Physicochemical properties of rock salt-type ordered Sr2MMoO6 (M = Mg,Mn, Fe,Co, Ni) double perovskites

B-site rock salt-type ordered Sr₂MMoO₆ (M = Mg, Mn, Fe, Co, Ni) oxides have been systematically investigated in terms of their crystal structure, oxygen non-stoichiometry, transport properties, thermal expansion and chemical stability. Structural evolution on temperature of the studied materials was interpreted on a basis of octahedra rotation, with I4/m → Fm-3m phase transition. In the case of P2₁/n → I4/m transformation recorded for Sr₂MnMoO₆, a region of coexistence of two phases was observed. The Fe-containing compound was found to possess very high electrical conductivity, around 1000 S cm⁻¹ in 5 vol.% H₂ in Ar, and small negative Seebeck coefficient. Studies show that Mn- and Fe-containing compounds are stable only in reducing conditions, Co- and Ni-containing ones are stable only in oxidizing conditions, while Sr₂MgMoO₆ remains stable in a wide range of oxygen partial pressures. Recorded power density of a button-type SOFC with Sr₂FeMoO₆-based anode was 0.32 W cm⁻² at 900 °C.     

Dielectric behaviors of (Nb2O5)(1-x)xTiO2 ceramics

The dielectric constants (K) of ceramics composition (Nb₂O₅)(_1-x): xTiO₂, (x = 0.05, 0.08, 0.11) have been measured over the temperature range from -180 °C to 180 °C in the frequency range 100 Hz-IMHz. For all three samples, the temperature dependence of the dielectric constants at various frequencies shows the dielectric dispersion and the frequency dependent dielectric losses are observed, suggesting the several relaxation phenomena. Dielectric loss data are analyzed by Arrhenius formalism indicating the presence of relaxation type dielectric mechanisms in (Nb₂O₅)(_1-x): xTiO₂ ceramics.     

Use of Nb2O5 as nickel passivating agent: characterisation of the Ni/Nb2O5/SiO2 system

Niobia-modified silica was used as support for nickel-based catalysts. Catalysts with different nickel loading were prepared by successive incipient-wetness impregnation of toluenic nickel octanoate solutions. The samples were characterised after both calcination and reduction by TEM-EDX techniques and tested in the ethane hydrogenolysis. Their catalytic behaviour was compared to that of related silica-supported nickel catalysts. A high suppression of hydrogenolysis activity (93–99%) was determined for the niobia-modified silica-supported nickel catalysts which do not show nickel particles on silica.     

(MMgBO3)n (n = 1, M = Li,Na, K,Rb and n = 4, M = Cs): An investigation on the structure transition and optical properties

In crystal engineering, modification of the crystal structure by cation substitution is one of the most important and efficient methods. This paper reveals the origin of the structure transition of a series of (MMgBO₃)_n (n = 1, M = Li, Na, K, Rb and n = 4, M = Cs) compounds. A force-equilibrium model was established with the comparison of different structures of these compounds. After substituted by other alkali metal cations with different atomic radii, the changed bond between oxygen and cations leads to structural transition. The electronic and optical properties of NaMgBO₃, KMgBO₃ and RbMgBO₃ were also investigated by using DFT methods to give more comprehension of these compounds.     

Correlation between structural characteristics and microwave dielectric properties of walstromite BaCa2M3O9 (M = Si,Ge) ceramics

Novel BaCa₂M₃O₉ (M = Si, Ge) microwave dielectric ceramics were prepared via solid-state reaction with sintering at 1125°C–1275°C for 5 h. Single-phase BaCa₂M₃O₉ (M = Si, Ge) ceramics were obtained according to stoichiometry. The single-phase BaCa₂Ge₃O₉ ceramic was confirmed through Rietveld refinement and high-resolution transmission electron microscopy/selected area electron diffraction and synthesized for the first time. The BaCa₂M₃O₉ (M = Si, Ge) exhibited a triclinic structure with a P$\overline{1}$ space group and good microwave dielectric properties. The ε_r, Q × f, and τ_f values of BaCa₂M₃O₉ (M = Si, Ge) ceramics are mostly dominated by the relative density, ionic polarizability, relative covalence, and bond energy of M–O bond, respectively. A high Q × f value (61 800 GHz at 16.3 GHz) was obtained in BaCa₂Ge₃O₉ ceramic due to its high r_c (Ge–O) and low intrinsic dielectric loss. The BaCa₂Si₃O₉ ceramic exhibited small |τ_f| value (‒36.4 ppm/°C) due to its large E_Si-O. Excellent microwave dielectric properties (ε_r = 8.31, Q × f = 61 800 GHz, and τ_f = ‒58.7 ppm/°C) were obtained for the BaCa₂Ge₃O₉ ceramic.     

Microwave Dielectric Ceramics Li2MO4−TiO2 (M = Mo,W) with Low Sintering Temperatures

In this work, novel series of (1 ? x)Li₂MO₄–xTiO₂ (M = Mo, W; x = 0.3, 0.4, 0.45, 0.5, 0.6) ceramics were developed for microwave dielectric application. They were prepared via the mixed‐oxide process and the phase composition, microstructures, sintering behaviors, and microwave dielectric properties were investigated. The X‐ray diffraction (XRD) pattern and scanning electron microscope analysis indicated that the Li₂MO₄ (M = Mo, W) did not react with rutile TiO₂ and a stable two‐phase composite system Li₂MO₄–TiO₂ (M = Mo, W) was formed. The XRD pattern of cofired ceramics revealed that some parts of Li₂MoO₄ phase and very small part of Li₂WO₄ phase react with Ag to form Ag₂MoO₄ phase and Ag₂WO₄ phase, respectively. At x = 0.45–0.5, temperature stable microwave dielectric materials with low sintering temperature (700°C–730°C) were obtained: ε_r = 10.6–11.0, Qf = 30 060–32 800 GHz, and temperature coefficient of resonant frequency ~0 ppm/°C.     

Comparison of H2 production from ethanol and ethylene glycol on M/Pt(1 1 1) (M = Ni, Fe, Ti) bimetallic surfaces

The reactions of ethylene glycol and ethanol have been studied on Fe/Pt(1 1 1) and Ti/Pt(1 1 1) bimetallic surfaces utilizing temperature programmed desorption (TPD). These results are compared to our previous studies on Ni/Pt(1 1 1) to illustrate the trend in the reforming activity on 3d-Pt bimetallic surfaces. The oxygenates decomposed on these surfaces to produce mainly H₂ and CO. The bimetallic surfaces were prepared by thermal evaporation of Fe or Ti onto Pt(1 1 1), using Auger electron spectroscopy (AES) to monitor surface compositions. Surfaces prepared by deposition of a monolayer of Fe or Ti on Pt(1 1 1), designated Fe–Pt–Pt(1 1 1) or Ti–Pt–Pt(1 1 1), displayed higher reforming activity for both ethylene glycol and ethanol than the corresponding subsurface monolayer Pt–Fe–Pt(1 1 1) and Pt–Ti–Pt(1 1 1) structures or clean Pt(1 1 1). The reforming yield increased as the surface d-band center, calculated from density functional theory (DFT), shifted closer to the Fermi level. The reforming selectivity of oxygenates, especially ethanol, began to decrease as the d-band center shifted closer to the Fermi level. Combining results in the current work with previous studies on Ni/Pt(1 1 1), a general criterion can be formulated for selecting 3d-Pt bimetallic surfaces with desirable reforming activity and selectivity.     

Effect of B‐site cation on the catalytic activity of La1−xCaxBO3 (B = Fe,Ni) perovskite‐type oxides for toluene combustion

Background The effect of the B cation on the surface properties and catalytic activity in the total combustion of toluene over La_1?xCa_xBO₃ (B = Fe,Ni) perovskite‐type oxides was studied. Result For the La_1?xCa_xFeO₃ series, the perovskite structure was maintained in the range of substitution studied. A completely different behaviour was observed for the La_1?xCa_xNiO₃ series. A Brownmillerite‐type structure (La₂Ni₂O₅) with a large degree of phase segregation as well dispersed mixed oxides was observed upon the substitution of La for Ca. In the Fe series, the catalytic activity in the total combustion of toluene showed that the insertion of calcium ions into the perovskite lattice resulted in higher activity relative to the unsubstituted LaFeO₃ perovskite. In contrast, for the Ni series, substitution results in solids with lower activity than the pure LaNiO₃ perovskite. Conclusion For the Fe series, higher activity and stability are attributed to a synergy between Fe⁴⁺/Fe³⁺ and the oxygen vacancies generated by the calcium substitution. For the Ni counterpart, the structural modification leads to a lower activity of substituted solids compared with the pure LaNiO₃ perovskite, indicating that Ni³⁺ ions are the active sites for toluene oxidation. Copyright © 2011 Society of Chemical Industry     

Colloidal Sol–Gel Synthesis and Photocatalytic Activity of Nanoparticulate Nb2O5 Sols

A detailed study of a novel synthesis via colloidal sol–gel route for obtaining nanoparticulate Nb₂O₅ was performed. Parameters such as temperature and H⁺:Nb⁵⁺ and Nb⁵⁺:H₂O molar ratios were controlled in order to determine the best conditions of synthesis. Moreover, particle size distribution, zeta potential, structure by X‐ray diffraction, and the photocatalytic activity of the particulate sols were also evaluated. The obtained results indicate that the colloidal sol–gel synthesis is a good alternative for obtaining Nb₂O₅ either as stable nanoparticulate sol or as a nanosized powder. Nb₂O₅ amorphous nanoparticles with an average size of 20 nm were obtained by controlling the synthesis variables. The heat‐treatment process allowed the formation of Nb₂O₅ with orthorhombic structure that transforms at higher temperatures to monoclinic phase. The highest photocatalytic activity was observed under λ = 365 nm, the smallest UV energy used in the experimental tests.     

Oxidation of alkylaromatics to benzylic ketones using TBHP as an oxidant over LaMO3 (M = Cr,Co, Fe,Mn, Ni) perovskites

A series of LaMO₃ (M = Cr, Co, Fe, Mn, Ni) perovskites were synthesized by citrate precursor decomposition method and characterized by XRD technique. The catalytic activity of the synthesized perovskites was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using TBHP as an oxidant. Of various perovskites screened, LaCrO₃ exhibited remarkable catalytic activity providing the oxidation of alkylarenes selectively at the benzylic position. The LaCrO₃/TBHP catalytic system provides excellent yields of the desired ketones under solvent-free conditions and the catalyst can be successfully used up to six consecutive cycles with no significant loss in activity.     

Effects of Al substitution for Ni and Mn on the electrochemical properties of LiNi0.5Mn1.5O4

The effects of Al substitution for Ni or (and) Mn in LiNi_0.5Mn_1.5O₄ spinel on the structures and electrochemical properties are investigated. Powders of LiNi_0.5Mn_1.5O₄, Li_0.95Ni_0.45Mn_1.5Al_0.05O₄, LiNi_0.475Mn_1.475Al_0.05O₄ and Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ are synthesized by a thermopolymerization method. Their structures and electrochemical properties are studied by X-ray powder diffraction, scanning electron microscopy, infrared spectroscopy, cyclic voltammetry and galvanostatic charge–discharge testing. The introduction of Al in these LiNi_0.5Mn_1.5O₄ samples has resulted in structure variation, and greatly improved their cyclic performance and rate capability. The effects of Al substitutions for Ni and Mn in the LiNi_0.5Mn_1.5O₄ are different. Compared with LiNi_0.5Mn_1.5O₄, Li_0.95Ni_0.45Mn_1.5Al_0.05O₄ demonstrates higher specific capacity at room temperature but faster capacity fading at elevated temperatures. Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ displays a lower discharge capacity but better capacity retention at 55 °C. Moreover, the cyclic performance and rate capability of the Ni-substituted Li_0.95Ni_0.45Mn_1.5Al_0.05O₄, Ni/Mn co-substituted LiNi_0.475Mn_1.475Al_0.05O₄ and Mn-substituted Li_1.05Ni_0.5Mn_1.45Al_0.05O₄ at room temperature are similar, and have improved substantially compared with the Al-free LiNi_0.5Mn_1.5O₄ sample.     

Microwave dielectric properties of ATiO3 (A = Ni,Mg, Co,Mn) ceramics

Relationships between structural characteristics and microwave dielectric properties of ATiO₃ (A = Ni, Mg, Co, Mn) with ilmenite structure were investigated. The oxygen octahedral distortion was dependent on the type of A-site ions which affected to the temperature coefficient of the resonant frequency (TCF) of ATiO₃ ceramics. The quality factor (Qf) of ATiO₃ (A = Mn, Ni, Co) specimens was appreciably lowered than that of MgTiO₃ specimens due to the degree of covalency of cation–oxygen ion bond and the ordering of A-site ions. Also, the dielectric constant (K) was dependent on the electronic oxide polarizabilities of ATiO₃ ceramics.     

Effect of cation vacancies on the optical and dielectric properties of KSr2Nb5O15: A first-principles study

Using first-principles calculations, the effect of cation vacancies on the electronic structures and optical characters of KSr₂Nb₅O₁₅ (KSN) lead-free ferroelectrics are investigated. The calculated dielectric properties are demonstrated by the experimental results. The cation vacancies involve K⁺ vacancies (KSN-K), Sr²⁺ vacancies (KSN-Sr), and coexisting K⁺ and Sr²⁺ vacancies (KSN-K&Sr). When these cation vacancies exist in KSN, the unit cell volumes decrease, leading to phase transition from tetragonal to orthorhombic, and the cation vacancies show strong effects on the band gap of KSN, declining by 1.46%-9.46%. The optical properties including the static dielectric constants, refraction, and extinction coefficient of KSN-K, KSN-Sr, and KSN-K&Sr increase more than those of KSN without vacancies, but the reflectivity and loss function decrease. All structures with cation vacancies are mainly refractive in the 0-4 eV photon energy range and are reflective at 5-8 eV. The refractivity increases and reflectivity decreases after vacancies occur. KSN-Sr has the largest static dielectric constant while KSN-K&Sr has the smallest values. The dielectric constant can be adjusted in the range of 25% by controlling the cation vacancies. The calculated dielectric properties are in good agreement with the experimental results. The results pave the way to regulate the optical and dielectric properties of lead-free ferroelectrics by controlling different cation vacancies.     

Microwave dielectric properties of (Zn1/3B2/3)0.5(Ti0.8M0.2)0.5O2 (B = Nb5+, Ta5+, M = Sn4+, Ge4+) ceramics

Dielectric properties of (Zn_1/3Nb_(2?x)/3Ta_x/3)_0.5(Ti_0.8Sn_0.1Ge_0.1)_0.5O₂ (x = 0, 1, 2) and/or (Zn_1/3Nb_1/3Ta_l/3)_0.5(Ti_0.8Sn_0.2(l?y)Ge_0.2y)_0.5O₂ (y = 0, 0.5, 1) were investigated at the microwave frequencies. For the compositions with single phase of rutile structure, the dielectric constant (K) of specimens was not only dependent on the dielectric polarizabilities, but also on the bond length ratio of apical bond (d_apical) to equatorial bond (d_equatorial) of oxygen octahedron in the unit cell. Temperature coefficients of the resonant frequencies (TCF) of the specimens with B = Nb⁵⁺ and/or M = Sn⁴⁺ was larger than those with B = Ta⁵⁺ and/or M = Ge⁴⁺. These results could be attributed to the changes of the degree of oxygen octahedral distortion. Quality factors (Qf) of the specimens with B = Ta⁵⁺ and/or M = Sn⁴⁺ were larger than those with B = Nb⁵⁺ and/or M = Ge⁴⁺.     

Investigation of anisotropic mechanical properties of textured KSr2Nb5O15 ceramics via ab‐initio calculation and nanoindentation

Anisotropic mechanical properties of KSr₂Nb₅O₁₅ (KSN) crystals were investigated through first‐principles calculations based on density functional theory. These properties were experimentally verified via nanoindentation on textured KSN ceramics fabricated, using a reactive template grain growth method. Nanoindentation was performed in directions parallel and perpendicular to the [001] direction of samples consisting of highly oriented grains with tetragonal symmetry. Calculations revealed that Nb‐O yields a relatively strong covalent effect and Nb‐O octahedral distortions induce spontaneous polarization in the KSN crystal. The measured indentation modulus values concurred with the predictions, based on the calculated elastic constants, as indicated by an anisotropic ratio of ~10% between the 2 tested orientations. The hardness exhibited negligible anisotropy. However, the predictions revealed a pronounced anisotropy of the Young's modulus (ratio of ~40% between the [100] direction and a direction tilted by ~45° from the [001] toward the [100]).     

钴掺杂对Li1.2Mn0.6Ni0.2O2电化学性能的影响

利用共沉淀法制备了锂离子电池正极材料Li1.2Mn0.6Ni0.2O2和Li1.2Mn0.588Ni0.196Co0.016O2,并利用XRD、SEM和充放电测试对其晶体结构、形貌和电化学性能进行了表征.XRD结果表明:掺杂钴材料后,材料的层状结构保持完整,阳离子混排程度降低.电化学性能测试结果表明:掺钴材料的首次充放电效率和倍率放电性能明显优于Li1.2Mn0.6Ni0.2O2,且表现出较优的循环性能,其1、2、5C放电比容量分别为230.3、215.6、155.6 mA·h/g,1 C下循环50次的容量保持率为90.9％.     

Syntheses,structural characterization and luminescent properties of M2(ATPA)3(DMF)2(H2O)2 (M = Nd,Sm, Eu,Gd, Tb,Dy; ATPA = 2-aminoterephthalate,DMF = N,N-dimethylformamide)

Six isomorphous metal–organic frameworks: M₂(ATPA)₃(DMF)₂(H₂O)₂ (M = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6); ATPA = 2-aminoterephthalate, DMF = N,N-dimethylformamide), were synthesized by the self-assembly of lanthanide ions, 2-aminoterephthalate, DMF and H₂O. Their crystal structures determined by X-ray crystallography reveal interpenetrating frameworks. Compounds 2, 3, 5 and 6 exhibit mainly ligand luminescence at room temperature while 3 exhibits enhanced Eu³⁺ emissions at 77 K. Magnetic studies indicate 4 is paramagnetic with slight Gd³⁺–Gd³⁺ couplings.     

Oxidation of alkylaromatics to benzylic ketones using TBHP as an oxidant over LaMO3 (M = Cr, Co, Fe, Mn, Ni) perovskites

A series of LaMO₃ (M = Cr, Co, Fe, Mn, Ni) perovskites were synthesized by citrate precursor decomposition method and characterized by XRD technique. The catalytic activity of the synthesized perovskites was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using TBHP as an oxidant. Of various perovskites screened, LaCrO₃ exhibited remarkable catalytic activity providing the oxidation of alkylarenes selectively at the benzylic position. The LaCrO₃/TBHP catalytic system provides excellent yields of the desired ketones under solvent-free conditions and the catalyst can be successfully used up to six consecutive cycles with no significant loss in activity.     

Activity and stability of Pd/MMnOx (M = Co,Ni, Fe and Cu) supported on cordierite as CO oxidation catalysts

Several catalysts of the general formula, MMnO_x (M = Co, Ni, Fe and Cu), were synthesised through the impregnation method; their activities were shown to be enhanced by the addition of a small amount of Pd (0.01–0.1 wt%). These catalysts exhibit different activities for the catalytic oxidation of CO, due to the different valence states of various transition metal oxides. The introduction of Pd prominently enhanced both the reduction and oxidation capabilities of the catalysts. These catalysts were optimised for oxidation activities by designing orthogonal experiments. Based upon the catalysts’ properties, the stability of these samples and their ability to resist steam over Pd/CoMnO_x/cordierite were investigated.