The influence of iron-based powder catalysts on diamond synthesis is primarily reflected in their particle size and uniformity.
Here is a detailed description:
Role and Advantages of the Catalyst
    Iron-based powder catalysts are used in domestic six-sided top presses for synthesizing diamond single crystals under high-temperature and high-pressure conditions.
    The static high-temperature high-pressure method is currently the main synthesis method for large-scale industrial production of diamonds. Compared to flake catalysts, powder catalysts have a clear technical advantage in synthesizing high-grade diamonds, and typically yield a higher single output.
    During diamond synthesis, melting and agglomeration of the catalyst occur, and the particle size of the catalyst directly affects this agglomeration process.
Influence of Catalyst Particle Size on Diamond Synthesis
    The finer the catalyst particle size:
        The particle size of the synthesized diamond single crystals is more concentrated and their uniformity is better.
        For example, finer catalysts with particle sizes of 250-325 mesh and 325-400 mesh produce crystals with very uniform particle size, mainly concentrated between 0.3-0.5 mm.
        Finer particle size catalysts have a relatively lower optimal synthesis temperature, which may be due to the larger surface activity of fine particle catalysts and their lower eutectic point with graphite.
    The coarser the catalyst particle size:
        The uniformity of the synthesized diamond single crystals' particle size is poorer.
        The crystal particle size is non-uniform, with a few abnormally large crystals still present, such as those ranging from 0.6-1.0 mm, while most other crystals are concentrated between 0.3-0.5 mm.
        During diamond formation, larger catalyst particles can lead to the agglomeration of multiple catalyst particles, resulting in abnormally large diamond particles and causing non-uniformity in the synthesized diamond's particle size.
Influence of Catalyst Particle Size Uniformity on Diamond Synthesis
    The more uniform the catalyst particle size:
        The particle size distribution of the synthesized diamond single crystals is more concentrated.
        When sieved powder catalysts with relatively uniform particle sizes are used, the synthesized diamond crystals' particle sizes are relatively concentrated.
        Experimental results show that crystals synthesized with very uniform catalysts (especially 325-400 mesh catalysts) have a highly concentrated particle size of 0.3-0.5 mm, with complete crystal habits, mainly hexa-octahedral.
    Non-uniform catalyst particle size:
        If unsieved catalysts with dispersed particle sizes are used, the synthesized diamond crystals will have a very non-uniform particle size.
        The main characteristics are the appearance of abnormally large crystals and polycrystalline spheres. This may be related to the uniformity of the powder catalyst's particle size.
Explanation of the Agglomeration Mechanism
    The authors believe that the finer and more uniform the catalyst particles, for the same mixing ratio of graphite-catalyst system, the smaller the spacing between catalyst particles.
    Under high temperature and high pressure, catalyst agglomeration is more likely to occur in the early stage of diamond growth, making the size of the agglomerated radius almost uniform throughout the reaction process, thus resulting in more uniform diamond particle sizes.
    Conversely, the coarser the catalyst particles, the longer the time for two catalyst particles to agglomerate, and multiple catalyst particles may agglomerate, leading to the appearance of abnormally large particles and causing non-uniformity in the synthesized diamond's particle size.
Catalyst Preparation and Morphological Characteristics
    The iron-based powder catalysts used in the experiments were prepared using the rapid solidification gas atomization method.
    During gas atomization, due to the action of the gas field, powders collide with each other. Small-sized particles may adhere to larger-sized powder particles because the larger particles cool slower and have not fully solidified.
    The larger the powder particles, the more severe the adhesion phenomenon and the rougher the surface. This explains the dispersed particle size of unsieved catalysts. To study the effect of particle size uniformity, the initial powder catalyst was finely sieved to obtain 6 types of catalysts with relatively uniform particle sizes.
In summary, the finer and more uniform the particle size of iron-based powder catalysts, the more conducive it is to synthesizing diamond single crystals with concentrated particle size, complete morphology, and good uniformity.