Bi 2 O 3 Solubility of Bi-based Pyrochlores and Related Phases
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Bismuth oxide solubility in Bi2O3–ZnO–Nb2O5 and Bi2O3–MgO–Nb2O5 (BMN) pyrochlore systems was investigated and the solubility limits were identified. Differences in the solubility behavior between the two Bi-pyrochlore systems investigated were analyzed and explained through crystal chemistry concepts. For the BMN system, the dielectric constant was enhanced, and a systematic shift occurred in the dielectric relaxation loss peak to higher temperatures with increases in the excess bismuth are reported in the BMN system. The relationship between solubility and the shift in the dielectric loss curve was analyzed under the scope of possible relaxation mechanisms.
I. INTRODUCTION
Low-fire bismuth-based ceramics have been studied in the past by several researchers.1–10 Among the many systems investigated, the Bi2O3–ZnO–Nb2O5 (BZN) pyrochlore system has received special attention. There are two main phases in the system: a cubic pyrochlore phase Bi3/2ZnNb3/2O7 (dielectric constant approximately 150, temperature coefficient of capacitance approximately −400 ppm/°C) and a monoclinic zirconolitelike phase Bi2Zn2/3Nb4/3O7 (dielectric constant approximately 80, temperature coefficient of capacitance approximately 200 ppm/°C).6–11 Therefore, a two–phase composite with negative or positive, but almost zero temperature coefficient of capacitance (NP0) characteristics (±15 ppm/°C between −55 and 125 °C) is attainable within the system. In addition to this, low sintering temperatures (below 950 °C) and relatively high dielectric constants make the BZN system very appealing for manufacturing low-fire, high-frequency, multilayer devices such as capacitors with internal silver electrodes. Recent studies have investigated the formation, stability, processing windows, and crystallographic characterization of the main phases in the BZN system.10–13 In particular, Nino et al. synthesized a series of ceramic batches with excess and deficient ZnO with respect to the base formulations. Phase stability was analyzed and the corresponding solubility limits were reported.10 These results have provided useful information for the manufacturing of dielectric devices based on BZN ceramics cofirable with silver electrodes. In this respect, very a)
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J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 15 Feb 2015
recent publications from both industry and academia have reported successful manufacturing of prototype devices using these materials.14,15 Prototypes reported include low p ass inductor-capacitor (LC) filters and a BZN NP0 dielectric compatible with commercial low-temperature cofired ceramic tape systems, amongst others. In spite of all these achievements, many questions still remain to be answered about Bi-based pyrochlore systems. To expand our knowledge on the processing windows and solubility limits of Bi-based pyrochlore
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