1. Reduction in Effective Contact Area and Nucleation Rate:

     Pores, especially surface open pores, are one of the fundamental characteristics of powder catalysts and are considered a measure of their reactivity.

    The presence of surface open pores is capable of increasing the contact area between the catalyst and graphite and the diamond nucleation rate per unit volume within the high-temperature, high-pressure synthesis chamber.

    Therefore, the absence of surface open pores implies a reduction in the effective working area between the catalyst and the graphite. Correspondingly, this leads to a decrease in the diamond nucleation rate.

2. Detrimental to High-Yield Diamond Synthesis (Lower Output):

    Surface open pores are beneficial for increasing the unit output (yield) of diamond.

    In the experimental comparison, the synthesis was conducted using the same round molybdenum sheet, comparing the front side of the Ni2Mn powder catalyst which was rich in open pores (the upper half) with the relatively smooth, non-porous back side (the lower half).

    The experimental results indicate that the diamond yield in the lower half (the area without open pores) was lower than that in the upper half, which was rich in open pores. In small-batch synthesis experiments, the diamond yield in the upper half was approximately 9.86% higher than in the lower half.

    Furthermore, the diamond nucleation density in the region corresponding to the Ni2Mn powder catalyst without open pores (the lower half) was lower than that in the upper half, which contained open pores.

3. No Obvious Influence on Crystal Shape:

    The presence or absence of open pores on the surface of the powder catalyst has no significant influence on the crystal shape of the synthetic diamond.

    Under the experimental conditions, the synthetic diamonds, whether produced by Ni2Mn powder catalyst with open pores or without open pores, exhibited crystal shapes predominantly composed of equiaxed crystals.

Summary of the Mechanistic Contrast:

According to the open pore mechanism model, the existence of pores essentially enlarges the effective contact area between the catalyst and the graphite, thereby improving the unit yield of synthetic diamond.

For catalysts with open pores, fine graphite powder (blocks) may fall into and fill the open pores under external pressure. When heating, the catalyst powder shrinks due to surface tension, leading to some of the filling graphite powder being encapsulated, forming "encapsulated graphite" within the catalyst powder. This allows the carbon source to diffuse into the solvent catalyst not only from the outer surface but also from the "interior of the catalyst powder". When the carbon concentration reaches "supersaturation," the supersaturated carbon can precipitate as diamond structure on the surface layer of the catalyst powder, or crystallize and precipitate diamond from the "interior of the catalyst powder," thereby significantly increasing the nucleation rate.

For catalysts without surface open pores, this mechanism that relies on "encapsulated graphite" to enhance internal carbon source diffusion and increase the internal nucleation rate is diminished or eliminated, consequently leading to reduced nucleation density and lower final output.