Deformation at small-scale: insights from experiments and simulations

Resume : The trend of miniaturization has revealed exciting novel material properties at small length scales. This demands miniaturized testing techniques to determine for instance the residual stresses and fracture properties in miniaturized systems such as thin films. In this presentation, we will review recent developments regarding the measurement of residual stresses and miniaturized fracture properties using in situ SEM experiments. In detail we will discuss challenges and benefits of the presented approach by examine the residual stresses and fracture properties of single layer and multilayer thin film systems. Further, we address possible limitations regarding the data analysis as well as the development of accompanied finite element modeling for the determination of crack driving forces. The presented work shall give an approach to understand novel material properties resulting due to the ongoing trend of miniaturization.

Resume : Good mechanical and electrical properties place copper among the materials, which are the most frequently used for operation in extreme conditions. One of such application is to use copper as a construction material for high gradient accelerating structure in linear particle accelerators. A major problem in the design of such accelerators is that the high electric field causes electric discharges in form of arcs to appear inside the structures, despite an ultra high vacuum. One hypothesis for these arcs are that they are triggered by nano-sized tips on the copper (Cu) surface. In order to study the feasibility of the tip hypothesis, it becomes important to know under which conditions such tips are stable and for how long. We have for this purpose developed a new Kinetic Monte Carlo model designed especially for studying the surface evolution of metals and Cu in particular by considering the atom migration jumps, described by tabulated migration barriers calculated using the Nudged Elastic Band method. We have validated our model by comparing with Molecular Dynamics simulations of diffusion of Cu tips at different kinds of surfaces and different temperatures. We obtained good agreements. Using the model, we found a significant stability of 13 nm high tips at room temperature are stable for hours. At temperatures above 800 K, we found that the same tip will disappear in less than a microsecond, indicating a strong temperature dependence on the flattening mechanism.

Resume : We used a combination of Monte Carlo (MC) and molecular dynamics MD to study Cu(x)Ag(1-x)|Ni multilayers with 5 nm and 25 nm layer width. We find that Cu-Ni multilayers form a semi-coherent interface with a network of partial dislocations arranged in a regular triangular pattern. Silver is then alloyed to the copper layers in order to tune the lattices misfit in a controlled way. A combination of MC and MD was used to equilibrate Cu(x)Ag(1-x)|Ni with x=0%, 5% and 10% towards the thermodynamic equilibrium. We find segregation of Ag within the Cu layers at 300 K and 600 K and mixing of Cu and Ni at 600 K in good agreement with the experimental binary phase diagrams of the Cu-Ag and Cu-Ni systems. Ag segregates preferentially at the nodes of the triangular misfit pattern. The equilibrated structures were then sheared parallel and perpendicular to the normal of the bilayer interface. We generally find (1) initial sliding at the Cu|Ni interface within the dislocation network, (2) followed by emission of partial dislocations into the Cu layer, and finally (3) an increase of yield stress with increase of interface roughness and silver content.

Resume : Nanowires show unique deformation behaviour resulting from the nanoscale size effect. This specific behaviour can cause unexpected structural reorganizations of atoms inducing the creation of novel functional materials as well as innovative heterostructures. We report on a stress induced martensitic phase transformation in Ge nanowires attributed to the size effect.<111>-oriented Ge nanowires with standard diamond structure (3C) undergo plastic deformation under external stress leading to a phase transformation toward the hexagonal 2H-allotrope. The obtained nanostructures exhibit Ge-2H nano-domains heterogeneously embedded along their length. This polytypism was successfully achieved from both bottom-up and top-down fabrication approaches with diameters ranging from 40 to 230 nm. Uncommon phases of Ge have been of high interest for researchers for the past two decades. Theoretical studies predict in particular a direct small gap in Ge-2H. Thereby, this novel heterostructure 2H/3C in Ge nanowires can pave the way to exciting applications of group-IV material for next-generation devices. For instance, the periodic formation of phase boundaries could induce a strong reduction of thermal conductivity while the electronic conductivity is conserved therefore enhanced thermoelectric properties are expected. Understanding the origin of this metastable phase formation is critical to evaluate the potential of those technological applications. The generation of the 2H allotrope was observed in both silicon and germanium upon hot indentation. The cubic to hexagonal diamond phase transformation was assigned to a martensitic phase transformation where a mechanism of local relaxation stress in the region of twin intersection was proposed. Structural (HRTEM) and physical characterizations have been performed in our transformed nanowires. We have studied various key parameters such as the diameters of the nanowires and the temperature of the transformation. A strain-induced martensitic transformation could account for the transformation in nanowires with a threshold temperature of 300° C; below this temperature the stress induces plastic deformation. In the case of nanowires twin-twin interaction seems yet not to be a relevant mechanism. Raman shift evidence both a residual stress within the nanostructures and the presence of the 2H phase.

Resume : MIT - Dept. of Materials Science and Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139-4307 USA

Resume : Metal oxide nanowires have attracted considerable attention during past decades. The fundamental physical and chemical properties of these nanostructures have been widely investigated. Furthermore, the interfacial reactions of these nanostructures and metals also affect the performance of electronic devices that are one of the main applications of nanomaterials. With in situ transmission electron microscopy (TEM), we could provide essential information for the interface dynamics and phase transformation of nanomaterials. In the present work, we demonstrate the in situ TEM observation of the interfacial dynamics with In metal/SnO2 nanowire and Sn metal/In2O3 nanowire. Both metal oxide nanowires are fabricated by vapor-liquid-solid (VLS) method. The nanowires are dispersed on the grids with a 50 nm thick Si3N4 window. Afterwards, different metals with thicknesses of 10 to 25 nm were deposited directly by thermal deposition. The thermal treatment of the sample is carried out with the heating element built in the double tilt heating holder (JEOL EM-21240) and calibrated by the infrared pyrometer. Furthermore, we also use scanning electron microscopy and photoluminescence to analyze these diffusion phenomena and dynamics transformations. A number of efforts have been devoted to obtain fundamental information on the interfacial reaction of metal/metal oxide nanowires. The present experiments have been able to investigate in detail the mechanism of the interfacial dynamics of these nanostructures. Several different kinetic phenomena have been observed through ultra high vacuum (UHV) - in situ TEM system, such as reverse VLS process, metal oxide compound formation and diffusion with metal atoms. The diverse phenomena are correlated to the variation in temperature, structure size and vapor pressure.

Resume : Inspired by the interest raised by Porous Graphene (recently synthesized) and other structures, like the theoretically predicted two-dimensional carbon allotrope Biphenilene Carbon (BPC) and by the topological relation between Graphene and Carbon nanotubes, we propose and investigate a new class of structures, the Porous Nanotubes. They are based on carbon nanotubes architecture and constructed by rolling up Porous Graphene (or BPC membrane) in order to form a Nanotube like structure with an organized porous pattern. We investigate structural and electronic properties of this class of quasi-one-dimensional materials using complementary methodologies. The Reactive Force Field (ReaxFF), used for structural stability and thermal properties calculations and the so called Density Functional Tight Binding (DFTB) method, for electronic structure calculations. Taking into account the possibility of tailoring some physical properties of these materials by geometrical modifications, electronic structure calculations were carried at equilibrium geometry and under mechanical stress, caused by small-scale deformations. Two classes of "Porous Nanotubes" were considered, one derived from Porous Graphene membranes and the second one constructed by rolling up BPC membranes. Calculations were carried for several diameter sizes and chiralities. We investigate also the possible use of these Porous Nanotubes as molecular sieves for selective separation of diatomic gases, like H2 from a mixture.

Resume : Bulk crystalline materials can often be sorted in two categories: on one hand metallic materials that are ductile in a broad range of conditions, and on the other hand materials like semiconductors, ceramics and minerals, which are brittle at ambient conditions. However some oxides, although referred to as ceramics, are known to show some ductility even at ambient temperature, like magnesium oxide (MgO). More surprisingly strontium titanate (SrTiO3) not only has a usual ductile-to-brittle transition when cooled below 1300 K, but it becomes ductile again below 1000 K. This inverse transition, quite unusual for a ceramic, raises questions about the mechanisms responsible for the plastic deformation in this class of materials. Recent experimental and theoretical studies allow for a better understanding of the mechanical behavior of perovskite materials, emerging from the atomic-scale structure and mobility of dislocations. We will present three examples: strontium titanate (SrTiO3) which is cubic at all temperatures; potassium niobate (KNbO3), a ferroelectric perovskite that exists in four different phases depending on temperature; and magnesium silicate (MgSiO3), a mineral that is stable only at very high pressure. The slip systems, dislocations properties, and mechanical behavior of these materials will be presented and compared. It will be shown that although all these materials belong to the same structural family, they do not form an isomechanical group and each perovskite has specific properties of its own.