Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item DURABILITY AND OPTIMIZATION OF SOFC COMPOSITE CATHODES(2016) Painter, Albert Steven; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The combination of the conventional cathode material, La0.8Sr0.2MnO3-𝛿 (LSM), and exceptional oxygen ion conducting material, (Er0.2Bi0.8)2O3 (ESB), has shown promise as a potential candidate for low temperature solid oxide fuel cell (LT-SOFC) cathodes. Though the initial performance of this composite is encouraging, the long-term stability of LSM-ESB has yet to be investigated. Here electrochemical impedance spectroscopy (EIS) was used to in situ monitor the durability of LSM-ESB at typical LT-SOFC operation temperatures. The degradation rate as a function of aging time was extracted based on the EIS data. Post analysis suggests that below 600 °C the order-disorder transition of ESB limits the performance due to a decrease in the oxygen incorporation rate. Above 600°C, the formation of secondary phases, identified as Mn-Bi-O, is the major performance degradation mechanism. Furthermore, the relative particle size of the LSM to ESB was optimized to minimize long-term degradation in cathode performance.Item INCREASING DURABILITY OF HOT MIX ASPHALT PAVEMENTS DESIGNED WITH THE SUPERPAVE SYSTEM(2009) Karimi, Sahand Sasha; Goulias, Dimitrios G; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the implementation of the Superpave mix design method, state highway agencies have experienced significant problems in durability of Hot Mix Asphalt mixtures due to lower binder content. To get a better understanding of the HMA mix production and the current specifications used by MSHA, the following were examined: i) differences in HMA properties that have been observed between samples taken at the plant (QC) vs. behind the paver (QA), ii)possibility of defining a transfer function between QA and QC data and iii) the potential risk to both the agency and the contractors using simulation analysis and based on the current specifications and pay factor equations. For this purpose a simulation tool was developed. The F and t tests showed that the QA and QC are two different populations and cannot be related. The simulation analysis illustrated that the correlation among mixture parameters doesn't affect the long run average pay factor. In addition it was concluded that the newly adopted pay equations are fairly rewarding and penalizing the contractors for mixtures, but the density pay equation needs modification.Item Damage Initiation and Evolution in Voided and Unvoided Lead Free Solder Joints Under Cyclic Thermo-Mechanical Loading(2007-02-05) Jannesari Ladani, Leila; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of process-induced voids on the durability of Sn-Pb and Pb-free solder interconnects in electronic products is not clearly understood and researchers have reported conflicting findings. Studies have shown that depending on the size and location, voids are not always detrimental to reliability, and in fact, may sometimes even increase the durability of joints. This debate is more intensified in Pb-free solders; since voids are more common in Pb-free joints. Results of experimental studies are presented in this study to empirically explore the influence of voids on the durability of Ball Grid Array (BGA) Pb-free solder joints. In order to quantify the detailed influence of size, location, and volume fraction of voids, extensive modeling is conducted, using a continuum damage model (Energy Partitioning model), rather than the existing approaches, such as fracture mechanics, reported in the literature. The E-P approach is modified in this study by use of a successive initiation method, since depending on their location and size; voids may influence either the time to initiate cyclic fatigue damage or time to propagate fatigue damage, or both. Modeling results show competing interactions between void size and location, that results in a non-monotonic relationship between void size and durability. It also suggests that voids in general are not detrimental to reliability except when a large portion of the damage propagation path is covered with either a large void or with many small voids. In addition, this dissertation also addresses several fundamental issues in solder fatigue damage modeling. One objective is to use experimental data to identify the correct fatigue constants to be used when explicitly modeling fatigue damage propagation in Pb-free solders. Explicit modeling of damage propagation improves modeling accuracy across solder joints of vastly different architectures, since the joint geometry may have a strong influence on the ratio of initiation-life to propagation-life. Most solder fatigue models in the literature do not provide this capability since they predict failure based only on the damage accumulation rates during the first few cycles in the undamaged joint. Another objective is to incorporate into cyclic damage propagation models, the effect of material softening caused by cyclic micro-structural damage accumulation in Pb-free solder materials. In other words the model constants of the solder viscoplastic constitutive model are continuously updated with the help of experimental data, to include this cyclic softening effect as damage accumulates during the damage-propagation phase. The ability to model this damage evolution process increases the accuracy of durability predictions, and is not available in most current solder fatigue models reported in the literature. This mechanism-based microstructural damage evolution model, called the Energy Partitioning Damage Evolution (EPDE) model is developed and implemented in Finite Element Analysis of solder joints with the successive initiation technique and the results are provided here. Experimental results are used as guidance to calibrate the Energy Partitioning fatigue model constants, for use in successive initiation modeling with and without damage evolution. FEA results show 15% difference between the life predicted by averaging technique and successive initiation. This difference could significantly increase in the case of long joints such as thermal pads or die-attach, hence validating the use of successive initiation in these cases. The difference between using successive initiation with and without damage evolution is about 10%. Considering the small amount of effort that has to be made to update the constitutive properties for progressive degradation, it is recommended that softening be included whenever damage propagation needs to be explicitly modeled. However the damage evolution exponents and the corresponding E-P model constants obtained in this study, using successive initiation with damage evolution, are partially dependent on the specimen geometry. Hence, these constants may have to be re-calibrated for other geometries.