Understanding the mechanisms for fatigue crack initiation and propagation in micron-scale silicon (Si) is of great importance to assess and improve the reliability of Si based microelectromechanical systems (MEMS) in harsh environments. Accordingly, this investigation studies the fatigue properties of 10-micron-thick single-crystal Si (SCSi) films using kHz-frequency resonating structures under fully-reversed loading. Overall, the stress plays a major role on the fatigue properties: decreasing the stress amplitude from ~3-3.5 GPa to ~1.5-2 GPa results in an increase in lifetime from 10² to 10¹⁰ cycles, and a decrease in degradation rate by 4-5 orders of magnitude. In addition to stress, the influences of resonant frequency (4 vs. 40 kHz) and environment (30°C, 50%RH vs. 80°C, 30%RH and 80°C, 90%RH) on the resulting S-N curves and resonant frequency evolution are thoroughly investigated. In the high- to very high-cycle fatigue (HCF/VHCF) regime, both the frequency and environment strongly affect the fatigue properties. Damage accumulation rates are significantly higher in harsh environments. In 80°C, 90%RH the rates exceed by one to two orders of magnitude the values at 30°C, 50%RH for similar stress amplitudes. The separate influence of humidity, affecting the adsorbed water layer thickness, is also highlighted at 80°C: the decrease rates are measured up to one order of magnitude lower at 30%RH than at 90%RH. Moreover, a strong influence of frequency is detected. These observations bring further evidence supporting reaction-layer fatigue as a viable description of the HCF/VHCF behavior of micron-scale Si.

<p><p>This master thesis is about an investigation of anti-moisture methods for a nonhermeticelectronics enclosure containing a number of printed circuit boards(PCB) and placed in a severe climatic environment.</p><p>The relevant theoretical background was provided first. It included the impact of moisture on electronics, some useful psychrometrics concepts, heat transfer fundamentals, introduction of environmental test, temperature and humiditysensing techniques, computational fluid dynamics (CFD) modelling, antimoisture methods and commercial heaters in the current marketing.</p><p>Then a CFD modelling methodology was developed and validated based on experiment data. An extra heater was added to the enclosure to prevent water adsorption on printed circuit assemblies (PCA) surfaces. The heat dissipation and switch-on period strategies were parametrically studied in order to maintain the internal relative humidity below 60% in the vicinity of PCA surfaces, according to the relative humidity control method.</p><p>In the end, results obtained from the environmental tests and the CFD simulations were presented and analyzed. Conclusions and future work were also discussed.</p></p>

<p>Joints are critical parts of timber structures, transmitting static and dynamic forces between structural members. The ultimate behavior of a loaded building depends strongly on the structural configuration and the capacity of the joints. The collapse of a whole building or less extensive accidents that may occur is usually starting as a local failure inside or in the vicinity of a joint. Such serious failures have recently occurred in our Nordic countries. Especially the collapse of two large glued laminated timber structures clearly indicates the need of an improved joint design. The trend toward larger and more complex structures even further increases the importance of a safer design of the joints.</p><p>An aim of this partly experimental and partly numerically based thesis has been to investigate if steel-to-timber dowel joints are affected by moisture-induced stresses. The experimental results showed that the load-bearing capacity of the joints is reduced by such a moisture influence. Most of the decrease in load-bearing capacity observed was found in joints initially exposed to restrained shrinkage deformations caused by the presence of dowel fasteners in the joint area. The load-bearing capacity was, however, also found to decrease in joints exposed to an initial decrease in moisture without any fasteners present in the specimens during storage before loading. An explanation of this unexpected behavior is that moisture gradients cause tensile stresses. It is shown by numerical simulations that the moisture-induced stresses are so large that they may have a considerable influence on the joint behavior.</p><p>Use of contact-free measurement methods, used in some of the experimental tests, was in many ways found to be superior to traditional measurement techniques, but was also found to be a valuable complement to the numerical analysis performed. From numerical results obtained in combination with results from contact-free measurements several observations of considerable interest were made. For dowel-type joints loaded in tension parallel to the grain a strongly non-uniform strain distribution was found in the joint area. It was further observed that the shear and tensile strains were concentrated close to the fasteners in the joint area. These concentrations will influence the failure mode of the joint. A general observation was that the larger sized joints failed in a brittle manner.</p><p>Keywords: constraint stresses, contact-free measurement, dowel-type joints, humidity variations, moisture-induced deformations, timber structures</p>

This thesis consists of two different research problems. In the first one, the aim is to model and simulate a solar-powered, single-effect, absorption refrigeration system using a flat-plate solar collector and LiBr-H2O mixture as the working fluid. The cooling capacity and the coefficient of performance of the system are analyzed by varying all independent parameters, namely: evaporator pressure, condenser pressure, mass flow rate, LiBr concentration, and inlet generator temperature. The cooling performance of the system is compared with conventional vapor-compression systems for different refrigerants (R-134a, R-32, and R-22). The cooling performance is also assessed for a typical year in Tampa, Florida. Higher COP values are obtained for a lower LiBr concentration in the solution. The effects of evaporator and condenser pressures on the cooling capacity and cooling performance are found to be negligible. The LiBr-H2O solution shows higher cooling performance compared to other mixtures under the same absorption cooling cycle conditions. For typical year in Tampa, Florida, the model shows a constant coefficient of performance of 0.94. In the second problem, a numerical model is developed for a typical food retail store refrigeration/HVAC system to study the effects of indoor space conditions on supermarket energy consumption. Refrigerated display cases are normally rated at a store environment of 24ºC (75ºF) and a relative humidity of 55%. If the store can be maintained at lower relative humidity, significant quantities of refrigeration energy, defrost energy and anti-sweat heater energy can be saved. The numerical simulation is performed for a typical day in a standard store for each month of the year using the climate data for Tampa, Florida. This results in a 24 hour variation in the store relative humidity. Using these calculated hourly values of relative humidity for a typical 24 hour day, the store relative humidity distribution is calculated for a full year. The annual average supermarket relative humidity is found to be 51.1%. It is shown that for a 5% reduction in store relative humidity that the display case refrigeration load is reduced by 9.25%, and that results in total store energy load reduction of 4.84%. The results show good agreement with available experimental data.

Recent failures and severe corrosion distress of post-tensioned (PT) bridges in Florida have revealed corrosion of the 7-wire strands in tendons. Post-tensioned duct assemblies are fitted with multiple 7-wire steel strands and ducts are subsequently filled with grout. During construction, the length of time from the moment in which the strands have been inserted into the ducts, until the ducts are grouted, is referred to as the `ungrouted' period. During this phase, the steel strands are vulnerable to corrosion and consequently the length of this period is restricted (typically to 7 days) by construction guidelines. This investigation focuses on determining the extent of corrosion that may take place during that period, but limited to strands that were in the unstressed condition. Visual inspections and tensile testing were used to identify trends in corrosion development. Corrosion induced cracking mechanisms were also investigated via wire bending and metallographic cross section evaluation. Corrosion damage on unstressed strands during ungrouted periods of durations in the order of those otherwise currently prescribed did not appear to seriously degrade mechanical performance as measured by standardized tests. However the presence of stress in the ungrouted period, as is normally the case, may activate other mechanisms (e.g., EAC) that require further investigation. As expected in the unstressed condition, no evidence of transverse cracking was observed.

Continuously reinforced concrete pavement (CRCP) is a portland cement concrete (PCC) pavement structure with a continuous longitudinal steel layout. CRCP is forming a major portion of PCC roadway systems in the state of Texas due to its low life cycle cost, ease of maintenance, and durable nature. While the overall performance of CRCP is proven to be excellent, some performance problems are still found as a form of distresses such as punchout and spalling. The current pavement design guide states that these distresses are closely related with the early-age behavior characteristics of CRCP, and various measures are underway to develop to improve the long-term performance of CRCP in terms of initial material design and use, structural design, and quality control. Understanding the current issues that pavement engineers and researchers face, the primary objective of this dissertation research focuses on sound understanding of the early-age structural behavior characteristics of CRCP and its effect on the long-term performance to provide reliable design and analysis criteria for CRCP. To achieve this main objective, characterizing the early-age structural response in CRCP was a core task of this study. For this purpose, a zero-stress temperature (ZST), one of the design and construction variables considered to have most significant effects on CRCP behavior and performance, was evaluated. As a beginning point of the entire framework, a series of field experiments were conducted in four new PCC pavement construction projects in the state of Texas to evaluate the actual structural response in early-age CRCP since a laboratory experiment would have a critical limitation in simulating the restraint conditions that exist in actual CRCP. To expand this core task to various parametric categories, a computer-aided parametric simulation was performed using valid numerical models. Based on data sets obtained from the parametric investigation, a statistical model to quantify the early-age structural response of CRCP was proposed to implement in codes of practice and pavement design guides. A secondary task was to identify a correlation between the early-age structural response and the long-term performance of CRCP structures. Since the experimental and analytical investigations tended to provide quite localized information for the time-dependent behavior of CRCP, the overall performance of CRCP could not be properly identified solely based on those results. To overcome this limitation, extensive field condition surveys were performed in seven different old CRCP sections with known material and early-age temperature history to find the implications of early-age behavior characteristics on the long-term performance of CRCP from a macroscopic point of view. It is expected that this research effort will provide pavement engineers and researchers with useful information to understand the actual time-dependent behavior of CRCP and a solid foundation to improve the sustainability of CRCP structures. / text

Single-walled carbon nanotubes are naturally-forming nanostructures that have attracted considerable recent research interest due to their unique opto-electronic properties and comparative ease of fabrication. Two-thirds of nanotube species are semiconductors due to symmetry conditions imposed by their pseudo-one-dimensional tubular structure, and exhibit band-gap photoluminescence when isolated from their environment. Despite their elegant structural simplicity, fundamental properties of carbon nanotubes, such as their intrinsic quantum efficiency, non-linear excitonic recombination mechanisms, and the role of environmental effects, continue to be disputed in the literature. The design of an apparatus capable of observing nanotube photoluminescence is presented, along with conclusive proof of the observation of a single (9,8)-chirality nanotube in the form of spectral, spatial, and polarization-dependent measurements. The dependance of the excitation and emission spectra of a single nanotube on the excitation intensity is explored and the emission spectra found to be described by a Gaussian peak function, in contrast to previously-reported results. The unexpected ability to cause redshifts in the emission spectrum via the ambient humidity is discovered, which has consequences on experimental best practices. Photoluminescence quantum efficiencies are measured to be 4±2% and 13±6% for two different nanotubes. This is at the high end of the range for comparable literature results, and supports the validity of a recent literature value for the effective atomic absorption coefficient for carbon, AC=1.6×10^−3nm^2, which is ten times greater than previous literature values. Pulsed power dependence studies show that the PL emission undergoes ‘hard’ saturation at an excitation intensity of 0.5×10^12photons/pulse/cm2, which is at least 100 times lower than previous reports and provides insight into non-linear decay dynamics. A novel theoretical model is developed to explain this saturation process, which yields an absorption co-efficient of AC=1.2±0.3×10^−3nm^2 as a fit parameter. Time-resolved photoluminescence dynamics are explored using femtosecond excitation correlation spectroscopy. Results suggest that the one-body decay processes are bi-exponential, with time constants of 31±4ps and 313±61ps, but also highlight the limitations of this technique in observing the expected very rapid (~1 ps) two-body Auger recombination process. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-09-26 16:23:40.81

No description available.

The active ventilation of grain with the optimum drying regimen was the main issue of the work. The moisture of the harvested grain in Lithuania often exceeds the conditional one. Thus the grain should by dried before the storage. The active ventilation is mostly used in Lithuania to dry the grain, it was the topical point in this work. There was analyzed meteorology of the topical months, were maked the calculating equations, whitch we can prognosticate these meteorological facts: temperature and relative humidity. The maked mathematical model defines metathesis moisture‘s between ambient air flow and grain. Also there were the algorithm of model‘s realisation‘s and the computer programme with Visual basic 6.0 maked. Adequacy of mathematical model‘s was rated with results of experimental researches and calculated results.

Radio frequency identification (RFID) and sensing are two key technologies enabling the Internet of Things (IoT). Development of RFID tags augmented with sensing capabilities (RFID sensor tags) would allow a variety of new applications, leading to a new paradigm of the IoT. Chipless RFID sensor technology offers a low-cost solution by eliminating the need of an integrated circuit (IC) chip, and is hence highly desired for many applications. On the other hand, printing technologies have revolutionized the world of electronics, enabling cost-effective manufacturing of large-area and flexible electronics. By means of printing technologies, chipless RFID sensor tags could be made flexible and lightweight at a very low cost, lending themselves to the realization of ubiquitous intelligence in the IoT era. This thesis investigated three construction methods of printable chipless RFID humidity sensor tags, with focus on the incorporation of the sensing function. In the first method, wireless sensing based on backscatter modulation was separately realized by loading an antenna with a humidity-sensing resistor. An RFID sensor tag could then be constructed by combining the wireless sensor with a chipless RFID tag. In the second method, a chipless RFID sensor tag was built up by introducing a delay line between the antenna and the resistor. Based on time-domain reflectometry (TDR), the tag encoded ID in the delay time between its structural-mode and antenna-mode scattering pulse, and performed the sensing function by modulating the amplitude of the antenna-mode pulse. In both of the above methods, a resistive-type humidity-sensing material was required. Multi-walled carbon nanotubes (MWCNTs) presented themselves as promising candidate due to their outstanding electrical, structural and mechanical properties. MWCNTs functionalized (f-MWCNTs) by acid treatment demonstrated high sensitivity and fast response to relative humidity (RH), owing to the presence of carboxylic acid groups. The f-MWCNTs also exhibited superior mechanical flexibility, as their resistance and sensitivity remained almost stable under either tensile or compressive stress. Moreover, an inkjet printing process was developed for the f-MWCNTs starting from ink formulation to device fabrication. By applying the f-MWCNTs, a flexible humidity sensor based on backscatter modulation was thereby presented. The operating frequency range of the sensor was significantly enhanced by adjusting the parasitic capacitance in the f-MWCNTs resistor. A fully-printed time-coded chipless RFID humidity sensor tag was also demonstrated. In addition, a multi-parameter sensor based on TDR was proposed.The sensor concept was verified by theoretical analysis and circuit simulation. In the third method, frequency-spectrum signature was utilized considering its advantages such as coding capacity, miniaturization, and immunity to noise. As signal collision problem is inherently challenging in chipless RFID sensor systems, short-range identification and sensing applications are believed to embody the core values of the chipless RFID sensor technology. Therefore a chipless RFID humidity sensor tag based on near-field inductive coupling was proposed. The tag was composed of two planar inductor-capacitor (LC) resonators, one for identification, and the other one for sensing. Moreover, paper was proposed to serve as humidity-sensing substrate for the sensor resonator on accounts of its porous and absorptive features. Both inkjet paper and ordinary packaging paper were studied. A commercial UV-coated packaging paper was proven to be a viable and more robust alternative to expensive inkjet paper as substrate for inkjet-printed metal conductors. The LC resonators printed on paper substrates showed excellent sensitivity and reasonable response time to humidity in terms of resonant frequency. Particularly, the resonator printed on the UV-coated packaging paper exhibited the largest sensitivity from 20% to 70% RH, demonstrating the possibilities of directly printing the sensor tag on traditional packages to realize intelligent packaging at an ultra-low cost. / <p>QC 20150326</p>