This disclosure relates generally to methods for making synthetic diamond crystals and to resulting synthetic diamond crystals.

US patent number 7,368,013 discloses a method for synthesising super-abrasive particles. The method including forming a growth precursor, coating at least part of the growth precursor with a layer of adhesive material, arranging crystalline seeds in a predetermined pattern on the adhesive layer and subjected to conditions for growth of the particles. The growth precursor can be a substantially homogenous mixture of particulate raw material and particulate catalyst material; it can include a raw material and a particulate catalyst material; or alternatively it can comprise layers of raw material and catalyst material. The crystalline seeds can have a diameter of from about 30 microns to about 500 microns. Alternatively, the crystalline seeds can have a diameter from about 10 microns to about 50 microns. It is disclosed that crystalline seeds can have an average diameter from about 0.05 to about 0.2 times the average diameter of the desired grown super-abrasive particles. There is a need to provide a relatively efficient method for making relatively high quality synthetic diamond crystals produced in batch manufacturing processes, particularly but not exclusively relatively large diamond crystals.

Viewed from a first aspect there is provided a method of manufacturing a plurality of synthetic diamond crystals, the method including providing a reaction compact comprising a plurality of diamond seed particles dispersed within a matrix comprising a carbon source such as graphite and catalyst material for promoting diamond growth; in which the combined surface area of the diamond seed particles is at least about 0.01 square cm or at least about 0.015 square cm per cubic cm of the reaction compact volume (assumed for this calculation to be at maximum theoretical density); the method further including subjecting the reaction compact to an ultra-high pressure and high temperature at which the catalyst material is molten and diamond is more thermodynamically stable than the carbon source, for a sufficient period of time to grow the diamond crystals on the diamond seed particles; and treating the reaction compact to recover the diamond crystals. Various versions and arrangements are envisaged for the method by this disclosure.

The diamond seed particles may be substantially uniformly dispersed in the reaction compact. The diamond seed particles may be sufficiently uniformly dispersed within the reaction compact so that the substantially all of the grown diamond crystals will be substantially homogeneously spaced apart from each other. This will reduce the risk of diamond crystals interfering with each other during their growth and is likely to increase the yield of substantially euhedral grown diamond crystals.

The reaction compact may comprise graphite powder substantially uniformly blended with catalyst material powder, or the graphite and the catalyst material may be at least partly segregated within the reaction compact. For example, the ratio of the graphite to the catalyst material may vary throughout the reaction compact or within the local vicinity of each diamond seed particle. In an example arrangement, the graphite and the catalyst material may be arranged as alternate layers within the local vicinity of each diamond seed particle. The carbon source and the catalyst material may be arranged in concentrations and or layers in the vicinity of each seed particle in such a way as to limit the movement of the growing diamond crystal by buoyancy effects.

The seed particles may be dispersed in the reaction compact such that each diamond crystal can growth over the entire surface of each respective seed particle. For example, each seed particle may be surrounded by a substantially uniform blend of carbon source powder and catalyst material powder so that when the catalyst material is in the molten state, the carbon source can dissolve and carbon can migrate to the growing seed and become attached to it in the diamond crystalline form. Each of the grown diamond crystals may include a seed particle within its volume, the seed particle being completely enclosed within the grown diamond crystal. Since it will be very difficult or impossible to remove the seed particles from the diamond crystals, it will likely be desirable to minimise the occurrence of defects that may arise in the vicinity of the seed at an early stage of the crystal growth.

The method may include providing a plurality of discs at least some of which comprise the carbon source and at least some of which comprise the catalyst material, arranging the seed particles on the discs, stacking the discs on top of one another and compacting the stacked discs to provide the reaction compact.

The method may include encapsulating each seed particle in a respective pellet comprising the carbon source and the catalyst material to provide a plurality of pellets, and combining and compacting the pellets to provide a reaction compact.

The method may include encapsulating the plurality of diamond seed particles within pellets comprising graphite powder and powder of catalyst material for diamond, in which each seed is encased within each respective pellet and substantially all the pellets contain only one diamond seed particle. The pellets may be substantially spherical and can be combined and compacted together to provide a solid reaction compact. A reaction compact thus produced is likely to comprise diamond seed particles arranged in a three dimensional honeycomb like arrangement. In various examples the pellets may be substantially spherical, ovoid, flattened spherical or polygonal.

The method may include coating each seed particle with coating structure comprising catalyst material for diamond, or the seeds may be uncoated.

The seed particles may have substantially the same size and or shape. At least about 80 per cent of the seed particles may be substantially euhedral diamond crystals, which may be substantially convex and have smooth crystal facets. At least about 80 per cent of the seed particles may be substantially free of pitted surfaces or other substantial crystallographic defects. The seed particles may be at least about 1 , at least about 10 microns, at least about 40 microns or at least about 100 microns. The seed particles may be at most about 1 ,000 microns, at most about 100 microns or at most about 50 microns. In some examples, the seed particles may have a mean size in the range of about 1 micron to about 100 microns. The standard deviation of the size distribution of the seed particles may be at most about 20 per cent of the mean size of the seed particles.

The mean size of the diamond particles may be at least about 40 U.S. Mesh (about 0.4 millimetres) or at least about 30 U.S. Mesh (about 0.6 millimetres). In some examples, the method may be used for growing a plurality of diamond crystals having a mean size of at least about 0.75 mm or 20 U.S. Mesh (about 0.8 millimetres), or at least about 2 mm or 16 U.S. Mesh (about 1.2 millimetres), in which the diamond seed particles have a mean size of about 50 microns or at least about 100 microns and are dispersed substantially uniformly in the reaction compact, which may comprise graphite powder and catalyst metal powder, such as powder iron, nickel, cobalt and or manganese.

Some versions of the disclosed method may provide substantially octahedral and relatively large synthetic diamond crystals, having U.S. Mesh size of at least about 30 (about 0.6 millimetres). The method is likely to allow substantially unconstrained growth of the crystal in all directions. In some examples the grown diamond crystals may have substantially octahedral shape and mean size, as measured along a side of a crystal, of at least about 400 microns, at least about 800 microns or at least about 1 ,000 microns. At least about 80 per cent of the diamond crystals may be substantially octahedral in crystal habit, having a morphology index of at least 7 and a mean size as measured along a side of a crystal of at least about 400 microns.

The method may include heating the reaction compact to melt the catalyst material at a pressure at which diamond is less thermodynamically stable than the carbon source, allowing sufficient time for the catalyst material substantially throughout the volume of the reaction compact to melt, and then increasing the pressure so that the pressure throughout the reaction compact is sufficiently high for diamond to be more thermodynamically stable than the carbon source. This is likely to have the aspect of reducing the width of the distribution of grown diamond crystal size and or quality.

Some versions of the method may include subjecting the reaction compact to an ultra-high pressure and high temperature by means of a belt type press, in which reaction compact is placed in a chamber defined by a die and subjected to opposing forces applied at opposite ends of the reaction compact. The reaction compact may be assembled into a capsule suitable for a belt type ultra-high pressure press, the capsule being generally cylindrical and having a pair of opposite ends. The opposing forces may be applied by means of a pair of oppositely travelling anvils impinging on the ends of the capsule, substantially on the same longitudinal axis by in opposite directions. Such arrangements may involve some of the applied force impinging on the die as well as on the reaction compact in order to reduce the risk of material from the reaction compact or components of the capsule from escaping from the chamber. Gaskets may be placed between each anvil and the respective end of the die and the gaskets may be squeezed between the anvils and the die to effect a seal for the contents of the chamber. Since not all the force applied by the anvils is transferred onto the reaction compact it is likely to be difficult to control the actual pressure within the reaction compact. This may be particularly problematic at the early stage of the crystal growth process at which even a relatively small excess of pressure is likely to result in excessively rapid growth of diamond material on the seed particles and excessive inclusion of catalyst material in the grown diamond crystal. The method according to this disclosure is likely to mitigate against early stage growth instability.

The catalyst material may comprise iron, nickel, cobalt or manganese or mixtures including two or more of these. In particular, powders comprising at least two of these metals may be blended together substantially in a eutectic ratio so that the combined materials will melt at a relatively lower temperature to form a molten alloy. For example the reaction compact may comprise iron and nickel powder in the mass ratio 70:30, or it may comprise cobalt and iron powder in the mass ratio 80:20.

Viewed from a second aspect, there is provided a plurality of diamond crystals made by a method as claimed in any of the preceding claims, in which at least 80 per cent of the diamond crystals are substantially octahedral in crystal habit, having a morphology index of at least 7 and a mean size as measured along a side of the crystal of at least 400 microns. In some examples, the plurality may comprise at least about 1 ,000 diamond crystals. In some examples, the plurality of diamond crystals may have been provided directly from the reaction compact (for example, by means of digestion in acid), without a step of removal of diamond crystals or introduction of diamond crystals from another source.

Non-limiting examples will be described with reference to the accompanying drawings, of which

Fig. 1 shows four of eight archetypal cubo-octahedral crystal shapes of synthetic diamond. As used herein, euhedral crystals are well-formed with substantially sharp, easily recognised faces. The crystal shapes of synthetic diamond and cBN crystals is discussed by Bailey and Hedges (M.W. Bailey and L.K. Hedges, 1995, "Crystal morphology identification of diamond and ABN", Industrial Diamond Review, vol. 1/95, pages 1 1 to 14) using a morphology index, which describes the basic characteristics of crystal shapes in terms of the growth of different crystal faces or planes.

With reference to Fig. 1 , synthetic diamond crystals may have a range of shapes from cubic, corresponding to a morphology index of 0, to octahedral, corresponding to a morphology index of 8, and various shapes between these comprising octahedral and cubic faces. It may also be possible for a diamond crystal to have mixed morphology, in which a part of the crystal may have a different morphology index from another part of the crystal. Diamond crystals may also exhibit minor faces in addition to and between the cubic and or octahedral faces, and may also exhibit crystallographic twinning. The surfaces of synthetic diamond crystals may be smooth or exhibit various kinds of features, such as striations, dendritic micro-formations or pitting, which may arise form etching of the diamond surface or from the environment of the crystal during its synthesis. Crystals may also exhibit deformations arising from growth competition and or physical interaction with other crystals during synthesis, and or from mechanical degradation of the crystal after synthesis.

In some example arrangements, the seed particles may be provided coated, clad and or encapsulated as disclosed for example in United States patent number 5,980,982 or United States patent application publication number 2008/219914. In some example arrangements the seed particles may be placed according to a predetermined pattern onto layers comprising catalyst material and or raw material, as disclosed in United States patent number 7,585,366 or 5,980,982. Diamond crystals grown according to the disclosed method are likely to have the aspect of reduced content of impurities and inclusions surrounding the seed particles on which they are grown. Since impurities and inclusions tend to have a significant effect on the strength of diamond crystals, the crystals made using this method are likely to have substantially enhanced strength. While wanting not to be bound by a particular theory, the observed technical effect may result from the seed particles having a combined surface area selected such that the window of pressure through which the reaction compact must pass is sufficiently large so that it can be routinely achieved while limiting the risk of excessively high linear growth rates and consequent unstable growth conditions. The faster the growth of the diamond, the greater the likelihood that impurities and catalyst material will be included in the grown diamond crystal as a result of the crystal growth instability that tends to occur when crystals grow too fast. The initial rate of the diamond crystals on the seed particles is expected to depend on the surface area of the seed particles. By selecting the seed particles and dispersing them in the reaction compact such that their combined surface area is according to this disclosure, defects in the grown diamond crystal around the seed particle can be substantially reduced. This effect appears to occur regardless of the size of the seed particles, all else being equal. In some examples, the largest seed particles possible may be used (all else being equal) in order more readily to control their dispersion within the reaction compact, the seed particles not being so large that their size is relatively too close to that of the intended grown crystals. If the combined surface are of the seeds is substantially less than 0.01 square cm per cubic cm of reaction compact volume, the initial growth of diamond is likely to be too fast for the benefits to be observed. Further enhancement of the quality of the grown diamond crystals will likely be achieved if the seed particles are of good quality, and in particular if they are generally euhedral and convex, and or if the seed particles are substantially uniformly dispersed in the reaction compact.

Non-limiting examples are provided below further to illustrate the disclosure. Example 1 A plurality of synthetic diamond crystals having mean size of about 85 microns and substantially euhedral cu bo-octahedral crystal habit were provided as seed particles. Each seed particle was encapsulated within a respective pellet comprising graphite powder, nickel powder and iron powder according to the mass ratio 35:20:45 to provide a pellet weighing about 9 mg. The diamond seed particles and the thickness of the pellet shell surrounding them were selected such that once the pellets are compacted together and treated to provide a reaction compact having substantially the maximum theoretic density, the combined surface area of the diamond seed particles would be about 0.1 square cm per cubic cm of the reaction compact. Each pellet contained a single seed particle substantially at its centre and was initially held together by means of a PVA binder solution, which was subsequently removed by heat treatment in which the pellets were partially sintered. The heat treatment involved passing the pellets through a continuous furnace in a reducing atmosphere at a temperature of about 1 ,050 degrees Celsius. The pellets were compacted to form a cylindrical reaction compact, configured for a capsule for a belt type ultra-high pressure furnace (or press) and the reaction compact was heat treated in a reducing atmosphere to reduce oxide compounds.

The reaction compact was assembled into the capsule and subjected to an ultra-high pressure of about 5.5 GPa and a high temperature of about 1 ,300 degrees Celsius for a period of about 120 minutes, after which the conditions were reduced to ambient levels. The synthesis conditions were selected for growing diamond crystals having a U.S. mesh size band of 20/25 (about 0.7 to about 0.8 millimetres). The reaction compact containing a plurality of grown synthetic diamond crystals (being approximately equal in number to the seed particles in the reaction compact prior to synthesis, taking into account some degree of spontaneous nucleation) was removed from the capsule and the diamond crystals recovered by an acid digestion method. The diamond crystals were sieved into U.S. mesh sizes and classified into high and low quality bands in terms of metallic inclusion content and shape quality. The yield of high quality grown diamond crystals within the target size band of 20/25 U.S. Mesh (about 0.7 to about 0.8 millimetres) was about 0.79 carats per cubic cm of reaction compact (assumed for the purpose of the calculation to have maximum theoretical density).

Example 2

Pellets were manufactured as described in Example 1 , except that the seed crystals had a mean size of about 24 microns. A reaction compact was made from the pellets as described above, except that the surface area of diamond seed crystals was about 0.01 square cm per cubic cm of the reaction compact (assumed to be at its theoretical full density). The reaction compact was subjected to the same pressure and temperature cycle as described above, after which the reference grown diamond crystals were recovered, sieved into U.S. mesh sizes and classified into high and low quality bands in terms of metallic inclusion content and shape quality. The yield of high quality grown diamond crystals within the target size band of 20/25 U.S. Mesh (about 0.7 to about 0.8 millimetres) was about 0.26 carats per cubic cm of reaction compact.

Example 3 A plurality of synthetic diamond crystals having mean size of about 85 microns and substantially euhedral cu bo-octahedral crystal habit were provided as seed particles. Each seed particle was encapsulated within a respective pellet comprising graphite powder, nickel powder and iron powder according to the mass ratio 35:20:45 to provide a pellet weighing about 15 milligrams (mg). The diamond seed particles and the thickness of the pellet shell surrounding them were selected such that once the pellets are compacted together and treated to provide a reaction compact having substantially the maximum theoretic density, the combined surface area of the diamond seed particles would be about 0.06 square cm per cubic cm of the reaction compact (assuming maximum theoretical density). Each pellet contained a single seed particle substantially at its centre and was initially held together by means of a PVA binder solution, which was subsequently removed by heat treatment in which the pellets were partially sintered. The heat treatment involved passing the pellets through a continuous furnace in a reducing atmosphere at a temperature of about 1 ,050 degrees Celsius. The pellets were compacted to form a cylindrical reaction compact, configured for a capsule for a belt type ultra-high pressure furnace (or press) and the reaction compact was heat treated in a reducing atmosphere to reduce oxide compounds.

The reaction compact was assembled into the capsule and subjected to an ultra-high pressure of about 5.5 GPa and a high temperature of about 1 ,300 degrees Celsius for a period of about 180 minutes, after which the conditions were reduced to ambient levels. The synthesis conditions were selected for growing diamond crystals having a US mesh size band of 18/20 (about 0.8 to about 1 .0 millimetres). The reaction compact containing a plurality of grown synthetic diamond crystals (being approximately equal in number to the seed particles in the reaction compact prior to synthesis) was removed from the capsule and the diamond crystals recovered by an acid digestion method. The diamond crystals were sieved into US mesh sizes and classified into high and low quality bands in terms of metallic inclusion content and shape quality. The yield of high quality grown diamond crystals within the target size band of 20/25 US Mesh (about 0.7 to about 0.8 millimetres) was about 0.35 carats per cubic cm of reaction compact, assumed to have maximum theoretical density.

Example 4 Pellets were manufactured as described in Example 3, except that the seed crystals had a mean size of about 24 microns. A reaction compact was made from the pellets as described above, except that the surface area of diamond seed crystals was about 0.01 square cm per cubic cm of the reaction compact (assumed to be at its theoretical full density). The reaction compact was subjected to the same pressure and temperature cycle as described above, after which the reference grown diamond crystals were recovered, sieved into US mesh sizes and classified into high and low quality bands in terms of metallic inclusion content and shape quality. The yield of high quality grown diamond crystals within the target size band of 18/20 US Mesh (about 0.8 to about 1 .0 millimetres) was about 0.1 1 carats per cubic cm of reaction compact.

Example 5

A plurality of synthetic diamond crystals having mean size of about 85 microns and substantially euhedral cu bo-octahedral crystal habit were provided as seed particles. Each seed particle was encapsulated within a respective pellet comprising graphite powder, nickel powder and iron powder according to the mass ratio 35:20:45 to provide a pellet weighing about 24 mg. The diamond seed particles and the thickness of the pellet shell surrounding them were selected such that once the pellets are compacted together and treated to provide a reaction compact having substantially the maximum theoretic density, the combined surface area of the diamond seed particles would be about 0.04 square cm per cubic cm of the reaction compact (assuming maximum theoretical density). Each pellet contained a single seed particle substantially at its centre and was initially held together by means of a PVA binder solution, which was subsequently removed by heat treatment in which the pellets were partially sintered. The heat treatment involved passing the pellets through a continuous furnace in a reducing atmosphere at a temperature of about 1 ,050 degrees Celsius. The pellets were compacted to form a cylindrical reaction compact, configured for a capsule for a belt type ultra-high pressure furnace (or press) and the reaction compact was heat treated in a reducing atmosphere to reduce oxide compounds.

The reaction compact was assembled into the capsule and subjected to an ultra-high pressure of about 5.5 GPa and a high temperature of about 1 ,300 degrees Celsius for a period of about 210 minutes, after which the conditions were reduced to ambient levels. The synthesis conditions were selected for growing diamond crystals having a US mesh size band of 16/18 (about 1 .0 to about 1 .2 millimetres). The reaction compact containing a plurality of grown synthetic diamond crystals (being approximately equal in number to the seed particles in the reaction compact prior to synthesis) was removed from the capsule and the diamond crystals recovered by an acid digestion method. The diamond crystals were sieved into US mesh sizes and classified into high and low quality bands in terms of metallic inclusion content and shape quality. Certain terms and concepts as used herein are briefly explained below.

A catalyst material for diamond is capable of promoting the growth, sintering and or intergrowth of diamond particles. Although the term "catalyst" is used, it is not intended to indicate that the promotion of the growth, sintering and or intergrowth of particles of diamond material is limited or even involves a catalytic function. For example, a catalyst material may promote the growth of diamond crystals principally or exclusively by dissolving a source of carbon and transporting atoms or molecules comprising the source to a seed particle or partially grown crystal to which it becomes attached. A catalyst material as used herein may also be referred to as a "solvent / catalyst" material. Examples of catalyst material for diamond include iron, nickel, cobalt and manganese or certain alloys including any of these, which are capable of promoting the growth of diamond crystals from a source of carbon such as graphite at ultra-high pressure and high temperature at which diamond is thermodynamically more stable than graphite and the catalyst material is in the liquid state. The catalytic effect is likely to be particularly strong when the catalyst material is in the liquid state.

Crystal habit refers to the visible external shape of a crystalline material. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernible habit seen). For example, the shape of synthetic diamond may be substantially defined by various combinations of (1 1 1 ) and (100) surfaces. Eight- sided diamond crystals having only (1 1 1 ) surfaces may be referred to as having octahedral habit and six-sided diamond crystals having only (100) surfaces may be referred to as having cubic habit. Diamond crystals may have both (100) and (1 1 1 ) surfaces and may be referred to as having cubo-octahedral habit. A diamond morphology index has been developed to assign a integer from 0 (completely cubic habit) to 8 (completely octahedral habit) according to the crystal habit.

Particulate material may be divided into size bands according to the mesh of sieves through which they may pass and by which they may be retained. There are various mesh size standards, including U.S. Mesh. Where particles are at least about 50 microns or even less than about 100 microns it may be more convenient to measure and report there size in microns and U.S. Mesh size may be used for relatively coarser particles.