**3.2 Ultra-high pressure and ultra-high temperature (UHPHT) technology**

Ultra-high pressure and ultra-high temperature technology is a cutting-edge technology. At present, the technology is mainly focused on the research of nano-polycrystalline diamond (NPD). However, their industrial applications are limited by tiny sample sizes. This chapter will introduce new UHPHT techniques to create centimeter-sized samples that are large enough for industrial and scientific applications. Expanding the sample chamber is an important goal in UHPHT device development, and its record is constantly being refreshed. In 2003, Irifune et al. successfully synthesized millimeter-scale nanocrystalline polycrystalline diamonds using a hexagonal large-cavity hydrostatic pressure device [8, 15] under high pressure conditions of about 15 GPa. After nearly 10 years of improvement, the size of synthetic NPD has been increased to the centimeter level. Large tonnage high pressure devices are required to obtain larger sample sizes and to ensure reasonable high-pressure efficiency. The high-pressure occurrence efficiency is mainly affected by the load loss in the transmission process, whether it is the mechanical structure of the assembly or the strength of the final stage of anvil material. Please refer to the reference [12] for more details in NPD development and their performance. In a combination of multi-stage loading, for the first time a two-stage loading device that is integrated directly into the first six-sided cubic pressure chamber was developed [16], eliminating the intermediate conversion process of loading by a single axis. Compared to type 2–6-8 loading based on belt press technology, tri-axial loading significantly improves the transmission efficiency of loads. The utility of a new type of superhard material, such as NPD and cubic boron nitride, depends heavily on the sample size and pressure required for bulk materials, typically around 14 GPa. Therefore, the challenge in developing a large cavity static pressure device should be to increase the pressure limit while expanding the cavity. Ultra-high-pressure technology is based on a hinge six-sided cubic press, with a single cylinder load capacity of about 50MN (5000 tons) in the centimeter cavity. On the other hand, the technology capable of producing large tonnage six-sided cubic presses is cost-effective, which will expand its range of applications. Millimeter-scale samples are still limited to the physical properties studied. The application prospect of centimeter-level samples in comprehensive physical characterization and tool device preparation is broad. Therefore, the ability to integrate the production of centimeter-level samples on a single-axis (double-sided) press and the development of centimeter-level high-pressure chambers with pressures greater than 14 GPa are of great significance for the high-pressure research and application of new superhard materials.
