The Graphitization of Diamond

8/3/2019 The Graphitization of Diamond

http://slidepdf.com/reader/full/the-graphitization-of-diamond 1/16

The Graphitization of Diamond

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

1962 Proc. Phys. Soc. 80 648

(http://iopscience.iop.org/0370-1328/80/3/310)

Download details:

IP Address: 141.217.20.120

The article was downloaded on 25/01/2012 at 17:40

Please note that terms and conditions apply.

View the table of contents for this issue, or go to the journal homepage for more

Home Search Collections Journals About Contact us My IOPscience

http://iopscience.iop.org/page/terms

http://iopscience.iop.org/0370-1328/80/3

http://iopscience.iop.org/0370-1328

http://iopscience.iop.org/

http://iopscience.iop.org/search

http://iopscience.iop.org/collections

http://iopscience.iop.org/journals

http://iopscience.iop.org/page/aboutioppublishing

http://iopscience.iop.org/contact

http://iopscience.iop.org/myiopscience

http://iopscience.iop.org/myiopscience

http://iopscience.iop.org/contact

http://iopscience.iop.org/page/aboutioppublishing

http://iopscience.iop.org/journals

http://iopscience.iop.org/collections

http://iopscience.iop.org/search

http://iopscience.iop.org/

http://iopscience.iop.org/0370-1328

http://iopscience.iop.org/0370-1328/80/3

http://iopscience.iop.org/page/terms



P R O C . PHYS. SOC., 1 9 6 2 , VOL. 80

The Graphitization ofDiamond

BY V. R. H O W E S

J. J. Thomson Physical Laboratory, University of Reading

Communicated by R . W. Ditchburn; M S . received 16th March 1962,in revisedform 16th April 1962

Abstract. In experiments on diamond octahedra at temperatures of 1700 'c a d

above in high vacua, graphite has been observed to develop both internally a d

on the surfaces. The surface graphite that has been observed is different fromthat associated with etching, and is independent of the gaseous environment. Its

development increases with t h e and rapidly increases with temperature. The

internal graphite always has associated with it an intense strain in the surrounding

diamond. It has varied in appearance from isolated particles to orientated distri-

butions of crystallographic microscopic inclusions. Some diamonds have remained

completely free from internal changes. For the diamonds which have developed

inclusions the changes have occurred suddenly and discontinuously, the internal

graphite generally not extending with time or increasing temperature. It is thought

that both phenomena are due to a physical phase transformation, and that defects,

of some particular kind which are present inside many but not all diamonds,

initiate the transformation which has given rise to the internal changes that have

been observed.

5 1. I N T R O D U C T I O N

N the experiments to be described, natural diamonds have been carefully examinedI ith an optical microscope before and after being heated to temperatures within the

range 1400-2100 OC. A vacuum of 5 x 10-6 m H g or better has usually been main-

tained du rin g the heat treatment. T hi s allows th e changes th at occur to be studied

without the obscuring and complicating effects of etching by the surrounding gases.The results are drawn from experiments on thirty-five diamonds, seventy-five furnaceruns having been carried ou t on different diamonds fo r different times and temperatures,The diamonds were small octahedra; thus the only faces observed were (111) faces

except where (110) faces developed with th e heat treatm ent. T h e weight of each

diamond was approximately carat.Friedel and Ribaud (1924) first reported the development of a black layer on the

surface of diamond at temperatures between 1500 O c and 1800 O c , which was bright and

hard and dissimilar to the graphite associated with the 'burning' of diamonds.

T he y also reported the total transformation of diamond to graphite at a temperameof around 1900 OC. They found that a large and sudden change occurred in the b kfringence pattern jus t before th e bursting and graphitization of th e stone. Seal (1958)studied the surfaces of octahedral diamonds after heating up to 1800 OC and observedraised triangular pyramids of graphite, but the vacuum in which the diamonds Were

heated was only lO-4mmHg and no mention is made of th e appearance after de.

graphitization . Seal also reports evidence of graphitization occurring at points just

below the diamond surface and b ursting through to t he surface.

The present work shows that on all the diamonds heated above 1700O C

graphitewas found on the surfaces. It s formation was independent of the surrounding gas

pressure. T h e graphite started to form at points, and then spread and increasedfobuild a layer similar to tha t described by Friedel an d R ibaud. Many of the diamonds$

648



The Graphitization of Diamond 649

but sipficantly not all, developed black inclusions upon heating. The se ranged from;solated partic les to distributions of crysta llographically orien tated microscopic in-

clusions. T h e structure , tim e and temperature dependence, and othe r properties of

both the inclusions and the surface graph ite have been observed.

$ 2. A P P A R A T U S A N D P R O C E D U R E

The furnace used was of the design described by Haworth (1958) and consistedessentially of a cylindrical tungsten element, surrounded by radiation shields, withwater-cooled electrodes, inside a water-cooled vacuum chamber. The temperature wasmeasured with an optical pyrometer which had been checked against standard meltingpoints to allow for possible non-black-body conditions; measurements were made with

estimated error of i 0 OC. T he diamond specimen, placed between two flat diamonds,172s held in a tungsten clip which was balanced on top of a tungsten pedestal inside theelement. T hu s only diamond and tungsten were in the hot zone of the furnace. A thigh temperatures it was found th at diamond was attacked by tungsten vapour, tungstencarbide being formed. T h e two diamond flats were used to protect the top and bottomfaces of the octahedral specimen from this attack, so tha t these faces and the interiorcould be examined free from the interfering effect of the tungsten reaction.

Careful microscopic examination was ca rried out to select diamonds with no unusualsurface

features, and no visible internal flaws or inclusions. For this examination, illumin-ation at right angles to the direction of observation was used, together with an objectivelens which had just sufficient working distance to be able to focus through from face toface of the diamonds available. A maximum limit to the magnification of 500 wasobtainable, so that with careful scanning one could be sure of detecting inclusions ofsize 1 micron o r bigger.

Before being put into the furnace the diamonds were boiled in concentrated nitricacid, washed in distilled water, and refluxed in alcohol vapour; after the acid treatmentthe diamonds were handled only with g lass tweezers.

Because of the importance of obtaining as high a vacuum as possible, the vacuumfurnace was pumped down overnight before each experiment, a liquid air trap wasused during each experiment, and the heating up was carefully controlled to allow foroutgassing. I n th is way a vacuum of 5 x 10-6 mmHg or better was maintained. How-ever, for temperatures greater than 2000 O c the rapidity of g raphitization made it

necessary to keep the times very short, SO that some pressure rise was inevitable; theprocedure th en followed is described later. After an experiment the temperaturecould be reduced to below 1400 OC in less tha n 30 seconds.

After the furnace runs a study was m ade of th e surfaces of the diamond specimens.

Then the stones were boiled in a strong degraphitizing agent consisting of 3 parts60% perchloric ac id, 1 part concentrated nitric acid and 4 arts concentrated sulphuricacid. Finally, a thorough examination of surface and interior was carried out.

$ 3 . T H E S U R F A C E PHENOMENA

3 . 1 . The Exposed and the Protected Faces

After experim ents at 1700 Oc and above, x-ray exam ination has shown the presence

Ofa tungsten carbide layer on the specimen surfaces th at had been exposed during theheating, and a graphite layer on the protected surfaces. T he former layer had a roughetch appearance showing no regular structure. Every diamond that has been used i n

at tem peratures of 1700 Oc and above has show n the form ation of black6



650 V.R.Howes

nuclei on the protected faces. These nuclei have extended and increased in numberwith time of heating, until by interweaving they have formed a continuous black layerof graphite. T he observations described below have been made on the protected faces

of the diamond specimens.

3.2. Time and Temperature Dependence

After experiments at 1400 OC for periods of up to 20 hours duration and at 1600 oc

lasting up to 5 hours, n o nuclei could be detected on th e surfaces examined using t6e

highest optical microscope magnifications. At 1700 O c th e black nuclei form slowly, notbeing detectable with less than 15 minutes heating. After 30 minutes they are Seen

clearly as isolated propeller shapes-figures l(a) and (b) show the same area of surfaceafter 15 and 45 minutes. Figure l(e) shows the whole of another diamond face

approximately half its area still clear after 120 minutes. At higher temperatures thegraphitization increases rapidly; figure l(c) shows the same area as figure l(b) &er a

further 5 minutes at 1800 OC. After 5 minutes at 1900 OC the surface was completelycovered, the interwoven appearance of the layer is seen in figure l(f)where some of

th e graphite has been removed.

3.3. The Nuclei

These grow as propeller shapes as seen in figure 2(a), with the three blades being

in the (112) directions parallel to the (111) plane of the diam ond. The orientation

with respect to the triangular face of th e diamond octahedra can be seen in figure l(e).

Side branches develop from the main branches, again in (112) directions, formingcomplex nuclei as in figure 2(b).

Although nucleation of these isolated propellers does occur all over the surfaces,steps, such as the edges of growth sheets, trigons, and ring cracks, do form preferentialsites. T he nuclei which the n develop have single or double blades extending away

from th e step edge in (112) directions. The nuclei also appear on (111) cleavagefaces when cleaved diamonds are heated, single blades occurring densely at the cleavage

steps in all three (112) directions according to the direction of the step, as seenfigure 2(c). Some isolated nuclei were also observed. On the cleavage face of another

diamond a twin boundary was observed, but after heating no nucleation had occurredat this boundary, although graphite blades were profusely clustered along the cleavagesteps crossing it.

In order to see if the graphite propeller nuclei were atmosphere dependent, thepressure was increased up to 5 pmHg during a furnace run of 15 minutes at 1700 C.

No increase in number or size of nuclei was observed but only the rough etch attack

shown in figure l(d). This can be compared with figure l ( a ) for which the pressure

did not rise above 0.01 pmHg. At 1800 O c with an air pressure of 5 pm Hg, the intenseetch attack on the diamond and the graphite by the air left only an extremely roughblack surface. However, after th e matt black graph ite layer du e to the etching readonhad been removed by degraphitization, propeller-shaped markings could be seen On

th e surface beneath. Another experiment done without using th e liquid air trap on

the vacuum system, so that the oil vapour pressure inside the furnace was Sfighds

increased, gave no visible change in the appearance of the diamond surfaces.When t he diam ond specimens were boiled in th e degraphitization agent it was noted

th at the propeller-shaped nuclei were very little changed. Figures 2(a) and 2(b) are Of

surfaces that have undergone this treatment and it can be seen that the graphitebabeen removed only at the smallest propellers and at th e very edges of t he larger ones)

an imprint in the diamond surface remaining.



65 1he Graphitization of Diamond

Figure 1. (a), ( b ) , (c) x 141, part of an octahedral diamond face after 1 5 minutes

at 1700 O c , after another 30 minutes at 1700 Oc, after another 5 minutes at 1800 "crespectively; (d) 141, another face exposed to air at 5 pmHg pressure for 1 5 minutes

at 1700 "c; e ) x 60, the whole of another face after 120 minutes at 1700 Oc; (f)x 372, a face after 5 minutes at 1900 O c when the surface graphite has been partially

cleaned off.



652 V . R . Howes

Figure 2. (a) , (h) x 10.50, surface graphite nuclei; (c) x4.50, nuclei forming at

cleavage steps on a cleavage face o f diamond.

3.4. The Layer

The graphite layer that was formed on th e (111) faces of the diamond octahedra

was smooth and compact, and was extremely difficult to remove with the degraphitiz-ation agent used; even prolonged boiling in the agent removed only small patches. A t

these patches the thickness of the layer was measured, by the fine focusing control on

the microscope, to be of the order of 1 micrometre. An attempt was made to polish

the layer off the surface using a fine alumina polishing powder, and figure l(f) hows

the appearance afterwards. T h e exposed diamon d surface had a grooved pattern

corresponding exactly to the structure of the graphite layer.

T h e graphite layer was studied by reflection electron diffraction, which revealed

that the graphite had a grain size of the o rder of 100A. 'fhere was definite evidence of

a preferred orientation such that thec

axis and the a axis of the graphite were in theplane of \the surface, i .e. were orthogonal t o th e diam ond (111) direction. But there

was also evidence of randomly orientated graphite being present as well.

5 4. T H E I N T E R N A L P H E N O M E N A

4.1. Results fo r Thirty-Jive DiamondsIn the initial examination of all diamonds available, the proportion of rejections

amongst gem quality diamonds was high. For example, in one batch of forty-five em




quality octahedra only nineteen had no visible inclusions. Many of th e inclusions seenin the rest were fa r too small to b e detected by eye or with a hand lens.

Of the thirty-five diamonds free from visible inclusions tha t w ere used in the experi-ments, sixteen contained black inclusions after heating at temperatures above 1500 OC.

Of he rest, thirteen diamonds remained quite clear, no inclusions being visible withb e most careful examination, and for six diamonds observation of the interior was no tpossible because of the surface graphitization tha t had occurred. I n fu rther experimentsat 1800 O c and 1900 OC for the diamonds which had remained clear at 1700 O c , noneshowed inclusions after the higher temperature treatment. Fo r those in which inclusionshad been found after heating at 1700 OC, none showed additional new inclusions at th ehigher temperatures, and in only tw o special cases had the original inclusions extended.

For seven of the sixteen diamonds affected internally by the heat treatment, theinclusions had shapes and orientations crystallographically related to the diamond.

The other nine diamonds had inclusions that were irregular and non-crystallographic.

4.2. The Non-crystallographic Inclusions

The non-crystallographic black inclusions that have been produced were generallyless than 100 microns across, irregular and sometimes jagged. Th ey varied in num berper stone from one to ten or more, and in thre e stones at least occurred only just belowan octahedral diamond surface. Intense localized strain patterns w ere detected a t theseparticles, as seen fo r three of them in one diamond in figure 3(a),where the micrographwas taken with the polarizer and an analyser in crossed positions in the microscope.

The small particles did not change in appearance or increase in number whenfurther heated at 1700 OC. Similarly, when the diamonds containing such inclusionswere heated at 1800 OC and 1900 Oc , the inclusions were not observably changed orincreased in number. Also the strain patterns remained unchanged.

Large irregular inclusions were found in tw o diamonds after they had been treatedfor 15 minutes at 1700 OC. Figure 3(b) shows six inclusions in the interior of one of

these diamonds. Figure 3(c) shows the associated extreme state of strain which, incontrast to that for the small particles, extends throughout the crystal. It was also

found for this particular diamond that some of the inclusions extended with furtherheating at 1700 OC, and with heating at higher temperatures.

4.3. The Crystallographic Inclusions

After being heated above 1500 O c , three diamonds ex?iiibited needle-like inclusionscrystallographically orientated with respect to the octahedral diamond, and four dia-monds ha d inclusions in th e form of platelets.

The black needles, from 5 to 50 microns in length, were in all six ( 1 1 0 ) directionslying in (111) layers approximately 10 microns below the octahedral faces. Some of

these needles can be seen in figure 4(a) ; he three ( 1 1 0 ) directions parallel to thesurface are clearly seen. T he other three ( 1 1 0 ) directions project as ( 1 1 2 ) directionson a ( 1 1 1 ) plane, and these needles appear short and blurred since each one cannotall be in focus at once. For one of these diamonds, needle inclusions were found approxi-mately 10 microns below all eight faces. Distributions varied from a few needles onlyto a complete uniform layer, and from complete irregularity, as seen in figure 3 0 , oorientated triangular distributions, one of which is seen in figure 3(d) . There was noapparent relation between the distributions on opposite or adjacent faces, and noinclusions at all were visible in the interior of the stone deeper than 20 microns belowthe faces. Each needle had an intense localized strain associated with it, so that thelayers of inclusions gave com plicated strain patterns like that seen in figure 3(e) for the



654 V . R. Howes

Figure 3. (a) 78 , stra in pat tern associa ted with par t ic le inclusions; (h) x27,

an d (c) x 30 inclusions inside a diamond and th e gene ral asso ciated strain pattern;(4, e ) x 33, needle-like inclusions just below a surface, and the associated strain

P a t t e r n ; (f) x 33, an ir regular distr ib utio n of needle-l ike inclusions.




Figure 4. (a) x 249, the needle-like inclusions; (h) x 420, a layer of small platelet

inclusions just below a surface; (c) x 108, two trails of platelet inclusions occurring

through a diamond; (d) x 150, central distribution of platelet inclusions found in

one diamond; (e) x7.8 , a diamond after 5 minutes at 1900 ' before degraphitiz-

ation; (f) x7.2, a diamond after 12 hours at 1700 O c after degraphitization.



656 V.R.Howes

distribution seen in figure 3(4. Away from the needles th e diamond was nearly strain.free.

Another diamond showed a layer of discontinuous inclusions about 10 microns

below one (111) face after being heated for 15 minutes at 1700 OC. But in this casethere was nothing visible beneath the other seven faces, and the inclusions Were veqsmall platelets, 2 to 5 microns across, as seen in figure 4(b).T h e largest of th e plateletscan be seen to be hexagonal and they have sides in (110) directions. Another diamondshowed similar platelet inclusions just below the surface but only occurring in smallgroups or singly. Platelets were found in two other diamonds after heating at 1700 OC,

I n one, several trails of small platelets extending a considerable distance through thestone could be seen, two of which are shown in figure 4(c) ; in the other an extensiveformation of platelets was found centred in the middle of th e diamond. Figure 4(d)

shows this distribution which was approximately 6 m m across comparing with the1+mm leng th of edge of the triangular face of th e octahedral diam ond; the individualplatelets were of the order of 10 microns across. Both the platelets and the boundaryof th e distribution as observed were hexagonal with sides parallel to t he (110) directionsof t he diamond. Microscopic observations at different levels in th e diamond, and thefact that the distribution appeared the same through all eight faces, have led to the

conclusion tha t it consists of four hexagonal-shaped layers lying on the four (111)planes passing through the centre of the formation, each layer comprising many smallhexagonal platelets. Figure 5 illustrates the composition diagram matically. The linear

2 / ”4 J

CO o !

Figure 5. Four hexagonal distributions of equal density on the four (111)planes

through a point as seen in a (111) direction.

inclusions, which also can be seen in figures 4(b) and 4(d), have thickness a d heirlengths lie in (110) directions. Measurem ents of the ratio of the width to the lengthof some of th e larger ones show tha t they are very probably projected views of hexagonal

platelets lying on th e other three (111)planes. Thus the layers are of hexagonal plateletslying in all four possible (111) planes randomly.It was found that both types of crystallographic inclusions were in no way altered

either fu rth er heating at 1700 OC , or heating at higher temperatures of 1800 OC or 1900 c-




4.4. Attempted Correlations

In trying to find an indication of the cause of the internal changes that have beendescribed, several phenomena related t o diamond were compared with the production

of hternal inclusions upon heating, bu t no correlations were fou nd :(i) Some type I diamonds gave inclusions, some did not.(5)T h e inclusions had no association with th e surface graphite nuclei.(iii) Some diamonds having intense strain patterns before treatment developed

inclusions, some did not.(iv) A diamond showing an intense fluorescent centre when exposed to ultra-violet

fight remained free from internal graphite after heating a t 1900 OC , the centre notbekg visibly changed.

(v) A twinned diamond gave no internal graphite after heating.(vi) After an irradiation with approximately 1017 fast neutrons[cm2 another diamondbecame transparent again after heating at 1700 O c bu t showed no inclusions.

3 5 . M A C R O S C O P I C C H A N G E S

5.1. Change in Shape

Graphite formed a t the (110) edges of the diamond octahedra much m ore rapidlythan on the (111) faces. T h e edges became encrusted and rough in contrast to the

smooth black faces, as can be seen in figure 4(e) which shows a diamond that had beenheated for 5 minutes at 1900 OC. The encrusted graphite at the edges was dissolvedquite easily in the degraphitizing agent, revealing rough flat areas beneath. Anotherdiamond, which had been heated a t 1 7 0 0 " ~or 12 hours and then degraphitized,showed that these areas were i n fact dodecahedral faces. Figure 4f3 shows this stonewith a remnant (111) face surrounded by three (110) faces which have become pre-dominant. Th ese (110) faces were rough and black, with nothing like the interwovenpropeller structure seen on th e (111) faces. Most, b ut not all, of the eight (111) facesstill remained, bu t th ere w ere no cubic faces visible.

Although the encrusted graphite on the edges readily dissolved in the degraphitizingagent, the (110) faces exposed remained quite as black as the (111) faces. T ha t this

was due to a thin layer only was proved by the clear diamond visible underneath wherefragments had fractured off at the edges. A diamond tha t was superficially graphitized,but still with no internal inclusions after 10 minutes at 1900 OC, was transformed com-pletely to graphite after 30 minutes at 1900 OC .

5.2. Expmiments above 2000 OC

Two diamonds, selected from those that had previously shown no visible internalchanges when heated to 1700 O c , were heated above 2000 OC (2010 OC and 2120 "c ) foras short a time as experimentally possible (up from 1700 OC and down again in threeminutes). Both were very heavily graphitized superficially. After boiling in the de-graphitization solution the diamonds were found, although in fragments, showing tha tcomplete graphitization had not occurred. Of the faces of the fragments some werecleavage, some original (111), and some (110). The original (111) faces and the (110)faces had the usual appearance of graphitization with a trace of additional rough etch.The cleavage (111) faces were often almost clear with a few graphite spikes and in

Some cases again a trace of rough etch. It would seem that the diamonds had fracturedat the end of the experiments, probably owing to th e rapid cooling, SO tha t little graphit-Gation occurred on the cleavage faces before the temperature dropped below 1700 OC.



658 V.R . Homes

It is also a possibility that they did not completely break up until the boiling in the

d e g r a p h ih ti o n solution. At the maximum temperature the pressure in the furnace

had risen to .$pmHg, so that th e rough etch effect was almost certainly due to somegas

etching occurring before the pressure dropped again as th e temperature was reduced,

$ 6 . D I S C U S S I O N

6.1. Surface Graphitization

T he surface graphite that forms on th e octahedral diamond faces dur ing the high

temperature high vacuum experiments described is different in many ways from that

formed during gas etching of octahedral diamond surfaces. T h e latter has been des.

cribed by Evans and Sauter (1961), where it was concluded t ha t the formation was due

to the deposition of carbon on the diamond surface as a result of surface chemicalreactions in the presence of the etchant gases. During etching, the graphite layerobtained is continuous over the surface and varies from thin grey to thick black accordingto the gas pressure and temperature. No distinct nuclei are visible, and th e black layerhas a matt and porous appearance. It is easily removed by the usual degraphitizationagents. T h e graphite layer was found to be crystallographicallyorientated with respectto the diamond, the c axis of the graphite being perpendicular to the (111) diamondfaces. T hu s the two types of graphite layer differ in formation, i n macroscopic and

microscopic appearance, in adherence to the surface, and in crystallographic orientation,In the experiments described, the formation of the surface graphite was found to be

independent of th e surrounding atmosphere. T h e difficulty of removing the graphite,together with the observation of the imprint of its structure in the diamond beneath,indicates that the graphite was produced in the surface rather than superficially on it.

At normal pressures diamond is th e thermodynamically unstable form of carbon. Theabove evidence indicates that the surface graphite formed at 1700 OC and above in high

vacuum is due to a physical phase change occurring at the surface, transforming the

carbon from the diamond to the graphite structure.

This surface graphitization has been shown experimentally to occur more rapidlyat the (110) edges than on the (111) faces of the octahedrons of diamond, leading to

the development of a dodecahedron from an original natural octahedron. A possibleexplanation of this is as follows: when the temperature is increased so that diamondnears the point at which it will become unstable and transform to graphite, atoms less

strongly attached will break free from the diam ond structure before those more stronglyattached. Thu s random fluctuations will be important if they preferentially weakenthe bonding of surrounding atoms. If one atom from a (111) face randomly becomesfree from the diamond lattice (three bonds being broken) it leaves triply bonded atomaround it, which would the n be susceptible only to similar random action. But if one

atom from a (110) edge randomly becomes free (three bonds being broken) then the

atoms on either side are only doubly bonded, so that they become more susceptibleto transformation tha n o ther atoms. Each atom becoming fre e leaves the next along the

edge only doubly bonded. Thu s for a temperature range where normal triply bondedatoms are just stable apart from random fluctuations, but doubly bonded atoms are

unstable, the freeing of one atom at an edge leads to the transformation of the wholerow of edge atoms. T his would lead to th e &st sign of graphitization occurring at the

(110) edges of an octahedral diamond, and to th e continued preferential transformationa t these edges.

It is to be noted tha t th e graphite nuclei which occur on the octahedral faces havebeen observed to form preferentially at steps on th e surfaces, which do contain (110)




&s. Th e electron-digraction results fo r the g raphite layer show that both randomly,ientated graphite and specifically orientated graphite are present; that for the latterhe hexagonal rings of the graphite structure lie perpendicular to the diamond (111)surface; and th at the a axis of th e graphite is in the surface. Since the a spacing of t h e

graphite (2.47A) is very close to the atomic spacing in the (110) direction for diamond( 2 . 5 2 ~ ) , t is a possibility that this forms the basis of the transformation. Fig ure 6

I 2

Figure 6. T w o puckered hexagons within the diamond lattice.

shows two ways of forming flat graphite-type hexagonal rings from the Puckeredhexagonal rings of the diamond structure. Suppose atoms A B C D E F form onepuckered ring in the surface (111) plane of a diamond octahedron, and at0ms G H I

occur below C and E to complete another puckered hexagon C D E G 1H. f the bondsmarked 1 are broken, th en th e hexagon A B C D E F becomes free to flatten, giving agraphite-type hexagon ring parallel to the diamond (111)plane. Fo r graphite formingon this principle, the c axis would be perpendicular to the (111) diamond plane (this is

the orientation of the graphite found on diamond octahedral faces after etching withOxygen). If the bonds marked 2 are broken, then hexagon C D E G IH can flattenmerely by the swinging up of atom D and the swinging down of atom 1, atoms c , E ,G and H having to move only very slightly. Graphite forming on this basis wouldhave the c axis and the a a i s (direction c 3 ) n the (111)d i m " plane-which theorientation of the surface graphite formed in he experiments that have been described.In this model the a direction of the graphite hexagon is the (110) direction of thediamond structure, and a threefold symmetry would be expected in the developmentof graphite across the (111) diamond surface involving the three (110) directions.

However, if it is considered that the formation of a flat graphite-type hexagon ringWeakens the bonds from the atoms involved to other atoms around them, the type ofPropagation that might be expected is illustrated in figure 7 . Here th e atoms are shownU-I a puckered (111)plane of diamond, the filled-in circles representing the lower atoms6 t h the fou rth bond going down, the open circles representing the upper atoms withthe fourth bond coming up. Suppose a flat hexagon first starts to form on atoms Ba d c, and the two beneath the m (atom A breaking bond 1 and swinging into the newPosition as shown). Then if it is considered that this weakens bonds marked 2, it is

more likely that atoms D and E will swing into positions as shown to form furth er flat

hexagonal rings, rather than into positions between B and F, and c and G respectively.At the same time, atoms H nd 1 re 1;kely t o swing into the ir new positions as shown.



660 V.R.Howes

The propagation of graphite-type hexagon rings will then be in the directions markedby the large mo w s, which are (112) directions in the (111) diamond plane. Thismechanism, although giving a development with orientation agreeing with that found

experimentally, does not immediately give normal graphite, since the large value of

the c spacing of norm al graphite (6.8 A) has not been taken into account. But by selective

development of some of th e initial hexagon sheets, and a small relative shift of &ematesheets, the normal graphite structure could be obtained. T h e extra carbon atoms

Figure 7. The puckered (111) plane of diamond showing a possible developmentof perpendicular flat hexagon rings.

between the graphite sheets could form, after migration, the randomly orientatedgraphite also detected-it is to be noted that the graphite grain size is very small (1001).The model described is no more than an attempt to understand the adhesion and

orientation of the graphite found on the diamond surfaces, particularly the orientationin (112) directions rather tha n (110) directions.

T h e surface graphitization tha t occurs has been seen to increase with time and with

higher temperatures, so th at eventually the diamonds would be completely transformedto graphite. Fo r the size of diamond used in the experiments described there will be

a time t, for each temperature T, when all the diamond has been graphitized by th e

surface reaction.Thu s, for T = 1700 Oc , t > 12 hours

T = 1900 OC, t > 10 minutes b ut < 30 minutes

T = 2100 OC, t > 10 seconds but < 3 minutes.It can be seen that considering just the surface graphitization alone, diamond is

very rapidly transformed to graphite at temperatures around 2000 OC.

6.2. Internal Graphitization

For the intemal inclusions that have been observed after heating diamonds to

1500 O C and above, the black material has not been directly analysed since it occursinside the diamond octahedra and usually in discontinuous arrays of small particles.




However, in three cases where the inclusions have been non-crystallographic and inmo cases for crystallographic inclusions (one for platelets and one for needles), the

of black material occurring were much greater tha n the order of total impuritycontent, excluding nitrogen, that has been reported by Kaiser and Bond (1959). Sincethe black material appears similar in all the diamonds showing inclusions, and certainlyha the same form for those diamonds where it occurs crystallographically, it wouldSeem very likely tha t all the material that has been observed has been carbon occurring

as graphite.The nucleation of the internal graphite may very well be associated with impurities

in the diamonds. Som e diamonds remain free from internal graphite, while for theothers which do develop inclusions du rin g heating, th e graphite occurs d iscontinuously.These facts lead to the conclusion that the internal graphitization must be initiated at

defects of some kind inside th e stones. T h e s u e of these defects must be som ethingless than 1 micron, and different forms of defects may explain the various forms ofthe inclusions-lumps, particles, needles, platelets. For example, the source defect fo ra platelet must be effectively two-dimensional since nothing but single platelets havebeen observed (if the defects were points o r three-dimensional, then 4 111)planes couldequally well be involved in the graphitization and multiple platelets should be observed ;if the defects were one-dimensional th en 2 (111) planes could be involved and doubleplatelets should be seen). I t is to be noted that no exam ples have been found of d iamondsthat have developed both types of crystallographic inclusions.

It is possible that the defects, associated with graphitization, simply supply theextra space required for the initial transformation from the diamond to the graphitestructure. Alternatively, a reaction may occur at the defects a t the high temperaturesobtained, which allows the required energy for transforming the carbon from themetastable to the stable state. The fact that the graphitization a t the surface is stillvery slow at 1700 OC, while the internal graphite develops very rapidly, suggests thesecond possibility . Also th e negative results following th e irradiation experim entsindicate that th e firs t possib ility is unlikely .

The graphite form ed distorts the surrounding diamond causing the intense localizedstrain that has been detected for all observed particles, platelets or needles of graphite.The impression is th at a stimulated phase change occurs from th e defect, and graphiterapidly develops until the pressure built up in the distorted diamond rises to a valuefor which diamond is again non-transformable. Bridgman (1947) has shown th at suchan inhibition of graphitization of diamond can occur below the theoretical thermo-dynamic equ ilibrium pressure. T hus th e process is not reversible, and what graphiteis formed will remain when the temperature is reduced. It has been shown experi-mentally tha t after furth er heating at a higher temperature no ex tension of the internal

graphite distributions was observed, bu t it has been impossible to prove whether initialheating a t 1900 O c instead of 1700 O c would have given larger inclusions.The frequent occurrence of the internal graphite in (111) plane layers parallel to

the octahedral faces indicates a possible association with the growth process, the defectsassociated with graphitization being trapped between growth layers. T he presence ofthe layers ju st beneath the diamond faces and not deeper has alternative explanations;first, that the defects associated with graphitization are trapped only in the last stagesofthe growth. T h e second explanation is that, in a general distribution, developinggraphite can distend th e diamond more easily near a free surface, SO growing to a visible

size before the critical pressure is built up in the surrounding diamond. However, itSeems unlikely th a t th e central symmetrical formation of platelets that was foundOnce could be directly connected with the growth of the crystd, or that an exactly



662 V.R.Howes

similar pattern of defects associated with graphitization existed before th e diamond was

heated. A more likely explanation is that a central inclusion or fault, within a generaldistribution of defects, produced a strain effect on all the (111) planes through the

inclusion when the diamond was heated, th is effect accelerating graphitization at thedefects on these planes.T he development of a graphite platelet from a defect can be explained by the

flattening out of the kinked hexagon rings in the (111) planes of the diamond lattice,If this were to happen uniformly, spreading from a central ring, then a flat hexagonsheet would be formed with (110) edges, thus having the same orientation as &eplatelets. However, no mechanism can be offered at the moment for the developmentof the needle inclusions.

T h e fact tha t diamond was still found after an experiment at 2100 OC is notable in&at up to th is temperature at least spontaneous transformation from diamond to graphitedoes not occur. The increasingly rapid progressive surface graphitization that has beenseen to occur at temperatures above 1900 OC indicates that in practice this process is

likely to transform a diamond before the necessary temperature is attained for thespontaneous transformation.

The bursting and disintegration of diamonds that have been reported by Friedeland Ribaud (1924) at temperatures around 1900 OC might well have been brought about

by microscopic inclusions already present, or by internal graphite inclusions formed as

th e diamonds were heated. Th eir observations of a sudden change in birefringencefollowed by the bursting of the stone, could be caused by th e strain associated with the

inclusions at th e high temperatures involved. It was to avoid these possibilities, experi-enced in preliminary experiments, tha t very careful initial examination of the diamondsms carried out, and that before going to higher temperatures the diamonds werealways first heated at 1700 OC in order to check whether or not internal graphite formed.

A C K N O W L E D G M E N T S

The author would like to thank Dr. T. Evans for useful discussion of the work;also Dr. R. C. Newman of Associated Electrical Industries, Aldermaston Court, for

carrying ou t th e reflection electron diffraction examinations and for helpful discussionsof the results obtained, and Dr. K. Robinson for carrying out the x-ray examination.

Thanks are due to Professor R. W. Ditchburn fo r his encouragement of the work,which has been financed by a grant from th e Industrial Distributors (1946) Ltd. Dr.J. F. H. Custers, Director of the Diamond Research Laboratory, Johannesburg, kindlysupplied the diamonds used in the experiments.

R E F E R E N C E S

BRIDGMAN,. W., 1947, J. Chem. Phys., 15, 92.EVANS, ., and SAUTER,. H., 1961, Phil. Mug., ,429.FRIEDEL,., and RIBAUD,G., 1924, Bull. Soc. Prang. Minkr., 47, 94.HAWORTH, . W., 1958, J. Inst. Met. , 87, 269.KAISER,W., and BOND,W. L., 1959, Phys. Rev., 115, 857.SEAL, ., 1958, Nature, Lond., 182, 1264.

The Graphitization of Diamond

Documents

Transcript of The Graphitization of Diamond