In Situ Formation And Properties Of Sol-gel Derived Ferroelectric/ferrimagnetic Composite Materials

Ferroelectric/ferromagnetic (ME) composite materials have both capacitance and inductance in one material, which can be used to minimize the size of the electric components. ME composite materials have drawn more and more attention because of the bright application future in the recent years. With the development of the high effiency of the electric products, the ME composite materials with high dielectric constant and high permeability will become an important research hotspot in the functional material area. The introduction of percolation theory into ME composite system can increase the dielectric constant of the composite materials. However, the percolation threshold is quite low (less than 0.2) in the normal percolation system. The permeability of the composite in which percolation effect happens is very low due to the compound law. Thus the ME composite material with super high dielectric constant as well as high permeability can not be obtained in the normal percolative system. If the percolation threshold is increased through a certain method, that is to say, if percolation effect happens when the ferrite content is very high, the permeability can be maintained while the dielectric constant is increased significanty. This provides a very good idea for the research of high properties ME composite material of high properties.The purpose of this thesis is to prepare PTO/NFO composite powder, thin films, ceramics and BTO/NZFO composite ceramics using sol-gel in situ method successfully, to research the formation of PTO and NFO crystalline phases in the composite system, to introduce percolation theory into ME composite ceramics, improve the percolation threshold and reduce the loss through forming the package structure of NFO grains by PTO phase obtaining the composite ceramics with super high dielectric constant and high permeability, to research the mechanism of increased dielectric constant in composite ceramics and explore its theoretical basis.This thesis reviewed the research progress of ferroelectromagnetic material, ME composite material and percolative composite ceramics, compared the properties and preparation method of ME composite material, concluded the merit and shortcoming of existing ferroelectromagnetic material and the factors which affect its dielectric and magnetic properties. The preparation, microscopic structure, conductance, dielectric property and magnetic property of PTO/NFO composite powder, PTO/NFO composite thin films, PTO/NFO composite ceramics and BTO/NZFO composite ceramics were analysed by XRD, SEM, Precision Impedance Analyzer (Agilent 4294A,16451B) , Resistance Analyzer (Keithley 6571A) and so on. The formation and mechanism and the affecting mechanism of the phase structure on the dielectric properties and magnetic properties, the mechanism of the increased dielectric constant, the mechanism of the increased percolation threshold are investigated in detail in the thesis.The aim of this work is to fabricate PTO/NFO ferroelectric/ferrimagnetic composite thin films and percolative PTO/NFO and BTO/NZFO composite ceramics with super high percoaltion threshold in situ by sol-gel process. The phase formation, the dielectric behavor and magnetic behavor of PTO/NFO and BTO/NZFO composites have been studied.(1) PTO/NFO biphase ME composite powders are formed in situ by sol-gel method. The effect of annealing temperature and annealing time on the formation and magnetic properties of the powder composite is studied. PTO/NFO biphase composite is formed by controling the annealed temperature under 700℃-750℃. PTO and NFO phases exist in the system in the form of solid solution. The system tends to form more stable phases with increasing the annealing temperature and time. The magnetization and coercive force are related with the component of the composite system. The increase in NFO phase content increases the magnetization of the composite. The non-magnetic phase PTO blocks the magnetization of the magnetic fields of NFO leading to the increase of the coercive force of the composite. PTO / NFO biphase composite and PTO / NFO / PFO / TiO₂ are obtained by sol-gel process and in situ formation method in the system with different PTO and NFO stoichiometric proportion.(2) PTO/NFO biphase ME composite thin films are formed in situ by sol-gel method by controling the annealed temperature under 800℃-900℃. The effect of annealing temperature on the formation, dielectric properties and magnetic properties of the composite thin films is studied. PTO and NFO phases exist in the system in the form of solid solution. The interface between PTO and NFO phases induces space polarization besides defections in the composite thin films. The frequency where the dipole polarization relaxation happens increases because of the introduction of NFO phase in the composites. The polarization of the PTO phase is nailed by non-ferroelectric phase NFO causing the increase of the coercive force of the PTO/NFO composite thin films.(3) Percolative PTO/NFO biphase ME composite ceramics are formed in situ by sol-gel method. The effect of the pre-sintered temperature and sintering temperature on the formation of the two phases is studied. Compact and good-formation ceramics are obtained by controling the presintered temperature of 550℃and sintered temperature of 1150℃. PTO and NFO phases exist as solid solution in the system. The structure of NFO phase packed by PTO phase is obtained by controling the homogeneous distribution and the growing sequence of PTO and NFO phases. As a result, the percolation threshold is increased significantly. The significant dielectric constant and comparatively high initial permeability of the composite are 9432 and 6 respectively. (4) The dielectric behavor and magnetic behavor of the percolative PTO/NFO ceramics are investigated. The dielectric behavor fits "Hybrid dielectric model", which is caused by space charge polarization and dipole polarization and fit "micro-capacitance" model. The intial permeability of the composite fits MG equation in the large range of NFO volume content between 0 and 0.85 because of the packed stucture, and fits BH equation better when NFO volume content is higher than 0.85. The BH equation is developed because of the decrease of the NFO initial permeability caused by the doping of non-magnetic ions. The experimental results fit the developed BH equation very well. The spin-glass-like phenomenon is found in PTO/NFO composite ceramics. The Tirr is increased greatly because of the blocking of PTO phase on NFO phase. The grain size of NFO crystalline particles is decreased to nanometers by doping PTO phase leading to the supermagnetic properties of the composite ceramics.(5) Percolative BTO/NZFO biphase ME composite ceramics are formed in situ by sol-gel method. The effect of the sintering temperature on the formation of the two phases is studied. Compact and good-formation ceramics are obtained by controling the presintered temperature of 750℃and sintered temperature of 1300℃. BTO and NZFO phases exist as solid solution in the system. The structure of NZFO phase packed by BTO phase is obtained by controling the homogeneous distribution and the growing sequence of BTO and NZFO phases. Besides, amorphous phase in formed on the surface of NZFO phase. Thus, the percolation threshold is increased furture, and the dielectric loss is decreased. The significant dielectric constant and comparatively high initial permeability of the composite are 75000 and 162 respectively. The dielectric behavor of the percolative BTO/NFO ceramics fits "Hybrid dielectric model". The intial permeability of the composite fits MG equation in the large range of NFO volume content between 0 and 0.6 because of the packed stucture, and fits BH equation better when NFO volume content is higher than 0.6. The experimental results fit the developed BH equation very well.