1,064 research outputs found
Studies of the use of high-temperature nuclear heat from an HTGR for hydrogen production
The results of a study which surveyed various methods of hydrogen production using nuclear and fossil energy are presented. A description of these methods is provided, and efficiencies are calculated for each case. The process designs of systems that utilize the heat from a general atomic high temperature gas cooled reactor with a steam methane reformer and feed the reformer with substitute natural gas manufactured from coal, using reforming temperatures, are presented. The capital costs for these systems and the resultant hydrogen production price for these cases are discussed along with a research and development program
Optical shield: measuring viscosity of turbid fluids using optical tweezers
The viscosity of a fluid can be measured by tracking the motion of a suspended micron-sized particle trapped by optical tweezers. However, when the particle density is high, additional particles entering the trap compromise the tracking procedure and degrade the accuracy of the measurement. In this work we introduce an additional LaguerreâGaussian, i.e. annular, beam surrounding the trap, acting as an optical shield to exclude contaminating particles
Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere
Einstein realised that the fluctuations of a Brownian particle can be used to
ascertain properties of its environment. A large number of experiments have
since exploited the Brownian motion of colloidal particles for studies of
dissipative processes, providing insight into soft matter physics, and leading
to applications from energy harvesting to medical imaging. Here we use
optically levitated nanospheres that are heated to investigate the
non-equilibrium properties of the gas surrounding them. Analysing the sphere's
Brownian motion allows us to determine the temperature of the centre-of-mass
motion of the sphere, its surface temperature and the heated gas temperature in
two spatial dimensions. We observe asymmetric heating of the sphere and gas,
with temperatures reaching the melting point of the material. This method
offers new opportunities for accurate temperature measurements with spatial
resolution on the nanoscale, and a new means for testing non-equilibrium
thermodynamicsComment: 5 pages, 4 figures, supplementary material available upon reques
Control over phase separation and nucleation using a laser-tweezing potential
Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a poorly understood practice. Although there are now numerous examples of control using laser-induced nucleation, the absence of physical understanding is preventing progress. Here we show that the proximity of a liquidâliquid critical point or the corresponding binodal line can be used by a laser-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in a liquid mixture using a low-power laser diode confirm the effect. Phase separation and nucleation using a laser-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter
Power spectrum analysis for optical tweezers. II: Laser wavelength dependence of parasitic filtering, and how to achieve high bandwidth
In a typical optical tweezers detection system, the position of a trapped object is determined from laser light impinging on a quadrant photodiode. When the laser is infrared and the photodiode is of silicon, they can act together as an unintended low-pass filter. This parasicit effect is due to the high transparency of silicon to near-infrared light. A simple model that accounts for this phenomenon is here solved for frequencies up to 100 kHz and for laser wavelengths between 750 and 1064 nm. The solution is applied to experimental data in the same range, and is demonstrated to give this detection system of optical tweezers a bandwidth, accuracy, and precision that are limited only by the data acquisition board's bandwidth and bandpass ripples, here 96.7 kHz and 0.005 dB, respectively. ©2006 American Institute of Physic
Exact solutions of closed string theory
We review explicitly known exact solutions with Minkowski signature in
closed bosonic string theory. Classical string solutions with space-time
interpretation are represented by conformal sigma models. Two large
(intersecting) classes of solutions are described by gauged WZW models and
`chiral null models' (models with conserved chiral null current). The latter
class includes plane-wave type backgrounds (admitting a covariantly constant
null Killing vector) and backgrounds with two null Killing vectors (e.g.,
fundamental string solution). chiral null models describe some exact
solutions with electromagnetic fields, for example, extreme electric
black holes, charged fundamental strings and their generalisations. In
addition, there exists a class of conformal models representing axially
symmetric stationary magnetic flux tube backgrounds (including, in particular,
the dilatonic Melvin solution). In contrast to spherically symmetric chiral
null models for which the corresponding conformal field theory is not known
explicitly, the magnetic flux tube models (together with some non-semisimple
WZW models) are among the first examples of solvable unitary conformal string
models with non-trivial curved space-time interpretation. For these
models one is able to express the quantum hamiltonian in terms of free fields
and to find explicitly the physical spectrum and string partition function.Comment: 50 pages, harvma
Commensurate Scale Relations in Quantum Chromodynamics
We use the BLM method to show that perturbatively-calculable observables in
QCD can be related to each other without renormalization scale or scheme
ambiguity. We define and study the commensurate scale relations. We show that
the commensurate scales satisfy the renormalization group transitivity rule
which ensures that predictions in PQCD are independent of the choice of an
intermediate renormalization scheme. We generalize the BLM procedure to higher
order. The application of this procedure to relate known physical observables
in QCD gives surprisingly simple results. In particular, the annihilation ratio
and the Bjorken sum rule for polarized electroproduction are
related through simple coefficients, which reinforces the idea of a hidden
symmetry between these two observables.Comment: 35 pages (RevTeX), one PostScript figure included at the end.
SLAC-PUB-6481, UMD Preprint #94-13
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Isotopic tracers of gold deposition in Paleozoic limestones, Southern Nevada
Strontium isotopic analyses of barren and mineralized Paleozoic carbonate rocks show that hydrothermal fluids added radiogenic strontium ({sup 87}Sr) to the mineralized zones. At Bare Mountain, samples collected from mineralized areas have {delta}{sup 87}Sr{sub t} values ranging from +3.0 to +23.0, whereas unmineralized carbonate rocks have {delta}{sup 87}Sr, values of {minus}0.6 to +2.9. In other ranges, {delta}{sup 87}Sr, values of the unmineralized carbonate rocks are even lower and virtually indistinguishable from primary marine values. This correlation of elevated {delta}{sup 87}Sr{sub t} values with mineralized zones provides a useful technique for assessing the mineral potential of the Paleozoic basement beneath Yucca Mountain, and may find broader use in mineral exploration in the Basin and Range province as a whole
Enterprise Education Competitions: A Theoretically Flawed Intervention?
The demand for including enterprise in the education system, at all levels and for all pupils is now a global phenomenon. Within this context, the use of competitions and competitive learning activities is presented as a popular and effective vehicle for learning. The purpose of this chapter is to illustrate how a realist method of enquiry â which utilises theory as the unit of analysis â can shed new light on the assumed and unintended outcomes of enterprise education competitions. The case developed here is that there are inherent flaws in assuming that competitions will âworkâ in the ways set out in policy and guidance. Some of the most prevalent stated outcomes â that competitions will motivate and reward young people, that they will enable the development of entrepreneurial skills, and that learners will be inspired by their peers â are challenged by theory from psychology and education. The issue at stake is that the expansion of enterprise education policy into primary and secondary education increases the likelihood that more learners will be sheep dipped in competitions, and competitive activities, without a clear recognition of the potential unintended effects. In this chapter, we employ a realist-informed approach to critically evaluate the theoretical basis that underpins the use of competitions and competitive learning activities in school-based enterprise education. We believe that our findings and subsequent recommendations will provide those who promote and practice the use of competitions with a richer, more sophisticated picture of the potential flaws within such activities.Peer reviewedFinal Published versio
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