Au Nanofluids Research Articles

Seed-Assisted Hydrothermal Synthesis of Monodispersed Au@C Core–Shell Nanostructures for Enhancing Thermal Diffusivity of Water-Based Nanofluids

Au@C core–shell nanostructures (Au@C-NS) were synthesized through a low-temperature seed-assisted hydrothermal approach using glucose as carbon source. The material characterization and chemical analysis confirm that the synthesis method allows to obtain uniform core–shell nanostructures constituted by a crystalline metal core and an amorphous carbon shell. Depending on the synthesis conditions, their average size ranges from 146 nm to 342 nm with relative standard deviation as low as 7 %. It is proposed that the characteristic monodispersity results due to a high nucleation rate of the carbon phase at the liquid–solid interface. The obtained monodisperse Au@C-NS were used to prepare water-based nanofluids with superior heat transport properties. The thermal lens analysis shows that the thermal diffusivity of Au@C nanofluids is 9.5 % and 31.3 % higher than their Au nanofluids counterparts and pure water, respectively, at particle concentration of 285 × 1011 ml−1. Phonon-related interactions at the metal cores and carbon shells interfaces are proposed as the heat transport mechanism behind the thermal diffusivity enhancement of the Au@C water-based nanofluids.

Effect of morphological evolution and aggregation of plasmonic core-shell nanostructures on solar thermal conversion.

Achieving high solar energy absorption based on nanofluids (NFs) needs further study in solar photothermal conversion technology. In this work, we performed COMSOL simulations to investigate the solar energy absorption using a core-shell nanostructure composed of the Au core and shell with different materials. The influence of the radius of the Au core, the materials of the shell, and the shell thickness on the solar absorption efficiency factor (SAEF) are systematically studied. The results show that the SAEF of the Au@Li nanoparticle with ratio of 0.5 has the highest SAEF of 1.4779, increasing 1.99 times compared to that of the bare Au nanoparticle of 0.74326 with the same radius. Moreover, the optical properties, electric field distribution, and SAEF of the Au@Li dimer are further evaluated to demonstrate the aggregation effects on SAEF. We find that the SAEF of the Au@Li dimer reaches the maximum of 4.34 with a distance around 1nm, where the LSPR coupling effect in the nanogap is sharply enhanced 700 times irradiated by light with wavelength of 760nm. Finally, the direct absorber solar collector performance demonstrates that Au@Li dimer NFs can collect 93% of solar energy compared to 54% for Au@Li NFs and 51% for Au NFs. This work provides the possibility to achieve more efficient solar thermal conversion, and may have potential applications in efficient solar energy harvesting and utilization.

Investigations of stable surface-modified gold nanofluids optical filters based on optical optimization for photovoltaic/thermal systems

Nanofluid filters have become increasingly popular for solar photovoltaic/thermal (PV/T) systems due to their tunable optical and thermal properties, especially for some plasmonic nanofluids. However, towards optimizing the characteristics of the plasmonic nanofluids filters, by identifying the best nanofluid parameters and improving their long-term dispersion stability, more researches need to be conducted. In this work, the effects of nanoparticles concentration, optical thickness and nanoparticle size on performance of Au nanofluids filtered PV/T systems are investigated theoretically. The PV/T systems can obtain optimal merit function (MF) of 1.367 and 1.357 for Au/water nanofluid and Au/ethylene glycol (EG) nanofluid, respectively. Then, surface-modified Au nanoparticles capped with polyethylene glycol (PEG) or polyvinylpyrrolidone (PVP) polymers are prepared and dispersed in water and EG, respectively. The stability of the stabilizer coated Au nanofluids upon long-term ambient storage, continuous heating, cyclic flowing and solar radiation is discussed. Au + PVP/EG nanofluids exhibit more excellent thermal stability than that of Au + PVP/water. To study the effectiveness of the optimal Au + PVP/EG nanofluids filtered PV/T system, indoor flowing experiments are conducted. The optimal MF of 1.95 and exergy efficiency of 13.98% is achieved filtered by Au + PVP/EG nanofluid with concentration of 29.2 ppm when mass flow rate is 2 mL/min.

Molecular dynamics simulations of the microscopic mechanism of thermal conductivity enhancement of ethylene Glycol–Water–Au nanofluids

With an increase of the heat load of internal combustion engines, the heat dissipation of the equipment increases accordingly. Traditional ethylene glycol (EG)–water coolants are unable to meet the heat dissipation demands of high heat load internal combustion engines. Nanofluids have better heat transfer performances than conventional fluids, suggesting they can be used to solve the problem of the insufficient heat dissipation capacity of internal combustion engines. In this paper, EG–water–Au nanofluids are formed by adding Au nanoparticles with a volume fraction of 1.5% to EG–water solutions. These are used to investigate the effect of the EG mass concentration on the thermal conductivity of the EG–water solutions and their nanofluids, and to analyze the microscopic mechanism of thermal conductivity enhancement of the nanofluids. It is found that the thermal conductivity of the EG–water solutions are positively correlated with the water content of the system, and the higher the water content of the system, the better the diffusion performance of the system components. The thermal conductivity of the EG–water solutions increase for all EG mass concentrations after the addition of Au nanoparticles, up to 17.20%. The presence of an adsorption layer of base liquid molecules on the surface of the nanoparticles is an important reason for the improved thermal conductivity of the nanofluids. According to the analysis of molecular radius of gyration, end-to-end distance and intermolecular interaction energy in the systems, it is found that the addition of the nanoparticles has no significant effect on the conformation of EG molecular chain, and the van der Waals interaction energy in the nanofluid systems is slightly enhanced. Through an analysis of the radial distribution function and density distribution of the nanofluid systems, it is found that the base liquid molecules that adsorb on the surface of the nanoparticles are stratified. The high-density interval of EG molecules is (0.65, 1.25) nm, while water molecules mostly gather in the form of clusters in the (1.15, 1.65) nm interval, and the arrangement of base liquid molecules in the adsorption layer is similar to the orderly arrangement of solids. The properties of some components in the nanofluids show a solid-like microstructure, which makes the thermal conductivity of the nanofluids higher than that of the base solution. These research results can provide an important reference for improving the heat transfer performance of traditional alcohol–water coolants.

Optical properties optimization of plasmonic nanofluid to enhance the performance of spectral splitting photovoltaic/thermal systems

Over the past few years, a growing interest has surfaced about the PV/T systems based on nanofluid filter. Some palsmonic nanofluids including Ag, Au and Cu nanoparticles are proposed as the optical filter of PV/T system. To identify the best nanofluid parameters for Si and GaAs solar cell, an optical model of the nanofluids based on Mie scattering theory and Lambert Beer law are developed. Results show that the absorption peak of Ag nanoparticle displays a red-shift trend with the increase of particle diameter and refractive index of base fluid. Adjusting the optical path-length is one efficient way to regulate the optical properties. A maximum merit function (MF) of 1.375 can be achieved for Si cells when Ag/propylene glycol (PG)+CoSO4 nanofluid is used as the optical filter with nanoparticle diameter of 20 nm, concentration of 20 mg/L and optical path-length of 22 mm. The investigation of the best Ag nanofluid filter for GaAs cells reveals that PG base fluid with Ag nanoparticle diameter of 20 nm, concentration of 1 mg/L and optical path-length of 60 mm can achieve a MF value of 1.342. The optimized MF values for Cu and Au nanofluids are slight higher than that for Ag nanofluid.

Molecular Dynamics Simulations of the Microscopic Mechanism of Thermal Conductivity Enhancement of Ethylene Glycol–Water–Au Nanofluids

Molecular Dynamics Simulations of the Microscopic Mechanism of Thermal Conductivity Enhancement of Ethylene Glycol–Water–Au Nanofluids

Transparent nanofluids with high thermal conductivity for improved convective thermal management of optoelectronic devices

ABSTRACT We report the development of transparent high-thermal-conductivity nanofluids for enhanced flow cooling of solid-state laser crystal rods. The transparent nanofluids were prepared by dispersing citrate-stabilized Au nanoparticles in water and strictly controlling the particle size and loading. The Au nanofluids have shown stable dispersion, high optical transparency at the excitation wavelength of the YAG solid-state laser (808 nm), and 50% increase of thermal conductivity. Experimental measurement and theoretical simulation of circulating flow cooling show that the heat dissipation capacity of the lasering rod could be increased by more than 20% by using the Au nanofluids to replace pure water coolant.Abbreviations: Au: Gold; NP: Nanoparticle; Au NP: Gold nanoparticle; SEM: Scanning electron microscopy

Shape-dependent solar thermal conversion properties of plasmonic Au nanoparticles under different light filter conditions

In order to provide a relationship between nanoparticle (NP) parameters and irradiated lights when designing solar heating devices, shape-dependent solar thermal conversion properties of Au nanofluids at different irradiated lights were investigated experimentally and theoretically. Firstly, quasi-sphere and thorny Au NPs were successfully synthesized by a seed-mediated method, and the peak absorbance wavelength of which was located in ranges of 879–553 nm and 899–594 nm, respectively. Experimental results showed that the solar thermal conversion efficiency of Au nanofluids decreased with increasing NP size, which decreased from 86.0% to 58.6% when the NP size increased from 8.5 nm to 138.9 nm. Theoretical results indicated that for these large NPs, the scattering effect plays a major role in the solar thermal conversion process, and using the extinction coefficient to calculate the conversion efficiency may cause a substantial error. Furtherly it can be found that the solar thermal conversion efficiency of thorny Au nanofluids can achieve even 10% more than that of the quasi-sphere Au nanofluids. In addition, the temperature difference between the nanofluids and base fluid water under different incident light using a filter was found to be smaller than that of full solar spectra irradiation. The maximum efficiency of Au nanofluids was ∼61.7%, the enhancement of which reached ∼37.1% compared with water at an irradiation light with the wavelength of 500 nm.

Optofluidic and strain measurements induced by polarization-resolved nanosecond pulses in gold-based nanofluids

The influence of an optical fringe patterns on the viscoelastic properties exhibited by Au nanofluids samples was evaluated. A sensitive interferometric technique for analyzing optofluidic effects in different samples was implemented. The assistance of the plasmonic characteristics in Au nanostructures allows measuring particular mechano-optical effects at 532 nm wavelength by a Fabry-Perot interferometer to explore multiaxial strains. A representative volume of the sample was studied to determine the stability and maximum viscoelastic properties exhibited by the nanostructures. A vectorial two-wave configuration allows controlling the maximum strain induced in the sample. The oscillating nature of the colloid was examined by using interferometric optical signals reflected from a remnant drop pending at the end of an optical fiber. Nanosecond pulses were used to induce inelastic optofluidic effects. The mechanical parameters were approximated by a nonlinear second order system activated by a Dirac delta functions.

Investigation into Au nanofluids for solar photothermal conversion

We herein investigate the solar photothermal conversion performances of Au nanofluids both experimentally and theoretically. In the experiments, the Au nanofluids are loaded into a beaker and are exposed to solar radiation. The solar photothermal conversion efficiency performances of the Au nanofluids using different nanoparticle (NP) volume fractions and solar intensities are discussed. Experimental results show that the equilibrium temperature increases as the NP volume fraction and solar intensity increase, although the extent of variation in the photothermal conversion efficiency declines under these conditions. Furthermore, a theoretical analysis based on the Rayleigh scattering approximation and the Beer-Lambert Law to predict the solar absorption efficiency is discussed. A good agreement between the calculated absorption efficiency and the experimental solar photothermal conversion efficiency is reported. The absorption efficiency increases exponentially with increasing collector height and NP volume fraction. Moreover, the optimization of NP selection and collector design for direct absorption solar collectors without consideration of the temperature field is of particular scientific importance.

Preparation of Au nanoparticles in a non-polar medium: obtaining high-efficiency nanofluids for concentrating solar power. An experimental and theoretical perspective

Au nanofluids with enhanced thermal properties for use in CSP.

물-기반 금나노유체의 분산안정성이 열전도도에 미치는 영향

This paper experimentally reports the effect of suspension stability on the thermal conductivity of water-based Au nanofluids. For this purpose, the water-based Au nanofluids are prepared by the one-step method called electro-chemical method with volume fraction of 0.0005%. The thermal conductivity of water-based Au nanofluids is measured from <TEX>$22^{\circ}C$</TEX> to <TEX>$42^{\circ}C$</TEX> using the transient hot wire method. To quantify the suspension stability of Au nanofluids, the suspension stability of nanofluids is evaluated using the in-house developed laser scattering system at a fixed wavelength of 632.8nm with the elapsed time. Based on the experimental results, the both thermal conductivity and suspension stability of water-based Au nanofluids are gradually decreased according to the time. These results experimentally show that the suspension stability of water-based Au nanofluids is the one of the important factor of thermal conductivity.

Enhancement of photo-thermal conversion using gold nanofluids with different particle sizes

This work describes an experimental study of the particle size dependence of gold nanofluids during photo-thermal conversion in a direct absorption solar collector (DASC). Au nanoparticles (NPs) with different sizes were synthesized using a seed mediated method. Au NPs play a significant role in enhancing the solar light absorption with respect to a pure base fluid at a very low concentration due to the localized surface plasmon resonance effect. Experimental results of the photo-thermal conversion showed that the photo-thermal conversion efficiency of Au nanofluids obtained an average enhancement of 19.9% and 21.3% for a cube shaped DASC and a flat shaped DASC, respectively, compared with H2O at a relatively low mass fraction (∼0.000008% weight). Reducing the Au NP size led to the enhancement of the photo-thermal conversion efficiency under the present experimental conditions, which could be an effective way for the modification of optical properties and thermodynamic characteristics. However, the size of Au NPs did not significantly influence the efficiency for the cube shaped DASC. The cube shaped DASC model usually had the higher efficiency than the flat DASC model using the same working fluids since the heat loss percentage of the cube shaped DASC was lower.

Conduction and convection heat transfer characteristics of water–Au nanofluid in a cubic enclosure with differentially heated side walls

Abstract The present work deals with the steady-state natural convection in a cubic enclosure filled with the water–Au nanofluid. The enclosure is heated on the vertical and cooled from the adjacent wall, while the other walls are adiabatic. The governing differential equations have been solved by the standard finite volume method and the hydrodynamic and thermal fields were coupled together using the Boussinesq approximation. The effects of the volume fraction of nanoparticles in the range 0 % ⩽ φ ⩽ 5 % on the heat transfer characteristics of Au nanofluids are investigated for the nominal values of base-fluid Rayleigh number 10 1 ⩽ Ra bf ⩽ 10 6 . It is shown that adding nanoparticles in a base-fluid delays the onset of convection. Contrary to what is argued by many authors, we show by numerical simulations that the mean Nusselt number Nu ‾ values for nanofluids φ > 0 % are smaller than those obtained in the case of pure fluid with the same nominal value of Rayleigh number Ra bf due to the weakening of convective transport.

Conduction and convection heat transfer characteristics of water-based au nanofluids in a square cavity with differentially heated side walls subjected to constant temperatures

The present work deals with the natural convection in a square cavity filled with the water-based Au nanofluid. The cavity is heated on the vertical and cooled from the adjacent wall, while the other two horizontal walls are adiabatic. The governing differential equations have been solved by the standard finite volume method and the hydrodynamic and thermal fields were coupled together using the Boussinesq approximation. The main objective of this study is to investigate the influence of the nanoparticles? volume fraction on the heat transfer characteristics of Au nanofluids at the given base fluid?s (i.e. water) Rayleigh number. Accurate results are presented over a wide range of the base fluid Rayleigh number and the volume fraction of Au nanoparticles. It is shown that adding nanoparticles in a base fluid delays the onset of convection. Contrary to what is argued by many authors, we show by numerical simulations that the use of nanofluids can reduce the heat transfer rate instead of increasing it.

Thermal diffusivity measurement of Au nanofluids of very low concentration by using photoflash technique

In this paper, the application of photoflash technique to measuring the thermal diffusivity of gold nanofluids of very low concentration at room temperature was presented. The nanofluid samples were prepared from the pulse laser ablation procedure. The thermal diffusivity was obtained by fitting the theoretical temperature signal to the experimental data, and it was found to increase linearly from 1.47×10−3cm2s−1 to 1.68×10−3cm2s−1 as the concentration increased from 1.11mg/L to 3.18mg/L. The increase in thermal diffusivity in these multidispersed nanofluids was attributed to the higher nanoparticle concentration as well as to the increasing presence of the smaller size nanoparticles.

Preparation and Thermal Stability of Oil-based Au Nanofluids

Two cationic surfactants,N-cetyl-N-(hydroxyethyl)-N,N-dimethyl ammonium bromide(CHDAB) and 1,4-bis(cetyldimethylammonium) propane dibromide(G16-4-16),were used as surface-modified reagents for preparation of hydrophobic gold nanoparticles from reduction of HAuCl4 with KBH4 in mixture of petroleum ether/n-butanol/H2O.The results show that gemini surfactant G16-4-16 is better for coating and dispersing,and the gold nanoparticles coated by the gemini surfactant G16-4-16 have a narrow size distribution with an average diameter of 5.2 nm.Stable nanofluids containing gold nanoparticles with hydrocarbon(toluene) as base liquids were prepared.The thermal stability of the nanofluids as a function of time was monitored using UV-Vis absorption method.It is shown that the gold nanofluid has good thermal stability,which has a stable time of 20 h at 130 ℃.The thermal conductivity of the gold nanofluids was performed by point heat source method.The enhancements of thermal conductivity of the gold nanofluids containing 1.5%(mass fraction) gold nanoparticles can reach 17%.

Tunable optical properties of Au nanofluids under electric field

Using a repeating deoxidization method, we prepared Au nanoparticles with different mean sizes of 11 nm, 35 nm and 58 nm. Then the Au nanoparticles were dispersed in mineral oil to form Au nanofluids by the surfactant-assisted phase transition technique. Under electric field, the optical properties of the Au nanofluids were measured. It was found that the Au nanofluids exhibit significant electric field-induced birefringence and the birefringence index can be adjusted by changing the external electric field strength. Furthermore, the birefringence index is influenced by the Au concentration and particle size. Within the suitable particle concentration range, the index decreases with the increase of Au concentration and particle size. Finally, we further discuss the electric field-induced birefringence phenomena of the Au nanofluids according to the structure transformation mechanism of electrorheological fluids under electric field.

Study of Ag and Au Nanoparticles Synthesized by ArcDischarge in Deionized Water

The paper presents a study of Ag and Au nanofluids synthesized by the arc discharge method (ADM) in deionized water. The metallic Ag nanoparticle (Ag0) and ionic Ag (Ag+) have played an important role in the battle against germs which are becoming more drug‐resistant every year. Our study indicates that Ag nanoparticle suspension (SNPS) fabricated by using ADM without added surfactants exclusively contains the metallic Ag nanoparticle and ionic Ag. Besides that, the ADM in deionized water has also been employed for the fabrication process of Au nanoparticles. The experimental results indicate that the prepared Ag nanoparticles can react with the dissolved H2CO3 in deionized water, leading to the formation of Ag2CO3. Significantly different to Ag, the prepared Au nanoparticles with their surfaces bonded by oxygen are suspended in deionized water by the formation of hydrogen bonded with the neighboring water molecules.