Electrochemical and thermal characterization of doped ceria electrolyte with lanthanum and zirconium

Nanocomposites electrolytes consisting of La³⁺ and Zr⁴⁺ doped with ceria labelled as La_0.2 Ce_0.8 O_2-δ (LDC), Zr_0.2Ce_0.8O_2-δ (ZDC) and Zr_0.2La_0.2Ce_0.6O_2-δ (ZLDC) have been synthesized via a co-precipitation route. DC conductivity was studied with a four-probe method in the range of temperature 450–650 °C and maximum conductivity was found to be 0.81 × 10^?2 S.cm^?1 (LDC) > 0.32 × 10^?2 S.cm^?1 (ZLDC) > 0.15 × 10^?2 S.cm^?1 (ZDC) at a temperature of 650 °C, respectively. Further, electric behavior of doped and co-doped ceria electrolytes was investigated by A.C electrochemical impedance spectroscopy (frequency range ~ 0.1 Hz?4 MHz). The phase/structural identification of the material prepared was studied using X-ray diffraction and found ceria to possess a cubic fluorite structure. Scanning electron microscopy (SEM) was carried out to study its morphology and particle size (~ 90–120 nm). Thermal behavior on its change in weight and length with the temperature were studied by thermogravimetric analysis (TGA) and dilatometry respectively. Furthermore, thermal expansion coefficients (TECs) of prepared electrolytes are calculated and found as follows: 13.4 × 10^?6 °C^?1, 13.6 × 10^?6 °C^?1and 15.3 × 10^?6 °C^?1 for LDC, ZDC and ZLDC, respectively, in the temperature range 150–1150 °C.     

The preparation and properties of La3.5Ru4O13 and La2RuO5

The stability of the La_3.5Ru₄O₁₃ and La₂RuO₅ compounds in the La–Ru–O system in various atmospheres and various temperature ranges was investigated by thermal analysis, X-ray diffraction analysis and electron microscopy. The La_3.5Ru₄O₁₃ compound is stable in oxidizing and neutral atmospheres (N₂ with 10 ppm O₂), while La₂RuO₅ is partially reduced in a neutral atmosphere to form La₂RuO_4.6. In a reducing atmosphere both compounds decompose into metallic Ru and La₂O₃. The thermal expansion coefficients of La₂RuO₅ and La_3.5Ru₄O₁₃ at 800 °C are 11.2 × 10⁻⁶ K⁻¹ and 9.3 × 10⁻⁶ K⁻¹, respectively. The specific electrical resistivity for La_3.5Ru₄O₁₃ is relatively independent of temperature and is 2 × 10⁻² Ω cm at 800 °C, while for La₂RuO₅ it decreases with increasing temperature and is 1 Ω cm at 800 °C.     

Fine structural analysis and phase transition behavior for Li-modified Na0.5K0.5NbO3 lead-free piezoelectric ceramics

The fine crystal structure of Li_x(Na_0.5K_0.5)_1?xNbO₃ ceramics has been studied by means of Nb-K edge extended X-ray absorption fine structure (EXAFS) and X-ray internal strain measurement technique in the vicinity of the compositions showing a polymorphic phase boundary (PPB) between orthorhombic and tetragonal structures. The anisotropic distortion of the NbO₆ octahedral initially occurred when x was increased from 0.050 to 0.053, prior to the completion of the phase transition from orthorhombic to tetragonal symmetry. EXAFS clearly revealed that the bond distance of Nb–O1 with [0 0 1] configuration was increased, and that of Nb–O2 with [1 1 0] configuration was oppositely decreased in the NbO₆ octahedral. In the vicinity of the PPB compositions, the internal strain η_(0 1 1) also increased from 4.5 × 10^?3 to the maximum value of 12.0 × 10^?3 in the narrow x range from 0.040 to 0.055, then decreased to 3.2 × 10^?3 at x = 0.06. On the other hand, the η_(1 0 0) increases from 1.5 × 10^?3 to the maximum value of 2.9 × 10^?3 in the next narrow x range from 0.055 to 0.060. The variation of η_(1 0 0) differed in Li dependence from that of η_(0 1 1), which indicates that a large anisotropic strain remains in the crystal lattice in the PPB compositions.     

Characteristics of highly (001) oriented (K,Na)NbO3 films grown on LaNiO3 bottom electrodes by RF magnetron sputtering

(K,Na)NbO₃ ferroelectric films were grown on LaNiO₃ coated silicon substrates by RF magnetron sputtering. The conductive LaNiO₃ films acted as seed layers and induced the highly (001) oriented perovskite (K,Na)NbO₃ films. Such films exhibit saturated hysteresis loops and have a remnant polarization (2P_r) of 23 μC/cm², and coercive field (2E_c) of 139 kV/cm. The films showed a fatigue-free behavior up to 10⁹ switching cycles. A high tunability of 65.7% (@300 kV/cm) was obtained in the films. The leakage current density of the films is about 6.0×10^?8 A/cm² at an electric field of 50 kV/cm.     

Thermal expansion of EB-PVD yttria stabilized zirconia

Yttria stabilized zirconia (YSZ) layers, suited for thermal barrier coatings (TBCs) in turbine engines, were produced by electron-beam physical vapour deposition (EB-PVD) on metallic (Ni-based alloys) ceramic (Al₂O₃) substrates (typically at 1000 °C) using ingot compositions of about 4 mol% Y₂O₃–ZrO₂. Employing powdered samples, removed from the surface of the as-deposited YSZ layers, precision X-ray diffraction data sets were recorded between 60 and 900 °C and the lattice parameters of the metastable, tetragonal (t′) and the cubic (c) phases were refined using Rietveld's method. It was unambiguously verified that phase (c) was present in all investigated samples as a minority phase already in the as-deposited state.The thermal expansion coefficients of the tetragonal phase (t′) (α₁₁ = 9.3(2) × 10^?6 K^?1, α₃₃ = 10.8(1) × 10^?6 K^?1) and the cubic phase (c) (α₁₁ = 8.5(2) × 10^?6 K^?1) were evaluated from the refined temperature dependent lattice parameter values of this study, and turned out to be close to each other, but significantly different from the coefficient of the monoclinic phase (m) (α₁₁ = 9.0 × 10^?6 K^?1, α₂₂ = 1.2 × 10^?6 K^?1, α₃₃ = 11.9 × 10^?6 K^?1, α₁₃ = 0.0 × 10^?6 K^?1), which was derived from temperature dependent lattice parameter measurements published by Touloukian et al.⁵ The expansion coefficient of the monoclinic phase (m) exhibited a very pronounced anisotropy in contrast to the tetragonal phases (t′) and (t). Our values for (t′) and (c) were in excellent agreement with that of other tetragonal and cubic phases from the literature with quite different Y₂O₃ contents.     

Optical properties of ferroelectric lanthanum lithium niobate

Fine structures (ferroelectric domains), ferroelectricity and Second Harmonic Generation results were found and studied as a function of laser linearly and circularly polarization dependent polarized excitations in ferroelectric stoichiometric La_0.05Li_0.85NbO₃ nanocrystals ceramic material. Scanning Electron Microscope images are taken as a comparison to Second Harmonic Generation intensity profiles revealed fine structures. By using laser polar measured response we are able to find the angle orientation from 0° to 90° angles of ferroelectric domains with highly good definition contrast obtained in blue/gray colors. It shows ferroelectric hysteresis loops at room temperature with a polarization saturation of (0.247 μC/cm²), remnant polarization of (0.15 μC/cm²) and coercitivity field of (1.31 kV/cm). X-ray Diffraction, Atomic Force Microscope, Raman spectroscopy and X-ray Photoelectron Spectroscopy, revealed well formed of ferroelectric ABO₃ perovskite crystal structure, piezoelectric image response indicate ferroelectric pattern domain structure, new vibrations modes on LaO₆, LiO₆ and NbO₆ octahedral sites and binding energies of electronic structure of La⁵⁷, Nb⁴¹, O⁸, Li³ from the surface of the ferroelectric stoichiometric La_0.05Li_0.85NbO₃ nanocrystals ceramic material, respectively.     

Effect of Ce and Yb co-doping on conductivity of LaNbO4

La_1−x−yCe_xYb_yNbO₄ specimens with various Ce and Yb contents were prepared by solid reactions, and their crystal structure, element valence, sinterability and conductivity were investigated. LaNbO₄-type single phase was formed at 1200 °C in air, and the lattice of La_1−x−yCe_xYb_yNbO₄ was distorted from that of LaNbO₄ to various extents, depending on the added amount of Ce and Yb. Both La and Nb remained the same valence as they are in LaNbO₄; Ce⁴⁺, Ce³⁺ and Yb³⁺ were detected at room temperature. Highly densified La_1−x−yCe_xYb_yNbO₄ specimens were achieved by sintering at above 1215 °C in air with conductivity 1–2 orders higher than that of pure LaNbO₄ in dry air, wet air and wet 5% H₂–N₂ atmospheres. The conductivity changed with testing atmosphere owing to the competition of electron and proton conduction, and maximal value 4.7 × 10⁻⁴ S cm⁻¹ was obtained in wet air at 900 °C.     

A comparative study to evaluate the corrosion performance of Zr incorporated Cr3C2-(NiCr) coating at 900 °C

In this study zirconium incorporated Cr₃C₂-(NiCr) coating has been sprayed on three superalloys viz. Superni 718, Superni 600 and Superco 605 using D-gun technique. A comparative study has been carried out to check the cyclic oxidation in air and hot corrosion in simulated incinerator environment (40%Na₂SO₄-40%K₂SO₄-10%NaCl-10%KCl) for the coated specimens at 900 °C for 100 cycles. Oxidation kinetics has been established for all the specimens using weight change measurements. Corrosion products have been characterized using X-ray diffractometer (XRD) and scanning electron microscopy/energy-dispersive analysis (SEM/EDAX). Cr₃C₂-(NiCr) + 0.2%wtZr coating provides very good corrosion resistance in air oxidation for all the three coated superalloys. As all the three coated superalloys shows parabolic behaviour with parabolic rate constant as 0.07 × 10^?10 (g² cm^?4 s^?1) for Superni 718, 0.43 × 10^?10 (g² cm^?4 s^?1) for Superni 600 and 0.3 × 10^?10 (g² cm^?4 s^?1) for Superco 605 This coating is also effective in the molten salt environment but coating on Co-based superalloy Superco 605 did not perform satisfactorily. The parabolic rate constants for coated Superni 718 is 0.61 × 10^?10 (g² cm^?4 s^?1), for coated Superni 600 is 6.72 × 10^?10 (g² cm^?4 s^?1) and for coated Superco 605 is 17.5 × 10^?10 (g² cm^?4 s^?1).     

Improving the Li-ion conductivity and air stability of cubic Li7La3Zr2O12 by the co-doping of Nb,Y on the Zr site

The elements Nb and Y were simultaneously substituted to the Zr sites of an Li₇La₃Zr₂O₁₂ (LLZO) electrolyte to improve its Li-ion conductivity and air stability. Samples of Li₇La₃Zr_2-2xNb_xY_xO₁₂ were fabricated using a solid-state reaction method. The results show that the introduction of Nb and Y can stabilise cubic-phase LLZO. The total conductivity of Li₇La₃ZrNb_0.5Y_0.5O₁₂ electrolyte can reach 8.29 × 10^?4 S cm^?1 at 30 °C when sintered at 1230 °C for only 15 h. Surprisingly, the conductivity of Li₇La₃ZrNb_0.5Y_0.5O₁₂ can be maintained at 6.91 × 10^?4 S cm^?1 after exposure to air for 1.5 months, indicating excellent air stability. Furthermore, a LiFePO₄/Li₇La₃ZrNb_0.5Y_0.5O₁₂/Li cell displayed stable charge/discharge and cycling performance at ambient temperature, suggesting there is potential to use Li₇La₃ZrNb_0.5Y_0.5O₁₂ electrolyte in Li-ion batteries. Additionally, the effects of varying the co-doping amount and dwelling time on the Li-ion conductivity of Li₇La₃Zr_2-2xNb_xY_xO₁₂ were investigated.     

Study of structure and conductivity of Li3/8Sr7/16-3x/2LaxZr1/4Nb3/4O3 solid electrolytes

Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized using the conventional solid-state reaction method. In order to increase the vacancy concentration, La³⁺ was doped on the Sr²⁺ site. Crystal structures of doped samples were characterized by X-ray diffraction. Except, perovskite-type Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0, 0.05, 0.10, 0.15) samples were fabricated by heat treatment at 1250 °C, 1275 °C, 1275 °C and 1275 °C, respectively, for 15 h. Lattice sizes decreased with the increase of doping amounts because of the smaller ion radius of La³⁺ compared to that of Sr²⁺. Ionic conductivities of the samples were measured by AC impedance spectroscopy. The results showed that the ionic conductivity increases at first and then decreases with raising doping amounts and sintering temperatures. So the optimized composition Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0.05) sintered at 1275 °C was selected with the highest total conductivity of 3.33 × 10^?5 S cm^?1at 30 °C and an activation energy of 0.27 eV. Additionally, potentiostatic polarization test was used to evaluate the electronic conductivity. The optimal composition Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0.05) as a possible Li-ion conducting solid electrolyte has an electronic conductivity of only 8.39 × 10^?9 S cm^?1.     

Oxy-apatite reaction sintering of colloidal and classic ceramic processed powders

Oxy-apatites are promising electrolyte materials for intermediate-temperature solid oxide fuel cells, IT–SOFC. However the requirements of electrolytes make necessary the preparation of dense films with the appropriated composition to show good electrical properties; in this way, colloidal processing is a key issue.This work involves the application of colloidal processing for four oxygen-excess oxy-apatites, La_9.67(Si₆O₂₄)O_2.5, La₁₀(Si₆O₂₄)O₃, La₁₀(Si_5.5Al_0.5O₂₄)O_2.75 and La₁₀(Si₅Al₁O₂₄)O_2.5 and their characterization (phases, microstructure and electrical properties). The results have been compared with those obtained by classical ceramic method to assure the same composition without loosing properties. Samples with the desired compositions were obtained by reaction sintering of La₂O₃, SiO₂ and Al₂O₃. La₁₀(Si_5.5Al_0.5O₂₄)O_2.75 prepared by colloidal processing and heated at 1923 K showed the highest conductivity value, 3.0 × 10^?2 S cm^?1 at 973 K. Furthermore, its residual porosity was very low. On the other hand, La₁₀(Si₆O₂₄)O₃ stoichiometry was tried by colloidal and ceramic methods under several experimental conditions. Unfortunately, the obtained oxy-apatite seems to have slightly lower lanthanum content. In spite of previous reports claiming the preparation of stoichiometric La₁₀(Si₆O₂₄)O₃, this study cannot support these findings.     

Influence of doping with various cations on electrical conductivity of apatite-type neodymium silicates

Apatite-type neodymium silicates doped with various cations at the Si site, Nd₁₀Si₅BO_27?δ (B=Mg, Al, Fe, Si), were synthesized via the high-temperature solid state reaction process. X-ray diffraction and complex impedance analysis were used to investigate the microstructure and electrical properties of Nd₁₀Si₅BO_27?δ ceramics. All Nd₁₀Si₅BO_27?δ ceramics consist of a hexagonal apatite structure with a space group P63/m and a small amount of second phase Nd₂SiO₅. Neodymium silicates doped with Mg²⁺ or Al³⁺ cations at the Si site have an enhanced total conductivity as contrasted with undoped Nd₁₀Si₆O₂₇ ceramic at all temperature levels. However, doping with Fe³⁺ cations at the Si site has a little effect on improving the total conductivity above 873 K. The enhanced oxide-ion conductivity in a hexagonal apatite-type structure depends upon the diffusion of interstitial oxide-ion through oxygen vacancies induced by the Mg²⁺ or Al³⁺ substitution to the Si⁴⁺ site and through the channels between the SiO₄ tetrahedron and Nd³⁺ cations. At 773 K, the highest total conductivity is 4.19×10^?5 S cm^?1 for Nd₁₀Si₅MgO₂₆ ceramic. At 1073 K, Nd₁₀Si₅AlO_26.5 silicate has a total conductivity of 1.55×10^?3 S cm^?1, which is two orders of magnitude higher than that of undoped Nd₁₀Si₆O₂₇.     

Cellulose templating method for the preparation of La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM) solid oxide electrolyte

La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) materials are synthesized with a fast and facile cellulose templating method for the first time and characterized by XRD, EIS, Archimedes method and SEM–EDS. LSGM powders with a phase purity of 91.7 mol% are obtained after the calcination at 1300 °C for 12 h. SEM–EDS results indicate possible decomposition and reconstruction of the LSGM phase due to the diffusion of Sr-rich species to the grain boundaries for the sample sintered at 1500 °C for 6 h. Maximum conductivity value is found to be 4.2 × 10^?2 S cm^?1 at 800 °C for the sample calcined at 1300 °C for 12 h and sintered at 1400 °C for 6 h. Phase purity, stability and relative density are the important factors for obtaining high performance LSGM electrolytes. Therefore, cellulose templating method is a promising candidate for the preparation of LSGM electrolytes.     

Fabrication of dense Ce0.9Mg0.1P2O7-PmOn composites by microwave heating for application as electrolyte in intermediate-temperature fuel cells

In the present work, we report a method of fabrication of dense 10 mol% Mg²⁺-doped cerium pyrophosphate-phosphate (Ce_0.9Mg_0.1P₂O₇-P_mO_n; CMP-P) composites by microwave heat-treatment of the preformed Ce_0.9Mg_0.1P₂O₇ substrates in the presence of phosphoric acid. The composite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The microwave heating at 375 °C for 5 min resulted in the formation of dense CMP-P composites which retained most of the pyrophosphate phase. The electrical conductivity was extracted from the EIS data and for the CMP-P composite prepared by H₃PO₄ loading for 10 h and microwave heat-treatment for 5 min it was found to be >10^?2 S m^?1 in 100–250 °C range with a maximum of 0.062 S cm^?1 at 190 °C, which was significant for its application as electrolyte in intermediate temperature fuel cells.     

Fabrication and thermal expansion properties of ZrW2O8/Zr2WP2O12 composites

ZrW₂O₈/Zr₂WP₂O₁₂ composites were fabricated by sintering ZrW₂O₈–Zr₂WP₂O₁₂ powder mixtures at 1473 K for 1 h, and their negative thermal expansion properties were investigated. The relative density of sintered pure-phase ZrW₂O₈ was 72.3%, while that of the sintered composites was 88.4–92.3%. In the composites, the observed hysteresis in the thermal expansion data was small because of the small difference between the CTEs of ZrW₂O₈ and Zr₂WP₂O₁₂. The CTE of the composites was negative and increased with the Zr₂WP₂O₁₂ content. When the Zr₂WP₂O₁₂ volume fraction in the composites was increased from 0 to 75 vol%, the CTEs of the composites increased from ?9.1 × 10^?6 to ?3.1 × 10^?6 K^?1 and from ?5.0 × 10^?6 to ?1.9 × 10^?6 K^?1 over the temperature ranges of 323–373 and 473–673 K, respectively. In composites with Zr₂WP₂O₁₂ volume fractions of 0–25 vol%, the experimentally obtained CTE values were in good agreement with the calculated values obtained by assuming mixed law behavior.     

Diffusion coefficients and electrical conductivities for calcium chloride aqueous solutions at 298.15 K and 310.15 K

Experimental values for mutual diffusion coefficients (interdiffusion coefficients) for CaCl₂ aqueous solutions at 298.15 K and 310.15 K, not available in the literature, were experimentally determined between 0.005 mol dm^?3 and 0.1 mol dm^?3. The measurements were performed by using a conductimetric open-ended capillary cell. Values at infinitesimal concentration, D⁰, were also obtained. These data were analysed using the Onsager–Fuoss and Gordon models. In addition, molar conductance data were measured at 310.15 K for CaCl₂ aqueous solutions at the same concentration range and the value at infinitesimal concentration determined. Afterwards, it was split into the ionic limiting values. By replacing them into the Nernst equation, diffusion coefficients at infinitesimal concentration were derived (1.335 × 10^?9 m² s^?1 and 1.659 × 10^?9 m² s^?1) at both temperatures (298.15 K and 310.15 K, respectively). They agree well with the extrapolated experimental ones (1.312 × 10^?9 m² s^?1 and 1.613 × 10^?9 m² s^?1).     

Microstructures and microwave dielectric properties of (1 − y)La1−xSmx(Mg0.5Sn0.5)O3–yCa0.8Sm0.4/3TiO3 ceramics

The (1 ? y)La_1?xSm_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that La_1?xSm_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little La₂Sn₂O₇ as the second phase. An apparent density of 6.59 g/cm³, a dielectric constant (?_r) of 19.9, a quality factor (Q × f) of 70,200 GHz, and a temperature coefficient of resonant frequency (τ_f) of ?77 ppm/°C were obtained when the La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1500 °C for 4 h. The temperature coefficient of resonant frequency (τ_f) increased from ?77 to +6 ppm/°C as y increased from 0 to 0.6 when the (1 ? y)La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramics were sintered at 1500 °C for 4 h. 0.425La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–0.575Ca_0.8Sm_0.4/3TiO₃ ceramic that was sintered at 1500 °C for 4 h had a τ_f of ?3 ppm/ °C.     

Lithium ion conductivity in the solid electrolytes (Li0.25La0.25)1−xM0.5xNbO3 (M=Sr,Ba, Ca,x=0.125) with perovskite-type structure

Perovskite-type structured solid electrolytes with the general formula (Li_0.25La_0.25)_1−xM_0.5xNbO₃ (M=Sr, Ba, Ca, x=0.125) have been prepared by solid-state reaction. Their crystal structure and ionic conductivity were examined by means of X-ray diffraction analysis (XRD), scanning electron microscope (SEM), and alternating current (AC) impedance technique. All sintered compounds are isostructural with the parent compound Li_0.5La_0.5Nb₂O₆. Some impurity phase is detected at the grain boundary in the Ba- and Ca-substituted compounds. The substitution of partial Li⁺ by alkaline earth metal ions has responsibility for the cell volume expansion as determined by the XRD data. The densification is accelerated, with the overall porosity and grain boundary minimized as Sr²⁺ ions are doped. Among the investigated compounds, the perovskite (Li_0.25La_0.25)_0.875Sr_0.0625NbO₃ shows a remarkable ionic conductivity of 1.02×10⁻⁵ S/cm at room temperature (20 °C) and the lowest activation energy of 0.34 eV in comparison with 0.38 eV and 0.44 eV for the corresponding Ba- and Ca-doped samples, respectively. It is identified that the enhancement of ionic conductivity is attributed to a reduction in activation energy for ionic conduction which is related to an increase in the cell volume.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Basicity–FAME yield correlations in metal cation modified MgAl mixed oxides for biodiesel synthesis

In this paper Zn^2 +, Co^2 +, Fe^3 + and La^3 + were respectively introduced into Mg₃Al₁ mixed oxides to prepare Mg₁Zn₂Al₁, Mg₁Co₂Al₁, Mg₃Al_0.6Fe_0.4 and Mg₃Al_0.6La_0.4 oxides by calcinations at 773 K. The XRD, XPS, BET, ICP-AES and CO₂-TPD characterizations of samples exhibited that the effects of the substitution of trivalent cations (Fe^3 +, La^3 +) for Al^3 + on the basicity of samples were stronger than the substitution of divalent cations (Zn^2 +, Co^2 +) for Mg^2 + because of high O^2 − concentration which were coincident with the FAME yield for these catalysts. La^3 + modified catalyst exhibited highest activity in transesterification.