HAN Liqiang ¹, LI Xiulan ¹, XU Jialing ²

(1. Weiqi Electronics Technology Co., Ltd., Mianyang 621000, Sichuan, China; 2. School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621000, Sichuan, China)

Extended abstract:[Background and purposes] With the rapid rise of strategic emerging industries, such as 5G communications, artificial intelligence and new energy vehicles, electronic devices are rapidly evolving toward high performance, multifunctionality and extreme environmental adaptability. This places increasingly demanding challenges and requirements on the performance of dielectric materials for multilayer ceramic capacitor (MLCC). The Ba_6−3xLn_8+2xTi₁₈O₅₄ ceramic system, with tungsten bronze structure, has emerged as a highly promising dielectric material for MLCC, due to its high dielectric constant, low dielectric loss and excellent temperature stability. In this study, a two-step sintering method was employed to prepare Ba(Sm_0.1La_0.5Bi_0.4)₂Ti_3.8(Al_0.5Nb_0.5)_0.2O₁₂ (BST-BLAN) ceramics. Using COMSOL software, the electric field distribution and breakdown path evolution of the ceramics with different microstructures were simulated, systematically revealing the influence of sintering processes on material structure and performance.[Methods] The raw materials were mixed and ball-milled for 4 h with a planetary ball mill, dried and pre-sintered at 1100 ℃ for 4 h. After secondary ball-milling for 8 h, the powder was granulated and pressed into 10 mm×2 mm green bodies. The green bodies were sintered with a two-step method. They were first heated at 5 ℃·min⁻¹ to 650 ℃ and held for 2 h to remove PVA, then heated at 10 ℃·min⁻¹ to T1, held for 1 min, and cooled to T2 (170 or 200 ℃ below T1). After holding for 2 h, 12 h, or 24 h, the samples were cooled down with the furnace. The sintered ceramic samples underwent grinding, polishing, ultrasonic cleaning, electrode coating and powder milling before characterization with XRD, SEM, dielectric property and breakdown strength. Simultaneously, COMSOL software was employed to simulate electric field distribution and breakdown path evolution of the ceramics with different microstructures.[Results] Average grain size of the two-step sintered samples decreased significantly. With increasing T1, the density first increased then decreased, reaching maximum value at 1230 ℃. At 1260 ℃, the second phase disappeared, due to the fostered Bi³⁺ volatilization. With increasing T1, the dielectric constant exhibited a parallel trend to the variation of density, peaking at 157 at 1230 ℃, higher than that of the conventionally sintered samples. Conversely, dielectric loss followed an opposite pattern, reaching a minimum of 0.04% at 1230 ℃. The absolute value of the temperature coefficient decreased slightly with rising T1. Breakdown strength was significantly improved when using the two-step sintering method. It is first increased and then decreased with rising T1, reaching the maximum value of 10.2 kV·mm⁻¹ at 1230 ℃. Increasing T2 and prolonging holding time both cause Bi³⁺ volatilization to outweigh grain growth effects, reducing relative density. The maximum relative density of 98.7% is achieved in the sample sintered at T2=1030 ℃ for 12 h. Dielectric constant exhibits a similar trend to density, reaching the maximum value of 161 at T2=1030 ℃ for 12 h. Conversely, dielectric loss follows an opposite trend, reaching the minimum value of 0.028% at T2=1030 ℃ for 12 h. Temperature stability showed slight improvement at T2=1030 ℃ for 12 h. With varying T2 and holding times, density and porosity were the primary factors influencing breakdown strength. Breakdown strength followed the same trend as density, reaching the maximum level of 13.0 kV·mm⁻¹ at T2=1030 ℃ for 12 h.[Conclusions] Two-step sintering did not cause significant changes in phases of the ceramics. Compared to conventional sintering, it resulted in finer, more uniform grains and higher grain boundary density. Appropriately increasing T2 temperature and holding time reduces internal defects and enhances density, thereby improving dielectric properties and breakdown strength. However, excessive increases in these parameters lead to adverse effects due to the accelerated volatilization of Bi³⁺.

Key words: BST-based ceramics; two-step sintering method; microstructure; dielectric properties; breakdown strength.