Optical and Electrical properties of KNN-SBN:Ho/xYb Transmittance Luminescent Ferroelectric Ceramics

Transparent luminescent ceramics 0.94(K0.5Na0.5)NbO3-0.06Sr(Bi0.5Nb0.5)O3: 0.1Ho2O3/xYb2O3 (KNN-SBN:Ho/xYb, x=0.5, 1.0, 1.5, 2.0) have been synthesized using a solid-state reaction method.All the ceramic materials exhibit a pseudo-cubic structure without heterogeneous phases at room temperature. In addition, the microstructure of KNN-SBN:Ho/xYb transparent luminescent ceramics is very dense, and at the same time, good optical transparency is obtained, with a transmittance of 70.19% when x is 0.5. KNN-SBN:Ho/xYb transparent luminescent ceramics exhibit upconversion luminescence properties, and the best upconversion luminescence intensity is obtained when x is 1.5. The KNN-SBN:Ho/xYb transparent luminescent ceramics are ferroelectrics with good ferroelectric properties under an applied electric field of 120 kV·cm−1, and the maximum polarization intensity is the highest when x is 0.5, which is 21.32 μC·cm−2.


Introduction
Transparent ferroelectric ceramics and optically stimulated luminescent ceramics have attracted significant in recent years due to their remarkable applications in display devices, sensors, and actuators [1][2][3].Among them, transparent ferroelectric ceramics and optically stimulated luminescent ceramics based on (K0.5Na0.5)NbO3(KNN) have good research prospects due to their high piezoelectric properties, high Curie temperature, and lead-free characteristics that are harmless to human tissues [4][5][6].
Currently, there has been extensive research on the transparent ferroelectric and luminescent properties of lead-free ceramics based on KNN.Wang et al. prepared Er 3+ /Yb 3+ co-doped KNN upconversion luminescent ceramics and studied the effect of different Yb 3+ contents on the upconversion luminescent properties, but they did not exhibit transparent properties [7].Sun et al. doped rare earth Ho 3+ into KNN-SYN ceramics, and although the prepared ceramics showed certain upconversion luminescent properties, they had low transparency [8].So far, there have been few reports on simultaneously improving the excellent transparency and luminescent properties.There are many factors that affect transparency, such as phase structure, grain size, and ceramic density.In addition, the introduction of the second component of ABO3 type has a great influence on the phase structure and grain size, and the divalent ions in the A and B positions can effectively control the grain growth of ceramics.For example, Sr 2+ can improve the ferroelectric properties of KNN ceramics [9], and Bi2O3, as a sintering agent, can effectively reduce the sintering temperature of ceramics, decrease the grain size, and increase the ceramic density [10].While considering transparency, the introduction of Ho 3+ can absorb and emit light of specific wavelengths, giving the ceramics photoluminescent properties [11].Moreover, Yb 3+ serves as a luminescent sensitizer, and by adjusting its concentration, the spectral characteristics of luminescent ceramics can be modulated [12], allowing for control of the emission wavelength and intensity of the luminescent ceramics.

Experimental
KNN-SBN:Ho/xYb ceramic samples were synthesized using traditional solid reaction technique.K2CO3, Na2CO3, Nb2O5, SrCO3, Bi2O3, Ho2O3, and Yb2O3 were accurately weighed according to the stoichiometric ratio.These powders were then loaded into a ball mill jar along with an appropriate amount of anhydrous ethanol and zirconia grinding balls for 24 hours.The resulting slurry was calcined at 860°C for 2 hours to obtain ceramic powder.Subsequently, the calcined powder was subjected to a second round of ball milling for 24 hours under the same conditions.The milled sample was mixed with a 7% PVA binder solution and pressed into green pellets at a pressure of 50 MPa.The green pellets were then sintered at 1180°C for 2 hours to prepare the ceramic samples.
The steady-state and transient fluorescence spectrometer (Edinburgh Company) utilizes a 980nm diode laser as the light source to measure the upconversion emission spectra of the samples.The transmittance of the samples in the range of 200-1100 nm is tested using a UV-visible spectrophotometer.The phase structure of the samples is analyzed using X-ray diffraction technique (Bruck D8 Advance X, Germany) with a diffraction angle range of 20°-80°.The surface morphology of the samples is observed using a field-emission scanning electron microscope (Quanta 450, USA).The burned silver electrode samples are subjected to voltages of 30-200 kV at room temperature using a ferroelectric testing system to measure the P-E hysteresis loops of the samples.

Results and Discussion
Figure 1 shows the X-ray diffraction (XRD) patterns of KNN-SBN:Ho/xYb transparent luminescent ceramics at room temperature.It can be observed that there are no hetero peak and splitting peak in the XRD patterns of all ceramics under different Yb 3+ content, indicating a pseudocubic structure.Figure 1b is the enlarged view of Figure 1a in the range of 45°-46.5°,showing that all ceramic samples exhibit single peaks, indicating a highly symmetric pseudocubic structure of KNN-SBN:Ho/xYb transparent luminescent ceramics.Additionally, with the increase of Yb 3+ doping, the XRD diffraction peaks show noticeable shifting, which is attributed to the replacement of Nb 5+ by Ho 3+ and Yb 3+ , leading to lattice expansion and the diffraction peak shifting to higher angles.As the Yb 3+ content increases, K + and Na + are replaced by Yb 3+ , resulting in lattice contraction and the diffraction peak shifting to lower angles.Figure 2 presents the scanning electron microscope (SEM) images of the surface of KNN-SBN:Ho/xYb transparent luminescent ceramics.It can be seen that the microstructure of all ceramic samples is dense, with uniform grain distribution and small grain size.This is because in the sintering process of ceramic samples, the doped Bi 3+ aggregates near the grain boundaries, hindering the movement and migration of grain boundaries, thus suppressing the growth of grains.Among them, the grain size is the smallest when x is 0.5.Figure 3 shows the measured transmittance of KNN-SBN:Ho/xYb transparent luminescent ceramics in the range of 200-1100 nm.It can be observed that the transmittance of KNN-SBN:Ho/xYb transparent luminescent ceramics increases with the increase of incident light wavelength, reaching the maximum transmittance at 1100 nm wavelength.When x is 0.5, the transmittance at 1100 nm wavelength reaches 70.19%.With the increase of Yb 3+ doping, the optical transmittance of ceramic samples gradually decreases, and when x is 2.0, the optical transmittance at 1100 nm wavelength decreases to 47.94%.This is because when x is 0.5, KNN-SBN:Ho/xYb transparent luminescent ceramics have the smallest grain size, highest density, fewer pores and defects, resulting in better optical transmittance.Figure 4a shows the upconversion fluorescence spectra of KNN-SBN:Ho/xYb transparent luminescent ceramics under excitation of 980 nm light source.It can be observed that there are two green fluorescence peaks at 530 nm and 550 nm, and a red fluorescence peak at 650 nm.The luminescence intensity of the ceramic samples increases with the increase of Yb 3+ doping.With the continuous increase of Yb 3+ content, when x is 1.5, the luminescence intensity reaches its maximum.When the x exceeds 1.5, concentration quenching phenomenon occurs, and the luminescence intensity decreases with further increase of Yb 3+ content.Figure 4b shows the excitation transition process of Ho 3+ and Yb 3+ .It can be observed that Yb 3+ is excited from the ground state by absorbing light, from the energy level of 2 F7/2 to 2 F5/2, transferring energy from the excited state Yb 3+ to the ground state 5 I8 of Ho 3+ , forming the energy level of 5 I6.Through non-radiative process, some electrons decay from the 5 I6 level to the 5 I7 level.During the lifetime of the 5 I6 and 5 I7 levels, the energy is transferred from Yb 3+ to Ho 3+ , and thus the distribution is achieved in the energy levels of 5 F5 and ( 5 F4, 5 S2).Finally, the electrons relax from the energy levels of ( 5 F4, 5 S2) and 5 F5 to the energy levels of 5 I8 and 5 I7, corresponding to the emission of green light at 530 nm and 550 nm, and red light at 650 nm and 750 nm.When x is 1.5, all the green and red emissions reach their maximum intensity.

Figure 5
Figure5shows the ferroelectric P-E hysteresis loops KNN-SBN:Ho/xYb transparent luminescent ceramics at room temperature.Under the maximum electric field of 120 kV•cm -1 , all ceramics exhibit saturated hysteresis loops, indicating that all ceramics are ferroelectric materials with certain ferroelectric properties.At the same time, with the increase of Yb 3+ content, both the residual polarization (Pr) and coercive field (Ec) of the ceramic samples decrease.When x is 0.5, KNN-SBN:Ho/xYb transparent luminescent ceramics achieve the maximum polarization intensity, with a maximum polarization intensity of 21.32 μC•cm -2 .