Vaddiraju Intellectual Merit: The proposal uses solid-state thermoelectric modules to convert automobile waste heat directly into clean electricity without contributing additional greenhouse gas emissions. Suitable thermoelectric devices require: 1) high conversion efficiencies, 2) optimized form factor, 3) high stability, and 4) tunability for optimized system design. For widespread use of thermoelectric modules in automobiles, a zT>3 is needed, which is not possible with the current state-of-the-art devices. Recent theoretical predictions indicate that one-dimensional nanostructures (nanowires) are useful for the fabrication of highly efficient thermoelectric modules. Fabrication of thermoelectric devices and modules with zT>3 performance requires answering two overarching questions: Q1) what are the sizes and chemical compositions of inorganic nanowires required for achieving zT>3 performance? Q2) how can these nanowires be integrated on a large-scale into thermoelectric devices and modules?
This proposal?s hypothesis is that thermoelectric devices with zT>3 can be fabricated through an ?out-of-the-box? approach that utilizes organic and inorganic materials in unison, performed through homogeneous ?molecular wiring? of inorganic nanowires either to each other or though heterogeneous ?wiring? to organic semiconductor thin films. These two approaches are expected to solve the elusive problem of large-scale integration of nanomaterials while also providing the necessary flexibility for judicious selection of both the chemical components for high thermoelectric performance. The final goal of demonstrating zT>3 performance in large (> 1inch2) inorganic-organic hybrid TE devices will be realized by: 1) Using well-known chemical vapor deposition techniques, modified to this application, to synthesize both inorganic nanowires and organic thin films, followed by assembling them using ?molecular wiring? into cells of various sizes ranging from a few nm2 to a few cm2. 2) Systematically studying the effect of inorganic nanowire size and organic conducting polymer thin film chemistry and thickness on their individual thermoelectric performance, and also on their performance when used in unison as ?molecular wired? inorganic-organic hybrids.
A second advantage offered by the proposed ?molecular wired? assemblies is enhanced stability against air and moisture-assisted degradation and also against high temperature degradation. This is owed to the saturation of all dangling bonds in the hybrids, leaving no room for both oxygen/moisture adsorption and reaction. This complete saturation of dangling bonds is also expected to make the hybrids stable at very high temperatures. A large temperature difference, as high as 800oC, is available for electricity generation in automobiles. Hence, systematic investigation of the thermoelectric performance of the hybrids over a wide temperature range of 25-800oC will be performed to evaluate the temperature range over which stable zT>3 performance could be realized in them.
Finally, assembling individual TE devices into modules requires a metal that exhibits low resistance to charge transfer when brought into contact with the TE cells. Hence, the last aspect of the proposed study is to determine the type of the metal required for assembling individual TE devices into TE modules without lowering their performance. Systematic studies of the contact resistance of metal-hybrid junctions will be performed by varying the material of the metal in contact with the hybrids (using transfer length method). The type of the metal required for assembling individual TE devices into modules exhibiting zT>3 performance will be deduced.
Broader Impact: The educational impact of the proposed work will be the training of high school, undergraduate and graduate students through various avenues. 1) The PI, in collaboration with Mr. Berkan kaya, Assistant Principal at Harmony Science Academy, proposes to train high school students from Harmony Science Academy in Houston, TX and teach them techniques of nanomaterials synthesis and characterization. Small projects aimed at fabrication of energy conversion devices will be designed and the results of their work will be presented at various science fairs and competitions, such as ?International Sustainable World (Engineering, Environment, Energy) (www.isweeep.org). 2) The PI is also a volunteer for the NASA?s Motivating Undergraduates in Science and Technology (http://mustmentor.org) project. Through this avenue, the PI proposes to recruit undergraduate students and train them on the techniques for the fabrication of thermoelectrics and further motivate them to pursue graduate education. 3) Training of graduate students will include development of a new class useful for students across many disciplines. This class tentatively scheduled to be offered in spring 2011 is entitled ?Nanomaterials for Energy Conversion?.
. Intellectual Merit: Deployment of thermoelectric modules in automobiles and scavenging some of the heat lost through the exhaust as waste is a possible route to enhance their fuel efficiencies. This requires enhancing the heat-to-electricity conversion efficiencies of thermoelectric devices beyond that is possible by the current state-of-the-art, commercially-available devices. This in turn requires enhancing the figure of merit, zT, of thermoelectric materials. As the zT of materials is directly proportional to their electrical conductivities (σ) and inversely proportional to their thermal conductivities (κ), good thermoelectric materials should simultaneously be electrical conductors and thermal insulators (Nature Materials, 7, 105-114, 2008). Fabricating materials in nanostructured format is a possible pathway for accomplishing this task. These nanostructured materials allow for selectively reducing the lattice thermal conductivities portion (κl) of their thermal conductivities, without sacrificing their electrical conductivities. In this context, the intellectual merit of this project is as follows: the use of inorganic nanowires, in conjunction with electrically conducting conjugated organic molecules, is expected to provide the necessary degrees of freedom for enhancing the zT values of inorganic-organic hybrids composed of conjugated molecular wired inorganic nanowires. Here, single-crystalline nanowires are expected to provide a path for increasing electrical conductivity in the hybrids. Similarly, conjugated linker molecules that bind the nanowires together provide for enhanced electrical conductivity across nanowire interfaces. At the same time, the smaller nanowire diameters aid in lowering the κl. This is aided by the low κ of the organic molecules. The research work performed in the PI’s lab with the support of this grant resulted in 10 peer-reviewed publications and two Ph.D. dissertations. One of the publications describes a method for the mass production of both unfunctionalized and organic molecule functionalized nanowires (Physical Chemistry Chemical Physics, 15, 6260-6267, 2013). A second paper indicated that thin nanowire morphology not only allows for tuning thermal and electrical transport through materials, but also makes them amenable for consolidation into highly dense pellets (Nanotechnology, 25, 145401, 2014). The mechanical flexibility afforded by thin nanowires aids in their consolidation, while simultaneously ensuring that their morphology is not destroyed in the process, a fact that was hitherto unknown. The use of nanowire morphology, along with the use of doping, led to enhanced thermoelectric performance in materials. Pellets composed of copper doped Zn3P2 nanowires were observed to exhibit zT values of 0.23, much higher than that reported previously (Nanotechnology, 25, 125402, 2014). A zT value of 0.6 was obtained in Al and Ga doped ZnO nanowire pellets, higher than that observed previously in bulk ZnO (ACS Applied Materials & Interfaces, 6, 14923-14930, 2014). In addition, a phase transformation strategy for transforming silicon nanowires into either polycrystalline Mg2Si nanowires (Materials Letters, 100, 106-110, 2013), or single-crystalline Mg2Si nanowires (Chemistry of Materials, 26, 2814-2819, 2014) was developed. This strategy was also extended for assembling Mg2Si nanowires via welding. Finally, a strategy involving the non-conformal decoration of semiconductor nanowires with BN to make them stable against water- and acid-assisted degradation, without majorly altering their electrical and electronic properties, was developed (Physical Chemistry Chemical Physics, 16, 16150-16157, 2014). More recently, the simultaneously consolidation and alignment of nanowires into ingots using equal channel angular extrusion (ECAE) was accomplished (Materials Research Express, 2, 015013, 2015). Broader Impacts: This grant supported the education and training of three PhD students. The first student, Dr. Lance Brockway graduated with a Ph.D. degree (chemical engineering) in May 2014. A second student, Dr. Yongmin Kang, graduated with a Ph.D. degree (Materials Science and Engineering) in December 2014. The support also allowed a third student, Mr. Venkata Vasiraju, travel to the Conn Center for Renewable Energy at the University of Louisville and learn the fundamentals of materials characterization. This support also allowed the PI forge collaboration with Dr. Fleurial’s group at NASA’s Jet Propulsion laboratory (JPL) and facilitated the training of Dr. Brockway on the fundamentals of thermoelectric device fabrication. A chemical vapor deposition (CVD) chamber procured through this support aided the experimental training of students enrolled in the ‘Nanomaterials for energy conversion’ class taught by the PI. Overall, this CVD reactor was also used to teach more than 20 undergraduate students fundamental principles underlying CVD and its use for mass producing nanowires in the PI’s laboratory. Finally, a high school student from A&M consolidated high school in College Station, Texas was taught the fundamental concepts underlying renewable energy generation from nanomaterials. He was also provided hands-on experience with the operation of the CVD chamber and its utility in the production of Zn3P2 nanowires. In addition to the dissemination of the results in peer-reviewed publications, dissemination was also performed via presentations at various conferences, including meetings of the American Institute of Chemical Engineers and the Electrochemical Society.
