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1.
Nieminen JO Vesanen PT Zevenhoven KC Dabek J Hassel J Luomahaara J Penttilä JS Ilmoniemi RJ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,212(1):154-160
In ultra-low-field magnetic resonance imaging (ULF MRI), superconductive sensors are used to detect MRI signals typically in fields on the order of 10-100 μT. Despite the highly sensitive detectors, it is necessary to prepolarize the sample in a stronger magnetic field on the order of 10-100 mT, which has to be switched off rapidly in a few milliseconds before signal acquisition. In addition, external magnetic interference is commonly reduced by situating the ULF-MRI system inside a magnetically shielded room (MSR). With typical dipolar polarizing coil designs, the stray field induces strong eddy currents in the conductive layers of the MSR. These eddy currents cause significant secondary magnetic fields that may distort the spin dynamics of the sample, exceed the dynamic range of the sensors, and prevent simultaneous magnetoencephalography and MRI acquisitions. In this paper, we describe a method to design self-shielded polarizing coils for ULF MRI. The experimental results show that with a simple self-shielded polarizing coil, the magnetic fields caused by the eddy currents are largely reduced. With the presented shielding technique, ULF-MRI devices can utilize stronger and spatially broader polarizing fields than achievable with unshielded polarizing coils. 相似文献
2.
Detection of phase biaxiality in liquid crystals by use of the quadrupole shift in 131Xe NMR spectra
An experimental method to unambiguously distinguish between uniaxial and biaxial liquid crystal phases is introduced. The method is based on the second order quadrupole shift (SOQS) observable in 131Xe NMR spectra of xenon dissolved in liquid crystals. It is shown that besides revealing the biaxiality, the 131Xe SOQS offers a novel method to determine the tilt angle in smectic C phases. As an example, the 131Xe SOQS in a ferroelectric liquid crystal is reported. It yields up a biaxial phase in between isotropic and smectic C phases. 相似文献
3.
A one dimensional, nonlinear, singular integral equation was recently shown to be equivalent to Suhl's dispersion equations for the Kondo-problem of a half-spin magnetic impurity in a finite magnetic field. We investigate this integral equation further analytically and numerically and obtain numerical solutions which we use for a calculation of transport coefficients. The normal part of the scattering potential of the magnetic impurity is included via ans-wave phase shift. The transport coefficients are universal functions of the ratiosT/T
K andB/B
K of the temperatureT and the zero magnetic field Kondo-temperatureT
K and of the magnetic inductionB and the Kondo magnetic inductionB
K. We find maxima in the electrical and thermal resistivities as functions ofT/T
K forBB
K. These are typical Kondo phenomena, and can be influenced by. Interference of and the phases of Kondo-scattering amplitudes leads to dramatic effects in the thermopower and the Hall coefficient.SFB 125The numerical part of this work was performed at the Institut für Festkörperforschung, Kernforschungsanlage Jülich, F.R. Germany 相似文献
4.
Tanskanen H Khriachtchev L Lundell J Räsänen M 《The Journal of chemical physics》2004,121(17):8291-8298
We investigate the formation mechanism of HXeCCXeH in a Xe matrix. Our experimental results show that the HXeCCXeH molecules are formed in the secondary reactions involving HXeCC radicals. The experimental data on the formation of HXeCCXeH is fully explained based on the model involving the HXeCC+Xe+H-->HXeCCXeH reaction. This reaction is the first case when a noble-gas hydride molecule is formed from another noble-gas molecule. In addition, we investigate the (12)C/(13)C isotope effect on the vibrational properties of organo-noble-gas hydrides (HKrCCH, HXeCCH, HXeCC, and HXeCCXeH) in noble-gas matrixes. The present experimental results and ab initio calculations on carbon isotope shifts of the vibrational modes support the previous assignments of these molecules. Upon (12)C to (13)C isotope substitution, we observed a pronounced effect on the H-Kr stretching mode of HKrCCH (downshift of 1.0-3.6 cm(-1), depending on the matrix site) and a small anomalous shift (+0.1 cm(-1)) of the H-Xe stretching mode of HXeCCH and HXeCCXeH. 相似文献
5.
Tero Tuuttila Manu Lahtinen Juhani Huuskonen Kari Rissanen 《Thermochimica Acta》2010,497(1-2):109-116
The thermal properties of twelve Janus-type dendrimers up to the second generation were evaluated by termogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Compounds consist of the dendritic bisMPA based polyester moieties, and either 3,4-bis-dodecyloxybenzoic acid, 3,5-bis-dodecyloxybenzoic acid or 3,4,5-tris-dodecyloxybenzoic acid moieties, attached to opposite sides of the pentaerythritol core. The thermal stability of the compounds was evaluated by TGA, displaying onset decomposition temperatures (Td) at around 250 °C. DSC measurements upon heating and cooling confirmed that OH terminated Janus dendrimers featuring large polarity difference in opposite sides display liquid crystalline phases with exception of 3,5-type G1 dendrimer; while acetonide terminated dendrimers displayed merely melting transitions. Dendrimers having terminal alkyl chains at positions 3,4 or 3,4,5 in aromatic moieties exhibited enantiotropic mesophases. However, the thermal behavior of the dendrimers with 3,5-substitution pattern was different: the 3,5-type G1 dendrimer exhibit a lack of mesomorphic transition, and in the case of the 3,5-type G2 dendrimer, the mesophase was absent in the first heating scan but was observed during the subsequent cooling and heating scans at the rate of 10 °C/min. 相似文献
6.
Analysis of ALD-processed thin films by ion-beam techniques 总被引:1,自引:0,他引:1
Putkonen M Sajavaara T Niinistö L Keinonen J 《Analytical and bioanalytical chemistry》2005,382(8):1791-1799
This review introduces the possibilities of ion-beam techniques for the analysis of thin films and thin-film structures processed by atomic layer deposition (ALD). The characteristic features of ALD are also presented. The analytical techniques discussed include RBS, NRA and ERDA with its variants, viz. the TOF-ERDA and HI-ERDA. The thin film examples are taken from flat-panel display technology (TFEL structures) and the semiconductor industry (high-k insulators).Dedicated to the memory of Wilhelm Fresenius 相似文献
7.
It is well known and important that if u ≥ 0 is subharmonic on a domain Ω in ℝ
n
and p > 0, then there is a constant C(n,p) ≥ 1 such that for each open ball B(x,r) ⊂ Ω. The definition of a relatively new function class, quasi-nearly subharmonic functions, is based on such a generalized
mean value inequality. It is pointed out that the obtained function class is natural. It has important and interesting properties
and, at the same time, it is large: In addition to nonnegative subharmonic functions, it includes, among others, Hervé’s nearly
subharmonic functions, functions satisfying certain natural growth conditions, especially certain eigenfunctions, polyharmonic
functions and generalizations of convex functions. Further, some of the basic properties of quasi-nearly subharmonic functions
are stated in a unified form. Moreover, a characterization of quasi-nearly subharmonic functions with the aid of the quasihyperbolic
metric and two weighted boundary limit results are given.
相似文献
8.
Summary. In this paper, we consider the problem of designing plate-bending elements which are free of shear locking. This phenomenon is known to afflict several elements for the Reissner-Mindlin plate model when the thickness of the plate is small, due to the inability of the approximating subspaces to satisfy the Kirchhoff constraint. To avoid locking, a “reduction operator” is often applied to the stress, to modify the variational formulation and reduce
the effect of this constraint. We investigate the conditions required on such reduction operators to ensure that the approximability
and consistency errors are of the right order. A set of sufficient conditions is presented, under which optimal errors can
be obtained – these are derived directly, without transforming the problem via a Hemholtz decomposition, or considering it
as a mixed method. Our analysis explicitly takes into account boundary layers and their resolution, and we prove, via an asymptotic
analysis, that convergence of the finite element approximations will occur uniformly as , even on quasiuniform meshes. The analysis is carried out in the case of a free boundary, where the boundary layer is known
to be strong. We also propose and analyze a simple post-processing scheme for the shear stress. Our general theory is used
to analyze the well-known MITC elements for the Reissner-Mindlin plate. As we show, the theory makes it possible to analyze
both straight and curved elements. We also analyze some other elements.
Received June 19, 1995 相似文献
9.
A. Craig W. H. Jacobson A. Meyer N. Welwart A. Seuthe H. N. Marr B. S. Evans G. E. F. Lundell J. A. Scherrer E. Stelling F. J. Mück A. Kling A. Lassieur L. Bertiaux C. C. Bissett P. de Pauw J. Heslinga A. Bartsch R. E. Lee W. H. Fegeley F. H. Reichel und E. Saz 《Fresenius' Journal of Analytical Chemistry》1927,71(1-2):83-90
Ohne Zusammenfassung 相似文献
10.
Patrick J. Whitham Dennis P. Strommen Sandra Lundell Lisa D. Lau Rene Rodriguez 《Plasma Chemistry and Plasma Processing》2014,34(4):755-766
The plasma enhanced chemical vapor depositions of germanium chalcogenide thin films from germanium tetrachloride, hydrogen sulfide and alkyl chalcogenides were studied to determine the viability of these reagents for thin film deposition. Hydrogen sulfide is a commonly used reagent for this technique and was used to determine optimal reaction conditions for thin film deposition. Germanium tetrachloride, alkylsulfides and alkylselenides were also employed because of their lower potential toxicities and higher availabilities compared to their more typical congeners: germane, hydrogen sulfide and hydrogen selenide in the formation of germanium chalcogenides. Alkylsulfides were found to be unsuitable for the deposition of germanium sulfides, however alkylselenide precursors were used successfully for the deposition of germanium selenides. The relative mass flow rates, reactor pressure, substrate temperature and plasma power density were studied for their effects on germanium chalcogenide deposition. These parameters affected the composition, deposition rate, film quality, and spectroscopic properties of the deposited films. 相似文献