Daniel Cullen

Observation of sulfate crystallites in Vostok accretion ice

Abstract Controlled sublimation in a low-vacuum scanning electron microscope has been used to reveal unusual crystalline features in the triple junctions of accretion ice from the Antarctic ice sheet above Lake Vostok. Energy dispersive X-ray microanalysis showed that the features were composed mostly of magnesium, sulfur and oxygen, although small amounts of sodium and calcium were also occasionally observed. Synchrotron X-ray topography showed that the ice grains contained dislocation densities ranging from ∼1×10 7 to >1×10 10 m −2 .

SEM/EDS observations of impurities in polar ice: artifacts or not?

ABSTRACT . A series of experiments was undertaken to determine the origin of fila-ments found in grainboundaries and impurity spots found in grain interiors of polar iceduring observation in the scanning electron microscope. It is shown that although thefilamentsareartifacts,theydemonstratethepresenceofimpuritiessegregatedtothegrainboundaryplanes.Itisalsodemonstratedthattheimpuritiesobservedinthegraininteriorreside there and were not transported fromthe grainboundaries during specimen prep-arationorobservation. INTRODUCTION Inanumberofrecentpapers(CullenandBaker,2000,2001,2002; Baker and Cullen, 2002; Iliescu and others, 2002;Bakerand others, in press; Obbard and others, inpress), wehaveusedsecondaryelectron (SE)imagingandenergy-dis-persive (X-ray) spectroscopy (EDS) in a low-vacuum scan-ning electron microscope (LVSEM) to examine the typesandmicrostructurallocationsofimpuritiesinpolarice-corespecimens held at 158K. Specimen preparation involvedshaving the surface flat in a cold room at 253K and thenallowingtheicetosublimateintheLVSEMattemperaturesof 158^213K. The sublimation caused the impurities to beconcentrated, after which the elements present could be

The structure and chemistry of 94 m Greenland Ice Sheet Project 2 ice

Abstract Optical microscopy, X-ray topography, scanning electron microscopy and X-ray spectroscopy have been used for microstructural analysis of ice from 94 m depth from Greenland Ice Sheet Project 2. the ice had a high density of air bubbles, a 2 mm grain-size and contained a high dislocation density in most grains (>1×1010 m–2), although a few lower-dislocation-density grains were present. the main impurities, Na and Cl, were found in several grain boundaries and triple points, often as filaments. Na and Cl were also found in the lattice, but to a lesser extent, along with S, Mg, K and Ca. These observations are compared to previous studies of dislocations and impurity locations in natural ice.

Preliminary microstructural and microchemical observations on pond and river accretion ice

Abstract Scanning electron microscopy and X-ray microanalysis were employed to investigate the morphology of impurities present in pond and river freshwater ice. Several structural features, 0.4 to roughly 1.2 mm in diameter, with distinct morphological characteristics were identified in the pond ice. The features reside both in the interior of the grains as well as at grain boundaries. In river ice, the impurity aggregates were significantly smaller than in the pond ice (the majority with sizes ⪝20 μm) with no particular shape and were concentrated in the grain boundaries.

X-ray Topographic Observations of Dislocation/Grain Boundary Interactions in Ice

Abstract Dislocation/grain-boundary (GB) interactions have been studied in situ in polycrystalline ice using synchrotron X-ray topography in the temperature range 0° to –15°C GBs were observed to act both as sources of lattice dislocations and as strong obstacles to dislocation motion. Dislocations were observed to form pile-ups at GBs upon loading. Generally the basal slip system with the highest Schmid factor was found to be the most active, and dislocations were emitted from GB facets as semi-hexagonal loops in order to relieve the stress build-up from GB sliding. When the relative orientation of two adjacent grains and the orientation of the GB between them with respect to the loading direction discouraged GB sliding, thus suppressing dislocation nucleation at the GB, dislocations originating in one grain piled up at the GB and led to slip transmission through the GB The latter geometrical arrangement is rarely encountered, suggesting that slip transmission through grain boundaries in ice is a rare event. When basal slip was suppressed, i.e. when the loading direction lay in the basal plane, slip occurred by the glide of a fast edge segment on non-basal planes.

Dislocation motion around loaded notches in ice single-crystals

Abstract Synchrotron X-ray topography has been used to study dislocation behavior around notches in single-crystals of ice during in-situ deformation at a constant strain-rate. During deformation at −8°C a dislocation-depleted zone (DDZ) formed above the notch while a vertical array of dislocations formed below the notch. Modeling the interaction between basal plane dislocation loops and the notch stress field suggested that this DDZ arose from dislocations gliding completely through the specimen. Dislocation activity around the notch at −8°C was much greater than that observed at −40°C and −60°C.

Dislocation motion around loaded notches in ice single crystals

Synchrotron X-ray topography has been used to study dislocation behavior around a notch in single crystal ice during in-situ deformation at a constant strain-rate of 1 × 10 −8 s −1 and a temperature of -8 °C. During deformation a dislocation depleted zone (DDZ) formed above the notch. Modeling the interaction between basal plane dislocation loops and the notch stress field suggested that this DDZ arose from dislocations gliding completely through the specimen.