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Nanomechanics of defects in solids: applications to nanolayers, nanoparticles, nanocrystals and biomaterials

Final Report Summary - MATERIALS NANOMECH (Nanomechanics of defects in solids: applications to nanolayers, nanoparticles, nanocrystals and biomaterials)

The main aim of the project was to develop a general nanomechanics of defects framework for the understanding and prediction of structure-properties relationships of nanoscale materials, components, and devices.

The following case tasks were planned as continuation of the main stage of the project:

(1) Modelling of crystal lattice defects in nanoscale metal particles and nanorods with special attention to five-fold symmetry of these objects.
(2) Mechanics of crystal lattice defects in core / shell nanostructures and nanoscale thin film electronic materials.
(3) Modelling of structural defects (in particular disclinations) in ultrafine grained bulk nanostructures for the clarification of the mechanisms of their plastic deformation and fracture.
(4) Theory of structure defects in crystalline nanotubes and their influence on mechanical properties of both carbon nanotubes and protein membrane nanotubes.
(5) Experimental study of mechanical properties of bulk nanocrystalline metal materials.
(6) Comparison of the results delivered from modelling (i.e. 3) and obtained in experiments (i.e. 5).

The scientific results on the above tasks are included in four published and one accepted articles in scientific journals, as well as in the monograph under preparation. The highlights of the project are listed below.

Crystalline pentagonal nano- and microrods (PRs) and pentagonal nano- and microparticles (PPs) with five-fold symmetry have been investigated [1]. It was demonstrated that structure of PRs and PPs and their elastic distortions could be characterised in the framework of the disclination approach. It is the disclination-induced stresses relaxation that 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 was demonstrated. A brief review of existing models of stress relaxation in PRs and PPs was presented. We developed a new model of nanowhisker (NW) 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. Demonstrated calculations provide a qualitative evidence for the formation of NW-pore pair in a material with disclinations, in particular in PRs and PPs. We propose the development of the given work as elaboration of a theoretical explanation as well as experimental estimation of the critical size of PR and PP as well as the NW radius for relaxation of mechanical stresses.

A complete analytical solution of the plane elasticity problem for the concentrated force acting at the half-space weakened by a circular hole was presented in [2]. The biharmonic stress function used for the derivation of stresses and strains in the half-space with a hole and the associated biharmonic function that allows one to determine the displacement field were described. Both functions are given in the form of Fourier series with the compact coefficients. It was shown that the found analytical formulas of surface displacements give the way to find the circular hole diameter and position when the applied force and elastic modules of the material are known. It is expected that the examination of surface displacements caused by an applied force will be used for the determination of the parameters of the inhomogeneities, voids and cracks placed in the near-surface layers of elastic bodies. The described method [2] can be applied for the objects (defects) that modify the elastic fields caused by the applied force, i.e. for defects possessing the inhomogeneity property. It proposed to use the results of this theoretical work for the interpretation of nanoindentation experiments.

The results on stress relaxation in nano-sized objects [1] and the results on plane elasticity problem caused by nano-sized defects [2] will be included in the planned monograph by Romanov and Aifantis [3].

Numerical simulations of stress-deformed state in thin structured films of gallium nitride on sapphire substrate with open porosity were performed by finite element method [4]. The stress intensity factor K1 was calculated for physical model considering a crack on GaN / sapphire boundary near an open pore. This permits one to analyse the possibility of an initial crack propagation. Stress redistribution in GaN film caused by an ordered open pore array was estimated on the basis of performed elastic fields calculations. The formation of the patterned porous structure was demonstrated to result in a substantial stress drop in GaN films grown on sapphire substrates and corresponding improvements of crystal and optical quality of produced templates.

More recently, analysis of stress relaxation have been performed for GaN / sapphire heterostructure with patterned nanocolumn interlayer [5]. It was found that the analysed structure with patterned nanocolumn interlayer results in up to 15 % drop in average hydrostatic stress in GaN films. Relaxation becomes stronger with overgrown layer thickness decreasing and with relative column diameter increasing. Tensile sigma_zz stress component was demonstrated to present in vicinity of a nanocolumn. Length of initial semicircular crack in nanocolumns for propagation up to the complete layer self-separation while cooling (according to K1c criteria) was estimated.

The structure - physical properties relationship of nanoscale materials were studied on the example of biomorphic porous SiC and SiC / Si composites which were produced on basis of wood-derived carbon through an infiltration of molten silicon [6]. The heat capacity at constant pressure of these materials has been measured for the first time in the temperature range 5 - 300 K. It was shown that in highly porous bio-SiC the surface heat capacity contributes much into experimentally measured value of the heat capacity in the temperature range 5 - 60 K. Influence of free surface on the heat capacity substantially decreases in the composites SiC/Si: the more the content of Si (the pore filling) the smaller the contribution of the surface heat capacity.

[1] A. E. Romanov, A.A. Vikarchuk, A.L. Kolesnikova, L.M. Dorogin, I. Kink and E.C. Aifantis, Structural transformations in nano- and microobjects triggered by disclinations. Journal of Materials Research 27, 545 - 551, 2012.

[2] A. V. Proskura, A. B. Freidin, A. L. Kolesnikova, N. F. Morozov, A. E. Romanov, Identification of defects in a solid body on the base of surface displacements, Materials Physics and Mechanics 15, 9-25, 2012.

[3] A. E. Romanov and E. C. Aifantis, Defects at the Nanoscale, in preparation.

[4] I. Ivukin, D. Artemiev, V. Bougrov, M. Odnoblyudov, and A. Romanov, Modelling of stress-deformed state in thin structured gallium nitride films on sapphire substrates, Physics of Solid State, 54, N12, 2012.

[5] D. M. Artemiev, V. E. Bougrov, M. A. Odnoblyudov, and A. E. Romanov, Mechanical stress control in GaN films on sapphire substrate via patterned nanocolumn interlayer formation, Physics Solid State (c), accepted, 2013.

[6] I. A. Smirnov, B. I. Smirnov, T. S. Orlova, D. Wlosewicz, A. Hackemer, H. Misiorek, J. Mucha, A. Jezowski, J. Ramirez-Rico, J. Martinez-Fernandez, Heat capacity of bio-SiC and ecoceramic SiC/Si produced on the basis of eucalyptus, beech and sapele wood. Physics of the Solid State, 55, N2, 2013.