How pyrolytic boron nitride crucibles are produced

Pyrolytic boron nitride (PBN) crucibles are essential components for high-temperature, inert or vacuum environments, and they offer excellent thermal properties, chemical resistance, and inertness. They have become a popular choice for processes such as crystal growth and epitaxy that require high temperatures.

The production of these crucibles is a multi-step process that involves a unique technique called chemical vapor deposition (CVD). The CVD process involves the deposition of thin films of PBN on a substrate that acts as an engine in the production.

The first step in producing PBN crucibles is the preparation of the mold. The mold is commonly made from silicon carbide or graphite and polished to achieve a smooth surface. Before coating with a layer of silicon nitride, that can act as a barrier for precursor gases, it is cleaned thoroughly to eliminate contamination that could interfere with the final product.

The second step involves the preparation of the precursor gas, which is composed of boron trichloride (bcl3) and ammonia (NH3). These two gases are mixed together in a reactor chamber and heated to a temperature of around 2000°c, which causes the gases to react and form hexagonal boron nitride (h-BN).

The third step is the deposition of the PBN layer. Once the precursor gas is prepared, it is fed into the reactor chamber above the mold. As the gas flows over the mold, it reacts and deposits a layer of h-BN on the surface of the mold. The deposition process can take several hours depending on the thickness of the desired PBN layer. A single cycle typically results in 30 to 40 microns thick layer of PBN being deposited onto the mold.

The fourth step involves removing the crucible from the mold. After the PBN layer has been deposited on the mold, the reactor is cooled down, and the mold is removed from the chamber. Precision cutting is then used to extract the crucibles so as to avoid any damage to the PBN layer.

Finally, the fifth step in producing PBN crucibles is post-processing. The crucibles must be cleaned and polished to remove impurities or imperfections in the PBN layer. Various techniques such as plasma etching, mechanical polishing, or chemical treatment can be used for this purpose. After post-treatment, the PBN crucible can be used for high-temperature processes and it has a long operational life span.

In conclusion, pyrolytic boron nitride crucibles are produced using the CVD process, where a precursor gas containing boron is applied. Due to the nature of the CVD process, PBN crucibles are very pure, with very little contaminations. So, it could be used to prepare high purity materials.