Abstract
The powder synthesis of barium zirconate titanate (BZT) (BaZrTiO3) from the mechanochemical activation of BaCO3, ZrO2, and TiO2 was studied. The grinding effect, by using a planetary ball milling, on the crystallization temperature of BZT powders was analyzed. X-ray diffractometry, differential thermal analysis, thermogravimetric analysis, and scanning electronic microscopy (SEM) were used as characterization methods. The crystallization behavior of powders activated by high-energy grinding and the effect of grinding time on the BZT crystallization were analyzed. After grinding by 4 h, the BaZr(0.05)Ti(0.95)O3 sample was almost fully crystallized at . The results of dielectric and ferroelectric properties show that high-energy ball milling is a practical and promising way to prepare BZT ceramics.
1. Introduction
Ferroelectric materials based in Pb have been studied for many years for its important applications in piezoelectric, pyroelectric, and optical devices [1–5]. Lead-free compositions can be of great interest for environmentally friendly applications such as actuators and dielectrics for capacitors. BaTiO3 is the most widely used ferroelectric material in ceramic capacitors and is known for its large electrochemical coupling factor [2, 3]. Within BaTiO3 family, barium zirconate titanate (Ba(ZrTi)O3, BZT) has become one of the most attractive materials because it has been reported that the zirconium substitutions into the titanium lattices enhance the dielectric and piezoelectric properties [6, 7]. Furthermore, Curie temperature can be shifted below room temperature by doping with Zr [8, 9]. However, under common preparation conditions, BZT ceramics have very high sintering temperature that does not fit the industrial requirements. In order to decrease the sintering temperature, it is necessary to produce powders with fine particle sizes and homogeneous distribution.
Mechanochemical activation by mixing of high energy (high-energy ball milling) has become important to produce solid-state reactions at lower temperatures than those used in conventional methods of powder preparation [10–14]. This mechanical technique is unique because of its advantages: (i) low cost of starting materials, (ii) low operating temperature, and (iii) higher sinterability of milled powders.
The purpose of this work is to reach low energy cost in the synthesis of BZT ceramics without expense of their electrical properties. The effects of increasing the milling time of the powders and, on the other hand, of lowering the powder calcination temperature and/or sintering temperature of the pellets are analyzed.
2. Experimental
BZT ceramics were prepared via conventionally mixed-oxide method by high-energy ball milling. Commercial powders of barium carbonate (BaCO3) (99.98% Sigma-Aldrich), titanium dioxide (TiO2) (99.98% Sigma-Aldrich), and zirconium oxide (ZrO2) (99.9% Sigma-Aldrich), were used as starting materials with the nominal composition of BaZr0.05Ti0.95O3. The precursors were ground in a planetary ball milling (Torrey Hills Technologies ND 0.4L) during 1, 2, and 4 h, into a zirconia jars. Simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) measurements were carried out on the as-obtained powder from 20°C to 1200°C at 10°C/min heating rate, with 50 mL/min of O2 flux into alumina crucibles, using a Shimadzu DTG-60 analyzer. The mixed powders were calcined between 600°C and 1200°C for 2 h in the air. Afterwards, powders were analyzed by X-ray diffraction (XRD-Philips PW1700 diffractometer), scanning electron microscopy (SEM Philips 505), and high-resolution SEM (FEI QUANTA 200).
The powders with two weight percent PVB binder were uniaxially pressed (200 MPa) into green pellets with 10 mm of diameter and 1 mm of thickness. The sintering behavior of these pellets was monitored at constant heating rate (5°C/min) in a Theta Dilatronic dilatometer. Structural analyses of sintered samples at 1350°C and 1400°C were performed by XRD and SEM.
To measure the electric properties, the surfaces were polished, and silver electrodes were painted on both sides of the pellets. The capacitance and dielectric loss of the samples were measured with a HP 4192A impedance analyzer, and the hysteresis loops were examined by using a conventional Sawyer-Tower tester at 50 Hz.
3. Results and Discussion
Figures 1 and 2 show the DTA-TGA curves of the oxides mixture milled for 1 and 4 h, respectively, when they were heated from room temperature to 1200°C. Three endothermic peaks (labeled as (1), (2), and (3)) are observed in both DTA curves. The first two peaks match with weight losses, whereas no weight loss is seen for the last one. It is well known [15, 16] that the first two peaks correspond to reactions of BaCO3 decomposition and synthesis of BaTiO3, including the formation of a transient Ba2TiO4 phase. These reactions are as follows: where the CO2 emission causes the first weight loss realized in the TGA graph (around 600°C), where again there is weight loss by the CO2 emission, and which has no weight loss associated.
By comparing both figures, it is noted that the peak (1) is broader and deeper in Figure 2 which means further development of reaction (1) in the sample milled by 4 h and matches with the greater mass loss (about double) observed in the TGA curve. This behavior also agrees with the smaller depth of peak (2) in Figure 2 since more BaCO3 has decomposed and also with the 800°C XRD patterns in Figures 3(a) and 3(b), where it is quite clear that there is fewer remaining BaCO3 in the sample milled for 4 h.
(a)
(b)
It is noteworthy that the formation of Ba2TiO4 phase advances at the temperature range between 600°C and 900°C due to the diffusive nature of this reaction [15]. The peak (2) in Figure 1 is quite sharp since superposes the allotropic transition of BaCO3 (at 827°C) [17] with reaction (2). As it is observed, this peak in Figure 2 is not so deep because that transition is hindered by the lattice deformation resulting of the greater milling time.
On the other hand, Figure 2 shows an increase in the mass loss rate (see the derivative of the TGA curve from the inset) because of the higher reactivity of the powders and the faster kinetic of the reaction. Furthermore, the mass loss finishes at 885°C in the BZT-4 h sample and at 920°C for the BZT-1 h one. By revising DTA graphs, only reaction (3) could be present from these temperatures until the maximum of hump (4). This temperature defines the synthesis of BaTiO3 after accomplishment of all the reactions above mentioned.
Berbenni et al. [18] have obtained a final plateau in its TG curve at 730°C by applying high-energy milling for 159 h on BZT powders. By analyzing the temperatures where the mass loss is over, Berbenni’s results and ours indicate that crystallization starts at lower temperatures if more energy from milling is delivered. That is to say, by increasing the milling time the powders will have more reactivity because of their smaller particle size and hence higher surface area. By comparing the end of hump (4) in both DTA curves, it is clear that the sample ground for 4 h has completely crystallized at 1040°C. The sample milled for 1 h seems to be completely crystallized at 1200°C. Finally, from Figures 1 and 2, it can be expected that Ba2TiO4 and BZT coexist only in samples calcined in temperatures ranging between 700°C and 900°C.
XRD patterns of both samples (Figures 3(a) and 3(b)) are on good agreement with the DT and TG analysis. The diffractogram of 600°C for the 4 h milled sample shows the main peak of BZT sharper and higher than the one corresponding to the 1 h milled sample. The 800°C pattern for the 1 h ground sample (Figure 3(a)) shows clear presence of BaCO3 peaks yet (23.89°, 27.71°, 34.08°, 34.58°, 41.98°, 42.95°, and 46.77°) while they have disappeared in the 4 h ground sample (Figure 3(b)).
Furthermore, in the 800°C-4 h ground XRD pattern (Figure 3(b)), the BZT seems to be practically crystallized; just some peaks of BaCO3 and the strongest of cubic BaZrO3 are weakly seen. At 1000°C, there only remains the main peak of BaZrO3, whereas, at 1200°C, the sample is fully crystallized. Even though the DTA and TGA graphics have suggested possible presence of Ba2TiO4 phase between 800°C and 900°C, this phase has not been detected in the XRD patterns. It is worth noting that XRD measurements were carried out on samples previously calcined for 2 h, while thermal measurements have a quite different thermal evolution (10°C/min). The calcination for 2 h could have increased the Ba2TiO4 diffusion, and there it will be transformed completely into BaTiO3 [15].
Figure 4 shows the milling time effect on powders calcined at 1200°C for 2 h. The peaks are broad, showing poorly crystallized phases with amorphous phases for unmilled powders. They become stronger and sharper when the samples are milled. It is observed that the unground sample presents peaks corresponding to BaCO3 and BaZrO3, and 62.67°). After grinding by 1 hour, the samples did not exhibit BaCO3, but, however, the BaZrO3 content decreased gradually as the milling time increased. Finally, a BZT structure free of spurious phases was obtained after 4 h of grinding and 2 h of calcination at 1200°C. The crystallite sizes of powders calcined at different times were calculated from the X-ray broadening of (110) peaks, using the Scherrer equation [19]. Crystallite sizes for powders treated at 1 h, 2 h, and 4 h are 32.9, 33.8, and 35.7 nm, respectively. This indicates that BZT crystallizes and grows as the milling time increases.
The curves of linear shrinkage and its derivative with respect to temperature , both as functions of temperature, are shown in Figure 5. The first contraction between 750°C and 900°C indicates powder agglomeration during the sintering due to processes inside the agglomerates and between them [20]. Densification then increases sharply (~20%) in the range from 1250°C up to 1400°C. From these data, the 1350°C and 1400°C values were set as suitable temperatures for the sintering of ceramic pellets.
The SEM from Figure 6(a) indicates that the average grain size of ceramic is 250 nm, which means that grains have not grown in the sintering process with respect to the initial size of the crystallites in the powders (Figure 6(b)). However, certain porosity can be seen in the micrograph presumably induced by a binder evaporation and/or by a residual inhomogeneity in the shape of the calcined crystallites. Indeed, if the calcined crystallites are irregularly shaped prior to pressing, greater porosity will result in the sintered ceramic.
(a)
(b)
Figure 7 shows the XRD patterns of BZT pellets sintered at 1350°C and 1400°C. BZT pellets sintered at 1400°C display full crystallization in a perovskite phase, while small peaks of secondary phase are noted in the sample sintered at 1350°C. Moreover, larger grain growth is detected in the 1400°C sample. Precisely for both ceramics, crystallite sizes of 56.3 nm and 58.9 nm were calculated from half-width of (011) peaks.
The sintering temperature has significant influence on the electrical properties as displayed in Figure 8. The behavior of the dielectric constant with frequency [1 kHz–1 MHz] is similar in both samples (1350°C and 1400°C), but their values are higher, in all this range, for the sample sintered at 1400°C. The comparison between both samples in the range 1 kHz–10 kHz is reported at the inset. Furthermore, the dielectric loss in both samples decreases with the increasing frequency in this range, this value being lower for the sample sintered at 1400°C. These two behaviors come from a further densification at higher temperature.
Figure 9(a) shows the dielectric constant evolution of the BZT sample (sintered at 1400°C) with the temperature, where the three curves correspond to measurements at 10, 50, and 100 kHz. The ceramic presents two abrupt changes in dielectric constant corresponding to the orthorhombic-tetragonal (~60°C) and tetragonal-cubic (~125°C) phase transitions, displaying a typical ferroelectric behavior [21–23]. Besides, it is not observed a shift (Curie temperature) with frequency, according to a nonrelaxor ferroelectric.
(a)
(b)
The Curie temperature increases slightly as does the sintering temperature; it can be seen in Figure 9(b). Moreover, the orthorhombic to tetragonal transition phase around 60°C is more evident in the sample sintered at 1400°C.
The hysteresis cycle measured at 50 Hz and room temperature, with the sample inside of silicon oil, is shown in Figure 10. Typical performances of normal ferroelectrics, according to the designed ceramic composition, were observed. The sample sintered at 1400°C shows a remnant polarization of 35 C/cm2 and a coercive electric field of about 2.5 kV/cm, while the 1350°C sintered ceramic shows lower values in its ferroelectric properties.
These results can be compared to the dielectric constant and the ferroelectric P-E hysteresis loops previously reported. Nanakorn et al. [23] have obtained 25 C/cm2 for remnant polarization and about 3000 as dielectric constant in ceramics of BZT . Their procedure involved ball milling in acetone during 24 h, calcination at 1200°C for 2 h, milling for 24 h in ethanol, and sintering at 1300°C–1450°C for 2 h. Similar method has been used by Yu et al. [24] (except for sintering at 1500°C–1560°C from 2 to 15 h) and they have found values of 13.3 C/cm2 and for BaZr(0.05)Ti(0.95)O3 ceramics.
4. Conclusions
The purpose of reaching low energy cost in the synthesis of BZT ceramics was achieved. By high-energy milling, suitable crystallization of BaZr0.05Ti0.95O3 powders was achieved at 800°C due to the increasing in their reactivity. From these powders, BZT ceramics were sintered for 2 h at 1350°C and 1400°C. BZT ceramics with high values of remnant polarization (35 C/cm2) and good values of dielectric constant (1700 at room temperature) were obtained. These results show an efficient way in the synthesis of BZT ceramics as compared to other proceedings reported in the literature.
Moreover, several common features of these ceramics were found in our samples, summarized as follows. The dielectric constant drops when the frequency rises at the range from 1 kHz to 1 MHz. The Curie temperature does not change with frequency but grows slightly with sintering temperature increasing. Furthermore, sintering at 1400°C improves the dielectric properties (rise of and decreasing of D) with regard to sintering at 1350°C.