Study On Mechanical Problems Of Low-dimensional Nano Materials

Since the 21st century,nano science and technology has been greatly developed,which has a great impact on the development of human science and technology,as well as people's daily life.The study of nanotechnology has spread all over the fields,such as electronics,materials,information,energy,environment and healthcare.Among them,the mechanical problem of nano materials has always been a hot issue in nano science and technology.Different from macro materials,nano materials have surface effect,small size effect,quantum size effect,macro quantum tunneling effect and dielectric confinement effect,so they have new phenomena in many disciplines including mechanics.In this context,we take single crystal nanowires and novel carbon materials as the research object,and use molecular dynamics simulations to explore the internal mechanism of special vibration phenomenon of single crystal nanowires,such as body centered cubic iron nanowires and face centered cubic copper nanowires,and predict the basic mechanical properties of novel carbon materials,such as single layer diamond nanoribbons and carbon nitrogen nanothreads.This paper mainly includes the following parts.In the first part,the vibrational properties of body centered cubic Fe nanowires have been studied.Based on the atomic structure of nanowires,an effective discrete spring mass model is proposed to show the relationship between the lattice arrangement and vibration properties.For one-dimensional vibration,it is found that the vibration frequency of nanowires increases nonlinearly and slowly with the rise of the initial actuation amplitude.In addition,the simulation results of the nanowires with different lengths and heights reveal that the vibration frequency of slender nanowires is similar to that of Euler-Bernoulli beam,while the frequency of thick nanowires is similar to that of Timoshenko beams.For the beat phenomenon,its dynamic characteristics are revealed by time history of displacements and model analysis.In addition,it is observed that the vibration phenomenon of nanowires may occur when the actuation is applied along the fundamental direction,which is related to the plastic deformation.Finally,by analyzing the relationship between the excitation frequencies in the two fundamental directions,the excitation mechanism of the beat phenomenon is preliminarily explored.In this part,the vibration properties of the nanowires are explored comprehensively,which lays the foundation for the nanomechanical system.In the second part,the general existences of flexural mode doublets of single crystal nanowires have been studied.Based on in silico studies and Timoshenko beam theory,this part finds that the flexural mode doublets of nanowires can be effectively controlled through the proper selection of their growth direction.It is found that metallic nanowires with directional-independent shear modulus possess a single flexural mode.However,nanowires with directional-dependent shear modulus naturally exhibit flexural mode doublets,which do not disappear even with increasing slenderness ratio.Further studies show that such feature generally exist in other nanowires,such as Si nanowires.Mimicking a pendulum configuration as used in a nanowire-based scanning force microscopy,the cantilevered<110>Si nanowire demonstrates zeptogram mass resolution and a force sensitivity down to the order of 10^-24 N/Hz^-1/2(yN/Hz^-1/2)in both transverse directions.The findings in this part open up a new and facile avenue to fabricate2D vectorial force sensors,which could enable ultra-sensitive and novel detection devices/systems for 2D effects,such as the anisotropy strength of atomic bonds.In the third part,the tensile and vibration properties of single-layer diamond–diamane have been studied.Diamane nanoribbons?DNRs?with different atomic stacking structures have similar tensile stiffness.Temperature has little effect on the tensile stiffness of DNR,but has great impact on its failure stress/strain.With the increase of temperature,the failure stress/strain of DNR decreases significantly.Compared with graphene,DNR resonator is found to possess a higher natural frequency and a larger quality factor on the order of 10⁵.Unlike the bilayer graphene,DNR resonator has a high in-plane stiffness and is free from the influence of the warped edge.As a result,the natural frequency of the DNR resonator experiences a remarkable increase when it is under pre-tensile strain,in the meanwhile,its quality factor maintains a high magnitude with a figure of merit on the order of 10¹⁵.Moreover,it is found that diamane has a much higher quality factor than other 2D materials,such as MoS₂ and monolayer graphene,suggesting that it experiences less thermoelastic damping dissipation.Furthermore,a remarkable impact to DNRs'vibration properties is found when the surface hydrogenation distributed unevenly in the structure due to unbalanced strains,e.g.,a patterned hydrogenation only on one surface of the nanoribbon.Overall,this part suggests that diamane nanoribbon possess extraordinary vibrational characteristics,which could be harnessed to develop ultra-sensitive resonator-based nanosensors.In the last part,the mechanical properties of ultrathin carbon nitride nanothreads?CN-NTHs?have been studied.Comparing with the carbon nanothreads?C-NTHs?,the presence of nitrogen atoms is found to enhance the tensile and bending stiffness of the nanothreads.Specifically,the CN-NTHs in the polymer I group and tube?3.0?group are found to possess a higher failure strain than their C-NTH counterparts.While,the CN-NTH in the polytwistane group has a smaller failure strain compared with the corresponding C-NTH.Simulation results show that the length of the C-C bonds is re-distributed due to the introduction of nitrogen atoms,which affects the stress or strain distribution during the tensile deformation.Based on the geometrical relationship,the C-C bonds and C-N bonds can be divided into different groups in C-NTHs and CN-NTHs.Within the same group,the C-N bonds are found to share the same changing tendency?either stretching or compressing?under the tensile deformation as the C-C bonds.Particularly,for the polymer I and tube?3,0?groups,the presence of nitrogen atoms introduces extra flexibility to the structure that delays the failure of the structure,i.e.,resulting in a larger failure strain.This part provides a comprehensive understanding of the mechanical properties of carbon nitride nanothreads,which is able to shed lights on their potential applications such as fibers or reinforcements for nanocomposites.