Several metal plates with different thickness including copper, iron, aluminum, and stainless steel have been drilled in the surroundings of air and water, respectively, by a Q-switched pulsed Nd:yttrium–aluminum–garnet laser. It is observed that for the same metal plate less energy is needed to drill a hole in water than that in air, and the surface morphology of hole drilled in water is improved greatly than that in air by comparison of the scanning electron micrographs. The underlying mechanisms behind the efficiency and quality enhancement in water are further investigated by means of optical beam deflection technique. The experimental results show that due to the water confinement the peak amplitude and duration of the laser-ablation-generated impact underwater is much larger than that in air. During the underwater laser drilling, besides laser ablation effect, both the first and second liquid-jet-induced impulses by cavitation bubble collapse in the vicinity of a solid boundary are also observed and their amplitudes are, respectively, about 12.4 and 5.2 times that of the laser ablation impact in air. Cavitation bubbles are the special dynamic phenomenon occurring in liquids. Therefore, it is concluded that in-air-drilling laser ablation-produced impact is a dominant mechanism; while during laser underwater drilling, it is the result of a combination of ablation-produced impact effect and liquid-jet-induced impact, especially the latter. Thus, the efficiency and quality of laser processing in the surrounding water can be greatly increased and improved compared with that in air.
Vertical profiles of instantaneous cohesive suspension concentration, obtained from an acoustic suspended sediment monitor in the Changjiang Estuary, indicate that near-bed high-concentration suspensions consist of upper and lower high-concentration suspensions, separated by a natural breakpoint at ca. 2 g l\. Acoustic images revealed near-bed high-frequency resuspension events of a few seconds, which contribute mainly to the formation of the lower near-bed high-concentration suspension. Upper and lower lutoclines are also indicative of re-entrainment of lower and upper high concentration suspension, respectively. Near-bed high-frequency resuspension is caused by turbulence, while re-entrainment is probably attributable to internal wave activity.
Quasi-phase-matching optical parametric and cascaded parametric processes in a two-component quasiperiodic superlattice were studied in theory and experiment. This letter demonstrates how to obtain red at 666 nm and blue at 443 nm simultaneously from the superlattice using a 532 nm laser as a pump through these two processes mentioned above. The result confirms that some nonlinear frequency conversion processes occurring in a high-dimension χ(2) nonlinear photonic crystal may be efficiently achieved in such a one-dimension quasiperiodic optical superlattice.
The nanoscale structural, chemical, and electronic properties of artificial engineered superlattice thin films consisting of superconducting Co-doped BaFe2As2 (Ba-122) and nonsuperconducting SrTi03 (STO) layers are determined by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy. The bonding of Ba-122/STO occurring between As (Ba) and SrO (Ti02) terminated layers has been identified. The thin STO (3 unit ceil) insertion layers are a mixture of cations (Ba, Sr, Fe, and Ti) and rich in oxygen vacancies and the Ba-122 layers (10 unit cell) are free of vertical second phases. Our results explain why these superlattices show anisotropic transport response to an external magnetic field, i.e., strong ab-axis pinning (enhancing critical current density) and no c-axis pinning, which is opposite to single layer Ba-122 thin films. These findings reveal physical and chemical properties of superconducting/nonsuperconducting heterostructures and provide important insights into engineering of superconducting devices.High-quality epitaxial heterostructures consisting of su perconducting and nonsuperconducting layers have been attracting extensive research interest because their novel properties make them very attractive candidates for a variety of applications, such as Josephson junctions, superconducting quantum interference devices (SQUIDs), and other field-effect devices [1][2][3][4][5][6][7]. Epitaxial pnictide thin films, however, have so far been hard to produce due to the volatile elements in the ironbased superconductor phase and therefore it is difficult to con trol the stoichiometry of the deposited films [8,9]. By employ ing epitaxial SrTi03 (STO) templates, we were able to grow high-quality epitaxial Co-doped BaFe2As2 (Ba-122) thin films on (La,Sr)(Al,Ta)03 (LSAT) substrates [10][11][12]. Recently, we reported the growth and properties of artificially engineered superlattices of Co-doped BaFe2As2/SrTi0 3 (Ba-122/STO) and Co-doped BaFe2 As2/undoped BaFe2 As2 [12], The former was possible due to their similar structures of which bulk Ba-122 has a tetragonal structure with lattice parameter a = 3.962 A, c = 13.017 A [13,14], while cubic STO has a lattice parameter with a -3.905 A. As a result of small misfit (1.4%) at the interface, the films showed excellent epitaxial growth with the use of the oxide templates.Previously we reported that single layer Ba-122 thin films showed strong c-axis pinning effect which enhances the 'Present address: PACS number(s): 68.37.-d, 73.21.-b, 74.25.-q in-field performance of the critical current density [10,11].High-resolution TEM results indicated that the presence of the c-axis aligned self-assembled oxide nanopillars in Ba-122 thin film [ 15] accounts for the c-axis pinning effect. In contrast, superlattice structure of Ba-122/STO shows very strong abaxis pinning rather than c-axis pinning. The different transport properties in these thin film devices essentially originate from the nature of defects, i.e., heterointerfaces and se...
Two kinds of cadmium sulfide (CdS) particulate films have been generated in situ by exposing stearic acid (SA) Langmuir monolayer at the air−aqueous CdCl2 interface to hydrogen sulfide (H2S) gas: particulate films composed of oriented rodlike nanocrystals (A-type particulate films), and those of dotlike nanocrystals which formed a stripelike domain with straight edges aligned with 6-fold symmetry (B-type particulate films). The SA- coated CdS particulate films were transferred to a solid substrate and examined by transmission electron microscopy (TEM) and photoacoustic spectroscopy (PAS). The dark field image and the transmission electron diffraction of the TEM were used to study the morphology and growth mechanisms of the CdS particulate films in detail. The PAS of the CdS particulate films composed of the oriented rodlike nanocrystals shows a new peak at 417.5 nm. This peak implies some new physical phenomena corresponding to the ordered alignment of the semiconductor nanocrystals, which exists in the ordered nanosystems. The novel synthesis method described here leads to the fabrication of the highly oriented semiconductor quantum wires and provides a new method to investigate the structure of the Langmuir monolayer at the air−water interface.
Thermoacoustic effect, as a subprocess of the photoacoustic effect, can also convert heat to acoustic wave by a thermoacoustic prime mover, and can also pump heat by acoustic wave in a thermoacoustic refrigerator. The present study focuses on a thermoacoustically driven pulse tube refrigerator, utilizing sound from a thermoacoustic prime mover to drive a pulse tube refrigerator. Experiments emphasize on the characteristics of both the thermoacoustic prime mover and the combined refrigeration system, including resonant frequency, onset temperature, pressure amplitude, and refrigeration temperature, etc. A cryogenic temperature lower than 120 K has been achieved from the present system. Much effort is also devoted to the influence of some working gases, such as helium–argon mixture, on the performance of the system.
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