全文获取类型
收费全文 | 113498篇 |
免费 | 29417篇 |
国内免费 | 7326篇 |
专业分类
化学 | 101752篇 |
晶体学 | 705篇 |
力学 | 6768篇 |
综合类 | 311篇 |
数学 | 10821篇 |
物理学 | 29884篇 |
出版年
2024年 | 578篇 |
2023年 | 5048篇 |
2022年 | 4038篇 |
2021年 | 5082篇 |
2020年 | 7199篇 |
2019年 | 5050篇 |
2018年 | 4584篇 |
2017年 | 2862篇 |
2016年 | 8654篇 |
2015年 | 8549篇 |
2014年 | 8719篇 |
2013年 | 10090篇 |
2012年 | 9705篇 |
2011年 | 7667篇 |
2010年 | 7671篇 |
2009年 | 7434篇 |
2008年 | 5568篇 |
2007年 | 4832篇 |
2006年 | 3847篇 |
2005年 | 3195篇 |
2004年 | 2469篇 |
2003年 | 1957篇 |
2002年 | 1784篇 |
2001年 | 1508篇 |
2000年 | 1373篇 |
1999年 | 1559篇 |
1998年 | 1326篇 |
1997年 | 1371篇 |
1996年 | 1365篇 |
1995年 | 1188篇 |
1994年 | 1004篇 |
1993年 | 972篇 |
1992年 | 770篇 |
1991年 | 646篇 |
1990年 | 556篇 |
1989年 | 459篇 |
1988年 | 449篇 |
1987年 | 372篇 |
1986年 | 328篇 |
1985年 | 301篇 |
1977年 | 349篇 |
1976年 | 391篇 |
1975年 | 479篇 |
1974年 | 486篇 |
1973年 | 311篇 |
1972年 | 380篇 |
1971年 | 359篇 |
1970年 | 545篇 |
1969年 | 414篇 |
1968年 | 460篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
The bifunctional chiral thiourea-tertiary amine organocatalysts have been applied to a direct asymmetric vinylogous Michael addition of α,α-dicyanoolefins to nitroolefins with 2-10 mol % catalyst loadings. The electronic properties of the thiourea-based catalysts have significant influences on this reaction. Moderate to excellent enantioselectivities (57-95% ee) have been achieved with low to good isolated yields through fine tuning the structures of the bifunctional organocatalysts. Much better ees were obtained for some α,α-dicyanoolefinic substrates compared with that catalyzed by modified cinchona alkaloids. 相似文献
902.
为增强有机锗化合物的生物活性,合成了一系列有机锗氨基酸酯类倍半氧化物,并重点研究了它们的快原子轰击质谱(FABMS)。研究结果表明:该系列化合物的各碎片离子峰与其组成及结构均有很好的对应关系及很强的规律性 相似文献
903.
Chen H Aleksandrov A Chen Y Zha S Liu M Orlando TM 《The journal of physical chemistry. B》2005,109(22):11257-11262
Polycrystalline gadolinia-doped ceria (GDC) surfaces were studied using low-energy (5-400 eV) electron stimulated desorption (ESD). H(+), O(+), and H(3)O(+) were the primary cationic desorption products with H(+) as the dominant channel. H(+), H(3)O(+), and O(+) have a 22 eV threshold followed by a yield change around 40 eV. H(+) also has an additional yield change approximately 75 eV and O(+) has an additional change approximately 150 eV. The O(+) ESD yield change approximately 150 eV may indicate bond breaking of Gd-O and the involvement of oxygen vacancies. The H(+) and H(3)O(+) threshold data collectively indicate the presence of hydroxyl groups and chemisorbed water molecules on the GDC surfaces. ESD temperature dependence measurements show that the interaction of water with GDC surface defect sites, mainly oxygen vacancies, influences the desorption of H(+), O(+), and H(3)O(+). The temperature dependence of the O(+) ESD at 400 eV incident electron energy yields a 0.21 eV activation energy. This is close to the energy needed for oxygen vacancy production next to a pair of Ce(3+) on a CeO(2) surface. These results may indicate a correlation between the O(+) ESD yield and oxygen vacancy density on GDC surfaces and a potential correlation of O(+) ESD and GDC ionic conductivity. 相似文献
904.
Surface plasmon resonance (SPR) has been successfully applied for the simple, rapid, and label-free assay of various biomolecules. This assay evaluates a novel wavelength modulation SPR biosensor for the detection of tetanus toxin. The wavelength modulation SPR biosensor is designed based on fixing the incident angle of light and measuring the reflected intensities in the resonance wavelength range spanning 400-800 nm simultaneously. Tetanus toxin (TeNT), one of the most potent toxins known, is synthesized as a 150 kDa single polypeptide chain. The SPR biosensor has been shown to be capable of directly detecting concentration of tetanus toxin as low as 0.028 Lf ml−1. Under selected experimental conditions, the SPR biosensor has a good reproducibility, sensitivity and reversibility. The results illustrate how wavelength modulation SPR biosensor can be used to detect biomolecular interactions. 相似文献
905.
Petra A. M. van Koppen Andreas J. Illies Shuying Liu Michael T. Bowers 《Journal of mass spectrometry : JMS》1982,17(9):399-402
The effect of the precursor ion internal energy on the branching ratios obtained from collision induced dissociation fragmentation patterns was examined for [NH3]+ and [C2H4N]+. The ion internal energy was changed by varying both the chemical ionization reagent gas and the ion source pressure. Effects observed in the collision induced dissociation fragmentation patterns as a function of the ion source pressure are explained by the reaction exothermicities and by collisional deactivation of internally excited ions (at high pressure). 相似文献
906.
Dipl.-Ing. Dr. O. Hinterhofer 《Monatshefte für Chemie / Chemical Monthly》1974,105(2):279-290
The selective chlorination of a m/p-xylene mixture, followed by distillation of the unreacted p-xylene, leaves a residue containing up to 90% of monochlorinated m-xylenes. m-Xylene is recovered from the latter by heterogeneous catalytic hydrogenolysis in the gas-phase. It was found that the hydrogenolysis on certain noble metal catalysts proceeds according to an ionic reaction mechanism at temperatures below a definite temperature range. At temperatures above this range hydrogenolysis follows a radical reaction mechanism. 相似文献
907.
Haifang Liu Yuanqiang Sun Zhaohui Li Ran Yang Jie Yang Aaron Albert Aryee Xiaoge Zhang Jia Ge Lingbo Qu Yuehe Lin 《中国化学快报》2019,30(9):1647-1651
Intracellular pH plays a significant role in various biological processes, including cell proliferation, apoptosis, metabolism, enzyme activity and homeostasis. In this work, a novel design strategy for the preparation of pH responsive carbon dots (CDs-pH) for ratiometric intracellular imaging was reported. By using SciFinder database, fluorescent CDs-pH with the required pKa value of 6.84 were rationally designed, which is vital important for precise sensing of intracellular pH. As a result, the synthesized CDs-pH demonstrated robust ability to test pH fluctuations within the physiological range of 5.4-7.4. The CDs-pH was further utilized for fluorescent ratiometric imaging of pH in living HeLa cells, effectively avoided the influence of autofluorescence from native cellular species. Moreover, real-time monitoring of intracellular pH fluctuation under heat shock was successfully realized. This SciFinder-guided design strategy is simple and flexible, which has a great potential to be used for the development of other types of CDs for various applications. 相似文献
908.
ISSIe山和W6flSChllh山子1960年首次用pdCI。和二苯基磷合成出黄色的二聚络合物,后被H。yieY*确认为具有pRZ的桥式结构.此后一系列该类结构的化合物被合成并引起了广泛的兴趣.磷可以和两个甚至多个金属原子络合构成多核金属络合物,在催化反应中有重要应用.磷原子存在多种电子态,其3d空轨道也能参与成键.P乃1的NMR谱是研究含磷化合物立体结构的有力工具.P-31与其它核素之间的耦合常数以及P-31的化学位移对阐明化合物的立体构象以及电子结构具有重要作用.近年来,West饲用计算机神经网络技术预测P-:if的化学位移,但仅… 相似文献
909.
Zhanglin Liu Dong Tian Fei Shen Lulu Long Yanzong Zhang Gang Yang Yongmei Zeng Jing Zhang Jinsong He Ying Zhu Shihuai Deng 《中国化学快报》1990,30(12):2221-2224
Four distinct biochars were employed to remove three typical pollutants, meanwhile, path analysis, a multi-statistical regression method, was performed to elucidate the dominant factors of biochar adsorption. This work can provide a new insight to prepare a targeted biochar as adsorbents. 相似文献
910.
Jian X. Gao Jian C. Wang Chong L. Song T. Liu Tian D. Hu Ya N. Xie J. Zhang G. Wang H. Yang 《Journal of solution chemistry》2006,35(1):113-119
NMR spectroscopy was used to show that the symmetry of the crown ether bis(C6) is increased by an increase of the alkali metal
cation radius. The EXAFS spectrum demonstrates that a seven oxygen atom coordinated configuration is present in the bis(C6)/Cs+/NPME system, where NPME denotes o-nitrophenylmethyl ether. The seventh oxygen in this complex, besides the six crown ether oxygens of bis(C6), may come either
from a H2O molecule or an NO3− ion. 相似文献