Journal
J. Am. Ceram. Soc., 94 [3] 776–781 (2011) DOI: 10.1111/j.1551-2916.2010.04138.x r 2010 The American Ceramic Society
Effects of Ultrasonic Irradiation on the Structural and Electrical Properties of Lead-Free 0.94(K0.5Na0.5)NbO3–0.06LiNbO3 Ceramics Chee Kiang Ivan Tan, and Kui Yaow Institute of Materials Research and Engineering, ASTAR (Agency for Science, Technology and Research), Singapore 117602, Singapore
Jan Ma School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore Temasek Laboratories@NTU, Research TechnoPlaza, Singapore 637553, Singapore
sensitivity of the raw powders (typically carbonates) to moisture is very high, which leads to the presence of hygroscopic secondary phases. There are also problems that arise during heat treatment when the alkali oxides evaporate, which lead to stoichiometric deviations, and poor density and properties of the ceramics.7 Recently, Guo et al.8 showed that the sintering behavior of KNN can be improved by introducing LiNbO3 (LN) as an additive. They investigated the alkali niobate-based solid solution of (1x)(K0.5Na0.5)NbO3–x(LiNbO3) (KNN–LN) and reported that an MPB existed at about 5–7 mol% LiNbO3 and reported excellent piezoelectric properties and high Curie temperature. Subsequently, there have been quite a number of investigations on this promising system due to the improved piezoelectric properties and also high Curie temperature.9–18 However, at least two disadvantages are still restricting its practical applications: first, the alkali elements’ volatility during calcination and sintering process remain an issue; Second, most of the solid-state ceramic processing involves long hours of ball milling (typically longer than 24 h), which is time consuming, not cost effective, and potentially introduces uncontrolled contaminations. Therefore, special attention should be paid on controlling the alkali volatility during heat treatment and eliminating long hours of mechanical milling for processing KNN–LN ceramics. It should be noted that using high-intensity ultrasound can affect the surface morphology of solid-state reaction products considerably. The reaction rates are reasonably fast and the reaction temperatures are also reduced as reported for some materials.19 However, research on the ultrasonic irradiation method of KNN-based ceramics from solid-state reaction has not been reported so far. The objective of this work was to investigate the effects of ultrasonic irradiation on the structural and properties of KNN– LN ceramic and obtain KNN–LN piezoelectric ceramic by a low-cost and fast ultrasonic irradiation method without using any isostatic pressing process.
Lead-free piezoelectric ceramics of 0.94(K0.5Na0.5)NbO3– 0.06LiNbO3 (0.94KNN–0.06LN) were prepared by an ultrasonic irradiation method. X-ray diffraction, thermal analyses, and X-ray photoelectron spectroscopy were used to evaluate the effects of different ultrasonic powers on the structural and electrical properties of the 0.94KNN–0.06LN ceramics. The results revealed that the ultrasonic irradiation facilitated the decomposition of starting carbonates, promoted the perovskite phase formation and the transformation from orthorhombic to tetragonal phase, and resulted in improved ceramic density. 0.94KNN–0.06LN ceramic samples with a relative density of 94% were obtained after the ultrasonic irradiation processing under air sintering, which exhibited a piezoelectric constant of 184 pC/N. This study showed that ultrasonic irradiation was an effective method for producing high-quality KNN-based leadfree ceramics with significantly reduced processing time. I. Introduction BECAUSE the discovery of PbZrO3–PbTiO3 (PZT) in the 1950s, lead-based piezoelectric ceramics have been widely used in sensors, actuators, and many other electronic devices. However, these traditional PZT ceramics contain 460% lead by weight, which is detrimental to human health and environment due to the high toxicity of lead. As of July 1, 2006, the Europe legislations have restricted the use of certain hazardous substances in electrical and electronic equipment (RoHS) and are gradually requiring the removal of lead.1 There are similar activities and regulations emerging all over the world. Therefore, there is an urgent need to find alternative materials to replace existing leadbased piezoelectric materials, in particular those with properties comparable to PZT. In 2004, Saito et al.2 reported that potassium sodium niobate (KNN) modified by Li, Sb, Ta has comparable piezoelectric properties with those of PZT at room temperature. Over the past several years, intensive efforts have been made to explore KNN family. As pure KNN ceramics are difficult to be sintered to high density, they often have poor electric properties.3 Phase stability of the KNN is limited to 11401C and therefore it is not possible to achieve high density by further increasing the sintering temperature.4,5 By using the hot-pressing method, KNN with improved density exhibits a piezoelectric constant of d33 5 160 pC/N.6 However, this method is not cost effective and is unsuitable for industrial production. Furthermore, the
II. Experimental Procedure The material compositions studied are 0.94(K0.5Na0.5)NbO3— 0.06LiNbO3 (0.94KNN–0.06LN) with 1 mol% of excess K, Na, K2CO3 (99.5%), Na2CO3 (99.0%), Li2CO3 (99.999%), and Nb2O5 (99.9%) (Alfa Aesar, Karlsruhe, Germany) were used as the starting raw materials. From our previous unpublished work, 1 mol% excess K, Na to compensate their loss during heat treatment could lead to improved electrical properties as compared with nonexcess compositions. As the three carbonate powders are moisture sensitive, they were first dried before use to avoid compositional errors. The dried powders were then stoichiometrically weighed and subsequently dispersed in ethanol in a beaker and treated for 30 min with ultrasonic irradiation
S. Zhang—contributing editor
Manuscript No. 28042. Received May 24, 2010; approved August 17, 2010. w Author to whom correspondence should be addressed. e-mail:
[email protected]. edu.sg
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using a direct-immersion titanium horn (Sonics VCX500, 20 kHz, 500 W, Sonics & Materials Inc., Newtown, CT with different ultrasonic powers of 5, 13, 28, and 52 W for four samples, respectively). During the ultrasonic treatment, the temperature was increased and controlled by a temperature thermocouple at 601C. After the ultrasonications were finished, the slurry was dried and crushed using an agate mortar and pestle before it was calcined at 8501C for 5 h in an alumina crucible. Only the calcined powders were ball milled for 4 h using planetary milling. After the powders were ground and mixed with 4 wt% polyvinyl butyral as a binder, they were uniaxially pressed into disks with a diameter of 10 mm and a thickness of about 1.8 mm without using cold isostatic pressing (CIP). The disk samples were sintered at 10501C for 2 h in air. Bulk densities of the sintered disk samples were measured using mass and geometrical dimension. The crystal structures were determined by X-ray diffraction (XRD) (D8-ADVANCED, Bruker AXS GmbH, Karlsruhe, Germany). The weight loss analysis was conducted using thermogravimetry (TGA-DSC, SDT2960, TA Instruments, New Castle, DE). The individual ions valence states were characterized by X-ray photoelectron spectroscopy (XPS) (VG ESCALAB 2201-XL Imaging System, East Sussex, U.K.) using AlKa source (1486.6 eV) with the binding energies (BEs) referenced to the C1s at 284.5 eV. The morphology of polished surfaces after thermal etching as examined using a scanning electron microscope (SEM) (JEOL model JSM6360LA, Tokyo, Japan). For electrical characterization, the disk samples were coated with silver paste on both sides after being polished, and then fired at 5501C. Dielectric constant and dielectric loss were characterized with an impedance analyzer (HP4294A, Agilent Technologies Inc., Hyogo, Japan). All samples were poled at 1201C for 30 min under a direct current field of 40 kV/cm for piezoelectric measurement. The piezoelectric constant, d33 was measured using a piezo-d33 meter (ZJ-4B, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China).
III. Results and Discussion Figure 1(a) shows the XRD patterns of the 0.94KNN–0.06LN ceramic samples sintered at 10501C for 2 h. The peaks of orthorhombic and tetragonal structure were indexed in terms of KNbO3 (JCPDS No. 32-0822) and (JCPDS No. 71-0945), respectively. It can be seen from Fig. 1(a) that a single-phase perovskite structure was formed with ultrasonic power of 52 W—(labeled as 52W-US), whereas a small amount of secondary phase of Li3NbO4 was detected in the samples with ultrasonic power at 5, 13, and 28 W (labeled as 5W-US, 13W-US, and 28W-US, respectively). The zoomed XRD patterns of the ceramics in the range of 2y from 441 to 581 are shown in Fig. 1(b). It can be clearly seen when the ultrasonic power increases from 5 W to 52 W, the orthorhombic peaks of (220), (002), (130), (221), (112), (131), (311), and (202) transforms to tetragonal peaks of (002), (200), (102), (201), (210), (112), and (211), which illustrate the gradual transformation from orthorhombic to tetragonal phase in the ceramics derived from the increased ultrasonic power. In the literature study, it is reported that the orthorhombic and tetragonal phase coexisted for 0.94KNN–0.06LN ceramic with Li substitution in the lattice, and the tetragonal phase was characterized by (002)/(200) peak splitting at about 451.8 The results here thus indicate that increasing the ultrasonic power caused the structural phase transition due to an increased solubility of Li substitution, which should result in improved properties as expected from the solid solution of KNN with LN. Figure 2 shows the XRD patterns of 0.94KNN–0.06LN ceramic samples calcined at 4501C for 5 h. As the decomposition of carbonates starts at 4201C as mentioned in the literature,20 all the samples were calcined at 4501C to examine the phase evolution at the initial reaction stage. The XRD peak at 321 in 2y range can be referred for perovskite phase detection as the peak
Fig. 1. X-ray diffraction patterns of the 0.94KNN–0.06LN ceramic samples sintered at 10501C for 2 h, (a) 2y in the range of 101–651, and (b) 2y in the range of 441–581.
at 221 in 2y could be overlapped with Nb2O5 peaks. As shown in Fig. 2, perovskite phase formation was promoted for 28W-US and 52W-US at 4501C. Whereas, for 5W-US and 13W-US
Fig. 2. X-ray diffraction patterns of 0.94KNN–0.06LN powders calcined at 4501C for 5 h.
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Fig. 5. Thermogravimetric curves of 0.94KNN–0.06LN powders after the ultrasonic irradiation. Fig. 3. X-ray photoelectron spectroscopic spectra of K 2p for 0.94KNN–0.06LN ceramic powders calcined for 5 h at 4501C.
sample, a large amount of intermediate phase of K4Nb6O17, Na2Nb4O11, Nb2O5, and some residue carbonates are detected. It is important here to take note that ultrasonic irradiation energy increased the reaction rate significantly and promoted the perovskite phase formation. Figure 3 shows XPS spectra of K 2p for 0.94KNN–0.06LN ceramic powders calcined for 5 h at 4501C. The samples calcined
Fig. 4. X-ray photoelectron spectroscopic spectra of O 1s for the 0.94KNN–0.06LN powders calcined for 5 h at (a) 4501C and (b) 5501C.
at 4501C for 5W-US and 13W-US showed two peaks for K 2p, labeled as K 2p01/2 and K 2p03/2 which were attributed to K ions before being crystallized into the perovskite phase. It has been reported that the XPS peaks for the K4Nb6O17 phase as present in the XRD result in Fig. 2 have the BE of K 2p01/2 at 295 eV and K 2p03/2, at 292 eV.21 The K in noncrystalline oxide structure also exhibited XPS peaks corresponding to these two BEs in the literature.22 In contrast, there were additional two peaks for samples 28W-US and 52W-US, labeled as K 2p1/2 and K 2p3/2, which shifted by B1.5–1.6 eV toward the lower BE from K 2p01/2 and K 2p03/2, respectively. These additional two peaks at lower BEs are also reported previously to appear accordingly with KNN perovskite phase formation.22 Therefore, the additional two peaks detected are an indication of the crystallization of perovskite structure, which can be confirmed by XRD results in Fig. 2. However, 5W-US and 13W-US samples did not clearly exhibit peaks of K 2p1/2 and K 2p3/2, which indicated that perovskite formation is delayed to higher temperature. Therefore, the results indicated that the perovskite phase formation was promoted at lower temperature with the increasing ultrasonic irradiation power. Figure 4(a) shows the XPS spectra of O 1s for the 0.94KNN– 0.06LN powders calcined for 5 h at 4501C. By analyzing the C 5 O peaks, we can obtain the information about the decomposition of carbonates in the samples. It can be noted that by the increasing ultrasonic power from 5 to 52 W, the C 5 O peaks were reduced greatly and a further reduction was observed at 5501C, as shown in Fig. 4(b). Comparing the XPS data on C 5 O peaks for the different samples, clearly high ultrasonic power led to decomposition of carbonates at lower temperature, and therefore the perovskite phase formation was promoted. Figure 5 presents the TG curves of 0.94KNN–0.06LN powders after the ultrasonic irradiation and dried at 1201C. According to the literature, the decomposition of the hydrated sodium carbonate to sodium carbonate occurs at a temperature between 1001 and 1801C,23 and the decomposition of hydrated potassium carbonate also occurs below 2001C.24 As observed in Fig. 5, for the powders treated with an increased ultrasonic power, including 13W-US, 28W-US, and 52W-US, weight loss below 2231C was below 1.4%, which was attributed from the elimination of water. However, 5W-US sample had much higher water absorption, which was reflected by the large weight loss of 4.5% below 2231C. It seemed that the high-power ultrasonic irradiation could help to suppress the hydration of the carbonates or facilitate the dehydration. The theoretically calculated weight loss from all the carbonate decomposition is 11.55%. From the TG curves in Fig. 5, the weight loss after the complete dehydration at 2231C for 5W-US, 13W-US, 28W-US, and 52W-US samples are 14.27%, 11.77%, 11.66%, and 11.54%, respectively. The overall weight loss after
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Fig. 6. Bulk density and weight loss of 0.94KNN–0.06LN ceramic samples sintered at 10501C for 2 h.
dehydration matched well with the calculated one from the carbonate decomposition when the ultrasonic power increased. However, it was noted that there was a significant additional weight loss for 5W-US sample. The larger weight loss with decreasing ultrasonic irradiation power implied that there was a certain amount of alkali composition loss during heat treatment. For the TG curves in Fig. 5, the slope becomes steeper in the temperature range of carbonate decomposition from 4001 to 7001C as the ultrasonic power increases; this indicates the faster carbonate decomposition process for the powder treated with ultrasonic irradiation of higher power, which leads to earlier perovskite phase formation as confirmed by XRD results in Fig. 2. The results indicate that high-power ultrasonic irradiation treatment accelerated the decomposition of carbonates and promoted the perovskite phase formation at lower temperature. This is very crucial because the alkali compositions are much more volatile at the moderate temperature before they are crystallized into the perovskite phase than after the crystallization at
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high temperature, as our group observed before.22 As shown in Fig. 5, higher power of the ultrasonic irradiation consistently resulted in smaller overall weight loss due to the suppressed alkali loss, although faster weight loss due to carbonate decomposition. The weight loss stopped at lowered temperature from 8151 to 7111C for 5W-US and 52W-US samples, respectively. Figure 6 shows the bulk density and weight loss of 0.94KNN0.06LN ceramic samples sintered at 10501C for 2 h. As shown in Fig. 6, the bulk density of 0.94KNN–0.06LN increased with increasing ultrasonic power, reaching the value of 4.24 g/cm3 at the ultrasonic energy of 52 W. Compared with the theoretical density of K0.5Na0.5NbO3 of 4.51 g/cm3, the relative density was 94%. The weight differences in samples were recorded before and after sintering, and the weight loss during the sintering process decreased with the increasing ultrasonic power. This decrease in weight loss during sintering further indicated that low volatility of alkali metal ions can be achieved by using ultrasonic irradiation with high ultrasonic power and dense ceramic can be achieved without using CIP. Figure 7 shows the surface morphology after polishing and thermal etching for 0.94KNN–0.06LN ceramic samples sintered at 10501C for 2 h. It can be seen that the average grain size for 5W-US and 13W-US as shown in Figs. 7(a) and (b) is 30 mm, and the average grain size was reduced to 22 and 14 mm for 28W-US and 52W-US as shown in Figs. 7(c) and (d). To estimate the average grain size, linear straight lines were drawn across the micrograph and the numbers of grains intercepted with the lines were counted, with the length of the straight line divided by the number of grains intercepted. Large pores exist in 5W-US and 13W-US, but only smaller pores are noted in 28WUS and 52W-US with improved dense morphology. Therefore, it is clear that the ceramic samples derived from ultrasonic irradiation with higher power, 28W-US and 52W-US, exhibited a denser and more homogenous morphology than those processed with lower power irradiations, 5W-US and 13W-US. The density, dielectric, and piezoelectric properties of 0.94KNN–0.06LN ceramic samples sintered at 10501C are summarized in Table I. The relative density of the samples was increased from 89.3% to 94% with increasing ultrasonic power to 52 W. For 28W-US and 52W-US, the piezoelectric properties
Fig. 7. Polished surface morphology after thermal etching of 0.94KNN–0.06LN ceramic samples sintered at 10501C for 2 h, (a) 5W-US, (b) 13W-US, (c) 28W-US, and (d) 52W-US.
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Table I. Density, Grain size, Weight loss, Piezoelectric, and Dielectric Properties of 0.94KNN–0.06LN Ceramic Samples Sintered at 10501C Samples
Ball milled 5W-US 13W-US 28W-US 52W-US
Density (g/cm3)
Relative density (%)
Grain size (mm)
Weight lossw (%)
d33 (pC/N)
Dielectric constant at 1 kHz
Loss tangent at 1 kHz
4.05 4.03 4.05 4.19 4.24
89.8 89.3 89.8 92.9 94.0
19 30 30 22 14
1.44 1.39 1.4 0.86 0.12
80 — — 130 184
280 — — 370 630
0.03 4.15 0.55 0.06 0.01
w
The weight loss indicates the weight difference in percentage of the pellets before and after sintering.
were improved significantly. The d33 value for 52W-US ultrasonic samples was 184 pC/N, with dielectric constant and loss of 630 and 0.01, respectively. Du et al.25 once reported on an optimized 0.94KNN–0.06LN composition using 24 h of ball milling (versus our 30 min ultrasonic irradiation) for the powder mixing and achieved a highest piezoelectric of 215 pC/N and dielectric constant of 530 without CIP. For a comparison, we cited a previous batch of 0.94KNN–0.06LN from the conventional ball-milling method using planetary ball milling for 24 h for the raw powders and ball milling for another 12 h after calcination without CIP. The sample was also sintered at 10501C for 2 h, and the results are labeled as Ball milled in Table I. Despite the long and tedious ball-milling process, the ball-milled sample had a low density of 89.8% and poor piezoelectric properties, which could be due to the absence of CIP to achieve high ceramic density. These results showed that high density and competitive electrical properties could be achieved with fast and effective ultrasonic irradiation method for 0.94KNN–0.06LN ceramic. For 5W-US and 13W-US ultrasonic samples, the piezoelectric properties could not be tested because the ceramics were unable to be poled due to the high conductivity, possibly as a result of incomplete mixing and the low density. The measured dielectric constant data are not included in Table I for these two samples in consideration of their high dielectric loss.
eliminated uncontrolled contamination arising during the intensive long-time ball-milling process.
V. Conclusion Lead-free piezoelectric ceramics of 0.94KNN–0.06LN were prepared by an ultrasonic irradiation method. Our experimental results revealed that the ultrasonic irradiation facilitated the decomposition of the starting carbonates, promoted the perovskite phase formation and the desired coexistence of the orthorhombic and tetragonal phases, and improved ceramic density. The 0.94KNN–0.06LN ceramic samples with a relative density of 94% were obtained after the ultrasonic irradiation processing under normal air sintering, which exhibited a piezoelectric constant of 184 pC/N. This study showed that ultrasonic irradiation was an effective method for producing high-quality KNN-based lead-free ceramics with significantly reduced processing time.
Acknowledgment The authors would like to thank Ms June Ong Lay Ting for her technical supports in XPS.
References 1
IV. Discussion The present study revealed the effectiveness of ultrasonic irradiation to process the raw powders for forming 0.94KNN– 0.06LN ceramic instead of the time-consuming conventional ball-milling method. According to the XRD data in Fig. 1, a single-phase perovskite structure was achieved for 52W-US ultrasonic sample with the desired coexistence of orthorhombic and tetragonal phases. The promotion of single-phase perovskite structure through high ultrasonic power can be due to cavitation bubbles accelerating the solid particles to high speed and increasing the particles collision rate, which could lead to a tremendous rise of local temperatures of B5000 K and pressures of B1000 bar.26,27 Therefore, the mass transport processes are amplified, which could increase the diffusion rate for forming the solid solution of the 0.94KNN–0.06LN ceramic. The important factors that affect the piezoelectric properties of 0.94KNN–0.06LN is the composition control and coexistence of orthorhombic and tetragonal phase at room temperature. The strong alkali volatility at an elevated temperature often make composition control difficult, and serious alkali loss led to the orthorhombic phase only with deficient potassium and sodium.28 Our results indicated that high-intensity ultrasonic irradiation accelerated the decomposition of carbonates and promoted the perovskite phase formation at substantially lower temperature. Once the alkali compositions were crystallized into the perovskite structure, they were much more stable. That is why the ceramic with improved density, solubility of Li, and electrical properties were obtained with KNN–LN powder treated with the high-intensity ultrasonic irradiation. The duration for the ultrasonic irradiation treatment of the raw materials powder was only 30 min, in contrast to the typical 24-h ball milling. This not only saved processing time but also
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