1,252 research outputs found
An investigation of the dissolution of hafnium-free zirconium in hydrofluoric acid and the effect of fluoride additions
Hafnium-free zirconium has assumed great importance in commercial use in recent years. When economical methods were developed in 1950 to remove hafnium from Zr, a metal was produced which not only exhibited good corrosion resistance and desirable physical properties, but also had a very low absorption cross section for thermal neutrons. Such a metal could obviously be utilized to advantage as a structural material for nuclear reactors. This application is the most important one for Zr at present.
Good corrosion resistance to most mineral acids is one of the outstanding non-nuclear properties of Zr. Hf-free Zr has good corrosion resistance in all mineral acids with exception of hydrofluoric acid, concentrated sulfuric and phosphoric acids, and aqua regia. The resistance of Zr to attack in concentrated nitric acid is excellent, with but negligible attack occurring in fuming nitric acid. The corrosion of Zr in hydrochloric acid is particularly dependent on the purity of the metal. It has been shown that high carbon graphite melted Zr is severely embrittled in HCl because of selective attack of the carbides. In general, Hf-free Zr (containing less than 0.1% Hf) is more corrosion resistant than Zr containing the usual 2.5% Hf. Small percentages of other impurities in Zr such as carbon, nitrogen, and oxygen will also decrease corrosion resistance.
It has been stated that Zr has poor corrosion resistance in HF. It was proposed that a quantitative investigation of the dissolution of Zr in HF be made to provide additional data to that already gathered on the corrosion of Zr in other mineral acids. Since the rate of dissolution of Zr in HF is greater than in any other mineral acid, a study of the rate and mechanism of the reaction and possible passivation effects might be a valuable addition to present knowledge --Introduction, pages 1-2
Self-tuning of threshold for a two-state system
A two-state system (TSS) under time-periodic perturbations (to be regarded as
input signals) is studied in connection with self-tuning (ST) of threshold and
stochastic resonance (SR). By ST, we observe the improvement of signal-to-noise
ratio (SNR) in a weak noise region. Analytic approach to a tuning equation
reveals that SNR improvement is possible also for a large noise region and this
is demonstrated by Monte Carlo simulations of hopping processes in a TSS. ST
and SR are discussed from a little more physical point of energy transfer
(dissipation) rate, which behaves in a similar way as SNR. Finally ST is
considered briefly for a double-well potential system (DWPS), which is closely
related to the TSS
Constraints on Fluid Dynamics from Equilibrium Partition Functions
We study the thermal partition function of quantum field theories on
arbitrary stationary background spacetime, and with arbitrary stationary
background gauge fields, in the long wavelength expansion. We demonstrate that
the equations of relativistic hydrodynamics are significantly constrained by
the requirement of consistency with any partition function. In examples at low
orders in the derivative expansion we demonstrate that these constraints
coincide precisely with the equalities between hydrodynamical transport
coefficients that follow from the local form of the second law of
thermodynamics. In particular we recover the results of Son and Surowka on the
chiral magnetic and chiral vorticity flows, starting from a local partition
function that manifestly reproduces the field theory anomaly, without making
any reference to an entropy current. We conjecture that the relations between
transport coefficients that follow from the second law of thermodynamics agree
to all orders in the derivative expansion with the constraints described in
this paper.Comment: Typos corrected, References adde
Multifractal characterization of stochastic resonance
We use a multifractal formalism to study the effect of stochastic resonance
in a noisy bistable system driven by various input signals. To characterize the
response of a stochastic bistable system we introduce a new measure based on
the calculation of a singularity spectrum for a return time sequence. We use
wavelet transform modulus maxima method for the singularity spectrum
computations. It is shown that the degree of multifractality defined as a width
of singularity spectrum can be successfully used as a measure of complexity
both in the case of periodic and aperiodic (stochastic or chaotic) input
signals. We show that in the case of periodic driving force singularity
spectrum can change its structure qualitatively becoming monofractal in the
regime of stochastic synchronization. This fact allows us to consider the
degree of multifractality as a new measure of stochastic synchronization also.
Moreover, our calculations have shown that the effect of stochastic resonance
can be catched by this measure even from a very short return time sequence. We
use also the proposed approach to characterize the noise-enhanced dynamics of a
coupled stochastic neurons model.Comment: 10 pages, 21 EPS-figures, RevTe
Constraints on Superfluid Hydrodynamics from Equilibrium Partition Functions
Following up on recent work in the context of ordinary fluids, we study the
equilibrium partition function of a 3+1 dimensional superfluid on an arbitrary
stationary background spacetime, and with arbitrary stationary background gauge
fields, in the long wavelength expansion. We argue that this partition function
is generated by a 3 dimensional Euclidean effective action for the massless
Goldstone field. We parameterize the general form of this action at first order
in the derivative expansion. We demonstrate that the constitutive relations of
relativistic superfluid hydrodynamics are significantly constrained by the
requirement of consistency with such an effective action. At first order in the
derivative expansion we demonstrate that the resultant constraints on
constitutive relations coincide precisely with the equalities between
hydrodynamical transport coefficients recently derived from the second law of
thermodynamics.Comment: 46 page
Evaluation of be-38 percent al alloy final report, 27 jun. 1964 - 28 feb. 1965
Mechanical properties, microstructural features, and general metallurgical quality of beryllium- aluminum allo
Collective dynamics of two-mode stochastic oscillators
We study a system of two-mode stochastic oscillators coupled through their
collective output. As a function of a relevant parameter four qualitatively
distinct regimes of collective behavior are observed. In an extended region of
the parameter space the periodicity of the collective output is enhanced by the
considered coupling. This system can be used as a new model to describe
synchronization-like phenomena in systems of units with two or more oscillation
modes. The model can also explain how periodic dynamics can be generated by
coupling largely stochastic units. Similar systems could be responsible for the
emergence of rhythmic behavior in complex biological or sociological systems.Comment: 4 pages, RevTex, 5 figure
Hydrodynamics in 1+1 dimensions with gravitational anomalies
The constraints imposed on hydrodynamics by the structure of gauge and
gravitational anomalies are studied in two dimensions. By explicit integration
of the consistent gravitational anomaly, we derive the equilibrium partition
function at second derivative order. This partition function is then used to
compute the parity-violating part of the covariant energy-momentum tensor and
the transport coefficients.Comment: 9 pages, JHEP format.v2; added comments and references, matching
published versio
Exact Solution for the Time Evolution of Network Rewiring Models
We consider the rewiring of a bipartite graph using a mixture of random and
preferential attachment. The full mean field equations for the degree
distribution and its generating function are given. The exact solution of these
equations for all finite parameter values at any time is found in terms of
standard functions. It is demonstrated that these solutions are an excellent
fit to numerical simulations of the model. We discuss the relationship between
our model and several others in the literature including examples of Urn,
Backgammon, and Balls-in-Boxes models, the Watts and Strogatz rewiring problem
and some models of zero range processes. Our model is also equivalent to those
used in various applications including cultural transmission, family name and
gene frequencies, glasses, and wealth distributions. Finally some Voter models
and an example of a Minority game also show features described by our model.Comment: This version contains a few footnotes not in published Phys.Rev.E
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