全文获取类型
收费全文 | 15273篇 |
免费 | 2816篇 |
国内免费 | 3745篇 |
专业分类
化学 | 11905篇 |
晶体学 | 378篇 |
力学 | 950篇 |
综合类 | 299篇 |
数学 | 2026篇 |
物理学 | 6276篇 |
出版年
2024年 | 41篇 |
2023年 | 175篇 |
2022年 | 423篇 |
2021年 | 480篇 |
2020年 | 536篇 |
2019年 | 590篇 |
2018年 | 532篇 |
2017年 | 529篇 |
2016年 | 721篇 |
2015年 | 786篇 |
2014年 | 990篇 |
2013年 | 1229篇 |
2012年 | 1233篇 |
2011年 | 1375篇 |
2010年 | 1154篇 |
2009年 | 1185篇 |
2008年 | 1364篇 |
2007年 | 1208篇 |
2006年 | 1068篇 |
2005年 | 981篇 |
2004年 | 805篇 |
2003年 | 591篇 |
2002年 | 764篇 |
2001年 | 684篇 |
2000年 | 566篇 |
1999年 | 334篇 |
1998年 | 221篇 |
1997年 | 163篇 |
1996年 | 172篇 |
1995年 | 135篇 |
1994年 | 120篇 |
1993年 | 128篇 |
1992年 | 82篇 |
1991年 | 80篇 |
1990年 | 46篇 |
1989年 | 58篇 |
1988年 | 49篇 |
1987年 | 40篇 |
1986年 | 29篇 |
1985年 | 31篇 |
1984年 | 23篇 |
1983年 | 24篇 |
1982年 | 11篇 |
1981年 | 25篇 |
1980年 | 7篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1974年 | 5篇 |
1965年 | 3篇 |
1959年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Metabolic profiles of dioscin in rats revealed by ultra‐performance liquid chromatography quadrupole time‐of‐flight mass spectrometry 下载免费PDF全文
He Zhu Jin‐Di Xu Qian Mao Hong Shen Ming Kong Jian‐Ping Chen Song‐Lin Li 《Biomedical chromatography : BMC》2015,29(9):1415-1421
Dioscin (DIS), one of the most abundant bioactive steroidal saponins in Dioscorea sp., is used as a complementary medicine to treat coronary disease and angina pectoris in China. Although the pharmacological activities and pharmacokinetics of DIS have been well demonstrated, information regarding the final metabolic fates is very limited. This study investigated the in vivo metabolic profiles of DIS after oral administration by ultra‐performance liquid chromatography quadrupole time‐of‐flight mass spectrometry method. The structures of the metabolites were identified and tentatively characterized by means of comparing the molecular mass, retention time and fragmentation pattern of the analytes with those of the parent compound. A total of eight metabolites, including seven phase I and one phase II metabolites, were detected and tentatively identified for the first time. Oxidation, deglycosylation and glucuronidation were found to be the major metabolic processes of the compound in rats. In addition, a possible metabolic pathway on the biotransformation of DIS in vivo was proposed. This study provides valuable and new information on the metabolism of DIS, which will be helpful for further understanding its mechanism of action. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
3.
4.
5.
6.
7.
A lithium(I) coordination polymer has been formed from LiClO4 and the 2,2′‐bipyrimidine (bpym) ligand in which each square pyramidal lithium(I) atom is coordinated in the basal plane by four nitrogen donor atoms derived from two bpym ligands and one water molecule at the apical position. These are connected into a layer structure via hydrogen‐bonding interactions involving the perchlorate anions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
8.
BP神经网络在上海住宅市场需求预测中的应用 总被引:5,自引:0,他引:5
人工神经网络是近期发展最快的人工智能领域研究成果之一 ,本文在介绍 BP神经网络的有关原理的基础上 ,建立了一个上海住宅市场的 BP神经网络模型 ,并通过该模型对上海住宅市场的需求进行了预测和分析 .分析结果表明人工神经网络方法在住宅市场需求预测中的应用是可行的并且是有效的 . 相似文献
9.
有限循环群的Fuzzy子群的等价类数 总被引:4,自引:2,他引:2
有限循环群G的F子群可以有无数个.但是.若当两个F子群的水平集构成的集合相等就称其等价的话,那么其等价类数是有限的。通过研究群的合成群列、商群列以及数的因数列和极大因数列找出了有限循环群的极大F子群和F子群的等价类数的求解公式.并给出二者之间的关系式. 相似文献
10.
Ren‐Shen Lee Jen‐Ming Yang Tz‐Feng Lin 《Journal of polymer science. Part A, Polymer chemistry》2004,42(10):2303-2312
Water‐soluble poly(ester‐carbonate) having pendent amino and carboxylic groups on the main‐chain carbon is reported for the first time. This article describes the melt ring‐opening/condensation reaction of trans‐4‐hydroxy‐N‐benzyloxycarbonyl‐L ‐proline (N‐CBz‐Hpr) with 5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one (MBC) at a wide range of molar fractions. The influence of reaction conditions such as catalyst concentration, polymerization time, and temperature on the number average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the copolymers was investigated. The polymerizations were carried out in bulk at 110 °C with 3 wt % stannous octoate as a catalyst for 16 h. The poly(ester‐carbonate)s obtained were characterized by Fourier transform infrared spectroscopy, 1H NMR, differential scanning calorimetry, and gel permeation chromatography. The copolymers synthesized exhibited moderate molecular weights (Mn = 6000–14,700 g mol?1) with reasonable molecular weight distributions (Mw/Mn = 1.11–2.23). The values of the glass‐transition temperature (Tg) of the copolymers depended on the molar fractions of cyclic carbonate. When the MBC content decreased from 76 to 12 mol %, the Tg increased from 16 to 48 °C. The relationship between the poly(N‐CBz‐Hpr‐co‐MBC) Tg and the compositions was in approximation with the Fox equation. In vitro degradation of these poly(N‐CBz‐Hpr‐co‐MBC)s was evaluated from weight‐loss measurements and the change of Mn and Mw/Mn. Debenzylation of 3 by catalytic hydrogenation led to the corresponding linear poly(ester‐carbonate), 4 , with pendent amino and carboxylic groups. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2303–2312, 2004 相似文献