Direct Current Method Research Articles

A Metallogenic Model for Bauxite Deposits and Geophysical Prospecting Methods: Using the Sedimentary Type in Northern China as an Example

An ideal metallogenic and prospecting model has important guiding significance for aluminum ore development and geophysical exploration. Previous research in this field only focused on ore body evaluations and metallogenic belts. Selection of reasonable geophysical methods for exploration of bauxite deposits is still challenging. To solve these problems, a new metallogenic model based on the new geological information is established firstly. The ore-bearing rock and ore bodies of sedimentary bauxite deposits are strictly controlled by the paleo-topographic conditions. The undulating shapes of the paleo-topography control the scales and shapes of ore-bearing rock series and bauxite ore bodies. Then, different geophysical methods applied for exploration of bauxite deposit are analyzed based on the petrophysical measurement. For the areas with thick overburden and large buried depths, it was found that fixed-loop transient electromagnetic (TEM), high-precision gravity, and seismic exploration methods were most effective. In regard to the areas with thin overburden and small buried depths, direct current (DC) sounding, high-precision gravity, seismic exploration, and TEM methods were considered to be effective. However, for the areas with very shallow buried depths, DC sounding and seismic exploration methods were deemed to be more suitable. This study selected the actual exploration processes of the Wanggudong bauxite mining area in western Henan as an example, and the application of a DC sounding method for sedimentary-type bauxite was successfully introduced.

A Dual Reference Frame Multistep Direct Model Predictive Current Control With a Disturbance Observer for SPMSM Drives

The parameter mismatch problem has a great impact on the control performance of model predictive control, which is however unavoidable during the operation. In order to improve the system robustness against the parameter mismatches and disturbances, an improved direct model predictive current control with a disturbance observer is proposed in this article, where the disturbance observer is realized by an incremental moving horizon estimator. Moreover, another concern raised from the applications of direct model predictive current control is the computational burden, especially for the long-horizon implementations. Therefore, a dual reference frame solution for the surface permanent-magnet synchronous motor (SPMSM) is proposed in this article to allocate a great proportion of heavy computations required for the optimization problem to the offline preparation, which can reduce the computational burden by almost 50% on average for a prediction horizon of five time-steps. Besides, the parameter mismatch effects of individual electrical parameters on the control performance of the model predictive direct current control method are investigated and quantified via simulations. A five-step direct model predictive current control is implemented on a dSPACE system with a sampling frequency of 20 kHz to validate the effectiveness of the proposed scheme with an SPMSM drive system.

An Improved Current Detecting Method for STATCOM Considering Unbalanced Compensation

Background: Static Synchronous Compensator plays an essential role in governing the reactive power and harmonics of power systems. The accuracy and real-time performance of reactive power and harmonic detection are necessary to ensure balancing compensation effectiveness. The traditional dq current detection method can only extract positive sequence components in the positive sequence rotational reference frame and extract negative sequence components in the negative sequence rotational reference frame. Objective: This paper aims to give a practical and straightforward current detection solution to enhance the compensator performance and simplify the control system by extracting the positive sequence, negative sequence, and harmonics in the positive sequence rotational reference frame. Methods: Firstly, a grid-connected four-quadrant converter with an Inductance-Capacitance- Inductance (LCL) filter balancing compensation topology is constructed. Then an improved current detection algorithm is proposed to extract the positive sequence, negative sequence, and harmonics in the positive rotational reference frame separately. Finally, the Direct Current (DC) side voltage design method is discussed to avoid the overmodulation effects. Results: Matlab simulation and a prototype 500kVA Static Synchronous Compensator (STATCOM) experiment are implemented to demonstrate the proposed method. Conclusion: The proposed strategy can effectively improve the compensation accuracy, and the system structure is simplified while the stability and reliability of the whole system are ensured.

Comparison of the structural and optic al properties of amorphous silicon thin films prepared by direct current, bipolar pulse, and high-power impulse magnetron sputtering methods

This paper presents a comparative study of amorphous silicon (a-Si) thin films deposited by three different techniques: direct current magnetron sputtering (DCMS), bipolar pulse magnetron sputtering (bipolar pulse MS) and high-power impulse magnetron sputtering (HiPIMS). The structurafl properties of the a-Si films with a thickness of ∼112 nm were characterized by X-ray reflectivity and Raman spectroscopy. The surface morphology, optical and mechanical properties of a-Si films with a thickness of ∼400 nm were investigated using a field emission secondary electron microscope, an atomic force microscope, spectroscopic ellipsometry and nanoindentation. It was found that for the same time-averaged discharge power, the deposition rate of the HiPIMS method was lower compared to DCMS and bipolar pulse MS. The a-Si film prepared by HiPIMS method had the lowest root mean square roughness of 0.65 nm and the highest density of 2.21 g/cm3. These results are consistent with the highest refractive index (400–700 nm) of 3.66 and a hardness of 9.09±0.19 GPa in the film, indicating that the HiPIMS method produces denser films than the DCMS and bipolar pulse MS methods. The amorphous silicon thin films fabricated by HiPIMS have the potential to improve the structural and mechanical properties for high performance thin film semiconductor applications.

Experimental Study on Failure Law and Mechanism of Red Sandstone under Ultrasonic Vibration Excitation

Rock breakage is inevitable for creating openings in underground engineering operations. Ultrasonic vibration has been attracting extensive attention for such a practice considering its outstanding performance in rock breakage. In order to understand the fundamental failure mechanism of rocks subjected to ultrasonic vibrations, based on P-wave monitoring and the direct current electric method, we captured the evolution of the failure process of the red sandstone. In addition, we fundamentally analyse the failure mechanisms of the red sandstone using numerical simulation and microscopy scans. It was found that extensive fractures were initiated due to the ultrasonic vibration and the fractures propagated downwards forming a conical shape. The apparent resistivity became as high as 320000 Ω being 16 times the initial resistivity. The fracture propagated downwards as deep as 41 mm. The maximum damage parameter on the testing sample could be as high as 0.68, and it completely failed after 140 s of ultrasonic vibration duration. As a result of numerical simulation, it was found that the microfractures and pores in the testing sample were activated due to the stress wave resulting from the ultrasonic vibration leading to the fracture propagation and eventually complete failure. Through comparing the performance of uniaxial compressive loading and ultrasonic vibration techniques in rock damage, it was concluded the latter has a much higher capacity and competence in rock breakage.

Three-dimensional numerical modeling of direct current resistivity methods in mixed space-wavenumber domain

At present, in order to obtain highly accurate solution for complex (geo-electric) conditions, it is necessary to divide the model precisely in the numerical modeling of DC resistivity method, which results in greatly increases the amount of computation and memory. To solve this problem, we propose a 3D numerical simulation method in a mixed space-wavenumber domain. The method transforms the partial differential equation of DC abnormal potential into many independent 1D differential equations with different wavenumbers by performing a 2D Fourier transform along the horizontal direction. It decomposes a large-scale 3D numerical modeling problem into many 1D numerical modeling problems, which greatly reduces the computation and memory requirements of modeling, and each 1D differential equation is independent, so it has high parallelism. The vertical direction is reserved as the spatial domain, which is convenient for the subdivision, taking into account the calculation accuracy and efficiency. A finite-element algorithm combined with a chasing method is used to solve the 1D differential equations with different wavenumbers, and a contraction operator based on electromagnetic integral equation method is used to the iteration of the algorithm for the first time. Taking full advantage of the rapidity of Fourier transform, the high efficiency of solving 1D differential equations and the high parallelism of 1D problems with different wavenumbers, it can greatly reduce the amount of calculation and storage, thus greatly improves the efficiency of the algorithm. The accuracy and convergence of the algorithm are verified by using the model of a sphere in half space. Based on the model with five cubes in half space, the efficiency of the algorithm is verified by comparing with the academic Gimli package based on the finite element method. In addition, a parallel algorithm based on OpenMP is used to verify the parallelism of the proposed algorithm. The examples results show that, the 3D numerical modeling method of DC resistivity in the mixed space-wavenumber domain has the characteristics of high efficiency, high precision and highly parallel. This method provides a new way for 3D numerical simulation of DC resistivity method.

Direct current electrochemical method for removal and recovery of heavy metals from water using straw biochar electrode

Treatment of heavy-metal pollution in industrial wastewater is critical in protecting human health and the environment. Current methods for heavy-metal treatment have many limitations. For large quantities of industrial wastewater, the common methods such as adsorption and chemical precipitation can not meet the needs of actual production because of their high cost and limited removal efficiency. Electrochemical deposition is a rapidly developing clean treatment technology in recent years. The preparation of good electrode materials is one of the keys to the development of this technology. Firstly, by adjusting the pyrolysis temperature, rice straw was transformed into conductive biochar at 900 °C and CO2 atmosphere; then, we proposed a new concept of electrode preparation, that is, straw biochar electrode was prepared by thermosetting method. In this innovative method, a three-dimensional circular straw biochar electrode with high strength was successfully prepared by thermosetting method, phenolic resin as molding agent and conductive straw biochar as main raw material; using the electrode as cathode, cadmium polluted wastewater was treated by DC electrodeposition. The preliminary experimental results show that when the current density is 7.80 mA/cm2, the removal rate of cadmium in the solution reaches 76.60% in 240 min. Furthermore, the electrode is separated from cadmium by acid pickling to realize the effective recovery of cadmium and electrode regeneration. After regeneration, the removal rate of cadmium still reached 74.16%. Our work is very important for the treatment of cadmium ion pollution in water. The electrode can directly and effectively removes cadmium ions from wastewater. After pickling, the deposited cadmium can be effectively separated from the electrode, and the separated electrode can continue to be recycled. This study provides a simple and clean method for the treatment of cadmium containing wastewater, which can effectively reduce the cost of treating heavy metal wastewater by electrochemical deposition, and has important research and industrial significance.

Electrodeposition of Thick and Crack-Free Fe-Cr-Ni Coatings from a Cr (III) Electrolyte

The electrodeposition of iron-nickel-chromium coatings is a more environmentally friendly and economical alternative to hard-chrome coatings made from chromium (VI) electrolytes and stainless-steel bulk materials. The aim of the study was to develop a suitable deposition method for thick and low-crack Fe-Cr-Ni coatings. Iron-nickel-chromium coatings were electrodeposited using a more ecological chromium (III) electrolyte with direct current (DC), stepped direct current, and pulse current (PC). The influence of the deposition method on the electrolyte aging, the alloy composition of the coating, and their microstructure was investigated. Corrosion studies of the Fe-Cr-Ni coatings in 3.5% NaCl solution were performed using polarization tests. Furthermore, hardness measurements and scratch tests were carried out to determine the adhesion strength. Phase analyses were performed by X-ray diffraction, and the chemical composition and microstructure were characterized by scanning electron microscopy. Using the stepped DC and PC method, crack-free Fe-Cr-Ni coatings were successfully deposited.

Bi@C Core-Shell Nanostructure as Anode for Sodium-Ion Batteries from One-Step Arc Discharge

Bismuth has a great potential as the anode for sodium-ion batteries (SIBs) because of the layered structure with large interlayer space, long mean free path, and high volumetric capacity. In this paper, carbon-coated bismuth (Bi@C) core-shell nanostructure has been in situ synthesized by one-step direct current (DC) arc discharge method for the first time. The Bi@C nanocomposite without further decoration or any other treatment exhibits superior electrochemical performance as anode for SIBs and delivers stable capacities of 356, 342, 334, 327, 325, 323, and 318[Formula: see text]mA[Formula: see text]h[Formula: see text]g[Formula: see text] at the current densities of 0.2, 0.5, 1, 3, 5, 7 and 10[Formula: see text]A[Formula: see text]g[Formula: see text], respectively. There is no obvious decay after 630 cycles with a capacity of 291[Formula: see text]mA[Formula: see text]h[Formula: see text]g[Formula: see text] at 10[Formula: see text]A[Formula: see text]g[Formula: see text], showing the excellent rate capability and long cycle stability in SIBs. Electrochemical tests proved that the ultra-fast charge–discharge performance is related to the fast redox kinetics of Na[Formula: see text] in layered structure of bismuth crystal. The Na[Formula: see text] storage mechanism of Bi@C nanocomposite anode was also investigated by cyclic voltammetry and in situ XRD. Based on experimental data, this paper proves that the arc discharge method is a simple and efficient method for preparing nanoscale electrode materials.

Determination of Cracks using Multiple DC Potential Drop Measurements – Experimental Verification of an Advanced Model

The Direct Current Potential Drop Method (DCPDM) is a frequently used technique for crack detection and crack length measurement in fatigue experiments. Recent investigations have shown that with multiple potential drop measurements the location of crack initiation can be determined in single edge notch specimens (Wiehler and Bär, 2020), (Bär and Nahbein, 2022) and in round bars (Hartweg and Bär, 2019). In the present work a more detailed investigation of crack initiation and propagation is undertaken on round bars to advance the geometrical model by Hartweg and Bär (2019). Three potential probes were equipped on notched steel bars – at 0° (U1), at 120° (U2) and at 240° (U3). The three potentials were measured simultaneously during the fatigue tests using amplifiers of the control electronics. Moreover, the introduction of overloads was used to mark the crack front on the fracture surface in defined intervals to compare crack and size calculated from the potential drop measurements with the real crack size and geometry.In the case of an initiated single crack, the experimental set-up delivers different potentials depending on the distance of the probes to the crack. The geometrical model can be used not only for the determination of the crack angle like performed by Hartweg and Bär (2019) but also to calculate the crack size. On the measured data a function is fitted that allows the calculation of the crack length independent of the location of the crack initiation site. The experimental results have shown that it is possible to determine a crack with a size of less than 1 % of the specimen cross-sectional area.

Fracture mechanical investigation of preformed metal sheets using a novel CC-specimen

Joining is used in many areas of manufacturing where components are formed from individual parts or sheets into complex structures. A simple and common method applied in industry to join dissimilar and coated materials is clinching. This joining process does not require additional joining parts, which saves weight to reduce energy requirements and harmful emissions due to moving masses. In addition, the joining process is carried out without pre-punching and is therefore time-efficient. Damage caused by the joining process or by operational loading can lead to cracks near the joint, to crack propagation due to time-varying loading and subsequently to the complete failure of the component. To avoid these damage cases, knowledge about the fracture mechanical behavior of the clinched connection is required.Within the scope of this paper, a novel CC-specimen for fracture mechanical investigation of preformed metal sheets is developed and investigated. The new specimen geometry is taken directly from a clinched joint in order to identify a possible influence of the outer forming area of the joint on the fatigue crack growth rate. The clinched joint is placed at three different positions and is moved from the radius to the center of the C-shape at 3 mm intervals. In order to investigate the crack growth rate of this new specimen, it is necessary to numerically determine the geometry factor function for the stress intensity factor as well as the calibration curve for the crack length measurement with the direct current potential drop method beforehand. Finally, the results of the crack growth rate tests using the CC-specimen with different positions of the clinched joint are compared with the results of the base material.

Automated crack length measurement for mixed mode fatigue cracks using digital image correlation

It is well known that mode II leads to a kinking and mode III to a twisting of a fatigue crack. On the surfaces of an SEN specimen, an overlapping of these two crack loading modes occurs. Superimposed with mode I, this results in very complex fatigue crack paths and, in some cases, multiple crack branches. Thus, established methods for mode I crack growth measurement cannot be utilized. Therefore, the digital image correlation (DIC) technique is used to determine the crack length of mixed mode cracks on the surface of a specimen by means of the maximum principal strain. By Gehri et al. (2020) a concept is described to obtain the crack path by morphological thinning of the area on the surface, where the maximum principal strain exceeds a threshold value. In this paper, an extension of the provided MATLAB script is proposed that includes two criteria for measuring the crack length versus the number of load cycles (a-N-curve). For the first criterion, the crack opening is used to determine the current crack tip position and to obtain the crack length as a function of the number of load cycles. In the second criterion, a threshold value of the maximum principal strain is used for determining the number of cycles at the detected crack length. The new evaluation routine with both criteria is initially calibrated on mode I experiments using the direct current potential drop method. Moreover, mixed mode experiments under tension-torsion loading are conducted, to validate the method and compare the two proposed criteria. The experimental results in terms of the a-N-curves obtained with the marker load technique are in good agreement with the results using the presented method. Thus, the extended tool provides an evaluation routine to determine a-N-data for mixed mode fatigue crack growth tests independently from the crack orientation and possible branching.

Investigation of Crack Formation and Propagation in AA7475 using Multiple Potential Drop Measurement

The direct current potential drop method (DCPD) is a well-known method for crack detection and crack length measurement in fatigue experiments. Recent Investigations have shown that with multiple potential drop measurements the location of crack initiation can be determined in round bars (Hartweg and Bär, 2019) and single edge notched specimens (Wiehler and Bär, 2020).In this work a more detailed investigation of crack initiation and propagation is undertaken on single-edge notched specimens. The specimens were equipped with three potential probes – on the frontside (Ufront), on the backside (Uback) and on the narrow side (Unarrow) of the specimen. During the fatigue tests all three potentials were measured simultaneously using amplifiers of the control electronics. The crack front was marked on the fracture surface by introducing overloads in defined intervals to allow a direct comparison between the measured potential drop and the real crack location and crack front geometry.In a first step relative Potentials Pi (i= front, back and narrow) were calculated by normalizing the actual potential on the potential of the crack-free specimen. To localize the crack initiation site and to eliminate temperature effects quotients of the potentials were formed by dividing Pfront and Pback by Pnarrow, respectively. By comparing the runs of the potential quotients plotted over the cycle number, the time of crack initiation and the location of the crack initiation site can be determined. In addition, the run of the quotients gives information about the shape of the crack front of long fatigue cracks.

Effect of the sputtering time on the characteristics of p-SnOx:Na films prepared by the reactive direct current magnetron sputtering method

The fabrication of n- and p-type transparent oxide semiconductor films is critical in the production of transparent electrical and optoelectronic components on the p‒n junction, particularly the p-type, because the maximum of the valence band formed by the 2p orbitals of oxygen and their local (narrow) dispersions have formed deep levels, making difficult in doping of acceptor impurities in p-type oxide semiconductors. Among the p-type transparent oxide semiconductor films that have been studied so far, the p-type SnOx film which had outstanding electrical conductivity and optical transparency, is a good candidate for constructing a p‒n junction with another n-type semiconductor in homogeneous or heterogeneous form. In this paper, a Na-doped SnOx film was produced from Sn + 3 at% Na alloy target using the reactive sputtering process at a temperature of 200 0C. XRD, XPS, AFM, Hall, and UV-Visible techniques were used to evaluate the effect of sputtering duration on the structural and electro-optical properties of SnOx:Na films. The results of the survey showed that when the sputtering time increased from 7 minutes to 14 minutes, 21 minutes, and 35 minutes, the film thickness increased, resulting in an increase in the crystal size, a decrease in the grain boundary, and an increase in carrier mobility, all of which lead to a reduction in the film's resistivity. However, as the film thickness increased, the transmittance dropped and the optical bandgap shrinked.

Prediction of Conductive Anomalies Ahead of the Tunnel by the 3D-Resitivity Forward Modeling in the Whole Space

Water inrush in tunneling poses serious harm to safe construction, causing economic losses and casualties. The prediction of water hazards before tunnel excavations becomes an urgent task for governments or enterprises to ensure security. The three-dimensional (3D) direct current (DC) resistivity method is widely used in the forward-probing of tunnels because of its low cost and highly sensitive response to water-bearing structures. However, the different sizes of the tunnel will distort the distribution of the potential field, which causes an inaccurate prediction of water-bearing structures in front of the tunnels. Some studies have pointed out that the tunnel effect must be considered in the quantitative interpretation of the data. However, there is rarely a predicted model considering the tunnel effect to be reported in geophysical literature. We developed a predicted model algorithm by considering the tunnel effect for forward-probing in tunnels. The algorithm is proven to be feasible using a slab analytic model. By simulating a large number of models with different tunnel sizes, we propose an equation, which considers the tunnel effect and can predict the water-bearing structures ahead of the tunnel face. The Monte Carlo method is used to evaluate the quality of the predicted model by simulating and comparing 10,000 random models. The results show that the proposed method is accurate to forecast the water-rich structures with small errors.

Combined RBFN based MPPT and d-axis stator current control for permanent magnet synchronous generators

This paper proposes a new radial basis function neural network maximum power point tracking controller based on a differential evolution algorithm for machine side converter of permanent magnet synchronous generator wind turbine under variable wind speed. Direct axis stator current control methods of permanent magnet synchronous machine are reviewed shortly. A combined radial basis function neural network-based network maximum power point tracking method and d axis stator current control techniques including zero d axis stator current, unity power factor, and constant stator flux-linkage have been implemented to control the machine side converter of permanent magnet synchronous generator wind turbine. The dynamic performance of the proposed approach is assessed under different operating conditions through a simulation model based on MATLAB. It has been seen that the radial basis function neural network controller can not only track well the maximum power point but also can be reduced costly.

Retraction Note to: Structural and electrochemical impedance spectroscopic studies on reactive magnetron sputtered titanium oxynitride (TiON) thin films

Titanium oxynitride films were deposited onto commercially pure titanium substrates by direct current reactive magnetron sputtering method using Ti targets and an Ar–N2–O2 mixture discharge gas. The X-ray photoelectron spectroscopy survey spectra on the etched surfaces of TiON films exhibited the characteristic Ti 2p, N 1s, and O 1s peaks at the corresponding binding energies 454.5, 397.0, and 530.7 eV, respectively. The surface topography of these coatings was studied using atomic force microscopy. The characteristic Raman peaks at 200 and 641 cm−1 for the TiN bonds and at 148, 398 ,and 518 cm−1 for the TiO2 bonds were observed from the Laser Raman spectrometer. The potentiodynamic polarization studies in simulated bodily fluid were performed and the results are reported in this article.

Peculiarities of specular infrared reflection spectra of ZnO-based ceramics

Undoped and Mn-doped ZnO ceramics were theoretically and experimentally investigated using specular infrared reflection method. It was shown that infrared reflection spectra can be modeled using the parameters explored for ZnO single crystals. For ceramic samples, it was shown that ZnO grains with orientation of the C-axis along the normal to the electric field ( ) give the main contribution to IR reflection spectra. It has been ascertained that the surface roughness is manifested in these spectra mainly within the range 450…550 cm–1 giving negligible effect for the frequencies above longitudinal phonon frequency. This allowed the electrophysical parameters of ZnO crystallites to be evaluated. In the case of undoped ceramics, the obtained results were found to be consistent with the values of direct current measurements. This finding supports the utility of infrared spectroscopy for determination of the electrophysical parameters of polycrystalline ceramic materials. For Mn-doped ceramic samples, the conductivity value measured using the direct current method was found to be essentially lower than those determined from simulation of infrared reflection spectra. This phenomenon was explained by barrier formation at the grain boundaries in Mn-doped ZnO ceramics.

Evaluation of the passivity limits in austenitic stainless steel exposed to H2S-containing brines using point defect model analysis

ABSTRACT This study evaluates the effects of temperature, H2S, and Cl− concentration on the onset of localized corrosion, in UNS S31603 stainless steel by evaluating the changes in the behavior of the passive layer. This is accomplished experimentally by using direct current electrochemical methods to study the passive layer formed by the stainless steel, in equilibrium with a gas phase at 2.8 MPa (400 psi) containing up to 60% mol of H2S (bal. CO2) at temperatures up to 150°C. The results indicate that the increase in the likelihood of both metastable and stable pitting on the stainless steel is consistent with the increase in the metal cation diffusivity as postulated by the point defect theory. In this context, the solution temperature was found to exert a larger effect than the H2S activity.

Numerical modeling of fault structures in the Kurai basin of Gorny Altai based on data from direct current methods

The southern site is located in the junction zone of the Southwestern and Eshtykel bench, where vertical electrical soundings were performed, and a preliminary fault-block depth model was built based on the results of field data interpretation using a horizontally layered model. Comparison of geoelectric and seismological data showed that the epicenters of significant earthquakes (M> 4) are concentrated in the identified faults. In the central site, three profiles of electrotomography were made through a bench, well expressed in the relief. Three-dimensional modeling was used to verify and clarify the structural features of both sections. Modeling is performed using programs EMF_DC3Dmod (GPU) - an accelerated version of the program EMF_DC3Dmod for vertical electrical sounding and SCALA-48 (GPU) for the method of electrotomography. The study is relevant for the tasks of geodynamics, seismic zoning and seismic hazard assessment.