Thermophysical and electrical properties of the MO based NF
2.2 Experimental process
2.2.1 Characterization of NPs and MO
As the primary particles size of the nanopowder are approximately in the form of large spherical agglomerates so the surface modifications of the NPs are very much necessary to get enhanced dispersion behavior in the base fluid. The specification of the h-BN NP is presented in Table. 2.1. The surface of h-BN particle is modified into 2D nanosheets by exfoliation process [23, 24]. In this process, the pure 5gm h-BN powder is mixed with isopropyl alcohol (IPA) of 300 ml and then extensively sonicated for 3 hours using the probe sonicator. To maintain the solution at room temperature, ice cubes are kept around the beaker so to avoid rise in temperature of the semi prepared sample. The solution is then centrifuged at 1500 RPM for 30 minutes after sonication, and the collected sample is vacuum filtered and dried to get the Eh-BN particles. The IPA is observed to be a better polar solvent to peel off the pure h-BN powder [24]. The process flow diagram is shown in Figure 2.1.
Figure 2.1: Exfoliation process of the h-BN power.
Table 2.1: Specifications of h-BN NP and MO
Characteristics Specification
Purity 98 %
Distribution size of NPs 1 μm
Density 2.29 g/cm³
Dielectric Constant 3-4
Thermal Conductivity 300 W/m-K at 25oC
Electrical Resistivity 1015 Ω-cm
Thermal expansion coefficient 4×10-6/oC Chemical composition of MO CnH2n+2
Paraffinic structure MO Straight and branched chain
The pure and Eh-BN powders are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The XRD pattern of a material is like a unique identification of that material. The powder diffraction method is thus suitable for characterizing and identifying the polycrystalline phases. In this study, XRD apparatus (Model-Rigaku, TTRAX III diffractometer, supply: 55 kV, 250 mA) is used for confirming the reduction of the size of pure h-BN by using the Scherrer equation. It is confirmed from Figure. 2.2 that the size of Eh-BN particle is decreased. The size of the particle calculated from the Scherrer equation (2.1) is as follows:
(2.1) Where λ (=1.5406Å) is the wavelength of the X-rays, LC is the average thickness of one multilayer, β is the full width at half maximum (FWHM) of the peak in radians and θ is the
0.89
c
cos
L
Bragg angle. As the FWHM value of Eh-BN is 0.0034 radian which is more than that of h-BN powder i.e. 0.0028, which suggests that the size of bulk h-BN is reduced after exfoliation. The peaks observed for Eh-BN is matching with JCPDS data (JCPDS No: 34-0421) [23].
Figure 2.2: XRD pattern of (a) bulk h-BN, (b) exfoliated h-BN powders, and (c) is the image of Eh-BN powder.
It is observed from the studies that the Eh-BN powder show the maximum exposure at (002) planes [25-27]. FESEM image of bulk h-BN and Eh-BN are shown in Figure. 2.3 (a) and (b), where the particle size is reduced from 1 μm to the range of 150-300 nm due to an exfoliation process. This reduction of particle size is also supported by the results obtained from the XRD pattern in Figure. 2.2. The average size of the Eh-BN powder is observed to be 50-100 nm as per TEM analysis. It is observed from Figure. 2.4 (b) that around 10 to 12 layers of h-BN nanosheets are present in Eh-BN. Only 5gm of Eh-BN nanosheets are obtained that for the 8 gm of pure h-BN powder. It confirms that the size of the particles is reduced from 1 μm to 200-300 nm as shown in in Figure. 2.4. Enhancement of surface area to volume ratio occurs is because of exfoliation of the h-BN-NPs.
(a) (b)
Figure 2.3: FESEM images of (a) procured pure h-BN from Sigma-Aldrich, of average particle size of 1μm and (b) Exfoliated h-BN nanosheets average particle size of 300nm.
(a) (b)
Figure 2.4: TEM images of (a) a pure h-BN of single particle size of 0.2 μm and (b) an exfoliated h-BN nanosheets of average of 10 to 12 2D sheets having average diameter of 50 nm.
2.2.2 Preparation of NF
The NF samples are prepared by taking MO as a base fluid with the dispersion of Eh-BN NPs. Bulk h-BN powder of size 1 µm procured from Sigma-Aldrich is exfoliated into 2-D nanosheets for the enhancement of its aspect ratio and high surface area for better heat transfer capabilities. The exfoliation process of 1μm size h-BN (shown in Figure 2.1) has been carried
out and the hexagonal structure of h-BN particle is modified to 2D nanosheets with the particles size of 150-300 nm.
Figure. 2.5: preparation of NF as per two step method.
The selection of the NP concertation is based on the investigation of the dispersion stability of the NF. Figure. 2.5 shows the two-step method to prepare the Eh-BN/MO NFs at five different nanofiller concentrations from 0.01-0.1wt.%. For fully dispersed NF, the mixture NP and MO are sonicated for an hour using probe Sonicator. The dispersed NF are kept in a shaking incubator at a temperature of 60o C and 120 RPM to avoid moisture and sedimentation of the NP in the liquid. Again the prepared NF is kept in the vacuum oven for one day at room temperature and high pressure for further moisture evaporation. The moisture measurement for the NF is performed using Karl Fischer titration (KFT) method. Upon confirming the moisture limit of the prepared samples meeting the specified limit of insulating oil standard, further experimental analysis has been performed.