Deposition, Corrosion and Coloration of Tungsten Trioxide Electrochromic Thin Films PDF Download
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Languages : en
Pages :
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Languages : en
Pages :
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Author: Sey-shing Sun
Publisher:
ISBN:
Category : Thin films
Languages : en
Pages : 282
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Electrochromism (reversible color change of a material, induced by an electric field) has been investigated in tungsten trioxide thin films. It is known that electrochromic device lifetime is generally limited by corrosion of WO3 films in an acid electrolyte. Two mechanisms of corrosion were observed, viz., general dissolution and interfacial delamination. It was found that the dissolution of WO3 films in acid could be attributed to high concentration of thermodynamically unstable species (W02 and W2O5) in the as-deposited films. These species were identified by X-ray photoelectron spectroscopy data and are consistent with Rutherford backscattering spectroscopy data which showed an oxygen deficiency (0/W = 2.76). The Pourbaix diagram for tungsten indicated that WO3 was the thermodynamically stable specie for the present storage condition (pH = 0.5, E = 0.4 Vshe). A corrosion mechanism was proposed consisting of dissolution of WO2 and W2O5 and precipitation of crystalline WO3 and its hydrates. Interfacial delamination occurred when WO3 and its hydrates precipitated back onto the original films. The oxygen content in the WO3 films was increased by oxygen backfilling during evaporation. Dissolution and interfacial delamination of the oxygen enriched films were reduced to negligible rates due to reduced concentration of WO2 and W2O5 . However, the electrochromic properties were degraded by oxygen enrichment. For example, increased resistivity and decreased optical efficiency in the oxygen enriched films resulted in slower coloration speed. The resistivity increase and decreased optical efficiency were explained by postulating an increased density of inactive electron trapping sites. The porosity of the films could be increased by deposition at high background pressure, resulting in increased surface area and absorbed water. The bleaching speeds and self-erasure rates were increased since the rates of removal of protons were increased by the increases in porosity and absorbed water. In another approach to increase the electrochromic device lifetime, the electrolyte was modified. Devices using a solution of LiClO4 in propylene carbonate exhibited excellent lifetime. Switching speeds were increased by increased porosity, deposition of MgF2 overlayers, and more conductive indium-tin-oxide layers. In addition, solid state electrochromic devices using a hydrated MgF3 film were fabricated.
Author: Abeer Atef Al Mohtar
Publisher:
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Category :
Languages : en
Pages : 124
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A low-pressure plasma system operating at microwave frequencies (2.45 GHz) in pure oxygen has been developed to perform surface treatment of materials. The aim of this work is to use this plasma reactor to induce the formation of the electrochromic material tungsten trioxide, WO3, starting from tungsten thin films. The experimental procedure is to deposit pure, metallic tungsten thin films, with thicknesses ranging between 15 and 85 nm, using Pulsed Laser Deposition. The films are then exposed to the oxygen plasma in order to induce oxidation of the films. The plasma has been previously characterized using Langmuir probes and optical spectroscopy in order to determine ion densities and atomic oxygen generation respectively. A parametric study has been performed on the films as a function of gas pressure, exposure time and substrate temperature, in order to determine the optimal conditions for tungsten oxidation. A variety of materials characterization techniques, such as X-Ray Diffraction, Rutherford Backscattering Spectrometry and Atomic Force Microscopy have been used to study the properties of the synthesized material. Our results show that operation of the plasma at high pressure (250 mTorr) and high power (400 W) with the substrate held at a temperature of 500 oC is suitable for obtaining high quality WO3 thin films. The time behavior of the oxidation process was investigated as well. An analysis and discussion of these results is presented.
Author:
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Category : Dissertations, Academic
Languages : en
Pages : 532
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Author: Kuo-Chuan Ho
Publisher: The Electrochemical Society
ISBN: 9781566771740
Category : Science
Languages : en
Pages : 434
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Author: Syed Ali Shamoel Zaidi
Publisher:
ISBN:
Category : Electrochromic devices
Languages : en
Pages : 128
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In order to fabricate layered electrochromic devices capable of reversibly changing color in response to applied voltage, pulsed laser deposition was used to deposit thin films of tungsten oxide on fluorine doped tin oxide substrates.The thin films were deposited at room temperature in an O2 atmosphere of the order of 10−1 mbar on FTO/glass substrates using a KrF pulsed laser source. XRD analysis confirmed the resulting films to be amorphous and UV-Vis spectroscopy confirmed transparency. Raman shift confirms non-crystalline WO3 presence. Electrochromic devices were connected to a cyclic voltammetry system to regulate voltage sweeps, investigate charge transport, response time and reversibility of color/bleached states. Scanning electron microscopy was performed on the films post device operation to examine the surface topography and composition. Raman spectroscopy and XRD was performed post device films to investigate any changes in the films. Li+ ion conducting glasses of composition xLi2SO4 - (1-x)[0.50Li2O-0.5(2NH4H2PO2)) were characterized using resistivity measurements and neutron diffraction experiments to evaluate them as suitable substitutes for the electrolyte in the electrochromic devices. Formation of crystallites above 400°C has been observed which is understood to hamper Li+ ion mobility, resulting in reduced performance of the electrolyte. The acid based electrochromic devices have exhibited superior reversibility.
Author:
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Category : Dissertation abstracts
Languages : en
Pages : 540
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ISBN:
Category :
Languages : en
Pages : 312
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Author: Aravinda Ramaswamy Haranahalli
Publisher:
ISBN:
Category : Thin layer chromatography
Languages : en
Pages : 584
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Author: Vincent David Barth
Publisher:
ISBN:
Category : Tungsten
Languages : en
Pages : 192
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The report presents a detailed review of available information on the oxidation of W and its alloys. W is relatively inert below 700 C. As the temperature is increased above this level, however, oxidation becomes progressively more rapid, reaching catastrophic rates at temperatures around 1200 C and above. Various theories for the mechanism and rates of W oxidation at different temperatures are reviewed, and the effect of pressure and water vapor on the stability of W oxides is discussed in detail. The elevatedtemperature reactions of W with other materials, such as refractory oxides, and with gases other than oxygen also are covered. Information on the protection of W by alloying and coating is included. (Author).
