Amelioration on energy storage performance of KNN-based transparent ceramics by optimizing the polarization and breakdown strength

Transparent ceramic capacitors have broad application prospects in electronic devices due to their excellent optical transparency and energy storage properties. However, the low polarizability and high remnant polarization of the existing transparent dielectric ceramics limit the promotion of energy storage performance. Here, Bi(Li_0.5Nb_0.5)O₃ (BLN) was chosen to modify the (K_0.5Na_0.5)NbO₃ (KNN)-based ceramics to optimize the optical transmittance and energy storage characteristics simultaneously. On the one hand, the grain growth is inhibited, contributing to the improved breakdown strength and transmittance. On the other hand, the doping BLN could reduce the polar nanoregions size, which makes them respond more quickly to the external electric field and, thus, improves the energy storage efficiency. As a consequence, 0.95KNN–0.05BLN ceramic possesses the excellent W_rec of 4.39 J/cm³, η of 81.4%, and transparency of 77.9% with an average grain size of ∼109 nm. This work opens up a paradigm to develop a transparent pulse capacitor.     

Enhanced optical transmittance and energy-storage performance in NaNbO3-modified Bi0.5Na0.5TiO3 ceramics

Dielectric ceramics with both excellent energy storage and optical transmittance have attracted much attention in recent years. However, the transparent Pb-free energy-storage ceramics were rare reported. In this work, we prepared transparent relaxor ferroelectric ceramics (1 − x)Bi_0.5Na_0.5TiO₃–xNaNbO₃ (BNT–xNN) by conventional solid-state reaction method. We find the NN-doping can enhance the polarization and breakdown strength of BNT by suppressing the grain growth and restrained the reduction of Ti⁴⁺ to Ti³⁺. As a result, a high recoverable energy-storage density of 5.14 J/cm³ and its energy efficiency of 79.65% are achieved in BNT–0.5NN ceramic at 286 kV/cm. Furthermore, NN-doping can promote the densification to improve the optical transmittance of BNT, rising from ∼26% (x = 0.2) to ∼32% (x = 0.5) in the visible light region. These characteristics demonstrate the potential application of BNT–xNN as transparent energy-storage dielectric ceramics.     

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.     

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.     

Ho3+-doped (K,Na)NbO3-based multifunctional transparent ceramics with superior optical temperature sensing performance

Ho³⁺-doped (K_0.5Na_0.5)NbO₃-based transparent ceramics have been prepared via pressureless solid-state method. The ceramics possess moderate optical transparency with the energy band gap of ~2.9 eV and submicron-sized grains (<500 nm). The temperature-dependent dielectric properties and ferroelectric polarization-electric field hysteresis loops demonstrate that the ceramics own relaxor-like characteristics. The up-conversion photoluminescence and optical temperature sensing properties of the ceramics have been investigated. The temperature dependence of photoluminescence provides a fluorescent method to detect phase transitions, which can be expanded to other ferroelectric systems. The outstanding optical temperature sensitivity (~0.0075/K at 430 K) of the ceramic is higher than many other rare-earth-doped ceramics or glasses. These results suggest that the Ho³⁺-doped (K_0.5Na_0.5)NbO₃-based transparent ceramics are promising lead-free transparent materials for multifunctional applications, especially in temperature sensing devices.     

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.     

Large recoverable energy storage density and low sintering temperature in potassium‐sodium niobate‐based ceramics for multilayer pulsed power capacitors

Multilayer pulsed power ceramic capacitors require that dielectric ceramics possess not only large recoverable energy storage density (W_rec) but also low sintering temperature (<950°C) for using the inexpensive metals as the electrodes. However, lead‐free bulk ceramics usually show low W_rec (<2 J/cm³) and high sintering temperature (>1150°C), limiting their applications in multilayer pulsed power ceramic capacitors. In this work, large W_rec (~4.02 J/cm³ at 400 kV/cm) and low sintering temperature (940°C) are simultaneously achieved in 0.9(K_0.5Na_0.5)NbO₃–0.1Bi(Mg_2/3Nb_1/3)O₃–1.0 mol% CuO ceramics prepared using transition liquid phase sintering. W_rec of 4.02 J/cm³ is 2‐3 times as large as the reported value of other (Bi_0.5Na_0.5)TiO₃ and BaTiO₃‐based lead‐free bulk ceramics. The results reveal that 0.9(K_0.5Na_0.5)NbO₃–0.1Bi(Mg_2/3Nb_1/3)O₃–1.0 mol% CuO ceramics are promising candidates for fabricating multilayer pulsed power ceramic capacitors.     

Fabrication of transparent electro-optic (K0.5Na0.5)1−xLixNb1−xBixO3 lead-free ceramics

Lead-free transparent electro-optic ceramics (K_0.5Na_0.5)_1?xLi_xNb_1?xBi_xO₃ have been fabricated by hot-press sintering. Owing to the effective suppression of grain growth, the Li and Bi co-modified ceramics generally possess a dense and fine-grained structure. The co-modification also causes the ceramics to transform into a nearly cubic structure with minimal optical anisotropy. A diffuse phase transformation is also induced, causing the ceramics to become more relaxor-like and contain more polar nano-regions. These would reduce the light scattering by the grains, at the grain boundaries and at the domain walls, respectively, and thus making the ceramics become optically transparent. For the ceramic modified with 5 mol% Li⁺ and Bi⁵⁺, the optical transmittance reaches a high value of 60% in the near-IR region. The ceramics also exhibit a strong linear EO response, giving a large effective linear EO coefficient in the range of 120–200 pm/V.     

Transparency of K0.5N0.5NbO3–Sr(Mg1/3Nb2/3)O3 lead-free ceramics modulated by relaxor behavior and grain size

High transparency was obtained in (1−x)(K_0.5Na_0.5)NbO₃–xSr(Mg_1/3Nb_2/3)O₃ (x=0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08) lead-free ceramics by pressure-less sintering procedure. The effects of Sr(Mg_1/3Nb_2/3)O₃ content on the microstructure, phase transition, optical properties and electrical properties were studied in detail. The X-ray diffraction results showed that the crystal structure of ceramics gradually transformed from orthorhombic phase into pseudo-cubic phase with doping of Sr(Mg_1/3Nb_2/3)O₃. The fine grain microstructure with clear grain boundary was observed in all compositions, while the grain size exhibited significant composition dependence. It was found that a more uniform distribution with smaller grain size was favorable to high optical transmittance, owing to the decreased scattering by grains and grain boundaries. In addition, a strong diffuse phase transformation in KNN-based ceramics induced by Sr(Mg_1/3Nb_2/3)O₃ doping, causing the ceramics become more relaxor-like and transparent. The transmittance and electric properties results indicated that the 0.95(K_0.5Na_0.5)NbO₃–0.05 Sr(Mg_1/3Nb_2/3)O₃ ceramics exhibited higher transmittance (60% in the near-IR region) accompanied with better electrical properties (ε_m=2104, P_r=5.0 μC/cm², d₃₃=92 pC/N).     

Smart white lighting and multi-mode optical modulations via photochromism in Dy-doped KNN-based transparent ceramics

Dy³⁺-doped K_0.5Na_0.5NbO₃-Ba(Sc_0.5Nb_0.5)O₃ transparent ferroelectric ceramics, a novel smart white-lighting (WL) photochromic (PC) material, were prepared by conventional pressureless sintering. The ceramics exhibit high optical transmittance (~67% at 1800 nm), good photoluminescence (PL) performance, high-quality WL, and moderate electrical properties. Meanwhile, high-contrast modulations of optical transmittance and PL intensity (with the maximal modulating values of 20.5% and 57.4%, respectively) have been realized via PC behavior by alternating the illumination of UV light and thermal stimulus. In particular, the reversible modulations of WL intensity, color coordinates, correlated color temperature, and color purity have been achieved through the PC regulation to the intensity ratio of yellow/blue emission. Definitively, based on the effect of intrinsic defects and extrinsic defects (Ba⁴⁺/Sc^3-x) on the PL properties, the PC mechanism in KNN-based material has been further complemented. The transparent ceramics should be a promising material in white LEDs and optoelectronic multifunctional devices.     

Improved transmittance and ferroelectric properties realized in KNN ceramics via SAN modification

Transparent, Sr(Al_0.5Nb_0.5)O₃‐modified K_0.5Na_0.5NbO₃ (KNN) ceramics were successfully fabricated by a solid‐state pressureless sintering method in this work. The obtained microstructure, transmittance, and electrical properties were characterized in detail. Our results indicated that the modification by Sr(Al_0.5Nb_0.5)O₃ significantly limited the grain growth behavior of KNN, resulting in dense ceramics with submicron grain size (<0.5 μm) and small pore size. Consequently, the ceramic with the 0.96K_0.5Na_0.5NbO₃‐0.04Sr(Al_0.5Nb_0.5)O₃ composition showed superior transmittance and electrical properties: T = 55% in the visible region (0.78 μm), d₃₃ = 105 pC/N, ε_r = 1021, and P_r = 15.1 μC/cm², which were significantly higher than those of pure KNN. Our findings implied that the addition of Sr(Al_0.5Nb_0.5)O₃ could be a good strategy to obtain superior transmittance and electrical properties in KNN and may shed light on other ferroelectric systems.     

Outstanding optical temperature sensitivity and dual-mode temperature-dependent photoluminescence in Ho3+-doped (K,Na)NbO3–SrTiO3 transparent ceramics

High optical temperature sensing properties based on rare-earth-doped (K,Na)NbO₃-based ferro-/piezoelectrics have attracted much attention due to their potential application in novel optoelectronic devices. Here, we fabricated Ho³⁺-doped (K_0.5Na_0.5)NbO₃–SrTiO₃ transparent ceramics by conventional pressureless sintering. Their microstructures, transmittances, up-conversion photoluminescence, and optical temperature sensing properties have been characterized in details. Because of the cubic-like phase, dense, and fine-grained structure as well as relaxor-like feature, the ceramics exhibit high transmittance (~70%) in the near-infrared region. Owing to Ho³⁺, green and red up-conversion emissions have been observed, which can be easily modulated by temperature. The ceramics have stable emission colors (<200°C) and superior temperature-modulating emission color-tunable performance (>200°C). Furthermore, the temperature sensing behavior based on the thermally coupled levels (⁵F₄, ⁵S₂) of Ho³⁺ has been analyzed by a fluorescence intensity ratio technique. The transparent ceramics possess outstanding optical temperature sensitivity (~0.0096/K at 550 K), higher than most rare-earth-doped materials (e.g., ceramics, glasses, and phosphors).     

Highly responsive photochromic behavior with large coloration contrast in Ba/Sm co-doped (K0.5Na0.5)NbO3 transparent ceramics

Ferroelectrics that simultaneously possess optical transparency and photochromic (PC) behavior have attracted extensive attention for multi-functionality. However, inability to achieve both rapid and large coloration contrast in photo-stimulated ferroelectrics limits their practical application. In this work, we propose a new strategy for realizing rapid photochromism by constructing the intermediate trap level (T₂) in Ba/Sm co-doped (K_0.5Na_0.5)NbO₃ (KNN) ferroelectric transparent ceramics. Specifically, rapid photo-response time of about 2 s was achieved, and the modulation ratios of transmittance and luminescence intensity were 37.6% and 72.6% within 2 s for the ceramics. This highly responsive PC behavior with large coloration contrast is expected to broaden the application of PC materials in optical devices, e.g. photo-sensitive glasses and rewritable information displays.     

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.     

Simultaneous realization of high transparency and piezoelectricity in low symmetry KNN-based ceramics

Transparent piezoelectric ceramic, as a lead-free multifunctional ceramic, is in dire need of development for future high-tech industries. However, excellent piezoelectricity and high transmittance are usually hard to achieve simultaneously, mainly due to the two mutual restricting factors (phase structure and grain size). In this work, we report that high piezoelectricity and transmittance can be obtained simultaneously in K_0.5Na_0.5NbO₃ ceramics via Sr(Sc_0.5Nb_0.5)O₃ (SSN) modification. The superior piezoelectric performance comes from the retain of orthorhombic phase structure at room temperature (RT); while the high transparency (>70% at 780 nm) can be attributed to the improved relative density and reduced grain size via SSN modification. Remarkably, in the sample with 0.05SSN modification, we realized a comprehensively high transmittance (73% at 780 nm) accompanied by a superior piezoelectric constant (d₃₃ = 101 pC/N), which outperform other reported KNN-based transparent ceramics to our best knowledge. Our results may provide insight for further developing the transparent piezoelectric ceramics by controlling the grain size and phase structure.     

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.     

Energy storage performance of SrSc0.5Nb0.5O3 modified (Bi,Na)TiO3-based ceramic under low electric fields

Dielectric ceramics with a high recoverable energy density (W_rec) and high efficiency are desirable for the development of pulsed power capacitors under low electric fields. In this study, through the introduction of SrSc_0.5Nb_0.5O₃ into (Bi_0.5Na_0.5Ti_0.95Al_0.025Nb_0.025O₃) [(1-x)BNTA-xSSN], a considerable recoverable energy storage density (W_rec) of approximately 2.7 J/cm³ and energy storage efficiency (η) of approximately 76 % at 210 kV/cm are achieved at x = .1; additionally, η is further improved to 85 % at x = .2. Moreover, η and W_rec of .9BNTA-.1SSN exhibit outstanding stability (thermal and frequency stability) at 150 kV/cm, which is superior to that of other lead-free ceramics. The excellent energy storage performance is attributed to the increased relaxation degree and the formation of ferroelectric nanodomains, whereas the enhanced E_b is ascribed to the increased electrical resistivity and decreased grain size upon modification. These results indicate the potential of (1-x)BNTA-xSSN as an ideal candidate for energy-storage applications.     

Simultaneous high transmittance and large tunability of up-conversion photoluminescence in Er3+ doped KNN-based ceramics

0.98(0.94K_0.51Na_0.5NbO₃-0.06SrZrO₃)-0.02Li_0.5La_0.5TiO₃+x mol Er³⁺ ceramics were prepared using a conventional solid-state reaction method. The transmittance of the sample x = 0.5% sintered at 1210 °C reaches 56% in at the wave length of 780 nm and 73% at the wave length of 2000 nm (the thickness of the sample is 0.3 mm). Under 980 nm excitation, two typical emission bands are obtained, which are the green emission band at 510 nm–580 nm and the red emission band at 645–695 nm. Under the irradiation of visible light, the up-conversion luminescence intensity of the sample is significantly reduced, showing a luminescence quenching characteristic. The PL quenching degree (ΔR) of up-conversion emission is up to 78.2% for the sample x = 0.5% sintered at 1210 °C. Besides, a behavior of photosensitive resistance is achieved at room temperature and 350 °C, which suggests that this system has a great application prospect in optical information storage.     

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.     

Optical temperature sensor based on upconversion luminescence of Er3+ doped GdTaO4 phosphors

For the development of optical temperature sensor, a series of GdTaO₄ phosphors with various Er³⁺-doping concentrations (0, 1, 5, 10, 25, 35, 50 mol%) were synthesized by a solid-state reaction method. The monoclinic crystalline structure of the prepared samples was determined by X-ray diffraction (XRD). Under excitations of 980 and 1550 nm lasers, the multi-photon-excited green and red upconversion (UC) luminescence emissions of Er³⁺ were studied, and the critical quenching concentration of Er³⁺-doped GdTaO₄ phosphor was derived to be 25 mol%. By changing the pump power of laser, it was found that the two-photon and three-photon population processes happened for the UC emissions of Er³⁺-doped GdTaO₄ phosphors excited by 980 and 1550 nm lasers, respectively. Furthermore, based on the change of thermo-responsive green UC luminescence intensity corresponding to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ with temperature, the optical temperature sensing properties of Er³⁺-doped GdTaO₄ phosphor were investigated under excitations of 980 and 1550 nm lasers by using the fluorescence intensity ratio (FIR) technique. It was obtained that the maximum absolute sensitivity (S_A) and relative sensitivity (S_R) of Er³⁺-doped GdTaO₄ phosphors are as high as 0.0041 K⁻¹ at 475 K and 0.0112 K⁻¹ at 293 K, respectively. These significant results suggest that the Er³⁺-doped GdTaO₄ phosphors are a promising candidate for optical temperature sensor.