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1.
Inductively coupled plasma (ICP)-assisted sputtering with an internal coil enabled deposition of stoichiometric crystalline vanadium dioxide (VO 2) films on a sapphire (Al 2O 3) (001) substrate under widely various deposition conditions. The films showed a metal-insulator (M-I) transition around temperatures of 68 °C with several orders of change in resistivity. Particularly, low-temperature (250 °C) growth of VO 2 film with two orders transition decade was achieved in ICP-assisted sputtering, in contrast with conventional sputtering, which required 400 °C for VO 2 growth. Rutherford back scattering (RBS) measurements revealed that the VO 2 film prepared by ICP-assisted sputtering was exactly stoichiometric, containing no impurity atoms from the inserted coil material. The ICP-assisted sputtering was examined in comparison to conventional sputtering in view of plasma characteristics. 相似文献
2.
For metal-to-insulator transition (MIT) in vanadium oxide thin film, a thermodynamically stable vanadium dioxide (VO 2) phase is essential. In VO 2 films sputter-deposited on a quartz substrate from a V 2O 5 target, a radio-frequency (RF) magnetron sputter system at working pressure of 7 mTorr is used. Due to the lower sputtering yield of oxygen compared to vanadium leading to oxygen-ion deficiency, the reduction of V ions is resulted to compensate charge with the oxygen ions. Under lower working pressures, the deposition rate increases, but a simultaneous oxygen-ion deficiency causes the destabilization of VO 2. To prevent this, titanium oxide co-deposition is suggested to enrich the oxygen source. When TiO 2 is used, it is found that the Ti ion has a stable +4 charge state so that the use of extra oxygen in sputtering prevents the destabilization of VO 2. However, this is not the case for TiO. For the latter, Ti ions are oxidized from the +2 state to the +3 and +4 states, and V ions with less oxidation potential are reduced to +3 or so. Pure VO 2 thin film exhibits MIT at 66 °C and a large resistivity ratio of four orders of magnitude from 30 to 90 °C. The (V 2O 5 + TiO 2) system under working pressure as low as 5 mTorr yields fairly good films comparable to pure VO 2 deposited at 7 mTorr, whereas the use of TiO yields films with MIT absent or considerably weakened. 相似文献
3.
We present the structural and physical characterization of vanadium dioxide (VO 2) thin films prepared by reactive electron beam evaporation from a vanadium target under oxygen atmosphere. We correlate the experimental parameters (substrate temperature, oxygen flow) with the films structural properties under a radiofrequency incident power fixed to 50 W. Most of the obtained layers exhibit monocrystalline structures matching that of the monoclinic VO 2 phase. The temperature dependence of the electrical resistivity and optical transmission for the obtained films show that they present thermoelectric and thermochromic properties, with a phase transition temperature around 68 °C. The results show that for specific experimental conditions the VO 2 layers exhibit sharp changes in electrical and optical properties across the phase transition. 相似文献
4.
Thermochromic VO 2 nanorods were prepared via thermal conversion of the metastable VO 2–B phase synthesized by hydrothermal methods. We observe an increased thermochromic transition temperature to ∼75–80 °C by variable-temperature infrared spectroscopy. Nano- and sub-micron structures of other vanadium oxides (V 3O 7, (NH 4) 0.5V 2O 5, and V 2O 5) were obtained simply by varying the starting materials in the hydrothermal synthesis. We also obtained nanostructures of the high temperature tetragonal rutile phase of VO 2 by thermolysis of single-source vanadium (IV) precursors. 相似文献
5.
A novel process was developed for synthesizing pure thermochromic vanadium dioxide (VO 2) by thermal reduction of vanadium pentoxide (V 2O 5) in ammonia gas. The process of thermal reduction of V 2O 5 was optimized by both experiments and modeling of thermodynamic parameters. The product VO 2 was characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TG), and differential scanning calorimetry (DSC). The experimental results indicated that pure thermochromic VO 2 crystal particles were successfully synthesized. The phase transition temperature of the VO 2 is approximately 342.6 K and the enthalpy of phase transition is 44.90 J/g. 相似文献
6.
Thermochromic VO 2 thin films presenting a phase change at Tc = 68 °C and having variable thickness were deposited on silicon substrates (Si-001) by radio-frequency sputtering. These thin films were obtained from optimized reduction of low cost V 2O 5 targets. Depending on deposition conditions, a non-thermochromic metastable VO 2 phase might also be obtained. The thermochromic thin films were characterized by X-ray diffraction, atomic force microscopy, ellipsometry techniques, Fourier transform infrared spectrometry and optical emissivity analyses. In the wavelength range 0.3 to 25 μm, the optical transmittance of the thermochromic films exhibited a large variation between 25 and 100 °C due to the phase transition at Tc: the contrast in transmittance (difference between the transmittance values to 25 °C and 100 °C) first increased with film thickness, then reached a maximum value. A model taking into account the optical properties of both types of VO 2 film fully justified such a maximum value. The n and k optical indexes were calculated from transmittance and reflectance spectra. A significant contrast in emissivity due to the phase transition was also observed between 25 and 100 °C. 相似文献
7.
A new cubic perovskite-like vanadium bronze of composition Na 0.25Cu 0.75VO 3, with a lattice parameter of 7.2517(1) Å, is obtained by reacting NaVO 3, V 2O 5, and Cu 2O at 1100°C and 8 GPa. It exhibits metallic conduction, with a sharp change in resistivity above 200°C, and, presumably, undergoes a phase transition near 280°C. The thermal properties of the new vanadium bronze are studied in air and helium at normal pressure. 相似文献
8.
The vanadium oxide (VO 2) films have been prepared on SiO 2/Si substrates by using a modified Ion Beam Enhanced Deposition (IBED) method. During the film deposition, high doses of Ar + and H + ions have been implanted into the deposited films from the implanted beam. The resistance change of the VO 2 films with temperature has been measured and the phase transition process has been observed by using the X-ray Diffraction technique. The phase transition of the IBED VO 2 films starts at a low temperature of 48 °C and ends at a high temperature of 78 °C. It is found that the phase transition characteristics can be adjusted by changing the annealing temperature or the time and the phase transition characteristics of the IBED VO 2 films depend on the quantity and location of argon atoms in the film matrix. 相似文献
9.
Magnetite (Fe 3O 4) thin films are prepared by pulsed laser deposition using an α-Fe 2O 3 target on silicon (111) substrate in the substrate temperature range of 350 °C to 550 °C. X-ray diffraction (XRD) measurement shows that the film deposited at 450 °C is a single phase Fe 3O 4 film oriented along [111] direction. However, the film grown at 350 °C reveals mixed oxide phases (FeO and Fe 3O 4), while the film deposited at 550 °C is a polycrystalline Fe 3O 4. X-ray photoelectron spectroscopy study confirms the XRD findings. Raman measurements reveal identical spectra for all the films deposited at different substrate temperatures. We observe abrupt increase in the resistivity behavior of all the films around Verwey transition temperature ( TV) (125 K-120 K) though the transition is broader in the film deposited at 350 °C. We observe that the optimized temperature for the growth of Fe 3O 4 film on Si is 450 °C. The electrical transport behavior follows Shklovskii and Efros variable range hopping type conduction mechanism below TV for the film deposited at 450 °C possibly due to the granular growth of the film. 相似文献
10.
A method for the synthesis of vanadium dioxide (VO 2) nanoparticles in nanoporous silicate glass matrices with a pore size of 17 and 7 nm has been developed. According to this, vanadium pentoxide (V 2O 5) nanoparticles are initially grown in the pores, and then V 2O 5 is reduced to VO 2 in hydrogen. The optical transmission spectra of 1-mm-thick VO 2-modified glass samples have been measured. The temperature dependence of the transmission coefficient has been studied in the course of the semiconductor-metal phase transition in VO 2 nanoparticles. 相似文献
11.
Mg xCu 3−xV 2O 6(OH) 4·2H 2O ( x ∼ 1), with similar crystal structure as volborthite Cu 3V 2O 7(OH) 2·2H 2O, was successfully prepared by a soft chemistry technique. The method consists of mixing magnesium nitrate and copper nitrate with a boiling solution of vanadium oxide (obtained by reacting V 2O 5 with few mL of 30 vol.% H 2O 2 followed by addition of distilled water). When ammonium hydroxide NH 4OH 10% was added (pH 7.8), a green yellowish precipitate was obtained. Using X-ray powder diffraction data, its crystal structure has been determined by Rietveld refinement. Compared to volborthite, the vanadium coordination changes from tetrahedral VO 4 to trigonal bipyramidal VO 5, and magnesium replaces copper, preferably, in the less distorted octahedron. At 300 °C, the phase formed is similar to the high pressure (HP) monoclinic Cu 3V 2O 8 phase. However at higher temperature, 600 °C, the phase obtained is different from known Cu 3V 2O 8 phases. 相似文献
12.
In this study, magnesium-cobalt ferrite (Mg0.85Co0.15Fe2O4) powder was synthesized using a solid-state synthesis method, followed by the liquid sintering using 0.50–3.00 wt% vanadium oxide (V2O5) at 1050 °C for 2 h. X-ray diffraction (XRD) studies confirmed the formation of spinel ferrite. Microstructure studies revealed that by increasing the amount of V2O5 from 0.50 to 3.00 wt%, the average grain size was reduced from 15.9?±?5.9 to 7.0?±?2.5 μm and the samples were highly densified. V2O5 promoted the sintering process and reduced the dielectric constant (ε′), loss tangent (tanδ), and increased electrical resistivity. A magnesium-cobalt ferrite sample with 25.4 dielectric constant, 0.078 loss tangent, and 9.0?×?105 Ω.cm resistivity at 1 MHz was achieved using 3.00 wt% V2O5. Increasing V2O5 content caused increasing coercivity (Hc) from 89 to 129 Oe. Moreover, the maximum saturation magnetization (Ms) value of 26.8 emu/g was obtained for the sample containing 1.50 wt% V2O5. The small dielectric loss tangent of the samples at 1 MHz suggests applications of these ceramics in microwave devices. 相似文献
13.
The phase and structure evolution during synthesis of vanadium carbide (V 8C 7) nanopowders by thermal processing of the precursor were investigated using X-ray diffraction (XRD). The morphology of the reactant was characterized by transmission electron microscopy (TEM). Simultaneous thermogravimetric and differential thermal analysis (TG-DTA) were made on the precursor. The results indicate phase evolution sequences are NH 4VO 3→V 2O 5→VO 2→V 5O 9+V 4O 7→V 2O 3→VC 1−X→V 8C 7. The single phase V 8C 7 powders can be prepared at ∼1100 °C for 1 h, and the powders show good dispersion and are mainly composed of uniformly sized spherical particles with the majority diameters of 20-50 nm. 相似文献
14.
In the present work using V 2O 5 and MoO 3 powders as precursors, a novel method, the inorganic sol-gel method, was developed to synthesize Mo 6+ doped vanadium dioxide (VO 2) thin films. The structure, valence state, phase transition temperature, magnitude of resistivity change and change in optical transmittance below and above the phase transition of these films are determined by XRD, XPS, four-point probe equipment and spectrophotometer. The results showed that the main chemical composition of the films was VO 2, the structure of MoO 3 in the films didn't change, and the phase transition temperature of the VO 2 was obviously lowered with increasing MoO 3 doped concentration. The magnitude of resistivity change and change in optical transmittance below and above phase transition were also decreased, of which the magnitude of resistivity change was more distinct. However, when the MoO 3 concentration was 5 wt%, the magnitude of resistivity change of doped thin films still reached more than 2 orders, and the change in optical transmittance below and above phase transition was maintained. Analysis showed that the VO 2 doped films formed local energy level, and then reduced the forbidden band gap of VO 2 as the donor defect changing its optical and electrical properties and lowering the phase transition temperature. 相似文献
15.
X-ray diffraction and optical microscopy data are presented which demonstrate that substoichiometric vanadium oxide (VO 0.57-VO 0.97) consists of a cubic phase with the B1 structure (sp. gr. Fm \(\bar 3\) m) and an ordered monoclinic phase of composition V 14O 6 (sp. gr. C2/ m). The content of the latter phase decreases with increasing oxygen content. The superstoichiometric vanadium oxide VO 1.29 is shown to contain trace amounts of V 52O 64. Vickers microhardness data for nonstoichiometric vanadium oxides in the range VO 0.57-VO 1.29 show that, with increasing oxygen content, their H V has a tendency to decrease, from 18 to 12 GPa. Their microhardness is shown for the first time to have a maximum near the stoichiometric composition VO 1.00. 相似文献
16.
Thin films of vanadium oxide have been prepared from an aqueous solution system of (V 2O 5–HF aq.) with the addition of aluminium metal by a novel wet-preparation process which is called liquid-phase deposition (LPD).
From X-ray diffraction measurements, the as-deposited film was found to be amorphous and it was then crystallized to V 2O 5 by calcination at 400 °C under an air flow. In contrast, the monoclinic VO 2 phase was obtained when the deposited film was calcined under a nitrogen atmosphere. The deposited film showed excellent
adherence to the substrate and was characterized by a homogeneous flat surface. The deposited VO 2 film exhibited a reversible semiconductor–metal phase transition around 70 °C and its transition behaviour depended on the
way in which the film was prepared.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
17.
Large-scale VO 2(B) nanobelts have been synthesized by hydrothermal strategy via one-step method using V 2O 5 as vanadium source and C 6H 5-(CH 2) n-NH 2 with n = 2 and 4 (2-phenylethylamine and 4-phenylbutylamine) as structure-directing templates. The composition and morphology of the nanobelts were established by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The as-obtained VO 2(B) nanobelts have a length of 3-10 μm, a wideness of 100-375 nm and a thickness of 30-66 nm. 相似文献
18.
A nanostructured vanadium dioxide (VO 2) thin film showing a low metal-insulator transition temperature of 30 °C has been fabricated through reactive ion beam sputtering followed by thermal annealing. The thin film was grown on borosilicate glass substrate at the temperature of 280 °C with a Si 3N 4 buffer layer. Both scanning electron microscopy and atomic force microscopy images have been taken to investigate the configuration of VO 2 thin film. The average height of the crystallite is 20 nm and the grain size ranges from 40 nm to 100 nm. The transmittance measured from low to high temperatures also reveals that the film possesses excellent switching property in infrared light at critical transition temperature, with switching efficiency of 52% at 2600 nm. This experiment paves the way of VO 2 thin film's application in smart windows. 相似文献
19.
Vanadium oxide ( V) and silver-doped vanadium oxide ( Ag- V) powders were prepared via sol–gel processing. Structural evolution and bactericidal activity was examined as a function of temperature ranging from 250, 350, 450 and 550 °C. Powders were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Raman spectroscopy. Results from all techniques showed vanadium pentoxide (V 2O 5) is the predominant phase regardless of heat treatment temperature or the addition of silver (Ag). XRD analysis suggests Ag is present as AgCl in samples heat treated to 250, 350, and 450 °C and as AgV 6O 15 at 550 °C. Bactericidal activity was evaluated against Escherichia coli using the agar disk diffusion method considering both Ag-V and undoped, V powders. While the addition of Ag significantly increased bactericidal properties, the specific Ag valency, or crystal structure and morphology formed at higher temperatures, had little effect on functionality. 相似文献
20.
Vanadium dioxide thin films were prepared by an atmospheric-pressure chemical vapour deposition method. The raw material was vanadium(III) acetylacetonate. Polycrystalline thin films were obtained at a reaction temperature of 500°C. Slow post-deposition cooling of the deposits on a substrate of fused quartz or sapphire single crystal yields vanadium dioxide films which are not mixed with other phases, i.e. V 3O 7 or V 4O 9. Optical and electrical switching behaviours strongly depend on film thickness. At a film thickness of about 300 nm the transition temperature showed a minimum value of 44 °C. 相似文献
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