The interfacial contact and charge conduction between the electrolyte and cathode have a direct impact on the performance and efficiency of single cells, which is a crucial area in both fundamental research and technological development of SOFCs. The following perspectives for future development are provided.

The development of novel electrolytes and electrode materials represents an effective strategy to address interfacial issues. Concurrently, the exploration of innovative interfacial structural designs can enhance the performance and stability within existing material systems. Future research endeavors should prioritize meeting the practical demands of industrial applications. Furthermore, it is essential to optimize the preparation process. The quality of the interface between the electrolyte and the cathode directly affects the overall performance of SOFCs. Therefore, optimizing the preparation process is of crucial importance. Currently, coating technologies such as pulsed laser deposition (PLD) and chemical vapor deposition (CVD) can achieve a precise preparation of specific compositions and structures. In the future, it is necessary to focus on the research and development of new high-performance, low-cost thin film preparation technologies with large-scale manufacturing capabilities to further improve interface quality. In addition, there is a focus on interdisciplinary research. Various in-situ spectroscopic characterization methods, machine learning and artificial intelligence technologies will also contribute to SOFCs research, including material performance prediction, parameter optimization design, and operating condition performance simulation. This can promote the connection between theoretical models and practical applications, driving SOFCs technology toward higher efficiency and lower costs. Finally, despite significant progress in lab-scale research, the commercialization of SOFCs still faces some challenges. In the next phase, it is necessary to further optimize the cell preparation process and operating conditions, such as electrolyte structure, composite electrode materials, and sintering processes. Reducing costs, improving cell reliability and lifespan can promote SOFCs widespread application in the energy sector.

Visible-light lasers have applications in biology, medicine, and detection. Compared to conventional nonlinear frequency doubling methods for obtaining visible light lasers, it is possible to prepare laser crystals that directly emit a visible light via doping with rare-earth elements such as Pr³⁺, Sm³⁺, Tb³⁺, and Dy³⁺. For yellow light lasers, doping with Dy³⁺ is more suitable. The existing Dy³⁺-doped laser crystals are still at the theoretical stage, with a few experimental cases. Recently, a high-quality Dy³⁺-doped borate crystal GdMgB₅O₁₀ (Dy³⁺:GMB) is prepared to produce a yellow light laser at a wavelength of 580 nm. In this study, a plane-plane cavity laser was assembled using a 3 mm×3 mm×20 mm Y-cut Dy³⁺:GMB crystal with a 444 nm blue LD pump, achieving a laser output at 579.3 nm. The maximum output power reached 161 mW with a slope efficiency of 3.7% as using different output couplers at different transmission rates.

In this experiment, a 444 nm blue LD with a maximum power of 12 W was used as a pump source. The model of the blue LD pump was LSR444CP4-12W. The pump output a spatial light directly without fiber coupling and used air cooling to maintain a temperature of 25 ℃. The beam size at the output of the pump was 4.3 mm horizontally and 4.1 mm vertically. The beam remained parallel within 0.5 m and could be approximated as a parallel light. A plano-convex lens with a focal length of 150 mm was used. A Y-cut Dy:GMB crystal with the dimensions of 3 mm×3 mm×20 mm was used. The doping concentration of Dy³⁺ in the crystal was 6.08%, and the end faces of the crystal were polished but not coated for the blue wavelength band. The Dy:GMB crystal was wrapped in indium foil and placed in a water-cooled copper block, with the copper block's water cooling set to 20 ℃. The polarization direction of the pump light was vertical. Based on the polarization bsorption efficiency of the Dy:GMB crystal along different axes, the crystal was positioned in an optical path. The pump light absorption efficiency of the crystal was measured under non-lasing conditions. When the crystal was placed, the pump light absorption efficiency was 55%. The laser resonator consisted of two coated plane mirrors. IM was the input mirror for the pump light, with coating arameters that could reduce reflection and increase transmission at 444 nm. The measured transmission at 444 nm was 96%. It also had a high reflectivity at 579 nm with a reflectivity of≥99.8%. OC was the output coupler mirror, and the coated mirrors with 579 nm transmission rates of 1% and 2% were used. A long-pass edge filter with a cutoff wavelength of 450 nm was placed at the laser output to effectively reduce the impact of pump light on power measurements. After the output power measurement, a Glan prism was used to analyze the polarization of the output laser.

The output wavelength of the experimental laser is 579.3 nm, without other wavelengths detected. In the experiment with an OC transmission rate of 1%, a laser generation begins when the crystal absorbes 1.94 W of pump power, achieving a maximum output power of 161 mW and a slope efficiency of 3.7%. In the experiment with an OC transmission rate of 2%, a laser generation starts at an absorption power of 2.81 W, having a maximum output power of 129 mW with a slightly lower slope efficiency of 3.6%. No saturation in laser output power occurs in either set of experiments. The laser threshold and output power using an OC transmission rate of 1% are slightly greater than those using an OC transmission rate of 2%, which is consistent with the results reported in a previous study. Based on the slope efficiency, it is estimated that under the same pump absorption power, the maximum output power of the laser in this experiment can exceed 628 mW. There are three methods to improve the output power and slope efficiency of the laser via comparing the performance of the crystal in two separate experiments. The first method is to use a higher-power and higher-quality pump to enhance the absorption efficiency of the Dy:GMB crystal. The second method is to employ different types of laser cavities and optimize the coating parameters of the IM and OC, as well as shape the pump beam to achieve better spatial mode matching between the pump light and the laser cavity. The third method is to optimize the growth process to obtain X-cut or Z-cut crystals of sufficient length with varying Dy doping concentrations, as the emission cross-section of the Dy:GMB crystal is maximum at 578.5 nm along the E//Y direction.

A 579.3 nm yellow laser output with a maximum power of 161 mW and a slope efficiency of 3.7% was obtained using a plane-plane cavity laser setup with Dy:GMB crystal pumped by a 444 nm blue LD. An application potential of the crystal could be achieved via comparing the initial laser experiments with the same crystal using blue LDs at different wavelengths. This study could provide insights for future improvements in the output power and slope efficiency of lasers constructed with Dy:GMB crystals.

Lithium-ion batteries (LIBs) are widely applied in electric vehicles and portable electronics due to their high energy density and environmentally friendly characteristics. However, large numbers of LIBs would be retired in the future because of their limited lifespan (5-8 a). Used batteries contain numerous organic materials and valuable elements. They cause serious pollution and resource waste in the absence of suitable treatment. Therefore, typical pre-treatments are carried out to obtain used cathode or anode materials. A series of recycling methods are explored to utilize spent electrode materials, such as element-extracting leaching and direct regeneration. Compared to element-extracting leaching, direct regeneration displays lower energy consumption and higher element recovery, making it regarded as a next-generation approach. In the pre-treatment process, however, the obtained electrode materials are mixed (e.g., LiFePO₄ and graphite), resulting in low leaching efficiency and inferior regeneration performance. Also, direct regeneration processes still suffer from long treatment durations and high energy requirements. It is thus important for recycling used batteries to explore highly effective separation and regeneration methods.

Based on the particle size of raw material, three graded products involved coarse fraction (i.e., +0.21 mm), medium fraction (i.e., -0.21 mm - +0.15 mm), and fine fraction (i.e., -0.15 mm). Magnetic separation experiments on each fraction were conducted, separately. The coarse fraction was processed at a pulsation frequency of 200 times/min and a background magnetic field intensity of 1.0 T, the medium fraction at 240 times/min and 1.2 T, and the fine fraction at 260 times/min and 1.6 T. 5 g of pre-heating waste graphite was placed in 20 mL of hydrochloric acid solution as a stripping solution. After stirring for 5 h, the mixture was washed for three times with deionized water and then further cleaned for three times with an ethanol solution, enabling the short-process regeneration of graphite material.

The pre-classification, magnetic separation, and "surface peeling" regeneration are applied to transform mixed electrode materials into regenerated products. Used LiFePO₄ typically exhibits large bulk sizes, while used graphite particles are comparatively smaller. At the particle size of < 0.075 mm, the carbon content reaches 86.35%. Evidently, simple sieving effectively separates the mixed components. The resulting tailings primarily consist of carbonaceous material (96.53%) and impurities after magnetic separation. To address the detrimental effects of residual impurities, a "surface peeling" regeneration method is developed for used graphite. Increasing the number of treatment cycles progressively tailors surface/near-surface properties, while removing impurities. The related analysis reveals that high-temperature active atmospheres promote an impurity evolution, while acid solutions facilitate their removal. Based on the water-gas shift reaction principles, active carbon atoms react with water, eliminating degraded surface/near-surface layers to yield regenerated graphite. The physicochemical characterization indicates that the regenerated samples exhibit high graphitization, smooth surfaces, and reduced particle sizes. As LIB anodes, these materials deliver a capacity of 320 mA·h·g^-1 at 1.0 C after 300 cycles, with an initial Coulombic efficiency of >80%. At 2.0 C, a capacity retention remains 280 mA·h·g^-1 for over 300 cycles. Increased "surface peeling" cycles shortens activation time due to the optimized surface/near-surface properties, enhancing lithium-ion diffusion. The kinetic analysis indicates that excessive "surface peeling" treatments damage surface structures, while increasing pseudocapacitive contributions. At 0.9 mV·s^-1, pseudocapacitive contributions reach 75%, with a lithium-ion diffusion coefficient of 4.0 × 10^-11 cm²·s^-1, demonstrating a significant physicochemical evolution.

For the promising potential of used LIB graphite, effective recovery methods (i.e., from separation of mixed materials to graphite regeneration) were used. Used LIB materials consisted of large LiFePO₄ bulks and small graphite particles. For pre-classification, the mixed materials were segregated by particle size ranges, i.e., larger fractions were predominantly LiFePO₄, while smaller fractions were primarily graphite. At particle sizes of < 0.075 mm, carbon content reached 86.35%, with LiFePO₄ (12.03%) and some impurities. The magnetic separation increased carbon content to >96%, achieving an effective separation of mixed electrode materials. Subsequently, used graphite was regenerated by "surface peeling" strategie, removing impurities (i.e., conductive carbon, Al, Cu, and LiFePO₄) and tailoring surface/near-surface properties. The optimized sample with a reduced particle size exhibited smooth surfaces and high graphitization. As LIB anodes, these materials delivered the electrochemical performance up to 323 mA·h·g^-1 at 1.0 C. The analysis revealed that tailored diffusion channels played a key role in enhancing reversible lithium-ion storage and deep insertion behaviors. This work could provide effective all-process recycling strategies from pre-classification and separation to regeneration, while elucidating the regeneration mechanism of used graphite.

This study was to investigate the influence of mixed alkali effect on the structure and properties of a R₂O-CaO-Al₂O₃-SiO₂ glass system, to provide theoretical support and practical guidance for optimizing the formulation of high-level radioactive liquid waste (HLW) glass vitrification technology. The safe disposal of HLW is a critical aspect for the sustainable development of the nuclear industry. The Joule-heated melter vitrification as an internationally engineered and implemented method could immobilize radioactive waste via melting HLW with base glass frit in an electric furnace to form a stable vitrified product for long-term containment.

Glass samples with varying Z-values (i.e., Z = n(Na₂O)/(n(Na₂O) + n(Li₂O)) were prepared by a high-temperature melting method. Their structure and properties were systematically analyzed by infrared spectroscopy, Raman spectroscopy, and high-temperature resistivity analysis.

The structural characterization revealed that the silicate tetrahedral units Q² and Q³ are the primary structural units influencing glass properties. The structure exhibits regular changes as Z-value increases. At Z=0.2, the proportions of Q³ and Q⁴ reach the maximum values, while Q¹ and Q² reach the minimum values. At Z=0.6, Q¹, Q³, and Q⁴ proportions reach the maximum values, while Q² reaches the minimum value, indicating the most compact glass structure and the most significant mixed-alkali effect.

The results of performance tests demonstrate that the physical and thermal properties both are governed due to the mixed-alkali effect The density and thermal expansion coefficient both increase with increasing Z-value. The density reaches the maximum value as Z=0.6, while the thermal expansion coefficient has the minimum value. The glass transition temperature (T_g) initially decreases and then increases, reaching its minimum as Z=0.2. The high-temperature viscosity has the maximum value as Z=0.4 and the minimum value as Z=0.6.

For the electrical properties in single-alkali systems, the resistivity at the melting temperature (T₂) decreases in an order of K₂O >Na₂O > Li₂O, while it at the forming temperature (T₄) decreases in an order of K₂O > Li₂O > Na₂O. In the mixed-alkali system, the resistivity reaches the maximum value at Z of 0.6-0.8, being correlated to the "blocking effect" of alkali ions. The activation energy for electrical conduction changes nonlinearly with Z-value. In single-alkali systems, it increases in the order of Na₂O < Li₂O < K₂O.

This study clarified the mixed-alkali effect on the R₂O-CaO-Al₂O₃-SiO₂ glasses, particularly highlighting the low viscosity as Z = 0.6 and the high resistivity characteristics as Z = 0.6-0.8. These findings could provide a clear guidance for glass formulation design.

Sodium-ion batteries (NIBs) have attracted much attention as promising alternatives to lithium-ion batteries due to the natural abundance of sodium, low cost, and enhanced safety. Among various cathode materials, layered transition metal oxides, particularly O3-type structures, are considered highly attractive due to their high specific capacity, moderate operating voltage, and facile synthesis. High-nickel ternary oxides such as NaNi_0.6Co_0.2Mn_0.2O₂ (NNCM622) exhibit superior capacity and economic viability, making them candidates for next-generation NIBs. However, the electrochemical performance of these materials is highly sensitive to synthesis conditions, especially the heat treatment process, significantly affecting the crystallinity, morphology, and structural stability. This work was to investigate the influence of calcination temperature on the structure, morphology, and electrochemical properties of O3-type NNCM622 cathode materials synthesized via a conventional solid-state reaction.

Method In the preparation of O3-type NNCM622 cathode materials, stoichiometric amounts of Ni_0.6Co_0.2Mn_0.2(OH)₂ precursor and NaOH (with a 5% excess of sodium source) were thoroughly mixed and subsequently calcined in an oxygen atmosphere at 700, 750 ℃ and 800 ℃ for 24 hours, yielding the final products denoted as NNCM622-700, NNCM622-750, and NNCM622-800, respectively.

The X-ray diffraction(XRD) patterns show that all the samples crystallize in an O3-type layered structure (space group R-3m). The clear splitting of the (006)/(012) and (018)/(110) diffraction peaks indicates a well-developed layered structure. The Rietveld refinement reveals that the sample calcined at 750 ℃(i.e., NNCM622-750) possesses the lowest refinement discrepancy factor (R_wp = 3.374%) and optimal crystallinity. In contrast, the sample NNCM622-700 has a poorer crystallinity due to incomplete reaction at a lower temperature, while the sample NNCM622-800 likely suffers from sodium volatilization and potential structural distortions at a higher temperature. The X-ray photoelectron spectra (XPS) on the sample NNCM622-750 indicate the coexistence of Ni²⁺/Ni³⁺, Co, and Mn in the material, which is consistent with the expected valence states for this composition. The inductively coupled plasma optical emission spectra (ICP-OES) show that the elemental compositions of all the samples are close to the theoretical stoichiometry.

The images of scanning electron microscopy (SEM) reveal that all the samples consist of spherical secondary particles with the sizes of 2-10 μm formed due to the agglomeration of numerous primary particles. The sample NNCM622-700 has larger primary particles due to insufficient reaction, whereas the sample NNCM622-800 exhibits over-sintering and particle cracking. The images of transmission electron microscopy (TEM) and the mapping of energy dispersive X-ray spectroscopy(EDS) of the sample NNCM622-750 further confirm its well-crystallized layered structure with an interplanar spacing of 0.263 nm, corresponding to the (102) plane, and a homogeneous distribution of elements Na, Ni, Co, and Mn.

The electrochemical performance is evaluated in half-cells versus sodium metal. The NNCM622-750 cathode has a high initial discharge capacity of 150.5 mA·h·g^-1 at 0.1 C (1 C = 150 mA·h·g^-1) within the voltage window of 1.5-4.2 V, with a high initial Coulombic efficiency of 86.5%. This cathode also demonstrates a superior rate capability, retaining discharge capacities of 106, 95 mA·h·g^-1 and 85.9 mA·h·g^-1 at 1 C, 2 C, and 5 C, respectively. After 100 cycles at 1 C, this cathode maintains 55.3% of its initial capacity, showing a better cycling stability than the other two samples. In contrast, the NNCM622-700 and NNCM622-800 cathodes deliver lower initial capacities (i.e., 119.8 mA·h·g^-1 and 97.8 mA·h·g^-1, respectively) and suffer from more rapid capacity decay. This performance degradation is attributed to incomplete sodiation (for NNCM622-700) and sodium loss/structural defects (for NNCM622-800), increasing the electrode impedance and hindering Na⁺ diffusion.

The cyclic voltammetry (CV) curves of the NNCM622-750 cathode shows a pair of well-defined redox peaks around 2.3/2.7 V, corresponding to the Ni²⁺/N⁴⁺ redox reaction. The minimum peak potential separation (ΔV = 0.4 V) for NNCM622-750 indicates the lowest polarization and optimum reaction reversibility among the three samples. The electrochemical impedance spectra (EIS) reveal that the NNCM622-750 electrode has the minimum charge-transfer resistance (i.e., R_ct = 327.1 Ω). Furthermore, based on the Warburg coefficient derived from the EIS data, the maximum Na⁺ diffusion coefficient (D_Na⁺) for NNCM622-750 (i.e., 7.53×10^-15 cm²·s^-1) can be obtained, compared to that for NNCM622-700 (i.e., 5.12×10^-15 cm²·s^-1) and NNCM622-800 (i.e., 1.92×10^-17 cm²·s^-1). These results corroborate its superior rate performance.

The nickel-rich O3-type NNCM622 cathode materials were synthesized by a high-temperature solid-state reaction method, and their microstructures and electrochemical properties were investigated. The O3-type NNCM622 cathode material prepared under calcination at 750 ℃ for 24 h exhibited a superior crystallinity, a well-defined layered structure, and the minimum Na⁺ diffusion barrier due to its excellent structural and compositional synergy. As a result, the NNCM622-750 cathode delivered the maximum initial discharge specific capacity (i.e., 150.5 mA·h·g^-1 at 0.1 C with a coulombic efficiency of 86.5%), superior rate performance (i.e., 85.9 mA·h·g^-1 at 5 C), and relatively high cycling stability (i.e., capacity retention of 55.3% after 100 cycles at 1 C). An appropriate calcination temperature could enhance the reversibility and cycling performance of the cathode material, thereby improving the structural stability and electrochemical properties of nickel-rich O3-type NNCM622 cathodes. This work could present a simple and low-cost preparation process for synthesizing nickel-rich O3-type NNCM622 cathode materials with superior electrochemical performance, offering valuable insights for addressing the challenges associated with heat treatment in the development of next-generation layered cathode materials for sodium-ion batteries.

For a challenge of global water scarcity, the development of efficient and sustainable freshwater production technologies is urgently needed. Solar-driven interfacial water evaporation (SIE) technology has attracted recent attention due to its eco-friendly and low-energy-consumption characteristics. However, the inherent intermittency of solar energy availability, both temporally and spatially, prevents SIE systems to maintain a high efficiency during nighttime, severely limiting further performance improvements. The introduction of phase change materials (i.e., PCMs) is generally considered as an effective approach to address the intermittency of solar water evaporation systems and enhance evaporation efficiency. Nevertheless, leakage issues associated with PCMs during their phase transition process significantly restrict their application scope. To overcome this problem, researchers are dedicated to developing porous mineral-based supporting materials. However, the performance of mineral carriers encapsulating PCMs in some existing studies still requires a further enhancement. Furthermore, the problem of low thermal conductivity in composite PCMs also hinders their application progress in SIE systems.

Halloysite-based aerogels represent a highly promising supporting material capable of preparing composite PCMs with high loading rates, good stability, and large thermal energy storage density. Carbon fiber can be incorporated at different ratios to enhance the thermal conductivity of the composite PCMs. This study was to investigate the thermal properties, photothermal conversion efficiency, and water evaporation rate within an SIE system for the composite PCMs, and analyze the enhancement mechanisms responsible for the improved water evaporation performance.

Dopamine was firstly used to coat halloysite to obtain dopamine-coated halloysite powder (HNTP), and then 1 g of the prepared HNTP powder was added into 40 mL of deionized water. After stirring uniformly, the mixture was ultrasonicated for 30 min. Afterwards, 1 g of sodium alginate (SA) was added to the resulting suspension under continuous stirring until the SA was completely dissolved. The mixture was evenly divided into four portions and transferred into molds. The samples were snap-frozen in a freezer until completely solidified and then freeze-dried (lyophilized) for 72 h. The aerogels were then washed with anhydrous ethanol and dried in an oven, ultimately yielding the halloysite-based aerogel HA. Halloysite-based aerogels with different carbon fiber contents were prepared via replacing HNTs proportionally with carbon fiber (CF). These aerogels were named as HCA 4∶1 and HCA 3∶2 based on a mass ratio of halloysite to carbon fiber.

The composite phase change materials (PCMs) were fabricated by a vacuum impregnation method. Myristic acid (MA) was placed in an oven and heated beyond its phase transition temperature until it completely melted into its liquid state. The prepared aerogels (i.e., HA, HCA 4∶1, HCA 3∶2) were then submerged in a liquid MA. Under vacuum conditions, the MA gradually infiltrated into the pores of the aerogels. After complete adsorption, the composite phase change materials (i.e., MA@HA, MA@HCA 4∶1, and MA@HCA 3∶2) were obtained.

The SEM images of the halloysite aerogel (HA) clearly reveal its abundant porous structure. HNTs are uniformly distributed within the three-dimensional network framework. This highly porous structure provides ample space for encapsulating the phase change material. The results of leakage tests confirm a high phase change material loading rate of up to 84% within the composite. A significant supercooling reduction also occurs. The latent heat of the sample MA@HCA 3∶2 measured is 140.65 J/g. The XRD patterns and FTIR spectra show no emergence of new peaks, indicating a superior compatibility between the MA and the aerogel without chemical reactions. This ensures the core thermal energy storage functionality of the MA remains unaffected. To enhance the thermal conductivity of MA@HA, carbon fiber (CF) is introduced as a thermal conductivity enhancer. The results measured by a thermal conductivity analyzer demonstrate that compared to MA@HA, the thermal conductivity of the composites with the incorporation of carbon fiber is increased by 158%. This significant improvement is attributed to a high intrinsic thermal conductivity of carbon fiber, which forms a continuous and oriented thermally conductive network within the composite, providing highly efficient pathways for heat transfer.

The composite PCMs can efficiently store thermal energy through sunlight absorption during the day and release this stored heat to drive water evaporation during the night due to their superior photothermal conversion and thermal conductivity properties. This effectively mitigates the bottleneck of conventional SIE technology, which relies solely on daytime solar energy for high-efficiency water production. Based on this principle, a novel SIE evaporator incorporating the halloysite aerogel composite PCMs can be designed. The results show that under one sun illumination after 3-h energy storage, the water evaporation rate of the MA@HCA 3∶2 sample is 4.21 times greater than that of pure water. The mechanisms behind this enhanced water evaporation performance are investigated. This work provides valuable insights and a reference basis for enhancing the efficiency of solar energy utilization and improving the effectiveness of seawater desalination.

This study fabricated a highly porous aerogel using halloysite for encapsulating myristic acid (MA). Carbon fiber was incorporated as a thermal conductivity enhancer to modify the composite phase change materials (PCMs). The resulting composite PCMs exhibited a high loading rate, significantly reduced supercooling, and markedly improved photothermal conversion and thermal conductivity properties. The photothermal conversion model designed based on this strategy ensured that more of the heat stored within the PCMs could be utilized for water evaporation. This enabled an efficient operation under various conditions, including nighttime and low-light environments, achieving all-weather high-efficiency evaporation in the solar-driven interfacial evaporator. These results could demonstrate promising applications for enhancing solar energy utilization efficiency and seawater desalination performance.

Mid-infrared (MIR) lasers operating within a spectral region of 2.7-3.0 μm have a significant importance in applications such as laser surgery and remote sensing. Trivalent erbium (Er³⁺) ions are among the most efficient activators for MIR lasers, leveraging the ⁴I_11/2→⁴I_13/2 transition. Nevertheless, the realization of high-power and efficient laser output is often hindered due to the self-terminating effect and substantial thermal loading in Er³⁺-doped gain medium. This study was to investigate the high-performance Er³⁺-doped Gd₃Ga₅O₁₂ (GGG) and Lu₃Ga₅O₁₂ (LuGG) laser crystals with the low phonon energy and high thermal conductivity.

High-quality single crystals of Er: GGG, Er/Pr: GGG, and Er: LuGG were grown by the Czochralski method in an optimized atmosphere (i.e., 98% N₂ + 2% O₂) with iridium crucibles and (111)-oriented seeds. Raw materials underwent multi-step sintering to guarantee the phase purity. Comprehensive spectroscopic analysis was conducted on the polished samples. The room-temperature absorption spectra were determined by a model Lambda 900 spectrophotometer. The fluorescence spectra at the mid-infrared (MIR: 2500-3000 nm), near-infrared (NIR: 950-1750 nm) and upconversion (UC: 500-700 nm) bands were measured under 965/967 nm excitation by an optical parametric oscillator (OPO) or a xenon lamp using models FLS920 and FSP920 fluorescence spectrometers. The fluorescence decay curves for the ⁴I_11/2 and ⁴I_13/2 energy levels were recorded to determine the fluorescence lifetime. The laser performance was assessed under two pumping schemes (i.e., a xenon lamp and a 965 nm fiber-coupled laser diode (LD)). The crystals were fabricated into laser gain media of various sizes and doping profiles, including a bonded configuration (undoped GGG/56% Er: GGG) to alleviate thermal lensing. The continuous-wave (CW) performance was evaluated via input-output power curves, slope efficiency, laser threshold, and beam quality factor (M²). The passive Q-switching was implemented with two-dimensional materials (i.e., graphene and a Bi₂Te₃/graphene heterostructure) as saturable absorbers. The main pulsed laser metrics (i.e., pulse train profile, average output power, pulse width, repetition rate, and peak power) were systematically determined.

The as-grown crystals display a high optical quality with intense and broad absorption bands centered at 965 nm. The 30% Er, 0.5% Pr: GGG composition has a notably high absorption cross-section of 5.13×10^-21 cm² and a full width at half maximum (FWHM) of 15 nm, indicating a superior spectral overlap with commercial 980 nm InGaAs laser diodes and enabling efficient pump coupling. The incorporation of Pr³⁺ ions can enhance the MIR fluorescence emission based on a cross-relaxation energy transfer mechanism, i.e., ⁴I_13/2 (Er³⁺) + ³H₄ (Pr³⁺) → ⁴I_15/2 (Er³⁺) + ³F₄ (Pr³⁺). This process effectively depopulates the lower laser level (⁴I_13/2), leading to a reversal of the intrinsic lifetime ratio between metastable states. Specifically, a ratio of τ(⁴I_11/2)/τ(⁴I_13/2) of 2.348 appears in the sample 30% Er, 0.5% Pr:GGG, which is critical for overcoming the self-termination barrier in Er³⁺ -based~3 μm lasers.

Under 965 nm LD pumping, a CW laser emission at 2.75 μm is attained. The 56% Er: GGG crystal has a maximum output power of 325 mW with a slope efficiency of 16.15%. The bonded GGG/56% Er: GGG structure improves the thermal management, increasing the maximum power to 453 mW and the slope efficiency to 17.1%, while also improving beam quality (i.e., M² reduces from 1.68 to 1.48). The Er/Pr: GGG crystal (11% Er, 0.4% Pr) shows a superior thermal performance, reaching 372 mW output and a higher damage threshold, due to the lower Er³⁺ concentration and efficient energy transfer.

The passive Q-switching with graphene and Bi₂Te₃/graphene absorbers produce impressive pulsed outcomes. The graphene-based Q-switched bonded GGG/Er, Pr: GGG/GGG crystal has a maximum average power of 186 mW, a pulse width of 360 ns, and a repetition rate of 120.5 kHz, corresponding to a pulse energy of 1.54 μJ and a peak power exceeding 4.28 W. The Bi₂Te₃/graphene Q-switched Er: LuGG laser can deliver a higher average power of 274 mW with a shorter pulse width of 340 ns at 135 kHz, yielding a pulse energy of 2.03 μJ and a peak power of > 5.97 W. These findings affirm a potential of these materials in high-performance pulsed MIR lasers.

This work established Er³⁺-doped GGG and LuGG crystals as highly promising gain media for efficient ~2.75 μm mid-infrared lasers. The strategies of Pr³⁺ co-doping and crystal bonding proved effective in mitigating the self-terminating effect and thermal load, leading to the improvement of substantial performance. The effective operation in both continuous-wave and passively Q-switched regimes could underscore the applicability of these crystals in practical high-power MIR laser systems.

Pure NBT exhibits the frequency-dependent Curie temperature, indicating that its dielectric properties are affected by temperature, measurement frequency, and material processing condition. To enhance the dielectric and relaxor properties of NBT, various dopants such as Sr, K, Li and Bi are incorporated into the NBT structure. These modifications significantly alter the dielectric constant and relaxation behavior, demonstrating a dominant influence of dopant on the material properties. Among these, the solid solution of BaTiO₃ (BT) with NBT is widely investigated due to its ability to stabilize the perovskite structure and improve dielectric performance. However, the temperature-dependent stability of dielectric properties remains a critical challenge for high-temperature applications.

In this study, (1-x)(0.75Na_0.5Bi_0.5TiO₃-0.25BaTiO₃) -xBaZrO₃ (NBT-BT-xBZ, x = 0, 0.08, 0.14, and 0.20) ceramics were prepared by a solid-state reaction method. The effect of BaZrO₃ (BZ) addition on the structural, dielectric, and energy storage properties was systematically investigated. In addition, the phase transition and relaxation behaviors were also analyzed based on the modified Curie-Weiss law, Vogel-Fulcher relation, and Lorentz-type empirical law.

The starting materials were powders of high purity Na₂CO₃, Bi₂O₃, TiO₂, BaCO₃, and ZrO₂. The powders were weighed according to a stoichiometric ratio (with 1% excess of Na and Bi) and ground with ethanol in a ball mill at 300 r/min for more than 12 h, and the weight ratio of raw material to ethanol and zirconium balls was 1∶1∶2. The dried material was heat-treated at 850 ℃ for 2 h to promote the formation of NBT-BT-BZ. After further grinding for 12 h, the samples were mixed with a small amount of polyvinyl alcohol (PVA). The samples were sintered in air at 1150 ℃ for 2 h and cooled to room temperature.

The phase composition of the ceramic samples was determined by an model D8 ADVANDCE X-ray diffractometer (D8 ADVANDCEXRD, Bruker AXS Ltd., Germany) with Cu target Kα rays, at X-ray wavelength λ of 1.540 6 Å, 2θ in the range of 10° to 80°, applied voltage of 40 kV, and a current of 500 mA. A silver paste was coated on the two surfaces as electrodes and heat-treated at 700℃ for 10 min. The dielectric properties of the ceramic samples were determined at different frequencies by a model DMS-1000 high-temperature dielectric temperature spectroscope (BALAB Tech. Co., China) with at a ramp rate of 3(°)/min in a temperature range from room temperature to 450 ℃. The overdamped (200 Ω) discharge tests for bulk ceramic samples were performed by a model CFD-005 discharge tester (Gogo (GG) Instruments Technology, China)).

The XRD patterns indicate that all the ceramic samples have a perovskite structure without any detectable secondary phase, proving that zirconium ions can completely enter the lattice and form a solid solution. Based on the locally magnified XRD peaks, the XRD peak shape shifts towards lower angles as a whole as the BZ content increases. This indicates that the overall volume of the crystal cell shows an expansion as the Zr ions replace Ti ions due to different ionic radii of Zr and Ti ions. The SEM images show that the grain size gradually increases with increasing the BZ content. The addition of BZ promotes the grain growth. However, this gradually slows down with the increase of content up to x of 0.20. The limited grain size variation appears in the latter two samples. All the samples show a relatively dense morphology.

The Curie temperature of the NBT-0.25BT ceramic samples is 256 ℃, which is similar to the reported results. The Curie temperature decreases gradually with the increase of BZ additive, and the dielectric temperature spectrum flattens out, indicating that the enhanced structural and temperature stability of the NBT-BT-BZ ceramics. The maximum values of all dielectric constants correspond to temperatures that increase with frequency, indicating a dielectric relaxor behavior. A frequency dispersion is accompanied at near the Curie temperature, which can be ascribed to the thermal evolution of the tetragonal polar nanoregions (PNRs) and the mixing effect of the transition from tripartite to tetragonal PNRs.

The γ values obtained from the experimental data at 100 kHz are 1.79, 1.83, 1.89, and 1.92 for NBT-BT, NBT-BT-0.08BZ, NBT-BT-0.14BZ, and NBT-BT-0.20BZ, respectively. The γ value increases gradually with the addition of the BZ content, showing an enhanced relaxation of the NBT-BT-BZ ceramics. The comparison of discharge current curves and energy density of all the ceramics indicate that the addition of BZ significantly improves the discharge current and energy storage performance. A high discharging energy density (W_d) of 1.6 J·cm^-3 with a fast discharging speed (τ_0.9) of 75 ns is obtained for the ceramic samples with x of 0.14. This can be attributed to an increased relaxation as the BZ content increases.

NBT-BT-BZ ceramics were prepared by a solid-state reaction method. The XRD patterns revealed a phase transition from a tetragonal phase to a pseudocubic phase as the BZ content increased. The dielectric relaxation behavior of the ceramics could be described by three empirical laws (i.e., modified Curie-Weiss law, Vogel-Fulcher relation and Lorentz-type empirical law). The dielectric relaxation followed the modified Curie-Weiss law and the Vogel-Fulcher relationship. The parameters γ and E_a, which were obtained to evaluate the relaxation behavior, increased at a higher BZ content. The Lorentz-type relationship effectively described the temperature dependence of the dielectric constant on both the low- and high-temperature sides within a specific temperature range for all the ceramics.