全文获取类型
收费全文 | 127193篇 |
免费 | 1634篇 |
国内免费 | 461篇 |
专业分类
化学 | 70286篇 |
晶体学 | 2042篇 |
力学 | 4989篇 |
综合类 | 5篇 |
数学 | 12436篇 |
物理学 | 39530篇 |
出版年
2016年 | 1342篇 |
2015年 | 1062篇 |
2014年 | 1405篇 |
2013年 | 4906篇 |
2012年 | 3502篇 |
2011年 | 4544篇 |
2010年 | 2844篇 |
2009年 | 2594篇 |
2008年 | 4058篇 |
2007年 | 4127篇 |
2006年 | 4161篇 |
2005年 | 4077篇 |
2004年 | 3560篇 |
2003年 | 3228篇 |
2002年 | 3139篇 |
2001年 | 3612篇 |
2000年 | 2718篇 |
1999年 | 2229篇 |
1998年 | 1931篇 |
1997年 | 1909篇 |
1996年 | 1836篇 |
1995年 | 1818篇 |
1994年 | 1587篇 |
1993年 | 1579篇 |
1992年 | 1780篇 |
1991年 | 1775篇 |
1990年 | 1700篇 |
1989年 | 1702篇 |
1988年 | 1695篇 |
1987年 | 1688篇 |
1986年 | 1597篇 |
1985年 | 2155篇 |
1984年 | 2280篇 |
1983年 | 1898篇 |
1982年 | 2219篇 |
1981年 | 2035篇 |
1980年 | 2085篇 |
1979年 | 2065篇 |
1978年 | 2196篇 |
1977年 | 2101篇 |
1976年 | 2117篇 |
1975年 | 2057篇 |
1974年 | 1898篇 |
1973年 | 2114篇 |
1972年 | 1285篇 |
1971年 | 977篇 |
1970年 | 912篇 |
1969年 | 923篇 |
1968年 | 1050篇 |
1967年 | 1101篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Ralf Freund Christos Allagiannis Peter Schnholzer Karl Bernauer 《Helvetica chimica acta》1994,77(3):615-621
A crude hydrate 6 and a crystalline hemiacetal 7 of glyoxylamide 4 were prepared from crotonamide 5 (Scheme 2). Particularly hemiacetal 7 , but also 6 and the ‘dimer’ 8 (obtained from 7 ) may serve as homochiral auxiliaries. The structure of 8 was determined by X-ray analysis. By arenesulfonyl halides, tryptimines 12–14 of 4 were diastereoselectively transformed into spirotricycles 15–17 and 19 . 相似文献
42.
To investigate local ordering and segregation phenomenon in a Ni91Pt9-alloy after sputtering and annealing a 3D optical atom probe (OAP) has been used. The specimen tips have been prepared from polycrystalline samples. To sputter the samples a separate preparation chamber with a scannable Ar-sputter-gun is connected to the OAP vessel. When necessary, the sample can be electrically heated to induce segregation and cure the altered layer. After a heat treatment of a Ni91 at. %Pt 9 at.% specimen at 1100 K the surface of a (111)-oriented specimen is enriched in platinum by a factor of two in relation to the bulk. The phenomenon of short-range ordering has been investigated on the surface and in the subsurface volume. A 3D reconstruction of this annealed NiPt specimen shows regions with high concentration of platinum that gives an indication at short-range ordering. Uniform sputtering of the tip without a heat treatment induces a decisive depletion of Pt on the surface and the following subatomic layers. The atom-probe results of specimens in thermal equilibrium are in close agreement to further surface sensitive results obtained from Ion Scattering Spectroscopy (ISS) and Auger Electron Spectroscopy (AES). 相似文献
43.
44.
In this paper, the authors studied certain properties of the estimate of Liang and Krishnaiah (1985, J. Multivariate Anal. 16, 162–172) for multivariate binary density. An alternative shrinkage estimate is also obtained. The above results are generalized to general orthonormal systems. 相似文献
45.
W.-H. Chen 《Archiv der Mathematik》2002,79(5):396-400
Consider the class of closed Riemannian manifolds M of dimension dim(M) \geqq 3 \dim(M) \geqq 3 , Ricci curvature Ric(M) \geqq -(n - 1) \textrm{Ric}(M) \geqq -(n - 1) , diameter diam(M) < D and almost maximal volume. We show that the isomorphism types of fundamental groups characterize the diffeomorphism types of manifolds in such a class. In particular, it can be viewed as a generalization of the well-known Mostows rigidity theorem and a finiteness theorem. 相似文献
46.
I. L. Davies K. E. Markides M. L. Lee M. W. Raynor K. D. Bartle 《Journal of separation science》1989,12(4):193-207
Although coupled liquid chromatographygas chromatography (LC-GC) was first demonstrated ten years ago, only in the last few years has there been a sudden surge of interest in the technique. Approximately 70% of the total number of LC-GC applications have been published in the last two years (1987–88) alone. This review categorizes LC-GC publications into four main application areas: fossil fuels, foods, environmental samples, biologiical/pharmaceutical samples, and miscellaneous samples. Multidimensional separations carried out using other coupled-column chromatographic techniques (such as supercritical fluid chromatography (SFC) with GC, and on-line trace enrichment-GC) have also been included in this review. 相似文献
47.
Critical problems of the present approach of electrolytes for Solid Oxide Fuel Cells (SOFCs) for commercialization are discussed.
Major progress is expected from the development of materials based on the “SEA (Single Element Arrangement)” concept. The
galvanic cell consists in this case basically of a single chemically homogeneous compound, which functions as electrodes at
high and low activity and as electrolyte at intermediate activities of the electroactive component.
In view of the large structural flexibility of the chemical nature of the constitutents, we explored perovskite (ABO3)-type compounds to be used as SEAs for SOFCs. Results of studies on Pr-substituted LSGM and Fe-substituted SrSnO3 perovskite-type oxides are presented. For instance, SrSn1-xFexO3-δ with x=0.1 exhibits p and n-type electronic conduction at the cathode and anode sides of the SOFC, respectively, while oxide
ion conduction prevails at intermediate oxygen partial pressures. The SEA concept is also applicable for other devices in
the field of Ionics. 相似文献
48.
Electrical resistivity of U3Tein4, U2Te3 (cubic) and UTe3 has been measured over temperature range 4.2–300 K. The two former compounds appear to be semimetallic conductors while the last one has semiconducting character. The results are discussed in the terms of available magnetic data. 相似文献
49.
We describe a new algorithm which uses the trajectories of a discrete dynamical system to sample the domain of an unconstrained objective function in search of global minima. The algorithm is unusually adept at avoiding nonoptimal local minima and successfully converging to a global minimum. Trajectories generated by the algorithm for objective functions with many local minima exhibit chaotic behavior, in the sense that they are extremely sensitive to changes in initial conditions and system parameters. In this context, chaos seems to have a beneficial effect: failure to converge to a global minimum from a given initial point can often be rectified by making arbitrarily small changes in the system parameters. 相似文献
50.
Steven D. Bull Simon W. Epstein Nadeam Mujtaba Edward D. Savory Juan A. Tamayo 《Tetrahedron》2006,62(33):7911-7925
Enolates of (S)-N,N′-bis-(p-methoxybenzyl)-3-iso-propylpiperazine-2,5-dione exhibit high levels of enantiodiscrimination in alkylations with (RS)-1-aryl-1-bromoethanes and (RS)-2-bromoesters, affording substituted diketopiperazines containing two new stereogenic centres in high de. Deprotection and hydrolysis of the resultant substituted diketopiperazines provides a route to the asymmetric synthesis of homochiral methyl 2-amino-3-aryl-butanoates and 3-methyl-aspartates in high de and ee. 相似文献