Kinetics of hydrogen evolution on submicron size Co, Ni, Pd and Co–Ni alloy powder electrodes by d.c. polarization and a.c. impedance studies

Submicron size Co, Ni and Co–Ni alloy powders have been synthesized by the polyol method using the corresponding metal malonates and Pd powder by reduction of PdO _x in methanol. The kinetics of the hydrogen evolution reaction (HER) in 6 M KOH electrolyte have been studied on electrodes made from the pressed powders. The d.c. polarization measurements have resulted in a value close to 120 mV decade^–1 for the Tafel slope, suggesting that the HER follows the Volmer–Heyrovsky mechanism. The values of exchange current density (i _o) are in the range 1–10 mA cm^–2 for electrodes fabricated in the study. The a.c. impedance spectra measured at several potentials in the HER region showed a single semicircle in the Nyquist plots. Exchange current density (i _o) and energy transfer coefficient () have been calculated by employing a nonlinear least square-fitting program.     

Admittance—Frequency Response in Zinc Oxide Varistor Ceramics

The lumped parameter/complex plane analysis technique revealed several contributions to the terminal admittance of the ZnO—Bi₂O₃ based varistor grain-boundary ac response. The terminal capacitance has been elucidated via the multiple trapping phenomena, a barrier layer polarization, and a resonance effect in the frequency range 10⁻²≤ f ≤ 10⁹ Hz. The characterization of the trapping relaxation behavior near ∼ 10⁵ Hz (∼ 10⁻⁶ s) provided a better understanding of a previously reported loss-peak. The possible nonuniformity in this trapping activity associated with its conductance term observed via the depression angle of a semicircular relaxation in the complex capacitance ( C *) plane has been postulated.     

偏振调制微波光子信号处理

微波光子信号处理具有带宽大、调谐性好、对电磁干扰不敏感、易于实现并行系统等优点,在过去数十年中受到了学术界的重视。相比于传统的电光相位调制和电光强度调制,偏振调制将待调制信息转换到光的一个二维参量（偏振）上,从而在实现信号处理时可提供更多的自由度和灵活性。在偏振调制器后连接一个检偏器,通过调节检偏器的检偏角去掉偏振调制的一个维度,即可实现相位调制和强度调制,因而任何基于相位调制和强度调制的微波光子信号处理都能基于偏振调制实现。同时,将偏振调制器的输出分成多路,每一个支路上都插入一个检偏器,则基于一个偏振调制器就可同时实现多个调制方式,从而进一步实现并行信号处理或多功能一体信号处理。此外基于单边带偏振调制,可利用非常简单的装置实现幅相无耦合的微波移相器,进而实现相位编码、复抽头系数滤波器、光控相控阵等信号产生或处理功能。本文建立了偏振调制的基本数学模型,并对基于偏振调制的多种微波光子信号处理功能进行了原理分析。     

Experimental Evaluation of the Acceptor-States Compensation in Positive-Temperature-Coefficient-Type Barium Titanate

Experimental evidence shows that the acceptor-state levels in Sb-doped positive-temperature-coefficient-type BaTiO₃ are compensated up to a critical acceptor-state density. Using the slope of the natural logarithm of the resistivity with respect to 1/ T , instead of maximum resistivity as a measure for the acceptor-state density, it is possible to estimate this critical value. The value obtained (4.2 × 10¹⁷ m⁻²) is believed to be the first reported estimate based on experimental data. It is in good agreement with the estimate of 6 × 10¹⁷ m⁻² (first reported by Jonker) obtained from the spontaneous polarization of BaTiO₃. This shows that the ferroelectric behavior of BaTiO₃ is indeed a feasible explanation for the low resistivity below the Curie point, as proposed by Jonker.     

十二烷基－二甲基（2－羟基）乙基氯化铵系列季铵盐的缓蚀性能

通过失重法和极化曲线、线性极化两种电化学方法，研究了十二烷基二甲基（２－羟基）乙基氯化铵（１２２）、十四烷基二甲基（２－羟基）乙基氯化铵（１４２）和十六烷基二甲基（２－羟基）乙基氯化铵（１６２）对２０＃碳钢在５％盐酸中的缓蚀作用。并与新洁尔灭（十二烷基二甲基苄基溴化铵）进行了比较。结果表明，它们在酸性介质中具有很好的缓蚀作用。     

Mathematical modeling of reverse osmosis systems

The present investigation pertains to modeling of seawater desalination system. A simulation model was developed and verified for a small-scale reverse osmosis system. The proposed model combines material balances on the feed tank, membrane module andproduct tank with membrane mass transfer models. Finally a comprehensive simulation model has been developed incorporating the effect of mass transfer inhibition The model is non-linear differential equation representing the feed concentration as a function of operating time and space. The solution of the simultaneous differential equations was obtained using the fourth order Runge-Kutta method, due to self starting and stability. The model was verified using the experimental data from the literature [17,24]. Parameter sensitivity was carried out to select the proper step size. The simulation was run for over 1000 11 enabling a prediction of operational performance at high overall system recoveries.     

Electrochemical performance of gel-cast NiO-SDC composite anodes in low-temperature SOFCs

NiO-Ce_0.8Sm_0.2O_1.9 (SDC) composites were synthesized using gel-casting technique. The electrochemical performance of the gel-cast (GC) Ni-SDC cermet as anode was investigated contrast with that fabricated from traditional mechanical mixing (MM) technique using fuel cells with about 35 μm-thick SDC electrolyte and Sm_0.5Sr_0.5CoO₃-SDC cathode. Maximum power density of the cell with GC anode achieved 491 mW cm⁻² at 600 °C, over 100 mW cm⁻² larger than that with MM anode, inferring high catalytic activity of the GC anode. Impedance measurements on the fuel cell at open circuit showed that the anodic interfacial polarization resistance of the GC anode was 0.1 Ω cm² lower than that of the MM anode. Long-term stability of the cell with GC anode in hydrogen was also performed, which showed that it can stabilize at least 7 days.     

Performance of Methanol Oxidation Catalysts with Varying Pt:Ru Ratio as a Function of Temperature

This paper describes the effects of varying the Pt to Ru ratio in carbon-supported catalysts for methanol oxidation as a function of temperature. Previously these effects were studied in isolation, but now it is shown that the composition of a given catalyst as a function of temperature is extremely important for its activity towards methanol oxidation. Platinum rich 3:2 atomic ratio catalysts perform better than a 1:1 catalyst at 25 °C, where only Pt is believed to be active towards methanol dehydrogenation, since this process is a highly thermally activated process on Ru sites. This result is reversed at 65 °C, where the 1:1 catalyst displays much higher currents across the entire range of polarization. This may result from methanol dehydrogenation occurring on both Ru and Pt sites at higher temperatures. At an intermediate temperature, 45 °C, the 3:2 catalyst is seen to perform better at lower current values, while the 1:1 catalyst is superior at higher current densities, with the crossover occurring at 62 A g^–1. As a consequence, when designing fuel cell catalysts, the composition of the catalyst employed should be tailored with respect to the exact operating conditions, in order to promote optimum fuel cell performance.     

Dielectric behavior of PVDF/POMA blends that have a low doped POMA content

The real (ε′) and imaginary (ε″) components of the complex permittivity of blends of PVDF [poly(vinylidene fluoride)] with POMA [poly(o‐methoxyaniline)] doped with toluenosulfonic acid (TSA) containing 1, 2.5, and 5 wt % POMA–TSA were determined in the frequency interval between 10² and 3 × 10⁶ Hz and in the temperature range from ?120 up to 120°C. It was observed that the values of ε′ and ε″ had a greater increase with the POMA–TSA content and with a temperature in the region of frequencies below 10 kHz. This effect decreased with frequency and it was attributed to interfacial polarization. This polarization was caused by the blend heterogeneity, formed by conductive POMA–TSA agglomerates dispersed in an insulating matrix of PVDF. The equation of Maxwell–Garnett, modified by Cohen, was used to evaluate the permittivity and conductivity behavior of POMA–TSA in the blends. A strong decrease was observed in POMA–TSA conductivity in the blend, which was bigger the lower the POMA–TSA content in the blend. This decrease could have been caused either by the POMA dedoping during the blend preparation process or by its dispersion into the insulating matrix. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 752–758, 2003