January_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 8 2 8 (1000 kg.m -3 ), which is why it floats on water. Work on freshwater ice has mostly focused on polar ice and labo- ratory-grown ice with less work on river and lake ice, and little on accretion ice. For freshwater ice, one would like to relate the grain size and orientation, soluble impurities, particles, and dis- location density to the flow behavior and texture development in ice sheets. This would enable the flow of ice sheets to be modeled on a fundamen- tal physics-based level rather than on a semi-empirical basis as it is now. Key features of the behavior of ice under load is that critical resolved shear stress for non-basal slip is an order of magni- tude greater than for slip on the basal plane [1] , and that dynamic recrystalliza- tion in ice occurs at low strains of the or- der of 5-10%. Understanding the flow of ice sheets also feeds into the interpreta- tion of paleoclimate derived from gases in the air bubbles within the ice sheets. For sea ice, there is great interest in relating the mechanical properties to the microstructure so that the interac- tion of sea ice with ships and structures can be modeled. Further, determining the brine network connectivity is essen- tial for determining the permeability of sea ice, which is itself important for un- derstanding air-ice-ocean interactions. MICROSCOPY TECHNIQUES Ice has been characterized using both optical microscopy and scanning electron microscopy (SEM) techniques. The grain structure of ice and c -axis direction can be determined optical- ly: A thin section (0.5-1 mm thick) of ice is viewed in transmission between crossed polarized gratings. If no spec- imen is present, then no light passes through the crossed gratings. Ice is bire- fringent and rotates the light. Thus, dif- ferently oriented ice crystals or grains exhibit different colors; when viewed directly along the c -axis, the ice crystal appears black. This feature can be used to determine the c -axis orientation of all the crystals in a thin section using a so-called Rigsby or universal stage on which the sample is rotated and tilted until each appears black. The orienta- tions of all crystals determined in this way can be used to construct a pole figure of the c -axis of the crystals, but the method does not allow the a -axis to be determined. This process is rath- er tedious and automated ice analyzers have been developed that can rapid- ly produce c -axis pole figures [2] . For sea ice, in addition to columnar grains, brine pockets that are formed when salt is rejected from the freezing water can also be observed in optical images. While optical microscopy can re- veal brine pockets in ice, it does not provide a good 3D picture of their dis- tribution. Micro computed tomogra- phy (CT) can provide 3D information because the x-ray attenuation between ice and brine is sufficiently different. Fig. 2 shows a micro CT reconstruc- tion of sea ice in which the ice, brine pockets, and air pockets can be clear- ly seen [3] . The micro CT can be used to quantify the detailed topological char- acteristics of brine networks [4,5] . For freshwater ice, a micro CT can be used to show the distribution of dust par- ticles, tephra, and air bubbles as long as they are >5 µm. Micro CT cannot be used to image the ice crystals because the x-ray attenuation does not depend on grain orientation. However, this is an area where diffraction-contrast syn- chrotron x-ray tomography has the po- tential to relate the grain structure and orientation directly to the mechanical properties of ice using in-situ deforma- tion experiments. The use of an SEM to examine ice dates back to the early work of Cross in 1969 [6] . In 1989, Schulson et al. used an SEM to examine fracture surfac- es in strained ice [7] , while in 1988, Mul- vaney, Wolff, and Oates were the first to use SEM combined with energy disper- sive x-ray spectroscopy (EDS) to deter- mine the location of soluble impurities in ice [8] . They coated the ice with alu- minum to prevent charging, which has the disadvantages of the difficulty of the aluminum coating itself and the problem that the aluminum absorbs x-rays generated in the specimen. In 2000, Cullen and Baker demonstrated that EDS data could be obtained from Fig. 2 — Micro CT reconstructions of a sea ice specimen viewed from above. The brine (green) and air (red) are shown separately and then together; ice appears black. Volume is 7.4 x 7.4 x 14.5 mm [26] . Fig. 3 — Secondary electron image shows three grains and a triple junction in ice from a depth of 2563 m from the GISP2 core, which was drilled at Summit, Greenland. EDS data are from the NaCl filament that was present along the grain boundaries. The oxygen peak is from the ice. Courtesy of D. Cullen.