Preparation And Physical Properties Of?K,Na?NbO₃-based Lead-Free Piezoelectric Ceramics

Piezoelectric ceramics are a kind of functional materials,which can realize the conversion between electric energy and mechanical energy.Pb(Zr,TiO₃-based?abbreviated as PZT-based?ceramics have been widely used in sensors,actuators,ultrasonic transducers and other piezoelectric devices due to their excellent piezoelectric properties and temperature stabilities.However,because PZT-based ceramics contain a large amount of volatile and toxic lead,relevant laws and regulations were issued by many countries to restrict the use of lead-containing substances.Therefore,it is an urgent and significant task to develop environmentally friendly lead-free piezoelectric ceramic materials.?K,Na?NbO₃-based?abbreviated as KNN-based?lead-free piezoceramics,as a promising potential alternative to the traditional PZT-based ceramics,have attracted extensive attention in recent years.Pure KNN lead-free ceramics possess high Curie temperature and excellent thermal stability.However,compared with the practically used PZT-based ceramics,there are still two large problems in KNN lead-free ceramics:Firstly,it is difficult to prepare a highly dense KNN ceramic with good quality and good repeatability through conventional sintering because of the high volatile property of alkali elements.Secondly,the piezoelectric properties of conventionally prepared KNN ceramics are not good enough,since the piezoelectric constant d₃₃ and the planar electromechanical coupling coefficient k_p at room temperature are only about 125 pC/N and 40%,respectively.Thus,the KNN ceramics are insufficient for practical application as alternatives of PZT ceramics in most cases.In recent years,a lot of efforts have been made to improve the fabrication and piezoelectric properties of the KNN lead-free ceramics.In 2004,a large d₃₃ value of 416 pC/N was reported in textured Li-,Ta-and Sb-doped KNN ceramics by Saito et al.,which caused a widespread concern.Subsequently,a large number of studies on compositional modification disclosed that the excellent piezoelectric properties are mainly ascribed to the shift of orthorhombic-tetragonal phase transition temperature?TO-T?to room temperature.Unfortunately,this method also causes the poor thermal stability.Recently,a big breakthrough was made by a research group at Sichuan University in the KNN-based ceramic with rhombohedral-tetragonal?R-T?phase boundary,the d₃₃ value has reached a high level of 570 pC/N.However,subsequent studies brought a new argument that the best piezoelectric properties in some KNN-based ceramics are achieved not at the R-T phase boundary but in the region of rhombohedral-orthogonal-tetragonal phase coexistence.Therefore,the structural origin and piezoelectric temperature stability for such kind of KNN-based ceramics still need to be further studied.On the other hand,a variety of sintering methods were attempted to prepare KNN-based ceramics.For example,as early as 1962,Jaeger et al.have reported that the d₃₃ and k_p values in the hot-press sintered KNN ceramic at room temperature were respectively improved to 160 pC/N and 0.45 with the relative density up to 99%.This indicates that hot-pressing sintering is an effective method to improve the density and piezoelectric properties of KNN ceramics.However,no researches on the preparation of KNN-based ceramics with hot-pressing sintering had been subsequently reported for several decades from then.In addition,as a comparatively new and effective sintering method,the two-step sintering technology has been widely used for preparing some highly dense nano-oxide materials like Y₂O₃,ZnO and Al₂O₃,etc,but only very limited attempts were made for preparing the KNN-based ceramics.Under the abovementioned circumstances,this dissertation mainly carried out the investigations on KNN-based lead-free piezoelectric ceramics from the aspects of partilcle-size effects of raw-material powders,sintering,phase transitions,microstructure,domain structure,piezoelectric properties and piezoeletric temperature stabilities.The main research contents and results are as follows:1.Partilcle-size effects of raw-material powders and influences of poling conditions on the piezoelectric properties of KNN-based ceramics were investigated.The results indicate that particle sizes of raw materials can be effectively reduced by pre-milling and both the powder uniformity of raw-material mixture and the piezoelectric properties of the resultant Li-doped KNN-based ceramics are largely improved.In addition,it is proved that the optimum poling condition is in the vicinity of phase transition point under a large poling electric field for 30 min.The author observes that the morphology of each individual raw material is quite different,and particle sizes of most raw materials are basically below 1 ?m,while the particle sizes of LiCO₃ powder are as large as 20 ?m.Interstingly,the particle sizes of Li₂CO₃ powder can be reduced to about 2 ?m by pre-milling.When raw materials are treated by pre-milling before mixing,a well-dispersed mixture powder with finer particle sizes can be obtained after the conventional 1st ball-milling.In this way,the maximum piezoelectric constant d33 obtained in the(K_0.45Na_0.55)_0.98Li_0.02Nb_0.76Ta_0.18Sb_0.06O₃ ceramic was increased from 345 pC/N to 393 pC/N.However,both two kinds of(K_0.45Na_0.55)_0.98Li_0.02Nb_0.76Ta_0.18Sb_0.06O₃ ceramics prepared from either untreated or treated Li₂CO₃ powder by pre-milling show an obviously bimodal grain distribution in microstructure and simple parallel stripes in domain structure2.A series of dense(K_0.45Na_0.55)_0.98Li_0.02Nb_0.82-xTa_0.18Sb_xO₃(x = 0.025 and 0.06,abbreviated as KNNLTS_0.025 and KNNLTS_0.06)ceramics were prepared by hot-press sintering,and the effects of sintering conditions and annealing conditions in hot-press sintering on piezoelectric properties,dielectric properties,crystal structures and microstructures of KNNLTS_0.025 and KNNLTS_0.06 ceramics were investigated The maximum d₃₃ value of 414 pC/N is obtained in the hot-pressed ceramic,which is higher than 393 pC/N obtained in the conventionally sintered one.Furthermore,the author found an amazing physical phenomenon that the orthorhombic-tetragonal phase transition temperature?TO-T?can be greatly increased by hot-pressing sintering and can be further increased after annealing.It is found that the To-T of KNNLTS_0.025 ceramic increases from 50? to 90? when changing conventional sintering to hot-press sintering and further increased to 112? after a subsequent annealing treatment.According to the analysis results of ?'-temperature dependence,XRD and microstructure,the phase transition behaviors of the hot-pressed ceramics are mainly caused by internal stress and grain-size effect.Furthermore,in the hot-pressing sintering of KNNLTS_0.06 ceramics,the author found that the microstructure is very sensitive to the temperature and dwelling time.3.KNNLTS_0.06 ceramics were prepared by both two-step sintering?TSS?and conventional sintering?CS?,respectively.The effects of sintering mathods and sintering conditions on dielectric properties,piezoelectric properties,ferroelectric properties,microstructure and domain structure of KNNLTS_0.06 ceramics were investigated.The results indicate that piezoelectric properties,microstructure and domain structure changed significantly with sintering methods and sintering conditions,and the authore believed that the increase of the relative density and the hierarchical nano-domain structure are two key factors to the improvement of piezoelectric properties.The relative density of KNNLTS_0.06 ceramics was improved from 94.8%to 97.2%when changing sintering mathods from CS to TSS,consequently,the d₃₃ value was enhanced largely from 393 pC/N to 455 pC/N.The microstructure in the ceramics prepared by CS showed obviously bimodal grain distribution,the small grain size is less than 1 ?m,and the average large grain size is about 3.6 m.However,with the dwell time extension,the ceramics prepared by TSS possess a dense microstructure of changed from bimodal grain distribution to uniform and large grains.The excellent piezoelectric properties of d₃₃ = 455 pC/N,k_p= 0.54,k₃₃=0.67 were obtained at room temperature in the KNNLTS_0.06-TSS ceramic with the average grain size of about 12.1 ?m.Furthermore,the authors found that in addition to the contribution of O-T phase transition,the enhancement of piezoelectric properties in KNNLTS_0.06-TSS ceramic also benefits from high density,large dielectric constant and the remanent polarization,and derived from uniform and dense microstructure and unique domain structure,especially the hierarchical nano-domain structure in wide parallel stripe structure4.The effects of Bi_0.5Na_0.5ZrO₃?abbreviated as BNZ?content on dielectric properties,piezoelectric properties,crystal structure,microstructure and domain structure of the?0.994-x?(K_0.4Na_0.6)Nb_0.965Sb_0.045O₃-0.006BiFeO₃-xBi_0.5Na_0.5ZrO₃?abbreviated as KNNS-BF-xBNZ?ceramics were investigated,and the origin of strong piezoelectric properties was discussed.The major research results include:?1?with the increase of BNZ content,the orthorhombic-tetragonal phase transition temperature?TO-T?and Curie temperature?TC?of the KNNS-BF-xBNZ ceramics decrease gradually.However,the rhombohedral-orthorhombic phase transition temperature?TR-O?remains nearly unchanged,which is obviously different from the gradual increase of TR-O in the KNN systems without Sb⁵⁺ doping.?2?Rhombohedral-orthogonal-tetragonal phase coexistence is constructed at room temperature,and the best piezoelectric properties of d₃₃ = 550 pC/N,k_p = 0.52,kt =0.46 and k₃₃ = 0.64 are obtained at x = 0.03.This d₃₃ value is comparable to those record values ever obtained in the KNN-based ceramics so far.Moreover,the result of "the best piezoelectric properties occurring in the rhombohedral-orthogonal-tetragonal phase coexistence region" obtained in this thesis is obviously different from the earlier reported conclusion that the rhombohedral-tetragonal phase boundary possesses the best performance.?3?Theoretical fitting of XRD profiles shows that the ceramic with x = 0.03 has the volume fractions rhombohedral-orthogonal-tetragonal phases of 20.4:50.9:28.7.?4?KNNS-BF-xBNZ ceramics possess the large grain sizes and the grain sizes become relatively uniform with the increase of BNZ content.The domain patterns are relatively simple,mainly composed of a set or several sets of parallel stripes across most or even the whole grain.It's worth noting that there are smaller parallel nano-stripes in the wide parallel bands,and the width of these nano-domains is about 65 nm.These nano-domains are believed to play an important role in the piezoelectric properties of KNN-based ceramics.?5?KNNS-BF-0.03BNZ ceramic not only has excellent piezoelectric properties(d₃₃ = 550 pC/N,k_p = 0.52)but also has good thermal-ageing stability and time-ageing stability.5.The 0.95K_0.575Na_0.425Nb_0.965Sb_0.035O₃-0.02BaZrO₃-0.03Bi_0.5K_0.5HfO₃ ceramic with the rhombohedral-tetragonal phase structure was prepared by two-step sintering,and a large room-temperature piezoelectric constant d₃₃?530 pC/N was obtained.Domain structure and piezoelectric temperature stability was investigated.The results indicate that this ceramic has dense microstructure and simple domain patterns.The average grain size is about 7.8 ?m,and domain patterns in the poled ceramic are mainly parallel long stripes,and the domain widths are 160?400 nm.This ceramic has poor piezoelectric temperature stability,thermal-ageing stability and time stability.