Thermal Conductivity Performance of 2D h-BN/MoS2/-Hybrid Nanostructures Used on Natural and Synthetic Esters

In this paper, the thermal conductivity behavior of synthetic and natural esters reinforced with 2D nanostructures—single hexagonal boron nitride (h-BN), single molybdenum disulfide (MoS2), and hybrid h-BN/MOS2—were studied and compared to each other. As a basis for the synthesis of nanofluids, three biodegradable insulating lubricants were used: FR3TM and VG-100 were used as natural esters and MIDEL 7131 as a synthetic ester. Two-dimensional nanosheets of h-BN, MoS2, and their hybrid nanofillers (50/50 ratio percent) were incorporated into matrix lubricants without surfactants or additives. Nanofluids were prepared at 0.01, 0.05, 0.10, 0.15, and 0.25 weight percent of filler fraction. The experimental results revealed improvements in thermal conductivity in the range of 20–32% at 323 K with the addition of 2D nanostructures, and a synergistic behavior was observed for the hybrid h-BN/MoS2 nanostructures.


Introduction
For over a century, fossil fuels have been the major contributor to the energy sector. Challenges to mitigate pollution, global warming, and more will be critical. This, together with the imminent scarcity of oil reserves, increases in oil prices, rises in lubricant disposal costs, as examples, will promote the development of alternative sources of energy as well as novel technologies to attend the current and future world energy needs with sustainability and efficiency [1][2][3].
Petroleum-based fluids and lubricants are applied as lubricants and dielectric and coolant material in electrical or electronic devices such as power systems (transformers), machinery, and the automotive applied boron nitride (BN) to a vegetable oil and found that the thermal conductivity was increased in comparison with the pure oil. The use of BN allowed the lubricant to be less viscous at higher temperatures which further enhanced the temperature dissipation. Salehirad and Nikje [5] reported similar results applying h-BN to a mineral oil: increased thermal conductivity, lower viscosity, and better insulation capability when compared to the pure mineral oil. Li et al. [46] developed ethylene glycol/boron nitride nanofluids, varying the size of the nanoparticles. It was demonstrated that nanofluids containing larger nanoparticles have higher improvements in thermal conductivity than smaller nanoparticles. Ilhan et al. [47] reported an enhancement of thermal conductivity for h-BN nanofluids of up to 26%, 22% and 16% at 3 vol.% for water, water/ethylene glycol mixture, and ethylene-glycol-based nanofluids, respectively.
Molybdenum disulfide (MoS 2 ) involves very optimal thermal and chemical stabilities. It demonstrates applications as a catalyst and lubricant due to its unique properties like anisotropy, photocorrosion resistance and chemical inertness [48,49]. MoS 2 nanostructures have good thermo-physical features and great anti-friction properties, making it another promising material as reinforcement for the cooling purposes of nanofluids. Diverse factors such as the composition and loading of nanoparticles, suspension stability, base fluid composition, nanofluids preparation method, surface modifier, and surfactant application may influence the thermo-physical properties of nanofluids, including viscosity and thermal transport [50][51][52][53][54][55]. Su et al. [52] investigated stability and thermal transport behavior of MoS 2, water-based, and oil-based nanofluids at various concentrations (0.01 wt.% to 0.5 wt.%). It was observed that by increasing the filler fraction, the thermal conductivity increased. Furthermore, the thermal conductivity improvement in MoS 2 and water-based nanofluids was higher than for oil-based nanofluids, which according to Su et al. was due to a better dispersion and suspension stability achieved on water-based fluid. Zeng et al. found that thermal conductivity of dibenzyl-toluene-based MoS 2 nanofluids increased from 17.5 to 37.5% as the temperature increased from 40 to 180 • C [53].
In this work, the effect of incorporation and homogeneous dispersion of single h-BN, MoS 2 , and hybrid h-BN/MoS 2 mixture at various concentrations (by weight) within synthetic and natural esters, aims to improve their thermal conductivity performance, which is determined at various evaluating temperatures (up to 323 K). The contributions of this work rely on the aspect of biodegradable materials suitable for industrial applications, such as energy and metal-mechanic field. The nanofluids with only h-BN or MoS 2 nanofiller components were compared with their hybrid h-BN/MoS 2 combination.

Materials and Methods
In our study, synthetic esters Midel 7131 (M&I Materials-Manchester, UK) and natural esters-Envirotemp ® FR3™ (Cargill Industrial Specialties-Minneapolis, MN, USA) and VG-100 (Prolec GE International, Apodaca, México) [56] (Table 1)-were used as base materials to develop different nanofluids with h-BN, MoS 2 , and h-BN/MoS 2 nanostructures (Table 2) at various concentrations of 0.01, 0.05, 0.10, 0.15, and 0.25 wt.%. The nanofluids with the hybrid composition h-BN/MoS 2 were used at a mass ratio of 1:1 between h-BN and MoS 2 . Nanofillers were obtained by wet exfoliation in tetrahydrofuran (THF), using probe ultrasound for 5 h at an amplitude of 35%. The exfoliated material was filtered and dried in a vacuum oven for 24 h at 80 • C. To prepare the nanofluids, a two-step method to homogeneously disperse the 2D nanostructures within the esters was used. Glass containers were prepared with each set of fluids at different filler fractions: 0.01, 0.05, 0.10, 0.15, and 0.25 wt.% of the nanostructures for each set of fluid. Extended water bath sonication (~4-5 h) was used (Branson ultrasonic homogenizer model 5510-Danbury, CT, USA, 40 kHz). To avoid the nanostructures' agglomeration and quick sedimentation, the temperature of the water bath was maintained constant at room temperature (24 • C). Samples were maintained on a drawer for at least 2 weeks without significant sedimentation. Experimental evaluations were performed after 3 days of sample preparation. This process assists in obtaining the stable homogeneous nanofluids, which are further evaluated. Nanomaterials' morphology and thickness were analyzed by atomic force microscopy (AFM) using an Asylum Research MFP-3D-AS (Belo Horizonte, MG, Brazil), operated in contact mode. An Olympus AC240TS probe (Belo Horizonte, MG, Brazil) and a 70 kHz resonant frequency were used. Specimens were prepared by ultrasonically dispersing the 2D nanostructures in isopropanol (IPA) and water mixture. This was followed by the deposition of dispersion drops on mica, which was then dried. A Quanta™ 200 FEG-FEI microscope (Belo Horizonte, MG, Brazil), operated under a vacuum with a 30.0 kV accelerating voltage, was used to obtain SEM images. The 2D nanostructures were prepared by dispersing the material in IPA/water mixture using an ultrasonic water bath (2 h); subsequently, the sample was cast onto a silicon substrate and evaporated in air at room temperature. The chemical composition of the 2D nanostructures was investigated (EDS) using a FEI QUANTA™ 200 microscope (Belo Horizonte, MG, Brazil). To evaluate the homogeneity of the mixture between them, EDS mapping was performed.
SEM was also performed to characterize the microstructure of the h-BN/MoS 2 mixture. Figure 1a-d shows SEM images of h-BN, MoS 2 , and h-BN/MoS 2 nanosheets with different morphologies and sizes. Figure 1a shows a typical SEM image of a large-size h-BN sample, which consists of thin and crumpled sheets with a lateral dimension of about~5-20 µm. Figure  Raman spectra of the h-BN nanostructures were obtained by a Raman Confocal Spectrometer WITec, model Alpha 300R (Sao Paulo, SP, Brazil), using a laser (532 nm) as the excitation source. The MoS 2 nanosheets on the Raman spectrum showed the main characteristic bands of this nanostructure ( Figure 3a). The first one, in 382 cm −1 , is attributed to E 2g vibrational mode, and the second (in 410 cm −1 ) is related to A 1g mode. On the other hand, these modes correspond to the in-plane vibrations of sulfur atoms in one direction and molybdenum atoms in another one, and to out-of-plane vibrations (A 1g ) of sulfur atoms. For the sample containing h-BN, a spectrum with only one intense band in 1372 cm −1 , in relation to the E 2g vibrational mode, was observed. In relation to the mixture of these nanofillers, the same three bands from MoS 2 and h-BN nanosheets were presented, without another additional band or significative shifting from original positions related from the original nanomaterials. [37,63].
1a-d shows SEM images of h-BN, MoS2, and h-BN/MoS2 nanosheets with different morphologies and sizes. Figure 1a shows a typical SEM image of a large-size h-BN sample, which consists of thin and crumpled sheets with a lateral dimension of about ~5-20 μm. Figure 1b shows the morphology of MoS2. Figure 1c,d shows a typical SEM image of the h-BN/MoS2 mixture, which keeps its features between its single components. Finally, Figure 1e-f presents the AFM image of h-BN/MoS2 and its corresponding height profile (thickness approximately 2.3 nm).  Raman spectra of the h-BN nanostructures were obtained by a Raman Confocal Spectrometer WITec, model Alpha 300R (Sao Paulo, SP, Brazil), using a laser (532 nm) as the excitation source. The MoS2 nanosheets on the Raman spectrum showed the main characteristic bands of this nanostructure (Figure 3a). The first one, in 382 cm −1 , is attributed to E2g vibrational mode, and the second (in 410 cm −1 ) is related to A1g mode. On the other hand, these modes correspond to the in-plane vibrations of sulfur atoms in one direction and molybdenum atoms in another one, and to out-of-plane vibrations (A1g) of sulfur atoms. For the sample containing h-BN, a spectrum with only one intense band in 1372 cm −1 , in relation to the E2g vibrational mode, was observed. In relation to the mixture of these nanofillers, the same three bands from MoS2 and h-BN nanosheets were presented, without another additional band or significative shifting from original positions related from the original nanomaterials. [37,63].  (002) and (100) planes (Figure 3b) [38]. The mixture presented the same peaks related to both nanofillers, without additional peaks or significative shifting from the original MoS2 or h-BN diffraction spectra.  The h-BN/MoS2 mixture can be also confirmed by EDS maps obtained from the SEM image (Figure 2a), which displays the homogeneity of the mixture of h-BN and MoS2. Elements B, N, Mo, and S are uniformly distributed in the whole area according to Figure 2b-e.
Raman spectra of the h-BN nanostructures were obtained by a Raman Confocal Spectrometer WITec, model Alpha 300R (Sao Paulo, SP, Brazil), using a laser (532 nm) as the excitation source. The MoS2 nanosheets on the Raman spectrum showed the main characteristic bands of this nanostructure (Figure 3a). The first one, in 382 cm −1 , is attributed to E2g vibrational mode, and the second (in 410 cm −1 ) is related to A1g mode. On the other hand, these modes correspond to the in-plane vibrations of sulfur atoms in one direction and molybdenum atoms in another one, and to out-of-plane vibrations (A1g) of sulfur atoms. For the sample containing h-BN, a spectrum with only one intense band in 1372 cm −1 , in relation to the E2g vibrational mode, was observed. In relation to the mixture of these nanofillers, the same three bands from MoS2 and h-BN nanosheets were presented, without another additional band or significative shifting from original positions related from the original nanomaterials. [37,63]. X-ray diffraction (XRD) evaluation was performed using a Shimadzu XRD-7000 system (Sao Paulo, SP, Brazil) employing Cu Kα radiation (λ = 1.5418 Å). The XRD patterns were obtained in the 2θ range between 4° and 80° at a scanning rate of 4° min −1 . These results were found to be fully matched with a JCPDS 37-1492 card. The XRD spectrum of MoS2 presented diffraction peaks in 2θ equal to 14.4°, 32.8°, 39.6°, 49.8°, and 60.3° related to (002), (100), (103), (105), and (110) planes, respectively. The diffractogram of h-BN depicted peaks in 2θ equal to 26.7° and 41.7° from the (002) and (100) planes (Figure 3b) [38]. The mixture presented the same peaks related to both nanofillers, without additional peaks or significative shifting from the original MoS2 or h-BN diffraction spectra.   (Figure 3b) [38]. The mixture presented the same peaks related to both nanofillers, without additional peaks or significative shifting from the original MoS 2 or h-BN diffraction spectra.
Thermal conductivity measurements were carried out on nanofluids at various nanostructure concentrations according to the transient hot-wire (THW) technique, with a KD2 Pro device. A temperature-dependence scan was also performed, maintaining a thermal equilibrium for at least 10 min before each set of evaluations. A minimum of 7 measurements were taken for each set of experiments to report average values with standard deviation as error bars.

Results
Improvements in thermal conductivity in synthetic and natural ester systems have been obtained for different types of nanofillers. Figure 4 depicts the temperature-dependent thermal conductivity performance of Midel 7131 nanofluids with h-BN, MoS 2 , and h-BN/MoS 2 at various filler fractions. The synthetic ester did not show significant temperature dependency (less than 1% at 50 • C): actually, a decrement in thermal conductivity was observed as temperature was increased, similar to other authors' findings [64,65]. In general, the thermal conductivity of the evaluated nanofluids gradually increased for all the fillers and concentrations studied as temperature was also increased. Thermal conductivity measurements were carried out on nanofluids at various nanostructure concentrations according to the transient hot-wire (THW) technique, with a KD2 Pro device. A temperature-dependence scan was also performed, maintaining a thermal equilibrium for at least 10 min before each set of evaluations. A minimum of 7 measurements were taken for each set of experiments to report average values with standard deviation as error bars.

Results
Improvements in thermal conductivity in synthetic and natural ester systems have been obtained for different types of nanofillers. Figure 4 depicts the temperature-dependent thermal conductivity performance of Midel 7131 nanofluids with h-BN, MoS2, and h-BN/MoS2 at various filler fractions. The synthetic ester did not show significant temperature dependency (less than 1% at 50 °C): actually, a decrement in thermal conductivity was observed as temperature was increased, similar to other authors' findings [64,65]. In general, the thermal conductivity of the evaluated nanofluids gradually increased for all the fillers and concentrations studied as temperature was also increased. Figure 4a depicts the effect of h-BN on the thermal conductivity of Midel 7131. As filler fraction and temperature is increased, thermal conductivity also increases. For instance, at 323 K, improvements of 12, 14, 15 and 19% are obtained at 0.01, 0.05, 0.10, and 0.15 wt.%, respectively. A maximum enhancement of 24.2% in thermal conductivity was observed at 0.25 wt.% at 323 K. A similar improvement trend was observed for the nanofluids with MoS2 addition (Figure 4b

Conclusions
Thermal transport phenomena in nanofluids was influenced by physical and chemical characteristics of the base fluid, as well as interactions with the reinforcement nanostructures. Environmentally-friendly nanofluids based on single h-BN, single MoS2, and hybrid h-BN/MoS2 nanostructures were developed at various filler fractions, and temperature-dependence evaluations were performed for thermal conductivity. In general, all nanofluids showed a temperaturedependence in thermal conductivity performance, indicating the role of the interaction of The addition of MoS 2 within natural esters showed an improvement trend similar to h-BN (Figures 5b and 6b). In this case, thermal conductivity enhancement of 10,14,16,19 and 21% at 0.01, 0.05, 0.10, 0.15, and 0.25 wt.% was observed for FR3 fluid (Figure 5b). Moreover, VG-100 nanofluids showed a slightly higher enhancement, reaching a maximum of 23% at 0.25 wt.% at 323 K (Figure 6b).
The incorporation of 2D nanostructures significantly improves the performance of the nanofluids. h-BN contributes to enhance thermal conductivity and MoS 2 also contributes to the fluid's reinforcement and may help to obtain an homogenous dispersion of the nanofillers due to its well-known lubricant properties [66,67]. It is suggested that due to the low filler fraction used, the observed enhancement on thermal conductivity is due to molecular interactions (collisions) between the fluid and the 2D nanostructures [7,14,37,53,68,69]. However, observing thermal conductivity behavior during the temperature-dependent evaluations indicates that this increase is based on the percolation mechanism as well as the contribution of the Brownian motion of the sheet-like nanostructures [70][71][72]. As the concentration of 2D nanostructures is increased in the synthetic or natural esters, the chance of phonons to get scattered in the contiguous nanostructures increases proportionally, which leads to enhanced contact conductance [73]. Consequently, thermal conduction channels are formed, which may increase the thermal conductivity due to the percolation mechanism.
The heat transfer between colliding nanostructures may increase the thermal conductivity of the nanofluids. Particularly, a higher temperature corresponds to a more intense Brownian motion [38]. Thus, at 323 K, the thermal conductivity of nanofluids is more apparent than at the other evaluating temperatures. Moreover, liquid layering at the nanostructures/fluid can also contribute to the improvement of thermal conductivity behavior [74][75][76].
In the hybrid nanofluids, it was observed that at room temperature, the effect of coupling h-BN and MoS 2 was similar as a single component reinforcement, either h-BN or MoS 2 . Moreover, as evaluating temperature increased, the value of thermal conductivity was higher than the single component nanofluids [77,78]. h-BN contributed to the increase of the thermal conductivity as well as MoS 2 , promoting the dispersion of the nanostructures due to their lubricant properties [38,79].

Conclusions
Thermal transport phenomena in nanofluids was influenced by physical and chemical characteristics of the base fluid, as well as interactions with the reinforcement nanostructures. Environmentally-friendly nanofluids based on single h-BN, single MoS 2 , and hybrid h-BN/MoS 2 nanostructures were developed at various filler fractions, and temperature-dependence evaluations were performed for thermal conductivity. In general, all nanofluids showed a temperature-dependence in thermal conductivity performance, indicating the role of the interaction of nanostructures among synthetic and natural esters. The addition and homogeneous dispersion of these 2D nanomaterials within conventional esters showed significant positive results on the effective thermal conductivity performance. Thermal conductivity improved in the range of 21-23% for single reinforcement of h-BN or MoS 2 on natural esters and 23-27% for synthetic esters at 323 K, respectively. Furthermore, for the hybrid h-BN/MoS 2 nanofluids, an improvement of 30-32% was observed for natural esters and 32% for synthetic esters at 323 K, respectively. Hence, the hybrid h-BN/MoS 2 nanostructures dispersed in conventional esters induced a high thermal conductivity, suggesting that the hybrid h-BN/MoS 2 have highly desirable multifunctional features for advanced materials applications.
This research showed the potential of an interesting synergistic effect resulting from the contribution of two 2D nanostructures for synthetic and natural esters. Increased environmental awareness is the main driving force for the development of novel technologies, such as the use of biodegradable fluids and lubricants in environmentally-sensitive areas, which have great potential to succeed in industrial applications.