Investigation On Physical Properties Of Carbon Nanocoils And Their Applications For Stretchable Strain Sensors

Benefiting from their unique helical morphology and polycrystalline-amorphous internal structure,carbon nanocoils(CNCs)exhibit unique mechanical,thermal,electrical and optical properties.Researchers have done sufficient works to investigate the electrical properties of CNCs and proposed the electron hopping model.For mechanical properties,there are no appropriate theory and model to describe the mechanical vibration of CNCs.As a result of experimental difficulty,preliminary results for thermal conductivity measurement have been reported.Futhermore,it is still unclear that how internal structure will affect the physical properties of CNCs,especially the mechanical and thermal properties.This thesis focuses on the mechanical,thermal and electrical properties of CNCs,and tries the explain these physical properties from the essence of the internal strcture.In chapter 2,the mechanical properties of single CNCs was studied through an electromechanical vibration.Using a classical continuum theory and a method of material mechanics,we proposed a formula for accurately describing the mechanical vibration and a model for calculating the Young's modulus of quasi 1-dimensional helical nanowires.The Young's modulus of CNCs was calculated to be ranged from several to several tens of GPa.Through microstructure characterization,we discovered the negative relation between graphitization degree and Young's modulus of CNCs.In chapter 3,the thermal diffusity,thermal conductivity,specific heat and electrical conductivity of CNCs,and their dependence on temperature from 290 to 10 K were systematically measured through a transient electro-thermal technique.A peak around 75 K of thermal conductivity was observed.It was found that the specific heat was an intrinsic attribute,whose temperature dependence was independent from the crystallinity of CNCs.Using a concept of thermal reffusivity(reciprocal of thermal diffusivity),we calculated the domain size for phonon transport of CNCs to be about 3 nm,which is close to the real grain size.Larger domain size determines larger thermal diffusivity and electrical conductivity.On the basis of previous two chapters,in chapter 4,we obtained the linearly quantitative relations between Young's modulus,thermal diffusivity and electrical conductivity through fitting.By means of theoretical deduction,simple proportional relations between these paramters and graphite grain size were revealed,achieveing one-to-one correspondence between mechanical,thermal,electrical and structural properties.Futher research observed that alcohol molecule can diffuse into CNCs through defects and interspaces,which leads to change of physical and chemical structures.The research of this chapter confirmed the roles of graphite grains as localized states and quantum dots.Owing to their unique helical structures,CNCs show obvious application advantages in stretchable sensors.In chapter 5,we combined CNCs with gold nanofilm,achieveing a micro/nano scale stretchable strain sensing unit based on the helical gold nanotube film.Gold nanofilm was deposited onto the surface of a single CNC using magnetron sputtering.The structure regulation,electron transport and response to strains of this composite structure were investigated.The strain range and gauge factor of this sensing unit reach 10%and 5 respectively.The real time resistance response of the sensing unit under cyclic tenstion was detected using a four-probe method.To meet the measurement needs,we developed a flexible electrical probe based on bundled carbon nanotube(CNT),by extracting CNT bundle from vertical CNT array using a tungsten probe.The geometric morphology and performance of the probe were modified and optimized using electricity,laser and alcohol.Its application advantages in resistance measurement of nanowire,dynamic and nondestructive tests were demonstrated.In chapter 6,a super stretchable and highly sensitive wearable strain sensor was fabricated by peeling off a film of CNCs networks using a stretchable polydimethylsilane substrate.We systematically studied electrical responses to tension,pressure and bending of the sensor.The strain range,maximum gauge factor and response time of the sensor is 260%,192 and 12 ms respectively.Its application potentials in various strain detection,such as human pulse,slight vibration and impact of weight,were demonstrated.The works of the thesis filled the gaps and blanks in the research of mechanical and thermal properties,drawing a theoretical image of the physical properties of CNCs from the essence of internal structure.Taking advantages of the unique helical morphology and excellent mechanical and electrical properties of CNCs,we provided sensitive,economical and stable strectchable strain sensing units for both the micro/nanoscale and macroscale applications.