Electrochromism and local order in amorphous WO3

WO₃ films prepared under different conditions (evaporation, reactive sputtering and spraying of aqueous solutions of metatungstic acid) differ by orders of magnitude in their electrochromic sensitivity. Diffuse X-ray studies show the evaporated and sputtered films to be amorphous and to consists of a disordered network of corner sharing WO₆ octahedra. Sprayed films have different degrees of crystallinity depending on spraying conditions. From differential scanning calorimetry we conclude that the crystal water present in most films strongly affects the local order of the corner sharing octahedra. We find that crystal water not only provides a high ionic conductivity which is conditional for a fast electrochromic reaction but also stabilises electrocatalytically active surface sites for fast hydrogen or Li exchange with the adjacent electrolyte.     

非晶态WO3薄膜电致变色特性的研究

采用射频溅射三氧化钨粉末靶的技术，在不同的氧分压条件下沉积得到非晶态WO3电致变色薄膜，分析得知氧分压为1∶10的样品变色性能更好些.采用x射线衍射(XRD)，原子力显微镜(AFM)，伏安特性曲线和分光光度计分析所制备薄膜的特性.将薄膜在15mol/L的LiClO4的丙稀碳酸脂（PC）溶液进行电化学反应.发现氧分压在1∶10的情况下沉积得到的薄膜呈非晶态，薄膜有较多的孔隙，这有利于Li+的抽取，进而显示出很好的变色性能.x射线光电子能谱(XPS)成分分析表明WO3薄膜在原态中只有W和O两种原子电色反应后 关键词： 三氧化钨薄膜 非晶 射频溅射 电致变色     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be ?2×10^?4 eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm^?3 when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Calculation of radon diffusion coefficient and diffusion length for different building construction materials

The diffusion of radon in dwellings is a process determined by the radon concentration gradient across the building material structure between the radon source and the surrounding air, and can be a significant contributor to indoor radon inflow. Radon can originate from the deeply buried deposit beneath homes and can migrate to the surface of earth. Radon emanates to the surfaces mainly by diffusion processes from the point of origin following α-decay of ²²⁶Ra in underground soil and building materials used, in the construction of floors, walls, and ceilings. In the present study radon diffusion through some building materials viz. coarse sand and stone dust of different grain size has been carried out using LR-115 type II solid-state nuclear track detectors (SSNTDs). The radon diffusion coefficients and diffusion lengths through these building construction materials have been calculated. The effect of grain size on radon diffusion through these building materials shows the decrease in radon diffusion with decrease in grain size.     

Comparison of electrochromic amorphous and crystalline electron beam deposited WO3 thin films

Tungsten oxide (WO₃) thin films were prepared by an electron beam deposition technique. Films were deposited onto fluorine-doped tin oxide (FTO)-coated glass substrates maintained at 523 K. The as-deposited films were found to be amorphous and crystallized after annealing at 673 K. The electrochromic and optical properties, structure, and morphology are strongly dependent on the annealing conditions. Cyclic voltammetry (C-V) was carried out in the potential range −1 to +1 V. Before and after colouration, the films were characterized by measuring transmittance and reflectance. The colouration efficiencies at 630 nm are about 39.4 cm² C⁻¹ and 122.2 cm² C⁻¹ for amorphous and crystalline films, respectively. An investigation of self-bleaching for the coloured film revealed that the film fades gradually over time.     

Morphologically different WO3 nanocrystals in photoelectrochemical water oxidation

Different morphologies of WO₃ nanocrystals such as nanorods and nanoplates have been obtained under hydrothermal conditions using ammonium metatungstate as the precursor in presence of different organic acids such as citric, oxalic, and tartaric acid in the reaction medium. Detailed characterization of the crystal structure, particle morphology, and optical band gap of the synthesized powders have been done by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and solid-state UV–visible spectroscopy study. The as-synthesized materials are WO₃ hydrates with orthorhombic phase which transform to the hexagonal WO₃ through dehydration upon heating at 350 °C. The resultant products are crystalline with nanoscale dimensions. Finally, the photoactivity of the synthesized materials annealed at 500 °C has been compared employing in photoelectrochemical water oxidation under the illumination of AM 1.5G simulated solar light (100 mWcm⁻²). The photocurrent measurements upon irradiation of light exhibit obvious photocatalytic activity with a photocurrent of about 0.77, 0.61, and 0.65 mAcm⁻² for the WO₃ film derived with the oxalic acid, tartaric, and citric acid assisting agents, respectively, at 1.8 V versus Ag/AgCl electrode.     

Calculation of the effective diffusion coefficient for random wear surface migration on different scales

The paper considers the contact interaction of crystalline solids under shear deformation in the context of molecular dynamics. The interatomic interaction is specified by a potential calculated using the embedded atom method. The peculiarities of structural changes in a contact zone are studied for various materials of the contact pair. Based on the data extracted, the effective diffusion coefficient was estimated for random migration of the contact zone in a direction perpendicular to applied shear strains. The calculation results agree well with data of a microscopic contact model built around the method of movable cellular automata.     

Jump diffusion models and the evolution of financial prices

We analyze a stochastic model to describe the evolution of financial prices. We consider the stochastic term as a sum of the Wiener noise and a jump process. We point to the effects of the jumps on the return time evolution, a central concern of the econophysics literature. The presence of jumps suggests that the process can be described by an infinitely divisible characteristic function belonging to the De Finetti class. We then extend the De Finetti functions to a generalized nonlinear model and show the model to be capable of explaining return behavior.     

Mechanism of boron diffusion in amorphous silicon

We have elucidated the mechanism for B migration in the amorphous (a-) Si network. B diffusivity in a-Si is much higher than in crystalline Si; it is transient and increases with B concentration up to 2 x 10(20) B/cm(3). At higher density, B atoms in a-Si quickly precipitate. B diffusion is indirect, mediated by dangling bonds (DB) present in a-Si. The density of DB is enhanced by B accommodation in the a-Si network and decreases because of a-Si relaxation. Accurate data simulations allow one to extract the DB diffusivity, whose activation energy is 2.6 eV. Implications of these results are discussed.     

Mechanisms of ion-beam-enhanced diffusion in amorphous silicon

We have investigated ion-beam-enhanced diffusion of Au in undoped and B doped amorphous Si. The diffusion coefficients depend linearly on ion flux and exibit an Arrhenius-like temperature dependence with an activation energy of 0.37 eV in the temperature range 200–350° C. Moreover the diffusivity is enhanced by a factor of 5 by B-doping at a concentration of 1×10²⁰ atoms/cm³. A similar enhancement is observed in thermal diffusion of Au which has an activation energy of 1.5 eV. On the basis of these results a model for the ion-beam-enhanced diffusion of Au is proposed where the high density of defects present in amorphous Si act as traps for the fast moving interstitial Au atoms. The effectiveness of this trapping process can be changed by the high concentration of mobile defects generated by the beam and also by a change in the charge state of the traps induced by the presence of B.     

Calculation of the diffusion coefficient in crystals

The diffusion coefficient for vacancies is calculated on the basis of the Kubo formula. Here no assumptions are made about the character of the migration of the atom into the vacant site, and the calculation is carried out on the basis of a Hamiltonian proposed earlier by the author. A general expression is obtained for the temperature dependence of the diffusion coefficient that is valid in a wide temperature range. The case T » T_D (T_D is the Debye temperature) is especially analyzed, where the diffusion process is in the main connected with over-barrier jumping. The corresponding temperature dependence is determined by the expression T^3/2exp (–U/_BT, where U is the height of the potential barrier that separates neighboring sites. The dependence of the diffusion coefficient on the mass of the diffusing atom is also investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 60–65, June, 1985.     

Electrical properties of Cu-O monolayers intercalated into crystalline graphite

Electrical properties of Cu-O monolayers intercalated into crystalline graphite have been studied at microwave frequencies (up to 1 GHz) in the temperature range 80–400 K. As the temperature increases above 300 K, the resistance of the starting graphite samples increases because of oxygen desorption. Heating a sample containing intercalated Cu-O layers results in a transition from the metallic to the semiconducting (or insulating) state in the 95–130 K interval. At T=8 K, the samples exhibit microwave absorption typical of superconductors (this effect is not observed at T=260 K). Fiz. Tverd. Tela (St. Petersburg) 39, 97–100 (January 1997)     

Polarity sensitive electro-optical response in liquid crystal devices due to ionic diffusion in WO3 thin films

The present work is aimed to a deeper understanding of the polarity sensitive electro-optical response observed in a liquid crystal cell with asymmetric insertion of a thin film of tungsten trioxide, deposited by r.f. sputtering on a transparent ITO substrate. After preliminary structural characterization of the films, the electro-optical response of the liquid crystal cells containing a WO₃ layer has been analyzed. The voltage thresholds of the Frédericksz transition are increased or decreased for the anodic or the cathodic charge of the tungsten trioxide film, respectively. In such a way a polarity-sensitive electro-optical response is generated, with the same frequency of the applied field. Impedance and cyclic voltammetry measurements have been performed on liquid crystal cells having WO₃ films inserted as electrode, for as-deposited films as well as for annealed films at increasing temperatures. A strong decrease of the ionic current is observed in the cell where the film underwent the highest temperature treatment, above 300 °C. All the measurements suggest that a charge redistribution occurs for the hydrogen ions, always present in the films, during the intercalation-deintercalation processes induced by the applied voltage, and it is responsible for the changes of the effective electric field acting on the liquid crystal layer. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Calculation of the diffusion coefficient in acoustic turbulence

The first (Born) approximation commonly used to calculate the diffusion coefficient D_T of a passive scalar in acoustic turbulence is shown to be insufficient. Even for a small main parameter—the Mach number, M?1—the next approximation gives a larger contribution to D_T than does the first approximation, but negative in sign. We present a procedure for correctly calculating D_T based on the solution of a nonlinear DIA (direct interaction approximation) equation for the mean Green’s function of the problem. We include an additional term in the general formula for D_T that directly describes the compressibility of acoustic turbulence. This term has not been known previously and has been disregarded even in the Born approximation. A positive value was obtained for D_T=CM³u₀/p₀. The spectrum E(x) was assumed to be smooth at distances Δ x～M²?1.     

The diffusion coefficient of zinc in NaCl crystals

Measurements of the temperature and concentration dependence of the diffusion coefficient of divalent zinc were made to explain the mechanism of the process and to check the current theoretical conception. The measurements were executed on pure NaCl crystals. A knee in the curve of the logarithm of the diffusion coefficient versus the inverse temperature shows the diffusion is connected to two processes of different activation energy in accordance with the theoretical conception. The theoretically expected concentration dependence, however, has not been found, nor does the knee correspond to the expectations.     

Determination of diffusion coefficient in melts by the method of stationary diffusion source

This method eliminates some shortcomings encountered with other methods currently used for determination of the diffusion coefficients in melts. There are, with reasonable accuracy, fulfilled the starting and boundary conditions of a diffusion equation, and the diffusion process is not disturbed by melt flowing. This method considers a diffusion source at the boundary of a diffusion range and, when fulfilling certain conditions, both the solution of the diffustion equation and the evaluation of the diffusion coefficients is very simple. The required diffusion source, having a constant power, can be simulated by contacting the saturated gases of a substance, the diffusion of which is being investigated, with melt surface in which the diffusion takes place. The method takes into account application of radionuclides and this article brings about some criteria for application of that method. Simplicity is the main feature of the experimental arrangement.This work has been carried out in the scope of the State Research Task III-5-8 Equilibrium States of Metallurgical Systems.