Interfacial engineering in heterogeneous NiCuZn ferrite/(Mg,Ca)TiO₃ 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 interlocking brazing, for the fabrication of robust NiCuZn ferrite (NCZF)/(Mg,Ca)TiO₃ (MCT) joints, overcoming the limitations inherent to conventional adhesive bonding. The addition of low-melting-point ZnO–B₂O₃–SiO₂ (ZBS) 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 is just 29 μm. Subsequently, brazed substrates were fabricated on the basis of the wettability of the La₂O₃–CaO–ZnO–B₂O₃–SiO₂ (LCZBS) glass. The ceramic boundaries undergo localized dissolution by the erosive interaction of molten glass, whereas the width of the brazing seam increases with brazing temperature (T_b), with the narrowest transition region of 30 μm. Ti⁴⁺ ions from MCT diffused through the glass network, forming a ZnO–TiO₂ enrichment interfacial layer at the NCZF boundary. Moreover, the Mg₂TiO₄ whiskers grow into the solder region through in situ reactions at the 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 substrates. By elucidating the distinct interfacial regulatory 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.