Surface and electrical studies of CuO:V2O5 thin films

Results from the studies of multicomponent CuO:V₂O₅ bulk material and thermally evaporated thin films of highly conducting bulk composition prepared at different substrate temperatures are thus compared and discussed. The electronic conductivity is enhanced on increase in the substrate temperature T_s and reaches a maximum value of 12.3 × 10⁻⁶Ω⁻¹ cm⁻¹ for T_s = 423 K. X-ray photoelectron spectroscopy studies indicate an increase in the reduced states of vanadium and copper ions in going from the bulk glass to the thin film. Dynamic secondary-ion mass spectroscopy studies on thin films over a depth of 3000 Å show a strong dependence of T_s on the Cu-to-V intensity ratio. Even though stoichiometric values for thin films are achievable by varying the T_s, the oxidation states of Cu in these films are predominantly monovalent. The electrical behaviors of these materials and their thin film counterparts are finally being discussed in relation to the surface analysis data.     

Highly stable nonlinear behavior against impulse current of ZnO-Pr6O11-CoO-Cr2O3-Y2O3 varistors

The nonlinear electrical behavior against the impulse stress (400-1200 A) in the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃ varistors has been investigated with sintering time. The sintering time did have a significant effect on the clamp ratio, which exhibits a surge protection capability. The varistors sintered for 1 h exhibited the best clamp characteristics, in which the clamp voltage ratio was in the range of K = 1.61-1.92 at a lower impulse current region (5-50 A) and was in the range of K = 2.06-2.55 at a higher single impulse current region (400-1200 A). The best electrical stability against the multi-impulse aging stress of 1200 A was also obtained from the varistors sintered for 2 h, where %ΔE_{1 mA} =−1.6%, %Δα =1.3%, and %ΔJ_L =−4.9%.     

A study of thin films of V2O5 containing molybdenum from an evaporation boat

An attempt was made to produce thin films of vanadium oxide by evaporating V₂O₅ in vacuum using molybdenum boats. Following analysis of the films by X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, it was found that the films contained a large amount of molybdenum (atomic ratio of Mo:V>1). Films were chemically inhomogeneous along the direction of growth such that the value of the atomic ratio decreased from the substrate side of the film to its interface with the air. However, a study of the optical properties of the films revealed that they were optically homogeneous. The films went through a semiconductor-to-metal phase transition at a temperature of approximately 200 °C. When annealed in vacuum at a temperature of 275 °C, it was found that, (a) the films remained amorphous, (b) there was a loss of oxygen leading to an increase in their electrical conductivity, (c) their thickness decreased leading to a larger refractive index of the films, and (d) their band gap energy shifted to a higher photon energy by approximately 0.1 eV.     

Impact of substrate temperature on the microstructure, electrical and optical properties of sputtered nanoparticle V2O5 thin films

Applying reactive direct current (DC) magnetron sputtering method, nanoparticle vanadium pentoxide thin films were deposited onto glass slides and KBr substrates at different substrate temperatures. The films were characterized by X-ray photoelectron spectroscopy and atomic force microscope. Infrared spectra were recorded with a Fourier transform infrared spectrophotometer. It was found that, excepting the compositions, the film growth and vanadium oxygen bonds were strongly affected by the substrate temperature. Electrical measurements indicated that the square resistances of films showed an exponential decrease from 46 MΩ/□ to 33 kΩ/□ with substrate temperature increasing from 433 K to 593 K, and that the square resistance-temperature curves of films exhibited typical semiconducting behavior. Optical investigations were carried out in the near infrared and ultraviolet-visible range. Transmittance varied from about 95 to 55% in near-infrared range when the substrate temperature was elevated. In ultraviolet-visible range, optical band gaps and refractive indexes of films were deduced according to the transmission and reflection spectra.     

The effect of titanium on the lithium intercalation capacity of V2O5 thin films

The effect of Ti incorporation on lithium intercalation capacity of V₂O₅ thin films prepared by spin coating using metalorganic (MO) and inorganic sol-gel (SG) precursors on indium tin oxide coated glass substrates have been investigated. Earlier studies show that V₂O₅-TiO₂ oxide system has a higher cyclic stability than V₂O₅. However, there is controversy concerning the capacity of these mixed phases. We observe that upon incorporation of 5% Ti in MO films the lithium intercalation capacity decreases from 47 mC/cm² to 27 mC/cm², while for the SG films, the capacity increases from 14 mC/cm² to 27 mC/cm². We attribute this difference in the lithium intercalation capacity of the 5% Ti doped V₂O₅ films prepared by MO and SG precursors to the variation in the nonstoichiometry and the particle size. We find that it is essential to have a critical V:O ratio to achieve high intercalation capacity. Any deviations from this critical V:O ratio leads to a decrease in capacity and films having similar nonstoichiometry have similar values of intercalation capacity and diffusion coefficient.     

Influence of Dy/Bi dual doping on thermoelectric performance of CaMnO3 ceramics

Ca_1−2xDy_xBi_xMnO₃ (x = 0.02, 0.05, 0.08 and 0.10) ceramics have been synthesized by solid state reaction method. Samples with relative densities up to 98% have been obtained. Microstructures have been characterized by XRD and SEM. Thermoelectric properties are evaluated between 300 and 1100 K. Both electrical resistivity and Seebeck coefficient decrease with increasing Dy and Bi content. Thermal conductivity increases with doping, increasing up to x = 0.08. The highest power factor of 420 μW (K² m)⁻¹ is obtained at x = 0.02, resulting in the highest dimensionless figure of merit (ZT) of 0.21 at 973 K. This value is four times as much as that of undoped CaMnO₃ ceramics. All examples show good self-compatibility with weak temperature dependence.     

Synthesis and structural characterization of mesoporous V2O5 thin films for electrochromic applications

Mesoporous vanadium pentoxide (V₂O₅) films have been synthesized by hydrolysis of vanadium tri-isopropoxide (VO(OC₃H₇)₃) in the presence of polyethylene glycol (PEG) as a structure-directing agent. The structure, the stoichiometry and the morphology of the films have been studied as a function of the thermal annealing by X-ray diffraction (XRD), micro-Raman spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. XRD patterns and Raman spectra show the presence of two previously unreported crystalline phases. The PEG:V₂O₅ molar ratio affects the temperature of phase formation, the amount and even the order in which the phases appear. The morphological characterization underlines the role of the surfactant to promote porous networks, formed by micrometric clusters of controlled shapes and patterns embedded in a homogeneous host matrix.     

Transport properties and thermal expansion of La2Mo2O9-based solid electrolytes

The oxygen ion transference numbers of La_1.7Bi_0.3Mo₂O₉, La₂Mo_1.7W_0.3O₉ and La₂Mo_1.95V_0.05O₉ ceramics, determined by modified faradaic efficiency and e.m.f. methods at 973-1173 K, vary in the range 0.995-0.977 in air, decreasing when temperature increases. The activation energies for the ionic and electronic transport are 61-71 kJ/mol and 123-141 kJ/mol, respectively. Reducing oxygen chemical potential leads to increasing n-type electronic contribution to the total conductivity, which remains, however, essentially p(O₂)-independent down to oxygen pressures of 10⁻⁴-10⁻³ atm and exhibits reversible drop on further reduction, probably due to phase decomposition. Doping La₂Mo₂O₉ with calcium results in segregation of a CaMoO₄-based phase, accompanied with increasing electronic transport. The average thermal expansion coefficients of La₂Mo₂O₉-based materials, calculated from dilatometric data in air, are (14.4-14.8) × 10⁻⁶ K⁻¹ at 300-700 K and (16.4-22.5) × 10⁻⁶ K⁻¹ at 700-1070 K.     

Pulsed laser deposition of (MoO3)1 − x(V2O5)x thin films: Preparation, characterization and gasochromic studies

In this study we demonstrate a new composite oxide thin films of (MoO₃)_1 − x(V₂O₅)_x, x = 0, 0.01, 0.03, and 0.05, fabricated by pulsed laser deposition (PLD). The performance of platinum (Pt) catalyst activated hydrogen gas sensor with modified (MoO₃)_1 − x(V₂O₅)_x thin films were investigated. The thickness of the (MoO₃)_1 − x(V₂O₅)_x thin film is about 600-650 nm and its surface has a uniform morphology. Our results show that the gasochromic sensors prepared by (MoO₃)_0.99(V₂O₅)_0.01 thin film exhibited excellent hydrogen sensibility. The response and recovery time are in the range of 9-15 min for coloration and bleaching at room temperature under H₂ atmosphere. The results also show that (MoO₃)_1 − x(V₂O₅)_x/Pt (x = 0.01, 0.03, 0.05) thin films perform better gasochromic capability than the pristine MoO₃/Pt sample.     

Optical bistability for ZnO–Nb2O5–TeO2 glasses

We examined the optical nonlinear properties of ZnO–Nb₂O₅–TeO₂. The absorption and Raman spectra were measured, the third-order nonlinear susceptibility was determined by degenerated four wave mixing technique. The magnitude of χ⁽³⁾ is about 1.0 × 10⁻¹² esu, larger than that of silica glasses, and the optical bistability was observed in a Fabry–Perot cavity.     

A low-temperature reactive sintering process for the 5Li2O–1Nb2O5–5TiO2 microwave dielectric ceramics

The B₂O₃-doped 5Li₂O–1Nb₂O₅–5TiO₂ composite microwave dielectric ceramics prepared by conventional and low-temperature single-step reactive sintering processes were investigated in the study. Without any calcinations involved, the Nb₂O₅ mixture of Li₂CO₃ and TiO₂ was pressed and sintered directly in the reactive sintering process. More uniform and finer grains could be obtained in the 5Li₂O–1Nb₂O₅–5TiO₂ ceramics by reactive sintering process, which could effectively save energy and manufacturing cost. And relatively good microwave dielectric properties of _r = 41, Q × f = 9885 GHz and τ_f = 43.6 ppm/°C could be obtained for the 1 wt.% B₂O₃-doped ceramics reactively sintered at 900 °C.     

Structural and optical properties of pulsed laser deposited V2O5 thin films

Pulsed laser deposited nanocrystalline V₂O₅ thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and optical spectroscopy. The films were deposited on amorphous glass substrates, keeping the O₂ partial pressure at 13.33 Pa and the substrate temperature at 220 °C. The characteristics of the films were changed by varying the laser fluence and repetition rate. XRD revealed that films are nanocrystalline with an orthorhombic structure. XPS shows the sub-stoichiometry of the films, that generally relies on the fact that during the formation process of V₂O₅ films, lower valence oxides are also created. From the HRTEM images, we observed the size evolution and distribution characteristics of the clusters in the function of the laser fluence. From the spectral transmittance we determined the absorption edge using the Tauc plot. Calculation of the Bohr radius for V₂O₅ is also reported.     

Low temperature chemical vapor deposition of nanocrystalline V2O5 thin films

Spatially uniform, carbon-free thin films of V₂O₅ were deposited on silicon by chemical vapor deposition using vanadium oxide triisopropoxide and water as gaseous precursors, in the temperature range of 100-300 °C. Films with substantial crystallinity were obtained for deposition temperatures as low as 180 °C. The “neat” chemistry that nominally leaves no fragments of ligand or water in the solid promotes film purity and reduces the deposition temperature needed for crystallization. Such deposition temperatures also open up additional possibilities for using crystalline vanadia on fragile substrates such as polymers for electronics and optical applications.     

Structural and optical properties of (100 − x)(Li2B4O7) − x(SrO–Bi2O3–0.7Nb2O5–0.3V2O5) glasses and glass nanocrystal composites

Glasses of various compositions in the system (100 − x)(Li₂B₄O₇) − x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (10  x  60, in molar ratio) were prepared by splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses (DTA). The amorphous nature of the as-quenched glasses and crystallinity of glass nanocrystal composites were confirmed by X-ray powder diffraction studies. Glass composites comprising strontium bismuth niobate doped with vanadium (SrBi₂(Nb_0.7V_0.3)₂O_9−δ (SBVN)) nanocrystallites were obtained by controlled heat-treatment of the as-quenched glasses at 783 K for 6 h. High resolution transmission electron microscopy (HRTEM) of the glass nanocrystal composites (heat-treated at 783 K/6 h) confirm the presence of rod shaped crystallites of SBVN embedded in Li₂B₄O₇ glass matrix. The optical transmission spectra of these glasses and glass nanocrystal composites of various compositions were recorded in the wavelength range 190–900 nm. Various optical parameters such as optical band gap (E_opt), Urbach energy (ΔE), refractive index (n), optical dielectric constant and ratio of carrier concentration to the effective mass (N/m^*) were determined. The effects of composition of the glasses and glass nanocrystal composites on these parameters were studied.     

Ohmic contacts and n-type doping on TixCr2 − xO3 films and the temperature dependence of their transport properties

A procedure to dope n-type Cr_2 − xTi_xO₃ thin films is proposed. Besides doping the material, at the same time the method forms ohmic contacts on Ti_xCr_2 − xO₃ films. It consists on the deposition of 10 nm Ti and 50 nm Au, followed by thermal annealing at 1000 °C for 20 min in N₂ atmosphere. Ohmic contacts were formed on three samples with different composition: x = 0.17, 0.41 and 1.07 in a van der Pauw geometry for Hall effect measurements. These measurements are done between 35 K and 373 K. All samples showed n-type nature, with a charge carrier density (n) on the order of 10²⁰ cm^− 3, decreasing as x increased. As a function of temperature, n shows a minimum around 150 K, while the mobilities have an almost constant value of 11, 28 and 7 cm²V^− 1 s^− 1 for x = 0.17, 0.41 and 1.07, respectively.     

IR spectra of VO2 and V2O3

IR spectra of the tetragonal modification of VO₂ and of the trigonal form of V₂O₃ are recorded at room temperature and compared with that of V₂O₅. The investigated samples of the two lower-valent vanadium oxides, obtained on temperature-programmed reduction treatment, were also characterized with diffuse reflectance and electron-paramagnetic resonance spectra. The effect of atmospheric oxygen on these materials was revealed with XPS measurements and also studied with IR spectra.     

High energy density lithium ion batteries using Li2.6Co0.4 − xCuxN (anode) and Cu0.04V2O5 (cathode) electrode materials

Li_2.6Co_0.4 - xCu_xN (x = 0, 0.15) anode materials were prepared by conventional solid state reaction. Between both materials, Li_2.6Co_0.25Cu_0.15N exhibited better capacity retention than that of Li_2.6Co_0.4N. According to electrochemical impedance spectroscopy, the better cycling behavior of Li_2.6Co_0.25Cu_0.15N has been attributed to the improvement in interfacial compatibility between the electrode and electrolyte interface. A possible explanation to this was given. Li_2.6Co_0.4 - xCu_xN/Cu_0.04V₂O₅ full-cells were assembled to investigate the reliability of Li_2.6Co_0.4 - xCu_xN anode materials in practical applications. The Li_2.6Co_0.25Cu_0.15N/Cu_0.04V₂O₅ cell delivered a specific capacity of 260 mA h g^− 1, and a specific energy of 505.7 mW h g^− 1, which was much higher than that of C/LiCoO₂ lithium ion batteries.     

Conversion of V2O5·xH2O into orthorhombic V2O5 single-crystalline nanobelts

Orthorhombic V₂O₅ single-crystalline nanobelts have been synthesized by hydrothermal treating V₂O₅·xH₂O precipitate derived from aqueous solution of V₂O₅ and H₂O₂. The synthetic method is facile, fast, environmental friendly, and easy to scale up. The V₂O₅ single-crystalline nanobelts are 30-80 nm in width, 30-40 nm in thickness, and lengths up to several tens of micrometers. The V₂O₅·xH₂O precursor is crucial for the formation of orthorhombic V₂O₅ single-crystalline nanobelts. The influences of synthetic parameters, such as reaction time and reaction temperature, on the crystal structures and morphologies of the resulting products have been investigated. Time-dependent experiments show that V₂O₅·xH₂O are dehydrated gradually and converted into orthorhombic V₂O₅ single-crystalline nanobelts. High reaction temperature also favors the formation of orthorhombic V₂O₅ nanobelts.     

Structural and impedance properties of KBa2V5O15 ceramics

The polycrystalline sample of KBa₂V₅O₁₅ ceramics was prepared by a mixed oxide method at low temperature (i.e., at 560 °C). The formation of the compound was confirmed using an X-ray diffraction technique at room temperature. Scanning electron micrograph of the material showed uniform grain distribution on the surface of the sample. Detailed studies of dielectric properties of the compound as a function of temperature at different frequencies suggest that the compound has a dielectric anomaly of ferroelectric to paraelectric type at 323 °C, and exhibits diffuse phase transition. Electrical properties of the material were analyzed using a complex impedance technique. The Nyquists plot showed the presence of both grain (>10³ Hz) and the grain boundary (<10³ Hz) effects in the material. Studies of electrical conductivity over a wide temperature range suggest that the compound exhibits the negative temperature coefficient of resistance behavior. The ac conductivity spectrum was found to obey Jonscher's universal power law.     

Molten salt synthesis and thermoelectric properties of Ca2Co2O5

A molten salt method was applied to synthesize Ca₂Co₂O₅ powder by using Co₃O₄ and CaCO₃ as raw materials and CaCO₃-CaCl₂ as eutectic salt. The formation process and molten salt mechanism of Ca₂Co₂O₅ were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that sheet-like particles can be obtained at 963 K. Moreover, the electrical conductivity and the Seebeck coefficient of the specimen were measured from 323 to 963 K. The properties increased with increasing temperature and the highest value of power factor (2.0 × 10^− 4 W m^− 1 K^− 2) was obtained at 963 K, which indicates that molten salt synthesis is a promising method to prepare high thermoelectric performance material.