Study of Spark Plasma Sintering and Properties of Multiferroic Bismuth Ferrite Ceramics

Author:Wang Ting

Supervisor:song shen hua


Degree Year:2019





It is in urgent need to improve the performance of electronic information materials in order to adapt to China’s manufacturing of 2025,quantum communication and 5G communication technology,etc.BiFeO3,an excellent candidate of lead-free multiferroic materials,has high Curie and Neel temperatures.Due to the multiferroic coupling,BiFeO3 shows exciting choices for the application of spintronics,information storage device,etc.However,there are still remaining several drawbacks:a high leakage current density resulting from Bi evaporation and oxygen vacancies,weak magnetic properties coming from G-type antiferromagnetic spin ordering,the formation of impurities(e.g.,Bi2Fe4O9 and Bi25FeO39)due to their thermo-dynamic unstable properties,etc.In the present thesis,high-density and high-purity BiFeO3 ceramics were fabricated by sol-gel method combined with spark plasma sintering at a low temperature.Followed by,Bi3+ions at A-sites of BiFeO3 were substituted by rare earth element Nd/Sm,Fe3+ions at B-sites of BiFeO3 were replaced by high-valence transition metal elements and A-sites and B-sites were co-doped by different elements(Ba-Zr,Sr-Zr,Nd-Nb,and Sm-Nb).The effect of the doping elements and contents on structural,dielectric,ferroelectric and magnetic properties of ceramics was investigated.The phase transitions and structure-properties relationship were discussed in detail.The leakage current mechanism was also illustrated.The sol was prepared with the concentration of precursor of 0.01 mol/L and mole ratio of complex agent to metal ions of 1:1.The sol was heated until all solvent evaporated and calcined at 500°C.Well-crystallinity and phase-pure BiFeO3powders with a size of approximately 100 nm were harvested.BiFeO3 ceramics were prepared by spark plasma sintering(SPS)at 700°C with holding time of 5 min.After the SPS,the as-prepared pellets were annealed at 650°C for 4 h under atmospheres of oxygen.The relative density reaches 96%of the theoretical density.The obtained remnant polarization and magnetization are about 0.41μC/cm2 and5.67?10-4 emu/g,respectively.The crystal structure transforms from rhombohedral to orthorhombic structure with doping rare earth elements Nd/Sm.The Goldsmith tolerance factor of Sm-doped ceramics is lower than that of Nd-doped ceramics,indicating that the Jahn-Teller distortion resulting from Sm-doping is stronger.Rare earth elements doping can reduce the grain size and improve the compactness.The dielectric constant increases and dielectric loss and leakage current density decrease with Nd/Sm doping.For Nd-doped samples,the remnant polarization increases from 0.41to 1.45μC/cm2 and remnant magnetization increases from 5.67?10-4 to 0.26 emu/g.The maximum remanent polarization and magnetization for the Sm-doped samples are approximately 3.45μC/cm2 and 0.28 emu/g,respectively.The better ferroelectric and magnetic properties of Sm-doped ceramics are derived from smaller ionic radius and higher Bohr spin magnetic moment.BiFe1-y-y By O3(B:Zr,Nb;y=0,0.02,0.05,0.08,and 0.10)ceramics were successfully prepared.The maximum solid solubility of Zr/Nb in BiFeO3 is 5%.For Zr-doped ceramics,the crystal structure changes to the coexistence of R3c and Pm-3m with y=0.05.For Nb-doped ceramics,the structure starts to change with y=0.02.The grain size of Zr/Nb-doped ceramics is less than 1μm.The dielectric loss of Nb-doped ceramics reduces to 0.007,which can effectively reduce the heat of component.The leakage current density is remarkably decreased by doping high-valence elements.When y=0.05,both Zr-doped and Nb-doped ceramics have the optimal ferroelectric and magnetic properties.With further increasing the doping content,there are impurities present in the samples,which leads to the degraded ferroelectric and magnetic properties.The leakage current mechanism for undoped BiFeO3 ceramics is space-charge-limited-current,while that of Zr/Nb doped ceramics is between Ohmic conduction and space-charge-limited-current mechanismX-ray diffraction and TOPAS refinement results indicate that all Ba-Zr co-doped samples are R3c and the crystal structure of Sr-Zr co-doped ceramics changes from R3c to coexistence of R3c and Pm-3m.Scanning electron microscopy results reveal that there are some macroscopic void present in samples.The ferroelectric hysteresis loop is not preformed due to the increase of oxygen vacancies and leakage current density,as evidenced by X-ray photoelectron spectroscopy and leakage current density curves.The magnetic hysteresis loops of Ba-Zr co-doped ceramics show soft magnetism and saturation magnetization,while the loops of Sr-Zr co-doped ceramics have almost no change.The leakage current mechanism for Ba-Zr and Sr-Zr co-doped ceramics is mixed with space-charge-limited-current and Fowler-Nordheim tunneling mechanism.The crystal structure of Nd-Nb and Sm-Nb co-doped ceramics transforms from R3c to Pnma.The lattice parameter a increases and c decreases with doping.The grain size significantly decreases and becomes homogeneous with Nd-Nb and Sm-Nb co-doping.The leakage current density of the co-doped samples is reduced by about three orders of magnitude compared with the pure BiFeO3 ceramics.The leakage current mechanism for the two types of co-doped ceramics is mixed with space-charge-limited-current and Ohmic conduction mechanism.The remnant polarization of Nd-Nb co-doped ceramics increases from 0.41 to 3.12μC/cm2,showing a 667%growth,while the maximum remnant polarization of Sm-Nb co-doped ceramics is 5.85μC/cm2,which is about fourteen times larger than that of undoped BiFeO3 ceramics.The maximum remanent magnetization for Nd-Nb and Sm-Nb co-doped ceramics are approximately 0.15 emu/g and 0.19 emu/g,respectively.The enhanced ferroelectricity is related to crystal structure distortion and reduced leakage current density,while the improved magnetization is originated from the suppression of cycloidal spin structure and 4f spin-oribits of rare earth elements.These results will point out a way to improve the properties of BiFeO3materials by the site engineering and doped element types.