全文获取类型
收费全文 | 119560篇 |
免费 | 21856篇 |
国内免费 | 34004篇 |
专业分类
化学 | 87425篇 |
晶体学 | 3945篇 |
力学 | 8208篇 |
综合类 | 3886篇 |
数学 | 19470篇 |
物理学 | 52486篇 |
出版年
2024年 | 248篇 |
2023年 | 1300篇 |
2022年 | 3713篇 |
2021年 | 3910篇 |
2020年 | 4022篇 |
2019年 | 4050篇 |
2018年 | 3673篇 |
2017年 | 4857篇 |
2016年 | 4454篇 |
2015年 | 5821篇 |
2014年 | 7011篇 |
2013年 | 9315篇 |
2012年 | 9305篇 |
2011年 | 10273篇 |
2010年 | 9433篇 |
2009年 | 9854篇 |
2008年 | 10705篇 |
2007年 | 9544篇 |
2006年 | 9427篇 |
2005年 | 8030篇 |
2004年 | 6522篇 |
2003年 | 4850篇 |
2002年 | 4952篇 |
2001年 | 4932篇 |
2000年 | 4864篇 |
1999年 | 3011篇 |
1998年 | 1713篇 |
1997年 | 1372篇 |
1996年 | 1348篇 |
1995年 | 1172篇 |
1994年 | 1184篇 |
1993年 | 1188篇 |
1992年 | 1033篇 |
1991年 | 835篇 |
1990年 | 702篇 |
1989年 | 717篇 |
1988年 | 645篇 |
1987年 | 594篇 |
1986年 | 527篇 |
1985年 | 461篇 |
1984年 | 456篇 |
1983年 | 389篇 |
1982年 | 340篇 |
1981年 | 346篇 |
1980年 | 305篇 |
1979年 | 314篇 |
1978年 | 224篇 |
1977年 | 196篇 |
1976年 | 160篇 |
1975年 | 171篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
211.
非等间距序列建模过程中存在的问题及改进 总被引:6,自引:0,他引:6
在研究非等间距序列的基础上 ,本文将非等间距累加运用于非等间距序列的建模过程中 ,使原有模型得到改进 .实例表明 ,改进后模型的精度有了很大提高 相似文献
212.
Nanograins are characterized by a typical grain size from lto 100 nm.Mclecular dynamics aimubations have been carried out for the nanograin sphere with the diameters from 1.45 to 10.12 nm.We study the influence of grain size on structure and diffusion properties of the nanograins.The results reveal that as the grain size is reduced,the fraction of grain surface increases significantly,and the surface width is approximately constant;the diffusicn coefficlent is increased sharply,and the relation of the diffusion coefficient and the grain size is close to exponential relation below 10nm. 相似文献
213.
WANG Mingwei LI Bo FENG Kun RAO Jun KANG Zihua LI Xiujuan LILi WANG Minghong FAN Mingjie 《核工业西南物理研究院年报(英文版)》2004,(1):77-79
in order to study non-indutive plasma current production, the lower hybrid current drive(LHCD) experiment on the HL-2A tokamak is carried out. Simultaneously a microcomputer has been used to control the whole LHCD system. During the experiment this year, we can monitor and protect the LHCD system by use of the microcomputer control system, which will imediately switch off the microwave power to be launched into the tokamak if the plasma is disrupted. All this ensure that the microwave is injected into the equipment correctly. 相似文献
214.
车前子中多糖含量的测定 总被引:12,自引:1,他引:11
采用蒽酮-硫酸法比色测定,用精制车前子多糖测得车前子多糖对葡萄糖的换算因子,对我国同属不同品种的车前子不同采集时间及炮制前后其多糖含量进行了比较研究。结果表明:此测定方法简便可行。供试液在8h内显色稳定,重现性较好,平均回收率为97.8%,RSD=1.41%(n=5)。江西吉安产大粒车前子生品多糖含量在9.15%~9.83%之间,炮制后其多糖含量在6.29%~6.81%之间;南昌新祺周车前GAP标准基地的大粒车前子生品多糖含量为7.85%~8.38%,炮制后其多糖含量为6.01%~6.64%;东北产的小粒车前子生品多糖含量为6.61%,炮制后其多糖含量为6.45%。 相似文献
215.
A new prediction technique is proposed for chaotic time series. The usefulness of the technique is thatit can kick off some false neighbor points which are not suitable for the local estimation of the dynamics systems. Atime-delayed embedding is used to reconstruct the underlying attractor, and the prediction model is based on the timeevolution of the topological neighboring in the phase space. We use a feedforward neural network to approximate thelocal dominant Lyapunov exponent, and choose the spatial neighbors by the Lyapunov exponent. The model is testedfor the Mackey-Glass equation and the convection amplitude of lorenz systems. The results indicate that this predictiontechnique can improve the prediction of chaotic time series. 相似文献
216.
A rigorous finite element analysis procedure is developed to study the effect of the inner structures on the acoustic absorption of multi-layer absorbers. To do this, four types of basic inner structure compartments adopted in the multi-layer absorbers are selected for analysis. These compartments are composed of porous materials inlaid with perforated plates of various shapes, say, triangle, semicircle, convex rectangle and plate shapes. As it is different from the conventional finite element analysis for the acoustic system in the literature, the perforated plates are simulated by appropriate equivalent boundary conditions, depending on their thickness, hole radius, hole pitch and porosity and the air contained in the holes. A large number of total degrees of freedom generated from meshing the air in the holes of perforated plates are thus avoided. The results reveal that the inner structures of the multi-layer absorbers will influence the acoustic absorption at some frequency bands significantly. Based on those features, the multi-layer absorber with a novel inner structure is then designed and manufactured. Both the finite element and experimental results show that its acoustic absorption would be distinctly promoted. 相似文献
217.
218.
219.
研究了Fe(acac)3-Al(i-Bu)3-8-羟基喹啉(acac=乙酰丙酮)催化体系催化丙烯酸丁酯(BA)聚合,考察了聚合规律,用凝胶渗透色谱研究了聚合物分子量和分子量分布.动力学研究表明聚合反应对单体浓度呈一级关系,表观活化能为13.9kJ/mol. 相似文献
220.
A lithium(I) coordination polymer has been formed from LiClO4 and the 2,2′‐bipyrimidine (bpym) ligand in which each square pyramidal lithium(I) atom is coordinated in the basal plane by four nitrogen donor atoms derived from two bpym ligands and one water molecule at the apical position. These are connected into a layer structure via hydrogen‐bonding interactions involving the perchlorate anions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献