Analysis On Mechanical/Electrical Properties Of Wire-structures Under Coupling Effects

Wire structures have wide application prospects in varied fields, including nanodevices, nanoelectromechanical systems (NEMS), and thermonuclear experimental reactor magnet system. The size dependent mechanical properties, and electro-mechanical coupling effects of which are involved in the applications due to gaining relevance in the operation performance criteria and controlling functionality of the actual device. Therefore, theoretical studies on the mechanical properties of metal nanowires, the electric field effects in the metal nanowire/ZnO nanowire based NEMS, and the strain effects in the critical current density of composite superconducting wires are carried on in this thesis.Firstly, from the viewpoint of the atom binding energy, a Young's modulus model is proposed for the widely used fee metal nanowires, which allows for an analytical study on the elastic behavior from the perspective of atomic interactions and easy application in engineering practice. By examining the bond-strength enhancing arising from the spontaneous bond relaxation, a scale function is obtained and a simple formula characterizing the Young's modulus of the fee metal nanowires is given, which indicate the key role the bond-strength enhancing plays in the size-dependent elastic properties. There is good qualitative agreement between theoretical predictions and experimental observations. The number of material parameters needed for the low dimensional material Young's modulus modeling in this paper is reduced by comparing with the continuum modeling approach.Secondly, elastic properties of metal nanowires in a transverse electric field are investigated using molecular dynamics simulations and classical electrostatics theory. The negative pressure on the surfaces stemmed from the transverse electric field modifies the Young's modulus and Poisson's ratio of nanowires significantly. The simulation results reveal the decrease in Young's modulus and the increase in Poisson's ratio as a result of the negative pressure. With the increase of the applied voltage and decreasing the distance between the nanowire and the ground plane, the electromechanical coupling effect on elastic properties of nanowires becomes more remarkable.Thirdly, within the framework of continuum mechanics, the self-equilibrium strain field of nanowire-structure on substrate under the surface stress is derived. An inadequate understanding of the physics responsible for the observed stiffness change of micro/nano cantilevers originating from surface stress in the previous literatures is demonstrated. The mechanical behavior of nanowire under different loading modes is studied; the elastic modulus measurement systems of nanowires using an electric-field-induced resonance flexure method are carefully analyzed. The resonance frequency dependence on the surface stress, the residual strain, as well as the electric field is obtained. On the basis of the analysis, the agreement of the theoretical predictions and experimental observations of different trends of the Young's modulus of ZnO nanowires varying with the diameter is achieved, a unified explanation is made.Finally, a scaling law for the three-dimensional strain dependence of the critical current of Nb3Sn conductors is established, which considers the strain effects on the electron density of state at the Fermi level. The scaling law, in the degenerate form, describes well experimental data, such as critical properties measurements of one-dimensional superconducting wires and two-dimensional superconducting tapes under applied strain. And there is consistency in the form between the degenerate formula and the one-dimensional empirical relation for the variation of critical current density with the axial strain. Taking the critical current measurements of Nb3Sn wires under tensile loading mode as example, a preliminary study is conducted to investigate the structure effects on the strain field distribution in the superconducting filaments.In conclusion, through this paper, we can get further understanding of the size dependent Young's modulus of nanowire and the electromechanical effects in the nanowire-based systems. It lays the foundation for the realization of NEMS systems and the improvement of the nanomechanical test systems. At the same time, studying the effects of strain on the critical current density of Nb3Sn composite wires would provide theoretical guidance to engineering application of superconductors.