Nature of the stable oxide layer formed on an aluminum surface by work function measurements

The interaction of oxygen with clean aluminum results in formation of a stable oxide layer on the surface of the metal. This stable layer has a lower work function than that of clean aluminum. The nature of this stable layer is studied by measurement of work function changes. Heating of the stable layer, formed at room temperature and low oxygen pressures' (~10^?8 torr), in ultra-high vacuum resulted in a further decrease in the work function. The extent of this additional decrease was a function of temperature and the changes in work function caused by heating were irreversible in nature. At high oxygen pressures (p_O2 > 10^?2 torr) the effect of growth of incorporated oxide on the net limiting work function change appears to be small and the reversible changes in the work function are mainly a result of the variation in the amount of surface oxide. At 250°C, the contribution of incorporated oxide to net limiting work function change was dependent on oxygen pressure when it was formed at low oxygen pressures (P < 10^?7 torr). However, when the incorporated oxide was formed at oxygen pressures above 10^?7 torr, its contribution to the limiting work function change and, perhaps, the structure of the incorporated oxide itself were nearly independent of oxygen pressure. The difference in limiting work function change at various oxygen pressures was mainly because of the difference in the limiting amounts of surface oxide. Similar behavior is expected at room temperature.     

Hydrogen absorption and re-emission characteristics of metal-oxide bi-layer composite materials measured by ion beam analysis

Hydrogen absorption and emission characteristics of Pt-Li₂ZrO₃ bi-layer materials exposed to normal air at room temperature have been studied by means of elastic recoil detection analysis(ERD), Rutherford backscattering spectroscopy (RBS), weight gain measurement (WGM) and thermal desorption spectroscopy (TDS). The Pt/Li₂ZrO₃/Pt sandwich specimens have been found to absorb H at the Pt surface from H₂O vapor, store it in Li₂ZrO₃, and emit 80% of it as H₂ gas, when they are heated at 100 °C for 10 min. Data obtained by WGM shows that the weight gain increases linearly with increasing the exposure time. TDS analysis also shows that the main species of gases re-emitted are H₂ and H₂O. Moreover, the hydrogen storage rate in Li₂ZrO₃ is shown to be controlled by the hydrogen absorption rate at the Pt surface, based on the hydrogen absorption and storage model proposed. The maximum storage capacity of Li₂ZrO₃ has been estimated to be 0.15 Nl/cm³ from the saturation hydrogen concentration, (1/2)H/Li₂ZrO₃, measured by means of the ERD technique.     

Structural and electrical properties of γ-alumina - lithium sulphate films

Li₂SO₄ as a model ionic conductor has received very much attention over several decades. Especially, in recent years Li₂SO₄ and Li₂SO₄-Al₂O₃ have been mentioned as promising proton conducting electrolytes for applications such as intermediate temperature fuel cells and novel cogeneration systems regarding H₂S handling devices. This has encouraged us to strive towards further improvement of the properties of the materials to meet the demands of the applications. In order to improve the properties of this system, a new process, a suspension technique, has been recently developed to prepare nanostructured powder and thin film Li₂SO₄-Al₂O₃ membranes. The powders and thin films have a well crystallised structure composed of two phases, Li₂SO₄ and γ-Al₂O₃, and excellent mechanical strength. The thin film thickness is in the scale of a few to several mm with a smooth and shining surface and a homogeneous macroscopic structure. It is a very interesting phenomenon that all samples show no significant conductivity increase at the temperature of the phase transition (∼ 577 °C) from β to α phase of pure Li₂SO₄. This transition has important significance for applications. The conductivity of the two-phase film materials has been greatly enhanced, where the xLi₂SO₄-(1-x)Al₂O₃ (x=58) sample shows the highest conductivity, about 1 S/cm at 600 °C; the activation energy decreases with increasing Li₂SO₄ content. These results agree with the so called composite effect for the conductivity enhancement observed earlier for two-phase bulk materials. Based on the four-step proton conducting mechanism in sulphate-based materials, this work may propose a new mechanism. The protons might jump in a water network associated with the water molecular re-orientation, which is accompanied with the single proton jump of the four-step transportation among SO ₄ ²⁻ groups from one Li₂SO₄ molecule to another. The former mechanism occur in the interfacial region between the Li₂SO₄ and the Al₂O₃ grains, while the latter occur in the bulk of the Li₂SO₄ grains. These thin film materials are intended for use as proton conducting ceramic membranes in applications such as desulphurisation and fuel cell co-generation plants. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

First applicability of ZnGa2O4 : Ge, Li, Mn phosphor for a plasma display panel

The green emission intensity of ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ excited by the vacuum ultraviolet line of 147 nm reaches 70% of commercial green Zn₂SiO₄:Mn²⁺. The vacuum ultraviolet excitation spectra consist of four peaks. In a plasma display test bed filled with Ar and Ne plasma discharged by a radio-frequency generator of 13.6 MHz, ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ and commercial Zn₂SiO₄:Mn²⁺ phosphor screens show a linear increase in luminance with increasing self bias voltages. Increasing gas pressures cause the luminance to increase. Also, on increasing the self bias voltages and the gas pressures, the current densities of ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ phosphor screens are increased; this is the same behavior as that of the commercial phosphor.     

A comparative study on the dielectric and ac conductivity of ionically conducting lithium silicate ceramics

Lithium silicate ceramics were prepared via solid-state reaction technique. Two ceramics with different molar ratios of Li₂CO₃/SiO₂ (designed as L₁S and L₂S) were prepared for dielectric studies. X-ray diffraction pattern showed that L₁S ceramic is obtained as single-phase composition, whereas L₂S ceramic exhibit multiphase with Li₄SiO₄ as a major phase. Dielectric studies of the compounds shows a strong frequency dispersion of permittivity in the low frequency region followed by a nearly frequency independent behavior in the high frequency region. Dielectric loss (tan δ) seems to be reduced at higher frequencies after reaching the instrumental saturation. The present study reveals that high dielectric compositions can be realized by Li₂O-rich system (i.e., L₂S ceramic). From the dielectric studies, we can prescribe L₁S ceramic as a good dielectric material since it possesses low dielectric loss. The values of activation energies suggest the presence of singly ionized oxygen vacancies in the conduction process.     

Preparation and characteristics of Li4Ti5O12 with various dopants as anode electrode for hybrid supercapacitor

Spinel-Li₄Ti₅O₁₂ is successfully synthesized by a solid phase synthesis. The Li₄Ti₅O₁₂ powders with various dopants (Al³⁺, Cr³⁺, Mg²⁺) synthesized at 800 °C are in accordance with the Li₄Ti₅O₁₂ cubic spinel phase structure. The dopants are inserted into the lattice structure of Li₄Ti₅O₁₂ without causing any changes in structural characteristics. In order to study the effect on various dopants, the hybrid supercapacitor is prepared by using un-doped Li₄Ti₅O₁₂ and doped Li₄Ti₅O₁₂ in this work. The electrochemical performance of the hybrid supercapacitor is characterized by impedance spectroscopy and cycle performance. The results show Cr³⁺ and Mg²⁺ dopants enhance the conductivity of Li₄Ti₅O₁₂. Also, Al³⁺ substitution improves the reversible capacity and cycling stability of Li₄Ti₅O₁₂. It is found that effect of dopant on the electrochemical performance of Li₄Ti₅O₁₂ as electrode material for hybrid supercapacitor where the EDLC and the Li ion secondary battery coexist in one cell system.     

Relationship between the structural and catalytic properties of mechanosynthesized lithiated manganese oxides

The relationship between the structural and catalytic properties of lithiated spinel manganese oxides was investigated by means of X-ray diffraction, Infrared and Xanes spectroscopies, thermogravimetric analysis, and by evaluating two catalytic oxidation tests, namely the carbon black combustion and the toluene conversion. Li-Mn-O catalysts were prepared from stoichiometric (Li₂O + MnO₂) mixtures, either by the classical high temperature ceramic method or by mechanochemistry. For both catalytic tests, some spectacular temperature reductions were measured as a function of grinding. A remarkable decrease of 210 °C (from 650 °C to 440 °C) in the carbon black combustion temperature was obtained when using mechanosynthesized Li-Mn-O spinel prepared from a mixture of Li₂O and MnO₂ ground for 3 hours, whereas a 100 % toluene conversion rate was achieved for a temperature lower than 200 °C for the 5 hours milled ceramic LiMn₂O₄ while the as-made ceramic was inactive. The enhancement of the performances (i.e. decrease in carbon black combustion temperature Tc and decrease in toluene conversion temperature T_95%) is due both to an increase in grain boundaries and in specific BET surface area and to the nano-crystallite size nature of the material. Besides, the spinel stoichiometry (both in oxygen or in cations) reflected by the lattice parameter variation plays a significant role in the catalytic reaction mechanism.     

Li3VO4-Li4TO4(T=Ge,Si)固溶体离子导体7Li的NMR研究

在150—573K温度范围内,研究了固溶体Li₃VO₄-Li₄TO₄(T=Ge,Si)系统不同成分的⁷Li的NMR谱。发现γ_II相固溶体室温⁷Li的NMR线宽和自旋晶格弛豫时间T₁的值都比Li₄GeO₄,Li₄SiO₄和Li₃VO₄小约一个数量级。这表明在γ_II相固溶体离子导体中,Li⁺离子运动有可能比固溶前有数量级增长。同时还发现⁷Li的电四极分裂伴线数随成分和温度而异,以及伴线强度百分比依赖于温度。这反映γ_II相的不同成分中,间隙Li⁺离子占有的不等价位置个数不同,而Li⁺离子在每个不等价位置上的占有率又随温度而变化。 关键词：     

ArF excimer laser deposition of wide-band gap solid electrolytes for thin film batteries

High quality solid electrolyte thin films was grown by pulsed laser deposition (PLD) using a high photon energy ArF excimer laser. Various amorphous thin films were successfully deposited on glass substrates from oxide targets; such as Li₃PO₄, LiBO₂, Li₂SiO₃, Li₂CO₃, Li₂SO₄, Li₂ZrO₃, LiAlO₂, Li₂WO₄ and Ohara glass ceramics. The morphology, optical property and ionic conductivity of these films were examined by optical microscope, UV–VIS spectroscopy and impedance analysis. Dramatic improvement of the film morphology was observed by using a high photon energy laser, while the broken film with many droplets was obtained by using lower ones. Ionic conductivity of the films was examined by impedance spectroscopy and dc polarization method. For example, an ionic conductivity of a Li₃PO₄ film was 4.6 × 10^? 6 S cm^? 1 at 25 °C with activation energy of 0.57 eV. Electronic conductivity measurements revealed that most of the films were pure lithium ion conductors, while a Li₂WO₄ film was a mixed conductor.     

Mechanism of LiFePO4 solid-phase synthesis using iron (II) oxalate and ammonium dihydrophosphate as precursors

An experimental study of the thermolysis mechanism of FeC₂O₄, NH₄H₂PO₄, Li₂CO₃, and citric acid from the viewpoint of the usage of a mixture of these compounds in lithium power engineering for the solid-state synthesis of LiFePO₄ and its composite with carbon LiFePO₄/C as well as comparison of experimental data with thermodynamic calculations were made in the temperature range from 25 up to 1,000 °C. The oxides Fe₃O₄, Fe₂O₃, and FeO were detected as the intermediate products of thermolysis of ferrous oxalate in these conditions. Various paths of oxalate decomposition may well proceed concurrently with the predomination of this or that path under slight changes in the experimental conditions. The formation of orthorhombic lithium phosphate Li₃PO₄ is detected just in a blend grinded at room temperature, and Li₃PO₄ and NH₄PO₃ are the basis of triphylite synthesis at increased temperatures (up to 800 °C). A new phase of single-substituted anhydrous lithium citrate C₆H₇O₇Li is formed at room temperature if citric acid C₆H₈O₇?H₂O is used as an organic precursor. The thermal treatment, at which citric acid can form a carbon coating with a maximum conductivity, was estimated experimentally. To identify the products of chemical reactions, structural characterization, and comparative analysis of samples synthesized at several temperatures, a set of techniques was used, namely TG with gas release analysis, Mossbauer spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, surface microanalysis, laser diffraction analyses. Galvanostatic cycling was used to study the electrochemical properties of the LiFePO₄/C electrode material.     

Effects of Li2CO3 as a secondary lithium source on the LiFePO4/C composites prepared via solid-state method

Li₂CO₃ was used as the secondary lithium source for the synthesis of LiFePO₄/C composites via a solid-state reaction method by adopting Li₃PO₄ as the main lithium source. The main purpose of using Li₂CO₃ is to compensate for the partial lithium loss during the sintering while reducing the usage of excess Li₃PO₄. In this study, the effects of Li₂CO₃ amount on the phase, structural and electrochemical properties of LiFePO₄/C material were systematically investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), constant-current charge–discharge test and cyclic voltammetry (CV). The results showed that by adding an appropriate amount of Li₂CO₃, the impurities, e.g. Li₃PO₄, normally appearing in the final product, could be excluded. It was found that LiFePO₄/C with Li₂CO₃ in 6% excess (vs. stoichiometric LiFePO₄) exhibited the best electrochemical performance, which delivered initial discharge capacities of 141.7, 125.2, 119.9 and 108.9 mAh g^?1, respectively, at 0.5, 1, 2 and 5C rates. The capacity was reduced to 113.4 mAh g^?1 after 50 cycles at 2C rate, with capacity retention rate of 94.6%.     

Lithium ion conductive glass and its application to solid state batteries

Three kinds of coin-type battery, In-Li_x / Li_1−xCoO₂, Li_4/3+xTi_5/3O₄ / Li_1−xCoO₂, and Li_2+xFeS₂ / Li_1−xCoO₂, were fabricated with a Li⁺ ion conductive glass as an electrolyte, and their properties were investigated. They show excellent performance thanks to the solid electrolyte. Iron sulfide is found to be an excellent electrode material in solid state rechargeable batteries. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Mass spectrometric and thermodynamic studies of vapor-phase growth of In(1−x)GaxP

A time-of-flight mass spectrometer was coupled directly to an open tube vapor growth system. The vapor composition and the processes occurring during the deposition of In_(1?x)Ga_xP alloys were studied. It was found that the vapor species present in this system are InCl, GaCl, HCl, PH₃, P₂, P₄ and H₂. The deviations from the chemical equilibrium in the system were determined and measured.A chemical equilibrium model was set and used to calculate the equilibrium partial pressures of all species under a wide range of experimental conditions. These calculations were used to predict successfully the proper conditions for growth of desired In_(1?x)Ga_xP alloys.     

Surface charging at the (1 0 0) surface of Cu doped and undoped Li2B4O7

We have compared the photovoltaic charging of the (1 0 0) surface termination for Cu doped and undoped Li₂B₄O₇. While the surface charging at the (1 0 0) surface of Li₂B₄O₇ is significantly greater than observed at (1 1 0) surface, the Cu doping plays a role in reducing the surface photovoltage effects. With Cu doping of Li₂B₄O₇, the surface photovoltaic charging is much diminished at the (1 0 0) surface. The density of states observed with combined photoemission and inverse photoemission remains similar to that observed for the undoped material, except in the vicinity of the conduction band edge.     

New Li+ ion conductors in the system Li4SiO4−Li3AsO4

In the system Li₄SiO₄-Li₃AsO₄, Li₄SiO₄ forms a short range of solid solutions containing up to 14 to 20% Li₃AsO ₄, depending on temperature, and γ-Li₃AsO₄ forms a more extensive range of solid solutions containing up to ≈55% Li₄SiO₄. The Li₄SiO₄-Li₃AsO₄ phase diagram has been determined and is of binary eutectic character. The ac conductivity of polycrystalline samples was measured over the range 0 to at least 300°C for nine different compositions. The two solid solution series have much higher conductivity than the pure end-members; maximum conductivity was observed in the γ-Li₃AsO₄ solid solutions containing ≈40 to 55% Li₄SiO₄, with values of $\text{≈2×10}{}^{\mathrm{?6}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ at 20°C rising to ≈0.02 Ω^?1 cm^?1 at 300°C. These values are comparable to those found in the system Li₄SiO₄-Li₃PO₄. The variation with composition of the Arrhenius prefactor and activation energy has been interpreted in terms of the mechanisms of conduction. Li₃AsO₄ is a poor conductor essentially because the number of mobile Li⁺ ions is very small. This number, and hence the conductivity, increases dramatically on forming solid solutions with Li₄SiO₄, by the creation of interstitial Li⁺ ions. At ≈40 to 55% Li₄SiO₄, the number of mobile Li⁺ ions appears to be optimised. An explanation for the change in activation energy of conduction at ≈290°C in Li₄SiO₄ and at higher temperatures in Li₄SiO₄ solid solutions is given in terms of order-disorder of the Li⁺ ions.     

Synthesis,characterization, and cell performance of Li<Subscript>0.5</Subscript>FeV<Subscript>1.5</Subscript>O<Subscript>4</Subscript>

A Li_0.5FeV_1.5O₄ sample was synthesized using sol-gel route. The X-ray diffraction study indicates formation of spinel phase (with Fd3m space group) for this sample. LiO₄, LiO₆, and V-O bonds were identified from the Raman spectrum, while LiO₄ and Fe-O bonds were identified from the FTIR spectrum of this sample phase. The FESEM study indicates formation of inhomogeneous grains. The surface area of 74.39 m²/g was estimated from the Brunauer-Emmett-Teller (BET) surface area analysis technique. The cyclic voltammetry study of Li_0.5FeV_1.5O₄ indicates an anodic peak at 2.1 V while a cathodic peak at 1.98 V. The charge-discharge study exhibits two voltage plateaus respectively at 2.1 and at 4 V. Stable electrochemical capacity of 40 mAh/g for Li_0.5FeV_1.5O₄ was found for 30 cycles. The electrochemical impedance spectroscopy study indicates smaller bulk resistance and higher ionic diffusion, i.e., less Warburg impedance for this phase. An energy density of 89 Wh/kg, a power density of 33 W/kg, and a 90% Coulombic efficiency was achieved with relatively good cyclic stability from Li_0.5FeV_1.5O₄.     

Effects of duty ratio at low frequency on growth mechanism of micro-plasma oxidation ceramic coatings on Ti alloy

The aim of this work is to study the effects of duty ratio on the growth mechanism of the ceramic coatings on Ti-6Al-4V alloy prepared by pulsed single-polar MPO at 50 Hz in NaAlO₂ solution. The phase composition of the coatings was studied by X-ray diffraction, and the morphology and the element distribution in the coating were examined through scanning electron microscopy and energy dispersive spectroscopy. The thickness of the coatings was measured by eddy current coating thickness gauge. The corrosion resistance of the coated samples was examined by linear sweep voltammetry technique in 3.5% NaCl solution. The changes of the duty ratio (D) of the anode process led to the changes of the mode of the spark discharge during the pulsed single-polar MPO process, which further influenced the structure and the morphology of the ceramic coatings. The coatings prepared at D = 10% were composed of a large amount of Al₂TiO₅ and a little γ-Al₂O₃ while the coatings prepared at D = 45% were mainly composed of α-Al₂O₃ and γ-Al₂O₃. The coating thickness and the roughness were both increased with the increasing D due to the formation of Al₂O₃. The formation of Al₂TiO₅ resulted from the spark discharge due to the breakdown of the oxide film, while the formation of Al₂O₃ resulted from the spark discharge due to the breakdown of the vapor envelope. The ceramic coatings improved the corrosion resistance of Ti-6Al-4V alloy. And the surface morphology and the coating thickness determined the corrosion resistance of the coated samples prepared at D = 45% was better than that of the coated samples prepared at D = 10%.     

Anode materials for lithium ion batteries in the Li-Zn-P system

This paper presents an exhaustive study of the Li-Zn-P system through the synthesis and electrochemical characterisation of several binary and ternary phases: Li₉ZnP₄, LiZnP, Zn₃P₂ (α and β), ZnP₂ (α and β), LiZn, and LiZn₄. Three synthetic routes have been used to prepare these materials: ceramic synthesis and ball milling without and with annealing. Li-Zn-P system phases have been evaluated through X-ray diffraction and electrochemical reactivity towards lithium. Exhibiting high specific capacities at potentials close to 0.7 V vs. Li⁺/Li, some of these materials are promising as negative electrode materials for lithium ion battery. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

A new composite material Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>–SnO<Subscript>2</Subscript> for lithium-ion batteries

A new Li₄Ti₅O₁₂–SnO₂ composite anode material for lithium-ion batteries has been prepared by loading SnO₂ on Li₄Ti₅O₁₂ to obtain composite material with improved electrochemical performance relative to Li₄Ti₅O₁₂ and SnO₂. The composite material was characterized by X-ray diffraction and scanning electron microscopy. The results indicated that SnO₂ particles have encapsulated on the surface of the Li₄Ti₅O₁₂ uniformly and tightly. Electrochemical results indicated that the Li₄Ti₅O₁₂–SnO₂ composite material increases the reversible capacity of Li₄Ti₅O₁₂ and has good cycling reliability. At a current rate of 0.5 mA/cm², the material delivered a discharge capacity of 236 mAh/g after 16 cycles. It suggests the existence of synergistic interaction between Li₄Ti₅O₁₂ and SnO₂ and that the capacity of the composite is not a simple weighted sum of the capacities of the individual components. In the composite material, SnO₂ can act as a bridge between the spinel particles to reduce the interparticle resistance and as a good material for the Li intercalation/deintercalation. Thus, electrochemical performance of the Li₄Ti₅O₁₂ spinel can be improved by the surface modification with SnO₂, and the stability of Li₄Ti₅O₁₂ also serves to buffer the internal stress caused by the volume changes in lithium insertion and extraction reactions.     

Deposition of duplex Al2O3/aluminum coatings on steel using a combined technique of arc spraying and plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) is a cost-effective technique that can be used to prepare ceramic coatings on metals such as Ti, Al, Mg, Nb, etc., and their alloys, but this promising technique cannot be used to modify the surface properties of steels, which are the most widely used materials in engineering. In order to prepare metallurgically bonded ceramic coatings on steels, a combined technique of arc spraying and plasma electrolytic oxidation (PEO) was adopted. In this work, metallurgically bonded ceramic coatings on steels were obtained using this method. We firstly prepared aluminum coatings on steels by arc spraying, and then obtained the metallurgically bonded ceramic coatings on aluminum coatings by PEO. The characteristics of duplex coatings were analyzed by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The corrosion and wear resistance of the ceramic coatings were also studied. The results show that, duplex Al₂O₃/aluminum coatings have been deposited on steel substrate after the combined treatment. The ceramic coatings are mainly composed of α-Al₂O₃, γ-Al₂O₃, θ-Al₂O₃ and some amorphous phase. The duplex coatings show favorable corrosion and wear resistance properties. The investigations indicate that the combination of arc spraying and plasma electrolytic oxidation proves a promising technique for surface modification of steels for protective purposes.