Effect of ?- Al2O3 nanoparticles on thermal conductivity of SiO2 based hybrid nanofluids

Development of nanosciences has allowed production of nanoparticles from various materials with different types. Nanoparticles feature greater surface to volume ratio and mobility, which gives them special abilities. According to the unique properties of nanoparticles such as electronical and optical, magnetic, mechanical and thermal, they can be used in wide variety of applications including drug delivery, food processing and packing, coatings, adsorption, photo-catalytic and heat transfer fluids.  Nanofluids are made by dispersing different nanoparticles, including elementary particles (such as Si, Cu, Al, Ag and Au), metal or non-metal oxides (ZnO, SiO2, MgO, Al2O3, TiO2, CuO) and organic materials (carbon nanotubes (CNT) and multi walled carbon nanotubes (MWCNT) and graphene) with nanometer size range, in a base fluid such as alcohol, distilled water, ethylene glycol (EG), engine oil and other conventional fluids [1]. Nanofluids are a new class of fluids engineered by dispersing nanometer-sized materials (nanoparticles, nanofibers, nanotubes, nanowires, nanorods, nanosheet, or droplets) in base fluids.In recent years, nanofluids have attracted more and more attention. The main driving force for nanofluids research lies in a wide range of applications [2]. The nano-materials attain ultra-high and fascinating features like viscosity, thermal conductivity and surface tension in contrast to the base materials. The inclusion of nanoparticles and base fluids improves the thermal capability of base liquids more precisely.  This substantial issue is solved by utilizing the nanoparticles as an energy source with immersion with base fluids [3]. Nanofluids have been used instead of conventional fluids to enhance the heat transfer rate or prevent overheating in heat exchange applications. Numerous investigators reported that conventional cooling fluids, such as water, oil, glycol and ethylene, which are commonly used in heat exchangers have low thermal conductivity compared to metallic and non-metallic nanoparticles [4]. With the prominence of the thermal conductivity advantages of nanofluids, nanofluids are widely used in automotive, electronics, solar energy, etc.Other than applications in conventional fields, nanofluids also have important applications in medicine. In the

medical field, it is used to transport drugs and destroy cancer cells. In the process of oil exploitation, the efficiency of oil exploitation can be improved. With wider applications, a number of factors have been found to affect the thermal conductivity of nanofluids, such as particle size and shape, particle concentration, particle aggregation, fluid stability, temperature and magnetic field [5]. Adding a small amount of solid nanoparticles to the base fluid increases the mass transfer coefficient. Some researchers believe that the mainly responsible mechanism in the improvement of mass transfer process, is Brownian motion of suspended nanoparticles which is similar to the heat transfer enhancement in nanofluid [6]. Nanofluid stability is the most important requirement to guarantee the conservation of enhanced thermophysical properties. Large surface area to volume ratio can result in aggregation and sedimentation of nanoparticles,which can compromise the target operation. To obtain a stable nanofluid, two methods are generally used: chemical methods and physical methods. The former includes the use of surfactants, surface modification and pH adjustment. The latter, instead, consist of ultrasonic agitation, homogenization and ball milling. To date, there is no standard method to analyze nanofluids stability. Many papers checked stability only through visual inspection, while a smaller number of studies also reported a zeta potential analysis. Zeta potential is defined as the potential difference between the nanoparticles surface and the base fluid stationary layer that is attached to nanoparticles. Nanofluids with high absolute zeta potential are electrically stable, while nanofluids with low absolute zeta potential are unstable [7]. All the steps of production, use and waste disposal of nanoparticles may lead to their release into water, soil and air. So, investigation of their uptake, bio accumulation, bio tranformation and the risks posed by nanomaterials is urgently needed. There is also growing need to develop technologies for soil protection and remediation. Phytoremediation techniques, are eco-friendly and less invasive, more cost effective and restorative compared to conventional methods. Aluminum oxide nanoparticles (Al₂O₃-NPs) are among the most widely used nanosized materials. Aluminum toxicity to plants is well known, while only a few studies have been conducted to investigate the phytotoxicity of Al₂O₃ nanoparticles [8].

METHODS AND MATERIAL

SiO₂ nanoparticles were produced by using sol-gel metod. Analytical grade Tetraethyl Ortho-silicate (TEOS, 99%), absolute ethanol (C₂H₅OH, 99.9%), and ammonium hydroxide (NH₄OH, 30%) were utilized without further purification.

A certain amount of Ethanol was taken in 20 ml of distilled water.The solution was stirred using a magnetic stirrer at 300 rpm for 10 minutes. Then Tetraethyl Orthosilicate (TEOS) was added drop wise for two hours.  Then Ammonium Hydroxide was added until the pH of 11 was achieved and the solution was then stirred for five hours.Post that the solution was centrifuged and subsequently washed with  ethanol thrice. Then it was placed in oven at 70^o C for 24 hours. The solid thus obtained was then calcinated at 500^oC for two hours. Post calcination the solid was crushed to obtain SiO₂ nanopowder. The XRD pattern of SiO₂ was obtained using X-ray diffractometer. TGA Analysis was carried out as well. The surface morphology of particles was measured by Transmission Electron Microscope (TEM: JEOL-2010) with an accelerating voltage of 100kV.

RESULT AND DISCUSSION

Fig 1 shows XRD pattern of synthesized nano-powder after calcination.

Fig 1 XRD pattern of SiO₂ nanoparticles

Fig 2. TEM images of nanoparticles post calcination.

Fig 3. TGA Analysis of SiO₂ nanoparticles

Fig 4 - Variation of Thermal conductivity with different sizes of α- Al₂O₃ nanoparticles

Fig shows variation of zeta potential with addition of α- Al₂O_3.  SiO₂ 1,2 and 3 depict SiO2 mixed with the respective 3 sizes of α- Al₂O₃