电化学双石英晶体阻抗分析新方法

电化学石英晶体微天平已被广泛应用于电分析化学与电化学领域[1] .电化学现场的石英晶体阻抗分析法能动态实时地提供电化学扰动下石英晶体谐振的多维信息 ,已用于研究电沉积与溶出[2 ,3] 、高聚物性质[4～ 6 ] 、耗尽层内溶液粘密度效应与金电极腐蚀[7,8] 等 .然而 ,以往的石英晶体阻抗分析均只涉及到一个石英晶体 ,同时对两个晶体 (分别作为工作电极和对电极 )谐振的阻抗分析未见报道 .本文以蔗糖溶液粘密度效应和氨性介质中铜沉积 /溶出过程为例 ,研究电化学双石英晶体阻抗分析新方法 .1　实验部分电化学双石英晶体阻抗分析系统由 HP43 9…     

Applicability and power of complex nonlinear least squares for the analysis of impedance and admittance data

An analysis is made of the ability of nonlinear complex least squares fitting to yield useful estimates of parameter values occurring in equivalent circuits. In addition, three-dimensional perspective plots are presented which show the full frequency response of either impedance or admittance data. Three different kinds of data are studied using these two methods.For a given set of four circuit elements (two resistors and two capacitors) artificially computer-generated impedance and admittance values are constructed for a Voigt circuit having two time constants. These exact results are then contaminated either by digit round-off or addition of zero mean, normally distributed random noise. A complex least squares analysis is then made on the contaminated data and the resulting estimates of the circuit parameters and their associated standard errors are compared to the exact starting values. For the case when one of the capacitors is vanishing small, the optimal range of measurement over the resulting semi-circle in the complex impdance plane is discussed in detail. For this case the accuracy of input data required to resolve the values of the two resistors when they are separated in size by two orders of magnitude is determined. The other extreme of two time constant which are not well separated is also studied. Again one obtains approximate estimates of both the range of frequencies and in the inherent accuracy of the data necessary to adequate resolve all four circuit parameters.The actual frequency dependent admittance of real ladder network of resistors and capacitors having three different time constant was measured on a Solartron type 1172 response analyzer. These data were then analyzed using the complex least squares procedure and the fitted circuit parameters were all in close agreement with their nominal experimental values.Finally, the admittance response of β-PbF₂ at 474 K was measured on the Solartron 1172. A complex least squares analysis of these data yields a good fit when an equivalent circuit employing a constant phase element is used. A three-dimensional perspective plot showing very clearly the agreement between fitted and measured values of the impedance is shown.     

从头计算中Fm(z)积分系列计算的最优方案——向上和向下结合的递推法

本文阐明了量子化学从头计算中的Gauss积分函数Fm(z)用递推公式计算时误差传递和变化的定量规律.提出了向上和向下递推相结合进行Fm(z)系列计算的最优方案,它保证绝对误差在递推过程中只减不增,对递推基项无附加的精度要求,可用于改进各种Fm(z)程序.本法与Schaad公式相结合,用于P、d、f和g型Gauss函数的计算,可分别节约加法运算40％以上和乘法、指数运算60%以上.     

Phasor transform to extract glucose and ascorbic acid data in an amperometric sensor

A method for separating the signals from glucose and ascorbic acid on a single recognition surface using an ac immittance technique is presented. It is proposed that each oxidation process can be represented by a unique vector based on psi and YO, and that the concentration of each analyte can be determined by monitoring the change in the admittance magnitude in the direction of the characteristic angle for that particular species. The total Faradaic admittance (YF,total) for all electroactive species present is given by a linear combination of the independent vectors from the different species. In the system tested, the analytes are glucose and ascorbic acid, the former being estimated via the measurand, hydrogen peroxide. Thus, one of the electroactive species (hydrogen peroxide) is not a bulk solution species, but is 'generated' in the enzyme matrix. The admittance measurements from ascorbic acid and the enzyme-generated hydrogen peroxide showed the characteristic phase angles of each oxidation signal, allowing for good spatial resolution. The behaviour of each of these analytes is presented and calibration curves tested. Based on the calibration curves and the basis vectors, samples containing both glucose and ascorbic acid were measured by transforming the measured total admittance from the complex Cartesian space into 'analyte space', where the X-Y axes are given by the basis vectors ?EGHP,GOD and ?AA,GOD, respectively.     

Continuous-time random-walk theory of interfering diffusion and chemical reaction with an application to electrochemical impedance spectra of oxidized Zr-1%Nb

A microscopic theory is developed for the interplay of diffusion and chemical reaction and the results are compared with electrode impedance measurements on an oxide electrode. The theory is based on the ideas of continuous-time random walk and accounts for the interference of diffusion and recombination of the charge carriers in the oxide. The treatment results in a dispersive diffusivity with two time constants, one of them corresponding to the random walk, the other to the reaction. Combining this diffusivity with the Warburg electrode admittance expression, which refers to cases where the rate-limiting step is diffusion in a semi-infinite medium bounded by a plane, an admittance function is obtained. The phase angle is found to be higher than 45 degrees distinguishing it from the Gerischer impedance which was developed for a related problem. The oxides were produced by hydrothermal oxidation of Zr-l%Nb alloy, a metal used as cladding material for nuclear fuel elements. The electrode impedance spectra of Zr/Zr-oxide electrodes in aqueous SO(3) (2-) solutions were taken at various anodic voltages between 1 Hz and 100 kHz and temperatures between 278 and 333 K. The theoretical admittance functions could be successfully compared with the observed spectra. Both the functional forms and the fitted parameter values support our theory which is also in keeping with Macdonald's point-defect model.     

Admittance as a useful tool for investigation of solute–solvent interactions at the double layer

The application of admittance measurements, an often neglected topic in impedance spectroscopy, to further our understanding of solute–solvent interactions at the double layer is presented by comparing aqueous solutions of sodium acetate and protonated lidocaine acetate. Unlike impedance, the overlap of concentration-dependent hydration cospheres and the potential- and frequency-dependent orientation effects of water molecules around the ions at or near the double layer could be manifested in admittance data. Admittance, but not impedance, also allows identification of interactions between two dipoles, such as amino and carbonyl groups, by changing frequencies and applied potentials. The effects manifest themselves by exhibiting cathodic and anodic shifts with frequency changes. All the admittance data could be reasonably explained by utilizing the concept of “potential induced and water structure-enforced ion-pair formation”.     

Neural networks for Nyquist plots prediction during corrosion inhibition of a pipeline steel

This paper presents a predictive model for electrochemical impedance Nyquist plots using artificial neural network. The proposed model obtains predictions of imaginary impedance based on the real part of the impedance as a function of time. The model takes into account the variations of the real impedance and immersion time of steel in a corrosive environment, considering constant carboxyamido-imidazoline corrosion inhibitor concentrations (5 and 25 ppm). For the network, the Levenberg–Marquardt learning algorithm, the hyperbolic tangent sigmoid transfer function, and the linear transfer function were used. The best-fitting training data set was obtained with five neurons in the hidden layer for 5 ppm of inhibitor and two neurons in the hidden layer for 25 ppm of inhibitor, which made it possible to predict the efficiency with accuracy at least as good as that of the theoretical error, over the whole theoretical range. On the validation data set, simulations and theoretical data test were in good agreement with an R value of 0.984 for 5 ppm and 0.994 for 25 ppm of inhibitor. The developed model can be used for the prediction of the real and imaginary parts of the impedance as a function of time for short simulation times.     

Small-signal ac response theory for systems exhibiting diffusion and trapping of an electroactive species

A theoretical expression is derived for the interfacial admittance of an electrochemical system in which a product species diffuses into (or along internal surfaces of) an electrode subject to reversible trapping at one or more types of stationary sites in the electrode. The admittance obtained in a generalization of earlier results for diffusion without trapping and is exactly representable by an equivalent electrical circuit involving a lossy transmission line. Representative numerical data are given for the impedance of this distributed element. Trapping effects are found to be significant primarily when the number of trapped atoms is comparable to the number of mobile atoms. The results obtained may therefore be of greatest interest in the case of polycrystalline or amorphous electrodes.     

Diffusion processes in silver-conducting solid electrolyte in terms of the Grafov-Ukshe model of adsorption relaxation of double layer

The results of impedance studies of an interface formed by solid electrolyte RbAg₄I₅ with iodine vapors and certain iodine complexes are compared with the results on the diffusion of minority charge carriers and color centers. The impedance results are analyzed based on the Grafov-Ukshe model for the adsorption relaxation of the double layer (ARDL) [1]. The obtained kinetic parameters agree within the experimental error with the diffusion coefficients found by independent methods. It is shown that the experimental data of impedance measurements agree with the results of calculations in terms of the Grafov-Ukshe ARDL model.     

Improved intermolecular SCF theory and the BSSE problem

Analytical and numerical studies are performed concerning the exclusion of the basis set superposition error (BSSE ) from the SCF calculations of intermolecular interactions. Based on these studies a new procedure is proposed, which consists of the following steps: (1) determine the orbitals by the SCF scheme based on the recent “chemical Hamiltonian approach” (CHA-SCF method), i.e., excluding the delocalization effects caused by BSSE , and then (2) calculate the usual energy expectation value. (This gives results superior to those obtained by the previous nonsymmetric CHA energy formula.) The actual numerical calculations performed for different simple systems (He₂, water dimer) by using various basis sets indicate that the CHA/CE (CHA with “conventional energy” formula) potential curves are well-balanced and are close to those obtained by the Boys–Bernardi (BB ) method and usually (but not necessarily) go slightly beyond the latter. So our method gives results better than (or close to) those given by the BB method by performing only a single ～N⁴ calculation at each geometrical arrangement of the system.     

Simultaneous Impedance Analysis of Three Parallel Piezoelectric Quartz Crystals for Electrochemical Depletion Layer Effect Study

Simultaneous impedance analysis of three one-face sealed resonating piezoelectric quartz crystals (PQCs) in parallel is proposed through admittance measurements of the three PQCs on one impedance analyzer and then non-linear fitting according to the parallel combination of three Butterworth-Van Dyke circuits. Responses of each PQC obtained from the three-PQC mode agreed well with those measured separately in series sucrose aqueous solutions. This novel method has been used for the study of depletion-layer effect during ferri-/ferrocyanide electrochemical reactions.     

Assessment of Defects and Collapsed Sites in Self-Assembled Alkanethiol Monolayer on Gold by Cyclic Voltammetry and A.C.Impedance

Thespontaneousself-assemblytechniqueprovidesaconvenientandpowerfultooltopreparecompactmonolayerswithwell-definedcomposition,structureandthicknessl.However,theexistenceofstrUcturaldefectsanddisorderinself-assemblymayinfluenceitspropertiesinmanyfields.Soitisincreasinglyclearthatthecharacterizationofdefectsisanimportantissuewhichneedstobeaddressedinmanyapplications.Thepurposeofthisworkistostudythedefectsand"collapsed"sitesinAulthiolmonolayerbycyclicvoltanunetryandelectrochemicalimpedancespectros…     

Interatomic distance measurement in solid-state NMR between a spin- 1/2 and a spin- 5/2 using a universal REAPDOR curve

A universal curve for the solid-state NMR REAPDOR experiment on an isolated spin-1/2-spin-5/2 pair is proposed that provides a simple means to measure their interatomic distance. REAPDOR data were obtained at three separate REAPDOR experiments using different values of the rotor spinning frequency. All points were fitted simultaneously to the universal formula without a need for full density matrix calculations. The 13C-17O distance of 2.45 A was measured between the C6 carbon and the 17O label in a tyrosine sample. The error of 8% in the dipolar coupling (Dfit = 278 Hz) is well within the 15% theoretical tolerance of this curve.     

A comprehensive analysis of impedance of the electrochemical cell

A comprehensive impedance characteristics of two electrodes electrochemical cell has been presented. In this method a multisinusoidal current excitation signal is used. The change of potential of both the electrodes are all registered as a function of time. The proposed method gives the possibility of determining the impedance of both electrodes individually as well as the impedance of a two-electrode system. Additional application of short time Fourier transform of time registers allows the determination of changes in the measured impedance values over time. In order to present the possibilities of the proposed technology it was applied to a process of charging commercially available electrochemical cell NiCd. The new measurement methodology allows understanding the dynamics of processes occurring in a electrochemical cell. This is the basis for the development of effective and affordable electro-catalysts. Thanks to results obtained with Dynamic Electrochemical Impedance spectroscopy (DEIS) method it is possible to understand the mechanisms and kinetics of processes occurring in electrochemical cells while charging.     

Synthesis, Characterization, Spectroscopy and Study of Antimicrobial Properties of Two New Substituted Trichloroaluminate（Ⅲ） Complexes [AlCl3X]- （X = SCN-, CN-）

The synthesis and characterization of two new aluminate(Ⅲ) complexes with general formula K[AlCl3X] are reported. These compounds derived from aluminate trichloride and related salts. Potassium trichlorothiocyanoaluminate, PCTA, and potassium trichlorocyanatoalu-minate, PCCA, are two new ionic aluminate complexes. They can be easily synthesized in a nearly quantitative yield by using the direct reaction of AlCl3 and KX. The complexes were characterized by physico-chemical and spectroscopic methods. Theoretical calculations have been used for the extraction of structural and spectroscopic data of these new synthesized complexes. The antibacterial activities of such compounds were studied against the Staphylococcus aureus, Escherichia coli, Staphylococcus Epidermidis, Estreptococo B and Shigella.     

Phenomenological model for the interpretation of impedance/admittance spectroscopy results in polymer light-emitting electrochemical cells

A phenomenological model has been developed to account for the results of impedance/admittance spectroscopy measurements from light-emitting electrochemical cells (LECs) comprising a polymer electrolyte and two different conjugated polymers used as organic semiconductor. The application of a d.c. offset bias superimposed to the a.c. modulation voltage was used to observe the transition from the behavior prior to device operation and after the formation of the electrochemical p-i-n junction. The analysis of the whole device “conductivity” as a function of the applied bias and of the frequency was used to support the assumptions considered to develop the model. The results show that the device, after the p-i-n junction formation, can be considered as composed by two highly conductive electrochemically doped (n and p) regions and a thin (few tens nanometers), insulating layer, where the electrical current is dominated by electronic charge carrier injection via tunneling through a rectangular energy barrier. Before the p-i-n junction formation, there is no doping of semiconductor material, and the device electrical properties are dominated by the intrinsic electronic charge carriers in the organic semiconductor. Results from devices made of organic semiconductors with different band gap energy and different layer thicknesses are used to corroborate the proposed model.     

Investigation of intermolecular interactions based on the atomic statistical model

Possibilities of improving individual contributions to the statistical interaction energy (kinetic and exchange) are examined. A new method of calculating statistical interaction energies is proposed. The exchange term is calculated using a suitably modified second-order gradient correction. For the kinetic contribution the accurate formula corresponding to the first-order perturbation theory is applied. The calculations have been carried out for several pairs of noble gas atoms.     

Bringing about matrix sparsity in linear-scaling electronic structure calculations

The performance of linear-scaling electronic structure calculations depends critically on matrix sparsity. This article gives an overview of different strategies for removal of small matrix elements, with emphasis on schemes that allow for rigorous control of errors. In particular, a novel scheme is proposed that has significantly smaller computational overhead compared with the Euclidean norm-based truncation scheme of Rubensson et al. (J Comput Chem 2009, 30, 974) while still achieving the desired asymptotic behavior required for linear scaling. Small matrix elements are removed while ensuring that the Euclidean norm of the error matrix stays below a desired value, so that the resulting error in the occupied subspace can be controlled. The efficiency of the new scheme is investigated in benchmark calculations for water clusters including up to 6523 water molecules. Furthermore, the foundation of matrix sparsity is investigated. This includes a study of the decay of matrix element magnitude with distance between basis function centers for different molecular systems and different methods. The studied methods include Hartree–Fock and density functional theory using both pure and hybrid functionals. The relation between band gap and decay properties of the density matrix is also discussed.     

Electrohydrodynamical impedance on a rotating disk electrode: Part II. Non-steady mass transfer in an Ostwald fluid

The electrohydrodynamical impedance on a rotating disk defined as the ratio of the mass-transfer rate response to a small sine-wave modulation of the angular velocity has been calculated for an Ostwald fluid. This impedance, expressed as a function of the modulation frequency suitably reduced by means of the rheological parameters, is then identical to the expression derived for a newtonian fluid. An excellent agreement was found between these calculations and experimental data obtained for a viscous flow of a polyethylene oxide solution of high molecular weight.     

Prediction of Programmed-temperature Retention Values of Naphthas by Artificial Neural Networks

Abstract

It is proposed for the first time a method of prediction of the programmed-temperature retention times of components of naphthas in capillary gas chromatography using artificial neural networks. People are used to predict the programmed-temperature retention time using many formulas such as the integral formula, which requires that four parameters must be determined by calculation or experiments. However the results obtained by the formula are not so good to meet the demand of industry. In order to predict retention time accurately and conveniently, artificial neural networks using five-fold cross-validation and leave-20%-out methods have been applied. Only two parameters: density and isothermal retention index were used as input vectors. The average RMS error for predicted values of five different networks was 0.18, whereas the RMS error of predictions by the integral formula was 0.69. Obviously, the predictions by neural networks were much better than predictions by the formula, and neural networks need fewer parameters than the formula. So neural networks can successfully and conveniently solve the problem of predictions of programmed-temperature retention times, and provide useful data for analysis of naphthas in petrochemical industry.