1. Catalyst classification and preparation methods

Catalyst materials generally use transition metal elements or their alloys. According to its shape, it can be divided into: flake, powder, granular, filament, tube, foam, etc. At present, powder catalysts are mainly used, and flake catalysts have been eliminated.

The manufacturing methods of powder catalysts mainly include physical and chemical methods and mechanical crushing methods. Physical chemical methods use heating, electrolysis or chemical reactions to prepare powders from metal compounds, such as reduction methods, electrolysis methods and carbonyl methods. Mechanical method is a method of breaking solid or liquid metal (or alloy) into powder, and its chemical composition basically does not change. Typical methods include vortex grinding, ball milling and atomization.

In terms of preparing powder catalysts for diamond synthesis, mechanical powdering is more common. For example, adding certain trace elements (Si, C, etc.) to the catalyst alloy to make it brittle, and then crushing it with a ball mill can obtain a catalyst alloy of about 200 mesh. powder. Atomization powdering has advantages in the large-scale production of powder catalysts. This method can easily produce high-quality metal catalyst powder with the required ingredients, high purity, uniform structure and excellent process performance, and the shape, size and particle size distribution of the powder obtained by atomization can be adjusted within the scope. In addition, atomized powder can also use cheap raw materials (scrap metal, etc.). Its process flow is short and the equipment is simple, so the overall cost is low.

The key technology for powder catalyst production is to control the particle size and shape of the powder, especially the oxygen content in the powder catalyst. Judging from the current application of powder catalysts to synthesize diamond, atomized powder catalysts have great advantages. Using vacuum/inert gas atomization process, high-quality catalyst alloy powder with low oxygen content, uniform composition and fine structure can be prepared. However, water atomized powder catalysts have the advantage of low cost, and although the catalyst prepared by water atomization has high oxygen content, it can be effectively deoxidized after being mixed with graphite powder and subjected to vacuum heat treatment, so its development prospects are broader.

2.Characteristics of powder catalyst

Powder catalyst has the following characteristics:

(1) Large specific surface area and large contact area, which is conducive to crystal growth

A basic characteristic of powder catalysts is their large specific surface area. The rich specific surface is conducive to the contact between the catalyst and graphite, which greatly increases the reaction area between the catalyst and graphite under high temperature and high pressure conditions. If the powder catalyst can be fully mixed with graphite powder, the synthesis yield can be greatly increased.

As we all know, a uniform temperature field and pressure field are necessary conditions for the growth of high-quality diamond. The uniform mixing of powder catalyst and graphite can produce a relatively uniform temperature field and pressure field, creating good conditions for diamond growth.

(2) Uniform structure and high purity

Using powder catalyst, the content of key elements in each part is uniform and uniform, which can ensure uniform diamond growth. Since the powder catalyst is prepared using the atomization method, the cooling rate is very fast during the process from molten metal liquid to solid powder, and the deviation of each component in the catalyst can be controlled to about one thousandth.

(3) Low melting point

As the particle size of the catalyst particles decreases, its melting temperature decreases significantly. And the degree of melting point reduction is inversely proportional to the particle size of the powder. Taking advantage of this feature, the nucleation density and growth rate of diamond in the synthesis cavity can be effectively controlled through the reasonable matching of different catalyst particle sizes and the adjustment of the synthesis process.

(4) High resource utilization rate

Powder catalyst is used to produce diamond. The weight percentage of the catalyst in the synthesis cavity is about 30%, and the utilization rate is relatively high. Its composition is also easy to adjust. This significantly reduces the use of catalysts in the diamond industry.

Although powder catalyst has the above advantages, it also has the following disadvantages:

(1) The surface is susceptible to contamination

The geometric size of the powder catalyst material is very small, and its surface adsorption is greatly improved. Dust and oil will cause great pollution to it. In particular, powder catalysts are very easily oxidized. If the oxygen content in the catalyst cannot be reduced, the catalytic performance of the powder catalyst will be greatly affected.

(2) Mixing is not easy to be uniform

Since the particle sizes of graphite and catalyst are very small, and the specific gravity of the two is very different (the specific gravity of the catalyst is about 8, and the specific gravity of graphite is 2.25), it is difficult to mix the two evenly. Although adding an appropriate amount of binder during mixing can improve the uniformity of the mixture, incomplete post-processing will affect the synthesis of diamond.

In addition, if the oxidation problems of powdered catalysts during production, storage, transportation and application are not well solved, it will cause instability in the powdered catalyst synthesis process.