Theoretical Study on Ferroelectric Phase Transition and Development of BaTi1-xHfxO3-based Ceramics with High Piezoelectric Coefficient
Author:Yin Hong Mei
Supervisor:huang yi neng
Lead-free piezoelectric ceramics instead of lead-based ones is the inevitable requirement of social economic development and environmental protection,and it is also an important task of material science research-At present,the short-term goal of lead-free piezoelectric ceranics research is to find ceramic systems with large piezoelectric coefficient(d33).Nearly 20 years,some systems with d33 close to or higher than that of lead zirconate-titanate ceramics,has been found in barium titanate-based perovskite-type ceramics.It is one of the lead-free piezoelectrics that are expected to replace lead-based ceramics in the future.At present,large d33 materials are the ferroelectrics those are in ferroelectric phase at room temperature,and the properties and applications of these ones are more or less related to ferroelectric phase transition(FPT).However,However,for triglycine sulfate,a typical ferroelectric with 2nd-order phase transition,the existing theoretical models still have obvious deviations to the experimental results of the spontaneous polarization and specific heat.In this study,a new model with neglectable tunneling effect is proposed based on the coaling between spontaneous polarization and spontaneous strain in ferroelectrics,and The spontaneous polarization and specific heat of ferroelectric phase transition predicted by the model are in better agreement with the corresponding data of TGS.In addition,the model predicts a new type of ferroelectrics in which a phase transition and a phase-like transition coexists.It is found that the piezoelectric properties of ceramics are affected by phase corrposition,grain size(g),grain boundary and domain structure,etc.Up to now,the study on the co-effect of multiphase coexistence and g on the high d33 in a specific system has not been studied yet.In this paper,BaTi0.98Hf0.02O3(BTH)ceramics were prepared by the conventional solid-state reaction technique at series of sintering temperatures(Ts),and the room temperature d33,hysteresis loop,phase compositions,Raman spectra,g and telperature-dependent dielectric spectra of BTH ceramics were measured and analyzed.The effect of gand phase conpositions on piezoelectric properties of simples was researched.The results show that:1)The grain size of the ceramics is 0.9,21.3,21.6,and 37.2 μm,respectively,and the corresponding phase compositions are the tetragonal-orthogonal(T-O),tetragonal-orthogonal-rhombohedral(T-O-R),tetragonal-orthogonal(T-O),and tetragonal-orthogonal-rhombohedral(T-O-R),while the piezoelectric coefficient is 475,352,258,and 327 pC/N,ie.;2)With increase of g,d33 decreases first and then increases,and when have the same phase compositions,the larger g is,the smaller d33 is;3)When g≤21.6 μm,d33 of the ceramics increases with the decrease of g.When g≥21.6 μm,ceramics with T-O-R phase have larger d33.Therefore,the phase compositions and g of the ceramics are interrelated,and they co-affect the d33.Besides,two approaches were used to increase the d33 values of BaTi1-xHfxO3-based Ba0.85Ca0.15Hf0.1Ti0.9O3 ceramics to over 600 pC/N.The one is to change the calcining temperature(Tcal)and Ts of ceramics,the other is to dope rare earth Ho ions.The details are as follows.Pure Ba0.85Ca0.15Hf0.1Ti0.9O3(BCHT)ceramics were prepared by the conventional solid-state reaction technique at series of Tcal and Ts.And based on the test results of bulk density,room temperature hysteresis loop,d33 and surface morphology of BCHT ceramics,the Tcal and Ts at the maximum of d33 were obtained,which were 1250℃C and 50 ℃,respectively.The bulk density and d33 of the ceramics are higher when Ts=1400(Tcal=1150,1200,1250 ℃),1450,and 1500 ℃,and they are ferroelectric at room temperature.Especially when Ts= 1500 ℃,the whole ceramic d33 is higher than other ones,and the grain size of the ceramics is15.6,22,24.5,and25μm,respectively,the corresponding phase compositions are the T-R,T-O-R,O-R,and T-O-R,while the d33 is 601,413,367,and 387 pC/N i.e.,it decreases first andthen increases slightly as the g goes up.When g ≥21.6 μm,the ceramics with T-O-Rphase have larger d33 than of O-R phase ones,and when have the same phasecompositions(T-O-R phase),the larger g is,the smaller d33 is.It can be seen thatsimilar to BTH ceramics,the phase compositions and g of the ceramics are interrelated,and they co-affect the d33.Among all BCHT ferroelectric ceramics,Ts= 1500 ℃,T’cal = 1250℃,the ceramics have maximum d33,corresponding to larger 2Pr andsmaller 2Ec,whose values are 601 pC/N,20.4μC/cm2 and 2.6 kV/cm,respectively,and kp is 0.537.On the basis of the above work,0.02-0.8 mol%Ho203-dopedBa0.85Ca0.15Hf0.1Ti0.9O3 ceramics were prepared also by the conventional solid-statereaction technique.The phase diagram of this non-Pb system,determined by measuringdielectric permittivity(ε*= ε’-iε")versus temperature curves was studied.Meanwhile,by means of XRD,dielectric spectroscopy,scanning electron microscopy(SEM),Raman spectroscopy,ferroelectricity and other means of material analysis,themicro-origin of high d33 of the ceramics were analysed,too.The results show that:1)All of the Ho2O3-doped Ba0.85Ca0.15Hf0.1Ti0.9O3 ceramics,have a perovskite phasewithout visible impurity phase,which means that Ho3+ entered intoBa0.85Ca0.15Hf0.1Ti0.9O3 lattice;2)The phase diagram of Ba0.85Ca0.15Hf0.1Ti0.9O3+x mol%Ho2O3 shows that cubic-tetragonal phase transition temperature(near 370 K)do notbeen changed with H02O3 doping,and there is a tetragonal-orthorhombic phasesboundary in ferroelectric phase at x =0.2;3)The the largest d33 of the ceramic withT-O-R phases coexistence was achieved at x =0.1,which the value is 657 pC/N,and corresponding kp,2Pr,2EC,and g are 0.54,19.6 μC/cm2、9 kV/cm,and 20.1 μm,respectively.Multiphase coexistence near the tetragonal-orthorhombic phase boundarymight be the origin of high d33 for this system.