Results 1 - 10 of 17972
Results 1 - 10 of 17972. Search took: 0.033 seconds
|Sort by: date | relevance|
[en] We describe measurements we have made of the emittance of a high-current, moderate-energy ion beam after transport through a permanent-magnet electrostatic plasma lens. The results indicate the absence of emittance growth due to the lens, when the lens is adjusted for optimal beam focusing. The measured emittance for a 16 keV Cu2+ ion beam formed by a vacuum arc ion source was about 0.4 π · mm · mrad at a beam current of 50 mA rising more-or-less linearly to 1.5 π · mm · mrad at 250 mA, and was conserved in beam transport through the lens. These results have significance for the application of high-current ion sources and the electrostatic plasma lens to particle accelerator injection
[en] Here, the equilibrium shape of 6H-SiC nanostructures and their surfaces were studied by analyzing nano-void (~10 nm) shapes, which were introduced in monocrystalline 6H-SiC by high-temperature neutron irradiation, using transmission electron microscopy. The nano-voids were determined to be irregular icosahedrons truncated with six (1¯100), twelve (1¯103), one smaller top-basal, and one larger bottom-basal planes, which suggests that (1¯100) and (1¯103) are the next stable surface class after the basal planes. The relatively frequent absence of the (1¯100) surface in the nano-voids indicated that the (1¯103¯) surface type is energetically rather stable. These non-basal surfaces were found not to be atomically flat due to the creation of nanofacets with half unit-cell height in the c-axis. The (1¯100) and (1¯103) surfaces were classified as two and four face types according to their possible nanofacets and surface termination, respectively. We also discuss the surface energy difference between the (1¯103¯) and (1¯103) face types in relation to the energy balance within the equilibrium, but irregular, polyhedron, in which the (1¯103) surface had double the surface energy of the (1¯103¯) surface (~3900 erg/cm"2).
[en] We have examined the influence of bismuth (Bi) and nitrogen (N) fluxes on N and Bi incorporation during molecular-beam epitaxy of GaAs1-x-yNxBiy alloys. The incorporation of Bi is found to be independent of N flux, while the total N incorporation and the fraction of N atoms occupying non-substitutional lattice sites increase with increasing Bi flux. A comparison of channeling nuclear reaction analysis along the , , and  directions with Monte Carlo-Molecular Dynamics simulations indicates that the non-substitutional N primarily incorporate as (N-As)As interstitial complexes. We discuss the influence of Bi adatoms on the formation of arsenic-terminated -oriented step-edges and the resulting enhancement in total N incorporation via the formation of additional (N-As)As.
[en] Monitoring and control of gaseous combustion process are critically important in advanced energy systems such as power plants, gas turbines, and automotive engines. However, very limited gas sensing solutions are available in the market for such application due to the inherent high temperature of combustion gaseous atmosphere. In this study, we fabricated and demonstrated high-performance metal oxide based nanorod array sensors assisted with ultra-violet (UV) illumination for in situ and real-time high-temperature gas detection. Without UV-illumination, it was found surface decoration of either 5 nm LSFO or 1 nm Pt nanoparticles can enhance the sensitivity over CO at 500 °C by an order of magnitude. Under the 254 nm UV illumination, CO gas-sensing performance of Ga2O3-based nanorod array sensors was further enhanced with the sensitivity boosted by 125 %, and the response time reduced by 30 % for La0.8Sr0.2FeO3(LSFO)-decorated sample. The UV-enhanced detecting of CO might be due to the increased population of photo-induced electron-hole pairs. While for LSFO-decorated nanorod array sensor under UV illumination, the enhancement is through a combination of sensitizing effect and photocurrent effect.
[en] Relaxor ferroelectric behavior occurs in many disordered ferroelectric materials but is not well understood at the atomic level. Recent experiments and theoretical arguments indicate that Anderson localization of phonons instigates relaxor behavior by driving the formation of polar nanoregions (PNRs). Here, we use inelastic neutron scattering to observe phonon localization in relaxor ferroelectric PZN-5%PT (0.95[Pb(Zn_1_/_3 Nb_2_/_3)O_3]–0.05PbTiO_3) and detect additional features of the localization process. In the lead, up to phonon localization on cooling, the local resonant modes that drive phonon localization increase in number. The increase in resonant scattering centers is attributed to a known increase in the number of locally off centered Pb atoms on cooling. The transition to phonon localization occurs when these random scattering centers increase to a concentration where the Ioffe-Regel criterion is satisfied for localizing the phonon. Finally, we also model the effects of damped mode coupling on the observed phonons and phonon localization structure.
[en] Here, we present compelling experimental results of the optical characteristics of transparent oxide CuGaO2 and related CuGa1-xFexO2 (with 0.00 ≤ x ≤ 0.05) alloys, whereby the forbidden electronic transitions for CuGaO2 become permissible in the presence of B-site (Ga sites) alloying with Fe. Our computational structural results imply a correlation between the global strain on the system and a decreased optical absorption edge. However, herein, we show that the relatively ordered CuGa1-xFexO2 (for 0.00 ≤ x ≤ 0.04) structures exhibit much weaker vis-absorption compared to the relatively disordered CuGa0.95Fe0.05O2.
[en] In controlling the thermal properties of the surrounding environment, we provide insight into the underlying mechanisms driving the widely used laser direct write method for additive manufacturing. In this study, we find that the onset of silver nitrate reduction for the formation of direct write structures directly corresponds to the calculated steady-state temperature rises associated with both continuous wave and high-repetition rate, ultrafast pulsed laser systems. Furthermore, varying the geometry of the heat affected zone, which is controllable based on in-plane thermal diffusion in the substrate, and laser power, allows for control of the written geometries without any prior substrate preparation. In conclusion, these findings allow for the advance of rapid manufacturing of micro- and nanoscale structures with minimal material constraints through consideration of the laser-controllable thermal transport in ionic liquid/substrate media.
[en] In this study, CCD-based thermoreflectance imaging and finite element modeling are used to study the two-dimensional (2D) temperature profile of a junction-down broad-area diode laser facet subject to back-irradiance. By determining the temperature rise in the active region (ΔΤAR) at different diode laser optical powers, back-irradiance reflectance levels, and back-irradiance spot locations, we find that ΔΤAR increases by nearly a factor of three when the back-irradiance spot is centered in the absorbing substrate approximately 5 μm away from the active region, a distance roughly equal to half of the back-irradiance spot FWHM (9 μm). This corroborates prior work studying the relationship between the back-irradiance spot location and catastrophic optical damage, suggesting a strong thermal basis for reduced laser lifetime in the presence of back-irradiance for diode lasers fabricated on absorbing substrates.
[en] Here, the ability to create optical materials with arbitrary index distributions would prove transformative for optics design and applications. However, current fabrication techniques for graded index (GRIN) materials rely on diffusion profiles and therefore are unable to realize arbitrary distribution GRIN design. Here, we demonstrate the laser direct writing of graded index structures in protein-based hydrogels using multiphoton lithography. We show index changes spanning a range of 10"–"2, which is comparable with laser densified glass and polymer systems. Further, we demonstrate the conversion of these written density variation structures into SiO_2, opening up the possibility of transforming GRIN hydrogels to a wide range of material systems.