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   324190篇
  免费   2374篇
  国内免费   1046篇
化学   150268篇
晶体学   4998篇
力学   17660篇
综合类   7篇
数学   55290篇
物理学   99387篇
  2021年   2672篇
  2020年   2896篇
  2019年   3285篇
  2018年   12197篇
  2017年   12150篇
  2016年   10623篇
  2015年   3925篇
  2014年   5790篇
  2013年   12910篇
  2012年   12639篇
  2011年   19752篇
  2010年   13009篇
  2009年   13298篇
  2008年   15489篇
  2007年   17061篇
  2006年   9926篇
  2005年   9458篇
  2004年   8950篇
  2003年   8350篇
  2002年   7701篇
  2001年   8466篇
  2000年   6384篇
  1999年   4847篇
  1998年   4097篇
  1997年   3933篇
  1996年   3746篇
  1995年   3201篇
  1994年   3103篇
  1993年   3032篇
  1992年   3310篇
  1991年   3537篇
  1990年   3311篇
  1989年   3257篇
  1988年   3110篇
  1987年   3162篇
  1986年   3031篇
  1985年   3740篇
  1984年   3791篇
  1983年   3096篇
  1982年   3239篇
  1981年   3030篇
  1980年   2986篇
  1979年   3177篇
  1978年   3160篇
  1977年   3091篇
  1976年   3154篇
  1975年   2996篇
  1974年   2916篇
  1973年   2966篇
  1972年   2104篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Nickel nanoparticles were deposited on the surface of highly oriented pyrolytical graphite by laser electrodispersion and precipitation from a solution of reverse micelles. The particles were studied using scanning tunneling microscopy and spectroscopy as well as Auger spectroscopy. The elemental composition of the nanoparticles and specific features of the electronic structure of their surface were determined. Adsorption of molecular hydrogen was also studied.  相似文献   
32.
33.
The synthesis of an octavalent bolaamphiphile with D-glucose moiety as branching unit has been designed and accomplished via 1,3-dipolar cycloaddition of bis(tetra-O-propargylglucoside) and 8 equiv of 2-azidoethyl β-D-lactoside.  相似文献   
34.
A study of the Li2FeSiO4/C cathode material doped with Mn demonstrated that introduction of polymethyl methacrylate results in a substantial decrease in the particle size and increase in the specific surface area of the cathode material. Polymethyl methacrylate strongly improves the cyclic stability of the cathode material. The discharge capacity after the first cycle was 218 mA h g–1, and that upon stabilization of the structure of the cathode material, 170 mA h g–1.  相似文献   
35.
The structures of the O‐glycosyltransferase LanGT2 and the engineered, C? C bond‐forming variant LanGT2S8Ac show how the replacement of a single loop can change the functionality of the enzyme. Crystal structures of the enzymes in complex with a nonhydrolyzable nucleotide‐sugar analogue revealed that there is a conformational transition to create the binding sites for the aglycon substrate. This induced‐fit transition was explored by molecular docking experiments with various aglycon substrates.  相似文献   
36.
The group 6 molybdenum(II) cyclopentadienyl amidinate (CPAM) bis(carbonyl) complex [Cp*Mo{N(iPr)C(Ph)N(iPr)}(CO)2] (Cp*=η5‐C5Me5) serves as a precatalyst for the high‐yielding photocatalytic production of COS from CO and S8 under near‐ambient conditions (e.g., 10 psi, 25 °C). Further documented is the isolation and structural characterization of several key transition‐metal intermediates which collectively support a novel molybdenum(IV)‐based catalytic cycle as being operative. Finally, in the presence of an excess amount of a primary amine, it is demonstrated that this catalytic system can be successfully used for the “on‐demand” generation and utilization of COS as a chemical reagent for the synthesis of ureas.  相似文献   
37.
Two new interesting entangled structures, namely, [Ni1.5(L)(bpy)2(H2O)3] n · 3nH2O (I) and [Cd3(L)2(bbi)2]n · nH2O (II)(where H3L is 4-(4-carboxyphenoxy)-phthalic acid, bpy is 4,4′-bipyridine, and bbi is 1,1′-(1,4-butanediyl)bis(imidazole)) have been synthesized and characterized by elemental analysis (EA), infrared spectra (IR), X-ray powder diffraction (XRPD), solid fluorescence and thermogravimetric analysis (TGA). Single-crystal X-ray diffraction analysis revealed that complex I possesses a 3D self-penetrating framework constructed from ladder-like and fishbone-like subunits. Complex II shows a 3D framework of two-fold interpenetration assembled from trinuclear Cd(II) clusters bridged by bbi and L3? ligands.  相似文献   
38.
This study proposes changes to the design and evaluation of the CO2 headspace test, which is used as a simple method for assessment of the complete biodegradability of surfactants. It presents a modified equation for the calculation of biodegradation. It is proposed that the solution of 7 mol L?1 sodium hydroxide commonly used in the process involving alkalinisation of the vial contents be replaced with 15 mol L?1 sodium hydroxide. The use of the higher hydroxide concentration leads to a significant reduction in the value of the blank CO2 headspace test.  相似文献   
39.
Based on rational design of the transition state analog inhibitors of glycosyltransferases, four model glycomimetics of this type, viz. benzyl 2-thio-α-dfructofuranoside 1-diethylphosphate (XIa), its β-anomer (XIb), and their ethyl 2-thio analogs — α-anomer (XIIa) and β-anomer (XIIb), were synthesized. In addition, fourteen precursors arising during the synthesis of the desired final model compounds (XI and XII), partially or fully acetylated benzyl and/or ethyl 2-thiofructofuranoside 1-diethyl phosphates, were isolated and characterized with the aim to prepare complete series of glycomimetics, representing donor UDP-GlcNAc designated for biological assays on human GnT’s, viz. GnT-I, Core2GnT, and GnT-V.  相似文献   
40.
Potential protective effects of elderberry (Sambucus nigra) extracts against oxidative degradation of hyaluronan (HA) were detected in vitro. To induce free-radical-mediated HA degradation, Weissberger’s biogenic oxidative system, which mimics the situation of acute inflammation, was applied. Time- and dose-dependent changes of dynamic viscosity of the HA solutions in the presence and absence of two elderberry extracts produced in 2006 and 2012 were recorded by rotational viscometry (RV). Radical scavenging capacity of both extracts was investigated by the spectrocolorimetric ABTS [2,2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt] assay and the “inverted” ABTS assay. Oxygen consumption of the system oxidizing HA either in the absence or presence of the elderberry extracts was determined. The results of RV revealed that an addition of the newer extract (2012) promoted the inhibition of HA degradation more markedly compared to the older extract (2006). The same effect of both extracts on ABTS·+ scavenging was observed. Inverted ABTS assay demonstrated that colorful substances were not responsible for the radical-scavenging activity. Results of oximetry demonstrated that the effect of the extract from 2006 was more significant than that of the extract from 2012.  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