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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tace20 Journal of Asian Ceramic Societies ISSN: (Print) (Online) Journal homepage: www.tandfonline.com/journals/tace20 Structure and electrical properties of [(Na0.825K0.175)0.5Bi0.5]1+xTiO3 piezoceramics with A- site nonstoichiometry Xiaoming Chen, Yunwen Liao & Guorong Li To cite this article: Xiaoming Chen, Yunwen Liao & Guorong Li (2020) Structure and electrical properties of [(Na0.825K0.175)0.5Bi0.5]1+xTiO3 piezoceramics with A-site nonstoichiometry, Journal of Asian Ceramic Societies, 8:4, 1036-1042, DOI: 10.1080/21870764.2020.1806192 To link to this article: https://doi.org/10.1080/21870764.2020.1806192 © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan. Published online: 18 Aug 2020. 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The effects of Bi3+, Na+ and K+ content on the structure and electrical properties of 0.825Bi0.5Na0.5TiO3-0.175Bi0.5K0.5TiO3 piezocera- mics were investigated.The XRD patterns demonstrated that BKNT-x piezoceramics form a solid solution without a secondary phase. Meanwhile, SEM micrographs indicated that excess [(Na0.825K0.175)0.5Bi0.5]2+ suppresses grain growth and that BKNT-x ceramics become consider- ably denser. BKNT-0.002 ceramics offer the following enhanced properties: d33 = 154 pC/N, kp = 33%, εr = 1087, tanδ = 0.036, and Qm = 116. This improvement of properties is most likely related to the dense structure and the maintenance of morphotropic phase boundaries through the introduction of a proper A-site cation excess. For ceramics containing volatile elements, stoichiometric control is an efficacious method of enhancing performance. ARTICLE HISTORY Received 3 March 2020 Accepted 2 August 2020 KEYWORDS Structure; X-ray diffraction; piezoelectric properties; Bi0.5 Na0.5TiO3; ceramics; oxygen vacancy 1. Introduction To date, lead-free piezoceramics have received a great amount of attention from researchers due to their envir- onmental friendliness. Smolenski et al [1]. reported Na0.5 Bi0.5TiO3 (BNT) in 1960. At room temperature, rhombo- hedral BNT ceramic shows a high Curie point and rela- tively good electrical properties [2,3], which have made BNT ceramics become one of promising lead-free cera- mics. BNT ceramics have a large coercive field Ec (73kv/ cm), however, that makes ceramic poling difficult [4,5]. A great deal of modification work has been conducted since the early 1990s. Other approaches have been proved to be helpful in the poling of the ceramics, as well as, in improving their electrical properties, include the formation of new solid solutions, such as, Bi0.5(Kx Na1-x-yLiy)0.5TiO3 [6], BaTiO3(BT)-BNT [3,7], BNT-Na NbO3 [8], K0.5Bi0.5TiO3(BKT)-BNT [5], BT- BNT-BKT [9], BNT- BaTiO3-NaNbO3 [10], BKT-BNT-SrTiO3 [11], Bi0.5(K0.20 Na0.70Li0.10)0.5TiO3 [12], Bi0.5(KxNa1-x-Agy)0.5 TiO3 [13], and BNT-BT-(Na0.5K0.5)NbO3 [14], and doping with metal oxides, such as, Dy2O3-doped (Na0.7K0.3)0.5Bi0.5 TiO3 [15], Gd2O3-doped (Na0.5 Bi0.5)0.94Ba0.06TiO3 [16], Li2O-doped0.825BNT-0.175BKT [17], La2O3-doped0. 92BNT-0.08BT [18]. Among these systems, BKT-BNT (BNKT) was observed by Sasaki et al [5]. They found that it has relatively strong piezoelectric performance owing to the existence of morphotropic phase bound- aries (MPB, x=0.16-0.2). However, properties of BNT- based piezoceramics are usually affected by the volatilization of A-site cations for the sintering process. Zuo R et al [19]. revealed that the sintering behavior of (Na0.5Bi0.5)1+xTiO3 ceramics is sensitive to variants of A-site cations that contribute to enhancing its electrical properties(x=0.01, d33=83pC/N). Sung et al [20,21]. revealed that the effect of Bi nonstoichiometry on d33 and Td of BNT is different from the results of Na non- stoichiometry. Wu et al [22]. found that a proper Na/K excess shows good electrical properties in 0.85 BNT- 0.15BKT systems (Pr=13.6 μC/cm2, d33* = 56 pm/V). Prasertpalichat [23] demonstrated that changes in Bi/ Na stoichiometry have a significant influence on the electrical properties of (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics. Chu et al [24]. also observed that changes in the A-site cations can effectively enhance the electrical properties of nonstoichiometric (Bi0.5Na0.5)0.92Ba0.08TiO3 systems (d33=149 pC/N, ε= 1230). Ni et al [25]. reported that Bi0.505(K0.18Na0.82)0.485TiO3 ceramics show relatively good piezoelectric properties when tuned with A-site vacancies (d33=151pC/N, Pr=29.5μC/cm2). In order to clarify nonstoichiometry effects, therefore, it is signifi- cant to investigate the connection between the proper- ties and the amount of A-site cations in BNT-based piezoceramics. In this case, a type of lead-free [(Na0.825K0.175)0.5 Bi0.5]1+xTiO3 piezoceramics was synthesized using a conventional mixed-oxide method. The influence of [(Na0.825K0.175)0.5Bi0.5]2+ nonstoichiometry on the microstructure, crystal phase and electrical properties CONTACT Xiaoming Chen chen-xm123@163.com Key Laboratory of Light Metal Materials Processing Technology of Guizhou Province, Guizhou Institute of Technology, Guiyang 550003, China JOURNAL OF ASIAN CERAMIC SOCIETIES 2020, VOL. 8, NO. 4, 1036–1042 https://doi.org/10.1080/21870764.2020.1806192 © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. http://www.tandfonline.com https://crossmark.crossref.org/dialog/?doi=10.1080/21870764.2020.1806192&domain=pdf&date_stamp=2020-12-05 of (Na0.825K0.175)0.5Bi0.5TiO3 (BKNT) piezoceramics was investigated in detail. 2. Experimental procedure Piezoceramics [(Na0.825K0.175)0.5Bi0.5]1+xTiO3 (x = 0, 0.002, 0.004, 0.006,0.009) were synthesized via a conventional solid state process. Bi2O3 (Chenguang Chemical Co., Ltd, Qishan, China, AR, 99.9%), K2CO3 (Xilong Chemical Co., Ltd, Shantou, China, AR, 99.8%), Na2CO3 (Xilong Chemical Co., Ltd, Shantou, China, AR, 99.9%), TiO2 (Zhongxing Electronic Materials Co., Ltd, Xiantao, China, AR, 99.5%) powders were used as raw materials with their water removed at 120 oC/24 h. According to the desired stoichiometry, all the pow- ders were blended with anhydrous ethanol, and then ball-milled at a rate of 350 rotations/min for 7–8 h. When the powders become dry under 75 oC conditions, the mixture was calcined at 850 °C/2 h. The heating rate was 3 min/oC. Re-milled samples were blended with polyvinyl alcohol (5 wt%), and then pressed into discs measuring 10 mm in diameter and 1.2 mm in thickness under 20 Mpa pressure. The obtained samples were sintered at 1180 oC/2 h at a heating rate of 5 min/oC. In order to form electrodes, a coating of silver conducting paste (PC-Ag-8080, Xizhi Electronic Materials Co., Ltd, China) was applied to two sides of polished compositions, which were fired at 600 °C. In a silicone oil bath (80 oC), the annealed composi- tions (9 mm in diameter, 1 mm in thickness) were poled with a DC electric field (4.5 kV/mm) for 45 min. The sintered compositions had been determined by XRD (Cu Kα radiation, Rigaku Dmax/RB). The lattice parameters of the BKNT-x ceramics were refined by the Rietveld refinement method [26] and MAUD soft- ware (version 2.94) [27–29]. The weighted profile resi- dual factor (Rwp) was less than 15%, indicating good agreement of refinement. The density of the composi- tions was calculated by the Archimedes method. The microstructures of the samples were characterized by SEM (JSM-6510, Japan). The average grain size was obtained by measuring the number of grains at the intersection lines. The measurement of piezoelectric properties was carried out with a d33 meter (ZJ-3A, Institute of Acoustics, China). The dielectric properties of the BKNT-x piezoceramics were measured with an impedance analyzer (Agilent 4294A,1 KHz) at 25–500 ° C. The kp and Qm were calculated by use of Onoe’s formula [30], employing the resonance and antireso- nance frequencies method as a basis. 3. Results and discussion Figure 1(a) displays the XRD patterns of BKNT-x cera- mics. All the compositions possess typical diffraction peaks displayed by an ABO3-type structure. There is no impurity phase in these solid solutions. Ni et al [25]. observed that when x � 0.02, second phase Bi2Ti2O7 forms in (K0.18Na0.82)0.5–3xBi0.5+xTiO3 ceramics. Babu et al. [31] also demonstrated that excess K2CO3 (x ≤ 0.8 mol%) makes up for the volatility of K+ to maintain the phase purity of (Na0.8K0.2)0.5Bi0.5TiO3 piezoceramics. Seo et al [32]. observed, moreover, that Ba0.06(Bi0.5Na0.5)0.94+x TiO3 (−0.01 ≤ x ≤ 0.02) piezoceramics possess a homo- geneous structure with phase purity. Our results are consistent with previous reports, which showed that excess Bi3+, Na+, and K+(x ≤ 0.009) may keep (Na0.825 Figure 1. (a)XRD Patterns of various compositions in the BKNT-x ceramics; (b) and (c) The expanded XRD patterns of BKNT-x ceramics in the range of 39–47°. JOURNAL OF ASIAN CERAMIC SOCIETIES 1037 K0.175)+/Bi3+ at 1:1 and maintain the phase purity of BKNT-x ceramics after the A-site cations of BKNT-x ceramics partly volatilize in the sintering process. Sasaki et al [5]. reported that (Na1-xKx)0.5Bi0.5TiO3 ceramics (BNKT) exhibit high performance due to the R-T MPB (0.16 ≤ x ≤ 0.2), which has a similar pseudo- cubic structure [33]. To investigate the phase structure of BKNT-x ceramics, the magnified XRD patterns are given in the 2ɵ range of 39–47°, as shown in Figure 1(b, c). According to the R3 c space group (COD ID: 2103295) and P4bm space group (COD ID: 2102068) [34,35], the structure could be designated by the fol- lowing peaks: rhombohedral (003) and tetragonal (201) peaks at 40°, as well as rhombohedral(202) and tetragonal (002) peaks at 46°, repectively. The peak splitting reveals that BKNT-x ceramics are still in the rhombohedral(R)-tetragonal(T) phase transition range. In addition, the (002) peak moves toward lower angles with increases in x. The lattice parameters (aT, cT) and cell volume of the tetragonal phase of BKNT-x ceramics increase slightly with changes in the content of A-site cations, as shown in Table 1. In the sintering process, oxygen vacancies of BKNT ceramics are usually gener- ated due to the volatilization of A-site cations. The addition of excess A-site cations (Bi3+, Na+, and K+, hereinafter BKN) balances the effect of BKN loss, which leads to a reduction in the number of oxygen vacancies. It is generally known that large numbers of oxygen vacancies can cause unit cells to shrink and lead to a shift of the diffraction peaks to high angles [36–39]. Therefore, the low-angle movement of (002) peaks for BKNT-x ceramics indicates a decrease in the number of oxygen vacancies. Figure 2 displays surface micrographs of BKNT-x ceramics sintered at 1180 oC. The crystal grains of BKNT-x piezoceramics show a regular square shape. With increases in BKN, the average grain size decreases slightly. The mean crystallite sizes are about 1.28, 1.25, 0.98, 0.76 μm, respectively. This can probably be ascribed to the collection of excess BNK at the A-site lattice of ABO3 perovskite structure due to volatiliza- tion of A-site cations (Bi3+, Na+, and K+) at high sinter- ing temperatures.This behavior can reduce oxygen vacancies, while the existence of the defects is condu- cive to mass transport, accelerating the grain growth [40]. As a result, crystallite growth is restrained. This is similar to a previous report on (Bi0.5+xNa0.5)0.94Ba0.06 TiO3 systems with A-site nonstoichiometry [41]. It can be observed, from Figure 3, that with the concentra- tion of BKN, the relative densities of BKNT-x piezocera- mics reach a maximum, and then descend, as was achieved by the theoretical density of (Na0.8K0.2)0.5 Bi0.5TiO3 (5.889 g/cm3) [42]. Appropriate BKN excesses are able to decrease the evaporation of volatile A-site cations, and contribute to an improvement in the den- sity of BKNT-x ceramics [43]. Bi2O3 with its melting Table 1. Lattice Parameters and lattice volume of BKNT-x ceramics. x aT /Å cT /Å VT/Å3 0 5.51298 3.89562 118.39938 0.002 5.51347 3.89552 118.41739 0.004 5.51477 3.89960 118.59732 0.006 5.51489 3.89980 118.60856 0.009 5.51685 3.89855 118.65484 Figure 2. SEM surface micrographs of BKNT-x ceramics with various compositions sintered at 1180 oC for 2 hours. 1038 X. CHEN ET AL. point of 825°C usually decreases sintering tempera- tures by forming a liquid phase [44]. With further increases in BKN(x > 0.004), the content of bismuth oxide goes up, which leads to lower sintering tempera- tures of BKNT-x piezoceramics. For this reason, the ceramics can’t be fully sintered at the same sintering temperature (1180 oC), resulting in decreased com- pactness. The relative densities of BKNT-x piezocera- mics are more than 95% of the theoretical values when 0.002 � x � 0.004. A dense microstructure is particu- larly advantageous to improvement of the electrical properties of BKNT-x ceramics. Figure 4 displays the temperature dependance of εr and tanδ for unpoled BKNT-x ceramics under different frequencies. BKNT-x ceramics have two dielectric anomalies: a relaxor antiferroelectric-antiferroelectric phase transformation and an antiferroelectric- paraelectric phase transformation corresponding to TRE and Tm, respectively, which are consistent with NBT-based piezoceramics [5,45–48]. A phenomenon of mergence is exhibited by the different εr-T curves of BKNT-x piezoceramics, where the temperature is described as TRE. Frequency has a great effect on εr when T< TRE. Ma et al. reported that nanodomains with a short-range order stimulate the relaxor behavior of BNT-basedceramics and suggested a new term, “relaxor antiferroelectric,” to describe the phenom- enon of frequency dispersion [46]. With further incre- ments in temperature, the relative permittivity εr reaches a maximum value at the second dielectric anomaly, where the temperature is expressed as Tm. Ma et al. identified a long-range antiferroelectric(AFE) order presents in BNT-based ceramics at between TRE and Tm, and showed that the paraelectric phase is located at temperatures above Tm [46]. As soon as the temperature rises above Tm, tanδ increases greatly due to the considerable leakage conduction [49]. In addition, pure BKNT ceramics exhibit larger values for dielectric constants and loss tangents than BKNT-x ceramics at high temperatures (T > Tm) and low fre- quencies, which can be ascribed to the fact that Figure 3. Relative density of BKNT-x ceramics sintered at 1180 oC / 2 h. Figure 4. Temperature dependence of relative permittivity and dielectric loss for BKNT-x ceramics (1180°C/2 h). JOURNAL OF ASIAN CERAMIC SOCIETIES 1039 A-site vacancies and oxygen vacancies form at pre- paration process and lead to the appearance of fre- quency dispersion for pure BKNT ceramics [46]. Figure 5 shows the electrical properties of BKNT-x piezoceramics. When the content of BKN is 0.2 mol%, the maximum values of the piezoelectric constant d33 and electromechanical coupling factor kp reach 154 pC/N and 33%, respectively. The comprehensive per- formance of BKNT-x piezoceramics is slightly superior to those of NBT-based ceramics with nonstoichiometry (see Table 2). This can be ascribed to the following reasons: a proper BKN excess improves the compact- ability of BKNT-x piezoceramics which makes the polar- ization of ceramics more completely under a broad electrical field [17]. Moreover, an excess amount of BKN can make up for the volatilization of A-site cations and maintain the R-T phase fractions of compositions around MPB, which improves the piezoelectricity of BKNT-x ceramics. Analogous to the close dependence of BKN content and d33, the maximum value of the relative dielectric constant εr (1087) occurs at x=0.002. Meanwhile, the dielectric loss tanδ initially decreases (x<0.002) and reaches a minimum value (0.0362), and then, finally increases with the concentration of BKN. Qm gradually decreases with the increases in BKN. This can be ascribed to the decrement of oxygen vacancies caused by the compensation of excess BKN for the loss of Bi3+, Na+ and K+. The temperature dependence of the kp of BKNT-x ceramics is demonstrated in Figure 6. The kp of BKNT-x ceramics shows a decreasing trend with increases in the annealing temperature. The kp changes signifi- cantly as the annealing temperature increases up to 134 oC (x=0.004, kp=0.03), and then decreases as the annealing temperature increases further. When the annealing temperature is 134-140 oC, the piezoelectric responses of BKNT-0.004 ceramics almost vanish. In other words, the depolarization temperature (Td) of BKNT-0.004 ceramics is about 134 oC, while pure BKNT ceramics have a high Td of around 140 oC, as reported by Sapper [50]. As a result, the addition of excess BKN has a negative effect on Td. On the other hand, variations in Td can reflect changes in the oxygen vacancies. The depolarization temperature of ceramics increases at a high oxygen vacancy concentration owing to a strong coaction between the oxygen vacan- cies and the domains [41]. Therefore, use of moderate amounts of BKN to compensate for the loss of Na+, Bi3+ Figure 5. Piezoelectric and dielectric properties of BKNT-x ceramics. Table 2. Comparison of the properties of different NBT-based ceramics with nonstoichiometry. systems d33 (pC/ N) kp Td(oC) References 0.94(Na0.5Bi0.52)TiO3-0.06BaTiO3 118 0.24 29 [41] 0.8(Na0.5Bi0.5)TiO3-0.2(K0.5Bi0.5)TiO3 + 0.8 mol% K2CO3 196 - - [31] Bi0.51(Na0.82K0.18)0.5TiO3 145 0.31 ~125 [25] Bi0.505(Na0.82K0.18)0.485TiO3 151 0.298 101 [25] [(Na0.825K0.175)0.5Bi0.5]1.002TiO3 154 0.33 137 This work Figure 6. The temperature dependency of kp for BKNT-x ceramics. 1040 X. CHEN ET AL. and K+ during the sintering step contributes to a decrease in oxygen vacancies that causes Td to shift to a low temperature. 4. Conclusions The influence of BKN on the structure, crystal phase and electrical properties of (Na0.825K0.175)0.5Bi0.5TiO3 piezo- ceramics, which were synthesized by conventional sin- tering, were systematically researched. All the BKNT-x piezoceramics have typical diffraction peaks with pure perovskite structures. Excess BKN compensates for the loss of the A-site cations, and leads to a decrease in oxygen vacancies which restrain the growth of crystal- lites. All the compositions are well sintered with dense microstructure at 1180 oC/2 h when 0.002 � x � 0.004. BKNT-x piezoceramics with BKN compensation achieve high d33 (154 pC/N) and kp (33%). The electrical proper- ties of BKNT-x ceramics have been enhanced by the introduction of excess BKN, which maintains the struc- tural density and the R-T phase fractions of the samples at around MPB. 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Conclusions Disclosure statement Funding References
