J. Weissmüller

A general theory of curved deformable interfaces in solids at equilibrium

Abstract We discuss the deformation of a curved interface between solid phases, assuming small strains in the bulk phases and neglecting accretion at the interfaces. Such assumptions are relevant to the deformation of solid microstructures when atomic diffusion and the formation of defects such as dislocations are negligible. We base our theory on a constitutive equation giving the (excess) free energy ψ of the interface when the interfacial limits of the displacement fields in the abutting phases as well as the limits of the displacement gradients are known. Using general considerations of frame invariance, we show that ψ can depend on these quantities at most through: firstly the normal and tangential components of the jump in displacement at the interface (stretch and slip), secondly the average of the projected strain in the tangent plane (average tangential strain), thirdly the tangential component of the jump in the projected displacement gradient at the interface (relative tangential strain and rel...

Surface-chemistry-driven actuation in nanoporous gold

Although actuation in biological systems is exclusively powered by chemical energy, this concept has not been realized in man-made actuator technologies, as these rely on generating heat or electricity first. Here, we demonstrate that surface-chemistry-driven actuation can be realized in high-surface-area materials such as nanoporous gold. For example, we achieve reversible strain amplitudes of the order of a few tenths of a per cent by alternating exposure of nanoporous Au to ozone and carbon monoxide. The effect can be explained by adsorbate-induced changes of the surface stress, and can be used to convert chemical energy directly into a mechanical response, thus opening the door to surface-chemistry-driven actuator and sensor technologies.

Alloy effects in nanostructures

Abstract Thermal stability and microstructure of nanocrystalline alloys are determined by the interaction of the chemical components with the topological defects. An important aspect of this interaction is the reduction of the grain boundary specific energy by grain boundary segregation. Based on very general thermodynamic considerations, this work presents a concept for the stabilization of nanocrystalline solids against grain-growth by grain boundary segregation. It is predicted that, for alloy systems with a large heat of segregation, the poly- or nanocrystalline alloy is in a metastable state for a particular grain-size which decreases with increasing concentration of the solute element. In the metastable state, the specific grain boundary energy is zero. A narrowing of the glass-forming region in glass-forming alloy systems is predicted when the metastable polycrystal can form.

Approaching the theoretical strength in nanoporous Au

The mechanical properties of nanoporous Au have been investigated by uniaxial compression. Micron-sized columns were machined in the surface of nanoporous Au using a focused Ga+ beam and compressed with a flat punch in a nanoindenter. Using scaling laws for foams, the yield strength of the 15nm diameter ligaments is estimated to be 1.5GPa, close to the theoretical strength of Au. This value agrees well with extrapolations of the yield strength of submicron, fully dense gold columns and shows that in addition to foam density and structure, the absolute size of ligaments and cell walls can be used to tailor foam properties.The mechanical properties of nanoporous Au have been investigated by uniaxial compression. Micron-sized columns were machined in the surface of nanoporous Au using a focused Ga+ beam and compressed with a flat punch in a nanoindenter. Using scaling laws for foams, the yield strength of the 15nm diameter ligaments is estimated to be 1.5GPa, close to the theoretical strength of Au. This value agrees well with extrapolations of the yield strength of submicron, fully dense gold columns and shows that in addition to foam density and structure, the absolute size of ligaments and cell walls can be used to tailor foam properties.

Nanoporous Au-Pt alloys as large strain electrochemical actuators.

Nanoporous Au-Pt alloys with pore- and ligament size down to few nanometers were fabricated by dealloying Ag-Au-Pt. Owing to the small structure size and large specific surface area, the surface stress and its variation give rise to significant stress and strain in the bulk of these materials. In fact, dilatometry experiments find electrochemical actuation with large reversible strain amplitude. The linear strain reaches approximately 1.3% and strain energy density is up to 6.0 MJ/m(3). The associated stresses may approach the elastic limit of the alloy.

Mean stresses in microstructures due to interface stresses: A generalization of a capillary equation for solids

Abstract We derive a general expression for the mean stress in a solid induced by the stresses of areal and linear defects such as grain boundaries and triple junction lines. The mean stress is a function of measurable stereological quantities, weighted areas of interfaces and lengths of the linear defects, not their curvatures. Generally the stress is non-hydrostatic. For anisotropic and textured microstructures, the mean stress depends on stereological moments of the interface orientation distribution function, but otherwise it is independent of the geometry of the microstructure. In two limiting cases, isolated spherical particles (in a matrix or not) and layer structures, the solution for the stress reduces to results which have been derived previously. These solutions can be used to determine average interface stresses from experimental lattice constant data. Although the specific interface area is always the important quantity in this relation, for the spheres this area/volume reduces to the particle curvatures commonly used in the literature for many microstructures.

Nanocrystalline materials: a way to solids with tunable electronic structures and properties?

It is the purpose of this paper to point out that nanometer-sized structures may open the way to modify the electronic structure (e.g. the charge carrier density) and the related properties of solids. The modification may be achieved by means of externally applied electric fields or by internal electric fields. These fields affect the charge carrier density in a surface region which in the case of nanostructured solids may attain a substantial fraction of the total volume. Tuning the charge carrier density by means of an externally applied electric field may be realized, for example, by immersing a chain (or a network of chains) of interconnected, nanometer-sized particles into an electrolyte. Similarly, the electronic structure of nanocomposites is proposed to deviate from the one of coarse-grained materials if a large volume fraction of the material consists of electronically modified regions at interphase boundaries. Experimental observations supporting these ideas are discussed.

ALLOY THERMODYNAMICS IN NANOSTRUCTURES

The importance of the interactions between alloy atoms and topological defects for the thermodynamic properties of nanostructured alloys is pointed out. The McLean model for grain boundary segregation is extended to yield an expression for the total Gibbs free energy of an alloy polycrystal. This provides a simple conceptual basis for a qualitative discussion of the thermodynamic properties of nanocrystalline alloys. It is demonstrated that certain alloy poly- or nanocrystals may reach a metastable state, where the alloy is stable with respect to variation of its total grain boundary area.

Microstructure evolution during rolling of inert-gas condensed palladium

During cold-rolling of nanocrystalline Pd consolidated from clusters we observed a strong increase in stacking fault density, conclusive evidence for lattice dislocation activity. However, the absence of texture and the retention of an equiaxed grain shape even after large deformation suggested grain boundary sliding and grain rotation as concurring processes. The rate of tensile creep at 313 K and at low stress is in agreement with predictions for Coble creep.

A material with electrically tunable strength and flow stress.

Changes in a layer of oxygen absorbed onto nanoporous gold reversibly affect its mechanical properties. The selection of a structural material requires a compromise between strength and ductility. The material properties will then be set by the choice of alloy composition and microstructure during synthesis and processing, although the requirements may change during service life. Materials design strategies that allow for a recoverable tuning of the mechanical properties would thus be desirable, either in response to external control signals or in the form of a spontaneous adaptation, for instance in self-healing. We have designed a material that has a hybrid nanostructure consisting of a strong metal backbone that is interpenetrated by an electrolyte as the second component. By polarizing the internal interface via an applied electric potential, we accomplish fast and repeatable tuning of yield strength, flow stress, and ductility. The concept allows the user to select, for instance, a soft and ductile state for processing and a high-strength state for service as a structural material.