Electrical properties of lead-free 0.98(Na0.5K0.5)NbO3-0.02Ba(Zr0.52Ti0.48)O3 piezoelectric ceramics by optimizing sintering temperature
© Lee et al; licensee Springer. 2012
Received: 9 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Lead-free 0.98(Na0.5K0.5)NbO3-0.02Ba(Zr0.52Ti0.48)O3 [0.98NKN-0.02BZT] ceramics were fabricated by the conventional mixed oxide method with sintering temperature at 1,080°C to 1,120°C. The results indicate that the sintering temperature obviously influences the structural and electrical properties of the sample. For the 0.98NKN-0.02BZT ceramics sintered at 1,080°C to 1,120°C, the bulk density increased with increasing sintering temperature and showed a maximum value at a sintering temperature of 1,090°C. The dielectric constant, piezoelectric constant [d33], electromechanical coupling coefficient [kp], and remnant polarization [Pr] increased with increasing sintering temperature, which might be related to the increase in the relative density. However, the samples would be deteriorated when they are sintered above the optimum temperature. High piezoelectric properties of d33 = 217 pC/N, kp = 41%, dielectric constant = 1,951, and ferroelectric properties of Pr = 10.3 μC/cm2 were obtained for the 0.98NKN-0.02BZT ceramics sintered at 1,090°C for 4 h.
KeywordsNKN-BZT lead-free sintering temperature piezoelectric properties
Lead-based perovskites have been extensively used in industries as sensors, actuators, and transducers due to their outstanding electrical properties. However, the PbO in these materials presents an environmental problem. The studies are now focused on discovering an alternative for lead-based materials. Potassium sodium niobate ((Na, K)NbO3) materials are thought to be one of the candidates as substitute systems [1–3]. When hot-pressed, (Na0.5K0.5)NbO3 [NKN] ceramics have been reported to possess high phase transition temperature [Tc] (approximately 420°C), high remnant polarization [Pr] (approximately 33 μC/cm2), large piezoelectric longitudinal response [d33] (approximately 160 pC/N), and high planar coupling coefficients [kp] (approximately 45%) [4–6]. However, conventionally sintered NKN ceramics show relatively lower electrical properties (d33 = 70 pC/N, kp = 25%) due to the difficulty of getting a high density of pure NKN ceramics . To compensate for these problems, NKN-based ceramics (e.g., solid solutions of NKN-LiNbO3, NKN-LiTaO3, NKN-LiSbO3, NKN-Li(Ta,Sb)O3, NKN-BaTiO3, NKN-SrTiO3, NKN-Ba(Zr,Ti)O3, and NKN-CaTiO3) have received significant attention largely for two reasons: (1) piezoelectric properties exist over an extensive range of temperature and (2) several possibilities for substitution and additions. Among them, Ba(Zr0.52Ti0.48)O3 [BZT] ceramics possess very strong piezoelectric properties (d33 is approximately 236 pC/N) and ferroelectric properties (Pr is approximately 13 to 18 μC/cm2). BZT has the advantage of exhibiting improved piezoelectric properties. However, it has a low Curie temperature (about 100°C), which limits its practical application as a piezoelectric material. In view of the high Curie temperature of NKN ceramics, the NKN-BZT binary system is of much value as a piezoelectric material [8–12]. In this paper, we have fabricated a 0.98(Na0.5K0.5)NbO3-0.02Ba(Zr0.52Ti0.48) [0.98NKN-0.02BZT] solid solution by a conventional ceramics technique, and the influence of sintering temperatures on the dielectric and piezoelectric properties of the 0.98NKN-0.02BZT ceramics was investigated.
The chemical molecular formula used in this experiment for the perovskite ceramics with (Na, K, Ba) complex A-sites and (Nb, Zr, Ti) complex B-sites is 0.98(Na0.5K0.5)NbO3-0.02Ba(Zr0.52Ti0.48) ceramics. For specimens prepared by the conventional mixed oxide method from Na2CO3, K2CO3, Nb2O5, BaCO3, ZrO2, and TiO2 as the staring materials, these powders were separately dried in an oven at 100°C for 4 h. They were ball-milled for 24 h using zirconia balls in alcohol. After drying at 110°C for 24 h, the powders were calcined at 850°C for 5 h. The calcined powders were pressed into disk samples of φ = 12 mm. The samples were sintered at 1,080°C to 1,120°C for 4 h. After the samples were polished up to 1.0-mm thick, Ag paste was screen-printed on the surfaces as electrodes and then fired at 400°C for 10 min. We used X-ray diffraction [XRD] and scanning electron microscopy [SEM] to analyze the crystalline and microstructures. The dielectric properties were measured using an LCR meter (PM6306, Fluke, Test Equipment Connection Corporation, Lake Mary, FL, USA). Hysteresis loops of the samples were measured by a Sawyer-Tower circuit. The samples were poled under a DC field of 4 kV/mm for 20 min. The d33 was measured by a d33 meter (DT-3300, Channel Products Inc., Chesterland, OH, USA). The kp was calculated by measuring the antiresonance and resonance frequencies. The relative density of the sintered samples was measured by the Archimedes method.
Results and discussion
where G is the average grain size at the time, n, the kinetic grain growth exponent, t, the sintering time, K0, a constant, Q, the apparent activation energy, R, the gas constant, and T, the absolute temperature. It can be inferred that increasing sintering temperature improves the grain growth. However, with an increasing sintering temperature above 1,090°C, the microstructure was inhomogeneous and the grain size becomes exceptionally huge. These can be the reason for the deterioration of the relative bulk density over 1,090°C as shown in the SEM images.
In conclusion, the lead-free 0.98NKN-0.02BZT ceramics with a perovskite structure have been sintered at various sintering temperatures. The effects of the sintering temperatures on the structural and electrical properties were investigated. Increasing sintering temperatures improve the grain growth, densification, and electrical properties in effect. However, with an increasing sintering temperature above 1,090°C, the structural and electrical properties have significantly deteriorated. The obtained d33 is 217 pC/N, which is the highest value in the 0.98NKN-0.02BZT system. The equivalent properties of Tc, kp, Pr, and dielectric constant values are 411°C, 0.41, 10.3 μC/cm2, and 1,951, respectively. Therefore, 0.98NKN-0.02BZT ceramics is a potential candidate for lead-free piezoelectric ceramics.
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