Substrate temperature effects on the structural,compositional, and electrical properties of VO2 thin films deposited by pulsed laser deposition

The vanadium dioxide (VO₂) thin films were deposited on silicon (100) substrate using the pulsed laser deposition technique. The thin films were deposited at different substrate temperatures (500°C, 600°C, 700°C, and 800°C) while keeping all the other parameters constant. X‐ray diffraction confirmed the crystalline VO₂ (B) and VO₂ (M) phase formation at different substrate temperatures. X‐ray photoelectron spectroscopy analysis showed the presence of V⁴⁺ and V⁵⁺ charge states in all the deposited thin films which confirms that the deposited films mainly consist of VO₂ and V₂O₅. An increase in the VO₂/V₂O₅ ratio has been observed in the films deposited at higher substrate temperatures (700°C and 800°C). Scanning electron microscope micrographs revealed different surface morphologies of the thin films deposited at different substrate temperatures. The electrical properties showed the sharp semiconductor to metal transition behavior with approximately 2 orders of magnitude for the VO₂ thin film deposited at 800°C. The transition temperature for heating and cooling cycles as low as 46.2°C and 42°C, respectively, has been observed which is related to the smaller difference in the interplanar spacing between the as‐deposited thin film and the standard rutile VO₂ as well as to the lattice strain of approximately −1.2%.     

Phase formation in the Ag3VO4-ScVO4 quasi-binary system

Subsolidus phase formation in the Ag₃VO₄-ScVO₄ quasi-binary system was studied using X-ray powder diffraction and DTA in air under atmospheric pressure over the entire range of component concentrations in 5 mol % steps between 20 and 800°C. Two compounds were found to form: Ag₃Sc(VO₄)₂ and Ag₃Sc₂(VO₄)₃, both melting incongruently at 750 ± 5 and 960 ± 5°C, respectively. A T-x diagram of the Ag₃VO₄-ScVO₄ quasi-binary system was constructed. A eutectic (T _m = 450 ± 5°C) is between the compounds Ag₃VO₄ and Ag₃Sc(VO₄)₂; Ag₃VO₄ concentration is ～5 mol %.     

Substitutions of neodymium for strontium in the Sr5(VO4)3OH structure

Neodymium-for-strontium isomorphic substitutions in synthetic hydroxovanadate Sr_{5 ? x} Nd _x (VO₄)₃(OH)_{1 ? x} O _x were studied from 600 to 800°C using powder and single-crystal X-ray diffraction (the Rietveld technique) and IR spectroscopy. Single-phase solid solutions at 800°C are formed in the range up to x = 0.14. At higher x, a Sr₃(VO₄)₂ phase exists along with the apatite solid solution. In the range of 0 < x ≤ 0.14, the intensity of the IR bands due to the stretching and libration vibrations of OH groups decreases with increasing x. Single-crystal X-ray diffraction shows that neodymium mostly substitutes for strontium atoms at the Sr(1) sites in the solid solution with x = 0.14.     

pH-Metrische Studien an den binären Komplexen von Oxovanadium(IV) mit Hippur- und Anthranilsäure und Pyridin-2-aldoxim

pH-metric studies on the interaction of oxovanadium(IV) with hippuric and anthranilic acids and pyridine-2-aldoxime indicate the formation of monohydroxo derivatives of 1:1 chelates. The equilibrium constants for the reaction, VO²⁺+HA+H₂O?VO(OH)A+2H⁺ have been calculated as 4.47±0.07 and 6.32±0.05 in the 1:1 VO²⁺-hippuric or anthranilic acid systems resp. and for the reaction, VO²⁺+H₂ A ⁺+H₂O?VO(OH)A+3H⁺ as 8.40±0.09 in the 1:1 VO²⁺-pyridine-2-aldoxime hydrochloride system at 30±0.5°C (μ=0.1-KNO₃).     

Darstellung und elektronenmikroskopische Untersuchung neuer Verbindungen des Typs Ln3MO4Cl5 (Ln = La?Nd; M = Ge,V)

Preparation and Electronmicroscopic Investigation of New Compounds Ln₃MO₄Cl₅ (Ln = La? Nd; M = Ge, V) By heating mixtures of LnOCl, LnCl₃ und GeO₂ (2:1:1) in evacuated silica tubes (Pt-shells inside) the compounds Ln₃GeO₄Cl₅ (Ln = La? Nd) were prepared. The case that the temperature of preparation (La: T = 900°C, 8d; Ce: T = 800°C, 9d; Pr, Nd: T = 650°C, 13 d) had to be reduced from Ln = La to Ln = Nd indicates a decreasing thermodynamic stability in this direction. The compound La₃VO₄Cl₅ was prepared by heating (900°C, 8d) a mixture (2:1:1) of LaOCl, LaCl₃ and VO₂ and was investigated by electronmicroscopic techniques.     

Isomorphic substitution of neodymium and gadolinium for calcium in synthetic hydroxovanadate

Isomorphic substitution of neodymium and gadolinium for calcium in synthetic hydroxovanadate Ca_{5 ? x} M _x (VO₄)₃(OH)_{1 ? x} O _x (M = Nd, Gd) is studied in the range 700–1000°C using X-ray powder diffraction, single-crystal X-ray diffraction (Rietveld technique), and IR spectroscopy. Single-phase solid solutions at 800°C are formed with x ≤ 0.35 for M(III) = Nd and x ≤ 0.3 for M(III) = Gd. With high x, the apatite solid solution coexists with Ca₃(VO₄)₂, Nd₂O₃, and X phases. With increasing x in the homogeneous region, the intensity of the bands of stretching vibrations and librations of OH groups decrease. Single-crystal X-ray diffraction shows that neodymium and gadolinium substitute for calcium in solid solutions mostly in Ca(2) positions.     

A Novel Route for the Easy Production of Thermochromic VO2 Nanoparticles

In this work, a simple, fast and dry method for the fabrication of a thermochromic product with a high load of VO₂(M1) consisting of the controlled heat treatment of pure vanadium nanoparticles in air is presented. After a complete design of experiments, it is concluded that the most direct way to attain the maximum transformation of V into VO₂(M1) consists of one cycle with a fast heating ramp of 42 °C s⁻¹, followed by keeping 700 °C for 530–600 seconds, and a subsequent cooling at 0.05 °C s⁻¹. Careful examination of these results lead to a second optimum, even more suitable for industrial production (quicker and less energy-intensive because of its lower temperatures and shorter times), consisting of subjecting V to two consecutive cycles of temperatures and times (625 °C for 5 minutes) with similar preheating (42 °C s⁻¹) but a much faster postcooling (∼ 8 °C s⁻¹). These green reactions only use the power for heating a tube open to atmosphere and a vanadium precursor; without assistance of reactive gases or catalysts, and no special vacuum or pressure requirements. The best products present similar thermochromic properties but higher thermal stability than commercial VO₂ particles. These methods can be combined with VO₂ doping.     

Synthesis of Sb(VO3)3 and study of ternary system xNH4VO3?+?(1???x)(NH4)2HPO4?+?Sb2O3

Sb(VO₃)₃ has been synthesized by interaction between NH₄VO₃ and Sb₂O_3. The compound crystallizes in monoclinic system with lattice parameters: a = 17.150; b = 15.940; c = 14.600 Å and angle ?? = 90.50°. The scanning electronic microscopy shows thin flat plates measuring ~20 ??m along with detritus material. The synthesis was simulated by thermal analysis and the final product identified by X-ray diffraction. Thermal analyses of the ternary system xNH₄VO₃ + (1 ? x)(NH₄)₂HPO₄ + Sb₂O₃ lead to the formation of Sb(VO₃)₃ and SbPO₄ at 500 °C. At high temperature (900 °C), SbVO₄, SbOPO₄, VO and SbP₅O₁₄ are formed. The data of thermal analysis match with the composition of intermediate and final products. No solid solutions containing simultaneously PO ₄ ^?3 and VO ₄ ^?3 ions have been found.     

Synthese en milieu hydrothermal et caractérisation de l'oxyhydroxyde de vanadium V3+OOH et d'une nouvelle variété allotropique du dioxyde VO2

Vanadium oxyhydroxide containing mostly trivalent vanadium was synthesized under hydrothermal conditions at 200°C and 2 k bar by hydrolysis of NaVO₃ previously reduced under hydrogen. The structure of VOOH is of diaspore type, isotypic with A1OOH. Oxidation of VOOH at 80°C in air gives in a few days a new metastable phase of vanadium dioxide VO₂; the structure of this new phase is also of the diaspore type, the transformation VOOH → VO₂ being topotactic. Characterization of these phases was done by X-ray diffraction, thermogravimetry, and magnetic, and electrical measurements. Both phases show semiconductor behavior and no electronic transition is observed in VO₂ (diaspore) up to 200°C.     

Supercapacitors based on carbide-derived carbons synthesised using HCl and Cl2 as reactants

Micro- and mesoporous carbide-derived carbons (CDCs) were synthesised from TiC powder via a gas-phase reaction using HCl and Cl₂ within the temperature range of 700–1,100 °C. Analysis of X-ray diffraction results show that TiC-CDCs consist mainly of graphitic crystallites. The first-order Raman spectra showed the graphite-like absorption peaks at ~1,577 cm^?1 and the disorder-induced peaks at ~1,338 cm^?1. The low-temperature N₂ sorption experiments were performed, and specific surface areas up to 1,214 and 1,544 m²?g^?1 were obtained for TiC-CDC (HCl) synthesised at T?=?800 °C and TiC-CDC (Cl₂) synthesised at T?=?900 °C, respectively. For the TiC-CDC powders synthesised, a bimodal pore size distribution has been established with the first maximum in the region up to 1.5 nm and the second maximum from 2 to 4 nm. The energy-related properties of supercapacitors based on 1 M (C₂H₅)₃CH₃NBF₄ in acetonitrile and TiC-CDC (Cl₂) and TiC-CDC (HCl) as electrode materials were also investigated by cyclic voltammetry, impedance spectroscopy, galvanostatic charge/discharge and constant power methods. The specific energy, calculated at U?=?3.0 V, are maximal for TiC-CDC (Cl₂ 800 °C) and TiC-CDC (HCl 900 °C), which are 43.1 and 31.1 W?h?kg^?1, respectively. The specific power, calculated at cell potential U?=?3.0 V, are maximal for TiC-CDC (Cl₂ 1,000 °C) and TiC-CDC (HCl 1,000 °C), which are 805.2 and 847.5 kW?kg^?1, respectively. The Ragone plots for CDCs prepared by using Cl₂ or HCl are quite similar, and at high power loads, the TiC-CDC material synthesised using Cl₂ at 900 °C, i.e. the material with optimal pore structure, delivers the highest power at constant energy.     

On the oxidation behaviour of monolithic TiB2 and Al2O3-TiB2 and Si3N4-TiB2 composites

In view of the susceptibility of TiB₂ to oxidation, the thermal stability of monolithic TiB₂ and of Al₂O₃-30 vol% TiB₂ and Si₃N₄-20 vol% TiB₂ composites was investigated. The temperature at which TiB₂ ceramic starts to oxidize is about 400°C, oxidation kinetics being controlled by diffusion up toT≈900°C and in the first stage of the oxidation at 1000°C and 1100°C (up to 800 min and 500 min respectively), and by a linear law at higher temperatures and for longer periods. Weight gains in the Al₂O₃-TiB₂ composite can be detected only at temperatures above ≈700°C and the rate governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism atT=700°C andT=800°C and by two-dimensional diffusion at higher temperatures. Concerning the Si₃N₄-TiB₂ composite, three different oxidation behaviours related to the temperature were observed, i.e. up to ≈1000°C the reaction detected regards only the second phase; at ≈1000<T<≈1200°C, the diffusion of O₂ or N₂ through an oxide layer is proposed as the rate-governing step; atT〉=1200°C, a linear kinetic indicates the formation of a non protective scale.     

Doping effects of Yb3+ on the crystal structures,nanoparticle properties and electrical behaviors of ZrO2 derived from a facile urea-based hydrothermal route

Weakly-agglomerated nanocrystalline (ZrO₂)_1−x(Yb₂O₃)_x (x=0.02–0.2) powders with high surface area (109–151 m² g⁻¹) were synthesized by a two-step hydrothermal process in the presence of urea: a stock solution of metal nitrates and urea was heated at 80 °C for 24 h and then at 180 °C for 48 h. For x=0.04–0.2, the as-derived powders were an assembly of uniform nanoparticles with well-defined edges in the size between 6.1–8.4 nm. Before and after calcination at 800 °C, the lattice parameters, microstrain and surface area of the (ZrO₂)_1−x(Yb₂O₃)_x samples tended to increase with Yb³⁺ concentration; while, the average crystallite size decreased correspondingly. In the Arrhenius plots over the measurement temperature range of 400–800 °C, the bulk ionic conductivity of the compacts sintered at 1400 °C for 24 h showed a maximum value at the composition of x=0.08 in cubic structure, with an activation energy of 0.89 eV. At 800 °C, σ_b=0.049 S cm⁻¹ for x=0.08.     

The thermal decomposition of (NH4)4V2O11 in a nitrogen atmosphere and the kinetics of the first decomposition step

Although the reaction products are unstable at the reaction temperatures, at a heating rate of 2 deg·min^?1 ammonium peroxo vanadate, (NH₄)₄V₂O₁₁, decomposes to (NH₄)[VO (O₂)₂ (NH₃)] (above 93°C); this in turn decomposes to (NH₄) [VO₃ (NH₃)] (above 106°C) and then to ammonium metavanadate (above 145°C). On further heating vanadium pentoxide is formed above 320°C. The first decomposition reaction occurs in a single step and the Avrami-Erofeev equation withn=2 fits the decomposition data best. An activation energy of 148.8 kJ·mol^?1 and a ln(A) value of 42.2 are calculated for this reaction by the isothermal analysis method. An average value of 144 kJ·mol^?1 is calculated for the first decomposition reaction using the dynamic heating data and the transformation-degree dependence of temperature at different heating rates.     

Ultrathin free-standing electrospun carbon nanofibers web as the electrode of the vanadium flow batteries

Ultrathin free-standing electrospun carbon nanofiber web(ECNFW) used for the electrodes of the vanadium flow battery(VFB) has been fabricated by the electrospinning technique followed by the carbonization process in this study to reduce the ohmic polarization of the VFB. The microstructure, surface chemistry and electrochemical performance of ECNFW carbonized at various temperatures from 800 to 1400 °C have been investigated. The results show that ECNFW carbonized at 1100 °C exhibits the highest electrocatalytic activity toward the V~(2+)/V~(3+)redox reaction, and its electrocatalytic activity decreases along with the increase of carbonization temperature due to the drooping of the surface functional groups.While for the VO~(2+)/VO_2~+redox couple, the electrocatalytic activity of ECNFW carbonized above 1100 °C barely changes as the carbonization temperature rises. It indicates that the surface functional groups could function as the reaction sites for the V~(2+)/V~(3+)redox couple, but have not any catalytic effect for the VO~(2+)/VO_2~+redox couple. And the single cell test result suggests that ECNFW carbonized at 1100 °C is a promising material as the VFB electrode and the VFB with ECNFW electrodes obtains a super low internal resistance of 250 mΩ cm~2.     

Li, K‖Br, VO3 ternary mutual system

Phase equilibria in the Li, K‖Br, VO₃ ternary mutual system were studied by differential thermal analysis. The composition square of the ternary mutual system is divided into two phase triangles, LiVO₃-KBr-KVO₃ and LiBr-LiVO₃-KBr. The ternary eutectics E ₁ at 331°C and E ₂ at 330°C have the compositions 40.0 mol % LiVO₃, 6.0 mol % KBr, 54.0 mol % KVO₃ and 58.0 mol % LiBr, 3.2 mol % LiVO₃, 38.8 mol % KBr, respectively. The fields of phases crystallizing in the system were delimited.     

Thermal stability of the manganese-nickel mixed ferrite and iron phases in the Mn0.5Ni0.5Fe2O4/Fe composite/nanocomposite powder

The composite/nanocomposite powders of Mn_0.5Ni_0.5Fe₂O₄/Fe type were synthesized starting from nanocrystalline Mn_0.5Ni_0.5Fe₂O₄ (D = 7 nm) (obtained by ceramic method and mechanical milling) and commercial Fe powders. The composites, Mn_0.5Ni_0.5Fe₂O₄/Fe, were milled for up to 120 min and subjected to heat treatment at 600 °C and 800 °C for 2 h. The manganese-nickel ferrite/iron composite samples were subjected to differential scanning calorimetry (DSC) up to 900 °C for thermal stability investigations. The composite component phases evolution during mechanical milling and heat treatments were investigated by X-ray diffraction technique. The present phases in Mn_0.5Ni_0.5Fe₂O₄/Fe composite are stable up to 400–450 °C. In the temperature range of 450-600 °C, the interdiffusion phenomena occurs leading to the formation of Fe_1?xMn_xFe₂O₄/Ni–Fe composite type. The new formed ferrite of Fe_1?xMn_xFe₂O₄ type presents an increased lattice parameter as a result of the substitution of nickel cations into the spinel structure by iron ones. Further increases of the temperature lead to the ferrite phase partial reduction and the formation of wustite-FeO type phase. The spinel structure presents incipient recrystallization phenomena after both heat treatments (600 °C and 800 °C). The mean crystallites size of the ferrite after heat treatment at 800 °C is about 75 nm. After DSC treatment at 900 °C, the composite material consists in Fe_1?xMn_xFe₂O₄, Ni structure, FeO, and (NiO)_0.25(MnO)_0.75 phases.     

Phase Transformations in the Mn–Ga–O System Depending on the Preparation Conditions

Manganese-gallium samples with cation ratios Mn:Ga = 1:2, 1.5:1.5, and 2:1 are synthesized by coprecipitation with subsequent annealing in air in a temperature range 600–1200 °C. Powder XRD, TEM, and BET methods are used to study the physicochemical characteristics of the samples. It is found that in the air at the annealing temperature of 600 °C finely dispersed low-temperature Mn_3–xGa_xO₄ spinels primarily form in all series, but in the whole temperature range (600–1200 °C) the system is multiphase. Annealing at 800–1200 °C leads to an increase in the concentration of simple oxides (β-Mn₃O₄ and β-Ga₂O₃). Only simple α-Mn₂O₃ and β-Ga₂O₃ oxides exist in a Mn:Ga = 2:1 series at 800 °C. In the sample with a cation ratio Mn:Ga = 1.5:1.5 annealed in air at 1000 °C, the formation of a superstructure based on the spinel structure is found.     

Synthesis and characterization of PrBaCo2 − x Ni x O5 + δ cathodes for intermediate temperature SOFCs

Nickel-substituted layered perovskite PrBaCo_{2 ? x} Ni _x O_{5 + δ} (PBCN) powders with various proportions of nickel (x?=?0, 0.1, 0.2, and 0.3, abbreviated as PBCN-0, PBCN-1, PBCN-2, and PBCN-3, respectively) are investigated as potential cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) based on the yttria-stabilized zirconia (YSZ) electrolyte. It is found that PBCN-1 has the highest electrical conductivity of 1,397 S cm^?1 at 400 °C. Substitution of Co by Ni decreases the thermal expansion coefficient (TEC) clearly. The average TEC at the temperature range of 35–900 °C decreases from 22.8?×?10^?6 K^?1 for PBCN-0 to 18.9?×?10^?6 K^?1 for PBCN-3. The polarization resistances of PBCN samples on YSZ electrolyte at 800 °C are 0.053, 0.048, 0.052, and 0.042 Ω cm² for PBCN-0, PBCN-1, PBCN-2, and PBCN-3, respectively. The single fuel cell with the configuration of PBCN-3/YSZ/Pt delivers the highest power densities of 100, 185, 360, 495, and 660 mW cm^?2 at 600, 650, 700, 750, and 800 °C, respectively.     

Thermal stability of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of an α-Bi₂O₃ sample revealed staged phase transitions in the range 720–800°C (at 720, 780, and 800°C) and the elimination of oxygen to the composition Bi₂O_2.967 during heating to 895°C in air at 16 K/min. In dynamic vacuum (p = 1.33 Pa) at 780–800°C, Bi₂O₃ consecutively transforms to a phase with the cubic γ-Bi₂O₃ structure and tetragonal Bi₂O_2.3?2.4. In the latter, electron diffraction in a transmission electron microscope (ED/TEM) shows a superstructure with the superstructure vector q ₁₁₀ ≈ 1/9, which indicates an ordered arrangement of oxygen vacancies.     

Electrode performance and X-ray absorption spectroscopy of La0.8Sr0.2Mn1−x Cu x O3 (0 ≤ x ≤ 0.2)–GDC composite cathodes for SOFCs

Electrochemical properties of composite cathodes consisting of La_0.8Sr_0.2Mn_1?x Cu _x O₃ (LSMCu, 0?≤?x?≤?0.2) and Ce_0.8Gd_0.2O_2?x (GDC) were determined by impedance spectroscopy, and conduction mechanism for the composite cathodes was investigated by a near-edge X-ray absorption fine-structure analysis (NEXAFS). LSMCu–GDC cathodes showed lower polarization resistance (R _p) than LSM–GDC up to 750 °C, whereas they exhibited better performance at higher temperature (≥800 °C). The best performance was achieved with the LSMCu10–GDC cathode: 0.27 and 0.08?Ω cm² at 800 °C and 850 °C, respectively. NEXAFS and refinement results confirmed that Cu doping caused the oxidation of Mn³⁺ to Mn⁴⁺ and lattice contraction. This additional Mn⁴⁺ can lead to the formation of oxygen vacancies when Mn⁴⁺ is converted to Mn³⁺ at relatively high temperatures (above 600 °C). This in turn contributes to improved oxygen ion transport in LSM. The LSMCu–GDC composite cathode can thus be considered a suitable potential cathode for SOFC applications.