Physical Properties Of Lead-Free Piezoelectric Alkaline Niobate Ceramics

The Pb(Zr,Ti)O₃-based(PZT-based)piezoelectric ceramics have been widely used for actuators,sensors,transducers and other electronic devices in practical applications for many years because of their excellent electrical properties.However, due to the toxicity of lead oxide that is largely used during the production process, PZT ceramics are in the list poisonous materials whose use will be restricted by regulations.There is an increasing demand to replace them with the lead-free alternatives.Recently,significant progresses have been reached in KNN-based ceramics,which showed that they may be the promising candidates as the lead-free piezoelectric materials.Basically,recent investigations on KNN-based ceramics start from the first two results published in 2004.Conventionally-sintered(K_0.50Na_0.50)_0.94Li_0.06NbO₃, (K_0.50Na_0.50)_0.97Li_0.03Nb_0.80Ta_0.20O₃ and(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃ ceramics were shown to have the high d₃₃values of 235,230 and 300 pC/N, respectively,while those textured ceramics with the latter compositions were revealed to have the even higher d₃₃values of 373 pC/N and 416 pC/N.Since then,a lot of researches have been carried out on KNN-based ceramics.Interestingly,most of the recent investigations on KNN-based ceramics have been carried out based on the nearly equal molar ratio of K to Na.This situation might be explained as that these researches were influenced to a great extent by the historical results.It was reported that KNN ceramics prepared by the conventional solid-state reaction method exhibit a k_p peak around x=0.50 and the d₃₃value of 80 pC/N at x=0.50. Later,KNN ceramics with high relative densities were prepared using the hot pressing method,and the results were generally in agreement with the ones obtained in those conventional ceramics,except in that the k_p values are improved and the d₃₃ value at x=0.50 is particularly enhanced to 160 pC/N.Thereafter,the nearly equal molar ratio of K/Na has been widely believed to be the essential condition to obtain good piezoelectric properties in the KNN-based ceramics,which is frequently ascribed to the existence of a MPB that separates two orthorhombic phases with a slight discontinuity in lattice parameters at x=0.475.However,on the close examination of the earlier published results,we find that there exists one serious problem;i.e.the conclusion was made being based on very questionable experiment where the compositional interval of K/Na ratio is adopted as large as 10 mol%.In consideration of the importance,the alkali compositional ratio issue was reexamined in this study.(K,Na)NbO₃ and(K,Na,Li)(Nb,Ta)O₃ ceramics were prepared by conventional solid-state reaction method.Microstructures,crystalline structures,phase transitions,dielectric and piezoelectric properties were investigated. For application purpose,the temperature dependence and thermal ageing character of some(K,Na,Li)(Nb,Ta)O₃ ceramics were studied.(1)The K/Na ratio influence on the physical properties in(K_xNa_1-x)NbO₃ (abbreviated as KNN)ceramics was investigated in the compositional range of x=0.10-0.80.The lattice parameter discontinuity previously reported at x=0.475 is not confirmed,while the existence of a change in the line slopes of lattice parameter variation with x is disclosed in the vicinity of x=0.35.Within x=0.40-0.60,d₍330 and k_p are nearly independent of the K/Na ratio and all the KNN ceramics have the high piezoelectric values of d₃₃～125 pC/N and k_p～0.41,respectively.From analysis,we suggest that the high piezoelectric properties observed in the KNN ceramics nearby x=0.50 are closely related to their microstructures of well-faceted and uniformly distributed large grains rather than the widely-believed MPB effect of lattice parameter discontinuity.The result indicates that the popularly believed concept "equal molar ratio of K/Na or nearly so is the essential condition in obtaining the excellent piezoelectric properties" might be wrong,and should be instructive to the further design of new lead-free KNN-based piezoelectric ceramics.(2)The influence of Li content on physical properties of (K_0.50Na_0.50)_1-yLi_yNb_0.80Ta_0.20O₃ ceramics was studied.It is found that the ceramics with y=0.020 and 0.025 have an orthorhombic structure,whereas those with y=0.035 or more exhibit a tetragonal phase.The ceramics with y=0.030 shows the coexistence of the orthorhombic and the tetragonal phases and also high piezoelectric properties at room temperature.Li-element is considered to have the effects of enhancing the ceramic sinterability.(3)The effects of K/Na ratio on physical properties of (K_xNa_1-x)_1-yLi_yNb_0.80Ta_0.20O₃(abbreviated as KNLNT,with y=0.03 or 0.04) ceramics were investigated.It has been found that all the KNLNT ceramics possess the very similar microstructures,which can be characterized by the relatively high density and the faceted grains of nearly cubic shape.By comparison with KNN ceramics and Li-modified KNN ceramics,Ta-element is disclosed to have the effect of suppressing the grain growth.From XRD analysis,it was clear that the ceramics with y=0.30 or 0.35 has the orthorhombic structure,whereas those with K=0.40～0.60 show the tetragonal structure.With further increasing K(y＞0.60),the KNLNT ceramics show again the orthorhombic structure.Thus,there exist two phase boundaries of orthorhombic and tetragonal structures in KNLNT systems at room temperature.The phase transition temperature(T_O-T)between orthorhombic and tetragonal phases decreases with the increase of K content,which is unexpected and is in contrast with the behavior found in KNN system.All the (K_xNa_1-x)_0.97Li_0.03Nb_0.80Ta_0.20O₃ ceramics with x=0.40～0.60 exhibit the high performance of d₃₃～230 pC/N and k_p～48.5%.Among the KNLNT ceramics,the one with y=0.40 and x=0.40 shows the particularly high piezoelectric properties of d₃₃=254 pC/N,k_p=51.5%,k_t=49.4%and k₃₃=66.6%with the low value of dielectric loss tanδ= 1.5%.(4)The examination of temperature dependence and thermal ageing character was performed for some KNLNT ceramics.Some compositions such as (K_0.40Na_0.60)_0.96Li_0.04Nb_0.80Ta_0.20O₃,(K_0.50Na₀₅₀)_0.96Li_0.04Nb_0.80Ta_0.20O₃, (K_0.50Na₀₅₀)_0.97Li_0.03Nb_0.80Ta_0.20O₃ were found to have the good temperature independence over the temperature range of-40℃to 120℃and very excellent thermal ageing invariability in the severe test(where specimens were exposed to the temperatures as low as -120℃to as high as 300℃for 1 h).The result indicates that some of the KNLNT ceramics are very promising lead-free piezoelectric material suitable for practical applications.