Hexagonal boron nitride (α-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 π-bonds in the BN structure.

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.

 

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 α-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% B₂O₃. The glassy B₂O₃ cannot be converted to BN. The B₂O₃ 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 B₂O₃ as a sintering agent. α-BN also is the starting material for the synthesis of cubic boron nitride (β-BN), a dense, diamond-like modification, with zinc blende structure. β-BN is the second hardest material known.

Use	Consuption (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

 

 

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