Claims:I/We Claim:

1. A method to improve the optical properties of single crystal CVD diamond 1, said method comprising the steps of:
a. placing a container 2 containing single crystal CVD diamond 1 into an enclosed chamber 4;
b. maintaining a pressure in the range of 800 to 100000 psi in the enclosed chamber 4 by supplying gas from a gas container 8 into the enclosed chamber 4 using a gas line 6, a pressure pump 5 and a pressure control assembly 7;
c. raising a temperature of single crystal CVD diamond 1 up to 1500 to 1800 0C using a heater 3 while maintaining set pressure in the enclosed chamber 4;
d. maintaining single crystal CVD diamond 1 at set temperature and pressure in enclosed chamber 4 and;
e. cooling of single crystal CVD diamond 1 to an ambient temperature while maintaining set pressure in enclosed chamber 4.
2. The method to improve the optical properties of single crystal CVD diamond 1 as claimed in claim 1 wherein nitrogen or argon gas is used.
3. The method to improve the optical properties of single crystal CVD diamond 1 as claimed in claim 1 wherein the single crystal CVD diamond 1 is maintained at set temperature and pressure in the enclosed chamber 4 for a time period of 12 to 40 hours.
4. The method to improve the optical properties of single crystal CVD diamond 1 as claimed in claim 1 wherein the single crystal CVD diamond 1 is cooled to the ambient temperature over the time period of 3 to 9 hours.
5. The method to improve the optical properties of single crystal CVD diamond 1 as claimed in claim 1 wherein plurality of the single crystal CVD diamond are placed in the container 3.
, Description:FIELD OF THE INVENTION:

[001] This invention relates to a method for improving the optical properties of synthetic diamond. More particularly, the invention relates to the method of annealing of single crystal chemical vapor deposition (CVD) diamond at moderate temperature and pressure in controlled environment.

[002] The word moderate temperature is used to indicate that temperature range used for annealing in this invention is in lower range of temperatures used in known High Pressure High Temperature (HPHT) and Low Pressure High Temperature (LPHT) method of annealing. The word moderate pressure is used to indicate that pressure range used for annealing in this invention is lower than that used in known High Pressure High Temperature (HPHT) method of annealing and higher than that used in known Low Pressure High Temperature (LPHT) method of annealing.

[003] Some common applications demanding optical grade single crystal CVD diamonds includes but not limited to lenses for high optical transmission of light and gem quality diamond for jewelry.

BACKGROUND OF THE INVENTION:

[004] Various characteristics of diamond such as its extreme hardness, strength, optical and thermal properties and chemical inertness in different aggressive environments make it suitable as a gemstone and for modern industrial applications.

[005] Inherent variability and scarcity present in natural diamond limits its use in engineering applications. Production of consistently engineered synthetic diamond on commercial scale is possible now a days because of research and development in process synthesis. Exceptional covalent crystal diamond was produced first time in 1950s using High Pressure and High Temperature and later in the 1980s using Chemical Vapour Deposition (CVD).
[006] Majority of the synthetic diamond used in practice are made by High Pressure and High Temperature methods. In High Pressure and High Temperature methods, pressure and temperature is maintained at a level to manipulate natural thermodynamic conditions in which diamond is formed. In High Pressure and High Temperature methods, a molten metal solvent or catalyst is used to reduce the large kinetic barrier and to act as transport media for dissolved carbon. Because of incorporation of nitrogen in to the diamond lattice from the atmosphere and growth material, synthetic diamond grown in this way has a yellow hue. Most High Pressure and High Temperature grown diamond falls into type Ib classification.

[007] Synthetic diamond grown by Chemical Vapour Deposition (CVD) method exploits the relatively small difference in stability between two allotropes (SP2 & SP3) of carbon. In Chemical Vapour Deposition (CVD), growth of diamond is achieved by imparting energy into gas phase carbon containing precursor molecules. Microwave energy can be used to create plasma that deposits carbon upon a seed diamond to form a diamond. The plasma can be heated by microwaves, radio frequency, lasers, direct current, hot filament and chemical reactions. The nucleation and growth of continuous diamond requires a substrate with refractory characteristics, stable carbide formation and a low thermal expansion coefficient. After nearly 4 decades of research into CVD diamond growth, microwave plasma enhanced CVD diamond synthesis emerged as a commercial synthesis method in the1990s. The growth rates and control over purity of this method lends itself to manufacturing high quality, free-standing, polycrystalline and single crystal CVD diamond. Most synthetic diamonds manufactured by CVD process falls into type IIa classification. Typical impurities present in CVD diamond includes graphite, hydrogen and very small amount of other materials such as nitrogen, phosphorous and boron. Structural defects including dislocations, twins, point defects etc. present in the material further degrade the optical properties of the CVD diamond and as a result CVD diamond is often opaque and dark brown in colour. In comparison to natural diamond, single crystal CVD diamond is with less bulk defects and because of this reason probability of graphitization is lower as graphite formation usually starts at discrete nucleation centers such as inclusions, boundaries and cracks. Though CVD method for producing synthetic diamond is in existence from more than 50 years, manufacturing of gem quality synthetic diamonds on a commercial scale is not fully developed.

[008] Single crystal CVD diamond are observed to be with narrower x-ray rocking curves than natural brown diamond while exhibiting extremely high fracture toughness. Single crystal CVD diamonds are also found to be with extremely low intensity of dislocations and is regarded as high quality brown diamonds.

[009] Today 4Cs of diamond quality (i.e. Colour, Clarity, Cut and Carat weight) is the universal method for assessing the quality of any diamond anywhere in the world. Various treatment to enhance one or more Cs of a diamond especially colour and clarity of the diamond are of interest and they are known since long. Even natural diamonds are treated by humans since long before the invention of methods for manufacturing of synthetic diamonds. Various treatments used to give specific colour to diamonds or to enhance optical quality of the diamond includes high energy irradiation, high energy irradiation combined with heating, surface layering with other coloured material, High Pressure High Temperature (HPHT) annealing, High Pressure High Temperature (HPHT) annealing combined with high energy irradiation, high temperature treatment and Low Pressure High Temperature (LPHT) annealing.

DESCRIPTION OF THE RELATED ART:

[010] Prior work to improve the optical properties of single crystal CVD diamond shows that high temperature treatment at ambient pressure is usually performed in the temperature range of 700 to 1600 0C. It is well reported that high temperature treatment at ambient pressure by heating the diamond above 850 0C significantly degrade the quality of diamond and heating of diamond above 1600 0C considerably destroys the diamond integrity due to formation of crack as a result of loss of bonded hydrogen and graphitization (i.e. conversion of diamond carbon to graphite). In order to prevent graphitization of diamond at high temperature (i.e. at temperature greater than 1600 0C), high pressure annealing is used. U.S. Pat. No. US RE41, 189 E discloses High Pressure High Temperature (HPHT) annealing method of improving the optical quality of CVD diamond. In case of High Pressure High Temperature (HPHT) annealing, single crystal CVD diamond is heated at least up to a temperature of 1500 0C preferably in the range of 1800 to 2900 0C maintaining pressure of at least 4 GPa. In case of High Pressure High Temperature (HPHT) annealing process there is no clarity on medium gas to be used to apply pressure on single crystal CVD diamond. Time period for which single crystal CVD diamond is maintained at temperature and pressure condition varies from 1 minute to 1 hour. There is no clarity about the cooling rate of CVD diamond from high temperature to ambient temperature.

[011] U.S. Pat. Application Pub. No. US/ 2009/0110626 A1 and U.S. Pat. Application Pub. No. 2010/0104494 A1 discloses Low Pressure High Temperature (LPHT) annealing method of improving optical properties of single crystal CVD diamond. Reported work shows that single crystal CVD diamond has been successfully annealed without graphitization by heating single crystal CVD diamond in the temperature range of 1400 to 2200 0C at pressure in the range of 150 to 300 torr in hydrogen environment. Time period for which single crystal CVD diamond is subjected to specified temperature and pressure condition varies from 0.1 to 720 minutes. There is no clarity on cooling rate to be used to cool the single crystal CVD diamond to the ambient temperature. Single crystal CVD diamond annealed using Low Pressure High Temperature (LPHT) needs further chemical processing to remove surface graphite formed during annealing. Contradiction is observed in the findings about Low Pressure High Temperature (LPHT) annealing of single crystal CVD diamond reported in prior art. In vacuum or inert environments the measurable rates of direct transformation of diamond to graphite have been found at 1700 0C or higher temperatures (Khomich et al., Diamond and Related Materials, 10(2001) 546-551). Annealing by heating single crystal CVD diamond in vacuum turns diamond to dark colour at temperature as low as 850 0C (Mitra and Gleason, Diamond and Related Materials, 2(1993) 126-129).

[012] High Pressure High Temperature (HPHT) annealing method used for treatment of single crystal CVD diamond is costly. Disintegration or rapturing of diamond may be observed at temperature around 1900 0C when diamond is heated to this temperature at very high pressure (i.e. greater than 5 GPa), if structural defects are present in diamond. Processing of diamond at such high temperature and extremely high pressure is very risky and hazardous and needs very high level of safety precautions and costly processing equipment. Graphitization temperature of the diamond can be increased by reducing pressure below atmospheric pressure but again to generate high level of vacuum is time consuming and costly. In presence of structural defects, bursting of diamond may be observed at temperature around 1900 0C when diamond is heated to this temperature at low pressure (i.e. vacuum). For the known High Pressure High Temperature (HPHT) and Low Pressure High Temperature (LPHT) annealing methods, diamond is heated rapidly from ambient temperature to high temperature at set pressure condition, maintained at high temperature at set pressure for very short time (ranging from fraction of second to few minutes) and then rapidly cooled down to the ambient temperature. Thermal stresses are generated in the diamond material because of short heating and cooling cycles resulting into; (i) thermal cracking of the diamond, (ii) generation of residual stresses and associated straining of material degrading optical and mechanical properties of material and, (iii) non-homogeneous material structure with colour and property variation in diamond. Plastic strains present in the diamond before annealing may not be fully relieved in case of short heating and cooling cycles. Further, in case of High pressure High Temperature (HPHT) annealing process when diamond is again brought back to ambient pressure at the end of the process by rapidly decreasing pressure to ambient pressure, there is a possibility that diamond having surface crack may fail when pressure is reduced. This is because when diamond with surface crack is subjected to high pressure, high pressure medium is entrapped into surface micro cracks and on reduction of pressure entrapped may not have been able to escape fast enough resulting into high pressure gradient on the surface of material and bursting the material.

[013] Hydrogen was used to maintain the reducing atmosphere in most of the Low Pressure High Temperature (LPHT) annealing processes of diamond. In an oxygen-free medium, diamond-to-graphite transformation occurs at high temperatures, because the energy barrier between diamond and graphite per one atom is 3.5 eV. Thermodynamic analysis indicates that the direct diamond-to-graphite transition should occur at temperatures higher than 4000 0C but in reality, it is observed even at 900 C. This occurs due to the interaction with residual gases in the vacuum chamber, first and foremost, with oxygen. The thickness of the graphite layer increases with the increase of residual gases pressure (Khmelnitsky and Gippius, Phase Transitions, 87(2004) 175-192). To prevent graphitization of single crystal CVD diamond at moderate temperature and moderate pressure used in the present invention, nitrogen or argon gas is used as a gas medium to apply the pressure.

OBJECTS OF THE INVENTION:

[014] The principal object of this invention is to improve the optical properties of the single crystal CVD diamond. Another object of this invention is to brighten or remove colour from single crystal CVD diamond. A further object of this invention is to minimize or remove defects from the single crystal CVD diamond. Another object of this invention is to produce gem quality synthetic stone on a commercial scale by processing single crystal CVD diamond. Further object of this invention is to prevent graphitization of single crystal CVD diamond while processing by controlling process parameters. Yet another object of this invention is to minimize the cost associated with enhancement of optical quality of single crystal diamond by processing multiple diamonds in a single process cycle. Further object of this invention is to minimize the cost and increase safety by limiting the maximum temperature and maximum pressure used in processing the single crystal CVD diamond.

SUMMARY OF THE INVENTION:

[015] Broadly stated, the present invention relates to a method for improving the optical properties of single crystal CVD diamond by annealing at moderate temperature and pressure in controlled environment that substantially obviates one or more problems due to limitations or disadvantages of the related prior art. To achieve this and other objects of the invention as mentioned in the foregoing section, the method to improve the optical property of single crystal CVD diamond summarized as:
a. Container is filled with the multiple single crystal CVD diamonds and placed inside the enclosed chamber.
b. Pressure is raised in the enclosed chamber up to desired level in the range of 800 to 100000 psi using gas medium such as nitrogen or argon.
c. After raising and maintaining the pressure in the enclosed chamber at desired level, single crystal CVD diamonds placed in the container are heated up to desired temperature in the range of 1500 to 1800 0C.
d. Single crystal CVD diamonds placed in the container are maintained at set temperature and pressure conditions in enclosed chamber for the time period in the range of 12 to 40 hours.
e. Then after single crystal CVD diamonds are cooled to ambient temperature while maintaining set pressure inside the enclosed chamber over the time period ranging from 3 to 9 hours.
f. The single crystal CVD diamonds so processed are tested for checking optical quality.
g. Processed single crystal CVD diamonds in which desired optical quality results are not achieved are reprocessed again as per steps a to f.

[016] The greatest improvements in optical quality will be observed for single crystal CVD diamonds that are partially translucent and free from defects such as surface pits, microscopic inclusions etc. Optical properties of such single crystal CVD diamonds may be improved up to a level that can be polished and faceted to produce gem quality diamond. Method disclosed in present invention can be applied to polycrystalline synthetic diamonds as well as to natural diamonds.

BRIEF DESCRIPTION OF THE DRAWINGS:

[017] The accompanying drawings are included to provide clear understanding of the present invention along with detail description and it constitute a part of this complete specification.
[018] Fig. 1 - schematic diagram of a processing unit used to process single crystal CVD diamonds as per the present invention.
Fig. 2(a) and 2(b) – Result of FTIR Spectra before and after annealing respectively to CVD diamond as per the present invention
Fig. 3(a) an 3(b) – Result of LRS Spectra before and after annealing respectively of CVD diamond as per the present invention
Fig. 4(a) and 4(b) – Result of UV-VSI spectra before and after annealing respectively of CVD diamond as per the present invention

[019] List of designations/ reference numbers in figure
1 Single crystal CVD diamonds
2 Container
3 Heater
4 Enclosed chamber
5 Pressure pump
6 Gas line
7 Pressure control assembly
8 Gas container

DETAILED DESCRIPTION OF THE INVENTION:

[020] The preferred embodiment describes a method of present invention in detail with the help of the schematic diagram of a processing unit used to process single crystal CVD diamonds as per the present invention as shown in Fig. 1. The method to improve the optical properties of single crystal CVD diamond its features and advantages are explained systematically in more detail using technical data and example.

[021] Container 2 is filled with the multiple single crystal CVD diamonds 1 (up to 1000 caret) and placed inside the enclosed chamber 4. Pressure is raised in the enclosed chamber 4 and set in the range of 800 to 100000 psi using gas medium. Argon or nitrogen gas is used as a gas medium in enclosed chamber 4. Gas from the gas container 8 is supplied into enclosed chamber 4with the help of pressure pump 5 and gas line 6 and pressure in the enclosed chamber 4 is maintained at set value using pressure control assembly 7. After raising and maintaining the pressure in the enclosed chamber 4 at set value in the range of 800 to 100000 psi, single crystal CVD diamonds 1 placed in the container 2 are heated up to the temperature in the range of 1500 to 1800 0C using heater 3 provided in the enclosed chamber 4. Single crystal CVD diamonds 1 placed in the container 2 are maintained at set temperature and pressure conditions in enclosed chamber 4 for the time period of 12 to 40 hours. Then after single crystal CVD diamonds 1 are cooled to ambient temperature while maintaining set pressure inside the enclosed chamber 4 over the time period of 3 to 9 hours.

[022 Improvement in various optical properties of single crystal CVD diamond by practicing the invention as per the methodology disclosed in the detail description is reported with the help of the following example.

EXAMPLE:

[023] As grown single crystal CVD diamond cube of approximately 10 mm3 size with light brown colour was placed into the container. Pressure was raised to 45000 psi in the enclosed chamber. Argon gas was used as a medium gas in enclosed chamber. Temperature of single crystal CVD diamond was raised to 1750 0C. Single crystal CVD diamond was maintained at this pressure and temperature condition (i.e. 45000 psi and 1750 0C respectively) for 28 hours. Then after single crystal CVD diamond was cooled to ambient temperature and pressure in the enclosed chamber was reduced to ambient pressure over the time period of 7 hours.

[024] Sample of single crystal CVD diamond was characterized before and after annealing by Fourier Transform Infrared (FTIR) spectroscopy, Laser Raman Spectroscopy (LRS) and Ultraviolet-visible (UV-VIS) spectroscopy. Table 1 shows the results of spectroscopy. After annealing, light brown colour of single crystal CVD diamond was turning it into near colour less and will more clear and brighter.
Test Sample No. Type of sample Sample weight (in carat) Shape/ Cut FTIR spectra peaks observed LRS spectra peaks observed UV-VIS spectra peaks
observed
1 Before annealing 0.72 Square 3123 wavenumber (cm-1) 3110, 1424, 1333
Raman shift/ cm-1 270 nm
2 After annealing 0.72 Square Above mentioned spectra peaks not observed 1400, 1333
Raman shift/ cm-1 Above mentioned spectra peaks not observed

[025] It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, examples recited herein are principally intended expressly to be for explanatory purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and implementations of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

[026] Although implementations of present subject matter have been described in language specific to structural features and/or methods, it is to be understood that the present subject matter is not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed and explained in the context of a few example implementations of the above-mentioned aspects of the present subject matter.