全文获取类型
收费全文 | 12596篇 |
免费 | 1787篇 |
国内免费 | 1478篇 |
专业分类
化学 | 7126篇 |
晶体学 | 87篇 |
力学 | 604篇 |
综合类 | 84篇 |
数学 | 1014篇 |
物理学 | 3536篇 |
无线电 | 3410篇 |
出版年
2024年 | 55篇 |
2023年 | 306篇 |
2022年 | 351篇 |
2021年 | 436篇 |
2020年 | 447篇 |
2019年 | 404篇 |
2018年 | 316篇 |
2017年 | 326篇 |
2016年 | 518篇 |
2015年 | 473篇 |
2014年 | 693篇 |
2013年 | 823篇 |
2012年 | 995篇 |
2011年 | 1048篇 |
2010年 | 761篇 |
2009年 | 721篇 |
2008年 | 868篇 |
2007年 | 733篇 |
2006年 | 648篇 |
2005年 | 627篇 |
2004年 | 452篇 |
2003年 | 463篇 |
2002年 | 485篇 |
2001年 | 393篇 |
2000年 | 298篇 |
1999年 | 293篇 |
1998年 | 268篇 |
1997年 | 225篇 |
1996年 | 209篇 |
1995年 | 172篇 |
1994年 | 196篇 |
1993年 | 130篇 |
1992年 | 140篇 |
1991年 | 117篇 |
1990年 | 89篇 |
1989年 | 90篇 |
1988年 | 52篇 |
1987年 | 32篇 |
1986年 | 41篇 |
1985年 | 31篇 |
1984年 | 21篇 |
1983年 | 21篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 8篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1974年 | 9篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
显示雷达视频是雷达显示终端的主要任务,本文介绍了一种视频融合的设计,采用stratixⅡ公司的可编程逻辑器件EP2S30F484C4及QuartusⅡ5.1开发系统实现。由于采用该器件,减少了设备体积,同时也使设备的可靠性和设计的灵活性大大提高。 相似文献
993.
A new planar-chiral bidentate phosphaferrocene ligand (2) has been synthesized and structurally characterized. The derived rhodium complex, [Rh(cod)(2)]BF(4), serves as an effective catalyst for asymmetric isomerizations of allylic alcohols to aldehydes, furnishing improved yields, scope, and enantioselectivities relative to previously reported methods. The catalyst is air-stable and can be recovered at the end of the reaction. Mechanistic studies establish that the isomerization proceeds via an intramolecular 1,3-hydrogen migration and that the catalyst differentiates between the enantiotopic C1 hydrogens. 相似文献
994.
We give a Newton type rational interpolation formula (Theorem 2.2). It contains as a special case the original Newton interpolation, as well as the interpolation formula of Liu, which allows to recover many important classical q-series identities. We show in particular that some bibasic identities are a consequence of our formula. 相似文献
995.
Tianwen Chen Fengfu Fu Zhixin Chen Danzhen Li Lan Zhang Guonan Chen 《Journal of chromatography. A》2009,1216(15):3217-3222
The intermediates of photodegradation and microbial degradation of pirimicarb insecticide were investigated by liquid chromatography coupled with ion-trap mass spectrometry (LC–IT-MS). Different intermediates were detected in the photodegradation and microbial degradation of pirimicarb. In the photodegradation of pirimicarb in aqueous solution 2-dimethylamino-5,6-dimethyl-4-hydroxypyrimidine (MW = 167), 2-methylamino-5,6-dimethylpyrimidin-4-yl-dimethylcarbamate (MW = 224) and 2-formylamino-5,6-dimethylpyrimidin-4-yl-dimethylcarbamate (MW = 252) were the main products. It was found that 2-dimethylamino-5,6-dimethyl-4-hydroxypyrimidine (MW = 167) was the major product in the microbial degradation of pirimicarb in soil. 相似文献
996.
A rationally designed 4-phenoxy substituted prolinamide phenols as an efficient hydrophobic organocatalyst for direct asymmetric aldol reaction in water has been developed. High yield (up to 99%), diastereoselectivity (up to 99:1), and enantioselectivity (up to 97%) were obtained under optimal condition. The influence of substituent groups on the reactivity of catalysts was studied in detail. 相似文献
997.
Guo‐Xu Zhang Ying Xie Hai‐Tao Yu Hong‐Gang Fu 《Journal of computational chemistry》2009,30(12):1785-1798
The structural and electronic properties of five terminations of cubic lead titanate (PbTiO3) (110) polar surface were investigated by first‐principles total‐energy calculations using a periodic slab model. On the PbTiO termination, an anomalous filling of conduction band was observed, whereas on the O2 termination, two surface oxygen atoms formed a peroxo group, demonstrating that the electronic structures of the two stoichiometric terminations undergo significant changes with respect to bulk materials. However, for the three nonstoichiometric TiO‐, Pb‐, and O‐terminated surfaces, their electronic structures are very similar to bulk. Charge redistribution results for the five terminations confirmed that electronic structure and surface composition changes are responsible for their polarity compensation. However, which mechanism actually dominates the stabilization process depends upon energetic considerations. A thermodynamic stability diagram suggested that the two stoichiometric terminations are unstable; however, the three nonstoichiometric terminations can be stabilized in some given regions. Furthermore, this study indicates that the very different stabilities and surface states filling behaviors of the PbTiO3 (110) polar surface with respect to SrTiO3 and BaTiO3 ones seem to originate from the partially covalent characteristics of Pb O pairs. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009 相似文献
998.
Wenjin Yang Dingcai Wu Ruowen Fu 《Colloids and surfaces. A, Physicochemical and engineering aspects》2008,312(2-3):118-124
This paper focuses on how the surface chemical groups of carbon aerogels (CAs) affect their adsorption properties of basic dyes. First of all, the surface structures of CAs were tailored by controlling carbonization temperatures and oxidation treatments including HNO3 oxidation, H2O2 oxidation and hot air oxidation. The pore structures of samples were investigated by N2 adsorption–desorption analysis. In addition, their surface chemistries were characterized by Boehm titration, elemental analysis and XPS analysis. Subsequently, the adsorption capacities of the typical basic dyes on them were determined by UV Spectroscopy. The results show that the surface chemistry of CAs plays a key role in dye adsorption performance. HNO3 oxidation can produce much acidic groups on the surface of sample, such as carboxyl, lactone and phenolic groups, resulting in an increase in the adsorption amounts of basic dyes. 相似文献
999.
Xia Li Min Lu Qi Wu De‐shui Lv Xian‐Fu Lin 《Journal of polymer science. Part A, Polymer chemistry》2008,46(1):117-126
To develop designed polymer–drug conjugates, where the rate of drug liberation and hepatoma cell targeting function could be rationally and widely controlled, we facilely synthesized a series of novel, galactose‐functionalized polymer–acyclovir conjugates with different linkers and first reported the effect of the linker structure including the type of acyclovir‐linked bond (an ester bond or an amide bond) and relative length of the linker between acyclovir and the polymer main chain on release rate and targeting ability of conjugates. In vitro release studies showed that the cumulative released acyclovir from these polymer–acyclovir conjugates was between 24 and 65% in pH 1.2 glycine solution after 7 days. The ester bond more easily underwent hydrolysis than the amide bond. The longer the relative linker length was, the faster the acyclovir was released. The cell recognition experiments visualized using confocal laser scanning microscopy exhibited that the resultant galactose‐functionalized polymer–acyclovir conjugates had evident targeting to hepG2 cells, and targeting ability was also in connection with the relative length of linker. By choosing appropriate linker, cellular internalization of acyclovir could be well achieved. We consider these results to be helpful for the design of multifunctional polymeric prodrugs, in which the required release rate and targeting ability could be rationally controlled by predetermined molecular architecture. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 117–126, 2008 相似文献
1000.
Qiang Fu Chao Liu Wencheng Lin Junlian Huang 《Journal of polymer science. Part A, Polymer chemistry》2008,46(20):6770-6779
Heterograft copolymers poly(4‐glycidyloxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl‐co‐ ethylene oxide)‐graft‐polystyrene and poly(tert‐butyl acrylate) (poly (GTEMPO‐co‐EO)‐g‐PS/PtBA) were synthesized in one‐pot by atom transfer nitroxide radical coupling (ATNRC) reaction via “graft onto.” The main chain was prepared by the anionic ring‐opening copolymerization of ethylene oxide (EO) and 4‐glycidyloxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (GTEMPO) first, then the polystyrene and poly (tert‐butyl acrylate) with bromine end (PS‐Br, PtBA‐Br) were prepared by atom transfer radical polymerization (ATRP). When three of them were mixed each other in the presence of CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) at 90 °C, the formed secondary carbon radicals at the PS and PtBA chain ends were quickly trapped by nitroxide radicals on poly(GTEMPO‐co‐EO). The heterograft copolymers were well defined by 1H NMR, size exclusion chromatography, fourier transform infrared, and differential scanning calorimetry in detail. It was found that the density of GTEMPO groups on main chain poly(GTEMPO‐co‐EO), the molecular weights of PS/PtBA side chains, and the structure of macroradicals can exert the great effects on the graft efficiency. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6770–6779, 2008 相似文献