Synthesis of high density sub-10 µm (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3-xCeO2 lead-free ceramics using a two-step sintering technique

ABSTRACT (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3-xCeO2, (BCZTCe) lead-free piezoelectric ceramics were processed conventionally using a two-step sintering technique. The results suggest that two-step sintering is an effective technique to acquire a high density (99%) homogeneous microstructure with sub-10 μm grain size. The low CeO2 content (0.07 wt.%) facilitates good functional properties at low sintering conditions of T1 = 1400°C/30 min & T2 = 1275°C/4 h, in which d33 = 353 ± 7 pC/N, kp= 40%, εr= 3393 ± 100, tan δ =0.039, TC = 96.5ºC, Pr = 11.45 μC/cm2, EC = 2.32 kV/cm and a large strain of 0.18%. These results indicate that BCZTCe ceramics are a promising lead-free piezoelectric substitute for room temperature device applications.


Introduction
Piezoelectric materials are smart functional materials that can convert energy (mechanical to electrical or vice versa) and are used in a variety of electronic devices [1][2][3][4]. Among these lead zirconate titanate (PZT) and related materials are dominant in terms of use of their piezoelectric properties in sensors, actuators, fuel injectors, transducers, etc [1,3]. Due to the lead (Pb) toxicity and its non-environmentally friendly nature, a search has begun to replace PZT and related systems [3,4]. Lead-derived materials exhibit a morphotropic phase boundary (MPB), which generates enhanced ferroelectric and piezoelectric properties. Hence, work on lead-free systems has also concentrated on developing systems with an MPB.
(Ba 0.85 Ca 0.15 ) (Zr 0.1 T i0. 9 ) O 3 (BCZT) lead-free material system developed by Liu and Ren in 2009 showed excellent piezoelectric properties: d 33~6 20 (pC/N [5]. Several groups have tried to reproduce these results since then but could not achieve the same results with pure BCZT and other solid solutions (d 33 < 470 pC/N) [6][7][8][9][10][11]. A possible reason behind their inferior results may be the stoichiometric ratio, method of synthesis and sintering techniques used for the preparation of raw powders and the final sintered body. The method used in previous publications does not guarantee stoichiometric homogeneity during processing, which can cause fluctuation of the piezoelectric properties. These techniques include the solid-state reaction method [12], hot pressing method [13], modified pechini method [14] and the application of various processing parameters such as calcination temperatures [15].
Piezoelectric constants fluctuate as 500-650 (pC/N) from these methods which is mainly due to fluctuations in the initial particle size, density and pore size distribution.
There are always experimental complications in synthesizing a series of dense BaTiO 3 (BT) based leadfree ceramics with different desired values such as an average grain size using a sole source of BT powder as well as a single sintering technique [26]. Accordingly, different kinds of BT powder and sintering techniques have usually been combined, such as those described by Arlt et al. [27]. This may distress the consistency of the experimental results owing to the different characters of the obtained BT ceramics. To this approach, it is highly desirable in studying grain-size effects (sub-10 µm) to use a group of BT ceramics (Such as BCZT) that offers a very high final ceramic density and a uniform grain-size distribution in their microstructures in order to obtain good functional properties, in particular.
The present study is a follow-up to our previous study in which we prepared (Ba 0.85 Ca 0.15 ) (Zr 0.1 T i0.9 ) O 3 -xCeO 2 ceramics by the conventional method and conventional single-step sintering [28]. In that study, we found that a grain size of~13 µm and density of~95% were suitable for obtaining high piezoelectric properties (d 33 > 500 pC/N) at a lower sintering temperature of 1350°C/4 h while doping with CeO 2 [26]. Nonetheless, reports showing the effects of sub-10 µm grain sizes on the functional properties of (Ba 0.85 Ca 0.15 ) (Zr 0.1 T i0.9 ) O 3 ceramics are rare. However, reducing the grain size (<10 µm) while maintaining fairly good properties for BCZT ceramics is a challenge [29,30]. Thus, a two-step sintering technique was introduced in order to further reduce the grain size of these ceramics and study the effects on their functional properties. The main aim of the current study is to maintain grain size (<10 μm) with a homogeneous microstructure and obtain very high-density ceramics together with good piezoelectric, dielectric and ferroelectric properties. (Ba 0.85 Ca 0.15 ) (Zr 0.1 T i0.9 ) O 3 -xCeO 2 (x = 0, 0.02, 0.04, 0.07 wt.%) were prepared using a two-step sintering (TSS) technique. The microstructural, piezoelectric, dielectric and ferroelectric properties were systematically studied. , UK) were added during the last hour of milling. The particle sizes were measured using a particle size analyzer (Gracell, Sympa Tec, Germany) and the average particle sizes of all batches were~2 µm. Dried powder was then sieved (300 mesh, VWR, England) and uniaxially pressed at 155 MPa (Instron, 5507, England) into the green bodies using a 13 mm diameter cylindrical steel die (P.T. No. 3000, Specac, UK). The green bodies were finally sintered at different temperatures using a two-step sintering technique (TSS). In the first step the furnace is programmed for a rapid rampup rate 10°C/min to a set temperature T 1 (1350, 1400, 1450°C) with 30 min dwell in order to ensure the uniformity of the heat atmosphere inside the furnace chamber. The furnace was then cooled rapidly at 30°C/min to a lower temperature T 2 . This temperature was set at 1275°C to prevent further grain growth and maintained for 4 h. Finally, the temperature was ramped down to room temperature at a rate of 10°C/min. The relative densities were measured by the Archimedes method using the theoretical density (5.80 g/cm 3 ) from XRD results. The phase structure of the sintered discs was examined by X-ray diffraction (Equinox 3000, INEL, France) with Cu-Kα radiation (λ = 1.54178 Å). A chromium (Cr)gold (Au) coating was sputtered as electrodes using a sputter coater (K575X, Emitech, UK), and the samples were poled in silicon oil at 3 kV/mm for 10 min. The dielectric properties were measured with an impedance analyzer (4294, Agilent, USA) at 1 kHz. The piezoelectric constant was measured with a Berlincourt d 33 meter (YE2730A, Sinocera, China). The surface morphologies were observed by scanning electron microscopy (JEOL 6060LV, England). Ferroelectric P-E and S-E hysteresis were measured with an aixACCT Systems (GmbH, Germany) at 1 Hz.

Microstructures, density and grain size of the samples
The microstructures of all samples of sintered BCZT-xCeO 2 ceramics were observed by SEM under sintering conditions of T 1 -1400°C/30 min and T 2 -1275°C/4 h as shown in Figure 1. All the CeO 2 concentrations were added in wt.% to the BCZT ceramics. It is evident that CeO 2 incorporation up to x = 0.07% causes high densification and uniform microstructures. As the sintering temperature increased further (T 1 = 1450°C/30 min), the samples with x = 0-0.04% became somewhat denser but sample x = 0.07% started to become porous, as seen in Figure 2. From that point on, we designated the optimum sintering conditions, i.e. T 1 -1400°C/30 min and T 2 -1275°C/4 h, as the temperatures at which the maximum relative density with a uniform microstructure were obtained.
The relative densities of BCZT-xCeO 2 ceramics treated under different sintering conditions are shown in Figure 3(a), which is in agreement with the microstructural analysis. As the Ce concentration increases, the relative density increases, and is achieved maximum (99%) of theoretical density (t.d. = 5.80 g/cm 3 calculated from cell parameters), for sample x = 0.07 wt.%, under optimum sintering conditions, T 1 -1400°C/30 min and T 2 -1275°C/4 h. For pure BCZT (x = 0) however, a low relative density was observed (~94% of t.d.), even under higher sintering conditions. i.e. T 1 -1450°C/ 30 min and T 2 -1275°C/4 h. This clearly suggests that CeO 2 is an effective dopant for achieving high densification of BCZT ceramics at low temperature. The grain size calculations were conducted by the linear intercept method, as shown in Figure 3(b). The average grain size was observed to be~8 µm for all the samples under different sintering conditions, indicating that the TSS technique could be useful in grain-size reduction and maintenance of a uniform microstructure. The average grain size for x = 0.07% under sintering conditions of T 1 -1400°C & T 2 -1275°C was 7.96 μm, which is slightly higher than that of such other lead-free piezoelectric materials as KNN and BNT (<3 μm) [4,31,32] but relatively lower than that of pure BCZT (grain size 8 μm) with almost 100% densification [33,34].
3.2. X-ray diffraction analysis and temperature-dependent relative permittivity     provided by Rietveld analysis. All the reflections show an additional peak on the right side of the peaks corresponding to the Kα2 reflection. It was therefore concluded that the low CeO 2 content (0-0.07%) did not affect the crystal structure of the BCZT ceramics. It is clear however that the tetragonality of the sintered samples increased with increase in the concentration of CeO 2 . The cell parameters established by Rietveld refinement are listed in Table 1. We do not have the direct evidence of the orthorhombic phase in the XRD analysis as we obtained evidence from a temperaturedependent relative permittivity plot as discussed below. The reason is possibly the temperature at which the XRD measurement was conducted. We measured the XRD patterns at room temperature (21ºC) while the orthorhombic-tetragonal (O-T) phase transition was found at around 32ºC by the temperature dependent relative permittivity plot. Temperature dependent XRD measurement may therefore be conducted to analyze more precise phase identification which in future studies. Figure 4(d) depicts the temperature dependent relative permittivity of BCZT-xCeO 2 ceramics measured at 1 kHz. All specimens undergo two-phase transitions, i.e. the orthorhombic-tetragonal phase transition (T O-T ) near 32ºC and the tetragonalcubic phase transition (T C ) at around 100ºC. Similar results were obtained in our previous study [30]. A small amount of CeO 2 doping (x = 0.02-0.07%) did not change the crystal structure but significantly increased the relative permittivity (ε r ). The highest value for ε r was observed as 15,067 for x = 0.07%, compared to 6659 found for x = 0%. The enhancement of relative permittivity may be attributed to the uniform grain sizes and highly dense microstructure caused by a small addition of CeO 2 . T C decreased slightly from 103ºC to nearly 96ºC due to the compositional adjustment of Ce ions in the BCZT lattice. Moreover, the addition of CeO 2 led to a larger c/a ratio, as shown in Figure 5, which demonstrates that the tetragonality of these ceramics rises and phase boundary moves.  Figure 6(a,b), that the planar coupling coefficient (k p ) and piezoelectric constant (d 33 ) increased with increase in the Ce concentration and achieved the maximum for the sample x = 0.07%, while the dissipation factor (tan δ) was reduced. The d 33 was altered from 295 ± 15 pC/N of x = 0% to 353 ± 7 pC/N of x = 0.07% and the dissipation factor was reduced from 0.088 to 0.039 units. In addition, k p increased from 23.2 ± 2% to 40 ± 1%, while relative permittivity increased from 1906 ± 80 to 3393 ± 100 at room temperature. This improvement in d 33 , k p , and ε r   and the reduction in tan δ were ascribed to the final ceramic density and uniform grain sizes. The piezoelectric and dielectric properties are summarized in Table 2.

Piezoelectric and dielectric properties at room temperature
Previously, CeO 2 was used as an additive to PZT and BNBT materials [35,36], and it generally showed some effects on enhancing d 33 and reducing tan δ at the same time. CeO 2 showed the same double effect here. The mechanism of CeO 2 on BCZT ceramics is more complex and is described in our previous study in detail [30]. Moreover, Ce ions possibly exist in two valance states, i.e. Ce 3+ of the ionic radius 0.13 nm and Ce 4+ of the ionic radius 0.087 nm.  [37]. They presented the basis for showing the possibility that one of the few cases that the Ce 3+ donor charge could be compensated by a reduction of Ti 4+ to Ti 3+ (electronic compensation) by creation of ionizing vacancies at the Ti sites or by a combination of both mechanisms [38,39]. This results in suppressing the movement of domains that cause a reduction in the dissipation factor (tan δ) [40].

Ferroelectric properties at room temperature
All ceramics show a polarization-electric (P-E) field loop as well as a current-electric field loop at sintering conditions of T 1 -1400°C & T 2 -1275°C, as depicted in Figure 7(a-d).
For pure BCZT (x = 0%) a small remnant polarization (P r = 4.22 μC/cm 2 ) is observed with a relatively large coercive field (E C ) of 3.17 kV/cm. As the Ce content increased up to x = 0.07%, a large remnant polarization (P r = 11.61 μC/ cm 2 ) with a low E C = 2.32 kV/cm was observed. A low E C suggest a soft nature of these ceramics, which indicates that Ce 3+ acts as a donor dopant. The high P r , may be attributable to large internal polarizability and large electromechanical coupling [41]. In addition, a sharp loop squareness generally indicates better homogeneity of grain sizes [41] as can be seen in Figure 7(d). Furthermore, I-E plot depicted in Figure 7(a-d), shows the switching process of these ceramics. For pure BCZT (x = 0%), the current peak intensity is low and broad, which indicates that the domain switching was not easy. A further increase in the Ce concentration (x = 0.02, 0.04%) provides some improvement in the current peak behavior and a sharp current peak was observed for the sample x = 0.07%. The width of the current peak gives the value of E C at which remnant polarization could be obtained for ceramics [42,43]. Figure 8 represents the electric field induced strain loops for these ceramics. For sample x = 0.07%, a large strain (~0.18%) was observed under a 4 kV/mm positive electric field as compared to that of pure BCZT (x = 0%) as 0.11%. Such high bipolar strain values in a soft piezoelectric material may be attributable to easy domain motion. The ferroelectric properties are summarized in Table 3.

Conclusions
In this study, A sub-10 µm grain size and high-density (99%) uniform microstructure were obtained while maintaining good ferroelectric, dielectric and piezoelectric properties for BCZTCe ceramics via a two-step sintering technique. It has been reported that with TSS, grain size can be controlled (~8 µm) with a uniform microstructure. Addition of a small amount of CeO 2  significantly reduced the sintering temperature as well as helping densification (relative density~99%) which led to maintenance of good properties potentially suited to the fabrication of technological devices. The optimal properties were obtained for x = 0.07 wt.% at sintering conditions of T 1 = 1400ºC & T 2 = 1275ºC/4 h, in which d 33 = 353 ± 7 pC/N, k p = 40%, ε r = 3393 ± 100, tan δ =0.039, T C = 96.5ºC, P r = 11.45 μC/cm 2 and E C = 2.32 kV/cm with an average grain size~7-8 μm. These results strongly suggest that TSS is an effective sintering method for grain-size reduction with a uniform microstructure and that CeO 2 is a suitable dopant for reducing the sintering temperature as well as achieving enhanced density of BCZT ceramics.