Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: II. Effect of solvent and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> concentration

The effect of solvent (aqueous solutions of HClO₄ and H₂SO₄) on the low-temperature (T < 273 K) reaction of U(IV) with XeF₂, accompanied by formation of uranyl ion in the electronically excited state *(UO ₂ ²⁺ ), was examined. In the course of heating of 0.2–8.31 M HClO₄ solutions after their quick cooling to 77 K, the chemiluminescent reaction occurs at a detectable rate only at T > 220 K. In the similar experiments performed with 0.05–0.5 M H₂SO₄ as solvent, the emission appears at a considerably lower temperature, 165–175 K. The chemiluminescence (CL) intensity increases in the course of exothermic phase transitions occurring when the temperature of H₂SO₄ solutions (at [H₂SO₄] > 0.05 M) is changed. The increase in the CL intensity in the course of a phase transition is associated with the appearance in a multicomponent frozen system of a juvenile surface of H₂SO₄ crystal hydrates. The lack of CL at T < 220 K in HClO₄ solutions is associated with the fact that heating of the samples after their quick cooling to 77 K is not accompanied by formation of crystalline phases exhibiting catalytic properties toward the oxidation of U(IV) with XeF₂.     

Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: III. Effect of solvent deuteration

The effect of the solvent deuteration on the reaction of U(IV) with XeF₂ in solutions of H₂SO₄ in H₂O and D₂SO₄ in D₂O, occurring in the course of heating these solutions after their quick cooling (10–15 K s^?1) to 77 K, was examined. The effect is manifested in essentially different temperature dependences of the chemiluminescence accompanying the oxidation of U(IV) with XeF₂. The results obtained are interpreted within the framework of the concept that the low-temperature reaction of U(IV) with XeF₂ is catalyzed by the juvenile surface of crystal hydrates of sulfuric and deuterosulfuric acids, formed in the course of low-temperature phase transitions. The isotope effect is due to different kinetics of the phase transitions in solutions of H₂SO₄ in H₂O and D₂SO₄ in D₂O. In the course of crystallization of supercooled H₂SO₄ solutions, depending on the mode of temperature variation, either one or two eutectics appreciably differing in the physical properties are formed. In the course of crystallization of supercooled solutions of D₂SO₄ in D₂O, one stable eutectic is formed in each sample, and its characteristics depend on the thermal history of the sample. The eutectics melt in the course of heating of the frozen solution at temperatures differing by only 3–4 K.     

Formation of excited uranyl ion in oxidation of U(IV) with xenon difluoride in aqueous solutions of sulfuric and deuterated sulfuric acids

Kinetic features of the chemiluminescence accompanying oxidation of U(IV) with xenon difluoride in solutions of 0.2 M H₂SO₄ in H₂O and 0.2 M of D₂SO₄ in D₂O at temperatures within 283–313 K and concentrations (M) 10^?6 ≤ [U(IV)] ≤ 10^?4 and 10^?4 ≤ [XeF₂] ≤ 10^?3 were examined. The shape of the kinetic curve depends on the concentration of the reactants and the solution temperature. At [U(IV)] = 10^?6 and [XeF₂] = 10^?4 M, the curve exhibits a maximum, more prominent at low temperatures. At relatively high concentration ([U(IV)] = 10^?4 and [XeF₂] = 10^?3 M), self-acceleration of the reaction is observed: The mechanism of oxidation of U(IV) turns into a branched-chain mode owing to a higher concentration in the solution of radicals ^·OH, HO ₂ ^· , and XeF ₂ ^· , whose additional amounts are yielded by hydrolytic reduction of XeF₂. A kinetic isotope effect of the solvent k _H/k _D was revealed, which attains a maximum of 1.8 at 283 K. The activation energies of the oxidation of U(IV) with xenon difluoride in H₂SO₄ and D₂SO₄ solutions were estimated at E _a,H = 12 and E _a,D = 13 kcal mol^?1, respectively. The occurrence of the isotope effect is an indirect evidence of participation in the reaction of the OH (or OD) groups. The rate of hydrolytic reduction of XeF₂ in deuterated solvent (0.2 M D₂SO₄ in D₂O) in its photochemical stage is several times lower, and the luminescence accompanying the reaction is by an order of magnitude smaller than that in 0.2 M H₂SO₄.     

Graphite intercalation in the graphite-H<Subscript>2</Subscript>SO<Subscript>4</Subscript>-R (R = H<Subscript>2</Subscript>O,C<Subscript>2</Subscript>H<Subscript>5</Subscript>OH,C<Subscript>2</Subscript>H<Subscript>5</Subscript>COOH) systems

The electrochemical behavior of graphite in polar solvent-H₂SO₄ electrolytes is studied in a wide range of H₂SO₄ concentrations. The results demonstrate that, with decreasing H₂SO₄ concentration, the charging curves become smoother and shift to higher potentials, the stage index increases, and intercalation compounds are more difficult to obtain. At H₂SO₄ concentrations of 50% and lower, graphite polarization is accompanied by a significant overoxidation, as evidenced by the anomalously small intercalate layer thicknesses: 7.75–7.85 Å. Anodic polarization of graphite in electrolytes consisting of H₂SO₄ and a polar solvent (H₂O and C₂H₅OH) follows the same mechanism as in the case of the formation of graphite bisulfate. In going from water to C₂H₅OH, a less polar solvent, the intercalation threshold increases from 30 to 70% H₂SO₄. It is shown using a set of characterization techniques that, in the graphite-H₂SO₄-R (R = H₂O, C₂H₅OH) systems, the solvent is not intercalated into graphite. Stage I–III ternary graphite intercalation compounds (TGICs) are synthesized for the first time in the graphite-H₂SO₄-C₂H₅COOH system: stage I TGICs at H₂SO₄ concentrations above 70%, stage II in the range 30–70% H₂SO₄, and stage III at H₂SO₄ concentrations down to 10%. The intercalate layer thickness in the TGICs is 7.94 Å. The mechanism of TGIC formation in this system is shown to differ from those in mixtures of H₂SO₄ and other organic acids. Thermal analysis in combination with spectroscopic analysis of gaseous products provides clear evidence for intercalation of propionic acid into the TGIC and indicates that the thermal stability of this compound is lower than that of graphite bisulfate.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 162–169.Original Russian Text Copyright © 2005 by Shornikova, Sorokina, Maksimova, Avdeev.     

Etching behavior of CdTe in aqueous H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HI-C<Subscript>6</Subscript>H<Subscript>8</Subscript>O<Subscript>7</Subscript> solutions

This paper examines the dissolution behavior of the (111)A, (111)B, (110), and (100) surfaces of CdTe single crystals in aqueous H₂O₂-HI-C₆H₈O₇ (citric acid) solutions. We have determined the dissolution rate of the crystals as a function of temperature and solution concentration, located the composition regions of polishing and selective etchants, and studied the microstructure and roughness of surfaces polished with optimized etchants. The etching behavior of CdTe is shown to depend on its crystallographic orientation.     

Molybdenum dissolution in mixtures of H<Subscript>2</Subscript>O<Subscript>2</Subscript> and concentrated HNO<Subscript>3</Subscript> and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> in the presence of tungsten

The oxidation of molybdenum and tungsten in hydrogen peroxide solutions and the effect of H₂O₂ on the selectivity of Mo and W dissolution in mixtures of concentrated nitric and sulfuric acids have been studied.     

Stability limits of graphite intercalation compounds in the systems graphite-HNO<Subscript>3</Subscript>(H<Subscript>2</Subscript>SO<Subscript>4</Subscript>)-H<Subscript>2</Subscript>O-KMnO<Subscript>4</Subscript>

The formation of binary graphite intercalation compounds (GICs) with nitric and sulfuric acids in the presence of a strong oxidant has been studied by x-ray diffraction and potentiometry in a wide range of acid concentrations. The redox potential of the oxidizing solution and the intercalation ability of the acid are shown to influence the stage number (the number of graphite layers between two successive intercalate layers) of the forming GIC and the concentration ranges of GIC formation. The (\(E_{H_2 } \))-H ₀ (redox potential of the oxidizing solution-Hammett function of the acid) stability fields of graphite nitrate and graphite bisulfate are presented. Our results are the first to demonstrate that KMnO₄ extends the concentration ranges of GIC formation and reduces the threshold acid concentration for the synthesis of binary GICs (to 40%).     

Study on titania nanotube arrays prepared by titanium anodization in NH<Subscript>4</Subscript>F/H<Subscript>2</Subscript>SO<Subscript>4</Subscript> solution

In this paper, the formation of titania nanotube arrays was investigated in NH₄F/H₂SO₄ electrolyte. Under optimized electrolyte conditions, the titania nanotube arrays with an inner diameter of about 120 nm and a length of about 300 nm was obtained. During the formation process, the variety of current was observed. The current–time curve implied that the evolvement process of titania nanotube arrays include three stages. The stability of titania nanotube arrays at elevated temperatures was studied. The as-prepared titania nanotube arrays is amorphous, crystallized in anatase and rutile as the rising of the temperature. The samples were characterized by ESEM and XRD.     

Corrosion resistance of amorphous Fe<Subscript>82</Subscript>P<Subscript>16</Subscript>Si<Subscript>2</Subscript> in 0.1 M Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>

A soft-magnetic amorphous Fe-P-Si alloy prepared using ferrophosphorus waste was tested for corrosion in 0.1 M Na₂SO₄. In a nonequilibrium state, the Fe₈₂P₁₆Si₂ alloy interacts with the medium, but annealing and relaxation reduce the interaction, without influencing the magnetic properties of the alloy. The corrosion resistance of the alloy is comparable to that of Finemet (Fe-Si-B-Nb-Cu) materials.     

Structural phase transitions and piezoelectric anomalies in ordered Sc<Subscript>0·5</Subscript>Ga<Subscript>0·5</Subscript>N alloys

Local-density approximation calculations (LDA) within density functional theory (DFT) and Berry phase approach within modern theory of polarization are performed to predict the structural and piezoelectric properties of ordered Sc_0·5Ga_0·5N alloys under compressive and tensile in-plane strain. This alloy is found to exhibit a tremendous piezoelectric response, associated with a phase transition from nonpolar p6₃/mcc(D _6h) space group to a polar p6₃ mc(C _6v) structure, at fixed Ga and Sc compositions when continuously changing the experimental accessible parameters (i.e. the compressive and tensile strain). The mechanism of the effects behind such anomalous behaviour is revealed and explained.     

Etching behavior of GaAs,GaSb, InAs,and InSb in aqueous H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HBr-ethylene glycol solutions

The chemical interaction of GaAs, GaSb, InAs, and InSb single crystals with H₂O₂-HBr-ethylene glycol bromine-releasing etchants has been studied under reproducible hydrodynamic conditions. We have located boundaries between regions of polishing and nonpolishing solutions, assessed the surface condition of the crystals using microstructural analysis and surface profiling, and optimized the compositions of the polishing solutions and dynamic chemical polishing conditions for the materials studied.     

Phase equilibria and properties of solid solutions in the Tl<Subscript>5</Subscript>Te<Subscript>3</Subscript>-Tl<Subscript>9</Subscript>BiTe<Subscript>6</Subscript>-Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>I system

Phase equilibria in the Tl₅Te₃-Tl₉BiTe₆-Tl₅Te₂I system, containing ternary structural analogs of Tl₅Te₃, have been studied by differential thermal analysis, X-ray diffraction, microhardness tests, and emf measurements using concentration cells with a thallium electrode. A number of T-x sections, the 760- and 800-K sections, and the liquidus and solidus projections of the phase diagram are constructed, and the composition dependences of lattice parameters, microhardness, and emf are obtained. The results demonstrate that, despite the presence of a liquid-liquid miscibility gap, the system contains a continuous series of solid solutions isostructural with Tl₅Te₃.     

The calculation of liquid-vapor phase equilibrium in H<Subscript>2</Subscript>O-H<Subscript>2</Subscript>O<Subscript>2</Subscript> two-component system

The dependences of total pressure and equilibrium composition of vapors on temperature and mole fraction of hydroperoxide in solution are calculated for a water-hydroperoxide two-component two-phase system. It is assumed that the relaxation to concentration equilibrium in gas at the surface of solution is attained quite rapidly compared to the half-life of hydroperoxide. Three independent approximate methods are used to calculate the total pressure. The ideal gas approximation for the gas phase is not used in two of these methods. Approximation formulas are suggested, which explicitly express the dependence of pressure and composition of gas on relative concentration of hydroperoxide in solution and on temperature. The accuracy of the methods and the range of their validity are discussed.     

Effect of Cl<Superscript>−</Superscript>on the corrosive wear of AISI 321 stainless steel in H<Subscript>2</Subscript>SO<Subscript>4</Subscript> solution

The effect of Cl⁻ on the corrosive wear behaviour of AISI 321 stainless steel in H₂SO₄ solution was studied via the corrosive wear rate, the load bearing capacity of passive film and the relationship between pitting and corrosive wear. There is a critical load at natural potential, below which the corrosive wear rate is slightly lowered by Cl⁻, while above which is increased. At natural potential there are more pits at low load than that at a higher one in the wear tracks and the pits are also deeper. The load bearing capacity is lowered by Cl⁻ at passive region and then the corrosive wear rate increased.     

Degradation of bisphenol A by combining ozone with UV and H<Subscript>2</Subscript>O<Subscript>2</Subscript> in aqueous solutions: mechanism and optimization

A laboratory-scale study on the abatement of bisphenol A (BPA) was performed by combining O₃ with H₂O₂ and UV (O₃/H₂O₂/UV), an ozone-based advanced oxidation processes technique (AOP). This work aimed to (1) evaluate the removal of BPA with O₃/H₂O₂/UV, and to compare the degradation efficiency with other ozone-based AOPs (such as O₃ alone, O₃/H₂O₂, and O₃/UV), (2) structurally optimize BPA abatement by using a central composite design (CCD) for experimental design purposes and/or a response surface methodology to find the optimum, and (3) identify the degradation pathways, and main intermediate products, formed during BPA abatement with O₃/H₂O₂/UV. The degradation pathways of BPA degradation were revealed by O₃/H₂O₂/UV on the basis of evidences of intermediate generation. The effect of initial pH, ozone, and H₂O₂ dose during BPA abatement was studied in detail. By increasing each of these three parameters, an enhancement of the BPA degradation efficiency is mostly observed. BPA can be degraded completely when a sufficiently high ozone dose is applied. However, excess H₂O₂, as a scavenger of HO·, has a negative effect on BPA abatement, resulting in a decrease in the BPA’s degradation efficiency. For example, the removal decreased from 64 to 58% by enhancing the H₂O₂ initial dose from 0.5 to 0.75 mmol/L (at an initial pH and ozone dose of, respectively, 7 and 0.1 mg/L). The results confirmed that combining ozone with H₂O₂ and UV was a more efficient method than the other three ozone-based AOPs on the removal of BPA. Therefore, this method could be further applied for the treatment of real wastewaters containing BPA and other micropollutants.     

Reaction of Np(VI) with H<Subscript>2</Subscript>O<Subscript>2</Subscript> in carbonate solutions

The kinetics and stoichiometry of the reaction of Np(VI) with H₂O₂ in carbonate solutions were studied by spectrophotometry. In the range 1–0.02 M Na₂CO₃, the reaction 2Np(VI) + H₂O₂ = 2Np(V) + O₂ occurs, as Δ[Np(VI)]/Δ[H₂O₂] ≈ 2. In Na₂CO₃ + NaHCO₃ solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H₂O₂ follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H₂O₂ concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na₂CO₃ solution containing 1 mM H₂O₂, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H₂O₂] = 1 mM, k passes through a minimum at [Na₂CO₃] = 0.5–0.1 M. The activation energy in a 0.5 M Na₂CO₃ solution is 48 kJ mol⁻¹.     

Crystallization from high-temperature solutions in the K<Subscript>2</Subscript>O-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied general trends of crystallization from high-temperature solutions in the K₂O-P₂O₅-V₂O₅-Bi₂O₃ system at P/V = 0.5?2.0, K/(P + V) = 0.7?1.4, and Bi₂O₃ contents from 25 to 50 wt % and identified the stability regions of BiPO₄, K₃Bi₅(PO₄)₆, K₂Bi₃O(PO₄)₃, and K₃Bi₂(PO₄)_{3 ? x} (VO₄) _x (x = 0?3) solid solutions. The synthesized compounds have been characterized by X-ray powder diffraction and IR spectroscopy, and the structure of two solid solutions has been determined by single-crystal X-ray diffraction (sp. gr. C ₂/c): K₃Bi₂(PO₄)₂(VO₄), a = 13.8857(8), b = 13.5432(5), c = 6.8679(4) Å, β = 114.031(7)°; K₃Bi₂(PO₄)_1.25(VO₄)_1.75, a = 13.907(4), b = 13.615(2), c = 6.956(2) Å, β = 113.52(4)°.     

Formal oxidation potentials of actinide couples in K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript> solutions

The formal oxidation potentials of the M(VI)/M(V), M(V)/M(IV), and M(IV)/M(III) couples for actinides from U to No and of the M(IV)/M(III) couples for some actinides in 1 M H⁺ or 1 M Na⁺ (pH ～5–5.5) solutions containing K₁₀P₂W₁₇O₆₁ were calculated from the data on stability of complexes of f element ions with the unsaturated heteropolytungstate anion P₂W₁₇O ₆₁ ^10? . In some cases, the previously accepted values were subjected to major revision, especially the potentials of the An(V)/An(IV) couples. Problems arising in measuring the potentials of the couples involving Np(III) and Pu(III) which react with the heteropolyanion to form a heteropoly blue are discussed. The potentials of some M(III)/M(II) couples are estimated.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.