IE45102B1 - Process for converting type ib nitrogen in a diamond crystal into type ia nitrogen - Google Patents

Abstract

Process for converting type Ib nitrogen included in diamond to type Ia nitrogen It consists in subjecting this crystal to an annealing temperature ranging from 1500 ° C. to about 2200 ° C. under a pressure which prevents significant graphitization of this crystal at this annealing temperature for a period of time sufficient to convert at least about 20 ° C. % of the total amount of type Ib nitrogen present in the crystal in type Ia nitrogen. Application to the manufacture of abrasive.

Description

This invention relates to the annealing of synthetic or natural diamondtype lb or natural diamond mixed type Ib=±a to convert at least a portion of type lb nitrogen to type la nitrogen.

- Diamonds, whether synthetic or natural,, are generally 5 classified Into four main types: 2a, lb, Ha, and lib. These types are aost easily distinguished by infrared and ultraviolet spectra and somstlass by electron paramagnetic resonance (EER). Type la and lb diamonds contain dissolved nitrogen; in Ia diamonds, most of the nitrogen is not EPR active and appears to be in aggregated form; in lb diamonds most of ths nitrogen is EER active, and is atomically dispersed. Types Ua and Hb dianonds do not contain appreciable nitrogen. Each type of diamond has typical infrared and ultraviolet spectra with characteristic features.

Tha large majority of. synthesised diamonds are type lb, but type Ha diamonds can easily be made either by excluding nitrogen from the diamond growing media or by U3ing appropriate nitrogen getters.

Ths large majority of natural diamonds esamlnsa are type la. Ms.type la diamonds have been synthetizsd thus, far in the laboratory. Natural type Is/diamond crystals can have a variety of colors, with '20 vsES^nbsing’paleryellcf^ to colorless. Such a diamond costal can also be a ccsshlnation of pale yellow and colorless areas ss well as exhibit local variations in its characteristic color in different parts of the crystal. Ordinarily, it has a. rounded dodecahedral or octahedral morphology.

-. There are two forms of., type 2h natural diamond, an A band form ' and a B band fox®, and these forms are distinguishable by their infrared, ; visible , and ultraviolet absorption spectra. Usually, however, these two forms are most easily differentiated by their infrared, spectra wherein Oil the A band form has its main absorption band coming at 1280 cm-1 and ths B band fowl has its main abaaption band coming at 1175 er1. While each, form of type ώ appears to be thermodynamically more stable than type Es diamond, the present invention thus far produced only the A Laid form · Whieh ie hereinafter referred to broadly as type la.

Synthetic diamonds are substantially toe sane as natural diamonds hut there era enough differences between than to distinguish between the natural and synthetic crystals. These differences-are mainly in morphology, surface appearance, hipurity inclusions and the nature of impurity inpetfecelons such as the different fora® of nitrogen. As found, natural diamond crystals most frequently nave curved edges and convex faces. On the other hand, synthesised diamond crystals, as grown, have stozp edges., flat and relatively smooth faces. Depending on the «irditians of growM, sy.' tetie type lb crystals have octahedral or cubo-15 octahedral rcipfeology, the iatfcer sometimes having small (113) faces.

Also, depending cn thi conditions of growth, a synthetic type lb crystal aay have no definite morphology ind can have & viiSg variety of shapes with sea» substantially distorted fran the octahedral or cUbo-oetahedral regular ehspea, as well as In extreme esses, highly irregular particles of no particular shape. Inpurity inclusions in synthetic diar®nds are aetal catalysts «hsreas in natural diamonds they arc „ variety of minerals, and these inpurity inclusions are detectable by several techniques such as elecurasi diffraction analysis or X-ray analysis.

Less than IS of natural diamonds are type lb, and usually natural diamonds of a nixed type Ib-Ia are found which can range widely in type lb nitrogen content. Ordinarily, natural type lb diamond crystal has a morphology exhibited in natural type Ia diamond crystals. 65102 Those skilled in the art vri.ll gain a further· snd better understanding of the present invention from the detailed description set forth belowj considered in conjunction xvith the figures accompanying and forming ε part of ths specification In which: Figure 1 represents the phase diagram of carbon showing ths dissand-graphite equilibrium line and the shaded area defines ths Region of Conversion tfidch encompasses the required annealing temperatures and the corresponding annealing pressures of the present process.

Figure 2 is a sectional view of a preferred reaction vessel 10 for carrying out the present invention.

Figure 3 shows a typical infrared absorption spectrum of ths region of interest for a type la natural diamond crystal.

Figure ;S shows a typical infrared absorption spectnas of ths region of interest for a synthetic type Ta diamond crystal.

- Figure 5 shows infrared absorption spectra of the region of interest of a synthetic type lb crystal taken before and after it was annealed in accordance with the present process stowing that the annealing resulted in a conversion significantly higher than 20/ of type Ib nitrogen to type la nitrogen. 2Q Figure 5 stows infrared absolution spectra of a natural mixed type Ib-Ia diamond crystal taken before and after it was annealed in accordance xvith the present process shox-ring that the annealing resxilted is a conversion significantly higher than 20/ of type If) nitrogen to type la nitrogen.

According to the present process., natural or synthetic diamonds type Ib or natural diamond mixed type Ib-Ia is annealed to convert type Ib nitrogen to type Ia nitrogen. Briefly stated,, the present process coisorises annealing type Ib synthetic diamond crystal,type Ib natural diamond crystal or mixed type Ib-Ia natural diamond crystal at m «1 Ο 3 annealing tenperature ranging ίΥχί about 153CeC to 2200°C under a pressure which prevents significant grsphitlaation of the diamond during annealing to convert at least 20? of the total amount of type lb nitrogen present in the crystal to type Ia nitrogen.

In the present process the diamond crystal can be wholly type Wsynthetic srto/sr natural or mixed type Jb-la or mixtures of the foregoing, Ihe aizcd type crystal can range iii type lb nitrogen content from about 99? to 1? of the total aoKsib of nitrogen present in the - crystal, In the present process the amount: of «ewsica of type lb nitrogen to type Ί& nitrogen is osseisdnaale hy a nunswr of conventional techniques. Ilia most frequently used technique Is one where it is revealed by the differences or changes to the absorption spectra of the Zb or mixed type Γο-Ιϋ crystal, token before and after snnealing.

Specifically, spectra token of tta type lb or mixed type Ib-Ia crystal at room tenperature by means of spectrometers in a conventional rannsf steiag the ultraviolet, visible and infrared absorption spectra of the crystal. Aits? tte crystal is annealed, spectra ars taken of it again at room tenperaiare showing its ultraviolet, visible and infrared absorption spectra. Erom a comparison of the changes to these spectra, the amount of conversion of type lb nitrogen to type Ia nitrogen, i.e.s ths percent of the total amount of type IS nitrogen present to the crystal converted to type la nitrogen, is determinable in a conventional manner.

Synthetic type lb diamond crystal and natural diamond crystal , have a color depending on the amount of nitrogen dissolved in ths crystal. Ihe color of the crystal, ranges from a green to a greenish-yellow to a yellow with the maximum or largest amount of dissolved nitrogen producing the greenish-yellow color. likewise, the amount of nitrogen dissolved in the crystal determines the intensity of the yellow color which can range from a deep golden yellow to a pale yellow with the deep golden yellow s5lOa indicating substantially mors dissolved nitrogen than the pale yellow,, 2h additions tha Ib diamond crystal and natural diamond crystal can exhibit a sflxture of greenish-yell©/ and/or yellow colors or shades,, i.e./ it can exhibit local variations in Its characteristic color and intensity, which indicates regions of varying nitrogen content. In the present process there is no limitation on the size of the diamond crystal. Specifically, the minimum size of the crystal can be one Mcron or less and the maximum size is limited only by the capacity of the annealing equipment. Star most present applications, the Ib crystal size ranges fecan 0.25 Millimeter to 6 millimeters. The size of the diamond crystal given herein is that measured along the longest edge dimension of the crystal.

Hie present annealing process is carried out in high taxpsrature-hlgh pressure apparatus normally used for synthesizing diamonds by application of high temperatures and pressures to a suitable reaction mass or specimen.

One preferred form of a high pressuz's-higii temperature apparatus in which the present invention ean be carried out is disclosed by ϋ. S. Patent Kb. 2,9^1,248 - Hall ' and it is also disclosed in numerous other patents and 20 publications. Those skilled in the art are well acquainted with this ’’belt-type” apparatus and, for this reasons ths apparatus is not illustrated. Essentially, the apparatus consists of a pair of eeaanted tungsten carbide punches disposed to either side of an intermediate belt or die member of the same material. Ths space between the two punches and Che die is occupied by the reaction vessel and surrounding gasket/insulafcien assemblies therefc-r, High pressures are generated in ths reaction vessel from the compressive . forces caused by the relative movement of the so-asially disposed punches toward each other within the die. Keans are provided for heating the reaction. mass ia tha reaction vessel during the applies tie.” of pressure.

There are, cf course, various other apparatuses ’ capable of providing the required pressures and temperatures 10 that can be employed within th® scope of this invention such as tetrahedral typ<..., eufele types and spherical types. Operational techniques for applying high pressures and temperatures in tiie apparatuses useful ia the present process are well known to thosa skilled in the super15 pressure art.

Various reaction vessel configurations which provide £<-r indirect or direct heating ef the reaction mass are disclosed in the patent literatura and are useful in carrying out the present annealisu process, These reaction vessels usually consist or a plurality of interfitting cylindrical members and ead plugs or dises for containing tha reaction mss in the centermost cylinder.

Zn the indirectly heated type of reaction vessel one of the cylindrical members is made of graphite which is heated by ths passage of electric current therethrough and which thereby heats the reaction mass. la the directly heated type o£ reaction vessel, the reaction E233 is electrically conductive, thereby eliminating the need for an electrically conductive graphite cylinder, cad electric current is passed directly through ths reaction mass gs best it» U.S. Patent 2,541,248 = Hall discloses an Gsbedlmsnt of a reaction vessel wherein th© reaction ’ spaeiEsn is indirectly heated, as well as the alternative embodiment for directly heating the reaction specimen when it is electrically conductive.

U.S.Patent No. 3,031,269 - Sovenkerk discloses a reaction vessel for indirect heating of the reaction mass.

IS Specifically, the outer element of the reaction vessel io a hollow pyrophyllite cylinder, positioned concentrically within and adjacent t© the pyrophyllite cylinder is a graphite electrical resistance heater tube, and within ths graphite tube there is concentrically 2® positioned an alumina cylinder whieh holds fhe reaction ©aos or specimen. .

Ths directly heated embodiment of fhe reaction vessel is preferred in the present process, and a particularly preferred form is shown in Figure 2. Specifically, 2S this reaction vessel includes a hollow outer cylinder 3 8· 33103 made of non-conducting material such es pyrophyllite. Positioned concentrically within and adjacent pyrophllite cylinder 3 ia ceramic cylinder 4 preferably made of alumina. Charge element or insert assembly 5 is adapted to fit concentrically in ceramic cylinder 4 and is dimensioned for a close fit with cylinder 4, Charge element 5 is ssstprised of graphite red 1 and graphite rod 6 wherein the graphite is of spectroscopic purity. Graphite red 6 has hole 8 which is drilled te fit closely around diamond crystal 9, i.e., the ditacnd to be annealed, Sisaond crystal S should not project outside o£ hole 3 since such projection would prevent a close contiguous fit of rot ? with rod 2od 7 should be in electrical contact' with rod 6 at surface 10. Preferably, the cop surface of diamond 9 is flush with surface 10 ex graphite rod 6, Any spacs between diamond crystal 9 and hole 8 Is preferably filled with an electrically conducting material, such as graphite sawder of spectroscopic purity, to promote passage of sha electric current and thereby promote heating of diamond 9. Slestrieally conducting circular metallic discs 1 and 2 close the ends of graphite rods 6, 7, and cylinders 3 and 4.

Discs 1 and 2 are preferably made cf a metal such as nickel or tantalum and must be in electrical contact with graphite rods 7 and -5, respectively. Since graphite rod 6 is electrically conducting and diamond crystal 9 ia not elec= tri,cally conducting, ths highest temperatures ara attained and maintainsd at tha thinnest portions of graphite rod 6, e.g., ths area of graphite rad δ surrounding diamond crystal 9.

After· assembly of the reaction vessel and introduction thereof into ths high pressure-high temperature apparatus within ths gaslcet/insulation assemblies, preferably pressure Is raised first and then ths temperature. Ths rates of increase of pressure or teeaerature are not critical. When pressure and temperature are at a' level in ths Region of Conversion defined in Figure 1, they are held at that level for a period of time sufficient to attain the desired conversion of at least about 20/ of the diamond’s Ib type nitrogen to type Is, nitrogen. VSisn desired conversion is attained, the electrical power which heats the dlSEEnd crystal is shut off and the crystal eools to about room temperature quickly, usually in about one minute·. Generally, when the crystal has cooled to below 50°C, the pressure is then released, preferably at a rate of about 10 Zdlobsra per minute to atmospheric pressure.

Si ths present annealing process, ths diamond crystal is annealed at a temperature ranging fraa I5oo°c to 22oo°c.

Annealing tasperatures lower than 15OO°C are not operable or take too long a period of annealing time to be practical. Annealing temgeratures higher than2200°C providexso significant advantage. Annealing ts^eratures ranging from 16OQ°C to 2000°C are preferred since they are not too difficult to attain, do not require excessively high pressures and since they induce high rates of conversion.

The pressure used in the'present process need only be sufficient to maintain the diamond stable at the annealing teasgsrature. Specifically, io sSl 02 it is a pressure which prevents graphitizatien cr prevents significant graphitization of the diamond crystal at the simesling temperature. The shaded area of Figure· 1 defines the Region of Conversion which defines the operable teaperatures and corresponding annealing operable pressures of the present process. 55» diamond-graphite equilibrium Itos as well as pressure and tesperature calibrations at suoh suparorassuras are not definitely known. 5as tooffond-graphite equilitedia line shown in Figure 1 is the best approximation known at present for disinond-graphits equilibria!!. Preferably, the present process is carried out at or above this diamond-graphite equilibrium line. The shaded area in Figure 1 Of the Region of Conversion below tho diamonu-siapiiite equilibrium line is a tolerance zone which shows the lower* pressures which are operable in the present process for limited periods of time. For example, for the minimum pressures shown by the tolerance sone, the mqxtam period of annealing tins is about ons hour* without si^lflcsnt graphitization of the diamond crystal occurring. If annealing times longer than ons hour are used, then the pressure applied in the tolerance zone should be closer to the diamondgraphite equilibrium line. &s ehCRi in Figure 1 by fcl» Region of Conversion, an annealing tssperatwes of about 1500°C requires a pressure of at least about 48 kilebars, at 16QO°C the pressure should be at lesst about 51 kilobars and preferably about 61 kilobars, at 2000°C She pressure should be at least about 63 kUobars and preferably about 74 ldlobars, and at a tenperature of about ?200'C the pressure should bs at least 70 kilebars and preferably about 80 kilobars.

Annealing time, I.e. the period of time at annealing temperators snd pressure, Is determinable eapirieally and can range frcm about one minute to about 50 hours, and preferably up to about 20 hours. Usually it ranges from about 10 minutes to about 5 hours. Specifically, annealing time depends largely on annealing temperature, the kind of lb crystal being annealed as determined by its nitrogen content, and the extent or degree of conversion of the type lb nitrogen to Ih nitrogen required. With rising annealing temperatures, the rate of conversion of type lb nitrogen to type la nitrogen increases si^iificantly, i.e. more than five times in going from 16OQ°C to 2200°C. Sis mechanism of the present process is not understood but it is believed that the rate of conversion of lb nitrogen to Ia nitrogen does not differ significantly between a crystal of high nitrogen content and one of low nitrogen content, but the period of annealing' time at a given annealing temperature to leave essentially the same amount of type lb nitrogen in each crystal doss differ since the Γο crystal rith ths higher nitrogen content has rare nitrogen to convert to type Ia thereby requiring a longer emsallng time.

• Khile ths detailed mechanism of the conversion proeess is not understood., annealing, experiments have shorn that the activation energy for the process is approximately 83 ldlocalori.es/mole (3.6 eV)„ She extent of conversion of. a lb nitrogen to lh nitrogen is 20 deterasinahle enpirically by a number of known methods in the art. For esanple, most of tha type la nitrogen crystal is ES inactive (Electron Paramagnetic Resonance) whereas the dissolved nitrogen in type Xb nitrogen is EPR active. Also, types la and lb nitrogen each have typical infrared, visible and ultraviolet spectra with characteristic features which are z identifiable in infrared, visible and ultraviolet spectra of a crystal of Mxsd type la and lb.

Sraferably, to determine· satisfactory annealing times and temperatures for a particular kind of crystal, e.g., a crystal containing i 02 a certain amount of dissolved nitrogen as reflected by its infrared, visible and ultrevicOet spectra sad ths Intensity of its color, the crystal should preferably be initially produced in the form of a platelet polished on both sides so that the spectra taken thereof are well-defined. ite platelet is then annealed at a given annealing tesp^ature for a e&rtsin period of tine and aits? each annealing run, ite infirared, visible and w.trovlolet- spectra era taken. A ecnpsrison of spectra taken before and after annealing Inii2ate3 the extent of , conversion to type Ia. Also, additional earoarisors of such spectra 10 with EEK spectra of the crystal before sad after annealing are another indication of the extent of conversion to type Ia. Once the time for annealing this particular kind of crystal has been deteradnsd to attain a certain conversion te type Ihj, auefc annealing time and annealing 1 teaperature can be used for tin asae kind of crystal. e.g. a crystal 15 containing substantially the same amount of dissolved .To nitrogen, regardless of its size or Sir.:®, to attain ths earns or sifestantially the same degree of conversion to type la nitrogen.

Also, after the reaction rates are determined by esparimants on a parfcieuthr kina of crystal, it is -possible to estimate tha correct ao aaasaSlsg times which would leave a specified amount of type Ib nitrogen in tee crystal for crystals bavins ?- wide range of typa 3b nitrogen eoneentratianr. initislly. iii the present process from at least 20$ up to about IOCS of the total- amount of type Ib nitrogen present in the crystal is converted co type la nitrogen. However, regardless of annealing conditions a residue of type Γο nitrogen in an amount of less than 1$ of the total nitrogen present In the crystal will always remain in the crystal and such type Ib nitrogen residue can be as low as 0,001$, or lower, of the total amount of nitrogen present in the crystal. Initial conversion to type la nitrogen lower than 202 of tha total amount of type lb nitrogen present in the crystal may not effect ths physical properties of the crystal significantly for most applications. tee extent or degree of'conversion of type lb nitrogen to type Ia nitrogen depends largely on the particular properties desired. te ths annealed crystal produced by the present process which contains both types la and lb, type la appears to be uniformly distributed throughout type lb.

As a result of the present process, at least a portion of the crystal undergoes some change in color or shade, i.e. te a greenishyellow crystal at least a portion changes toward the yellow or for a yellow crystal a portion becomes at least a shade lighter yellow, the extent of which depends on the extent of its conversion to type la. Also, when substantially all or all of the type lb nitrogen is converted to type Ia nitrogens the result is a very pale yellow and/or a colorless crystal which has many use3 as jewelry, and which frequently is of gem quality.

Sjs annealed diamond crystals produced by the present process are useful as abrasives. The abrasive industry requires numerous types of abrasive materials to carry out various grinding or machining operations, the requirements of which are determined largely by the properties of the material being machined and, to some extent, the results desired. ¥br certain operations te the abrasive industry, synthetic type lb crystal has been satisfactory and for other operations natural type la crystal has been satisfactory. However, as a result of the present invention, the abrasive Industry now has available a crystal which is a mixture of types lb end Ia, the caspositlon of which can be controlled to produce crystals with graded physical properties over a wide range to adjust the crystal to the particular abrasive use to which it is applied. Specifically, with increasing degrees of conversion of type lb to type la, the czystsl changes to abrasive properties, usually csecadng harder and stronger. As a result. a nixed type Xb-Xa crystal ea» be produced having optimum properties for a particular abrasive use.

Vhen substantially all or all of the type lb nitrogen in the 5 ' crystal is converted in the present process, the resulting type Ia crystal is also highly useful as an abrasive, In ths eiss of syiivheiic diamond eiystal, since it ret2to3 tie “cxpholosr of the tyzs 2fc crystal frtsa which it kss produced, it is a type la crystal with a wrphDloa’ not found to nature, I ]0 ®» annealed diamond crystals of the present process are also «sefUi ae Jewelry, especially those of gem quality.

The morphology of the as- 2h S’ioh instance wneie ths present ansaalsd crystal- polished or abolished, -~is a shape which does not reveal it to fee either synthetic or natural, it esn he so ideitified by a known licjjt scattering technique. Specifically, this technique conprises examining the crystal wider a microscope by staining a beam of light at an angle trireon and observing the scattered light reflected from scattering centers normally present to synthetic diamond, but such scattering centers, and resulting scattered light, are not known to have been seen in natural type la tiiaiionc crystal.

The invention is further illustrated by the following examples which are tabulated in Table I sad wherein the procedure ms as follows unless otherwise stated in Sable I. in EXfiMPIES 1 - 2A Type To synthetic diamond crystals were prepared ia a high pressure-high temperature apparatus of the ’’belt-type” disclosed in U. S. Patent Ko. 2,941,248 - Hall.

Xn aaeh ejsample the type Xb diamond crystal was at least partly polished in a conventional wanner using a seaifs. Tho resulting plate had a significantly S uniform thickness which ranged from about 1/2 sm to about 1 ms. The siaa of the plate given in Tabla X is its maximum width.

Each type Xb diamond crystal was annealed in a reaction vessel as shown in Figure 2. Graphite rods 6 and 7 were of spectroscopic purity and af ths same sise, each was 80 ails in diameter and 223 ails in length. &, hole S was drilled in rod 6 to a sise to fit closely around around the particular diamond being annealed and any space hatween the diamond crystal and inner surface of hole 8 was filled with graphite powder of spectroscopic purity. Xn all of tha examples the diamond crystal did not protrude from the drilled hole 8 and electrical contact between rods β and 7 was maintained as shown in Figure 2. Saramic cylinder 4 ' was mad® of alumina and had an inner diameter of about 80 ails and a wall thickness of sQ mils. Gylinder 3 was mads ©f pyrophyllite and had an inner diameter sf about 200 mils and s wall thickness of 75 ails. Metallic diss members 1 , and 2 ware circular, of tha same sise, oaeh with a diameter ©f 350 mils and a thickness of 10 tails, and made of tantalum.

Ths discs wars in electrical ceatact with reds δ and 7 as. shews in Figure 2. To carry out the present annealing Qt 15 ί Ο 2 process, this reaction vessel wae case in the belt-type apparatus disclosed in U.S. Patent Ko. 2,941,248 - Hall.

Absorption spectra ranging from the ultraviolet through the infrared were made of the diamond crystals at room temperature before and after they were annealed.

Electron paramagnetic resonance (SPR) spectra were made of tha diamond crystals st roes totapsrature before and after ths crystals were annealed, With respect to infrared spectra massuremants, although the 1130 band in type Ih crystals is normally used to characterize the Io crystal., ia tha present instance, for purposes sf accuracy, the 1345 caT^· band which is correlated te type ib nitrogen was used to determine the conversion of the fcjpe ib nitrogen to type X«t nitrogen. ’ · 1? Table Ϊ Illustrates the present invention. Ths decrease ia EFR, and infeared intensities in She type Sb diamonds of Table X was used te monitor ths eoavsrsion of typo Xb nitrogen to type Xa nitrogen,, Th© alternatives that a decrease in these type Xb diaaond EF'S. and infrared intensities could also occur if tha Xb nitrogen was diffusing out of th© crystal with no conversion So type Xa nitrogen or changing to nitrogen of yet another type wars ruled out for two reasons. The first reason is qualitative in that typo Xa infrared absorption bands do appear, hessc there is some conversion t© type Xa nitrogen. The second reason is quantitative in that fro® tha type Xb aad/or type Xa absorption bands present one eaa calculate, based en publishes data, using standard techniques, the total IS amount of nitrogen present. Fes the present annealing experiments this nitrogen content of any eaa diamond remains constant, within experimental error. For isstaaea, Example 1 had an initial content of only type Xb nitrogen ©f 365 ppm. After the annealing treatment, tha infrared absorption spectra and EE’S, fcr Examples 1 and 1& showed that the type Xb nitrogen decreased fee 35 percent sad 39 percent, respectively, of its original content as shown in Table I, yet the total nitrogen was 364 ppm, the same as before the annealing pfdbesa. Hence, na I change occurred in the total nitrogen content despite the fact that the final type Xb nitrogen was approximately ia. dSi ο 2 to 39 percent of that· originally present, lhersfore, tha change in Intensity of the type lb absorption band at 13^3 cm is a good indication that type lb nitrogen is being converted tc type 2a nitrogen and is aofc diffusing out of the diamond ‘ or being converted to nitrogen of yet another type.

A a.: sa %sge Ib-Ia natural diamond crystal was usad„ Eis procedure used was identical to that set forth above Sa Esanles 1 - 2A (exceptions are noted in Table. I). » conversion of the type Γο niferecsK to type la nitrogen was verified by the same iaafchod as ret forth for Etesples 1, IA.

Examples 3 and 3ft shon-iS that tha type 3b ritregan decreased to J?5 percent sad 55 percent, respectively, of its original content as ύνΑΐί in Table I, yet the V-tal nitrogen kss 135 ppm, the sues as before the annealing proesss. dsnci·. no change occurred in tfcs total nitrogen content despite the that ti» final type lb nitrogen «as approxiE&tely 45 to 55 ps'-ueni. of that originally present.

It is understood that the present annealing precess can be carried out with, the seme diamond crystal more than one time to additionally increase the amount of type Ia nicregon therein. For exsnple, a mixed type Ib-Ia annealed diamond crystal produced by fche present ‘ process can bs annealed in accordance with the present process to convert an additional aacaafc of type Ib nitrogen to t-pa Is idtrogsn.

Claims (13)

1. CLAIMS :1. to annealing process for converting typa i'c nitrogen to diamond crystal (whether wholly Ib or in mixed Ia-Ib type diamond crystal) to type Ia nitrogen which comprises sub5 jecting said crystal to an annealing temperature ranging-from 1500¾ 220Q°C under a pressure which prevents sigiificant graphitization of said crystal at said annealing temperature for a pssdcd of time sufficient to convert at least about 20? of tha total amount of typa

2. ) nitrogen present to said crystal to type la nitrogen. 10 2. to annealing process according to Claim 1 wherein said annealing temperature ranges from l6OO°c to 2000°c.

3. to annealing process according to Cleia 2 wherein said annealing: tsapsrature is about 1SOO°C.

4. to annealing process according to Claim 1 wherein at 15 least 50? of the total amount,of type Ib nitrogen present in the crystal is converted to type to nitrogen.

5. « to annealing process according to Claim 1 wherein the resulting annealed crystal Is mixed type Ib-to which is further annealed to accordance with the process of Claim 1 to convert sn additional 20 amount of type Ib nitrogen to type la nitrogen.

6. A synthetic diamond type Ib crystal whereto at least 20? of the total amount of nitrogen present to ths crystal is type la.

7. A synthetic diamond crystal according to Claim 6 whereto at least 50? of ths total amount of nitrogen present to the crystal is 25 type to.

8. A synthetic diamond crystal according to Claim 6 wherein more than 99? of the total amount of nitrogen present in the crystal Is type to.

9. A synthetic diamond crystal according to Claim 6 whereto 30 said crystal has an octahedral morphology.

10. A synthetic diamond crystal according to Claim 6 whereto said crystal has a cubo-octahedral morphology. .

11. A synthetic diamond crystal as claimed in Claim S, substantially as hereinbefore described,

12. A process as claimed in Claim 1, substantially as hereinbefore described.

13. A process for annealing diamond crystals, substantially as described in any one of the examples.