Jiale Zhang,Jianjun Chen,Jinyu Guo,Hao Chen,Longbo Xue,Chenyu Li
In this study, silicon nitride (Si 3 N 4 ) powders with high α-phase content and rod-like crystal morphologies were synthesized via self-propagating high-temperature synthesis (SHS) in a low-pressure nitrogen atmosphere (0.2 MPa), using silicon and Si 3 N 4 powders as reactants, with magnesium powder serving as an igniter. The influence of different ratios of silicon to silicon nitride on the phase composition, crystal morphology, and sintering activity of the synthesized Si 3 N 4 powders was systematically investigated. The results demonstrated that controlling the silicon-to-silicon nitride ratio under low-pressure conditions significantly affects the formation of α-phase Si 3 N 4 and the resulting microstructure of the powders. A low-pressure nitrogen atmosphere was adopted based on kinetic and thermodynamic analyses, providing a feasible environment for SHS. Notably, in situ formation of magnesium nitride (MgSiN 2 ) during the reaction not only acted as an effective sintering aid but also improved the thermal conductivity of the resulting Si 3 N 4 ceramic. The powder produced from a reaction using a raw material composition of 65% Si minimizes both silicon infiltration and melting to the greatest extent, and consequently, the sintered silicon nitride ceramics exhibit the highest thermal conductivity. The low-pressure environment effectively moderated the reaction rate and temperature, promoting the formation of fine α-phase Si 3 N 4 particles with rod-like morphologies. This innovative method offers a cost-effective and environmentally friendly approach for producing high-quality Si 3 N 4 powders with high sintering activity.
Zhensheng Shen,Feiping Long,Tian Ma,Huan Li,An Li,Qian Feng,Jiuyi Liu,Yingqiang Sun
Graphical Minimize to enhance : Heteropolyacids enhance bio-oil yield by catalyzing the hydrolysis of cellular compounds and reducing their nitrogen content. This is done by avoiding Maillard reactions between the intermediates during the hydrolysis of carbohydrates and proteins. Hydrothermal liquefaction (HTL) of microalgae for biofuel production is suffering from low bio-oil yield and high heteroatomic compositions owing to their low efficiency and selectivity to hydrolysis of cellular compounds. Hereby we report Keggin-type (Mo−V−P) heteropolyacids (HPAs)-catalyzed HTL of microalgae for efficient low-nitrogen biocrude production. The increases of reaction temperature, reaction time, and vanadium substitution degrees of HPAs are favorable to biocrude yield initially, whereas a significant decrease of biocrude yield is observed owing to the enhanced oxidation of carbohydrates above the optimum reaction conditions. The maximum biocrude yield of HPAs-catalyzed HTL of microalgae is 29.95 % at reaction temperature of 300 °C, reaction time of 2 h, and 5 wt% of HPA-4, which is about 19.66 % higher than that of control with 71.17 % less N-containing compounds, including 1,3-propanediamine, 1-pentanamine, and 2, 2’-heptamethylene-di-2-imidazoline than that of control. This work reveals that HPAs with Brønsted acidity and reversible redox properties are capable of both enhancing biocrude production via catalyzing the hydrolysis of cellular compounds and reducing their nitrogen content through avoiding the Maillard reactions between the intermediates of hydrolysis of carbohydrates and proteins. HPAs-catalyzed HTL is an efficient strategy to produce low N-containing biofuels, possibly paving the way of their direct use in modern motors.
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