Processing And Properties Optimization Of Titanate-based Lead-free Piezoelectric Ceramics

To date,the most widely used piezoelectric ceramics belong to Pb(Zr,Ti)O₃ family owning to its superior piezoelectric properties.However,the hazardous lead(Pb)presents in these materials has raised increasing environmental concerns,and developing alternative lead-free piezoelectric materials that can eventually replace the lead-based ones is currently motivated by environmental regulations.Perovskite Ba TiO₃ and Bi_0.5Na_0.5TiO₃-based ceramics were considered promising candidates for replacing lead-based piezoelectric ceramics.However,their properties should be improved further for practical applications.In this thesis,the properties of these titanate-based lead-free piezoelectric ceramics have been tailored by doping,changing stoichiometry and forming solutions with other phases.The microstructure,crystal structure and defects on the dielectric,piezoelectric and ferroelectric properties of the ceramics were systematically investigated.Firstly,MnO₂-doped Ba_0.925Ca_0.075TiO₃ ceramics(abbreviated as BCT)were synthesized by conventional solid-state reaction method.The effects of MnO₂addition on the structure,microstructure and electrical properties including the dielectric,piezoelectric and ferroelectric behavior of the ceramics were systematically investigated.XRD results revealed that all the ceramics possessed a perovskite structure with tetragonal symmetry and the tetragonality decreased with the addition of MnO₂.The density of the BCT ceramics was improved after the addition of small amount of MnO₂.However the grain size growth started to be inhibited when 1%mol or more MnO₂was added.Both the Curie temperature(T_c)and orthorhombic-tetragonal phase transition temperature(T_o-t)were shifted downward after the addition of MnO₂.The doping of MnO₂ can obviously enhance the mechanical quality factor(Q_m)and decrease the dielectric loss(tan?).High mechanical quality factors(Q_m?1500)and low dielectric losses(tan??0.3%)were found when the doping corcerntration reached 1%mol,meanwhile,the piezoelectric and electromechanical properties were found to decrease compared with the pure BCT,exhibiting a typical characteristic of“hard”behavior.The internal bias field(E_i)played a key role on the“hardening”of BCT ceramics,which was believed to be the result of defect dipoles(acceptor-V_O^··)formed by the acceptor dopant ions(Mn²⁺/Mn³⁺)and oxygen vacancies,leading to the piezoelectric“hardening”effect by stabilizing and pinning the domain-wall motion.In order to further optimize the piezoelectric and electromechanical properties of the“hard”BCT ceramics,the effects of excess Ba²⁺on the crystal structure,microstructure,and electrical properties of the 0.75%mol MnO₂-doped BCT ceramics were systematically investigated.Both the crystal structure and microstructure of the ceramics was significantly affected by adding the extra Ba²⁺ions.The grain size of the ceramics decreased from?30?m to?2?m when the excess of Ba²⁺reached 2.5%mol.The tetragonality deceased and unit cell volume increased dramatically when the excess of Ba²⁺reached 1.5%mol,indicating that some Ca²⁺ions were pushed from Ba site to Ti site of BCT.The internal bias field was also significantly decreased when the the excess of Ba²⁺reached 1.5%mol due to the decrease of tetragonality.The piezoelectric coefficient(d₃₃)was enhanced from 135 p C/N to 230 p C/N with the increase of excess of Ba²⁺,due to the decrease of grain size.The Q_m values were more than 1000 when the the excess of Ba²⁺was within 1%mol due to the high internal bias fields(E_i=3-4 k V/cm)and decreased significantly to?300 after the excess of Ba²⁺reached1.5%mol,due to the low internal bias fields(E_i?1 k V/cm).The defects relaxation behavior in the acceptor Mn and Co-doped BCT ceramics were comparatively studied by using the thermally stimulated depolarization current(TSDC)method.A defect dipole relaxation peak was found in the Mn-doped BCT ceramics,and the activation energy of the defect dipole relaxation was estimated at 0.7 e V.Both a defect dipole relaxation peak and a high temperature oxygen vocation relaxation peak were indentified for the Co-doped BCT,and the defect dipole relaxation energy was estimated at 0.55 e V.The internal bias field in the Co-doped BCT with higher concentration of defect dipoles was found larger than that of Mn-doped BCT.However,the internal bias field in the Mn-doped BCT showed much more stable behavior under the cycling of electric field due to its higher activation energy of its defect dipole relaxation.(1-x)Bi_0.5Na_0.5TiO₃–x Bi_0.2Sr_0.7TiO₃(BNT–BST100x)solid solutions with x=0-0.35were synthesized by conventional solid-state reaction method.The crystal structure and electric properties of the solid solutions were investigated systematically.With the increase of BST content,the crystal structure of the solid solutions gradually changed from rhombohedral to cubic,the laminated domain configuration disappeared,and the dielectric relaxation peak T_d moved from high temperature to room temperature.When the BST content reached23%mol,the ferroelectric and piezoelectric properties almost disappeared but a large electric field-induced strain(S=0.36%)under an electric field of 8 k V/cm was observed.At the rhombohedral-cubic morphtropic phase boundary,the large electric-field induced strain effect was originated from the non-polar relaxor phase to the polar ferroelectric phase transition induced by the electric field.(1-x)(K_0.5Na_0.5)_0.95Li_0.05Nb O₃–x Bi_0.5(K_0.15Na_0.85)_0.5TiO₃(KNLN–BNKT)solid solutions with x=0-0.03 were synthesized by conventional solid-state reaction method.The crystal structure and electric properties of the solid solutions were investigated systematically.X–Ray diffraction analysis revealed that the KNLN–BKNT solid solutions possessed pure perovskite structure,showing symmetries of orthorhombic at x<0.01 and tetragonal at x?0.015 at room temperature.A morphtropic phase boundary(MPB)that coexistence of orthorhombic and tetragonal phases was identified at x=0.01,where enhanced piezoelectric properties was found because of the increased polarizability.