全文获取类型
收费全文 | 12565篇 |
免费 | 1286篇 |
国内免费 | 1165篇 |
专业分类
化学 | 10061篇 |
晶体学 | 170篇 |
力学 | 395篇 |
综合类 | 91篇 |
数学 | 1257篇 |
物理学 | 3042篇 |
出版年
2023年 | 105篇 |
2022年 | 189篇 |
2021年 | 330篇 |
2020年 | 332篇 |
2019年 | 354篇 |
2018年 | 286篇 |
2017年 | 309篇 |
2016年 | 530篇 |
2015年 | 511篇 |
2014年 | 611篇 |
2013年 | 845篇 |
2012年 | 997篇 |
2011年 | 1156篇 |
2010年 | 784篇 |
2009年 | 719篇 |
2008年 | 938篇 |
2007年 | 772篇 |
2006年 | 744篇 |
2005年 | 633篇 |
2004年 | 539篇 |
2003年 | 482篇 |
2002年 | 560篇 |
2001年 | 355篇 |
2000年 | 291篇 |
1999年 | 231篇 |
1998年 | 156篇 |
1997年 | 145篇 |
1996年 | 108篇 |
1995年 | 96篇 |
1994年 | 70篇 |
1993年 | 72篇 |
1992年 | 78篇 |
1991年 | 67篇 |
1990年 | 57篇 |
1989年 | 50篇 |
1988年 | 43篇 |
1987年 | 35篇 |
1986年 | 35篇 |
1985年 | 47篇 |
1984年 | 30篇 |
1983年 | 27篇 |
1982年 | 33篇 |
1981年 | 19篇 |
1980年 | 20篇 |
1979年 | 17篇 |
1978年 | 23篇 |
1977年 | 26篇 |
1976年 | 14篇 |
1975年 | 15篇 |
1970年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
The image depth of integral imaging is enhanced by doubling the number of central depth planes by use of optical path control. To accomplish this, the optical path lengths are changed by controlling whether reflections occur behind the lens array. We propose three schemes that use mirrors, a combination of beam splitters and polarizers, and polarization beam splitters, respectively. In experiments we implement the systems that are completely electronically controllable, are compact, and provide two central depth planes with 50.4-mm separation. 相似文献
962.
Endoscopes employing a single optical fiber may have advantages over conventional fiber-bundle or CCD array imaging techniques, including the potential for greater flexibility and miniaturization. Although single-mode fibers can provide superior resolution compared with multimode fibers, they are prone to increased speckle noise and suffer from limited optical throughput and reduced depth of field. We demonstrate the use of a double-clad fiber for single-mode illumination and multimode detection to achieve high-resolution, reduced-speckle imaging with high optical throughput and a large depth of field. 相似文献
963.
Nassif N Cense B Park BH Yun SH Chen TC Bouma BE Tearney GJ de Boer JF 《Optics letters》2004,29(5):480-482
An ultrahigh-speed spectral domain optical coherence tomography (SD-OCT) system is presented that achieves acquisition rates of 29,300 depth profiles/s. The sensitivity of SD-OCT and time domain OCT (TD-OCT) are experimentally compared, demonstrating a 21.7-dB improvement of SD-OCT over TD-OCT. In vivo images of the human retina are presented, demonstrating the ability to acquire high-quality structural images with an axial resolution of 6 microm at ultrahigh speed and with an ocular exposure level of less than 600 microW. 相似文献
964.
Highly sulfated cyclosophoraoses (HS-Cys) were synthesized by the chemical modification of a family of neutral cyclosophoraoses isolated from Rhizobium leguminosarum bv. trifolii. The HS-Cys were then analytically characterized using Fourier transform-infrared spectroscopy and elemental analysis. These HS-Cys were successfully used as a novel chiral additive, in low-pH aqueous background electrolytes, for capillary electrophoretic separation of five basic chiral drugs such as arterenol, atenolol, isoproterenol, propranolol, and metoprolol. 相似文献
965.
E. A. Kuznetsova M. V. Postnova S. G. Koshel’ N. V. Lebedeva T. A. Yun’kova G. N. Koshel’ 《Russian Journal of Organic Chemistry》2004,40(9):1288-1290
Cyclohexylphenols were converted into hydroxybiphenyls via dehydration over large-scale palladium catalysts. The process involves slow reaction of the substrate with active centers of the catalyst with subsequent fast product formation and hydrogen desorption.Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 9, 2004, pp. 1337–1339.Original Russian Text Copyright © 2004 by Kuznetsova, Postnova, S. Koshel, Lebedeva, Yunkova, G. Koshel. 相似文献
966.
A simple and accurate pulsed-gradient nuclear magnetic resonance technique for measuring coefficients of self-diffusion in liquid crystals is described. It is based on exciting sharp response signals with long weak pulses. The method uses an extremely weak radio-frequency field, which eliminates the problem of radio-frequency heating of the sample. The temperature dependencies of coefficients of self-diffusion for two liquid crystals, 5CB (4-pentyl-4(')-cyanobiphenyl) and EBBA (N-(4-ethoxybenzylidene)-4-butylaniline), are presented. 相似文献
967.
Ashihara S Shimura T Kuroda K Yu NE Kurimura S Kitamura K Ro JH Cha M Taira T 《Optics letters》2003,28(16):1442-1444
Large nonlinear phase shifts were generated with femtosecond pulses at 1560 nm through cascaded quadratic interactions in periodically poled MgO-doped LiNbO3. The off-diagonal component of the nonlinear coefficient was utilized for simultaneous quasi phase matching and group-velocity matching. The effective nonlinear refractive index was varied from -2.9 x 10(-14) to +3.3 x 10(-14) cm2/W by tuning the phase-mismatch conditions. 相似文献
968.
The intracavity laser absorption spectra (ICLAS) of dideuteroacetylene, C2D2, and acetylene, C2H2, have been recorded between 1.03 and 0.99 μm with a vertical external cavity surface emitting laser (VECSEL) leading to the observation of seven and six bands, for C2D2 and C2H2 respectively, most of them newly reported. The strong ν1+3ν3 band of C2D2 at is found accompanied by the two Π-Π hot bands with v4=1 and v5=1 lower state and by the ν2+3ν3+2ν4 band near . This last band results from an intensity transfer from the ν1+3ν3 band induced by the 1/244 anharmonic interaction. The ν1+3ν3 band of , present in natural abundance in the sample, could also be detected at in full agreement with local mode model predictions. The different bands of both C2H2 and C2D2 were found mostly unperturbed and the spectroscopic parameters retrieved from the rovibrational analyses agree satisfactorily with the predictions of the respective effective Hamiltonian models. 相似文献
969.
Optical conductivity spectra sigma(omega) were used to investigate the effect of orbital ordering on the electronic structure of Ca2RuO4. Our LDA+U calculation predicts Ru 4d(xy) ferro-orbital ordering at the ground state, and well explains the present sigma(omega) as well as the reported O 1s x-ray absorption spectra. Variation of temperature (T) causes a large change of spectral weight over several eV as well as collapse of a charge gap accompanied by elongation of the c-axis Ru-O bond length. These results clearly indicate that the d(xy) orbital ordering plays a crucial role in the metal-insulator transition and the T-dependent electronic structure on a large energy scale. 相似文献
970.
Sim TB Kang SH Lee KS Lee WK Yun H Dong Y Ha HJ 《The Journal of organic chemistry》2003,68(1):104-108
5-Funtionalized enantiomerically pure oxazolidin-2-ones were prepared in one pot from commercially available chiral aziridines bearing an electron-withdrawing group at C-2 with retention of the configuration in high yields by regioselective aziridine ring-opening followed by intramolecular cyclization. 相似文献