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Diamond synthesis at ultra-low pressures
Diamond synthesis at ultra-low pressures Boris Deryczgin a n d D m i t r i F e d o s e e v
Research reports and p a t e n t specifications t h r o u g h o u t the world are c o n c e r n e d with the p r o b l e m o f synthetic d i a m o n d p r o d u c t i o n , mainly for industrial and technological uses and only occasionally as jewellery. The discovery o f ' w h i s k e r ' crystals has imparted new direction to this work, the p r o d u c t i o n o f crystalline structures with notable properties, which are p r o d u c e d at the start o f the synthesis process.
Both processes have advantages and limitations resnltmg from tire particula~ crystallization conditions. [n addition the necessity fo~ special catalysts complicates the synthesia of pure crystals- inclusions of forei H substances are found--and tile high process rate either complicates control of the synthesis or makes it completely impossible.
In 1939 the Soviet physicist O. Leipunskii discovered in lus theoretical investigations that the synthesis of diamonds at high pressures and temperatures was in principle possible. His calculations showed that at a pressure of 100 000 atm and temperatures up to 2 000°C, diamonds should be formed from graphite.
On the other hand, the lower the pressure the longer the duration of the process and the more easily it can be controlled. For this reason research workers have attempted and are attempting to effect siHificant reductions in the pressure. Reference to this possibility was also made in the paper by O. Leipunskii.
Tile American researcher Bundy and co-workers were tile first to produce synthetic diamonds in 1955. Subsequently the problem of diamond synthesis at high pressures and temperatures was successfully solved in the USSR also under the direction of Academician L. Vereshchagin. A considerable contribution to the development of the diamond synthesis technique and of industrial processes for synthetic diamond production has been made by the temn under the direction of Doctor of Technical Sciences V. Bakul at the Institute for Ultra-ftard Materials at Kiev. In this Institute the new synthetic ultra-hard materials Kubonite and Slavutich have recently been developed, and these are in some respects not inferior to diamonds.
ttigh pressures are, however, not the only means of effecting diamond synthesis. There are other possible approaches. If the 'seed crystal' of a diamond is available, growth of the diamond can occur under conditions which should give rise only to the formation of graphite - as the thermodynamically stabilized form of carbon - but in no circumstances to that of dianmnd proper. In spite of this diamond crystallization which is thermodynamically unstable at low pressmes does occur.
The diamond synthesis process has been successfully developed in many countries. For this synthesis powerful presses and a large number of ha~d alloys are required.
With the new method of synthesis the processes occur at the boundary between two phases -solid and liquid or solid and gaseous-and are governed by the boundary surface phenomena which fall within the scope of physical chemistry. It is not surprising therefore that this process was developed in tile Department for Boundary Surface Phenomena of the Institute of Physical (Themistry.
High pressures and temperatures can be produced simultaneously either for a limited time (minutes) in a small volume or for a millionth of a second in a large volume using an explosive technique.
66
Selected Works - 3
I
~I
1
~
~
~ ~ . :
~
.
.
.
.
.
.
'.. . . .
Fig. 1
The new approach is extremely attractive. It enables the synthesis to be undertaken without tfigh pressures and without metal additives; the apparatus can consist of conventional materials. The dimensions of the reaction chambers in which the synthesis is carried out are, however, unrestricted. The process of diamond synthesis frown gas at extreme pressures can be explained either by the molecular mechanism of crystal growth or on the hypothesis of the formation of a new phase, ie nucleation. Let us assume that the concentration of carbon atoms in the neighbourhood of a seed crystal exceeds the corresponding equifibrium concentration. In this case the excess carbon atoms are deposited on the diamond surface. The probability that a carbon seed (a diamond or graphite seed) will form in the general volume is far less than that it will form on the surface of a diamond seed crystal. In other words, the homogeneous nucleation rate is far lower than the heterogeneous nucleation rate. During the deposition process the carbon is subject to the field of force of the seed crystal. As a result, under certain conditions, the process will follow the same course as the 'bricklaying' operation which previously led to the formation of the seed crystal itself. Owing to the presence of this 'base' the carbon atoms which have been deposited from the carboncontaining gas are obliged to range themselves in the prescribed order and not in the form of the graphite lattice. The orientating effect of the base, leading to the growth of the crystal modification, which is isomorphic to or similar in structure to the lattice of the base, is designated epitaxy. In the case of growth of diamond on diamond we are concerned with autoepitaxy (homo-epitaxy). If the supersaturation with carbon is too great and the deposition of
67
carbon atoms from the gas or the solution proceeds too rapidly, conditions may arise under which the growth and formation of graphite seeds, as the thermodynamicallypreferred carbon modification, predominates in spite of the influence of the surface forces. Initially, the growth of the diamond and the graphite will occur at different points on the surface simultaneously but subsequently, if the carbon supersaturation is sufficient, the graphite will cover the whole surface of the diamond crystal, the growth of which then stops (Figure 1). The conditions under which the marked carbon deposition can be avoided are given by the theory of crystal formation and growth. The probability of the occurrence of a stable (critical) seed of a new phase - in the case of graphite with which we are here concerned-capable of further free growth becomes progressively smaller the greater the work required for its production. It is therefore preferable to select a low level of supersaturation, which must nevertheless be higher than the minimum requirement. As we approach this limit below which the diamond cannot grow but only becomes dissolved in the gaseous or liquid metal phase, we reduce the growth rate of the seed crystal. In this case, in spite of the presence of the already formed crystal - i t can happen in certain growth stages of the diamond faces, at the start of the growth of a new layer of the 'brick w a l l ' - t h a t the growth rate is restricted by the requirement for work to be expended on the formation of a twodimensional critical diamond seed of the thickness of 1 atom. With a very low level of supersaturation with carbon atoms, the probability of seed formation is so smaU that the growth of the crystal proceeds accompanied by the formation o f dislocations. At certain points on the crystal surface, however, it may proceed freely, no two-dimensional seeds being required. Tiffs growth may be compared with climbing a spiral staircase whicli has no landings.
In tlus way under certain conditions the work required for the formation of the critical two-dimensional seed on the surface of the monocrystal is either zero or very small. The work of formation of a three-dimensional critical graphite seed on the same surface however is, in comparison to the energy of thermal movement of the atoms, relatively large. In this case the epitaxial growth of the diamond will be accompanied by extremely small deposits of graphite. A considerable number of papers at the present time are devoted to epitaxial diamond synthesis, the majority of which are in the form of patents filed in different countries. Let us consider the most important of these. Eversole proposed growing diamond seed powder frown carbon-containing gases with methyl groups (-CH3). The process is carried out tinder pressures up to 70 mm Hg; the best results, however, were obtained using methane at a pressure of approx, t m m Hg and temperatures up to I 100°C. In order to obtain greater accuracy in the determination of the growth rate, use was made of powdered diamonds with a high specific surface area, so making it possible to record
B. V. Derjaguin
68
'
inert m e d i u m - which may be a v a c u u m - from the graphite heated to 4 000°C on to the diamond seed crystal. in all the papers leferred to the linear growth rate was no higher than 100 ,~ per hour. It was frequently considerably less. The discovery of the crystal of diamond was of fundamental importance in this connection.
-
Fig. 2
reliably the weight gain of the new diamond phase with linear crystallization rates of some t0 -8 cm per hour. The removal of the carbon which possessed no diamond structure was effected by Eversole either in an acid mixture, or at pressures up to 50 atm and temperatures of 1 000°C in a hydrogen atmosphere. As a result of the different kinetic activities of the diamonds and the black carbon and their differing specific surface area, under these conditions the graphite is evaporated more rapidly. The interaction of the graphite and the diamond with hydrogen and also the process of epitaxial diamond growth was further studied by Angus and his co-workers. In order to investigate the morphology of the diamond nronocrystals which we had grown, the growth process of tile seed diamond monocrystals was carried out under conditions similar to those described above. The diamond phase grown in tiffs way was examined under the electron microscope. Figure 2 shows the surface of the seed crystal face of a diamond and Figure 3 the same face after the growth of tile continuous layer.
The wtffsker crystal of diamond was first grown and investigated by tile authors of the present paper in conjunctions with V. Ryabov, V. Lukyanovich, B. Spitsyn, A. Lavrentiev, L. Builov and A. Kochergina. Its most important properties were described in the literature. Whiske~ crystals of various substances have attracted the attention of research workers to a progressively increasing extent; in view of their unique properties they have been described as the 'constructional material of the future'. These properties include the monocrystalline structure, the ability to retain elasticity and strength at high temperatures and their ability after deformation due to heat treatment to completely resume their original shape. Their strength comes close to the theoretical value. Steel whiskers with a diameter of 2,u and a length of 2 mm have for example a tensffe strength of 1 2 0 0 - 1 3 0 0 k p / m m 2, whereas the tensile strength of normal steel is 18-23 kp/mm 2. Tire use of a sapphire filament alloyed with niobium (50% by weight) increases the strength of the products by a factor of 4 in relation to pure niobium. Up to now whisker crystals of all substances have been grown in their thermodynamic stability range. The discovery of whisker crystals of the diamond is of value m two respects. In the first place it demonstrated the possibility of their growth under conditions in which the crystallizing substance is present in a metastable phase. Secondly, it demonstrated tile possibility in principle of diamond growth from carbon-contairung gases at low pressures with
~
.
.
A French patent describes a wide variety of synthesis procedures for diamond modifications using chemical transport reactions. In order to avoid parasitic deposition of non-diamond carbon, the concentration of carbon-containing gases Surrounding the diamond should not deviate too greatly from the equilibrium concentration during diamond formation. The theory of this process was investigated by the authors of tile present paper using methods of nnnequilibrium thermodynamics. Of particular interest is the paten~ by Brinkman which is, strictly speaking, the continuation of his patent of 1957. This patent describes the process for epitaxial diamond synthesis from a solution of carbon in metals, the carbon source being graphite, which acts as the seed crystal at a higher temperature. The patent by tire same authors describes tile tlansport of the carbon atoms thr~mgh all
b)g.
,~
~
2 ¸
•
.
Selected Works - 3
Fig. 4
high linear growth rates. These growth rates are several times higher than the linear growth rates of diamond powder (a few .~., in some cases even tens of A per hour). For these experiments a modified radiation heating apparatus based on an ultra-high-pressure xenon valve was used. This apparatus was developed in the Institute of Physical Chemistry under the direction of V. Sasorov. The apparatus makes it possible to focus the valve radiation on the surface of the diamond seed crystal which is secured by a special rhenium holder and mounted in a spherical quartz reactor.
69
of the diamond (the base) prior to the experiment. Special experiments were therefore carried out on the growth of diamond whiskers in liquid metal droplets. This method is known as the VLS process (from tile English vapour-liquidsolid: ie gas-liquid-crystal). Carbon-containing gas decomposes on the surface of the liquid metal, the carbon is dissolved in the metal, diffuses through the metal and becomes deposited on the surface of the seed crystal. Diamond whisker crystals with cooled metal droplets on their tips were obtained. Positive results were obtained with metals which easily dissolved carbon and produce a good wetting effect on the diamond, such as nickel, iron and manganese; with molten g01d no such structures were observed. The VLS process is clearly one - but not the only - m e t h o d of elucidating the growth mechanism of the diamond whisker crystal. The use of an intermediate metal phase for diamond growth is of fundamental importance. The work of formation of a three-dimensional graphite seed in a metal layer is under certain conditions considerably greater than the work of formation of a two-dimensional critical diamond seed. The reason for this is that the surface energies at the graphitemetal and graphite-diamond interfaces are large, thus considerably increasing the work of formation of the graphite seed and preventing its deposition. A question of particular importance is the recently observed transformation of the whisker crystals into spheroid structures, frequently with crystal facets (isometric crystals). The authors and their co-workers have been able during the experiment itself to observe under the microscope the growth of the whisker crystal, the end of its growth period and the gradual increase in the thickness of
The whisker crystal was obtained as a result of an attempt to accelerate the process of epitaxial diamond synthesis. The mean growth rate of the diamond whisker crystal was I0 # per hour, but in some cases reached 50-250"# per hour. It may be assumed that this is still not the maximum possible linear growth rate. Figure 4 shows the whisker crystal of a diamond 1.5 ~ in length which is grown on face (Ill) of the diamond crystal. Experiments have shown that on irradiation with an electron stream the wlfisker ~.rystals himinesce in the same way as the base. For more accurate identification of the structures obtained after one of the experiments the crystals were carefully separated from the base, ground and placed on the slide of an electron microscope. The results of electron diffraction investigations enabled us to record the following face intervals in A: 2-06, 1-27, 1-068, 0-920, 0-825, 0-749, 0.680. According to special tables, however, the face intervals in the diamond in A are: 2.05, 1.26, 1-072, 0-883, 0-813, 0-721, 0-684. The electron diffraction diagrams obtained by measurement are an indication of the monocrystalline character of the filamentary crystals. It was found in some experiments that the whisker crystals have at their tips dark spherical structures. The reason for this may be the deposition of metal particles on the surface
~,g. 5
70
B.V.
the crystal. The result of this complex process was au isometric crystal with clearly-marked facets and with a mean cross-section of 0.1 ram. This crystal was examined by micro-X-ray diffraction analysis. The X-ray bundle, with a diameter of 12/.t, passed through the tip, the centre and the foot of the crystal which had been grown. The investigation showed that it was a monocrystal. Figure 5 shows an isometric diamond crystal which has been made visible in the Stereoscan microscope.
Dedaguin Deryagin, B. V., Builov, L. L. et al. (1969)Krystallografiya
14 (3) Deryagin, B. V. (1969) New Scientist 44 673 Deryagin, B. V. and Fedoseev, D. V. (1969) Papers delivered to 22nd International Congress on Theoretical and Applied Chemistry, Sydney, Australia I)eryagin, B. V., Lopatina, G. G. et al. (1968) Zhurnal fizicheskoi khimii 42 (9) Deryagin, B. V., Lyutsay, V. G. et al. (1970) Doklady AN SSSR 190 (1)
References Leipunskii, O. 1. (1939) Uspekhi khimii 8 (10) Bundy, F. R. et aL (1955) Nature 176 (4471 ) 5 l 'Pravda' (24 October 1961) Eversole, W. G. US Patent No. 3 030 187 and 3 030 t88 Angus, D. C. et al. (1968) J. AppL Phys. 39 (6) Deryagin, B. V., Ryabov, V. A. et al. (1969) Second AUUnion Symposium on Growth and Synthesis Processes. Report theses, p. 34. Novosibirsk French Patent (1964) No. 1 366 544 Deryagin, B. V., Fedoseev, D. V. and Spitsyn, B. V. (1968) J. Crystal Growth 3 (4) 111 Brinkman, D. A. et al. US Patent No. 3 142 539 Brinkman, D. A. et al. US Patent No. 3 175 885 Deryagin, B. V., Fedoseev, D. V. et al. (1968)Doklady AN SSSR 181 (5) Idem (1968)J. Crystal Growth 2 (6)
Boris Deryagin is Corresponding Member of the USSR Academy of Sciences and Head of the Department of Boundary Surface Phenomena at the Institute of Physical Chemistry of the USSR Academy of Sciences. He is a known expert in the field of physical chemistry and a specialist in colloid chemistry, aerosols and molecular physics. In 1958 he was awarded the Lomonosov Prize of the USSR Academy of Sciences. He is a member of the Faraday Society and an Honorary Doctor of Clarkson College, USA. Dmitri Fedoseev is doctor of chemical sciences. He is a Group Leader in the Department of Boundary Surface Phenonrena at the Institute of Physical Chemistry. He is a specialist in the field of physical and chemical kinetics and solid body physics.
Research reports and p a t e n t specifications t h r o u g h o u t the world are c o n c e r n e d with the p r o b l e m o f synthetic d i a m o n d p r o d u c t i o n , mainly for industrial and technological uses and only occasionally as jewellery. The discovery o f ' w h i s k e r ' crystals has imparted new direction to this work, the p r o d u c t i o n o f crystalline structures with notable properties, which are p r o d u c e d at the start o f the synthesis process.
Both processes have advantages and limitations resnltmg from tire particula~ crystallization conditions. [n addition the necessity fo~ special catalysts complicates the synthesia of pure crystals- inclusions of forei H substances are found--and tile high process rate either complicates control of the synthesis or makes it completely impossible.
In 1939 the Soviet physicist O. Leipunskii discovered in lus theoretical investigations that the synthesis of diamonds at high pressures and temperatures was in principle possible. His calculations showed that at a pressure of 100 000 atm and temperatures up to 2 000°C, diamonds should be formed from graphite.
On the other hand, the lower the pressure the longer the duration of the process and the more easily it can be controlled. For this reason research workers have attempted and are attempting to effect siHificant reductions in the pressure. Reference to this possibility was also made in the paper by O. Leipunskii.
Tile American researcher Bundy and co-workers were tile first to produce synthetic diamonds in 1955. Subsequently the problem of diamond synthesis at high pressures and temperatures was successfully solved in the USSR also under the direction of Academician L. Vereshchagin. A considerable contribution to the development of the diamond synthesis technique and of industrial processes for synthetic diamond production has been made by the temn under the direction of Doctor of Technical Sciences V. Bakul at the Institute for Ultra-ftard Materials at Kiev. In this Institute the new synthetic ultra-hard materials Kubonite and Slavutich have recently been developed, and these are in some respects not inferior to diamonds.
ttigh pressures are, however, not the only means of effecting diamond synthesis. There are other possible approaches. If the 'seed crystal' of a diamond is available, growth of the diamond can occur under conditions which should give rise only to the formation of graphite - as the thermodynamically stabilized form of carbon - but in no circumstances to that of dianmnd proper. In spite of this diamond crystallization which is thermodynamically unstable at low pressmes does occur.
The diamond synthesis process has been successfully developed in many countries. For this synthesis powerful presses and a large number of ha~d alloys are required.
With the new method of synthesis the processes occur at the boundary between two phases -solid and liquid or solid and gaseous-and are governed by the boundary surface phenomena which fall within the scope of physical chemistry. It is not surprising therefore that this process was developed in tile Department for Boundary Surface Phenomena of the Institute of Physical (Themistry.
High pressures and temperatures can be produced simultaneously either for a limited time (minutes) in a small volume or for a millionth of a second in a large volume using an explosive technique.
66
Selected Works - 3
I
~I
1
~
~
~ ~ . :
~
.
.
.
.
.
.
'.. . . .
Fig. 1
The new approach is extremely attractive. It enables the synthesis to be undertaken without tfigh pressures and without metal additives; the apparatus can consist of conventional materials. The dimensions of the reaction chambers in which the synthesis is carried out are, however, unrestricted. The process of diamond synthesis frown gas at extreme pressures can be explained either by the molecular mechanism of crystal growth or on the hypothesis of the formation of a new phase, ie nucleation. Let us assume that the concentration of carbon atoms in the neighbourhood of a seed crystal exceeds the corresponding equifibrium concentration. In this case the excess carbon atoms are deposited on the diamond surface. The probability that a carbon seed (a diamond or graphite seed) will form in the general volume is far less than that it will form on the surface of a diamond seed crystal. In other words, the homogeneous nucleation rate is far lower than the heterogeneous nucleation rate. During the deposition process the carbon is subject to the field of force of the seed crystal. As a result, under certain conditions, the process will follow the same course as the 'bricklaying' operation which previously led to the formation of the seed crystal itself. Owing to the presence of this 'base' the carbon atoms which have been deposited from the carboncontaining gas are obliged to range themselves in the prescribed order and not in the form of the graphite lattice. The orientating effect of the base, leading to the growth of the crystal modification, which is isomorphic to or similar in structure to the lattice of the base, is designated epitaxy. In the case of growth of diamond on diamond we are concerned with autoepitaxy (homo-epitaxy). If the supersaturation with carbon is too great and the deposition of
67
carbon atoms from the gas or the solution proceeds too rapidly, conditions may arise under which the growth and formation of graphite seeds, as the thermodynamicallypreferred carbon modification, predominates in spite of the influence of the surface forces. Initially, the growth of the diamond and the graphite will occur at different points on the surface simultaneously but subsequently, if the carbon supersaturation is sufficient, the graphite will cover the whole surface of the diamond crystal, the growth of which then stops (Figure 1). The conditions under which the marked carbon deposition can be avoided are given by the theory of crystal formation and growth. The probability of the occurrence of a stable (critical) seed of a new phase - in the case of graphite with which we are here concerned-capable of further free growth becomes progressively smaller the greater the work required for its production. It is therefore preferable to select a low level of supersaturation, which must nevertheless be higher than the minimum requirement. As we approach this limit below which the diamond cannot grow but only becomes dissolved in the gaseous or liquid metal phase, we reduce the growth rate of the seed crystal. In this case, in spite of the presence of the already formed crystal - i t can happen in certain growth stages of the diamond faces, at the start of the growth of a new layer of the 'brick w a l l ' - t h a t the growth rate is restricted by the requirement for work to be expended on the formation of a twodimensional critical diamond seed of the thickness of 1 atom. With a very low level of supersaturation with carbon atoms, the probability of seed formation is so smaU that the growth of the crystal proceeds accompanied by the formation o f dislocations. At certain points on the crystal surface, however, it may proceed freely, no two-dimensional seeds being required. Tiffs growth may be compared with climbing a spiral staircase whicli has no landings.
In tlus way under certain conditions the work required for the formation of the critical two-dimensional seed on the surface of the monocrystal is either zero or very small. The work of formation of a three-dimensional critical graphite seed on the same surface however is, in comparison to the energy of thermal movement of the atoms, relatively large. In this case the epitaxial growth of the diamond will be accompanied by extremely small deposits of graphite. A considerable number of papers at the present time are devoted to epitaxial diamond synthesis, the majority of which are in the form of patents filed in different countries. Let us consider the most important of these. Eversole proposed growing diamond seed powder frown carbon-containing gases with methyl groups (-CH3). The process is carried out tinder pressures up to 70 mm Hg; the best results, however, were obtained using methane at a pressure of approx, t m m Hg and temperatures up to I 100°C. In order to obtain greater accuracy in the determination of the growth rate, use was made of powdered diamonds with a high specific surface area, so making it possible to record
B. V. Derjaguin
68
'
inert m e d i u m - which may be a v a c u u m - from the graphite heated to 4 000°C on to the diamond seed crystal. in all the papers leferred to the linear growth rate was no higher than 100 ,~ per hour. It was frequently considerably less. The discovery of the crystal of diamond was of fundamental importance in this connection.
-
Fig. 2
reliably the weight gain of the new diamond phase with linear crystallization rates of some t0 -8 cm per hour. The removal of the carbon which possessed no diamond structure was effected by Eversole either in an acid mixture, or at pressures up to 50 atm and temperatures of 1 000°C in a hydrogen atmosphere. As a result of the different kinetic activities of the diamonds and the black carbon and their differing specific surface area, under these conditions the graphite is evaporated more rapidly. The interaction of the graphite and the diamond with hydrogen and also the process of epitaxial diamond growth was further studied by Angus and his co-workers. In order to investigate the morphology of the diamond nronocrystals which we had grown, the growth process of tile seed diamond monocrystals was carried out under conditions similar to those described above. The diamond phase grown in tiffs way was examined under the electron microscope. Figure 2 shows the surface of the seed crystal face of a diamond and Figure 3 the same face after the growth of tile continuous layer.
The wtffsker crystal of diamond was first grown and investigated by tile authors of the present paper in conjunctions with V. Ryabov, V. Lukyanovich, B. Spitsyn, A. Lavrentiev, L. Builov and A. Kochergina. Its most important properties were described in the literature. Whiske~ crystals of various substances have attracted the attention of research workers to a progressively increasing extent; in view of their unique properties they have been described as the 'constructional material of the future'. These properties include the monocrystalline structure, the ability to retain elasticity and strength at high temperatures and their ability after deformation due to heat treatment to completely resume their original shape. Their strength comes close to the theoretical value. Steel whiskers with a diameter of 2,u and a length of 2 mm have for example a tensffe strength of 1 2 0 0 - 1 3 0 0 k p / m m 2, whereas the tensile strength of normal steel is 18-23 kp/mm 2. Tire use of a sapphire filament alloyed with niobium (50% by weight) increases the strength of the products by a factor of 4 in relation to pure niobium. Up to now whisker crystals of all substances have been grown in their thermodynamic stability range. The discovery of whisker crystals of the diamond is of value m two respects. In the first place it demonstrated the possibility of their growth under conditions in which the crystallizing substance is present in a metastable phase. Secondly, it demonstrated tile possibility in principle of diamond growth from carbon-contairung gases at low pressures with
~
.
.
A French patent describes a wide variety of synthesis procedures for diamond modifications using chemical transport reactions. In order to avoid parasitic deposition of non-diamond carbon, the concentration of carbon-containing gases Surrounding the diamond should not deviate too greatly from the equilibrium concentration during diamond formation. The theory of this process was investigated by the authors of tile present paper using methods of nnnequilibrium thermodynamics. Of particular interest is the paten~ by Brinkman which is, strictly speaking, the continuation of his patent of 1957. This patent describes the process for epitaxial diamond synthesis from a solution of carbon in metals, the carbon source being graphite, which acts as the seed crystal at a higher temperature. The patent by tire same authors describes tile tlansport of the carbon atoms thr~mgh all
b)g.
,~
~
2 ¸
•
.
Selected Works - 3
Fig. 4
high linear growth rates. These growth rates are several times higher than the linear growth rates of diamond powder (a few .~., in some cases even tens of A per hour). For these experiments a modified radiation heating apparatus based on an ultra-high-pressure xenon valve was used. This apparatus was developed in the Institute of Physical Chemistry under the direction of V. Sasorov. The apparatus makes it possible to focus the valve radiation on the surface of the diamond seed crystal which is secured by a special rhenium holder and mounted in a spherical quartz reactor.
69
of the diamond (the base) prior to the experiment. Special experiments were therefore carried out on the growth of diamond whiskers in liquid metal droplets. This method is known as the VLS process (from tile English vapour-liquidsolid: ie gas-liquid-crystal). Carbon-containing gas decomposes on the surface of the liquid metal, the carbon is dissolved in the metal, diffuses through the metal and becomes deposited on the surface of the seed crystal. Diamond whisker crystals with cooled metal droplets on their tips were obtained. Positive results were obtained with metals which easily dissolved carbon and produce a good wetting effect on the diamond, such as nickel, iron and manganese; with molten g01d no such structures were observed. The VLS process is clearly one - but not the only - m e t h o d of elucidating the growth mechanism of the diamond whisker crystal. The use of an intermediate metal phase for diamond growth is of fundamental importance. The work of formation of a three-dimensional graphite seed in a metal layer is under certain conditions considerably greater than the work of formation of a two-dimensional critical diamond seed. The reason for this is that the surface energies at the graphitemetal and graphite-diamond interfaces are large, thus considerably increasing the work of formation of the graphite seed and preventing its deposition. A question of particular importance is the recently observed transformation of the whisker crystals into spheroid structures, frequently with crystal facets (isometric crystals). The authors and their co-workers have been able during the experiment itself to observe under the microscope the growth of the whisker crystal, the end of its growth period and the gradual increase in the thickness of
The whisker crystal was obtained as a result of an attempt to accelerate the process of epitaxial diamond synthesis. The mean growth rate of the diamond whisker crystal was I0 # per hour, but in some cases reached 50-250"# per hour. It may be assumed that this is still not the maximum possible linear growth rate. Figure 4 shows the whisker crystal of a diamond 1.5 ~ in length which is grown on face (Ill) of the diamond crystal. Experiments have shown that on irradiation with an electron stream the wlfisker ~.rystals himinesce in the same way as the base. For more accurate identification of the structures obtained after one of the experiments the crystals were carefully separated from the base, ground and placed on the slide of an electron microscope. The results of electron diffraction investigations enabled us to record the following face intervals in A: 2-06, 1-27, 1-068, 0-920, 0-825, 0-749, 0.680. According to special tables, however, the face intervals in the diamond in A are: 2.05, 1.26, 1-072, 0-883, 0-813, 0-721, 0-684. The electron diffraction diagrams obtained by measurement are an indication of the monocrystalline character of the filamentary crystals. It was found in some experiments that the whisker crystals have at their tips dark spherical structures. The reason for this may be the deposition of metal particles on the surface
~,g. 5
70
B.V.
the crystal. The result of this complex process was au isometric crystal with clearly-marked facets and with a mean cross-section of 0.1 ram. This crystal was examined by micro-X-ray diffraction analysis. The X-ray bundle, with a diameter of 12/.t, passed through the tip, the centre and the foot of the crystal which had been grown. The investigation showed that it was a monocrystal. Figure 5 shows an isometric diamond crystal which has been made visible in the Stereoscan microscope.
Dedaguin Deryagin, B. V., Builov, L. L. et al. (1969)Krystallografiya
14 (3) Deryagin, B. V. (1969) New Scientist 44 673 Deryagin, B. V. and Fedoseev, D. V. (1969) Papers delivered to 22nd International Congress on Theoretical and Applied Chemistry, Sydney, Australia I)eryagin, B. V., Lopatina, G. G. et al. (1968) Zhurnal fizicheskoi khimii 42 (9) Deryagin, B. V., Lyutsay, V. G. et al. (1970) Doklady AN SSSR 190 (1)
References Leipunskii, O. 1. (1939) Uspekhi khimii 8 (10) Bundy, F. R. et aL (1955) Nature 176 (4471 ) 5 l 'Pravda' (24 October 1961) Eversole, W. G. US Patent No. 3 030 187 and 3 030 t88 Angus, D. C. et al. (1968) J. AppL Phys. 39 (6) Deryagin, B. V., Ryabov, V. A. et al. (1969) Second AUUnion Symposium on Growth and Synthesis Processes. Report theses, p. 34. Novosibirsk French Patent (1964) No. 1 366 544 Deryagin, B. V., Fedoseev, D. V. and Spitsyn, B. V. (1968) J. Crystal Growth 3 (4) 111 Brinkman, D. A. et al. US Patent No. 3 142 539 Brinkman, D. A. et al. US Patent No. 3 175 885 Deryagin, B. V., Fedoseev, D. V. et al. (1968)Doklady AN SSSR 181 (5) Idem (1968)J. Crystal Growth 2 (6)
Boris Deryagin is Corresponding Member of the USSR Academy of Sciences and Head of the Department of Boundary Surface Phenomena at the Institute of Physical Chemistry of the USSR Academy of Sciences. He is a known expert in the field of physical chemistry and a specialist in colloid chemistry, aerosols and molecular physics. In 1958 he was awarded the Lomonosov Prize of the USSR Academy of Sciences. He is a member of the Faraday Society and an Honorary Doctor of Clarkson College, USA. Dmitri Fedoseev is doctor of chemical sciences. He is a Group Leader in the Department of Boundary Surface Phenonrena at the Institute of Physical Chemistry. He is a specialist in the field of physical and chemical kinetics and solid body physics.