Nonuniform Potential Distribution Research Articles

Influence of the vapor shield potential control on the post-arc sheath development in vacuum interrupters

In tank-type multi-break vacuum circuit breakers (VCBs) and gas-insulated switchgear (GIS), the non-uniform distribution of vapor shield potential in the vacuum interrupters impacts the post-arc process significantly. To explore the effect of the vapor shield potential on the development of the post-arc sheath, a particle-in-cell model with shield potential control is proposed to obtain two-dimensional distributions of post-arc particles. Based on the synthetic test circuit and probe array, the test platform of the post-arc characteristic diagnostic is set up to verify the influence of vapor shield potential distribution on the post-arc sheath. The extinguishing process of the vacuum arc is also observed by the high-speed CMOS camera. Simulation and experimental results show that at shield potential of 0.5 transient recovery voltage, the post-arc sheath develops most rapidly and the distribution of cathode spots is the most uniform. These findings can provide valuable insights for the design of vacuum interrupters and the control of shield potential in tank-type GIS.

“Anomalous” diffusion in multi-layer silicide systems

The transition metal silicide coatings have excellent oxidation resistance. In the high-temperature oxidization environment, the “anomalous” diffusion phenomenon of the reverse concentration gradient occurs in the multilayer silicide coating structure, which has a significant impact on the coating degradation. This study is to explore the physical mechanism of “anomalous” diffusion phase transformation in the MoSi2–NbSi2 bilayer silicide coating on the Nb alloy substrate. Through vacuum annealing experiments, combined with micro-Raman spectroscopy and electron probe microanalysis measurements, the diffuisonal phase transformation of multilayer silicide coating in the oxygen-free environment at high temperatures was studied. By decoupling the oxidation and diffusion, the experiments indicate that high-temperature oxidation is not the dominant driving factor for the “anomalous” diffusion of atoms. The thermodynamic analysis reveals that the reduction in the nucleation barrier of the silicide-poor layer due to multicomponent solid solution and the non-uniform distribution of component chemical potential provide the driving force for the “anomalous” diffusion growth. Based on the diffusion kinetic modeling, the simulation of diffusion-controlled phase transformation in multilayer silicide coating was carried out, and the effect of tracer diffusion coefficients on the growth of the silicide-poor phase was analyzed. The research will have guiding significance for the recognition of failure mechanisms of silicide coating systems and performance improvement.

(Invited) Numerical Simulation of Ohmic Impedance

Impedance spectroscopy measurements are often confounded by high-frequency phenomena associated with frequency-dependent ohmic resistance. Newman introduced the concept of frequency dispersion to account for the effect of the disk electrode geometry on the impedance response [1]. The phenomenon described by Newman in terms of frequency-dependent capacitance and ohmic resistance may be considered as an ohmic impedance, a transfer function associated with the resistance of the electrolyte to passage of current under the influence of geometry-induced (and frequency-dependent) non-uniform current and potential distributions [2].Two kinds of models are described in the presentation. Finite-element models account for the influence of geometry on the impedance response for a disk electrode. These models demonstrate the ohmic impedance. The second model is an interpretation model used to extract properties from impedance data. For most cases, data are truncated to avoid being confounded by ohmic impedance. On rare instances, the ohmic impedance may be extracted by regression to data.Gharbi et al. [3] suggested that the ohmic impedance follows the form of the Havriliak-Negami equation. The Havriliak-Negami equation was found to provide a good fit to the ohmic impedance calculated for different electrode geometries, including interdigitated electrodes [4] as well as disk electrodes [3].The ohmic impedance was obtained by Levenberg-Marquardt regression of a process model that accounted for the ohmic impedance [5]. The experiments were performed using ferri/ferrocyanide redox species (10 mM each) in a 0.5 M KCl solution. The working electrode consisted of a 5-mm-diameter Au disk rotating at 800 rpm. A Pt gauze (4 cm2) was used as the counterelectrode, a saturated Ag/AgCl electrode was used as a reference electrode, and the temperature of double-walled electrochemical cell was held at 25 ±0.1°C.The process model was expressed in terms of an ohmic impedance and an expression that accounted for the influence of a local constant-phase element on the faradaic reaction and the convective diffusion impedance, expressed in terms of a series expansion following the work of Tribollet and Newman [6]. Regressions were weighted by the experimentally determined error structure of the data, and the resulting parameters characteristic of the ohmic impedance were in excellent agreement with numerical simulations.References J. S. Newman, J. Electrochem. Soc., 117 (1970), 198.V. Huang, V. Vivier, M. E. Orazem, N. Pébère, and B. Tribollet, J. Electrochem. Soc., 154 (2007), C81-C88.O. Gharbi, A. Dizon, M. E. Orazem, M. T.T. Tran, B. Tribollet, and V. Vivier, Electrochim. Acta, 320 (2019), 134609.A. Dizon and M. E. Orazem, Electrochim. Acta, 337 (2019), 135000.W. Watson and M. E. Orazem, EIS: Measurement Model Program, ECSArXiv, 2020, https://ecsarxiv.org/kze9x/.B. Tribollet and J. S. Newman, J. Electrochem. Soc., 130 (1983), 2016.

Graphene‐on‐Silicon Hybrid Field‐Effect Transistors

AbstractThe combination of graphene and silicon in hybrid electronic devices has attracted increasing attention over the last decade. Here, a unique technology of graphene‐on‐silicon heterostructures as solution‐gated transistors for bioelectronics applications is presented. The proposed graphene‐on‐silicon field‐effect transistors (GoSFETs) are fabricated by exploiting various conformations of channel doping and dimensions. The fabricated devices demonstrate hybrid behavior with features specific to both graphene and silicon, which are rationalized via a comprehensive physics‐based compact model which is purposely implemented and validated against measured data. The developed theory corroborates that the device hybrid behavior can be explained in terms of two independent silicon and graphene carrier transport channels, which are, however, strongly electrostatically coupled. Although GoSFET transconductance and carrier mobility are found to be lower than in conventional silicon or graphene field‐effect transistors, it is observed that the combination of both materials within the hybrid channel contributes uniquely to the electrical response. Specifically, it is found that the graphene sheet acts as a shield for the silicon channel, giving rise to a nonuniform potential distribution along it, which impacts the transport, especially at the subthreshold region, due to non‐negligible diffusion current.

Control of the piezoelectric and flexoelectric homogenized properties of architected materials by tuning their inner topology

The present contribution aims to study the effect the inner topology of periodic architected materials on their effective piezoelectric and flexoelectric properties. An analysis is performed to study the sensitivity of the effective flexoelectric moduli to the geometrical variables of the considered unit cells of periodic lattice materials. More specifically, hexagonal, re-entrant auxetic, and rectangular unit cells are investigated in terms of their relative piezoelectric and flexoelectric response. The base material obeys a piezoelectric behavior, and the homogenized electromechanical properties are evaluated using the extended Hill–Mandel macrohomogeneity condition articulated with variational principles in the context of periodic multiphysical homogenization. The developed modeling framework of coupled electromechanical phenomena accounts for the higher gradient effects modeled by flexoelectricity that are induced by the strong gradient of properties that exist at the microlevel of the lattice unit cell. Regular hexagonal, re-entrant hexagonal and rectangular lattice unit cells are mutually compared in terms of their flexoelectric properties. The regular hexagonal UC shows the highest piezoelectric moduli, whereas the flexoelectric moduli are found to be maximum for the re-entrant UC. The coupling between mechanical and electrical fields is attested by the non-uniform distribution of the electric potential within the three studied architected materials subjected to a uniform strain.

Vibration modes of flexoelectric circular plate

Beams, plates, and shells, as the fundamental mechanical structures, are widely used in microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) as sensors, actuators, energy harvesters, and among others. Deeply understand the electromechanical coupling of these dielectric structures is of crucial for designing, fabricating, and optimizing practice devices in these systems. Herein we demonstrate the electromechanical coupling in flexoelectric circular plate, in which higher-order strain gradients were considered to extend the classical electromechanical properties to isotropic materials, in which the non-uniform distribution of the electric potential along the radial direction was considered. Analytical solutions for the vibration modes of the flexoelectric circular plates showed that the dynamic modes were totally different from the piezoelectric circular plates owing to the inversion symmetry breaking by the strain gradient. The electromechanical coupling dynamic modes are sensitive to bending, twisting modes owing to the sensitivity of the flexoelectric effect to bending. This work provides a fundamental understanding of the electromechanical coupling in flexoelectric circular plate, which is helpful in designing novel flexoelectric circular plate-based devices, such as flexoelectric mirrors.

Characterization of CVD-synthesized graphene films transferred on different substrates using the scanning probe microscopy electrical techniques

The use of local electrical characteristics of graphene films prepared by chemical vapor deposition (CVD) method on Cu and transferred on different substrates such as glass, SiO 2 /Si and Al 2 O 3 substrates was discussed. The topography, Kelvin probe force microscopy (KPFM), scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM) were compared for graphene on the three different dielectric substrates. The surface potentials of graphene films were investigated using KPFM. It was found that graphene films exhibited nonuniform surface potential distribution depending on the sort of the underlying substrate used, the number of graphene layers and the presence of graphene film defects such as edges, wrinkles etc. The EFM results demonstrated that the local electrostatic properties of graphene film on Al 2 O 3 substrate differ significantly compared to that on glass and SiO 2 /Si substrates. The cross-correlation coefficients between topographical image and SCM- and EFM-images were estimated in order to compare these techniques. It was shown that the techniques can bring complementary information for the characterization of graphene films on different substrates relevant for applications. • The local electrical properties of graphene films were investigated by KPFM, SCM and EFM techniques. • The quantitative values of local dielectric properties are sensitive to the underlying substrate. • Kelvin probe force microscopy. • Scanning capacitance microscopy. • Electrostatic force microscopy.

Method to Predict the Non-Uniform Potential Distribution in Random Electrical Machine Windings under Pulse Voltage Stress

This article describes a practical method for predicting the distribution of electric potential inside an electrical machine’s winding based on design data. It broadens the understanding of winding impedance in terms of inter-winding behavior and allows to properly design an electrical machine’s insulation system during the development phase. The predictions are made based on an frequency-dependent equivalent circuit of the electrical machine which is validated by measurements in the time domain and the frequency domain. Element parameters for the equivalent circuit are derived from two-dimensional field simulations. The results demonstrate a non-uniform potential distribution and demonstrate that the potential difference between individual turns and between turns and the stator core exceeds the expected values. The findings also show a link between winding impedance and potential oscillations inside the winding. Additionally, the article provides an overview of the chronological progression of turn-based models and shows how asynchronous multiprocessing is used to accelerate the solution process of the equivalent circuit.

Geometric gradient assisted control of the triboelectric effect in a smart brake system for self-powered mechanical abrasion monitoring

The triboelectric effect is proposed as a fundamental principle of self-power sensing and energy harvesting, and the application in commercially available machine components is spotlighted to cost-effectively monitor the status of components in a self-powered manner. In this study, the method of natural control of the triboelectric effect with the conventional brake system, which is one of the core parts of a moving machine to prevent the damage of the vehicle and establish user safety, is explored with the development of the geometric Gradient contact layer assisted smart Brake pad with the Triboelectric effect-based design Strategy (G-BTS). A physically asymmetric composite material is employed to uniquely cause spatially non-uniform electric potential distribution and provides the method to naturally perceive the level of the mechanical abrasion. The concept is further extended to demonstrate the real-time brake abrasion level monitoring system, which does not require any external power source to generate monitoring signals. The direct perception of the status of the brake pad in real time in this study can contribute to the prevention of damage and achievement of an efficient diagnosis of the machine in the future. • Use of physically asymmetric composite material to naturally control the triboelectric effect. • Develop Spatially non-uniform electric potential distribution in the smart brake system. • Naturally perceive the level of the mechanical abrasion via geometric gradient contact layer. • Demonstrate energy harvesting as well as self-powered and real-time abrasion monitoring.

Influence of Ohmic Impedance on Impedance Spectroscopy

Impedance spectroscopy measurements are often confounded by high-frequency phenomena associated with frequency-dependent ohmic resistance. Newman introduced the concept of frequency dispersion to account for the effect of the disk electrode geometry on the impedance response.1 The phenomenon described by Newman in terms of frequency-dependent capacitance and ohmic resistance may be considered as an ohmic impedance, a transfer function associated with the resistance of the electrolyte to passage of current under the influence of geometry-induced (and frequency-dependent) non-uniform current and potential distributions.2 Gharbi et al.3 suggested that the ohmic impedance follows the form of the Havriliak-Negami equation. The Havriliak-Negami equation was found to provide a good fit to the ohmic impedance calculated for different electrode geometries, including interdigitated electrodes4 as well as disk electrodes.3 The ohmic impedance was obtained by Levenberg-Marquardt regression of a process model that accounted for the ohmic impedance.5 The experiments were performed using ferri/ferrocyanide redox species (10 mM each) in a 0.5 M KCl solution. The working electrode consisted of a 5-mm-diameter Au disk rotating at 800 rpm. A Pt gauze (4 cm2) was used as the counterelectrode, a saturated Ag/AgCl electrode was used as a reference electrode, and the temperature of double-walled electrochemical cell was held at 25 ±0.1°C.The process model was expressed in terms of an ohmic impedance and an expression that accounted for the influence of a local constant-phase element on the faradaic reaction and the convective diffusion impedance, expressed in terms of a series expansion following the work of Tribollet and Newman.6 Regressions were weighted by the experimentally determined error structure of the data, and the resulting parameters characteristic of the ohmic impedance were in excellent agreement with numerical simulations.References S. Newman, J. Electrochem. Soc., 117 (1970), 198.Huang, V. Vivier, M. E. Orazem, N. Pébère, and B. Tribollet, J. Electrochem. Soc., 154 (2007), C81-C88.Gharbi, A. Dizon, M. E. Orazem, M. T.T. Tran, B. Tribollet, and V. Vivier, Electrochim. Acta, 320 (2019), 134609.Dizon and M. E. Orazem, Electrochim. Acta, 337 (2019), 135000.Watson and M. E. Orazem, EIS: Measurement Model Program, ECSArXiv, 2020, https://ecsarxiv.org/kze9x/.Tribollet and J. S. Newman, J. Electrochem. Soc., 130 (1983), 2016.

The Sensitivity Optimization Guided Imaging Method for Electrical Capacitance Tomography

In this study, we propose a new sensor structure with driving electrodes and a new excitation mode in electrical capacitance tomography (ECT) to improve: 1) the low sensitivity distribution in the central area caused by low potential distribution in the central area and 2) the nonuniform sensitivity distribution caused by the nonuniform potential distribution. An imaging method based on sensitivity map optimization is derived through electromagnetic field analysis and numerical simulation. In addition, the change rule of the optimal driving voltage is studied when the excitation voltage of the excitation electrodes and the length of the driving electrodes change. The numerical simulation and experimental results show that the imaging method proposed in this article can distinctively alter the potential distribution and sensitivity distribution in the central area by selecting the optimal driving voltage, which brings significant improvement in image reconstruction.

Mixing control in a continuous-flow microreactor using electro-osmotic flow

In recent years, pharmaceutical production has been stimulating the gradual miniaturization of continuous-flow chemical reactors. This process eventually resulted in the emergence of a new generation of microreactors. The advantages of these new reactor types are the flexible production that allows us to quickly reconfigure the scheme, small reactant quantities used for the synthesis, the control of the main reaction parameters with high accuracy. Nevertheless, a decrease in the thickness of the channels where the species contact and react forces us to search for new non-mechanical mechanisms for mixing. This problem is relevant for the slow reaction occurring in a slot where diffusion alone cannot provide mixing at reasonable distances from the entrance. It is also true for the fast reaction that takes place in a frontal manner. In this work, we consider the efficiency of mixing the reactants induced by electro-osmotic flow in a Hele-Shaw configuration with non-uniform zeta potential distribution. As a test reaction, we take the neutralization reaction with simple albeit non-linear kinetics. The reaction occurs between two miscible solutions, which are initially separated in space and come into contact in a continuous-flow microreactor. The reaction proceeds frontally, which prevents the efficient mixing of the reactants due to diffusion. Using direct numerical simulations of 2D and 3D flows, we demonstrate that the zeta potential applied to boundaries can effectively control the mixing rate of fluids by lengthening the front of the reaction. This approach makes it possible to increase the yield of the reaction product.

Interfacial Potentials in a Thin Film All-Solid-State Li-Ion Battery Probed By in Operando KPFM

Designing smaller, safer, cheaper, and more stable batteries necessitates thorough understanding of the electrochemical processes governing their operation at multiple length scales. In all-solid-state power sources the electrolyte is no-flammable and can be made ultimately thin, making them safer and more compact. Yet, experimental evidence suggests that the multiple internal interfaces in their layered structure give rise to high impedances, limiting their performance. Studying these nanoscopic interfaces requires microscopic tools. Here, we employ in operando Kelvin Probe Force Microscopy (KPFM) to quantitatively measure the potential distribution in a solid-state Li-ion battery as a function of its state of charge. The battery was fabricated by sequentially depositing thin layers of Pt (110-130 nm), LiCoO2 (280-420 nm), LIPON (1100-1200 nm), Si (50-240 nm) Cu or Pt (150-200 nm) onto a Si/SiO2 wafer (oxide thickness 100 nm). The fabricated battery was cleaved in an Ar atmosphere to expose the stacked layers, mounted on a holder, wired, and safely transferred without exposing to air into a dual-beam instrument that combines a scanning electron microscope (SEM), a Ga-ion focused ion beam (FIB) and an atomic force microscope (AFM) in one vacuum chamber (residual pressure of 10-4 Pa). The stacked battery was milled to expose a cross-section of the layers, and imaged using SEM and KPFM, while cycling the battery. The acquired potential maps reveal a highly non-uniform interelectrode potential distribution, with most of the potential drop occurring at the electrolyte-Si anode interface in the pristine battery. During the first charge, the potential distribution gradually changes, revealing complex polarization within the LIPON layer due to Li-ion redistribution. Cycling the battery at high rate significantly decreased its capacity, although the capacity loss can be recovered. KPFM imaging allowed the detection of the interface responsible for this capacity loss. Li distribution in the battery was also measured using Neutron Depth Profiling as a function of the state of charge. The acquired data was compared to first principles calculations shedding light onto the interfacial Li-ion transport in the battery and its reversibility.ES acknowledges support under the Cooperative Research Agreement between the University of Maryland and the National Institute of Standards and Technology Center for Nanoscale Science and Technology, Award 70NANB14H209, through the University of Maryland.

Physical Properties Obtained from Measurement Model Analysis of Impedance Measurements

This paper demonstrates the application of the Voigt measurement model to extract capacitance, ohmic resistance, and polarization resistance values from impedance data. The systems explored include a Randles circuit, films with exponential and power-law distributions of resistivity, systems exhibiting geometric capacitance, and systems showing geometry-induced frequency dispersion. The present work demonstrates that the measurement model serves as a useful means to provide quantitative estimates for parameters relevant to impedance spectroscopy data, including ohmic resistance, polarization resistance, and capacitance.The ability to extract capacitance by use of the measurement model is important, because most impedance systems show distributed-time-constant behavior that is often fit by use of the CPE. The CPE parameters are related to the capacitance of the electrode, but extraction of a capacitance requires different approaches depending on the nature of the underlying distribution. For example, the Brug formula[1] is used in cases of a surface distribution of time constants, and the power-law model[2],[3] applies for a distribution through a film. Not all distributed-time-constant behaviors are represented by a CPE, and formulas like the Brug and Power-Law equations do not exist for such systems. Furthermore, the measurement model provides a unique ability to extract the high-frequency and low-frequency ohmic resistances for systems exhibiting an ohmic impedance.[4] The application of the measurement model to extract parameters for experimental impedance data depends on the nature of the electrochemical system under investigation. For systems that have a uniform current and potential on the electrode surface, the ohmic resistance and capacitance may be extracted by regression of the measurement model to the part of the impedance spectrum that is found to be consistent with the Kramers-Kronig relations. The polarization resistance corresponding to the zero-frequency limit may also be estimated. For systems that have a nonuniform current and potential distribution, a characteristic frequency can be identified above which the impedance is influenced by the associated frequency dispersion. This characteristic frequency may be estimated for general systems using values of capacitance and ohmic resistance estimated by regression of the measurement model to the impedance spectrum. The estimate for capacitance may be updated be eliminating the frequencies above the characteristic frequency. This is an iterative process is completed when the estimated characteristic frequency is larger than the maximum frequency used in the regression. The iterative process will yield estimates for capacitance and low-frequency ohmic resistance. The iterative process may fail to yield a suitable convergence for cases in which the charge-transfer resistance is smaller than the ohmic resistance.Used wisely, the measurement model may be a powerful tool for extracting the capacitance, ohmic resistance, and polarization resistance for impedance data. It can be applied both for systems exhibiting normal distributions of time constants and, if applied to frequencies sufficiently below the characteristic frequency for geometry-induced dispersion, for systems showing surface distributions of time-constants. Used in this way, the measurement model provides a powerful complement to the development of system-specific process models.

Method for the Prediction of the Potential Distribution in Electrical Machine Windings Under Pulse Voltage Stress

This article provides an applicable method to predict the distribution of the electric potential inside the winding of an electrical machine with a reasonable set of parameters. It extends the understanding of the winding impedance in terms of the inter-winding behavior and gives us the opportunity to properly design the insulation system during the development phase of an electrical machine. Predictions are backed up by measurements and an in-depth look at the measurement setup. The results proof nonuniform potential distribution and show that the potential difference between individual turns exceeds the amplitude of the phase voltage. Our findings also establish an interrelation between the winding impedance and the potential oscillations inside the winding. The article provides an introduction to the fundamental process of voltage stress on electrical machine windings and discusses drivers for upcoming challenges in the field of winding insulation and partial discharges.

Boosting Reaction Homogeneity in High-Energy Lithium-Ion Battery Cathode Materials.

Conventional nickel-rich cathode materials suffer from reaction heterogeneity during electrochemical cycling particularly at high temperature, because of their polycrystalline properties and secondary particle morphology. Despite intensive research on the morphological evolution of polycrystalline nickel-rich materials, its practical investigation at the electrode and cell levels is still rarely discussed. Herein, an intrinsic limitation of polycrystalline nickel-rich cathode materials in high-energy full-cells is discovered under industrial electrode-fabrication conditions. Owing to their highly unstable chemo-mechanical properties, even after the first cycle, nickel-rich materials are degraded in the longitudinal direction of the high-energy electrode. This inhomogeneous degradation behavior of nickel-rich materials at the electrode level originates from the overutilization of active materials on the surface side, causing a severe non-uniform potential distribution during long-term cycling. In addition, this phenomenon continuously lowers the reversibility of lithium ions. Consequently, considering the degradation of polycrystalline nickel-rich materials, this study suggests the adoption of a robust single-crystalline LiNi0.8 Co0.1 Mn0.1 O2 as a feasible alternative, to effectively suppress the localized overutilization of active materials. Such an adoption can stabilize the electrochemical performance of high-energy lithium-ion cells, in which superior capacity retention above ≈80% after 1000 cycles at 45 °C is demonstrated.

A Flexible Touch-Sensing Sensor Based on the Theory of Non-Uniform Gradient Potential Distribution

Tactile perception is very important for the physical interaction between robots and the external environments. Based on the theory of non-uniform gradient potential distribution (GPD), a novel non-array flexible touch-sensing sensor is proposed in this paper. It can detect whether there is a touch, and further to know where the touch took place. The sensor uses an indium-tin oxide polyethylene terephthalate (ITO-PET) film as signal generation layer. The electric fields in two different directions are constructed by time-sharing in the ITO-PET conductive plane, and the potential distribution over the ITO-PET film has the characteristics of uniqueness and determinacy. The touching position can be worked out by detecting the potentials of the touching point in two directions, respectively. In order to establish the relation between the position and the potential value effectively, quickly and accurately, the radical basis function (RBF) neural network is used to construct the electric field model and calculate the touching position coordinate. A detection circuit was designed to detect the touching position of the sensor sample. The simulation and experimental results show that the detection principle proposed in this paper is feasible, and the real-time detection of single point touching position can be realized. The touch-sensing sensor is simple in structure, thin in thickness and soft in flexibility. It can cover the surface of the robot and obtain external touching information in real time.

Biocompatibility of Porous SHS-TiNi

Implants made of porous SHS-TiNi alloys are successfully used in medicine to replace solid tissues of the human body. Self-propagating synthesis reaction of TiNi alloy was carried out through layer-by-layer combustion. XRD analysis of the phase composition and structural parameters of porous Ni50Ti50 alloy, as well as microscopic studies, were carried out. The structural methods employed in the study showed that the surface of porous SHS-TiNi alloys is a complex of dense layers of amorphous-nanocrystalline intermetallic oxycarbonitrides saturated with O, N, C intercalation impurities. The study of the surface layer S showed that the layer S consists of three layers: the foam layer F and two sublayers. Samples were studied for the nonuniform potential distribution in the cross section of interpore partitions. It was shown that they correlate with the structural phase inhomogeneity of the SHS-TiNi alloy. The structural studies carried out using different methods allowed us to reliably establish the presence of surface nonmetallic phases in the form of surface films and grain boundary inclusions formed during the self-propagating reaction synthesis of the porous TiNi alloy. High biochemical compatibility is ensured by specific surface layers of the porous alloy formed in the process of its metallurgy, which do not require additional surface modification.

Analytical study of AC electroosmotic mixing in 2-dimensional microchannel with time periodic surface potential.

This work reported an analytic study of AC electroosmotic flows with a view to control the degree of mixing in a rectangular microchannel. Only with spatially non-uniform zeta potential distribution, fluid particles travel back and forth along a vortical flow field developed inside a microchannel. Although complex patterns of electroosmotic vortical flows can be obtained by various types of non-uniform zeta potential distributions, fluid particles always follow regular paths due to a laminar flow limit. To further facilitate the mixing of sample fluid, we propose a scheme that the zeta potential distribution was temporally non-uniform as well. General solutions for both the double layer potential distribution and the AC electroosmotic flow field are analytically determined by solving the unsteady Stokes equation with an electrostatic body force. As an illustrative example, we consider a case where two different types of non-uniform zeta potential distributions alternate with each other and the effects of both the AC frequency and the frequency of the alternation of the two zeta potential distributions on flow characteristics are examined using the Poincaré sections. Conclusively, one can either enhance or prevent mixing compared to a static electroosmotic flow, which is in line with previously demonstrated experimental works. Thus, the results presented would be an effective mean for controllable electroosmotic flow in a microfluidic platform.

Mathematical Model for the Electrokinetic Instability of Electrolyte Flow

We study a one-dimensional model of the dilute aqueous solution of KCl in the electric field. Our model is based on a set of Nernst-Planck-Poisson equations and includes the incompressible fluid velocity as a parameter. We demonstrate instability of the linear electric potential variation for the uniform ion distribution and compare analytical results with numerical solutions. The developed model successfully describes the stability loss of the steady state solution and demonstrates the emerging of spatially non-uniform distribution of the electric potential. However, this model should be generalized by accounting for the convective movement via the addition of the Navier-Stokes equations in order to substantially extend its application field.