Pure V2O3 Research Articles

Investigation on characteristic of vanadium trioxide insulation mixed with metal powder for rare-earth barium copper oxide coils

This study examined the turn-to-turn contact resistance (R ct) between rare-earth barium copper oxide (REBCO) tapes and layers of vanadium trioxide (V2O3) and V2O3 mixed with metal powder mixture. V2O3 in single crystal structure was electrically characterised to exhibit resistivity with negative temperature dependence, allowing the turn-to-turn insulation to self-regulate the current bypass between REBCO tapes. To facilitate effective quench protection of V2O3-insulated REBCO magnets above the metal-insulator transition temperature (T rt), R ct must be further reduced to a level similar to those of non- and metal as insulated (NI and MI) REBCO magnets. Thus, we explored the mixing of conductive metal powders such as molybdenum (Mo) with V2O3 paste and investigated the transition properties of R ct. The resistance versus temperature characteristics, microscopic morphologies of the V2O3 layers, and thermal conductivity (k v) were appropriately assessed to determine the effects of mixing the metal powder with V2O3. The R ct of virgin V2O3 exhibited variations of 107–105 μΩ cm2 under 77–293 K. As the mixing concentration of the metal powder was increased, the reduction magnitude on R ct increased for > T rt (approximately 150 K). Furthermore, the transition slope became gentler for a wider temperature range of < T rt. For metal powder concentrations exceeding 50 wt%, R ct decreased by approximately 2 orders of magnitude (∼103 μΩ cm2) for > 150 K compared with that for virgin V2O3 paste. Moreover, compared to that of pure V2O3, k v demonstrated a remarkable increase of approximately 352% at 91 K for Mo powder mixed at a concentration of 60 wt%. The improved electrical and thermal properties of the V2O3 insulation layer owing to the mixing of metal powders can help REBCO magnets operate in an insulated state under normal conditions and effectively convert to a non-insulated state under quenching.

Positive and Negative Pressure Regimes in Anisotropically Strained V2O3 Films

AbstractThe metal‐insulator phase transitions in V2O3 are considered archetypal manifestations of Mott physics. Despite decades of research, the effects of doping, pressure, and anisotropic strains on the transitions are still debated. To understand how these parameters control the transitions, anisotropically strained pure V2O3 films are explored with nearly the same contraction along the c‐axis, but different degrees of ab‐plane expansion. With small ab‐plane expansion, the films behave similar to bulk V2O3 under hydrostatic pressure. However, with large ab‐plane expansion, the films are driven into the “negative pressure” regime, similar to that of Cr‐doped V2O3, exhibiting clear coexistence of paramagnetic insulator and paramagnetic metal phases between 180–500 K. This shows that c‐axis contraction alone, or an increase in c/a ratio is insufficient for inducing “negative pressure” effects. Actually, c‐axis contraction alone destabilizes the two insulating phases of V2O3, whereas a‐axis expansion tends to stabilize them. The effects of strain are modeled using density functional theory providing good agreement with experimental results. The findings show that chemical pressure alone cannot account for the phase diagram of (V1−xCrx)2O3. This work enables to manipulate a Mott transition above room temperature, thereby expanding the opportunities for applications of V2O3 in novel electronics.

Low-temperature electrical transport and tunable optical properties of Mo-doped V2O3 thin films

This work reports the structural and low-temperature electrical transport properties of Mo doped V2O3 single-layer thin films (i.e., with compositions between Mo and V2O3 [V2−xMoxO3−δ (x = 0, 0.05–0.1)]) deposited on alumina (Al2O3) substrates by sol–gel dip-coating technique. The crystallinity of the V2O3 films is modified with the high concentration of Mo doping and exhibits a relatively defective structure compared to that of pure V2O3 films. The low temperature (i.e., over a temperature range of 253.15 to 273.15 K) electrical hysteresis curves reveal that the triggering of metal–insulator transition at low temperatures. The metal–insulator transition (MIT) temperature (Tc) values are found to be 258.49 and 261.11 K for 8 and 10 mol% Mo doped V2O3 films, respectively, which are higher than that of undoped V2O3 (Tc = 253.09 K). The experimental results contribute a promising method to develop a novel material for high-performance low-temperature engineering applications.

The fabrication of V2O3 by using NH4VO3 as vanadium source without extra reductant under sealed condition

NH3 produced by the thermolysis of NH4VO3 is a good reductant for the fabrication of low valence vanadium oxides. In this study, the thermodynamics of V–O and V–O–NH3 was analyzed to provide a theoretical basis for the preparation of V2O3 using NH4VO3. Based on this analysis, the thermolysis processes of NH4VO3 under different conditions (air, nitrogen and sealed atmosphere) were investigated. Compared with the air and nitrogen conditions, the sealed condition fully utilises NH3 escaping from NH4VO3 and is more suitable for the low-cost fabrication of V2O3. First, the only product of thermolysis under the dynamic air condition was V2O5 due to the high oxygen partial pressure. Second, the fabrication of VO2 under the static air and nitrogen conditions was feasible with a high temperature (＞625 ​°C). The formation of VO2 in these conditions is explained by the instability of V2O5, which decomposed into VO2 at a temperature higher than 625 ​°C. Finally, under the sealed condition at 700 ​°C, pure V2O3 was obtained and NH4VO3 underwent the following phase transition: NH4VO3→ (NH4)4(V6O16) → V4O7→ V3O5→V2O3.

Hydrothermal precipitation of V3+ ions in V-bearing oxalic acid leachate to prepare V2O3

In our previous study, a novel method of direct leaching of V3+ ions in iron vanadate spinel minerals by oxalic acid synergized with Fe powder under hydrothermal conditions was developed. In the present study, we attempted to prepare V2O3 from this V-bearing oxalic acid leachate. First,the dissolution and precipitation behavior of V3+ ions in the V2O3-H2C2O4-H2O system were investigated at various mass ratios of V2O3 to H2C2O4 and temperatures. Then, according to the hydrothermal features of this system, two routes of preparing V2O3 were designed. In Route A, a certain amount of pure V2O3 reagent was added into the pregnant leachate as a nucleating agent and to adjust the ratio of V3+ to C2O42− ions in solution, and V2O3 was prepared from the V-bearing oxalic acid leachate by a one-step hydrothermal process. In Route B, a precursor composed of hybrid vanadium oxides was initially obtained from the V-bearing oxalic acid leachate by hydrothermal precipitation, and then the precursor was reduced in a 95% Ar + 5% H2 atmosphere at 700 °C to prepare the V2O3 product. Whether by route A or B, a V2O3 product with high crystallinity was obtained from the V-bearing oxalic acid leachate, and a higher extent of vanadium precipitation was achieved.

Room temperature Mott metal–insulator transition in V2O3 compounds induced via strain-engineering

Vanadium sesquioxide (V2O3) is an archetypal Mott insulator in which the atomic positions and electron correlations change as temperature, pressure, and doping are varied, giving rise to different structural, magnetic, or electronic phase transitions. Remarkably, the isostructural Mott transition in Cr-doped V2O3 between paramagnetic metallic and insulating phase observed in bulk has been elusive in thin film compounds so far. Here, via continuous lattice deformations induced by heteroepitaxy, we demonstrate a room temperature Mott metal–insulator transition in 1.5% Cr-doped and pure V2O3 thin films. By means of a controlled epitaxial strain, not only the structure but also the intrinsic electronic and optical properties of the thin films are stabilized at different intermediate states between the metallic and insulating phases, inaccessible in bulk materials. This leads to films with unique features such as a colossal change in room temperature resistivity (ΔR/R up to 100 000%) and a broad range of optical constant values as consequence of a strain-modulated bandgap. We propose a new phase diagram for pure and Cr-doped V2O3 thin films with the engineered in-plane lattice constant as a tunable parameter. Our results demonstrate that controlling phase transitions in correlated systems by epitaxial strain offers a radical new approach to create the next generation of Mott devices.

Synthesis of Walnut Shaped V₂O₃ Particles and Its Photocatalytic Activity for Methylene Blue Degradation Under Visible Light Irradiation.

In this study, walnut-shaped V₂O₃ particles with high photocatalytic activity in the visible light were synthesized by hydrothermal process. The V₂O₃ samples synthesized with the various temperature conditions of the hydrothermal process were characterized using XRD, SEM, TEM, UV-Visible spectrometer and N₂gas adsorption/desorption analysis. For investigating the photocatalytic performance of synthesized V₂O₃ particles in the visible light condition, photodegradation experiments of methylene blue (MB) solution under artificial sunlight irradiation was conducted. As a result, the V₂O₃ hydrothermal-synthesized at 280 °C was composed of pure V₂O₃ crystal structure and showed high photocatalytic activity for the degradation of MB dye in visible light.

Fabrication of pure V2O3 powders by reducing V2O5 powders with CO-CO2 mixed gases

Vanadium (III) oxide (V2O3) can be widely used in the fields of metal ceramics, catalysts, temperature sensors, and cathode materials. In this paper, V2O3 powders are prepared via a gas-based reduction by the mixed gases of CO and CO2 in the temperature range of 783–963 K. The effects of the CO percentage and reaction temperature on the phase transition and morphology evolution of the final products are investigated. The results reveal that the reduction mechanism are different at low and high temperatures. At a temperature lower than the melting point of V2O5 (943 K), the phase transformation sequence can be described as follows: V2O5→V6O13→VO2→V2O3. The reduction process is dominated by a pseudomorphic transformation mechanism, under which the morphology of the produced V2O3 particles roughly keeps the original morphology of V2O5 particles. As the temperature is higher than the melting point of V2O5, the reduction process becomes a gas-liquid reaction, and the reaction rate obviously decreases. Compared with the V2O3 prepared at low temperatures, the morphology becomes much more irregular and compact. It is also found from the experiments that the introduction of CO2 can restrain the carbon deposition reaction and assist in increasing the purity of the V2O3 powders.

Synthesis and characterization of metastable transition metal oxides and oxide nitrides

Abstract New routes to vanadium sesquioxide and tantalum oxide nitride (γ- and δ-phase) are presented. Phase pure V2O3 with bixbyite-type structure, a metastable polymorph, was obtained from vanadium fluoride hydrates at ~750 K. It crystallizes in the cubic crystal system in space group I a 3 ¯ $Ia\bar 3$ with lattice parameter a=939.30(5) pm. The catalytical properties of the corresponding oxide nitride phases and their oxidation and reduction solid-state kinetics were investigated. The preparation of γ-TaON as a phase pure sample can be realized by ammonolysis of X-ray amorphous tantalum oxide precursors at 1073 K. This metastable tantalum oxide nitride crystallizes in the monoclinic VO2(B)-type structure in space group C2/m. The same precursors can be used to synthesize the δ-modification with an anatase-type structure at 1023 K. It crystallizes in the tetragonal crystal system in space group I41/amd. A maximum yield of 82 m % could be obtained. The fundamental band gaps of the synthesized and of other metastable TaON polymorphs were calculated from first principles using the GW method. The present results are compared to experimental data and to previous calculations at hybrid DFT level.

Preparation and characterization of V2O3 micro-crystals via a one-step hydrothermal process

Abstract Phase pure V2O3 micro-crystals with a hexagonal dipyramid morphology were fabricated for the first time via a facile one-step hydrothermal method. The crystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). A hexagonal dipyramid structure of V2O3 enclosed by well-grown {012} facets was obtained by hydrothermally reducing VO(acac)2 precursor with N2H4·H2O at 220 °C for 48 h. The results indicated that V2O3 can be well crystallized up to micron size with distinguished facets by only one step hydrothermal treatment. The formation mechanism and morphology evolution for V2O3 micro-crystals were discussed. Based on our experiments, the V2O3 nuclei formed and grew by a phase transformation through a dissolution–recrystallization process of VOOH, and the formation of the hexagonal dipyramids was ascribed to the specific adsorption of Hacac to the {012} facets restraining the growth in the directions normal to the {012} facets. The present work provides a facile method for preparing phase pure V2O3 micro-crystals with hexagonal dipyramid morphology, which can be used as a new powder material for ceramic fabrication.

Effects of electronic correlation, physical structure, and surface termination on the electronic structure of V2O3nanowires

We report on a density functional theory (DFT) study of the electronic structure of vanadium sesquioxide (V2O3) in both bulk and nanowire form. In particular, our study focuses on the role of spin polarization and electronic correlations, as computed within the local (spin) density approximation (L(S)DA) and the LDA + U formalism. As expected for a mean-field approach such as DFT, our optimized bulk V2O3 structure is shown to be metallic in nature, while an adequate choice of the Hubbard U parameter (U = 4 eV) is enough to open the band gap, making the system insulating. However, this formalism predicts a nonmagnetic insulator, as opposed to the experimentally observed antiferromagnetic structure, to be the ground state. The electronic structure of the V2O3 nanowire system is more complex, and it strongly depends on the surface termination of the structures. Our results show that non-spin-polarized LDA calculations of � 001� -grown nanowires are metallic in nature. However, LSDA predicts that some surface terminations are half-metals, with a large band gap opening for one of the spins. When LSDA + U was used to study the nanowire model with a closed-shell oxygen surface termination, we observe insulating behavior with no net magnetic moment, with a 104 meV band gap. This is consistent with the experimentally observed gap recently reported in the literature for similar wires. To experimentally address the surface structure of these nanowires, we perform surface specific nano-Auger electron spectroscopy on as-synthesized V2O3 nanowires. Our experimental results show a higher O:V peak ratio (1.93:1) than expected for pure V2O3, thereby suggesting higher oxygen content at the surface of the nanowires. From our results, we conclude that oxygen termination is likely the termination for our as-synthesized V2O3 nanowires.

Synthesis and characterization of belt-like VO2(B)@carbon and V2O3@carbon core–shell structured composites

VO2(B) nanobelts encapsulated into carbon core–shell structured composite (VO2(B)@C) was successfully synthesized using VO2(B) nanobelts as the cores and glucose as the shells via an environmental hydrothermal method. The “adsorption carbonization growth” (ACG) mechanism was proposed to explain the formation of VO2(B)@C. The influence of synthetic parameters, such as, the reaction temperature, concentration of glucose and reaction time, was systematically investigated to control the fabrication of VO2(B)@C. Subsequently, belt-like V2O3@C core–shell structured composite was prepared by the heating treatment of VO2(B)@C. Furthermore, the electrochemical properties of this novel belt-like V2O3@C core–shell structured composite was studied, indicating that it exhibits an initial discharge capacity of as high as 162mAh/g and the discharge capacity is still 104, 89, 67mAh/g after 30, 50, and 89 cycles, which is much higher than pure V2O3. The phase transition properties of V2O3 powder and V2O3@C composite were investigated by DSC, which revealed that V2O3@C has the same phase transition properties of V2O3. The as-obtained V2O3@C composite has potential applications to the cathode and functional positive temperature coefficient (PTC) materials.

Neutron scattering study of the spin waves and magnetic structures in pure and metal-deficient V2O3

We have carried out inelastic neutron scattering measurements of the spin waves propagating along the (00ℓ) trigonal axis and along the (hh0) axis in the antiferromagnetic phase of pure V2O3. We find an energy gap in the spin wave spectrum at the zone center at 4.75 meV. We obtain exchange parameters and the anisotropy energy from this data using a nearest neighbor and next nearest neighbor Heisenberg Hamiltonian Model. Elastic neutron scattering on V2O3.0198 has shown two distinct transitions. Magnetic ordering occurs at ∼100 K and the magnetic moment reaches ∼0.5μB at 50 K where the metal-insulator (M–I) transition occurs and the moment saturates at 0.9μB. This contrasts sharply with pure V2O3 in which the magnetic and M-I transitions coincide and with higher oxygen concentrations for which the M-I transition is completely suppressed. In addition, magnetic intensity observed at the 3/2,3/2,3 position in both pure and metal-deficient specimens suggests a magnetic contribution from the oxygen sites.

Lattice parameter anomalies in V2O3 at high temperature

The rhombohedral lattice parameters of pure V2O3 show anomalous behaviour in the high temperature region. The change in the rate of expansion of the rhombohedral angle αR occurs at 533 K where the lattice constant aR reaches a minimum. The sharp dip at 533 K in the aR curve can be correlated with the peak observed in recent small angle scattering of neutrons and might be due to some dynamic magneto elastic effect.