Hydrothermal Synthesis and Densification
Yu-Dong Hou,f Lei Hou, Ting-Ting Zhang, Man-Kang Zhu, Hao Wang, and Hui Yan Department of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, China
The sol-gel-hydrothermal processing of (Nao.8Ko.2)o.5Bio.sTi03 (NKBT) nanowires as well as their densification behavior were investigated. The morphology and structure analyses indicated that the sol-gel-hydrothermal route led to the formation of phase-pure perovskite NKBT nanowires with diameters of 5080 nm and lengths of 1.5-2 pm, and the processing temperature was as low as 160°C, but the conventional sol-gel route tended to lead to the formation of NKBT agglomerated porous structured nanopowders, and the processing temperature was higher than 650°C. It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowires at a low temperature. Owing to the better packing efficiency and therefore a good sinterability of the freestanding nanowhiskers, the pressed pellets made by NKBT nanowires showed >98% theoretical density at 1100°C for 2 h. The sol-gel-hydrothermal-derived ceramics have typical characteristics of relaxor ferroelectrics, and the piezoelectric properties were better than the ceramics prepared by the conventional sol-gel and solid-state reaction.
I. Introduction
At present, lead zirconate titanate (PZT)-based ceramics are the most widely used in electronic devices due to their high piezoelectric performance. However, the pollutant of toxic lead during the fabrication and waste of products cause a crucial environmental problem. Therefore, there is an increasing interest in developing lead-free piezoelectric ceramics to replace PZT-based ceramics and to minimize lead pollution. It is well known that covalency between unoccupied states of the Pb6d in the perovskite structure, and Op states favor ferroelectric ground states.1 Compared with Pb2+, Bi3+ ions were in an isoelectronic state and also showed a long pair effect, which encouraged studies of Na0.5Bio.5Ti03 (NBT) as an alternative to PZT ceramics.
Sodium bismuth titanate, NBT, which was found by Smolensk» et al.,2 is a kind of perovskite-type relaxor ferroelectric with a Curie temperature Tc = 320°C. At room temperature, it has a rhombohedral structure (a = 0.389 nm, a = 89.6°), and shows a relatively large remanent polarization (Pr = 38 |iC/cm2). However, it is difficult to pole NBT due to the high coercive field (Ec = 7.3 kV/mm), making it difficult to obtain the desirable piezoelectric properties. In addition, unlike PZT ceramics, NBT has no morphotropic phase boundary (MPB), which plays a very important role in PZT ceramics. The electromechanical properties show a maximum over a compositional range around the MPB of PZT, which can be attributed to an increase in
J. Nino—contributing editor
Manuscript No. 22459. Received November 8, 2006; approved January 29, 2007. This work was supported by the National Natural Science Foundation of China (Grant No. 60601020) and the Natural Science Foundation of Beijing (Grant No. 4072006). fAuthor to whom correspondence should be addressed, e-mail: ydhou@bjut.edu.cn
the number of possible spontaneous polarization directions for the compositions near the MPB due to the coexistence of rhombohedral and tetragonal phases. Therefore, NBT-based solid solutions that have an MPB structure and can be poled easily have been studied recently.3-5 Among the NBT-based binary lead-free piezoelectric systems, (Nao.sKoJo.sBio.sTiOj (NKBT) systems have shown good piezoelectric performance and have been the subject of the most extensive investigations because the composition was close to the MPB and the ceramics can be poled easily in a relatively low dc field (4-5 kV/mm). NKBT powders are traditionally prepared by a solid-state reaction.6-8 In case of solid-state reactions, the starting materials are oxides or carbonates of Bi, Na, K, and Ti. The particle size of these starting materials is in the micrometer or submicrometer range. A perovskite phase-forming temperature of 800°C and above is needed so that the components of the mixture have sufficient thermal energy to overcome the atomic/ionic diffusion barriers for the reaction. In most cases, the resulting powders are not nanocrystalline and undergo drastic agglomeration and yield an inhomogeneous particle size as a result of the high-temperature treatment; therefore, they are unsuitable for enhancing the dielectric and piezoelectric properties of ceramics for high-performance uses. In comparison with a conventional solid-state reaction, the sol-gel process has some advantages, including excellent compositional control, and homogeneity at the molecular level due to the mixing of liquid precursors. However, sol-gel was regarded as a solid rather than solution process because sol-gel-derived precipitates are amorphous in nature and calcinations in air are inevitable for the formation of the crystalline material. The sol-gel-hydrothermal processing represents an alternative to the calcinations for the crystallization of an objective compound under mild temperatures. As a novel method to prepare oxide powders, the sol-gel-hydrothermal technique has the double advantage of both sol-gel and hydrothermal syntheses and has become attractive in the last decade due to its high degree of crystallinity, well-controlled morphology, high purity, and narrow particle size distribution of the prepared powders.9'10 In our previous work, Ko.5Bio.5Ti03 (KBT) nanowires with good sinterability were successfully synthesized by the solgel-hydrothermal technique, and the dielectric properties of the derived KBT ceramics were superior to that prepared by all the other methods reported previously.11'12 Thus, the direct generation of NKBT nanowires with good sinterability at a low temperature is of considerable interest.
In the present work, the sol-gel-hydrothermal process was presented as a new route to produce NKBT nanowires at a temperature below 200°C, which is comparatively lower than that synthesized by the normal sol-gel route, which requires a temperature at least 650°C. The nanowires were characterized by X-ray diffraction (XRD), FT-IR, Raman, and transmission electron microscope (TEM) analysis. The densification behavior, the final microstructure of the sintered material, and the electric properties of the densified disks were investigated in detail. To the best of our knowledge, this is the first time that the synthesis of NKBT nanowires has been reported, and the electric properties of sol-gel-hydrothermal-derived ceramics were superior to that prepared by other methods.
II. Experimental Procedure
The raw materials used in the present work were analytical-grade bismuth nitrate pentahydrate (Bi(N03)2 • 5H20), sodium nitrate (NaN03), potassium nitrate (KN03), tetrabutyl titanate (Ti(OC4H9)4), acetic acid (CH3COOH), and ethanol (CH3CH2OH). Firstly, bismuth nitrate pentahydrate was dissolved in acetic acid; potassium nitrate and sodium nitrate were dissolved in C02-free distilled water, respectively. The mixture was then introduced into a prepared solution of a stoichiometric amount of tetrabutyl titanate in ethanol. After stirring vigorously for 2 h, a thin yellow homogeneous sol was formed. Then, the sol was heated at 80°C for 12 h to prepare a dry gel. The obtained gel precursor was added to a NaOH solution to form a suspension. The initial concentration of the NaOH solution was \()M. The as-prepared mixture was poured into a Teflon vessel, and then subjected to hydrothermal treatment at an appropriate temperature under auto-generated pressure for 48 h. After cooling, the product was filtered, washed with distilled water, and dried at ambient temperature. To compare the results with the conventional sol-gel process, a part of the gel was calcined at different temperatures from 300° to 800°C for 2 h in air. The crystal phase of the powders was determined using X-ray diffractometry (XRD; Model D8 Advance, Bruker AXS, Karlsruhe, Germany) in the 9-26 mode with graphite-monochromatized CuAjx radiation (X = 0.154178 nm). The powder morphology was observed on a transmission electron microscope (TEM; Model JEM-2000 F, JEOL, Tokyo, Japan) and scanning electron microscope (SEM; Model S-3500N, Hitachi, Tokyo, Japan). Fourier transform infrared absorption spectra of the powders were obtained using an FT-IR apparatus (Model NEXUS670, Nicolet, Madison, WI). Raman scattering spectra of powders were recorded at room temperature from a Raman spectrometer (Model T64000, Jobin-Yvon, Paris, France) under backscattering geometry. Excitation was taken as the 488 nm line of an Ar+ laser with a 50 mW output power.
For the sintering experiments, the obtained powders were pressed into pellets with a diameter of 12 mm under an isostatic pressure of 150 MPa. Conventional sintering was performed at 50°C temperature intervals between 1000° and 1200°C for 2 h in a sealed alumina crucible. The bulk densities of the sintered pellets were measured by the Archimedes method. The microstructure of the sintered pellets was observed using SEM; (Model S-3500N, Hitachi, Tokyo, Japan) on the fracture side and free-top surfaces of the pellets. When the fracture surface did not reveal clear grain boundaries, the polished and thermally etched surface was observed. To measure the electrical properties, silver paste was coated on both sides of the sintered pellets and fired at 560°C for 30 min to form electrodes. The dielectric property and its dependence on temperature were measured using a precision LCR meter (Agilent 4284A, Agilent Technologies Inc., Palo Alto, CA) with an automated temperature controller. Before testing the piezoelectric properties, the specimens were poled in a silicone oil bath at 120°C by applying a dc field of 5 kV/mm for 30 min and aged for 24 h. The piezoelectric coefficient (i/33) was measured using a quasi-static piezoelectric d3} meter (Model ZJ-3D, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China). The electromechanical coupling factor (kp) and the mechanical quality factor (gm) were estimated by the resonance and anti-resonance technique using an impedance analyzer (Agilent 4294A, Agilent Technologies Inc.).
III. Results and Discussion
Figure 1 shows the XRD patterns of powders hydrothermally treated at different temperatures. As can be seen from the XRD pattern for the sample synthesized at 100°C, peaks corresponding to the perovskite phase had begun to appear, but the peaks were ill defined, which was indicative of the low crystallinity of this phase. Well-crystallized phases of NKBT were obtained for the samples hydrothermally treated at 160°C and above. The inset in Fig. 1 shows the fine scanning XRD patterns in 20 = 37°
- Fig. 1. X-ray diffraction (XRD) patterns of powders hydrothermally treated at different temperatures: ■, perovskite. Inset: Fine scanning XRD patterns in 29 = 37°—48° for Nao sBi0 sTiC>3 (NBT), (Na„.8Ko.2)o.5Bio.5Ti03 NKBT, and K(15Bi,15TiO, (KBT) powders hydrothermally treated at 160°C.
to 48° for NBT, NKBT, and KBT powders hydrothermally treated at 160°C. It can clearly be seen that (111) and (200) peak positions of NKBT shift to high degrees compared with that of pure KBT, which can be attributed to the larger radius of K+ (1.33 A) than that of Na+ (1.02 A) in the A site of the AB03 structure.5 To compare the results with the conventional sol-gel process, a part of the gel was calcined at different temperatures from 300° to 800°C for 2 h in air, and the XRD results are shown in Fig. 2. As can be seen in the figure, an amorphous phase was formed at a temperature below 400°C. When the temperature was increased to 450°C, some diffraction peaks corresponding to the Bi2Ti207 pyrochlore phase appeared. However, for the powders heated at 550°C, an obvious change was observed in the XRD patterns. The characteristic peaks of the NKBT perovskite phase appeared. After further increasing the temperature to 650°C and above, only a pure NKBT phase could be observed and there was no evidence of a second phase. The above results revealed that a temperature as high as 650°C is needed for the transformation from the pyrochlore to perovskite phase in a conventional sol-gel process, which is about 500°C higher than that required for the sol gel -hydrothermal technique.
Figures 3(a) and (b) show the images of NKBT samples synthesized by the sol gel hydrothermal and conventional sol-gel
Fig. 2. X-ray diffraction patterns of the dried gel heated at different temperatures: ■ , perovskite; •, pyrochlore.
2 Theta (Degree)
Fig. 2. X-ray diffraction patterns of the dried gel heated at different temperatures: ■ , perovskite; •, pyrochlore.
Fig. 3. (a) Transmission electron spectroscopy (TEM) photograph of (Nao.slCo.ito.sBio.sTiOj (NKBT) powders synthesized by the sol-gel-hydrothermal method at 160°C. Inset: A typical NKBT nanowire and its SAED pattern, (b) Scanning electron microscope photograph of NKBT powders synthesized by the conventional sol-gel method at 650°C. Inset: High-magnification TEM image of powders.
method, respectively. From Fig. 3(a), it can clearly be observed that the sample synthesized at 160°C by the sol-gel-hydrother-mal process demonstrates the morphology of nano-sized wirelike fibers, which are monodispersed and are not fused to one another. Furthermore, each nanowire is uniform in width along its entire length, with diameters of 50-80 nm and lengths of 1.5-2 nm. The inset in Fig. 3(a) shows the SAED pattern recorded from an isolated nanowire. The diffraction spots have been well indexed to the planes of NKBT, confirming the formation of a
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