全文获取类型
收费全文 | 24017篇 |
免费 | 4142篇 |
国内免费 | 2807篇 |
专业分类
化学 | 16968篇 |
晶体学 | 309篇 |
力学 | 1581篇 |
综合类 | 195篇 |
数学 | 2714篇 |
物理学 | 9199篇 |
出版年
2024年 | 99篇 |
2023年 | 489篇 |
2022年 | 850篇 |
2021年 | 907篇 |
2020年 | 917篇 |
2019年 | 944篇 |
2018年 | 807篇 |
2017年 | 790篇 |
2016年 | 1132篇 |
2015年 | 1133篇 |
2014年 | 1374篇 |
2013年 | 1838篇 |
2012年 | 2157篇 |
2011年 | 2329篇 |
2010年 | 1564篇 |
2009年 | 1515篇 |
2008年 | 1611篇 |
2007年 | 1454篇 |
2006年 | 1300篇 |
2005年 | 1035篇 |
2004年 | 882篇 |
2003年 | 685篇 |
2002年 | 644篇 |
2001年 | 550篇 |
2000年 | 515篇 |
1999年 | 437篇 |
1998年 | 388篇 |
1997年 | 340篇 |
1996年 | 346篇 |
1995年 | 282篇 |
1994年 | 275篇 |
1993年 | 223篇 |
1992年 | 205篇 |
1991年 | 162篇 |
1990年 | 162篇 |
1989年 | 136篇 |
1988年 | 87篇 |
1987年 | 75篇 |
1986年 | 76篇 |
1985年 | 62篇 |
1984年 | 47篇 |
1983年 | 30篇 |
1982年 | 21篇 |
1981年 | 13篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1977年 | 6篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
以高炉水淬渣合成的Ca-α-Sialon粉体为原料采用无压烧结技术制备了(Ca/Y)-α-Sialon陶瓷。通过用排水法、三点弯曲实验法、金相显微镜法、XRD法等手段研究了(Ca/Y)-α-Sialon陶瓷的烧结致密化过程、材料的力学性能、显微组织、相组成和材料的断裂特征。结果表明,适量的Y2O3促进材料的烧结致密化和提高材料的力学性能,但Y2O3过量(大于10%)时对材料的烧结和力学性能不利。掺杂Y^3+的(Ca/Y)-α-Sialon呈柱状晶,随着Y2O3含量的增加和烧结温度的提高,(Ca/Y)-α-Sialon呈柱状晶出现粗化和等轴化。含10%Y2O3的材料在1700℃烧结时可获得较高的力学性能。 相似文献
112.
JieLiang Bi-qianLiu 《高分子科学》2005,(1):83-92
Plasma-induced surface graft copolymerization of acrylic acid on polypropylene non-woven fabric (PP-g-AA) and polypropylene membrane were reported. The extents of grafting were controlled by the plasma and polymerization condition. Hexadecyltrimethyl ammonium bromide was then coupled with the carboxyl group of PP-g-AA to obtain a polyion complex (PIC). At last, CF4 plasma was used to give PICs hydrophobic property. The moisture regain and water-repellency of the processed PICs was investigated. The surfaces were characterized using ATR FT-IR and XPS. The result indicates that the products have very high ability to adsorb moisture, even better than cotton fiber. At the same time, the products show excellent hydrophobic property, which can‘t be wetted by those reagents whose surface tensions were higher than 327mN/m. 相似文献
113.
Dependence of Performance of Organic Light-emitting Devices on Sheet Resistance of Indium-tin-oxide Anodes 总被引:2,自引:0,他引:2
ZHOU Liang ZHANG Hong-jie YU Jiang-bo MENG Qing-guo PENG Chun-yun LIU Feng-yi DENG Rui-ping PENG Ze-ping LI Zhe-feng 《高等学校化学研究》2006,22(4):427-431
IntroductionIndium-tin-oxide(ITO) has been widely used asthe anode material in organic light-emitting devices(OLEDs) because of its high transmittance in the visi-ble region and low electrical resistivity. In the pastyears, many investigations focused on … 相似文献
114.
A capillary electrophoresis (CE) method was developed for the separation of heparin oligosaccharides compatible to study the interactions between the oligosaccharides and granulocyte-colony stimulating factor (G-CSF). Unfractionated heparin was eliminitively degraded to heparin oligosaccharides by an endolytic heparinase. The degraded smaller oligosaccharides (M(r) < 1000) were baseline-separated by CE under a 50 mM phosphate buffer (pH 9.0) in 10 min. Standard heparin disaccharides and larger oligosaccharides (1000 < M(r) < 8000) were all separated under optimized separation conditions. Compared with standard heparin disaccharides, smaller oligosaccharides contained one nonsulfated, two monosulfated, and two disulfated disaccharides, but trisulfated disaccharides were not found. The smaller oligosaccharides were also identified and molecular mass was deduced by electrospray ionization-mass spectrometry (ESI-MS). Furthermore, interactions between G-CSF and the oligosaccharides were studied by using capillary zone electrophoresis (CZE) under the above separation conditions. It was found that larger oligosaccharides could interact with G-CSF while smaller oligosaccharides were not observed to bind to G-CSF under the experimental conditions. In conclusion, the purified heparinase could selectively degrade heparin into oligosaccharides and the interaction between G-CSF and heparin was correlated with the chain length of heparin. 相似文献
115.
采用乙酸乙酯为环型和线型聚苯乙烯的非良溶剂, 确定了Mark-Houwink方程分别为[η]_r=1.91×10~(-2)·M~(0.607)和[η]_l=4.06×10~(-2)·M~(0.572)。研究了环型和线型聚苯乙烯的激基缔合物的荧光效应。通过稀溶液中环型和线型高分子线团在柔顺性上存在着差异, 解释了二者之间在荧光效应方面的差异, 并根据实验结果,确定了(I_E/I_M)c→0~M关系式分别为(I_E/I_M)r,c→0=1.91+3.40×10~(-6)M和(I_E/I_M)l,c→0=2.15+3.05×10~(-6)M。 相似文献
116.
Yuanchun Hu Xinmiao Liang Junwei Wang Zhuo Zheng Xinquan Hu 《Tetrahedron: Asymmetry》2003,14(24):3907-3915
A series of new phosphite–pyridine ligands, based on the H8-binaphthyl backbone, were synthesized and employed in the copper-catalyzed enantioselective 1,4-conjugate addition of diethylzinc to acyclic enones. Ligands derived from (S)-H8-NOBIN provided better results than their parent ligands in the reaction. Ligand L1 provided excellent ees for trans-4-aryl-3-buten-2-ones (up to 97.8% ee) as substrates. Ligand L2 was very efficient for various para-chalcones, and up to 97.2% ee was achieved. 相似文献
117.
Ammonia-treated activated carbon has been studied as a support of Ru-Ba catalyst for ammonia synthesis. It is shown that the
introduction of nitrogen leads to a decrease of ammonia synthesis activity for the catalysts with a low Ba/Ru molar ratio,
while no significant changes are obtained for the catalysts with a high Ba/Ru molar ratio, confirming that electronegative
impurities suppress the activity in ammonia synthesis and consume part of the promoters.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
118.
Xing Yong Wang Jing Nan Cui Wei Min Ren Feng Li Chun Liang Lu Xu Hong Qian State Key Laboratory of Fine Chemicals Dalian University of Technology Dalian China School of Chemical Engineering Technology China University of Mining & Technology Xuzhou China Shanghai Key Laboratory of Chemical Biology East China University of Science Technology Shanghai China 《中国化学快报》2007,18(6):681-684
Baker's yeast mediated reduction of acenaphthenequinone within 4-10 h afforded mono-hydroxyacenaphthenone mainly with low enantioselectivity, the substrate and mono-hydroxyacenaphthenone product almost converted to dihydroxyacenaphthene after 48 h. By control of the reaction time and in the presence of DMF as co-solvent, the reduction of 6-substituted acenaphthenequinones under vigorous agitation afforded the corresponding 2-hydroxyacenaphthenones in 24-84% yields with 10-93% ee. 相似文献
119.
Xi-Shang Sun Xing-Heng Wang Hai-Yan Tao Liang Wei Chun-Jiang Wang 《Chemical science》2020,11(40):10984
In this study, we developed an efficient Ir-catalyzed cascade umpolung allylation/2-aza-Cope rearrangement of tertiary α-trifluoromethyl α-amino acid derivatives for the preparation of a variety of quaternary α-trifluoromethyl α-amino acids in high yields with excellent enantioselectivities. The umpolung reactivity empowered by the activation of the key isatin-ketoimine moiety obviates the intractable enantioselectivity control in Pd-catalyzed asymmetric linear α-allylation. In combination with quasi parallel kinetic resolution or kinetic resolution, the generality of this method is further demonstrated by the first preparation of enantioenriched quaternary trifluoromethyl β-, γ-, δ- and ε-amino acid derivatives.In this study, we developed an efficient Ir-catalyzed cascade umpolung allylation/2-aza-Cope rearrangement for the preparation of a variety of quaternary trifluoromethyl α-ε-amino acids in high yields with excellent enantioselectivities. 相似文献
120.
Adsorption, desorption and activities of acid phosphatase on various colloidal particles from an Ultisol 总被引:1,自引:0,他引:1
Adsorption, desorption and activity of acid phosphatase on various soil colloidal particles and pure clay minerals were studied. Higher adsorption amounts and low percentage of desorption of acid phosphatase were found on fine soil clays (<0.2 μm). Electrostatic force and ligand exchange are the major driving forces that are involved in the adsorption of enzymes on soil clays. More enzyme molecules were adsorbed on soil clays in the presence of organic components. However, enzymes on organic clays were more easily released. One-third of the enzyme on goethite was adsorbed via ligand exchange process. Some other interactions, such as van der Waals force, hydrophobic force and hydrogen bonding may be more important in the adsorption of enzyme on kaolinite and the enzyme in this system cannot be easily removed. Coarse clays (0.2–2 μm) and inorganic soil clays had higher affinities for enzyme molecules than fine clays and organic clays, respectively. The activity of enzyme bound on soil clays was inhibited and the thermal stability was increased in the presence of organic matter. Data obtained in this study are helpful for a better understanding of the interactions of enzymes with inorganic and organic constituents in soil and associated environments. 相似文献