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Spiral Growth Manufacturing (SGM) – A Continuous Additive Manufacturing Technology for Processing Metal Powder by Selective Laser Melting
Spiral growth manufacturing is a new innovative powder based rapid manufacturing
technique. The innovation exists in the methodology in which powder layers are deposited.
Unlike other pre-placed powder systems, the deposited layers move relative to the location at
which they are processed. This is made possible by a rotating build drum into which powder is
deposited, in spiralled layers, from a stationary hopper. With this configuration powder can be
continuously deposited and levelled and simultaneously processed, eliminating delays in the
build cycle. Stainless steel and cobalt-chrome powder is selectively melted using a 100W flash
lamp pumped Nd:YAG laser. This paper reports on factors affecting build rate and on build
strategies for creating a number of axis-symmetric thin and thick walled cylinders. Experimental
results suggest that build rate for thin walled structures bonded to a substrate will ultimately be
governed by tangential movements of the powder particles when frictional forces are not
sufficient to accelerate the particles along a curved path, provided that enough laser power is
available for melting. Even melt pool balling, which is evident when melting one layer at high
speeds, diminishes in multiple layer builds due to re-melting and infilling.Mechanical Engineerin
Thermodynamics of the frustrated - Heisenberg ferromagnet on the body-centered cubic lattice with arbitrary spin
We use the spin-rotation-invariant Green's function method as well as the
high-temperature expansion to discuss the thermodynamic properties of the
frustrated spin- - Heisenberg magnet on the body-centered
cubic lattice. We consider ferromagnetic nearest-neighbor bonds and
antiferromagnetic next-nearest-neighbor bonds and arbitrary spin
. We find that the transition point between the ferromagnetic ground
state and the antiferromagnetic one is nearly independent of the spin ,
i.e., it is very close to the classical transition point . At finite temperatures we focus on the parameter regime
with a ferromagnetic ground-state. We calculate the Curie
temperature and derive an empirical formula describing the
influence of the frustration parameter and spin on . We find
that the Curie temperature monotonically decreases with increasing frustration
, where very close to the -curve exhibits a
fast decay which is well described by a logarithmic term
. To characterize the magnetic ordering
below and above , we calculate the spin-spin correlation functions
, the spontaneous
magnetization, the uniform static susceptibility as well as the
correlation length . Moreover, we discuss the specific heat and the
temperature dependence of the excitation spectrum. As approaching the
transition point some unusual features were found, such as negative
spin-spin correlations at temperatures above even though the ground state
is ferromagnetic or an increase of the spin stiffness with growing temperature.Comment: 19 pages, 10 figures, version as in EPJ
A theoretical analysis of the current-voltage characteristics of solar cells
The current-voltage characteristics and efficiencies of solar cells are discussed. For one solar cell structure detailed curves are presented which include carrier densities, current densities, potential, and quasi-Fermi levels at different voltage levels both with and without optically generated carriers (AMO conditions). In addition some results are presented concerning the influence of various parameter variations such as lifetime, cell thickness, and high-low junction width on solar cell performance
A theoretical analysis of the current-voltage characteristics of solar cells
Various mechanisms which limit the conversion efficiency of silicon solar cells were studied. The effects of changes in solar cell geometry such as layer thickness on performance were examined. The effects of various antireflecting layers were also examined. It was found that any single film antireflecting layer results in a significant surface loss of photons. The use of surface texturing techniques or low loss antireflecting layers can enhance by several percentage points the conversion efficiency of silicon cells. The basic differences between n(+)-p-p(+) and p(+)-n-n(+) cells are treated. A significant part of the study was devoted to the importance of surface region lifetime and heavy doping effects on efficiency. Heavy doping bandgap reduction effects are enhanced by low surface layer lifetimes, and conversely, the reduction in solar cell efficiency due to low surface layer lifetime is further enhanced by heavy doping effects. A series of computer studies is reported which seeks to determine the best cell structure and doping levels for maximum efficiency
A theoretical study of heterojunction and graded band gap type solar cells
The work performed concentrated on including multisun effects, high temperature effects, and electron irradiation effects into the computer analysis program for heterojunction and graded bandgap solar cells. These objectives were accomplished and the program is now available for such calculations
Solving the characteristic initial value problem for colliding plane gravitational and electromagnetic waves
A method is presented for solving the characteristic initial value problem
for the collision and subsequent nonlinear interaction of plane gravitational
or gravitational and electromagnetic waves in a Minkowski background. This
method generalizes the monodromy transform approach to fields with nonanalytic
behaviour on the characteristics inherent to waves with distinct wave fronts.
The crux of the method is in a reformulation of the main nonlinear symmetry
reduced field equations as linear integral equations whose solutions are
determined by generalized (``dynamical'') monodromy data which evolve from data
specified on the initial characteristics (the wavefronts).Comment: 4 pages, RevTe
ARXPS-studies ofcˆ-axis textured YBa2Cu3Ox-films
YBa2Cu3Ox sputter deposited cold on MgO grows in O2 annealing epitaxially to a transparent, superconducting film with Tc 80K. The unscraped surfaces of these films are smooth showing XPS lines changing with photoelectron take-off angle. This enhanced data base allows to separate the different chemical compounds (hydroxide, peroxide, carbonate, carboxyle, cuprate, graphite ...) and to obtain their spatial distribution. This yields the compounds, their amount and distribution making up the cinder growing with O2-anneal at internal and external surfaces. The cinder stoichiometry gives insights in the chemistry going on in O2 annealing. Below the cinder the signature ofcˆ-axis oriented YBa2Cu3Ox is identified, showing that a Ba-oxide layer forms the stable surface. This coats insulating CuO2 and Y-oxide layers yielding so an intrinsic dead layer
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