PSI - Issue 14

Neelanchali Asija Bhalla et al. / Procedia Structural Integrity 14 (2019) 564–570 Neelanchali et al/ Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Fumed silica (FS) is a rheologically significant material. Its suspensions in low molecular weight oligomers are extensively used in electronic and photonic applications. Since FS particles possess high specific surface area, their low weight fraction dispersions exhibit shear thickening behavior thereby making them potential candidate for soft body armor applications. The suspensions and gels of FS are also being extensively used for polymer electrolytes in Lithium batteries as given by Raghavan et. al (1997). The FS suspensions in low molecular weight polar solvents are being used in microelectronics and photonics, Galindo et al. (2010). To understand the rheological behavior of fumed silica it is imperative to study its unique physical structure. Fumed silica is an amorphous nonporous silica prepared by the flame hydrolysis process, Sanchez (2006). Morphologically, it comprises of finely divided amorphous silicon dioxide particles containing different categories of sizes – primary spherical particles around 10-30 nm in diameter fused together in stable aggregates of approximately 100-250 nm. These aggregates finally build up to form large micron sized agglomerates called clusters, Nandi et. al (2013). Consequently, fumed silica exhibits high specific surface area (>100 m 2 /g). The surface of fumed silica is inherently hydrophilic due to the presence of hydroxyl groups. When FS is dispersed in a polar solvent, the surface hydroxyls form hydrogen bonds with the solvent molecules, thus resulting in a non-flocculated stable dispersion, as described by Raghavan (1997). There is substantial literature which explains the steady and dynamic shear rheology of FS dispersions (Raghavan 2000; Raghavan 1997; Park et.al. 2007; Fischer et. al. 2006; Zhang et. al. 2008; Kang et. al 2010; Khandavalli et. al 2014; Shan et.al 2015), but there is dearth of literature which explores the effect of high strain rate on the shear rheology of FS dispersions. In this study, an attempt is made to cover this aspect. Two grades of FS were chosen for the study – Aerosil A-130 and A-150 respectively, where the numerical figure depicts the specific surface area of fumed silica. The fumed silica suspensions were synthesized using ultrasonic homogenization technique. The low strain rate testing of the synthesized colloidal dispersions was accomplished on cone and plate rheometer, whereas the high strain rate study was conducted on custom-built Split Hopkinson Pressure Bar (SHPB) apparatus. 2. Materials and Methods 2.1. Materials Fumed silica was obtained from Evonik Industries. Material was in the form of white fluffy powder, having a tapped density of 50 g/L approx. and SiO 2 content ≥ 99.8 wt.%. The two grades of FS selected for the study were Aerosil A-130 and A-150, respectively. Material specifications are given in Table 1.

Table 1: Specifications of fumed silica Aerosil FS

Average primary particle size (nm)

Specific Surface Area, as per BET method (m 2 /g)

A-130 A-150

130 ± 25 150 ± 15

16 nm 14 nm

Poly Propylene Glycol (PPG) 400 was used as the dispersion medium for its good thermal stability and non toxicity. It was obtained from Qualikems Fine Chemicals. Ethyl alcohol was used as a solvent in ultrasonication process. It was procured from Sigma-Aldrich Chemicals. All the materials were used as received, without any further purification.