Senior Research Abstracts 2010

Poster Presentations at the Festival of Science, April 23, 2010

1.
Alex Fisher ’10.   Characterization of
Dielectric and Metallic Surfaces Using
Ellipsometry.
Faculty Sponsor: Catherine Jahncke, Physics

Ellipsometry
is an optical technique which utilizes the polarization property of
light to measure the complex refractive index of thin films and bulk
materials as well as thin film thickness. This technique works by
measuring the polarization of light reflected off a surface near an
angle of incidence where the reflected light is minimized. A home-built
ellipsometer was used to measure the complex refractive index of
different surfaces. The dielectric surfaces measured were samples of
BK7, sapphire, and quartz. The expected refractive index of BK7 is
1.515, which compared to the measured value of 1.498, gave a percent
error of 1.1%. Adjusting for variations in the incident angle, the
indices of sapphire and quartz were measured to be 1.740-0.076i and
1.448, which gave percent errors of 1.5% and 0.7% when compared to their
expected values of 1.766 and 1.458, respectively. Thin films of
aluminum and gold were also measured, but produced significant percent
errors with respect to the expected indices, likely due to oxide layers
and non-homogeneous surface topography.

2. Richards (Chad)
Miller ’10
Efficiency of PEM Fuel Cells.     Faculty
Sponsor: Daniel Koon, Physics

This experiment tried to model a
PEM FC as a simple battery in an attempt to characterize its efficiency
as a result of certain parameters. It was found that the EMF of a FC
behaves like a battery when placed in series, but the internal
resistance does not. A thinner membrane results in lower EMFs (roughly
15%) and the internal resistance goes up. It is certain that the
efficiency of the PEM FCs decreases as the temperature increases. The
efficiencies found in this experiment (39.9% to 59.7%) are subject to
question. The reason for the uncertainty is that the model of a simple
battery used to approximate the fuel cells proved to be a questionable
substitute.

3. David Tersegno ’10   Growth and
Viscosity of Non-Newtonian Corn Starch Structures under
Vibration with Embedded
Ball Bearings and Dissolved Salt.  

Faculty Sponsor: Brian Watson, Physics

A fluid's viscosity is a
measure of how much it resists flow against a shear stress. Most fluids
have a constant viscosity regardless of their environment. Non-Newtonian
fluids are an exception in that their viscosity changes with the amount
of applied force. Corn starch mixed with water is an example of a
dilatant, or shear-thickening fluid. Under low pressures it flows
easily, but quickly hardens when it is pressed. Vibrating a sample of
corn starch mixture alternates high and low stress environments and
allows it to flow and jam in a way that creates unusual structures.
Stable finger-like columns can grow up out of the surface, while holes
in the fluid can form and spread as the fluid crawls up the side of the
container. We duplicated these effects and compared them to other
investigators' observations. The non-Newtonian effects are the result of
corn starch particles jamming under pressure. We added ball bearings to
the mixture, creating a macroscopic jamming condition. The mixture
created the unusual surface effects more easily, drawing the bearings
into the fingers and eventually ejecting a ball. When saltwater was used
to create the mixture, the viscosity was observed to decrease
significantly. We have examined how the viscosity and pressure
relationship varies with salt concentration for this specific
non-Newtonian Fluid.

4.  Dave Tersegno '10.   Detection
of Atmospheric Nitrous Oxide Using a 4.54 µm Quantum Cascade Laser. 
Brian
Watson, Faculty Sponsor.

Nitrous oxide (N2O) is the third most
important anthropogenic greenhouse gas. About half of the current N2O
generation on the Earth is due to human activity, with most of that from
agriculture. It has seen an increase from 270 ppbv to 320 ppbv since
the beginning of modern industry. Its long lifetime of about 120 years
contributes to its influence on the greenhouse effect. N2O’s small
mixing ratio, relative to other greenhouse gases, along with its even
mixture in the atmosphere due to its long lifetime make it difficult to
take meaningfully fine measurements, on the order of single ppbv or
less. Gas chromotography methods, which involve taking a sample from the
air, are accurate and precise but have poor time resolution and are
labor intensive. Our goal is to make a sensor that takes sensitive and
fast measurements in situ so that we can accurately determine the
distribution of fluxes in space and time.

We first characterized the
laser by determining its wavelength tuning rates with respect to these
operating voltages and temperatures. The laser was then tuned to a
single absorption peak. Scanning over this peak, the beam was passed
through a cell of N2O. The observed width and height of the peak on this
direct absorption signal were checked as the concentration and pressure
in the cell were varied. For high concentrations, one could determine
the amount of nitrous oxide in the cell from the size of this absorption
line.

We then applied wavelength modulation spectroscopy, adding a
40 kHz sinusoid to the laser voltage on the lower frequency signal. A
lock-in amplifier is used to give a second derivative line shape of the
peak, allowing us to measure atmospheric concentrations of N2O. Ambient
measurements will be discussed along with future work on long term
stability, precision, and accuracy.