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991.
Principles and recent developments in ultrasound contrast agents 总被引:6,自引:1,他引:5
The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications. 相似文献
992.
Liquid crystal televisions have been employed as spatial light modulators in a variety of optical image processing applications. We have used such devices to develop techniques in speckle metrology for the electronic addition of speckle patterns and the display of speckle correlation, and also for the real-time display of object motion. Factors which affect the performance in terms of the useful working range and the accuracy of displacement measurement are discussed. Some modifications to the LCTV which should increase the potential of these devices as spatial light modulators, when used in both amplitude and phase modulation, are considered briefly. 相似文献
993.
994.
Ram C. Maurya Deen D. Mishra Surendra K. Jaiswal Sudipta Mukherjee 《Transition Metal Chemistry》1992,17(5):381-383
Some new low-spin hexacoordinated cyanonitrosyl complexes of Cr1 of the type [Cr(NO)(CN)2(L)2(H2)], where L is an aromatic aldehyde oxime or Mannich base, have been prepared by the interaction or K3[Cr(NO)(CN)5]· H2O with L in aqueous AcOH, and characterized by a range of physico-chemical techniques, I.r. data suggest that all the oxime
derivatives and Mannich bases act as monodentate ligands by coordinating through the aromatic nitrogen.
TMC 2564 相似文献
995.
G. K. Hubler C. N. Waddell E. P. Donovan J. M. Zavada 《Optical and Quantum Electronics》1991,23(7):S883-S893
Precision reflection measurements were performed on GaAs/AlAs superlattices of the same composition but different layer spacings. Nonlinear-least-squares fits to the data were performed to a single layer. Measurements were extracted for the superlattice thickness, thickness of a disturbed interface layer between the superlattice and substrate, the uniformity in composition and/or spacing and the composition. It was demonstrated that these nondestructive measurements in the infrared region (3000 to 12 000 cm–1) in conjunction with a simple single layer model are capable of accurately yielding the above quantities with high precision. 相似文献
996.
The theory for the calculation of the frequency-dependent hyperpolarizabilities β(?2ω; 0, ω), β(?ω; 0, ω), and β(0; ω, ?ω) is discussed. New relations between these tensors are derived for those wave functions that obey the time-dependent Hellmann–Feynman theorem (e.g., the self-consistent field [SCF] or the exact wave function). Using second-order Møller–Plesset perturbation theory (MP2), expressions are obtained for the hyperpolarizabilities in terms of derivatives of appropriately defined linear polarizability tensors with respect to a static electric field. Results are presented for ammonia and formaldehyde for the optical Kerr effect and for secondharmonic generation. These results indicate that it is desirable to determine the frequency-dependent contribution to the hyperopolarizability at the MP2 rather than the SCF level of theory, in cases where the static hyperpolarizability has a large contribution from electron correlation and/or where the frequency-dependent contribution may be more significant, such as for secondharmonic generation. 相似文献
997.
998.
Ferric ferrocyanides synthesized from different sources of ferric cations and ferrocyanide anions and those derived from ferrous ferricyanide, ferrous ferrocyanide and ferric ferricyanide have been studied using Mössbauer and XRD techniques. Structural information of the effect of the provenance on the properties of the final compound is obtained. Significant correlations between isomer shift and quadrupole splitting of inner and outer iron cations have been observed. 相似文献
999.
An interlaminar-shear-stress analysis developed earlier by Tsai et al. (1990, Micro-cracking-Induced Damage in Composites) for a [φm/θn], bi-directional composite laminate is used to solve the case of a cross-ply [0m/90n]x laminate with the 90° layer only or both layers cracked under pure shear loading. Strains, forces and laminate shear modulus reduction due to matrix cracking were obtained. Experimental results for shear modulus as a function of crack densities were obtained by a simple shear test and they agree very well with the theoretical prediction. 相似文献
1000.
Techniques of uniformization are used to prove that it is not consistent that the Whitehead groups of cardinality ℵ1 are exactly the strongly ℵ1-free groups. Some consequences of the assumption that every strongly ℵ1-free group of cardinality ℵ1 is Whitehead are derived. Other results about uniformization are also proved.
Research partially supported by NSERC grant #9848.
Research partially supported by the BSF. The authors thank Rutgers University for its support. Publication #441. 相似文献