Membranes based on phosphotungstic acid and polybenzimidazole for fuel cell application

Composite membranes based on phosphotungstic acid (PWA) adsorbed on silica (SiO₂) and polybenzimidazole (PBI) have been prepared and their physico-chemical properties have been studied. The membranes with high tensile strength and thickness of less than 30 μm can be cast. They are chemically stable in boiling water and thermally stable in air up to 400°C. Proton conductivity is influenced by the temperature (range: 30–100°C), relative humidity and PWA loading in the membrane. Maximum conductivity of 3.0×10⁻³ S/cm is obtained at 100% relative humidity and 100°C with membrane containing 60 wt.% PWA/SiO₂ in PBI. Conductivity measurements performed at higher temperatures, in the range from 90°C to 150°C, give almost stable values of 1.4–1.5×10⁻³ S/cm at 100% relative humidity.     

Kinetics of the NCO radical reacting with atoms and selected molecules

Rate constants for the reaction of isocyanate radicals (NCO) in its electronic ground state ( ²Π) with oxygen atoms were determined at 2.5 Torr total pressure in the temperature range 302–757 K. Excimer laser photolysis (ELP) of chlorine isocyanate (ClNCO) produced NCO radicals detected by laser-induced fluorescence (LIF). The reaction NCO + O exhibits a negative temperature dependence, described by the two-parameter equation: k_NCO+O(T) = (4.3_−2.2^+3.2) × 10⁻⁸ × T^{−1.14_−0.12+0.08} cm³ molecule⁻¹ s⁻¹. Measurements at 298 K and total pressures of 2.5 and 9.9 Torr, respectively, indicated a slight pressure dependence. For the reaction of NCO radicals with hydrogen atoms, the rate constant k_NCO+H = (2.2 ± 1.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹was obtained at 298 K and a total pressure of 2.6 Torr for the first time by a direct measurement. From a single measurement k = (3.8 ± 1.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ was determined at 548 K and 2.4 Torr total pressure. In addition, rate constants for the reactions of NCO radicals with molecular oxygen (O₂), carbon dioxide (CO₂), molecular hydrogen (H₂), and carbon monoxide (CO), which is a dissociation product of CO₂ in a microwave discharge, were measured at two different temperatures. At room temperature these reactions were slow and at the detection limit of the ELP/LIF technique. However, at elevated temperatures at least the rate constants of the reactions NCO + O₂ and NCO + H₂ become significantly larger and, therefore, should be taken into account, when modeling combustion processes under certain conditions.     

Electrical and optical properties of F-doped textured SnO2 films deposited by APCVD

This paper presents the structural, electrical and optical properties of transparent conducting F-doped textured SnO₂ films prepared by atmosphere pressure chemical vapour deposition (APCVD). Polycrystalline SnO₂:F films having a variable preferred orientation have been obtained with resistivity as low as 5 × 10⁻⁴ Ωcm, with carrier concentrations between 3.5 × 10²⁰ and 7 × 10²⁰ cm⁻³, and Hall mobilities from 15.7 to 20.1 cm²/V/s. The average transmittance (including diffusion transmittance) is as high as 94% in the wavelength range of the visible spectrum and the maximum infrared reflectance reaches 92% for a film 655 nm thick. The figure of merit ƒ_TC = T10/_sh, (7.12 × 10⁻² S) of these films is the highest amongst the results reported on doped SnO₂ films.     

Electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 hydrogen storage alloy

The electrochemical properties of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 hydrogen storage alloy have been studied through chronopotentiometric, chronoamperometric and cyclic voltammogram measurements. The maximum capacity value obtained was 265 mAh g⁻¹ at rate C/6 and the capacity decrease was recorded by 1.5% after 30 cycles. The values of the hydrogen diffusion coefficient D_H obtained through cyclic volammogram and chronoamperometric techniques were, respectively, 7.01 × 10⁻⁸ cm² s⁻¹ and 4.23 × 10⁻¹¹ cm² s⁻¹.     

Effect of rare earth oxides for improvement of MCFC

The solubility of rare earth metal oxides and their effect on the NiO solubility have been discussed to stabilize the cathode of molten carbonate fuel cells. The solubility of Ho, Yb, and Nd oxides were 4.4 × 10⁻⁴, 3.4 × 10⁻⁴, and 1.3 × 10⁻³ (mole fraction) at 923 K, respectively. The solubilities of NiO in (Li_0.52/Na_0.48)₂CO₃ with the saturated Ho, Yb, and Nd were 1.57 × 10⁻⁵, 1.41 × 10⁻⁵, and 9.5 × 10⁻⁶, respectively. Among these three, Nd, which has the highest solubility in the carbonates, reduced the NiO solubility most; although, the La reduced the NiO solubility more than Nd.

The logarithm of the solubility of the rare earth metal oxides has a linear relation to the Coulomb force ratio between the rare earth metal and the alkaline metal. Following this relation, the La should have the highest solubility among all the lanthanides. The basicity which NiO solubility closely relates has a linear relationship to the Coulomb force parameter of the melts. Based on these two models, the La would be the best additive to reduce the NiO solubility in Li/Na eutectic carbonate melt, among all the lanthanides.     

Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK_th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK_th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK_th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level.

In terms of yield strength σ_y, the threshold stress intensity level could be given by the expression: ΔK_th = 8·74 − 3·42 × 10⁻³(σ_y).

At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√m. It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.     

Optical and electrochemical characteristics of niobium oxide films prepared by sol-gel process and magnetron sputtering A comparison

Electrochromic niobia (Nb₂0₅) coatings were prepared by the sot-gel spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated for the process fabrication of sol-gel spin coated Nb₂0₅ films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sot-gel deposited and magnetron sputtered films heat treated at temperatures below 450°C, were amorphous, whereas those heat treated at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was Nb₂0₅. The refractive index and electrochromic coloration were found to depend on the preparation technique. Both films showed low absorption and high transparency in the visible range. We found that the n, k values of the sot-gel deposited films to be lower than for the sputtered films. The n and k values were n = 1.82 and k = 3 × 10⁻³, and n = 2.28 and K = 4 × 10⁻³ at 530 urn for sot-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiC10₄/propylene carbonate solution. Using the electrochemical measurements and X-ray photoelectron spectroscopy, the probable electrode reaction with the lithiation and delithiation is Nb₂O₅ + x Li⁺ + x e⁻ ↔ Li_xNb₂0₅. Cyclic voltametric (CV) measurements showed that both Nb₂0₅ films exhibits electrochemical reversibility beyond 1200 cycles without change in performance. “In situ” optical measurement revealed that those films exhibit an electrochromic effect in the spectral range 300 < λ < 2100 nm but remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for 250 nm thick electrodes. Spectroelectrochemical measurements showed that spin coated films were essentially electrochemically equivalent to those prepared by d.c. magnetron sputter deposition.     

Long-term thermal cycling of Phlogopite mica-based compressive seals for solid oxide fuel cells

Planar solid oxide fuel cells (SOFCs) require sealants to function properly in harsh environments at elevated temperatures. The SOFC stacks are expected to experience multiple thermal cycles (perhaps thousands of cycles for some applications) during their lifetime service in stationary or transportation applications. As a result, thermal cycle stability is considered a top priority for SOFC sealant development. In previous work, we have developed a hybrid mica-based compressive seal with very low leak rates of 2–4 × 10⁻² to 10⁻³ sccm cm⁻¹ at 800 °C, and showed stable leak rates over limited thermal cycles. In this paper we present results of long-term thermal cycle testing (>1000 thermal cycles) of Phlogopite mica-based compressive seals. Open-circuit voltage (OCV) was measured on a 2 in. × 2 in. 8-YSZ plate with the hybrid Phlogopite mica seals during thermal cycling in a dual environment (2.75% H₂/Ar versus air). During two long-term cycling tests, the measured OCVs were found to be consistent with the calculated Nernst voltages. The hybrid mica seal showed excellent thermal cycle stability over 1000 thermal cycles and can be considered a strong candidate for SOFC applications.     

Low temperature μc-Si film growth using a CaF2 seed layer

This paper describes low temperature thin film Si growth by remote plasma chemical vapor deposition system for photovoltaic device applications. Using CaF₂/glass substrate, we were able to achieve an improved μc-Si film at a low process temperature of 300°C. The μc-Si film on CaF₂/glass substrate shows that a crystalline volume fraction of 65% and dark conductivity of 1.65×10⁻⁸ S/cm with the growth conditions of 50 W, 300°C, 88 mTorr, and SiH₄/H₂=1.2%. XRD analysis on μc-Si/CaF₂/glass showed crystalline film growth in (1 1 1) and (2 2 0) planes. Grain size was enlarged as large as 700 Å for a μc-Si/CaF₂/glass structure. Activation energy of μc-Si film was given as 0.49 eV. The μc-Si films exhibited dark- and photo-conductivity ratio of 124.     

Preparation and properties of transparent conducting zinc oxide and aluminium-doped zinc oxide films prepared by evaporating method

Undoped and aluminium-doped zinc oxide films have been prepared by thermal evaporation of zinc acetate [Zn(CH₃COO)₂ 2H₂O] and aluminium chloride [AlCl₃] onto a heated glass substrate. The structural and optoelectrical properties of the films have been studied. The effects of heat treatment for the as-deposited films in air and vaccum are investigated. Highly transparent films with conductivity as low as 2×10⁻³ Ω cm can be produced by controlling the deposition parameters. The electron carrier densities are in the range 0.2–7×10¹⁹ cm⁻³ with mobilities of 22–58 cm² V⁻¹ s⁻¹.     

The effect of annealing on the electrochemical properties of Zr0.5Ti0.5Mn0.5V0.3Co0.2Ni1.1 alloy electrodes

The annealing treatment was found to result in the improvement in the cyclic stability but the degradation of discharge capacity, activation and high-rate dischargeability for Zr_0.5Ti_0.5Mn_0.5V_0.3Co_0.2Ni_1.1 alloy electrode. A lower discharge potential in the annealed alloy electrode was found owing to a more homogeneous alloy, which is consistent with the pressure–composition isotherms (P–C–T) measurement. We found that the annealed alloy also had lower and flatter pressure plateaus, and larger pressure hysteresis. At high discharge rates, the hydrogen diffusion in the bulk of the alloy was the rate-determining step. The diffusion coefficients for hydrogen in the annealed and as-cast alloys were calculated to be 1.4×10⁻¹² cm² s⁻¹ and 4.3×10⁻¹² cm² s⁻¹, respectively. The lowering of high-rate discharge capacity can be ascribed to the reason that the hydrogen diffusion coefficient is lower due to homogeneous microstructure in the annealed alloy.     

Thermodynamic evaluation of water splitting by a cation-excessive (Ni, Mn) ferrite

Water-splitting potential by cation-excessive (Ni, Mn) Ferrite, Ni_{0.5(1 + )}Fe_{1.99(1 + )}O₄ was evaluated using the standard Gibbs free energy change (ΔG°) for the cation-excessive ferrite formation in different O₂ partial pressures. The cation-excessive degree ranged from 0.026 to 0.16 in pO₂ values of 7.9 × 10⁻⁷ to 1.0 × 10⁻¹. From the relation between value of (Ni, Mn) ferrite and oxygen partial pressure, equilibrium constant K(th) was determined. Furthermore ΔH°s for O₂ releasing and water-splitting reactions with cation-excessive (Ni, Mn) ferrite were evaluated from the van't Hoff plot and compared with that for magnetite-wüstite system; where ΔH°s were assumed to be the same values for both (Ni, Mn) ferrite and magnetite-wüstite system: +300 kJ for O₂ releasing and −35 −63 kJ for water-splitting. ΔG°s evaluated for water-splitting with cation-excessive (Ni, Mn) ferrite and wüstite were −38 kJ and −35 kJ, respectively, at 298K. It suggests that water splitting with cation-excessive (Ni, Mn) ferrite proceed easily compared with that with wüstite. ΔS°s for water-splitting are −0.93 kJ K⁻¹ for the former and −0.83 kJ K⁻¹ for the latter. H₂ generation rates by reaction with H₂O for (Ni, Mn) ferrite were studied at temperatures of 573 K-1073 K. It reached the maximum at 1000 K while it proceeds preferentially below 830 K from thermodynamics.     

Operation of a commercial solar gel pond

The solar gel pond is an innovative concept which overcomes many of the shortcomings of the conventional salt-gradient solar pond. In this paper, the design, construction and start-up details of a commercial scale pond (400 m²), built for a publishing company in Albuquerque, New Mexico will be presented. A pond with trapezoidal cross section was designed so that shadowing could be minimized and also the ratio of surface area to the volume of the storage zone could be maximized. The publishing company required a minimum of 1 GJ/day (1MBTU/day). Generally it has been noted that in ponds with large volume a lesser percentage of retained energy is lost as edge losses. Based on the above consideration a pond size of 400 m² and 5 m deep with a gel thickness of 60 cm and a mass flow rate (for the heat extraction loop) of 1 × 10⁻⁴ to 1 × 10⁻³ kg/m²·sec was determined to be the optimum size for the publishing company's needs. Two to seven percent salt water was used mainly to keep the gel bags floating on the surface. Tedlar bags were used to contain the gel. During the first year of operation, while the pond was still heating up, the pond obtained a temperature of 60°C and the gel showed no signs of degradation.     

Electrochemistry of poly(vinylferrocene) modified electrodes in aqueous acidic media

A cyclic voltammetric study of the electrochemistry and chemical stability of the poly(vinylferrocene) (PVFc) redox couple, coated on a gold substrate, in aqueous solutions of H₂SO₄, HClO₄ and HCl was carried out. It was found that the anodic peak potential (E_pa) did not depend on the acid concentration in the range (1.0 × 10⁻² to 1.0 × 10⁻⁷ mol L⁻¹). However, the E_pa values shifted linearly to less positive potentials when investigated in more concentrated acid solutions in the range 1–5 mol L⁻¹. The slope of the E_pa versus acid concentration graph was found to be in the order H₂SO₄ > HCl > HClO₄. In this regard PVFc behaved very similar to 1,1′-bis(11-mercaptoundecyl)ferrocene (Fc(C₁₁SH)₂) except for its chemical stability. In H₂SO₄ media the PVFc was found to be much less stable than 1,1′-Fc(C₁₁SH)₂. The dependence of E_pa on acid concentration could be used to monitor state of charge of lead-acid batteries. However, for this application Fc(C₁₁SH)₂ would be a better choice because of its superior chemical stability.     

Singlet methylene removal by saturated and unsaturated hydrocarbons

The technique of laser flash phyotolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, ¹CH₂ (ã¹A₁), with various saturated and unsaturated hydrocarbons. The removal rate constants for CH₄, C₂H₆, C₃H₈, C₂H₄, C₃H₆, C₂H₂, CH₂CCH₂, and C₆H₆ were found to be in excellent agreement with previously reported results. Removal rate constants were also measured for n-C₄H₁₀, i-C₄H₁₀, n-C₅H₁₂, c-C₃H₆, c-C₆H₁₂, 1-C₄H₈, cis-2-C₄H₈, trans-2-C₄H₈, and 1-C₄H₆, and determined to be (3.17 ± 0.15), (2.53 ± 0.11), (3.35 ± 0.24), (1.63 ± 0.08), (3.77 ± 0.21), (3.80 ± 0.20), (3.67 ± 0.16), (3.43 ± 0.16) and (4.05 ± 0.18) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively. This series of hydrocarbons forms the basis of a larger series of compounds containing a wide variety of organic functional groups. The removal rate constants are reported here, both as a series within its own right, and as a reference point for future work.     

Preparation and electrochemical properties of double-metal nitrides containing lithium

A study is reported of the preparation, chemical composition, and crystal structure of binary compounds of Li₃N with other nitrides, i.e., Mg₃N₂, AlN, BN, and Si₃N₄. Most of the crystal structures are related to an antifluorite system. Except for LiMgN, the compounds are pure lithium ion conductors. A new compound, Li₈SiN₄, has the highest lithium ion conductivity (viz., 1 × 10⁻³ S m⁻¹ at 298 K) of the double-metal nitrides investigated.     

Impact of air ions on mass transfer from liquids

Evaporation rates and drying times of liquids exposed to relatively high fluxes of unipolar air ions of either sign, produced by corona electrodes, were studied with a beta-ray gauge apparatus. Liquids of constant volume, exposed to air ions, dried 1.72 to 4.80 times faster than the corresponding control liquids under the same laboratory conditions. An analysis of the time-transmission curves from the beta-ray gauge showed mass transfer rates of 1.90 × 10⁻³ and 8.5 × 10⁻³ g min⁻¹ for control and treated (2.44 × 10¹² positive ions cm⁻² s⁻¹) water, respectively. Air ions were observed to cause circulatory and vibrational movements in the treated liquids. These movements contributed to the enhancement in the mass transfer rates from organic and inorganic liquids studied. Polarity of the molecules may also determine the rapidity at which a liquid will evaporate when exposed to a high flux of air ions.     

A new electrode for a poly(pyrrole)-based rechargeable battery

The use of dodecylbenzenesulfonate-doped poly(pyrrole) films, PPYDBS, as a secondary battery electrode was studied. The redox and morphologic properties of these films are suitable for battery application. Films were synthesized by electrolysis of pyrrole and sodium dodecylbenzenesulfonate aqueous solutions with a current density of 1.0 mA cm⁻² and were switched in LiC1O₄ 1.0 M propylene carbonate solutions (PC) by cyclic voltammetry. In these experiments an apparent diffusion coefficient of 3.7x10⁻⁹ cm²s⁻¹ has been found. Charge/discharge tests at ±50, ±100, ±150 and ±200 μA cm⁻² were done for a PPYDBS/ LiC1O₄, PC/Li battery. The open-circuit voltage was 3.2 V after 30 h, the specific capacity 53 A h kg⁻¹ and the energy density 154 W h kg⁻¹. These values indicate that this polymer can be used as an electrode in a rechargeable battery.     

Synthesis and characterization of CuInSe2 thin films from Cu, In and Se stacked layers using a closed graphite box

Various techniques have been used to produce CuInSe₂ but the problem of producing films with the desired properties for efficient device fabrication over large areas has always persisted. The Stacked Elemental Layer (SEL) technique has been demonstrated as a method for producing films over a large area, but the films normally annealed in vacuum or in Se ambient, mostly exhibited poor morphology with small grain sizes which result in poor devices. A method of synthesizing CuInSe₂ films by annealing or selenization of the Cu, In and Se elemental layers using a closed graphite box was developed. SEM, EDX, XRD, spectrophotometric and Hall measurements were used to characterize all annealed films. Results have shown single phase chalcopyrite films with improved crystal sizes of about 4 μm The film composition varied from Cu-rich to In-rich with electrical resistivities of 10⁻³ to 10⁴ Ωcm, cattier concentrations of 5 × 10¹⁵ to 10¹⁷ cm⁻³ and mobilities of 0.6 to 7.8 cm² V⁻¹ s⁻¹ An energy band gap of 0.99 eV and 1.02 eV was obtained for a Cu-rich and near stoichiometric In-rich films respectively. Heterojunction devices using the structure ZnO/CdS/CuInSe₂ were fabricated with electrical conversion efficiencies of 6.5%.     

High temperature rate coefficient for the reaction of O(P) with H2 obtained by the resonance absorption of O and H atoms

The reaction of O(³P) with H₂ has been studied behind reflected shock waves in the temperature range of 1713–3532K at total pressures of about 1.4–2.0 bar by Atomic Resonance Absorption Spectroscopy using mixtures of N₂O and H₂ highly diluted in Ar. The O atoms were generated by the fast thermal decomposition of N₂O and the reaction with H₂ was followed by monitoring the time dependent O and H atom concentrations in the postshock reaction zone. For the experimental conditions chosen, the measured O and H atom concentrations were primarily sensitive to the well-known N₂O dissociation and to the studied reaction and hence its rate coefficient could be deduced. The measured rate coefficient data are fitted by the least-squares method to obtain the following three parameter expression: K₄=3.72×10⁶(T/K)^2.17exp(−4080K/T)cm³ mol⁻¹8⁻, which is in excellent agreement with the recent ab initio calculations for the rate coefficient of this reaction in the overlapping temperature range. The present result is also compared to the experimental results reported by earlier investigators.