全文获取类型
收费全文 | 18369篇 |
免费 | 2742篇 |
国内免费 | 3289篇 |
学科分类
数理化 | 24400篇 |
出版年
2024年 | 65篇 |
2023年 | 311篇 |
2022年 | 609篇 |
2021年 | 683篇 |
2020年 | 697篇 |
2019年 | 730篇 |
2018年 | 625篇 |
2017年 | 611篇 |
2016年 | 895篇 |
2015年 | 910篇 |
2014年 | 1118篇 |
2013年 | 1521篇 |
2012年 | 1675篇 |
2011年 | 1746篇 |
2010年 | 1307篇 |
2009年 | 1203篇 |
2008年 | 1367篇 |
2007年 | 1225篇 |
2006年 | 1098篇 |
2005年 | 911篇 |
2004年 | 777篇 |
2003年 | 637篇 |
2002年 | 601篇 |
2001年 | 529篇 |
2000年 | 429篇 |
1999年 | 364篇 |
1998年 | 266篇 |
1997年 | 204篇 |
1996年 | 203篇 |
1995年 | 177篇 |
1994年 | 153篇 |
1993年 | 125篇 |
1992年 | 87篇 |
1991年 | 98篇 |
1990年 | 99篇 |
1989年 | 52篇 |
1988年 | 50篇 |
1987年 | 30篇 |
1986年 | 42篇 |
1985年 | 42篇 |
1984年 | 29篇 |
1983年 | 26篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1944年 | 4篇 |
1942年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
A Minor New Flavone from Scutellaria baicalensis Georgi 总被引:1,自引:0,他引:1
A new flavone,6,2′-dihydroxy-5,7,8,6′-tetramethoxyflavone,was isolated from the roots of Scutellaria baicalensis. Its structure was established on the basis of spectral evidences. 相似文献
103.
近些年来,无铅低熔点填料日渐引起人们的兴趣,且将得到广泛的应用。Cu-Zn-Sn合金正是无铅低熔点填料的一种基本合金。该体系相图虽有一些报道,但他们的研究只涉及富铜区(>50Wt%)的液相限和等温截面(500℃)。我们颇感兴趣的富Sn区的液相限,尚未见文献报道,因此测定该体系富Sn区的液相限是必要的。关于Cu-Zn-Sn体系的三个相关二元体系,已有文献报道,也较成熟。对于Cu-Zn体系,富Zn区600℃以下存在两个转熔反应,其转熔点分别为Q(~88Wt%Zn,598℃),W(97.5-98.5Wt%Zn,424℃);Cu-Sn体系富Sn区有一个转熔反应,其转熔点 相似文献
104.
105.
Wei Ge ZHANG Ying Hua JIN Ping QI Kai BAO Jin Guang LIN Nai Li WANG Xin Shen YAO 《中国化学快报》2006,17(1):9-11
Erythromycin has been extensively used in the treatment of bacterial infections for over 50 years1. In addition to the antimicrobial effect, the quite unique antiinflammatory activity of the erythromycin derivatives has attracted much attention as new the… 相似文献
106.
A series of 2-benzyl-1,3-dicabonyl derivatives was synthesized.Their insulin-sensitizing activity was evaluated in 3T3-L1 preadipocyte cells.Compounds3,26 and 27 were found to possess strong insulin-sensitizing activity in vitro and were selected for further hypoglycemic evaluation in vivo. 相似文献
107.
108.
As a natural diterpenoid, crotophorbolone possesses a challenging trans,trans-5/7/6 framework decorated with six contiguous stereogenic centers and is structurally and biogenetically related to tigliane-type diterpenoids with intriguing bioactivities such as phorbol and prostratin. Based on the convergent strategy, we completed an eighteen-step total synthesis of crotophorbolone starting from (−)-carvone and (+)-dimethyl-2,3-O-isopropylidene-l-tartrate. The key elements of the synthesis involve expedient installation of the six-membered ring and the five-membered ring with multiple functional groups at an early stage, cyclization of the seven-membered ring through alkenylation of the ketone between the five-membered ring and the six-membered ring, functional group-sensitive ring-closing metathesis and final selective introduction of hydroxyls at C20 and C4.Convergent total synthesis of crotophorbolone was accomplished in 18 longest linear steps. Observation of unexpected thermodynamic stability of a cis,trans-5/7/6 tricycle would benefit synthetic design of tigliane- and daphnane-related diterpenoids. 相似文献
109.
YUAN Liwei WANG Yanqiu WANG Li BAI Jiling & HE Guozhong State Key Laboratory of Molecular Reaction Dynamics Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian China 《中国科学B辑(英文版)》2004,47(4)
Dichlorobenzene is a precursor molecule of di-oxins, which are some of the most toxic chemicals and are suspected of being mutagenic with the structure of benzene rings containing different numbers of chlorine atoms. They are seriously harmful to the health of human. Therefore, it is necessary to study these aryl chemicals especially dichlorobenzene molecules in great detail. In recent years, a number of groups have investi-gated the molecules of aryl halide. The near ultraviolet absorption sp… 相似文献
110.
A non-aqueous reversed phase HPLC was developed for determining alpha-tocopherol in Sea buckthorn oil capsule without the need for saponification. A reversed phase column (Alltima C(18), 4.6 x 250 mm, 5 microm) was used with a mobile phase of methanol-acetonitrile (95 : 5, v/v) and flow rate of 1 ml/min. The contents in capsule were extracted with n-hexane. Detection wavelength was set at 292 nm. Each analysis requires no longer than 20 min. The linearity range for alpha-tocopherol was 9.4-47.0 microg/ml. The detection limit was 0.94 microg/ml. The mean recovery was 95.82 (RSD 2.3%). This method is suitable for quantitative analysis of alpha-tocopherol in Sea buckthorn oil or its Traditional Chinese Medicinal preparation. 相似文献