Photo-intercalation: Possible application in solar energy devices

Theoretical considerations and preliminary photo-electrochemical experiments with ZrSe₂ indicate the possibility of converting and simultaneously storing solar energy by means of light driven electrochemical reactions producing intercalation compounds of layer-type semiconducting material. A precondition is that the intercalated compound maintains a semiconducting behaviour and that its ionic properties complement in a favourable way. Promising substrates were identified inp-type zirconium-and hafniumdichalcogenides, but also TiS₂ would be useful if it could be madep-conducting. Solar cells based on photo-intercalation — if they could be developed for practical use — would not only be simple [sandwich ofp-conducting layer-type semiconductor (e.g. ZrS₂)/ionic conductor (wet or solid)/metal of intercalating species (e.g. Cu)], but also more convenient to use in irregular sunlight than conventional devices. Some thermodynamic properties and attainable efficiencies of this new type of solar cell are discussed as well as difficulties which would have to be surmounted.     

Ion transfer in solid solutions of Cu<Subscript>2</Subscript>Se and Ag<Subscript>2</Subscript>Se superionic conductors

The cationic conductivities of Cu₂Se and Ag₂Se superionic conductor solid solutions in the composition region from Cu₂Se to Cu_0.7Ag_1.3Se are measured. It is demonstrated that the activation energy of ionic conduction depends only slightly on the chemical composition, varies from 0.14 to 0.17 eV, and exhibits a weakly pronounced maximum for the Ag_0.44Cu_1.56Se solid solution. The ionic Seebeck coefficients are measured for the Ag_0.23Cu_1.757Se solid solution. The heat of cation transfer in this solution is found to be equal to 0.144±0.014 eV from the Seebeck coefficients.     

Transport in dispersed ionic conductors: Effect of insulating particle size

Dispersed ionic conductors are random mixtures of a solid salt, e.g. AgI, LiI, with fine particles of an insulating second phase, like Al₂O₃ or SiO₂. These composites can show a dramatic increase in ionic conductivity compared to the pure homogeneous system. Generally, this observation is attributed to an increased conductivity along the internal interface between the conducting salt and the insulating material. In this work a three-component random resistor network (RRN) model for dispersed ionic conductors is reviewed. In the model, the ionic conductor is represented by normally conducting bonds, the insulating material by non-conducting bonds and the interface between the two phases by highly conducting bonds. A special feature of the model is the existence of two critical concentrations of the insulating phase, p′_c and p″_c , for interface percolation and bulk conduction, respectively, where critical transport properties corresponding to conductor/superconductor and conductor/insulator networks are predicted. The model describes satisfactorily the dependence on composition of the conductivity and activation energy of dispersed ionic conductors. Furthermore, the observed effect on the conductivity of the size of dispersed particles can be described qualitatively well by a generalized version of the RRN model, which in addition predicts a sensitive dependence of the critical thresholds on particle size. Non-universality features in the critical exponents for the conductivity are also discussed within a continuum percolation analog of the model.     

Investigation of the influence of solid ionic conductor admixtures on the performance of gas sensors based on tin oxide

Tin oxide is a well-known and widely used gas sensitive material for detection of toxic and hazardous gas components in air. Different admixtures such as catalysts, aliovalent ions and solid ionic conductors were added to improve the sensitivity, selectivity and stability of these sensors. For material preparation a sol-gel route is usually used and catalysts are added before sintering to improve the sensitivity and to favour the sensors to specific gas components. In this work we investigated the influence of added solid ionic conductors on the gas sensitivity of tin oxide based ceramic admixtures dependent on the relative parts of the oxidic sodium ionic conductor, the ionic conductivity and the grain size in relation to the tin oxide grains. Almost ideal model substances for these studies are solid electrolytes with the NASICON (Na_1+xZr₂Si_xP_3−xO₁₂) structure. They can be prepared in the sol-gel route and in a solid state reaction as well resulting in different powder grain sizes. The sodium ionic conductivity of the solid electrolytes in the composition range 0≤×≤3 varies by several orders of magnitude. Both materials, the NASICON and the tin oxide, were characterized with respect to morphology and structure by BET and XRD measurements. Powders of tin oxide were homogeneously mixed with different NASICON powders, pressed as discs, sintered, and contacted with gold electrodes for measurements of the conductivity in air at various temperatures (200°C≤T≤400 °C). For preparation of sensor layers the powder mixtures were transferred to a paste, screen printed on an alumina substrate with interdigital gold electrodes and sintered for gas sensitivity measurements. A dramatic sensitivity enhancement of the composite to alcohol and significant sensitivity reduction to other gases like CO, H₂, NH₃, CH₄, C₃H₈ was found. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Laser Raman spectral studies under d.c. bias on fast proton transport in molybdic acid (MoO3· 2H2O)

Laser Raman spectroscopy on samples across which a d.c. field has been applied is suggested as a new technique for studying ionic solids. As an example, the laser Raman spectra of a solid state proton conductor MoO₃ · 2H₂Ois reported with the d.c. field “on”. Two new peaks at 795 and 414 cm⁻¹ appear in the laser Raman spectra and their intensities grow with increasing time and applied d.c. voltage. Their appearance has been explained by assuming that H₃O⁺ ions are created by the d.c. field as the mobile charged ionic species. By monitoring the peak intensities of the 795 and 414cm⁻¹ bands at different places in the sample between the cathode and the anode, the diffusion coefficient has been estimated.     

Current-voltage curves from a Bi2O3Y2O3 oxygen ion conductor

The solid ionic conductor cell Bi₂O₃Y₂O₃ was used to the current-voltage behaviour under different temperatures and voltage scan rate, as is usually done in cyclic voltammetry in three-electrode cells using liquid electrolytes. The result shows that the reactions are different at the electrodes and a hysteresis effect is presented at low temperatures and high voltage scanning rates.     

Study on ionic conductivity and dielectric properties of PEO-based solid nanocomposite polymer electrolytes

The ionic conductivity and dielectric properties of the solid nanocomposite polymer electrolytes formed by dispersing a low particle-sized TiO₂ ceramic filler in a poly (ethylene oxide) (PEO)-AgNO₃ matrix are presented and discussed. The solid nanocomposite polymer electrolytes are prepared by hot press method. The optimum conducting solid polymer electrolyte of polymer PEO and salt AgNO₃ is used as host matrix and TiO₂ as filler. From the filler concentration-dependent conductivity study, the maximum ionic conductivity at room temperature is obtained for 10 wt% of TiO₂. The real part of impedance (Z′) and imaginary part of impedance (Z″) are analyzed using an LCR meter. The dielectric properties of the highest conducting solid polymer electrolyte are analyzed using dielectric permittivity (ε′), dielectric loss (ε″), loss tangent (tan δ), real part of the electric modulus (M′), and imaginary part of the electric modulus (M″). It is observed that the dielectric constant (ε′) increases sharply towards the lower frequencies due to the electrode polarization effect. The maxima of the loss tangent (tan δ) shift towards higher frequencies with increasing temperature. The peaks observed in the imaginary part of the electric modulus (M″) due to conductivity relaxation shows that the material is ionic conductor. The enhancement in ionic conductivity is observed when nanosized TiO₂ is added into the solid polymer electrolyte.     

Electroluminescence of the electrolyte/n-ZnSe junction under anodic polarization

The electroluminescent properties of the electrolyte/ZnSe junction give important information on electron transfer across the interface. Adsorption of OH^? groups on it, that enables electron injection into the semiconductor, accounts for a luminescence at 2.0 eV, produced by an impact-ionization mechanism already observed in solid state diodes. Addition of Cu²⁺ ions in the electrolyte gives an additional luminescene at an energy of 2.2 eV. This phenomenon is connected to the adsorption of Cu⁰ and Cu²⁺ onto the semiconductor surface in relation with a corresponding energy level Cu^X_Zn in the semiconductor bulk.     

AgI(Cr2O3)混合离子导体改性的机理研究

本文研究了Agl(Cr₂O₃)混合离子导体的红外吸收光谱、近紫外、可见反射光谱,透射电子显微镜的显微形貌及成份分析,发现混合离子导体的微观结构与纯Agl,Cr₂O₃不同。由透射电子显微镜的显微形貌得出不同于他人的介质模型。提出电介质引起的变形极化,导致了微观精细结构改变的观点。并对其物性改变的机理给予了讨论。 关键词：     

Fundamental Concepts of Electron Spin Resonance Technique in the Determination of the Crystal Field Structure of O− 2 Ions in TiO2

Abstract

The basic principles of Electron Spin Resonance (ESR) as applicable in crystal field characterization of paramagnetic species has been outlined. Fundamental concepts of the precessional motion of electrons and their magnetic moments at resonance were developed. The theory of ESR based on the response of unpaired electron(s) as they undergo spin-spin or spin-lattice relaxation when subjected to strong external magnetic fields was examined. Ions of the O^? ₂ group adsorbed on TiO₂ were studied using a Varian Spectrometer. The resulting spectral diagrams obtained were used in calculating the g-factors which gave results for orthorhombic crystal symmetry for O^? ₂ ions in TiO₂.     

Nanowire and nanocable junction capacitances: Results for metal and semiconducting oxides

Formulas are derived for Schottky junction and p-n junction capacitances of nanowires and nanocables (which includes the hollow case of nanotubes). Evaluation of these expressions for the very unusual metal oxide, metallic conductor RuO₂, demonstrates that capacitance values occur in the aF range. Furthermore, for recently fabricated conductor RuO₂/SiO₂ nanocables as reported in the literature, we show that the junction capacitances are significantly lower than the intrinsic quantum capacitance. These formulas are of great interest for device applications, and the capacitance findings could also have huge implications for charge storage and energy conversion applications.     

ϑ-Sensors: A new concept for advanced solid-state ionic gas sensors

A new principle of solid state electrochemical sensors based on the kinetics of controlled chemical reactions of the gas with the electroactive species of a solid electrolyte is presented and demonstrated for the measurement of CO₂ partial pressures. The reaction may be for many gases modified by the formation of intermediate product phases.A periodic voltage or current signal is applied to the electrolyte. The current corresponds to alternating chemical reactions at the electrodes in contact with the gas. The new concept avoids the often complicated application of reference electrodes and shows characteristic signals for different types of gases and allows in this way to overcome problems of cross-sensitivity. The experimental results show high accuracy with an approximately linear relationship between the current and the CO₂ partial pressure and also show extraordinarily fast response.     

Photochemistry of solid C60 with tunable infrared radiation

60 with trains of picosecond infrared (IR) pulses, tuned over the 8–15 μm range, is studied. At some specific wavelengths, white-light emission as well as ejection of ionic species from the solid is observed. The spectral characteristics of the white-light emission resemble those of a black body. The mass distribution of the ejected ionic species shows substantial amounts of C₆₀ coalescence products. Unexpectedly, all these processes only occur at wavelengths where solid C₆₀ is relatively transparent. No white-light emission nor ejection of ionic species is observed when being resonant with an IR-allowed transition of C₆₀. It is concluded that regular C₆₀ is not the chromophore for the observed processes, and that sequential absorption of single photons by a strong absorber that is dilute in the crystal takes place. Plausible chromophores are sites that are intercalated with alkali metals. Accumulation of energy at these sites leads to fullerene coalescence in the solid, ion ejection, and white-light emission, ultimately resulting in the destruction of the C₆₀ molecules. Received: 14 April 1998/Accepted: 22 April 1998     

Adsorption of acetylene and ethylene on a clean Ir(111) surface

The adsorption of C₂H₂ and C₂H₄ on Ir(111) is studied within the temperature range 180–500 K by the HREELS and XPS methods. The absolute concentration of hydrocarbon coverage is estimated by XPS. Data are obtained on the kinetics of adsorption of the two gases at different temperatures. It is established by HREELS studies that at 180 K C₂H₄ forms ethylidyne (CCH₃ whereas C₂H₂ is adsorbed as CCH and ethylidyne species. At 300 K both kinds of species are found on the Ir(111) surface after C₂H₂ or C₂H₄ exposures. The ethylidyne decomposes completely to CCH at 500 K, which can be accompanied by polymerization of adsorbed hydrocarbon species.     

Scanning photocurrent and PL imaging of a frozen polymer p–i–n junction

Polymer light‐emitting electrochemical cells (LECs) are two‐terminal, solid state devices with a mixed ionic/electronic conductor as the active layer. Once activated by a DC voltage or current, a doping‐induced homojunction dictates the electrical and optical response of the LEC, making it highly unique and attractive among organic devices. However, the depletion width, a fundamental parameter of any semiconductor homojunction, has never been determined experimentally for a static LEC junction. In this study, we apply spatially resolved photocurrent and photoluminescence (PL) scanning to an extremely large planar LEC that had been turned on to emit strongly then subsequently frozen. These concerted scanning and imaging studies depict a p–i–n junction structure in which the peak built‐in electric field lies at the interface between the intrinsic region and the p‐doped region. The corresponding 18 μm depletion width is very small compared to the 700 μm interelectrode spacing.

     

CO2-laser-induced breakdown in mono- and diatomic gases

Several monoatomic and homonuclear diatomic gases absorb energy from a focused CO₂-laser photon field. It has been established that the pressure threshold for the energy absorption correlates qualitatively with the known ionization potentials of those gases. The simplified phenomenological theory of the CO₂-laser-induced dielectric breakdown of gases is invoked to explain this phenomenon. In the H₂–D₂ system, the formation of HD is observed under these conditions. The examination of the reaction yields for HD formation demonstrates that the system studied does not reach equilibrium under our experimental conditions. Considerations regarding kinetics of primary processes reveal that ionic species, created originally via an inverse bremsstrahlung mechanism, are converted into atomic transients in fast ionic association processes. The latter species initiate chain reactions with surrounding molecules of substrates leading to the formation of HD. Simple kinetic analysis based on a non-steady-state assumption permitted the derivation of an expression for the yield of HD formation. This equation was fitted to the experimental data assuming that the temperature of the reaction rises with an increase of the amount of D₂ in the mixture. Some other aspects regarding the behavior of this system in a focused CO₂ laser beam are also discussed.U.S. Department of Energy Document No. DE-AS02-76ER03416-37     

Enhanced conductivity in ionic conductor-insulator composites: numerical models in two and three dimensions

We describe a two-dimensional (2D) and a three-dimensional (3D) percolation model for ionic conductor-insulator composites such as copper(I) bromide-titanium dioxide (CuBr-TiO₂) or lithium iodide-alumina (LiI-Al₂O₃). These composites present an enhanced conductivity closely related to the insulator concentration. This effect is explained by the formation of highly conducting space charge regions near the phase boundaries which are represented by good conductor bonds. Our numerical model takes into account grain size and correlation effects. The dimension has a leading role for the conduction properties. In the 2D case, the good conductor bonds do not percolate, whatever the insulator concentration, and the maximum conductivity of the composite samples is of the same order as that of the ionic conductor grains. The behavior of the system is very different in the 3D case where, for a large domain of composition, the good conductors percolate through the regions between the conductor grains. For the CuBr-TiO₂ composites the conductivity versus composition curve is bell-shaped. Conversely, in the LiI-Al₂O₃ system, a linear relation between the conductivity and the insulator volume fraction is obtained in the experiments. Our model gives a plausible interpretation of the conductivity in both systems. Received 10 April 2001     

OPE molecular junction as a hydrogen gas sensor

Oligo(phenylene ethynylene) (OPE) molecular junction has been suggested as a H₂ molecule sensor based on calculations using the first principles of density–functional theory and non-equilibrium Green's function. The electronic transport properties of the OPE molecule between two Au electrodes with or without adsorbed H₂ molecules are investigated. Results show that the adsorbed H₂ molecule significantly changes the characteristics of the current–voltage curve of the OPE molecular junction. The pure OPE molecular junction exhibits a significant negative differential resistance, but this kind of phenomenon will disappear or weaken after hydrogen molecules are adsorbed. The conductance of the junction also obviously decreases in the bias range of [−0.4, 0.4] V after adsorbing H₂ molecules. These effects can be used to design a H₂ molecule sensor.     

Materials design and optimization for thin-film microbatteries

Considerable effort has been invested in developing thin-film solid microbatteries as possible integrated components in microelectronics. Advances have been made particularly in the engineering of lithium/amorphous inorganic electrolyte/layered compound cells fabricated using various evaporation techniques. The main features of these cells are influenced by the characteristics of the fast ionic conductor and the insertion compound layers which are discussed in detail in this work. Design and optimization of lithium-microbattery components are discussed. Different electrochemical systems using lithium borate-glass films as solid electrolytes have been fabricated and characterized. Thin-film active cathodic materials include transition-metal dichalcogenides such as TiS₂, MoS₂, non-transition-metal chalcogenides such as InSe and transition-metal oxides such as MoO₃, V₂O₅, V₆O₁₃ films. Particular attention is paid to the structural and transport properties of these materials and their behaviour in electrochemical lithium cells are presented. Thermodynamics and kinetics are studied as functions of the growth conditions of thin-film components. The physical properties are discussed in relation to the electrochemical cell behavior and battery performance. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept. 1994.     

Pipe-sphere model for enhancement of ionic conductivity in composite solid electrolytes

It is well-known that the ionic conductivity of a superionic conductor when dispersed with an insulator shows a remarkable enhancement. In this work we suggest that the contribution coming from grain-boundaries and dislocations is primarily responsible for this phenomenon in a number of cases. We propose a simple theoretical model for such composites and with the aid of the Effective Medium Theory (EMT) under self-consistent scheme we estimate the effective conductivity as a function of insulator volume fraction and particle size for four composites, namely CaF₂-Al₂O₃, CuCl-Al₂O₃, Sr(NO₃)₂-Al₂O₃ and SrCl₂-Al₂O₃. This model is applicable to composites where enhancement is observed for a very low insulator volume fraction and other prevalent models are inadequate. The results exhibit a good qualitative fit to the experimental data and all characterisitic experimental observations.