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.     

A new method of determining the thermodynamic parameters of metal hydride electrode materials

Thermodynamic parameters of the hydriding reaction of a hydride electrode can be obtained by two different methods. The results show that the change of enthalpy ΔH, change of entropy ΔS and hydrogen pressure calculated from e-c-T curves of electrochemical reactions have much larger inherent errors compared with those obtained from p-c-T curves of alloys in hydriding reactions with hydrogen gas (gaseous method). The hydrogen storage capacities determined by gaseous and electrochemical methods are also different, owing to the low utilization factor, and diffusion of hydrogen into air and formation of hydrogen gas bubbles in electrolyte in an unsealed battery. ΔH_e⁰ and ΔS_e⁰ as well as the relations between ΔH_e⁰, ΔS_e⁰ of the electrochemical reaction and gaseous ΔH_p⁰, ΔS_p⁰ have also been derived on the basis of the Nernst equation. Much more accurate results have thus been assured.     

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.     

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.     

Hydrogen evolution reaction on amorphous Ni–S–Co alloy in alkaline medium

Using an orthogonal design, i.e. the CoCl₂ concentration, the current density, the temperature and the pH value as the factors, Ni–S–Co alloy was obtained through electrodeposition in a modified Watts bath by adding thiourea (TU) and CoCl₂ into the bath. The structure and morphology were determined by XRD and SEM. The electrochemical properties, including the hydrogen evolution overvoltages η (h.o.v.), the Tafel slope b, the exchange current density i⁰ and the activation energy ΔH^* of amorphous Ni–S–Co cathode were measured. The effect of heat treatment under various temperatures on its structure and electrochemical properties were determined. The results indicate that the structure of Ni–S–Co alloy obtained under the optimum conditions is amorphous and it shows much higher electrochemical activity than other electrodes including amorphous Ni–S electrode due to the higher sulfur content in such electrode, and heat treatment is harmful to its electrochemical activity.     

Physical metallurgy of FeTi-hydride and its behaviour in a hydrogen storage container

FeTi-samples have been prepared and characterized by metallography, X-ray diffraction, neutron diffraction and specific heat measurements. The conditions for the presence of the ordered CsCl structure at room temperature have been investigated. This material has been used to study FeTi-hydrides, especially the structure, phase transition and the heat of reaction as a function of the hydrogen concentration. The heat of reaction is 30 kJ/mole H₂ for 0.05 < c < 0.5 (c = atomic ratio: atoms H/atoms metal). The heat of solution (c → 0) is approx. 130 kJ/mole H₂. The FeTi material has been used to build and operate a small hydrogen storage container (1 Nm³H₂) as an alternative to high pressure hydrogen gas containers in the laboratory.     

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.     

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.     

The principles of the detection of flammable atmospheres by catalytic devices

Most instruments used for measuring the explosibility of fuel/air atmospheres use catalytic oxidation as a method of measurement. The detailed mechanism of this method has been examined and equation describing the output from these devices have been derived. The output V_(LEL) at the lower explosive limit of a fuel has the general form

V_(LEL)=K D₁₂ δH[LEL],

where D₁₂, ΔH, and [LEL] are respectively the diffusion coefficient, heat of oxidation, and the lower explosive limit of the fuel in air, and K is a constant. Calculations have been made which enable the responses to explosive gas/air mixtures to be predicted and correction factors to be derived for practical devices. A new method for the measurement of explosiveness is discussed based on the empirical correlation between the heat of oxidation of the fuel and its lower explosive limit.     

Synthesis and characterization of a highly stable amorphous silicon hydride as the product of a catalytic helium–hydrogen plasma reaction

A novel highly stable surface coating SiH(1/p) which comprised high-binding-energy hydride ions was synthesized by a microwave plasma reaction of a mixture of silane, hydrogen, and helium wherein it is proposed that He⁺ served as a catalyst with atomic hydrogen to form the highly stable hydride ions. Novel silicon hydride was identified by time of flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). The ToF-SIMS identified the coatings as hydride by the large SiH⁺ peak in the positive spectrum and the dominant H⁻ in the negative spectrum. XPS identified the H content of the SiH coatings as hydride ions, H⁻(1/4), H⁻(1/9), and H⁻(1/11) corresponding to peaks at 11, 43, and 55 eV, respectively. The silicon hydride surface was remarkably stable to air as shown by XPS. The highly stable amorphous silicon hydride coating may advance the production of integrated circuits and microdevices by resisting the oxygen passivation of the surface and possibly altering the dielectric constant and band gap to increase device performance.

The plasma which formed SiH(1/p) showed a number of extraordinary features. Novel emission lines with energies of q·13.6 eV where q=1,2,3,4,6,7,8,9, or 11 were previously observed by extreme ultraviolet spectroscopy recorded on microwave discharges of helium with 2% hydrogen (Int. J. Hydrogen Energy 27 (3) 301–322). These lines matched H(1/p), fractional Rydberg states of atomic hydrogen where p is an integer, formed by a resonant nonradiative energy transfer to He⁺ acting as a catalyst. The average hydrogen atom temperature of the helium–hydrogen plasma was measured to be 180–210 eV versus ≈3 eV for pure hydrogen. Using water bath calorimetry, excess power was observed from the helium–hydrogen plasma compared to control krypton plasma. For example, for an input of 8.1 W, the total plasma power of the helium–hydrogen plasma measured by water bath calorimetry was 30.0 W corresponding to 21.9 W of excess power in 3 cm³. The excess power density and energy balance were high, 7.3 W/cm³ and −2.9×10⁴ kJ/molH₂, respectively. This catalytic plasma reaction may represent a new hydrogen energy source and a new field of hydrogen chemistry.     

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.     

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.     

Kinetics of hydrogen absorption and desorption by ternary LaNi5-type intermetallic compounds

Kinetic rules of formation and decomposition of the hydrides derived from LaNi₅-type compounds are determined. Comparison between LaNi₅H₆ and more stable hydrides such as LaNi_5-χM_χ (with M = Al, Cu, Ti) is made in terms of pressure, number and time of cycles. Measurements are performed by a differential volumetric device functioning under constant pressure, during hydrogen absorption and desorption. The influence on the kinetics and hydrogen capacities of the gaseous impurities contained in industrial hydrogen (H₂O, CO₂, CH₄) is studied as a function of the number of cycles. It shows a good resistance to poisoning of the above mentioned compounds.     

Synthesis of LiCoO2 from cobalt—organic acid complexes and its electrode behaviour in a lithium secondary battery

Well-crystallized, layered LiCoO₂ has been prepared by heating cobalt—organic acid complexes (such as malic acid and succinic acid) at 900 °C in air after preheating at 400 °C (2 h) and at 650 °C (6 h). LiCoO₂ obtained by this method shows a high (003) peak intensity and low (104) or (101) intensities in X-ray diffraction (XRD). The first discharge capacity of LiCoO₂ obtained from this method in ester-based electrolyte is 132 mA h g⁻¹ on cycling between 4.3 and 3.7 V. The value is larger than that obtained by the conventional method. X-ray diffraction studies and open-circuit voltage curves show the presence of at least two types of reaction. A two-phase reaction occurs in the region of 0.71<χ <1.0 in Li_xCoO₂. The lithiation proceeds as a homogeneous reaction together with expansion of the c-axis in the region of 0.47<χ<0.71. The expansion of the c-axis againstΔχ at x=0.56 corresponds well with the voltage jump observed in the charge/discharge curves.     

The total emissivities of luminous and non-luminous flames

Total emissivities of CO₂-H₂O and CO₂-H₂O-soot mixtures arising in oil and gas combustion, p_w/p_c = 1 and 2 respectively, have been computed from the statistical band model and experimental spectral data for the gases and from the optical constants of soot using the Mie theory. For CO₂-H₂O mixtures the emissivity has been fitted by a one-clear three-gray gas model, suitable for use in the zone method of calculating radiative heat transfer, in the temperature range 1200–2400°K, accuracy 0.5 per cent for a 1000-fold range of p_cL, whereby

It is shown that the total emissivity of luminous CO₂-H₂O-soot mixtures can also be represented by a three-gray gas model with constant absorption coefficients K_m,n where,

and the weighting coefficients a_m,n are again linearly temperature dependent. At constant gas composition the K_m,n are related to the soot concentration, c, by

K_m,n = exp[q_1,n + q_2,n·c]

so that the remaining coefficients, a_m,n for luminous flame gases may be determined without recourse to trial and error.     

Thermodynamics of hydrogen absorption by Pd---Sb and Pd---Bi alloys

The thermodynamic parameters for absorption of hydrogen by Pd---Sb and Pd---Bi solid solution alloys containing up to 10.0 at.% Sb and Bi have been determined from measurements of pressure-composition isotherms at temperatures between 273 and 433 K and hydrogen pressures up to 1000 Torr. The hydrogen-free Pd lattice is expanded by additions of Sb and Bi, and at the same alloy composition the lattice expansion of Pd---Bi alloys is larger than that of Pd---Sb alloys. The dilute phase solubilities in both alloys, as relected by the relative chemical potential of dissolved hydrogen at infinite dilution, Δμ_H⁰, increases with increasing Sb and Bi contents; where the solubilities in Pd---Bi alloys are larger than those in Pd---Sb alloys. The relative partial molar enthalpy, ΔH_H⁰, at infinite dilution in both alloy systems increases in exothermicity with increasing Sb and Bi contents, and at the same solute metal content, the ΔH_H⁰, values for Pd---Bi alloys are more exothermic. The standard free energy change, ΔG_plat⁰, for β-hydride formation in Pd---Bi alloys decreases with Bi content, whereas the ΔG_plat⁰ value for Pd---Sb alloys increases with Sb content, although only by a small amount, despite the fact that the alloys have expanded lattices compared to Pd. The higher pressure solubilities in both alloys decrease with increase of the solute metal contents.     

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.     

Solar radiation estimates from common parameters

In tropical countries, precipitable water, W, is not a good basis for predicting solar radiation because of the narrow range of fluctuation in this parameter. Relative humidity, however, does fluctuate over a larger range and has been used in this study. The following empirical relationships between solar-radiation intensity, H, ratio of hours of bright sunshine to twelve hours, S, and relative humidity of the environment, R, have been obtained by statistical methods applied to the available data.^*

H=490S^0.357R^−0.262

H=460e^0.607(S−R)

H=464+265S−248R

These equations give better estimates of solar-radiation intensity than the single-parameter relationship like

H=a+bS

Charts have been constructed from the empirical relations to simplify the use of the expressions.     

Effect of temperature dependence of electrical resistivity on the cooling performance of a single thermoelectric element

The coefficients of performance (COP) φ₀ and φ for a single thermoelectric (TE) element welded with two metal plates were calculated as functions of temperature difference (ΔT) and thermoelectric figure of merit (ZT) from the conventional thermal rate equations and the new thermal rate ones proposed here, respectively. We made an attempt to take the differences in the Seebeck coefficient , electrical resistivity ρ and thermal conductivity κ of TE materials at the hot and cold sides of a TE element into the thermal rate equations on the assumption that their TE properties change linearly with temperature. However, the difference in κ was neglected even in the new thermal rate equations because its temperature dependence was too small when φ was applied to the high-performance Bi–Te alloys. The normalized temperature dependences at 300 K of and ρ were denoted by A and B, respectively. The term of A in the thermal rate equations was canceled out by the Thomson coefficient, but that of B remained. When B > 0 K⁻¹, φ/φ₀ is enhanced more significantly with an increase of B at larger ΔT and lower ZT, and it reached about 1.20 at ΔT = 80 K for Bi–Te alloys with B ≈ 5 × 10⁻³ K⁻¹. It was thus found that the COP of a cooling module is also affected strongly by B as well as ZT.     

An empirical method for estimating sunshine from total cloud amount

A new empirical method has been developed for computing sunshine S from total cloud amount. The equation used is of the form S = 1 − f₁ + f₂, where f₁ and f₂ are, respectively, functions of total cloud amount and latitude correction. The results obtained with this equation are compared with observed values and are found to be in good agreement. Tables 1 and 2 present these results.

Assuming extreme conditions, the maximum possible deviations of total solar radiation, net radiation and evaporation (or potential evapotranspiration), using the formulae of Reddy[1–4] are discussed for the case where calculated values of S deviate 0·10 from observed values of S. The formula can be usefully employed with considerable accuracy in water and energy budget studies.