United States Patent [191
`Strong et a1.‘
`
`[111
`[45]
`
`4,124,690
`Nov. 7, 1978
`
`[54] ANNEALING TYPE IB OR MIXED TYPE
`IBJA NATURAL DIAMOND CRYSTAL
`
`[75] Inventors; Herbert M. Strong, Schenectady;
`Richard M. Chrenko, Scotia; Roy E.
`Tuft, Gullderland Center, all of NY.
`[73] Assignee: General Electric Company,
`Schenectady, NY.
`[21] Appl. No.: 856,905
`[22] Filed:
`Dec. 2, 1977
`
`[63]
`
`Related US. Application Data
`Continuation of Ser. No. 707,299, Jul. 21, 1976,
`abandoned.
`
`[51] Int. Cl.2 ............................................ .. C01B 31/06
`
`[52] US. Cl. . . . . . . . . . . . . . . .
`
`. . . . .. 423/446; 51/307
`
`[58] Field of Search ......................... .. 423/446; 51/307
`[56]
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,083,080
`
`3/1963 Bovenkerk ......................... .. 423/446
`
`3,141,746
`3,297,407
`3,423,177
`3,574,580
`4,073,380
`
`Delai ..................................... .. 51/307
`7/ 1964
`1/1967 Wentorf
`423/446
`l/ 1969 Bovenkerk ..... ..
`423/446
`4/1971
`Stromberg et al.
`........ .. 51/307
`2/1978
`Strong et all.
`423/446 X
`OTHER PUBLICATIONS
`
`Klyuev et al, Sov. Phys. Solid State, vol. 16, No. 11,
`May, 1975, pp. 2118-2121.
`Primary Examiner-Edward J. Meros
`Attorney, Agent, or Firm——Jane M. Binkowski; Joseph T.
`Cohen; Charles T. Watts
`[57]
`ABSTRACI‘
`Type lb or mixed type Ib-Ia natural diamond crystal is
`annealed at an annealing temperature ranging from
`about 1500° C to about 2200° C under a pressure which
`prevents signi?cant graphitization of the diamond dur
`ing the annealing to convert at least 20% of the total
`amount of type Ib nitrogen present in the crystal to type
`Ia nitrogen.
`
`3 Claims, 3 Drawing Figures
`
`DIAMOND STABLE REGION
`
`REGION OF CONVERSION
`
`6
`
`//
`
`TOLERANCE ZONE
`
`DIAMOND-GRAPHITE EQUILIBRIUM LINE
`
`80~
`
`5
`
`E
`
`2O_
`
`GRAPHITE STABLE REGION
`
`0
`
`|
`
`I000
`
`l
`
`I500
`TEMPERATURE "C
`
`2000
`
`2500
`
`1
`
`TIFFANY 1014
`
`

`
`US.
`Patent Nov. 7, 1978
`
`Sheet 1 of 3
`
`4,124,690 '
`
`80
`
`03 O I
`
`PRESSURE IN KILOBARS a O
`
`20
`
`DIAMOND STABLE REGION
`
`REGION OF CONVERSION
`
`I
`
`‘
`
`‘
`
`DIAMOND-GRAPHITE EQUILIBRIUM LINE
`
`GRAPHITE STABLE REGION
`
`1
`
`I000
`
`l
`
`I500
`TEMPERATURE "0
`
`l
`
`2000
`
`I
`
`2500
`
`2
`
`

`
`US. Patent Nov. 7, 1978
`
`Sheet 2 of3
`
`4,124,690
`
`,//
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`W////
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`////
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`% 4
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`\\\
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`AV////I////////////
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`llz
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`3
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`

`
`US Patent Nov. 7, 1978
`
`Sheet 3 of3
`
`4,124,690
`
`FIG. 3
`
`80
`
`R E r F A
`
`A
`
`E
`
`M M
`A F
`
`L m
`
`m. M N A
`
`_ _ _ w o m
`
`I400
`
`l 300
`
`I200
`
`IIOO
`
`IOOO
`
`WAVE NUMBER crvr'
`
`4
`
`

`
`1
`
`ANNEALING TYPE IB 0R MIXED TYPE lB-IA
`-
`NA ~
`DIAMOND CRYSTAL
`
`This is a continuation, of application Ser. No.
`707,299, ?led July 21, 1976, now abandoned.
`This invention relates to the annealing of natural
`diamond type lb or mixed type Ib-Ia to convert at least
`a portion of type Ib nitrogen to type Ia nitrogen.
`Diamonds are generally classi?ed into four main
`types: Ia, Ib,.IIa, and IIb. These types are most easily
`distinguished by infrared and ultraviolet spectra and
`sometimes by electron paramagnetic resonance (BPR).
`Type Ia and Ib diamonds contain dissolved nitrogen; in
`la diamonds, most of the nitrogen is not EPR active and
`appears to be in aggregated form; in Ib diamonds most
`of the nitrogen is EPR active, and is atomically dis
`persed. Types 11a and 11b diamonds do not contain
`appreciable nitrogen. Each type of diamond has typical
`infrared and ultraviolet spectra with characteristic fea
`
`5
`
`15
`
`20
`
`tures.
`
`.
`
`‘4,124,690
`2
`I
`the‘ corresponding annealing pressures of the present
`process.
`‘
`FIG. 2 is a sectional view of a preferred reaction
`vessel for carrying out the present invention.
`FIG. 3 shows infrared absorption spectra of a natural
`mixed type Ib-Ia diamond crystal taken before and after
`it was annealed in accordance with the present process
`showing that the annealing resulted in a conversion
`signi?cantly higher than 20% of type Ib nitrogen to
`type Ia nitrogen.
`According to the present process, type Ib or mixed
`type Ib-Ia natural diamond is annealed to convert type
`lb nitrogen to type Ia nitrogen. Brie?y stated, the pres
`ent process comprises annealing type lb or mixed type
`Ib-Ia natural diamond crystal at an annealing tempera
`ture ranging from about 1500° C. to about 2200° C.
`under a pressure which prevents signi?cant graphitiza
`tion of the diamond crystal during annealing to convert
`at least about 20% of the total amount of type Ib nitro
`gen present in the crystal to type Ia nitrogen.
`In the present process the diamond crystal can be
`wholly type lb or mixed type Ib-Ia. The mixed type
`crystal can range in type Ib nitrogen content from about
`99% to about 1% of the total amount of nitrogen pres
`ent in the crystal.
`In the present process the amount of conversion of
`type Ib nitrogen to type Ia nitrogen is determinable by
`a number of conventional techniques. The most fre
`quently used technique is one where it is revealed by the
`differences or changes in the absorption spectra of the
`Ib crystal taken before and after annealing. Speci?cally,
`spectra are taken of the type lb or mixed type Ib-Ia
`crystal at room temperature by means of spectrometers
`in a conventional manner showing the ultraviolet, visi
`ble and infrared absorption spectra of the cyrstal. After
`the crystal is annealed, spectra are taken of it again at
`room temperature showing its ultraviolet, visible and
`infrared absorption spectra. From a comparison of the
`changes in these ‘spectra, the amount of conversion of
`type Ib nitrogen to type Ia nitrogen is determinable in a
`conventional manner. Speci?cally, the percent of the
`total amount of type Ib nitrogen present in the crystal
`converted to type Ia nitrogen is determinable.
`The present natural diamond crystals have a color
`depending largely on the amount of type Ib nitrogen
`dissolved therein. The color of the crystals usually
`range from a greenish-yellow to a yellow with the maxi
`mum or largest amount of dissolved type Ib nitrogen
`producing the greenish-yellow color. Likewise, the
`amount of type Ib nitrogen dissolved in the crystal
`_largely determines the intensity of the yellow color
`which can range from a deep golden yellow to a pale
`yellow with the deep golden yellow indicating substan
`tially more type Ib dissolved nitrogen than the pale
`yellow. In addition, the present natural diamond crys
`tals. can exhibit a mixture of greenish-yellow and/or
`yellow colors or shades, i.e. they can exhibit local varia
`tions in their characteristic color and intensity, which in
`general indicates regions of varying type Ib nitrogen
`
`The large majority of synthesized diamonds are type
`Ib, but type 1121 diamonds can easily be made either by
`excluding nitrogen from the diamond growing media or
`by using appropriate nitrogen getters.
`'
`The large majority of natural diamonds examined are
`type Ia. No Type Ia diamonds have been synthesized
`thus far in the laboratory. Natural type Ia diamond
`crystals can have a variety of colors, with many being a
`pale yellow to colorless. Such a diamond crystal can
`also be a combination of pale yellow color and colorless
`‘areas as well as exhibit local variations in its characteris
`tic color in different parts of the crystal. Ordinarily, it
`‘has a rounded dodecahedral or octahedral morphology.
`Less than 1% of natural diamonds are type Ib, and
`usually natural diamonds of a mixed type Ib-Ia are
`found which can range widely in type Ib nitrogen con
`tent. Ordinarily, natural type Ib diamond crystal has a
`morphology exhibited in natural type Ia diamond crys
`tals.
`I
`Synthetic diamonds are substantially the same as
`natural diamonds but there are enough differences be
`tween them to distinguish between the natural and syn
`thetic crystals. These differences are mainly in mor
`phology, surface appearance, impurity inclusions and
`the nature of impurity imperfections such as the differ
`ent forms of nitrogen. As found, natural diamond crys- '
`tals most frequently have curved edges and convex
`‘faces. On the other hand, synthetized diamond crystals,
`as grown, have sharp edges, ?at and relatively smooth
`faces. Depending on the conditions of growth, synthetic
`type Ib crystals have octahedral or cubo-octahedral
`morphology, the latter sometimes having small (113)
`faces. Impurity inclusions in synthetic diamonds are
`metal catalysts whereas in natural diamonds they are a
`variety of minerals, and these impurity inclusions are
`detectable by several techniques such as electron dif
`fraction analysis or X-ray analysis.
`Those skilled in the art will gain a further and better
`understanding of the present invention from the de
`tailed description set forth below, considered in con
`junction with the ?gures accompanying and‘fo‘rming a '
`part of the speci?cation in which:
`FIG. 1 represents the phase diagram of carbon show
`ing the diamond-graphite equilibrium line _ and the
`shaded area de?nes the Region of Conversion which
`encompasses the required annealing temperatures and
`
`25
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`30
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`35
`
`40
`
`50
`
`55
`
`content.
`
`I
`
`In the present process there is no limitation on the
`size of the diamond crystals. Speci?cally, the minimum
`size of the crystals can be one micron or less and the
`maximum size is limited only by the capacity of the
`annealing equipment. For most present applications, the
`crystal size ranges from about 0.25 millimeter to about 6
`millimeters. The size of the diamond crystal given
`
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`30
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`4,124,690
`4
`3
`of non-conducting material such as pyrophyllite. Posi
`herein is that measured along the longest edge dimen
`tioned concentrically within and adjacent pyrophyllite
`sion of the crystal.
`cylinder 3 is ceramic cylinder 4 preferably made of
`The present annealing process is carried out in high
`alumina. Charge element or insert assembly 5 is adapted
`temperature-high pressure apparatus normally used for
`synthesizing diamonds by application of high tempera
`to ?t concentrically in ceramic cylinder 4 and is dimen
`sioned for a close ?t with cylinder 4. Charge element 5
`tures and pressures to a suitable reaction mass or speci
`is comprised of graphite rod 7 and graphite rod 6
`men.
`wherein the graphite is of spectroscopic purity. Graph
`One preferred form of a high pressure-high tempera
`ite rod 6 has hole 8 which is drilled to ?t closely around
`ture apparatus in which the present invention can be
`diamond crystal 9, i.e., the diamond to be annealed.
`carried out is disclosed by US. Pat. No. 2,941,248 —
`Diamond crystal 9 should not project outside of hole 8
`Hall which, by reference, is incorporated herein, and it
`since such projection would prevent a close contiguous
`is also disclosed in numerous other patents and publica
`?t of rod 7 with rod 6. Rod 7 should be in electrical
`tions. Those skilled in the art are well acquainted with
`contact with rod 6 at surface 10. Preferably, the top
`this “belt-type” apparatus and, for this reason, the appa
`surface of diamond 9 is ?ush with surface 10 of graphite
`ratus is not illustrated. Essentially, the apparatus con
`rod 6. Any space between diamond crystal 9 and hole 8
`sists of a pair of cemented tungsten carbide punches
`is preferably ?lled with an electrically conducting mate
`disposed to either side of an intermediate belt or die
`rial, such as graphite powder of spectroscopic purity, to
`member of the same material. The space between the
`promote passage of the electric current and thereby
`two punches and the die is occupied by the reaction
`promote heating of diamond 9. Electrically conducting
`vessel and surrounding gasket insulation assemblies
`circular metallic discs 1 and 2 close the ends of graphite
`therefor. High pressures are generated in the reaction
`rods 6, 7, and cylinders 3 and 4. Discs 1 and 2 are prefer
`vessel from the compressive forces caused by the rela
`tive movement of the co-axially disposed punches
`ably made of a metal such as nickel or tantalum and
`must be in electrical contact with graphite rods 7 and 6,
`toward each other within the die. Means are provided
`respectively. Since graphite rod 6 is electrically con
`for heating the reaction mass in the reaction vessel dur
`ducting and diamond crystal 9 is not electrically con- '
`ing the application of pressure.
`ducting, the highest temperatures are attained and main
`There are, of course, various other apparatuses capa
`tained at the thinnest portions of graphite rod 6, e. g., the
`ble of providing the required pressures and tempera
`area of graphite rod 6 surrounding diamond crystal 9.
`tures that can be employed within the scope of this
`After assembly of the reaction vessel and introduc
`invention such as tetrahedral types, cubic types and
`tion thereof into the high pressure-high temperature
`spherical types. Operational techniques for applying
`apparatus within the gasket/insulation assemblies, pref
`high pressures and temperatures in the apparatuses use
`erably pressure is raised ?rst and then the temperature.
`ful in the present process are well known to those
`The rates of increase of pressure or temperature are not
`skilled in the superpressure art.
`Various reaction vessel con?gurations which provide
`critical. When pressure and temperature are at a level in
`the Region of Conversion in FIG. 1, they are held at
`for indirect or direct heating of the reaction mass are
`that level for a period of time suf?cient to attain the
`disclosed in the patent literature and are useful in carry
`desired conversion of at least about 20% of the
`ing out the present annealing process. These reaction
`diamond’s Ib type nitrogen to type Ia nitrogen. When
`vessels usually consist of a plurality of inter?tting cylin
`the desired conversion is attained, the electrical power
`drical members and end plugs or discs for containing
`which heats the diamond crystal is shut off and the
`the reaction mass in the centermost cylinder. In the
`crystal cools to about room temperature quickly usually
`indirectly heated type of reaction vessel one of the
`in about 1 minute. Generally, when the crystal has
`cylindrical members is made of graphite which is heated
`cooled to below 50° C., the pressure is then released,
`by the passage of electric current therethrough and
`preferably at a rate of about 10 kilobars per minute to
`which thereby heats the reaction mass. In the directly
`atmospheric pressure.
`heated type of reaction vessel, the reaction mass is elec
`In the present annealing process the diamond crystal
`trically conductive, hereby eliminating the need for an
`electrically conductive graphite cylinder, and electric
`is annealed at a temperature ranging from about 1500" '
`C. to about 2200° C. Annealing temperatures lower than
`current is passed directly through the reaction mass to
`about 1500° C. are not operable or take too long a per
`heat it.
`iod of annealing time to be practical. Annealing temper
`U.S. Pat. No. 2,941,248 — Hall discloses an embodi
`atures higher than 2200° C. provide no signi?cant ad
`ment of a reaction vessel wherein the reaction specimen
`vantage. Annealing temperatures ranging from 1600° C.
`‘is indirectly heated, as well as the alternative embodi
`to 2000“ C. are preferred since they are not too dif?cult
`ment for directly heating the reaction specimen when it
`to attain, do not require excessively high pressures and
`is electrically conductive.
`since they induce high rates of conversion. 7
`U.S. Pat. No. 3,031,269 Bovenkerk which, by refer
`The pressure used in the present process need only be
`ence, is incorporated herein, discloses a reaction vessel
`sufficient to maintain the diamond stable at the anneal
`for indirect heating of the reaction mass. Speci?cally,
`ing temperature. Speci?cally, it is a pressure which‘
`the outer element of the reaction vessel is a hollow
`prevents graphitization or prevents signi?cant graphiti
`pyrophyllite cylinder, positioned concentrically within
`zation of the diamond crystal at the annealing tempera
`and adjacent to the pyrophyllite cylinder is a graphite
`ture. The shaded area of FIG. 1 de?nes the Region of
`electrical resistance heater tube, and within the graphite
`Conversion which de?nes the operable temperatures
`tube there is concentrically positioned an alumina cylin
`and corresponding annealing operable pressures of the
`der which holds the reaction mass or specimen.
`present process. The diamond-graphite equilibrium line
`' The directly heated embodiment of the reaction ves
`as well as pressure and temperature calibrations at such
`sel is preferred in the present process, and a particularly
`superpressures are not de?nitely known. The diamond
`preferred form is shown in FIG. 2. Speci?cally, this
`graphite equilibrium line shown in FIG. 1 is the best
`reaction vessel includes a hollow outer cylinder 3 made
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`approximation known at present for diamond-graphite
`equilibrium. Preferably, the present process is carried
`out at or above this diamond-graphite equilibrium line.
`The shaded area in FIG. 1 of the Region of Conversion
`below the diamond-graphite equilibrium line'is a toler
`ance 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 zone, the maximum period of ~ annealing
`time is about one hour without signi?cant graphitization
`of the diamond crystal occurring. If annealing times
`longer than one hour are used, then the pressure applied
`in the tolerance zone should be closer to the diamond
`graphite equilibrium line.
`1
`As shown in FIG. 1 by the Region of Conversion, an
`annealing temperature of about 1500‘ C. requires a pres
`sure of at least about 48 kilobars, at 1600° C. the pres
`sure should be at least about 51 kilobars and preferably
`about 61 kilobars, at 2000° C. the pressure should be at
`least about 63 kilobars and preferably about 74 kilobars,
`and at a temperature of about 2200° C. the pressure
`should be at least about 70 kilobars and preferably about
`80 kilobars.
`Annealing time, i.e. the period of time at annealing
`temperature and pressure, is determinable empirically
`and can range from 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. Speci?cally,
`annealing time depends largely on annealing tempera
`ture, the kind of crystal being annealed as determined
`by its type Ib nitrogen content, and the extent or de
`grees of conversion of the type Ib nitrogen to Ia nitro
`gen required. With rising annealing temperatures, the
`rate of conversion of type Ib nitrogen to type Ia nitro
`gen increases signi?cantly, i.e. more than ?ve times in
`going from 1600" C. to 2200° C. The mechanism-of the
`present process is not understood but it is believed that
`the rate of conversion of Ib nitrogen to Ia nitrogen does
`not differ signi?cantly between a crystal of high type Ib
`nitrogen content and one of low type Ib nitrogen con
`tent, but the period of annealing time at a given anneal
`ing temperature to leave essentially the same amount of
`type Ib nitrogen in each crystal does differ since the
`crystal with the higher type Ib nitrogen content has
`more nitrogen to convert to type ‘Ia nitrogen thereby
`requiring a longer annealing time.
`While the detailed mechanism of the conversion pro- I
`cess is not understood, annealing experiments have
`shown that the activation energy for the process is
`approximately 83 kilo-calories/mole (3.6 eV).
`The extent of conversion of the type Ib nitrogen to la
`nitrogen is determinable empirically by a number of ‘
`known methods in the art. For example, type Ia nitro
`gen is EPR inactive (Electron Paramagnetic Reso
`nance) whereas type Ib nitrogen is EPR active. Also,
`types Ia and lb nitrogen each have typical infrared,
`visible and ultraviolet spectra with characteristic fea
`tures which are identi?able in infrared, visible and ultra
`violet spectra of a crystal of mixed type Ia and lb.
`Preferably, to determine satisfactory annealing times
`and temperatures in the present process for a particular
`kind of crystal, e.g., a crystal containing a certain
`amount of dissolved type Ib nitrogen as re?ected by its
`infrared, visible and ultraviolet spectra and the 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-def'med. The
`platelet is then annealed at a given annealing tempera
`
`6
`ture for a certain period of time and after each annealing
`run, its infrared, visible and ultraviolet spectra are
`taken. A comparison of spectra taken before and after
`annealing indicates the extent of conversion to type Ia.
`Also, additional comparisons of such spectra with EPR
`spectra of the crystal before and 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 determined to attain a certain conver~
`sion to type Ia, such annealing time and annealing tem
`perature can be used for the same kind of crystal, e.g. a
`crystal containing substantially the same amount of
`dissolved type Ib nitrogen, regardless of its size or
`shape, to attain the same or substantially the same de
`gree of conversion to type Ia nitrogen.
`Also, after the reaction rates are determined by ex
`periments on a particular kind of crystal, it is possible to
`estimate the correct annealing times which would leave
`a speci?ed amount of type Ib nitrogen in the crystal for
`crystals having a wide range of type Ib nitrogen con
`centrations initially.
`In the present process from at least about 20% up to
`about 100% of the total amount of type Ib nitrogen
`present in the crystal is converted to type Ia nitrogen.
`However, regardless of annealing conditions a residue
`of type Ib 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 nitro
`gen present in the crystal. A conversion to type Ia nitro
`gen lower than 20% of the total amount of type Ib
`nitrogen present in the crystal may not effect the physi
`cal properties of the crystal signi?cantly for most appli
`cations. The extent or degree of conversion of type Ib
`nitrogen to type Ia nitrogen depends largely on the
`particular properties desired. In the annealed crystal
`produced by the present process which contains both
`types Ia and lb, type Ia appears to be uniformly distrib
`uted throughout type Ib.
`As a result of the present process, at least a portion of
`the crystal undergoes some change in color or shade,
`i.e. in a greenish-yellow 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 Ia. Also, when substantially all or all of the type
`Ib nitrogen is converted to type Ia nitrogen, the result is
`a very pale yellow and/or a colorless crystal which has
`many uses as jewelry, and which frequently is of gem
`quality.
`The annealed diamond crystals produced by the pres
`ent process are useful as abrasives. The abrasive indus
`try 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. For certain operations in the
`abrasive industry, synthetic type Ib crystal has been
`satisfactory and for other operations natural type Ia
`crystal has been satisfactory. However, as a result of the
`present invention, the abrasive industry now has avail
`able a natural crystal which is a mixture of types ,Ib and
`Ia, the composition of which can be controlled to pro
`duce crystals with graded physical properties over a
`wide range to adjust the crystal to the particular abra
`sive use to which it is applied. Speci?cally, with increas
`ing degrees of conversion of type Ib to type la, the
`crystal changes ‘in abrasive properties, usually becom
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`4, 124,690
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`With respect to infrared spectra measurements, al
`ing harder and stronger. As a result, a mixed type Ib-Ia
`crystal can be produced having optimum properties for
`though the 1130 cm-1 band in type Ib crystals is nor
`mally used to characterize the Ib crystal, in the present
`a particular abrasive use.
`When substantially all or all of the type Ib nitrogen in
`instance, for purposes of accuracy, the 1345 cm-1 band
`the crystal is converted in the present process, the resul
`which is correlated to type Ib nitrogen was used to
`determine the conversion of the type Ib nitrogen to type
`tant natural type Ia crystal is also highly useful as an
`Ia nitrogen.
`abrasive.
`'
`The annealed diamond crystals of the present process
`There are two forms of type Ia diamond, an A band
`are also useful as jewelry, especially those of gem qual
`form and a B band form, and these forms are distin
`ity.
`guishable by their infrared, visible and ultraviolet ab
`sorption spectra. Usually, however, these two forms are
`In such instance where the present annealed crystal,
`most easily differentiated by their infrared spectra
`polished or unpolished, has a shape which does not
`wherein the A band form has its main absorption band
`reveal ‘it to be natural, it can be identi?ed'as a natural
`diamond by a known light scattering technique. Speci?
`coming at 1280 cm‘1 and the B band form has its main
`cally, this technique comprises examining the crystal
`absorption band coming at 1175 cm— 1. While each form
`under a microscope by shining a beam of light at an
`of type Ia appears to be thermodynamically more stable
`than type Ib-diamond, the present example produced
`angle thereon and observing the scattered light re
`?ected from scattering centers normally present in syn
`only an increase in the A band form which is herein
`thetic diamond but such scattering centers, and result
`referred to broadly as type Ia.
`TABLE I
`EVIDENCE FOR CONVERSION TO TYPE Ia
`INFRARED
`E P R
`VISIBLE-UV
`Absorption
`Total % Transmission APPEARANCE
`Converted at
`0F
`to Ia
`4500 A
`CRYSTAL
`
`10
`
`Intensity
`
`Size Treat-
`Ex.
`No. (mm) ment
`
`ANNEALING CONDITION
`Tempera-
`ture
`(° C)
`
`Total %
`Intensity
`Pressure Time At 1343 cfn‘l Converted
`(Kilobars) (Min)
`(cm_ )
`to la
`
`1
`1A
`
`1.0 None
`Annealed
`
`—
`1875
`
`—
`60
`
`—
`30
`
`1.91
`.85
`
`——
`55
`
`120
`66
`
`— Low
`45
`Signi?cantly
`higher
`than non
`annealed Ex. 1
`
`Yellow
`Lighter Yellow
`than Ex. 1
`
`35
`
`45
`
`ing scattered light, are not known to have been seen in
`natural type Ia or mixed type Ia-Ib diamond crystal.
`The invention is further illustrated by the following
`exmples which are tabulated in Table I and wherein the
`procedure was as follows unless otherwise stated:
`A mixed type Ib-Ia natural diamond crystal was used.
`It was at least partly polished in a conventional manner
`using a scaife. The resulting plate had a signi?cantly
`uniform thickness which was about % mm. The size of
`4-0
`the plate given in Table I is its maximum width.
`The crystal was annealed in a reaction vessel as
`shown in FIG. 2. Graphite rods 6 and 7 were of spectro
`scopic purity and of the same size, each was 80 mils in
`diameter and 225 mils in length. A hole 8 was drilled in
`rod 6 to a size to ?t closely around the diamond crystal
`and any space between the diamond crystal and inner
`surface of hole 8 was ?lled with graphite powder of
`spectroscopic purity. The diamond crystal did not pro
`trude from the drilled hole 8 and electrical contact
`between rods 6 and 7 was maintained as shown in FIG.
`2. Ceramic cylinder 4 was made of alumina and had an
`inner diameter of about 80 mils and a wall thickness of
`60 mils. Cylinder 3 was made of pyrophyllite and had an
`inner diameter of about 200 mils and a wall thickness of
`55
`75 mils. Metallic disc members 1 and 2 were circular, of
`the same size, each with a diameter of 350 mils and a
`thickness of 10 mils, and made of tantalum. The discs
`were in electrical contact with rods 6 and 7 as shown in
`FIG. 2. -To carry out the present annealing process, this
`reaction vessel was used in the “belt-type” apparatus
`disclosed in U.S. Pat. No. 2,941,248 — Hall.
`Absorption spectra ranging from the ultraviolet
`through the infrared were made of the diamond crystal
`at room temperature before and after it was annealed.
`Electron paramagnetic resonance (EPR) spectra
`were made of the diamond crystal at room temperature
`before and after the crystal was annealed.
`
`60
`
`65
`
`Table I illustrates the present invention. The decrease
`in EPR and infrared intensities in the natural diamond
`of Table l was used to monitor the conversion of its
`type Ib nitrogen to type Ia'nitrogen. The alternatives
`that a decrease in these type Ib diamond EPR and infra
`red intensities could also occur if the 1b, nitrogen was
`diffusing out of the crystal with no conversion to type
`Ia nitrogen or changing to nitrogen of yet another type
`were ruled out for two reasons. The ?rst reason is quali
`tative in that type Ia infrared absorption bands increase,
`hence there is some conversion to type Ia nitrogen. The
`second reason is quantitative in that from the type Ib
`and type Ia absorption bands present one can calculate,
`based on published data, using standard techniques, the
`total amount of nitrogen present. For the present an
`nealing experiment this nitrogen content of the diamond
`of Table I remains constant, within experimental error.
`For instance, Example 1 had an initial total nitrogen
`content of 133 ppm. After the annealing the infrared
`absorption spectra and EPR for Examples 1 and 1A
`showed that the type Ib nitrogen decreased to 45 per
`cent and 55 percent, respectively, of its original content
`as shown in Table I, yet the total nitrogen was 135 ppm,
`the same as before the annealing process. Hence, no
`change occurred in the total nitrogen content despite
`the fact that the ?nal type Ib nitrogen was approxi
`mately 45 to 55 percent of that originally present.
`Therefore, the change in intensity of the type Ib ab
`sorption band ‘at 1343 cm_1 is a good indication that
`type Ib nitrogen is being converted to type Ia nitrogen
`and is not diffusing out of the diamond or being con
`verted to nitrogen of yet another type.
`It is understood that the present annealing process
`can be carried out with the same diamond crystal more
`than one time to additionally increase the amount of
`type Ia nitrogen therein. For example, a mixed type
`Ib-Ia annealed diamond crystal produced by the present
`
`8
`
`

`
`_ 4,124,690
`
`10
`
`15
`
`9
`process can be annealed in accordance with the present
`process to convert an additional amount of type Ib
`nitrogen to type ‘Ia nitrogen.
`In copending US. patent application Ser. No. 707,298
`entitled “Annealing Synthetic Diamond Type Ib” ?led
`July 21, 1976 in the names of Herbert M. Strong, Rich
`ard M. Chrenko and Roy E. Tuft and assigned to the
`assignee hereof, and which by reference is made part of
`a disclosure of the present application, there is disclosed
`the annealing of type Ib synthetic diamond crystal at an
`annealing temperature ranging from about 1500" C to
`about 2200° C under a pressure which prevents signi?
`cant graphitization of the diamond during the annealing
`to convert at least about 20% of the total amount of
`type Ib nitrogen present in the crystal to type Ia nitro
`gen and produce an annealed crystal wherein at least
`about 20% of the total nitrogen present is type Ia nitro
`gen.
`What is claimed is:
`1. An annealing process for converting type Ib nitro
`gen to type Ia nitrogen in natural diamond type Ib crys
`tal or natural diamond mixed type Ib-Ia crystal, each of
`said crystals having a minimum size of one micron as
`measured along the longest edge dimension of the crys
`tal, which consists essentially of subjecting a specimen
`consisting of said crystal to an annealing temperature
`
`10
`ranging from about 1500° C. to about 2200° C. under at
`least a pressure which prevents signi?cant graphitiza
`tion of said crystal at said annealing temperature for a
`period of time ranging from about one minute to about
`50 hours and suf?cient to convert at least about 20% of
`the total amount of type lb nitrogen present in said
`crystal to type Ia nitrogen, the minimum pressure rang
`ing from 48 kilobars at said annealing temperature of
`1500“ C. ‘to a minimum pressure of 70