【作者】 张勇；

【导师】 张林；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2017， 博士

【摘要】 气凝胶是一种新型的纳米多孔材料,具有密度低、孔隙率高、比表面积大、表面化学性质可调等许多优良的物理化学性能,可用作吸附材料、超级电容器材料、承重材料、保温隔热材料及惯性约束聚变(the Inertial Confinement Fusion,ICF)靶丸材料等,具有广泛应用前景。气凝胶主要包括无机气凝胶、有机气凝胶及碳气凝胶等。三聚氰胺甲醛(Melamine Formaldehyde,MF)气凝胶属于有机气凝胶,具有化学结构稳定、机械力学强度较高、反应单体官能度高、不易发生氧化反应而变色等优势。但同时由于MF溶胶-凝胶过程属于小分子反应体系,导致传统方法制备的MF气凝胶凝胶化时间长,所得气凝胶密度偏大,机械强度高而韧性差,不利于成型加工。近几年国内外对于MF气凝胶的研究进展缓慢,本论文通过在MF体系中引入淀粉、壳聚糖及石墨烯等制备高分子基改性的MF复合气凝胶,并对其制备工艺、改性机理与结构进行研究。首先,在MF体系中引入淀粉,制备得到淀粉/MF复合气凝胶,考察了反应物浓度、反应物配比、体系pH值、反应温度等参数对复合体系溶胶-凝胶过程、气凝胶微观形貌、理化特性等的影响,结果表明,淀粉在复合体系中以物理交联的方式与MF形成互穿三维网状多孔结构,复合体系中引入5 wt%淀粉时,凝胶化时间缩短至7 h,且能够降低凝胶在溶剂交换与超临界干燥过程中的体积收缩(最低收缩为0.35%),最终制得密度为38 mg/cm~3的复合气凝胶。与纯MF气凝胶相比,该复合气凝胶的孔径分布更加均匀,机械韧性得到明显改善,压缩形变可达79%。为进一步缩短凝胶化时间并增强凝胶网络结构,在MF体系中引入具有活性氨基的壳聚糖大分子,制备出密度低于10 mg/cm~3具有化学交联作用的互穿网络结构的壳聚糖/MF复合气凝胶。探讨了反应物浓度、反应物配比、体系pH值、反应温度等对溶胶-凝胶过程及气凝胶微观形貌与结构的影响规律。研究发现,当引入10 wt%的壳聚糖时,所得气凝胶的综合性能最好,凝胶时间可缩短至10 min,最低收缩率仅3.5%,比表面积高达392.2 m~2/g。与淀粉/MF气凝胶相比,反应物配比在较大范围内变化,均可获得机械韧性优良的壳聚糖/MF复合气凝胶。为了使气凝胶在保持优良机械韧性的同时具有更高的杨氏模量,考虑到石墨烯具有优异的力学性能,在MF/壳聚糖体系基础上进一步加入石墨烯,制备出壳聚糖/石墨烯/MF三元体系新型复合气凝胶。研究了石墨烯加入量对体系凝胶化时间、微观形貌、比表面积及机械性能的影响。石墨烯中含有大量活性官能团,可进一步加强复合体系中的化学交联作用,在保持低体积收缩率的同时进一步缩短凝胶化时间。石墨烯具有优异的力学性能,其杨氏模量约为1 TPa。因此,在壳聚糖/MF体系中引入石墨烯能有效提高气凝胶的杨氏模量。研究结果表明,该三元复合气凝胶具有良好机械韧性的同时,其杨氏模量较之壳聚糖/MF二元复合气凝胶得到进一步的提高。这种方法为制备综合性能优良的MF复合气凝胶提供了新的可能性。此外,将该三元复合气凝胶进行高温碳化处理,成功制得相应的碳气凝胶。研究表明,相应的碳气凝胶材料具有优良的电化学性能和吸附性能。更多还原

【Abstract】 Aerogel is a new kind of nanoporous material with many excellent physical and chemical properties,such as low density,high porosity,high specific surface area,adjustable surface chemical properties and so on.Due to these unique properties,the aerogels have been applied widely in many fields,such as adsorption materials,super capacitors,bearing materials,insulation materials and the inertial confinement fusion(ICF)target.Aerogels mainly include inorganic aerogels,organic aerogels and carbon aerogels.Melamine formaldehyde(MF)aerogels belong to organic aerogels.Compared with resorcinol formaldehyde aerogels,there are a few reports on MF aerogels in recent years.MF aerogel has its own advantages:chemical structure is stable;it does not change color because of oxidation reaction;the reaction monomer has high degree of functionality.But at the same time,the MF aerogel also has its disadvantages:the gelation time of MF aerogels prepared by using traditional method is too long;the density of MF aerogels is higher than that of other organic aerogels;the mechanical strength is higher,but the toughness is poor,these will went against enginery processing.In this paper,the natural polymer modified MF aerogels were prepared by introducing starch,chitosan and graphene into MF system,and the preparation process,structure and properties of the modified MF aerogels were studied.Starch/MF composite aerogels were prepared through physical cross-linking between MF and starch by adding starch into the MF system.The effects of the concentration of reactants,the ratio of reactants,the pH value of the system and the reaction temperature on the gelation process,the microstructure and the specific surface area of the aerogel were investigated.It was found that the gelation time was greatly reduced(7 h)when a small amount(5%)of starch was added to the reactants.Meanwhile,the prepared starch/MF composite aerogels with a density of 38 mg/cm~3 had a extremely low volume shrinkage(0.35%)during the solvent exchange and supercritical drying.Compared with the pure MF aerogel,the pore size distribution of the composite aerogel was more uniform,and the mechanical toughness was improved obviously(compressive deformation up to 79%).In order to further shorten the gelation time and strengthen the network structur e of gels,chitosan/MF composite aerogel with the density less than 10 mg/cm~3 was p repared by adding the chitosan containing amino groups into the MF system.The effects of reactant concentration,reactant ratio,pH value and reaction temperature on the sol-gel process and the microstructure and morphology were discussed.It was found that the comprehensive performance of chitosan/MF aerogels was best when the content of chitosan in reactants was 10 wt%.The gelation time was reduced to 10 min,and the prepared aerogel had many excellent properties,such as low shrinkage(3.5%),high surface area(392.2 m2/g)and best mechanical toughness.Compared with the starch/MF aerogel,the chitosan/MF composite aerogel prepared under different reactant ratios all had excellent mechanical toughness.In order to achieve the purpose that the composite aerogel has both higher Young’s modulus and good mechanical toughness,the chitosan/graphene/MF composite aerogel was prepared by adding graphene into the chitosan/MF system.The effects of the addition of graphene on the gelation time,morphology and specific surface area were studied.Graphene contains a large number of active groups,which could further enhance the chemical crosslinking of the composite system.It was found the gelation time was further shortened and the composite aerogels had lower volume shrinkage.Graphene has excellent mechanical properties and its young’s modulus is about 1 TPa.Therefore,the introduction of graphene in chitosan/MF system could effectively improve the young’s modulus of composite aerogels.Compared with chitosan/MF aerogel,the three element composite aerogel had good mechanical toughness and its young’s modulus was further improved.This method provides a new possibility for the preparation of MF composite aerogels with excellent comprehensive properties.The carbon aerogels were prepared by the thermal annealing process,and the structure and properties of the carbon aerogels were studied.The results showed that the carbon aerogels had excellent electrochemical properties and adsorption properties.更多还原