Dynamics of premelted films: Frost heave in a capillary

Polycrystallinity Enhances Stress Buildup around Ice

Damage caused by freezing wet, porous materials is a widespread problem but is hard to predict or control. Here, we show that polycrystallinity significantly speeds up the stress buildup process that underpins this damage. Unfrozen water in grain-boundary grooves feeds ice growth at temperatures below the freezing temperature, leading to fast stress buildup. These stresses can build up to levels that can easily break many brittle materials. The dynamics of the process are very variable, which we ascribe to local differences in ice-grain orientation and to the surprising mobility of many grooves-which further accelerates stress buildup. Our Letter will help understand how freezing damage occurs and in developing accurate models and effective damage-mitigation strategies.

Impact of solid-liquid interfacial thermodynamics on phase-change memory RESET scaling

A model of the RESET melting process in conventional phase-change memory (PCM) devices is constructed in which the Gibbs-Thomson (GT) effect, representing local equilibrium at the solid-liquid interface, is included as an interfacial condition for the electro-thermal model of the PCM device. A comparison is made between the GT model and a commonly used model in which the interfacial temperature is fixed at the bulk melting temperature of the PCM material. The model is applied to conventional PCM designs in which a dome-shaped liquid/amorphous region is formed. Two families of solutions are computed representing steady state liquid regions, distinguished by their thermodynamic aspects. There is a family of solutions representing a hypothetical liquid nucleation process, and a family of larger steady-state liquid solutions representing the limit of the melting process. These 'melting limits' enable calculation of minima in voltage and corresponding current required for the RESET process. In this PCM configuration, the GT effect constrains the equilibrium solid-liquid interface temperature to remain above the bulk melting temperature during melting. The magnitude of this temperature difference increases with decreasing device size scale, thus requiring an increase in the required voltage and current needed for RESET compared to the case in which the interface temperature is approximated by the bulk melting temperature. This increase becomes substantial for active device dimensions in the <20 nm range. The impact of this phenomena on PCM device design is discussed.

External Field Assisted Freeze Casting

Freeze casting under external fields (magnetic, electric, or acoustic) produces porous materials having local, regional, and global microstructural order in specific directions. In freeze casting, porosity is typically formed by the directional solidification of a liquid colloidal suspension. Adding external fields to the process allows for structured nucleation of ice and manipulation of particles during solidification. External control over the distribution of particles is governed by a competition of forces between constitutional supercooling and electromagnetism or acoustic radiation. Here, we review studies that apply external fields to create porous ceramics with different microstructural patterns, gradients, and anisotropic alignments. The resulting materials possess distinct gradient, core–shell, ring, helical, or long-range alignment and enhanced anisotropic mechanical properties.

Confinement effects in premelting dynamics

We examine the effects of confinement on the dynamics of premelted films driven by thermomolecular pressure gradients. Our approach is to modify a well-studied setting in which the thermomolecular pressure gradient is driven by a temperature gradient parallel to an interfacially premelted elastic wall. The modification treats the increase in viscosity associated with the thinning of films, studied in a wide variety of materials, using a power law and we examine the consequent evolution of the confining elastic wall. We treat (1) a range of interactions that are known to underlie interfacial premelting and (2) a constant temperature gradient wherein the thermomolecular pressure gradient is a constant. The difference between the cases with and without the proximity effect arises in the volume flux of premelted liquid. The proximity effect increases the viscosity as the film thickness decreases thereby requiring the thermomolecular pressure driven flux to be accommodated at higher temperatures where the premelted film thickness is the largest. Implications for experiment and observations of frost heave are discussed.

Surface transport in premelted films with application to grain-boundary grooving

We present a new model of surface transport in premelted films that is applicable to a wide range of materials close to their melting points. We illustrate its use by applying it to the evolution of a grain-boundary groove in a high vapor pressure material and show that Mullins's classical equation describing transport driven by gradients in surface curvature is reproduced asymptotically. The microscopic contact angle at the groove root is found to be modified over a thin boundary layer, and the apparent contact angle is determined. An explicit transport coefficient is derived that governs the evolution rate of systems controlled by surface transport through premelted films. The transport coefficient is found to depend on temperature and diverges as the bulk melting temperature is approached.

Interfacial premelting and the thermomolecular force: thermodynamic buoyancy

The presence of a substrate can alter the equilibrium state of another material near their common boundary. Examples include wetting and interfacial premelting. In the latter case, temperature gradients induce spatial variations in the thickness of the premelted film that reflect changes in the strength of the repulsion between the substrate and the solid. We show that the net thermomolecular force on a macroscopic substrate is equivalent to a thermodynamic buoyancy force-proportional to the mass of solid that can occupy the volume enclosed by the substrate and the temperature gradient.

Frost heave in physisorbed films: vapor flow and substrate effects

The magnitude of vapor flow accompanying the surface-melted liquid flow in physisorbed multilayer films due to thermomolecular pressures was estimated. It was found that the vapor flow is significant compared to the flow of the surface-melted liquid in thick films. As the flows continue, the film thickness profile evolves into one determined by a dynamic equilibrium of vapor pressure. The flow of surface-melted liquid stops when the dynamic equilibrium is reached.