钙钛矿型氧化物催化剂SrTiO3的甲烷氧化偶联反应性能的研究

碱土元素氧化物是一类具有较好催化活性和选择性的甲烷氧化偶联(OCM)催化剂[1,2]。Filkova[3]等研究了不同的碱土金属氧化物促进的Nd2O3催化剂的OCM催化性能,发现在所有碱土金属氧化物-Nd2O3催化剂中,SrO-Nd2O3催化剂具有最高的生成C2烃的选择性,这可能因为SrO较强的碱性以及Sr     

Thermal and structural characterization of SrTi1-xNdxO3

Sr(Ti,Nd)O₃ was synthesized in order to evaluate the influence of the amount of neodymium on the thermal and structural properties of SrTiO₃. The synthesis was carried out using the polymeric precursor method. A small mass gain was observed for the SrTiO₃ and SrTi_0.98Nd_0.02O₃ samples accompanied by an exothermic peak in the DTA curves. Other steps at higher temperatures are assigned to the combustion of the organic material and carbonate. Elimination of defects by previous calcination of the precursors is responsible by the short and long range ordering of the perovskite. Cubic phase was obtained for undoped and doped SrTiO₃.     

Electrochemical study of niobium fluoride and oxyfluoride complexes in molten LiF–KF–K2NbF7 bath

The reduction of niobium in an equimolar molten mixture of LiF and KF has been investigated using a cyclic voltammetric technique. A two-step mechanism of reduction of niobium in LiF–KF melt is proposed. The influence of oxide ions on the reduction has been studied and it was found to strongly influence the redox properties and structure of niobium ions in the melt. The presence of oxide ions caused the formation of the mono-oxyfluoride complex [Nb(V)OF₅]²⁻. The formation of the mono-oxyfluoride complex was not complete at 720°C. Free oxide ions caused an appearance of di-oxyfluoride complex. In the redundant oxide ions the di-oxyfluoride complex was converted to [Nb(IV)O₂F]⁻ by a redox reaction.     

The Preparation and Electrical Properties of SrTiO3-Based Capacitor-Varistor Double-Function Ceramics

SrTiO₃ powders were prepared using a sol–gel process. The electrical properties and the microstructure of doped SrTiO₃-based capacitor-varistor double functions ceramics prepared by the sol–gel process were compared with those of ceramics prepared via the solid-state reaction method using X-ray diffraction, scanning electron microscopy, and measurements of dielectric constants and current-voltage characteristics. The sol–gel based materials exhibited enhanced capacitor-varistor properties compared to those prepared by a conventional solid-state method. These enhanced properties are attributed to the smaller, more uniform grain structure obtained in the sol–gel-derived material, which promotes the formation of an effective core-shell structure in which an acceptor-doped layer encapsulate the donor-doped SrTiO₃ grains. The effect of such microstructures on electrical properties is discussed.     

High-throughput first-principle calculations of the structural,mechanical, and electronic properties of cubic XTiO3 (X = Ca,Sr, Ba,Pb) ceramics under high pressure

High-throughput first-principle calculations are implemented to study the structural, mechanical, and electronic properties of cubic XTiO₃ (X = Ca, Sr, Ba, Pb) ceramics under high pressure. The effects of applied pressure on physical parameters, such as elastic constants, bulk modulus, Young's modulus, shear modulus, ductile-brittle transition, elastic anisotropy, Poisson's ratio, and band gap, are investigated. Results indicate that high pressure improves the resistance to bulk, elastic, and shear deformation for XTiO₃ ceramics. Pugh's ratios B/G reveal that CaTiO₃ and PbTiO₃ ceramics are ductile, but SrTiO₃ and BaTiO₃ ceramics are brittle under the ground state. The brittle-to-ductile transition pressures are 24.26 GPa for SrTiO₃ and 43.23 GPa for BaTiO₃. Under high pressure, the strong anisotropy promotes the cross-slip process of screw dislocations, and then enhances the plasticity of XTiO₃ ceramics. Meanwhile, XTiO₃ (X = Ca, Sr, Ba) is intrinsically an indirect-gap ceramic, but PbTiO₃ is a direct-gap ceramic. High pressure increases the band gap of XTiO₃ (X = Ca, Sr, Ba) ceramic, but decreases that of PbTiO₃ ceramic. This work is helpful for designing and applying XTiO₃ ceramics under high pressure.     

The Primary Origin of Excellent Dielectric Properties of (Co,Nb) Co-Doped TiO2 Ceramics: Electron-Pinned Defect Dipoles vs. Internal Barrier Layer Capacitor Effect

(Co, Nb) co-doped rutile TiO₂ (CoNTO) nanoparticles with low dopant concentrations were prepared using a wet chemistry method. A pure rutile TiO₂ phase with a dense microstructure and homogeneous dispersion of the dopants was obtained. By co-doping rutile TiO₂ with 0.5 at.% (Co, Nb), a very high dielectric permittivity of ε′ ≈ 36,105 and a low loss tangent of tanδ ≈ 0.04 were achieved. The sample–electrode contact and resistive outer-surface layer (surface barrier layer capacitor) have a significant impact on the dielectric response in the CoNTO ceramics. The density functional theory calculation shows that the 2Co atoms are located near the oxygen vacancy, creating a triangle-shaped 2CoV_oTi complex defect. On the other hand, the substitution of TiO₂ with Nb atoms can form a diamond-shaped 2Nb2Ti complex defect. These two types of complex defects are far away from each other. Therefore, the electron-pinned defect dipoles cannot be considered the primary origins of the dielectric response in the CoNTO ceramics. Impedance spectroscopy shows that the CoNTO ceramics are electrically heterogeneous, comprised of insulating and semiconducting regions. Thus, the dielectric properties of the CoNTO ceramics are attributed to the interfacial polarization at the internal insulating layers with very high resistivity, giving rise to a low loss tangent.     

Preparation and Dielectric Properties of Mn-Doped Ba0.6Sr0.4TiO3-MgTiO3 Composite Ceramics

Barium strontium titanate (Ba_0.6Sr_0.4TiO₃, BST) nano-powders were prepared using Ba(NO₃)₂, Sr(NO₃)₂, oxalic acid dehydrate, and tetrabutyl titanate (Ti(OC4H9)4) as precursors by the chemical co-precipitation method. The product was characterized by thermogravimetry-differential scanning calorimetry (TG-DSC) thermal analyses, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The experimental results indicated that the resulting Ba_0.6Sr_0.4TiO₃ nano-powders were homogeneous with agglomerated nature. The Ba_0.6Sr_0.4TiO₃-MgTiO₃ (BST-MT) bulk composite ceramics doped by Mn were obtained by the traditional solid phase method. The XRD patterns demonstrated that Mn-doped BST was unable to change the perovskite crystalline structure of BST materials. SEM photographs revealed that the crystalline grains became larger with increasing the content of doping Mn (<1.5% (x, molar fraction)) and then the size of grains decreased after the Mn content exceeded 1.5% in the BST ceramics, suggesting the effect of Mn doping on the morphologies of BST-MT composites. The dielectric properties of BST-MT composite ceramics doped with 0.1%-2.0% (x) Mn were investigated systematically. Two effects of Mn doping on the dielectric properties of the BST-MT composite ceramics were observed. At low Mn doping concentrations (<1.5%), Mn mainly acted as an acceptor dopant to replace Ti at the B site of ABO3 perovskite structure, leading to a diffused phase transition. It was also observed that the grain size increased drastically as the Mn content increased and thus caused the decrease of dielectric loss. At higher Mn doping concentrations (>1.5%), the grain size decreased and the suppression of permittivity and the drastic increase of the dielectric losses were observed, which indicated a “composite” mixing effect.     

Chemical stability of Ba(Ce<Subscript>1−x</Subscript>Ti<Subscript>x</Subscript>)<Subscript>1−y</Subscript>Y<Subscript>y</Subscript>O<Subscript>3</Subscript> proton-conducting solid electrolytes

The purpose of this work was to investigate the influence of titanium and yttrium dopants on chemical stability of selected Ba(Ce_1−xTi_x)_1−yY_yO₃ compounds. The presented results are the part of wider research concerning the crystallographic structure, microstructure, electrical and transport properties of these groups of materials. Samples of Ba(Ce_1−xTi_x)_1−yY_yO₃ with x=0.05, 0.07, 0.10, 0.15, 0.20, 0.30 and y=0.05, 0.10, 0.20 (for x=0.05) were prepared by solid-state reaction method. Initially, differential thermal analysis (DTA) and thermogravimetry (TG) were used for optimization of preparation conditions. Subsequently, DTA-TG-MS (mass spectrometry) techniques were applied for evaluation of the stability of prepared materials in the presence of CO₂. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results were used to determine the phase composition, structure and microstructure of materials and to assist the interpretation of DTA-TG-MS results. The strong influence of Ti and Y dopants contents (x and y) on the properties was found. The introduction of Ti dopant led to the improvement of chemical stability against CO₂. The lower Ti concentration the better resistance against CO₂ corrosion was observed. Doping by Y had the opposite effect; the decrease of chemical stability was determined. In this case the higher Y dopant concentration the better resistance was observed. The attempt to correlate the influence of dopant on structure and chemical stability was also presented.     

Structure and real composition of undoped and Cr- and Ni-doped Sr<Subscript>0.61</Subscript>Ba<Subscript>0.39</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> single crystals

Strontium barium niobate crystals with congruent melting composition Sr_0.61Ba_0.39Nb₂O₆ (SBN-61), both nominally pure and doped with Cr³⁺ и Ni³⁺ ions, have been investigated by neutron diffraction. Different strontium and barium contents as well as their different distribution over the Sr1, of Sr2 and Ba2 crystallographic sites of SBN-61 structure, caused by introduction of dopants, have been revealed. Coordination polyhedra of cations have been established based on the analysis of cation–anion internuclear distances together with the calculation of bond-valence sums for cations, which are equal to their formal charge. It was found that the Nb1 and Nb2 atoms are located in distorted octahedra with quadfurcated (the Nb1O₆ polyhedron) or bifurcated (the Nb2O₆ polyhedron) vertices, and the Sr1 atoms are located in a cuboctahedron with bifurcated vertices in the base plane. Different polyhedra have been revealed for the Sr2 and Ba2 atoms: Sr2 atoms are coordinated by 15 oxygen atoms to form a highly distorted five-capped pentagonal prism, whereas Ba2 atoms are located in a highly distorted three-capped trigonal prism with a coordination number 9. Comparison of interatomic and internuclear distances, determined by X-ray and neutron diffraction analyses, respectively, allowed to reveal a highly pronounced shift of electron density in Nb1 and Sr2 polyhedra, responsible for the covalent bond and properties of crystals. Location of Cr³⁺ и Ni³⁺ dopant ions in the SBN-61 structure as well as their formal charges has been discussed.     

XAS investigation of tantalum and niobium in nanostructured TiO2 anatase

Sol-gel routes were used to prepare Ta 10 at% and Nb 5 at% and 10 at% doped titania nanosized powders. When fired between 410°C and 850°C the doped titania powders are in the anatase phase; further heating up to 1050°C is required to obtain the rutile phase. The presence of dopant atoms delays the rate of transformation as compared with pure titania powders. Doping also affects the rate of grain growth and increases the conductance response to gas. To better understand the role played by dopant atoms in inhibiting both phase transformation to rutile and grain growth, X-ray Absorption Spectroscopy measurements were performed at the L_III-L_I absorption edges of Ta and Nb K absorption edge. Analysis was restricted to the anatase phase because the transformation to rutile phase, obtained by firing at 1050°C, is accompanied by the formation of undesired Ta and Nb oxides (Ta₂O₅ and Nb₂TiO₇, respectively). Extended X-ray Absorption Fine Structure and X-ray Absorption Near-Edge Spectroscopy analysis results indicate that in nanostructured anatase both tantalum and niobium atoms substitute Ti cations with +5 valence state.     

Annealing of thin B/Nb<Subscript>2</Subscript>N bilayers,B/Nb bilayers and Nb/B/Nb trilayers via rapid thermal processing (RTP)

B/Nb and B/Nb₂N bilayers and Nb/B/Nb trilayers of about 550 nm total thickness have been deposited on Si(100) wafers with 100 nm thermally grown oxide. Nb and B layers were deposited by magnetron sputtering. Nb₂N layers were prepared by nitridation of Nb films via rapid thermal processing (RTP). The samples were annealed subsequently at temperatures between 600 and 1,200 °C in an RTP system under Ar or NH₃ gas flow to study interdiffusion and reactivity of niobium, boron and nitrogen. Formation of phases was investigated by X-ray diffraction (XRD); surface morphology and roughness were studied via scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. Elemental depth profiles of selected samples were recorded by secondary ion mass spectrometry (SIMS). Annealing of the B/Nb bilayers and Nb/B/Nb trilayers under Ar leads to the formation of Nb₃B₂ at 1,200 °C at the B/Nb interface. At lower temperatures the high oxygen content in the boron layer is supposed to hinder the formation of borides due to formation of glass-like boron oxides. In NH₃ several niobium nitrides are formed but no boride phases. Here again the reactivity of boron with niobium is suppressed by the high oxygen content and boron oxide formation. During annealing of the B/Nb₂N bilayers no borides were formed indicating that well-formed Nb₂N is an effective diffusion barrier for B.     

Oriented growth of Srn+1TinO3n+1 Ruddlesden-Popper phases in chemical solution deposited thin films

Oriented thin films of perovskite-related Sr_n+1Ti_nO_3n+1 Ruddlesden-Popper phases (n=1, 2, 3) were grown on (001) single-crystalline SrTiO₃ substrates. Preparation of the films was carried out by wet chemical deposition from metalorganic Sr-Ti solutions (rich in Sr) and subsequent conversion into the crystalline state by thermal treatment in air atmosphere at a maximum temperature of 700 °C. Solutions were prepared by a modified Pechini method. The films were investigated by wide-angle X-ray scattering and high-resolution transmission electron microscopy. The phase content of powders prepared from the dried solutions and annealed under similar conditions differed from that present in the films, i.e. only polycrystalline SrTiO₃ was detected together with oxides of Ti and Sr.     

Preparation and Characteristics of Nb<Superscript>5+</Superscript>, Ta<Superscript>5+</Superscript>/TiO<Subscript>2</Subscript> Nanoscale Powders by Sol–Gel Process Using TiCl<Subscript>3</Subscript>

Nanoscale TiO₂ powders doping with niobium and tantalum were prepared using TiCl₃ as a source matter. Characterization of the materials was performed by Thermoanalys, particle size, XRD, BET, FTIR, Magnetic Susceptibility. The influence of niobium and tantalum ions on the phase transition was studied, the changes in the crystal size and microstain distributions obtained at 400C were analyzed. The results show that the substitutes of Nb^{5 +}, Ta^{5 +} for Ti⁴⁺ in the anatase structure cause distortions and improve to form rutile. When the dopant content is over certain molar percent, biphase reappears. The IR spectra and magnetic susceptibility indicate the Nb–Nb (or Ta–Ta) bonds along c-axis in rutile by two Nb^{5 +} (Ta^{5 +}) ions located in sites adjacent along the c-axis appear with the dopant content. The magnetic characteristics at rutile showed a weak paramagnetism.     

Synthesis,structure and magnetic properties of a trinuclear π-cyclopentadienylniobium complex containing the formate,hydroxylic and oxygen bridges

The trinuclear complex [C₅H₅NbOOCH]₃ (OH)₂ (O)₂ was prepared by heating C₅H₅Nb(CO)₃PPh₃ with anhydrous HCOOH in xylene. The niobium atoms are located at the vertices of an almost regular triangle with short Nb?Nb distance equal to 3.14». The electronic configuration of niobium is d¹. Analysis of the magnetic properties and location of the niobium atoms indicated that they bind only by the formate, hydroxylic and oxygen bridges instead of by direct NbNb bonds.     

Oxidizing valve metals: effect of electronic properties of the oxide films

Photocurrents emerging during the formation of anodic oxide films (AOF) on such valve metals as W, Ti, Zr, Nb, Ta are measured during an increase (direct run) and a decrease (reverse run of voltammetric curves) in the anodic potential. Capacitances of AOF formed at certain potentials are measured at potentials below the AOF formation potential. Effect of semiconductor properties on the AOF growth is considered through the formation of a Schottky barrier at the oxide/electrolyte interface. Calculated thicknesses of AOF and the depleted layer are compared. The donor-concentration drop in AOF with the distance from the metal/oxide interface is a condition for the growth of thick semiconductor oxide films. The measured potential dependence of the semiconductor-film capacitance is used to plot the donor concentration drop as a function of the distance from the Nb₂O₅/Nb interface in the oxide layer on a niobium electrode. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Compositional evolution of zirconium and niobium in the process of high-temperature nitridation of Zr–Nb alloys

The mechanism of formation of zirconium nitride-based ceramics in the single-stage high-temperature nitridation of Zr–Nb alloys with different niobium contents (0.1–10 at%) in a wide temperature range was determined using an oxidative constructing approach. In the process of nitridation, the segregation of a metallic niobium phase occurred with the subsequent formation of NbN. This should be taken into account when producing new high-temperature materials with unique functional properties     

Determination of impurities in strontium titanate ceramics by inductively coupled plasma mass spectrometry

The determination of impurities in SrTiO₃ by ICPMS was investigated. The sample was decomposed with hydrochloric and hydrofluoric acids in a PTFE pressure vessel. The internal standard method using Au was selected to eliminate an ion count suppression by the Sr and Ti matrix. Impurities at sub g/ml level in SrTiO₃ were determined. The detection limits were in the range of 0.008 to 0.01 g/g.     

Site‐Selected Doping of Upconversion Luminescent Er3+ into SrTiO3 for Visible‐Light‐Driven Photocatalytic H2 or O2 Evolution

A series of upconversion luminescent erbium‐doped SrTiO₃ (ABO₃‐type) photocatalysts with different initial molar ratios of Sr/Ti have been prepared by a facile polymerized complex method. Er³⁺ ions, which were gradually transferred from the A to the B site with increasing Sr/Ti, enabled the absorption of visible light and the generation of high‐energy excited states populated by upconversion processes. The local internal fields arising from the dipole moments of the distorted BO₆ octahedra promoted energy transfer from the high‐energy excited states of Er³⁺ with B‐site occupancy to the host SrTiO₃ and thus enhanced the band‐to‐band transition of the host SrTiO₃. Consequently, the erbium‐doped SrTiO₃ species with B‐site occupancy showed higher photocatalytic activity than those with A‐site occupancy for visible‐light‐driven H₂ or O₂ evolution in the presence of the corresponding sacrificial reagents. The results generally suggest that the introduction of upconversion luminescent agents into host semiconductors is a promising approach to simultaneously harnessing low‐energy photons and maintaining redox ability for photocatalytic H₂ and O₂ evolution and that the site occupancy of doped elements in ABO₃‐type perovskite oxides greatly determines the photocatalytic activity.     

First‐principles calculation of electronic structure of V‐doped anatase TiO2

The energetic and electronic structures of V‐doped anatase TiO₂ have been investigated systematically by the GGA+U approach, including replacement of Ti by V in the absence and presence of oxygen vacancies and the presence of an interstitial site. It was found that V should exist as a V⁴⁺ ion in the replacement of Ti in the anatase lattice, the electron transitions of which to the conduction band from V 3d states are responsible for the experimentally observed visible light absorption. The influence of V dopant concentration on the electronic and magnetic properties is also discussed, such as the influence of the U value in systems containing oxygen vacancies and spin flip phenomena for interstitial V‐doping.     

锰掺杂Ba0.6Sr0.4TiO3-MgTiO3复相陶瓷的制备和介电性能

以分析纯的Ba(NO3)2、Sr(NO3)2、草酸和钛酸丁酯为原料, 采用草酸盐共沉淀法制备了钛酸锶钡（Ba0.6Sr0.4TiO3, BST）纳米粉体. XRD和SEM分析结果表明, 该方法制备出立方相的Ba0.6Sr0.4TiO3粉体, 平均粒径小于100 nm, 具有较高的烧结活性. 用传统固相法制备了锰掺杂钛酸锶钡-钛酸镁(Ba0.6Sr0.4TiO3-MgTiO3, BST-MT)复相陶瓷, 系统研究了掺杂0.1%-2.0%(x, 摩尔分数, 下同)锰对钛酸锶钡-钛酸镁复相陶瓷微观形貌和介电性能的影响机理. 结果表明, 当锰的掺杂量小于1.5%时, Mn作为受主掺杂取代占据钙钛矿ABO3的B位, 因此导致居里点略微向高温偏移和相变扩散的发生, 锰的掺杂导致晶格畸变, 促进了晶粒生长, 使晶界相比例下降, 因此介电损耗随着锰掺杂量的增大而减小; 当锰的掺杂量为1.5%时, 介电损耗达到最小值, 继续增大掺杂量, 介电常数下降, 介电损耗上升.