Synthesis And Electromagnetic Performance Enhancement Of Sol-Gel Derived Percolative BaTiO₃//Ni_0.5Zn_0.5Fe₂O₄ Ceramic Composite

The miniaturization, integration and multi-functionality of modern electronic devices bring new challenges to materials science. Multiferroic materials possess two or more basic ferroic properties such as ferroelectricty, ferromagnetism and ferroelasticity, and the coupling effects between the two constituent phases, thus become appealing to scientists in recent years. Ferroelectric/ferromagnetic composites are typical multiferroic materials that exhibit both ferroelectricity and ferromagnetism simultaneously, together with giant permittivity and high permeability, hence can be potentially used in next-generation devices.BaTi03(BTO)/Ni0.5Zn0.5Fe2O4(NZFO) ceramic composite is a typical ferroelectric/ferromagnetic composite. On one hand, it is a percolative system. In the neighborhood of percolation threshold fc, the system experiences a second-order phase transition. Accoding to percolation theory, giant permittivity could be obtained in the composite due to the nonlinear critical behavior near fc. Similarly, there propbaly exist some other critical behaviors in respect of the magnetic properties of the composite accompanied by the formation of percolation networks. Such behaviors are worth being investigated because it is helpful to reveal the physical mechanisms behind them and establish a clear physical view on the interactions between ferroelectric phase and ferromagnetic phase. On the other hand, the formation of conductive percolation networks gives rise to an increment in dielectric loss, thus seriously undermining the comprehensive performance of the composite, which is not beneficial for applications. Up to now, the investigations on low-loss percolative composite are mainly focused on polymer-based materials, but the research on ceramic-based composites is still scarce.In this work, the microstructure-property relation in BTO/NZFO ceramic composite was revealed and some trials were conducrted to reduce the dielectric loss via investigating the physical mechanisms, controlling topological microstructure and introducing barrier layer into the composite. The influence of thermal treatment conditions on the formation and microstructure of BTO/NZFO ceramic composite was investigated, and the optimal densification conditions were determined. Meanwhile, the effect of thermal treatment conditions on the grain size, interface amount and intergrain connectivity of the composite was also revealed, making the relation between microstructure and properties more clear. On the basis of these results, related theoretical models have been modified in order to attain wider generality.The critical behavior of the magnetic properties of BTO/NZFO ceramic composite induced by percolation was also observed in this work. For conventionally sintered samples, the percolation threshold fc was found to be 0.55, at which the topological microstructure of the composite exhibits a sudden transition by transforming non-magnetic BTO matrix to magnetic NZFO matrix, leading to the occurrence of critical behavior of microstructure-sensitive magnetic properties. For example, the initial permeability of the composite starts to increase significantly above fc, and the coercivity of the composite reaches a minimum value at it. However, the parameters that are not sensitive to microstructure such as saturation magnetization are not influenced by such topological microstructure transition.The influence of the semi-conductive behavior of NZFO phase on the dielectric properties of BTO/NZFO ceramic composite was also investigated. On one hand, the grain size of NZFO phase in the composite increases with increasing NZFO content, resulting in an increment in conductivity that is proportional to grain size. Therefore, the conductivity of the composite is mainly controlled by NZFO phase above fc. On the other hand, as NZFO phase exhibits giant extrinsic permittivity itself, the value of percolation threshold (fc=0.93) obtained from the nonlinear change of effective permittivity is not the same as its actual value (fc=0.55～0.65). Such effect is considered as pseudo-percolation effect caused by the semi-conductive behavior of NZFO phase.To solve the problem of high dielectric loss in BTO/NZFO ceramic composite, two solutions have been proposed:one is Critical-Sintering-Rapid-Cooling (CSRC) method and the other is Low-Activity Precursor Co-sintering (LAPC) method. These two methods can be used to prepare novel composite with insulating barrier layers. The formation, dielectric and magnetic properties of the composite have been investigated systematically. In CSRC method, by using the disparity between the melting points of the constituent phases and their particular calcination characteristics, the insulating BTO phase was successfully controlled to distribute continuously along the grain boundaries of conductive NZFO particles, thus cutting off the percolation networks. Results showed that the sample prepared by this method exhibited an initial permeability of～106, approximately 70% the value of single-phased NZFO ferrite. Also, the effective permittivity of the sample exceeds 40000, with a saturation magnetization of 75.9 emu/g and a relatively low dielectric loss of 0.30. Results also showed that when the in situ formed BTO barrier layer is amorphous phase, the temperature dependence of the dielelectric properties of the composite exhibited an anomalous behavior compared with conventionally sintered samples. For instance, after the introduction of in situ barrier layer, the activation energy of conductivity decreases from 0.838 eV to 0.582 eV; meanwhile, the temperature spectrum of effective permittivity has broadened. However, the activation energy of relaxation in the composite has not changed, maintaining the same value of 0.204 eV.In LAPC method,2PbO-B2O3 (PBO) glass was used as insulating barrier layer to decrease the dielectric loss of the composite. Results showed that the formation of unwanted impurity phase in the composite can be effectively inhibited by using low-activity composite powder precursor to co-sinter with PBO glass. The introduction of PBO glass barrier layers can effectively cut off the conductive paths formed by the ferrite particles, hence restraining the transportion of charger carriers across the grain boundries, leading to a reduction in dielectric loss, especially in low frequencies. It showed that the minimum dielectric losses of the samples of x=0.1～0.6 were all below 0.1, while that of x=0.7～0.8 was below 0.3. However, excessive amount of PBO glass in the composite can cause impairment to magnetic properties. With the increase in PBO content, the initial permeability of the composite decreases linearly. As a result, the content of PBO glass should be controlled so that the magnetic properties of the composite are not undermined too seriously.