Seminar: Characterization of Microstructure of Nuclear Materials using Laser Ultrasonics with Applications to In-pile Measurements

Abstract: 

In condensed matter physics, an ultrasonic wave is defined as a collective oscillation of atoms that compose a crystalline lattice. In this sense, ultrasonic waves are a natural choice to study lattice imperfections, such as dislocations, point defects, grain boundaries, and nanometer-sized cracks. In a perfect single crystal, the velocity of an acoustic wave is a measure of the parabolic term in the expansion of the interatomic potential. By measuring ultrasonic velocities along different crystallographic directions, one is able to determine all of the components of the single crystal elastic stiffness tensor. Measurement of the ultrasonic velocity change with temperature reveals information about the cubic term in the expansion of the interatomic potential. Ultrasonic waves are also strongly influenced by material microstructure. This interaction between ultrasonic waves and the long range elastic fields associated with individual microstructural features is mediated by lattice anharmonicity (i.e., the cubic term in the expansion of the interatomic potential). For example, the ultrasonic velocity is a function of dislocation density and pining length for single crystals containing dislocations. This mechanism has been attributed to scattering of ultrasonic waves due to the anharmonicity of the dislocation strain field. 

The ultrasonic velocity in polycrystals is determined by the polycrystalline averaged elastic stiffness tensor. Measurements in polycrystals can reveal important information about deformation-induced grain texture. It is also possible to relate attenuation to crystal defects. This field of condensed matter physics is referred to as internal friction. Early internal friction measurements involved sub-audio and audio frequency excitations of torsional pendulums. Internal friction measurements were extended to MHz range in the early 1950s using piezoelectric transducers.  The larger frequency range made possible the validation of competing theories.  Interest in internal friction started to wane in the 1960s, however, due to several factors. First, issues associated with the separation of small intrinsic attenuation from other forms of extrinsic attenuation, such as mechanical coupling to the environment, were never fully resolved. Second, owing to the large number of internal friction mechanisms, materials, and competing models, a unified interpretation of data was often difficult to obtain. Lastly, materials scientists and condensed matter physicists started to rely more heavily on electron microscopy to image crystalline defects.

Presentation:  Many of the drawbacks to using ultrasound to characterize microstructure can be traced back to ultrasonic transduction methods that employ piezoelectric transducers. Transducers are typically large and do not afford high spatial resolution. In most cases, a couplant is required, which necessarily introduces reproducibility issues. The use of couplants also put severe restrictions on scanning and limit high-temperature operation. Additionally, piezoelectric transducers are limited to the low MHz frequency range and rarely have a flat frequency response. Many of these shortcomings can be alleviated or partially alleviated by using laser ultrasonic techniques.  Laser ultrasound employs short laser pulses to generated MHz to GHz frequency ultrasonic waves and a second laser is used to monitor the ultrasonic propagation.  This presentation is focused on applications of laser ultrasonics to the characterization of nuclear materials and will include the following topics:

1. Laser ultrasound to characterize corrosion films on zirconium cladding
2. Laser Resonant ultrasound to characterize thermally driven texture evolution
3. Laser ultrasound to monitor phase transformation of UZr
4. Coherent acoustic phonon spectroscopy to measure lattice anharmonicity
5. In-pile laser ultrasound to monitor phase transformation

Bio:

David Hurley received a Ph.D. in Materials Science and Engineering from Johns Hopkins University and is currently a Laboratory Fellow at Idaho National Laboratory.  He is the director of the Center for Thermal Energy Transport under Irradiation, an Energy Frontier Research Center devoted to first principle understanding of thermal energy transport in 5f electron materials under irradiation. He is also the technical coordinator for the In-Pile Instrumentation Program.  Dr. Hurley has authored over 65 peer-reviewed publications involving theoretical and experimental studies of laser-based characterization of materials. Since coming to INL, he has focused on characterizing material behavior in extreme environments. Dr. Hurley’s research background and expertise encompass elements of physics, mechanical engineering and materials science. This middle ground between science and engineering has given him a unique perspective on many materials issues facing the nuclear industry. Connecting microstructure to mechanical properties of nuclear fuel provides an important example of this perspective. As part of this effort his group at INL contributed significantly to the foundation of a new field of mechanical characterization termed laser resonant ultrasonic spectroscopy. On the science side he has focused on the development of a first-principles understanding of the effect of irradiation-induced defects on thermal transport in oxide and metallic nuclear fuels. One of his more notable accomplishments in this area involved leading a team that isolated thermal transport signatures of specific microstructural features. Examples include the first measurement of the Kapitza resistance of a bicrystal boundary and the identification of a cooperative effect between oxygen vacancies and grain boundaries in oxide fuel.