全文获取类型
收费全文 | 2439篇 |
免费 | 80篇 |
国内免费 | 25篇 |
专业分类
化学 | 1625篇 |
晶体学 | 9篇 |
力学 | 54篇 |
数学 | 496篇 |
物理学 | 360篇 |
出版年
2023年 | 20篇 |
2022年 | 67篇 |
2021年 | 61篇 |
2020年 | 59篇 |
2019年 | 52篇 |
2018年 | 57篇 |
2017年 | 52篇 |
2016年 | 94篇 |
2015年 | 62篇 |
2014年 | 93篇 |
2013年 | 179篇 |
2012年 | 165篇 |
2011年 | 189篇 |
2010年 | 136篇 |
2009年 | 105篇 |
2008年 | 147篇 |
2007年 | 150篇 |
2006年 | 136篇 |
2005年 | 117篇 |
2004年 | 87篇 |
2003年 | 71篇 |
2002年 | 67篇 |
2001年 | 29篇 |
2000年 | 29篇 |
1999年 | 12篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 28篇 |
1995年 | 11篇 |
1994年 | 13篇 |
1993年 | 15篇 |
1992年 | 11篇 |
1991年 | 18篇 |
1990年 | 10篇 |
1989年 | 7篇 |
1988年 | 13篇 |
1987年 | 7篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有2544条查询结果,搜索用时 0 毫秒
1.
The partition coefficient of strontium upon its extraction from the aqueous solutions of picric acid into chloroform containing DC-18-crown-6, B-15-crown-5 after their exposition to gamma radiation has been studied. A significant decrease of the partition coefficient in the studied range of doses 10–70 kGy was observed. This effect can be attributed to the radiolytic products of chloroform. The radiation destruction as well as dehydrogenation of crown ethers were not observed. 相似文献
2.
[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions. 相似文献
3.
Rafael Sorkin 《International Journal of Theoretical Physics》1978,17(5):369-376
It is proved without resort to calculus methods that every continuous group multiplier for R can be reduced to the identity by a continuous remultiplication. The method introduced may generalize to infinitedimensional Abelian groups such as occur in analyzing the projective representations of the Bondi-Metzner-Sachs (BMS) group. 相似文献
4.
Belén Abarca Rafael Ballesteros Gurnos Jones 《Journal of heterocyclic chemistry》1984,21(6):1585-1588
A synthesis of the methylthieno[3,2-c]cyclohepteno[b]indole 7 from 2-methylthieno[3,2-b]cycloheptanone 3 is described. Unsuccessful attempts to prepare the isomeric thienocycloheptenoindole system present in formula 2 , from the dihydrobenzothiophenone 8 , and from derivatives of 5-(2-thienyl)-4-oxopentanoic acid, 17 and 18 , and from N-benzylcycloheptindol-1-one 22 were unsuccessful. The preparation of 4,5-dihydro-2-phenyl-1-thienylmethyl-3H-pyridazin-3-one 20 and of the 5-aminopyrazole 21 are reported. 相似文献
5.
Rafael Rojas-Hernndez Juan Luis Díaz-de-Len-Santiago Grettel Barcel-Alonso Jorge Bautista-Lpez Valentin Trujillo-Mora Julio Csar Salgado-Ramírez 《Entropy (Basel, Switzerland)》2022,24(7)
This paper introduces a new method of compressing digital images by using the Difference Transform applied in medical imaging. The Difference Transform algorithm performs the decorrelation process of image data, and in this way improves the encoding process, achieving a file with a smaller size than the original. The proposed method proves to be competitive and in many cases better than the standards used for medical images such as TIFF or PNG. In addition, the Difference Transform can replace other transforms like Cosine or Wavelet. 相似文献
6.
Different types of nonredundant sampling patterns are shown to guarantee completeness of the basis formed by the sampled partial derivatives of Zernike polynomials, commonly used to reconstruct the wavefront from its slopes (wavefront sensing). In the ideal noise-free case, this enables one to recover double the number of modes J than sampling points I (critical sampling J=2I). With real data, noise amplification makes the optimal number of modes lower I相似文献
7.
The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 μm. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 μm. In straight waveguides more than 90% of the transmission loss is due to scattering. 相似文献
8.
We present results of mathematical relations existing between the Mueller matrix obtained for an in-plane of incidence scattering geometry (plane Mueller matrix, PMM) and the Mueller matrix obtained for an out-the-plane of incidence scattering geometry (conical Mueller matrix, CMM), for light scattered from a rough surface. We obtain a similarity relation between the CMM and the PMM for one- (1-D) and for two-dimensional (2-D) surfaces. This similarity relation implies that the PMM and the CMM have the same determinant, trace and eigenvalues for 1-D and 2-D surfaces, respectively. We can say that measurements made in the conical geometry are “Similarity Equivalent” to those in the in-plane geometry for both kind of surfaces, respectively. 相似文献
9.
10.