Diffusion of H bearing species in silicate glasses at low temperatures - development of a newexperimental technique

在相对较低的温度(低于200摄氏度)下，H在玻璃中的传输与许多应用有关，如考古文物的Obisidian测年、古气候研究、高放射性核废料的储存、火山灰的产生、玄武岩的生物地球化学风化及其对各种地球化学循环的影响、设计玻璃的机械性能、了解玻璃电极在化学分析中的行为以及一些光学数据存储设备的理解。在这些条件下，很难通过实验测量含氢物种的扩散系数，因为水是最常用的氢的来源，通过溶解-沉淀反应与玻璃反应，很难分离扩散过程。低温下缓慢的扩散速度增加了进一步的复杂性。然而，在较高温度下测量的扩散系数的外推不足以描述低温过程产生的浓度梯度的观测形状。我们已经发展了几个新的实验方面，特别是(A)在不存在游离H2O的情况下生产含H的非晶态薄膜作为氢源的能力，以及(B)在纳米尺度上以高空间分辨率测量与物种无关的低浓度H的能力。在这个项目中，它打算利用这些发展来探索含氢物种在迄今无法进入的条件下的扩散。一个具体的目标是表征在这些条件下氢扩散的成分依赖性，并了解导致不同于在更高温度下观察到的行为的扩散机制的变化。

The transport of H in glasses at relatively low temperatures (below 200 °C) is relevant for a number of applications such as obisidian dating of archaeological artefacts, palaeoclimate studies, storage of high level nuclear waste, production of volcanic ash, biogeochemical weathering of basalts and its implications for various geochemical cycles, engineering the mechanical properties of glasses, understanding the behaviour of glass electrodes in chemical analysis, and the understanding of some optical data storage devices.Experimental measurement of diffusion coefficients of H-bearing species has been difficult to impossible at these conditions because water, the most commonly used source of H, reacts with glass by dissolution - precipitation reactions and it is difficult to isolate the process of diffusion. Sluggish diffusion rates at low temperatures add further complications. Yet, extrapolation of diffusion coefficients measured at higher temperatures is shown to be inadequate for describing the observed shapes of concentration gradients produced by low temperature processes. We have developed several new experimental aspects, and in particular (a) the ability to produce H-bearing amorphous thin films to act as a source of H without the presence of free H2O, and (b) the ability to measure low concentrations of H, independent of the speciation, with a high spatial resolution on the nanometer scale. In this project it is intended to use these developments to explore the diffusion of H-bearing species at conditions that have been inaccessible so far. A specific goal is to characterize the compositional dependence of H-diffusion at these conditions and understand the change of diffusion mechanism that leads to a different behaviour from those observed at higher temperatures.