Reversible Na+-extraction/insertion in nitrogen-doped graphene-encapsulated Na3V2(PO4)2F3@C electrode for advanced Na-ion battery

NASICON-structured sodium vanadium fluorophosphate has caused widespread concern for sodium energy conversion and storage because of its high voltage platform and high theoretical energy density. However, the inferior electrical conductivity is still a big problem, which greatly prevent the applications of Na₃V₂(PO₄)₂F₃ material. Herein, the nitrogen-doped graphene-encapsulated Na₃V₂(PO₄)₂F₃@C (NG-NVPF@C) has been prepared using the sol-gel approach. The physical and electrochemical performances for the resulted NG-NVPF@C composite have been systematically characterized and compared with that of Na₃V₂(PO₄)₂F₃@C (NVPF@C) in this study. The electrochemical tests demonstrate that the as-fabricated NG-NVPF@C displays higher capacity, superior rate property and better cyclic life than NVPF@C. It displays the discharge capacity of 108.6 mAh g⁻¹ at 5C. Moreover, it also possesses the high capacity of 101.6 mAh g⁻¹ at 10C over 300 cycles with the capacity retention of about 96.5%. The improved properties of NG-NVPF@C electrode are assigned to the constructed conductive network by nitrogen-doped graphene, which can modify the conductivity of Na₃V₂(PO₄)₂F₃.     

Growth,microstructure, energy–storage and dielectric performances of chemical–solution NBT–based thin films: Effect of sodium nonstoichimometry

Na_0.5+δBi_0.5(Ti_0.96W_0.01Ni_0.03)O₃ thin films with various Na contents (abbreviated as Na_.5+δBTWN, δ?=?? 3.0, ??1.5, 0, 1.5%) were fabricated on ITO/glass substrates using a chemical–solution process. The effects of Na nonstoichiometry on the microstructure, insulating, ferroelectric and dielectric performances are investigated. The pure perovskite phase can be obtained in Na_0.5BTWN and Na_0.515BTWN, while for Na_0.470BTWN or Na_0.485BTWN, the main composition contains secondary phase of TiO₂. The grain size increases from 30?nm at δ?=?? 3.0% to 55?nm at δ?=?0%, then decreases to 52?nm with δ?=?1.5%. The leakage current of Na_0.485BTWN sample is reduced dramatically in comparison with Na_0.5+δBTWN (δ?=?? 3.0, 0, 1.5%). The big recoverable energy–storage density of 63.1?J/cm² and high energy–storage efficiency of 55.0% can be obtained for Na_0.485BTWN due to the improved electric break–down strength and large difference value between the remanent polarization and maximum polarization. Enhanced dielectricity is achieved in Na_0.485BTWN with a high tunability of 36.0% and a figure of merit of 4.0 at 450?kV/cm and 500?kHz. These results demonstrated that the crystallization, micrographs and energy storage and dielectric properties of Na_0.5Bi_0.5TiO₃ are highly sensitive to low levels of Na–site nonstoichiometry.     

Formation mechanism of elongated β–Si3N4 crystals in Fe–Si3N4 composite via flash combustion

Elongated β–Si₃N₄ crystals have a significant influence on the mechanical property of Fe–Si₃N₄ composite. In this paper, the formation mechanism of elongated β–Si₃N₄ crystals in Fe–Si₃N₄ composite was investigated. During the preparation process, β–Si₃N₄ crystals developed in a spiral and layer growth mechanism in the dense areas. They kept growing from the dense areas and formed radially distributed elongated crystals with hexagonal prismatic morphology as time went on. As for the formation mechanism, the (100) crystal plane of β–Si₃N₄ from Si-N-O melt is mainly the vicinal crystal planes growth with different angles from the (100) crystal plane. At the later stage, the crystallization and the diffusion forces in Si-N-O molten phase decreased. However, the short range diffusion remained active and resulted in the gradient distribution of N content near the boundary. With the temperature decreasing, the disappearance of the short range diffusion implied the end of the crystallization process of the elongated β–Si₃N₄ crystals.     

Characterization of zirconia specimens fabricated by ceramic on-demand extrusion

The Ceramic On-Demand Extrusion (CODE) process is a novel additive manufacturing method for fabricating dense (~99% of theoretical density) ceramic components from aqueous, high solids loading pastes (>50?vol%). In this study, 3?mol% Y₂O₃ stabilized zirconia (3YSZ) specimens were fabricated using the CODE process. The specimens were then dried in a humidity-controlled environmental chamber and afterwards sintered under atmospheric conditions. Mechanical properties of the sintered specimens were examined using ASTM standard test techniques, including density, Young’s modulus, flexural strength, Weibull modulus, fracture toughness, and Vickers hardness. The microstructure was analyzed and grain size measured using scanning electron microscopy. The results were compared with those from Direct Inkjet Printing, Selective Laser Sintering, Lithography-based Ceramic Manufacturing (LCM), and other extrusion-based processes, and indicated that zirconia specimens produced by CODE exhibit superior mechanical properties among the additive manufacturing processes. Several sample components were produced to demonstrate CODE’s capability for fabricating geometrically complex ceramic components. The surface roughness of these components was also examined.     

Advanced visible-light photocatalytic property of biologically structured carbon/ceria hybrid multilayer membranes prepared by bamboo leaves

Biologically structured carbon/cerium dioxide materials are synthesized by biological templates. The microscopic morphology, structure and the effects of different oxidation temperatures on materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) ultraviolet-visible light spectrum (UV–Vis) and X-ray Photoelectron Spectroscopy (XPS). Moreover, by splitting water under visible light irradiation, the hydrogen production is measured to test the photocatalytic property of these materials. The results show that materials made with bamboo biological templates which are immersed in 0.1 mol L^?1 of cerium nitrate solution, then carbonizated in nitrogen (700 °C) and oxidized in air (500–600 °C), can obtain the biological structure of bamboo leaves. The product is in the composition of hybrid multilayer membrane, which one is carbon membrane form plant cell carbonation and another is ceria membrane by nanoparticle self assembly. The best oxidation temperature is 550 °C and the band gap of carbon/cerium dioxide materials synthesized at this optimum oxidation temperature could be reduced to 2.75 eV. After exposure to visible light for 6 h, the optimal hydrogen production is about 302 μmol g^?1, which is much higher than that of pure CeO₂.     

Polyvinyl alcohol thin film reinforced by green synthesized zirconia nanoparticles

The focus of this work is to prepare polyvinyl alcohol (PVA) thin film reinforced by green synthesized zirconia nanoparticles. In order to do so, firstly, zirconia nanoparticles were synthesized by the rosemary extract-assisted sol-gel process as both template and stabilizing agents. The results showed that the as-obtained sample with zirconium salt to rosemary extract ratio of 1:4 had a semi-spherical morphology with the mean particle size of 12–17?nm. This nanoparticle was added as reinforcement with different ratios to the polyvinyl alcohol matrix. The mechanical property of the as-prepared nanocomposites revealed that the elastic modulus of 1?wt% ZrO2-PVA sample was about 5.5 times higher than pure PVA thin film.     

Effect of donor W and acceptor Ni codoping at Ti site on the structure and electrical properties of Na0.5Bi0.5TiO3 thin film

Pure Na_0.5Bi_0.5TiO₃ (NBT), donor W⁶⁺ doped NBT (NBTW), acceptor Ni²⁺ doped NBT (NBTNi), as well as donor W⁶⁺ and acceptor Ni²⁺ codoped NBT (NBTWNi) polycrystalline films are fabricated on indium tin oxide (ITO)/glass substrates via a chemical solution deposition method. The roles of aliovalent-ion substitution on the crystallinity, ferroelectric and dielectric properties of NBT film are mainly investigated. With the introduction of aliovalent-ion, the surface of the doped film becomes more uniform and the leakage current is reduced. Well saturated polarization-electric field (P-E) loops can be observed in W⁶⁺ and Ni²⁺ codoped NBT film due to its lowest leakage currents compared to those of other films. Also, the effect of voltage and frequency on the capacitance-voltage (C-V) curve and the dielectric tunability for the NBTWNi film is discussed. The ferroelectric and dielectric properties are largely improved in NBTWNi film, which can be ascribed to the synergetic effect of high-valence W⁶⁺ and low-valence Ni²⁺ ions. The cooperation between the acceptor and donor cations can effectively eliminate the mobile oxygen vacancies in NBT films.     

Influence of CLVD thermal gradient on the deposition behavior,microstructure and properties of C/C-ZrC composites

Chemical liquid vapor deposition (CLVD) is a rapid preparation method, but it is rarely involved in fabrication of C/C-UHTCs composites and its technology parameters are hardly discussed. In the present study, C/C-ZrC samples were prepared by CLVD process, and the effects of thermal gradient on the deposition behavior, microstructure and properties were investigated. Results exhibited the density, ZrC content and uniformity of the composites increased as the thermal gradient decreased from 30.4 to 9.8?°C/mm, indicating the deposition behavior was improved gradually. When the thermal gradient was 30.4?°C/mm, the deposition behavior of the specimen was poor, which resulted in the high porosity, small numbers of ZrC blocks and uneven distribution. Therefore, the specimen had a low flexural strength with brittle fracture and poor ablation resistance. As the thermal gradient decrease to 9.8?°C/mm, there was an excellent deposition in the composites, and the composites possessed large amounts of ZrC particles and their dispersion were improved remarkably. In this case, the composites displayed a non-brittle fracture with high strength, and the linear and mass ablation rates were reduced, which indicated an improvement of anti-ablation property. Nevertheless, the deposition was deteriorated evidently when the thermal gradient reached to 0?°C/mm. The density, ZrC content and uniformity of the sample became poor, leading to the decline of mechanical property and ablation resistance.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.