Structure,Ferroelectricity/Piezoelectricity And Magnetism Of BiFeO₃-based Lead-free Piezoelectric And Single Phase Room-temperature Multiferroic Ceramics

Multiferroic materials refers to the materials that possess two or more ferroics properties （ferroelectricity, ferromagnetism, ferroelasticity and ferrotoroidicity）. Among them, multiferroic materials possessed simultaneously ferroelectricity and ferromagnetism and are extensively investigated.As rare single-phase room temperature multiferroic materials, BiFeO₃ possesses high Curie temperature（Tc=830℃） and Neel temperature （TN=370℃）, which suggests that it has potential applications in functional devices. However, for the BiFeO₃ materials, the difficulty of preparation of pure phase, large leakage current and the structure of anti-ferromagneticity with G-type limit the multifunctional applications of BiFeO₃-based materials. Therefore, there is a hot research topic in current BiFeO₃ materials, which focus on improving the electrical insulation and suppressing the formation of impurity phases, and thus the enhanced electrical/magnetic properties are obtained. In this work, we fabricated BiFeO₃-based mulferroic materials by compound- (NdCoO₃ with magnetism and Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ with non-magnetism), oxide- and Er³⁺ ions-doping. The composition and sintering condition dependences on the structure, multiferroic properties and and magnetoelectric effect of the ceramics were studied systematically, and the relationship between structure and properties of the materials has also been discussed. The main research contents and conclusions are listed as following:（1） Novel （0.75-x）BiFeO₃-0.25BaTiO₃-xNdCoO₃+1 mol% MnO₂ mulferroic materials were synthesised by traditional sintering technique, and the effects of NdCoO₃ doping on the structure, multiferroic properties and magnetoelectric effect were investigated. All the ceramics possess typical perovskite structure, the structure changed from rhombohedral to tetragonal phase are observed with x increasing from 0 to 0.08, and the morphotropic phase boundary of rhomb ohedral-tetragonal phase is formed at x0.01-0.03. SEM graphs indicate that a small amount of NdCoO₃ doping can promote grain growth, however, grain growth was inhibited at high doping level. Good piezoelectricity, strong ferroelectricity, high Curie temperature （Tc～486-605） and excellent the temperature stability of piezoelectricity are obtained at morphotropic phase boundary. The ceramic with x= 0.08 possesses the strongest magnetism （M_r= 0.4229 emu/g and M_s= 2.7186 emu/g）, is about six times compared with un-doped ceramics. The ceramic with x= 0.02 shows superior magnetoelectric effect （α33=750.1 mV·cm-1·Oe-1）, meanwhile, the practical application potential of the current ceramic was preliminarily confirmed by imitating the current dectection.（2） New （0.75-x）BiFeO₃-0.25BaTiO₃-xBi_0.5(Na_0.8K_0.2)_0.5TiO₃+1 mol% MnO₂ mulferroic materials were prepared by traditional sintering technique. The effects of composition and sintering condition on the phase structure, microstructure, electric insulation properties, piezoelectricity and ferroelectricity were systematically studied. The XRD results show that all the ceramics possess perovskite structure, and as x increases, the structure of the ceramics transformed from rhombohedral to tetragonal phase. When x=0.01, the morphotropic phase boundary of rhombohedral-tetragonal phase is formed. The SEM results show that grain growth was promoted when a small amount of Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ doping, however, grain growth was inhibited when excess.The studies suggest that the temperature has an important effect on the structure and electrical properties of the ceramics.The optimal sintering temperature of the ceramic with high piezoelectricity is 960℃.The ceramic with x=0.01 possesses large geain size, good density, high resistivity, enhanced ferroelectricity （P_r=11.0 μC/cm²） and excellent piezoelectricity (d₃₃=114 pC/N).（3） Novel 0.725BiFe_0.96Sc_0.0403-0.275BaTiO₃+x mol% MnO₂ mulferroic materials were fabricated,and the structure, electric/magnetic properties, and magnetoelectric effect were systematically studied.All the ceramics possess perovskite structure, andthe structure of the ceramics transformed from rhombohedral to tetragonal phase as x increases;the morphotropic phase boundary of rhombohedral-tetragonal phase is formed at x=0.5-1.0.The doping of MnO₂ inhibits the grain growth of the ceramics.The addition of a small amount of MnO₂ decreases the oxygen vacancy concentration and thus improves the insulation properties of the ceramics. The ceramics with x=0.5-1.0 possess good insulation properties (R 1.2-1.7×10¹⁰ Q-cm), high Curie temperature （590-596℃）, strong ferroelectricity/piezoelectricity (P_r=13.1-17.6 μC/cm²,d₃₃=123-143 pC/N,kp 0.34-0.35)and excellent the temperature stability of ferro-/piezoelectricity. This is related to the contribution of Mn ions.Surprisingly, the ceramic with x=7.0 possess superior ferromagnetism （M_r=0.4946 emu/g and M_s=1.0298 emu/g）, which is about 1000 times compared with un-doped ceramics.The excellent magnetoelectric effect（α33 =429.57 mV·cm-1·Oe-1）was obtained at x=0.02.（4） New 0.75Bi1-xErxFeO₃-0.25BaTiO₃+lmol% MnO₂ mulferroic materials were synthesised, and the structure, ferroelectric/piezoelectric and magnetic properties were systematically studied. All the ceramics possess perovskite structure, and the structure of the ceramics transformed from rhombohedral to tetragonal phase as x increases. SEM graphs show that a small amount of Er³⁺doping can promote grain growth.The ceramics with x=0.125-0.15 possess good electrical insulation (R=2.75-2.83×10¹⁰ Ω·cm) and strong ferroelectricity （P_r=13.44-14.21 μC/cm²）.The M_r of 0.4111 emu/g and M_s of 1.4519 emu/g were observed in the ceramic with x=0.15,which are 380% and 722% compared un-doped ceramics.