全文获取类型
收费全文 | 154816篇 |
免费 | 29114篇 |
国内免费 | 14956篇 |
专业分类
化学 | 120552篇 |
晶体学 | 1809篇 |
力学 | 8413篇 |
综合类 | 861篇 |
数学 | 16355篇 |
物理学 | 50896篇 |
出版年
2024年 | 421篇 |
2023年 | 2458篇 |
2022年 | 4282篇 |
2021年 | 4601篇 |
2020年 | 5872篇 |
2019年 | 6851篇 |
2018年 | 4983篇 |
2017年 | 4469篇 |
2016年 | 8850篇 |
2015年 | 8797篇 |
2014年 | 10064篇 |
2013年 | 12762篇 |
2012年 | 13608篇 |
2011年 | 13323篇 |
2010年 | 10260篇 |
2009年 | 9871篇 |
2008年 | 10005篇 |
2007年 | 8846篇 |
2006年 | 7910篇 |
2005年 | 7017篇 |
2004年 | 5450篇 |
2003年 | 4521篇 |
2002年 | 4964篇 |
2001年 | 3913篇 |
2000年 | 3472篇 |
1999年 | 2874篇 |
1998年 | 2254篇 |
1997年 | 2056篇 |
1996年 | 2186篇 |
1995年 | 1784篇 |
1994年 | 1710篇 |
1993年 | 1376篇 |
1992年 | 1240篇 |
1991年 | 1137篇 |
1990年 | 925篇 |
1989年 | 659篇 |
1988年 | 555篇 |
1987年 | 431篇 |
1986年 | 421篇 |
1985年 | 382篇 |
1984年 | 298篇 |
1983年 | 187篇 |
1982年 | 163篇 |
1981年 | 121篇 |
1980年 | 88篇 |
1979年 | 50篇 |
1978年 | 38篇 |
1976年 | 41篇 |
1974年 | 47篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A series of 4,6-disubstituted pyrimidines were synthesized via Suzuki and Kumada coupling reaction of 4,6-dichloropyrimidine. 相似文献
992.
1-Propyl-3-methylimidazolium chloride ([C3MIm]Cl) ionic liquid and sulfamic acid (NH2SO3H), as a synergetic catalytic medium, were used for the transesterification of acetoacetate with alcohols of different structures. It shows the good ability for the chemoselective transesterificatin of β-ketoesters and maintains its catalytic activity in the reuse. 相似文献
993.
Brian Ingalls Eduardo D. Sontag Yuan Wang 《Proceedings of the American Mathematical Society》2003,131(2):487-499
The fundamental relaxation result for Lipschitz differential inclusions is the Filippov-Wazewski Relaxation Theorem, which provides approximations of trajectories of a relaxed inclusion on finite intervals. A complementary result is presented, which provides approximations on infinite intervals, but does not guarantee that the approximation and the reference trajectory satisfy the same initial condition.
994.
在变缓冲层高迁移率晶体管(MM_HEMT)器件中,二维电子气的输运性质对器件性能起着决定作用.通过低温下二维电子气横向电阻的量子振荡现象,结合变温度的Hall测量,系统研究了不同In组分沟道MM_HEMT器件中子带电子迁移率和浓度随温度的变化关系.结果表明,沟道中In组分为0.65的样品,材料电学性能最好,In组分高于0.65的样品,严重的晶格失配将产生位错,引起迁移率下降,大大影响材料和器件的性能.
关键词:
变缓冲层高迁移率晶体管
Shubnikov_de Hass 振荡 相似文献
995.
有机化合物电子光谱中的助色基及其作用机理探讨(Ⅰ) 总被引:1,自引:1,他引:0
本文就电子光谱中助色基的作用进行了讨论 ,认为不能一概地认为助色基能使与之相连的生色基最大吸收波长统统发生红移 ,而应具体问题具体分析 :对于n→π 来说 ,一方面助色基的p轨道和生色基的最低π 反键空轨道相互作用 ,使π 反键轨道能量上升 ,另一方面助色基的p轨道和生色基的n轨道正交 ,n轨道能量基本保持不变 ,因此n→π 跃迁能增加 ,吸收波长发生蓝移。对于π→π 跃迁来说 ,助色基的p轨道和生色基的最高π成键轨道相互作用生成的新的最高π成键轨道 ,能量虽然有所升高 ,但升高的程度与π 反键轨道升高的程度相比 ,在一些化合物中π轨道升高程度较大 ,因而π→π 跃迁能减小 ,吸收波长发生红移 ;在另一些化合物中π轨道升高程度较小 ,因而π→π 跃迁能增加 ,吸收波长发生蓝移。 相似文献
996.
997.
Xiaohua He Hailiang Zhang Deyue Yan Xiayu Wang 《Journal of polymer science. Part A, Polymer chemistry》2003,41(18):2854-2864
A series of side‐chain liquid‐crystalline (LC) homopolymers of poly[6‐(4‐methoxy‐4′‐oxy‐azobenzene) hexyl methacrylate] with different degrees of polymerization were synthesized by atom transfer radical polymerization (ATRP), which were prepared with a wide range of number‐average molecular weights from 5.1 × 103 to 20.6 × 103 with narrow polydispersities of around 1.17. Thermal investigation showed that the homopolymers exhibit two mesophases, a smectic phase, and a nematic phase, and the phase‐transition temperatures of the homopolymers increase clearly with increasing molecular weights. A series of novel LC coil triblock copolymers with narrow polydispersities was synthesized by ATRP, and their thermotropic phase behavior was investigated with differential scanning calorimetry and polarized optical microscopy. The LC coil triblocks were designed to have an LC conformation of poly[6‐(4‐methoxy‐4′‐oxy‐azobenzene) hexyl methacrylate] with a wide range of molecular weights from 3.5 × 103 to 1.7 × 104 and the coil conformation of poly(ethylene glycol) (PEG) (number‐average molecular weight: 6000 or 12,000) segment. Their characterization was investigated with 1H NMR, Fourier transform infrared spectra, and gel permeation chromatography. Triblock copolymers exhibited a crystalline phase, a smectic phase, and a nematic phase. The phase‐transition temperatures from the smectic to nematic phase and from the nematic to isotropic phase increased, and the crystallization of PEG depressed with increasing molecular weight of the LC block. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2854–2864, 2003 相似文献
998.
In this article a new technique was developed to fabricate scaffolds with a unique microstructure by solid–liquid separation in combination with particulate-leaching. Firstly, the effects of polymer concentration, quenching temperature on the porous morphology and the mechanical property of obtained scaffolds during solid–liquid separation have been investigated. Then, salt granules as porogen were introduced into the solid–liquid phase separation to produce the unique pore structure of the scaffold. The pore diameter of the scaffold could be controlled with the particulate size and the wall of pores possessed special microstructure, which enhanced the pore interconnectivity. The cell culture results confirmed that a good interconnectivity of the scaffold prepared by the improved solid–liquid separation was useful for nutrition transportation and cell proliferation. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
999.
1000.
Elastic electron scattering on the exotic light nucleus28S is investigated in the plane wave Born approximation. The variation of the squared form factors of28S with momentum transfer is compared with that of32S. It is found that the behavior of the form factors near the second minimum (with a moderate momentum transfer) is sensitive
to the alteration of the charge density distribution of halo protons in28S. This indicates that elastic electron scattering can be a good probe of the structure of proton-halo nuclei. 相似文献