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 & Microstructure. The organization has 5817 authors who have published 9157 publications receiving 292559 citations. The organization is also known as: Wright-Patterson AFB & FFO.

Widely tunable midinfrared difference frequency generation in orientation-patterned GaAs pumped with a femtosecond Tm-fiber system

Abstract: We demonstrate a midinfrared source tunable from 6.7 to 12.7 μm via difference frequency generation (DFG) in orientation-patterned GaAs, with 1.3 mW average output power. The input pulses are generated via Raman self-frequency shift of a femtosecond Tm-doped-fiber laser system in a fluoride fiber. We numerically model the DFG process and show good agreement between simulations and experiments. We use this numerical model to show an improved design using longer pump pulses.

Electromagnetic Computation in Scattering of Electromagnetic Waves by Random Rough Surface and Dense Media in Microwave Remote Sensing of Land Surfaces

Abstract: Active and passive microwave remote sensing has been used for monitoring the soil moisture and snow water equivalent. In the interactions of microwaves with bare soil, the effects are determined by scattering of electromagnetic waves by random rough surfaces. In the interactions of microwaves with terrestrial snow, the effects are determined by volume scattering of dense media characterized by densely packed particles. In this paper, we review the electromagnetic full-wave simulations that we have conducted for such problems. In volume scattering problems, one needs many densely packed scatterers in a random medium sample to simulate the physical solutions. In random rough surface scattering problems, one needs many valleys and peaks in the sample surface. In random media and rough surface problems, the geometric characterizations of the media and computer generations of statistical samples of the media are also challenges besides electromagnetic computations. In the scattering of waves by soil surfaces, we consider the soil to be a lossy dielectric medium. The random rough surface is characterized by Gaussian random processes with exponential correlation functions. Surfaces of exponential correlation functions have fine-scale structures that cause significant radar backscattering in active microwave remote sensing. Fine-scale features also cause increase in emission in passive microwave remote sensing. We apply Monte Carlo simulations of solving full 3-D Maxwell's equations for such a problem. A hybrid UV/PBTG/SMCG method is developed to accelerate method of moment solutions. The results are illustrated for coherent waves and incoherent waves. We also illustrate bistatic scattering, backscattering, and emissivity which are signatures measured in microwave remote sensing. For the case of scattering by terrestrial snow, snow is a dense medium with densely packed ice grains. We have used two models: densely packed particles and bicontinuous media. For the case of densely packed particles, we used the Metropolis shuffling method to simulate the positions of particles. The particles are also allowed to have adhesive properties. The Foldy-Lax equations of multiple scattering are used to study scattering from the densely packed spherical particles. The results are illustrated for the coherent waves and incoherent waves. For the case of bicontinuous media, the method developed by Cahn is applied to construct the interfaces from a large number of stochastic sinusoidal waves with random phases and directions. The volume scattering problem is then solved by using CGS-FFT. We illustrate the results of frequency and polarization dependence of such dense media scattering.

Lubrication using a microstructurally engineered oxide: performance and mechanisms

Abstract: Oxide coatings have the potential to lubricate over a wide range of environmental conditions. However, oxides are typically brittle, form abrasive wear debris, and have high friction. ZnO is no exception; hot-pressed 1–2 µm ZnO has a friction coefficient of about 0.6 and causes extensive wear on steel counterfaces. Microstructural engineering may be used to permit plastic deformation and the formation of lubricious transfer films. The work presented here focuses on controlling the microstructure and chemistry within ZnO to provide low-friction and long-life coatings (e.g., µ=0.1−0.2, 1M+ sliding cycles). Coatings having a (0001) columnar texture with good crystallinity along the c-axis wear quickly and generate substantial wear debris. Depositions that create a (0001) texture with a mosaic substructure within the columns deform plastically. Here, nanocrystalline structures may enhance grain boundary sliding and contribute to plastic deformation and low friction. Dislocation motion within ZnO is enhanced by oxygen adsorption, which may further reduce friction by lowering shear strength. In addition, it is likely that defects arising from oxygen deficiency and the high surface-to-volume ratio of nanostructures, promote adsorption of water and/or oxygen. The adsorbed species can reduce friction through passivation of dangling or strained bonds. The complex interaction of mechanical and surface chemical effects result in millions of dry sliding cycles on nanostructured coatings in 50% RH air. In addition, the coatings have low friction in vacuum. Coating characterization and performance are discussed and a mechanism to explain the tribological properties is proposed.