Low temperature stabilization of zirconia by Mn through Co-precipitated hydroxide gel route
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Stabilization of zirconia into cubic phase is achieved by partly substituting Zr 4+ with Mn 4+ ions (5-30 mole %) via hydroxide gel formation and subsequent calcination at 773 K and is supported by XRD and IR data. A linear correlation between the lattice parameter and the Mn content confirms the incorporation of Mn into ZrO 2 . The XPS and TPR results provide some evidence for the presence of Mn 4+ ions in these samples which have a surface area of about 100 m2 g" 1 and are stable in the cubic phase up to 973 K. On reduction above 973 K, the cubic phase is stabilized probably by Mn 2+ ions.
Among various oxides of the group IV B, ZrO 2 is unique in its properties, such as permeability, thermal shock resistance, and ionic conductivity, and finds application as catalysts, gas sensors, gas turbines, thermal barrier coatings, etc.1"* The three polymorphs of zirconia, viz., monoclinic, cubic, and tetragonal, have been well characterized.5 Among them, the cubic form is known to play a significant role in the field of ceramics and refractory oxides. Apart from this, it can also act as a good catalyst.6""8 For all these reasons, considerable work has been done to stabilize zirconia into cubic phase. The various polymorphs of zirconia can be made stable or metastable even at room temperature by manipulating the method of preparation or by the addition of some metal oxides. Solid solutions of zirconia and various metal ions have been widely investigated. It has been reported that the addition of alkaline earth oxides such as CaO and MgO, group III B metal oxides such as Sc 2 O 3 and Y 2 O 3 , and rare earth oxides such as La 2 O 3 and Ce 2 O 3 stabilizes zirconia into cubic (fluorite) phase.1'9'10 Recently, Co(o)-, Ru(iv)-, and La(m)-stabilized zirconia solid solutions have been reported.11'12 Although the cubic phase stabilization of zirconia using other transition metal oxides such as MnO, NiO, Cr 2 O 3 , Mn 2 O 3 , CuO, and Fe 2 O 3 had earlier been reported,1'13 they required either a very high temperature (1275 K) of calcination or the presence of some other metal oxide, such as TiO 2 as an adduct. To tailor ZrO 2 for catalytic applications, a lower calcination temperature is preferable in order to get a high surface area, apart from the creation of oxygen ion vacancy and a stable single phase. Presently, this has been achieved by stabilizing ZrO 2 with Mn 4+ ions using a hydroxide gel method. The synthesis of 5-30 mole % Mn-substituted cubic zirconia and characterization using XRD, IR, TPR, and XPS analysis is reported in this paper. The Mn 4+ stabilized zirconia was obtained by a coprecipitation method using high purity zirconyl nitrate J. Mater. Res., Vol. 9, No. 4, Apr 1994 http://journals.cambridge.org
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(99.9%, Loba Chemie) and manganese acetate (99.9%, Loba Chemie). Required quantities of solutions of these two salts were mixed and the hydroxides were precipitated at room temperature by a slow addition of tetramethylammonium hydroxide (25% aqueous solution) with constant stirring until t
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