Hector E. Medina

Method for Generating and Realizing Replicates of Randomly Roughened Surfaces, Tested on Poly Methyl (Methacrylate)

In experimental studies the need of having replicates of specimens is crucial. This requirement becomes even more critical when performing statistical analysis in combination with destructive tests. We report a novel technique to generate and realize random rough surfaces at different stages of degradation based on the central limit theorem, with either linear or nonlinear growth. To materialize the technique, the realization of surfaces is carried out using poly(methyl methacrylate) (PMMA) as substrate, a CO2 laser system as etcher, and degradation is assumed to be linear. To substantiate this method, profiling, imaging analysis, and nanoindentation characterizations techniques are utilized. Though the depths of surfaces developed here grow to Gaussian, the present method can be adapted to any other topology, as long as the surface is mathematically or digitally represented. A successful application of this method using PMMA has been previously reported. The extension to other polymeric materials is dependent solely on the etching process to be used. This method finds direct applications in the forensic sciences, where the safeguarding of evidence material is crucial, allowing for replicate surrogates for destructive testing purposes. Other applications include tribology, in vitro testing of sun protecting factor, and coatings studies.

Neutron Exposure and Electrical Analysis of P-Type Boron-Doped Silicon Resistors and Transistors

Neutron detectors are deployed at ports of entry across the world to monitor people and cargos for smuggled nuclear materials and are often incorporated in nuclear power plant designed to monitor power levels and ensure safe operations. With the supply of Helium-3 rapidly decreasing and due to increase of terrorism threats it is vital to U.S. national security that a viable alternative material be identified, and a new neutron detector design made available, especially for portal monitoring applications. The interest to study Boron-10 as an alternative to helium-3, due to the vast natural supply that the United States possesses and its nontoxic characteristics, is increasing in different research arenas. Our work consists on taking advantage of the near 20% of boron-10 present in naturally occurring boron which (just as phosphorus) is used to dope semiconductors. Boron doped semiconductor wafers were characterized using four-point probe techniques. Resistors and transistors made with various levels of boron concentration were exposed to a thermal flux at various fluencies at Los Alamos National Laboratory. The reaction, 10B+n → Li + α, caused by neutron irradiation, introduces impurities in the silicon lattice thus producing measurable differences in electronic parameters. These changes are likely to be proportional to the fluence of the source, and hence to the neutron flux. The results show that for irradiated resistors possessing very high values of boron concentration there is a significant reduction in resistivity. This trend is not seen for medium or low values of boron. Additionally, there was no observation of significant changes in other electronic parameters such as threshold voltage or trans-conductance, for the transistors exposed and tested.Copyright

Being Educated in a New Nuclear Engineering Program: A Graduate Student Perspective

Due to the advent of a dramatic increase in the demand for nuclear professionals, a number of universities are either strengthening their existing nuclear engineering programs or starting them for the first time. Following this trend, Virginia Commonwealth University, in Richmond, Virginia, in 2009 began to offer a program in Nuclear Engineering within the Department of Mechanical Engineering. As a student within the PhD program, the author presents his perspective — as well as observations from some undergraduate students — of being educated in a new Nuclear Engineering program. From his perspective, the author presents some mechanisms that have made this new program evolve. Additionally, herein are included some ideas applied by the author in order to carry out successful research — while overcoming the limitations of a new program: spontaneous and embedded innovation, networking, and creativity. It is hoped that the present paper will provide positive feedback to faculty members and motivate students, in any new educational program, particularly, in this renaissance of education, in the nuclear engineering field.Copyright

Generation and Mechanical Analysis of Repeatable Random Rough Surfaces

Natural weathering and wear of structural materials in service nearly always generate surface roughness, and follow the Central Limit Theorem prediction for surface topology. This study couples experimental and statistical theory, to extend knowledge of life of materials from initial service surface conditions through surface damage accumulation. Statistical moments and other parameters were correlated with fracture locations probability (H/N), versus auto correlation length, and depth. As the surface grows to a full Gaussian, H/N increases its dependence on profile’s average and RMS roughness, and derived parameters. This dependence shows an asymptotic limit behavior that approaches agreeably Griffith’s crack criterion, though with multiple fracture locations. Importantly, a Transitional Region was observed, below which the probable location of fracture is uncorrelated to the parameters studied. This is because introduced roughness is insufficient to compete with impurities, internal and external manufacturing flaws, and scratches, due to handling and machining, on the samples.Copyright