Ac Technique Research Articles

Effect of Temperature on Passive Film Characteristics of LPBF (Laser Powder-Bed Fusion) Processing on UNS-S31603.

The effect of temperature on the localized corrosion resistance and passive film characteristics of laser powder-bed fusion (LPBF) 316L (UNS S31603) was studied in a buffered 3.5 wt% NaCl solution at 25, 50, and 75 °C. DC techniques such as cyclic potentiodynamic polarization showed lower passive current densities, high breakdown potentials, and a higher resistance to initial breakdown compared with wrought 316L samples at all temperatures. However, LPBF 316L was more susceptible to metastable pitting at potentials before film breakdown and higher damage accumulation post film breakdown. AC techniques, such as Mott-Schottky analysis and electrochemical impedance spectroscopy, showed that the formed passive film was more robust on the LPBF 316L samples at all temperatures, accounting for the higher initial resistance to pitting. However, with increasing temperatures, the film formed had an increasing concentration of defect density. Passive compositions at the various test temperatures studied using X-ray photoelectron spectroscopy (XPS) showed that the LPBF samples showed higher amounts of Cr and Fe oxides and hydroxides compared with the wrought samples, which made the passive films on the LPBF samples more compact and protective. Investigation of the pits formed on the LPBF showed the preferential regions of attack were the melt-pool boundaries and cell interiors due to their being depleted of Cr and Mo when compared with the boundaries and matrix.

Determination of the dynamic Young's modulus of quantum materials in piezoactuator-driven uniaxial pressure cells using a low-frequency AC method.

We report on a new technique for measuring the dynamic Young's modulus, E, of quantum materials at low temperatures as a function of static tuning strain, ϵ, in piezoactuator-driven pressure cells. In addition to a static tuning of stress and strain, we apply a small-amplitude, finite-frequency AC (1Hz ≲ ω ≲ 1000Hz) uniaxial stress, σac, to the sample and measure the resulting AC strain, ϵac, using a capacitive sensor to obtain the associated modulus E. We demonstrate the performance of the new technique through proof-of-principle experiments on the unconventional superconductor Sr2RuO4, which is known for its rich temperature-strain phase diagram. In particular, we show that the magnitude of E, measured using this AC technique at low frequencies, exhibits a pronounced nonlinear elasticity, which is in very good agreement with previous Young's modulus measurements on Sr2RuO4 under [1 0 0] strain using a DC method [Noad etal., Science 382, 447-450 (2023)]. By combining the new AC Young's modulus measurements with AC elastocaloric measurements in a single measurement, we demonstrate that these AC techniques are powerful in detecting small anomalies in the elastic properties of quantum materials. Finally, using the case of Sr2RuO4 as an example, we demonstrate how the imaginary component of the modulus can provide additional information about the nature of ordered phases.

Comparison of PetrifilmTM AC and Pour plate techniques used for the heterotrophic aerobic bacterial count in water.

Heterotrophic bacteria (HPC) are commonly found in water samples. While these bacterial counts do not necessarily indicate a health hazard, high counts provide a good indication of the efficiency of water disinfection and integrity of distribution systems. The aim of this study was to compare the PetrifimTM AC method to the Pour Plate technique for the testing of HPC in water samples. Artificially contaminated (192 samples) and natural water samples (25) were processed using two methods. Both methods accurately detected high, medium and low counts of HPC, producing average Z scores between -2 and+2. Paired-wise student t-test and correlation coefficient showed nonsignificant differences between the results of two methods. Acceptable repeatability and reproducibility was obtained using both the methods. Uncertainty of measurement for PetrifilmTM AC and Pour Plate method was found to be 2.9% and 5.4% respectively. PetrifilmTM AC proved to be robust at 33°C and 37°C. In conclusion, PetrifimTM AC, which is easy to process, read and less time consuming, proved to be comparable to the conventional Pour Plate method in establishing HPC in water. In addition, PetrifimTM AC requires less space for the processing and incubation, generate small volume of waste for disposal and requires no equipment, except for the incubator.

La0.6Sr0.4MnO3-Based Fuel Electrode Materials for Solid Oxide Electrolysis Cells Operating under Steam, CO2, and Co-Electrolysis Conditions

The conventional Ni–YSZ (yttria-stabilized zirconia) fuel electrode experiences severe degradation due to Ni- agglomeration and migration away from the electrolyte. Therefore, herein, we have considered Ni free electrodes, i.e., La0.6Sr0.4MnO3-δ (LSM)-based perovskite oxides as fuel electrodes. The LSM perovskite phase transforms into a Ruddlesden–Popper LSM (RP-LSM) phase with exsolved MnOx under reducing atmospheres. The RP-LSM is mainly interesting due to its good electrical conductivity, redox stability, and acceptable electrochemical behaviour. In this work, we synthesized the LSM powder and characterized it using several methods including X-ray diffraction (XRD), thermogravimetry analyses (TGA), four-probe conductivity, and scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDX). Finally, the electrolyte-supported single cells were fabricated and electrochemically characterized using AC and DC techniques under electrolysis conditions. In addition to pure LSM fuel electrodes, we have also investigated the electrochemical behaviour of LSM + YSZ (50:50) and LSM + GDC (50:50) composite fuel electrodes. The single cells containing LSM and LSM + GDC fuel electrodes show higher cell performance than LSM + YSZ. For instance, current densities of 1, 1.03, and 0.51 A·cm−2 at 1.5 V are obtained for LSM, LSM + GDC, and LSM + YSZ fuel electrodes containing single cells, respectively, with a 50% N2 and 50% H2O feed gas mixture. Moreover, the performance of the cell was also investigated under co-electrolysis with 50% CO2 and 50% H2O and under direct CO2 electrolysis conditions with 100% CO2 fuel gas.

Large Area Solid Oxide Electrolyzer Stack Development for Cost-Effective Hydrogen Production

This presentation will provide an overview of the SOEC stack development effort at the Pacific Northwest National Laboratory aiming to construct and operate efficient high temperature electrolyzers for hydrogen and syngas production. Electrode-supported cells with a 300 cm2 active area were fabricated using state of the art materials: Ni-YSZ hydrogen electrode, LSCF oxygen electrode, YSZ electrolyte, and a ceria barrier layer. Stack repeat units consisting of cells, metal frames and interconnects with protective coatings were assembled into several short stacks and tested at 750oC. Initial stack shakedown tests revealed multiple sealing and assembling issues that were timely addressed. Short duration, 200-300 hours parametric tests were then performed to obtain realistic steam utilizations, 50-70%, and hydrogen production rates as a function of operating voltage/current. The electrochemical performance was monitored using dc and ac techniques to determine degradation rates and ohmic and polarization losses.

Exploring the dielectric polarization and ionic conduction mechanisms in sodium-containing silicate and borosilicate glasses

The dielectric and electrical behavior of a sodium silicate glass (soda-lime-silica; SLS) and a sodium borosilicate glass (D263T) are systematically and comprehensively characterized by a combination of ac and dc techniques at various temperatures and electric fields. This study reveals that Na+ migration primarily influences the dielectric loss, energy efficiency, and leakage current in these glasses, with a less detrimental effect in D263T due to its higher activation energy for ionic conduction. Moreover, this study identifies an additional relaxation mechanism contributing to the leakage current in D263T which was not observed in SLS. The origin of this mechanism has not been extensively explored, and it is believed to arise from a more limited migration of Na+ ions associated with the short migration and accumulation of a second and less mobile ion near the anode.

On-site Insulation Tests for MV cable using Very Low Frequency and Damped AC Techniques

MV cables are vital components in urban power networks. Several insulation test techniques are proposed for guaranteeing the operation of MV cables. In this paper, two kinds of off-line testing techniques are introduced, namely Tanδ measurement using VLF and PD detection using DAC. Firstly, the operation principle of each technique is introduced. Then, a simulation model was built in Simulink platform to demonstrate the operation processes of detecting the defects in the cable. At last, an on-site case using both VLF and DAC techniques is introduced for presenting the functions of different techniques.

Spectroscopic ellipsometry and raman spectroscopy of Bi1-xSbxTeI solid solutions with x≤0.1

Spectroscopic ellipsometry supported by reflectivity measurements and Raman spectroscopy are applied at room temperature to n-type Rashba semiconductors BiTeI and Bi1-xSbxTeI with nominal compositions x = 0.05 and 0.1. Complementary four probe Hall measurements are made using standard ac technique and the concentration of free carriers is determined. The Raman spectra, including TO-LO resonances are found to occupy a frequency range below 250 cm−1 in the studied materials. The pseudodielectric function, retrieved at different angles of incidence is analyzed focusing on the free carrier absorption (intraband transitions) and the optical transitions between the Rashba-split branches of the conduction band (intersubband vertical transitions). The former transitions with screened plasma frequency anchored to the zero-crossing point of the real part of the pseudodielectric function are described within a simple Drude model and the important parameters such as electron effective mass, electron mobility and high frequency dielectric constant are obtained. Anchored to the Rashba energy, the intersubband transitions obtained for each material by refining the pseudodielectric function from Drude contribution, appear in the imaginary part as a broad peak in the photon energy range between 0.15 and 0.4 eV. A noticeable red shift of this peak for Bi0.9Sb0.1TeI as compared to BiTeI is proposed to be a manifestation of the reduction of the spin splitting after part of Bi atoms is replaced by lighter Sb atoms.

Identification of Photoacidic Behavior Using AC and Open-Circuit Photoelectrochemical Techniques

Photoacids are molecules whose acidity increases through absorption of light. When the excited-state lifetime of a photoacid is sufficiently long, proton transfer from its thermally equilibrated electronic excited state results in a transient change in pH and/or pOH, which is commonly detected using spectroscopic techniques. Herein we expand this measurement toolkit by introducing alternating AC and open-circuit photoelectrochemical techniques that characterize photoacidic behavior from a model photoacid, the sodium salt of 8-hydroxypyrene-1,3,6-trisulfonate, dissolved in aqueous solutions in a thin-pathlength two-electrode cell. Continuous illumination of protonated photoacids in their electronic ground state results in significant and reproducible changes in low-frequency impedance and open-circuit potential. When these molecules are made to be non-acidic, via deprotonation using more alkaline pH conditions or methoxylation via synthesis, electrochemical data measured in the dark and under illumination are nearly identical. Best fits of AC electrochemical data to a simplified equivalent circuit support that photoelectrochemical responses are likely due to changes in local proton concentration at the electrode∣electrolyte interface, and not changes in proton flux due to mass transfer, as previously suggested. Collectively, our results provide further insight into the utility of these photoelectrochemical techniques to probe photoacidic behavior.

Anodization of medical grade stainless steel for improved corrosion resistance and nanostructure formation targeting biomedical applications

Stainless steel is extensively used in various biomedical engineering and hospital applications, including surgical equipment and furniture. Strong adhesion of bacteria and viruses on metal surfaces can restrict long-term utilization for biomedical applications. This study aims to develop an improved electrochemical etching protocol for the anodization of 316 L grade stainless steel (10 × 15 mm) to fabricate nanostructures for biomedical and hospital applications. Anodizing conditions were optimized using two different electrolyte solutions; HNO3: H2SO4 (1:1) and HNO3, by varying applied potential, electrolyte concentration and anodizing time. Morphology and topography of the anodized surfaces were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPFM). AC and DC electrochemical techniques were used to further characterize the corrosion behaviour of the nanostructured surfaces. Electrochemical optimization produced two different nanostructured surfaces with the anodizing conditions of (1) 50% HNO3 at 0.465 A/cm2 for 1 min (surface 1), and (2) 0.5 M HNO3 + H2SO4 (1:1) at 0.366 A/cm2 for 5 min (surface 2). Both processes produced nanoscale surface roughness with varying corrosion susceptibility. Surfaces anodized using 50% HNO3 comprised of ‘hierarchical roughness’ with dense spikes (10 – 20 nm in diameter), covering rock candy-like protrusion (10 - 15 µm diameter). Whereas the second set of conditions produced single scale roughness with a terrace-like topography with nanoscale ridges of 34.8 ± 1.2 nm in width atop microscale hills. Surface 2 possessed improved corrosion resistance through the formation an oxide film, while the surface 1 was more susceptible to corrosion. Overall, this study demonstrates the importance of the careful optimization of electrochemical surface treatment for medical grade stainless steel in terms of roughness of nanostructures and corrosion susceptibility.

Alternating Current Techniques for a Better Understanding of Photoelectrocatalysts

Several spectroscopic and electroanalytical techniques are in use when it comes to the investigation of interfaces in photoelectrocatalysis (PEC). Among them, AC techniques are the least used ones except for electrochemical impedance analysis despite having several advantages over DC electroanalytical techniques. Though AC electrolysis of water was known in the early years of the 20th century, it was not given significant attention either. In this Perspective, we discuss how these barely used yet powerful AC techniques, namely, (1) Mott–Schottky in impedance and photoelectrochemical impedance analyses (equivalent circuit analysis, capacitance spectroscopy), (2) intensity-modulated photocurrent spectroscopy (IMPS), (3) FS-HA-FT-AC voltammetry, and (4) paired electrolysis, can lead us ahead in the way of further fathoming interfaces in PEC with highlights on prospects of these AC techniques for energy conversion reactions.

Temperature-Modulated Pyroelectricity Measurements of a Thin Ferroelectric Crystal with In-Plane Polarization and the Thermal Analysis Based on One-Dimensional Layer Models

A temperature-modulated pyroelectricity measurement system for a small single crystal is developed and applied to standard sample measurements performed on a thin single crystal of lithium niobate. The modulation measurement is based on the AC technique, in which the temperature of the sample is periodically oscillated, and the synchronized pyroelectric signal is extracted using a lock-in amplifier. Temperature modulation is applied by irradiating periodic light on the sample placed in the heat exchange gas. To apply this technique to the transparent reference sample, a commercially available black resin is coated on the sample’s surface to absorb the light energy and transmits it to the specimen. The experimental results are analyzed using a two-layer heat transfer model to verify the effect of the light-absorbing layer as well as the radiative temperature modulation system.

Forschungszentrum Jülich – Current Activities in SOC Development

Understanding degradation mechanisms in SOCs remains an ongoing issue for its mitigation and to reach levels that will bring the technology closer to commercialization. Benefiting from our experience in performing long-term tests on stacks (up to 100,000 h) post-test examination of the components of several of these stacks gave a better understanding about various degradation effects. Among others the chromium poisoning of La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) and La0.6Sr0.4CoO3-δ (LSC) air electrodes used over the past decade in the cells will be presented.In parallel alternative air electrode materials for low-temperature application based on La2Ni1-xCoxO4+δ (LNCO) are being developed and the influence of A- and B-site substitution on the structural, physico-chemical and electrochemical properties of the materials is investigated. First promising results of a short-stack with Ni/YSZ substrate cells with La2Ni0.8Co0.2O4+δ air electrodes will be presented.Based on the earlier experience obtained from the operation of a 5 kW (in FC-mode) reversible system (rSOC), stacks and supporting components were adapted to scale-up to a 10/40 kW (FC/EL)-class system. This included among others the development of a scalable steam generator of which a prototype was tested separately. This allows for the optimization of the efficiency of the system, especially in electrolysis mode. The 10/40 kW class rSOC system will contain four 20-layer sub-stacks in the planar-type window frame stack-design, each with four standard-sized cells (10 × 10 cm²) in a 2 × 2 array per layer.Our activities focus more and more on investigating high-temperature electrolysis as a first step in future Power-to-X value chains using renewable energies, focusing on CO2-valorization through co-electrolysis of H2O and CO2 for syngas production as well as pure CO2-electrolysis. The performance and stability of cells and short-stacks were investigated with DC and AC techniques for various feed gas compositions. Especially the analysis of the Electrochemical Impedance Spectroscopy (EIS) measurements performed on State-of-the-Art cells in the boundary region between pure CO2-electrolysis and co-electrolysis with low steam content in the feed gas led to a better insight in the role of the reverse water gas shift reaction (RWGS) in the conversion of CO2 under these conditions.For the (co-)electrolysis case a model of the multi-physics phenomena occurring on cell level was developed using the COMSOL platform. The model is fully coupled including electrochemical, chemical, heat and transport phenomena occurring at single cell level. On stack level a steady-state, 3-D, non-isothermal, and homogenized model for rSOC stacks has been developed from an extension of previous studies and implemented into the open-source library OpenFOAM.

Investigation of Electrode Kinetics of Porous La-Sr-Co-Fe-oxide (LSCF) Electrodes on Yttria-Stabilized Zirconia (YSZ) Electrolyte Using Alternating Current (AC) and Direct Current (DC) Methods

Lanthanum strontium cobalt iron oxide (LSCF) is commonly used as a cathode in solid oxide fuel cells (SOFCs), because it is a mixed ionic-electronic conductor with reasonable oxygen ion conductivity and high electronic conductivity. Yttria stabilized zirconia (YSZ) is used as an electrolyte in SOFCs with good oxygen ion conductivity. AC techniques are used to test the performance of SOFCs. But electrode processes at the cathode and the anode cannot be studied separately using two-electrode electrical impedance spectroscopy (EIS). To overcome this problem, two-electrode EIS with three probes and DC tests were conducted. An LSCF/8YSZ/LSCF symmetrical bar-shaped cell was made, and platinum strip electrodes were applied as probes for EIS and DC measurements. Impedance spectra across the cathode and the platinum strip electrode and across the anode and the platinum strip electrode were measured separately. The sum was evaluated to see if it matches the EIS spectra across the cathode and the anode. The polarity was switched to study how it affects the electrode processes. The polarization resistances of the electrodes were also measured by a DC method separately. EIS and DC measurements are in good agreement. Results indicate the two electrodes need not be identical.

Highly anti–corrosive treatment of low–carbon steel

This paper describes the use of a combination of two specific layers to protect the surface of low–carbon steel against electrochemical corrosion phenomena, which has not been published in the scientific literature so far. The layers include a black oxide film and siloxane coating, which are formed in the steel blackening and sol–gel processes, respectively. The so–obtained samples were subjected to surface morphology and physicochemical analysis (atomic force microscopy and Fourier-transform infrared spectroscopy) as well as electrochemical measurements, including AC and DC techniques. The physicochemical and surface morphology analysis allowed to prove the presence and adhesion of the siloxane coating to the surface of the black oxide layer, while the results of the electrochemical tests show that low–carbon steel modified this way is characterized by remarkable anti–corrosive properties, comparable to those of stainless steel.

Microstructures and Electrical Properties of Carbon/Silica Nanocomposites: Influence of the Silica Nanoparticles

In this work, the inorganic/organic a Carbon-Silica (C-S) nanocomposites in additional amorphous carbon matrix was successfully obtained through the sol–gel method, the two components are mixed in an effort to blend the best properties of both. The acquired C-S nanocomposites were characterized by X-ray diffraction (XRD). It was observed by Transmission Electronic Microscopy that nanoparticles with sizes from 14[Formula: see text]nm to 20[Formula: see text]nm. After heat treatment, we explore the electrical properties of the obtained C-S material. The current–voltage ([Formula: see text]–[Formula: see text]) and alternating current (ac) techniques in the temperature range (80–300[Formula: see text]K) was examined.

A Low-Cost DAC System for On-Site PD Testing of HV Cables Based on ACRF and Capacitor Bank

Damped AC (DAC) technique has proven to be one of the most acceptable techniques for on-site partial discharge (PD) detection. However, the expensive high-voltage switches, used in the DAC system, impede the further spread of the technique. This letter presents a low-cost DAC system for PD test of high-voltage cables. The DAC voltage is generated by the frequency-tuned series resonant technique and two cost-effective switches which have relatively low rated voltage. Since the system uses a capacitor bank, high input power is not necessary. This letter describes the design and implementation of the system. An experimental study and an on-site test validate the effectiveness of the proposed system.

Application of a Langmuir probe AC technique for reliable access to the low energy range of electron energy distribution functions in low pressure plasmas

The electron energy distribution function (EEDF) in low pressure plasmas is typically evaluated by using the second derivative d2I/dV2 of a Langmuir probe I–V characteristic (Druyvesteyn formula). Since measured probe characteristics are inherently noisy, two-time numerical differentiation requires data smoothing techniques. This leads to a dependence on the employed filtering technique, and information particularly in the region near the plasma potential can easily get lost. As an alternative to numerical differentiation of noisy probe data, a well-known AC probe technique is adopted to measure d2I/dV2 directly. This is done by superimposing a sinusoidal AC voltage of 13 kHz on the probe DC bias and performing a Fourier analysis of the current response. Parameters such as the modulation amplitude (up to 1.5 V) and the number of applied sine oscillations per voltage step of the DC ramp are carefully chosen by systematic parameter variations. The AC system is successfully benchmarked in argon and applied to hydrogen plasmas at a laboratory inductively coupled plasma experiment (4–10 Pa gas pressure, 300–1000 W RF power). It is shown that the EEDF is reliably accessible with high accuracy and stability in the low energy range. Hence, a trustworthy determination of basic plasma parameters by integration of the EEDF can be provided.

Protective Layers Based on Carbon Paint To Yield High-Quality Large-Area Molecular Junctions with Low Contact Resistance.

A major obstacle for transforming large-area molecular junctions into a viable technology is the deposition of a top, metallic contact over the self-assembled monolayer (SAM) without chemically damaging the molecules and preventing an interface-limited charge transport. Often a thin conducting layer is softly deposited over the SAM to protect it during the deposition of the metal electrode which requires conditions under which organic molecules are not stable. We report a new protective layer based on carbon paint which is highly conductive and has metallic-like behavior. Junctions made of SAMs of n-alkanethiolates supported by Au were characterized with both dc and ac techniques, revealing that carbon paint protective layers provide a solution to three well-known challenges in molecular junctions: series resistance of the leads, poor interface conductance, and low effective contact area related to the roughness of the interfaces. Transport is constant with coherent tunneling down to 10 K, indicating the carbon paint does not add spurious thermally activated components. The junctions have both high reproducibility and good stability against bias stressing. Finally, normalized differential conductance analysis of the tunneling characteristics of the junctions as a function of molecular length reveals that the scaling voltage changes with molecular length, indicating a significant voltage drop on the molecules rather than on the molecule–electrode interface. There is a clear inverse dependence of the scaling voltage on length, which we deduced has a tunneling barrier height of close to 2 eV. The paper establishes the reliability of carbon paint protective layers and provides a procedure for discriminating genuine molecular effects from interfacial contributions.

Tackling Challenges in Seebeck Coefficient Measurement of Ultra‐High Resistance Samples with an AC Technique

AbstractSeebeck coefficient is a widely studied semiconductor property. Conventional Seebeck coefficient measurements are based on DC voltage measurement. Normally this is performed on samples with moderate resistances (e.g., below a few MΩ level). Certain semiconductors are intrinsic and highly resistive. Many examples can be found in optical and photovoltaic materials. The hybrid halide perovskites that have gained extensive attention recently are a good example. Despite great attention from the materials and physics communities, few successful studies exist of the Seebeck coefficient of these compounds, for example CH3NH3PbI3. An AC‐technique‐based Seebeck coefficient measurement is reported, which makes high‐quality Seebeck voltage measurements on samples with resistances up to the 100 GΩ level. This is achieved through a specifically designed setup to enhance sample isolation and increase capacitive impedance. As a demonstration, Seebeck coefficient measurement of a CH3NH3PbI3 thin film is performed at dark, with sample resistance 150 GΩ, and found S = +550 µV K−1. The strategy reported could be applied to the studies of fundamental transport parameters of all intrinsic semiconductors that have not been feasible.