Li Li, Ph.D - US grants

Axel Schulzgen
University of Arizona

Intellectual Merit: The objective of this research is to combine experiments and rigorous theoretical modeling to develop innovative fiber-based devices with nanoscale structures to provide novel performance and complex functionality. By incorporating structures with sub-wavelength feature sizes into the optical fiber itself, highly integrated next generation photonic devices will be demonstrated. Devices that will be modeled, fabricated, and tested include monolithic multicore fiber lasers, photonic crystal fiber with integrated functionalized nanostructures, and optical fiber with chiral nanostructures inside the mode guiding area. Computation of electromagnetic response and modal behavior of these nanostructured waveguides will help to advance predictive capabilities for photonic devices with integrated nanostructures. Sophisticated nanofiber technology will be developed to fabricate well-defined, optically addressable nanostructures at rather low cost with possible applications in fields far beyond optics and photonics.

Broader Impact: The research will be a groundbreaking study that combines nanometer-scale science with optical fiber technology. This approach has a potential to enable unprecedented optical integration by introducing fiber that is a multifunctional photonic device by itself and promises large impact on industry engaged in optics and photonics. The proposed work will provide an excellent opportunity for students to be trained in the vital research fields of nano-science, fiber optics, and laser physics. Special attention will be paid to integrate this research program into established educational outreach activities with the Navajo Nation and educationally disadvantaged students. This program will provide opportunities for students from underrepresented groups to participate in science and engineering education and in world-class research.

The main goal of this joint project is to further develop the experimental techniques of studying plastic deformation under deep Earth conditions. When a large force (stress) is applied to minerals or rocks under shallow Earth conditions, they will be deformed by brittle fracture. In the deep interior of Earth, temperature is higher and then plastic deformation becomes possible. This plastic deformation helps material circulation by convection that cools Earth and causes most of geological activities including mountain building and deep circulation of water and other materials. However, to date very little is known on the plastic flow properties of materials under deep Earth conditions due mainly to the technical difficulties. For example, in the deep interior of Earth, not only is temperature high, but also pressure is high. Usually pressure suppresses atomic motion and hence plastic deformation becomes difficult under high-pressure conditions. Does the role of pressure become more important than temperature and hence the viscosity of materials increases with depth? Also most of minerals undergo a series of phase transformations. How do these phase transformations affect the plastic properties? These issues are critical to our understanding of the dynamics and evolution of Earth and other terrestrial planets.

Despite its importance, almost nothing was known about these deep earth deformation as recently as ~ten years ago. Recognizing this need, the investigators started a group effort to develop new techniques of plastic deformation under deep Earth conditions in 2002. Based on the studies during the previous funding periods, they have made major progress including the development of new types of deformation apparatus and the improvements to the stress (and strain) measurements using synchrotron x-ray sources. As a result, we can now conduct quantitative deformation experiments to ~20 GPa and ~2000 K. However, these conditions correspond only to the depth of ~500 km. Earth's mantle extends to ~2900 km. Also, there has been very poor control of water content in materials previously studied. In this new phase of technical development, the team of investigators will focus on (i) extending the maximum pressure to ~30 GPa and higher (~1000 km depth), (ii) improving the control of chemical environment (such as water fugacity) under high-pressure conditions, and (iii) improving the stress measurements through the use of new hardware and theory. These developments will allow investigation of the plastic properties of Earth materials to the conditions equivalent to the shallow part of the lower mantle under well-controlled chemical environment. Applications of these techniques will shed important new light into our understanding of dynamics of whole Earth. The project is a collaboration among teams at four institutions, and will provide enhanced infrastructure to the experimental geophysics community, including new facilities at national synchrotron beamlines that will be available to the broader community. The developments will include training and mentoring of graduate students and post doctoral scholars.

The solid portion of the Earth is extremely dynamic on a million year time scale. Plastic flow of the solid leaves a history in the form of alignment of the crystalline building blocks of rocks. Unraveling this history requires understanding the processes that create this fabric through experiments at the pressure and temperature conditions of the Earth's interior. Here we capitalize on new facilities that have been developed at the National Synchrotron Light Source to probe mineral systems subjected to the deforming conditions of the deep Earth. The goal is to better understand these processes, to define the time scale for changing the induced fabric, and to define the control of the pressure and temperature conditions on the efficiency of producing these fabrics.

The dynamic history of the Earth can now be constrained by the anisotropy of seismic wave velocities. Texture in the rocks of the deep Earth is understood to give rise to this anisotropy and it is the plastic flow of the rocks that creates the texture. This breakthrough in understanding was enabled by laboratory investigations of rock deformation at mantle conditions. The former approaches generally resolve the end-product of large deformation. The efficiency of fabric production as a function of the environmental variables is still not well defined. The degree to which small strains will create elastic anisotropies has not been experimentally quantified. We lack a clear understanding of the interactions of the grains during the texture formation such as the relative roles of recrystallization and dislocation glide. New studies need to occur at mantle pressures and temperatures to assure that the proper processes are active. The research of this proposal will provide important information about plastic deformation of solids in general. It will provide metrics that can be used to evaluate the amount of deformation and the mechanism of deformation. This provides potential tools for assessing the failure state of structural materials in engineering applications. Preliminary experiments with sinusoidal stress fields indicate that the techniques proposed here have a strong probability of success and are well defined by theoretical models.

The temperature within the Earth is high enough to melt rocks as evidenced by volcanoes and volcanic rocks. Here we wish to address the question of how important partial melting is in defining the mechanical properties of regions within the mantle. The association of low shear wave velocity and low viscosity with partial melting has been part of the lore since the early days of plate tectonic theory. The strong rigid lithosphere defined the plates that were lubricated from below by the plastically weak aesthenosphere. The seismic verification came from the ubiquitous low velocity zone (at least under oceanic plates). The implicit assertions of this model are 1) velocity and viscosity are both affected by small amounts of partial melting, 2) shear modulus is more affected than bulk modulus, and 3) the top of the low velocity zone marks the onset of melting and the bottom of the low velocity zone is thus defined by the disappearance of partial melting and the increase of both viscosity and velocity. Laboratory studies, however, have not fully endorsed this view of the material response to partial melting. While partial melting is often associated with low seismic shear wave velocity zone (LVZ); laboratory studies have suggested that partial melting may not be effective in creating the large velocity variation in the upper mantle.

Our main potential contribution to this problem comes from our capability to assess these questions over the pressure range of 0 - 15 GPa and 700 - 2000K that are relevant to the conditions of the low velocity zone. Previous studies have been capable of pressures of up to 0.3 GPa. The low velocity zone in the upper mantle is found at 3 - 7 GPa and the upper mantle itself extends to about 14 GPa. Thus, our study will be the first that can analyze these properties at the conditions where the Earth phenomena are found. We will use the facilities at the National Synchrotron Light Source where we have developed high-pressure equipment capable of achieving these goals. The use of X-rays to probe the sample provides a piezometer to measure stress and images to measure strain. Our system can operate in a DC mode to generate steady state flow and AC at frequencies of 0.1 - 0.001 Hz. This research will provide a learning experience for students including graduate students in the preparation of their PhD and undergraduates students in the focus of a summer research project. The results will be used throughout the Earth sciences as fundamental information on the behavior of the Earth's interior. The tools that are developed will be made available to other researchers for studying strength related properties of materials. These tools (software and hardware) will immediately become part of the COMPRES operated synchrotron facility and available to all in the community that wish to use them. The tools developed here will be useful for studying material properties under extreme environments and will be useful to develop better industrial materials.

The goal of this research program is to develop and utilize experimental capabilities for studying the plastic properties of rocks at conditions of the deep Earth. Over geologic time we see that continents have been ripped apart with plate boundaries punctuated by earthquakes and volcanoes. However, over the vast regions of the Earth, these processes proceed smoothly and slowly. While earthquakes express the dynamic character of Earth deformation, the slow movement of the continents provides the driving force. The enabling process for this large-scale motion is the plastic deformation of rocks throughout the Earth's mantle. The foundation of plate tectonics rests on the contention that rocks deform slowly but surely at the high pressure and temperature of the deep Earth. This research program is to continue to build experimental capabilities to quantify the plastic character of rocks as a function of depth in the Earth. This program works at the juncture of high-pressure apparatus development and national synchrotron facilities that can provide intense x-ray probes. This union promises experimental capabilities that increase the depth range of the Earth that we can access, with high precision measurement, by a factor of 100 from previous studies. The data that will come from this program will enable testing and modifying of models of Earth evolution. These deformation facilities enable new directions in Earth material research at mantle pressure and temperature including elastic wave attenuation at seismic frequencies, reaction kinetics, thermal diffusivity, and relationship of lattice preferred orientation to deformation geometry, which links seismic anisotropy to flow history. They also provide a potential facility and technical knowhow for studying material strength and plasticity at extreme conditions such as those generated in the next generation power plants.

Stress, strain, pressure, and temperature are the primary variables that need to be measured during a deformation experiment. With the aid of the national synchrotrons (the Advanced Photon Source and the National Synchrotron Light Source), the investigators have developed the tools to make these measurements. They have also built the first generation of high-pressure apparatus for introducing 'large - volume high pressure' technology into deformation machines. They are now able to make accurate rheology experiments at pressures 1 to 2 orders of magnitude higher than could be achieved 10 years ago. The next phase is to take full advantage of the current hydrostatic high-pressure equipment, including advanced technologies for making polycrystalline diamonds, to reach lower mantle conditions. The goals of this program are to 1) increase the pressure range for deformation experiments to 30 - 40 GPa, well into the lower mantle, 2) improve measurement resolution of stress and strain with a combination of hardware and software developments, 3) enable simultaneous measurements of a sample properties such as preferred orientation of grains and acoustic velocity, 4) explore advanced techniques such as those developed by the synchrotron community but may be useful to earth science goals. These are often high risk, but high return tools such as white beam Laue diffraction that could yield very detailed information about the individual grains within a polycrystal.

The Earth is a dynamic evolving body, driven by heat sources deep within and enabled by the plastic character of the materials that make up the Earth. As observers on the outer skin of the Earth, even after four and a half billion years, we see constant reminders of this evolution in the form of earthquakes and volcanoes. Even the distinction between ocean and continent owes its origin to this dynamic process. This study is focused on the enabler of this process - the plastic nature of what we consider hard rocks. Over long times at elevated pressure and temperature, rocks behave as liquids. In recent years, tools have evolved that allow us to measure the viscous character of these hard objects at pressures and temperatures that one finds deep in the Earth. These experiments are built around a synchrotron facility that has been built at a national lab. The investigators will use both the National Synchrotron Light Source II at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory. This study will help our understanding of this dynamic evolution of the Earth. A graduate student will be supported and will participate in this research.

MgSiO3 in the perovskite crystal structure (bridgmanite) is the most abundant mineral in the Earth and it's calcium counterpart (CaSiO3) is among the five most abundant minerals. The plastic nature of these minerals defines the fluid character of the deep Earth. These properties are both pressure and temperature dependent, and as of yet, unknown. Knowledge of these properties along with proposed temperature models can help constrain dynamics of the Earth evolution and current state. The plastic properties of these minerals is particularly interesting because these minerals are composed of silicon in six coordination with oxygen, while the bulk of materials studied to date have silicon in four coordination with oxygen. The investigating team proposes to study the mechanical properties of these minerals at conditions of pressure and temperature appropriate to the deep Earth. They will focus on defining a quantitative flow law that expresses the plastic properties of these materials. The plasticity of lower mantle minerals at lower mantle conditions of pressure and temperature has been beyond the experimental reach. This has left us with little mineral-based knowledge of the effective viscosity of the lower mantle at the conditions of the anticipated geotherm. Development of new deformation equipment enables a new effort to characterize the quantitative flow law of the high pressure phases at the extremes of pressure and temperature of the top of the lower mantle. This proposal is to study both the Mg and the Ca end-member silicate perovskite at these conditions. This is made possible by a new facility being installed at the new synchrotron, NSLS II, at the Brookhaven National Laboratories. The new facility will provide world class synchrotron X-ray light for these experiments with a new DT25 guideblock in a 1000 ton hydraulic press with deformation capabilities. Using standard X-ray diffraction and imaging tools, the team will be able to define stress and strain rate in the sample at the extreme conditions.

The temperature within the Earth is high enough to melt rocks as evidenced by volcanoes and volcanic rocks. It is thereby probable that parts of the Earth's crust and mantle are partially molten even though they are dominantly solid. The goal of this proposal is to improve the understanding of the relationship between the state of partial melting and changes in elastic properties of the media. The focus will be the upper mantle of the Earth. Since elastic properties quantitatively define the speed that seismic waves propagate through the media, the results of this research will enable better associations between maps of seismic speed variations and degrees of partial melting. The approach of this program is to probe the physics and chemistry basics of melting and mechanics yielding a deeper understanding of the underlying principles in order to provide a robust model that can then be applied to a wide variety of environments. While the focus of the proposal is the upper mantle region of the Earth, an improved model will also be relevant to the crust. Indeed, the results could also have applications in defining this relationship in industrial settings in detecting incipient stages of melting of various materials. This program provides a cutting-edge learning experience for students including graduate students in the preparation of their PhD and undergraduate students in the focus of a summer research project. The results will be used throughout the Earth sciences as fundamental information on the behavior of the Earth's interior. The tools that are developed will be made available to other researchers for studying strength related properties of materials. These tools (software and hardware) will immediately become part of the COMPRES-operated synchrotron facility and available to all in the community that wish to use them. The tools developed here will be useful for studying material properties under extreme environments and will be useful to develop better industrial materials.

The authors have recently demonstrated that the current models of the mechanical properties of solid/melt combinations are inadequate and have proposed that the interaction between the solid and melt has a significant effect on the elastic properties. The new model, "dynamic melting," changes the predicted effect of melting on the relationship between the bulk modulus and the shear modulus. In the dynamic melting model, the kinetics of melting introduces a change in the frequency dependence of the elastic properties. The current proposal is to carefully test the dynamic melting model by studying a broader range of chemistry of the material and alter the experimental conditions and protocols in order to compare the predicted changes with the experimentally observed changes. The experiments are made possible by new high pressure capabilities on multi-anvil high pressure apparatus installed at synchrotron light sources. These systems enable the generation of pressure and temperature of the upper mantle on samples that can then be axially compressed, with the synchrotron yielding images of the sample with very high precision measurements of length, and diffraction profiles that yield state of stress.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.