ULTRASONIC SPECTROSCOPY

Department of Physics

PROJECTS

1. HYDROGEN IN METALS
         Hydrogen is absorbed readily into a large number of metals. Often a hydrogen-metal system is formed wherein the hydrogen occupies an interstitial site in the host lattice and diffuses by making transitions among these sites. A detailed understanding of the interactions and motion of hydrogen in metals is of considerable importance for both technological and fundamental reasons. The most important technological application of these materials at present is as an electrode in metal hydride batteries. Hydrogen embrittlement of metals is a concern in many situations.. New potential applications of hydrogen-metal systems continue to appear. The most exciting ones at present have to do with metal-insulator transitions resulting in switchable mirrors, alteration in electronic structure leading to switchable magnetic properties, and the general behavior of hydrogen in multilayers. In all of these cases the diffusion of hydrogen and the elastic interaction of hydrogen with the host lattice are of fundamental importance.
        Resonant ultrasound spectroscopy is being applied to the study of hydrogen motion and hydrogen-induced elasticity effects in a number of hydrogen-metal systems. Several different studies are underway: (i) Many of the hydrogen-metal systems of current interest exhibit motion with two or more characteristic frequencies. There is often a rapid local motion and a slower long range motion. The rapid local motion, involving closely spaced intersititial sites is likely to be dominated by quantum effects at rather easily achievable temperatures. This unusual motion is being studied by means of ultrasonic attenuation measurements in rare- earth metals and Laves-phase compounds. (ii) The elastic constants of the Laves-phase materials have been found to have an anomalous dependence on temperature and hydrogen concentration. Measurements over an extended range of temperatures and hydrogen concentrations are under way to characterize this behavior. These effects appear to be due to an unusual electronic structure. The Laves-phase work is carried out in collaboration with Dr. A. V. Skripov of the Urals Branch of the Academy of Science, Ekaterinburg, Russia. (iii) The elastic modulii of the technologically important alloy system LaAlxNi5-x are being measured with the goal of understanding relations between atomic bonding properties and hydrogen absorption characteristics. This work is carred out in collaboration with Professor. Isaac Jacob of theBen-Gurion University, Israel.
   
   
2. QUASICRYSTALS
         Quasicrystals are an unusual form of matter which have long-range order, yet do not have translational periodicity. Instead of the usual crystallographic translation periodicity, they have non-crystallogrpahic rotational symmetries. The structure and stability of these materials are still open questions. Resonant ultrasound spectroscopy is being used to measure the elastic constants of titanium-based quasicrystals. The elastic constants are directly related to atomic potentials, but have been little explored in the titanium-based materials due to experimental difficulties. These titanium-based quasicrystals and their crystalline approximants have been found to absorb considerable amounts of hydrogen. The hydrogen dynamics are being explored by means of ultrasonic attenuation measurements. This work is being carried out in collaboration with Professor Ken Kelton of Washington University, St. Louis, MO.
   
   
3. PHASE TRANSITIONS
        Elastic constants are a sensitive indicator of phase transitons, and the temperature dependence of the elastic constants at the transition often provide information about the nature of the transition. It is planned to study magnetic transitions in Ce-based compounds. Although the elastic constants have not been measured in these materials, many thermodynamic properties have been found to behave anomalously. This work will be carried out in collaboration with Professor Peter Riedi of St. Andrews University, Scotland.
   
   
4. TEXTURE IN ALLOYS
         As a result of processing, many polycrystalline alloys show preferential orientation of the grains. This non-randomness results in anisotropic elasticity. Such anisotropy has many practical implications. RUS is being used to investigate the elasticity of such materials. This work is being carried out in collaboration with Dr. G. Alers and Dr. H. Ledbetter of NIST in Boulder. CO.