Mechanochemical Synthesis of Lead Magnesium Niobate Ceramics in Iron Media
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INTRODUCTION
LEAD magnesium niobate (PMN) belongs to a family of complex perovskites with broad and large dielectric permittivity maxima at -8 C and 1 kHz, whose position shifts toward higher temperatures with frequency within a range of 20 C.[1] The PMN also has very large electrostrictive and field-induced piezoelectric coefficients, low hysteresis losses,[2,3] and other properties, which make PMN and its modifications suitable for applications such as chip capacitors, piezoelectric transducers, and actuators.[2,4] The lead iron niobate (PFN) also belongs to the group of perovskite materials. It is interesting because of the extremely high dielectric constant, but also significantly higher Curie temperature of 112 C.[2] The PMN-PFN is also considered as an attractive material with interesting possible applications.[2,3] The synthesis of lead-based perovskite ceramics is always followed with the problem of stable cubic pyrochlore phase formation. Therefore, methods to obtain pyrochlore-free ceramics were widely studied and several methods were proposed: the columbite/ wolframite method,[5,6] simplified wolframite,[7] reaction sintering,[8] and mechanochemical synthesis.[1,9] In conventional solid-state and Columbite methods, the loss of PbO is inevitable when the multiple steps of phaseforming calcinations and subsequent sintering are carried out at high temperatures. Mechanochemical synthesis in high-energy mills could be a possible Z. BRANKOVIC´ and G. BRANKOVIC´, Senior Researchers, and V. VUKOTIC´, Postdoctoral Student, are with the Institute for Multidisciplinary Research, Belgrade University, Kneza Visˇ eslava 1a, 11000 Belgrade, Serbia. Contact e-mail: [email protected] R. TARARAM, Postdoctoral Student, and J.A. VARELA, Professor, are with the Instituto de Quimica, UNESP, 14800 Araraquara, SP, Brazil. Manuscript submitted May 3, 2007. Article published onlined February 14, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
solution to these problems. In this method, chemical reaction occurs during intensive milling of the precursor powders and steps of phase-forming calcinations can be avoided. It has to be emphasized that mechanochemical synthesis is usually connected with the problem of contamination from the milling media. Usually, a small amount of material from a milling media incorporates into the processed powder. The amount of incorporated material depends on the milling conditions (time and intensity), as well as on the mechanical properties of the milling media and processed material. Although this amount is usually small (about 1 wt pct), it can have a dramatic influence on some properties, especially on the electrical parameters. Because of this fact, it is extremely important to determine the amount of incorporated material and the method of incorporation (incorporation into crystal lattice, formation of new phase through solid-state reaction with material being processed, or formation of simple mixture of powders). During the mechanochemical synthesis of PMN in iron media, the small amount of PF
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