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15 Feb 2010

Volume 107, Issue 4, Articles (04xxxx)

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J. Appl. Phys. 107, 041101 (2010); http://dx.doi.org/10.1063/1.3318287 (13 pages)

Y. C. Tao and J. G. Hu
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back to top Interdisciplinary and General Physics

Vascular structures for volumetric cooling and mechanical strength

K.-M. Wang, S. Lorente, and A. Bejan

J. Appl. Phys. 107, 044901 (2010); http://dx.doi.org/10.1063/1.3294697 (9 pages) | Cited 2 times

Online Publication Date: 17 February 2010

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When solid material is removed in order to create flow channels in a load carrying structure, the strength of the structure decreases. On the other hand, a structure with channels is lighter and easier to transport as part of a vehicle. Here, we show that this trade off can be used for benefit, to design a vascular mechanical structure. When the total amount of solid is fixed and the sizes, shapes, and positions of the channels can vary, it is possible to morph the flow architecture such that it endows the mechanical structure with maximum strength. The result is a multifunctional structure that offers not only mechanical strength but also new capabilities necessary for volumetric functionalities such as self-healing and self-cooling. We illustrate the generation of such designs for strength and fluid flow for several classes of vasculatures: parallel channels, trees with one, two, and three bifurcation levels. The flow regime in every channel is laminar and fully developed. In each case, we found that it is possible to select not only the channel dimensions but also their positions such that the entire structure offers more strength and less flow resistance when the total volume (or weight) and the total channel volume are fixed. We show that the minimized peak stress is smaller when the channel volume (ϕ) is smaller and the vasculature is more complex, i.e., with more levels of bifurcation. Diminishing returns are reached in both directions, decreasing ϕ and increasing complexity. For example, when ϕ = 0.02 the minimized peak stress of a design with one bifurcation level is only 0.2% greater than the peak stress in the optimized vascular design with two levels of bifurcation.
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61.66.Bi Elemental solids
62.20.-x Mechanical properties of solids

Effects of shielding gas compositions on arc plasma and metal transfer in gas metal arc welding

Z. H. Rao, S. M. Liao, and H. L. Tsai

J. Appl. Phys. 107, 044902 (2010); http://dx.doi.org/10.1063/1.3291121 (11 pages) | Cited 3 times

Online Publication Date: 22 February 2010

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This article presents the effects of shielding gas compositions on the transient transport phenomena, including the distributions of temperature, flow velocity, current density, and electromagnetic force in the arc and the metal, and arc pressure in gas metal arc welding of mild steel at a constant current input. The shielding gas considered includes pure argon, 75% Ar, 50% Ar, and 25% Ar with the balance of helium. It is found that the shielding gas composition has significant influences on the arc characteristics; droplet formation, detachment, transfer, and impingement onto the workpiece; and weld pool dynamics and weld bead profile. As helium increases in the shielding gas, the droplet size increases but the droplet detachment frequency decreases. For helium-rich gases, the current converges at the workpiece with a “ring” shape which produces non-Gaussian-like distributions of arc pressure and temperature along the workpiece surface. Detailed explanations to the physics of the very complex but interesting transport phenomena are given.
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89.20.Kk Engineering
52.77.Fv High-pressure, high-current plasmas (plasma spray, arc welding, etc.)
81.20.Vj Joining; welding

Surface chemical analysis and ab initio investigations of CsI coated C fiber cathodes for high power microwave sources

Vasilios Vlahos, Dane Morgan, Matthew LaCour, Ken Golby, Don Shiffler, and John H. Booske

J. Appl. Phys. 107, 044903 (2010); http://dx.doi.org/10.1063/1.3304923 (7 pages) | Cited 2 times

Online Publication Date: 24 February 2010

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CsI coated C fiber cathodes are promising electron emitters utilized in field emission applications. Ab initio calculations, in conjunction with experimental investigations on CsI-spray coated C fiber cathodes, were performed in order to better understand the origin of the low turn-on E-field obtained, as compared to uncoated C fibers. One possible mechanism for lowering the turn-on E-field is surface dipole layers reducing the work function. Ab initio modeling revealed that surface monolayers of Cs, CsI, Cs2O, and CsO are all capable of producing low work function C fiber cathodes (1 eV<Φ<1.5 eV), yielding a reduction in the turn-on E-field by as much as ten times, when compared to the bare fiber. Although a CsI-containing aqueous solution is spray deposited on the C fiber surface, energy dispersive x-ray spectroscopy and scanning auger microscopy measurements show coabsorption of Cs and I into the fiber interior and Cs and O on the fiber surface, with no surface I. It is therefore proposed that a cesium oxide (CsxOy) surface coating is responsible, at least in part, for the low turn E-field and superior emission characteristics of this type of fiber cathode. This CsxOy layer could be formed during preconditioning heating. CsxOy surface layers cannot only lower the fiber work function by the formation of surface dipoles (if they are thin enough) but may also enhance surface emission through their ability to emit secondary electrons due to a process of grazing electron impact. These multiple electron emission processes may explain the reported 10–100 fold reduction in the turn-on E-field of coated C fibers.
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79.70.+q Field emission, ionization, evaporation, and desorption
73.30.+y Surface double layers, Schottky barriers, and work functions
73.20.At Surface states, band structure, electron density of states
71.15.-m Methods of electronic structure calculations

Formation and conversion of defect centers in low water peak single mode optical fiber induced by gamma rays irradiation

J. X. Wen, W. Y. Luo, Z. Y. Xiao, T. Y. Wang, Z. Y. Chen, and X. L. Zeng

J. Appl. Phys. 107, 044904 (2010); http://dx.doi.org/10.1063/1.3273363 (5 pages) | Cited 3 times

Online Publication Date: 24 February 2010

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The formation and conversion processes of defect centers in low water peak single mode optical (LWPSM) fiber irradiated with gamma rays were investigated at room temperature using electron spin resonance. Germanium electron center (GEC) and self-trapped hole center (STH) occur when the fibers are irradiated with 1 and 5 kGy cumulative doses, respectively. With the increase in irradiation doses, the GEC defect centers disappear, and new defect centers such as E′ centers (Si and Ge) and nonbridge oxygen hole centers (NBOHCs) generate. The generation of GEC and STH is attributed to the electron transfer, which is completely balanced. This is the main reason that radiation-induced attenuation (RIA) of the LWPSM fiber is only 10 dB/km at communication window. The new defect centers come from the conversion of GEC and STH to E′ centers and NBOHC, and the conversion processes cause bond cleavage, which is the root cause that the RIA of the LWPSM fiber significantly increases up to 180 dB/km at working window. Furthermore, the concentration of new defect centers is saturated easily even by increasing cumulative doses.
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61.80.Ed γ-ray effects
76.30.Mi Color centers and other defects
42.81.-i Fiber optics

Theoretical study of the factor of merit of porous silicon based optical biosensors

J. Charrier and M. Dribek

J. Appl. Phys. 107, 044905 (2010); http://dx.doi.org/10.1063/1.3295906 (10 pages) | Cited 4 times

Online Publication Date: 25 February 2010

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Porous silicon is an attractive material for label-free optical biosensors because of its biocompatibility, its large internal surface area, its open pore network, and its widely tunable refractive index. Many structures using this material and exploring reflectometry can be used for biosensing. The sensor performances and sensitivity depends on the parameters of the porous silicon layers and its thermal treatment such as porosity, pore size, oxidation degree, and used wavelength. A theoretical framework to model the reflectance spectra of three optical nanostructures (monolayer, Bragg mirror, and microcavity based on porous silicon layers) before and after the functionalization step is used to study the merit parameters for each device. Based on this theoretical work, optimized conditions to fabricate glucagon biosensors are proposed. A microcavity formed by a period constituted of two porous layers of porosities equal to 95% and 65% with a pore size of 60 and 51 nm, respectively, and with 40% oxidation degree allows a significant redshift to be obtained. The value of minimum detectable coating thickness for a detection system capable of resolving a wavelength shift of 0.1 nm is about 5×10−3 nm.
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87.85.-d Biomedical engineering
42.79.Dj Gratings
42.79.Wc Optical coatings
42.70.-a Optical materials
61.43.Gt Powders, porous materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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