Robert Leisure

Professor
B.S. Western Kentucky University, 1960; Ph.D., Washington University, 1967.
Fellow of the Institute of Physics (UK) and of the Acoustical Society of America.

Ultrasonic Studies of Condensed Matter

Ultrasound is used as a type of spectroscopy to study properties of condensed matter. The existence of both longitudinal and transverse modes in the elastic wave spectrum, and the dependence of those modes on crystalline symmetry and direction, gives ultrasonic spectroscopy the power to obtain a rich variety of information about any excitation or defect which is coupled to lattice vibrations.

A more recently developed technique is that of resonant ultrasound spectroscopy (RUS). With this technique a large number of vibrational eigenmodes of a small sample are excited using continuous waves. From the spectrum of eigenmodes, the complete set of elastic constants may be obtained, as well as measurements of the ultrasonic loss associated with each mode. RUS is particularly valuable for the study of samples for which only small single crystals are available, and for the study of materials undergoing phase transitions. Examples of materials recently under study in our laboratory are described below.

Nanocrystalline Materials

In contrast to the usual polycrystalline materials, nanocrystalline materials have a substantial fraction of the atoms near a grain boundary. As a result, it is expected that many properties, including mechanical ones, may be substantially different in nanocrystalline materials as compared to their polycrystalline counterparts. This difference suggests the possibility to design materials with desired properties. At this time, the available bulk nanocrystalline solids have at least one dimension rather small, < 1 mm. This small size makes it difficult, if not impossible, to obtain reliable elastic constant measurements with conventional techniques. However, with resonant ultrasound, we have been able to make reliable measurements on very small samples (see the nanocrystalline sample on the penny). Our results show that nanocrystalline Pd is substantially softer than polycrystalline Pd.

Quasicrystals

Quasicrystalline materials, discovered in 1984, lack the translational periodicity of ordinary crystals, but yet are highly ordered. In particular, they have rotational symmetries forbidden for crystals made of repeating unit cells. Exploring the structure and interatomic interactions of these materials is an active field of research. Elastic constants, being the second derivative of internal energy with respect to strain, are sensitive to both the local structure and the interatomic potentials. Only small samples are available for many of the interesting quasicrystals. Our capability to make quantitative measurements on these very small samples allowed us to determine the elastic constants of icosahedral phase Ti-Zr-Ni materials, one of the largest classes of quasicrstals, and of potential use for hydrogen storage. The results indicate that the interatomic bonding in the Ti-Zr-Ni i-phase materials differs substantially from many of the other i-phase quasicrystals. Our ultrasonic results did not support certain literature reports of exceptionally low shear modulus and a correspondingly high Poisson’s ratio for the Ti-based materials.

Hydrogen-storage Materials

The efficient and economical storage of hydrogen is an unsolved technological problem preventing the widespread use of hydrogen in transportation systems. One hope is to store hydrogen in metals or intermetallic compounds. The elastic constants are important for understanding hydrogen absorption. It is well-known that there is an attractive elastic interaction between dissolved H atoms that depends on the bulk modulus, yet the shear modulus also seems to play a role. We have recently succeeded in making elastic constant measurements on the technologically important materials LaNi5 and its alloys. The success was due to the preparation of high density materials using hot isostatic pressing (HIP). The hope, and real progress is being made, is to relate the elastic properties to the hydrogen storage capacity of these materials. Success in understanding these effects could result in important insights in choosing materials with the desired hydrogen storage properties (e.g., high capacity, low weight, low cost).

Selected Publications

• R.G. Leisure, S. Kern, F.R. Drymiotis, H. Ledbetter, A. Migliori, and J.A. Mydosh, "Complete Elastic Tensor Through the First-Order Transformation in U₂Rh₃Si₅," Phys. Rev. Lett. 95, 075506 (2005).

• D. S. Agosta, R. G. Leisure, J. J. Adams, Y. T. Shen, and K. F. Kelton, "Elastic moduli of a Ti-Zr-Ni i-phase quasicrystal as a function of temperature," Phil. Mag. 87, 1 (2007).

• D. S. Agosta, R. G. Leisure, K. Foster, J. Markmann, and J. J. Adams, "Elastic moduli and internal friction of nanocrystalline Pd and PdSi as a function of temperature," Phil. Mag. 88, 949 (2008).

• Hassel Ledbetter, Robert G. Leisure, C. Pantea, and J. B. Betts, "Diamond’s elastic stiffnesses from 322 K to 10 K," J. Appl. Phys. 104, 053512 (2008).