Fig2“Nonsense is that which does not fit into the prearranged patterns which we have superimposed on reality…Nonsense is nonsense only when we have not yet found that point of view from which it makes sense.”
― Gary Zukav
. . .and in these works we seek that perspective.
Glasses are unique states of solid matter where the atoms are not arranged in a repeating structure (crystal) but rather have liquid-like atomic order. A glass is usually formed by cooling a liquid and bypassing crystallization, however, some systems form glasses very easily while other must be cooled from a high temperature liquid very rapidly. Traditionally, good glass formers have been distinguished by the behavior of their viscosity with temperature. We’re working on an idea that the glass-forming ability may also manifest in the atomic structural evolution of the liquid. This has been indirectly postulated in a couple of different forms but we think there may be direct evidence based on X-ray diffraction studies.
To study high temperature metallic liquids is to study something whose temperature may exceed 2000°C, that would immediately oxidize several nanometers deep from the surface if exposed to air, and reflects nearly 75% of the incident light off its surface making it an amazingly challenging class of systems to attempt to study. Additionally, while equilibrium thermodynamics would has us believe that a system can never be cooled below its melting temperature, we know better as glasses (like window glass) exist all around us, suggesting that if we’re very careful, and ask very nicely, we might very gently coax a liquid below its melting temperature, called supercooling, thus allowing us to probe its properties as it nears the glass transition temperature. Such challenges invite innovation and the electrostatic levitator is one such technique. My research group works to extend this and other advanced sample environment technologies to study metals, ceramics, insulators and ionic liquids.
Amorphous materials lack long-range atomic periodicity but they do not lack structure. In fact, glasses of all kinds have been shown to exhibit a high degree of short-range order (on the order of a single bond length) and some have been shown to form extended atomic structures on the length scale of several nanometers. Some of my initial work into this area led to our realization that even binary metallic glasses, which are generally poor glass formers have extensive medium range order. In one example, shown here, it was discovered that extended ordering in a Zr-Pd alloy occurs, but this order is difficult to detect. This extended ordering may be at the heart of understanding the differences in glass-forming ability between systems. We work with advanced modelling techniques including reverse Monte Carlo simulations of experimental diffraction data, Molecular Dynamics simulations, and empirical modeling to attempt to connect atomic order on multiple scales with thermophysical properties and glass formation.
When a glass is formed and then heated above the glass transition temperature, it devitrifies, or crystallizes. Now, because of the unique thermal dissipation properties of metallic glasses, it is extremely difficult to mechanically weld or fuse two glasses together. However, if one can locally heat the surface of a metallic glass, the top few hundred nanometers crystallize, while the bulk remains amorphous and retains the excellent mechanical properties of the glass. It has been shown, with limited success, that these crystallized layers can then be bonded together. The applications for this process are truly vast: Consider, as an example, the ability to weld and aluminum-based glass to the leading edge of a projectile. The glass is light, corrosion resistant and has superior fracture toughness. However, detailed understandings of the devitrification mechanisms at the surface are required. My research group explores the engineering challenge from a fundamental physics perspective- we conduct X-ray diffraction measurements on Bulk Metallic Glasses (BMGs) that are cheap, relatively easy to produce and have tremendous potential for application. By understand the atomic structural evolution of the glass and the crystallizing phase we can match BMGs with candidate conventional alloys for diffusion bonding.