Development of fuel cell electrodes, Electrode improvement and life testing, tasks 1 and 3 Final report, 30 Jun. 1966 - 30 Apr. 1968

Volt-ampere characteristics improvement and life testing of electrodes for hydrogen oxygen fuel cell

A Checkout Language for Future Space Vehicles

To support an increased emphasis on automated checkout of future space vehicles, a procedureoriented computer language is required. This language needs to be more user-oriented and needs to have a more complete set of capabilities than existing languages. Such a language, named TOTAL, was developed under contract to NASAKSC. This paper presents an overall view of the language in terms of its major characteristics as derived from the basic design objectives

Development of Silicon-Modified 48 Wt. % U-Al Alloys for Aluminum Plate-Type Fuel Elements

The casting procedures, mechanical working characteristics, composite fuel plate fabrication, and fuel element assembly methods for a nominal 48 wt.% U- 3 wt.% Si -AP alloy are examined. It was found that localized clad thinning, resulting from dogboning'' of the fuel alloy, is essentially eliminated in composite Plates when this alloy is used in combination with 5154 Al frames and 1100 Al covers. However, fuel component dimensions are difficult to maintain within commonly accepted tolersnces in the brazing prccess because of the marked differences in the thermal expansion of the fuel alloy and the containment materials. The mechanical properties, irradiation studies, corrosfon testing, pneumatic pressure testing, and chemical reprocessing of the Si-modified alloy are also discussed. (auth

17O NMR study of q=0 spin excitations in a nearly ideal S=1/2 1D Heisenberg antiferromagnet, Sr2CuO3, up to 800 K

We used 17O NMR to probe the uniform (wavevector q=0) electron spin excitations up to 800 K in Sr2CuO3 and separate the q=0 from the q=\pm\pi/a staggered components. Our results support the logarithmic decrease of the uniform spin susceptibility below T ~ 0.015J, where J=2200 K. From measurement of the dynamical spin susceptibility for q=0 by the spin-lattice relaxation rate 1/T_{1}, we demonstrate that the q=0 mode of spin transport is ballistic at the T=0 limit, but has a diffusion-like contribution at finite temperatures even for T << J.Comment: Submitted to Phys. Rev. Lett. 4 pages, 4 figure

Critical dynamics of a spin-5/2 2D isotropic antiferromagnet

We report a neutron scattering study of the dynamic spin correlations in Rb$_2$MnF$_4$, a two-dimensional spin-5/2 antiferromagnet. By tuning an external magnetic field to the value for the spin-flop line, we reduce the effective spin anisotropy to essentially zero, thereby obtaining a nearly ideal two-dimensional isotropic antiferromagnet. From the shape of the quasielastic peak as a function of temperature, we demonstrate dynamic scaling for this system and find a value for the dynamical exponent $z$. We compare these results to theoretical predictions for the dynamic behavior of the two-dimensional Heisenberg model, in which deviations from $z=1$ provide a measure of the corrections to scaling.Comment: 5 pages, 4 figures. Submitted to Physical Review B, Rapid Communication

Nuclear spin relaxation rates in two-leg spin ladders

Using the transfer-matrix DMRG method, we study the nuclear spin relaxation rate 1/T_1 in the two-leg s=1/2 ladder as function of the inter-chain (J_{\perp}) and intra-chain (J_{|}) couplings. In particular, we separate the q_y=0 and \pi contributions and show that the later contribute significantly to the copper relaxation rate ^{63}(1/T_1) in the experimentally relevant coupling and temperature range. We compare our results to both theoretical predictions and experimental measures on ladder materials.Comment: Few modifications from the previous version 4 pages, 5 figures, accepted for publication in PR

63Cu NQR evidence of dimensional crossover to anisotropic 2d regime in S= 1/2 three-leg ladder Sr2Cu3O5

We probed spin-spin correlations up to 725 K with 63Cu NQR in the S= 1/2 three-leg ladder Sr2Cu3O5. We present experimental evidence that below 300 K, weak inter-ladder coupling causes dimensional crossover of the spin-spin correlation length \xi from quasi-1d (\xi ~ 1/T) to anisotropic 2d regime (\xi \~ exp[2\pi\rho_{s}/T], where 2\pi\rho_{s} = 290 +/- 30 K is the effective spin stiffness). This is the first experimental verification of the renormalized classical behavior of the anisotropic non-linear sigma model in 2d, which has been recently proposed for the striped phase in high T_{c} cuprates.Comment: 4 pages, 3 figure

Direct inversion of S-P differential arrival-times for Vp/Vs ratio in SE Asia

Open Access via Jisc Wiley agreementPeer reviewedPublisher PD

Electrodynamics of a Coulomb Glass in n-type Silicon

Optical measurements of the real and imaginary frequency dependent conductivity of uncompensated n-type silicon are reported. The experiments are done in the quantum limit, $\hbar\omega > k_{B}T$, across a broad doping range on the insulating side of the Metal-Insulator transition (MIT). The observed low energy linear frequency dependence shows characteristics consistent with theories of a Coulomb glass, but discrepancies exist in the relative magnitudes of the real and imaginary components. At higher energies we observe a crossover to a quadratic frequency dependence that is sharper than expected over the entire dopant range. The concentration dependence gives evidence that the Coulomb interaction energy is the relevant energy scale that determines this crossover.Comment: 5 pages, 4 figures; accepted for publication in Phys. Rev. Let