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   7879篇
  免费   621篇
  国内免费   650篇
化学   3595篇
晶体学   57篇
力学   309篇
综合类   33篇
数学   429篇
物理学   1529篇
无线电   3198篇
  2024年   51篇
  2023年   487篇
  2022年   184篇
  2021年   334篇
  2020年   258篇
  2019年   251篇
  2018年   190篇
  2017年   396篇
  2016年   486篇
  2015年   395篇
  2014年   622篇
  2013年   557篇
  2012年   532篇
  2011年   485篇
  2010年   380篇
  2009年   414篇
  2008年   379篇
  2007年   422篇
  2006年   341篇
  2005年   292篇
  2004年   273篇
  2003年   183篇
  2002年   166篇
  2001年   135篇
  2000年   115篇
  1999年   118篇
  1998年   92篇
  1997年   93篇
  1996年   84篇
  1995年   65篇
  1994年   49篇
  1993年   57篇
  1992年   36篇
  1991年   37篇
  1990年   28篇
  1989年   20篇
  1988年   37篇
  1987年   14篇
  1986年   10篇
  1985年   8篇
  1984年   15篇
  1982年   4篇
  1981年   12篇
  1980年   7篇
  1979年   10篇
  1978年   7篇
  1977年   4篇
  1976年   5篇
  1975年   2篇
  1974年   2篇
排序方式: 共有9150条查询结果,搜索用时 10 毫秒
961.
962.
The salt N-benzylmethylammonium dihydrogenmonophosphate monohydrate is monoclinic with the following unit cell dimensions: a = 6.356(1)Å, b = 8.385(7)Å, c = 11.472(5)Å, = 104.32(1)°, space group P21 with Z = 2. The structure consists of infinite parallel two-dimensional [110] planes built of mutually connected ions and water molecules by strong O–H···O and N–H···O hydrogen bonding. There are no contacts other than normal van der Waals interactions between the layers.  相似文献   
963.
The design, fabrication and operation of a range of functional power converter circuits, based on diode-configured organic field-effect transistors as the rectifying unit and capable of transforming a high AC input voltage to a selectable DC voltage, are presented. The converter functionality is demonstrated by selecting and tuning its constituents so that it can effectively drive a low-voltage organic electronic device, a light-emitting electrochemical cell (LEC), when connected to high-voltage AC mains. It is established that the preferred converter circuit for this task comprises an organic full-wave rectifier and a regulation resistor but is void of a smoothing capacitor, and that such a circuit connected to the AC mains (230 V, 50 Hz) successfully can drive an LEC to bright luminance (360 cd m−2) and high efficiency (6.4 cd A−1).  相似文献   
964.
This paper is concerned with the following one-dimensional nonlinear system of equations:
(0.1)  相似文献   
965.
Pinhua Li  Yicheng Zhang 《Tetrahedron》2008,64(48):10825-10830
A novel SiO2-NHC-Cu(I) 3b was developed and used as a highly efficient catalyst for [3+2] cycloaddition of organic azides and terminal alkynes. In the presence of SiO2-NHC-Cu(I) 3b (1 mol %), the reactions of terminal alkynes with organic azides underwent smoothly to generate the corresponding regiospecific 1,4-disubstituted 1,2,3-triazoles in excellent yields under solvent-free reaction conditions at room temperature. Furthermore, catalyst 3b was quantitatively recovered from the reaction mixture by a simple filtration and reused for 10 cycles without loss of its activity.  相似文献   
966.
Functionalization of pentacene at the 6- and 13-positions affords versatile building blocks for oligomer and polymer formation. Di- and trimeric materials are synthesized using unsymmetrical building block 18, while symmetrical diol monomer 17 allows for the synthesis of polymers. The materials reported herein are soluble in common organic solvents and air-stable. UV-vis and fluorescence spectroscopic properties have been investigated. Solid-state X-ray crystallography of building blocks 17 and 19 shows that these derivatives can π-stack with significant acene face-to-face interactions with spacing of less than 3.5 Å.  相似文献   
967.
Vivek Polshettiwar 《Tetrahedron》2008,64(20):4637-4643
The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica was prepared using sol-gel method and its application in Heck and Suzuki reactions was demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, with excellent yield, without any change in catalytic activity for at least five reaction cycles, with negligible Pd concentration in the end product.  相似文献   
968.
The full reaction photosynthesis of H2O2 that can combine water-oxidation and oxygen-reduction without sacrificial agents is highly demanded to maximize the light-utilization and overcome the complex reaction-process of anthraquinone-oxidation. Here, a kind of oxidation-reduction molecular junction covalent-organic-framework (TTF-BT-COF) has been synthesized through the covalent-coupling of tetrathiafulvalene (photo-oxidation site) and benzothiazole (photo-reduction site), which presents visible-light-adsorption region, effective electron-hole separation-efficiency and photo-redox sites that enables full reaction generation of H2O2. Specifically, a record-high yield (TTF-BT-COF, ≈276 000 μM h−1 g−1) for H2O2 photosynthesis without sacrificial agents has been achieved among porous crystalline photocatalysts. This is the first work that can design oxidation-reduction molecular junction COFs for full reaction photosynthesis of H2O2, which might extend the scope of COFs in H2O2 production.  相似文献   
969.
Heavy-atom integration into thermally activated delayed fluorescence (TADF) molecule could significantly promote the reverse intersystem crossing (RISC) process. However, simultaneously achieving high efficiency, small roll-off, narrowband emission and good operational lifetime remains a big challenge for the corresponding organic light-emitting diodes (OLEDs). Herein, we report a pure green multi-resonance TADF molecule BN-STO by introducing a peripheral heavy atom selenium onto the parent BN-Cz molecule. The organic light-emitting diode device based on BN-STO exhibited state-of-the-art performance with a maximum external quantum efficiency (EQE) of 40.1 %, power efficiency (PE) of 176.9 lm W−1, well-suppressed efficiency roll-off and pure green gamut. This work reveals a feasible strategy to reach a balance between fast RISC process and narrow full width at half maximum (FWHM) of MR-TADF by heavy atom effect.  相似文献   
970.
The ethanol/water separation challenge highlights the adsorption capacity/selectivity trade-off problem. We show that the target guest can serve as a gating component of the host to block the undesired guest, giving molecular sieving effect for the adsorbent possessing large pores. Two hydrophilic/water-stable metal azolate frameworks were designed to compare the effects of gating and pore-opening flexibility. Large amounts (up to 28.7 mmol g−1) of ethanol with fuel-grade (99.5 %+) and even higher purities (99.9999 %+) can be produced in a single adsorption process from not only 95 : 5 but also 10 : 90 ethanol/water mixtures. More interestingly, the pore-opening adsorbent possessing large pore apertures showed not only high water adsorption capacity but also exceptionally high water/ethanol selectivity characteristic of molecular sieving. Computational simulations demonstrated the critical role of guest-anchoring aperture for the guest-dominated gating process.  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