Hydrothermal synthesis, diamond

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Perhaps the most dramatic example of hydrothermal synthesis of gems is the recent synthesis of diamond from graphite in scHzO at 800 °C and 1700 bar... 

As described in more detail in the contribution on hydrothermal synthesis of diamond in this book, hydrothermal reactions start for fine powders at 700°C, but a significant etching and dissolution of large diamond grains is observed only at F > 800°C [51]. 

In this chapter, we explain why hydrothermal synthesis deserves special consideration and present a review of the current state of the art in hydrothermal synthesis of diamond as a potential future method of making diamond or hard diamond-like sp -carbon. 

This publication is a good example of the development of analytical techniques such as high resolution electron microscopy lattice imaging [108] and micrometre-area Raman spectroscopy (microRaman) [109] which permit the researcher to determine that diamond is present in crystallites of submicrometre, perhaps even nanometre size. Early scientists like Moissan [110] and Hannay [111] performed experiments in the pressure and temperature ranges now being cited as feasible for hydrothermal synthesis of diamond [38, 112, 113]. Had they been equipped to use the analytical techniques now available, they may have been proven correct in their claims, but in a particle size regime much finer than they were able to inspect. 

R. Roy, D. Ravichandran, A. Badzian, and E. Breval, Attempted hydrothermal synthesis of diamond by hydrolysis of 3-SiC powder. DiamondRel. Mater, 5, 973-976,1996. 

A. Szymanski, E. Abgarowicz, A. Bakon, A. Niedbalska, R. Slalcinski, and J. Sentek, Diamond formed at low pressures and temperatures through liquid phase hydrothermal synthesis. DiamondRel. Mater., 4, 234-235, 1995. 

Figure 1. P T diagram of the low-pressure, low-temperature labile equilibriums of carbon solution 1 = graphite-diamond equilibrium line, 2 = glassy carbon-diamond transition line, 3 = range of pneumatolytic hydrothermal processes, 4=oxidative corrosion of diamond, 5 = anticipated area of diamond hydrosynthesis, 6 and 7 = diamond synthesis from glassy carbon precursors, 8 = low-pressure, low-temperature hydrothermal homoepitaxy of diamond. Reproduced from [15] with permission from A. Szymanski.

The discussion of hydrothermal diamond synthesis is divided into two sections, dealing with synthesis from C-H-0 liquids and synthesis based on decomposition of silicon carbide, respectively. Both start with thermodynamic calculations in order to demonstrate the theoretical possibility of carbon formation before the experimental findings are summarized. Naturally, equilibrium calculations do not consider kinetic limitations. 

Successful hydrothermal diamond synthesis was carried out in autoclaves filled with a specially prepared carbon enriched water solution , the composition of which was not disclosed [15,45,46]. The carbon precursor should be fine-grained diamond, vitreous carbon or emulsion of crude oil and water [29]. The presence of free radical catalysts was mentioned and paragenetic crystallization of quartz needles and diamond indicate the presence of silicon [45]. The synthesis was described as a sol/gel colloidal process working in the range 200-600°C and 100-200 MPa. Healing and joining of diamond crystals was reported. After 21 days at 400°C and 170 MPa, thin colorless films of polycrystalline diamond were obtained on (111) surfaces of seed crystals (Fig. 3c). With a reported size of 15-40 pm, these are the largest diamond crystals from hydrothermal experiments. 

Comparison of CVD- and Hydrothermal Diamond Synthesis from the SiC-H20 System... 

Current understanding of the diamond synthesis under hydrothermal conditions and quantity of diamond produced are comparable with those of CVD synthesis of diamond at the end of the 1950s or the beginning of the 1960s. Thus, we are at the beginning of the road and further research is strongly recommended. 

N. Yamasaki, K. Yokosawa, S. Korablov, and K. Tohjt, Synthesis of diamond particles under alkaline hydrothermal conditions. Solid State Phenomena, 114, 271-276, 2006. 

S. Korablov, K. Yokosawa, T. Sasaki, D. Korablov, A. Kawasaki, K. loku, H. Ishida, and N. Yamasaki, Synthesis of diamond from a chlorinated organic substance under hydrothermal conditions. J. Mater. Set, 42, 7939-7949, 2007. 

There are no obstacles in searching for novel approaches to diamond synthesis by studying new growth processes electrolysis, hydrothermal (15) and laser assisted. Graphite has been transformed to diamond by laser process (16,17) and some laser assisted CVD processes were... 

---