Enhanced piezoelectricity and non-contact optical temperature sensing performance in Er3+ ion modified (K,Na)NbO3-based multifunctional ceramics

Rare earth ions doped ferroelectrics have attracted wide attentions due to their multifunction characteristics with both ferroelectric/piezoelectric properties and intriguing photoluminescence performance, which show great prospects for future multifunctional devices. In this work, a novel rare earth Er³⁺ ion modified potassium-sodium niobate (KNN) based ceramics were elaborately designed and prepared by the conventional solid-state reaction. The microstructure, phase structure, electric properties and photoluminescence performance of the Er³⁺ ion modified KNN-based ceramics were systematically investigated. Enhanced piezoelectricity (a considerable d₃₃ of exceeding 300 pC/N and a large d₃₃* up to 500 p.m./V) was realized through optimizing the substitution of BaZrO₃ by (Er_0.5,Na_0.5)ZrO₃. Both down-conversion and up-conversion photoluminescence emissions were detected in the optimal composition. The temperature-dependent upconversion emissions of the optimal Er³⁺ modified ceramic sample in the temperature range of 303–573K were verified to be applicable for non-contact optical temperature sensing with a maximum sensitivity S_a of 0.0028 K^-1 and a peak relative sensitivity S_r of 0.96% K⁻¹. Moreover, low-temperature sensing performance with a maximum S_r of 16.7% K⁻¹ in the temperature range of 80–280K was also presented based on the temperature-dependent down-conversion emissions. With both decent electrical properties and intriguing photoluminescence performance, the Er³⁺-modified KNN-based ferroelectrics exhibit good application potential in the future multifunctional optoelectronic devices.     

Dy/Sr-codoped (K0.5Na0.5)NbO3 multifunctional transparent-ferroelectric ceramics with adjustable white-light emission

Traditional solid-state luminescent materials (e.g., phosphors and microcrystalline glasses) have attracted extensive attention due to their high energy efficiency and long lifespan. However, their application scope is limited by complex compositions and manufacturing processes, as well as single-function. In this study, solid-state reaction was utilized to prepare transparent-ferroelectric (K_0.5Na_0.5)NbO₃ (KNN) ceramics by simply codoping with Dy³⁺ and Sr²⁺, providing possibilities for large-scale production. Ceramics exhibit high optical transmittance, high-quality white-light emission and relaxor ferroelectric feature. Under irradiation of UV light, photochromic behavior occurring in the ceramics, resulting in modulating the optical performances, especially quality of emitted white light. Multifunctionality can expand latent applications of KNN-based ceramics from white light-emitting diodes to modulated optoelectronic devices.     

Photochromism-induced light scattering and photoswithing in Er doped (K,Na)NbO3 transparent ceramics

The (K_0.5Na_0.5)_0.95Li_0.05Nb_0.95Bi_0.05O₃-1mol% Er₂O₃ transparent ceramics were prepared by a pressureless sintering method. The fabricated transparent ceramics not only exhibit high optical transmittance (85.3%) due to dense microstructure (nanoscale size grains), but also show the photochromism-induced light scattering and reversible upconversion (UC)-switching properties by visible light irradiation. Upon 407 nm light irradiation, the optical transmittance intensity is significantly decreased, showing a strong light scattering (ΔAbs = 28.3%). The light scatting degree can be quantitatively reflected by Er³⁺ ion UC emission, and could be recovered to its initial optical transmittance based on photochromic reactions. Meanwhile, the transparent ceramic is found to maintain good energy storage properties with higher W (5.87 J/cm³) and W_rec (1.96 J/cm³) under a higher electric field of 260 kV/cm. These results suggest that Er³⁺ doped (K_0.5Na_0.5)NbO₃ transparent ceramics are promising for the modulation of light scattering and the design of photoelectric multifunction devices.     

Electrocaloric effect and pyroelectric performance in (K,Na)NbO3-based lead-free ceramics

The issue of how to achieve an electrocaloric effect (ECE) and pyroelectric effect in a material simultaneously remains to be a challenge for developing practical solid-state cooling devices and RF-detectors. Here, we structure a polymorphic phase transition (PPT) region by doping modification in KNN-based ceramics, which are developed to achieve the ECE. The direct measured ECE and pyroelectric properties are investigated in lead-free (1-x)K_0.5Na_0.5NbO₃-xBi_0.5Na_0.5ZrO₃ (KNN-xBNZ) ceramics. The adiabatic temperature change (∆T) of 0.22 K at 100°C, 0.14 K at 70°C and 0.16 K at 30°C can be obtained under an electric field of 35 kV cm^–1 for x = 0.03, 0.04 and 0.05, respectively. In addition, the temperature dependence of pyroelectric coefficient (p) is established for all compositions via the Byer-Roundy method. A large p of 454.46 × 10^–4 C m^–2 K^–1 is detected at Curie temperature (T_C) in the ceramics with x = 0.03. Achieving electrocaloric effect and pyroelectric performance simultaneously may shed light and provide a feasible design scheme for developing practically useful electrocaloric and pyroelectric materials.     

Lead‐free (K,Na)NbO3‐based ceramics with high optical transparency and large energy storage ability

Highly transparent lead‐free (1‐x)K_0.5Na_0.5NbO₃–xSr(Zn_1/3Nb_2/3)O₃ (KNN–xSZN) ferroelectric ceramics have been synthesized via a conventional pressureless sintering method. All samples are optically clear, showing high transmittance in the visible and near‐infrared regions (~70% and ~80% at 0.5 mm of thickness, respectively). This exceptionally good transmittance is due to the pseudo‐cubic phase structure as well as the dense and fine‐grained microstructure. In addition, a high energy storage density of 3.0 J/cm³ has been achieved for the 0.94K_0.5Na_0.5NbO₃–0.06Sr(Zn_1/3Nb_2/3)O₃ ceramics with submicron‐sized grains (~136 nm). The main reason is likely to be the typical relaxor‐like behavior characterized by diffuse phase transition, in addition to the dense and fine‐grained microstructure. This study demonstrates that the 0.94K_0.5Na_0.5NbO₃–0.06Sr(Zn_1/3Nb_2/3)O₃ ceramic is a promising candidate of lead‐free transparent ferroelectric ceramics for new areas beyond transparent electronic device applications.     

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.     

The absorption and emission properties of highly transparent ZrO2-doped Yb3+: Y2O3 ceramics

Ultra-highly transparent ZrO₂-doped Yb³⁺: Y₂O₃ ceramics were prepared by slip casting and vacuum pressureless sintering and the transmittance reached the highest value of 80.9% for the sample doped with 8.0 at% Yb³⁺. There are three main absorption peaks at 905, 950, and 976 nm, corresponding to the transition from the lowest level of field splitting of ²F_7/2 crystal to every splitting energy levels of ²F_5/2 crystal field. We analyzed the absorption and emission spectra of transparent Yb³⁺: Y₂O₃ from the energy level structure of Yb³⁺, and the transmission, absorption, and emission spectra were systematically studied. There are three main absorption peaks at 905, 950, and 976 nm and four emission peaks at 1076, 1031, 1013, and 977 nm, respectively. The emission peaks at 977 and 1013 nm broaden and vanish for 8.0 and 10.0 at% Yb³⁺-doped Y₂O₃, which may be related to the change of Y₂O₃ crystal field caused by high concentration.     

Synthesis and luminescence characterization of Pr3+, Gd3+ co-doped SrF2 transparent ceramics

Pr³⁺, Gd³⁺ co-doped SrF₂ transparent ceramic, as the potential material for visible luminescent applications, was prepared by hot-pressing of precursor nanopowders. The microstructure, phase compositions, and in-line transmittance, as well as the photoluminescence properties were investigated systematically. Highly optical quality Pr,Gd:SrF₂ transparent ceramic with nearly pore-free microstructure was obtained at 800°C for 1.5 hours. The average in-line transmittance of the x at.% Pr, 6 at.% Gd:SrF₂ (x = 0.2, 0.5, 1.0, 2.0) transparent ceramics reached to 87.3 % in the infrared region. The photoluminescence spectra presented intense visible light emissions under the excitation of 444 nm, the main intrinsic emission bands located at 483 and 605 nm, which were attributed to the transitions of Pr³⁺: ³P₀ → ³H₄ and ¹D₂ → ³H₄, respectively. With the co-doping of Gd³⁺ ions, the emission intensity of the Pr:SrF₂ transparent ceramic was greatly enhanced. All the emission bands of x at.% Pr, 6 at.% Gd:SrF₂ transparent ceramics exhibited the highest luminescence intensity with the 1.0 at.% Pr³⁺ doping concentrations, whereas the lifetimes decreased dramatically with the Pr³⁺ doping contents increasing from 0.2 to 2.0 at.% due to its intense concentration quenching effect. The 1 at.% Pr, 6 at.% Gd:SrF₂ transparent ceramic is a promising material for visible luminescent device 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.     

Diffused and successive phase transitions of (K,Na)NbO3-based ceramics with high strain and temperature insensitivity

Phase boundaries realize enhanced piezoelectricity in lead-free (K, Na)NbO₃-based ceramics but suffer from the weakness of undesirable temperature sensitivity. Here, an effective method is designed to develop temperature-insensitive piezoelectricity (small signal piezo-coefficient [d₃₃] and large signal piezo-coefficient []) in KNN-based piezoceramics by constructing the diffused and successive phase transitions, which results in a broadness of the optimal temperature range of the electrical properties. The room-temperature value in KNN-based ceramics modified with BaZrO₃ and (Bi_0.5Na_0.5)HfO₃ reaches as high as 540 (±10) pm V⁻¹, which is higher than PZT-5H and most reported KNN-based systems. Notably, superior temperature insensitivity of the and P_r values is also observed among the diffused and successive phase transitions region (20-100°C), with <5% fluctuation. In addition, the in situ temperature-dependent d₃₃ measurement shows a high-temperature reliability and less fluctuation (<15%) in a wide temperature range (20-120°C). These results open a new window for further development of highly temperature-insensitive lead-free piezoceramics.     

Y3NbO7 transparent ceramic series for high refractive index optical lenses

A₂B₂O₇ and A₃BO₇ transparent ceramic families are potential materials for optical lenses because of their high refractive index. Although nonstoichiometry is widely present in these material families, its effect on refractive index and optical properties has not yet been fully studied. In this study, optical properties are reported for the Y₃NbO₇ transparent ceramic series, Y_1−xNb_xO_1.5+x (x = 0.20, 0.22, 0.24, 0.25, 0.26), which were fabricated by a pressureless pre-sintering and a hot isostatic pressing post-sintering treatment. The refractive index increases from 2.04 to 2.10 (at 587.6 nm) as the Nb content x increases, which is mainly attributed to the variation in the oxygen ion/vacancy ratio. The Abbe number is larger than 40, showing a decreasing trend as the Nb content x increases. The specimen with x = 0.24 has the highest inline transmittance, which were 62% and 76% at 587.6 and 2000 nm, respectively, for a 1-mm-thick specimen. Through the approach of nonstoichiometry, Y_1−xNb_xO_1.5+x series exhibit balanced properties of refractive index, Abbe number, and transmittance, which can be considered as a promising candidate for high refractive index optical lenses.     

Enhanced electromechanical properties of CaZrO3-modified (K0.5Na0.5)NbO3-based lead-free ceramics

Pairing of large strain response and high d₃₃ with high T_c in (K_0.5Na_0.5)NbO₃-based materials is of high significance in practical applications for piezoelectric actuators. Here, we report remarkable enhancement in the electromechanical properties for (1-x)(K_0.52Na_0.48) (Nb_0.95Sb_0.05)O₃-xCaZrO₃ (KNNS-xCZ) lead-free ceramics through the construction of a rhombohedral (R)-tetragonal (T) phase boundary. We investigated the correlation between the composition-driven phase boundary and resulting ferroelectric, piezoelectric, and strain properties in KNNS-xCZ ceramics. The KNNS-xCZ ceramics with x=0.02 exhibited a large strain response of 0.23% while keeping a relatively large d₃₃ of 237pC/N, which was mainly ascribed to the coexistence of R and T phases confirmed by the XRD and dielectric results. It was found that pairing of large strain response and high d₃₃ in KNN-based materials was achieved. As a consequence, we believe that this study opens the possibility to achieve high-performance lead-free electromechanical compounds for piezoelectric actuators applications.     

Developing "smart windows" for optical storage and temperature sensing based on KNN transparent ceramics

Smart windows have attracted considerable attention due to their wide applications in optical data storage, switchable sunroof and temperature sensing. The development strategy for smart windows is focused on performance design, enhancement and integration. However, developing integrated multi-functional smart windows in a single material remains a challenge. In this work, we have successfully prepared (K_0.5Na_0.5)_0.95Ba_0.04Er_0.01NbO₃ (4Ba-1Er-KNN) transparent ceramics for potential applications of temperature detection and optical information storage in smart windows. With alternating ultraviolet (UV) illumination and 300 °C thermal stimulation, the prepared 4Ba-1Er-KNN ceramics can not only achieve non-destructive luminescence readout, but also exhibits an ultra-high photochromic (PC) contrast with rapid response time of 1 s. Furthermore, based on the up-conversion (UC) photoluminescence (PL) intensity ratio of Er³⁺: ²H_11/2/⁴S_3/2 thermally coupled levels, excellent low-temperature sensing performance with the maximum relative sensitivity of 0.023 K⁻¹ at 213 K is obtained. The integration between UC PL, PC response and temperature sensing performance makes it possible to develop multi-functional smart windows.     

Optical and electrical properties of pressureless sintered transparent (K0.37Na0.63)NbO3-based ceramics

Lead-free (1−x)(K_0.37Na_0.63)NbO₃-xCa(Sc_0.5Nb_0.5)O₃ (x=0.050, 0.070, 0.090, 0.095 and 0.100) transparent ferroelectric ceramics have been fabricated by pressureless sintering procedure. Transmittance of 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics sintered in sealed alumina crucible was 15% higher than those sintered unsealed in air. By increasing the content of Ca(Sc_0.5Nb_0.5)O₃, the phase structure of (K_0.37Na_0.63)NbO₃ ceramics transformed from orthorhombic to tetragonal symmetry first and then to pseudo cubic symmetry. The 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics exhibited high density (98%), high transmittance (60%) in the near-IR region and relatively good electrical properties (ε_r=1914, tanδ=0.037, T_c=147 °C, P_r=6.88 μC/cm², E_c=8.49 kV/cm). Meanwhile, the introduction of Ca(Sc_0.5Nb_0.5)O₃ induced a composition fluctuation in the (K_0.37Na_0.63)NbO₃ lattice and made the ceramics more relaxor-like, which would lead to a further reduction of light scattering. These results demonstrated that 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ could be promising lead-free transparent ferroelectric ceramics.     

Perfectly transparent pore-free Nd3+-doped Sr9GdF21 polycrystalline ceramics elaborated from single-crystal ceramization

An Nd³⁺-doped Sr₉GdF₂₁ (SGF) transparent pore-free polycrystalline ceramic is produced by the ceramization of single crystals. The average transmittance (T_A) of the Nd³⁺-doped SGF ceramic (8-mm thick) is about 90.6% in the visible region and more than 92% in the near-IR region. The fracture toughness of the Nd³⁺-doped SGF ceramic is up to 1.28 MPa m^1/2, which is significantly higher than that of similar types of ceramics. Gd³⁺ ions, acting as both buffer ions and pinning ions, effectively improve the optical and mechanical properties of the SGF ceramic, implying that the sample is a promising candidate for high-power lasers. Moreover, the SGF ceramic shows a high mid- and far-infrared (up to 9 μm) transmittance, low phonon energy, and high density (ρ = 4.8466 g/cm³), which pave the way for a wide range of applications such as infrared windows, mid- and far-infrared lasers, scintillators, and in photonics.     

Effect of sintering temperature on the depolarization behavior of (Bi0.5Na0.5)TiO3-based ceramics

(Bi_0.5Na_0.5)TiO₃ (BNT)-based ferroelectric ceramics have drawn extensive attention because of their excellent electrical properties and interesting depolarization behavior. However, the poor thermal stability of electrical properties limits their practical application. In this work, the effect of sintering temperature (T_s) on the depolarization behavior of BNT-based ceramics was systematically investigated. It is found that the depolarization temperature T_d determined from pyroelectric measurement tends to decrease with increasing T_s, which indicates that lower T_s defers the ferroelectric-relaxor (FE-RE) phase transition. However, for the samples sintered at higher T_s (such as 1180°C), although the T_d is reduced, the thermal stability is better compared with the sample sintered at lower T_s (1100°C) because the diffuse behavior of the FE-RE phase transition is suppressed. According to these results, we propose that the thermal stability of electrical properties for BNT-based ceramics is not only related to high T_d, but also to the diffuse degree of phase transition.     

Production and optical properties of Ce3+-activated and Lu3+-stabilized transparent gadolinium aluminate garnet ceramics

We first report the novel Ce³⁺-activated and Lu³⁺-stabilized gadolinium aluminate garnet (GAG) transparent ceramics derived from their precipitation precursors via a facile co-precipitation strategy using ammonium hydrogen carbonate (AHC) as the precipitant. The resulting precursors in liquid phase were substantially homogeneous solid solutions and could directly convert into sinterable garnet powders via pyrolysis. Substituting 35 at.% of Lu³⁺ for Gd³⁺ was effective to stabilize the cubic GAG garnet structure and transparent (Gd,Lu)₃Al₅O₁₂:Ce ceramics were successfully fabricated by vacuum sintering at 1715°C. The ceramic transparency was improved by optimizing the particle processing conditions and the best sample had an in-line transmittance of ~70% at 580 nm (Ce³⁺ emission center) and over 80% in partial infrared region with a fine average grain size of ~4.5 μm. Transparent (Gd,Lu)₃Al₅O₁₂:Ce ceramics have a short critical wavelength (<200 nm) and a maximal infrared cut-off at ~6.6 μm. Both the (Gd,Lu)₃Al₅O₁₂:Ce phosphor powder and the transparent ceramic exhibited characteristic yellow emission of Ce³⁺ with strong broad emission bands from 490 to 750 nm upon UV excitation into two groups of broad bands around 340 and 470 nm. The photoluminescence and photoluminescence excitation intensities as well as the quantum yield were greatly enhanced via high-temperature densification. Both the phosphor powder and ceramic bulk had short effective fluorescence lifetimes.     

Fabrication and characterizations of Cr3+-doped ZnGa2O4 transparent ceramics with persistent luminescence

0.5 at.% Cr:ZnGa₂O₄ precursor was synthesized by the co-precipitation method with nitrates as raw materials, using ammonium carbonate as the precipitant. Low-agglomerated Cr:ZnGa₂O₄ powders with an average particle size of 43 nm were obtained by calcining the precursor at 900℃ for 4 h. Using the powders as starting materials, 0.5 at.% Cr:ZnGa₂O₄ ceramics with an average grain size of about 515 nm were prepared by presintering at 1150℃ for 5 h in air and HIP post-treatment at 1100℃ for 3 h under 200 MPa Ar. The in-line transmittance of 0.5 at.% Cr:ZnGa₂O₄ ceramics with a thickness of 1.3 mm reaches 59.5% at the wavelength of 700 nm. The Cr:ZnGa₂O₄ ceramics can be effectively excited by visible light and produce persistent luminescence at 700 nm. For Cr:ZnGa₂O₄ transparent ceramics, the brightness of afterglow was larger than 0.32 mcd/m² after 30 min, which is far superior to that of Cr:ZnGa₂O₄ persistent luminescence powders.     

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.     

Polycrystalline Ho: LuAG laser ceramics: Fabrication,microstructure, and optical characterization

Ho:Lu₃Al₅O₁₂(LuAG) transparent ceramics are potential 2 μm eye‐safe laser materials. Polycrystalline 0.8 at.% Ho:LuAG ceramics with high optical quality were successfully fabricated by solid‐state reactive sintering of high‐purity oxide powders. The microstructure, the optical transmission, the spectrum characteristic, and the laser performance were investigated in this paper. The average grain size of Ho:LuAG ceramics vacuum sintered at 1830°C for 30 hour is about 14 μm. The in‐line transmittance of the sample is measured to be 81.7% and 82.0% at 1000 and 2250 nm, respectively. The absorption and the emission cross sections are calculated to be 0.88 × 10^?20 cm² at 1906 nm and 1.26 × 10^?20 cm² at 2094 nm. Using a thulium‐doped yttrium‐lithium‐fluoride (Tm:YLF) laser with the central wavelength of 1907.5 nm as the pump source, 2.67 W continuous wave (CW) laser operation at 2100.74 nm was obtained with a slope efficiency of 26.5%. The beam quality factor M² was calculated to be 1.1, which indicated nearly diffraction‐limited beam propagation and the laser was the fundamental TEM₀₀ Gaussian mode.