Remarkable Thermal Performance Enhancement of Micro Heat Pipes with Graphene-Nanoplatelet Nano-Wicks.

The ultrafast water permeation property of graphene nanoplatelets (GNPs) synergically enhances the evaporation and water circulation processes in a micro heat pipe (MHP). An MHP is a promising phase-change heat-transfer device capable of transferring large amounts of heat energy efficiently. The hydrophobic, atomically smooth carbon walls of GNPs nanostructures provide a network of nanocapillaries that allows water molecules to intercalate frictionlessly among the graphene layers. Together with the attraction force of the oxygenated functional groups, a series of hydrophobic and hydrophilic surfaces are formed that significantly improve the water circulation rate. The intercalation of water molecules encourages the formation of water-thin film for film-wise evaporation. The effect of nano-wick thickness on the thermal performance of the MHP is investigated. A thinner GNP nano-wick is more favorable to film-wise evaporation while a thicker nano-wick promotes a higher water circulation rate from the condenser to the evaporator, leading to the existence of an optimal thickness. By benchmarking with the uncoated MHP, the thermal conductance of an MHP with a 46.9-µm GNP nano-wick manifests a maximum enhancement of 128%. This study provides insights on the feasible implementation of GNP nano-wicks into a highly efficient micro-scale electronics cooling device for environmental sustainability.

Nonporous, Strong, Stretchable, and Transparent Electrospun Aromatic Polyurea Nanocomposites as Potential Anticorrosion Coating Films.

This study, for the first time, focused on the fabrication of nonporous polyurea thin films (~200 microns) using the electrospinning method as a novel approach for coating applications. Multi-walled carbon nanotubes (MWCNTs) and hydrophilic-fumed nanosilica (HFNS) were added separately into electrospun polyurea films as nano-reinforcing fillers for the enhancement of properties. Neat polyurea films demonstrated a tensile strength of 14 MPa with an elongation of 360%. At a loading of 0.2% of MWCNTs, the highest tensile strength of 21 MPa and elongation of 402% were obtained, while the water contact angle remained almost unchanged (89°). Surface morphology analysis indicated that the production of polyurea fibers during electrospinning bonded together upon curing, leading to a nonporous film. Neat polyurea exhibited high thermal resistance with a degradation temperature of 380 °C. Upon reinforcement with 0.2% of MWCNTs and 0.4% of HFNS, it increased by ~7 °C. The storage modulus increased by 42 MPa with the addition of 0.2% of MWCNTs, implying a superior viscoelasticity of polyurea nanocomposite films. The results were benchmarked with anti-corrosive polymer coatings from the literature, revealing that the production of nonporous polyurea coatings with robust strength, elasticity, and thermal properties was achieved. Electrospun polyurea coatings are promising candidates as flexible anti-corrosive coatings for heat exchanges and electrical wires.

Influence of substrate on ultrafast water transport property of multilayer graphene coatings.

The ultrafast water transport in graphene nanoplatelets (GNPs) coating is attributed to the low friction passages formed by pristine graphene and the hydrophilic functional groups which provide a strong interaction force to the water molecules. Here, we examine the influence of the supporting substrate on the ultrafast water transport property of multilayer graphene coatings experimentally and by computational modelling. Thermally cured GNPs manifesting ultrafast water permeation are coated on different substrate materials, namely aluminium, copper, iron and glass. The physical and chemical structures of the GNPs coatings which are affected by the substrate materials are characterized using various spectroscopy techniques. Experimentally, the water permeation and absorption tests evidence the significant influence of the substrate on the rapid water permeation property of GNPs-coating. The water transport rates of the GNPs coatings correspond to the wettability and the free surface energy of their substrates where the most hydrophilic substrate induces the highest water transport rate. In addition, we conduct molecular dynamics (MD) simulations to investigate the transport rate of water molecules through multilayer GNPs adjacent to different substrate materials. The MD simulations results agree well with the experimental results inferring the strong influence of the substrate materials on the fast water transport of GNPs. Therefore, selection of substrate has to be taken into consideration when the GNPs-coating is placed into applications.

Effective micro-spray cooling for light-emitting diode with graphene nanoporous layers.

A graphene nanoplatelet (GNP) coating is utilized as a functionalized surface in enhancing the evaporation rate of micro-spray cooling for light-emitting diodes (LEDs). In micro-spray cooling, water is atomized into micro-sized droplets to reduce the surface energy and to increase the surface area for evaporation. The GNP coating facilitates the effective filmwise evaporation through the attribute of fast water permeation. The oxygenated functional groups of GNPs provide the driving force that initiates the intercalation of water molecules through the carbon nanostructure. The water molecules slip through the frictionless passages between the hydrophobic carbon walls, resulting an effective filmwise evaporation. The enhancement of evaporation leads to an enormous temperature reduction of 61.3 °C. The performance of the LED is greatly enhanced: a maximum increase in illuminance of 25% and an extension of power rating from 9 W to 12 W can be achieved. With the application of GNP coating, the high-temperature region is eliminated while maintaining the LED surface temperature for optimal operation. This study paves the way for employing the effective hybrid spray-evaporation-nanostructure technique in the development of a compact, low-power-consumption cooling system.

Acoustically enhanced heat transport.

We investigate the enhancement of heat transfer in the nucleate boiling regime by inducing high frequency acoustic waves (f ∼ 10(6) Hz) on the heated surface. In the experiments, liquid droplets (deionized water) are dispensed directly onto a heated, vibrating substrate. At lower vibration amplitudes (ξs ∼ 10(-9) m), the improved heat transfer is mainly due to the detachment of vapor bubbles from the heated surface and the induced thermal mixing. Upon increasing the vibration amplitude (ξs ∼ 10(-8) m), the heat transfer becomes more substantial due to the rapid bursting of vapor bubbles happening at the liquid-air interface as a consequence of capillary waves travelling in the thin liquid film between the vapor bubble and the air. Further increases then lead to rapid atomization that continues to enhance the heat transfer. An acoustic wave displacement amplitude on the order of 10(-8) m with 10(6) Hz order frequencies is observed to produce an improvement of up to 50% reduction in the surface temperature over the case without acoustic excitation.

Acoustically-controlled Leidenfrost droplets.

Suppressing the Leidenfrost effect can significantly improve heat transfer from a heated substrate to a droplet above it. In this work, we demonstrate that by generating high frequency acoustic wave in the droplet, at sufficient vibration displacement amplitudes, the Leidenfrost effect can be suppressed due to the acoustic radiation pressure exerted on the liquid-vapor interface; strong capillary waves are observed at the liquid-vapor interface and subsequently leads to contact between the liquid and the heated substrate. Using this technique, with 10(5)Hz vibration frequency and 10(-6)m displacement amplitude of the acoustic transducer, a maximum of 45% reduction of the initial temperature (T0∼200-300°C) of the heated substrate can be achieved with a single droplet of volume 10(-5)l.

Enhanced Evaporation Strength through Fast Water Permeation in Graphene-Oxide Deposition.

The unique characteristic of fast water permeation in laminated graphene oxide (GO) sheets has facilitated the development of ultrathin and ultrafast nanofiltration membranes. Here we report the application of fast water permeation property of immersed GO deposition for enhancing the performance of a GO/water nanofluid charged two-phase closed thermosyphon (TPCT). By benchmarking its performance against a silver oxide/water nanofluid charged TPCT, the enhancement of evaporation strength is found to be essentially attributed to the fast water permeation property of GO deposition instead of the enhanced surface wettability of the deposited layer. The expansion of interlayer distance between the graphitic planes of GO deposited layer enables intercalation of bilayer water for fast water permeation. The capillary force attributed to the frictionless interaction between the atomically smooth, hydrophobic carbon structures and the well-ordered hydrogen bonds of water molecules is sufficiently strong to overcome the gravitational force. As a result, a thin water film is formed on the GO deposited layers, inducing filmwise evaporation which is more effective than its interfacial counterpart, appreciably enhanced the overall performance of TPCT. This study paves the way for a promising start of employing the fast water permeation property of GO in thermal applications.