11,136 research outputs found
Hierarchical macro-nanoporous metals for leakage-free high-thermal conductivity shape-stabilized phase change materials
Impregnation of Phase Change Materials (PCMs) into a porous medium is a
promising way to stabilize their shape and improve thermal conductivity which
are essential for thermal energy storage and thermal management of small-size
applications, such as electronic devices or batteries. However, in these
composites a general understanding of how leakage is related to the
characteristics of the porous material is still lacking. As a result, the
energy density and the antileakage capability are often antagonistically
coupled. In this work we overcome the current limitations, showing that a high
energy density can be reached together with superior anti-leakage performance
by using hierarchical macro-nanoporous metals for PCMs impregnation. By
analyzing capillary phenomena and synthesizing a new type of material, it was
demonstrated that a hierarchical trimodal macro-nanoporous metal (copper)
provides superior antileakage capability (due to strong capillary forces of
nanopores), high energy density (90vol% of PCM load due to macropores) and
improves the charging/discharging kinetics, due to a three-fold enhancement of
thermal conductivity. It was further demonstrated by CFD simulations that such
a composite can be used for thermal management of a battery pack and unlike
pure PCM it is capable of maintaining the maximum temperature below the safety
limit. The present results pave the way for the application of hierarchical
macro-nanoporous metals for high-energy density, leakage-free, and
shape-stabilized PCMs with enhanced thermal conductivity. These innovative
composites can significantly facilitate the thermal management of compact
systems such as electronic devices or high-power batteries by improving their
efficiency, durability and sustainabilit
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On emerging micro- and nanoscale thermofluidic technologies
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.This paper highlights examples of my current research in heat transfer and fluidics at the interface of energy applications and micro- and nanoscale technologies. It is not the scope of this paper to present an
exhaustive account of all current and past activities related to its title. It is rather an account of current research in
my laboratory in this area, containing both the underlying scientific challenges as well as the hoped final outcome in terms of applications. To this end, examples from the areas of energy conversion, as well as energy
transport will be discussed. In the area of energy conversion an original, deformable, direct methanol microfuel cell will be presented made of lightweight, flexible, polymer-based materials. A basic understanding and control of two-phase flows (in this case methanol and carbon dioxide) in microchannels as well as novel materials processing and microfabrication methods are directly related to the performance of such energy conversion devices. In the area of energy conservation and reuse, examples from the information technology are employed. Specifically, new concepts of liquid (water) cooling of chips reaching heat removal rates in excess of 700 W/cm2 in domains with restricted heights of the order of one mm will be presented. One additional advantage of using water to cool high density electronics is energy reuse, due to the potentially much higher exergy content of the coolant compared to air cooled technologies. The last part of the paper focuses on the employment of functional nanostructures such as carbon nanotubes and nanowires of conductive and semiconductive
materials for the efficient transport of electricity and heat and the need for the development of novel technologies for the manufacturing, characterization as well as handling of such nanostructures
Heat conduction tuning using the wave nature of phonons
The world communicates to our senses of vision, hearing and touch in the
language of waves, as the light, sound, and even heat essentially consist of
microscopic vibrations of different media. The wave nature of light and sound
has been extensively investigated over the past century and is now widely used
in modern technology. But the wave nature of heat has been the subject of
mostly theoretical studies, as its experimental demonstration, let alone
practical use, remains challenging due to the extremely short wavelengths of
these waves. Here we show a possibility to use the wave nature of heat for
thermal conductivity tuning via spatial short-range order in phononic crystal
nanostructures. Our experimental and theoretical results suggest that
interference of thermal phonons occurs in strictly periodic nanostructures and
slows the propagation of heat. This finding broadens the methodology of heat
transfer engineering by expanding its territory to the wave nature of heat
Effects of porosity and contaminant on evaporation from nanopores
Evaporation from nanopores, owing to its high mass/heat fluxes and high heat transfer coefficients, have found widespread applications in various industrial process, including electronics cooling, solar steam generation, membrane distillation and power generation. To further improve the performance of these nanopore-evaporation-associated processes, it is necessary to experimentally quantify the ultimate transport limit of evaporation from nanopores and understand its dependence on nanoscale confinement and operating conditions. This ultimate transport limit has now been widely accepted to be dictated by evaporation kinetics at the liquid-vapor interface, which is very difficult to quantify experimentally due to the ultra-small evaporation rates from single nanopores. To overcome this challenge, a new measurement approach based on a hybrid nanochannel-nanopore device design has been developed recently. This measurement approach can accurately measure evaporation rates/fluxes from single nanopore and has been used to investigate the effect of nanopore diameter on kinetic-limited evaporation flux. Although this study provides us new fundamental understanding about how nanoscale confinements change evaporation from nanopore, the effects of contaminant and pore porosity, which to some extent determines the practical performance of evaporation from nanopores, have remained elusive. Such lacking understanding has prevented us from developing optimized evaporative nanoporous structures for practical applications.
This works aims to investigate the effects of porosity and contaminant on kinetic-limited evaporation flux by experimentally measuring kinetic-limited evaporation rates from nanopore arrays. A modified hybrid nanochannel-nanopore device design is used to achieve this goal. In this modified device design, a nanopore array is directly connected to a 2-D nanochannel and the total evaporation rate from the nanopore array is measured by tracking meniscus receding in the nanochannel during a drying/evaporation process. Using this modified device design, we measured the kinetic-limited evaporation rates from 3x3 nanopore arrays with different interval distances ranging from 200 nm to 1 ÎĽm. To facilitate comparison between different devices, the total evaporation rates were converted to evaporation fluxes based on the nanopore projected area. Our results showed that that porosity or nanopore interval distance has negligible effect on the kinetic-limited evaporation flux. We also performed evaporation experiment using water with impurity and studied the effect of contaminant on kinetic-limit evaporation flux. It was observed that the contaminants in water can significantly reduce the kinetic-limited evaporation flux in nanopores and the contaminant effect becomes much more obvious in smaller nanopore due to contaminant-accumulation-induced pore blockage
Electrical power dissipation in carbon nanotubes on single crystal quartz and amorphous SiO2
Heat dissipation in electrically biased semiconducting carbon nanotubes
(CNTs) on single crystal quartz and amorphous SiO2 is examined with temperature
profiles obtained by spatially resolved Raman spectroscopy. Despite the
differences in phonon velocities, thermal conductivity and van der Waals
interactions with CNTs, on average, heat dissipation into single crystal quartz
and amorphous SiO2 is found to be similar. Large temperature gradients and
local hot spots often observed underscore the complexity of CNT temperature
profiles and may be accountable for the similarities observed
Direct electronic measurement of Peltier cooling and heating in graphene
Thermoelectric effects allow the generation of electrical power from waste
heat and the electrical control of cooling and heating. Remarkably, these
effects are also highly sensitive to the asymmetry in the density of states
around the Fermi energy and can therefore be exploited as probes of distortions
in the electronic structure at the nanoscale. Here we consider two-dimensional
graphene as an excellent nanoscale carbon material for exploring the
interaction between electronic and thermal transport phenomena, by presenting a
direct and quantitative measurement of the Peltier component to electronic
cooling and heating in graphene. Thanks to an architecture including nanoscale
thermometers, we detected Peltier component modulation of up to 15 mK for
currents of 20 A at room temperature and observed a full reversal between
Peltier cooling and heating for electron and hole regimes. This fundamental
thermodynamic property is a complementary tool for the study of nanoscale
thermoelectric transport in two-dimensional materials.Comment: Final version published in Nature Communications under a Creative
Commons Attribution 4.0 International Licens
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