全文获取类型
收费全文 | 33207篇 |
免费 | 4400篇 |
国内免费 | 3067篇 |
专业分类
化学 | 22672篇 |
晶体学 | 327篇 |
力学 | 2017篇 |
综合类 | 246篇 |
数学 | 3778篇 |
物理学 | 11634篇 |
出版年
2024年 | 85篇 |
2023年 | 549篇 |
2022年 | 987篇 |
2021年 | 1078篇 |
2020年 | 1125篇 |
2019年 | 1107篇 |
2018年 | 923篇 |
2017年 | 838篇 |
2016年 | 1356篇 |
2015年 | 1416篇 |
2014年 | 1618篇 |
2013年 | 2234篇 |
2012年 | 2659篇 |
2011年 | 2952篇 |
2010年 | 1874篇 |
2009年 | 1840篇 |
2008年 | 1968篇 |
2007年 | 1839篇 |
2006年 | 1754篇 |
2005年 | 1479篇 |
2004年 | 1347篇 |
2003年 | 1064篇 |
2002年 | 970篇 |
2001年 | 827篇 |
2000年 | 719篇 |
1999年 | 631篇 |
1998年 | 539篇 |
1997年 | 462篇 |
1996年 | 510篇 |
1995年 | 450篇 |
1994年 | 447篇 |
1993年 | 355篇 |
1992年 | 361篇 |
1991年 | 299篇 |
1990年 | 284篇 |
1989年 | 234篇 |
1988年 | 181篇 |
1987年 | 150篇 |
1986年 | 144篇 |
1985年 | 169篇 |
1984年 | 111篇 |
1983年 | 112篇 |
1982年 | 85篇 |
1981年 | 65篇 |
1980年 | 61篇 |
1979年 | 42篇 |
1977年 | 42篇 |
1975年 | 40篇 |
1974年 | 49篇 |
1973年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
63.
Synthesis, single-crystal structure determination, and magnetic properties are reported for manganese carbodiimide, MnNCN. The presumably unstable but inert phase adopts the trigonal system (R3m) with a = 3.3583(4) A, c = 14.347(2) A, V = 140.13(3) A3, and Z = 3. Divalent manganese is octahedrally coordinated by nitrogen atoms at 2.26 A, and the NCN(2-) unit adopts the linear [N=C=N](2-) carbodiimide shape with two C=N double bonds of 1.23 A. MnNCN contains high-spin Mn(II) with five unpaired electrons and behaves like an antiferromagnet with an ordering temperature below 30 K. 相似文献
64.
Cai H Chen T Wang X Schultz AJ Koetzle TF Xue Z 《Chemical communications (Cambridge, England)》2002,(3):230-231
Reactions of Ta(NMe2)5 with D2SiR'Ph (R' = Me, Ph) were found to give a dideuteride eta 2-imine complex (Me2N)3Ta(mu-D)2(mu-N-eta 2-N,C-CH2NMe)Ta(NMe2)3(1-d2) through C-H activation of an amide ligand via beta-H abstraction, and the structure of 1 was confirmed by single crystal neutron and X-ray diffraction. 相似文献
65.
A solid-phase microextraction (SPME) device was used as a time-weighted average sampler for n-valeraldehyde. The SPME device was first modified to improve the wearer's acceptance as a passive sampler. Then a poly(dimethylsiloxane)-divinylbenzene fiber was used and O-2,3,4,5,6-(pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA) was loaded onto the fiber. Vapors of known concentrations around the threshold limit values time-weighted average of n-valeraldehyde and specific relative humidities (RHs) were generated by syringe pumps in a dynamic generation system. n-Valeraldehyde vapors in gas bags were also generated. An exposure chamber was designed to allow measurement of face velocities, temperatures, exposing vapor concentrations, and RHs. Gas chromatography with flame ionization detection was used for sample analysis. The appropriate adsorption time for SPME coating PFBHA was determined to be 2 min and the desorption time for oxime formed after sampling was optimized to be 2 min. The experimental sampling constant was found to be (3.86+/-0.13) x 10(-2) cm3/min and face velocity was not expect to have effect on the sampler. 相似文献
66.
<正> 苯乙烯(简称St)-4(甲基丙烯酸)2,2,6,6-四甲基哌啶醇酯(简称MTMP)共聚物(简称PDS)是七十年代发展起来的一种新型高效光稳剂。本文用IR研究了一系列自由基聚合的PDS共聚物,找出了MTMP对序列结构敏感的吸收峰,发现MTMP的1152.5厘米~(-1)峰位置与三单元序列分布F_(SMS)值有一定线性关系,还对通过测量MTMP羰基伸缩振动ν_(C=O)峰位置来计算二单元序列分布P_(mm)的关系进行了讨论。 相似文献
67.
Separation and determination of podophyllum lignans by micellar electrokinetic chromatography 总被引:2,自引:0,他引:2
A micellar electrokinetic chromatography method was established for the quantitative analysis of seven podophyllum lignans in Podophyllum emodi Wall. var. chinesis sprague. The optimum buffer system was 10 mM NaH2PO4-5 mM borate-100 mM sodium dodecylsulfate-30% isopropanol (pH 7.20). Voltage was 18 kV and detection at 214 nm. The second derivative chromatogram was used to determine a low-content component and those not fully separated from adjacent ones. The RSD values of migration times and peak areas were <2.2 and <5.5%, respectively. The effects of several CE parameters on the resolutions were studied systematically. 相似文献
68.
69.
This paper is the study of the fluorescence enhancement of Eu3+-1-ethyl-6-fluoro-4-oxo-7-(1-piperazinyl)-1,8-naphthyridin-3-carbonic acid (enoxacin, EFLX) system by surfactants. It was
found that sodium dodecylbenzenesulfonate (SDBS) exhibits great enhancement on the fluorescence of the Eu-EFLX system. The
molar ratio is 1:2:1 for Eu:EFLX: SDBS. Under the optimum conditions, the fluorescence intensity is a linear function of europium
in the range of 1.0 × 10−8 ∼ 5.0 × 10−6 mol/L, the detection limit is 1.0 × 10−9 mol/L. The application of the Eu-EFLX-SDBS system for the determination of trace europium in rare earth samples gave satisfactory
results.
Received October 19, 2000. Revision August 10, 2001. 相似文献
70.
The Suzuki cross-coupling methodology provides a facile synthetic approach for the modular preparation of meso-tetraaryl cofacial bisporphyrins anchored by xanthene and dibenzofuran. This synthetic method furnishes cofacial bisporphyrin templates with enhanced steric and electronic protection from mu-oxo formation and oxidative degradation. The ability of these platforms to support multielectron oxidation chemistry mediated by proton-coupled electron transfer (PCET) is demonstrated by their reactivity for the catalytic disproportionation of hydrogen peroxide to oxygen and water. 相似文献