Synthesis of SiO2 Aerogel Having Low Thermal Conductivity
隔熱材 ； 二氧化矽 ； 氣凝膠 ； 低熱傳導係數 ； Aerogrl ； Insulator ； Low K ； SiO2
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氣凝膠(aerogel)是ㄧ種具有奈米級多孔性(porous)的材料，ㄧ般以二氧化矽(Silica, SiO2)為主要結構，構成3-D立體網狀的型式，氣凝膠的多孔結構中，空氣佔有80%以上的體積，因此密度極低，被稱為「被凍結的煙」(frozen smoke)20。因為氣凝膠中，二氧化矽的粒子極小，所以一般氣凝膠呈現透明或是半透明。由於空氣的折射率(refractive index)為1，並且具有極佳的絕熱性質，使的富含空氣的氣凝膠擁有低折射率與低熱傳導係數(thermal conductivity)。 在早期的文獻中，二氧化矽氣凝膠大多使用超臨界乾燥程序進行乾燥，這將需要極高的成本，也使的製程上具有危險性。因此在近幾年，多數的研究團隊，開始傾向使用常壓乾燥的方式製備二氧化矽氣凝膠，不過因為在常壓下乾燥，乾燥的條件與方法非常的嚴苛，製備而得的產品在物理性質上，一直無法與超臨界乾燥所製備的氣凝膠相提並論，因此仍然還處於研究的階段。本研究將以過去文獻中表面改質為基礎，企圖開發一個較為理想的方法，改善以往常壓乾燥製備的方式，以獲得具有低密度、高孔隙率的二氧化矽氣凝膠，並針對氣凝膠在隔熱性質上的應用做探討。 經過一年之努力，氧化矽氣凝膠保溫片之熱傳導係數已可降到0.0058W/MK。然而目前之條件仍非最佳條件，主要是添加之輻射熱遮斷劑碳粉使用奈米碳管，價格太貴，在未來實用上仍有障礙，但使用奈米碳管已提供了一個方向：碳粉也希望市高孔隙之碳粉，其孔洞管徑也希望保持在奈米級，如此不但限制了碳粉中氣體之對流現象，也因碳粉對紅外線之吸收而使輻射遮蔽達到最大效果，未來將再努力尋找合適之一般碳粉，使保溫片成本降至最低。
Aerogel, a nano-sized porous material, has a three-dimensional network structure which is mainly composed of silica. In this porous structure, above 80% volume of the aerogel is occupied by air, so the density of aerogel is extremely low and is called “frozen smoke”. Because the silica particles comprising the aerogel are very small, the appearance of aerogel is always transparent or semi-transparent. Due to the low refractive index and excellent capacity for heat insulation of air, the aerogel filled with a great deal of air has the low refractive index and low thermal conductivity. In the early literatures, the researchers had been almost used the supercritical fluids (SCF) for drying the silica aerogel. It would take much cost to achieve the drying process and the drying process was also hazardous. Therefore, in these years, most research groups attempted to use the ambient pressure drying for prepare the silica aerogel. However, the drying condition and method were very difficult under the ambient pressure, and the physical properties of product made from the ambient pressure drying could not achieve to the level with that made from the SCF drying. In this research, we attempt to develop an ideal method on a basis of the surface modification to improve the former ambient pressure drying method. Then, we try to prepare the silica aerogel having low density and high porosity and probe into the heat insulation application of aerogel. According to the adjustment of process factors, the thermal conductivity of silica aerogel preservation piece could be reduced to 0.0058W/M2K. However, the resent processing condition is still not the optimal condition due to the addition of the carbon powder irradiation obstruct agent. The carbon powder is composed of carbon nanotubes that are too expensive and it will be an inhibitor in the future application. But the use of nanotubes also offers a new way: the carbon powder can also have high porosity powder and their pore diameter can be kept in the nanometer size. And then, it can not only confine the fluid convection phenomenon in the carbon powders, but also get the most excellent irradiation shielding effect due to the absorbance of infrared rays form the carbon powder. We will search the other suitable carbon powder to reduce the cost of preservation piece.
理學院 > 化學研究所