Solubility and stability of diamond in cobalt under 5 GPa
Introduction
In 1797, English chemist Smithson Tennant deduced that diamond comprises carbon atoms. Since then, the transformation of graphite and other non-diamond carbon from carbonaceous materials to precious diamond via the simulation of the tremendous pressure and temperature conditions that natural diamond growing inside the earth is subjected to is considered possible [1]. Since the first successful experiment on artificial diamond reported by General Electric, a significant number of synthetic methods for the fabrication of diamond have been explored by researchers [2,3]. The synthesis of diamond is mainly based on three techniques. The first technique is conducted under relatively low static high pressure and high temperature (HPHT), using molten or nearly molten metal (generally a metal/or their alloys selected from group VIII of the periodic table of elements, mainly comprising Fe, Co, and Ni) as ‘solvent/catalysts’ for the non-diamond carbon from carbonaceous materials to diamond conversion, for example, the temperature gradient method (TGM). The second technique involves the synthesis of diamond from graphite and other carbonaceous materials directly under relatively high static or dynamic pressures and temperatures, for example, the shock compression method. The third technique involves the synthesis of diamond at atmospheric pressures by adding carbon atoms successively to an initial substrate, and is called the chemical vapour deposition (CVD) method [[4], [5], [6], [7], [8], [9], [10], [11]]. Compared with the CVD method, diamond growth under HPHT is a reasonable choice for research on the mechanism of natural diamond growth [12]. Since the successful synthesis of diamond with transition metal as a solvent catalyst in 1955, the conversion of graphite into diamond through a catalyst or solvent method has been extensively researched [4]. Today, vast quantities of diamond crystals are mainly produced through the HPHT method using a molten or nearly molten metal as solvent/catalyst. However, understanding the formation process of diamonds is a longstanding and enigmatic problem, because the direct observation of the growth process of diamond using the existing technical means is difficult under HPHT conditions. The role of metals as solvents, catalysts, or both is now intensely debated. When metal acts as a solvent, the solubility difference between graphite and diamond stimulates the transformation of graphite into diamond. When metals act as catalysts, the activation energy of graphite to diamond is reduced, thereby enabling the conversion of graphite into diamond [4].
A succession of experiments aimed at determining the mechanism of transformation to diamonds by dissolution has resulted in interesting ideas, some of which exhibit persistent challenges [[13], [14], [15], [16], [17], [18], [19], [20], [21]]. In addition, a severe lack of solubility data has hindered the progress of elucidating the aforementioned mechanism. Although Andreyev and Belousov [22] claimed that the solubility of diamonds in the melting of metals under HPHT has been investigated, experimental information and quantitative data on the solubility of diamond in the melting of metal under high pressure is lacking [22]. Additional research is required for the in-depth understanding of the aforementioned mechanism.
This study describes a series of experiments, wherein the solubility of diamond in cobalt was measured quantitatively under a diamond-stable region pressure of 5 GPa and temperatures ranging from 1000 to 1300 °C. The experimental phenomena were analysed based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) to elucidate the catalytic or solvent concepts of diamond growth. Because high-pressure diamond synthesis using cobalt as a catalyst/solvent is a suitable technique, and a binary metal‑carbon system is the simplest from a thermodynamic point of view [[23], [24], [25]], we selected the diamond‑cobalt system for the study of the dissolution behaviour of diamond at HPHT. Moreover, we hope to create significant guidelines for the growth of single diamond crystals [[26], [27], [28], [29]].
Section snippets
Preparation of starting materials
Commercially available synthetic monocrystalline diamonds (grain size 1.5–2 mm, Hubei Dishi Superhard Material Co. Ltd., China) were treated with aqua regia to remove residual impurities. The typical SEM images of the initial diamond crystals are shown in Fig. 1(a) and (b). The purchased cobalt powder (grain size 1–2 μm, 99.5% pure, Shanghai Aladdin Biochemical Technology Co. Ltd., China) was treated in a hydrogen reduction atmosphere at 900 °C for 0.5 h before the HPHT experiments. The SEM
Stability of diamond in cobalt at 5 GPa
The pressure and temperature range of the diamond-stable region in the phase diagram of carbon is a two-dimensional area: a temperature range wherein diamond will not convert to graphite at a fixed pressure point, and a pressure range wherein diamond does not graphitise at a fixed temperature point [[32], [33], [34], [35], [36]]. We performed experiments at different temperatures over a certain period of time under a pressure of 5 GPa. After HPHT treatments, the XRD beam was reflected on the
Conclusion
The diamond solubility in cobalt at 5 GPa pressure is approximately 46.16 ppm at 1100 °C, 76.36 ppm at 1200 °C, and 140.83 ppm at 1300 °C. The diamond growth region at 5 GPa forms at temperatures exceeding 1000 °C and below 1350 °C. Our experimental results show that the diamond formation in metal solvents occurs through the dissolution and precipitation of carbon atoms. The results obtained in our experiments contribute to elucidating the mechanism of diamond crystal growth in metal solvents.
CRediT authorship contribution statement
Yi Tian: Writing - Original Draft, Validation, Formal analysis, Investigation, Resources, Data Curation, Visualization. Junpu Wang: Formal analysis. Jiawei zhang: Visualization. Shixue Guan: Visualization. Lu Zhang: Investigation. Binbin Wu: Visualization. Yuzhu Su: Resources. Mengyang Huang: Investigation. Li Zhou: Investigation. Duanwei He: Conceptualization, Methodology, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key R&D Program of China (grant no. 2018YFA0305900).
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