The difference between pyrolytic boron nitride and hot pressed boron nitride

Boron nitride is an advanced ceramic material, the most common of which are cubic boron nitride (c-BN) and hexagonal boron nitride (h-BN). c-BN is similar to diamond and is mainly used to make cutting tools; H-BN is a white powder with a graphite like layered structure and lattice parameters, also known as white graphite. It has excellent thermal conductivity and insulation properties, making it the most highly valued crystal form. The difference between PBN and HBN ceramics as follows:

Hot pressed boron nitride

Hot pressed boron nitride is produced by using hot pressing sintering technology. Hot pressed boron nitride is an excellent electrical insulator with excellent lubricity and high-temperature stability, which can maintain lubricity and inertness even at extremely high temperatures. Boron nitride has relatively poor mechanical properties, but it has high thermal capacity, excellent thermal conductivity, outstanding dielectric strength, and ease of processing. In an inert atmosphere, boron nitride can withstand temperatures exceeding 2000℃, making it an ideal high-temperature thermal insulation material. By utilizing the excellent chemical stability of boron nitride ceramics, they can be used as crucibles for melting and evaporating metals, boats, liquid metal transport pipes, crucibles for synthesizing GaAs crystals, rocket nozzles, high-power device bases, pipelines for melting metals, pump parts, cast steel molds, insulation materials, etc.

Pyrolytic boron nitride

The preparation process of pyrolytic boron nitride is significantly different from the former, which uses chemical vapor deposition technology to deposit ammonia and boron halides by chemical vapor deposition (CVD) under high temperature and high vacuum conditions. It can be deposited into PBN thin plate materials or directly deposited into PBN final products such as pipes, rings, or thin-walled containers.

This boron nitride prepared by high-temperature pyrolysis reaction has high purity, high thermal conductivity, high mechanical strength, good electrical insulation and non-toxic properties, chemical inertness, and excellent structure and properties, making it an ideal container for element purification, compound and compound semiconductor crystal growth. The main applications include OLED evaporation units, semiconductor single crystal growth (VGF, LEC) crucibles, molecular beam epitaxy (MBE) evaporation crucibles, MOCVD heaters, polycrystalline synthesis boats, insulation plates for high temperature and high vacuum equipment, etc.

The main characteristic of pyrolytic boron nitride is its extremely high purity, which can reach up to 99.999% or more (hot pressed boron nitride usually has a purity of only about 99%). The main reason for this is that its preparation process does not require the addition of any sintering agents. Therefore, pyrolytic boron nitride has many unique characteristics, such as excellent chemical and thermal stability, no pores, and good density (its density is close to the theoretical density value of the material).

Comparison of application performance between PBN and HBN ceramics:

There are many similarities between pyrolytic and hot pressed boron nitride in applications, such as evaporation crucibles, melting crucibles, insulation boards, etc. In practical applications, there may be differences due to their different properties. The total impurities of PBN products are usually less than 100 ppm, which means the purity is not less than 99.99%. Such high purity makes PBN crucibles more favored by the semiconductor industry, and can be used as OLED evaporation units, semiconductor single crystal growth (VGF, LEC) crucibles, etc. Thanks to its high density and purity, PBN is also a widely used material in vacuum processes, such as insulation boards for high-temperature and high vacuum equipment.

However, in some applications, cost needs to be considered. When melting metals, PBN crucibles have high density and no pores, making it difficult for molten metals to penetrate into the crucible walls. When using small crucibles to melt titanium and its alloys, even when cooled to room temperature in the furnace, they are easily poured out and do not bond. PBN is expensive and generally compact in size, because even if using PBN crucibles has better results, their cost is generally considered in industrial production.

In addition, hot pressed boron nitride is easy to process, making it easier to obtain the desired shape and size according to requirements; Secondly, it is more cost-effective and suitable for use as insulation components for high-temperature furnaces, thermocouple protection tubes, crucibles or molds for molten metals, amorphous ribbon nozzles, and powder metal atomization nozzles, among other high-temperature components; Finally, some hot pressed boron nitride ceramics can be used to replace PBN, but the important prerequisite for achieving it is to control the impurity content including oxygen, silicon, aluminum, etc in the powder raw materials, with oxygen content being the main one.