以超臨界流體技術製備生質柴油及改善電阻式隨機存取記憶體的效能

Preparation of Biodiesel and Improvement of RRAM (Resistant Random Access Memory) Performance by Supercritical Fluid Technology

林華經(Hua-Ching Lin)

指導教授 ： 談駿嵩

國立清華大學/工學院/化學工程學系/博士(2015年)

摘要

在本論文中，我們分別以“與超臨界流體反應”及“在超臨界流體中反應”兩種不同型式的超臨界流體反應來進行製備生質柴油及改善電阻式隨機存取記憶體（RRAM）效能的研究。

在第一章中，我們對超臨界流體的發展歷史、特性及其應用做一個簡單的介紹。

在第二章中，我們首先檢視反應器金屬表面的催化作用。自Saka及Kusdiana提出了利用超臨界流體技術製備生質柴油的方法後，不需使用催化劑一直是被公認為此製程的重要優勢之一。然而，我們的實驗結果顯示：在超臨界甲醇與植物油的轉酯化反應中仍然因受到不鏽鋼反應器金屬表面的催化作用而使得反應速率加快，得到很高的轉化率；當不鏽鋼反應器金屬表面的催化活性喪失後，轉化率將降至極低。
然後進行觸媒的篩選，在我們所篩選的數種金屬氧化物中，MnO2對此轉酯化反應具有最好的催化效果。在MnO2的催化作用下，植物油可在較溫和的操作條件下幾乎完全轉化為生質柴油；而在沒有觸媒催化的相同條件下，其轉化率則相當低。
接著我們以超臨界/次臨界甲醇和椰子油及痲瘋籽油於連續式反應器中進行轉酯化，並於添加或不添加觸媒及共溶劑的情況下，分別對溫度、壓力、滯留時間及醇/油莫耳比等操作變數對轉酯化反應的影響逐一做系統性的探討。
由實驗結果可知：無論是否存在觸媒，溫度對轉酯化反應的影響皆遠大於壓力的影響；且壓力對轉酯化反應的影響只有在當壓力遠小於甲醇的臨界壓力且醇/油混合物未形成單一均相時方為顯著。由視窗反應器目視觀察發現：在200 °C及4.14 MPa時，甲醇/椰子油混合物形成單一均相；且其視活化能亦由在低溫（180 °C以下）時之107.7 kJ/mol減少為在高溫（220 °C以上）時的35.3 kJ/mol，變得更有利於此轉酯化反應的進行。以上結果顯示：此轉酯化反應並不一定需要在超臨界甲醇、或是醇/油混合物的超臨界狀態下進行，而是只要有足夠高的溫度及壓力可以使得醇/油混合物形成均一相即可。
在高溫操作時的最佳滯留時間與溫度成反比；溫度越高，則所需滯留時間越短。另在我們實驗所採用的12/1到60/1的醇/油莫耳比範圍中，轉化率及視反應速率k皆隨著醇油比的增加而增加，例如：當甲醇/椰子油莫耳比從12/1增加到60/1（5倍）時，其視反應速率k值亦自0.00476 s–1增為0.02118 s–1（4.45倍）；我們並未發現有所謂最佳醇油比的存在。
實驗最後，我們於連續式操作系統中加入共溶劑，其結果是對轉酯化反應無明顯影響甚至產生負面效果。這是由於共溶劑的加入，使得反應系統內的通量體積增加，造成反應物的濃度降低及滯留時間減少，這兩個不利因素會與因加入共溶劑而造成的醇油互溶度增加作用互相抵消。

在第三章中，我們藉由超臨界流體處理技術來降低氧化矽基電阻式記憶體的操作電流。在120 oC的溫度下，利用超臨界二氧化碳的低黏度、低表面張力及高擴散性，使得水分子能輕易進入薄膜內部，進行晶界懸鍵的修補；由此使得電阻式記憶體薄膜中的傳導路徑變得不連續，而電阻式記憶體的導通電阻也因為薄膜中缺陷數目的減少而變大，造成操作電流顯著的下降。隨著元件操作功耗的降低，元件因為焦耳熱所引起的退化也會被改善。因此，超臨界流體處理技術可以改善電阻式記憶體的切換特性及操作效能，這對電阻式記憶體在次世代非揮發式記憶體的應用發展上有著極大的助益。
在實驗中，我們利用超臨界二氧化碳來修補錫摻雜的二氧化矽（Sn:SiO2）薄膜的懸空鍵。經過超臨界流體處理的Sn:SiO2薄膜發生鈍化使得錫形成不連續的金屬絲，造成電流下降。此外，我們也利用此技術對以ITO透明導電薄膜作為電極的電阻式記憶體做處理，能有效降低其消耗功率及操作電壓。
最後，我們以超臨界二氧化碳處理技術來調控TaN薄膜電阻之阻抗溫度係數（TCR）。經退火處理後，TaN薄膜電阻之阻抗溫度係數會由負的變成正的，但再經過超臨界二氧化碳處理之後，正的阻抗溫度係數又會變回負的。藉由超臨界流體技術與熱退火方式的組合最佳化，可以將TaN薄膜電阻器的阻抗溫度係數控制到接近於零，以適合汽車電子或其他高溫惡劣環境之應用。

在第四章中，則是對本論文的內容作一總結。

關鍵字

超臨界流體 ； 生質柴油 ； 轉酯化 ； 電阻式隨機存取記憶體

並列摘要

In this dissertation, both types of supercritical fluid reactions, “reaction with supercritical fluid” and “reaction in supercritical fluid”, were adopted for the study of preparation of biodiesel and improvement of resistant random access memory (RRAM) performance, respectively.

In Chapter 1, a brief introduction to the development, properties, and applications of supercritical fluid was made.

In the beginning of Chapter 2, the catalytic effect of metal reactor surface was investigated. Ever since Saka and Kusdiana proposed the method of preparation of biodiesel by supercritical fluid technology, “non-catalytic” has been recognized as one of the most advantages of this process. Nevertheless, our experimental results showed that, in the transesterification of vegetable oils with supercritical methanol, the reaction rate was indeed accelerated by the catalytic effect of stainless-steel reactor surface, resulting in a high conversion; after the deactivation of this catalytic ability, the biodiesel yield was decreased.
Then we went on the screening of catalysts. Among various metal oxides tested in this study, MnO2 was found to be the most effective catalyst. The presence of MnO2 was essential for the complete conversion of vegetable oils to biodiesel under mild conditions; the conversion was relatively low during catalyst-free operation.
Thereafter, transesterification of supercritical/subcritical methanol with coconut oil and jatropha oil were conducted in a continuous operation system. With or without the addition of catalyst and co-solvent, the effects of the operating variables, namely the temperature, pressure, residence time, methanol-to-oil molar ratio, on the yield of biodiesel were systematically examined.
Our experimental results indicated that: regardless of the presence of catalyst, the effect of temperature on transesterification was more pronounced than that of pressure; the latter was apparent only at pressures far below the critical pressure of methanol and before the formation of a homogeneous liquid phase from the methanol/oil mixture. Through visual observation in a windowed-reactor, at 200 °C and 4.14 MPa, the methanol/coconut oil mixture formed a homogeneous liquid phase; the apparent activation energy decreased from 107.7 kJ/mol at temperatures below 180 °C to 35.3 kJ/mol at temperatures above 220 °C, more favorable for transesterification. The obtained results revealed that this transesterification does not necessarily have to be performed in supercritical methanol, nor in supercritical methanol/oil mixtures, but only at temperatures and pressures where a homogeneous liquid phase exists.
The optimal residence time for the transesterification was dependent on the reaction temperature; higher temperatures required shorter residence times. The FAME yield and the apparent rate constant k both increased upon increasing the molar ratio, for example, when the molar ratio of methanol to coconut oil increased from 12/1 to 60/1 (fivefold), the apparent rate constant (k) also increased from 0.00476 to 0.02118 s–1 (4.45-fold); we did not, however, observe an optimal molar ratio within the range from 12 to 60.
The effect of co-solvent in a continuous operation mode was investigated at the end of Chapter 2, and the experimental results showed that the effect of co-solvent on transesterification was negligible or even negative. The addition of co-solvent might enhance the miscibility between oils and methanol; on the other hand, it also increased the flux volume of transesterification , resulting in decreases in both the concentrations of reactants and residence time. Therefore, the overall effect of co-solvent in a continuous operation mode might be negligible or even negative.

In Chapter 3, the operating current of silicon oxide-based RRAM was reduced by supercritical fluid processing (SFP) technology. At a temperature of 120 oC, with the facilities of low viscosity, low surface tension and high diffusivity of supercritical carbon dioxide, the water molecules could easily diffuse into the film and repair the dangling bonds of grain boundary; by SFP, the conduction path of RRAM film became discontinuous and its conduction resistance also increased due to the reduce of defects in the film, resulting in a significant decline in operating current. With the reduction of operation power consumption of RRAM, the degradation of IC caused by the Joule heat would therefore be improved. Thus, SFP techniques can improve the switching characteristics of RRAM and its operation performance, showing a great benefit on the development and applications of RRAM as next-generation non-volatile memory.
In the experiment, the dangling bonds of Tin-doped Silica (Sn:SiO2) film were repaired by supercritical carbon dioxide (SCCO2). A discontinuous metal filament would be formed in Sn:SiO2 film through SCCO2 passivation process, causing the device current declined. In addition, we also use this technique to treat the RRAM with ITO transparent conductive electrode to effectively reduce the power consumption and operating voltage of device.
At last, SCCO2 treatment technology was used to manipulate the temperature coefficient of resistance (TCR) of TaN thin-film resistors. After annealing process, the TCR value of TaN film resistor was changed from negative to positive; by SCCO2 treatment, the positive TCR value turned back to negative again. Through optimization of supercritical fluid technology combined with thermal annealing method, the TCR value of TaN thin-film resistor could be modulated to close to zero, making it conform the requirements of a stricter specification for car-used electronic applications or other harsh environments of high temperature.

In Chapter 4, a summary of the content in this dissertation was made.

並列關鍵字

Supercritical Fluid ； Biodiesel ； Transesterification ； Resistant Random Access Memory

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