| As one of the most promising ferroelectric ceramics system, the tungsten bronze (TB) compounds have received considerable interest owing to their superior electrooptic, piezoelectric, ferroelectric, and pyroelectric properties.Among these tungsten bronze compounds, sodium strontium niobate Sr2NaNbsO15-based ceramics with filled tungsten bronze structure have received increasing interests in recent years. The tungsten bronze niobate ceramics are known to be difficult in obtaining high density and good electrical properties by traditional sintering methods. In this work, the two-step solid state reaction method and traditional sintering process were used to prepare densitified Sr2K0.1Na0.9Nb5015(SKNN) ceramics. The effects of preparing conditions, substitution for B sites, and dopant on phase structure, microstructure and electrical properties were studied to restrain abnormal grain growth, improve the density, reduce the sintering temperature and optimize the electrical properties.Firstly, the effects of preparing conditions on microstructure, phase structure, and electrical properties were studied in detail. The results showed that when the calcining temperature and sintering temperature are1180℃and1340℃, the obtained ceramics all exhibited tetragonal tungsten bronze (TB) structure structure. The ceramics with a near-theoretical density and a uniform and fine-grained microstructure were obtained, exhibiting outstanding electrical properties: εr=1221,εm=1701, Tc=270℃, tanδ=0.013, Pr=5.55μC/cm2, and Ec=15.7kV/cm.Then, Ta and Sb were used to substitute for Nb5+in B sites. The effects of Ta and Sb contents on microstructure, phase structure, dielectric and ferroelectric properties of the ceramics were studied in detail. Besides, the underlying mechanism for variations of the structures and properties due to Ta and Sb substitutions were also discussed.When Ta substituted for Nb5+, the crystal structure changed from tetragonal (4mm) to tetragonal (4/mmm) phase with increasing Ta content according to the XRD results. Substitution of Ta led to an increase in the sintering temperature, inhibited the grain growth, and prevented the abnormal grain growth behavior. Curie temperature Tc associated to the ferroelectric phase transition decreased drastically from270℃to-35℃with increasing Ta content, further confirming that the ceramics with higher Ta contents were of paraelectric (4/mmm) phase at room temperature. Dielectric and ferroelectric properties of the ceramics decreased and even vanished due to the increasing crystal structure symmetry caused by the change in point group. The mechanism is recognized as:it is the effect of the covalence of the M-O bonds (M5Nb, Ta) that leads to a large decrease in Tc. In relatively covalent compositions such as the niobates, the increase in covalence arising from the more diffuse character of the wave function of Ta. A larger M-O overlap for M=Ta, leads to a decline in Tc owing to the hardening or rigidification of the crystalline network, which thus cancels the ferroelectric distortion. Then it is reasonable that the compounds with higher Ta content are of tetragonal (4/mmm) structure at room temperature. When Sb substituted for Nb5T the obtained ceramics all exhibited tetroganal tungsten bronze structure. With increasing Sb content, the grain size became smaller, but the anisotropic morphology became more obvious. It was also found that Sb substitution could low the sintering temperature, but also deteriorated the electrical properties.Although both Ta and Sb substitutions could inhibit grain rowth to avoid abnormal grain growth (≥50mm) along with crack generation which were frequently encountered in TB ferroelectric ceramics, the electrical properties were deteriorated drastically. Then LiSbO3dopant was added in Sr2K0.1Nao.9Nb5015. The effects of LiSbO3content were studied in order to improve the dielectric properties of the ceramics and decrease the sintering temperature. The results showed that:all samples exhibited single tetragonal tungsten bronze structure. The addition of LiSbO3promoted densification of ceramics. The SEM micrographs showed that with increasing LiSbO3content the average grain size increased, but no abnormal grain growth could be found. The addition of LiSbO3could also decrease the calcing and sintering temperatures from1180℃to1160℃and from1340℃to1320℃, respectively. All the ceramics showed an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics according to the modified Curie-Weiss law. When LiSbO3=0.04mol, the electrical properties of the ceramics were better:εr=1492, εr=2062, Pr=6.84μC/cm2, Ec=17.86kV/cm, and Tc=241℃... |
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