The large-chamber synthesis process for the cubic press (such as 38mm and 40mm chambers), in contrast to traditional small-chamber processes (such as 28mm and 30mm chambers), faces challenges because the expansion of the chamber size alters the state of the pressure and temperature fields within the synthesis cavity. Optimizing the pressure and temperature control modes to fully exploit the advantages of large chambers has become the primary challenge for diamond factories.

I. Pressure Field Challenges: Increased Pressure Gradient and Difficulty Maintaining Stable Conditions

The growth of high-quality diamond single crystals requires relatively stable pressure conditions. However, the expansion of the chamber, combined with phase changes during the synthesis process, contributes to the challenge of generating a greater pressure gradient inside the synthesis cavity.

1. Increased Pressure Gradient Due to Transmission Loss The enlargement of the synthesis chamber inevitably leads to an increase in the pressure difference (i.e., the pressure gradient increases) between the outer shell and the core of the synthesis rod, which is caused by pressure transmission loss.

2. Volume Contraction and Pressure Drop from High Temperature and High Pressure Phase Changes The diamond growth process also causes changes in the pressure field. Under high temperature and high pressure, the transformation of graphite into diamond, using pyrophyllite as the pressure-transmitting medium, involves a series of phase changes:

The pyrophyllite mineral phase change produces kyanite and coesite.

The graphite phase change produces diamond. Because the specific gravity of these phase-change products is high, volume shrinkage occurs before and after the phase change, resulting in a drop in internal pressure within the synthesis chamber.

3. Poorer Pressure Transmission Due to Increased Rigidity After the phase change, the friction coefficient and strength of the pyrophyllite increase, causing it to become rigid. This rigidity negatively affects the effectiveness of pressure transmission and pressure boosting. All these factors together lead to a greater pressure gradient inside the cavity.

Core Challenge: The challenge for large-chamber processes is how to better reduce pressure loss and compensate for the pressure gradient.

II. Temperature Field Challenges: Maintaining Uniformity and Stability

The temperature field within the synthesis cavity is affected by two factors: heating and heat dissipation. Although the enlargement of the chamber objectively provides conditions for forming a more balanced and stable temperature field—meaning the spatial proportion that meets the required temperature conditions for the growth of high-quality diamonds will be greater in large chambers—practical control challenges persist.

1. Generation of the Temperature Gradient The temperature field in the synthesis cavity is established through direct electric heating. The temperature gradient is generally considered to be generated by heat dissipation, and the temperature is expected to decrease gradually from the rod core outward.

2. Difficulty in Maintaining Equilibrium In actual production, the phenomenon of asynchronous diamond growth between the rod core and the exterior often occurs, and the temperature gradient is cited as one of the causes. The critical challenge for temperature control is to realize or approach the balance point between heating and heat dissipation (i.e., a "heat preservation state") by adjusting the heating power appropriately and timely within the limited synthesis duration. Achieving this heat preservation state helps effectively reduce temperature changes and the temperature gradient.