A relatively new area that involves silicon-containing materials is the synthesis of “ultrastructure” materials (i.e., materials in which structure can be controlled at the level of 100 Å). An example is the “sol-gel” hydrolysis of alkoxysilanes (organosilicates) to give silica, SiO2. The reaction is complicated, involving polymerization and branching, but the overall reaction may be written . . . Si(OR4 + 2H2O → SiO2 + 4ROH (9.1) . . . where the Si(OR)4 organometallic species is typically tetraethoxysilane such as tetraethylorthosilicate (TEOS, with R being C2H5). In this application, the precursor compound is hydrolyzed and then condensed to yield branched polymers. Eventually a continuous swollen gel is formed. The gel is dried at moderately low temperatures to remove volatile species, and then it is fired into a porous ceramic object that can then be densified and machined into a final ceramic part. The production of ceramics by this novel route triggered interest in the ceramics community because of advantages over the conventional powder-processing approach to ceramics. Advantages include (i) the higher purity of the starting materials, (ii) the relatively low temperatures required, (iii) the possibility of controlling the ultrastructure to reduce the microscopic flaws that lead to failure, (iv) the ease with which ceramic coatings can be formed, and (v) the ease with which ceramic alloys can be prepared (e.g., by hydrolyzing solutions of both silicates and titanates). The sol-gel approach has been used to form ceramic-like phases in a variety of polymers. Poly(dimethylsiloxane) (PDMS) is the most popular. PDMS is relatively weak and stands to benefit most from reinforcement. PDMS is easily absorbs the precursor materials generally used in the solgel process. Nearly monodisperse silica microparticles can be obtained using siloxane elastomer mixtures. In some cases, the PDMS has been part of a copolymer, with ureas, imides, amideimides, and dianilines. In other approaches, the particle surface is modified, for example, with a polysiloxane. Siloxane/silica nanocomposites have also been used as “culture-stone-protective materials.” Sol-gel hydrolysis and condensation can be carried out within a polymeric matrix to generate particles of the ceramic material, typically with an average diameter of a few hundred angstroms.
Ultrasonic Evaluation of Rough and Porous Ceramic Coatings with a Dual-Element Large Aperture Lensless Line-Focus Transducer
