Dissertation com jun 2006 (4 Ergebnisse)
- Softcover
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Taschenbuch. Zustand: Neu. Neuware - This ex post facto study was used to compare community college transcripts of African American students who participated in a developmental reading course at a multi-campus, urban community college and those who declined to participate in the developmental reading program.The study examined 5…13 records to determine if participation in a developmental reading course in a community college improved achievement and persistence for African American college students.Results discuss the positive relationship of overall and third semester achievement to enrollment in a developmental reading course as shown in the student QPA.Neither enrollment in a developmental reading course nor reading level was related to persistence as measured by the number of semesters enrolled in college.
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Taschenbuch. Zustand: Neu. Neuware - Groundwater represents an important supply source for municipal and irrigation uses in Far West Texas. The City of El Paso receives about 50 percent of its municipal water supply from surface water and 50 percent from local groundwater. Groundwater pumping in El Paso is from the Hueco Bolson…and the Mesilla Bolson. Historic groundwater pumping in the Texas portion of the Mesilla Bolson has not resulted in significant changes in groundwater levels or groundwater quality in existing wells.Historic pumping in the Hueco Bolson has resulted in lowered groundwater levels and brackish groundwater intrusion. The groundwater level declines have resulted in the intrusion of brackish groundwater into areas that historically pumped fresh groundwater. A 1979 assessment concluded that El Paso would deplete fresh groundwater in the Hueco Bolson by 2030 if groundwater pumping continued to increase. Partly as a result of the 1979 assessment, El Paso reduced its groundwater pumping from the Hueco Bolson by increasing surface water diversions from the Rio Grande, increasing conservation efforts, and increasing reclaimed water use. As a result, groundwater levels in many parts of the Hueco Bolson have stabilized.Brackish groundwater intrusion remains an issue, and is being addressed with a brackish groundwater desalination plant, currently under construction. The new wells and existing wells that will supply this desalination plant will also assist in the management of brackish groundwater intrusion by intercepting the brackish groundwater before it can flow towards existing fresh groundwater wells.The 2006 Far West Texas Regional Water Plan contemplates a groundwater transfer project to meet increasing demands in El Paso County beginning about 2030, mostly from the Dell City area in Hudspeth County, Texas. Groundwater management in the Dell City area is governed by a groundwater conservation district that has established limits on groundwater pumping based on existing and historic uses. Groundwater pumping for irrigation began in the Dell City area in 1948, and groundwater levels have been essentially stable for since the 1980s. Future planning for a groundwater transfer project will require a detailed understanding of the hydrogeology of the groundwater in the Dell City area.
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Taschenbuch. Zustand: Neu. Neuware - For many applications, including power generation, aerospace and the automobile industry, high temperature wear provides serious difficulties where two or more surfaces move or slide relative to one another. In aerospace, for example, demands for more powerful, efficient engines operating at…ever higher temperatures, mean that conventional lubrication is no longer sufficient to prevent direct contact between metallic sliding surfaces, accelerating wear. However, one high temperature phenomenon observed to reduce metallic contact, and thus wear and friction, is the formation of 'glazes',essentially compacted oxide wear debris layers that sinter together to form wear resistant surfaces. This thesis studies the nature of wear encountered with four different combinations of Superalloys, slid together using a 'block-on-cylinder' configuration (Nimonic 80A and Incoloy MA956 as block / sample materials; Stellite 6 and Incoloy 800HT as cylinder / counterface materials) simulating car (automobile) engine 'valve-on-valve-seat' wear. Initially this study concentrates on the combined effects of sliding speed (either 0.314 m/s or 0.905 m/s, supplementing previous testing at 0.654 m/s) and temperature (between room temperature and 750°C) - by altering either or both of these variables, the nature of the wear process can be radically altered, encouraging or suppressing wear protective oxide or 'glaze' layer formation. Extensive characterisation is conducted of the 'glaze' layers during this study, using a wide range of tools including optical microscopy, SEM, EDX (spot, mapping and Autopoint), XRD (including Glancing Angle) and micro-hardness. On selected samples, TEM and STM show these 'glaze' layers to be nano-structured (nano-crystalline), with an estimated grain size of as little as 2 to 10 nm.
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Taschenbuch. Zustand: Neu. Neuware.
