Structural transformations in nano- and microobjects triggered by disclinations
- PDF / 335,967 Bytes
- 7 Pages / 584.957 x 782.986 pts Page_size
- 56 Downloads / 161 Views
Anatoly A. Vikarchuk Togliatti State University, 445667 Togliatti, Russia
Anna L. Kolesnikova Institute of Problems of Mechanical Engineering, Russian Academy of Sciences, 199178 St. Petersburg, Russia
Leonid M. Dorogina) and Ilmar Kink Institute of Physics, University of Tartu, 51014 Tartu, Estonia
Elias C. Aifantis Aristotle University of Thessaloniki, GR 54124 Thessaloniki, Greece (Received 20 July 2011; accepted 3 October 2011)
Crystalline pentagonal nano- and microrods (PRs) and pentagonal nano- and microparticles (PPs) with 5-fold symmetry are studied. Structure of PRs and PPs and their elastic distortions are characterized in the framework of the disclination approach. Relaxation of mechanical stresses due to disclinations causes structural transformations in PRs and PPs. Experimental evidence of such transformations, namely, the appearance of internal cavities and pores, and growth of whiskers in copper PRs and PPs grown in the process of electrodeposition is demonstrated. A brief review of existing models of stress relaxation in PRs and PPs is presented. We discuss a new model of nanowhisker growth based on the nucleation of two dislocation loops of opposite signs near the surface of the crystal with disclination. As a result, vacancy-type dislocation loop remains in the material and serves as a nucleus for cavity, while the interstitial loop comes to the free surface and contributes to whisker growth.
I. INTRODUCTION
It is well known that small crystalline particles and rods produced from materials with face-centered cubic (FCC) crystal lattice, such as Cu, Ag, Au, often demonstrate axes of 5-fold symmetry, e.g., see Refs. 1–8. During FCC crystal growth low-energy {111}-type twin boundaries (TBs) can be formed in the particle body. These TBs divide a particle or a rod into mutually misoriented crystalline regions, i.e., twins. The presence of twins is crucial for the emergence of pentagonal particles (PPs) and pentagonal rods (PRs) with unusual “noncrystallographic” axes of rotational symmetry. Typically, for experimentally observed PPs and PRs diameter varies from 10 nm to 10 lm.3,6,9 In an ideal case, low-energy TBs are the only defects in PPs and PRs. Nanoscale PPs and PRs can be defect free providing particular physical and mechanical properties, e.g., strong anisotropy, high mechanical strength and hardness similar to those observed for monocrystalline whiskers.4 Another
important feature of PPs and PRs is their enhanced catalytic activity,3,9 which is dictated by the crystallography of their surface. PPs of icosahedral and decahedral shapes are bounded by {111}-type facets, and PRs have {100}-type planes as side facets and {111}-type planes as cap facets. This specific crystallography of cap ends was also shown to be responsible for the interactions and assembling of PRs.9 The structure and elastic distortion of PPs and PRs crystal lattice can be characterized in the framework of the disclination approach as it was first proposed in Ref. 10 and then developed in full details in Refs. 3, 10–12.
Data Loading...
