Daryoosh Vashaee, Ph.D. - US grants

0933763
Vashaee

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

Thermoelectric materials have the potential to directly convert waste heat into electrical energy. Nanocomposite bulk thermoelectric materials can be fabricated quickly and inexpensively, and in a form that is compatible with existing thermoelectric device configurations. Unfortunately, existing theory of thermoelectricity is not able to
predict correctly the thermoelectric properties of such structures, and often hundreds of samples might need to be grown and measured, in the quest to identify optimum nanostructures in terms of high efficiency and low cost.

Intellectual Merit: This proposal aims to develop a capability to predict the relevant properties of thermoelectric nanocomposite materials. A nonequilibrium Green?s function technique will be developed to account for the natural coupling of scattering of phonons and electrons, as well as other quantum effects. Issues such as crystal orientation and strain will be accounted for to determine their effects on the bulk thermoelectric properties. Model predictions will be compared with measurements of thermoelectric properties of synthesized nanocomposites to optimize the nanoscale features of the material, the material composition, the doping concentration(s), and the material processing parameters to improve the efficiency of such inexpensive thermoelectric materials.

Broader Impact: Development of highly efficient, inexpensive thermoelectric materials is a key to reduce both energy consumption and harmful emissions on a large scale. The research will be integrated into new graduate courses to be developed at Oklahoma State-Tulsa. Undergraduate students will be involved in the research. Recruitment of students from underrepresented groups in conjunction with the Oklahoma Louis Stokes Alliance will be pursued. Participation in an educational program at the Tulsa Children?s Museum for K12 students will occur.

Amorphous based materials can possess fundamentally different electrical and thermal properties than crystalline or nanocrystalline forms of the same material. Although, amorphous materials have found applications and continue to show promise for modern technologies, charge carrier and phonon transport in these materials remain a point of dispute. The lack of long- and short-range order in amorphous materials leads to complicated interplay between structure and energy transport. In this project a novel class of electronic materials based on bulk amorphous structures in the form of amorphous-crystalline nanocomposites will be developed and their thermal and electrical properties will be tailored. The application will be focused on thermoelectric materials, but the results are expected to produce new science applicable to other functional materials including optical and magnetic materials. Parallel to the research endeavors, an educational plan will be implemented which incorporates and develops a new teaching initiative in the upper-division undergraduate curriculum, involves undergraduates in research, promotes student international collaborative research, exposes the field of energy materials to the general public, and provides a resource web-site for advanced thermoelectric material studies. The available resources in the Oklahoma Louis Stokes Alliance for Minority Participation (OK-LSAMP) and Multicultural Engineering Program (MEP) programs will be used for expanding the participation of minority students and the recruitment of high school students.

This research plan addresses the essential need for a physical description of charge and phonon transport in amorphous based materials, characterization of their structural dependencies, and application of this understanding to enhance the thermoelectric performance of amorphous and the more complex structure of amorphous-crystalline nanocomposite materials. The focus will be especially in the regime where the carriers energy remains at non-equilibrium state due to the consecutive passage through material domains with different equilibrium energy distribution of carriers. The multi-mode transport of charge carriers in extended and localized states in disordered multi component amorphous-crystalline nanocomposite structures will be addressed. This is a new scientific problem with many unresolved scientific questions. Further understanding of charge carrier and phonon transport in such amorphous based materials will directly impact their material design and offer novel material structures for electronic applications. Parallel to theoretical studies, an efficient and scalable top-down approach for synthesizing such structures will be developed. The material is processed in a single transversal mode microwave cavity that provides an extraordinary route to create a new state of amorphous materials in a rather quick and convenient way. The decrystallization process happens by merely subjecting the solid material to a strong E or H field in the cavity. The method results in in-situ formation of such structures, which is not possible by conventional bulk processing methods. This unique capability opens a new landscape for engineering non-equilibrium structures.

Thermoelectric materials are materials that can produce an electric current when two sides of the material are exposed to different temperatures or vice-versa (i.e., supplying an electric voltage and current to a thermoelectric material can change the temperature of its surfaces). Thermoelectric materials are a promising technology for a range of application from electric power generation to heating and cooling. Most electric power today is produced from processes where fuel is burned to produce heat that turns water into steam that is then used to turn electric power generators, and low temperature steam is exhausted as waste. Such processes are relatively inefficient, converting only about 30 to 40 percent of the fuel's energy into useful electric power. Thermoelectric materials could be used to convert this wasted thermal energy into useful electric power. The team will design and construct high efficiency thermoelectric materials and devices from composite materials constructed from combinations of amorphous (i.e., non-crystalline, molecularly disordered) and crystalline materials that can maximize the material's electrical conductivity while minimizing the material's thermal conductivity, an ideal combination for effective thermoelectricity. The project will have broad educational impacts through both development of nanotechnology related university-level coursework and through the direct involvement of underrepresented students in the research.

The primary focus of the work will be on the use of composite silicide (i.e., materials that combine silicon with other elements such as metals) to create highly efficient thermoelectrics. The target of more efficient thermoelectric materials is to achieve a large thermoelectric effect so that large amounts of electric power can be generated from relatively small temperature differences between waste heat sources and the environment. This project is a computationally guided material design effort which encompasses both theoretical and experimental aspects of amorphous based materials. The program addresses the multi-mode transport of charge carriers in extended and localized states, along with phonon transport properties in disordered multi-component amorphous structures. It is expected that new material structures based on amorphous-crystalline composites of silicide alloys developed in this work should result in significant nonlinear enhancement of the thermoelectric power factor, along with the reduction of the thermal conductivity of the materials. This research concept is a nanoscale effect that happens only if the energy distribution function of the carriers does not relax to that of the bulk material in the crystallites. This state requires crystallite sizes of sub-10 nm in most thermoelectric materials, which is often difficult to reach with the existing material processing methods. This project will develop a new material synthesis method based on field decrystallization in a microwave cavity that can produce non-equilibrium silicide materials. The effect of hydrogenation on thermoelectric properties will be investigated for the first time, and the scalability of the material growth technique will be demonstrated

Title:
A novel high performance thin and light three-dimensional thermoelectric generator harvesting human body heat for powering wearable devices

Nontechnical Abstract:
Thermoelectric generators can convert body heat to electrical energy providing a continuous source of energy for low power electronics. Small and light weight thermoelectric generators can be integrated into wearable devices making battery-less devices a reality. Studies have shown that a large fraction of the users stop using their wearables after a few months partly caused by the need for frequent charging. The solution is to make such wearables self-powered eliminating the need for recharging or replacing the batteries. Such a feat would also enable the use of wearables in clinical applications. For instance, self-powered devices would allow physicians to monitor continuously the state of their elderly patients after they are discharged from the hospital. In addition to wellness and health monitoring, connected networks of self-powered sensors could inform decisions in industrial manufacturing, precision agriculture, environmental monitoring, surveying and civil engineering, and of course smart and connected homes.
To date, commercial thermoelectric devices are fabricated in similar way as they were made fifty years ago. They are bulky with only dozens of millimeter-scale elements per device, as such, their output voltage is too low (a few milli-volts) and their form factor is not appropriate for wearable applications. The objective of this research is to make a novel device architecture that enables integration and stacking of thousands of microscale thermoelectric elements per centimeter square. The new device is thin and light weight, and can generate several volts from body heat appropriate for various wearable applications. Moreover, the fabrication process will be wafer-scale relying on mature industry compatible processes, which makes it a viable technology for commercialization.

Technical Abstract:
The objective of this research is to develop a novel three-dimensional thin-film thermoelectric generator suitable for body heat harvesting and powering wearable sensors and electronics. A conventional thermoelectric generator consists of only a dozen of millimeter-scale elements, which cannot generate sufficient voltage from body heat. A novel device architecture is proposed that enables fabrication of high efficiency thin film thermoelectric devices consisting of several thousands of microscale thermoelectric elements per square centimeter. Therefore, the new device can generate >1000X larger output voltage. It achieves this performance enhancement thanks to an entirely new device architecture, which allows stacking of thin film elements in a three-dimensional construction, and self-vacuum-sealing that minimizes parasitic heat losses. The approach not only enables vacuum encapsulation, but also allows making a thin-film device in which the thermoelectric length is independent of the thickness of the deposited film. This is a significant improvement over conventional architectures since optimization of the electrical and thermal resistances of the elements can be achieved independent of the film thickness.
Industry compatible, wafer-scale micro-fabrication process will be used to fabricate the proposed three-dimensional thermoelectric generator on inexpensive silicon wafers. The fabrication will rely on mature processes and techniques used in micro-electro-mechanical-systems integration. Furthermore, in the path to achieving this goal, (a) bismuth telluride based thermoelectrics will be grown on silicon oxide/silicon substrate and characterized, (b) a comprehensive three-dimensional model will be developed to better understand and optimize the device architecture and minimize the parasitic heat losses, and (c) the device will be characterized for human body heat harvesting and wearable applications.

Thermoelectric materials can generate electricity in the presence of a temperature difference or work in a reverse mode providing cooling when an electric current is passed through the material. The thermoelectric technology, which used to be primarily based on alloys of bismuth telluride for Peltier cooling modules, or silicon-germanium for radioisotope thermoelectric generators used in NASA spacecraft, has expanded over the last two decades to a wide range of materials for power generation, cooling, or infrared detection and imaging applications. Power generation from low-grade heat sources, such as waste heat at industry, ambient heat, buildings, or body heat, has particularly taken much attention. Waste heat recovery can significantly reduce the use of fossil fuels and help prevent a worldwide energy crisis. As such, thermoelectric materials research is currently an area of intense research. Until now, most of the efforts and progress have been on the direct conversion of heat into electricity, with the progress approaching a plateau. This proposal investigates an alternate route based on converting heat into the thermal fluctuation of magnetization that can, in turn, convert into electricity. This approach offers a parallel path to boost energy conversion efficiency, leading to a promising direction towards low-cost, high efficiency, and versatile thermoelectric technology.

The project team plans to design and synthesize a new class of thermoelectric materials that can overcome the fundamental limits imposed by Fermi-Dirac statistics on charge carriers by utilizing paramagnons - bosonic quasi-particles that can play as a new independent variable not limited to the counter-balancing nature of the parameters that enter zT. Just as in the discovery of the spin-Seebeck effect, which led to the new area of spincaloritronics, where the spin angular momentum is transferred to the electrons, the project team designs materials where the local thermal fluctuations of magnetization in the paramagnetic state (i.e., paramagnons) transfer their linear momentum to electrons and increase the thermopower. The proposal envisions three major thrusts: (i) understand the physics of electron-paramagnon interactions and identify the key material parameters through multiscale modeling, (ii) design multi-phase magnetic materials and synthesize them based on the theoretical understandings and the available experimental data, (iii) synthesize such materials, characterize and study them, and provide feedback to the design procedure for optimization. The emphasis will be placed on engineering these effects and designing high-performance commercially scalable compounds. This transdisciplinary work will open a new way to design high-performance thermoelectrics. At the same time, the study proposed here will provide data and information critical to studying the dynamics of short-lived local magnetic order, which is now at the forefront of the development of spin-dynamic theories in general.

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.

Semiconductor technology is highly globalized, and its future relies on innovations in materials and their applications in novel device structures. Interfaces between different parts of semiconductor devices are of paramount importance in micro-, nano-, and optoelectronics devices and heterogeneous systems. Therefore, fundamental and applied research on semiconductor interfaces is critical for emerging technologies that impact a wide range of industrial sectors. This award supports the planning phase of the proposed Center for Interface Sciences for Emerging Devices & Systems (CISEDS), which is composed of North Carolina State University and Purdue University. The mission of CISEDS is to produce fundamental science to address electrical and thermal interface challenges of upcoming devices in virtually all sectors of the semiconductor industry, including low and high-power electronics, power transmission, communications, energy, and medicine. The industry is well aware that unresolved or undiscovered challenges related to interfaces can significantly limit achievable device performance and reliability. The Center aims to work with various industries to identify and address key interface challenges that limit the performance or lifetime of the devices. The research that will be conducted under CISEDS has the potential for high economic and societal impact and will provide a robust educational framework to train next-generation U.S. graduate students for the industry. The Center will endeavor to broaden the participation of underserved and underrepresented student groups by leveraging the existing relationships and actively building new ones with local minority-serving institutions and organizations on the local campuses.<br/><br/>There has been significant progress in semiconductor materials, devices, and manufacturing over the last couple of decades. These advances were often made possible by inserting new materials into mature device structures. The semiconductor industry learned that to achieve the desired performance and device reliability, it is critically important to understand the scientific challenges in interfacing these new materials with those currently used in existing devices (e.g., metals, insulators, and other semiconductors). Quite often, such interfaces share similar science. As such, four thrust areas were selected as an initial research focus for the proposed Center: 1) Energy Conversion Devices, 2) Flexible/Stretchable Electronics, 3) Si & (Ultra) Wide Bandgap (WBG) Materials & Devices, and 4) 2D Materials & Devices. The fundamental science governing the interfaces, and the critical impact of interfaces on semiconductor device performance, are the backbone of the Center. The fundamental work on interfaces will enable advances within a given material/device technology as well as integration of various material systems into multifunctional components.<br/><br/>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.