Preparation And Properties Research On Single-phase Multiferroics And Magnetoelectrical Composites

It just undergoes ten years that the multiferroics have become one of the forefrontresearches in Materials Science and Condensed Matter Physics from its renaissance.The main reasons are that multiferroics are those materials in which two or more ferroicorders （ferromagnetic, ferroelectric or ferroelastic） coexist simultaneously and cancoupled between with themselves, therefore they can have a great important potentialapplications in magnetomechanical actuators, electric-field-controlled ferromagneticresponse devices, transducers with magnetically modulated piezoelectricity,multiple-state memory elements, electrical field controlled magnetic random accessmemory, high-frequency magnetoelectrical converters/filters and other high-tech areas;Additionally, multiferroics also contain a wealth of physical phenomena and materialsscience issues, thus can have an important contribution to the development of basicphysical science. For these reasons, we have paid our attention to one or two branchesin this broad emerging field. The main research contents and results are as follows：（1） The hydrothermal method was used to prepare single-phase BiFeO₃micronpowder particles under the conditions of low temperature and low alkalinity；Studieshave shown that different reaction temperature, alkali concentration and reaction timefor the hydrothermal reaction have a significant impact on the powder products. XPSdata analysis showed that Fe²⁺and Fe³⁺ions are coexist in pure phase BiFeO₃sample(reaction temperature170°C,16hours,[OH^-]_NaOH=0.07M). By the test of ZFC and FCmagnetization curves, results showed that the BiFeO₃powder samples showparamagnetic behavior between290k³00k. When T<290k, BiFeO₃powder sampleswith a weak ferromagnetism. At the room and lower temperatures, the non-linear characteristics in hysteresis loop of BiFeO₃powder can show that differ from themagnetization behavior of BiFeO₃bulk ceramics.（2） High anisotropy GdMn₂O₅, SmMn₂O₅and YbMn₂O₅nanocrystalline grains andnano-rods （all of them are of orthogonal crystal structure） have been synthesizedsuccessfully by hydrothermal method, and can find that the concentration of mineralizerhave a major impact on the morphology and size of the products. By the experiments’results, the formation mechanism of high anisotropy of these nano-structured can bethought to be the "Dissolution-Crystallization". For the analytic results from themagnetic properties of the nanopowders synthesized at the different alkali concentration,the different crystal morphology can has a great influence on its magnetic properties.（3） We also have systematically investigated the evolution laws of product phase,crystal structure parameters and morphology of the ferroelectrics Pb(Zr_0.52Ti_0.48)O₃,BaTiO₃and ferrite CoFe₂O₄（doped） powder synthesized under different hydrothermalconditions. The formation mechanisms of ferroelectric and ferromagnetic nanoparticlesproducts also have been discussed, and thought that the "Dissolution-Recrystallization"mechanism in which to play a major role. Here, we emphasize the fabrication ofgadolinium-doped CoFe₂O₄nanoparticles under hydrothermal conditions andinvestigation of the effect of gadolinium doping on the structural parameters andmagnetic properties of cobalt ferrite nanoparticles.（4） The hydrothermal synthesis and traditional ceramic sintering process have beenused to successfully fabricate PZT-CFO magnetoelectrical composites （ME composites）.The impacts of sintering process parameters upon the crystal structure and morphologyof the ME composite materials have been discussed. The electrical properties, magneticproperties and ME coupling effects of the ME composite materials also have beencarefully studied. The test results of ME voltage coefficientα_ME（dE/dH） and MEsusceptibility coefficientα"_me（dH/dE） can show that, the0.80PZT-0.20CFO MEcomposite material （sintering at1100℃for4h） has a more significant ME couplingeffect, the maximumα_ME（dE/dH） andα"_me（dH/dE） were226mV cm^-1 Oe^-1and1.15×10^-8s/m, respectively.（5） By the hydrothermal method combined with traditional ceramic sintering process,the BTO-CFO ME composites have been prepared successfully. The impacts of sintering process parameters upon the crystal structure and morphology of the MEcomposite materials have also been discussed. The electrical properties, magneticproperties and ME coupling effects of the ME composite materials also have beencarefully studied. The magnetic voltage coefficient results of the ME composites showthat the polarization processing strategy has a profound effect on the ME coupling effect,for0.55BTO-0.45CFO ME composite material （sintering at1100℃for4h）, themagnetic voltage coefficientα_ME（dE/dH） reaches a maximum value (⁴9.6mV cm^-1 Oe^-1) at the frequency of75KHz.（6） The Sol-Gel process and hydrothermal techniques were used to successfullyprepare core-shell-structured BTO-CFO nano-composite powders, and then make theprepared nano-composite powders as raw materials to fabricate the core-shell-structuredBTO-CFO ME composites by traditional ceramic sintering process. For the ferritecontent of x=0.15,0.30and0.45, the maximum ME voltage coefficientα_ME（dE/dH）values of the core-shell-structured of ME composite samples （sintering at1150℃for4h）are17.1mV cm^-1 Oe^-1,15.3mV cm^-1 Oe^-1and14.4mV cm^-1 Oe^-1at the frequency of80KHz, respectively. By the improved polarization treatment, the piezoelectriccoefficient d33values of the core-shell-structured ME composite materials （sintering at1100℃for4h） have been enhanced significantly, and for the ferrite content of x=0.15,0.30and0.45, the maximum magnetic voltage coefficientα_ME（dE/dH） values ofthe core-shell-structured ME composites are31.2mV cm^-1 Oe^-1,23.7mV cm^-1 Oe^-1and6.12mV cm^-1 Oe^-1at the frequency of87KHz, respectively. At the frequencies of11.9KHz and70KHz, the ME susceptibility coefficientα ’’me（dH/dE） of thecore-shell-structured0.85BTO-0.15CFO ME composite materials （sintering at1150℃for4h） are8.59×10^-9s/m and6.25×10^-9s/m, respectively.