-solvent metal through the layer of material or (in the case of a solid disc) through an appropriately sized hole. The selected group of materials consists of cobalt, iron, manganese, titanium, chromium, tungsten, vanadium, niobium, tantalum, zirconium, alloys of the preceding recitation of metals, natural mica, polycrystalline high-density alumina, alumina powder, quartz, silica glass, hexagonal boron nitride crystals, cubic boron nitride crystals, wurtzite-structure boron nitride crystals and silicon carbide protected with one of the metals of the platinum family. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     This invention will be better understood from the following description and drawing in which: 
     FIG. 1 illustrates one exemplary high pressure, high temperature apparatus useful in the practice of this invention; 
     FIG. 2 illustrates in an enlarged view a preferred reaction vessel construction assembled in accordance with this invention; 
     FIG. 3 is an even larger scale view of the vicinity of the diamond seed material shown in FIG. 2 and 
     FIG. 4 shows the relation between the new diamond growth and the diamond seed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One preferred form of a high pressure, high temperature apparatus in which the reaction vessel of the instant invention may be employed is the subject of the aforementioned U.S. Pat. No. 2,941,248--Hall and is schematically illustrated in FIG. 1. 
     In FIG. 1, apparatus 10 includes a pair of cemented tungsten carbide punches 11 and 11&#39; and an intermediate belt or die member 12 of the same material. Die member 12 defines a centrally-located aperture and in combination with punches 11, 11&#39; defines two annular volumes. Between punch 11 and the die 12 and between punch 11&#39; and the die 12 there are included gasket/insulating assemblies 13, 13&#39;, each comprising a pair of thermally insulating and electrically non-conducting pyrophyllite members 14 and 16 and an intermediate metallic gasket 17. The aforementioned assemblies 13, 13&#39; together with end cap assemblies 19, 19&#39; and electrically conductive metal end discs 21, 21&#39; serve to define the volume 22 occupied by reaction vessel 30. Each end cap assembly comprises a pyrophyllite plug, or disc, 23 surrounded by an electrical conducting ring 24. 
     Reaction vessel 30 (FIG. 2) is of the general type disclosed in U.S. Pat. No. 3,030,662--Strong (incorporated by reference) modified by the addition of steel retaining rings 31 and 32. Hollow cylinder 33 is preferably made of pure sodium chloride, but may be made of other material such as talc. 
     Broad criteria for the selection of the material for cylinder 33 are that the material (a) not be converted under pressure to a stronger and stiffer state as by phase transformation and/or compaction and (b) be substantially free of volume discontinuities appearing under the application of high temperatures and pressures, as occurs for example with pyrophyllite and porous alumina. The materials meeting the criteria set forth in U.S. Pat. No. 3,030,662 (column 1, line 59 through column 2, line 2) are useful for preparing cylinder 33. Positioned concentrically within and adjacent cylinder 33 is a graphite electrical resistance heater tube 34. Within graphite heater tube 34 there is in turn concentrically positioned cylindrical salt liner plug 36 upon which are positioned hollow salt cylinder 37 and its contents. 
     Operational techniques for applying both high pressures and high temperatures in this apparatus are well known to those skilled in the art. The foregoing description relates to merely one high pressure, high temperature apparatus. Various other apparatuses are capable of providing the required pressures and temperatures that may be employed within the scope of this invention. Pressures, temperatures, metallic catalyst-solvents and calibrating techniques are disclosed in the aforementioned patents incorporated by reference. 
     The bottom end of cylinder 37 encloses the embedment disc 38 having at least one diamond seed 39 embedded therein. As shown, seed 39 is located in a portion of disc 38 projecting from surface 40 thereof a sufficient distance to present the exposed face of seed 39 through hole 41 in nucleation suppressing layer 42 made up of a layer of particulate material or a solid disc. Hole 41 is filled with the salt projection presenting the exposed upper surface of diamond seed material 39 (preferably a cube face) into contact with the undersurface of plug 43 of metallic catalyst-solvent. The thickness of plug 43 helps determine the temperature difference prevailing in the cell. With a thicker slug, the temperature difference is greater. Also located within salt cylinder 37 are the nutrient supply 44 and salt cylinder 46 disposed thereover. The vertical dimension of plug 43 also affects the temperature gradient. 
     Pressure-transmitting members 36, 37, 38 and 46 are made of material meeting the same criteria as the material for cylinder 33. All of parts 33, 36, 37, 38 and 46 are dried in vacuum for at least 24 hours at 124° C. before assembly. Other combinations of shapes for the pressure-transmitting members 36, 37, 38 and 46 may, of course, be employed. However, the arrangement of these parts shown in FIG. 2 has been found to be the most convenient to prepare and assemble. 
     When reaction vessel 30 is disposed in space 22, heater tube 34 forms electrical contact between end discs 21, 21&#39; so that heat may be controllably applied during conduct of the process. 
     Nucleation suppressing layer 42 is composed of a material different from the catalyst-solvent employed and selected from the group consisting of cobalt, iron, manganese, titanium, chromium, tungsten, vanadium, niobium, tantalum, zirconium, alloys of the preceding metals, natural mica, polycrystalline high-density alumina, powdered alumina, quartz, silica glass, hexagonal boron nitride crystals, cubic boron nitride crystals, wurtzite-structure boron nitride crystals and silicon carbide protected with one of the metals of the platinum family. Silicon carbide particles are preferably mixed with an inert material, such as, sodium chloride and formed as a solid disc having the upper surface thereof (in contact with the underside of plug 43) covered with a thin layer of one of the platinum family metals. 
     When a metallic disc is used with a hole in it, the ratio of the diameter of the hole to the largest dimension of the seed should be in the range of 1.5:1 to 5:1. The thickness of the nucleation suppressing layer 42, would range from about 1 to about 10 mils. The natural mica, e.g. muscovite should first be fired at about 800° C. for 12-15 hours. The preferred thickness of mica is about 2-3 mils. 
     The nutrient material 44 may be composed of diamond, diamond plus graphite or may be entirely of graphite, if desired. The graphite occupies any void space. It is preferred that the nutrient contain mostly diamond in order to reduce the volume shrinkage that can result during conduct of the process. In conduct of the process any graphite present at operating temperatures and pressures converts to diamond before going into solution in the catalyst-solvent metal. Thus, the pressure loss is minimized so that the overall pressure remains in the diamond-stable region at the operating temperature. 
     Enough of the surface of the underside of catalyst-solvent metal plug 43 is covered by layer 42 to provide an environment adjacent the seed 39 to suppress spontaneous diamond nucleation for a considerable distance around diamond seed 39. Preferably the entire underside of plug 43 around diamond seed 39 is covered by layer 42, but if less than the rest of the entire surface is covered, the layer 42 should extend at least 50% greater distance in all directions from the seed than the lateral growth dimension desired for the diamond. If the disc 42 is made of one of the metallic materials listed above, some space must exist between the diamond seed 39 and the wall of hole 41 into which the material of disc 38 will extend. This relation is shown more clearly in FIG. 3. 
     The exact mechanism (or mechanisms) by which discs, or layers, of these diamond nucleation suppressor materials located in the manner described function to reduce or eliminate diamond nucleation in the vicinity of the diamond seed 39 is not known for certain. However, it has been found that in this way diamond nucleation can be held back at least until the seeded growth becomes quite large, well formed and capable of accepting the full carbon flux presented thereto during operation at temperature differentials with which in identical systems without the nucleation suppression disc, spurious diamond nucleation resulted in a clustered mass of diamond growth. 
     As is shown in FIG. 4 the developing new diamond projects into bath 43 (FIG. 4 is drawn for an arrangement in which layer 42 is dissolved by the catalyst-solvent metal) as it grows. After termination of the run and reduction of temperature and pressure to permit removal of the reaction vessel 30, the new diamond growth embedded in the solidified metallic catalyst-solvent 43 readily detaches from the seeding site(s). The diamond(s) so prepared is easily removed by breaking open the mass 43. Any recess or surface roughness may then be polished away. 
     In each of the following examples the reaction vessel configuration provided a temperature differential in the 20°-30° C. range, the nutrient consisted of 1 part by weight SP-1 graphite and 3 parts by weight 325 mesh diamond prepared by the thin film method, seeds used were 1/4 to 1/2 mm, the catalyst-solvent is 70Ni30Fe and temperatures were measured using a Pt/Pt 10 Rh thermocouple: 
     EXAMPLE 1 
     [Run 815] 
     
         ______________________________________Pressure               57 kbTemperature (14.0-14.2 mv)                  1430-1450° C.Nutrient               210 mgmNucleation Suppressing Layer                  NoneTime                   24 hours______________________________________ 
    
     At least 10 yellow diamond crystals grew together in a cluster. The 1/2 mm seed had dissolved a little and had grown back in. The crystals were either octahedra or cubo-octahedra. 
     EXAMPLE 2 
     [Run 816] 
     
         ______________________________________Pressure              57 kbTemperature (14.0-14.2 mv)                 1430-1450° C.Nutrient              210 mgmNucleation Suppressing Layer                 5 mil Fe disc with                 an 80 mil hole                 (as in FIG. 2)Time                  5 hours, 40 min.______________________________________ 
    
     Only one yellow diamond crystal grew developing from the diamond seed. There was no spurious nucleation of diamond. The crystal was an octahedron with small cube faces at the points. 
     EXAMPLE 3 
     [Run 817] 
     
         ______________________________________Pressure             57 kbTemperature (14.0-14.2 mv)                1430-1450° C.Nutrient             210 mgmNucleation Suppressing Layer                as in Example 2 but                slightly smaller in                diameter than plug                43Time                 311/2 hoursWeight of Seeded Growth                43.7 mgm______________________________________ 
    
     Very nice single seeded growth developed, well-shaped, symmetrical and relatively flaw-free. The crystal was a yellow octahedron with small cube faces at the points. A small diamond crystal developed where the underside of plug 43 was not covered with the Fe disc 42. This experiment confirmed the nucleation suppressing capabilities of Fe. There was, however, partial dissolution of the seed before new growth began. 
     EXAMPLE 4 
     [Run 818] 
     Pressure, temperature and nutrient weight were the same as in EXAMPLE 1 and no nucleation suppressing layer was employed. The time was 241/2 hours. As in EXAMPLE 1, a cluster of yellow crystals developed from spontaneous nucleation. The seed grew to about 2×2 mm with a diamond &#34;barnacle&#34; attached thereto. Also five other individual small crystals grew from spontaneous nucleation. 
     EXAMPLE 5 
     [Run 906] 
     
         ______________________________________Pressure              58 kbTemperature (14.0-14.2 mv)                 1430-1450° C.Nutrient              200 mgmNucleation Suppressing Layer                 1 mil Ti disc as in                 FIG. 2Time                  43 hoursWeight of Seeded Growth                 147.6 mgm______________________________________ 
    
     A single light yellow crystal was formed from the seed. There was no spontaneous nucleation of diamond. The flaw content was minor. The crystal had a very low nitrogen content. 
     Experiments with different reaction vessel constructions have verified the excellent nucleation suppressing capabilities of cobalt and natural mica and the useful nucleation suppressing capabilities of tungsten. In the same manner the lack of utility of synthetic mica, platinum, nickel and molybdenum as nucleation suppressing materials has been demonstrated. 
     Designations of the diamond seed are schematic and no attempt has been made to show the preferred disposition.