Fuel quality and operational issues for polymer electrolyte membrane (PEM) fuel cells

Polymer electrolyte membrane (PEM) fuel cells are susceptible to degradation due to the catalyst poisoning caused by CO present in the fuel above certain limits. Although the amount of CO in the fuel may be within the permissible limit, the fuel composition (% CO₂, CH₄, CO and H₂O) and the operating conditions of the cell (level of gas humidification, cell temperature and pressure) can be such that the equilibrium CO content inside the cell may exceed the permissible limit leading to a degradation of the fuel cell performance. In this study, 50 cm² active area PEM fuel cells were operated at 55–60 °C for periods up to 250 hours to study the effect of methane, carbon dioxide and water in the hydrogen fuel mix on the cell performance (stability of voltage and power output). Furthermore, the stability of fuel cells was also studied during operation of cells in a cyclic dead end / flow through configuration, both with and without the presence of carbon dioxide in the hydrogen stream. The presence of methane up to 10% in the hydrogen stream showed a negligible degradation in the cell performance. The presence of carbon dioxide in the hydrogen stream even at 1–2% level was found to degrade the cell performance. However, this degradation was found to disappear by bleeding only about 0.2% oxygen into the fuel stream.     

Structure and thermoelectric properties of CrSi2 crystallized from a tin solution—melt

The thermoelectric properties of CrSi₂ single crystals grown from a tin solution—melt are studied. A correlation is found between the unit cell parameters of the CrSi₂ crystals, their thermoelectric properties, and solution—melt cooling conditions.     

The Self-avoiding Walk Spanning a Strip

Populations of unicellular organisms that grow under constant environmental conditions are considered theoretically. The size distribution of these cells is calculated analytically, both for the usual process of binary division, in which one mother cell produces always two daughter cells, and for the more complex process of multiple division, in which one mother cell can produce 2 ⁿ daughter cells with n=1,2,3,… . The latter mode of division is inspired by the unicellular algae Chlamydomonas reinhardtii. The uniform response of the whole population to different environmental conditions is encoded in the individual rates of growth and division of the cells. The analytical treatment of the problem is based on size-dependent rules for cell growth and stochastic transition processes for cell division. The comparison between binary and multiple division shows that these different division processes lead to qualitatively different results for the size distribution and the population growth rates.     

Different types of nonlinear convective oscillations in a multilayer system under the joint action of buoyancy and thermocapillary effect

The nonlinear development of oscillatory instability under the joint action of buoyant and thermocapillary effects in a multilayer system, is investigated. The nonlinear convective regimes are studied by the finite difference method. Two different types of boundary conditions – periodic boundary conditions and rigid heat-insulated lateral walls, are considered. It is found that in the case of periodic boundary conditions, the competition of both mechanisms of instability may lead to the development of specific types of flow: buoyant-thermocapillary traveling wave and pulsating traveling wave. In the case of rigid heat-insulated boundaries, various types of nonlinear flows – symmetric and asymmetric oscillations, have been found.     

Modelling of solid oxide proton conducting reactor-cells: thermodynamics and kinetics

A model is prepared for hydro- and dehydrogenation reactions that are carried out in solid electrolyte cell-reactors. Hydrogen is supplied in the form of H⁺ through the proton conducting wall and the thermodynamic activity of hydrogen on the catalyst surface is controlled electrochemically. Three reactor types are considered, a plug flow reactor, a well-mixed reactor and a “single-chamber” cell in which both electrodes are exposed to the same reacting mixture. Under certain conditions, the yield of the cell-reactor is orders of magnitude higher than that of the corresponding conventional catalytic reactor. The key parameters that affect the product yield as well as the conditions that must be met, are discussed. Experimental results for the reaction of methane dimerization studied with a strontia-ceria-ytterbia electrolyte on palladium electrodes, are presented and compared to model predictions. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Theoretical study on the behaviour of metal-p-n-Si Schottky barrier solar cell

The performance of Au-p-n-Si Schottky barrier solar cell has been investigated theoretically following Li’s work on GaAs. The behaviour of the barrier height as a function of carrier densities in then andp regions and thep layer thickness is investigated. The photovoltaic cell characteristics are worked out and conditions for maximum efficiency obtained. Communication No. 2376     

Forced convectional heat transfer in solid oxide fuel cells: An analytical treatment

The heat transfer within a fuel cell at steady state conditions is considered here for one dimensional geometry. Analytical solution for the heat transfer equation accompanied by the appropriate boundary conditions is obtained. The heat transfer coefficient is also estimated for the case of ideal heat exchange. It was found that the geometrical characteristics of the cell that are strongly related with its electrical ones (namely, the ohmic resistance and the current developed), are favourable parameters for the maximization of the heat transfer. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Dynamics of twist effect in a dual-frequency nematic liquid crystal

The dynamics of the electrooptical 90° twist effect in a dual-frequency nematic liquid crystal is investigated for wavelengths of 0.65 and 1.55μ m. It is shown that the boundary conditions of the interaction between the phases affect the optical threshold of the twist effect, the contrast, and the working voltage range. The switching time of the twist effect from the off to the on state upon a variation of the amplitude of a rectangular dc voltage pulse from 15 to 50 V changes from 1.5 to 0.3 ms for a thickness of the nematic crystal layer of about 7 μ m. The minimal time of switching from the “on” to the “off” state was 3 ms in the case when relaxation of molecules in a cell with asymmetric boundary conditions was controlled electrically. The dynamic range of transmittance variation at a wavelength of 1.55 μm extended to 30 dB.     

Alterations in the Notch4 pathway in cerebral endothelial cells by the HIV aspartyl protease inhibitor,nelfinavir

Background  

Aspartyl protease inhibitors (PIs) used to treat HIV belong to an important group of drugs that influence significantly endothelial cell functioning and angiogenic capacity, although specific mechanisms are poorly understood. Recently, PIs, particularly Nelfinavir, were reported to disrupt Notch signaling in the HIV-related endothelial cell neoplasm, Kaposi's sarcoma. Given the importance of maintaining proper cerebral endothelial cell signaling at the blood brain barrier during HIV infection, we considered potential signaling pathways such as Notch, that may be vulnerable to dysregulation during exposure to PI-based anti-retroviral regimens. Notch processing by γ-secretase results in cleavage of the notch intracellular domain that travels to the nucleus to regulate expression of genes such as vascular endothelial cell growth factor and NFκB that are critical in endothelial cell functioning. Since, the effects of HIV PIs on γ-secretase substrate pathways in cerebral endothelial cell signaling have not been addressed, we sought to determine the effects of HIV PIs on Notch and amyloid precursor protein.     

Nanoindentation mapping of a wood-adhesive bond

A mapping experiment of a wood phenol–resorcinol–formaldehyde adhesive bond was performed by means of grid nanoindentation. The variability of the modulus of elasticity and the hardness was evaluated for an area of 17 μm by 90 μm. Overall, the modulus of elasticity of the adhesive was clearly lower than the modulus of wood cell walls, whereas the hardness of the adhesive was slightly higher compared to cell walls. A very slight trend of decreasing modulus of elasticity was found with increasing distance from the immediate bond line. However, the trend was superimposed by a high variability of the modulus of elasticity in dependence on the position in the wood cell wall. The unexpectedly high variation of the modulus between 12 and 24 GPa may be explained by the interaction between the helical orientation of the cellulose microfibrils in the S2 layer of the wood cell wall and the geometry of the three-sided Berkovich type indenter pyramid used. Corresponding to the very slight decrease in modulus with increasing distance from the bond line, a similar but clearer trend was found for hardness. Both trends of changing mechanical properties of wood cell walls with varying distance from the bond line are attributed to effects of adhesive penetration into the wood cell wall. PACS 81.70.-q     

Waveguide co-laser with an intracavity optothermal cell for gas analysis systems

We investigate the possibilities for intracavity optothermal detection of microimpurities of gases. The generation regimes of a frequency-tunable wavequide CO-laser with an intracavity absorbing cell that contains NH₃ and NO₂ gases as absorbers are investigated experimentally. The conditions for stationary generation of such a laser are discussed. It is shown that the intracavity siting of the optothermal cell in a waveguide CO-laser makes it possible to increase the detecting ability of a gas analyzer by more than an order of magnitude as compared to the case of its external position. For NO₂ and NH₃ gases the magnitude of the minimum detected concentrations is ≈10 and 30 ppb, respectively. Moscow State Engineering Physics Institute, 31, Kashirskoe shosse, Moscow, 115409, Russia. Translated from Zhurnal Prikladni Spektroskopii, Vol. 64, No. 5, pp. 686–690, September–October, 1997.     

Dislocational kinetics in the intense deformation of fcc single crystals

The stability of the dislocational subsystem of fcc single crystals deformed in dynamic conditions is investigated. It is shown that, depending on the deformational conditions, the system may have one or two steady states, one of which is ρ _s ⁽¹⁾ . When the system has one trivial steady state, it may be stable or unstable. In some conditions, a second unstable point ρ _s ⁽²⁾ appears; in this case, ρ _s ⁽¹⁾ is stable. Tomsk State Architectural and Building Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–48, August, 1997.     

Effects of various solar indices on accuracy of Earth’s thermospheric neutral density models

Four kinds of solar indices F10.7, E10.7, S10, Mg10 and four thermospheric neutral density models, i.e., CIRA72, DTM94, NRLMSISE00 and JB2006, are discussed. The CHAMP accelerometer data are used to calculate thermospheric total mass density. Based on the comparison of the model densities with CHAMP observations, the effects of various indices on the model accuracy are detected. It is found that under quiet and moderate solar conditions (F10.7<160), all of the models’ errors are reduced about 15% by using E...     

Redshift in Hubble's constant

A topological field theory with Bogomol'nyi solitons is examined. The Bogomol'nyi solitons have much in common with the instanton in Yang-Mills theory; consequently we called them ‘topological instantons.’ When periodic boundary conditions are imposed, the field theory comments indirectly on the speed of light within the theory. In this particular model the speed of light is not a universal constant. This may or may not be relevant to the current debate in astronomy and cosmology over the large values of the Hubble constant obtained by the latest generation of ground- and space-based telescopes. An experiment is proposed to detect spatial variation in the speed of light.     

On sufficient stability conditions of the Couette — Poiseuille flow of monodisperse mixture

The stability of the Couette — Poiseuille flow of a monodisperse mixture is considered. Sufficient stability conditions are derived. Results of the computation of the spectrum are presented. A considerable stabilization of the flow with particles admixture to small disturbances is observed. It is found that the regions of instability generation may have complex geometry. The influence of the main velocity profile and admixture parameters on the stability conditions is considered.     

A mathematical model of the evolution of free radical concentrations in a cell under ionizing radiation

A probabilistic mathematical model that describes the evolution of the free radical concentration in a cell under ionizing radiation is proposed. The model is of a stochastic character and allows one to describe the generation of free radicals in the process of radiolysis, as well as the interaction between the radicals of different types and between the radicals and the cell constituents (the latter is responsible for radiobiological effect). The possibility of the presence in the cell of a certain equilibrium quantity of radicals of nonradiative origin at the moment that the irradiation begins is taken into account. The results thus obtained show that under certain values of the model’s free parameters the total number of free radicals in a cell under radiation may decrease. This effect can explain the “positive” influence of small doses of ionizing radiation.     

Spectral properties of backward stimulated scattering in liquid carbon disulfide

The spectral structure of backward stimulated scattering from a 10 cm-long CS₂-liquid cell is investigated by using Q-switched 10-ns and 532-nm laser pulses with different spectral linewidths. Under a narrow spectral line (∼0.1 cm⁻¹) pump condition, very strong sharp lines near the pump wavelength (λ ₀) position and the first-order stimulated Raman scattering (λ _s1) position can be observed. However, under a wide line (≈1 cm⁻¹) pump condition, only a strong and superbroadening spectral band can be observed mainly in the red-shift side of the pump wavelength. The different spectral features under these two conditions can be explained by a competition between stimulated Brillouin, Raman, and Rayleigh-Kerr scattering. Under both pump conditions, the broadening spectral distributions are not consistent with the predictions given by stimulated Rayleigh-wing scattering theories, but can be interpreted well utilizing the theoretical model of stimulated Rayleigh-Kerr scattering. Zh. éksp. Teor. Fiz. 112, 1563–1573 (November 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.