Jan 6 2004
Background
Boron Nitride exhibits a hexagonal structure and is sometimes known as white graphite, due to its lubricity, anisotropic properties, heat resistance, and high thermal conductivity. This latter property, combined with a low thermal expansion, leads to excellent resistance to thermal shock.
All three forms, Hot Pressed, Pyrolytic and Hot Isostatic Pressed, can be used at very high temperatures. However, in an oxidising atmosphere the maximum use temperature is 850-900 °C.
Hot Pressed and Hot Isostatic Pressed Boron Nitride are produced by densification of powder, whereas Pyrolitic Boron Nitride is produced by Chemical Vapour Deposition (CVD), depositing onto graphite at temperatures in excess of 1800 °C.
Boron Nitride differs from graphite in having a high dielectric strength and low dielectric constant and is often used as an electrical insulator at very high temperatures.
Chemically inert, Boron Nitride is used as crucibles for molten metals. Boron Nitride is non-toxic and transparent to microwaves. It may be machined to close tolerances.
Key Properties
The key properties of Boron Nitride are tabulated below.
Table 1. Key properties
| Chemical Resistance |
| Acids - concentrated |
Fair |
| Acids - dilute |
Fair |
| Alkalis |
Fair |
| Halogens |
Poor |
| Metals |
Good |
| Electrical Properties |
| Dielectric constant |
4.3 |
| Dielectric strength ( kV.mm-1 ) |
40-200 |
| Volume resistivity ( Ohm.cm ) |
1011-1014 @ 25 °C |
| Mechanical Properties |
| Compressive strength ( MPa ) |
30-120 |
| Shear strength ( MPa ) |
12-25 |
| Tensile modulus ( GPa ) |
20-35 |
| Physical Properties |
| Apparent porosity ( % ) |
2-15 |
| Density ( g.cm-3 ) |
1.9-2.2 |
| Thermal Properties |
| Coefficient of thermal expansion ( x10-6 K-1 ) |
1.0-36 @ 20-1000C |
| Specific heat ( J.K-1.kg-1 ) |
800-2000 @ 25 °C |
| Sublimation point (°C ) |
2600-2800 |
| Thermal conductivity ( W.m-1.K-1 ) |
15-50 @ 20 °C |
| Upper continuous use temperature (°C ) |
950-2500 |
About Goodfellow
Goodfellow supplies metals, ceramics and other materials to meet the research, development and specialist production requirements of science and industry worldwide.
Source: Goodfellow
For more information on this source please visit Goodfellow.