Boron nitride is an interesting advanced ceramic material. Although it’s also an electric insulator, low coefficient thermal expansion and has great corrosion resistance, the hardness is quite low (only 4, similar with graphite). Compared to more conventional alumina and zirconia, it's a machinable material due to the low hardness. Pyrolytic boron nitride is a thin (1~3mm) boron nitride material produced by CVD process.
When normal hot pressed boron nitride acts more like graphite, except for the electric conductivity, pyrolytic boron nitride has quite a lot different properties due to the structure. As the material is “grown” from chemicals around in gas phase, PBN by nature has a layered structure. The strong, thin and paralleled slices of PBN give this material some flexibility, which is abnormal for most of ceramics, including hot pressed boron nitride. Although thin and semi-transparent, PBN is not that brittle as it looks like. It is actually quite hard to break a PBN thick disc.
*It takes the quite large force to take this 3mm PBN disc apart, and it’s significantly layered before broken.
Hot Pressed Boron Nitride material is used to make Boron Nitride crucible for growing single crystals in most cases. For this application, it would be a serious issue if the slices tear apart from each other. As the crystal growth process requires accurate temperate control, the lower thermal conductivity caused by the layered structure will be harmful. During the manufacturing, it is common to build up some internal force inside the PBN material, especially on the tips, and this force is the major reason for layered defects for PBN products. That's why the VGF crucible is harder to make and cost considerably more than simple shape crucibles.
It is quite simple to examine if a PBN part is layered. As the material is usually quite thin, it is semitransparent. Under strong lights, there will be the shadow at the layered area. Though it looks normal from outside, layered products should be treated as unqualified products.
*Layered crucible tip. It can be detected with the LED on a cell phone
For more information, please visit http://www.samaterials.com/
When normal hot pressed boron nitride acts more like graphite, except for the electric conductivity, pyrolytic boron nitride has quite a lot different properties due to the structure. As the material is “grown” from chemicals around in gas phase, PBN by nature has a layered structure. The strong, thin and paralleled slices of PBN give this material some flexibility, which is abnormal for most of ceramics, including hot pressed boron nitride. Although thin and semi-transparent, PBN is not that brittle as it looks like. It is actually quite hard to break a PBN thick disc.
*It takes the quite large force to take this 3mm PBN disc apart, and it’s significantly layered before broken.
Hot Pressed Boron Nitride material is used to make Boron Nitride crucible for growing single crystals in most cases. For this application, it would be a serious issue if the slices tear apart from each other. As the crystal growth process requires accurate temperate control, the lower thermal conductivity caused by the layered structure will be harmful. During the manufacturing, it is common to build up some internal force inside the PBN material, especially on the tips, and this force is the major reason for layered defects for PBN products. That's why the VGF crucible is harder to make and cost considerably more than simple shape crucibles.
It is quite simple to examine if a PBN part is layered. As the material is usually quite thin, it is semitransparent. Under strong lights, there will be the shadow at the layered area. Though it looks normal from outside, layered products should be treated as unqualified products.
*Layered crucible tip. It can be detected with the LED on a cell phone
For more information, please visit http://www.samaterials.com/
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