Vacuum Gap Research Articles

Thermal Analysis of a New Sliding Smart Window Integrated with Vacuum Insulation, Photovoltaic, and Phase Change Material

A zero-energy building (ZEB) requires an innovative integration of technologies, in which windows play a paramount role in energy reduction, storage, and generation. This study contributes to four innovative designs of sliding smart windows. It integrates air-gap (AG), phase change material (PCM), photovoltaic (PV), and vacuum glazing (VG) technologies. These smart sliding windows are proposed to generate electricity along with achieving efficient thermal insulations and heat storage simultaneously. A two-dimensional multiphysics thermal model that couples the PCM melting and solidification model, PV model, natural convection in the cavity, and the surface-to-surface radiation model in the vacuum gap are developed for the first time. The model is validated with data in the literature. The transient simulations were carried out to investigate the thermo-electrical performance of a window with an area of 1 m by 1 m for the meteorological conditions of Kuwait city on the 10th of June 2018, where the window was oriented to south direction. The results showed that the total solar heat energy gain per unit window area is 2.6 kWh, 0.02 kWh, 0.22 kWh, 1.48 kWh, and 0.2 kWh for the double AG, AG + PV + PCM + VG, PV + PCM + VG, AG + PV + PCM, and the ventilated AG + PV + PCM + VG, respectively. The results elucidate the advantages of the integration of VG in this integrated sliding smart window. The daily generated PV electrical energy in these systems is around 1.3 kWh, 1.43 kWh, and 1.38 kWh for the base case with double AG, PV + PCM + VG, and the ventilated AG + PV + PCM + VG respectively per unit window area.

Computation of intermolecular forces that carry sound across vacuum gaps

A recently understood ability of sound to cross narrow vacuum gaps between material bodies has numerous implications in modern technology, where it opens additional channels for the transfer of heat and acoustic signals between narrowly separated objects. Acoustic vibrations in a body are characterized by perturbations of its surface and of its material density, which affect molecules of another body via intermolecular forces. Methods of continuum mechanics allow the description of the acoustic waves initiated by external body forces, including van der Waals forces. The expressions for such forces have been known for many years, but their applications to configurations relevant to sound transmission are often based on restrictive approximations and asymptotic expansions, which are rarely satisfied in practical situations, where amplitudes of molecular displacements may be in a picometer range. This paper develops a method of accurate computations of van der Waals forces caused by such perturbations with scales comparable with intermolecular distances in a material. The accuracy of the developed approach is limited only by calculation errors, and the scope of its applications is not only limited to acoustic perturbations but also includes the description of van der Waals forces caused by arbitrary molecular density perturbations, by corrugated surfaces, contaminated materials, etc.

Thermophysical study of surface phonon polaritons in multilayer systems for heat dissipation

Heat dissipation is an important concern for micro devices which could be overheated in their working process. In the present work, aiming to find new methods for heat dissipation, the thermophysical properties induced by surface phonon polaritons in multilayer systems were probed. The specific heat capacity, expansion coefficient, thermal conductivities along in-plane and also cross-plane directions in a multilayer system which is composed of SiC films separated by vacuum gaps were all calculated and discussed. Results turn out that: The polariton-induced specific heat is much smaller than that contributed by phonons because of the large difference of the density of states. The thermal conductivity along in-plane direction could be as high as about 104 W m−1 K−1 for the SiC film with a thickness of 0.1 mm, which is almost 1012 times larger than that along cross-plane direction. This large difference of thermal conductivities suggests a potential application of the multilayer systems for heat dissipation and also a long-distance thermal energy transport. Additionally, the expansion coefficient caused by polaritons is several orders of magnitude lower than that caused by phonons, which will benefit the method by utilizing polaritons to enhance heat transfer.

Super-Planckian near-field heat transfer between hyperbolic metamaterials

Heat transfer between closely-spaced objects may be greatly enhanced with efficiency far beyond than the classic blackbody radiation limit under the participation of evanescent waves. Metamaterials with high density of optical surface states have been particularly explored to improve the heat flux upper limit but mostly confined in theoretical discussions due to the experimental challenges. In the near-field context, the effective medium theory (EMT) as widely used in the discussions is also lack of practical inspections. In this work, we managed to fabricate two 2 × 2 cm2 sized high-quality infrared hyperbolic metamaterials made of silicon nanorod array and studied their near-field thermophotonic interaction using home-made setup. At the 500 nm vacuum gap, a strong heat flux density of 830 W/m2 is observed, which is 4.7 times larger than the blackbody value. A local uniaxial model with parameters retrieved from the full-wave simulation was built to reproduce the measured results, which unambiguously validated the widely-adopted semi-empirical condition on the relationship between vacuum gap and lattice constant for the application of the EMT in the near-field regime. These results are believed essential for the future development of metamaterials for novel thermophotonic devices.

Fundamental limitations of the mode temperature concept in strongly coupled systems

Abstract We theoretically analyze heat exchange between two quantum systems in interaction with external thermostats. We show that in the strong coupling limit the widely used concept of mode temperatures loses its thermodynamic foundation and therefore cannot be employed to make a valid statement on cooling and heating in such systems; instead, the incorrectly applied concept may result in a severe misinterpretation of the underlying physics. We illustrate these general conclusions by discussing recent experimental results reported on the nanoscale heat transfer through quantum fluctuations between two nanomechanical membranes separated by a vacuum gap.

Significant enhancement of near-field radiative heat transfer between black phosphorus-covered hyperbolic metamaterial

Hyperbolic metamaterial, a novel type of anisotropic bulk material, can enhance the near-field radiation heat transfer (NFRHT) via supporting unbounded hyperbolic phonon polaritons (HPPs). A cutting-edge two-dimensional material, black phosphorous (BP), also gives a rise to promising performance in the NFRHT due to its excitation of surface plasmon polaritons (SPPs). Here, by coupling SPPs with HPPs, we have proposed a hybrid heterostructure that consists of a single top-covered BP layer with doped-Si nanowire arrays (D-SiNWs) and explored the NFRHT between two heterostructures separated by a vacuum gap. The hybrid heterostructure enables a significant enhancement of NFRHT contributed to the excited hybridized modes of BP plasmons and HPPs. The SPPs of BP expand the effective range of HPPs to the high-k space, resulting in a heat transfer coefficient that is 2.9 times and 1.7 times larger than that of two D-SiNWs and two monolayer BPs, respectively. Influences of electron density of BP are conclusively clarified from the profile of the plasma dispersion relation. Thus, it is expected that the proposed configuration can be used to regulate the near-field heat transfer in a tunable fashion and shed light on an invaluable technique toward the design of two-dimensional materials for thermal and electrical applications.

Pulsed-power-driven cylindrical aluminium liner implosions with a high aspect ratio at an 8 MA facility

First experimental investigations of foil aluminium liner implosions with a high aspect ratio of ∼ 103 at an 8 MA pulsed-power facility in China, motivated by possible application for magneto-inertial fusion, will be reported. Key diagnostics including UV cameras, micro-magnetic probes and x-ray pinhole imaging systems were fielded. Quasi-monochromatic UV images and optical streak data showed localized plasma formation at breakdown positions, suggesting that vacuum gaps in contact play a crucial role in long-time and non-uniform initiation. In liner ablation, quasi-periodic striation patterns were directly observed in UV self-emission images and were found to be azimuthally correlated with Rayleigh–Taylor instabilities. An m ≠ 0 azimuthal helical mode was obtained for the unmagnetized liner, which could be attributed to the large surface roughness that may act as an inherently and randomly seeded instability. Precursor plasma formation was confirmed by the observed x-ray emission on-axis at −70 ns by the end-on x-ray framing camera. Further quantitative current measurements taken by micro magnetic probes suggested that the current division due to precursor plasma was approximately 20% of the load current at −66 ns. Side-on imaging diagnostics indicated an evident liner implosion, however with a relatively low convergence ratio of ∼ 3.2 at stagnation. For the observed emission rings with much faster velocity in end-on x-ray images, possible mechanisms were discussed.

Near field radiative heat transfer between macro-scale metallic surfaces at cryogenic temperatures

Knowledge of radiative heat transfer between bodies at various temperatures is essential for efficient design of cryogenic devices. Radiative far-field heat transfer is commonly taken into account. Nevertheless, when the distance d between components of a device becomes small, smaller than the characteristic wavelength of Planck’s far field thermal radiation, an additional heat transfer caused by thermal electromagnetic near-field starts to play a role. At cryogenic temperatures and micrometric distances, this near field heat transfer can exceed the far-field one by orders of magnitude.We report experimental results on near-field and far-field heat fluxes q transferred across a plane parallel vacuum gap d between pair of identical copper (RRR = 10) and tungsten (RRR = 1.5) samples. The heat flux q was measured over the distances d = 1–100 μm and for various temperatures T2 = 15–80 K of the hot sample and the temperature T1 down to 5 K of the cold one. We compare the copper and tungsten data with previously published results for normal metals Nb and NbN. For each pair of identical samples, the measured values of near-field thermal conductance of vacuum gap, KT = q/(T2 − T1), collapse into nearly a single dependence on the gap size d. Thus for specific metallic surfaces, this relation enables estimate the near field heat flux at cryogenic temperatures over micrometric distances.

Double short vacuum gaps in series under lightning impulse voltage: Estimation of breakdown voltage

The objective of this paper is to propose a method for quantitatively estimating the breakdown voltage of double short vacuum gaps in series under the lightning impulse voltage. The study found a quantitative relationship between the breakdown voltage and the maximum electric field strength at the cathode. This relationship is validated by previous experimental works when the total gap distance is less than 20 mm.

Extending the Thermal Near Field Through Compensation in Hyperbolic Waveguides

A promising method to leverage near-field power densities without the use of nanoscale vacuum gaps is through hyperbolic metamaterial (HMM) waveguides. When placed between a hot and cold reservoir, an ideal HMM can transmit surface waves across several microns, enabling an extension of near-field enhancements. However, when accounting for transmission loss due to realistic levels of absorption within the waveguide, previous studies have shown that the enhancements are significantly curtailed at wide separations. In our study, we investigate the role of internal sources within nonisothermal HMMs. We demonstrate that, in some cases, the emission from the HMM accounts for over 90% of the total radiative heat transfer to the receiver, and that these additional sources can largely compensate for optical losses associated with decreased transmission from the emitter to the receiver. Lastly, we investigate the spectral transport in a realistic three-body system that has mismatching optical properties between the boundaries (emitter and receiver) and the waveguide (HMM). Our model shows that the near-field thermal transport remains spectrally selective to the boundaries, even as major radiative contributions come from the waveguide. This work may enable the design of nonisothermal emitter-waveguide-receiver systems that transmit near-field power levels over wider separations.

Surface-Plasmon-Enhanced Near-Field Radiative Heat Transfer between Planar Surfaces with a Thin-Film Plasmonic Coupler

In last decade, there have been enormous efforts to experimentally show the near-field enhancement of radiative heat transfer between planar structures. Several recent experiments also have striven to achieve further enhanced heat transfer with the excitation of coupled surface polaritons by introducing nanostructures on both emitter and the receiver; however, these symmetric structures are hardly employed in real-world applications. Here, we demonstrate substantially increased near-field radiative heat transfer between asymmetric structures (i.e., doped Si and SiO$_2$) by using a thin Ti film as a plasmonic coupler. The measured near-field enhancement at vacuum gap of 380 nm is found to be 3.5 times greater than that for the case without the coupler. The enhancement mechanism is thoroughly elucidated for both polarizations and a dimensionless parameter, which can quantify the coupling strength of the surface polaritons at vacuum, is suggested. As a thin film can be readily used in many engineering applications, this study will facilitate the development of the high-performance engineering devices exploiting the near-field thermal radiation.

Super-Planckian radiative heat transfer between macroscale metallic surfaces due to near-field and thin-film effects

In this study, we demonstrate that the radiative heat transfer between metallic planar surfaces exceeds the blackbody limit by employing the near-field and thin-film effects over macroscale surfaces. Nanosized polystyrene particles were used to create a nanometer gap between aluminum thin films of different thicknesses from 80 nm to 13 nm coated on 5 × 5 mm2 silicon chips, while the vacuum gap spacing is fitted from the near-field measurement with bare silicon samples. The near-field radiative heat flux between 13-nm-thick Al thin films at 215 nm gap distance is measured to be 6.4 times over the blackbody limit and 420 times to the far-field radiative heat transfer between metallic surfaces under a temperature difference of 65 K with the receiver at room temperature. The experimental results are validated by theoretical calculation based on fluctuational electrodynamics, and the heat enhancement is explained by non-resonant electromagnetic coupling within the subwavelength vacuum gap and resonant coupling inside the nanometric Al thin film with s polarized waves. This work will facilitate the applications of near-field radiation in thermal power conversion, radiative refrigeration, and noncontact heat control where metallic materials are involved.

Electronic structure of 9 quintuple layers Bi2Se3 within Density Functional Theory

Bismuth Selenide (Bi2Se3) is a well-known 3D-type topological insulator. The interior part of this material is insulating, while its surface state is metallic which is protected by time-reversal symmetry. The experimental study of this material by varying its thickness reveals that the metallic surface state starts to appear at a minimum thickness of six Quintuple Layer (QL). This study was performed within the Density Functional Theory (DFT) in which a vacuum gap is added to the multilayer structure in order to capture the surface states of the material. While a previous study has explored the electronic structure of this material upto a thickness of 8 QL, a more detailed information of how the system evolves from this thickness to a fully bulk limit has not much been explored. With this motivation, we propose to calculate the electronic structure of this material with thickness of 9 QL. We perform the DFT calculation using Quantum Espresso (QE) package. Our results show that the band structure of this system reveals the surface metallic states with many additional bands as compared to those of thinner layer systems.

Near-field radiative heat transfer between nanowire-based dual uniaxial magneto-dielectric metamaterials

Over past several years, much attention has been paid on near-field radiation with super-Planckian heat flux due to resonant coupling of surface polaritons and non-resonant hyperbolic modes but mainly with non-magnetic materials. With nanowire-based magneto-dielectric metamaterials, this work theoretically investigates the near-field radiative heat transfer associated with both uniaxial magnetic and electric responses. Maxwell–Garnett effective medium theory is used to obtain the dual uniaxial effective permeability and permittivity of the magneto-dielectric metamaterials. With the fluctuational electrodynamics, multiple resonant and non-resonant modes associated with both magnetic and electric responses that spectrally enhance the radiative heat transfer, such as magnetic and electric hyperbolic modes, magnetic and electric surface polaritons, mu- and epsilon-near-pole modes, are identified and elucidated at different wave polarizations. Effects of filling ratio, magnetic and electric scattering rates, and vacuum gap distance are also studied, and it is found that the spectral and total heat fluxes of s-polarized waves, which are usually neglected for non-magnetic materials, are much enhanced and comparable with that of p-polarized waves. The results here will deepen the fundamental understanding in magneto-dielectric metamaterials and facilitate their applications in the area of near-field thermal radiation.

Thermal transport across nanometre gaps: Phonon transmission vs. air conduction

Heat transfer between two surfaces separated by a nanometre gap is important for a number of applications ranging from spaced head disk systems, scanning thermal microscopy and thermal transport in aerogels. At these separation distances, near field radiative heat transfer competes with heat transfer mediated by phonons. Here we quantity the contribution of phonon assisted heat transfer between apolar solids using lattice dynamics combined with ab-initio calculations. We clearly demonstrate that phonons dominate heat transfer for subnanometre gaps. Strikingly, we conclude that even in the situation where the gap is filled with air molecules, phonons provide the dominant energy channel between the two solids nearly in contact. Our results predict orders of magnitude enhanced phonon heat transfer compared to previous works and bring forward a methodology to analyse phonon transmission across nanoscale vacuum gaps between apolar materials.

Near-field radiative thermal modulation between Weyl semimetal slabs

We propose a thermal modulation structure composed of two identical Weyl semimetal (WSM) slabs separated by a nanoscale vacuum gap. Following Maxwell's equations and the dielectric tensor of the WSM, we derive the reflection matrix at the WSM/vacuum interface. Based on fluctuation electrodynamics, the effects of the Fermi level, the number of Weyl nodes, the separation between Weyl nodes and the thickness of vacuum gap on the heat transfer coefficient and the thermal modulation contrast are numerically discussed. It is found that the WSM possesses stronger near-field radiative ability in the low frequency region, where the coupled surface plasmon polaritons between the WSM slabs play a dominant role. By continuously tuning the Fermi level from 0.01 to 0.15 eV, a large modulation contrast as high as 58.5 is obtained at T = 300 K, while in the low temperature interval [50,160] K, the minimum modulation contrast can exceed 198 for a vacuum gap of 20 nm. The obtained results might be helpful in designing a WSM-based thermal modulator with giant modulation contrasts.

Study of high DC voltage breakdown between stainless steel electrodes separated by long vacuum gaps

High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER neutral beam injector (NBI) (Toigo et al 2017 New J. Phys. 19 085004). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER NBI (De Lorenzi et al 2011 Fusion Eng. Des. 86 742–5). A voltage up to 800kVDC was achieved in the HVPTF during the experimental campaigns with a sphere-plane configurations having variable gap length (from 30 to 150 mm) and pressure ranging from high vacuum (10–7 mbar) to 10–3 mbar in argon. Such an experimental campaign represents one of the few examples where voltages higher than 500–550 kV DC are sustained by a single vacuum gap between electrodes (Rohrbach 1971 CERN Report 71-5) and therefore constitutes additional experience to improve the knowledge of voltage holding across large vacuum gaps. The results in high vacuum indicate that at the beginning of voltage conditioning the breakdown events occur at the same cathodic electric field irrespective of the electrode geometry. However, after sufficient conditioning time, the breakdown voltage distribution seems to depend also on the electric field at the anode and on the total voltage between electrodes. A benchmark between a numerical tool previously developed to predict the voltage holding in high vacuum (voltage holding prediction model, VHPM (Pilan et al 2011 Fusion Eng. Des. 86 742–5)) and the experimental results is also reported and discussed.

Design of an indium arsenide cell for near-field thermophotovoltaic devices

An indium arsenide photovoltaic cell with gold front contacts is designed for use in a near-field thermophotovoltaic (NF-TPV) device consisting of millimeter-size surfaces separated by a nanosize vacuum gap. The device operates with a doped silicon radiator maintained at a temperature of 800 K. The architecture of the photovoltaic cell, including the emitter and base thicknesses, the doping level of the base, and the front contact grid parameters, are optimized for maximizing NF-TPV power output. This is accomplished by solving radiation and charge transport in the cell via fluctuational electrodynamics and the minority charge carrier continuity equations, in addition to accounting for the shading losses due to the front contacts and additional series resistance losses introduced by the front contacts and the substrate. The results reveal that these additional loss mechanisms negatively affect NF-TPV performance in a non-negligible manner, and that the maximum power output is a trade-off between shading losses and series resistance losses introduced by the front contacts. For instance, when the cell is optimized for a 1 x 1 mm2 device operating at a vacuum gap of 100 nm, the losses introduced by the front contacts reduce the maximum power output by a factor of ~ 2.5 compared to the idealized case when no front contact grid is present. If the optimized grid for the 1 x 1 mm2 device is scaled up for a 5 x 5 mm2 device, the maximum power output is only increased by a factor of ~ 1.08 with respect to the 1 x 1 mm2 case despite an increase of the surface area by a factor of 25. This work demonstrates that the photovoltaic cell in a NF-TPV device must be designed not only for a specific radiator temperature, but also for specific gap thickness and device surface area.

Self-amplification of radiation from an electron bunch inside a waveguide filled with periodic medium

We investigate the radiation from a bunch of relativistic electrons moving along the cylindrical waveguide axis, assuming that the waveguide is partially loaded by a medium with periodic dielectric permittivity and magnetic permeability. The spectral distribution of the radiation energy flux through the cross section of the waveguide is studied at large distances from the medium. The analysis is based on the corresponding exact solution of Maxwell equations for the case of a single electron moving along the waveguide axis. The results of numerical calculations are presented in the special case of layered medium consisting of a finite number of dielectric plates separated by vacuum gaps. We show that under certain conditions on the problem parameters the quasi-coherent Cherenkov radiation generated by the electron bunch inside the plates is self-amplified at certain waveguide modes. A visual explanation of this phenomenon is provided that reproduces the main features to rather good accuracy.

Physics of Pair Producing Gaps in Black Hole Magnetospheres. II. General Relativity

Abstract This is the second paper in a series where we examine the physics of pair producing gaps in low-luminosity accreting supermassive black hole systems. In this paper, we carry out time-dependent self-consistent fully general relativistic 1D particle-in-cell simulations of the gap, including full inverse Compton scattering and photon tracking. Similar to the previous paper, we find a highly time-dependent solution where a macroscopic vacuum gap can open quasiperiodically, producing bursts of pairs and high energy radiation. We present the light curve, particle spectrum, and photon spectrum from this process. Using an empirical scaling relation, we rescale the parameters to the inferred values at the base of the jet in M87, and find that the observed TeV flares could potentially be explained by this model under certain parameter assumptions.