全文获取类型
收费全文 | 30835篇 |
免费 | 737篇 |
国内免费 | 186篇 |
专业分类
化学 | 21235篇 |
晶体学 | 244篇 |
力学 | 766篇 |
综合类 | 1篇 |
数学 | 4979篇 |
物理学 | 4533篇 |
出版年
2021年 | 223篇 |
2020年 | 275篇 |
2019年 | 370篇 |
2018年 | 278篇 |
2017年 | 247篇 |
2016年 | 544篇 |
2015年 | 440篇 |
2014年 | 551篇 |
2013年 | 1517篇 |
2012年 | 1452篇 |
2011年 | 1844篇 |
2010年 | 952篇 |
2009年 | 778篇 |
2008年 | 1681篇 |
2007年 | 1729篇 |
2006年 | 1705篇 |
2005年 | 1721篇 |
2004年 | 1513篇 |
2003年 | 1253篇 |
2002年 | 1247篇 |
2001年 | 443篇 |
2000年 | 373篇 |
1999年 | 301篇 |
1998年 | 345篇 |
1997年 | 379篇 |
1996年 | 498篇 |
1995年 | 352篇 |
1994年 | 330篇 |
1993年 | 338篇 |
1992年 | 325篇 |
1991年 | 286篇 |
1990年 | 257篇 |
1989年 | 270篇 |
1988年 | 297篇 |
1987年 | 281篇 |
1986年 | 264篇 |
1985年 | 428篇 |
1984年 | 483篇 |
1983年 | 351篇 |
1982年 | 465篇 |
1981年 | 440篇 |
1980年 | 375篇 |
1979年 | 359篇 |
1978年 | 385篇 |
1977年 | 350篇 |
1976年 | 301篇 |
1975年 | 297篇 |
1974年 | 276篇 |
1973年 | 250篇 |
1972年 | 148篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
61.
There are few techniques available to numerically solve sixth-order boundary-value problems with two-point boundary conditions. In this paper we show that the Sinc-Galerkin method is a very effective tool in numerically solving such problems. The method is then tested on examples with homogeneous and nonhomogeneous boundary conditions and a comparison with the modified decomposition method is made. It is shown that the Sinc-Galerkin method yields better results.
62.
Uptake of trace elements into fish otoliths is governed by several factors such as life histories and environment in addition to stock and species differences. In an attempt to elucidate the elemental signatures of rare earth elements (REEs) in otoliths, a solid phase extraction (SPE) protocol was used in combination with electrothermal vaporization (ETV) as a sample introduction procedure for the determinations by inductively coupled plasma quadrupole mass spectrometry (ICP-MS). Effects of various parameters, such as carrier gas flow rate, atomization temperature and chemical modification, were examined for optimization of the conditions by ETV-ICP-MS. Atomization was achieved at 2800 °C. Lower temperatures (i.e. 2600 °C) resulted in severe memory problems due to incomplete atomization. Palladium was used as a chemical modifier. It was found that an increase in Pd concentration up to 0.5 μg in the injection volume (70 μl) led up to four-fold enhancement in the integrated signals. This phenomenon is attributed to the carrier effect of Pd rather than the stabilization since no significant losses were observed for high temperature drying around 700 °C even in the absence of Pd. Preconcentration was performed on-line at pH 5 by using a mini-column of Toyopearl AF-Chelate 650M chelating resin, which also eliminated the calcium matrix of otolith solutions. After preconcentration of 6.4 ml of solution, the concentrate was collected in 0.65 ml of 0.5% (v/v) HNO3 in autosampler cups, and then analyzed by ETV-ICP-MS. The method was validated with the analysis of a fish otolith certified reference material (CRM) of emperor snapper, and then applied to samples. Results obtained from otoliths of fish captured in the same habitat indicated that otolith rare earth element concentrations are more dependent on environmental conditions of the habitat than on species differences. 相似文献
63.
64.
We complete our previous(1, 2) demonstration that there is a family of new solutions to the photon and Dirac equations using spatial and temporal circles and four-vector behaviour of the Dirac bispinor. We analyse one solution for a bound state, which is equivalent to the attractive two-body interaction between a charged point particle and a second, which remains at rest. We show this yields energy and angular momentum eigenvalues that are identical to those found by the usual method of solving of the Dirac equation,(4) including fine structure. We complete our previous derivation(2) of QED from a set of rules for the two-body interaction and generalise these. We show that QED may be decomposed into a two-body interaction at every point in spacetime. 相似文献
65.
Jacques Rouzaud MatthewD. Jones Robert Raja BrianF.G. Johnson John MeurigThomas MelindaJ. Duer 《Helvetica chimica acta》2003,86(5):1753-1759
A set of new, air‐stable, RhI‐based heterogeneous asymmetric hydrogenation catalysts have been synthesised, characterised, and tested. Individual members of this new family all exhibit good enantioselectivity. 相似文献
66.
Arthur G. Maki John C. Grecu Brenda Winnewisser Manfred Winnewisser 《Journal of Molecular Spectroscopy》2003,222(2):198-212
High-resolution spectra of 15N12C12C15N and 14N13C13C14N have been measured and analyzed from 200 to 3600 cm−1. All the vibrational levels below 900 cm−1 have been observed and characterized. The Fermi resonance between ν2 and 2ν4 has been studied and the resonance constant has been determined for several cases. Several Σ− states have been directly observed for the first time for each isotopomer, the (0001111)0f, (0011111)0f, and (0002222)0f states. The pattern of the energy levels for clusters of l-type resonance coupled levels, such as 0001131,3, has been determined for cyanogen for the first time. Among other things this involved the determination of the vibrational l-type resonance constant, r45. Many of the power series constants, αi and xij, and higher order constants have been determined. 相似文献
67.
John M. Brown 《Journal of organometallic chemistry》2004,689(24):4006-4015
An overview of the contribution of NMR to the development of our understanding of homogeneous catalysis is presented, with an emphasis on work from the author’s research group. 相似文献
68.
69.
The selection of the branching variable can greatly affect the speed of the branch and bound solution of a mixed-integer or
integer linear program. Traditional approaches to branching variable selection rely on estimating the effect of the candidate
variables on the objective function. We present a new approach that relies on estimating the impact of the candidate variables
on the active constraints in the current LP relaxation. We apply this method to the problem of finding the first feasible
solution as quickly as possible. Empirical experiments demonstrate a significant improvement compared to a state-of-the art
commercial MIP solver. 相似文献
70.
If a Horn set I has a single satisfying truth assignment or model then that model is said to be unique for I. The question of determining whether a unique model exists for a given Horn set I is shown to be solved in O((L)*L) time, where L is the sum of the lengths of the clauses in I and is the inverse Ackermann function. It is also shown that if LA*log (A) where A is the number of distinct proposition letters then unique satisfiability can be determined in O(L) time. 相似文献