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Rare Earth-Doped Phosphors for White LEDs Chapter 278 83 The last synthetic method is called diimide route, which is known to be advantageous to produce high-purity material. First, silicon diimide Si(NH)2 is obtained by reaction between SiCl4 and NH3. Decomposition of Si(NH)2 by heating produces amorphous Si3N4. Another heating is required to obtain crystalline Si3N4. SiCl4 ðlÞ + 6NH3 ðgÞ ! SiðNHÞ2 ðsÞ + 4NH4 ClðsÞ ðat 0°CÞ 3SiðNHÞ2 ! Si3N4ðamorphÞ + N2ðgÞ + 3H2ðgÞ ðat 1000°CÞ Aluminum nitride (AlN) is synthesized by either direct nitridation of metallic aluminum or carbothermal reaction (Ramisetty et al, 2014). Direct nitridation of metallic aluminum is a self-sustaining reaction, beginning from ignition of aluminum fine powder in a nitrogen-containing atmosphere. This synthetic method is advantageous with respect to low energy consumption. But the obtained material agglomerates substantially and must be ground mechanically into a fine powder. 2Alðs=lÞ + N2ðgÞ ! 2AlNðsÞ + heat Carbothermal reaction is conducted as follows: powders of Al2O3 and carbon are mixed and heated at 1400–1800°C. Al2 O3 ðsÞ + 3CðsÞ + N2 ðgÞ + heat ! 2AlNðsÞ + 3COðgÞ This method produces high-quality materials, whereas manufacturing cost and environmental load is high because a lot of carbon-containing by-product (CO and CO2) are generated. Alkaline-earth nitrides are obtained by heating alkaline-earth metals in a nitrogen-containing atmosphere. Alternatively they can also be synthesized by reaction with NH3, but in this case, small amount of hydrides are generated as by-products (Ehrich, 1963). Lanthanide nitrides are also obtained by reacting metallic lanthanides and N2 or NH3 (Kieffer et al., 1972). Carbothermal reaction of lanthanide oxides does not yield lanthanide nitrides, but lanthanide carbides. The nitrides can then be obtained by reacting the carbides with NH3 (Nakagawa et al., 1997). It is reported that lanthanide nitrides can also be synthesized by the reaction of lanthanide chlorides and lithium azide (Li3N) (Fitzmaurice et al., 1994). 5.2.3 Solid-State Reaction of Nitride Raw Materials Most common synthetic methods of (oxy)nitride phosphors are also solid-state reactions. Rare earth-activated a-sialon phosphor, one of the oldest (oxy) nitride phosphors, was synthesized as follows: A mixture of a-Si3N4, g-Al2O3, AlN, (Y2O3, CaCO3, or Ca3N2), and (Tb4O7, CeO2, or Eu2O3) was introduced into a molybdenum crucible and heated at 1700°C for 2 h in a 5% H2+95% N2 atmosphere (van Krevel et al., 2002). Xie et al. (2004a)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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