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The effect of an iron powder catalyst clad with a Fe2O3 layer on the nucleation of diamonds
Diamond & Related Materials 15 (2006) 1369 – 1373 www.elsevier.com/locate/diamond
The effect of an iron powder catalyst clad with a Fe2O3 layer on the nucleation of diamonds J.M. Qin a,b, H.A. Ma a, L.X. Chen a, Y. Tian a, C.Y. Zang a, G.Z. Ren a, Q.F. Guan a,d, X. Jia a,c,* a
d
National Lab of Superhard Materials, Jilin University, Changchun 130012, China b Inner Monglia Unversity for Nationalities, Tongliao 028043, China c Henan Polytechnic University, Jiaozuo 454000, China Department of Materials Science and Engineering, Jilin University, Changchun 130012, China
Received 29 March 2005; received in revised form 1 September 2005; accepted 10 October 2005 Available online 28 December 2005
Abstract In this paper, we have researched the effect of a Fe2O3 clad layer covering iron catalyst particles on the nucleation of a diamond under high pressure and high temperature (HPHT). The Fe2O3 layer is deoxidized by graphite to form Fe3O4 and FeO at 5.7 GPa and 1600 -C. At the same time the iron melts and is exuded through the Fe2O3 layer, and then contacts with graphite, to cause the nucleation and growth of a diamond. Compared with a pure iron catalyst, the Fe2O3 clad layer distinctly restrains the nucleation of the diamond. Furthermore, the thicker the clad layer becomes, the more obvious this effect is. The Mo¨ssbauer spectrum, X-ray diffraction (XRD) and a scanning electronic microscope (SEM) were used to characterize and analyze the experimental samples. D 2005 Elsevier B.V. All rights reserved. Keywords: HPHT; Fe2O3 clad layer; Diamond; Nucleation
1. Introduction Since the diamond was first synthesized from a metal catalyst and graphite using the hydrostatic HPHT method in 1954 by G.E. [1], the industrialization of diamond production has a history of half a century. The categories of catalyst have extended from transition metals to alloys of all kinds of metals and even to non-metal catalysts [2 – 6]. Sumiya et al. have researched a series of metal oxide catalysts to synthesize diamonds [7]. Although they confirmed the formation of the diamond at 7.7 GPa and 1750 -C by a-Fe2O3, Fe3O4 etc., the formation mechanism using these metal oxide catalysts have not been clarified. Furthermore, metal powder catalysts are prone to oxidization in the process of synthesizing an industrial diamond, which will decrease the output and degrade the quality of the resulting diamond. In order to study the effect of the Fe2O3 clad layer covering iron catalyst particles on the nucleation of diamonds and the synthesis mechanism of the * Corresponding author. National Lab of Superhard Materials, Jilin University, Changchun 130012, China. Fax: +86 431 5168858. E-mail addresses: [email protected], [email protected] (X. Jia). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.10.033
diamond, we synthesized diamonds using graphite and an iron powder catalyst covered with a Fe2O3 clad layer. It has been found that the Fe2O3 clad layer is deoxidized partially by graphite to form Fe3O4 and FeO at 5.7 GPa and 1600 -C, and the Fe2O3 clad layer restrains the nucleation of the diamond, which is more noticeable when the clad layer is thicker. The Mo¨ssbauer spectrum, X-ray diffraction and SEM were used to characterize the experimental samples and investigate the diamond formation mechanism. 2. Experimental details The experiments were performed using a China-type SPD 6 1200 cubic high-pressure apparatus with an anvil top face of 27.5 mm. The iron powder catalyst covered with an Fe2O3 clad layer was prepared by baking a-Fe (purity 99.9%, mesh 400) in air at 400 -C for between naught and 4 h. After mixing the prepared iron catalyst having the Fe2O3 clad layer with crystalline flake graphite powder (mesh 400) in a certain proportion, the powder mixture was pre-pressed into the desired shape, and placed in the high pressures ample assembly (Fig. 1). For comparison, a sample of pure a-Fe2O3 (purity
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J.M. Qin et al. / Diamond & Related Materials 15 (2006) 1369 – 1373
Fig. 1. Schematic illustration of the sample assembly for high pressure experiments.
99.0%, mesh 200) without graphite and a second sample of mixture of a-Fe2O3 containing graphite in a certain proportion were also prepared for HPHT experiments. The synthesis pressure was 5.7 GPa, and the temperature was 1600 -C. The synthesis time of all the experiments was 15 min. An Optical microscope, SEM, Mo¨ssbauer spectrum and X-ray diffraction were used to characterize and analyze the samples recovered from the HPHT experiments. 3. Results and discussion 3.1. The preparation and analysis of iron powder with Fe2O3 clad layer The powder of a-Fe was baked in air for 1, 1.6, 2.4 and 4 h, and iron powders with Fe2O3 clad layer of various thicknesses were prepared. By optical microscope, the oxidized samples changed from silver gray to reddish-brown, which showed that the oxide layer was formed on the surface of the iron particles. Fig. 2 shows SEM photographs of catalyst iron powder (a) before and (b) after baking for 4 h. All the particles on the unbaked powder of a-Fe are certainly smaller than the 40 Am mesh size shown in (a). On the baked powder, however, some particles grew up to 50 Am as shown in (b). The growth of the particle size must be caused by oxidation on their surface. Although the detailed process of oxidation on the particle surface is not clear, the rate of oxidation can be evaluated by Mo¨ssbauer spectrum. The Mo¨ssbauer spectrum of pure a-Fe powder and Fe powder covered by a Fe2O3 clad layer prepared at different baking times are shown in Fig. 3. Arrows indicate
large peaks of a-Fe and small peaks of Fe2O3 for each spectrum. The relative content of both phases can be evaluated based on the relative area of these peaks [8]. The evaluated content for each of the baking times is shown in the inset table of Fig. 3. The content of Fe2O3 on the surface of iron increases as the baking time is prolonged, which show that the Fe2O3 layer on the surface of iron thickens as the baking time increases. Although the detailed mechanism of oxidation is not clear at present, we can show the progress of oxidation from the Mo¨ssbauer measurements. Fig. 4 illustrates a plotted relation of the content of Fe2O3 to the baking time. The content of Fe2O3 increases nearly linearly, until it shows a sign of decrease in the rate of oxidation at around 4 h. 3.2. The effect of the Fe2O3 clad layer on the nucleation of diamonds Fig. 5 shows SEM photographs of fracture sections of samples which were treated at the same HPHT condition. We can find diamond crystals and their imprints in each picture, easily recognized by their crystal morphology having 111 and 100 growth facets. The quantity of diamonds in the field having the same area is in the order of (a) > (b) > (c) > (d). This means that the nucleation density decreases as the baking time (i.e. thickness of the Fe2O3 layer) increases. This fact indicates that the Fe2O3 clad layer on the surface of the Fe catalyst obviously restrains the nucleation of diamonds. 3.3. Interaction between Fe2O3 and graphite Sumiya et al. synthesized diamonds using a-Fe2O3 as a catalyst at 7.7 GPa and 1750 -C [7]. However, the cause of the formation has never been positively identified. In order to investigate the role of Fe2O3 on the formation of a diamond, we studied the behaviors of pure Fe2O3, and also a mixture of Fe2O3 and graphite at 5.7 GPa, 1600 -C. In Fig. 6, X-ray diffraction patterns of pure Fe2O3 (Fig. 6b) and a mixture of Fe2O3 and graphite (Fig. 6c) are shown. Both were treated as described above. As can be seen from Fig. 6, the phase structure of pure Fe2O3 treated at 5.7 GPa and 1600 -C does not change from that of the original Fe2O3 (Fig. 6a). However, using a mixture of Fe2O3 and graphite treated at the same condition of 5.7 GPa and 1600 -C, Fe3O4 and FeO are
Fig. 2. SEM photographs of powder catalyst. (a): original pure iron powder; (b): powder after baking for 4 h.
J.M. Qin et al. / Diamond & Related Materials 15 (2006) 1369 – 1373
1371
Fig. 3. Mo¨ssbauer spectra of iron powder baked for various periods (1, 2, 3 and 4 correspond to the baking times of 1, 1.6, 2.4, and 4 h, respectively).
produced, with neither diamonds nor metallic iron being formed. The above results prove that pure Fe2O3 does not decompose under the condition of 5.7 GPa, 1600 -C. Fe2O3 mixed with graphite is deoxidized to form Fe3O4 and FeO, but cannot be deoxidized to pure iron. This result is not consistent with that of Sumiya et al. [7]. The difference must be caused by the difference in treating conditions. However, we can confirm that Fe2O3 cannot be deoxidized by graphite to free iron at least at the condition of 5.7 GPa and 1600 -C, which is typical for the industrial diamond synthesis. 3.4. The mechanism of diamond formation using graphite and iron powder wrapped in Fe2O3 An iron oxide catalyst cannot contribute to diamond formation using the experimental conditions described in this paper. However, diamonds can be formed using a baked iron
Fig. 4. The dependence of the content of Fe2O3 on the baking time.
catalyst, although the quantity is degraded. We must therefore explain a possible mechanism of diamond formation using a baked catalyst. Some additional behaviors of iron and Fe2O3 should first be noted. In Fig. 7 are shown SEM photographs of (a) Fe powder and (c) Fe2O3 powder, both treated at 5.7 GPa and 1600 -C, together with (b) untreated Fe2O3. Comparison of Figs. 6a and 2a shows that the initial grains of pure iron congregate together during the HPHT treatment, suggesting that iron is easily sintered and easily deformed under high stress. On the other hand, Fe2O3 grains grow into large euhedral grains without being fused together. When a grain of iron wrapped in Fe2O3 is exposed to high pressure and high temperature, the Fe2O3 on the surface of iron ought to promote Fe2O3 grain growth, while the inner iron partially deforms. This cracks the Fe2O3, creating small holes in the Fe2O3 layer, allowing contact between graphite and the inner iron. Even if the contact area is very small, a small quantity of Fe– C liquid can be formed there, especially when the treating temperature exceeds the eutectic melting point of Fe and C. This is a reasonable assumption, as unbaked Fe and graphite certainly produce diamonds under present HPHT conditions. Once a liquid phase region is formed, it plays the role of a dissolved carbon diffusion path from graphite to the inner iron, and the liquid region within the particle grows larger. Because the melted phase has a larger volume than that of the solidified state, the melted iron (containing carbon) partially exudes out through the Fe2O3 layer, inducing nucleation and growth of a diamond. If a thicker Fe2O3 layer covers the iron particle, cracking in the Fe2O3 layer mentioned above decreases, thus decreasing the quantity of melted iron, which can exude out of the Fe2O3 layer, further reducing the possibility of nucleation of a diamond. The exuded Fe – C liquid may gather to form a small pool where nucleation and growth of a diamond can occur. However, the pool does not always have a sufficient volume
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Fig. 5. SEM photographs of formed diamond particles (a, b, c and d correspond to the samples synthesized with Fe catalyst baked for 0, 1.6, 2.4 and 4 h, respectively).
Fig. 7. The SEM photographs of a-Fe and Fe2O3 treated at HPHT (a: a-Fe treated at 5.7 GPa and 1600 -C; b: Fe2O3 without HPHT treatment; c: Fe2O3 treated at 5.7 GPa and 1600 -C).
to grow a completely habited crystal. In industrial diamond production, it is common for catalyst deficiencies (even in a plate shape) to result incompletely habited particles. Oxidized powder catalyst has a tendency of producing degraded quality diamonds as described in the Introduction. The above process is consistent with current knowledge of crystal growth, as well as mechanisms of diamond formation using oxidized iron powder catalysts. 4. Conclusion
Fig. 6. The X-ray diffraction patterns of Fe2O3 and mixture of Fe2O3 and graphite (a: untreated pure Fe2O3 sample; b: Fe2O3 treated at 5.7 GPa and 1600 -C; c: the mixture of Fe2O3 and graphite treated at the condition of 5.7 GPa and 1600 -C).
We studied the effect of a Fe2O3 clad layer on the nucleation of diamonds. The results show that the Fe2O3 layer restrains nucleation of diamonds compared to iron powder catalysts without the Fe2O3 clad layer. The restraining effect becomes more noticeable as the catalyst particle is covered with a progressively thicker layer of Fe2O3. The oxide, Fe2O3 is not reduced by graphite to free iron under present HPHT conditions typical for the industrial diamond production. The Fe2O3 layer is not a catalyst, but plays the role of limiting contact between iron and graphite.
J.M. Qin et al. / Diamond & Related Materials 15 (2006) 1369 – 1373
The mechanism of forming diamonds using the oxidationtreated powder catalysts is considered as follows. The Fe2O3 layer covering the iron particle is recrystallized into larger grains when heated at high pressure, while iron within the particle is sintered and partially deformed, resulting in local contact of graphite with the inner iron. This causes limited Fe – C melt, which grows with supply of dissolved carbon from graphite through diffusion in the liquid phase. Finally, the Fe – C melt exudes out of the Fe2O3 layer and contacts with graphite to cause nucleation and growth of a diamond. Acknowledgments This work is partly supported by Natural Science Funds of China (50172018) and Henan Polytechnic University Funds for Changjiang Scholar.
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References [1] F.P. Bundy, H.T. Hall, H.M. Strong, R.H. Wentorf Jr., Nature 176 (1955) 51. [2] H.B. Bovenkerk, F.P. Bundy, H.T. Hall, H.T. Strong, R.H. Wentrorf Jr., Nature 184 (1959) 1094. [3] M. Wakatsuki, Jpn. J. Appl. Phys. 5 (1966) 337. [4] M. Akaishi, H. Kanda, S. Yamaoka, J. Cryst. Growth 104 (1990) 578. [5] M. Akaishi, H. Kanda, S. Yamaoka, Jpn. J. Appl. Phys. 29 (1990) L1172. [6] M. Akaishi, H. Kanda, S. Yamaoka, Science 259 (1993) 1592. [7] H. Sumiya, S. Satoh, Int. J. Refract. Met. Hard Mater. 17 (1999) 345. [8] S. Iwama, T. Fukaya, K. Tanaka, K. Ohshita, Y. Sakai, Nano Struct. Mater. 12 (1999) 241.
The effect of an iron powder catalyst clad with a Fe2O3 layer on the nucleation of diamonds J.M. Qin a,b, H.A. Ma a, L.X. Chen a, Y. Tian a, C.Y. Zang a, G.Z. Ren a, Q.F. Guan a,d, X. Jia a,c,* a
d
National Lab of Superhard Materials, Jilin University, Changchun 130012, China b Inner Monglia Unversity for Nationalities, Tongliao 028043, China c Henan Polytechnic University, Jiaozuo 454000, China Department of Materials Science and Engineering, Jilin University, Changchun 130012, China
Received 29 March 2005; received in revised form 1 September 2005; accepted 10 October 2005 Available online 28 December 2005
Abstract In this paper, we have researched the effect of a Fe2O3 clad layer covering iron catalyst particles on the nucleation of a diamond under high pressure and high temperature (HPHT). The Fe2O3 layer is deoxidized by graphite to form Fe3O4 and FeO at 5.7 GPa and 1600 -C. At the same time the iron melts and is exuded through the Fe2O3 layer, and then contacts with graphite, to cause the nucleation and growth of a diamond. Compared with a pure iron catalyst, the Fe2O3 clad layer distinctly restrains the nucleation of the diamond. Furthermore, the thicker the clad layer becomes, the more obvious this effect is. The Mo¨ssbauer spectrum, X-ray diffraction (XRD) and a scanning electronic microscope (SEM) were used to characterize and analyze the experimental samples. D 2005 Elsevier B.V. All rights reserved. Keywords: HPHT; Fe2O3 clad layer; Diamond; Nucleation
1. Introduction Since the diamond was first synthesized from a metal catalyst and graphite using the hydrostatic HPHT method in 1954 by G.E. [1], the industrialization of diamond production has a history of half a century. The categories of catalyst have extended from transition metals to alloys of all kinds of metals and even to non-metal catalysts [2 – 6]. Sumiya et al. have researched a series of metal oxide catalysts to synthesize diamonds [7]. Although they confirmed the formation of the diamond at 7.7 GPa and 1750 -C by a-Fe2O3, Fe3O4 etc., the formation mechanism using these metal oxide catalysts have not been clarified. Furthermore, metal powder catalysts are prone to oxidization in the process of synthesizing an industrial diamond, which will decrease the output and degrade the quality of the resulting diamond. In order to study the effect of the Fe2O3 clad layer covering iron catalyst particles on the nucleation of diamonds and the synthesis mechanism of the * Corresponding author. National Lab of Superhard Materials, Jilin University, Changchun 130012, China. Fax: +86 431 5168858. E-mail addresses: [email protected], [email protected] (X. Jia). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.10.033
diamond, we synthesized diamonds using graphite and an iron powder catalyst covered with a Fe2O3 clad layer. It has been found that the Fe2O3 clad layer is deoxidized partially by graphite to form Fe3O4 and FeO at 5.7 GPa and 1600 -C, and the Fe2O3 clad layer restrains the nucleation of the diamond, which is more noticeable when the clad layer is thicker. The Mo¨ssbauer spectrum, X-ray diffraction and SEM were used to characterize the experimental samples and investigate the diamond formation mechanism. 2. Experimental details The experiments were performed using a China-type SPD 6 1200 cubic high-pressure apparatus with an anvil top face of 27.5 mm. The iron powder catalyst covered with an Fe2O3 clad layer was prepared by baking a-Fe (purity 99.9%, mesh 400) in air at 400 -C for between naught and 4 h. After mixing the prepared iron catalyst having the Fe2O3 clad layer with crystalline flake graphite powder (mesh 400) in a certain proportion, the powder mixture was pre-pressed into the desired shape, and placed in the high pressures ample assembly (Fig. 1). For comparison, a sample of pure a-Fe2O3 (purity
1370
J.M. Qin et al. / Diamond & Related Materials 15 (2006) 1369 – 1373
Fig. 1. Schematic illustration of the sample assembly for high pressure experiments.
99.0%, mesh 200) without graphite and a second sample of mixture of a-Fe2O3 containing graphite in a certain proportion were also prepared for HPHT experiments. The synthesis pressure was 5.7 GPa, and the temperature was 1600 -C. The synthesis time of all the experiments was 15 min. An Optical microscope, SEM, Mo¨ssbauer spectrum and X-ray diffraction were used to characterize and analyze the samples recovered from the HPHT experiments. 3. Results and discussion 3.1. The preparation and analysis of iron powder with Fe2O3 clad layer The powder of a-Fe was baked in air for 1, 1.6, 2.4 and 4 h, and iron powders with Fe2O3 clad layer of various thicknesses were prepared. By optical microscope, the oxidized samples changed from silver gray to reddish-brown, which showed that the oxide layer was formed on the surface of the iron particles. Fig. 2 shows SEM photographs of catalyst iron powder (a) before and (b) after baking for 4 h. All the particles on the unbaked powder of a-Fe are certainly smaller than the 40 Am mesh size shown in (a). On the baked powder, however, some particles grew up to 50 Am as shown in (b). The growth of the particle size must be caused by oxidation on their surface. Although the detailed process of oxidation on the particle surface is not clear, the rate of oxidation can be evaluated by Mo¨ssbauer spectrum. The Mo¨ssbauer spectrum of pure a-Fe powder and Fe powder covered by a Fe2O3 clad layer prepared at different baking times are shown in Fig. 3. Arrows indicate
large peaks of a-Fe and small peaks of Fe2O3 for each spectrum. The relative content of both phases can be evaluated based on the relative area of these peaks [8]. The evaluated content for each of the baking times is shown in the inset table of Fig. 3. The content of Fe2O3 on the surface of iron increases as the baking time is prolonged, which show that the Fe2O3 layer on the surface of iron thickens as the baking time increases. Although the detailed mechanism of oxidation is not clear at present, we can show the progress of oxidation from the Mo¨ssbauer measurements. Fig. 4 illustrates a plotted relation of the content of Fe2O3 to the baking time. The content of Fe2O3 increases nearly linearly, until it shows a sign of decrease in the rate of oxidation at around 4 h. 3.2. The effect of the Fe2O3 clad layer on the nucleation of diamonds Fig. 5 shows SEM photographs of fracture sections of samples which were treated at the same HPHT condition. We can find diamond crystals and their imprints in each picture, easily recognized by their crystal morphology having 111 and 100 growth facets. The quantity of diamonds in the field having the same area is in the order of (a) > (b) > (c) > (d). This means that the nucleation density decreases as the baking time (i.e. thickness of the Fe2O3 layer) increases. This fact indicates that the Fe2O3 clad layer on the surface of the Fe catalyst obviously restrains the nucleation of diamonds. 3.3. Interaction between Fe2O3 and graphite Sumiya et al. synthesized diamonds using a-Fe2O3 as a catalyst at 7.7 GPa and 1750 -C [7]. However, the cause of the formation has never been positively identified. In order to investigate the role of Fe2O3 on the formation of a diamond, we studied the behaviors of pure Fe2O3, and also a mixture of Fe2O3 and graphite at 5.7 GPa, 1600 -C. In Fig. 6, X-ray diffraction patterns of pure Fe2O3 (Fig. 6b) and a mixture of Fe2O3 and graphite (Fig. 6c) are shown. Both were treated as described above. As can be seen from Fig. 6, the phase structure of pure Fe2O3 treated at 5.7 GPa and 1600 -C does not change from that of the original Fe2O3 (Fig. 6a). However, using a mixture of Fe2O3 and graphite treated at the same condition of 5.7 GPa and 1600 -C, Fe3O4 and FeO are
Fig. 2. SEM photographs of powder catalyst. (a): original pure iron powder; (b): powder after baking for 4 h.
J.M. Qin et al. / Diamond & Related Materials 15 (2006) 1369 – 1373
1371
Fig. 3. Mo¨ssbauer spectra of iron powder baked for various periods (1, 2, 3 and 4 correspond to the baking times of 1, 1.6, 2.4, and 4 h, respectively).
produced, with neither diamonds nor metallic iron being formed. The above results prove that pure Fe2O3 does not decompose under the condition of 5.7 GPa, 1600 -C. Fe2O3 mixed with graphite is deoxidized to form Fe3O4 and FeO, but cannot be deoxidized to pure iron. This result is not consistent with that of Sumiya et al. [7]. The difference must be caused by the difference in treating conditions. However, we can confirm that Fe2O3 cannot be deoxidized by graphite to free iron at least at the condition of 5.7 GPa and 1600 -C, which is typical for the industrial diamond synthesis. 3.4. The mechanism of diamond formation using graphite and iron powder wrapped in Fe2O3 An iron oxide catalyst cannot contribute to diamond formation using the experimental conditions described in this paper. However, diamonds can be formed using a baked iron
Fig. 4. The dependence of the content of Fe2O3 on the baking time.
catalyst, although the quantity is degraded. We must therefore explain a possible mechanism of diamond formation using a baked catalyst. Some additional behaviors of iron and Fe2O3 should first be noted. In Fig. 7 are shown SEM photographs of (a) Fe powder and (c) Fe2O3 powder, both treated at 5.7 GPa and 1600 -C, together with (b) untreated Fe2O3. Comparison of Figs. 6a and 2a shows that the initial grains of pure iron congregate together during the HPHT treatment, suggesting that iron is easily sintered and easily deformed under high stress. On the other hand, Fe2O3 grains grow into large euhedral grains without being fused together. When a grain of iron wrapped in Fe2O3 is exposed to high pressure and high temperature, the Fe2O3 on the surface of iron ought to promote Fe2O3 grain growth, while the inner iron partially deforms. This cracks the Fe2O3, creating small holes in the Fe2O3 layer, allowing contact between graphite and the inner iron. Even if the contact area is very small, a small quantity of Fe– C liquid can be formed there, especially when the treating temperature exceeds the eutectic melting point of Fe and C. This is a reasonable assumption, as unbaked Fe and graphite certainly produce diamonds under present HPHT conditions. Once a liquid phase region is formed, it plays the role of a dissolved carbon diffusion path from graphite to the inner iron, and the liquid region within the particle grows larger. Because the melted phase has a larger volume than that of the solidified state, the melted iron (containing carbon) partially exudes out through the Fe2O3 layer, inducing nucleation and growth of a diamond. If a thicker Fe2O3 layer covers the iron particle, cracking in the Fe2O3 layer mentioned above decreases, thus decreasing the quantity of melted iron, which can exude out of the Fe2O3 layer, further reducing the possibility of nucleation of a diamond. The exuded Fe – C liquid may gather to form a small pool where nucleation and growth of a diamond can occur. However, the pool does not always have a sufficient volume
1372
J.M. Qin et al. / Diamond & Related Materials 15 (2006) 1369 – 1373
Fig. 5. SEM photographs of formed diamond particles (a, b, c and d correspond to the samples synthesized with Fe catalyst baked for 0, 1.6, 2.4 and 4 h, respectively).
Fig. 7. The SEM photographs of a-Fe and Fe2O3 treated at HPHT (a: a-Fe treated at 5.7 GPa and 1600 -C; b: Fe2O3 without HPHT treatment; c: Fe2O3 treated at 5.7 GPa and 1600 -C).
to grow a completely habited crystal. In industrial diamond production, it is common for catalyst deficiencies (even in a plate shape) to result incompletely habited particles. Oxidized powder catalyst has a tendency of producing degraded quality diamonds as described in the Introduction. The above process is consistent with current knowledge of crystal growth, as well as mechanisms of diamond formation using oxidized iron powder catalysts. 4. Conclusion
Fig. 6. The X-ray diffraction patterns of Fe2O3 and mixture of Fe2O3 and graphite (a: untreated pure Fe2O3 sample; b: Fe2O3 treated at 5.7 GPa and 1600 -C; c: the mixture of Fe2O3 and graphite treated at the condition of 5.7 GPa and 1600 -C).
We studied the effect of a Fe2O3 clad layer on the nucleation of diamonds. The results show that the Fe2O3 layer restrains nucleation of diamonds compared to iron powder catalysts without the Fe2O3 clad layer. The restraining effect becomes more noticeable as the catalyst particle is covered with a progressively thicker layer of Fe2O3. The oxide, Fe2O3 is not reduced by graphite to free iron under present HPHT conditions typical for the industrial diamond production. The Fe2O3 layer is not a catalyst, but plays the role of limiting contact between iron and graphite.
J.M. Qin et al. / Diamond & Related Materials 15 (2006) 1369 – 1373
The mechanism of forming diamonds using the oxidationtreated powder catalysts is considered as follows. The Fe2O3 layer covering the iron particle is recrystallized into larger grains when heated at high pressure, while iron within the particle is sintered and partially deformed, resulting in local contact of graphite with the inner iron. This causes limited Fe – C melt, which grows with supply of dissolved carbon from graphite through diffusion in the liquid phase. Finally, the Fe – C melt exudes out of the Fe2O3 layer and contacts with graphite to cause nucleation and growth of a diamond. Acknowledgments This work is partly supported by Natural Science Funds of China (50172018) and Henan Polytechnic University Funds for Changjiang Scholar.
1373
References [1] F.P. Bundy, H.T. Hall, H.M. Strong, R.H. Wentorf Jr., Nature 176 (1955) 51. [2] H.B. Bovenkerk, F.P. Bundy, H.T. Hall, H.T. Strong, R.H. Wentrorf Jr., Nature 184 (1959) 1094. [3] M. Wakatsuki, Jpn. J. Appl. Phys. 5 (1966) 337. [4] M. Akaishi, H. Kanda, S. Yamaoka, J. Cryst. Growth 104 (1990) 578. [5] M. Akaishi, H. Kanda, S. Yamaoka, Jpn. J. Appl. Phys. 29 (1990) L1172. [6] M. Akaishi, H. Kanda, S. Yamaoka, Science 259 (1993) 1592. [7] H. Sumiya, S. Satoh, Int. J. Refract. Met. Hard Mater. 17 (1999) 345. [8] S. Iwama, T. Fukaya, K. Tanaka, K. Ohshita, Y. Sakai, Nano Struct. Mater. 12 (1999) 241.