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461.
We show that there is a thermoinduced contribution to the magnetic moment of nanoparticles of antiferromagnetic materials. It arises from thermal excitations of the uniform spin-precession mode, and it has the unusual property that its magnitude increases with increasing temperature. This has the consequence that antiferromagnetism is nonexistent in nanoparticles at finite temperatures and it explains magnetic anomalies, which recently have been reported in a number of studies of nanoparticles of antiferromagnetic materials. 相似文献
462.
BACKGROUND AND GOAL: More than 60% of all myocardial infarction is caused by rupture of a vulnerable plaque. A vulnerable plaque can be described as a large, soft lipid pool covered by a thin fibrous cap. Plaque material composition, geometry, and inflammation caused by infiltration of macrophages are considered as major determinants for plaque rupture. For diagnostic purposes, these determinants may be obtained from elastograms (i.e. radial strain images), which are derived from intravascular ultrasound (IVUS) measurements. IVUS elastograms, however, cannot be interpreted directly as tissue component images, because radial strain depends upon plaque geometry, plaque material properties, and used catheter position. To understand and quantify the influence of these parameters upon measured IVUS elastograms, they were varied in a finite element model (FEM) that simulates IVUS elastograms of vulnerable plaques. MATERIALS AND METHODS: IVUS elastography measurements were performed on a vessel mimicking phantom, with a soft plaque embedded in a hard wall, and an atherosclerotic human coronary artery containing a vulnerable plaque. Next, FEMs were created to simulate IVUS elastograms of the same objects. In these FEMs the following parameters were varied: Young's modulus (E), Poisson's ratio (nu) in range 0.49-0.4999, catheter position (translation of 0.8 mm), and cap thickness (t) in range 50-350 microm. Hereby the resulting peak radial strain (PRS) was determined and visualized. RESULTS: Measured static E for phantom was 4.2 kPa for plaque and 16.8 kPa for wall.Variation of E-wall in range 8.4-33.2 kPa and/or E-plaque in range 2.1-8.4 kPa using the phantom FEM, gave a PRS variation of 1.6%, i.e. from 1.7% up to almost 3.3%; for variation in nu this was only 0.07%, i.e. from 2.37% up to 2.44%. Variation of E-lipid in range 6.25-400 kPa and E-cap in range 700-2300 kPa using the artery FEM, gave a PRS variation of 3.1%, i.e. from 0.6% up to 3.7%. The PRS was higher for lower E-lipid and E-cap; it was located at a shoulder of the lipid pool. Variation of nu gave only a variation of 0.17%. Variation of t and E-cap resulted in a PRS variation of 1.4%, i.e. from 0.3% up to 1.7%; thinner and weaker caps gave higher PRS. Catheter position variation changed radial strain value. CONCLUSIONS: Measured IVUS elastograms of vulnerable plaques depend highly upon the Young's modulus of lipid and cap, but not upon the Poisson's ratio. Different catheter positions result in different IVUS elastograms, but the diagnostically important high strain regions at the lipid shoulders are often still detectable. PRS increases when cap weakens or cap thickness decreases. 相似文献
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464.
Jacques van Wonterghem Steen Mørup Per Helvig Christensen Michel D. Bentzon Jørgen Villadsen 《Hyperfine Interactions》1988,41(1):721-724
An iron-tin catalyst has been prepared by impregnation of a silica-support with a solution of organic salts of the metals in chloroform followed by reduction in hydrogen. Using a combined57Co and Ba119SnO3 source, the catalyst was studied simultaneously with both iron and tin Mössbauer spectroscopy. Reduction at 848 K of a catalyst prepared with an Fe∶Sn ratio equal to 1∶1 resulted in the formation of ordered FeSn alloy particles with an average diameter of about 20 nm. The particles were not affected by high-temperature treatment in an H2/CO gas mixture. 相似文献
465.
In an area dominated by clayey till the distribution of nitrate and the Fe(II)/Fe(III) ratio have been measured below arable
land till a depth of 20 metres. In the subsoil the nitrate (NO−
3) occurs in measurable quantities in the upper oxidised zone (until c. 3 m) and the Fe(II)/Fe(III) ratio is low. At the transition
to the reduced zone the nitrate content decreases to below the detection limit and the Fe(II)/Fe(III) ratio is considerably
increased. The results indicate that the Fe(II) in the clay minerals is oxidised by the nitrate which thereby is reduced. 相似文献
466.
Jacques Van Wonterghem Steen Mørup Stuart W. Charles Stephen Wells Jørgen Villadsen 《Hyperfine Interactions》1986,27(1-4):333-336
A ferrofluid has been prepared by thermal decomposition of Fe(CO)5 in an organic liquid. Mössbauer spectroscopy and x-ray diffractometry studies show that the ferrofluid contains small particles of a metallic glass. The particles are only partly oxidized even after exposure of the ferrofluid to air at room temperature for 30 days. 相似文献
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470.
Steen Krenk 《Applied Mathematical Modelling》1979,3(2):137-142
The formulation of stress concentration problems of plane anisotropic elasticity in terms of integral equations is discussed. First the singular solutions of a concentrated force and a dislocation are formulated so that they remain valid in the case of double roots. The distribution of singularities on smooth curves is then treated, and general formulae for the stress discontinuities are given. Finally, the integral equations arising from stress boundary conditions are discussed, and the characteristics and numerical solution of one of the types are treated in detail. 相似文献