22,562 research outputs found
Thermochemical energy storage for a lunar base
A thermochemical solar energy storage concept involving the reversible reaction CaO + H2O yields Ca(OH)2 is proposed as a power system element for a lunar base. The operation and components of such a system are described. The CaO/H2O system is capable of generating electric power during both the day and night. Mass of the required amount of CaO is neglected since it is obtained from lunar soil. Potential technical problems, such as reactor design and lunar soil processing, are reviewed
"Economics in Context: The Need for a New Textbook"
economics education, textbooks, economic theory
X-ray induced persistent photoconductivity in Si-doped AlGaAs
We demonstrate that X-ray irradiation can be used to induce an
insulator-metal transition in Si-doped AlGaAs, a
semiconductor with {\it DX} centers. The excitation mechanism of the {\it DX}
centers into their shallow donor state was revealed by studying the
photoconductance along with fluorescence. The photoconductance as a function of
incident X-ray energy exhibits an edge both at the Ga and As K-edge, implying
that core-hole excitation of Ga and As are efficient primary steps for the
excitation of {\it DX} centers. A high quantum yield () suggests that
the excitation is indirect and nonlocal, due to secondary electrons, holes, and
fluorescence photons.Comment: 3 pages of text, 6 figures. An error in Fig.5 was detected, so we
corrected i
A time lens for high resolution neutron time of flight spectrometers
We examine in analytic and numeric ways the imaging effects of temporal
neutron lenses created by traveling magnetic fields. For fields of parabolic
shape we derive the imaging equations, investigate the time-magnification, the
evolution of the phase space element, the gain factor and the effect of finite
beam size. The main aberration effects are calculated numerically. The system
is technologically feasible and should convert neutron time of flight
instruments from pinhole- to imaging configuration in time, thus enhancing
intensity and/or time resolution. New fields of application for high resolution
spectrometry may be opened.Comment: 8 pages, 11 figure
Evaporation and growth of crystals - propagation of step density compression waves at vicinal surfaces
We studied the step dynamics during crystal sublimation and growth in the
limit of fast surface diffusion and slow kinetics of atom attachment-detachment
at the steps. For this limit we formulate a model free of the quasi-static
approximation in the calculation of the adatom concentration on the terraces at
the crystal surface. Such a model provides a relatively simple way to study the
linear stability of a step train in a presence of step-step repulsion and an
absence of destabilizing factors (as Schwoebel effect, surface electromigration
etc.). The central result is that a critical velocity of the steps in the train
exists which separates the stability and instability regimes. When the step
velocity exceeds its critical value the plot of these trajectories manifests
clear space and time periodicity (step density compression waves propagate on
the vicinal surface). This ordered motion of the steps is preceded by a
relatively short transition period of disordered step dynamics.Comment: 18 pages, 6 figure
- …