- Email: [email protected]
The photoluminescence and piezoelectric properties of Eu2O3 doped KNN-based ceramics
Journal of Alloys and Compounds 829 (2020) 154518
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The photoluminescence and piezoelectric properties of Eu2O3 doped KNN-based ceramics Yuzhi Zhai , Juan Du *, Chong Chen , Wei Li , Jigong Hao School of Materials Science and Engineering, Liaocheng University, Liaocheng, 252059, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2019 Received in revised form 22 February 2020 Accepted 24 February 2020 Available online 27 February 2020
For researching lead-free piezoelectric ceramics, (K0.43Na0.52Li0.05)Nb0.91Sb0.045Ta0.045O3-x%mol Eu2O3 ceramics are prepared via solid state reaction method. Piezoelectric, photoluminescence properties of the ceramics are studied. The typical effects of “soft” doping are observed. Well piezoelectric and ferroelectric properties are achieved at x ¼ 0.3(d33 ¼ 252±6 pC/N, Pr ¼ 24mc/cm2, Strain ¼ Smax/Emax ¼ 0.166% at 4 kV/mm). Comparable d33 are achieved after P-E test for sample x ¼ 0.1e0.3. Better strain (0.177% at 4 kV/mm) and temperature stability are obtained when x ¼ 0.4. Photoluminescence are introduced by Eu3þ, and better photoluminescence with emission wavenumber 591 nm and 701 nm are detected when x ¼ 0.5. © 2020 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Piezoelectric properties KNN-Based ceramics Temperature stability
1. Introduction Piezoelectric material is a kind of material which realizes the mutual conversion between mechanical energy and electrical energy. Nowadays, piezoelectric ceramics are widely used in actuators, transducers, igniters etc. At present, lead-based piezoelectric ceramics still occupy a dominant position in the market. However, with the development of society, the toxicity of lead limits the production and application of lead-containing materials in the direction of sustainable green development [1e3]. Therefore, leadfree piezoelectric ceramics become a hot research topic. Potassium-sodium niobate (abbreviate KNN) piezoelectric ceramics have attracted much attention since discovery. KNN piezoelectric ceramics have high Curie temperature (TC~410 C), high piezoelectric constant (d33~160 pC/N) and electromechanical coupling factor (Kt~47%) which are the potential materials to replace lead based piezoelectric ceramics [4]. Pure KNN ceramics has a polymorphic phase transition (PPT) from orthorhombic phase to tetragonal phase [5]. Experiences demonstrate that this transition could be arised near room temperature through doping ions and other compounds of perovskite structure (abbreviate ABO3) [6e10]. Meanwhile, orthorhombic phase and tetragonal phase are concomitant, this region is called polymorphic phase boundary
(abbreviate PPB). Better piezoelectric performances are usually achieved in the region. Although, some dopants could bring outstanding piezoelectric performances, the Curie temperature is much reduced [11]. Co-doping of stibium, lithium and tantalum could form PPB near room temperature and hold Curie temperature around 300 C [12]. Rare earth ions are often used as activators or sensitizers of luminescent materials to achieve outstanding luminescent properties. Therefore, piezoelectric ceramics could be used as host and doped with rare earth ions to realize luminescence [13,14]. This kind of material has both piezoelectric and luminescent properties. In fact, with ferroelectric perovskite crystals as host, rare earth luminescent materials generally do not have satisfactory luminescence intensity [15]. Therefore, it is interesting and meaningful to design a kind of electro-optic multifunctional material with good piezoelectric and luminescence performance. Otherwise, some researchers proposed that photoluminescence performance could be a “probe” to reflect the change of crystal structure in the system [16,17]. This may be helpful for the study of crystal structure. Lead free luminescent ferroelectric ceramics (K0.43Na0.52Li0.05) Nb0.91Sb0.045Ta0.045O3-x%mol Eu2O3 (abbreviate KNLNST-xEu) were obtained. The piezoelectric and photoluminescence properties of the ceramics were studied. 2. Methods
* Corresponding author. E-mail address: [email protected] (J. Du). https://doi.org/10.1016/j.jallcom.2020.154518 0925-8388/© 2020 Elsevier B.V. All rights reserved.
Piezoelectric ceramics (K0.43Na0.52Li0.05)Nb0.91Sb0.045Ta0.045O3-x
2
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
%mol Eu2O3 (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) were prepared via traditional solid state reaction method. Raw materials are K2CO3 (99%), Na2CO3 (99.8%), Li2CO3 (99.99%), Nb2O5 (99.5%), Sb2O3 (99%), Ta2O5 (99.99%), and Eu2O3 (99.99%). After calculating and weighing, raw materials with absolute ethyl alcohol were milled by ZrO2 balls for 15 h. Then they were compacted and calcined at 850 C for 4 h. After ball milling again, calcinate was pressed into wafers with diameter of 12 mm and thickness of 1 mm, using polyvinyl alcohol aqueous (PVA) solution as binder. After getting rid of binder at 600 C, wafers were sintered at 1100 Ce1120 C. For electrical characterization, silver electrodes of both sides were formed at 650 C for 20 min. The crystal structures were detected by X-ray diffraction (XRD) (D8 Advance, Bruker Inc., Germany) using Cu ka (l ¼ 0.154 nm) as the X-ray source. Dielectric performance as a function of temperature was measured by an Agilent 4294 A precision impedance analyzer (Agilent Inc., USA). All ceramics were polarized in silicon oil under direct current (DC) electric field of 4 kV/mm at room temperature. The polarizing behavior under alternative current (AC) electric field was investigated after measuring the P-E loops at 4 kV/mm and room temperature. The d33 values of all samples were obtained by d33 m. The exciting and emission spectra were detected by fluorescence spectrometer (FLS980). Tested samples are polished and have similar thicknesses (the difference is less than 0.02 mm). We use Xe lamp, the step of the detection is 1.00 nm, and the dwell time is 0.2s. Detector is red photomultiplier. 3. Results and discussion To study the distortion of crystal lattice introduced by Eu3þ, the X-ray diffraction test is implemented. Fig. 1(a) shows the XRD patterns of all samples x ¼ 0.0e0.5. All the diffraction lines exhibit typical perovskite structure, and no secondary phase is observed. Fig. 1 (b) shows the diffraction peaks around 32 and 45.5 . We can see that with the increase of doping amount the diffraction peaks shift to larger angle. Actually, all peaks in Fig. 1 (a) have the same behavior. These imply the dopant have entered the crystal lattice and brought distortion. According to the Bragg diffraction formula 2dsinq ¼ nl, the diffraction angles become larger after doping,
Fig. 1. (a) XRD patterns of KNLNST-xEu ceramics (x ¼ 0.0e0.5), (b) Enlarged XRD patterns around 32 and 45.5 .
which indicates that the cell shrinks. The radius of Eu3þ (coordination number (CN) ¼ 8, ionic radius (Ri) ¼ 1.07 Å) is smaller than those of Kþ (CN ¼ 12, Ri ¼ 1.64 Å), Naþ (CN ¼ 12, Ri ¼ 1.39 Å) and larger than that of Nb5þ (CN ¼ 6, Ri ¼ 0.64 Å). What’s more, the Ri of Eu3þ should larger than 1.07 when CN is more than 8. Therefore, the Eu3þ ion should enter the A sites of ABO3 structure. That is to say, Eu3þ ion plays the role of “soft” doping. “Soft” doping is also called donor doping that is low valence ion replaced by high valence ion [18]. Dielectric constant would have abnormal change when undergoing phase transition for piezoelectric ceramics. Fig. 2 shows dielectric constants and loss (Tand) of all samples at different temperature. For sample x ¼ 0.0, there are two permittivity peaks at 50 C and 340 C, respectively. The former corresponds to the phase transition from orthorhombic phase (O) to tetragonal phase (T), and the latter is about another one from tetragonal phase to cubic phase. As shown, the Eu ions could increase the phase transition temperature of O-T (TO-T) and decrease the Curie temperature (TC). Demonstrably, the dielectric constants and loss increase with the increase of Eu content. These typical “soft” effects are induced by the substitution of high valence ions for low valence ions. This kind of non-equivalent substitution brings A-sites vacancies in this system. A-sites vacancies can relax stress for the orientation of polarization. As a result, the dielectric constants and loss increase [17]. Fig. 3 shows the ε-T and Tand-T curves at different frequencies. No frequency dispersion is observed for all samples. The frequency stability of dielectric constant and loss becomes worse with the increase of doping amount. It is reported that full polarization may be constructed under AC electric field [19]. So the d33 values of all samples are measured after P-E tests instantly and after 48 h, respectively. Similarly, the d33 values of all samples are obtained via DC polarization. These d33 values are shown in Fig. 4. We can see that the sample x ¼ 0.3 has the highest d33 value (252±6 pC/N). The values fast decrease when x˃0.3. This may be contributed by the increase of phase transition temperature. The orthorhombic-tetragonal phase transition temperature is farther and farther away from room temperature when x˃0.3, resulting in the decrease of electrical properties of samples. Noticeable, the d33 values yield by AC field is comparable with those by DC field when 0.1 x 0.4. Thus AC polarization may be a timesaving and convenient polarization method for ceramics. However the d33 values from the two ways have palpable difference when
Fig. 2. Dielectric constants and loss (Tand) of KNLNST-xEu ceramics (x ¼ 0.0e0.5) at different temperatures.
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
3
Fig. 3. The frequency dependence of the dielectric constant and loss of x ¼ 0.0, x ¼ 0.1 and x ¼ 0.5 samples.
Fig. 5. P-E hysteresis loops of KNLNST-xEu ceramics. Fig. 4. d33 values of KNLNST-xEu ceramics obtained after DC polarization and after P-E tests.
x ¼ 0.0 and 0.5. This needs further study. Fig. 5 shows the P-E loops of all samples at 10 Hz. After doping, the saturated polarization and remanent polarization are enhanced. The samples x ¼ 0.3 and 0.4 have the largest remain polarization. Meanwhile the coercive field (EC) is degraded after doping. Fig. 6 shows the coercive fields of KNLNST-xEu ceramics at different temperature. We can see that the EC decreases with the increase of doping contents. The degradation of EC is another typical behavior of “soft” doping [18]. A-site vacancies are regarded to be the originator of this behavior. All EC values first increase with raising temperature. They reach their maximum when the temperature rises to a certain value. These particular values of temperature agree with the orthorhombic-tetragonal phase transition temperatures of the samples. Then, EC values decrease gradually with the further increase of temperature. In addition, EC has the maximum value at TO-T. This is probably due to the existence of phase boundary which improves the resistance of polarization reorientation. Unipolar strain is often used in practical applications. The unipolar strains of KNLNST-xEu ceramics and its temperature stability are studied. Temperature stability of unipolar strain of KNLNST-xEu
Fig. 6. Coercive fields of KNLNST-xEu ceramics at different temperatures.
ceramics under 4 kV/mm are shown in Fig. 7 and the inset shows unipolar strain of KNLNST-xEu ceramics at room temperature under 4 kV/mm. It is seen that the sample x ¼ 0.4 has better strain
4
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
Fig. 7. Temperature stability of unipolar strain of KNLNST-xEu ceramics under 4 kV/ mm, inset: unipolar strain of KNLNST-xEu ceramics at room temperature under 4 kV/ mm.
(0.177%) and better temperature stability. The strain remains 100% at 50 C and 87.5% at 100 C. There are two reasons for the enhancement of temperature stability of the x ¼ 0.4 sample [20e23]. First, as we know, the PPB is a temperature-sensitivity phase boundary. After doping 0.4 mol% Eu3þ, the PPB shifts from room temperature to a higher temperature (about 75 C). As a result, the temperature stability of the x ¼ 0.4 sample is improved. Second, sharp and narrow phase transition in εr-T curves results in strong temperature dependence, whereas broad and gentle one causes weak temperature dependence. A-site cation disorder is always regarded as the main derivation of diffuse phase transition. After doping 0.4 mol% Eu3þ, the wide phase transition peaks and the increased diffuseness of phase transition indicate that the temperature stability of the sample is enhanced. 4. Luminescence Rare earth ions Eu3þ are always used in photoluminescence system. There should be a bi-functional material if Eu3þ doped in piezoelectric matrix as a luminescence center [7]. The photoluminescence spectra with different dopant contents are shown in Fig. 8(a). Fig. 8(b) shows the partial energy level of Eu3þ [24]. We
Fig. 9. The photoluminescence spectra (lEx ¼ 465 nm, lEm ¼ 616 nm) of KNLNST-xEu ceramics.
can see that all the samples have an absorption band between 398 nm and 406 nm corresponding to the electronic excitation of O to ion at B sites within the BO3 groups, and a weak but sharp absorption peak at 465 nm (7F0/5D2) [25]. The emission spectra excited by 465 nm are shown in Fig. 9. There are four main emission peaks which locate at 598 nm (5D0/7F1), 616 nm (5D0/7F2), 681 nm (5D0/7F4) and 701 nm (5D0/7F4), respectively. Among them, the emission peak at 701 nm with red color is the strongest one. The presence of the host lattice excitation bands indicates that there is energy transfer from host lattice to Eu ions in these compounds [25]. Compared Figs. 8 and 9, we can notice that the energy transfer has most contribution for emission. The transition 5 D0/7F1 belongs to magnetic dipole transition. It is related to Eu ions located symmetrical center position. The transition 5D0/7F2 is electrical dipole transition [26]. It depends on the Eu ions without inversion symmetry. As can be seen from Fig. 8, both these two peaks are of high intensity, and transition 5D0/7F1 is stronger. These indicate that Eu ions have two kinds of lattice sites and symmetrical center positions are more. Relative strengths of transitions 5D0/7F1 and 5D0/7F2 which are excited by 404 nm and 465 nm respectively as a function of Eu contents are shown in Fig. 10. They have same trend varied with Eu contents at the two
Fig. 8. (a) The photoluminescence spectra (lex ¼ 404 nm, lem ¼ 598 nm) of KNLNST-xEu ceramics, (b) the partial energy level of Eu3þ.
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
5
Technology Program (No. J17KA005).
References
Fig. 10. The ratios of IF1 and IF2 at exciting wavenumber of 404 nm and 465 nm at different x contents, respectively.
excited wavenumbers. The relative strength first enhanced with increasing Eu contents. It reaches top when Eu content is 0.4%mol, and then degrade with more Eu contents. It means that Eu ions occupied more sites with inversion symmetry than noninversion. The amount of Eu sites without inversion symmetry begins to increase when doping contents are more than 0.4%mol. 5. Conclusions Piezoelectric luminescence ceramics K0.43Na0.52Li0.05Nb0.91 Sb0.045Ta0.045-x%mol Eu2O3 are prepared via solid state reaction method. The dielectric constants and loss increase and the coercive filed decreases with the increase of Eu3þ contents. These behaviors are induced by the A-site vacancies produced by non-equivalent substitution. AC polarization method should be further studied. Well piezoelectric properties (d33 ¼ 252±6 pC/N, Pr ¼ 24mc/cm2, Strain ¼ Smax/Emax ¼ 0.166% at 4 kV/mm, x ¼ 0.3) are achieved. Temperature stability is enhanced after doping Eu3þ. Red luminescence is introduced by Eu3þ. As a kind of electro-optical multifunctional material, KNLNST-xEu2O3 ceramic has potential applicability. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Yuzhi Zhai: Writing - original draft. Juan Du: Writing - original draft. Chong Chen: Writing - original draft. Wei Li: Formal analysis, Writing - original draft. Jigong Hao: Formal analysis, Writing original draft. Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province of China (Grant Nos. ZR2018MEM011, ZR201709250374, ZR2017MEM019 and ZR2016EMM02), the National Key R&D Program of China (NO.2016YFB0402701), the Key R & D project of Shandong Province (No. 2017GGX202008) and the Project of Shandong Province Higher Educational Science and
[1] L. Zheng, J. Wang, X. Huo, R. Wang, S. Sang, S. Li, P. Zheng, W. Cao, Temperature dependence of dielectric and electromechanical properties of (K,Na)(Nb,Ta)O3 single crystal and corresponding domain structure evolution, J. Appl. Phys. 116 (2014), 044105. [2] H. Tian, B. Yao, P. Tan, Z. Zhou, G. Shi, D. Gong, R. Zhang, Double-loop hysteresis in tetragonal KTa0.58Nb0.42O3 correlated to recoverable reorientations of the asymmetric polar domains, Appl. Phys. Lett. 106 (2015), 102903. [3] L. Zheng, X. Yi, S. Zhang, W. Jiang, B. Yang, R. Zhang, W. Cao, Complete set of material constants of 0.95(Na0.5Bi0.5)TiO3-0.05BaTiO3 lead-free piezoelectric single crystal and the delineation of extrinsic contributions, Appl. Phys. Lett. 103 (2013), 122905. [4] Y. Dai, X. Zhang, K. Chen, Morphotropic phase boundary and electrical properties of K1xNaxNbO3 lead-free ceramics, Appl. Phys. Lett. 94 (2009), 042905. [5] L. Liu, Y. Huang, Y. Li, L. Fang, H. Dammak, H. Fan, M. Thi, Orthorhombic to tetragonal structural phase transition in Na0.5K0.5NbO3-based ceramics, Mater. Lett. 68 (2012) 300e302. [6] Y. Zhai, Y. Sun, J. Du, J. Jin, Z. Zhang, The effects of terbium oxide on phase transition and electronic properties of potassium-sodium niobate-based ceramics, J. Alloys Compd. 776 (2019) 984e992. [7] J. Hao, Z. Xu, R. Chu, W. Li, J. Du, Bright reddish-orange emission and good piezoelectric properties of Sm2O3-modified(K0.5Na0.5)NbO3-based lead-free piezoelectric ceramics, J. Appl. Phys. 117 (2015), 194104. [8] F. Li, Z. Tan, J. Xing, L. Jiang, B. Wu, J. Wu, D. Xiao, J. Zhu, Investigation of new lead free (1x)KNNSexBKZH piezo-ceramics with ReOeT phase boundary, J. Mater. Sci. Mater. Electron. 28 (2017) 8803e8809. [9] L. Xie, J. Xing, Z. Tan, L. Jiang, Q. Chen, J. Wu, W. Zhang, D. Xiao, J. Zhu, Comprehensive investigation of structural and electrical properties of (Bi, Na) CoZrO3-doped KNN ceramics, J. Alloys Compd. 758 (2018) 15e24. [10] R. Sumang, S. Chootin, T. Bongarn, Phasetransition electrical properties and large strain response in lead-free (1-x-y)BNT-xBKT-yKNN ceramics, Integrated Ferroelectrics Int. J. 195 (2019) 119e130. [11] X. Wang, J. Wu, D. Xiao, J. Zhu, X. Cheng, T. Zheng B, Zhang, X. Lou, X. Wang, Giant piezoelectricity in potassium-sodium niobate lead-free ceramics, J. Am. Chem. Soc. 136 (2014) 2905e2910. [12] B. Ming, J. Wang, P. Qi, G. Zang, Piezoelectric properties of (Li, Sb, Ta) modified (Na,K)NbO lead-free ceramics, J. Appl. Phys. 101 (2007), 054103. [13] J. Lin, Q. Lu, J. Xu, X. Wu, C. Lin, T. Lin, C. Chen, L. Luo, Outstanding optical temperature sensitivity and dual-mode temperature-dependent photoluminescence in Ho3þ-doped (K,Na)NbO3eSrTiO3 transparent ceramics, J. Am. Chem. Soc. 102 (2019) 4710e4720. [14] J. Hao, Z. Xu, R. Chu, S. Chu, W. Li, P. Fu, J. Du, C. Hu, Bright upconversion emission and large strain in Er/Sb-codoped (Bi0.5Na0.5)0.945Ba0.065TiO3 ceramics, Mater. Lett. 193 (2017) 138e141. [15] J. Du, Z. Xu, R. Chu, J. Hao, W. Li, G. Jinang, P. Zheng, Rare-earth doped (K0.5Na0.5)NbO3 multifunctional ceramics, J. Mater. Sci. Mater. Electron. 28 (2017) 5288e5294. [16] J. Wang, L. Luo, The NbO6 octahedral distortion and phase structural transition of Eu3þ-doped K0.5Na0.5NbO3exLiNbO3 ferroelectric ceramics, J. Am. Ceram. Soc. 101 (2018) 400e407. [17] Z. Geng, K. Li, X. Li, D. Shi, Fabrication and photoluminescence of Eu-doped KNN-based transparent ceramics, J. Mater. Sci. 52 (2017) 2285e2295. [18] T. Shrout, S. Zhang, Lead-free piezoelectric ceramics: alternatives for PZT? J. Electroceram. 19 (2007) 113e126. [19] H. Tao, J. Wu, New poling method for piezoelectric ceramics, J. Mater. Chem. C. 5 (2017) 1601e1606. [20] C. Zhou, J. Zhang, W. Yao, D. Liu, W. Su, Highly temperature-stable piezoelectric properties of 0.96(K0.48Na0.52)(Nb0.96Sb0.04)O3-0.03BaZrO30.01(Bi0.50Na0.50)ZrO3 ceramic in common usage temperature range, Scripta Mater. 162 (2019) 86e89. [21] W. Yang, L. Wang, H. Li, J. Han, H. Xiu, Z. Zhou, Piezoelectric properties and diffusion phase transition around PPT of La-doped (Na0.52K0.44Li0.04) Nb0.8Ta0.2O3 lead-free piezoelectric ceramics, Physica B 498 (2016) 59e64. [22] W. Yang, P. Li, F. Li, X. Liu, B. Shen, J. Zhai, Enhanced piezoelectric performance and thermal stability of alkali niobate-based ceramics, Ceram. Int. 45 (2019) 2275e2280. [23] X. Lv, J. Wu, C. Zhao, D. Xiao, J. Zhu, Z. Zhang, C. Zhang, X. Zhang, Enhancing temperature stability in potassium-sodium niobate ceramics through phase boundary and composition design, J. Eur. Ceram. Soc. 39 (2019) 305e315. [24] K. Binnemans, Interpretation of europium(III) spectra, Coord. Chem. Rev. 295 (2018) 1e45. [25] H. Li, H.K. Yang, J.H. Jeong, K. Jang, H.S. Lee, S.S. Yi, Solegel synthesis, structure and photoluminescence properties of nanocrystalline Lu2MoO6:Eu, Mater. Res. Bull. 46 (2011) 1352e1358. [26] H. Li, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, K. Jang, H.S. Lee, S.S. Yi, Colorconversion and photoluminescence properties of Ba2MgW(Mo)O6:Eu phosphor, J. Alloys Compd. 509 (2011) 8788e8793.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The photoluminescence and piezoelectric properties of Eu2O3 doped KNN-based ceramics Yuzhi Zhai , Juan Du *, Chong Chen , Wei Li , Jigong Hao School of Materials Science and Engineering, Liaocheng University, Liaocheng, 252059, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2019 Received in revised form 22 February 2020 Accepted 24 February 2020 Available online 27 February 2020
For researching lead-free piezoelectric ceramics, (K0.43Na0.52Li0.05)Nb0.91Sb0.045Ta0.045O3-x%mol Eu2O3 ceramics are prepared via solid state reaction method. Piezoelectric, photoluminescence properties of the ceramics are studied. The typical effects of “soft” doping are observed. Well piezoelectric and ferroelectric properties are achieved at x ¼ 0.3(d33 ¼ 252±6 pC/N, Pr ¼ 24mc/cm2, Strain ¼ Smax/Emax ¼ 0.166% at 4 kV/mm). Comparable d33 are achieved after P-E test for sample x ¼ 0.1e0.3. Better strain (0.177% at 4 kV/mm) and temperature stability are obtained when x ¼ 0.4. Photoluminescence are introduced by Eu3þ, and better photoluminescence with emission wavenumber 591 nm and 701 nm are detected when x ¼ 0.5. © 2020 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Piezoelectric properties KNN-Based ceramics Temperature stability
1. Introduction Piezoelectric material is a kind of material which realizes the mutual conversion between mechanical energy and electrical energy. Nowadays, piezoelectric ceramics are widely used in actuators, transducers, igniters etc. At present, lead-based piezoelectric ceramics still occupy a dominant position in the market. However, with the development of society, the toxicity of lead limits the production and application of lead-containing materials in the direction of sustainable green development [1e3]. Therefore, leadfree piezoelectric ceramics become a hot research topic. Potassium-sodium niobate (abbreviate KNN) piezoelectric ceramics have attracted much attention since discovery. KNN piezoelectric ceramics have high Curie temperature (TC~410 C), high piezoelectric constant (d33~160 pC/N) and electromechanical coupling factor (Kt~47%) which are the potential materials to replace lead based piezoelectric ceramics [4]. Pure KNN ceramics has a polymorphic phase transition (PPT) from orthorhombic phase to tetragonal phase [5]. Experiences demonstrate that this transition could be arised near room temperature through doping ions and other compounds of perovskite structure (abbreviate ABO3) [6e10]. Meanwhile, orthorhombic phase and tetragonal phase are concomitant, this region is called polymorphic phase boundary
(abbreviate PPB). Better piezoelectric performances are usually achieved in the region. Although, some dopants could bring outstanding piezoelectric performances, the Curie temperature is much reduced [11]. Co-doping of stibium, lithium and tantalum could form PPB near room temperature and hold Curie temperature around 300 C [12]. Rare earth ions are often used as activators or sensitizers of luminescent materials to achieve outstanding luminescent properties. Therefore, piezoelectric ceramics could be used as host and doped with rare earth ions to realize luminescence [13,14]. This kind of material has both piezoelectric and luminescent properties. In fact, with ferroelectric perovskite crystals as host, rare earth luminescent materials generally do not have satisfactory luminescence intensity [15]. Therefore, it is interesting and meaningful to design a kind of electro-optic multifunctional material with good piezoelectric and luminescence performance. Otherwise, some researchers proposed that photoluminescence performance could be a “probe” to reflect the change of crystal structure in the system [16,17]. This may be helpful for the study of crystal structure. Lead free luminescent ferroelectric ceramics (K0.43Na0.52Li0.05) Nb0.91Sb0.045Ta0.045O3-x%mol Eu2O3 (abbreviate KNLNST-xEu) were obtained. The piezoelectric and photoluminescence properties of the ceramics were studied. 2. Methods
* Corresponding author. E-mail address: [email protected] (J. Du). https://doi.org/10.1016/j.jallcom.2020.154518 0925-8388/© 2020 Elsevier B.V. All rights reserved.
Piezoelectric ceramics (K0.43Na0.52Li0.05)Nb0.91Sb0.045Ta0.045O3-x
2
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
%mol Eu2O3 (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) were prepared via traditional solid state reaction method. Raw materials are K2CO3 (99%), Na2CO3 (99.8%), Li2CO3 (99.99%), Nb2O5 (99.5%), Sb2O3 (99%), Ta2O5 (99.99%), and Eu2O3 (99.99%). After calculating and weighing, raw materials with absolute ethyl alcohol were milled by ZrO2 balls for 15 h. Then they were compacted and calcined at 850 C for 4 h. After ball milling again, calcinate was pressed into wafers with diameter of 12 mm and thickness of 1 mm, using polyvinyl alcohol aqueous (PVA) solution as binder. After getting rid of binder at 600 C, wafers were sintered at 1100 Ce1120 C. For electrical characterization, silver electrodes of both sides were formed at 650 C for 20 min. The crystal structures were detected by X-ray diffraction (XRD) (D8 Advance, Bruker Inc., Germany) using Cu ka (l ¼ 0.154 nm) as the X-ray source. Dielectric performance as a function of temperature was measured by an Agilent 4294 A precision impedance analyzer (Agilent Inc., USA). All ceramics were polarized in silicon oil under direct current (DC) electric field of 4 kV/mm at room temperature. The polarizing behavior under alternative current (AC) electric field was investigated after measuring the P-E loops at 4 kV/mm and room temperature. The d33 values of all samples were obtained by d33 m. The exciting and emission spectra were detected by fluorescence spectrometer (FLS980). Tested samples are polished and have similar thicknesses (the difference is less than 0.02 mm). We use Xe lamp, the step of the detection is 1.00 nm, and the dwell time is 0.2s. Detector is red photomultiplier. 3. Results and discussion To study the distortion of crystal lattice introduced by Eu3þ, the X-ray diffraction test is implemented. Fig. 1(a) shows the XRD patterns of all samples x ¼ 0.0e0.5. All the diffraction lines exhibit typical perovskite structure, and no secondary phase is observed. Fig. 1 (b) shows the diffraction peaks around 32 and 45.5 . We can see that with the increase of doping amount the diffraction peaks shift to larger angle. Actually, all peaks in Fig. 1 (a) have the same behavior. These imply the dopant have entered the crystal lattice and brought distortion. According to the Bragg diffraction formula 2dsinq ¼ nl, the diffraction angles become larger after doping,
Fig. 1. (a) XRD patterns of KNLNST-xEu ceramics (x ¼ 0.0e0.5), (b) Enlarged XRD patterns around 32 and 45.5 .
which indicates that the cell shrinks. The radius of Eu3þ (coordination number (CN) ¼ 8, ionic radius (Ri) ¼ 1.07 Å) is smaller than those of Kþ (CN ¼ 12, Ri ¼ 1.64 Å), Naþ (CN ¼ 12, Ri ¼ 1.39 Å) and larger than that of Nb5þ (CN ¼ 6, Ri ¼ 0.64 Å). What’s more, the Ri of Eu3þ should larger than 1.07 when CN is more than 8. Therefore, the Eu3þ ion should enter the A sites of ABO3 structure. That is to say, Eu3þ ion plays the role of “soft” doping. “Soft” doping is also called donor doping that is low valence ion replaced by high valence ion [18]. Dielectric constant would have abnormal change when undergoing phase transition for piezoelectric ceramics. Fig. 2 shows dielectric constants and loss (Tand) of all samples at different temperature. For sample x ¼ 0.0, there are two permittivity peaks at 50 C and 340 C, respectively. The former corresponds to the phase transition from orthorhombic phase (O) to tetragonal phase (T), and the latter is about another one from tetragonal phase to cubic phase. As shown, the Eu ions could increase the phase transition temperature of O-T (TO-T) and decrease the Curie temperature (TC). Demonstrably, the dielectric constants and loss increase with the increase of Eu content. These typical “soft” effects are induced by the substitution of high valence ions for low valence ions. This kind of non-equivalent substitution brings A-sites vacancies in this system. A-sites vacancies can relax stress for the orientation of polarization. As a result, the dielectric constants and loss increase [17]. Fig. 3 shows the ε-T and Tand-T curves at different frequencies. No frequency dispersion is observed for all samples. The frequency stability of dielectric constant and loss becomes worse with the increase of doping amount. It is reported that full polarization may be constructed under AC electric field [19]. So the d33 values of all samples are measured after P-E tests instantly and after 48 h, respectively. Similarly, the d33 values of all samples are obtained via DC polarization. These d33 values are shown in Fig. 4. We can see that the sample x ¼ 0.3 has the highest d33 value (252±6 pC/N). The values fast decrease when x˃0.3. This may be contributed by the increase of phase transition temperature. The orthorhombic-tetragonal phase transition temperature is farther and farther away from room temperature when x˃0.3, resulting in the decrease of electrical properties of samples. Noticeable, the d33 values yield by AC field is comparable with those by DC field when 0.1 x 0.4. Thus AC polarization may be a timesaving and convenient polarization method for ceramics. However the d33 values from the two ways have palpable difference when
Fig. 2. Dielectric constants and loss (Tand) of KNLNST-xEu ceramics (x ¼ 0.0e0.5) at different temperatures.
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
3
Fig. 3. The frequency dependence of the dielectric constant and loss of x ¼ 0.0, x ¼ 0.1 and x ¼ 0.5 samples.
Fig. 5. P-E hysteresis loops of KNLNST-xEu ceramics. Fig. 4. d33 values of KNLNST-xEu ceramics obtained after DC polarization and after P-E tests.
x ¼ 0.0 and 0.5. This needs further study. Fig. 5 shows the P-E loops of all samples at 10 Hz. After doping, the saturated polarization and remanent polarization are enhanced. The samples x ¼ 0.3 and 0.4 have the largest remain polarization. Meanwhile the coercive field (EC) is degraded after doping. Fig. 6 shows the coercive fields of KNLNST-xEu ceramics at different temperature. We can see that the EC decreases with the increase of doping contents. The degradation of EC is another typical behavior of “soft” doping [18]. A-site vacancies are regarded to be the originator of this behavior. All EC values first increase with raising temperature. They reach their maximum when the temperature rises to a certain value. These particular values of temperature agree with the orthorhombic-tetragonal phase transition temperatures of the samples. Then, EC values decrease gradually with the further increase of temperature. In addition, EC has the maximum value at TO-T. This is probably due to the existence of phase boundary which improves the resistance of polarization reorientation. Unipolar strain is often used in practical applications. The unipolar strains of KNLNST-xEu ceramics and its temperature stability are studied. Temperature stability of unipolar strain of KNLNST-xEu
Fig. 6. Coercive fields of KNLNST-xEu ceramics at different temperatures.
ceramics under 4 kV/mm are shown in Fig. 7 and the inset shows unipolar strain of KNLNST-xEu ceramics at room temperature under 4 kV/mm. It is seen that the sample x ¼ 0.4 has better strain
4
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
Fig. 7. Temperature stability of unipolar strain of KNLNST-xEu ceramics under 4 kV/ mm, inset: unipolar strain of KNLNST-xEu ceramics at room temperature under 4 kV/ mm.
(0.177%) and better temperature stability. The strain remains 100% at 50 C and 87.5% at 100 C. There are two reasons for the enhancement of temperature stability of the x ¼ 0.4 sample [20e23]. First, as we know, the PPB is a temperature-sensitivity phase boundary. After doping 0.4 mol% Eu3þ, the PPB shifts from room temperature to a higher temperature (about 75 C). As a result, the temperature stability of the x ¼ 0.4 sample is improved. Second, sharp and narrow phase transition in εr-T curves results in strong temperature dependence, whereas broad and gentle one causes weak temperature dependence. A-site cation disorder is always regarded as the main derivation of diffuse phase transition. After doping 0.4 mol% Eu3þ, the wide phase transition peaks and the increased diffuseness of phase transition indicate that the temperature stability of the sample is enhanced. 4. Luminescence Rare earth ions Eu3þ are always used in photoluminescence system. There should be a bi-functional material if Eu3þ doped in piezoelectric matrix as a luminescence center [7]. The photoluminescence spectra with different dopant contents are shown in Fig. 8(a). Fig. 8(b) shows the partial energy level of Eu3þ [24]. We
Fig. 9. The photoluminescence spectra (lEx ¼ 465 nm, lEm ¼ 616 nm) of KNLNST-xEu ceramics.
can see that all the samples have an absorption band between 398 nm and 406 nm corresponding to the electronic excitation of O to ion at B sites within the BO3 groups, and a weak but sharp absorption peak at 465 nm (7F0/5D2) [25]. The emission spectra excited by 465 nm are shown in Fig. 9. There are four main emission peaks which locate at 598 nm (5D0/7F1), 616 nm (5D0/7F2), 681 nm (5D0/7F4) and 701 nm (5D0/7F4), respectively. Among them, the emission peak at 701 nm with red color is the strongest one. The presence of the host lattice excitation bands indicates that there is energy transfer from host lattice to Eu ions in these compounds [25]. Compared Figs. 8 and 9, we can notice that the energy transfer has most contribution for emission. The transition 5 D0/7F1 belongs to magnetic dipole transition. It is related to Eu ions located symmetrical center position. The transition 5D0/7F2 is electrical dipole transition [26]. It depends on the Eu ions without inversion symmetry. As can be seen from Fig. 8, both these two peaks are of high intensity, and transition 5D0/7F1 is stronger. These indicate that Eu ions have two kinds of lattice sites and symmetrical center positions are more. Relative strengths of transitions 5D0/7F1 and 5D0/7F2 which are excited by 404 nm and 465 nm respectively as a function of Eu contents are shown in Fig. 10. They have same trend varied with Eu contents at the two
Fig. 8. (a) The photoluminescence spectra (lex ¼ 404 nm, lem ¼ 598 nm) of KNLNST-xEu ceramics, (b) the partial energy level of Eu3þ.
Y. Zhai et al. / Journal of Alloys and Compounds 829 (2020) 154518
5
Technology Program (No. J17KA005).
References
Fig. 10. The ratios of IF1 and IF2 at exciting wavenumber of 404 nm and 465 nm at different x contents, respectively.
excited wavenumbers. The relative strength first enhanced with increasing Eu contents. It reaches top when Eu content is 0.4%mol, and then degrade with more Eu contents. It means that Eu ions occupied more sites with inversion symmetry than noninversion. The amount of Eu sites without inversion symmetry begins to increase when doping contents are more than 0.4%mol. 5. Conclusions Piezoelectric luminescence ceramics K0.43Na0.52Li0.05Nb0.91 Sb0.045Ta0.045-x%mol Eu2O3 are prepared via solid state reaction method. The dielectric constants and loss increase and the coercive filed decreases with the increase of Eu3þ contents. These behaviors are induced by the A-site vacancies produced by non-equivalent substitution. AC polarization method should be further studied. Well piezoelectric properties (d33 ¼ 252±6 pC/N, Pr ¼ 24mc/cm2, Strain ¼ Smax/Emax ¼ 0.166% at 4 kV/mm, x ¼ 0.3) are achieved. Temperature stability is enhanced after doping Eu3þ. Red luminescence is introduced by Eu3þ. As a kind of electro-optical multifunctional material, KNLNST-xEu2O3 ceramic has potential applicability. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Yuzhi Zhai: Writing - original draft. Juan Du: Writing - original draft. Chong Chen: Writing - original draft. Wei Li: Formal analysis, Writing - original draft. Jigong Hao: Formal analysis, Writing original draft. Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province of China (Grant Nos. ZR2018MEM011, ZR201709250374, ZR2017MEM019 and ZR2016EMM02), the National Key R&D Program of China (NO.2016YFB0402701), the Key R & D project of Shandong Province (No. 2017GGX202008) and the Project of Shandong Province Higher Educational Science and
[1] L. Zheng, J. Wang, X. Huo, R. Wang, S. Sang, S. Li, P. Zheng, W. Cao, Temperature dependence of dielectric and electromechanical properties of (K,Na)(Nb,Ta)O3 single crystal and corresponding domain structure evolution, J. Appl. Phys. 116 (2014), 044105. [2] H. Tian, B. Yao, P. Tan, Z. Zhou, G. Shi, D. Gong, R. Zhang, Double-loop hysteresis in tetragonal KTa0.58Nb0.42O3 correlated to recoverable reorientations of the asymmetric polar domains, Appl. Phys. Lett. 106 (2015), 102903. [3] L. Zheng, X. Yi, S. Zhang, W. Jiang, B. Yang, R. Zhang, W. Cao, Complete set of material constants of 0.95(Na0.5Bi0.5)TiO3-0.05BaTiO3 lead-free piezoelectric single crystal and the delineation of extrinsic contributions, Appl. Phys. Lett. 103 (2013), 122905. [4] Y. Dai, X. Zhang, K. Chen, Morphotropic phase boundary and electrical properties of K1xNaxNbO3 lead-free ceramics, Appl. Phys. Lett. 94 (2009), 042905. [5] L. Liu, Y. Huang, Y. Li, L. Fang, H. Dammak, H. Fan, M. Thi, Orthorhombic to tetragonal structural phase transition in Na0.5K0.5NbO3-based ceramics, Mater. Lett. 68 (2012) 300e302. [6] Y. Zhai, Y. Sun, J. Du, J. Jin, Z. Zhang, The effects of terbium oxide on phase transition and electronic properties of potassium-sodium niobate-based ceramics, J. Alloys Compd. 776 (2019) 984e992. [7] J. Hao, Z. Xu, R. Chu, W. Li, J. Du, Bright reddish-orange emission and good piezoelectric properties of Sm2O3-modified(K0.5Na0.5)NbO3-based lead-free piezoelectric ceramics, J. Appl. Phys. 117 (2015), 194104. [8] F. Li, Z. Tan, J. Xing, L. Jiang, B. Wu, J. Wu, D. Xiao, J. Zhu, Investigation of new lead free (1x)KNNSexBKZH piezo-ceramics with ReOeT phase boundary, J. Mater. Sci. Mater. Electron. 28 (2017) 8803e8809. [9] L. Xie, J. Xing, Z. Tan, L. Jiang, Q. Chen, J. Wu, W. Zhang, D. Xiao, J. Zhu, Comprehensive investigation of structural and electrical properties of (Bi, Na) CoZrO3-doped KNN ceramics, J. Alloys Compd. 758 (2018) 15e24. [10] R. Sumang, S. Chootin, T. Bongarn, Phasetransition electrical properties and large strain response in lead-free (1-x-y)BNT-xBKT-yKNN ceramics, Integrated Ferroelectrics Int. J. 195 (2019) 119e130. [11] X. Wang, J. Wu, D. Xiao, J. Zhu, X. Cheng, T. Zheng B, Zhang, X. Lou, X. Wang, Giant piezoelectricity in potassium-sodium niobate lead-free ceramics, J. Am. Chem. Soc. 136 (2014) 2905e2910. [12] B. Ming, J. Wang, P. Qi, G. Zang, Piezoelectric properties of (Li, Sb, Ta) modified (Na,K)NbO lead-free ceramics, J. Appl. Phys. 101 (2007), 054103. [13] J. Lin, Q. Lu, J. Xu, X. Wu, C. Lin, T. Lin, C. Chen, L. Luo, Outstanding optical temperature sensitivity and dual-mode temperature-dependent photoluminescence in Ho3þ-doped (K,Na)NbO3eSrTiO3 transparent ceramics, J. Am. Chem. Soc. 102 (2019) 4710e4720. [14] J. Hao, Z. Xu, R. Chu, S. Chu, W. Li, P. Fu, J. Du, C. Hu, Bright upconversion emission and large strain in Er/Sb-codoped (Bi0.5Na0.5)0.945Ba0.065TiO3 ceramics, Mater. Lett. 193 (2017) 138e141. [15] J. Du, Z. Xu, R. Chu, J. Hao, W. Li, G. Jinang, P. Zheng, Rare-earth doped (K0.5Na0.5)NbO3 multifunctional ceramics, J. Mater. Sci. Mater. Electron. 28 (2017) 5288e5294. [16] J. Wang, L. Luo, The NbO6 octahedral distortion and phase structural transition of Eu3þ-doped K0.5Na0.5NbO3exLiNbO3 ferroelectric ceramics, J. Am. Ceram. Soc. 101 (2018) 400e407. [17] Z. Geng, K. Li, X. Li, D. Shi, Fabrication and photoluminescence of Eu-doped KNN-based transparent ceramics, J. Mater. Sci. 52 (2017) 2285e2295. [18] T. Shrout, S. Zhang, Lead-free piezoelectric ceramics: alternatives for PZT? J. Electroceram. 19 (2007) 113e126. [19] H. Tao, J. Wu, New poling method for piezoelectric ceramics, J. Mater. Chem. C. 5 (2017) 1601e1606. [20] C. Zhou, J. Zhang, W. Yao, D. Liu, W. Su, Highly temperature-stable piezoelectric properties of 0.96(K0.48Na0.52)(Nb0.96Sb0.04)O3-0.03BaZrO30.01(Bi0.50Na0.50)ZrO3 ceramic in common usage temperature range, Scripta Mater. 162 (2019) 86e89. [21] W. Yang, L. Wang, H. Li, J. Han, H. Xiu, Z. Zhou, Piezoelectric properties and diffusion phase transition around PPT of La-doped (Na0.52K0.44Li0.04) Nb0.8Ta0.2O3 lead-free piezoelectric ceramics, Physica B 498 (2016) 59e64. [22] W. Yang, P. Li, F. Li, X. Liu, B. Shen, J. Zhai, Enhanced piezoelectric performance and thermal stability of alkali niobate-based ceramics, Ceram. Int. 45 (2019) 2275e2280. [23] X. Lv, J. Wu, C. Zhao, D. Xiao, J. Zhu, Z. Zhang, C. Zhang, X. Zhang, Enhancing temperature stability in potassium-sodium niobate ceramics through phase boundary and composition design, J. Eur. Ceram. Soc. 39 (2019) 305e315. [24] K. Binnemans, Interpretation of europium(III) spectra, Coord. Chem. Rev. 295 (2018) 1e45. [25] H. Li, H.K. Yang, J.H. Jeong, K. Jang, H.S. Lee, S.S. Yi, Solegel synthesis, structure and photoluminescence properties of nanocrystalline Lu2MoO6:Eu, Mater. Res. Bull. 46 (2011) 1352e1358. [26] H. Li, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, K. Jang, H.S. Lee, S.S. Yi, Colorconversion and photoluminescence properties of Ba2MgW(Mo)O6:Eu phosphor, J. Alloys Compd. 509 (2011) 8788e8793.