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181.
Dmitry V. Peryshkov 《Journal of fluorine chemistry》2010,131(11):1252-1256
The anhydrous salt K2B12F12 crystallized from aqueous solution and its structure was determined by single crystal X-ray diffraction. The Ni2In-type structure it exhibits is rare for an A2X ionic compound at 25 °C and 1 atm., consisting of an expanded hexagonal close-packed array of B12F122− centroids (cent?cent distances: 7.204-8.236 Å) with half of the K+ ions filling all of the Oh holes and half of the K+ ions filling all of the D3h trigonal holes in the close-packed layers that are midway between two “empty” Td holes. The structure is also unusual in that the bond-valence sum for the K+ ions in Oh holes is less than or equal to 0.73 (the bond-valence sum for the other type of K+ ion is 1.16). A variation of the Ni2In structure is exhibited by the previously published monohydrate Cs2(H2O)B12F12, for which an improved structure is also reported here. For K2B12F12: monoclinic, C2/c, a = 8.2072(8), b = 14.2818(7), c = 11.3441(9) Å, β = 92.832(5)°, Z = 4, T = 120(2) K. For Cs2(H2O)B12F12: orthorhombic, P212121, a = 9.7475(4), b = 10.2579(4), c = 15.0549(5) Å, Z = 4, T = 110(1) K. 相似文献
182.
We propose a simple method to impose both no-slip boundary conditions at fluid-wall interfaces and at outflow boundaries in fully developed regions for Dissipative Particle Dynamics (DPD) fluid systems. The procedure to enforce the no-slip condition is based on a velocity-dependent shear force, which is a generalized force to represent the presence of the solid-wall particles and to maintain locally thermodynamic consistency. We show that this method can be implemented in both steady and time-dependent fluid systems and compare the DPD results with the continuum limit (Navier-Stokes) results. We also develop a force-adaptive method to impose the outflow boundary conditions for fully developed flow with unspecified outflow velocity profile or pressure value. We study flows over the backward-facing step and in idealized arterial bifurcations using a combination of the two new boundary methods with different flow rates. Finally, we explore the applicability of the outflow method in time-dependent flow systems. The outflow boundary method works well for systems with Womersley number of O(1), i.e., when the pressure and flowrate at the outflow are approximately in-phase. 相似文献
183.
The measurements of the Sherman function in elastic electron-cadmium scattering by Bartsch et al. [J. Phys. B 25, 1511 (1992)] have been in serious disagreement with scattering theories for nearly two decades. The recently developed relativistic convergent close-coupling method is applied to the problem and found to be in excellent agreement with experiment over the complete energy range measured. The unusually rapid variation in the spin asymmetry parameter in the vicinity of 4 eV projectile energy is now explained in terms of unitarity of the close-coupling formalism. 相似文献
184.
Vladislav G. Bagrov Dmitry M. Gitman Alexander D. Levin 《Journal of Russian Laser Research》2011,32(4):317-321
We consider an example where coherent states appear naturally (dynamically). We study the interaction between an arbitrary
quantized matter with a quantized electromagnetic field. Using an approximation that is similar to the Hartree–Fock one and
a variational principle, we demonstrate that the electromagnetic field is created by the charged matter exactly in Glauber
coherent states with parameters determined by mean currents of the matter. 相似文献
185.
Radiation damping on 600 and 800 MHz cryoprobes was investigated. The phase angle β between a vector 90° phase shifted to the precessing magnetization and the rf field induced in the coil was found to depend markedly on whether an FID was being acquired or not. The magnitude of the radiation damping field was sufficiently strong to restore 95% of the equilibrium water magnetization of a 90% H2O sample in a 5 mm sample tube within about 5 ms following a 165° pulse. This can be exploited in water flip-back versions of NOESY and TOCSY experiments of proteins, but care must be taken to limit the effect of the radiation damping field from the water on the Ha protons. Long water-selective pulses can be applied only following corrections. We developed a program for correcting pulse shapes if β is non-zero. The WATERGATE scheme is shown to be insensitive to imperfections introduced by radiation damping. 相似文献
186.
Dmitry Grishenkov Leif KariLars-Åke Brodin Torkel B. BrismarGaio Paradossi 《Ultrasonics》2011,51(1):40-48
The focus of contrast-enhanced ultrasound research has developed beyond visualizing the blood pool and its flow to new areas such as perfusion imaging, drug and gene therapy, and targeted imaging. In this work comparison between the application of polymer- and phospholipid-shelled ultrasound contrast agents (UCAs) for characterization of the capillary microcirculation is reported. All experiments are carried out using a microtube as a vessel phantom. The first set of experiments evaluates the optimal concentration level where backscattered signal from microbubbles depends on concentration linearly. For the polymer-shelled UCAs the optimal concentration level is reached at a value of about 2 × 104 MB/ml, whereas for the phospholipid-shelled UCAs the optimal level is found at about 1 × 105 MB/ml.Despite the fact that the polymer shell occupies 30% of the radius of microbubble, compared to 0.2% of the phospholipid-shelled bubble, approximately 5-fold lower concentration of the polymer UCA is needed for investigation compared to phospholipid-shelled analogues. In the second set of experiments, destruction/replenishment method with varied time intervals ranging from 2 ms to 3 s between destructive and monitoring pulses is employed. The dependence of the peak-to-peak amplitude of backscattered wave versus pulse interval is fitted with an exponential function of the time γ = A(1 − exp(−βt)) where A represents capillary volume and the time constant β represents velocity of the flow. Taking into account that backscattered signal is linearly proportional to the microbubble concentration, for both types of the UCAs it is observed that capillary volume is linearly proportional to the concentration of the microbubbles, but the estimation of the flow velocity is not affected by the change of the concentration. Using the single capillary model, for the phospholipid-shelled UCA a delay of about 0.2-0.3 s in evaluation of the perfusion characteristics is found while polymer-shelled UCA provide response immediately. The latter at the concentration lower than 3.6 × 105 MB/ml have no statistically significant delay (p < 0.01), do not cause any attenuation of the backscattered signal or saturation of the receiving part of the system. In conclusion, these results suggest that the novel polymer-shelled microbubbles have a potential to be used for perfusion evaluation. 相似文献
187.
We present the numerical study of the statistical properties of the partially coherent quasi-CW high-Q cavity Raman fiber laser. The statistical properties are different for the radiation generated at one or many cavity passes. It is found that rare extreme events are generated at the far spectral wings of the spectrum. The mechanism of the extreme events generation is a turbulent-like four-wave mixing of numerous longitudinal generation modes. 相似文献
188.
We theoretically calculate the electromagnetic response of metallic rod dimers for the arbitrary planar arrangement of rods in the dimer. It is shown that dimers without an in-plane symmetry axis exhibit elliptical dichroism and act as "atoms" in planar chiral metamaterials. Because of a very simple geometry of the rod dimer, such planar metamaterials are much easier to fabricate than conventional split-ring or gammadion-type structures and lend themselves to a simple analytical treatment based on a coupled dipole model. Dependencies of the metamaterial's directional asymmetry on the dimer's geometry are established analytically and confirmed in numerical simulations. 相似文献
189.
We present a variety of dissipative solitons and breathing modes in a medium with localized gain and homogeneous linear dissipation. The system possesses a number of unusual properties, like exponentially localized modes in both focusing and defocusing media, existence of modes in focusing media at negative propagation constant values, simultaneous existence of stable symmetric and antisymmetric localized modes when the gain landscape possesses two local maxima, as well as the existence of stable breathing solutions. 相似文献
190.
We study theoretically the possibility of performing temporal differentiation of optical signals using a resonant diffraction grating. We demonstrate that the resonant grating allows the calculation of the first-order derivative of an optical signal envelope in the vicinity of waveguide resonant frequencies in the zeroth transmitted diffraction order. The grating is shown to allow the calculation of the fractional derivative of order 1/2 in the vicinity of Rayleigh-Wood anomalies. Numerical simulations based on the rigorous coupled-wave analysis of Maxwell's equations demonstrate the high-quality differentiation of optical signals with temporal features in the picosecond range. 相似文献