Preparation of Plate‐Like Sodium Niobate Particles by Hydrothermal Method

Sodium niobate NaNbO₃ hydrate (NN‐hydrate) particles with a plate‐like morphology were prepared at 140°C for 2 h in 12 mol/L of NaOH by the hydrothermal method. Bar‐like Na₈Nb₆O₁₉·13H₂O particles were synthesized at as low as 100°C for 2 h. This work demonstrates that by carefully optimizing the reaction condition, we can selectively fabricate niobate structures, including the bar‐like, plate‐like, fibers and cube particles through a direct reaction between NaOH solution and Nb₂O₅. It was found that Nb₆O₁₉^8? formed was an important premise for formation of the NN‐hydrate, and lower [OH^–] was not favorable in preparing the NN‐hydrate as there was an optimum [OH^?]. Through researching effects of the reaction temperature, time, concentration of NaOH, and content of Nb₂O₅ on the NN‐hydrate structure and evolution, the formation mechanism from solid reactants to the intermediate were investigated. After calcining at 800°C, the synthesized NN‐hydrate particles made a phase almost transform to the perovskite NaNbO₃, and the morphology of these calcined particles was still plate‐like.     

One‐Step Surfactant‐Free Hydrothermal Synthesis of Platelike Sodium Niobate Template Powders

A one‐step surfactant‐free hydrothermal route is developed to prepare platelike NaNbO₃ template powders. At optimal KOH concentration, pure platelike NaNbO₃ with rhombohedral structure (width and thickness of 20 and 2 μm, respectively) is obtained at 200°C for 16 h. After calcination at 600°C for 4 h, the structure of the hydrothermally synthesized NaNbO₃ changes from rhombohedral to orthorhombic, whereas the initial platelike morphology is maintained. Such characteristics in terms of phase structure, elemental composition, and morphology render our hydrothermally synthesized NaNbO₃ suitable for textured ceramic fabrications.     

Near/mid‐infrared emission and energy transfer of Er/Tm‐Yb co‐doped β‐NaYF4 microcrystals

The well‐formed high quality β‐NaYF₄:Er³⁺/Tm³⁺, Yb³⁺ microcrystals with near/mid‐infrared (NIR/MIR) emission are synthesized by the solvothermal method. Obvious 1.4 μm, 1.8 μm emissions, and 1.5 μm emission are observed in as‐prepared β‐NaYF₄:Tm³⁺, Yb³⁺ and β‐NaYF₄:Er³⁺, Yb³⁺ microcrystals, respectively. To obtain MIR emission, the as‐prepared β‐NaYF₄:Er³⁺, Yb³⁺ microcrystals are heat‐treated at different temperature schedule and atmosphere, it demonstrates there is great effect on the morphology and crystal structure when heat‐treated at different temperature, while little effect under different heat‐treated atmosphere. Subsequently, after heat‐treatment at 575°C in air, owing to the efficient elimination of internal defects and partly surface hydroxyl/citrate groups, an obvious 2.7 μm MIR emission is successfully detected in heat‐treated β‐NaYF₄:Er³⁺, Yb³⁺ microcrystals for the first time.     

Mid‐IR to Visible Photoluminescence,Dielectric, and Ferroelectric Properties of Er‐Doped KNLN Ceramics

Two mole percentage Er‐doped (K_0.5Na_0.5)_1 ? xLi_xNbO₃ ceramics have been prepared and their dielectric, ferroelectric, and photoluminescence (PL) properties have been investigated. Under an excitation of 980 nm, the ceramics exhibit intense up‐conversion luminescent emission at 548 nm (green), weak emission at 660 nm (red) as well as strong down‐conversion luminescent emission in near‐infrared (NIR) (1.40–1.65 μm) and mid‐infrared (2.60–2.85 μm) regions. Probably due to the induced structure distortion and reduced local symmetry, the PL intensities of the green, red as well as mid‐infrared emissions are enhanced by the doping of Li⁺. Our results show that the Li‐doping is effective in establishing a dynamic circulatory energy process to further enhance the PL intensity of the mid‐infrared emission at the expense of the NIR emission. At the optimum doping level of Li⁺ (~6 mol%), the full bandwidth at half maximum of the mid‐infrared emission reaches a very large value of ~250 nm. The ceramics also exhibit good ferroelectric properties, and thus they should have great potential for multifunctional optoelectronic applications.     

Hydrothermal synthesis of morphology-controlled KNbO3, NaNbO3, and (K,Na)NbO3 powders

NaNbO₃, (K,Na)NbO₃ and KNbO₃ powders were synthesized using (1− y) NaOH–y KOH solutions ([OH⁻] =7.5–15 M) with y=0, 0.78, and 1 at 200 °C by the hydrothermal method, respectively. Their compositions, structures, and morphologies were analysed. Both of the synthesized NaNbO₃ and KNbO₃ powders had sub-micron- or micron-sized grains. The [OH⁻] drastically influenced the size and morphology of the KNbO₃ particles but did not influence those of the NaNbO₃ particles. In contrast, the morphology of the (K,Na)NbO₃ particles, which were aggregates of nano-grains, was influenced by the hydrothermal-treatment time rather than [OH⁻]. Moreover, their composition and phase were influenced by both annealing and the hydrothermal-treatment time, and their formation mechanism was discussed by comparison with those of KNbO₃ and NaNbO₃ particles. The present synthetic strategy enables tailoring the compositions, morphologies, and structures of the niobate products to different applications by controlling the process parameters.     

Two‐step Synthesis of Platelike Potassium Sodium Niobate Template Particles by Hydrothermal Method

Two‐step hydrothermal synthesis of platelike potassium sodium niobate (K, Na)NbO₃ (KNN) template particles was investigated. Platelike K₄Na₄Nb₆O₁₉·9H₂O (KNN‐hydrate) particles were synthesized in 4 mol/L aqueous alkali at 150°C by the sodium dodecyl benzene sulfonate (SDBS) surfactant‐assisted hydrothermal method, which were used as crystal nucleus in the second step of hydrothermal synthesis. The two‐step synthesized KNN‐hydrate particles with 0.6 μm thickness and 7 μm width were prepared at 80°C after 10 h of the second step. After calcination of the KNN‐hydrate particle at 600°C, platelike KNN particles were obtained, which were used as templates for textured ceramics. Particles obtained by the two‐step synthesis showed regular morphology and uniform distribution, with a marked improvement in grain size.     

Effect of Electrodeposition Parameters on the Microstructure and Corrosion Behavior of ‎DCPD Coatings on Biodegradable Mg–Ca–Zn Alloy

In this study, DCPD (Brushite, CaHPO₄.2H₂O) coatings were prepared on the surface of a Mg–Ca–Zn alloy using different current density (0.15–1.2 mA/cm²) and deposition time (5–90 min). The results revealed that DCPD with needle‐like morphology was observed for the current density between 0.15 and 0.4 mA/cm²?, whereas ?plate‐like morphology was obtained at current density above 0.8 mA/cm². The results showed that surface roughness increased with increasing current density. The lowest corrosion rate of 0.14 mm/year was obtained for the dense and uniform DCPD coating ?at 0.4 mA/cm², while further increase has deleterious effect on the corrosion resistance.     

Temperature‐insensitive piezoelectricity in lead‐free NaNbO3‐based ceramics

In this work, we report a lead‐free piezoelectric ceramic of (0.9‐x)NaNbO₃‐0.1BaTiO₃‐xBaZrO₃, and the effects of BaZrO₃ on the phase structure, microstructure, electrical properties and temperature stability are investigated. A morphotropic phase boundary‐like region consisting of rhombohedral (R) and tetragonal (T) phases is constructed in the compositions with x = 0.035‐0.04. More importantly, in situ temperature independence of the piezoelectric effect {piezoelectric constant (d₃₃) and strain} can be achieved below the Curie temperature (T_c). Intriguingly, the electric field‐induced strain is still observed at T ≥ T_c due to the combined actions of the electrostrictive effect and the electric field‐induced phase transition. We believe that NaNbO₃‐based ceramics of this type have potential for applications in actuators and sensors.     

CW CO2‐Laser‐Induced Formation of Fulgurite on Lime–Pozzolan Mortar

A glassy material similar to fulgurites (fusion of the soil which has been struck by lightning) was prepared by continuous wave (CW) CO₂ laser (λ = 10.6 μm) ablation of lime–pozzolan mortar at medium‐vacuum conditions and atmospheric pressure. In all the irradiated samples, the determined surface temperature is higher than the melting temperature of mortar (1556 K), so the surface is melted and converted into an amorphous glassy when cooled. The samples were studied combining laser‐induced breakdown spectroscopy (LIBS) and Raman spectroscopy. The emission induced by the CW CO₂ laser is mainly due to electronic relaxation of Na, K, Si, Si⁺, Ca, O, N, and CaOH species along with an intense continuum due to blackbody emission. The emission induced on both natural and produced fulgurite is mostly due to electronic relaxation of Ca, Ca⁺, Si, Si⁺, Si²⁺, Si³⁺, H, Na, K, Mg, N, O, CaOH, and OH species with different relative intensities in some of them. Raman spectra show that the glassy formed material is similar to natural fulgurites, with the main difference arising from portlandite formed over the surface of the lime–pozzolan mortar. As the laser power increases, less density SiO₂ glass is formed with more Q⁴ and Q¹ units present.     

Well‐functioned photovoltaics based on nanofibers composed of PBDT‐TIPS‐DTNT‐DT and graphenic precursors thermally modified by polythiophene,polyaniline and polypyrrole

Reduced graphene oxide nanosheets modified by conductive polymers including polythiophene (GPTh), polyaniline (GPANI) and polypyrrole (GPPy) were prepared using the graphene oxide as both substrate and chemical oxidant. UV–visible and Raman analyses confirmed that the graphene oxide simultaneously produced the reduced graphene oxide and polymerized the conjugated polymers. The prepared nanostructures were subsequently electrospun in mixing with poly(3‐hexylthiophene) (P3HT)/phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) and poly[bis(triisopropylsilylethynyl)benzodithiophene‐bis(decyltetradecylthien)naphthobisthiadiazole] (PBDT‐TIPS‐DTNT‐DT)/PC₇₁BM components and embedded in the active layers of photovoltaic devices to improve the charge mobility and efficiency. The GPTh/PBDT‐TIPS‐DTNT‐DT/PC₇₁BM devices demonstrated better photovoltaic features (J_sc = 11.72 mA cm^?2, FF = 61%, V_oc = 0.68 V, PCE = 4.86%, μ_h = 8.7 × 10^?3 cm² V^–1 s^?1 and μ_e = 1.3 × 10^?2 cm² V^–1 s^?1) than the GPPy/PBDT‐TIPS‐DTNT‐DT/PC₇₁BM (J_sc = 10.30 mA cm^?2, FF = 60%, V_oc = 0.66 V, PCE = 4.08%, μ_h = 1.4 × 10^?3 cm² V^–1 s^?1 and μ_e = 8.9 × 10^?3 cm² V^–1 s^?1) and GPANI/PBDT‐TIPS‐DTNT‐DT/PC₇₁BM (J_sc = 10.48 mA cm^?2, FF = 59%, V_oc = 0.65 V, PCE = 4.02%, μ_h = 8.6 × 10^?4 cm² V^–1 s^?1 and μ_e = 7.8 × 10^?3 cm² V^–1 s^?1) systems, assigned to the greater compatibility of PTh in the nano‐hybrids and the thiophenic conjugated polymers in the bulk of the nanofibers and active thin films. Furthermore, the PBDT‐TIPS‐DTNT‐DT polymer chains (3.35%–5.04%) acted better than the P3HT chains (2.01%–3.76%) because of more complicated conductive structures. © 2019 Society of Chemical Industry     

CaO‐containing LaCO3OH nanogears and their luminescence and de‐NOx properties

Large‐scale, uniform, monodisperse LaCO₃OH cherry‐blossom‐like nanogears and/or nanocubes have been synthesized under hydrothermal reaction conditions. Upon the addition of only 5 mol% Ca²⁺ ions into a La nitrate salts solution with pH 8.5, LaCO₃OH crystals with novel cubic or nanogear structures are formed in the hexagonal phase. The hydrothermal reactions were carried out without the addition of a template or catalysts. Both 24 hour and 48 hour hydrothermal reactions yield 100% pure LaCO₃OH with no irregular particles. We examined the photoluminescence properties of the as‐synthesized powders of the pure LaCO₃OH nanogears and found one broad emission band centered at 394 nm after excitation at λ  =  280 nm. The NO reduction activity was also examined over highly dispersed CaO‐containing La₂O₃ obtained after calcination the LaCO₃OH at 800^○C for 2 hours. The CaO‐containing La₂O₃ catalysts showed good stability for NO reduction with CH₄ in the presence of O₂ and H₂O vapor.     

Dielectric Properties of Ba0.8Ca0.2TiO3–Bi(Mg0.5,Ti0.5)O3–NaNbO3 Ceramics

Ceramics in the system 0.45Ba_0.8Ca_0.2TiO₃–(0.55?x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃, x = 0–0.02 were fabricated by a conventional solid‐state reaction route. X‐ray powder diffraction indicated cubic or pseudocubic symmetry for all samples. The parent 0.45Ba_0.8Ca_0.2TiO₃–0.55Bi(Mg_0.5Ti_0.5)O₃ composition is a relaxor dielectric with a near‐stable temperature coefficient of relative permittivity, ε_r = 950 ± 10% across the temperature range 80°C–600°C. Incorporation of NaNbO₃ at x = 0.2 extends the lower working temperature to ≤25°C, with ε_r = 575% ± 15% from temperatures ≤25°C to >400°C, and tan δ < 0.025 from 25°C to 400°C. Values of dc resistivity ranged from ~10⁹ Ω·m at 250°C to ~10⁶ Ω·m at 500°C. The properties suggest that this material may be of interest for high‐temperature capacitor applications.     

Phase transition and morphology evolution of superfine NaNbO3 particles synthesized by the hydrothermal method with the assistance of K+

NaNbO₃ (NN) is a well-known perovskite-type dielectric material. However, its phase transition is a complex process and the phase transition mechanism is insufficiently investigated yet. Therefore, in the work, the NN superfine particles were synthesized by a hydrothermal method with the assistance of K⁺ and the microstructures of NN were carefully investigated to understand the strain-driven phase transition and morphology evolution of NN. The results suggest that K⁺ leads to strong planar bending of NbO₆ by influencing the Na ⁺ vibration, which accounts for the O-R phase transition as the K⁺ increases. Once the R phase is triggered, the O-R phase transition proceeds spontaneously based on phonon calculations of orthorhombic (O) NN and R NN. The antiferroelectric nature of rhombohedral (R) NN with the space group R–3H is confirmed through charge density distribution. Additionally, the morphology evolution is deduced on the TEM analysis basis.     

Infrared‐Transparent Y2O3–MgO Nanocomposites Fabricated by the Glucose Sol–Gel Combustion and Hot‐Pressing Technique

A glucose sol–gel combustion method has been developed to synthesize composite nanopowders with equal volume fractions of Y₂O₃ and MgO. The synthesis involves the generation of precursor foam containing Y³⁺ and Mg²⁺ cations via the chemical and thermal degradation of glucose molecules in aqueous solutions. Subsequent calcination of the foam gave the composite nanopowders uniform composition and surface areas of 44–62 m²/g depending on the relative amount of glucose. Then the nanopowder with an average particle size of 19 nm was consolidated by the hot‐pressing technique with different sintering temperatures. The fabricated nanocomposite is mid‐infrared transparent as the result of fine grains, narrow grain size distribution, and uniform phase domains. The transmittance increases with increase in the sintering temperature and reaches 83.5% at 3–5 μm mid‐infrared wave range once the temperature reaches 1350°C, which is close to the theoretical value of 85%. And it is noteworthy that the cutoff wavelength reaches 9.6 μm, which is superior to those of spinel, AlON, and sapphire. And the Vickers hardness of the sample reaches 10.0 ± 0.1 GPa, which is significantly higher than those of the coarse grained single‐phase MgO and Y₂O₃. The results indicate that the glucose sol–gel combustion and hot‐pressing technique is an effective method to fabricate infrared transparent Y₂O₃–MgO nanocomposites.     

Temperature‐Stable Relative Permittivity from −70°C to 500°C in (Ba0.8Ca0.2)TiO3–Bi(Mg0.5Ti0.5)O3–NaNbO3 Ceramics

Temperature‐stable relaxor dielectrics have been developed in the solid solution system: 0.45Ba_0.8Ca_0.2TiO₃–(0.55 ? x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃. Ceramics of composition x = 0 have a relative permittivity ?_r = 950 ± 15% over a wide temperature range from +70°C to 600°C. Modification with NaNbO₃ at x = 0.2 decreases the lower limiting temperature to ?70°C, but also decreases relative permittivity such that ?_r ~ 600 ± 15% over the temperature range ?70°C to 500°C. For composition x = 0.3, the low‐temperature dispersion in loss tangent, tan δ, (at 1 kHz) shifts to lower temperature, giving tan δ values ≤0.02 across the temperature range ?60°C to 300°C in combination with ?_r ~ 550 ± 15%. Values of dc resistivity for all samples are of the order of 10¹⁰ Ω m at 250°C and 10⁷ Ω m at 400°C.     

Microstructure and Cell Performance of NiO–YSZ Composite Anode‐Supported Solid Oxide Fuel Cells Using Graphite as Anode Pore‐Former: Effects of Graphite Particle Size and HNO3 Treatment

The influences of graphite particle size and HNO₃ treatment on microstructure and cell performance of the NiO–YSZ anode‐supported solid oxide fuel cells were investigated. The peak power density for the cells using 1, 3, and 6 μm graphite was 679, 603, and 549 mW/cm², respectively. HNO₃ treatment was an efficient approach to improve the wettability of graphite in water, to modify the microstructure and to enhance the cell performance. The peak power density for the cell using HNO₃‐treated graphite (1 μm) was 766 mW/cm², approximately 13% higher than that of the cell using pristine graphite.     

Properties of an Infrared‐Transparent MgO:Y2O3 Nanocomposite

A 50:50 vol% MgO–Y₂O₃ nanocomposite with ~150 nm grain size was prepared in an attempt to make 3–5 μm infrared‐transmitting windows with increased durability and thermal shock resistance. Flexure strength of the composite at 21°C is 679 MPa for 0.88 cm² under load. Hardness is consistent with that of the constituents with similar grain size. For 3‐mm‐thick material at 4.85 μm, the total scatter loss is 1.5%, forward scatter is 0.2%, and absorptance is 1.8%. Optical scatter below 2 μm is 100%. Variable intensity OH absorption (~6% absorptance) is observed near 3 μm. The refractive index is ~0.4% below the volume‐fraction‐weighted average of those of the constituents. Thermal expansion is equal to the volume‐fraction‐weighted average of expansion of the constituents. Specific heat capacity is equal to the mass‐fraction‐weighted average of heat capacities of the constituents. Thermal conductivity lies between those of the constituents up to 1200 K. Elastic constants lie between those of the constituents. The Hasselman mild thermal shock resistance parameter for the composite is twice as great as that of common 3–5 μm window materials, but half as great as that of c‐plane sapphire.     

Hot Pressing of Infrared‐Transparent Y2O3–MgO Nanocomposites Using Sol–Gel Combustion Synthesized Powders

A sol–gel combustion method has been used to synthesize Y₂O₃–50 vol%MgO composite nanopowders. Solutions of the precursor nitrates were mixed with citric acid and ethylene glycol, heated from 200°C to a predetermined temperature gradually, giving nanocrystalline ceramic powders. The influence of the ratio of yttrium nitrate to the whole precursor mixture and the holding temperature on the properties of the composite nanopowder was investigated using a combination of thermal analysis, X‐ray diffraction, specific surface area analysis, and scanning electron microscopy techniques. When the ratio of yttrium nitrate to the whole precursor mixture reaches 22.5 mol%, the average particle size of synthesized composite nanopowder is 13 nm and the specific surface area is 45.9 m²/g. Then the synthesized Y₂O₃–MgO composite nanopowder was consolidated by the hot‐pressing technique at 1200°C with different dwell time. As a result, the nanocomposite ceramic prepared with a dwell time of 60 min got the highest transmittance of 75% at 5 μm wavelength. The cut‐off wavelength of Y₂O₃–MgO nanocomposite ceramic reaches 9.8 μm, which is superior to other mid‐IR transparent materials. In addition, the fabricated sample is more or less transparent in visible wavelengths and the transmittance at 0.8 μm is as high as 14.5%.     

Novel Er3+/Ho3+‐codoped glass‐ceramic fibers for broadband tunable mid‐infrared fiber lasers

It is well recognized that a widely wavelength‐tunable mid‐infrared (MIR) fiber laser plays an important role in the development of compact and efficient coherent sources in the MIR range. Herein, the optimizing Er/Ho ratio for enhancement of broadband tunable MIR emission covering 2.6‐2.95 μm in the Er³⁺/Ho³⁺‐codoped transparent borosilicate glass‐ceramic (GC) fibers containing NaYF₄ nanocrystals under 980 nm excitation was investigated. Specifically, the obtained GC fibers with controllable crystallization and well fsd‐maintained structures were prepared by the novel melt‐in‐tube approach. Owing to the effective energy transfer between Er³⁺ and Ho³⁺ after crystallization, the 2.7 μm MIR emission was obviously enhanced and the emission region showed a notable extension from 2.6‐2.82 μm to 2.6‐2.95 μm after the addition of Ho³⁺. Importantly, we conducted a theoretical simulation and calculation related to the MIR laser performance, signifying that the GC fiber may be a promising candidate for MIR fiber laser. Furthermore, the melt‐in‐tube approach will provide a versatile strategy for the preparation of diverse optical functional GC fibers.     

Direct Laser Patterning of β‐BaB2O4 Crystals with High Orientation in the Inside of Glass Fiber

The laser‐induced crystallization method was applied to pattern β‐BaB₂O₄ crystals (β‐BBO) in the inside of bulk glass plate and fibers of 8Sm₂O₃–42BaO–50B₂O₃, in which the focal position of continuous wave (CW) Yb:YVO₄ fiber lasers (power: 0.9–1.0 W, scanning speed: 2 μm/s) with a wavelength of 1080 nm was moved gradually from the surface to the inside. It was confirmed from micro‐Raman scattering spectra, second harmonic generation, and transmission electron microscope observations that β‐BBO crystals are patterned in the inside of glass plate and fibers (diameter: 110 μm) and are highly oriented that is c‐axis orientation, along the laser scanning direction. This study proposes the direct and simple processing for the fabrication of glass fibers consisting of optical functional crystal cores.