全文获取类型
收费全文 | 23801篇 |
免费 | 1729篇 |
国内免费 | 2205篇 |
专业分类
化学 | 23378篇 |
晶体学 | 430篇 |
力学 | 697篇 |
综合类 | 86篇 |
数学 | 149篇 |
物理学 | 2995篇 |
出版年
2024年 | 26篇 |
2023年 | 240篇 |
2022年 | 489篇 |
2021年 | 502篇 |
2020年 | 726篇 |
2019年 | 750篇 |
2018年 | 773篇 |
2017年 | 1033篇 |
2016年 | 1255篇 |
2015年 | 967篇 |
2014年 | 983篇 |
2013年 | 2415篇 |
2012年 | 1546篇 |
2011年 | 1313篇 |
2010年 | 1219篇 |
2009年 | 1445篇 |
2008年 | 1244篇 |
2007年 | 1322篇 |
2006年 | 1142篇 |
2005年 | 955篇 |
2004年 | 861篇 |
2003年 | 745篇 |
2002年 | 611篇 |
2001年 | 537篇 |
2000年 | 540篇 |
1999年 | 464篇 |
1998年 | 433篇 |
1997年 | 384篇 |
1996年 | 364篇 |
1995年 | 380篇 |
1994年 | 304篇 |
1993年 | 245篇 |
1992年 | 201篇 |
1991年 | 163篇 |
1990年 | 159篇 |
1989年 | 142篇 |
1988年 | 168篇 |
1987年 | 175篇 |
1986年 | 103篇 |
1985年 | 86篇 |
1984年 | 94篇 |
1983年 | 24篇 |
1982年 | 63篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 28篇 |
1978年 | 15篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
An analytical method is presented to investigate the bending-torsion vibration characteristics of a cylinder with an arbitrary cross-section and partially submerged in water. The compressibility and the free surface waves of the water are considered simultaneously in the analysis. The exact solution of structure–water interaction is obtained mathematically. Firstly, the analytical expression of the velocity potential of the water is derived by using the method of separation of variables. The unknown coefficients in the velocity potential are determined by the longitudinal and circumferential Fourier expansions along the outer surface of the cylinder and are expressed in the form of integral equations including the unknown dynamic bending deflection and torsional angle of the cylinder. Secondly, the force and torque acting on the cylinder per unit length, provided by the water, are obtained by integrating the water dynamic pressure along the circumference of the cylinder. The general solution of bending-torsion vibration of the cylinder under the water dynamic pressure is derived analytically. The integral equations included in the velocity potential of the water can be solved exactly. Finally, the eigenfrequency equation of cylinder–water interaction is obtained by means of the boundary conditions of the cylinder. Some numerical examples for elliptical columns partially submerged in water are provided to show the application of the present method. 相似文献
72.
73.
Yangyang Jiang Chen Guo Huizhou Liu 《中国颗粒学报》2007,5(1):130-133
A magnetically rotational reactor (MRR) has been developed and used in absorbing benzene emissions. The MRR has a permanent magnet core and uses magnetic ionic liquid [bmim]FeCl4 as absorbent. Benzene emissions were carried by N2 into the MRR and were absorbed by the magnetic ionic liquid. The rotation of the permanent magnet core provided impetus for the agitation of the magnetic ionic liquid, enhancing mass transfer and making benzene better dispersed in the absorbent. 0.68 g benzene emissions could be absorbed by a gram of [bmim]FeCl4, 0.27 and 0.40 g/g higher than that by [bmim]PF6 and [bmim]BF4, respectively. The absorption rate increased with increasing rotation rate of the permanent magnet. 相似文献
74.
Sarah Josefsson Rikard Westbom Lennart Mathiasson Erland Bjrklund 《Analytica chimica acta》2006,560(1-2):94-102
Pressurized liquid extractions were performed on eight sediments in order to investigate if a modified US EPA method (100 °C, 100 bar, n-heptane/acetone (1:1), 2 × 5 min) provided exhaustive extractions of polychlorinated biphenyls (PCBs) from sediment, and to study if the extractability of PCBs from the different sediments was affected by characteristics of the sediment. The recovery from the eight native sediments, contaminated in nature, was between 96.4% and 98.9%, as an average of the recoveries from 10 PCB congeners. Hundred percent recovery was defined as the sum of two consecutive extractions (2 × 5 min each) at the stated conditions. The recoveries of the individual congeners were above 94%, except for one congener in one sediment, which had a recovery of 92%. When the recoveries and different characteristics of the sediments were compared, no correlation appeared between recoveries and sediment PCB concentration, total organic carbon (TOC), soot carbon (SC) or amorphous carbon (AC). The fact that carbon did not influence the extractions was somewhat surprising, since previous experiments have indicated a connection. Instead, statistically significant (p < 0.05) correlations were observed for water content and carbon/nitrogen (C/N) ratio. The decrease in recoveries with decreased water content was attributed to less access of the solvent to the analytes due to less matrix swelling. The lowered recoveries with increased C/N ratio can indicate that a difference in structure of the organic matter exists, which influences the binding strength between the analytes and the matrix. The difference in structure can possibly be explained by different origin of the organic matter or by aging effects. Overall the method was found to be exhaustive and the excellent recoveries show that sediment characteristics do not influence the extractions markedly. 相似文献
75.
《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(13):3419-3419
76.
Fernando M. Lanas Janete H. Y. Vilegas Srgio Martins Elaine A. F. Gobato 《Journal of separation science》1994,17(4):237-244
An overview is presented of the analytical approaches developed by our research group over the last ten years for analysis of alternative fuel, both biomass and fossil. The alternative fuels are analyzed successively by PLC-8 (preparative liquid chromatography–group-type) fractionation and high resolution gas chromatography. Some of the possibilities for fractionation and characterization of alternative fuels are herein exemplified with sugar cane bagasse pyrolysis products. 相似文献
77.
Thermotropic copolyester fibers of oxynaphthoate and oxybenzoate have been subjected to conditions that promote solid-state polymerization as well as annealing. The annealing process causes the crystals to perfect with a simultaneous increase in heat of fusion and melting temperature. Solid-state polymerization, a reaction rate-controlled process, causes the polymer viscosity average molecular weight to increase by chain extension from about 14,000 g/mole to more than 87,000 g/mole with a simultaneous impressive increase in tenacity from about 10 g/d (1.2 GPa) to almost 30 g/d (3.7 GPa). To understand the changes in mechanical properties, we have modeled the fiber structure as short rod-like molecules poorly bonded to a continuous matrix of parallel molecules. Lengthening of the reinforcing molecules facilitates better transfer of load from matrix to molecules, resulting in higher tenacity fibers. © 1994 John Wiley & Sons, Inc. 相似文献
78.
The question about the definition of the “internal pressure” concept is being discussed. It is shown that the previously found differential relation between the cohesion energy density and the internal pressure is one of the examples of an absolutely general interrelation between definitely connected differential functions. It is ascertained that the ratio (the internal pressure divided by the cohesion energy density) is a structuresensitive parameter inherent to the calorific (thermal) equation of a liquid state. 相似文献
79.
A new, high performance liquid chromatography method has been developed for the separation of monovinyl- and divinyl-protochlorophyllides, using commercially available, C30 reverse phase column and isocratic elution. This method can be used both for analytical applications and preparative scale purification of monovinyl- and divinyl-protochlorophyllides using the same column where submilimolar concentrations of the crude protochlorophyllide extract can be separated in one run. The purity of the obtained protochlorophyllides was demonstrated by spectroscopic methods, as well by the formation of aggregates in toluene. 相似文献
80.
We investigated the features of the glass transition relaxation of two room temperature ionic liquids using DSC. An important
observation was that the heat capacity jump, that is the signature of the glass transition relaxation, shows a particularly
strong value in this type of new and promising materials, candidates for a range of applications. This suggests a high degree
of molecular mobility in the supercooled liquid state. The study of the influence of the heating rate on the temperature location
of the glass transition signal, allowed the determination of the activation energy at the glass transition temperature, and
the calculation of the fragility index of these two ionic glass-formers. It was concluded that this kind of materials belong
to the class of relatively strong glass-forming systems.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献