The effects of Y2O3, La2O3 and Nd2O3 on the sintering, microstructure and mechanical properties of magnesia refractories were investigated. Addition of rare earth oxide (ReO) to magnesia refractories increases the bulk density, decreases the porosity and improves the mechanical strength of the refractories. The improved sinterability was attributable to the vacancies generation associated with the solid-solution reactions between MgO and ReO. In the samples with ReO, rare earth silicate phases form at magnesia grain boundaries, providing additional bonding between magnesia grains and between magnesia grains and matrix. Consequently, the samples with ReO showed much higher high temperature strengths than those without ReO.
The diamond honing oilstone was fabricated by hot-pressing at 550–650 °C and 25 MPa pressure for 4 min, using Cu–Sn based alloys as binder metal and uncoated or W-coated diamond grains as abrasive material. The microstructures and phase compositions of the honing oilstone were examined and analyzed by SEM and XRD. Effects of the oilstone composition, sintering temperature and volume fraction of diamond grains on the mechanical properties of diamond honing oilstone were investigated. The experimental results show that the interfaces between the diamond grains and metal matrices of all the oilstone samples are smooth and no defects are observed in the metal matrices. The bending strength and rockwell hardness of the honing stones increase with the sintering temperature increasing from 550 °C to 650 °C, and the bending strength decrease with the increase of diamond grains faction. The minimum grinding ratio is obtained as the diamond was W-coated, which can be attributed to the improved interfacial bonding derived from the W coating.
With the extended applications of hexagonal silicon carbide (h-SiC) in the various fields, particularly in the application of the electronic devices, more and more attentions have been focused on the micro structures as well as their physical properties of h-SiC surface. In this study, we have performed the first principal calculations to compare the formation energies of four typical defects (Vc, Vsi, CI and SiI) on the 4H-SiC (0001) surface as well as in the interior layers. Due to the surface reconstruction and the reduced lattice constrain, the optimized structures of the defects on/near the 4H-SiC (0001) surface are quite different from the ones in the deeper layers. The distinguished formation energies as function of chemical potential indicate that we may control the defects concentrations in different layers by tuning the environmental conditions. This theoretical work provides a significant understanding to the formation mechanism of the point defects on the 4H-SiC surface, and paves a way to the modification of the SiC surface via electron irradiation or ion implantation with micro-defects introduced.
The wetting and spreading of molten Cu, Ag and Ag-28Cu (wt.%) on the WC-Co cemented carbide were investigated by the sessile drop technique at different temperatures, and the interfacial behaviors of the metal/WC-Co couples were analyzed by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The experimental results showed that the testing temperature and the composition of drop can play a key role in the wetting and spreading, and the good wettability with contact angle of less than 40o can be obtained. Moreover, the strongest interactions can be produced between the molten Cu and the WC-Co substrate at the higher temperature of 1100 oC due to the strong interdiffusion and solid solution between Cu and Co, resulting in the lowest equilibrium contact angle of ~0o. In addition, smooth and clean interfaces, without formation of visible interfacial reaction layer, were observed in all the three wetting systems. This work may also provide guidance to brazing of the cemented carbide.
The coalescence of the fullerenes encapsulated in the host single-walled carbon nanotube (SWNT) with bend junction is explored theoretically by energy driven kinetic Monte Carlo (EDKMC) method. Despite the lower productivity of successful coalescence (with clear identified chiralitys), there is still a possibility to form the inner tube with bend junction which can copy the separated pentagon and heptagon from the host tube exactly with the chiralitys at the two sides clearly identified. The statistic to ~20 successfully coalesced inner tubes with bend junctions shows that the chiral angle differences (CAD) between the two sides > 20 o, which is determined by the minimization of the formation energies of the junctions. Therefore, the chirality distribution of the inner tube may be effectively narrowed by tuning the CAD of the bend host tube, which may provide an alternative way to the application of specific control to the chiralitys.
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