Nb-doped 0.90BaTiO 3 -0.10(Bi 0.5 Na 0.5 )TiO 3 temperature-insensitive ceramics with novel core-shell structure were sintered at low temperature by the conventional solid-state reaction method. The beneficial role of Nb in facilitating the formation of core-shell structure because of chemical inhomogeneity is verified, which is responsible for the weak temperature dependence of dielectric properties. Temperature dependence of permittivity measured at different frequencies shows high frequency dispersion at low temperature, while without relaxor characteristic at high temperature. The Vogel-Fulcher model was adopted to study the relaxor behaviour of Nb-doped 0.90BaTiO 3 -0.10(Bi 0.5 Na 0.5 )TiO 3 ceramics at low temperature. The samples with an addition of 1.5 mol% Nb 2 O 5 provide a temperature coefficient of capacitance meeting the requirements of the X9R characteristic, and result exhibits an optimum dielectric behaviour of ε r ∼1900, tanδ ∼1.8% at room temperature, making the material a promising candidate for high temperature applications.
IntroductionMultilayer ceramic capacitors (MLCCs) have been widely used in many kinds of electronic products since they were first developed in 1960s. 1,2 The capability of its withstanding temperature being over 150°C is quite crucial and essential for their application in oil drilling, aerospace and other fields. To integrate the power transistors and smart power devices into the electromechanical actuator, it is also important for MLCC power devices to be able to operate at 175-200°C. 2 However, neither the X7R (−55 to 125°C, ΔC/C 25°C ≤ ±15%) nor the X8R-type (−55 to 150°C, ΔC/C 25°C ≤ ±15%) MLCCs defined by the Electronic Industries Association (EIA) standards are competent as their ceiling temperatures are 125°C and 150°C , respectively. 3-6 Therefore, it is necessary to exploit a material used for X9R-type (−55 to 200°C, ΔC/C 25°C ≤ ±15%) MLCCs, then developing a new temperatureinsensitive material that can withstand higher temperature (≥200°C) and show weak temperature dependence of properties, might be the only alternative approach.As well known, most high-temperature dielectrics are based on the barium titanate (BaTiO 3 ) materials.However, it is difficult to prepare pure BaTiO 3 (BT) ceramics with weak temperature dependence due to the dramatic variation of permittivity above the Curie temperature (T c = 130°C). In order to obtain hightemperature BaTiO 3 -based dielectrics, the first step is to escalate T c point. It has been reported that T c can be raised up by replacing Ba sites with smaller ionic radius elements, but such effect is not sufficient. 2,7 Moreover, though Pb ions are very beneficial in raising T c , the use of lead-based ceramics has been restricted due to its toxicity. Another way to increase T c is to combine BT with materials with high T c . (Bi 0.5 Na 0.5 )TiO 3 (BNT) is an attractive candidate material because of its strong ferroelectricity at room temperature and relatively high T c (∼320°C). 8,9 Additionally, BNT can form solid solution w...