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SYNTHESIS AND PROCESSING: GENERAL METHODS 27 conventional coating materials. Sputtering also makes possible the deposition of new coating materials with complex compositions that cannot be made by evaporation. To allow coating of plastics and other temperature-sensitive substrates, reactive magnetron sputtering can be used. Magnetic confinement of secondary electrons minimizes substrate heating. Increased adatom energy at the substrate plus increased chemical reactivity in a gas discharge allow fabrication of dense, adherent, and fully reacted dielectric compounds without external substrate heating. By the use of multiple sources, one can control composition and thickness of the ultrafine multilayered structures. Substrate temperature, bias, and deposition rate in the appropriate combination can be utilized to control the microstructures from the microcrystalline to the glassy state. Some examples of magnetron sputtered optical coatings on polymeric substrates are given in Table 3. TABLE 3 Magnetron Reactively Sputtered Coating Materials and Measured Properties. Coating Material Refractive Index at 550 nm Absorption Edge (nm) TiO2 2.26 350 Nb2O5 2.22 330 V2O2 2.20 (700 nm) 475 Ta2O5 2.13 275 ZrO2 2.04 235 SnO2 1.93 315 Al2O3 1.58 <200 SiO2 1.46 <200 Si3N4 1.95 220 AlN 1.94 220 CRYOCHEMICAL SYNTHESIS. Although a number of methods exist for making ultrafine-grain (diameter <100 nm) starting materials, freeze-drying methods (Schnettler et al., 1968) have several very important advantages. The most prominent advantage is that very little change in technique is required over an unlimited range of materials of varying crystal structure, and control is afforded over particle sizes from very reactive fine-grain materials of 5 nm
