PSI - Issue 23

Sergiy Kotrechko et al. / Procedia Structural Integrity 23 (2019) 310–315 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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the nature of the time dependence of strength (Fig. 4). Thus, presence of IZ gives rise to the existence of two failure mechanisms for nanoelements: ( i ) high-energy, when thermal fluctuations of significant magnitude are required for breaking the bond, and ( ii ) low-energy, when much smaller fluctuations are sufficient for bond breaking ( un C    ). For the carbyne – graphene nanoelement with a ten-atom carbyne chain, transition from the first to the second mechanism occurs at a force field level exceeding 70% of the contact bond strength un F ( un f F F   0 70 . ). According to the data obtained (Fig. 4), the waiting time for fluctuations un C    (low-energy mechanism ) doesn't exceed tenths of a second, i.e. when un f . F F   0 70 the fast failure should be observed. Thus, the existence of IZ results in the limitation of the upper level of strength of nanoelements, which for CGNs can't exceed 70% of the bond strength. 3. “ Even-odd ” effects for lifetime A specific feature of nanoelements is the existence of quantum-mechanical effects for their long-term strength. This effect is manifested in the dependence of the life-time on whether the number of atoms in the carbyne chain is even or odd [Kotrechko et al. (2019)]. In Fig. 5 such dependences are given for a temperature of 750K. Technique of DFT-calculations is described by Timoshevskii et al. (2015) and Kotrechko et al. (2017). The reasons for this is the quantum-mechanical nature of the binding energy 0 E and the magnitude of the instability force un F . The probability of contact bond break depends exponentially on the magnitude of above characteristics; therefore, for the lifetime of these nanoelements, an “even - odd” effect is observed . Specific feature of the appearance of the "even-odd" effect for the lifetime of CGN is the inversion of the "even odd" effect when the force field is applied. According to the data in Fig. 5, in the absence of a force field, CGNs with "even" chains have a longer lifetime, and under the effect of the force field an opposite regularity is observed, that is, CGNs with carbyne chains containing an odd number of atoms become more stable and reliable. The transition itself from the first to the second regularity occurs approximately at un f F F   0 3 . . Inversion of the "even odd" effect is due to a change in the mechanism of the fluctuation-induced contact bond break, namely, due to the transition from the “ high-energy ” mechanism of bond break (at 0  f F ) to the “ low-energy ” one (at ). A detailed analysis of this effect is given in [Kotrechko et al. (2019)]. In general, the “e ven- odd” effect and its inversion in the force field are of interest from a physical point of view and can have practical significance in the elements of a straintronics. In particular, from the applied point of view, this means that in the absence of a force field, or at its small values, nanoelements with an even number of atoms should be utilised, and nanoelements that will be stretched during the work [Cahangirov et al. (2010)], on the contrary, should contain carbyne chains with odd number of atoms. All this requires further research. 4. Conclusions 1. Dependence of the critical fluctuation of an atomic bond break, С  , on the level of the applied force is a key feature of the fluctuation-induced breaking of the atomic bond in nanoelements under the action of a force field. Accounting for this feature enables to predict the lifetime of nanoelements. 2. Existence of a region where the existence of equilibrium states of atoms is impossible (IZ) is a key factor determining the lifetime of nanoelements consisting of 1D- and 2D-nanostructures and, particularly, carbyne – graphene nanoelements. IZ appears due to the redistribution of potential energy accumulated in the stretched nanoelement during the instability of the weakest (contact) bond. Under conditions of fluctuation-induced breaking of bonds, this gives rise to the fact that fluctuations can play the role of a “trigger” to release the potential energy accumulated in the system, which is spent on the contact bond break. This situation is realised when the level of the acting force f F lies above the lower boundary of the IZ, R F . This results in a catastrophic decrease (by many orders of magnitude) in the lifetime of nanoelements. In general, depending on the ratio of the level of the acting un f F F    . ) ( . 0 71 0 74