The Ferroelectric-Ferromagnetic Composite Ceramics with High Permittivity and High Permeability in Hyper-Frequency

Abstract

With the rapid development of portable electronic products and wireless technology, many electronic devices have evolved into collections of highly integrated systems for multiple functionality, faster operating speed, higher reliability, and reduced sizes. This demands the multifunctional integrated components, serving as both inductor and capacitor. As a result, low temperature co-fired ceramics (LTCC) with integrated capacitive ferroelectrics and inductive ferrites has been regarded as a feasible solution through complex circuit designs. However, in the multilayer LTCC structure consisting of ferroelectrics and ferrites layers, there are always many undesirable defects, such as cracks, pores and cambers, owing to the co-firing mismatch between different material layers, which will damage the property and reliability of end products (Hsu & Jean, 2005). A single material with both inductance and capacitance are desired for true integration in one element. For example, if the materials with both high permeability and permittivity are used in the anti electromagnetic interference (EMI) filters, the size of components can be dramatically minimized compared to that of conventional filters composed of discrete inductors and capacitors. Because little single-phase material in nature can meet such needs (Hill, 1999), the development of ferroelectric-ferromagnetic composite ceramics are greatly motivated. Many material systems, such as BaTiO3 / NiCuZn ferrite, BaTiO3 / MgCuZn ferrite, Pb(Zr0.52Ti0.48)O3 / NiCuZn ferrite, Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3-PbTiO3 / NiCuZn ferrite and Bi2(Zn1/3Nb2/3)2O7 / NiCuZn ferrite, were investigated and found exhibit fine dielectric and magnetic properties. In these reports, spinel ferrites, such as NiCuZn ferrite, were always used as the magnetic phase of composite ceramics, because they are mature materials for LTCC inductive components. However, the cut-off frequency of spinel ferrites is limited below 100MHz by the cubic crystal structure, so the resulting composite ceramics can not be used in hyper-frequency or higher frequency range. To keep up with the trend towards higher frequency for electronic technology, hexagonal ferrites, including Y-type hexagonal ferrite Ba2Me2Fe12O22 and Z-type hexagonal ferrite Ba3Me2Fe24O41 (Me=divalent transition metal), should be used in the composite ceramics. Co2Z hexagonal ferrite has high permeability and low loss in hyper-frequency, but the very high sintering temperature (>1300oC) works against its application in LTCC. Y-type hexagonal ferrite has a bit lower permeability, but the excellent sintering behavior makes it a