| Magnetoelectric (ME) multiferroics is a family of materials that the electric and magnetic order exist simultaneously. As a representative system. BaTiO3/CoFe2O4 system has been investigated widely. The ME effect of CoFe2O4/BaTiO3 bulk composite is comparatively weak, which is more originate from the inter-diffusion and electric leakage. Learning the phase transition and inter-diffusion of ions for the BaTiO3/CoFe2O4 bulk composite in sintering process is very important to improve the ME effect. In this work, three composite modes of CoFe2O4/BaTiO3 composite system were created by changing the order of BaTiO3 powders addition of sol-gel route for CoFe2O4, those are appropriate stoichiometric proportion of nitrate solution of CoFe2O4 mixed with BaTiO3 (CoFe2O4(L)/BaTiO3), self-propagating precursor of CoFe2O4 mixed with BaTiO3 (CoFe204(P)/BaTiO3) and the made-up CoFe2O4 mixed with BaTiOs (CoFe2O4(E)/BaTiO3), BaTiO3 and CoFe2O4 were prepared by sol-gel method, respectively. The differences in microstructure and ferroelectric properties among three modes of CoFe2O4/BaTiO3 bulk composites after sintered were investigated by changing the sintering temperature. The solid solubility of rare earth elements in strontium ferrite (SrFe12O19) structure is limited. Higher levels of earth element doping could cause the formation of REFeO3 and the appearance of in situ symbiotic magnetoelectric composite of SrFe12O19/REFeO3 system. In this work, the Sr1-xRExFe12O19 (RE=Pr, Dy; x=0, 0.15,0.25,0.5) have been prepared by sol-gel auto-combustion and microwave-assisted calcination method. The soild solubility, magnetic properties and microstructure of the Pr3+ and Dy3+ doping in SrFe12O19 structure has been investigated. The in situ symbiotic Ternery composite system of SrFe12O19/REFeO3/α-Fe2O3 was systhesized with higher Pr3+ and Dy3+ substitution. The growth mechanism, magnetic properties, microstructure of the ternery composite system was also investigated. The main conclusions are as follows:1. The diffusion of Co2+ in sintering process is closely related to the sintering temperature and composite way. The stabilized existence of BaTiO3 with hexagonal (HBTO) at RT after sintering derives from the ions diffusion. The ions diffusion degree increases in composite bulk in order CoFe2O4(L)/BaTiO3>CoFe2O4(P)/BaTiO3>CoFe2O4(E)/BaTiO3 after sintering at 1200℃. While, the blocked ions diffusion would cause the formation of BaFe12O19 in CoFe2O4(E)/BaTiO3 bulk.2. The polarization of all the CoFe2O4(L)/BaTiO3 bulk with different molar ratios is less than that of CoFe2O4(P)/BaTiC>3 and CoFe2O4(E)/BaTiO3 with same molar ratio, which is more derived from two reasons, first, the lower sintering temperature would lead to poor density; second, the serious interfacial polarization and heterogeneous conduction caused by the relatively matching grain size of two phases and better interface contact.3. CoFe2O4(E)/BaTiO3 bulks with different molar ratio have relatively better polarization and lower current leakage. The threshold ferromagnetic phase content of percolation effect in bulk composite is no more than 40mol% in CoFe2O4(E)/BaTiO3 bulk composite. The maximum polarization of BaTiC>3/CoFe04 bulk is obtained in CoFe2O4(E)/BaTiO3 (1:9) system with 5.24μC/cm2.4. The saturated solubility of Pr3+ in magnetoplumbite structure is about 0.25. The co-existence of SrFe12O19/PrFeO3/α-Fe2O3 was systheszed with the substitution of Pr3+at x=0.5. The grain size, distribution and structure of the phases have a great difference, thus cause bad exchange coupling interaction and lower magnetization. The highest coercivity of 6717Oe is received with the Dy3+ ions substitution content up to at x= 0.5. The synchronized precipitation of the DyFeO3 and SrFe12O19 contributs to the fine grains, homogeneous phase distribution, as well as excellent exchange coupling interaction among SrFe12O19 DyFeO3 and α-Fe2O3 phases. |
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