Amorphous ceramic fibers

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

In principle these compounds offer access to materials with AliCh-SiCL and Al203 2Si02 stoichiometries. The latter stoichiometry is equivalent to the Al[OSi(OBu-t)3 (OBu-t)] precursor. The major drawbacks with these materials are their air and moisture sensitivity, and the cost of the starting materials. Although the idealized stoichiometries of the above ceramics products are not those of crystalline aluminosilicates, amorphous aluminosilicate glasses are often important in optical applications or in scratch-resistant coatings. Furthermore, they may offer potential for CVD-type applications. There still remains considerable need for simple precursors to crystalline aluminosilicates, especially for structural applications. Dense, phase pure crystalline ceramic materials are desired for optimal mechanical properties, e.g. ceramic fibers for composite manufacture. 

The last quarter of the twentieth century saw tremendous advances in the processing of continuous, fine diameter ceramic fibers. Figure 6.4 provides a summary of some of the important synthetic ceramic fibers that are available commercially. We have included in Fig. 6.4 two elemental fibers, carbon and boron, while we have excluded the amorphous, silica-based glasses. Two main categories of synthetic ceramic fibers are oxide and nonoxides. A prime example of oxide fibers is alumina while that of nonoxide fibers is silicon carbide. An important subclass of oxide fibers are silica-based glass fibers and we devote a separate chapter to them because of their commercial importance (see chapter 7). There are also some borderline ceramic fibers such as the elemental boron and carbon fibers. Boron fiber is described in this chapter while carbon fiber is described separately, because of its commercial importance, in Chapter 8. 

The earliest report of fibers containing silicon and nitrogen is by Verbeek (11). Chlorosilanes and amines are used to prepare polycarbosilazane polymers, which are converted into amorphous SiCN-containing ceramic fibers (equation 7). 

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 . ... 

The advantages of the polymeric route to ceramic fibers include the ability to control morphology (amorphous or crystalline and control of crystalline size) and the ability to prepare continuous, fine-diameter fibers (<30 pm) suitable for weaving and knitting. One of the unique advantages of this method is the ability to prepare metastable compositions unobtainable by conventional methods. 

TADB-derived ceramic fibers, with the idealized composition SiBNsC, do not reach the E-modulus of the most advanced SiC fibers at room temperature. However, they are clearly superior to the latter in a crucial point, namely the drop of the E-modulus and of the creep resistance at high temperatures. SiC fibers already lose a large part of their mechanical strength below 1400 °C, as can be measured by creep resistance. These limitations are fundamental in nature, since they are related to grain boundary sliding and thus to the crystallinity of SiC. In contrast, amorphous SiBNsC fibers do not show any grain boundaries and, moreover, the concentration of microstructural flaws is extremely low. 

Composites may be identified and classified many hundreds of ways. There are aggregate-cement matrix (concrete), aluminum film-plastic matrix, asbestos fiber-concrete matrix, carbon-carbon matrix, carbon fiber-carbon matrix, cellulose fiber-lignin/silicic matrix, ceramic fiber-matrix ceramic (CMC), ceramic fiber-metal matrix, ceramic-metal matrix (cermet), concrete-plastic matrix, fibrous-ceramic matrix, fibrous-metal matrix, fibrous-plastic matrix, flexible reinforced plastic, glass ceramic-amorphous glass matrix, laminar-layers of different metals, laminar-layer of glass-plastic (safety glass), laminar-layer of reinforced plastic, laminar-layers of unreinforced plastic. 

FIGURE 1-6 Single filament of boron nitride-coated Nippon Carbon Nicalon non-oxide ceramic fiber. Nonoxide fibers discussed in this report include polycrystalline SiC fibers and multiphase (amorphous or crystalline) fibers consisting of B,C,N,Ti, or Si. Current manufacturers include Bayer, Dow Corning, Nippon Carbon, Textron, Tonen, and Ube. Source Dow Coming Corporation. 

The performance objectives described in the previous chapters limit the material choices of ceramic fibers for CMCs to polycrystalline oxides (e.g., AI2O3, mullite), non-oxides (e.g., SiC, Si3N4), and amorphous Si-C-N-B-0 compositions. Singlecrystal monofilaments have certain performance advantages, but their cost is prohibitive. Therefore, they are not discussed in this chapter. 

Pyrolysis of poly(organoborosilazane) (entry 6) under argon at 1,050 °C gives an amorphous ceramics, which resist crystallization up to 1,700 °C and thermally degradation up to 2,200 °C [210]. It should be noticed that the ratio of ceramic elements (B Si N) in the ceramic chars is about the same as that in the polymer precursors, illustrating the importance to control the ratio of ceramic elements in the preceramic polymers. Ceramic fibers can be obtained from this type of polymer for high temperature application... 

The modulus or stiffness of a fiber reflects its structural or internal order [3]. Figure 5 compares the fiber modulus of silica-alumina and calcia-alumina based composition ranging from 100% silica (or calcia) to 100% alumina. The fiber modulus increases with increasing structural order, I.e., from 41 to 125 GPa for amorphous glass fibers to 125 to 250 GPa for nanocrystalline glass fibers, 250 to 400 GPa for polycrystalline ceramic fibers, and 405 to 410... 

Furthermore, the differences in the strengths of amorphous glass, nanocrystalline glass ceramic and polycrystalline ceramic fibers are attributable to differences in the internal order of the fiber structure. For example, the presence of a uniformly distributed minor nanocrystalline (second) phase in a major amorphous (primary) phase of a glass-ceramic... 

A liquid phase, as opposed to a vapor or solid phase, includes dispersions, solutions and melts. Several processes, which yield continuous inorganic fibers directly from the melt, have been discussed in Chapter 4. Only one generic process, dry spinning, is known to yield one specific amorphous oxide fiber directly from a liquid phase other than that of a melt. All other processes which start with a liquid phase (see Chapters 8-12) yield first a solid, non-functional precursor or green fiber, and then a functional, nano- or polycrystailine ceramic fiber. Such refractory ceramic fibers are therefore directly derived from a solid phase, a precursor or a green fiber, and only indirectly from a liquid phase. 

PCS precursor fibers are converted to SiC based ceramic fibers by pyroiysis in an inert atmosphere at 1200-1300°C [6] [8] [13] [18] [28] [46-47]. This conversion is accompanied by evolution of gas (Figure 3) and weight loss. A large fraction of the organic bonds break at 800-900°C, and the pyrolytic residue is an amorphous, still hydrogenated, Si-C (or Si-C-0)... 

Titanium can be introduced in PCS precursors as titanium tetrabutoxide, Ti(OC4H9)4, to yield Si-C-O(Ti) ceramic fibers such as Tyranno (Equation 6) (48). Fibers produced from such PCS(Ti) precursors have a slightly higher pyrolysis temperature (Tp) than that previously mentioned for their pure PCS counterparts [49]. As a result, they also retain their amorphous state and hence their tensile strength to a slightly higher temperature. 

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