Interfacial engineering in heterogeneous NiCuZn ferrite/(Mg,Ca)TiO ₃ ceramic composite circulator substrates

Heterogeneous nested composite substrates significantly enhance circulator performance over single-ferrite substrates. This study establishes two distinct interface-engineering strategies, diffusion-suppressed co-firing and mechanically-interlocked brazing, for fabricating robust NiCuZn ferrite (NCZF)/(Mg,Ca)TiO₃ (MCT) joints, overcoming the limitations inherent to conventional adhesive bonding. The low-melting ZnO-B₂O₃-SiO₂ addition tailored the sintering behavior and dielectric properties of MCT, enabling co-firing with NCZF at 1050 °C. Thermal shrinkage adjustment effectively suppresses ion interdiffusion driven by compressive stresses from radial extrusion of the outer ring contraction, and the width of the transition region just 29 μm. Subsequently, brazed substrates were fabricated based on the wettability of La₂O₃-CaO-ZnO-B₂O₃-SiO₂ glass. The ceramic boundaries undergo localized dissolution by erosive interaction of molten glass, while the width of the brazing seam increases with brazing temperature, with a narrowest transition region of 30 μm. Ti⁴⁺ ions from MCT diffused through the glass network, forming a ZnO-TiO₂ enrichment interfacial layer at NCZF boundary. Meanwhile, Mg₂TiO₄ whiskers grow into the solder region through in situ reactions at MCT interface, forming a mechanically interlocked architecture. This structure serves as the primary contributor to the superior shear strength of brazed substrates relative to co-fired. By elucidating the distinct interfacial regulation mechanisms in co-firing and brazing, this study establishes a foundation for precision interface design in high-reliability composite substrates, supporting the development of high-performance circulators for microwave applications.