SUMMARY: Widely accepted physics and experimental work indicates that graphite can be directly and quickly converted to diamond by exposing the graphite to sufficiently high temperatures and pressures. Experience also demonstrates that in order to grow large crystals, one must use a hydrostatic pressure. A design is made for a containment vessel having a working volume of 0.40 liters, a hydrostatic pressure of 10 G Pa and operating temperature of approximately 3000 K. The design permits continuous measurement and control of the chamber pressure and the diamond-graphite interface temperature. The containment vessel is designed for a specific five kilo-ton press owned by the U.S. Army. The working chamber is 40 mm in diameter by 320 mm long. It will hold a graphite rod 25 mm in diameter by 300 mm long weighing 265 to 300 gm, depending upon the source of the graphite. The expected diamond crystals will be in the range of 20 mm in diameter by 250 mm in length with a weight of about 1,000 carats. The diamond will have a purity of 10 ppm which is equal to the best investment grade diamonds and suitable for laser and infrared windows. Higher purity levels may be obtained using specially grown pyrolytic graphite or high purity lamp black powder. However the use of powder will substantially reduce the weight of the diamond crystal. The diamond can also be grown as semiconductor material by doping with boron or nitrogen.

There are five key aspects of this proposed design that set it apart from previous approaches. (1) Placement of the working chamber, high pressure piston and cylinder, inside of a secondary high pressure piston and cylinder. (2) Employment of cemented carbides (cermets) under hydrostatic pressure for high strength. (3) The replacement of cobalt in the cermet, as the binding metal, with rhenium which provides superior strength, a high melting temperature and an unusually high bulk modulus of elasticity. (4) The use of argon as the pressurizing fluid. Argon is chemically inert, the atoms are too large to displace carbon in the diamond lattice and it is a liquid at temperatures above 1500 K at the chamber operating pressure. (5) Diamond windows to transmit optical signals for control and diagnostics and to admit the laser beam used for heating.

Heat losses will be held to acceptable levels by inserting the graphite in a clean tungsten "can" to get a low emissive surface and coating the cermet surfaces with highly reflective gold metal. Argon is a poor conductor of heat at the operating conditions, which will both reduce the operating temperature of the surrounding pressure vessels and make the process controllable.

The diamond will be grown using the following process. The graphite will first be given a vacuum bake out to eliminate contaminants. The graphite will be sealed in a tungsten foil "can" evacuated. The graphite will be placed in the pressure chamber and the chamber will be evacuated to eliminate air. The containment vessels will then be pressurized to 1.38 G Pa (200,000 psi) with argon. The argon will be filled from one end and frozen from the opposite end using chilled water to remove the heat energy. All of the void regions will be filled with solid argon at a pressure of 1.38 G Pa. The inner containment vessel will then be pressurized to 10 G Pa by moving one piston inward. The secondary containment vessel will be held at 2.9 G Pa by monitoring the pressure and bleeding off the argon, as needed, to hold the desired 2.9 G Pa. When the desired chamber pressures have been reached, a three kW infrared laser beam will be passed through a diamond window to affect internal heating. The laser beam will melt a hole in the metal can, then pass through a diamond seed crystal and be absorbed in the adjacent graphite. It is expected that the resulting combination of controlled hydrostatic pressure and laser induced temperature will

1

provide the conditions necessary to produce a single crystal diamond. In making the conversion from graphite to diamond, the carbon volume will be reduced to two thirds of the original volume. This is a potential problem area that must be addressed experimentally.

The consulting services of Dr. Francis Bundy and Bill Newhall have been obtained for a review of this work and for assistance in future hardware work. Bundy headed the diamond growth research for General Electric for over 25 years, has much experience with both success and failures and is the source of much of our present knowledge of high pressure and temperature growth of diamond.

Bill Newhall is a brilliant, experienced design engineer in the area of high pressure equipment and CEO of Harwood Engineering. When Bridgman retired from Harvard, Newhall hired his research workers and thus brought much of Bridgman's expertise into the commercial market. Bill has 43 patents on the design of high pressure equipment. His patented designs are on every piece of high pressure equipment the author has seen. Harwood Engineering supplies a wide variety of equipment including gas handling systems, valves and plumbing with industrial ratings of 1.4 G Pa service.

Bundy and Newhall have raised some troublesome, but resolvable questions which have been addressed in this study. The original planned design used a secondary pressure vessel cylinder made of high quality, fracture tough precipitation hardening stainless steel which would be stressed to 50 % of it's yield strength. Bundy used a precipitation hardened steel cylinder on the belt press which failed after three days of loading at a level of 75 % of yield. Harwood Engineering has expressed similar concerns based on their experience. Based on this input, the cylinder will be made of heat treated 5160 and 4340 steel which are interference fit together. Both men have used 4340 extensively with good results.

Bundy once used an inner cylinder of WC-20Co cermet in the belt apparatus. The cermet failed after a day of use. He attributes the failure to (1) the tensile strength of cermets being a short time property (viscoelastic ?) and (2) the large mismatch in the modulus of elasticity between steel and cermets. The author expects that it is not a viscoelastic problem but rather a design problem. Bundy indicates that the GE scientists highly stressed the cermets, to 90 % or more of the yield stress. The mismatch in the moduli of elasticity would cause a disproportionate share of the operational loads to be carried by the cermet. It appears that they did not have a strong preload in compression on the cermet. If they did not properly account for the differences in the moduli of elasticity, one would reasonably expect a failure. If there was a modest flaw or stress concentration in the cermet, it would also cause a failure. Bundy was aware of the Bridgman experiment where he measured a superposed tensile strength in cermet equal to twice the compressive load. This result may have been used without a full understanding which could have resulted in the failure. We plan to use the inner cermet cylinder as originally planned. If it fails, we will increase the confining pressure and reduce the tensile stress until a successful solution is obtained.

BACKGROUND: The concept of growing large single crystal diamonds is to place a large quantity of carbon in the form of graphite in an environment of high pressure and temperature,

2

such that the graphite is driven into the diamond region of the phase diagram. Since diamond is the stable form, with time, the carbon will change it's lattice from graphite to diamond. The reality of the problem is not so simple because the carbon atom must have sufficient energy to break the hexagonal graphite bonds before it can reattach itself in the face centered cubic lattice of diamond. The activation energy required for the conversion is given by Burns & Davies1 as 728 k J per gm mole. This is approximately equal to the energy required to vaporize graphite. Zhang et.al.2 give the activation energy as 120 k J per gm mole which is approximately the energy required to melt carbon. The energy required to make the transformation is the difference between the activation energy and the Gibbs free energy. The Gibbs free energy is generally four or more orders of magnitude smaller than the activation energy, so the activation energy is a very large barrier that must be crossed.

In the mid 1950s, Tracy Hall3 invented the belt press apparatus while an employee at General Electric (GE), see Figure (1). He worked for a pressure of 13 G Pa and he computed that he needed a temperature in excess of 3000 K to cross the energy barrier. He surrounded the graphite with a soft mineral which acted as an electrical and thermal insulator and a pressure transmitting medium. The graphite was heated by discharging a capacitor bank through the graphite to obtain resistive heating to the required temperature for time of order milliseconds. He obtained diamond, but it was in the form of small cubes suitable for use as an abrasive. Many investigators in the field of

3

synthesizing diamond believe that a Swedish group was first, however Hall published his results and the Swedes did not. The Swedish group invented a cubic apparatus which had six hydraulic cylinders pushing each of the six faces of a cube together to obtain the needed pressure. The patent rights to this press were purchased by DeBeers. A recent improvement is holding one face stationary, using cam surfaces to move four of the faces and a single hydraulic cylinder to drive the system. The cubic apparatus is the more efficient of the two and is widely used in the production of industrial diamond today. Hall soon learned that one could reduce the temperature and pressure by a factor of two by including a catalyst. This is of great importance because it allows operation of the WC-3Co pistons at a stress below the normal yield value of 5.8 G Pa. Many companies are commercially synthesizing industrial diamond using the above concepts. The DeBeers cartel is the largest producer. The synthesized diamond is superior to natural diamond by all scientific measures and as such fetches a higher market price.

In 19864 a Japanese group demonstrated that diamond could be grown at pressures below one bar, in the presence of hydrogen as long as the activation energy was provided. This has led to rapid growth in synthesizing chemical vapor deposition (CVD) diamond. CVD diamond is a

4

polycrystalline film which can be grown to mm thickness with sufficient time. This knowledge has been used by others to grow industrial diamond. Scientists at two U. S. National laboratories5, 6 demonstrated growth of diamond crystals using the conditions produced by a detonation shock.

The GE scientists concluded that hydrostatic pressure would be required to grow "large" single crystal diamonds, after a decade of research. They were limited to pressure of about five G Pa by the strength of the piston and limited to temperatures in the area of 2000 K. These conditions are not sufficient to directly convert graphite to diamond. The solution was to use a liquid metal to dissolve the graphite and thus avoid the "activation energy" problem. By keeping the modest temperature and pressure combination in the diamond region of the carbon phase diagram, a single diamond crystal could be grown on a seed crystal. Today there is widespread research and commercial production of "large" single crystal diamonds using the catalyst process. The world record diamond grown by this process is approximately 17 carats in weight7. At the present time, these diamonds cost more to grow than the market price of good quality natural diamond. The commercial market for synthesized single crystal diamond is single point diamond cutting tools. These diamonds fetch a premium price, as the synthesized diamonds are sharper, stronger and last longer than the best natural diamonds. Major participants in this technology include DeBeers, Sumitomo Electric and Mashushita Electric. The operating temperatures are slowly increasing with time in the hope that the growth rate will rise appreciably. If they can get a few more hundred degrees of temperature, they can eliminate the metal solvent and go for direct conversion of graphite to diamond.

The only true piston and cylinder containment vessel used to successfully grow diamond, that the author is aware of, was made by George Kennedy8 at UCLA under contract to ONR. His operating conditions were 6 G Pa (5.3-5.8 actual performance) and 1523 K. This is in the diamond region of the carbon phase diagram, but the temperature is far too low to provide the required activation energy. He interspersed invar with the graphite to act as a catalyst. The system produced industrial diamond just like the GE process. The writer does not have ready access to all of the Kennedy reports but persons that did follow his work, such as Bundy et. al.9 uniformly report that Kennedy "broke a lot of carbide" and had piston failures which "welded" the piston to the cylinder.

Since this study proposes using a piston and cylinder containment vessel, a bit of analysis is of value to differentiate the two designs. Kennedy's design was a simple cylinder and single piston. He reported that 5.6 G Pa pressure is required to make diamond. He also reported that the cermet crushes at a compressive stress of 4.8 to 5.5 G Pa10 which puts his operating stress at or above his highest measured strength. He observed that the failure was in shear with the break being along a 45 degree angle to the axis of symmetry. He reasoned that if the piston was too short to allow a 45 degree break, that he would pick up additional strength. He succeeded in making the system work while breaking a lot of WC-Co parts. Schwarzkoff et. al.11 report a cermet compressive strength of 5.8 G Pa . If the contacting surfaces between the piston and cylinder had a low coefficient of friction, the axially loaded piston would expand radially, based on Poisson's ratio, and the resultant radial force generated by the interference with the cylinder wall would provide additional compressive strength to the piston as explained in Appendix A. If there was inadequate radial force, the piston could be expected to suffer compression failure and the axial force would drive

5

the piston into the sides of the cylinder. This appears to be the case because it was found that the failed piston had to be machined out of the cylinder. Some of these conclusions are influenced by discussions and advice received from Francis Bundy who had first hand knowledge of Kennedy's work and provided technical assistance. Bundy suggested that the cylinders and pistons be coated with "Teflon" to provide low friction surfaces and to carefully provide the proper clearance between the piston and cylinder. Kennedy found that the oxygen in air reacted with the carbon to form carbon dioxide which appeared as gas bubbles in the diamond giving it a milky appearance. He added powdered aluminum as an oxygen getter which eliminated the carbon dioxide problem. The resulting diamonds were "clear", which the author understands to mean "not opaque or translucent", and yellow in color. The yellow color indicates that some nitrogen was included as atomic nitrogen in the diamond lattice replacing carbon atoms. The yellow diamond is N type semiconductor material. This result is in contrast to the CVD technology which currently is experiencing trouble getting N type doping.

Many key elements of this report were developed prior to this contract and are company proprietary information. The following items are identified, however the author should be consulted before any part of this paper is released. The use of argon as the fluid medium and the use of solid argon to facilitate obtaining the desired ten G Pa chamber with minimal seal problems and piston stroke length. The use of WC-Re in lieu of WC-Co is proprietary information. The unusually high tensile strength of WC in the presence of biaxial compression is a key element of the design, however the mathematical modeling of increase in strength was performed as a part of this contract. It is strongly recommended that this information not be released because it will allow competitors to rapidly advance in the large crystal diamond field (once they know that WC can hold more than 5.8 G Pa, the current accepted limitation). A few of the international competitors have more than a score of presses of five kilo-tons or greater capacity that could quickly be put into service on this technology. At the present time, Clay's does not plan to patent the technology as the patents educate the competition, few countries adhere to patent law and those that do make exceptions when they feel that it is in their best financial interests to do so.

PHYSICS OF DIAMOND GROWTH: A widely accepted pressure vs. temperature phase diagram for carbon is illustrated in figure (2)12. It might be expected that if one has carbon at pressure and temperature conditions that place it in the diamond or graphite region of the phase diagram, that the carbon will have the corresponding lattice. This is not the case. The hexagon rings of graphite are formed with strong double co-valent bonds of carbon as are all of the face centered cubic bonds of diamond. In order to make the transition from one lattice to the other, the energy must be sufficiently high to break the double covalent bonds so that the carbon atoms can be reformed in the new lattice. The activation energy is variously reported to be 120 -728 k J per gm mole. This requires a temperature of order 3000 K. An exact temperature can not be specified as the energy of individual atoms will approximate a Gaussian distribution, so one has a statistical probability that a particular atom has sufficient energy to make the transition. This is illustrated in figure (3) taken from Zhang et. al.13 which illustrates their computed probability of transition in the two regions. The metal catalyst work is in the area of 10-6 and the proposed work is in the area of 10-3. Bundy14 clearly showed how graphite can be directly converted to diamond by using the proper temperature and pressure combination. The conversion is rapid at a pressure of 12.5 G Pa and temperature of 3400 K. By "rapid" he means in a time interval of order one millisecond. The

6

temperature-pressure combinations that Bundy experimentally demonstrated produced rapid conversion to diamond is illustrated in figure (2). Bundy's experimental work was performed on the "belt" apparatus which may conceal the true pressure requirement. The pistons are made of Carboloy grade 999 which in generic terms is a cermet made from fine particles of WC that are cemented with cobalt metal at a weight proportion of 97 % WC and 3 % Co. The compressive strength of the cermet is 5.8 G Pa as indicated previously, while Bundy reported pressures up to 16 G Pa. An elevated compressive stress is achieved by making the piston ends in the shape of truncated cones. The pressure acting on the conical surface provides a compressive radial stress on the piston, which increases the compressive strength as explained in Appendix A. The word "pressure" used in conjunction with the belt press really implies a rheological stress, not a hydrostatic pressure. All information available to the author indicates that the reported pressures were actually achieved within experimental accuracy of measurement. But, when the conversion is made from graphite to diamond, the carbon volume is reduced to 67 % of the graphite volume. The conical piston ends and the thin nickel foil seal do not permit the piston to move forward to replace the lost volume, so as the conversion to diamond proceeds, the compressive stress level is reduced. Bundy reports rapid "partial" conversion and rapid "total" conversion of the graphite to diamond. The author believes that the conditions for "partial" conversion would be "total" conversion for conditions of steady state

hydrostatic pressure. Bundy and his co-workers made independent measurements of the compressive stress level in the region of the graphite and the writer believes the reported starting

7

values are valid. Bundy was able to rapidly convert diamond to graphite or graphite to diamond by a small change in the pressure. This experimental data confirms the validity of the carbon phase diagram and the activation energy. Bundy noted that the temperature required for the activation energy was below the melting temperature of carbon, which favors the 120 k J reported by Zhang.

In summary, there have been many approaches used to successfully produce small diamonds. All of these methods have been limited from making large, high purity diamonds by three inadequacies. (1) insufficient, sustained hydrostatic pressure, (2) insufficient temperature to overcome the activation energy and (3) the presence of catalysts which produce impurities and inclusions. The methodology proposed by the author overcomes these obstacles and will result in the technology to produce diamonds many times larger and purer than previously produced.

Proposed Process: The proposed process uses hydrostatic pressure and a temperature sufficient to make the direct conversion to diamond. The containment vessel is a high pressure cermet piston and cylinder vessel located inside of a secondary high pressure piston and cylinder vessel. The hydrostatic pressure of the secondary pressure vessel is designed to increase the strength of the inner cermet components. This compares favorably with Kennedy's work which operated with a factor of safety a little above or below one depending on the dynamics of the interference fit between the piston and cylinder, under load. If one believes that the short piston increased the cermet strength by a factor of the square root of two, he had a factor of safety of 1.25. In view of the amount of broken carbide, the author believes the former number.

8

The proposed concept is to pressurize the graphite and diamond to a steady value of 10 G Pa with liquid argon. Both chambers will be filled with argon to a pressure of 1.38 G Pa and the argon frozen, using a pump-concentrator unit. The 5 kilo-ton press force will then be used to advance the piston forward to compress the argon and raise the pressure in both chambers. The argon in the secondary chamber will be bled off, as needed, to hold the secondary chamber at 2.9 G Pa. Once the 10 G Pa main chamber pressure is achieved, the graphite will be heated using a variable power three kW laser beam that is passed through a diamond window in the end of the lower piston. The temperature will be monitored using a two frequency optical pyrometer and the same beam path as the laser beam. Diamond has a very small emissivity in the region of wavelength monitored by the optical pyrometer, so the measured temperature will be dominated by the

9

temperature of the graphite at the diamond-graphite interface. A diamond window in the end of the second piston will be used to monitor the chamber pressure and the temperature of the top of the graphite rod. The temperature measurement will be used to determine when the graphite has been "fully" converted to diamond and the laser beam should be turned off. A third diamond window will be placed in the end of a large piston to measure and control the secondary chamber pressure.

Impurities: Obtaining pure diamond will require eliminating nitrogen and boron from the system to ensure that all atoms in the diamond lattice are carbon. The initial plan is to perform a vacuum bakeout of the graphite rod at a temperature in excess of 2000 K. The graphite rod will then be placed in a vacuum tight, tungsten foil "can" to be assembled and welded in a vacuum chamber. After the graphite package is placed in the pressure vessel, the pressure vessel will be vacuum purged and back filled with argon. When the vessel reaches a pressure of 10 G Pa, a three kW laser beam will be used to melt a hole through the cover of the tungsten can so that heat can be applied directly to the graphite. The argon, at a temperature of 1500 K or higher, will be in liquid form and will provide a hydrostatic pressure. The close spacing of the carbon atoms in cubic diamond will not permit an atom as large as argon to displace a carbon atom. If the conversion to diamond is done rapidly, bubbles of argon gas will be trapped as inclusions. If the conversion process proceeds slowly, the argon will be pushed aside and excluded from the crystal.

This is the process that Haller15 uses in purifying germanium crystals to a purity of a few parts in 1015. He slowly passes a melt zone through a large cylindrical germanium crystal. The slow recrystallination process allows the impurities to be excluded from the crystal and be pushed forward through the melt zone to the end of the crystal. While a remelt process on a diamond crystal is expected to be difficult because of it's high thermal conductivity, the effect should be achieved by growing the crystal slowly. It may be possible to do a moving melt zone purification, as the thermal conductivity of diamond at temperatures in the region of 2500 K may not be large. This possibility will be investigated when large crystals and facilities are available.

For special applications it is desirable to grow diamond doped with nitrogen and or boron to obtain color and optical or electrical properties. This can be achieved by using high purity lamp black (graphite) which is mixed with powdered boron, for blue or green, or left pure for yellow. The tungsten can will be filled and packed with the lamp black, then given the vacuum bakeout and sealed. For blue diamond, the pressure vessel will be backfilled with argon, as above. For yellow or green diamond, the vessel will be backfilled with nitrogen to a pressure of approximately 0.3 G Pa then further filled to the design 1.38 G Pa with argon. The partial pressure of nitrogen will control the intensity of the yellow color.

The graphite material package requires special handling and processing to obtain high purity diamond. There are two elements, boron and nitrogen, that can replace carbon in the diamond lattice. Hydrogen is also present inside, but not part of, the lattice in natural diamond. All other elements and compounds are inclusions in diamond. Boron impurities make diamond a P type semiconductor and give diamond a blue color. Nitrogen, as atomic nitrogen, make diamond a N type semiconductor and give diamond a yellow color. Most natural diamonds contain non-trivial quantities of inclusions of nitrogen gas in the form of N2 which weakens the diamond but does not

10

color or reduce it's electrical resistivity. Natural diamonds are sorted under the illumination of an ultraviolet light. Diamonds containing atomic nitrogen fluoresce yellow and are classified as "type I" diamond. Diamonds that do not fluoresce yellow are called "type II" diamond. Type IIa diamond has few impurities and type IIb diamond has substitutional boron in the diamond lattice but negligible amounts of nitrogen. These diamonds are classified by the color of the fluorescence even though they may appear to be clear under white light. Kennedy's work clearly demonstrated substitutional nitrogen in diamond. He saw the problem of oxygen attacking the carbon and had a procedure of vacuum bakeout of the graphite followed by a nitrogen backfill. There was a second vacuum purge followed by a backfill of nitrogen. He reported that his small diamond cubes were yellow in color. His results are consistent with our present knowledge of the chemistry and physics of diamond.

Diamond Windows: Diamond windows will be used for instrumentation of the high pressure apparatus and insertion of laser beams. Two varieties of variable power, pulsed lasers could be used. A three kW neodymium laser that carries the beam in a 0.6 mm optical fiber and has a termination lens that is attached directly to the piston would be ideal. The laser beam would always be properly pointed and collimated. However, this laser is very expensive costing about twice as much as a carbon dioxide laser of the same power. The carbon dioxide laser is the second choice, but the beam must be sent through the air. Each time the pressure apparatus is opened, one must ensure that it is replaced in the press in the same location and orientation each time. The laser beam path will need to be carefully adjusted to get the beam to reach the diamond window without hitting the cermet wall first. A 45 degree mirror within the piston will turn the beam up to the diamond window and just above the window will be a diamond seed crystal pressed against the graphite rod. The laser beam is expected to have a 1.5 mm diameter at the focus point which will be at the diamond window. The window must pass a 1.06 micron wavelength beam for the neodymium laser and 10.6 micron wavelength for the carbon dioxide laser. All diamonds that appear to be optically clear to the eye will freely pass the 1.06 micron beam with little absorption, as the wavelength is between the two and three phonon absorption energy. The 10.06 micron beam is below the one phonon absorption band, but there is continuous absorption into the 10.06 region, so the window must be a type IIa diamond. Type IIa diamond is free from easily detected atomic nitrogen and boron and as such can not absorb one phonon energy16. The type IIa diamond would provide superior performance for either laser, but is required for the longer wavelength. An illustration of experimentally measured infrared absorption of diamond is illustrated in figure (4).

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The diamond window must be strong enough to support the 10 G Pa chamber pressure on one side and ambient pressure on the other side. The compressive strength of diamond is so large that it is not of concern It is well known that a natural diamond can suffer lattice slip at high temperature and shear stress. Lattice slip is demonstrated at temperatures above 1900 K. This is to be expected considering that the diamond lattice softening temperature is 2200 K. The maximum predicted operating temperature of the diamond window is 525 K . Ruoff17 predicts a lattice slip strength of 35 G Pa for type IIa diamond at 300 K. At a temperature of 1337 K the measured strength is 0.7 and 1.5 G Pa for type Ib and Ia diamond respectively. The author is using an allowable stress of 0.63 G Pa on type Ia and IIa diamonds.

Materials for Pressure Vessel: The pressure vessel that holds a 10 G Pa pressure must have pistons that can carry a 10 G Pa compressive stress and a cylinder that can hold a 10 G Pa compressive radial stress on the inside surface and a greater than 10 G Pa tensile hoop stress on the same inside surface. The piston and cylinder materials must be able to sustain this load with a substantial factor of safety. A survey of possible materials revealed no material capable of handling these stresses.

Cemented tungsten carbide appears to be the best material for use. If the pistons are made of WC-3Co they will have a failure compressive stress of 5.8 G Pa. If the cylinder is made of WC-20Co it may have a tensile strength as high as 3.7 G Pa. The tensile strength can be improved by pre-stressing the cylinder in compression. The usual method of pre-stressing a cylinder is to interference fit one or more cylinders on the outside of the cylinder. Kennedy8 reported that a confining pressure of 1.7 G Pa would raise the tensile strength to 5.4 G Pa. He put insulation on the inside surface of the cylinder so that there would be no degradation due to elevated operating temperatures. This explains why all of the vessels using hydrostatic pressure to grow single crystal diamond are operated at five G Pa. That is the material limitation.

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If a large hydrostatic confining pressure is placed on a cermet it will increase the ductility and elastic limit of the Co18. Bridgman also noted a three fold increase in the flexure strength of WC-3Co under a hydrostatic pressure of 2.9 G Pa which he said explained how his cermet cylinder held an internal pressure of 10 G Pa. In a later paper19 Bridgman measured a pure tensile strength in WC-3Co of 2.75 G Pa under a confining pressure of 2.7 G Pa. He expressed considerable surprise at the unexpected high tensile strength, repeated the test before he believed the results and gave no explanation for the phenomena.

Part of the work done under this contract was to model the cermet to see if the unusual strength properties could be explained from basic physical principles. The cermet analysis is Appendix A of this report. The tensile strength is given by...

(1)

where is the tensile yield strength of the cermet, is the yield shear stress of the binder metal and is the volume fraction of the cermet that is metal. Note that metal fraction is normally given as the fraction or percent of the total weight. In order to be able to change the binder metal, the modeling equation uses "volume" fraction. If the cermet is used at one atmosphere, there is also a coefficient to account for the fraction volume of gas in the cermet. If the confining pressure is well above the tensile strength of the binder metal, the gas bubbles will be compressed to a negligible volume and the coefficient will be one as assumed in equation (1). The shear yield strength of the binder metal is given by...

(2)

where is the reported tensile strength of the binder metal at one atmosphere of pressure and the design temperature, the other sigmas are the two remaining orthogonal stresses and is the coefficient of friction. Note that a compressive stress has a negative value. The compressive strength of a cylindrical piston with a confining pressure on the cylindrical wall is given by...

(3)

These equations accurately reflect the performance of manufactured cermets as well as the measured results of Bridgman. They are used for the design of the high pressure apparatus and the computation of the factors of safety in this report.

Argon: Argon is used to convey the hydrostatic pressure because it is chemically inert, the atoms are too large to displace a carbon atom in the diamond lattice and argon is a very poor conductor of heat at the design operating pressure. Halving the volume of argon by compression increases the pressure by a factor of 1.985 for a gas and 19.438 for a solid or liquid. The available press has an available travel of 150 mm, so one needs to use the piston stroke to compress solid or liquid

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argon, not the gas phase. The author plans to modify the hydraulic cylinder on the press to get a 200 mm stroke. A pump and concentrator system will be purchased that will pump argon gas from ordinary compressed gas bottles to a pressure of 1.38 G Pa. Under ordinary operating conditions, the pump stalls out at a pressure of 1.28 G Pa due to the argon freezing to a solid in the lines. The lines will be heated or the pumping rate so that the design pressure can be reached. The argon pumping system is a standard, "turn key" product sold by Harwood Engineering. The system, including tubing and valves are rated for 1.38 G Pa service.

HARDWARE DESIGN: The physics of the conversion of graphite to diamond is well established and experimentally confirmed by many scientists throughout the world. Many organizations have spent scores of millions of dollars in the quest of large, inexpensive diamonds. They all believe that they are limited to hydrostatic pressures below the compressive yield strength of WC-3Co, as nearly as the writer can ascertain from their activities and reports. The real question is; can the hardware be made that can reliably achieve the required pressure and temperature for controlled, direct conversion of graphite to diamond? A scale drawing of the press and pressure vessel is illustrated in figure (5).

Containment Vessel Design: The containment vessel is a high pressure and temperature cermet piston and cylinder vessel that is inside of a secondary pressure vessel that is filled with argon to an initial pressure of 1.38 G Pa and compressed to 2.9 G Pa. A general arrangement scale drawing of a cross section of the apparatus is illustrated in figure (6). The cylinder is made of WC-37Re which has a volume fraction of metal of 0.306 which corresponds to WC-20Co. The pistons are made of WC-6.8Re which have a metal volume fraction of 0.0515 and corresponds to WC-3Co. The reason for selecting Re as the binder metal over Co can best be illustrated by listing some of the properties and noting that rhenium has the highest shear strength of any known material20. Rhenium, compared to cobalt for the same conditions, gives half the linear strain, one fortieth of the bulk compression, three times the strength at room temperature and it holds it's strength better than cobalt at elevated temperatures. In addition, under ordinary conditions, ReC is not made. Other high temperature metals can not be used as they combine chemically with the tungsten carbide. For example, tungsten has the highest melting temperature but where tungsten bicarbide is substantially inferior to the monocarbide. The steel end cushions will be made of 4340

14

15

E K TmeltG Pa G Pa G Pa G Pa K

Re (anneal.) 460 7,667 0.49 2.41 1.0 3,453Co 211 183 0.287 0.8-0.875 0.1 1,768

Table (1). Selected Properties of Rhenium and Cobalt.

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steel. The secondary cylinder will be made of a 5160 steel cylinder that is interference fit into a 4043 steel cylinder. A fourth cylinder will be placed around the others as a safety sleeve and to provide a means of lifting the containment vessel.

Cylinders: The calculations required to determine the dimensions and tolerances for manufacturing the cylinders are extensive, so are not included here. Calculations are presented which reveal the maximum stresses involved in the design, how the stresses are handled and the factors of safety involved. The stresses in cylinders are computed from the Lamé equations following the notation of Roark21 where an "a" subscript is data referenced on the outer wall and a "b" subscript to the inner wall. The dimensions and loads for the cermet cylinder are; G Pa., G Pa., G Pa., Da = 120 mm, Db = 40 mm, G Pa., =0.49 and

.

(4)

G Pa (5)

G Pa (6)

G Pa (7)

(8)

Which gives a factor of safety of two, a generous value for pressures at this level.

The secondary cylinder could also be made of cermet with a confining pressure. However, this would reduce the working chamber to an unacceptably small bore. No ordinary metal could be used for the secondary chamber cylinder, so two cylinders shall be used which are interference fit together. The interference fit on the inner cylinder is 0.3 percent of the diameter. The second cylinder inside diameter is 122 mm, outer diameter 361 mm, Young's modulus of 200 G Pa and Poisson's ratio of 0.29. The calculations are as follows...

(9)

The change in the outer diameter or radius under the pressure of the interference fit is given by...

(10)

Solving equation (10) for the pressure of the interference fit, one obtains...

17

M Pa (11)

The compressive hoop stress on the inside surface due to the interference fit is ...

M Pa (12)

The compressive stress on the outside surface is...

M Pa (13)

The change in the outside diameter was specified as 0.003 times 361 mm which is minus 1.08 mm. The change in the inside radius is...

mm (14)

The third cylinder has an inside diameter of 358.1 mm, an outside diameter of 864 mm, Young's modulus of 200 G Pa and a Poisson's ratio of 0.29. The inside hoop stress is...

M Pa (15)

The outside hoop stress is...

M Pa (16)

The inside change of diameter is given by...

mm (17)

The outside change in diameter is given by...

mm (18)

When the secondary chamber is pressurized, the two interference fit cylinders will act as a single unit. The pair have an inside diameter of 121.15 mm an outside diameter of 865.1 mm with R = 0.0196. The design argon pressure will superimpose a hoop stress on the inside surface of ...

18

G Pa (19)

The superimposed hoop stress imposed at the interference, D = 360 mm, is ...

G Pa (20)

The superimposed hoop stress on the outside wall is ...

G Pa (21)

The net hoop stress on the second cylinder's inside and outside surfaces are and M Pa respectively. The extreme

stresses experienced by the second cylinder are -1,404 M Pa and +1,404 M Pa. The 0.2 % yield strength of the metal is 1,793 M Pa which provides a factor of safety of 1.28.

The net hoop stress on the third cylinder's inside and outside surfaces are and M Pa.. The 0.2 % yield strength of the metal is 1,675 M Pa which provides a minimum factor of safety of 1.34.

Prior to writing this report, this cylinder was planned to be fabricated from VascoMax steel which is a particularly good, fracture tough, precipitation hardening stainless steel. The generic name is "maraging steel". Bundy tried it as a tensile member in the GE belt press, Kennedy tried it as a pressure cylinder and Harwood Engineering tried it as a hydraulic cylinder. All three report that the steel suffered unexpected catastrophic failure. It appears that the failure is due to water aggressively attacking the steel, a phenomenon that is not widely discussed. Bundy and Newhall use 4340 steel extensively.

Pistons: The small diameter piston is analyzed using equations (1-3). In the vicinity of the diamond window and seal, and for a temperature of 525 K, the strength is given by...

G Pa (22)

G Pa (23)

G Pa (24)

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The axial stress in the small piston is approximately ten G Pa which gives a factor of safety of four. A finite element analysis was performed on the piston as there is a "square" hole in the center of the end to hold the diamond window and a short length of reduced diameter at the end with a 45 degree conical chamfer to the full diameter to accommodate the piston seal. The piston was modeled with sharp corners, while the actual piston will have rounded corners to minimize stress concentrations. The computed maximum and minimum principle stresses are +15.06 and -13.70 G Pa. The factors of safety are 0.53 and 2.6 respectively. The space for the diamond window needs to be carefully modeled to determine how to properly radius the corners and probably change from a square cross section to a "round" one. Behind the seal, the radial pressure changes from 10 to 2.7 G Pa. Making these changes in equations (21-22) one obtains a compressive strength of 25.38 G Pa which provides a factor of safety of 2.5 which is acceptable.

The end of the large piston is made of the same WC-6.8Re cermet. The temperature of the piston end will be below 400 K for a Re tensile strength of 2.3 G Pa. The end and side pressure, to the

seal, is 2.7 G Pa. The shear yield strength is G Pa. The compressive yield strength is...

G Pa (25)

which will handle the 10 G Pa compressive stress of the small diameter piston entering the larger piston. There is no confining pressure past the seal. The yield shear stress is

G Pa. The corresponding compressive yield stress is 8.867 G Pa. This is a critical design area as the ten G Pa of the small diameter is just getting distributed. The shape must start spreading the load prior to entering the larger piston. The average compressive stress on the piston is the total piston force divided by the area which is given by

G Pa (26)

The factor of safety is 2.29. The cermet will then be increased in diameter to 250 mm in diameter where the force is carried by a steel cylinder. The compressive stress at this diameter is 1.02 G Pa which can be carried by the steel.

Piston-Cylinder Clearance: It is necessary to compute and machine an air gap between the pistons and cylinder which allows the pistons to expand outward under axial load without getting an interference fit which would cause binding between the piston and cylinder. The first problem is the large piston. Two fits must be computed; the WC-6.8Re piston inside of the unpressurized steel cylinder and inside the pressurized steel cylinder. The increase in diameter for the two piston sections are...

mm (27)

mm (28)

20

The diameter increase of the pressurized portion of the steel cylinder is given by...

mm (29)

The steel expands under pressure more than the piston , so the clearance criterion is on the unpressurized cylinder. Make the piston undersized by 0.40 mm for piston expansion. The steel cylinder inside diameter will be 121.15 mm and the piston 120.75 mm. In the actual design, the piston will be made smaller to allow for machining tolerances. Under pressure, the gap between the piston and cylinder will be 0.40 mm machined plus 0.140 mm the piston shrinks plus 2.173 mm that the steel expands. The triangular cross section back up ring on the piston seal must bridge the 1.36 mm radial gap.

The small piston can be analyzed in a similar manner. The axial stress is 10 G Pa and the radial stress is either 2.7 or 10 G Pa. Using equation (26), the diameter change of the small piston is...

mm (30)

mm (31)

In the high pressure region, the cermet cylinder increases in diameter according to...

mm (32)

In the low pressure region, all pressures are the same, so the inside diameter decreases as...

mm (33)

In the low pressure region the piston grows 0.127 mm and the cylinder shrinks -0.0034 mm for an interference of 0.130 mm. In the high pressure region the piston shrinks -0.294 mm and the cylinder grows 0.799 mm for a radial air gap of 0.547 mm plus the initial gap plus the tolerance allowance. Make the cermet cylinder 40.2 mm in diameter and the piston 40 mm in diameter. The back up ring with triangular cross section must bridge the radial air gap of 0.65 mm.

Piston Seals: The piston seal design concept is illustrated in figure (7). It is based on a patent of Bill Newhall. This design is widely used in high pressure work. The metal ring with a triangular cross section is usually made of brass. The ring should have a small modulus of elasticity so that

21

it can readily expand to fill the gap between the piston and cylinder. A force balance on the ring reveals that the compressive stress on all three sides is the same, if there is no air gap and negligible friction. The maximum shear stress that the metal can hold is given by...

(34)

The small piston design will use an O-ring seal size Parker # 2-219 with a piston shoulder diameter of 33 mm and a back up ring of 3.6 mm on a side and one mm of extra thickness. A radial gap of 0.65 mm will leave a thickness of 3.85 mm to hold the shear stress. The force is the unsupported area times the applied pressure difference. The pressure required to stretch the titanium ring can be approximated as being half the force required to stretch a rectangular ring of twice the cross section. The square of the radius ratios is R = 0.68, E = 110 G Pa and the radius change divided by the initial radius is 0.0325.

G Pa (35)

The equivalent end pressure required to expand the ring is 28 % greater than this as the axial depth is 4.6 mm and the radial depth is 3.6 mm. The equivalent end pressure is 0.538 G Pa. The force on the unsupported area is...

M N (36)

The shear stress is the force divided by the area.

G Pa (37)

The tensile strength of an acceptable brass alloy is 0.7 G Pa and the pressure is ten G Pa. Using equation (32), the shear strength is 6.18 G Pa which provides a factor of safety of 5.52.

22

The large piston seal design will be similar to the one above. The O-ring seal will be Parker size #2-425 The shoulder on the piston will be 113 mm in diameter and the back up ring will be 4.0 mm on a side with an extra two mm of axial depth. The square of the radius ratio is R = 0.872, E = 110 G Pa and the increase in radius divided by the initial radius is 0.0228. The end pressure required to stretch the back up ring is given by...

G Pa (38)

The unsupported force on the back-up ring is...

M N (39)

The shear stress on the back-up ring is given by the force divided by the shear area.

G Pa (40)

The yield shear stress is...

G Pa (41)

The ratio of the two shear stresses gives a factor of safety of 2.6.

Windows: The failure of diamonds as windows in a pressure chamber is by lattice slip. Type IIa diamond is predicted to fail at a shear stress of 35 G Pa at 300 K and common yellow, type Ia, yields at a stress of 1.5 G Pa at a temperature of 1373 K. The writer has little additional information to base the design. The window design is illustrated in figure (8). The design is for a six mm depth and minimal cutting and polishing. The shear stress is given by...

G Pa (42)

This gives a factor of safety of 2.4 for yellow diamond at a temperature of 1373 K. A type IIa diamond will have a greater factor of safety. The window for the laser beam will be a type IIa window and the two windows for measuring pressure and temperature will be type Ia diamond.

23

It is expected that a type Ia window in combination with a 10.6 micron wavelength laser beam would get too hot to function well. The coefficient of absorption is about 1.5 cm-1 for type Ia diamonds. The energy transmitted is given by...

Watts (43)

This means that if one had 3,000 W of energy enter the diamond, 1,780 W would be absorbed in the window. This is not acceptable. If the wavelength was 1.06 microns, the absorbtivity would drop to the region of 0.1 cm-1 which would be acceptable.

An exact analysis has not been done on the temperature of the diamond window passing the laser beam. An estimate of the problem is given below. First, one desires that all of the incident energy enter the window. If the project is hard pressed for funds, the diamond will not be coated. The index of refraction of air is 1.0 and the index of refraction for diamond at 10.6 microns is 2.380522. The fraction of energy reflected off the front face is given by...

(44)

If there is $20k available for giving the diamond a coating of barium fluoride, n = 1.474 at 10.6 microns, the fraction of incident energy reflected will be ...

(45)

24

The fraction of the entering energy that is absorbed by the window in the initial pass is given by...

(46)

The index of refraction of liquid argon at 10 G Pa is 1.4823. The fraction of energy reflected off the second face, as given by equation (37), is...

(47)

It does not seem reasonable to put an anti-reflective coating on the inside face of the diamond as it will be "scrubbed" with a ten G Pa pressure of solid argon. It is doubtful that the coating would remain intact. Of the 5.44 % of energy reflected off the inside face of the window, 4.7% will be absorbed in the return trip through the window. Most of the reflected energy will not hit the 1.5 mm diameter window but rather the Re gasket which is opaque. There will also be incident radiation from the hot graphite/ diamond rod. To minimize reflection, the Re gasket will be coated with gold to give it a low emissive coating. As an approximation, lets assume that all of the reflected and absorbed energy will be absorbed in the window, or ten percent of the power of the laser beam. For a 3,000 Watt laser beam, that is 300 Watts of energy absorbed in the window. The 300 Watts of energy absorbed in the window plus the radiant heat and heat conducted to the end of the piston yields an equilibrium temperature of approximately 525 K. This is an acceptable temperature for strength considerations and an acceptable loss of laser power that does not reach the graphite. If one were using a 1.06 micron laser beam, the results would be about the same except that one could consider using a yellow diamond for the window.

If the diamond is not given an anti-reflective coating, 17 % of the laser energy will be reflected off the front face and 10 % of the remaining energy will be lost which means the (1-0.17)(1.-0.1) = 0.75 of the energy will reach the graphite. If the front face of the diamond is coated, this number rises to 0.90.

Laser Heat: Laser heat of 3 kW is needed to reach the required conversion temperature and to accommodate the heat losses. The laser will be a pulse laser of variable power. The two candidate lasers are carbon dioxide and neodymium. The laser beam will be focused and passed horizontally into the lower cermet piston. A smooth, tapered, small diameter hole will be electrical discharge machined (DM) into the piston to provide a beam path. One hole will be perpendicular to the axis of symmetry and intersecting the center line of the piston. This hole will be machined contiguous to the large end of the piston. A second hole will be machined along the centerline from the large end to the small end. A shallow hole will be milled from the small end of the piston to accommodate the diamond window. The beam path will be wetted with "Liquid Gold" and fired to get a highly reflective, low emissivity coating around the beam path. A shallow hole will be milled in the large end of the piston to accommodate the 45 degree mirror and the water cooling lines for the mirror. The pistons and cylinders will be oriented with the centerline of rotation vertical. The laser beam will be brought into the piston as a horizontal beam and turned upward with a 45 degree mirror at the centerline. The beam will travel upward through the piston

25

and through the window. An optical pyrometer will use the same path for measuring the temperature of the graphite-diamond interface.

Laser heat is very expensive as is the electric bill, but resistive heating is not considered to be a viable solution to providing heat. Resistive heating provides uniform heating throughout the graphite rod. Once the graphite begins the conversion to diamond, the diamond will have a resistivity of order 1015 ohm cm. The electrical current will not be carried by the diamond, so the graphite must be flash heated or one must settle for a diamond of small length. The laser permits controlled heating throughout the entire process.

Pressure measurement: The pressure in the chambers will be measured using the ruby fluorescence technology. Yellow diamond windows in the ends of the pistons will be used for measuring pressure. A small ruby crystal will reside on the window. The ruby will be radiated with a miliwatt blue laser beam which will cause it to fluorescence red. A fiber optic will pick up the fluorescent light and carry it to a spectrometer which will detect the frequency of the two primary lines of fluorescence. The Hertz separation of the two beams determines the pressure on the ruby. The planned operation is for the press to apply it's design force of 44 mega-Newtons force, five thousand tons. The pressure in the secondary chamber will be continuously measured and the argon pressure held to 2.70 G Pa. This will provide a primary chamber pressure of ten G Pa according to the design. The central window will be used to measure the pressure in the primary chamber to ensure that the design pressure is actually being achieved. The secondary chamber could use a Manganin cell to accurately measure the pressure .

Controlling Pressure and Temperature: The pressure in the hydraulic cylinder will be held at 172 M Pa (25,000 psi), the design operating pressure, within the accuracy allowed by the elasticity of the press frame and containment vessel and the volume provided by a single stroke of the concentrator piston. A small amount of argon can be put into the hydraulic system, if necessary, to provide an elastic volume which will provide a small pressure deviation capability. The secondary chamber can be pumped to a pressure of 1.38 G Pa, then compressed with the piston. Remote controlled valves can bleed off the argon, as needed, to hold a constant pressure of 2.7 G Pa. This pressure can be held almost as accurate as it can be measured. A small orifice will be used to limit the rate of argon flow. The argon system will be purchased from Harwood Engineering. It is a standard item that they supply to industry. There is little risk and no new scientific frontiers to push back in this portion of the hardware.

Temperatures will be read with two color optical pyrometers that are accurate to 0.75 percent of full scale, ±26 K, and reproducible to 0.05 percent of full scale, ± 2 K. Two color optical pyrometers are used as one is able to eliminate the effects of the emissivity. The temperature is determined from the difference between the power received of the two channels. If a 10.6 micron laser is used, the thermometers will use wavelengths of 0.7 and 1.08 microns. The optical pyrometer will be purchased with a PID controller. The desired diamond-graphite interface temperature will be set in the controller along with the desired proportional and integral control settings. The PID controller will have a 4 to 20 mA output signal that will control the power of the laser. A "proportional" setting will quickly bring the temperature to near the control setting. A proportional setting always has an off set, so an "integral" setting will be used to slowly bring

26

the temperature to the exact setting. There will be no fast transients except during the initial heat up, which may be done manually, so the "differential" circuit will probably not be used.

A second optical pyrometer will be used to monitor the temperature at the top of the graphite rod. This temperature reading will be used to ensure that no thermal damage is done to the top piston and to turn off the growth process at the completion of graphite conversion.

Graphite Package: Many suppliers offer graphite rods with a guaranteed purity of less than 10 ppm by weight of ash upon burning. They are supplying natural graphite that has been purified by baking in a furnace at 1773 K. We plan to resistively or inductively heat the graphite to 2500 K under environmental pressure of less than 10-6 mm of Hg. This is above the boiling temperature of many contaminants. In particular, it should eliminate water and air, the largest contaminants. The chamber will be back filled with argon, an inert gas. To obtain pure diamond, it would be beneficial to place the graphite into a tungsten can while still under vacuum. Then weld a cover onto the can, while still under vacuum. The financial compromise approach is the argon back fill. The graphite would then be exposed to the air for as little time as is practical. In both cases, a diamond seed crystal is placed on the bottom of the graphite at the center. The laser used for heat is capable of welding the tungsten can and cover. It is primarily a problem of tooling costs and figuring out a reliable welding procedure.

In growing blue diamond an air tight can is needed. The graphite will be a fine powder called "lampblack". Powdered boron will be mixed into the lampblack to get the dopant uniformly dispersed. This powder will have a huge surface area which will be covered with air and small pores for removal of the air. A long, high temperature bake out will be required to remove the oxygen and nitrogen. A back fill of argon in the vacuum chamber would introduce a large quantity of argon within the carbon. It would be best if the argon did not come into contact with the graphite powder until it is compressed to a specific gravity of about 2.5 at a pressure of 10 G Pa. Then when the argon comes into contact with the graphite it will be a dense liquid. If growing fancy green or yellow diamond, back filling the vacuum chamber with pure nitrogen would be beneficial as it is the desired dopant. The pressure chamber will be further filled with nitrogen to achieve the desired yellow color density. In all cases, it is necessary to place a diamond seed crystal on the bottom of the graphite, in the laser beam's path, to ensure that one gets a particular crystal orientation and a good start for a single crystal.

GROWTH PROCESS: The writer believes that growing a nearly perfect crystal requires a diamond-graphite interface that is approximately planer across the entire graphite cylinder with the center preceding the periphery. The growth front must move sufficiently slowly to allow the contaminants to be pushed ahead of the growth surface. Mechanical vibration on a macro scale may enhance the growth process. A 25 mm diameter graphite rod will produce a diamond 21 mm in diameter under ideal conditions. This requires migration of the carbon atoms radially toward the center. In the early stages it will be necessary to use small, natural diamond seed crystals. After diamond synthesized in the laboratory is available, large diameter seed crystals will be used to spread the heat and quickly establish growth across the whole graphite cylinder. The optimum growth temperature and speed can not be determined by other than experimental means. We estimate that a week will be adequate for growing a 1,000 carat diamond. This estimate is

27

influenced by the rate of passing a melt front through a germanium crystal for purification purposes. This estimate may be pessimistic, as the carbon atoms freed from the graphite lattice will have many times the velocity of the germanium atoms which are being melted and frozen. This is due to the high kinetic energy level required to break the graphite bonds and the lower mass of the carbon atom.

The pressure instrumentation will allow the pressure to be measured to order of 10 ppm of the operating pressure. It appears that the temperature can be measured plus or minus about 3,000 ppm of the operating temperature, or within 18 degrees Kelvin of the temperature setting. The limiting factor on controlling temperature and pressure will be the PID controllers which will have four and a half digits of reading capability. It is the writer's experience that the process can be controlled to plus or minus two integers of the least significant digit. For pressure, that means 20-1

of the accuracy of the instrument reading. For temperature, it means plus or minus two degrees Kelvin on the controller with a reading reproducibility of nine degrees Kelvin, so the controller would appear to have more accuracy than it actually has. If the temperature controller were a three and a half digit controller, then the controller would be the limiting component, controlling temperature to an accuracy of half the reading ability. In the final analysis, the growing of the crystals will be an art form that must be learned experimentally.

SUMMARY: The physics of growing diamond from graphite is well established both theoretically and experimentally. The experimental results are in close agreement with theoretical calculations. High pressure equipment which can directly convert graphite to diamond use a soft mineral and rheological stress to reach the required pressures. The mineral with friction between particles ensures that only small crystals will be grown. In converting from graphite to diamond, one third of the carbon volume is lost. As a local region starts the transition to diamond, the loss of volume coupled with friction provide a loss of high compressive stress contiguous to the diamond crystal which terminates the conversion process. An activation energy estimated at 120 k J per gram mole is required to break the graphite hex bonds so that the atoms can be reformed as cubic diamond. The high activation energy requires environmental conditions of order ten G Pa pressure and 3,000 K temperature. Early in the successful diamond growth research, it was discovered that the required pressure and temperature could be reduced by a factor of two by including metal in the graphite which dissolve graphite and accept it as an alloying metal. This concept was applied to using liquid metal as a solvent and a hydrostatic pressure fluid to grow "large", single crystal diamonds. This growth process is called diamond catalyst growth. The largest diamond grown by this process weighs 17 carats. These diamonds currently cost more to grow that to purchase a competing natural stone of similar weight and quality. There is currently much activity in the growth of CVD diamond which by nature is a polycrystalline, porous, thin film. This process is done at pressures below atmospheric.

Clay's proposes to demonstrate "proof of process" of growing large high quality single crystal diamond by building pilot plant size equipment and growing 1,000 carat single crystal diamonds. Clay's process is to provide hydrostatic pressure of ten G Pa with sufficient temperature to directly convert graphite to diamond. The design call for the growth process to be slowed down and controlled to provide the needed time for the growth of a high quality crystal. The containment vessel is a cylinder and piston design with the primary chamber placed inside of a secondary

28

vessel. The pressurizing fluid is argon which is chemically inert and conducts heat poorly at the operating conditions. Access to the high pressure chamber is through diamond windows using electromagnetic radiation. The high pressure will be obtained through the force of a 44 MN press. Arrangements have been made to borrow the press from the U. S. Army. Heat will be obtained by directing the focused beam of a variable power 3 k W infrared laser beam through one of the diamond windows. The beam will pass through a diamond seed crystal which is resting on the graphite. The temperature of the diamond-graphite interface will be continuously measured with an optical pyrometer and this information will be used to vary the laser power to obtain the desired growth temperature. A Manganin cell will be used to measure the pressure in the secondary pressure chamber and to control the movement of the 44 MN hydraulic cylinder. A second diamond window will be used to measure the temperature of the graphite rod and detect when the conversion process to diamond is complete.

To the best of the writer's knowledge, all competitors are limited to hydrostatic pressures below six G Pa because the crushing stress for WC-3Co is 5.8 G Pa. A key technology advantage available to Clay's is an understanding of how cermets get their unusual strength. The cermet modeling is contained in Appendix A. The key element is to not treat a cermet as a homogenous isotropic material. Once one analyzes the material as hard WC crystals held together with thin layers of metals, the unusual strength properties are understood and predicted. This knowledge has been used to design a 10 G Pa pressure chamber for growing large bulk diamond crystals. The design chamber is 40 mm in diameter by 320 mm long for a working volume of 0.4 liters.

Making pure diamond crystals requires the removal of air and other contaminants from the graphite and the pressure chamber. Oxygen makes the diamond milky in appearance due to the carbon dioxide bubbles in the diamond. Nitrogen atoms replace carbon atoms in the diamond lattice turning the diamond yellow in color and making it an N semiconductor. Boron can also replace carbon atoms making the diamond blue in color and a P semiconductor. Hydrogen may act to enhance the conversion from graphite to diamond. The Hydrogen atoms are so small they can roam relatively freely within the diamond lattice without being a part of it. All other elements and chemical compounds are inclusions in diamond.

All significant aspects of the design of the hardware have been made. The key elements of the design are a part of this report. It appears that the equipment can be readily made and successfully operated. The principle investigator has designed, manufactured and successfully put into operation highly technical equipment which performed tasks that were considered impossible by textbook and accepted wisdom of the masses. In almost every instance technical problems with hardware arose which were unexpected. In all cases, solutions were found. The task of growing large, single crystal diamonds will undoubtedly have problems also.

Areas that the writer considers potential problem areas include the fabrication of the rhenium based cermets, weaknesses in the cermet modeling and focusing and properly directing the laser beam. A general problem will be getting timely and quality products and workmanship from vendors.

29

The Re based cermet is a problem because it must be fabricated at the Lawrence Livermore National Laboratory as they have the required experience and probably the only autoclave capable of sintering the designed cermet. If the cermet becomes a large problem we can default to WC-xCo cermet parts. The cobalt based cermet will require using a smaller graphite package and may require the use of a insulating ceramic liner in the cermet cylinder. We can default to cobalt parts, but the crystals will be substantially smaller.

We believe that the cermet modeling is valid and correct. It not only explains the unusual high strength properties familiar to persons designing and using cermets, but it also correctly predicts the high pressure results of the flexure and tensile tests of Bridgman performed under 2.4 and 2.9 G Pa of hydrostatic pressure. If the analysis is in error, we will reduce the diameter of the working chamber and increase the pressure in the secondary chamber to relieve the strength requirements of the cermet.

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1Burns, R.C. & Davis, G.J.,"The Properties of Natural and Synthetic Diamond", J.E. Field, Editor, Academic Press,1992, pp 396-398.2Zhang, Y., Zhang, F. & Chen, C., A Study of Phase Transformation Between Diamond and Graphite in the P-T Diagram of Carbon, Carbon, Vol 32, No. 8, pp 1415-1518, 1994.3Hall, Tracy, private communication.4Kirk-Othmer "Encyclopedia of Chemical Technology", 4th ed., Vol. 4, John Wiley & Sons.5Badziag, P., W. S. Verwoerd, W. P. Ellis & N. R. Greiner, "Nanometre-Sized Diamonds are More Stable than Graphite", Nature, Vol. 343, 18 January 1990, pp 244-245.6Erskine, D.J., & W.J. Nellis, "Shock-Induced Martensitic Phase Transformation of Oriented Graphite to Diamond", Nature, Vol. 349, 24 January 1991.7Seals, Michael, private communication.8Kennedy, George, "Diamond Synthesis and the Bonding of Polycrystalline Diamond Masses", UCLA Inst. of Geo. and Planetary Phys.,AD A048259.9Bundy, Francis, private communications.10Kennedy, G., Rev. Sci. Instuments 38, #11, pp 1590-1592, Nov. 1967.11Schwarzkoff, P., R. Kieffer, W. Leszynski & F. Benesovsky, "Cemented Carbides". Macmillan Company pp 139, 1960.12Bundy, F. P., "Pressure-Temperature Phase Diagram of Elemental Carbon", Physica A 156, pp 169-178, 1989.13 Same as reference 2.14 Bundy, F. P., "Direct Conversion of Graphite to Diamond in Static Pressure Apparatus", J. of Chem. Physics, Vol. 18, No. 3, pp 631-643, 1963.15 Haller, Eugene E., Private communication during a tour of his crystal growth and purification facilities at the Lawrence Berkeley Laboratory.16Lax, M. & Burstein, E., "Infrared Lattice Absorption in Ionic and Homopolar Crystals", Phys. Rev., Vol. 97, pp 39-52, 1955.17Ruoff, A.L., in "High Pressure Science and Technology", Vol. 2, Timmerhaus & Barker eds., Plenum Press, New York, pp 525-548, 1979.18Bridgman, P.W., "The Tensile Properties of Several Special Steels and Certain Other materials under Pressure", J. App. Physics, Vol. 17, pp 201-212, 1946.19Bridgman, P.W., "The Effect of Hydrostatic Pressure on the Fracture of Btrittle Substances", J App. Physics, Vol. 18, pp 246-259, 1947.20Jeanloz, R., B. K. Godwal & C. Meade, Static Strength and Equation of State of Rhenium at Ultra-High Pressures, Nature, Vol. 349, pp 687-689, 1991.21Roark, R.J. & W.C. Young, Formulas for Stress and Strain, fifth ed., pp 504-505, McGraw-Hill, New York, 1975.22"The Properties of Natural and Synthetic Diamond", J.E. Field ed., Academic Press, pp 684, 1992.23Grimsditch, M. et.al., "Refractive Index Determination in Diamond Anvile Cells: Results for Argon", J. Appl. Phys. Vol. 60, pp 3479-3481, 1985.