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|Relaxor dielectrics for multilayer capacitors with temperature-stable permittivity up to 250°C|
|Keywords: relaxor dielectrics, high temperature, sintering aids|
|High demands for integration and efficiency of electronics in technical and chemical process monitoring require new electronic components operable in harsh environments. Environment-friendly and commercially available capacitors are restricted to working temperatures up to 150-200°C (X9R). Nowadays there is a lack for reliable dielectric materials for capacitor applications well above these temperatures, hence new dielectric materials are necessary to satisfy those demands. A new class of complex perovskite ceramics (“weakly coupled relaxors“) exhibits medium permittivity with low dielectric losses. Another particular feature is a stable trend of permittivity over a wide temperature range. Those desirable characteristics are achieved with cation substitution of the A- and B-site into a standard ferroelectric perovskite BaTiO3. We report on the synthesis of complex perovskite-type compositions 0.45(Ba0.8Ca0.2)TiO3-0.55- xBi(Mg0.5Ti0.5)O3-xNaNbO3 and (100-x) (0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3)- xCaZrO3 solid solutions via conventional solid-state reaction route. Phase formation and crystal structure properties were examined through X-ray diffraction methods and indicates a cubic or pseudocubic structure for all compositions between x=0-0.3. Analysis of the shrinkage behavior shows minimum sintering temperatures around 1150°C for obtaining dense samples with relative densities over 95% of the theoretical value. The frequency and temperature dependent dielectric response was characterized between 30-300°C using impedance spectroscopy. Both material systems show relaxor-like, frequency- dependent properties with a varying temperature for the maximum of the permittivity Tm. At temperatures above Tm all samples exhibit a stable and frequency-independent relative permittivity between 400 and 1800 matched with a low dielectric loss (tanδ below 0.02). Some compositions show a stable permittivity around 500- 600 with a maximum deviation of +-15% at 1 kHz over the whole measured temperature range. Selected compositions could be suited for application at temperatures between room temperature and well above 200°C. Selected relaxor compositions were processed to fabricate multilayer ceramic capacitors (MLCCs). The sintering behavior was modified through sintering aids, e.g. Li2CO3 and CuO. It was necessary to lower the sintering temperature from around 1150°C to match co-fire temperatures with noble metal electrodes between 900-1050°C. Through the addition of 0,75 wt% - 1 wt% of Li2CO3 the sintering temperature could be lowered below 900°C, enabling co-firing with commercial silver/palladium electrode pastes, as well as potential integration in complex multilayer LTCC modules. It will be shown, that the low-temperature cofiring allows fabrication of multilayer structures with good dielectric response. Green tapes of these compositions were fabricated via tape casting and multilayer laminates were sintered at 900°C and evaluated. First tests of multilayer capacitor structures will be reported.|
|Thomas Schulz, scientific assistant
University of Applied Sciences Jena