The Effect of MWNTs Surface State on Micro-structure and Resistance Switching Properties of Polyimide Matrix Nanocomposite Films

Author:Liu Zuo Zuo

Supervisor:yin jing hua


Degree Year:2018





With the vigorous development of information technology,it requires electrical storage devices to have non-volatile characteristics,high storage capacity,fast response speed,and low manufacturing cost.However,Polymer memory has the characteristics of fast operation speed,high storage density,low power consumption,flexible structure,simple manufacturing process,low cost,etc.,and has become an important direction for the development of a new generation of memory devices.In recent years,since the carbon atoms of the carbon nanotubes are bonded in a sp~2hybrid manner,each carbon atom is connected to other three carbon atoms by a sigma bond,and theπelectron forms a delocalizedπbond with theπelectrons of other carbon atoms,making them unique electrical properties.Carbon nanotube polymer composite material storage devices have attracted widespread attention,and related research has been conducted and remarkable progress has been done.In general,high surface energy carbon nanotubes tend to agglomerate and have poor compatibility with the polymer,resulting in poor dispersion of the carbon nanotubes in the polymer matrix.Improving the dispersion of carbon nanotubes in polymers is one of the important research topics for polymer-based memory devices.Moreover,the conventional carbon nanotube/polymer-based memory devices have problems such as a single storage form and a small switching current ratio.In order to improve the switching current ratio of memory devices,to achieve different types of storage and to improve the current stability of memory devices,this paper deals with the preparation of carbon nanotube/polyimide-based composite films with different surface states and electrical storage by processing multi-walled carbon nanotubes.In this paper,different surface states of carbon nanotubes/polyimide matrix composite films and electrical storage devices are prepared by treating the surfaces of carbon nanotubes(MWNTs).The effect of the surface state of MWNTs on the microstructure of polyimide-based films and resistive characteristics of memory devices was studied.MWNTs/PI,end-group MWNTs/PI and SC-MWNTs/PI doped MWNTs with different surface states and composition were prepared by in-situ polymerization method.The microstructure of the composite films was characterized by XRD,FT-IR,SEM,synchrotron radiation small angle X-ray scattering and TEM.The effect of surface states of different MWNTs on the microstructure of composite films was investigated.The results of the study indicate that the surface state structure of the end-group and spinning carbonized affects the energy level distribution of the interface layer and the molecular orbits.The molecular orbital energy levels of composite films are divided into three types:single-energy type,double-energy type,and single-double mixed type;Interfacial layers and single-energy level molecular orbital energy levels are formed between pure MWNTs and hydroxyl MWNTs and PI matrix;An interface layer is formed between the carboxylated MWNTs and the PI matrix,which has a two-level molecular orbital energy level.A dual interface layer is formed between the spinning carbonized SC-MWNTs and the PI matrix,with a single-double hybrid molecular orbital energy level.The influence of the different surface states of MWNTs on the dielectric properties of polyimide-based composite thin films and the resistive switching characteristics of the memory devices was investigated using electrochemical analyzers,broadband dielectric spectroscopy,and electrical bistable methods.The research results show that the single-energy level achieves second order step storage,which is consistent with I~V~2 current characteristics of SCLC in high conduction state.The dual-level type achieves three-level variable-level storage,and the high conduction state meets I~V ohmic current characteristics,and the switching current ratio is small.The single-double hybrid type achieves three-level step memory,which satisfies I~V~2 current characteristics in the high-conduction state,and the switching current ratio is large.The composite film of the three-order variable memory device exhibits negative dielectric characteristics,and has a dual-level structure and deep and shallow traps.According to the experimental results and the molecular orbital energy level and the distribution characteristics of the interface layer,this paper establishes the interface charge transfer models of single-energy level,dual-energy level,and single-dual hybrid three-storage devices,respectively,through different types of composite film interface structures and molecular orbital energy level.The distribution of orbital energy levels,analysis of single and dual interface layer effects of storage devices,and charge transfer mechanisms provide theoretical basis for the preparation of different storage forms of polyimide-based memory devices.