Abstract
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.