Achieving simultaneously high selectivity and high rate in the coupling reaction of CO 2 with poorly reacting oxetanes remains a major challenge. Here is described the selective and nearly quantitative conversion of the coupling reaction of oxetanes with CO 2 into six-membered cyclic organic carbonates (COCs) when a binary metal-free system composed of commercially available alkyl borane and onium iodide salts is used under 10 bar CO 2 pressure between 90 and 110 o C. Kinetic investigations provide quantitatively the enthalpy and entropy of activation [ΔH ‡ = 6.7 ± 1.2 kcal/mol and ΔS ‡ = -57 ± 4 cal/(mol•K)] of the back-biting, cyclic formation reaction. In addition to forming borate complexes with the anions responsible for the CO 2 /oxetane coupling reaction, these alkyl boranes activate the cyclic ethers as unambiguously confirmed by DFT studies. Upon selecting onium salts other than iodide-based ones, in particular those with poor leaving ability, the process is driven towards chain growth and the formation of linear polycarbonates. This metal-free system also exhibits both versatility and an activity comparable to that of metal catalysts (turnover frequency values of 14 -124 h -1 ) for the synthesis of various fivemembered COCs from epoxides and CO 2 .

The stirring assisted brazing of SiCp/A356 composites with a large size of diameter in air was investigated. A first stirring was applied in brazing of the composites at 470°C. Then, it was continuously heated up to 510°C. Then, a secondary stirring with a large size (2.4mm) diameter was introduced in process. It was found that a composite joint reinforced with SiC particles can be obtained by stirring at 510°C. The experiment result shows that micro-hardness of joint brazed by 2nd stirring is obviously higher than that of joint brazed without 2nd stirring. Moreover, micro-hardness of the upper part of joint is slightly higher than that of the bottom part.

The effect of high welding speed on the macro-appearance and microstructure of joints were investigated in this paper. Experimental results are shown that bonding at one side of joint interface is obviously better than that at another side of bond interface. When welding speed is up to 72mm/min, a discontinuous joining interface is found. It is indicated that increasing of welding speed will be not in favor of disruption of oxide film on surface of composites.