Employing sound waves to investigate inner structure of materials

A multi-institutional team is investigating how sound waves generated by light can be used to observe materials’ inner structure remotely and non-destructively. Being able to see how various materials evolve over time and under extremes of stress, temperature and radiation is important to a range of energy industries. 

Mechanical and Aerospace Engineering Professor Marat Khafizov and graduate student Yuzhou Wang are collaborating with researchers from Idaho National Laboratory. The team reported their progress in a Nature Communications paper, “Imaging grain microstructure in a model ceramic energy material with optically generated coherent acoustic phonons,” the last word referring to vibrations in crystals.

The research demonstrated a new method to measure crystallite orientation in a transparent ceramic material. Khafizov said the basic idea behind this approach is “similar to dropping a rock on the surface of water and watching wave ripples propagate on the surface.”

While waves on the surface of the water are visible to the eye, the acoustic waves resonating inside the solid material are a result of atomic vibrations. These require a different approach to detect.

“Instead of a rock, we use a very short laser pulse focused through a microscope objective to launch the wave inside a solid material,” said Khafizov, co-author of the paper. “To observe the ripples, we use a different laser pulse.”

The second laser pulse is used to detect any changes in optical transparency of the material that can occur as the wave ripples through the solid. The unique aspect of measuring these waves in solids is that unlike liquids, which only produce longitudinal waves, solids can produce three different types of waves. These wave types are determined by the different directions atoms vibrate as the wave propagates.

Much of the hands-on work was performed by Wang, a nuclear engineering graduate student. Other portions of the measurements were done by Wang at experimental facilities in Idaho National Laboratory (INL) during his summer internship. Wang took these measurements and brought them back to Columbus for interpretation as part of his PhD dissertation.

INL Directorate Fellow David Hurley and professors at Le Mans University in France helped establish a foundational basis for the experimental and theoretical components. The team at Ohio State led interpretation of the experimental observations that allow measurement of grain orientation.

Khafizov said the chance to perform research with INL is a valuable experience for Ohio State students. Alongside scientific leaders in laser ultrasonics, graduate students worked to recognize and identify new information from the experiments. The opportunity to spend time at INL and work with their researchers is made possible in part by Ohio State’s participation in the INL National University Consortium.

The results of this research are already influencing new research. For instance, studying a material's grain microstructure as temperature changes could lead to better understanding of what limits its thermal conductivity—the degree to which it conducts heat. Such information could be important in designing higher performing materials for tomorrow’s reactors, fuel cells and photovoltaics.

According to Khafizov, the research could ultimately benefit multiple fields. It could allow development of ceramic materials that advance clean and secure energy systems, or be applied to defense needs by providing optical ceramics for high powered lasers and ceramic armor.

modified version of original article by Sam Cejda, Department of Mechanical and Aerospace Engineering