Narrow Results

In the present work, a series of samples were prepared by pressureless sintering from the starting materials of Si₃N₄ (α and/or β phases), SiO₂ and Li₂CO₃. The phase transformation was studied with emphasis on the influence of sintering temperature and Li₂CO₃ content. Related phase transformation was observed and analyzed by SEM and XRD and probable mechanisms were given, which will be helpful for the further explanation to the oxidation mechanism of Si₃N₄ based ceramics at elevated temperature.

Reactions of the carbon-chain radicals are of great importance in the combustion and astrophysical processes. The kinetics of the butadiynyl radical, C₄H, has received recent attention. While there has been sufficient knowledge concerning the oxidation of the ethynyl radical, C₂H, oxidation of the higher even-numbered members C_2nH (n > 1) is hardly known. In this paper, to enrich the C₄H-chemistry, we report the first study of the oxidation mechanism of C₄H. At the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level, the potential energy surface (PES) survey is presented covering various product channels P₁(CO + HC₃O) (-152.7 kcal/mol), P₂(C₃H + CO₂) (-117.9), P₃(HCO + C₃O) (-108.5), P₄(HC₄O+³O) (-45.2), and P₅(OH + C₄O) (-33.2) accompanied by the master equation rate constant calculations. Despite the similarity in the PES, the kinetics of C₄H+³O₂ differs dramatically from that of the analogous C₂H+³O₂ reaction. For the C₄H+³O₂ reaction, the O-abstraction product P₄(HC₄O+³O) is almost the exclusive product, whereas the lowest C,O-exchange product P₁(CO + HC₃O) and other products have little importance. By contrast, the C₂H+³O₂ reaction favors the C,O-exchange product HCO + CO. Being overall barrierless and mainly associated with the molecular → atomic oxygen conversion, the C₄H+³O₂ reaction should play an important role in the soot formation and interstellar chemistry where C₄H is involved.

The oxidation mechanism of diethyl ethers by NO₂ was carried out using density functional theory (DFT) at the B3LYP/6-31+G (d, p) level. The oxidation process of ether follows four steps. First, the diethyl ether reacts with NO₂ to produce HNO₂ and diethyl ether radical with an energy barrier of 20.62 kcal ⋅ mol^-1. Then, the diethyl ether radical formed in the first step directly combines with NO₂ to form CH₃CH(ONO)OCH₂CH₃. In the third step, the CH₃CH(ONO)OCH₂CH₃ was further decomposed into the CH₃CH₂ONO and CH₃CHO with a moderately high energy barrier of 32.87 kcal ⋅ mol^-1. Finally, the CH₃CH₂ONO continues to react with NO₂ to yield CH₃CHO, HNO₂ and NO with an energy barrier of 28.13 kcal ⋅ mol^-1. The calculated oxidation mechanism agrees well with Nishiguchi and Okamoto's experiment and proposal.