The use of copper (Cu) electrodes in multilayer ceramic capacitors (MLCCs) is highly attractive due to its low cost and electrical resistivity compared with nickel (Ni). However, Cu easily oxidizes and diffuses into BaTiO3 (BT) during high-temperature sintering, degrading dielectric properties and breakdown strength. In this study, 24 GHz millimeter-wave (MMW) irradiation was employed to investigate and control Cu diffusion in BT ceramics. Pre-sintered dense BT was coated with Cu paste and subsequently heated either conventionally (Conv.) or under MMW irradiation at 950–1050 °C. MMW irradiation significantly suppressed Cu diffusion into BT, increasing the activation energy for diffusion from 282 to 340 kJ mol−1. Under Conv. heating, oxygen-vacancy accumulation stabilizes mixed-valence Cu states in the CuO grain-boundary phase, promoting long-range Cu diffusion. In contrast, enhanced oxygen-vacancy mobility under MMW irradiation suppresses such defect-rich states, resulting in reduced Cu diffusion. These results demonstrate that MMW irradiation provides a nonthermal pathway to selectively control cation diffusion, offering a promising strategy for developing reliable Cu-based MLCCs.
Teranishi et al. (Mon,) studied this question.