Fiber boron nitride

A boron nitride fiber can be very competitive commercially with carbon fiber. It has about the same density (2.2 g/cm ) as the carbon fiber, but has greater oxidation resistance and excellent dielectric properties. A method of converting boric oxide precursor fibers into boron nitride fibers has been developed (Economy and Anderson, 1967). Melt spun boric oxide precursor fiber is nitrided with ammonia according to the following reaction ... 

Wider use of fiber-reinforced ceramic matrix composites for high temperature structural applications is hindered by several factors including (1) absence of a low cost, thermally stable fiber, (2) decrease in toughness caused by oxidation of the commonly used carbon and boron nitride fiber-matrix interface coatings, and (3) composite fabrication (consolidation) processes that are expensive or degrade the fiber. This chapter addresses how these shortcomings may be overcome by CVD and chemical vapor infiltration (CVI). Much of this chapter is based on recent experimental research at Georgia Tech. 

S. T. Gonczy, E. P. Butler, N. R. Khasgiwale, L. Tsakalakos, W. R. Cannon, and S. C. Danforth, Blackglas-Nicalon composites with CVD boron nitride fiber interface coatings,... 

G.S. Corman and K.L. Luthra, Silicon-Doped Boron Nitride Fiber Coatings for Melt Infiltrated Composites, presented at the 24 annual Conference on Composites, Materials and Stmctures, Cocoa Beach, FL, January 24-28,2000. 

The review is in part based on our own work related to the poly(alkylaminoborazines)-and poly(borylborazines)-derived boron nitride fibers, but will also cover a comprehensive state of the art including the published literature in this field. 

In the 1970s, Tanigushi et al. reported the preparation of boron nitride fibers from a melt-spinnable poly[B-aminoborazine] (Eq. 14.4) [25], However, Uttle information has been provided and the lack of reproducibihty in the melt-spinning process rendered the production of BN fibers difficult. 

Green fibers derived from melt-spinnable polymers have been cured in an ammonia atmosphere, then pyrolyzed in ammonia and nitrogen atmospheres to generate at 1800 °C boron nitride fibers with excellent mechanical properties. It was interesting to note that boron nitride fibers, with controlled mechanical properties and crystallinity, could be produced through simply changing the meltspinning conditions (Table 14.1) [35]. 

Table 14.1 Boron nitride fibers with controlled mechaniceil properties and crystaUinity...

Boron nitride fibers have been prepared in the laboratory by chemical vapor infiltration of boron oxide glass fibers with ammonia (Equation 8), a process that may alternatively be considered to be a nitridation of B2O3 precursor fibers [31]. The precursor fibers, in turn, are melt spun at a low temperature (480 C) from a viscous melt. Thus, the nitridation of a boron oxide fiber could alternatively be considered to be derived from a solid precursor fiber, a topic otherwise discussed in Chapters 8 to 12. In this process, the final step is the chemical conversion of a given precursor fiber at a high temperature in a highly reactive vapor phase environment. 

A boron nitride fiber derived from a borazine precursor fiber has recently been reported [32]. As with boron nitride fibers from B2O3, the final heat treatment of boron nitride fibers from borazine must be carried out in a reactive atmosphere (also ammonia) to chemically complete the conversion of the precursor fiber. This nitridation step renders the reaction sequence a chemical vapor infiltration process. 

Despite its low molecular weight, the precursor can be melt spun at 100°C in air, and yields a continuous precursor fiber. This green fiber is heat treated in ammonia at lOOO C. Chemical vapor infiltration and the resulting reaction convert carbonaceous materials in the fiber into volatiles. A short heat treatment at 1800°C in nitrogen removes all residual volatiles and it stabilizes the boron nitride fiber and its microtexture. The final boron nitride fiber is white and has a density of 2.05 glom ... 

J. Economy and R. V. Anderson, Boron nitride fibers. Journal of Polymer Science, Part C, 19,283-297 (1967). 

Y. Kimura, Y. Kubo and N. Hayashi, High-performance boron nitride fibers from poly(borazine) preceramics. Composites Science and Technology, 51,173-179 (1994). 

Hollow boron nitride fibers are being studied as potential wave guides for transmitting high power IR radiation from CO2 lasers [6]. 

B(NH2)-N(CeH5H3 is used to prepare boron nitride fibers by copyrolysis with 1% H3BO3 (260°C) and extrusion of the resultant borazine polymer. The fibers are converted to boron nitride by firing at 1800°C in a nitrogen atmosphere [40] see also [31 to 34]. 

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