Highly sinter-active nanocrystalline RE 2 O 3 (RE = Gd, Eu, Dy) by a combustion process, and role of oxidant-to-fuel rat
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Highly sinter-active powders of RE2O3 [rare earth (RE) ⳱ Gd, Eu, Dy] have been prepared using the corresponding metal nitrates as the oxidants, and glycine and citric acid as the fuels. Two different oxidant-to-fuel ratios, namely stoichiometric ratio and fuel-deficient ratio were used to explore the possibility of preparing different crystallographic modifications. By a careful control of oxidant-to-fuel ratio, nanocrystalline Eu2O3 and Gd2O3 could be prepared in cubic (C-type) as well as monoclinic (B-type) modifications. However, the high-temperature monoclinic modification could not be obtained for Dy2O3 due to a very high C-to-B-type phase transition temperature. The crystallite size, surface area, and sintering behavior were also studied for powders prepared using different oxidant-to-fuel ratios, and the results showed a remarkable correlation between different fuel contents and powder properties. Some of these powders resulted in pellets of nearly theoretical density. The sintered microstructure was studied by scanning electron microscopy.
I. INTRODUCTION
The synthesis and study of nanocrystalline materials has gained tremendous momentum in the recent past. The nanocrystalline ceramics have far superior powder properties, such as large surface area and higher sinterability. Another motivation for preparing the nanocrystalline materials is to tune the various properties, which usually show a gradual transition from solid-state matter to molecular structures as the particle size is decreased. Such materials usually show greatly modified optical, electronic catalytic,1 and magnetic2 properties compared to their microcrystalline counterparts. Another important aspect concerning nanocrystalline ceramics is the preparation of various modifications of a given material by tailoring the processing conditions. In fact, there are a number of investigations in which the metastable phases were obtained by converting a given material into its nanostate.3 Rare-earth (RE) sesquioxides have a wide range of applications as, for example, refractory materials, phosphors, or catalysts.4 RE oxides like Gd2O3, Eu2O3, and Dy2O3 are common neutron absorbers, and are used as burnable poison in nuclear reactors.5 The RE sesquioxides are known to exist in several modifications depending on ionic size and temperature. Adachi and Imanaka,6 in their a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0101 J. Mater. Res., Vol. 22, No. 3, Mar 2007
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review article on binary RE oxides, lucidly elaborated on the structure of RE sesquioxides as a function of temperature as well as ionic size. At temperatures below 2000 °C, three types of polymorphs, designated as A (hexagonal), B (monoclinic), and C (cubic) have been reported in RE2O3 depending upon the RE ionic size. In general, on going from La to Lu, the structure of RE2O3 changes from A-type to B-type to C-type. Foex and Traverse7 have reported that the stability of C-type modification in
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