We investigate the related energy transfer mechanisms in organic layer/substrate systems.
The interactions of femtosecond laser pulses with materials, especially layered systems, are of interest to many fields. Femtosecond laser pulses are shorter than the time it takes for electrons in irradiated materials to establish a thermodynamic equilibrium with phonons. As result, the laser pulse produces effects very different from those observed with longer pulses or continuous wave lasers. For this reason, femtosecond laser-based methods are used for production of microfluidic, microelectromechanical, and data storage systems. In addition to materials processing applications, ultra-short pulsed lasers are used to probe chemical reactions using chemical spectroscopic methods. Although there have been many studies on femtosecond laser pulse interactions with metal, semiconducting and dielectric substrates and a few studies on metals with overlayers and adsorbed molecules, there has not yet been a comprehensive study into the effects of substrate electronic and thermal properties on femtosecond laser pulse interactions with organic overlayer-substrate systems. In this research, we are investigating the related energy transfer mechanisms.
Sponsor: US Department of Energy
This research, a collaborative effort with the Johns Hopkins Applied Physics Laboratory, seeks to understand the characteristics of material dispersal that occurs when solid rocket motors fail. During such conflagrative events, fuel can react with materials in the payload and release species whose subsequent fate and transport must be understood for effective rocket design. Under this program, aerosols from fires and related surrogates are being analyzed for elemental content and material structure to support remote optical detection of fire products. This information will be used for simulations of these types of events as well as for design of fault tolerant payload modules.
Under this program, a range of ultrasonic nondestructive methods will be developed to assess microstructural characteristics of nuclear grade graphites including those subjected to thermal cycling and/or neutron irradiation. Since laser ultrasonics methods have proven to be effective for characterizing nuclear grade graphites, these will be used and will be correlated with microstructure by characterizing materials using standard techniques including x-ray diffraction, scanning electron microscopy, transmission electron microscopy and porosimetry. Under this program, three different laser-based ultrasonics approaches to bulk, microstructural characterization of nominally isotropic, nuclear grade graphites will be pursued: effective medium, multimode ultrasonics for assessment of microcrack and void densities; shear birefringence measurements for determination of microcrack density and orientation distribution; ultrasonic scattering correlation measurements for defect distribution determination.
The importance of this overall investigation is that it will allow direct identification of different types of microstructural defects within the bulk of graphite bodies and will allow for monitoring of defect density changes brought about by thermal cycling and/or radiation-induced damage. The ability to nondestructively assess the microstructural state of nuclear-grade graphites is critical to qualifying these materials for current and future nuclear reactors.