Reversible upconversion switching for Ho/Yb codoped (K,Na)NbO3 ceramics with excellent luminescence readout capability

Luminescent readout capability for photochromic materials plays a critical role in 3D optical data storage applications, especially for inorganic photochromic materials in the solid‐state form. In our previous studies, we found that the luminescent readout capability can be improved using two or multiple‐photon excited luminescent mode (upconversion), which can effectively decrease the destruction degree of the excitation energies to the stored information during the luminescent “reading” process. However, the luminescent readout performance is unsatisfactory owing to the absence of nondestructive luminescence readout capability. Herein, we report a new solid‐state photochromic material with excellent upconversion readout capability: Ho³⁺/Yb³⁺ codoped (K,Na)NbO_3. Upon 407 nm light irradiation, the luminescent switching contrast (ΔR_t) is up to 78%. Particularly, the materials almost have no any re‐absorption to 980 nm light, exhibiting extremely low destruction to information recording points. The luminescent readout intensity retains 96% after constant 980 nm irradiation for 4 minutes at a high pumping power of 1W, which is superior to our previously reported results (Er/Yb codoped Bi_2.5Na_0.5Nb₂O₉ materials). This work would help to further develop new inorganic photochromic materials with high performance to satisfy the requirements for optical storage devices.     

Defect‐driven evolution of piezoelectric and ferroelectric properties in CuSb2O6‐doped K0.5Na0.5NbO3 lead‐free ceramics

Defect greatly affects the microscopic structure and electrical properties of perovskite piezoelectric ceramics, but the microscopic mechanism of defect‐driven macroscopic properties in the materials is not still completely comprehended. In this work, K_0.5Na_0.5NbO₃+x mol CuSb₂O₆ lead‐free piezoelectric ceramics were fabricated by a solid‐state reaction method and the defect‐driven evolution of piezoelectric and ferroelectric properties was studied. The addition of CuSb₂O₆ induces the formation of dimeric (DC1) and trimeric (DC2) defect dipoles. At low doping concentration of CuSb₂O₆ (0.5‐1.0 mol%), DC1 and DC2 coexist in the ceramics and harden the ceramics, inducing a constricted double P‐E loop and high Q_m of 895 at x=0.01. However, DC2 becomes more dominant in the ceramics with high concentration of CuSb₂O₆ (≥1.5 mol%) and thus leads to softening behavior of piezoelectricity and ferroelectricity as compared to the ceramic with x=0.01, giving a single slanted P‐E loop and relatively low Q_m of 206 at x=0.025. All ceramics exhibit relatively high d₃₃ of 106‐126 pC/N. Our study shows that the piezoelectricity and ferroelectricity of K_0.5Na_0.5NbO₃ ceramics can be tailored by controlling defect structure of the materials.     

Piezoelectric properties of (Na0.5K0.5)(Nb1‐xSbx)O3‐SrTiO3 ceramics with tetragonal‐pseudocubic PPB structure

CuO‐added 0.96(Na_0.5K_0.5)(Nb_1‐xSb_x)O₃‐0.04SrTiO₃ ceramics sintered at the low temperature of 960°C for 10 hours showed dense microstructures and high relative densities. The specimens with 0.0 ≤  x ≤ 0.04 had orthorhombic‐tetragonal polymorphic phase boundary (PPB) structure. Tetragonal‐pseudocubic PPB structure was observed in specimens with 0.05 ≤  x ≤ 0.07, while the specimen with x = 0.08 has a pseudocubic structure. The structural variation in the specimens is explained by the decreases in the orthorhombic‐tetragonal transition temperature and Curie temperature with the addition of Sb⁵⁺ ions. The specimens with 0.05 ≤  x ≤ 0.07, which have tetragonal‐pseudocubic PPB structure, had large electric field‐induced strains of 0.14%‐0.016%. Moreover, these specimens also showed increased d₃₃ values between 280 pC/N and 358 pC/N. In particular, the specimen with x = 0.055 showed particularly enhanced piezoelectric properties: d₃₃ of 358 pC/N, k_p of 0.45, and the electric field‐induced strain of 0.16% at 4.5 kV/mm.     

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.     

High Piezoelectric Voltage Coefficient in Structured Lead‐Free (K,Na,Li)NbO3 Particulate—Epoxy Composites

A high‐voltage coefficient has been found in lead‐free piezoelectric particulate composites based on epoxy with lead‐free (K_0.50Na_0.50)_0.94Li_0.06NbO₃ (KNLN) piezoceramic particles with a natural cubic morphology. The KNLN powder used in the composites has been prepared using a new solid‐state double calcination processing route. These particles were subsequently used to create random and structured KNLN‐epoxy composites. Using dielectrophoresis, these natural cubical KNLN particles were structured into one‐dimensional chains inside the epoxy matrix. Composites produced with these powders showed piezoelectric properties about a factor of 2 higher than those of composites processed with conventionally calcined KNLN powders. The dielectrophoretically structured KNLN‐epoxy composites with optimized particle size and morphology showed excellent piezoelectric properties, which can replace lead containing piezoelectric composites for sensor and energy harvesting applications in future.     

Properties and structures of nonstoichiometric (K,Na)NbO3‐based lead‐free ceramics

The 0.968[(K_0.48Na_0.52)]Nb_0.95+xSb_0.05O₃–0.032(Bi_0.5Na_0.5)ZrO₃ [KNN_xS–BNZ] lead‐free ceramics with nonstoichiometric niobium ion were fabricated via conventional solid‐state sintering technique and their piezoelectric, dielectric and ferroelectric properties were investigated. When x = 0.010, enhanced piezoelectric properties (d₃₃ ≈ 421 pC/N and k_p ≈ 0.47) were obtained due to the construction of rhombohendral—tetragonal phase boundary near room temperature. The KNN_xS–BNZ ceramics possesses enhanced Curie temperature (T_c) with improved piezoelectric constant. A large d₃₃ of ~421 pC/N and a high T_c ~256°C can be simultaneously induced in the ceramics with x = 0.010. Especially, good thermal stability was observed in a broad temperature range. The results indicated that our work could benefit development of KNN‐based ceramics and widen their application range.     

Defect suppression in CaZrO3‐modified (K,Na)NbO3‐based lead‐free piezoceramic by sintering atmosphere control

During high‐temperature crystal growth, lattice defects will inevitably form inside piezoelectric materials, which can be a hindrance for performance optimization. Through appropriate atmosphere control during sintering, defect levels inside the piezoelectric material can be regulated. Herein, CaZrO₃‐modified (K, Na)NbO₃‐based lead‐free piezoelectric ceramics with a nominal composition of 0.95(Na_0.49K_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃‐0.05CaZrO₃ are produced by sintering in an oxygen‐rich atmosphere. Compared with an air‐sintered sample, the piezoelectric constant of the oxygen‐sintered sample has greatly improved 15% up to 390 pC/N, which is comparable to commercial lead‐based counterparts. In addition, the planar electromechanical coupling factor k_p is enhanced from 0.46 to 0.52. A qualitative model related to defect engineering is proposed to support the experimental observations. Our results indicate the feasibility of purposely optimizing the piezoelectric performance by sintering atmosphere control.     

Electromechanical properties of CaZrO3 modified (K,Na)NbO3‐based lead‐free piezoceramics under uniaxial stress conditions

Piezoelectric actuators are typically preloaded with a modest mechanical compressive stress during actuation to reduce cracking and allow for operation in the dynamic range. In addition, actuators are required to carry out mechanical work during operation, resulting in a nonlinear relationship between stress and actuation voltage. In fact, mechanical loading can significantly impact the electromechanical performance of lead‐free piezoelectrics. Herein, we report the dependence of electromechanical properties of CaZrO₃ modified (K,Na)NbO₃‐based lead‐free piezoceramics on uniaxial compressive stress, comparing to their lead‐based counterparts. It is demonstrated that increased non‐180° domain switching enhances the strain output at a moderate stress of approximately ?50 MPa from room temperature to 150°C. Larger uniaxial stress, however, is found to suppress ferroelectric domain switching, resulting in the continuous strain and polarization decrease.     

Orthorhombic‐pseudocubic phase transition and piezoelectric properties of (Na0.5K0.5)(Nb1−xSbx)‐SrZrO3 ceramics

0.96(Na_0.5K_0.5)(Nb_1?xSb_x)‐0.04SrZrO₃ ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic‐tetragonal transition temperature (T_O‐T) and Curie temperature (T_C) decreased with the addition of Sb₂O₅. The decrease in T_C was considerable compared to that in T_O‐T, and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic‐pseudocubic transition as opposed to that for orthorhombic‐tetragonal transition appeared at 115°C‐78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03. However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na_0.5K_0.5)NbO₃‐based lead‐free piezoelectric ceramics.     

Structural,magnetic and electrical properties of K0.5Na0.5NbO3 doped NiZnCo ferrite for high frequency device

Ni_0.6Zn_0.4Co_0.2Fe_1.8O₄ ferrites doped with x wt.% K_0.5Na_0.5NbO₃ (0.00≤x≤1.00) were successfully prepared by solid-state reaction method. The lattice parameters a decreased and the diffraction peak of (311) shifted to higher angle with the increase of K_0.5Na_0.5NbO₃ (KNN) content. The grain size D initially increased to 3.12 μm (x = 0.50) and then reduced to 2.66 μm (x=1.00). The study also showed the addition of KNN effectively improved magnetic and electrical properties of NiZnCo ferrites. The saturation magnetization Ms decreased from 60.59 to 46.11 emu/g and the coercivity Hc overall showed a decreasing trend from 84.64–67.00 Oe. The dielectric constant ε´ of prepared samples increased when x≤0.75, then decreased when x>0.75, and all prepared samples had low loss tangent tanδ at high frequency. In addition, all samples exhibited high resistivity ρ and activation energy E_ρ.     

Effects of Bi0.5Na0.5HfO3 addition on the phase structure and piezoelectric properties of (K,Na)NbO3‐based ceramics

Lead‐free perovskite (1‐x)(K_0.48Na_0.48Li_0.04)Nb_0.95Sb_0.05O₃‐x(Bi_0.5Na_0.5)HfO₃ piezoelectric ceramics were prepared by a traditional ceramic fabrication method. An investigation was conducted to assess the effects of (Bi_0.5Na_0.5)HfO₃ content on the crystal structure, microstructure, phase‐transition temperatures, and piezoelectric properties of the ceramics. The X‐ray diffraction results, combined with the temperature dependence of dielectric properties, revealed that the ceramics experienced a structural transition from an orthorhombic phase to a tetragonal phase with the addition of (Bi_0.5Na_0.5)HfO₃, and a coexistence of orthorhombic and tetragonal phases was identified in the composition range of 0.005≤x≤0.015. An obviously improved piezoelectric activity was obtained for the ceramics with compositions near the orthorhombic‐tetragonal phase boundary, among which the composition x=0.005 exhibited the maximum values of piezoelectric constant d₃₃, and planar and thickness electromechanical coupling coefficients (k_p and k_t) of 246 pC/N, 0.435, and 0.554, respectively. Furthermore, the Curie temperature of the ceramics was found decreasing with the increase in (Bi_0.5Na_0.5)HfO₃ content, but still maintaining above 300°C for the phase boundary compositions. These results indicate that the ceramics are promising lead‐free candidate materials for piezoelectric applications.     

Broadband luminescent ferroelectric biocompatible Er:(Na,K)NbO3 nanofibers

Dense homogeneous fabric composed from continuous bead‐free erbium‐doped sodium potassium niobate (Er:NKN) 100 μm long and 100‐200 nm in diameter nanofibers was sintered by sol‐gel calcination assisted electrospinning technique. X‐ray diffraction revealed preferential cube‐on‐cube [001]‐directional growth of fibers containing predominantly monoclinic Na_0.35K_0.65NbO₃‐type phase and significantly less of tetragonal NbO₂, cubic Er₂O₃, and monoclinic ErNbO₄ phases. Er doping with the concentration of 2 at.% provides readily detectable room‐temperature broad‐band photoluminescence (PL) centered at λ_PL = 0.55 and 0.98 μm being pumped, respectively, with 532 and 785 nm lasers. Impedance spectroscopy and static electrical tests revealed ferroelectric properties, electric field induced resistance switching and strong rectification effect in nanoporous sandwich Au/Er:NKN/Pt capacitive cell. Memristor‐type current‐voltage (I‐V) characteristics originate from the electrochemical migration of oxygen vacancies at the n‐type NKN oxide/high work function Pt cathode junction interface.     

Tunable upconversion luminescence in new Ho3+/Yb3+-doped SrBi4Ti4O15 photochromic ceramics for switching application

Upconversion (UC) luminescence modulation is quite important in controlling and processing light for active components of light sources, photoswitches, optical memories, and optical sensing devices. In this work, we reported one kind of novel phosphor, Ho³⁺/Yb³⁺-doped SrBi₄Ti₄O₁₅ ceramics, which displayed both strong UC luminescence and obvious photochromic (PC) reaction. The UC luminescence, PC effect, and the modulation of UC performance based on PC behavior were investigated in detail. By alternating visible light irradiation and thermal stimulus, the UC luminescence could be reversibly regulated. Meanwhile, the modulation was unveiled to tightly rely on the irradiation time and thermal treatment processes. Excellent reproducibility was also achieved. In addition, as an alternative method to thermal treatment, the manipulation of luminescence by electric field was also explored. Finally, the mechanism related to the UC luminescence manipulation was illustrated. The results indicate that these samples could be potentially utilized in optical data storage and anti-counterfeiting security fields.     

Effect of Li2O on the defect polarization in CuO‐added (K0.9Na0.1)NbO3 piezoelectric ceramics

CuO‐added (Li_xK_0.9?xNa_0.1)NbO₃ [C(L_xK_0.9?xN_0.1)N] ceramics with 0.0≤x≤0.05 were well‐sintered at 960°C for 6 hours. The lattice parameters of the specimens decreased with the addition of Li₂O. Defect polarization (P_D) formed between Cu²⁺ions and oxygen vacancies. Double polarization vs electric field (P‐E) hysteresis and sprout‐shaped strain vs electric field (S‐E) curves were observed in these specimens with a large strain of 0.16% at 7.0 kV/mm, possibly owing to the presence of P_D. When the P‐E curve was measured at temperatures higher than 75°C, the C(K_0.9N_0.1)N ceramic exhibited a normal P‐E hysteresis curve, whereas the C(L_0.04K_0.86N_0.1)N ceramic maintained the double P‐E hysteresis curve up to 125°C, indicating that Li₂O increased the thermal stability of P_D. The latter specimen also showed the sprout shaped S‐E curve with a strain of 0.15% at 7.0 kV/mm after 10⁴ cycles of a high electric field of 7.0 kV/mm.     

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.     

Enhancing energy harvesting potential of (K,Na,Li)NbO3‐epoxy composites via Li substitution

In this study, the influence of Li substitution on the piezoelectric performance of lead‐free K_0.5Na_0.5NbO₃ (KNN)‐epoxy composites is explored. KNN piezoceramic particles modified with 0‐12 mol% of Li are prepared via a double calcination technique, resulting in a perovskite particulate which transitions from an orthorhombic to tetragonal crystal structure between 6 and 9 mol% of Li, and contains a minor nonperovskite second phase from 6 mol%. A cuboid particle morphology is evident in all cases, though tetragonal KNN‐based particles have formed with serrated edges and fractures. The particles are dispersed at 10 vol% in an epoxy matrix to develop both random and dielectrophoretically structured (K,Na,Li)NbO₃‐epoxy composites. The dielectric constant of the composites appears almost independent of Li content, while the piezoelectric charge constant of structured composites peaks before the polymorphic phase transition, at 3 mol% of Li. The peak in performance can be attributed to the increased primary particle size of the composition in combination with its single phase orthorhombic crystal structure. The enhancement of the energy harvesting figure of merit, derived from substituting 3 mol% of Li in the KNN particulate, makes these composites an interesting choice for flexible energy generators.     

Thermal stability and local atomic structure of (Na0.5K0.5)NbO3 doped BaTiO3 ceramics

Dielectric properties of x(Na_0.5K_0.5)NbO₃–(1−x)BaTiO₃ (x=0.00 and 0.06) specimens were investigated in terms of changes in local atomic structure, according to the phase transition by elevating the overall temperature. A 0.06(Na_0.5K_0.5)NbO₃–0.94BaTiO₃ (NKN–BT) specimen exhibited enhanced temperature stability along with an increased dielectric constant. The degree of reduction in tetragonality (c/a) at the Curie temperature was smaller in NKN–BT compared to that in pure BaTiO₃, as calculated by Rietveld refinement. From a comparison of the pre-edge region in the Ti K-edge, it was determined that the off-center displacement of the Ti atom was also raised to 13.4% through NKN substitution, with a change in local orientation from the [001] to the [111] directions. The substitution by NKN, which has a different ionic radius and electrical charge compared with BaTiO₃, causes structural distortion of the TiO₆ octahedra in the NKN–BT lattice, resulting in local polarization. These structural changes lead to the temperature stability of the dielectric constant and an overall improvement in the electrical properties of BaTiO₃.     

The NbO6 octahedral distortion and phase structural transition of Eu3+‐doped K0.5Na0.5NbO3–xLiNbO3 ferroelectric ceramics

A small quantity of Eu³⁺ ions were doped in the lead‐free ferroelectric K_0.5Na_0.5NbO₃–xLiNbO₃ (KNN–xLN, 0 ≤ x ≤ 0.08) ceramics to investigate the NbO₆ octahedral distortion induced by the increasing LN content. In addition, the phase structure, ferroelectric, and photoluminescence properties of K_0.5Na_0.5NbO₃–xLiNbO₃:0.006Eu³⁺ (KNN–xLN:0.006Eu³⁺) lead‐free piezoelectric ceramics were characterized. All the X‐ray diffraction, Raman spectra, dielectric constant vs temperature measurements and the photoluminescence of Eu³⁺ ions demonstrated that the prepared ceramics undergo a polymorphic phase transition (PPT, from orthorhombic to tetragonal phase transformation) with the rising LN content, and the PPT region locates at 0.05 ≤ x ≤ 0.06. The ferroelectric properties, Raman intensity ratios and photoluminescence intensity ratios show similar variations with the increasing LN content, all with a maximum value achieved at the PPT region. We believe that the close relationship among the ferroelectric properties, Raman intensity ratios, and photoluminescence intensity ratios is caused by the NbO₆ octahedral distortion. The photoluminescence of Eu³⁺ ion was discussed basing on the crystal‐symmetry principle and Judd‐Ofelt theory.     

Large Piezoelectricity and Ferroelectricity in Mn‐Doped (Bi0.5Na0.5)TiO3‐BaTiO3 Thin Film Prepared by Pulsed Laser Deposition

Mn‐doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (MnBNBT) thin films were prepared on SrRuO₃ (SRO)‐coated (001) SrTiO₃ (STO) single crystal substrates by pulsed laser deposition under different processing conditions. Structural characterization (i.e., XRD and TEM) confirms the epitaxial growth of STO/SRO/MnBNBT heterostructures. Through the judicious control of deposition temperature, the defect level within the films can be finely tuned. The MnBNBT thin film deposited at the optimized temperature exhibits superior ferroelectric and piezoelectric responses with remanent polarization P_r of 33.0 μC/cm² and piezoelectric coefficient d₃₃ of 120.0 ± 20 pm/V.     

Ferroelectric K0.5Na0.5NbO3 catalysts for dye wastewater degradation

K_0.5Na_0.5NbO₃ (KNN) particles were prepared by a solid–state method. X–ray diffraction, scanning electron microscopy, UV–visible spectrophotometry, and electrochemical impedance spectroscopy were used to study the structure, morphology, and properties of the samples. The obtained KNN is a ferroelectric material with orthorhombic perovskite structure at room temperature. The KNN particles can be used as piezo/photo–bicatalysts for degrading organic pollutants by utilizing vibrational and solar energy; the catalytic activity of the particles can be significantly improved owing to their polarization under an applied electric field. Poled KNN particles show a bicatalytic degradation ratio of rhodamine B (RhB) dye reaching 92% after 60 min. The results indicate that the KNN particles can be applied as attractive ferroelectric catalysts for organic pollutant degradation.