M.Tech. / Among the materials being used for gas sensors, metal oxides are the most important materials because of their potential to detect many gases at low concentrations. Nevertheless, sensors made of metal oxide need to be operated at high temperatures (above 200°C) and have a weak sel ectivity. In order to overcome this difficulty, the materials are being investigated for gas sensing applications. Carbon nanotubes (CNTs) are promising materials with unique properties, such as high electrical conductivity, mechanical strength, nanometer–scale sizes, and high aspect ratio. Their adsorption ability and high surface area make them attractive as gas sensing materials, which have been intensively studied. CNTs can be used solely or combined with metals and oxides materials in order to constitute efficient gas sensors. In the present research, multi–walled CNTs (MWCNTs) were coated with tin dioxide (SnO2) and incorporated into two epoxy resins with widely different mechanical properties in order to study the effect of CNTs on the morphology, mechanical, electrical, and sensing properties of the composites. In the MWCNT/polymer composite study, Epon 828 was used as the polymer matrix and D–2000 (which gives rubbery composites) and T–403 (which gives glassy composites) as the hardeners. Composite were prepared with 0.1 wt.% MWCNTs in an epoxy matrix. Pristine MWCNTs (MWCNTs not treated with any acid and therefore used as received) and SnO2–MWCNTs were used for comparison and a two–step curing procedure was used with initial temperature set at 75°C for 3 hours, followed by additional 3 hours at 125°C. The sample s were characterized for morphology, mechanical, thermo–mechanical and electrical properties using scanning electron microscopy (SEM), an Instron tensile tester, dynamic mechanical analysis (DMA) and Cascade Microtech four–point probe, respectively. In both cases, strong covalent bonds were created as a bridge between the CNTs and matrix, but due to differences in viscosity, the nanotubes dispersion was much better in the rubbery epoxy resin than in the glassy epoxy resin. A 77% increase in tensile modulus was observed in the rubbery system using 0.1 wt.% SnO2–MWCNTs compared to the neat rubbery epoxy. As for the glassy epoxy based composite, only a 3% improvement in tensile modulus could be observed. In addition to the mechanical properties, the presence of CNTs has demonstrated a material with high vi electrical conductivity. But for the surface measurements during the gas sensing analysis, the conductivity was very low for the composites to be used for this application as envisioned. MWCNTs coated with SnO2 nanoparticles used in the present study, were synthesized by a microwave synthesis method. The composite samples were characterized by X–ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR) and Brunauer–Emmet–Teller (BET) surface area analysis. These techniques gave evidence for surface and chemical modifications of the synthesized composites. The results showed microwave synthesis to be a very efficient method in producing CNTs that are densely coated and well dispersed with SnO2 nanoparticles in a very short time (total reaction time of 10 minutes). Microwave synthesis is particularly interesting because of the energy used, the higher temperature homogeneity and the shorter reaction times led to nanoparticles with high crystallinity and a narrow particle size distribution. Controlling the morphology by varying synthesis conditions such as temperature, pressure and time is also possible.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:8885 |
| Date | 31 July 2012 |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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