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Upconversion luminescence and electrical properties of (K,Er) co-modified Na0·5Bi4·5Ti4O15 high-temperature piezoceramics
Journal Pre-proof Upconversion luminescence and electrical properties of (K,Er) co-modified Na0.5Bi 4.5
Ti4O15 high-temperature piezoceramics
Zhihao Zhang, Junyan Li, Lulu Liu, Jinghan Sun, Jigong Hao, Wei Li PII:
S0921-4526(19)30800-2
DOI:
https://doi.org/10.1016/j.physb.2019.411920
Reference:
PHYSB 411920
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
05 September 2019
Accepted Date:
30 November 2019
Please cite this article as: Zhihao Zhang, Junyan Li, Lulu Liu, Jinghan Sun, Jigong Hao, Wei Li, Upconversion luminescence and electrical properties of (K,Er) co-modified Na0.5Bi4.5Ti4O15 hightemperature piezoceramics, Physica B: Physics of Condensed Matter (2019), https://doi.org/10. 1016/j.physb.2019.411920
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Upconversion luminescence and electrical properties of (K,Er) comodified Na0.5Bi4.5Ti4O15 high-temperature piezoceramics Zhihao Zhang1, Junyan Li1, Lulu Liu1, Jinghan Sun1, Jigong Hao*, Wei Li* College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China
Abstract In this work, (Na,Bi)0.5-x(K,Er)xBi4Ti4O15 (NBT-xK/Er) multifunctional ceramics with a strong
green
upconversion
emission
were
prepared.
The
structural
and
electrical/luminescence properties of the ceramics were studied. Dense microstructures with single bismuth-oxide layered phase structure formed in the NBT-xK/Er samples. After K/Er doping, the samples showed good electrical properties while simultaneously obtaining a bright upconversion green photoluminescence under excitation at 980 nm. The emission intensity is related to doping concentration. At x = 0.008, the maximum emission intensity was achieved. Meanwhile, a good electrical performance (piezoelectric constant: ~22 pC/N, remnant polarization: ~16.4 C/cm2, and Curie temperature: ~626 °C) was simultaneously achieved. The samples showed good temperature stability, which make them appropriate candidates for high-temperature piezoelectric applications. The results indicate that the prepared materials feature market potential and application foreground in new multifunctional devices. Keywords: Ceramics; Ferroelectrics; Electrical properties; Luminescence
________________________ The authors contributed equally to this work. * Corresponding author. Email address: [email protected] (J. Hao); [email protected] (W. Li) 1
Journal Pre-proof 1. Introduction With the development of advanced optoelectronic technology, exploring the multifunctional properties of materials and the integration and coupling effects between different properties has become a research hotspot in recent years. As a kind of multifunctional materials, ferroelectrics are attractive for a wide range of applications [1]. To date, novel multiple functions have been discovered along with the development of multifunctional ferroelectrics. Luminescent properties are induced after introducing trivalent rare-earth (RE) elements into ferroelectrics, enriching the multifunctional category of ferroelectric materials [2]. Among various ferroelectric materials, the Aurivillius compounds with bismuthlayer structured ferroelectrics (BLSFs) are important ferroelectric materials composed of Bi oxide layer (Bi2O2)2+ and perovskite-like layer (Am-1BmO3m+1)2- arranged regularly and alternately along the c axis. BLSFs feature advantages, such as low dielectric constant, high Curie temperature, low aging rate, high resistivity, and high dielectric breakdown strength. Therefore, BLSFs present wide application prospects in piezoelectric fields given their high temperature and frequency [3]. However, one drawback of BLSFs is the high coercive field and leakage current, which cause difficulty in their complete polarization [4,5]. Thus, the ideal levels of ferroelectric/piezoelectric properties cannot be achieved. Another drawback of BLSFs is the deviation of stoichiometric ratio from the original composition, thereby producing oxygen vacancies that affect the electrical properties of the material; this formation of oxygen vacancies is mainly induced by the easy volatilization of Bi at high temperatures 2
Journal Pre-proof during sintering [6]. Researchers mainly adopted doping at A-sites to replace Bi3+ of BLSFs with RE ions to improve their electrical properties [7-10]. Such method can effectively inhibit Bi3+ volatilization and oxygen vacancy formation, thereby considerably improving the electrical properties of BLSFs [7,8]. Furthermore, RE ions are recognized as luminescent active ions, and they exhibit excellent luminescent properties when excited by ultraviolet light [2]. Therefore, as RE ions are introduced into the matrix of BLSFs materials, the new BLSFs components present abundant absorption and fluorescence emission spectra, thereby realizing the multifunction of BLSF materials. Na0.5Bi4.5Ti4O15 (NBT) belongs to the Aurivillius family of BLSFs with m = 4 [7]. At room temperature, NBT exhibits an orthorhombic phase. With increasing temperature, NBT undergoes a ferroelectric orthorhombic to paraelectric tetragonal phase transition at the Curie temperature Tc of 655 °C [11]. NBT shows considerable potential for high-temperature applications given its high Curie temperature. In the present work, (K, Er) was introduced into NBT-based ceramics to further enhance their electric properties and realize their luminescence/electrical multifunctional features. In the NBT lattice, Bi3+ and Na+ occupy A-sites. In view of their radius and electrovalence, Er3+ and K+ are considered substitutes in occupying the A-sites of NBT ceramics lattice filled by the Bi3+ and Na+ sites of the NBT lattice. After K/Er doping, the samples showed improved electrical properties while simultaneously obtaining a bright upconversion green photoluminescence. 2. Experimental procedure 3
Journal Pre-proof (Na,Bi)0.5-x(K,Er)xBi4Ti4O15 (NBT-xK/Er, x=0.000, 0.004, 0.008, 0.012) ceramics were prepared via a conventional solid-state reaction method. Na2CO3 (99.5%), K2CO3 (99%), Bi2O3 (99.975%), TiO2 (99.6%), and Er2O3 (99.9%) were used as raw materials. The raw materials were placed in an oven and dried for 2 h to remove moisture, and they were accurately weighed in accordance with the stoichiometric ratio of their composition. The weighed raw materials were ball-milled for 12 h by using ZrO2 balls and alcohol as medium. The dried mixture was pressed into pieces under a certain pressure and calcined at 850 °C for 2 h. After calcination, the powders were ground for 12 h by using the above milling equipment. After drying, an appropriate amount of polyvinyl alcohol (PVA) binder was added to the powder. The mixture was pressed to form discs with a diameter of 10 mm and a thickness of approximately 0.4–0.6 mm under a certain pressure. Finally, the samples were sintered at 1130 °C for 3 h after burning off PVA. The polished ceramics were coated with silver slurry and heated at 850 °C for 20 min to form electrodes. The phase structure of samples was analyzed using a D8 Advance X-ray powder diffractometer (Bruker Company, Germany). The micro-morphology of ceramics was analyzed by field emission electron microscopy (FE-SEM, Carl Zeiss Company, Germany). The ferroelectric properties were analyzed using a ferroelectric test system (TF Analyzer 2000 FE-Module, aixACCT, Germany). The dielectric spectrum and impedance spectra of the samples were measured by a Concept 80 broadband dielectric spectrometer (Novocontrol, Germany). For piezoelectric measurements, the samples were placed in hot silicon oil (about 180 °C) under an electric field of 50–70 kV/cm for 4
Journal Pre-proof 20 min. Then, the piezoelectric constant d33 of the polarized samples was measured using a quasi-static d33 measuring instrument (YE2730A, Sinocera Piezotronics, Inc., China). The photoluminescence properties of the ceramic samples were tested using a FLS920 fluorescence spectrometer (FLS920, Edinburgh Instruments, UK). 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the NBT-xK/Er ceramics. All NBTxK/Er ceramics have formed typical BLSF structures without impurity phases. The peaks of the ceramics can be indexed in accordance with the orthorhombic space group, A21am. This finding indicates that K+ and RE Er3+ have completely diffused into the NBT’s crystal lattice. Thus, a homogeneous solid solution was formed. In this work, K+ and Er3+ were considered substitutes occupying the A-sites of the NBT ceramics lattice. The radius of Er3+ (1.062 Å, CN=12) is close to that of Bi3+ (1.30Å, CN=12), whereas the radius of K+ (1.64Å, CN=12) is close to that of Na+ (1.39Å, CN=12). In view of the electrovalence and radius, Er3+ is considered a substitute occupying the Bi3+ sites and K+ occupying the Na+ sites of the NBT lattice. Fig. 2 shows the FE-SEM of NBT-xK/Er ceramics. All NBT-xK/Er ceramics showed the typical morphological characteristics of BLSF ceramics with uniform and dense microstructure stacked by plate-like grains [12]. The microstructure with platelike grain morphology further indicates that the grain growth of BLSF ceramics is anisotropic [13]. The surface energy of the grains perpendicular to the c-axis of the BLSF ceramics is lower than that along the c-axis [14]. Thus, in the direction of a–b plane, the growth rate of the ceramic grains is fast, whereas in the direction of the c5
Journal Pre-proof axis, the growth rate of the ceramic grains is slow. Therefore, the thickness of the platelike grain is considerably smaller than the length, leading to the highly structurally anisotropy of the grains [15]. In addition, it can be noted that the average grain size of the ceramic samples doped with K/Er exhibited no remarkable change (the average grain size is listed in Table 1), indicating that K/Er doping caused no substantial effect on the micro-morphology of NBT ceramics. Table 1 summarizes the relative densities of the NBT-xK/Er samples obtained via Archimedes’ method. All ceramics featured a high relative density (>93.6%), indicating that all samples have been well sintered at 1130 °C/3 h. Fig. 3 shows the dielectric spectrum of the NBT-xK/Er ceramics at the test frequency of 10 kHz. As shown in the figure, all NBT-xK/Er samples showed high Curie temperatures (625 °C–633 °C). Meanwhile, the samples presented a low dielectric loss tanδ of 1% at room temperature. At 550 °C test temperatures, tanδ remained considerably low (<3%). Thus, the NBT-xK/Er system features a high-temperature stability and is suitable for high-temperature applications [7,8,16]. This result can be further confirmed by the thermal annealing behavior of the NBT-xK/Er ceramics in the flowing measurements [17]. Fig. 4 shows the complex impedance plots [Z–Z] of the NBT-xK/Er ceramics at the test frequency of 10-2–2×107 Hz at different temperatures (500 °C–700 °C). An impedance diagram has been widely used to clarify the electrical conduction behavior of polycrystalline ceramics. In general, the electrical conduction behavior in ceramics is influenced by the grain, grain boundary, and electrode interface [18]. For the studied 6
Journal Pre-proof NBT-xK/Er samples, two semicircular arcs were observed in the [Z–Z] curves for all samples. These arcs showed that all samples contained two electrical components in the equivalent R–C circuit [19]. The large and small semicircles can be assigned to the grain boundary response and grain effect, respectively, and can be modeled by equivalent circuits RgbCgb and RbCb. [20]. With the increase in temperature, the semicircles decreased in size owing to the increased conductivity. The [Z–Z] data indicate that the conductivity σ of the samples can be detected. [Z–Z] Fig. 5 shows the temperature dependence of the conductivity of the NBT-xK/Er ceramics. On the basis of the conductivity as a function of temperature, the activation energy (Ea) obtained from the conductivity data can be detected using the following equation: σ =σ0 exp(-Ea/kT),
(1)
where Ea is the activation energy, σ and σ0 are the dc conductivity and the preexponential constant, respectively, k is Boltzmann’s constant, and T is the absolute temperature [21]. The solid lines represent the best least-squares fitting of Eq. (1). The achieved Ea of NBT-xK/Er was in the range of 0.73–0.76 eV. In ABO3 perovskite materials, the Ea for oxygen vacancies is in the range of 0.5–2 eV depending on concentration [22]. For A- and B-site cations, the Ea approximate 4 and 12 eV, respectively [23]. Thus, the oxygen vacancies dominate the conductivity in the present NBT-xK/Er system given that the achieved Ea is close to that of oxygen vacancy migration. Fig. 6(A) shows the P–E hysteresis loops of the NBT-xK/Er ceramics recorded at 7
Journal Pre-proof 180 °C and 10 Hz. The hysteresis loops of all compositions have reached a saturation state under the electric field of 130 kV/cm at 180 °C. From the P–E hysteresis loops, the remnant polarization (2Pr) was achieved, as summarized in Fig. 6(B). The value of 2Pr increased with the doping contents of K/Er. At x = 0.004. The samples achieved the maximum 2Pr (~16.4 C/cm2), which is higher than that of pure NBT ceramics (~14.4 C/cm2). This finding implies that the ferroelectric properties of NBT ceramics were promoted by K/Er doping. In the present work, Bi3+ volatilization was inhibited by K/Er doping, thereby decreasing the amount of defects, and this effect may be useful in significantly enhancing ferroelectric properties. Meanwhile, K/Er doping also improved the piezoelectric properties of NBT ceramics, as shown in Fig. 6(B). The piezoelectric constant d33 respectively reached 16, 18, 22, and 19 pC/N for samples with x = 0, 0.004, 0.008, 0.012. The enhancement of piezoelectric properties of ceramics induced by a proper amount of K/Er doping (x = 0.008) can be also attributed to the effective reduction of Bi3+ volatilization at the A-site by doping K/Er, benefitting the polarization of ceramics. This finding is supported by the enhanced relative density of 0.8 mol% K/Er-doped ceramics in Table 1. In addition, when Er doping was further increased from 0.008 to 0.012, d33 decreased from 22 pC/N down to 19 pC/N. This phenomenon is attributed to the decreased densification of the ceramics as shown by the decreased relative density of the 0.012 K/Er-modified sample (Table 1). Good thermal stability is crucial for BLSF materials [1] because practical applications demand high reliability during thermal shock aside from outstanding electrical properties at room temperature. In the present work, NBT-xK/Er ceramics 8
Journal Pre-proof showed good temperature stability, as shown in Fig. 6(C), in which the values of d33 of the NBT-xK/Er ceramics are plotted against the annealing temperature. At an annealing temperature of 450 °C, the values of d33 of the NBT-xK/Er ceramics showed a negligible change without evident decrease. The d33 values dropped with the increase in annealing temperature from 450 °C to 625 °C. d33 reached zero near the Curie temperature of 625 °C. The results reveal that the NBT-xK/Er ceramics exhibit good resistance to thermal annealing, making the prepared materials appropriate candidates for high-temperature piezoelectric applications [7,8]. The piezoelectric/ferroelectric and photoluminescence properties can be realized in RE-doped ferroelectrics hosts. Integrating the above properties expands the application fields of ferroelectrics as a multifunctional device [1, 24]. In the present work, Er3+ was selected as doping ion, exhibiting a strong green upconversion fluorescence [24,25]. Thus, NBT-xK/Er may exhibit multifunctional characteristics including upconversion luminescence performance and good piezoelectric/ferroelectric properties. Fig. 7(a) shows the upconversion emission spectrum of the NBT-xK/Er ceramics under 980 nm optical excitation. The inset shows the variation in the intensity of the green emission band (4S3/2→4I15/2 transition emission at 550 nm) with K/Er doping. Fig. 7(b) shows the typical energy level diagram for Er3+. The upconversion luminescence process mainly consists of several parts: ground state absorption, excited state absorption (ESA), energy transfer upconversion, radiative transition, and multiphonon [26]. Under 980 nm excitation, Er3+ are excited initially from the ground state 4I15/2 to the 4I11/2 state after absorbing a 980 nm photon. Subsequently, the Er3+ is further excited 9
Journal Pre-proof from the 4I11/2 state to the 4F7/2 state after a photon is absorbed via ESA. Then, the Er3+ at the 4F7/2 state are relaxed to the ground state with a radiative transition of strong green (2H11/2→4I15/2, at 526 nm; 4S3/2→4I15/2, at 549 nm for the present NBT-xK/Er system) and weak red (4F9/2→4I15/2, at 663 nm for the present NBT-xK/Er system) emission bands, respectively [26]. On the basis of the typical energy level diagram for Er3+, the emission spectra of the ceramics in the present NBT-xK/Er system exhibited three emission peaks under the excitation of 980 nm: a relatively weak red emission peak (at 663 nm, corresponding to 4F
4 9/2→ I15/2
transition) and two strong green emission peaks (at 526 and 549 nm,
corresponding to 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions, respectively). Compared with the intensity of green emission, the intensity of red emission was considerably weaker for the ceramics. As a result, the whole spectrum pumped by 980 nm LD exhibited a bright green color. Furthermore, the intensity of the emission peak increased with the increase in K/Er doping, and the maximum emission was obtained at the doping level of 0.008 mol. The sample with 0.012 mol K/Er content showed a decreased emission intensity. This finding can be attributed to the concentration-quenching effect [27,28]. In the present work, when Er3+ content was lower than 0.008, the average distance between the adjacent Er3+ was remarkable, and the interaction between Er3+ ions was remarkably weak. As a result, no concentration-quenching effect occurred, leading to the increased upconversion luminescence intensity with increasing Er3+ concentration. However, with the further increment in Er3+ concentration up to 0.012, the distance between Er3+ ions decreased, and a fraction of energy migrated to the quenchers, 10
Journal Pre-proof leading to the decreased upconversion luminescence intensity [24]. 4. Conclusion In summary, good electrical properties and strong green upconversion emission were achieved in K/Er-modified NBT ceramics. When the K/Er doping level was 0.008 mol, an optimal electrical performance with a large d33 of 22 pC/N, large 2Pr of 16.4 C/cm2, and high Tc of 626 °C was achieved. The sample showed exceptionally good temperature stability, making the materials appropriate candidates for high-temperature piezoelectric applications. Furthermore, the K/Er-modified NBT ceramics exhibited a strong green upconversion emission aside from their good piezoelectric/ferroelectric properties. These results suggest that the K/Er-modified NBT system features market potential and application foreground in new multifunctional devices. Acknowledgments This work was supported by the National Key R&D Program of China (No. 2016YFB0402701), the Research Foundation of Liaocheng University (No. 318011906). The National Undergraduate Training Programs for Innovation and Entrepreneurship of China (No. 201910447002). The Undergraduate Training Programs for Innovation and Entrepreneurship of Liaocheng University (Nos. CXCY2019y002; CXCY2018061). References [1] J.G. Hao, W. Li, J.W. Zhai, H. Chen, Progress in high-strain perovskite piezoelectric ceramics, Mater. Sci. Eng. R 135 (2019) 1-57. [2] J. Zhang, X. Wang, G. Marriott, C. Xu, Trap-controlled mechanoluminescent materials, Prog. Mater. Sci. 103 (2019) 678-742. [3] C.M. Wang, L. Zhao, Y. Liu, R.L. Withers, S. Zhang, Q. Wang, The temperature-dependent piezoelectric and electromechanical properties of cobalt-modified sodium bismuth titanate, Ceram. 11
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Figure captions Fig. 1. shows the XRD patterns of NBT-xK/Er ceramics. Fig. 2. Field emission electron microscopy of NBT-xK/Er ceramics samples sintered at 1130oC for 3h. Fig. 3. Temperature dependence of (a) dielectric constant εr and (b) dielectric loss tanδ of NBT-xK/Er ceramics at the test frequency 10 kHz. Fig. 4. The complex impedance plots [Z–Z] of NBT-xK/Er ceramics in the test frequency of (10-2~2×107Hz) at different temperatures (500~700oC). Fig. 5. The temperature dependence of the conductivity of the NBT-xK/Er ceramics measured in the temperature range from 500 to 700°C. Fig. 6. The P-E hysteresis loops of NBT-xK/Er ceramics with (a) x = 0, (b) x = 0.004, (c) x = 0.008 and (d) x = 0.012, recorded at 180oC and 10 Hz. (e) Composition dependence of the remnant polarization (2Pr) and piezoelectric constant (d33) for NBTxK/Er ceramics. (f) The dependence of d33 of NBT-xK/Er ceramics on annealing temperature. Fig. 7. (a) Upconversion emission spectrum of NBT-xK/Er ceramics under 980 nm optical excitation [Inset shows the intensity of green emission band at 550 nm (4S3/2→4I15/2 transition emission)]. (b) The energy level diagram for Er3+ ions. 14
Journal Pre-proof Conflict of Interest Form We declared that we have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Journal Pre-proof Authors’contributions All authors read and approved the manuscript. Jigong Hao and Wei Li conceived and designed the study. Zhihao
Zhang,
Junyan
Li,
Lulu
Liu,
Jinghan
Sun
performed
the
experiments/measurements and wrote the paper. In the revised manuscript, they revised the manuscript according to the reviewer’s comments. Jigong Hao and Wei Li reviewed and revised the manuscript.
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(a)
(b)
Journal Pre-proof Table 1 Average grain sizes and relative densities of NBT-xK/Er samples Sample x=0 x = 0.004 x = 0.008 x = 0.012
Sintering conditions 1130oC, 3h 1130oC, 3h 1130oC, 3h 1130oC, 3h
Average grain size (m) 5.46 5.26 5.11 5.82
Relative density (%) 95.6 95.2 95.9 93.6
Ti4O15 high-temperature piezoceramics
Zhihao Zhang, Junyan Li, Lulu Liu, Jinghan Sun, Jigong Hao, Wei Li PII:
S0921-4526(19)30800-2
DOI:
https://doi.org/10.1016/j.physb.2019.411920
Reference:
PHYSB 411920
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
05 September 2019
Accepted Date:
30 November 2019
Please cite this article as: Zhihao Zhang, Junyan Li, Lulu Liu, Jinghan Sun, Jigong Hao, Wei Li, Upconversion luminescence and electrical properties of (K,Er) co-modified Na0.5Bi4.5Ti4O15 hightemperature piezoceramics, Physica B: Physics of Condensed Matter (2019), https://doi.org/10. 1016/j.physb.2019.411920
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Upconversion luminescence and electrical properties of (K,Er) comodified Na0.5Bi4.5Ti4O15 high-temperature piezoceramics Zhihao Zhang1, Junyan Li1, Lulu Liu1, Jinghan Sun1, Jigong Hao*, Wei Li* College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China
Abstract In this work, (Na,Bi)0.5-x(K,Er)xBi4Ti4O15 (NBT-xK/Er) multifunctional ceramics with a strong
green
upconversion
emission
were
prepared.
The
structural
and
electrical/luminescence properties of the ceramics were studied. Dense microstructures with single bismuth-oxide layered phase structure formed in the NBT-xK/Er samples. After K/Er doping, the samples showed good electrical properties while simultaneously obtaining a bright upconversion green photoluminescence under excitation at 980 nm. The emission intensity is related to doping concentration. At x = 0.008, the maximum emission intensity was achieved. Meanwhile, a good electrical performance (piezoelectric constant: ~22 pC/N, remnant polarization: ~16.4 C/cm2, and Curie temperature: ~626 °C) was simultaneously achieved. The samples showed good temperature stability, which make them appropriate candidates for high-temperature piezoelectric applications. The results indicate that the prepared materials feature market potential and application foreground in new multifunctional devices. Keywords: Ceramics; Ferroelectrics; Electrical properties; Luminescence
________________________ The authors contributed equally to this work. * Corresponding author. Email address: [email protected] (J. Hao); [email protected] (W. Li) 1
Journal Pre-proof 1. Introduction With the development of advanced optoelectronic technology, exploring the multifunctional properties of materials and the integration and coupling effects between different properties has become a research hotspot in recent years. As a kind of multifunctional materials, ferroelectrics are attractive for a wide range of applications [1]. To date, novel multiple functions have been discovered along with the development of multifunctional ferroelectrics. Luminescent properties are induced after introducing trivalent rare-earth (RE) elements into ferroelectrics, enriching the multifunctional category of ferroelectric materials [2]. Among various ferroelectric materials, the Aurivillius compounds with bismuthlayer structured ferroelectrics (BLSFs) are important ferroelectric materials composed of Bi oxide layer (Bi2O2)2+ and perovskite-like layer (Am-1BmO3m+1)2- arranged regularly and alternately along the c axis. BLSFs feature advantages, such as low dielectric constant, high Curie temperature, low aging rate, high resistivity, and high dielectric breakdown strength. Therefore, BLSFs present wide application prospects in piezoelectric fields given their high temperature and frequency [3]. However, one drawback of BLSFs is the high coercive field and leakage current, which cause difficulty in their complete polarization [4,5]. Thus, the ideal levels of ferroelectric/piezoelectric properties cannot be achieved. Another drawback of BLSFs is the deviation of stoichiometric ratio from the original composition, thereby producing oxygen vacancies that affect the electrical properties of the material; this formation of oxygen vacancies is mainly induced by the easy volatilization of Bi at high temperatures 2
Journal Pre-proof during sintering [6]. Researchers mainly adopted doping at A-sites to replace Bi3+ of BLSFs with RE ions to improve their electrical properties [7-10]. Such method can effectively inhibit Bi3+ volatilization and oxygen vacancy formation, thereby considerably improving the electrical properties of BLSFs [7,8]. Furthermore, RE ions are recognized as luminescent active ions, and they exhibit excellent luminescent properties when excited by ultraviolet light [2]. Therefore, as RE ions are introduced into the matrix of BLSFs materials, the new BLSFs components present abundant absorption and fluorescence emission spectra, thereby realizing the multifunction of BLSF materials. Na0.5Bi4.5Ti4O15 (NBT) belongs to the Aurivillius family of BLSFs with m = 4 [7]. At room temperature, NBT exhibits an orthorhombic phase. With increasing temperature, NBT undergoes a ferroelectric orthorhombic to paraelectric tetragonal phase transition at the Curie temperature Tc of 655 °C [11]. NBT shows considerable potential for high-temperature applications given its high Curie temperature. In the present work, (K, Er) was introduced into NBT-based ceramics to further enhance their electric properties and realize their luminescence/electrical multifunctional features. In the NBT lattice, Bi3+ and Na+ occupy A-sites. In view of their radius and electrovalence, Er3+ and K+ are considered substitutes in occupying the A-sites of NBT ceramics lattice filled by the Bi3+ and Na+ sites of the NBT lattice. After K/Er doping, the samples showed improved electrical properties while simultaneously obtaining a bright upconversion green photoluminescence. 2. Experimental procedure 3
Journal Pre-proof (Na,Bi)0.5-x(K,Er)xBi4Ti4O15 (NBT-xK/Er, x=0.000, 0.004, 0.008, 0.012) ceramics were prepared via a conventional solid-state reaction method. Na2CO3 (99.5%), K2CO3 (99%), Bi2O3 (99.975%), TiO2 (99.6%), and Er2O3 (99.9%) were used as raw materials. The raw materials were placed in an oven and dried for 2 h to remove moisture, and they were accurately weighed in accordance with the stoichiometric ratio of their composition. The weighed raw materials were ball-milled for 12 h by using ZrO2 balls and alcohol as medium. The dried mixture was pressed into pieces under a certain pressure and calcined at 850 °C for 2 h. After calcination, the powders were ground for 12 h by using the above milling equipment. After drying, an appropriate amount of polyvinyl alcohol (PVA) binder was added to the powder. The mixture was pressed to form discs with a diameter of 10 mm and a thickness of approximately 0.4–0.6 mm under a certain pressure. Finally, the samples were sintered at 1130 °C for 3 h after burning off PVA. The polished ceramics were coated with silver slurry and heated at 850 °C for 20 min to form electrodes. The phase structure of samples was analyzed using a D8 Advance X-ray powder diffractometer (Bruker Company, Germany). The micro-morphology of ceramics was analyzed by field emission electron microscopy (FE-SEM, Carl Zeiss Company, Germany). The ferroelectric properties were analyzed using a ferroelectric test system (TF Analyzer 2000 FE-Module, aixACCT, Germany). The dielectric spectrum and impedance spectra of the samples were measured by a Concept 80 broadband dielectric spectrometer (Novocontrol, Germany). For piezoelectric measurements, the samples were placed in hot silicon oil (about 180 °C) under an electric field of 50–70 kV/cm for 4
Journal Pre-proof 20 min. Then, the piezoelectric constant d33 of the polarized samples was measured using a quasi-static d33 measuring instrument (YE2730A, Sinocera Piezotronics, Inc., China). The photoluminescence properties of the ceramic samples were tested using a FLS920 fluorescence spectrometer (FLS920, Edinburgh Instruments, UK). 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the NBT-xK/Er ceramics. All NBTxK/Er ceramics have formed typical BLSF structures without impurity phases. The peaks of the ceramics can be indexed in accordance with the orthorhombic space group, A21am. This finding indicates that K+ and RE Er3+ have completely diffused into the NBT’s crystal lattice. Thus, a homogeneous solid solution was formed. In this work, K+ and Er3+ were considered substitutes occupying the A-sites of the NBT ceramics lattice. The radius of Er3+ (1.062 Å, CN=12) is close to that of Bi3+ (1.30Å, CN=12), whereas the radius of K+ (1.64Å, CN=12) is close to that of Na+ (1.39Å, CN=12). In view of the electrovalence and radius, Er3+ is considered a substitute occupying the Bi3+ sites and K+ occupying the Na+ sites of the NBT lattice. Fig. 2 shows the FE-SEM of NBT-xK/Er ceramics. All NBT-xK/Er ceramics showed the typical morphological characteristics of BLSF ceramics with uniform and dense microstructure stacked by plate-like grains [12]. The microstructure with platelike grain morphology further indicates that the grain growth of BLSF ceramics is anisotropic [13]. The surface energy of the grains perpendicular to the c-axis of the BLSF ceramics is lower than that along the c-axis [14]. Thus, in the direction of a–b plane, the growth rate of the ceramic grains is fast, whereas in the direction of the c5
Journal Pre-proof axis, the growth rate of the ceramic grains is slow. Therefore, the thickness of the platelike grain is considerably smaller than the length, leading to the highly structurally anisotropy of the grains [15]. In addition, it can be noted that the average grain size of the ceramic samples doped with K/Er exhibited no remarkable change (the average grain size is listed in Table 1), indicating that K/Er doping caused no substantial effect on the micro-morphology of NBT ceramics. Table 1 summarizes the relative densities of the NBT-xK/Er samples obtained via Archimedes’ method. All ceramics featured a high relative density (>93.6%), indicating that all samples have been well sintered at 1130 °C/3 h. Fig. 3 shows the dielectric spectrum of the NBT-xK/Er ceramics at the test frequency of 10 kHz. As shown in the figure, all NBT-xK/Er samples showed high Curie temperatures (625 °C–633 °C). Meanwhile, the samples presented a low dielectric loss tanδ of 1% at room temperature. At 550 °C test temperatures, tanδ remained considerably low (<3%). Thus, the NBT-xK/Er system features a high-temperature stability and is suitable for high-temperature applications [7,8,16]. This result can be further confirmed by the thermal annealing behavior of the NBT-xK/Er ceramics in the flowing measurements [17]. Fig. 4 shows the complex impedance plots [Z–Z] of the NBT-xK/Er ceramics at the test frequency of 10-2–2×107 Hz at different temperatures (500 °C–700 °C). An impedance diagram has been widely used to clarify the electrical conduction behavior of polycrystalline ceramics. In general, the electrical conduction behavior in ceramics is influenced by the grain, grain boundary, and electrode interface [18]. For the studied 6
Journal Pre-proof NBT-xK/Er samples, two semicircular arcs were observed in the [Z–Z] curves for all samples. These arcs showed that all samples contained two electrical components in the equivalent R–C circuit [19]. The large and small semicircles can be assigned to the grain boundary response and grain effect, respectively, and can be modeled by equivalent circuits RgbCgb and RbCb. [20]. With the increase in temperature, the semicircles decreased in size owing to the increased conductivity. The [Z–Z] data indicate that the conductivity σ of the samples can be detected. [Z–Z] Fig. 5 shows the temperature dependence of the conductivity of the NBT-xK/Er ceramics. On the basis of the conductivity as a function of temperature, the activation energy (Ea) obtained from the conductivity data can be detected using the following equation: σ =σ0 exp(-Ea/kT),
(1)
where Ea is the activation energy, σ and σ0 are the dc conductivity and the preexponential constant, respectively, k is Boltzmann’s constant, and T is the absolute temperature [21]. The solid lines represent the best least-squares fitting of Eq. (1). The achieved Ea of NBT-xK/Er was in the range of 0.73–0.76 eV. In ABO3 perovskite materials, the Ea for oxygen vacancies is in the range of 0.5–2 eV depending on concentration [22]. For A- and B-site cations, the Ea approximate 4 and 12 eV, respectively [23]. Thus, the oxygen vacancies dominate the conductivity in the present NBT-xK/Er system given that the achieved Ea is close to that of oxygen vacancy migration. Fig. 6(A) shows the P–E hysteresis loops of the NBT-xK/Er ceramics recorded at 7
Journal Pre-proof 180 °C and 10 Hz. The hysteresis loops of all compositions have reached a saturation state under the electric field of 130 kV/cm at 180 °C. From the P–E hysteresis loops, the remnant polarization (2Pr) was achieved, as summarized in Fig. 6(B). The value of 2Pr increased with the doping contents of K/Er. At x = 0.004. The samples achieved the maximum 2Pr (~16.4 C/cm2), which is higher than that of pure NBT ceramics (~14.4 C/cm2). This finding implies that the ferroelectric properties of NBT ceramics were promoted by K/Er doping. In the present work, Bi3+ volatilization was inhibited by K/Er doping, thereby decreasing the amount of defects, and this effect may be useful in significantly enhancing ferroelectric properties. Meanwhile, K/Er doping also improved the piezoelectric properties of NBT ceramics, as shown in Fig. 6(B). The piezoelectric constant d33 respectively reached 16, 18, 22, and 19 pC/N for samples with x = 0, 0.004, 0.008, 0.012. The enhancement of piezoelectric properties of ceramics induced by a proper amount of K/Er doping (x = 0.008) can be also attributed to the effective reduction of Bi3+ volatilization at the A-site by doping K/Er, benefitting the polarization of ceramics. This finding is supported by the enhanced relative density of 0.8 mol% K/Er-doped ceramics in Table 1. In addition, when Er doping was further increased from 0.008 to 0.012, d33 decreased from 22 pC/N down to 19 pC/N. This phenomenon is attributed to the decreased densification of the ceramics as shown by the decreased relative density of the 0.012 K/Er-modified sample (Table 1). Good thermal stability is crucial for BLSF materials [1] because practical applications demand high reliability during thermal shock aside from outstanding electrical properties at room temperature. In the present work, NBT-xK/Er ceramics 8
Journal Pre-proof showed good temperature stability, as shown in Fig. 6(C), in which the values of d33 of the NBT-xK/Er ceramics are plotted against the annealing temperature. At an annealing temperature of 450 °C, the values of d33 of the NBT-xK/Er ceramics showed a negligible change without evident decrease. The d33 values dropped with the increase in annealing temperature from 450 °C to 625 °C. d33 reached zero near the Curie temperature of 625 °C. The results reveal that the NBT-xK/Er ceramics exhibit good resistance to thermal annealing, making the prepared materials appropriate candidates for high-temperature piezoelectric applications [7,8]. The piezoelectric/ferroelectric and photoluminescence properties can be realized in RE-doped ferroelectrics hosts. Integrating the above properties expands the application fields of ferroelectrics as a multifunctional device [1, 24]. In the present work, Er3+ was selected as doping ion, exhibiting a strong green upconversion fluorescence [24,25]. Thus, NBT-xK/Er may exhibit multifunctional characteristics including upconversion luminescence performance and good piezoelectric/ferroelectric properties. Fig. 7(a) shows the upconversion emission spectrum of the NBT-xK/Er ceramics under 980 nm optical excitation. The inset shows the variation in the intensity of the green emission band (4S3/2→4I15/2 transition emission at 550 nm) with K/Er doping. Fig. 7(b) shows the typical energy level diagram for Er3+. The upconversion luminescence process mainly consists of several parts: ground state absorption, excited state absorption (ESA), energy transfer upconversion, radiative transition, and multiphonon [26]. Under 980 nm excitation, Er3+ are excited initially from the ground state 4I15/2 to the 4I11/2 state after absorbing a 980 nm photon. Subsequently, the Er3+ is further excited 9
Journal Pre-proof from the 4I11/2 state to the 4F7/2 state after a photon is absorbed via ESA. Then, the Er3+ at the 4F7/2 state are relaxed to the ground state with a radiative transition of strong green (2H11/2→4I15/2, at 526 nm; 4S3/2→4I15/2, at 549 nm for the present NBT-xK/Er system) and weak red (4F9/2→4I15/2, at 663 nm for the present NBT-xK/Er system) emission bands, respectively [26]. On the basis of the typical energy level diagram for Er3+, the emission spectra of the ceramics in the present NBT-xK/Er system exhibited three emission peaks under the excitation of 980 nm: a relatively weak red emission peak (at 663 nm, corresponding to 4F
4 9/2→ I15/2
transition) and two strong green emission peaks (at 526 and 549 nm,
corresponding to 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions, respectively). Compared with the intensity of green emission, the intensity of red emission was considerably weaker for the ceramics. As a result, the whole spectrum pumped by 980 nm LD exhibited a bright green color. Furthermore, the intensity of the emission peak increased with the increase in K/Er doping, and the maximum emission was obtained at the doping level of 0.008 mol. The sample with 0.012 mol K/Er content showed a decreased emission intensity. This finding can be attributed to the concentration-quenching effect [27,28]. In the present work, when Er3+ content was lower than 0.008, the average distance between the adjacent Er3+ was remarkable, and the interaction between Er3+ ions was remarkably weak. As a result, no concentration-quenching effect occurred, leading to the increased upconversion luminescence intensity with increasing Er3+ concentration. However, with the further increment in Er3+ concentration up to 0.012, the distance between Er3+ ions decreased, and a fraction of energy migrated to the quenchers, 10
Journal Pre-proof leading to the decreased upconversion luminescence intensity [24]. 4. Conclusion In summary, good electrical properties and strong green upconversion emission were achieved in K/Er-modified NBT ceramics. When the K/Er doping level was 0.008 mol, an optimal electrical performance with a large d33 of 22 pC/N, large 2Pr of 16.4 C/cm2, and high Tc of 626 °C was achieved. The sample showed exceptionally good temperature stability, making the materials appropriate candidates for high-temperature piezoelectric applications. Furthermore, the K/Er-modified NBT ceramics exhibited a strong green upconversion emission aside from their good piezoelectric/ferroelectric properties. These results suggest that the K/Er-modified NBT system features market potential and application foreground in new multifunctional devices. Acknowledgments This work was supported by the National Key R&D Program of China (No. 2016YFB0402701), the Research Foundation of Liaocheng University (No. 318011906). The National Undergraduate Training Programs for Innovation and Entrepreneurship of China (No. 201910447002). The Undergraduate Training Programs for Innovation and Entrepreneurship of Liaocheng University (Nos. CXCY2019y002; CXCY2018061). References [1] J.G. Hao, W. Li, J.W. Zhai, H. Chen, Progress in high-strain perovskite piezoelectric ceramics, Mater. Sci. Eng. R 135 (2019) 1-57. [2] J. Zhang, X. Wang, G. Marriott, C. Xu, Trap-controlled mechanoluminescent materials, Prog. Mater. Sci. 103 (2019) 678-742. [3] C.M. Wang, L. Zhao, Y. Liu, R.L. Withers, S. Zhang, Q. Wang, The temperature-dependent piezoelectric and electromechanical properties of cobalt-modified sodium bismuth titanate, Ceram. 11
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Figure captions Fig. 1. shows the XRD patterns of NBT-xK/Er ceramics. Fig. 2. Field emission electron microscopy of NBT-xK/Er ceramics samples sintered at 1130oC for 3h. Fig. 3. Temperature dependence of (a) dielectric constant εr and (b) dielectric loss tanδ of NBT-xK/Er ceramics at the test frequency 10 kHz. Fig. 4. The complex impedance plots [Z–Z] of NBT-xK/Er ceramics in the test frequency of (10-2~2×107Hz) at different temperatures (500~700oC). Fig. 5. The temperature dependence of the conductivity of the NBT-xK/Er ceramics measured in the temperature range from 500 to 700°C. Fig. 6. The P-E hysteresis loops of NBT-xK/Er ceramics with (a) x = 0, (b) x = 0.004, (c) x = 0.008 and (d) x = 0.012, recorded at 180oC and 10 Hz. (e) Composition dependence of the remnant polarization (2Pr) and piezoelectric constant (d33) for NBTxK/Er ceramics. (f) The dependence of d33 of NBT-xK/Er ceramics on annealing temperature. Fig. 7. (a) Upconversion emission spectrum of NBT-xK/Er ceramics under 980 nm optical excitation [Inset shows the intensity of green emission band at 550 nm (4S3/2→4I15/2 transition emission)]. (b) The energy level diagram for Er3+ ions. 14
Journal Pre-proof Conflict of Interest Form We declared that we have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Journal Pre-proof Authors’contributions All authors read and approved the manuscript. Jigong Hao and Wei Li conceived and designed the study. Zhihao
Zhang,
Junyan
Li,
Lulu
Liu,
Jinghan
Sun
performed
the
experiments/measurements and wrote the paper. In the revised manuscript, they revised the manuscript according to the reviewer’s comments. Jigong Hao and Wei Li reviewed and revised the manuscript.
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(a)
(b)
Journal Pre-proof Table 1 Average grain sizes and relative densities of NBT-xK/Er samples Sample x=0 x = 0.004 x = 0.008 x = 0.012
Sintering conditions 1130oC, 3h 1130oC, 3h 1130oC, 3h 1130oC, 3h
Average grain size (m) 5.46 5.26 5.11 5.82
Relative density (%) 95.6 95.2 95.9 93.6