Breakdown Limit Research Articles

On the impact of self-clearing on electroactive polymer (EAP) actuators

Electroactive polymer (EAP)-based actuators have large potential for a wide array of applications; however, their practical implementation is still a challenge because of the requirement of high driving voltage, which most often leads to premature defect-driven electrical breakdown. Polymer-based capacitors have the ability to clear defects with partial electrical breakdown and subsequent removal of a localized electrode section near the defect. In this study, this process, which is known as self-clearing, is adopted for EAP technologies. We report a methodical approach to self-clear an EAP, more specifically P(VDF-TrFE-CTFE) terpolymer, to delay premature defect-driven electrical breakdown of the terpolymer actuators at high operating electric fields. Breakdown results show that electrical breakdown strength is improved up to 18% in comparison to a control sample after self-clearing. Furthermore, the electromechanical performance in terms of blocked force and free displacement of P(VDF-TrFE-CTFE) terpolymer-based bending actuators are examined after self-clearing and precleared samples show improved blocked force, free displacement and maximum sustainable electric field compared to control samples. The study demonstrates that controlled self-clearing of EAPs improves the breakdown limit and reliability of the EAP actuators for practical applications without impeding their electromechanical performance.

Charge density as a driving factor of discharge formation in GEM-based detectors

We report on discharge probability studies with a single Gas Electron Multiplier (GEM) under irradiation with alpha particles in Ar- and Ne-based gas mixtures. The discharge probability as a function of the GEM absolute gain is measured for various distances between an alpha source and the GEM. We observe that the discharge probability is the highest when the charge deposit occurs in the closest vicinity of the GEM holes, and that the breakdown limit is lower for argon mixtures than for neon mixtures.Our experimental findings are in line with the well-grounded hypothesis of the charge density being the limiting factor of GEM stability against discharges. A detailed comparison of the measurements with GEANT4 simulations allowed us to extract the critical charge density leading to the formation of a spark in a GEM hole. This number is found to be within the range of (5−9)×106 electrons after amplification, and it depends on the gas mixture.

Achieving ultrahigh triboelectric charge density for efficient energy harvesting

With its light weight, low cost and high efficiency even at low operation frequency, the triboelectric nanogenerator is considered a potential solution for self-powered sensor networks and large-scale renewable blue energy. As an energy harvester, its output power density and efficiency are dictated by the triboelectric charge density. Here we report a method for increasing the triboelectric charge density by coupling surface polarization from triboelectrification and hysteretic dielectric polarization from ferroelectric material in vacuum (P ~ 10−6 torr). Without the constraint of air breakdown, a triboelectric charge density of 1003 µC m−2, which is close to the limit of dielectric breakdown, is attained. Our findings establish an optimization methodology for triboelectric nanogenerators and enable their more promising usage in applications ranging from powering electronic devices to harvesting large-scale blue energy.

Wellbore Geomechanics of Extended Drilling Margin and Engineered Lost-Circulation Solutions

SummaryWellbore tensile failure is a known consequence of drilling with excessive mud weight, which can cause costly events of lost circulation. Despite the successful use of lost-circulation materials (LCMs) in treating lost-circulation events of the drilling operations, extensions of wellbore-stability models to the case of a fractured and LCM-treated wellbore have not been published. This paper presents an extension of the conventional wellbore-stability analysis to such circumstances. The proposed wellbore geomechanics solution revisits the criteria for breakdown of a fractured wellbore to identify an extended margin for the equivalent circulation density (ECD) of drilling. An analytical approach is taken to solve for the related multiscale and nonlinear problem of the three-way mechanical interaction between the wellbore, fracture wings, and LCM aggregate. The criteria for unstable propagation of existing near-wellbore fractures, together with those for initiating secondary fractures from the wellbore, are obtained. Results suggest that, in many circumstances, the occurrence of both incidents can be prevented, if the LCM blend is properly engineered to recover certain depositional and mechanical properties at downhole conditions. Under such optimal design conditions, the maximum ECD to which the breakdown limit of a permeable formation could be enhanced is predicted.

PCB Embedded Semiconductors for Low-Voltage Power Electronic Applications

Power conversion applications in the low voltage (LV) range (≤ 1.2 kV)—such as three-phase inverters—are required to operate at higher efficiencies, higher ambient temperatures, increasingly smaller form factor, and higher power density. Up to now, most research has focused on voltages up to 650 V for printed circuit board (PCB) embedded power electronics. This research evaluates a novel three-phase invertor module based on six insulated gate bipolar transistors and six diodes rated to 1.2 kV and 25 A each. This unique module is compared to the Semikron MiniSKiiP 23AC126V1. This paper considers some key details of the PCB embedding assembly process, a comparative switching performance assessment, measurement of thermal resistance, comparative lifetime, and electric insulation. First, a detailed outline of the package is presented including the top- and bottom-side metallization and the copper interconnect technology. The switching performances of both modules are compared for turn-ON and turn-OFF currents for a waveform at 600 V and 25 A at 150 °C. A finite-element-method thermal simulation demonstrates up to 44% lower thermal resistance for the PCB embedded package than that of the traditional wire-bonded direct bonded copper (DBC) package for an identical applied current and cooling condition. Furthermore, both packages are active power cycled to failure with the PCB embedded package demonstrating superior lifetime to the traditional DBC module. Finally, the maximum breakdown limit and the onset of partial discharge with the embedded PCB module are reported for both aged and non-aged conditions. The overall findings identify the promising application of PCB embedded power electronics for LV power conversion.

High Antiferromagnetic Domain Wall Velocity Induced by Néel Spin-Orbit Torques.

We demonstrate the possibility to drive an antiferromagnetic domain wall at high velocities by fieldlike Néel spin-orbit torques. Such torques arise from current-induced local fields that alternate their orientation on each sublattice of the antiferromagnet and whose orientation depends primarily on the current direction, giving them their fieldlike character. The domain wall velocities that can be achieved by this mechanism are 2 orders of magnitude greater than the ones in ferromagnets. This arises from the efficiency of the staggered spin-orbit fields to couple to the order parameter and from the exchange-enhanced phenomena in antiferromagnetic texture dynamics, which leads to a low domain wall effective mass and the absence of a Walker breakdown limit. In addition, because of its nature, the staggered spin-orbit field can lift the degeneracy between two 180° rotated states in a collinear antiferromagnet, and it provides a force that can move such walls and control the switching of the states.

Experimental Demonstration of Time-Dependent Breakdown in GaN-Based Light Emitting Diodes

This letter reports the first experimental demonstration of time-dependent breakdown in GaN-based light-emitting diodes (LEDs); based on a number of constant voltage stress tests, carried out below the breakdown limit of the devices, we show that: 1) when submitted to reverse-bias stress, the LEDs show a gradual increase in current, well correlated with an increase in the breakdown luminescence signal; 2) for sufficiently long stress times, the LEDs can reach a catastrophic (sudden) breakdown, which leads to the failure of the devices; 3) the breakdown process is time-dependent and the time to failure (TTF) has an exponential dependence on stress voltage; and 4) TTF is Weibull distributed. The results presented within this letter demonstrate that the GaN-based heterostructures can show a TDDB-like behavior, and can be useful for the interpretation of the degradation data of LEDs and HEMTs.

Side-illuminated tip-enhanced Raman study of edge phonon in graphene at the electrical breakdown limit

Nanoscale integration of graphene into a circuit requires a stable performance under high current density. However, the effects of the current density that approach the electronic breakdown limit of graphene are not well understood. We explored the effects of a high current density, close to the electronic breakdown limit of 10 A/cm (∼3.0 × 108 A/cm2), on graphene, using tip-enhanced Raman scattering. The results showed that the high current density induces Raman bands at 1456 and 1530 cm−1, which were assigned to edge-phonon modes originating from zigzag and armchair edges. This led us to conclude that C–C bonds are cleaved due to the high current density, leaving edge structures behind, which were detected through the observation of localized phonons.

Large exchange-dominated domain wall velocities in antiferromagnetically coupled nanowires

Magnetic nanowires supporting field- and current-driven domain wall motion are envisioned for methods of information storage and processing. A major obstacle for their practical use is the domain-wall velocity, which is traditionally limited for low fields and currents due to the Walker breakdown occurring when the driving component reaches a critical threshold value. We show through numerical and analytical modeling that the Walker breakdown limit can be extended or completely eliminated in antiferromagnetically coupled magnetic nanowires. These coupled nanowires allow for large domain-wall velocities driven by field and/or current as compared to conventional nanowires.

From Organized High-Throughput Data to Phenomenological Theory using Machine Learning: The Example of Dielectric Breakdown

Understanding the behavior (and failure) of dielectric insulators experiencing extreme electric fields is critical to the operation of present and emerging electrical and electronic devices. Despite its importance, the development of a predictive theory of dielectric breakdown has remained a challenge, owing to the complex multiscale nature of this process. Here, we focus on the intrinsic dielectric breakdown field of insulators—the theoretical limit of breakdown determined purely by the chemistry of the material, i.e., the elements the material is composed of, the atomic-level structure, and the bonding. Starting from a benchmark data set (generated from laborious first-principles computations) of the intrinsic dielectric breakdown field of a variety of model insulators, simple predictive phenomenological models of dielectric breakdown are distilled using advanced statistical or machine learning schemes, revealing key correlations and analytical relationships between the breakdown field and easily access...

Structural evolution of graphene in air at the electrical breakdown limit

In application of graphene to real electronics, understanding the mechanism of the electrical breakdown of the graphene in harsh environments should precede many activities in tamed conditions. In this article, we report the unusual structural evolution of microbridge graphene in air near the electrical current-breakdown limit. In-situ micro-Raman study revealed that Joule heating near the electrical breakdown gave rise to a substantial structural evolution: a previously unknown broad amorphous-like and partially reversible phase at an on- and off-current of ∼3.0 × 108 A/cm2, which finally drove the phase to the electrical current-breakdown. Our calculations suggest that the phase originates from the broken symmetry caused by defect formations during Joule heating. In particular, these formations are bonds of carbon-oxygen and vacancies-oxygen. A collection of energetically favorable vacancies-oxygen pairs results in porous graphene, and its evolution can be the key to understanding how the breakdown starts and propagates in graphene under high current density in air.

Measuring single-shot, picosecond optical damage threshold in Ge, Si, and sapphire with a 51-µm laser

Optical photonic structures driven by picosecond, GW-class lasers are emerging as promising novel sources of electron beams and high quality X-rays. Due to quadratic dependence on wavelength of the laser ponderomotive potential, the performance of such sources scales very favorably towards longer drive laser wavelengths. However, to take full advantage of photonic structures at mid-IR spectral region, it is important to determine optical breakdown limits of common optical materials. To this end, an experimental study was carried out at a wavelength of 5 µm, using a frequency-doubled CO2 laser source, with 5 ps pulse length. Single-shot optical breakdowns were detected and characterized at different laser intensities, and damage threshold values of 0.2, 0.3, and 7.0 J/cm2, were established for Ge, Si, and sapphire, respectively. The measured damage threshold values were stable and repeatable within individual data sets, and across varying experimental conditions.

Which is better, electrostatic or piezoelectric energy harvesting systems?

This paper answers the often asked, and until now inadequately answered, question of which MEMS compatible transducer type achieves the best power density in an energy harvesting system. This question is usually poorly answered because of the number of variables which must be taken into account and the multi-domain nature of the modelling and optimisation. The work here includes models of the mechanics, transducer and the power processing circuits (e.g. rectification and battery management) which in turn include detailed semiconductor models. It is shown that electrostatic harvesters perform better than piezoelectric harvesters at low accelerations, due to lower energy losses, and the reverse is generally true at high accelerations. At very high accelerations using MEMS-scale devices the dielectric breakdown limit in piezoelectric energy harvesters severely decreases their performance thus electrostatics are again preferred. Using the insights gained in this comparison, the optimal transduction mechanism can be chosen as a function of harvesting operating frequency, acceleration and device size.

A High-Power Low-Loss Continuously Tunable Bandpass Filter With Transversely Biased Ferrite-Loaded Coaxial Resonators

This paper presents a technology for high-power low-loss continuously tunable RF filters demonstrated by the example of a two-pole coupled-resonator filter. The resonators are shortened coaxial cavities loaded with ferrite inserts, where an externally applied transverse dc magnetic bias controls the center frequency. The filter is analyzed by simulations that take into account the inhomogeneity of the bias field. A prototype is manufactured and tested, exhibiting a tuning span of 41% in which the unloaded Q factor varies in the range from 1223 to 2425. Good correspondance is observed between simulation and measurement results. Furthermore, the power-handling capability of the filter is discussed and the ionization breakdown limit under normal conditions is estimated to be 800 W. The filter's linearity is characterized by a standard two-tone test, where the level of the third-order passive intermodulation product is observed to be -53.1 dBm at an input fundamental tone level of 2 × 43 dBm.

Frequency-tunable continuous-wave terahertz sources based on GaAs plasmonic photomixers

We present frequency-tunable, continuous-wave terahertz sources based on GaAs plasmonic photomixers, which offer high terahertz radiation power levels at 50% radiation duty cycle. The use of plasmonic contact electrodes enhances photomixer quantum efficiency while maintaining its ultrafast operation by concentrating a large number of photocarriers in close proximity to the device contact electrodes. Additionally, the relatively high thermal conductivity and high resistivity of GaAs allow operation under high optical pump power levels and long duty cycles without reaching the thermal breakdown limit of the photomixer. We experimentally demonstrate continuous-wave terahertz radiation with a radiation frequency tuning range of more than 2 THz and a record-high radiation power of 17 μW at 1 THz through plasmonic photomixers fabricated on a low temperature grown GaAs substrate at 50% radiation duty cycle.

Unbiased continuous wave terahertz photomixer emitters with dis-similar Schottky barriers.

We are introducing a new bias free CW terahertz photomixer emitter array. Each emitter consists of an asymmetric metal-semiconductor-metal (MSM) that is made of two side by side dis-similar Schottky contacts, on a thin layer of low temperature grown (LTG) GaAs, with barrier heights of difference (ΔΦ(B)) and a finite lateral spacing (s). Simulations show that when an appropriately designed structure is irradiated by two coherent optical beams of different center wavelengths, whose frequency difference (∆f) falls in a desired THz band, the built-in field between the two dis-similar potential barriers can accelerate the photogenerated carriers that are modulated by ∆ω, making each pitch in the array to act as a CW THz emitter, effectively. We also show the permissible values of s and ΔΦ(B) pairs, for which the strengths of the built-in electric field maxima fall below that of the critical of 50 V/μm- i.e., the breakdown limit for the LTG-GaAs layer. Moreover, we calculate the THz radiation power per emitter in an array. Among many potential applications for these bias free THz emitters their use in endoscopic imaging without a need for hazardous external biasing circuitry that reduces the patient health risk, could be the most important one. A hybrid numerical simulation method is used to design an optimum emitter pitch, radiating at 0.5 THz.

The effect of uni/bipolar charge injection on EHD conduction pumping

Abstract Electrohydrodynamics (EHD) conduction pumping takes advantage of Coulomb force generated by externally applied electric field and dissociated charges from electrolytes present in the working fluid. With the electric field maintained below the DC breakdown limit (i.e voltage required for charge injection), EHD conduction generated flow relies primarily upon the asymmetry of the electrodes where the flow is always generated toward the specific direction regardless of the electrodes polarity. The charge distribution induced by the process of dissociation may be altered by charge injection, potentially present at the electrodes' surfaces. The charge injection could occur, for example, because of the electrode surface roughness. This paper is a numerical investigation to quantify the impact of the charge injection on the performance of EHD conduction pump. The numerical domain comprises a coplanar asymmetric electrode pair embedded against a 2-D channel wall where the EHD conduction induced liquid flow is expected to be generated from the narrower electrode toward the wider electrode in the absence of charge injection. The electric field, net charge density, and electric body force distributions are presented in the absence and presence of charge injection. In addition, the electrically generated net flow is calculated for several operating conditions.

Wavefront correction in the extreme ultraviolet wavelength range using piezoelectric thin films.

A new scheme for wavefront correction in the extreme ultraviolet wavelength range is presented. The central feature of the scheme is the successful growth of crystalline piezoelectric thin films with the desired orientation on an amorphous glass substrate. The piezoelectric films show a high piezoelectric coefficient of 250 pm/V. Using wavefront calculations we show that the grown films would enable high-quality wavefront correction, based on a stroke of 25 nm, with voltages that are well below the electrical breakdown limit of the piezoelectric film.

Understanding avalanches in a Micromegas from single-electron response measurement

Avalanche fluctuations set a limit to the energy and position resolutions that can be reached by gaseous detectors. This paper presents a method based on a laser test-bench to measure the absolute gain and the relative gain variance of a Micro-Pattern Gaseous Detector from its single-electron response. A Micromegas detector was operated with three binary gas mixtures, composed of 5% isobutane as a quencher, with argon, neon or helium, at atmospheric pressure. The anode signals were read out by low-noise, high-gain Cremat CR-110 charge preamplifiers to enable single-electron detection down to gain of 5× 103 for the first time. The argon mixture shows the lowest gain at a given amplification field together with the lowest breakdown limit, which is at a gain of 2×104 an order of magnitude lower than that of neon or helium. For each gas, the relative gain variance f is almost unchanged in the range of amplification field studied. It was found that f is twice higher (f~0.6) in argon than in the two other mixtures. This hierarchy of gain and relative gain variance agrees with predictions of analytic models, based on gas ionisation yields, and a Monte-Carlo model included in the simulation software Magboltz version 10.1.

Pulsed actuation avoids failure in dielectric elastomer artificial muscles

Dielectric elastomer actuators (DEAs) are a class of artificial muscles capable of large linear strains (well over 100%), and with high energy density, and low cost and weight. One of the most prominent failure modes of a DEA is electrical breakdown, which can damage the device permanently, limiting its deformation capability. Breakdown is also common, since to maximize energy output, devices often operate near the breakdown limit. Elucidating breakdown mechanisms, as well as finding ways to prevent it, are of intense research interest. We show that by applying short electrical pulses, one could minimize the exposure of the DEAs to high leakage current, which is one of the main mechanisms for electrical breakdown. This allows one to operate at significantly higher potentials than the DC breakdown voltage. By applying pulses, we demonstrate up to 81.7% area strain repeatedly, at voltages more than twice the DC breakdown limit, without the risk of failure. The pulsed operation mode of DEAs accommodating higher voltages than possible with DC represents an opportunity for potential applications, safer and simpler device designs, and a technique for further study of DEA breakdown mechanisms.