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Influences of additive sulfur on the synthesis of industrial diamonds, using Ni70Mn25Co5 alloy powder as catalyst
Diamond & Related Materials 15 (2006) 1318 – 1322 www.elsevier.com/locate/diamond
Influences of additive sulfur on the synthesis of industrial diamonds, using Ni70Mn25Co5 alloy powder as catalyst Lin Zhou a, Xiaopeng Jia a,b,*, Hongan Ma a, Lixue Chen a, Weili Guo a, Yantao Li a a
National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China b Henan Polytechnic University, Jiaozuo 454000, China
Received 19 April 2005; received in revised form 25 September 2005; accepted 9 October 2005 Available online 18 January 2006
Abstract Sulfur was introduced to the synthesis of industrial diamonds using Ni70Mn25Co5 alloy powder as catalyst. Optical microscope, XRD, XPS, and SEM were utilized for observation and detection. The influences of additive sulfur on the synthesis have been investigated from several aspects such as the diamond-forming conditions, the composition of synthesized sample and inclusions in diamond, and the morphology character of diamonds etc. The results indicate that the formation of MnS, which should have been in a solid state during the synthesis, has an important influence on the synthesis of industrial diamonds. D 2005 Elsevier B.V. All rights reserved. Keywords: Additive sulfur; Ni70Mn25Co5 alloy powder; Diamond; MnS
1. Introduction In the field of industrial diamonds, the study on catalyst is an extremely important respect. Different catalysts have different characters, which directly influence the diamondforming conditions, the morphology character of diamonds, and the mechanical performance of diamonds, and so on. Adding some trace elements to catalyst can change the characters of catalyst remarkably. Thus the categories of catalyst will be increased and the applied range of industrial diamonds will be widened. It is well known that diamond is a wide-bandgap semiconductor with unsurpassed physical and chemical properties. When doped, semiconducting diamond can lead to the realization of electronic and optoelectronic devices with exceptional properties. One of the major issues in diamond electronics is the search for a useful n-type dopant. Several impurities are candidates to act as donors in diamond when occupying the proper lattice site. In analogy to donors in * Corresponding author. National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China. Fax: +86 431 5168858. E-mail addresses: [email protected] (L. Zhou), [email protected] (X. Jia). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.10.013
silicon, group V (N, P) and VI (S) elements are expected to create shallow donor levels in diamond. But in fact nitrogen distorts the lattice of diamond leading to a deep level of 1.7 eV [1]. And phosphor has been shown to have a rather deep donor level in diamond (E a ¨0.5 eV) and the electron mobility values achieve so far for CVD-grown P-containing diamond layers are still rather low. The efforts therefore focus on sulfur. Computer simulations for diamond containing different dopant impurities show that substitutional S in diamond should have a rather shallow donor level (0.2 eV) [2]. Utilizing CVD-grown method, researchers have launched a large number of work in this respect [3 – 5]. However, this prediction has not been experimentally confirmed yet [6]. In the periodic table of elements, sulfur is a light element more close to carbon element. So it is relatively easy for sulfur atom to enter into the lattice of diamond. Under the conditions of 8 –8.5 GPa and 1600– 1800 -C, sulfur can act as catalyst for the synthesis of diamonds alone [7]. In this paper, we added a small amount of simple substance sulfur powder to Ni70Mn25Co5 catalyst. The influences of additive sulfur on the synthesis of diamonds had been studied. Optical microscope was applied to identify the formation of diamond. The synthesized sample was characterized by X-ray diffraction. XPS was used to determine the composition of the
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after the release of the applied pressure. The identification of diamond formation was conducted by optical microscope. The experimental conditions and results are presented in Fig. 2. It is easy to find out that with the increase of sulfur content in Ni70Mn25Co5-graphite-sulfur system, the diamond-forming temperature interval at certain pressure has a tendency to narrow down, while the minimum pressure at certain temperature for diamond formation tends to rise. This indicates that the diamond-forming V-shape region should move in the case of some change happens to the sulfur content in the system. Fig. 1. Sample assembly sketch of high pressure experiment.
inclusions in diamonds. Utilizing scanning electron microscope (SEM), we investigated the morphology of diamonds. 2. Experimental details The experiments were carried out in China-made SPD 6 1200 cubic high-pressure apparatus under the conditions of 5.5 – 5.9 GPa and 1400 – 1600 -C. The experimental raw materials are Ni70Mn25Co5 alloy powder and natural crystalline flake graphite powder (both of them are 200 mesh in granularity and higher than 99.9% in purity). The additive is a sedimented sulfur, a kind of much fine powder. Compared to other sulfur, it presents a larger active surface for the same amount of sulfur. It is above 99% in purity. Ni70Mn25Co5 alloy powder, graphite powder and sulfur powder were uniformly mixed according to mass ratios. The mass ratio of catalyst to graphite was 1 : 1 in the mixture. And there were three kinds of mass percentage compositions of sulfur powder, which were 0.5%, 1% and 2.5%, respectively. Synthesis sample was prepared by pressing the mixture of certain mass into the shape of cylinder. To contrast with that, we also prepared some samples without additive sulfur. Fig. 1 shows the high pressure sample assembly. After the synthesis under different pressure and temperature conditions, the synthesized samples were examined with optical microscope and X-ray diffractometer (XRD). When it is for the purpose of obtaining single diamond crystals, the synthesized samples were treated in the boiling mixture of H2SO4 and HNO3. And then scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were applied for observation and detection.
3.2. X-ray diffraction of the synthesized sample After synthesis, the synthesized sample whose initial sulfur content is 2.5 wt.% is characterized by X-ray diffraction (see Fig. 3). The results of XRD indicate that there is a new compound MnS in the sample. The melting point of MnS is 1615 -C at normal state, while the temperature of diamond formation in this paper is about 1400 –1600 -C. So we infer that MnS should have been in solid state under the conditions of diamond forming due to its higher melting point. 3.3. XPS measurement of the inclusions in synthetic diamonds Based on the XRD results that MnS exists in the synthesized sample, we used X-ray photoelectron spectrosco-
3. Results and discussions 3.1. Influences of additive sulfur on synthesis conditions To investigate the influences of additive sulfur on diamondforming conditions, we carried out the experiments under the conditions of 5.5 –5.9 GPa and 1400 –1600 -C. The synthesis systems were first compressed to certain pressure and then heated to certain temperature. Ten minutes later, quenching was achieved by shutting off the electric power supply under highpressure conditions. And the synthesis samples were recovered
Fig. 2. Experimental results under different pressure and temperature conditions. Solid square means diamond crystallization and open square means absence of diamond.
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Fig. 5. Scheme of the movement of diamond-forming V-shapes region with the introduction of additive sulfur. Fig. 3. X-ray diffraction results of the synthesized sample whose sulfur content is 2.5 wt.%.
py (XPS) measurement to determine the composition of inclusions in diamonds for the purpose of further characterizing the state of MnS during the synthesis. Diamonds, which
Fig. 4. XPS narrow scan spectra of Mn2p and S2p.
had been synthesized in 2.5 wt.% sulfur-contained system, were selected for XPS measurement. After a series of thorough acid treatment and purification, diamonds were smashed into powder. And then XPS measurement was carried out. XPS narrow scan spectra of Mn2p and S2p regions are shown in Fig. 4. The Mn2p spectrum indicates Mn2+ and the S2p spectrum indicates S2 and S – O bond. As for S –O bond, we think that there are two approaches, the oxidation of raw materials and the oxygen molecules sealed in the sample assembly, for the introduction of oxygen. XPS measurement shows that there are no other positive ions in the inclusions except for Mn2+. So we think that it is MnS that have entered into diamond. On the other hand, though inclusions can not be distributed evenly in diamond powder so that we can not carry on accurate quantitative analysis to it, we have also roughly calculated the number percentage of MnS to the atomic carbon of diamond. It is about 0.22%. Such higher content of MnS in diamond confirms our inference that MnS should be in solid state, rather than dissolve in the melted alloy during the synthesis.
Fig. 6. Mn – Ni phase diagram.
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enhancement of the melting point of the catalyst, which would probably further make the catalyst-diamond eutectic line move to the higher-temperature region. Accordingly, in Fig. 5, the eutectic line is moved from AB to AVB.V As a result of that, the V-shape region is moved along the D –G equilibrium line towards the higher-temperature region in the P – T phase diagram of carbon. 3.5. Color and morphology of diamond crystals
Fig. 7. Representative SEM photographs of diamond crystals. The sulfur content of (a), (b), (c) and (d) are 0, 0.5, 1 and 2.5 wt.%, respectively.
3.4. Explanation for the movement of V-shape region In the P –T phase diagram of carbon, the district for diamond formation is a V-shape region bounded by the D – G equilibrium line and the metal-diamond eutectic line (Fig. 5). For different catalysts, the metal-diamond eutectic lines are different, so are the corresponding V-shape regions. In this paper, because solid MnS was formed during the synthesis, the content of manganese element in the Ni70Mn25Co5 alloy was reduced. Thus the stoichiometry of alloy composition was changed. The character of the catalyst was different from the one before the synthesis, and so was the metal-diamond eutectic line. Because the essential compositions of Ni70Mn25Co5 alloy catalyst are nickel and manganese, we can consult Mn –Ni phase diagram (Fig. 6) to carry on qualitative analysis to the relation between the melting point of catalyst and the stoichiometry of its components. From Mn –Ni phase diagram, we can find out that when the mass percentage composition of nickel in the alloy is more than 40%, the melting point of the alloy tends to rise with the reduction of manganese content. As we have already mentioned above, the content of manganese in the alloy was reduced due to the formation of solid MnS. Based on the analysis to Mn – Ni phase diagram, this should have caused the
Based on the above experimental results that the introduction of additive sulfur makes the diamond-forming V-shape region move, we accordingly adjusted the temperature and pressure conditions to the optimal synthesis conditions for each kind of samples, and then carried out the synthesis. The synthesis time of 15 min was applied. We obtained single diamond crystals by treating the synthesized samples with a series of acid treatment and purification. Utilizing optical microscope, we observed that there is a little inclusion in the diamonds from sulfur-contained system. And the more sulfur there is in the synthesis system, the more inclusions there are in the synthetic diamonds. On the other hand, with the increase of sulfur content in the synthesis system, the color of diamonds turns from yellow to green, dark green and black gradually. Scanning electronic microscope (SEM) was employed to observe the surface morphology of diamonds. Fig. 7 shows four representative SEM photographs of diamonds. We can obviously find out that the integrality of diamonds has been destroyed due to the introduction of sulfur. The synthetic diamonds from no-sulfur system are intact. But when a small quantity of sulfur has been introduced to the system, some pits begin to appear on the surface of diamonds. And with the increase of sulfur content in the system, the appearance of pits behaves more and more seriously on the crystal surface. In addition, though the synthesis time is identical, some differences still exist in the sizes of the synthetic diamonds from different sulfur-contained system. In detail, the size of diamonds tends to be reduced with the increase of the sulfur content in the system. That is to say, the introduction of additive sulfur reduced the growth rate of diamonds. The solvent theory indicates that metal catalyst keeps a melted state under the conditions of diamond forming. Once the conditions are suitable, diamond or graphite will be dissolved in the melted metal. Then the solution, of which
Fig. 8. Sketch map of the transport of carbon during the synthesis.
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the solute and solvent are carbon and metal respectively, comes into being. In the course of the growth of industry diamond, diamond surface is wrapped up with a thin metal layer. Outside the two sides of the metal layer, there are graphite and diamond, respectively (Fig. 8). The temperature gradient, which acts between the two sides of the metal layer, can be neglected. And the solubility of graphite in the liquid metal is greater than that of diamond under the same conditions. On the one side of the metal layer, graphite is dissolved in the metal. This enhances the concentration of carbon element in the solution. While on the other side of the metal layer, carbon is deposited on diamond when the concentration of the carbon element in the solution is higher than that in diamond-metal saturated solution. This prevents the concentration of carbon element in the solution from further rising. Thus the concentration gradient of carbon element appears between the two sides of the metal layer. And this offers the transport of carbon element with driving force. It is a continuous course that graphite is constantly dissolved in metal and the carbon elements in the solution are deposited on diamond. Based on some knowledge on crystal growth, we know that certain crystal surfaces may have a function of choosing and adsorbing impurities. And impurities influence the interface processes of crystal growth [8]. In this article, solid MnS was formed during the synthesis. However, not all the additive sulfur has participated in the formation of MnS. Some sulfur dissolved in the melted alloy, or adsorbed to diamond surface. This could have made the interface processes of diamond growth slow down. So the growth rate of diamond was reduced. On the other hand, the introduction of sulfur makes the stoichiometry of alloy composition change. This equals to change the character of catalyst, which directly influences the growth rate of diamond. That is to say, this is another approach by which the introduction of sulfur influenced the growth rate of diamond. The solid MnS might have adsorbed to diamond surface during the growth of diamond (Fig. 7). With the further growth of diamond, some MnS particles were entirely embedded in diamond and formed the inclusions, while another part of MnS
particles was not completely surrounded by atomic carbon. After acid treatment, these MnS particles were removed from diamond surface and some pits appeared on crystal surface. When the sulfur content is increased, the formation amount of MnS should be increased accordingly, and so do the sulfur amount dissolved in the melted alloy. This will further reduce the growth rate of diamond, and then cause the size of crystal to be reduced. On the other hand, with the increase of sulfur content in synthetic system, the appearance of pits on diamond surface would be more serious. 4. Conclusions When additive sulfur is introduced to the synthesis of industrial diamonds using Ni70Mn25Co5 alloy as catalyst, with the increase of sulfur content in the system, the color of diamond crystals turns from yellow to black via green and dark green gradually, and the size and integrality of crystal are reduced by degrees. On the other hand, the temperature interval at certain pressure for diamond formation has a tendency to narrow down, while the minimum pressure at certain temperature tends to rise. We infer that the formation of MnS, which should have been in a solid state during the synthesis, has largely influenced the synthesis and growth of diamonds. References [1] E. Gheeraert, N. Casanova, A. Tajani, A. Deneuville, E. Bustarret, J.A. Garrido, C.E. Nebel, M. Stutzmann, Diamond Relat. Mater. 11 (2002) 289. [2] R. Kalish, Carbon 37 (1999) 781. [3] Isao Sakaguchi, Mikka N. -Gamo, Yuko Kikuchi, Eiji Yasu, Hajime Haneda, Toshimitsu Suzuki, Toshihiro Ando, Phys. Rev., B 60 (1999) R2139. [4] R. Kalish, A. Reznik, C. Uzan-Saguy, C. Cytermann, Appl. Phys. Lett. 76 (2000) 757. [5] K. Nakazawa, M. Tachiki, H. Kawarada, A. Kawamura, K. Horiuchi, T. Ishikura, Appl. Phys. Lett. 82 (13) (March 31, 2003) 2074. [6] R. Kalish, Diamond Relat. Mater. 10 (2001) 1749. [7] K. Sato, T. Katsura, J. Cryst. Growth 223 (2001) 189. [8] D. Elwell, H.J. Scheel, Crystal Growth from High-Temperature Solutions, Academic Press, London, 1975.
Influences of additive sulfur on the synthesis of industrial diamonds, using Ni70Mn25Co5 alloy powder as catalyst Lin Zhou a, Xiaopeng Jia a,b,*, Hongan Ma a, Lixue Chen a, Weili Guo a, Yantao Li a a
National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China b Henan Polytechnic University, Jiaozuo 454000, China
Received 19 April 2005; received in revised form 25 September 2005; accepted 9 October 2005 Available online 18 January 2006
Abstract Sulfur was introduced to the synthesis of industrial diamonds using Ni70Mn25Co5 alloy powder as catalyst. Optical microscope, XRD, XPS, and SEM were utilized for observation and detection. The influences of additive sulfur on the synthesis have been investigated from several aspects such as the diamond-forming conditions, the composition of synthesized sample and inclusions in diamond, and the morphology character of diamonds etc. The results indicate that the formation of MnS, which should have been in a solid state during the synthesis, has an important influence on the synthesis of industrial diamonds. D 2005 Elsevier B.V. All rights reserved. Keywords: Additive sulfur; Ni70Mn25Co5 alloy powder; Diamond; MnS
1. Introduction In the field of industrial diamonds, the study on catalyst is an extremely important respect. Different catalysts have different characters, which directly influence the diamondforming conditions, the morphology character of diamonds, and the mechanical performance of diamonds, and so on. Adding some trace elements to catalyst can change the characters of catalyst remarkably. Thus the categories of catalyst will be increased and the applied range of industrial diamonds will be widened. It is well known that diamond is a wide-bandgap semiconductor with unsurpassed physical and chemical properties. When doped, semiconducting diamond can lead to the realization of electronic and optoelectronic devices with exceptional properties. One of the major issues in diamond electronics is the search for a useful n-type dopant. Several impurities are candidates to act as donors in diamond when occupying the proper lattice site. In analogy to donors in * Corresponding author. National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China. Fax: +86 431 5168858. E-mail addresses: [email protected] (L. Zhou), [email protected] (X. Jia). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.10.013
silicon, group V (N, P) and VI (S) elements are expected to create shallow donor levels in diamond. But in fact nitrogen distorts the lattice of diamond leading to a deep level of 1.7 eV [1]. And phosphor has been shown to have a rather deep donor level in diamond (E a ¨0.5 eV) and the electron mobility values achieve so far for CVD-grown P-containing diamond layers are still rather low. The efforts therefore focus on sulfur. Computer simulations for diamond containing different dopant impurities show that substitutional S in diamond should have a rather shallow donor level (0.2 eV) [2]. Utilizing CVD-grown method, researchers have launched a large number of work in this respect [3 – 5]. However, this prediction has not been experimentally confirmed yet [6]. In the periodic table of elements, sulfur is a light element more close to carbon element. So it is relatively easy for sulfur atom to enter into the lattice of diamond. Under the conditions of 8 –8.5 GPa and 1600– 1800 -C, sulfur can act as catalyst for the synthesis of diamonds alone [7]. In this paper, we added a small amount of simple substance sulfur powder to Ni70Mn25Co5 catalyst. The influences of additive sulfur on the synthesis of diamonds had been studied. Optical microscope was applied to identify the formation of diamond. The synthesized sample was characterized by X-ray diffraction. XPS was used to determine the composition of the
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after the release of the applied pressure. The identification of diamond formation was conducted by optical microscope. The experimental conditions and results are presented in Fig. 2. It is easy to find out that with the increase of sulfur content in Ni70Mn25Co5-graphite-sulfur system, the diamond-forming temperature interval at certain pressure has a tendency to narrow down, while the minimum pressure at certain temperature for diamond formation tends to rise. This indicates that the diamond-forming V-shape region should move in the case of some change happens to the sulfur content in the system. Fig. 1. Sample assembly sketch of high pressure experiment.
inclusions in diamonds. Utilizing scanning electron microscope (SEM), we investigated the morphology of diamonds. 2. Experimental details The experiments were carried out in China-made SPD 6 1200 cubic high-pressure apparatus under the conditions of 5.5 – 5.9 GPa and 1400 – 1600 -C. The experimental raw materials are Ni70Mn25Co5 alloy powder and natural crystalline flake graphite powder (both of them are 200 mesh in granularity and higher than 99.9% in purity). The additive is a sedimented sulfur, a kind of much fine powder. Compared to other sulfur, it presents a larger active surface for the same amount of sulfur. It is above 99% in purity. Ni70Mn25Co5 alloy powder, graphite powder and sulfur powder were uniformly mixed according to mass ratios. The mass ratio of catalyst to graphite was 1 : 1 in the mixture. And there were three kinds of mass percentage compositions of sulfur powder, which were 0.5%, 1% and 2.5%, respectively. Synthesis sample was prepared by pressing the mixture of certain mass into the shape of cylinder. To contrast with that, we also prepared some samples without additive sulfur. Fig. 1 shows the high pressure sample assembly. After the synthesis under different pressure and temperature conditions, the synthesized samples were examined with optical microscope and X-ray diffractometer (XRD). When it is for the purpose of obtaining single diamond crystals, the synthesized samples were treated in the boiling mixture of H2SO4 and HNO3. And then scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were applied for observation and detection.
3.2. X-ray diffraction of the synthesized sample After synthesis, the synthesized sample whose initial sulfur content is 2.5 wt.% is characterized by X-ray diffraction (see Fig. 3). The results of XRD indicate that there is a new compound MnS in the sample. The melting point of MnS is 1615 -C at normal state, while the temperature of diamond formation in this paper is about 1400 –1600 -C. So we infer that MnS should have been in solid state under the conditions of diamond forming due to its higher melting point. 3.3. XPS measurement of the inclusions in synthetic diamonds Based on the XRD results that MnS exists in the synthesized sample, we used X-ray photoelectron spectrosco-
3. Results and discussions 3.1. Influences of additive sulfur on synthesis conditions To investigate the influences of additive sulfur on diamondforming conditions, we carried out the experiments under the conditions of 5.5 –5.9 GPa and 1400 –1600 -C. The synthesis systems were first compressed to certain pressure and then heated to certain temperature. Ten minutes later, quenching was achieved by shutting off the electric power supply under highpressure conditions. And the synthesis samples were recovered
Fig. 2. Experimental results under different pressure and temperature conditions. Solid square means diamond crystallization and open square means absence of diamond.
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Fig. 5. Scheme of the movement of diamond-forming V-shapes region with the introduction of additive sulfur. Fig. 3. X-ray diffraction results of the synthesized sample whose sulfur content is 2.5 wt.%.
py (XPS) measurement to determine the composition of inclusions in diamonds for the purpose of further characterizing the state of MnS during the synthesis. Diamonds, which
Fig. 4. XPS narrow scan spectra of Mn2p and S2p.
had been synthesized in 2.5 wt.% sulfur-contained system, were selected for XPS measurement. After a series of thorough acid treatment and purification, diamonds were smashed into powder. And then XPS measurement was carried out. XPS narrow scan spectra of Mn2p and S2p regions are shown in Fig. 4. The Mn2p spectrum indicates Mn2+ and the S2p spectrum indicates S2 and S – O bond. As for S –O bond, we think that there are two approaches, the oxidation of raw materials and the oxygen molecules sealed in the sample assembly, for the introduction of oxygen. XPS measurement shows that there are no other positive ions in the inclusions except for Mn2+. So we think that it is MnS that have entered into diamond. On the other hand, though inclusions can not be distributed evenly in diamond powder so that we can not carry on accurate quantitative analysis to it, we have also roughly calculated the number percentage of MnS to the atomic carbon of diamond. It is about 0.22%. Such higher content of MnS in diamond confirms our inference that MnS should be in solid state, rather than dissolve in the melted alloy during the synthesis.
Fig. 6. Mn – Ni phase diagram.
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enhancement of the melting point of the catalyst, which would probably further make the catalyst-diamond eutectic line move to the higher-temperature region. Accordingly, in Fig. 5, the eutectic line is moved from AB to AVB.V As a result of that, the V-shape region is moved along the D –G equilibrium line towards the higher-temperature region in the P – T phase diagram of carbon. 3.5. Color and morphology of diamond crystals
Fig. 7. Representative SEM photographs of diamond crystals. The sulfur content of (a), (b), (c) and (d) are 0, 0.5, 1 and 2.5 wt.%, respectively.
3.4. Explanation for the movement of V-shape region In the P –T phase diagram of carbon, the district for diamond formation is a V-shape region bounded by the D – G equilibrium line and the metal-diamond eutectic line (Fig. 5). For different catalysts, the metal-diamond eutectic lines are different, so are the corresponding V-shape regions. In this paper, because solid MnS was formed during the synthesis, the content of manganese element in the Ni70Mn25Co5 alloy was reduced. Thus the stoichiometry of alloy composition was changed. The character of the catalyst was different from the one before the synthesis, and so was the metal-diamond eutectic line. Because the essential compositions of Ni70Mn25Co5 alloy catalyst are nickel and manganese, we can consult Mn –Ni phase diagram (Fig. 6) to carry on qualitative analysis to the relation between the melting point of catalyst and the stoichiometry of its components. From Mn –Ni phase diagram, we can find out that when the mass percentage composition of nickel in the alloy is more than 40%, the melting point of the alloy tends to rise with the reduction of manganese content. As we have already mentioned above, the content of manganese in the alloy was reduced due to the formation of solid MnS. Based on the analysis to Mn – Ni phase diagram, this should have caused the
Based on the above experimental results that the introduction of additive sulfur makes the diamond-forming V-shape region move, we accordingly adjusted the temperature and pressure conditions to the optimal synthesis conditions for each kind of samples, and then carried out the synthesis. The synthesis time of 15 min was applied. We obtained single diamond crystals by treating the synthesized samples with a series of acid treatment and purification. Utilizing optical microscope, we observed that there is a little inclusion in the diamonds from sulfur-contained system. And the more sulfur there is in the synthesis system, the more inclusions there are in the synthetic diamonds. On the other hand, with the increase of sulfur content in the synthesis system, the color of diamonds turns from yellow to green, dark green and black gradually. Scanning electronic microscope (SEM) was employed to observe the surface morphology of diamonds. Fig. 7 shows four representative SEM photographs of diamonds. We can obviously find out that the integrality of diamonds has been destroyed due to the introduction of sulfur. The synthetic diamonds from no-sulfur system are intact. But when a small quantity of sulfur has been introduced to the system, some pits begin to appear on the surface of diamonds. And with the increase of sulfur content in the system, the appearance of pits behaves more and more seriously on the crystal surface. In addition, though the synthesis time is identical, some differences still exist in the sizes of the synthetic diamonds from different sulfur-contained system. In detail, the size of diamonds tends to be reduced with the increase of the sulfur content in the system. That is to say, the introduction of additive sulfur reduced the growth rate of diamonds. The solvent theory indicates that metal catalyst keeps a melted state under the conditions of diamond forming. Once the conditions are suitable, diamond or graphite will be dissolved in the melted metal. Then the solution, of which
Fig. 8. Sketch map of the transport of carbon during the synthesis.
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the solute and solvent are carbon and metal respectively, comes into being. In the course of the growth of industry diamond, diamond surface is wrapped up with a thin metal layer. Outside the two sides of the metal layer, there are graphite and diamond, respectively (Fig. 8). The temperature gradient, which acts between the two sides of the metal layer, can be neglected. And the solubility of graphite in the liquid metal is greater than that of diamond under the same conditions. On the one side of the metal layer, graphite is dissolved in the metal. This enhances the concentration of carbon element in the solution. While on the other side of the metal layer, carbon is deposited on diamond when the concentration of the carbon element in the solution is higher than that in diamond-metal saturated solution. This prevents the concentration of carbon element in the solution from further rising. Thus the concentration gradient of carbon element appears between the two sides of the metal layer. And this offers the transport of carbon element with driving force. It is a continuous course that graphite is constantly dissolved in metal and the carbon elements in the solution are deposited on diamond. Based on some knowledge on crystal growth, we know that certain crystal surfaces may have a function of choosing and adsorbing impurities. And impurities influence the interface processes of crystal growth [8]. In this article, solid MnS was formed during the synthesis. However, not all the additive sulfur has participated in the formation of MnS. Some sulfur dissolved in the melted alloy, or adsorbed to diamond surface. This could have made the interface processes of diamond growth slow down. So the growth rate of diamond was reduced. On the other hand, the introduction of sulfur makes the stoichiometry of alloy composition change. This equals to change the character of catalyst, which directly influences the growth rate of diamond. That is to say, this is another approach by which the introduction of sulfur influenced the growth rate of diamond. The solid MnS might have adsorbed to diamond surface during the growth of diamond (Fig. 7). With the further growth of diamond, some MnS particles were entirely embedded in diamond and formed the inclusions, while another part of MnS
particles was not completely surrounded by atomic carbon. After acid treatment, these MnS particles were removed from diamond surface and some pits appeared on crystal surface. When the sulfur content is increased, the formation amount of MnS should be increased accordingly, and so do the sulfur amount dissolved in the melted alloy. This will further reduce the growth rate of diamond, and then cause the size of crystal to be reduced. On the other hand, with the increase of sulfur content in synthetic system, the appearance of pits on diamond surface would be more serious. 4. Conclusions When additive sulfur is introduced to the synthesis of industrial diamonds using Ni70Mn25Co5 alloy as catalyst, with the increase of sulfur content in the system, the color of diamond crystals turns from yellow to black via green and dark green gradually, and the size and integrality of crystal are reduced by degrees. On the other hand, the temperature interval at certain pressure for diamond formation has a tendency to narrow down, while the minimum pressure at certain temperature tends to rise. We infer that the formation of MnS, which should have been in a solid state during the synthesis, has largely influenced the synthesis and growth of diamonds. References [1] E. Gheeraert, N. Casanova, A. Tajani, A. Deneuville, E. Bustarret, J.A. Garrido, C.E. Nebel, M. Stutzmann, Diamond Relat. Mater. 11 (2002) 289. [2] R. Kalish, Carbon 37 (1999) 781. [3] Isao Sakaguchi, Mikka N. -Gamo, Yuko Kikuchi, Eiji Yasu, Hajime Haneda, Toshimitsu Suzuki, Toshihiro Ando, Phys. Rev., B 60 (1999) R2139. [4] R. Kalish, A. Reznik, C. Uzan-Saguy, C. Cytermann, Appl. Phys. Lett. 76 (2000) 757. [5] K. Nakazawa, M. Tachiki, H. Kawarada, A. Kawamura, K. Horiuchi, T. Ishikura, Appl. Phys. Lett. 82 (13) (March 31, 2003) 2074. [6] R. Kalish, Diamond Relat. Mater. 10 (2001) 1749. [7] K. Sato, T. Katsura, J. Cryst. Growth 223 (2001) 189. [8] D. Elwell, H.J. Scheel, Crystal Growth from High-Temperature Solutions, Academic Press, London, 1975.