Thermal conductivities of nanofluids are expected to be higher than common heat transfer fluids. The use of metal nanoparticles has not been intensely investigated for heat transfer applications due to lack of stability. Here we present an experimental study on the effect of silver nanoparticles (Ag NPs) which are stabilized with surfactants, on the thermal conductivity of water, ethylene glycol and hexane. Hydrophilic Ag NPs were synthesized in aqueous medium with using gum arabic as surfactant and oleic acid/oleylamine were used to stabilize Ag NPs in the organic phase. The enhancement up to 10 per cent in effective thermal conductivity of hexane and ethylene glycol was achieved with addition of Ag NPs at considerably low concentrations (i.e. 2 and 1 per cent, by weight, for hexane and ethylene glycol respectively). However, almost 10 per cent of deterioration was recorded at effective thermal conductivity of water when Ag NPs were added at 1 per cent (by wt). Considerable amount of Gum Arabic in the medium is shown to be the major contributor to this fall, causing lowering of thermal conductivity of water. Same particles performed much better in ethylene glycol where the stabilizer does not lower the thermal conductivity of the base fluid. Also thermal conductivity of nanofluids was found to be temperature independent except water based Ag nanofluids above a threshold concentration. This temperature dependency is suggested to be due to inhibition of hydrogen bonding among water molecules in the presence of high amounts of gum arabic.

Convective heat transfer can be enhanced by altering flow type, boundary conditions and corresponding thermal conductivity of the heat transfer fluid used. Common heat transfer fluids have low thermal conductivities at room temperature1 leading to inefficiency in heat transfer applications. Relative thermal conductivities of fluids containing solid particles have been investigated both experimentally and theoretically since the theory of Maxwell.2 However, the particles used in these studies cover a size range in micrometers which cause clogging and consequent settling of the particles as a result of insufficient stability.3 Modern technology provides scientists to work with nano-sized particles that are expected to have superior properties over micro-sized particles and traditional base fluids. The most important properties that differentiate nanoparticles from micro-sized particles are their large surface area to volume ratio where the heat transfer will occur3 and smaller size which will prevent clogging.

Previously, the use of nanofluids for heat transfer applications was demonstrated by suspending metallic NPs in heat transfer fluids by Choi et al.1 They did an experimental work by suspending carbon nanotubes in synthetic oil at 1 per cent (v/v) ratio based on the theoretical studies of Batchelor & O’Brien4 and Hamilton & Crosser5 and 150 per cent enhancement over the corresponding thermal conductivity of the base fluid was reported. Several studies cover the factors affecting heat transfer in nanofluids such as; NP type and size,6 base fluid, the role of surfactant7 and ambient conditions like pH and temperature.8 

Metal oxides such as aluminum oxide (Al2O3),3 copper oxide (CuO)9 and titanium dioxide (TiO2)9,10 constitute the major research area on effective thermal conductivity. Bulk metals, on the other hand, have relatively higher thermal conductivity than their oxides and copper (Cu),1 iron (Fe), silver (Ag),1 and gold (Au)11 have been the most useful metallic NPs.

In this work, thermal conductivities of Ag NPs in several solvents were investigated using transient hot wire technique (THW). Nanofluids were prepared via two step method; which includes the synthesis of nanoparticles followed by the addition of these particles in heat transfer solvents. Nanoparticles yielded an increase in thermal conductivities of oil based solvent (hexane and ethylene glycol), whereas deterioration in effective thermal conductivity was recorded when particles were suspended in water.

Water soluble Ag NPs (Ag/w) are synthesized using AgNO3 (Fluka, 99.5%) as metal source, NaBH4 (Merck, 98%) and gum arabic (C6H1005, Merck, Pure) as reducing and dispersing agents, respectively.12,13 In a typical synthesis, aqueous gum arabic solution (3%, m/v) is prepared in 50 ml distilled water and is stirred at 60 °C. After 30 minutes, 3.5 ml of 0.1 M aqueous solution of AgNO3 was added to the flask under continuous stirring. The solution was stirred for an additional 15 minutes. 3.5 ml of 0.1 M NaBH4 solution was then injected into the flask and the reaction temperature was elevated to 85 °C. The solution was left for 3 hours under continuous agitation at 85 °C. It should be noted that all the solutions were freshly prepared prior to the experiment.

An inexpensive and reproducible method for the synthesis of Ag NPs in organic phase (Ag/o) was previously proposed by Osterloh et al.14 The method comprises the reduction of silver salt via addition of organic surfactants with hydrocarbon chains such as oleylamine and oleic acid. The method is also known to produce organoamine protected nanoparticles with low polydispersity (6.9 %). In a typical synthesis, 0.9 mmol of silver acetate (ChemPur GmbH, 99 %) were dispersed in the refluxing solvent of 150 ml toluene (AnalaR, NORMAPUR®, min. 99.5%, bp: 110 °C). 7.5 mL of oleylamine (Fluka, technical, ≥ 70 %, GC) and oleic acid (Aldrich, 65.0-88.0%, GC) solution with 1:1 volumetric ratio was added to the solution. After 24 h, the solution was cooled down and left at room temperature.

Colloidal solutions of the synthesized hydrophilic (Ag/w) and hydrophobic (Ag/o) Ag NPs were centrifuged 3 times with methanol (Sigma Aldrich, CHROMASOLV®, ≥ 99.9%) at 6000 rpm for 10 minutes. Resulted precipitates were collected and left to dry in an oven at 32 °C overnight.

TEM images for Ag/w NPs and Ag/o NPs were collected with JEOL JEM-ARM200CFEG UHR-Transmission Electron Microscope with an accelerating voltage of 200 kV. Nanoparticle suspensions were dropped onto carbon coated 200 mesh copper grids and dried at ambient conditions. Images obtained from microscope were processed by ImageJ.15,16

To prepare nanofluids, the obtained nanoparticle powders were then dissolved in hexane, distilled water and ethylene glycol. Nanoparticle concentrations in solvents were altered from 0.037 to 0.99; 0.25 to 1 and 1 to 2 per cent (by weight) for water, ethylene glycol (BDH, VWR Analytical, < 99%) and hexane (AnalaR, NORMAPUR®) respectively. Mass fractions of the solutions were converted to volume fractions with taking the densities constant as 10.5 g/ml for Ag NPs,17 0.655 g/ml for hexane, 1.115 g/ml for ethylene glycol and 0.998 g/ml for distilled water which were measured at 25 °C by densometer (Anton Paar DMA 4100 M). The nanoparticle concentration and solvent combinations used in this study are listed in Table I.

TABLE I.

Prepared nanofluids for thermal conductivity experiments.

   NP Concentration (per cent) 
Nanofluid sample Solvent type NP type Weight (by wt) Volume (by vol) 
Ag/o-Hex-1 Hexane Ag/o 0.063 
Ag/o-Hex-2 0.127 
Ag/w-DW-0.037 Distilled Water Ag/w 0.037 0.0035 
Ag/w-DW-0.11 0.11 0.011 
Ag/w-DW-0.33 0.33 0.033 
Ag/w-DW-0.66 0.66 0.066 
Ag/w-DW-0.99 0.99 0.099 
Ag/w-EG-0.25 Ethylene glycol Ag/w 0.25 0.027 
Ag/w-EG-0.50 0.50 0.054 
Ag/w-EG-0.75 0.75 0.079 
Ag/w-EG-1 0.107 
   NP Concentration (per cent) 
Nanofluid sample Solvent type NP type Weight (by wt) Volume (by vol) 
Ag/o-Hex-1 Hexane Ag/o 0.063 
Ag/o-Hex-2 0.127 
Ag/w-DW-0.037 Distilled Water Ag/w 0.037 0.0035 
Ag/w-DW-0.11 0.11 0.011 
Ag/w-DW-0.33 0.33 0.033 
Ag/w-DW-0.66 0.66 0.066 
Ag/w-DW-0.99 0.99 0.099 
Ag/w-EG-0.25 Ethylene glycol Ag/w 0.25 0.027 
Ag/w-EG-0.50 0.50 0.054 
Ag/w-EG-0.75 0.75 0.079 
Ag/w-EG-1 0.107 

Transient hot wire technique (THW) was used to measure the thermal conductivity of samples. Flucon GmBH Lambda thermal conductivity meter equipped with PSL Systemtechnik LabTemp 30190 temperature controller, which is able to measure thermal conductivity over a temperature range of -30 °C to 190 °C, was used. The temperature was kept constant circulating tap water as cooling medium. Thermal conductivity values were recorded in mW/m.K for each 10 °C intervals with an accuracy of 0.1 °C. Only small amounts of sample (ca. 50 ml) are sufficient to execute reliable measurements. The agglomerations of nanoparticles were observed for the nanofluids with higher concentrations, thus the nanofluids with higher Ag concentrations than presented could not be studied. Each measurement was repeated for at least 3 times.

After the synthesis of hydrophilic Ag NPs (Ag/w), size analysis by counting 330 particles from TEM micrographs showed that the green route synthesis gave roughly spherical (first eccentricity is 0.46) particles in the range of 3 nm to 17 nm having a size distribution of 7.3 ± 2.5 nm. The TEM images and the corresponding size histogram are given in Fig. 1. Based on DLS analysis, Ag NPs displayed good colloidal properties in water. Light scattering profile of Ag NPs in water has a single peak denoting that they dispersed well in water (See supplementary materialfor Fig. SI.1.a).

FIG. 1.

TEM images and size distribution of Ag/w NPs synthesized in water (Green route synthesis).

FIG. 1.

TEM images and size distribution of Ag/w NPs synthesized in water (Green route synthesis).

Close modal

The hydrophobic particles obtained were also roughly spherical (first eccentricity is 0.43) with sizes in the range of 4 and 17 nm having a size distribution of 8.4 ± 2.0 nm as shown in Fig. 2. Again, the light scattering profile of Ag NPs in oil phase represents a single peak which suggests a homogeneous colloidal dispersion (See supplementary material for Fig. SI.1.b). However, as these light scattering analyses were performed by using rather diluted samples, i.e. at low concentrations, it is not possible to disregard a probable aggregation of nanoparticles in concentrated samples.

FIG. 2.

TEM images and size distribution of Ag/o NPs synthesized in toluene (Osterloh method).

FIG. 2.

TEM images and size distribution of Ag/o NPs synthesized in toluene (Osterloh method).

Close modal

1. Enhanced effective thermal conductivity for Ag/o-Hex and Ag/w-EG nanofluids

Fig. 3.a shows the increase in thermal conductivity of hexane with addition of Ag10/o NPs at 1 and 2 per cent (by wt) and Fig. 3.b shows the increase in thermal conductivity of ethylene glycol with addition of Ag/w NPs at 0.25 to 1 per cent (by wt). Previously, the increase in effective thermal conductivity of ethylene glycol was recorded by 10, 16 and 18 per cent for nanofluids containing 1000, 5000 and 10000 ppm of Ag NPs coated with poly (acryl-amide-co-acrylic acid)18 and also the lack of stability in Ag NPs has found to have an effect on thermal conductivity.19 However the temperature dependency of Ag-ethylene glycol nanofluids has not been investigated.

FIG. 3.

(a) Thermal conductivity of Hex/Ag10 nanofluids at different concentrations as a function of temperature. (b) Thermal conductivity of EG/Ag10 nanofluids at different concentrations as a function of temperature.

FIG. 3.

(a) Thermal conductivity of Hex/Ag10 nanofluids at different concentrations as a function of temperature. (b) Thermal conductivity of EG/Ag10 nanofluids at different concentrations as a function of temperature.

Close modal

Relative thermal conductivity is defined as the ratio of thermal conductivity of nanofluid to pure solvent (keff/kBF). In Fig. 4.a and 4.b, it is clearly seen that the addition of Ag NPs of comparable size at similar volume concentrations to EG and hexane yields approximately similar results. It should be noted that although the particle sizes are very similar, surface coatings are different to facilitate nanoparticle suspension in different media of varying polarity. The coating material alone (oleic acid, oleylamine and gum Arabic) is shown not to have an effect on the thermal conductivity of the base fluid (Fig. SI.2 and Fig. SI.3 of the supplementary material), therefore all the enhancement in thermal conductivity can be attributed to the presence of the nanoparticles. The enhancement in relative thermal conductivity with addition of Ag NPs to hexane and ethylene glycol is mainly due to relatively high thermal conductivity of Ag NPs with respect to base fluids (kHex1 and kEG1 is recorded as 0.117 and 0.253 W/m.K, whereas it is 429 W/m.K for kAg1). The interactions between carrier fluid molecules and solid particles, thermal diffusion and Brownian motion of NPs are responsible for enhanced heat transfer. On the other hand, deceleration in rate of increase in thermal conductivity is seen for higher concentrations. It is attributed to agglomerations of particles. As the nanoparticles aggregate, some of them will settle; consequently the particle concentration adjacent to hot wire (where the measurements are taken) decreases. Settling of particles can be readily observed for higher nanoparticle concentrations that support this argument.

FIG. 4.

Relative thermal conductivities of Ag/o NPs in hexane and Ag/w NPs in ethylene glycol at a. 20 °C; b. 40 °C.

FIG. 4.

Relative thermal conductivities of Ag/o NPs in hexane and Ag/w NPs in ethylene glycol at a. 20 °C; b. 40 °C.

Close modal

2. Deterioration in relative thermal conductivity of Ag/w-DW nanofluids

Although, there are several studies in the literature that cover thermal conductivity of aqueous nanofluids with metal oxides like CuO, Al2O3 and iron oxide (magnetite), it is rare to find an article on the effect of metals, either itself or nanoparticle of bulk metals, on the thermal conductivity of water. Previous works involving enhanced thermal conductivity of water usually comprises addition of CuO,20–22 Al2O36,22–24 and iron oxide25,26 to water and all agreed on enhancement in thermal conductivity upon addition of NPs.

In recent researches those investigate thermal conductivity of Au and Ag dispersed aqueous nanofluids; nanoparticles were capped with several surfactants; i.e., citrate,7,11 poly (acryl-amide-co-acrylic acid),18 poly (vinylpyrrolidone) (PVP).27 Most of them found a considerable increase in thermal conductivity of nanofluids,7,18 except no considerable affect was observed for TC of water when citrate coated AuNPs at a concentration of 0.25 x 10-3 per cent (by vol) were added at 40 °C11.

Deterioration in thermal conductivity was only found and published in the work of Altan et al., in which effective thermal conductivity was measured for aqueous magnetic fluids.28 In this work, effect of gum arabic coated Ag NPs on the thermal conductivity of water is investigated and data in FIG. 5 is obtained.

FIG. 5.

Thermal conductivity of Ag10/w NPs dispersed in water at different concentrations as a function of temperature.

FIG. 5.

Thermal conductivity of Ag10/w NPs dispersed in water at different concentrations as a function of temperature.

Close modal

Fig. 5 demonstrates the change in thermal conductivity of water with temperature for nanofluids having different Ag/w concentrations.

There are mainly two phenomena that are rather unusual in this plot. Upon addition of Gum Arabic coated 10 nm AgNPs, the thermal conductivity of the nanofluid is found to be lower than that of the base fluid. Considering that the thermal conductivity of silver (kAg= 429 W/m.K) is almost 700 times of water (kwater = 0.613 W/m.K),1 this result is rather unexpected. A fall in effective thermal conductivity of magnetic nanofluids was first recorded by Altan et al.28 They tried to relate this decline with interfacial thermal resistance (R). In classical EMT models, usually the interfacial thermal resistance is neglected by analyzing particles and fluids separately. The insufficiency of these models is derived from under-predicting presence of interfacial thermal resistance in between the nanoparticles and the surrounding fluid molecules. The interfacial thermal resistance is referred to the equivalent thickness (h). Although there are not many studies that cover the interfacial conductance of silver water system, it has been widely estimated as 200 MW/m2.K for all nanoparticle-water systems.29 Therefore, h is calculated to be 3.07 nm for Ag NPs with a diameter of 7.3 ± 2.5 nm. The model developed by Putnam et al.30 suggests effective thermal conductivity by imposing the interfacial thermal resistance. However, keff calculated from Putnam’s EMT model does not fit the experimental data recorded. At all temperatures, the keff value is calculated to be higher than that of water even though the interfacial resistance is taken into consideration.

It should be noted that when the same exact particles (Ag/w) of the same concentration (1 wt%) were suspended in ethylene glycol, approximately 10% enhancement of thermal conductivity was achieved, whereas 10% deterioration was observed when they were suspended in water. In both cases, in order to stabilize the Ag NPs, Gum Arabic is used as a surfactant. After the removal of excess reagents from the synthesis of Ag NPs in water, the mass ratio of Gum Arabic to Ag molecules in reaction yield is predicted to be 42:1; which means the mass of Gum Arabic layer is considerably high relative to the mass of nanoparticles. When Gum Arabic is dissolved in water, the thermal conductivity is found to be lower (0.540 W/m.K for 8 wt% Gum Arabic in water at 20 °C) than that of pure water (0.595 W/m.K at 20 °C). On the other hand, the thermal conductivity of Gum Arabic in ethylene glycol is almost the same as that of pure ethylene glycol. As there is considerable amount of Gum Arabic in the medium around the particles, the lowering of thermal conductivity in water can be attributed to the low thermal conductivity of the stabilizer in water. However, when the surfactant itself does not lower the thermal conductivity of the medium, nanoparticles can indeed enhance the thermal conductivity.

The other interesting behavior comes from the temperature dependence of thermal conductivity upon addition of nanoparticles. Thermal conductivity of water was recorded as 595.5 ± 1.50 mW/m.K at 20 °C and found to elevate to 627.3 ± 2.46 mW/m.K at 60 °C. Water based nanofluids having Ag/w NPs up to a concentration of 0.33 per cent (by wt) displayed a similar temperature dependency trend with pure water. An increase in thermal conductivity with elevating temperature is observed for just three nanofluids with low nanoparticle concentrations (Ag/w-DW-0.037, Ag/w-DW-0.11 and Ag/w-DW-0.33). However, for the nanofluids having Ag/w concentration of 0.66 and 0.99 per cent (by wt.), a decrease in thermal conductivity was observed with increasing temperature. To summarize, for lower nanoparticle concentrations, the temperature dependence of the nanofluids resemble that of water where thermal conductivity increases with temperature. However, for higher concentrations, thermal conductivity is observed to decrease with increasing temperature, trend resembling that of other solvents.

To sum up all data, normalized graphs showing temperature dependence of relative thermal conductivity are presented (Fig. 6) in order to see the effect of temperature on relative thermal conductivity of hexane, ethylene glycol and water. The data for nanofluids are normalized to thermal conductivity of pure solvents at that temperature in order to neglect the temperature dependency of base fluid and only nanofluids with the highest amount of Ag NP are presented.

FIG. 6.

Temperature dependency of effective thermal conductivity of nanofluids.

FIG. 6.

Temperature dependency of effective thermal conductivity of nanofluids.

Close modal

It is well reported in the literature24 that thermal conductivity rise with increasing temperature comes from the base fluid rather than from behavior associated with the nanoparticles. It is often reported that the enhancement in nanofluids relative to base fluids is essentially temperature independent, although some reported otherwise.22 

Data showed that temperature has no effect on thermal conductivity of nanofluids of hexane and ethylene glycol regardless of nanoparticle concentration. However, the thermal conductivity of water based nanofluids was found to depend on temperature significantly for higher nanoparticle concentrations (Ag/w-DW-0.66 and Ag/w-DW-0.99).

Initially, the unusual trend of thermal conductivity with temperature for pure water should be noted. Water, unlike polar solvents, has increasing thermal conductivity with temperature up until 130 °C. Instead of the energy being transferred between molecules, it is stored in the hydrogen bonding fluctuations within the increasingly large clusters that occur at lower temperatures. Therefore, unlike other solvents, the thermal conductivity in water increases with increasing temperature. When nanoparticles are stabilized in aqueous medium with Gum Arabic, the colloidal particle has several –OH groups exposed to the medium. These colloidal particles reduce the extensive hydrogen bonding network of water, being alternative hydrogen bonding sites. This possibly hinders the formation of so called large clusters that are responsible for the unusual thermal conductivity trend of pure water. Therefore, above a critical colloidal particle concentration, the temperature dependency of thermal conductivity resembles that of other fluids, which is the observed trend in Figs. 5 and 6.

Although the effect of metal oxides on thermal conductivity of base fluids is intensively studied, there is a lack of knowledge on nanoparticles of bulk metals due to the difficulty of stabilization. Here we investigated the thermal conductivity of common heat transfer fluids; water, ethylene glycol and hexane; with the addition of relatively low concentrations of Ag NPs. At 20 °C, the addition of Ag10/o NPs to hexane made an enhancement of 3.1 and 11.3 per cent at volume concentrations of 0.063 and 0.127 per cent, respectively. 9.7 per cent of increase was recorded for EG based nanofluid with Ag10/w concentration of 0.107 per cent (by vol). The increase in keff is attributed to Brownian motion and thermal diffusion of NPs added, also the enhancement in TC can be achieved with increasing NP concentration. However, the rate of increase in TC slows down at higher NP concentration, which is due to the possible NP agglomerations.

On the other hand, the effective thermal conductivities of Ag10/w-0.037 and Ag10/w-0.99 were found to decrease by 2 and 11 per cent at 20 °C, respectively. These particles are stabilized with Gum Arabic. When Gum Arabic alone is solubilized in ethylene glycol, its presence does not alter the thermal conductivity of this fluid. However, the same stabilizer solubilized in water reduces the thermal conductivity of water significantly. When there is enhancement of thermal conductivity upon addition of the very same particles to ethylene glycol, the observed deterioration of thermal conductivity in the presence of Gum Arabic coated AgNPs in water is attributed to the presence of the stabilizer, which reduces the thermal conductivity of the base fluid.

An interesting temperature dependency of nanofluids is also observed. Thermal conductivity of water increases with temperature, contrary to most solvents. Upon addition of low concentrations of AgNPs, the temperature trend of water was maintained; however above a threshold AgNP concentration, the temperature dependency of the nanofluids resembled that of other fluids. This reversal of temperature dependency is explained in terms of inhibition of hydrogen bonding among water molecules due to the presence of Gum Arabic, which provides alternative hydrogen bonding sites.

See supplementary material for further size characterization of AuNPs and AgNPs by DLS and thermal conductivity measurements of nanoparticle coating materials in the relevant base fluids with respect to temperature.

We would like to thank Prof. Mehmet Ali Gulgun and Melike Mercan Yildizhan for TEM imaging experiments which were performed at Sabancı University Nanotechnology Research and Application Center. Authors of the manuscript “The Effect of Functionalized Silver Nanoparticles over the Thermal Conductivity of Base Fluids” hereby confirm that the corresponding content of this article has no conflict of interest.

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Supplementary Material