首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   37712篇
  免费   8287篇
  国内免费   1399篇
化学   42125篇
晶体学   356篇
力学   471篇
数学   2117篇
物理学   2329篇
  2023年   29篇
  2022年   56篇
  2021年   183篇
  2020年   1243篇
  2019年   2610篇
  2018年   1024篇
  2017年   653篇
  2016年   3395篇
  2015年   3556篇
  2014年   3471篇
  2013年   4148篇
  2012年   3061篇
  2011年   2294篇
  2010年   2863篇
  2009年   2851篇
  2008年   2459篇
  2007年   1847篇
  2006年   1547篇
  2005年   1700篇
  2004年   1477篇
  2003年   1352篇
  2002年   2044篇
  2001年   1353篇
  2000年   1269篇
  1999年   373篇
  1998年   54篇
  1997年   52篇
  1996年   39篇
  1995年   31篇
  1994年   29篇
  1993年   23篇
  1992年   24篇
  1991年   24篇
  1990年   22篇
  1989年   18篇
  1988年   13篇
  1987年   14篇
  1986年   19篇
  1985年   18篇
  1984年   13篇
  1983年   16篇
  1982年   16篇
  1981年   6篇
  1980年   15篇
  1979年   12篇
  1978年   10篇
  1977年   12篇
  1976年   9篇
  1975年   6篇
  1974年   8篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
921.
The specific binding ability of DNA–lipid micelles (DLMs) can be increased by the introduction of an aptamer. However, supramolecular micellar structures based on self‐assemblies of amphiphilic DLMs are expected to demonstrate low stability when interacting with cell membranes under certain conditions, which could lead to a reduction in selectivity for targeting cancer cells. We herein report a straightforward cross‐linking strategy that relies on a methacrylamide branch to link aptamer and lipid segments. By an efficient photoinduced polymerization process, covalently linked aptamer–lipid units help stabilize the micelle structure and enhance aptamer probe stability, further improving the targeting ability of the resulting nanoassembly. Besides the development of a facile cross‐linking method, this study clarifies the relationship between aptamer–lipid concentration and the corresponding binding ability.  相似文献   
922.
Exploring materials with regulated local structures and understanding how the atomic motifs govern the reactivity and durability of catalysts are a critical challenge for designing advanced catalysts. Herein we report the tuning of the local atomic structure of nickel–iron layered double hydroxides (NiFe‐LDHs) by partially substituting Ni2+ with Fe2+ to introduce Fe‐O‐Fe moieties. These Fe2+‐containing NiFe‐LDHs exhibit enhanced oxygen evolution reaction (OER) activity with an ultralow overpotential of 195 mV at the current density of 10 mA cm?2, which is among the best OER catalytic performance to date. In‐situ X‐ray absorption, Raman, and electrochemical analysis jointly reveal that the Fe‐O‐Fe motifs could stabilize high‐valent metal sites at low overpotentials, thereby enhancing the OER activity. These results reveal the importance of tuning the local atomic structure for designing high efficiency electrocatalysts.  相似文献   
923.
Alcohol dehydrogenases can act as powerful catalysts in the preparation of optically pure γ‐hydroxy‐δ‐lactones by means of an enantioconvergent dynamic redox isomerization of readily available Achmatowicz‐type pyranones. Imitating the traditionally metal‐mediated “borrowing hydrogen” approach to shuffle hydrides across molecular architectures and interconvert functional groups, this chemoinspired and purely biocatalytic interpretation effectively expands the enzymatic toolbox and provides new opportunities in the assembly of multienzyme cascades and tailor‐made cellular factories.  相似文献   
924.
925.
A nitrogen‐rich compound, ReN8?x N2, was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser‐heated diamond anvil cell. Single‐crystal X‐ray diffraction revealed that the crystal structure, which is based on the ReN8 framework, has rectangular‐shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, ReN8?x N2 is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [?N=N?] that constitute the framework have not been previously observed in any compound. Ab initio calculations on ReN8?x N2 provide strong support for the experimental results and conclusions.  相似文献   
926.
The efficient, 12–14 step (LLS) total synthesis of (?)‐halenaquinone has been achieved. Key steps in the synthetic sequence include: (a) proline sulfonamide‐catalyzed, Yamada–Otani reaction to establish the C6 all‐carbon quaternary stereocenter, (b) multiple, novel palladium‐mediated oxidative cyclizations to introduce the furan moiety, and (c) oxidative Bergman cyclization to form the final quinone ring.  相似文献   
927.
The ligands L1 and L2 both form separable dinuclear double‐stranded helicate and mesocate complexes with RuII. In contrast to clinically approved platinates, the helicate isomer of [Ru2( L1 )2]4+ was preferentially cytotoxic to isogenic cells (HCT116 p53?/?), which lack the critical tumour suppressor gene. The mesocate isomer shows the reverse selectivity, with the achiral isomer being preferentially cytotoxic towards HCT116 p53+/+. Other structurally similar RuII‐containing dinuclear complexes showed very little cytotoxic activity. This study demonstrates that alterations in ligand or isomer can have profound effects on cytotoxicity towards cancer cells of different p53 status and suggests that selectivity can be “tuned” to either genotype. In the search for compounds that can target difficult‐to‐treat tumours that lack the p53 tumour suppressor gene, [Ru2( L1 )2]4+ is a promising compound for further development.  相似文献   
928.
A scalable flow reactor is demonstrated for enantioselective and regioselective rhodium carbene reactions (cyclopropanation and C?H functionalization) by developing cascade reaction methods employing a microfluidic flow reactor system containing immobilized dirhodium catalysts in conjunction with the flow synthesis of diazo compounds. This allows the utilization of the energetic diazo compounds in a safe manner and the recycling of the dirhodium catalysts multiple times. This approach is amenable to application in a bulk‐scale synthesis employing asymmetric C?H functionalization by stacking multiple fibers in one reactor module. The products from this sequential flow–flow reactor are compared with a conventional batch reactor or flow–batch reactor in terms of yield, regioselectivity, and enantioselectivity.  相似文献   
929.
Highly crystalline and (hydro)thermally stable zeolites with extra‐large pores [≥14‐ring (14‐R)] are desirable as catalysts. A novel zeolite, ECNU‐9, with an intersecting 14*12‐R channel system was rationally designed and synthesized by a building block strategy, in which the interlayer expansion of a two‐dimensional silicate structure was realized by combining organic amine assisted layer‐stacking reorganization and subsequent silylation with a square‐shaped single 4‐ring (S4R) silane, 1,3,5,7‐tetramethylcyclotetrasiloxane (TMCS). The PLS‐3 precursor was disassembled into building blocks and then intercalated with flexible and removable organic amine pillars to offer enough interlayer spacing for accommodating TMCS molecules. The additionally introduced building blocks interconnected the neighboring layers to construct new 14‐R and 12‐R pores. ECNU‐9 possesses a well‐ordered structure with a novel topology. The corresponding Ti‐ECNU‐9, with tetrahedral Ti ions in the framework, showed superior catalytic performance in the selective epoxidation of bulky alkenes.  相似文献   
930.
Methylammonium (CH3NH3+) and formamidinium ((NH2)2CH+) based lead iodide perovskites are currently the two commonly used organic–inorganic lead iodide perovskites. There are still no alternative organic cations that can produce perovskites with band gaps spanning the visible spectrum (that is, <1.7 eV) for solar cell applications. Now, a new perovskite using large propane‐1,3‐diammonium cation (1,3‐Pr(NH3)22+) with a chemical structure of (1,3‐Pr(NH3)2)0.5PbI3 is demonstrated. X‐ray diffraction (XRD) shows that the new perovskite exhibits a three‐dimensional tetragonal phase. The band gap of the new perovskite is about 1.6 eV, which is desirable for photovoltaic applications. A (1,3‐Pr(NH3)2)0.5PbI3 perovskite solar cell (PSC) yields a power conversion efficiency (PCE) of 5.1 %. More importantly, this perovskite is composed of a large hydrophobic cation that provides better moisture resistance compared to CH3NH3PbI3 perovskite.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号