Conventional Direct Current Research Articles

High-performance photon-driven DC motor system

Direct current (DC) motors are crucial in drones, robotics, and electrical devices. Conventionally, the DC motor is driven by a switching electricity converter, which utilizes electrical energy to drive mechanical motion. However, the rapid on-off switching actions in the switching electricity converter would cause electromagnetic interference (EMI), impairing the functionality of drive systems. Here, we propose a photon-driven DC motor system based on photonic converter, which utilizes optical energy to drive mechanical motion, and therefore avoids EMI derived from electrical switching and immunizes against EMI during electrical energy transmission in conventional switching electricity-driven DC motor system. The operation principle and power modulation-based speed control are also presented for the proposed photon-driven DC motor system. The experiments demonstrate that the motor accurately follows the speed reference and withstands load disturbances. This innovation opens new potential for DC motor applications by improving electromagnetic compatibility.

Radiofrequency-transparent local B0 shimming coils using float traps.

Static field (B0) inhomogeneities present a major challenge in high-field MRI. Multicoil shimming using independent, local, direct-current (DC) shim coils has emerged as a powerful and flexible technique to address this issue. However, many-turn DC coils can lead to significant mutual coupling with radiofrequency (RF) coils, causing transmit field (B1 +) distortions and signal-to-noise ratio degradation. We introduce an innovative RF-transparent DC coil that performs B0 shimming while minimizing RF performance impact. The design incorporates float traps to maintain high RF impedance, allowing flexible placement relative to the RF coil without compromising signal-to-noise ratio or affecting B1 +. We fabricated square-shaped DC coils with float traps for 3T MRI and compared them with conventional DC coils. To demonstrate high ΔB0/Amp efficiency, we conducted a B0 shimming experiment around a metal hip implant. Bench tests and MRI experimental results demonstrated that the RF-transparent DC coil effectively minimized RF interference, preserved signal-to-noise ratio, and maintained B1 +, even when placed near the RF receive coil. Additionally, the DC coil significantly improved B0 homogeneity near metal implants and substantially reduced image distortion. The RF-transparent DC coil offers a flexible, effective solution for managing B0 inhomogeneities, paving the way for integrating multiturn DC coils in clinical MRI settings without extensive hardware modifications.

Bidirectional rotating direct-current triboelectric nanogenerator with self-adaptive mechanical switching for harvesting reciprocating motion

Abstract Amid the growing interest in triboelectric nanogenerators (TENGs) as novel energy-harvesting devices, several studies have focused on direct current (DC) TENGs to generate a stable DC output for operating electronic devices. However, owing to the working mechanisms of conventional DC TENGs, generating a stable DC output from reciprocating motion remains a challenge. Accordingly, we propose a bidirectional rotating DC TENG (BiR-TENG), which can generate DC outputs, regardless of the direction of rotation, from reciprocating motions. The distinct design of the BiR-TENG enables the mechanical rectification of the alternating current output into a rotational-direction-dependent DC output. Furthermore, it allows the conversion of the rotational-direction-dependent DC output into a unidirectional DC output by adapting the configurations depending on the rotational direction. Owing to these tailored design strategies and subsequent optimizations, the BiR-TENG could generate an effective unidirectional DC output. Applications of the BiR-TENG for the reciprocating motions of swinging doors and waves were demonstrated by harnessing this output. This study demonstrates the potential of the BiR-TENG design strategy as an effective and versatile solution for energy harvesting from reciprocating motions, highlighting the suitability of DC outputs as an energy source for electronic devices.

Significant texture and wear resistance improvement of TiN coatings using pulsed DC magnetron sputtering

Titanium nitride (TiN) coatings fabricated through reactive sputtering feature a suite of parameters capable of altering the microstructure and properties. Here, we explore the influence of bipolar pulsed direct current (DC) magnetron sputtering on microstructure evolution of TiN coating, in comparison to conventional DC sputtering. The implementation of a pulsed DC voltage profile promotes a drastic texture change from randomly oriented polycrystals to (111) textured TiN coatings across the full range of pulse frequencies on various Si substrates, including amorphous SiO2. Additionally, higher pulse frequencies promote significant grain size reduction, accompanied with corresponding increases in hardness and wear resistance. The potential mechanism for this microstructural and texture evolution is also explored.

Electrochemical oxidation degradation of Rhodamine B dye on boron-doped diamond electrode: Input mode of power attenuation

Electrochemical oxidation is a rapid and efficient technology to remove organic matter from wastewater, but the most obvious defects are low current efficiency and high energy consumption. This paper presents a novel power supply mode with power attenuation. The attenuated power is periodically input to the reaction system to improve the average current efficiency and reduce energy consumption. The removal efficiency, current efficiency and energy consumption of treating Rhodamine B were compared between the two modes of power attenuation and the conventional direct current. Applying current attenuation in the step-Mode A1, 98.58% Rhodamine B was decomposed and the COD removal efficiency was 91.47%. The current efficiency reached 18.49% and electrocal energy consumption was only 44.58 kWh∙m−3. Under the conditions of the same removal efficiency of Rhodamine B, the current efficiency of Mode A1 was 2.04 times larger than that of Mode D1, and the energy consumption was only 45.41% of Mode D1. Five cycles of experiments show that the boron-doped diamond electrode has obvious stability and reusability in the electrochemical oxidation process. ·OH was the main active substance involved in Rhodamine B degradation. The power attenuation mode to treat Rhodamine B dyes electrochemically will be the most promising saving energy method for efficient processing dyes wastewater in the future.

Energy-saving actuation signal for magnetorheological valve train by leveraging hysteresis effect

• Analysis of electromagnetic hysteresis effect in magnetorheological valve train. • Customized pulse actuation signal for leveraging the hysteresis effect in magnetorheological valve train. • Pulse signal optimization for energy saving operation of VVL that improves the engine efficiency. • Customized pulse signal saves actuation energy of magnetorheological valve train. Smart, adaptive, and effective magnetorheological (MR) technology-based devices are replacing conventional brakes, clutches, valves, and dampers. MR valve improves engine performance by enabling variable valve actuation. Constructing an MR valve train is simpler than other active valve systems. However, similar to other electromagnetic valves, the MR valve train consumes considerable electrical energy in controlling the resistance force of the MR fluid such that the valve can remain partially open or fully closed. Thus, an engine equipped with an MR valve expends additional fuel for the actuation of the valve system. In this study, instead of conventional direct current (DC) actuation, an energy-saving customized pulse (CP) signal is proposed for valve actuation by leveraging the electromagnetic hysteresis effect. The applied CP signal causes fluctuations in the generated magnetic flux, resistance force, and consequent valve position. Therefore, the variation in MR resistance force at various duty cycles and negative half cycles was investigated and compared with the required force to perform variable valve lift (VVL). Simulations indicated that a 35% duty cycle with 75% current during the negative half cycle was able to maintain the required force but results in negligible deviation in the resistance force which doesn’t affect the VVL operation. Therefore, the proposed method is advantageous over conventional DC input, conferring actuation energy-savings of approximately 28%.

DC Drive Adaptive Speed Controller Based on Hyperstability Theory

The scope of this research is to develop a hyperstable adaptive control system of a direct current (DC) drive speed for effective load torque attenuation. The proposed speed controller is based on the model reference adaptive control framework and integrated into the conventional DC drive cascade control system. Its main features are as follows: (1) the boundedness of the control action signal, as well as the armature current control loop non-stationarities, are taken into consideration with the help of the reference model hedging technique; (2) its inputs include only measurable signals, thus there is no need to use any kind of state estimators; (3) it attenuates the disturbances, which are matched with its control action signal, particularly, the inertia moment non-stationarity and load torque. The asymptotic hyperstability of the obtained DC drive control system is proven with the help of Lyapunov’s theorems and Popov’s criterion. The numerical experiments corroborate the obtained results. They include the demonstration of disadvantages of the conventional cascade control system under conditions of the drive parameters’ non-stationarity and advantages of the proposed solution for different disturbance types and amplitudes.

Less energy and material consumption in an electrocoagulation system using AC waveform instead of DC for nickel removal: Process optimization through RSM

This study pursues two major aims, first, to compare the electrocoagulation (EC) system for Ni2+ removal from aqueous solution using direct current (DC) versus alternating current (AC), and second, to optimize the process for each mode. For this purpose, four variables, viz., current density, initial nickel concentration, initial pH of the solution, and reaction time, alongside three responses, namely remaining nickel, energy consumption, and electrode (aluminum) consumption, were considered. Accordingly, 30 experiments designed by the response surface methodology (RSM) based on the central composite design (CCD) were carried out for each type of current. The statistical analysis revealed that the quadratic models proposed by the Design Expert are reliable to predict the outcomes. As quantitative results for the DC and AC modes, the average amount of the remaining nickel was 44.06 and 43.91 mg/L, energy consumption was 34.9 and 29.2 kWh/kg Ni removed, and electrode consumption was 2.3 and 1.2 kg Al/kg Ni removed, respectively. Thus, the transition from conventional DC to AC waveform coagulation led to a more than 16% reduction in energy consumption and approximately 47% depletion in electrode consumption.

Bipolar high‐power impulse magnetron sputtering synthesis of high‐entropy carbides

AbstractIn this study, we report high‐entropy carbides synthesis with reactive bipolar high‐power impulse magnetron sputtering (HiPIMS). Uncontrolled microstructure and stoichiometry development with reactive gas flow rate are major limitations of conventional direct current (DC) and radio frequency (RF) magnetron sputtering of multicomponent carbides. With HiPIMS these chemically disordered crystals structurally and compositionally transform from a carbon‐deficient metallic (C/M < 1), to a stoichiometric ceramic zone (C/M ∼ 1), and to a nanocomposite embodiment (C/M > 1), as a function of the carbon content during HiPIMS deposition. X‐ray diffraction, X‐ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and nanoindentation hardness measurements are combined to demonstrate the three regions of synthesis domain. HiPIMS provides access to metallic, ceramic, and composite carbides with great control over the microstructure and stoichiometry, which is elusive in case of conventional DC and RF magnetron sputtering. Notably, the stoichiometric ceramic zone maintains a constant carbon to metal ratio (C/M ∼ 1) over an extended amount of methane flow before transitioning to a nanocomposite microstructure (C/M > 1). The transition zone breadth depends on materials affinity for carbon that correlates with valence electron concentration (VEC). As such, synthesis conditions for new high‐entropy carbides can be understood and predicted based on VEC.

New polyamide 6 nanofibrous sorbents produced via alternating current electrospinning for the on-line solid phase extraction of small molecules in chromatography systems

Nanofibrous materials provide a large extraction capacity owing to their unique structure. Although most research conducted to date has employed nanofibers produced via the direct current (DC) electrospinning method, the resulting nanofibers have disadvantages that limit their practical use. We focused on the investigation of the electrospinning method using alternating current (AC), firstly used for production of the nanofibrous material as sorbent for extraction. The applicability of the new material was reported as a case study for extraction of bisphenol A (BPA), bisphenol S (BPS), butylparaben (BTP), and fenoxycarb (FXC) in high pressure liquid chromatography system. The nanofibrous samples were produced from polyamide 6 (PA 6) by AC electrospinning. The properties of AC electrospun PA 6 material were compared to the nanofibers produced from the same polymer via conventional DC electrospinning. Our results demonstrated the unique ratio of high porosity, fiber diameter, and specific surface area of AC PA 6 material that led to very good stability in high-pressure flow system and absence of undesired flow and back pressure fluctuations. Good repeatability of cartridge packing with RSD 5.22% for AC material and 16.61 % for DC material, and high extraction efficiency ranging from 95 to 145% was achieved. Efficient on-line extraction procedure enabled the limits of detection ranging from 30 to 60 μg L-1 using both nanofibrous materials and UV detector. The slopes of matrix matched calibration curves demonstrated good selectivity of both extraction materials for BPA, BPS, and BTP. We confirmed the usefulness of our novel approach to nanofiber sorbents for SPE that met the exacting demands in the field of advanced flow extraction methods.

A new direct current circuit breaker with current regeneration capability

Direct current (DC) power systems are becoming very popular due to better control ability and equipment reliability thanks to the continuous development of power electronics. A DC circuit breaker (DCCB) used for current interruption in a DC network is a major part of the system. It plays the vital role of isolating networks during fault clearing as well as during normal load switching. Breaking the DC current is a major challenge as it does not have any natural zero crossing points like the AC current has. In addition, energy stored in the network inductances during normal operation opposes the instantaneous current breaking. Hence, all the conventional DC circuit breaker topologies use lossy elements to dissipate this stored energy as heat during the current breaking operation. However, it is possible to store this energy and reuse it later by developing an improvised topology. In this paper, the prospects of energy recovery and reuse in DC circuit breakers have been studied, and a new topology with regenerative current breaking capability has been proposed. This new topology can feed the stored energy of the network back into the same network after breaking the current and thus can improve the overall system efficiency.

High-performance photonic transformers for DC voltage conversion

Direct current (DC) converters play an essential role in electronic circuits. Conventional high-efficiency DC voltage converters, especially step-up type, rely on switching operation, where energy is periodically stored within and released from inductors and/or capacitors connected in a variety of circuit topologies. Since these energy storage components, especially inductors, are fundamentally difficult to scale down, miniaturization of switching converters proves challenging. Furthermore, the resulting switching currents produce significant electromagnetic noise. To overcome the limitations of switching converters, photonic transformers, where voltage conversion is achieved through light emission and detection processes, have been demonstrated. However, the demonstrated efficiency is significantly below that of the switching converter. Here we perform a detailed balance analysis and show that with a monolithically integrated design that enables efficient photon transport, the photonic transformer can operate with a near-unity conversion efficiency and high voltage conversion ratio. We validate the theory with a transformer constructed with off-the-shelf discrete components. Our experiment showcases near noiseless operation and a voltage conversion ratio that is significantly higher than obtained in previous photonic transformers. Our findings point to the possibility of a high-performance optical solution to miniaturizing DC power converters and improving the electromagnetic compatibility and quality of electrical power.

Cutaneous sensation of electrical stimulation waveforms

BackgroundSkin sensation is the primary factor limiting the intensity of transcranial electrical stimulation (tES). It is well established that different waveforms generate different sensations, yet transcranial stimulation has been limited to a relatively small number of prototypical waveforms. ObjectiveWe explore whether alternative stimulation waveforms could substantially reduce skin sensation and thus allow for stronger intensities in tES. MethodsWe systematically tested a range of waveforms in a series of 6 exploratory experiments stimulating human adults on the forearm and in one instance on the head. Subjects were asked to rate skin sensation level on a numerical scale from “none” to “extreme”. ResultsHigh frequency (>1 kHz) monophasic square wave stimulation was found to decrease in sensation with increasing duty cycle, baseline, and frequency, but the sensation was never lower than for constant current stimulation. For the purpose of injecting a net direct current (DC), a constant current is optimal. For stimulation with alternating current (AC), sensation decreased with increasing frequency, consistent with previous reports. Amplitude modulation did not reduce sensation below stimulation with constant AC amplitude, and biphasic square waveforms produced higher sensation levels than biphasic sinusoidal waveforms. Furthermore, for DC stimulation, sensation levels on the arm were similar to those reported on the head. ConclusionOur comparisons of various waveforms for monophasic and biphasic stimulation indicate that conventional DC and AC waveforms may provide the lowest skin sensations levels for transcutaneous electrical stimulation. These results are likely generalizable to tES applications.

Expanding Bias-instability of MEMS Silicon Oscillating Accelerometer Utilizing AC Polarization and Self-Compensation.

This paper presents a MEMS (Micro-Electro-Mechanical System) Silicon Oscillating Accelerometer (SOA) with AC (alternating current) polarization to expand its bias-instability limited by the up-converted 1/f noise from front-end transimpedance amplifier (TIA). In contrast to the conventional DC (direct current) scheme, AC polarization breaks the trade-off between input transistor gate size and white noise floor of TIA, a relative low input loading capacitance can be implemented for low noise consideration. Besides, a self-compensation technique combining polarization source and reference in automatic-gain-control (AGC) is put forward. It cancels the 1/f noise and drift introduced by the polarization source itself, which applies to both DC and AC polarization cases. The experimental result indicates the proposed AC polarization and self-compensation strategy expand the bias-instability of studied SOA from 2.58 μg to 0.51 μg with a full scale of ± 30 g, a 155.6 dB dynamic range is realized in this work.

Optimal Energy Management of DC Microgrid System using Dynamic Programming

Because voltage regulation of direct current(DC) microgrid is controlled by active power, it is closely related to optimal energy management. In a conventional DC microgrid, the microgrid scheduling is defined as the optimization problem considering the cost of power sources, demand response(DR) and electric vehicle(EV) that make up the system. In this case, the voltage stability is considered separately in power flow analysis. However, since the voltage control is related to economy, it is reasonable to consider it together with the optimal scheduling problem. Considering both the voltage control and the optimal management problem has the issue of increasing the complexity. Hence, this paper proposes optimal operation strategy using dynamic programming(DP) in DC microgrid system to reduce the complexity of the problem. This study is a preliminary study to consider voltage control and economics, and it uses DP to define a day-ahead optimal scheduling problem. The proposed method is verified by DC microgrid system configured by using MATLAB. The result of optimal energy management shows that the proposed strategy reduces complexity and makes reasonable results.

Electric and magnetic fields synergistically enhancing high power impulse magnetron sputtering deposition of vanadium coatings

The high power impulse magnetron plasmas (HIPIMS) were employed as a novel deposition technique, which had the benefit of providing a high ionization degree. In contrast, due to the ion return effect, the deposition rate was relatively low compared to the conventional direct current. In this paper, the external electric field and external magnetic field enhanced simultaneously the HIPIMS ((E-MF)-HIPIMS) to achieve higher deposition rate of the coatings. The vanadium coatings were deposited by the (E-MF)-HIPIMS at equivalent average target powers. The substrate peak ion current density measured in the (E-MF)-HIPIMS mode was increased by a factor of 4 to the discovered current density for the HIPIMS conditions when the anode voltage was set to 70 V. The enhanced ion flux bombardment from the highly ionized (E-MF)-HIPIMS plasma led to the formation of a smoother surface and a denser structure. At the same average target power for the vanadium coatings depositions, the (E-MF)-HIPIMS exhibited higher deposition rates compared to the conventional HIPIMS by approximately 73%. The (E-MF)-HIPIMS sputtered vanadium coating exhibited significantly higher-sized film and substrate interface bonding strengths than compared to the HIPIMS by the Rockwell adhesion testing. In addition, the (E-MF)-HIPIMS sputtered vanadium coating exhibited an improved friction coefficient and improved corrosion resistance compared to the substrate and the HIPIMS sample. The improvement occurred due to the enhanced particle ionization and intense ion bombardment.

Epitaxial growth of Cu(001) thin films onto Si(001) using a single-step HiPIMS process

We report on a new route to grow epitaxial copper (Cu) ultra-thin films (up to 150 nm thick) at ambient temperature on Si(001) wafers covered with native oxide without any prior chemical etching or plasma cleaning of the substrate. It consists of a single-step deposition process using high power impulse magnetron sputtering (HiPIMS) and substrate biasing. For a direct current (DC) substrate bias voltage of −130 V, Cu/Si heteroepitaxial growth is achieved by HiPIMS following the Cu(001) [100]//Si(001) [110] orientation, while under the same average deposition conditions, but using conventional DC magnetron sputtering, polycrystalline Cu films with [111] preferred orientation are deposited. In addition, the intrinsic stress has been measured in situ during growth by real-time monitoring of the wafer curvature. For this particular HiPIMS case, the stress is slightly compressive (−0.1 GPa), but almost fully relaxes after growth is terminated. As a result of epitaxy, the Cu surface morphology exhibits a regular pattern consisting of square-shaped mounds with a lateral size of typically 150 nm. For all samples, X-ray diffraction pole figures and scanning/transmission electron microscopy reveal the formation of extensive twinning of the Cu {111} planes.

Nanoelectronic Heterodyne Sensor: A New Electronic Sensing Paradigm

Nanoelectronic devices based on nanomaterials such as nanowires, carbon nanotubes, graphene, and other 2D nanomaterials offer extremely large surface-to-volume ratios, high carrier mobility, low power consumption, and high compatibility for integration with modern electronic technologies. These distinct advantages promise great potential for nanoelectronic devices as next generation chemical and biological sensors. Currently, majority of existing nanoelectronic sensors are direct current (DC) sensors, which rely ubiquitously on detection of conductance change associated with molecular adsorption. However, despite the simplicity of the conventional DC sensing technology, it also has severe limitations such as the Debye screening effect in ionic solutions, and the speed-sensitivity trade-off for the detection of charge-neutral molecules. Hence, the development of nanoelectronic sensors calls for new sensing platform technologies that can truly showcase the advantages of electronic sensors. In this Account, we will summarize recent efforts from our group on the development of a new electronic sensing paradigm, the nanoelectronic heterodyne sensors. Unlike conventional charge-detection based sensors, the heterodyne sensor explores the frequency mixing response between molecular dipoles and a nanoscale transistor. As an example, we first discuss the capability of heterodyne sensing in gas sensing applications by using graphene devices. Rapid (down to 0.1 s) and sensitive (down to 1 ppb) detection of a wide range of vapor analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronic sensors. Furthermore, the heterodyne sensing technique enables electrical probing and tuning of the noncovalent physisorption of polar molecules on graphene surface for the first time. These results provide insight into small molecule-nanomaterial interaction dynamics and signify the ability to electrically tailor interactions, which can lead to rational designs of complex chemical processes for catalysis and drug discovery. Finally, we discuss the application of heterodyne sensing in solution for chemical and biological sensors by using carbon nanotube devices. The fundamental ionic screening effect can be mitigated by operating carbon nanotube field effect transistor as a heterodyne biosensor. Electrical detection of streptavidin binding to biotin in 100 mM buffer solution can be achieved at a frequency beyond 1 MHz. The results should promise a new biosensing platform for point-of-care detection, where biosensors functioning directly in physiologically relevant condition are desired.

Sustainable Electrochemical Machining for Metal Recovery, Elimination of Waste, and Minimization of Water Usage

Faraday is developing and installing a β-scale Recycling Electrochemical Machining ((R)ECM) system at the U.S. Army Benet Laboratory. The (R)ECM system is sized to recover up to 0.5 m3 per year of metal from an electrochemical machining operation. The metals of initial interest include: 1) beryllium-free nickel-copper alloy (C18000), 2) Cr-Mo alloy steel (SAE4150), 3) nickel alloy (IN718) and 4) tantalum tungsten (Ta10W) alloy. The (R)ECM system couples an electrochemical machining unit operation with an electrowinning (EW) unit operation for recovery of metals, elimination of waste, and minimization of water usage. Consequently, the RECM system is consistent with the U.S. Army’s “Vision for Net Zero”[1]. Electrochemical machining (ECM) is suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. Conventional, direct-current (DC) ECM typically operates with neutral salt electrolytes whereby the machined material is generally precipitated from the machining electrolyte as metal oxide/hydroxide sludge. This sludge is generally transported off-site and disposed in a land-fill. Depending on the alloying components in the machined metal, the sludge is often designated as a hazardous material. Furthermore, the rifling of a gun barrel by ECM generates ~350 gal of centrifuged sludge per ~250 in3 of material removed [2]. This represents greater than 300X volume multiplier of waste sludge relative to the amount of material removed. As recently noted, the generation and disposal of the sludge is a major impediment to the widespread use and implementation of ECM operations [3]. Faraday has previously demonstrated the ability of pulse current/pulse reverse current (PC/PRC) waveforms electrochemically machining materials while achieving a high quality surface finish (Figure 1) [4],[5]. Smooth surfaces are typically not possible in conventional DC ECM. The PRC waveforms control the ECM process and consequently ECM with PRC is adaptable to a variety of electrolytes. To develop a (R)ECM process, we select an electrolyte whereby the machined material is soluble and avoids sludge formation by precipitation. The ECM unit operation is coupled to an EW unit operation where the machined material is collected as metal. The ECM and EW unit operations are adjusted to keep the soluble metal concentration in a range where the ECM process is not adversely effected and the EW operation is efficient and economical. Consequently, metals are recovered, waste is avoided and water usage is minimized. In the presentation, we will summarize the initial electrolyte selection studies for C18000, SAE4150, IN718 and Ta10W materials. In additionally, we will demonstrate integrated operation of the (R)ECM process where the soluble metal concentration is maintained in a pre-determined range by 1) simultaneous (R)ECM operation by adjusting the ECM unit operation in the presence of constant EW unit operation, 2) simultaneous (R)ECM operation by adjusting the EW unit operation in the presence of constant ECM unit operation, and 3) sequential operation of the ECM and EW unit operations. Finally, the status (R)ECM system design and installation at Benet Laboratories will be reviewed.

Surface Finishing of Strongly Passive Biomedical Materials, such as Nitinol, in Low- and Non-Acid Electrolytes

A process for surface finishing of biomedical alloys is described. For strongly passivating metals, electropolishing under direct current (DC) electric fields in a simple electrolyte can lead to a roughened surface. To remove the passive film, aggressive chemicals are added to the electrolyte, for example, hydrofluoric acid may be used to depassivate the surface.[1]A pulse/pulse reverse waveform can be used to electropolish passive materials, by tuning anodic pulses to control mass transport and current distribution, interspersed with cathodic pulses to depassivate the surface.[2]-,[3],[4],[5]Off-times inserted between the anodic and cathodic pulses facilitate replenishment of reacting species and removal of by-products and heat. The cathodic pulse eliminates the need for HF or other chemicals to remove the surface oxide. While the exact mechanism of depassivation is unknown at this stage, we speculate that the cathodic pulses remove the oxide either by direct electrochemical reduction or indirect chemical reduction, and restore the virgin metal surface prior to the next anodic pulse. The amplitude of the cathodic pulses required for depassivation is material specific, and appears to be based on the strength of the passive film.The inclusion of cathodic pulses and off-times suggests that the overall process would be slower than a conventional DC process, which is undesirable for industrial implementation. However, the maximum current density during the anodic pulse is higher than the DC limiting current density. Therefore, the overall removal rate of a pulsed process can rival or exceed that of a DC process despite a duty cycle that is less than 100%, while enjoying enhanced process performance.Studies in near-neutral aqueous solutions have demonstrated the feasibility of eliminating toxic solutions from manufacturing of biomedical devices. The best processing conditions to date for Nitinol in 17 w/w% H2SO4 has resulted in a final surface Ra of 0.19 µm. We have electropolished Ti-15Mo and Ti-Al-Nb alloys in 10 w/w% H2SO4 or 150 g/L Na2SO4to an Ra of 0.12 µm and 0.23 µm, respectively. We have done preliminary work on Ti-6Al-4V, achieving a near-mirror like surface finish.For electropolishing of stainless steels in an aqueous NaCl-NaNO3 electrolyte, a waveform sequence of short anodic pulses with interspersed cathodic pulses for 30 s followed by a waveform with long anodic pulses and interspersed cathodic pulses for 15 s has achieved a final Raof 0.026 µm (Figure 1: SS316 surface before and after FARADAYIC® Electropolishing). Finally, we have electropolished molybdenum to a mirror-like surface finish, and investigated on cobalt-chrome alloys and rhenium alloys.In summary, we have described a pulse reverse electropolishing process that may be applied to biomedical alloys using low viscosity aqueous electrolytes, devoid of aggressive chemicals for depassivating the surface. Consequently, the safety and chemical handling and disposal issues associated with pulse reverse electropolishing are minimized. In addition, the need for active chilling is minimized by the appropriate use of off-times and we anticipate that the process reproducibility and robustness are greatly enhanced. Acknowledgements: The authors acknowledge the financial support of Faraday corporate, NIH Grant No.1 R43 HL095216-01A1, DOE P.O. No. 594128 and DOE Contract No. DE-SC0004588.