Lead-Free KNbO3:x

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Lead-free KNbO3: xZnO Composite Ceramics Xiang Lv, Zhuoyun Li, Jiagang Wu, Dingquan Xiao, and Jianguo Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11677 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016

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Lead-free KNbO3: xZnO Composite Ceramics Xiang Lv§, Zhuoyun Li§, Jiagang Wu∗, Dingquan Xiao and Jianguo Zhu Department of Materials Science, Sichuan University, 610064, Chengdu, People’s Republic of China

ABSTRACT: It is a tough issue to develop dense and water resistant KNbO3 ceramics duo to high evaporation and hygroscopicity of K2O. Here, KNbO3: xZnO composite ceramics were used to successfully solve this problem, where ZnO particles were randomly distributed into KNbO3 matrix. The addition of ZnO hardly affects phase structure of KNbO3, and moreover the enhancement of electrical properties, thermal stability and aging characteristics was observed in KNbO3: xZnO composite ceramics. The composites possessed the maximum d33 of 120±5 pC/N, which is superior to pure KNbO3 (d33=80 pC/N). More importantly, a strong water resistance and an aging-free characteristic were observed in KNbO3: 0.4ZnO. It is the first time for KNbO3 ceramics to simultaneously improve electrical properties and resolve the water-absorbing. We believe that this composite ceramics are promising for practical applications.

KEYWORDS: KNbO3, ZnO, Composite ceramics, Electrical properties, Water resistance

*

Corresponding author’s e-mail address: [email protected] and [email protected] 1


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1. INTRODUCTION KNbO3 (KN) material is among the most promising candidates because of its high Curie temperature (TC) and good piezoelectric properties.1-5 Previously, it was reported that the electromechanical coupling factor in the thickness-extensional mode (kt) as high as 0.69 which is the highest value in lead-free piezoelectric materials, and a good piezoelectric constant (d31~52 pC/N) were observed in KN single crystal.6,7 However, it is too difficult to develop dense KN ceramics duo to high evaporation of K2O,8,9 and furthermore a strong water-absorbing seriously inhibits KN ceramics’ applications.10,11 Until now, few reports were involved to successfully resolve these disadvantages.

Previously, many attempts were used to improve the density and electrical properties of KN ceramics, such as new sintering methods, sintering aids, additives, and so on.12-16 For example, KN ceramics fabricating by sol-gel process possessed a high density of 99% and low dielectric loss.12 K4CuNb8O23 as sintering aid can improve the sinterability of KN ceramics due to the formation of liquid phases, leading to the enhancement of electrical properties.13 In addition, both density and electrical properties (from 80 pC/N to 122 pC/N) were promoted in KN ceramics by introducing the additives, such as LaFeO3, BiFeO3, MnO2 and so on.14-17 Unfortunately, although these efforts can improve the density and electrical properties of KN ceramics, an obvious water-absorbing still continues.12-16

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According to the previous studies, the strong water-absorbing of KN mainly stems from porous microstructure and deliquescence of alkalis.6-17 Therefore, an idea came out, that is, if we reduced the porosity and covered the pores and grains with other additives or fillers which possess a strong water resistance, how the performance of KN ceramics will be? In order to achieve this conception, constructing composite ceramics may be the best choice.18,19 Furthermore, the filler should basically meet the requirements, that is, i) forming liquid phases during sintering process; ii) improving the sinterability; iii) enhancing electrical properties. According to the previous investigations, ZnO as sintering aid was widely doped to piezoelectric ceramics.18,19,29-31 For example, a dense microstructure without deliquescence as well as an improved d33 (~121 pC/N) were developed in ZnO-modified KNN ceramics due to the presence of liquid phases.29 In addition, ZnO was also widely used to improve the sinterability of piezoceramics.30,31 Very recent, the improved electrical properties and thermal stability were observed in the BNT-6BT: ZnO and KNN: ZnO composite ceramics due to the unique microstructure and compensatory electric field induced by ZnO.18,19 Thus, ZnO should be a good candidate as filler.

In this work, KNbO3: xZnO (x is the mole ratio of ZnO to KNbO3 and x=0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) composite ceramics were prepared by a two-step method. KNbO3: xZnO composite ceramics were successfully developed, in which ZnO particles were randomly distributed in KN matrix. Their phase structure, microstructure, electrical properties, deliquescence, thermal stability and aging 3


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characteristic were systematically investigated. The addition of ZnO resulted in the enhancement of electrical properties, thermal stability and aging characteristic in KN ceramics. More importantly, there is a strong water resistance in KN: 0.4ZnO composite ceramics. The related physical mechanism was investigated.

2. EXPERIMENTAL PROCEDURE KNbO3: xZnO (x is the mole ratio of ZnO to KNbO3 and x=0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) composite ceramics were fabricated by two-step method. The raw materials were K2CO3 (99%), Nb2O5 (99.5%) and ZnO (99%). First, pure KNbO3 ceramic powder was synthesized by the conventional fabrication method, with the calcination at 800 oC for 4 h.32 Then, the calcined powder was mixed with ZnO according to the mole ratio. Subsequently, the green disks were fabricated along with the conventional fabricated procedure and sintered at 850-1020 oC for 1-6 h.32 The rest fabrication process (e.g., forming electrodes and poling process) and related measurement procedures of a series of properties (e.g., crystallinel structure, microstructure morphology, dielectricity, ferroelectricity, and piezoelectricity) were detailedly described in our previous publication.32 In order to investigate the water-absorbing, the composite ceramics with different x values were immersed into water and the photographs were recorded per 20 h. For checking the thermal stability, the poled KNbO3: 0.4ZnO composite ceramics were heated at each temperature for 30 min and then their d33 values were measured by using d33-meter. At last, in order to measure aging characteristic, d33 values of the poled KNbO3: 0.4ZnO composite ceramics were 4



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recorded weekly.

3. RESULTS AND DISCUSSION The phase structure of the ceramics was characterized by XRD patterns, as shown in Fig. 1. There is a pure phase for pure KN and secondary phase (PDF# 97-0206, ZnO) was formed after the addition of ZnO, as marked by red diamonds [Fig. 1(a)].20 In addition, the intensity of secondary phase (ZnO) gradually enhanced because of its increasing contents. However, it is worth noting that core phase of KN: xZnO almost remained unchanged despite of the addition of ZnO [Figs. 1(b) and (c)], indicating that Zn did not effectively diffuse into the lattice of KN matrix.

To further investigate the effects of ZnO on phase structure of KN ceramics, their temperature (-150-200 oC and 25-450 oC) dependence of dielectric constant (εr-T) curves were measured, as shown in Figs. 2(a) and (b). One can see from Fig. 2(a) that there is only one abnormal dielectric peak for all ceramics, which corresponded to the rhombohedral-orthorhombic phase transition (TR-O).5 With an increase of ZnO contents, their TR-O values almost remained unchanged regardless of the change of peak intensity, which suggested that doping ZnO to KN has little effect on their TR-O. Both orthorhombic-tetragonal phase transition temperature (TO-T) and tetragonal-cubic phase transition (TC) slightly fluctuated after the addition of ZnO [Fig. 2(b)], that is, their TO-T values slightly increased and TC values slowly dropped. After deriving TR-O, TO-T and TC values from Figs. 2(a) and (b), its phase diagram was established [Fig. 5


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2(c)]. It was observed that the addition of ZnO hardly affects phase structure of KN ceramics, which was consistent with the results of XRD patterns [Fig. 1]. Therefore, considering the results of both XRD patterns and εr-T curves, it can be concluded that the composite ceramics consisting of KN and ZnO were formed instead of solid solutions between KN and ZnO.

Since KN: xZnO composite ceramics were formed, it is necessary to figure out the existence form and distribution of ZnO in KN matrix. In order to achieve this goal, we firstly characterized surface morphologies of KN: xZnO composite ceramics, as shown in Fig. 3. It was found that their grain sizes were strongly dependent on ZnO contents. For a pure KN, there is a porous surface morphology consisting of “large” and “small” grains [Fig. 3(a)]. After doping with ZnO, both grain sizes and porosities were sharply reduced, and then there is a dense surface morphology consisting of only “small” grains in the ceramics with x≥0.3 [Figs. 3(d)-(f)]. The statistics of grain size distribution corresponding to Fig. 3 was conducted, as shown in Fig. 4. A wide grain size distribution was observed for a pure KN [Fig. 4(a)]. With an increase of ZnO contents, their grain size distribution became narrowed and shifted to small size range. In addition, there were decreased average grain sizes from 4.17 µm to 1.1 µm as x increased from 0 to 0.6, indicating that the addition of ZnO can strongly affect grain growths of KN ceramics. Their decreased grain size was mainly attributed to the considerable addition of ZnO, which formed the liquid phase and inhibited grain growth during sintering process,18,19 as discussed in Fig. 10. 6



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Subsequently, we checked the elements (Zn, Nb, K, O) distribution in the ceramics with x=0.4. As shown in Fig. 5(b), Zn was detected in the whole area corresponding to Fig. 5(a). Furthermore, Zn displayed an obvious enrichment in the areas marked by green circles [Fig. 5(a)]. Both Nb and K showed an appreciable loss in the areas in which Zn got enriched. The similar distribution was also observed in KNN: ZnO composite ceramics.19 Furthermore, the linear elements (Zn, O, Nb, K) distribution was also measured, as shown in Fig. 6. In the selected line range, all elements except for O show an alternately “enhanced” and “reduced” distribution. Taking Zn as the example, the selected line range can be divided into six segments, as marked by the Roman numerals [Fig. 6(f)]. An alternate distribution of Zn can be easily observed, that is, the segments I, III and V show an obvious existence of Zn, while the segments II, IV and VI show a considerable lack of Zn. Therefore, both surface mappings and linear distribution confirmed that ZnO possesses two existing forms in composite ceramics, that is, part ZnO gathered with each other at grain boundaries acting as the sintering aid, and the rest of ZnO was randomly distributed in KN ceramic matrix.

In order to check the morphology of ZnO, the back scattering of polished composite ceramics with x=0.4 was performed, as shown in Fig. 7. One can observe from Figs. 7(a) and (g) that some white dots or areas were observed, as marked by green ellipses. Two of them were selected for linear elements distribution analysis [Figs. 7(b)-(f) and 7(h)-(l)]. As shown in Figs. 7(b)-(e), it was found that the count of Zn greatly 7


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increases in the segment crossing area 1, while there is an obvious lack in two sides of segment without crossing area 1. An inverse distribution was observed for K and Nb [Figs. 7(b) and (c)] and the energy dispersive spectrometer (EDS) also confirms the existence of four elements [Fig. 7(f)]. Furthermore, very similar linear elements distribution and EDS pattern were also found in area 2 [Figs. 7(h)-(l)], strongly demonstrating that these white areas with the scale of ~1 µm were ZnO particles. Therefore, the composite ceramics of this work consist of KNbO3 ceramic matrix and ZnO particles, and ZnO particles were randomly distributed into KN ceramic matrix. In addition, the EDS patterns of the ceramics with x≥0.05 were also measured, as shown in Fig. 8. One can see from Fig. 8(a) that Zn can be detected in the ceramics with x≥0.05. With an increase of x, the characteristic peaks belonging to Zn gradually increase [Figs. 8(b) and (c)], which was consistent with the change of XRD patterns [Fig. 1(a)].

As we know, a pure KNbO3 is easily deliquesced due to the high water-absorbing.21,22 In this work, we checked the water-absorbing of KN: xZnO ceramics, as shown in Fig. 9. One can see from Fig. 9(a) that pure KN ceramics were immediately deliquesced as soon as the sample was immersed into water. However, the composite ceramics with x=0.2~0.6 almost keep unchanged even if the immersion time is more than 60 hours, suggesting a strong water resistance. Therefore, the construction of the composite ceramics is a promising way to resolve the water-absorbing of KN ceramics.

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To illuminate the physical mechanism why KN: xZnO show a strong water resistance, a physical model was proposed [Fig. 10]. For a pure KN, the powders gathered with each other and grew up during sintering process, and then the porous ceramics were obtained because of different grain growth rates along different directions, as proved in Fig. 3(a). After doping with ZnO, the way of grain growth was changed. Some ZnO particles became liquid phase during the first stage of sintering process,18,19 and then the liquid ZnO can restrain the flow of KN powders and thus inhibit grain growths. Finally, ZnO particles gathered with each other and occupied the gaps among grains. Therefore, it can be expected that two forms were involved into ZnO, as proved by the surface elements mappings [Fig. 5] and linear elements distribution [Fig. 6]. During the cooling process, the ZnO as a liquid phase was solidified, forming an effective waterproofer. As a result, a dense composite ceramic consisting of KN matrix and ZnO particles was obtained. Therefore, both dense microstructure and strong water resistance of solidified ZnO phase are responsible for the excellent water resistance of composite ceramics.18,19,29-31

To investigate electrical properties of KN: xZnO, their d33, kp, εr and tan δ values were measured, as shown in Fig. 11. As shown in Figs. 11(a) and (b), piezoelectric and dielectric properties of KN ceramics were greatly improved by introducing ZnO. With an increase of x, both d33 and kp firstly increased and then decreased, reaching the peak values (e.g., d33=120±5 pC/N and kp=0.42±0.03) for x=0.4. A similar change tendency was also observed for εr, while tan δ slightly fluctuated. Here, the 9


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improvement in piezoelectric and dielectric properties was mainly attributed to dense microstructure–induced by the addition of ZnO.18,19 According to the previous works, both sintering temperatures and dwell time played an important role in electrical properties of a ceramic.23 As shown in Figs. 11(c) and (d), piezoelectric and dielectric properties of KN: 0.4ZnO were first slightly enhanced and then greatly deteriorated as their sintering temperatures increased. There is a wide sintering temperature range (850 oC~980 oC), which is much lower than other reported values (960 oC~1040 oC), indicating that the addition of ZnO can effectively improve the sinterability of KN.8-16,24,25 The deteriorated electrical properties of the composite ceramics sintered at high temperatures were mainly caused by the serious volatilization of potassium.26 As shown in Figs. 11(e) and (f), piezoelectric properties of KN: 0.4ZnO were strongly dependent on dwell time. As the dwell time increased, both d33 and kp firstly elevated and then dropped, reaching the maximum for 4 hours. However, dielectric properties of KN: 0.4ZnO show an independent change against dwell time. εr firstly kept unchanged and then sharply reduced, while tan δ showed a slight change. Hence, appropriate sintering temperature and dwell time are other key factors to effectively improve electrical properties of KN ceramics.8-11, 23

In order to evaluate the effects of ZnO on their ferroelectric properties, polarization hysteresis (P-E) loops of KN: xZnO were measured, as shown in Fig. 12(a). All composite ceramics exhibited a typical ferroelectric loop. Their remanent polarization (Pr) and coercive field (EC) were shown in Fig. 12(b). Pr firstly increased and then 10



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reduced as x increased, reaching the maximum value of 19 µC/cm2 at x=0.05 and 0.1 [Fig. 12(b)]. Enhanced Pr for x=0.05 and 0.1 was mainly attributed to dense microstructure caused by ZnO acting as sintering aid. However, reduced Pr for x≥0.2 was attributed to high ZnO content. With an increase of x, the proportion of KN in the composite ceramics was reduced, leading to the reduced Pr. The similar effect was also observed in KNN: xZnO composite ceramics.19 Although a degraded ferroelectricity was observed, but it is worth noting that the ferroelectricity of composite ceramics with 0≤x≤0.30 was still better than that of pure K0.5Na0.5NbO3 ceramics, as confirmed in Fig. 12(b). As a result, constructing composite ceramic is indeed a useful way to improve the performance of KNbO3 ceramics.18,19

It is difficult to measure the strain properties of a pure KN because of their high water-absorbing and instability. In this work, a strong water resistance and good stability (e.g., thermal stability and aging stability) [Fig.14] was observed in KN: xZnO composite ceramics, which made it possible to measure the strain properties. The unipolar strain curve of KN: 0.4ZnO was shown in Fig. 13. The curve displayed a typical unipolar “butterfly” form. Furthermore, a remanent strain value of 0.068% was obtained, which was larger than pure KNN ceramics (

Lead-Free KNbO3:x

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