IL23569A - Process for the production of synthetic diamonds - Google Patents

Description

'jiiuji im TJinrn '*n PATENT ATTORNEYS · D ' D ] Π 3 ' D T 1 JJ PATENTS AND DESIGNS ORDINANCE SPECIFICATION A prooess for the production of synthetic diamonds_ I (we)j!.I.DTJ POUT DB NEMOURS & COMPANY, a corporation organized and existing under the laws of the State of Delaware, U.S.A., of 10th and Market Streets, Wilmington, Delaware, U.S.A., do hereby declare the nature of. this invention and in what manner the same is to be performed, to be particularly described- and ascertained in and by the · following statement: — The present invention concerns improvements in and relating to synthetic diamonds.

It has been suggested in British Patent Specification No. 822,565 that synthetic diamonds be formed from graphite by the detonation of a shaped charge of explosive so as to drive a high-velocity jet onto a target containing the graphite , but no commercial process, involving shock treatment, has yet been developed for synthesizing diamonds of adequately large particle size and in sufficient yield.

According to the present invention, there is provided a process for converting graphite into diamond, comprising forming an intimate mixture of graphite and a cooling medium capable of cooling the resulting diamond rapidly to less than the rapid atmospheric graphitization temperature, and passing therethrough a shock wave of intensity sufficiently high to convert at least some of the graphite to diamond but sufficiently low to enable the cooling medium to cool the resulting diamond to below the rapid atmospheric graphitization temperature (RAGT).

J The shock wave may be obtained by detonation of a high explosive in contact with the graphite mixture or preferably by driving a projectile plate against the mixture as described hereinafter. The high shocking pressure in the graphite normally exists only for a very short time, e.g. 0.1 to 10 microseconds, depending on the means used to develop the pressure, and then falls rapidly, probably following an exponential curve to atmospheric pressure. The shocking raises the temperature of the graphite to the diamond conversion tem work done as the carbon changes from its large volume graphite phase to its smaller volume diamond phase, the diamond formed is even hotter than the graphite and, in prior art processes, has remained, for some time after the shocking, at temperatures above its stability temperature at atmospheric pressure. By way of contrast, in the process of the present invention, however, the diamond is cooled to stable conditions as rapidly as possible, so as to prevent or reduce the possibility of reversion to graphite at atmospheric pressure or low superatmospheric pressures, e.g. of 100 kilobars, or less. The high shocking pressure is maintained for such a short period of time that it is not sufficient in practice to delay the cooling of the graphite-diamond assembly until after the shocking treatment, e.g. by subsequent drowning of the assembly in water, even if the force of the explosion is used to drive the assembly into the water, but it is essential, according to the present invention, to have the cooling medium present throughout the shocking. The significance of these new process conditions is emphasized by the fact that it is possible to obtain thereby diamond of unique crystal form.

Solids are preferred cooling media, especially metals, such as iron, copper, nickel, aluminium, manganese, magnesium, tungsten, titanium, niobium and alloys such as steel and brass. Desirable qualities for a coo-Hng medium are a high shock impedance, e.g. of the order of 3 x 10 dyne-sec/cc or more, a high heat capacity, e.g. of the order of 0.1 cal./g/°C. or more,high thermal conductivity, e.g. of the order of 0.1 cal./sec./cm°C. or more at room temperature, low porosity, divided by the volume exclusive of pores containing graphite, and generally an overall density of at least 1 , preferably 85 , and normal desiderata such as low cost and good handling qualities. A material that would be completely gaseous under the shocking conditions would almost certainly become too hot to be effective , but some gas, e.g. in the , pores, may be present. Choice of a suitable cooling medium for any particular operation will depend on balancing several factors, as will appear hereinafter, e.g. some media, such as aluminium and tungsten, may be useful inspite of lacking one desirable characteristic, such as a high heat capacity or a high shock impedance. The "shock impedance" of a material is defined as the change in pressure applied to the material divided by the change in velocity of the material resulting from the pressure. When the material is compressed by a shock wave, the shock impedance is equal to the initial density of the material times the velocity of the shock wave passed through it, and thus varies with pressure. Generally, media of shock impedance below 7 x dyne-sec/cc have not given good cooling results.

A preferred intimate mixture of graphite and cooling medium is an integral solid that will not shatter on shocking, e.g. an integral piece of metal, such as cast iron, forming a skeleton of metal for a dispersion of the graphite.

If a particulate intimate mixture is used, it is preferred to have a low porosity or high density mixture, and it is desirable to compact the mixture, e.g. by explosive means, before synthesizing the diamond. The cooling will be more efficient if the graphite and cooling medium are uniformly The mixture will generally have an overall relative density of at least 70$, preferably at least 85$ of the theoretical density; overall relative density is pal So that the cooling medium can exert its beneficial cooling effect and cool the diamond rapidly below the rapid atmospheric graphitlzation temperature, the cooling medium should not itself be heated, during the shocking treatment, above this temperature. According to statements in the prior art, graphite, and not diamond, is the stable form of carbon at atmospheric pressure, but the rate of graphitizatlon is imperceptible below about 1000°C. Graphitlzation is believed to become rapid only at about 2000°C. at atmospheric pressure. Temperatures such as these are difficult to measure accurately, especially when the heating and/or cooling occurs over a period of the order of microseconds, as with explosive treatments. It is possible, however, to calculate the temperature to which the cooling media will rise on shocking according to the method of R. G. McQueen and S. P. Marsh, Journal of Applied Physics, Vol. 31 pages 1253-1269 (i960), and data is given therein for many media. Thus, if tungsten is used as cooling medium, a pressure of 2100 kilobars produces a satisfactorily low temperature of l800°C, whereas a pressure of even 1200 kilobars pro-duces a temperature of 2l 7°C. in tin, which is above the RAGT, and lower pressures have to be used, therefore, with tin.

The shocking temperature in the graphite must be adequate to permit diamond formation and will depend on the shocking pressure, the density of the graphite and the shock impedance of the cooling medium. Thus, with a high-impedance cooling medium and compact (i.e. low porosity) graphite (e.g. as in cast iron) a shock pressure of at least 750 kilobars, and preferably about 1000-2000 kilobars, should be introduced, these pressures being the average pressures introduced into the mixture, rather phite or a lower-impedance cooling medium will result in a higher temperature in the graphite, so that lower pressures may be introduced, e.. g. as low as 200 kilobars. The graphite is preferably of density at least 60$, especially at least 70$ of theoretical, although lower density graphite can be used if the cooling conditions are appropriate.

The shock impedance of the cooling medium is preferably higher than that of the graphite, since this results in building up the pressure stepwise in the graphite, owing to successive reflection of compressive shocks back into the particle from its interface with the cooling medium; thus, the peak pressure in the graphite can approach the pressure in the cooling medium, and the temperature in the graphite does not rise as high as it would by shocking to the same pressure in a single stage; any projectile plate used should be thicker, preferably at least 5 times as thick as the graphite particles in these circumstances. If the shock impedance of the cooling medium is less than that of the graphite, the graphite will attain its peak pressure in one step, this pressure is transiently higher, than that in the medium, and the shock reflections are rarefactions .

The rate of cooling required for the diamond will depend on the conditions of operation, any reduction of the time for which the diamond has to undergo high temperature and low pressure conditions giving a corresponding increase in diamond yield. Thus, maintenance of the high pressure for longer periods, use of lower diamond synthesis temperatures and formation of smaller diamonds (allowing faster economical particle size, and so one will try to arrange for optimum cooling conditions. The cooling time should desirably be less than 0.1 sec, and usually much shorter, e.g. of the order of 0.001 sec. or less. Large particle size graphite, especially of over 500 microns, will require especially efficient cooling. Generally it is preferred to use at most 65$ by volume of graphite in the intimate mixture with the cooling medium.

The high intensity shock pressure is preferably ob-tained by driving a projectile plate against the graphite mixture, or any container therefor, e.g. as described in the above-mentioned article by McQueen and Marsh, the pressure depending on the velocity and shock impedance of the plate and the shock impedance of the mixture. The shock pressure introduced into the mixture can be determined from the pressure-particle velocity relationships for the plate and for the mixture when a shock traverses the boundary between the two materials. The pressure and particle velocity are continuous across such interfaces, this con-tinuity being represented as the intersection of two Hugo-niot curves in the pressure-particle velocity plane (a curve which is the locus of all possible states which can be attained by shocking the material from the original state). One curve is for a forward-facing shock in the mixture starting at zero pressure and zero particle velocity, and the other curve is for a backward-facing shock in the projectile plate starting at zero pressure and particle velocity equal to the plate velocity. The shock pressure at the plate mixture interface then is given by the intersection are available in the published literature, and may be determined as described in the above-mentioned article by McQueen and Marsh. If only a small amount of graphite is present in the mixture, the shock impedance of the mixture will be similar to that of the cooling medium. For mixtures containing larger amounts of graphite, the Hugoniot curves can be ascertained or estimated by the weighted addition of specific volumes on the Hugoniot curves of graphite and the cooling medium at equal pressures. The Hugoniot curve for graphite is given by B. J. Alder and R. H. Christian in Physical Review Letters, Vol. 7, No. 10, p. 368 (1961).

A shock front becomes attenuated after penetrating a certain distance into a material. For example, in the < projectile plate technique, a shock front travels back into the projectile plate from the collision surface, reaches the back face of the plate, and is reflected back to the collision surface as a rarefaction, wave, this wave then entering the graphite mixture and subsequently overtaking and attenuating the shock wave therein. The depth at which this occurs depends on the thickness of the plate, and the difference between the shock velocities (Us) and the rarefaction velocities (Ur). The Ur/U3 ratio of the mixture increases with higher graphite content, greater porosity in the mixture, and higher conversions to diamond. Generally it is in the range of about 1.2 to 2, but it can be determined for the plate material and the mixture o, the ratio of the density of the material behind the shock to that of the material ahead of the shock, and the sonic velocity, C of the material on the Hugoniot curve, accordin to: Values for Ur and Us can be calculated from the published values for P/p 0 and CH, e.g. in the above-mentioned McQueen and Marsh article, estimated from Hugoniot curves, or measured, e.g. by smear or streak camera measurements.

Shock duration times for steel projectile plates (all such projectile plates mentioned herein are of low-carbon SAE 1010 steel, unless otherwise identified) of various thicknesses and velocities are shown in Table I.

TABLE I. 0.1 2 430 5.46 6.55 .85 0.1 3 710 6.50 7.80 .72 0.1 4 1070 7.84 9.40 .59 0.2 2 430 5.46 6.55 1.70 0.2 3 710 6.50 7.80 1.44 0.2 4 1070 7.84 9.40 1.18 It is generally desired to synthesize diamonds to a depth of at least half an inch, so that a shock duration of at least about half a microsecond will generally be employed, although longer durations of at least 3 microseconds are preferred. Durations longer than about 10 microseconds are likely to be difficult to obtain in practice.

If the intimate mixture is not self-supporting, or is" a solid which shatters or fragments extensively as a result of shocking, it may be necessary to confine the rnix- 30 ture in a shock-resistant container to hold the cooling a container or cover is used, its thickness must be considered in determining the shocking conditions, especially the duration and depth of the shock, and this cover or container should generally be as thin as possible, preferably from about 0.1 to about once the thickness of the projectile plate, and in no case thicker than about twice the thickness of the plate.

Table II shows the velocities to which 10-inch-diameter flat circular steel plates of different thicknesses can be driven (in a distance of about 1-1/2 inches) by varying thicknesses of 10-inch-diameter flat circular layers of the sheet explosive described in U.S. Patent 2 , 999, 743 (detonation velocity about 7500 m./sec.).

TABLE II.

Explosive Mass Plate Thickness Plate Velocity (g./sq. in.) (in.) (km./sec . ) 0.15 2.1 0.09 2.6 0.03 3.7 50 0.21 2.4 50 0.12 2.9 50 0.06 3.7 100 0.27 2 .9 100 0.15 3.5 100 0.09 4.1 The projectile plate may be driven by the plate . acceleration method described by Balchan & Cowan, Review of Scientific Instruments, Vol. 35, pages 937-944 (Aug. 1964 ) .

The intensity of the shock pressure obtained when nature of the explosive used, and how it is arranged and initiated. Explosives having high detonation velocities (at least 8000 m./sec.), such as RDX, HMX, the cyclotols (RDX TNT), octols (ΗΜΧ/ΤΝΤ), blasting gelatin, nitroglycol, nitroguanidine, PETN, and 70/30 nitrogen dioxide/nitrobenzene, may be used in the form of a layer and may be plane- wave initiated to introduce pressures of about 300 kilobars into high impedance materials. Preferably the shocking pressure introduced into the mixture is between 600 and 1200 kilobars. If necessary, therefore, the pressure may be increased by various arrangements and methods of initiating the explosive, e.g. so that two or more shock waves converge or intersect, or as suggested by J. Thouvenin and J. P.

Argous in Compt. rend., Vol. 258, pages 1725-7 (Feb. 10, 1964).

Suitable pressures for any particular mixture of graphite and cooling medium may be determined by subjecting the mixture, with the cooling medium at 100$ density, in. a suitable assembly, to a shock pressure, depending on the density of the graphite, increasing from 400 to 900 kilobars as the density increases from 70 to 100$, and determining the yield of diamond. Another sample of the same mixture is then subjected to a pressure about 200 kilobars higher (so long as the cooling medium will not attain a shocking temperature much over l800°C), and the diamond yield again determined. If the yields of diamond in the two runs are the same, the approximate area of optimum pressure for these conditions has been reached. If there is a significant increase in yield in the second run, more runs should Diamond particles are recovered from the resulting mixture after shocking by appropriate methods, e.g. first removing the cooling medium, e.g. by filtering or centri-fuging of liquid media or by dissolving away a solid medium, e.g. with a mineral acid for most metals, and then separating any unconverted graphite. The unconverted graphite may be completely oxidized to gaseous products, chiefly carbon dioxide, by heating the mixture with a lead oxide in air or oxygen at a temperature of 350 to 550°C, with-out substantially affecting the diamond, and this represents an important feature of the invention, and may be applied to the recovery of any diamond from mixtures thereof with graphite. This recovery has posed a problem in the prior art owing to the fact that both diamond and graphite are forms of carbon and have similar chemical properties, and the diamond particles are small and difficult to recover.

The oxidation catalyst may be PbO, Pb02, Pb203 or Pb30 , or a mixture thereof, or an oxygen-containing lead compound which is transformed to a lead oxide in situ, e.g. lead carbonate, hydroxide, nitrate or subacetate.

The particle size of the lead compound is preferably less than about 1 mm, especially less than 50 microns, to assure sufficient contact with the graphite, and similarly the graphite particle size is preferably less than 1 mm, especially less than 200 microns, and the graphite and catalyst should preferably be intimately mixed homogeneously.

The desirable amount of catalyst varies according to the amount of graphite, and ' inversely according to the heating time and temperature. There should generally be t le st of l ad oxide catal st based on the wei ht of The graphite/diamond mixture may contain most other components, e.g. such as are formed during the diamond synthesis , although chromium ion should not be present , since it catalyzes graphitizatlon of diamond.

The time required for complete oxidation of the graphite depends on the rate of gas flow, the compactness of the mixture, the chemical reactivity of the:rg hiite*'·.:'and whether a static system is used or whether the mixture is shaken or swirled during oxidation. The preferred temper-ature range of 38O to 4j50°C. requires reaction times of about 12 hours or more. As the temperature increases, there is a greater tendency towards graphitizatlon of the diamond, and yet one wishes to use the maximum practical temperature to accelerate the oxidation of the graphite.

After oxidation of the graphite, the diamond may be recovered by selectively dissolving out the lead oxide with an aqueous acid such as nitric, acetic, hydrochloric, citric or ethylenediaminetetraacetic acid.

The diamond resulting from the shock synthesis process of the present invention contains particles of varying size, at least some of which may be too small for certain end uses, e.g. diamond dust, or powder of particle size less than 10 microns , which it is desirable to agglomerate into larger particles for many uses such as in cutting and shaping wheels, saws and drill heads. Conventional sintering processes have until now proved inadequate for agglomerating diamond, and this is not surprising considering that the intended uses of the agglomerated diamond particles is the provision of a process for sintering any small diamond particles resulting from the above process to form diamond particles of larger particle size, involving forming a mass of the small diamond particles, or powder, of bulk density at least hOfo of the crystalline density of diamond, and subjecting the mass to a shock wave at a pressure sufficient to achieve the desired sintering effect. It will be appreciated that this shock sintering of diamond powder may be applied to natural diamond powder or synthetic diamond powder de-rived by methods other than the shock synthesis of the present invention.

Sphericity and uniformity of size of the diamond particles facilitate the making of a uniform product. The particle size of the starting diamond powder is preferably no greater than about 150 microns. Larger crystals may sinter to some degree, but generally do not give a strong dense product. The particle size will generally depend on the powder that is available, e.g. 1-10 microns if the powder is diamond dust .

The powder may consist only of pure diamond or other, preferably hard, materials, e.g. carbides, borides, nitrides and oxides may also be present and can give useful sintered products, and minor amounts of metals or graphite can also be present, but it is preferred to have less than 10 by weight of any such impurities, and that the surfaces of all the particles be free of contaminants such as grease or moisture which could Interfere with effective sintering of the cr stals Densities substantially lower than 4θ of the diamond crystalline density lead to non-uniformity in the distribution of voids in the product, and the collapse of large voids leads to high local temperatures on shocking and therefore graphitlzation. As a rule, the higher the density the higher the shock pressure required, and a higher strength product generally results from higher shock pressures, unless the pressure is so high as to give such a high temperature that graphitlzation becomes significant. There is generally no advantage in using more than about 2000 kilo bars, even in the case of high-density powder where less shock heating occurs, and with lower-density powder, the maximum pressure will be lower.

The method of producing the shock wave is similar to that for the shock synthesis of diamond, described above, the use of a projectile plate is preferred, and much of the same criteria apply. It is desirable to avoid scattering of the powder, for example by evacuating the space between the plate and the powder and/or by placing the powder in a suitable protective assembly. The shock pressure to which the diamond powder should be subjected is preferably at least 600 kilobars when the powder is at a density 40 that of the crystalline density of diamond, and this generally requires plate velocities of at least about 2.5 km./sec. with certain relatively high-shock-impedance (at least 10^ dyne -sec ./cc ) plate materials such as the various steels, copper, nickel, titanium, zinc, and their alloys, which are preferred.

The shock impedance of diamond powder at different presently available information on the pressure-particle velocity relationship for diamond. A Hugoniot curve for diamond is not available, but Alder and Christian in the above-mentioned article, giving the curve for graphite, indicate a conversion of graphite to diamond over a pressure range of 0.4-0.6 megabar. This portion. of the graphite curve can be used as the curve for diamond, provided that the initial density of the diamond is the same as that of the graphite used by Alder and Christian. The Hugoniot curve for diamond of initially different bulk density is estimated by using the Gruneisen equation, of state and assuming a constant ratio of the QriHneisen parameter to specific volume ( V /V) for diamond (the above-mentioned McQueen and Marsh article; and Modern Very High Pressure Techniques, Wentorf, R. H., Editor, Butter- worths, Washington (1962 ) , pages 200-207 ) .

For a particular pressure, P, and specific volume, V, in the shocked state, taken from the curve, the particle velocity of the diamond is calculated from the equation: Up = (P-P0) (V0-V) which is obtained by combining the mass conservation rela¬ and the momentum conservation relation in which V0 and P0 are volume and pressure ahead of the c s th reci rocal of the initial diamond densit By obtaining a number of Up values from different P-V combinations, the pressure-particle velocity curve for diamond can be plotted, and the shock pressure at the plate -diamond interface then is given for a particular V0 by the intersection of the pressure -particle velocity Hugoniot curves for the forward-facing and backward-facing shocks described above.

It will generally be necessary to have a shock duration of at least about 0.1 microsecond, and longer dura-tions are · generally preferred, particularly when relatively low shock pressures are employed, e.g. as long as about 5 microseconds. If a container for the powder is used, the thickness of the cover between the impact surface and the powder must again be considered in determining the shock duration, and should not generally be more than about twice the thickness of the projectile plate. The shock impedance of this cover is preferably at least about the same as that of the projectile plate, but a lower-shock-impedance material can be used if the cover is thin enough to permit a number of compressive reflections from the contacting surface to reinforce the shock. The cover is preferably fixed in place, e.g. by welding.

If the projectile plate thickness is such that shock duration is more than sufficient for the shock wave completely to traverse the sample, and if the remote surface of the sample container has a shock impedance higher than that of the unshocked diamond, any pressure generated in the sample by the shock reflected from such material must be considered. The same is true for further reflec- wave becomes attenuated. The shock pressure obtained upon reflection from the base of the container is obtained from the intersection on the pressure -particle velocity plane of the Hugoniot curve for a forward-facing shock in the base material starting at zero pressure and zero particle velocity with the Hugoniot curve of a backward-facing shock in the diamond (already shock-compressed) starting at the intersection of the pressure particle velocity curves for the diamond-cover plate interface or diamond-projectile plate interface, as the case may be. Pressure produced by any further reflection is estimated in an equivalent manner.

In computing the pressure produced by reflection, the Hugoniot for the compressed diamond powder through which the shock wave has just passed is estimated by using the Gruneisen equation as described above and assuming that the compressed powder is 100 dense thereby making, the ratio { '(, /V ) for such powder 3.16 g/cc, which, as already noted, is assumed to be constant.

Although it is not intended to limit the invention to any theory, it is believed that a reason for the success of the explosive sintering process is the high dif-fusivity induced by the high temperature and shear associated with the high-pressure shock wave introduced into the powder. Since the high pressure at any spot lasts only for a very short time, the undesired effects of a sustained high temperature, such as were associated with prior art processes, are avoided.

The shock-sintered diamond product thus obtained is a compact mass of particles of sintered diamond of various sizes mixed with some unsintered particles to an extent depending on the conditions used. The compact mass is readily broken up to give a loose mass of discrete diamond particles of sizes up to 1 mm and more, which can be sized by known methods, such as sieving.

The shock-sintered product has unique characteristics, being a polycrystalline diamond, containing diamond crystals sintered together, having a density of at least 80o of the crystal density of diamond (the precise density depending on the process conditions), and producing an X-ray diffraction pattern wherein the diffraction lines for diamond have a line broadening coefficient, K!, of . -2 -k from 4.5 x 10 to 7.5 x 10 as calculated from the expression: Κ' = β 003 Q , where β is the pure angular breadth in radians of a powder reflection free of all broadening due to the experimental method employed in observing it, is the wave length in Angstroms of the mono-chromatized CuKa radiation X-rays used to obtain the pattern, and 2Θ is the angle of deviation of the diffracted beam, i.e., the diffraction. The presence of diamond is established by comparison of the diffraction angles and the intensity of the reflection with those published for diamond. Although the diffraction pattern enables this identification to be made, the diffraction lines for diamond in this new sintered product are broader and less intense than those obtained with conventional diamond. These differences are believed to result from a lattice strain tic contributes to the line broadening is not known. These characteristics are generally advantageous , since they lead to a product of greater strength.

The pure breadth of a diffraction line is readily determined by procedures described in detail in standard texts such as X-ray Diffraction Procedure, Klug, H. P. and Alexander, L. E., New York, John Wiley & Sons, 1954.

Sintered diamonds of at least 50-micron size are preferred, e.g. for grinding and shaping operations. Shock-sintered particles appear to have rougher edges than conventional diamonds, making them more readily adapted for bonding in a tool.

The unique nature of the shock-sintered polycrys-talline diamond product is more particularly described with reference to Figures 7 through 10 of the attached drawings, in which: - FIGURES 7 and 8 are photomicrographs of polycrystal-line diamond made by sintering 1-micron natural diamond, the magnification in FIGURE 7 being 15, 200 times, and that in FIGURE 8, 64, 000 times; FIGURE 9 is the X-ray diffraction pattern of the product shown in FIGURES 7 and 8 ; and FIGURE 10 is the X-ray diffraction pattern of natural diamond.

FIGURE 7 reveals a high-density structure with extensive interparticle growth. Some small spherical voids are also present, probably indicating rapid self-diffusion during the sintering process. In FIGURE 8 , arrows S indicate the areas where intercrystalllne bonds have been form A comparison of the X-ray diffraction patterns shown in FIGURES 9 and 10 reveals that the sintered product of FIGURE exhibits much broader diffraction lines at the angles of greatest intensity than does unsintered natural diamond in FIGURE 10. In order to obtain the pattern in FIGURE 9 a higher X-ray intensity was used than for the diamond whose pattern is shown in FIGURE 10. With the same X-ray intensity the peaks in FIGURE 9 would be much lower.

An important aspect of the invention is the apparatus or assembly in which the shock synthesis and shock sintering may be effected, and this is discussed in greater detail with reference to FIGURES 1 through 6 of the attached drawings, in which like numbers are used throughout to denote like members, and in which: - FIGURE 1 is a vertical cross-sectional view of an assembly according to the invention; FIGURE 2 is a plan view of the assembly shown in FIGURE 1; FIGURE 5 is a vertical cross-sectional view of another assembly, according to the invention; FIGURES 4A, 4B and 5A through 5E are vertical cross -sectional views of parts of assemblies essentially similar to that shown in FIGURE 1; and FIGURE 6 is a vertical cross-sectional view of another assembly according to the invention, including means for initiating an explosive.

In FIGURES 1 and 2, the assembly is a solid cylindrical block 1_ having an exposed cylindrical cavity 2_, con- adjoining exposed surface of block 1_. Spaced above block 1_ over cavity 2 is a circular metal plate 4 having on its upper surface a cylindrical layer of detonating explosive 5, which, when initiated, propels plate 4 down so as to collide with the coplanar exposed surfaces of sample 3 and block 1_.

In FIGURE 3, block 1, cavity 2, and sample 3 are all hemispherical in configuration.

In FIGURE 4A, compact 3 does not extend to the ex-posed surface of block 1_, but is overlaid with a cover 6, the exposed surface of which is coplanar with the adjoining surface of block 1.

Sample 3 in FIGURE 4B is in the form of a bowl, i.e. an unfilled hemisphere, and the exposed sui ace thereof (to be contacted by a metal projectile plate) is hemispherical.

In all of FIGURES 5A through 5E, there is a sample 3 within a cup-shaped container 7 fitting snugly in cavity 2 in block 1. FIGURES 5B, 5C, and 5D also show a multi-membered block construction around container 7 to provide increased protection against loss of powder owing to the effect of tension set up by release waves. The container 7 has a base in FIGURES 5A, 5C, 5D, and 5E, but not in FIGURE 5B. In FIGURES 5B, 5C and 5E, further reinforcement around container 7 is provided by ring 9. In FIGURE 5B, ring 9 and surrounding external element 1A constitute block 1., and in FIGURES 5C and 5E, block 1 also includes In H$UB86, is a plane-wave generator affixed to the upper flat surface of a cylindrical layer of detonating explosive 5, the lower flat surface of which rests on plate which is substantially parallel to the upper surf ce of block 1. Plane-wave generator Ifi is initiated by blasting cap H having wires leading to a source of electricity. The layer of explosive £ is surrounded by a metal confining means 12 which may be used to increase the impulse given to plate &· The compact 2 is retained in container 2, supported by cylinder SL and external ring element 2A surrounding and fi. Cylinder and element together constitute block 1. Upon initiation of plane-wave generator 1Q by actuation of blasting cap 2J», explosive layer 5 is detonated along an entire flat surface thereof and a plane shock wave is thereby introduced into plate 4, propelling the plate toward block 1 so that substantially the entire flat lower surface of plate Δ collides simultaneously with the exposed coplanar surfaces of container 2 and powder compaot 2, (parallel collision). The apparatus of FIGURE 6 Can be employed to cause plate A to collide obliquely with the surface of container 2# ¾y positioning plate A at an appropriate angle to the block surfaoe and/or by using point or line initiation.

It will be appreciated that all the Figures show that the plate is driven down onto the sample, since this is preferred, but the assemblies could be set up so that the plate is driven in any other direction.

At least the peripheral portion, and preferably all of block 1, including the base, i.e. the place remote from the entry of the shock wave, should comprise one or more of lead, pi&ee-- eme e-£ em- be-e& ^-^--tt^-&h€X)^-w&*fi6-, and the greater the proportion of such metal in the block, the higher the ductility and density of the material surrounding the sample, and the better the sample retention. The width of this peripheral portion of the block, measured in a direction parallel to the surface contacted by the projectile plate, should preferably be at least half the minimum distance from the periphery of the impact area to the outside wall of the block, to provide the desired cushioning ef-feet, and thicker pieces are preferred, for safety. The container £, and, if desired, supporting members such as 8 and 9> within the block 1_, may conveniently be of a tough metal such as steel, titanium, niobium, nickel, aluminium or their alloys, to give added protection to thin, fragile samples.

When the assembly is made of a plurality of members, such members have matching surfaces, which cause the pieces to fit together as an integral unit. Exact fit is not necessary, however, and a space between members can be tolerated provided it is filled with a material such as grease .

The block 1, and the cavity 2 may be cylindrical, hemispherical, or like a parallelepiped, or the cross sections may be oval, triangular, hexagonal, or any convenient con iguration, and need not be the same or uniform in area and shape in any direction. The bottom surface (opposite the surface contacted by the projectile plate) should be such as to permit the assembly to rest securely on a base support, such as an anvil or on or in the (having detonation velocities, e.g. of about 7000 m/sec), an explosive loading of at least 25 g./sq. in. is required to drive a 0 , 06-in. -thick steel plate to a velocity of about 5 km./sec. With such a loading, the height of the block should generally be at least half the thickness of the explosive layer, and so should the distance between the contacting surfaces and the nearest outside surface of the block. For example, with an explosive loading range of 25 to 150 g./sq. in., and of thickness 1/2 to 6 in., the block should preferably have a height of 2 to 10 in. and the other distance of 1 to 6 in. In general, for the same explosive loading, a thinner and larger-surface -area explosive layer requires less height and more side area in the block relative to the thickness of the explosive layer than does a smaller-area, thicker layer.

The object sample may be in the form of a cylinder, solid hemisphere, bowl, cone, pyramid, parallelepiped, sphere, ellipsoid, or any other suitable shape, provided that the impacted surface conforms with that of the plate, and the sample fits snugly in the assembly. In some instances, e.g. when the1 sample is a powder compact, it is preferable, particularly if the density of the compact is not high, to provide a suitable cover layer 6 of a coherent solid material on top of the sample, and the projectile plate then contacts the upper surface of this cover 6 rather than the sample itself. The cover need not fit closely, but it is preferred in such instances to evacuate any space to avoid overheating. Metals, e.g.' iron, nickel, titanium, niobium, aluminium, copper and their alloys, are preferred for such a cover 6 , and the be of substantially uniform thickness, at least where it overlies the sample, to assure that all of the sample is subjected to a shock wave of substantially the same intensity. The thickness of the cover need be only about 1/16 in., and is preferably less than twice the plate thickness, to avoid shock attenuation problems. The cover may extend over the container J_, and even over part of the block 1, and is preferably unitary with or welded to the container.

The projectile plate 4 preferably has at least a moderately high shock impedance, e.g. at least about 10° dyne-sec ,/c ., and comprises a metal, such as steel, copper, aluminium or an alloy thereof. The plate 4 may be curved, to conform with the upper surface of the sample, but a flat plate and flat sample is preferred. To avoid edge lag effects, the contacting surface of the plate, and thus the impact area, preferably overlaps the sample. The plate should preferably be at least Ο.θβ in. thick, and, in practice, considering explosive requirements, is unlikely to be more than 6 in., a 1-in. thickness being generally adequate. The plate 4 should be spaced from the block l^ surface at least far enough to allow the plate to attain the velocity needed to generate the desired shocking pressure, generally about the plate thickness being required, although greater distances are often preferred to provide more efficient utilization of energy in some cases. As the spacing increases, however, the chance of introducing undesired effects, such as disrupting action on the plate, increases, but spacings up to 25 times the plate thickness have been used. Any convenient means may 23569/2 Th following Example farther illustrate the invention? all parte and percentages are by weight, unless otherwise indicated. The analysis for diamond yield is carried out by irst dissolving the metal cooling medium out of the sample piece (concentrated hydrochloric acid is used to dissolve Out iron), and the the silica with hydrofluoric acid, and weighing the remaining solid whioh is graphite and any diamond present. he graphite is then oxidized by heating in ai at 425°0. for 24 hours with lead oxide powder, the lead oxide is dissolved out with acetic acid, and the remaining solid, i.e. the diamond, is filtered off and washed with aqua regia before weighing and subjection to X-Hay diffraction tests to identify the material as diamond. Example 1 Referring to Figures 1 and 2, flat cylindrical disk of nodular cast iron, 6 in. in diameter, 2 in. thick and of density about 7g./oc, consisting o 3$ of non-porous graphite (density about 2.26 g./cc.) in the form of 20-50 micron spheres dispersed I a ferrite matrix, has molten lead cast around it tq form a coaxial cylindrical block 1, 22 in. in diameter and 10 in. in height. A 10-in. diamete circular steel plate 4, of 0.09-in. thickness, with a circular layer 2. of Composition Bt of the same diameter and thickness 5 in. on its upper surface, is spaced 1.5 in. from the block and held in place by means of wooden support blooks. The explosive layer 2 is initiated by a plane-wave generator on its upper surface so as to drive the plate A against the disk 2 with a shock wave of 1. megabars pressure, and fracture the disk 2· The disk 2 *>® sliced parallel to the impact surface into seven 0.25-inch thick sections. Pieces from each section are analyzed for dia- " mond content as described above. The pieces from the two top slices contain diamond as indicated in the following Table, whereas- the lower five slices contain substantially no diamond.

No. of Wt. (g) DiaWt. (g) Yield Piece Origin of Piece mond & Graof of Diaphite Diamond mond* 2 2A 2nd from top 4.31 0.97 ") 2B It II II 3.60 1.21 32$ 2C It II It 6.20 2.36 / * (Average ! of Pieces in Slice) The yield is about 520 cts./ft.2 of shocked surface, and about 40$ of the diamond recovered is larger than 10 microns in size.

Example 2 Example 1 is repeated with a cast iron disk 3 of thickness 1.5 in. and a plate 4 of thickness 0.22 in. introducing into the cast iron at its exposed surface a shock pressure of 900 kilobars. Pieces taken at random show a diamond yield of about 12 . (about 445 cts./ft.2 about 50$ of the diamonds being larger than 10 microns.

Example 3 Example 1 Is repeated with a 3-in. diameter, 21/32-in. -thick disk 3 of gray cast iron of density of about 7 g./oc, and consisting of 3$ of flake graphite in a pearl- ite matrix, the graphite flakes having a density of about 2 .26 g./cc. being 10-20 microns thick and up to several hundred microns long, an explosive charge 5 3.5 in. thick, and a plate (as in Example 2.) introducing a shock pressure of 780 kilobars. Diamond is obtained in 5$ yield as plates of up to several hundred microns in length.

Example 4 42 g. of Ceylon natural graphite powder, of particle size such as to pass a 220-mesh screen (average particle size of about 30 microns) are mixed thoroughly with 785 g. of iron powder, of particle size such as to pass a 325-mesh screen (average particle size of about 20 microns), and the mixture is loaded into a cylindrical cavity of diameter 4 in. and height 1.5 in., in a steel cup-shaped member, of diameter 6 in. and height 2 in., fitting snugly in a cavity in a lead block of diameter 22 in. and height 8 in. The powder is pressed under 40 tons pressure to form a flat disk. A 0.12-inch-thick steel cover plate is placed over the disk within the cavity, and the powder mixture is evacuated and sealed-off . A layer of the PETN sheet explosive described in U.S. Patent 2 , 999.» 743 and having a loading of 4 g./in. is positioned over the cover plate and is initiated at its center so as to compact the powder mixture to about 80$ overall density. The powder compact again being 5 in. thick, however, to introduce into the compact a shock wave at a pressure of 1000 kilobars. The shocked sample, which weighs 736 grams, is analyzed to show 17 g. of solids (graphite plus diamond) and 1.7 g. diamond (lOo yield).

Example 5 24 g. of spectrographic-grade artificial graphite powder and 89 g. of copper powder, both of particle size such as to pass a 325-mesh screen are mixed thoroughly and the mixture is loaded into a steel ring (6-in. outside diameter, 4-in. inside diameter, 2-in. height) and pressed therein under 40 tons pressure to form a 4-in. diameter, 0.9-in.-thick disk having a density of 5 g./cc. A 0.12-in.-thick steel cover plate is placed over the disk within the aperture in the ring, the powder mixture evacuated, and sealed off. The ring-compact assembly is surrounded snugly by a steel ring of outside diameter 8 in. and height 2 in., mounted snugly in a cavity in a lead block as in Example 4. The powder is compacted to a dens-ity of about 80$ as in Example 4, and then subjected to the explosive projectile plate assembly as in Example 4, but the explosive. layer is 3.25 in. thick and drives the plate 4 at a velocity of 3.4 km./sec., introducing into the compact a shock wave at a pressure of 950 kilo-bars. The shocked sample is treated with nitric acid to remove the copper, and analyzed to sho 17.74 g. solids (graphite plus diamond) and 1.99 g. diamond (10$ yield).

Example 6 β,4 g. of spectrographic -grade artificial graphite powder in the 0-10 micron particle size range and 91 .6 g. of 5-10 micron size aluminium powder are mixed thoroughly, and the mixture is hot-pressed at 500°C. and 2000 psi for 30 minutes to form eight 2-in. -diameter, 1-in. -thick compacts at 98$ theoretical density. The compacts are cast in lead in an 8-in. -diameter steel ring, in the cavity of a lead block as in Example 5. An explosive/projectile plate ■ assembly as described in Example 1 is employed, except that the plate is 0.162 in. thick. Detonation of the explosive drives the plate at a velocity of 3.82 km./sec.

After shocking, the sample is treated with hot caustic solution to remove the aluminium, and then analyzed, as described above, to show 3 g. of solids (graphite plus diamond), and 0.462 g. of diamond (1 . yield), i.e. 400 cts/ft2.

Example 7 Example 6 is repeated except that the powder mixture comprises 2 .7$ graphite and 97 .3$ nickel. The hot-pressing operation is carried out at 1200°C. and 4000 psl for 30 minutes. The density of the compacts is 93$. The amount of graphite plus diamond obtained is 2 .2 g., and the amount of diamond is Ο.387 gram (17.6$ yield), i.e., 490 cts/ft2.

The diamond prepared according to the preceding Examples contains only trace amounts of the matrix metal, i.e. iron, copper, ' aluminium, and nickel respectively. X-ray studies reveal that these diamonds have a unique crystal structure, i.e. hexagonal diamond having the lattice di 2/5, ζ;ί 2/5, 1/5, Zj 2/5, 1/5, 1/2 + z ; 1/5, 2/5, 1/2 - z; with z = l/l6. The carbon atoms are arranged in tetra-hedra with Interatomic distances and bond angles identical to those observed in well-known cubic diamond.

Hexagonal diamond generally constitutes up to about 50$, e.g. 50 to 50$, by weight of the diamond product prepared by shocking graphite dispersed in a continuous metal matrix cooling medium of this invention, with amounts down to about 5$ being detectable using current techniques. It is believed that, under ideal cooling conditions, 100$ of hexagonal diamond can be obtained. Diamond containing these hexagonal characteristics constitutes, of course, an important feature of the present invention. The character zing features of the powder diffraction diagram of such shock-synthesized diamond is the cubic (111) reflection at a plane spacing of 2.06l A, between a hexagonal (100) reflection at a spacing of 2.186 A and a hexagonal (101) reflection which forms a shoulder ranging from the cubic (ill) reflection up to 1.895 & spacing. The hexagonal (002) reflection coincides with the cubic (111) reflection and cannot be separated. Most particles of diamond shock-synthesized according to the above procedures are mixtures of cubic and hexagonal diamond phases in a texture orientation, the texture axis being a combination of the cubic (110) axis and the hexagonal (110) axis. The presence of stacking faults is indicated by the broadening of hexagonal reflections (hkl) with h + k 5n and 1 0.

The hexagonal diamond phase and stacking faults are believed to ehhance the chemical reactivity of the crystal, - 33 - 2^569 2 EXAMBILE β 108 g. of the graphite powder described in Example 4 are mixed thoroughl with 364 g» of steel shot (1 mm* in diameter). The mixture is loaded into a steel ring as described in Example 9 and pressed therein under 40 tons pressure to form a 4-in.-diameter, 0.526-in.-thick disc, having approximately 45 vol. ≠ steel having a density of about 7.7 g./cc. and 55 vol. # graphite, the particles being about 100$ dense, and the graphite density in the disc (computed from a graphite volume obtained by subtracting the volume of the steel shot, assumed to be at theoretical density, from the volume of the disc) being about 78.5$. covered with a 0.12-In.-thick cover plate, evacuated, sealed off, and surrounded by a steel ring mounted in lead block as described in Example 5. An explosive/projectile plate assembly as described i Example 1 is employed, the plate in this case being 0.167 in. thick, and the explosive being 5.5 in. thick. Detonation of the explosive drives the plate at a velocity of 3.75 km./sec. The amount of solids before oxidation. is 9.62 g., and the amount of diamond recovered therefrom is 2.53 g. (17·6# yield), 2 about 1090 cts./ft. The diamond appeared to be of the usual cubic form.

EXAMPLE 9 A. 39.6 g. of spectrographic-grade artificial graphite(less than 6 parte per million total impurities) and 5.29 g of yellow FbO, both of particle size such as to pass a 325-mesh soreen (openings t 44 microns), and 0.4 g. of 1 micron size natural diamond are mixed together by shaking with steel balls, and heated for 24 hours in a flat tray (powder depth* 3/8 inch) in a 425°C. furnace having a loosely fitting door to permit the circulation of air therein. The oxidized mix being decanted off, the solids are washed with 1000 ml. of distilled water, and treated with 50 ml. of >6 0 hydrochloric acid, the liquid being decanted off, and the solids are washed with 1000 ml, of distilled water, and dried to a constant weight of Ο.69 g.

B. Part A is repeated except that 2 g. of graphite, 2 g. of diamond, and 2 g, of PbO are used to give 2.84 g. of product.

C. Part A is repeated except that 58 g. of graphite, 2 g. of diamond, and 5.2 g. of PbO are used to give 2.53 g. of product.

All the products of A, B and C are shown by X-ray diffraction to be chiefly. diamond and the remainder graphit D. The products from Parts A, B, and C are combined and mixed with 2 .5 g. of PbO (yellow), and heated as described in Part A for 65 hours, and then treated with acid, etc., as described in Part A, to give 4.11 g. of diamond; no graphite is detected.

Example 10 In this Example and following Examples 11 and 12 , the shock pressures given are calculated according to the above description in connection with shock sintering, i.e. taking into account shock wave reflection, where applicable Carats of yellow-green natural diamond powder, In which all particles are in the 1-2 .5 micron size range, and having an impurity content of less than 4$, are pressed to form a 1-in. -diameter, 0.15 in. thick disc, of- bulk block of lead, diameter 24 In. and height 12 in., and having a cavity 2 , 2 in. deep and diameter 5 in., and rests on a steel anvil. Steel cup-shaped member 7 has walls 1 in. thick, and base 1 in. thick, steel ring 9 is 1 in. thick and 2 in. high, steel cover plate 6 is 1/8 in. thick, and closure member 8 is a lead cylinder, diameter 3 In. The projectile plate is 0.18 in. thick and 5 in. in diameter, and has on its upper surface a cylindrical layer of Composition B (6θ/4θ RDX/TNT containing 1% wax), 6 in. in height, en-cased in a 1/4-in. -thick steel sleeve. The projectile/plate is supported by wooden blocks 1.5 In. above cover plate 6. The explosive is initiated by a plane-wave generator, so as to drive the plate against cover plate 6 at an impact velocity of 3.4 kilometers per second, thereby introducing into the cover plate a shock wave at a pressure of about 70 kilobars, the duration of which is about 1.1 microseconds. The shock pressure at the cover plate -diamond interface is estimated to be about 250 kilobars and in the diamond powder is estimated to be about 870 kilobars, resulting from reflection of the shock wave (780 kilobars) from the bottom of the cavity in steel cup-shaped member 4 followed by a reflection from the cover plate before the shock wave is attenuated by the rarefaction wave reflected from the back surface of the relatively thick projectile plate. 90$ of the diamond powder is converted to discrete dark-gray sintered diamond particles, a large proportion of which are 1 mm. in length and which have a density of about 90 the crystalline density of diamond. They scratch glass to form readily visible scratches, whereas the diamond powder from which X-ray diffraction pattern shown in FIGURE 9. When an effort is made to comminute the sintered diamond by grinding in an agate mortar with an agate pestle , the agate chips severely and little comminution is achieved. When a cutting tool is made by metal-bonding the diamonds into a cutting tool head, the tool cuts a silicon carbide wheel with a ratio of silicon carbide wear to diamond wear greater than 100,000 to 1.

Example 11 Six g. (29.3 carats) of natural diamond powder in the 1-3 micron size range is cold pressed to form a 1-in.- diameter pellet having a density of 1.72 g/cc. The pellet is placed in an assembly substantially the same as that shown in FIGURE;- 5C utilizing a 0.12 in. thick cover plate and then is explosively compacted to a thickness of 0.137 In. and a density of 3.4 g/cc. by means of a 5-in. -diameter, 0.187-ln.- thick, steel plate driven to a velocity of 2.8 km,/sec» by a 2-in. -thick, 5-in. -diameter layer of Composition B. Using the same assembly, this compact is then shocked by means of a 0.09-in. -thick, 10-in . -diameter steel plate driven to an impact velocity of 4.5 km./sec. by 5-in. -thick, 10-in. - diameter Composition B. The ahock pressure in the diamond is estimated to be about 1100 kilobars. The sintered diamond particles have a density equal to the crystalline density of diamond (3.5 g/cc) and about 90$ of the particles are at least 1 mm. long.

Example 12 carats of diamond powder, synthesized by explo particle size is 8 microns) are cold pressed to form a 1-in. -diameter pellet having a density of 50$ of the crystalline density of diamond. The pellet is placed in an assembly substantially as shown in FIGURE 5C under a 0.12 in. -thick cover plate. The pellet is then shocked by means of a 0.l8 in. -thick steel plate , 5 in. in diameter driven to a velocity of 3.46 km./sec. by a 5 in. -diameter layer of Composition B having a density of 110 g./sq. in. The shock pressure is estimated to be slightly in excess of 870 kilobars including reflections as in Example 10. A large proportion of the sintered diamond particles have a length of about 1 mm with a density of about 90$ of the crystalline density of diamond. They readily scratch glass and tungsten carbide, and when an effort is made to comminute them in an agate mortar and pestle, the agate is severely chipped. A diamond point about 1/4 in. diameter and 1/2 in. long composed of the sintered diamond particles bonded in a polymer matrix readily grinds tungsten carbide.

Example 13 The apparatus shown in FIGURE 1 is used with a container J and block elements 1A, 8 and 9 shown in FIGURE 5C, as follows: a lead cylindrical block _1, diameter 22 in. and height 8 in., resting on a steel anvil; disc 3, diameter 4 in. and 0.5 in. thick, comprising 150 g. of synthetic graphite powder packed to a density of about 1.6 g./cc; steel container 7., walls 1 in. thick and base 1.5 in. thick; steel ring 9 1 in. thick, and steel cover plate 6 0.12 in. thick; steel projectile plate 4 0.06 in. thick and diameter 10 in.; lead cylinder 8 0.25 in. high; cavity 2, depth 2.12 in. and diameter 6 in. Plate 4 is ' in. and height 3.5 in., and comprising a blend of pentaerythritol tetranitrate in a 50/50 mixture of butyl rubber and a thermoplastic terpene resin (mixture of polymers of β-pinene having the formula an explosive loading of 84 g./sq, in., and initiated by a plane-wave generator as shown in FIGURE 6. Plate 4 strikes the block surface in a parallel position relative to the surface. After impact, the lead block is deformed, as is cover plate 6 and the side-walls of cup-shaped member 7. However, disc 3 is intact although cracked. The graphite is recovered completely, and contains diamonds. The lead block is re-worked to its original shape and size by melting and recasting.

Claims (9)

1. itWIilG HOW particularly described and ascertained the nature of Otir said invention and in what manner the same s to he e fo med, we declare that what ¾re claim Is:* 1. A process for converting graphite into diamond/, comprising forming an intimate mixture of graphite and a cooling medium capable of cooling the resulting diamond rapidly to less than the rapid atmospheric graphitization temperature, and passing therethrough a shock wave of intensity sufficiently high to convert at least some of the graphite to diamond but sufficiently low to enable the cooling medium to cool the resulting diamond to below the rapid atmospheric graphitization temperature.

2. A process according to Claim 1, wherein the graphite and cooling medium are uniformly mixed. 5. A process according to Claim 1 or 2, wherein the shock wave is obtained by driving a projectile plate against the mixture. 4. A process according to Claim 3, wherein the projectile plate is propelled by the force derived by detonation of an explosive. 5. A.process according to Claim 1 or 2, wherein the shock wave is obtained by detonation of a high explosive in substantially direct contact with the intimate mixture. 6.1 A process according to any of the preceding claims, wherein the intimate mixture is confined n a container, and the shock wave is directed against the outside of the container instead of directly against the intimate mixture . 7. A process according to any of the preceding claims , wherein the shock wave produces a high pressure for between 0.1 and 10 microseconds. 8. A process according to any of the preceding claims, wherein the shock wave Introduces into the mixture a pressure of at least 600 kilobars. 9. A process according to Claim 8, wherein the shock wave introduces a pressure of at least 750 kilo-bars . 10. A process according to Claim 9, wherein the shock wave Introduces into the mixture a pressure of at least 1000 kilobars. 11. A process according to any of the preceding claims, wherein the shock wave introduces into the mixture a pressure of up to 2000 kilobars. 12. A process according to Claim 11, wherein the shock wave introduces into the mixture a pressure of up to 1200 kilobars. 1

3. A process according to any of the preceding claims, wherein the intimate mixture contains up to 65$ by volume of graphite . 1

4. A process according to any of the preceding claims, wherein the graphite is of density at least 60o of theoretical. 1

5. A process according to any of the preceding claims, wherein the cooling medium has a shock impedance higher than that of the graphite. 1

6. A process according to Claims j5 and 15, 'wherein the projectile plate is at least 5 times as thick as the graphite particles. 1

7 . A process according to any of the preceding claims, wherein the cooling medium is a solid. 1

8. A process according to any of the preceding claims, wherein the cooling medium is a metal. 1

9. A process according to any of the preceding claims, wherein the cooling medium has a shock impedance of the order of 3 x 10 dyne-sec ./cc . or more. 20 . A process according to any of the preceding claims, wherein the cooling medium has a heat capacity of the order of 0.1 cal./g./°C. or more. 21 . A process according to any of the preceding claims, wherein the cooling medium has a thermal conductivity at room temperature of the order of 0.1 cal ,/sec ./cm°C. or more . 22 . A process according to any of the preceding claims, wherein the cooling medium has a density at least 85 of the theoretical density, in terms of the weight divided by the volume exclusive of pores containing graphite. 23. A process according to any of the preceding relative claims, wherein the cooling medium has an overall/density of at least 70$ . 24. A process according tp any of the preceding claims, wherein the cooling medium is any of the materials 25. A process according to any of the preceding claims, wherein the intimate mixture is an integral solid that will not shatter on shocking. 26. A process according to Claim 25 wherein the intimate mixture is cast iron. 27. A process according to any of Claims 1 to wherein the graphite and cooling medium used to form the intimate mixture are in particulate form. 28. A process according to Claim 27 , wherein the particulate intimate mixture is compacted before passing therethrough the shock wave . 29. A process according to Claim 28, wherein the compaction is carried out by explosive means. 30. A process according to any of the preceding claims, wherein the diamond resulting is cooled to below the rapid atmospheric graphitization temperature in less than 0.1 seconds. 31. A process according to Claim 30, wherein the diamond is cooled to less than the rapid atmospheric graphitization temperature in a period of the order of 0.001 seconds or less. 32. A process according to any of the preceding claims, wherein the maximum temperature induced into the cooling medium as a result of the shock wave is less than 2000°C. 33. A process according to Claim 3 , wherein the 3½ . A process for converting graphite into diamond, the graphite being in the form of an intimate mixture with a suitable cooling medium, involving the use of a shock wave, substantially as hereinbefore described. 35. A process for converting the graphite into diamond substantially as described in any of the foregoing Examples 1 to 8. 36. A process according to any of the preceding claims, carried out in an assembly as described and illustrated with reference to any of Figures 1 to 6 in the accompanying drawings. . 37. A process for selectively oxidizing graphite in a mixture thereof with diamond, such as is obtainable by a process according to any of the preceding claims, wherein the mixture is heated with a lead oxide in air or oxygen at a temperature of 350 to 550°C. 38. A process according to Claim 37 , wherein the lead oxide is formed in situ from an oxygen-containing lead compound. 39. process according to Claim 37 or 38, wherein the graphite and lead oxide are intimately mixed homogeneously. 40. A process according to any of Claims 37 to >9, wherein the particle size of the lead compound is less than 50 microns. 41 . A process according to any of Claims 37 to 40, wherein the particle size of the graphite is less than 200 42. A process according to any of Claims 37 to 4l, wherein there is used at least 10$ of lead oxide based on the weight of the graphite-diamond mixture. 43. A process according to any of Claims 37 to 42, wherein there is used up to 100 by weight of the lead oxide based on the weight of the graphite-diamond mixture. 44. A process according to any of Claims 37 to 43, wherein the temperature is 380 to 430°C. 45. Diamond prepared according to a process as1 claimed in any of Claims 1 to 44. 46. Hexagonal diamond, having lattice dimensions a = 2.524 A and c = 4.122 A. 47. Diamond whose powdered diffraction diagram shows a hexagonal (100) reflection at the spacing of 2.186 A. 48. Diamond containing at least 5 of the diamond claimed in Claim 46 or 47. 49. Diamond according to Claim 48, wherein the diamond as claimed in Claim 46 or 47 comprises at least 30$ by weight of the diamond. 50. Diamond according to Claim 48 or 49, whose powdered diffraction diagram shows the cubic (ill) reflection at the o spacing of 2.06l A, and a hexagonal (101) reflection which forms a shoulder ranging from the cubic (ill) reflection up to I.895 A. 51. Diamond according to any of Claims 48 to 50, wherein the mixture of cubic and hexagonal diamond phases 52 . A process for sintering together small diamond particles , such as are obtainable by the process claimed in any of Claims 1 to 44, to form larger particles , comprising forming a mass of the Bmall diamond particles of bulk density at least 40$ of the crystalline density of diamond, and subjecting the mass to a shock wave at a pressure sufficient to achieve the desired sintering effect. '53. A process according to Claim 2 , wherein the pressure is at least 600 kilobars. 54. A process according to Claim 52 or 53 J wherein the small diamond particles have an average particle size of .less than 150 microns. 55. A process according to any of Claims 52 to 5 , wherein the mass of small diamond particles contains less than \ by weight of impurities. 56. A process according to any of Claims 52 to 55, wherein the duration of the high pressure is at least 0.1 microseconds . 57 · A process according to any of Claims 52 to 6 , wherein the shock wave is obtained by driving a projectile plate against the mass. 58. A process according to Claim 57* wherein the projectile plate has a shock impedance of at least 10^ dyne-sec./ cc . 59 · A process according to Claim 57 or 58* wherein the velocity of the projectile plate on impact is at least βθ. A process according to any of Claims 52 to 56, wherein the shock wave is obtained by detonating an explosive in substantially direct contact with the mass of small diamond particles. 61. A process according to any of Claims 52 to 60, wherein the mass of small diamond particles are contained within a container which is subjected to the shock wave. 62. A process according to any of Claims 52 to 6l substantially as hereinbefore described. 63. A process for the shock-sintering of diamond substantially as described in any of the foregoing Example 10 to 12. 64. A process according to any of Claims 52 to 63, wherein there is used an assembly substantially as described and illustrated with reference to any of Figures 1 through 6 of the accompanying drawings. 65. Shock-sintered diamond prepared according to a process as claimed in any of Claims 52 to 64. ■ · ·':..].··.;. -.- Λ : ..· ■ v *· ■'· ■ ·;.ι·'· ' :·· · ■■■ ■ 66. Polycrystalline diamond, containing diamond crystals sintered together, having a density of at least 80$ of the crystal density of diamond, and producing an X-ray diffraction pattern wherein the diffraction lines for diamond have a line broadening coefficient, K', of -2 -4 from 4.5 x 10 to 7.5 x 10 as calculated from the expression: Κ' = β cos θ , where β is the pure angular breadth in radians ^f--a powder reflection free of all broadening due to the experimental method employed in observin it is the wave len th in An stroms of the mono tern, and 2Θ is the angle of deviation of the diffracted beam. , 67. Diamond according to Claim 65 or 66 of particle size at least 5C\ microns. 68. Diamond substantially as illustrated in Figure 7 or 8 of the accompanying drawings. 69. Diamond having an X-ray diffraction pattern substantially as shown in Figure 9 of the accompanying drawing 70. Apparatus for carrying out shock treatment, substantially as hereinbefore described, with particular reference to any of Figures 1 to 6 of the accompanying drawing Dated this 19th 4ay of May, 1965