Boron Fibre

Boron has properties borderline between metals and non-metals. It is a good conductor at high temperature. Again, it is chemically closer to silicon, for example, crystalline boron is inert chemically.

Boron is the first synthetic fibre, used since 1960 (Space Shuttle). It is very brittle to directly draw in the form of fibre. Hence usually Boron is deposited through CVD on fine (10-12 m m) Tungsten wire also some times on Carbon Core. The process is shown below: Hydrogen Gas is used to reduce Boron Trihalide:

High temperature around 1000 o C is required - hence Tungsten base is used as resistive heating of wire. The temperature is to be controlled during CVD since beyond 1300^oC unwanted crystal- form occurs in the wire. Core diffusion forms materials like WB₄, W₂B₅ etc developing unwanted increase in core thickness - hence SiC coating is used on the fibres.

Typically Born fibre has a density of about 2340 Kg/m³ , T_m - 2040^oC, strength - 3-4.5 Gpa, modulus of elasticity 400-480 Gpa. Boron fibre is quite expensive - It has been used in F14, F15, and space shuttle Columbia.