Specific features of reactions of Am(V) with reductants in weakly acidic solutions

In EDTA solutions with pH ??5 at 25°C, Am(V) in a concentration of 5 × 10^?4?3 × 10^?3 M slowly transforms into Am(III). The Am(V) reduction and Am(III) accumulation follow the zero-order rate law. In the range 60?C80°C, the reaction is faster. In some cases, an induction period is observed, disappearing in acetate buffer solutions. In the range pH 3?C7, the rate somewhat increases with pH. In an acetate buffer solution, an increase in [EDTA] accelerates the process. The activation energy is 47 kJ mol^?1. Zero reaction order with respect to [Am(V)] is observed in solutions of ascorbic and tartaric acids, of Li₂SO₃ (pH > 3), and of hydrazine. The process starts with the reaction of Am(V) with the reductant. The Am(III) ion formed in the reaction is in the excited state, *Am(III). On collision of *Am(III) with Am(V), the excitation is transferred to Am(V), and it reacts with the reductant: *Am(V) + reductant ?? Am(IV) + R₁ and then Am(IV) + reductant ?? *Am(III) + R₁, Am(V) + R₁ ?? Am(IV) + R₂. A branched chain reaction arises. The decay of radicals in side reactions keeps the system in the steady state; therefore, zero reaction order is observed.     

EDTA-assisted phase conversion synthesis of (Gd0.95RE0.05)PO4 nanowires (RE = Eu,Tb) and investigation of photoluminescence

Abstract

Hexagonal (Gd_0.95RE_0.05)PO₄·nH₂O nanowires ~300 nm in length and ~10 nm in diameter have been converted from (Gd_0.95RE_0.05)₂(OH)₅NO₃·nH₂O nanosheets (RE = Eu, Tb) in the presence of monoammonium phosphate (NH₄H₂PO₄) and ethylene diamine tetraacetic acid (EDTA). They were characterized by X-ray diffraction, thermogravimetry, electron microscopy, and Fourier transform infrared and photoluminescence spectroscopies. It is shown that EDTA played an essential role in the morphology development of the nanowires. The hydrothermal products obtained up to 180 °C are of a pure hexagonal phase, while monoclinic phosphate evolved as an impurity at 200 °C. The nanowires undergo hexagonal→monoclinic phase transformation upon calcination at ≥600 °C to yield a pure monoclinic phase at ~900 °C. The effects of calcination on morphology, excitation/emission, and fluorescence decay kinetics were investigated in detail with (Gd_0.95Eu_0.05)PO₄ as example. The abnormally strong ⁵D₀→⁷F₄ electric dipole Eu³⁺ emission in the hexagonal phosphates was ascribed to site distortion. The process of energy migration was also discussed for the optically active Gd³⁺ and Eu³⁺/Tb³⁺ ions.     

The Development of Microstructure and Ferroelectric Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3.96</Subscript>Nb<Subscript>0.04</Subscript>O<Subscript>12</Subscript> Thin Films

Bi₄Ti_3.96Nb_0.04O₁₂ thin films were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a sol–gel method and rapid thermal annealing. The effects of Nb-substitution and annealing temperature (500–800°C) on the microstructure and ferroelectric properties of bismuth titanate thin films were investigated. X-ray diffraction analysis reveals that the intensities of (117) peaks are relatively broad and weak at annealing temperatures smaller than 700°C. With the increase of annealing temperature from 500°C to 800°C, the grain size of Bi₄Ti_3.96Nb_0.04O₁₂ thin films increases. The Bi₄Ti_3.96Nb_0.04O₁₂ thin films annealed at 700°C exhibit the highest remanent polarization (2P _r), 36 μC/cm² and lowest coercive field (2E _c), 110 kV/cm. The improved ferroelectric properties can be attributed to the substitution of Nb⁵⁺ to Ti⁴⁺ in Bi₄Ti₃O₁₂ assisting the elimination of defects such as oxygen vacancy and vacancy complexes.     

Synthesis of hexagonal Al2−x Cr x O3 nanodisks by a thermal decomposition of mesoporous Cr4+:Al2O3 powders

Monodispersed hexagonal Al_2−x Cr _x O₃ nanodisks are synthesized through a reactive doping of Cr⁶⁺ cations in a hydrogenated mesoporous AlO(OH)·αH₂O powder followed by annealing at 1,200 °C in air. The reaction was carried out by a drop wise addition of an aqueous Cr⁶⁺ solution (0.5–1.0 M) to AlO(OH)·αH₂O, at room temperature. Al_2−x Cr _x O₃ nanostructure formation was controlled by the nucleation and growth from the intermediate amorphous mesoporous Cr⁴⁺:Al₂O₃ composites in the temperature range 400–1,000 °C. The nanodisks of ∼50 nm diameter and thickness of ∼16 nm is observed in the sample with x of 0.2 and similar nanodisks with a low dimension is observed at a higher value of x of 1.6 (after 2 h of heating at 1,200 °C). The Cr³⁺ ↔ Al³⁺ substitution, x ≤ 1.2, inhibits grain growth in small crystallites. The crystallites in x = 0.2 composition have 43 nm diameter while it is 15 nm in those with x = 1.2 composition.     

Preparation of nano-sized ABi2Nb2O9 (A?=?Ca2+, Sr2+, Ba2+) ferroelectric ceramics by soluble Nb(V) tartarate precursor and their dielectric characteristics after sintering

ABi₂Nb₂O₉ (A = Ca²⁺, Sr²⁺, Ba²⁺) were prepared by using an aqueous solution method from the mixture of water-soluble M-EDTA (M = Ca²⁺, Sr²⁺, Ba²⁺), Bi-EDTA, Nb-tartarate and TEA as the starting materials. A black fluffy mass resulted on heating the solution mixture at ∼200 °C. X-ray powder diffraction (XRD) showed that a single phase with the layered perovskite structure was formed after calcining the black fluffy mass at 600 °C. No intermediate phase was found during heat treatment at and above 600 °C. The particle size obtained from TEM lay between 20 and 30 nm. Structural electron microscopy (SEM) revealed that the average grain size after sintering at 900 °C for 4 h ranges from 1.2 to 1.67 μm with a relative compact density of ≥90%. A comparative study on dielectric constant and corresponding tangent loss were carried out at 1 kHz, 100 kHz, 1 MHz, and 5 MHz from which the Curie temperature (T _c) was found to be at 935, 450, and 170 °C with peak dielectric constants at 930, 595, and 545, respectively, for CBN (CaBi₂Nb₂O₉), SBN (SrBi₂Nb₂O₉), and BBN (BaBi₂Nb₂O₉) measured at frequency 100 kHz with very low tangent loss.     

Na2EDTA-assisted hydrothermal synthesis and luminescent properties of YVO4:Eu3+ with different morphologies in a wide pH range

Nano- and micro-scaled Eu-doped yttrium orthovanadate (YVO₄:Eu³⁺) powders had been fabricated via disodium ethylenediamine tetraacetate (Na₂EDTA)-assisted morphology controllable hydrothermal method in a wide pH range at 180 °C for 24 h. The as-synthesized samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL). The results showed that the pH value of the synthesis solution played a key role in the formation of the final products with different morphologies, including ball-like micro-spheres, micro-spheres composed of submicron cubes and flower-like structures containing nano-plates. The photoluminescence measurement revealed that the luminescent properties of the samples were changed by varying their morphologies. The significant ball-like micro-spheres of YVO₄:Eu³⁺ particles had been synthesized, and the luminescence intensity of them is the strongest one among all products.     

Rubidium ion conducting Rb2 ? 2xAl2 ? xAxO4 (A = Nb, Ta) solid electrolytes

Rb_{2 − 2x} Al_{2 − x} A _x O₄ (A = Nb, Ta) solid solutions have been synthesized, and their conductivity has been measured as a function of temperature and composition. The highest rubidium ion conductivity in the Rb_{2 − 2x} Al_{2 − x} A _x O₄ solid solutions is 3.16 × 10⁻³ S/cm at 300°C and ∼ 2 × 10⁻² S/cm at 700°C. The high rubidium ion conductivity of the synthesized solid electrolytes is mainly due to the formation of rubidium vacancies when Nb⁵⁺ or Ta⁵⁺ substitutes for Al³⁺ and to the specific features of the crystal structure of RbAlO₂.     

Photoluminescence properties of β-SiAlON:Yb2+, a novel green-emitting phosphor for white light-emitting diodes

Abstract

We have synthesized Yb²⁺-activated Si_6?zAl_zO_zN_8?z (0.05z2.3, 0.03 mol% Yb²⁺0.7 mol%) green phosphors by solid-state reaction at 1900 °C for 2 h under a nitrogen pressure of 1.0 MPa. Phase purity, photoluminescence and its thermal quenching were investigated. A single phase was obtained for all values of z and Yb²⁺ concentration. A distinct emission band was observed at 540 nm originating from the 5d–4f electronic transition in Yb²⁺ under 480 nm excitation. The photoluminescence properties mainly depended on the Yb²⁺ concentration and chemical composition of the matrix. The resultant phosphor showed high thermal stability, that is, the emission intensity at 150 °C was about 82% of that measured at room temperature. The experimental results indicate that β-SiAlON:Yb²⁺ is a potential green phosphor for white light-emitting diodes (LEDs), which use blue LEDs as the primary light source.     

Physico-chemical study of impregnated Cu and V species on CeO<Subscript>2</Subscript> support by thermal analysis,XRD, EPR, <Superscript>51</Superscript>V-MAS-NMR and XPS

CuVCe oxides were prepared by impregnation of copper and/or vanadium precursors on ceria support. The solids freshly prepared were calcined under air between 400 and 700 °C. Physico-chemical properties of these oxides were then studied using different techniques: Thermal Analysis (DSC/TG), X Ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR), ⁵¹V Magic Angle Spinning Nuclear Magnetic Resonance (⁵¹V-MAS-NMR) and X-Ray Photoelectron Spectroscopy (XPS). X-ray diffraction and thermal analysis revealed cerium orthovanadate phase formation during the calcination of the 1Cu1V10Ce solid. This CeVO₄ phase is not observed for the 10Cu1V10Ce sample. The EPR study revealed two well-resolved copper signals: the first corresponds to isolated Cu²⁺ species and the second to Cu²⁺ dimers. The ⁵¹V-MAS-NMR confirmed the presence of CeVO₄ phase for 1Cu1V10Ce sample and revealed polymeric V–O–V chains in interaction with a copper ceria matrix for 10Cu1V10Ce sample. Finally, the XPS study indicated high vanadium content on the solid surface. This phenomenon is enhanced by the copper content in the solid and could explain the absence of the CeVO₄ phase in 10Cu1V10Ce sample. Thus the ceria orthovanadate phase formation is inhibited by the presence of a high copper content in the solid.     

Nano-domain structure of Li<Subscript>4</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript> spinel

XRD-pure Li₄Mn₅O₁₂ spinels are obtained below 600 °C from oxalate and acetate precursors. The morphology consists of nanometric particles (about 25 nm) with a narrow particle size distribution. HRTEM and electron paramagnetic resonance (EPR) spectroscopy of Mn⁴⁺ are employed for local structure analysis. The HRTEM images recorded on nano-domains in Li₄Mn₅O₁₂ reveal its complex structure. HRTEM shows one-dimensional structure images, which are compatible with the (111) plane of the cubic spinel structure and the (001) plane of monoclinic Li₂MnO₃. For Li₄Mn₅O₁₂ compositions annealed between 400 and 800 °C, EPR spectroscopy shows the appearance of two types of Mn⁴⁺ ions having different metal environments: (i) Mn⁴⁺ ions surrounded by Li⁺ and Mn⁴⁺ and (ii) Mn⁴⁺ ions in Mn⁴⁺-rich environment. The composition of the Li⁺, Mn⁴⁺-shell around Mn⁴⁺ mimics the local environment of Mn⁴⁺ in monoclinic Li₂MnO₃, while the Mn⁴⁺-rich environment is related with that of the spinel phase. The structure of XRD-pure Li₄Mn₅O₁₂ comprises nano-domains with a Li₂MnO₃-like and a Li_4/3−x Mn_5/3+x O₄ composition rather than a single spinel phase with Li in tetrahedral and Li_1/3Mn_5/3 in octahedral spinel sites. The annealing of Li₄Mn₅O₁₂ at temperature higher than 600 °C leads to its decomposition into monoclinic Li₂MnO₃ and spinel Li_4/3−x Mn_5/3+x O₄.     

Oxidation of carbohydrazide with nitric acid

The kinetics of carbohydrazide oxidation with nitric acid in aqueous solutions was studied. In the range [HNO₃] = 3–7 M, the reaction rate is described by the equation −d[(NH₂NH)₂CO]/dt = k[(NH₂NH)₂CO][HNO₃] ⁿ , where n = 3.6 and 2.9 at 70 and 90°C, respectively. The constant k = (6.65 ± 0.23) × 10⁻⁴ l^2.9 mol^−2.9 h⁻¹ at 90°C, and the activation energy of the reaction in 7 M HNO₃ is 124 kJ mol⁻¹. The Fe(III) and especially Tc(VII) ions considerably accelerate the reaction, whereas the uranyl ions accelerate it insignificantly. The reaction mechanism in which the reactive species oxidizing carbohydrazide is nitronium ion NO₂⁺ was suggested.     

Electrical and magnetic studies of iron (III) vanadate

Iron(III) vanadate (FeVO₄) is an n-type semiconductor between 300 and 800 K. Electrical conduction in this phase occurs due to small deviation from oxygen stoichiometric composition. The mechanism of electrical transport is of a thermally activated hopping of charge carriers (electrons) on equivalent iron lattice sites. The FeVO₄ obeys Curie-Weiss law between 80 and 300 K. The measured magnetic moment (μ _eff) of Fe³⁺ ion in FeVO₄ is 5·270 BM at 298 K, which is lower than theμ _spin only value. The predominant exchange interactions are the weak 90° M-O-M superexchange and M-O-O-M super-super-exchange. The negative Weiss constantϑ=− 30 K of the phase indicated the possibility of an antiferromagnetic ordering of the iron(III) vanadate lattice. The IR absorption spectrum of FeVO₄ gave bands at 990, 900, 825 and 725 cm⁻¹ due to the presence of distorted VO₄ polyhedra of the lattice.     

Effect of cobalt content on microstructure and property of electroplated nickel‐cobalt alloy coatings

Nickel‐cobalt alloys were electrodeposited on copper sheets in sulfate bath containing 288.5 g/l NiSO₄·6H₂O, 30 g/l CoSO₄·7H₂O, 40 g/l HBO₃, 15 g/l NaCl and 0.08 g/l lauryl sodium sulfate. The effects of cobalt content on microstructure, microhardness, and wear resistance of electroplating nickel‐cobalt alloys were studied by using SEM and XRD techniques, and microhardness tester and wear tester. The relationship between the microhardness of nickel‐cobalt alloy coatings and heat treatment procedures was also investigated. The experimental results show that cobalt content (W_t) in coating increases with Co²⁺/(Co²⁺ + Ni²⁺)% (X) in plating solution. Fitted regression equation is as following: W_t = –0.7399 + 2.2847X – 0.0133X². The increase of cobalt content leads to that the longitudinal section morphology of coating transforms from the cone into sphericity and at last into the shape of willow leaf, and its structure transforms from face centered cubic (fcc) nickel solid solution into fcc cobalt solid solution and at last into hcp cobalt solid solution. The increase of cobalt content results in the increase of microhardness of nickel‐cobalt alloy coatings, and the hardness reaches a maximum value (363 HV) when cobalt content is 54.9%. After heat treatment at 400°C and 600°C, the microhardness of coatings begins to decrease except the coating containing 79.2% Co. Moreover, the wear resistance of electroplated coatings increases with the increase of cobalt content.     

Molecular beam epitaxial growth of high-quality InP layers from solid phosphorus using a valved phosphorus cracker cell

We report the molecular beam epitaxial (MBE) growth of epitaxial InP using a valved phosphorus cracker cell at a range of cracking-zone temperatures (T_cr = 875–950°C), V/III flux ratios (V/III = 1.2–9.3) and substrate temperatures (T_s = 360–500°C). The as-grown epitaxial InP on an InP (100) substrate is found to be n-type from Hall measurements. The background electron concentration and mobility exhibit a pronounced dependence on the cracking-zone temperature, V/III flux ratio and substrate temperature. Using a T_cr of 850°C, the highest 77 K electron mobility of 40 900 cm² (V s)⁻¹ is achieved at a V/III ratio of 2.3 and a T_s of 440°C. The correponding background electron concentration is 1.74 × 10¹⁵ cm⁻³. The photoluminescence (PL) spectra show two prominent peaks at 1.384 and 1.415 eV, with the intensity of the low-energy peak becoming stronger at higher cracking-zone temperatures. The lowest PL FWHM achieved at 5 K is 5.2 meV. Within the range of substrate temperatures investigated, the effect on the crystalline quality determined from X-ray diffraction (XRD) measurements is not significant.     

Catalytic decomposition of organic anions in alkaline radioactive waste: IV. Oxidation of glycolate with persulfate in the presence of Ru(III)

Oxidation of glycolate ions with Na₂S₂O₈ + RuCl₃ mixture in 0.2 M NaOH was studied by spectrophotometry. Glycolate is oxidizd to oxalate at 20–70°C. The reaction of glycolate with persulfate follows the first-order rate law with respect to [S₂O ₈ ^2? ], weakly depends on the glycolate concentration, and accelerates with increasing the Ru(III) content from 2 × 10^?5 to 1 × 10^?4 M. Further increase in the Ru(III) concentration does not affect the reaction rate. Probable reaction mechanism was considered.     

Combusion synthesis,structural and photo-physical characteristics of Eu<Superscript>2+</Superscript> and Dy<Superscript>3+</Superscript> co-doped SrAl<Subscript>2</Subscript>O<Subscript>4</Subscript> phosphor nanopowders

In this research, we reported the synthesis of Eu²⁺ and Dy³⁺ co-doped SrAl₂O₄ phosphor nanopowders with high brightness and long afterglow by urea-nitrate solution combustion synthesis (SCS) at 600 °C, followed by heating the resultant combustion ash at 1,200 °C in a weak reductive atmosphere (5% H₂ + 95% N₂). The broad-band UV-excited luminescence of the SrAl₂O₄: Eu²⁺, Dy³⁺ nanopowders was observed at λ _max = 517 nm due to transitions from the 4f⁶5d¹ to the 4f⁷ configuration of the emission center (Eu²⁺ ions). The excitation spectra consist of 240- and 254 nm broad peaks. Finally, it was found that the optimum ratio of urea is 2.5 times higher than theoretical quantities for the best emission condition of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor nanopowders.     

Coprecipitation of Pu(IV) with Fe(III) and Cr(III) hydroxides from nitrate-acetate solutions under hydrothermal conditions simulating deep disposal of liquid radioactive wastes

Precipitation of Fe(III), Cr(III), Ni(II), and Mn(II) from nitrate-acetate solutions and coprecipitation of Pu(IV) with Fe(III) and Cr(III) were studied. The degree of precipitation of 80–95% is attained for Fe(III) at 95–200°C and pH>0.5–0.6, and for Cr(III), at T=95°C and pH≥4.0 or T=200°C and pH≥1.0. The phase composition of the precipitates formed by thermal hydrolysis of iron nitrate in model solutions was analyzed. Depending on pH and temperature, the solid phase contains various modifications of Fe₂O₃, FeOOH, and amorphous phases. Noticeable coprecipitation of plutonium from nitrate-acetate solutions is observed at pH≥4, and it is incorporated in the precipitate only at formation of FeOOH. No coprecipitation of Pu(IV) with Fe₂O₃ was found. Under the given experimental conditions, plutonium in aqueous solutions occurs in the oxidation state +4 forming monoacetate (or, probably, hydroxo acetate) complexes.     

Nonhydrolytic sol-gel synthesis and characterization of YAG

Yttrium–aluminum oxides are interesting compounds, which are widely used as hosts for lasers and phosphors due to their stable physical and chemical properties. The manufacture of YAG has been investigated thoroughly. YAG powders are traditionally synthesized through the reaction of aluminum and yttrium powders at high temperatures. The work reported here involved an investigation into the preparation of YAG by a nonhydrolytic sol-gel route and the influence of heating time at low temperatures to obtain YAG from inorganic precursors (yttrium and aluminum chloride). AlCl₃, YCl₃ and ethanol were reflux reacted under an argon atmosphere. Europium III chloride was added as a structural probe. The powder was treated at 800 °C for 1, 2, 4, 8 and 16 h. The YAG structure was analyzed by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal analysis (TA) and photoluminescence (PL). The XRD revealed only peaks corresponding to the YAG phase. PL data showed that the YAG phase was formed in 2 min with samples pretreated at 50 °C. For the samples pretreated at 800 °C, the YAG phase appeared in 30 s. The excitation spectra presented a maximum of 394 nm corresponding to the ⁵L₆ level, while the emission spectra of Eu III ions showed characteristic transition bands arising from the ⁵D₀ → ⁷F _J (J = 1, 2, 3, 4) manifolds at maximum excitation. The magnetic dipole ⁵D₀ → ⁷F₁ transition exhibited greater intensity than the electric dipole ⁵D₀ → ⁷F₂ transition. This methodology proved efficient for obtaining YAG phase.     

Effects of Gd doping on the sintering and microwave dielectric properties of BiNbO<Subscript>4</Subscript> ceramics

Gd³⁺ was chosen as a substitute for Bi³⁺ in BiNbO₄ ceramics, and the substitution effects on the sintering performance and microwave dielectric properties were studied in this paper. The high temperature triclinic phase was observed only in the Bi_0.98Gd_0.02NbO₄ ceramics when sintered at 920 °C. Both bulk densities and dielectric constant (ε_r) increased with the sintering temperature, while decreased with the Gd content. The quality factor (Q) exhibited a correlation to the Gd content and the microstructures of Bi_1−x Gd _x NbO₄ ceramics. At the sintering temperature of 900 °C, Bi_0.992Gd_0.008NbO₄ ceramics exhibited microwave dielectric properties of ε_r ∼ 43.87, Q × f ∼ 16,852 GHz (at 4.3 GHz), and its temperature coefficient of resonant frequency (τ_f) was found to be near-to-zero.     

Effect of agglomeration during coprecipitation: Delayed spinellization of magnesium aluminate hydrate

Precipitation of magnesium aluminate hydrate with faster addition of ammonia at desired pH causes agglomeration. Agglomerated powder, without any further treatment, on calcination forms intermediate compounds at low temperatures (≤ 900°C). The intermediate compounds on further heat treatment (≥ 1000°C) decompose into MgO, MgAl₂O₄ and α-Al₂O₃. Effect of agglomeration and absorption of foreign ions such as Cl⁻, SO₄²⁻, and NH₄⁺ in complex compounds probably cause loss of Al³⁺ and Mg²⁺ ions during heat treatment, and stoichiometry changes. Powders prepared by continuous method with better control of process parameters than batch process yields better spinellization.