Ceramic fibers green

The spun polysilane fibers are cured by oxidation in air at a temperature of 160 - 200°C. The curing process is necessary to permit the conversion of the green fiber to the ceramic fiber without softening during pyrolysis. It is presumed that the oxidation mechanism results in the formation of Si-O-C and Si-O-Si bonds by the reaction of Si-CH3 and Si-Si in the polysilane. 

Besides the desired dehydrocoupling, Si-N bond cleavage of HPZ and formation of DEB-NHSiMej occurred. Nevertheless, the polymeric precursor had an ideal glass-transition temperature for melt spinning and stable melt viscosity. Melt spun Si-B-C-N-H green fibers were subsequently transformed into ceramic fibers by thermolysis at 1400°C. 

This so-called polymer route was introduced by Chantrell and Popper [10], who proposed the use of inorganic polymers as starting materials for the preparation of ceramics, opening up the rather ambitious perspective of easily shaping monolithic green bodies via this route. At the end of the 1960s, Winter et al. [11, 12] pioneered such a process for the production of Si/C/N fibers, and developed it to technical feasibility. Following the route developed by Yajima et al. [13, 14], ceramic fibers (SiC, Nicalon) have been available on... 

There are three basic steps in the manufacture of high performance, continuous ceramic fibers, all of which have important cost considerations (1) preparation of bulk, preceramic material to be spun (2) spinning the bulk material into a green fiber and (3) heat treating the spun green fiber to convert it into ceramic fiber. 

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. 

The most important application of boron is to make fibers or whiskers of single crystal or ceramic crystal. The addition of boron to metals, alloys, or other solids, imparts resistance to plastic flow, and thereby produces unusual strength in the material. Amorphous boron is used in rockets as an igniter, and in pyrotechnic flares to give green color. Many boron compounds, such as borax, boron hydrides, and boron hahdes, have important commercial applications (see individual compounds). 

In the last 10 years, significant advances in fibrous monolithic ceramics have been achieved. A variety of materials in the form of either oxide or nonoxide ceramic for cell and cell boundary have been investigated [1], As a result of these efforts, FMs are now commercially available from the ACR company [28], These FMs are fabricated by a coextrusion process. In addition, the green fiber composite can then be wound, woven, or braided into the shape of the desired component. The applications of these FMs involve solid hot gas containment tubes, rocket nozzles, body armor plates, and so forth. Such commercialization of FMs itself proves that these ceramic composites are the most promising structural components at elevated temperatures. 

Heat treating involves ramping the temperature of the green fiber to the temperature of its conversion to ceramic material, holding it at that temperature until the fibers are fully converted, and ramping the fiber temperature back down. The environment must be carefully controlled throughout this process. The time and temperature required to convert the fiber, the controlled environment, and the custom furnaces all make this an expensive process. 

---