Oxygen transport during formation and decomposition of AgNbO 3 and AgTaO 3
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A thermogravimetric method was used to analyze intermediate processes involved in the formation and decomposition of AgNbO3 and AgTaO3 perovskites. Critical parameters that control the kinetics of the formation are associated with oxygen transport. The Nb2O5 crystal structure has a capacity to trap molecular oxygen, which evolves during the decomposition of Ag2O that is present in a starting mixture. The formation of the perovskite phase involves a simultaneous reaction of three species: O2, Ag, and Nb2O5/Ta2O5. As the trapped molecular oxygen is in the immediate vicinity of the reaction site, the kinetics of the reaction is significantly accelerated. An absence of the molecular oxygen in the solid-state phase cannot be compensated for with an increase in a partial pressure of oxygen in the gas phase, that is, application of oxygen atmosphere.
I. INTRODUCTION
Ag(Nb,Ta)O3 solid solutions are perovskite materials with a highly flexible crystal structure, which can be modified to induce different ferroelectric states or, alternatively, to remain in a paraelectric state.1,2 Initially, research and development of these materials have targeted only passive electronic applications,3 while today the main interest has shifted toward their piezoelectric properties4,5 and voltage tunability of dielectric constant.6 The majority of current investigations on Ag(Nb,Ta)O3 are focused on technologies based on thick and thin films.7 These differ from those investigations in which Ag(Nb,Ta)O3 has been investigated and used as a bulk material. However, some fundamental questions regarding the solid-state chemistry of these materials remain equally important for the bulk chemistry and thin films. The production of Ag(Nb,Ta)O3 powder is fraught with problems related to the synthesis of the material.8 The formation of Ag(Nb,Ta)O3 is highly sensitive to firing conditions, starting materials, powder packing, and so forth, and sometimes fails without any obvious reason. Despite this, no detailed study of the formation mechanism and reaction kinetics has been reported. It is taken for granted that a high-oxygen vapor pressure is required during the synthesis of this material, although the critical mechanisms that control the kinetics of the reaction have
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0196 1650 J. Mater. Res., Vol. 22, No. 6, Jun 2007 http://journals.cambridge.org Downloaded: 11 Mar 2015
never been determined. Understanding the formation processes would allow an optimization of a powder production in terms of a synthesis of purer powders, reproducibility, economic efficiency, and optimal scale up of the production. In addition, understanding oxidation– reduction mechanisms of Ag species that are involved in the synthesis of AgNbO3, oxygen transport, kinetics, and the thermodynamics of these processes is essential for studies of the processes occurring during combustion of gels or a deposition of thin films. Usually, the solid-state synthesis of AgNbO3 starts from Nb2O
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