Reversible and color controllable emissions in Er3+/Pr3+-codoped K0.5Na0.5NbO3 ceramics with splendid photochromic properties for anti-counterfeiting applications

It is highly significant to develop multi-mode optical anti-counterfeiting materials to efficiently fight against counterfeit products. In this study, we chose ferroelectric K_0.5Na_0.5NbO₃ (KNN) with excellent photochromism properties as the host and rare-earth Er³⁺ and Pr³⁺ ions as dopants to prepare the Er³⁺/Pr³⁺-codoped KNN ceramics. The color-tunable emissions can be obtained from red-orange-yellow to green by controlling the excitation wavelength. Upon 980 nm excitation, the synthesized ceramics does not only have superior upconversion (UC) emission behaviors but also have good luminescence modulation properties based on the photochromism properties. It is found that the KNN:0.003Er³⁺/0.003Pr³⁺ sample with the optimal UC emission features shows a highest ΔR_t value of 74.52% when irradiated by 390 nm light for 5 min, whereas the KNN:0.005Er³⁺/0.003Pr³⁺ ceramics also exhibit a high ΔR_t value of 66.81% under 395 nm light irradiation. According to the XPS and EPR results, one knows that the mechanism of luminescence modulation is closely related to defects and traps caused by the oxygen vacancies. Furthermore, the optical information writing and erasing test is conducted, exhibiting a good reproducibility and fatigue resistance. These results reveal that the designed ceramics are appropriate for the anti-counterfeiting applications.     

Reversible luminescence modulation of Ho‐doped K0.5Na0.5NbO3 piezoelectrics with high luminescence contrast

A significant luminescence modulation behavior based on photochromic reactions was observed in Ho³⁺‐doped (Na_0.52K_0.48)_0.92Li_0.08NbO₃ ceramics, fabricated by the conventional solid‐state reaction method. Under visible light irradiation (407 nm) for 20 second, the samples changed pale gray from initial pale green, and returned to their original color by a thermal stimulus of 230°C for 10 minutes, showing typical photochromic phenomenon. Under 453 nm excitation, the samples exhibited strong green emission at 551 nm. Interestingly, their green emission intensity can be effectively tailored by controlling photochromic reaction processes (irradiation wavelength and time), and the luminescent modulation ratio (ΔR_t) reaches up to 77%. And, the ΔR_t value has no any obvious degradation after 10 cycles by alternating visible light irradiation and thermal stimulus, showing excellent reversibility. These results make it potential applications in many fields as a kind of multifunctional material.     

Structure,electrical and luminescent properties of the NBT:Er-0.3CT:Pr composite ceramics

A binary composite ceramic Na_0.5Bi_0.5TiO₃:0.002 Er-0.3CaTiO₃:0.003 Pr (NBT:Er-0.3CT:Pr) was fabricated by a conventional solid state reaction route. The effect of sintering temperature on the microstructure and phase structure of the ceramics has been investigated. And the basic electrical properties such as dielectric properties and ferroelectric performances of the ceramics were examined. Macro long-ranged ferroelectric order cannot be induced via electric field in the composite ceramics. And under local electric field using PFM, obvious ferroelectric switching is identified in the ceramics. Furthermore, the up-conversion luminescence, photochromic reaction, and luminescence modulation have been achieved in the ceramics. Additionally, photo-stimulated luminescence and thermal luminescence are obtained. The present study indicates that optical properties are tightly related to the sintering temperature. The prepared ceramic NBT:Er-0.3CT:Pr owns a piezoelectric performance and multi luminescence properties, which can widen the applications of ferroelectric ceramics.     

Poling effect on optical and dielectric properties of Pr3+-doped Na0.5Bi0.5TiO3 ferroelectric single crystal

In recent years, with increased environmental awareness, more and more research has been carried out for lead-free ferroelectric materials. Among such materials, Na_0.5Bi_0.5TiO₃ (NBT) single crystal has become a material of interest due to its high Curie temperature. In this study, Pr-doped NBT crystals were prepared and poled along different crystal orientations. The effects of different crystal orientations and poling on the optical properties of Pr-doped NBT crystals were investigated. The optical transmission of the crystal decreases after poling along the [001] orientation and increases after poling along the [111] orientation. The emission and excitation spectra show that the poled process enhances the crystal luminescence. The dielectric constant dependence on temperature for different crystal orientations was measured. The domain structure was measured to explain the changes in the optical properties. The optical properties of a Pr-doped NBT single crystal were modulated by poling along different crystal orientations, which is expected to improve the performance of optoelectronic devices based on such materials.     

Optical temperature sensing and luminescent switching properties in Pr/Er-doped (K0.5Na0.5)NbO3 materials

For optical temperature sensing materials, the emission and excitation bands are extremely critical to measure the temperature by fluorescence intensity ratio (FIR) technique. Singly Ln-doped optical temperature sensing materials exhibit very few emission bands, which greatly constraints their practical applications of FIR technique. Here, the fabricated Pr/Er co-doped (K_0.5Na_0.5)NbO₃ materials exhibited multi-color (red-green) and dual-mode (downshifting/upconversion) luminescence properties. The temperature sensitivity can be effectively tuned by choosing different emission or excitation bands. The optimized optical temperature sensitivity reached up to 0.0094 K⁻¹, much higher than that of most temperature sensing materials. Besides, the samples also showed excellent luminescence modulation properties based on the photochromic reaction. Under sunlight irradiation, the luminescent switching contrast (ΔR_t) of the samples reached more than 60%. These results may provide a guiding role in designing and modulating optical temperature sensing properties for multifunctional materials.     

Fabrication and upconversion luminescence properties of Er:SrF2 transparent ceramics compared with Er:CaF2

SrF₂ transparent ceramic is a promising upconversion material due to the low phonon energy. The effect of different sintering temperatures on Er:SrF₂ transparent ceramics was investigated. The suitable sintering temperature for Er:SrF₂ transparent ceramics was 900 °C by hot-pressed sintering in this study. High quality of Er:SrF₂ transparent ceramics with different doping concentrations were obtained. The upconversion luminescence spectra and decay behavior were compared between Er:SrF₂ and Er:CaF₂ transparent ceramics with different Er³⁺ doping concentration. The green emission of 5 at.% Er:SrF₂ ceramic was much stronger than that of 5 at.% Er:CaF₂ ceramic, while the red emission of Er:SrF₂ ceramic was almost the same as that of Er:CaF₂ ceramic. The upconversion luminescence lifetime of Er:SrF₂ transparent ceramics was longer than that of Er:CaF₂.All the results indicated Er:SrF₂ transparent ceramics was a candidate for green fluorescent upconversion materials.     

Luminescence modulation of the Eu3+ doped Srn+1SnnO3n+1 (n=1, 2, 5, ∞) ceramics based on photochromism and its application in anti-counterfeiting technology

The structure and optical properties of Sr_n+1Sn_nO_3n+1 ceramics greatly depend on the n value. Thus, we fabricated four compositions, namely Sr_n+1Sn_nO_3n+1:Eu³⁺ (n = 1, 2, 5, ∞) ceramics, and their crystal structure, photoluminescence, photochromism and luminesce modulation properties have been investigated. It is found that excellent photochromism and luminesce modulation properties are found in Sr₂SnO₄:Eu³⁺ and Sr₃Sn₂O₇:Eu³⁺ ceramics. After 280-nm light irradiation, the Sr₂SnO₄:Eu³⁺ ceramics transform into gray purple from primal white. Meanwhile, luminescence intensity decrement ratio ΔI_dec of the colored Sr₂SnO₄:Eu³⁺ reaches a high value of 80.8% under optimized irradiation wavelength. The decreased luminescence intensity of Eu³⁺ can be completely recovered via 450-nm light irradiation. The ΔI_dec of Sr₃Sn₂O₇:Eu³⁺ ceramic reaches 53.1%, and the decreased luminesce intensity can not be covered by light irradiation, only can be covered by a high temperature stimulus at 400 °C. Finally, we successfully fabricated a flexible membrane using Sr₂SnO₄:Eu³⁺ and PDMS for anti-counterfeiting applications.     

Structure and electrical properties of perovskite layer (1−x)Sr2Nb2O7‐x(Na0.5Bi0.5)TiO3 high‐temperature piezoceramics

The structure and electrical properties of perovskite layer structured (PLS) (1?x)Sr₂Nb₂O₇‐x(Na_0.5Bi_0.5)TiO₃ (SNO‐NBT) prepared by solid‐state reaction method are investigated. The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering, leading to better sinterability and higher bulk density up to 96.8%. The solid solution limit x in the SNO‐NBT system is below 0.03, over which Ti⁴⁺ is preferable to aggregate and results in the generation of secondary phase. After the modification by NBT, all SNO‐NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0 pC/N. The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density. This study not only demonstrates how to improve piezoelectricity by NBT addition, but also opens up a new direction to design PLS piezoceramics by introducing appropriate second phase.     

Novel strategy for the enhancement of anti-counterfeiting ability of photochromic ceramics: Sm3+ doped KSr2Nb5O15 textured ceramics with anisotropic luminescence modulation behavior

Sm³⁺ doped KSr₂Nb₅O₁₅ (KSN-Sm) textured ceramics with anisotropic photochromic and luminescence modulation behaviors provided a new strategy for the enhancement of anti-counterfeiting ability. The KSN-Sm textured ceramics were fabricated by the tape casting technology, which exhibited obvious grain-orientation, with Lotgering factor f_(00l) of 0.62. The textured sample possessed evident difference of reflectivity, photochromic, luminescent and luminescence modulation properties among various grain-orientated directions. The difference of luminescent emission intensity was over than 30 % and the luminescence modulation ratios △R_t are 75.3 % and 63.3 % along paralleled and vertical [00l] orientations, respectively. These optical anisotropies were attributed to the different refractive indexes, distributions of photochromic centers and energy transfer rates at various orientations. This work is hopeful to achieve the multidirectional data recording and enhancement of anti-counterfeiting ability of photochromic ceramics by the anisotropic properties of textured ceramics.     

Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics

Na_0.5Bi_0.5TiO₃ (NBT) based oxide-ion conductor ceramics have great potential applications in intermediate-temperature solid oxide fuel cells (SOFCs) and oxygen sensors. Na_0.5Bi_0.49Ti_1−xMg_xO_3−δ ceramics with x=0, 0.01, 0.02, 0.03, 0.05 and 0.08 were prepared by conventional solid-state reaction. XRD measurement and SEM analysis revealed the formation of pure perovskite structures without secondary phase. MgO doping greatly decreased the sintering temperature and inhibited grain growth. AC impedance spectroscopy measurement was adopted to measure the total conductivity, which was found to increase with MgO doping content ranging from 0 to 3 mol% and subsequently to decrease. High oxygen ionic conductivity σ_t=0.00629 S/cm was achieved for sample doped with 3 mol% MgO at 600 °C in air atmosphere.     

Significant photostrictive response in lead-free Bi0.5Na0.5TiO3 ceramics under visible light illumination

Photostrictive effect that has been discovered for over half a century is getting renewed interest in recent years in view of the great potential for the wireless remote-control technology. Compared with lead lanthanum zirconate titanate (PLZT) ferroelectrics that have been intensively studied, Bi_0.5Na_0.5TiO₃ (BNT) ceramics have a more environment-friendly prospect. In this study, we present a significant photostrictive response for Bi_0.5Na_0.5TiO₃ ceramics under visible light illumination. The ultraviolet–visible diffuse reflectance spectrum indicates that the BNT ceramic is a direct bandgap semiconductor (E_g = 2.93 eV) with a long tail absorbance till wavelength ~560 nm. Transmission electron microscopy analysis reveals nanoscale domains and fluctuated local structure in BNT ceramics. A significant photostriction ΔL/L about 0.08% was observed for BNT when irradiated by 405-nm laser at the power of 20 kW/m². The nonuniform shifts of the X-ray diffraction peaks under external laser illumination indicates that the photostriction is mainly attributed to the light-induced distortion of crystalline lattice. Such a big deformation approaching that of piezoelectrics/ferroelectrics induced by electric field shows great promise for photo-driven multifunctional applications.     

Reversible luminescence modulation and temperature-sensing properties of Pr3+/Yb3+ codoped K0.5Na0.5NbO3 ceramics

In this work, we have prepared a novel (K_0.5Na_0.5)_0.99-xPr_xYb_0.01NbO₃ (abbreviated as KNN:xPr³⁺/0.01Yb³⁺, x = 0.0006, 0.0008, 0.001, 0.002, 0.003, and 0.004) ceramics, which possess visible UC emissions, photochromic (PC) and optical thermometric properties. Under the excitation of a 980-nm diode laser, all the samples show the featured emissions of Pr³⁺ ions and the UC emission intensity is greatly dependent on the Pr³⁺ doping content. The optimal UC luminescence intensity is obtained at x = 0.001. All the prepared samples show a strong PC reaction, and a large luminescence quenching degree (ΔR_t) of 74.94% is found. The optical thermometric properties of both the irradiated and unirradiated KNN:0.001Pr³⁺/0.01Yb³⁺ ceramics in the temperature range of 123-573 K have been investigated via measuring the temperature-dependent UC emission spectra of green emissions, which originate from the two ³P₁ and ³P₀ thermally coupled levels. It has been found that the prepared samples have both excellent PC behaviors and temperature-sensing performances. These results suggest that the KNN:xPr³⁺/0.01Yb³⁺ ceramics are promising candidates for the applications in PC reaction and thermometers.     

Low-temperature sintering,structure transition and dielectrical properties of Ba0.85Ca0.15Ti0.9Zr0.1O3 with Na0.5Bi0.5TiO3 addition

Na_0.5Bi_0.5TiO₃-modified Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ ceramics were prepared by solid state route and their structural and dielectric properties were investigated. The sintering temperature of BCTZ ceramics has been significantly decreased from 1460 °C to 1280 °C with NBT addition. All samples showed a pure perovskite structure and a stable solid solution has been formed between BCTZ and NBT. Some tetragonal phase gradually transformed to rhombohedral or cubic phase with the addition of NBT. Dielectric peak gradually becomes broader, revealing that the diffuser behavior was enhanced. The prominent superimposed loss peaks related to thermally activated relaxation process. The values of activation energy of the relaxation process are 1.034, 1.285, 1.308 and 1.353 eV, which could be associated with the migration of oxygen vacancies.     

Enhanced piezoelectric property in Mn-doped K0.5Na0.5NbO3 ceramics via cold sintering process and KMnO4 solution

Through mixing the KMnO₄ solution with K_0.5Na_0.5NbO₃ (KNN) powders, cold sintering process (CSP) was employed to fabricate high-density Mn-doped KNN green pellets and ceramics. The microstructure, doping effect of Mn and electrical properties of these ceramics were studied in detail. Compared with conventional sintering (CS), the CSP supports the homogeneity of dopants and then promotes grain growth and ceramic densification; thus the Mn-doped KNN ceramics prepared by CSP show the obviously higher density and larger grain size. Besides, the less alkalis volatilization and oxygen vacancies result in more Mn³⁺ but less Mn⁴⁺ in CSP ceramics compared to CS ones, which endows the pinning effect and good poling characteristics in CSP ceramics. All the previous results contribute to the high dielectric constant and remnant polarization in CSP ceramics, which support the enhanced piezoelectric coefficient and are much superior than Mn-doped KNN ceramics prepared by CS. This work reveals that CSP can be a new doping strategy to perform chemical modification of electrical properties in KNN ceramics.     

Composition and microstructure of Na0.5Bi0.5TiO3 ceramics with excess Bi

In the present work, Na_0.5Bi_0.5TiO₃ (NBT) ceramics with the addition of excess Bi in two different ways – before calcination and before sintering – are considered, revealing how the excess Bi affects their microstructure and chemical content. Average grain size is seen to decrease, with the grain size distribution becoming less diffused at higher excess Bi concentrations. The reason for such a feature is the shift of the sintering temperature region where the abnormal grain growth starts to contribute towards higher temperatures. The influence of excess Bi is more pronounced in the case if it is added before calcination. It was discovered that a small amount of excess Bi helps to prevent the formation of Bi-deficient inclusions. While, high concentrations of excess Bi induce the formation of Bi-rich inclusions – most probably Na_0.5Bi_4.5Ti₄O₁₅. Possible mechanisms of formation of both types of inclusions are discussed in detail. Instead of Bi over-stoichiometry, elevated Na content and slightly lower O content were detected in the matrix grains of the sintered NBT ceramics prepared with excess Bi. These deviations increase upon increasing the added excess Bi concentration. The presence of another, Na-rich phase, is assumed, which could not be detected by X-ray diffraction or by energy-dispersive X-ray analysis.     

Cold sintering of the ceramic potassium sodium niobate, (K0.5Na0.5)NbO3, and influences on piezoelectric properties

Cold Sintering was applied to densify a Potassium-Sodium Niobate solid solution composition, 0.5KNbO₃-0.5NaNbO₃ (KNN); the process uses a transient chemical sintering aid, moderate pressure (400 MPa), and temperatures between 230–300 °C to obtain ceramics of ～92 to 96 % theoretical density. Typically, sintering temperatures between ～1000?1050 °C are required to density KNN using conventional methods. In this paper, the densification was investigated during heating, particularly the shrinkage in the first 60 min of the cold sintering process. The low-field dielectric and electrical properties of the resulting ceramics were found to be comparable to conventionally sintered KNN. Electric fields up to 80 kV/cm could be applied, however the ceramics showed pinched hysteresis loops, even after poling over a wide range of temperatures and electric fields. A Rayleigh analysis was used to investigate domain dynamics and high reversible permittivity. The irreversible behavior was an order of magnitude lower than in conventionally sintered KNN, likely associated with defect pinning of ferroelectric domains. A Transmission Electron Microscopy (TEM) study revealed a high density of line defects in most grains; dislocations in the grains limit poling and domain wall movement, thus suppressing both the piezoelectric properties and the hysteresis. Furthermore, TEM observations indicated crystalline grain boundaries that were faceted with terrace kink ledges. These observations point to the importance of the initial powder optimization and grain boundary diffusion when using cold sintering to prepare ceramics that are intended to show bulk cooperative properties such as ferroelectricity. The impact of limited high temperature homogenization of bulk diffusional processes is discussed.     

Combining effects of TiO6 octahedron rotations and random electric fields on structural and properties in Na0.5Bi0.5TiO3

The structural and dielectric properties of Na_0.5Bi_0.5TiO₃ (NBT) ceramics and crystals have been investigated and are compared to that of Pb(Zr_0.55Ti_0.45)O₃ (PZT55/45) and Pb(Mg_1/3Nb_2/3)_0.72Ti_0.28O₃ (PMNT 72/28) ceramics. X-ray diffraction (XRD) profiles for (100), (110), (111), (200), (220), and (222) (referred to cubic structure) reveal that the monoclinic structure with Cc space group exists both in the NBT single crystal and ceramics. The diffraction profile obtained with high resolution laboratory XRD for the NBT single crystal can be well described, using Cc model instead of R3c model. The dielectric constant of NBT below T_hump shows some similarity to that of PZT45/55 ceramics below 50°C in which oxygen octahedron rotations cause the frequency dispersion of the dielectric constant. The temperature-dependent dielectric constant for NBT can be deconvolved into two independent processes. The lower temperature process shows a typical relaxor characteristic and follows the Vogel-Fulcher relationship. The other process at higher temperature shows less frequency-dependent behavior. Comparing the dielectric constant of NBT with that of PZT55/45 and PMNT72/28 reveals that both oxygen octahedral rotations and random electric fields play an important role in the frequency dispersion of the dielectric constant for NBT relaxor feroelectric.     

Reversible photoluminescence modulation in praseodymium-doped bismuth titanate ceramics for information storage based on photochromic reaction

Photoluminescence materials with reversible luminescence regulation properties have great potential for applications in photoswitches, optical sensing, and high-density optical data storage devices. In this work, a series of Pr³⁺-doped Bi₄Ti₃O₁₂ (BiT) ceramics were synthesized by a high-temperature solid-phase reaction method. It is well known that BiT is the most famous one within the Aurivillius ferroelectric (FE) system. Previous works routinely focused on the FE-related properties of BiT-based compounds, but little attention has been devoted to the luminescence, especially the luminescence modulation performance. Here, a large and reversible photoluminescence modulation by alternating visible light irradiation and thermal stimulus based on photochromic (PC) process was achieved. Meanwhile, the influence of sintering temperature on the luminescence modulation behavior was also illustrated. The luminescence regulation properties of the Pr³⁺-doped BiT ceramics prepared at optimized sintering temperature were investigated in detail. The mechanisms connected to the luminescence, luminescence manipulation, and PC effect were discussed. It is believed that Pr³⁺-doped BiT materials can be utilized as a kind of ideal optical medium for future information storage device.     

Photoluminescence,thermoluminescence and reversible photoluminescence modulation of multifunctional optical materials Pr3+ doped KxNa1-xNbO3 ferroelectric ceramics

It is highly significant to develop multifunctional optical materials to meet the huge demand of modern optics. Usually, it is difficult to realize multiple optical properties in one single material. In this study, we choose ferroelectric (K_xNa_1-x)NbO₃:Pr³⁺ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) as hosts, and the rare earth ions Pr³⁺ are doped in them. For the first time, the integration of photoluminescence, photochromism, luminescence modulation and thermoluminescence and has been achieved in the Pr³⁺ doped (K_xNa_1-x)NbO₃:Pr³⁺ ferroelectric ceramics. Upon 337- or 448-nm light irradiation, all samples show strong red emissions centered at 610 nm. The photochromic reaction increases with the increasing K⁺ content in the (K_xNa_1-x)NbO₃:Pr³⁺ ceramics. A strong photochromic reaction has been found in the (K_0.5Na_0.5)NbO₃:Pr³⁺ ceramics. Accordingly, a large and reversible photoluminescence modulation (ΔR_t = 50.71%) is achieved via altering 395-nm-light irradiation and 200 °C thermal stimulus. All the prepared ceramics show a visible thermoluminescence when stimulated at 200 °C. The mechanisms of luminescence modulation and thermoluminescence are discussed. Present study could provide a feasible paradigm to realize multiple optical properties in one single material.     

Growth and electrical properties of Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 lead-free single crystals by the TSSG method

A series of NBT-KBT lead-free crystals with dimensions of Φ 35×10 mm were successfully grown by the TSSG method. The as-grown crystals possess rhombohedral perovskite structure at room temperature. The curves ε(T) for all crystals show two abnormal dielectric peaks. The depolarization temperatures T_d derived from the first peak of curves tan δ(T) vary with the KBT content, which are 130, 150, 140, and 115 °C respectively, for (100−x)NBT−xKBT (x=5, 8, 12, 15) crystals, being well consistent with the T_d obtained from the temperature dependence of k_t. A notable thermal hysteresis, ΔT≈35 °C, for ferroelectric-antiferroelectric phase transition was also disclosed for 92NBT-8KBT crystal. The investigation of orientation dependence for electrical properties disclosed the dielectric parameters show weak anisotropy. The piezoelectric constants (d₃₃) are 147, 175, 205, 238 pC/N and the values of k_t are 38%, 52%, 52%, 54%, respectively for (100−x)NBT−xKBT (x=5, 8, 12, 15) crystals.