Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

Probing the catalytic activity of M-N4−xOx embedded graphene for the oxygen reduction reaction by density functional theory

In this work, the detailed oxygen reduction reaction (ORR) catalytic performance of M-N_4−xO_x (M= Fe, Co, and Ni; x = 1–4) has been explored via the detailed density functional theory method. The results suggest that the formation energy of M-N_4−xO_x shows a good linear relationship with the number of doped O atoms. The adsorption manner of O₂ on M-N_4−xO_x changed from end-on (x = 1 and 2) to side-on (x = 3 and 4), and the adsorption strength gradually increased. Based on the results for binding strength of ORR intermediates and the Gibbs free energy of ORR steps on the studied catalysts, we screened out two highly active ORR catalysts, namely Co-N₃O₁ and Ni-N₂O₂, which possess very small overpotentials of 0.27 and 0.32 V, respectively. Such activities are higher than the precious Pt catalyst. Electronic structure analysis reveals one of the reasons for the higher activity of Co-N₃O₁ and Ni-N₂O₂ is that they have small energy gaps and moderate highest occupied molecular orbital energy levels. Furthermore, the results of the density of states reveal that the O doping can improve the electronic structure of the original catalyst to tune the adsorption of the ORR intermediates.     

Regeneration of thermally aged Pt-Rh/CexZr1−xO2-Al2O3 model three-way catalysts by oxychlorination treatments

Pt-Rh/Ce_xZr_1−xO₂-Al₂O₃ with 0.6 and 1.0 wt.% noble metal loadings were prepared and characterized for their metal dispersion with respect to Ce_xZr_1−xO₂-free Pt-Rh/Al₂O₃ in fresh, thermally aged and oxychlorinated states. Thermal ageing at 973 K led to loss of metal dispersion in all cases but to negligible effect on the dispersion of the Ce_xZr_1−xO₂ component where present. Oxychlorination was able to fully recover metal dispersion in all cases but led to different effects on the redox properties of Ce_xZr_1−xO₂ which appeared to be related to the metal loadings. Despite showing improved dispersion following regeneration, higher loaded catalyst showed no improvement in light-off performance for either NO reduction or CO oxidation and showed poorer oxygen storage (OSC) ability, particularly at higher temperatures. Lower loaded catalyst showed improved dispersion, improved OSC and reduced light-off temperatures for NO reduction and CO oxidation after oxychlorination compared to that in the thermally aged state.     

Electronic and structural properties of SnxTi1−xO2 solid solutions: a periodic DFT study

The structural and electronic properties of selected compositions of Sn_xTi_1−xO₂ solid solutions (x=0, 1/24, 1/16, 1/12, 1/8, 1/6, 1/4, 1/2, 3/4, 5/6, 7/8, 11/12, 15/16, 23/24 and 1) were investigated by means of periodic density functional theory (DFT) calculations at B3LYP level. The calculations show that the corresponding lattice parameters vary non-linearly with composition, supporting positive deviations from Vegard’s law in the Sn_xTi_1−xO₂ system. Our results also account for the fact that chemical decomposition in Sn_xTi_1−xO₂ system is dominated by composition fluctuations along [0 0 1] direction. A nearly continuous evolution of the direct band gap and the Fermi level with the growing value of x is predicted. Ti 3d states dominate the lower portion of the conduction band of Sn_xTi_1−xO₂ solid solutions. Sn substitution for Ti in TiO₂ increases the oxidation–reduction potential of the oxide as well as it renders the lowest energy transition to be indirect. These two effects can be the key factors controlling the rate for the photogenerated electron–hole recombination. These theoretical results are capable to explain the enhancement of photoactivity in Sn_xTi_1−xO₂ solid solutions.     

The homogeneity range and the microwave dielectric properties of the BaZn2Ti4O11 ceramics

Ceramics with a composition close to BaZn₂Ti₄O₁₁ were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn₂Ti₄O₁₁ was formed in the homogeneity range corresponding to the formula BaZn_2 − xTi₄O_11 − x (0 < x < 0.1). Densely sintered BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τ_f = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn₂Ti₄O₁₁-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO₃, hollandite-type solid solutions (Ba_xZn_xTi_8 − xO₁₆) and BaTi₄O₉ caused a decrease in Q × f values.     

Fabrication and thermoelectric properties of heavily rare-earth metal-doped SrO(SrTiO3)n (n = 1, 2) ceramics

To clarify the effect of substitutional electron doping on the thermoelectric figure of merit (ZT = S²σTκ⁻¹) of Ruddlesden–Popper phase SrO(SrTiO₃)_n (or Sr_n+1Ti_nO_3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ), of (Sr_1−xRE_x)_n+1Ti_nO_3n+1 (n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr_1−xRE_x)_n+1 Ti_nO_3n+1 evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the σ values simply depended on the dopant concentration, indicating that both La³⁺ and Nd³⁺ ions act as electron donors. The |S| values increased with temperature due to decrease in the chemical potential. Significant reduction of the κ values was observed as compared to cubic-perovskite SrTiO₃. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr_0.95La_0.05)₃Ti₂O₇.     

Synthesis, characterization and catalytic activity for NO–CO reaction of Pd–(La, Sr)2MnO4 system

La_xSr_2−xMnO₄ (0 ≤ x ≤ 0.8) oxides were synthesized and single-phase K₂NiF₄-type oxides were obtained in the range of 0.1 ≤ x < 0.5. The catalytic activity of La_xSr_2−xMnO₄ for NO–CO reaction increased with increasing x in the range of solubility limit of La. La_0.5Sr_1.5MnO₄ showed the highest activity among La_xSr_2−xMnO₄ prepared in this study, but its activity was inferior to perovskite-type La_0.5Sr_0.5MnO₃. Among the Pd-loaded catalysts, however, Pd/La_0.8Sr_1.2MnO₄ showed the higher activity and the selectivity to N₂ than Pd/La_0.5Sr_0.5MnO₃ and Pd/γ-Al₂O₃. The excellent catalytic performance of Pd/La_0.2Sr_1.2MnO₄ could be ascribable to the formation of SrPd₃O₄ which was detected by XRD in the catalyst but not in the other two catalysts.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Copper doped nickel aluminate: Synthesis,characterisation, optical and colour properties

Novel environment friendly inorganic nano green pigments, Ni_1-xCu_xAl₂O₄ (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) were successfully synthesised by simple, cost effective sol-gel method using citric acid as a gelling agent. Synthesised nano pigments were characterised by powder XRD, FT-IR, UV-DRS, SEM-EDX and TEM. Distribution of elements such as Ni, Cu, Al and O for the pigment Ni_0.98Cu_0.02Al₂O₄ was authenticated by elemental mapping analysis. The colour parameters were studied using CIE-LAB parameters. It is evident from the DRS measurement that the band gap energy of NiAl₂O₄ (3.11 eV) has been massively diminished to 2.63 eV when x=0.02, unexpectedly changed the colour of the pigment from cyan to green. Whilst x=0.1 the pigment colour has turned into grey and the corresponding band gap condensed into 2.17 eV. Effect of mineralisers like NaF, CaF₂, NH₄H₂PO₄ and Li₂CO₃ on the colour of Ni_0.98Cu_0.02Al₂O₄ was investigated.     

Superior performance of Ir-substituted hexaaluminate catalysts for N2O decomposition

Novel Ir-substituted hexaaluminate catalysts were developed for the first time and used for catalytic decomposition of high concentration of N₂O. The catalysts were prepared by one-pot precipitation and characterized by X-ray diffraction (XRD), N₂-adsorption, scanning electronic microscopy (SEM) and temperature-programmed reduction (H₂-TPR). The XRD results showed that only a limited amount of iridium was incorporated into the hexaaluminate lattice by substituting Al³⁺ to form BaIr_xFe_1−xAl₁₁O₁₉ after being calcined at 1200 °C, while the other part of iridium existed as IrO₂ phase. The activity tests for high concentration (30%, v/v) of N₂O decomposition demonstrated that the BaIr_xFe_1−xAl₁₁O₁₉ hexaaluminates exhibited much higher activities and stabilities than the Ir/Al₂O₃-1200, and the pre-reduction with H₂ was essential for activating the catalysts. By comparing BaIr_xFe_1−xAl₁₁O₁₉ with BaIr_xAl_12−xO₁₉ (x = 0–0.8), it was found that iridium was the active component in the N₂O decomposition and the framework iridium was more active than the large IrO₂ particles. On the other hand, Fe facilitated the formation of hexaaluminate as well as the incorporation of iridium into the framework.     

Barium-doped Pr2Ni0.6Cu0.4O4+δ with triple conducting characteristics as cathode for intermediate temperature proton conducting solid oxide fuel cell

Proton conducting solid oxide fuel cell (H-SOFC) is an emerging energy conversion device, with lower activation energy and higher energy utilization efficiency. However, the deficiency of highly active cathode materials still remains a major challenge for the development of H-SOFC. Therefore, in this work, K₂NiF₄-type cathode materials Pr_2-xBaNi_0.6Cu_0.4O_4+δ (x=0, 0.1, 0.2, 0.3), single-phase triple-conducting (e^-/O^2-/H⁺) oxides, are prepared for intermediate temperature H-SOFCs and exhibit good oxygen reduction reaction activity. The investigation demonstrates that doping Ba into Pr_2-xBaNi_0.6Cu_0.4O_4+δ can increase its electrochemical performance through enhancing electrical conductivity, oxygen vacancy concentration and proton conductivity. EIS tests are carried at 750℃ and the minimum polarization impedances are obtained when x=0.2, which are 0.068 Ω·cm² in air and 1.336 Ω·cm² in wet argon, respectively. The peak power density of the cell with Pr_1.8Ba_0.2Ni_0.6Cu_0.4O_4+δ cathode is 298 mW·cm^-2 at 750℃ in air with humidified hydrogen as fuel. Based on the above results, Ba-doped Pr_2-xBaNi_0.6Cu_0.4O_4+δ can be a good candidate material for SOFC cathode applications.     

Oxidation of unsymmetrical dimethylhydrazine over heterogeneous catalysts Solution of environmental problems of production, storage and disposal of highly toxic rocket fuels

The catalytic oxidation of unsymmetrical dimethylhydrazine (UDMH) by air has been studied in a vibro-fluidized catalyst bed laboratory kinetic setup over catalysts Cu_xMg_1−xCr₂O₄/γ-Al₂O₃, 32.9%Ir/γ-Al₂O₃ and β-Si₃N₄ in a temperature range 150–400 °C. The catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ was found to be optimal regarding high yields of CO₂ and low yields of NO_x. A probable mechanism of UDMH heterogeneous catalytic oxidation is proposed. Catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ has been further used in the pilot plant specially designed for the destruction of UDMH. Results of testing the main fluidized bed catalytic reactor for UDMH oxidation and the reactor for selective catalytic reduction of NO_x with NH₃ are presented. These results prove that the developed UDMH destruction technology is highly efficient and environmentally safe.     

XPS studies on the oxygen species of LaMn1−xCuxO3+λ

The electronic states of LaMn_1−xCu_xO_3+λ (x=0–0.4) have been studied with X-ray photoelectron spectroscopy (XPS). The valence states of substituted copper ions were Cu²⁺ and the manganese ions were a highly mixed state of Mn³⁺ and Mn⁴⁺. The nonstoichiometry and electronic state of lattice oxygen have been studied. The samples at x=0 and 0.1 had an excess of lattice oxygen but those at x=0.2–0.4 had lattice oxygen deficiency. A modified Auger parameter (Δ′) was used to evaluate the electronic states of oxygen ions. The Δ′ of lattice oxygen increased with increasing substitute quantity. This increase of Δ′ reflected the decrease of ionic bond character of lattice oxygen. The adsorbed oxygen species on LaMn_1−xCu_xO_3+λ was assigned mainly as O⁻ from the peak positions of spectra for the O 1s and O KLL levels, and the Δ′ of this O⁻ decreased with x. This decrease, i.e., the increase of ionic bond character of adsorbed oxygen was correlated well with the value of nonstoichiometry of lattice oxygen.

The rate of CO oxidation at 448 K was increased by the substitution till x=0.4. We consider that this enhancement of reactivity comes from the change of electronic state of adsorbed oxygen, O⁻ itself, i.e., a weak interaction between O⁻ and low coordinated metal site brings about a high reactivity.     

Preparation of CexZr1−xO2 (x = 0.75, 0.62) solid solution and its application in Pd-only three-way catalysts

Nanoparticles of Ce_xZr_1−xO₂ (x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H₂O₂ as an oxidant, and characterized by N₂ adsorption, XRD and H₂-TPR. Ce_xZr_1−xO₂ prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce_xZr_1−xO₂ increased, but thermal stability of Ce_xZr_1−xO₂ decreased. The surface area of Ce_0.62Zr_0.38O₂ was 41.2 m²/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO₂, and the reduction properties of Ce_xZr_1−xO₂ were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce_0.75Zr_0.25O₂ was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C₃H₈ was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C₃H₈ was induced by the steam reform and dissociation adsorption reaction of C₃H₈. Pd-only catalyst using Ce_0.75Zr_0.25O₂ as active washcoat showed high light off activity, the reaction temperatures (T₅₀) of 50% conversion of CO, C₃H₈ and NO were 180, 200 and 205 °C, respectively. However, the conversions of C₃H₈ and NO showed oscillation with continuously increasing the reaction temperature. The presence of La₂O₃ in washcoat decreased the light off activity and suppressed the oscillation of C₃H₈ and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La₂O₃ in active washcoat can enhance the thermal stability of catalyst significantly.     

Methane combustion on Mg-doped LaMnO3 perovskite catalysts

LaMn_1−xMg_xO₃ perovskite catalysts (x=0–0.5) were synthesised by the so-called “citrates method”, characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO₃ and LaMn_0.8Mg_0.2O₃). The major results obtained are: (i) Mg substitution in the basic LaMnO₃ perovskite has a positive effect on the catalytic activity only at low x values (x≤0.2); (ii) as opposed to the results of previous studies on the LaCr_1−xMg_xO₃ system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800–1200°C) and on the value of x; (iii) an Eley–Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO₃, the catalytic combustion over LaMn_0.8Mg_0.2O₃ seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.     

Kinetics of the water–gas shift reaction over nanostructured copper–ceria catalysts

Water–gas shift reaction was studied over two nanostructured Cu_xCe_1−xO_2−y catalysts: a Cu_0.1Ce_0.9O_2−y catalyst prepared by a sol–gel method and a Cu_0.2Ce_0.8O_2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al₂O₃ catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h⁻¹, respectively. Experimentally estimated activation energy, E_af, of the forward water–gas shift reaction at CO/H₂O = 1/3 was 51 kJ/mol over the Cu_0.1Ce_0.9O_2−y, 34 kJ/mol over the Cu_0.2Ce_0.8O_2−y and 47 kJ/mol over the CuO–ZnO–Al₂O₃ catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.     

Magnetic properties of Y–Fe–O ultrafine particles containing YFe(3+x)O1.5(4+x) synthesized by rf thermal plasma

Y–Fe–O ultrafine particles containing YFe_(3+x)O_1.5(4+x), -Fe₂O₃, and γ-Fe₂O₃(Fe₃O₄) were fabricated using a thermal plasma evaporation method with rf Ar–O₂. To determine if YFe_(3+x)O_1.5(4+x) in the particles is a ferri-, ferro-, or paramagnetic compound at room temperature (R.T.), the magnetic properties of these particles at R.T. were studied using X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Mössbauer spectrometry. VSM results showed that the saturation magnetization of particles at R.T. increased after the Curie point (CP) measurement at reduced pressure (4×10⁻³ Pa) from R.T. to an upper limit temperature higher than 460 °C. The saturation magnetization of particles at R.T. after the CP measurement at reduced pressure from R.T. to 700 °C was larger than that from R.T. to 600 °C. In the XRD patterns, the relative quantities of h-YFeO₃ and γ-Fe₂O₃(Fe₃O₄) to that of YFe_(3+x)O_1.5(4+x) increased after the CP measurement at reduced pressure from R.T. to 700 °C, indicating that the saturation magnetization at R.T. increased as the relative quantity of γ-Fe₂O₃(Fe₃O₄) increased. The relative quantities of h-YFeO₃ and γ-Fe₂O₃(Fe₃O₄) to that of YFe_(3+x)O_1.5(4+x) after the CP measurement depended on Fe/Y of the particles, indicating that the increase in saturation magnetization at R.T. after the Curie point measurement depended on the increase in relative quantity of γ-Fe₂O₃(Fe₃O₄). Mössbauer spectrometry before and after the CP measurements showed that YFe_(3+x)O_1.5(4+x) exhibited only a single type of quadrupole splitting and no magnetic splitting, indicating that YFe_(3+x)O_1.5(4+x) is a paramagnetic compound.     

Cr/V/Sb mixed oxide catalysts for the ammoxidation of propane to acrylonitrile: Part I: Nature of the V species

Rutile-type Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile, were prepared and characterized. For atomic ratios between components equal to Cr/V/Sb 1/x/1 and 1/x/2 the systems were monophasic, but different types of compounds formed depending on the ratio between the three metals. The compositional parameter which most affected the nature of the compound formed was the (Cr+V)/Sb atomic ratio. When this ratio was between 2 and ≈1, a rutile Cr³⁺/V⁴⁺/Sb⁵⁺ mixed oxide of composition Cr₁V_xSb₁O_4+2x developed (0<x<1), which in practice corresponds to a solid solution between 1 CrSbO₄ and x VO₂. When the (Cr+V)/Sb ratio was between 0.5 and ≈1, a rutile Cr³⁺/V³⁺/Sb³⁺/Sb⁵⁺ mixed oxide of composition CrV_xSb_1+x+2zO_4+4x+4z developed (0<x<1), which corresponds to a solid solution between 1 Cr³⁺Sb_z³⁺Sb_1+z⁵⁺O_4+4z and x VSbO₄. The distinction between the two classes of compounds was not clear-cut, and when the (Cr+V)/Sb atomic ratio was around 1, mixed oxides containing both V³⁺ and V⁴⁺ formed. Values of the (Cr+V)/Sb atomic ratio lower than ≈0.5 led to the additional formation of antimony oxide.     

EPR study of electrochemical lithium intercalation in V2O5 cathodes

The evolution of the cathode material of Li/V₂O₅ cells upon lithium intercalation is studied by Electron Paramagnetic Resonance (EPR) spectroscopy. Below one lithium per V₂O₅(x < 1), where the electrochemical process is completely reversible, V^IV ions of the - and δ-Li_x V₂O₅ phases are detected by EPR. Within this intercalation range, the shape of the EPR signal is dominated by the strong exchange interactions between magnetically concentrated V^IV ions of the Li_x V₂O₅ phases. The intercalation range x > 1 is characterized by a new EPR signal attributed to magnetically diluted vanadyl ions VO²⁺. It is shown that these species do not belong to the Li_x V₂O₅ matrix, but most probably originate from the liberation of surface vanadyl ions during the irreversible transformation of δ-Li_x V₂O₅ into γ-Li_x V₂O₅. On the basis of these results, a mechanism to explain the partial irreversibility of the electrochemical processes is proposed. A third EPR signal is also observed for x 1. This signal is attributed to an electron-hole defect in Li_x V₂O₅, originating from a local charge compensation of the vanadyl vacancies.     

Effect of glass additions on the sintering and microwave properties of composite dielectric ceramics based on BaO–Ln2O3–TiO2 (Ln = Nd, La)

Ten weight percent BBZS (Bi₂O₃, B₂O₃, ZnO and SiO₂) glass was added to x(Ba₄Nd_9.333Ti₁₈O₅₄) − (1 − x)(BaLa₄Ti₄O₁₅) (BNLT, 0 ≤ x ≤ 1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10 wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140 °C, depending on composition (x), an average reduction of sintering temperature by 350 °C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on _r and τ_f, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT − (1 − x)BLT)/0.1BBZS (x = 0.75) had _r  61, τ_f  38 ppm/°C and Qf  2305 GHz.