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 7 *Member of ASM International EXPLORING THE MICROSTRUCTURE OF ICE Characterizing ice and snow is important not only for building accurate climate models, but also for activities such as relating the mechanical properties of sea ice to its microstructure so that the interaction of ice with ships and structures can be better understood. Ian Baker, FASM,* Dartmouth College, Hanover, N.H. I ce exists inmany different crystal struc- tures, but at the temperatures andpres- sures on Earth ice exists naturally only in the hexagonal form referred to as Ice I h (lattice parameters of a = 0.45227 nm; c = 0.73671 nm), which belongs to the P 6 3 / mmc spacegroup (Fig. 1). Ice crystals are generally quite large (>1mm) because ice exists on Earth, even in Antarctica, quite close to itsmelting point. Three nat- urallyoccurring forms of ice include snow, firn (multiyear snow), and ice. This article describes the microstructural characteri- zation of the third form: Ice. The interest in characterizing the microstructure of ice is, as with many materials, to relate the microstructure to the mechanical properties, to exam- ine for instance how ice flows under its own weight in polar ice sheets. Keep in mind that two forms of ice should be distinguished—freshwater ice and sea ice. Freshwater ice forms via snow com- paction, by freezing the water in rivers or lakes, or by accretion onto the bot- tomof ice sheets that overlay subglacial lakes. Unusually among materials, at its melting point the density of ice (917 kg.m -3 ) is less than its liquid form Fig. 1 — Crystal structure of ice I h , showing the hydrogen (black) and oxygen (white) atoms in relation to the hexagonal unit cell (dotted lines). Courtesy of Victor F. Petrenko. Secondary electron images of snow crystals. Courtesy of Si Chen.