UxTh1-x(C2O4)2 Solid Characterisation Studies

Many advanced reprocessing schemes under development are aimed at co-processing and co-conversion of actinides, unlike current reprocessing plants that produce separate uranium and plutonium products. The most well developed option for the co-conversion stage is probably oxalate co-precipitation, followed by the thermal co-conversion to a mixed oxide product. It is thus envisaged that future processes will avoid separation of plutonium from uranium and instead allow part of the uranium to flow with the plutonium, resulting in co-precipitation as the oxalate, and finally co-conversion to a mixed uranium-plutonium oxide (MOX), which can be fabricated into recycled nuclear fuel for further energy generation.The co-crystallisation of uranium (IV) and plutonium (III) into a single oxalate structure ensures the homogenous distribution of the two actinides at the molecular scale. The joint conversion of uranium and plutonium to the oxide form makes it possible to remove the complicated step of blending and grinding the two distinct oxide powders, as currently employed for the purposes of MOX fuel fabrication. This concept can also be extended to other actinides, including minor actinides from partitioning processes such as SANEX (Selective Actinide Extraction) and GANEX (Grouped Actinide Extraction) processes or even a thorium containing product from recycle of thorium based fuels.A selection of U_xTh_1-x(C₂O₄)₂ solids at varying concentrations of uranium and thorium were prepared by oxalate co-precipitation. Uranium (VI) was conditioned electrochemically at -0.7 V to uranium (IV), in the presence of hydrazine. The reduced uranium (IV) in nitric acid was mixed with thorium nitrate solutions at different concentration ratios with oxalic acid. The mixed tetravalent uranium-thorium oxalate solid products have been characterised by Raman and IR spectroscopies. The influence of thorium substituted into the uranium oxalate structure was evaluated. Several vibrational modes were found to be affected by the variation in ionic radius appearing to be metal sensitive and therefore, provide the initial indication in the evaluation of the chemical composition.     

How synthesis parameters influence the quality of lead magnesioniobate PbMg1/3Nb2/3O3

Lead magnesioniobate PbMg_1/3Nb_2/3O₃ (PMN) has been prepared using lead oxide and magnesium niobate. Factors that influence the perovskite/pyrochlore ratio in the PMN structure have been studied. In order for the maximal amount of the perovskite phase to be obtained, synthesis should be carried out at 850?C900°C; an excess of MgO (1?C5 wt %) does not exert a positive effect. An excess of PbO (5?C7 wt %) allows one to obtain a phase that contains 85% PMN, with the perovskite structure. The perovskite lattice is found to be stabilized as a result of cationic substitutions (Yb, Lu) in the Mg-Nb sublattice due to the formation of solid solution Pb(Mg_1/3Nb_2/3)_{1 ? x} Yb(Lu) _x O₃, where 0 ?? x ?? 0.9.     

Reactions between YBa2Cu3O7−δ and La2O3 and SrCO3

Solid state reactions at 925°C between the high-T _c ceramic superconductor YBa₂Cu₃O_7?δ and La₂O₃ and SrCO₃, respectively, mixed in various molar ratiosr=MeO_n/YBa₂Cu₃O_7?δ, were studied using X-ray powder diffraction and scanning electron microscopy. The reaction between YBa₂Cu₃O_7?δ and La₂O₃ yielded (La_1?xBa_x)₂CuO_4?δ, withx≈0.075?0.10. La_2?xBa_1+xCu₂O_6?δ, withx≈0.2?0.25 and La-doped (Y_1?xLa_x)₂BaCuO₅, withx≈0.10?0.15. Forr=3.0, Y-doped La₂BaCuO₅ resulted also. The reaction between YBa₂Cu₃O_7?δ and SrCO₃ yielded (Sr_1?zBa_z)₂CuO₃, withz≈0.1, Y₂(Ba_1?zSr_z)CuO₅, withz=0.1?0.15, and a nonsuperconducting compound with an approximate composition of Y(Ba_0.5Sr_0.5)₅Cu_3.5O_10±δ. At values ofr≤2.0, unsubstituted YBa₂Cu₃O_7?delta was found in the reaction products.     

Synthesis of solid solutions of double-layered Ruddlesden-Popper phases in the Gd2O3-SrO-Fe2O3-Al2O3 system

The continuous Gd₂Sr(Al_{1 ? x} Fe _x )₂O₇ solid solution series was synthesized by a solid-state route over the entire concentration range (0 < x < 1). The processing stages of Gd₂Sr(Al_{1 ? x} Fe _x )₂O₇ series were investigated, and phase interaction schemes reflecting the multistaged formation of double-layered Ruddlesden-Popper solid solution are constructed. The closeness of the reactive mixture composition to the composition of individual compounds Gd₂SrAl₂O₇ or Gd₂SrFe₂O₇ is of importance for the realization of a particular way of Gd₂Sr(Al_{1 ? x} Fe _x )₂O₇ solid solutions formation.     

Study of the formation of the apatite-type phases La9.33+x(SiO4)6O2+3x/2 synthesized from a lanthanum oxycarbonate La2O2CO3

Lanthanum silicated apatites with nominal composition La_9.33+x(SiO₄)₆O_2+3x/2 (−0.2 < x < 0.27) have been successfully synthesized by solid state reaction using a new reagent La₂O₂CO₃ and amorphous SiO₂ precursors. The formation mechanism of La₂O₂CO₃ reagent, which cannot be purchased, has been followed by in-situ temperature depend XRD of La₂O₃ under CO₂ atmosphere. The stability of this reagent during the synthesis step allowed to limit the formation of secondary phase La₂Si₂O₇ and made the weighting of the reagent easier. High purity powders could be synthesized at the temperature of 1400 °C. Dense pellets (more than 98.5%) were obtained by isostatic pressing of powders calcined at 1200 °C and then sintered at 1550 °C. Traces of La₂SiO₅ secondary phase present in synthesized powder disappeared after densification and pure oxyapatite materials were obtained for all the compositions. Electrical measurements confirmed that conductivity behaviors of the sintered pellets were dependent to the oxygen over-stoichiometry. Indeed, a relatively high conductivity of 1 × 10⁻² S cm⁻¹ was exhibited at 800 °C for the nominal composition La_9.60(SiO₄)₆O_2.405 with low activation energy around 0.79 eV. The ionic conductivity properties were comparable with that of the earlier obtained materials.     

Effects of gamma irradiation on optical and structural properties of PbO–Bi2O3–B2O3 glasses

Optical and structural properties of xPbO·2xBi₂O₃(1−3x)B₂O₃ glasses of different composition have been studied using UV-VIS and FTIR spectroscopic techniques. Effects of gamma radiations on glass network and structural units have been studied by irradiating glass samples with a ⁶⁰Co radioisotope to the overall dose of 2.5 kGy. It is shown that irradiation causes compaction of the borate network by breaking the bonds between trigonal elements, which leads to a decrease in the optical band gap energy. Changes in the atomic structure before and after the irradiation are observed and explained.     

Hydrothermal crystal synthesis in the Ho2O3(Yb2O3)GeO2KFH2O systems

Crystallization in the Ho₂O₃(Yb₂O₃)GeO₂KFH₂O systems has been investigated under hydrothermal conditions. Crystallization fields of the crystalline phases have been determined. Single crystals of Ho₂Ge₂O₇, Yb₂Ge₂O₇ (two types), K₂HoF₅, K₂YbF₅, K_xYb_yGe_pO_q (P-type), Ho(OH)₃, Yb(OH)₃, and K₂Ge₄O₉ have been obtained. The germanates synthesized have been studied by X-ray analysis and infrared-spectroscopy. Diorthogermanate Yb₂Ge₂O₇ has been found to crystallize in two structural types; the first is characterized by the usual structure that is typical for rare-earth germanates, the second is new for germanates of rare-earth elements. High chemical resistance is typical of these crystals. The P-type germanate also has a new type of structure among rare earth germanates. Some suggestions are made as to the structure of these new germanates on the basis of X-ray and ir-spectroscopic data.     

Synergy effects in mixed Bi2O3, MoO3 and V2O5 catalysts for selective oxidation of propylene

In this work, the possible synergy effects between Bi₂O₃, MoO₃ and V₂O₅, and between Bi₂Mo₃O₁₂ and BiVO₄, were investigated. The catalytic activity of the ??mechanical mixture?? of these compounds was measured. The mixture containing 36.96?mol% Bi₂O₃, 39.13?mol% MoO₃ and 23.91?mol% V₂O₅ (21.43?mol% Bi₂Mo₃O₁₂ and 78.57?mol% BiVO₄), corresponding to the compound Bi_1?x/3V_1?x Mo _x O₄ with x?=?0.45 (Bi_0.85V_0.55Mo_0.45O₄), exhibited the highest activity for the selective oxidation of propylene to acrolein. The mixed sample prepared chemically by a sol?Cgel method possessed higher activity than that of mechanical mixtures.     

Synthesis and properties of titanium glycolate Ti(OCH2CH2O)2

A new efficient method for the synthesis of extended micro-and nano-sized crystals (whiskers, fibers) of titanium glycolate Ti(OCH₂CH₂O)₂ has been suggested. The method implies the reaction of hydrated titanium dioxide with ethylene glycol on heating in air. Thermolysis of Ti(OCH₂CH₂O)₂ in air gives titanium dioxide as anatase (400–500°C) and rutile (T > 700°C), the morphology of titanium glycolate crystals being inherited by the oxide. The pseudocrystals of the thermolysis product in an inert gas medium (T = 500–950°C) represent aglomeration of nano-sized titanium dioxide particles and amorphous carbon. At temperatures up to 1300°C, the formation of the TiO_2?x C _x phase with a rutile structure is probable. In a wet air environment, titanium glycolate is partially hydrolyzed to give TiO _x (OCH₂CH₂O)_2?2x (OH)_2x ·xH₂O (0 ≤ x ≤ 1) and on keeping in water at room temperature, ethylene glycol is completely displaced from the crystals. This process is also not accompanied by changes in the particle morphology.     

Synthesis and characterization of nanometric powders of UO2+x, (Th,U)O2+x and (La,U)O2+x

This paper describes a new way of preparing nanometric powders of uranium oxide, to fit the needs of studies on UO₂ oxidation, through the electrochemical reduction of U(VI) into U(IV). These powders can also be doped with radionuclides if necessary. The precipitation of oxides occurs in reducing and anoxic conditions. This original method makes it possible to synthesize nanometric UO₂ powders with a calibrated size, as well as the Th- and La-doped UO₂ powders with a predefined composition. The powder characterization by the X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron Microscopy shows the formation of spherical crystallites of UO_2+x, (Th,U)O_2+x and (La,U)O_2+x phases. The composition can be defined by the initial Th/(Th+U) and La/(La+U) ratios in solution and the particle size can be controlled by varying the pH.     

Layered manganese oxide with O2 structure,Li(2/3)+x(Ni1/3Mn2/3)O2 as cathode for Li-ion batteries

Using LiI as the reducing agent, the compound O2-Li_(2/3)+x(Ni_1/3Mn_2/3)O₂, x∼1/3 (O2(Li+x)) has been prepared from the O2-Li_2/3(Ni_1/3Mn_2/3)O₂ (O2(Li)). Cyclic voltammetry and voltage-capacity profiles of the O2(Li+x) phase are qualitatively different from that of O2(Li) phase. The first extraction capacity of O2(Li+x) at C/10 rate is 190 mAh/g corresponding to the removal of 2/3 mole of Li from the compound. At C/5 rate it delivers a reversible capacity of 158 mAh/g at 25 °C and 184 mAh/g at 50 °C (vs Li metal; voltage window 2.5–4.6 V). In Li-ion cells, with MCMB anode and O2(Li+x) as cathode, a discharge capacity of 140 mAh/g was obtained at C/5 rate in the voltage window 2.5–4.5 V (25 °C). The charge–discharge cycling performance and the cyclic voltammograms reveal that O2(Li) and O2(Li+x) do not convert to the spinel structure.     

Structural and Electrochemical Properties of Li2O-V2O5-B2O3-Bi2O3 Glass and Glass-Ceramic Cathodes for Lithium-Ion Batteries

In this study, 20Li₂O-60V₂O₅-(20 − x)B₂O₃-xBi₂O₃ (x = 5, 7.5, 10 mol%) glass materials have been prepared by the melt-quenching method, and the structure and morphology of the glass materials have been characterized by XRD, FTIR, Raman, and FE-SEM. The results show that the disordered network of the glass is mainly composed of structural motifs, such as VO₄, BO₃, BiO_3, and BiO₆. The electrochemical properties of the glass cathode material have been investigated by the galvanostatic charge-discharge method and cyclic voltammetry, and the results show that with the increases of Bi₂O₃ molar content, the amount of the VO₄ group increases, and the network structure of the glass becomes more stable. To further enhance the electrochemical properties, glass-ceramic materials have been obtained by heat treatment, and the effect of the heat treatment temperature on the structure and electrochemical properties of the glass has been studied. The results show that the initial discharge capacity of the glass-ceramic cathode obtained by heat treatment at 280 °C at a current density of 50 mA·g⁻¹ is 333.4 mAh·g⁻¹. In addition, after several cycles of charging and discharging at a high current density of 1000 mA·g⁻¹ and then 10 cycles at 50 mA·g⁻¹, its discharge capacity remains at approximately 300 mAh·g⁻¹ with a capacity retention rate of approximately 90.0%. The results indicate that a proper heat treatment temperature is crucial to improving the electrochemical properties of glass materials. This study provides an approach for the development of new glass cathode materials for lithium-ion batteries.     

Ferromagnetic and dielectric properties of lead free KNbO3-CoFe2O4 composites

Multiferroic (1-x)KNbO₃-(x)CoFe₂O₄ (x = 0.0, 0.25, 0.5, 0.75 and 1.0 mol) composites were prepared by solid state reaction method. X-ray diffraction results showed that the prepared (1-x)KNbO₃-(x)CoFe₂O₄ composites belong to orthorhombic system for x = 0.0 (perovskite KNbO₃), cubic system for x = 1.0 (spinel CoFe₂O₄) and mixed phase of KNbO₃ and CoFe₂O₄ for x = 0.25, 0.5 and 0.75. Nb-O and Fe-O stretching vibrational modes were observed in the Fourier transform and Raman spectral analyses, respectively. The HR-SEM analysis showed that the morphology of KNbO₃ and CoFe₂O₄ composites was significantly modified by CoFe₂O₄ content. Energy dispersive X-ray spectroscopy results confirmed the presence of K, Nb, Fe, Co and O in (1-x)KNbO₃-(x)CoFe₂O₄ composites. The ionic state of Nb, Co, Fe and O was examined by X-ray photoelectron spectroscopy analysis. The high value of coercivity (H_c = ∼981Oe) for x = 0.5 and the enhanced dielectric constant for x = 0.5 and 0.75 were observed.     

Structural and spectroscopic characterization of Al2−x Cr x O3 powders obtained by polymeric precursor method

Al₂O₃ and Al_2−x Cr _x O₃ (x = 0.01, 0.02 and 0.04) powders have been synthesized by the polymeric precursors method. A study of the structural evolution of crystalline phases corresponding to the obtained powders was accomplished through X-Ray Diffraction and UV-vis spectroscopy (reflectance spectra and CIEL^*a^*b^* color data). The obtained results allow to identify the γ-Al₂O₃ to α-Al₂O₃ phase transition. The single-phase α-Al₂O₃ powder was obtained after heat treatment at 1050 °C for 2 h. The results show that the green to red color transition and ruby luminescence lines observed for the powders of Al_2−x Cr _x O₃ are related to the γ to α-Al₂O₃ phase transition and the temperature and time range for such transition depends on the chromium content.     

Oxygen ionic transport in Bi2O3-based oxides: II. The Bi2O3–ZrO2–Y2O3 and Bi2O3–Nb2O5–Ho2O3 solid solutions

Electrical conductivity, fluorite-type cubic unit cell volume and thermal expansion of the (Bi_{1− x} Nb _x )_{1− y} Ho _y O_1.5+δ (x=0.05 and 0.08; y=0.10−0.15) and (Bi_{1− x} Zr _x )_{1− y} Y _y O_1.5+δ (x=0.05 and 0.07; y=0.15) solid solutions have been found to decrease regularly with increasing dopant content. Annealing at temperatures below 900 K leads to a phase decomposition and to a sharp decrease in conductivity of the ceramics. Oxygen ion transference numbers have been determined by the e.m.f. method and by Faradaic efficiency measurement to exceed 0.9. A new technique of studying Faradaic efficiency has been proposed and verified using (Bi_0.95Zr_0.05)_0.85Y_0.15O_1.5+δ and Zr_0.90Y_0.10O_1.95 ceramic samples. Received: 31 October 1997 / Accepted: 18 December 1997     

Studies regarding the formation from metal nitrates and diol of NiM 2 III O4 spinels, inside a silica matrix

The present study deals with preparation and characterization of spinel mixed oxide systems NiM ₂ ^III O₄, where M^III?=?Fe^III, Cr^III. In order to obtain 50% NiFe₂O₄/50% SiO₂ and 50% NiCr₂O₄/50% SiO₂ nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO₂ matrix, of some particular precursors, coordination compounds of the involved M^II and M^III cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140?°C of the gels (tetraethylorthosilicate?Cmetal nitrates?C1,3-propanediol?Cwater). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350?°C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe₂O₄ starting with 600?°C, and NiCr₂O₄ starting with 400?°C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300?°C, the spinelic phase ??-Fe₂O₃ is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr₂O_3+x ) is formed. In the range 380?C400?°C, Cr₂O_3+x turns into Cr₂O₃ which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.     

Phase relations in the dioxide-trioxide region of some 3d transition Metal-WO ternary systems

The phases occurring in the MnWO, FeWO, CoWO, and NiWO systems at 1373°K have been determined using X-ray diffraction and electron and optical microscopy. Experimentally most attention was given to the MnWO system, where it was found that Mn entered as the Mn²⁺ ion into the WO₃ host matrix and formed a perovskite-related bronze Mn_xWO₃. The highest observed x-value in the bronze is about 0.027. In addition a metastable θ_w(Mn) oxide with the Mo₅O₁₄ structure and a disordered oxide of overall composition approximately (Mn, W)O_2.82 were found. The FeWO system was similar to the MnWO system but significant differences occurred in the CoWO and NiWO systems where M_xWO₃ bronze phases were not observed to form at 1373°K. The stability of the M_xWO₃ and the θ_w(M) oxides formed are discussed in terms of the ionic size of the M ions involved. It is suggested that M_xWO₃ bronzes are metastable if these M ions are small.     

Synthesis,crystal structure modeling and thermal decomposition of yttrium propionate [Y2(CH3CH2COO)6·H2O]·3.5H2O

An yttrium propionate complex was synthesized and characterized for its application as precursor for Y₂O₃ based oxide thin films deposition and YBa₂Cu₃O_7 − x superconducting thin films. The TG–DTA and FT-IR analyses have revealed the formation of an yttrium propionate complex with the formula [Y₂(CH₃CH₂COO)₆·H₂O]·3.5H₂O. The molecular structure of the yttrium propionate complex was determined by modeling the FT-IR spectra. The coordination numbers for the yttrium ions are eight and nine, respectively being coordinated by bridging bimetallic triconnective and chelating bidentate propionate groups.The thermal decomposition of yttrium propionate has been investigated by thermogravimetric (TG) and differential thermal analysis (DTA) coupled with quadrupole mass spectrometry (QMS), X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FT-IR) techniques.     

Heat capacity of pure and doped V2O3 single crystals

Heat capacities have been measured for single crystals of V₂O₃, either pure or doped with 1 and 1.4 mole% Cr₂O₃ and Al₂O₃ over the temperature range 100–700°K. V₂O₃ undergoes a fairly sharp transition at low temperatures (～170°K) but fails to exhibit any thermal anomaly above 300°K. The thermal behavior of (M_xV_1?x)₂O₃, M = Cr, Al, is manifested by two transitions: one at low temperatures, 170–180°K for x = 0.01 and 180–190°K for x = 0.014, and the other at high temperatures. For x = 0.01, the high-temperature (HT) anomaly extended over the range 325–345°K (Cr-doped V₂O₃) and 345–365°K (Al-doped V₂O₃), respectively. The corresponding ranges for x = 0.014 were found to be 260–280°K and 270–290°K, respectively. Further, the HT anomaly was characterized by a large hysteresis (～50°K). The values of lattice heat capacity of pure and doped V₂O₃ were, however, found to be almost the same and could be empirically represented by the Debye (D)?Einstein (E) function $\text{D}\text{(}\text{580}\text{T}\text{) + 4}\text{E}\text{(}\text{θ}\text{T}\text{)}$ with θ values 430°K (T = 100–230°K) and 465°K (T > 230°K), respectively. Further, the enthalpy change ΔH associated with the HT anomaly in doped V₂O₃ (80 ≤ ΔH ≤ 510 J/mole) was 5–10 times smaller than the ΔH corresponding to the lower-temperature transition. The results cited here appear incompatible with the Mott transition model that has been invoked to explain the HT anomaly.     

Structural and electrical properties of (1−x)(Na1/2Bi1/2)TiO3-xPb(Mg1/3Nb2/3)O3 solid solution

Structural, dielectric and piezoelectric properties of (1−x)(Na_1/2Bi_1/2)TiO₃-xPb(Mg_1/3Nb_2/3)O₃ (NBT-xPMN) solid solution have been investigated. An addition of PMN into NBT transformed the structure of sintered samples from rhombohedral to pseudocubic phase where x is larger than 0.1. In calcined powders, however, the intermediate structure were observed between rhombohedral and cubic phases near x=0.1. The formation of solid solution between NBT and PMN modified the dielectric and piezoelectric properties of NBT to be suitable for high temperature dielectric and piezoelectric material. With increasing the content of PMN, the temperature-stability of ε_r(T) increased and the high temperature dielectric loss decreased. In addition, the piezoelectric property of NBT-xPMN was enhanced, for the decrease of coercive field and conductivity promoted the domain reversal under the high electric field of the poling process.