Precursor and processing effects on BaPbO 3 formation kinetics
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Guerman Popov Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854
Paul R. Mort Procter & Gamble Co., Cincinnati, Ohio 45217
Richard E. Rimana) Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854 (Received 10 May 2005; accepted 8 August 2005)
The synthesis of BaPbO3 from a wide range of mixtures containing metalorganic precursors, nitrate precursors, lead oxides, barium oxide and peroxide was investigated, and the kinetics was analyzed using the Johnson–Mehl–Avrami (JMA) equation. It was found that Ba and Pb stearate soaps and Pb oxalate that were used as metalorganic precursors formed BaCO3 and PbO or Pb3O4 after firing at 440 °C. The formation rate of BaPbO3 from a metalorganic precursor system is not higher than that from the conventional BaCO3–PbO system and does not depend on mixing methods or the kinds of metalorganic precursors but instead on the synthesis atmosphere. In the case of the BaCO3–PbO system, the Avrami exponent (n) is ∼1, indicating that the reaction is controlled by the phase-boundary-contraction interface reaction. For the BaO2–PbO2 system, n has two values ∼1 and ∼0.3, depending on the reaction temperature and time, indicating that the reaction is either controlled by the phase-boundary-contraction interface reaction or diffusion-controlled reaction. In the Ba nitrate–Pb nitrate system, phase-pure BaPbO3 is obtained at 550 °C, which is 250 °C lower than in the case of the BaCO3–PbO system. The value of n for the nitrate system is ∼1.5, indicating that the reaction is controlled by a three-dimensional (3D) diffusion-controlled nucleation mechanism. In the BaO–PbO system, the formation of BaPbO3 started at 350 °C by an exothermic reaction and the content of BaPbO3 in the product was ∼40 wt%, which is independent of reaction temperature as well as time in the temperature range of 350–500 °C.
I. INTRODUCTION
BaPbO3 is a perovskite-type compound with metallic conductivity and is reported as a promising oxide material for high-temperature thermoelectric energy conversion1 and superconductivity.2,3 The traditional way of preparing BaPbO3 is a solid-state reaction of barium carbonate and lead oxide at a relatively high temperature (∼980 °C).4 However, the high temperatures used for the solid-state reaction lead to lead oxide volatilization, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0071 584
J. Mater. Res., Vol. 21, No. 3, Mar 2006
which necessitates the use of excess lead oxide to obtain phase pure BaPbO3. With the aim of controlling the phase purity, it is preferred to synthesize the compound from a stoichiometric starting mixture at the lowest possible temperature to avoid the evaporation of PbO. For this reason, alternative methods have been proposed to synthesize BaPbO3 that would allow for lower synthesis temperature. The methods are based on chemical synthesis routes such as the nitrate solid
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