PSI - Issue 28

Kotrechko Sergiy et al. / Procedia Structural Integrity 28 (2020) 116–123 Author name / Structural Integrity Procedia 00 (2019) 000–000

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R F is the lower boundary of the instability zone;  is the waiting time for the contact bond break; 0  is the average time of an atom oscillation ( 0  =0.035 ps).

To obtain the theoretical dependence of the CGN lifetime and to compare it with the results of MD-simulation, we used the strain diagram obtained for the modified REBO2 potential. Dependence of the lower boundary of the instability region, R F , on the value of the applied force f F was described by the Eq. (1) at  = 0.94. According to the data obtained, the fluctuation model predicts well both the average waiting time for a contact bond break and the scatter limits for this value. As noted above, the potential of REBO2 was used only to verify the fluctuation model. To predict the lifetime of CGN, the fluctuation model has used strain diagrams obtained by DFT calculations [Kotrechko et al. (2019)]. According to the data obtained, the lifetime of carbyne-graphene nanoelements with ten atom carbyne chains is quite sufficient for their practical use at temperatures not exceeding 600 ÷ 1000 K (Fig. 5). Generally, transition from macro- to nano-objects is accompanied by the appearance of a qualitatively new phenomenon associated with the manifestation of quantum-mechanical effects. As was shown by Kotrechko et al. (2019), for the CGN lifetime this is manifested in the fact that the waiting time for a contact bond break depends on whether the number of atoms in a carbyne chain is even or odd (the “even-odd” effect). This effect can be considered as “indirect”, since it is due to the dependence of the binding energy 0 E and the contact bond strength un F on whether the number of atoms in the chain is even or odd. The CGN lifetime is a function of these parameters. But the most interesting thing is the inversion of the “even-odd” effect under the action of mechanical load. The inversion is manifested in the fact that in the absence of load ( f F = 0) CGNs containing an “even” chains have a longer lifetime as compared to a CGNs with an “odd” chains; however, with an increase in load, CGNs containing “odd” chains become more stable and durable. This transition occurs when un f F F   0 3 . [Kotrechko et al. (2019)]. Inversion of the “even-odd” effect is a consequence of transition from the “high-energy” mechanism of the contact bond break to the “low-energy” one with a load growth. From a practical point of view, this effect is of fundamental importance when using CGNs in straintronics. 5. Conclusions: 1. Existence of two mechanisms of the contact bond breaking - “low-energy” and “high-energy” is the key factor determining the regularities of the effect of mechanical load on the lifetime of carbyne-graphene nanoelements. In general, this is inherent in nanoelements consisting of combinations of nanostructures of different dimensionality. 2. Waiting time for a contact bond break at the "low-energy" mechanism doesn't exceed tenths of a second. This means that carbyne-graphene nanoelements can only be used when realising the “high-energy" mechanism. For this purpose, the maximum load can't exceed 70% of the contact bond strength. 3. Presence of an instability region (IZ) in the decreasing branch of the contact bond strain diagram is the reason for existence of two mechanisms of the contact bond break. 4. The quantum-mechanical nature of the CGN lifetime is manifested in its dependence on whether the number of atoms in a carbyne chain is even or odd. This effect is due to the fact that the probability of contact bond break depends on both the binding energy and the strength of the contact bond, which, in turn, depend on the even-or-odd number of atoms in carbyne. This means that the "even-odd" effect for lifetime is not direct, but induced. The phenomenon of inversion of the "even-odd" effect under the action of applied load is of practical interest. It can be used in straintronics elements. 5. In general, the lifetime of carbyne-graphene nano-elements is sufficient for their practical use at temperatures no higher than 600÷1000 K. Acknowledgements This work was supported by the National Academy of Sciences of Ukraine (grants’ number #0117U002131).