全文获取类型
收费全文 | 506565篇 |
免费 | 3570篇 |
国内免费 | 1137篇 |
专业分类
化学 | 248039篇 |
晶体学 | 7761篇 |
力学 | 25602篇 |
综合类 | 13篇 |
数学 | 70683篇 |
物理学 | 159174篇 |
出版年
2020年 | 3513篇 |
2019年 | 3890篇 |
2018年 | 12024篇 |
2017年 | 12670篇 |
2016年 | 10712篇 |
2015年 | 5183篇 |
2014年 | 7337篇 |
2013年 | 18185篇 |
2012年 | 17392篇 |
2011年 | 26542篇 |
2010年 | 17603篇 |
2009年 | 17382篇 |
2008年 | 23881篇 |
2007年 | 26841篇 |
2006年 | 15158篇 |
2005年 | 18609篇 |
2004年 | 15060篇 |
2003年 | 13951篇 |
2002年 | 12134篇 |
2001年 | 12382篇 |
2000年 | 9749篇 |
1999年 | 7369篇 |
1998年 | 6307篇 |
1997年 | 6238篇 |
1996年 | 5957篇 |
1995年 | 5509篇 |
1994年 | 5519篇 |
1993年 | 5151篇 |
1992年 | 5812篇 |
1991年 | 5883篇 |
1990年 | 5702篇 |
1989年 | 5399篇 |
1988年 | 5534篇 |
1987年 | 5468篇 |
1986年 | 5153篇 |
1985年 | 6839篇 |
1984年 | 7240篇 |
1983年 | 6018篇 |
1982年 | 6321篇 |
1981年 | 6220篇 |
1980年 | 6065篇 |
1979年 | 6134篇 |
1978年 | 6489篇 |
1977年 | 6375篇 |
1976年 | 6486篇 |
1975年 | 5979篇 |
1974年 | 6042篇 |
1973年 | 6311篇 |
1972年 | 4304篇 |
1971年 | 3521篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
The shape of lines in the radiospectroscopic (NMR and EPR) and dielectric spectra of materials formed by nanoparticles (hereafter, nanomaterials) is analyzed theoretically. The theory is developed in the framework of the core and shell model according to which a nanoparticle consists of two regions whose properties are affected and unaffected by the surface, respectively. The changes in the resonance frequency, the relaxation time, and the static permittivity due to the surface tension are taken into account, and the Gaussian and Lorentzian shapes of homogeneously broadened lines are considered. The inhomogeneous broadening of the spectral lines is examined for several types of nanoparticle size distributions. It is demonstrated that the splitting of the initial lines in the spectra of bulk systems into pairs of lines with a decrease in the particle size is a specific feature of the spectra of nanoparticles. The intensities and half-widths of the lines are investigated as functions of the parameters of the size distribution of nanoparticles. The results of theoretical calculations are compared with recent experimental data on the 17O and 25Mg NMR spectra of nanocrystalline MgO. The theoretical dependences of the intensity, the resonance frequency, and the half-width of the spectral lines are in good agreement with the experimental data. The proposed theory offers a satisfactory explanation of the behavior of the static permittivity in BaTiO3 ceramic materials with nanometer-sized grains. 相似文献
993.
A tight-binding (TB) hamiltonian for calcium is built with a high precision parametrization technique based on density functional calculations of the energy bands and the total energy at various lattice volumes. The new set of TB parameters is appropriate to study phenomena under pressures as high as 20 GPa. Specifically, both the metal to nonmetal transition at 4 GPa and the structural transition fcc to bcc at 19 GPa are well reproduced. These transitions and several static properties are in excellent agreement with experiments. Phonon frequencies, plasmon energy, melting temperature and the coefficient of thermal expansion were calculated with a molecular dynamics scheme of this TB hamiltonian. 相似文献
994.
Synchronized whistlers recorded at Varanasi 总被引:1,自引:0,他引:1
Some interesting events of synchronized whistlers recorded at low latitude station Varanasi during magnetic storm period of
the year 1977 are presented. The dynamic spectrum analysis shows that the component whistlers are Eckersley whistlers having
dispersion 10 s1/2 and 30 s1/2. An attempt has been made to explain the dynamic spectra using lightning discharge generated from magnetospheric sources 相似文献
995.
F.O. Borges G.H. Cavalcanti A.G. Trigueiros 《Journal of Quantitative Spectroscopy & Radiative Transfer》2003,78(1):119-129
The weighted oscillator strengths (gf) and the lifetimes presented in this work were carried out in a multi configuration Hartree-Fock relativistic (HFR) approach. In this calculation, the electrostatic parameters were optimized by a least-squares procedure, in order to improve the adjustment to experimental energy levels. This method produces gf-values that are in better agreement with intensity observations and lifetime values that are closer to the experimental ones. In this work, we presented all the experimentally known electric dipole S IX and S X spectral lines. 相似文献
996.
B. G. Tagiev U. F. Kasumov N. N. Musaeva R. B. Dzhabbarov 《Physics of the Solid State》2003,45(3):426-432
The current-voltage characteristics of Ca4Ga2S7: Eu3+ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca4Ga2S7: Eu3+ single crystals are high-resistance semiconductors with a resistivity of ~109 Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×1014 to 2.7×1015 cm?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×103 V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×104 V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level. 相似文献
997.
Given one or more vibrational modes of a membrane, the free vibration equation can be applied to infer the mass surface density. This paper considers determining the surface density of an inhomogeneous membrane from digitized holographic projections (interferograms) of the modeshapes. Spatially discrete numerical models of the membrane surface are presented, which can be used to solve both forward and inverse vibration problems. The accuracy of the discrete models is examined for exactly solvable free vibration problems involving inhomogeneous membranes. For the solution of the inverse problem, error estimates are given for the mass surface density deduced from modeshape interferograms. The practicability of the method is investigated using simulated experimental data for membranes with composite and continuously inhomogeneous density profiles. Strategies are discussed for reducing errors in the reconstructed densities. 相似文献
998.
999.
1000.
A gravitational field can be seen as the anholonomy of the tetrad fields. This is more explicit in the teleparallel approach, in which the gravitational field-strength is the torsion of the ensuing Weitzenböck connection. In a tetrad frame, that torsion is just the anholonomy of that frame. The infinitely many tetrad fields taking the Lorentz metric into a given Riemannian metric differ by point-dependent Lorentz transformations. Inertial frames constitute a smaller infinity of them, differing by fixed-point Lorentz transformations. Holonomic tetrads take the Lorentz metric into itself, and correspond to Minkowski flat spacetime. An accelerated frame is necessarily anholonomic and sees the electromagnetic field strength with an additional term. 相似文献