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.     

Relationship between preferential adsorption coefficient and intrinsic viscosity in ternary systems

Summary The relation proposed between the preferential adsorption coefficient, , and the intrinsic viscosity, ¦¦ ¦¦=¦¦+AK has been applied in this paper to previously published data. This equation is found to be valid in theta condi tions and far away from them. The obtained results are compared to the ones calculated with the Dondos-Benoit equation.     

[η]-M-relationship for polyacrylamide in aqueous 0.1 M Na2SO4 solution

Summary The intrinsic viscosity [] and the molecular weight M_W of 8 pure, unbranched polyacrylamide (PAAm) samples — prepared in our laboratory — were determined and lead to the following formula [] = 0,0194 · M_W ^0,70(cm³/g) Solvent: aqeous 0.1 M Na₂SO₄ solution; T=298K; M_W/M_n-2,5 . In addition a comparison to the []-M-relationships of PAAm in other solvents is given.     

Structure and thermal stability of zinc–nickel electrodeposits

ZnNi alloys were electrodeposited from a chloride bath on steel substrates. The effect of nickel bath concentration on chemical composition, structure and microstructure of the deposits is demonstrated. From 0 to 13 nickel, the phases obtained do not correspond to that reported on the thermodynamic phase diagram. It is shown that the substitution of zinc by nickel is responsible for the formation of distorted _d and _d phases corresponding to the supersaturated hexagonal phase of zinc and to the unsaturated cubic phase of Zn–Ni alloy, respectively. Differential scanning calorimetry indicates that the thermal instability of the alloys containing up to 13 wt of nickel, results from the crystallization of the phase from the _d and _d phases at around 200 °C and 250 °C, respectively.     

Correlation between configuration/conformation of zirconocenes on the stereoselectivity of the propylene polymerization reaction

Summary The catalytic performance (activity and polymer properties) of metallocenes with different symmetries in combination with methylaluminoxane (MAO) in the polymerization of propylene has been investigated at different temperatures, under standardized reaction conditions. The zirconocene rac-ethylene (⁵-1-indenyl) zirconium (IV) dichloride, with C₂ symmetry, produces isotatic polypropylene and isopropylidene(⁵-cyclopentadienyl (⁵-9-fluorenyl) zirconium (IV) dichloride, with C _S symmetry, syndiotactic polypropylene. The degree of the tacticity of these polymers increases with decreasing polymerization temperature. Only atactic polypropylene was formed with the unbridged zirconocenes bis(⁵-cyclopentadienyl) zirconium (IV) dichloride and bis(⁵-indenyl zirconium (IV) dichloride at any temperature investigated (10–60°C).     

The linear master dependence for polymer melts in the non-Newtonian range

Summary The multivariable power function · _o = B( _o)^b ₁· q^b ₂· G^b ₃ (symbols are explained in the text) is proposed as the general linear master dependence for the behaviour of linear and branched polymer melts in the non-Newtonian range. It can be applied for generation of any flow curve for a given polymer sample provided that constants B, b₁, b₂ and b₃ are known. It has been found, using the experimental data on polycarbonate, that B is approximately equal to 3×10^–5 being the universal constant, and b₁=0.77 for linear polymers. Lower values of b₁ indicate the existence of branched macromolecules, and the relationship of b₁ vs G can be used for determination of branching degrees from the rheological data.     

Comparison of the critical conditions for initiation of dendritic growth and powder formation in potentiostatic and galvanostatic copper electrodeposition

Critical current densities for the initiation of dendrite growth and powder formation in potentiostatic and galvanostatic deposition are determined. Induction times for dendritic growth formation in potentiostatic and galvanostatic deposition are discussed.Nomenclature C ₀ bulk concentration - D diffusion coefficient - F Faraday constant - h height of a protrusion - h _i height of ith protrusion - h ₀ initial height of a protrusion - i current density - i _c current density at which dendrites appear instantaneously - i _i minimal current density at which dendritic growth becomes possible - i _L limiting current density - i ⁰ initial current density - i ₀ exchange current density - k proportionality factor - n number of electrons - N number of protrusions - R _t tip radius - S electrode surface area - S ₀ initial electrode surface area - t time - t _i induction time - V molar volume - thickness of the diffusion layer - overpotential - _c critical overpotential of instantaneous dendritic growth - _c,t critical overpotential of dendritic growth following non-dendritic roughness amplification - _i critical overpotential for the initiation of dendritic growth - ₀ initial overpotential - 2.3 ₀ slope of the Tafel line - quantity defined by Equation 3 - surface tension - time constant     

A theory of pseudo cross-link

Summary The calculation of entanglement is reexamined and it is expressed by two factors nc² and n²c², n and c being the length of the polymer and its concentration, respectively. Both factors are concerned with the inter-coil interaction revealing in the melt viscosity so that is proportional to n^3.5c^4.5. At the melt temperature T_B, a critical chain length n_B is 256 and T_B is twice as high as t_A, i.e. a liquid-solid transition. At T_B the entropy of cross-linking becomes 2/3 times of that at T_A. The osmotic pressure increases in the semi-dilute solution by a factor of (c/c^*)^4/3, c^* being a critical concentration for overlapping. In the dilute solution in -solvents the cohesion of segments enables the independent coil to form an intra-coil linking even below c^* and induces the coil-globule transition at the -point to give rise to self-coaguration.     

On the possibility of estimating weak interactions of macromolecules in solutions from the experimental viscous flow activation energies data

Summary The initial viscosity and activation energy in viscous flow of the systems: water(W)-casein(C)-polysaccharide(PS) (gum arabic(G),de-xtran(D), extran sulfat (DS)) have been determined for various ionic strengths corresponding to total or limited thermodynamic compatibility of macrocomponents. Excess activation energy H ^E due to the protein-polysaccharide interactions has been calculated. It is positive for systems with total compatibility and negative for systems with limited compatibility. Moreover, it yields information on type of the protein-polysaccharide interactions. Negative H ^E means that repulsive forces are dominant, while positive H ^E means that attractive forces are dominant. Since the properties of the systems W-C-D and W-C-DS are similar, it is believed that C-D complexes can possibly be formed with an energy 2 kT(10 mJ/g).The authors express their sincere graditude to Konstantinov A.A., Kondratev V.P., Chernysh M.M. (Special Design Bureau of the Institute of Petroleum Chemical Synthesis, The USSR Academy of Sciences) and Dmitrienko A.P. (Institute of Organoelement compounds, The USSR Academy of Sciences) for their invaluable help.     

High resolution transmission electron microscopic (HRTEM) determination of the preferentially exposed faces onγ-Al2O3 andη-Al2O3

High resolution transmission electron microscopy has been used to examine the crystallites in -Al₂O₃ and -Al₂O₃. -Al₂O₃ has been found to be nearly hexagonally shaped crystallites whose identity distances and angles of fringe patterns indicate that (110) planes are preferentially exposed. -Al₂O₃ showed preferentially exposed surfaces of (100), (110) and super imposed (111)/(211). This information is valuable to the catalyst researcher trying to model the surfaces of these two supports.     

Mark-Houwink-Sakurada coefficients for conventional poly(methyl methacrylate) in tetrahydrofuran

Summary Intrinsic viscosities [] in tetrahydrofuran (THF) at 30°C are reported for ten narrow molecular weight distribution poly(methyl methacrylate) (PMMA) samples covering nearly two orders of magnitude in molecular weight. By combining the viscosity results with absolute weight-average molecular weights determined via low-angle laser light scattering (LALLS) experiments, we have accurately determined the Mark-Houwink-Sakurada (MHS) parameters for this polymer-solvent pair (K=7.56x10⁵ and a=0.731, when [] is in dL g^-1).     

Electrodeposition, composition and structure of Zn—Cr alloys

The effect of polyethylene glycol (PEG 1500) as additive and of deposition conditions on Zn—Cr alloy electrodeposition from an acidic sulfate electrolyte at room temperature, without agitation was investigated. PEG polarizes the overall cathodic reaction and inhibits Zn deposition. Cr codeposition with Zn starts at a cathodic potential of about –1,95 V vs Hg/Hg₂SO₄, which is reached at current density of about 20 A dm^–2 in galvanostatic conditions. Zn—Cr alloy coatings containing up to 28 at % Cr were obtained depending on the plating conditions. SEM observations showed an island-like structure, formed by the local growth of crystals, which covered the surface during further deposition. In the first stages of electrodeposition the powder diffraction spectra contain lines of b.c.c. -(Zn,Cr) phase (a 3.02 Å). After 30 s deposition time weak lines of Zn-based phase (a 2.67 Å, c 4.90 Å) appear, and become clearly visible in coatings deposited for 90 s. The average Cr content in the alloy coatings decreases with advancing deposition. The as-plated surface contains C in organic compounds and Zn(OH)₂. After 50 min sputtering, Zn and a mixture of Cr, Cr₂O₃ and Cr₇C₃ were found. The presence of organic C and O, probably from inclusions of PEG, were also detected.     

Investigation of water electrolysis by spectral analysis. I. Influence of the current density

The potential (or current) fluctuations observed under current (or potential) control during gas evolution were analysed by spectral analysis. The power spectral densities (psd) of these fluctuations were measured for hydrogen and oxygen evolution in acid and alkaline solutions at a platinum disk electrode of small diameter. Using a theoretical model, some parameters of the gas evolution were derived from the measured psd of the potential fluctuations, such as the average number of detached bubbles per time unit, the average radius of the detached bubbles and the gas evolution efficiency. The influence of the electrolysis current on these parameters was also investigated. The results of this first attempt at parameter derivation are discussed.Nomenclature b Tafel coefficient (V^–1), Equation 46 - C electrode double layer capacity (F) - e gas evolution efficiency (%) - f frequency (Hz) - f _p frequency of the peak in the psd _v and _i (Hz) - F Faraday constant, 96 487 C mol^–1 - l electrolysis current (A) - J electrolysis current density (mA cm^–2) - k slope of the linear potential increase (V s^–1), see Fig. 1 - n number of electrons involved in the reaction to form one molecule of the dissolved gas - r _b radius of a spherical glass ball (m) - r _e radius of the disk electrode (m) - R _e electrolyte resistance () - R _p polarization resistance () - R _t charge transfer resistance () - u ₁ distribution function of the time intervals between two successive bubble departures (s^–1) - v _g mean volume of gas evolved per unit time (m³ s^–1) - v _t gas equivalent volume produced in molecular form per unit time (m³ s^–1) - V ₀ gas molar volume, 24.5×10^–3 m³ at 298 K - x ₀ time pseudoperiod of bubbles evolution (s) - Z electrode electrochemical impedance () Greek characters _e dimensionless proportional factor (Equation 19) - slope of log /logJ and loge/logJ curves - number of bubbles evolved per unit time (s^–1) - _a activation overpotential (V) - _ci concentration overpotential of reacting ionic species (V) - _cs concentration overpotential of dissolved molecular gas (V) - _ohm ohmic overpotential (V) - _t total overpotential (V) - v parameter characteristic of the gas evolution pseudoperiodicity, Equation 13 (s^–1) - time constant of the double layer capacity change (s) - _v power spectral density (psd) of the potential fluctuations (V² Hz^–1) - _i power spectral density (psd) of the current fluctuations (A² Hz^–1) Special symbols spatial average of the overpotential _j over the electrode surface - time averaged value of - _j fluctuation of around - <> mean value of the total overpotential jump amplitude due to a bubble departure - <I> mean value of the current jump amplitude due to a bubble departure Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

Interaction of methane with surfaces of silica,aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (, and ) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactive ₁ band (2917 cm^–1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm^–1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, -alumina and HZSM-5 zeolite could exchange with CD₄ at temperatures higher than 773K, while those on -alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al₂O₃ (both and ) at temperatures higher than 473 K. The formate species would decompose to CO₂, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, -Al₂O₃ did not adsorb or react with methane in any case.     

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.     

Synthesis and Redox Properties of a π-Conjugated Cyclobutadienecobalt Polymer Containing Ferrocenyl Groups

The reaction of CpCo(PPh₃)₂, in which Cp= ⁵-cyclopentadienyl, with a -conjugated diacetylene, FcCC–o-C₆H₄–CCFc, in which Fc=ferrocenyl, was found to give a cyclobutadienecobalt mononuclear complex, { ⁴-C₄Fc₂(o-FcC₆H₄)₂}CoCp (1), the crystal structure of which was determined by X-ray crystallography. In contrast, the reaction of CpCo(PPh₃)₂ with FcCC–p-C₆H₄–CCFc affords a cyclobutadienecobalt polymer, [p-C₆H₄( ⁴-C₄Fc₂)CoCp] _n (2). The monocobalt complex 1 shows reversible 1e^– and 3e^– redox waves at E ⁰=0.116 and 0.350 V vs Ag/Ag⁺, and the polymer complex 2 shows two chemically reversible redox waves at E ⁰=0.143 and 0.219 V for the oxidation of the ferrocenyl moieties in the cyclic voltammogram. Crystal data are as follows: (1, C₆₅H₄₉CoFe₄), triclinic, space group P\={1} (No. 2), a=13.547(4), b=16.197(4), c=11.763(4) Å, =106.79(2), =97.93(3), =97.12(3), V=2410(1) ų, Z=2.     

Surface-mediated stereoselective hydrogenation of dienes over chromium subcarbonyls encaged in zeolite

Tricarbonyl chromium (0) species encaged in LiX or NaX was found to be highly efficient and stereoselective for the hydrogenation of butadiene at > 230 K. The selectivity to cis-2-butene depended both on the reaction temperature and butadiene pressure. The reaction mechanism is proposed, in which cis-2-butene is formed via (⁴-C₄H₆)Cr(CO)₃, while 1-butene via (²-C₄H₆)Cr(CO)₃ and (²-C₄H₆)₂Cr(CO)₃.     

Ferrocenylenesilylene Polymers as Coatings for Tapered Optical-Fiber Gas Sensors

In this paper we report the use of ferrocenylenesilylene polymers as coatings for tapered optical-fiber sensors. The principle of operation of this device is based upon environmentally induced changes in the refractive index of the polymer layer which change the power transmitted through the tapered fiber. The results for two sensor arrays fabricated using the ferrocenylenesilylene polymer [( ⁵-C₅H₄)Fe( ⁵-C₅H₄)MePhSi] _m and copolymer {[( ⁵-C₅H₄)Fe( ⁵-C₅H₄)SiPhMe] _n [( ⁵-C₅H₄)Fe( ⁵-C₅H₄)Me₂Si] _m are presented. We also show that the sensitivity of this device is a function of the taper beat length.     

A model of the electrochemical behaviour within a stress corrosion crack

A mathematical model of the electrochemical behaviour within a stress corrosion crack is proposed. Polarization field, crack geometry, surface condition inside the crack, electrochemical kinetics, solution properties and applied stress can be represented by the polarization potential and current, the electrochemical reactive equivalent resistance of the electrode, the change in electrolyte specific resistance and surface film equivalent resistance, respectively. The theoretical calculated results show that (i) when anodic polarization potential is applied, the change in the crack tip potential is small; (ii) when cathodic polarization potential is applied, the crack tip potential changes greatly with the applied potential; (iii) the longer the crack, the smaller the effect of the applied potential on the crack tip potential in both anodic polarization and cathodic polarization conditions. The calculated results are in good agreement with previous experimental results.Notation coordinate, from crack mouth (on the metal surface) to crack tip (cm) - y y = s _L L/(s ₀ – s _L) + L – , function of (cm) - y ₀ y ₀ = s _L L/(s ₀ – s _L) + L (cm) - V polarization potential (V) - galvanic potential of electrode (V) - ₁ galvanic potential of electrolyte (V) - t sample thickness (cm) - w sample width (cm) - S _L crack tip width (cm) - S _o crack mouth width (cm) - L crack length (cm) - s() crack width at position (cm) - _lo specific resistance of electrolyte, as a constant ( cm) - _s specific resistance of metal ( cm) - (, y) specific resistance of electrolyte, varies with potential and crack depth ( cm) - R _b (, y) electrochemical reactive equivalent resistance of electrode, varies with potential and crack depth () - R ₁ electrolyte resistance () - R _s metal resistance () - r(, y) surface film equivalent resistance, varies with potential and crack depth () - r _o surface film equivalent resistance, as a constant () - I _o total polarization current (A) - I net polarization current from integrating 0 to in Fig. 2 (A) - polarization overpotential (V) - _a anodic polarization overpotential (V) - _c cathodic polarization overpotential (V) - Euler's constant     

Proposed prediction method for the frictional pressure drop of bubble-filled electrolytes

A relationship is derived to predict the pressure drop in a two-phase flow system between gas evolving electrodes and in the pipes between the cells. The design equation (dp/dx)=[(1+) ⁿ /(1–)](dp _L/dx) only requires the flow rates of the gas and liquid and the single-phase (liquid) pressure drop to be known. The equation is compared with other theoretical and empirical prediction methods, and with experimental data.Nomenclature C geometry factor - d_B diameter of the departing bubbles (m) - d_h hydraulic diameter (m) - k_s wall roughness (m) - k _L multiplier - L length of electrode in flow direction (m) - n exponent in Equation 16 - p pressure (kg m^–1 s^–2) - Re Reynolds number - s interelectrode distance (m) - S cross-sectional flow area (m²) - V_G, V_L volumes of gas and liquid, respectively (m³) - volumetric flow rate of gas and liquid, respectively (m³ s^–1) - x coordinate in flow direction (m) - X parameter due to Equation 19 - viscosity (kg m^–1 s^–1) - fractional surface coverage - friction coefficient - density (kg m^–3) - volumetric gas fraction - Thorpe's multiplier, Equation 25 Indices A anode - C cathode - G gas - L liquid - T cell exit