Collaborative Research: The Impact of Impurities and Stress State on Polycrystalline Ice Deformation

极地冰盖的冰是地球上最纯净的物质之一，然而少量的杂质——比如酸——对冰的流动方式和从冰芯中了解过去的气候非常重要。该项目的目标是通过比较纯和硫酸掺杂样品的变形行为来了解这些酸对多晶冰变形的作用。之所以选择硫酸，一方面是因为它对解释过去的气候很重要，另一方面是因为它可以在低温下导致冰中的水脉。这项工作将侧重于酸在多晶冰中的位置、运动和影响，多晶冰比单晶冰更复杂。通过变形样品并进行微观结构表征，将评估酸对变形速率、晶粒演化和酸本身运动的作用。这项工作将培养一名达特茅斯学院的博士生，并将本科生引入华盛顿大学和达特茅斯学院的研究。尽管普遍使用的冰的本构关系通常被称为“格伦流动定律”，但存在显著的不确定性，特别是关于杂质的作用和定向织物的发展。本项目旨在通过对纯冰和掺硫酸冰进行变形试验和微观结构表征，改善冰的本构关系。该项目将重点研究硫酸对冰粘度、结构演变和扩散的影响。硫酸对多晶冰的力学性能既有直接影响，也有间接影响。直接影响改变位错速度和/或密度，间接影响改变晶粒尺寸和织构。这些影响的复杂性和相互作用意味着不可能仅仅通过检查冰芯标本来了解硫酸的影响。在这个项目中，该团队将变形四种类型的冰：实验室生长的冰样品掺杂了与自然浓度相似的硫酸，实验室生长的高纯度冰，分层掺杂和纯冰，以及来自南极冰芯的天然冰。变形将在单轴压缩和单轴剪切中进行。添加简单的剪切试验对于将实验室观测到的变形行为与极地冰盖的行为联系起来是至关重要的，在极地冰盖中，剪切应变主导着基底冰的冰运动。在变形至5%至25%的应变后，微观结构的发展将通过各种扫描电子显微镜技术，拉曼显微镜，基于同步加速器的纳米x射线荧光和离子色谱法进行评估。这些分析技术将允许确定1)杂质的偏析和运动，2)晶界迁移的速度，3)再结晶晶粒的数量；4)冰晶的完整方向。这些结果将使硫酸影响的微观结构模拟和冰芯扩散的数值模拟成为可能。最终结果将是更好地理解冰的变形，从而改进冰芯解释和冰盖建模。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The ice of the polar ice sheets is among the purest substances on Earth, yet the small amount of impurities --such as acids-- are important to how the ice flows and what can be learned from ice cores about past climate. The goal of this project is to understand the role of such acids on the deformation of polycrystalline ice by comparing the deformation behavior of pure and sulfuric acid-doped samples. Sulfuric acid was chosen both because of its importance for interpreting past climate and because it can lead to water veins in ice at low temperatures. This work will focus on the location, movement, and impact of acids in polycrystalline ice that are more complex than in single crystals of ice. By deforming samples and performing microstructural characterization, the role of acids on deformation rate, grain evolution, and the movement of the acids themselves, will be assessed. The work will lead to the education of a Ph.D. student at Dartmouth College, introduce undergraduate students to research at both the University of Washington and Dartmouth College. Despite the ubiquitous use of the constitutive relation for ice commonly referred to as "Glen's Flow Law", significant uncertainty exists particularly with regard to the role of impurities and the development of oriented fabrics. The aim of this project is to improve the constitutive relationship for ice by performing deformation tests and microstructural characterization of pure and sulfuric acid-doped ice. The project will focus on sulfuric acid's impact on ice viscosity, fabric evolution, and diffusivity. Sulfuric acid can have both direct and indirect effects on the mechanical properties of polycrystalline ice. The direct effects change the dislocation velocity and/or density, and the indirect effects change the grain size and fabric. The complexity and interaction of these effects means that it is not possible to understand the effects of sulfuric acid by simply examining ice core specimens. In this project, the team will deform four types of ice: lab-grown ice samples doped with similar-to-natural concentrations of sulfuric acid, lab-grown high-purity ice, layered doped and pure ice, and natural ice from Antarctic ice cores. Deformation will be performed in both uniaxial compression and simple shear. The addition of simple shear tests is critical for relating the laboratory-observed deformation behavior to the behavior of polar ice sheets where the shear strain dominates ice motion in basal ice. After deformation to strains from 5 percent up to 25 percent, the microstructural development will be assessed with methods including a variety of scanning electron microscope techniques, Raman microscopy, synchrotron-based Nano-X-ray fluorescence, and ion chromatography. These analysis techniques will allow the determination of 1) the segregation and movement of impurities, 2) the rate of grain-boundary migration, 3) the number of recrystallized grains; and 4) the full orientation of the ice crystals. The results will enable both microstructural modeling of the effects of sulfuric acid and numerical modeling of diffusion in ice cores. The net result will be a better understanding of ice deformation that improves ice-core interpretation and ice-sheet modeling.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.