Critical Power Density Research Articles

Performance of solid-oxide fuel cells operating with different sustainable fuel reformates

Solid oxide fuel cells (SOFCs) are recognized for their outstanding fuel flexibility and high efficiency in converting chemical energy into electrical energy. This research presents experiments on SOFCs fed with humidified hydrogen and hydrogen-rich gaseous reformates derived from methanol and ammonia: methanol steam reforming (MSR), methanol decomposition (MD), and ammonia decomposition (AD), at 700–850 °C. Results of SOFCs operating with MD reformate on different types of SOFCs, which to the best of our knowledge aren't available in the literature, are presented and compared with other reformate types for the first time. MD-reformate yielded the highest cell performance, with the least disparity in power density from humidified hydrogen, followed by AD-, and MSR-reformates. The study compares the direct internal reforming method, and the innovative Combined Electro-Thermo-Chemical cycle's potential. Introducing the concept of critical power density, the research evaluates fuel utilization across SOFC types. Scanning Electron Microscopy imaging and Energy-Dispersive X-ray Spectroscopy analyses confirm the viability of CO as a fuel, with no carbon deposits on the anodes when using MD-reformate. The findings demonstrate the suitability of using methanol- and ammonia-decomposition products in SOFCs and their compatibility with hybrid power generation cycles.

Diagnosis of Mechanoluminescent Crack Based on Double Deep Learning in Al 7075

The phenomenon of mechanoluminescence (ML) refers to the emission of light induced by mechanical stimulation applied to mechano-optical materials for example SrAl<sub>2</sub>O<sub>3</sub>:Eu,Dy (SAO). Numerous technologies on the basis of ML have been presented to visualize the stress or strain in various structures for the applications including structural health monitoring. As a result, extensive attention has been devoted to the design, synthesis, characteristics, optimizations, and applications of ML materials. However, challenges still remain in the standardization of ML measurement and evaluation, thereby commercially viable ML applications are currently unavailable. To overcome these difficulties, present study proposes ML measurement and evaluation techniques employing the ML fracture mechanics, finite element method, and dual deep learnings. For the effective normalization of visualized ML images under fixed initial ML intensity condition, continuous UV irradiation above the critical ML power density has been subjected to tensile and compact tension (CT) specimens. Therefore, Plastic Stress Intensity Factor (SIF) as well as crack tip stress field have been extracted successfully from normalized ML images based on ML fracture mechanics. To complement and verify the ML analysis, numerical FEM simulation and analytical ASTM calculation have been also provided. Finally, a double deep learning consists of Generative Adversarial Networks (GAN) and Convolutional Neural Networks (CNN) has been trained and tested for the standard evaluation of in-situ ML images.

Thermoelectric performance and mechanical analysis of inner-arc type leg applied in solar thermoelectric generator

The solar thermoelectric generator (STEG) is capable of converting solar energy directly into electricity. To solve the contradictory relationship between thermoelectric efficiency and thermal stress, a new type of internal-arc thermoelectric leg is proposed. A mathematical model of STEG is established, and the effects of leg structure on the thermoelectric performance and thermal stress of STEG are analyzed. The results show that the new thermoelectric leg has excellent power output, which is 10.82% higher than that of rectangular leg. At the same time, it has lower thermal stress, which is 3.82% lower than that of rectangular leg. As the indicator of STEG structure optimization, this study uses the critical power density that can reach 113.63 × 105W/m3 for the inner-arc type thermoelectric leg, which is 146.97% of the X-type leg. Taking into account the thermoelectric performance and thermal stress, the best configuration is X-type thermoelectric leg when the concentration ratio is between 0 ∼ 72.5, and the inner-arc type is preferred when the concentration ratio is between 72.5 ∼ 89.8. By conducting further optimization of the inner-arc type leg, it is possible to achieve an enhancement in the critical power density, amounting to a notable increase of 64.63%. These findings hold significant implications for guiding the structural design of STEG.

In situ monitoring of beam current in electron beam directed energy deposition based on adsorbed electrons

Electron beam directed energy deposition (EB-DED) is a promising and efficient additive manufacturing technology, but the vacuum environment challenges the in situ parameters monitoring. In this paper, an in situ beam current monitoring method is developed based on the absorbed electrons. A series of experiments show that there is a linear relationship between the absorbed electron current and the impinging beam current. However, this relationship only holds when the beam power density is relatively low. When the power density is high, the absorbed electron current will be lower than the theoretical value determined by the linear relationship. This is mainly due to the massive generation and ionization of metal vapor. The critical power density depends on the melting point of the material. Nonetheless, the deviation of the absorbed electron current at high power density can roughly determine the relative position between the focal spot and the workpiece surface. In addition, the slope of the linear relationship is material-dependent, so this method can also distinguish different materials.

Numerical Calculation Method of Spanwise Icing Characteristics and Performance Loss of Three-Dimensional Vertical Axis Wind Turbine Blades

Most icing studies use 2D or quasi-3D models, ignoring the effects of blade span and tip icing. In this paper, a new model for predicting 3D vertical axis wind turbine blades icing by supplementing the blade span is established, the icing characteristics of blades under multiple working conditions are simulated step by step from the time scale, and the unsteady icing laws are compared and analyzed. Ice properties and surface temperature distribution determine the corresponding deicing power. The analysis results show that the unfrozen water film flows in the spanwise direction of the blade, resulting in a smaller icing limit on the pressure surface than the two-dimensional model, which indicates that the chordwise anti-icing structure can be set in a small range. At the same time, the annular ridge ice formed at the end of the blade slows down the reduction rate of the lift-drag ratio of the blade, effectively reducing the influence of the leading edge ice shape on the aerodynamic performance of the blade. Finally, the critical ice melting power density required to reach equilibrium temperature at the leading edge of the blade is reduced by about 11% compared to the trailing edge due to the increased thermal resistance of the gas-to-blade matrix interface by the ice layer.

Laser melting and quenching effects on magnetic domains in quasi-two-dimensional ferromagnetic Cr5Te8 crystal

By utilizing magneto-optical microscopy combined with the polar magneto-optical Kerr effect (MOKE) techniques, the photo-induced melting and quenching effects on magnetic domains in a quasi-two-dimensional (quasi-2D) ferromagnetic Cr5Te8 crystal were investigated. With irradiation of a pulsed laser, the magnetic domain of the Cr5Te8 crystal can be melted at a critical laser power density (2.55 × 108 mW/cm2), which has been confirmed by both MOKE images and curves. Moreover, it is intriguing to find that after laser melting and quenching, the dendritic type domains of the Cr5Te8 crystal are broken and the regrown spherical type domains are isolated from the original dendrites. Simultaneously, the magnetization behavior as a function of external field was modulated. These findings suggest that laser melting and quenching is an effective technique to tailor the magnetic domains in quasi-2D magnets and it provides a viable route in the creation of optical spintronic devices.

Nature of Degradation in Semiconductor Lasers with Electronic Energy Pumping. Theoretical Background

. Catastrophic degradation takes place in case of reaching critical values of laser radiation density power in semiconductor lasers with electronically pumped energy made from single crystals of some compounds. It has been accompanied by mechanical destruction of the surface at resonator ends, an irreversible decrease in radiation power and an increase in generation threshold. Moreover, during the catastrophic degradation of semiconductor lasers under the action of intrinsic radiation, significant changes in the crystal structure occur within the single crystal: dislocation density reaches a value more 1012–1015 cm–2. It has been shown that initial density of dislocations and critical power density of the intrinsic radiation are inversely proportional. Thus, the degradation process of semiconductor lasers is directly related to generation and multiplication of dislocations during laser operation. Mechanical destruction of a crystal lattice occurs at critical values of laser radiation power and dislocation density. To clarify the proposed mechanism for the degradation of semiconductor lasers, it is necessary to take into account an effect of dislocations on optical properties of semiconductors. Typically, this effect is considered as follows: dislocations cause an appearance of a local deformation field and, in addition, form space-charge regions that surround a dislocation core in the form of a charged tube. The paper proposes a model of the phenomenon under study: large stresses arise in the dislocation core, leading to a displacement of individual atoms and deformation of the crystal lattice. Lattice deformation in the dislocation core leads to a local change in the width of a forbidden band. This change value is about 10–2 eV for a screw dislocation and 10–1 eV for a boundary dislocation. The mechanism of this change is that aforementioned deformation leads to a multiple rupture of electronic bonds and an increase in the electron concentration in the dislocation core to approximately value 1018 cm–3. The developed analytical model of the degradation mechanism allows to perform selection of a semiconductor and estimation of a laser operating mode under conditions of increased radiation power.

The role of two-stage phase formation for the solid-state runaway reaction in Al/Ni reactive multilayers

While extensively studied for heating rates below 1.7 K/s and above 1000 K/s, the solid-state phase transformations in Al/Ni reactive multilayers have not been examined at intermediate heating rates between 100 K/s and 1000 K/s. Combined nanocalorimetry and time-resolved synchrotron x-ray diffraction studies are utilized to address this range of heating rates for multilayers with an overall composition of 10 at. % Ni and a bilayer thickness of 220 nm. It was found that a two-stage phase formation of Al3Ni proceeds up to a heating rate of 1000 K/s. The two growth stages occur in the solid-state and are kinetically separated. The activation energy of the first growth stage is determined to be 137 kJ/mol, which agrees well with the literature data at low heating rates. At 1000 K/s, a transition to a runaway reaction is observed. Unusual for metallic multilayers, the reaction proceeds completely in the solid-state which is also known as “solid flame.” Using nanocalorimetry, a critical input power density for ignition of 5.8 × 104 W/cm3 was determined. The rapid succession of the two Al3Ni formation stages was identified as the underlying mechanism for the self-sustaining reaction.

A simple route to prepare anatase TiO2 film onto polyimide substrate by DC pulsed magnetron sputtering

TiO2 film was deposited onto polyimide substrate by DC pulsed magnetron sputtering with different sputtering power densities. XRD results showed that with increasing the sputtering power density from 0.83 W/cm2 to 5.00 W/cm2, the crystallinity of as-deposited TiO2 film improved monotonously. And 5.00 W/cm2 was determined as the critical sputtering power density in which the highest temperature the substrate could withstand. Then, XRD and TEM results showed that the as-deposited TiO2 film could have a better anatase structure by providing an auxiliary heating to the substrate at the highest temperature during TiO2 film deposition. Combining the sputtering parameters, as well as DC pulsed voltage waveform, the reason for as-deposited TiO2 film crystallization was investigated systematically.

Earthquake Initiation From Laboratory Observations and Implications for Foreshocks

AbstractThis paper reviews laboratory observations of earthquake initiation and describes new experiments on a 3‐m rock sample where the nucleation process is imaged in detail. Many of the laboratory observations are consistent with previous work that showed a slow and smoothly accelerating earthquake nucleation process that expands to a critical nucleation length scale Lc, before it rapidly accelerates to dynamic fault rupture. The experiments also highlight complexities not currently considered by most theoretical and numerical models. This includes a loading rate dependency where a “kick” above steady state produces smaller and more abrupt initiation. Heterogeneity of fault strength also causes abrupt initiation when creep fronts coalesce on a stuck patch that is somewhat stronger than the surrounding fault. Taken together, these two mechanisms suggest a rate‐dependent “cascade up” model for earthquake initiation. This model simultaneously accounts for foreshocks that are a by‐product of a larger nucleation process and similarities between initial P wave signatures of small and large earthquakes. A diversity of nucleation conditions are expected in the Earth's crust, ranging from slip limited environments with Lc < 1 m, to ignition‐limited environments with Lc > 10 km. In the latter case, Lc fails to fully characterize the initiation process since earthquakes nucleate not because a slipping patch reaches a critical length but because fault slip rate exceeds a critical power density needed to ignite dynamic rupture.

Calculation of the critical power density of natural light in a polymer optical fiber

This article discusses the advantages of polymer fiber over quartz one. The comparison is made between the absorption spectra of polymethylmethacrylate, perfluoropolymer and quartz fibers. The scheme of thermophysical processes occurring during the interaction of radiation with matter is considered. The formulas for calculating the threshold (critical) power density necessary for heating the surface to a given temperature for pulsed and continuous heating are derived. Further, the calculations of the critical power density and the allowable size of the area are made, with which the concentration of natural light in the POF could be determined.

Laser induced structural phase transitions in Cu3SbS4 thin films

We investigate the laser-induced structural phase transition of Cu3SbS4 to CuSbS2. The Cu3SbS4 thin films were synthesized by a two-stage process of sputtering and subsequent sulfurization. The as-grown films were characterized by x-ray diffraction and Raman spectroscopy to investigate the phase purity of the material. The Cu3SbS4 thin films were irradiated with 532 nm CW laser light at different power densities and in situ Raman spectroscopy was used to investigate the phase transitions. Experimental results show an evidence of local CuSbS2 phase formation from Cu3SbS4 phase in the laser annealed areas above a critical power density. Heat transfer equations solved using COMSOL Multiphysics were used to estimate the surface temperature during the laser irradiation process.

UV–VIS optical spectroscopy investigation on the kinetics of long-lived RONS produced by a surface DBD plasma source

The analysis of the gas phase chemistry of a cold atmospheric plasma is a fundamental step for a more thorough understanding of the effects it can induce on target substrates. This work aims at investigating, by means of optical spectroscopic techniques, the kinetics of O3, NO2 and NO3 produced by a Surface Dielectric Barrier Discharge. The phenomenon of discharge poisoning (or ozone quenching) in static ambient air was investigated varying the electrical power density applied to the plasma source. Kinetics of the reactive oxygen and nitrogen species production were obtained by means of time resolved UV/VIS optical absorption spectroscopy and highlighted how the discharge poisoning takes place once the applied power density ovecomes a critical value of 0.11 W cm−2. An ozone-enriched atmosphere (with a maximum O3 density around 3000–4000 ppm) is thus obtained when the source is operated below the critical power density, while a NOx-enriched atmosphere (highest concentrations of NO2 around 1250 ppm and NO3 around 35 ppm) is obtained at higher applied power densities. Moreover, since the production of NO, one of the most important quenchers of O3, is directly related to the vibrational energy of nitrogen molecules, the vibrational population of N2, determined by processing emission spectra of N2(B → C) band, was studied. Finally, considerations regarding both the energy cost of production of a reactive oxygen species and reactive nitrogen species atmosphere and the possibility of on-line monitoring its chemical composition, were presented in order to emphasize the potential of optical absorption spectroscopy techniques for the on-line control of plasma assisted industrial processes.

Critical Power Density: A Metric To Compare the Excitation Power Density Dependence of Photon Upconversion in Different Inorganic Host Materials

In photon upconversion (UC) based on triplet-triplet annihilation, the upconversion photoluminescent quantum yield (UC-PLQY) depends on the excitation power density in a way that can be described by a single figure of merit. This figure of merit, the threshold value, allows the excitation power density required for efficient UC-PLQY to be compared between different triplet-triplet annihilation systems. Here, we investigate the excitation power density dependence of two-photon UC processes in a series of four lanthanide-doped inorganic host materials (oxides, fluorides, and chlorides) all doped with 18 mol % Yb3+ sensitizer ions and 2 mol % Er3+ activator ions. We demonstrate that an analogous figure of merit, which we call the critical power density (CPD), accurately describes the UC power dependence of these samples. Better CPD values are obtained when the lifetime of the intermediate states is long. The UC-PLQY at the CPD is linked to the saturation UC-PLQY. Thus, a measurement of the UC-PLQY at this low power density can be used to estimate the theoretical saturation UC-PLQY in the absence of deleterious effects such as laser-induced heating. This is compared to another method to estimate the saturation based on the CPD model, namely, taking half of the level's PLQY under direct excitation. Our careful analysis of the upconversion spectra as a function of excitation power density gives several insights into the differing upconversion pathways in the hosts and proves to be a useful tool for their comparison.

Surface plasmon induced enhancement in selective laser melting processes

PurposeSelective laser melting (SLM) is an advanced rapid prototyping or additive manufacturing technology that uses high power density laser to fabricate metal/alloy components with minimal geometric constraints. The SLM process is multi-physics in nature and its study requires development of complex simulation tools. The purpose of this paper is to study – for the first time, to the best of the authors’ knowledge – the electromagnetic wave interactions and thermal processes in SLM based dense powder beds under the full-wave formalism and identify prospective metal powder bed particle distributions that can substantially improve the absorption rate, SLM volumetric deposition rate and thereby the overall build time.Design/methodology/approachWe present a self-consistent thermo-optical model of the laser-matter interactions pertaining to SLM. The complex electromagnetic interactions and thermal effects in the dense metal powder beds are investigated by means of full-wave finite difference simulations. The model allows for accurate simulations of the excitation of gap, bulk and surface electromagnetic resonance modes, the energy transport across the particles, time dependent local permittivity variations under the incident laser intensity, and the thermal effects (joule heating) due to electromagnetic energy dissipation.FindingsLocalized gap and surface plasmon polariton resonance effects are identified as possible mechanisms toward improved absorption in small and medium size titanium powder beds. Furthermore, the observed near homogeneous temperature distributions across the metal powders indicates fast thermalization processes and allows for development of simple analytical models to describe the dynamics of the SLM process.Originality/valueTo the best of the authors’ knowledge, for the first time the electromagnetic interactions and thermal processes with dense powder beds pertaining to SLM processes are investigated under full-wave formalism. Explicit description is provided for important SLM process parameters such as critical laser power density, saturation temperature and time to melt. Specific guidelines are presented for improved energy efficiency and optimization of the SLM process deposition rates.

METHOD FOR PRELIMINARY ESTIMATION OF THE CRITICAL POWER DENSITY IN LASER TECHNOLOGICAL PROCESSES

For a number of new laser technology processes, it is essential to plan an experimental plan for primary experimental engineering activities in terms of quality. The assessment of the critical power density to reach the melting or evaporation temperature of the surface with a suitable theoretical model is an important stage in the development of a particular manufacturing technology. With the help of numerical experiments, this report provides a method for pre-examining the influence of wavelength on the laser technological process. The calculations are performed with a specialized program, running MATLAB. A series of temperature fields were obtained at a change in power density and wavelength at laser impact for concrete types of structural steel. The temperature dependence of the optical and thermo-physical characteristics of the material is also reporded. The analysis is made for laser technology complexes working with lasers emitting in the ultraviolet, visible, near and distant infrared areas. For these wavelengths the critical power density of melting and evaporation is determined.

A study on welding mode transition by electrical detection of laser-induced plasma at varying energy levels

The transition between heat conduction and deep penetration laser welding is investigated by the electrical detection of laser-induced plasma with an electrical passive probe. A further improved physical model based on plasma sheath effect is established to clarify the mechanism of plasma electrical signal. Different electrical signals are detected at varying levels of laser input energy and analyzed in the time and frequency domains. The amplitudes and the fluctuation frequency of the collected signals are demonstrated to be effective in reflecting the electron temperature and the plasma oscillation features respectively. These assessments are identical with the experimental results obtained by a high-speed camera. Combined with the signal characteristics and the weld profiles at different laser powers, the critical state and laser power density of mode transition are discussed and identified by an analytic calculation model.

Stability of low ohmic thick-film resistors under pulsed operation

Durability of low ohmic thick-film resistors on pulse load with micro- and millisecond duration are described in this paper. Standard thick-film resistive compositions with sheet resistance of 10 Ohms/sq. (4311 and QS871 - DuPont; R400-10A - Heraeus) and 3 Ohms/sq. (QS870 - DuPont) were tested. Test resistors with resistance of 3 Ω were prepared on alumina substrates. Resistance changes were measured after 50 pulses at each voltage. Then the voltage was increased and the series of pulses were repeated until the resistance change exceeded 0.5%.Impact of long and short pulses was analyzed for selected pulse duration of 10 μs and 20 ms, respectively. The measurements were carried out for some selected resistors length. The resistors stressed by long pulses exhibited the highest durability, when the thermal interface between substrate and heatsink was filled by thermally conductive grease. In this case resistors made of 4311 and QS871 pastes can withstand the highest power density of about 19 W/mm2 and 18 W/mm2 for the samples with and without the overglaze, respectively. Short (microsecond) pulses had the least influence on overglazed resistors made of QS871 paste, for which the resistance change of +0.5% was observed at electric field intensity of about 85 ± 10 V/mm.Additionally, the critical electric field intensity and critical power density were determined for different pulse duration for selected resistor length of 2 mm. Pulse duration from 10 μs to 1 s with one point per decade were taken into account. The results shown, that overglazed resistors exhibit better durability for all tested resistive pastes for short pulses. However for the long pulses, the results are almost the same. This fact can be explained by limitation of heat transfer in the substrate.Repetitive pulse load was applied for selected samples. The results show that resistors exhibited very good stability after test with 0.1 million pulses with amplitude of 70% of critical electric field intensity values.

Phase field simulation of powder bed-based additive manufacturing

Quality control in parts built layer-by-layer via Additive Manufacturing (AM) process is still a challenge since critical features, such as the control in porosity, residual stress and deformation, surface roughness and microstructure, are yet to be fully addressed. In this work, we develop a phase field model to simulate powder bed-based AM, focusing on the effects of two major process parameters, the beam power and scanning speed, on the melt pool size and shape, porosity and grain structure. The model reproduces many important phenomena observed experimentally and reveals scaling relations for the depth and length of melt pool, the porosity and the grain density on process parameters. We find critical power densities below which the grain density or the porosity increases rapidly and the grain structure is controlled by different mechanisms in different power density regimes which allows for the possibility of controlling the grain structure. The present work could serve as a useful reference for accurate control of defects and microstructure in AM build.

Interfacial thermal conductance in graphene/black phosphorus heterogeneous structures

Graphene, as a passivation layer, can be used to protect the black phosphorus (BP) from the chemical reaction with surrounding oxygen and water. However, BP and graphene heterostructures have low efficiency of heat dissipation due to its intrinsic high thermal resistance at the interfaces. The accumulated energy from Joule heat has to be removed efficiently to avoid the malfunction of the devices. Therefore, it is of significance to investigate the interfacial thermal dissipation properties and manipulate the properties by interfacial engineering on demand. In this work, the interfacial thermal conductance between few-layer BP and graphene is studied extensively using molecular dynamics simulations. Two important parameters, Pcr, the critical heat power density of maintaining thermal stability, and Pmax, the maximum heat power density with which the system can be loaded, are identified. Our results show that interfacial thermal conductance can be effectively tuned in a wide range by external strains and interfacial defects. The compressive strain can enhance the interfacial thermal conductance by one order of magnitude, while interface defects give a two-fold increase. These findings could provide guidelines in heat dissipation and interfacial engineering for thermal conductance manipulation of BP-graphene heterostructure-based devices.