A reliable tool for the design of shape and size of Faraday segmented MHD generator channel

In Bajovi (Energy Conversion & Management 35 (4) (1994)), the quasi-one-dimensional (QID) electrodynamical part (the electric current density vector in a working fluid has only one component: j_y) of a model for the fluid flow in a Faraday ideally segmented (absence of the Hall electric current: I_x = 0) generator channel, legitimate regarding the equation of conservation of charges (div = 0 for the stationary electric current), was developed. In this paper, the expected characteristic of the model being sensitive to the shape and size of a channel is demonstrated. Using the values of the semi-empirical factor Δx of the model, obtained in Bajovi (Energy Conversion & Management 35 (12) (1994)), the dimensions of a new channel are designed. The experimental testing of the channel proves that the model is a reliable tool for the design of the shape and size of a Faraday segmented channel.

It is noticeable in the literature that all the existing electrodynamic two- and three-dimensional model calculations, which are necessary for optimization of the segmentation length of a channel, deal with channels of constant cross-section along the working fluid flow. The correct QID model provides the possibility for independent optimization of the shape and size of a channel from its segmentation distance, which makes further complications in multidimensional models nonessential.     

Preparation and properties of LiCoyMnxNi1−x−yO2 as a cathode for lithium ion batteries

The preparation of LiCo_yMn_xNi_1−x−yO₂ from LiOH·H₂O, Ni(OH)₂ and γ-MnOOH in air was studied in detail. Single-phase LiCo_yMn_xNi_1−x−yO₂ (0y0.3 and x=0.2) is obtained by heating at 830–900°C. The optimum heating temperatures are 850°C for y=0–0.1 and 900°C for y=0.2–0.3. Excess lithium (1z1.11 for y=0.2) and the Co doping level (0.05y0.2) do not significantly affect the discharge capacity of Li_zCo_yMn_0.2Ni_0.8−yO₂. The doping of Co into LiMn_0.2Ni_0.8O₂ accelerates the oxidation of the transition metal ion, and suppresses partial cation mixing. Since the valence of the manganese ion in LiMn_0.2Ni_0.8O₂ is determined to be 4, the formation of a solid solution between LiCo_yNi_1−yO₂ and Li₂MnO₃ is confirmed.     

THM growth and characterization of CuGaxIn1−xSe2 solid solutions

Growth of CuGa_xIn_1−xSe₂ single crystals by THM (traveling heater method) has been investigated. On the basis of the relation between the composition x of grown crystal CuGa_xIn_1−xSe₂ and y of CuGa_yIn_1−ySe₂ solute in 60 mol% In solution, THM growth of CuGa_xIn_1−xSe₂ (x = 0, 0.2, 0.4, 0.7, 1) was performed using a zone ingot which was prepared in advance. Bulk single crystals with 10 mm in diameter and 20–30 mm in length have been obtained by the THM growth, and their electrical and optical properties have been studied.     

Stability of some ternary and quaternary CeNi5-based and PrNi5-based hydride systems

The Ce_1−xR_xNi_2.5Cu_2.5 (R = La,Pr; 0.8 x 0.3) and PrNi_5−xM_x (M = Cu, Fe; 0.5 x 2.5) alloys were investigated for their hydriding characteristics in the temperature range 0–70°C and hydrogen pressure range 0.01–50 atm. The nonlinear behaviour of unit cell volume vs x in Ce_1–xLa_xNi_2.5Cu_2.5 suggests that both size and electronic effects are involved. The partial replacement of Ce by La and Ni by Cu in CeNi₅ causes a substantial reduction in the hydrogen sorption pressures without significantly impairing its hydrogen capacity. It was observed that Fe is more effective than Cu in stabilizing PrNi₅-H₂. The high values of the molar entropy of hydrogen of the β-hydrides studied, S^β_H, are attributed to extensive hydrogen disorder in the interstitial sites of the host lattice. A linear correlation between the hydride decomposition pressures (or free energy) and the unit cell volume. V_c, of the host alloys was observed. This behavior is helpful in predicting the stabilities of new hydrides in a given substitutional alloy series.     

On the properties of some special functions related to Bessel''s functions and their application in heat exchanger theory

Basic properties of Bs_n(x, y) and Bes_n(x, y) functions related to Bessel functions, are presented. The functions are defined by double power series:

Numerous formulae are given on the origin of identities, differential formulae as well as the ones for calculation of integrals, whose sub-integral functions comprise the foregoing special functions. Some applications of Bs_n(x, y) and Bes_n(x, y) functions are also given with regard to the theory of cross-flow recuperator and to heat transfer analysis of gas-cooled clinker beds. Some cases of boundary conditions and initial boundary conditions are considered. The solutions are of the so-called closed form which is by far better than those achieved by approximate methods. Bs_n(x, y) and Bes_n(x, y) functions may be applied to theoretical analyses of heat and mass exchangers, regenerators, ion exchangers and for different kinds of heavy equipment used in the chemical industry. These functions can also be applied to control and protect various physical processes.     

On the computer simulation of the P–C isotherms of ZrFe2 type hydrogen storage materials

This paper deals with computer simulation of the P–C isotherms of some ZrFe₂ type (Zr(Fe_1−xCr_x)₂, Zr_1−xTi_xFe_1.4Cr_0.6, Zr_1−2xMm_xTi_xFe_1.4Cr_0.6 : x00.4) of hydrogen storage materials. A feasible mathematical model has been developed to simulate the P–C isotherms. The randomized variables in the model applied for simulating the P–C isotherms of the above-mentioned ZrFe₂ type hydrogen storage materials correspond to change in enthalpy (ΔH) and entropy (ΔS) of hydride formation. Several ZrFe₂ type materials as in above have been synthesized and their P–C isotherms, enthalpy and entropy change has been evaluated experimentally in order to have input data for simulation. A special software was developed to simulate the P–C isotherms using the said model. A close match between the experimentally observed and simulated P–C isotherms for the above said ZrFe₂ type alloys has been obtained.     

Structural and electrochemical properties of Li1+xNi0.5Mn0.5O2+δ (0 ≤ x ≤ 0.7) cathode materials for lithium-ion batteries

A comparative analysis of the properties of LiNi_0.5Mn_0.5O₂ and Li_1+xNi_0.5Mn_0.5O₂ (0.2 ≤ x ≤ 0.7) powders, obtained by the freeze drying method, was performed. Lattice parameters of Li_1+xNi_0.5Mn_0.5O₂ decreased considerably with growing amounts of Li until x = 0.3; at x > 0.5 trace amounts of Li₂MnO₃ are observed by X-ray diffraction (XRD) patterns. X-ray photoelectron spectroscopy (XPS) analysis displayed an increase of Ni³⁺/Ni²⁺ ratio at 0.3 < x < 0.5, while Mn 2p spectra were almost identical in all samples. Rechargeable capacity values (V = 2.5–4.6 V) increased systematically with x reaching its maximum (185–190 mAh g⁻¹) at x = 0.5. Samples with superstoichiometric lithium content also demonstrated good C rate characteristics.     

High pressure droplet vaporization; effects of liquid-phase gas solubility

Numerical results are presented for an n-hexane droplet initially at 300 K evaporating into nitrogen, for ambient pressures, P_x, 1–100 atm and ambient temperatures, T_x, 500–1500 K. At low P_x (<30 atm), droplet lifetimes predicted with or without liquid-phase gas solubility are very close. At high P_x, the model neglecting solubility either underestimates the droplet lifetime (low T_x) or breaks down by failing to predict vapor-liquid equilibria (high T_x). At high enough P_x, heat-up is extremely important throughout the entire droplet lifetime. In a fuel rich environment, relatively low T_x, and high T_x, substantial condensation occurs before the onset of vaporization.     

Effect of substituting Mm with La on the electrochemical performances of Co-free LaxMm1−x(NiMnSiAlFe)4.9 (x = 0–1) electrode alloys prepared by casting and rapid quenching

In order to enhance the electrochemical capacity of the Co-free AB₅-type electrode alloy, Mm in the alloys was substituted with La and Co-free La_xMm_1−x(NiMnSiAlFe)_4.9 (x = 0, 0.45, 0.75, 1.0) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of the substituting Mm with La on the electrochemical performances of the as-cast and quenched alloys were investigated in detail. The obtained results show that substituting Mm with La can enhance markedly the capacities of the as-cast and quenched alloys. When the amount of substituting Mm with La, x increased from 0 to 1.0, the maximum capacity of the as-cast alloys at 0.2C rate increased from 273.45 to 304.47 mAh g⁻¹, and the capacity retaining rate (R_h) increased from 59.16 to 59.86%. The capacity of the as-quenched alloys with a quenching rate of 10 m s⁻¹ increased from 236.83 to 300.31 mAh g⁻¹, and the capacity retaining rate (R_h) decreased from 78.69 to 62.29%. The substituting Mm with La had an insignificant effect on the activation capabilities of the as-cast and quenched alloys.     

The calculation of wall and fluid temperatures for the incompressible turbulent boundary layer, with arbitrary distribution of wall heat flux

Numerical solution of the partial differential equation for heat transfer in the incompressible turbulent boundary layer has been obtained for uniform (q_w/ρC_pu_i)/√(c_f/2) and for Prandtl numbers 0–7, 1 and 7. The Spalding boundary-layer velocity law was assumed, and the Schmidt method of integration used. Boundary-layer temperature distributions up to x⁺ = 10⁶ are presented, together with the “Spalding function” St/√(c_f/2). A method is given for the application of the solutions to the case of arbitrary distribution of heat flux at the wall.     

IrxCo1−x (x = 0.3–1.0) alloy electrocatalysts, catalytic activities, and methanol tolerance in oxygen reduction reaction

Novel methanol-tolerant catalysts for oxygen reduction reaction (ORR), Ir_xCo_1−x/C (x = 0.3–1.0), were synthesized by a conventional impregnation method. These carbon-supported catalysts showed particle sizes of 2.7–5.0 nm. The catalyst activity and the catalyzed ORR kinetics were characterized by cyclic voltammetry and rotating disk electrode methods. Among these Ir_xCo_1−x/C catalysts, the alloy with a formula of Ir_xCo_1−x/C with x value in the range of 0.7–0.8 exhibited the highest mass and specific activities. Compared to a Pt/C catalyst, these alloy catalysts have much stronger methanol tolerance in terms of ORR onset potential (or open-circuit potential). Based on the rotating disk electrode measurements, it was confirmed that these Ir_xCo_1−x/C alloy catalysts could catalyze a complete four-electron transfer reaction of oxygen to water. These results strongly suggest that the novel Ir–Co metal alloy catalysts synthesized in this work could be promising for DMFC cathodes.     

The effect of wall conduction on the performance of regenerative heat exchangers

The effect of heat conduction in the wall on the effectiveness of rotary regenerators is described. In order to document the effect of wall conduction, it is assumed that the thermal conductance of the solid is finite, both parallel and perpendicular to the gas flow. Two parameters introduced for modeling conduction effects are the Biot numbers in the y(Bi_y) and x(Bi_x) directions. Our results show that for certain values of Bi_x and Bi_y, the effectiveness including wall conduction can deviate substantially from results obtained when wall conduction is neglected.     

Differential Ångstrom model for predicting insolation from hours of sunshine

The Ångstrom model for predicting insolation is limited in scope beacuse it gives equal weighting to sunshine hours recorded at any time of the day. The differential Ångstrom model presented in this paper removes this limitation and relates insolation, q_j, in the j^th hour to the sunshine duration, n_j, of the same period by the equation: q_j = a_j + b_j. By regression analysis of monthly data, the set of constants a_j and b_j for each hour of each month of the year can be determined. Thus, using the appropriate set of a and b regression coefficients, any sunshine data can be transformed to insolation. The sum of the equation over a day gives the daily insolation from which monthly means can be calculated. The method has been applied to the 1986 and 1988 sunshine data recorded at the University of Papua New Guinea to predict the observed insolation to within 3.5%. The differential Ångstrom method has applications in places which have much recorded data on hours of sunshine but have limited observed insolation data.     

Preparation of Culn(SxSe1−x)2 thin films by sulfurization and selenization

A CuIn(S_xSe_1−x)₂ alloy thin-film was prepared by selenization of CuInS₂: its composition ratio x can be controlled by the number of selenization cycles implemented. Crystallinity of the films was improved by annealing in vacuum. The resistivity of the film was about 1 Ω cm and increased by one to two orders of magnitude after KCN treatment. An 8.1 % efficiency solar cell was obtained by using this annealed alloy thin-film.     

Effect of Co content on performance of LiAl1/3−xCoxNi1/3Mn1/3O2 compounds for lithium-ion batteries

Layered LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ (0  x  1/3) compounds were studied via the combination of computational and experimental approach. The calculated voltage curve of LiNi_1/3Al_1/3Mn_1/3O₂ compound is presented, indicating it is of great potential for a cathode material of lithium-ion batteries. Unfortunately, it was found that the LiNi_1/3Al_1/3Mn_1/3O₂ compound without impurity phase could not be synthesized via a sol–gel process. To obtain a layered compound without impurity phase, partial of Al is replaced by Co in LiNi_1/3Al_1/3Mn_1/3O₂ compound in this study. Layered LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ (0  x  1/3) compounds were synthesized via sol–gel reaction at 900 °C under a oxygen stream. Single phase of the LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ in 1/6  x  1/3 region could be prepared successfully. The discharge capacity and conductivity increased with an increase in the Co-substitution content. The enhancement of the conductivity and phase purity by the introduction of Co content shows profound influence on the performance of the LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ compounds.     

Response function for transient heat conduction in semi-infinite media

The concept of response function is used to investigate the transient heat conduction in a semi-infinite medium, when the temperature of the surface is an arbitrary function of time. First, the response of the system to a unit step function of surface temperature T_s(t) is obtained. The temperature distribution in the medium corresponding to arbitrary surface temperature T_s(t) is then expressed in terms of the response function and convolution integral. The analytical solution of the convolution integral is obtained for a periodic variation of T_s(t). The resulting expression for T(x, t) is identical with that obtained by the usual periodic analysis. Numerical computations are carried out to investigate the dependence of the accuracy of numerical results on the upper limit of time (t_o) in the convolution integral.     

A new method for estimating solar radiation from bright sunshine data

Daily values of H/H₀, the ratio of total horizontal radiation to that outside the atmosphere has been correlated with s/S, bright sunshine as a fraction of daylength for 3 yr measurements in Adana and Ankara, Turkey. Using a maximum-likelihood quadratic fit, we show that monthly averages s/S and its standard deviation σ_s/S can be used to estimate the monthly average H/H₀ as

H/H₀ = 0.204 + 0.758s/S − 0.250[s/S² + σ²_s/S.

Comparison of the estimations of the above equation with measurements from different regions of Turkey indicate that less than 5 per cent relative error is possible. A further correlation σ²_s/S with s/S makes it possible to estimate H/H₀ with just the knowledge of s/S.     

Characterization of Cu(InxGa1−x)2Se3.5 thin films prepared by rf sputtering

Cu(In_xGa_1−x)₂Se_3.5 thin films were fabricated by rf sputtering from CuIn_xGa_1−xSe₂ and Na mixture target by controlling the mixture ratio. X-ray diffraction analyses show that the structure of Cu(In_xGa_1−x)₂Se_3.5, thin films is different from chalcopyrite structure: especially, CuIn₂Se_3.5 thin films have a defect chalcopyrite structure. The lattice parameters for Cu(In_xGa_1−x)₂Se_3.5 thin film are slightly smaller than those for CuIn_xGa_1−xSe₂ thin film and linearly decreased with increasing Ga content. The optical absorption coefficients for Cu(In_xGa_1−x)₂Se_3.5, thin films exceed 2 × 10⁴ cm⁻¹ in energy region above the fundamental band edge. The band gap for Cu(In_xGa_1−x)₂Se_3.5 thin films is larger than that for CuIn.Ga_1−x2Se₂ with the same Ga content and increased with increasing Ga content.     

Conditional probabilities of daily relative sunshine data and the dependence on the weather of the previous day

In this paper, the conditional probabilities p₁₁(m, x) and p₀₁(m, x) of the daily relative sunshine (DRS) are estimated for each month m, using available data for 30 years. p₁₁(m, x) (p₀₁(m, x)) is the probability that the DRS of a day for a certain month is greater than x, (o < x < 1), given that the DRS of the previous day is greater (smaller) than x. The empirical curves for p₁₁(m, x) and p₀₁(m, x) are fitted and the fitting parameters are estimated for each month. The results show a good agreement between the empirical and the calculated values. The vertical distance between the curves p₁₁(m, x) and p₀₁(m, x) shows a strong dependence of the weather on that of the previous day. A method to estimate a sequence of k consecutive days of “good” or “bad” weather is also given.     

Heat conduction in a semi-infinite solid subject to steady and non-steady periodic-type surface heat fluxes

An analytical solution for the temperature and heat flux distribution in the case of a semi-infinite solid of constant properties is investigated. The solutions are presented for time-dependent, surface heat fluxes of the forms: (i) Q₁(t) = Q₀(1+a cos ωt); and (ii) Q₂(t) = Q_o(1+bt cos ωt), where a and b are controlling factors of the periodic oscillations about the constant surface heat flux Q₀. The dimensionless (or reduced) temperature and heat flux solutions are presented in terms of decompositions C_r and S_r of the generalized representation of the incomplete Gamma function. It is demonstrated that the present analysis covers the limiting case for large times which is discussed in several textbooks, for the case of steady periodic-type surface heat fluxes. In addition, an illustrative example problem on heating of malignant tissues, making use of transient and long-time solutions, is also presented.