Preparation And Electronic Transport Properties Of Metals(Sn,Ag,Cu)/Carbon Nanotubes(CNTs) Nanocomposite Powders

As the development of electronic devices towards intelligence and minimization,conductive nanomaterials will be widely utilized in the fields of flexible electrodes,sensors,electronic packaging,microwave absorption and electromagnetic shielding.Metallic nanomaterials exhibit unique physical and chemical properties,which also bring some new challenges and opportunities for the integration of metals at nanoscale with carbon materials.The changes in electronic transport modes resulted by reducing the scale of bulk metals to nanometer,as well as the regulable electrical properties from compositing carbon nanotubes that together offer the important material foundations for new electronic components,which has become one of the research hotspots of electronic material field.In this thesis,conductive metals Sn,Ag and Cu as the research objects while carbon materials as composite components were selected to prepare core/shell structured nanoparticles as well as metal/carbon nanotube composite powders(Sn@CNTs,Ag@C/MWCNTs,Cu/CNTs)by using a high temperature plasma method.The nucleation and growth mechanisms of nanostructures under non-equilibrium conditions were deeply explored,in which the intrinsic electronic transport mode,the regulation mechanism of electrical properties of the composite system,as well as the effects of shell thickness,microstructure,morphology and interface characteristics on the electrical properties were revealed by the relationship between resistivity and temperature.On this basis,the electromagnetic parameters and absorbption properties of three metal/carbon nanotube composite powders were measured at microwave range.Moreover,the electromagnetic loss mechanism was also analyzed.The main contents and conclusions of this thesis are as follows:(1)The results indicate that in temperature range from the superconducting critical transition temperature T,to 300 K,both kinds of Sn nanoparticles with different shell thicknesses behave according to Bloch-Gruneisen mode by electron-phonon coupling.Thicker oxide shell can cause stronger scattering of electrons,thus hinders the conduction of electrons.At temperature below the superconducting critical transition temperature Tc,the superconduction phenomenon happens in two kinds of Sn nanoparticles resulting from the quantum fluctuation-induced tunneling,while higher Tc of Sn nanoparticles(3.98 K and 4.15 K for two kinds of Sn nanoparticles)than bulk Sn(3.73 K)are measured which is thought from an enhanced electron-phonon scattering at the surface of nanostructures.The nanoparticles of Ag and Cu also exhibit electron diffusion conduction in Bloch-Gruneisen mode through electron-phonon coupling in temperature range of 2-300 K.Due to the broken bonds at surface of nanoparticles and the special core-shell interfaces,a phenomenon of surface phonon softening is induced and leads to change of phonon mode as well its vibration frequency.Detected Debye temperatures(158 K and 296 K)in Ag and Cu nanoparticles are significantly lower than those of their bulks(235 K and 343 K).Average free paths for the electrons in Ag and Cu nanoparticles are 138 nm and 55 nm,respectively,one of the electronic transport characteristics for nanometer-sized particles.(2)It is found that the vapor-liquid-solid(VLS)mechanism is dominant on formation for the in-situ fabricated of Sn@CNT NRs,Ag@C/MWCNTs、Cu/CNTs nanocomposites powders.with the metal element acting as catalyst for growth of CNTs growth.Among all crystallographic planes of Sn crystal,the(100)plane has the highest atomic density(0.2695 atoms/A2)and the highest surface free energy.,which causes it thermodynamically unstable surrounding by the saturated carbon atoms from dissociation of methane molecules,and favors a preferential growth of Sn nanocrystals along the crystal direction of<]00>,finally form the Sn nanorods with one-dimensional structure:meanwhile.the catalysis from metallic Sn also promote diffusion of carbon atoms and growth of CNTs on surface of Sn nanorod.Metals of Ag and Cu also have stronger catalytic abilities,particularly the(111)facets with the lowest adatom diffusion barriers(Ag～0.20 eV,Cu～0.07 eV),which induce a remarkable adsorption-diffusion of C atoms on(111)facets and favors growth of CNTs along<111>direction,final products of Ag@C/MWCNTs and Cu/CNTs nanocomposites powders are obtained.(3)In the Ag@C/MWCNTs and Cu/CNTs composites,the electrons transport in general according to the variable range hopping(VRH)mode,a transition occurs from Mott-David(MD)to Shklovskii-Efros(SE)VRH mode at 5.4 K and 10.7 K for two kinds of nanocomposites,respectively.Detected Coulomb gap Ac are 0.05 meV and 0.14 meV for Ag@C/MWCNTs and Cu/CNTs composites,respectively.However,for Sn@CNT NRs nanocomposites,electrons transport in Mott-David(VRH)mode happens above 3.69 K.the superconducting critical temperature,below which occurs a semiconductor-superconductor transition(SST)due to Josephson junction in term of Sn/CNT/Sn layered structure,the electron coupling in pairs and subsequent tunneling through the Josephson junction will be concomitant.In fact,the resistance RN of junction comes from the electron reflection at the interfaces of core-shell,it is calculated as～0.5Ω.(4)Microwave absorptions properties of three kinds of nanocomposites(Sn@CNT NRs,Ag@C/MWCNTs and Cu/CNTs)are studied.The results show that the minimum reflection loss value of Sn@CNTs/paraffin composite is-43.5 dB at 10.2 GHz with effective absorption bandwidth of 3.5 GHz;the optimum absorption performance of Cu/CNTs/paraffin composite(-46.7 dB)appears at 7.8 GHz with effective bandwidth of 3.2 GHz;Ag@CNTs/paraffin composite displays a relatively weak absorption capability,that is the minimum reflection loss of-40.3 dB at 15.8 GHz with effective bandwidth of 2 GHz.All of three nanocomposites exhibit good absorption properties,mainly derived from the excellent dielectric loss capability of CNTs.Defects from bends and kinks of CNTs can work as the centers for dielectric polarizations to be the equivalent dipoles,which favors to lose and absorb the microwave energy.As the non-magnetic metals,Sn,Ag,and Cu components with strong conductive abilities can promote local electrical conductivity and improve multiple scattering of electromagnetic waves inside the absorbers,thereby provide much probability for absorption,also can be used to optimize the absorption properties.