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   45737篇
  免费   5335篇
  国内免费   237篇
化学   17546篇
晶体学   297篇
力学   3662篇
综合类   497篇
数学   10465篇
物理学   18842篇
  2023年   399篇
  2022年   709篇
  2021年   869篇
  2020年   1056篇
  2019年   1072篇
  2018年   920篇
  2017年   981篇
  2016年   1056篇
  2015年   804篇
  2014年   1480篇
  2013年   2860篇
  2012年   1788篇
  2011年   1999篇
  2010年   1586篇
  2009年   2680篇
  2008年   2936篇
  2007年   3316篇
  2006年   2794篇
  2005年   2184篇
  2004年   1814篇
  2003年   2134篇
  2002年   2103篇
  2001年   1813篇
  2000年   1731篇
  1999年   1435篇
  1998年   1400篇
  1997年   777篇
  1996年   739篇
  1995年   677篇
  1994年   741篇
  1993年   540篇
  1992年   618篇
  1991年   408篇
  1990年   385篇
  1989年   289篇
  1988年   254篇
  1987年   255篇
  1986年   226篇
  1985年   204篇
  1984年   210篇
  1983年   122篇
  1982年   172篇
  1981年   162篇
  1980年   105篇
  1979年   132篇
  1978年   100篇
  1977年   85篇
  1976年   39篇
  1974年   32篇
  1973年   34篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Two new aryl‐tetralin lignan glycosides, linderanosides A and B ( 1 and 2 , resp.), and a new dihydrobenzofuran neolignan glycoside, linderanoside C ( 3 ), together with five known lignan derivatives ( 4 – 8 ) were isolated from the trunk of Lindera glauca. The structures of these new compounds were determined through spectroscopic analyses, including extensive 2D‐NMR data and acid hydrolysis. The absolute configurations of the compounds were clarified by circular dichroism (CD) spectroscopic studies. Compounds 1 – 8 were evaluated for their cytotoxicity against A549 (non‐small cell lung adenocarcinoma), SK‐OV‐3 (ovarian cancer cells), A498 (human kidney epithelial cells), and HCT‐15 (colon cancer cells) human tumor cell lines using sulforhodamine B assays in vitro.  相似文献   
2.
Two new diterpenoids, named paecilomycine A ( 1 ) and paecilomycine B ( 2 ), including a novel skeleton with a five‐membered lactone ring, together with three known labdane diterpenoids, rel‐(1R,3S,4aS,5R,8aS)‐5‐[(3E)‐4‐carboxy‐3‐methylbut‐3‐en‐1‐yl]decahydro‐3‐hydroxy‐1,4a‐dimethyl‐6‐methylidenenaphthalene‐1‐carboxylic acid ( 3 ), botryosphaerin E ( 4 ), and agathic acid ( 5 ), were isolated from solid culture of the insect pathogenic fungi strain Paecilomyces sp. The structures of all compounds were established on the basis of comprehensive spectroscopic studies. The relative configurations of 1 and 2 were determined by single‐crystal X‐ray diffraction analyses.  相似文献   
3.
PdCuAu ternary alloy samples with different composition were synthesized on top of ZrO2‐modified porous stainless steel disks by the sequential electroless deposition technique. The structure, morphology and bulk composition of the samples were characterized by X‐ray diffraction (XRD), scanning electron microscopy and energy dispersive X‐ray spectroscopy (EDX). Complete alloy formation with a pure fcc phase for the Pd71Cu26Au3, Pd70Cu25Au5 and Pd67Cu24Au9 samples and a bcc structure for the Pd62Cu36Au2 and Pd60Cu37Au3 samples were obtained upon annealing at 500 °C for 120 h as revealed by XRD. A combination of low‐energy ion scattering (LEIS) and X‐ray photoelectron spectroscopy (XPS) was used to investigate the surface properties of the PdCuAu alloys. XPS results confirmed alloy formation under the annealing conditions. XPS analysis also revealed that the near‐surface regions of the alloys became enriched in Pd with respect to the bulk composition determined by EDX. In contrast, LEIS and angle‐resolved XPS analyses showed that the top‐most surface layers in all samples were copper‐rich compared with the bulk composition. This high Cu surface concentration could impart resistance to bulk sulfide formation to the PdCuAu alloy membranes. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
4.
The dynamics and mechanisms of proton dissociation and transfer in hydrated phosphoric acid (H3PO4) clusters under excess proton conditions were studied based on the concept of presolvation using the H3PO4–H3O+nH2O complexes (n = 1–3) as the model systems and ab initio calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations at the RIMP2/TZVP level as model calculations. The static results showed that the smallest, most stable intermediate complex for proton dissociation (n = 1) is formed in a low local‐dielectric constant environment (e.g., ε = 1), whereas proton transfer from the first to the second hydration shell is driven by fluctuations in the number of water molecules in a high local‐dielectric constant environment (e.g., ε = 78) through the Zundel complex in a linear H‐bond chain (n = 3). The two‐dimensional potential energy surfaces (2D‐PES) of the intermediate complex (n = 1) suggested three characteristic vibrational and 1H NMR frequencies associated with a proton moving on the oscillatory shuttling and structural diffusion paths, which can be used to monitor the dynamics of proton dissociation in the H‐bond clusters. The BOMD simulations over the temperature range of 298–430 K validated the proposed proton dissociation and transfer mechanisms by showing that good agreement between the theoretical and experimental data can be achieved with the proposed rate‐determining processes. The theoretical results suggest the roles played by the polar solvent and iterate that insights into the dynamics and mechanisms of proton transfer in the protonated H‐bond clusters can be obtained from intermediate complexes provided that an appropriate presolvation model is selected and that all of the important rate‐determining processes are included in the model calculations. © 2015 Wiley Periodicals, Inc.  相似文献   
5.
Methods of automatic computer analyses of images have large impotence in numerous physical problems with different kinds of radiations, which use such solid-state detectors as nuclear photographic emulsions, plastics and others. We described a method of expanding boundaries of dark areas for such computer analyses of micro-pictures. We demonstrated effectiveness of this method by the example of the search of microcrystals size distributions in two undeveloped nuclear emulsions and similar distribution of background grains in the developed emulsion.  相似文献   
6.
The non-resonant Raman spectra of homogeneous bundles of C60 peapods (C60 inserted in single-walled carbon nanotubes) are calculated in the framework of spectral moment method, together with a bond-polarizability model. The evolutions of the low wavenumber range of the Raman spectrum of homogeneous bundles of C60 peapods as a function of the nanotube diameter and the size of bundles are discussed. The effect of the C60 filling factor is investigated in detail. The results are compared to experimental Raman data measured on various samples of C60 peapods.  相似文献   
7.
Principal role of substrate types on the nonlinear optical properties of Au NP was investigated. Third harmonic generation (THG) studies were carried out for Au NP deposited on the Al-doped ZnO (AuNP/AZO) and Ga-doped ZnO (AuNP/GZO) substrates at fundamental wavelength of 20 ns Er:glass laser (generating at 1540 nm wavelength) during photostimulation by the 532 nm 15 ns laser pulses. Sizes of Au NP were 5 nm, 10 nm, 20 nm, and 30 nm. The output signal was registered at 513 nm. The photoinduced power density was increased from 0 up to 800 MW/cm2. So in our work we explore the role of the substrate on the optically stimulated non-linear optical properties during simultaneous photo stimulation near the inter-band transition. The results were studied depending on the type of substrate and the sizes of the deposited nanoparticles. The analysis was done within a framework of interaction between the photoinduced light and SPR wavelengths. Control of the photoinduced temperature was done.  相似文献   
8.
The interband and intraband radiation from the n-InGaAs/GaAs heterostructures with the double and triple tunnel coupled and selectively doped quantum wells (QWs), which is appeared under the lateral electric field and in the presence of hole injection from the anode contact, has been investigated. A steep increase of the interband radiation intensity was found at the fields of E≥1.7 kV/cm. This effect should be related to the big lifetime of the injected charge carriers (~10−6 s) which exceeds by three orders of magnitude the lifetime in the similar bulk direct-band semiconductor. Its reason lies in spatial separation of the injected holes and electrons between coupled wells, firstly, by the built-in transverse electric field between wells and, secondly, due to the real-space transfer of carriers heated by the lateral electric field from the wide well to the narrow δ-doped one. Furthermore, an increase of the carrier concentration due to injection leads to an increase of that transition intensity and, consequently, to an intensity increase of the radiative intersubband transitions of carriers in QWs which results in a steep intensity increase of the far (50–120 µm) infrared radiation.  相似文献   
9.
农作物冠层光谱信息检测技术及方法综述   总被引:2,自引:0,他引:2  
相比传统的化学方法及主观视觉测定植物生理信息指标,通过光谱辐射仪对农作物冠层信息的评估更简单、快速和精确。本文结合近年来农作物群体信息的获取方法,综述了不同类型光谱仪尤其以国际上应用最广泛的Cropscan多光谱辐射仪在农作物群体叶面积指数、生物量、氮素及叶绿素的预测,病虫害的监测及产量预测中的应用。总结了不同类型的的植被指数(VIs)、冠层光谱的获取及光谱分析方法,比较了不同农作物建立相关模型的回归系数。总体上建立的数学模型的相关系数较高,能实现对农作物各种生理信息等的检测。此外,将多光谱辐射仪与多种传感器相结合所得到的综合信息对于全面评价农作物生长情况具有重要指导价值。  相似文献   
10.
Gamma-ray irradiation effects on the photoresponsive thin-film devices based on the regioregular poly(3-hexylthiophene) (RR-P3HT) conjugated polymer have been studied by means of atomic force microscopy, UV–vis absorption, photoluminescence (PL), and time-of-flight measurements. As a result, increased light absorption in the red region and PL quenching induced by the irradiation were observed. Besides, enhancement of the electron/hole mobilities, attributable to improved ordering or increased nanodomain size of the P3HT thin films, was revealed.  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