Fig.
1. SEM micrograph (secondary-electron image) illustrating the platelike
particles of a refractory-grade BN powder and a sphere of molten B2O3
(center) that was not converted to BN during synthesis. |
Fig.
1 | Table 1 (chem. data) | Table
2 (production) | Table 3 (consumption)| Properties
| Synthesis | Production
| Consumption
Hexagonal
boron nitride (a-BN)
often is called white graphite, because it has a graphite-like hexagonal
layer structure. In contrast to graphite, however, BN is white and has
a high electrical resistance. The reason for these differenees is the absence
of the delocalized p-bonds
in the BN structure.
BN is a ceramic material with outstanding
chemical, thermal and electrical properties. It can be used as a high-temperature
solid lubricant, is stable at high temperatures and is not wetted by many
metallic melts, such as aluminium and magnesium. BN shows good thermal
shock resistance, high thermal conductivity and low dielectric constant.
Therefore, it is a nontoxic substitute for beryllia in certain electronic
applications.
BN does not
occur in nature. It was first prepared in 1842 by reacting molten boric
acid with potassium cyanide. In the 1950s, commercial production was initiated
by Union Carbide and Carborundum Co. Three general reactions are used for
the synthesis of a-BN: |
 |
| B2O3 + 2NH3
---> 2BN + 3H2O |
(T = 900°C) |
| B2O3 + CO(NH2)2
---> 2BN + CO2 +2H2O |
(T > 1000°C) |
| B2O3 + 3CaB6
+ 10N2 ---> 20BN +3CaO |
(T > 1500°C) |
These processes yield refractory
grades with concentrations of 92%-95% BN and 5%-7% B203.
The glassy B203 cannot be converted to BN. The B203
is removed by evaporation in a second step by reheating the product to
temperatures >1500°C. The remaining ceramic-grade product has BN concentrations
>98.5%. |
|
| |
|
Composition
|
by supplier |
(%) |
| Component |
A*
|
B*
|
C*
|
D*
|
| Boron |
42.2
|
43-44
|
43
|
42.5
|
| Nitrogen |
55.8
|
54-55
|
54.5
|
55.5
|
| Oxygen+ |
0.5
|
1.5
|
1.5
|
0.5-1.0
|
| Carbon |
0.2
|
NA
|
0.1
|
0.1
|
| B2O3 |
NA
|
0.1
|
0.1
|
0.15
|
| Metallic impurities |
0.1
|
0.2
|
0.2
|
0.1
|
Table 1. Commercially Available
Ceramic-Grade BN Powders
* NA is data is not available.
+Includes
oxygen content of B2O3. |
|
|
BN production
figures are not included in statistical reports. Estimates, however, show
production of 220-250 metric tons in 1994 for the western world. Companies
involved include Advanced Ceramics Corp. and Carborundum Co. in the United
States; Boride Ceramics & Composites Ltd. in the United Kingdom; Elektroschmelzwerk
Kempten GmbH and H. C. Starck GmbH & Co. KG in Germany; and Denki Kagaku
Kogyo, Kawasaki Steel Corp., Shin-Etsu Chemical Co. Ltd. and Showa Denko
KK in Japan.
Costs of BN powders are about $70-$80/kg
for refractory grades and $100-$120/kg for ceramic grades, with little
or no increase expected for the near future. High-purity grades can be
purchased at $200-$400/kg, depending on quality and size of order. |
|
|
|
|
Composition
|
by supplier
|
(%) |
| Component |
A*
|
B*
|
C
|
D
|
| Boron |
41.5
|
42-45
|
41.5
|
41.5
|
| Nitrogen |
53.5
|
50.5-53.4
|
51.5
|
52.0
|
| Oxygen+ |
3.5
|
5.0
|
7
|
5
|
| Carbon |
0.4
|
NA
|
0.1
|
0.2
|
| B2O3 |
NA
|
1.0
|
7
|
5
|
| Metallic impurities |
0.1
|
0.2
|
0.2
|
0.1
|
Table 2. Commercially Available
Refractory-Grade BN Powders
* NA is data is not available.
+Includes
oxygen content of B2O3. |
|
|
BN powders
are used for the production of TiB2/BN intermetallic composites
used as evaporator boats for vacuum metallization of plastics and glass.
Dense shapes, e.g., crucibles and break rings for horizontal continuous
steel casting, are made by hot isostatic pressing, using B203
as a sintering agent. a-BN
also is the starting material for the synthesis of cubic boron nitride
(ß-BN), a dense, diamond-like modification, with zinc blende structure.
ß-BN is the second hardest material known. |
|
| Use |
Consumption (metric tons)*
|
| TiB2/BN composites |
60-70
|
| Refractory applications |
20-30
|
| Break rings |
20-30
|
| Cubic boron nitride |
15-20
|
| HIPed shapes |
15-25
|
| Paints/coatings/lubricating agents |
25-35
|
| Electronic applications |
NA
|
Table 3. Boron Nitride Powder
1994 Estimated Consumption
*NA is data not available. |
|
|
The extremely good high-temperature
lubrication and nonwetting properties of BN are used in coatings in the
production of aluminium and magnesium castings as well as glass-forming
and superplastic forming of titanium sheets for aerospace and defense applications
These paints and lubricating coatings
are suspensions of BN in a carrier (mainly water for safety and environmental
considerations), often blended with binders and rheological additives.
Some formulations are available as aerosols. BN coatings are produced by
Advanced Ceramic Corp. and ZYP Coatings Inc. in the United States and by
Büro für angewandte Mineralogie and Elektroschmelzwerk Kempten
GmbH in Germany.
Stephan Rudolph, Büro für
angewandte Mineralogie
|
|
from: American Ceramic Society Bulletin,
74 (1995) 6, pp. 115-117 |
