Reactive yttrium aluminate garnet powder via coprecipitation using ammonium hydrogen carbonate as the precipitant
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(Received 3 January 2000; accepted 16 June 2000)
Ammonium hydrogen carbonate was used as the precipitant to synthesize yttrium aluminate garnet (YAG) precursors from a mixed solution of aluminum and yttrium nitrates via coprecipitation. The carbonate precursor, with an approximate composition of NH4AlY0.6(CO3)1.9(OH)2 ⭈ 0.8H2O, transformed to pure YAG at 900 °C without the formation of intermediate phases. Reactive YAG powder was produced by calcining the precursor at 1100 °C. The YAG powder densified to 99.8% of the theoretical density by vacuum sintering at 1500 °C for 2 h, and the sintered body showed transparency. Less agglomeration of the precursor and good dispersity of the resultant YAG powder were responsible for the excellent sinterability.
Yttrium aluminate garnet (YAG; Y3Al5O12) is an important ceramic material from both the mechanical and the optical points of view. YAG holds great potential for high-temperature applications because of its high melting point (–1970 °C),1 excellent chemical stability,1 and outstanding creep resistance.2 Besides, because YAG is intrinsically transparent and the dodecahedral sites of its structure can be partially or completely substituted with other cations such as Nd5+, Cr3+, Eu3+, etc.,3 it is also an ideal host material for phosphors and solid-state lasers. YAG powder is traditionally produced by a solid-state reaction between the component oxides, which requires repeated mechanical mixing and extensive heat treatment at temperatures as high as 1700 °C to eliminate yttrium aluminate monoclinic (YAM; Y4Al2O9) and yttrium aluminate perovskite (YAP; YAlO3) intermediate phases.4 Although the YAG formation temperature has been reduced to around 900 °C in recent years by adopting wetchemical methods, typically sol-gel processing5,6 and hydroxide coprecipitation,7,8 the sinterability of the produced YAG powders is still not good due to the severe agglomeration of the gel-like precursors during drying. Chemical composition and physical properties of the precursor could be manipulated by varying the precipitant as well as the precipitation conditions. Previous work showed that ammonium hydrogen carbonate (AHC) exceeds ammonium hydroxide solution for the production of less-agglomerated, well-sinterable alpha-alumina,9 and yttria10 powders via precipitation. The present work
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J. Mater. Res., Vol. 15, No. 9, Sep 2000 Downloaded: 12 Mar 2015
demonstrated that AHC is also a suitable precipitant for YAG synthesis: The derived carbonate precursor was loosely agglomerated and the resultant YAG powder showed good dispersity and excellent sinterability. The YAG powder under discussion was synthesized from a mixed solution of aluminum nitrate nonahydrate (>99.9% purity) and yttrium nitrate hexahydrate (99.99% purity) using a 1.5 M solution of AHC (ultrahigh purity) as the precipitant. All the chemicals were purchased from Kanto Chemical Co., Inc., Tokyo, Japan, and were used as
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