Well-sinterable Y 3 Al 5 O 12 Powder from Carbonate Precursor
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Carbonate precursors of Y3Al5O12 (YAG) were synthesized from a mixed solution of alum and yttrium nitrate using ammonium hydrogen carbonate as precipitant. Precipitation method (normal-strike or reverse-strike) and aging were found to have dramatic effects on cation homogeneity of the precursor, which in turn influenced the formation temperature of the YAG phase. Reactive YAG powders were produced from the reverse-strike-derived, as-synthesized precursors at temperatures 艋1200 °C. These powders densified to >98.0% of the theoretical density up to 1500 °C at a constant heating rate of 8 °C/min or to transparency by vacuum sintering at 1700 °C for 1 h without additives.
I. INTRODUCTION
Yttrium aluminum garnet, Y3Al5O12 (YAG) is an attractive ceramic material from the viewpoints of both its optical and mechanical properties. Recent work of Corman1 and Parthasarathy et al.2,3 identified that YAG possesses exceptionally high creep resistance and is the most temperature-resistant material among oxides. This enables YAG to hold great potential as a high-temperature structural material or as the reinforcing constituent of composites. Besides, since single-crystal YAG is optically transparent and the dodecahedral sites of its structure can be partially doped or completely substituted with other cations such as Nd5+, Cr3+, Eu3+, Tb3+ etc., it is also an ideal host material for solid-state lasers and phosphors.4–7 Typically, YAG powders are synthesized by a solidstate reaction between the individual component oxide powders.8–11 This method generally requires repeated mechanical mixing and extensive heat treatment at high temperatures to achieve the desired phase purity. Previous work10,11 revealed that prolonged heating of Al2O3 and Y2O3 mixtures between 1500 and 1600 °C does not produce pure YAG powder free from yttrium aluminum monoclinic, Y4Al2O9 (YAM) and yttrium aluminum perovskite, YAlO3 (YAP) intermediate compounds. So, unlike the processing of single-component oxides such as Al2O3, Y2O3, MgO, etc., high-level mixing of Al and Y elements must be made at the early stages of the processing of YAG powder to achieve lower formation temperature, higher phase purity, and excellent sinterability. It has long been accepted that the wet-chemical process offers advantages of good mixing of the starting materials and excellent chemical homogeneity of the final a)
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J. Mater. Res., Vol. 15, No. 7, Jul 2000
product. Several kinds of wet-chemical methods or wetchemical-related methods have been developed and successfully used for YAG synthesis. These methods include hydroxide coprecipitation,12–16 urea homogeneous precipitation,17,18 sol-gel processing,19–22 hydrothermal treatment,23 glycothermal treatment,24 pyrolysis of metal nitrate or sulfate mixtures,25–27 combustion synthesis,28,29 and a modified Pechini process.30 These methods proved effective in lowering YAG formation temperature and improving phase purity of the final product. Unfortunately, highl
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