Boron nitride ceramic yields

The introduction of small amounts of boron into precursors that produce silicon nitride have been known to improve the ceramic yields of silicon nitride and Si—B—C—N ceramics as first reported in 1986.110 Several reports have appeared in the past couple of years alone that utilize borazine precursors such as 2,4-diethylb-orazine and other cyclic boron precursors, such as pinacolborane, 1,3-dimethyl-1, 3-diaza-2-boracyclopentane, for their reactions with silanes, polysilazanes, and polysilylcarbodiimides for the high-yield production of Si—B—N—C ceramics.111... 

There is a great deal of potential interest in borazine as a precursor of boron nitride, since it offers the advantages of being a single source of boron and nitrogen with the correct B/N ratio and a high ceramic yield. In addition, borazine contains the elementary BN building block as its substituted derivatives. This is described later. 

In this work, boron was chosen as the active filler due to its considerable volume expansion (AV/Vo) on nitridation (B+0.5N2=BN, AV/Vo =142%). Influence of heat-treatment temperatures on the eompositions, ceramic yields and linear shrinkages as well as effects of boron on the mierostructure evolution and properties of composites were studied. 

Non-oxide preceramic polymers which are expected to yield, under convenient thermal and chemical conditions, boron-containing amorphous or crystallized ceramics including boron nitride (BN), boron carbide (B C), boron carbonitride (B-C-N), and boron silicon carbonitride Si-B-C-N. 

Borazine (Figure 14.1), which was originally synthesized in 1926 by Stock et al. [9], finds considerable interest as molecular candidate for the preparation of boron nitride, beause it offers the advantages of a source of boron and nitrogen elements with the correct atomic ratio and geometry to yield polymers (i.e., polyborazylene, then boron nitride in a high ceramic yield) [8]. 

Some of these compounds lead to poly(B-alkenylborazines) (Eq. 14.2) through thermally-induced polymerizations at moderate temperature, leading to boron nitride-based ceramic in ammonia and nitrogen atmosphere. Depending on the reaction conditions, it is interesting to note that either insoluble or soluble polymers are produced. The latter is particularly valuable for producing either boron nitride-based films or coated ceramics with both high ceramic and chemical yields. 

Interest in borazine-based polymers arises out of their possible utility as precursors for the preparation of boron nitride, a high value ceramic material. It was discovered that borazine readily undergoes a dehydropolymerization at 70-110 °C to afford a soluble polymer, polyborazylene, in about 80-90% yield (see Eq. 5.15) [30, 31]. 

P. J. Fazen, J. S. Beck, A. T. Lynch, E. E. Remsen, and L. G. Sneddon (1990), Thermally induced borazine dehydropolymerization reactions. Synthesis and ceramic conversion reactions of a new high-yield polymeric precursor to boron nitride, Chem. Mater. 2, 96. 

The poly(B-vinyllborazine) polymers satisfy a number of the criteria previously identified for preceramic materials and their use as precursors to boron nitride was investigated. It was found that depending upon the polymer and pyrolysis conditions, a variety of ceramic materials may be produced, ranging from black, high carbon content materials to white h-BN. In each case, however, the polymer/ceramic conversion was found to take place with both high ceramic and chemical yields and give materials with B/N ratios of 1.0. 

Poly(B-vinylborazine) shows excellent conversions to boron nitride, but because of it.s carbon backbone, it requires pyrolysis under an ammonia atmosphere in order to achieve low-carbon boron nitride. In an effort to develop new boron nitride precursors that give improved ceramic yields and/or do not require the use of ammonia during the ceramic conversion step, we initiated investigations of the syntheses of alternative types of borazinc based polymers. 

Poly(borazylene) polymers, such as shown below, composed of linked borazine rings analogous the organic poly(phenylene) polymers, were of particular interest because of their potentially high ceramic yields (95%) and their close structural relationship to boron nitride. 

Because of its composition, high yield synthesis, and excellent solubilities, poly(borazylene) would appear to be an ideal chemical precursor to boron nitride. Indeed, bulk pyrolys s of the polymer under either argon or ammonia to 1200 C were found to result in the formatit)ri of white boron nitride powders in excellent purities and ceramic yields 85-93% (theoretical ceramic yield, 95%). 

Ceramic-type materials that contain no organic linkage units can be prepared by the pyrolysis of cyclic or high polymeric aminophosphazenes. An example is shown in reaction (44). Under appropriate conditions, pyrolysis products that correspond to phosphorus-nitride are formed. Polyphosphazenes that contain both amino and borazine side groups yield phosphorus-nitrogen-boron ceramics following pyrolysis 94,95 The conversion of a formable polymer into a ceramic has many potential advantages for the controlled synthesis and fabrication of advanced ceramics. This principle is discussed in more detail in Chapter 9. 

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