Wireless clients are vulnerable to exploitation by evil twins due to flaws in the authentication process of 802.11 Wi-Fi networks. Current certificate-based wireless authentication protocols present a potential solution, but are limited in their ability to provide a secure and usable platform for certificate validation. Our work seeks to mitigate these limitations by exploring a client-side strategy for utilizing alternative trust models in wireless network authentication. We compile a taxonomy of various trust models for conducting certificate-based authentication of wireless networks and methodically evaluate each model according to desirable properties of security, usability, and deployability. We then build a platform for leveraging alternative certificate-based trust models in wireless networks, present a proof-of-concept using one of the most promising alternative validation models identified--a whitelisting and pinning hybrid--and examine its effectiveness at defending against evil twin attacks in 802.11 networks.

Fetal testosterone (FT) exposure influences sexual differentiation and may promote well-established sex differences in externalizing (EXT) behavior. Although puberty may be a critical period for these effects, it is unknown how FT exposure influences EXT as a function of pubertal development. We used a longitudinal, multi-sample design to test the relationships between two proxy indices of FT exposure and EXT as a function of age and pubertal development (approximately ages 6, 9, 11, 14, and 16). Twin data were used to approximate FT exposure (TT-FT) because testosterone is thought to cross the intrauterine membrane and cause variability in co-twin gonadal hormone exposure, with increasing exposure for males and participants with male co-twins. Increasing number of older siblings may also approximate increasing FT exposure (SI-FT), although existing research has yet to disentangle possible postnatal socialization effects from potential FT exposure using this variable. Given that biologically related siblings share a fetal and social environment while non-biologically related siblings simply share a social environment, we tested the independent effect of SI-FT on EXT using a sibling adoption design. Across four independent samples, SI-FT and TT-FT predicted externalizing for males alone. SI-FT predicted EXT over-and-above socialization influences and interacted with pubertal development in two independent samples, with elevated EXT for those in mid-late puberty that were exposed to increased FT. TT-FT predicted EXT differentially as a function of developmental period. Our data are consistent with the notion that exposure to FT promotes sexually differentiated, sexually selected behavior during reproductively relevant periods.

The need for lighter weight vehicles to improve fuel efficiency is becoming increasingly imperative. Sheet magnesium alloys offer the potential as a light weight material for use in the transportation industry due to their high specific strength and stiffness. In fact, magnesium alloys have the highest strength-to-weight ratio of all the common structural metals. Though the demand for light weight materials is present and sheet magnesium is available, the use of these materials in automotive applications has been rather limited due to high production costs and poor corrosion performance and formability. A promising process to produce wrought magnesium sheet in a more cost effective manner is Twin Roll Casting (TRC). In addition, enhanced corrosion resistance and ductility may be realized in these sheet alloys with the possible introduction of a clad layer during the TRC process thereby producing a laminate sheet where the surface properties are different from the core. The focus of this research was to investigate the potential of cladding magnesium alloy AZ31 material during the TRC process. As part of this research, a thermal fluid mathematical model of the TRC process was developed, which was then further refined to include the addition of a clad layer during the process. The TRC model was validated through experimental work conducted at the Pohang Institute of Science and Technology (POSTECH University), where TRC experiments of AZ31 were conducted under various casting conditions. The as-cast microstructure of the AZ31 sheets were characterized and measurements of the secondary dendrite arm spacing (SDAS) made at the mid-region were compared to predicted microstructures from the TRC model based on solidification history. The predicted SDAS matched with the measured values, thus, validating the model. Using the validated TRC model the feasibility of adding a clad layer was assessed and various simulations were conducted to observe the effects of cast speed, cast thickness, and clad material on the thermal history and temperature profile in both the clad and core domains. The material properties and clad thickness did not seem to impact the temperature profiles significantly, while the cast speed and initial temperature dictated whether or not the cast would be successful. Using these operational parameters a process window was created (based on the CANMET facility) to illustrate the feasibility of casting and cladding during TRC. This window is beneficial for future experimentation and understanding the effects of these casting parameters.

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¡@¡@We consider output voltage regulation of a novel twin-buck switching power converters with so-called zero voltage switching (ZVS) and zero current switching (ZCS). In order to observe the constraints imposed by ZVS and ZCS, it is necessary to adopt the pulse frequency modulation (PFM) technique, which lead to a switching system with aperiodic operating cycles. The control design is based on a sampled data model of the original switching dynamics and a linear quadratic criterion that takes the at-sampling behaviour into account. The applicability of the proposed controller is validated via numerical simulations written in MATLAB and SIMULINK. The controller is realized using Field Programmable Gate Array (FPGA). The experimental results indicate that the feedback system have good transient response and adequate robustness margin against source and load variation, which verify the applicability of the proposed control design approach.

Twin-screw multiphase pumps experience a severe decrease in efficiency, even the breakdown of pumping function, when operating under wet gas conditions. Additionally, field operations have revealed significant vibration and thermal issues which can lead to damage of the pump internals and expensive repairs and maintenance. There are limited models simulating the performance of twin-screw pump under these conditions. This project develops a pump-user oriented simulator to model the performance of twin-screw pumps under wet gas conditions. Experimental testing is conducted to verify the simulation results. Based on the simulations, an innovative solution is presented to improve the efficiency and prevent the breakdown of pumping function. A new model is developed based upon a previous Texas A&M twin-screw pump model. In this model, both the gas slip and liquid slip in the pump clearances are simulated. The mechanical model is coupled with a thermodynamic model to predict the pressure and temperature distribution along the screws. The comparison of experimental data and the predictions of both isothermal and non-isothermal models show a better match than previous models with Gas Volume Fraction (GVF) 95% and 98%. Compatible with the previous Texas A&M twin-screw pump model, this model can be used to simulate the twin-screw pump performance with GVF from 0% to 99%. Based on the effect of liquid viscosity, a novel solution is investigated with the newly developed model to improve the efficiency and reliability of twin-screw pump performance with GVF higher than 94%. The solution is to inject high viscosity liquid directly into the twin-screw pump. After the simulations of several different scenarios with various liquid injection rates and injection positions, we conclude that the volumetric efficiency increases with increasing liquid viscosity and injecting liquid in the suction is suggested.

Pools of mutants of minimal complexity but maximal coverage of genes of interest facilitate screening for genes under selection in a particular environment. Prior to this work, mutants were generated by random transposon insertions, which yielded highly complex pools for in vivo studies. Recent advances in polymerase chain reaction (PCR)-based mutagenesis in bacteria using the lambda red recombinase, as well as whole genome sequencing, enable a more directed approach for the generation of mutants. The lambda red approach was used to construct individual mutants in 1,023 Salmonella enterica serovar Typhimurium genes, including almost all genes found in Salmonella, but not in related genera. All the mutations were confirmed simultaneously using a novel amplification strategy to produce labeled ribonucleic acid (RNA) from a T7 RNA polymerase promoter, introduced during the construction of each mutant, followed by hybridization of this labeled RNA to a Typhimurium genome tiling array. To demonstrate the ability to identify fitness phenotypes using our pool of mutants, the pool was subjected to selection by intraperitoneal injection into BALB/c (Bagg Albino) mice and was recovered from the spleen. Changes in the representation of each mutant were monitored using T7 transcripts hybridized to a novel inexpensive minimal microarray. Among the top 120 statistically significant spleen colonization phenotypes, 51 were mutations in genes with no previously known role in this model. Fifteen phenotypes were tested using individual mutants in competitive assays and eleven were confirmed in individual mixed intraperitoneal infection in mice, including the first two examples of attenuation for sRNA mutants in Salmonella. We refer to our method as Array-Based Analysis of Cistrons Under Selection (ABACUS). Among the confirmed mutants identified in the ABACUS screen was a component of the twin arginine transport (Tat) system, tatC, required for transport of folded proteins across the cellular membrane. TatC is the highly conserved component necessary for recognition of the twin arginine containing signal sequence S/T-R-R-x-FL- K. We confirmed [delta] tatC mutants are defective for colonization of the liver and spleen in competitive infections with wild type ATCC14028 after intraperitoneal infection in Salmonella- susceptible (BALB/c). We also found that [delta] tatC mutants were defective for swimming motility, but not swarming motility, which was linked to the ability to elaborate flagellins on the bacterial surface under different conditions.

The focus of this research is the development of high strength, high conductivity copper films. Pure copper is soft and traditional strengthening mechanisms cause substantial decrease in conductivity. To address the challenge, epitaxial nanotwinned copper films are synthesized on HF etched Si (110) substrates. These films show high hardness (~ 2.8 GPa) due to high density of coherent twin boundaries (CTBs) which effectively block the motion of dislocations similar to grain boundaries (GBs). Resistivity of CTBs is calculated to be an order of magnitude lower than that of GBs. Hence, conductivity of nanotwinned copper is still comparable to that of pure copper. In addition, it is shown that average twin spacing can be controlled by adjusting deposition rate. Analytical studies together with experimental evidence show that nanotwins can improve the strength-to-resistivity ratio significantly in copper. In general, nanocrystalline metals suffer from low ductility. To study plastic deformation via rolling, thick polycrystalline nanotwinned copper foils are sputtered on SiO2 and then peeled off the substrate. Despite the high strength, room temperature rolling experiments show that nanotwinned copper films exhibit stable plastic flow with no shear localization or fracture even at thickness reduction of over 50%. Postdeformation studies of microstructure reveals that the plastic deformation is facilitated by the migration of CTBs normal to the twin boundary plane due to the glide of twinning dislocations in the twin plane. X-ray pole figure measurements show insignificant out of plane rotation as a result of 50% rolling thickness reduction. Thermal stability of nanocrystalline metals is also a concern. Free standing nanotwinned polycrystalline copper films show remarkable thermal stability after annealing at 800 degrees C. The driving force for twin growth is much lower than that for grain coarsening because the energy stored in CTBs is an order of magnitude lower than that of GBs. As a result, the average twin spacing stays below 20 nm after annealing. Such high thermal stability of nanotwins leads to the retention of hardness of 2.2 GPa. Low energy twin boundary may provide a unique way to achieve both high strength and high temperature thermal stability in certain metallic materials.

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The Northwest orebody, located in Pima County, Arizona, is a large tonnage skarn containing copper, silver, and molybdenum mineralization. The orebody is separated from the main orebodies of the Twin Buttes mine by the Twin Buttes fault. Sulfide mineralization and silicate alteration are systematically zoned along a westerly trend away from the fault and from a central intrusive complex. Original host rock chemistry was the major control on the silicate and sulfide mineralogy of the skarns. The highest grade ore occurs in a garnet-diopside alteration zone within beds of altered limestone and dolomite. Lower grade ores formed in cal-silicated siltstones, quartzites, and granitic rocks. The skarns exhibit a complex history of sequential veining and replacement. The alteration can be subdivided into an early thermal event, followed by three stages of hydrothermal alteration. The first stage of hydrothermal alteration is characterized by the formation of anhydrous Ca-Mg-Fe-Al silicates, predominantly garnet and diopside. Stage II is a period of sulfide mineralization. The copper sulfides were deposited when copper in the hydrothermal solutions reacted with Fe⁺⁺⁺₋ rich garnet to form chalcopyrite. Mineralization continued during Stage III as increasingly pyritic sulfide mineralization was deposited with hydrous silicate minerals, predominantly epidote and actinolite. Several points of evidence show that the mineralization in the Northwest orebody is related to the intrusive activity at the center of the Twin Buttes mine area and not to either the pre-mineral Ruby Star Granodiorite or to a buried intrusive at depth.