Controlled Assembly Of Nanomaterials Based On Ice-templated Method And Its Functional Applications In Electronic Devices

In the past half century,the development of nanotechnology has made remarkable progress,and various functional nanomaterials have been widely used in all walks of life.Especially with the arrival of the era of artificial intelligence and the rapid development of 5G communication technology,the application of nanomaterials in electronic devices will determine the development speed of the era of human intelligence.At present,the large size,high precision,high integration,low cost and high efficiency of nanomaterial assembly into macroscopic assemblies with hierarchaical ordered structure,and then to build a series of functional micro-nano integrated devices,is still the research focus and development direction of nanotechnology.In this paper,the assembly methods of various dimensions of nanomaterials are described in detail and comprehensively,and the functional applications of nanomaterial assembly in electronic devices are summarized.Among them,the ice-templated method as a controllable,large-scale,green nanomaterial assembly method,has the very broad prospects for development.However,to date,most icetemplated method applies only to the preparation of three-dimensional block material.Expanding its assembly scope,such as assembling nanomaterials in two-dimensional thin film space is still a challenge for the ice-templated method.On the other hand,expanding its assembly mode,such as introducing dynamic moving and pattern array temperature field into the preparation process,is still an important development direction of the ice-templated method in the future.Based on the above research background,aiming at the difficulties and challenges of ice-templated method,the development direction of this thesis is to assemble functional nanomaterials in a controllable and orderly way.We explore the principle and process of the ice-templated method in twodimensional and three-dimensional assembly.On the basis of the preparation technology of threedimensional block,the technology and theory system of the two-dimensional liquid membrane and pattern array temperature field is established.The orderly assembly and controllable preparation of nanomaterials in two-dimensional and three-dimensional space is realized,as well as the controllable preparation of three-dimensional honeycomb pattern.Moreover,we have expanded the practical application of functional nanomaterials in electronic devices.The main content of this paper is divided into the following three parts:Firstly,a two-dimensional ice-templated method was developed,and the specific application of the two-dimensional ice-templated method in assembling silver nanowires was described in detail from the perspectives of microcosmic principle,in-situ observation,structure regulation and process preparation.By directional freezing on a flexible substrate,the growth rate and morphology of ice crystals were controlled accurately,and the controllable ordered assembly of nanomaterials in two-dimensional liquid film system was realized.Large area of long range ordered silver nanowire arrays were prepared,which directly increased the contact area between silver nanowires and reduced the contact resistance fundamentally.With a relatively low dosage of AgNWs(4 μg·cm-2),the resulted flexible electrode simultaneously achieves high optical transmittance(91%)and low sheet resistance(20 Ω·sq-1).In addition,our electrode exhibits excellent durability during cyclic bending(～10000 times)and stretching(50%strain).We further demonstrate the potential applications of our flexible transparent electrode in both touch screen and electronic skin sensor which could monitor the sliding pressure and direction in real time.More importantly,we believe that our study represents a facile and low-cost approach to assemble various nanomaterials into large-area functional patterns for advanced flexible devices.Then,the three-dimensional ice-templated method(a bidirectional freezing technique)was used to construct the three-dimensional BNNS network with large area long-range aligned lamellar layers.Combining with the filling of epoxy resin,the BNNS/epoxy composite with excellent high anisotropy and thermal conductivity was prepared.A strategy to improve the thermal conductivity of polymer based thermal interface materials was proposed by optimizing the design of three-dimensional network structure of filler.The highly-organized 3D conductive network provides prolonged phonon pathways,yielding a much higher thermal conductivity(6.07 W/m·K)at a relatively low BN loading(15 vol%)comparing to the similar composites in the literature.Together with its excellent electrical resistivity(2×1012 Ω·cm)and thermal stability(glass transition temperature:120℃),our composite may find wide applications including TIM for the advanced electronic packaging technology.By comparison of BNNS/epoxy composites with different filler network,different layer density and thickness,the influence of structural parameters and three-dimensional thermal conductive network on the thermal conductivity coefficient of composite materials was systematically explored.Finally,we demonstrated our BNNS/epoxy composite as a TIM which is superior to commercial thermal conductive silicone pad,and as a thermal protection material which can effectively resist thermal shock temperature up to 500 degrees.Finally,a patterned array ice-templated method was developed.By building the patterned array temperature field and precise temperature control technology,we use the patterned array ice-templated method to assemble the graphene oxide nanometer piece and realize the preparation of the large-area honeycomb structure graphene aerogels.The aerogel has the ultra low density(0.08 mg/cm3),ultra-thin cell wall(100 nm),as well as the mechanical properties and excellent thrmal insulation performance(18 mW/m·K).At the same time,we show the potential application of the honeycomb structure graphene thin layer in flexible transparent heater device which was prepared by the patterned array ice-templated method.This method is a process that can be widely expanded,such as different material systems,different patterned array structures,different spatial dimensions,and thus can realize richer functional applications.