Hydrothermal synthesis of morphology-controlled KNbO3, NaNbO3, and (K,Na)NbO3 powders

Author’s Accepted Manuscript Hydrothermal synthesis of morphology-controlled KNbO3, NaNbO3, and (K,Na)NbO3 powders Guodong Shi, Junhan Wang, Hengli Wang, Zhanjun Wu, Huaping Wu www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)30371-1 http://dx.doi.org/10.1016/j.ceramint.2017.03.012 CERI14781

To appear in: Ceramics International Received date: 10 January 2017 Revised date: 27 February 2017 Accepted date: 2 March 2017 Cite this article as: Guodong Shi, Junhan Wang, Hengli Wang, Zhanjun Wu and Huaping Wu, Hydrothermal synthesis of morphology-controlled KNbO 3 NaNbO3, and (K,Na)NbO3 powders, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrothermal synthesis of morphology-controlled KNbO3, NaNbO3, and (K,Na)NbO3 powders Guodong Shia, Junhan Wanga, Hengli Wanga, Zhanjun Wua*, Huaping Wub a

State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116024, PR China b

Key Laboratory of E&M, Zhejiang University of Technology, Hangzhou 310014, PR China

[email protected] [email protected] *

Corresponding author: Tel/Fax: +86-411-84708646

Abstract： NaNbO3, (K,Na)NbO3 and KNbO3 powders were synthesized using (1 − y) NaOH–y KOH solutions ([OH−] = 7.5–15 M) with y = 0, 0.78, and 1 at 200 °C by the hydrothermal method, respectively. Their compositions, structures, and morphologies were analysed. Both of the synthesized NaNbO3 and KNbO3 powders had sub-micron- or micron-sized grains. The [OH−] drastically influenced the size and morphology of the KNbO3 particles but did not influence those of the NaNbO3 particles. In contrast, the morphology of the (K,Na)NbO3 particles, which were aggregates of nano-grains, was influenced by the hydrothermal-treatment time rather than [OH−]. Moreover, their composition and phase were influenced by both annealing and the hydrothermal-treatment time, and their formation mechanism was discussed by comparison with those of KNbO3 and NaNbO3 particles. The present synthetic strategy enables tailoring the compositions, morphologies, and structures of the niobate products to different applications by controlling the process parameters. Keywords: Lead-free piezoelectric ceramics; Niobates; Hydrothermal synthesis; Crystal growth; Phase evolution; Morphology.

1. Introduction Alkaline niobates have attracted considerable scientific attention for their excellent ferroelectric, piezoelectric, electromechanical, nonlinear optical, and ionic conductivity properties [1–6]. For example,

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they are potential substitutes for lead zirconium titanate (PZT) as high-performance piezoelectric ceramics, especially NaNbO3, KNbO3 and (K,Na)NbO3 (KNN). Among various piezoelectric materials, PZT is the most widely used because of its excellent electromechanical properties [7], but it is harmful to the environment and human health due to its high toxic lead oxide content. Therefore, researching lead-free piezoelectric ceramics as potential PZT substitutes is very urgent [6,8]. KNN is a combination of ferroelectric KNbO3 and antiferroelectric NaNbO3. For all compositions, NaNbO3 and KNbO3 can form a complete solid solution of KNN [9]. In particular, K0.5Na0.5NbO3, which forms a morphotropic phase boundary (MPB), shows very good piezoelectric properties [1]. Alkaline niobate powders are usually synthesized via a solid-state reaction, wherein niobium pentoxide is heated with potassium and/or sodium salt at temperatures of 800 °C or above [10–12]. However, high temperatures should be avoided during the preparation of alkaline niobates, because the volatilization of Na/K above 650 °C produces secondary phases and oxygen vacancies to the detriment of their properties [13]. Thus, the hydrothermal method is promising for synthesizing alkaline niobates because it requires a low temperature and yields crystalline powders without further heat treatments [14,15]. Recently, a series of crystalline KNbO3, NaNbO3, and KNN powders were successfully synthesized by the hydrothermal method at temperatures of about 200 °C, and the effects of hydrothermal parameters such as temperature, pressure, alkali concentration, and reactant ratio on their phase evolution and reaction mechanism were investigated [9,13,16–19]. However, thus far, the growth mechanism of the alkaline niobate crystals and the influence of [OH−] and the reaction time on the evolution of their morphology have not been systematically studied. Thus, further study is needed in order to control the composition, size, structure, and morphology of the products. In this study, we systematically investigated the influence of [OH−] over a wide range (7.5–15 M) on the hydrothermal synthesis of crystalline KNbO3, NaNbO3 and KNN powders at 200 °C. Their morphologies and growth mechanisms were compared. Moreover, the effects of low-temperature annealing and reaction time on the composition, phase, and morphology of KNN powders were also investigated. The present synthetic strategy enables us to tailor the compositions, morphologies and structures of the niobate products to different applications by controlling the process parameters.

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2. Materials and methods Analysis-grade Nb2O5 powders, NaOH, and KOH were adopted as raw materials. NaNbO3, KNbO3, and KNN ceramic powders were prepared by a hydrothermal synthesis method. A typical synthesis procedure was as follows. First, Nb2O5 (0.01 mol) was added into an aqueous solution of (1 − y) NaOH– y KOH (84 ml) with y = 0, 0.78, and 1 to prepare NaNbO3, KNN and KNbO3, respectively. The alkaline concentration (i.e., [OH−]) ranged from 7.5 to 15 M in the solutions. The mixture was stirred for 30 min and subsequently poured into a Teflon autoclave with a 70% filling ratio. Then, the autoclave was placed in an oven and heated at 200 °C for 12 h. After cooling, the products were filtered, washed with distilled water, and then dried at 80 °C for 12 h. In order to obtain homogeneous, single-phase KNN powders, the as-synthesized KNN powders were annealed at 600 °C. To investigate the hydrothermal reaction process and formation mechanism of the KNN powders, they were hydrothermally synthesized in 0.22 NaOH–0.78 KOH solutions with 9 M [OH−] for different times (1, 2, 4, 6, 8, and 12 h). The crystalline phases of the as-synthesized and annealed powders were analysed using X-ray diffractometry (XRD; D/max-2400, Rigaku, Japan) using a Cu K radiation. The composition of the powders was estimated by energy-dispersive spectroscopy (EDS; 51-XMX0013, Oxford, England). The microstructure and morphology were observed using scanning electron microscopy (SEM; QUANTA 450, FEI, USA), and high-resolution electron microscopy (HREM; Tecnai F30, FEI, USA) was used to study the structure of the KNN powders.

3. Results and discussion 3.1. Effects of the OH− ion concentration on NaNbO3 and KNbO3 powders Hydrothermal reactions of Nb2O5 with aqueous MOH (M = Na, K) were performed at 200 °C for 12 h with MOH concentrations ranging from 7.5 to 15 M. The XRD patterns of the reaction products of Nb2O5 with NaOH are shown in Fig. 1. The main diffraction peaks of all the products were assigned to the pure orthorhombic NaNbO3 phase (JCPDS 77-0873), indicating good crystallization. The XRD

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patterns of reaction products of Nb2O5 with KOH are shown in Fig. 2. When the KOH concentration was 7.5 M, the product was mainly composed of an impurity and orthorhombic KNbO3 (JCPDS 77-1098). The diffraction peaks of the impurity were indexed to potassium hexaniobate (K6H2[Nb6O19]·13H2O), whose X-ray diffraction patterns have not been published, as reported by Wang et al. and Santos et al. [17,18]. With the increase in the KOH concentration to 10 M and beyond, the impurity disappeared, and a pure, well-crystallized orthorhombic KNbO3 phase was obtained. According to previous works [16,18], an intermediate phase, sodium or potassium hexaniobate, forms first and then transforms into the MNbO3 (M = Na or K) phase during the hydrothermal reactions. In this study, potassium hexaniobate was present in the product synthesized in the 7.5 M KOH solution. However, it disappeared in the products synthesized with KOH concentrations of 10–15 M, which was attributed to the increase in [OH−], which accelerated the hydrothermal reaction speed and the transformation of the intermediate phase. In contrast, the XRD patterns of the powders recovered from synthesis in the 7.5 M NaOH solution did not show the presence of a sodium hexaniobate, which was attributed to the higher reaction speed to form NaNbO3 due to the higher reaction activity of Na+ than that of K+ [20]. The SEM micrographs show that the synthesized NaNbO3 powders consisted of cubic crystals with a grain size of less than 2.5 μm, and [OH−] did not obviously influence the cubic morphology and grain size. Representative SEM micrographs of the NaNbO3 powders are shown in Fig. 3. Fig. 4 shows SEM images of the KNbO3 powders synthesized with varying [OH−]: (a) 7.5 M, (b) 10 M, (c) 12.5 M, and (d) 15 M. The KNbO3 particles showed different morphologies at different [OH−], which agreed well by the results reported by Wang et al. [17]. Fig. 4a shows particles with irregular sizes and shapes. These particles were a mixture of potassium hexaniobate and KNbO3, as demonstrated by the XRD patterns in Fig. 2. Brick-like, porous KNbO3 crystals with a grain size of less than 1 μm are shown in Fig. 4b. When [OH−] was 12.5 M, rod-like KNbO3 crystals with rectangular sections were obtained, and they were several micrometres in length and about 500 nm in diameter (Fig. 4c). When [OH−] increased to 15 M, well-crystallized cubic KNbO3 crystals with a grain size of less than 2 μm were obtained (Fig. 4d) whose morphology was similar to that of the NaNbO3 crystals (Fig. 3). In previous studies, NaNbO3 and

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KNbO3 particles with similar morphologies were synthesized under similar hydrothermal reaction conditions and were proven to be single crystalline particles [17,19,21]. The above results indicated that increasing [OH−] tended to promote the formation of KNbO3 crystals and change their morphology. However, [OH−] had no effect on the phase or microstructure of the NaNbO3 crystals. Moreover, some growth steps were observed on the surfaces of the NaNbO3 and KNbO3 crystals (in Fig. 3 and 4), indicating that both NaNbO3 and KNbO3 crystals grew in the layer-by-layer mode. For sub-micron crystals grown from solutions, their morphology is mostly controlled by surface energy and tends to be equiaxial. When the crystals grow further, the different growth rates of their different crystal faces become a predominant factor in controlling the crystal morphology. Layer-by-layer growth of crystals occurs via the nucleation of new layers (i.e., the formation of stable two-dimensional nuclei) on the growth interface and lateral growth of the layers. The vertical growth rate of the growth interface is a function of the nucleation rate of new layers, which increases with an increase in the supersaturation in the solution. The morphology of microscale crystals is determined by the vertical and lateral growth rates [22]. As mentioned above, the increase in [OH−] promoted the reaction to form KNbO3. As a result, the supersaturation and the vertical growth rate increased, which might be a major reason for the change in morphology of the microscale KNbO3 crystals. The change in [OH−] exhibited little effect on the reaction to form NaNbO3 due to the relatively higher reaction activity of Na+ than that of K+. As a result, the growth of NaNbO3 crystals was not evidently affected by the increase of [OH-], and thus, their morphology remained unaltered.

3.2. Effects of the OH− ion concentration on (K,Na)NbO3 powders KNN lead-free piezoceramic powders were synthesized by a hydrothermal treatment at 200 °C for 12 h using the 0.22 NaOH–0.78 KOH solutions with varying [OH−] from 7.5 M to 15 M. The XRD patterns of the synthesized KNN samples are presented in Fig. 5. When [OH−] was 7.5 M, the product consisted mainly of an impurity and a Na-rich KNN phase whose patterns were similar to those of the orthorhombic NaNbO3. The XRD peaks of the impurity were isolated as K4Na4Nb6O19·9H2O (14-0360 in the PDF card), which were indexed to (K8–8xNa8x)Nb6O19·nH2O (a KNN-hydrate phase) because of

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the nonconstant K/Na ratio in previous studies [1,9,13]. When [OH−] was 9 M, the diffraction peaks belonging to (K8–8xNa8x)Nb6O19·nH2O were no longer present, and single-phase Na-rich KNN was obtained. With the increase in [OH−] to 10 M and higher, both Na-rich and K-rich KNN phases were observed in the XRD patterns. This indicated that the products were mixtures of a Na-rich KNN solid solution and a K-rich KNN solid solution rather than a single-phase solid solution. The K-value method [23] was adopted to ascertain the weight fraction of K-rich KNN in the two-phase mixtures based on their XRD patterns. As shown in Table 1, the weight fraction of the K-rich phase increased with the increase in [OH−]. It was also found that the two-phase mixtures could change into single-phase KNN after annealing. Fig. 6 shows that after annealing at 600 °C for 20 h, a single-phase Na-rich KNN solid solution was obtained from the as-synthesized powders prepared with the [OH−] of 10 M, and a single-phase K-rich KNN solid solution was obtained from the as-synthesized powders prepared with the [OH−] of 12.5 M and 15 M. As K and Na could not evaporate below 650 °C [13], the K/Na ratio in the annealed KNN particles should be consistent with the average K/Na ratio of the corresponding as-synthesized KNN particles. The K/Na molar ratio in the as-synthesized and annealed single-phase KNN powders was investigated by EDS. As shown in Table 1, the results indicated that the molar fraction of K gradually increased with the increase in [OH−]. The EDS results were in good agreement with the XRD results (Table 1), which indicated that the increase in [OH−] could promote more K to participate in the reaction to form KNN, thus resulting in a higher K/Na ratio. Therefore, the K/Na ratio in the single-phase KNN powders could be controlled by modifying [OH−] of the 0.22 NaOH–0.78 KOH solutions. As shown in Table 1, Na0.5K0.5NbO3, which was the MPB composition, could be obtained by a hydrothermal synthesis and subsequent annealing when [OH−] was between 10 M and 12.5 M in the starting alkaline solution. Fig. 7 shows the morphology of the KNN powders synthesized with varying [OH−] from 7.5 M to 15 M. All of the KNN particles were spherical aggregates of small crystallites, while the pure NaNbO3 and KNbO3 particles did not aggregate, as shown in Fig. 3 and 4. The spherical morphology of the KNN powders made them an ideal reinforcement additive for 0–3 type piezoelectric composites. Shi et al. [24,25] reported that the coalescence growth of crystallites occurs in two ways under hydrothermal

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conditions. The first type of coalescence growth is the recrystallization process, which is the mass transfer from small crystallites to large ones. In the second type of coalescence growth, crystallites connect with each other via specific structure-compatible surfaces to form aggregate particles with a special morphology. Evidently, in the present work, small KNN crystallites (Fig. 7) formed spherical aggregates via the second type of coalescence growth, while the pure NaNbO3 and KNbO3 particles (Fig. 3 and 4) formed via recrystallization. Spontaneous polarization is an important factor influencing the aggregation of small crystallites. NaNbO3 is antiferroelectric and has no intensity of spontaneous polarity because two contrary polar axes exist in a single domain. Thus, NaNbO3 crystallites do not easily aggregate [26]. In contrast, KNbO3 is a ferroelectric with spontaneous polarization because a single domain has only one single polar axis. The KNN solid solution, i.e., (K1-xNax)NbO3, is also ferroelectric over almost the whole composition range (i.e., x ranging from 0 to 0.99) and usually exhibits a larger spontaneous polarization than KNbO3 [27]. Interestingly, the KNN grains agglomerated, whereas the KNbO3 grains did not in this study. A larger spontaneous polarization increases attractions between small crystallites and thus promotes aggregation [24]. Because the driving force of the aggregation process is related to the reduction in surface energy, a higher surface energy also promotes aggregation [25,28,29]. Due to the smaller radius of Na+, substituting Na+ for K+ in the KNbO3 lattice can result in stronger interatomic bonds and thus a higher surface energy. Therefore, the KNN powders’ higher tendency to agglomerate than that of the KNbO3 powers could be attributed to the larger spontaneous polarization and surface energy. To determine the detailed structure of the KNN grain aggregates, the aggregates were ground by hand and then characterized by high-resolution transmission electron microscope (HRTEM). Fig. 8a–d shows a bright field TEM image and HRTEM images of the KNN powders synthesized with the [OH−] of 10 M. The HRTEM images and their fast Fourier transform (FFT) patterns confirmed that the KNN powders contained both the K-rich and Na-rich KNN phases. This result agreed well with the XRD results above (Fig. 5). It was also observed that the nano-sized grains connected with each other to form KNN aggregates. The grains ranged in size from several nanometres to tens of nanometres, thus proving that the spherical KNN aggregates formed via the second type of coalescence growth.

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3.3. Formation mechanism analysis of the perovskite (K,Na)NbO3 powders The phase evolution of the KNN powders hydrothermally synthesized from the 0.22 NaOH–0.78 KOH solution with 9 M [OH−] at 200 °C for different times was studied by XRD, as shown in Fig. 9. After the first hour of the hydrothermal treatment, the obtained powders were a mixture of a small amount of Nb2O5 and the (K8–8xNa8x)Nb6O19·nH2O phase. This indicated that Nb2O5 gradually reacted with the alkaline solution and changed into the (K8–8xNa8x)Nb6O19·nH2O phase during the hydrothermal heating procedure. When the hydrothermal treatment time increased to 2 h, the hexaniobate phase of (K8–8xNa8x)Nb6O19·nH2O disappeared, and only the perovskite phases of KNN were observed. This implied that the hexaniobate (K8–8xNa8x)Nb6O19·nH2O was an intermediate compound throughout the reaction of Nb2O5 and KOH/NaOH to produce KNN powders. The perovskite phases of KNN consisted of a dominant K-rich KNN phase and a minor Na-rich KNN phase. After all of the Nb2O5 was converted into KNN, the amount of Na-rich KNN phase increased in the obtained powders with the further increase in the processing time, while that of the K-rich KNN phase decreased. Finally, the Na-rich KNN phase became the majority phase in the product after 12 h of the hydrothermal treatment. Table 2 shows the phase evolution, weight fraction of K-rich KNN, and K/Na molar ratio in the as-synthesized KNN powders synthesized at 200 °C with 9 M [OH−] for different times. The results indicated that more K participated in the reaction to form KNN than Na in the early stage of the hydrothermal reaction because the K+ concentration was higher than the Na+ concentration in the solution. As the reaction proceeded, the K+ concentration in the solution decreased, and Na+ gradually substituted for K+ in the KNN product due to the higher reaction activity of Na+ than K+ to form MNbO3, which resulted in the Na-rich KNN phase becoming the majority phase after 12 h of hydrothermal treatment. As mentioned above, the weight fraction of K-rich KNN phase in the products synthesized for 12 h increased with the increasing [OH−] in the starting solutions with K/Na = 3.5:1 (see Table 1), which indicated that the substitution of Na+ for K+ became more difficult with the increase in [OH−]. Therefore, it could be concluded that the KNN formed according to the following procedure: (8 − 8x) KOH + 8x NaOH + 3 Nb2O5 + (n − 4) H2O → (K8-8xNa8x)Nb6O19·nH2O

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(1)

(K8-8xNa8x)Nb6O19·nH2O → 6 (K1-yNay)NbO3 + (n − 1) H2O + (2 − 8x + 6y) KOH + (8x − 6y) NaOH (2) (K1-yNay)NbO3 + z NaOH → (K1-y-zNay+z)NbO3 + z KOH

(3)

Evidently, KNN powders with different molar ratios of K-rich phase to Na-rich phase (i.e., different average ratios of K/Na) could also be synthesized by controlling the hydrothermal reaction time when [OH−] was constant in the starting solution. The SEM images of the KNN powders synthesized from the 0.22 NaOH–0.78 KOH solution with the [OH−] of 9 M at 200 °C for different times illustrate the evolution of the powder morphology during the hydrothermal reaction, as shown in Fig. 10. When the reaction time was 2 h, the KNN-grain aggregates were principally octahedral, as shown in Fig. 10a. The second type of coalescence growth mode involved oriented particle aggregation [28,29], which was termed conceptually as ‘oriented attachment’ by Penn et al., Banfield et al., and Wu et al. [30–32]. In this mechanism, small crystallites rotate to self-assemble with adjacent crystallites, thus sharing a common crystallographic orientation. The octahedral shape of the KNN-grain aggregates may be attributed to oriented attachment. A similar morphology has been reported in studies on BaMoO4 grain aggregates, which were caused by this mechanism [29,32]. As the reaction proceeded, the octahedrons gradually changed to spherical aggregates (Fig. 10b and c). When the reaction time increased to 12 h, almost all aggregates became spherical (Fig. 7b). After 2 h, all of the Nb2O5 reacted with KOH/NaOH, and the intermediate hexaniobate phase completely converted into perovskite KNN, as shown by the XRD patterns in Fig. 9. Therefore, we thought that the change in particle morphology occurred because the surface atoms tended to find new equilibrium positions in order to decrease the surface energy [29].

4. Conclusions NaNbO3, (K,Na)NbO3, and KNbO3 powders were hydrothermally synthesized in (1 − y) NaOH–y KOH solutions ([OH−] = 7.5–15 M) with y = 0, 0.78, and 1 at 200 °C for 12 h, respectively. The NaNbO3 and KNbO3 had sub-micron- or micron-sized grains which formed via the first type of coalescence growth, while the (K,Na)NbO3 particles were spherical aggregates of nano-grains which formed via the second type of coalescence growth. All of the NaNbO3 particles were cubes, independent

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of [OH−]. The morphology of the (K,Na)NbO3 nano-grain aggregates was also independent of [OH−] and changed from octahedral to spherical with an increase in the hydrothermal-reaction time from 2 h to 12 h. The morphology of the KNbO3 particles depended on [OH−] and was brick-like, rod-like and cubic when [OH−] was 10, 12.5, and 15 M, respectively. [OH−] had an important influence on the phase and composition of the (K,Na)NbO3 powders synthesized for 12 h. When [OH−] was 7.5 M, the powders mainly consisted of the Na-rich (K,Na)NbO3 phase, with a few intermediate hexaniobate species. When [OH−] increased to 9 M, only the Na-rich KNN phase was present. When [OH−] was more than 10 M, the Na-rich and K-rich KNN phases coexisted in the products. After annealing at 600 °C for 20 h, the two phases coexisting in the products converted into a completely single-phase Na-rich or K-rich solid solution, which depended on the [OH−] of the starting alkaline solution. Na0.5K0.5NbO3 with a MPB could be obtained by hydrothermal synthesis and subsequent annealing when [OH−] was between 10 M and 12.5 M. The reaction time was also an important factor influencing the phase and composition of the as-synthesized (K,Na)NbO3 product. Nb2O5 reacted with 0.22 NaOH–0.78 KOH, first forming an intermediate phase (K8-8xNa8x)Nb6O19·nH2O, which then tended to transform into a K-rich KNN phase. Finally, Na elements gradually substituted for K in the (K,Na)NbO3 phase with the further increase in reaction time.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51002019 and 91016024) and the Open-End Fund for Instruments of Dalian University of Technology.

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Fig. 1. XRD patterns of the products synthesized at 200 °C in the NaOH solutions with different [OH −]: (a) 7.5 M, (b) 10 M, (c) 12.5 M, and (d) 15 M. Fig. 2. XRD patterns of the products synthesized at 200 °C in the KOH solutions with different [OH−]: (a) 7.5 M, (b) 10 M, (c) 12.5 M, and (d) 15 M. Fig. 3. SEM micrographs of the products synthesized at 200 °C in the NaOH solutions with the [OH−] of (a) 7.5 M and (b) 15 M. Fig. 4. SEM micrographs of the products synthesized at 200 °C in the KOH solutions with different [OH−]: (a) 7.5 M, (b) 10 M, (c) 12.5 M, and (d) 15 M. Fig. 5. XRD patterns of the products synthesized at 200 °C for 12 h in the 0.22 NaOH–0.78 KOH solutions with different [OH−]: (a) 7.5 M, (b) 9 M, (c) 10 M, (d) 12.5 M, and (e) 15 M. Fig. 6. XRD patterns of the KNN samples synthesized at 200 °C for 12 h with the [OH −] of (a) 10 M, (b) 12.5 M and (c) 15 M after annealing at 600 °C for 20 h. Fig. 7. SEM micrographs of the products synthesized at 200 °C for 12 h in the 0.22 NaOH–0.78 KOH solutions with different [OH−]: (a and b) 7.5 M, (c and d) 9 M, (e and f) 10 M, (g and h) 12.5 M, and (I and K) 15 M. Fig. 8. TEM image (a) of the ground KNN grain aggregates synthesized from the starting solution with the [OH−] of 10 M at 200 °C for 12 h and HRTEM images of (b) the K-rich KNN phase and (c and d) the Na-rich KNN phase in the ground aggregates (insets are the corresponding FFT patterns). Fig. 9. XRD patterns of the products synthesized in the 0.22 NaOH–0.78 KOH solution with 9 M [OH−] at 200 °C for different times: (a) 1 h, (b) 2 h, (c) 4 h, (d) 6 h, (e) 8 h, and (f) 12 h. Fig. 10. SEM micrographs of the products synthesized in the 0.22 NaOH–0.78 KOH solution with 9 M [OH−] at 200 °C for different times: (a) 2 h, (b) 4 h, and (c) 6 h.

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Fig. 1 (d)

(c)

(b) (a)

Fig. 2 (d)

(c)

(b)

(a)

15

Fig. 3 (b)

(a)

1 µm

1 µm

Fig. 4 (a)

(b)

1 µm

1 µm

(d)

(c)

1 µm

1 µm

16

Fig. 5 (e)

(d) (c) (b) (a)

Fig. 6 (c)

(b)

(a)

17

Fig. 7 (a)

(b)

1 µm

2 µm (c)

(d)

2 µm

1 µm (f)

(e)

1 µm

2 µm (g)

(h)

2 µm

1 µm (k)

(I)

1 µm

2 µm

18

Fig. 8 (a)

Fig. 9

(f) (e) (d) (c) (b) (a)

19

Fig. 10 (b)

(a)

8 µm

2 µm

(c)

(d)

8 µm

2 µm

(e)

(f)

8 µm

2 µm

20

Table 1 XRD and EDS results of the as-synthesized and annealed KNN powders synthesized at 200 °C with different [OH−]. As-synthesized KNN powders

Annealed KNN powders

−

[OH ] Phase

Weight fraction of K-rich KNN

K/Na molar ratio

Phase

K/Na molar ratio

7.5 M

Mixture of Na-rich KNN and KNN-hydrate phase

—

—

—

—

9M

Na-rich KNN

≈ 0%

25:75

—

—

10 M

Mixture of Na-rich and K-rich KNN

38%

—

Na-rich KNN

40:60

12.5 M

Mixture of Na-rich and K-rich KNN

76%

—

K-rich KNN

61:39

15 M

Mixture of Na-rich and K-rich KNN

89%

—

K-rich KNN

69:31

Table 2 XRD and EDS results of the as-synthesized KNN powders synthesized at 200 °C for different times with 9 M [OH−]. Synthesis time

Phase

Weight fraction of K-rich KNN

Average K/Na molar ratio

1h

Mixture of Nb2O5 and (K8– 8xNa8x)Nb6O19·nH2O

—

—

2h

Mixture of Na-rich and K-rich KNN

78%

64:36

4h

Mixture of Na-rich and K-rich KNN

73%

59:41

6h

Mixture of Na-rich and K-rich KNN

54%

52:48

8h

Mixture of Na-rich and K-rich KNN

44%

43:57

12h

Na-rich KNN

≈ 0%

25:75

21