Previous studies on very high-cycle fatigue behavior of thin silicon films suggest a strong environmental dependence of the degradation mechanism, the precise nature of which is still subject to debate. In the present study, 2-micron-thick polycrystalline Si notched cantilever beam structures were used to investigate fatigue degradation in a high-temperature (80°C), high-humidity (90%RH) environment. The specimens were subjected to fully reversed sinusoidal loading at resonance (~40kHz) with stress amplitudes ranging from 1.46 to 1.6GPa, resulting in life-spans between 10⁶ and 10⁹ cycles. Comparison to a reference set of S-N data obtained at moderate environmental conditions (30°C and 50%RH) reveals a strong tendency for faster degradation with increasing temperature and humidity. The obtained damage accumulation rates in the 80°C, 90%RH environment exceed the reference by two orders of magnitude. Transmission electron microscopy (TEM) on vertical through-thickness slices reveals oxide thickening after cycling. The influence of ~20nm Al[subscript2]O₃ deposited on the surface of the fatigue specimens using Atomic Layer Deposition (ALD) technique was also studied. The presence of the alumina coating results in a higher fatigue resistance at 30°C and 50%RH, as well as a drastically different frequency evolution behavior. No oxide thickening was observed in the TEM for coated run-out specimens. A model is proposed to explain the different degradation behavior of the ALD-alumina coated samples. Thickened oxides after cycling appear consistent with the reaction-layer fatigue mechanism. Finite element modal analysis incorporating surface oxide layers and cracking was employed to relate the damage observed in TEM to the experimentally measured changes in resonant frequency. In conclusion, the reaction-layer mechanism seems capable of describing micron-scale polysilicon fatigue, even though the critical processes such as room-temperature, stress-assisted oxidation remain elusive.

<p>The role of nitric acid (HNO₃) on the atmospheric corrosion of metals has so far received little or no attention. However, the last decades of decreasing sulphur dioxide (SO₂) levels and unchanged HNO₃ levels in many industrialized countries have resulted in an increased interest in possible HNO₃-induced atmospheric corrosion effects. In this study a new method was developed for studying the corrosion effects of HNO₃ on metals at well-defined laboratory exposure conditions. The method has enabled studies to be performed on the influence of individual exposure parameters, namely HNO₃-concentration, air velocity, temperature and relative humidity, as well as comparisons with newly generated field exposure data.</p><p>The corrosion rate and deposition rate of HNO₃ on copper was shown to follow a linear increase with HNO3 concentration. The deposition velocity (Vd) of HNO₃ increased up to an air velocity of 11.8 cm s^-1. Only at a higher air velocity (35.4 cm s^-1) the Vd on copper was lower than the Vd on an ideal absorbent, implying the Vd of HNO3 at lower air velocities to be mass-transport limited.</p><p>Within the investigated temperature range of 15 to 35 ºC only a minor decrease in the HNO₃-induced copper corrosion rate could be observed. The effect of relative humidity (RH) was more evident. Already at 20 % RH a significant corrosion rate could be measured and at 65 % RH the Vd of HNO₃ on copper, zinc and carbon steel reached maximum and nearly ideal absorption conditions.</p><p>During identical exposure conditions in HNO_3-containing atmosphere, the corrosion rate of carbon steel was nearly three times higher than that of copper and zinc. The HNO₃-induced corrosion effect of copper, zinc and steel turned out to be significantly higher than that induced by SO2 alone or in combination with either NO₂ or O₃. This is mainly attributed to the much higher water solubility and reactivity of HNO3 compared to SO₂, NO₂ and O_3. Relative to SO₂, zinc exhibits the highest sensitivity to HNO₃, followed by copper, and carbon steel with the lowest sensitivity.</p><p>Extrapolation of laboratory data to an assumed average outdoor wind velocity of 3.6 m s-1 enabled a good comparison with field data. Despite the fact that ambient SO₂ levels are still much higher than HNO₃ levels, the results show that HNO₃ plays a significant role for the atmospheric corrosion of copper and zinc, but not for carbon steel. The results generated within this doctoral study emphasize the importance of further research on the influence of HNO₃ on degradation of other materials, e.g. stone and glass, as well as of other metals. </p>

Thesis (M.S.)--Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.

This study considers airflow simulations to evaluate the impact of different window air-conditioner locations on the thermal comfort in an office room (OR). This thesis compares the air distribution for an office room by using computational fluid dynamics (CFD) modeling to previously studied rooms. The air distribution was modeled on a typical office room window air conditioning unit, air supply from a high pressure on the top and the low pressure exhaust on the bottom considering the existing manufacturing ratios for surface areas. The discharge angle for the supply grill of the AC unit was varied from 20 to 40 degrees. The position of the air conditioner was also varied and studied at 60%, 75% and 90% of the total height of the room. In addition, the location of the occupant within the office room was varied, two locations were studied, one where the occupant is far from the unit and the other to closer to the AC unit at the middle of the room. Predictions of the air movement, room temperature, room relative humidity, comfort level, and distribution of contaminants within the office room are shown. Analysis of these simulations is discussed. Energy estimations are also performed and evaluated. The positions of the air-conditioner unit, the inlet angle and the occupant position in the office room have shown to have an important impact on supply controlling air quality and thermal comfort. Results are in good agreements with the experimental data.The primary function of a HVAC (heating refrigerating and air conditioning) system is the generation and maintenance of comfort for occupants in a conditioned space [1]. This work also provides a detailed analysis of three-dimensional mixed convective flow induced by a window air conditioning system. Using a three dimensional CFD simulation, several characteristics of human comfort are analyzed.

Prestressed concrete piles are commonly used to support over-water highway bridges in marine environments. The reinforcing steel within will ultimately be degraded via corrosion damage due to the penetration of chloride ions from sea water. The service life of these structures is, in part, dictated by the time required to diffuse chloride ions through the concrete cover and subsequently corrode the steel. Therefore, by slowing the rate of diffusion or increasing the chloride threshold of the steel (or both) an increased service life can be expected. This thesis focuses on the latter whereby stainless steel reinforcing alternatives were investigated to elevate the chloride threshold before corrosion begins. The designation "stainless" steel implies corrosion resistance. However, corrosion resistance in itself is not a sufficient condition to make it a suitable alternative for prestressed concrete applications. In this study, the corrosion susceptibility of stainless steel alloys was scrutinized with the understanding that high strength stainless steels are vulnerable to stress corrosion cracking (SCC). This investigation screened three candidate alloys that span the norms of stainless steel compositions: a common austenitic stainless steel with high nickel content (316L), a less common austenitic stainless steel with low nickel but high manganese (XM 29), and a duplex stainless steel with high chromium and an additional constituent, molybdenum (2205). Each alloy was subjected to two stress conditions imposed by varied mechanical fixtures then subjected to various forms of high chloride concentrations. The pH of these conditions was also varied and in one case simulated the high pH common to concrete pore water solutions. Elevated temperatures were used to accelerate the effects of these exposures. Results of Phase 1 showed that for exposure at 135oC (275oF) cracking of alloys 316 L and 2205 occurred after 1 hour while XM29 experience cracking after 24 hours. At 90oC (194oF) alloy 316L cracked after 4 hours; XM29 did not crack after 96 hours while 2205 did crack after 96 hours. The results were interpreted with an Arrhenius relationship between time to cracking and test temperature to extrapolate toward the anticipated service regime. Results of Phase 2 showed that SCC was less likely to initiate in high pH conditions than in low pH conditions at typical marine environment temperatures and chloride concentration. In these limited tests the SCC performance of XM29 was better relative to that of the other two alloys.

Given the increasing awareness of indoor air quality (IAQ) and the direct correlation to human health, passive removal materials (PRM) have become known as a potential strategy for reducing occupant exposure to indoor air pollutants (Lu 2013). In recent studies, untreated natural wool fiber has been recognized as a PRM for removing formaldehyde, sulfur dioxide and nitrogen dioxide. These are common volatile organic compounds (VOCs) emitted from sources, such as building materials, fixtures, furnishings and cleaning supplies (Darling et al. 2012). Test chamber studies have shown that wool fiber can irreversibly remove up to 67% of these VOC’s in an interior environment (Curling et al 2012). When the toxins come in physical contact with the fiber, a chemical reaction occurs due to the amino-acid side chains within the keratin molecule. Increase in air-tight buildings has recently become a concern with the rising popularity of sustainable building practices, causing occupant exposure to these indoor air pollutants to rise (Weschler 2009). Beyond known adverse health effects, such as eye irritation and respiratory issues, formaldehyde has been designated by the International Agency for Research on Cancer (IARC) as a human carcinogen and is the leading cause for Sick Building Syndrome (SBS) (World Health Organization 2010). The interior cortex of a wool fiber is hydrophilic – highly water absorbent, and can absorb 1/3 its weight in moisture. Wool fiber has a unique wicking property that allows the fiber to absorb water vapor from the air in a regulating sense; absorbing when there is an excess moisture level and releasing the gained moisture when the surrounding atmosphere is less humid. This provides passive humidity regulation in an indoor environment, stabilizing the comfort level of 20-50% relative humidity (RH) without requiring higher air-conditioning or ventilation rates (Bingelli 2010). Wool also has excellent properties for optimizing indoor acoustics, as it absorbs acoustic energy via the friction of air being moved through the tiny spaces between fibers and reduces traveling noise and reverberation (Bingelli 2010). In an untreated, natural roving state the density of wool is ideal for acoustic control in conversational speech situations where 70dB or lower is present, such as meeting rooms, lobbies and restaurants. With the consideration of these properties, wool has the capability to improve the indoor environment quality (IEQ) and the health of occupants through the absorption of indoor air pollutants, humidity regulation and acoustic control. As Australia and the USA are among the top 3 wool producing countries, I will be working specifically with locally sourced wool from New South Wales and Texas as a basis for a sustainable IEQ intervention installation model that may be applied to future projects. The wool was obtained from local, small-scale fiber farms that implement hand processing in an effort to reduce toxins, in addition to lowering the manufacturing energy and transportation emission requirements. The local supply chain model provides increased environmental, social, economic and human health benefits to the design. Individual installations based on the vernacular wool fiber atrributes and interior climate needs will greatly increase the overall spatial environment, while also serving as an aesthetically pleasing piece of art. / text

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Water is essential to life on Earth. Since water exists in all three phases (solid, liquid, and gas) on Earth, it exists in various reservoirs throughout the planet that compose the hydrologic cycle, and its movement through these reservoirs requires energy. Thus, water is a key component of the energy balance of the Earth. Despite its importance, its representation in modeling and dataset generation is problematic. Here, the depiction of three phenomena, ocean surface turbulent fluxes, humidity inversions, and groundwater, are assessed, and suggestions for improvements of their representations are made. First, ocean surface turbulent fluxes, including those of moisture (latent heat flux), heat (sensible heat flux), and momentum (wind stress), from reanalysis, satellite-derived, and combined products which are commonly used to produce climatologies and to evaluate global climate models are compared to in situ observations from ship cruises to ascertain which products are the least problematic. The National Aeronautics and Space Administration’s reanalysis, the Modern Era Retrospective Analysis for Research and Applications, is the least problematic for all three fluxes, while a couple of others are the least problematic for only one of the three fluxes. Also, the product biases are disaggregated into uncertainties from the grid cell mean quantities, or bulk variables, used plus the residual uncertainties which includes the algorithm uncertainties due to the parameterization used to relate the small-scale turbulent processes to the large-scale bulk variables. The latter contribute the most to the majority of product latent heat fluxes, while both uncertainties can contribute the most to product sensible heat fluxes and wind stress. Thus, both algorithms and bulk variables need to be improved in ocean surface flux datasets. Second, humidity inversion climatologies in five reanalyses are evaluated. Humidity inversions, similar to its thermal counterpart, are layers in which specific humidity increases with height rather than the usual decrease with height. These are especially persistent in the polar regions in autumn and winter. However, Arctic inversions are the strongest in summer corresponding to the time of year that low cloud cover is the highest. Comparing the reanalysis inversions to radiosonde observations reveals some problems with the realization of humidity inversions in reanalyses including the misrepresentation of the diurnal cycle and of the overproduction of inversions in areas outside the polar regions. Finally, the simulation of groundwater in the Community Land Model (CLM) as used in the Community Earth System Model is made more realistic by including variable soil thickness. Because the bottom of the model soil column is placed at effectively bedrock, the unconfined aquifer model currently used in CLM is removed and a zero bottom water flux is put in place. The removal of the unconfined aquifer allows the simulation of groundwater to not be treated separately from soil moisture. The model is most affected where the number of soil layers is reduced from the original constant 10 layers and largely unaffected where the number of soil layers is increased except for baseflow where the mean annual range in rainfall is large.

No description available.

Problemos aktualumas. Dabar beveik visi žemės ūkio darbai dirbami mobilios technikos pagalba. Netinkama traktoriaus kabinos šiluminė aplinka mažina darbo našumą, kokybę, iškyla galima rizika dėl sveikatos sutrikimo, pablogėjimo. Darbo tikslas. Išanalizuoti nuo ko priklauso šiluminė aplinka traktoriaus kabinoje, ištirti ir palyginti trijų skirtingų traktorių amžiaus ir markės šiluminės aplinkos parametrus su normuojamomis bei palyginti tarpusavyje ir pasiūlyti priemones šiluminei aplinkai traktorių kabinose gerinti. Tyrimai atlikti traktoriuose ,,John Deere 6330“ (2008 m.), ,,MTZ-820“ (1994 m.) ir ,,T-16M“ (1974 m.) kabinose. Gauta, kad oro judėjimo greičiai traktorių kabinose buvo iki 0,1 m/s, o leistinas yra iki 0,3 m/s. Pakankama šiluminės aplinkos oro temperatūra 17°C traktoriaus ,,John Deere 6330“ kabinoje buvo po 53 min., traktoriaus ,,MTZ-820“ kabinoje maksimali temperatūra buvo 13,5°C, o traktoriaus ,,T-16M“ kabinoje maksimali temperatūra buvo 10,6°C. Santykinis oro drėgnis viršijo pakankamos šiluminės aplinkos vertę iki 75% visuose traktoriuose. Traktoriaus ,,John Deere 6330“ kabinos priekinės sienelės atitvaros temperatūra siekė 22,26°C ir buvo 2,29 kartus didesnė negu traktoriaus ,,MTZ-820“ bei 31,8 kartus didesnė negu traktoriaus ,,T-16M“. Metalinę atitvarą iš kabinos vidaus pusės padengus veltiniu, šilumos srauto tankis per atitvarą į aplinką sumažėtų nuo 74,82 W/m2 iki 0,14 W/m2. / Problem. Now almost all the agricultural work performed in support of mobile equipment. Inappropriate tractor thermal environment reduces the productivity, quality and there is a potential risk for health problems, deterioration. Objective. To analyze the thermal environment depends on what the tractor's cab, to investigate and compare the three tractors of different ages and grades of thermal environmental parameters and compared with the regulated one another and to propose measures to the thermal environment of tractor cabs to improve. Investigations were carried out for tractors ,,John Deere 6330” (2008), ,,MTZ-820” (1994) and ,,T-16M” (1974) booths. Try the air speed of tractor cabs were up to 0,1 m/s and an allowance of up to 0,3 m/s. Sufficient heat the ambient air temperature of 17°C, a tractor ,,John Deere 6330” cab was after 53 minutes. Tractor ,,MTZ-820” deck the maximum temperature was 13,5°C and the tractor ,,T-16M” deck the maximum temperature was 10,6°C. Relative humidity above a sufficient amount of thermal environment by up to 75% on all tractors. Tractor ,,John Deere 6330” cab front wall of the envelope temperature was 22,26°C was 2,29 times higher than the tractor ,,MTZ-820“ and 31,8 times higher than the tractor ,,T-16M”. Metal guard from inside the cab sides felt coating, heat flux density through the guard into the environment would be reduced from74,82 W/m2 to 0,14 W/m2.