Enzymatic Hydrolysis of Pretreated Corn Stover at High Solids Loadings

Biochemically produced cellulosic ethanol holds promise for the future of renewable liquid transportation fuels. Cellulosic ethanol must demonstrate an economic and large-scale viability in order to realize its full potential, and therefore intermediate processes such as enzymatic cellulose digestion must be more fully understood. In this study, the digestibility of dilute sulfuric acid pretreated corn stover was explored with respect to pretreatment severity, particle size, and regimen of pH adjustment before enzymatic hydrolysis. A series of experiments were performed at 20% insoluble solids weight, which is considered a high solids concentration because the material has an appreciable yield stress. Conversion of cellulose to glucose and cellobiose by 20 mg protein/ g cellulose GC220 enzyme was assessed over a period of 7 days. Each sample consisted of 60 g of slurry in 125-mL cylindrical bottles rotated on mechanized rollers at 4 rpm incubated at 48.5°C. Enzymatic hydrolysis of pretreated insoluble solids suspended in DI water and citrate buffer was performed in duplicate with and without mechanical size reduction. GC220 enzyme is most effective at a pH near 5, thus necessitating the buffer in the insoluble solids samples. Pretreated whole biomass slurries include soluble and insoluble solids and have a pH around 1.5 – 2.0. Ammonium hydroxide was mixed into the whole slurry samples to increase the pH before enzyme addition. Results showed that increasing extent of pretreatment predicts increasing digestibility of the pretreated insoluble biomass samples in both cases. Despite particle size reduction of 10-40%, there was little difference in conversion between the two different particle size distribution materials. The results indicate that pretreatment is effective and the well-established correlation between smaller particle size and high digestibility is not a simple causal relationship. In all cases, whole slurry is more difficult to break down with enzymes than insoluble solids suspended in the equivalent amount of water, but ultimately this challenge will need to be overcome to efficiently convert biomass to fuels. This series of experiments begins to show the effect that process-relevant conditions have on cellulose digestibility. Further investigation into these operations can lead to modifications and improvements that may help cellulosic ethanol become the liquid fuel of the future.

Monte Carlo Simulation of Neutron Background Sources in the Measurement of the 12C(α,γ)16O Reaction Rate

The 12C(α, γ)16O reaction rate strongly affects the relative abundances of chemical elements, as well as when core collapse supernovae occur. There have been several attempts to measure the reaction rate, but the Coulomb barrier between the carbon nucleus and the α-particle inhibits direct measurement at stellar energies. In a proposed experiment, a water-filled bubble chamber will be used to measure the reverse reaction rate. This technique will accurately measure the reaction rate closer to stellar energies than previous experiments have accomplished. A potential background source is photoneutrons from the γ-ray beam collimator entering the bubble chamber and generating a false signal. To minimize this effect, a Monte Carlo simulation has been performed to compare the number of photoneutrons created in lead, copper, and aluminum collimators. It was found that 30 cm of copper would be an effective beam collimator by stopping 99.8% of γ-rays and generating no photoneutrons. The simulation also compared the effectiveness of concrete, polyethylene, and water as neutron shields. These simulations show that polyethylene consistently stops the most neutrons at relevant energies. Further simulation will be required to evaluate shielding materials for cosmic ray neutrons, which can also generate false signals.

Crystallographic Characterization of GaN Nanowires by Raman Spectral Image Mapping

Obtaining structural information of nano structured materials often requires electron microscopy for suficient spatial and crystallographic resolution. This study uses Raman spectral imaging to extract information regarding crystalline orientation and structure by non-invasive means. Seeking a correlation between crystallographic facet and favored Raman mode, Gallium Nitride (GaN) nanowires were imaged by confocal Raman microscopy with a 532nm laser, and scanning electron microscopy. Raman spectral maps containing pixel-by-pixel spectra were acquired. Comparison to scanning electron microscope (SEM) images revealed that for regularly-shaped wires at least 230nm in width, the E2 mode is observed more strongly in the [1 1 2] and [-1 -1 2] "smooth" facets of the wire, while the A1(TO) mode is only observed in the [0 0 1] "rough" facet, suggesting a strong surface-structure dependence of Raman signal that can be exploited for imaging. Further experimentation on irregular and small wires that exhibit only the E2 peak, on other favored modes in GaN, and with other group III/V nitrides is recommended.

Simulation and Optimization of the HINS Ion Source Extraction System

The heart of the High Intensity Neutrino Source (HINS) linear accelerator (linac) is a magnetron-type, circular aperture H􀀀 source, which is currently being tested at Fermilab. Although this prototype already delivers the beam current and emittance required by the HINS project, an exploration of whether or not the performance of the source could be improved was undertaken. To this end, the extraction geometry of the source was simulated with SIMION 8.0 and Finite Element Method Magnetics. The effects of changing the angle of the extraction cone (cone angle), the size of the gap between the extraction cone and the source plate (extraction gap), and the aperture of the extraction cone (extraction aperture) were studied. These parameters were chosen because we thought that they would have the greatest impact on space charge effects, which is a major source of emittance growth in this ion source. Based on the results of these simulations, four different configurations were ultimately tested in the ion source. The simulations indicated that the final emittance of the source should be significantly decreased by utilizing geometry with a 45 degree cone angle, a 4 mm extraction gap with extraction voltage of 25000 V, and a 3 mm extraction aperture. Subsequent emittance measurements on the ion source have confirmed this result. This new geometry also allows the source to output a higher current beam with the same duty factor.