Modulating conductivity type of cuprous oxide (Cu2O) films on copper foil in aqueous solution by comproportionation

Cuprous oxide (Cu₂O) is an attractive material for photoelectrochemical (PEC) hydrogen production or photovoltaic application, because of its appropriate band gap, low material cost and non-toxic. In this paper, Cu₂O films were obtained by comproportionation in acid cupric sulfate solutions with varying concentrations of potassium nitrate. Photoelectrochemical and electrochemical experiments, such as zero-bias photocurrent responses, voltammograms, and Mott-Schottky measurements, show that the Cu₂O films grown in low (≤0.75 mol dm^–3) and high (≥1.00 mol dm^–3) nitrate ion concentrations presented n-type and p-type conductivity, respectively. Open circuit potential and polarization behavior were monitored to investigate the mechanism of modulating conductivity type. Nitrate ions consume protons in the plating solution during comproportionation with different concentrations of nitrate ions creating different pH at the Cu₂O/solution interface. This gradient leads to the transformation of Cu₂O films conductivity changing from n-type to p-type with increasing the concentration of nitrate ions in the plating solution. This method could be used to fabricate homojunction electrode on metal substrate for PEC hydrogen production or photoelectric application.     

Enhanced performance and durability of lanthanum strontium cobalt ferrite by in-situ solvothermal modification

Due to the limitations of single material, the oxygen reduction kinetics may be slow, which is considered as a key challenge for developing the high-performance cathodes. In this work, an effective strategy is proposed, employing the in-situ solvothermal method, in which the LSCF, one of the most promising cathode materials, is decorated with Co₃O₄. After being treated at 180 °C in a neutral solution, the R_p of N-?Co₃O₄ @LSCF electrode is decreased by 37%, exhibiting reasonable stability for 120 h. Utilizing the DRT technique, finding the decorated Co₃O₄ mainly promotes the charge transfer and oxygen dissociation processes. The single cell with the N-?Co₃O₄ @LSCF cathode achieves a maximum power density of 803 mW cm^-?² at 750 °C, which is 25% higher than that for the untreated LSCF electrode. Our work demonstrates that such Co₃O₄ decorated composite obtained by the in-situ solvothermal treatment is a promising cathode material for solid oxide fuel cells.     

Dynamic response of a Li2O-Al2O3-SiO2 transparent glass-ceramic under shock compression

Li₂O-Al₂O₃-SiO₂ transparent glass-ceramics (LAS-TGCs) are promising candidates for transparent armour materials due to their excellent physical and mechanical capabilities. In this work, the dynamic behaviour of a LAS-TGC were further investigated using the planar impact technique and photon Doppler velocimetry. The shock stress, shock wave velocity, failure wave velocity and spall strength were obtained. In addition, the recompression signal, as a signature of failure waves, was observed to evolve into an oscillation signal as the impact stress decreases, indicating that the failure wave is gradually formed at a threshold. It has been noted that the failure wave velocity decreases with the increase in external loading and then turns upwards. The damage parameter of the LAS-TGC was assessed to be 0.410(5) under a shock stress of ~5.5 GPa, which is smaller than that of K9 glass. It is suggested that the LAS-TGC has better shock resistance than K9 glass.     

Joining of Ti-coated monolithic SiC using a SiCw/Ti3SiC2 filler by electric field-assisted sintering

Monolithic SiC, for the first time, was successfully joined using a SiC whisker-reinforced Ti₃SiC₂ composite (SiC_w/Ti₃SiC₂) filler via electric field-assisted sintering technique. A thin Ti coating layer was formed on the SiC surface to minimize the residual stress at the joint interface by transforming it into a TiC gradient layer. After optimizing process parameters, a joint strength higher than 250 MPa was obtained, which is higher than the other values reported in the literature. Failure occurred at the SiC base rather than the joining interface because of the improved joint strength by the incorporation of SiC_w. The addition up to 15 wt. % SiC_w in the filler layer improved the joint strength by various strengthening mechanisms. On the other hand, the joint strength was lower with 20 wt. % SiC_w addition, indicating the importance of thermal expansion mismatch between SiC_w and Ti₃SiC₂ to obtain a sound SiC joint.     

Synthesis and thermal expansion of chalcogenide MAX phase Hf2SeC

Thermal expansion of MAX phases along different directions tends to be different because of the anisotropy of hexagonal crystals. Herein, a new Hf₂SeC phase was synthesized and confirmed to be relatively isotropic, and the coefficients of thermal expansion (CTEs) along a and c directions were determined to be 9.73 μK^?1 and 10.18 μK^?1, respectively. The strong MS bond endowed Hf₂SC and Zr₂SC lower CTEs than those of Hf₂SeC and Zr₂SeC. The relationship between the thermal expansion anisotropy and the ratio of elastic stiffness constant c₁₁ and c₃₃ was established. This straightforward approximation can be used to roughly predict the thermal expansion anisotropy of MAX phases.     

Preparation of novel titanium-niobium-oxygen composite ceramic with excellent thermoelectric properties using the high-pressure and high-temperature method

Titanium oxide is a promising thermoelectric material because of its high stability and low cost. We synthesize novel titanium-niobium-oxygen composite ceramics directly from elementary substance Nb and TiO₂ under high-pressure and high-temperature (HPHT) in this work. Elemental substance Nb will reduce TiO₂ to Magnèli phase titanium oxide at high pressure and high temperature. In this process, elemental substance Nb is oxidized to various niobium oxides. The experimental results show that the composite ceramics have a special 'pore' microstructure, and their thermoelectric and mechanical properties are very prominent among metal oxide thermoelectric materials. After repeated tests, the optimum concentration sample zT value is 0.313 at 973 k, with a Vickers hardness of 7.06. This work provides a novel concept for improving the performance of TiO₂ by reducing it with metal elementary substances other than Ti.     

Joining of SiCf/SiC using a layered Ti3SiC2-SiCw and TiC gradient filler

A layered filler consisting of Ti₃SiC₂-SiC whiskers and TiC transition layer was used to join SiC_f/SiC. The effects of SiC_w reinforcement in Ti₃SiC₂ filler were examined after joining at 1400 or 1500 °C in terms of the microstructural evolution, joining strength, and oxidation/chemical resistances. The TiC transition layer formed by an in-situ reaction of Ti coating resulted in a decrease in thermal expansion mismatch between SiC_f/SiC and Ti₃SiC₂, revealing a sound joint without cracks formation. However, SiC_f/SiC joint without TiC layer showed formation of cracks and low joining strength. The incorporation of SiC_w in Ti₃SiC₂ filler showed an increase in joining strength, oxidation, and chemical etching resistance due to the strengthening effect. The Ti₃SiC₂ filler containing 10 wt.% SiC_w along with the formation of TiC was the optimal condition for joining of SiC_f/SiC at 1400 °C, showing the highest joining strength of 198 MPa as well as improved oxidation and chemical resistance.     

Synergy effects on blending Li-rich and classical layered cathode oxides with improved electrochemical performance

Blending different cathode materials to construct composites can be in compatibility with their individual advantages to exhibit better electrochemical performances. In this study, we blend Li-rich and classical layered cathode materials to realize the suppression of voltage decay. The electrochemical results indicate that the composite electrodes show better capacity retention exceeding 90% after 100 cycles. More importantly, the cumulative voltage decay of the composite materials with different ratio are 280 mV and 140 mV for 100 cycles’ duration, which are much lower than that of the single component with 390 mV in Li-rich layered cathode, respectively. Based on the ex situ X-ray diffraction, the blended composites show the structural origin of synergy effect, which are like a pair of parallel resistors to reciprocally buffer the crystal structure change during the charge and discharge process between Li-rich and classical layered cathode materials. Blending of layered cathode oxide materials with the synergy effect provide a possible approach to achieve more excellent electrochemical performances in lithium-ion batteries.