Effect of Y2O3 and Er2O3 co-dopants on phase stabilization of bismuth oxide

Bi₂O₃ compositions were prepared to investigate the effect of rare earth metal oxides as co-dopants on phase stability of bismuth oxide. Compositions containing 9-14 mol% of Y₂O₃ and Er₂O₃ were synthesized by solid state reaction. The structural characterization was carried out using X-ray powder diffraction. The XRD results show that the samples containing 12 and 14 mol% total dopants had cubic structure, whereas the samples with lower dopant concentrations were tetragonal. Comparing the lattice parameters of the cubic phases of (Bi₂O₃)_0.88(Y₂O₃)_0.06(Er₂O₃)_0.06 and (Bi₂O₃)_0.86(Y₂O₃)_0.07(Er₂O₃)_0.07 revealed that lattice parameter decreases by increasing the dopant concentration. The XRD pattern and the powder density results indicated the formation of solid solution in the studied systems. After annealing samples with cubic phase at 600 °C for various periods of time, phase transformation to tetragonal and rhombohedral occurs.     

Dehydration of 1,4-butanediol over supported rare earth oxide catalysts

Vapor-phase catalytic dehydration of 1,4-butanediol was investigated over rare earth oxides supported on ZrO₂. In the dehydration of 1,4-butanediol, 3-buten-1-ol was mainly produced, together with tetrahydrofuran (THF) and γ-butyrolactone. The heavy group of rare earth oxides, such as Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃, supported on monoclinic ZrO₂ showed higher selectivity to 3-buten-1-ol than pure monoclinic ZrO₂ and supported light rare earth oxides, such as La₂O₃ and Pr₆O₁₁. Supported Yb₂O₃ catalysts dispersed on other oxides, such as alumina, silica, and tetragonal ZrO₂ catalyze the formation of THF. X-ray diffraction (XRD) measurements reveal that cubic Yb₂O₃ crystallites dispersed on monoclinic ZrO₂ provide active sites in the dehydration of 1,4-butanediol to produce 3-buten-1-ol.     

Electrochemical synthesis and characterization of erbium oxide

Erbium oxide (Er₂O₃) has been used in a variety of microelectronic, optoelectronic, thermophotovoltaic, and biomedical applications, and especially in nuclear reactor systems. Electrochemical synthesis of rare earth oxides has conventionally been based upon base generation under the application of anodic or cathodic potential and subsequent sintering of as-deposited rare earth hydroxides. In this study, we investigated a direct, room-temperature electrochemical synthesis of Er₂O₃ onto titanium base metal by applying cathodic potentials. Iminodiacetate (IDA) ligand was added to form Er(IDA)₂⁻ complexes with Er³⁺ in a neutral electrolyte. A cathodic reaction for the direct deposition of Er₂O₃ from Er(IDA)₂⁻ was suggested as the mechanism of Er₂O₃ synthesis. The formation of cubic Er₂O₃ phase at all applied potentials was verified by means of X-ray diffractometry and X-ray photoelectron spectroscopy. The relationship between the diffusion characteristics of the reacting ions and the resulting microstructures of Er₂O₃ deposits was also studied as a function of applied potential.     

Electrochemical noise analysis of O2 evolution on PbO2 and PbO2-matrix composites containing Co or Ru oxides

Electrochemical noise measurements have been performed on various electrode materials during oxygen evolution reaction. Fluctuations of both electrode potential and electrolyte resistance, recorded under galvanostatic polarization, have been analysed in order to separate ohmic and non-ohmic contributions. The anode materials compared in this study were PbO₂ and composite oxides consisting of a PbO₂ matrix and variable amounts of catalytic dispersed phases (Co₃O₄, RuO₂ and hydrous Co oxides); the composite oxide layers were generally obtained by anodic codeposition of fine particles (typically 0.1-1 μm in size) with a PbO₂ matrix electrochemically grown by Pb²⁺ oxidation. The power spectral density (PSD) of the electrolyte resistance fluctuations is very similar for PbO₂ and composite electrodes, suggesting a similar gas evolution profile. On the contrary, the potential PSD is markedly different for PbO₂ and composites containing catalytic particles. In the latter case, the potential fluctuations are entirely due to ohmic effects, while non-ohmic components are clearly dominant in the case of O₂ evolution on pure PbO₂, probably because of the much higher activation overpotential. Furthermore, on PbO₂ electrodes a plateau was obtained at intermediate frequencies (0.1-10 Hz), which was tentatively explained by bubble coalescence phenomena.     

Selective electrodeposition of PbO2 on anodised-polycrystalline titanium

Electrochemical experiments on titanium electrodes were coupled with electron backscattered diffraction (EBSD) experiments. The substrates were thermally treated and electropolished in order to have flat and reproducible polycrystalline surfaces, leading to EBSD orientation mapping. Afterwards, the samples were anodised by a galvanostatic procedure. It was shown that electrodeposition of PbO₂ from a 0.5 M Pb(NO₃)₂+2.5 M HNO₃ solution occurs selectively on the near {0 0 0 1} grains, whereas lead electrodeposition occurs on all the grains, whatever their orientation. These results are discussed, taking into account the fact that on {0 0 0 1} grains, the oxide layers are thinner than on other grains. It was concluded that electrodeposition is observed locally on Ti/TiO₂ electrodes for (i) cathodic electrodeposition of metals at low overvoltage; (ii) anodic electrodeposition of PbO₂, in potentiostatic or galvanostatic conditions.     

Structure and properties of PbO2-CeO2 anodes on stainless steel

The lack of ideal anodes with excellent activity and stability is one of the critical problems in electrochemical oxidation for organic wastewater treatment. It is reported in this paper that the PbO₂-CeO₂ films electrodeposited on stainless steel were used as catalytic electrodes for treating antibiotic wastewater. The PbO₂-CeO₂ films on stainless steel were proved to be high stability, good activity and relatively low cost. Because of these properties, the films are more attractive than any other electrocatalytic materials among conventional dimensionally stable anodes (DSA). Experimental results showed that the PbO₂-CeO₂ electrode has a service life of 1100 h in 3 M H₂SO₄ solution under a current density of 1 A cm⁻² at 35 °C, compared with 300 h for PbO₂ under the same conditions. The X-ray diffraction (XRD) patterns and SEM images indicated that the PbO₂-CeO₂ films on stainless steel have a dense structure and the preferred crystalline orientation on the substrate surface was changed. Color and chemical oxygen demand (COD) of antibiotics wastewater were studied by electrolysis by using these electrodes as anode and stainless steel as cathode. The results indicated that the anodes have excellent activity in antibiotic wastewater treatment. The PbO₂-CeO₂ electrodes have high chemical stability which contributed by the superstable nature of the electrode, dense microstructure, good conductivity and the improvement of bonding with the stainless steel during electrodeposition.     

Phase stability and electric conductivity of Er2O3-Nb2O5 co-doped Bi2O3 electrolyte

Two oxides, Er₂O₃ and Nb₂O₃, are used to stabilize delta-phase Bi₂O₃ used as electrolyte of solid oxide fuel cell. Optimization of dopant ratio and total doping concentration (TDC) is determined by X-ray diffraction, and successfully reduce the TDC (Er + Nb) to 10-15 mol.%. Conductivities of different compositions are measured by two-probe method. The results show that highest conductivity appears at the minimum doping concentrations. Phase stability of ENSB samples with Er/Nb ratio of 2/1 and TDC of 10-20 mol.% at 650 °C up to 300 h is analyzed showing two newly formed (alpha- and gamma-) phases in the samples. Degradation of conductivity at 650 °C is studied in detail by DTA and TEM. The abnormity of lattice contraction of delta-phase is discussed.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Deactivation and regeneration of Pt anodes for the electro-oxidation of phenol

Electro-oxidation tests with different electrolytes (Na₂SO₄, NaCl, H₂SO₄) and anode types (Pt, Ti lined with Ir and Ta oxides, PbO₂, activated carbon) were performed on aqueous solutions containing phenol to assess the mechanism and nature of electrode deactivation phenomena. For the Pt electrode, the nature of the electro-deposited organic species was investigated by ATR-FTIR and FESEM-EDS analyses, which showed adsorption of intermediate oxidation products (e.g. benzoquinone, hydroquinone) is likely responsible for the early deactivation stages. Conversely, in the longer term, formation of polymeric films is promoted. Potentiostatic tests showed that anode regeneration can be achieved by anodic polarisation above 1.1 V (vs Hg/Hg₂SO₄). This reactivation was found to be easier in the presence of significant amounts of chloride ions. Conversely, the deactivated state is maintained for the Ti/IrO₂/Ta₂O₅ electrode even though anodic polarisation at high positive potentials is applied. Cyclic voltammetric curves on PbO₂ electrodes did not provide satisfactory results as the intensity of the lead-dioxide reduction peak was so high that peaks for phenol oxidation were hardly detectable. Finally, the activated carbon based electrode was found to be promising as it enables simultaneous adsorption of the organic pollutant and oxidation of the pollutant itself to constitute a sort of self-regenerating adsorber unit.     

Structure and electrical properties of Ca0.28Ba0.72Nb2O6 ceramics with addition of rare earth oxides (CeO2, La2O3)

Ca_0.28Ba_0.72Nb₂O₆ (CBN28) ceramics with addition of CeO₂ and La₂O₃, were prepared by the conventional ceramic fabrication technique. XRD results showed that the single tungsten bronze structure of CBN28 was not changed by adding CeO₂ or La₂O₃. SEM results indicated that both CeO₂ and La₂O₃ dopants were effective in inhibiting the grain growth and suppressing the anisotropic growth behavior in tungsten bronze structure. It was also found that both two kinds of dopants had remarkable effects on the dielectric and ferroelectric properties of CBN28 ceramics. Compared with CBN28 ceramics, the dielectric constant around room temperature ε_r, dielectric loss tan δ, the degree of diffuseness γ and coercive field E_c were all ameliorated when doping proper amount of CeO₂ or La₂O₃. The comprehensive electric performance was obtained in CBN28–0.3 wt% CeO₂ and CBN28–0.4 wt% La₂O₃ ceramics. Besides, the underlying mechanism for variations of the electrical properties due to different dopants was explained in this work.     

Electrochemical reduction behavior of U3O8 powder in a LiCl molten salt

The reduction path of the U₃O₈ powder vol-oxidized at 1200 °C has been determined by a series of electrochemical experiments in a 1 wt.% Li₂O/LiCl molten salt. Various reaction intermediates are observed by during electrolysis of U₃O₈. The formation of the metallic uranium is caused from two different reduction paths, a direct reduction of uranium oxide and an electro-lithiothermic reduction. As the uranium oxide is converted to the metallic uranium, the lithium metal is more actively formed in the cathode basket. The reducibility of the rare earth oxides with the U₃O₈ powder has been tested by constant voltage electrolysis. The results suggest the advanced vol-oxidation could lead to the enhancement in the reducibility of the rare earth fission products.     

Enhanced electrochemical performance and thermal stability of La2O3-coated LiCoO2

An enhanced electrochemical performance LiCoO₂ cathode was synthesized by coating with various wt.% of La₂O₃ to the LiCoO₂ particle surfaces by a polymeric method, followed by calcination at 923 K for 4 h in air. The surface-coated materials were characterized by XRD, TGA, SEM, TEM, BET and XPS/ESCA techniques. XRD patterns of La₂O₃-coated LiCoO₂ revealed that the coating did not affect the crystal structure, α-NaFeO₂, of the cathode material compared to pristine LiCoO₂. TEM images showed a compact coating layer on the surface of the core material that had an average thickness of about ∼15 nm. XPS data illustrated that the presence of two different environmental O 1s ions corresponds to the surface-coated La₂O₃ and core material. The electrochemical performance of the coated materials by galvanostatic cycling studies suggest that 2.0 wt.% coated La₂O₃ on LiCoO₂ improved cycle stability (284 cycles) by a factor of ∼7 times over the pristine LiCoO₂ cathode material and also demonstrated excellent cell cycle stability when charged at high voltages (4.4, 4.5 and 4.6 V). Impedance spectroscopy demonstrated that the enhanced performance of the coated materials is attributed to slower impedance growth during the charge-discharge processes. The DSC curve revealed that the exothermic peak corresponding to the release of oxygen at ∼464 K was significantly smaller for the La₂O₃-coated cathode material and recognized its high thermal stability.     

The effect of rare earth oxides on the pressureless liquid phase sintering of α-SiC

Silicon carbide (SiC) ceramics have been fabricated by pressureless liquid phase sintering with Al₂O₃ and rare-earth oxides (Lu₂O₃, Er₂O₃ and CeO₂) as sintering additives. The effect was investigated of the different types of rare earth oxides on the mechanical property, thermal conductivity and microstructure of pressureless liquid phase sintered SiC ceramics. The room temperature mechanical properties of the ceramics were affected by the type of rare earth oxides. The high temperature performances of the ceramics were influenced by the triple junction grain boundary phases. With well crystallized triple junction grain boundary phase, the SiC ceramic with Al₂O₃–Lu₂O₃ as sintering additive showed good high temperature (1300 °C) performance. With clean SiC grain boundary, the SiC ceramic with Al₂O₃–CeO₂ as sintering additive showed good room temperature thermal conductivity. By using appropriate rare earth oxide, targeted tailoring of the demanding properties of pressureless liquid phase sintered SiC ceramics can be achieved.     

Effect of rare earth oxides on the microstructure and properties of mullite/hBN composites

Mullite/hBN composites were fabricated with different rare earth oxides additives (ReO: Er₂O₃, CeO₂, La₂O₃, Lu₂O₃) by pressureless sintering at 1600 °C for 4 h. The impacts of ReO on the phase composition, microstructure, mechanical, dielectric and tribological properties of the composites were investigated. XRD results showed that all the ReO additives were beneficial to the formation of mullite phase. With the decrease in the ionic radius of the ReO, the mullite grains of the composite were refined while their mechanical properties were increased. The sample sintered with Lu₂O₃ showed the smallest grain size and the most excellent mechanical properties, e.g., its relative density, flexure strength, fracture toughness and hardness reached 93.7%, 222 MPa, 3.2 MPa m^1/2 and 6.02 GPa, respectively. Due to the different porosity and phase composition of the composites, the sample sintered with La₂O₃ had the lowest dielectric constant while the sample sintered with Er₂O₃ exhibited the lowest dielectric loss. Attributing to the highest hardness and fracture toughness, the sample sintered with Lu₂O₃ showed the best tribological properties.     

Effect of La-doping on the properties of CaCu3Ti4O12 dielectric ceramics

Nano-size Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ (χ = 0.00, 0.05, 0.10, 0.15 and 0.20) precursor powders were prepared via the sol–gel method and the citrate auto-ignition route and then processed into micro-crystal Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics under heat treatment. Characterization of the as-obtained ceramics with XRD and SEM showed an average grain sizes of ∼1–2 μm, indicating La³⁺ amount to have little impact on grain size. The room-temperature dielectric constant of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics sintered at 1000 °C was of the order of 10³–10⁴ despite the variation of χ values. Compared with CaCu₃Ti₄O₁₂, La³⁺-doped CaCu₃Ti₄O₁₂ showed a flatter dielectric constant curve related to frequency. It was found that the loss tangent of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics was less than 0.20 in ∼600–10⁵ Hz region, which rapidly decreased to a minimum value of 0.03 by La³⁺doping with χ = 0.05. Our measurement of the ceramics conductivities (σ) also indicated that the appropriate introduction of La³⁺ into CaCu₃Ti₄O₁₂ would distinctly result in its dielectric properties.     

Effects of promoters on catalytic activity and carbon deposition of Ni/γ‐Al2O3 catalysts in CO2 reforming of CH4

Catalytic reforming of methane with carbon dioxide was studied in a fixed‐bed reactor using unpromoted and promoted Ni/γ‐Al₂O₃ catalysts. The effects of promoters, such as alkali metal oxide (Na₂O), alkaline‐earth metal oxides (MgO, CaO) and rare‐earth metal oxides (La₂O₃, CeO₂), on the catalytic activity and stability in terms of coking resistance and coke reactivity were systematically examined. CaO‐, La₂O₃‐ and CeO₂‐promoted Ni/γ‐Al₂O₃ catalysts exhibited higher stability whereas MgO‐ and Na₂O‐promoted catalysts demonstrated lower activity and significant deactivation. Metal‐oxide promoters (Na₂O, MgO, La₂O₃, and CeO₂) suppressed the carbon deposition, primarily due to the enhanced basicities of the supports and highly reactive carbon species formed during the reaction. In contrast, CaO increased the carbon deposition; however, it promoted the carbon reactivity. © 2000 Society of Chemical Industry     

Fabrication and Upconversion Luminescent Properties of Er3+‐Doped and Er3+/Yb3+ Codoped La2O2CN2 Nanofibers

La₂O₂CN₂:Er³⁺and La₂O₂CN₂:Er³⁺/Yb³⁺ upconversion (UC) luminescence nanofibers were successfully fabricated via cyanamidation of the respective relevant La₂O₃:Er³⁺ and La₂O₃:Er³⁺/Yb³⁺ nanofibers which were obtained by calcining the electrospun composite nanofibers. The morphologies, structures, and properties of the nanofibers are investigated. The mean diameters of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are 179.46 ± 12.58 nm and 198.85 ± 17.07 nm, respectively. It is found that intense green and weak red emissions around 524, 542, and 658 nm corresponding to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_l5/2 energy levels transitions of Er³⁺ ions are observed for La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers under the excitation of a 980‐nm diode laser. Moreover, the emitting colors of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are all located in the green region. The upconversion luminescent mechanism and formation mechanism of the nanofibers are also proposed.     

Hot corrosion behaviors of gas tunnel type plasma sprayed La2Zr2O7 thermal barrier coatings

Gas tunnel type plasma sprayed free-standing La₂Zr₂O₇ coating specimens with a thickness of 300-400 μm were prepared under optimized operating conditions and were subjected to hot corrosion test in the presence of corrosive impurities such as V₂O₅, Na₂SO₄, and Na₂SO₄ + V₂O₅ mixtures (60:40 wt%) at two different temperatures for duration of 5 h, i.e. 1000 and 1350 K for V₂O₅ and Na₂SO₄ + V₂O₅ mixtures, 1200 and 1350 K for Na₂SO₄. For temperatures at 1350 K, the reaction mechanism of V₂O₅ and the mixture of Na₂SO₄ + V₂O₅ are similar and LaVO₄ is formed as the corrosive product, which leads to massive phase transformation from pyrochlore to tetragonal and monoclinic phases. Microstructural observations from planar reaction zone (PRZ) and melt infiltrated reaction zone (MIRZ) reveals that the present La₂Zr₂O₇ coating exhibits good hot corrosion resistance in V₂O₅ environment and moderate for the mixture of Na₂SO₄ + V₂O₅, but is worst in Na₂SO₄ environment.     

The characterisation of PbO2-coated electrodes prepared from aqueous methanesulfonic acid under controlled deposition conditions

A series of PbO₂ coatings on planar carbon substrates has been prepared by anodic deposition in aqueous methanesulfonic acid (MSA) under galvanostatic conditions. The effect of four experimental parameters, i.e., lead(II) methanesulfonate and MSA concentrations, current density, and temperature was analysed. Surface characterisation by XRD, SEM-EDX, and AFM has provided information about the structural (phase distribution, degree of crystallinity, and crystallite size), morphological (crystallite shape, degree of porosity), and tribological (surface roughness) properties of the PbO₂ coatings, respectively. Electrochemical studies based on linear and cyclic voltammetry allowed comparison between electrodes prepared in MSA and classical electrodes prepared in HNO₃. Pure α- or β-PbO₂ and α + β mixtures were obtained depending on the conditions, being temperature the most influential deposition parameter. A temperature rise caused a transition to pure β-PbO₂ and led to a higher degree of crystallinity with a progressive increase of crystallite size, always within the range of 10-30 nm, as well as to a remarkably higher roughness, from smooth (35-50 nm rms) to rough (up to 500 nm rms) surfaces. Low MSA and high lead(II) methanesulfonate concentrations were required to avoid the formation of excessively porous powdery coatings, as well as cracks, pits, and holes. Most of the coatings obtained in MSA were uniform, nanocrystalline, and moderately rough. Their electrocatalytic behaviour was comparable to that of the electrodes prepared in HNO₃, showing an O₂-overpotential of +0.66 V in 0.05 M Na₂SO₄ at pH 3.0. Such coatings can then be envisaged as suitable anodes for energy and water treatment applications. Prolonged electrolysis has shown their stability against leaching.     

Electrochemical incineration of chloranilic acid using Ti/IrO2, Pb/PbO2 and Si/BDD electrodes

The electrochemical oxidation of chloranilic acid (CAA) has been studied in acidic media at Pb/PbO₂, boron-doped diamond (Si/BDD) and Ti/IrO₂ electrodes by bulk electrolysis experiments under galvanostatic control. The obtained results have clearly shown that the electrode material is an important parameter for the optimization of such processes, deciding of their mechanism and of the oxidation products. It has been observed that the oxidation of CAA generates several intermediates eventually leading to its complete mineralization. Different current efficiencies were obtained at Pb/PbO₂ and BDD, depending on the applied current density in the range from 6.3 to 50 mA cm⁻². Also the effect of the temperature on Pb/PbO₂ and BDD electrodes was studied.UV spectrometric measurements were carried out at all anodic materials, with applied current density of 25 and 50 mA cm⁻². These results showed a faster CAA elimination at the BDD electrode. Finally, a mechanism for the electrochemical oxidation of CAA has been proposed according to the results obtained with the HPLC technique.