全文获取类型
收费全文 | 53955篇 |
免费 | 924篇 |
国内免费 | 413篇 |
专业分类
化学 | 28592篇 |
晶体学 | 470篇 |
力学 | 1848篇 |
综合类 | 12篇 |
数学 | 12122篇 |
物理学 | 12248篇 |
出版年
2023年 | 185篇 |
2022年 | 272篇 |
2021年 | 384篇 |
2020年 | 500篇 |
2019年 | 479篇 |
2018年 | 1486篇 |
2017年 | 1702篇 |
2016年 | 1419篇 |
2015年 | 1144篇 |
2014年 | 1187篇 |
2013年 | 1837篇 |
2012年 | 4657篇 |
2011年 | 4031篇 |
2010年 | 2773篇 |
2009年 | 2357篇 |
2008年 | 2002篇 |
2007年 | 1974篇 |
2006年 | 1948篇 |
2005年 | 5652篇 |
2004年 | 4965篇 |
2003年 | 3111篇 |
2002年 | 1351篇 |
2001年 | 712篇 |
2000年 | 452篇 |
1999年 | 535篇 |
1998年 | 466篇 |
1997年 | 403篇 |
1996年 | 391篇 |
1995年 | 329篇 |
1994年 | 360篇 |
1993年 | 319篇 |
1992年 | 428篇 |
1991年 | 343篇 |
1990年 | 351篇 |
1989年 | 291篇 |
1988年 | 293篇 |
1987年 | 228篇 |
1986年 | 202篇 |
1985年 | 308篇 |
1984年 | 285篇 |
1983年 | 209篇 |
1982年 | 218篇 |
1981年 | 203篇 |
1980年 | 194篇 |
1979年 | 193篇 |
1978年 | 219篇 |
1977年 | 198篇 |
1976年 | 215篇 |
1974年 | 195篇 |
1973年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
A 1.5-μm eye-safe, 3-D scanning, and compact Mie LIght Detection and Ranging (LIDAR) is presented. The transmitter of the LIDAR is based on a KTA optical parameter oscillator (OPO) resonator. For detecting return signals, an InGaAs APD is used. The all solid-state OPO laser transmitter has the feature of small volume and lightweight, which allows a 165-lb compact eye-safe scanning LIDAR to be constructed. A system simulation using our own model was conducted to direct the system development. A method to solve the problem with small active area APD detectors was developed and described. The preliminary field-test measurement results indicated that the LIDAR has the capability to detect aerosols and clouds in lower atmospheres up to three dimensions. 相似文献
2.
Helmut Schmidhammer Eva Scherb-Bukowiecki Thomas A. Mayer Peter Schnholzer 《Helvetica chimica acta》1994,77(6):1590-1594
The synthesis of benzazepine analogues of the opium alkaloid noscapine ( 1 ) is described. The benzazepines 2 and 3 were prepared starting from nornarceine ethyl ester ( 4 ; readily available from 1 ) in several steps. X-Ray analysis of compound 2 revealed that it is not a diastereosisomer mixture but a racemate of the threo-form and thus has the same configuration as 1 . 相似文献
3.
4.
We examine two criteria for balance of a gain graph, one based on binary cycles and one on circles. The graphs for which each criterion is valid depend on the set of allowed gain groups. The binary cycle test is invalid, except for forests, if any possible gain group has an element of odd order. Assuming all groups are allowed, or all abelian groups, or merely the cyclic group of order 3, we characterize, both constructively and by forbidden minors, the graphs for which the circle test is valid. It turns out that these three classes of groups have the same set of forbidden minors. The exact reason for the importance of the ternary cyclic group is not clear. © 2005 Wiley Periodicals, Inc. J Graph Theory 相似文献
5.
Hugh Thomas 《Order》2006,23(2-3):249-269
In this paper, we study lattices that posess both the properties of being extremal (in the sense of Markowsky) and of being left modular (in the sense of Blass and Sagan). We call such lattices trim and show that they posess some additional appealing properties, analogous to those of a distributive lattice. For example, trimness is preserved under taking intervals and suitable sublattices. Trim lattices satisfy a weakened form of modularity. The order complex of a trim lattice is contractible or homotopic to a sphere; the latter holds exactly if the maximum element of the lattice is a join of atoms. Any distributive lattice is trim, but trim lattices need not be graded. The main example of ungraded trim lattices are the Tamari lattices and generalizations of them. We show that the Cambrian lattices in types A and B defined by Reading are trim; we conjecture that all Cambrian lattices are trim. 相似文献
6.
Julia K Pruns Jens-Peter Vietzke Manfred Strassner Claudius Rapp Ulrich Hintze Wilfried A K?nig 《Rapid communications in mass spectrometry : RCM》2002,16(3):208-211
A new solvent-free sample preparation method using silver trifluoroacetate (AgTFA) was developed for the analysis of low molecular weight paraffins and microcrystalline waxes by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Experiments show that spectral quality can be enhanced by dispersing AgTFA directly in liquid paraffins without the use of additional solvents. This preparation mixture is applied directly to the MALDI probe. Solid waxes could be examined by melting prior to analysis. The method also provides sufficiently reproducible spectra that peak area ratios between mono- and bicyclic alkane peaks indicated variations in the cycloalkane content of paraffin samples. Dehydrogenation of hydrocarbons observed during the desorption/ionization process was studied by analysis of alkane standards. 相似文献
7.
The versatile behavior of a trihydrazidophosphoradamantane allowing the synthesis of a variety of neutral, mono- or di-cationic water-soluble molecules of potential interest for biphasic catalysis is reported. 相似文献
8.
9.
We build a metric space which is homeomorphic to a Cantor set but cannot be realized as the attractor of an iterated function system. We give also an example of a Cantor set K in R3 such that every homeomorphism f of R3 which preserves K coincides with the identity on K. 相似文献
10.
The synthesis of rigid spirocyclic bislactams derived from pyroglutamic acid has been established. 相似文献