全文获取类型
收费全文 | 349675篇 |
免费 | 31046篇 |
国内免费 | 25482篇 |
专业分类
化学 | 160570篇 |
晶体学 | 4049篇 |
力学 | 14897篇 |
综合类 | 34篇 |
数学 | 31404篇 |
物理学 | 121294篇 |
无线电 | 73955篇 |
出版年
2022年 | 2703篇 |
2021年 | 2903篇 |
2020年 | 3302篇 |
2019年 | 4492篇 |
2018年 | 4568篇 |
2017年 | 5205篇 |
2016年 | 7486篇 |
2015年 | 7346篇 |
2014年 | 8182篇 |
2013年 | 16732篇 |
2012年 | 15136篇 |
2011年 | 17903篇 |
2010年 | 16961篇 |
2009年 | 17347篇 |
2008年 | 15053篇 |
2007年 | 15011篇 |
2006年 | 14376篇 |
2005年 | 13591篇 |
2004年 | 13672篇 |
2003年 | 11643篇 |
2002年 | 11161篇 |
2001年 | 13379篇 |
2000年 | 10367篇 |
1999年 | 8677篇 |
1998年 | 7185篇 |
1997年 | 6990篇 |
1996年 | 7024篇 |
1995年 | 6885篇 |
1994年 | 6835篇 |
1993年 | 6865篇 |
1992年 | 6941篇 |
1991年 | 6561篇 |
1990年 | 5750篇 |
1989年 | 5589篇 |
1988年 | 5358篇 |
1987年 | 4171篇 |
1986年 | 3990篇 |
1985年 | 4832篇 |
1984年 | 4833篇 |
1983年 | 3446篇 |
1982年 | 4081篇 |
1981年 | 3793篇 |
1980年 | 3693篇 |
1979年 | 3488篇 |
1978年 | 3588篇 |
1977年 | 3640篇 |
1976年 | 3852篇 |
1975年 | 2943篇 |
1974年 | 2830篇 |
1973年 | 3131篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
241.
Erzs bet Ill s Etelka Tomb cz 《Colloids and surfaces. A, Physicochemical and engineering aspects》2003,230(1-3):99-109
The ionic strength dependence of humic acid (HA) adsorption on magnetite (Fe3O4) was investigated at pH 5, 8 and 9, where variable charged magnetite is positive, neutral and negative, respectively. The adsorption studies revealed that HA has high affinity to magnetite surface especially at lower pH, where interacting partners have opposite charges. However, in spite of electrostatic repulsion at pH 9 notable amounts of humate are adsorbed. Increasing ionic strength enhances HA adsorption at each pH due to charge screening. The dominant interaction is probably a ligand-exchange reaction, nevertheless the Coulombic contribution to the organic matter accumulation on oxide surface is also significant under acidic condition. The results from size exclusion chromatography demonstrate that the smaller size HA fractions enriched with functional groups are adsorbed preferentially on the surface of magnetite at pH 8 in dilute NaCl solution. 相似文献
242.
243.
Manfred Brun Peter Hubner Dieter Oelkrug 《Fresenius' Journal of Analytical Chemistry》1992,344(4-5):209-213
Summary A theoretical model is derived that describes the influence of lateral light diffusion in a scattering medium on the absorptivity of an absorber spot on top of the substrate. The model uses the lateral resolved reflectivity under point irradiation that has been analyzed experimentally with a scanning-micro-laser-reflectometer. The model allows quantification of the absorptivity by one single equation that contains only the mean radial diffusion length of light and the spot area. Experiment and theory are applied to typical substrates for thin layer chromatography (alumina, silica, cellulose). The diffusion lengths in these substrates are given and the absorptivities of the spots are calculated as a function of the spot area. 相似文献
244.
245.
The cellular neural network (CNN) architecture combines the best features from traditional fully-connected analog neural networks and digital cellular automata. The network can rapidly process continuous-valued (gray-scale) input signals (such as images) and perform many computation functions which traditionally were implemented in digital form. Here, we briefly introduce the the theory of CNN circuits, provide some examples of CNN applications to image processing, and discuss work toward a CNN implementation in custom CMOS VLSI. The role of analog computer-aided design (CAD) will be briefly presented as it relates to analog neural network implementation.This work is supported in part by the Office of Naval Research under Contract N00014-89-J1402, and the National Science Foundation under grant MIP-8912639. 相似文献
246.
Edison Munaf Toyohide Takeuchi Hiroki Haraguchi 《Fresenius' Journal of Analytical Chemistry》1992,342(1-2):154-156
Summary A continuous flow analysis is described for the determination of total mercury by cold vapor atomic absorption spectrometry. Organic mercury compounds such as methylmercury(II) chloride, ethylmercury(II) chloride and phenylmercury(II) chloride were decomposed by potassium peroxodisulphate with addition of ferric chloride as catalytic reagent. The reducing reagent used was tin(II) chloride in sodium hydroxide solution. With 1,000 mg Fe/l added in the decomposition process, we found that methylmercury(II) chloride and ethylmercury(II) chloride gave response signals similar to those of mercury(II) chloride. The proposed method was applied to the analysis of total mercury in waste water.
Permanent address: Department of Chemistry, Faculty of Mathematics and Natural Sciences, Andalas University, Padang, West Sumatra, Indonesia 相似文献
247.
A finite element solution of the Navier-Stokes equations for steady flow under the magnetic effect through a double-branched two-dimensional section of a three-dimensional model of the canine aorta is discussed. The numerical scheme involves transforming the physical co-ordinates to a curvilinear boundary-fitted co-ordinate system. The shear stress at the wall is calculated for a Reynolds number of 1000 with the branch-to-main aortic flow rate ratio as a parameter. The results are compared with earlier works involving experimental data and found to be in reasonable qualitative agreement. The steady flow, shear stress and branch flow under the effect of a magnetic field have been discussed in detail. 相似文献
248.
Yan R.-H. Lee K.F. Jeon D.Y. Kim Y.O. Park B.G. Pinto M.R. Rafferty C.S. Tennant D.M. Westerwick E.H. Chin G.M. Morris M.D. Early K. Mulgrew P. Mansfield W.M. Watts R.K. Voshchenkov A.M. Bokor J. Swartz R.G. Ourmazd A. 《Electron Device Letters, IEEE》1992,13(5):256-258
The authors report the implementation of deep-submicrometer Si MOSFETs that at room temperature have a unity-current-gain cutoff frequency (f T) of 89 GHz, for a drain-to-source bias of 1.5 V, a gate-to-source bias of 1 V, a gate oxide thickness of 40 Å, and a channel length of 0.15 μm. The fabrication procedure is mostly conventional, except for the e-beam defined gates. The speed performance is achieved through an intrinsic transit time of only 1.8 ps across the active device region 相似文献
249.
Trapping of net positive charge at low gate stress voltage, and of net negative charge at high gate stress voltage, is observed through changes in the gate-to-drain capacitance of the stressed junction. These observations can be explained in terms of electron trapping, hole trapping, and generation of acceptor-like interface states located in the upper half of the bandgap. Channel shortening is also observed and found to exhibit a logarithmic time dependence 相似文献
250.
Martin A. Bos Bertus Dunnewind Ton van Vliet 《Colloids and surfaces. B, Biointerfaces》2003,31(1-4):95-105
Interfacial rheological properties and their suitability for foam production and stability of two vegetable proteins were studied and compared to β-casein. Proteins used ranged from flexible to rigid/globular in the order of β-casein, gliadin and soy glycinin. Experiments were performed at pH 6.7. Network forming properties were characterised by the surface dilational modulus (determined with the ring trough) and the critical falling film length (Lstill) at which a stagnant protein film will break. Gliadin had the highest dilational modulus, followed by glycinin and β-casein, whereas glycinin formed the strongest film against fracture in the overflowing cylinder. The rate of decrease in the surface tension was studied at the air–water (Wilhelmy plate method) and the oil–water interface (bursting membrane) and the dynamic surface tension during compression and expansion in the caterpillar. Gliadin had the lowest equilibrium interfacial tensions and β-casein the lowest dynamic surface tension during expansion. Hardly any foam could be formed at a concentration of 0.1 g/l by shaking. At a concentration of 1.4 g/l most foam was formed by β-casein, followed by gliadin and glycinin. It seems that in the first place the rate of adsorption is important for foam formation. For the vegetable proteins, adsorption was slow. This resulted in lower foamability, especially for glycinin. 相似文献