Magnetotransport study of in situ magnetic field textured Dy1Ba2Cu3O7−δ superconductors

The electrical resistivity, thermoelectric power and the Nernst effect have been studied as a function of temperature and magnetic field for a typical superconductor DyBa₂Cu₃O_7– magnetically textured in situ at 1035°C. The three transport coefficients show hybrid microscopic features. In particular, we show the anisotropy with respect to the field direction (H//a,H//c) in all transport coefficients. We verify that Tinkham's law is obeyed for the broadening of the resistive transition in a magnetic field. Similarly we obtainanisotropic broadening exponents for each integrated excess property. From Arrhenius plots we obtain orders of magnitude for the activation energies characterizing each property. They are markedly different from each other.     

Non-isovalent alkali metal “substitution” in YBa2Cu3O7−y granular ceramics

The aim of this paper is to study the influence of non-isovalent doping in YBa₂Cu₃O_7–y in particular on its synthesis conditions and on the resistive properties both with and without a magnetic field. We concentrate on the study of possible alkali ions (Na, K, Cs) substitution at the barium sites. A low temperature sintering process is used in order to induce a reactive liquid phase. The final chemical composition is discussed as a function of the amount of the liquid phase. No alkali ion is substituted. Carbonate layers are present. However, this (lack of) substitution leads to induced vacancies and improved electrical transport properties which are as good as in highly pure materials. For conciseness the case of Na substitution only is illustrated. The use of such data in order to probe the microstructure is emphasized.     

Determination of the operating parameters of an evaporator crystallizer

This work involves the study of the solid/liquid equilibrium in the NaCl–NaOH–H₂O ternary system. The aim is to optimize the operating conditions of an evaporator–crystallizer used in a hydrazine extraction process. The latter is conducted at industrial scale using the Raschig method, a general method for preparing hydrazines for aerospace and pharmaceutical applications. The extraction process includes the use of a Kestner type evaporator–crystallizer to first isolate hydrazine and water by evaporation and then separate two salts: sodium hydroxide, recovered as an aqueous solution, and sodium chloride which crystallizes. However, problems related to the co-precipitation of NaCl–NaOH may be encountered in industry, leading to the clogging of installations. Therefore, it is necessary to ensure that only NaCl crystallizes while maintaining NaOH in liquid state. Knowledge of the equilibrium phases of the NaCl–NaOH–H₂O ternary system is thus required, particularly the liquid/solid phase. Phase equilibrium experiments were performed by isoplethic thermal analysis (ITA) and chemical titrations over the temperature range of 313 to 353 K at atmospheric pressure. ITA is an isobaric and isothermal method developed in our laboratory and based on the measurement of the thermal effects associated with transformations in a system (appearance or disappearance of a phase) when its composition is changed. The NaCl crystallization domain was fully determined and the operating parameters of the device were defined precisely as a function of temperature and composition of the Kestner loop.     

Synthesis of hydroxyethylhydrazine by the Raschig process and comparison with synthesis by the alkylation process

The Raschig synthesis of hydroxyethylhydrazine (HEH) is studied, that is, the reaction of monochloramine on ethanolamine. The formation of HEH is monitored by UV spectrometry, and the influence of temperature and pH is studied. The primary reaction is an SN₂‐type mechanism, whereas the main secondary reaction is the oxidation of HEH by monochloramine. This reaction is also monitored by UV spectrometry, and the oxidation product is identified by GC–MS analysis, showing the formation of hydroxyethylhydrazone. The reaction mechanisms and the rate constants were determined, and the results permit establishing the main reactions occurring during HEH synthesis. These reactions were validated in a concentrated medium, with the systematic study of the influence of the molar ratio p ([HEH]₀/[NH₂Cl]₀) and the final sodium hydroxide concentration and temperature. A comparison is made with the other synthesis process already published, that is, the alkylation of hydrazine by either chloroethanol or epoxide. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 331–344, 2011     

Kinetic modelling of synthesis of N-aminopiperidine from hydroxylamine-O-sulfonique acid and piperidine

A new route to synthesize N-aminopiperidine (NAPP) from hydroxylamine-O-sulfonique acid (HOSA) and piperidine was described. Kinetics of the reaction was investigated to optimize the conditions of the synthesis. Since the reaction is fast, this study was carried out in a diluted medium (10⁻³ to 10⁻² mol/l). To determine the concentration of the reaction product, NAPP was allowed to react with formaldehyde and the product was analysed by UV and HPLC techniques. The formation of NAPP is consistent with the first-order reaction to two reagents, governed by the nucleophilic substitution via SN2 mechanism. Oxidation of NAPP by HOSA was identified as the main secondary reaction which consistently reduced the yield of NAPP. A number of differential equations were elaborated and solution of these equations serves to predict the behavior of the system as a function of the reagent concentrations, pH and temperature. From the corresponding mathematical treatment a unique implicit expression was derived that characterizes the reaction medium. It was found that the [PP]₀/[HOSA]₀ molar ratio (p), the initial concentrations of [PP]₀ and [HOSA]₀, the ratio of rate constants k ₂/k ₁ and temperature are the only parameters that affect the yield of NAPP from HOSA. The results calculated from this model are in good agreement with the experimental data and they can be used to determine the optimal conditions of the reaction.     

Oxidation kinetics of methylhydrazine in a strictly single-phase medium studied in reconstituted air

The oxidation of methylhydrazine (monomethylhydrazine MMH) is studied in a strictly single-phase gaseous medium, in a reconstituted surrounding atmosphere. In order to work under such conditions, a specific apparatus was assembled to monitor the evolution of the reactants through time. The reaction kinetics was studied at 50°C to avoid condensation of the water formed and for O₂/MMH mole ratios between 1 and 4. Under these conditions, the partial pressures of O₂ were between 0.05 and 0.18 bar (4% MMH per volume). The main reaction products were monitored through time and identified by gas chromatography coupled with mass spectrometry (GC/MS). The products were: N₂, CH₄, CH₃-NH-N=CH₂ (formaldehyde monomethylhydrazone), NH₃, H₂O, CH₃OH, and CH₂=N-N=N-CH₃ (2,3,4-triazapenta-1,3-diene). The formation of nitrosamines was not observed under these experimental conditions. The rate laws related to the disappearance of the reagents were clearly established and can be described by 2 consecutive parallel reactions of order 2 whose rate constants are: k ₁ = 7.57 × 10⁻² bar⁻¹ min⁻¹ and k ₂ = 0.5 bar⁻¹ min⁻¹. Analysis of the products permitted establishing an approximated balance of the global reaction.     

Field percolation and high current density in 80/20 DyBa2Cu3O7 − x/Dy2BaCuO5 bulk magnetically textured composite ceramics

We measured the AC susceptibility of magnetically textured (123) 80%/211(20%) DyBaCuO composite in a special set-up in order to enhance the intergrain contribution. The synthesis process led to very clean “weak links” at grain boundaries. At the percolation threshold bulk shielding paths were such that the intergrain critical current density J_c was above 10⁵ A/cm². The field dependence of J_c was understood through an analytical form indicating a distribution of currents similar to the law of clusters at fracture/percolation thresholds.     

Determination of the isobaric liquid–liquid ternary system water–sodium hydroxide-1,3-diaminopropane for the demixing of diaminopropane

This study relates to a determination of liquid–liquid ternary system water–sodium hydroxide-1,3-diaminopropane to optimize the conditions of the isolation of hydrazine by liquid–liquid extraction, generated by the addition of solid sodium hydroxide. Three isotherms were established at 293, 313, and 323 K by Isoplethic Thermal Analysis, method developed in our laboratory and consists of the evolution of one or more physical properties of the system when its composition is modifying, and chemical titrations. The coordinates of the critical point were determined using the diameter method.     

Extraction optimization of organic compounds by demixing observed in the H<Subscript>2</Subscript>O–NaOH–piperidine ternary diagram

Three isothermal sections of the isobaric ternary system H₂O–NaOH–C₅H₁₀NH were determined by Isoplethic Thermal Analysis at 293, 313, and 323 K. Miscibility gaps were completely delimited and each critical point was calculated. This system is then characterized by a prevalent miscibility gap, three crystallization domains and two triphasic invariant domains. The relevant exploitation of this diagram so permits us to deduce the demixing temperature leading to the optimal transfer of the organic compounds in the light phase and also the composition of the organic phase recovered after this second step of extraction. Moreover, the addition of calculated quantities of sodium hydroxide aqueous solution allows to be in optimized conditions for the next distillation operations.