全文获取类型
收费全文 | 29024篇 |
免费 | 4885篇 |
国内免费 | 4101篇 |
专业分类
化学 | 21819篇 |
晶体学 | 483篇 |
力学 | 1675篇 |
综合类 | 334篇 |
数学 | 3032篇 |
物理学 | 10667篇 |
出版年
2024年 | 54篇 |
2023年 | 465篇 |
2022年 | 929篇 |
2021年 | 954篇 |
2020年 | 1154篇 |
2019年 | 1145篇 |
2018年 | 947篇 |
2017年 | 916篇 |
2016年 | 1401篇 |
2015年 | 1371篇 |
2014年 | 1632篇 |
2013年 | 2177篇 |
2012年 | 2706篇 |
2011年 | 2786篇 |
2010年 | 2013篇 |
2009年 | 1853篇 |
2008年 | 2094篇 |
2007年 | 1767篇 |
2006年 | 1636篇 |
2005年 | 1480篇 |
2004年 | 1169篇 |
2003年 | 987篇 |
2002年 | 1059篇 |
2001年 | 761篇 |
2000年 | 666篇 |
1999年 | 583篇 |
1998年 | 422篇 |
1997年 | 378篇 |
1996年 | 365篇 |
1995年 | 320篇 |
1994年 | 278篇 |
1993年 | 235篇 |
1992年 | 226篇 |
1991年 | 195篇 |
1990年 | 179篇 |
1989年 | 137篇 |
1988年 | 115篇 |
1987年 | 73篇 |
1986年 | 82篇 |
1985年 | 88篇 |
1984年 | 53篇 |
1983年 | 44篇 |
1982年 | 32篇 |
1981年 | 28篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1974年 | 6篇 |
1966年 | 2篇 |
1957年 | 8篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
In the presence of montmorillonite K 10, 5(6)-unsaturated sterols (1) were heated at refluxing temperature in dichloromethane to provide 5(6)/5′(6′)-unsaturated 3β,3′β-disteryl ethers (4) in 69–73% yield. The mechanism of the reaction was discussed. 相似文献
992.
Jin Kai Li Baowei Wang Kaixing Luo Huijuan Gong Zhijun Wu Wenfei 《Research on Chemical Intermediates》2018,44(11):6933-6943
Research on Chemical Intermediates - A rare-earth concentrate obtained from Bayan Obo tailings was used as catalyst for low-temperature selective catalytic reduction (SCR) of NOx. X-ray powder... 相似文献
993.
Ji‐Hoon Kim Sunyoung Lee In‐Nam Kang Moo‐Jin Park Do‐Hoon Hwang 《Journal of polymer science. Part A, Polymer chemistry》2012,50(16):3415-3424
An alternating copolymer composed of heal‐to‐tail‐structured 3,4′‐dihexyl‐2,2′‐bithiophene (DHBT) and pyrene units [poly(DHBT‐alt‐PYR)] was synthesized using a Stille coupling reaction for use in photovoltaic devices as a p‐type donor. For the reduction of the bandgap energy of poly(DHBT‐alt‐PYR), 4,7‐bis(3′‐hexyl‐2,2′‐bithiophen‐5‐yl)benzo[c][1,2,5]thiadiazole (BHBTBT) units were introduced into the polymer. Poly(DHBT‐co‐PYR‐co‐BHBTBT)s were synthesized using the same polymerization reaction. The synthesized polymers were soluble in common organic solvents and formed smooth thin films after spin casting. The optical bandgap energies of the polymers were obtained from the onset absorption wavelengths. The measured optical bandgap energy of poly(DHBT‐alt‐PYR) was 2.47 eV. As the BHBTBT content in the ter‐polymers increased, the optical bandgap energies of the resulting polymers decreased. The bandgap energies of poly(50DHBT‐co‐40PYR‐co‐10BHBTBT) and poly(50DHBT‐co‐20PYR‐co‐30BHBTBT) were 1.84 and 1.73 eV, respectively. Photovoltaic devices were fabricated with a typical sandwich structure of ITO/PEDOT:PSS/active layer/LiF/Al using the polymers as electron donors and [6,6]‐phenyl C71‐butyric acid methyl ester as the electron acceptor. The device using poly(50DHBT‐co‐20PYR‐co‐30BHBTBT) showed the best performance among the fabricated devices, with an open‐circuit voltage, short‐circuit current, fill factor, and maximum power conversion efficiency of 0.68 V, 5.54 mA/cm2, 0.35, and 1.31%, respectively. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
994.
Abstract: α-(Benzotriazol-1-yl)hydrazones 2a-d and 13a-c were prepared by refluxing the corresponding α-(benzotriazol-1-yl)ketones with p-tosyl hydrazide or benzenesulfonyl hydrazide. Treatment of 2a-b with n-butyllithium in the presence of TMEDA gave benzotriazines 6a-b, while lithiation of 13a-c resulted in indole derivatives 16a-c, depending on the structure of the hydrazones. 相似文献
995.
Shixu Liu Junxiu Meng Wei Zhang Shengbiao Wan Tao Jiang 《Journal of heterocyclic chemistry》2016,53(4):1194-1199
Six novel analogues of bioactive natural indolo[3,2‐b]quinoline alkaloid glycoside jusbetonin were designed and synthesized, employing polyphosphoric acid mediated cyclization and two different amidation strategies on introduction of tetra‐O‐acetyl‐β‐d ‐glucosamine to tetracyclic carboxylic acids. 相似文献
996.
997.
998.
999.
In this paper, a tri-neuron BAM neural network model with multiple delays is considered. We show that the connection topology of the network plays a fundamental role in classifying the rich dynamics and bifurcation phenomena. There is a wide range of different dynamical behaviors which can be produced by varying the coupling strength. By choosing the connected weights c 21 and c 31 (the connection weights through the neurons from J-layer to I-layer) as bifurcation parameters, the critical values where a Bogdanov–Takens bifurcation occurs are derived. Then, by computing the normal forms for the system, the bifurcation diagrams are obtained. Furthermore, some interesting phenomena, such as saddle-node bifurcation, pitchfork bifurcation, homoclinic bifurcation, heteroclinic bifurcation and double limit cycle bifurcation are found by choosing the different connection strengths. Some numerical simulations are given to support the analytic results. 相似文献
1000.
Xiongchao Lin Keiko Ideta Jin Miyawaki Yusuke Nishiyama Isao Mochida Seong‐Ho Yoon 《Magnetic resonance in chemistry : MRC》2012,50(4):289-294
A general strategy of structural analysis of alumina silicate by combining various solid‐state NMR measurements such as single pulse, multi‐quantum magic angle spinning, double‐quantum homo‐nuclear correlation under magic angle spinning (DQ‐MAS), and cross‐polarization hetero‐nuclear correlation (CP‐HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for 1H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of 27Al in single pulse, DQ‐MAS, and even in triple‐quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ‐MAS methods for homo‐nuclear correlations between both 27Al–27Al and 29Si–29Si and by CP‐HETCOR for hetero‐nuclear correlations between 27Al–29Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina‐silicate minerals. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献