Research Index / Materials Science / Motivation - cBN

Boron nitride is a molecule composed of two elements: boron (B) and nitrogen (N). The boron and nitrogen atoms bond to each other in one of four different arrangements in space: hexagonal, rhombic, wurtzitic, and cubic (see sketch). Each arrangement is called a crystal phase. For carbon, the hexagonal crystal phase is more commonly known as graphite and the cubic crystal phase is known as diamond. The properties of the hexagonal and cubic crystal phases of boron nitride are analogous to those of graphite and diamond, respectively. In fact, cubic boron nitride has a hardness second only to diamond. This and other properties of cubic boron nitride (cBN) are listed below, along with applications of these properties.

Properties Applications
Extremely hard
#2 to diamond
Cutting & grinding tools
Protective optical coatings
Transparent
in the visible
High resistivity:
108 ohm.m
High temperature / high
frequency semiconductor
devices
High thermal
conductivity:
1300 W/m.C
Wide Bandgap:
~6 eV
UV photodiode
Blue solid-state laser?

The production of good-quality cubic-phase boron nitride is hindered by a number of problems. Stoichiometry, the ratio of boron to nitride atoms (which should be 1:1 for boron nitride), is often a major problem in that the films produced are deficient in nitrogen. This problem is caused by the fact that nitrogen atoms prefer to form in pairs and produce nitrogen gas (N2) which is relatively inert and is thus lost to the production of the boron nitride film. The deficiency in nitrogen means that the film is not a perfect crystal. In some crystal locations where a nitrogen atom should exist, there is a boron atom or no atom at all (a vacancy). Extra boron atoms and vacancies distort the atomic bonding and thus changes the crystal structure. It will no longer be purely cubic in structure since any one of the four crystal structures depends on the correct ratio of boron to nitrogen atoms and the correct bonding of the atoms.


Research Index / Materials Science / Motivation - cBN

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