Microstructure and corrosion resistance of vanadium films deposited at different target-substrate distance by HPPMS

High power pulsed magnetron sputtering（HPPMS）, a novel physical vapor deposition technology, was applied to prepare vanadium films on aluminum alloy substrate in this paper. The influence of target–substrate distance（Dt–s）（ranging from 8 to 20 cm） on phase structure, surface morphology, deposition rate, and corrosion resistance of vanadium films was investigated. The results show that the vanadium films are textured with a preferential orientation in the（111） direction except for that fabricated at 20 cm. With Dt–sincreasing, the intensity of（111） diffraction peak of the films decreases and there exists a proper distance leading to the minimum surface roughness of 0.65 nm. The deposition rate decreases with Dt–sincreasing. All the V-coated aluminum samples possess better corrosion resistance than the control sample. The sample fabricated at Dt–sof 12 cm demonstrates the best corrosion resistance with the corrosion potential increasing by 0.19 V and the corrosion current decreasing by an order of magnitude compared with that of the substrate. The samples gain further improvement in corrosion resistance after annealing, and if compared with that of annealed aluminum alloy, then the corrosion potential of the sample fabricated at 20 cm increases by 0.415 V and the corrosion current decreases by two orders of magnitude after annealed at 200 °C. If the annealing temperature further rises to 300 °C, then the corrosion resistance of samples increases less obviously than that of the control sample.

Effect of source temperature on phase and metal–insulator transition temperature of vanadium oxide films grown by atomic layer deposition

Vanadium oxide films were grown by atomic layer deposition using the tetrakis[ethylmethylamino] vanadium as the vanadium precursor and H2O as the oxide source. The effect of the source temperature on the quality of vanadium oxide films and valence state was investigated. The crystallinity, surface morphology, film thickness, and photoelectric properties of the films were characterized by x-ray diffraction, atomic force microscope, scanning electron microscope, I–V characteristics curves, and UV–visible spectrophotometer. By varying the source temperature, the content of V6O11, VO2, and V6O13 in the vanadium oxide film increased, that is, as the temperature increased, the average oxidation state generally decreased to a lower value, which is attributed to the rising of the vapor pressure and the change of the ionization degree for organometallics. Meanwhile, the root-mean-square roughness decreased and the metal–insulator transition temperature reduced. Our study is great significance for the fabrication of vanadium oxide films by atomic layer deposition.

Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film

Vanadium dioxide(VO_(2))is a strongly correlated material,and it has become known due to its sharp metal-insulator transition(MIT)near room temperature.Understanding the thermal properties and their change across MIT of VO_(2)thin film is important for the applications of this material in various devices.Here,the changes in thermal conductivity of epitaxial and polycrystalline VO_(2)thin film across MIT are probed by the time-domain thermoreflectance(TDTR)method.The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO_(2)film to attenuate the impact from extra thermal interfaces.It is demonstrated that the method is feasible for the VO_(2)films with thickness values larger than 100 nm and beyond the phase transition region.The observed reasonable thermal conductivity change rates across MIT of VO_(2)thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate,which is different from the reported behavior of single crystal VO_(2)nanowires.The recovery of the relationship between thermal conductivity and electrical conductivity in VO_(2)film may be attributed to the increasing elastic electron scattering weight,caused by the defects in the film.This work demonstrates the possibility and limitation of investigating the thermal properties of VO_(2)thin films by the TDTR method without depositing any metal thermoreflectance layer.

Electrochemical properties of vanadium pentoxide xerogel films

Vanadium pentoxide xerogel（VXG） films were prepared by rapid quenching, then corn type 2016 size lithium rechargeable batteries were assembled and tested with the VXG film electrodes and lithium anodes. Electrochemical impedance spectroscopy（EIS） analysis result reveals the expected response for intercalation, except that there is almost no Warburg （diffusion） component. Analyses results of cyclic voltammetry（CV）, constant discharge （CD） and discharge-charge（DC） indicate that the sample achieves a high initial discharge specific capacity of approximate 400 mA·h/g and a corresponding efficiency of 97 % in the voltage diapason of 1.5 - 4.0 V with a draining current of 60 mA/g. Its preservation ratio of capacity still keeps as high as 85 % even after 100 cycles. The good electrochemical performance indicates that VXG film material is a promising cathode for lithium rechargeable batteries.

Microstructure and Oxidation Resistance of V Thin Films Deposited by Magnetron Sputtering at Room Temperature

Vanadium films were deposited on Si(100)substrates at room temperature by direct current(DC)magnetron sputtering.The microstructure and surface morphology were studied using scanning electron microscopy(SEM)and atomic force microscope(AFM).The oxidation resistance of films in air was studied using X-ray photoelectron spectroscopy(XPS)and transmission electron microscopy(TEM).The results showed that the amorphous vanadium film with a flatter surface had higher oxidation resistance than the crystalline film when exposed to atmosphere.The rapid formation of the thin oxide layer of amorphous vanadium film could protect the film from sustained oxidation,and the relative reasons were discussed.

Electrochemical impedance spectra of V_2O_5 xerogel films with intercalation of lithium ion

Vanadium pentoxide xerogel films used for lithium rechargeable batteries were prepared from crystalline c-V2O5 by melt quenching method,then the electrochemical process of lithium intercalation into vanadium pentoxide xerogel films was simulated with an equivalent circuit model, which was derived from the mechanism of electrode reactions. Measured electrochemical impedance spectra at various electrode potentials were analyzed by using the complex non-linear least-squares fitting method. The results show that impedance spectra consist of 2 high-to- medium frequency depressed arcs and a low frequency straight line. The high frequency arc is attributed to the absorption reaction of lithium ions into the oxide film, the medium frequency arc is attributed to the charge transfer reaction at the vanadium oxide/electrolyte interface and the low frequency is characterized by a straight line with a phase angle of 45° corresponding to the diffusion of lithium ion through vanadium oxide phase. The experimental and calculated results are compared and discussed focusing on the electrochemical performance and the state of charge of the electrode. Moreover, the high consistence of the fitted values of the model to the experimental data indicates that this mathematical model does give a satisfying description of the intercalation process of vanadium pentoxide xerogel films.

Infrared Thermochromic Properties of VO_2 Thin Films Prepared through Aqueous Sol-gel Process

The stoichiometric vanadium（IV） oxide thin films were obtained by controlling the temperature, time and pressure of annealing. The thermochromic phase transition and the IR thermochromic property of 400 nm and 900 nm VO2 thin films in the 7.5 μm-14 μm region were discussed. The derived VO2 thin film samples were characterized by Raman, XRD, XPS, AFM, SEM, and DSC. The resistance and infrared emissivity of VO2 thin films under different temperature were measured, and the thermal images of films were obtained using infrared imager. The results show that the VO2 thin film annealed at 550 ℃ for 10 hours through aqueous sol-gel process is pure and uniform. The 900 nm VO2 thin film exhibits better IR thermochromic property than the 400 nm VO2 thin film. The resistance of 900 nm VO2 film can change by 4 orders of magnitude and the emissivity can change by 0.6 during the phase transition, suggesting the outstanding IR thermochromic property. The derived VO2 thin film can control its infrared radiation intensity and lower its apparent temperature actively when the real temperature increases, which may be applied in the field of energy saving, thermal control and camouflage.