Crucible is a cup-shaped vessel used in the chemical industry, which was first used in alchemy experiments and is used to hold liquid or solid for high temperature heating, which is the basis for ensuring the smooth progress of chemical reactions.
Crucible material requirements heat resistance, strong, and at high temperatures are not easy to chemical reaction, the earliest crucible in history is made of clay, development to modern times has been able to melt or change its contents of any material, such as quartz ceramics, corundum, boron nitride, zirconia, graphite and platinum, nickel, chromium and other metals.
Boron Nitride is an advanced ceramic material with broad application prospects, which is composed of nitrogen atoms and boron atoms crystal, chemical composition of 43.6% boron and 56.4% nitrogen, with four different variants: Hexagonal boron nitride (HBN), cubic boron nitride (CBN), Rhombic boron nitride (RBN), wurtzite boron nitride (WBN).
Because the boron nitride material has a comparable coefficient of thermal expansion with quartz, but the thermal conductivity is 10 times that of the latter, the thermal shock resistance is quite good, which can reduce the risk of rupture caused by rapid changes in temperature, and there is no problem with several cycles at 20~1200 ° C.
In addition, boron nitride and acid, alkali, glass and most metals do not react, mechanical strength is low, only slightly higher than graphite, but there will be no load softening phenomenon at high temperatures, can be processed by the general metal processing machine, so it is indeed suitable for melting, evaporation of metal crucible, utensils, liquid metal conveying pipe and casting steel mold.
Boron nitride crucible currently on the market is divided into two types:
PBN Crucible
Usually, boron containing gas (BCl3 or B2H6) is used as a raw material and is produced by chemical vapor deposition method, but because B2H6 is highly toxic, BCl3 is currently used as a raw material. The boron-containing gas is pyrolyzed (1500-1800 ℃) and reacts with NH3 in a high-temperature reaction chamber to form boron nitride solids, and the chemical equation is as follows. Because there is a pyrolysis reaction in the reaction, it is also called the pyrolytic boron nitride crucible (commonly known as PBN crucible).
The growth process of PBN material is similar to "falling snow", that is, the hexagonal BN snow flakes grown in the reaction are constantly piled on the heated graphite matrix (core mold), with the extension of time, the accumulation layer is thickened, that is, the PBN shell is formed, and the mold is removed from the independent and pure PBN components, and the PBN coating is left on it.
Because the PBN crucible does not have to go through the traditional hot pressing sintering process, without adding any sintering agent, it has a high purity (99.99% or more), and the use temperature under vacuum is as high as 1800 degrees, and the use temperature under atmosphere protection is up to 2100 degrees, (generally with nitrogen or argon). It is mostly used for evaporation/molecular beam epitaxy (MBE)/GaAs growth. In addition, due to the slow deposition rate, the price of PBN crucible is quite expensive (mostly small size crucible).
BN crucible
Sintered BN crucible is the use of hexagonal crystal system of boron nitride and sintering aid (Y2O3, etc.) as raw materials, after forming by high temperature sintering production, also has good heat resistance, thermal stability, thermal conductivity, high temperature dielectric strength, can resist most of the corrosion of molten metal.
However, because the sintered BN crucible contains a sintering aid (1~6wt%), the purity is not as high as the PBN crucible. However, the price is relatively cheap, suitable for the production of large-size crucible, which is used in inert gases such as argon or nitrogen, the maximum temperature is 2800℃; The stability in oxygen is poor and can only be used below 900 ° C.
Application of Boron Nitride Crucible
Although the cost of boron nitride crucible will be higher, because it has good thermal shock characteristics, corrosion resistance, lubrication, high temperature insulation and high temperature non-reactivity advantages, it is quite practical in specific fields.
For example, between the excellent chemical stability of P-BN and the above high temperature insulation properties, high thermal conductivity, low thermal expansion properties, it is very suitable for use as a material in the semiconductor process and other strict environmental conditions, such as gallium arsenide, gallium phosphide, indium phosphide.
At the same time, because the boron nitride crucible has good machining performance, extremely high temperature resistance and dielectric strength, it can also be used to make insulating materials or glass fixture for various heaters, heating tube sleeves and high temperature, high frequency and high pressure heat dissipation materials.
