- Email: [email protected]
Excellent optical transparency of potassium-sodium niobate-based lead-free relaxor ceramics induced by fine grains
Journal of the European Ceramic Society 39 (2019) 3684–3692
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Excellent optical transparency of potassium-sodium niobate-based lead-free relaxor ceramics induced by fine grains
T
Xiaolian Chaoa, , Xiaodan Rena, Xiaoshuai Zhanga, Zhanhui Penga, Juanjuan Wangb, ⁎ Pengfei Liangc, Di Wua, Zupei Yanga, ⁎
a
Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710062, Shaanxi, China b School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, Shaanxi, China c School of Physics and Information Technology, Shaanxi Normal University, Xi’an, 710062, Shaanxi, China
ARTICLE INFO
ABSTRACT
Keywords: Transparent ceramics Grain size Relaxor behavior Electrical properties
Here we report a lead-free multifunctional material (1-x)(K0.5Na0.5)NbO3-xBa(Mg1/3Nb2/3)O3 prepared by pressure-less sintering procedure. X-ray diffraction indicates a gradual crystal structure transformation with increasing x. Microstructural observation demonstrates that the addition of Ba(Mg1/3Nb2/3)O3 additive reduces the size of grains with clear grain boundary, which is favorable for a high optical transmittance. The relaxor characteristics of the ceramics could lead to further enhancement of the transparency, owing to the low defects and weak light scattering. Notably, the ceramics with 0.04 ≤ x ≤ 0.06 all show good transparency over 70% in visible region and 80% in infrared region.
1. Introduction
The transmittance of transparent ceramics with a thickness t can be expressed by the following formula: [17]
Transparent ceramics are one of the most valuable and widely used functional materials, drawing growing interest nowadays [1–3]. Lead lanthanum zirconate titanate (PLZT), as a typical representative of transparent ferroelectric ceramics, has great potential in optical devices and transparent electronic device, such as color filters, light shutters, image storage, devices modulators, displays, e-readers, and smart phones, due to the excellent optical and electrical properties. However, the pollution of lead-based ceramics to the environment and the threat to human health, wihch make the preparation of a sustainable environmentally friendly material has become the most urgent and challenging issue for the development of optoelectronic device [4–6]. Among the transparent ferroelectric ceramics, KNN-based lead-free ceramics have drawn intensive attentions [6–14]. For example, the KNN-based ceramics with relatively high transmittance of 60% in both visible and near-IR region, and very high electro-optic coefficient were reported by Kwork et al. [15]. Du et al also reported (K0.5Na0.5)NbO3-Sr (Sc0.5Nb0.5)O3-based ceramics exhibiting an excellent transmittance of 60% in visible region along with a decent energy storage density [16]. However, the transmittance of those KNN-based ceramics still cannot match that of conventional lead-based ceramics, such as PLZT based ceramics, which possess an over 70% transmittance in visible region.
R2) exp (
T = (1
t)
(1)
where t is the sample thickness a, β is the scattering coefficient neglecting the absorption (generally extinction coefficient) and R is the reflectivity. The scattering coefficient β can be calculated by the given Eq. (2): [18]
=
p 4 3
r3
CS
(2)
where CS is the scattering cross section of the spherical pores, r is the pore radius, p is the porosity volume fraction and β is the scattering coefficient. According Eqs. (1) and Eq. (2), previous studies exhibited that a high-transmittance ceramic shall possess high density, absence of pores and second phase, isotropic lattice structure, small grain size, all of which are needed to reduce some light scattering [19,20]. Moreover, homogeneous and uniform grains can also reduce the light scattering, as confirmed by R Apetz [21]. Despite of its negative effect on transmittance as mentioned above, the addition of secondary compound was found capable of modulating the grain size distribution and reducing the grain size in KNN-based
⁎ Corresponding authors at: Key Laboratory for Macromolecular Science of Shaanxi Province, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an, 710062, Shaanxi, China. E-mail addresses: [email protected] (X. Chao), [email protected] (Z. Yang).
https://doi.org/10.1016/j.jeurceramsoc.2019.04.053 Received 15 March 2019; Received in revised form 25 April 2019; Accepted 29 April 2019 Available online 30 April 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
ceramics. For instance, the grain size of KNN-based ceramics changes from 3 to 4 μm to 0.6 μm after doping with Bi0.5(Na0.925Li0.075)0.5ZrO3, and the transparency have an obviously improved [22]. Geng et al reported that [(K0.5Na0.5)1-2x(Sr0.75 Ba0.25)x]0.93Li0.07Nb0.93Bi0.07O3 ceramics have uniform and fine grain size, as well as a high transmittance of 60% in the near-IR region [10]. Yang et al. reported that KNN-based ceramics could achieve an excellent transparency (74% in visible region, comparable to that of PLZT) upon the addition of proper LaBiO3, resulting from the reduced grain size (to ˜100 nm) [14]. Besides, our previous work [23] by adding secondary component Sr(Mg1/3Nb2/3)O3 to KNN ceramics was prepared, this results indicated that the transparency is very poor (60% in the near-IR region) due to the relatively large grain size. Inspired by the doping of PZT-based ceramics at A-site to inhibit grain growth [24], we incorporated Ba2+ with ionic radii less than K+ and Na+ into KNN-based ceramics in the hope of achieving grain refinement, thereby obtained a ceramic that combines the excellent electrical and superior optical properties. Fig. 2. Optical transmittance spectra of the KNN-xBaMN ceramics.
2. Experimental procedures
(3)
( hv ) 2 = A(hv-Eg )
High transparent ceramics with a nominal composition of (1-x) (K0.5Na0.5)NbO3-xBa(Mg1/3Nb2/3)O3 (KNN-xBaMN) with 0.01 ≤ x ≤ 0.09 were synthesized by the conventional solid-state method as outlined in our previous work [23,25]. The starting raw materials are Na2CO3 (Ourchem 99.99%), K2CO3 (Ourchem 99.99%), SrCO3 (Aladdin 99.99%), Nb2O5 (Sinopharm 99.99%) and MgO (Aladdin 99.99%). The dried powders were calcined at 900 °C for 5 h, and sintered 1200 °C for 6 h soaking period in air in covered alumina crucibles. Other intermediate steps were also carried out as outlined in previous work, including milling, granulation, pressing into a late-shape extrusion and so on [23]. Finally, the phase structure, optical transmittance, mass density, microstructures, ferroelectric performance and dielectric property were measured in the same manner as reported previously [23,25]. In particular, the samples used for the measurement of optical transparency required to be ground to a thickness of 0.5 mm and polished to a parallel and smooth surface using the diamond spray polishing agent with 1 μm.
where h is Planck’s constant, ν is the photon frequency and A is a constant. Through the transmittance T, the absorption coefficient can be determined, as shown in the following formula:
=
1 1 ln t T
(4) 2
where t is the thickness of the sample. From the plotting (αhν) versus hν and extrapolating the linear portion of the curve to zero, the Eg of KNN-xBaMN ceramics can be determined. The calculated Eg of the KNNxBaMN ceramics as a function of BaMN content are summarized in Fig. 3(a–i). Visibly, the Eg values goes up initially and then decreases slightly. The values of Eg reaches a maximum value of 3.12 eV at KNN0.06BaMN ceramic, suggesting BaMN plays an important role in interband transition. Furthermore, the excellent transparent ceramics with x = 0.06 accompany with higher Eg values (3.12 eV) than the reported values of KNN nanorods (3.09 eV) [26]. In KNN perovskite ceramics, the electronic occupied state of NbO6 octahedra determines the bandgap energy value, as also reported by Liu et al. [27]. In KNN-xBaMN ceramics, the addition of smaller alkaline earth cation (Ba2+) changes the electron occupied state and enlarges the band-gaps [28]. The FE-SEM images for the fractured surfaces of KNN-xBaMN ceramics are shown in Fig. 4(a–e). Many little pores can be observed when x = 0.01, this may be caused by the higher sinter temperature (1200 °C). According to Eqs. (1) and (2), when the crystal structure is isotropic, the optical scattering by pore in ceramics should be considered as the major factor to weaken the optical transparency of ceramics. Thus, the ceramic of KNN-0.01BaMN shows very low transparency (< 40%) in the visible wavelength. With increasing x content, the amount of pores decreases, and the densification degree of fracture surface tends to be higher. The Fig. 4(f) shows the dependence of relative density on x content for the KNN-xBaMN ceramics. The relative density increases initially with increasing x from 0.01 to 0.06 where the highest value over 99% is achieved, and then decreases after x > 0.06. The high relative density and low porosity are beneficial for the transparency; thus, the KNN-xBaMN ceramics with x from 0.03 to 0.06 possess relatively higher transparency values. SEM images of polished and thermally etched cross-sections of the
3. Results and discussion Fig. 1 shows an intuitive photograph of the KNN-xBaMN ceramics with a thickness of 0.5 mm. It can be seen from the figure that with the increase of x content from 0.01 to 0.06, the letters under the samples gradually become clear, which indicated that the ceramics have changed from translucency to optical transparency. The transmission spectrum in Fig. 2 also verifies the enhancement of optical transparency. With increasing x from 0.01 to 0.09, the transmittance of ceramics have an increases initially and then decreases in 200 ˜ 2000 nm. In particular, the ceramics with 0.04 ≤ x ≤ 0.06 exhibit superior transmittance over 80% in the infrared region and 70% in the visible region, which is comparable to the highest ever value of reported ceramics [9,11,12,14,16]. As is apparent from Fig. 2, the transmittance of the KNN-xBaMN ceramic is zero below 400 nm, which could be caused by the inter-band transition. From the absorption spectra, the optical band gap energy Eg can be estimated according to the Tauc equation [10]. For direct transition, the relationship the between absorption coefficient α and Eg can be expressed as:
Fig. 1. Photograph of the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09.
3685
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 3. Plots of (αhν)2 versus hν, energy band gap Eg for the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09.
Fig. 4. (a–e) SEM images on fracture of the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.03; (c) x = 0.05; (d) x = 0.06; (e) x = 0.08, (f) Relative density of the KNN-xBaMN ceramics.
3686
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 5. SEM images of polished and thermally etched cross-sections of the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09 (g) Dependence of the mean grain size on the composition x.
KNN-xBaMN pellets are present in Fig. 5(a–i). As observed, the addition of BaMN has a crucial effect on the microstructure of KNN-xBaMN ceramics, especially on the grain size. Sample with x = 0.01 exhibits porous microstructure and large grains; as x increases, the pellet seems more dense and the grains become fine and uniform. The grain size distribution of each KNN-xBaMN ceramic is shown in the inset of each figure. And we showed the relationship between the average grain size of KNN-xBaMN ceramics as a function of BaMN content in Fig. 5(g). It can been found that the average grain size trend to decrease as x increases. Furthermore, the KNN-0.01BaMN ceramic has a very broad grain distribution range from 150 nm to 1 μm, while that of the ceramics with x ≥ 0.03 is only from 50 nm to 300 nm. The gradual grain size reduction may be caused by the Ba2+ doping on K+ or Na+ site, which is called “donor doping” and can suppress grain growth. Similar results were already observed in many La-doped perovskite-type ceramics [29–33]. The KNN-0.06BaMN have the glorious transmittance value of 85% in the near infrared wavelength due to the smallest grain size along with uniform size distribution close to 100 ± 25 nm. Fig. 6(a) display the XRD patterns of KNN-xBaMN ceramics with 2θ from 20 to 80°. All ceramics exhibit pure perovskite structure without second phase formation. In order to analyze the effect of BaMN content on the phase structure of ceramics, the characteristic diffraction peaks near 45° of the sample were amplified to Fig. 6(b). The (200) reflection of KNN-xBaMN ceramics with x ≤ 0.02 is split into two diffraction peaks of (202)O/(020)O. It means that the ceramics exhibits an orthorhombic phase structure at room temperature. Obviously, the two diffraction peaks merge into one sharp (200) diffraction peak with x increases, indicateding that ceramics are transformed into the pseudocubic phase structures with high symmetry and similar lattice constants at room temperature. Moreover, the calculated lattice parameters of KNN-xBaMN ceramics are used to further study the effect of BaMN content on phase structure, as shown in Fig. 6(c). The a, b, c values of ceramics with x ≤ 0.02 is quite different from each other, suggesting the ceramics are at low-symmetry orthorhombic phase. As x ≥ 0.03, a, b, c values are getting closer, indicating the ceramics may have pseudocubic structure. Especially, a, b, c value of the ceramic with x = 0.06 is almost the same, suggesting this ceramic is nearly cubic. This explains the observed only one diffraction index (XRD) and lowest light scattering among all ceramics. Fig. 7(a–g) represents the dielectric loss and dielectric constant of temperature-dependent KNN-xBaMN ceramics under various measuring frequencies. For the ceramics with x = 0.02, it is obvious that two dielectric peaks representing the phase transitions from orthorhombic to tetragonal (TO-T) and from tetragonal to cubic (TT-C) are observed at
Fig. 6. XRD patters for the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09.
181 °C and 375 °C, respectively. The temperature of TO-T increases while that of TT-C decreases with x increasing, eventually merging into one peak after x ≥ 0.03, Fig. 7(c–h). The maximum dielectric constant (εm), dielectric loss and the temperature of the maximum dielectric constant (Tm) of the KNN-xBaMN ceramics at the frequency of 1 MHz are counted as a function of x content in Fig. 7(i). It could be due to the dramatic reduction of grain size that Tm and εm are observed to decrease with the increase of component content from Fig. 7(i). In addition, the phenomenon that εm decreases with increasing frequency was named 3687
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 7. (a–g) Temperature dependence of dielectric constant of the KNN-xBaMN ceramics measured under various measuring frequencies: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.05; (e) x = 0.06; (f) x = 0.08; (g) x = 0.09, (h) Temperature dependence of dielectric constant and dielectric loss at 100 kHz of the KNNxBaMN ceramics, and (i) Temperature at maximum dielectric constant (Tm), the max dielectric constant (εm) and the dielectric loss at 100 kHz for the KNN-xBaMN ceramics.
diffuse phase transition caused (DPT) by heterogeneous doping, which can aggravate the fluctuation of composition [9,16,22,27]. The εm peaks shift toward higher temperatures with increasing the measured frequency suggests the ceramics transform from ferroelectric ceramics to relaxor ceramics. In generally, the reciprocal of the dielectric constant and temperature obey the Uchino and Nomura function regard as a modified CurieWeiss law, which is used to characterize relaxor ferroelectrics [34]:
1
1 m
=
(T
Tm) C
which can reduce the scattering of light at the grain boundaries and weaken the optical anisotropy. Impedance spectra at different frequencies were used to better elucidate the relaxation mechanism of KNN-xBaMN ceramics. The complex plane plots of KNN-0.06BaMN ceramics measured at 300 °C–540 °C is shown in Fig. 9(a). As observed, the impedance pattern almost appears a linear straight line when the test temperature less than 420 °C. However, as the measuring temperature increases, the image transforms into a semicircle of a depression centered below the Z' axis. This phenomenon indicates that the ceramic exhibits a non-Debye type relaxation with multiple relaxation times [36,37]. Moreover, the intercept of the semi-circular arc on the z-axis, that is, the resistance of the ceramic, gradually approaches the origin with the rise of temperature. It is further confirm the negative temperature coefficient of resistance (NTCR) behavior of the ceramic. In Generally, when two semicircles appear in the impedance spectrum, they correspond to the response of grain and grain boundary respectively [38]. The complex impedance spectra of KNN-xBaMN ceramics measured at 540 °C are displayed in Fig. 9(b), all the nine ceramics consist of a semi-circular arc, which indicates that the ceramics are electrically homogeneous structure and mainly correspond to grain response [25,38,39]. From the imaginary parts diagram of the electric modulus (Fig. 9(c)) for the KNN0.06BaMN at different temperatures, we can see that there is only one peak of the electrical modulus in all temperature ranges, which indicates that only one carrier contributes to the dielectric response, that is, the grain response. With the increase of test temperature, the peak value of electrical modulus moves towards high frequency and increases, which indicates that ceramics undergo a temperature-dependent relaxation process, i.e. thermal activation process. When the frequency range is lower than M peak, it corresponds to the long-range migration of ions, while when the frequency range is higher than M peak, the jump of ions is confined to the short ranges [40].
(5)
Where γ gives information on the character of the phase transition, C is the Curie-like constant. The value of γ from 1 for normal ferroelectrics to 2 for typical relaxor ferroelectrics. Fig. 8(a–i) shows the plots of log (1/ε−1/εm) as a function of log(T–Tm) at 1 MkHz for the KNN-xBaMN ceramics, and the γ value was determined by slope of the fitting curves. The γ value of the KNN-0.01BaMN ceramics is close to 1, suggesting the ceramics is closer to typical ferroelectrics. With increasing x content, γ increases dramatically, indicating that the ceramics become more relaxor-like. In particular, the γ value of ceramics with 0.04 ≤ x ≤ 0.07 are 1.95, 1.97, 1.99, 1.98, respectively, suggesting these ceramics have already transformed into relaxor ceramics. Generally speaking, polar nano-regions (PNRs) caused by component fluctuations are the direct reason for obtaining relatively large γ values [3]. In KNN-xBaMN ceramics, the introduction of Ba2+ ions with radius less than K+ and Na+ increases the fluctuation of local components and the disorder degree of local electric field and local elastic field, and then induces the formation of PNRs or nanostructure domains, which effectively enhances the relaxation behavior of ceramics [2,3,35]. Furthermore, Nb5+ (0.64 Å) and Mg2+ (0.89 Å) with different ionic radii also have similar mass effects on B-sites of KNN lattice. The high transparency of KNN-xBaMN ceramics is due to the pseudo-cubic phase structure with high symmetry formed by the appearance of a large number of PNRs, 3688
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 8. Log(1/ε−1/εm) as a function of log(T−Tm) at 1 MHz for the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09. Fig. 9. a) Complex impedance spectra for the KNN-0.06BaMN ceramics measured at different temperatures; (b) complex impedance spectra for the KNN-xBaMN ceramics measured at540 °C; (c) imaginary parts of the electric modulus for the KNN-0.06BaMN at different temperatures; (d) the equivalent circuit model instead of a pure capacitor for representing the electrical response in the ceramics.
3689
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 10. ln(fmax) as a function of 1000/T for the KNN-xBaMN ceramics: (a) x = 0.02; (b) x = 0.04; (c) x = 0.05; (d) x = 0.06; (e) x = 0.08; (f) x = 0.09.
Fig. 11. (a) P-E hysteresis loops of KNN-xBaMN ceramics; (b) Remnant polarization (Pr) and coercive field (Ec) of KNN-xBaMN ceramics as a function of x content.
Furthermore, as shown in the Fig. 9(d), the constant phase element (CPE) is usually used to represent the electrical response in ceramics instead of pure capacitors in the equivalent circuit model. The activation energy can be obtained by fitting the peak values of the electrical modulus (f max) at different temperatures in the electrical modulus map as follows [41]:
fmax = f0 exp
Ea kT
where k is the Boltzmann constant, T is the absolute temperature, Ea is the activation energy associated with the relaxation process and f0 is the pre-exponential factor. The plots of ln(fmax) as a function of 1000/T for the KNN-xBaMN ceramics and linear fittings are shown in Fig. 10. The activation energies (Ea) obtained from the slopes of fitting lines are estimated to be 1.13–1.83 eV. The higher value of activation energy, fewer defects exist in the ceramics. It is known that the value of activation energy is inversely proportional to the defect concentration [42].
(6) 3690
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 12. (a) Variations in P–E hysteresis loops of KNN–0.06BaMN ceramics at room temperature under different electric fields; (b) P–E hysteresis loops at their critical breakdown strength for the KNN-xBaMN ceramics; (c) Total energy storage density (W) and recoverable energy storage density (Wrec) of KNN-xBaMN ceramics as a function of x content.
For transparent ceramics, the effect of defects on the transmittance is inevitable [27]. The smaller the number of defects, the lower the optical scattering and the better the transmittance in the ceramics [43]. In addition, the Ea value of KNN-xBaMN ceramics is about half of the band gap energy in Fig. 3, which indicates that the ceramics have intrinsic band-type conduction. The room temperature P-E hysteresis loops of KNN-xBaMN transparent ceramics are demonstrated in Fig. 11(a). All P-E loops possess a slim-like shape and become almost linear with increasing BaMN content. Since slim P–E loop is a typical characteristic of relaxor ferroelectric ceramics, the very slim P-E loops of ceramics with x ≥ 0.03 further confirm their relaxor behaviors. This phenomenon also displays that the ferroelectric properties of KNN–BaMN ceramics exhabits a deteriorating trend with increasing BaMN content. From the Fig. 11(b), as x increase, the remnant polarization (Pr) of the KNN-xBaMN have a significant decrease. Nevertheless, the ceramics with 0.03 ≤ x ≤ 0.06 can obtain excellent transparency (over 80% in the infrared wavelength and 70% in the visible region) accompanied with relatively high electrical properties (over 2 μC/cm2). The P–E hysteresis loops of KNN-0.06BaMN ceramics under different electric fields at room temperature is depicted in Fig. 12(a). In general, the electrical energy storage density W of the relaxed ferroelectric ceramic can be calculated by the hysteresis loop [12,44]. It can be seen from the Fig. 12(a) that the maximum polarization of KNN0.06BaMN ceramics increases from 7.2–13.4 μC/cm2 with electric fields increasing from 50 to 100 kV/cm, which indicates that higher energy density can be obtained by enhancing the dielectric strength of ceramics. Fig. 12(b) gives the P–E hysteresis loops of KNN-xBaMN under the highest dielectric breakdown strength at room temperature. The dielectric strength of the ceramics gets enhanced with increasing x, which may be due to the contribution of fine and dense grains. The calculated total energy storage density (W) and recoverable energy storage density (Wrec) of KNN-xBaMN ceramics are listed in Fig. 12(c). As seen, the W and Wrec have an obviously enhancement with the increase of component content. Notably, the KNN-0.08BaMN ceramics possess the highest values of Wrec ˜ 0.8 J/cm3. It could due to the maximum breakdown strength (Eb) at x = 0.08, which further proves that it is feasible to obtain a high Wrec by increasing Eb [2]
grain and lower defects are considered to be the reasons for the excellent transmittance of this system. Acknowledgments This work was supported by National Science Foundation of China (NSFC) (Grant No. 51607108, 51872177, 51572163, 51577111) and the Fundamental Research Funds for the Central Universities (Program No. GK201903017, GK201802007 and No.GK201701011). References [1] P. Günter, Coherent light amplification and optical phase conjugation with photo refractive materials, Phys. Rep. 4 (1982) 199–299. [2] Z.T. Yang, F. Gao, H.L. Du, L. Jin, L.L. Yan, Q.Y. Hu, Q. Yu, S.B. Ying, X.Y. Wei, X. Zhuo, Y.J. Wang, Grain size engineered lead-free ceramics with both large energy storage density and ultrahigh mechanical properties, Nano Energy 58 (2019) 768–777. [3] X. Lv, J.G. Wu, Effects of a phase engineering strategy on the strain properties in KNN-based ceramics, J. Mater. Chem. C 7 (2019) 2037. [4] G.R. Li, W. Ruan, J.T. Zeng, H.R. Zeng, L.Y. Zheng, L.S. Kamzina, Y. Kopylov, V. Kravchenko, The effect of domain structures on the transparency of PMN–PT transparent ceramics, Opt. Mater. 35 (2013) 722–726. [5] M. Veithen, X. Gonze, P. Ghosez, First-principles study of the electro-optic effect in ferroelectric oxides, Phys. Rev. Lett. 93 (2004) 187401. [6] X. Zeng, X. He, W. Cheng, P. Qiu, B. Xia, Effect of Dy substitution on ferroelectric, optical and electro-optic properties of transparent Pb0.90La0.10(Zr0.65Ti0.35)O3 ceramics, Ceram. Int. 40 (2014) 6197–6202. [7] T. Zheng, J. Wu, D. Xiao, J. Zhu, Recent development in lead-free perovskite piezoelectric bulk materials, Prog. Mater. Sci. 98 (2018) 552–624. [8] M. Kosec, V. Bobnar, M. Hrovat, J. Bernard, B. Malic, J. Holc, New lead-free relaxors based on the K0.5Na0.5NbO3–SrTiO3 solid solution, J. Mater. Res. 19 (2004) 1849–1854. [9] F. Li, K.W. Kwok, Fabrication of transparent electro-optic (K0.5Na0.5)1−xLixNb1−xBixO3 lead-free ceramics, J. Eur. Ceram. Soc. 33 (2013) 123–130. [10] D. Damjanovic, N. Klein, J. Li, V. Porokhonskyy, What can be expected from leadfree piezoelectric materials? Funct. Mater. Lett. 3 (2010) 5–13. [11] D. Yang, C. Ma, Z. Yang, L. Wei, X. Chao, Z. Yang, J. Yang, Optical and electrical properties of pressureless sintered transparent (K 0.37Na0.63)NbO3-based ceramics, Ceram. Int. 42 (2016) 4648–4657. [12] B. Qu, H. Du, Z. Yang, Lead-free relaxor ferroelectric ceramics with high optical transparency and energy storage ability, J. Mater. Chem. C 4 (2016) 1795–1803. [13] Q. Chai, X. Zhao, P. Liang, D. Wu, X. Chao, Z. Yang, Excellent near-infrared transparency realized in low-symmetry orthorhombic (K,Na)NbO3-based submicron ceramics, Scr. Mater. 154 (2018) 64–67. [14] Z. Yang, X. Zhang, D. Yang, B. Yang, X. Chao, L. Wei, Z. Yang, R.J. Xie, Excellent transmittance induced phase transition and grain size modulation in lead-free (1-x) (K0.5Na0.5)NbO3-xLaBiO3 ceramics, J. Am. Ceram. Soc. 99 (2016) 2055–2062. [15] K.W. Kwok, F. Li, D. Lin, A novel lead-free transparent ceramic with high electrooptic coefficient, Funct. Mater. Lett. 04 (2011) 237–240. [16] H. Du, W. Zhou, D. Zhu, L. Fa, S. Qu, Y. Li, Z. Pei, Sintering characteristic, microstructure, and dielectric relaxor behavior of (K0.5Na0.5)NbO3-(Bi0.5Na0.5)TiO3 lead-free ceramics, J. Am. Ceram. Soc. 91 (2008) 2903–2909. [17] R.M.J.G.J. Peelen, Light scattering by pores in poly-crystalline materials, J. Appl. Phys. 45 (1974) 216–220. [18] U. Anselmi-Tamburini, J.N. Woolman, Z.A. Munir, Transparent nanometric cubic and tetragonal zirconia obtained by high pressure pulsed electric current sintering, Adv. Funct. Mater. 17 (2007) 3267–3273. [19] Y.J. Wu, N. Wang, S.Y. Wu, X.M. Chen, Transparent barium strontium titanate ceramics prepared by spark plasma sintering, J. Am. Ceram. Soc. 94 (2011) 1343–1345.
4. Conclusions The (1-x)(K0.5Na0.5)NbO3-xBa(Mg1/3Nb2/3)O3 (x from 0.01 to 0.09) relaxor ceramics with high transmittance have been achieved by pressure-less sintering procedure. The phase was found transforming from orthorhombic phase to pseudo-cubic phase with increasing x by XRD results. SEM results showed that the porosity of the ceramics was gradually eliminated, and that the grain size decreased significantly owing to the “donor doping” effect of Ba2+. The lower defects with increasing x, in favor of less light scattering, was also analyzed by activation energy of the ceramics. Overall, the pseudo-cubic phase, fine and uniform 3691
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al. [20] Y. Bai, H. Jantunen, J. Juuti, Energy harvesting research: the road from single source to multisource, Adv. Mater. 30 (2018) 1707271. [21] R. Apetz, M.P.B. van Bruggen, Transparent alumina: a light-scattering model, J. Am. Ceram. Soc. 86 (2003) 480–486. [22] R. Zuo, X. Fang, C. Ye, Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics, Appl. Phys. Lett. 90 (2007) 092904. [23] X. Zhang, D. Yang, Z. Yang, X. Zhao, Q. Chai, X. Chao, L. Wei, Z. Yang, Transparency of K0.5N0.5NbO3 –Sr(Mg1/3Nb2/3)O3 lead-free ceramics modulated by relaxor behavior and grain size, Ceram. Int. 42 (2016) 17963–17971. [24] M.V.C. Moure, J.F. Fernández, P. Durán, Microstructural and piezoelectric properties of fine grained PZT ceramics doped with donor and/ or acceptor cations, Ferroelectrics 127 (1992) 113–118. [25] Q. Chai, X. Zhao, X. Chao, Z. Yang, Enhanced transmittance and piezoelectricity of transparent K0.5Na0.5NbO3 ceramics with Ca(Zn1/3Nb2/3)O3 additives, RSC Adv. 7 (2017) 28428–28437. [26] Z. Wang, H. Gu, Y. Hu, K. Yang, M. Hu, D. Zhou, J. Guan, Synthesis, growth mechanism and optical properties of (K,Na)NbO3 nanostructures, CrystEngComm 12 (2010) 3157–3162. [27] Z. Liu, H. Fan, Y. Zhao, G. Dong, W. Jo, Optical and tunable dielectric properties of K0.5Na0.5NbO3-SrTiO3 ceramics, J. Am. Ceram. Soc. 99 (2016) 146–151. [28] S. Lee, W.H. Woodford, C.A. Randall, Crystal and defect chemistry influences on band gap trends in alkaline earth perovskites, J. Am. Ceram. Soc. 92 (2008) 201909. [29] X. Vendrell, J.E. García, E. Cerdeiras, D.A. Ochoa, F. Rubio-Marcos, J.F. Fernández, L. Mestres, Effect of lanthanide doping on structural, microstructural and functional properties of K0.5Na0.5NbO3 lead-free piezoceramics, Ceram. Int. 42 (2016) 17530–17538. [30] D. Gao, K.W. Kwok, D. Lin, H.L.W. Chan, Microstructure and electrical properties of La-modified K0.5Na0.5NbO3 lead-free piezoelectric ceramics, J. Phys. D: Appl. Phys. 42 (2009) 035411. [31] X. Diez-Betriu, J.E. Garcia, C. Ostos, A.U. Boya, D.A. Ochoa, L. Mestres, R. Perez, Phase transition characteristics and dielectric properties of rare-earth (La, Pr, Nd, Gd) doped Ba(Zr0.09Ti0.91)O3 ceramics, Mater. Chem. Phys. 125 (2011) 493–499. [32] H.D. Li, C.D. Feng, P.H. Xiang, Electrical properties of La3+-doped (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics, J. Appl. Phys. 42 (2003) 7387–7391.
[33] M.D. Durruthy-Rodríguez, L.D. Pérez-Fernández, A. Peláiz-Barranco, F. CalderónPiñar, Structural and dielectric characteristics of donor dopants in A and B places of perovskite ceramic PZT 54/46, Appl. Phys. A 95 (2009) 423–428. [34] S.N.K. Uchino, L.E. Cross, S.J. Jang, R.E. Newnham, Electrostrictive effect in lead magnesium niobate single crystals, J. Appl. Phys. 51 (1980) 1142–1145. [35] N.T. Liu, R.H. Liang, X.B. Zhao, C.H. Xu, Z.Y. Zhou, X.L. Dong, Novel bismuth ferrite-based lead-free ceramics with high energy and power density, J. Am. Ceram. Soc. 101 (2018) 3259–3265. [36] B.K. Barick, R.N.P. Choudhary, D.K. Pradhan, Dielectric and impedance spectroscopy of zirconium modified (Na0.5Bi0.5)TiO3 ceramics, J. Phys. D: Appl. Phys. 39 (2013) 5695–5704. [37] B.K. Barick, K.K. Mishra, A.K. Arora, R.N.P. Choudhary, D.K. Pradhan, Impedance and Raman spectroscopic studies of (Na0.5Bi0.5)TiO3, J. Phys. D: Appl. Phys. 44 (2011) 355402. [38] P. Li, J. Zhai, B. Shen, S. Zhang, X. Li, F. Zhu, X. Zhang, Ultrahigh piezoelectric properties in textured (K,Na)NbO3-based lead-free ceramics, Adv. Mater. 30 (2018). [39] Z. Tang, Y. Huang, K. Wu, J. Li, Significantly enhanced breakdown field in Ca1xSrxCu3Ti4O12 ceramics by tailoring donor densities, J. Eur. Ceram. Soc. 38 (2018) 1569–1575. [40] C. Long, H. Fan, M. Li, Q. Li, Effect of lanthanum and tungsten co-substitution on the structure and properties of new Aurivillius oxides Na0.5La0.5Bi2Nb2-xWxO9, CrystEngComm 14 (2012) 7201–7208. [41] J. Hou, Y. Qu, R. Vaish, D. Krsmanovic, R.V. Kumar, Effect of Sb substitution on the structural and electrical properties of Bi4Ti3−2xNbxTaxO12 ceramics, J. Am. Ceram. Soc. 94 (2011) 2523–2529. [42] F. Yang, P. Wu, D.C. Sinclair, Suppression of electrical conductivity and switching of conduction mechanisms in ‘stoichiometric’ (Na0.5Bi0.5TiO3)1−x(BiAlO3)x (0 ≤ x ≤ 0.08) solid solutions, J. Mater. Chem. C 5 (2017) 7243–7252. [43] X.D. Ren, Q.Z. Chai, X.M. Zhao, Z.H. Peng, D. Wu, P.F. Liang, L.L. Wei, Z.P. Yang, X.L. Chao, Relaxor behaviors and electric response in transparent 0.95(K0.5Na0.5NbO3)-0.05Ca(ZrxZnyNbz)1.025O3 ceramics with low-symmetric structure, Ceram. Int. 45 (2019) 3961–3968. [44] J. Li, F. Li, Z. Xu, S. Zhang, Multilayer lead-free ceramic capacitors with ultrahigh energy density and efficiency, Adv. Mater. 30 (2018) 1802155.
3692
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Excellent optical transparency of potassium-sodium niobate-based lead-free relaxor ceramics induced by fine grains
T
Xiaolian Chaoa, , Xiaodan Rena, Xiaoshuai Zhanga, Zhanhui Penga, Juanjuan Wangb, ⁎ Pengfei Liangc, Di Wua, Zupei Yanga, ⁎
a
Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710062, Shaanxi, China b School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, Shaanxi, China c School of Physics and Information Technology, Shaanxi Normal University, Xi’an, 710062, Shaanxi, China
ARTICLE INFO
ABSTRACT
Keywords: Transparent ceramics Grain size Relaxor behavior Electrical properties
Here we report a lead-free multifunctional material (1-x)(K0.5Na0.5)NbO3-xBa(Mg1/3Nb2/3)O3 prepared by pressure-less sintering procedure. X-ray diffraction indicates a gradual crystal structure transformation with increasing x. Microstructural observation demonstrates that the addition of Ba(Mg1/3Nb2/3)O3 additive reduces the size of grains with clear grain boundary, which is favorable for a high optical transmittance. The relaxor characteristics of the ceramics could lead to further enhancement of the transparency, owing to the low defects and weak light scattering. Notably, the ceramics with 0.04 ≤ x ≤ 0.06 all show good transparency over 70% in visible region and 80% in infrared region.
1. Introduction
The transmittance of transparent ceramics with a thickness t can be expressed by the following formula: [17]
Transparent ceramics are one of the most valuable and widely used functional materials, drawing growing interest nowadays [1–3]. Lead lanthanum zirconate titanate (PLZT), as a typical representative of transparent ferroelectric ceramics, has great potential in optical devices and transparent electronic device, such as color filters, light shutters, image storage, devices modulators, displays, e-readers, and smart phones, due to the excellent optical and electrical properties. However, the pollution of lead-based ceramics to the environment and the threat to human health, wihch make the preparation of a sustainable environmentally friendly material has become the most urgent and challenging issue for the development of optoelectronic device [4–6]. Among the transparent ferroelectric ceramics, KNN-based lead-free ceramics have drawn intensive attentions [6–14]. For example, the KNN-based ceramics with relatively high transmittance of 60% in both visible and near-IR region, and very high electro-optic coefficient were reported by Kwork et al. [15]. Du et al also reported (K0.5Na0.5)NbO3-Sr (Sc0.5Nb0.5)O3-based ceramics exhibiting an excellent transmittance of 60% in visible region along with a decent energy storage density [16]. However, the transmittance of those KNN-based ceramics still cannot match that of conventional lead-based ceramics, such as PLZT based ceramics, which possess an over 70% transmittance in visible region.
R2) exp (
T = (1
t)
(1)
where t is the sample thickness a, β is the scattering coefficient neglecting the absorption (generally extinction coefficient) and R is the reflectivity. The scattering coefficient β can be calculated by the given Eq. (2): [18]
=
p 4 3
r3
CS
(2)
where CS is the scattering cross section of the spherical pores, r is the pore radius, p is the porosity volume fraction and β is the scattering coefficient. According Eqs. (1) and Eq. (2), previous studies exhibited that a high-transmittance ceramic shall possess high density, absence of pores and second phase, isotropic lattice structure, small grain size, all of which are needed to reduce some light scattering [19,20]. Moreover, homogeneous and uniform grains can also reduce the light scattering, as confirmed by R Apetz [21]. Despite of its negative effect on transmittance as mentioned above, the addition of secondary compound was found capable of modulating the grain size distribution and reducing the grain size in KNN-based
⁎ Corresponding authors at: Key Laboratory for Macromolecular Science of Shaanxi Province, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an, 710062, Shaanxi, China. E-mail addresses: [email protected] (X. Chao), [email protected] (Z. Yang).
https://doi.org/10.1016/j.jeurceramsoc.2019.04.053 Received 15 March 2019; Received in revised form 25 April 2019; Accepted 29 April 2019 Available online 30 April 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
ceramics. For instance, the grain size of KNN-based ceramics changes from 3 to 4 μm to 0.6 μm after doping with Bi0.5(Na0.925Li0.075)0.5ZrO3, and the transparency have an obviously improved [22]. Geng et al reported that [(K0.5Na0.5)1-2x(Sr0.75 Ba0.25)x]0.93Li0.07Nb0.93Bi0.07O3 ceramics have uniform and fine grain size, as well as a high transmittance of 60% in the near-IR region [10]. Yang et al. reported that KNN-based ceramics could achieve an excellent transparency (74% in visible region, comparable to that of PLZT) upon the addition of proper LaBiO3, resulting from the reduced grain size (to ˜100 nm) [14]. Besides, our previous work [23] by adding secondary component Sr(Mg1/3Nb2/3)O3 to KNN ceramics was prepared, this results indicated that the transparency is very poor (60% in the near-IR region) due to the relatively large grain size. Inspired by the doping of PZT-based ceramics at A-site to inhibit grain growth [24], we incorporated Ba2+ with ionic radii less than K+ and Na+ into KNN-based ceramics in the hope of achieving grain refinement, thereby obtained a ceramic that combines the excellent electrical and superior optical properties. Fig. 2. Optical transmittance spectra of the KNN-xBaMN ceramics.
2. Experimental procedures
(3)
( hv ) 2 = A(hv-Eg )
High transparent ceramics with a nominal composition of (1-x) (K0.5Na0.5)NbO3-xBa(Mg1/3Nb2/3)O3 (KNN-xBaMN) with 0.01 ≤ x ≤ 0.09 were synthesized by the conventional solid-state method as outlined in our previous work [23,25]. The starting raw materials are Na2CO3 (Ourchem 99.99%), K2CO3 (Ourchem 99.99%), SrCO3 (Aladdin 99.99%), Nb2O5 (Sinopharm 99.99%) and MgO (Aladdin 99.99%). The dried powders were calcined at 900 °C for 5 h, and sintered 1200 °C for 6 h soaking period in air in covered alumina crucibles. Other intermediate steps were also carried out as outlined in previous work, including milling, granulation, pressing into a late-shape extrusion and so on [23]. Finally, the phase structure, optical transmittance, mass density, microstructures, ferroelectric performance and dielectric property were measured in the same manner as reported previously [23,25]. In particular, the samples used for the measurement of optical transparency required to be ground to a thickness of 0.5 mm and polished to a parallel and smooth surface using the diamond spray polishing agent with 1 μm.
where h is Planck’s constant, ν is the photon frequency and A is a constant. Through the transmittance T, the absorption coefficient can be determined, as shown in the following formula:
=
1 1 ln t T
(4) 2
where t is the thickness of the sample. From the plotting (αhν) versus hν and extrapolating the linear portion of the curve to zero, the Eg of KNN-xBaMN ceramics can be determined. The calculated Eg of the KNNxBaMN ceramics as a function of BaMN content are summarized in Fig. 3(a–i). Visibly, the Eg values goes up initially and then decreases slightly. The values of Eg reaches a maximum value of 3.12 eV at KNN0.06BaMN ceramic, suggesting BaMN plays an important role in interband transition. Furthermore, the excellent transparent ceramics with x = 0.06 accompany with higher Eg values (3.12 eV) than the reported values of KNN nanorods (3.09 eV) [26]. In KNN perovskite ceramics, the electronic occupied state of NbO6 octahedra determines the bandgap energy value, as also reported by Liu et al. [27]. In KNN-xBaMN ceramics, the addition of smaller alkaline earth cation (Ba2+) changes the electron occupied state and enlarges the band-gaps [28]. The FE-SEM images for the fractured surfaces of KNN-xBaMN ceramics are shown in Fig. 4(a–e). Many little pores can be observed when x = 0.01, this may be caused by the higher sinter temperature (1200 °C). According to Eqs. (1) and (2), when the crystal structure is isotropic, the optical scattering by pore in ceramics should be considered as the major factor to weaken the optical transparency of ceramics. Thus, the ceramic of KNN-0.01BaMN shows very low transparency (< 40%) in the visible wavelength. With increasing x content, the amount of pores decreases, and the densification degree of fracture surface tends to be higher. The Fig. 4(f) shows the dependence of relative density on x content for the KNN-xBaMN ceramics. The relative density increases initially with increasing x from 0.01 to 0.06 where the highest value over 99% is achieved, and then decreases after x > 0.06. The high relative density and low porosity are beneficial for the transparency; thus, the KNN-xBaMN ceramics with x from 0.03 to 0.06 possess relatively higher transparency values. SEM images of polished and thermally etched cross-sections of the
3. Results and discussion Fig. 1 shows an intuitive photograph of the KNN-xBaMN ceramics with a thickness of 0.5 mm. It can be seen from the figure that with the increase of x content from 0.01 to 0.06, the letters under the samples gradually become clear, which indicated that the ceramics have changed from translucency to optical transparency. The transmission spectrum in Fig. 2 also verifies the enhancement of optical transparency. With increasing x from 0.01 to 0.09, the transmittance of ceramics have an increases initially and then decreases in 200 ˜ 2000 nm. In particular, the ceramics with 0.04 ≤ x ≤ 0.06 exhibit superior transmittance over 80% in the infrared region and 70% in the visible region, which is comparable to the highest ever value of reported ceramics [9,11,12,14,16]. As is apparent from Fig. 2, the transmittance of the KNN-xBaMN ceramic is zero below 400 nm, which could be caused by the inter-band transition. From the absorption spectra, the optical band gap energy Eg can be estimated according to the Tauc equation [10]. For direct transition, the relationship the between absorption coefficient α and Eg can be expressed as:
Fig. 1. Photograph of the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09.
3685
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 3. Plots of (αhν)2 versus hν, energy band gap Eg for the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09.
Fig. 4. (a–e) SEM images on fracture of the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.03; (c) x = 0.05; (d) x = 0.06; (e) x = 0.08, (f) Relative density of the KNN-xBaMN ceramics.
3686
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 5. SEM images of polished and thermally etched cross-sections of the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09 (g) Dependence of the mean grain size on the composition x.
KNN-xBaMN pellets are present in Fig. 5(a–i). As observed, the addition of BaMN has a crucial effect on the microstructure of KNN-xBaMN ceramics, especially on the grain size. Sample with x = 0.01 exhibits porous microstructure and large grains; as x increases, the pellet seems more dense and the grains become fine and uniform. The grain size distribution of each KNN-xBaMN ceramic is shown in the inset of each figure. And we showed the relationship between the average grain size of KNN-xBaMN ceramics as a function of BaMN content in Fig. 5(g). It can been found that the average grain size trend to decrease as x increases. Furthermore, the KNN-0.01BaMN ceramic has a very broad grain distribution range from 150 nm to 1 μm, while that of the ceramics with x ≥ 0.03 is only from 50 nm to 300 nm. The gradual grain size reduction may be caused by the Ba2+ doping on K+ or Na+ site, which is called “donor doping” and can suppress grain growth. Similar results were already observed in many La-doped perovskite-type ceramics [29–33]. The KNN-0.06BaMN have the glorious transmittance value of 85% in the near infrared wavelength due to the smallest grain size along with uniform size distribution close to 100 ± 25 nm. Fig. 6(a) display the XRD patterns of KNN-xBaMN ceramics with 2θ from 20 to 80°. All ceramics exhibit pure perovskite structure without second phase formation. In order to analyze the effect of BaMN content on the phase structure of ceramics, the characteristic diffraction peaks near 45° of the sample were amplified to Fig. 6(b). The (200) reflection of KNN-xBaMN ceramics with x ≤ 0.02 is split into two diffraction peaks of (202)O/(020)O. It means that the ceramics exhibits an orthorhombic phase structure at room temperature. Obviously, the two diffraction peaks merge into one sharp (200) diffraction peak with x increases, indicateding that ceramics are transformed into the pseudocubic phase structures with high symmetry and similar lattice constants at room temperature. Moreover, the calculated lattice parameters of KNN-xBaMN ceramics are used to further study the effect of BaMN content on phase structure, as shown in Fig. 6(c). The a, b, c values of ceramics with x ≤ 0.02 is quite different from each other, suggesting the ceramics are at low-symmetry orthorhombic phase. As x ≥ 0.03, a, b, c values are getting closer, indicating the ceramics may have pseudocubic structure. Especially, a, b, c value of the ceramic with x = 0.06 is almost the same, suggesting this ceramic is nearly cubic. This explains the observed only one diffraction index (XRD) and lowest light scattering among all ceramics. Fig. 7(a–g) represents the dielectric loss and dielectric constant of temperature-dependent KNN-xBaMN ceramics under various measuring frequencies. For the ceramics with x = 0.02, it is obvious that two dielectric peaks representing the phase transitions from orthorhombic to tetragonal (TO-T) and from tetragonal to cubic (TT-C) are observed at
Fig. 6. XRD patters for the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09.
181 °C and 375 °C, respectively. The temperature of TO-T increases while that of TT-C decreases with x increasing, eventually merging into one peak after x ≥ 0.03, Fig. 7(c–h). The maximum dielectric constant (εm), dielectric loss and the temperature of the maximum dielectric constant (Tm) of the KNN-xBaMN ceramics at the frequency of 1 MHz are counted as a function of x content in Fig. 7(i). It could be due to the dramatic reduction of grain size that Tm and εm are observed to decrease with the increase of component content from Fig. 7(i). In addition, the phenomenon that εm decreases with increasing frequency was named 3687
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 7. (a–g) Temperature dependence of dielectric constant of the KNN-xBaMN ceramics measured under various measuring frequencies: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.05; (e) x = 0.06; (f) x = 0.08; (g) x = 0.09, (h) Temperature dependence of dielectric constant and dielectric loss at 100 kHz of the KNNxBaMN ceramics, and (i) Temperature at maximum dielectric constant (Tm), the max dielectric constant (εm) and the dielectric loss at 100 kHz for the KNN-xBaMN ceramics.
diffuse phase transition caused (DPT) by heterogeneous doping, which can aggravate the fluctuation of composition [9,16,22,27]. The εm peaks shift toward higher temperatures with increasing the measured frequency suggests the ceramics transform from ferroelectric ceramics to relaxor ceramics. In generally, the reciprocal of the dielectric constant and temperature obey the Uchino and Nomura function regard as a modified CurieWeiss law, which is used to characterize relaxor ferroelectrics [34]:
1
1 m
=
(T
Tm) C
which can reduce the scattering of light at the grain boundaries and weaken the optical anisotropy. Impedance spectra at different frequencies were used to better elucidate the relaxation mechanism of KNN-xBaMN ceramics. The complex plane plots of KNN-0.06BaMN ceramics measured at 300 °C–540 °C is shown in Fig. 9(a). As observed, the impedance pattern almost appears a linear straight line when the test temperature less than 420 °C. However, as the measuring temperature increases, the image transforms into a semicircle of a depression centered below the Z' axis. This phenomenon indicates that the ceramic exhibits a non-Debye type relaxation with multiple relaxation times [36,37]. Moreover, the intercept of the semi-circular arc on the z-axis, that is, the resistance of the ceramic, gradually approaches the origin with the rise of temperature. It is further confirm the negative temperature coefficient of resistance (NTCR) behavior of the ceramic. In Generally, when two semicircles appear in the impedance spectrum, they correspond to the response of grain and grain boundary respectively [38]. The complex impedance spectra of KNN-xBaMN ceramics measured at 540 °C are displayed in Fig. 9(b), all the nine ceramics consist of a semi-circular arc, which indicates that the ceramics are electrically homogeneous structure and mainly correspond to grain response [25,38,39]. From the imaginary parts diagram of the electric modulus (Fig. 9(c)) for the KNN0.06BaMN at different temperatures, we can see that there is only one peak of the electrical modulus in all temperature ranges, which indicates that only one carrier contributes to the dielectric response, that is, the grain response. With the increase of test temperature, the peak value of electrical modulus moves towards high frequency and increases, which indicates that ceramics undergo a temperature-dependent relaxation process, i.e. thermal activation process. When the frequency range is lower than M peak, it corresponds to the long-range migration of ions, while when the frequency range is higher than M peak, the jump of ions is confined to the short ranges [40].
(5)
Where γ gives information on the character of the phase transition, C is the Curie-like constant. The value of γ from 1 for normal ferroelectrics to 2 for typical relaxor ferroelectrics. Fig. 8(a–i) shows the plots of log (1/ε−1/εm) as a function of log(T–Tm) at 1 MkHz for the KNN-xBaMN ceramics, and the γ value was determined by slope of the fitting curves. The γ value of the KNN-0.01BaMN ceramics is close to 1, suggesting the ceramics is closer to typical ferroelectrics. With increasing x content, γ increases dramatically, indicating that the ceramics become more relaxor-like. In particular, the γ value of ceramics with 0.04 ≤ x ≤ 0.07 are 1.95, 1.97, 1.99, 1.98, respectively, suggesting these ceramics have already transformed into relaxor ceramics. Generally speaking, polar nano-regions (PNRs) caused by component fluctuations are the direct reason for obtaining relatively large γ values [3]. In KNN-xBaMN ceramics, the introduction of Ba2+ ions with radius less than K+ and Na+ increases the fluctuation of local components and the disorder degree of local electric field and local elastic field, and then induces the formation of PNRs or nanostructure domains, which effectively enhances the relaxation behavior of ceramics [2,3,35]. Furthermore, Nb5+ (0.64 Å) and Mg2+ (0.89 Å) with different ionic radii also have similar mass effects on B-sites of KNN lattice. The high transparency of KNN-xBaMN ceramics is due to the pseudo-cubic phase structure with high symmetry formed by the appearance of a large number of PNRs, 3688
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 8. Log(1/ε−1/εm) as a function of log(T−Tm) at 1 MHz for the KNN-xBaMN ceramics: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03; (d) x = 0.04; (e) x = 0.05; (f) x = 0.06; (g) x = 0.07; (h) x = 0.08; (i) x = 0.09. Fig. 9. a) Complex impedance spectra for the KNN-0.06BaMN ceramics measured at different temperatures; (b) complex impedance spectra for the KNN-xBaMN ceramics measured at540 °C; (c) imaginary parts of the electric modulus for the KNN-0.06BaMN at different temperatures; (d) the equivalent circuit model instead of a pure capacitor for representing the electrical response in the ceramics.
3689
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 10. ln(fmax) as a function of 1000/T for the KNN-xBaMN ceramics: (a) x = 0.02; (b) x = 0.04; (c) x = 0.05; (d) x = 0.06; (e) x = 0.08; (f) x = 0.09.
Fig. 11. (a) P-E hysteresis loops of KNN-xBaMN ceramics; (b) Remnant polarization (Pr) and coercive field (Ec) of KNN-xBaMN ceramics as a function of x content.
Furthermore, as shown in the Fig. 9(d), the constant phase element (CPE) is usually used to represent the electrical response in ceramics instead of pure capacitors in the equivalent circuit model. The activation energy can be obtained by fitting the peak values of the electrical modulus (f max) at different temperatures in the electrical modulus map as follows [41]:
fmax = f0 exp
Ea kT
where k is the Boltzmann constant, T is the absolute temperature, Ea is the activation energy associated with the relaxation process and f0 is the pre-exponential factor. The plots of ln(fmax) as a function of 1000/T for the KNN-xBaMN ceramics and linear fittings are shown in Fig. 10. The activation energies (Ea) obtained from the slopes of fitting lines are estimated to be 1.13–1.83 eV. The higher value of activation energy, fewer defects exist in the ceramics. It is known that the value of activation energy is inversely proportional to the defect concentration [42].
(6) 3690
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al.
Fig. 12. (a) Variations in P–E hysteresis loops of KNN–0.06BaMN ceramics at room temperature under different electric fields; (b) P–E hysteresis loops at their critical breakdown strength for the KNN-xBaMN ceramics; (c) Total energy storage density (W) and recoverable energy storage density (Wrec) of KNN-xBaMN ceramics as a function of x content.
For transparent ceramics, the effect of defects on the transmittance is inevitable [27]. The smaller the number of defects, the lower the optical scattering and the better the transmittance in the ceramics [43]. In addition, the Ea value of KNN-xBaMN ceramics is about half of the band gap energy in Fig. 3, which indicates that the ceramics have intrinsic band-type conduction. The room temperature P-E hysteresis loops of KNN-xBaMN transparent ceramics are demonstrated in Fig. 11(a). All P-E loops possess a slim-like shape and become almost linear with increasing BaMN content. Since slim P–E loop is a typical characteristic of relaxor ferroelectric ceramics, the very slim P-E loops of ceramics with x ≥ 0.03 further confirm their relaxor behaviors. This phenomenon also displays that the ferroelectric properties of KNN–BaMN ceramics exhabits a deteriorating trend with increasing BaMN content. From the Fig. 11(b), as x increase, the remnant polarization (Pr) of the KNN-xBaMN have a significant decrease. Nevertheless, the ceramics with 0.03 ≤ x ≤ 0.06 can obtain excellent transparency (over 80% in the infrared wavelength and 70% in the visible region) accompanied with relatively high electrical properties (over 2 μC/cm2). The P–E hysteresis loops of KNN-0.06BaMN ceramics under different electric fields at room temperature is depicted in Fig. 12(a). In general, the electrical energy storage density W of the relaxed ferroelectric ceramic can be calculated by the hysteresis loop [12,44]. It can be seen from the Fig. 12(a) that the maximum polarization of KNN0.06BaMN ceramics increases from 7.2–13.4 μC/cm2 with electric fields increasing from 50 to 100 kV/cm, which indicates that higher energy density can be obtained by enhancing the dielectric strength of ceramics. Fig. 12(b) gives the P–E hysteresis loops of KNN-xBaMN under the highest dielectric breakdown strength at room temperature. The dielectric strength of the ceramics gets enhanced with increasing x, which may be due to the contribution of fine and dense grains. The calculated total energy storage density (W) and recoverable energy storage density (Wrec) of KNN-xBaMN ceramics are listed in Fig. 12(c). As seen, the W and Wrec have an obviously enhancement with the increase of component content. Notably, the KNN-0.08BaMN ceramics possess the highest values of Wrec ˜ 0.8 J/cm3. It could due to the maximum breakdown strength (Eb) at x = 0.08, which further proves that it is feasible to obtain a high Wrec by increasing Eb [2]
grain and lower defects are considered to be the reasons for the excellent transmittance of this system. Acknowledgments This work was supported by National Science Foundation of China (NSFC) (Grant No. 51607108, 51872177, 51572163, 51577111) and the Fundamental Research Funds for the Central Universities (Program No. GK201903017, GK201802007 and No.GK201701011). References [1] P. Günter, Coherent light amplification and optical phase conjugation with photo refractive materials, Phys. Rep. 4 (1982) 199–299. [2] Z.T. Yang, F. Gao, H.L. Du, L. Jin, L.L. Yan, Q.Y. Hu, Q. Yu, S.B. Ying, X.Y. Wei, X. Zhuo, Y.J. Wang, Grain size engineered lead-free ceramics with both large energy storage density and ultrahigh mechanical properties, Nano Energy 58 (2019) 768–777. [3] X. Lv, J.G. Wu, Effects of a phase engineering strategy on the strain properties in KNN-based ceramics, J. Mater. Chem. C 7 (2019) 2037. [4] G.R. Li, W. Ruan, J.T. Zeng, H.R. Zeng, L.Y. Zheng, L.S. Kamzina, Y. Kopylov, V. Kravchenko, The effect of domain structures on the transparency of PMN–PT transparent ceramics, Opt. Mater. 35 (2013) 722–726. [5] M. Veithen, X. Gonze, P. Ghosez, First-principles study of the electro-optic effect in ferroelectric oxides, Phys. Rev. Lett. 93 (2004) 187401. [6] X. Zeng, X. He, W. Cheng, P. Qiu, B. Xia, Effect of Dy substitution on ferroelectric, optical and electro-optic properties of transparent Pb0.90La0.10(Zr0.65Ti0.35)O3 ceramics, Ceram. Int. 40 (2014) 6197–6202. [7] T. Zheng, J. Wu, D. Xiao, J. Zhu, Recent development in lead-free perovskite piezoelectric bulk materials, Prog. Mater. Sci. 98 (2018) 552–624. [8] M. Kosec, V. Bobnar, M. Hrovat, J. Bernard, B. Malic, J. Holc, New lead-free relaxors based on the K0.5Na0.5NbO3–SrTiO3 solid solution, J. Mater. Res. 19 (2004) 1849–1854. [9] F. Li, K.W. Kwok, Fabrication of transparent electro-optic (K0.5Na0.5)1−xLixNb1−xBixO3 lead-free ceramics, J. Eur. Ceram. Soc. 33 (2013) 123–130. [10] D. Damjanovic, N. Klein, J. Li, V. Porokhonskyy, What can be expected from leadfree piezoelectric materials? Funct. Mater. Lett. 3 (2010) 5–13. [11] D. Yang, C. Ma, Z. Yang, L. Wei, X. Chao, Z. Yang, J. Yang, Optical and electrical properties of pressureless sintered transparent (K 0.37Na0.63)NbO3-based ceramics, Ceram. Int. 42 (2016) 4648–4657. [12] B. Qu, H. Du, Z. Yang, Lead-free relaxor ferroelectric ceramics with high optical transparency and energy storage ability, J. Mater. Chem. C 4 (2016) 1795–1803. [13] Q. Chai, X. Zhao, P. Liang, D. Wu, X. Chao, Z. Yang, Excellent near-infrared transparency realized in low-symmetry orthorhombic (K,Na)NbO3-based submicron ceramics, Scr. Mater. 154 (2018) 64–67. [14] Z. Yang, X. Zhang, D. Yang, B. Yang, X. Chao, L. Wei, Z. Yang, R.J. Xie, Excellent transmittance induced phase transition and grain size modulation in lead-free (1-x) (K0.5Na0.5)NbO3-xLaBiO3 ceramics, J. Am. Ceram. Soc. 99 (2016) 2055–2062. [15] K.W. Kwok, F. Li, D. Lin, A novel lead-free transparent ceramic with high electrooptic coefficient, Funct. Mater. Lett. 04 (2011) 237–240. [16] H. Du, W. Zhou, D. Zhu, L. Fa, S. Qu, Y. Li, Z. Pei, Sintering characteristic, microstructure, and dielectric relaxor behavior of (K0.5Na0.5)NbO3-(Bi0.5Na0.5)TiO3 lead-free ceramics, J. Am. Ceram. Soc. 91 (2008) 2903–2909. [17] R.M.J.G.J. Peelen, Light scattering by pores in poly-crystalline materials, J. Appl. Phys. 45 (1974) 216–220. [18] U. Anselmi-Tamburini, J.N. Woolman, Z.A. Munir, Transparent nanometric cubic and tetragonal zirconia obtained by high pressure pulsed electric current sintering, Adv. Funct. Mater. 17 (2007) 3267–3273. [19] Y.J. Wu, N. Wang, S.Y. Wu, X.M. Chen, Transparent barium strontium titanate ceramics prepared by spark plasma sintering, J. Am. Ceram. Soc. 94 (2011) 1343–1345.
4. Conclusions The (1-x)(K0.5Na0.5)NbO3-xBa(Mg1/3Nb2/3)O3 (x from 0.01 to 0.09) relaxor ceramics with high transmittance have been achieved by pressure-less sintering procedure. The phase was found transforming from orthorhombic phase to pseudo-cubic phase with increasing x by XRD results. SEM results showed that the porosity of the ceramics was gradually eliminated, and that the grain size decreased significantly owing to the “donor doping” effect of Ba2+. The lower defects with increasing x, in favor of less light scattering, was also analyzed by activation energy of the ceramics. Overall, the pseudo-cubic phase, fine and uniform 3691
Journal of the European Ceramic Society 39 (2019) 3684–3692
X. Chao, et al. [20] Y. Bai, H. Jantunen, J. Juuti, Energy harvesting research: the road from single source to multisource, Adv. Mater. 30 (2018) 1707271. [21] R. Apetz, M.P.B. van Bruggen, Transparent alumina: a light-scattering model, J. Am. Ceram. Soc. 86 (2003) 480–486. [22] R. Zuo, X. Fang, C. Ye, Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics, Appl. Phys. Lett. 90 (2007) 092904. [23] X. Zhang, D. Yang, Z. Yang, X. Zhao, Q. Chai, X. Chao, L. Wei, Z. Yang, Transparency of K0.5N0.5NbO3 –Sr(Mg1/3Nb2/3)O3 lead-free ceramics modulated by relaxor behavior and grain size, Ceram. Int. 42 (2016) 17963–17971. [24] M.V.C. Moure, J.F. Fernández, P. Durán, Microstructural and piezoelectric properties of fine grained PZT ceramics doped with donor and/ or acceptor cations, Ferroelectrics 127 (1992) 113–118. [25] Q. Chai, X. Zhao, X. Chao, Z. Yang, Enhanced transmittance and piezoelectricity of transparent K0.5Na0.5NbO3 ceramics with Ca(Zn1/3Nb2/3)O3 additives, RSC Adv. 7 (2017) 28428–28437. [26] Z. Wang, H. Gu, Y. Hu, K. Yang, M. Hu, D. Zhou, J. Guan, Synthesis, growth mechanism and optical properties of (K,Na)NbO3 nanostructures, CrystEngComm 12 (2010) 3157–3162. [27] Z. Liu, H. Fan, Y. Zhao, G. Dong, W. Jo, Optical and tunable dielectric properties of K0.5Na0.5NbO3-SrTiO3 ceramics, J. Am. Ceram. Soc. 99 (2016) 146–151. [28] S. Lee, W.H. Woodford, C.A. Randall, Crystal and defect chemistry influences on band gap trends in alkaline earth perovskites, J. Am. Ceram. Soc. 92 (2008) 201909. [29] X. Vendrell, J.E. García, E. Cerdeiras, D.A. Ochoa, F. Rubio-Marcos, J.F. Fernández, L. Mestres, Effect of lanthanide doping on structural, microstructural and functional properties of K0.5Na0.5NbO3 lead-free piezoceramics, Ceram. Int. 42 (2016) 17530–17538. [30] D. Gao, K.W. Kwok, D. Lin, H.L.W. Chan, Microstructure and electrical properties of La-modified K0.5Na0.5NbO3 lead-free piezoelectric ceramics, J. Phys. D: Appl. Phys. 42 (2009) 035411. [31] X. Diez-Betriu, J.E. Garcia, C. Ostos, A.U. Boya, D.A. Ochoa, L. Mestres, R. Perez, Phase transition characteristics and dielectric properties of rare-earth (La, Pr, Nd, Gd) doped Ba(Zr0.09Ti0.91)O3 ceramics, Mater. Chem. Phys. 125 (2011) 493–499. [32] H.D. Li, C.D. Feng, P.H. Xiang, Electrical properties of La3+-doped (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics, J. Appl. Phys. 42 (2003) 7387–7391.
[33] M.D. Durruthy-Rodríguez, L.D. Pérez-Fernández, A. Peláiz-Barranco, F. CalderónPiñar, Structural and dielectric characteristics of donor dopants in A and B places of perovskite ceramic PZT 54/46, Appl. Phys. A 95 (2009) 423–428. [34] S.N.K. Uchino, L.E. Cross, S.J. Jang, R.E. Newnham, Electrostrictive effect in lead magnesium niobate single crystals, J. Appl. Phys. 51 (1980) 1142–1145. [35] N.T. Liu, R.H. Liang, X.B. Zhao, C.H. Xu, Z.Y. Zhou, X.L. Dong, Novel bismuth ferrite-based lead-free ceramics with high energy and power density, J. Am. Ceram. Soc. 101 (2018) 3259–3265. [36] B.K. Barick, R.N.P. Choudhary, D.K. Pradhan, Dielectric and impedance spectroscopy of zirconium modified (Na0.5Bi0.5)TiO3 ceramics, J. Phys. D: Appl. Phys. 39 (2013) 5695–5704. [37] B.K. Barick, K.K. Mishra, A.K. Arora, R.N.P. Choudhary, D.K. Pradhan, Impedance and Raman spectroscopic studies of (Na0.5Bi0.5)TiO3, J. Phys. D: Appl. Phys. 44 (2011) 355402. [38] P. Li, J. Zhai, B. Shen, S. Zhang, X. Li, F. Zhu, X. Zhang, Ultrahigh piezoelectric properties in textured (K,Na)NbO3-based lead-free ceramics, Adv. Mater. 30 (2018). [39] Z. Tang, Y. Huang, K. Wu, J. Li, Significantly enhanced breakdown field in Ca1xSrxCu3Ti4O12 ceramics by tailoring donor densities, J. Eur. Ceram. Soc. 38 (2018) 1569–1575. [40] C. Long, H. Fan, M. Li, Q. Li, Effect of lanthanum and tungsten co-substitution on the structure and properties of new Aurivillius oxides Na0.5La0.5Bi2Nb2-xWxO9, CrystEngComm 14 (2012) 7201–7208. [41] J. Hou, Y. Qu, R. Vaish, D. Krsmanovic, R.V. Kumar, Effect of Sb substitution on the structural and electrical properties of Bi4Ti3−2xNbxTaxO12 ceramics, J. Am. Ceram. Soc. 94 (2011) 2523–2529. [42] F. Yang, P. Wu, D.C. Sinclair, Suppression of electrical conductivity and switching of conduction mechanisms in ‘stoichiometric’ (Na0.5Bi0.5TiO3)1−x(BiAlO3)x (0 ≤ x ≤ 0.08) solid solutions, J. Mater. Chem. C 5 (2017) 7243–7252. [43] X.D. Ren, Q.Z. Chai, X.M. Zhao, Z.H. Peng, D. Wu, P.F. Liang, L.L. Wei, Z.P. Yang, X.L. Chao, Relaxor behaviors and electric response in transparent 0.95(K0.5Na0.5NbO3)-0.05Ca(ZrxZnyNbz)1.025O3 ceramics with low-symmetric structure, Ceram. Int. 45 (2019) 3961–3968. [44] J. Li, F. Li, Z. Xu, S. Zhang, Multilayer lead-free ceramic capacitors with ultrahigh energy density and efficiency, Adv. Mater. 30 (2018) 1802155.
3692