Structure and electrical properties of BCZT ceramics derived from microwave-assisted sol–gel-hydrothermal synthesized powders

A novel microwave-assisted sol–gel-hydrothermal method was employed to rapidly synthesize Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) powders. The effects of reaction time on the structure, crystallinity, purity and morphology of the products were investigated. The results of XRD, FTIR, SEM and TEM indicated that BCZT powders could be obtained in even 60 min at a low synthesis temperature of 180 °C, which were well-crystallized with stoichiometric composition and uniform grain size (~ 85 nm). BCZT ceramic derived from the rapidly-synthesized powders had a dense microstructure and good electrical properties (εm = 9579, d33 = 496 pC/N, 2Pr = 25.22 µC/cm2, 2Ec = 7.52 kV/cm). The significant electrical properties were closely related to the high activity of the BCZT powders, resulting from the rapid microwave-assisted sol–gel-hydrothermal process.

In the present study, a novel technique, MSGH, was employed to rapidly synthesize crystalline BCZT powders with high purity and activity rapidly (60 min) at low temperature (180 °C) for the first time. The effects of reaction time on the structure, crystallinity, purity and morphology of BCZT powders were studied. Furthermore, followed by traditional sintering method, electrical properties of MSGH derived BCZT ceramics were measured, so as to verify the high activity of the powders.

Experimental
Ba 0.85 Ca 0.15 Zr 0.1 Ti 0.9 O 3 (BCZT) powders were synthesized by the microwave-assisted sol-gel-hydrothermal method. Firstly, BCZT gel was formed through sol-gel process by using barium acetate (BaC 4 O 4 ) as the raw materials. The acetic acid solution was mixed with the raw materials and kept stirring at 60 °C for 30 min. The solution turned into sol and then transformed to gel. After dried overnight and grounded, the formed gel was introduced to a NaOH aqueous solution with a concentration of 4 M. The precursor solutions were then sealed and placed in the microwave hydrothermal equipment (JUPITER BF, SINEO, China). The reaction was carried out under 300 W microwave and a synthesis temperature of 180 °C. The precipitate was centrifuged and washed with distilled water and absolute ethanol for several times to remove the soluble impurities. After drying, the BCZT powders were obtained. Finally, the MSGH-derived BCZT powders were pressed into disks, and further sintered at 1400 °C for 2 h to produce BCZT ceramics by traditional sintering method.
The phase structure of the samples was identified by XRD (Rigaku, Ultima III) with Cu-K α radiation, FTIR (Thermo, Nicolet-6700) with the KBr in the range from 400 to 4000 cm −1 and Raman spectra (Renishaw, INVIA), respectively. The microstructure was characterized by SEM (Quanta, FEG250) and TEM (JEOL, JEM-2100F). The piezoelectric and dielectric properties were measured by quasi-static d 33 meter (Institute of Acoustics, ZJ-3AN) and precision LCR meter (Agilent, E4980A). The ferroelectric property was obtained by a ferroelectric test system using a precision LC unit (Radiant, Premier II) at room temperature with a frequency of 10 Hz. Figure 1 shows the XRD patterns of BCZT powders synthesized at 180 °C for various reaction time of 15 min, 30 min, 45 min and 60 min. It can be seen that all the powders exhibit perovskite structure with no impurities, suggesting a solid solution formed by doping Ca and Zr into BaTiO 3 lattice 17 . The inset of Fig. 1 shows the FWHM and powder size of the BCZT powders as a function of reaction time, calculated from the XRD data. Clearly, a better crystallinity could be observed with increasing the reaction time, since the value of FWMH decreases gradually. Also, the powder size calculated by Scherrer formula is estimated to be 29 nm, 46 nm, 59 nm and 78 nm, respectively. Figure 2 shows the FTIR spectra of the BCZT powders synthesized at 180 °C for various reaction time of 15 min, 30 min, 45 min and 60 min. No obvious impurity peaks corresponding to CO 3 2− (around 690, 860 and 1750 cm −1 ) or X-O-C (X = Ti, Zr) groups (around 1120 cm −1 ) can be noted with the extending reaction time [18][19][20] . Meanwhile, for all the samples, the absorption peaks corresponding to O-H (around 1640 cm −1 and 3000-3600 cm −1 ) as well as -COOH (around 1420 cm −1 ) disappear nearly and the peaks corresponding to Z-O (Z = Ba, Ca, Ti, Zr) were quite significant 13 , implying the high purity and crystallinity of MSGH derived BCZT powders 12 .  Figure 4 shows the TEM images of BCZT powders obtained by MSGH at 180 °C for 60 min. It is clear from Fig. 4a that the boundaries of the particulates are clear and the powder size from TEM image is around 90 nm, which is basically consistent with SEM. Moreover, the powder size obtained here is slightly smaller compared with our previous work (around 105 nm by sol-gel method and 93 nm by sol-gel-hydrothermal method 15,21 ) and other literature (around 100 nm by hydrothermal method 22 ). Furthermore, from Fig. 4b, fringe features of crystal lattice with a fringe spacing of 0.287 nm corresponding to (110) planes can be observed clearly, proving the good crystallinity of the powders synthesized at 180 °C for 60 min 7 .

Results and discussion
Compared with our previous work about low temperature synthetizing BCZT powders by sol-gel-hydrothermal method, MSGH can shorten the reaction time greatly (about 11 h shorter) 15 , which can be attributed to the introduction of microwave in enhancing the reaction efficiency 16 . In addition, compared with other conventional methods, MSGH has a shorter reaction time than solid-state reaction (> 2 h), sol-gel method (> 2 h) and hydrothermal method (> 10 h) 2,7,8 , indicating the advantages of MSGH in rapid synthesis of powders.
BCZT powders synthesized at 180 °C for 60 min were further sintered at 1400 °C to prepare BCZT ceramics. Figure 5 shows the XRD patterns of BCZT ceramic. All the diffraction peaks show a typical perovskite structure with no impurities, suggesting the high purity of the BCZT ceramic 23,24 . Besides, the sharp diffraction peaks of the XRD pattern indicate great crystallinity of BCZT ceramic sintered at 1400 °C.
In order to confirm the phase structures of the obtained BCZT ceramics, Raman spectra are measured ranging from 125 cm −1 to 775 cm −1 , which is shown in Fig. 6. Modes at 149, 197, 292, 525, 730 cm −1 can be clearly observed respectively. Modes at about 149 and 197 cm −1 reflect the existence of the rhombohedral phase 25 , while modes at about 292, 525 and 730 cm −1 prove the existence of the tetragonal phase 26 , which agrees well with previous reports 27,28 . Thus, the result of Raman spectra indicates he coexistence of two phases in MPB structure of BCZT ceramics. Figure 7 shows the SEM image and EDS spectra of BCZT ceramics. The SEM image shows a homogenous distribution of grains with a dense microstructure and the grain size is measured to be 20-30 μm. The density of the obtained BCZT ceramics is 5.57 g/cm 3 , which is better than other related reports 29,30 . As shown in the EDS spectrums, all elements belonging to BCZT ceramics are uniformly distributed throughout the observed  www.nature.com/scientificreports/ area, without any significant element enrichment areas. Compared with solid-state reaction and sol-gel derived BCZT ceramics, MSGH derived BCZT ceramics have a lower sintering temperature (1400 °C) than those in the related reports (1450-1600 °C) 7,31-33 , which may be attributed to the high activity of BCZT powders prepared by MSGH 34,35 . Figure 8a shows the temperature vs. dielectric constant (ε r ) for BCZT ceramics measured at 1 kHz, 10 kHz, 100 kHz and 1000 kHz, respectively. The T C and ε m is measured to be 83.61 °C and 9579 respectively under 1 kHz frequency, and the ε m obtained here is slightly higher than that in other related reports 8,36 . However, an interesting phenomenon can be observed that T C measured in this work is lower than that in other literature 3,7 , which may be attributed to the larger grain size 37 . Duo to the ferroelectric transition of BCZT ceramics, two dielectric anomalies can be observed, which is related to the phase transition of rhombohedral-tetragonal and tetragonal-cubic 2 . Moreover, a relaxor ferroelectrics phenomenon of strong frequency dispersion and diffuse phase transition could be observed clearly, which is manifested as that not only ε r decreases but also T C moves to higher temperatures area with the increasing frequency 31 . Furthermore, the temperature and frequencydependence of dielectric loss (tanδ) of the BCZT ceramic is also shown in the Fig. 8a. The relatively low tanδ observed in this work may be ascribed to the less cavities in the dense BCZT ceramic and the lower electron diffusion in the grain boundaries 38,39 .
It is known that the dielectric constant of a normal ferroelectric above the Curie temperature follows the Curie-Weiss law which can be described by:  www.nature.com/scientificreports/   www.nature.com/scientificreports/ where T 0 is the Curie-Weiss temperature and C is the Curie-Weiss constant. Figure 8b shows the plots of temperature vs. dielectric constant (10 4 /ε r at 100 kHz) fitted to the Curie-Weiss law. For BCZT ceramics obtained through MSGH, T 0 = 83.45 °C and C = 1.33 × 10 5 °C were obtained. The C value obtained here is close to the existing literature on BaTiO 3 (approximately 10 5 °C), indicates a displacive type phase transition in BCZT ceramics 32 . Moreover, a deviation of dielectric constant from the Curie-Weiss law starting at T C can be seen. The parameter ΔT m which is often used to characterized the degree of the deviation from the Curie-Weiss law and a relaxor-like behavior can be defined as 32,40 : where T CW denotes the temperature at which the dielectric constant starts to deviate from the Curie-Weiss law and T m represents the temperature at which dielectric constant reaches its maximum. A narrower dielectric peak of BCZT ceramic which indicates a weaker diffuse phase transition behavior can be clearly observed as the value of ΔT m (29.54 °C) is lower than that in other literature 41 .
To further describe dielectric behavior of BCZT ceramic, a modified empirical expression proposed by Uchino and Nomura can be given as 42 : where C is the Curie-Weiss constant, and γ is a constant implying the degree of diffuse phase transition (1 < γ < 2). A normal Curie-Weiss ferroelectrics can be observed at γ = 1 and an ideal relaxor ferroelectric can be observed at γ = 2. The inset in Fig. 8b shows the plot of ln (T − T m ) vs. ln (1/ε r -1/ε m ). Depending on the slope of the fitting curve, the value of γ is fitted to be 1.524, reflecting a characteristic of relaxor ferroelectric to some degree.
The P-E hysteresis loops of the BCZT ceramics measured under different electric fields are shown in Fig. 9. The inset shows electric fields vs. remnant polarization (2P r ) and coercive field (2E c ). With the increasing electrical fields, 2P r as well as 2E c increases gradually and the hysteresis loops tend to be saturated. Well saturated loops with large 2P r (25.22 µC/cm 2 ) and moderate 2E c (7.52 kV/cm) were obtained under 30 kV/cm electric field.
Immersed in a silicone oil bath, the BCZT samples coated with silver electrodes were subjected to electrical poling under a certain poling condition (poling field = 35 kV/cm, poling temperature = 300 K, poling time = 30 min). After poling process, the piezoelectric coefficient (d 33 ) of the BCZT ceramics are measured, giving the value of 496 pC/N. The large piezoelectric property may be related to the MPB structure in BCZT ceramic and the release of the stress in the domains wall. This may promote the lateral movement of domain walls, even the reorientation and the growth of domains during poling process 43 .

Conclusion
In summary, Ba 0.85 Ca 0.15 Zr 0.1 Ti 0.9 O 3 (BCZT) powders were synthesized rapidly by a novel microwave-assisted sol-gel-hydrothermal method (MSGH). The reaction time was shortened to 60 min even at a lower synthesis temperature of 180 °C, as compared with the sol-gel process or sol-gel-hydrothermal methods. BCZT powders were well-crystallized and compositional uniform with fine grains (~ 85 nm). The BCZT ceramic derived from www.nature.com/scientificreports/ the MSGH-synthesized powders had a dense structure (density 5.57 g/cm 3 ) as well as excellent electrical properties (ε m = 9579, d 33 = 496 pC/N, 2P r = 25.22 µC/cm 2 , E c = 7.52 kV/cm), which was attributed to the high activity of the powders rapidly synthesized by MSGH.