Relationship Between Dielectric Dispersion Characteristics And Electrical Properties Of Barium Titanate-Based Lead-Free Ferroelectric Ceramics

Ferroelectric materials are extensively used in military,high-tech and civil industries owning to their rich electrical properties and complex coupling effects.Therein,relaxor ferroelectrics are capable of excellent temperature stable dielectric property,large electrostriction,high energy storage properties and desired electrocaloric effect,standing out in the fields of ceramic capacitors,microdisplacement drivers,transducers,high power density pulsed capacitors,and solidstate cooling devices and so on.At present,the dominant lead-containing materials are subjected to a stricter restriction due to the rising environmental concerns,and leadfree materials have become a research hotspot.BaTiO₃（BT）-based ceramics are of great importance in lead-free perovskite-type dielectric,piezoelectric and ferroelectric materials due to their high dielectric constant,low dielectric loss,large polarization and high insulation resistivity,et al.Last decade,constructing phase boundary in BT-based materials to realize giant piezoelectricity(d₃₃>600 pC/N)has got widely attention.In addition to the piezoelectricity,various electrical properties such as electrostrictive,energy storage and electrocaloric properties are concerned closely with the behavior of dielectric dispersion or relaxation in BT-based ceramics.However,it is unclear how the dielectric relaxation properties and diffuse phase transition influence these electrical properties.Besides,compared with lead-based relaxor ferroelectrics,BT-based relaxor ferroelectrics yield a number of unique and interesting features,such as diffuse phase transition as an intermediate state between ferroelectric and relaxor states,relaxor characteristic in compositions without the nominal charge disorder,and re-entrant behavior,et al.Therefore,it is of great scientific significance to study the dielectric dispersion characteristics and their correlation with electrical properties of BT-based ceramics.Usually,doping is an important way to obtain diffuse phase transition and dielectric relaxation behavior in BT-based materials,and the local structure inhomogeneity caused by doping is the main reason for the formation of polar nanoregions（PNRs）.This dissertation focuses on A-site and/or B-site ion doping tuning diffuse ferroelectric-ferroelectric or ferroelectric-paraelectric phase transitions and dielectric relaxation characteristic in BT-based materials,therefore exploring the relationship between composition,structure and electrical properties,emphatically studying the performance of electrostrictive strain,dielectric energy storage,electrocaloric effect and related physical mechanism.This thesis mainly comprises the following research results:（1）The electrostrictive properties of BT-based ceramics were enhanced by the preferred distribution of A-site Li⁺-Ho³⁺ ion-pairs.BT-based materials were developed by the co-doping of Li⁺ and Ho³⁺ with smaller ionic radii at A site,and the distribution of Li⁺ and Ho³⁺ in BT unit cell was examined.(Li⁺,Ho³⁺)co-doped BT ceramics exhibited pure perovskite when the doping content was lower than 3 mol%.Furthermore,the preferentially [001] direction distribution of Li⁺-Ho³⁺ ion-pairs occupying the neighboring Ba sites in the same lattice of BT were detected by XRD analysis,which shows the position of（002）and（200）characteristic peaks and their change in the ratio of peak intensity.And Li⁺-Ho³⁺ ion-pairs in BT ceramics were confirmed by Raman spectra,XPS analysis,aging test and orientation behavior under the combined stimulation of thermo & electric-field.The existence of Li⁺-Ho³⁺ ionpairs destroyed the long-range order of ferroelectric domains in BT,inducing diffuse ferroelectric-ferroelectric and ferroelectric-paraelectric phase transition,and it would reduce electric field-induced strain hysteresis and remnant strain,thereby enhancing the positioning accuracy and repeatability of actuators.Besides,the strong lattice distortion along [001] direction around Li⁺-Ho³⁺ ion-pairs would limit active space for Ti site,and it may result in smaller polarization per unit magnitude of electric field but a larger strain per unit magnitude of polarization,therefore an enhanced longitudinal electrostrictive coefficient(Q₃₃=0.06 m⁴/C²),which was superior to other lead-bearing and lead-free electrostrictive materials(Q₃₃=0.02～0.04 m⁴/C²).Utilizing A-site Li⁺-Ho³⁺ ion-pairs doping,inducing diffuse phase transition and lattice distortion along [001] direction collaboratively realized large electrostrain（0.12%～0.2%）with thermal stability（20～150 ℃）,low hysteresis（<10%）and outstanding Q₃₃ in BT-based ceramics.（2）The co-doping of monovalent ions and trivalent ions on A site of BT-based ceramics were investigated,and the regulation rules of doping on the dielectric and electrical properties were established.Firstly,Ho³⁺,Y³⁺,Yb³⁺,Gd³⁺,Sm³⁺ and Pr³⁺ were seleted to co-dope BT with K⁺,finding that the influence of different rare earth ions on Curie temperature of BT-based materials was different: the smaller the ionic radius was,the higher the TC was.Therein,the TC of（K,Ho）and（K,Y）co-doped BT were enhanced to 144 ℃ and 139 ℃,respectively.In addition,when the ionic radius of trivalent ion increased,the diffuseness parameter of ceramic increased,which was related to the different occupancy situation of rare earth ions with different sizes in the lattice.Rare earth ions with larger radius tended to occupy A site of BT lattice,while rare earth ions with smaller radius tended to occupy B site of BT,leading to the variance in <R> effect,<σ2> effect and electrovalance mismatch effect,thus having different influences on TC.Besides,the larger electrostrain and smaller strain hysteresis were obtained simultaneously in ceramics with smaller TC.Furthermore,the effects of different monovalent ions（Li or Na）and small radius trivalent ions（Ho,Y or Yb）on the dielectric properties and temperature stability of electrostrain of BT-based ceramics were also investigated.The co-doing of（Li,Ho）,（Li,Y）,（Na,Ho）and（Na,Y）can increase TC without inducing the dielectric dispersion behavior in BT.And the temperature stability of electrostrain and Q₃₃（0.059～0.070 m⁴/C²）can be improved by such types of co-doping.The electrostrain properties of BT-（Li/Na,Ho/Y）T ceramic were stable in the temperature range from room temperature（RT）to 130 ℃,which solved the defect of strain instability with increasing temperature in BT-based ceramics.（3）Lead-free BT-based relaxor ferroelectrics were conceived by co-doping of K⁺ in A site and Nb⁵⁺ in B site to construct the relaxor crossover region,targeting multifunctional applications of energy storage capacitors and electrostrictive actuators.Guided by the characteristics of universal relaxor ferroelectrics,dielectric properties and electrical properties were different in different temperature regions.Therefore,in Ba_1-xK_xTi_1-xNb_xO₃ ceramics,the regulation of doping content of K⁺ and Nb⁵⁺ resulted in different relaxation state: when x≤0.02,the ceramics with Tm larger than 60 ℃ were composed of chunks of ferroelectric domains on the order of 1 μm together with few nanodomains,exhibiting large P_max,large Pr,and distinct strain hysteresis due to the switching and motion of domains under an applied electric field.When x=0.04,the composition with Tm near RT,which corresponded with the beginning of dielectric relaxation behavior,was defined as relaxor crossover region.Besides,the coexistence of nanodomains and PNRs being highly dynamic,which can return to the original state easily after the removal of the electric field,yielded the major contribution to large P_max,small Pr and ultra-low strain hysteresis.When x≥0.06,Tm of the ceramics were lower than 0 ℃.At RT,only PNRs existed in the ceramics,which exhibited non-polar state macroscopically.Therefore,P_max and Pr were relatively small,and the electrostrain only originated from the electrostrictive effect.In addition,the doping of K⁺ and Nb⁵⁺ hindered the grain growth of ceramics,resulting in an average grain size of about 250 nm.Finally,fine-grained BT-0.04 KN with crossover region represented the combination of excellent Wrec of 2.03 J/cm3,sky-high η of 94.5%,breakdown strength of 300 k V/cm,together with superior temperature stability of Wrec（without a drop from 25 ℃ to 130 ℃）.Notably,BT-0.04 KN ceramic also exhibited superior electrostrictive properties: high Q₃₃ of 0.049 m⁴/C²,and a large pure electrostrictive strain(S_max=0.13%)can be obtained at a medium electric field of 80 k V/cm.（4）By adjusting the doping sites（A or B site）of Ce in BT ceramics,two components with distinct dielectric properties can be obtained,leading to superior electrocaloric effect and large electrostrain,respectively.The rare earth element Ce,as an amphoteric dopant,can preferentially occupy A or B site of BT lattice according to the starting Ba/Ti ratio of the component.Ba-site doping of Ce³⁺ in BT shifted TC quickly to RT with high-k sharp peak behavior.In conjunction of A-site size mismatch effect and defect structure concerning Ba vacancies,the local chemical stress field around the dopants drove a second-order-like phase transition near RT in a specific composition（x=0.04）,optimizing the performance and temperature span of electrocaloric properties of BT-based ceramics: electrocaloric adiabatic temperature change ΔT_max ～1.35 K was obtained in wide temperature range（ΔT>1 K,37～70 ℃）with high electrocaloric coefficient ΔT/ΔE of 0.4 K mm k V-1.Furthermore,typical direct ECE measurement results by a modified DSC were consistent with indirect method,proving the validity of electrocaloric data from the indirect approach.Besides,Ti-site doping of Ce⁴⁺ in BT induced diffuse phase transition with a degraded _maximum permittivity but a maintained Tm,leading to multiphase coexistence and the formation of nanosize domains,which benefited large electrostrain（0.15%～0.18%）with ultra-low hysteresis and good temperature stability（RT～100 ℃）.The diffuse phase transition originated from the orthorhombic phase distortion of CeO₆ breaking the long-range order of ferroelectric domains,lattice stress resulting from the inconsistency of ionic radius of Ce⁴⁺ and Ti⁴⁺,and grain size effect.In addition,the effects of different doping sites of Ce in BT on the phase structure（macroscopic）,domain structure（mesoscopic）and defect mechanism（microscopic）of the system were studied in detail,hoping to opens up a brand-new route to understand the doping mechanism in BT-based materials.