全文获取类型
收费全文 | 6645篇 |
免费 | 244篇 |
国内免费 | 39篇 |
专业分类
化学 | 4507篇 |
晶体学 | 36篇 |
力学 | 239篇 |
数学 | 956篇 |
物理学 | 1190篇 |
出版年
2023年 | 57篇 |
2022年 | 133篇 |
2021年 | 177篇 |
2020年 | 158篇 |
2019年 | 152篇 |
2018年 | 126篇 |
2017年 | 105篇 |
2016年 | 265篇 |
2015年 | 195篇 |
2014年 | 244篇 |
2013年 | 409篇 |
2012年 | 453篇 |
2011年 | 544篇 |
2010年 | 358篇 |
2009年 | 323篇 |
2008年 | 478篇 |
2007年 | 413篇 |
2006年 | 345篇 |
2005年 | 341篇 |
2004年 | 307篇 |
2003年 | 209篇 |
2002年 | 220篇 |
2001年 | 90篇 |
2000年 | 81篇 |
1999年 | 55篇 |
1998年 | 54篇 |
1997年 | 49篇 |
1996年 | 63篇 |
1995年 | 42篇 |
1994年 | 45篇 |
1993年 | 31篇 |
1992年 | 51篇 |
1991年 | 38篇 |
1990年 | 42篇 |
1989年 | 22篇 |
1988年 | 29篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 28篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1973年 | 6篇 |
1972年 | 7篇 |
排序方式: 共有6928条查询结果,搜索用时 31 毫秒
31.
In this paper we address a two-dimensional (2D) orthogonal packing problem, where a fixed set of small rectangles has to be placed on a larger stock rectangle in such a way that the amount of trim loss is minimized. The algorithm we propose hybridizes a placement procedure with a genetic algorithm based on random keys. The approach is tested on a set of instances taken from the literature and compared with other approaches. The computation results validate the quality of the solutions and the effectiveness of the proposed algorithm. 相似文献
32.
Alternatives based on polynomial expansions of gaussian basis set exponents are introduced and evaluated. The formulas presented here outperform methods based upon the even-tempered formula or combinations of it. They closely match the performance of other methods based upon larger polynomial expansions of the logarithm of the exponents using the same or one less parameter per orbital angular symmetry. 相似文献
33.
Julio Penin Santos Fernando J.G. Landgraf Geraldo Caixeta Guimarães 《Journal of magnetism and magnetic materials》2006
The paper describes effect of texture on the anisotropy of magnetic properties and shows that an Epstein strip arrangement including strips cut in a direction 45° to the rolling direction gives property values that are closer to the average values than the traditional RD+TD arrangement. 相似文献
34.
Marcos F. de Campos Marília Emura Fernando J.G. Landgraf 《Journal of magnetism and magnetic materials》2006
Electrical steels, when submitted to operation, present continuous decrease of their magnetic properties, depending on the carbon content. This effect is attributed to the increase of the size of carbides, a process also known as coarsening or Ostwald Ripening. Loss separation can offer a better understanding of this phenomenon. Experimental results show that all effect of aging is inside the hysteresis loss component, with the excess losses unaffected. The carbon content in electrical steels should be less than 25 ppm to avoid magnetic aging. 相似文献
35.
Ricardo de Souza Pereira Nivaldo AntÔnio Parizotto Vitor Baranauskas 《Applied biochemistry and biotechnology》1996,59(2):135-143
Different strains of baker’s yeast(Saccharomyces cerevisiae) were imaged with an atomic force microscope (AFM). The images of uncoated and nonfixed samples were reproducible with high-constrast
and nanometer-resolution. Molecules from the polysaccharide surface of the cell wall were pictured and the distance of atoms
was measured. The preparation of samples was easy, suggesting that AFM is a useful tool in this type of analyses. 相似文献
36.
Fernando Torres 《Semigroup Forum》1997,55(3):364-379
This work was realized while the author was ICTP (Trieste - Italy) with a grant from the International Atomic Energy Agency
and UNESCO. 相似文献
37.
By means of exponential laws we show a Bianchi Type I model of the universe where we define overall deceleration and Hubble's parameters that have constant values. We employ a convenient relation for defining the radius of the universe. Pressure and density have constant values, too. 相似文献
38.
G. Carelli N. Ioli A. Moretti D. Pereira F. Strumia R. Densing 《Applied physics. B, Lasers and optics》1988,45(2):97-100
FIR laser lines close to the N+
3
P
2–3
P
1 transition at 122 m can be used as a local oscillator for heterodyne detectors. The frequencies of some candidate laser lines from CH2F2,13CH3OH, CD3OD, and CD3OH have been measured and the output power of the most interesting lines has been optimized. 相似文献
39.
A novel experimental procedure to obtain α-benzyl-β-keto esters from the Heck reaction between arenediazonium tetrafluoroborate salts and Baylis-Hillman (BH) adducts in the presence of Pd(OAc)2 as catalyst is described. The methodology is simple, straightforward and the use of arenediazonium salts as opposed to conventional arylhalides/triflates over the usual Heck reaction is highlighted. 相似文献
40.
Eliana M. Alhadeff Andrea M. Salgado Nei Pereira Jr. Belkis Valdman 《Applied biochemistry and biotechnology》2004,113(1-3):125-136
An automated flow injection analysis (FIA) system for quantifying ethanol was developed using alcohol oxidase, horseradish
peroxidase, 4-aminophenazone, and phenol. A colorimetric detection method was developed using two different methods of analysis,
with free and immobilized enzymes. The system with free enzymes permitted analysis of standard ethanol solution in a range
of 0.05–1.0 g of ethanol/L without external dilution, a sampling frequency of 15 analyses/h, and relative SD of 3.5%. A new
system was designed consisting of a microreactor with a 0.91-mL internal volume filled with alcohol oxidase immobilized on
glass beads and an addition of free peroxidase, adapted in an FIA line, for continued reuse. This integrated biosensor-FIA
system is being used for quality control of biofuels, gasohol, and hydrated ethanol. The FIA system integrated with the microreactor
showed a calibration curve in the range of 0.05–1.5 g of ethanol/L, and good results were obtained compared with the ethanol
content measured by high-performance liquid chromatography and gas chromatography standard methods. 相似文献