High-pressure high-temperature (HPHT) synthesis is a well-established method for producing bulk diamond and cubic boron nitride (cBN) crystals. These materials exhibit exceptional hardness, thermal conductivity, and electronic properties, making them valuable for industrial and scientific applications. The process involves subjecting carbon or boron nitride precursors to extreme pressures and temperatures in the presence of catalyst metals, facilitating the transformation into their crystalline forms. This article details the HPHT synthesis process, including catalyst selection, anvil designs, and phase stability, while contrasting it with chemical vapor deposition (CVD) diamond growth.

The HPHT synthesis of diamond typically employs a carbon source, such as graphite, alongside a metal catalyst, including nickel (Ni), iron (Fe), or cobalt (Co). These catalysts lower the activation energy required for the conversion of graphite to diamond by dissolving carbon and precipitating it in the diamond phase. The process occurs within a pressure range of 5 to 7 GPa and temperatures between 1300 and 1600°C. The phase stability of carbon under these conditions is governed by the carbon phase diagram, where diamond becomes the thermodynamically stable phase at high pressures and temperatures. The catalyst metals form a molten solvent that enables carbon atoms to rearrange into the diamond lattice structure.

Similarly, cubic boron nitride (cBN) is synthesized from hexagonal boron nitride (hBN) using alkali or alkaline earth metals as catalysts, such as lithium (Li) or magnesium (Mg). The synthesis conditions for cBN are slightly more demanding, requiring pressures of 5 to 8 GPa and temperatures of 1500 to 2000°C. The phase stability of boron nitride is analogous to carbon, with hBN being the stable phase at ambient conditions and cBN forming under HPHT conditions. The catalyst facilitates the dissolution of hBN and subsequent precipitation of cBN, similar to the diamond synthesis mechanism.

The anvil designs used in HPHT synthesis are critical for achieving the necessary pressure and temperature conditions. The most common apparatus is the belt-type press, which employs two anvils to compress the sample chamber radially. Another widely used design is the cubic press, which utilizes six anvils to apply uniform pressure from multiple directions. The toroidal anvil design is also employed for larger volume synthesis, offering improved pressure distribution. These anvils are typically made from tungsten carbide (WC) or sintered diamond due to their high compressive strength and thermal stability. The sample chamber, often constructed from pyrophyllite or other pressure-transmitting materials, houses the carbon or boron nitride precursor along with the catalyst.

The growth process begins with the assembly of the precursor material and catalyst within the sample chamber. The chamber is then subjected to increasing pressure and temperature until the desired conditions are reached. The catalyst melts, dissolving the precursor material, and upon supersaturation, diamond or cBN crystals nucleate and grow. The growth duration varies from several minutes to hours, depending on the desired crystal size and quality. Post-synthesis, the material is slowly cooled and depressurized to preserve the crystalline structure.

In contrast, CVD diamond growth occurs at lower pressures (typically below 1 atm) and moderate temperatures (700 to 1000°C). The process involves the dissociation of hydrocarbon gases, such as methane (CH4), in a hydrogen (H2) plasma, leading to the deposition of carbon atoms onto a substrate. The absence of a metal catalyst in CVD growth results in higher purity diamonds, as metallic inclusions are avoided. However, CVD-grown diamonds are typically limited to thin films or small crystals, whereas HPHT synthesis produces bulk crystals suitable for industrial applications like cutting tools and abrasives.

The choice between HPHT and CVD methods depends on the intended application. HPHT is favored for producing large, high-quality single crystals, while CVD excels in creating ultra-pure films for electronic or optical applications. The HPHT process also allows for the incorporation of dopants, such as boron or nitrogen, to modify the electrical properties of diamond or cBN. For instance, boron-doped diamond exhibits p-type semiconducting behavior, making it useful for electronic devices.

Phase stability diagrams for carbon and boron nitride provide critical insights into the HPHT synthesis conditions. The carbon phase diagram shows that graphite is stable at low pressures, while diamond becomes favorable above approximately 2.5 GPa at room temperature. At higher temperatures, the transition pressure increases slightly. The boron nitride phase diagram exhibits a similar trend, with hBN stable at ambient conditions and cBN forming above 5 GPa. The presence of catalysts shifts these phase boundaries, reducing the kinetic barriers for transformation.

The HPHT synthesis of diamond and cBN has evolved significantly over the decades, with advancements in anvil designs and catalyst optimization enabling larger and higher-quality crystals. Despite the progress, challenges remain, such as minimizing defects and controlling crystal morphology. Future developments may focus on alternative catalyst systems or hybrid techniques combining HPHT and CVD for tailored material properties.

In summary, HPHT synthesis is a robust method for producing bulk diamond and cBN crystals, leveraging extreme pressures, temperatures, and catalyst metals to achieve phase transformation. The process contrasts with CVD growth in terms of pressure regimes, scalability, and crystal quality. Understanding the phase stability and growth mechanisms is essential for optimizing synthesis conditions and expanding the applications of these advanced materials.