Investigation of the stability of nanofluids based on water and carbon nanoparticles synthesized by the electric arc method

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Introduction
The possibility of application of nanofluids as effective heat carriers is a hot topic for numerous studies around the world.Nanofluids are a base liquid with solid nanoparticles evenly distributed in it.The addition of nanoparticles to liquid heat carriers significantly changes their thermophysical properties, which can significantly increase their efficiency [1][2][3].
In the rapidly developing field of solar energy nanofluids can be used as efficient heat carriers and absorbers of solar heat instead of traditional heat carriers to increase the efficiency of various types of solar energy systems [4][5][6][7][8].The practical application of such coolants is limited by the problem of obtaining stable nanofluids.In particular, carbon nanoparticles have a hydrophobic surface, and in order to obtain a stable water-based nanofluid, various methods are applied to change the surface properties of the nanoparticles.For carbon nanoparticles, the most commonly used method is to add a surfactant to the nanofluid, which forms an additional layer with a hydrophilic surface on the particle surface [9,10].Various methods of functionalization and modification of the surface of carbon nanoparticles, which are more often used for carbon nanotubes, are also widespread [11,12].

Synthesis of nanoparticles
In this study, nanofluids were obtained with a two-stage method.At the first stage, the plasma-chemical synthesis of nanomaterials was carried out, as a result of which nanoparticles of carbon globules and graphene flakes were obtained.The electric arc reactor for synthesis consisted of a hermetically sealed vacuum chamber containing two electrodes.Helium was used as a buffer gas in the reactor chamber.For the synthesis of carbon globules, the helium pressure was 100 Torr.The helium pressure for the synthesis of graphene flakes was 12 Torr.The electrodes were connected to a direct current source.The cathode, made of graphite, had the shape of a cylinder with a diameter of 20 mm and a length of 12 mm.The anode was a cylindrical rod 80 mm long and 8 mm in diameter.For the synthesis of carbon globules, a pure graphite anode was used; for the synthesis of graphene flakes, a composite anode filled with a powder with a mixture of silicon and graphite with a mass ratio of 1:2 was used.The current strength in the experiments was 100 A. An electric arc discharge was ignited between the electrodes.The voltage between the electrodes was maintained at a constant value of 23 V.After the synthesis, the obtained materials were collected from the surface of the water-cooled screen.The scheme and principle of operation of the E3S Web of Conferences 459, 08005 (2023) https://doi.org/10.1051/e3sconf/202345908005XXXIX Siberian Thermophysical Seminar electric arc reactor for the synthesis of nanomaterials used in this work are described in more detail in [13].
The resulting nanomaterials were characterized with Raman spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM).Raman spectroscopy was performed using a LabRAM HR Evolution spectrometer (Horiba-JobinYvon, Germany), which uses radiation with a wavelength of 512 nm.SEM was carried out using an SN-3400N (Hitachi, Japan) and JSM 6700F (JEOL, Japan) microscope.TEM was performed using a LIBRA 120 microscope (Carl Zeiss, Germany).

Stability of nanofluids
The second stage of obtaining nanofluids includes the stabilization of nanoparticles in the base liquid by adding a surfactant and processing the sample in ultrasound.Ultrapure water obtained using a Millipore Direct Q 3 UV water purification system was used as the base fluid in this work.The anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant neonol AF 9-12 were used as stabilizers.Nanofluid samples were processed in an ultrasonic bath with controlled heating Stegler 6DT with an ultrasonic disperser frequency of 40 kHz and a power of 180 W. The treatment was carried out for two hours at a temperature of 50 °C.
The stability of nanofluids was determined by the change in the spectra of radiation passing through the sample with time using an SF-2000 spectrophotometer.The spectral range of the SF-2000 spectrophotometer is 190 -1000 nm, the resolution limit is 1 nm.The measurements were carried out in quartz cuvettes for the spectrophotometer.
As a parameter characterizing the stability of the nanofluid, we chose the change in the transmittance of the sample with time.The transmittance was determined by the ratio of the intensity of radiation transmitted through the nanofluid under study to the intensity of radiation transmitted through the base liquid.The transmittance was quantitatively estimated by integrating the transmittance spectrum.For a stable nanofluid based on carbon nanoparticles, the intensity of light transmitted through the sample is zero in the entire spectrum range, and the transmittance is correspondingly zero.When the processes of aggregation and sedimentation occur in the nanofluid, precipitation is observed in the sample and the intensity of the light transmitted through the sample increases.Thus, the value of the transmittance increases, which indicates a stability violation of the nanofluid.The stability of the sample can be monitored by repeating the measurements over time.

Electric arc synthesis of nanoparticles
The presence of silicon in the composition of the anode leads to the formation of the structure of graphene flakes [ ].
TEM and SEM studies showed that the materials obtained with spraying a graphite anode were aggregation of chains of globular particles (Fig. 1, 2).The Raman spectrum of carbon globules is shown in Fig. 4. The spectrum contains intense peaks D and G, and sights in the region of the 2D peak can also be seen.The G peak refers to bond vibrations of sp2 hybridized carbon atoms, and the D peak refers to radial vibrations of hexagonal rings in the graphite structure, which appears only at the edges and defects of the graphite structure.The 2D peak, which is the second mode of the D peak, is associated with the graphene structure and may appear in low dimensional carbon structures where at the particle edges the carbon structure may exhibit some graphene properties.On the Raman spectrum of the material containing graphene flakes, in addition to the carbon D and G peaks, there is also an intense 2D peak, which confirms the presence of a graphene structure in the material.As shown with TEM studies of materials containing graphene, almost the entire material consists of crumpled graphene flakes, on the surface of which nanoparticles of silicon carbide SiC are located (Fig. 5).Based on SEM images (Fig. 6), the dimensions of graphene flakes were measured and a distribution histogram was constructed, shown in Fig. 7. Taking into account that the structure of graphene flakes can be turned towards the electron beam of the microscope, the largest size of the particle image was chosen to measure the particle size of graphene flakes.The average flake size in the material was determined to be 50 nm.According to the results of previous works [15], the number of layers in the flake structure can be from 1 to 7.

Stability of nanofluids
To obtain stable nanofluids, it is necessary to minimize the processes of aggregation and sedimentation of nanoparticles.The primary sign of a stability violation of the nanofluids is the precipitation of particles.However, it should be noted that the precipitation of an insignificant precipitate is acceptable, since a small amount of sufficiently large particles is always present in the powder of nanoparticles, which is also typical for the materials synthesized in this study (Fig. 3, 7).Also, the stability violation of the nanofluids is evidenced by the stratification of the sample or the inhomogeneous distribution of particles due to the formation of various aggregative structures in it, which can be determined with optical methods.

Nanofluids with the addition of SDS
For each type of carbon nanoparticles, nanofluids were obtained with mass concentrations: 0.02 %; 0.1 %; 0.2 %.The mass concentration of SDS in each sample was 1 %.The study of the transmission spectra of the samples was carried out for a month.The presence of sediment and stratification was visually controlled for all nanofluids at each stage of the stability analysis.
For nanofluids based on carbon globules, the absorption spectra are shown in Figures 8, 9, and 10.The absorption spectra for samples with concentrations of 0.1 % and 0.2 % did not change during the month; for the sample with a concentration of 0.02 %, an increase was registered after a month intensity in the region of near infrared radiation up to 1.5 %.For nanofluids based on graphene nanoparticles, the absorption spectra are shown in Figures 11, 12, and 13.The absorption spectra for these samples was not changing during the month.The presence of sediment of nanoparticles at the bottom of the samples was also monitored for a month.In nanofluids with concentrations of 0.2 % and 0.1 %, a precipitate appeared within a month.In nanofluids with a concentration of 0.02 %, precipitate had not been observed.
The appearance of a precipitate in nanofluids with concentrations of 0.1 % and 0.2 % within a month, with no changes in the transmission spectra, indicates an excessive concentration of nanoparticles.Increasing the addition of SDS for these nanofluids in order to avoid the appearance of a precipitate is not advisable, since the solubility limit (critical micelle concentration) will be exceeded, which for sodium dodecyl sulfate is ~ 1 g per 100 ml [16].For a concentration of 0.02 %, a stable nanofluid based on grapheme flakes was obtained.For nanofluids based on water and carbon globules with a concentration of 0.02 %, a stable nanofluid for 1 month was obtained.

Nanofluids with the addition of neonol AF 9-12
Two nanofluids were also obtained based on water and carbon globules with a concentration of 0.02 %, and based on water and graphene flakes with a concentration of 0.02 %.Neonol AF 9-12 at a concentration of 2 % was added as a surfactant to both samples.The presence of sediment and stratification was visually controlled for nanofluids at each stage of the stability analysis.
In nanofluid based on carbon black globules, sample delamination was observed on the second day.The nanofluid based on graphene flakes remained stable for a month; the transmission spectra are shown in Fig. 14  For particles of different sizes and shapes at the same mass concentration, the effective surface area differs significantly, therefore, in order to obtain a stable nanofluid, the surfactant concentration for each type of particles should be also differ.For this reason, at the same concentrations of carbon nanoparticles and neonol AF 9-12, a stable nanofluid was obtained only on the basis of graphene nanoparticles, and the surfactant concentration for carbon globules turned out to be insufficient.

Conclusions
As a result of arc discharge synthesis with spraying electrodes with various compositions in a helium medium, the materials containing carbon globules and graphene flakes were obtained, and their structures were studied.
The study was made of the influence of the concentration of carbon nanoparticles and the type of surfactants on the stability of nanofluids.
For carbon globules and water, the mass concentrations of nanoparticles and sodium dodecyl sulfate were determined to obtain a nanofluid based on them stable for 1 month, which are 0.02 % and 1 %, respectively.It was shown that the use of neonol AF 9-12 at a concentration of 2 % did not lead to the stabilization of carbon globules with a mass concentration of 0.02 % in water.
For graphene flakes, the mass concentrations of nanoparticles and stabilizers to obtain a water-based nanofluid stable for 1 month are: 0.02 % graphene flakes and 1 % SDS and 0.02 % graphene flakes and 2 % neonol AF 9-12, respectively.

Fig. 3
Fig. 3 shows a histogram of the size distribution of carbon globules.The distribution histogram is approximated by a lognormal distribution.The average size of carbon globules is 14 nm.

Fig. 8 .
Fig. 8.Light transmission spectra for a nanofluid based on water, 0.2 % carbon globules and 1 % SDS.For comparison, the transmission spectrum for water is shown.

Fig. 9 .
Fig. 9. Light transmission spectra for a nanofluid based on water, 0.1 % carbon globules and 1 % SDS.For comparison, the transmission spectrum for water is shown.

Fig. 11 .
Fig. 11.Light transmission spectra for a nanofluid based on water, 0.2 % graphene flakes and 1 % SDS.For comparison, the transmission spectrum for water is shown.

Fig. 12 .
Fig. 12.Light transmission spectra for a nanofluid based on water, 0.1 % graphene flakes and 1 % SDS.For comparison, the transmission spectrum for water is shown.

Fig. 13 .
Fig. 13.Light transmission spectra for a nanofluid based on water, 0.02 % graphene flakes and 1 % SDS.For comparison, the transmission spectrum for water is shown.
. During the month, an insignificant amount of sediment fell out in the sample compared to the initial sample.The formation of a precipitate in the absence of a change in the transmission spectra may indicate an excessive concentration of nanoparticles.