Shape-controlled synthesis of silver nanostructures for high-thermal conductivity nanofluids
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Shape-controlled synthesis of silver nanostructures for high-thermal conductivity nanofluids Glorimar Garcia1, Oscar Perales-Perez2, Majid Ahmadi3, and Maxime J-F Guinel3 1
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University of Puerto Rico, Department of Mechanical Engineering, P.O. Box 9045, Mayagüez, P.R. 00681-9045. University of Puerto Rico, Department of Engineering Science and Materials, P.O. Box 9044, Mayagüez, P.R. 00681-9044. 3 University of Puerto Rico, Department of Physics, P.O. Box 70377, San Juan, P.R. 00936-8377.
ABSTRACT A nanofluid is a solid-liquid composite material consisting of a stable suspension of nanometric particles in a conventional refrigerant liquid expected to exhibit enhanced heat transfer properties. Elemental silver (Ag) was selected in this research because of its high electrical and thermal conductivity that are likely to be dependent on the crystal size and shape at the nanoscale. Accordingly, we have synthesized highly monodisperse silver nanowires and nanocrystals by reducing silver nitrate solutions with ethylene glycol in presence of polyvinylpyrrolidone, (PVP). The shape-control in the silver nanostructures was achieved by a proper selection of the type and level of chloride salts, e.g. KCl and CaCl2, and specific PVP/Ag mole ratios in starting solutions. The development of the metal phase was confirmed by X-ray diffractometry. Transmission electron microscopy analyses evidenced the formation of silver nanowires exhibiting a very uniform thickness that could be tuned in the 40-130nm range. UV-vis measurements evidenced the plasmon peak at ~387nm and clear shoulders at ~357nm that are indicative of the formation of elongated nanostructures. INTRODUCTION The high heat transfer efficiency exhibited by metals encourages the preparation of stable liquid suspensions of metallic nanostructures, nanofluids, with enhanced cooling performance. However, the development of nanofluid-based technology in thermal management systems still require a systematic study of the mechanisms behind the sizeand shape-controlled synthesis of the metallic particles at nanoscale and their suitable suspension in conventional refrigerant liquids [1-3]. In this regard, silver nanostructures are being considered very promising materials for nanofluid preparation based on their morphological features and expected influence on the corresponding structural, optical and thermal conductivity [3, 4]. On this basis, the present work was focused on the study of the size- and shape-controlled synthesis of silver nanostructures via a polyol process in presence of chloride species, which allowed the formation of wires, rods, or nanoparticles.
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EXPERIMENTAL Materials synthesis Ethylene glycol (99.8% purity, anhydrous) was used as a reducing agent and solvent. AgNO3 (ACS, 99.9+ %, Alfa Aesar) was the metal precursor and poly-vinyl pyrrolidone (PVP) (99.8%, Sigma Aldrich) to prevent aggregation of the silver particles. The PVP: Ag mole ratio was kept constant at 1.7:1 for all experiments [3]. Two 3-mL ethylene glycol solutions of AgNO3 and PVP were sim
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