11,536 research outputs found
Casting copper to tungsten for high-power arc lamp cathodes
Voids forming at interface when copper is cast onto tungsten can be eliminated by adding wetting agent during casting process. Small amount of copper and nickel are cast onto thoriated tungsten insert, insert is recast with more copper to form electrode. Good thermal conductance results in long-lived cathode
Fabrication of Thin Film Heat Flux Sensors
Prototype thin film heat flux sensors have been constructed and tested. The sensors can be applied to propulsion system materials and components. The sensors can provide steady state and fast transient heat flux information. Fabrication of the sensor does not require any matching of the mounting surface. Heat flux is proportional to the temperature difference across the upper and lower surfaces of an insulation material. The sensor consists of an array of thermocouples on the upper and lower surfaces of a thin insulating layer. The thermocouples for the sensor are connected in a thermopile arrangement. A 100 thermocouple pair heat flux sensor has been fabricated on silicon wafers. The sensor produced an output voltage of 200-400 microvolts when exposed to a hot air heat gun. A 20 element thermocouple pair heat flux sensor has been fabricated on aluminum oxide sheet. Thermocouples are Pt-Pt/Rh with silicon dioxide as the insulating material. This sensor produced an output of 28 microvolts when exposed to the radiation of a furnace operating at 1000 C. Work is also underway to put this type of heat flux sensor on metal surfaces
Computer program for pulsed thermocouples with corrections for radiation effects
A pulsed thermocouple was used for measuring gas temperatures above the melting point of common thermocouples. This was done by allowing the thermocouple to heat until it approaches its melting point and then turning on the protective cooling gas. This method required a computer to extrapolate the thermocouple data to the higher gas temperatures. A method that includes the effect of radiation in the extrapolation is described. Computations of gas temperature are provided, along with the estimate of the final thermocouple wire temperature. Results from tests on high temperature combustor research rigs are presented
Revisiting the double-binary-pulsar probe of non-dynamical Chern-Simons gravity
One of the popular modifications to the theory of general relativity is
non-dynamical Chern-Simons (CS) gravity, in which the metric is coupled to an
externally prescribed scalar field. Setting accurate constraints to the
parameters of the theory is important owing to their implications for the
scalar field and/or the underlying fundamental theory. The current best
constraints rely on measurements of the periastron precession rate in the
double-binary-pulsar system and place a very tight bound on the characteristic
CS lengthscale k_cs^{-1} <~ 3*10^{-9} km. This paper considers several effects
that were not accounted for when deriving this bound and lead to a substantial
suppression of the predicted rate of periastron precession. It is shown, in
particular, that the point mass approximation for extended test bodies does not
apply in this case. The constraint to the characteristic CS lengthscale is
revised to k_cs^{-1} <~ 0.4 km, eight orders of magnitude weaker than what was
previously found.Comment: 12 pages, 4 figures, to be submitted to PRD. Comments are welcom
Comparing scalar-tensor gravity and f(R)-gravity in the Newtonian limit
Recently, a strong debate has been pursued about the Newtonian limit (i.e.
small velocity and weak field) of fourth order gravity models. According to
some authors, the Newtonian limit of -gravity is equivalent to the one of
Brans-Dicke gravity with , so that the PPN parameters of these
models turn out to be ill defined. In this paper, we carefully discuss this
point considering that fourth order gravity models are dynamically equivalent
to the O'Hanlon Lagrangian. This is a special case of scalar-tensor gravity
characterized only by self-interaction potential and that, in the Newtonian
limit, this implies a non-standard behavior that cannot be compared with the
usual PPN limit of General Relativity.
The result turns out to be completely different from the one of Brans-Dicke
theory and in particular suggests that it is misleading to consider the PPN
parameters of this theory with in order to characterize the
homologous quantities of -gravity. Finally the solutions at Newtonian
level, obtained in the Jordan frame for a -gravity, reinterpreted as a
scalar-tensor theory, are linked to those in the Einstein frame.Comment: 9 page
Singularity problem in f(R) model with non-minimal coupling
We consider the non-minimal coupling between matter and the geometry in the
f(R) theory. In the new theory which we established, a new scalar has
been defined and we give it a certain stability condition. We intend to take a
closer look at the dark energy oscillating behavior in the de-Sitter universe
and the matter era, from which we derive the oscillating frequency, and the
oscillating condition. More importantly, we present the condition of coupling
form that the singularity can be solved. We discuss several specific coupling
forms, and find logarithmic coupling with an oscillating period in the matter era , can improve singularity in the early
universe. The result of numerical calculation verifies our theoretic
calculation about the oscillating frequency. Considering two toy models, we
find the cosmic evolution in the coupling model is nearly the same as that in
the normal f(R) theory when . We also discuss the local tests of the
non-minimal coupling f(R) model, and show the constraint on the coupling form.Comment: 13 pages, 4 figure
Epitaxial growth of 6H silicon carbide in the temperature range 1320 C to 1390 C
High-quality epitaxial layers of 6H SiC have been grown on 6H SiC substrates with the grown direction perpendicular to the crystal c-axis. The growth was by chemical vapor deposition from methyltrichlorosilane (CH3SiCl3) in hydrogen at temperatures in the range of 1320 to 1390 C. Epitaxial layers up to 80 microns thick were grown at rates of 0.4 microns/min. Attempts at growth on the (0001) plane of 6H SiC substrates under similar conditions resulted in polycrystalline cubic SiC layers. Optical and X-ray diffraction techniques were used to characterize the grown layers
Laboratory process control using natural language commands from a personal computer
PC software is described which provides flexible natural language process control capability with an IBM PC or compatible machine. Hardware requirements include the PC, and suitable hardware interfaces to all controlled devices. Software required includes the Microsoft Disk Operating System (MS-DOS) operating system, a PC-based FORTRAN-77 compiler, and user-written device drivers. Instructions for use of the software are given as well as a description of an application of the system
Mine water outbreak and stability risks : examples and challenges from England and Wales
Abstract Although their frequency of occurrence is rare, the sudden outbreak of mine water from abandoned mines, or collapse of waste rock stores can be environmentally significant and represent significant postclosure legacies. This paper reports on a national survey of abandoned non-coal mine sites where concerns over mine water outbreak or stability are apparent across England and Wales. A range of respondents across environmental regulators and local authorities were consulted to populate a geodatabase. Outbreak risk was highlighted as a documented or suspected concern at 19 mine sites. Typical issues were related to adit blockages and associated perched mine water alongside issues of sudden ingress of surface waters into mines under high flow conditions. The majority of the responses concerning stability issues (72 sites in total) were related to fluvial erosion of riparian waste rock heaps. While successful management of such issues is highlighted in some cases, these are generally isolated examples. In both cases, the fact that stability or outbreak issues are often caused or exacerbated by extreme rainfall events highlights a potential future management issue with the predicted effects of climate change in north west Europe
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