Many potential applications have been proposed for silicon carbide (SiC)
nanostructures, including high-strength composites, nanosensors and nanodevices.
Limitations in processing are an important barrier that has to be overcome in order to
develop these applications.
The aim of this study is to explore the possibility of fabricating SiC nanotubes
and to improve the elaboration process of SiC nanofibers prepared by using polymer
blend and spinning techniques.
In the general introduction, the structure of SiC and the recent uses of
polycarbosilane (PCS) as a SiC precursor for nanofibers are discussed. A review of the synthesizing methods for SiC nanotubes and nanofibers and their possible applications
are presented.
The present work proposes the use of polymer blend technique in order to
prepare the SiC nanotubes and nanofibers. Due to the versatility of the polymer blend
industrially, it could be used to mass-production of nanostructures. The polymer blend
technique presented is based on the use of a precursor polymer with a ceramic residue and a thermally decomposable polymer without carbon residue after heating.
Chapter 1 presents an attempt to prepare nanotubes from core/shell particles by
using polymer blend and wet-spinning techniques. Polystyrene (PS) was used as a core decomposable polymer and polydimethylsilylvinylacetylene (PDMSVA) as the shell silicon carbide precursor polymer. The core/shell particles were prepared by emulsion
polymerization. They were subjected to wet-spinning, stretched mechanically six times their original length, stabilized by UV and also in air, and finally heat-treated in an inert atmosphere. Nanotubes were prepared although the silicon carbide precursor
polymer (PDMSVA) was insufficiently stabilized.
Preparation of nanotubes from poly(methyl methacrylate) / polycarbosilane
(PMMA/PCS) and poly(methyl methacrylate) / polyacrylonitrile / polycarbosilane
(PMMA/PAN/PCS) core/shell particles by the aforementioned techniques is presented in
Chapter 2. The core/shell particles were prepared by the spray drying method. They were subjected to wet-spinning, stretched, stabilized and heat-treated. Few nanotubes were obtained from the PMMA/PAN/PCS heat-treated sample because the stretching
technique was not efficient in order to elongate all the core/shell particles.
Recently, SiC fibers have found many applications in composites at higher
temperatures in oxidizing environments. One of the most important factors related to the
chemical composition of these fibers is the oxygen content because it decreases the
mechanical properties of the fibers used for such applications.
In this study, how to improve the preparation process in order to obtain SiC
nanofibers with low oxygen content was sought.
Chapter 3 refers to preparation and study of the oxidation behavior of nanofibers
derived from polycarbosilane by using polymer blend and melt-spinning techniques.
Polycarbosilane (PCS) and novolac-type phenol-formaldehyde resin (PF) were dissolved in tetrahydrofuran (THF) and after removal of THF, the polymer blend was melt-spun.
The polymer blend fibers were soaked in an acid solution in order to cure the phenolic matrix. The stabilized polymer blend fibers were heated at 1000 oC under a nitrogen atmosphere and kept in nitric acid solution to remove the matrix carbon.
The oxidation behavior was characterized. A part of the nanofibers was further heated at
1500 oC. Nanofibers of several 100 nm in diameter were prepared. A large amount of
oxygen was introduced into the nanofibers during the oxidation process. After heating at 1500 oC, the nanofibers changed from an amorphous phase to β-SiC.
In Chapter 4, the microstructural changes of nanofibers were examined. The preparation procedure was the same as that of Chapter 3, but the nanofibers were heated
to two different temperatures, 1000 oC and 1200 oC. The nanofibers heated at 1200 oC
were straight, longer and had a smoother surface compared with the nanofibers heated at 1000 oC. The nanofibers heated at 1200 oC had higher resistance to oxidation by nitric acid treatment than the nanofibers heated at 1000 oC, revealing the importance of the heat treatment temperature.
Finally, the general conclusions are presented.学位記番号：工博甲31
