BN fibers

BN fibers can be made by decomposition of hydrated cellulose impregnated with boric acid or ammonium tetraborate (NH4B407) in ammonia and nitrogen atmosphere at elevated temperatures ( 1000 °C) [148, 149]. 

Another method is to extrude borazine network polymers ([-B(NH2)-N(C6H5)-]3) with addition of B203 at T 300 °C into fibers and subsequently firing in nitrogen stream at 1800 °C [150]. 

BN fibers are used for reinforcing ceramic materials (e.g., Al203, Si3N4, SiC) to enhance mechanical properties as well as to extend the range of possible applications. They serve as reinforcement of organic polymers (e.g., epoxides, polyether-polyketones, polyphenylensulfides) which exhibit good thermal conductivity and low thermal expansion. 

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Development of practical and low cost separators has been an active area of ceU development. CeU separators must be compatible with molten lithium, restricting the choice to ceramic materials. Early work employed boron nitride [10043-11-5] BN, but a more desirable separator has been developed using magnesium oxide [1309-48-4], MgO, or a composite ofMgO powder—BN fibers. Corrosion studies have shown that low carbon steel or... 

Table 3 Mechanical Properties of the BN Fibers Prepared from Different Polymers...

The polymer-to-ceramic conversion was performed in flowing ammonia to 1000°C (heating rate 25°C/h), then in flowing nitrogen to 1800°C (heating rate 100°C/h).29 30 Some of the properties of the final BN fibers are summarized in Table 4. 

Boron nitride (BN) is a versatile ceramic that is found and used in various crystallographic and geometrical forms. Although there are chemical similarities, the conditions for the synthesis and fabrication of each form (hexagonal, cubic, polymorphs of BN, fibers, two-dimensional forms and composites) have different requirements and are... 

Hollow BN fibers are made by depositing a coating of BN on carbon fibers, heating in an oxidizing atmosphere to form CO or CO2, and leaving a BN coating. The reaction between BF3 and NH3 at 1200-1400°C may be used . Yarn and BN/BN composites can be made with similar techniques (see 17.3.5.6). 

A review of the development of high performance BN fibers has appeared . BN fibers have also been prepared by nitriding boron filaments . The boron fibers... 

Multidimensional BN fiber infiltrated (matrix) composites are analogous to carbon-carbon composites and overcome the brittleness and low to moderate thermal shock resistance of hot-pressed BN. With the availability of BN fibers (see 17.3.5.4), a weave of three or more dimensions is possible. Chemical vapor deposition may produce a turbostatic BN matrix that is sensitive to H2O, while liquid impregnation requires multiple reimpregnation cycles h One scheme is to impregnate a BN woven fiber preform with H3BO3, dry, sinter in NH3, and hot-press. 

In another approach, 3-6 mm lengths of BN fiber are blended with a hot aqueous solution saturated with H3BO3 and urea. This mixture is spread over a fine screen to... 

BN fibers are used in aluminum matrices because they are one of the few materials easily wet by molten aluminum. hBN-AlN composites are described ". Gel methods resulted in an hAlN-turbostratic hBN composite. zBN-based composites, e.g., with Al, Si, Ti nitride matrices, were reported. A nanostructure composite was made by low temperature conversion of an aqueous solution of simple Al, B, and N-containing compounds to intermediate precursor (gel) materials. BN-AIN composite fabrication is also described . 

Cylindrically converging shock waves on powders were used to make mixtures of diamonds and hBN. BN fiber reinforced Zr02 was described . A nanostructured composite of magnetic particles of FOj N in a nonmagnetic matrix of BN is made via an inorganic geP. Fabrication of BN-B4 composites was reported . Consolidation of novel sintered composites formed from high pressure crystallization of amorphous ceramics was also described. The literature discusses other ceramics reinforced with BN fibers, as welF . ... 

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