Elastic behaviour of RbHSeO4 at the superionic phase transition investigated by brillouin scattering

Brillouin investigations have been performed over the temperature range 160-200°C, which includes the superionic phase transition near ~178°C. Strong discontinuities are obtained for the three elastic constants C_ll, C₂₂ and C₃₃ at the superionic transition. In the superionic phase, two longitudinal Brillouin doublets can be observed for every direction of the acoustic wave vector. This fact can be attributed to the presence of two kinds of regions above the transition. Besides, systematic broadenings of these longitudinal lines are observed in the superionic phase. The results are qualitatively discussed in connection with theoretical expectations.     

Free convection mass transfer at horizontal electrodes

The free convection mass-transfer behaviour of upward-facing horizontal surfaces has been investigated experimentally using an electrochemical technique involving the measurement of limiting currents for the deposition of copper on copper electrodes from acidified cupric sulphate solutions. The data are well correlated by the equations

Sh = 0·64(Sc . Gr)^0·25 for 3 × 10⁴ < ScGr < 2·5 × 10⁷ and

Sh = 0·16(Sc . Gr)^0·33 for 2·5 × 10⁷ < ScGr < 10¹². Electrode diameters varied between 0·1 and 12 cm and cupric sulphate concentrations between 0·01 and 0·3 M.

The data compare favourably with previous experimental results for the corresponding heat-transfer case.     

Structural phase transition and high temperature phase structure of Friedels salt, 3CaO · Al2O3 · CaCl2 · 10H2O

Friedels salt, the chlorinated compound 3CaO · Al₂O₃ · CaCl₂ · 10H₂O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group R c with lattice parameters of a = 5.7358(6)Å and c = 46.849(9)Å (Z = 3 and D_x = 2.111 g/cm³). The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition [Ca₂Al(OH)₆]⁺ and negatively charged layers of composition [Cl⁻,2H₂O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules.     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

Conductivity of Na5+xYAlxSi4-xO12 ceramics

A series of ceramics samples, Na_5+xYAl_xSi_4-xO₁₂, has been prepared by a solid state reaction with the starting materials of SiO₂, Y₂O₃, Al₂O₃ and Na₂CO₃. Their crystalline structure and morphology have been studied by the determination of XRD, IR, TG, DTA and SEM. Their conductivity has been measured by means of the complex impedance method. The dependence of the conductivity and density of the samples on the amount of the added Al₂O₃ and the reaction between the conductivity and the temperature have been discussed. When x = 0, the density of the sintering sample is 90% T.D., and the conductivity is 1·48 x 10⁻¹ (ωcm)⁻¹ at 300°C; when x = 0·1, the density is up to 97% T.D., and the conductivity up to 1·74 x 10⁻¹ (ω cm)⁻¹ at 300°C.     

Alkali metal ions transfer across a water/1,2-dichloroethane interface facilitated by a novel monoaza-B15C5 derivative

In this paper, a novel monoaza-B15C5 derivative, N-(2-tosylamino)-isopentyl-monoaza-15-crown-5 (L), is used as an ionophore to facilitate alkali metal cations transfer across a water/1,2-dichloroethane (W/DCE) interface. Well-defined voltammetric behaviors are observed at the polarized W/DCE interfaces supported at micro- and nano-pipets except Cs⁺. The diffusion coefficient of this ionophore in the DCE phase is calculated to be equal to (3.3±0.2)×10⁻⁶ cm² s⁻¹. The experimental results indicate that a 1:1 (metal:ionophore) complex is formed at the interface with a TIC/TID mechanism. The selectivity of this ionophore towards alkali ions follows the sequence Na⁺>Li⁺>K⁺>Rb⁺>Cs⁺. The logarithm of the association constants (log β₁^o) of the LiL⁺, NaL⁺, KL⁺ and RbL⁺ complexes in the DCE phase are calculated to be 10.6, 11.6, 9.0 and 7.1, respectively. The kinetic parameters are determined by steady-state voltammograms using nanopipets. The standard rate constants (k⁰) for Li⁺, Na⁺, K⁺ and Rb⁺ transfers facilitated by L are 0.54±0.05, 0.63±0.09, 0.51±0.04 and 0.46±0.06 cm s⁻¹, respectively. The pH values of aqueous solution have little effect on the electrochemical behaviors of these facilitated processes. The results predicate that this new type of ionophore might be useful to fabricate electrochemical sensor of sodium ion.     

Propriétés chimiques et électrochimiques de l'eau dans les carbonates alcalins fondus

After recalling the acid—base and redox properties of the solvent species in fused alkaline carbonates, it is shown through potentiometric and thermogravimetric investigations that the acid—base system exchanging O²⁻ H₂O/2OH⁻ is weak, with an equilibrium constant K_A = p_CO2 × [OH⁻]²/p_H2O with pK_A = 1·5 ± 0·2. This result makes it possible to analyse the chemical transformation of fused carbonates into hydroxides under the influence of water vapour. The redox properties of hydrogen and its derivatives are next discussed with underlying thermodynamic considerations, taking into account the chemical and electrochemical experimental data.

With all of these data in mind, the operation of fuel cells containing fused carbonate electrolytes is explained. A new type of fuel cell is suggested, in which polarization due to proton transport to the electrodes is suppressed by the buffer effect of the acid—base pair H₂O/2OH⁻. Performances of such devices are compared to that of cells designed with a combustive mixture of oxygen and carbon dioxide.

Zusammenfassung

Nach einem Überblick auf das Säure/Basen- und Redox-Verhalten der Komponenten des Lösungsmittels in Schmelzen von Alkali-Karbonaten wird mittels potentiometrischer und thermogravimetrischer Untersuchungen gezeigt, dass das Säure/Basenpaar H₂O/2OH⁻, welches O²-austausct, eine Gleichgewichtskonstante K_A = P_CO2 × [OH⁻]²/P_H2O mit einem pKA = 1·5 ± 0·2 besitzt, also in diesem Lösungsmittel bei 600°C sehr schwach ist. Dieses Ergebnis erlaubt die Aufklärung der chemischen Umsetzung von Karbonaten in Schmelzen zu den entsprechenden Hydroxyden bei Einwirkung von Wasserdampf. Die Redoxeigenschaften des Wasserstoffs und seiner Verbindungen werden im folgenden auf Grund von thermodynamischen Betrachtungen diskutiert, indem von den vorgängig erhaltenen Resultaten und experimentellen chemischen und elektrochemischen Daten Gebrauch gemacht wird. Schliesslich wird die Gesamtheit der Ergebnisse verwendet, um eine Untersuchung des Verhaltens von Brennstoffzellen mit Karbonatschmelzen durchzuführen. Man beschreibt einen neuen Typ dieser Reihe, bei welchem die auf den Protonentransport zurückzuführenden Polarisationserscheinungen infolge des Puffereffektes des vorhandenen Säure/Basenpaares H₂O/2OH⁻ unterdrückt werden. Das Verhalten dieser Zellen wird mit demjenigen von Brennstoffzellen auf der Basis von Sauerstoff und Kohlendioxyd als Brennstoffen verglichen.     

Estimation of particle size distribution parameters in animal lungs

A constant of specific solubility of 2·5 × 10⁻⁸ g cm⁻²day⁻¹ was determined for fused aluminosilicate particles, by observing in vivo retention kinetics after intravenous injection into rats. Studies over the past years in this laboratory, in which dogs and rats have inhaled labeled aerosols of these particles, have shown retention half-lives in the lung of 460 and 285 days, respectively. By applying these values for solubility and half-life to Mercer's theory of dissolution from the deep lung, the initial distribution of particles deposited in the pulmonary regions of dogs and rats following inhalation was calculated. From an inhaled aerosol with a mass median diameter, D_m, of 1·0 μm and σ_o = 1·7, a distribution described by D_m = 0·51 μm and σ_o's ranging from 1·16–1·48 was estimated to have been deposited in the Beagle dog lung. and a D_m =< 0·32 μm and σ_o's ranging from 1·18–1·29 was similarly calculated for rats.     

Kinetics of Fischer–Tropsch synthesis on titania-supported cobalt

Rate data have been obtained for CO hydrogenation on a well-characterized 11.7% Co/TiO₂ catalyst in a differential fixed bed reactor at 20 atm, 180–240°C, and 5% conversion over a range of reactant partial pressures. The resulting kinetic parameters can be used to model precisely and accurately the kinetics of this reaction within this range of conditions. Turnover frequencies and rate constants determined from this study are in very good to excellent agreement with those obtained in previous studies of other cobalt catalysts, when the data are normalized to the same conditions of temperature and partial pressures of the reactants. Based on this comparison CO conversion and the partial pressure of product water apparently have little effect on specific rate per catalytic site. The data of this study are fitted fairly well by a simple power law expression of the form −r_CO=kP_H2^0.74P_CO^−0.24, where k=5.1×10⁻³ s⁻¹ at 200°C, P=10 atm, and H₂/CO=2/1; however, they are best fitted by a simple Langmuir–Hinshelwood (LH) rate form −r_CO=aP_H2^0.74P_CO/(1+bP_CO)² similar to that proposed by Yates and Satterfield.     

Electrochemical characterization of 8-hydroxyquinoline-5-sulphonate/aluminium(III) aqueous solutions

8-Hydroxyquinoline-5-sulphonate/Al(III) aqueous solutions were studied both by potentiometric titrations and voltammetric measurements, in order to obtain the number, the stoichiometry and the stability constants of the complexes formed at equilibrium, and to evaluate the redox and (electro)kinetic properties of the free ligand and of the metal/ligand complexes. The complexes formed in 0.2 m (Na)Cl aqueous solution (stability log beta values ± standard deviation) are AlL⁺ (8.95 ± 0.05), AlL₂⁻ (17.43 ± 0.03) and AlL₃³⁻ (24.58 ± 0.05), where “L” denotes the free ligand in the completely deprotonated form (L²⁻, pK_a1 = 3.910 ± 0.008, pK_a2 = 8.319 ± 0.004). AlL₃³⁻ is the predominant Al(III) species in a very wide range of pH, metal and ligand concentrations and metal-to-ligand ratios. The free ligand shows an oxidation wave at 0.62 V versus SCE. The proposed oxidation mechanism includes a first reversible one-electron oxidation of the ligand, followed by a coupling reaction and by a second reversible one-electron oxidation, and finally by a decomposition reaction. The addition of Al(III) lowers the intensity of the oxidation wave due to the formation of the redox-inactive complex AlL₃³⁻. A residual low signal was attributed to the free ligand produced by the complex dissociation, AlL₃³⁻ = AlL₂⁻ + L²⁻. All the kinetic parameters involved in the ligand oxidation and in the complex disruption were calculated on the basis of the agreement between experimental and simulated linear sweep and cyclic voltammetries. Correctness of the mechanisms proposed was further confirmed “a posteriori” by the agreement between potentiometric and linear sweep voltammetric results. The low residual signal observed in the presence of fully formed complex was attributed to the free ligand produced by the complex dissociation, having a kinetic constant estimated 0.2 s⁻¹.     

Mass transfer by free convection at horizontal electrodes

Limiting currents were measured in an unstirred cell at horizontal cathodes facing upward. Electrolyte composition ranged from 0·01 to 0·7 M CuSO₄ in 1·5 M H₂SO₄. Cathode sizes varied from 0.1–30 by 10 cm, and the free height above the electrode from 1–16 cm. Limiting currents for deposition of copper ranged from 0·68–129 mA/cm². For electrode width larger than 20 mm the data is well represented by the general correlation

Nu′ = 0.19 (Sc. Gr) , where Nu′, Sc, and Gr are the Nusselt number for mass transfer, the Schmidt, and Grashof numbers, respectively. The experimental range used in the correlation included 10⁸ < (Sc. Gr) < 1·4 × 10¹² The results indicate that the boundary layer is turbulent, as it is in the case of the analogous heat-transfer model.     

Reversible heat effects in high temperature batteries: Transported entropy and thomson coefficients in cells with β″-alumina

The transported entropy and the Thomson coefficient for charge conducting ions are needed to predict reversible heat effects in batteries. Transported entropies and Thomson coefficients have been calculated from Seebeck coefficients of the cell Fe(s, T₁)|Me| β″ alumina | Me | Fe(s, T₂) for Na and K (Me). The result is S*_Na⁺ = 56 ± 3 J K⁻¹ mol⁻¹ at 500 K, with a Thomson coefficient τ_Na⁺ = 30 ± 2 J K⁻¹ mol⁻¹ in the temperature interval 333–773 K. The transported entropy of Na⁺ did not change by freezing Na at 370. The results for K⁺ are identical to those of Na⁺ within the accuracy of the experiments. The Thomson coefficient derived from measurements at different values of T₁ was consistent with the observed variation in emf with ΔT for a given T₁. The reversible heat changes at the electrodes have been calculated for sodium sulphur and potassium sulphur batteries. During discharge both batteries produce a net reversible heat, the production always being largest at the alkali metal anode. At the cathode, the heat effect becomes relatively small when the composition of Na and S is within the one phase region. A change in composition from the one phase to the two phase region is expected to lead to changes in local temperature gradients. The systems were described by the electric work method, a method which has practical advantages compared to other electrochemical methods.     

Low-temperature H2 and N2 transport through thin Pd66Cu34Hx layers

The single gas H₂ and N₂ permeability of a 4 μm thick dense fcc-Pd₆₆Cu₃₄ layer has been studied between room temperature and 510 °C and at pressure differences up to 400 kPa. Above 50 °C the H₂ flux exhibits an Arrhenius-type temperature dependence with J_H2=(5.2±0.3) mol m⁻² s⁻¹ exp[(−21.3 ± 0.2) kJ mol⁻¹/(R·T)]. The hydrogen transport rate is controlled by the bulk diffusion although the pressure dependence of the H₂ flux deviates slightly from Sieverts’ law. A sudden increase of the H₂ flux below 50 °C is attributed to embrittlement.     

Optical and electro-optical properties of (CH3)4NHgBrI2 crystals

(CH₃)NH_gBrI₂ is an electro-optic material which shows a structural phase transition with a symmetry change from orthorhombic to monoclinic at T = 108°C upon heating. In this contribution we report first experimental data on the optical properties (refractive indices, absorption coefficients), on the elastic and electro-optical properties. It is shown, that the dispersion of the refractive indices is adequately described by a single term Sellmeier oscillator model. Measurements of the temperature dependence of the birefringence and of the elastic constants show pronounced changes at the structural phase transition. The biaxial crystal with the three main indices n_a = 1.761±0.001, n_b = 1.802±0.001 and n_c = 1.755±0.001 at room temperature and a wavelength λ=633nm becomes optically uniaxial at T = (50±0.5)°C with n_a=n_c=1.750, due to different temperature dependences n_a(T) and n_b(T). Room temperature electro-optic coefficients and the corresponding half-wave voltages have been determined, and indicate that the electro-optic activity of (CH₃)₄NH_gBrI₂ is about two orders of magnitude smaller than in KH₂PO₄.     

Aerosol measurement by laser doppler spectroscopy—II. Operational limits, effects of polydispersity, and applications

The theoretical basis and the results of a computer simulation are presented which describe the operational limits of size and concentration for aerosol sizing by laser Doppler spectroscopy LDS,. This analysis suggests that a state of the art LDS system has the capability of sizing 0·03 μm diameter particles when the number concentration is 10⁸ cm⁻³ or greater and 0·2 μm diameter for coocentrations as low as 100 particles cm⁻³. An evaluation of the effect on the laser Doppler spectroscopy measurements of a polydisperse aerosol having a log normal size distribution is presented and methods for combining these measurements with other averaged measurements to determine both count median diameter (CMD) and geometric standard deviation (δ_g) are proposed. For aerosols having log normal distributions with 0·3 < CMD < 3 μm and 1·0 < δ_g < 2·0, laser Doppler spectroscopy is able to measure the surface area median diameter within ± 15 per cent, independent of polydispersity. Applications of LDS to aerosol sizing are evaluated and its advantages and disadvantages relative to other sizing methods are discussed.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.     

Comparative study of lithium ion conductors in the system Li1+xAlxA2−x (PO4)3 with A=Ti or Ge and 0≤x≤0·7 for use as Li sensitive membranes

Preparations and physico-chemical characterizations of NASICON-type compounds in the system Li_1+xAl_xA_2−x^IV(PO₄)₃ (A^IV=Ti or Ge) are described. Ceramics have been fabricated by sol-gel and co-grinding processes for use as ionosensitive membrane for Li⁺ selective electrodes. The structural and electrical characteristics of the pellets have been examined. Solid solutions are obtained with Al/Ti and Al/Ge substitutions in the range 0≤x≤0·6. A minimum of the rhombohedral c parameter appears for x about 0·1 for both solutions. The grain ionic conductivity has been characterized only in the case of Ge-based compounds. It is related to the carrier concentration and the structural properties of the NASICON covalent skeleton. The results confirm that the Ti-based framework is more calibrated to Li⁺ migration than the Ge-based one. A grain conductivity of 10⁻³ S cm⁻¹ is obtained at 25°C in the case of Li_1·3Al_0·3Ti_1·7(PO₄)₃. A total conductivity of about 6×10⁻⁵ S cm⁻¹ is measured on sintered pellets because of grain boundary effects. The use of such ceramics in ISE devices has shown that the most confined unit cell (i.e. in Ge-based materials) is more appropriate for selectivity effect, although it is less conductive.©     

Hallspannungsmessungen in wässrigen elektrolyten

In measuring the Hall effect in aqueous solutions some aspects of electrochemistry and electrode kinetics have to be considered. The described apparatus is based on a constant magnetic field and an alternating electrical field of variable frequency and on three Hall electrodes. The hall mobility in 0·1 N HNO₃, 0·01 N AgNO₃ at 20°C is 2·58 × 10⁻³ cm/Vs.

Résumé

Quelques aspects électrochimiques et électrocinétiques des mesures de l'effet Hall dans les solutions aqueuses sont discutés. L'apparaillage décrit en détail fonctionne au moyen d'un champ magnétique continu, d'un champ électrique alternatif de fréquence variable et avec trois sondes d'Hall. La mobilité d'Hall trouvée dans un électrolyte de 1/10 n HNO₃, 1/100 n AgNO₃ est = 2,58 · 10⁻³ cm²/Vs à 20°C.     

Deactivation behavior of ruthenium promoted Co/γ-Al2O3 catalysts in Fischer–Tropsch synthesis

Heterogeneous catalytic hydrogenation of cyclohexene, catalyzed by Pt/Al₂O₃, was carried out in solutions of 2-hydroxyethylammonium formate (a room temperature ionic liquid, RTIL) mixed with methanol, ethanol and propan-2-ol at 25 °C. The rate constants of the reaction in ionic liquid alcohol mixtures were higher than alcohol alone. First-order rate constant of the reaction in the RTIL relative to propan-2-ol is approximately 28. Furthermore, the rate constant of the reaction increases with the mole fraction of the ionic liquid. Single-parameter correlations of log k vs. normalized polarity parameter (), hydrogen-bond acceptor basicity (β) and hydrogen-bond donor acidity () do not give acceptable results in the solutions. In addition, log k does not show an acceptable dual-parameter correlation with π* (dipolarity/polarizibility, one of the Kamlet–Taft parameters for solvent that shows the dipolarity of the solvent) and , π* and β and and β. However single-parameter correlation of log k vs. π^* gives reasonable results. The increase of the reaction rate with π^* is attributed to the non-polar nature of the reactants.     

Study of the elastic properties of glycine phosphite crystals (Gly·h3PO3) by Brillouin scattering

The results concerning the elastic properties of glycinium phosphite (GPI) crystals obtained by Brillouin scattering method are reported. The temperature measurements of the relative change AVN of the phase velocity for acoustic modes propagating in q = [100], q = [010], q = [110] and q = [011] directions have been performed in the temperature range 300 K - 170 K. Obtained results confirm the existence of the ferroelectric phase transition (P2₁/a - P2₁) at 224 K.