Utilization of multiple-stage sintering to control Ni electrode continuity in ultrathin Ni-BaTiO ₃ multilayer capacitors

Microstructural control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the present-day challenges for maintaining an increase in capacitive volumetric efficiency. The present paper continues a series of investigations aimed at understanding and controlling the microstructural stability of ultrathin Ni electrodes in MLCCs. Here, a kinetic approach based on the control of sintering profiles is used. Ni-BaTiO ₃ MLCC chips (0805-type with 300 active layers) are nonisothermally sintered up to 900°-1300°C with different heating rates in the range from 200° to 3000°C/h. In general, the continuity of the Ni electrodes increases with heating rate. However, a strong nonlinear dependence of Ni electrode continuity on sintering temperature is observed. It is concluded that a low-melting interfacial liquid (Ni,Ba,Ti) alloy layer initiates at temperatures between 1000° and 1100°C when the Ni electrodes are under tension. This interfacial liquid phase accelerates a stress-induced diffusion and is the key cause of the severe electrode discontinuities during heating. At higher temperatures (above 1100°C), where compressive stresses are active, the interfacial liquid alloy layer facilitates some recovery of the Ni electrode microstructure. The formation of the interfacial liquid alloy layer can be kinetically controlled using fast-heating rates, which improves the Ni electrode continuity.