【作者】 徐强；

【导师】 秦家强；

【作者基本信息】 四川大学 ， 材料学， 2021， 硕士

【摘要】 聚酰亚胺（PI）气凝胶是一种综合性能优异的高性能气凝胶材料,具有低的密度、高的孔隙率、大的比表面积、优异的机械性能、低的热导率和介电常数、良好的热稳定性等优点,受到了研究者们的广泛关注。制备PI气凝胶的主要方法包括超临界CO₂干燥法和冷冻干燥法。利用超临界CO₂干燥法制备的PI气凝胶通常呈现纳米纤维缠结的三维网络结构;冷冻干燥法制备的PI气凝胶通常具有无序的蜂窝状结构。PI气凝胶的性能及应用与其微观结构紧密相关,因此,对于PI气凝胶的微观结构的调控显得尤为关键。目前,PI气凝胶主要应用在保温隔热、油水分离、气体吸附、介电材料等领域,在其他领域的应用还有待拓展。本文通过特殊的定向冷冻方法,设计并调控冷冻工艺,制备得到了一系列微观结构有序的,各向异性的PI气凝胶。从PI气凝胶的结构设计出发,利用针状结构和层状结构的特点,分别与CNT和r GO复合制备了针状结构的PI/CNT复合气凝胶和层状结构的PI/r GO复合气凝胶,探索在电磁屏蔽材料和柔性压力传感器领域的应用。具体研究工作分为以下三个部分:（1）以水溶性聚酰胺酸盐作为前驱体,以水作为溶剂,利用径向定向冷冻方法,制备得到了有序的,各向异性的,径向结构的PI气凝胶。通过改变固含量,调控所得PI气凝胶的微观结构和性能。随着固含量的增加,PI气凝胶的微观结构由中心对称放射片状结构逐渐转变为中心对称针状结构,并且孔径逐渐减小,孔壁厚度逐渐增加。随着固含量的增加,PI气凝胶的密度增加,收缩率和孔隙率降低,力学性能提高。由于各向异性的微观结构,PI气凝胶表现出各向异性的力学性能,径向方向上的压缩强度和模量高于轴向方向。与常规冷冻方法制备的无序蜂窝状结构的PI气凝胶相比,径向结构的PI气凝胶具有更好的弹性和抗疲劳性能。PI-3.5气凝胶在轴向的热导率为35 m W m^-1 K^-1,在径向的热导率为52 m W m^-1 K^-1,表明各向异性的热传导性能。（2）以水溶性聚酰胺酸盐作为前驱体,以CNT作为导电材料,以水作为溶剂,利用径向定向冷冻方法,制备得到了有序的,各向异性的,径向结构的PI/CNT复合气凝胶,用作电磁屏蔽材料。通过调节固含量和CNT比例,进而调控所得PI/CNT复合气凝胶的微观结构和性能。PI/CNT复合气凝胶呈现中心对称针状结构,随着PI含量或固含量的增加,孔径更小且均匀。PI/CNT复合气凝胶表现出各向异性的力学性能,电性能和电磁屏蔽性能。PI/CNT复合气凝胶在径向方向的压缩强度和模量高于轴向方向。PI/CNT复合气凝胶在径向方向的电导率大于轴向方向。PI/CNT复合气凝胶在轴向方向的电磁屏蔽效率高于径向方向。当固含量为5.47 wt%时,PI/CNT复合气凝胶的密度均为36.2 mg/cm³,随着CNT比例增加,电磁屏蔽效率提高。当CNT的比例为66.7 wt%时,随着固含量的增加,电磁屏蔽效率提高,比屏蔽效率降低。PI/CNT复合气凝胶表现出以吸收占主导的屏蔽机理。（3）以水溶性聚酰胺酸盐作为前驱体,以r GO作为导电材料,以水作为溶剂,利用双向定向冷冻方法,制备得到了有序的,各向异性的大长径比蜂窝结构的PI/r GO复合气凝胶,用作柔性压力传感器。这种独特的大长径比蜂窝结构由定向排列的片层和相互连接的桥组成,是通过在双向温度梯度下适当控制冰晶的成核和生长而实现的。当r GO比例增加,PI/r GO复合气凝胶的电导率和力学强度提高。得益于独特的微观结构,PI/r GO复合气凝胶具有优异的弹性和抗疲劳性能。PI/r GO-10气凝胶传感器表现出高的灵敏度(1.33 k Pa^-1),超低的检测极限（3 Pa）,宽的检测范围（80%应变）,快的响应时间（60 ms）,优异的循环稳定性（超过1000次循环）。PI/r GO-10气凝胶传感器在恶劣的环境条件下也具有出色的传感性能。我们使用气凝胶传感器进行了多项人体运动检测,包括手腕脉搏、吹气、手指弯曲、肘部弯曲、行走和跑步。更多还原

【Abstract】 Polyimide（PI）aerogel is a high performance aerogel with excellent comprehensive properties.It has many advantages such as low density,high porosity,large specific surface area,excellent mechanical properties,low thermal conductivity,dielectric constant,and good thermal stability.The main methods to prepare PI aerogels include supercritical CO₂ drying and freeze-drying.The PI aerogel prepared by supercritical CO₂ drying usually presents a three-dimensional network structure of nanofiber entanglement.The PI aerogels prepared by freeze drying usually have disordered honeycomb structures.The properties and applications of PI aerogels are closely related to their microstructure.Therefore,the regulation of the microstructure of PI aerogels is particularly critical.At present,PI aerogels are mainly used in thermal insulation,oil and water separation,gas adsorption,dielectric materials and other fields,and the application in other fields needs to be expanded.In this paper,a series of microstructured ordered anisotropic PI aerogels were prepared through the special directional freezing method.Starting from the structural design of PI aerogels and using the characteristics of needle structure and lamellar structure,PI/CNT composite aerogels with needle like structure and layered PI/r GO composite aerogels were prepared with CNT and r GO respectively,and the applications in electromagnetic shielding materials and flexible pressure sensors were explored.The specific research work is divided into the following three parts:（1）An ordered,anisotropic,radial structure PI aerogel was prepared by radial directional freezing using water-soluble polyamic acid salt as precursor and water as solvent.The microstructure and properties of PI aerogels were controlled by changing the solid content.With the increase of solid content,the microstructure of PI aerogels gradually changed from the central symmetric radiate structure to the centrosymmetric needle like structure,and the pore size gradually decreased,and the thickness of the pore wall increased gradually.With the increase of solid content,the density of PI aerogels increased,shrinkage and porosity decreased,and mechanical properties increased.Because of the anisotropic microstructure,the PI aerogels exhibit anisotropic mechanical properties,and the compressive strength and modulus in the radial direction are higher than the axial direction.Compared with the PI aerogel prepared by conventional freezing method,the PI aerogel with radial structure has better elasticity and fatigue resistance.The axial thermal conductivity of PI-3.5 aerogel is 35 m W m^-1 K^-1,and the radial thermal conductivity is 52 m W m^-1K^-1,indicating anisotropic thermal conductivity.（2）Using water-soluble polyamic acid salt as precursor,CNT as conductive material and water as solvent,an ordered,anisotropic,radial structure PI/CNT composite aerogel was prepared by radial directional freezing method,which was used as electromagnetic shielding material.The microstructure and properties of PI/CNT composite aerogels were controlled by adjusting the solid content and CNT ratio.The PI/CNT composite aerogels exhibit a centrosymmetric needle like structure.With the increase of PI content or solid content,the pore size is smaller and even.PI/CNT composite aerogels exhibit anisotropic mechanical properties,electrical properties and electromagnetic shielding properties.The compressive strength and modulus of PI/CNT composite aerogels in radial direction are higher than that in axial direction.The conductivity of PI/CNT composite aerogels in radial direction is greater than that in axial direction.The electromagnetic shielding efficiency of PI/CNT composite aerogels in axial direction is higher than that in radial direction.When the solid content is 5.47 wt%,the density of PI/CNT composite aerogel is 36.2 mg/cm³.With the increase of CNT ratio,the electromagnetic shielding efficiency increases.When the proportion of CNT is 66.7wt%,with the increase of solid content,the electromagnetic shielding efficiency increases and the specific shielding efficiency decreases.PI/CNT composite aerogels showed absorption dominated shielding mechanism.（3）A PI/r GO composite aerogel with ordered and anisotropic large aspect ratio honeycomb structure was prepared by using bidirectional freezing method using water-soluble polyamic acid salt as precursor and r GO as conductive material and water as solvent.It was used as a flexible pressure sensor.This unique honeycomb structure with large aspect ratio is composed of oriented lamellae and interconnected bridges,which is realized by properly controlling the nucleation and growth of ice crystals under bidirectional temperature gradient.When r GO ratio increases,the conductivity and mechanical strength of PI/r GO composite aerogels increase.Thanks to its unique microstructure,PI/r GO composite aerogel has excellent elasticity and fatigue resistance.The PI/r GO-10 aerogel sensor showed high sensitivity(1.33 k Pa^-1),ultra-low detection limit（3 Pa）,wide detection range（80%strain）,fast response time（60 ms）,and excellent cycle stability（more than 1000 cycles）.PI/r GO-10 aerogel sensor also has excellent sensing performance under harsh environmental conditions.We used the aerogel sensor to detect a number of human movements,including wrist pulse,blow,finger bending,elbow bending,walking and running.更多还原