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Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature
Diamond & Related Materials 16 (2007) 1665 – 1669 www.elsevier.com/locate/diamond
Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature M.G. Zhang a,b , F. Peng a,⁎, C. Chen a , D.W. He a a
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China b Physics Department of Baoji University of Arts and Sciences, Baoji 721007, China
Received 10 July 2006; received in revised form 9 February 2007; accepted 19 February 2007 Available online 28 February 2007
Abstract Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature (HPHT) was observed with graphite as carbon source. The thickness of the electroplating nickel film which acts as a catalyst/solvent ranges from 54.6 μm to 255.6 μm. The relationship between the Ni film thickness and diamond growth rate is investigated. When the nickel film thickness is from 90 μm to 129 μm, diamond crystals can nearly grow up to three times as large as the original seeds at ∼ 5.8 GPa and ∼ 1460 °C within 14 min. The mechanism of the crystal growth with nickelplated diamond seeds under HPHT is discussed. The results and techniques might be useful for high quality saw-grade diamonds production and large diamond single crystal growth. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond crystal growth; High pressure and high temperature; Nickel-plated seeds
1. Introduction Diamonds can be synthesized with various techniques [1–3], but the static-pressure-catalyst method is still the most popular way for industrial diamond grits production. In this method, the starting graphite and metallic catalyst are treated at HPHTeither in a form of powder mixture or in a layer-by-layer assembly. In both cases, the control of nucleation is critical, which can directly affect the grain size, morphology, quality and yield of diamond [4–6]. The temperature difference method using diamond seeds has also been applied for large diamond single crystal growth as reported in several literatures [7–9]. When the graphite and metallic catalyst are used as starting materials for diamond grits synthesis, there always exists a molten metallic film with a thickness from several tens to hundreds of microns surrounding diamond crystals in the course of diamond growth at HPHT [10–12]. It is generally believed that the metallic film plays a critical role in the graphite– diamond transformation. As the solubility of the graphite carbon is higher than that of diamond carbon in the liquid metallic film, the graphite-form carbon atoms/clusters dissolve into the molten ⁎ Corresponding author. Tel.: +86 28 81691619; fax: +86 28 85405519. E-mail address: [email protected] (F. Peng). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.02.013
metallic film and precipitate on the seeds in a diamond form [13,14]. Xu et al. [10,13] have studied the effect of microstructure and different metallic films on diamond growth at HPHT, but there is no reported data about the influence of metallic film thickness on diamond crystal growth. In this work, we use nickelelectroplated diamonds as seeds to investigate the effect of the Ni film thickness during diamond growth process at HPHT. 2. Experimental The nickel film electroplating on diamond seeds was performed with a lab-designed facility. The main components of the facility are thermostat water bath, electric engine, and conical flask. In order to make the diamond seeds conductive, the diamonds were treated in a chemical plating nickel solution before electroplating. The two electrodes were inserted in the electrolyte. During the electroplating process, the diamond seeds were kept rolling with the conical flask. The rotation speed of the conical flask ranged from 5 to 15 rpm. The pH value of the electrolyte was 4.0–4.5, and the current density was 1.0–1.5 A·cm− 2. The temperature was kept at 45–50 °C. With the parameters as mentioned above, the thickness of nickel film could be controlled by electroplating time. When electroplating was completed, the diamond seeds were
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Fig. 1. (a) Optical image of nickel-plated diamond seeds before HPHT treatments. (b) Optical image of the plated nickel film shell before HPHT treatments. The diamond seeds have been taken out from the plated nickel film shells.
completely covered by plated nickel film as show in Fig. 1 (a). The plated nickel film on the diamond seeds could be knocked down and then the diamonds were taken out from the broken nickel shells carefully. In this way, we could check the thickness and uniformity of the plated nickel film. It can be seen from Fig. 1 (b), the scanning electron microscope (SEM) observation shows that the plated nickel film had a good uniformity. The HPHT experiments are carried out with a DS6×8MN cubic press. The cell temperature was measured with a PtRh6%– PtRh30% thermocouple, and pressure was calibrated with a method of silver melting-point [15]. The accuracy of the pressure and temperature measurements is estimated to be ±0.1 GPa and ±10 °C. The details of the starting materials we used in this work are: graphite disc (Φ 8 mm, 99.9% purity), nickel disc (Φ 8 mm, 99.5% purity), and diamond seeds (0.28 ± 0.02 mm, 60 mesh). The electroplated nickel film on diamond seeds has a thickness of 54.6 μm, 67.1 μm, 93.1 μm, 129 μm, 165.3 μm, 174.9 μm, and 255.6 μm, respectively. Details on HPHT experimental conditions and main results are summarized in Table 1. After the samples were treated at 5.7–5.8 GPa and 1445–1457 °C for 14 min, the temperature was decreased to less than 200 °C by shutting off the heating power, and then slowly decompressing the pressure to ambient pressure. The pressure–temperature–time (P–T–t) path and parameters for the press were controlled by a programmable controller. Fig. 2 shows the cell assembly for diamond growth experiments at HPHT. The nickel-plated diamond seeds are
placed in the pre-drilled holes in the graphite discs (as shown in Fig. 3), and then assembled layer by layer. The nickel disc in the cell assembly was used as a criterion reference to see whether the homogeneous nucleation and growth of diamonds happened under HPHT. The nickel film covering on the grown diamond seeds after HPHT treatments was still uniform no matter if the original plated film was thick or thin. The nickel film was dissolved by heating HNO3, and the residual graphite was removed in a boiling acid mixture of H2SO4 + HNO3 (80:20 in volume ratio). The grown diamonds were observed under optical microscope and SEM. Nine diamond seeds were placed on each graphite disc for HPHT treatment. We have checked the average size of the diamond seeds using optical microscope and SEM before and after HPHT treatment. Special attention has been paid to make sure that the size for each original diamond seed is about the same (0.28 ± 0.02 mm). In this way, we could estimate the size change for the diamond seeds after HPHT treatment according to the SEM and optical observations. Part of the nickel film on HPHT treated diamond seeds was also knocked down to investigate the structure by X-ray diffraction (XRD) or the microstructure by SEM. 3. Results and discussion Fig. 1 (b) shows the image of the plated nickel film shells, from which the diamond seeds have been taken out. The Ni film
Table 1 A list of details on HPHT experimental conditions and main results Sample No.
Thickness of nickel film (μm)
Pressure/GPa
Temperature/ °C
Keep time/min.
Granularmetric analysis
Linear growth speed (μm/min.)
Diamond seed 1 2 3 4 5 6 7
– 54.6 67.1 93.1 129 163.5 174.9 255.6
– 5.8 5.7 5.8 5.8 5.8 5.8 5.8
– 1452 1445 1457 1457 1462 1457 1457
– 12 12 14 14 14 12 12
0.28 mm Unchanged A little change 0.7 mm epigranular 0.6 mm epigranular 0.5–0.6 mm relative epigranular 0.4–0.5 mm unequigranular 0.4–0.5 mm unequigranular
– 0 Nearly 0 30 23 16–23 10–18 10–18
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Fig. 2. Sample assembly for diamond crystal growth at HPHT: 1, graphite disc; 2, nickel sheet; 3, electroplated diamond seeds; 4, copper sheet; 5, NaCl–20% ZrO2 (in weight ratio)-pressure medium; 6, graphite heater; 7, pyrophyllite.
surrounding diamond seeds has a relative uniform thickness. As shown in Fig. 4, the plated nickel film is in a form of single crystalline phase without any observable impurities according to the X-ray diffraction data. The diamond seeds were still uniformly covered with the nickel film after HPHT treatment according our SEM observation. Growth of diamond seeds were observed with the P–T–t conditions as listed in Table 1. Fig. 5 shows the SEM pictures of the original diamond seeds and HPHT treated diamonds with different nickel film thickness. The grown diamonds usually have a cube-octahedron, octahedron or twinned morphology. For most of the grown diamonds, the new-grown diamond develops epitaxially on the crystal planes of seeds by twodimensional nucleation, which is controlled by a layer growth mechanism. The new-grown diamond layer on the surface of original diamond crystals generally developed very well. Both the original seeds and new-grown diamond layers are transparent and have a light yellow color, but there is a very thin interlayer with a slightly dark yellow color between the original seeds and new-grown diamond layers, so the original seeds could be observed through the new-grown diamond layers under an optical microscope. According to the morphology and crystal plane orientation of grown diamonds, there exists a coherent growth relation between diamond seeds and new-grown layers. As shown in Fig. 5, the diamond seeds electroplated nickel film with a thickness of 67.1 μm started growth and became about 10%–15% bigger than the original diamond seeds after HPHT treatment. When the plated nickel film thickness was 93.1 μm and 129 μm, the grain size of grown diamonds was about three times as big as the original seeds. The diamond seeds covered by electroplating nickel film with thickness of 165.3 μm, 174.9 μm, 255.6 μm were also observed to grow, but
Fig. 3. Distribution of diamond seeds on graphite disc: 1, electroplated diamond seed; 2, graphite disc.
Fig. 4. X-ray diffraction pattern of nickel-plated film.
the grown diamonds had a worse morphology and smaller grain size as compared to those diamonds grown from the electroplated seeds with a nickel film thickness of 93.1 μm and 129 μm. The relationship between the growth rate of original seeds and thickness of nickel film was plotted in Fig. 6. There is an optimized nickel film thickness of about 100 μm for diamond seeds growth under our experimental conditions. It was reported by Strong H. M and Hanneman [2] that the growth rate of diamonds were determined by diffusion of carbon in the molten metallic film which surrounded the diamonds at HPHT. Considering good thermal conduction of molten metallic film, we could see that the dissolving region of graphite had the same temperature with the growth region of diamond in molten metallic film. The growth rate of diamond can be expressed by a diffusivity equation: dm=dt ¼ D S DC=L…
ð1Þ
Here dm / dt is the mass change rate of diamond crystal, and D is the diffusion coefficient of carbon in molten metallic film, S is the surface component of diffusion normal to direction of divergent current, L is the thickness of metallic film, ΔC is the differential concentration of carbon between graphite region side and diamond region side in molten metallic film. From Eq. (1), we could see that the growth rate of diamond was determined by the diffusion of carbon through the molten metallic film onto the growing surface. Thus, metallic film was also a critical factor, which could affect the growth rate. When the electroplating nickel film on diamond seeds was thin (b60 μm), the diffusion of carbon atoms/clusters through the molten metallic film was very fast, and the carbon atoms/clusters precipitated from the metallic film in a form of recrystallized graphite on the diamond seeds surface. It was found that both the thin and thick nickel film recovered from the HPHT treated diamond have the same structure from the XRD data. However, the SEM observation of the inner surface gives the different distribution of the recrystallized graphite. The recrystallized graphite on the inner surface of the thin Ni film is significantly more as compared to those for the thick Ni film. Fig. 7 (a) shows the SEM image of the inner surface of thin nickel film (54.6 μm) after HPHT
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Fig. 5. SEM image of grown diamond with nickel film of different thickness: (a) diamond seeds before plating; (b) grown diamonds with the nickel film thickness of 54.6 μm; (c) grown diamonds with the nickel film thickness of 67.1 μm; (d) grown diamonds with the nickel film thickness of 93.1 μm; (e) grown diamonds with the nickel film thickness of 129 μm; (f) grown diamonds with the nickel film thickness of 163.5 μm; (g) grown diamonds with the nickel film thickness of 174.9 μm; (h) grown diamonds with the nickel film thickness of 255.6 μm.
treatment. The energy dispersive X-ray analysis (Fig. 7 (b)) tells that the chemical composition of the dark zooms (dots) is mostly carbon. Those dark dots were also found to be very soft when checked with a needle under optical microscopy. This indicates that the dark dots are recrystallized graphite instead of diamond. The similar results were also reported in Ref. [10] from the analysis of components in the metallic film after HPHT treatments. The recrystallized graphite needs a larger driven force for diamond formation as compared to the starting graphite [16]. Therefore, when the nickel film was too thin, the
recrystallized graphite could quickly surround the diamond seeds, absorb more carbon atoms/clusters which pass through the nickel film, and prevent the diamond seeds from growing further. On the other hand, when Ni film on diamond seeds were thick (N150 μm), the L in Eq. (1) becomes large and leads to a small growth rate for the diamond seeds. The optimal thickness of the Ni film on diamond seeds was about 100 μm for large and high quality crystal growth under our experimental conditions. As discussed above, the growth rate for the diamond seeds is affected by the diffusion of carbon atoms/clusters through the melted Ni film during HPHT conditions, therefore, it decreases with an increase of thickness of the Ni film. However, the recalescence of the melted Ni film during the cooling down at high pressure could delay the growth process for the diamond seeds, and significantly lead to a continuous growth even after the heating power was shut off, especially when the nickel film thickness is large. This might be the reason that the growth rate does not continuously decrease with an increase of thickness of nickel film after the nickel film thickness reaches to 165.3 μm, and the line for diamond seeds growth rate vs. Ni film thickness becomes flat or even rises at Ni film thickness larger than ∼250 μm in Fig. 6. 4. Conclusions
Fig. 6. Relationship between growth rate of diamond seeds and the thickness of nickel film.
Epitaxial growth on nickel-plated diamond seeds at HPHT was observed with graphite as carbon source. The optimal thickness of the electroplating nickel film on diamond seeds
M.G. Zhang et al. / Diamond & Related Materials 16 (2007) 1665–1669
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Fig. 7. (a) SEM image of inner surface of nickel film (54.6 mm) after HPHT treatments. (b) Energy dispersive X-ray analysis results for the marked area as shown in (a).
was about 100 μm for large diamond crystal growth, and growth rate was 23–30 μm/min at ∼ 5.8 GPa and ∼ 1457 °C. For most of the grown diamonds, the new-grown diamond developed epitaxially on the crystal planes of seeds by two-dimensional nucleation and grew by a layer growth mechanism. The results and techniques might be useful for high quality saw-grade diamonds production and large diamond single crystal growth. Acknowledgments This work is supported by the Natural Science Foundation of China (Grant No. 50572067) and Si Chuan Yi Jing ChangYun Super-hard Materials Co., Ltd (Grant No. 05H187). References [1] F.P. Bundy, J. Chem. Phys. 38 (1963) 631. [2] H.M. Strong, R.E. Hanneman, J. Chem. Phys. 46 (1967) 3668. [3] H. Kanda, M. Akaishi, S. Yamaoka, Appl. Phys. Lett. 65 (1994) 784.
[4] M.D. Horton, G.R. Peterson, Proc. Tech. Sym. on Diamonds in the 80s, Industrial Diamond Association of America, New Jersey, 1980, p. 171. [5] R.H. Wentorf, Adv. Chem. Phys. 9 (1965) 365. [6] H.E'. Bovenkerk, Progr. Very High Pressure Res., Proc. Int.Conf., 1961, p. 58. [7] R.H. Wentorf, J. Phys. Chem. 75 (1971) 1833. [8] R.C. Burns, S. Saito, O. Fukunaga, M. Yoshikawa, Science and Technology of New Diamond, KTK Scientific Pub lishers/Terra Scientific Publishing Company, Tokyo, 1990, p. 197. [9] R.C. Burns, J.O. Hansen, R.A. Spits, M. Sibanda, C.M. Welbourn, D.L. Welch, Diamond Relat. Mater. 8 (1991) 1433. [10] B. Xu, M.S. Li, L.W. Yin, J.J. Cui, Y.X. Liu, J.H. Gong, Chin. Sci. Bull. 47 (2002) 1258. [11] S. Mourachov, V.P. Poliako, Diamond Relat. Mater. 7 (1998) 309. [12] L.W. Yin, M.S. Li, Z.G. Gong, Y.J. Bai, F.Z. Li, Z.Y. Hao, Appl. Phys., A Mater. Sci. Process. 76 (2003) 1061. [13] B. Xu, M.S. Li, L.W. Yin, J. Mater. Sci. Technol. 19 (2003) 113. [14] Z.Y. Hao, Y.F. Chen, G.T. Zou, Synthetic Diamond, Jinlin University Press, Changchun, 1996, p. 209. [15] P.W. Mirwald, I.C. Getting, G.C. Kennedy, J. Geophys. Res. 80 (1975) 1519. [16] M. Wakatsuki, Rev. High Press. Sci. Technol. 7 (1998) 951.
Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature M.G. Zhang a,b , F. Peng a,⁎, C. Chen a , D.W. He a a
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China b Physics Department of Baoji University of Arts and Sciences, Baoji 721007, China
Received 10 July 2006; received in revised form 9 February 2007; accepted 19 February 2007 Available online 28 February 2007
Abstract Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature (HPHT) was observed with graphite as carbon source. The thickness of the electroplating nickel film which acts as a catalyst/solvent ranges from 54.6 μm to 255.6 μm. The relationship between the Ni film thickness and diamond growth rate is investigated. When the nickel film thickness is from 90 μm to 129 μm, diamond crystals can nearly grow up to three times as large as the original seeds at ∼ 5.8 GPa and ∼ 1460 °C within 14 min. The mechanism of the crystal growth with nickelplated diamond seeds under HPHT is discussed. The results and techniques might be useful for high quality saw-grade diamonds production and large diamond single crystal growth. © 2007 Elsevier B.V. All rights reserved. Keywords: Diamond crystal growth; High pressure and high temperature; Nickel-plated seeds
1. Introduction Diamonds can be synthesized with various techniques [1–3], but the static-pressure-catalyst method is still the most popular way for industrial diamond grits production. In this method, the starting graphite and metallic catalyst are treated at HPHTeither in a form of powder mixture or in a layer-by-layer assembly. In both cases, the control of nucleation is critical, which can directly affect the grain size, morphology, quality and yield of diamond [4–6]. The temperature difference method using diamond seeds has also been applied for large diamond single crystal growth as reported in several literatures [7–9]. When the graphite and metallic catalyst are used as starting materials for diamond grits synthesis, there always exists a molten metallic film with a thickness from several tens to hundreds of microns surrounding diamond crystals in the course of diamond growth at HPHT [10–12]. It is generally believed that the metallic film plays a critical role in the graphite– diamond transformation. As the solubility of the graphite carbon is higher than that of diamond carbon in the liquid metallic film, the graphite-form carbon atoms/clusters dissolve into the molten ⁎ Corresponding author. Tel.: +86 28 81691619; fax: +86 28 85405519. E-mail address: [email protected] (F. Peng). 0925-9635/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.02.013
metallic film and precipitate on the seeds in a diamond form [13,14]. Xu et al. [10,13] have studied the effect of microstructure and different metallic films on diamond growth at HPHT, but there is no reported data about the influence of metallic film thickness on diamond crystal growth. In this work, we use nickelelectroplated diamonds as seeds to investigate the effect of the Ni film thickness during diamond growth process at HPHT. 2. Experimental The nickel film electroplating on diamond seeds was performed with a lab-designed facility. The main components of the facility are thermostat water bath, electric engine, and conical flask. In order to make the diamond seeds conductive, the diamonds were treated in a chemical plating nickel solution before electroplating. The two electrodes were inserted in the electrolyte. During the electroplating process, the diamond seeds were kept rolling with the conical flask. The rotation speed of the conical flask ranged from 5 to 15 rpm. The pH value of the electrolyte was 4.0–4.5, and the current density was 1.0–1.5 A·cm− 2. The temperature was kept at 45–50 °C. With the parameters as mentioned above, the thickness of nickel film could be controlled by electroplating time. When electroplating was completed, the diamond seeds were
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Fig. 1. (a) Optical image of nickel-plated diamond seeds before HPHT treatments. (b) Optical image of the plated nickel film shell before HPHT treatments. The diamond seeds have been taken out from the plated nickel film shells.
completely covered by plated nickel film as show in Fig. 1 (a). The plated nickel film on the diamond seeds could be knocked down and then the diamonds were taken out from the broken nickel shells carefully. In this way, we could check the thickness and uniformity of the plated nickel film. It can be seen from Fig. 1 (b), the scanning electron microscope (SEM) observation shows that the plated nickel film had a good uniformity. The HPHT experiments are carried out with a DS6×8MN cubic press. The cell temperature was measured with a PtRh6%– PtRh30% thermocouple, and pressure was calibrated with a method of silver melting-point [15]. The accuracy of the pressure and temperature measurements is estimated to be ±0.1 GPa and ±10 °C. The details of the starting materials we used in this work are: graphite disc (Φ 8 mm, 99.9% purity), nickel disc (Φ 8 mm, 99.5% purity), and diamond seeds (0.28 ± 0.02 mm, 60 mesh). The electroplated nickel film on diamond seeds has a thickness of 54.6 μm, 67.1 μm, 93.1 μm, 129 μm, 165.3 μm, 174.9 μm, and 255.6 μm, respectively. Details on HPHT experimental conditions and main results are summarized in Table 1. After the samples were treated at 5.7–5.8 GPa and 1445–1457 °C for 14 min, the temperature was decreased to less than 200 °C by shutting off the heating power, and then slowly decompressing the pressure to ambient pressure. The pressure–temperature–time (P–T–t) path and parameters for the press were controlled by a programmable controller. Fig. 2 shows the cell assembly for diamond growth experiments at HPHT. The nickel-plated diamond seeds are
placed in the pre-drilled holes in the graphite discs (as shown in Fig. 3), and then assembled layer by layer. The nickel disc in the cell assembly was used as a criterion reference to see whether the homogeneous nucleation and growth of diamonds happened under HPHT. The nickel film covering on the grown diamond seeds after HPHT treatments was still uniform no matter if the original plated film was thick or thin. The nickel film was dissolved by heating HNO3, and the residual graphite was removed in a boiling acid mixture of H2SO4 + HNO3 (80:20 in volume ratio). The grown diamonds were observed under optical microscope and SEM. Nine diamond seeds were placed on each graphite disc for HPHT treatment. We have checked the average size of the diamond seeds using optical microscope and SEM before and after HPHT treatment. Special attention has been paid to make sure that the size for each original diamond seed is about the same (0.28 ± 0.02 mm). In this way, we could estimate the size change for the diamond seeds after HPHT treatment according to the SEM and optical observations. Part of the nickel film on HPHT treated diamond seeds was also knocked down to investigate the structure by X-ray diffraction (XRD) or the microstructure by SEM. 3. Results and discussion Fig. 1 (b) shows the image of the plated nickel film shells, from which the diamond seeds have been taken out. The Ni film
Table 1 A list of details on HPHT experimental conditions and main results Sample No.
Thickness of nickel film (μm)
Pressure/GPa
Temperature/ °C
Keep time/min.
Granularmetric analysis
Linear growth speed (μm/min.)
Diamond seed 1 2 3 4 5 6 7
– 54.6 67.1 93.1 129 163.5 174.9 255.6
– 5.8 5.7 5.8 5.8 5.8 5.8 5.8
– 1452 1445 1457 1457 1462 1457 1457
– 12 12 14 14 14 12 12
0.28 mm Unchanged A little change 0.7 mm epigranular 0.6 mm epigranular 0.5–0.6 mm relative epigranular 0.4–0.5 mm unequigranular 0.4–0.5 mm unequigranular
– 0 Nearly 0 30 23 16–23 10–18 10–18
M.G. Zhang et al. / Diamond & Related Materials 16 (2007) 1665–1669
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Fig. 2. Sample assembly for diamond crystal growth at HPHT: 1, graphite disc; 2, nickel sheet; 3, electroplated diamond seeds; 4, copper sheet; 5, NaCl–20% ZrO2 (in weight ratio)-pressure medium; 6, graphite heater; 7, pyrophyllite.
surrounding diamond seeds has a relative uniform thickness. As shown in Fig. 4, the plated nickel film is in a form of single crystalline phase without any observable impurities according to the X-ray diffraction data. The diamond seeds were still uniformly covered with the nickel film after HPHT treatment according our SEM observation. Growth of diamond seeds were observed with the P–T–t conditions as listed in Table 1. Fig. 5 shows the SEM pictures of the original diamond seeds and HPHT treated diamonds with different nickel film thickness. The grown diamonds usually have a cube-octahedron, octahedron or twinned morphology. For most of the grown diamonds, the new-grown diamond develops epitaxially on the crystal planes of seeds by twodimensional nucleation, which is controlled by a layer growth mechanism. The new-grown diamond layer on the surface of original diamond crystals generally developed very well. Both the original seeds and new-grown diamond layers are transparent and have a light yellow color, but there is a very thin interlayer with a slightly dark yellow color between the original seeds and new-grown diamond layers, so the original seeds could be observed through the new-grown diamond layers under an optical microscope. According to the morphology and crystal plane orientation of grown diamonds, there exists a coherent growth relation between diamond seeds and new-grown layers. As shown in Fig. 5, the diamond seeds electroplated nickel film with a thickness of 67.1 μm started growth and became about 10%–15% bigger than the original diamond seeds after HPHT treatment. When the plated nickel film thickness was 93.1 μm and 129 μm, the grain size of grown diamonds was about three times as big as the original seeds. The diamond seeds covered by electroplating nickel film with thickness of 165.3 μm, 174.9 μm, 255.6 μm were also observed to grow, but
Fig. 3. Distribution of diamond seeds on graphite disc: 1, electroplated diamond seed; 2, graphite disc.
Fig. 4. X-ray diffraction pattern of nickel-plated film.
the grown diamonds had a worse morphology and smaller grain size as compared to those diamonds grown from the electroplated seeds with a nickel film thickness of 93.1 μm and 129 μm. The relationship between the growth rate of original seeds and thickness of nickel film was plotted in Fig. 6. There is an optimized nickel film thickness of about 100 μm for diamond seeds growth under our experimental conditions. It was reported by Strong H. M and Hanneman [2] that the growth rate of diamonds were determined by diffusion of carbon in the molten metallic film which surrounded the diamonds at HPHT. Considering good thermal conduction of molten metallic film, we could see that the dissolving region of graphite had the same temperature with the growth region of diamond in molten metallic film. The growth rate of diamond can be expressed by a diffusivity equation: dm=dt ¼ D S DC=L…
ð1Þ
Here dm / dt is the mass change rate of diamond crystal, and D is the diffusion coefficient of carbon in molten metallic film, S is the surface component of diffusion normal to direction of divergent current, L is the thickness of metallic film, ΔC is the differential concentration of carbon between graphite region side and diamond region side in molten metallic film. From Eq. (1), we could see that the growth rate of diamond was determined by the diffusion of carbon through the molten metallic film onto the growing surface. Thus, metallic film was also a critical factor, which could affect the growth rate. When the electroplating nickel film on diamond seeds was thin (b60 μm), the diffusion of carbon atoms/clusters through the molten metallic film was very fast, and the carbon atoms/clusters precipitated from the metallic film in a form of recrystallized graphite on the diamond seeds surface. It was found that both the thin and thick nickel film recovered from the HPHT treated diamond have the same structure from the XRD data. However, the SEM observation of the inner surface gives the different distribution of the recrystallized graphite. The recrystallized graphite on the inner surface of the thin Ni film is significantly more as compared to those for the thick Ni film. Fig. 7 (a) shows the SEM image of the inner surface of thin nickel film (54.6 μm) after HPHT
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Fig. 5. SEM image of grown diamond with nickel film of different thickness: (a) diamond seeds before plating; (b) grown diamonds with the nickel film thickness of 54.6 μm; (c) grown diamonds with the nickel film thickness of 67.1 μm; (d) grown diamonds with the nickel film thickness of 93.1 μm; (e) grown diamonds with the nickel film thickness of 129 μm; (f) grown diamonds with the nickel film thickness of 163.5 μm; (g) grown diamonds with the nickel film thickness of 174.9 μm; (h) grown diamonds with the nickel film thickness of 255.6 μm.
treatment. The energy dispersive X-ray analysis (Fig. 7 (b)) tells that the chemical composition of the dark zooms (dots) is mostly carbon. Those dark dots were also found to be very soft when checked with a needle under optical microscopy. This indicates that the dark dots are recrystallized graphite instead of diamond. The similar results were also reported in Ref. [10] from the analysis of components in the metallic film after HPHT treatments. The recrystallized graphite needs a larger driven force for diamond formation as compared to the starting graphite [16]. Therefore, when the nickel film was too thin, the
recrystallized graphite could quickly surround the diamond seeds, absorb more carbon atoms/clusters which pass through the nickel film, and prevent the diamond seeds from growing further. On the other hand, when Ni film on diamond seeds were thick (N150 μm), the L in Eq. (1) becomes large and leads to a small growth rate for the diamond seeds. The optimal thickness of the Ni film on diamond seeds was about 100 μm for large and high quality crystal growth under our experimental conditions. As discussed above, the growth rate for the diamond seeds is affected by the diffusion of carbon atoms/clusters through the melted Ni film during HPHT conditions, therefore, it decreases with an increase of thickness of the Ni film. However, the recalescence of the melted Ni film during the cooling down at high pressure could delay the growth process for the diamond seeds, and significantly lead to a continuous growth even after the heating power was shut off, especially when the nickel film thickness is large. This might be the reason that the growth rate does not continuously decrease with an increase of thickness of nickel film after the nickel film thickness reaches to 165.3 μm, and the line for diamond seeds growth rate vs. Ni film thickness becomes flat or even rises at Ni film thickness larger than ∼250 μm in Fig. 6. 4. Conclusions
Fig. 6. Relationship between growth rate of diamond seeds and the thickness of nickel film.
Epitaxial growth on nickel-plated diamond seeds at HPHT was observed with graphite as carbon source. The optimal thickness of the electroplating nickel film on diamond seeds
M.G. Zhang et al. / Diamond & Related Materials 16 (2007) 1665–1669
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Fig. 7. (a) SEM image of inner surface of nickel film (54.6 mm) after HPHT treatments. (b) Energy dispersive X-ray analysis results for the marked area as shown in (a).
was about 100 μm for large diamond crystal growth, and growth rate was 23–30 μm/min at ∼ 5.8 GPa and ∼ 1457 °C. For most of the grown diamonds, the new-grown diamond developed epitaxially on the crystal planes of seeds by two-dimensional nucleation and grew by a layer growth mechanism. The results and techniques might be useful for high quality saw-grade diamonds production and large diamond single crystal growth. Acknowledgments This work is supported by the Natural Science Foundation of China (Grant No. 50572067) and Si Chuan Yi Jing ChangYun Super-hard Materials Co., Ltd (Grant No. 05H187). References [1] F.P. Bundy, J. Chem. Phys. 38 (1963) 631. [2] H.M. Strong, R.E. Hanneman, J. Chem. Phys. 46 (1967) 3668. [3] H. Kanda, M. Akaishi, S. Yamaoka, Appl. Phys. Lett. 65 (1994) 784.
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