Preparation Of Si₃N₄-SiC?BN? Porous Ceramics By Foam-Gelcasting/Catalytic Nitridation Method

Porous ceramics are widely used as high-temperature insulation materials,gas and/or liquid filtration,purification separation,and catalyst supports due to their special pore-structure.In this work,firstly,high-purity?-Si₃N₄ powder was synthesized using high-purity Si powder as raw materials by a foam-gelcasting method and/or a molten salt-assisted foam-gelcasting method.The effects of the type and amount of molten salt,and the nitridation conditions on the as-synthesized powders were studied.Based on the density functional theory?DFT?calculation,the formation process of Si₃N₄ was also studied.Secondly,Si₃N₄,Si₃N₄/SiC and Si₃N₄/BN porous ceramics were respectively prepared by foam-gelcasting catalytic nitridation and in-situ catalytic nitridation method.Effects of catalysis routes,types,amount of catalyst addition and the reaction temperatures on the room-temperature physical properties of as-prepared porous ceramics,and the adsorption performance of Si₃N₄/BN foams over the drug wastewater were also investigated.Based on the results of DFT calculation,the mechanism of impurity Fe on low-temperature catalytic nitridation of Si₃N₄ was discussed.Finally,BN porous samples were fabricated via foam-gelcasting or direct gel-casting route using melamine and boric acid as the main starting materials,and the effects of the type and amount of dopants on the adsorption property of as-obtained porous products over drug wastewater were studied.To study the mechanism of drug adsorption,the models of the adsorption of drug molecules on the surface of BN and nano-graphite plates?NGP?were constructed based on density functional theory.The results indicated that:?1??-Si₃N₄ powders were prepared via foam-gelcasting or molten salt-assisted foam-gelcasting method using high-purity Si powders as raw materials,hexadecyl trimethyl ammonium bromide?CTAB?as foaming agent.The obtained powders with the relative content of?-Si₃N₄ in the final powders up to 97.5 wt%were synthesized at1623 K for 7 h with adding 0.15 wt%CTAB as foaming agent;DFT calculation results reveal that nitrogen atom enters the Si cell to occupy its octahedral vacancies can decrease the formation energy,increase stability of the structure,and then contribute to forming SiN₄ tetrahedron,finally promote Si₃N₄ formation.?2?Si₃N₄ porous ceramics were prepared via a combined foam-gelcasting and catalytic nitridation route using commercial Si powders as raw materials at 1473 K for 5h.When the solid content is 12.5 wt%,the porosity and compressive strength of the obtained porous samples are 93.8%and 1.1 MPa,and the thermal conductivity at 323 K is as low as 0.074 W/?m·K?.Moreover,synergistic catalytic nitridation can improve the mechanical properties of the final samples,and the catalytic effect of Fe is better than Fe?NO₃?₃·9H₂O.?3?Porous Si₃N₄/SiC ceramics were successfully prepared at 1473 K for 2 h,via a combined foam-gelcasting and catalytic nitridation route using commercial Si and SiC as the main raw materials.As-prepared Si₃N₄/SiC porous ceramics contained 71.5%porosity and had compressive strengths of 12.4 MPa.DFT calculations indicate that N₂molecules are adsorbed on Fe or FeSi catalysts leading to N?N bond length lengthened and the bonding strength weakened,ultimately promoting the low-temperature nitridation of Si.Compared with the conventional heating method,microwave heating method promotes the formation of Si₃N₄ and reduces the reaction temperature of Si to Si₃N₄ by about 100 K.?4?Si₃N₄/BN porous ceramics were prepared via a combined foam-gelcasting and catalytic nitridation route using Si powders,melamine and boric acid as raw materials.And the maximum adsorption capacities over ciprofloxacin?CIP?and atenolol?ATL?of Si₃N₄/BN porous samples contained 4 wt%of commercial Si obtained at 1523 K/5 h are25.3 mg/g and 21.9 mg/g.The maximum adsorption capacity of Si₃N₄/BN porous ceramics mainly depends on its specific surface area.?5?BN porous ceramics were prepared via a foam-gelcasting method using melamine and boric acid as raw materials.The normalized adsorption capacity of the as-prepared BN samples over the drugs CIP and ATL were respectively 1.90 mg/m² and 1.58 mg/m²,which are comparable or superior to the current adsorption materials in published literatures.DFT calculation results reveal that the adsorption is a process of selective adsorption of drug functional groups on the surface BN,and BN has a strong adsorption effect on hydroxyl?-OH?and carbonyl?-C=O?over the drugs.The drug wastewater adsorption of the doped BN porous sample is related to the type of dopants,among of which nano graphite plate?NGP?as the dopant has the best normalized adsorption capacity.Based on the DFT calculations,it can be attributed to the co-selective adsorption of NGP and BN.