Unipolar Waveforms Research Articles

In-situ incorporation of polydopamine surface modified bioactive glass nanoparticles into a plasma electrolytic oxidation coating on biodegradable AZ31 Mg alloy: A study of bioactivity and corrosion performance

Magnesium alloys are promising candidates for next-generation biodegradable biomaterials. However, their poor corrosion resistance has limited their medical applications. In this study, to improve the bioactivity performance and control the degradation rate of an AZ31 Mg alloy, anodic oxide coatings containing bioactive glass (BG) were created on the alloy using plasma electrolytic oxidation (PEO). First, BG nanoparticles were synthesized through the sol-gel method and surface-modified with polydopamine (BG/PDA). These nanoparticles were then dispersed in an alkaline phosphate electrolyte and incorporated into the coatings grown using unipolar and bipolar waveforms. According to EDS results, incorporation of BG/PDA nanoparticles was higher when the bipolar waveform was applied. Electrochemical impedance spectroscopy revealed that the coatings significantly enhanced the corrosion resistance of the AZ31 Mg alloy, with the BG/PDA nanoparticle-containing coatings demonstrating the lowest degradation rate during long-term immersion. These coatings achieved the highest faradic resistance of ∼ 12 kΩ.cm² after 28 days of immersion. The incorporation of BG and BG/PDA nanoparticles enhanced the bioactivity performance of the coatings, facilitating hydroxyapatite formation on the coating surface. Based on the results, the newly developed BG-coated alloy shows promising potential for biomedical applications.

Scalable high-repetition-rate sub-half-cycle terahertz pulses from spatially indirect interband transitions

Intense phase-locked terahertz (THz) pulses are the bedrock of THz lightwave electronics, where the carrier field creates a transient bias to control electrons on sub-cycle time scales. Key applications such as THz scanning tunnelling microscopy or electronic devices operating at optical clock rates call for ultimately short, almost unipolar waveforms, at megahertz (MHz) repetition rates. Here, we present a flexible and scalable scheme for the generation of strong phase-locked THz pulses based on shift currents in type-II-aligned epitaxial semiconductor heterostructures. The measured THz waveforms exhibit only 0.45 optical cycles at their centre frequency within the full width at half maximum of the intensity envelope, peak fields above 1.1 kV cm−1 and spectral components up to the mid-infrared, at a repetition rate of 4 MHz. The only positive half-cycle of this waveform exceeds all negative half-cycles by almost four times, which is unexpected from shift currents alone. Our detailed analysis reveals that local charging dynamics induces the pronounced positive THz-emission peak as electrons and holes approach charge neutrality after separation by the optical pump pulse, also enabling ultrabroadband operation. Our unipolar emitters mark a milestone for flexibly scalable, next-generation high-repetition-rate sources of intense and strongly asymmetric electric field transients.

Propagation Vectors Facilitate Differentiation Between Conduction Block, Slow Conduction, and Wavefront Collision.

[Figure: see text].

The effects of anodic amplitude and waveform of applied voltage on characterization and corrosion performance of the coatings grown by plasma electrolytic oxidation on AZ91 Mg alloy from an aluminate bath

In this study, plasma electrolytic oxidation (PEO) was used to treat AZ91 magnesium alloy in an aluminate electrolyte to increase its corrosion resistance. The PEO process was performed at 350 and 400 V using unipolar and bipolar waveforms. Using unipolar waveform, current density reached zero after a while, but for bipolar waveforms, the current density responded during the PEO process indicating on nonstop coating growth. X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy were applied to characterize the coatings. The coatings produced using unipolar waveform revealed a net-like surface morphology, while a crater-like morphology was observed for bipolar coatings. The presence of MgF2 in all coatings was confirmed whereas the bipolar waveform with higher cathodic duty cycle produced an increased coating thickness with higher content of MgF2. The results of corrosion tests in 3.5 wt% sodium chloride solution were in agreement with the changes observed in morphology and structure, where the samples coated at 400 V using the bipolar waveforms were able to exhibit high corrosion performance. In this way, the coating produced by the bipolar waveform with the higher cathodic duty cycle at the higher voltage (400 V) revealed the highest corrosion performance up to 28 days of immersion, where there was no sign of underlying corrosion in this case.

Effect of Pulse Current Mode on Microstructure, Composition and Corrosion Performance of the Coatings Produced by Plasma Electrolytic Oxidation on AZ31 Mg Alloy

Plasma electrolytic oxidation (PEO) coatings were grown on AZ31 Mg alloy in a silicate-based electrolyte containing KF using unipolar and bipolar (usual and soft-sparking) waveforms. The coatings were dual-layered consisting of MgO, MgF2 and Mg2SiO4 phases. Surface morphology of the coatings was a net-like (scaffold) containing a micro-pores network, micro-cracks and granules of oxide compounds. Deep pores were observed in the coating produced by unipolar and usual bipolar waveforms. The soft-sparking eliminated the deep pores and produced the lowest porosity in the coatings. It was found that the corrosion performance of the coatings evaluated using EIS in 3.5 wt. % NaCl solution is mostly determined by the inner layer resistance, because of its higher compactness. After 4 days of immersion, the inner layer resistances were almost the same for all coatings. However, the coatings produced by unipolar and usual bipolar waveforms showed sharp decays in inner layer resistances after 1 week and even the barrier effect of outer layer was lost for the unipolar-produced coating after 3 weeks. The low-frequency inductive loops appeared after a 3-week immersion for all coatings indicated that the substrate was under local corrosion attack. However, both coatings produced by soft-sparking waveforms provided the highest corrosion performance.

Head orientation and electrode placement potentially influence fetal scalp ECG waveform.

BackgroundFetal monitoring based on electrocardiographic (ECG) morphology is obtained from a single unipolar fetal scalp electrode. Ideally, it should be obtained from multiple leads, as ECG waveform depends on alignment between electrode and electrical heart axis. This alignment is unknown in fetuses. Besides, fetuses are surrounded by conductive media, which may influence ECG waveform. We explored the influence of electrode position and head orientation on ECG waveforms of unipolar and bipolar scalp ECGs recorded in air and in conductive medium.MethodsWe recorded ECGs in one adult subject at five different scalp positions in five different head orientations both in dry and immersed conditions. The ratio between T-amplitude and QRS-amplitude (T/QRS ratio) of unipolar and bipolar scalp ECGs was determined and compared between all conditions.ResultsIn the dry condition, we observed in the unipolar leads little to no difference between different electrode positions (maximal T/QRS difference 0.00–0.01) and minor differences between head orientations (0.02–0.03), whereas bipolar leads showed no recognizable ECG signal at all. During the immersed condition, we found variation in the unipolar leads, both between electrode positions (maximal T/QRS difference 0.02–0.05) and between head orientations (0.03–0.06). Bipolar leads showed different ECG signals in contrasting head orientations.ConclusionsBoth unipolar and bipolar scalp lead-derived ECG waveforms are influenced by electrode position and head orientation when the subject is submerged in a conductive medium. Fetal monitoring based on single scalp lead ECG waveform might be suboptimal, as it lacks correction for fetal head orientation and electrode position.

Synergistic effect of W incorporation and pulsed current mode on wear and tribocorrosion resistance of coatings grown by plasma electrolytic oxidation on 7075 Al alloy

Ceramic coatings were grown by plasma electrolytic oxidation on 7075 Al alloy using unipolar and bipolar pulsed current waveforms with 20 and 40% cathodic duty cycles, from a silicate-based bath without and with the addition of Na2WO4. Pancake-like morphology was dominant on the coatings grown by unipolar waveform, while the bipolar waveforms promoted volcano-like morphology, increased the roughness of the coating surface and the formation of more compact layers. The coatings produced using the bipolar waveforms provided higher resistances toward both tribocorrosion and dry sliding conditions, while further improvement was achieved by the presence of tungsten. The coatings produced in tungstate containing bath using the bipolar waveform with 40% cathodic duty cycle provided the best performance in both sliding conditions, showing ∼90% reduction in volume loss comparing to the coating produced in additive-free electrolyte using unipolar waveform.

Optical Pulse-Drive for the Pulse-Driven AC Josephson Voltage Standard

We developed an optical pulse-drive for the operation of the Josephson Arbitrary Waveform Synthesizer (JAWS) using a fast photodiode (PD) operated at 4 K, close to the JAWS chip. The optical pulses are transmitted to the PD by an easily removable optical fiber attached to it. A bare-lensed PD is mounted by flip-chip technique to a custom-made silicon-carrier chip. This carrier chip is equipped with coplanar waveguides to transmit the electrical pulses from the PD to the JAWS chip mounted on a separate printed circtuit board (PCB). The main components of this optical setup are a laser source, a high-speed Mach–Zehnder modulator, and the modulator driver. The waveform pattern is supplied by a commercial pulse pattern generator providing up to 15 GHz electrical return-to-zero (RTZ)-pulses. Unipolar sinusoidal waveforms were synthesized. Using a JAWS array with 3000 junctions, an effective output voltage of 6.6 mV root mean square (RMS) at the maximum available clock-frequency of 15 GHz was achieved. Higher harmonics were suppressed by more than 90 dBc at laser-bias operation margins of more than 1 mA.

An Optoelectronic Pulse Drive for Quantum Voltage Synthesizer

An optoelectronic pulse drive system based on commercially available telecoms optoelectronic components and a field-programmable gate array has been developed. In order to allow the use of ac-coupled optoelectronic components, a method of adding and subtracting complementary pulses was developed. The system was used to drive a Josephson junction array to synthesize quantum-accurate voltage waveforms. These waveforms will be used in voltage waveform metrology applications. Results from the synthesis of sinusoidal unipolar voltage waveforms from 3 to 300 kHz are presented. The margin of operation was measured and shown to require an optical pulse height stability better than 0.15 mW. The use of delta–sigma code with repeated sections, suitable for a quantum voltage digitizer was also examined. A test code corresponding to a delta–sigma feedback loop rate of 250 MHz was used to synthesize a 3.125 kHz sinusoidal waveform.

Electrochemically-induced TiO2 incorporation for enhancing corrosion and tribocorrosion resistance of PEO coating on 7075 Al alloy

7075 Al alloy was PEO-treated in a silicate based electrolyte containing 3 g l-1 potassium titanyl oxalate using unipolar and bipolar pulsed current waveforms. The coating formed by the bipolar waveform with the wider cathodic pulses showed volcano-like surface morphology with no evidence of large pores at the metal/coating interface. It revealed the highest corrosion performance due to the synergistic effect of TiO2 incorporation and structural/morphological features along with a better passivation behavior indicating no pitting susceptibility. For this coating, the tribocorrosion tests showed no potential drop during sliding under 1 N with the lowest volume loss of 0.022 mm3.

Influence of impulse voltage repetition frequency on RPDIV in partial vacuum

The reliability of low-voltage inverter-fed motors is highly dependent on the inception of partial discharges. The effect of impulsive waveform parameters must be investigated to predict the repetitive partial discharge inception voltage (RPDIV) and define test procedures that can indicate properly the behavior in service of the insulation system. This paper focuses on the RPDIV of magnet wires using twisted pairs subjected to repetitive unipolar impulsive voltage waveforms. The effects of supply frequency (5 to 200 kHz) at pressure levels that are typical for aircraft (20 to 100 kPa) is examined. Results show that RPDIV steadily decreases with frequency up to 100 kHz where it reaches a plateau. This behavior is explained as an effect of the oscillations that inevitably exist in the applied voltage waveform. Therefore, a conservative estimate of the RPDIV could be achieved by raising the supply frequency well above the operation frequency. In the experiments, the RPDIV is decreasing linearly with pressure. If this behavior could be confirmed for other insulation systems, the design of systems working at pressures typical of aircraft would result relatively easy.

Influence of cathodic duty cycle on the properties of tungsten containing Al2O3/TiO2 PEO nano-composite coatings

Current waveforms with different cathodic duty cycles of 0, 20 and 40% were applied to coat 7075 aluminum alloy in a silicate based electrolyte containing titania nano-particles. Using a constant current mode, the recorded voltage-time response showed that the required voltage for PEO coating process is lower in the presence of sodium tungstate as additive. By increasing the cathodic duty cycle, pancake surface morphology was converted to crater-like. Phase and chemical composition of the coatings were investigated by using grazing angle XRD and SEM/EDS. The tungsten concentration decreased linearly with the increase of cathodic duty cycle, which is dissimilar to the titania content. It was proposed that physical entrapping has been associated with electrophoretic incorporation of titania nano-particles, while the adsorption mechanism of tungstate anions is purely electrophoretic. Corrosion performance of the coatings was evaluated by potentiodynamic polarization and EIS measurements. Addition of sodium tungstate was detrimental for corrosion performance of the coating in the case of unipolar waveform while the corrosion resistance was improved when bipolar waveforms were used. Long-term corrosion behavior of the coatings was investigated by EIS up to 16 weeks. The results showed remarkable difference between corrosion performance of the coatings produced by unipolar and bipolar waveforms, but the difference was negligible for the coatings produced by the bipolar waveforms with different cathodic duty ratios.

The effect of pulse waveforms on surface morphology, composition and corrosion behavior of Al2O3 and Al2O3/TiO2 nano-composite PEO coatings on 7075 aluminum alloy

Plasma electrolytic oxidation was employed to produce Al2O3 and Al2O3/TiO2 composite coatings from a silicate based electrolyte on 7075 aluminum alloy using unipolar and bipolar waveforms with cathodic duty cycle of 20 and 40%. The results showed that the surface morphology of the coatings is dependent on the applied waveform. Pancake like morphology was converted to crater like by altering waveform from unipolar to bipolar. Higher thickness, lower porosity, and thus, higher corrosion protection were achieved using the bipolar waveform at higher cathodic duty cycle of 40%. The incorporation of TiO2 nano-particles in the coatings decreased the thickness, increased the micro-cracks and widened the micro-pores on coating surface when unipolar waveform was applied. It was found that TiO2 nano-particles have been incorporated into the coatings in their original crystalline structure, i.e. rutile, which was categorized in the “inert incorporation” mode and this incorporation has not changed the matrix micro-structure, i.e. γ-alumina. In addition, the incorporated amounts of TiO2 nano-particles were constant and showed no reasonable relation with the applied waveform. The corrosion results indicated that although the composite coating produced at unipolar waveform shows the highest corrosion resistance at short periods of immersion due to repairing mechanism, it degrades at a higher rate. However, for the composite coatings produced using the bipolar waveform with the higher cathodic duty cycle of 40%, the maximum corrosion protection was achieved at long term immersion. Repairing mechanism is plugging the micro-pores in the inner compact layer which was found more effective for the coatings with lower porosity. Accordingly, it was concluded that the corrosion protection of the coatings with higher “intrinsic resistance” get more benefit from the repairing mechanism too.

Dynamic electromigration modeling for transient stress evolution and recovery under time-dependent current and temperature stressing

Electromigration (EM) has been considered to be the dominant back end of line (BEOL) reliability issue for current and future VLSI technologies. Current EM reliability analysis is overloaded by over-conservative and simplified EM models. Particularly the transient recovery effect in the EM-induced stress evolution kinetics has never been modeled properly in all the existing analytical EM models. In this article, we propose a new physics-based dynamic compact EM model, which for the first time, can accurately predict the transient hydrostatic stress recovery effect in a confined metal wire. The evolution of preexisted void volume and the corresponding metal line resistance change can be further derived based on the stress evolution. The new dynamic EM model is based on the direct analytical solution of one-dimensional Korhonen's equation with load driven by any unipolar or bipolar current waveforms under varying temperature. We show that the EM recovery effect can be quite significant even under unidirectional current loads. This recovery/healing process is sensitive to temperature, and higher temperatures lead to faster and more complete recovery. Such effect can be further exploited to extend the lifetime of the interconnect wires if the chip current or power can be properly regulated and managed. As a result, the new dynamic EM model can be incorporated with existing dynamic thermal/power/reliability management and optimization approaches, devoted to reliability-aware optimization at multiple system levels (chip/server/rack/data centers). Presented results show that the proposed EM model agrees very well with the numerical analysis results under any time-varying current density and temperature profiles.

Converter With Four Quadrant Switches for EDM Applications

This paper presents a high-frequency rectifier stage using four quadrant switches as synchronous rectifiers for an electrical discharge machining (EDM) power supply. The EDM impulse generator is a series-parallel resonant inverter operating in current-mode. The device specifications and drive requirements of the converter are studied with the objective of generating unipolar and bipolar current waveforms suitable for different types of EDM operations.

Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer–Tower circuit

Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance—which is, however, still considerably larger than the sample capacitance—is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed.

Bipolar Multiplexed Pulse Width Modulation

This paper presents generation and detection of two-channel bipolar multiplexed pulse width modulated signal. The positive half cycles of a polar triangular wave of frequency fc are modulated in amplitude by one message signal and the negative half cycles are modulated in amplitude by another message signal. The two modulated polar triangular waves, each carrying a distinct message signal, are added to generate a carrier whose positive envelope resembles one message signal while negative envelope resembles second message signal. This is a bipolar division multiplexed (BDM) signal in which the initial carrier considered is polar triangular wave. The BDM signal is passed simultaneously through positive and negative clipper circuits. The clippers output unipolar trapezoidal waveforms of opposite polarity. Each polar trapezoidal waveform is differentiated to generate a sequence of positive and negative pulses. Positive pulses alone are considered from the pulse train obtained by differentiating positive trapezoidal signal while negative pulses alone are considered from the pulse train obtained by differentiating the negative trapezoidal signal. The two sets of pulses are added to form Bipolar multiplexed pulse width modulated signal. The message signal recovery involves demultiplexing the pulse trains, clipping their amplitude levels to avoid the effect of additive noise followed by low pass filtering. The proposed scheme offers multiplexing feature with inherent synchronizing gap resulting in no requirement for synchronizing pulses. The proposed scheme requires more bandwidth and hence better noise performance when compared to conventional multiplexed PWM scheme. The proposed scheme is better suited when the dynamic range of the message signal is in the order of a few tens of millivolts.

Reliability Enhancement by Suitable Actuation Waveforms for Capacitive RF MEMS Switches in III–V Technology

In this paper, the reliability of shunt capacitive radio frequency microelectromechanical systems switches developed on GaAs substrate using a III-V technology fabrication process, which is fully compatible with standard monolithic microwave integrated circuit fabrication, is investigated. A comprehensive cycling test is carried out under the application of different unipolar and bipolar polarization waveforms in order to infer how the reliability of the realized capacitive switches, which is still limited with respect to the silicon-based devices due to the less consolidation of the III-V technology, can be improved. Under the application of unipolar waveforms, the switches show a short lifetime and a no correct deactuation for positive pulses longer than 10 ms probably due to the charging phenomena occurring in the dielectric layer underneath the moveable membrane. These charging effects are found to vanish under the application of a waveform including consecutive positive and negative voltage pulses, provided that proper durations of the positive and negative voltage pulses are used. Specifically, a correct switch deactuation and a lifetime longer than 1 million cycles, being this value limited by the duration of the used testing excitation, are achieved by applying a 1-kHz waveform with 20-μs-long positive and negative consecutive pulses.

One-step unipolar pulse electrodeposition of nickel hexacyanoferrate/chitosan/carbon nanotubes film and its application in hydrogen peroxide sensor

Unipolar pulse waveforms consist of an applied anode potential during the on-period and an open-circuit potential during the off-period. Unipolar pulse electrodeposition (UPED) was used to fabricate nickel hexacyanoferrate/chitosan/carbon nanotubes (NiHCF/CS/CNTs) nanocomposite films with controllable structure on the electrode surface of a hydrogen peroxide (H2O2) sensor. One-step electrodeposition of NiHCF/CS/CNTs film with insoluble-structure NiHCF nanoparticles was performed, and the whole procedure took only several minutes. The morphology and the composition of the NiHCF/CS/CNTs film were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDS). With the introduction of CNTs, the NiHCF/CS/CNTs system formed showed synergy between CNTs and NiHCF with a significant improvement of redox activity of NiHCF due to the excellent electron-transfer ability of CNTs. Electrochemical experiments revealed that the modified electrode allowed low potential (−0.2V) detection of H2O2 and showed high electrocatalytic activity towards the reduction of H2O2. The linear range for the detection of H2O2 was 0.04–5.6mM with a high sensitivity of 654mAM−1cm−2 and a rapid response (less than 2s). The detection limit for H2O2 was as low as 2.8×10−7M (S/N=3).

Unipolar pulse electrodeposition of nickel hexacyanoferrate thin films with controllable structure on platinum substrates

Unipolar pulse electrodeposition was used to prepare nickel hexacyanoferrate (NiHCF) films on platinum substrates with controllable structure. The unipolar pulse waveforms consisted of an applied cathodic potential during the on-period and the open-circuit potential during the off-period. The effects of the pulse deposition parameters on the electrochemical properties of NiHCF films were investigated by potential cycling in K+-containing electrolytes. The morphology and composition of NiHCF films were characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy. The charge transfer and the stability of the film electrodes were also examined by cyclic voltammetry (CV). Experimental results show that the cathodic pulse potential is the primary factor influencing the composition of the NiHCF films. The structure of NiHCF films prepared with a pulse potential of 0.7V SCE appears close to that of the “insoluble” form with a single reversible CV peak at lower voltage and a unit cell with a stoichiometry close to KNi4II[FeIII(CN)6]3. These films exhibit good stability and high dynamics for charge transport. The application of more negative cathodic potential pulses produces less dense and robust NiHCF films, with a structure closer to the “soluble” form and an approximate stoichiometry K8Ni4II[FeII(CN)6]4.