Preparation and characterization of silica-Ag composite nanofluids

Abstract

열 교환기용 유체의 열전도도 향상을 위해 입자크기가 mm 또는 um의 금속입자를 첨가하는 연구가 진행되었지만 입자의 침전, 관내의 압력 증가, 막힘 현상, 큰 입자의 운동량에 의한 장치 마모 등 많은 단점이 있어서 상용화에 실패하였다. 그러나 nm크기의 입자가 분산된 나노유체는 마이크로 유체의 단점을 보완하고 이론적 예측보다 높은 열전도도를 나타내기에 많은 관심을 끌고 있다. 본 연구에서는 직접산화법으로 촉매량을 조절하여 입자크기 10 nm, 60 nm, 그리고 가수분해법으로 촉매량을 조절하여 10 nm, 70~80 nm 크기의 분산안정성이 뛰어난 콜로이달 실리카를 제조하였다. 그리고 제조한 콜로이달 실리카를 베이스로 은 입자를 코팅하여 (평균 10 nm) 실리카-은 복합입자 나노유체를 제조하였다. 제조한 나노유체에 대한 특성은 형상, Ag의 코팅량, 입자의 크기, 그리고 나노유체의 안정성과 열전도도를 분석하였다. 그리고 복합입자의 평균입경은 콜로이달 실리카보다 증가하였다. 콜로이달 실리카와 TEOS로 제조된 실리카를 코어로 하는 실리카-은 복합입자 나노유체는 안정하지만 직접산화로 제조된 실리카를 코어로 하는 실리카-은 복합입자 나노유체는 불안정하였다. 콜로이달 실리카는 상온에서 60도 범위에서 낮을수록, 입자 크기가 작을수록, 0.2 ~ 30 wt%범위에서 농도가 증가할수록 열전도도가 증가하였다. 0.2 ~ 3 wt% 실리카-은 복합입자 나노유체는 온도가 증가하고 농도가 증가할수록 열전도도가 증가하였다. | Solid particles in heat exchanger fluid is expected to increase thermal conductivity of fluid. The solid particles having high thermal conductivity in ㎛ or mm size in the fluids may lead shortcomings, such as increasing pressure drop, clogging the device, and precipitation of particles, so that it ended to failure to commercialize. Nanofluids having nm-sized paticles have been attracted to increase the thermal conductivity higher than the theoretical prediction as well as to overcome the shortcomings. In this study, Colloidal silica samples in two different sizes of 10 nm and 60-80 nm are prepared by direct oxidation of silicon and by hydrolysis and condensation of TEOS(tetraethly orthosilicate). And silica-Ag composite nanofluids was prepared from the colloidal silica with coating of 10 nm size silver. The nanofluids were characterized by their size, morphology, stability, silver coated on the silica surface and thermal condutivity. The particle size increased synthesis than ever before. Stability of colloidal silica and Ag-SiO2 from TEOS higher than Ag-SiO2 from direct oxidation of silicon. The thermal conductivity of colloidal silica were increased with lowering temperature, reducing size, and increasing solid contents. The Thermal conductivity of silica-Ag composite nanofluids were increased with increasing solid contents temperature.; Solid particles in heat exchanger fluid is expected to increase thermal conductivity of fluid. The solid particles having high thermal conductivity in ㎛ or mm size in the fluids may lead shortcomings, such as increasing pressure drop, clogging the device, and precipitation of particles, so that it ended to failure to commercialize. Nanofluids having nm-sized paticles have been attracted to increase the thermal conductivity higher than the theoretical prediction as well as to overcome the shortcomings. In this study, Colloidal silica samples in two different sizes of 10 nm and 60-80 nm are prepared by direct oxidation of silicon and by hydrolysis and condensation of TEOS(tetraethly orthosilicate). And silica-Ag composite nanofluids was prepared from the colloidal silica with coating of 10 nm size silver. The nanofluids were characterized by their size, morphology, stability, silver coated on the silica surface and thermal condutivity. The particle size increased synthesis than ever before. Stability of colloidal silica and Ag-SiO2 from TEOS higher than Ag-SiO2 from direct oxidation of silicon. The thermal conductivity of colloidal silica were increased with lowering temperature, reducing size, and increasing solid contents. The Thermal conductivity of silica-Ag composite nanofluids were increased with increasing solid contents temperature.