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   118835篇
  免费   9259篇
  国内免费   7537篇
化学   43301篇
晶体学   1943篇
力学   4673篇
综合类   1077篇
数学   34803篇
物理学   49834篇
  2023年   580篇
  2022年   650篇
  2021年   1794篇
  2020年   1663篇
  2019年   1952篇
  2018年   1775篇
  2017年   1579篇
  2016年   1577篇
  2015年   1565篇
  2014年   2755篇
  2013年   4926篇
  2012年   2986篇
  2011年   3771篇
  2010年   4199篇
  2009年   9221篇
  2008年   10277篇
  2007年   8668篇
  2006年   7579篇
  2005年   5379篇
  2004年   5040篇
  2003年   4992篇
  2002年   8064篇
  2001年   4704篇
  2000年   4328篇
  1999年   3878篇
  1998年   3287篇
  1997年   2444篇
  1996年   2258篇
  1995年   2652篇
  1994年   2570篇
  1993年   2062篇
  1992年   1565篇
  1991年   1122篇
  1990年   926篇
  1989年   814篇
  1988年   800篇
  1987年   597篇
  1986年   398篇
  1985年   1288篇
  1984年   883篇
  1983年   635篇
  1982年   930篇
  1981年   1085篇
  1980年   989篇
  1979年   866篇
  1978年   772篇
  1977年   654篇
  1976年   630篇
  1974年   380篇
  1973年   520篇
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
11.
This paper presents the effect of insecticides on the composition of the surface compounds of one of the most harmful insects, Tenebrio molitor, by analysis using GC–MS. As a result of the use of insecticides, the composition of the chemical compounds on the surface of insects changes, depending on the insecticides used. The most numerous groups of the marked compounds were fatty acids, alkanes, esters and sterols. The content of the identified compounds in the larvae increased at both 24 and 48 h after the application of insecticides, in comparison with the control samples. The content of identified compounds in the samples taken from the females increased 24, 48 and 72 h after the application of insecticides in comparison with the control samples. By contrast, in samples prepared from males, the content of identified compounds decreased 24 h after the application of insecticides, compared with the control samples. The highest content of chemical compounds was for fatty acids and alkanes after the use of insecticides. The content of fatty acids after the application of the insecticide with deltamethrin was 62.1 ± 3.3–466.9 ± 5.9 μg/g, and after the application of the insecticide with cyfluthrin was 49.9 ± 1.9–458.3 ± 4.2 μg/g. However, the content of alkanes after the use of deltamethrin was 115.6 ± 4.2–4672.0 ± 32.1 μg/g, and after the use of cyfluthrin was 189.4 ± 3.8–3975.0 ± 10.2 μg/g.  相似文献   
12.
This study aimed to investigate the chemical composition of Tribulus terrestris L. fruit (TT) extract named TT15 and its protective effect against ischemic stroke (IS) as well as corresponding mechanisms. The chemical composition of TT15 was analyzed by liquid chromatography-mass spectrometry (LC-MS), and the compound identification was conducted via searching the in-house database. The LC-MS-based multi-omics approach was applied to search the differential metabolites and differential proteins in rat brain tissue and to explore the biomarker and molecular mechanism of TT15 against middle cerebral artery occlusion (MCAO). A total of 20 compounds were identified from TT15, mainly including alkaloids, flavonoids, phenols, quinones, and esters. These 20 compounds significantly affected the metabolism of 44 metabolites and the expression of 51 proteins. Joint pathway analysis showed that these metabolites and proteins were mainly involved in the response to elevated platelet cytosolic Ca2+ and platelet activation, which inferred that TT15 may exert a protective effect against cerebral ischemic injury via regulating platelet function. This study provides useful information for further exploration of the mechanisms of TT extract against IS.  相似文献   
13.
Combustion processes in porous media have been used by the petroleum engineering industry to extract heavy oil from reservoirs. This study focuses on a one-dimensional nonlinear hybrid system consisting of n reaction–diffusion–convection equations coupled with n ordinary differential equations, which models a combustion front moving through a porous medium with n parallel layers. The state variables are the temperature and fuel concentration in each layer. Coupling occurs in both the reaction function and differential operator coefficients. We prove the existence of a classical solution, first locally and then globally over time, to an initial and boundary value problem for the corresponding system. The proof uses a new approach for combustion problems in porous media. The local solution is obtained by defining an operator in a set of Hölder continuous functions and using Schauder’s fixed-point theorem to find a fixed point as the desired solution. Using Zorn’s lemma, we extend the local solution to a global solution, proving that the first-order spatial derivative of the temperature in each layer is a bounded function.  相似文献   
14.
We survey some unsolvable conjectures in finite p-groups and their research progress.  相似文献   
15.
16.
分子张力作为空间设计的重要组成部分正成为调控有机半导体的重要手段。由于分子内产生的拉伸张力、扭曲/弯曲张力以及空间张力而导致p轨道排布重组和构型构象结构发生变化,最近各种几何与拓扑结构的高张力有机半导体材料相继被报道,这使得高张力有机半导体材料成为有机电子领域研究的焦点。为了进一步梳理分子张力在有机半导体材料中扮演的角色与价值,该综述从分子张力的类型、实验与理论量化以及可视化出发,总结了高张力共轭芳烃的分子设计策略、与其光电性能分子张力之间的关系,以及这类新兴材料在光电领域的应用。最后,对高张力共轭芳烃的研究前景进行了展望,阐述了该类材料所面临的机遇与挑战。  相似文献   
17.
In this paper, we define interval‐valued left‐sided and right‐sided generalized fractional double integrals. We establish inequalities of Hermite‐Hadamard like for coordinated interval‐valued convex functions by applying our newly defined integrals.  相似文献   
18.
19.
CRANAD-2 is a fluorogenic curcumin derivative used for near-infrared detection and imaging in vivo of amyloid aggregates, which are involved in neurodegenerative diseases. We explore the performance of CRANAD-2 in two super-resolution imaging techniques, namely stimulated emission depletion (STED) and single-molecule localization microscopy (SMLM), with markedly different fluorophore requirements. By conveniently adapting the concentration of CRANAD-2, which transiently binds to amyloid fibrils, we show that it performs well in both techniques, achieving a resolution in the range of 45–55 nm. Correlation of SMLM with atomic force microscopy (AFM) validates the resolution of fine features in the reconstructed super-resolved image. The good performance and versatility of CRANAD-2 provides a powerful tool for near-infrared nanoscopic imaging of amyloids in vitro and in vivo.  相似文献   
20.
We solve a combinatorial question concerning eigenvalues of the universal intertwining endomorphism of a subset representation.  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