The paper presents a theoretical model of the catalytic centers formation processes during annealing of multilayer nanosized metal films for carbon nanotubes growth. The approach to the description of the model is based on the mass transfer processes under the influence of mechanical thermoelastic stresses, which arise due to the difference in the thermal expansion coefficients of the substrate materials and nanosized metal layers. The thermal stress gradient resulting from annealing creates a drop in the chemical potential over the thickness of the film structure. This leads to the initiation of diffusion mass transfer between the inner and outer surfaces of the films. As a result, the outer surface begins to corrugate and fragment, creating separate islands, which serve as the basis for the catalytic centers formation. Experimental research on the formation of catalytic centers in the structure of Ni/Cr/Si was carried out. It is demonstrated that the proposed model allows to predict the geometric dimensions of the catalytic centers before growing carbon nanotubes. The results can be used to create micro-and nanoelectronics devices based on carbon nanotube arrays.
It is shown that the non-uniform elastic strain is the memristive switching origin in carbon nanotubes (CNT). The dependence of the resistance ratio in high-and low-resistance states of the non-uniformly strained CNT on the value strain is obtained. The process of the strain redistribution and its effect on the conductivity of CNT under action of the external electric field strength is studied. The obtained results can be used to develop memristor structures with reproducible parameters based on non-uniformly strained of carbon nanotubes.
Recent studies reveal that carbon nanostructures show anomalous piezoelectric properties when the central symmetry of their structure is violated. Particular focus is given to carbon nanotubes (CNTs) with initial significant curvature of the graphene sheet surface, which leads to an asymmetric redistribution of the electron density. This paper presents the results of studies on the piezoelectric properties of aligned multi-walled CNTs. An original technique for evaluating the effective piezoelectric coefficient of CNTs is presented. For the first time, in this study, we investigate the influence of the growth temperature and thickness of the catalytic Ni layer on the value of the piezoelectric coefficient of CNTs. We establish the relationship between the effective piezoelectric coefficient of CNTs and their defectiveness and diameter, which determines the curvature of the graphene sheet surface. The calculated values of the effective piezoelectric coefficient of CNTs are shown to be between 0.019 and 0.413 C/m2, depending on the degree of their defectiveness and diameter.
We studied the influence of the synthesis temperature on geometric parameters and structural perfection for vertically aligned carbon nanotubes (VACNT). We established that a synthetic temperature of 750 • C allows one to obtain the lowest concentration of defects in VACNT, with a diameter of 44±3 nm and a height of 80±9 nm. When temperature is increased up to 800 • C, an increase of the VACNT geometric dimensions was observed, which may be due to an increase in the catalytic centers (CCs) migration rate and their integration into larger centers. Also, at 800 • C, the concentration of defects in the nanotubes was increased due to the violation of carbon bonds during the acceleration of the acetylene desorption process from the surface of the sample.
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