Fabrication, Characterization and Thermo- physical Property Evaluation of SiC Nanofluids for Heat Transfer Applications

Nanofluids (NFs) are nanotechnology-based colloidal suspensions fabricated by suspending nanoparticles (NPs) in a base liquid. These fluids have shown potential to improve the heat transfer prop- erties of conventional heat transfer fluids. In this study we report in detail on fabrication, characterization and thermo-physical property evaluation of SiC NFs, prepared using SiC NPs with different crystal structures, for heat transfer applications. For this purpose, a series of SiC NFs containing SiC NPs with different crys- tal structure (�-SiC and �-SiC) were fabricated in a water (W)/ethylene glycol (EG) mixture (50/50 wt% ratio). Physicochemical properties of NPs/NFs were characterized by using various techniques, such as pow- der X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fouriertransform infrared spectroscopy (FTIR), dynamic light scattering (DLS) and Zeta potential analysis. Thermo-physical properties including thermal conductivity (TC) and viscosity for NFs containing SiC particles (�- and�- phase) weremeasured. The results show among all suspensions NFs fabricated with�-SiC particles have more favorable thermo-physical properties compared to the NFs fabricated with�-SiC.The observed dif- ference is attributed to combination of several factors, including crystal structure (�- vs. �-), sample purity, and residual chemicals exhibited on SiCNFs. A TC enhancement of �20% while 14% increased viscosity were obtained for NFs containing 9 wt% of particular type of�-SiC NPs indicating promising capability of this kind of NFs for further heat transfer characteristics investigation.


Introduction
Heat transfer fluids are involved in many industrial processes to remove excess heat. Conventional heat transfer fluids such as water (W), ethylene glycol (EG) and their mixture (W/EG) have shown poor thermal conductivity (TC) characteristics. On the other hand, developing effective cooling methods for high-tech ap-plications such as transportation, microelectronics, cutting fluid, energy supply, manufacturing and biomedical applications have been prioritized to accelerate further development of these kinds of technologies [1][2][3][4][5][6]. Hence, conventional working fluids for cooling are being replaced with newer fluids with enhanced TC than the pure and conventional liquids. Pioneer investigations of Choi [7], Lee and Choi [8] and Masuda et al. [9] reported that the effective TC of suspensions containing suspended nanoparticles (NPs) can be higher than those of normally used industrial heat transfer liquids. Such kind of new fluids, called as nanofluids (NFs), is a new class of dispersions containing nano-sized solid particles (NPs) dispersed in conventional heat transfer fluids as base liquid [7]; and are considered to be new enhanced heat transfer fluids, since they might offer appealing possibilities due to their improved heat transfer performance [10][11][12][13][14]. Consequently, there is a demand for effective and novel heat transfer fluids to solve these challenges. During last decade, there has been a rapid development in NFs preparation methods. There are two major techniques, which are usually utilized for making NFs: one-step method and two-step method. In the one-step method [10,15], NPs are directly prepared in the base liquid, thus forming the NFs in a combined process. Usually, physical vapor deposition (PVD) technique [10], solution chemical method [15] or microwave-assisted route [16] is used for one-step preparation of NFs. There are some advantages in this preparation method, including minimum agglomeration and better stability. In the two-step fabrication method [17][18], NPs are first synthesized/acquired, followed by dispersing them into the base liquid (such as W or mixture of W and EG) by techniques including high shear [19][20] and ultrasound [21]. Currently, availability of NPs from commercial sources and different suppliers has attractedresearchers to this method, since they can have several choices for a certain nanoparticle, which allow effective and comparative research.
To date, several kinds of solid particles/NPs and base liquids have been utilized in order to fabricate NFs for efficient heat transfer applications: NPs of metals such as Ag [22], Cu [23] and Au [24], oxides such as Fe 3 O 4 , mesoporous SiO 2 , CuO and Al 2 O 3 [25][26][27][28] and carbon based materials such as CNTs [29] are some examples in this regard. Among various materials used for preparing NFs, SiC is one of the promising materials to fabricate efficient NFs for cooling applications. It has one Although few studies have been performed on the use of SiC NF for heat transfer applications, there is no comparative study onthe effect of SiC crystal structure on the thermo-physical properties of NFs containing α-SiC and β-SiC NPs. Therefore, the objective of this work is to fabricate SiC NFs with different SiCcrystalline phases in order to investigate comparatively physicochemical and thermo-physical properties including TC, and viscosity of SiC NFs in W/EG as the base liquid.

Materials and methods
Silicon Carbide (SiC) particles with different crystal structures of alpha type (α-SiC) and beta type (β-SiC)were purchased from various suppliers/institutes including Superior Graphite (USA), PlasmaChem GmbH (Germany) and ENEA (Italy). Ethylene glycol (EG) and sodium hydroxide (NaOH) were acquired from Sigma Aldrich and Merck KGaA (Germany). All the reagents were used as received without further purification. The materials are abbreviated for ease as shown/listed in Table 2.
Fabrication of α-SiC NF and β-SiC NF via twostep method A series of NFs were fabricated by dispersing a known weight of α-SiC and β-SiC NPs in W/EG (50:50) base liquid, via two-step method. The suspension was sonicated for 15 min and the pH value was adjusted to ∼9.5 using NaOH in order to obtain stable NFs containing 9 wt% α-SiC and β-SiC NPs. All NFs (Table  3) were stable without any visual precipitation within a week. In order to study the real effect of α-SiC and β-SiC NPs on the thermo-physical properties of NFs, NPs were used as received and the use of surfactant/surface modifiers was avoided. Fabricated NFs were evaluated for TC and viscosity properties. Formulating NFs for obtaining optimum thermo-physical properties such as optimal TC enhancement, minimum impact on viscosity and longer stability is always desired. In this case, several factors such as selection of optimum particle loading play essential role. In this study, the optimum particle loading (9 wt%) was selected after an extensive test campaign of NFs with various NP concentration, where optimizing basis was maximizing the TC enhancement of NFs with minimal impact of NPs addition on viscosity. Moreover, by doing the literature survey about the relevant research (Table 1) concentration of 9 wt% was selected in order to achieve NFs with optimal properties. Therefore, all NFs were made with NPs loading of 9 wt%. As Table 1 shows, at higher  concentrations, although NFs with higher TC was achieved, very high viscosity values were observed for NFs, which is unfavorable for their use in practical heat transfer applications.

Characterization techniques
Microstructure and morphology of α-SiC and β-SiC particles were evaluated by using scanning electron microscopy (SEM; FEG-HR Zeiss-Ultra 55). Transmission electron microscopy (TEM) analysis of the particles was performed using JEOL 2100 at 200 kV acceleration voltage. Nicolet Avatar IR 360 spectrophotometer, in the range of 500-4000 cm −1 was used for Fourier transform infrared spectroscopy (FTIR) analysis of solid particles and solid/liquid samples. Powder X-ray diffraction (XRD) was performed on a Philips X'pert pro super diffractometer with Cu Kα source (λ=1.5418Å). Zeta potential analysis of α-SiC and β-SiC particles was performed for evaluating NFs stability region. Average solvodynamic particle size distribution of β-SiC particles was evaluated by Beckmann-Coulter DelsaNano C system. TC of NFs was measured by using TPS 2500 instrument (HotDisk model 2500), which works based on the Transient Plane Source (TPS) method. The validity of the TPS instrument was checked by comparing with a standard source for thermodynamic properties of water (IAPWS reference) [36]. Compared to the reference the accuracy of measurement for distilled water was within 2% [36]. Finally, viscosity of NFs was evaluated using a DV-II+ Pro-Brookfield viscometer.

XRD analysis
There are about 250 crystalline forms for Silicon carbide material [37]. Figure 1(a) displays β-SiC, which is formed at temperatures below 1700 • C, has a cubic crystal structure [38], while as Fig. 1(b) shows the schematics of α-SiC, which is formed at temperatures higher than 1700 • C, has a hexagonal crystal structure. In order to identify the crystal structure of SiC NPs, XRD analysis was performed. Figure 1

Morphology analysis
SEM micrographs of α-SiC and β-SiC NPs are presented in Fig. 2(a)-(d). Figure 2(a) and 2(c) represent the spherical morphology of β-SiCwith estimated primary particle size of (60±10) nm and (30±10) nm for β1-SiC and β2-SiC, respectivelywhile Fig. 2(b) and 2(d) present the SEM micrographs of α1-SiC and α2-SiC, respectively. Since there is a very wide size dispersion for both α1 and α2-SiC NPs, it is rather difficult to perform size estimation from the micrographs; however, a rough estimate (by counting > 200 particles) showed the primary particle size of (115±35) nm and (85±20) nm for α1-SiC and α2-SiC, respectively. SiC NPs with α-type structure shows larger particle size than β-type SiC NPs, which may influence the thermophysical properties of the resultant NFs. 20 Fig. 1 The unit cell (crystal structure) and XRD pattern of (a, c) β-SiC and (b, d) α-SiC particles, respectively.  show the TEM micrographs of β-SiC and α-SiC NPs, respectively. Near spherical morphology for β-SiC NPs was observed ( Fig. 3(a) and 3(c)) while hexagonal morphology for α-SiC particles were clearly visible ( Fig. 3(b) and 3(d)). The morphology of α-SiC particles may be dominated by the anisotropic crystal structure, allowing the crystal to grow in certain directions more than the other directions (a.k.a. Crystal habit). Compared to the SEM micrographs presented in Fig. 2(a) and 2(c), β-SiC NPsin TEM mi-crographs highly agglomerated but much smaller particles were observed. Selected area electron diffraction (SAED) pattern, shown in inset images in Fig. 3(c) and 3(d), were indexed for cubic structure β-SiC (ICDD No: 01-074-2307) and hexagonal crystal structure α-SiC (ICDD No: 01-073-1663), respectively. These electron diffraction results reveal purity and good crystallinity of β-SiC NPs and α-SiC particles. Moreover, there is good match between SAED pattern and XRD analysis.

Dynamic light scattering (DLS) analysis
DLS analysis was performed to estimate the dispersed size of SiC NPs in liquid media, in order to understand the influence of effective size of dispersed NPs in the liquid media. DLS analysis results are shown in Fig. 4(a) and 4(b) for both α and β-SiC in pure water and W/EG media. Figure 4(a) displays DLS results for α1-SiC and α2-SiC in pure water and W/EG mixture as base liquids. We included pure water in our DLS analysis, since it is also one of the most commonly used base liquids. A wide range of particle size distribution (150-4500 nm) with an average peak value of ∼1290 nm was obtained for α1-SiC particles in water media. For α2-SiC in water media, a much wider range of size between 30-10000 nm, with an average peak value of ∼1800 nm has been obtained, indicating strong aggregation of particles in water base liquid. When it comes to W/EG media, both in case of α1and α2-SiC, smaller average sizes with narrower size distribution was observed when compared to the water media, indicating W/EG can affect significantly the dispersion property of SiC particles. EG may act as dispersant and covalently bond onto the particles surface, which allow stabilization of smaller aggregates/agglomerates as compared to pure water media. In comparison, α2-SiC NPs showed larger dispersed particle size than that of α1-SiC. The average dispersed size of β-SiC NPs in both liquids are presented in Fig. 4(b). When β-SiC NPs were dispersed in pure water it is seen that, the size of β1-SiC NPs is in the range of ∼100-800 nm with an average diameter of 340 nm while the size estimation for β2-SiC in water media showed the range of 130-450 nm with an average hydrodynamic diameter of ∼260 nm. A narrow NPs size distribution with smaller average hydrodynamic size was obtained for β2-SiC NPs. In W/EG media, the size of β1-SiC NPs is in the range of 100-450 nm with a peak average of 245 nm while those range and average number are 100-200 nm and 145 nm for β2-SiC NPs. A narrower NPs size distribution with a smaller average dispersed size is estimated for β2-SiC NPs. In the same way, as observed for α-SiC NPs, the presence of EG reduces the aggregate/agglomerate size in the suspensions. Looking at the all results presented in Fig. 4(a) and 4(b), β-SiC NPs have smaller dispersed size than α-SiC NPs, both the NPs exhibit smaller average solvodynamic size in W/EG base liquid compared to the pure water media, indicating a better dispersability in W/EG media. A comparison between SEM, TEM and DLS results for β-SiC NPs display that for all the α and β-SiC NPs the primary particle size obtained from SEM and TEM micrographs is less than the predicted sizes from DLS method. This difference may be due to the aggregation/agglomeration of α-SiC and β-SiC NPs in the base liquid media, in addition to the adsorbed liquid layer on particles' surface.

FTIR analysis
In order to study the surface characteristics of particles, FTIR analysis was carried out. FTIR spectra for all "as-received" SiC particles are presented in Fig. 5. The absorption band between 860 cm −1 and 760 cm −1 is attributed to the Si-C bond. For all the SiC particles, the absorption between 1100 cm −1 and 1000 cm −1 are assigned to Si-O-Si or Si-O-C vibrations, respectively. For α1-SiC particles theband at 1200 cm −1 is attributed to the Si-C. α2-SiC particles show absorption bands at 1180 cm −1 and 1380 cm −1 , which can be attributed to the Si-C and amorphous carbon, respectively. Moreover, in both α1-SiC and α2-SiC particles, there are two bands between 1525 and 1620 cm −1 , attributed to the C= =O groups, which may be due to the un-reacted precursor or residual chemicals due to the method used for their fabrication. By looking at β1-SiC, in addition to Si-C bond Si-O-Si at 780 and 1050 cm −1 , respectively. There are two more bands at 1200 and 1315 cm −1 . The first band is assigned to Si-C and the second one to amorphous carbon, respectively. As Fig. 5 shows, compared to other samples, β1-SiC and α2-SiC (both are from the same supplier) present shoulder around 800 cm −1 , which may be attributed to the presence of O at the interface due to the synthesis procedure of high-temperature reduction of SiO 2 , based on information provided by the supplier [39].

Zeta potential analysis
Characterizing NPs dispersions and understanding the role of various parameters, which may affect colloidal properties, are important for any NFs. In the literature, very limited investigations can be found on NFs that report on the effect of pH, particle surface chemistry, primary particle size and crystal phase on properties such as Isoelectric Point [40][41][42][43][44][45][46]. The pH of a suspension has important role not only on the rheological property of suspension but also in terms of fabrication of stable suspension, which is related to the electrostatic charge on particles'. It is well known that for fabricating stable NFs, the pH value of the NF must be far from the Isoelectric Point (IEP), where the overall charge on the particles becomes zero. If the pH gets close to the IEP the particles tend to agglomerate and finally precipitate, since there are not enough repulsive forces between them. When the pH is set far from the IEP the absolute electrical charge on particles is increased, which cause repulsive interaction between particles due the collision. In order to identify the optimum pH values for stable NFs formulation, Zeta potential analyses were performed for both the α-SiC and β-SiC particles in the pH region from 2 to 10. The results are presented in Fig. 6 where the IEP obtained for the α-SiC and β-SiC particles show great variations, one very important common point of exhibiting highly negatively charged particles in the pH region of ∼9.5 and 10. Therefore, the pH of NFs was adjusted at 9.5 to obtain stable suspensions. Figure 6 indicates that β-SiC NPs has higher IEP values compared to the α-SiC. The α1-SiC and α2-SiC particles have very close IEP values in the pH range of 2-3, while β-SiC NPs display similar IEP values in the pH range of 5-6. The possible reasons for this observation may be due to different synthesis methods of SiC particles, and impurities present, resulting in different surface chemistry of SiC particle with αand βtype. As FT-IR analy-sis revealed (Fig. 5), different SiC NPs exhibited different surface chemistry. By doing a comparison between FT-IR test results and Zeta potential analysis (Fig. 6), one can see that having lower IEP for α2-SiC than α1-SiC may be due to higher silica content (more intensive Si-O-Si bands in FTIR) of α2-SiC, which shifts the IEP to more acidic pH region (IEP of SiO 2 is about 2) [47]. The β2-SiC NPs exhibit slightly lower IEP than β1-SiC NPs. Although β2-SiC showed higher Si-O-Si bands in FTIR, it exhibited lower IEP due to probably the high content of Si (assessed from XRD). There are few studies about the NPs' size effect on suspension properties, particularly IEP. The size of particles may not only affect the IEP of the suspension but also the surface reactivity, adsorption affinity and photocatalytic activity of NPs [45][46][48][49]. As NPs size decreases, the percentage of surface atom/molecule increases extensively. Particle electronic structure, surface defect density and surface sorption sites can be also changed [50][51]. Therefore, both surface reactivity and NPs IEP can become dependent on particle size. There is no report in literature about the impact of SiC NPs crystal on suspension properties such as IEP. Although there is no systematic study about size effect on SiC suspension properties yet, there are several reports on TiO 2 suspensions and the influence of materials crystal structure on suspension properties [42,46]. Kosmulski [42] reported on the effect of crystal phase of TiO 2 on IEP and showed that the IEP is rather independent to the crystal structure (rutile vs anatase). The same observation was also repeated by Suttiponparnit et al. [46] when their experimental efforts confirmed that the IEP of TiO 2 NPs was insensitive for the crystal structure of TiO 2 .
Suttiponparnit et al. [46] studied the role of primary NP size on TiO 2 dispersions with different crystal structures (anatase and rutile) and showed that when primary particle size increased, the IEP decreased. In our work, with a focus on size effect on SiC NPs, it can be seen that at the same particle loading of SiC with α type when the primary NP size increased the IEP also increased (primary size has not been calculated for α-SiC as the particles are non-spherical, though the magnitude of BET surface area is used for size comparison; large BET surface area revealing smaller primary particles -c/o Table 1). The same point is valid for β-SiC, for which an increase of IEP was observed with increasing primary NP size. A two-three orders of magnitude difference between the IEP values of α-SiC and β-SiC NPs may also be attributed to be sensitive to the SiC NPs crystal structure/phase. Our results on IEP of SiC NFs showed that IEP may be affected by the primary size of dispersed particles, impurities involved, and different surface chemistry (due to the fabrication method of particles) of NPs.

TC and viscosity measurements of NFs
Thermo-physical properties of NFs including TC and viscosity were performed for the all fabricated NFs at 20 • C. In order to evaluate the TC of NFs, 9 wt% NFs with α-SiC and β-SiC and W/EG as base liquid were prepared and TPS method was used to test the TC of NFs. The TC evaluation results, measured at 20 • C, are listed in Table 4, showing higher TC values (K nf ) of all SiC NFs than the base liquid W/EG. Moreover, Table  4 shows that TC of NF with α type SiC are higher than that of β types. The reason is not clear, however, several factors may play the role for facing this difference. It may be due to the dissimilarcrystal structureof SiC NPs which also have different magnitude of TC (360 W/m·K for β-and 490 W/m·K for αtype) [30]. This point indicates the possibility of influence of SiC NPs crystal phase on TC of NFs.The size of NPs may also have a role. The TC results clearly displayed that for all type of SiC NPs the TC values increase with increasing NP size, which is compatible with the literature [52].
Even sample impurityand different surface chemistry of SiC NPs, as observed from XRD and FTIR analysis,may also result in different TC for NFs. Table 4 also shows that the α1-SiC NF exhibited higher TC values than α2-SiC NF. As a result, NF with α1-SiC showed the highest TC values, while NF containing β2-SiC revealed the minimum TC value which may be due to the all above mentioned reasons such as having largest and smallest particle size for α1-SiC NPs and β2-SiC NPs, respectively. Table 4 lists the relative TC, defined as the ratio of TC of SiC NFs (K nf ) over the TC of base liquid (K bl ). The maximum TC enhancement of 20% was obtained for α1-SiC NF at 20 • C. Timofeeva et al. [35] presented several results for α-SiC NFs with W/EG base liquids (Table 1). In the best case they reported 17% TC enhancement for the particles with average size of 90 nm. A comparison between the TC enhancements of α1-SiC NF-W/EG (present study) at 20 • C and that reported by Timofeeva et al. [35], where they used commercial NFs, shows that NFs for the present work has higher TC even at ∼4 wt% lower particle loading, indicating higher effective thermal performance of NF presented in this work.
It is important to clarify that having greater relative TC value only is not enough for utilizing a NF as effective coolant. In order to choose the efficient NF with optimum characteristics for heat transfer applications, not only TC but also viscosity must be evaluated. The internal resistance of a fluid to flow is described by viscosity [53], which plays an important role in all thermal applications involving flowing fluids [54]. This property is expected to be higher when compared to their base liquids, but this increase makes a negative impact on the pumping power and heat transfer coefficient. These parameters are very essential in practical heat transfer applications. For instance, in laminar flow, the pressure drop is directly proportional to the viscosity. Moreover, Prandtl and Reynolds numbers are influenced by viscosity of fluids and heat transfer coefficient is a function of these numbers. Therefore, viscosity is as important as TC in engineering systems involving fluid flow [55]. This is an important criterion for the use of this type of NFs in convective heat transfer applications. The viscosity tests were carried out at 20 • C for all NFs with W/EG as the base liquid,where all the samples exhibited Newtonian behavior. The results are listed in Table 4, which show that all NF with αtype SiCparticles have lower viscosity values than NF containing βtype SiCparticles. Three possible reasons can be enlisted for the observed difference as the effect of the crystal structure or difference in primary particle size(BET surface area) for α-SiC NF and β-SiC NPs and even to the level of impurities as observed from XRD and FTIR analysis. The smaller surface area of α-SiC particles ( Table  2) results in a smaller contact (solid-liquid interface) area between α-SiC particles and W/EG base liquid and therefore exhibit smaller viscosity value than β-SiC at the same particle loading. The increased viscosity values for α1-SiC and α2-SiC particles are very close as the surface area for both particles are almost the same. Secondly, for the same reason α2-SiC NF exhibited the lowest and β2-SiC NFs the highest viscosity values at 20 • C. The relative viscosity, which is defined viscosity of NF (μ nf ) to the viscosity of base liquid (μ bl ), are listed in Table 4. The minimum increase of ∼12% in viscosity was achieved for SiC NF with 9 wt% α2-SiC particle loading, which has small surface area (larger average particle size), while the maximum increase of ∼60% in viscosity was obtained for SiC NF with β2-SiC at the same particle loading. NFs containing β2-SiC exhibit higher increase in viscosity compared to the β1-SiC NF at the same NPs loading may be due to its smaller size, and impurity content, which provides a larger surface area per unit volume. A direct comparison between increase of viscosity in this work and the reported values in the literature (Table 1) is not possible because of having different factors such as particle loadings, particle size and dissimilar temperature. Selecting NFs with similar particle loadings, Won Lee et al. [32] reported 7.2% TC enhancement while increase in viscosity was 102% for a water based β-SiC NF with ∼9.5 wt% particle loading indicating that although it exhibit nearly the same relative TC compared to the worst case in this study (β2-SiC NF with 9 wt% NPs loading), shows 42% more increase in viscosity. Our findings show favorable thermo-physical properties (higher TC enhancement with lower increase in viscosity) compared to all series of water based α-SiC NFs reported by Timofeeva et al. [34] (Table 3). They have utilized commercial water based NFs, prepared by dispersing α-SiC NPs, and reported 7-12.5% TC enhancement with 17.5-60% increase in viscosity of the NFs containing 13 wt% of SiC NPs with different sizes. Timofeeva et al. [35] also reported on α-SiC NFs with W/EG showing 17% increase in viscosity at 13 wt% SiC NPs loading ( Table 1). The observed differences between this work and Timofeeva et al [34,35] may be due to the different surface chemistry of α-SiC particles (IEP= 4 [34]), type of the base liquid, or additives, besides different NPs loading. The increased viscosity will result in use of a greater pumping power, which might counterweigh the benefits of higher TC enhancement values. The tradeoff between thermo-physical properties including increases in viscosity values and relative TC is very essential in order to utilize the NFs for heat transfer applications. Figure 7 summarizes the comparison between TC enhancement and viscosity increase, for all fabricated NFs, where both α-SiC NFs show that the TC increase is higher than the viscosity increase at 20 • C, while reverse behavior is observed for β-SiC NFs. This may imply that α-SiC NFs formulations can be proper candidates as efficient NFs for convective heat transfer ap-plications. Moreover, a comparison between α1-SiC NF and α2-SiC NF shows that although α1-SiC NF has 2% higher viscosity increase, it exhibits 2% greater TC enhancement value indicating a promising capability of this NF as efficient coolant in heat transfer applications. Among all the fabricated NFs in this work, NFs fabricated by α1-SiC particle is the optimum choice, which is therefore selected for further thermal characteristics investigations, and results are to be reported elsewhere.

Conclusions
We presented on the fabrication and evaluation of highly stable SiCNFs for heat transfer applications. The NFs were prepared by dispersing SiCparticles with different crystal structure in W/EG base liquid, using colloidal stabilization strategies. A detailed physico-chemical evaluation showed different characteristics of SiC NPs, including crystal structure, primary/dispersed particle size, surface functionality, surface charge, and purity levels of SiCNPs used. Interpretation of the results from different analytical techniques showed that IEP of the SiC NPs, and the viscosity of NFs, may be affected by the primary size, surface chemistry of NPs (due to the different NPs fabrication routes), as they may also be dependent on SiC crystal structure (α-vs. βtype). Thermo-physical properties, TC and viscosity, of SiC NFs were performed at 9 wt% NPs loading at 20 • C. TC enhancement of the NF, over the base liquid, due to the presence of SiC particles are observed for NFs containing both αand β-type SiC-NPs. W/EG based NFs with α-SiC exhibited higher TC than that with β-SiC, which may be attributed to the effect of the crystal structure (as α-type has higher TC value), or the phase purity of the as-received materials. Among all fabricated W/EG based NFs containing SiCwith different crystal structure (α-vs. βtype), α1-SiC NF displayed the highest TC enhancement of 20%,while only 14% increase in viscosity,revealing its promising characteristics for heat transfer applications.