In recent years, the continuous development of 3D reconstruction methods for porous media has promoted the integrated evolution of image modeling, performance prediction, and multiscale microstructure analysis. Although traditional reconstruction approaches achieve satisfactory matches for low-order statistical descriptors, they often struggle to accurately reproduce realistic geometric morphology and topological characteristics across multiple scales. Meanwhile, traditional generative adversarial network (GAN) methods exhibit remarkable advantages in handling complex porous structures (e.g., cement-based materials), yet are prone to issues such as mode collapse and gradient saturation. To address the aforementioned challenges, a high-fidelity 3D reconstruction method based on deep convolutional generative adversarial networks (DCGANs) is proposed in this paper, incorporating an autoencoder module within the generator to enhance multiscale feature representation. The model introduces a porosity constraint term in the adversarial learning framework and integrates Wasserstein generative adversarial networks with gradient penalty (WGAN-GP), significantly improving training stability and convergence performance. Trained on CT images of new silicoaluminate cement paste at different curing ages, the model demonstrates good performance in both geometric morphology and statistical feature consistency of the reconstructed results.

The samples were prepared using the new silicoaluminate (limestone and calcined clay cement) system with 55% (mass fraction) Portland cement, 30% calcined clay, 15% limestone powder, and a water-to-binder ratio of 0.42. High-resolution micro-CT images were obtained at curing ages of 3, 28, 90 d, and 180 d. The images were preprocessed using Gaussian filtering and threshold segmentation to obtain binarized pore structures. A DCGAN architecture was developed by integrating 3D generative adversarial networks (GANs) with an autoencoder module. The framework incorporated gradient penalty to establish a WGAN-GP based on the Wasserstein distance, thereby constraining the Lipschitz continuity of the discriminator. A porosity constraint term was introduced into the adversarial learning framework to ensure global statistical consistency of the reconstructed microstructures. The reconstructed 3D volumes were validated against experimental microstructures using Minkowski functionals (porosity, surface area, Euler characteristic) and spatial correlation descriptors including the two-point correlation function (S2), lineal-path function, two-point cluster function, and surface correlation function.

The results demonstrate that the proposed model effectively reconstructs pore morphology and distribution characteristics across different curing ages, with the porosity of reconstructed volumes showing good consistency with measured samples. The introduction of a gradient penalty term in the loss function enhanced the training stability and convergence efficiency of the model. By incorporating porosity as a regularization term in the adversarial learning framework, the model was effectively guided to learn the porosity evolution trends across different curing ages, thereby improving the consistency between generated structures and experimental data. The generator employed a combined ReLU and GELU activation mechanism, which contributed to stable generator training. The integration of Instance Normalization in the discriminator further improved the quality of generated results and the generalization capability of the model. Comparative analyses based on Minkowski functionals and statistical descriptors revealed that the generated structures exhibit high agreement with the original CT images in terms of geometric morphology, connectivity, and spatial correlation. Compared to conventional reconstruction methods, the proposed approach improves computational efficiency by approximately 150% while maintaining structural fidelity.

This study constructed an efficient 3D generative adversarial network (GAN) model based on CT images of cement paste, achieving high-precision 3D reconstruction of pore structures at different curing ages. The proposed model demonstrates good stability and robustness during training and achieves high-quality reconstruction of pore structures in cement-based materials while effectively preserving structural diversity and morphological realism.

Determining the glass content in fly ash is crucial for characterizing its compositional properties (i.e., glass content, crystal types and quantities, and glass phase composition) and serves as a fundamental parameter for regulating and optimizing its engineering performance (i.e., fresh workability, volume stability, hydration process and products, anti-permeability, and mechanical performance). The glass content of four low-calcium and four high-calcium fly ash samples can be quantitatively analyzed by three methods, i.e., the alkaline-acid two-step dissolution method (NaOH followed by HNO₃), the acid dissolution method (HF), and quantitative X-ray diffraction (QXRD) method. A comparison of the three methods demonstrates that QXRD offers superior testing precision and characterizes the full components of fly ash, unlike the two dissolution methods. Although the two dissolution methods can provide a rough estimation of glass content, their precision is relatively poor. This low precision is attributed to the dissolution of crystalline phases (like quartz and mullite), the formation of new crystalline phases (such as hydromagnesite and tobermorite) during the dissolution process, and the residual undissolved glass phases. Consequently, QXRD can be recommended for the quantitative analysis of fly ash glass content.

The glass content of eight different fly ash samples was measured by three characterization methods, i.e., the alkaline-acid two-step dissolution method (NaOH followed by HNO₃), the acid dissolution method (HF), and quantitative X-ray diffraction (QXRD). A comparison of the results revealed a significant discrepancy between these three testing methods, highlighting a need for further research. To investigate the dissolution power of the NaOH and HNO₃ solutions, two additional methods (i.e., the alkaline dissolution method (NaOH) and the acid dissolution method (HNO₃) were adopted. The respective dissolving capabilities toward both the crystalline and glassy phases of the eight fly ash samples were ultimately disclosed via using QXRD to analyze the dissolving effect of these four dissolution methods.

The dissolving characteristics of the four dissolution methods (i.e., alkaline, HF acid, HNO₃ acid, and two-step) on the crystalline and glass phases of fly ash are analyzed by linear regression and the Pearson correlation method, based on the measured glass and crystal contents remaining after each treatment. The linear regression between the crystal content of the raw fly ash and the residual crystal content after dissolution demonstrates that the crystalline phases of fly ash are dissolvable by both alkaline and/or acid solutions. Furthermore, the results confirm that glass phase residue remains after all alkaline and/or acid treatments, indicating that none of the dissolution methods can achieve a precise selective dissolution of the fly ash's glassy phase. The Pearson correlation method is employed to investigate the dissolving tendency of the crystalline and glass phases in the different solutions. The resulting fly ash component-Pearson correlation coefficient diagram effectively reveals the dissolution characteristics of fly ash components in various alkaline and/or acid solution, thereby clearly explaining the underlying dissolution mechanism and demonstrating the infeasibility of using these alkaline and/or acid dissolution methods for accurately determining the fly ash's glass content.

The quantitative analysis of the glass content in four low-calcium and four high-calcium fly ash samples was conducted by three methods, i.e., the alkaline-acid two-step dissolution method (NaOH followed by HNO₃), the acid dissolution method (HF), and quantitative X-ray diffraction (QXRD). For the three methods, QXRD demonstrated superior precision and comprehensiveness, determining the glass content and the types and quantities of crystalline phases, whereas the two dissolution methods showed a lower precision. The QXRD analysis of the dissolution residues revealed that using alkaline (NaOH) and/or acid (HNO₃, HF) solutions failed to achieve an ideal selective dissolution of the fly ash's glass and crystalline phases. Furthermore, new crystalline phases were sometimes generated during the dissolution process, fundamentally limiting the accuracy of using these dissolution methods for quantitative analysis of glass content. Based on the results of QXRD tests on both the raw fly ash and the alkaline/acid dissolution residues, the error sources were associated with using alkaline and/or acid dissolution methods for determining fly ash glass content. The application of the Pearson correlation method and the resulting interpretation of the reaction mechanisms and products between fly ash components and the NaOH, HNO₃, and HF solutions mechanistically revealed the dissolution process and mechanism of fly ash, thereby fundamentally explaining the infeasibility of accurately quantifying fly ash glass content by alkaline and/or acid dissolution methods.

Dielectric capacitors are widely used in pulsed power and electronic systems due to their high power density, long lifespan, and low cost. However, their energy density is considerably lower than that of batteries. Dielectric materials are the core components for enhancing capacitor performance. Although conventional lead-based ceramics (e.g., PZT) exhibit the superior properties, their toxicity drives the development of environmentally friendly lead-free alternatives as a major research focus. Bismuth-based perovskite materials, due to the similar electronic configurations of Bi³⁺ and Pb²⁺, demonstrate ferroelectric properties comparable to those of lead-based materials. Among them, bismuth potassium titanate (i.e., K_0.5Bi_0.5TiO₃, KBT) stands out as a highly promising lead-free energy storage candidate due to its combination of high theoretical polarization and a wide bandgap. However, a pure KBT is a typical ferroelectric with a high remnant polarization, which limits its energy storage performance. In general, introducing a paraelectric material into a ferroelectric to form a relaxor ferroelectric can disrupt a long-range order and refine ferroelectric domains, thereby improving energy storage characteristics. This study selected quantum paraelectric SrTiO₃ (ST) to form a solid solution with KBT. The effect of ST content on the crystal structure, microstructure, dielectric, and ferroelectric properties was investigated. The ceramics maintained a pure perovskite phase with increasing ST content, showing a shift of the dielectric peak to lower temperatures with an enhanced frequency dispersion and gradually slimmer polarization-electric field hysteresis loops. The optimal composition (i.e., 0.6KBT-0.4ST) achieved a recoverable energy storage density of 3.64 J/cm³ and an energy efficiency of 70% in an electric field of 210 kV/cm, along with good temperature and frequency stability. The results indicated that KBT-ST is a promising class of high-performance lead-free dielectric materials for energy storage applications.

In this study, (1-x)KBT-xST ceramics (where x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by a conventional solid-state sintering method. The starting raw materials were Bi₂O₃ (≥99%), K₂CO₃ (≥99%), TiO₂ (≥99%), and SrCO₃ (≥99%), from Aladdin Co., China. Prior to weighing, K₂CO₃ powder was dried in a muffle furnace at 200 ℃ for 2 h to remove absorbed moisture. The raw materials were then weighed according to the stoichiometric ratio and placed in a nylon milling jar with zirconia grinding balls. Isopropanol was added as a medium in grinding, and the raw materials were wet ground for 12 h. After grinding, the slurry was poured out, dried, and calcined at 850 ℃ for 2 h. The calcined powder with isopropanol was further ground for 22 h. Afterwards, the slurry with 0.2% of binder was ground for additional 2 h. The slurry obtained after further grinding was dried, sieved, and pressed into green ceramic pellets with a diameter of 10 mm and a thickness of 1-2 mm. The green pellets were placed in a muffle furnace, heated at a rate of 2 ℃/min to 600 ℃, and held for 6 h to allow binder burnout. After debinding, the pellets were embedded in an alumina crucible with calcined powder of the same composition (without binder) and sintered at 1050-1400 ℃ (with higher sintering temperatures corresponding to higher ST content) for 2 h. The sintered ceramics were then ground and polished. Silver paste was applied on the both sides of the ceramics and heat-treated at 850 ℃ for 20 min to form silver electrodes. Alternatively, gold electrodes were deposited on the both sides by an ion sputtering coater. The phase composition of the ceramic powders was analyzed by a model Smartlab X-ray diffractometer (XRD) (Rigaku Co., Japan). The Raman spectra were determined on well-polished ceramic samples by a model Alpha 300R Raman imaging microscope (WITec Co., Germany). The microstructure of the materials was characterized by a model Regulus 8230 high-resolution cold-field emission scanning electron microscope (Hitachi Co., Japan). The temperature-dependent dielectric properties were measured by a model IM3536 LCR meter (HIOKI Co., Japan) equipped with a model HCP421V-MP heating stage (INSTEC Co., USA). The polarization-electric field (P-E) hysteresis loops of the ceramics were tested by a model Multiferroic II ferroelectric analyzer (Radiant Co., USA).

The results show that the (1-x)K_0.5Bi_0.5TiO₃-xSrTiO₃(KBT-ST) lead-free dielectric ceramic system is synthesized by a conventional solid-state reaction method. The effect of SrTiO₃ incorporation on the microstructure, phase evolution, and electrical properties of the materials is systematically investigated.

The XRD patterns and SEM images confirm the formation of a single perovskite phase and a dense microstructure for all the compositions. The temperature stability of the dielectric constant is significantly enhanced, accompanied by a shift of the dielectric peak towards lower temperatures, as SrTiO₃ content increases. The gradual slimming of polarization-electric field hysteresis loops indicates a transformation from a typical ferroelectric to a relaxor ferroelectric state. The optimal composition (i.e., 0.6KBT-0.4ST) exhibits a high recoverable energy storage density of 3.64 J/cm³ and an energy efficiency of 70 % in an electric field of 210 kV/cm. This work demonstrates that KBT-ST ceramics constitute a highly promising lead-free dielectric system for high-performance energy storage applications.

In this work, KBT-ST lead-free ceramics were prepared by a solid-state sintering method. Their structural evolution, properties, and potential for dielectric energy storage applications were systematically investigated. Single-phase

perovskite-structured KBT-ST solid solutions were successfully synthesized. The lattice contraction occurred, and the incorporation of Sr²⁺ effectively introduced A-site compositional disorder as the ST content increased. The addition of ST effectively suppressed a long-range ferroelectric order, thus inducing a transition from a ferroelectric to a relaxor ferroelectric state. This transition was evidenced by enhanced frequency dispersion of the dielectric peak, slimmer polarization-electric field hysteresis loops, and improved temperature stability of the dielectric constant. The optimal energy storage performance was achieved in the composition with x of 0.4, which exhibited the optimal overall energy storage properties. The introduction of Sr²⁺ produced a synergistic effect. It enhanced a local polarization, and suppressed a long-range ferroelectric order, promoting the formation of an ergodic relaxor state. The effects both contributed to a reduction in (P_max-P_r). The high breakdown strength of 0.6KBT-0.4ST, combined with the high polarization contributed by the hybridization between Bi 6s and O 2p orbitals, collectively yielding a high recoverable energy density of 3.64 J/cm³ and a high energy efficiency of 70 %. Furthermore, this composition demonstrated superior cycling stability and temperature stability in its energy storage performance. In summary, A-site modification of KBT ceramics through SrTiO₃ solid solution was an effective strategy for optimizing their energy storage properties. This study could present a promising lead-free energy storage ceramic system and provide a valuable reference for designing high-performance relaxor ferroelectrics via tuning local structural disorder.

With the rapid development of new energy, pulsed power systems and other fields, there is an urgent demand for energy storage devices with a high power density, a fast charge-discharge rate and a long service life. Inorganic thin-film dielectric capacitors, featuring microsecond-level discharge speed and high safety, face a challenge of synergistically improving energy density and efficiency (e.g., the existing PZT thin films have an efficiency of <50% due to the large remanent polarization). As a widely studied material, PZT has an excellent polarization but a large leakage current. The existing optimizations include doping and heterostructure construction. This paper was to fabricate a "PZT/MgO" multilayer heterostructure by a sol-gel method. MgO was introduced to obtain a slim hysteresis loop, and the effect of annealing temperature on the phase structure, microstructure, dielectric and energy storage properties was investigated.

0.5 mol/L PbZr_0.52Ti_0.48O₃ (PZT) and MgO thin film precursor solutions were prepared by a sol-gel method, aged at room temperature in dry darkness for 24 h, and then spin-coated on Pt/Ti/SiO₂/Si substrates at 4000 r/min for 30 s. The wet films were dried at 200 ℃ for 5 min and pyrolyzed at 450 ℃ for 10 min. The process was repeated to obtain "PZT/MgO" multilayer films, which were annealed in O₂ at 500-600 ℃ for 10 min.

The crystal structure was analyzed by X-ray diffraction (XRD, Rigaku Co., Japan). The surface/cross-sectional morphologies were determined by scanning electron microscopy (SEM, Thermo Scientific Co., USA). The leakage current was characterized by a Keithley 2400 analyzer (Keithley Co., USA). Au electrodes with the diameter of 200 μm were sputtered, and the P-E loops, breakdown strength (E_b), dielectric properties, and stability were determined by an aixACCT TF Analyzer 3000 (TF Co., Germany). Results and discussion The results show that the characteristic peaks of the perovskite phase of PZT and MgO phase are intensified, the full width at half maximum (FWHM) is narrowed, and the crystallinity is improved with the increase of temperature when the PZT/MgO multilayer heterostructure thin films are annealed at 500-600 ℃ for 10 min. The SEM images show that the thin film annealed at 550 ℃ for 10 min has a dense and defect-free surface, a tight cross-sectional structure without delamination, and a total thickness of approximately 250 nm. In terms of dielectric properties, the dielectric constant and dielectric loss change slightly when annealed at 500-550 ℃, but the both increases when the temperature exceeds 550 ℃. For energy storage performance, the optimal annealing temperature is 550 ℃. The recoverable energy density is 66.0 J/cm³ and the energy storage efficiency is 68.9% in an electric field of 6.77 MV/cm. At 550 ℃, the leakage current of the thin film is ≤ 10^-7 A/cm² in an electric field of 0-2.4 MV/cm, with the Weibull coefficient β of 19.93, indicating a high breakdown stability. Regarding the stability, the energy storage performance fluctuates slightly at 1-30 kHz in an electric field of 4 MV/cm. After 10⁸ charge-discharge cycles, the attenuation of W_rec (recoverable energy density) and η (energy storage efficiency) is extremely small. W_rec increases with the electric field ranging from 1 MV/cm to 6 MV/cm, and the fitting result shows that n ≈ 1.86, presenting the quasi-linear polarization characteristics.

The surface of PZT/MgO multilayer film annealed at 550 ℃ for 10 min was dense without obvious delamination, and the thickness was approximately 250 nm, exhibiting good dielectric properties. In an electric field of 6.77 MV/cm, this film had a recoverable energy density of 66.0 J/cm³ and an energy storage efficiency of 68.9%, with a leakage current ≤ 10^-7 A/cm² (the Weibull coefficient β of 19.93). In addition, the film also had excellent frequency and cycling stability. W_rec increased with the electric field, and the fitting result showed that n ≈ 1.86, presenting the quasi-linear polarization characteristics.

With the evolution of optical communication from 5G to 6G, core optical devices face the increasing performance demands. Electro-optic (E-O) materials have attracted much attention due to their fast modulation speed, wide bandwidth, and good stability. Conventional materials such as KDP single crystals are hygroscopic and possess a low E-O coefficient, while LN single crystals have a limited damage threshold, thus hindering their suitability for high-power applications. Binary PMN-PT relaxor ferroelectrics exhibit excellent E-O properties, but suffer from a poor temperature stability. In contrast, the ternary PIN-PMN-PT system combines a high Curie temperature with outstanding ferroelectric and piezoelectric performance. However, the related research on its E-O properties remains limited. Previous studies mostly focused on strong ferroelectric materials exhibiting the primary E-O (Pockels) effect, which typically show low extinction ratios (<30 dB), insufficient for optical communication devices that require high extinction ratios (>30 dB). This work fabricated a series of transparent E-O ceramics with a weakened ferroelectricity in the xPIN-(85-x)PMN-15PT ceramics (i.e., x=0, 5, 10, 15, 20), to achieve both a high extinction ratio and good E-O performance through composition design. The results demonstrated that the composition with 5% PIN could exhibit optimal overall performance (i.e., a quadratic EO coefficient of 51.0×10^-16 m²/V², an extinction ratio of 31.87 dB, and a transmittance of 66.87% at 1550 nm). A modulation system based on this material was constructed, verifying its potential for applications such as optical switch. This study was to systematically investigate the influence of PIN content on the phase structure, transmittance, ferroelectric properties, and E-O performance, providing valuable insights for the future design of ternary PIN-PMN-PT E-O materials.

The ternary PIN-PMN-PT transparent electro-optic ceramics were prepared by a columbite precursor method with high-purity oxide raw materials (MgO, Nb₂O₅, In₂O₃, PbO, TiO₂, and La₂O₃, purity ≥ 99.9%). First, MgO, In₂O₃, and Nb₂O₅ were mixed by grinding in a high-energy ball mill and then calcined at 1100 ℃ to synthesize MgNb₂O₆ and InNbO₄. Subsequently, these precursors were mixed with La₂O₃, PbO and TiO₂ by milling and calcined at 850 ℃ to obtain PIN-PMN-PT powder. After dry pressing into green bodies, sintering was carried out in oxygen atmosphere at 1235 ℃ for 10 h to achieve densification. This was followed by hot-press sintering at 1200 ℃ and 100 MPa for 3 h to further eliminate pores, ultimately yielding highly dense transparent ceramics.

A systematic characterization was performed on the 0.5La-xPIN-(85-x)PMN-15PT (x=0, 5, 10, 15, 20) transparent electro-optic ceramics. After cutting and polishing, the transmittance, refractive index were measured by ultraviolet-visible-near infrared spectroscopy and spectroscopic ellipsometry, respectively. The quadratic electro-optic coefficient of the material was determined by a single-beam compensation method with a 632.8 nm He-Ne laser. The phase structure and microstructure were analyzed by X-ray diffraction and scanning electron microscopy. Finally, the ferroelectric properties were investigated via measuring the polarization-electric field hysteresis loops by a ferroelectric test system after depositing gold electrodes on the samples.

A series of 0.5La-xPIN-(85-x)PMN-15PT (x=0, 5, 10, 15, 20) transparent ceramics are fabricated. The effect of PIN solid-solution ratio on the transmittance, phase structure, microstructure, ferroelectricity, and electro-optic properties of the samples is systematically investigated. The transmittance of the ceramic sintered at 1235 ℃ gradually increases at a higher PIN content. The 0.5La-20PIN-65PMN-15PT transparent ceramic exhibits the optimum transmittance of 69.11% at 1550 nm, approaching a theoretical upper limit of 69.38%. All the compositions display a pseudocubic phase structure, allowing optical losses due to the birefringence to be neglected. The samples show dense microstructures with relative densities of 99.8%, 99.84%, 99.85%, 99.88%, and 99.95%, respectively, nearly eliminating scattering losses caused by internal pores. The average grain sizes are 6.63, 5.98, 6.41, 7.33 μm and 7.54 μm, respectively. The coercive field and remnant polarization of the samples both increase with increasing PIN content, indicating an enhanced ferroelectricity, which is attributed to the stronger ferroelectric nature of PIN, compared to PMN. The T_m also gradually increases, benefiting from a higher Curie temperature of PIN (90 ℃) relative to PMN (-10 ℃). The maximum and room-temperature dielectric constants both show a monotonic decreasing trend, which is related to a reduction in the relaxor behavior of the samples. The measured electro-optic coefficients at room temperature first increase and then decrease with PIN content, reaching a maximum value of 51.0×10^-16 m²/V² for the 0.5La-5PIN-80PMN-15PT ceramic. This is likely associated with domain sizes constrained by grain dimensions. The extinction ratios of the samples are 33.47, 31.87, 28.77, 23.87 dB and 22.08 dB, respectively, showing a gradual decrease influenced by the variation in ferroelectricity.

Based on the PIN-PMN-PT transparent electro-optic ceramics, an electro-optic modulation system is designed and constructed. Modulation experiments are conducted using a 250 kHz high-frequency AC signal and an audio signal. The modulated optical signal closely matches the original input signal, confirming a feasibility of applying PIN-PMN-PT transparent electro-optic ceramics in the field of optical communications.

La-doped PIN-PMN-PT transparent electro-optic ceramics were fabricated by a two-step sintering method, and the influence of PIN content on the transmittance and electro-optic coefficient was investigated. The 0.5La-20PIN-65PMN-15PT composition exhibited the optimum transmittance of 69.11% at 1550 nm, approaching the theoretical limit, which could be attributed to its pure perovskite phase, pseudocubic structure, and dense microstructure. The 0.5La-5PIN-80PMN-15PT composition demonstrated an optimal electro-optic performance with a transmittance of 66.87% and a high extinction ratio (31.87 dB), while exhibiting a significantly high electro-optic coefficient of 51×10^-16 m²/V², surpassing most reported PMN-PT-based ceramics. Its fine-grained structure facilitated domain switching. An electro-optic modulator based on this ceramic system achieved 200 kHz sinusoidal signal modulation and radio-frequency signal transmission, confirming its promising potential for optical communication applications.

(1-x%)(K_0.5Na_0.5NbO₃)-x%(Sr_0.7Bi_0.2TiO₃) (x = 1, 4, 7, 10, and 15 molar fraction) lead-free ferroelectric ceramics, denoted as xSBT, were synthesized by a conventional solid-state reaction method and a subsequent pressureless sintering. The effect of SBT incorporation on the microstructure, phase evolution, optical properties, and energy storage performance was investigated. The introduction of SBT effectively refined the grain size and promoted densification, resulting in a high optical transmittance of 72% and 58% in the visible region for the 7SBT and 10SBT ceramics, respectively. SBT doping induced a transition from a normal ferroelectric to a relaxor ferroelectric state in the KNN-based matrix, with the 7SBT and 10SBT compositions exhibiting pronounced relaxor characteristics. The results of ferroelectric performance tests revealed recoverable energy storage densities of approximately 1.05 J/cm³ at 150 kV/cm for 7SBT and 3.61 J/cm³ at 300 kV/cm for 10SBT, accompanied by an energy storage efficiency of 52%. This outstanding comprehensive performance (energy storage density and optical transmittance) demonstrated significant advantages over other reported lead-free ferroelectric ceramic systems, thereby providing both a material system and theoretical foundation for developing high-performance lead-free ferroelectric transparent dielectrics for optoelectronic integrated applications.

(1-x%)(K_0.5Na_0.5NbO₃) -x%(Sr_0.7Bi_0.2TiO₃) (x = 1, 4, 7, 10, and 15) ceramics, denoted as xSBT, were fabricated with high-purity oxides and carbonates as starting materials by a conventional solid-state reaction method and a subsequnt pressureless sintering. The phase structure was analyzed by X-ray diffraction (XRD). The microstructure and elemental distribution were examined by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The structural evolution was determined by Raman spectroscopy. The optical transmittance was measured by ultraviolet-visible-near infrared spectrophotometry. The temperature-dependent relative dielectric permittivity (ε_r) was evaluated by an impedance analyzer and the polarization-electric field (P-E) hysteresis loops were measured by a ferroelectric test system to assess energy storage performance. Results and discussion The SEM images demonstrate that the introduction of xSBT effectively refines the grain size and promotes the microstructural densification. The grain size is significantly reduced from approximately 290 nm (as x = 4) to below 150 nm (as x = 7 and 10), effectively minimizing a light scattering. Consequently, the 7SBT and 10SBT ceramics achieve a high optical transmittance of approximately 72% and 58% at 800 nm in the visible light region, respectively.The XRD patterns and XRD Rietveld refinement results reveal a continuous phase structural evolution with increasing x content, i.e., from an orthorhombic phase (x = 1) to a tetragonal phase (x = 4), then to a coexistence of tetragonal and cubic phases (x = 7, 10), and finally to a pure cubic phase (x = 15). This progression signifies a structural transition from low-symmetry orthorhombic to high-symmetry cubic phases. The enhanced structural symmetry, along with chemical disorder induced by heterovalent substitutions of Sr²⁺/Bi³⁺/Ti⁴⁺, effectively can disrupte the long-range ferroelectric order, facilitating the formation of polar nanoregions (PNRs) and leading to a pronounced relaxor behavior. This is quantitatively supported by the significant increase in the diffuseness coefficient (γ) to 1.49 and 1.56 for x = 7 and 10, respectively. Correspondingly, the P-E loops evolve from a saturated square shape for 1SBT to a slim shape for 10SBT. As a result, the 7SBT and 10SBT compositions deliver recoverable energy storage densities (W_rec) of 1.05 J/cm³ in 150 kV/cm and 3.61 J/cm³ in 300 kV/cm, respectively, with an efficiency (η) of 52%. The combination of high optical transmittance and superior energy storage performance makes the xSBT ceramics a promising lead-free material system for integrated optoelectronic applications.

This study achieved the grain refinement and structural densification in KNN-based ceramics via doping SBT components, resulting in lead-free ceramics with a high optical transparency. The optical transmittance of the 7SBT and 10SBT ceramics in the visible light region reached 72% and 58%, respectively. Furthermore, the increased crystal symmetry and introduced chemical disorder by SBT doping disrupted the long-range ferroelectric order of the KNN matrix, thereby inducing pronounced relaxor characteristics. These modifications contributed to excellent ferroelectric energy storage properties in the 7SBT and 10SBT transparent ceramics. The 7SBT and 10SBT ceramics maintained an energy storage efficiency of 52% in electric fields of 150 kV/cm and 300 kV/cm, respectively, while achieving recoverable energy storage densities of approximately 1.05 J/cm³ and 3.61 J/cm³. Their outstanding overall performance (i.e., energy storage density and optical transparency) demonstrated significant advantages among reported lead-free ferroelectric transparent ceramics. This work could provide important theoretical and experimental foundations for advancing key materials in next-generation optoelectronic integrated systems and open pathways for designing novel multifunctional lead-free ferroelectric ceramics. The main limitation could lie in the substantial room for improvement in the energy storage efficiency of the current system. Future work could involve more precise composition control and interfacial engineering to reduce dielectric loss and leakage. In addition, the stability of electrical performance across wide temperature ranges and high frequencies could also require a systematic evaluation.

Piezoelectric ceramics are critical components in sensors, actuators, energy converters, and micro-electromechanical systems. The lead-based compositions dominate in the existing market. However, the release of lead during their lifecycle poses severe environmental and human-being health risks, which contradicts global sustainable development goals. Therefore, developing high-performance lead-free alternatives becomes a paramount research priority. Among lead-free alternatives, potassium sodium niobate ((K,Na)NbO₃, KNN)-based ceramics stand out due to their low driving voltage, large electrostrain, and minimal hysteresis, making them performance-competitive with conventional PZT for precision actuators. Their performance is typically enhanced by constructing polymorphic phase boundaries and regulating microstructure. However, the existing doping strategies inadvertently degrade thermal stability, which remains a key bottleneck for practical applications. Rare-earth ions act as effective modifiers in KNN-based piezoceramics. They enhance a relaxor behavior, and improve thermal stability by shifting rhombohedral-tetragonal (R-T) phase transition below room temperature, and introduce photoluminescence due to their abundant energy levels. The integration of electrostrain and luminescence in KNN-based systems is thus significant, paving a way for developing novel bimodal functional materials for next-generation optoelectronic devices. In this work, the 0.96(K_0.48Na_0.52)(Nb_0.95Sb_0.05)O₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃(0.96KNNS-0.04BNKZ) composition with a polymorphic phase transition characteristic was chosen as a matrix, and rare-earth oxides (Dy₂O₃ and Eu₂O₃) served as modifiers. The effect of rare-earth oxides doping on the microstructure, ferroelectric/electrostrain properties, and photoluminescence performance of the 0.96KNNS-0.04BNKZ ceramics was investigated.

0.96KNNS-0.04BNKZ-x%Dy/Eu (x = 0, 0.05, 0.10, 0.20, 0.40) lead-free ferroelectric ceramics were synthesized by a solid-state reaction method. The stoichiometric raw materials were weighed according to the formula. They were then ground in ethanol in a ball mill with zirconia balls for 24 h. The powders were calcined at 850 ℃ for 4 h, then ground again for 24 h. After drying, the powders were mixed with 10% polyvinyl alcohol (PVA) as a binder and then granulated. Subsequently, disc-shaped green compacts (with the sizes of 10 mm in diameter and 0.5 mm in thickness) were pressed and subjected to a debinding process at 550 ℃. The ceramics were obtained after sintering at 1130-1200 ℃ for 2 h. After polishing, silver electrodes were fired at 650 ℃ for electrical testing. The ceramics were sintered at at 1130 ℃ to 1200 ℃ for 2 h, depending on the doping levels. After polishing, silver electrodes were fired on the surfaces at 650 ℃ for electrical characterization. The phase structure was analyzed by an X-ray diffractometer (D8 advance). The surface morphology was determined by a field emission scanning electron microscope (Merlin compact, Carl Zeiss Co., Germany). The temperature-dependent dielectric constant was measured by a broadband dielectric spectrometer (Novocontrol concept 40). The ferroelectric and electric-field-induced strain were characterized by a ferroelectric analyzer (TF 2000) coupled with a laser interferometer (SP-S 120/500). The photoluminescence properties were evaluated by a fluorescence spectrometer (FLS920).

Dy/Eu-doped 0.96KNNS-0.04BNKZ ceramics exhibit a single perovskite structure. At low doping levels (i.e., x ≤ 0.10%), the rhombohedral-tetragonal (R-T) coexisting phase maintains, while a higher doping (i.e., x ≥ 0.20%) induces a transition to the pseudocubic (P_c) phase, with no secondary phases appears in all the samples. All the ceramics present dense and uniform microstructures. The average grain size gradually decreases with increasing Dy/Eu doping, indicating that rare-earth ions effectively inhibit grain growth. In terms of electrical properties, a moderate Dy/Eu doping (i.e., Dy: x=0.10%; Eu: x=0.05%) enhances ferroelectricity, as reflected by improved P-E loop squareness and increased saturated polarization (P_max). The 0.10%Dy and 0.05%Eu samples achieve an optimal electrostrain performance, delivering unipolar strains (S_uni) of 0.257% and 0.241% at 70 kV/cm, which are accompanied by large piezoelectric strain coefficients (d₃₃^*) of 367 pm/V and 344 pm/V, respectively. An appropriate doping optimizes the domain structure and facilitates domain switching in an external electric field, thereby improving both ferroelectric and electrostrain properties. In contrast, excessive doping leads to a severe lattice distortion, notably octahedral tilting, which pins domain walls and hinders their reorientation in response to the electric field, ultimately compromising ferroelectric and electrostrain performance. Furthermore, the 0.05% Dy- and 0.05% Eu-doped ceramics exhibit excellent thermal stability, showing less than 10% variation in electrostrain at 25-100 ℃, which demonstrates their great potential for use in piezoelectric actuators. In addition, Dy/Eu doping introduces a pronounced photoluminescence to the ceramics. Under blue-light excitation, Dy³⁺-doped samples yield an intense yellow emission, whereas Eu³⁺-doped samples produce a bright orange-red emission.

The introduction of Dy/Eu could promote a phase transition from a mixed R-T phase to a P_c phase. An appropriate doping effectively enhanced the ferroelectric and electrostrain properties of the materials. Specifically, the 0.10% Dy- and 0.05% Eu-doped ceramics achieved unipolar strains (S_uni) of 0.257% and 0.241% at an electric field of 70 kV/cm, with corresponding piezoelectric strain coefficients (d₃₃^*) of 367 pm/V and 344 pm/V, respectively. Furthermore, the 0.05% Dy- and 0.05% Eu-doped samples exhibited excellent thermal stability, with a variation in electrostrain of less than 10% at 25-100 ℃, demonstrating a great potential for piezoelectric actuator applications. In addition, Dy/Eu doping imparted outstanding photoluminescence properties to the ceramics. Under blue-light excitation, Dy³⁺-doped samples exhibited an intense yellow emission, while Eu³⁺-doped samples displayed a bright orange-red emission. The combination of electrical and luminescent characteristics could make this material a promising candidate for optoelectronic coupling devices.

As an important branch of lead-free piezoelectric materials, ceramics with a tetragonal tungsten bronze (TTB) structure have a high spontaneous polarization strength, high Curie temperature (T_C), and beneficial dielectric properties, making them particularly useful in ferroelectric applications. Depending on which sites are filled, the TTB structure can be divided into three categories, i.e., the "stuffed" structure, the "filled" structure and the "unfilled" structure. As a typical "filled" TTB compound, the Sr₂NaNb₅O₁₅ (SNN) ceramics can be regulated through compositional design to exhibit a wide range of structures and functional characteristics, which have a broad development potential in the fields of dielectric energy storage, multiferroic materials, nonlinear optics, and pyroelectric sensors. However, SNN ceramics require relatively high sintering temperatures, and the secondary phase Na_0.5Sr_0.25NbO₃ (NSN) readily forms during the sintering process, which adversely affects the electrical properties. Although some previous studies used the methods of eliminating the impurity phase, there could be few studies on the relationship between impurities and the properties of SNN ceramics. In this paper, SNN ceramics (i.e., Sr_1-xNa_xNb₅O₁₅, 0.56 ≤ x ≤ 1.06) were prepared with the SrNb₂O₆ (SN)-NaNbO₃ (NN) binary solid solution system, to examine the relationship among the presence of the secondary phases and the ceramic composition, as well as the sintering temperature.

SN and NN powders were synthesized by a solid-state method with reagent-grade SrCO₃ (99.99%), Na₂CO₃ (99.99%), and Nb₂O₅ (99.99%) as raw materials. The starting raw materials were pre-dried at 120 ℃ for 24 h, and then weighed according to the target stoichiometric ratios. The powders were thoroughly mixed in ethanol in a planetary ball mill with ZrO₂ balls at 401 r/min for 20 h. The mixtures were dried at 80 ℃ for 24 h, and then calcined at their respective suitable temperatures. Subsequently, the SN and NN powders were obtained by separately grinding the calcined products in a mortar after natural cooling. Afterwards, SNN ceramics (Sr_1-xNa_xNb₅O₁₅, 0.56 ≤ x ≤ 1.06) were synthesized with polycrystalline SN and NN according to the desired SNN compositions, calcined at 1220 ℃ for 6 h, and uniaxially compacted into pellets with a binder (namely 5% polyvinyl alcohol). The binder was removed at a high temperature. Finally, SNN ceramics were prepared by sintering at 1280-1340 ℃ for 4 h.

The effect of the secondary phases on the structure of SNN ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and field emission transmission electron microscopy (FETEM). Furthermore, the ceramics were ground and polished down to a thickness of about 0.6 mm, coated with silver electrodes, and fired at 840 ℃ for 30 min. Their dielectric characteristics and piezoelectric coefficients d₃₃ were measured by the LCR meter and quasi-static piezoelectric meter. The ceramics were also ground and polished down to a thickness of about 0.2 mm and coated with gold electrodes to record the room-temperature ferroelectric hysteresis loops by a Radiant Precision Workstation.

The required composition and sintering temperature ranges of the SNN pure phase are determined. For Na⁺contents (x) of 0.56-0.68, SN-NN can be solidly dissolved to obtain a pure phase at high temperatures (i.e., > 1280 ℃). As x = 0.71 and 0.80, SN-NN can be solidly dissolved at 1200-1360 ℃. At the Na⁺ content of 0.88, the SN-NN solid solution can be obtained, but it decomposes at higher temperatures (> 1320 ℃), accompanying the formation of the secondary NN phase. As the Na⁺ content further increases, it cannot be solidly dissolved at 1200-1360 ℃.

The maximum density is obtained for SNN ceramics sintered at 1340 ℃, and the change in Na⁺ content has little effect on the microstructure. The T_C of SNN ceramics increases from 272 ℃ as x = 0.56 to 323 ℃ as x = 0.88 because the crystal structure changes from "unfilled" to "filled" as the Na⁺ content increases, thus affecting the degree of crystal distortion. T_C gradually decreases because a minor amount of NN appears at x of 0.88. The dielectric constant (ε_r) of SNN ceramics initially increases and then decreases, exhibiting a maximum of 1664 as x = 0.80. The dielectric loss (tanδ) remains at a low value at this point. The remnant polarization P_r (11.49 μC/cm²) and coercive field E_c (20.99 kV/cm) are both nearly maximized as x = 0.80 for SNN ceramics due to the "filled" TTB structure, and the maximum d₃₃ value of 34 pC/N is obtained for SNN ceramics as x = 0.80, which is consistent with the optimal composition for ferroelectric properties.

Based on the XRD patterns, the ceramics with a stable, pure TTB phase could be obtained by sintering at 1200-1360 ℃ when x = 0.71-0.80, and the existence of this phase depended on the ceramic composition and the sintering temperature. As the Na⁺ content increased, T_C initially increased because the filling degree of the TTB structure increased, and then gradually decreasesd when x > 0.88 because the increase in Na⁺ content induced the formation of the secondary NN phase. When x=0.71-0.88, the electrical properties could be optimal. The ceramic with x of 0.80 demonstrated a high ferroelectric performance (i.e., P_r = 11.49 μC/cm², and E_c = 20.99 kV/cm). This study could provide certain theoretical basis and technical support for the subsequent studies of SNN ceramics.