Preceramic Inorganic Polymers

Abstract

One of the most important interfaces in materials science is the one between polymers and ceramics. Ceramics can be viewed as highly cross-linked polymer systems, with the three-dimensional network providing strength, rigidity, and resistance to high temperatures. Although not generally recognized as such, a few ceramics exist that are totally organic (i.e., carbon-based). Melamine-formaldehyde resins, phenolformaldehyde materials, and carbon fibers are well-known examples. However, totally inorganic ceramics are more widely known, many of which are based on the elements silicon, aluminum, or boron combined with oxygen, carbon, or nitrogen. Among the inorganic ceramics, two different classes can be recognized—oxide ceramics and non-oxide materials. The oxide ceramics frequently include silicate structures, and these are relatively low melting materials. The non-oxide ceramics, such as silicon carbide, silicon nitride, aluminum nitride, and boron nitride are some of the highest melting substances known. Non-oxide ceramics are often so high melting that they are difficult to shape and fabricate by the melt- or powder-fusion techniques that are common for oxide materials. One major use for inorganic-organic polymers and oligomers is as sacrificial intermediates for pyrolytic conversion to ceramics. The logic is as follows. Linear, branched, or cyclolinear polymers or oligomers can be fabricated easily by solution- or melt-fabrication techniques. If a polymeric material that has been shaped and fabricated in this way is then cross-linked and pyrolyzed in an inert atmosphere to drive off the organic components (typically, the side groups), the resultant residue may be a totally inorganic ceramic in the shape of the original fabricated article. Thus, ceramic fibers, films, coatings, and shaped objects may by accessible without recourse to the ultra-high temperatures needed for melting of the ceramic material itself. Note, however, that although the final shape of the object may be retained during pyrolysis, the size will be diminished due to the loss of volatile material. If the pyrolysis takes place too quickly, this contraction process may cause cracking of the material and loss of strength.