Vacuum Gap Research Articles

Lightening Impulse Breakdown of Vacuum Gaps in Series—Part II: Bridging Resistor

In part I of this series article, it was found that a negative charge process after a partial breakdown (PB) could reduce the breakdown voltage of vacuum gaps in series. This part proposes the use of a bridging resistor to mitigate this negative charge process and improve the breakdown voltage of the entire arrangement. Two commercial vacuum interrupters (VIs) of the same model were connected in series, with each VI having a grading capacitor connected in parallel. A bridging resistor was then bridged between two floating potential middle points: at the middle point between the VIs and at the middle point between the grading capacitors. Experiments were performed using various gap distance arrangements and resistance values. A noncontact measurement method using an electric field sensor was proposed to measure the floating potentials of the two VIs. Experimental results showed that the negative charge process could be mitigated using the bridging resistor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g}$ </tex-math></inline-formula> by blocking the discharge current from the grading capacitor into the broken-down VI. The voltage distribution between the two VIs became uneven because of the bridging resistor, but the breakdown voltage of the series-connected gaps increased significantly, regardless of the resistance value. The increase rate was influenced by the gap distance arrangement. With a symmetrical gap distance arrangement for the two VIs, a maximum increase rate of 37.4% was reached when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g} =40\,\,\text{k}\Omega $ </tex-math></inline-formula> . With asymmetrical arrangements, when smaller gaps shared higher voltages, the increase rate reached a maximum of 52.8% with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g} =70\,\,\text{k}\Omega $ </tex-math></inline-formula> , but it only reached a maximum of 38.9% when larger gaps shared higher voltages with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g} =10\,\,\text{k}\Omega $ </tex-math></inline-formula> .

Lightening Impulse Breakdown of Vacuum Gaps in Series—Part I: Partial Breakdown

Partial breakdown (PB) has been reported in breakdown of vacuum gaps in series but has not been fully understood. The objective of this article is to investigate the influence of PB on breakdown of vacuum gaps in series under lightening impulse voltages. Two commercial vacuum interrupters (VIs) of the same model were connected in series with various gap distance arrangements. Experimental results show that four breakdown types could be classified, i.e., repeatedly PB without final full breakdown of the series gaps (type I), two PBs before final full breakdown (type II), one PB before final full breakdown (type III), and no PB before final full breakdown (type IV). Breakdown voltage of the vacuum gaps in series decreases with the increasing difference between the two gaps and could be even lower than breakdown voltage of the larger gap alone. This is explained by analyzing the three stages of transients induced by a PB. A model was established to simulate transients induced by PB of the vacuum gaps in series. The simulated results could match the experimental results well. An evolution process from a PB to the four breakdown types was deduced. The influence of PB on insulation coordination and breakdown voltage of the vacuum gaps in series was analyzed according to the abovementioned analysis.

Acoustic wave tunneling across a vacuum gap between two piezoelectric crystals with arbitrary symmetry and orientation

It is not widely appreciated that an acoustic wave can ``jump'' or ``tunnel'' across a vacuum gap between two piezoelectric solids, nor has the general case been formulated or studied in detail. Here, we remedy that situation, by presenting a general formalism and approach to study such an acoustic tunneling effect between two arbitrarily oriented anisotropic piezoelectric semi-infinite crystals. The approach allows one to solve for the reflection and transmission coefficients of all the partial-wave modes, and is amenable to practical numerical or even analytical implementation, as we demonstrate by a few chosen examples. The formalism can be used in the future for quantitative studies of the tunneling effect in connection not only with the manipulation of acoustic waves, but with many other areas of physics of vibrations such as heat transport, for example.

Parametric optimization of a novel solar concentrating photovoltaic-near field thermophotovoltaic hybrid system based on cascade utilization of full-spectrum solar energy

Based on cascade utilization of full-spectrum solar energy, a novel solar concentrating photovoltaic and near-field thermophotovoltaic hybrid system (CPV-NFTPVS) is proposed to achieve efficient solar power generation. The full-spectrum solar energy is split into two parts by spectrum splitter: The high-frequency part is provided to concentrating photovoltaic module while the rest to near-field thermophotovoltaic module. The effect of operating parameters such as concentration ratio, split wavelength, applied voltage, and vacuum gap on the energy conversion performance of the hybrid system is investigated. Starting with a concentration ratio of only 210, the efficiency of the hybrid system exceeds the maximum efficiency of near-field solar thermophotovoltaic system (NF-STPVS) with a concentration ratio of 2000. Considering the technical difficulties of the ultrahigh concentration ratio and ultrahigh cooling rate, a concentration ratio of 1000 is a practical choice for the hybrid system and its efficiency reaches 56.20%, which is about 1.12 times higher than the maximum efficiency of NF-STPVS. The law of the optimal value of the operating parameters is further explored and it's found that the optimal gap distance is always 10 nm and the optimal split wavelength needs to be adjusted according to the concentration ratio. The significant value of CPV-NFTPVS based on cascade utilization in solar power generation is indicated and practical application of the near-field thermophotovoltaic is guided.

Optical Investigation of Excited Species Distribution in Low-Current Vacuum Discharges

It is well-known that when high currents flow through a vacuum gap, anode spots are formed on the anode surface. However, anode flare phenomena are also observed in low-current vacuum discharges. The objective of this article is to experimentally observe the spatial distribution of excited Cu atoms and ions generated from a single cathode spot (CS) and to interpret the mechanism of the anode flare at low currents (40–80 A). Optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) techniques have been used for investigating low-current vacuum arc discharge confined to axial magnetic fields (AMFs). The effects of AMFs and arc currents on the axial distribution of the radiation intensity were investigated for different copper ionization states (Cu I and Cu II). The highest luminance was found near the electrodes for both excited neutral copper atoms and ions, and only weak spectral lines of excited Ni atoms were detected in the near-anode region. With increasing the external AMF, a bright “spot” appeared near the anode surface. In the visible light range, excited atoms were the main contributor to anode flare in low-current vacuum discharge. As measured by LIF, the density of copper atoms near the anode increased from 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}$ </tex-math></inline-formula> to 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}\,\,\text{m}^{-3}$ </tex-math></inline-formula> when the applied magnetic fields increased from 31 to 74 mT. The anode flare contains far more excited atoms from the cathode material than the anode material. They are the results of cathodic plasma expansion to reach the anode after charge neutralization and reflection on the anode surface. For the single CS, the observed fact that a tiny number of atoms leave the anode surface and participate in the anode flare is more likely the result of a sputtering effect rather than thermal evaporation by cathode electron stream.

Effect of Coaxial Electrode Geometry on the Electric Field Enhancement Factor for a High Voltage Vacuum Gap

We present an experimental analysis of the change in the electric field enhancement factor with varying gap size and penetration depth (P.D) of cathode into anode for a coaxial vacuum gap, diagnosed using Fowler–Nordheim analysis and optical imaging via scanning electron microscope (SEM) and time integrated Digital single lens reflex camera (DSLR). Data were collected on the Coaxial Gap Breakdown Machine (240 A, 25 kV, 150 ns, ~0.1 Hz). Experiments using five different gap sizes at nine different P.Ds are compared over runs comprising 50 shots for each case. The results show a strong link between enhancement factor and gap size, with P.D and surface topology. For large gap sizes, 150, 330, and 700 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \mathrm {m}$ </tex-math></inline-formula> , the average enhancement factor value increases with increasing P.D. For smaller gap sizes, 50 and 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \mathrm {m}$ </tex-math></inline-formula> , the average enhancement factor decreases with P.D. SEM imaging before and after plasma formation for each gap size allows for quantifying surface finish, microprotrusion growth, average blast diameter, and an estimation of the surface area breakdowns occupy. Time integrated DSLR imaging analysis of the gap at each shot allows for a determination of the distribution of breakdowns about the circumference of the gap for each case tested. The Fowler–Nordheim analysis allows for a quantitative analysis of the surface roughness of all gap sizes tested. Results show that for large gap sizes, the gap geometry and increasing area of breakdown is the main cause for increasing average enhancement factor. For small gap sizes, the dominant driving factor for small average enhancement factors—that subsequently decrease with P.D—is significant changes in surface topology due to an increased number of breakdowns.

Enhancement and Manipulation of Near-Field Thermal Radiation Using Hybrid Hyperbolic Polaritons

Owing to a high electromagnetic confinement and a strong photonic density of states, hyperbolic surface plasmon polaritons (HSPPs) provide a fascinating promise for applications in thermal photonics. In this work, we theoretically predict a possibility for the improvement of the near-field radiative heat transfer on the basis of tailoring the electromagnetic state of hyperbolic metasurfaces by the uniaxial hyperbolic substrate. By using the photonic tunneling coefficient and the polaritons dispersion, we present a comprehensive study of the hybrid effect of the hyperbolic substrate on HSPPs. We find that due to the hybrid effect of the hyperbolic substrate, the anisotropy surface state of hyperbolic metasurfaces would undergo significant deformations and even topological transition. Moreover, we systematically exhibit the evolution of such hybrid hyperbolic mode with different thicknesses of the hyperbolic substrate and analyze the thickness effect on radiative properties of the hybrid system. It is shown that the resulting heat transfer with the assistance of the hybrid hyperbolic mode by optimizing the substrate parameters is many times stronger than that of monolayer hyperbolic metasurface at the same vacuum gap. Taken together, our results provide a platform to tailor 2D hyperbolic plasmons as a potential strategy toward passive or active control of the near-field heat transfer, and the hybrid hyperbolic mode presented here may facilitate the system design for near-field energy harvesting, thermal imaging, and radiative cooling applications.

A Compact, Low-Field, Broadband Matching Section for Externally Powered X-Band Dielectric-Loaded Accelerating Structures

It has been technically challenging to efficiently couple external radio frequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures, especially when the DLA structure has a high dielectric constant. This article presents a novel design of matching section for coupling the RF power from a circular waveguide to an <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>-band DLA structure with a dielectric constant <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{\text {r}}=16.66$ </tex-math></inline-formula> and a loss tangent <inline-formula> <tex-math notation="LaTeX">$\tan \delta =3.43\times {10}^{-5}$ </tex-math></inline-formula>. It consists of a compact dielectric disk with a width of 2.035 mm and a base angle of 60&#x00B0;, resulting in a broadband coupling at a low RF field, which has the potential to survive in the high-power environment. To prevent a sharp dielectric corner break, a 45&#x00B0; chamfer is also added. A microscale vacuum gap, caused by metallic clamping between the thin coating and the outer thick copper jacket, is also studied in detail. Through optimization, most of RF power is coupled into the DLA structure, with no enhancement of peak electromagnetic fields above those of the structure itself. Tolerance studies on the geometrical parameters and mechanical design of the full-assembly structure are also carried out as a reference for fabrication.

Evidences of accumulation points: Effect of high voltage DC conditioning on concave electrodes insulated by large vacuum gaps

Counterintuitive experimental evidences have been observed during High Voltage Direct Current (HVDC) tests of two concave, axial-symmetric, electrodes insulated by large vacuum gaps of 3 and 7 cm with voltages from 150 to 370 kVdc. The dissipation of microdischarge power during the conditioning procedure occurs mostly on the anodic side in a region close to the axis of the system where the electric field is at a minimum, far from the positions where the breakdowns have been observed. The analyses of the phenomena are carried out by comparing the temporal evolution of voltages, currents, pressure, measurements of x-ray energy spectra, and images from infrared and visible light cameras. Numerical simulations, based on ray-tracing algorithm, correctly identify the positions where the power dissipation of microdischarges occurs. A mutual exchange of charged particles in the electrostatic field between electrodes seems a reasonable physical mechanism to interpret the observations. These findings suggest a new perspective to review the current literature and interpret new results considering geometric details which were so far omitted: the areas with the most intense electric field, typically located on the surfaces of the electrodes under test, are not necessarily the sole surfaces involved in the HVDC conditioning in high vacuum.

Thermophotovoltaic Energy Conversion in Far-to-Near-Field Transition Regime

Recent experimental studies on near-field thermophotovoltaic (TPV) energy conversion have mainly focused on enhancing performance via photon tunneling of evanescent waves. In the sub-micron gap, however, there exist peculiar phenomena caused by the interference of propagating waves, which is seldom observed due to the dramatic increase of the radiation by evanescent waves in full spectrum range. Here, we experimentally demonstrate the oscillatory nature of near-field TPV energy conversion in the far-to-near-field transition regime (250-2600 nm), where evanescent and propagating modes are comparable due to the selective spectral response by the PV cell. Noticeably, it was possible to produce the same amount of photocurrent at different vacuum gaps of 870 and 322 nm, which is 10\% larger than the far-field value. Considering the great challenges in maintaining nanoscale vacuum gap in practical devices, this study suggests an alternative approach to the design of a TPV system that will outperform conventional far-field counterparts.

Active Control of Near-Field Radiative Heat Transfer Between InSb and Graphene Multilayered Magneto-Optical Metamaterials

We theoretically investigate the active control of the near-field radiative heat transfer (NFRHT) between two multilayered metamaterials consisting of alternating two kinds of magneto-optical materials, i.e., graphene and InSb. The active control of NFRHT can be realized by applying an external perpendicular magnetic field. We found that, when the chemical potential of graphene μ is 0.05 eV or 0.1 eV, the surface magneto-plasmon polaritons (MPPs) gradually split, and one part moves toward higher frequencies with the increase of the magnetic field. When H = 7 T, the relative thermal magnetoresistance ratio is as high as 82.2% compared with the zero field for μ = 0.05 eV. And most importantly, due to the strong coupling between the MPPs of graphene and the surface modes of InSb, when μ = 0.7 eV, the heat flux reaches 286% compared with the zero field by changing the magnetic field to 1 T. In addition, the effect of the thicknesses of InSb, period number and vacuum gap on heat transfer are also investigated. Through the combined effect of the external magnetic field, the chemical potential of graphene and other factors on the local surface electromagnetic modes, the active control of the NFRHT between two multilayered metamaterials can be realized.

A Three‐Terminal Hybrid Thermionic‐Photovoltaic Energy Converter

AbstractHybrid thermionic‐photovoltaics (TIPV) are solid‐state thermal‐to‐electric energy converters that rely on the non‐isothermal transport of photons and electrons through a vacuum gap. In contrast to pure thermionic converters, the absorption of photons in a photovoltaic anode produces an electrochemical potential that can be delivered as electricity, ultimately boosting the power generation capacity of the device. In this work, the proof of concept of a three‐terminal TIPV converter where thermionic and photogenerated currents are collected independently is reported. Thermionic electrons are injected in the conduction band of a semiconducting anode (n‐type InP), from where they are directly extracted. Photogenerated electrons are also extracted from the conduction band of the anode, but they are then reinjected in the valence band through an independent hole‐selective contact (p‐type InGaAs). By using a low workfunction engineered anode (BaFx/InP) and cathode (ScxOy/W) a maximum power generation capacity of 125.6 and 0.35 mW cm−2 for PV and thermionic sub‐devices, respectively, is demonstrated, operating at 1400 °C. This proof of concept paves the way for the development of efficient hybrid thermionic and photovoltaic converters for the direct conversion of heat into electricity, and subsequently contributes to finding an efficient alternative to thermoelectric generators.

Effect of nonlocal electrical conductivity on near-field radiative heat transfer between graphene sheets

Graphene's near-field radiative heat transfer is determined from its electrical conductivity, commonly modeled using the local Kubo and Drude formulas. In this letter, we analyze the non-locality of graphene's electrical conductivity using the Lindhard model combined with the Mermin relaxation time approximation. We also study how the variation of electrical conductivity with wavevector affects near-field radiative conductance between two graphene sheets separated by a vacuum gap. It is shown that the variation of electrical conductivity with wavevector, $k_{\rho}$, is appreciable for $k_{\rho}$s greater than $100k_0$, where $k_0$ is the magnitude of the wavevector in the free space. The Kubo electrical conductivity provides an accurate estimation of the spectral radiative conductance between two graphene sheets except for around the surface-plasmon-polariton frequency of graphene and at separation gaps smaller than 20 nm where there is a non-negligible contribution from modes with $k_{\rho}>100k_0$ to the radiative conductance. The Drude formula proves to be inaccurate for modeling the electrical conductivity and radiative conductance of graphene except for at temperatures much below the Fermi temperature and frequencies much smaller than $2{\mu}_c/{\hbar}$, where ${\mu}_c$ and ${\hbar}$ are the chemical potential and reduced Planck's constant, respectively. It is also shown that the electronic scattering processes should be considered in the Lindhard model properly, such that the local electron number is conserved. A substitution of ${\omega}$ by ${\omega}+i{\gamma}$ (${\omega}$, $i$, and ${\gamma}$ being the angular frequency, imaginary unit, and scattering rate, respectively) in the collisionless Lindhard model does not satisfy the conservation of the local electron number and results in significant errors in computing graphene's electrical conductivity and radiative conductance.

Comprehensive analysis of an optimized near-field tandem thermophotovoltaic converter

It is well known that performance of a thermophotovoltaic (TPV) device can be enhanced if the vacuum gap between the thermal emitter and the TPV cell becomes nanoscale due to the photon tunneling of evanescent waves. Having multiple bandgaps, multi-junction TPV cells have received attention as an alternative way to improve its performance by selectively absorbing the spectral radiation in each subcell. In this work, we comprehensively analyze the optimized near-field tandem TPV system consisting of the thin-ITO-covered tungsten emitter (at 1500 K) and GaInAsSb/InAs monolithic interconnected tandem TPV cell (at 300 K). We develop a simulation model by coupling the near-field radiation solved by fluctuational electrodynamics and the diffusion-recombination-based charge transport equations. The optimal configuration of the near-field tandem TPV system obtained by the genetic algorithm achieves the electrical power output of 8.41 W/cm$^2$ and the conversion efficiency of 35.6\% at the vacuum gap of 100 nm. We show that two resonance modes (i.e., surface plasmon polaritons supported by the ITO-vacuum interface and the confined waveguide mode in the tandem TPV cell) greatly contribute to the enhanced performance of the optimized system. We also show that the near-field tandem TPV system is superior to the single-cell-based near-field TPV system in both power output and conversion efficiency through loss analysis. Interestingly, the optimization performed with the objective function of the conversion efficiency leads to the current matching condition for the tandem TPV system regardless of the vacuum gap distances.

Species separation in polystyrene shock release evidenced by molecular-dynamics simulations and laser-drive experiments

Material shock release generally happens in the targets of high-energy-density (HED) and inertial confinement fusion (ICF) experiments but has been challenging to study experimentally, theoretically, or computationally. Here, we report extensive studies of polystyrene (CH) shock release by employing large-scale nonequilibrium molecular dynamics and laser-drive experiments at various shock strengths. Our experimental design prevents radiation preheating of the sample and employs a witness foil to investigate the release of shocked CH across a vacuum gap. We observe earlier acceleration of the foil by the release of CH under stronger shocks as well as reflectivity changes in the interferometry data before the foil moves, which is strong evidence of hydrogen streaming ahead of carbon at the release front, consistent with findings from our simulations. Furthermore, our calculations show that lighter species or hydrogen isotopes can carry more mass by one to two orders of magnitude to farther distances during the release and that only less than 0.1 times thermal expansion as predicted by hydrodynamics is needed to explain the high velocities and large scale lengths of low-density plasmas observed in radiation-preheated CH release experiments. These results highlight the significant role of species separation in the shock release of compounds. This process shall be considered, and its potential effects shall be clarified, in the design, interpretation, and analysis of future HED and ICF experiments.

Highly Sensitive Direct‐Conversion Vacuum Flat‐Panel X‐Ray Detectors Formed by Ga2O3‐ZnO Heterojunction Cold Cathode and ZnS Target and their Photoelectron Multiplication Mechanism

AbstractX‐ray detectors are significantly applied in medical diagnoses, industrial inspections, and scientific researches, but the detection sensitivity is limited by the material, device structure, and detection mechanism. This article proposes a direct‐conversion flat‐panel X‐ray detector formed by Ga2O3‐ZnO heterojunction cold cathode and ZnS target to achieve highly sensitive X‐ray detection. The field emission efficiency of Ga2O3‐ZnO heterojunction cold cathode is improved by optimizing the spacing between the adjacent circular pattern in patterned ZnO nanowire arrays. The electron–hole pairs generated by X‐ray photons are multiplied in ZnS target by the electron bombardment induced photoconductivity effect, resulting in a high internal gain (1 × 104%) and a high X‐ray detection sensitivity (7.1 × 102 μCGyair−1 cm−2) for 6 keV X‐ray at 9.2 V μm−1 electric field. In addition, the proposed detector has low dark current density (50 pA mm−2 at 9.2 V μm–1 electric field), good stability (0.2% fluctuation for 60 s), and fast response speed (rise time: 5.3 ms) owing to a built‐in electric field in Ga2O3‐ZnO heterojunction, a ballistic electron transportation in vacuum gap, and a high resistance in ZnS thin film.

Effective Approximation Method for Nanogratings-induced Near-Field Radiative Heat Transfer.

Nanoscale radiative thermal transport between a pair of metamaterial gratings is studied within this work. The effective medium theory (EMT), a traditional method to calculate the near-field radiative heat transfer (NFRHT) between nanograting structures, does not account for the surface pattern effects of nanostructures. Here, we introduce the effective approximation NFRHT method that considers the effects of surface patterns on the NFRHT. Meanwhile, we calculate the heat flux between a pair of silica (SiO2) nanogratings with various separation distances, lateral displacements, and grating heights with respect to one another. Numerical calculations show that when compared with the EMT method, here the effective approximation method is more suitable for analyzing the NFRHT between a pair of relatively displaced nanogratings. Furthermore, it is demonstrated that compared with the result based on the EMT method, it is possible to realize an inverse heat flux trend with respect to the nanograting height between nanogratings without modifying the vacuum gap calculated by this effective approximation NFRHT method, which verifies that the NFRHT between the side faces of gratings greatly affects the NFRHT between a pair of nanogratings. By taking advantage of this effective approximation NFRHT method, the NFRHT in complex micro/nano-electromechanical devices can be accurately predicted and analyzed.

Automated quality control of vacuum insulated glazing by convolutional neural network image classification

Vacuum insulated glazing (VIG) is a highly thermally insulating window technology, which boasts an extremely thin profile and lower weight as compared to gas-filled insulated glazing units of equivalent performance. The VIG is a double-pane configuration with a submillimeter vacuum gap maintained by small pillars positioned in between the panes, which can damage the glass during manufacturing, transportation and installation. For the purpose of automatically classifying the damage, we have developed, trained, and tested a deep learning model using convolutional neural networks. We employ the state-of-the-art methods Grad-CAM and Score-CAM of explainable Artificial Intelligence (XAI) to provide an understanding of the internal mechanisms and were able to show that our classifier outperforms ResNet50V2 for identification of crack locations and geometry. Further analysis of our model's predictive capabilities demonstrates its superiority over state-of-the-art ResNet models in terms of convergence speed, accuracy, precision at 100% recall and AUC for ROC.

First-principles calculations of phonon transport across a vacuum gap

Phonon transport across a vacuum gap separating intrinsic silicon crystals is predicted via the atomistic Green's function method combined with first-principles calculations of all interatomic force constants. The overlap of electron wave functions in the vacuum gap generates weak covalent interaction between the silicon surfaces, thus creating a pathway for phonons. Phonon transport, dominated by acoustic modes, exceeds near-field radiation for vacuum gaps smaller than ~ 1 nm. The first-principles-based approach proposed in this work is critical to accurately quantify the contribution of phonon transport to heat transfer in the extreme near field.

Motion-induced spin transfer

We propose a spin transport induced by inertial motion. Our system is composed of two host media and a narrow vacuum gap in between. One of the hosts is sliding at a constant speed relative to the other. This mechanical motion causes the Doppler effect, which shifts the density of states and the nonequilibrium distribution function in the moving medium. Those shifts induce the difference in the distribution function between the two media, and they result in tunneling spin current. The spin current is calculated from the Schwinger-Keldysh formalism with a spin tunneling Hamiltonian. This scheme does not require temperature difference, voltage, or chemical potential.