Wright-Patterson Air Force Base

About: Wright-Patterson Air Force Base is a other organization based out in Wright-Patterson AFB, Ohio, United States. It is known for research contribution in the topics: Laser & Mach number. The organization has 5817 authors who have published 9157 publications receiving 292559 citations. The organization is also known as: Wright-Patterson AFB & FFO.

CONTROLLED SPUTTERING OF METALS BY LOW-ENERGY Hg IONS

Abstract: Sputtering has been studied under well controlled conditions, i.e., low gas pressure (1 micron), defined energy (up to 300 ev) and angle of incidence of the bombarding ions, and high ion-current density (10 ma/${\mathrm{cm}}^{2}$), by immersing the target in a low-pressure Hg plasma of high density created in a pool-type vacuum arc.It turned out that threshold energies (minimum ion energy for sputtering to ensue) are considerably lower under oblique than under normal incidence, and the atoms are ejected away from the direction of incidence. Thresholds were measured for twenty-three metals under normal incident Hg-ion bombardment. They follow, without exception a simple law: The product of the momentum transferred at threshold from ion to target surface atom and the sound velocity of the target material is proportional to the heat of sublimation of the target material. The threshold energies to be expected accordingly in the case of rare gas ion bombardment under normal incidence are tabulated. The yield (atoms per ion) $\mathrm{vs}$ ion energy was carefully measured for the case of polycrystalline Pt. When metal single crystals were sputtered, it was discovered that atoms are preferentially ejected in the directions of closest packing, that is [110] in fcc, [111] in bcc and diamond lattice. Deposits sputtered from plane low-index surfaces of single crystals therefore form characteristic patterns. Their study reveals more details: At low ion energy, only such atoms are sputtered which have no obstructing neighbors in the way of a close-packed direction. At higher ion energy, additional atoms are set free from positions where neighbor atoms interfere. Such atoms deviate in a characteristic way from the directions of close packing. Most atoms sputtered in close-packed directions which are parallel to the surface are trapped again, causing growth of oriented hillocks. The possibility for crystal growth by sputtering was demonstrated in another experiment. Some characteristic features of the etch effects caused by sputtering are described.The basic process in sputtering at low ion energies is one of momentum transfer. The important parameters with respect to the gas discharge are ion energy, angle of incidence, and atomic weight of ion; on the target side they are the atomic weight, the elastic constants, crystal structure and orientation, the heat of sublimation, dislocations, and surface roughness. Many details need durther clarification, and the studies are being continued.

Heterogeneity of crystallographic texture in friction stir welds of aluminum

Abstract: Over the past decade, friction stir welding (FSW) has rapidly become an important industrial joining process, particularly in the aluminum industry Included among the advantages of FSW are such important attributes as improved weld strength and the elimination of cracking and porosity During the friction stir process, the metal undergoes a tortuous deformation path that is not yet fully understood The crystallographic texture that evolves during FSW contains sharp spatial gradients that undoubtedly influence the integrity of the weld and surrounding region in subsequent performance The locally measured textures are discussed in the context of the material flow required to produce such textures, ultimately resulting in an estimate of the flow field present during FSW

Fundamental studies of Au contacts in MEMS RF switches

Abstract: Microelectromechanical systems (MEMS) radio frequency (RF) switches hold great promise in a myriad of commercial, aerospace, and military applications. However, there is little understanding of the factors determining the performance and reliability of these devices. Fundamental studies of hot-switched gold (Au) contacts were conducted using a micro/nanoadhesion apparatus as a switch simulator. Experiments were conducted in a well defined air environment under precisely controlled operating conditions. Fundamental properties were connected to performance with an emphasis on the effects of contact force and electric current on contact resistance (R), microadhesion, and reliability/durability. Electric current had the most profound effect on switch performance. Observations at low current (1–10 μA) include: (1) slightly higher R; (2) asperity creep; (3) high adhesion after rapid switching; (4) switch bouncing; and (5) reasonable durability. Conversely, observations at high current (1–10 mA) include: (1) slightly lower R; (2) melting; (3) no measurable adhesion; (4) less propensity for switch bouncing; (5) necking of contacts; and (6) poor reliability and durability due to switch shorting. Low current behavior was dominated by the propensity to form smooth surface contacts by hammering, which led to high van der Waals force. High current behavior was dominated by the formation of Au nanowires that bridge the contact during separation. Data suggest the presence of an adventitious film containing carbon and oxygen. Aging of the contacts in air was found to reduce adhesion.

Short-time windows of correlation between large-scale functional brain networks predict vigilance intraindividually and interindividually.

Abstract: A better understanding of how behavioral performance emerges from interacting brain systems may come from analysis of functional networks using functional magnetic resonance imaging. Recent studies comparing such networks with human behavior have begun to identify these relationships, but few have used a time scale small enough to relate their findings to variation within a single individual's behavior. In the present experiment we examined the relationship between a psychomotor vigilance task and the interacting default mode and task positive networks. Two time-localized comparative metrics were calculated: difference between the two networks' signals at various time points around each instance of the stimulus (peristimulus times) and correlation within a 12.3-s window centered at each peristimulus time. Correlation between networks was also calculated within entire resting-state functional imaging runs from the same individuals. These metrics were compared with response speed on both an intraindividual and an interindividual basis. In most cases, a greater difference or more anticorrelation between networks was significantly related to faster performance. While interindividual analysis showed this result generally, using intraindividual analysis it was isolated to peristimulus times 4 to 8 s before the detected target. Within that peristimulus time span, the effect was stronger for individuals who tended to have faster response times. These results suggest that the relationship between functional networks and behavior can be better understood by using shorter time windows and also by considering both intraindividual and interindividual variability. Hum Brain Mapp 34:3280–3298, 2013. © 2012 Wiley Periodicals, Inc.

Polynomial Chaos Expansion with Latin Hypercube Sampling for Estimating Response Variability

Abstract: A computationally efficient procedure for quantifying uncertainty and finding significant parameters of uncertainty models is presented. To deal with the random nature of input parameters of structural models, several efficient probabilistic methods are investigated. Specifically, the polynomial chaos expansion with Latin hypercube sampling is used to represent the response of an uncertain system. Latin hypercube sampling is employed for evaluating the generalized Fourier coefficients of the polynomial chaos expansion. Because the key challenge in uncertainty analysis is to find the most significant components that drive response variability, analysis of variance is employed to find the significant parameters of the approximation model. Several analytical examples and a large finite element model of a joined-wing are used to verify the effectiveness of this procedure.