DOI: 10.1021/cg901168s
Experimental Evidence for Nucleation and Growth Mechanism of Diamond by Seed-Assisted Method at High Pressure and High Temperature
2010, Vol. 10 2895–2900
Xiaobing Liu, Xiaopeng Jia, Xinkai Guo, Zhuangfei Zhang, and Hong-an Ma* State Key Lab of Superhard Materials, Jilin University, Changchun 130012, P. R. China Received September 23, 2009; Revised Manuscript Received June 5, 2010
ABSTRACT: In this paper, the diamond growth mechanism at high-pressure and high-temperature (HPHT) conditions from solvent-graphite system was investigated by growing diamond on different seeds and tracking the particular shapes of the seeds before and after treated under HPHT conditions. According to the results, we established a direct correlation between the morphology of the diamond and the original shape of the seeds. The crystallization of carbon phases (diamond-graphite) in the Fe-Ni-C system is depicted in detail in the P-T diagram. Experimental results show the synthetic pressure obviously decreases when diamond seed crystals are added into the original synthetic system, which confirms that the energy barrier of the diamond nucleation is higher than that of growth. In addition, we detected in our experiments that the diamond growth at HPHT conditions belongs to two-dimensional growth. Furthermore, we also found crystal direction and original shape of seed play important roles in the formation of diamond morphology in the early growth stage and the synthetic temperature will further affect the crystal shape in the following growth process.
1. Introduction It is well-known that diamond, with its outstanding physical and chemical properties, is a promising material for mechanical and electronic applications.1-3 There has been a long history for people to make use of natural diamond, but it is still unclear about the genesis of natural diamond. In 1797, Smithson Tennant proved beyond any doubt that diamond is made of carbon absolutely.4 Since diamond was synthesized successfully using transition metals as solvent-catalyst in 1955,5 a large amount of research has been devoted to investigating the forming processes of and on natural diamonds.6-13 And they probably can be classified into two categories: the first route is to choose many inorganic minerals as catalysts to simulate the forming environments of natural diamond, including carbonates, oxides, hydroxides, chlorides, phosphorus, and numbers of other compounds,14-17 and the other route concerns the effects of the minor elements and impurity diffusion on the diamond growth process, especially the nitrogen,10,13,18,19 and effects of hydrogenation.20-22 On the basis of the previous experiments, the essential dependence of diamond morphology and nucleation peculiarities on the composition of crystallization medium was established. However, there is still a lack of general estimates for the mechanism of diamond nucleation, growth model, and formation of crystal morphology. Recently, seed-assisted synthesis has been performed to accelerate the crystal nucleation and growth and also to help in the study on the crystal growth mechanism.23-27 Sung et al.23 reported that the diamond seeds could improve the distribution of nuclei and specific diamond shapes could be formed on the seeds under high pressure and high temperature (HPHT). However, the formation process is still unclear, since the growth process of diamond is difficult to be observed directly under the HPHT conditions. Furthermore, J. Hirmke et al.27 have investigated the influence of seed particles in hot filament chemical vapor deposition (CVD) and proposed that imperfections in the r 2010 American Chemical Society
rough surface of seeds did not impede the development of flat smooth surface, when the grain size of seeds is not larger than 5 μm. Unfortunately, the diamonds grown on the seeds with a grain size larger than 5 μm have poor qualities, such as undefined morphology with twining planes and shapes, which is detrimental to the research on diamond formation. In contrast to the CVD diamond growth, the diamond growth using catalyst under HPHT is confirmed to be the reasonable alternative to the research on growth mechanism of natural diamond.28 Thus, more attention should be paid on the study of the diamond epitaxial growth on some larger seeds by HPHT process to get a further understanding of the diamond growth mechanism. In this work, to obviously distinguish the crystal seeds from the new grown diamonds, we choose boron-doped diamonds, black in color, as seed crystals. The size of the seeds is about 0.1 mm. In our work, we also could not produce evidence on the nucleation process; nevertheless, we argued that the energy barrier of the diamond spontaneous nucleation is different from that of growth on seed from the thermodynamic point of view. Having in mind, the synthesis conditions for diamond spontaneous nucleation and epitaxial growth were all studied in this work, and a quantitative result was obtained. In addition, we separated some diamond fracture surface to examine the interface between the grown diamond and seed crystals, and it is expected to give some clues to the early growth stage. Furthermore, we established a direct correlation between the morphology of the diamond and the original shape of the seeds by tracking particular shapes of the seeds before and after treatment under HPHT conditions. Our ultimate goal is to provide some valuable information and evidence about the nucleation and growth of diamond at HPHT conditions and pave the way for the future study. 2. Experimental Procedures Experiments of diamond crystallization were carried out in a china-type large volume cubic high-pressure apparatus (CHPA) Published on Web 06/18/2010
pubs.acs.org/crystal
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Crystal Growth & Design, Vol. 10, No. 7, 2010
Liu et al.
Table 1. Experimental Results run (N)
P (GPa)
T (°C)
time (min)
diamond growth in system (N)
diamond growth in system (M)
S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 S-10 S-11 S-12 S-13 S-14 S-15 S-16 S-17
4.6 4.6 4.6 4.7 4.7 4.7 4.8 4.8 4.8 5.0 5.0 5.0 5.0 5.0 5.2 5.2 5.2
1340 1350 1360 1340 1350 1360 1350 1360 1370 1360 1370 1380 1390 1400 1380 1430 1500
20 20 20 30 30 30 15 15 15 12 12 12 12 12 30 30 30
/ þ / / þ þ þ þ þ þ þ þ þ þ þ þ þ
/ / / / / / / þ / / þ þ þ þ þ þ þ
(SPD-6 1670) with sample chamber of 38 mm edge length. The temperature was measured in each experiment using a Pt RH/Pt Rh6 thermocouple, whose junction was placed near crystallization sample. Pressure was calibrated at room temperature by the change in resistance of standard substances and at temperature by the graphite-diamond equilibrium. The starting materials were the high purity graphite rod (99% purity) as carbon source and Fe-Ni alloy powder (200 mesh) as solventcatalyst. The graphite power and Fe-Ni alloy (1:1, weight ratio) were mixed with and without diamond seed crystals for 4 h and then were machined into samples for synthesizing diamond, respectively. Growth runs were performed a fixed pressure in the range of 4.6-5.4 GPa and temperature of 1300-1500 °C with holding different time. After experiments, crystallization sample column were first cracked and dissolved in hot acids of mixture of H2SO4 and HNO3 to remove the remaining graphite and metal catalyst. An optical microscope and scanning electron microscopy (SEM) are explored to identify the recovered diamond morphology. Else, Raman spectroscopy is another most commonly used investigative technique to determine the quality and characteristics of diamond because of its fast and nondestructive advantage.29-31 Thus, the interface between the seed crystal and the grown diamond was directly observed using the SEM and laser Raman spectra. By combining the three common and convenient techniques, a general trend between the grown diamond and seed crystal is established in this work.
3. Results and Discussion 3.1. P-T Diagram of Diamond Synthesized at HPHT Conditions. We all know that, in the P-T phase diagram of carbon, the district for diamond synthesis is a V-shape region bounded by diamond-graphite equilibrium line and metaldiamond eutectic line in the metal solvent-carbon system.32 We investigated this district in detail in the Fe-Ni-C system performed at 4.6-5.4 GPa and temperatures ranging from 1300 to 1500 °C. Spontaneous diamonds crystallized in solventgraphite system (denoted as M) and diamond epitaxial growth on seeds in solvent-graphite system with seed addictive (denoted as N), were both obtained by the film growth (FG) processes. The experimental results are summarized in Table 1. From Table 1, at 4.6 GPa, there is no diamond spontaneous nucleation in system M at the temperature of 1340-1360 °C (S-1-S-3), while the diamond seed can exist steadily and grow up at a low growth rate of 60-80 μm/h at 1350 °C in system N (S-2). In experiments performed at 4.7 GPa and relative temperature of 1350 (S-5) and 1360 °C (S-6), metastable regrown graphite and diamond growth on seed crystal were found in system N. More importantly, besides the diamond growth on seed, some spontaneous nucleation of diamond was found at 1350 °C in system N in the run S-5. However, spontaneous
Gr-Dm a% transformation (N/M)
diamond morphology
size (mm) (N/M)
