Abstract
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.