以超重力旋轉盤反應器製備銅及氧化銅微粒

Abstract

氧化銅為重要的陶瓷材料，其用途廣泛，可用於人造雨之成核劑、塗佈於太陽能板、或作為P型半導體與高溫超導體之材料。將氧化銅奈米粒子分散在流體中，可製成奈米流體，以提升流體之熱傳導係數。銅則為最重要的金屬之一，由於其導電度與導熱度佳，且價格上比銀、金等貴金屬更為經濟，故被廣泛用於電工業、電鍍業與半導體業，亦可作為觸媒。 文獻中製備奈米氧化銅之方法甚多，物理法最常見者為氣相凝結法，是利用電弧使銅塊材氣化成蒸氣，再使此蒸氣與較冷的液體接觸而凝固出均勻的奈米顆粒；而化學法則包括聲波化學法、溶膠凝膠法、水熱法及固相反應法等，係使用不同之化學反應製備出奈米氧化銅顆粒。銅微粒之製備方法除了電鍍法之外，以使用聯胺(N2H4)與硼氫化鈉(NaBH4)作為還原劑之化學還原法為主，而無電鍍法則多使用甲醛作為還原劑。由以上可知，能成功製備奈米氧化銅與銅之方法甚多，但這些方法多為實驗室規模，不但常需使用對環境有害之有機化合物，而耗時、耗能且生產速率有限等缺點皆使其不利於工業化。 近年來發展出之超重力技術則可克服以上缺點。其可分為旋轉填充床與旋轉盤反應器，由於其旋轉時強大的離心力能提供高度的混合效率，因此能使反應器內各位置之過飽和度高且均勻，有助於產生粒徑小且均勻的粉體，且其有產量大、操作時間短等優點，故相當有利於放大至工業化生產。而本實驗室在以超重力系統製備微粉方面，已探討了鹽類、難溶藥物以及金屬等物系，配合結晶理論並選擇適當的操作條件後，均可得到優異的微粒化成果。 本研究以超重力旋轉盤反應器製備氧化銅與銅微粒。氧化銅方面，以硫酸銅與碳酸鈉之液－液相反應製備出氧化銅前驅物，再將之煅燒至500°C生成氧化銅。在操作變數方面，發現硫酸銅濃度小於0.10 M、轉盤轉速大於1000 rpm、兩液體流量小於3.0 L/min、反應pH值於6左右可得到較小的氧化銅奈米粒子。硫酸銅濃度與碳酸鈉濃度為0.10 M、轉速4000 rpm、兩液體流量為3.0 L/min時產能可達34.6 kg/day，且體積平均粒徑在65 nm以下，而以場發射槍電子顯微鏡觀察為20-30 nm之球狀氧化銅粒子。以六偏磷酸鈉作為分散劑，將氧化銅製成CuO-水奈米流體，其熱傳導係數較文獻與理論計算值為高，於0.4 vol.%時可使熱傳導係數提升10.8 %。 銅微粒方面，利用較弱之還原劑葡萄糖或稀硝酸製備銅粒子。以葡萄糖作為還原劑時，於反應溫度約80°C、循環時間為15 min、NaOH與glucose濃度分別為1.0 M與0.1 M、Cu(OH)2濃度為0.02 M時，添加兩倍銅重量之PVP(poly vinylpyrrolidone)可得到最小的粒子，約100-300 nm，晶貌為多面體。另外，於旋轉盤反應器製備出的粒子小於以攪拌槽製備者。硝酸還原法方面，反應可於常溫下進行且可以連續式操作。其中添加六偏磷酸鈉作為反應物Cu2O之分散劑，而以1.1 g/L之PVP作為副添加劑時可得到類球狀、大小約100-300 nm的銅粒子，產率為79.2 %。硝酸濃度小於0.32 M時，反應速率會變慢使產率下降；超過0.64 M時，則因氧化力增強會產生部分的氧化銅(CuO)。將硝酸還原法產生之含銅廢液回收後，可於旋轉盤反應器中與氫氧化鈉或碳酸鈉反應，將之再製成氧化銅。

Copper(II) oxide is an important ceramic material that has many applications, such as ice nucleating agent for artificial rain, coating on solar panel, p-type semiconductor, and high-temperature superconductor. Copper oxide nanoparticles can be dispersed into fluids to become nanofluids, which can enhance the thermal conductivity of fluids. Copper is one of the most important metals, which is wildly used in electric industry, electroplating, and semiconductor because of it is cheaper than other noble metals such as silver and gold. It can be also used as a catalyst. There are many methods for preparing copper oxide nanoparticles, and the most common physical one is the gas-condensation method, in which copper raw material is evaporated by a high-temperature arc and then the vapor is condensed by contacting with cold liquid to become uniform nanoparticles. The chemical synthetic methods including sonochemical, sol-gel, hydrothermal, and solid-state method, are to synthesize copper oxide via various chemical reactions. For preparing fine particles of copper, besides the electroplating method, most chemical reduction methods using hydrazine and sodium borohydride as reducing agents, and formaldehyde is also used as reducing agent for the electroless copper deposition. Although these methods are available for producing copper oxide and copper particles, most of the synthesizing methods stay in the laboratory, and toxic organic compounds are usually used. Moreover, the problems associated with energy-consumption, time-consumption, and slow production rate make them difficult to apply in industry. The high-gravity technique (HiGee) has been developing in recent years, and it can overcome the problems illustrated above. Two types of equipment, i.e., the rotating packed-bed reactor (RPBR) and spinning disk reactor (SDR), have been applied in this regard. As the packed-bed or disk is rotating, the high centrifugal force can be generated and thus a uniform and high supersaturation through micromixing is achieved. As a result, small and uniform particles can be obtained. Moreover, the short operating time and mass production rate are also advantageous to scale-up for industrial production. In our laboratory, powders of several chemicals including salts, drugs, and metals have been investigated, and they were all successfully micronized using high-gravity technique by applying crystallization theories and choosing optimal operating variables. The aim of this research is to synthesize fine powder of copper oxide and copper using a spinning disk reactor. For synthesizing copper oxide, the precursors of copper oxide were first prepared in a continuous mode through a liquid-liquid reaction using copper(II) sulfate and sodium carbonate as reactant. Then, the precursor particles were calcined up to 500°C to obtain copper oxide nanoparticles. Among the effects of operating variables, smaller copper oxide particles were obtained under reactant concentrations lower than 0.1 M, rotation speed higher than 1000 rpm, flow rates of reactant solutions lower than 3.0 L/min, and pH of slurry around 6. As the reactant concentrations were both 0.1 M, rotation speed was 4000 rpm, and flow rates were both 3.0 L/min, a production rate of 34.6 kg CuO/day can be achieved. The volume mean size of the product particles was smaller than 65 nm and the primary particle size was 20-30 nm observed under a field emission gun scanning electron microscope. Finally, a CuO-water nanofluid was prepared using sodium hexametaphosphate as the dispersant. The effective thermal conductivity of the nanofluid prepared in this study was higher than that reported in literature and that by theoretical calculation. The best result in the improvement of thermal conductivity was 10.8% when the solid content was 0.4 vol.%. For preparing copper fine powders, weak-reductant glucose or dilute nitric acid was used as the reducing agent. As glucose was used, the smallest copper particles were obtained under the temperature of 80°C, recycle time of 15 min, weight ratio of PVP/Cu=2. The concentrations of Cu(OH)2, NaOH, and glucose were 0.02 M, 1.0 M, and 0.1 M, respectively. The morphology of copper particles were polyhydral and the size was around 100-300 nm. Furthermore, the size of copper particle synthesized using an SDR was smaller than that using a stirred tank. As nitric acid was used as the reducing agent, continuous operating mode can be used, and the reaction can proceed at room temperature. Sodium hexametaphosphate was added for dispersing the reactant, Cu2O. Spherical copper particles with size around 100-300 nm can be obtained with PVP(polyvinylpyrrolidone) as the co-additive in a concentration of 1.1 g/L PVP, and the yield was 79.2 %. When the concentration of nitric acid was lower than 0.32 M, the yield of copper particles decreased because of the slower reaction rate. However, as the concentration of nitric acid was higher than 0.64 M, the copper oxide was obtained because of a higher oxidation ability. Finally, the waste solution containing copper(II) ions, which was produced from nitric reduction process, can be recycled to react with NaOH or Na2CO3 to produce copper oxide particles using the SDR.