Tribological Impact of SiC Encapsulation of Released Polycrystalline Silicon Microstructures

A method for coating released polysilicon microstructures with thin, uniform and conformal coatings of SiC derived from the single source precursor, 1,3-disilabutane (DSB) has been developed. This coating method has been successfully applied to micromechanical test devices which allow evaluation of friction and wear properties of the coating. Here, data on the coefficient of static friction of SiC coatings produced from DSB is presented. Also, a comparative wear study for devices which have been oxidized, treated with an anti-adhesion coating, and SiC coated is shown. Wear is examined by scanning electron microscopy (SEM) on devices which have been cycled repetitively under a nominal load. It is found that the application of a few nanometers-thin SiC coating provides exceptional wear resistance as well as significant reduction in friction on the microscale.     

二次硬化合金钢中多组元强化相M2C碳化物的粗化动力学研究

高Co—Ni超高强度合金钢在482℃等温回火处理，首先析出渗碳体，回火30min后，针状合金碳化物M2C以共格方式从基体α—Fe中析出．回火时间延长到10h以上，M2C仍与基体间保持良好的共格关系，该共格关系会随过时效而失去．高分辨电镜观察表明，M2C的尺寸r和回火时间t接近r^2αt关系，明显偏离扩散控制的三次方关系的经典LSW粗化理论，对相近的合金钢材料有关M2C析出动力学实验数据进行分析得到相似的结论．这不仅和第二相碳化物的形态有关，还与多组元扩散的体系相关，表明经典理论不完全适合多组分的碳化物粗化行为从经典扩散理论出发，考虑针状沉淀相多元扩散问题，得到的针状相粒子粗化方程与实验结果吻合.     

Effect of coral sand powders and seawater salinity on the impact mechanical properties of cemented coral sand

Coral sand is one kind of the important building materials in coral reef engineering practice. The use of cement as a stabilizing agent can significantly improve the mechanical properties of coral sands and is widely applied in the subbase engineering construction in coral reef islands. Cement-stabilized coral sand structures may contain high contents of fine coral particles and salinity because of the high crushability of coral sands and the existence of seawater surrounding them. In this study, the effects of coral sand powders and seawater salinity on the dynamic mechanical properties of cemented coral sand (CCS) were investigated through the split Hopkinson pressure bar (SHPB) tests and Scanning Electron Microscope (SEM) analysis. It was found that the strength (i.e., the peak stress) of CCS specimens increased firstly and then decreased with the increase of powder content. The specimens reached the maximum peak stress when 3% powder content was included. The initial improvement of CCS strength was attributed to the pore-filling effect of coral powders, namely, the micro pores of the CCS specimens could be more effectively filled with higher percentages of coral powders being used in the experiments. However, excessive coral powders resulted in the reduction of specimen strength because these powders could easily be cemented into agglomerates by absorbing water from the specimens. These agglomerates could reduce the cementation strength between the coarse coral particles and the cement. Meanwhile, the peak stress of CCS specimens was found to be negatively correlated with the average strain rate and the ultimate strain. The degree of specimen fracture was found to be correlated with the amount of specific energy absorption during the tests. Furthermore, the “sulfate attack” caused by the inclusion of salinity of water had different influences on the CCS specimens with different coral powder contents. The ettringite and gypsum produced in “sulfate attack” could fill the pores and lead to cracking of the specimens, significantly affecting the specimen strength.     

Studies on influence of hydrogen and carbon monoxide concentration on reduction progression behavior of molybdenum oxide catalyst

Temperature programmed reduction (TPR) analysis was applied to investigate the chemical reduction progression behavior of molybdenum oxide (MoO₃) catalyst. The composition and morphology of the reduced phases were characterized by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM). The reduction progression of MoO₃ catalyst was attained with different reductant types and concentration (10% H₂/N₂, 10% and 20% CO/N₂ (%, v/v)). Two different modes of reduction process were applied. The first approach of reduction involved non-isothermal mode reduction up to 700 °C, while the second approach of reduction involved the isothermal mode reduction for 60 min at 700 °C. Hydrogen temperature programmed reduction (H₂-TPR) results showed the reduction progression of three-stage reduction of MoO₃ (Mo⁶⁺ → Mo⁵⁺ → Mo⁴⁺ → Mo⁰) with Mo⁵⁺ and Mo⁴⁺. XRD analysis confirmed the formation of Mo₄O₁₁ phase as an intermediate phase followed by MoO₂ phase. After 60 min of isothermal reduction, peaks of metallic molybdenum (Mo) appeared. Whereas, FESEM analysis showed porous crater-like structure on the surface cracks of MoO₂ layer which led to the growth of Mo phase. Meanwhile, the reduction of MoO₃ catalyst in 10% carbon monoxide (CO) showed the formation of unstable intermediate phase of Mo₉O₂₆ at the early stage of reduction. Furthermore, by increasing 20% CO led to the carburization of MoO₂ phase, resulted in the formation of Mo₂C rather than the formation of metallic Mo, as confirmed by XPS analysis. Therefore, the presented study shows that hydrogen gave better reducibility due to smaller molecular size, which contributed to high diffusion rate and achieved deeper penetration into the MoO₃ catalyst compared to carbon monoxide reductant. Hence, the reduction of MoO₃ in carbon monoxide atmosphere promoted the formation of Mo₂C which was in agreement with the thermodynamic assessment.     

Storage and permeation of hydrogen molecule,atom and ions (H+ and H−) through silicon carbide nanotube; a DFT approach

The barriers for the encapsulation and decapsulation of hydrogen ions (cationic hydrogen and hydride), atom, and molecule through silicon carbide nanotube are thoroughly studied. DFT method is selected to measure the kinetic barriers for the passage of hydrogen atom, ions and molecule through nanotube via scanning potential energy surface. The kinetic barriers for the passage (encapsulation and decapsulation) of hydrogen are very important to understand the mechanism of hydrogen storage and release. The barriers for the permeation of H, H⁺ and H^? across SiC nanosheet are lower compared to hydrogen molecule (H₂). The exohedral and endohedral adsorption of hydrogen ions (cation and anion), atom and exohedral hydrogen molecule on silicon carbide are exothermic in nature. Whereas the encapsulation of hydrogen molecule in silicon carbide is endothermic. Electronic properties are analyzed through measurement of energy gap between highest occupied and lowest unoccupied molecular orbitals gap (G_H-L) and the density of state (DOS) spectra. The G_H-L analysis reveals that endohedral complexes have more pronounced effect on electronic properties compared to exohedral complexes. The SiC nanotube has highly favorable properties for storage and release of hydrogen ions, and atom.     

The influence of the processing route on the fragmentation of (Nb,Ti)C stringers and its role on mechanical properties and hydrogen embrittlement of nickel based alloy 718

The nickel-base superalloy 718 is a precipitation hardened alloy widely used in the nuclear fuel assembly of pressurized water reactors (PWR). However, the alloy can experience failure due to hydrogen embrittlement (HE). The processing route can influence the microstructure of the material and, therefore, the HE degree. In particular, the size and distribution of the (Nb,Ti)C particles can be affected by the processing. In this regard, the objective of this work was to analyze the influence of cold and hot deformation processing routes on the development of the microstructure, and the consequences on mechanical properties and hydrogen embrittlement. Tensile samples were hydrogenated through gaseous charging and compared to non-hydrogenated samples. Characterization was performed via scanning and transmission electron microscopies, as well as electron backscattered diffraction. The processing was effective to promote significant variations in average grain size and length fraction of special Σ3ⁿ boundaries, as well as reduction of average (Nb,Ti)C particle size, being these changes more intense for the cold-rolled route. For the mechanical properties, on one side, the cold-rolled route presented the highest increase in ductility for non-hydrogenated samples, while, on the other side, had the highest degree of embrittlement under hydrogen. This dual behavior was attributed to the interaction of hydrogen with the (Nb,Ti)C particles and stringers and its ensuing influence on the fracture processes.     

Super strong B4C ceramics prepared by dynamic sinter forging

In this study, super strong B₄C ceramic was prepared by dynamic sinter forging (DSF). The flexural strength and hardness of the sample are 935 MPa and 39.2 GPa, respectively. The strength is much higher than any value previously reported, while the hardness is one of the highest ever reported. The superior mechanical properties are attributed to the particle rearrangement and plastic deformation generated by the oscillatory pressure during dynamic hot forging. The current results suggest that DSF is a promising technique for preparing high-performance B₄C ceramics.     

从硬质合金典型材质发展看中国硬质合金工业的技术进步

中国是世界硬质合金的生产与消耗大国,中国硬质合金工业的技术现状令世人关注.本文从硬质合金典型材质与关键原料制备技术及其微观组织结构与性能特性等方面,展示了近年来中国硬质合金工业的主要技术进展.包括紫钨工艺制备的超细、纳米W粉与WC粉,紫钨工艺与WC-Co纳米复合粉工艺制备的超细晶硬质合金,超粗晶硬质合金以及原位激发自润...