全文获取类型
收费全文 | 3244篇 |
免费 | 507篇 |
国内免费 | 382篇 |
专业分类
化学 | 2438篇 |
晶体学 | 56篇 |
力学 | 188篇 |
综合类 | 45篇 |
数学 | 354篇 |
物理学 | 1052篇 |
出版年
2024年 | 4篇 |
2023年 | 61篇 |
2022年 | 72篇 |
2021年 | 109篇 |
2020年 | 147篇 |
2019年 | 131篇 |
2018年 | 103篇 |
2017年 | 114篇 |
2016年 | 152篇 |
2015年 | 147篇 |
2014年 | 205篇 |
2013年 | 238篇 |
2012年 | 259篇 |
2011年 | 303篇 |
2010年 | 214篇 |
2009年 | 181篇 |
2008年 | 189篇 |
2007年 | 191篇 |
2006年 | 170篇 |
2005年 | 150篇 |
2004年 | 125篇 |
2003年 | 95篇 |
2002年 | 97篇 |
2001年 | 87篇 |
2000年 | 75篇 |
1999年 | 78篇 |
1998年 | 72篇 |
1997年 | 46篇 |
1996年 | 53篇 |
1995年 | 40篇 |
1994年 | 42篇 |
1993年 | 27篇 |
1992年 | 36篇 |
1991年 | 15篇 |
1990年 | 23篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 5篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有4133条查询结果,搜索用时 15 毫秒
991.
使用液相包覆工艺对LiNi_(0.8)Co_(0.15)Al_(0.05)O_2(NCA)材料进行FePO_4包覆改性,利用FePO_4优异的结构稳定性与热稳定性,对NCA的长期可靠性与安全性能进行改良。重点研究FePO_4包覆对NCA材料的改性效果,以及不同包覆量造成的NCA材料电化学性能差异。表面包覆的FePO_4保护层,能够防止NCA材料与电解液直接接触发生副反应,抑制长期循环过程中过渡金属离子的溶出,保持结构的长期稳定性。当包覆量为1.0%(w/w)时,NCA材料表现出最优的综合性能,充放电循环800次后,容量保持率依然高达95%,25℃下存储100 d后,容量保持率也高于95%,达到了兼顾能量密度、使用寿命及安全性能的理想效果。 相似文献
992.
We reported here an efficient regioselective protection strategy for xylose with two-step successive protections on 4- and 2-OH. This method enables the expeditious preparation of various xylose building blocks (3a–c, 5, 6). The rapid selective 2,4-protection especially facilitates the synthesis of 3-substituted xyloside structures. By this approach, typical arabino-α-1,3-xyloside disaccharide and trisaccharide, as fragment structure units from arabinoxylans, have been successfully synthesized. 相似文献
993.
994.
Recent developments in solid-phase microextraction for on-site sampling and sample preparation 总被引:1,自引:0,他引:1
Chunfeng DuanZheng Shen Dapeng WuYafeng Guan 《Trends in analytical chemistry : TRAC》2011,30(10):1568-1574
On-site sampling and sample preparation favor portable, solventless or even solvent-free techniques. Solid-phase microextraction (SPME) has these advantages. This review focuses on developments between 2007 and early 2011 in microextraction techniques for on-site sampling and sample preparation, including fiber SPME, stir-bar sorptive extraction (SBSE), thin-film microextraction (TFME) and different types of in-needle SPME. The major trends in on-site applications of SPME appear to be fiber and thin-film SPME, microextraction by packed sorbent (MEPS) and the sorbent-packed needle-trap device (NTD). We discuss and compare several aspects of these types of SPME in on-site applications. We also describe sorbent phases for SPME that benefit on-site applications. Finally, we provide a perspective on SPME-based techniques for on-site applications. 相似文献
995.
996.
Guan Sun Ruolin Chen Yujie J. Ding Hieu P. T. Nguyen Zetian Mi 《Laser \u0026amp; Photonics Reviews》2015,9(1):105-113
The terahertz (THz) radiation from InGaN/GaN dot‐in‐a‐wire nanostructures has been investigated. A submicrowatt THz signal is generated with just ten vertically stacked InGaN quantum dots (QDs) in each GaN nanowire. Based on the experimental results and analysis, a single quantum wire is expected to generate an output power as high as 10 pW, corresponding to 1 pW per dot. These structures are among the most efficient three‐dimensional quantum‐confined nanostructures for the THz emission. By applying a reverse bias along the wires in a light‐emitting device (LED) consisting of such nanostructures, the THz output power is increased more than fourfold. Based on THz and photoluminescence (PL) experiments, the mechanism for the THz emission is attributed to dipole radiation induced by internal electric fields and enhanced by external fields.
997.
Hengheng Zhu Weiwei He Chuanlang Zhan Xiao LiZisheng Guan Fengqi GuoJiannian Yao 《Tetrahedron》2011,67(44):8458-8464
In this article, we report a new approach toward synthesis of pyridine-imide oligomers (PIOs). Using this approach, both dimer and trimer were one-pot synthesized from acylation of monomeric monoamide with monomeric dichloride. The yield of trimer was dependent on the alkoxyl terminals: it was 30% for methoyl group, whereas it was 95% for 3-chloro-1-propoxyl terminal. Acylation of dimeric monoamide with monomeric dichloride produced trimer, tetramer, and pentamer in a yield of 34%, 33%, and 28%, respectively. The synthesis was proposed to be mediated through an exchange between pyridine-2-carboxamide and pyridine-2-carbonyl chloride, both forming intramolecular or intermolecular hydrogen-bonds between pyridine-nitrogen and pyridine-2-amide hydrogen atoms. Crystal structure from three trimers with different terminal groups was reported. Analysis on the crystal structures revealed that these three trimers had different local conformations. The different local conformations were originated from the structural tunability of the imide unit in either the coplanarity or bond parameters. 相似文献
998.
999.
通过对平落4井雷口坡组段的岩心进行锶同位素组成的实验分析,研究了含卤碳酸盐地层物源贡献与沉积环境的演化,以及后期深部热液活动可能对富钾卤水形成所施加的影响.结果表明,区内海相碳酸盐w(87Sr)/w(86 Sr)值的变化,主要受二叠纪末峨眉山玄武岩浆喷发和印支期隆升导致四川盆地由"洋盆"向"孤盆"演化的地质构造背景所制... 相似文献
1000.
Liuchun Zheng Chuncheng Li Weiguo Huang Xi Huang Dong Zhang Guohu Guan Yaonan Xiao Dujing Wang 《先进技术聚合物》2011,22(2):279-285
Block copolymers demonstrate excellent thermal and mechanical properties superior to their corresponding random copolymers and homopolymers. However, it is difficult to synthesize block copolymers comprising of different polyester segments by copolycondensation due to the serious transesterification reaction. In this study, multiblock copolymers comprising of two different polyester segments, i.e. crystallizable poly(butylene succinate) (PBS) and amorphous poly(1,2‐propylene succinate) (PPSu), were synthesized by chain‐extension with hexamethylene diisocyanate (HDI). Amorphous PPSu segment was incorporated to improve the impact strength of PBS. The copolymers were characterized by GPC, laser light scattering (LLS), NMR, DSC, and mechanical testing. The results of 13C NMR spectra suggest that multiblock copolymers with regular sequential structure have been successfully synthesized. The data of DSC and mechanical testing indicate that block copolymers possess excellent thermal and mechanical properties with satisfactory tensile strength and extraordinary impact strength achieving upto 1900% of pure PBS. The influence of PPSu ratio and chain length of both the segments on the thermal and mechanical properties was investigated. The incorporation of an amorphous soft segment PPSu imparts high‐impact resistance to the copolymers without obviously decreasing the melting point (Tm). The favorable mechanical and thermal properties of the copolymers also depend on their regular sequential structure. At the same time, the introduction of amorphous PPSu segment enhances the enzymatic degradation rate of the multiblock copolymers. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献