Browsing by Subject "Graphite"

Now showing 1 - 7 of 7

Compression/injection molding of bipolar plates for proton exchange membrane fuel cells
(2009-12) Devaraj, Vikram; Beaman, Joseph J.; Koo, Joseph
Fuel cells are electrochemical energy conversion devices that convert chemical energy to electrical energy efficiently. Bipolar plates form an integral part of a fuel cell and their high manufacturing cost and low production rate have hindered the commercialization of fuel cells. Bipolar plates require high electrical conductivity, strength, chemical resistance and thermal conductivity. This thesis presents efforts to manufacture bipolar plates which meet these requirements using compression or injection molding. Compression or injection molding processes allow cost-effective, large-scale manufacturing of bipolar plates. A variety of material systems for the fabrication of bipolar plates are processed, molded and characterized.
Development of an experimental facility for biaxial compressive testing of fiber composite materials
(1991) Kotziapashis, Andreas Evangelou, 1962-; Kyriakides, S.
The purpose of this study was the development of a testing facility and test specimen for the characterization of laminated composites under biaxial compressive stress states. A biaxial testing facility capable of applying almost arbitrary stress paths in the in-plane compressive stress regime, to circular cylindrical test specimens, was designed, assembled and calibrated. The required biaxial load is achieved by applying combined axial compression and external pressure to the cylinder. The performance of the biaxial facility was verified by performing several exploratory experiments on Aluminum and Graphite/Epoxy specimens. Test specimens and testing procedures were designed such that material failure rather than structural failure would prevail under a prescribed loading path. A circular cylindrical shell was selected over the other possible biaxial test specimens, for its relatively simple manufacture, its potential in achieving a relatively homogeneous stress state within the test section and a boundary region that is relatively free of stress concentrations. An experiment was performed on a circular cylindrical Graphite/Epoxy specimen designed to fail by material failure. The test was conducted under hydrostatic loading. Failure of the specimen occurred at a pressure of 9035 psi. Post failure evaluation of the specimen confirmed that failure was the result of local buckling or “kinking” of the hoop fibers at the outer layers of the specimen
Direct Feedback Control of Gas-Phase Laser-Induced Deposition
(1996) Maxwell, J. L.; Pegna, J.; Messia, D.; DeAngelis, D.
Three-dimensional laser Chemical vapor deposition (3D-LCVD) or SALD, was used to prototype metallic and ceramic microstructures. Iron, nickel, and steel metal forms were grown from organic and halogen based precursors. Through the simultaneous use of multiple precursors, specific nickel-iron based alloys were produced. By observing the emission spectra during growth, a measure ofthe volumetric growth rate, was obtained. Direct, PID control ofthe process was then possible using the growth rate measurement as real-time feedback. Calibrated infrared photographs of evolving microstructures were taken at various wavelengths, giving a measure of the temperature gradient over the growth zone. While radiation contributes to heat losses at high temperatures, enhanced convection is the dominant heat transfer mechanism due to the small dimensions of the heated area. Enhanced growth rates, induced by convective flow, were also observed. The heat and mass transfer coefficients were determinedfor various processing conditions, and compare well with experimental data. Axi-symmetric rods may also be grown in both the kinetic and transport-limited regimes, and a systematic study of the precursor pressure and deposit temperature during growth yielded distinct growth regimes, influn, ced by the interplay of heat losses and diffusive transport.
Four-probe thermal transport measurements of few-layer graphene and ultrathin graphite
(2020-08-17) Ou, Eric; Shi, Li, Ph. D.; Hall, Matthew; Tutuc, Emanuel; Wang, Yaguo
The unique combination of mechanical, electrical, and thermal properties of graphite and its derivatives, such as graphene and carbon nanotubes, make graphitic materials desirable for a number of technological applications as well as a platform for studying various transport phenomena, especially at the nanoscale. Although it has been more than a decade since graphene was first successfully isolated, discrepancies between the results of theoretical and experimental studies have not yet been resolved and the answers to many fundamental questions concerning the details of thermal transport in graphene are still subject to debate. The presence of unknown contact thermal resistance has limited prior two-probe thermal transport measurements of suspended graphene samples. This work utilizes a four-probe thermal measurement technique to measure few-layer graphene and ultrathin graphite samples. This technique has the ability to measure the intrinsic thermal conductance of suspended samples and to isolate the contact thermal resistance between the sample and measurement device. By eliminating error due to contact thermal resistance and developing a clean method for transferring thin-film samples, the true intrinsic thermal properties of graphene can be realized, potentially leading to the observation of unique transport phenomena such as hydrodynamic phonon transport.
Nanoscale graphene for RF circuits and systems
(2013-08) Parrish, Kristen Nguyen; Akinwande, Deji
Increased challenges in CMOS scaling have motivated the development of alternatives to silicon circuit technologies, including graphene transistor development. In this work, we present a circuit simulator model for graphene FETs, developed to both fit measured data and predict new behaviors, motivating future research. The model is implemented in Agilent ADS, a circuit level simulator that is commonly used for non-standard transistor technologies, for use with parameter variation analyses, as well as easy integration with CMOS design kits. We present conclusions drawn from the model, including analyses on the effects of contact resistance and oxide scaling. We have also derived a quantum-capacitance limited model, used to intuit intrinsic behaviors of graphene transistors, as well as outline upper bounds on performance. Additionally, the ideal frequency doubler has been examined and compared with graphene, and performance limits for graphene frequency multipliers are elucidated. Performance as a demodulator is also discussed. We leverage this advancement in modeling research to advance circuit- and system-level research using graphene transistor technology. We first explore the development of a GHz planar carbon antenna for use on an RF frontend. This research is further developed in work towards the first standalone carbon radio on flexible plastics. A front end receiver, comprised of an integrated carbon antenna, transmission lines, and a graphene transistor for demodulation, are all fabricated onto one plastic substrate, to be interfaced with speakers for a full radio demo. This complete system will motivate further research on graphene-on-plastic systems.
One-dimensional electron systems on graphene edges
(2007-12) Hill, Jason Edward, 1978-; MacDonald, Allan H.
In this dissertation several aspects on one-dimensional edge states in grapheme are studied. First, a background in the history and development of graphitic forms is presented. Then some novel features found in two-dimensional bulk graphene are presented. Here, some focus is given to the chiral nature of the Dirac equation and the symmetries found in the grahene. Magnetism and interactions in graphene is also briefly discussed. Finally, the graphene nanoribbon with its two typical edges: armchair and zigzag is introduced. Gaps due to finite-size effects are studied. Next, the problem of determining the zigzag ground state is presented. Later, we develop this in an attempt to add the Coulomb interaction to the zigzag flat-band states. These nanoribbons can be stimulated with a tight-binding code on a lattice model in which many different effects can be added, including an A/B sublattice asymmetry, spin-orbit coupling and external fields. The lowest Landau level solutions in the different ribbon orientations is of particular current interest. This is done in the context of understanding new physics and developing novel applications of graphene nanoribbon devices. Adding spin-orbit to a graphene ribbon Hamiltonian leads to current carrying electronic states localized on the sample edges. These states can appear on both zigzag and armchair edges in the semi-finite limit and differ qualitatively in dispersion and spin-polarization from the well known zigzag edge states that occur in models that do not include spin-orbit coupling. We investigate the properties of these states both analytically and numerically using lattice and continuum models with intrinsic and Rashba spin-orbit coupling and spin-independent gap producing terms. A brief discussion of the Berry curvature and topological numbers of graphene with spin-orbit coupling also follows.
Optical measurements of the thermodynamic response in graphite due to extreme strain
(2021-05-07) Montaño, Raul David; Wang, Yaguo
Upon the discovery of graphene by micromechanical exfoliation, a boom in the microelectronics community has taken place. Contrasting to their bulk counterparts, two dimensional (2D) materials are held together in layered order with weak van der Waals (vdW) forces and strongly in-plane with covalent bonds. This wide range of emerging materials have been enthusiastically researched as they can possess unique band gaps, favorable to the semiconductor industry for emerging technologies in MEMS design. In relation to the weak interlayer bonds holding new materials together, they act differently in the cross- and in-plane directions with respect to their thermal and electrical performance. These intrinsic 2D material properties can further be enhanced by external factors such as temperature, electric, and magnetic fields. Vast efforts in the scientific community have proved large developments in using strain to control optical, thermal, and electronic properties of semiconductors, acting with higher performance compared to their bulk crystal forms. In this work, a diamond anvil cell (DAC) was implemented to impose a large compressive strain on a thin graphite film in order to induce strong interlayer covalent bonds. Measured with picosecond transient thermoreflectance (ps-TTR), an increase in cross-plane thermal conductivity (κ⏊) was observed to be ~180% with strains up to ~20 GPa. Furthermore, Raman spectroscopy was employed to monitor a phase transformation from graphite into a quasi-diamond film as a shift in the G/2D graphite peaks indicated a change in material structure.