Evolution of nanoscale glasses

纳米级玻璃的演变

• 批准号: RGPIN-2014-06458
• 负责人: Schiettekatte, François
• 金额: $ 3.5万
• 依托单位: Université de Montréal
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2014
• 资助国家: 加拿大
• 起止时间: 2014-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=567818
• 关键词: Evolution; nanoscale; glasses

Glass exists everywhere in nature and in technology, from solidified lava to our windows or Pyrex cookware, to Gorilla glass on our smartphones or in the rewritable layer of DVDs. And yet, it remains one of the unsolved problems of physics. Or more accurately, we do not know precisely the atomic mechanisms underlying their evolution. Glasses can be described to some extent as an arrested liquid. Contrarily to crystals where the atoms or molecules are positioned in a well-defined geometrical order, the atoms and molecules in a glass tend to be positioned randomly, like those in a liquid, but almost immobile, as in solids and contrarily to liquids. The problem comes from the “almost” part of it. Near but just below the melting point, Tm, the atoms or molecules can move around on a small scale while dragging with them the other atoms, so viscosity is small, and the glass blower has to be careful not to drop the gather of glass he holds at the end of his blowpipe. As the glass cools off, the movement of an atom requires that an increasing number of other atoms move at the same time, therefore with increasing viscosity, until the piece of glass reaches a temperature, called the glass transition Tg, where everything seems to stop. Or is it what we think does happens. In fact, even below Tg, things still move, at least if the glass is brought to Tg at a rate that is not infinitely slow. This is called relaxation. What are the processes occurring near and below Tg remain unidentified, as well as their spatial extent. The goal of this research program is to identify them in the following way. By bombarding materials that form glass with very heavy atoms accelerated at a low energy, we will produce very small regions of glass. The ions impact will each form a zone of glass on a layer of material, or will transform into glass the impacted nanoparticles, which we will have previously deposited on the surface. This will be done at the surface of a nanocalorimeter, a device made of a thin layer membrane on which runs a metallic strip, which serves both as a heater and thermometer. Hence, the device will be used to measure the heat released by the nanoparticles as the relaxation process goes on. Because of their small size (few nanometers across), the relaxation process cannot involve so many atoms. Furthermore, they are small enough that an atomistic simulation of the complete particle can be carried out. By comparing the heat measured to the evolution of the energy in the simulation, we should be able to find what are the relaxation mechanisms in the material. The material that will be investigated will be the GeO2, the more easily measurable brother of SiO2, which is what most windows and rocks are made of, and Ge2Sb2Te5, a glass at the basis of rewritable DVDs and that behaves very differently than GeO2 and SiO2.

玻璃在自然界和科技中无处不在，从固化的熔岩到我们的窗户或Pyrex厨具，再到我们智能手机上的大猩猩玻璃或DVD的可重写层。然而，它仍然是物理学中尚未解决的问题之一。或者更准确地说，我们并不确切地知道它们进化背后的原子机制。在某种程度上，玻璃可以被描述为一种滞留的液体。与原子或分子以明确的几何顺序排列的晶体相反，玻璃杯中的原子和分子往往是随机放置的，就像那些在液体中的原子和分子一样，但几乎是不动的，就像在固体中一样，与液体相反。问题来自它的“几乎”部分。在熔点Tm附近，原子或分子可以在小范围内移动，同时拖动其他原子，因此粘度很小，玻璃吹风机必须小心，不要掉下他吹管末端的一堆玻璃。随着玻璃冷却，原子的运动需要越来越多的其他原子同时运动，因此粘度增加，直到玻璃达到一个温度，称为玻璃化转变Tg，在那里一切似乎都停止了。或者是我们所认为的事情发生了。事实上，即使在Tg以下，物体仍然在运动，至少如果玻璃以不是无限慢的速度被带到Tg的话。这叫做放松。在Tg附近和Tg以下发生了什么过程仍未确定，以及它们的空间范围。这项研究计划的目标是通过以下方式识别它们。通过用低能量加速的非常重的原子轰击形成玻璃的材料，我们将产生非常小的玻璃区域。离子撞击将在一层材料上形成一个玻璃区，或者将受影响的纳米颗粒转化为玻璃，我们将先前将其沉积在表面上。这将在纳米热量计的表面完成，这是一种由一层薄膜制成的装置，在其上运行一条金属条，金属条既可以用作加热器，也可以用作温度计。因此，该装置将被用来测量随着弛豫过程的进行，纳米颗粒释放的热量。由于它们的尺寸很小(几个纳米宽)，弛豫过程不可能涉及这么多原子。此外，它们足够小，可以对整个粒子进行原子模拟。通过将测量的热与模拟中的能量演化进行比较，我们应该能够找到材料中的松弛机制是什么。将被研究的材料将是GeO2和Ge2Sb2Te5，前者是二氧化硅的兄弟，更容易测量，后者是大多数窗户和岩石的原料；Ge2Sb2Te5是一种基于可重写DVD的玻璃，其行为与GeO2和SiO_2非常不同。