The story of nano-polycrystalline diamond (NPD) begins in the middle of 1980s, when I was working as a post doc at the Research School of Earth Sciences, Australian National University (ANU) in Canberra. I started a new project on phase transitions and associated density changes in oceanic crust compositions to explore the behaviors of the oceanic lithosphere subducted deep into the mantle using multianvil high pressure apparatus.
One day in 1985, the high pressure run was unsuccessful, and I had to terminate it after a few minutes at a pressure of 15 GPa and at a temperature of probably about 1800 K. The temperature was very uncertain because of the unstable heating due to malfunctioning of the heater-thermocouple system. The recovered cell assembly was a total mess and the sample had gone somewhere, but I found a small piece of colorless and transparent glass-like material in the wreckage of the cell. At the first glance, I thought it was diamond, because I used graphite as a capsule for the powdered rock sample. However, I did not examine this material in further detail simply because my main target was different from diamond.
Soon after I took up an associate professor position at Ehime University in 1989, I started to reproduce this glassy material with some students under P, T conditions of 15–25 GPa and 1500–2300 K using a graphite rod as the starting material. We succeeded in synthesizing almost pure diamond by direct conversion of graphite in the early 90s, but the resultant diamond was always dark gray or black in color and quite different from what I saw in Canberra. As my students lost interest in working on this rather stupid and dream-like subject of direct conversion of graphite to transparent diamond, I was forced to continue this project alone since then.
In 1995, I changed the heating system to produce further higher temperatures, as there was some chance that the temperature of the run in 1985 was much higher than I thought due to the unstable heating and thermocouple failure. I made a run at 22 GPa and at a temperature of 2500 K for 5 minutes, and finally succeeded in reproducing the transparent material obviously converted from the graphite rod. The sample was so transparent and colorless (partly because the thickness of the sample was only about 0.3 mm) that I initially thought it was single crystal diamond as I had never seen such transparent polycrystalline diamond, but X-ray diffraction clearly showed the polycrystalline nature of this diamond.
Since then, I and some students worked hard to specify the P, T conditions where such transparent polycrystalline diamond can be formed. The results were first presented by one of the students as a poster paper in the annual meeting of the Japan Society of High Pressure Science and Technology (JSHPST) held in 2001, which happened to be the year when the Geodynamics Research Center (GRC) was founded at Ehime University. The presentation caught attention of an expert in synthesis of single crystal diamond at a major company, Sumitomo Electric Industries (SEI), and a collaboration started between GRC and SEI to further explore the nature of this highly transparent polycrystalline diamond. The polycrystalline diamond was then found to have a peculiar texture with a mixture of granular and lamellar diamond crystals in the nano-regime (<100 nm). More interestingly, this nano-polycrystalline diamond (NPD) exhibited extremely high hardness, even harder than any single crystal diamond. The results were first reported in Nature in 2003, followed by a number of related articles in various scientific journals.
As the NPD samples at this stage had dimensions of only ∼1 mm with many cracks, efforts have been made to synthesize larger NPD samples with higher quality, particularly after a large-volume multianvil apparatus operated in a 3000-ton hydraulic press (ORANGE-3000) was installed at GRC in 2003, followed by even a much larger apparatus with a 6000-ton press (BOTCHAN-6000) in 2009. The collaboration between GRC and SEI worked well and NPD samples with dimensions up to about 10 mm in both diameter and length without any visible cracks and impurities are now available using BOTCHAN-6000. The NPD was successfully commercialized by SEI as some special industrial tools in 2011, while it has been provided to the high pressure science community worldwide by GRC within the framework of joint research collaboration in the Joint Usage/Research Center, PRIUS, founded in 2013.
We had the first international symposium on scientific applications of NPD at GRC in March 2019 (Science and Technology of Nano-Polycrystalline Diamond, NPSTD-2019), where some new techniques and results were presented using advantages of the outstanding nature of NPD, such as nano-polycrystallinity, ultra-hardness, high thermal resistance, high transparency, relatively large size, ease of machinability by laser and so on, over single crystal diamond. At the same time, the problems and limitations of NPD were extensively discussed in the symposium for its further improvement.
This special volume of High Pressure Research is basically the proceedings of NPSTD 2019 with some additional contributed papers on related topics. Personally, I am going to officially retire from my current position at GRC in March 2020, at the same time when this volume is published, and am very happy to see that NPD has been used for many new devices and techniques in a variety of research fields, such as high pressure mineral physics, solid state physics, inorganic chemistry, materials science and so on 35 years after it was first witnessed. I hope this volume will contribute to the further developments of the NPD technology and advancement of cutting-edge studies in these and wider research fields relevant to high pressure science.
Finally, I would like to sincerely thank the authors and the reviewers for their great efforts in publishing this special volume, and also thank those who contributed to the NPD technology at various stages of its development. Thanks are also due to the Ministry of Education, Culture, Sports, Science and Technology (MEXT) for supporting the Premier Research Institute for Ultrahigh-pressure Science (PRIUS) and to Japan Society for Promotion of Science (JSPS) for Grant-in-Aid for Scientific Research on Innovative Areas (No. 15H05829).