(Bi1/2K1/2)TiO3-SrTiO3 solid solution ceramics for high temperature capacitor applications
Multilayer ceramic capacitors (MLCCs) have been widely used in many modern electronic devices such as smartphones, personal computers, digital cameras, etc. Today, there is a growing demand for MLCCs capable of operating at temperatures in excess of 200°C in many industrial sectors, such as the oil and gas industry, the aerospace industry, and the automotive industry. For example, SiC-based power devices for automotive applications are developed for temperatures up to 300°C, requiring their peripheral components, including MLCCs, to withstand the same harsh environment. For such high-temperature applications, dielectric materials for MLCCs should have high and temperature-stable dielectric constants, low dielectric losses, and high recoverable energy densities. BaTiO3 (BT)-based ferroelectrics are the most widely used dielectric materials for traditional MLCCs, but are not suitable for high-temperature applications because BT has a low Curie temperature at 130 °C and its dielectric constant drops sharply at higher temperatures.
In the search for alternative dielectric materials for high-temperature applications, researchers have so far investigated a number of material systems. Among them, relaxor ferroelectrics have attracted great interest due to their high dielectric constants and wide temperature dependence. Historically, Pb-based relaxors such as Pb(Mg1/3Nb2/3)O3 were first widely investigated for MLCC in the 1980s, but the toxicity of Pb limited their practical application in electronic devices. In this decade, Bi-containing lead-free relaxors have become the backbone of high-temperature MLCC dielectric materials. Among many Bi-containing relaxor compositions, sodium bismuth titanate (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions have attracted much attention due to their ability to modulate their electrical properties through compositional modifications. For example, BNT-NaNbO3 solid solutions have been reported to show high-temperature stable dielectric constants over a wide temperature range of -60-400°C. On the other hand, the BNT-(Sr0.7Bi0.2)TiO3 system can obtain a huge recoverable energy storage density due to its anti-Fe electrolysis-like response. Although BNT-based solid solution relaxation has been studied extensively so far, its dielectric constant is generally below 2000, which is lower than that of Pb-based relaxation. Therefore, there is a need to investigate a new material system to achieve better dielectric properties.
In this paper, to measure the dielectric and ferroelectric properties of ceramic samples, silver conductive paste was applied to the polished surface of the samples and fired in air at 600°C for 10 minutes. The temperature dependence of the dielectric constant and the tangent of the dielectric loss angle (tan δ) were measured using an LCR meter (ZM2371, NF Corp.) in the temperature range from room temperature to 400°C. The temperature dependence of the dielectric constant and the tangent of the dielectric loss angle (tan δ) were measured using an LCR meter (ZM2371, NF Corp.). To measure the polarization (P) versus electric field (E) curves, ceramic samples were cut into small square plates of approximately 3 × 3 mm2 size and electrodeposited by Au sputtering. The P-E curves were then measured using a ferroelectric test system (FCE10-B, TOYO Corp.) equipped with a high-voltage amplifier by applying a delta voltage wave with a frequency of 0.5 Hz at temperatures up to 150 °C (HEOP-5B6, Matsuda Precision Corp.).
Figure 7 shows the P-E curves of BKST-x ceramics measured at room temperature. The BKST-0.0 shows a significant hysteresis due to a residual polarization (Pr) of 25 μC cm-2 for the ferroelectric polarization switching. The asymmetric shape about its diagonal implies that the sample is still ferroelectric. As the ST content increases, the hysteresis becomes thinner and Pr decreases. BKST-0.4 and BKST-0.5 show slim nonlinear P-E curves, which are attributed to their relaxation behavior. This observation is consistent with the stability of the R state implied by the temperature dependence of the small-field dielectric properties. Such fine P-E curves at large electric fields are important for capacitor applications, as hysteresis leads to energy loss during operation.
Author: MinamiShiga ,Manabu Hagiwara,Shinobu Fujihara
Institution: Department of AppliedChemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi,Kohoku-ku, Yokohama, 223-8522, Japan
Published: April 15, 2020; Revised August 17, 2020; September 21, 2020
Key words: powder: chemicals, dielectric properties, capacitors, lead-free relaxants
Journal: Ceramics International
Article source website: (Bi1/2K1/2)TiO3-SrTiO3solid-solution ceramics for high-temperature capacitor applications -ScienceDirect