Project #1: Catalytic Microreactors for Hydrogen Generation and Failure Analysis of Solid Oxide Fuel Cells
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DESCRIPTION: This project involves design, fabrication, and theoretical and experimental analysis of novel transient MEMS reactors for hydrogen production via heterogeneous catalysis with in-situ hydrogen separation and integration with PEFC micro fuel cells. These are: (1) a forced flow unsteady-state catalytic reactor operating in the reverse-flow mode, input composition cycling mode, and input temperature cycling mode, and (2) a catalytic reactor exploiting the transient and steady-state spatial reaction patterns by using fractal structuring of the catalytically active surface. The advantages offered by these systems are the following: (a) rapid heat and mass transport, (b) non-equilibrium surface chemistry, (c) energy and chemical conversions at the optimal locations, and (d) high productivity and selectivity (per unit volume). The catalytic microreactors described constitute a new area of research with enormous potential applications in the power generation, chemical, pharmaceutical, aerospace and transportation industries as well as in the biotechnology and military. The examples are the distributed power generators for the space station and Navy warships, power sources for compact remote sensors and instruments, automotive exhaust clean-up systems, and others.
We are developing a unique concept for highly integrated hydrogen generator capable of hydrogen generation from liquid hydrocarbon fuels and in-situ separation and storage of high purity hydrogen to be used as a feedstock for low temperature PEFC energy sources. We have also developed an advanced theoretical methodology which combines the continuous fluid flow, heat and mass transfer models of the process with the statistical mechanical model for adsorption equilibrium within a unified computational framework. Both slip and no-slip flows have been considered depending upon the dimensions of the system. Our analysis has demonstrated the vital importance of appropriate modeling of non-linear near-wall interactions between heat/mass transport and adsorption/desorption for correct predictions of the overall system dynamics. The fundamental scaling relations for characteristic time constants for the system transient response have been also obtained and verified by detailed parametric simulations.
Cost of heterogeneous catalytic systems such as chemical and biological reactors, chemical sensors, and electrochemical fuel cells is primarily determined by the amount of the chemically active catalyst, usually highly expensive noble metals or alloys, that needs to be loaded to achieve desired rate of chemical conversion. This problem has been traditionally addressed by continuous search for less expensive and equally efficient catalyst materials. We have developed an alternative approach for significant reduction of the catalyst loading by optimal structuring of active catalyst surfaces in the heterogeneous chemical reactors and electrochemical sensors and fuel cells. Intuition tells us that any decrease in the active (i.e., loaded with catalyst) surface area should result in an equivalent decrease in the reaction yield and efficiency. However, our findings counter this by showing that the active surface and hence the catalyst loading can be reduced drastically in the diffusion-limited heterogeneous reaction systems, while the conversion rate remains essentially unchanged by using fractals for spatial distribution of the catalyst load. The results indicate that introduction of periodic singularities into the boundary conditions through fractal structuring of the active surface allows a 76% reduction in the catalyst loading while losing only 2.25% of the original rate of chemical conversion. The effect of fractal structuring of the active surface is the most profound to the design of micro/nano scale systems, for which the Peclet (Pe) and Rayleigh (Ra) numbers are intrinsically small due to the small characteristic length scale and body forces. Thus, introduction of periodic singularities into the mass transfer boundary condition is ideally suited for drastically lowering the cost of MEMS devices for chemical and biological processing as well as micro/nanoporous electrodes for fuel cells, and it can be exploited to its fullest extent by utilizing the advances in our abilities to manipulate matter on the micro/nano scale.
Finally, we are also working on the model-based optimal design and failure prediction in the solid oxide fuel cells. This work involves both theoretical and experimental aspects, and is accomplished in collaboration with a number of national labs (NETL, PNNL) and SOFC manufacturers.
Our goals are (1) to develop novel design configurations for transient MEMS reactors and identify integration schemes for development of highly efficient, intimately integrated, and controllable hydrogen generators, (2) to quantify interactions between heat and mass transfer and strong surface chemical/phase transformations (i.e., adsorption, catalysis, evaporation) in the Knudsen layer, (3) to access the effect of non-equilibrium surface chemistry on the near-wall flow and heat/mass transfer in catalytic microreactors, (4) to develop an optimal control strategies for the chemical microreactors under consideration, and (5) to validate theoretical models developed through detailed experimentation.
SPONSOR: Air Products & Chemicals, Inc. (completed), DOE (completed), NASA (completed), GT Strategic Energy Institute (current).
COLLABORATION: Prof. Levent Degertekin (GT ME), Prof. Jianmin Qu (GT ME), and Dr. Comas Haynes (GTRI)
STUDENTS: Mark Meacham (Ph.D.), Logan McLeod (Ph.D.), Mark Varady (Ph.D.), David Damm (Ph.D.), and Roger Lang (M.S.)
RELEVANT PUBLICATIONS/PATENTS:
Project #2:
Portable, Solid-State NanoReactor for Low Temperature, Energy Efficient Photocatalytic Disinfection of Air/Water Streams"
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DESCRIPTION: This project focuses on a new approach to photocatalytic disinfection of air and water streams through the exploitation of the unique structural and optical properties of semiconductor oxide nanocatalysts incorporated into the light trapping pores of porous silicon. This approach to photocatalysis could potentially pave the way to the development of a uniquely powerful technology for reliable, energy efficient, and inexpensive destruction of a large group of pathogenic bacteria, viruses, protozoa, and fungi which can present a significant threat to society. Of no lesser importance is that the developed technology will be scalable and suitable for integration with existing air/water purification systems for personal (e.g., emergency workers and military personnel) and stationary (e.g., hospitals and residential buildings) use. The porous silicon (PS) structure (see left figure) whose micron-size open pores are decorated by clusters of silica or titania nanospheres 5-30 nm in diameter (see middle figure), by themselves or impregnated with a noble metal, forming appropriate size photocatalyst quantum dots (QD), constitute a conceptually novel heterogeneous photocatalytic microreactor with possible superior performance (see right figure). Its success is based on four factors. First, micro-PS can provide a uniform and highly effective (i.e., tightly localized) illumination of the photocatalyst everywhere using the UV-VIS light generated through PS electroluminescence or photoluminescence. Second, PS, especially in the form of a hybrid macroporous-nanoporous structure, can provide a large specific (i.e., per unit volume) surface area, owing to its highly porous structure and enhanced by the additional surface area made available via attached catalyst-based nanostructures. Third, photocatalysts in the form of nanoparticles have been shown to often possess superior catalytic properties as compared to a conventional bulk catalyst by virtue of the electronic modification of the catalyst through size quantization . Finally, because of the small characteristic size of the PS pores forming the reactor flow network, we can essentially eliminate diffusion limitations (~1/D^2, where D is the pore diameter) on the rate of the heterogeneous chemical reaction. This mode can achieve reaction kinetics at its intrinsic rate while maintaining sufficient reactor throughput using parallel chemical processing in many identical reactor units.
Our current efforts include: (1) using porous silicon with a well-controlled geometry and surface chemical composition which, at the same time, acts as a light-trapping and reactor-supporting structure and as an internal light source for photocatalysis as we exploit its luminescence; (2) producing TiO2-xNx, silica (SiO2), and SiO2-xNx nanospheres and agglomeration within the micropores of the PS reactor to form photocatalyst quantum dots, (3) gold impregnating these nanostructures using surface-hydroxyl group-metal complexation and coating the nanostructures using electroless deposition techniques to enhance the quantum efficiency of photooxidation , (4) characterizing experimentally the PS photocatalytic reactor and its key elements, and (5) demonstrating reactor performance using selected model analytes.
SPONSOR: NSF NIRT (current)
COLLABORATION: Prof. James Gole (GT Physics), Drs. Mark King and Susan Ray (Emory Medical School and Grady Hospital)
STUDENTS: Andrew Ogden (Ph.D.)
RELEVANT PUBLICATIONS/PATENTS:
Project #3
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"Fluid Mechanics, Mass Transport, and Electrochemistry of Biochemical Interface Imaging Using AFM-Integrated Scanning Electrochemical and Optical Nanoprobes"
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DESCRIPTION: The present project focuses on an innovative multidimensional approach for the investigation of cell communication processes at the molecular level. In this project, novel, innovative and interdisciplinary research is emphasized with focus on the application of integrated scanning nanoprobe sensing systems. Scanning probe microscopy (SPM) techniques provide powerful means for obtaining chemical, topographical and optical information with high spatial resolution. Each technique - atomic force microscopy (AFM), scanning nearfield optical microscopy (SNOM) and scanning electrochemical microscopy (SECM) - is designed to provide a specific kind of data. To date, none of these individual techniques provide simultaneous information with high selectivity and sensitivity on multiple parameters correlated in space and time, information required for in situ investigations of complex biological systems and heterogeneous matrices.
In collaboration with Professor Mizaikoff's group at GT Chemistry, we work on developing the combinations of SECM-SNOM and SECM-AFM-SNOM, along with inverted confocal microscopy and IR spectroscopy, and investigate the application of these combined techniques for localization of exocytotic events at the cell surface. With the developed instrumentation, a unique set of multiple analytical parameters correlated in space and time will be obtained, which is critical to the investigation of complex biosystems and biological processes. Our part in this project is in development of process simulation and visualization tools for quantitative interpretation of the nanoprobe images.
The mathematical modeling and simulation of the electrochemical and physical processes taking place during the scanning process is essential for optimizing the design of integrated SECM-AFM scanning nanoprobes, as well as for interpretation of the imaging data. A dynamic electrochemical process involves the transport of reactants (reduced and oxidized species) and their intermediates between a metal electrode and a studied conducting or insulating substrate/interface. This includes adsorption/desorption of reactants, surface diffusion and phase formation, interfacial electron transfer(s), and possibly homogeneous reactions in the solution. During the experimental study of reaction kinetics and mechanisms, the challenge is to identify quantitatively the relative importance of each of these steps. Accordingly, quantitative interpretation of the experimental results demands rigorous modeling of the underlying physicochemical phenomena. The key feature of dynamic electrochemistry is that the mass flux of reactants at the electrode/solution interface is a direct measure of the interfacial reaction rate and, if the mass transport is sufficiently high and well defined (i.e., calculable), the kinetics of the interfacial and/or solution processes can be measured unambiguously. Thus, a carefully validated mathematical model of the mass transport process should become the key tool for quantitative evaluation of the cellular events investigated in the project.
The complex transport phenomena underlying SECM-AFM require visualization of multiple dependent variables (e.g., ion concentrations, velocity components, tip current, and others) recorded on the multitude of spatial and time scales, and the commercial visualization software programs are generally not able to provide satisfactory environment to handle such multivariable-multiscale datasets in an integrated framework. To this end, we propose to develop an interactive, hierarchical data flow visualization tool capable of visualizing and comparing the data obtained using macroscale continuous simulations and that from the microscale (atomic) level simulations by building on our latest advances in this area.
SPONSOR: NSF (completed), NIH (current)
COLLABORATION: Dr. Christine Kranz, Prof. Boris Mizaikoff (GT Chemistry), Prof. Doug Eaton (Emory)
STUDENTS: Audric Saillard (M.S.), Youg Koo Kwon (Ph.D.), Dr. Vladimir Zarnitsyn (Post-doc), and Dr. Peter Kottke (Post-doc)
RELEVANT PUBLICATIONS:
Project #4
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"Model-Based Optimal Design of (Bio)Chemical Sensors and Microfluidic Drug/Gene Delivery Devices"
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DESCRIPTION: In this project, a model-based methodology for optimal design of polymer coated chemical sensors is developed and is illustrated for the example of infrared evanescent field chemical sensors. The methodology is based on rigorous and computationally efficient modeling of combined fluid mechanics and mass transfer. A simple algebraic equation for the optimal size of the sensor flow cell is developed to guide sensor design and validated by extensive CFD simulations. Based upon these calculations, optimized geometries of the sensor flow cell are proposed to further improve the response time of chemical sensors.
Independent of the specific transduction mechanism, all chemical sensors generate signals upon molecular interaction of their selective chemical recognition interface with the desired target analyte. To increase the detection threshold, the analyte of interest is usually preconcentrated by various techniques, utilizing hydrophobic polymer layers as preferred implementation in many sensing applications operated in aqueous environments. In depth understanding of analyte enrichment in the polymers due to bulk solvation effects has been a major issue of physical and analytical chemistry over several decades prompted. In the chemical sensing community, major efforts have recently been invested into modeling and optimization of polymer-based enrichment layers, whereas optimization of the sensor flow cell geometry and mass transport was almost completely untouched. This is a surprising fact, in particular since data acquisition for chemical sensors can be a complex process; thus, appropriate simulation models of chemical sensors can serve as highly valuable tool for sensor design and data interpretation.
During decade only few relatively simplistic models have been developed for different types of chemical sensors, ranging from fiber-optic chemical sensor to dopamine biosensor and thermoelectric gas sensors. It is widely believed that miniaturization of the cross-sectional area of the sensor flow cell results in reduction of the response time, as well as threshold of detection. This "intuitive" rule-of-thumb is questioned in this project, aiming at establishing a sound theoretical basis for optimal design of the flow cell for polymer coated chemical sensors. In addition, multicomponent mass transfer effects are rigorously modeled to establish validity limits for commonly used pseudo-binary diffusion approximation for treating mixtures containing multiple analytes of interest.
SPONSOR: ALCOA Fellowship & GT Research Foundation (completed)
COLLABORATION: Prof. Boris Mizaikoff (GT Chemistry), Prof. Gole (GT Physics), Prof. Charlie Hao (Emory/Winship Cancer Institute), and Drs. Mark Papania and Rotta (CDC).
STUDENTS: Cynthia Phillips (M.S.)
RELEVANT PUBLICATIONS/PATENTS:
Project #5
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"Thermal Management of Next Generation Terascale Integrated Circuits and Interconnects"
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DESCRIPTION: Thermal management is a systems issue, spanning several orders in length scales from the interconnects and transistors to an entire data center. The industry has traditionally addressed these issues in segments, isolating the junction-to-case thermal management problem from the case-to-ambient problem, etc. This situation prevents truly optimal system-level solutions and consumes the entire chip backside for thermal management using bulky metal heat sinks and fans, making it difficult to develop novel strategies that release surface area for optical and RF I/O. The ITRS projects high performance single chip powers to approach 288 W by 2012, which yields heat fluxes that simply cannot be accommodated using this disjointed approach. Furthermore, the integration of multiple circuit functionalities onto a single chip, i.e., RF, optical, CMOS, MEMS, etc., the demands for highly localized temperature control and hotspot cooling cannot be accommodated using a traditional package centric approach. The best approach for satisfying the future needs is to use a multiscale integrated approach. These imply heat fluxes of ~100 W/cm2 at the chip level and 10 kW/ft2 at the data center floor level. For the portables, while the heat fluxes are lower, the space and battery power constraints, as well as the more stringent touch temperature limits are driving a search for new solutions. Along with the increase in power, there are a number of other emerging trends driving the need for unique thermal solutions: (i) Highly non-uniform power dissipations on the CPU chip due to integration of cache memory with the processor, (ii) Partial or no access to the top surface of the die due to optical interconnects, (iii) Increasing importance of Joule heating in interconnects.
The focus of our current efforts is on the development and assessment of three classes of thermal management devices for both stationary high performance and portable systems. The single phase and phase-change based micro and nano fluidic schemes are being developed that effectively utilize through-the-board thermal pathways. Additionally, schemes for the effective delivery and utilization of liquefied air in an open loop flow arrangement to achieve both effective thermal management, as well as the performance benefits of low temperature operation are being explored. We also explore the use of carbon nanostructures for localized chip-centric thermal management and mist impingement cooling at the chip level using micromachinned ultrasonic acoustic atomization.
SPONSOR: Interconnect Focus Center/MARCO/DARPA (current)
COLLABORATION: Prof. Yogendra Joshi (GT ME), Prof. Paul Kohl (GT ChE), Prof. J. Meindl (GT ECE)
STUDENTS: Stephane Launay (Post-doc), Xianjin Wei (Post-doc), Prajesh Bhattacharya (Post-doc), Yoon Jo Kim (Post-doc), Shivesh Suman (Ph.D.), Robert Wadell (M.S.), Shankar Narayanan (Ph.D.), Vivek Sahu (M.S.)
RELEVANT PUBLICATIONS/PATENTS:
Project #6
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"Electron, Mass, and Heat Transport in E-Beam and Laser-Jet Chemical Vapor Deposition of Nanostructured Materials and Thin Film Coatings"
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DESCRIPTION: In this project, we would like to determine via experimentation and thermal/mass transport analysis the conditions required for controlled growth of patterned arrays of carbon nanotubes and nanofibers. The influence of temperature, reagent flow rate concentration, and catalyst/substrate type will be investigated using statistically designed and analyzed experiments. Temperature is often the most significant process variable for any form of CVD, including Combustion (CCVD) and Laser (LCVD), and its measurement has been particularly troublesome in LCVD because of the small size of the heated spot.
Optimization and precise control of the shape and mechanical properties of deposited nanostructures can be achieved only if the transport phenomena underlying the gas-jet LCVD process are fundamentally understood and appropriate simulation tools are developed and experimentally validated. The complexity of the physical situation is underscored by the fact that CCVD and LCVD are truly multiphysics and multiscale processes. They includes jet impingement convective heat and mass transfer, heat transfer by conduction within a substrate, radiation heat transfer between the substrate and the reactor walls, and finally the homogeneous and heterogeneous chemical reactions on the substrate resulting in material deposition and accompanied by the volumetric heat release. Since the deposition rates follow an Arrhenius relationship that is exponential with respect to temperature, it is critical to understand and quantify the local temperature field in the vicinity of the deposition spot. In a laser-heated process such as pyrolytic LCVD, the temperature field not only varies by an order of magnitude over the diameter of the laser spot (i.e., a micrometer scale),4 but it is also directly influenced by the heat transfer processes occurring on the reactor macroscale (i.e., a meter scale)5. This example signifies the importance as well as the challenges of the multiscale integrated thermal/fluid modeling of the CVD process.
Over the past few years, we have developed the series of models of increasing complexity based on the Navier-Stokes equations of motion to simulate the fluid flow, thermal, and concentration fields in the LCVD reactor. Our on-going efforts focus on including the effect of surface-grown nanostructures on the modification of the surface radiadive properties of the substrate during deposition process as well as on rigorous modeling of radiative transfer in the CVD chamber.
SPONSOR: NSF NIRT (current)
COLLABORATION: Profs. Jack Lackey (GT ME), Thom Orlando (GT Chem), and ZL Wang (GT MSE)
STUDENTS: Ben White (Ph.D.) and Konrad Rykaczewski (Ph.D.)
RELEVANT PUBLICATIONS:
Project 7: Scanning Mass Spectrometry (SMS) Probe and AMUSE (Array of Micromachined UltraSonic Electrospray) Ion Source for Mass Spectrometry
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DESCRIPTION: Mass Spectroscopy (MS) has become the technology of choice to meet today's unprecedented demand for accurate bioanalytical measurements, including protein identification. Although MS can be used to analyze any biological sample, it must be first converted to gas-phase ions before it can be introduced into a mass spectrometer for analysis. It is transfer of a very small liquid sample (proteins are very expensive and often very difficult to produce in sizable quantities) into a gas-phase ions that is currently considered to be a bottleneck to high throughput proteomics.
Electrospray ionization (ESI) is a technique developed in early 1990th by Fenn (Noble Prize in Chemistry, 2002) to generate a spray gas-phase ions by applying high voltage (from several hundreds volts and up to a few thousands kilovolts relative to the ground electrode of the MS interface) to a small capillary through which the liquid solution is pumped. The high electric field ionizes the fluid forming the converging cone of the exiting jet which eventually breaks into many small droplets when the repulsive Coulombic forces overcome the surface tension. Because of the focusing effect associated with the spraying the electrically charged fluid, the size of the electrospray cone and thus of the formed droplets is in a few tens of nanometers range although the inner diameter of the capillary is in the micrometer range.
We are developing a unique drop-on-demand electrospray chip that utilizes the ultrasonic waves to drive the flow and capable of operating with the smallest reagent samples, resulting in the highest sensitivity, and potentially requiring much lower voltages for efficient ionization. The electrospray generation microchip relies on our patented MEMS based ultrasonic aerosolizing technology and offers potentially low-cost, disposable solution to the problem of producing charged liquid droplets of size and uniformity required for effective protein analysis. Further, since Taylor cone formation is not required for atomization in the proposed device, potentially much lower operating voltages would be needed for ion formation, leading to much gentler atomization process and reduction in molecule fragmentation. Our initial experiments demonstrate possible ion formation at applied voltages of the order of 100V rather than 1-4 kV required for conventional electrospray sources. Further, an array of individually addressable electrodes can be used for actuation of each droplet generator in the array separately, therby allowing multiplexing and parallel analysis of multiple analyte samples requiring different electric potential for ionization.
SPONSOR: NIH (current)
COLLABORATION: Profs. F. Levent Degertekin (GT ME), Facundo Fernandez (GT Chemistry), and David Muddiman (NCSU & Mayo Clinic)
STUDENTS: Tom Forbes (Ph.D.)
RELEVANT PUBLICATIONS/PATENTS:
Project X: Creativity and Rigor in Research: Could We Marry
Them and How?
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DESCRIPTION: The project title says it all...
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SPONSOR: Foundation for Unfundable Science (current)
COLLABORATION: Students and post-docs at MITf-Lab
RELEVANT PUBLICATIONS:
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