140 research outputs found
Mitigating Electronic Current in Molten Flux for the Magnesium SOM Process
The solid oxide membrane (SOM) process has been used at 1423 K to 1473 K (1150 °C to 1200 °C) to produce magnesium metal by the direct electrolysis of magnesium oxide. MgO is dissolved in a molten MgF[subscript 2]-CaF[subscript 2] ionic flux. An oxygen-ion-conducting membrane, made from yttria-stabilized zirconia (YSZ), separates the cathode and the flux from the anode. During electrolysis, magnesium ions are reduced at the cathode, and Mg[subscript (g)] is bubbled out of the flux into a separate condenser. The flux has a small solubility for magnesium metal which imparts electronic conductivity to the flux. The electronic conductivity decreases the process current efficiency and also degrades the YSZ membrane. Operating the electrolysis cell at low total pressures is shown to be an effective method of reducing the electronic conductivity of the flux. A two steel electrode method for measuring the electronic transference number in the flux was used to quantify the fraction of electronic current in the flux before and after SOM process operation. Potentiodynamic scans, potentiostatic electrolyses, and AC impedance spectroscopy were also used to characterize the SOM process under different operating conditions.National Science Foundation (U.S.) (Grant No. 102663)United States. Dept. of Energy (Grant No. DE-EE0005547
Universality in sandpiles
We perform extensive numerical simulations of different versions of the
sandpile model. We find that previous claims about universality classes are
unfounded, since the method previously employed to analyze the data suffered a
systematic bias. We identify the correct scaling behavior and conclude that
sandpiles with stochastic and deterministic toppling rules belong to the same
universality class.Comment: 4 pages, 4 ps figures; submitted to Phys. Rev.
Universality Classes in Isotropic, Abelian and non-Abelian, Sandpile Models
Universality in isotropic, abelian and non-abelian, sandpile models is
examined using extensive numerical simulations. To characterize the critical
behavior we employ an extended set of critical exponents, geometric features of
the avalanches, as well as scaling functions describing the time evolution of
average quantities such as the area and size during the avalanche. Comparing
between the abelian Bak-Tang-Wiesenfeld model [P. Bak, C. Tang and K.
Wiensenfeld, Phys. Rev. Lett. 59, 381 (1987)], and the non-abelian models
introduced by Manna [S. S. Manna, J. Phys. A. 24, L363 (1991)] and Zhang [Y. C.
Zhang, Phys. Rev. Lett. 63, 470 (1989)] we find strong indications that each
one of these models belongs to a distinct universality class.Comment: 18 pages of text, RevTeX, additional 8 figures in 12 PS file
The b quark low-scale running mass from Upsilon sum rules
The b quark low-scale running mass m_kin is determined from an analysis of
the Upsilon sum rules in the next-to-next-to-leading order (NNLO). It is
demonstrated that using this mass one can significantly improve the convergence
of the perturbation series for the spectral density moments. We obtain m_kin(1
GeV) = 4.56 \pm 0.06 GeV. Using this result we derive the value of the MS-bar
mass m: m(m) = 4.20 \pm 0.1 GeV. Contrary to the low-scale running mass, the
pole mass of the b quark cannot be reliably determined from the sum rules. As a
byproduct of our study we find the NNLO analytical expression for the cross
section e+e- --> Q\bar Q of the quark antiquark pair production in the
threshold region, as well as the energy levels and the wave functions at the
origin for the ^1S_3 bound states of Q\bar Q.Comment: 22 pages, Late
Scaling of waves in the Bak-Tang-Wiesenfeld sandpile model
We study probability distributions of waves of topplings in the
Bak-Tang-Wiesenfeld model on hypercubic lattices for dimensions D>=2. Waves
represent relaxation processes which do not contain multiple toppling events.
We investigate bulk and boundary waves by means of their correspondence to
spanning trees, and by extensive numerical simulations. While the scaling
behavior of avalanches is complex and usually not governed by simple scaling
laws, we show that the probability distributions for waves display clear power
law asymptotic behavior in perfect agreement with the analytical predictions.
Critical exponents are obtained for the distributions of radius, area, and
duration, of bulk and boundary waves. Relations between them and fractal
dimensions of waves are derived. We confirm that the upper critical dimension
D_u of the model is 4, and calculate logarithmic corrections to the scaling
behavior of waves in D=4. In addition we present analytical estimates for bulk
avalanches in dimensions D>=4 and simulation data for avalanches in D<=3. For
D=2 they seem not easy to interpret.Comment: 12 pages, 17 figures, submitted to Phys. Rev.
Driving, conservation and absorbing states in sandpiles
We use a phenomenological field theory, reflecting the symmetries and
conservation laws of sandpiles, to compare the driven dissipative sandpile,
widely studied in the context of self-organized criticality, with the
corresponding fixed-energy model. The latter displays an absorbing-state phase
transition with upper critical dimension . We show that the driven model
exhibits a fundamentally different approach to the critical point, and compute
a subset of critical exponents. We present numerical simulations in support of
our theoretical predictions.Comment: 12 pages, 2 figures; revised version with substantial changes and
improvement
Field theory of absorbing phase transitions with a non-diffusive conserved field
We investigate the critical behavior of a reaction-diffusion system
exhibiting a continuous absorbing-state phase transition. The
reaction-diffusion system strictly conserves the total density of particles,
represented as a non-diffusive conserved field, and allows an infinite number
of absorbing configurations. Numerical results show that it belongs to a wide
universality class that also includes stochastic sandpile models. We derive
microscopically the field theory representing this universality class.Comment: 13 pages, 1 eps figure, RevTex styl
Dynamically Driven Renormalization Group Applied to Sandpile Models
The general framework for the renormalization group analysis of
self-organized critical sandpile models is formulated. The usual real space
renormalization scheme for lattice models when applied to nonequilibrium
dynamical models must be supplemented by feedback relations coming from the
stationarity conditions. On the basis of these ideas the Dynamically Driven
Renormalization Group is applied to describe the boundary and bulk critical
behavior of sandpile models. A detailed description of the branching nature of
sandpile avalanches is given in terms of the generating functions of the
underlying branching process.Comment: 18 RevTeX pages, 5 figure
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