Observation of "ghost" islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

We observe "ghost" islands formed on terraces during homoepitaxial nucleation of GaN. We attribute the ghost islands to intermediate nucleation states, which can be driven into "normal" islands by scanning tunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface. The excess Ga also affect island number density: by increasing Ga coverage, the island density first decreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coverages above 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.

Synthesis Gas Generation by Chemical-Looping Reforming Using Ce-Based Oxygen Carriers Modified with Fe, Cu, and Mn Oxides

Chemical-looping reforming (CLR) is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. It involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from combustion air to the fuel. Composite oxygen carriers of cerium oxide added with Fe, Cu, and Mn oxides were prepared by co-precipitation and investigated in a thermogravimetric analyzer and a fixed-bed reactor using methane as fuel and air as oxidizing gas. It was revealed that the addition of transition-metal oxides into cerium oxide can improve the reactivity of the Ce-based oxygen carrier. The three kinds of mixed oxides showed high CO and H-2 selectivity at above 800 degrees C. As for the Ce-Fe-O oxygen carrier, methane was converted to synthesis gas at a H-2/CO molar ratio close to 2:1 at a temperature of 800-900 degrees C; however, the methane thermolysis reaction was found on Ce-Cu-O and Ce-Mn-O oxygen carriers at 850-900 degrees C. Among the three kinds of oxygen carriers, Ce-Fe-O presented the best performance for methane CLR. On Ce-Fe-O oxygen carriers, the CO and H-2 selectivity decreased as the Fe content increased in the carrier particles. An optimal range of the Ce/Fe molar ratio is Ce/Fe > 1 for Ce-Fe-O oxygen carriers. Scanning electron microscopy (SEM) analysis revealed that the microstructure of the Ce-Fe-O oxides was not dramatically changed before and after 20 cyclic reactions. A small amount of Fe3C was found in the reacted Ce-Fe-O oxides by X-ray diffraction (XRD) analysis.

板栗壳对水中Cu~(2+)吸附性能的研究

研究食品加工剩余物板栗壳对水中Cu2+的吸附性能,为其用于含铜废水的处理提供理论依据。【方法】研究吸附质溶液pH、Cu2+质量浓度、吸附剂用量、粒径、吸附温度和时间对板栗壳吸附Cu2+效果的影响,探讨吸剂和吸附剂循环利用次数对解吸和再生的影响;并采用穿透曲线和洗脱曲线对动态吸附进行了分析。【结果】吸附质溶液pH值为6、Cu2+起始质量浓度为20 mg/L、吸附剂粒径为0.25 mm时的吸附效果较好,该吸附为放热过程,升高温度虽然可以加快吸附进程,但却降低了吸附量和去除率。Na+和Ca2+对Cu2+的解吸置换能力较弱,0.1mol/L HCl可使96.1%的Cu2+得以解吸回收。通过Thomas模型预测,在固定床柱吸附条件下饱和吸附量为10.94mg/g。【结论】板栗壳对水中Cu2+的吸附性能较好,因而具有很好的应用前景。