Effect of Particle Shape, Silica Coating and Laser Irradiation on Thermal Conductivity of Some Metal Oxide Based Nanofluids

Abstract

This thesis represents an effect of particle shape, silica coating and laser irradiation on thermal conductivity of some metal oxide nanoparticles in basefluids. Heat conduction of the metal oxide nanoparticles varied as a function of geometric morphology and surface area of elongated nanoparticles. Dispersion stability of the nanoparticles in basefluids is a major problem in nanofluids; therefore, nanoparticles have been coated with a thin layer of SiO2 for improved dispersion stability and thermal conductivity. Metal oxide (TiO2, CuO and WO3) nanoparticles in basefluids have also been irradiated by UV-Vis light LASER irradiation and investigated their effect on thermal conductivity. The whole work is divided into five chapters which are described below. Chapter 1: Introduction and Literature: The first chapter provides brief introduction about thermal conductivity, nanofluids, metal oxide’s size and shape, core/shell structure and importance of silica coating and LASER irradiation. Literature review, research gap, objectives, experimental section and characterization techniques are also incorporated in this chapter. Chapter 2: Section A: Shape dependent thermal conductivity of TiO2-ethylene glycol and de-ionized water based suspension: This section demonstrates the importance of various shapes and crystal phases of TiO2 nanostructures such as TiO2 P-25 (70:30 anatase and rutile), as-prepared nanorods (pure anatase) and sodium titanate nanotubes (orthorhombic Na2Ti2O5.H2O crystal) on the thermal conductivity of ethylene glycol and de-ionized water. It was observed that TiO2 nanorods (L×W = 81-134 nm × 8-13 nm and surface area = 79 m2g-1) always showed higher thermal conductivity than porous nanotubes (L×W = 85-115 nm × 9-12 nm and surface area = 176 m2g-1) and commercial TiO2 P-25 (30-55 nm surface area = 56 m2g-1),) which was explained by their differences in crystallinity, crystal phases, compactness, surface exposed atoms, surface area and much greater mean free path of longitudinal phonon vibrations along its lateral dimensions. The subsequent effect of sonication time from 5-10 h results into the breakdown of TiO2 nanorods cluster (42 to 28 nm) with the instantaneous increase in negative zeta potential values from -31 to -45 mV, respectively, which seems to be an additional cause for the enhancement in its thermal conductivity. xxiii Section B: SiO2@TiO2 nanocomposites for enhanced thermal conductivity and dispersion stability in de-ionized water: This section presents the synthesis of bare and SiO2 coated TiO2 nanoparticles and investigate their effect on thermal conductivity of de-ionized water; as prepared nanoparticles were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy etc. These nanoparticles were dispersed in de-ionized water at various volume fractions (0.01%) and their thermophysical (density, thermal conductivity, refractive index etc) properties were studied. The experimental results showed that a thin layer of SiO2 coating (3-6 nm) over TiO2 nanostructures exhibit superior dispersion (0.5 vol%) stability as evident by steady zeta potential (-30 ↔ -36 mV), no significant change in particle-size (95↔133 nm) distribution, density (1.001↔0.998 g/cm3) and refractive index (1.336↔1.333) etc. Thin Si-OH layer over surface imparts superior hydrophilicity, larger surface area for effective solute-solvent (SiO2@TiO2-H2O) interaction for improved colloidal stability. Thereby, thermal conductivity is found to be quite stable (0.625↔0.614 W/m.K) up to 2-3 months, whereas aqueous suspension of bare TiO2 particles quickly settles down. Depending on the thickness of SiO2 layer and volume fraction of SiO2@TiO2, a maximum of 8-10% increment of thermal conductivity was achieved at 0.01 vol.%. Chapter 3: Section A: Anisotropic CuO nanostructures of different size and shape exhibit thermal conductivity superior than typical bulk powder: This work demonstrates the preparation of monoclinic crystalline CuO nanospheres (5-10 nm), nanorods (L × W = 100-140 nm × 30-40 nm) and nanowires (200-210 nm × 2-5 nm) for the study of thermal conductivity when dispersed in de-ionized water and ethylene glycol (0.005-0.1 vol.%). It has been observed that CuO nanorods and nanowires having surface area 53 and 61 m2/g, respectively, always displayed higher thermal conductivity than CuO nanospheres possessing lower surface area (41 m2/g) which attributed to the differences in their per-particle surface area, percentage of surface exposed atoms, anisotropic lengthy shape and large phonon-mean-free paths. The experimental results revealed higher thermal conductivity than obtained from theoretical models due to particle shape effect as expected from Hamilton-Crosser equation. It has also been found that density is directly proportional to thermal conductivity and increases with the increase in volume fraction. The decrease in aggregated particle size (130–104 nm) and an increase in zeta potential xxiv value (−32 to −37 mV) of CuO nanospheres cause more stability of CuO dispersion with 3-6 h of sonication. Section B: SiO2@CuO nanocomposites for enhanced thermal conductivity and dispersion stability in de-ionized water: This research demonstrates the synthesis of bare and SiO2 coated CuO nanoparticles and investigate their effect on thermal conductivity of de-ionized water. The samples were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy etc. The nanoparticles were dispersed (0.01 vol.%) in de-ionized water and sonicated for half an hour before the measurement of thermophysical properties. The experimental results showed that a thin layer of SiO2 coating (2-6 nm) over CuO nanoparticles display superior dispersion stability as marked by steady zeta potential (-31 ↔ -40 mV) and density (1.003↔1.002 g/cm3). Thin Si-OH layer over surface imparts superior hydrophilicity, larger surface area for effective solute-solvent (SiO2@CuOH2O) interaction for improved colloidal stability. Thereby, thermal conductivity is found to be quite stable (0.625↔0.614 W/m.K) up to 2-3 months, whereas aqueous suspension of bare CuO nanoparticles quickly settles down. Depending on the thickness of SiO2 layer and concentration of SiO2@CuO, a maximum of 8-10% enhancement in thermal conductivity was achieved. Chapter 4: Section A: WO3 nanostructures of different size and shape for improved dispersion stability and thermal conductivity in aqueous suspension: This work presents the preparation of different anisotropic (cubic, spherical and rod shaped) nanoparticles of WO3 (monoclinic and hexagonal crystal structure) and studied their relative thermal conductivity in de-ionized water and ethylene glycol. Experimental results showed that thermal conductivity increases (7-12%) with the increase in volume fraction (0.01-1%) and density. WO3 nanorods having surface area 61 m2g-1 showed higher (10-12%) increment in thermal conductivity than WO3 anisotropic nanoparticles (6-8%) possessing lower surface area 41 m2g-1 which attributes to the differences in their surface exposed atoms, long phonon mean free path and lengthy shape factor etc. The results also showed that choice of stabilizer for better thermal conductivity and dispersion stability depend upon the nature of stabilizers and nanoparticle’s interaction with stabilizer. Sodium dodecyl sulfate was found to be best stabilizer for WO3-de-ionized suspension as compared to other stabilizers (CTAB, Triton-x-100, PVP, PVA and Oleic acid). The dispersion behavior of WO3-de-ionized water suspension was also investigated under different xxv pH values (2-12). The point of zero charge (1.5-2) of WO3-de-ionized water suspension was identified in terms of its colloidal stability. Section B: A thin layer of SiO2 coating for highly improved dispersion stability and thermal conductivity of WO3-H2O suspension: Long term dispersion stability for an improved thermal conductivity is a challenging issue that needs to be solved for heat transfer applications. Hence, this research investigated that a thin layer of SiO2 coating (2-5 nm) over WO3 nanostructures (SiO2@WO3) of different shapes exhibited superior dispersion (0.01%) stability for longer duration as evident by steady zeta potential (-30 ↔ -60.70 mV), no significant change in particle-size (139↔147 nm) distribution, density (1.001↔0.988 g/cm3) and refractive index (1.335↔1.332) etc. Thin Si-OH layer over WO3 surface imparts superior hydrophilicity, larger surface area for effective solute-solvent (SiO2@WO3-H2O) interaction for improved colloidal stability showing no sedimentation and color change of SiO2@WO3 dispersion (0.01%) even after 3 days due to repulsive interaction between negatively charged Si-O- particles. Thereby, thermal conductivity is found to be quite stable (0.631↔0.618 W/m.K) up to 3 days, whereas aqueous suspension of bare WO3 particles quickly settle down and thermal conductivity rapidly decreased to a value of 0.584 W/m.K. Depending on the thickness of SiO2 layer and volume fraction of SiO2@WO3, a maximum of 8-10% increment of thermal conductivity was achieved where anisotropic WO3 displayed always higher enhancement in (~5%) thermal conductivity than typical spherical nanoparticles. Chapter 5 Phase-dependent Thermophysical Properties of α-and γ-Al2O3 in Aqueous Suspension: This study demonstrates the thermal conductivity and viscosity of as prepared crystalline α-Al2O3 and amorphous γ-Al2O3 nanoparticles, having size in the range of 30-50 nm. The α-Al2O3 and γ-Al2O3 aqueous suspension exhibited ~10% and 6% enhancement in thermal conductivity of de-ionized water, but α-Al2O3 showed (~4-6%) higher thermal conductivity than γ-Al2O3 aqueous suspension. This is ascribed to better crystallinity of α-Al2O3 phase having regular and long order arrangement of atoms which favours rapid transfer of phonon vibration from one atom to another than amorphous γ-Al2O3 phase with irregular atomic arrays, and thereby decreases the heat transfer rate. Ultra-sonication helps in the breakdown of large clusters with an increase in the dispersion stability and thermal conductivity as verified by particle size distribution and zeta potential measurements. The viscosity of both (α, γ-Al2O3 phase) aqueous xxvi suspensions is higher than de-ionized water. Viscosity is inversely proportional to thermal conductivity, which increased with increase in concentration and decrease with increase in temperature. The Al2O3 aqueous suspension showed Newtonian characteristics at lower concentration (0.05 vol.%).