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排序方式: 共有217条查询结果,搜索用时 15 毫秒
1.
The effects of chemical heat treatments of a semianthracite char (AC) on the composition of the mineral fraction of the material are investigated. The starting char was first treated with a mixture of LiCl/KCl or LiCl/KCl/CaO at 743, 873 or 1173 K and the products obtained were then washed thoroughly with distilled water. A small fraction of these samples were treated with 10−3 M HCl solution. The composition changes were studied by X-ray diffraction. The predominant mineral components initially present in the starting char are quartz, mullite, muscovite and/or kaolinite and oldhamite. The treatments of AC resulted in significant changes in the mineral fraction of the material, in particular when LiCl/KCl/CaO was used. In this case, spurrite, γ-calcium orthosilicate and gehlenite were formed, which were eliminated by treatment with 10−3 M HCl solution. 相似文献
2.
R. Jakob P. Kroll M. Schürmann W. Schweiger 《Zeitschrift für Physik A Hadrons and Nuclei》1993,347(2):109-116
We present an alternative parameterization of the quark-diquark model of baryons which particularly takes care of the most recent proton electric form-factor data from the E136 experiment at SLAC. In addition to electromagnetic form factors of the nucleon, for which good agreement with data is achieved, we discuss the weak axial vector form factor of the nucleon as well as electromagnetic form factors of and hyperons. Technical advance in calculating the pertinent analytic expressions within perturbative quantum chromodynamics is gained by formulating the wave function of the quark-diquark system in a covariant way. Finally, we also comment on the influence of Sudakov corrections within the scope of the diquark model.Sponsored in part by the Bundesministerium für Forschung und Technologie, Germany, under contract number 06 Wu 737 相似文献
3.
Jun-Lin Yuan Zhi-Jun Zhang Xiao-Jun Wang Hao-Hong Chen Guo-Bin Zhang 《Journal of solid state chemistry》2007,180(4):1365-1371
Three rare earth borosilicate oxyapatites, RE5Si2BO13 (RE=La, Gd, Y), were synthesized via wet chemical method, of which RE5Si2BO13 (RE=Gd, Y) were first reported in this work. In the three oxyapatites, [BO4] and [SiO4] share the [TO4] tetrahedral oxyanion site, and RE3+ ions occupy all metal sites. The differential scanning calorimetry-thermo gravimetry measurements and high temperature powder X-ray diffraction pattern revealed a vitrification process within 300-1200 °C, which was due to the glass-forming nature of borosilicates. From the VUV excitation spectra of Eu3+ and Tb3+ in RE5Si2BO13, the optical band gaps were found to be 6.31, 6.54 and 6.72 eV for RE5Si2BO13 (RE=La, Gd, Y), respectively. The emission and excitation bands of Eu3+ and Tb3+ are discussed relating with their coordination environments. Among the three hosts, Y5Si2BO13 would be the best for Eu3+ and Tb3+-doped phosphors. 相似文献
4.
E. García-Matres J.L Martínez J. Rodríguez-Carvajal 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(1):59-70
A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group
Immm, a
≈ 3.8?, b
≈ 5.8?, c
≈ 11.3?) is structurally characterised by the presence of flattened NiO6 octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist
above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously
for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures
of R2BaNiO5 ( R
=
Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic
structure characterised by the temperature-independent propagation vector
= (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except
for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R3+ cations are close to the free-ion value. The magnetic moment of Ni2+ is around 1.4
, the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects
and covalency. The thermal evolution of the magnetic structures from T
N
down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R3+, seems to occur below and very close to T
N
in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot
be neglected.
Received 19 July 2001 相似文献
5.
G. P. Gupta A. Z. Msezane 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,49(2):157-160
Excitation energies from ground state for 97 fine-structure levels as well as oscillator strengths and radiative decay rates
for all electric-dipole-allowed and intercombination transitions among the fine-structure levels of the terms belonging to
the (1s
22s
22p
6)3s
23p, 3s
3
p
2, 3s
23d, 3p
3, 3s3p3d, 3p
23d, 3s3d
2, 3s
24s, 3s
24p, 3s
24d, 3s
24f, and 3s3p4s configurations of Co XV are calculated, using extensive configuration-interaction (CI) wave functions, obtained with the
CIV3 computer code of Hibbert. The important relativistic effects in intermediate coupling are included through the Breit-Pauli
approximation via spin-orbit, spin-other-orbit, spin-spin, Darwin and mass correction terms. Small adjustments to the diagonal
elements of the Hamiltonian matrices have been made. Our calculated excitation energies, including their ordering, are in
excellent agreement with the experimental results and the experimentally compiled energy values of the National Institute
for Standards and Technology (NIST) wherever available. The mixing among several fine-structure levels is found to be very
strong, with most of the strongly mixed levels belonging to the (1s
22s
22p
6)3p
23d and 3s3d
2 configurations. The strong mixing among several fine-structure levels makes it very difficult to identify them uniquely.
Perhaps, that may be the reason for the lack of both experimental and theoretical results for these levels. We believe that
our extensive calculated values can guide experimentalists in identifying the fine-structure levels in their future work.
From our radiative decay rates we have also calculated radiative lifetimes of some fine-structure levels. In this calculation
we also predict new data for several fine-structure levels where no other theoretical and/or experimental results are available. 相似文献
6.
Blue-to-Orange Tunable Luminescence from Europium Doped Yttrium--Silicon--Oxide--Nitride Phosphors 下载免费PDF全文
Europium-doped yttrium-silicon-oxide-nitride phosphors are synthesized by carbothermal reduction and nitridation method. The crystal structure of the phosphors changed gradually from oxide Y2Si2O7 to nitride YSi3N5 state with increasing dosage of Si3N4 and carbon powder. The Y2Si2O7:Eu phosphor shows a blue emission at 465 nm with 300 nm excitation and a characteristic red emission of Eu^3+ at 612 nm with 230 nm excitation. The YSi3N5:EU phosphor shows a broad emission band centred at 595nm with some sharp peaks of Eu^3+ with 325nm excitation. The absorption of the studied phosphors increases from 450 to 700hm with an increment in nitrogen content. Blue-to-orange tunable luminescence is observed with 390 nm excitation. 相似文献
7.
Theoretical and experimental studies of the acoustic scattering by a finite linear grating of elastic cylindrical shells are performed. It is observed that a resonant interaction takes place at low frequency when the shells are very close to each other. This phenomenon can be clearly associated to the Scholte-Stoneley wave that propagates around a single shell. It is shown that each resonance of the Scholte-Stoneley wave is split up into N resonances when N shells compose the grating. 相似文献
8.
The new complex indium rhenium and scandium rhenium oxides, In6ReO12 and Sc6ReO12, have been synthesized as single phases in sealed silica tubes and by high-pressure high-temperature syntheses, and their crystal structures have been determined by single crystal X-ray diffraction.The compounds crystallize in a rhombohedral structure related to the distorted fluorite structure like Ln6ReO12 for some rare earth elements, S. G.: R-3, Z=3, aH= 9.248(2) Å, cH=8.720(2) Å for Sc6ReO12 and aH=9.492(1) Å, cH=8.933(1) Å for In6ReO12. A maximum in magnetization is observed for Sc6ReO12 at T(Mmax)=1.89(2) K, whereas ferromagnetic ordering is found for In6ReO12 by a pronounced increase in the temperature dependence of magnetization at TC=7.5(5) K. The magnetic moment per rhenium ion in In6ReO12 and Sc6ReO12 is 0.84(1) and 0.65(1) μB, respectively, derived from the paramagnetic regions. 相似文献
9.
D. Mikhailova H. Ehrenberg D. Trots G. Brey H. Fuess 《Journal of solid state chemistry》2009,182(6):1506-951
Mixed chromium-rhenium oxides, CrxRe1−xO2 with 0.31?x?0.66, have been synthesized for the first time by high-pressure high-temperature synthesis and in evacuated quartz tubes. The crystal structures of the compounds have been determined by single crystal and powder X-ray diffraction. Depending on synthesis conditions (pressure and temperature) these phases crystallize either in a tetragonal structure (P42/mnm) with statistical distribution of metal ions on one site (rutile-type), with cation ordering along c-axis (trirutile-type), or in a monoclinic rutile-like structure (C2/m) with ordering of Cr- and Re-cations and metallic Re-Re bonds. The “a” parameter of the tetragonal unit cell increases with increasing Re content whereas the “c” parameter decreases, indicating a strengthening of the Re-Re bond. The thermal stability of tetragonal CrxRe1−xO2 in Ar-atmosphere depends on the Re-content, decomposition is observed at 1241 K for x=0.34, but already at 966 K for x=0.5. The thermal expansion of CrxRe1−xO2 is anisotropic with a larger expansion coefficient in the “c” direction. Tetragonal CrxRe1−xO2 with 0.31?x<0.54 order antiferromagnetically at low temperatures with TN depending on the Cr-content x. 相似文献
10.
In 1907 Lord Rayleigh published a paper on the dynamic theory of gratings. In this paper he presented a rigorous approach for solving plane wave scattering on periodic surfaces. Moreover he derived explicit expressions for a perfectly conducting sinusoidal surface, and for perpendicular incidence of the electromagnetic plane wave. This paper was criticized by Lippmann in 1953 for he assumed Rayleigh’s approach to be incomplete. Since this time there have been published several arguments, proofs, and discussions concerning the correctness and the range of validity of Rayleigh’s approach not only for plane wave scattering on gratings but also for light scattering on nonspherical structures, in general. In the paper at hand we will discuss the different point of views on what is called “Rayleigh’s hypothesis” as well as the relevance of a found theoretical limit for its validity. Furthermore we present a numerical treatment of the original scattering problem of a p-polarized plane wave perpendicularly incident on a perfectly conducting sinusoidal surface (i.e., the scalar Dirichlet problem). In doing so we emphasizes the near-field solution especially within the grooves of the grating up to points on the surface, and below the surface. Two different Green’s function formulations of Huygens’ principle are used as starting points. One of this formulation results in the general T-matrix approach which is considered to be affected by Rayleigh’s hypothesis especially for near-field calculations. The other formulation provides a conventional boundary integral equation which is in accordance with Lippmann’s point of view and free of problems with Rayleigh’s hypothesis. But the obtained results show that Lippmann’s argumentation do not withstand a critical numerical analysis, and that the independence of least-squares approaches from Rayleigh’s hypothesis, as understood and proven by Millar, seems to hold also for certain methods which does not fit into such an approach. 相似文献