全文获取类型
收费全文 | 4459篇 |
免费 | 727篇 |
国内免费 | 551篇 |
专业分类
化学 | 1258篇 |
晶体学 | 93篇 |
力学 | 382篇 |
综合类 | 52篇 |
数学 | 1429篇 |
物理学 | 2523篇 |
出版年
2024年 | 14篇 |
2023年 | 53篇 |
2022年 | 150篇 |
2021年 | 109篇 |
2020年 | 135篇 |
2019年 | 108篇 |
2018年 | 112篇 |
2017年 | 115篇 |
2016年 | 134篇 |
2015年 | 127篇 |
2014年 | 209篇 |
2013年 | 349篇 |
2012年 | 214篇 |
2011年 | 216篇 |
2010年 | 228篇 |
2009年 | 292篇 |
2008年 | 268篇 |
2007年 | 281篇 |
2006年 | 252篇 |
2005年 | 240篇 |
2004年 | 201篇 |
2003年 | 206篇 |
2002年 | 246篇 |
2001年 | 184篇 |
2000年 | 182篇 |
1999年 | 169篇 |
1998年 | 156篇 |
1997年 | 116篇 |
1996年 | 73篇 |
1995年 | 76篇 |
1994年 | 71篇 |
1993年 | 53篇 |
1992年 | 42篇 |
1991年 | 40篇 |
1990年 | 31篇 |
1989年 | 30篇 |
1988年 | 31篇 |
1987年 | 28篇 |
1986年 | 32篇 |
1985年 | 36篇 |
1984年 | 18篇 |
1983年 | 10篇 |
1982年 | 23篇 |
1981年 | 14篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 12篇 |
1972年 | 4篇 |
排序方式: 共有5737条查询结果,搜索用时 15 毫秒
91.
M.L. Foo Q. Huang Wei-Li Lee I.S. Hagemann N.P. Ong 《Journal of solid state chemistry》2006,179(2):563-572
The synthesis, structure, and physical properties of five R-type Ru ferrites with chemical formula BaMRu5O11 (M=Li and Cu) and BaM′2Ru4O11 (M′=Mn, Fe and Co) are reported. All the ferrites crystallize in space group P63/mmc and consist of layers of edge sharing octahedra interconnected by pairs of face sharing octahedra and isolated trigonal bipyramids. For M=Li and Cu, the ferrites are paramagnetic metals with the M atoms found on the trigonal bipyramid sites exclusively. For M′=Mn, Fe and Co, the ferrites are soft ferromagnetic metals. For M′=Mn, the Mn atoms are mixed randomly with Ru atoms on different sites. The magnetic structure for BaMn2Ru4O11 is reported. 相似文献
92.
The lattice model of associated solutions was used to calculate the concentration dependences of the chemical shifts of13C magnetic nuclei in the acrylamide molecule in water—DMSO solvent. Each pair of bonds between the acrylamide group containing
a magnetic nucleus and the neighboring solvent molecule was assumed to additively contribute to the change in the chemical
shift of the nucleus in question. The new approach affords the same accuracy in describing the experimental data at any ratios
of the solution components. It was shown that the chemical shifts and the excess heat of mixing of the water—DMSO binary solution
can be simultaneously described using the same set of energy parameters of the model. The model makes it possible to employ
experimental data on chemical shifts for studying intermolecular interactions in solutions.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1983–1987, October, 1998. 相似文献
93.
Kim C. Pich Roger Bishop Donald C. Craig Ian G. Dance A. David Rae Marcia L. Scudder 《Structural chemistry》1993,4(1):41-51
In a further exploration of the structural factors causing alicyclic diols such asexo-2,exo-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (1) to adopt the helical tubuland inclusion host structure, the title compound (8) has been synthesized and its crystal structure determined. This 9-oxa analogue of (1) has a totally different structure [C10H18O3; orthorhombic;P21212;a 18.989(4),b 19.064(3),c 14.256(3) Å;Z 20; with finalR 0.065] due to involvement of the ether oxygen atom in the hydrogen bonding network. The structure of the diol (8) approximatesP¯421
c symmetry, and 16 of the 20 molecules per cell create a hostlike structure of this symmetry, which consists of tightly bound hydrogen bonded pillars parallel toc. The remaining guestlike molecules occupy inequivalent pseudo ¯4 sites separated byc/2 to form weakly bound columns parallel toc. Hydrogen bonds also occur between the pillars and columns. An ordered structure with sensible intermolecular contacts can be formed inP212121 with thec axis doubled. A difference betweena andb axial lengths correlates with a difference in occupancies of pseudo-¯4-related sites for the guest-like component of the structure. 相似文献
94.
Some gauge invariant atomic orbitals-coupled-perturbed Hartree-Fock (GIAO-CPHF) calculations were performed for seven indolizine derivatives and their monoprotonated forms. Chemical shift, molecular geometry, and charge distribution data are reported for each molecule. The calculations support the results of nuclear magnetic resonance (NMR) spectroscopy measurements showing that protonation occurs preferentially at N1. The good agreement between the calculated and observed 13C and 15N chemical shifts show that such calculations can be used for chemical shift assignment purposes. Cation structures and probable sites for electrophilic reaction or second protonation are also discussed. 相似文献
95.
This paper is considering the problem of traveling wave solutions (TWS) for a susceptible-exposed-infectious-recovered (SEIR) epidemic model with discrete diffusion. The threshold condition for the existence and nonexistence of TWS is obtained. More specifically, such kind of solutions are governed by the threshold number ?0. We can find a critical wave speed c? if ?0 > 1, by employing the Schauder's fixed point theorem, limiting argument and two-sided Laplace transform, we confirm that there exists TWS for c > c?, while there exists no TWS for c < c?. We also obtain the nonexistence of TWS for ?0 ≤ 1. At last, we give some biological explanations from the epidemiological perspective. 相似文献
96.
O. Penrose 《Journal of statistical physics》1995,78(1-2):267-283
The grand potentialP(z)/kT of the cluster model at fugacityz, neglecting interactions between clusters, is defined by a power series
n
Q
n
z
n
, whereQ
n
, which depends on the temperatureT, is the partition function of a cluster of sizen. At low temperatures this series has a finite radius of convergencez
s
. Some theorems are proved showing that ifQ
n
, considered as a function ofn, is the Laplace transform of a function with suitable properties, thenP(z) can be analytically continued into the complexz plane cut along the real axis fromz
s
to + and that (a) the imaginary part ofP(z) on the cut is (apart from a relatively unimportant prefactor) equal to the rate of nucleation of the corresponding metastable state, as given by Becker-Döring theory, and (b) the real part ofP(z) on the cut is approximately equal to the metastable grand potential as calculated by truncating the divergent power series at its smallest term. 相似文献
97.
In this paper we discuss the existence of generic long-range correlations in spatially homogeneous and stable equilibrium states of closed lattice gas automata whose stochastic collision rules violate the symmetry conditions of detailed balance and in addition satisfy local conservation laws. Such correlations occur even though the collision rules are strictly local and invariant under all symmetries of the lattice. First a phenomenological (Langevin equation) approach is discussed. Next we present a theoretical analysis on the basis of an approximate microscopic (ring kinetic) theory. This theory is used to calculate the amplitude ofr
– tails in the spatial correlations, and the result is compared with computer simulations. 相似文献
98.
New calculations to over ten million time steps have revealed a more complex diffusive behavior than previously reported of a point particle on a square and triangular lattice randomly occupied by mirror or rotator scatterers. For the square lattice fully occupied by mirrors where extended closed particle orbits occur, anomalous diffusion was still found. However, for a not fully occupied lattice the superdiffusion, first noticed by Owczarek and Prellberg for a particular concentration, obtains for all concentrations. For the square lattice occupied by rotators and the triangular lattice occupied by mirrors or rotators, an absence of diffusion (trapping) was found for all concentrations, except on critical lines, where anomalous diffusion (extended closed orbits) occurs and hyperscaling holds for all closed orbits withuniversal exponentsd
f
=7/4 and =15/7. Only one point on these critical lines can be related to a corresponding percolation problem. The questions arise therefore whether the other critical points can be mapped onto a new percolation-like problem and of the dynamical significance of hyperscaling. 相似文献
99.
We investigate the statistics of the numberN(R, S) of lattice pointsnZ
2, in an annular domain (R, w)=(R+w)A\RA, whereR, w>0. HereA is a fixed convex set with smooth boundary andw is chosen so that the area of (R, w) isS. The statistics comes fromR being taken as random (with a smooth density) in some interval [c
1
T,c
2,T],c
2>c
1>0. We find that in the limitT the variance and distribution of N=N(R; S)–S depend strongly on howS grows withT. There is a saturation regimeS/T, asT, in which the fluctuations in N coming from the two boundaries of are independent. Then there is a scaling regime,S/Tz, 0<z<, in which the distribution depends onz in an almost periodic way going to a Gaussian asz0. The variance in this limit approachesz for genericA, but can be larger for degenerate cases. The former behavior is what one would expect from the Poisson limit of a distribution for annuli of finite area. 相似文献
100.
F. den Hollander 《Journal of statistical physics》1994,75(5-6):891-918
Some fifteen years ago, Shuler formulated three conjectures relating to the large-time asymptotic properties of a nearest-neighbor random walk on 2 that is allowed to make horizontal steps everywhere but vertical steps only on a random fraction of the columns. We give a proof of his conjectures for the situation where the column distribution is stationary and satisfies a certain mixing codition. We also prove a strong form of scaling to anisotropic Brownian motion as well as a local limit theorem. The main ingredient of the proofs is a large-deviation estimate for the number of visits to a random set made by a simple random walk on . We briefly discuss extensions to higher dimension and to other types of random walk.Dedicated to Prof. K. E. Shuler on the occasion of his 70th birthday, celebrated at a Symposium in his honor on July 13, 1992, at the University of California at San Diego, La Jolla, California. 相似文献