Effect of distortions in the ideal profile of solid phase conductivity on performance efficiency of porouselectrodes

The effect of distortions in the ideal profile of the solid phase conductivity s(x) on porous electrode (PE) performance efficiency has been analysed using a one-dimensional model and an overall polarization curve involving both main and side reactions. Although an ideal electrically equally accessible PE is provided only by a hyperbolic profile s(x) passing through the middle of the electrode (the point s=L), it has been shown that a noticeable improvement in performance can be obtained using a rough approximation with any monotonic function providing a decrease in conductivity in the direction from the back current supply to the front plane of the PE, including functions not necessarily passing through the point s=L in the middle of the electrode. This permits less rigid requirements for the s(x) profile reproduction accuracy and simplification of experimental data acquisition.     

Water vapor absorption and aqueous retention of hydrolyzed starch polyacrylonitrile graft copolymers — “superabsorbent starch”

Summary Starch copolymers with polyacrylonitrile grafts were prepared using the Mn³⁺ initiation method and different amounts of acrylonitrile. The product was hydrolyzed in aqueous alkali (nitrile groups to amide and carboxyl groups) to enhance the hydrophilic character of the copolymer and achieve superabsorbent properties. At relative humidities (RH) up to 95–97 % the modified starch shows normal water vapor absorption like cellulose and other polysaccharides. When in contact with liquid distilled water, the modified starch forms a superabsorbent gel which retains very large amounts of water (up to 800 g/g). At low and high pH and in aqueous salt solutions (NaCl, CaCl₂ and synthetic urine) the water retention is strongly reduced. This is interpreted as due to the ionic strength (salt effect) and for Ca²⁺ ions probably also crosslinking by salt formation with carboxyl groups on adjacent chains or chain segments.     

A parameter characterizing the geometry of expanded metal. II. Natural convection mass transfer at expanded metal electrodes

Free convective mass transfer rates at vertical electrodes of expanded metal were measured by the electrochemical method. Electrode height and electrolyte concentration were varied and the dependence of the expanded metal on the geometry and on the mesh orientation with respect to the vertical direction was investigated. A single equation was developed to correlate all the results. Besides the generalized dimensionless groups for natural convection the correlation includes a parameter characterizing the geometry of the expanded metal. The correlation also represents free convective mass transfer results obtained by other investigators with vertical mesh electrodes.Nomenclature a width of narrow space - A mean mesh aperture - c ₀ bulk concentration - d cavity diameter - d _p particle diameter - D diffusivity - g acceleration due to gravity - Gr Grashof number =gh³/v² - h electrode height - H cavity depth - k mass transfer coefficient - LD long dimension of expanded metal - R _h hydraulic radius - Sc Schmidt number=/D - SD small dimension of expanded metal - Sh Sherwood number=kh/D - void fraction - kinematic viscosity - density - electrode area per unit volume - electrode area per unit net area     

Properties ofSrMO 3-δ(M=Fe,Co) as oxygen electrodes in alkaline solution

Strontium ferrates and cobaltates with compositions SrFeO_3- (0.060.40) and SrCoO_3- (0.040.30) were synthesized. The dependence of the oxygen electrode properties on the value was examined in 1 mol dm^–3 KOH solution. In the SrFeO_3- series, the samples with 0.24<<0.29, showed the highest activity in both oxygen evolution and reduction reactions. In contrast, no strong dependence on the value was observed in SrCoO_3-, which also showed a high catalytic activity for oxygen evolution.     

The role of Ni2+ diffusion on the reducibility of NiO/MgO system: A combined TRP-XPS study

The progressive dissolution of nickel ions in the lattice of MgO matrix, promoted by the calcination temperature, has been monitored using XPS. The reducibility of Ni/MgO catalysts is affected by the modification of the surface chemical composition. TPR peaks have been correlated with several NiO forms. Both XPS and TPR results indicate a different behaviour pattern for the supported and physically mixed NiO/MgO systems, and this has been attributed to the different available contact area between NiO and MgO. The formation of both NiO rich and MgO rich NiO-MgO solid solutions, with the tendency to evolve towards the bulk solid solution, has been invoked to explain the substantial changes in the TPR pattern of the different air calcined systems.     

Mechanistic role of enolate ions in “group transfer polymerization”

Summary A new dissociative mechanism involving enolate anion intermediates is proposed for the process currently categorized as nucleophile-catalyzed group transfer polymerization (GTP). A rapid, reversible complexation of small concentrations of enolate anions with silyl ketene acetals (reversible termination) is proposed to explain the living nature of these polymerizations and the role of the silyl ketene acetals in GTP. It is proposed that these polymerizations do not involve group transfer in the chain-propagating step. These mechanistic conclusions are based on studies of the tetrabutylammonium 9-methyl-fluorenide- and methyllithium-initiated polymerizations of methyl methacrylate at ambient temperatures in the presence and absence of silyl ketene acetal.     

Corrosion behaviour of Zn–Co compositionally modulated multilayers electrodeposited from single and dual baths

The corrosion behaviour of Zn–Co compositionally modulated multilayers (CMM) was studied using corrosion potential measurement and the neutral salt spray (NSS) method. It was established that the corrosion potentials of the CMM, deposited from a single bath, are significantly more positive than those deposited from a dual bath. The best corrosion resistance was found for multilayers consisting of four sublayers, each 3.0m thick, deposited both from a dual bath and from a single bath (with an oversublayer of Zn–Co1%).     

Synthesis of statistical polydiene networks by reaction of liquid polyenes with telechelic siloxanes

The present investigation concerns the synthesis of statistical networks similar to vulcanized rubber. We have used liquid precursor polymers: polybutadiene (PB) and polyisoprene (PI) with telechelic siloxanes. The reactivity of the polydienes can be classified as follows: PB units 1,2 PI units 3,4 PI units 1,4 0. The network formation is a function of the ratio R = nb, of pendent double bonds/nb. of SiH. For R 1 the networks are rather hard and brittle, for 8 R 10 they are soft and elastic, whereas for R 16 the gels obtained are very soft and sticky.     

Postinduction Processes During Thermal Explosion in the Systems “Porous Material–Reactive Gas–Solid Product”

A complete solution of the unsteadystate filtration problem of thermal explosion incorporating the postinduction period is given for the first time. The paper describes a study of the temperaturefield dynamics, poregas pressure, and the degree of condensedphase conversion versus the reactivegas deficiency in a reactive porous material. Focus is on the formation and propagation of frontal regimes of exothermic chemical reactions (their number, direction, and velocity of propagation, degree of condensedphase conversion at the front). The study revealed double selfignition phenomena and combustionwave propagation regimes with incomplete conversion at the front. A surface regime of thermal explosion limited by gas filtration from the outside was considered. The regularities in the dynamics of the exothermic chemical reaction found in the present study allow one to qualitatively control hightemperature synthesis under thermal explosion conditions.     

Specific features of the phase transformations occurring during the sintering process of the beta-alumina based ceramics and their properties

Conclusions During the sintering process of beta-alumina obtained using the method of plasmochemical synthesis, the structural transformation occurs and it is accompanied by the separation of the excess Na₂O and the formation of an intergranular meltlike phase that is saturated with sodium and is characterized by high dielectric properties.The quantity of the meltlike phase and the magnitude of the /( + ) ratio determine the degree of stabilization (stability) and the final resistivity of the material and depend on the initial chemical composition and the heating rate maintained during the sintering process.It was shown that when the duration of residence of the material in the hot zone of the furnace is increased or when the specimens sintered maintaining high heating rates are subjected to additional heat treatment, one observes the occurrance of the reverse structural transformation that is characterized by an increase of the phase ratio /( + ) 1, a reduction in the quantity of the meltlike phase right up to its complete disappearance, and a decrease of the electrical resistivity of the material.In the materials having a large excess quantity of sodium oxide, the process of crystallization of the intergranular NaAlO₂ phase occurs simultaneously with the second phase transformation.The best ceramic and electrophysical parameters were obtained when sintering the material contained 7.9% Na₂O.Translated from Ogneupory, No. 3, pp. 13–18, March, 1990.     

Suppression of the α → β-nickel hydroxide transformation in concentrated alkali: Role of dissolved cations

The presence of dissolved cations such as Al and Zn in alkaline electrolyte (6 M KOH) suppresses the -nickel hydroxide transformation. The uptake of Al (10 mol%) and Zn (30 mol%) exhibited by the active material likely stabilizes the -phase. Dissolved Al is deleterious to the performance of the nickel hydroxide electrode, whereas, dissolved Zn enhances the specific discharge capacity of nickel hydroxide by approximately 25% showing that the mode of metal uptake is different in the two cases.     

Polymerization of lactams

Summary The following limit (ceiling) temperatures of polymerization were extrapolated from kinetic data for the anionic polymerization of 2-pyrrolidone initiated with potassium salt of 2-pyrrolidone and the initiation systems of optimum compositions: 66°C for the nonactivated polymerization, 68°C and 73°C for the polymerization accelerated with 1-(1-pyr-rolin-2-yl)-2-pyrrolidone and carbon dioxide, respectively, 73°C and 76°C for the polymerization activated with N-benzoyl-2-pyrrolidone and N-acetyl-2-pyrrolidone,respectively.     

Nature and corrodability of thin oxide films formed on niobium in NaOH solutions

The corrodability of thin oxide films formed on niobium either naturally or anodically was investigated in NaOH solutions of different concentrations (0.05–3.0 M) using impedance and potential measurements. Naturally formed films were found to thicken in NaOH solutions of concentrations 0.1 M, while they were subjected to dissolution in NaOH solutions of concentrations1.0 M. The dissolution behaviour of anodically formed oxide films on niobium was found to depend on the thickness and alkali concentration. Anodically formed thin films (4 nm resist dissolution in NaOH solutions of concentrations0.1 M, partially dissolve at [NaOH]0.5 and 1.0 M and are subject to aggressive dissolution at higher concentrations. Thick films (50 nm) resist dissolution at [NaOH]1.0 M, while they dissolve at higher alkali concentrations. The corrosion potential was found to depend on the alkali concentration and the oxide film thickness. The impedance of the oxide film was found to be purely capacitive and to increase with increasing film thickness.     

Nucleation and growth kinetics of Ag7NO11 on a platinum single crystal electrode

The process of electrochemical deposition of Ag₇NO₁₁ on a platinum single crystal anode from 5M AgNO₃ is investigated. The nucleation and growth rates are measured at different constant overpotentials. The orientation of the Ag₇NO₁₁ crystals with respect to the substrate surface is determined in the case of deposition on glassy carbon and on the 100, 111 and 110 faces of a platinum single crystal.     

Crystal structure and morphology of electrodeposited zinc-iron binary alloys

Deposits of zinc-iron alloy have been prepared galvanostatically from a sulphate bath and the crystal structure has been determined by X-ray diffraction and transmission electron microscopy measurements. The electrodeposited zinc-iron alloys have metastable structures and the individual phases coexist over wide composition ranges. The phases are identified as (10073 at % zinc), (8748 at % zinc), ₁(7862 at % zinc) and (620 at % zinc). Thec andc/a in the h.c.p. lattice of the -phase decrease continuously with decrease of zinc concentrations, and the latter changes from 1.86 to 1.60 (a andc are the lattice constants of the -phase in the direction of thea- andc-axes, respectively). The -phase particles exhibit a hexagonal plate-like morphology which is thin in the direction of thec-axis. The morphology of the electrodeposits changes from plate-like to pyramidal shape when fine -phase particles (100 nm) start to form surrounding the -phase platelets, and then to lenticular or granular in the /₁ duplex region. The -phase forms in the low zinc concentration region and changes the electrodeposits to a fine cuboidal morphology.     

Heterogeneous asymmetric reactions. 23. Enantioselective hydrogenation of ethyl pyruvate over cinchonine- and α-isocinchonine-modified platinum catalysts

The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and -isocinchonine-modified Pt/Al₂O₃ catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmoldm^–3, 1 bar H₂, 23°C, 6% AcOH in toluene), and 76% ee in the case of -isocinchonine (0.14 mmoldm^–3, 1 bar H₂, –10°C, 6% AcOH in toluene). Since -isocinchonine of rigid structure exists only in anti-open conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of anti-open conformation in these cinchona-modified enantioselective hydrogenations.     

Structure of the Reaction Zone of a Steady Detonation Wave in Dinitrodiazapentane

The massvelocity profiles in 2,4dinitro2,4diazapentane samples of different densities were registered by a laser interferometer and the electromagnetic technique. The reaction time was shown to reach 300 nsec and weakly depend on density, while the pressure in the chemical spike can be twice as high as that at the Chapman–Jouguet point. No special features due to diamond formation were observed in the massvelocity profiles registered in the chemical reaction zone.     

Physicochemical properties of electrodepositedβ-lead dioxide: Effect of deposition current density

The influence of current density on the coulometric efficiency of -PbO₂ deposition in 0.5 M cM lead nitrate, the nonstoichiometry, impurity of -PbO₂ and voltammetric double layer capacitance have been studied. While the coulometric efficiency is about 95% at current densities less than 30 mA cm^–2, it decreases at higher current densities. The oxygen deficiency, , in -PbO_2- has been found to be invariant with the current density. X-ray diffraction studies provide a linear decrease in the weight percent of -PbO₂ as an impurity in the -PbO₂ with increase in current density, and the -PbO₂ is found to be absent at 100mA cm^–2 or higher. The estimated double layer capacitance from the cyclic voltammograms recorded in the potential range 0.70–1.10V, increases with deposition current density, indicating enhanced surface area.     

Soldier defense secretions of malaysian free-ranging termite of the genusLacessititermes (Isoptera,Nasutitermitinae)

Soldiers of free-ranging termites of the genusLacessititermes (Isoptera, Nasutitermitinae) secrete from their frontal glands a mixture of monoterpenes, sesquiterpenes, and diterpenes.Lacessititermes ransoneti, L. laborator, andL. species A produce species-specific secretions, the composition being most complex forL. laborator. Apart from known mono- and dihydroxytrinervitadienes, the following new diterpenes were isolated and tentatively assigned as trinervita-1(15),8(19)-dien-2,3,9,14-tetraol 2,3,14-O-triacetate, trinervita-1(15),8(19)-dien-2,3,9,14-tetraol 2,3,14-O-triacetate, 2,3,9, 14-tetraacetoxy-1 (15), 8(19)-trinervitadiene, and 2,3,11,13-tetraacetoxy-1(15),8(19)-trinervitadiene. Data on intragenus chemical variations were subjected to canonical discriminant analysis and genetic distances among the species were calculated to depict intragenus identities and affinities.     

Model Detonation of Solid Explosives by the Homogeneous Mechanism

A twophase detonation model of solid porous explosives, which takes into account the compression of solidstate particles and the presence of a solid component in detonation products, is developed. The homogeneous detonation mechanism based on the Arrheniustype reaction is studied. Detonation is initiated by the region of highpressure and hightemperature gases modeling the detonator explosion. A numerical experiment confirms that there exists an initiationpressure limit below which no homogeneous mechanism of detonation is active. The detonation wave has the leading front in which the explosive is precompacted and the gas in the porous volume is compressed up to a pressure of 100 GPa without any significant change in particle density. Then the gas compresses the particles themselves up to the pressure and temperature of the thermal explosion. As a result, the leading front is followed (after a certain delay) by a narrow reaction zone where detonation products are formed with a further increase in pressure.