全文获取类型
收费全文 | 217258篇 |
免费 | 4988篇 |
国内免费 | 1284篇 |
专业分类
化学 | 122033篇 |
晶体学 | 2710篇 |
力学 | 9220篇 |
综合类 | 37篇 |
数学 | 25332篇 |
物理学 | 64198篇 |
出版年
2021年 | 1605篇 |
2020年 | 1915篇 |
2019年 | 1882篇 |
2018年 | 2207篇 |
2017年 | 2035篇 |
2016年 | 3899篇 |
2015年 | 3357篇 |
2014年 | 3967篇 |
2013年 | 10554篇 |
2012年 | 9634篇 |
2011年 | 11349篇 |
2010年 | 6932篇 |
2009年 | 6772篇 |
2008年 | 10161篇 |
2007年 | 10150篇 |
2006年 | 9553篇 |
2005年 | 8928篇 |
2004年 | 7900篇 |
2003年 | 6716篇 |
2002年 | 6434篇 |
2001年 | 6834篇 |
2000年 | 5254篇 |
1999年 | 4035篇 |
1998年 | 3070篇 |
1997年 | 3017篇 |
1996年 | 3059篇 |
1995年 | 2594篇 |
1994年 | 2518篇 |
1993年 | 2392篇 |
1992年 | 2722篇 |
1991年 | 2632篇 |
1990年 | 2334篇 |
1989年 | 2296篇 |
1988年 | 2273篇 |
1987年 | 2199篇 |
1986年 | 2080篇 |
1985年 | 3096篇 |
1984年 | 3051篇 |
1983年 | 2416篇 |
1982年 | 2670篇 |
1981年 | 2535篇 |
1980年 | 2468篇 |
1979年 | 2408篇 |
1978年 | 2470篇 |
1977年 | 2377篇 |
1976年 | 2364篇 |
1975年 | 2333篇 |
1974年 | 2235篇 |
1973年 | 2313篇 |
1972年 | 1293篇 |
排序方式: 共有10000条查询结果,搜索用时 687 毫秒
971.
The sample is vaporized from tungsten filament coils (150 W) and transported by an argon stream to the cell of a modified hydride furnace for atomic absorption spectrometry (a.a.s.). The system provides almost the same sensitivity for elements with low appearance temperatures (e.g., Bi, Cd, Pb, Tl, Zn) as graphite-furnace a.a.s. The detection limits are between 0.1 and 5 ng ml?1, depending on the element. 相似文献
972.
自由曲面光学产品设计、制造与检测的工艺流程,通常采取试凑法逐次逼近。由于加工 检测 再加工,循环往复,既费时,成本又高,产生了瓶颈问题。为了解决此弊端,本文运用虚拟制造技术,提出光学虚拟制造的基本构想,即虚拟制造系统结构模型,给出光学系统虚拟原型的构成和光学系统成像质量虚拟检测系统的构成,讨论光学成像质量的仿真检测以及敏度分析方法。研究结果表明:运用虚拟制造与检测技术,可缩短研发周期,降低成本,优化工艺并提高产品质量。 相似文献
973.
Apurba Bhattacharya Nitin C. Patel Ritesh Tichkule Jiejun Wu 《Tetrahedron letters》2006,47(4):565-567
A simple, surfactant-mediated, one-pot, solvent-free dealkylative cleavage of aryl ethers and esters followed by subsequent optional trans-alkylation under essentially neutral conditions has been developed. 相似文献
974.
Aryl substituted α-carbonyl bromo-alkynes undergo facile cyclizations to the corresponding substituted 4-methylene-tetrahydrofurans using indium(I) iodide in acetonitrile under sonication in high yields. The reaction is predicted to proceed via a radical process initiated by InI and a plausible radical pathway is suggested. 相似文献
975.
Using a density-functional method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials and the generalized gradient approximation for exchange and correlation, we found that at 0 K the atoms of an Fe monolayer on the Ni (111) surface occupy hcp rather than fcc sites, in keeping with previous predictions made using the ab initio all-electron full-potential linearized augmented plane wave method with the local spin density approximation. 相似文献
976.
Let { X n} be a Markov chain that is either f -mixing or satisfies the Poisson equation.In this note we obtain the convergence rate under L 1 -criterion for bounded functions of the X k 's. And in the hidden Markov model setup { (X n ,Y n ) }we study the kernel estimate of the density of the observed variables { Y n }when a 'stable' status is reached. 相似文献
977.
R. Euler B. Fricke T. Morović W. -D. Sepp A. Rosén 《Zeitschrift für Physik A Hadrons and Nuclei》1980,297(2):101-104
Total energy SCF calculations were performed for noble gas difluorides in a relativistic procedure and compared with analogous non-relativistic calculations. The discrete variational method with numerical basis functions was used. Rather smooth potential energy curves could be obtained. The theoretical Kr-F and Xe-F bond distances were calculated to be 3.5 a.u. and 3.6 a.u. which should be compared with the experimental values of 3.54 a.u. and 3.7 a.u. Although the dissociation energies are off by a factor of about five it was found that ArF2 may be a stable molecule. Theoretical ionization energies for the outer levels reproduce the experimental values for KrF2 and XeF2 to within 2 eV. 相似文献
978.
In this paper, an interior point algorithm based on trust region techniques is proposed for solving nonlinear optimization problems with linear equality constraints and nonnegative variables. Unlike those existing interior-point trust region methods, this proposed method does not require that a general quadratic subproblem with a trust region bound be solved at each iteration. Instead, a system of linear equations is solved to get a search direction, and then a linesearch of Armijo type is performed in this direction to obtain a new iteration point. From a computational point of view, this approach may in general reduce a computational effort, and thus improve the computational efficiency. Under suitable conditions, it is proven that any accumulation of the sequence generated by the algorithm satisfies the first-order optimality condition. 相似文献
979.
W. Rattanasakulthong C. Sirisathitkul 《Applied Physics A: Materials Science & Processing》2006,83(1):77-81
Thermal and magnetotransport properties of mechanically alloyed Co30Cu70 powders of three different milling times are studied. Both milling and annealing bring about oxidation of the samples. The powders show exothermic behaviors corresponding to the lattice-recovery process. Different milling times and annealing temperatures give rise to different exothermic peaks because of oxidation and the cluster distribution. At room temperature, the compressed powders show, on average, up to 11% giant magnetoresistance (GMR) under 10 kOe with sharp switching. The results may be understood in terms of spin-dependent scattering across Co/oxide/Cu interfaces. Annealing reduces the resistance but does not promote the GMR. By annealing at 400 °C, the GMR is entirely suppressed as the magnetic content is largely replaced by oxides. PACS 72.15.Eb; 75.47.De; 75.47.Np 相似文献
980.
We build explicitly an infinite number of equilibrium solutions of unloaded Marguerre–von Kármán membrane shells. This construction is based upon the existence of three elementary solutions, together with the solution of a Monge–Ampère equation associated with a partition of the reference configuration of the shell. To cite this article: A. Léger, B. Miara, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 649–654. 相似文献