The influence of the tunnel probability on the anodic oxygen evolution and other redox reactions at oxide covered platinum electrodes

Potentiostatic and galvanostatic pulse measurements were carried out to investigate the anodic oxygen evolution at platinum electrodes in 1N H₂SO₄ in dependence on the oxide layer thickness d and the electrode potential ε. The thickness d (1·5–10 Å) was obtained from cathodic charging curves. Further, the temperature dependence (0°–81°C) was evaluated from Bowden's measurements. Summarizing, the current i_o2 follows the relation, log i = A - (E⁰_a - αFη)/2·3 RT - d/d_o. The experimental activation energy E_o = E^o_a = αFη decreases linearly with increasing overvoltage η. The linear decrease of log i with increasing d, which is given by the term d/d_o, is correlated to the probability of the quantum mechanical tunnel transition of the electron from adsorbed ions, OH^?_ad or O^2?_ad respectively, through the oxide layer to the metal. Similar effects of the oxide layer thickness on the current density were observed in the case of the oxygen evolution at iridium, the CO-oxidation on platinum, and the reduction of Cl^? and Ce⁴⁺ at platinum. In these cases a rate determining electron transfer through the oxide layer is also assumed.     

Preparation and crystallite growth measurement of active magnesium aluminate spinel

MgAl₂O₄ was prepared by cocrystallizing and decomposing Al(NO₃)₃.9H₂O and Mg(NO₃)₂.6H₂O mixture. The crystallite growth of the resulting powder was studied in the temperature range 500 – 1000 °C. It was found that the equation governing the process is D² – D²₀ = KT. Two activation energies for crystallite growth were obtained, 22 kcal and 44 kcal for the temperature ranges 500 – 785 °C and 785 – 1000 °C respectively. It was noted that rapid increase in crystallite size occurred on soaking at a temperature > 800 °C.     

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho₂Zr₂O₇) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho₂Zr₂O₇ pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10⁻³ Scm⁻¹) and grain boundary (2.50⨯10⁻³ Scm⁻¹) conductivities of Ho₂Zr₂O₇ prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.     

Experimental study and DFT calculations of improved giant dielectric properties of Ni2+/Ta5+ co-doped TiO2 by engineering defects and internal interfaces

High-performance ceramics with chemical formula (Ni_1/3Ta_2/3)_xTi_1?xO₂ with excellent dielectric properties are demonstrated. The dopants of Ni²⁺ and Ta⁵⁺ in TiO₂ caused the formation of oxygen vacancies and free electrons. The (Ni_1/3Ta_2/3)_xTi_1?xO₂ exhibited low loss tangent of 0.046 and a high dielectric permittivity of 3.5–4.5 × 10⁴ with a very weak dependence on temperature (?60 to 200 °C). Broadband dielectric spectroscopy shows at least four dominant sources in the dielectric relaxation response in the temperature range of ? 253–210 °C. DFT calculations indicate the formation of defect clusters, which are the largest contributors to the dielectric response, and these are found to be dominant even at temperatures down to ? 253 °C. Both grain boundary and surface layer mechanisms in the ceramics contribute to the dielectric response at the relatively high temperatures. The sample–electrode contact effect associated with oxygen vacancy diffusion is dominant at high temperatures above 150 °C.     

Study of ion solvation and ion association in polymer gel electrolyte with polyethylene oxide-co-polypropylene oxide as a plasticizer

Using a molal conductance method, ion solvation and ion association in polytriethylene glycol dimethacrylate (PTREGD)–LiClO₄ gel electrolytes with amorphous ethylene oxide-co-propylene oxide (EO-co-PO, M̄_n = 1750) as the plasticizer were investigated. It was found that the fraction of solute existing as single ions (α_i) and ion pairs (α_p) decreases, while that of triple ions (α_t) increases linearly with increasing salt concentration. The dependence of these fractions on molecular weight of plasticizer was also examined. It was shown that α_i and α_t increase and α_p decreases with increasing molecular weight. The result of temperature dependence of these fractions was very interesting: when the temperature is lower than 55°C, α_i increases while α_p and α_t decrease with increasing temperature; however, when the temperature is higher than 55°C, the reverse is true.     

Synthesis of LiBGeO4 using compositional design and its dielectric behaviors at RF and microwave frequencies

Borates are promising candidates as dielectric substrate materials in low temperature cofired ceramics technology (LTCC) due to their relative low sintering temperatures and relative permittivities compared to their counterparts. However, synthesizing borates having single-phase is still challenging because of the volatility and hydrophilicity of boron resources. In this work, a compositional design was utilized to synthesize single-phase LiBGeO₄ ceramics over a broad temperature range from 600 to 840 °C. Radio-frequency dielectric behaviours featured a strong temperature dependence, especially at high temperatures (>400 °C), which is related to the thermally activated polarizations. LiBGeO₄ ceramic sintered at 820 °C has optimum microwave dielectric properties with the relative permittivity (ε_r) of 6.28, a quality factor (Q × f) of 21,620 GHz, and a temperature coefficient of resonance frequency (τ_f) of -88.7 ppm/^°C. LiBGeO₄ also showed chemical inertness when cofired with silver (Ag), provided an evidence for its utilization in LTCC technology. Overall, this work provides a strategy for facile synthesis of phase pure borates, via the proposed two-step process to obtain stable boron resources.     

High-temperature fracture toughness of SiC–Mo5(Si,Al)3C composites

The fracture toughness (K_1C) of melt-infiltrated SiC–Mo₅(Si,Al)₃C composites was measured from room temperature to 1400°C using the indentation strength method at 1 atm argon atmosphere. The fracture toughness was found to increase from 3.6 MPa·m^1/2 at room temperature to 7.7 MPa·m^1/2 at 1400°C. This increase was mainly attributed to the plastic deformation of the infiltrated Mo₅(Si,Al)₃C phases at high temperatures, which act as ductile toughening inclusions. The influence of annealing temperatures and atmospheres on K_1C was studied. The effect of indentation load on K_1C was also analyzed.     

The structural,dielectric, electrocaloric,and energy storage properties of lead-free Ba0·90Ca0·10Zr0·15Ti0·85O3

Lead-free ferroelectric materials with Electrocaloric Effect (ECE) and recoverable energy (W_rec) represent a major breakthrough in the technological race for manufacturing low-cost and eco-friendly smart multifunctional devices. Here, guided by the benefits resulting from successive ferroelectric-ferroelectric (F–F) and ferroelectric-pseudo cubic (F-PC) transitions including broad operational temperature span, lead-free Ba_0·90Ca_0·10Zr_0·15Ti_0·85O₃ (BCZT) ferroelectric ceramic located near the phase convergence region was prepared using the conventional solid-state solution. In this study, the systematic investigation of structural and microstructural properties via X-Ray Powder Diffraction (XRPD), scanning electron microscopy (SEM), temperature-dependence Raman spectroscopy and dielectric properties confirm the formation of a pure perovskite material which undergoes consecutive R3c-R3m, R3m-Amm2, Amm2-P4mm (F–F) phase transitions followed by (F-PC) transition in a wide temperature range centered around room temperature (RT). Polarization-Electric field (P-E) measurements, revealed the coexistence of Positive Electro-Caloric Effect (PECE) and Negative Electro-Caloric Effect (NECE) in a wide temperature span of [-40 °C, 95 °C]. More interestingly, we reached a large absolute temperature change (ΔT) and EC coefficient (ξ) values of (5.505 °C, 1.847 KmmkV^?1), (0.861 °C, 0.289 KmmkV^?1) and (1.897 °C, 0.637 Kmm kV^?1) registered respectively at about ?5, 42.5 and 74.5 °C under an electric field of 29.80 kVcm^?1. This work will promote further studies on lead-free BCZT ceramics towards multifunctional environmentally-friendly material promising for solid-state cooling technology.     

Kinetic study of ethylene polymerization by highly active silica supported TiCL4/MgCl2 catalysts

The kinetics of ethylene polymerization with TiCl₄/MgCl₂/SiO₂ has been investigated in the range of temperatures between 40 and 90°C and in the range of ethylene pressures between 4 and 12.4 kg/cm². The role of MgCl₂ was discussed from the dependence of the Mg/Ti ratio on the catalytic activity. The polymerzation rate was first order with respect to the monomer concentration and the dependence of the polymerization rate on the concentration of Al(C₂H₅)₃ could be described by the Langmuir–Hinshelwood mechanism. The dependence of initial rate and the time to reach the maximum polymerization rate on the concentration of Al(C₂H₅)₃ was also discussed. Polymerization rates as a function of the polymerization temperature showed a maximum and the activation energy was 11.8 kcal/mol between 50 and 80°C. The polymerization rate decreased with the increase of hydrogen partial pressure. The active site concentration (C^*) was 1.9 × 10^?2 mol/mol Ti by the inhibition method with carbon monoxide.     

Study on conformational transition phenomena of poly(methyl methacrylate) in acetonitrile near theta conditions

The coil–globule transition for poly(methyl methacrylate) (PMMA) has been studied in a theta solvent, acetonitrile (Θ = 45 °C). The viscosity of PMMA was measured as a function of temperature in the range 26–55 °C. The contraction and expansion of the molecular chains are determined using the measured viscosity values. The temperature dependence of the intrinsic viscosity can be represented by a master curve in a versus |τ|M _w^1/2 (g^1/2 mol^−1/2) plot, where τ = |T − Θ|/T is the reduced temperature and M_w‐is the weight‐average molecular weight. A universal plot of reduced viscosity versus reduced blob parameter (N/N_c) shows the attainment of the collapsed state below the theta temperature. The dimensions of PMMA in acetonitrile (M_w = 3.15 × 10⁶ g mol⁻¹) decrease to 63 % at 26 °C of those in the unperturbed state. The results in this work are compared with those previously published. © 2000 Society of Chemical Industry     

Effect of Ni substitute in off-stoichiometric Bi(Pb)-Sr-Ca-Cu(Ni)-O superconductor. Excess conductivity,XRD analysis and thermal behaviour

Two phases (2223 and 2212) are identified in Bi_1.8Pb_0.3Sr₂Ca₂(Cu_1-xNi_x)_3.3O_y superconductor system, sintered at 847?°C for 322?h, in partial nitrogen atmosphere. The volume fraction of 2223-phase is strongly dependent on Ni doping: 78.37% for x?=?0.002, 70.29% for x?=?0.005% and 51.13% for x?=?0.015. The unit cells of 2223 and 2212 phases were indexed as tetragonal structures, having different lattice constants. Plots of resistance versus temperature (four points method) on cooling to 77?K, evidenced that the critical temperature for the transition to the superconductor phase, T_c, is linearly decreasing from 106.21 to 93.47?K when the Ni content is varying from x?=?0.002 to x?=?0.03. From log-log plots of the excess-conductivity we calculated the cross-over temperatures between 3D and 2D dimensionality as well as from 2D to SWF (short wavelength fluctuation) behaviour, the coupling factor and the coherence length for all the samples. Thermal analysis of the resulting samples (after the last sintering) was performed by heating each sample from room temperature (RT) to 1000?°C at a rate of 10?K?min^?1 in dynamic air atmosphere (150?cm³ min^?1). A clear dependence on Ni content is seen by TG and DSC, but a relative thermal equilibrium between the two phases, 2223 and 2212, in RT-869?°C range, is observed. Strong endothermic effects (melting accompanied by small decomposition processes) begin at around 869?°C for all Ni doped samples. The results for the specific heat capacities, calculated from DSC plots, are also presented. Contribution of the crystal lattice to the estimated specific heat capacity was in conformity with the Einstein model, the Einstein temperature values being dependent on Ni content.     

Ellipsometry of silver electrodes in base solutions under different potential controlled perturbation conditions

The ellipsometric and reflectance response at 5461 Å of silver in 0.1 M NaOH at 25°C, is studied in the Ag(0)/Ag(I) potential range when the electrode has been subjected to different complex potentiodynamic perturbations. The optical response depends remarkably on the potential sweep rate, on the accumulated anodic charge and on the number of potential cycles. The data of the complex anodic film can be interpreted in terms of a single film model in two limiting cases defined in terms of the anodic charge involved as thin and thick films, respectively: (i) for thin anodic films (Q_a < 50 mC cm^?2) ?_nf - 1.50 –0.12 i and (ii) for thick anodic films (Q_a > 50 mC cm^?2)?_nf = 1.50 ? 0.22 i. Time dependence of the optical parameters of both reformed silver and anodic films are shown. Optical results correlates with electrochemical data recently reported and are discussed in terms of different degree of hydrations for each type of anodic film.     

Crazing and the stress dependence of creep in two glassy polymers: Polystyrene and a poly(styrene–acrylonitrile) copolymer

The stress dependences of crazing and of tensile creep were studied at 30.5° and 80°C in polystyrene (M _v = 2.7×10⁵) and in a poly(styrene–acrylonitrile) copolymer (73.5% styrene, M _v = 2.35×10⁵) at four stresses in the interval from 0.6 to 1.5×10⁸ dynes/cm². Material failure was observed in all cases for the polystyrene and in no cases for the copolymer. Crazing was found to occur at all stresses for polystyrene, the spacing between craze lines decreasing with increasing stress and temperature. A much higher stress level for the onset of crazing was found for the copolymer. An inverse stress dependence of the compliance was observed for polystyrene at 30.5°C, i.e., the compliance decreased with increasing stress. This behavior was partially reversed at 80°C below 10² sec and became a positive stress dependence at long times. The stress dependence of the compliance for the SAN copolymer was partially reversed at 30.5°C. At 80°C, the stress dependence was positive for stresses ≥0.9×10⁸ dynes/cm². The present results suggest that in the copolymer there may exist an enhanced local mobility which alters the stress dependence observed in pure polystyrene and which enhances the ability of the material to deform without failure. This concept is discussed further in light of the stress dependence of the compliance and of crazing in these materials and appears to be consistent with our previous studies of the stress dependence of creep and of the stress dependence of whitening in ABS systems.     

Cyclic voltammetry of monel 400 in lithium hydroxide solution at elevated temperatures

The electrochemistry of Monel 400 in 1 mole/kg^?1 LiOH solution at 25, 125 and 250°C has been investigated using the technique of cyclic voltammetry. The general electrochemical behaviour is found to most closely approximate to that of the major component, nickel, although expansion of the current scale reveals anodic and cathodic peaks which probably arise from redox processes involving copper. The general similarity to nickel can be rationalized in terms of either the d electron theory for cupronickel alloys or phase separation, the latter being favoured in the present study. At 25°C the majority of charge consumed on sweeping the potential in the positive direction is involved in the formation of an oxide film at potentials close to oxygen evolution. This process is no longer observed at 250°C, due to a sharp reduction in the oxygen evolution overpotential with temperature. The majority of charge consumed on cyclic sweeping at this temperature is attributed to active dissolution of the nickel component of the alloy to form HNiO^?₂ (or Ni(OH)^?₂ (or Ni(OH)^?₃) at potentials slightly positive to the hydrogen evolution region.     

Supersaturation of oxygen in acidic solution in the vicinity of an oxygen-evolving platinum anode

The concentration of O₂ in 1 M H₂SO₄ solution in the vicinity of the O₂-evolving smooth Pt anode was measured as a function of anodic cdi_a using the galvanostatic potential—transient method.The solution near the O₂-evolving anode was supersaturated with O₂. When the anodic current was interrupted, supersaturated concentration C^* decreased at a rate proportional to C^*  C₀, where C₀ is the solubility of O₂ in the electrolyte at 1 atm. The rate constant of the decay of the supersaturation under the open circuit condition was measured to be 0.069 sec^?1 at 25°C.At i_a < 40 mA/cm² there was a linear relation between log(C^*  C₀) and log i_a. At i_a > 200 mA/cm², however, the supersaturation exhibited a limiting value of 9.0 × 10^?2 mol/l.     

Electrospinning synthesis and negative thermal expansion of one-dimensional Sc2Mo3O12 nanofibers

Orthorhombic Sc₂(MoO₄)₃ nanofibers have been prepared by ethylene glycol assisted electrospinning method. The effects of annealing temperature, precursor concentration, spinning distance and solvent on the preparation of Sc₂(MoO₄)₃ nanofibers were characterized by XRD, SEM, HRTEM, EDX and high-temperature XRD. XRD analysis shows as-prepared nanofibers are amorphous. Orthorhombic Sc₂(MoO₄)₃ nanofibers can be fabricated after annealing at different temperatures in 500–800 °C for 2 h. The crystallinity of Sc₂(MoO₄)₃ nanofibers improves and the nanofiber diameter decreases gradually as the annealing temperature increases. However, the nanofiber structure was destroyed at the annealing temperature above 700 °C. Higher precursor concentration results in a slight increase of diameter and decrease in destroying temperature of Sc₂(MoO₄)₃ nanofibers. Spinning distance also affects the diameter of nanofibers, and the nanofiber diameter decreases as the distance increases. One-dimensional orthorhombic Sc₂(MoO₄)₃ nanofibers exhibit anisotropic negative thermal expansion. In 25–700 °C, the coefficients of thermal expansion (CTE) of α_a, α_b and α_c are ?5.81 × 10^?6 °C^?1, 4.80 × 10^?6 °C^?1 and -4.33 × 10^?6 °C^?1, and the α_l of Sc₂(MoO₄)₃ nanofibers is ?1.83 × 10^?6 °C^?1.     

Preparation and Oxygen Permeation Properties of BaCrOx Coated Ba0.5Sr0.5Co0.8Fe0.2O3- Tubular Membrane

Barium-chromium oxide (BaCrO_x) coated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) tubular membranes were successfully prepared and evaluated for oxygen separation applications under high pressure–temperature conditions. The oxygen permeation flux was measured in accordance with the temperature, air pressure, and retentate flow rate, ranging from 750–950°C, 3–9 atm, and 200–1000 mL/min, respectively. The permeation testing on the BaCrO_x coated BSCF tubular membranes showed that the oxygen flux increased as the temperature, pressure, and retentate flow rate increased. The oxygen permeation flux was 5.7 mL/(min cm²) with temperature, pressure, and retentate flow rate of 900°C, 9 atm, and 1000 mL/min, respectively. The temperature dependence of the oxygen permeation process is further investigated, and the Arrhenius pre-exponential factor, as well as the apparent activation energy, is determined.     

Evaluation of Fe and Mn co-doped layered perovskite PrBaCo2/3Fe2/3Mn1/2O5+δ as a novel cathode for intermediate-temperature solid-oxide fuel cell

A B-site cation-deficient double-perovskite oxide, PrBaCo_2/3Fe_2/3Mn_1/2O_5+δ (PBCFM2), was successfully synthesized by a sol-gel method and systematically investigated as an efficient cathode for IT-SOFC. The PBCFM2 exhibits good thermally stability and broad chemical compatibility at high temperature. Appropriate substitution of Mn and Fe for Co dramatically decreases the thermal expansion coefficient (TEC) from 21.5 × 10^–6 K^–1 for PrBaCo₂O_5+δ to 17.8 × 10^–6 K^–1 to PBCFM2 at a temperature range of 30–1000 °C. The temperature dependence of conductivity of the PBCFM2 was tested from 300 °C to 850 °C and then confirmed using the p-type small-polaron transport mode. The maximum conductivity value was 72 S cm^–1 at 600 °C. When using 300 µm of Sm_0.2Ce_0.8O_1.9 (SDC) as an electrolyte, the area specific resistance (ASR) and peak power density values were 0.028 Ω cm² and 588 mV cm^–2 at 800 °C, respectively. The activity and performance of the PBCFM2 cathode are further improved by impregnation with nano-sized SDC particles. The composite cathode with two times impregnation provided the optimal nano-scale SDC loading and microstructure where the ASR and peak power densities were 0.023 Ω cm² and 621 mV cm^–2 at 800 °C, respectively. Our preliminary results lead us to propose that PBCFM and its composite cathodes are good candidate cathodes for IT-SOFC.     

Synthesis,thermal degradation and dielectric properties of poly[2-hydroxy,3-(1-naphthyloxy)propyl methacrylate]

2-Hydroxy-3-(1-naphthyloxy)propyl methacrylate (NOPMA) monomer was synthesized from reaction of 2-[(2-naphthyloxy)methyl]oxirane with methacrylic acid in the presence of pyridine. The polymerization of NOPMA was carried out by free radical polymerization method in the presence of AIBN at 60 °C. The structure of monomer and polymer was characterized by ¹H-NMR, ¹³C-NMR and FT-IR spectroscopy techniques. The glass transition temperature and average-molecular weights of poly(NOPMA) were measured using differential scanning calorimetry and gel permeation chromatography, respectively. The thermal degradation behavior of poly(NOPMA) has been investigated by FT-IR studies of the partially degraded polymer and thermogravimetry. The cold ring fractions (CRFs) were collected at two different temperatures, initially fraction-1 (CRF₁) is from room temperature to 320 °C, and the other fraction-2 (CRF₂) is from 320 to 500 °C. The volatile products of the degradation were trapped at ?195 °C (in liquid nitrogen). All the fractions were characterized by FT-IR, ¹H and ¹³C-NMR spectroscopic techniques, and the cold ring fractions (CRFs) were also characterized by GC–MS. For the degradation of polymer, the major compound between products of CRFs is α-naphthol. The GC–MS, FT-IR and NMR data showed that depolymerization corresponding to monomer was not prominent below 320 °C in the thermal degradation of poly(NOPMA). The mode of thermal degradation containing formation of the major products was identified. The dielectric permittivity (ε′), the loss factor (ε″) and conductivity (σ_ac) were measured using a dielectric analyzer in the frequency range of 50 Hz to 20 kHz.     

Syndiospecific styrene polymerization by (tert-BuC5H4)TiCl2(O-2,6-iPr2C6H3) – borate catalyst system

(^tBuC₅H₄)TiCl₂(O-2,6-ⁱPr₂C₆H₃) (2) exhibited relatively high catalytic activity for syndiospecific polymerization of styrene at 25 °C if both [PhMe₂NH]B(C₆F₅)₄ and a mixture of AlⁱBu₃/Al(n-C₈H₁₇)₃ were used as the cocatalyst. Effects of both organoaluminum and organoboron compounds were explored, and the effect of cocatalyst was different from that observed in 1-hexene polymerization catalyzed by Cp*TiCl₂(O-2,6-ⁱPr₂C₆H₃) (1). Resultant syndiotactic polystyrene possessed narrow molecular weight distribution under the optimized conditions, and the M_w values were unchanged during the time course.