Multiseed Method for High Quality Sheet Cubic Diamonds Synthesis

Dec 6, 2011 - diamonds synthesis by a temperature gradient method (TGM) under high ... For the cubic diamond crystal, the radial growth rate is much...
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Multiseed Method for High Quality Sheet Cubic Diamonds Synthesis: An Effective Solution for Scientific Research and Commercial Production Meihua Hu, Hong-an Ma, Bingmin Yan, Yong Li, Zhanchang Li, Zhenxiang Zhou, and Xiaopeng Jia* State Key Laboratory of Superhard Materials, Jilin University, Changchun, 130012, P. R. China ABSTRACT: A multiseed method for high quality sheet cubic diamonds synthesis by a temperature gradient method (TGM) under high pressure and high temperature (HPHT) conditions for scientific research and commercial production has been designed and discussed. Multiseed method diamond synthesis can accommodate much larger sample volumes than a single seed method as the high pressure apparatus volume increases. For the cubic diamond crystal, the radial growth rate is much higher than the axial growth rate, and the whole growth rate of diamond crystals have been effectively increased because of more seeds embedded in a growth cell. Computer simulation and experimental results indicate that the enhanced diamond growth cell designed for the multiseed method to synthesize large diamonds is effective and suitable. Our proposed method saves the cost associated with the growth cell and increases the efficiency to adapt the large sample volume for the commercial production.

1. INTRODUCTION The unique physical and chemical properties of diamond make it an ideal material for a variety of applications that could revolutionize science and technology in the 21st century. Many companies, such as General Electric, Element Six, and Sumitomo Electric, which produce large diamond single crystals for commercial purposes, all use a reaction cell in a high pressure and high temperature apparatus, although the design details of their respective equipment differ.1−7 When various transition metals (Fe, Ni, Co, Mn, and so on) are used as solvent/catalyst, typical growth conditions are pressures in the range of 5.0−7.0 GPa and temperatures in the range of 1250− 1800 °C.5−11 Generally, the temperature gradient method (TGM) under high pressures and high temperatures (HPHT) to synthesize large diamond crystals with gem-quality has been used for several decades and is an effective way for scientific research and commercial production. High quality large cubic diamonds with sheet shape, which are composed of a (100) crystal face, can be widely used in many fields. Many precision and ultraprecision machining tools need regular cubic shape and no inclusions diamond crystals.13,14 It has also been used for the heat spreader for the high power laser and windows for infrared instruments. Diamond synthesis by microwave plasma assisted chemical vapor deposition has made much progress, but first of all, sheet cubic diamonds after polishing must be used for the substrate of the chemical vapor deposition process.15,16 During the diamond growth process, the morphology of diamond changes from cubic crystal with the dominated (100) crystal faces, cuboctahedral crystal with dominated (100) and (111) crystal faces, to octahedral crystals with the dominated © 2011 American Chemical Society

(111) crystal faces as the synthetic temperature increases. However, compared with the growth of the conventional cuboctahedral diamond crystal, the growth of cubic crystals is more difficult. It is reported that the synthetic region of high quality cubic diamond crystal is only about 10 °C.8 So, the growth conditions of pressure, temperature, and assembly need more precision. Meanwhile, for the development of HPHT synthesis for diamond and cubic boron nitride (c-BN), it is well-known that to enhance the sample volume of the high pressure and high temperature growth cell and save the cost of production are the top priorities. As the sample volume of high pressure and high temperature growth cell increases, the utilization of high pressure and high temperature growth cell has become a choke point to synthesize the diamond by TGM under HPHT for scientific research and commercial production.17 Generally, the conventional synthesis process, in which the single diamond seed is embedded in a sample cell, is considered too expensive to translate into commercial production at this time. In this work, in order to save the cost associated with the diamond growth process, and thus develop our understanding of the diamond growth mechanism in a melt solvent/catalyst-C system, synthetic assembly and multiseed method for the high quality cubic diamonds synthesis have been designed, and a series of experimental and computer simulation by the multiseed method on diamond growth were undertaken in the Fe−Ni alloy−carbon system under HPHT. Received: November 3, 2011 Revised: December 5, 2011 Published: December 6, 2011 518

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2. EXPERIMENTAL SECTION

single diamond seed, if the diffusion of carbon will be too fast, carbon transported from the carbon source will nucleate spontaneously or become regrown graphite in excess of what growing crystals can accommodate. At the same time, many metallic inclusions were easily trapped in the diamond crystals. While, in the enhanced cell, there are more diamond seeds in the bottom of growth cell, the absorption ability of the multiseed method is larger than that of the single seed method. Therefore, the experimental result indicates that the diamond crystal growth by the multiseed method in the enhanced cell needs a much higher carbon solubility difference than that in the conventional cell. Meanwhile, the pressure and temperature of the growing diamond seed are also critical factors, because pressure and temperature are transmitted from the edge of the growth cell to inner; the pressure and temperature gradients exist along the radial and vertical directions. So, based on the above, the diamond growth cell has been designed for the next experiments. 3.2. Growth of High Quality Cubic Diamond Crystal by Multiseed Method. As we all know, in the P−T phase diagram of carbon, the district for diamond growth is a V-shape region bounded by a diamond−graphite equilibrium line and solvent/carbon eutectic melting line in the metal solvent− carbon system. As shown in Figure 2, it should be noted that

The synthetic experiments were performed in a China type SPD6×1200 cubic-anvil high pressure apparatus (CHPA) with an anvil top face of 27.5 × 27.5 mm2. The sample assembly for synthesis of diamond by TGM is shown in Figure 1. Scalelike purity graphite

Figure 1. Sample assembly for diamond growth (a, b, and c). powder (mesh 200) and Fe−Ni alloy (64:36 by wt %) were used as the carbon source and the catalyst/solvent, respectively. The temperature gradient could be changed freely by adjusting the structure of the assembly. In addition, well faceted {100} crystal face of 0.6 × 0.6 mm in size was used as the diamond growth face. The sample assembly for diamond growth is shown in Figure 1a. The ranges of the synthetic pressures and temperatures were 5.5 to 5.7 GPa and 1250 to 1350 °C, respectively. The pressure was estimated by the oil press load, which was calibrated by a curve that was established on the pressure-induced phase transitions of Bi, Tl, and Ba. The temperature was determined by a Pt6%Rh−Pt30%Rh thermocouple.18 The collected samples in these experiments were put into boiling acids of HNO3 and H2SO4 to remove the impurities remaining on the surfaces of the crystals. Then, the diamond crystals were obtained. Optical microscope (OM) is used to observe the crystal morphology and inclusions in the diamonds. The distribution of carbon concentration in the melted catalyst/ solvent of the growth cell was simulated by the finite element method (FEM). Using solid 69 and fluid 142 cells in FEM software, thermalelectrical-fluid finite element analyses were carried out in the whole model of the growth cell, respectively. Finite element model, boundary conditions, and material parameters were originated from other previous reports.2,19−22 The computer model is based on an axisymmetric approach accounting for heat transfer, carbon mass transport, and convection in catalyst solvent. The growth cell was heated when the voltage was set on the end of two anvils along the vertical direction in CHPA.17

Figure 2. Growth regions of high quality diamond crystal (Region A: T and P for sheet cubic diamond crystals by the multiseed method).

3. RESULTS AND DISCUSSION 3.1. Design of Multiseed Synthesis Cell. As shown in Figure 1a, based on the TGM of the diamond growth process, graphite powder is placed at the high temperature region, and diamond seeds are embedded at the low temperature region. The driving force of diamond growth is provided by the solubility difference resulting from the temperature gradient in the growth cell. An appropriate solubility difference is needed for the growth of diamond. That is to say, an appropriate temperature gradient plays the key role for the growth of large diamonds by TGM. The temperature gradient was changed by adjusting the structure of the assembly. As shown in Figure 1, we choose the conventional cell for the single diamond seed growth (Figure 1b) and the enhanced cell for the multiseed diamonds growth (Figure 1c). In the diamond growth cell, the higher temperature gradient will lead to the higher carbon concentration gradient. Thus, for the conventional cell with

the P−T regions of diamond synthesis in the metallic solvent/ catalyst−carbon systems appear significantly broader with the increase of the pressures. Experiments for the diamond growth in the Fe−Ni−C system were performed at a pressure of 5.7 GPa and temperatures ranging from 1250 to 1350 °C. High quality sheet cubic diamond crystals can be obtained in region A. The region is about from 1270 to 1295 °C. As shown in Figure 3a, large high quality cubic type Ib diamond crystals have been synthesized. From the optical micrographs of the synthetic diamond crystals in Figure 3, the morphologies of the synthetic diamonds were cubic shape with the dominated (100) crystal face and have a yellow color with no metal inclusions. The concentration of nitrogen impurity in diamond crystals is approximately 260−300 ppm. Furthermore, it should be noted that the morphologies of the synthetic diamonds are regular, smooth surfaces, and with few or no metal inclusions. All the cubic diamond crystals show a sheet shape and are composed of the dominated (100) crystal faces. 519

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Figure 3. OM photographs of large diamond crystals synthesized from different systems at 5.7 GPa and 1285 °C.

Figure 4. Diamond growth rate of the single diamond seed method and multiseed method at 5.7 GPa and 1280−1320 °C.

For example, one of the cubic diamond crystal surfaces in Figure 1a has been polished, which is shown in Figure 3b,c; the radial size is about 3.5 mm, while the axial size is only 1.3 mm. We believe that the growth conditions are low temperature Vshape region and suitable for the growth of the (100) crystal face, and the radial growth rate is enlarged but the axial growth rate is restrained. It is clear that the radial growth rate of the cubic diamond is much higher than the axial growth rate. The growth rate of multiseed method is discussed in the next section. 3.3. Growth Rate of Diamond by Multiseed Method. Usually, the diamond growth appears to take place to pile up layer upon layer. The growth process is explained in other earlier reports.2 Because several diamond seeds are used in the multiseed method, there are more carbon absorption surfaces than the single seed method, which may influence the growth rate significantly. The growth rate is determined by the carbon diffusion on the surface of the growing diamond crystal through the metallic film. The growth rate of diamond crystal can be expressed by a diffusivity equation:23

show in Figure 3, the diamond crystals synthesized by the multiseed method have same quality compared to the single seed method. It should be pointed out that, when the pressure, temperature, and dimension of the growth cell are constant, the size of the grown diamond crystal is inversely proportional to the quantity of diamond seed by the multiseed method. 3.4. Growth Mechanism Simulation by the FEM. In order to analyze the diamond growth mechanism of the multiseed method, FEM was used to simulate the carbon concentration in the growth process of diamond crystal. As shown in Figure 5, the carbon concentration of the conven-

dw/dt = DAΔC /L

(1)

where dw/dt is the mass change rate of the diamond crystal, D is the diffusion coefficient of carbon in solvent/catalyst metallic film, which is affected by the component of the solvent/catalyst, A is the effective area for growth, L is the thickness of the solvent/catalyst metallic film, which is a constant for high quality diamond crystal growth, ΔC is the different carbon concentrations between diamond seed and carbon source of the melted solvent/catalyst metallic film. Thus, the carbon concentration in the solvent/catalyst metallic film between diamond seed and carbon source is a critical factor, which could affect the growth rate. Take a single diamond seed and six diamond seeds in a growth cell as examples: a series of experiments on growth rates were performed, respectively. The experimental results on diamond crystal growth rates by the single seed method and multiseed method are presented in Figure 4. As shown in Figure 4, it is clearly seen that the diamond growth rate of the multiseed method is much larger than that of the single seed method. It should be noted that the one diamond growth rate of the multiseed method is not more rapid than that of the single seed method, but the whole growth rate of the multiseed method is much higher than that of single seed method. Meanwhile, as the OM photographs of large diamond crystals

Figure 5. Carbon concentration distributions of the theoretical calculation results in the solvent of the conventional growth cell (a) and the enhanced growth cell (b).

tional growth cell with single seed (Figure 5a) and the enhanced growth cell with multiseed (Figure 5b) were simulated, respectively. Molecular diffusion and convection provide carbon transport from the source to the crystal surface. It is observed obviously that the carbon concentration field has symmetrical distribution along the vertical direction regardless of the single seed method or multiseed method. The carbon concentration modes varied with different growth cells because of the change of solubility gradient in the solvent fluid. In fact, on the basis of our experimental results, the morphologies of diamond crystals synthesized by the multiseed method show individual differences about four tips on the cubic diamond crystal (as shown in Figure 3). Carbon concentration values in this figure are the molar ratio of carbon to catalyst solvent. Theoretical calculation results suggest that the solubility difference of the enhanced growth cell is much higher than that of the conventional growth cell. We believe that the higher solubility difference results from the higher temperature gradient in the designed enhanced growth cell. Generally, this 520

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(10) Kanda, H.; Akaishi, M.; Yamaoka, S. Appl. Phys. Lett. 1994, 65, 784−786. (11) Lin, I. C.; Lin, C. J.; Tuan, W. H. Diamond Relat. Mater. 2011, 20, 42−47. (12) Han, Q. G.; Liu, B.; Hu, M. H.; Li, Z. C.; Jia, X. P.; Li, M. Z.; Ma, H. A.; Li, S. S.; Xiao, H. Y.; Li, Y. Cryst. Growth Des. 2011, 11, 1000−1005. (13) Zong, W. J.; Sun, T.; Li, D.; Cheng, K. Diamond Relat. Mater. 2009, 18, 642−650. (14) Anthony, T. R. Diamond Relat. Mater. 1995, 4, 1346−1352. (15) Meng, Y. F.; Yan, C. S.; Lai, J.; Krasnicki, S.; Shu, H.; Yu, T.; Liang, Q.; Mao, H. K.; Hemley, R. J. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 17620−17625. (16) Yan, C. S.; Vohra, Y. K.; Mao, H. K.; Hemley, R. J. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 12523−12525. (17) Han, Q. G.; Ma, H. A.; Huang, G. F.; Zhang, C.; Li, Z. C.; Jia, X. P. Rev. Sci. Instrum. 2009, 80, 096107(3). (18) Ma, H. A.; Jia, X. P.; Chen, L. X.; Zhu, P. W.; Guo, W. L.; Guo, X. B.; Wang, Y. D.; Li, S. Q.; Zou, G. T.; Zhang, G.; Philip, Bex. J. Phys: Condens. Matter. 2002, 14, 11269−11273. (19) Sato, Y.; Sugisawa, K.; Aoki, D.; Yamamura, T. Meas. Sci. Technol. 2005, 16, 363−371. (20) Demina, S. E.; Kalaev, V. V.; Lysakovskyi, V. V.; Serga, M. A.; Kovalenko, T. V.; Ivahnenko, S. A. J. Cryst. Growth 2009, 311, 680− 683. (21) Han, Q. G.; Li, M. Z.; Jia, X. P.; Ma, H. A.; Li, Y. F. Diamond Relat. Mater. 2011, 20, 969−973. (22) Li, R.; Ma, H. A.; Han, Q. G.; Liang, Z. Z.; Yin, B. H.; Liu, W. Q.; Jia, X. High Pressure Res. 2007, 27, 249−257. (23) Zhang, M. G.; Peng, F.; Chen, C.; He, D. W. Diamond Relat. Mater. 2007, 16, 1665−1669.

carbon concentration gradient will cause carbon to diffuse downward to the diamond seed surface. If the carbon concentration gradients are higher and the diamond seed absorption capability is not strengthened (e.g., single diamond seed in growth cell), the surplus carbon will nucleate spontaneously or become regrown graphite. On the contrary, if the diamond seed absorption capability is strengthened, but the carbon concentration gradient is not high enough, the diamond seed will graphitize. On the basis of the above, the diamond growth mechanism can be explained as follows: with the increase of concentration gradients, the carbon transportation rate in the solvent/catalyst becomes high and more carbon is transported to the surface of the diamond seed. For the multiseed method, more diamond seeds are embedded in the growth cell; that is to say, the absorption area is enlarged. In the end, the whole growth rate is enlarged. Above all, the adjustment of the diamond growth cell is one of the most important in the growth process.

4. CONCLUSIONS High quality sheet cubic diamonds were successfully synthesized by the multiseed method from the Fe−Ni alloy−carbon system under high pressure and high temperature conditions. This solution can effectively save the costs associated with the sample volume of growth cell and increase the production efficiency for scientific research and commercial production. The growth rate of the multiseed method is much higher that of the single seed in the growth cell. Numerical simulation results were consistent with the experiments obtained and explain the diamond growth mechanism of the multiseed method. This multiseed method of diamond growth not only provides an effective way to improve diamond production but also proposes a route for the future study of diamond growth cell assembly.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 431 85168858.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 50572032 and 51172089) and the Program for New Century Excellent Talents in University.

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dx.doi.org/10.1021/cg2014539 | Cryst. Growth Des. 2012, 12, 518−521