全文获取类型
收费全文 | 5918篇 |
免费 | 470篇 |
国内免费 | 603篇 |
专业分类
化学 | 757篇 |
晶体学 | 15篇 |
力学 | 741篇 |
综合类 | 87篇 |
数学 | 4178篇 |
物理学 | 1213篇 |
出版年
2024年 | 14篇 |
2023年 | 74篇 |
2022年 | 156篇 |
2021年 | 132篇 |
2020年 | 194篇 |
2019年 | 176篇 |
2018年 | 160篇 |
2017年 | 191篇 |
2016年 | 181篇 |
2015年 | 136篇 |
2014年 | 231篇 |
2013年 | 443篇 |
2012年 | 276篇 |
2011年 | 357篇 |
2010年 | 304篇 |
2009年 | 460篇 |
2008年 | 423篇 |
2007年 | 399篇 |
2006年 | 356篇 |
2005年 | 328篇 |
2004年 | 242篇 |
2003年 | 235篇 |
2002年 | 203篇 |
2001年 | 162篇 |
2000年 | 172篇 |
1999年 | 159篇 |
1998年 | 179篇 |
1997年 | 129篇 |
1996年 | 100篇 |
1995年 | 53篇 |
1994年 | 51篇 |
1993年 | 39篇 |
1992年 | 43篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 22篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 12篇 |
1984年 | 18篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1974年 | 3篇 |
1972年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有6991条查询结果,搜索用时 0 毫秒
931.
Summary Hydrogen flame-ionization detectors (FIDs) are the most widely used type of detector in gas chromatography. The FID signals
is proportional to the number of carbon atoms in a hydrocarbon molecule; the presence of heteroatoms usually reduces the signal.
If the extent of the signal-reducing effect of heteroatoms were known, it would be possible to measure compounds which are
not available as pure standards, or cannot be prepared, or their preparation is very expensive. The sensitivity of a detector
to an organic molecule containing heteroatoms is referred to normal hydrocarbons by means of the effective carbon-atom number
(ECN) value. By use of the values of increments inECN for heteroatoms and functional groups, theECN can be calculated for any organic molecule. For this, exact values of theECN increments are needed, and the effects of different factors on the increments must be known. In this study a wide range of
homologues of normal paraffins, alcohols, amines, and esters was investigated, with emphasis on differences between the behaviour
of lower and higher homologues. Studies were extended to theECN values of ketones, and aromatic and halogenated compounds. For all types of compound investigated the difference between
the actual carbon number and the calculated effective carbon number (dECN) was compared with literature data, and an attempt was made to interpret the differences. 相似文献
932.
N. K. Bakirov M. V. Snachev V. E. Bening 《Moscow University Computational Mathematics and Cybernetics》2009,33(1):32-37
An exact formula for the mean number of claims served by a one-channel queuing system (QS) with rejections on a given time interval is obtained, which makes it possible, in particular, to calculate exactly the QS efficiency indexes on an arbitrary time interval. 相似文献
933.
V. P. Maslov 《Theoretical and Mathematical Physics》2005,145(3):1749-1752
A rigorous derivation of the Gibbs and Bose-Einstein distributions on a finite set of energies is stated, in essence, as a
theorem in number theory. If there are not very many particles, then the discussion of this question reduces to an exact theorem.
__________
Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 145, No. 3, pp. 433–436, December, 2005. 相似文献
934.
Eric S. Egge 《Discrete Mathematics》2006,306(6):552-563
Gire, West, and Kremer have found ten classes of restricted permutations counted by the large Schröder numbers, no two of which are trivially Wilf-equivalent. In this paper we enumerate eleven classes of restricted signed permutations counted by the large Schröder numbers, no two of which are trivially Wilf-equivalent. We obtain five of these enumerations by elementary methods, five by displaying isomorphisms with the classical Schröder generating tree, and one by giving an isomorphism with a new Schröder generating tree. When combined with a result of Egge and a computer search, this completes the classification of restricted signed permutations counted by the large Schröder numbers in which the set of restrictions consists of two patterns of length 2 and two of length 3. 相似文献
935.
Przemysaw Grzegorzewski 《Fuzzy Sets and Systems》2008,159(11):1354-1364
Fuzzy number approximation by trapezoidal fuzzy numbers which preserves the expected interval is discussed. Algorithms for calculating the proper approximations are proposed and some properties of the approximation operators are discussed. It is shown that an adequate approximation operator might be chosen through the comparisons of some characteristics of the fuzzy number, like its ambiguity, width, its value and weighted expected value. 相似文献
936.
Chaos,transport and mesh convergence for fluid mixing 总被引:1,自引:0,他引:1
Chaotic mixing of distinct fluids produces a convoluted structure to the interface separating these fluids. For miscible fluids (as considered here), this interface is defined as a 50% mass concentration isosurface. For shock wave induced (Richtmyer-Meshkov) instabilities, we find the interface to be increasingly complex as the computational mesh is refined. This interfacial chaos is cut off by viscosity, or by the computational mesh if the Kolmogorov scale is small relative to the mesh. In a regime of converged interface statistics, we then examine mixing, i.e. concentration statistics, regularized by mass diffusion. For Schmidt numbers significantly larger than unity, typical of a liquid or dense plasma, additional mesh refinement is normally needed to overcome numerical mass diffusion and to achieve a converged solution of the mixing problem. However, with the benefit of front tracking and with an algorithm that allows limited interface diffusion, we can assure convergence uniformly in the Schmidt number. We show that different solutions result from variation of the Schmidt number. We propose subgrid viscosity and mass diffusion parameterizations which might allow converged solutions at realistic grid levels. 相似文献
937.
We systematically analyze the experimental data of alpha decay in even-even heavy nuclei far from stabilityand find that the Geiger-Nuttall law breaks for an isotopic chain when its neutron number is across a magic numberor there is a deformed subshell. This break can be used to identify new magic numbers of superheavy nuclei. It is alsodiscovered that there is a new linear relation between the logarithm of half-life and the reciprocal of the square root ofdecay energy for N = 126 and N = 152 isotones. It could be a new law of alpha decay for nuclei with magic neutronnumbers but the physics behind it is to be explored. The significance of these researches for the search of new elementsis discussed. 相似文献
938.
939.
By means of the theory of modular forms the formulas for a number of representations of positive integers by two positive quaternary quadratic forms of steps 36 and 60 and by all positive diagonal quadratic forms with seven variables of step 8 are obtain. 相似文献
940.
We describe a method of creating an infinite family of crossing‐critical graphs from a single small planar map, the tile, by gluing together many copies of the tile together in a circular fashion. This method yields all known infinite families of k‐crossing‐critical graphs. Furthermore, the method yields new infinite families, which extend from (4,6) to (3.5,6) the interval of rationals r for which there is, for some k, an infinite sequence of k‐crossing‐critical graphs all having average degree r. © 2003 Wiley Periodicals, Inc. J Graph Theory 42: 332–341, 2003 相似文献