| Piezoelectric ceramics, as a kind of vital functional materials, is playing a moreand more important role as electronic components become high-tech, accurate andadvanced today. Niobate piezoelectric ceramics is believed to be the most promotedceramics to substitute (Pb, Zr) TiO3lead-free piezoelectric ceramics (PZT) for itshigh Curie temperature, excellent electrical, electromechanical coupling andferroelectrical properties. However, niobate piezoelectric ceramics can hardl y be wellsintered, and exhibits bad temperature stabilit y, which restricts its application.Nowadays, researches of niobate piezoelectric ceramics are mostly focus on themodification of (Na, K) NbO3(KNN) ceramics by doping some other elements,among which the most effective ones are Li, Sb, Ta. Usually, they are doped eitherseparately or integrally in the forms of LiNbO3and LiTaO3composite. Due to thecomplexity of both doping amount and variety, there are no unified results achieved.Thus, Li(Nb, Ta)O3(LN-LT),(Na0.53K0.47)Nb1-xTaxO3(KNNTx),(Na0.53K0.47)Nb1-xSbxO3(KNNSx),(Na0.53K0.47)1-xLixNb0.8Ta0.2O3(KNNLT) and(Na0.53K0.47)1-xLixNb0.91Sb0.09O3(KNNLS) ceramic powders were prepared by solid-phase synthesis. Then they were compacted into disks and sintered between1080℃~1360℃. Subsequently, the effects of a wide range of Sb, Ta substitution for KNNceramics on microstructure and electric properties were systematically studied in thispaper, so as the effects of the Li/Ta ratio and Li/Sb ratio on the microstructure andperformance of KNN ceramics. Furthermore, the influence rules of the ratio of dopingelements, atomic size, electrone gativity on the microstructure of the piezoelectricceramic were discussed. The main conclusions are as follows.(1) The production of second phase can be effectively restrained when doped withTa. However, the optimized sintering temperature was increased with the increasingof the content of Ta. LN-LT ceramics showed typical ferroelectric properties. And itsCurie temperature (TC) can reach600℃. Although the TCof LiNbO3ceramicsdecreased sharply when Ta was introduced, there brought no much difference with anyfurther increase of Ta. The polarization (P–E) hysteresis loops of LN-LT ceramicspresented "shuttle" shape with large residual polarization and coercive field. Takingboth the dielectric and ferroelectric properties into consideration, the LN-LT ceramicswith x=0.5can obtain comprehensive performance with high Curie temperature, relatively good.low dielectric loss and high remaining polarization value.(2) Both Ta and Sb induced the increase of the optimized sintering temperature ofKNN ceramics. Ta made a greater difference on sintering temperature than Sb for thebest sintering temperature range (1160℃~1260℃) of KNNTx ceramics was largerthan that of KNNSx ceramics(1160℃~1200℃). Besides, compared with Ta, itseemed to be difficult for Sb to diffuse into the lattice of KNN with a solid solubilityof no more than0.1mol%. There appeared transition from orthogonal phase totetragonal phase (PPT) with x(Ta)=0.2~0.3and x(Sb)=0.1, where the ceramicsperformed well for that the values of d33, kp, tanδ and TCreached93~89pC/N,26.08~25.98%,2.51~2.61%,226℃~239℃, respectively for KNNTx ceramics, andcorresponding values were99pC/N,19.87%,3.01%,159℃respectively for KNNSxceramics. What’s more, the TO-Tand TCof KNNTx ceramics were significantlyreduced from214℃and400℃to214℃and145℃when mixed with Ta. However,there appeared only one dielectric peak for KNNSx ceramics, corresponding to the TC,suggesting that the TO-Tmay have dropped below room temperature.(3) The influence rules of Li/Sb, Li/Ta ratio on KNN ceramics were compared.Although there showed no PPT for KNNLT ceramics, it presented goodcomprehensive performance when the ratio of Li/Ta was fixed at3/6, with d33=107pC/N, kp=38.8%, Qm=144, tanδ=2.42%. Different for KNNLS ceramics, thereappeared PPT when the ratio of Li/Sb was between3/9and7/9, with d33>100pC/N,kp and Qm hitting their peak, and tanδ showing low value. In addition, similar toKNNSx ceramics, there appeared only one dielectric peak in the spectrum for KNNLTceramics, corresponding to the TCwhich was restricted within235℃~255℃. TheTCof KNNLS ceramics rose at first, and then decreased after it reached the maximum(372℃) with Li/Sb=5/9. However, the TO-Tshowed no obvious change, and reachedthe lowest value (123℃) when the Li/Sb ratio was3/9. |
