Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology

Spray drying, binder jetting and chemical vapor infiltration (CVI) were used in combination for the first time to fabricate SiC whisker-reinforced SiC ceramic matrix composites (SiC_W/SiC). Granulated needle-shaped SiC_W was spray dried into SiC_W spherical particles to increase flowability and thereby increase printability. Then, binder jetting was employed to print a novel SiC_W preform with two-stage pores using the SiC_W spherical particles. The subsequent CVI technology produced pure, dense, and continuous SiC matrix with high modulus and strength. Consequently, SiC_W/SiC with appropriate mechanical properties was obtained. Finally, the challenges of the novel method and the ways to improve the mechanical properties of SiC_W/SiC are discussed.     

Large strain under low driving field in lead-free relaxor/ferroelectric composite ceramics

0.75(Na_0.5Bi_0.5)TiO₃–0.25SrTiO₃ lead-free incipient piezoceramic is a promising candidate for actuator applications due to their large reversible electromechanical strains at the relatively low driving field of 40 kV/cm. In order to further reduce the driving field of 0.75(Na_0.5Bi_0.5)TiO–0.25SrTiO₃ relaxor ceramic to meet the requirements for real actuators application, the relaxor/ferroelectric (RE/FE) 0-3 composite ceramics method was employed. The polarization and strain behaviors were examined as a function of the weight ratio of the relaxor/ferroelectric phases. It was found that 90 wt% 0.75(Na_0.5Bi_0.5)TiO₃–0.25SrTiO₃/10 wt% 0.96(Na_0.84K_0.16)_1/2Bi_1/2TiO₃–0.04SrTiO₃ RE/FE 0-3 type composite samples provided a high unipolar strain of 0.25% and the corresponding large-signal piezoelectric coefficient, d^*₃₃ of 833 pm/V at 30 kV/cm, which are 32% higher than the values of the pure 0.75(Na_0.5Bi_0.5)TiO₃–0.25SrTiO₃. The enhanced electric-field-induced strain at relatively lower field was attributed primarily to the reduction in the RE-FE phase transition electric field. It was also found that the RE/FE composite ceramics exhibited significantly reduced frequency dependence in the unipolar strain behavior at room temperature.     

Hollow graphene spheres coated separator as an efficient trap for soluble polysulfides in LiS battery

Hollow graphene spheres are successfully prepared and employed as the separator coating materials for lithium-sulfur batteries. The hollow graphene spheres coated separator has been proven an efficient trap to adsorb and block polysulfide, greatly alleviating the shuttle effect. In the case of using elemental sulfur as cathode active material and the weight of the diaphragm is only increased by 10.3%, the lithium-sulfur battery with hollow graphene spheres coated separator delivers a high initial specific capacity of 1172.3 mAh g⁻¹ at the current density of 0.2 C, and the discharge capacity remains at 829.6 mAh g⁻¹ after 200 cycles with a capacity decay of 0.146% per cycle, showing excellent electrochemical performance.     

Two-dimensional temperature distribution estimation for a cross-flow planar solid oxide fuel cell stack

The uniform temperature distribution of a cross-flow planar solid oxide fuel cell (SOFC) stack plays an essential role in stack thermal safety and electrical property. However, because of the strict requirements in stack sealing struture, it is hard to acquire the temperature inside the stack using thermal detection devices within an acceptable cost. Therefore, accurately estimating the two-dimensional (2-D) temperature distribution of the cross-flow stack is crucial for its thermal management. In this paper, Firstly, a 2-D mechanism model of a cross-flow planar SOFC stack is established. The stack is divided into 5*5 nodes along the gas flow directions, which can reflect the stack states with moderate computational burden. Then, experimental test data is utilized to modify and validate the stack model, guaranteeing the model accuracy as well as the reliability of model-based state estimator design. Finally, easily-measured stack inputs and outputs are selected, and a temperature distribution estimator combined with unscented kalman filter (UFK) approach is developed to achieve accurate and fast temperature distribution estimation of a cross-flow SOFC stack. Simulation results demonstrate that the UKF-based temperature distribution estimator can precisely and quickly achieve the temperature distribution estimation of the cross-flow stack under both static state and dynamic state changes and is applicable to cross-flow stacks with different size or cell number as well, the maximum estimated absolute error is less than 0.15 K with an absolute error rate of 0.015%, which indicates the developed estimator has good estimation performances.     

Structure variation and energy storage properties of acceptor-modified PBLZST antiferroelectric ceramics

Energy storage capacitors with high recoverable energy density and efficiency are greatly desired in pulse power system. In this study, the energy density and efficiency were enhanced in Mn-modified (Pb_0.93Ba_0.04La_0.02)(Zr_0.65Sn_0.3Ti_0.05)O₃ antiferroelectric ceramics via a conventional solid-state reaction process. The improvement was attributed to the change in the antiferroelectric-to-ferroelectric phase transition electric field (E_F) and the ferroelectric-to-antiferroelectric phase transition electric field (E_A) with a small Mn addition. Mn ions as acceptors, which gave rise to the structure variation, significantly influenced the microstructures, dielectric properties and energy storage performance of the antiferroelectric ceramics. A maximum recoverable energy density of 2.64 J/cm³ with an efficiency of 73% was achieved when x = 0.005, which was 40% higher than that (1.84 J/cm³, 68%) of the pure ceramic counterparts. The results demonstrate that the acceptor modification is an effective way to improve the energy storage density and efficiency of antiferroelectric ceramics by inducing a structure variation and the (Pb_0.93Ba_0.04La_0.02)(Zr_0.65Sn_0.3Ti_0.05)O₃-xMn₂O₃ antiferroelectric ceramics are a promising energy storage material with high-power density.     

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.     

Dielectric response and energy storage efficiency of low content TiO2-polymer matrix nanocomposites

TiO₂/epoxy nanocomposites were prepared at different filler concentrations varying from 3 to 12 phr (parts per hundred resin per weight). The dispersion of TiO₂ was examined by Scanning Electron Microscopy and proved to be adequate. Differential Scanning Calorimetry was implemented to determine the glass to rubber transition temperature of the polymer matrix. The dielectric analysis was performed via Broadband Dielectric Spectroscopy in a wide frequency and temperature range. Five different mechanisms were observed in the spectra of the examined composites which are identified, in terms of increasing temperature at constant frequency, as γ, β, Intermediate Dipolar Effect (IDE), α and Interfacial Polarization (IP) relaxation modes. The activation energies of all relaxation modes were calculated. Finally, the dielectric response of the TiO₂ nanocomposites compared to that of the TiO₂ microcomposites reveals that the former exhibit significantly higher energy storage efficiency even at lower TiO₂ concentration than the corresponding of the microcomposites.     

A review on integrating nano-carbons into polyanion phosphates and silicates for rechargeable lithium batteries

Lithium metal phosphate (Li₂MPO₄) and silicates (Li₂MSiO₄) (where M = Fe, Mn, and Co) are promising polyanion cathodes for rechargeable lithium batteries, owing to the inherent merits such as low cost, decent electrochemical property, and high stability. However, these merits have often been undermined by insufficient energy and power delivery due to poor Li extraction/insertion kinetics. It is generally believed that the extremely low conductivity, i.e. ∼10⁻⁹ s cm⁻¹ for phosphates and 10⁻¹²–10⁻¹⁶ s cm⁻¹ for silicates at room temperature, in combination with slow Li ion diffusion could account for such sluggish Li cycling kinetics. To address this critical issue, it is essential to integrate well-defined nano-carbons such as one-dimensional (1D) carbon nanotube (CNT), two-dimensional (2D) graphene, and their three-dimensional (3D) assembly into polyanion materials. By constructing hybrid architectures, integrated composites could afford much improved activity towards Li storage versus the bare ones. In this short review, we summarize recent advance in integrating CNT, graphene, and their 3D assemblies into LiMPO₄ and Li₂MSiO₄ cathodes, with particular emphasis on how the cathodes interact with carbon materials and their mechanism. We also conclude some general rules to engineer such integration structures to maximize their utilization towards Li storage.     

Aid of a metallic functional layer on Ni/YSZ anode for Direct Methane Fuel Cell

Aid of a metallic overlayer to nickel/yttrium-stabilized-zirconia (Ni/YSZ) anode is investigated in Direct Methane Fuel Cell. Copper modified nickel metallic overlayer shows high activity for fuel cell performance and good stability to coking in methane atmosphere. The copper-nickel overlayer provides advantages of material compatibility with the substrate and catalytic function on copper-modified nickel sites. The results suggest that the overlayer is effective for decomposition of methane and tolerant to coking by removal of deposited carbon via oxidation and gasification reaction.