Largely enhanced piezoelectric and luminescent properties of Er doped BST ceramics

Abstract

Multifunctional Ba_1−xTi_0.96Sn_0.04O₃ + x mol% Er (BST-Er) ceramics were prepared using a solid state reaction method. The large piezoelectric coefficient (d₃₃ = 400 pC N⁻¹) and field induced strain (S = 0.175%) were obtained for the BST-Er ceramics at x = 0.5. The polymorphic phase transitions (PPT) from the orthorhombic phase to the tetragonal phase, which contributes to the high piezoelectricities, was identified at room temperature (RT). There are bright green emission bands centered at 520 and 550 nm in up-conversion (UC) luminescence spectra measured under 980 nm laser excitation, which correspond to the radiative transitions from ²H_11/2 and ⁴S_3/2 to ⁴I_15/2, respectively. The fluorescence intensity ratio of green UC emissions at 520 and 550 nm was investigated in the temperature range of 233–413 K. The maximum sensing sensitivity was found to be 0.0033 K⁻¹. The results reveal that the BST-Er ceramics with simple composition are promising multifunctional sensing materials.

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Introduction

There is an urgent demand for high performance lead-free piezoelectrics to substitute for the current lead zirconate titanate (PZT) family. Recently, a triple point type morphotropic phase boundary (MPB) in a lead-free Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)TiO₃ (BCZT) system was reported to show piezoelectricity as excellent as soft PZT at room temperature (RT).¹

The high piezoelectric response of BCZT is attributed to the existence of MPB or polymorphic phase transitions (PPT), because the phase coexistence at MPB or PPT can cause an easy polarization rotation for the dipole, by an external stress or electric field.^2–4 Both intrinsic and extrinsic piezoelectric contributions are believed to be associated with the outstanding piezoelectric properties of MPB or PPT.^5,6 Therefore, two-phase or multiphase coexistence near RT have been designed to enhance the piezoelectric properties. Yao et al.⁷ reported that a large piezoelectric coefficient (d₃₃ ∼ 697 pC N⁻¹) was obtained at a quasi-quadruple point, where cubic–tetragonal–orthorhombic–rhombohedral phases coexisted together. In addition, the BCZT ceramics textured by a template-grain growth method,⁸ or sintered using ZnO as the sintering aid,⁹ showed improved piezoelectric properties.

The BCZT ceramics possess superior piezoelectric properties. Moreover, it is expected that the BCZT ceramics could exhibit good dielectric properties,¹⁰ electrocaloric effects¹¹ and optical properties,¹² and hold potential in various applications. With the development of a functional material and device, the materials which could realize multiple functions are quite desirable. It is reported that rare earth element (Pr and Er) doped ferroelectrics have both improved photoluminescence and electric properties.^13,14 The Er doped BCZT ceramics exhibit good piezoelectric properties and photoluminescence properties.¹⁵ Therefore, according to the theories of BCZT, the Ba_1−xTi_0.96Sn_0.04O₃ + x mol% Er (BST-Er) ceramics were designed to obtain high piezoelectric properties and up-conversion (UC) luminescent properties. In this work, the piezoelectric properties and UC luminescent properties of the BST-Er ceramics were investigated. The mechanism could result in designing multifunctional systems that are close to application.

Experimental procedure

The Ba_1−xTi_0.96Sn_0.04O₃ + x mol% Er (x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0) ceramics (BST-Er) were prepared using a conventional solid-state reaction technique. Raw materials of BaCO₃ (99.0%), SnO₂ (99.0%), TiO₂ (99.5%) and Er₂O₃ (99.0%) were mixed according to a predetermined ratio with the addition of alcohol. The mixed raw materials were dried at 80 °C for 10 h and then calcined at 1200 °C for 4 h. Thereafter, calcined powders were remixed and pressed into 12 mm-diameter pellets and sintered at 1400 °C for 4 h in air. Phase structure was examined using an X-ray diffraction meter with Cu K_α1 radiation (λ = 1.5406 Å) (XRD, D8 Advance, Bruker Inc., Germany). The dielectric properties were measured using a precision impedance analyzer (4294A Agilent Inc., USA) at 100 kHz. Electric-field-induced polarization (P–E) and strain (S–E) measurements were carried out using an aix-ACCT TF2000FE-HV ferroelectric test unit (aix-ACCT Inc., Germany) connected with a miniature plane mirror interferometer and an accessory laser interferometer vibrometer (SP-S 120/500; SIOS Mebtechnik GmbH, Ilmenau, Germany). The piezoelectric constant was measured using a quasi-static d₃₃ meter (YE2730 SINOCERA, China). Photoluminescence spectra were recorded using a fluorescence spectrophotometer (F-7000, HITACHI, Japan) under the excitation of a 980 nm laser diode. The temperature of the samples was controlled using a temperature-controlled stage (Linkam HFS600E-PB2). Luminescence photographs were taken using a common digital camera in the dark.

Results and discussion

Fig. 1 shows XRD patterns of the BST-Er ceramics in the 2θ range of 20–70°. It suggests that all the samples have a single phase of perovskite structure at RT. To determine the dependence of phase structure on Er content, fine-scanning XRD patterns for all compositions at around 2θ of 45° and 66° are provided in the insets of Fig. 1. It can be seen that the BST-Er ceramic at x = 0.2 has an orthorhombic phase, characterized by the single peak of (200) and (220) at around 2θ of 45° and 66°, respectively. The BST-Er ceramics become tetragonal phase, featured with the splitting of (002)/(200) and (202)/(220) peaks, with the increase in Er content (0.4 ≤ x ≤ 0.6). The BST-Er ceramics have a single tetragonal phase, when x ≥ 0.8. Therefore, it can be concluded that the tetragonal phase and the orthorhombic phase coexist in BST-Er ceramics at 0.4 ≤ x ≤ 0.6 at RT.

Fig. 1 XRD patterns of the BST-Er ceramics.

Fig. 2 shows the temperature-dependent dielectric constant (ε_r–T) of the BST-Er ceramics measured within the temperature range of 243 K to 393 K (100 kHz), and the inset is the T_O–T and T_R–O as a function of Er content. It can be seen that all the BST-Er ceramics have three remarkable ε_r peaks, which correspond to phase transitions of tetragonal–cubic (T_C), orthorhombic–tetragonal (T_O–T) and rhombohedral–orthorhombic (T_R–O).^16–18 The T_C of all the BST-Er ceramics are at around 353 K. The T_O–T are 305 K, 297 K, 294 K, 290 K, 284 K and 279 K, for the BST-Er ceramics at x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0 respectively. The T_R–O are 279 K, 268 K, 266 K, 264 K, 258 K and 253 K, for the BST-Er ceramics at x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0 respectively. The rate of the decrease of T_O–T and T_R–O with the addition of Er are −32 K mol⁻¹% and −31 K mol⁻¹%. This result indicates that Er addition does not strongly affect the T_C, but pushes the T_O–T and T_R–O to a lower temperature in the BST-Er system. Both of the T_O–T and T_R–O shift to a low temperature, while the T_O–T of the BST-Er ceramic at x = 0.5 is just at RT. It also confirms that the PPT of the orthorhombic–tetragonal phases for the BST-Er ceramic at x = 0.5 occurs at RT.

Fig. 2 Temperature-dependent dielectric constant (ε_r–T) of the BST-Er ceramics measured within the temperature range of 243 K to 393 K (100 kHz) (inset is the T_O–T and T_R–O as a function of Er content).

Fig. 3 shows (a) P–E hysteresis loops (inset is the d₃₃) and (b) bipolar strain curves of the BST-Er ceramics. All the samples exhibit good square-shaped hysteresis loop and butterfly shaped strain hysteresis loop, which is typical for ferroelectrics. The piezoelectric constant ∼ d₃₃, remnant polarization ∼ P_r and strain ∼ S are 320 pC N⁻¹, 8.4 μC cm⁻² and 0.135%, respectively, for the BST-Er ceramic at x = 0.2. With an increase in Er content, the highest d₃₃ (400 pC N⁻¹), P_r (10.7 μC cm⁻²) and S (0.175%), are obtained for the BST-Er ceramic at the PPT composition of x = 0.5. On one hand, the high piezoelectric response can be ascribed to the enhancement of polarization rotation that facilitated at the PPT, which possesses a flat Gibbs free energy profile of phases.³ On the other hand, a contributor for the high piezoelectric response lies on the extrinsic piezoelectric activity, i.e., the displacement of domain walls and the motion of interphase boundaries at the PPT region.^6,19 The piezoelectric response and ferroelectric properties decrease sharply with the further addition of Er content (x = 0.8 and 1.0). The reduced piezoelectric properties are due to the PPT being shifted below RT with Er doping.²⁰

Fig. 3 (a) P–E hysteresis loops (inset is the d₃₃) and (b) bipolar strain curves of the BST-Er ceramics.

For infra-red sensing applications, the photoluminescence performances of the BST-Er ceramics require investigation. Fig. 4 shows UC emission spectral patterns of BST-Er ceramics under laser 980 nm excitation at RT (the insets are the photograph and dependence of the intensity (ln I) of the bands centered at 520 and 550 nm as a function of the excitation power (ln P) of the BST-Er (x = 0.8) ceramic). The BST-Er ceramics exhibit strong green UC emission that can be easily seen by the naked eye at RT. The RT UC emission intensities increase with Er content up to x = 0.8, and then decrease because of concentration quenching,¹⁴ indicating that the maximum emission intensity occurs at x = 0.8 in the BST-Er ceramics. The UC efficiency of the BST-Er ceramics depends heavily on the doping concentration of the activators or luminous centers. On the one hand, the high concentration of Er³⁺ ions can increase the ions number of absorbing pumping source and promote energy transfer between neighboring Er³⁺ ions excited to an intermediate energy level. On the other hand, BST-Er ceramics show an energy quenching effect due to non-radiative relaxation with increasing Er concentration. The two effects depend on the nature of the host material and the microscopic distribution of the Er³⁺ ions in the host material.²¹ Therefore, the UC emission intensity peaks at x = 0.8 for the BST-Er ceramics. The UC emission consists of two strong green bands centered at 520 and 550 nm corresponding to ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ion. Photoluminescence intensity I is approximately proportional to the power of the pumping laser, I ∝ Pⁿ, where n is the number of photons involved in the pumping mechanism. The slope value n equals 1.7 for the green emissions, indicating that the two photon-excitation mechanism is dominant in the green emission.¹³

Fig. 4 UC emission spectral patterns of BST-xEr under 980 nm laser excitation at RT. The insets show the photograph and dependence of the intensity (ln I) of the bands centered at 520 and 550 nm, as a function of the excitation power (ln P) of the BST-Er (x = 0.8) ceramic.

The variation of green UC emissions of the BST-Er (x = 0.8) ceramic within the temperature range of 233 to 413 K is depicted in Fig. 5(a). The positions of the green emission peaks do not change with temperature, whereas the relative intensity of I₅₂₀ increases and the relative intensity of I₅₅₀ decreases with increasing temperature. Consequently, the relative intensity ratio R increases with the increase of temperature, as shown in Fig. 5(b).

ΔE is the energy gap between the ²H_11/2 and ⁴S_3/2 levels, and k is the Boltzmann constant. At low temperatures, populating the ²H_11/2 level by thermal agitation (TAG) is difficult due to the low TAG energy,²² and the relative intensity of I₅₂₀ is weak. According to the UC emission spectra (see Fig. 4 inset), the energy gap between the ²H_11/2 and ⁴S_3/2 levels is narrow. At high temperatures, the ²H_11/2 level can be populated from the ²S_3/2 level by TAG because of the low energy separation. Therefore, the relative intensity of I₅₂₀ becomes strong and the relative intensity of I₅₅₀ becomes weak. The thermal coupling characteristic enables the BST-Er ceramics to have potential applications in temperature sensing by a fluorescence intensity ratio (FIR) technique. The temperature dependence of emissions at 520 and 550 nm in the range of 233–413 K (Fig. 5(c)) shows a clear rise in FIR value with temperature, reaching a maximum value when the temperature approaches the maximal experiment temperature 413 K. The investigation of sensing sensitivity is important to understand the temperature response,²³ and it can be defined from

Fig. 5 Temperature sensing performances of the BST-Er ferroelectric ceramic: (a) UC emission spectra patterns at different temperatures; (b) FIR relative to temperature; (c) sensor sensitivity as a function of temperature.

The sensitivity as a function of temperature reaches its maximum value of 0.0044 K⁻¹ at 413 K. The optical temperature sensing performances of BST-Er ceramics are comparable to that of commonly used oxides, fluorides, and glasses materials.¹²

Conclusions

Multifunctional BST-Er ceramics were prepared by a conventional solid-sate reaction technique. Greatly enhanced piezoelectric properties of d₃₃ = 400 pC N⁻¹ and S = 0.175% and ferroelectric properties of P_r = 10.7 μC cm⁻², were obtained for the BST-Er ceramics at x = 0.5. The PPT region from the orthorhombic phase to tetragonal phase, which contribute to the high piezoelectricity, were identified at RT for the composition at around x = 0.5. With the increase in Er content, T_C was maintained at about 353 K, while the other two PPT shifted towards a low temperature. Moreover, UC emissions at 520 nm and 550 nm (²H_11/2/⁴S_3/2 → ⁴I_15/2) were investigated under 980 nm laser excitation in the temperature range of 233–413 K. The value of FIR for I₅₂₀/I₅₅₀ increased gradually with increasing temperature, and a maximum sensitivity of 0.0033 K⁻¹ was obtained. This work may provide an effective mechanism of designing high-performance piezoelectric materials and UC luminescence materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51502127, 51402144, 51372110 and 51302124), the State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (No. KF201510), the Project of Shandong Province Higher Educational Science and Technology Program (No. J14LA10 and J14LA11) and the Natural Science Foundation of Shandong Province of China (No. ZR2014JL030).

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