The Thermopower and Resistivity of Nearly Magnetic Dilute Alloys

<p>In some nearly magnetic dilute alloys, in which the host and impurity are transition metals of similar electronic structure, the thermopower is observed to form a "giant" peak at about the spin fluctuation temperature Tsf deduced from resistivity measurements. Two explanations for these peaks have been postulated: the first is that the peaks are a diffusion thermopower component involving scattering off localized spin fluctuations (LSF) at the impurity sites; the second is that they are an LSF drag effect. We examine the thermopower and resistively of two nearly magnetic alloy systems: Rh(Fe) and Pt(Ni). In the first part of this thesis we describe measurements of the low temperature thermopower and resistivity of several Rh(Fe) alloys to clarify discrepancies in previous measurements and we show, by using a modified Nordheim-Gorter analysis, that the observed thermopower peaks are a diffusion and not a drag effect. In the second part of the thesis we describe measurements of the low temperature thermopower and resistivity of Pt (Ni), for which no previous data had been available. The Pt(Ni) samples are manufactured as thin, evaporated films on glass substrates. However, due to the difficulty encountered in controlling the very high residual resistivity of these samples, we are not able to draw definite conclusions regarding either the thermopower or the resistivity.</p>

The Thermopower and Resistivity of Nearly Magnetic Dilute Alloys

<p>In some nearly magnetic dilute alloys, in which the host and impurity are transition metals of similar electronic structure, the thermopower is observed to form a "giant" peak at about the spin fluctuation temperature Tsf deduced from resistivity measurements. Two explanations for these peaks have been postulated: the first is that the peaks are a diffusion thermopower component involving scattering off localized spin fluctuations (LSF) at the impurity sites; the second is that they are an LSF drag effect. We examine the thermopower and resistively of two nearly magnetic alloy systems: Rh(Fe) and Pt(Ni). In the first part of this thesis we describe measurements of the low temperature thermopower and resistivity of several Rh(Fe) alloys to clarify discrepancies in previous measurements and we show, by using a modified Nordheim-Gorter analysis, that the observed thermopower peaks are a diffusion and not a drag effect. In the second part of the thesis we describe measurements of the low temperature thermopower and resistivity of Pt (Ni), for which no previous data had been available. The Pt(Ni) samples are manufactured as thin, evaporated films on glass substrates. However, due to the difficulty encountered in controlling the very high residual resistivity of these samples, we are not able to draw definite conclusions regarding either the thermopower or the resistivity.</p>

Respirometric assays of two different MBR (microfiltration and ultrafiltration) to obtain kinetic and stoichiometric parameters

A comparison of two different medium scale MBRs (ultrafiltration and microfiltration) using respirometric methods has been achieved. The ultrafiltration membrane plant (0.034 µm pore size) maintained recirculation sludge flow at seven times the influent flow, and membranes were backwashed every 5 min and chemically cleaned weekly. The microfiltration membrane plant (0.4 µm pore size) maintained recirculation sludge flow at four times the influent flow, membrane-relax was applied after the production phase and membranes were chemically cleaned in the event of high trans-membrane pressure. Both technologies showed a similar performance with regard to heterotrophic kinetic and stoichiometric parameters and organic matter effluent concentrations. The influent was characterized by means of its COD fractions and the average removal percentages for COD concentrations were around 97% for both plants in spite of influent COD fluctuation, temperature variations and sludge retention time (SRT) evolution. Both SRT evolution and temperature affect the heterotrophic yield (YH) and the decay coefficient (bH) in the same range for both plants. YH values of over 0.8 mg COD/mg COD were obtained during the unsteady periods, while under steady state conditions these values fell to less than 0.4 mg COD/mg COD. bH by contrast reached values of less than 0.05 d−1.

EFFECTS OF QUANTUM AND THERMAL FLUCTUATIONS ON COLLECTIVE PINNING AND CRITICAL CURRENT DENSITY IN TYPE-II SUPERCONDUCTING BULK MATERIALS

The effects of quantum and thermal fluctuations on collective pinning and critical current density Jc are investigated for bulk type-II superconductors by utilizing quantum statistics. It is shown that for a constant magnetic field, Jc is nearly independent of temperature in the quantum limit; however, in the classical limit, Jc decreases weakly with increasing temperature when T < Tdp (depinning temperature); when Tdp < T < Tf (boundary fluctuation temperature), Jc is power-law-decaying, and when T > Tf, Jc decays exponentially. For constant temperature, Jc first decreases, then increases after reaching a maximum, and finally decreases again. These results are in agreement with the experiments.

Rheology of granular materials: dynamics in a stress landscape

We present a framework for analysing the rheology of dense driven granular materials, based on a recent proposal of a stress-based ensemble. In this ensemble, fluctuations in a granular system near jamming are controlled by a temperature-like parameter, the angoricity, which is conjugate to the stress of the system. In this paper, we develop a model for slowly driven granular materials based on the stress ensemble and the idea of a landscape in stress space. The idea of an activated process driven by the angoricity has been shown by Behringer et al . (Behringer et al. 2008 Phys. Rev. Lett. 101 , 268301) to describe the logarithmic strengthening of granular materials. Just as in the soft glassy rheology (SGR) picture, our model represents the evolution of a small patch of granular material (a mesoscopic region) in a stress-based trap landscape. The angoricity plays the role of the fluctuation temperature in the SGR. We determine (i) the constitutive equation, (ii) the yield stress, and (iii) the distribution of stress dissipated during granular shearing experiments, and compare these predictions with the experiments of Hartley & Behringer (Hartley & Behringer 2003 Nature 421 , 928–931.).

An Advanced Mixing Tee Mock-Up Called “The Skin of the Fluid” Designed to Qualify the Numerical LES Analyses Applied to the Thermal Evaluation

Nowadays, the LES (Large Eddy Simulation) models have reached an advanced stage in the thermal hydraulic numerical analyses. Particularly, in the mixing tee geometry, the LES allows the thermal, temperature fluctuations cartography — both in amplitude and frequency — to be evaluated. However, for the purpose of mechanical studies a better determination of the fluctuation temperature which impacts the wall would be of significant interest, magnitude of the fluctuation being directly linked to the lifetime of the Tee. This demand steps of experimental qualifications based on cartographic comparisons between numerical and experimental results, not compatible with basic current experiments proposing only local measurements. A mixing tee mock-up (␀ 50 mm) called “the skin of the fluid” was designed to show the real time observation of the temperature fluctuations in the mixing area with a digital infrared device. With its low Biot number the mock-up is quasi permeable to the fluctuation and does not attenuate the temperature amplitude at high frequency, consequently, the infrared image represents the temperature fluctuations which impacts the wall. An existent LES numerical analysis performed with the TRIO U code (developed by the CEA) on the same geometry is shown for a first comparative approach.

ON THE MAGNETOACOUSTIC OSCILLATIONS IN HEAVY FERMION COMPOUNDS

Using the Green's function technique and MFA for the slave bosons approach, we consider the influence of the magnetic field on the oscillations of the sound velocity in cerium-based heavy fermion compounds. The quasiparticle-phonon interaction is assumed to result from the volume dependence of the spin fluctuation temperature T *. We obtain magnetic oscillations of the sound velocity at finite temperature. The mass enhancement and the renormalised Fermi energy are the same as those obtained for the magnetic de Haas-van Alphen effect.