Vacuum Gap Research Articles

Spatial profiles of photon chemical potential in near-field thermophotovoltaic cells

Emitted photons stemming from the radiative recombination of electron–hole pairs carry chemical potential in radiative energy converters. This luminescent effect can substantially alter the local net photogeneration in near-field thermophotovoltaic cells. Several assumptions involving the luminescent effect are commonly made in modeling photovoltaic devices; in particular, the photon chemical potential is assumed to be zero or a constant prescribed by the bias voltage. The significance of photon chemical potential depends upon the emitter temperature, the semiconductor properties, and the injection level. Hence, these assumptions are questionable in near-field thermophotovoltaic devices with nanoscale separation distances between the emitter and the cell. In the present work, an iterative solver that combines fluctuational electrodynamics with the drift-diffusion model is developed to tackle the coupled photon and charge transport problem, enabling the determination of the spatial profile of photon chemical potential beyond the detailed balance approach. The difference between the results obtained by allowing the photon chemical potential to vary spatially and by assuming a constant value demonstrates the limitations of the conventional approaches. This study is critically important for performance evaluation of near-field thermophotovoltaic systems with nanoscale vacuum gaps.

Electrical Insulation Behavior of High Vacuum under Variable Frequency Alternating Excitations

This paper presents results leading towards a superior understanding of the insulating properties of high vacuum by focusing mainly on the pre-breakdown and breakdown behavior of unbridged vacuum gaps under variable frequency alternating excitations. The supply frequency variation is from 30-300 Hz for a fixed inter-electrode spacing of 0.5 mm. The pre-breakdown (PBD) current magnitudes and the breakdown voltages are inversely proportional. The behavior is unpredictable over a specific range of frequencies. A pronounced breakdown voltage peak occurs at 130 Hz with stainless steel electrode pair, whereas this peak is less prominent with copper and aluminum electrodes. Fowler-Nordheim (F-N) emission, ion assisted field (I-F) emission and thermal interplay due to quasi-continuous shower of anode micro-particles predominantly neutral, lend support to the experimental observations. It is surmised that the apparent cause for time delay between voltage and current peaks is not only due to the presence of neutral molecules in the gap, but also principally due to their transit time.

Quasi-Casimir coupling induced phonon heat transfer across a vacuum gap

In vacuum, thermal energy is transported by photons (thermal radiation) but not phonons. Recent studies, however, indicated that phonon heat transfer across a vacuum gap is mediated by the quantum fluctuation of electromagnetic fields. Specifically, in the heat exchange between two objects separated by a nanoscale vacuum gap, phonons carry thermal energy more efficiently than photons. However, it remains unclear if phonons can propagate without electromagnetic fields. Here, we demonstrate that phonon transmission across a sub-nanometer vacuum gap can be induced by quasi-Casimir force subjected to the Lennard–Jones atoms using classical molecular dynamics simulation. The net heat flux across the vacuum gap increases exponentially as the gap distance decreases, owing to acoustic phonon transmission. The local heat flux, evaluated using the Irving–Kirkwood method, increases singularly at the interfacial layers, while that at the inner layers agrees well with the net heat flux. These findings provide evidence of the strong thermal resonance induced by quasi-Casimir coupling between the interfacial layers. Thus, we conclude that the quasi-Casimir coupling induced by intermolecular interaction is a heat transfer mode for phonon heat transfer across a vacuum gap in nanoscale.

High-Performance Ru 2 P Anodic Catalyst for Alkaline Polymer Electrolyte Fuel Cells

High-Performance Ru ₂ P Anodic Catalyst for Alkaline Polymer Electrolyte Fuel Cells

Near field versus far field in radiative heat transfer between two-dimensional metals

Using the standard fluctuational electrodynamics framework, we analytically calculate the radiative heat current between two thin metallic layers, separated by a vacuum gap. We analyse different contributions to the heat current (travelling or evanescent waves, transverse electric or magnetic polarization) and reveal the crucial qualitative role played by the dc conductivity of the metals as compared to the speed of light. For poorly conducting metals, the heat current may be dominated by evanescent waves even when the separation between the layers greatly exceeds the thermal photon wavelength, and the coupling is of electrostatic nature. For well-conducting metals, the evanescent contribution dominates at separations smaller than the thermal wavelength and is mainly due to magnetostatic coupling, in agreement with earlier works on bulk metals.

Polariton topological transition effects on radiative heat transfer

Twisted two-dimensional bilayer materials exhibit many exotic physical phenomena. Manipulating the twist angle between the two layers enables fine control of the physical structure, resulting in development of many novel physics, such as the magic-angle flat-band superconductivity, the formation of moire exciton and interlayer magnetism. Here, combined with analogous principles, we study theoretically the near-field radiative heat transfer (NFRHT) between two twisted hyperbolic systems. This two twisted hyperbolic systems are mirror images of each other. Each twisted hyperbolic system is composed of two graphene gratings, where there is an angle {\phi} between this two graphene gratings. By analyzing the photonic transmission coefficient as well as the plasmon dispersion relation of twisted hyperbolic system, we prove that the topological transitions of the surface state at a special angle (from open (hyperbolic) to closed (elliptical) contours) can modulate efficiently the radiative heat transfer. Meanwhile the role of the thickness of dielectric spacer and vacuum gap on the manipulating the topological transitions of the surface state and the NFRHT are also discussed. We predict the hysteresis effect of topological transitions at a larger vacuum gap, and demonstrate that as thickness of dielectric spacer increase, the transition from the enhancement effect of heat transfer caused by the twisted hyperbolic system to a suppression. This technology could novel mechanism and control method for NFRHT, and may open a promising pathway for highly efficient thermal management, energy harvesting, and subwavelength thermal imaging.

Control of non-equilibrium Casimir force

We introduce the concepts of the control theory into the study of non-equilibrium Casimir forces and radiative heat transfer. As an illustration, we consider the plate-plate system for non-equilibrium Casimir forces, where two dielectric plates having different temperatures are separated by a vacuum gap. Previous works have indicated that this system only supports an unstable mechanical equilibrium. Here, we prove analytically and confirm numerically that the equilibrium can be made stable through a feedback control process involving temperature measurement and modulation. Our results show that it should be fruitful to explore the concepts from control theory in the study of the Casimir force and radiative heat transfer.

Modulating Near-Field Radiative Heat Transfer through Thin Dirac Semimetal Films

ABSTRACT We propose a thermal modulation structure made of two identical SiO2 slabs coated by Dirac semimetal (DSM) films and separated by a nanoscale vacuum gap. The energy transmission probability reveals that the coupled surface plasmon polaritons (SPPs) between the two DSM films, and the surface phonon polaritons (SPhPs) supported by the SiO2 substrate can vary sensitively with the Fermi level, the degenerate factor of 3D Dirac points and the thickness of the DSM film, thus providing the possibilities for modulating the radiative heat transfer by tuning these parameters. Based on Maxwell’s equations incorporating fluctuational electrodynamics, the effects of these parameters on the heat transfer coefficient and the thermal modulation contrast are numerically analyzed. Under proper parameters, higher modulation contrasts are obtained by continuously tuning the Fermi level from 0.05 eV to 0.3 eV. The obtained results might be helpful in designing a DSM-based thermal modulator with higher modulation contrasts.

Lightning Impulse Long Delay Breakdown of Centimeter Vacuum Gaps

This paper presents an experimental investigation into the breakdown voltage fall time tf of lightning voltage breakdown of 2-4 centimeter vacuum gaps having a long breakdown delay time td(>30 μs). The results show that the average 50% breakdown voltage fall time t50 decreases with increasing vacuum gap. It is considered that the lightning impulse breakdown of the centimeter vacuum gaps with a long breakdown delay time may be initiated by the interaction between the cathode thermal processes and the micro-particles phenomena. The increase of the electrode diameter and the decrease of the electrode radius of curvature may contribute to the decrease of the breakdown voltage fall time.

Near-field radiative heat transfer enhancement in the thermophotovoltaic system using hyperbolic waveguides

As an energy conversion system, the near-field thermophotovoltaic (NTPV) device has attracted much attention. To improve the NTPV system output power, an intermediate modulator with the same plasmonic properties as thermal emitters can be intercalated into the two-body system. Surface modes are coupled in the vacuum gap between the modulator and the emitter. The modulator absorbs the enormous radiative energy produced by the coupling of surface modes and transfers it to the cell. However, due to the low penetration depth of surface modes and the loss of energy transmission in the intermediate slab, the direct heat exchange between the emitter and the cell in the three-body system is much smaller than that in the corresponding two-body system. In the present study, we design a hyperbolic metamaterial (HMM)-based three-body NTPV system. Ga-doped ZnO (GZO)/HfO2-multilayer-based HMMs serve as the emitter and the modulator, and GaSb works as the cell. The radiative heat flux from the emitter to the cell is enhanced by the weakly dissipating hyperbolic waveguide, which is attributed to the coupling of the vacuum-GZO surface plasmon polariton (SPP) and the SPPs at the GZO-HfO2 interfaces in the HMM modulator. Compared with the GZO slab emitter, the HMM emitter emits higher spectral heat flux located in the hyperbolic region. Different top layers of the HMM affect the coupling of surface modes. These findings provide a new way to enhance the three-body NTPV system output power.

Comprehensive energy balance analysis of photon-enhanced thermionic power generation considering concentrated solar absorption distribution

The present article reports a comprehensive energy balance analysis of a photon-enhanced thermionic emission (PETE) device when it is used for concentrated solar power (CSP) generation. To this end, we consider a realistic PETE device composed of a boron-doped silicon emitter on glass and a phosphorus-doped diamond collector on tungsten separated by the interelectrode vacuum gap. Depth-dependent spectral solar absorption and its photovoltaic and photothermal energy conversion processes are rigorously calculated to predict the PETE power output and energy conversion efficiency. Our calculation predicts that when optimized, the power output of the considered PETE device can reach 1.6 W/cm2 with the energy conversion efficiency of ~18% for 100× solar concentration, which is substantially lower than those predicted in previous works under ideal conditions. In addition, the photon-enhancement ratio is lower than 10 and decreases with the increasing solar concentration due to the photothermal heating of the emitter assembly, suggesting that PETE should be more suitable for low-to-medium CSP below ~100× concentration. These observations signify the importance of a rigorous energy balance analysis based on spectral and spatial solar absorption distribution for the accurate prediction of PETE power generation.

Nanomechanical Microwave Bolometry with Semiconducting Nanowires

Microwave loss of a semiconducting nanowire implemented in a cavity electromechanical system enables bolometric-microwave-power measurements at millikelvin temperatures. This hybrid system consists of a superconducting microwave cavity capacitively coupled to a suspended $\mathrm{In}\mathrm{As}$ nanowire. As a constant voltage is applied across a vacuum gap between the nanowire and the bottom electrode, the nanowire resistance embedded in the microwave resonator follows its own mechanical vibration. The mechanically modulated resistance of the nanowire contributes to microwave sidebands exhibiting its mechanical resonance. The presence of this resistive component provides the device with enhanced sensitivity to microwave dissipation. This dissipation localized at the nanowire results in a temperature increase, producing a frequency shift of the mechanical resonance sensitive to the microwave power. We employ a circuit model to demonstrate that semiconducting nanowires provide optimal resistances for the nanomechanical microwave bolometry. Our technique could benefit precision microwave measurements in quantum-device research.

Space–charge limited current in nanodiodes: Ballistic, collisional, and dynamical effects

This Perspective reviews the fundamental physics of space–charge interactions that are important in various media: vacuum gap, air gap, liquids, and solids including quantum materials. It outlines the critical and recent developments since a previous review paper on diode physics [Zhang et al. Appl. Phys. Rev. 4, 011304 (2017)] with particular emphasis on various theoretical aspects of the space–charge limited current (SCLC) model: physics at the nano-scale, time-dependent, and transient behaviors; higher-dimensional models; and transitions between electron emission mechanisms and material properties. While many studies focus on steady-state SCLC, the increasing importance of fast-rise time electric pulses, high frequency microwave and terahertz sources, and ultrafast lasers has motivated theoretical investigations in time-dependent SCLC. We particularly focus on recent studies in discrete particle effects, temporal phenomena, time-dependent photoemission to SCLC, and AC beam loading. Due to the reduction in the physical size and complicated geometries, we report recent studies in multi-dimensional SCLC, including finite particle effects, protrusive SCLC, novel techniques for exotic geometries, and fractional models. Due to the importance of using SCLC models in determining the mobility of organic materials, this paper shows the transition of the SCLC model between classical bulk solids and recent two-dimensional (2D) Dirac materials. Next, we describe some selected applications of SCLC in nanodiodes, including nanoscale vacuum-channel transistors, microplasma transistors, thermionic energy converters, and multipactor. Finally, we conclude by highlighting future directions in theoretical modeling and applications of SCLC.

Progress Toward High Power Output in Thermionic Energy Converters.

Thermionic energy converters are solid‐state heat engines that have the potential to produce electricity with efficiencies of over 30% and area‐specific power densities of 100 Wcm−2. Despite this prospect, no prototypes reported in the literature have achieved true efficiencies close to this target, and many of the most recent investigations report power densities on the order of mWcm−2 or less. These discrepancies stem in part from the low‐temperature (<1300 K) test conditions used to evaluate these devices, the large vacuum gap distances (25–100 µm) employed by these devices, and material challenges related to these devices' electrodes. This review will argue that, for feasible electrode work functions available today, efficient performance requires generating output power densities of >1 Wcm−2 and employing emitter temperatures of 1300 K or higher. With this result in mind, this review provides an overview of historical and current design architectures and comments on their capacity to realize the efficiency and power potential of thermionic energy converters. Also emphasized is the importance of using standardized efficiency metrics to report thermionic energy converter performance data.

Role of graphene on the enhancement of near-field radiative heat transfer between two homogeneous lossy media plates

In this paper, the near-field radiative heat transfer between two semi-infinite plates with different temperatures, which are composed of homogeneous lossy media, has been studied firstly. Attributed to the evanescent wave generated by total internal reflection, the near-field radiative heat transfer is much larger than the far-field radiative heat transfer. And, the closer the distance between the two plates is, the greater the heat transfer is. Then, a graphene layer has been used to cover on the surface of the plate with lower temperature to study the effect of graphene on the near-field radiative heat transfer. The results show that the existence of graphene can promote the near-field radiative heat transfer due to the excitation of surface plasmon polaritons on the surface of graphene. In addition, the heat transfer varies with chemical potential of graphene, which indicates that the near-field radiative heat transfer can be controlled by an external gate circuit. In the presence of graphene, the thermal resistance between the two plates ranges from 6.7×10−4 to 0.18 K/(W/m2) and increases with the increase of the thickness of vacuum gap between the plates.

Impact of DC bias on weak optical-field-driven electron emission in nano-vacuum-gap detectors

In this work, we investigate multiphoton and optical field tunneling emission from metallic surfaces with nanoscale vacuum gaps. Using time-dependent Schrödinger equation (TDSE) simulations, we find that the properties of the emitted photocurrent in such systems can be greatly altered by the application of only a few-volt direct current (DC) bias. We find that when coupled with expected plasmonic enhancements within the nanometer-scale metallic gaps, the application of this DC bias significantly reduces the threshold for the transition to optical-field-driven tunneling from the metal surface, and could sufficiently enhance the emitted photocurrents, to make it feasible to electronically tag fJ ultrafast pulses at room temperature. Given the petahertz-scale instantaneous response of the photocurrents, and the low effective capacitance of thin-film nanoantenna devices that enables &lt; 1 f s response time, detectors that exploit this bias-enhanced surface emission from nanoscale vacuum gaps could prove to be useful for communication, petahertz electronics, and ultrafast optical-field-resolved metrology.

Investigation on the Influence of Different External Magnetic Field Strength on the Dynamic Voltage Distribution of Double-Break Vacuum Gaps in Series

Previous researches show that grading capacitors truly improve the dynamic voltage distribution uniformity of multibreak vacuum circuit breakers (VCBs). However, large capacitors can reduce the breaking capacity. In order to obtain balanced voltage distribution with small capacitors, nonsynchronous external magnetic field was adopted. The Helmholtz coil was designed to generate a stable magnetic, and nonsynchronous magnetic field control system was designed to realize double-break nonsynchronous control. The 3-D models of Helmholtz coil and axial magnetic field (AMF) contact were simulated and analyzed in Maxwell. The external magnetic field (EMF) experiment platform for double-break vacuum gaps was set up. In the experiment, the strength of EMF on each break can be adjusted by the nonsynchronous magnetic field control system. The experimental results show that different EMF strengths on two vacuum breaks in series can improve the voltage distribution uniformity. Meanwhile, the magnetic field strength has the best matching parameters. The works in this article provide references for magnetic field regulation and interruption performance of VCBs.

Submicrometer-Gap Thermionic Power Generation Based on Comprehensive Modeling of Charge and Thermal Transport

Over the past years, thermionic energy conversion (TEC) with a reduced inter-electrode vacuum gap has been studied as an effective way to mitigate a large potential barrier due to space charge accumulation. However, existing theoretical models do not fully consider the fundamental aspects of thermionic emission when the inter-electrode gap shrinks to the nanoscale, which results in underestimation of thermionic power generation for such small gaps. The present work addresses this challenge by comprehensively modeling charge and thermal transport processes with specific consideration of nanoscale gap effects, such as image charge perturbation, electron tunneling, and near-field thermal radiation. Carefully conducted energy balance analysis reveals that if optimized, submicron-gap TEC can excel the micron-gap counterpart with $\sim$4 times the power output and ~5-10 % higher energy conversion efficiency. Moreover, the high-temperature collector of the submicron-gap TEC, which is due to thermionic and near-field radiative heat transfer, allows the addition of a bottom-cycle heat engine to further enhance the power and efficiency when combined. Electric field concentration due to engineered surface roughness is also examined as a potential approach to produce an additional increase in power generation. We believe that the present work provides a theoretical framework for submicron-gap thermionic power generation as a promising energy recycling scheme for high-quality heat sources.

On the radio frequency dependence of the pulse delay phenomenon in PSR B0943+10

ABSTRACT We report the result of measurements of a gradual shift of the integrated pulses towards later spin phase of the anomalous pulsar B0943+10 at high radio frequencies. We have used observations from the Arecibo Observatory and the GMRT at 327 and 325 MHz correspondingly. For the measurements, we have proposed a special method for calculating the correct positions of the partially merged two components of the pulse profile shape with significant temporal changes in their amplitude ratio. The exponential change in the pulse phase with an amplitude of 4 ms and characteristic time of about 1 h has been found. Comparison of our measurements at 325 and 327 MHz with those at the lower frequencies of 25–80, 62 and 112 MHz have shown that the character of the process does not depend on frequency across a wide frequency range. The result is very important for constraining the nature of the delay. It supports the assumption that the process results from changes in the vacuum gap near the surface of the pulsar. The further correlation between changes in the pulse phase and its intensity is discussed.

Near-field radiative transfer by bulk hyperbolic polaritons across vacuum gap

Thermal radiation has always been treated as a surface phenomenon. A recent study has demonstrated that new radiation channels, the hyperbolic phonon polaritons, can contribute to heat transfer inside bulk hyperbolic materials with a heat flux comparable to conduction. For near-field radiative transfer across a vacuum gap between two hyperbolic materials, hyperbolic phonon polaritons have been considered as surface modes that are responsible for heat transfer enhancement. Here, we analyze near-field radiative transfer due to hyperbolic phonon polaritons, driven by temperature gradient inside the bulk materials. We develop a mesoscale many-body scattering approach to account for the role of hyperbolic phonon polaritons in radiative transfer in the bulk and across a vacuum gap. Our study points out the equivalency between the bulk-generated mode and the surface mode in the absence of a temperature gradient in the material, and hence provide a unified framework for near-field radiative transfer by hyperbolic phonon polaritons. The results also elucidate contributions of the bulk-generated mode and the bulk temperature profile in the enhanced near-field radiative transfer.