Experimental Investigation of Electrical Conductivity and Permittivity of SC-TiO 2 -EG Nanofluids
© The Author(s) 2016
Received: 16 May 2016
Accepted: 16 August 2016
Published: 24 August 2016
The paper presents experimental studies of dielectric properties of nanofluids based on ethylene glycol and SC-TiO2 nanoparticles with average size of 15–40 nm with various mass concentrations. The dielectric permittivity both real part and imaginary part as a function of temperature and frequency were measured. Also, dependence ac conductivity on frequency, temperature, and mass concentration were investigated. Based on the curves of ac conductivity, dc conductivity was calculated, and 400 % enhancement in dc conductivity was exposed.
KeywordsDielectric properties Nanofluids Titanium oxide
In an era of continuous technology development, scientists and engineers constantly strive to achieve better and better results in various fields of science and technology. Regardless of whether it is an increase in energy efficiency or miniaturization of devices or medical applications, many of the researchers believe that the nanomaterials are the future of technique and industry. One of the most commonly studied group of nanomaterials are nanofluids. This name was established by Choi  in 1995. Nanofluids are identified as a suspension of solid particles with nanometric size (1–100 nm) in based fluids. As base fluids are usually used distilled water, oils, or alcohols, e.g. ethylene glycol. So much interests of nanofluids is due to their unusual thermophysical properties, completely different from the same materials in macro scale.
Nanofluids have a higher thermal conductivity, higher electrical conductivity, and absorption of light. Thanks to all of these properties, nanofluids have a huge potential in industry. The most frequently mentioned applications are all processes where there is a need to discharge large amounts of heat, for example: double pipe and multichannel heat exchanger [2–6], engine cooling, cooling of electronics, chillers, domestic refrigerator [7, 8], solar water heating [9–11], cooling, and heating in buildings .
Due to many of potential applications, nanofluids are extensively studied in many fields of science. The rheological properties of nanofluids were investigated by many researchers and were published a few review papers [13–15].
TiO2–EG nanofluids have been examined for rheological properties by Cabaleiro et al. , and they presented that this material exhibit a non-Newtonian shear-thinning nature, on the other hand Khedkar et al.  classified this material as Newtonian fluid. This shows that the physical properties of nanofluids require further experimental studies. Similar nature was presented by Tseng and Lin  for water-based TiO2 nanofluids. This material is extensively studied because of its interesting thermal properties [19–22]. Khedkar et. al.  presented results of experimental investigations and theoretical determination of thermal conductivity of TiO2–ethylene glycol nanofluids. They confirmed that thermal conductivity in these nanofluids increase linearly with volume concentration of nanoparticles.
Increase in thermal conductivity of TiO2 nanofluids (ethylene glycol water mixture) was also presented by Reddy and Rao .
Also, thermal properties of nanofluids were widely investigated both in experimental and theoretical field. Leong et al.  studied thermal properties of multiwalled carbon nanotube water based nanofluids. Ahammed et al.  investigated thermal conductivity of graphene–water nanofluid for various concentrations and temperatures. Palabiyik et al.  conducted experimental measurements of thermal conductivity of aluminium oxide (Al2O3) and titanium oxide (TiO2) propylene glycol-based nanofluids in temperature range form 293.15–353.15 K and reported non-linear increase of thermal conductivity with increasing in concentration. Murshed et al.  also studied TiO2 nanoparticle suspension in base fluid (water), and they also reported enhancement of thermal conductivity with increase in volume fraction of nanoparticles.
The electrical conductivity and dielectric properties are likewise investigated, but in comparison to the rheological or thermal properties, they are less popular among researchers. The electrical conductivity of silver and aluminium oxide nanofluids was studied by Maddah et al. . They reported significant increase in electrical conductivity as compared to base fluid which was water. Minea et al.  investigated electrical properties of aluminium oxides-water nanofluids too. They conducted measurements in temperature range from 298.15 to 343.15 K for various volume concentrations (1–4 %) and have found growth in electrical conductivity with increase in volume fraction and temperature. The maximum enhancement was achieved for 4 % volume concentration of nanoparticles in water at a temperature 333.15 K and it was 390.11 %.
The huge enhancement in electrical conductivity (almost 60000 % for 20 wt. % at 298.15 K) was observed for AlN nanoparticles suspended in ethylene glycol as reported in Ref. . Adio et al.  conducted research on electrical conductivity of MgO particles suspended in ethylene glycol. They reported enhancement in electrical conductivity more than 3000 %.
Angayarkanni and Philip  showed a 5218, 4366, 3142, and 8876 % increase in electrical conductivity, respectively, for α-Al2O3, SiO 2,γ-Al2O3, TiO2 water-based nanofluids. They explain such a large increase as an effect of double electric layer surrounding each particle and particle size effect. Nanosuspension of TiO2 particles in water-based fluid were also investigated by Sikdar et al. . As in previous cases, Sikdar et al. found an increase in electrical conductivity with increase in volume concentrations, but they also observed decrease in rate of increase with increasing in volume concentrations.
Dielectric properties of TiO2 nanofluids were studied by Subramaniyan et al.  for three TiO2 nanofluids with ethylene glycol, propylene glycol and water-based fluids for two frequencies (104 and 107 Hz) at 293.15 K. They reported increase in dielectric constant for all samples and the largest growth was observed for water-based nanofluid. These studies provide information in a narrow range of temperature and frequencies. To better understand phenomena occurring in nanofluids based on TiO2 nanoparticles, further experiments are needed in a wider range of temperatures and frequencies, and this is the main scope of this paper.
Nanoparticle preparation and characterization
Yellow powder of SC-TiO2 nanoparticles was synthesized using a solid phase method. The 10.4 g of metatitanic acid and 3.6 g of thiourea (provided by Wako Pure Chemical Industry) were triturated in an agate mortar to obtain a homogeneous mass, which was heated up at 773.15 K per 1 h. Based on our own experiments and published literature, thermolysis process takes place in the following steps: (1) endothermal process with a maximum at 408.15 K related to a dehydration methatitanic acid, (2) endothermic effects with a maximums of 448.15 and 468.15 K connected to the melting of thiourea, (3) exothermal process at 513.15 K linked to crystallisation of anatase with in presence of thiourea and simultaneous thermal degradation to the above described gaseous products, (4) exothermal processes with a maximums at 588.15 and 688.15 K showing the thermal oxidation of the organic residues, sulphur and continued crystallization of anatase. The loss of weight of thiourea and methatitanic acid on heating is completed at a temperature of 688.15 K, but optimum temperature for the sintering of sample is 773.15 K because in lower temperature, sulphur mixture could interact with anatase. The total weight loss in the temperature range 273.15–773.15 K is 9.4 wt.%.
The higher magnification imaging (Fig. 1 b) revealed that the observed crystal aggregates consist of many 15–40 nm sized SC-TiO2 nanoparticles which is almost 10 times smaller than in pure TiO2 powder.
The particle size and its distribution measured by a laser dynamic light scattering were much larger in comparison to obtained by microscopic methods. It happens because nanoparticles after doping by sulphur have a tendency to agglomerate into bigger aggregates, which are analyzed by DLS as complexes, not like a separate nanoparticles.
According to the XRD analysis performed to obtain phase composition, SC-TiO2 powder contains only one crystalline phase—the tetragonal modification of TiO2-anatase. In comparison to the pure TiO2 sample, a slight c lattice parameter and volume increase due to incorporation of additional elements. The phase identification of doped TiO2 was done by Siemens D5000 diffractometer in reflection mode with Cu target K α radiation. XRD patterns were recorded in the 2 θ range of [15; 100°] with a step size of 0.02° and a stay time of 1 s/step.
UV-VIS study performed by Evolution 300 UV-VIS spectrophotometer (Thermo Scientific) in the range from 300 to 800 nm suggests that SC-TiO2 nanoparticles got improved photocatalytic properties in comparison to pure TiO2. Recorded UV-VIS absorption spectra showed that the material is catalytically active in UV region and additional in some parts of VIS spectrum.
Raman spectroscopy was performed by Smart Raman DXR (Thermo Scientific) spectrometer. The semiconductor laser of 12 mW power and 780 nm wavelength was used as a light source. The obtained data confirmed the anatase crystal structure of doped TiO2 nanoparticles. The EPR analysis revealed that the sensitization process of the nanopowders doped by sulfur and carbon is a result of the formation of additional oxygen vacancies into TiO2 structure, which are essential for the photocatalytic activity of the material. According to XPS, carbon is in the form of elemental carbon compounds (Eb = 285.0 eV), and substances having a C-O bond (shoulder near 287 eV and a peak at 289 eV), which are adsorbed on the particle surface. Oxygen is included in the composition of oxides (Eb = 530.6 eV), and the carbon containing adsorbate (shoulder at 532 eV). The binding energy of titanium (459.0 ± 0.3 eV) and sulfur (169.0 ± 0.3 eV) core shell for S-TiO2 powders correspond to established data for TiO2 and MeSO4 compounds, respectively. In particular, sulphur is found exclusively in the oxidation state +6. The XPS results showed that sulphur and carbon are concentrated predominantly in the upper (periphery) layer of the nanoparticle. The quantification of the chemical elements detected by XPS analysis displays 2.8 % sulphur and 30.3 % carbon on the TiO2 nanoparticles surface, while the concentration of both elements drops to 0.17 % for S and for C detected in the particles volume. A detailed description of the preparation of nanoparticles is presented in Ref. .
The nanofluids used in experiment were prepared using a two-step method. First, an appropriate amount of nanoparticles and ethylene glycol has been weighed with the analytical balance WAS 220/X (Radwag, Radom, Poland). Then, samples were mechanically agitated for 30 min using Genius 3 Vortex (IKA, Staufen, Germany). The last step of preparation of samples was sonication in ultrasonic bath Emmi 60 HC (EMAG, Moerfelden-Walldorf, Germany) in order to break up the agglomerates remaining after the mechanical stirring. The time for sonication was 200 min. An ultrasound that we used has a power of about 350 W, and is equipped in six ultrawave generators with frequency about 45 kHz.
The dielectric properties of SC-TiO2-EG nanofluids were measured using BroadBand Dielectric Spectrometers Novocontrol Concept 80 System with temperature control system Quatro Cryosystem (NOVOCONTROL Technologies GmbH & Co. KG, Montabaur, Germany). This measuring stand allows to conduct measurements in broad range of frequencies and temperatures. Measurements were carried out in frequency range from 0.01 Hz to 10 MHz in 51 steps. Temperature was changed from 273.15 to 333.15 K with 5 K step, and stabilized at least 15 min before measurements. The samples were placed between two brass electrodes with diameter of 40 mm. The gap between electrodes was set by teflon fabric with 0.16 mm. The standard deviation of measurements has been determined on the basis of 10 measurements of dielectric constant of glycerine for 293.15, 298.15, 313.15, and 333.15 K. The results were compared with data published by Glycerine Producers’ Association . The maximum standard deviation of measurements was observed at 333.15 K and it was 6 %, and the minimum standard deviation of measurements was observed at 293.15 K and it was 5.6 %.
Results and discussion
DC conductivity and enhancement dc conductivity of SC-TiO2-EG nanofluids with various mass concentrations and temperatures
σ dc ×10−6[S/c m]
σ dc /σ 0[−]
The complex permittivity (ε ′,ε ′′), ac conductivity (σ ac ) and dc conductivity (σ dc ) in broad frequency range (0.01 Hz–10 MHz) and temperatures from 273.15 to 333.15 K for ethylene glycol with various mass concentrations of titanium oxide were investigated. Results indicate that frequency have strong influence on real part of complex permittivity in range below 10 kHz. Also, imaginary part of permittivity is strongly linked with the frequency in whole range. Based on the plateaus in ac conductivity, the dc conductivity was computed, and found almost 400 % increase in electrical conductivity for 20 wt.% at 278.15 K.
The authors wish to thank Dr. Andrzej Bak and Dr. Krystyna Chledowska for sharing the measuring station.
Reported research has been partially supported by project “Center for Innovation and Transfer of Natural Sciences and Engineering Knowledge” realized by the University of Rzeszów.
JF planned the experiments, performed the samples, conducted the studies, has made the processing and analysis of data, took an active part in the discussion of the results and preparation of the manuscript. AB, KB, YB, MC, IZ, KS, and JC performed and characterized the nanoparticles used in th experiment and took an active part in discussions of the results and preparation of the manuscript. GZ took an active part in the discussion of the results, preparation of the manuscript, and he coordinated the research. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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