Reoxidant Research Articles

Towards Better Phosphor Design: Effect of SiO2 Nanoparticles on Photoluminescence Enhancement of YAG:Ce

Rare-earth-doped yttrium aluminum garnet (YAG:RE) phosphors have good photoluminescence (PL) properties and are widely used in light-emitting diodes. However, the RE elements used in these phosphors are expensive and in short supply. It is therefore important to develop phosphors that contain smaller amounts of RE materials. One strategy is to produce nanocomposite phosphors in which a cheaper and more readily available material is used as a matrix for an RE oxide. In this study, we produced a YAG:Ce/SiO2 nanocomposite using a sol–gel method; poly(ethylene glycol) and urea were added to promote micelle formation and agglomeration, respectively. The nanocomposites were characterized using X-ray diffraction, scanning and transmission electron microscopies (TEM), and energy-dispersive X-ray spectroscopy. We determined the concentration of SiO2 that provided maximum PL enhancement, and used geometrical models as well as the characterization results to propose an explanation for this enhancement. Our results showed that an SiO2 concentration of 10 vol% provided a PL intensity 120% that of pure YAG:Ce. TEM analysis showed that SiO2 nanoparticles covered the voids between the single grain boundaries of the YAG:Ce crystals, thereby inhibiting light scattering, resulting in enhanced PL. This method will be useful for large-scale synthesis of low-RE, high-PL phosphors.

Effect of RE oxide on growth dynamics of primary austenite grain in hardfacing layer of medium-high carbon steel

The flux cored wires with different rare earth oxide additions for hardfacing the workpieces of medium-high carbon steel were developed. The microstructure of the hardfacing layer was observed using the optical microscopy. The average dimension of primary austenite grains in hardfacing layer was measured by image analyzer. The primary austenite grain growth activation energy and index were calculated according to Sellars's mode and Beck formula, respectively. Moreover, the effect of rare earth oxide on the growth dynamics of primary austenite grain was analyzed, and then discussed with the misfit theory. The experimental results showed that, by adding rare earth oxide, the average dimension of primary austenite grains in hardfacing layer of medium-high carbon steel decreased, and it was the smallest when the addition of rare earth oxide was 5.17 wt.%. Meanwhile, at this rare earth oxide addition, the primary austenite grain growth activating energy in hardfacing layer was the largest, while its index was the smallest. The calculated results indicated that the primary austenite grain could be refined because LaAlO3 as heterogeneous nuclei of γ-Fe was the most effective.

Room Temperature Magnetic Response of Sputter Deposited TbDyFe Films as a Function of the Deposition Parameters

Magnetostrictive thin films of the ternary compound (TbyDy1-y)xFe1-x have been prepared by magnetron sputtering from a Terfenol D target onto both room-temperature and high-temperature substrates (TS<550 °C). The aim is to select the deposition parameters of this highly magnetostrictive material in order to optimize its magnetic response. Films prepared on room-temperature substrates were amorphous and those deposited at high temperatures have a microstructure consisting of small grains of RFe2 compound and some RE oxide phases. Deposition temperatures around 550 °C promotes in-plane orientation of (220) and (311) planes of RFe2 phase. The magnetization measurements performed at room temperature showed that depending on the processing conditions the material changes from a soft magnetic behavior with perpendicular anisotropy to a two phase system with both hard and soft magnetic phases.

In situ investigation on the oxidation behavior of a RESn 3 film by transmission electron microscopy

An in situ ex-environmental transmission electron microscopy (TEM) study was carried out to investigate the oxidation behavior of RESn3 (RE = Nd, La, Ce) film in a room at 18 ± 2 °C with 20 ± 5% humidity. The original RESn3 compounds transformed into RE(OH)3 nanocrystals and Sn clusters. This involved the selective oxidation of RE in RESn3 and the atomic diffusion of Sn atoms. The whole oxidation behavior was discussed from the thermodynamic viewpoint, and a reaction formula was proposed to summarize the oxidation of RESn3 under different ambient conditions.

Surface modification of titanium alloy with laser cladding RE oxides reinforced Ti3Al–matrix composites

Ti3Al–matrix composites were prepared by laser cladding of the Al3Ti/TiB2/Al2O3 pre-placed powders on the Ti–6Al–4V alloy, which can improve the wear resistance of the substrate. With addition of the proper content of RE oxides (nano-Y2O3), this composite coating exhibited finer microstructure and better wear resistance. Nevertheless, excessive RE oxides could lead to the production of the micro-crack, and also decrease the temperature of the molten pool leading to the present of the un-melted TiB2 block, which can significantly decrease the wear resistance of this composite coating.

Wear Characterization of Thermal Sprayed and Fused Fe-Ni-Cr Alloy Coatings with the Addition of CeO<sub>2</sub>

Fe-Ni-Cr alloy powders with CeO2 were flame sprayed and fused on the surface of 1045 carbon steel substrate. The effect of CeO2 on microstructure and tribological behavior of coatings were studied experimentally by means of scanning electron microscopy (SEM), field emission gun scanning electron microscopy (FEGSEM), energy dispersive spectroscopy (EDS) and wear tests. The results show that an adhered oxide debris layer was formed on the worn surface in friction which contributed to decreased wear. Wear rate of the material increased with the load, but dramatically decreased at first and then slightly decreased the sliding speed. The friction coefficient of the material decreased slightly with the load, but increased with sliding speed at first, and then tended to be a constant value. Wear mechanism of the coatings was oxidation wear and a large amount of counterpart material was transferred to the coatings, the RE oxide in the debris layer contributes to the improvement in wear resistance.

Promotion of a copper–zinc catalyst with rare earth for the steam reforming of methanol at low temperatures

The co-precipitation method was used to prepare a reference catalyst composed of copper/zinc oxide (Cu/Zn), as well as a series of Cu/Zn catalysts promoted by monocomponent rare earth (Cu/mRE–Zn) and bicomponent rare earth (Cu/bRE–Zn). The prepared catalysts were tested in a packed-bed reactor in the production of hydrogen via the steam reforming of methanol (SRM). The experimental SRM performance of the prepared catalysts was compared according to three fundamental criteria: the minimum temperature required for 95% conversion of methanol ( T 95), the selectivity of CO in the hydrogen-rich gas produced ( S CO), and the decay rate ( k) at T 95. The catalysts followed the performance trend Cu/bRE–Zn > Cu/mRE–Zn > Cu/Zn in the SRM tests. Distinctly high SRM performance, i.e., low values for T 95 (210 °C), S CO (0.04%), and k (negligible in 16 h), was observed using the Cu/bRE–Zn catalyst. The high performance of Cu/bRE–Zn catalysts was attributed to good adhesion of RE oxides to both CuO and ZnO.

Thermodynamics of Modifying Effect and Experiment Research of Rare Earth in Plasma Jet Surface Metallurgy

The Fe-based composite coatings with RE oxides were prepared on low-carbon steel substrate by use of the plasma jet surface metallurgy, and the effect of RE on microstructure of coating was investigated. The result shows that the microstructure and properties with a proper amount of RE oxides are better than these of the coatings without RE oxides. In addition, the modifying effect of RE oxide on inclusions in metallurgical coating was studied by means of thermodynamics. The thermodynamics analysis shows that RE oxide (Ce2O3) can be reduced to RE by carbon, then the RE element can react with oxygen and sulfur to form the RE oxide-sulfide in metallurgical pool. As a result, the coating is purified and the solidification crack of coating can be restrained by deoxidization and desulphurization.

Synthesis under usual conditions, X-ray photoelectron spectroscopy and magnetic properties of Re 1− xMn xO 2 oxides with rutile structure

Solid solutions Re 1− x Mn x O 2 ( x = 0; 0.08; 0.21) with a tetragonal rutile-type structure (sp. gr. P4 2/mnm, z = 2) have been synthesized from NH 4ReO 4, KReO 4 and KMnO 4 in argon atmosphere at 450–520 °C in a quartz cell. According to XPS measurements, Mn in solid solutions Re 1− x Mn x O 2 is present as Mn 3+ and Mn 2+ cations, while the degree of oxidation of Re partly increases to penta- or hexavalent state. A transition of ReO 2 presumably in the antiferromagnetic state at 41 K was for the first time established by magnetic measurements. Along with the anomaly at 41 K, the second magnetic transitions at 6.5 K ( x = 0.08) and 11 K ( x = 0.21) was registered in these solid solutions.

RE 2O 3-promoted Pt–SO 42−/ZrO 2–Al 2O 3 catalyst in n-hexane hydroisomerization

Lanthanum, cerium and ytterbium nitrates used as RE oxide precursors, RE 2O 3-promoted PSZA catalysts for n-hexane hyroisomerization were prepared and RE 2O 3 promotion effects on catalytic activity and catalytic stability were investigated by N 2-adsorption, XRD, TG, FTIR, XPS and H 2-TPR. It was elucidated that the introduction of RE 2O 3 into Pt–SO 4 2−/ZrO 2–Al 2O 3 (PSZA) increased both the surface area and the number of active sites, resulting in a higher n-hexane isomerization activity. At the same time the catalyst stability has also been markedly increased by alleviating the sulfur loss and by stabilizing ZrO 2 tetragonal crystalline structure. In addition, XPS for the spent catalysts showed during n-hexane hydroisomerization reaction a relative small amount of S 6+ species was reduced to S 4+, and the amount of the most active sulfate entities decided the overall amount of n-hexane transformed, their reduction being, perhaps, retarded by the RE 2O 3 addition to PSZA. The promotion effects of RE oxides addition decreased in the order: La 2O 3 > Yb 2O 3 > Ce 2O 3.

Voltammetry of half-sandwich manganese group complexes of η6-PhC 3B 7H 9 and η5-C 60Bn 2PhH 2, two ligands that are cyclopentadienyl mimicks

The potentials of a series of one-electron oxidation and reduction reactions have been determined for manganese group half-sandwich complexes of the tricarbadecaboranyl ligand PhC 3B 7H 9 and the penta-organo fullerene ligand C 60Bn 2PhH 2 (Bn = benzyl). The anodic processes were studied in CH 2Cl 2 and the cathodic processes were studied in both CH 2Cl 2 and THF, the supporting electrolyte being [NBu 4][B(C 6F 5) 4]. The manganese complex Mn(CO) 2(PMe 3)(PhC 3B 7H 9) ( 1) is a member of a three-electron transfer series which includes oxidation to 1 + (0.51 V versus ferrocene) and successive reductions to 1 − (−1.66 V) and 1 2− (−1.77 V). Both the oxidation and reduction of the closely-related complex Mn(CO) 2(PPh 3)(PhC 3B 7H 9) ( 2) are chemically irreversible under slow-scan cyclic voltammetry conditions. The rhenium complex Re(CO) 2(PPh 3)(PhC 3B 7H 9) ( 3) oxidizes ( E 1/2 = 0.82 V versus ferrocene) to a radical cation which, unlike its cyclopentadienyl analogue, shows no evidence of dimerization. Oxidation of the fullerene-based complex Re(CO) 3(C 60Bn 2PhH 2) is more facile than that of its cyclopentadienyl analogue, in contrast to previous findings in this class of metal-fullerene derivatives. An electrochemical ligand factor, E L, of 0.63 is calculated for the PhC 3B 7H 9 ligand in manganese group half-sandwich complexes.

Mineral processing and extraction of rare earth elements from the Wadi Khamal Nelsonite Ore, Northwestern Saudi Arabia

A technological sample (50 kg) from Wadi Khamal Nelsonite ore was subjected to magnetic and flotation concentration techniques. Excellent recovery percentages of 72.95% and 71.22% were achieved by the dry/wet magnetic and flotation concentration techniques, respectively. The weight of the apatite concentrate reached a reasonable percentage of approximately 23.5% with an overall 40.23% P2O5 total content. Analytical data of the apatite concentrate after digestion in concentrated sulfuric acid revealed that the total content of the rare earth elements (REE) constitutes about 0.2% of the total apatite content. The REE content (0.2%) was partitioned between phosphoric acid liquor (65%) and gypsum precipitate (36%). The extraction of the REEs from the phosphoric acid liquor using oxalic acid and sodium carbonate–bicarbonate mixture (1:10 w/w) yielded the RE oxide cake which constitute about 1.2% (w/w). The produced rare earth oxide cake contains traces of various metal oxides, e.g., SrO, Na2O, etc. in addition to rare earth oxides. Attempts to determine quantitatively the constituents of the cake will be considered in future work.

Experimental and Theoretical Investigation into the Formation and Reactivity of M(Cp)(CO)2(CO2) (M = Mn or Re) in Liquid and Supercritical CO2 and the Effect of Different CO2 Coordination Modes on Reaction Rates with CO, H2, and N2

Nanosecond time-resolved infrared spectroscopy (TRIR) has been used to study the coordination of CO₂ to the metal centers by UV photolysis of M(Cp)(CO)₃ (M = Mn or Re) in liquid or supercritical CO₂ (scCO₂) solution, which led to the formation of the CO₂ complexes M(Cp)(CO)₂(CO₂). Differences between the positions of the ν(C−O) IR bands of the CO ligands in Mn(Cp)(CO)₂(CO₂) and Re(Cp)(CO)₂(CO₂) suggest that the CO₂ ligand has different coordination modes to the metal centers in these complexes. The kinetic data and the IR spectra of the CO₂, Xe, and heptane complexes provided evidence that the CO₂ coordination mode is η¹-O end-on bound in Mn(Cp)(CO)₂(CO₂), and η²-C,O side-on bound in Re(Cp)(CO)₂(CO₂). These different coordination modes lead to dramatic differences in reactivity with CO, H₂, and N₂, with the Re complexes being significantly less reactive. To provide more evidence for the nature of the CO₂ binding modes, a series of DFT calculations were performed at the B3LYP/SDD-6-311G** level. The calculations supported the experimentally proposed CO₂ coordination modes. A significant charge transfer from Re to CO₂ occurs, resulting in partial oxidation of Re.

Theoretical study of ignition-proof mechanism of RE element in Mg alloys

The atomic cluster models of α-Mg, liquid Mg and the interface between liquid/solid have been founded by computer program. The environment-sensitive embedding energy of RE elements in α-Mg, liquid Mg, liquid/solid interface has been calculated by recursion method. The atomic affinity energy between Mg, La, Y with O has been defined and calculated. The calculated results show that the solid solubility of La and Y is very small in α-Mg, because their higher environment-sensitive embedding energy leads to instability in α-Mg crystal. The RE elements diffuse to the liquid Mg, which has lower environment-sensitive embedding energy than the solid, and congregate on the surface of liquid Mg as the alloy solidifies. Because the atomic affinity energy of RE-O is lower than Mg-O(The atomic affinity energy between Mg, La, Y with O are Mg-O：-14.9338eV，La-O：-19.0608 eV，Y-O：-19.5050 eV, respectively), the RE elements congregating on the surface of liquid Mg will priorily combined with O, forming compact RE oxides, which prevent Mg alloys from burning.

Enhanced O 2 solubility by RE 2O 3 (RE = La, Gd) additions in molten carbonate electrolytes for MCFC

The solubilization of O 2 reactant gas in mixtures of molten alkali carbonate, which are to be used as electrolytes in the Molten Carbonate Fuel Cells, has been investigated as a function of melt composition, temperature and O 2–CO 2 gas atmosphere. The baseline content of dissolved oxygen in (52 + 48) mol% Li 2CO 3 + Na 2CO 3 and (62 + 38) mol% Li 2CO 3 + K 2CO 3 is markedly increased by the additions of 0.5 mol% rare-earth (RE = La and Gd) oxides to the melts. By a curve-fitting method the mechanisms through which oxygen dissolves into the melts are predicted. At 923 K the oxygen dissolution mechanism passes from a superoxide path, which is dominant in the additive-free melts, to either a prevalent peroxide path in the Li–Na melt or a mixture of superoxide and peroxide ions in the Li–K electrolyte. At lower temperature (873 K) RE oxide additives are effective in modifying the oxygen dissolution mechanism only for the Li–Na melts.

Microstructure and properties of coatings with rare earth formed by DC-plasma jet surface metallurgy

A new method of surface modification, DC-plasma jet surface metallurgy, is proposed in order to improve wear resistance of materials. This paper reports on an experiment, in which some metallurgical coatings of iron-based alloy with different contents of La 2O 3 and CeO 2 were prepared on AISI1020 steel using a homemade DC-plasma jet surface metallurgy equipment. A scanning electron microscope (SEM), energy dispersive X-ray (EDX) microanalysis and X-ray diffraction analysis (XRD) were employed to observe the microstructure and analyzed the chemical compositions of coatings. The effect of RE oxide (La 2O 3 and CeO 2) on microstructure and properties of coatings was investigated. The result shows that addition of a proper amount of RE oxide (La 2O 3 and CeO 2) can refine and purify the microstructure, significantly increase microhardness and enhance the wear resistance.

A study of effect of particle size on the oxidation of rhenium in the Re/γ-Al 2O 3 catalysts

The oxidation of the reduced Re/γ-Al 2O 3 catalysts with dispersion of Re from 0.25 to 0.74 and metal particle sizes of 1–9 nm, was examined over a temperature range 20–500 °C. The structure of the catalysts was characterised using H 2 chemisorption, O 2 uptake, BET and XPS and Raman spectroscopy. The O 2 uptake and XPS results showed that very small Re particles were in direct and strong contact with the oxygen atoms of the support. As a result, the O 2 uptakes were low at room temperature and even at 500 °C were lower than expected for oxidation of Re 0 to R 2O 7 oxide. On the other hand, XPS data indicated that at room temperature clusters and small Re particles were oxidised to Re 4+, Re 6+ and Re 7+ species. The O/Re ratios at temperatures up to 200 °C increased with the Re dispersion what implies additionally, high affinity of the highly dispersed Re to oxygen. Fraction of rhenium strongly interacting with the support amounts to 6 or 19% of total Re in the catalysts with average particle sizes of about 5 or 1.5 nm, respectively. At higher temperatures the process of oxidation of rhenium accelerates, with instantaneous sublimation of Re 2O 7 oxide and in accord to Raman data, its simultaneous adsorption as the ReO 4 species on γ-alumina. The O 2 uptake at 500 °C by the supported Re phase depends on the size of rhenium particles. Small Re particles could be oxidised to an oxidation level lower than heptvalent, while large particles to Re 2O 7. XPS and Raman data showed, however, that after treatment at 500 °C, independent on the particle size of the Re and its interaction with the support, whole rhenium was oxidised to Re 7+ species. This species forms some kind of surface complex with an Al O ReO 3 or Al (O ReO 3) 3 structure, which inhibits the surface migration of rhenium and its loss from the oxidised catalysts.

XPS study of oxidation of rhenium metal on γ-Al 2O 3 support

The oxidation of Re/ γ-Al 2O 3 catalysts, containing 1 and 10 wt% of rhenium, sintered in hydrogen was examined in the temperature range of 20–800 °C. The structures of the catalysts were investigated by XPS spectroscopy, TEM, and O 2 uptake measurements. The low-loaded catalyst comprises metallic particles with sizes of 1–4 nm ( d av=2.1 nm), while the high-loaded catalyst comprises particles with sizes of 1–9 nm ( d av=4.9 nm). Even short exposure to air at room temperature causes complete oxidation of small clusters of metallic Re, while larger particles are covered with very thin ReO x skin (undetected by TEM). XPS shows that the high-loaded catalyst still contains 94.5% of metallic Re, while the low-loaded catalyst contains only 60.5%. The remaining part of the Re is oxidized to Re 4+, Re 6+, and Re 7+ species. Oxidation at 150 °C causes enhanced formation of Re 4+–Re 7+ species and the amount of metallic Re quickly decreases to 33 and 2% for high- and low-loaded catalysts, respectively. This indicates a high affinity of the highly dispersed Re to oxygen. At this temperature, the Re/Al atomic ratio increases 2–4 times, indicating a large spreading of the oxide species on the support surface. Simultaneously, the average size of Re particles decreased as determined by TEM. At 300 °C, whole Re was oxidized mainly to Re 2O 7, though some amount to Re 4+ and Re 6+ species remained. The O 2 uptake measurements confirm oxidation of rhenium particles. For the high-loaded catalyst O 2 uptake attained a maximum level (O/Re=3.3) already at 300 °C, while for the low-loaded catalyst even at 500 °C the uptake (O/Re=2.98) is below the maximum level. XPS data showed, however, that at 500 °C, oxidation of rhenium to Re 2O 7 occurs for both catalysts. The Re/Al atomic ratio remains nearly constant after oxidation of both catalysts at 300–800 °C, indicating that Re 7+ species are firmly bonded to alumina surfaces even at 800 °C. A detailed mechanism of oxidation of Re particles with different sizes is proposed based on a quantitative analysis of the XPS, O 2 uptake, TEM, and previous Raman results.

Isotopic Measurements of Re, Os, Ir, and Ru by Quadrupole Negative Thermal Ionization Mass Spectrometry

A negative thermal ionization technique using a small and cost-efficient quadrupole mass spectrometer is presented. Intense ion beams of negatively charged oxides of Re, Os, Ir, and Ru were observed, and the principal molecular species formed in this technique are ReO4-, OsO3-, IrO2-, and RuO3-, respectively. The production of these ion species in the distinct mass ranges allows the isotopic analyses without any chemical separation of these elements each other. For 5 ng Os, all Os isotopic ratios were determined with the precision of better than ±0.5% except 184Os/192Os because of very low isotopic abundance of 184Os. For 30 ng Re, 10 ng Ir, and 30 ng Ru, isotopic measurements were also made with the precision of better than ±0.5%. The precision of the isotopic ratio measurements for Re, Os, Ir, and Ru is sufficient for the isotope dilution technique to determine the concentration of these elements in geological samples easily and cost-efficiently. In addition, an iridosmine from Hokkaido was analyzed for Os and Ir by direct loading technique, and 187Os/188Os ratio of 0.1229±0.0029 was obtained.

Oxidation and redispersion of a low-loaded Re/ γ-Al 2O 3 catalyst

The interaction of sintered low-loaded Re/ γ-Al 2O 3 catalysts with oxygen has been studied over a wide temperature range, 20–800 °C. The structure of the catalyst was characterized using different techniques: H 2 chemisorption, O 2 uptake, BET, TEM, and also by Raman and XPS spectroscopy. In the catalyst reduced in H 2 at 800 °C, Re is present as a nonuniform phase consisting of metallic particles with sizes of 1–4 nm and a certain amount of very small clusters (below 1 nm) undetectable by TEM. Oxidation at room temperature causes dissociative chemisorption of oxygen on rhenium with possible formation of a superficial oxide on the Re particles and complete oxidation of the clusters to Re 4+–Re 7+ species. After oxidation at 150 °C only 2% of total Re remains in the reduced state which implies high affinity of highly dispersed Re to oxygen. At 200–300 °C, oxidation of Re hastens giving a mixture of Re 4+, Re 6+, and Re 7+ ions, while at 500 °C, complete oxidation to Re 2O 7 occurs. At elevated temperatures, Re 2O 7 sublimates and instantaneously adsorbs on γ-alumina surface as monomeric ReO 4 species. At 500–800 °C, these species are strongly bound to alumina forming a kind of a surface compound with an Al–O–ReO 3 structure. This accounts for the low loss of Re observed even at 800 °C. Redispersion of rhenium was observed after a mild H 2 treatment of the oxidized rhenium catalyst.