12,091 research outputs found
Photoacoustic generation of focused quasi-unipolar pressure pulses
The photoacoustic effect was employed to generate short-duration quasi-unipolar acoustic pressure pulses in both planar and spherically focused geometries. In the focal region, the temporal profile of a pressure pulse can be approximated by the first derivative of the temporal profile near the front transducer surface, with a time-averaged value equal to zero. This approximation agreed with experimental results acquired from photoacoustic transducers with both rigid and free boundaries. For a free boundary, the acoustic pressure in the focal region is equal to the sum of a positive pressure that follows the spatial profile of the optical energy deposition in the medium and a negative pressure that follows the temporal profile of the laser pulse
Tungsten fibre reinforced Zr-based bulk metallic glass composites
A Zr-based bulk metallic glass (BMG) alloy with the composition (Zr55Al10Ni5Cu30)98.5Si1.5 was used as the base material to form BMG composites. Tungsten fiber reinforced BMG composites were successfully fabricated by pressure metal infiltration technique, with the volume fraction of the tungsten fiber ranging from 10% to 70%. Microstructure and mechanical properties of the BMG composites were investigated. Tungsten reinforcement significantly increased the material’s ductility by changing the compressive failure mode from single shear band propagation to multiple shear bands propagation, and transferring stress from matrix to tungsten fibers
Vegetation NDVI Linked to Temperature and Precipitation in the Upper Catchments of Yellow River
Vegetation in the upper catchment of Yellow River is critical for the ecological stability of the whole watershed. The dominant vegetation cover types in this region are grassland and forest, which can strongly influence the eco-environmental status of the whole watershed. The normalized difference vegetation index (NDVI) for grassland and forest has been calculated and its daily correlation models were deduced by Moderate Resolution Imaging Spectroradiometer products on 12 dates in 2000, 2003, and 2006. The responses of the NDVI values with the inter-annual grassland and forest to three climatic indices (i.e., yearly precipitation and highest and lowest temperature) were analyzed showing that, except for the lowest temperature, the yearly precipitation and highest temperature had close correlations with the NDVI values of the two vegetation communities. The value of correlation coefficients ranged from 0.815 to 0.951 (p <0.01). Furthermore, the interactions of NDVI values of vegetation with the climatic indicators at monthly interval were analyzed. The NDVI of vegetation and three climatic indices had strong positive correlations (larger than 0.733, p <0.01). The monthly correlations also provided the threshold values for the three climatic indictors, to be used for simulating vegetation growth grassland under different climate features, which is essential for the assessment of the vegetation growth and for regional environmental management
A method for describing large rotations with a combination of axial and transverse Euler vectors
In order to overcome the problem of “singular points”, a method has been developed for the kinematically accurate separation of a large rotation into an axial Euler vector and a transverse Euler vector. The proposal is based on the fact that in the problems of the rotor dynamics of machines consisting of shafts, gears, bearings, etc., the transverse rotation never reaches a value of 2π (a critical value for the Euler vector). The axial rotation is not limited in any way. A numerical dynamics example illustrating the method is presented. The result of the dynamics problem is checked by observing the law of conservation of total energy
Spin-quadrupole ordering of spin-3/2 ultracold fermionic atoms in optical lattices in the one-band Hubbard model
Based on a generalized one-band Hubbard model, we study magnetic properties
of Mott insulating states for ultracold spin-3/2 fermionic atoms in optical
lattices. When the \textit{s}-wave scattering lengths for the total spin
satisfy conditions , we apply a functional integral
approach to the half filled case, where the spin-quadrupole fluctuations
dominate. On a 2D square lattice, the saddle point solution yields a staggered
spin-quadrupole ordering at zero temperature with symmetry breaking from SO(5)
to SO(4). Both spin and spin-quadrupole static structure factors are
calculated, displaying highly anisotropic spin antiferromagnetic fluctuations
and antiferroquadrupole long-range correlations, respectively. When Gaussian
fluctuations around the saddle point are taken into account, spin-quadrupole
density waves with a linear dispersion are derived. Compared with the spin
density waves in the half filled spin-1/2 Hubbard model, the quadrupole density
wave velocity is saturated in the strong-coupling limit, and there are no
transverse spin-quadrupole mode couplings, as required by the SO(4) invariance
of the effective action. Finally, in the strong-coupling limit of the model
Hamiltonian, we derive the effective hyperfine spin-exchange interactions for
the Mott insulating phases in the quarter filled and half filled cases,
respectively.Comment: 12 pages, 5 figure
Simultaneous imaging of a lacZ-marked tumor and microvasculature morphology in vivo by dual-wavelength photoacoustic microscopy
Photoacoustic molecular imaging, combined with the reporter-gene technique, can provide a valuable tool for cancer research. The expression of the lacZ reporter gene can be imaged using photoacoustic imaging following the injection of X-gal, a colorimetric assay for the lacZ-encoded enzyme β-galactosidase. Dual-wavelength photoacoustic microscopy was used to non-invasively image the detailed morphology of a lacZ-marked 9L gliosarcoma and its surrounding microvasculature simultaneously in vivo, with a superior resolution on the order of 10 μm. Tumor-feeding vessels were found, and the expression level of lacZ in tumor was estimated. With future development of new absorption-enhancing reporter-gene systems, we anticipate this strategy can lead to a better understanding of the role of tumor metabolism in cancer initiation, progression, and metastasis, and in its response to therapy
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