Microstructures, electrical behavior and energy-storage properties of Ba0.06Na0.47Bi0.47TiO3-Ln1/3NbO3 (Ln = La, Nd, Sm) ceramics
Graphical abstract
Polarization hysteresis loops of BNBT-xLnN ceramics under 60kv/cm: (a) Ln = La; (b) Ln = Nd and (c) Ln = Sm. And (d) polarization hysteresis loops of BNBT-0.02LnN ceramics.
Introduction
Ferroelectric materials have great potential to be used for high energy density capacitors in electronic circuits, especially the pulse power circuits. However, it is difficult to apply the Na0.5Bi0.5TiO3(BNT) ceramics into applications due to the large leakage current. In order to obtain pinched P-E loops, many methods were adopted to enhance saturated polarization (Ps) while reduce remnant polarization (Pr). One common way is to dope weakly polar phase (such as BaTiO3, PbTiO3) [1], [2], [3], [4] or none-polar phase (such as NaNbO3, PbZrO3) [5], [6] to the BNT substrate. It was reported that BNT-0.06BT, whose composition near the morphotropic phase boundary (MPB) shown better ferroelectric, dielectric and other electric properties [3]. Few research reports assert that BNT-BT ceramic belonging to the region of MPB can be defined as ‘relaxor antiferroelectricity’ due to common feature of nanodomains [7]. Relaxor ferroelectrics have two essential features—diffuse phase transition and frequently dispersion. The former is characterized by a gradual process from ferroelectric to paraelectrics, in other words, relaxor ferroelectric has an unfixed Curie point. The latter is characterized by frequency dependence of the dielectric constant. Composition fluctuation theory (G.A. Smolensky) [8] and micro-macro domain transformation theory (Y. Xi) [9] are often used to explain relaxation feature mentioned before.
Doping with rare earth elements (La, Nd, Sm) allows to obtain materials with low dielectric loss and other excellent properties, therefore it has been widely adopted [10], [11], [12]. Recently, a subgroup of perovskite oxides, namely the A-site deficient Ln1/3NbO3 (Ln = La, Nd and Sm) oxides, was found to possess good microwave dielectric properties [13]. Besides, Ln1/3NbO3(Ln = La, Nd and Sm) are a kind of non-polar phase materials, which lack of domain structure would improve fatigue endurance property [14]. Thus, the solution of Ln1/3NbO3(Ln = La, Nd, Sm) in Ba0.06Na0.47Bi0.47TiO3 ceramics may be helpful for the weakness of the ferroelectric polar behavior and be an excellent alternative for energy storage properties. Then, the complex impedance spectroscopy and dielectric spectra were used to investigate dielectric relaxation, dispersion and AC impedance of these series of ceramics. Moreover, microstructure, conductivity and ferroelectricity were also studied.
Section snippets
Experiment
Single-phase powders of (1−x)Ba0.06Na0.47Bi0.47TiO3-xLn1/3NbO3(Ln = La, Nd and Sm, x = 0.01, 0.02, 0.03 and 0.04) were prepared by the mixed oxide route from appropriate quantities of high purity (≥99.9%) Bi2O3, Na2O, TiO2, Sm2O3, BaO and Nb2O5. The corresponding ceramics, for the sake of simplicity, are defined as BNBT-xLnN (Ln = La, Nd and Sm). The powders were mixed and ground in alcohol for 12 h with ZrO2 balls. Then, the mixtures were dried and calcined at 850 °C for 2 h. The resultant
Results and discussion
The structures of all the samples are characterized by x-ray diffraction (XRD), as shown in Fig. 1. There are no obvious second phases in the ceramics within the sensitivity of X-ray diffraction. Generally, Na0.5Bi0.5TiO3 is perovskite structure with rhombohedral symmetry which is characterized by (003)/(021) peaks at around 2θ of 40° [15], while the symmetry of BaTiO3 at room temperature is featured with splitting of (002)/(200) peaks at around 2θ of 46° [16]. For the BNBT-xLnN ceramics, an
Conclusions
The BNBT‒xLnN (Ln = La, Nd and Sm, x = 0.01–0.04) ceramics with different x content were fabricated by solid state reaction method, and the effects of x values on the microstructures, ferroelectric properties and impedance spectroscopy of the ceramics were studied. With the increasing LnN content, Pr and Ps decreased rapidly. In the system, the maximum recoverable energy storage density of BNBT-0.02LnN ceramic in a descending order as: 1.239 J/cm3 (Ln = Sm), 1.145 J/cm3 (Ln = Nd), 1.039 J/cm3
Acknowledgement
Financial supports of the National Natural Science Foundation of China (Grants No. 11464006, 61561011 and 51462005) and the Natural Science Foundation of Guangxi (Grants No. 2014GXNSFBA118254) are gratefully acknowledged by the authors.
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