全文获取类型
收费全文 | 37319篇 |
免费 | 3385篇 |
国内免费 | 6777篇 |
专业分类
化学 | 31559篇 |
晶体学 | 441篇 |
力学 | 6824篇 |
综合类 | 268篇 |
数学 | 2595篇 |
物理学 | 5794篇 |
出版年
2024年 | 53篇 |
2023年 | 360篇 |
2022年 | 786篇 |
2021年 | 913篇 |
2020年 | 1197篇 |
2019年 | 970篇 |
2018年 | 1031篇 |
2017年 | 1127篇 |
2016年 | 1480篇 |
2015年 | 1499篇 |
2014年 | 1895篇 |
2013年 | 3309篇 |
2012年 | 2292篇 |
2011年 | 2543篇 |
2010年 | 1999篇 |
2009年 | 2397篇 |
2008年 | 2417篇 |
2007年 | 2514篇 |
2006年 | 2442篇 |
2005年 | 2256篇 |
2004年 | 2227篇 |
2003年 | 1892篇 |
2002年 | 1360篇 |
2001年 | 972篇 |
2000年 | 885篇 |
1999年 | 853篇 |
1998年 | 806篇 |
1997年 | 747篇 |
1996年 | 635篇 |
1995年 | 587篇 |
1994年 | 521篇 |
1993年 | 486篇 |
1992年 | 470篇 |
1991年 | 294篇 |
1990年 | 233篇 |
1989年 | 201篇 |
1988年 | 177篇 |
1987年 | 109篇 |
1986年 | 97篇 |
1985年 | 114篇 |
1984年 | 97篇 |
1983年 | 47篇 |
1982年 | 85篇 |
1981年 | 31篇 |
1980年 | 19篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1976年 | 8篇 |
1971年 | 7篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
32.
Summary This paper is concerned with the problem of developing numerical integration algorithms for differential equations that, when
viewed as equations in some Euclidean space, naturally evolve on some embedded submanifold. It is desired to construct algorithms
whose iterates also evolve on the same manifold. These algorithms can therefore be viewed as integrating ordinary differential
equations on manifolds. The basic method “decouples” the computation of flows on the submanifold from the numerical integration
process. It is shown that two classes of single-step and multistep algorithms can be posed and analyzed theoretically, using
the concept of “freezing” the coefficients of differential operators obtained from the defining vector field. Explicit third-order
algorithms are derived, with additional equations augmenting those of their classical counterparts, obtained from “obstructions”
defined by nonvanishing Lie brackets. 相似文献
33.
34.
35.
以金属钇和异丙醇为原料,以HgCl2/I2为复合催化剂,通过对金属钇的机械加工以增加其比表面,并将异丙醇脱水使其含水量降低至0.05%,体系在82℃回流5h,经过滤、减压蒸馏,得到了白色海绵状异丙醇钇,其产率高达83%,合成时间比文献报道的缩短了19h,产率提高了8%。文章确定了催化剂的最佳用量为20gY加入60mg HgCl2/I2,研究了合成产率与HgCl2/I2催化剂和HgCl2催化剂的依赖关系及异丙醇中含水量对合成产率的影响,并对HgCl2/I2的催化作用机理进行了初步探讨。 相似文献
36.
David K Bisset 《国际流体数值方法杂志》2002,39(10):961-977
Turbulent flow simulation methods based on finite differences are attractive for their simplicity, flexibility and efficiency, but not always for accuracy or stability. This paper demonstrates that a good compromise is possible with the advected grid explicit (AGE) method. Starting from the same initial field as a previous spectral DNS, AGE method simulations of a planar turbulent wake were carried out as DNS, and then at three levels of reduced resolution. The latter cases were in a sense large‐eddy simulations (LES), although no specific sub‐grid‐scale model was used. Results for the two DNS methods, including variances and power spectra, were very similar, but the AGE simulation required much less computational effort. Small‐scale information was lost in the reduced resolution runs, but large‐scale mean and instantaneous properties were reproduced quite well, with further large reductions in computational effort. Quality of results becomes more sensitive to the value chosen for one of the AGE method parameters as resolution is reduced, from which it is inferred that the numerical stability procedure controlled by the parameter is acting in part as a sub‐grid‐scale model. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
37.
A. B. Mazo 《Fluid Dynamics》2002,37(6):913-918
Plane ideal incompressible flow in a rectangular channel partitioned by a thin permeable barrier (lattice) is considered. In flowing through the lattice the stream suddenly (jumpwise) changes direction and loses energy. The flow is assumed to be vortical; the vorticity is discontinuous on the lattice. A mathematical formulation of the problem for the stream function is proposed in the form of a nonlinear elliptic equation with coefficients discontinuous on the lattice line. A numerical solution is constructed using the finite-element iteration method. The results of the numerical simulation show how the flow velocity profile in the channel can be controlled by means of permeable barriers. 相似文献
38.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
39.
Non-stoichiometric ternary chalcogenides (Zn,Fe)S were prepared in the film form by pyrolytic spray deposition technique, using air/nitrogen as the carrier gas. The precursor solution comprised of ZnCl2, FeCl2 and thiourea. The depositions were carried out under optimum conditions of experimental parameters viz. carrier gas (air/nitrogen) flow rate, concentration of precursor constituents, nozzle substrate distance and temperature of quartz substrate. The deposited thin films were later sintered in argon at 1073 K for 120 min.The structural, compositional and optical properties of the sintered thin films were studied. X-ray diffraction studies of the thin films indicated the presence of (Zn,Fe)S solid solution with prominent cubic sphalerite phase while surface morphology as determined by scanning electron microscopy (SEM) revealed a granular structure.The chemical composition of the resulting thin films as analyzed by energy dispersive X-ray analysis (EDAX) reflected the composition of the precursor solutions from which the depositions were carried out with Fe at% values ranging from 0.4 up to 33.SEM micrographs of thin films reveal that the grain sizes of the thin films prepared using air as carrier gas and N2 as carrier gas are in the vicinity of 300 and 150 nm, respectively.The diffuse transmittance measurements for thin films, as a function of wavelength reveal the dependence of direct optical band gap on Fe content and type of phase. 相似文献
40.
The molding processes of polymer melts involve geometrically complex dies. Such dies are usually tapered or streamlined to achieve a maximum output rate under conditions of laminar flow. The model of a generalized second-grade fluid of power-law type is used and the results obtained are illustrated by examples of convergent flows in conical and wedge-shaped dies. 相似文献