Jane R. Blackford

Sintering and microstructure of ice: a review

Sintering of ice is driven by the thermodynamic requirement to decrease surface energy. The structural morphology of ice in nature has many forms—from snowflakes to glaciers. These forms and their evolution depend critically on the balance between the thermodynamic and kinetic factors involved. Ice is a crystalline material so scientific understanding and approaches from more conventional materials can be applied to ice. The early models of solid state ice sintering are based on power law models originally developed in metallurgy. For pressure sintering of ice, these are based on work on hot isostatic pressing of metals and ceramics. Recent advances in recognizing the grain boundary groove geometry between sintering ice particles require models that use new approaches in materials science. The newer models of sintering in materials science are beginning to incorporate more realistic processing conditions and microstructural complexity, and so there is much to be gained from applying these to ice in the future. The vapour pressure of ice is high, which causes it to sublime readily. The main mechanism for isothermal sintering of ice particles is by vapour diffusion; however other transport mechanisms certainly contribute. Plastic deformation with power law creep combined with recrystallization become important mechanisms in sintering with external pressure. Modern experimental techniques, low temperature scanning electron microscopy and x-ray tomography, are providing new insights into the evolution of microstructures in ice. Sintering in the presence of a small volume fraction of the liquid phase causes much higher bond growth rates. This may be important in natural snow which contains impurities that form a liquid phase. Knowledge of ice microstructure and sintering is beneficial in understanding mechanical behaviour in ice friction and the stability of snow slopes prone to avalanches. (Some figures in this article are in colour only in the electronic version)

Sub-structure characterization of experimentally and naturally deformed ice using cryo-EBSD.

In this work, we present first results of high‐resolution EBSD for ice with a spatial resolution down to 0.25 μm. The study highlights the potential of EBSD to significantly increase our understanding of deformation and annealing processes associated with the build‐up of internal stresses due to strain incompatibility between grains. Two polycrystalline samples were analyzed: a natural sample of polar ice from the Vostok ice core (Antarctica) and an experimentally deformed sample of laboratory grown columnar ice. In summary, we observe the following: (1) inhomogeneous deformation through the grains is translated into lattice distortions that are concentrated mainly at grain boundaries and triple junctions (natural and experimental sample), (2) these distortions may be continuous (natural and experimental sample) or may form distinct tilt boundaries and sub‐grains of 10–50 μm size (experimental sample). These form mainly by rearrangement of basal edge dislocations into low‐energy configurations (i.e. tilt boundaries) in various prism planes. Continuous lattice distortions originate from screw or mixed edge and screw dislocations lying in the basal plane.

Ice friction, wear features and their dependence on sliding velocity and temperature

Friction processes for ice samples sliding on steel have been determined by examining wear and debris morphology with low-temperature scanning electron microscopy and relating the processes to the velocity and temperature of formation. Friction experiments were carried out over a temperature range of −27 to −0.5°C and velocity range of 0.008–0.37 m s −1 . Data were used to develop a friction map. Low friction ( µ −1 ), and low temperature (–25.1oC)–high velocity (0.30 ms −1 ) is due to the presence of liquid water which lubricates the sliding interface. Diagnostic morphologies for lubricated sliding include the presence of residual liquid in wear grooves and the development of a consolidated mass of debris on the trailing side of the wear surface with distinct grain boundaries and spheroidal air bubbles. High friction (µ > 0.15) at low temperature (−24.5oC)–low velocity (0.03 m s −1 ) results from insufficient lubrication at the sliding interface, leading to plastic deformation. Diagnostic morphologies of plastic deformation include scuffing features on the wear surface and the accumulation of sheets of unconsolidated debris on the trailing edge of the wear surface.

Scale-free statistics of plasticity-induced surface steps on KCl single crystals

Experimental investigations of plastic flow have demonstrated temporal intermittency as deformation proceeds in a series of intermittent bursts with scale-free size distribution. In the present investigation, a corresponding spatial intermittency is demonstrated for plastic flow of KCl single crystals. Deformation bursts lead to large surface steps with a height distribution that is consistent with the distribution of strain increments in deformation of micron-size samples and the energy distribution of acoustic emission bursts observed in deformation of macroscopic single-crystal samples of a wide class of materials.

Experimental identification of dynamic tire friction potential on ice surfaces

Recently, it has been shown that the tire–ice friction is characterized by a significant dynamic potential for abrupt increases of wheel torque. With the aim to gain insight into the dynamic tire friction potential features and to provide a comprehensive set of experimental data for model validation, a detailed experimental investigation of the dynamic potential has been presented in this article. The experimental data have been collected by using an experimental electrical vehicle with an in-wheel motor. Influence of the following operating parameters has been analyzed: rate of change of applied wheel torque, time for which the tire stands still on ice before applying the abrupt transient, initial vehicle speed, and initial tire force. In order to check a possible correlation between the dynamic potential and the rubber–ice friction dynamics, an experimental analysis based on a pin-on-disc tribometer has also been presented.

Low-temperature-SEM study of dihedral angles in the ice-I/sulfuric acid partially molten system

The transport and mechanical properties of partially molten materials are influenced by the wetting behaviour of the melt with respect to the crystalline solid. The equilibrium microstructure of an ice + melt system was examined using low‐temperature scanning electron microscopy. The samples were prepared by spraying a liquid solution of H2O‐H2SO4 into liquid nitrogen and packing the frozen particulates into aluminium capsules. Samples were then sintered at –35 °C or –55 °C (above the eutectic temperature, TE=–62 °C) for various durations and were quenched in liquid nitrogen to capture the equilibrium microstructure. This paper reports the first quantitative measurements of dihedral angle in this system. The measured median dihedral angle between the solid and vitrified melt is approximately 26 ± 2° at –35 °C and increases slightly as temperature decreases and approaches the solidus (32 ± 3° at –55 °C).

Stress-induced optical anisotropy in polycrystalline copper studied by reflection anisotropy spectroscopy

The optical properties of polycrystalline copper subjected to tensile stress are monitored in situ and in real time using reflection anisotropy spectroscopy (RAS). It is shown that RAS allows investigation of the plastic regime. Here, in contrast to the Hookes law regime, the stress-induced RAS lineshape is found to be dependent on the applied stress. The optical anisotropy in the visible region of the electromagnetic spectrum is directly proportional to the mechanical strain. An intense RAS peak observed at a photon energy of 4.0 eV is observed to saturate at a stress approximately equal to the yield stress of copper. This work demonstrates the potential of RAS as a nanomechanics monitor of materials under mechanical stress.

Optimising Sweeping Techniques for Olympic Curlers

In the sport of curling players sweep the ice in the front of curling stones to increase the distance that the projectiles slide. Their vigorous sweeping raises the surface temperature of the ice thereby reducing its coefficient of friction. The change in ice temperature is dependent on the velocity that curlers sweep the ice, the downward force they apply and the pattern that is swept. The forces and velocities applied by Olympic level curlers were recorded on an instrumented brush. A numerical model was used to determine optimal sweeping pattern based on the curlers sweep force and velocity profiles.

Fracture toughness of snow: the influence of layered microstructure

Brittle failure of snow is of key importance in the release of dry snow slab avalanches. Here we report results of a series of experiments to measure the fracture toughness of soft snow slabs. Like the majority of materials, snow is found to be least tough in tension (mode I). For a density of 300 kg m 3 the critical stress 1/2 intensity factor for crack propagation is found to be about 1.5 kPa m . For both in-plane and anti-plane (mode Π and mode III) shear failure, critical stress intensities ranged between 2 and 3 kPa m l / 2 , typically about 40% larger than the mode I values. Fracture toughness is found to depend significantly on the layered snow microstructure as, even in apparently homogenous snow, cracks running parallel to the natural layering resulted in critical stress intensities about 20% below those for cracks running in the perpendicular direction. A weak snow layer (depth hoar) was found to have about 50% of the mode III fracture toughness of the surrounding snow. These findings point to the importance of properly accounting for layering, and snow microstructure in general, in deformation and fracture experiments on snow. K e y w o r d s : Fracture toughness, slab avalanche, snow, microstructure * Corresponding author. E-mail address: M. Zaiser@ed.ac.uk Tel: -44-131-6505671; Fax: -44-131-6513470