【作者】 罗朝霞；

【导师】 徐樑华； 刘力；

【作者基本信息】 北京化工大学 ， 材料科学与工程， 2022， 博士

【摘要】 介电弹性体（DE）是在外部电压作用下,发生内部电荷分离,同时电荷积聚在上下两层表面,形成Maxwell静电力,使弹性体薄片在平面方向发生横向扩展,而在厚度方向发生压缩的一类新型功能材料。由于介电弹性体能将电能转化为机械能,具有发生水平大形变的驱动能力,在人工肌肉、智能假肢、电子皮肤等方面有着重要应用。为了使驱动应变最大化,需要获得较大的电压感应应变。现有的大形变均发生在高压电场条件下（≥100k V/mm）,存在很大的安全隐患。而在低压电场下发生较大的形变非常困难,存在巨大挑战。本文将一维（1D）无机纤维[包括镀银SiO₂纤维、高介电钛酸钡（BaTiO₃）纤维和高电导率碳纳米管（CNTs）]和/或二维（2D）无机纳米片层材料[包括过渡金属钛碳化物（MXene）与石墨烯（GR）]掺杂到聚氨酯基体中,系统研究并分析了掺杂填料对复合材料宏观物理机械性能、电性能及电驱动性能的影响,为制备具有高介电常数、低模量、低介电损耗、高击穿强度、大驱动应变的高性能介电弹性体提供依据。论文主要研究内容如下:（1）通过溶胶凝胶结合静电纺丝的方法制备了SiO₂纳米纤维,采用聚多巴胺（PDA）对SiO₂纤维进行表面改性,制备了SiO₂/PDA纤维,进一步通过银镜反应制备了表面镀银SiO₂/PDA@Ag纤维。探究了三种纤维分别与TPU基体复合制备的介电复合材料的力学性能、介电性能的变化规律。结果表明,填加SiO₂、SiO₂/PDA、SiO₂/PDA@Ag纤维的TPU复合材料的模量得到明显提高,当纤维用量达20 wt%时,存在短纤维增强复合材料的应力屈服现象。TPU复合材料的介电常数随着SiO₂、SiO₂/PDA、SiO₂/PDA@Ag纤维用量的增加而增大,介电损耗较低,电力学敏感因子较小。其中填加1 wt%SiO₂/PDA@Ag纤维的TPU介电弹性体的电力学敏感因子比纯TPU基体增加了11%,电致形变能力相对增强。（2）通过静电纺丝的方法制备了BaTiO₃（BT）纳米纤维,使用PDA对其表面改性,制备了BT@PDA纤维。利用盐酸溶液刻蚀制备了二维MXene材料。探究了高介电常数BT@PDA纤维和高导电少层MXene共掺杂制备的TPU介电复合材料的介电性能和力学性能。结果表明,在协同效应作用下,共掺杂MXene/BT@PDA/TPU复合材料的介电常数比单一填加BT@PDA填料的介电常数明显提高,电力学敏感因子显著增强。MXene纳米片引起的界面极化和微电容结构,以及BT@PDA纤维引起的界面极化是提高共掺杂体系介电常数的主要原因。具有叠层结构的MXene纳米片,在含量0.5 wt%以内时,可抑制漏电流的传导,使TPU介电复合材料获得较高击穿强度。10³Hz时,0.5 wt%MXene/5 wt%BT@PDA/TPU复合材料的电力学敏感因子为11.38,介电常数（12.52）是纯TPU（ε=6.72）的1.86倍,具有相对良好的大变形能力。（3）研究了碳纳米管（CNTs）和CNTs@PDA分别与MXene共掺杂制备的MXene/CNTs/TPU、MXene/CNTs@PDA/TPU介电复合材料的力学性能及介电性能。结果表明,引入少量高导电性的MXene和CNTs、CNTs@PDA纤维,可在TPU基体中形成双掺杂填料网络结构,并且不增加复合材料小变形下的杨氏模量。发现MXene片层在TPU基体中所形成的微电容器结构,可强化界面极化,其少量填加时可获得适宜的击穿强度,抑制漏电流,对保护纵向电击穿发挥作用。共掺杂MXene/CNTs/TPU复合材料中,随着MXene/CNTs用量的增加,电力学敏感因子显著提升。当共掺杂体系中的MXene用量为1 wt%、CNTs用量为0.8 wt%时,TPU复合材料的电力学敏感因子达33.46,具有优异的机电效率值。（4）设计并研究了同样具有二维片层结构的石墨烯（GR）与MXene共掺杂制备MXene/GR/TPU介电复合材料,利用片层结构的纵向电流阻滞效应来获得较佳的击穿强度。结果表明,MXene/GR/TPU介电复合材料的断裂伸长率和拉伸强度随着MXene用量的增加而下降。与MXene/CNTs/TPU复合材料相比,两种片层材料共掺杂制备的MXene/GR/TPU复合材料,介电常数只少量增加。发现GR/TPU具有较高的击穿强度,MXene/GR/TPU介电复合材料的介电常数略有提升,击穿强度较高。其中,0.5 wt%MXene/1 wt%GR/TPU介电复合材料的电力学敏感因子为3.54,具有相对较好的大变形能力。更多还原

【Abstract】 Dielectric elastomer（DE）is a new type of functional material under the action of external voltage,which has internal charge separation and charge accumulation on the upper and lower surfaces to form Maxwell electrostatic force,which makes the elastomer sheet expand laterally in the plane direction and compress in the thickness direction.Because dielectric elastomer can convert electrical energy into mechanical energy and has the driving ability of large horizontal deformation,it has important applications in artificial muscle,intelligent prosthesis,electronic skin and so on.In order to maximize the driving strain,it is necessary to obtain a large voltage induced strain.However,the existing large deformation occurs under the condition of high-voltage electric field（100 k V/mm）,which has great potential safety hazards.However,it is very difficult to produce large deformation under low electric field,which is a great challenge for the current research.In this thesis,one-dimensional silica（SiO₂）fibers one-dimensional（1D）inorganic fibers[including silver plated SiO₂fibers,high dielectric barium titanate（BaTiO₃）fibers and high conductivity carbon nanotubes（CNTs）]and/or two-dimensional（2D）inorganic nano lamellar materials[including transition metal titanium carbide（MXene）and graphene（GR）]were doped into polyurethane matrix.The effects of doped fillers on the macro physical and mechanical properties,electrical properties and electrical driving properties of the composites were systematically studied and analyzed.It provides a basis for the preparation of high-performance dielectric elastomers with high dielectric constant,low modulus,low dielectric loss,high breakdown strength and large driving strain.The main research contents of this paper are as follows:（1）1D SiO₂fibers were prepared by sol-gel and electrospinning method.The surface of SiO₂fibers was modified by polydopamine,and SiO₂/PDA fibers were prepared.Furthermore,Silver coated SiO₂/PDA@Ag Fiber was prepared by silver mirror reaction.Mechanical and dielectric properties of the three dielectric composites were explored,after the three fibers were compounded with TPU matrix.The results show that the modulus of TPU composites with SiO₂,SiO₂/PDA and SiO₂/PDA@Ag fibers has been significantly improved.When the fiber content reaches 20 wt%,it shows the stress yield characteristics of short fiber reinforced composites on the curve.The dielectric constant of TPU composites increases with the increase of SiO₂fiber content,the dielectric loss is low.For SiO₂/TPU,SiO₂/PDA/TPU,SiO₂/PDA@Ag/TPU,the three types of TPU composites,due to the low dielectric constant and high Young’s modulus,the electrical sensitivity factor is small.However,the electrical sensitivity factor of 1 wt%SiO₂/PDA@Ag/TPU dielectric elastomer is 11%higher than that of pure TPU matrix,and the electric deformation ability is relatively increased.（2）1D BaTiO₃（BT）nanofibers were prepared by electrospinning,and their surface was modified by PDA,and BT@PDA fiber was prepared.Two dimensional MXene materials were prepared by etching in hydrochloric acid solution.The dielectric and mechanical properties of TPU dielectric composites prepared by high dielectric constant BaTiO₃（BT）fiber and high conductivity two-dimensional few-layer MXene were studied.The results show that the dielectric constant of codoped MXene/BT@PDA/TPU composite is significantly higher than that of single BT@PDA filler,and the electrical sensitivity factor is significantly increased.Interface polarization and micro-capacitor structure caused by MXene nanosheets,and BT@PDA fiber induced interfacial polarization is the main reason to improve the dielectric constant of codoped system.The laminated structure of MXene nanosheets can inhibit the conduction of leakage current and obtain appropriate relatively high breakdown strength when the content is less than 0.5 wt%.At 10³Hz,0.5wt%MXene/5 wt%BT@PDA/TPU composite has high electrical sensitivity factor（11.38）and dielectric constant is 12.52,which is 1.86 times higher than that of pure TPU（ε=6.72）,indicating that it has relatively larger deformation.（3）The mechanical properties and dielectric properties of MXene/CNTs/TPU and MXene/CNTs@PDA/TPU dielectric composites co-doped with high-conductivity carbon nanotubes（CNTs）,CNTs@PDA and MXene were studied.The results show that the introduction of a small amount of highly conductive MXene and highly conductive CNTs and CNTs@PDA fibers can form a double-doped filler network structure in the TPU matrix without increasing the Young’s modulus of the composite under small deformation.In the double doped system,complex physical and chemical bonding occurs between filler-filler particles and filler-TPU matrix are generated,which significantly improves the interfacial polarization properties.The micro-capacitor structure formed by MXene layer in TPU matrix can strengthen the interface polarization,obtain appropriate breakdown strength when a small amount is added,inhibit leakage current and play a role in protecting longitudinal electrical breakdown.In the codoped MXene/CNTs/TPU composites,the electrical sensitivity factor increases significantly with the increase of the amount of MXene/CNTs.When the amount of MXene in the codoped system is 1 wt%（0.8 wt%CNTs）,the electrical sensitivity factor of TPU composite is 33.46,which has excellent electromechanical efficiency.（4）MXene/GR/TPU dielectric composites were designed and prepared by codoped graphene（GR）with MXene,which also has two-dimensional lamellar structure,in order to obtain better breakdown strength by using the longitudinal current blocking effect of lamellar structure.The results show that the elongation at break of GR/TPU composites is lower when the amount of GR is higher.The elongation at break and tensile strength of MXene/GR/TPU dielectric composites decreased with the increase of MXene content.Compared with MXene/CNTs/TPU composites,MXene/GR/TPU composites doped with two kinds of two-dimensional materials not highly improved the expected dielectric constant.However,GR/TPU has high breakdown strength.Tthe dielectric constant of MXene/GR/TPU composite system can be increased slightly by doping GR and MXene in an appropriate proportion,and the breakdown strength was higher.The electrical sensitivity factor of 0.5 wt%MXene/1 wt%GR/TPU dielectric composite is 3.54,indicating that it has relatively larger deformation.更多还原