The Structure Evolution And Ferroelectric/ferromagnetic Properties Of Bi_3.15Nd_0.85Ti₃O₁₂/CoFe₂O₄ Composite Ceramics

Multiferroics have attracted a wide spread attention due to the couplings of two or more ferroic order parameters,especially between the ferroelectric and ferromagnetic orders,which show great potential application on the energy transition,information storages and so on.In general,multiferroics are divided into two kinds,namely,the single-phase intrinsic multiferroics and the multiferroic composites.The formers usually show their multiferroic properties only at very low temperatures.As to the multiferroic composites,stress/strain at the interfaces plays the key role in the coupling processes,however,its poor stability and endurance always result in the fatigue failure.In particular,the clamping effects from the rigid substrate make the stress/strain transfer more difficult in the multiferroic composite films.To solve those problems,Sol-gel method with multi metal ions was used to prepare stable complex Bi3.i5Nd0.85Ti3Oi2/CoFe2O4(BNdT/CFO)xerogel,and the BNdT/CFO composite ceramics with coherent-lattice interfacial bonding were designed,in which the magnetic CFO grains were uniformly embedded in the ferroelectric BNdT.This interesting composite form makes BNdT/CFO composites different from the classical multiferroic composites and the single-phase intrinsic multiferroics.In this paper,four aspects are focused:the crystallization behaviours of the complex xerogel,the formation mechanism of coherent-lattice interfaces,the densification mechanism and the electric/magnetic properties of the composite ceramics affected by the coherent-lattice interfaces.(Sub)nitrates replacing a part of metal alkoxieds with the help of ethanediol as solvent and glacial acetic acid as catalyst were successfully prepared stable BNdT/CFO composite gel.The in-situ self-separate composite ceramic powders can be obtained by the two-step calcining method.The start crystallization temperature of CFO phase in composites ceramics increases with smaller final grain size.The processes of heterogenous nucleation improve the crystallization of BNdT phase.The crystallization temperature reduces to 426? from the pure BNdT's 545?.The grain sizes of CFO and BNdT calcined at 600? are 80-100 nm and 50 nm,respectively.The crystallized powders keep good magnetic properties with 95.1 emu/g(H = 10000 Oe)saturated magnetic moment normalized by the CFO mass.As to the densification mechanism,chemical energy from the forming and breakage of coherent-lattice interfaces promote the sintering dynamics.The densification process can be divided as element diffusion,grain(phase)boundaries migration and liquid phase sintering.The two former kinetic processes as the early and middle solid phase sintering stages are in line with the Woolfrey-Bannister model and Genuist model,respectively.The sintering activation energy in composite is twice larger than that in pure BNdT.It can be found the CFO pins and impedes the grain boundary movement at the low temperature.The start critical temperature on the shrinkage curve increases from 790? in the pure BNdT ceramic to 895? in 0.8BNdT-0.2CFO composite ceramic.The morphology of BNdT grains change to spherical approximately and the temperature range of liquid BNdT phase becomes narrow,which is in favour of low temperature densification and lead to inhibition of the c-axis preferred orientation of BNdT.By the Rietveld analysis of XRD data,the value of I(200)/I(117)in the 0.8BNdT-0.2CFO composite ceramic is 0.43,which is larger than the value in pure BNdT(-0.3)indicating the improvement of ferroelectricity.Lots of Y-type grain boundaries observed on the chemical etching surface mean the 0.8BNdT-0.2CFO composite ceramic nearly completes the final densification at 1150?.Tested at various electric fields,the 0.8BNdT-0.2CFO composite ceramic approximately reaches the saturation polarization at 430 kV/cm,and the remanent polarization(Pr)is about 24 ?C/cm2.The coercive electric field in composite is higher than that in pure BNdT.The micro structure analysis show that CFO phase was homogeneously embedded in BNdT matrix with the appropriate sintering conditions.The BSE images of SEM show uniform element contrast on the whole surface of composite ceramic,and the coherent-lattice interfaces along the(200)ENdT/(130)CFO crystal planes are verified by the one-dimensional Fourier transform HRTEM images.The local symmetries of BNdT and CFO are broken due to the existence of coherent-lattice interfaces,which makes the vibration mode of CFO at 463 cm-1 disappeared.And a certain mode around 734-727 cm-1,which does not belong to the CFO or BNdT,comes out simultaneously.The interesting interfaces can also improve the ferroelectric properties,the composite ceramic with 30 mol.%CFO content has the maximum breakdown electric field intensity(?400 kV/cm),and its value of Pr is about 22.1?C/cm2.The BNdT phase has the 2/3/3 periodic arrays of TiO6 layers structure.There are two anomaly peaks in the temperature-dependent dielectric at around 370? and above 580?,the latter moves to higher temperature regions with the increase of CFO content.The coherent-lattice interfaces can change the morphology of BNdT grains to affect the conductive mechanism,it can be found that the grain boundary impedance and the plate boundary impedance integrate into one at high temperature.The thermal expansion measurement and the XRD tests at variable temperatures show good heat matching between CFO and BNdT.The Rietveld analysis of XRD tests at variable temperatures indicates that the orthogonal degree(alb)of BNdT in the composite ceramic has anomaly increasement at the temperature region from 480?and 610? revealing the phase transition strain of CFO can increase the Curie temperature of BNdT,which makes the BNdT better used in the high temperature filed.Ferroelectric domains were studied by the chemical etching,PFM and TEM,respectively.In composite ceramics,the super-lattice diffraction spots at(100)and(010)positions verify the existence of 90° domain of BNdT phase.90° and 180°domains are also observed using PFM.The 180° phase angle reversal with in-situ voltage loaded on the ceramic sample is identified.