There is a strong need for small, lightweight energy storage devices that can satisfy the ever increasing power and
energy demands of micro unmanned systems. Currently, most commercial and developmental micro unmanned systems
utilize commercial-off-the-shelf (COTS) lithium polymer batteries for their energy storage needs. While COTS lithium
polymer batteries are the industry norm, the weight of these batteries can account for up to 60% of the overall system
mass and the capacity of these batteries can limit mission durations to the order of only a few minutes. One method to
increase vehicle endurance without adding mass or sacrificing payload capabilities is to incorporate multiple system
functions into a single material or structure. For example, the body or chassis of a micro vehicle could be replaced with
a multifunctional material that would serve as both the vehicle structure and the on-board energy storage device.
In this paper we present recent progress towards the development of carbon nanotube (CNT)-based structural-energy
storage devices for micro unmanned systems. Randomly oriented and vertically aligned CNT-polymer composite
electrodes with varying degrees of flexibility are used as the primary building blocks for lightweight structural-supercapacitors.
For the purpose of this study, the mechanical properties of the CNT-based electrodes and the charge-discharge
behavior of the supercapacitor devices are examined. Because incorporating multifunctionality into a single
component often degrades the properties or performance of individual structures, the performance and property tradeoffs
of the CNT-based structural-energy storage devices will also be discussed.
We have used ab initio Hartree-Fock theory to characterize the microscopic structure and the nonlinear optical (NLO) properties of the over-coordinated oxygen hole center observed in radiation-exposed SiO2 films. Our calculations indicate that a proton (H+) forms a stable bond with a divalent oxygen atom in the Si-O-Si network at an equilibrium r(O-H) approximately equals 1.005 angstrom and also leads to an enhancement in the microscopic NLO response of the local structure by a factor of 4 or more. In the absence of the over-coordinating H+, the dipole, moment and the second-order NLO response of the Si- O-Si cluster is extremely small. Protonation of a bridging O atom distorts the electron charge cloud in the direction of the O-H bonding and also reduces the gap between the filled and the vacant energy levels. This leads to a substantial increase in the magnitude of the dipole moment vector and the component of the second-order NLO susceptibility along the O-H bond.
We present here our first-principles density functional theory (DFT) calculations of the potential energy and dipole moment function for Li+C60. We also present the results for the equilibrium geometry, dipole moment and polarizability dispersion obtained from ab initio Hartree-Fock (HF) calculations. The calculated equilibrium position of Li inside the C60 cage agrees with other theoretical calculations. The dipole moments calculated by the DFT and HF calculations, while differing from each other by almost a factor of two, are considerably smaller than a recent DFT result. The polarizability tensor calculated at lambda equals infinity (static) and at lambda equals 1064 nm are very nearly isotropic, indicating little deformation of the spherically symmetric charge distribution of C60. Quantitatively, the alpha values of the Li+C60 complex are very close to the corresponding value for the C60 molecule.
We present here the results for frequency-dependent linear polarizability, (alpha) , and second hyperpolarizability, (gamma) , coefficients of the icosahedral C60 molecule calculated from minimal basis ab initio and semiempirical methods in the framework of the time-dependent purturbation theory. The ab initio results of (alpha) are too small by a factor of two compared to their experimental counterpart. The semiempirical methods predict yet smaller values of (alpha) . The (gamma) coefficients obtained by the ab initio calculations are two orders of magnitude smaller than those derived from experiments. The corresponding values obtained by semiempirical calculations show improved agreement with the experiment. The ratio xxxx/xyyx for (gamma) (-3(omega) ;(omega) ,(omega) ,(omega) ) is calculated to be 1/3 up to an optical wavelength of 1.37 micrometers . The calculated values of (gamma) for the various third order effects follow the order: (gamma) (-3(omega) ;(omega) ,(omega) ,(omega) ) > (gamma) (-2(omega) ;0,(omega) ,(omega) ) > (gamma) (-(omega) ;(omega) ,(omega) ,-(omega) ) > (gamma) (-(omega) ;0,0,(omega) ) > (gamma) (0;0;0;0).
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