Effects of Er-doping on the Structure and Photoelectric Properties of 0.825K0.5Na0.5NbO3-0.175Sr (Yb0.5Nb0.5) O3 Ceramics

GUAN Yingwen, SUN Yabing, ZHAO Tianhao, SHI Shaoyang, FU Weining,LIU Shimin, WANG Hua,2＊, XU Jiwen,2

(1. School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China; 2. Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China)

Abstract: Multifunctional ceramics of 0.825K0.5Na0.5NbO3-0.175Sr(Yb0.5Nb0.5)O3- x%Er with ferroelectric, transparent and luminescent properties were obtained by doping Er. The effects of Er-doping on the structure, luminescence and electrical properties of the ceramics were studied. After Er doping, the ceramics have luminescent properties and the luminescence intensity reaches the maximum when x=0.75. The ceramics still have good light transmission, but the transmittance decreases with the increase of Er-doping content due to the change of phase structure from pseudo-cubic phase to three-phase orthogonality. The energy storage properties is also greatly improved, and the Wrec and the η of the ceramics reach the maximum value of 0.31 J/cm3 and 94.6% when x=0.25, respectively. At the same time, the maximum electro-induced strain value is 0.031%.

Key words: KNN-Sr(Yb0.5Nb0.5)O3; up-conversion luminescence; ferroelectric; multi-function

1 Introduction

With the development of society, the application of multifunctional ceramics is more and more extensive. Among them, multifunctional ferroelectric ceramics with ferroelectric, strain, energy storage and other properties are widely used in electronic components such as capacitors, transducers, and so on[1-3]. And that the demand for the functionality of materials is gradually increasing. As typical multifunctional ferroelectric ceramics, (Pb,La)(Zr,Ti)O3(PLZT) and Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT) ceramics exhibited excellent photoelectric effect, however, these ceramics contain toxic lead, which is harmful to the environment[4,5],so it is very important to explore new multi-functional materials.

Er element has the energy level structure, and it has a long life and plays an important part in up-conversion luminescence, but the photon absorption ability is poor. So rare earth elements Yb is used as a sensitizer to enhance its luminescence performance[19]. In this paper, the effects of rare earth Er on the structure,ferroelectric, strain, and luminescent properties of K0.5Na0.5NbO3-Sr(Yb0.5Nb0.5)O3transparent ferroelectric ceramics were studied.

2 Experimental

0.825KNN-0.175SYbN-x%Er (KNN-SYbNx%Er) (x= 0.25, 0.50, 0.75, 1.00, 1.50，2.00) ceramics were prepared by solid phase sintering method.The raw materials and purity were K2CO3(99.80%),Na2CO3(99.90%), Nb2O5(99.99%), SrCO3(99.90%),Yb2O3(99.99%) and Er2O3(99.99%). According to the stoichiometric ratio, the samples were first ground by ball mill with anhydrous ethanol for 24 h, then dried to complete dry at 70 ℃ and screened under 100 meshes,then burned at 950 ℃ for 5 h, and then pre-fired twice.The powders were mixed with a 7 wt% polyvinyl alcohol (PVA) binder. The sample was polished and cleaned to a thickness of 0.5 mm and sintered for 2 hours at 1 040 ℃ to 1 050 ℃. Finally, the silver paste was printed at 600 ℃ for 0.5 hours to obtain a silver electrode.After cooling to room temperature, took out the sample for electrical performance test.

X-ray diffraction (XRD, PixceD3D, Parak) was used to analyze the phase structure of the ceramics,while field emission scanning electron microscopy(FESEM, Quanta 450 FEG, FEI) was used to observe the surface morphology and grain size of the samples.The polarization-electric field (P-E) hysteresis loops of the samples were measured by a ferroelectric tester(TF Analyzer 2000HS, aixACCT). Photoluminescence properties of the ceramics were measured by a fluorescence spectrometer (SENS-9000A, Zhuoli).W1is the energy storage density of the sample,W2is the energy loss of the sample, whileW1+W2are the total amount of energy stored by charging. Energy storage densityW1and the energy storage efficiencyηcan be calculated by the following formula:

In the formula,Eis the electric field intensity andPis the polarization intensity.

3 Results and discussion

Fig.1 is the XRD diagram and the physical map of 0.825K0.5Na0.5NbO3-0.175Sr(Yb0.5Nb0.5)O3(KNNSYbN) ceramics doped with Er. As can be seen from Fig.1(a), all ceramic samples with different components are of single perovskite structure without hetero-peak generation, which indicates that the Er3+has been solid dissolved into the matrix ceramics to form a stable single perovskite phase. In order to further analyze the phase structure of ceramics, the diffraction peak (200) at the diffraction angle of 46° was amplified, as shown in Fig.1(b). As can be seen from Fig.1(b), when the doping amount of Er is relatively low, the ceramics still maintain a pseudo-cubic phase structure, and the diffraction peak is unimodal.With the increase of doping content, the diffraction peak of the ceramic gradually shifted to a lower angle, indicating that the introduction of Er caused the ceramic cell distortion and the cell enlargement. At the same time,when the doping content of Er isx=1.00, the diffraction peak of (200) of the ceramics shows a trend of splitting peaks, and with the continuous increase of doping content, the pillow-shoulder peaks begin to appear, which indicates that the Er makes the ceramics begin to change from pseudo-cubic phase structure to tripartite phase structure[20]. In addition, Er doped ceramics still have good light transmittance, as shown in the illustration in Fig.2 (0.3 mm thickness), but the light transmittance of Er doped ceramics is lower than that of Er-undoped KNN-Sr(Yb0.5Nb0.5)O3ceramics[15], and further weakens with the increase of doping amount, which is consistent with the weakening of the pseudo-cubic structure of ceramics by the addition of rare earth Er.

Fig.1 (a) XRD patterns of KNN-SYbN-x% Er ceramics; (b) An enlarged view of the diffraction peak (200) of the ceramic sample near 46°

Fig. 2 Physical drawings of ceramics

Fig.3 FESEM diagram of KNN-SYbN-x% Er: (a) x=0.25; (b) x=0.50; (c) x=0.75; (d) x=1.00; (e) x=1.50; (f) x=2.00

Fig.4 EDS spectra of KNN-SY6N-1%Er ceramics: (a) Energy spectra of 1arge grains on ceramic surface; (b) Energy spectrum of srmall grains on cerarnic surface

Fig.3 shows the surface morphology of KNNSYbN-x%Er ceramics observed under FESEM. As can be seen from Fig.3, whenx=0.25, the ceramic surface has a dense structure and its grains are small cubes with a size of 180-200 nm. As the doping content of Er increases, although the grains of ceramics are still relatively small, whenx=1.00, it can be observed that the grains on the surface of ceramics have a polyhedral structure of non-cubic phase, which may be caused by the change of the phase structure of ceramics caused by Er. In addition, with the increase of Er doping content,some unusually large grains were formed on the surface of the ceramics. In order to explore the composition of unusually large grains in ceramics, the ceramics with the composition pointx=1.00 were selected for EDS(energy dispersive X-ray spectroscopy) point scanning to analyze the composition of large grains and ordinary small grains. The results are shown in Fig.4. Fig.4(a)is the ceramic surface of larger particles EDS spectrum scan results. The energy spectrum shows that the large particle is made of the same composition as a ceramic.It shows that the large particles on the ceramic surface are still the ceramic ontology, not extraneous material.However, compared with the surface grains of the ceramic in Fig.4(b), although the composition elements of the two are the same, due to the different contents of Er, the element of the abnormally large grain in Fig.4(a) is higher, up to 1.6%, while the content of the ceramic grain Er in Fig.4(b) is only 0.82%. This result again shows that Er can promote the growth of ceramic grains and make the ceramic grains larger.

Fig.5 is theP-Ediagrams of KNN-SYbN-x%Er ceramics under electric fields from 10 kV/cm to 80 kV/cm. TheP-Ediagrams of all the ceramics in Fig. 5 are relatively slender, and their shapes are hyperbolic like relax-or ferroelectric. Relax-or ferroelectric has better energy storage performance due to its unique hysteresis loop. Therefore, it can be preliminarily judged that ceramics have good energy storage performance. In order to more intuitively explore the change of its energy storage performance, we calculated the energy storage performance of ceramics, and the results are shown in Fig.6. The energy storage density (Wrec) of ceramics is basically stable at about 0.3 J /cm3. When the doping content isx=0.25, the energy storage density reaches the maximum of 0.31 J /cm3. As the doping content of Er continues to increase, theWrecof ceramics does not change much. The minimumWrecof ceramics is 0.25 J/cm3. The energy storage efficiency (η) of all the ceramic samples is high. Whenx=0.25, theηof the ceramic samples reaches the maximum value of 94.6%,which is very higher than that of Er-undoped KNNSr(Yb0.5Nb0.5)O3ceramics[15], and then decreases with the increase of doping content.

Fig.5 P-E diagram of KNN-SYbN-x% Er under 10-80 kV/cm electric field: (a) x=0.25; (b) x=0.50; (c) x=0.75; (d) x=1.00; (e) x=1.50; (f)x=2.00

Fig.6 Energy storage density and energy storage efficiency of KNN-SYbN-x% Er ceramics under 80 kV/cm electric field

Fig.7 is the strain curve (S-E) of KNN-SYbNx%Er ceramics under an electric field of 80 kV/cm.All ceramics show a typical butterfly curve, which is caused by the reversal of the electrical domain inside ferroelectric ceramics under the action of an external electric field. Fig.7(a) shows that whenx=0.25, the positive strain of the ceramic reaches the maximum value of 0.031%, while the negative strain basically disappears. Meanwhile, the strain curve of the ceramic is also a symmetrical butterfly curve. With the increase of Er doping content, the maximum positive strain of ceramic decreases slowly, while the negative strain does not change significantly. Whenx=1.5, the maximum forward strain value reaches the lowest 0.017%. This is due to the fact that the introduction of Er reduces the reversal of the electrical domain inside the ceramics,and at the same time, the grain boundary hindrance effect caused by the internal grains of the ceramics is strengthened, which results in a downward trend of the strain value of the ceramics, indicating that the ceramics are relax-or ferroelectric bodies[21-23].

Fig.7 S-E diagram of KNN-SYbN-x%Er under 80 kV/cm electric field: (a) x=0.25; (b) x=0.50; (c) x=0.75; (d) x=1.00; (e) x=1.50; (f) x=2.00

Fig.8 (a) KNN-SYbN-x% Er ceramics up-conversion emission spectra; (b) Energy level diagram of Yb3+ and Er3+ ions in ceramics

Due to the unique energy level structure of Er,KNN-SYbN-x%Er ceramics have up-conversion luminescence properties under 980 nm laser excitation.Fig.8(a) is the emission spectrum of ceramics. It can be seen from Fig.8(a) that ceramics emit bright green light, and the emission wavelength is mainly around 550 and 650 nm. After the introduction of Er into KNN-SYbN ceramics, the energy level transitions are mainly2H11/2→4I15/2,4S3/2→4I15/2and4F9/2→4I15/2. In Fig. 8(a), the intensity of the emission wavelength of the ceramics is the highest at 670 nm, and the main energy level transition of the ceramics at this time is4F9/2→4I15/2. In order to have a clearer understanding of the energy level transition process, Fig.8(b) shows the corresponding energy level diagram. Because there is also a rare earth Yb3+ion in the ceramics, the energy gap between the2F5/2energy level and the4I11/2energy level of Er3+ion is very low. When Yb3+ion receives the energy of the excited light, its energy level changes from2F7/2to2F5/2through the radiation transition, and Er3+ion also absorbs photons. The transition from4I15/2energy level to4I11/2,4F9/2,4S3/2and so on is more difficult because the higher the energy level is, the more energy is needed. However, Yb is usually used as a sensitizer to enhance the luminescence of ceramics, because the energy levels of Yb3+and Er3+in the ceramics coincide partly. After absorbing the energy of photons,the Yb3+ion transitions from the ground state2F7/2to the excited state2F5/2.When the energy returns from the excited state to the ground state, the energy of the Yb3+ion transitions to the higher excited state in the way of radiation transition. So, the luminescent properties of ceramics are greatly improved. As shown in Fig. 8(a),the emission spectra of ceramics have splitting peaks at the4F9/2and2H11/2energy levels. Such obvious splitting peaks may be related to the Nb5+ion in the matrix material. Because the charge of Nb5+ion is relatively high,and its ion radius is relatively small, only 0.064 nm.The strong polarization effect in ceramics reduces the symmetry of the coordination field of Er3+ions, which causes Stark splitting of the energy level of rare earth ions in ceramics under the excitation of laser, leading to the existence of splitting peak[24]. Meanwhile, as shown in Fig.8(a), the intensity of the emission spectrum of ceramics first increases and then decreases with the increase of doping content. Whenx=0.75, the intensity of the emission spectrum of ceramics reaches the maximum and then decreases slowly. Excess Er causes cross relaxation inside the ceramics, which reduces the luminescence intensity of ceramics.

4 Conclusions

KNN-SYbN-x%Er multifunctional ceramics with ferroelectric, transparent and luminescence properties can be available by solid-state method. The addition of Er makes the crystal lattice of ceramics distorted,and the crystal cell of ceramics becomes larger. At the same time, with the increase of doping content (whenx=1.00), the phase structure of ceramics also changes,and the ceramics gradually change from pseudo-cubic phase to tripartite phase, and the transparency of ceramics also gradually decreases. The abnormal growth of grains on the surface of the ceramics indicates that the presence of Er makes the ceramics agglomerate in the sintering process and increases the porosity of the ceramic surface. When the doping content of Er is 0.25, the energy storage efficiency of ceramics is up to 94.6%. At the same time, the strain of the ceramic decreases gradually, and the maximum strain is 0.031%whenx=0.25. The introduction of Er makes the ceramics possess the up-conversion luminescence property,and the luminescence intensity increases with the increase of the content of Er. The luminescence intensity reaches the maximum atx=0.75. With the further addition of rare earth elements, the luminescence intensity of Er3+in the ceramics weakens due to crossing relaxation.