Sintering in Snow and the Possible Role of Soluble Impurities

四十年的积雪烧结理论与观测和其他材料的理论不一致。水蒸气从冰粒到冰颈的扩散受表面曲率半径的控制，一直被认为是干雪中主要的烧结机制。然而，在烧结过程中，键的详细几何形状和雪的收缩与蒸汽扩散不一致，表明晶界和表面扩散联合是主要机制。雪含有微量的可溶性杂质，扫描电子显微镜证实了它们在孔隙表面和分离结合雪颗粒的边界上的浓度。金属的活化烧结采用少量掺杂剂降低粉末的熔化温度、活化能和晶界能，从而提高烧结速度和强度。在雪中烧结是一个类似的活化烧结过程吗？对比烧结理论导致对几何的预测，为在不同条件下识别相关过程提供了明确的测试。对于纯冰和不纯冰，我们可以区分操作机制，并通过一系列决定性的烧结实验，在广泛的温度、密度、粒度和几何形状范围内确定控制过程。扫描电子显微镜将产生从压实雪水平到微米尺度的烧结过程的时间序列观察，用于在现场收集的雪晶体以及在实验室中生长和允许烧结的雪晶体。原子力显微镜将把地形成分的研究带到分子尺度。冰的光学和电子散射特性使我们能够跟踪晶粒取向、晶界几何形状和迁移，以及从初始粘附到最后烧结阶段的孔隙演化。额外的实验室测量将表征烧结引起的机械、体积和微观结构性能的变化。这项工作的意义超越了烧结理论，从了解冬季沉积期间的积雪稳定性和雪崩危险，到确定影响春季径流中溶质浓度的因素。

Four decades of snowpack sintering theory are at odds with observations and with theory for other materials. Vapor diffusion from ice grains to necks, controlled by surface radii of curvature, has long been considered the dominant sintering mechanism in dry snow. However, the detailed geometry of bonds and the shrinkage of snow during sintering are inconsistent with vapor diffusion and suggest combined grain boundary and surface diffusion as the dominant mechanisms. Snow contains minute quantities of soluble impurities, and scanning electron microscopy confirms their concentration on pore surfaces and at the boundaries separating bonded snow grains. Activated sintering of metals employs small quantities of dopants to reduce the powder melting temperature, activation energy and grain boundary energy, thereby increasing the rate and strength of sintering. Is sintering in snow a similar activated sintering process? Contrasting sintering theories lead to predictions about geometry that offer clear tests for identifying relevant processes under different conditions. For both pure and impure ice, we may distinguish between operative mechanisms and identify the controlling processes with a decisive set of sintering experiments across a wide range of temperatures, densities, particle sizes and geometries. Scanning electron microscopy will produce time-series observations of the sintering process from the compacted snow level down to the micrometer scale for snow crystals gathered in the field as well as those grown and allowed to sinter in the laboratory. Atomic force microscopy will carry the topographic component of the investigation down to molecular scale. Optical and electron scattering properties of ice allow us to track grain orientation, grain boundary geometry and migration, and pore evolution from initial adhesion through final stage sintering. Additional laboratory measurements will characterize changes in mechanical, bulk and microstructural properties due to sintering. This work has implications beyond sintering theory, from understanding snowpack stability and avalanche hazard during winter deposition to identifying factors affecting solute concentrations in the spring runoff.