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Thermal conductivity, viscosity and surface tension of nanofluids based on FeC nanoparticles
Powder Technology 284 (2015) 78–84
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Thermal conductivity, viscosity and surface tension of nanofluids based on FeC nanoparticles Angel Huminic a,⁎, Gabriela Huminic a,⁎, Claudiu Fleaca b, Florian Dumitrache b, Ion Morjan b a b
Mechanical Engineering Department, Transilvania University of Brasov, 29, Bulevardul Eroilor, Brasov 500036, Romania National Institute for Laser, Plasma and Radiation Physics, 409, Atomistilor Street, PO Box MG-36, Magurele, Bucharest 077125, Romania
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 23 May 2015 Accepted 16 June 2015 Available online 23 June 2015 Keywords: Nanofluids Laser pyrolysis Thermal conductivity Viscosity Surface tension
a b s t r a c t The main goal of the present study was to prepare and also to investigate the effects of both temperature and weight concentration on the thermo-physical properties of FeC/water nanofluids. The FeC nanoparticles were obtained by the laser pyrolysis technique. The TEM, XRD, EDX Raman and FT-IR spectroscopy techniques were used to characterize structure, purity and size of the nanoparticles. Thermal conductivity, viscosity and surface tension of FeC/water nanofluids were investigated within the range of the temperature of 10 °C to 70 °C, for three weight concentrations (0.1, 0.5 and 1.0 wt%). The experimental results show that the thermal conductivity of FeC/water nanofluids increases with the increase of both weight concentration of the nanoparticles and temperature, in all cases studied. The data also indicate that the influence of weight concentration of the nanoparticles on viscosity becomes less significant for temperatures above 55 °C. The surface tension of FeC/water nanofluids increases with the increase of the weight concentrations of the nanoparticles. In the cases of low concentrations of nanoparticles (0.1% and 0.5%), the surface tension of nanofluids was lower than the surface tension of the water. For a concentration of the nanoparticles of 1.0%, the surface tension of the nanofluids within the range of 10 °C to 40 °C was close to the surface tension of the water. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanofluids are stable colloidal solutions consisting of magnetic nanoparticles dispersed into a base fluid [1], with various applications, from mechanical engineering, to aerospace, to electronic packing to bioengineering [2]. The magnetic nanoparticles used in the magnetic nanofluids are metal materials (ferromagnetic materials), such as iron, cobalt and nickel, as well as their oxides, such as maghemite (Fe2O3), magnetite (Fe3O4), spinel-type ferrites, etc. [3]. In the last decade, researchers are focusing on the measurement of both thermal conductivity and viscosity of the magnetic fluids, in the absence or in the presence of magnetic fields, due to the unique magnetic properties of these fluids [2,4–10]. Studies concerning the measurement of the surface tension of the magnetic fluids not are available in the literature. Few studies were performed on the surface tension of the nanofluids [11–15]. Thus, Abareshi et al. [2] measured the thermal conductivity of a water-based magnetite nanofluid as a function of the particle volume fraction for different temperatures. They conclude that the thermal conductivity increased with the increase of both particle volume fraction
⁎ Corresponding authors. E-mail addresses: [email protected] (A. Huminic), [email protected] (G. Huminic).
http://dx.doi.org/10.1016/j.powtec.2015.06.040 0032-5910/© 2015 Elsevier B.V. All rights reserved.
and temperature. The highest thermal conductivity ratio reported was 11.5% at a particle volume fraction of 3% and 40 °C. Hong et al. [4] investigated the thermal conductivity of the nanofluids with various volume fractions of Fe nanoparticles in the ethylene glycol, and their results show the intensification of the thermal conductivity with the particle volume fraction, too. They also compared the thermal conductivity of the copper nanoparticles and the iron nanoparticles dispersed in the ethylene glycol and founded that the thermal conductivity of iron-based nanofluids was higher than of the copperbased nanofluid. Yu et al. [5] investigated the effects of the particle volume fraction on the thermal conductivity of a kerosene-based Fe3O4 magnetic nanofluid, prepared via a phase-transfer method. Their results show that the thermal conductivity ratio is increasing linearly with both volume fraction and temperature, up to 34.0% at 1 vol%. Khedkar et al. [6] conducted a study on thermal conductivity and viscosity of Fe3O4 nanoparticles in paraffin as a function of the particle volume fraction. Their experimental results also show that the thermal conductivity increases with the particle volume fraction, and the enhancement concerning the nanofluids with 0.1 volume fraction of Fe3O4 nanoparticles was 20%, having the base fluid at the room temperature as reference. Syam Sundar et al. [7] investigated the thermal conductivity enhancement of an ethylene glycol and water mixture-based magnetite (Fe3O4) nanofluid. Experiments were conducted within the range of
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2. Experimental procedure 2.1. Synthesis of FeC nanoparticles The nanopowders were synthesized using the laser pyrolysis technique from iron pentacarbonyl vapours carried by ethylene who also acts as the laser energy transfer agent [16,17]. Briefly, the CO2 continuous wave (λ = 10.6 μm) laser beam orthogonally intersects the reactive gas flow mixture, resulting in this way a flame, from which the nanoparticles emerge. These are further collected on a porous filter. For the FeC nanocomposites, the 60 sccm ethylene flow—the Fe(CO) 5 carrier—was injected through a thinner nozzle (1.8 mm diameter), whereas a 75 sccm C2H4 + 75 sccm H2 was introduced as a second annular flow (using a nozzle of 5 mm diameter) to form the carbonaceous protecting shell for the iron-based core. In these experiments, the reactive flows were confined by a 2500 sccm annular Ar flow passing through a nozzle of 14 mm diameter. The FeC nanopowders synthesis occurred at a pressure of 500 mbar and under higher laser power of 60 W.
Fe7 C3
Fe(Cx)@C
Fe3 C αFe
Intensity (a.u.)
the temperature from 20 °C to 60 °C and several volume concentration from 0.2% to 2.0%. They found the following enhancements of the thermal conductivity: 46% for 20:80% EG/W-based nanofluid, 42% for 40%:60% EG/W-based nanofluid, and 33% for 60%:40% EG/W-based nanofluid, at 2.0% particle volume concentration and a temperature of 60 °C. Syam Sundar et al. [8] measured the thermal conductivity and viscosity of a water-based magnetite (Fe3O4) nanofluid as a function of the particle volume fraction at various temperatures. Their results show that the thermal conductivity ratio increases with both particle volume fraction and temperature, too. High thermal conductivity enhancement of 48% was observed for a volume concentration of 2.0% and a temperature of 60 °C, compared to the distilled water. Djurek et al. [9] prepared maghemite and CoFe2O4 EG-stabilized nanoparticles dispersions in water (ferrofluids) at 65 and 120 g/l and studied the influence of the magnetic field on the thermal conductivity. Guo et al. [10] also reported an enhancement of the thermal conductivity with the volume fraction of the γ-Fe2O3 nanoparticles within a mixture of 45% water and 55% ethylene glycol. As a continuous subject of research, this study presents new aspects and results concerning thermo-physical properties of FeC/water nanofluids, which has not investigated till now. Therefore, the main goal of the current study is to prepare and also to investigate the effects of the temperature and weight concentration on the thermo-physical properties of FeC/water nanofluids. The FeC nanoparticles were obtained by the laser pyrolysis technique. The TEM, XRD, EDX Raman and FTIR spectroscopy techniques were used to characterize structure, purity and size of the nanoparticles. Thermal conductivity, viscosity and surface tension of FeC/water nanofluids were investigated within the range of temperature from 10 °C to 70 °C.
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25
30
35
40
45
50
55
60
65
2θ Fig. 1. X-ray diffraction patterns of iron-based cores with carbon-based shell.
Miller Index was not certain revealed. Also, the XRD pattern shows no evidence for (002) graphite peak, suggesting a disordered structure of the carbonaceous shell. The average grain size calculated from FWHM was evaluated at 10.0 nm for Fe7C3 (from the peak centred on 53.2°) and 7.1 nm for Fe3C (from 58.1° peak). The analysis with Raman spectroscopy Fe(Cx)C sample revealed a dominant presence of D and G band attributed to amorphous carbon phase, but there were no peaks ascribed to the iron oxides phases. This absence can be explained by the good passivation role played by the amorphous carbon shell for the iron-based core. Also, EDX analyses evaluate the following mediated elemental proportions: 42.7 at.% C, 13.2 at.% O and 44.1 at.% Fe. The existence of oxygen in the absence of iron oxide phases in this nanopowder points towards the prevalence of functional groups containing carbon–oxygen bonds such as carboxyl or alcohol/phenol (proposed also for the iron oxide-based sample) and/or ketone, aldehyde, ether and ester in the carbonaceous shells. This approach permits the
2.2. Characterization of FeC nanoparticles The iron-based core with C shell nanoparticles has been characterized using XRD, EDX Raman and FT-IR spectroscopy. Fig. 1 shows the XRD patterns along with phase identification, based on their relative maxima of as synthesized iron-based core with carbon shell nanoparticles. The three most probable phases (Fe7C3, Fe3C and αFe) have the positions around 2θ ~ 45° for the most intense maxima. The Fe7C3 phase with 00-017-0333 JCPDS code has distinct peaks at 2θ = 50.5° and 53.2° attributed to (202) and (220) crystallographic planes. Fe3C (00035-0772 JCPDS code) has a distinct peak at 2θ = 58.1° with Miller Index (301). The presence of α Fe crystalline phase (00-006-0696 JCPDS code) is unclear due to the fact that the diffraction absolute maximum placed at 2θ = 44.7° is obscured by the convoluted peak centred on 45°. Moreover, the αFe second peak located at 2θ = 65.1 for (200)
Fig. 2. TEM image of iron-based nanoparticle with core/ shell composite structure, also an HREM image with identified interplanar distances as insert.
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preservation of the nanoscale properties of zerovalent iron particles (magnetism or water and soil remediation [18,19]). TEM image of Fe-based composite nanoparticles is shown in Fig. 2. As depicted, some nanoparticles are organized as crosslinked chains, but some single nanoparticles with core shell composite structure are also highlighted. The insert in Fig. 2 shows an HREM image with examples of good individualized round nanoparticles where a crystalline core FexCy (αFe) and an amorphous or defective onion like carbon shell are identified. The particle diameter distribution is a rather sharp one with a maximum value of 14.2 nm. 2.3. Preparation of FeC nanofluids For this study, the nanofluids in 0.1, 0.5 and 1.0 wt.% concentrations were prepared. Due to the hydrophobic prevalent character of FeC nanoparticles surface (carbon-rich), they need to be functionalized with surfactants or hydrophilic polymers in order to disperse them in the water. Thus, low viscosity carboxymethyl cellulose sodium salt (CMCNa) was employed for the FeC composite nanoparticles stabilization. To obtain stable dispersion, the stabilization mechanism through steric repulsion was used. The concentration of the surfactant for each type of nanofluids was 3 g/l. In order to obtain homogeneous suspensions with a size of the aggregates as small as possible, there was employed a double ultrasonication: 10 hours at Elmasonic S40H bath followed by 3 hours under Hielscher UIP 1000hd sonotrode). In all cases, a temperature of 70 °C was maintained during ultrasonication. Following this procedure, no settlement of nanoparticles was observed after 6 months. The zeta potential and the mean hydrodynamic diameter were measured by dynamic light scattering analysis (DLS), as described in the paper of Dumitrache et al. [20]. 2.4. Thermal conductivity, dynamic viscosity and surface tension measurements Thermal conductivity was measured using a KD 2 Pro thermal properties analyzer. The device consists of a probe with 1.3 mm in diameter and 60 mm in length, a thermo-resistor and a microprocessor to control and measure the conduction in the probe. The instrument has a specified accuracy of ±5%. Before measurements, the calibration of the sensor needle was carried out by measuring the thermal conductivity of the distilled water at the room temperature of 20 °C. Thus, the measured value of the distilled water was 0.600 W/mK, which is in good agreement with the value in literature of 0.596 W/mK at a temperature of 293 K. The viscosity of the nanofluids was measured by a Brookfield programmable viscometer. A software was used for the complete external control of the viscometer. Before the measurements, the viscometer was calibrated with the distilled water, and the computed maximum uncertainty in the viscosity measurement was lower than 2%. The measurement of the surface tension of the nanofluids was performed by a Sigma force tensiometer, which is based on force measurements of the interaction of a probe with the surface of interface of two fluids. The Du Noüy ring method was employed. This method is based on the interaction of a platinum ring with the test surface. This instrument was computer controlled, and it was calibrated with the distilled water, and the computed maximum uncertainty was lower than 2%, too. In order to control the temperature of the nanofluids during the measurement process, a thermostatic vessel together with a thermostat bath Haake C10–P5/U having an accuracy of ± 0.04 °C in the controlling of the temperature within ranges of 0 °C to 100 °C were used.
3. Results and discussion 3.1. Thermal conductivity 3.1.1. Influence of the weight concentration on the thermal conductivity In the current study, the thermal conductivity was measured for three weight concentrations (0.1%, 0.5% and 1.0%) of FeC nanoparticles within the range of temperature from 15 °C to 70 °C. The effect of the weight concentrations on thermal conductivity of FeC nanofluids for various temperatures is shown in Fig. 3. The results indicate that the thermal conductivity of FeC/water nanofluids increases with the increase of the weight concentration of nanoparticles for all cases. Thus, the maximum thermal conductivity enhancement was found at 1.0% and 70 °C, respectively. One reason in the increasing of the thermal conductivity of FeC/water nanofluids is the higher thermal conductivity of the nanoparticles compared with the thermal conductivity of the base fluid. Also, at the higher concentrations of nanoparticle the interaction between particles increases, and this can lead to the improvement of the thermal conductivity. 3.1.2. Influence of the temperature on the thermal conductivity The effect of temperature on the thermal conductivity of FeC/water nanofluids is shown in Fig. 4 between 15 °C and 70 °C. For all studied cases, it can be seen an increase of the thermal conductivity of nanofluids with the temperature. It is known that the enhancement of the thermal conductivity of nanofluids contributes to several factors, as well the Brownian motion, that induce the micro-convection [21], the effects of nanoparticle aggregation [22], the interfacial layers of liquid to the liquid/particle interface [23–25], and also the combined effect of these factors [22]. The maximum relative thermal conductivity (100 ⋅ (knf − kw)/kw) was 24.1% for a concentration of 1.0 wt% and a temperature of 70 °C. At this temperature, the relative thermal conductivity was 15.3% and 19.5% for a concentration of the nanoparticles of 0.1% and 0.5%, respectively. Fig. 5 shows the comparison between the measured data and the predicted values using existing correlations from literature at standard temperature of 25 °C due to the lake of the experimental data concerning the thermo-physical properties of FeC/water nanofluids. The models of Bruggeman [26] and Murshed [27] were employed. The Bruggeman model allows the estimation of effective thermal conductivity of nanofluids using the following equation: keff;Bruggeman ¼
1 k pffiffiffiffi ½ð3ϕ−1Þks þ ð2−3ϕÞkb f þ b f Δ; 4 4
ð1Þ
Fig. 3. The effect of the weight concentration on thermal conductivity of FeC/water nanofluids.
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and the deviations between experimental data and this model being 9.4%, 12.8% and 15.8%, respectively. 3.2. Dynamic viscosity
Fig. 4. The thermal conductivity versus temperature at different weight concentrations of FeC nanoparticles.
where Δ is given by "
Δ ¼ ð3ϕ−1Þ2
ks kb f
2
# k s : þ ð2−3ϕÞ2 þ 2 2 þ 9ϕ−9ϕ2 kb f
ð2Þ
The model for predicting the effective thermal conductivity of nanofluids developed by Murshed et al. [27] is
keff;Murshed
" # ks 0:52ϕ ks kb f 1 þ 0:27ϕ4=3 −1 1þ −1 kb f 1−ϕ1=3 kb f ! ¼ ð3Þ 0:52ϕ 4=3 ks 1=3 −1 þ 0:27ϕ þ 0:27 1þϕ kb f 1−ϕ1=3
where kbf is the thermal conductivity of the base fluid, ks represents the thermal conductivity of the solid particles and φ is the volume concentration of the nanoparticles. In order to compare current results with those provided by theory, the weight concentrations were converted to the corresponding volume concentration. As shown in Fig. 5, the experimental data are in good agreement with the results of Murshed model. For all three concentrations of the nanoparticles, the deviations between experimental data and theoretical model were 7.3%, 3.1% and 3.3%, respectively. Also, the experimental results were reasonably predicted by Bruggeman model,
Fig. 5. Comparison between experimental data for FeC/water nanofluids and those calculated by theoretical models.
3.2.1. Influence of the weight concentration on the dynamic viscosity Together with the thermal conductivity, the viscosity represents one of the most important properties of the nanofluids. It is known that in engineering systems, a high viscosity can lead to faulty operation of these because the pumping power is proportional to the pressure drop, which is dependent to the fluid viscosity [28]. The effect of the weight concentrations on the dynamic viscosity of FeC nanofluids for various temperatures is shown in Fig. 6. The results indicate that the viscosity of FeC/water nanofluids increases with the increase of the weight concentration of the nanoparticles in all cases studied. For example, at standard temperature of 25 °C and 1.0% concentration of nanoparticles, the viscosity ratio defined as ratio between the viscosity of the nanofluids and the viscosity of the water was 1.34. Also, for same temperature and concentrations of nanoparticles of 0.1% and 0.5%, the dynamic viscosity enhancement was 1.12 and 1.26, respectively. 3.2.2. Influence of the temperature on the dynamic viscosity The effect of temperature on dynamic viscosity of the FeC/water nanofluids was studied, too. As shown in Fig. 7, the viscosity decreases with temperatures and increases with the weight concentrations of the nanoparticles, the latest aspect being more conspicuous for temperatures below 55 °C. Above this value, the influence of the increasing in weight concentration tends to be negligible for the cases studied. Similar results were observed by Turgut et al. [29], which studied the viscosity of TiO2/water nanofluids for a temperature within range of 13 °C to 55 °C and volume concentrations of 0.2% to 3%. The authors found that the influence of the volume concentration on viscosity decreases with the increase of temperature. In the study of Yang et al. [30], it is indicated that the main parameters that affect the viscosity of nanofluids are the content of surfactant and nanoparticles, the interaction between surfactant and nanoparticles and the dispersion type. As shown in Fig. 8, current results were compared with both experimental data and correlations available in literature at 30 °C. The models of Batchelor [31] and Wang [32] were considered, together with data for SiC/water nanofluids provided by Lee et al. [33], due to the lake of experimental data concerning the viscosity of FeC/water nanofluids. The classical Batchelor model was proposed in 1977:
μ n f ¼ μ b f 1 þ 2:5φ þ 6:5φ2 :
ð4Þ
Fig. 6. The effect of the weight concentration on dynamic viscosity of FeC/water nanofluids.
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Fig. 9. The effect of the weight concentration on surface tension of FeC/water nanofluids. Fig. 7. The effect of the temperatures on dynamic viscosity of FeC/water nanofluids.
Wang developed the following model for the viscosity of nanofluids:
μ n f ¼ μ b f 1 þ 7:3φ þ 123φ2 :
ð5Þ
As shown in Fig. 8, the viscosity of nanofluids computed with the models of Batchelor and Wang is lower than experimental values. The experimental results were in good agreement with the experimental data obtained by Lee for SiC/water nanofluids [33]. 3.3. Surface tension 3.3.1. Influence of the weight concentration on the surface tension Because the studies on the surface tension of nanofluids are limited in the literature, this work presents the study of the surface tension of FeC/water nanofluids on the weight concentration of nanoparticles. The surface tension of nanofluids was measured for same three weight concentrations of nanoparticles 0.1%, 0.5 and 1.0%, respectively. The influence of the weight concentration on surface tension is shown in Fig. 9. The results indicate that the surface tension of FeC/water nanofluids increases with the increase of the concentration of nanoparticles in all cases. For example, at temperature of 20 °C and 1.0% concentration of the nanoparticles, the surface tension ratio defined as ratio between the surface tension of the nanofluids and the surface tension of the water was 1.005. Also, for same temperature and concentrations of nanoparticles of 0.1% and 0.5%, respectively, the surface tension ratio
Fig. 8. Comparison between experimental data for FeC/water nanofluids and theoretical and experimental models.
was 0.82 and 0.99, respectively. The increasing surface tension together with the increasing concentration of the nanoparticles is due to van der Waals forces between particles at the liquid/gas interface, which leads to an increase of the surface free energy, and thus to the increasing of the surface tension [11]. 3.3.2. Influence of the temperature on surface tension The surface tension of FeC/water nanofluids was measured within the range of the temperature of 10 °C to 70 °C, for previously mentioned weight concentrations of the nanoparticles: 0.5%, 1.0% and 2.0%. Fig. 10 depicts the influence of the temperatures on surface tension. As shown, surface tension increases with the weight concentration of nanoparticles and decreases with temperature, for all cases studied. For concentrations of the nanoparticles of 0.1% and 0.5%, respectively, the surface tension of nanofluids was lower than the surface tension of the water. For a concentration of the nanoparticles of 1.0%, the surface tension of the nanofluids within the range of 10 °C to 40 °C was very close to the surface tension of the water. For temperatures above 40 °C, the surface tension of the nanofluids was higher than the surface tension of the water. In the work of Sohel et al. [12], it is stated that the addition of a small amount of surfactant into the base fluid leads to the reduction of the surface tension of both base fluid and nanofluid. On the other hand, in the work of Chen et al. [13], it is stated that the addition of the surfactants does not influence the surface tension of the nanofluids. Because previously reported results are yet contradictory concerning the increasing or decreasing of the surface tension, the reasons
Fig. 10. The effect of the temperature on surface tension of FeC/water nanofluids.
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in the surface tension behaviour could be the Brownian motion and the accumulation of the nanoparticles at the gas–liquid interface [14,15]. Also, Tanvir and Qiao [11] believe that the contradictions may arise from the use of different nanofluid/surfactant combinations and that the behaviour of each type of nanofluid upon the addition of surfactant is different. 3.4. Influence of the surfactant on properties of the nanofluid The influence of the surfactant on properties of the nanofluid was checked, too. Thus, it was found that the addition of the surfactants improves the stability of the nanofluids and also may influence the themophysical properties of the nanofluids. The thermo-physical properties of the base fluid with surfactant (water with CMCNa 3 g/l low viscosity) are presented in Table 1. As seen in table below, the thermal conductivity of the base fluid with surfactant (water with CMCNa 3 g/l low viscosity) is close to the value of the water. Thus, the increase in the thermal conductivity of the nanofluid with the concentration of the nanoparticles, as shown in Fig. 3, is due to the higher thermal conductivity of the nanoparticles. Concerning the viscosity of the base fluid with surfactant, it is higher than the viscosity of the water. Hence, both concentration of the nanoparticles and the viscosity of the surfactant are influencing the increasing of the viscosity of the nanofluid, see Fig. 6. Relating to the surface tension of the base fluid with surfactant, it is much lower that the value of the water, and consequently, it has a significant influence on the surface tensions of the nanofluid, which is lower than the value of the water, for nanoparticle concentrations of 0.1 wt% and 0.5 wt%, as shown in Fig. 10. For the cases studied, the surface tension of the nanofluid is higher than the value of the water for a nanoparticle concentration of 1.0 wt%. This could be exploited to increase the thermal performances of the thermosiphon heat pipes using nanofluids. 4. Conclusions In this study, synthesis, characterization and thermo-physical properties of nanofluids based on FeC nanoparticles were experimentally investigated. Nanopowders were synthesized using the laser pyrolysis technique from iron pentacarbonyl vapours carried by the ethylene, who also acts as a laser energy transfer agent. Iron-based core with C shell nanoparticles have been characterized using TEM, XRD, EDX Raman and FT-IR spectroscopy. Their aqueous suspensions in the presence of an additive (CMCNa) were prepared by double ultrasonication. Thermo-physical properties of FeC nanoparticles in distilled water have been experimentally determined as a function of the weight concentration and temperature. The experimental results showed that the thermal conductivity of FeC/water nanofluids increases with both weight concentration of nanoparticles and temperature in all cases studied. The maximum thermal conductivity enhancement was found at 1.0% and 70 °C, respectively. The viscosity of nanofluids increases with the increasing of weight concentrations and decreases with the increasing of temperatures. Current results indicate that at temperatures above 55 °C the influence of the weight concentration of nanoparticles was lower. The experimental results were compared with both theoretical and experimental correlations available in the literature.
Table 1 Thermo-physical properties of the base fluid at temperature 25 °C. Solution
Thermal conductivity (W/mK)
Viscosity (mPas)
Surface tension (N/m)
Water Water with CMCNa 3 g/l low viscosity
0.606 0.595
0.881 1.140
0.071972 0.057926
83
The surface tension of FeC/water nanofluids increases with the increase of weight concentrations. Also, at low concentrations of the nanoparticles (0.1% and 0.5%), the surface tension of the nanofluids was lower than the surface tension of the water, and at a concentration of nanoparticles of 1.0%, the surface tension of the nanofluids within the range of 10 °C to 40 °C was very close to the surface tension of water. It was found that the addition of the surfactants improves the stability of the nanofluids and also may influence the themo-physical properties of the nanofluids. Acknowledgements This work was supported by the Romanian National Authority for Scientific Research, CNCS—UEFISCDI, Project Number PN-II-ID-PCE2011-3-0275 and by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/134398. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2015.06.040. References [1] J.L. Viota, F. Gonzalez-Caballero, J.D.G. Duran, A.V. Delgado, Study of the colloidal stability of concentrated bimodal magnetic fluids, J. Colloid Interface Sci. 309 (2007) 135–139. [2] M. Abareshi, E. Goharshadi, S.M. Zebarjad, H.K. Fadafan, Youssefi Abbas, Fabrication, characterization and measurement of thermal conductivity of Fe3O4 nanofluids, J. Magn. Magn. Mater. 322 (2010) 3895–3901. [3] Innocent Nkurikiyimfura, Yanmin Wang, Zhidong Pan, Heat transfer enhancement by magnetic nanofluids—a review, Renew. Sust. Energ. Rev. 21 (2013) 548–561. [4] K.S. Hong, T.K. Hong, H.S. Yang, Thermal conductivity of Fe nanofluids depending on the cluster size of nanoparticles, Appl. Phys. Lett. 88 (2006) 031901. [5] W. Yu, H. Xie, L. Chen, Y. Li, Enhancement of thermal conductivity of kerosene-based Fe3O4 nanofluids prepared via phase-transfer method, Colloids Surf. A Physicochem. Eng. Asp. 355 (2010) 109–113. [6] R.S. Khedkar, A. Sai Kiran, S.S. Sonawane, K. Wasewar, S.S. Umre, Thermophysical characterization of paraffin based Fe3O4 nanofluids, Procedia Eng. 51 (2013) 342–346. [7] L. Syam Sundar, K. Singh Manoj, C.M. Sousa Antonio, Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid, Int. Commun. Heat Mass Transfer 49 (2013) 17–24. [8] L. Syam Sundar, K. Singh Manoj, C.M. Sousa Antonio, Investigation of thermal conductivity and viscosity of Fe3O4 nanofluid for heat transfer applications, Int. Commun. Heat Mass Transfer 44 (2013) 7–14. [9] I. Djurek, A. Znidarsik, A. Kosak, D. Djurek, Thermal conductivity measurements of the CoFe2O4 and γ-Fe2O3 based nanoparticle ferrofluids, Croat. Chem. Acta 80 (2007) 529–532. [10] Shou-Zhu Guo, Yang Li, Ji-Sen Jiang, Hua-Qing Xie, Nanofluids containing γ-Fe2O3 nanoparticles and their heat transfer enhancements, Nanoscale Res. Lett. 5 (2010) 1222–1227. [11] Saad Tanvir, Li Qiao, Surface tension of Nanofluid-type fuels containing suspended nanomaterials, Nanoscale Res. Lett. 7 (2012) 226. [12] S.M. Sohel Murshed, Denitsa Milanova, Ranganathan Kumar, An experimental study of surface tension-dependent pool boiling characteristics of carbon nanotubesnanofluids, 7th International Conference on Nanochannels, Microchannels, and Minichannels ASME 2009, (Paper No. ICNMM2009-82204) 2009, pp. 75–80. [13] R.H. Chen, Tran X. Phuoc, Donald Martello, Surface tension of evaporating nanofluid droplets, Int. J. Heat Mass Transf. 54 (2011) 2459–2466. [14] S.M.S. Murshed, S.H. Tan, N.T. Nguyen, Temperature dependence of interfacial properties and viscosity of nanofluids for droplet-based microfluidics, J. Phys. D. Appl. Phys. 41 (2008). [15] S. Vafaei, A. Purkayastha, A. Jain, G. Ramanath, T. Borca-Tasciuc, The effect of nanoparticles on the liquid-gas surface tension of Bi(2)Te(3) nanofluids, J. Colloid Interface Sci. 20 (2009). [16] F. Dumitrache, I. Morjan, R. Alexandrescu, R.E. Morjan, I. Voicu, I. Sandu, I. Soare, M. Ploscaru, C. Fleaca, V. Ciupina, G. Prodan, B. Rand, R. Brydson, A. Woodword, Nearly monodispersed carbon coated iron nanoparticles for the catalytic for growth of nanotubes/nanofibres, Diam. Relat. Mater. 13 (2) (2004) 362–370. [17] I. Morjan, R. Alexandrescu, I. Soare, F. Dumitrache, I. Sandu, I. Voicu, A. Crunteanu, E. Vasile, V. Ciupina, S. Martelli, Nanoscale powders of different iron oxide phases prepared by continuous laser irradiation of iron pentacarbonyl-containing gas precursors, Mater. Sci. Eng. C 23 (2003) 211–216. [18] M. Bystrzejewski, K. Pyrzynska, A. Huczo, H. Lange, Carbon-encapsulated magnetic nanoparticles as separable and mobile sorbents of heavy metal ions from aqueous solutions, Carbon 47 (2009) 1201–1204.
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[19] K. Mackenzie, A. Schierz, A. Georgi, F.K. Kopnike, Colloidal activated carbon and carbon-iron—novel materials for in-situ groundwater treatment, Glob. NEST J. 10 (2008) 54–61. [20] F. Dumitrache, C. Fleaca, R. Alexandrescu, I. Morjan, G. Huminic, A. Huminic, C. Daia, L. Vekas, Structural and thermophysical properties of aqueous nanofluids based on Fe@C and Fe2O3 nanoparticles synthesized by laser pyrolysis, E-MRS Spring Meeting, Strasburg, 2013. [21] S.P. Jang, S.U.S. Choi, Role of Brownian motion in the enhanced thermal conductivity of nanofluids, Appl. Phys. Lett. 84 (2004) 4316–4318. [22] Y. Feng, Y. Boming, P. Xu, M. Zou, The effective thermal conductivity of nanofluids based on nanolayer and aggregation of nanoparticles, J. Phys. D. Appl. Phys. 40 (2007) 3164–3171. [23] W. Yu, S.U.S. Choi, The role of interfacial layers in the enhanced thermal conductivity of nanofluids: a renovated Maxwell model, J. Nanoparticle Res. 5 (2003) 167–171. [24] H.Q. Xie, M. Fujii, X. Zhang, Effect of interfacial nanolayer on the effective thermal conductivity of nanoparticle–fluid mixture, Int. J. Heat Mass Transf. 48 (2005) 2926–2932. [25] W. Yu, S.U.S. Choi, The role of interfacial layers in the enhanced thermal conductivity of nanofluids: a renovated Hamilton–Crosser model, J. Nanoparticle Res. 6 (2004) 355–361. [26] D. Bruggeman, Berechnung verschiedener physikalischer konstanten von heterogenen substanzen, I. Dielektrizitatskonstanten und leitfahigkeiten der mischkorper aus isotropen substanzen, Ann. Phys. Leipzig 24 (1935) 636–679.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Thermal conductivity, viscosity and surface tension of nanofluids based on FeC nanoparticles Angel Huminic a,⁎, Gabriela Huminic a,⁎, Claudiu Fleaca b, Florian Dumitrache b, Ion Morjan b a b
Mechanical Engineering Department, Transilvania University of Brasov, 29, Bulevardul Eroilor, Brasov 500036, Romania National Institute for Laser, Plasma and Radiation Physics, 409, Atomistilor Street, PO Box MG-36, Magurele, Bucharest 077125, Romania
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 23 May 2015 Accepted 16 June 2015 Available online 23 June 2015 Keywords: Nanofluids Laser pyrolysis Thermal conductivity Viscosity Surface tension
a b s t r a c t The main goal of the present study was to prepare and also to investigate the effects of both temperature and weight concentration on the thermo-physical properties of FeC/water nanofluids. The FeC nanoparticles were obtained by the laser pyrolysis technique. The TEM, XRD, EDX Raman and FT-IR spectroscopy techniques were used to characterize structure, purity and size of the nanoparticles. Thermal conductivity, viscosity and surface tension of FeC/water nanofluids were investigated within the range of the temperature of 10 °C to 70 °C, for three weight concentrations (0.1, 0.5 and 1.0 wt%). The experimental results show that the thermal conductivity of FeC/water nanofluids increases with the increase of both weight concentration of the nanoparticles and temperature, in all cases studied. The data also indicate that the influence of weight concentration of the nanoparticles on viscosity becomes less significant for temperatures above 55 °C. The surface tension of FeC/water nanofluids increases with the increase of the weight concentrations of the nanoparticles. In the cases of low concentrations of nanoparticles (0.1% and 0.5%), the surface tension of nanofluids was lower than the surface tension of the water. For a concentration of the nanoparticles of 1.0%, the surface tension of the nanofluids within the range of 10 °C to 40 °C was close to the surface tension of the water. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetic nanofluids are stable colloidal solutions consisting of magnetic nanoparticles dispersed into a base fluid [1], with various applications, from mechanical engineering, to aerospace, to electronic packing to bioengineering [2]. The magnetic nanoparticles used in the magnetic nanofluids are metal materials (ferromagnetic materials), such as iron, cobalt and nickel, as well as their oxides, such as maghemite (Fe2O3), magnetite (Fe3O4), spinel-type ferrites, etc. [3]. In the last decade, researchers are focusing on the measurement of both thermal conductivity and viscosity of the magnetic fluids, in the absence or in the presence of magnetic fields, due to the unique magnetic properties of these fluids [2,4–10]. Studies concerning the measurement of the surface tension of the magnetic fluids not are available in the literature. Few studies were performed on the surface tension of the nanofluids [11–15]. Thus, Abareshi et al. [2] measured the thermal conductivity of a water-based magnetite nanofluid as a function of the particle volume fraction for different temperatures. They conclude that the thermal conductivity increased with the increase of both particle volume fraction
⁎ Corresponding authors. E-mail addresses: [email protected] (A. Huminic), [email protected] (G. Huminic).
http://dx.doi.org/10.1016/j.powtec.2015.06.040 0032-5910/© 2015 Elsevier B.V. All rights reserved.
and temperature. The highest thermal conductivity ratio reported was 11.5% at a particle volume fraction of 3% and 40 °C. Hong et al. [4] investigated the thermal conductivity of the nanofluids with various volume fractions of Fe nanoparticles in the ethylene glycol, and their results show the intensification of the thermal conductivity with the particle volume fraction, too. They also compared the thermal conductivity of the copper nanoparticles and the iron nanoparticles dispersed in the ethylene glycol and founded that the thermal conductivity of iron-based nanofluids was higher than of the copperbased nanofluid. Yu et al. [5] investigated the effects of the particle volume fraction on the thermal conductivity of a kerosene-based Fe3O4 magnetic nanofluid, prepared via a phase-transfer method. Their results show that the thermal conductivity ratio is increasing linearly with both volume fraction and temperature, up to 34.0% at 1 vol%. Khedkar et al. [6] conducted a study on thermal conductivity and viscosity of Fe3O4 nanoparticles in paraffin as a function of the particle volume fraction. Their experimental results also show that the thermal conductivity increases with the particle volume fraction, and the enhancement concerning the nanofluids with 0.1 volume fraction of Fe3O4 nanoparticles was 20%, having the base fluid at the room temperature as reference. Syam Sundar et al. [7] investigated the thermal conductivity enhancement of an ethylene glycol and water mixture-based magnetite (Fe3O4) nanofluid. Experiments were conducted within the range of
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2. Experimental procedure 2.1. Synthesis of FeC nanoparticles The nanopowders were synthesized using the laser pyrolysis technique from iron pentacarbonyl vapours carried by ethylene who also acts as the laser energy transfer agent [16,17]. Briefly, the CO2 continuous wave (λ = 10.6 μm) laser beam orthogonally intersects the reactive gas flow mixture, resulting in this way a flame, from which the nanoparticles emerge. These are further collected on a porous filter. For the FeC nanocomposites, the 60 sccm ethylene flow—the Fe(CO) 5 carrier—was injected through a thinner nozzle (1.8 mm diameter), whereas a 75 sccm C2H4 + 75 sccm H2 was introduced as a second annular flow (using a nozzle of 5 mm diameter) to form the carbonaceous protecting shell for the iron-based core. In these experiments, the reactive flows were confined by a 2500 sccm annular Ar flow passing through a nozzle of 14 mm diameter. The FeC nanopowders synthesis occurred at a pressure of 500 mbar and under higher laser power of 60 W.
Fe7 C3
Fe(Cx)@C
Fe3 C αFe
Intensity (a.u.)
the temperature from 20 °C to 60 °C and several volume concentration from 0.2% to 2.0%. They found the following enhancements of the thermal conductivity: 46% for 20:80% EG/W-based nanofluid, 42% for 40%:60% EG/W-based nanofluid, and 33% for 60%:40% EG/W-based nanofluid, at 2.0% particle volume concentration and a temperature of 60 °C. Syam Sundar et al. [8] measured the thermal conductivity and viscosity of a water-based magnetite (Fe3O4) nanofluid as a function of the particle volume fraction at various temperatures. Their results show that the thermal conductivity ratio increases with both particle volume fraction and temperature, too. High thermal conductivity enhancement of 48% was observed for a volume concentration of 2.0% and a temperature of 60 °C, compared to the distilled water. Djurek et al. [9] prepared maghemite and CoFe2O4 EG-stabilized nanoparticles dispersions in water (ferrofluids) at 65 and 120 g/l and studied the influence of the magnetic field on the thermal conductivity. Guo et al. [10] also reported an enhancement of the thermal conductivity with the volume fraction of the γ-Fe2O3 nanoparticles within a mixture of 45% water and 55% ethylene glycol. As a continuous subject of research, this study presents new aspects and results concerning thermo-physical properties of FeC/water nanofluids, which has not investigated till now. Therefore, the main goal of the current study is to prepare and also to investigate the effects of the temperature and weight concentration on the thermo-physical properties of FeC/water nanofluids. The FeC nanoparticles were obtained by the laser pyrolysis technique. The TEM, XRD, EDX Raman and FTIR spectroscopy techniques were used to characterize structure, purity and size of the nanoparticles. Thermal conductivity, viscosity and surface tension of FeC/water nanofluids were investigated within the range of temperature from 10 °C to 70 °C.
79
25
30
35
40
45
50
55
60
65
2θ Fig. 1. X-ray diffraction patterns of iron-based cores with carbon-based shell.
Miller Index was not certain revealed. Also, the XRD pattern shows no evidence for (002) graphite peak, suggesting a disordered structure of the carbonaceous shell. The average grain size calculated from FWHM was evaluated at 10.0 nm for Fe7C3 (from the peak centred on 53.2°) and 7.1 nm for Fe3C (from 58.1° peak). The analysis with Raman spectroscopy Fe(Cx)C sample revealed a dominant presence of D and G band attributed to amorphous carbon phase, but there were no peaks ascribed to the iron oxides phases. This absence can be explained by the good passivation role played by the amorphous carbon shell for the iron-based core. Also, EDX analyses evaluate the following mediated elemental proportions: 42.7 at.% C, 13.2 at.% O and 44.1 at.% Fe. The existence of oxygen in the absence of iron oxide phases in this nanopowder points towards the prevalence of functional groups containing carbon–oxygen bonds such as carboxyl or alcohol/phenol (proposed also for the iron oxide-based sample) and/or ketone, aldehyde, ether and ester in the carbonaceous shells. This approach permits the
2.2. Characterization of FeC nanoparticles The iron-based core with C shell nanoparticles has been characterized using XRD, EDX Raman and FT-IR spectroscopy. Fig. 1 shows the XRD patterns along with phase identification, based on their relative maxima of as synthesized iron-based core with carbon shell nanoparticles. The three most probable phases (Fe7C3, Fe3C and αFe) have the positions around 2θ ~ 45° for the most intense maxima. The Fe7C3 phase with 00-017-0333 JCPDS code has distinct peaks at 2θ = 50.5° and 53.2° attributed to (202) and (220) crystallographic planes. Fe3C (00035-0772 JCPDS code) has a distinct peak at 2θ = 58.1° with Miller Index (301). The presence of α Fe crystalline phase (00-006-0696 JCPDS code) is unclear due to the fact that the diffraction absolute maximum placed at 2θ = 44.7° is obscured by the convoluted peak centred on 45°. Moreover, the αFe second peak located at 2θ = 65.1 for (200)
Fig. 2. TEM image of iron-based nanoparticle with core/ shell composite structure, also an HREM image with identified interplanar distances as insert.
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preservation of the nanoscale properties of zerovalent iron particles (magnetism or water and soil remediation [18,19]). TEM image of Fe-based composite nanoparticles is shown in Fig. 2. As depicted, some nanoparticles are organized as crosslinked chains, but some single nanoparticles with core shell composite structure are also highlighted. The insert in Fig. 2 shows an HREM image with examples of good individualized round nanoparticles where a crystalline core FexCy (αFe) and an amorphous or defective onion like carbon shell are identified. The particle diameter distribution is a rather sharp one with a maximum value of 14.2 nm. 2.3. Preparation of FeC nanofluids For this study, the nanofluids in 0.1, 0.5 and 1.0 wt.% concentrations were prepared. Due to the hydrophobic prevalent character of FeC nanoparticles surface (carbon-rich), they need to be functionalized with surfactants or hydrophilic polymers in order to disperse them in the water. Thus, low viscosity carboxymethyl cellulose sodium salt (CMCNa) was employed for the FeC composite nanoparticles stabilization. To obtain stable dispersion, the stabilization mechanism through steric repulsion was used. The concentration of the surfactant for each type of nanofluids was 3 g/l. In order to obtain homogeneous suspensions with a size of the aggregates as small as possible, there was employed a double ultrasonication: 10 hours at Elmasonic S40H bath followed by 3 hours under Hielscher UIP 1000hd sonotrode). In all cases, a temperature of 70 °C was maintained during ultrasonication. Following this procedure, no settlement of nanoparticles was observed after 6 months. The zeta potential and the mean hydrodynamic diameter were measured by dynamic light scattering analysis (DLS), as described in the paper of Dumitrache et al. [20]. 2.4. Thermal conductivity, dynamic viscosity and surface tension measurements Thermal conductivity was measured using a KD 2 Pro thermal properties analyzer. The device consists of a probe with 1.3 mm in diameter and 60 mm in length, a thermo-resistor and a microprocessor to control and measure the conduction in the probe. The instrument has a specified accuracy of ±5%. Before measurements, the calibration of the sensor needle was carried out by measuring the thermal conductivity of the distilled water at the room temperature of 20 °C. Thus, the measured value of the distilled water was 0.600 W/mK, which is in good agreement with the value in literature of 0.596 W/mK at a temperature of 293 K. The viscosity of the nanofluids was measured by a Brookfield programmable viscometer. A software was used for the complete external control of the viscometer. Before the measurements, the viscometer was calibrated with the distilled water, and the computed maximum uncertainty in the viscosity measurement was lower than 2%. The measurement of the surface tension of the nanofluids was performed by a Sigma force tensiometer, which is based on force measurements of the interaction of a probe with the surface of interface of two fluids. The Du Noüy ring method was employed. This method is based on the interaction of a platinum ring with the test surface. This instrument was computer controlled, and it was calibrated with the distilled water, and the computed maximum uncertainty was lower than 2%, too. In order to control the temperature of the nanofluids during the measurement process, a thermostatic vessel together with a thermostat bath Haake C10–P5/U having an accuracy of ± 0.04 °C in the controlling of the temperature within ranges of 0 °C to 100 °C were used.
3. Results and discussion 3.1. Thermal conductivity 3.1.1. Influence of the weight concentration on the thermal conductivity In the current study, the thermal conductivity was measured for three weight concentrations (0.1%, 0.5% and 1.0%) of FeC nanoparticles within the range of temperature from 15 °C to 70 °C. The effect of the weight concentrations on thermal conductivity of FeC nanofluids for various temperatures is shown in Fig. 3. The results indicate that the thermal conductivity of FeC/water nanofluids increases with the increase of the weight concentration of nanoparticles for all cases. Thus, the maximum thermal conductivity enhancement was found at 1.0% and 70 °C, respectively. One reason in the increasing of the thermal conductivity of FeC/water nanofluids is the higher thermal conductivity of the nanoparticles compared with the thermal conductivity of the base fluid. Also, at the higher concentrations of nanoparticle the interaction between particles increases, and this can lead to the improvement of the thermal conductivity. 3.1.2. Influence of the temperature on the thermal conductivity The effect of temperature on the thermal conductivity of FeC/water nanofluids is shown in Fig. 4 between 15 °C and 70 °C. For all studied cases, it can be seen an increase of the thermal conductivity of nanofluids with the temperature. It is known that the enhancement of the thermal conductivity of nanofluids contributes to several factors, as well the Brownian motion, that induce the micro-convection [21], the effects of nanoparticle aggregation [22], the interfacial layers of liquid to the liquid/particle interface [23–25], and also the combined effect of these factors [22]. The maximum relative thermal conductivity (100 ⋅ (knf − kw)/kw) was 24.1% for a concentration of 1.0 wt% and a temperature of 70 °C. At this temperature, the relative thermal conductivity was 15.3% and 19.5% for a concentration of the nanoparticles of 0.1% and 0.5%, respectively. Fig. 5 shows the comparison between the measured data and the predicted values using existing correlations from literature at standard temperature of 25 °C due to the lake of the experimental data concerning the thermo-physical properties of FeC/water nanofluids. The models of Bruggeman [26] and Murshed [27] were employed. The Bruggeman model allows the estimation of effective thermal conductivity of nanofluids using the following equation: keff;Bruggeman ¼
1 k pffiffiffiffi ½ð3ϕ−1Þks þ ð2−3ϕÞkb f þ b f Δ; 4 4
ð1Þ
Fig. 3. The effect of the weight concentration on thermal conductivity of FeC/water nanofluids.
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81
and the deviations between experimental data and this model being 9.4%, 12.8% and 15.8%, respectively. 3.2. Dynamic viscosity
Fig. 4. The thermal conductivity versus temperature at different weight concentrations of FeC nanoparticles.
where Δ is given by "
Δ ¼ ð3ϕ−1Þ2
ks kb f
2
# k s : þ ð2−3ϕÞ2 þ 2 2 þ 9ϕ−9ϕ2 kb f
ð2Þ
The model for predicting the effective thermal conductivity of nanofluids developed by Murshed et al. [27] is
keff;Murshed
" # ks 0:52ϕ ks kb f 1 þ 0:27ϕ4=3 −1 1þ −1 kb f 1−ϕ1=3 kb f ! ¼ ð3Þ 0:52ϕ 4=3 ks 1=3 −1 þ 0:27ϕ þ 0:27 1þϕ kb f 1−ϕ1=3
where kbf is the thermal conductivity of the base fluid, ks represents the thermal conductivity of the solid particles and φ is the volume concentration of the nanoparticles. In order to compare current results with those provided by theory, the weight concentrations were converted to the corresponding volume concentration. As shown in Fig. 5, the experimental data are in good agreement with the results of Murshed model. For all three concentrations of the nanoparticles, the deviations between experimental data and theoretical model were 7.3%, 3.1% and 3.3%, respectively. Also, the experimental results were reasonably predicted by Bruggeman model,
Fig. 5. Comparison between experimental data for FeC/water nanofluids and those calculated by theoretical models.
3.2.1. Influence of the weight concentration on the dynamic viscosity Together with the thermal conductivity, the viscosity represents one of the most important properties of the nanofluids. It is known that in engineering systems, a high viscosity can lead to faulty operation of these because the pumping power is proportional to the pressure drop, which is dependent to the fluid viscosity [28]. The effect of the weight concentrations on the dynamic viscosity of FeC nanofluids for various temperatures is shown in Fig. 6. The results indicate that the viscosity of FeC/water nanofluids increases with the increase of the weight concentration of the nanoparticles in all cases studied. For example, at standard temperature of 25 °C and 1.0% concentration of nanoparticles, the viscosity ratio defined as ratio between the viscosity of the nanofluids and the viscosity of the water was 1.34. Also, for same temperature and concentrations of nanoparticles of 0.1% and 0.5%, the dynamic viscosity enhancement was 1.12 and 1.26, respectively. 3.2.2. Influence of the temperature on the dynamic viscosity The effect of temperature on dynamic viscosity of the FeC/water nanofluids was studied, too. As shown in Fig. 7, the viscosity decreases with temperatures and increases with the weight concentrations of the nanoparticles, the latest aspect being more conspicuous for temperatures below 55 °C. Above this value, the influence of the increasing in weight concentration tends to be negligible for the cases studied. Similar results were observed by Turgut et al. [29], which studied the viscosity of TiO2/water nanofluids for a temperature within range of 13 °C to 55 °C and volume concentrations of 0.2% to 3%. The authors found that the influence of the volume concentration on viscosity decreases with the increase of temperature. In the study of Yang et al. [30], it is indicated that the main parameters that affect the viscosity of nanofluids are the content of surfactant and nanoparticles, the interaction between surfactant and nanoparticles and the dispersion type. As shown in Fig. 8, current results were compared with both experimental data and correlations available in literature at 30 °C. The models of Batchelor [31] and Wang [32] were considered, together with data for SiC/water nanofluids provided by Lee et al. [33], due to the lake of experimental data concerning the viscosity of FeC/water nanofluids. The classical Batchelor model was proposed in 1977:
μ n f ¼ μ b f 1 þ 2:5φ þ 6:5φ2 :
ð4Þ
Fig. 6. The effect of the weight concentration on dynamic viscosity of FeC/water nanofluids.
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Fig. 9. The effect of the weight concentration on surface tension of FeC/water nanofluids. Fig. 7. The effect of the temperatures on dynamic viscosity of FeC/water nanofluids.
Wang developed the following model for the viscosity of nanofluids:
μ n f ¼ μ b f 1 þ 7:3φ þ 123φ2 :
ð5Þ
As shown in Fig. 8, the viscosity of nanofluids computed with the models of Batchelor and Wang is lower than experimental values. The experimental results were in good agreement with the experimental data obtained by Lee for SiC/water nanofluids [33]. 3.3. Surface tension 3.3.1. Influence of the weight concentration on the surface tension Because the studies on the surface tension of nanofluids are limited in the literature, this work presents the study of the surface tension of FeC/water nanofluids on the weight concentration of nanoparticles. The surface tension of nanofluids was measured for same three weight concentrations of nanoparticles 0.1%, 0.5 and 1.0%, respectively. The influence of the weight concentration on surface tension is shown in Fig. 9. The results indicate that the surface tension of FeC/water nanofluids increases with the increase of the concentration of nanoparticles in all cases. For example, at temperature of 20 °C and 1.0% concentration of the nanoparticles, the surface tension ratio defined as ratio between the surface tension of the nanofluids and the surface tension of the water was 1.005. Also, for same temperature and concentrations of nanoparticles of 0.1% and 0.5%, respectively, the surface tension ratio
Fig. 8. Comparison between experimental data for FeC/water nanofluids and theoretical and experimental models.
was 0.82 and 0.99, respectively. The increasing surface tension together with the increasing concentration of the nanoparticles is due to van der Waals forces between particles at the liquid/gas interface, which leads to an increase of the surface free energy, and thus to the increasing of the surface tension [11]. 3.3.2. Influence of the temperature on surface tension The surface tension of FeC/water nanofluids was measured within the range of the temperature of 10 °C to 70 °C, for previously mentioned weight concentrations of the nanoparticles: 0.5%, 1.0% and 2.0%. Fig. 10 depicts the influence of the temperatures on surface tension. As shown, surface tension increases with the weight concentration of nanoparticles and decreases with temperature, for all cases studied. For concentrations of the nanoparticles of 0.1% and 0.5%, respectively, the surface tension of nanofluids was lower than the surface tension of the water. For a concentration of the nanoparticles of 1.0%, the surface tension of the nanofluids within the range of 10 °C to 40 °C was very close to the surface tension of the water. For temperatures above 40 °C, the surface tension of the nanofluids was higher than the surface tension of the water. In the work of Sohel et al. [12], it is stated that the addition of a small amount of surfactant into the base fluid leads to the reduction of the surface tension of both base fluid and nanofluid. On the other hand, in the work of Chen et al. [13], it is stated that the addition of the surfactants does not influence the surface tension of the nanofluids. Because previously reported results are yet contradictory concerning the increasing or decreasing of the surface tension, the reasons
Fig. 10. The effect of the temperature on surface tension of FeC/water nanofluids.
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in the surface tension behaviour could be the Brownian motion and the accumulation of the nanoparticles at the gas–liquid interface [14,15]. Also, Tanvir and Qiao [11] believe that the contradictions may arise from the use of different nanofluid/surfactant combinations and that the behaviour of each type of nanofluid upon the addition of surfactant is different. 3.4. Influence of the surfactant on properties of the nanofluid The influence of the surfactant on properties of the nanofluid was checked, too. Thus, it was found that the addition of the surfactants improves the stability of the nanofluids and also may influence the themophysical properties of the nanofluids. The thermo-physical properties of the base fluid with surfactant (water with CMCNa 3 g/l low viscosity) are presented in Table 1. As seen in table below, the thermal conductivity of the base fluid with surfactant (water with CMCNa 3 g/l low viscosity) is close to the value of the water. Thus, the increase in the thermal conductivity of the nanofluid with the concentration of the nanoparticles, as shown in Fig. 3, is due to the higher thermal conductivity of the nanoparticles. Concerning the viscosity of the base fluid with surfactant, it is higher than the viscosity of the water. Hence, both concentration of the nanoparticles and the viscosity of the surfactant are influencing the increasing of the viscosity of the nanofluid, see Fig. 6. Relating to the surface tension of the base fluid with surfactant, it is much lower that the value of the water, and consequently, it has a significant influence on the surface tensions of the nanofluid, which is lower than the value of the water, for nanoparticle concentrations of 0.1 wt% and 0.5 wt%, as shown in Fig. 10. For the cases studied, the surface tension of the nanofluid is higher than the value of the water for a nanoparticle concentration of 1.0 wt%. This could be exploited to increase the thermal performances of the thermosiphon heat pipes using nanofluids. 4. Conclusions In this study, synthesis, characterization and thermo-physical properties of nanofluids based on FeC nanoparticles were experimentally investigated. Nanopowders were synthesized using the laser pyrolysis technique from iron pentacarbonyl vapours carried by the ethylene, who also acts as a laser energy transfer agent. Iron-based core with C shell nanoparticles have been characterized using TEM, XRD, EDX Raman and FT-IR spectroscopy. Their aqueous suspensions in the presence of an additive (CMCNa) were prepared by double ultrasonication. Thermo-physical properties of FeC nanoparticles in distilled water have been experimentally determined as a function of the weight concentration and temperature. The experimental results showed that the thermal conductivity of FeC/water nanofluids increases with both weight concentration of nanoparticles and temperature in all cases studied. The maximum thermal conductivity enhancement was found at 1.0% and 70 °C, respectively. The viscosity of nanofluids increases with the increasing of weight concentrations and decreases with the increasing of temperatures. Current results indicate that at temperatures above 55 °C the influence of the weight concentration of nanoparticles was lower. The experimental results were compared with both theoretical and experimental correlations available in the literature.
Table 1 Thermo-physical properties of the base fluid at temperature 25 °C. Solution
Thermal conductivity (W/mK)
Viscosity (mPas)
Surface tension (N/m)
Water Water with CMCNa 3 g/l low viscosity
0.606 0.595
0.881 1.140
0.071972 0.057926
83
The surface tension of FeC/water nanofluids increases with the increase of weight concentrations. Also, at low concentrations of the nanoparticles (0.1% and 0.5%), the surface tension of the nanofluids was lower than the surface tension of the water, and at a concentration of nanoparticles of 1.0%, the surface tension of the nanofluids within the range of 10 °C to 40 °C was very close to the surface tension of water. It was found that the addition of the surfactants improves the stability of the nanofluids and also may influence the themo-physical properties of the nanofluids. Acknowledgements This work was supported by the Romanian National Authority for Scientific Research, CNCS—UEFISCDI, Project Number PN-II-ID-PCE2011-3-0275 and by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/134398. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2015.06.040. References [1] J.L. Viota, F. Gonzalez-Caballero, J.D.G. Duran, A.V. Delgado, Study of the colloidal stability of concentrated bimodal magnetic fluids, J. Colloid Interface Sci. 309 (2007) 135–139. [2] M. Abareshi, E. Goharshadi, S.M. Zebarjad, H.K. Fadafan, Youssefi Abbas, Fabrication, characterization and measurement of thermal conductivity of Fe3O4 nanofluids, J. Magn. Magn. Mater. 322 (2010) 3895–3901. [3] Innocent Nkurikiyimfura, Yanmin Wang, Zhidong Pan, Heat transfer enhancement by magnetic nanofluids—a review, Renew. Sust. Energ. Rev. 21 (2013) 548–561. [4] K.S. Hong, T.K. Hong, H.S. Yang, Thermal conductivity of Fe nanofluids depending on the cluster size of nanoparticles, Appl. Phys. Lett. 88 (2006) 031901. [5] W. Yu, H. Xie, L. Chen, Y. Li, Enhancement of thermal conductivity of kerosene-based Fe3O4 nanofluids prepared via phase-transfer method, Colloids Surf. A Physicochem. Eng. Asp. 355 (2010) 109–113. [6] R.S. Khedkar, A. Sai Kiran, S.S. Sonawane, K. Wasewar, S.S. Umre, Thermophysical characterization of paraffin based Fe3O4 nanofluids, Procedia Eng. 51 (2013) 342–346. [7] L. Syam Sundar, K. Singh Manoj, C.M. Sousa Antonio, Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid, Int. Commun. Heat Mass Transfer 49 (2013) 17–24. [8] L. Syam Sundar, K. Singh Manoj, C.M. Sousa Antonio, Investigation of thermal conductivity and viscosity of Fe3O4 nanofluid for heat transfer applications, Int. Commun. Heat Mass Transfer 44 (2013) 7–14. [9] I. Djurek, A. Znidarsik, A. Kosak, D. Djurek, Thermal conductivity measurements of the CoFe2O4 and γ-Fe2O3 based nanoparticle ferrofluids, Croat. Chem. Acta 80 (2007) 529–532. [10] Shou-Zhu Guo, Yang Li, Ji-Sen Jiang, Hua-Qing Xie, Nanofluids containing γ-Fe2O3 nanoparticles and their heat transfer enhancements, Nanoscale Res. Lett. 5 (2010) 1222–1227. [11] Saad Tanvir, Li Qiao, Surface tension of Nanofluid-type fuels containing suspended nanomaterials, Nanoscale Res. Lett. 7 (2012) 226. [12] S.M. Sohel Murshed, Denitsa Milanova, Ranganathan Kumar, An experimental study of surface tension-dependent pool boiling characteristics of carbon nanotubesnanofluids, 7th International Conference on Nanochannels, Microchannels, and Minichannels ASME 2009, (Paper No. ICNMM2009-82204) 2009, pp. 75–80. [13] R.H. Chen, Tran X. Phuoc, Donald Martello, Surface tension of evaporating nanofluid droplets, Int. J. Heat Mass Transf. 54 (2011) 2459–2466. [14] S.M.S. Murshed, S.H. Tan, N.T. Nguyen, Temperature dependence of interfacial properties and viscosity of nanofluids for droplet-based microfluidics, J. Phys. D. Appl. Phys. 41 (2008). [15] S. Vafaei, A. Purkayastha, A. Jain, G. Ramanath, T. Borca-Tasciuc, The effect of nanoparticles on the liquid-gas surface tension of Bi(2)Te(3) nanofluids, J. Colloid Interface Sci. 20 (2009). [16] F. Dumitrache, I. Morjan, R. Alexandrescu, R.E. Morjan, I. Voicu, I. Sandu, I. Soare, M. Ploscaru, C. Fleaca, V. Ciupina, G. Prodan, B. Rand, R. Brydson, A. Woodword, Nearly monodispersed carbon coated iron nanoparticles for the catalytic for growth of nanotubes/nanofibres, Diam. Relat. Mater. 13 (2) (2004) 362–370. [17] I. Morjan, R. Alexandrescu, I. Soare, F. Dumitrache, I. Sandu, I. Voicu, A. Crunteanu, E. Vasile, V. Ciupina, S. Martelli, Nanoscale powders of different iron oxide phases prepared by continuous laser irradiation of iron pentacarbonyl-containing gas precursors, Mater. Sci. Eng. C 23 (2003) 211–216. [18] M. Bystrzejewski, K. Pyrzynska, A. Huczo, H. Lange, Carbon-encapsulated magnetic nanoparticles as separable and mobile sorbents of heavy metal ions from aqueous solutions, Carbon 47 (2009) 1201–1204.
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