共查询到20条相似文献,搜索用时 15 毫秒
1.
J. Bodzenta J. Mazur A. Ka?mierczak-Ba?ata 《Applied physics. B, Lasers and optics》2011,105(3):623-630
The thermal diffusivity and the thermal conductivity of compressed expanded graphite (CEG) samples were investigated by photothermal
measurements in two geometries differing by a place of temperature disturbance detection. This disturbance can be detected
on a surface opposite to the one at which the disturbance was generated (rear detection) or on the same surface (front detection).
A measurement based on the rear detection allowed us to determine the effective thermal diffusivity of the sample, while the
method with front detection gives the possibility of analysis of homogeneity of the sample. It is shown that the thermal diffusivity
of CEG strongly depends on its apparent density. Moreover, CEG samples reveal anisotropy of the thermal properties. The thermal
diffusivity in the direction parallel to the compacting axis is lower than the one in the direction perpendicular to it. The
parallel thermal diffusivity decreases with growing apparent density, while the perpendicular thermal diffusivity significantly
grows when the apparent density grows. The perpendicular thermal conductivity exhibits the same behavior as the perpendicular
thermal diffusivity. The parallel thermal conductivity slightly grows with growing density and then reaches a plateau. The
anisotropy of CEG samples grows with growing apparent density and vanishes for low-density samples. The photothermal measurement
with front signal detection revealed that the CEG samples are non-homogeneous in the direction of the compacting axis and
can be modeled by a two-layer system. 相似文献
2.
Katsunori Wakabayashi Mitsutaka Fujita Koichi Kusakabe Kyoka Nakada 《Czechoslovak Journal of Physics》1996,46(4):1865-1866
Anoble mechanism of spin polarization is proposed for finite graphite sheet with edge. For graphite ribbon with zigzag edge, there appear peculiar ‘edge states’. These localized states comprise nearly flat band at the Fermi level, which easily causes magnetic instability. Magnetic structure is suggested from Hartree-Fock analysis of the Hubbard model, where huge magnetic moments are induced at around both of edges by weak HubbardU and are coupled antiferromagnetically with each other. 相似文献
3.
X-ray photoemission spectra of vacuum cleaved LiC6, prepared from highly oriented pyrolytic graphite, provide a direct measure of the filling of the graphite π bands by electrons from Li. The resulting Fermi energy shift is in agreement with recent band structure calculations and indicates near unity charge transfer from Li. Core level spectra exhibit shifts compatible with the expected charge transger and line shapes showing strong asymmetries resulting from the metallic character of this compound. 相似文献
4.
A recursion method is used for calculating the electron-state density n(E) of carbon in diamond, graphite, C60 fullerene, and graphite nanopipes of various structures and diameters. The calculated n(E) for diamond, graphite, and fullerene are compared with experimental data. The distinctive features of the electron-state
density n(E) in graphite nanopipes are discussed.
Fiz. Tverd. Tela (St. Petersburg) 39, 1118–1121 (June 1997) 相似文献
5.
J. Kieser 《Zeitschrift für Physik B Condensed Matter》1977,26(1):1-10
Comprehending investigations of the electronic structure of polycrystalline as well as monocrystalline graphite have been performed by means of X-ray emission, self absorption and X-ray induced photoemission techniques. On the basis of these combined investigations a model for the determination of the relative cross sections for the photoemission process has been established and applied to graphite, where it yields
s
/
p
=32. The anisotropy and polarization of the CK-radiation of monocrystalline graphite is discussed in terms of the binding properties of the graphite lattice. The predictions are verified by measurements of the CK-emission employing a crystal monochromator which acts simultaneously as a nearly perfect analyzer for the polarization of the monochromatized radiation. By means of the self absorption technique the unoccupied part of the conduction band has been investigated. 相似文献
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10.
Using x-ray diffractometry and computer simulation, the process of graphite structure transformation under mechanical grinding
is investigated. It is established that all of main structural parameters vary over grinding time and are interrelated. The
major parameter, whose variation affects the changes of the other, is the average size of the regions of coherent scattering
along the crystallographic axis a, La. Reduction in the grain size is the main change under mechanical grinding, the change in the lattice parameters being its
consequence.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 29–34, August, 2006. 相似文献
11.
Exploiting the wavelet structure in compressed sensing MRI 总被引:1,自引:0,他引:1
Sparsity has been widely utilized in magnetic resonance imaging (MRI) to reduce k-space sampling. According to structured sparsity theories, fewer measurements are required for tree sparse data than the data only with standard sparsity. Intuitively, more accurate image reconstruction can be achieved with the same number of measurements by exploiting the wavelet tree structure in MRI. A novel algorithm is proposed in this article to reconstruct MR images from undersampled k-space data. In contrast to conventional compressed sensing MRI (CS-MRI) that only relies on the sparsity of MR images in wavelet or gradient domain, we exploit the wavelet tree structure to improve CS-MRI. This tree-based CS-MRI problem is decomposed into three simpler subproblems then each of the subproblems can be efficiently solved by an iterative scheme. Simulations and in vivo experiments demonstrate the significant improvement of the proposed method compared to conventional CS-MRI algorithms, and the feasibleness on MR data compared to existing tree-based imaging algorithms. 相似文献
12.
Summary The crystal size in accretions grown in an icing wind tunnel at ambient temperatures −18≥T
a
≥−27°C and different substrate temperatureT
s
<0°C is studied. For constantT
s
the mean area
of crystal sections shows an approximately exponential dependence onT
a
which may be written as
, where ΔT≃−T
a
represents the droplet supercooling. On the other hand, for fixedT
a
, the crystal size is observed to decrease markedly withT
s
. By considering previous results on the orientation of crystals forming the accretions, it is shown that the value of the
mean angle
formed by the crystalc-axis with the growth direction depends as well both onT
a
andT
s
. The exponential dependence of
andT
a
and the observed correspondence between the behaviour of
and
are correlated with a nucleation mechanism, accompanied by a process of orientation selection, which would be responsible
of both the crystal size and the orientation. The application of the crystal structure analysis to the determination of the
growth conditions of hailstones is discussed.
To speed up publication, the authors of this paper has agreed to not receive the proofs for correction.
Comisión Nacional de Energía Atómica and Consejo Nacional de Investigaciones Científicas y Técnicas. 相似文献
Riassunto Si studia la dimensione di un cristallo in accrezioni cresciute in una galleria a vento congelante a temperature ambienti −18≥T a ≥−27 °C e diversa temperatura di substratoT s <0 °C. PerT s costante, l’area media delle sezioni di cristallo mostra una dipendenza daT a approssimativamente esponenziale che potrebbe essere scritta così: , dove ΔT≃−T a rappresenta il superraffreddamento della gocciolina. D’altra parte, perT a fisso, si osserva che la dimensione del cristallo decresce marcatamente conT s . Considerando precedenti risultati sull’orientamento dei cristalli che formano le accrezioni, si mostra che anche il valore dell’angolo medio formato dall’assec del cristallo con la direzione di accrescimento dipende sia daT a che daT s . La dipendenza esponenziale di daT a e la corrispondenza osservata tra il comportamento di e di sono collegate con il meccanismo di nucleazione, accompagnato da un processo di selezione dell’orientamento, che dovrebbe essere responsabile sia della dimensione che dell’orientamento del cristallo. Si discute l’applicazione dell’analisi della struttura del cristallo alla determinzione delle condizioni di accrescimento dei chicchi di grandine.
To speed up publication, the authors of this paper has agreed to not receive the proofs for correction.
Comisión Nacional de Energía Atómica and Consejo Nacional de Investigaciones Científicas y Técnicas. 相似文献
13.
14.
N-m-tolyl phthalimide, C15NO2H11 crystallizes in the space group Cc with unit cell dimensions,a=8·54(1),b=19·89(2),c = 7·59(1)A, β=114·53(1)° andZ=4.V=1173(2)A3,D
m
=1·35(1),D
c
= 1·344 mg.m−3,M
r
=237 λCoKa=1·7903A. The structure was solved byMULTAN and refined to an R-factor of 0·116 for 632 counter reflections. The molecules are held together by van der Waal’s forces.
The angle between the tolyl plane and the plane through the phthalimide group is 53·4(4)°.
Contribution No. 607. 相似文献
15.
The crystal structure of DL-Serine H2SO4 · H2O is determined by single-crystal x-ray diffraction. The intensities of x-ray reflections are measured at three temperatures, namely, 295, 343, and 233 K. The crystal structure of the studied compound is refined using all three sets of intensities. It is demonstrated that, at a temperature of 233 K, the structure transforms into an incommensurately modulated state. This transformation is assumed to be responsible for the disappearance of the piezoresponse at this temperature. 相似文献
16.
The investigation of the electronic structure of polyacenacene (PAA) has been performed on the basis of the tight-binding SCF-MO (self-consistent field-molecular orbital) method. The predicted skeleton for PAA is of an aromatic structure without the band gap. Hence it can be considered that PAA will show the metallic property. In this connection one could regard PAA as a one-dimensional graphite. 相似文献
17.
A. V. Okotrub L. G. Bulusheva V. L. Kuznetsov D. V. Vyalikh M. V. Poyguin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2005,34(1-3):157-160
The electronic structure of samples produced by
nanodiamond annealing has been examined using ultra-soft X-ray emission and
X-ray absorption spectroscopy. Analysis of spectra of diamond/graphite
composites showed that carbon atoms constituting the nanoparticles are at
least in three states: diamond-like state, graphitic-like state and
interface carbon, characterized by high electron localization. Comparison
between theoretical spectra of the models and experimental spectra suggested
the latter states correspond to three-coordinated carbon atoms from diamond
surface. 相似文献
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19.
David A. Liberman 《Journal of Physics and Chemistry of Solids》1978,39(3):255-257
Recently there have been reports that magnesium oxide and other ionic solids have been converted into metallic conductors when subjected to pressures of about 1 Mbar. Electronic structure calculations for compressed magnesium oxide indicate that the gap between valence and conduction bands does not close until much higher pressures are reached—perhaps around 50 Mbar. 相似文献
20.
使用耦合波法以及特征矩阵法计算了周期膜堆结构以及介质压缩光栅的本征值,研究了介质型压缩光栅产生导模共振现象的条件,并计算了导模共振点附近光栅的衍射效率曲线。针对导模共振效应对光栅应用的影响,提出了一种避开该效应的方法。计算结果表明:当介质折射率呈周期变化时,取光栅周期为0.667 μm,深度为0.6 μm时,可以避开导模共振效应。 相似文献