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。长期以来，由表面曲率半径控制的水蒸气从冰粒扩散到颈部一直被认为是干燥雪中的主要烧结机制。然而，烧结过程中键的详细几何形状和雪花的收缩与蒸汽扩散不一致，表明晶界和表面扩散相结合是主要机制。雪中含有微量的可溶杂质，扫描电子显微镜证实了它们在孔隙表面和分隔结合雪粒的边界上的浓度。金属的活化烧结中加入少量的杂质，可以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。雪地烧结也是一种类似的活化烧结过程吗？对比烧结理论导致了对几何形状的预测，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，我们可以通过一套针对广泛的温度、密度、颗粒大小和几何形状的决定性的烧结实验来区分操作机制和确定控制过程。扫描电子显微镜将为野外采集的雪晶以及实验室中生长并允许烧结的雪晶产生从压实的雪层到微米级的烧结过程的时间序列观察。原子力显微镜将把研究的地形部分深入到分子尺度。冰的光学和电子散射特性使我们能够跟踪从最初的粘结到最后的烧结阶段的颗粒取向、晶界几何形状和迁移以及气孔的演变。更多的实验室测量将表征由于烧结而引起的机械、块体和微观结构特性的变化。这项工作的意义超越了烧结理论，从了解冬季沉积过程中积雪的稳定性和雪崩危险，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.