Structural regulation and polishing performance of dendritic mesoporous silica (D-mSiO2) supported with samarium-doped cerium oxide composites

Ceria (CeO₂) particles are prevalent polishing abrasive materials. Trivalent lanthanide ions are the popular category of dopants for enriched surface defects and thus improved physicochemical properties, since they are highly compatible with CeO₂ lattices. Herein, a series of dendritic-like mesoporous silica (D-mSiO₂)-supported samarium (Sm)-doped CeO₂ nanocrystals were synthesized via a facile chemical precipitation method. The relation of the structural characteristics and chemical mechanical polishing (CMP) performances were investigated to explore the effect of Sm-doping amounts on the D-mSiO₂/Sm_xCe_1?xO_2?δ (x = 0–1) composite abrasives. The involved low-modulus D-mSiO₂ cores aimed to eliminate surface scratch and damage, resulting from the optimized contact behavior between abrasives and surfaces. The trivalent cerium (Ce³⁺) and oxygen vacancy (V_O) at CeO₂ surfaces were expected to be reactive sites for the material removal process over SiO₂ films. The optimal oxide-CMP performances in terms of removal efficiency and surface quality were achieved by the 40% Sm-doped composite abrasives. It might be attributed to the high Ce³⁺ and V_O concentrations and the enhancement of tribochemical reactivity between CeO₂SiO₂ interfaces. Furthermore, the relationship between the surface chemistry, polishing performance as well as the actual role in oxide-CMP of the D-mSiO₂/Sm_xCe_1?xO_2?δ abrasives were also discussed.     

Co?N graphene encapsulated cobalt catalyst for H2O2 decomposition under acidic conditions

Hydrogen peroxide (H₂O₂) has been listed as one of the 100 most important chemicals in the world. However, huge amount of residual H₂O₂ is hard to timely decomposed into O₂ and H₂O under acidic condition, easily resulting in explosion hazard. Here, we reported a core–shell structure catalyst, that is graphene with Co N structure encapsulated Co nanoparticles. Co N graphene shell serves as the active site for the H₂O₂ decomposition, and Co core further enhance this decomposition. Benefiting from it, the H₂O₂ decomposition were close to 100% after 6 cycles without pH adjustment, which increased 6 orders of magnitude compared with no catalyst. At the same time, the O₂ generation reached 99.67% in 2 h with little metal leaching, and ·OH has been greatly inhibited to only 0.08%. This work can cleanly remove H₂O₂ with little deep oxidation and protect the process of H₂O₂ utilization to achieve a safer world.     

Core–Shell Photoanodes for Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water splitting into hydrogen and oxygen is a promising solution for the conversion and storage of solar energy. Because sluggish water oxidation is the bottleneck of water splitting, the design and preparation of an efficient photoanode is intensively investigated. Currently, all known photoanode materials suffer from at least one of the following drawbacks: ① low carriers separation efficiency; ② sluggish surface water oxidation reaction; ③ poor long-term stability; ④ insufficient water adsorption and gas desorption. Core–shell configurations can endow a photoanode with improved activity and stability by coating an overlayer that plays energetic, catalytic, and/or protective roles. The construction strategy has an important effect on the activity of a core–shell photoanode. Nonetheless, the mechanism for the improvement of performance is still ambiguous and is worthy of a closer examination. In this review, the successes and challenges of core–shell photoanodes for water oxidation, focusing on synthesis strategies as well as functionalities (facilitating carrier separation, surface reaction promotion, corrosion prevention, and bubble detachment) are explored. Finally, the perspectives of this class of materials in terms of new opportunities and efforts are discussed.     

Activated Carbon Supported CaO from Waste Shells as a Catalyst for Biodiesel Production

In the wake of increasing environmental constraints, this work is aimed at developing a catalyst purely prepared from waste biomass source as the raw material. The catalyst is investigated for its applicability in transesterification of vegetable oil with the objectives: (i) to use waste shells of mollusk as raw material for the preparation of activated carbon and CaO; (ii) to use it as heterogeneous catalyst in the transesterification of waste cooking oil; (iii) to optimize the different parameters affecting the transesterification reaction; and (iv) to study its reusability. Under optimized conditions it was observed that a conversion >90% was possible and the catalyst could be reused five times with a slight loss in activity. This study indicates that the biomass source could also be used as a potential raw material in the synthesis of environmentally benign catalysts.     

Electro-magneto temperature-dependent vibration analysis of functionally graded-carbon nanotube-reinforced piezoelectric Mindlin cylindrical shells resting on a temperature-dependent,orthotropic elastic medium

Vibration smart control analysis of a temperature-dependent functionally graded-carbon nanotube-reinforced piezoelectric cylindrical shell embedded in an orthotropic elastic medium is investigated. The mixture law is used for obtaining the material properties of the structure. The structure is subjected to a 2D magnetic field. Considering the first-order shear deformation theory, the motion equations are obtained. Based on an analytical method and differential quadrature method, the frequency is calculated. The effects of applied voltage, magnetic field, volume percent, and distribution type of carbon nanotubes, temperature, orthotropic elastic medium, and length to radius ratio of the shell are shown on system frequency.     

Numerical evaluation and the effects of geometrical and operational parameters on thermal performance of the shell and double coil tube heat exchanger

Heat exchangers are extensively used in various industries. In this study, the impact of geometric and flow parameters on the performance of a shell and double helical coil heat exchanger is studied numerically. The investigated geometric parameters include external coil pitch, internal coil pitch, internal coil diameter, and coil diameter. The influences of considered geometrical parameters are analyzed on the output temperature of the hot and cold fluid, convective heat transfer coefficient, pressure drop, and average Nusselt number. Water is considered as working fluid in both shell and tube. As an innovation, double helical coils are used instead of one in the heat exchanger. To compare the obtained results accurately, in each section, the heat transfer area (coil outer surface) is kept constant in all models. The results show that the geometrical parameters of double helical coils significantly affect the heat transfer rate.     

光学球罩数控抛光工艺关键技术研究

光学球罩被广泛应用于各类光电系统中,其面形精度制约着光电系统的成像质量。随着数控技术的发展,数控加工技术逐渐成为目前光学加工行业的主流发展方向。基于高质量的铣磨表面,为了能够在准球心高速抛光工艺中实现等去除量加工,本文对同步抛光技术进行了理论分析及数学模型推导,并通过实验证明,同步抛光技术可以有效地提升加工效率,获得高质量的光学表面。     

Room temperature response and enhanced hydrogen sensing in size selected Pd-C core-shell nanoparticles: Role of carbon shell and Pd-C interface

In the present work, the effect of carbon shell around size selected palladium (Pd) nanoparticles on hydrogen (H₂) sensing has been studied by investigating the sensing response of Pd-C core-shell nanoparticles having a fixed core size and different shell thickness. The H₂ sensing response of sensors based on Pd and Pd-C nanoparticles deposited on SiO₂ and graphene substrate has been measured over a temperature range of 25 °C–150 °C. It is observed that Pd-C nanoparticle sensor shows higher sensitivity with increase in shell thickness and faster response/recovery in comparison to that of Pd nanoparticle samples. Pd-C nanoparticles show room temperature H₂ sensitivity in contrast to Pd nanoparticles which respond only at higher temperatures. Role of carbon shell is also understood by investigating H₂ sensing properties of Pd and Pd-C nanoparticles on graphene substrates. These results show that higher catalytic activity and electronic interaction at Pd-C interface, a complete coverage and protection of Pd surface by carbon and presence of structural defects in nanoparticle core are important for room temperature and higher sensing response.     

三叶膨胀管换热器壳程强化传热的数值研究

三叶膨胀管是一种新型强化传热管,针对纵向流换热器特点,设计了三种不同管束结构参数的三叶膨胀管自支撑纵向流换热器。应用FLUENT软件及Realizable k-ε湍流模型,对三种不同结构参数的三叶膨胀管换热器壳程强化传热特性展开了数值模拟,并通过与实验数据的对比,验证了计算模型的可靠性。计算了不同壳程介质流速下,三叶膨胀管换热器壳程的换热系数与压降值,并获得了壳程流体流线以及相应的温度场、速度场和二次流分布图。结果发现,在壳程水流速一致的情况下,管束横向间距越大的三叶膨胀管换热器,壳程拥有更高的综合换热性能和更低的压降值,但相应地,换热系数也更低。流场分析显示,壳程流体流线呈现出三维纵向旋流形态,二次流的出现改变了速度场和温度场分布,二次流的强度随着管束横向间距的减小而增大。     

弹性地基处周期性圆形钢筋混凝土管道振动特性分析

针对弹性地基处周期性钢筋混凝土管道的波动特性,基于声子晶体理论和Flugge壳体理论,建立了圆形管道环径向轴对称波动微分方程,利用传递矩阵法建立了相邻胞元间的传递矩阵,数值分析了周期性管道结构的能带特性。结果表明,振动波在传播过程中存在禁带域和通带域,弹性地基对弯曲波在特定频率范围内的传播具有抑制作用,长度比的变化对周期性圆形混凝土管道禁带的幅值、宽度和个数影响显著,因此可通过调整结构尺寸参数改变结构中波的传播特性。