Study of adsorption of oxygen on β-Al2O3 + Au and β-Al2O3 + Pt: Work function measurements—proposition of a model

To improve the understanding of the electrochemical effects observed on an original potentiometric gas sensor, interactions of oxygen with the device were investigated. This gas sensor is made of a solid electrolyte (treated Na-β-alumina) associated with two metallic electrodes (gold and platinum) located in the same gas mixture. Adsorption of charged oxygen species, considered responsible for the electrical response developed by the sensor, was investigated by work function measurements. Results showed that charged oxygen species only form on partially gold or platinum covered solid electrolyte. Comparison of these results with those obtained in a previous calorimetric study of interactions between oxygen and the same materials suggests the existence of at least two different oxygen species adsorbed on the surface of the sensitive element. The first one, located on the solid electrolyte surface, is neutral and characterized by an endothermal reaction of formation. The second one is charged and probably produced at the gas/solid electrolyte/metallic electrode interface. A mechanism based on the concept of “three phase boundary” and similar to the “reverse spillover” phenomenon is proposed to account for the adsorption of these oxygen species.     

High temperature water electrolysis: The cathodic process at the cermet (Pt+La0.8Sr0.2CrO3)/Zirconia interface

The kinetics of the reduction of steam at the interface of a platinum containing cermet with Yttria Stabilized Zirconia (YSZ) electrolyte at high temperature has been investigated. The presence of an outer porous platinum layer deposited onto the underlying cermet layer results in a remarkable increase of the kinetic parameters of the overall reaction. However, the presence of metallic platinum embedded in the structure of the mixed oxide layer, restricting the section available for the diffusion of the reducible species, results in a worsening of the electrochemical kinetics.A comparison with other previously investigated electrodes has been performed and some hypotheses on the rate determining mechanism have been suggested.     

Prethermalization

Prethermalization of the equation of state and the kinetic temperature to their equilibrium values occurs on time scales dramatically shorter than the thermal equilibration time. This is a crucial ingredient for the understanding of collisions of heavy nuclei or other nonequilibrium phenomena in complex quantum and classical many body systems. We also compare the chemical equilibration time with other characteristic time scales.     

Computer simulation of ion images as an aid to indexing small crystallites

The magnitude of the sticking coefficient S of H₂ on polycrystalline platinum and its variation with temperature and coverage are compared with the exchange and equilibration f H₂ and D₂ under similar experimental conditions. The sticking coefficient measurements indicate the presence of at least two adsorbed states, a weakly bound precursor state and a more strongly bound state. The high temperature equilibration reaction is limited by the rate of adsorption into the precursor state. The low temperature exchange rate is limited by desorption from the strongly bound state and a model involving desorption from an immobile activated complex gives reasonable agreement with experiment. The heats of adsorption were determined at the coverage of interest. The low temperature equilibration reaction probably proceeds via mixing of H₂ and D₂ in the precursor state.     

固定化发光铂络合物的荧光特性

详细研究了固定化发光铂络合物的荧光特性。根据氧对金属铂络合物发射光的动态猝灭原理及高分子材料的非均一性特征，建立了荧光猝灭动力学模型，并推导出了一个定量描述氧对有机金属铂络合物发射光猝灭作用的通用表达式。该表达式克服了Ｓｔｅｒｎ－Ｖｏｌｍｅｒ方程容易产生负偏差的不足，并能很好地说明固定化发光铂络合物的荧光猝灭机理。     

Improvement of ammonia sensing properties of poly(pyrrole)–poly (n-methylpyrrole) composite by ion irradiation

In the present investigation we have electrochemically synthesized polypyrrole–poly (n-methylpyrrole) composite film with optimized process parameters (viz. concentration of monomers and dopant, applied current density, deposition time, pH of electrolyte etc.) on platinum substrate. The composite film of polypyrrole–poly (n-methylpyrrole) was subjected to electrical, spectral and morphological characterizations and its sensing response to various concentration of ammonia was also studied. Later, the synthesized composite films were irradiated under high vacuum (∼5×10⁻⁶ Torr) at room temperature with 85 MeV O⁷⁺ ion beam at various fluences from 1×10⁵ to 1×10⁷ ions/cm². We have observed remarkable improvements in electrical and morphological properties suitable for gas-sensing applications. The irradiated composite film was evaluated for the sensing of various concentrations of ammonia and excellent improvement in terms of sensitivity, lower detection limit and response time was observed.     

Study of solid electrolyte polarization by a complex admittance method

The polarization behavior of zirconia-yttria solid electrolyte specimens with platinum electrodes has been studied over a temperature range of 400° to 800°C and a wide range of oxygen partial pressures. The complex admittance of these specimens was determined over a frequency range from d.c. to 100kHz. An analysis of these data in the complex admittance plane indicated the presence of three polarizations: (1) an electrode polarization characterized by a double layer capacity and an effective resistance for the overall electrode reaction, O₂(gas) + 2e(platinum) O²⁻ (electrolyte); (2) a capacitive-resistive electrolyte polarization, probably corresponding to a partial blocking of oxygen ions at the electrolyte grain boundaries by an impurity phase there; and (3) a pure ohmic electrolyte polarization.     

Electrode reactions at oxygen,noble metal / stabilized zirconia interfaces

The polarization behaviour of electrodes of the type “oxygen, noble metal / stabilized zirconia”, comprising different zirconia-based materials as electrolyte, platinum or gold as metal component and an oxygen containing gas atmosphere, was investigated at elevated temperatures under equilibrium and non-equilibrium conditions by means of impedance spectroscopy. Massive metal contacts were used as part of the working electrodes. Under non-polarized conditions, the experimental results for platinum indicate a basically uniform reaction mechanism in a vast range of temperature and oxygen partial pressure, involving the surface diffusion of dissociatively adsorbed oxygen on platinum towards the electrochemical reaction sites on the electrolyte surface as rate-determining step. The experimental findings for gold are consistent with the occurrence of two competing reaction mechanisms, namely a charge transfer controlled process and a surface diffusion controlled process, each of them prevailing in different regimes of temperature and oxygen partial pressure. Under polarized conditions, a significant decrease of the polarization resistance takes place, followed by the onset of low frequency loops in the impedance spectra. In the case of cathodic polarization, the onset voltage can be correlated with the partial electron conductivity of the electrolyte, thus confirming the hypothesis of direct participation of electronic species of the electrolyte in the electrode reaction under biased conditions. At moderate temperatures, the polarization induced changes in the electrode properties exhibit a slow relaxation behaviour. This can be attributed to the successive annihilation of additional metastable electrochemical reaction sites having been created during the preceding polarization treatment. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy     

Improvement in performance of a hyoscine butylbromide potentiometric sensor using a new nanocomposite carbon paste: a comparison study with polymeric membrane sensor

Multiwalled carbon nanotubes and TiO₂ nanoparticles were used for the improvement of a hyoscine butylbromide (HYBB) potentiometric carbon paste electrode response. The electrode was based on the ammonium reineckate-hyoscine ion association complex as an ion-exchange site dissolved in 2-nitrophenyloctyl ether as pasting liquid. The characteristics of the suggested potentiometric nanocomposite-modified HYBB carbon paste electrode in comparison with the PVC membrane sensor show better responses in terms of sensitivity, dynamic concentration range, detection limit, Nernstain slope, low potential drift, response time, lifetime, and mechanical stability. The electrode exhibits good selectivity for HYB ion with respect to a large number of cations. For investigation of the effects of carbon nanotubes and TiO₂ nanoparticles on the conductivity property of the electrode surface, electrochemical impedance spectroscopy measurements were done. The prepared electrode was successfully applied to the determination of hyoscine butylbromide in pharmaceutical formulations and biological samples.     

表面活性剂增敏阻抑反应动力学光度法测定痕量铂

研究了在硫酸介质中 ,在乳化剂 OP存在下 ,痕量铂 ( )阻抑溴酸钾氧化酸性品红褪色的新指示反应 ,建立了表面活性剂增敏阻抑反应动力学光度法测定痕量铂新方法。最大吸收波长为 5 44 nm。铂浓度在 0～ 2 .2 μg· L- 1范围内与ΔA呈线性关系 ,检测限为 0 .0 6 μg· L- 1。方法灵敏度高、选择性好、操作简便 ,用于抗癌药物顺铂及患者血清中铂含量的测定 ,取得较好的结果。     

Anomalous scaling in surface roughness evaluation of electrodeposited nanocrystalline Pt thin films

Atomic force microscopy (AFM) is used to measure the surface roughness of crystalline Pt thin films as a function of film thickness and growth rate. Our films were electrodeposited on Au/Cr/glass substrates, under galvanostatic control (constant current density), from a single electrolyte containing Pt⁴⁺ ions. Crystalline structure of the films was confirmed by X-ray diffraction (XRD) technique. The effect of growth rate (deposition current density) and film thickness (deposition time) on the kinetic roughening of the films were studied using AFM and roughness calculation. The data is consistent with a rather complex behaviour known as “anomalous scaling” where both local and large scale roughnesses show power law dependence on the film thickness.     

Electrochemical promotion of a metal catalyst supported on a mixed-ionic conductor

It has been found that the catalytic activity and selectivity of a metal film deposited on a solid electrolyte could be enhanced dramatically and in a reversible way by applying an electrical current or potential between the metal catalyst and the counter electrode (also deposited on the electrolyte). This phenomenon is know as NEMCA [S. Bebelis, C.G. Vayenas, Journal of Catalysis, 118 (1989) 125–146.] or electrochemical promotion (EP) [J. Prichard, Nature, 343 (1990) 592.] of catalysis.Yttria-doped barium zirconate, BaZr_0.9Y_0.1O_3 − α (BZY), a known proton conductor, has been used in this study. It has been reported that proton conducting perovskites can, under the appropriate conditions, act also as oxide ion conductors. In mixed conducting systems the mechanism of conduction depends upon the gas atmosphere that to which the material is exposed. Therefore, the use of a mixed ionic (oxide ion and proton) conducting membrane as a support for a platinum catalyst may facilitate the tuning of the promotional behaviour of the catalyst by allowing the control of the conduction mechanism of the electrolyte. The conductivity of BZY under different atmospheres was measured and the presence of oxide ion conduction under the appropriate conditions was confirmed. Moreover, kinetic experiments on ethylene oxidation corroborated the findings from the conductivity measurements showing that the use of a mixed ionic conductor allows for the tuning of the reaction rate.     

Heat capacity of isolated clusters

The character of interaction between thermal (vibrational) and configurational cluster excitations is considered under adiabatic conditions when a cluster is a member of a microcanonical ensemble. The hierarchy of equilibration times determines the character of atomic equilibrium in the cluster. The behavior of atoms in the cluster can be characterized by two effective (mean) temperatures, corresponding to the solid and liquid aggregate states, because the typical time for equilibration of atomic motion is less than the transition time between aggregate states. If the cluster is considered for a time much longer than the typical dwell time in either phase, then it is convenient to characterize the system by only one temperature, which is determined from the statistical-thermodynamic long-time average. These three temperatures are not far apart, nor are the cluster heat capacities evaluated on the basis of these definitions of temperature. The heat capacity of a microcanonical ensemble may be negative for two coexisting phases if the mean temperature is defined in terms of the mean kinetic energy, rather than as the derivative of energy with respect to microcanonical entropy. However, if the configurational excitation energy is smaller than the total excitation energy separating the phases, then the two-state model predicts a positive heat capacity under either definition of temperature. Moreover, if the cluster is sufficiently large, then the maximum values of the microcanonical and canonical heat capacities are equal.     

用单色谱仪技术研究苯快速反应中碳生成的微观机理

利用单色谱仪、压力传感器和示波器等组成的单色谱瞬态测试系统研究了苯的快速反应光谱及点火特性.由于苯在冲击波的作用下极易发生高温分解,介绍了一种在入射激波条件下确定含能材料冲击点火延迟时间和防止提前触发的新方法。运用反应动力学程序模拟出主要生成物(C2)浓度随时间的变化关系,结合谱仪记录的该生成物随时间变化的信号曲线,对两条曲线的起跳点,最大波峰进行比较.模拟结果与测定的出现时间符合得较好,证明该单色谱瞬态测试系统能够较好的研究冲击发射光谱的时间分辨特性.把光谱实验和化学反应动力学相结合分析了苯在快速反应过程中碳的生成机理。     

Preparation of nano-structured Pt-YSZ composite and its application in oxygen potentiometric sensor

Nano-structured Platinum-Yttria Stabilized Zirconia (YSZ) composites for oxygen potentiometric sensors were directly prepared with carbon black and the precursors such as chloroplatinic acid, zirconyl nitrate and yttrium nitrate. The as-prepared Pt-YSZ composite consisted of cubic crystalline YSZ and Pt particles, and the particle sizes of Pt catalyst and YSZ electrolyte were about 25-35 and 5-10 nm, respectively. The Pt-YSZ composite electrodes exhibited excellent electrochemical performances when evaluated by EIS measurements. The introduction of the nano-structured Pt-YSZ composite into the oxygen potentiometric sensor can reduce sensor's operating temperature to be about 380 °C, and also can reduce sensor's response time to be about 5 s at 400 °C. The oxygen potentiometric sensors incorporating nano-structured Pt-YSZ composites exhibited longer lifetime than those employing pure Pt as the sensing electrodes.     

Nanoisland shape relaxation mechanism

The shape relaxation of supported crystallites is studied by means of kinetic Monte Carlo simulations, initialising the system with different configurations. At low temperature, when the facet nucleation limits the relaxation, the simulations show that the equilibration mechanism and the equilibration time scaling laws depend strongly on the initialisation. In this regime, the adhesion strongly increases the stability of intermediate configurations with large contact area. The relationship between the different equilibration pathways and the equilibration scaling laws is discussed considering the dependence of the nucleation barrier energy G^∗ on the particle energetics in the regions with the largest kinks and steps density.     

High-frequency thermal processes in harmonic crystals

We consider two high frequency thermal processes in uniformly heated harmonic crystals relaxing towards equilibrium: (i) equilibration of kinetic and potential energies and (ii) redistribution of energy among spatial directions. Equation describing these processes with deterministic initial conditions is derived. Solution of the equation shows that characteristic time of these processes is of the order of ten periods of atomic vibrations. After that time the system practically reaches the stationary state. It is shown analytically that in harmonic crystals temperature tensor is not isotropic even in the stationary state. As an example, harmonic triangular lattice is considered. Simple formula relating the stationary value of the temperature tensor and initial conditions is derived. The function describing equilibration of kinetic and potential energies is obtained. It is shown that the difference between the energies (Lagrangian) oscillates around zero. Amplitude of these oscillations decays inversely proportional to time. Analytical results are in a good agreement with numerical simulations.     

基于大型激光装置的材料塑性变形行为微介观研究进展

微介观尺度下材料结构的实时演化行为是决定其动态力学性能的关键因素。大型激光装置作为集加载和诊断能力为一体的综合实验平台,为高温、高压、高应变率等极端条件下材料动态力学性能的微介观尺度研究提供了重要支撑。随着激光功率密度和脉冲整形能力的不断提升,实验所能探索的压力（101～103 GPa）及应变率（106～1010 s?1）范围不断突破;而利用激光打靶产生的高亮X射线脉冲作为探测源,建立动态衍射和成像技术,可以实现高空间和时间分辨率下材料塑性变形机制的实时原位研究。简要介绍了基于大型激光装置的原位微介观实验技术及其在材料塑性变形行为研究中的应用,系统梳理了近二十年来具有代表性的研究成果,阐明了相关研究对推动材料动态响应多尺度物理建模的重要价值。     

Evaluation of solid electrolyte cells with a versatile electrochemical technique

Voltage losses in fuel cells and other solid electrolyte systems are due to several mass transport and kinetics processes at the electrode/electrolyte interface as well as to ohmic contributions from the electrolyte, electrodes, current collectors and contact resistances. Electrochemical impedance spectroscopy (EIS) has been in use for several decades in fuel cell research and is quite effective in determining the contribution of individual electrode and electrolyte processes. However, data acquisition and analysis can be time-consuming and the technique has many limitations whilst cell performance and operating conditions are varying rapidly with time especially when the cells are under current load. The galvanostatic current interruption (GCI) technique is fast and can be used under a wide range of operating as well as for rapidly varying loads and cell performance conditions. In this paper a totally new and very simple way of adapting commercially available equipment has been described to perform high quality, reliable and fast GCI measurements over a range of different currents in one sequence without having to use an electronic switch or a solid state relay or a separate fast data logging system. Its versatility has been demonstrated with a number of standard RC circuits simulating slow electrode and fast electrolyte processes and by evaluating a number of solid oxide fuel cell materials. The GCI technique has been shown to be able to determine the composition of all standard test circuits within ±1 % of those determined from the EIS technique and actual values of circuit components. The technique has been applied to investigating solid electrolyte cells and produced excellent results.     

High temperature electrochemical reduction of the water molecule at cerium dioxide electrodes

The high temperature cathodic reduction of the water molecule at the interface PtCeO_2?x Double Layer Electrode/Yttria Stabilized Zirconia has been investigated. Two different rate determining processes are involved depending on the current density. In the low current density range a Tafel's type kinetic behaviour was found, with an activation free enthalpy around 22 kcal/mole. The rate determining process has been identified in the transport of the oxygen atoms through the CeO_2?x layer. At higher currents a lack of any apparent additional, nonohmic contribution to the overvoltage was put into evidence.At any rate, such DLE resulted in a worsening of the energetic yield of the cathodic process with respect to the porous platinum/YSZ as well as Pt/cerium doped YSZ.