61,547 research outputs found
The effect of solid phase reactions on the ballistic properties of propellants
The combustion of NH4ClO4 composite propellants has been studied between 15 and 3000 psi. The emphasis in the program has been on determining the mechanisms by which the fuel components influence the burning rate of the composites. In order to have flexibility in the choice and concentration of the fuel component all combustion experiments were performed with pressed power strands. The fuels studied included those which affected the combustion mechanism of the composite primarily through their effect on: (1) the oxidizer decomposition mechanism and (2) the composite surface temperature. The combustion of pure and doped NH4ClO4 was studied using both pressed powder strands and pressed end burning motor grains. The experimental approach has been essentially a chemical one with emphasis on perturbing those reactions which occur on or immediately adjacent to the surface (zone of influence) of the composite
Final excitation energy of fission fragments
We study how the excitation energy of the fully accelerated fission fragments
is built up. It is stressed that only the intrinsic excitation energy available
before scission can be exchanged between the fission fragments to achieve
thermal equilibrium. This is in contradiction with most models used to
calculate prompt neutron emission where it is assumed that the total excitation
energy of the final fragments is shared between the fragments by the condition
of equal temperatures. We also study the intrinsic excitation-energy partition
according to a level density description with a transition from a
constant-temperature regime to a Fermi-gas regime. Complete or partial
excitation-energy sorting is found at energies well above the transition
energy.Comment: 8 pages, 3 figure
Detection of electrical spin injection by light-emitting diodes in top- and side-emission configuration
Detection of the degree of circular polarization of the electroluminescence
of a light-emitting diode fitted with a spin injecting contact (a spin-LED)
allows for a direct determination of the spin polarization of the injected
carriers. Here, we compare the detection efficiency of (Al,Ga)As spin-LEDs
fitted with a (Zn,Be,Mn)Se spin injector in top- and side-emission
configuration. In contrast with top emission, we cannot detect the electrical
spin injection in side emission from analysing the degree of circular
polarization of the electroluminescence. To reduce resonant optical pumping of
quantum-well excitons in the side emission, we have analysed structures with
mesa sizes as small as 1 micron.Comment: 15 pages with 3 figure
Viscous Withdrawal of Miscible Liquid Layers
In viscous withdrawal, a converging flow imposed in an upper layer of viscous
liquid entrains liquid from a lower, stably stratified layer. Using the idea
that a thin tendril is entrained by a local straining flow, we propose a
scaling law for the volume flux of liquid entrained from miscible liquid
layers. A long-wavelength model including only local information about the
withdrawal flow is degenerate, with multiple tendril solutions for one
withdrawal condition. Including information about the global geometry of the
withdrawal flow removes the degeneracy while introducing only a logarithmic
dependence on the global flow parameters into the scaling law.Comment: 4 pages, 4 figure
Voltage-Controlled Spin Selection in a Magnetic Resonant Tunnelling Diode
We have fabricated all II-VI semiconductor resonant tunneling diodes based on
the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the
quantum well, and studied their current-voltage characteristics. When subjected
to an external magnetic field the resulting spin splitting of the levels in the
quantum well leads to a splitting of the transmission resonance into two
separate peaks. This is interpreted as evidence of tunneling transport through
spin polarized levels, and could be the first step towards a voltage controlled
spin filter.Comment: To be published in Phys. Rev. Let
Large magnetoresistance effect due to spin-injection into a non-magnetic semiconductor
A novel magnetoresistance effect, due to the injection of a spin-polarized
electron current from a dilute magnetic into a non-magnetic semiconductor, is
presented. The effect results from the suppression of a spin channel in the
non-magnetic semiconductor and can theoretically yield a positive
magnetoresistance of 100%, when the spin flip length in the non-magnetic
semiconductor is sufficiently large. Experimentally, our devices exhibit up to
25% magnetoresistance.Comment: 3 figures, submitted for publicatio
Bound hole states in a ferromagnetic (Ga,Mn)As environment
A numerical technique is developed to solve the Luttinger-Kohn equation for
impurity states directly in k-space and is applied to calculate bound hole wave
functions in a ferromagnetic (Ga,Mn)As host. The rich properties of the band
structure of an arbitrarily strained, ferromagnetic zinc-blende semiconductor
yields various features which have direct impact on the detailed shape of a
valence band hole bound to an active impurity. The role of strain is discussed
on the basis of explicit calculations of bound hole states.Comment: 9 pages, 10 figure
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