External and internal dual-controls: Tunable cavity and Ru–O–Co bond bridge synergistically accelerate the RuCoCu-MOF/CF nanorods for urea-assisted energy-saving hydrogen production

Currently, there are still many limitations in the research of conductive metal-organic frameworks (MOFs) in the field of electrocatalysis. On the one hand, most MOFs have a solid construction, seriously hindering their mass transfer process. On the other hand, innate bonding is not conducive to the optimization of electronic structures and the excitation of intrinsic active sites. Therefore, external/internal dual-control of MOFs is urgently needed to break the shackles of their activity. Herein, the hollow RuCoCu-MOF/CF nanorods with tunable cavities are directionally constructed by a self-sacrificial template method. Benefiting from the exact morphological control and the unique Ru–O–Co bond bridge, RuCoCu-MOF/CF exhibits superior performances for alkaline hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Surprisingly, a record-breaking voltage of 1.402 V drives a current density of 10 mA cm⁻² for urea-assisted overall water splitting under alkaline conditions, greatly promoting the development of energy-efficient hydrogen production technology. This work firstly constructed the MOF-based self-supporting electrode with ultra-high urea-assisted hydrogen production and urea degradation performances via the dual controls of the cavity size and chemical bond bridge. This points out the direction for the development of unique integrated electrodes for both hydrogen production and decontamination.     

On the modelling of stress-dilatancy behavior in weakly cemented sands

A comprehensive study on the stress-dilatancy behavior of cemented sand and its modeling is presented. The effect of confining pressure, relative density, and cement content on stress-dilatancy behavior are studied from the published experimental results and an additional series of experiments performed in this study. To facilitate a contrast and comparison of stress-dilatancy behavior between these datasets, a normalized stress ratio is proposed which removes the effect of mineralogy and morphology of parent sand. A set of key insights were obtained from this comparative study which aided in improving the stress-dilatancy relation; for example, the effect of initial conditions on stress-dilatancy behavior was found to be captured by the ratio of cohesion intercept (or tensile strength) and mean effective stress before shearing. The limitations of stress transformation, often used in modelling of cemented sand, were also systematically studied by a set of carefully designed experiments; it was found to be only applicable before gross yielding of cementation. After gross yielding, it is necessary to take in account of the breakage of bonds/cementation. The gross yield locus was identified from 70 experimental datasets and a cohesion/bond degradation model was formulated to model the stress-dilatancy behavior of cemented sand. The efficacy of stress-dilatancy relations (after including the gross yield locus and bond degradation behavior) is evaluated from the experimental results; the Rowe's stress-dilatancy relation was found to be most suitable with the proposed bond/cohesion degradation model.     

Crystal structure,chemical bonding and infrared reflectivity of microwave dielectric SrIn2O4 ceramics

In this work, the orthorhombic structured SrIn₂O₄ ceramics with a space group Pnam were synthesized by a solid-state reaction method. A high relative density (96.5%) coupled with excellent microwave dielectric properties (ε_r ∼ 12.3, Q × f = 96,900 GHz, τ_f ∼ −61.6 ppm/°C) were obtained as sintered 1300 °C for 4 h. The bond valence analysis demonstrates that the large sized cation Sr²⁺ exhibits a compressed state, while the In³⁺ exhibits a weaken rattling effect. The P-V-L chemical bond theory analysis indicates that the In-O bonds play a key role in affecting the dielectric loss. The thermally conductivity activation energy E_dc (0.94 eV) of SrIn₂O₄ was obtained by the dielectric spectroscopy, indicating that the E_dc was contributed to the double ionized oxygen vacancies. Furthermore, the intrinsic dielectric properties (ε_r ∼ 11.2, Q × f = 148,900 GHz) of SrIn₂O₄ were obtained by infrared reflectivity spectroscopy.     

Effects of ionic polarizability and crystal structure on microwave dielectric properties of RE2O3 (RE = La,Eu) ceramics with opposite τf and high Q

Hexagonal La₂O₃ and monoclinic Eu₂O₃ ceramics were prepared, and their microwave dielectric properties were investigated. La₂O₃ sintered at 1400 °C exhibited promising microwave dielectric properties of ε_r = 18.6, Q×f = 71,400 GHz, and a negative τ_f of − 35.1 ppm/°C, while Eu₂O₃ sintered at 1500 °C possessed relative lower ε_r and Q×f values of 17.9 and 35,000 GHz, respectively, with an abnormally positive τ_f of + 19.6 ppm/°C. The difference in their microwave dielectric properties is mainly due to lattice-induced strain, which can be characterized by bond valence. To investigate the degradation of RE₂O₃ (RE = La, Eu) ceramics in air, a series of La_2−xEu_xO₃ (x = 0.5, 1, and 1.5) ceramics were prepared. The results of the present study suggest that the introduction of Eu³⁺ effectively prevents the decomposition of La₂O₃.     

Significant improvement on quality factor in Bi3+/Ta5+ codoped Ce2Zr3-3xBi1.5xTa1.5xMo9O36 microwave dielectric ceramics with ultra-low sintering temperatures

Molybdenum oxide-based ceramics have attracted intense interest due to ultra-low sintering temperatures. However, low quality factors (Q × f) hinder their practical applications. Although Q × f can be improved by ions doping, the sintering temperature is greatly increased. Accordingly, it is still a challenge to obtain high Q × f ceramics sintered at ultra-low temperatures (<660 °C). Herein, (Bi_0.5Ta_0.5)⁴⁺ ions are utilized to tackle this issue in the Ce₂Zr₃(MoO₄)₉ ceramic as a prototype. Density and scanning electron microscope (SEM) results uncover good sintering states, and X-ray diffraction (XRD) results reveal the formation of solid solutions. Interestingly, the Ce-O bonds exhibit a dominant contribution to the bond ionicity (f_i), while Mo-O bonds play an important role in the lattice energy (U), the bond energy (E) and the thermal expansion coefficient (α). The remarkable increase of Q × f can be interpreted by the enhancement of the packing fraction and the mean U of Mo-O bonds. Moreover, the variations of the dielectric constant (ε_r) and the temperature coefficient of the resonance frequency (τ_f) can be explained by the variations of the intrinsic parameters. More interestingly, a negative correlation between Q × f and τ_f is first found. Typically, the CZ_0.98B_0.02 ceramic sintered at 650 °C exhibits optimum microwave dielectric properties: ε_r = 9.92, Q × f = 110,670 GHz, and τ_f = −19.20 ppm °C⁻¹. Notably, Q × f of the Ce₂Zr_2.94Bi_0.03Ta_0.03Mo₉O₃₆ (CZ_0.98B_0.02) ceramic is about 6 times larger than that of the matrix while retaining a low sintering temperature of 650 °C and a low ε_r of 9.92, making it a promising candidate for ultra-low temperature cofired ceramics (ULTCC) applications.     

喷涂角度对HVOF喷涂WC-10Co-4Cr涂层性能的影响

随着超音速火焰喷涂技术的迅速发展,工业上需要耐磨处理的工件的外形复杂性问题越来越突出,喷涂过程中需要多种角度进行喷涂.本文用GTV-K2喷枪喷涂WC-lOCo-4Cr,对不同角度喷涂的涂层性能进行了研究.结果表明:(1)60(°)以下,随着喷涂角度的增加,粉末的沉积效率逐渐升高;(2)60(°)到90(°)之间,沉积效...     

自由电子能带模型中的平均键能与费米能级

根据半导体自由电子能带模型，采用能带本征值的布里渊区求和而得到费米能级的方法，其结果有助于了解异质结带阶理论计算中所采用的参考能级“平均键能 Ｅｍ”的物理意义