Low Temperature Synthesis Of Magnetoelectric Composites And Powders

With the rapid development of information and wireless technologies, thematerials which can meet the requirements of shrinking circuit dimension,multifunction, high level of passive integration are needed. LTCC is a classicalmethod for integration of components. Thus the magnetoelectric composites withlow sintering temperature and the related ultrafine powder are promisingcandidates for LTCC technology. In this thesis, some magnetoelectriccomposites with low sintering temperature were prepared. NaNbO3,K0.5Na0.5NbO3, LiNbO3and CoFe2O4ultrafine powder were synthesized bylow-temperature combustion method and molten salt synthesis method,respectively. The properties of the above composites and powders wereinvestigated, as well as some related mechanism.With Nb2O5, Na2CO3and K2CO3as starting materials and urea as fuel,NaNbO3and K0.5Na0.5NbO3powders were synthesized by low-temperaturecombustion method. The effect of calcining temperature, keeping time and theamount of urea on the phase composition and morphology of the powders wereinvestigated in detail. The results show that as the amount of urea is equivalent tothat of starting materials the single-phase orthorhombic NaNbO3andK0.5Na0.5NbO3powders can be obtained at550oC for6h. The as-preparedpowders are composed of well-developed and well-dispersed cubic grains. It canconclude that it is heat originated from the ignition of urea make the formation ofNaNbO3and K0.5Na0.5NbO3powders at lower temperatures.With Li2SO4-Na2SO4and NaCl-KCl as molten salt, LiNbO3and CoFe2O4powders were synthesized by molten salt synthesis method. The effect ofcalcining temperature, keeping time and the species and amount of urea on the phase composition and morphology of the powders were investigated in detail.The results show that LiNbO3powders can be rapid synthesized at650oC for5min in Li2SO4-Na2SO4molten salt. On the contrary single phase LiNbO3powders cannot be obtained in NaCl-KCl molten salt. Single phase LiNbO3powders can always be obtained with the ratio of starting materials and moltensalt ranged from2:1to1:2. Single phase CoFe2O4powders can be obtained at alower temperature in Li2SO4-Na2SO4molten salt than those in NaCl-KCl moltensalt due to the fact that the eutectic point of Li2SO4-Na2SO4molten salt is lowerthan that of NaCl-KCl molten salt. Single phase CoFe2O4powders can beobtained at700oC. With the increase of calcing temperature and prolongingof the keeping time the grain sizes of the as-prepared LiNbO3and CoFe2O4increase.Low loss flexible dielectric and magnetic composite with Ni0.8Zn0.2Fe2O4ferrite (NZO) ultrafine particles embedded in a Thermoplastic PolyolefinElastomer (POE) matrix was fabricated using the extrusion technology. Thedielectric and magnetic properties of the as-prepared composites with differentvolume fraction of ceramic fillers were investigated. The results indicate thatwith the increasing of the volume of ceramic fillers, the permittivity,permeability, dielectric loss and magnetic loss of the composites ‘all increase.The cut-off frequencies of the composites are all above1GHz. Because of thelow resistivity of NZO, the dielectric loss of the composites is high in lowfrequency range and decrease with frequency increasing. Good frequencystability of the electromagnetic properties within a wide frequency range wasobserved. With the increasing of the volume of ceramic filler, the tensile strengthincreases and elongation decreases. All the composites show very goodflexibility.0.7BiFeO3-0.3BaTiO3/BiY2Fe5O12(BFO-BT/BYIG) andLa0.1Bi0.9FeO3/BiY2Fe5O12(LBFO/YIG) composites were synthesized via theconventional solid-state reaction method. The phase composition and surfacemorphology of the composites were investigated using XRD and SEM,respectively. The two main phases can coexist. The composites possess denseand homogenous microstructure. The dielectric and magnetic properties of thecomposites were also investigated. The results show that the as-prepared composites with an excellent combination of ferroelectric and magneticproperties due to high resistivity of the composites.0.7BiFeO3-0.3BaTiO3/Y3Fe5O12(BFO-BT/YIG) composites weresynthesized via the hybrid processing route. The phase composition and surfacemorphology of the composites were investigated using XRD and SEM,respectively. The two main phases can coexist. The composites possess denseand homogenous microstructure. The dielectric and magnetic properties of thecomposites were also investigated. The results show that the as-preparedcomposites with an excellent combination of ferroelectric and magneticproperties due to high resistivity of the composites.Ba0.6Sr0.4TiO3/Ni0.37Cu0.20Zn0.43Fe1.92O3.88(BST/NiCuZn) composite bulksand thick films were prepared by the conventional solid-state reaction methodand screen printing method, respectively and sintered at low temperatures withthe addition of BaCu(B2O5). The phase composition and surface morphology ofthe composites were investigated using XRD and SEM, respectively. The twomain phases can coexist. The composites possess dense and homogenousmicrostructure. The dielectric and magnetic properties of the composites werealso investigated. The results show that the BST/NiCuZn composite bulks andthick films possess very high dielectric constants and permeabilities, and verylow loss.Bi3Ti4O12/Ni0.37Cu0.20Zn0.43Fe1.92O3.88(BTO/NiCuZn) composites with giantdielectric constant and high permeability were synthesized via the conventionalsolid-state reaction method. The phase composition and surface morphology ofthe composites were investigated using XRD and SEM, respectively. The twomain phases can coexist. The composites possess dense and homogenousmicrostructure. The dielectric and magnetic properties of the composites werealso studied. The results show that the BTO/NiCuZn composites have giantdielectric constants and very high permeabilities. The giant dielectric behavior ofthe BTO/NiCuZn composites is mainly attributed to the Maxwell-Wagnerpolarization.Ba(Fe0.5Nb0.5) O3/Ni0.37Cu0.20Zn0.43Fe1.92O3.88(BFN/NiCuZn) compositescan be sintered at low temperatures with the addition of BaCu(B2O5). The phasecomposition and surface morphology of the composites were investigated using XRD and SEM, respectively. The two main phases can coexist. The compositespossess dense and homogenous microstructure. The dielectric and magneticproperties of the composites were also studied. The results show that theBZN/NiCuZn composites possess very high dielectric constants andpermeabilities. The incorporation of NiCuZn can decrease the dielectric loss ofBFN.