Equilibrium temperature in aerated basins--comparison of two prediction models

This note presents and compares two models to predict the equilibrium temperature in aerated basins. They differ by their degree of complexity and therefore by the input data they require. Both models were able to estimate the temperature of an industrial aerated lagoon, the more complex model giving, in addition, a complete breakdown of the heat exchanges.     

Vanadium diphosphides as negative electrodes for secondary Li-ion batteries

We report on the Li electrochemical reactivity of amorphous and crystalline VP₂, synthesized by ball-milling and by 600 °C heat treatment of a ball-milled sample, respectively. The amorphous sample can reversibly react with 3.5 Li per formula unit as compared to solely 2.5 for the crystalline one. However in both cases there is a rapid capacity decay upon cycling that is more pronounced in the case of the crystalline sample. Complementary X-rays, HTREM and NMR tend to show that the Li reactivity mechanism differs from the classical conversion reactions since neither V nanoparticles nor the formation of Li₃P were detected, as opposed to some of the other MP₂ compounds (M = Ni or Cu). Besides structural phase variations within the 3d metal-based binary phosphide series, the possibility of a change in the nature of the redox centre upon lithiation from cation (M) to anion (P) is evoked.     

Prediction of alpha factor values for fine pore aeration systems.

The objective of this work was to analyse the impact of different geometric and operating parameters on the alpha factor value for fine bubble aeration systems equipped with EPDM membrane diffusers. Measurements have been performed on nitrifying plants operating under extended aeration and treating mainly domestic wastewater. Measurements performed on 14 nitrifying plants showed that, for domestic wastewater treatment under very low F/M ratios, the alpha factor is comprised between 0.44 and 0.98. A new composite variable (the Equivalent Contact Time, ECT) has been defined and makes it possible for a given aeration tank, knowing the MCRT, the clean water oxygen transfer coefficient and the supplied air flow rate, to predict the alpha factor value. ECT combines the effect on mass transfer of all generally accepted factors affecting oxygen transfer performances (air flow rate, diffuser submergence, horizontal flow).     

Integrated single and two-phase micro heat sinks under IGBT chips

Experiments have been performed to assess the feasibility of single and two-phase micro heat exchangers applied to the cooling of insulated gate bipolar transistor (IGBT) power components. After a brief recall of the principal characteristics of such heat exchangers, prototypes that have been built and tested are described. Then, the experimental measurements are compared to the predictions of the thermal and hydraulic performance with water and the inert fluorocarbon liquid (FC72) as coolant fluids     

Electrical conductivity of zinc ferrites near stoichiometry and manganese-zinc ferrites under vacuum or in the presence of oxygen

The electrical conductivity of zinc ferrites near stoichiometry and of manganese-zinc ferrites has been investigated as a function of temperature under vacuum and in the presence of oxygen. Under vacuum, the conductivity of these ferrites with iron excess is explained by the hopping mechanism, and with ZnO excess by the development of vacancies in octahedral sites of cation-deficient spinel. Activation energies and the transition temperatures are presented. During the oxidation in oxygen of Mn-Zn ferrites, the profile of the log σ= f(T) curves shows that the mechanism of electrical conduction in the temperature range 100 to 350 °C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 450 °C, the conductivity involves the hopping of electrons from octahedral sites of Mn³⁺ ions to octahedral sites of Mn⁴⁺ ions. Above 550 °C the oxidation of Mn²⁺ ions leads to a marked change in conductivity with the generation of new phases.     

Study of the oxidation kinetics of finely-divided magnetites IV. Influence of zinc substitution

The oxidation kinetics of zinc substituted magnetites (Fe²⁺_1?xZn²⁺_xFe³⁺₂)O^2?₄ (O < x < 1) into the γ defect phase of type x(Zn²⁺Fe³⁺₂O^2?₄), $\text{3}\text{2}\text{(1?}\text{x}\text{)}\text{Fe}{}^{\mathrm{3+}}{}_2\text{O}{}^{\mathrm{2?}}{}_3$ is found to be governed by the diffusion with a chemical diffusion coefficient depending on vacancy concentration. For compounds rich in zinc the diffusion coefficient is constant and the rate law written as v = k Pⁿ_(O2) with $\text{n}\text{=}\text{1}\text{4}$ is in agreement with a partial association of defects. For low substitution extents the diffusion coefficient varies with the transformation extent α and exponent n from the pressure law is equal to $\text{1}\text{2}$, which agrees with a total association of defects.     

Kinetics of the initial stage of oxidation in lacunar spinel formation electrical conductivity study of the transformation from γ(Fe3+1−yM3+y)2O2−3 to α(Fe3+1−yM3+y)2O2−3

The kinetics of the oxidation of aluminum or chromium-substituted magnetites to the lacunar phase γ, under very low oxygen pressure (PO₂ between 10^?6 and 10^?9 torr), in equilibrium conditions is found to be governed by the surface reaction. The transformation of the lacunar phase γ to the rhombohedral phase α, in these equilibrium conditions, was investigated by electrical conductivity. The lower the percentage of γ in solid solution, in the unoxidized spinel, the higher the temperature of the γ→α transformation.     

Some electrical properties of the defectiveγ phases of spinel structure during their transformation to the rhombohedral α phases

During the transformation of “cubic” iron sesquioxides, γ-Fe₂O₃, substituted by divalent or trivalent ions, to rhombohedral α phases, the electrical conductivity yields discontinuities and a change in plots of logσ againstf(1/T). The average temperature of these discontinuities is influenced by the particle size, and extent of oxidation and substitution. For divalent substituted defective spinels, the activation energy for the haematite precipitation appears to depend on the extent of substitution and the size of the particle.     

ESR characterization of Fe(III) ions in polycrystalline cation-deficient FeAl2O4 spinel

Polycrystalline samples of cation-deficient FeAl₂O₄ spinel with Fe³⁺ ions and vacancies on both octahedral and tetrahedral sites have been investigated by electron spin resonance (ESR). For samples annealing at 1100° C and slightly oxidized (⩽0.16 wt % Fe³⁺), the ESR spectrum originates from Fe³⁺ ions located in trigonally distorted octahedral sites ofα-Al₂O₃ formed during the annealing and also from Fe³⁺ ions introduced by oxidation at low temperature in octahedral sites of spinel structure. The samples with concentration between 0.16 and 1.6 wt % Fe³⁺ show the majority of Fe³⁺ ions to be on tetrahedral sites and the rhombic symmetry around the Fe³⁺ ions is attributed to the presence of cation vacancies.     

Predicting oxygen transfer of fine bubble diffused aeration systems--model issued from dimensional analysis

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (N(T)) The transfer number, which is written as a function of the oxygen transfer coefficient (k(L)a(20)), the gas superficial velocity (U(G)), the kinematic viscosity of water (nu) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. N(T) only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (S(p)), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers ("aerated area", S(a)) and the submergence of the diffusers (h)]. This analysis allowed to better describe the mass transfer in cylindrical tanks. Within the range of the parameters considered, the oxygen transfer coefficient (k(L)a(20)) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, k(L)a(20) decreases with the water depth (submergence of the diffusers). For a given water depth, k(L)a(20) increases with the number of diffusers, and, for an equal number of diffusers, with the total area of the zones covered by the diffusers. The latter result evidences the superiority of the total floor coverage over an arrangement whereby the diffusers are placed on separate grids. The specific standard oxygen transfer efficiency is independent of the air flow rate and the water depth, the drop in the k(L)a(20) being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (S(p)) and of the aerated area (S(a)) is the same as on the oxygen transfer coefficient.