·
.December,
1975
PRODUCTION OF DIAMONDS FROM GRAPHITE
USING EXPLOSIVE-DRIVEN IMPLOSIONS
8 JU1l!
1978
by
SUl'in~ PauJ.. Sbarma
.
'
'-.,
PRODUCTION OF DIAMONDS FROM GRAPHITE
USING EXPLOSIVE-DRIVEN IMPLOSIONS
December, 1975
by
Surinder Paul Sharma
UTIAS Technical No"te No. 196
CN ISSN
0082-5263'
.
Acknowledgements
I should like to express my appreciation for the oppElrtunity to· '
work on this projec't provided by Dr. I. I. Glass. Ris supervision and
advice throughout the course of this project are gratefully acknowledged.
Sincere 'thanks are offered to Professor U. R. Franklin, Department
of Metallurgy and Material Sciences, University of Toronto, for many helpful discussions and advice on X-ray diffraction analysis.
Dr. J. J. Gottlieb IS helpful discussions and reading of the
manuscript are very much appreciated.
I wishto thank Mr. P. C. Crouse for his invaluable help in
carrying out the experiments. Thanks are due to Ms. E. Mutterer and Ms.
A. Marshall for their assistance in conducting X-ray analyses. I acknowledge
wi th thanks the help recei ved from the s'taff of the DrIAS machine shop for
their technical assistance.
The moral support and stimulation given by my parents and brothers
were helpful and are very much appreciated.
I wishto sincerely thank Mrs. W. Dillon, Mrs. W. Ryan and Mrs.
D. Finlay for help in typing the manuscript.
This work was financially supported bythe United States Air Force Office of Scientific Research under Grant No. AF-AFOSR 72-2274c, and
the National Research Council of Canada under Grant No.' A2l62 . Their
Summary
The UTIAS Implosion Chamber Facility was used to synthetically
convert graphite powder into diamonds. Pure graphi te powder, enclosed in
a graphi te cartridge, was placed at the centre of the UTIAS Implosion Chamber and it was then subjected to an explosive-driven implosion which compressed the graphite powder to very high pressures and temperatures,
thereby producing diamonds. A number of tests including density, hardness,
scanning electron microscope, and conclusive X-ray diffraction tests were performed to confirm the presence of diamonds in the shocked graphite
powder. It was found that small amounts, 3 to 5 percent of the origiml
graphite powder, was converted to diamonds. The yield could probably be much improved by producing accurately centred implosions and a cartridge
of superior design.
1. 2.
3.
4.
6.
"'
.
TABLE OF CONTENTS Acknow1edgements Summary Tab1e of Contents INTRODUCTION1.1 Brief History of Diamonds
1.2 UnsuccessfuJ. Attempts at Making Diamonds 1.3 SuccessfuJ. Attempts at Making Diamonds 1.4 Scope of Present Work
CRYSTAL STRUCTURE AND PHYSICAL PROPERTIES OF DIAMOND AND GRAPHITE
PRASE DIAGRAM FOR GRAPHITE AND DIAMOND 3.1 Graphite-Diamond Equilibrium Diagram
3.2 Graphite-Diamond Equilibrium Line up to 12000
K
3.2.1 Va1ues of Thermodynamic Quantities 3.3 Graphite-Diamond Equilibrium Above 12000K
MECHANISM FOR GRAPHITE TO DIAMOND TRANSFORMATION
4.1 Solid-State Transformation of Graphite to Diamond 4.2 Crysta11ization ef Diamond from Liquid Carbon DrIAS IMPLOSION CHAMBER FACILITY
5.1 Graphite Cartridge 5.2 Exp10sive Gases 5.3 Ignition System 5.4 Exp10sive Liner 5 .5 Control Room
EXPERIMENTAL PROCEDURES AND RESULTS 6.1 Density Test
6.2 Hardness Test
6.3 Scanning Electron Microscepe Tests 6.3.1 Sample Preparation
6.3.2 Scanning Electron Micrographs of Graphite 6.4 X-Ray Analysis of Shocked Graphite Powder
ii iii iv 1 1 2
4
6
7
8
9
9
11 13 14 14 14 16 1617
17
17
18
18
18
19
19
20 20 216.4.1 The Debye-Scherrer Method 21
6.4.2 Specimen Preparation 21
6.4.3 Determination 0f Interplanar Spacings (d) 22 6.4.4 X-Ray Diffraction Patterns of Natural Diamond
DISCUSSIONS AND CONCLUSIONS REFERENCES
TABU: 1: THERMODYNAMIC DATA USED FOR CALCULATION OF
GRAPHITE-DIAMOND EQUILIBRIUM CURVE UP TO 12000K
TABLE 2: AN ACCOUNT OF VARIOUS RUNS
FIGURES 1 TO 42
APPENDIX A: X-RAY DIFFRACTION ANALYSIS
Page 24
1. INTRODUCTION
1.1 Brief History of Diamonds
Diamonds have been known to man since ancient times. Dravidians, Greeks, Phoenicians and other races used diamonds for ornaments as early as 600 B.C. Duringthe Mïddle Ages diamonds were regarded as a symbol of peace between husband and wi fe . Diamonds were believed to be able to avert calami ty , cure diseases, and ward off ·evil spirits . Diamonds were also thought to repel attacks of phantoms and made the sleep of the wearer free from nightmares. Princes and captains wore diamonds on their helmets , breastplates and sword handles because they believed that diamonds had the power of protection during battle. Diamonds als 0 had the abili ty to baffle magie arts and to cause
lawsui-ts to be settled in favour of the wearer. It was not explained what happened when both parties involved in the lawsui t wore diamonds. Tt was believed if a house, orchard, or vineyard were touched at each corner with a diamond,then it was supposed to be protected from lightning, storms and blight. Diamonds were used to render poison harmless and to avert madness.
The English word IIdiamondll is derived from the old French diamant,
itself derived from the Latin and Greek word lIadamasll (cxEa~cx~),
meaning lIinvinciblell
, usually a name for hard metal or stone. The weight of a diamond
is measured in carats, and one carat corresponds to approximately 200 milligrams. Notethat the Dravidians, who discovered diamonds in India about 2700 years ago, introduced the word carat, which originates from the carob tree's seeds that are very uniform in weight.
The exact origin of diamonds remains a mystery. One can only guess that diamonds were formed millions of years ago by a combination of extreme
conditions of high temperature and pressure under the Earth' s surface . Through the ages they were transported to or near the Earth's surface by volcanic
eruptions. Even today there is no clear pattern as to where diamonds might be found. Diamonds can be found in many common places such as in the bed of a river or stream as well as in mountains. They are usually embedded in a mixture of various rocks and minerals, which are all cemented together with bluish clay. Separation of diamonds from clay and rock. is a tedious and time-consuming process. ~so, the diamond yield is very poor, as only one-half of a carat of diamends is normally obtained from one ton of clay. The best diamond yields have been found to occur in South Africa, Tanzania, Angola, Siberia, South-West Africa, Brazil and India. Note tha t diamonds also exis t in space; small diamonds have been found in meteorites.
In 1729 two Jesuit missionaries found Brazilian natives playing a game with small shiny stones. The missionaries traded trinkets for these stones. Diamonds subsequently became prized possessions and diamond recovery on a commercial scale started in many regions of the world.
Until the late 1950' s all the diamonds used for industrial purposes were of natural origin and were obtained from diamond mines. Recently commercial processes have been developed and factories in the United States, Soviet Union, Sweden, Japan and South Africa manufacture diamond grit. Artificially-produced diamonds have found an increasing number of applications in modern society. Diamonds are being used in industry for various grinding and shaping tools, polishing abrasive wheels and gem stones, glass and metal cutting, and for
making wire dies. Diamond-tipped drills are used to facilitate prospecting, such as drilling for natural gas, oil and certain minerals. Diamonds are i:rr:q:>ortant in the aforementioned applications not only in that they are the harde st material but also because they are the best conductor of heat. Hence, diamond- tipped tools can be used at high cutting speeds without appreciable deformation. It is worth noting that manufactured diamonds are not of gem quality and therefore cannot replace natural diamonds for many uses.
1.2 Unsuccessful Atte:rr:q:>ts at Making Diamonds
The chemical co:rr:q:>osition of diamonds was unknown until 1797.
During that year Tennant (Ref. 1) discovered that diamond was an almost pure allotropic form of carbon. Following Tennant's discovery several atte:rr:q:>ts have been made to make diamonds. Most of the efforts were made during the half-century period of 1880-1930, and several claims of successful synthesis of diamonds were made.
In 1880, Hannay (Ref. 2), a Glasgow chemist (1855-1931), claimed that he could make diamonds by synthesis. He was searching for a solvent for alkali metals such as sodium and potassium. Hannay found that an inert
substance like paraffin would deco:rr:q:>ose when heated under pressure, especially when mixed with hydrogen gas and alkali metal. Carbon was freed from the paraffin while hydrogen combined with the alkali metal. This gave him -the idea that carbon might crystallize as diamond. Thick coiled tubes of wrought iron open only at one end were used to contain the high pressure and
te:rr:q:>era-ture gaseous mixte:rr:q:>era-ture. A tube was filled withthe reaction material and sealed with a blacksmi th' s weld on the open end of the tube. A large reverberatory furnace was used to raise the te:rr:q:>erature of the mixture. Several unsuccess-ful atte:rr:q:>ts were made. In most cases the tubes leaked "or exploded. On several occasions the furnace was co:rr:q:>letely destroyed. Af ter a long series of eighty experiments, Hannay claimed success in three of them. In the three successful experiments, he heated four milligrams of lithium and a mixture of 10 percent rectified bone oil and 90 percent paraffin spirit in his wrought iron tubes. Af ter several hours of heating the tubes were found to contain gas, some liquid, and some solid transparent crystals which were attached to the walls. Hannay claimed these crystals to be diamonds. These crystals had a density of 3.5 gm/cm3 and were 97.85 percent pure carbon. He submitted some of these crystals to the British Museum (12 of the crystals are still present in the British Museum, registered as BM 87756).
Also during the 1880's, Moissan (Ref. 3), carried out a large number of experiments in France. The discovery of small diamonds found in meteori tes suggested to him that if he selec ted a metal that would expand on solidifying, a solid outer skin would be formed as it cooled and very high pressures would be produced at the centre of the solidifying mass. Moissan thought this should produce te:rr:q:>eratures and pressures sufficient to form diamond from carbon. He studied the solubility of carbon in various metals like aluminum, chromium, iron, magnesium, manganese, platinum, silver and uranium. He finally claimed success when he dissolved carbon in pure iron in a carbon crucible. He used an electric furnace to increase the te:rr:q:>erature of the mixture. The molten mass of iron with absorbed carbon was quickly plunged into lead to form a rigid outer crust, then allowed to cool in air. Af ter the whole ingot solidified and cooled, it was placed in an acid bath and slowly eaten away to release the crystals. Moissan performed several tests on
his crystals. He found that some of the crystals had optical proper ties
similar -to those of diamond. Upon combustion the crystals gave off carbon
dioxide, indicating the presence of carbon. These crystals were found to scratch rUby, and to have a specific gravity of between 3 and 3.5. He also
examined them under a microscope . As aresult Moissan pronounced these crystals
to be diamond. None of his crystals survived for a conclusive X-ray diffraction
study. Note that various attempts have been made to repeat Moissan' s work. A
detailed account of these attempts is given by Leipunski (Ref. 4). For example, during the 1890' s Crookes (Ref. 5) claimed to have successfully repeated Moissan' s
work. He used high pressures up to 8000 atm and high temperature of 4000oK.
As a personal hobby, Parsons (Ref. 6), a shipbuilder and inventor
of a steam turbine, worked 30 years on the syrithesis of diamonds. He spent
thousands of dollars on equipment for repeating Hannay' s work. Hydraulic
presses for the production of high pressures were used. Parsons first claimed
success but later withèj.rew his claim.
In 1929, Hershey (Ref. 7) of McPherson College, Kansas City, claimed
to have made 50 diamonds by using a modified Moissan technique. His largest
diamond is said to be a record 2 x 1.5 x 1 mmo Because of its huge size doubts
exist as to its origin. However, Hershey has never withdrawn his claim of success.
In 1943 Bannister and Lonsdale (Ref. 8) examined Hannay' s 12 crystals
which are still present in the British Museum, London. An X-ray analysis was
made on the crystals~ Much to their surprise they found that 11 of the 12
crystals were diamonds. They also found that some of these diamonds were pure,
and others had a small amount of impuri ty. Furthermore, a Laue photograph taken
on one of the crystals revealed that it was of rare type 11. Bannister and
Lonsdale believed that Hannay possibly succeeded in making a small quantity of diamonds.
Furthermore, in the 1960's, Seal (Ref. 3) made an electron microscopie
examination of Hannay' s diamonds. From his examination he concluded that the
"Hannay di amonds " were probably genuine. He raises the ques tion, however, that someone could have "salted" Hannay' s reaction mixture.
Bridgman (Ref. 9) made a number of experiments in 1947 on making diamonds by using high pressures (15,000 to 30,000 atm) and high t emperatures
(30000K). Small diamonds were embedded in graphite to act as a seed catalyst.
On the basis of his experiments, Bridgman suggested that at temPeratures of
about 30000K and pressures greater than 30,000 atm, diamond crystals are
stable. From his experimental data it is clear that early claims to diamond
synthesis appear unrealis tic.
A number of other attempts at diamond making have been made by various scientists, and the first interesting account of such attempts is
given by Leipunski (Ref. 4). Early claims of diamond synthesis were
subse-quently reviewed by Professor Bridgman of Harvard University in 1947 (Ref. 9), and Professors Eyring and Cagle of the University of Utah in 1952 (Ref. 10). These authorities concluded from thermodynamic considerations alone that diamonds had never been made successfully in the laboratory. Leipunski
(Ref. 4) predicted that at a temperature of OOK the pressure required for
without a catalyst, then a pressure of 52,000 atm is required. From these calculations i t can be concluded that early claims to diamond synthesis we re unrealistic.
1.3 Successful Attempts at Making Diamonds
In 1954, Bundy, Hall, Strong and Wentorf (Ref. 11) succeeded in producing the first man-made diamonds. They used a "belt" apparatus (Fig. 1) developed at General Electric Research and Development Centre, Schenectady, New York. A large hydraulic press was used to force the carbloy pistons againstthe graphi te cartridge (containing graphite and nickel or iron as a cata1yst), thereby compressing the enclosed graphi te and producing a high pressure (above 50,000 atm). Simultaneously, the graphite mixture was raised
to a high temperature (above 20000K)". Both pressure and temperature were applied for about five minutes. Diamond sizes obtained by this process range from less than 100 microns to 1000 microns (1 mm) in diameter. The details of the apparatus can be found in U.S. Patent No. 2,941,248, issued in June, 1960. During 1955 a Swedish company (Ref. 12), Allmana Svenska Elektriska Aktiebolaget (ASEA) , announced the success of making diamonds under a milli-meter in size. A "multi-anvi1" device of cubic configuration was developed at ASEA. It consisted of a massive structural framework which supports and direds six centrally converging hydraulically powered anvils oriented 90 degrees to each other. Pressures between 80,000 to 90,000 atm could be achieved. Also, a high temperature up to 27000K was produced by igniting thermite. Thermite is a mixture of magnesium and barium peroxide, which produces a very high temperature upon ignition. Iron carbide was used as a catalyst in conjunction with graphite. Industrial diamonds are being produced in Sweden by using the multi-anvi1 apparatus.
DeCarli and Jamieson (Ref. 13) announced in 1961 that they produced diamonds with an explosive-driven shock wave. Graphite powder (carbon) was subjected to a high pressure of about 200 to 300 kilobars* for about 1 ~s by a shock wave. The principle of operation of the process is shown schematically in Fig. 2. The apparatus consists of a heavy steel base (anvil) and ring
having a small cavity in which graphite powder is placed and covered with a steel cover plate.
An
explosive charge is p1aced adjacent to a neighbouring plate. upon detonation of exp10sive charge, the expanding explosive gasesaccelerate the plate which s"trikes the cover plate. This impact produces a very strong shock wave in the cover plate and subsequently in the graphite, thereby creating high pressures and temperatures sufficient for the conversion of graphite to diamond. The process is being commercially used by the Allied Chemica1 Company of the U.S.A. Aggregates of randomly oriented diamond particles are obtained, ranging in sizes from 500 to 10000A (0.05 to 0.1 ~). The details of the process have been reported by DeCarli (Ref. 14).
In the same year, Alder and Christian (Ref. 15) reported evidence for conversion of graphite to diamond by using an explosi ve-dri ven shock wave. They found that the graphite-to-diamond transition starts at a pressure of about 180,000 atm. I t is interesting to note that their results showed that at 300,000 atm the low-density graphite showed considerable conversion to the diamond phase, while high-density graphite was not significantly converted to diamond. This shows that low density (1.5 - 1.8 gm/cc) should be used for converting graphite to diamonds.
*(1 bar
=
1.0197 Kg/cm2=
0.9869 atm=
14.5038 psi=
1 x 106 dynes/cm2.)4
In 1968 Cowan, Dunnington and Holtzman (Ref. 16) also developed a process for making diamonds from graphite by using an explosive-driven shock wave. The main advantage of this process is that the diamond yield is greater
compared with DeCarli's process. This is achieved by using a cooling medium (e,g., iron, copper, nickel or aluminum) in conjunction with graphite or any other form of carbon. The cooling medium (catalyst) qu{ckly lowers the achieved temperature af ter the passage of the shock wave, so that the reverse process of turning diamond into graphite (graphitization of diamond) is reduced substantially. The resulting diamond material is commercially available, and it is used for
polishing and lapping purposes.
In 1970 Rasquin and Estes (Rei'. 17) produced diamonds by using the apparatus shown in Fig. 3. The apparatus consists mainly of an exponentially tapered horn of solid hardened steel. A magnetic hammer is placed adjacent to the large end ef the exponential horn. A copper plate iS,positioned between the magnetic hammer and the horn. The small end of the horn fits into a cavity of an anvil. Pure graphite powder is placed in the cavity. The magnetic hammer is connected to a capacitor ~ank and voltage source that delivers an electric discharge in., the form of a fast rising current pulse for a few microseconds. The current operates the magnetic harnmer and produces a shock wave in the
exponential horn. The shock wave thus produced is amplified due to the converging shape of the norn, and a significant portion of the energy in the shock wave is simultaneously focussed at the small end of the horn. This concentrated energy is applied to the graphite powder enclosed in a cvaity of the anvil. A part of the graphite powder is conve~ted to diamonds. The yield and quality of diamonds depend on the energy applied to the capacitor ~ank.
In 1972, Garrett (R~f. 18) developed a device for diamond synthesis employing an imploding shock wave. The apparatus (Fig. 4) consists of two mating hemispherical parts forming a central spherical cavity which is filled with
graphite powder. Solid explosive is uniformly distributed around the periphery of the two mating hemispheres, which must be detonated simultaneously at many points on the explosive surface. High pressures and temperatures are produced by the imploding shock wave for a few micreseconds (0.4 to 10). These extreme conditions are sufficient for the conversion of graphite to diamend. Two
explosive layers, as shown in Fig. 4, can be used to produce a stronger imp loding shock wave generating greater pressures and temperatures in the graphite. Note that this implosion shock-wave device apparently has not been used conunerciallw
for diamond production. From similar work conducted at UTIAS (Ref. 19), it would appear that simultaneous detonation of many detonators is a problem.
In 1973, Professor I. I. Glass directed that the UTIAS explosive driven Implosion Chamber Facility (Ref. 19) be utilized to convert graphite ,into diamonds. For this purpose, the hemispherical implosion chamber was
modified to produce explosive-driven implosiens to focus on a piston that compressed graphite in a cylinder there~y producing diamonds.
The UTIAS hypervelocity projectile launcher was developed in the early 1~60's to facilitate simulation studies of meteoro~d impact and space-craft entry into a planetary atmosphere (Ref. 19). In the course of this development investigations were made of spherical deflagration and detonation wave and implosion phenomena (Refs. 20 to 21), initiation of both primary and secondary explosives in planar as well as spherical geometries (Refs. 23 and 24), gaseous detonation phenomena and explosive-driven implosion waves (Refs.
24 to 28). The operation of' the UTIAS launcher is illustrated schematically .
in Fig. 5. The stoichiometric mixture of' hydrogen and oxygen (2H2
+
02) isdetonated by an exploding wire at the centre of symmetry of the hemispherical chamber (20 cm diam). The resulting detonation wave moves outwards in the hydrogen-oxygen mixture and leaves behind a region filled with high-pressure, high-teIr!Perature reaction products (Fig. 5a). On reaching the hemispherical
wallof' the chamber, the de·tonation wave detonates the PEl'N (pentaerythrite
tetranitrate) explosive liner (6 rmn thick), which quickly produces an imploding
shock wave (Fig. 5b and 5c). This intense implosion ref'lects at the centre of
symmetry (Fig. 5d), and the resulting high-pressure, high-temperature pocket
of' gas accelerates the projec·tile down the barrel (0.56 cm diam). The
projectile subsequently leaves the barrel and enters a range tank where the ambient-gas conditions can be made to simulate an atmospheric entry.
Tt is worth notingthat the UTIAS launcher was previously also
modif'ied to accomrnodate a shock tube (2.5 cm diam) to f'acili tate a study on radiation from strong shock waves. Planar shock waves of about 20 km/sec
were produced in this facility (Ref. 29). As already noted, the dri ving- chamber
apparatus was modified again to facilitate ·the inves tigation of converting
graphit e to diamond. The stoichiometric hydrogen-oxygen mixture and solid PEl'N
explosi ve liner were utilized to produce astrong imploding shock wave. This
shock wave compressed graphite powder which was enclosed in a suitably designed
cartridge placed at the geometric cerrtre of' the implosion chamber. Only a
small amount (3 - 510) of graphite was converted to diamond by the
shock-compression process, mainly due to iIr.!Perf'ectly focussed implosions . The UTIAS IqJlosion Chamber Facility provides a means of doing research on shock waves in solids and their solid state phase transformation
under extreme conditions of' pressure and temperature . The main advantages of
this f'acility are tp.at it is saf'e to operate and it is re-usable. A disadvantage is that although it is re-usable a ntunber of secondary parts become distorted
af'ter every run, and have ·to be remachined, especially if' the iIr!Plosion is not
centred.
1.4 Scope of' Present Work
As an introductory part of the present work, the crystallographic structures of' graphite and diamond are reviewed in Sec. 2. Differences in their crystallographic structures account f'or the widely different properties
of' graphite and diamond. Thermodynamic considerations of the high-temperature
and high-pressure diamond f'ormation process are discussed wi th the aid of' a
graphite-diamond equilibrium diagram (phase diagram 0 f' carbon) in Sec.
3.
Someof' the mechanisms of transf'ormation of graphite to diamond are discussed in Sec. 4. Details of the experimental equipment and the operation of the UTIAS Implosion Chamber Facility are given in Sec. 5.
Various tests are required to demonstrate that diamonds are made
in the UTIAS Implosion Chamber Facility. Simple tests like densi ty, hardness
and microscopic examination on shocked-graphite powder alone do not necessarily conf'irm the presence of diamonds in the compressed graphite. As a convincing proof, however, an X-ray analysis of the shocked graphi te powder must be made.
Furthermore, scanning elec·tron microscope (SEM) tests were also done to study
some effects of' the shock wave on the graphi te powder. Complete details of
various tests are gi ven in Sec. 6. The results of these tests are discussed
and the concluding remarks given in Sec.
7.
2. CRYSTAL STRUCTURE ANTI PHYSICAL PROPERTIES OF DIAMOND AND GRAPHrrE
An understanding of the crystal structure and some physical
properties of graphite and diamond is important to the present work. Diamonds
and graphite are both crystals composed only of carbon atoms, as first
dis-covered by Terinant (Ref. 1) in 1797. However, their exact crystal structure
was unknown until 1913 when X-ray techniques were first used to study crystal
structure • The crys tar structures of diamond and graphi te are illustrated in
Figs. 6 and 7, respecti vely . The spatial arrangement of the carbon a toms in
each case are markedly different, and this difference accounts for the
radically different properties of graphite and diamond.
Chemically, diamond is an exceptionally pure form of carbon. The
main impurity in natural diamonds is generally nitrogen, which may account
for as much as 0.23 percent. Any other impurity is normally two orders of
magnitude smaller. But in the case of synthetic diamonds, impurities are
very conunon. For example, synthetic diamonds can contain up to 10 percent
nickel, and smaller amounts of aluminum, iron and magnesium are also present.
The amount of nitrogen present in synthetic diamonds is generally quite smalle
Diamond crystals n~rmally have a cubic structure (Fig. 6), each
side having a length of 3. 57Ä. One atom is located at each corner of the
cube, one is located in the centre of each face of the cube, and four
addi tional atoms are located in the cube interior, for a total of 18 atoms.
Each atom is therefore synnnetrically surrounded by four other atoms, thereby
forming a tetrahedron. The force of each interatomic bond acts over the
same length of 1.
54l?.
The four bonds to each atom are at an angle of 1090 •The interconnecting bonds for a number of carbon atoms in a three-dimensional
diamond lattice is illustrated in Fig. 8. Interwoven hexagons of carbon
atoms can Çl.lso be seen in the diamond structure illustrated in Fig. 8. These
hexagons are
2.06l?
apart.Graphite crystals (Fig. 7) have successive planes of hexagonally
arranged carbon atoms. Each carbon atom is bonded to three other atoms at
1200
• Within each plane the atoms are only 1.42l? apart and the bonding
energy is theref~re very strong. The parallel planes are separated b;y; a larger
distance of 3.35Ä. Owing to the short er interatomic distance of 1.42Ä for
graphite as compared to 1.541? for diamond, graphi te is harder than di\amond
wi thin the planes. However, becausethe separation distance of 3 .35Ä between
planes of graphite crystal is much larger than 2.061? for di amond , graphite is
actually we aker than diamond. The planes of graphite slide relatively easily
over each other. (For this reason graphite is a good dry lubricant.) The
planes in graphite crystal are stacked Buch that the atoms in alternating
planes lie directly above one another.
The specific gravity of diamond is always very nearly 3.52, varying
slightly from 3.514 to 3.518. The specific gravity of graphite is not nearly
as constant, varying from 2.2 to 2.3. Diamond is known to oe the hardest
materiaL Consequently, the hardness of diamond is taken to be 10 on the
Moh scale (l-taJ.c, 2-gypsum, 3-calcite, 4-fluorspar, 5-apatite, 6-feldspar,
7-quartz, 8-topaz, 9-corundum, 10-diamond). Note that the hardness of graphite
is less than one on a Moh scale.
Diamonds can oe classified into two categories, called type I and
are purer and are rare. Additi onally, type I dia.monds contain nitrogen as the main inq:lUrity, and type 11 dia.monds do not contain significant a.mounts
of nitrogen. Additional physical properties of graphite and dia.mond can be
found in Refs. 30 and 31.
3 .
PRASE DIAGRAM FOR GRAPHITE AND DIAMONDCarbon can exist in various forms including
tne
solid state of-graphite and dia.mond, as well as the liquid state depending on pressure and
temperature • The phase or equilibrium diagram (Fig. 9) shows the various
s-table regions of graphi te, dia.mond and liquid as a function of pressure and
-temperature . The region Ç)f solid carbon has been subdivided into regions
where carbon exists in the form of graphi te, dia.mond and solid carbon 111,
metastable graphite, and metastable dia.mond. Note that a metastable state
is one in which a substanee can exist but does not normally occur in nature.
Some of the "equilibrium" lines separating the various regions result from .
thermodynamic calculations and experiment al data, and others have been pro-posed for completeness.
For certain combinations of pressure and temperature graphi te exists in a stable form, whereas for different combinations of pressure and temperature
diamond is stable. As can be seen in Fig.
9,
graphi te is s table in region 1.However, diamond can also exist in this region in a metastable phase. In
region 2, diamond is stable but metastable graphi-te can also occur. Dia.mond
is the only phase that can exist in region
3.
Region4
represents the liquidphase of carbon. Region 5 has been proposed by Alder and Christian (Ref. 15) for a new form of carbon they call solid phase 111.
The first part of the line separating regions 1 and 2, from 0° to
12000
K, is based on quite accurate thermodynamic data. Berman and Simon (Ref.
32) extrapolated this line beyond 12000
K, and Bundy, Bovenkerk, Strong, Wentorf (Refs. 33 and 34) verified experimentally this extrapolation. Bundy et al
found thatthe slope of the line above 12000
K was equal to ~Q]O~kbarsjOK,
whereas Berman and Simon gave the value 0.0273 kbarsjOK. This difference is
small and within the experimental error.
Most of the other lines have been established by experirrents, but
not as accurately as the line separating regions 1 and 2. One difficulty is
-that the high-temperature carbon reacts with the metal container or even melts
it. Bundy (Ref. 35) established the graphite-dia.mond-liquid triple point while working on experiments on the melting of graphite at high pressures. The
triple point was estimated -to occur at about 125 kilobars and 41000
K. He also
established the melting line of graphi te by measuring the electric flash heatin,g. He found that the graphi te melting line changes slope at about 60-70 kilobars ,
that is, the melting point is higher in the mid-pressure range than i t is at
low and high pressures. The melting line of graphite starts at about 41000K,
rises to a maximum of 4600-47000
K at about 60-70 kilobars, then decreases to
4100-42000
K at the triple point. The high-pressure end of the melting of
diamond line (separating regions
3
and4)
was established by Alder andChristian (Ref. 15). They also proposed that at pressures above 600 to 700 kilobars, diamond collapses to a denser metallic carbon state called Solid
111, which has a density of 15 to 20 percent higher than dia.mond. Tt is
interestingto note toot Solid III has not as yet been shown to be harder than dia.mond.
The line separating regions 2 and 3 was established experimentally by DeCarli and Jamieson (Ref. 13) and Alder.and Christian (Ref. 15). In
order to produce diamonds wi th certainty, graphi te must be subjected to pressures and temperatures corresponding to region 3. otherwise, graphite exposed to pressures and temperatures corresponding to region 2 may not be transformed into diamond. For anormal atmospheric temperature it is clear that the pressure required is about 400 kbars. Early attempts at making diamonds by subjecting graphite to high pressures were unsuccessful because a high pressure of 400 kilobars was then unattainable.
3.1 Graphite-Diamond Equilibrium Diagram
The phase diagram for graphite and diamond has long been sought. In 1912, Pollitzer (Ref. 36) gave the first discussion on the graphite-diamond equilibrium diagram, based on the first two laws of thermodynamics. However, certain experimental data (e.g., heat of transi tion of graphi te to diamond) needed for the thermodynamic analysis were not correct, and his predicted equilibrium line was therefore inaccurate.
In 1920, Miss Miething (Ref. 37) used more reliable experimental data for her thermodynamic analysis. She calculated for the first time the current order of magnitude of the pressure under which diamond is stable. She also showed that at normal atmospheric pressure and temperature graphite is always the stable phase while diamond is not. Simon (Ref. 38) further modified the equilibrium diagram because he used more recent experimental data. As more accurate experimental data became available, further small changes to the equilibrium diagram were found. For example, in 1944 Prosen, Jessup,and Rossini (Ref. 39) surveyed all measurements and redetermined the heats of combUstion for graphite and diamond. They found a higher value of
the heat of transformation of graphite into di amond , but the implications were insignificant. "Their results are now generally accepted by researchers "
As noted previously, Bridgman (Ref. 9) performed some interesting experiments by embedding diamonds as seed crystals in graphite. He found from his experiments that at 30000
K and pressures above 30,000 atm diamond is stable.
Berman and Simon (Ref. 32) calculated the equilibrium diagram for graphite' and diamond up to l2000
K using new experimental data. They also pointed out that pressure under which diamond is stable at 30000
K should be much higher than Brid.gJ!1an estimated (30,000). Due to the lack of thermal data on diamond above l2000
K andthe values' of specific heat of graphite, they presented a linear extrapolation for the graphite-diamond equilibrium diagram above 12000
K (Fig. 10):
The extrapolation is accurate to wi thin
5
percent of the experimental value. 3.2 Graphite-Diamond Equilibrium Line Up to l2000K
At the end of the 19th century, Gibbs showed theoretically the conditions under which carbon would be transformed to diamond rather than to graphite. He showed that the thermodynamic or Gibbs potential for
graphite is much less than that for cliamond. Hence, under normal conditions diamond is thermodynamically unstable while graphi te is thermodynamically stable. Thus it is clear that only diamond will transform to graphite, and not vice versa, at nor mal temperature and pressure. Although from thermo-dynamic-potential considerations it is possible for cliamond to transform to graphite at orclinary temperature and pressure , yet i t does not. This is due to the fact that even though the thermodynamic potential is favourable for the transformation of cliamond to graphite ye-t thermodynamic stability is not. If the pressure can be raised high enough, then the graphite has the thermo-dynamic proper ties to make i t transform to cliamond at normal temperature. The thermodynamic potential can be derived from simple thermodynamic consider-ations. According to the first law of thermodynamics we have the fOllowing expression:
dQ = dU + PdV where, Q is the heat transmitted to the body
U denotes internal energy, V is the volume,
P denotes pressure. For a reversible process,
dB = dQ T where, T denotes the absolute temperature , and
S is the entropy.
Equation
3.1
can be written asTds
=
dU + Pdv If Pand T are constants , Eq.3.2
becomesd(U + PV - TS)
=
0 ord(H - TS)
=
0(3.1)
(3.2)
Thus at equilibrium, Gibbs potential (U - TS + PV) is a rru.nJ.mum. This shows that two phases (graphite and diamond) of carbon are in equilibrium with each other, if the clifference between the gibbs func"tion, m, is zero; that is:
m
=
Llli - TAs=
0(3.4)
Data necessary for calculating m(O,T), that is, the value of m at zero pres~ure and temperature T between 0 and 12000
K is available. At a fixed -temperature T,
M
varies with pressure accorcling to the following relation:hence at any higher pressure, P, 6G(P,T) is obtained as follows:
or
In this expres sion
6G(P,T) = 6G(O,T) +
J
P~VTdP
oLG(P,T) = 1m(O,T) - TAs(o,T) +
JP
~VTdP
o(3.6)
M(P, T) equals the difference in Gibbs free energies of graphi te and diamond at a specified pressure and terqperature,
M(O,T) equals the difference in heat of transformation of graphite into diamond at zero pressure and a terqperature, T,
~S( 0, T) is equal to the entropy difference between diamond and graphite at zero pressure and a terqperature, T,
equals the volume difference between diamond and graphite at a terqperature, T.
Values of Thermodynamic Quantities
M, the difference in heat of transformation of graphite into diamond cán be easily determined from the difference between the heats of combustion for graphite and diamond. At constant pressure its variation with terqperature can be obtained from the difference between the specific heats of graphite and diamond:
The value of M at zero pressure and terqperature 25°C (298°K) was measured by Prossen, Jessup and Rossini (Ref. 39). They measured experimentally the value
of M(0,298) equal to 453.2 ± 20.3 cal/gm atom. To obtain values of Lm(O,T) at other temperatures, a knowledge of specific heats over a wide range of terqperatures is required. Berman and Simon (Ref. 32) derived a value of M at zero pressure and OOKto 580 ± 20 cal/gm atom. Values of M at other terqperatures and pressures are given in Table 1.
~, the entropy difference between diamond and graphi te: (I) Entropy of graphite
DeSorbo and Tyler (Ref. 40) measured the entropy change between 13° and 3000
K. They gave the value of the graphite entropy at 298°K as 1.367 ± 0.005 cal/gm atom deg.
(II) Entropy of diamond
DeSorbo (Ref. 41) determined the entropy of diamond between 75° and 298°K. He gave the entropy value of diamond at 298°K as 0.568 ± 0.005 cal/gm atom deg. Values of ~(O,T) are given in Table I.
6V, the volume difference between diamond and graphi te:
(I)
Atomie volume of graphiteBerman and Simon (Ref. 33) gave limiting values of atomie volume for large graphi te crystals. They found the atomie velume of graphi te as 5.299 cc at 298°K, corresponding to a graphi te densi ty of 2.267 gml cc. The values of
the atomie volume at other temperatures are given in Table I. (n) Atomie volume of diamond
The atomie volume of diamond was determined by Straumanis and Aka (Ref. 42) in 1951 and by Thewlis and Davey (Ref. 43) in 1956. The values given by t hem differ only by 0 .• 02 percent. At 298°K the value is equal to 3.416 cc corresponding to a diamond density of 3.515 gm/cc. Values of the atomie volume at higher temperatures and zero pressure are taken from Thewlis and Davey' s results • Values of the atomie volume for diamond are gi ven in Table I.
For calculating m(
°
,T), that is, the value of the Gibbs potential at zero pressure and temperature TOK (0<
T<
12000K) , the necessary data is available. Substituting the values of AHTO,T) and ~(O,T) in Eq. 3.4 shows that at zero pressure, graphite isthe stable form of carbon at all temperatures. For any higher pressure , P, and at a temperature T, m(p ,T) can be obtained from Eq. 3.6. The volume difference between diamond' and graphite, 6V, is negative (because the atomie volume of diamond is consider-ably less than that of graphi te) • 6V is a function of both pressure and temperature . Thus for any temperature T, there is a pressure at which
m
can be reduced to zero, so tha·t graphite and diamond are in equilibrium. From Eq. 3.6 we have,m(p,T) - m(O,T) =
JP
6V(P,T) dP(3.8)
ok, compressibili ty of graphite and diamond.
(I)
The compressibility,k,
for diamond at room temperature is 1.8 x 10-7 cm2
Kg-l. This is so small that it can be assumed to be independent of temperature and pressure without introducing any error in the equilibrium calculations. Assuming the compressibility of· diamond as constant, th en for di.a.mond,o o
where k =
l/v
(ov/oP)T is the compressibility, assumed to be constant for diamond.(II)
The compressibility of graphite is not constant but decreases appreciablywith increasing pressure. Bridgman (Refs. 44 and 45) measured the compressibi-li ty of graphite at room temperature for pressures up to 100,000 atm. As the
compressibi1ity has a marked effect on the ca1cu1ations of the equilibrium diagram, it can be neg1ected. Some assumptions have to be made about the compressibilityof graphite. Beercroft and Sewnson (Ref. 46) assumed that the isothermal compressibi1ity of graphite can be described by the following
re1ation: I
1
(dV
~
k =
V OP
1
= A - BV (3.10)whe~e A and B are constants, having va1ues 1.67 x 10-5 Kg-1 cmF and 3.64 x
10-0 Kg-1 cm-1 , respective1y. From Eq. 3.10 we get,
dV
2OP
= AV - BV
Integrating Eq. 3.11,v -
AC exp(AP~- B(C
exp(AP-1)
where, C is a constant of integration and its va1ue is found equa1 to BV(O
T)
BV( 0,
T~
- ATo eva1uate the integra1
JPV(P,
T) for graphi te: oJ
;(P,T) dP-JP
ACexp~AP~
-
B(C
exp AP-1)
o 0=
]:.e{c
e?c(AP)-l }B
nC-1)
Substituting the va1ue of the constant
C in Eq. 3.14,
~V(P,T)
dP=
~ ~n(l
- (B/A) V(O,T) l-exp(AP)} o (3.11) . (3.12) (3.13) (3.14) (3.15)Thus the integra1 JPV
T dP in Eqs. 3.6 or 3.8 can be eva1uated using Eqs. 3.9 o
and 3.15 for various temperatures and pressures. Equation 3.8 can be solved for each temperature and pressure . The resu1ts are gi ven in Tab1e I and in Fig .. 10.
3.3 Graphite-Diamond Equilibrium Above 12000K
Due to the 1ack of therma1 data on graphi te and diamond above 1200{K, Berman and Simon (Rei'. 32) extrapo1ated the graphi te-diamond equilibrium diagram.
They assumed that the compressibility of diamond remains constant at its initial room temperature value (1.8 x 10-7 cm2 Kg-1), whi1e that of graphite varies with pressure according to Eq. 3.10. They a.lso assumed that the coefficients of
expansion for graphite and diamond remain the same as at l200oK. The following extrapoJ,ation equation above l2000
K is given by Berman and Simon (Refs. 32 and 47), or p = 7.1 + O. 027T (0 K), T
>
l200i~K kilobar or in engineering units:P
2 = 230,000 + 220T (OF), T>
l700°F lbs/in ,The linearly extrapolated curve for graphite-diamond is shown in Fig. 9. Bun dy , Bovenkerk, Strong and Wentorf (Ref. 33) verified by experiments the Berman-Simon extrapolation curve. Bundy et al found the slope of graphite-diamond equilibrium diagram as 0.0302 kilobar/degree as compared to 0.0273 kilobar/degree reported by Berman and Simon. This represents good agreement between the theoretical extrapolation and the experimental resul ts •
4. MECHANISM FOR GRAPHITE TO DIAMO:ND TRANSFORMATION
Diamonds can be formed from graphite in the following two basically different ways (Refs. 48 to 53): (a) solid-state transformation of graphite lattice to diamond lattice; (b) crystallization of diamond from liquid carbon in the presence of a molten metal catalyst.
4.1 Solid-State Transformation of Graphite to Diamond
In the solid-state transformation of the graphite lattice to the diamond lattice, it is believed that the graphi te is compressed and the relatively large interatomic distance between the hexagonal planes is sub-stantially reduced. Referring to Figs
6,
7 and 8, i t is seen tbat thehexagonal planes for graphite are 3
.35~
apartco~ared
wi th 2.06A for diamond. Also note that the individualpl~es
in the graphite atoms are closer (1.42R) than in the case of diamond (1.54Ä). Anders (Ref. 49) ,and Lipschutz and Anders (Ref. 50), proposed that the interatomic bonds between the hexagonal planes of graphite (Fig. 7) are broken and rearranged such that diamondcrys-tals (Fig. 6) are formed. They throught that a simple compression of graphite would convert i t to diamond. As the hexagonal planes in graphite are loosely interwoven by a larger dis tance (3. 35R) than in diamond (2. 06R), graphi te crystals oriented in such a way that their hexagonal planes are perpendicular to the direction of a shock wave, will convert to diamonds. To co~lete the transformation 'the hexagona;l. sheets (planes) must come closer to each other by a distance of about 1.29A. Furthermore, in individual planes the atoms have to increase their distance by about o.l2R.
4.2 Crys tallization of Diamond from Liquid Carbon
The actual transformation of liquid carbon to diamond on cooling in the presence of a molten metal catalyst is still not clear, but it is known that the catalyst present in carbon reduces the activation energy required for the formation of diamonds. There are several mechanisms proposed by various authorities (Refs. 51 to 53).
--- - - - .
Giardini and Tydings (Ref. 51) studied the mechanism of diamond formation under conditions of high static pressure and high temperature . They proposed that the graphite is dissolved in the metal catalyst to a state of supersaturation. This supersaturated solution decomposes due to thermodynamic instabili ty. Depending on the conditions of pressure and temperature , either graphite or diamond will crys tallize from the super-saturated solution. The conditions for the s table diamond region can be seen in the phase diagram for carbon (Fig.
9).
Thus, in order to be certain öf obtaining diamonds, i t is necessary that recrys tallizatiEln should take place under such pressures and temperatures that the diamond is of a more stable phase than graphite. They believe that a metal catalyst wouldreact wi th carbon to form metal carbides. These· me tal carbides have excess carbon and decompose to lower carbides, thus liberating free carbon in the form of diamond under suitable condi tions of high pressure and temperature .
A flow diagram (Fig. 11) shows tIE possible formation of diamond from a supersaturated solution of carbon in the presence Elf nickel and iron as catalysts.
Bokii and Volkov (Ref. 52) proposed that diamond would be formed through an intermediate carbon-metal compound. They thought that the presence of nitrogen, oxygen, carbon dioxide and water would react with
carbon in the presence of a metal catalyst, thus producing intermediate CElmpounds. These· intermediate compounds would decompose to diamond and free metal under high pressures and temperatures. ;For example, the following equations represent the formation of diamond in the presence of nickel via the intermediate compound carbonyl.
2C (graphite) + 2C0 2 <
low teSEerature ..
4co (4.1)
N· J. + 4co low temperature::o. Ni1 00 )4 (4.2) '<
l'Ji(CO)4 <:: high temperature N. J. + 4co (4.3) 4co high temperature .. 2C ( diamond) + 2C02 (4.4)
'<
These equations can be represented by only a single equation, as given below: high temperature>
c::
They did not rule out the possibili ty of crystallization of diamond from supersaturated carbon solution as discussed previously.
(4.5)
Vereshchagin et al (Ref. 53) emphasized that diamond does not cryst?llize from a molt en catalyst, but under thermodynamically stable cElnditions, graphite atoms would undergo a rearrangement tEl the diamond lattice. The atoms ef a metal catalyst act as internal sources of pressure and temperature and promote the formation of diamond bonds. The interlayer distance between the graphite layers is reduced while the atoms in the same plane are displacedto the larger distance of 1.54~, as noted above.
5 • UTIAS IMPLOSION CHAMBER F ACILITY
A
schematic diagram of the UTIAS Implosion Chamber Facility is givenin Fig. 12, shown previously in Fig. 5, for completeness for this special
application. The cylindrical graphite cartridge, explosive gases (2H2 + 02)
and explosive (PETN) liner in the 20-cm (8 in.) diam. hemispherical cavity are
indicated. The stoichiometric hydrogen-oxygen mixture is detonated at the centre of symmetry by means of an exploding wire. The outward-p.,ropagating detonation wave leaves behind it high-pressure and temperature reaction products. Ultimately the detonation wave (Fig. l2a) strikes the solid
explosive PETN liner and causes it to detonate (ideally, instantly and
simul taneously) over its surface . The detonation of the solid explosi ve
creates an inward moving or imploding shock wave (Fig. l2b). This strong shock wave is amplified as i t converges or implodes at the centre of symmetry of the UTIAS Implosion Chamber (Fig. l2c). The implosion wave reflects at the centre of symmetry and consequently produces a very pressure and high-temperature gas mixture (Fig. l2d).
The extremely high pressure associated wi th the imploding and reflecting shock wave compresses the graphite cartridge, thereby causing
some of the compressed graphite to be converted to diamond. The resulting
compressed graphite can be examined to determine how much diamond was produced.
Figure 13 illus tra'tes the various parts of the UTIAS Implosion
Chamber. Basically, it consis'ts of two massive discs, one containing the
hemispherical cavity and the o'ther the barrel. The front disc contains a
segmented cone which supports 'the barrel and a liner disc. The barrel
(Fig. 14) has a small cylindrical cavity at its centre. The graphi te
cartridge is secured in this cavity as shown in Fig. 14. A protector disc
is also fastened by four screws attached to the front disc. This replaceable disc protects the front disc from damage due to undesirable off-centre implo-sions. The massive back disc or the explosion chamber has a machined 20-cm
(8 in.) diam. hemispherical cavity to accommodate both explosive gases and
PETN liner. Both the front and rear discs are fas tened together by 32 s trong bolts. The front disc, segmented cone, barrel, graphite cartridge, liner and protector disc are shown in Fig. 15. Another picture showing the assembled UTIAS Implosion Chamber appears in Fig. 16.
The most important parts of the UTIAS Implosion Chamber are described in detail inthe following sections .
5.1 Graphite Cartridge
The graphite cartridge containing graphite powder installed in the cartridge holder is shown schematically in Fig. 14. The cartridge essentially consists of a tapered cylindrical piece of steel havi ng a cylindrical cavity
to house the graphite powder. The graphite powder is covered by a steel cover
plate which is welded in place. The graphite powder is compressed to the
desired density of
1.5
to 1.8 gm/cc. A hydraulic press is used to compressthe graphite powder before the cover plate is installed. The steel cover does
not allow the graphite to come into direct contact with the imploding gaseous
mixture. Af ter the graphite is compressed by the implöding shock-wave process,
the cartridge is opened and the graphi te is examined by performing various tests to determine the presence of diamonds.
5.2 Explosive Gases
As a first step the implosion chamber was pumped down to a partial vacuum of approximately 3 mm Hg. Then oxygen followed by hydrogen were pumped into the cha.m.ber. The proportions of hydrogen and oxygen were controlled such that a stoichiometrie mixture (2H2 + 02) was obtained at a final total pressure of 27.2 atm (400 psia).
Great care was taken when pumping hydrogen and oxygen gases in the implosion chamber. The gases were pumped very slowly to avoid damaging the fragile explosive liner. The gases were then allowed to mix for about ten minutes before they were detonated by an exploding wire.
5.3 Ignition System
In order to achieve a well-centred implosion, a special igni tion
sys"tem was employed to detonate the hydrogen and oxygen mixture. An exploding
copper wire (4 mil dia.m. x 1.25 mm long) was placed at the geometrie centre of the hemi..sphere where the graphite cartridge was also located. The wire was exploded by applying a sudden current discharge produced by a charged capacitor. The electronic circuit, shown in Fig. 17, consists basically of a 7.5 J.lf capacitor charged to 6 kV and a thyraton. The exploding wire success-fully detonates the hydrogen and oxygen mixture which subsequently detonates the explosive liner and produces a centred imploding shock wave about 20 percent of the time.
Tt is worth mentioning that the length of the exploding wire is important in achieving well-centred implosions. If the wire is too long, then off-centred implosions result. For successful implosions the wire length should be about 1.25 mm or less, which is near the practical limit of installa-tion.
5.4 Explosive Liner
The solid PETN explosive liner was mounted in a copper hemispherical liner having an internal diameter of 20 cm (8 in). The PEl'N explosive shell is about 6 mm thick and is made as uniform in thickness as possible to ensure that a good centred implosion can be obtained. Care must be taken in making a uniformly-thick PETN explosive liner.
The copper liner is first polished wi th sandpaper. Then a plastic foam is glued to the polished surface , and time is allowed for the glue to dry thoroughly. A PETN slurry is then prepared and pressed into the plastic foam to form a liner.
The superfine PETN explosive powder does not possess suffic~nt mechanical binding strength to form asolid hemispherical shel1. It is therefore first mixed with fine cotton linters and water. The resulting
PETN slurry consisting of fine powdered PETN (100 gm), water (250
gm),
andfine cotton linters (1.5 gm) is prepared carefully te ensure that the slurry is homogeneous . The cotton linters are first mixed with water and then the mixture is continuously stirred with an electrical stirring machine for a few minutes. Then PETN explosi ve powder is added to the mixture and s tirred continuously for at least an hour or until a final required consistency of
thick slurry is obtained which can be easily handled and installed in the capper liner. The wet PETN slurry is pushed into the plastic foam as carefully and homogeneously as possible to achieve a uniform thickness and density. The explosive liner is then allowed to dry very slowly. Four steps in the process of making the explosive liner are shown in Fig. 18. Obtaining a uniform thickness and homogeneous density of the solid PETN liner arevery important, otherwise off-centred implosions may result. The preparation of an explosive liner is very much an art which can only be per-fected with practice.
5.5 Control Room
The UTIAS Implosion Chamber was located in a blast room and aperated remotely from a control room for safety reasons . Personnel in the control room were protected in case of accidents by both the 30-cm thick concrete wall and 5-cm thick steel door separating the blast and control rooms. In addi tion the blast room has three blow-out walls in case of accidental explosions. All the operations including vacuum, pumping hydrogen and oxygen gases, firing and venting are carried out remotely from the control room. A picture of the
layout of the control room is shown in Fig. 19. Various interlocks incorporated in the layout of the control room allow only the proper sequence of aperations (Ref. 24).
6.
EXPERTMENTAL PROCEDURES ANTI RESULTSA total of fifteen runs (see Table 2) we:fe made to convert graphite powder to diamonds by using the high-pressure and high-temperature conditions produced in the UTIAS Implosion Chamber Facility. Most of the experiments were unsuccessful due to off-centred implosions . In order to eliminate -the effect of -the off-centred implosions various kinds of graphi te cartridges were used, as shown in Fig. 20. Graphite powder was compressed in the cavity of the graphi te cartridge -to a densi ty of 1. 5 - 1. 8 gm/ cc by using a hydraulic press. A cover plate was then welded in place. It acted like a piston and compressed -the graphite powder. Figure 21 shows a photograph of the graphite cartridges before and af ter the implpsion runs.
To a layman a polished diamond normally is a very hard material that sparkles under light. As other materials can also exhibi t this praperty i t can hardly be used to verify the existence of diamond. Consequently, several different methods, including density, hardness, scanning electron microscope and the very conclusive X-ray diffraction tests, were made to determine if the implosion process converted some of the graphite powder to diamond. The details and results of the various tests for verifying the presence of diamond in the shocked graphite will not be described.
6.1 Density Test
A special centri~uge can of ten be used to separate diamonds fr om a mixture of graphite and diamonds because graphi te and diamond have different densities of 2.23 gm/cc and 3.51 gm/cc, respectively. Af ter the implosion process the shocked graphi te powder was ground in a glass mortar such that the resulting powder had a mesh size of less than 300, or the particles were less than 50 microns in size. This extra fine powder was th en put in a test tube containing S-tetrabromoethane (having a density of 2.95 gm/cc, which lies between the densities of graphite and diamond).
The test tube was then placed in a centrifuge and run at about 20,000 RPM for a few minutes. Consequently, diamonds and mixtures of diamond and graphite containing
50%
or more diamond by weight would settle in the S-tetrabromoethane on centrifuging.On performing the centrifuge experiment it was found that a very small amount of shocked-graphite powder settled to the bottom of the test tube. Therefore, it is possible that some graphite was converted to diamond by the imploding shock-wave process. The density method does not conclusively establish that diamonds exist in the shocked graphi'te. There is a possibility that the settled material may consist entirely of dense graphi te having a density greater than that of S-tetrabromoethane.
6.2 Hardness Test
Diamond is the hardest known material. lts hardness on the Mohs' scale is taken to be 10. Therefore diamond should be able t0 scratch all the hardest known materials such as various carbides and sapphire.
The hardness test was carried out by using a glass slide (Mohs' hardness scale
4.5 - 6.5)
and a polished sapphire (Mohs' hardness scale9).
The shocked graphite powder was rubbed between a glass slide and a polisheds:~pphire. Af ter rubbing, fine scratches appeared on both the glass slide and the polished sapphire. These scratches are shown in the ph0tographs of Figs. 22 and 23, on a glass slide an~ a p0lished sapphire, respectively. These fine scratches, especially on a polished sapphire, give a g00d
indica-tion of the presence of diamonds in the shocked graphi te powder. Tt is interesting to note that the scratches on a glass slide (Fig. 22) are much deeper compared wi th the scratches on polished sapphire (Fig. 23).
During most of the explosive runs, the implosion was 'not centred and no trace of diamonds in the shocked graphite was found. However, it WÇl.S noticed that the hardness of the shocked graphite powder increased
considerably. Graphi te powder is a very soft material (Mohs' hardness scale less than 1) but af ter i t was subjected to even slightly off-centred implo-sions , the hardness was found to be considerably higher. The estimates were made using Mohs' hardness scale. Tt was found that the shocked graphite
(resulting from slightly off-centred implosions) was able to scratch fluorite (Mohs' hardness scale
4)
but not ~patite (Mohs' hardness scale5).
This showed that the hardness of the shocked graphite was between4
and5
on a Mohs' hardness scale. Precise measurements of hardness using a micro-hardness technique was not possible owing to the fineness of the graphite powder.lt is also worth men tioning that the shocked graphite los es the greasy feelof the original graphite powder. From the hardness test it can be stated that even though there were no traces of diamonds found in most
of the 0ff-centred explosive runs yet the hardness of the original graphite increased considerably.
6.3
Scanning Electron Microscope TestsA scanning electron microscope (SEM) (Cambridge Mark 2A) was used to study the unshocked and shocked graphite powder. The SEM detects and
