全文获取类型
收费全文 | 431篇 |
免费 | 14篇 |
国内免费 | 5篇 |
专业分类
化学 | 234篇 |
晶体学 | 7篇 |
力学 | 10篇 |
数学 | 45篇 |
物理学 | 60篇 |
无线电 | 94篇 |
出版年
2021年 | 8篇 |
2020年 | 6篇 |
2019年 | 4篇 |
2017年 | 5篇 |
2016年 | 3篇 |
2015年 | 8篇 |
2014年 | 8篇 |
2013年 | 11篇 |
2012年 | 16篇 |
2011年 | 13篇 |
2010年 | 6篇 |
2009年 | 9篇 |
2008年 | 10篇 |
2007年 | 16篇 |
2006年 | 13篇 |
2005年 | 8篇 |
2004年 | 14篇 |
2003年 | 19篇 |
2002年 | 18篇 |
2001年 | 13篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 10篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 13篇 |
1993年 | 6篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 16篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1972年 | 4篇 |
1971年 | 3篇 |
1970年 | 4篇 |
1920年 | 2篇 |
1916年 | 2篇 |
1914年 | 2篇 |
排序方式: 共有450条查询结果,搜索用时 31 毫秒
31.
32.
An improved design of the transport detector is described that uses a pre-oxidized titanium ribbon as the transport medium. The titanium ribbon has a high loading capacity that permits a large proportion of the total column eluent to be taken into the sensing system. The solute is sensed by pyrolysis and the subsequent detection of pyrolysis products by a miniature argon detector. The pyrolyzer and sensor system is designed to ensure that all the pyrolysis products enter the detector with minimum dilution and band dispersion. As a result, the sensitivity of the detector (or minimum detectable concentration) has been reduced by approximately two orders of magnitude compared with the original design. The sensitivity of the system described to sucrose is 8 x 10(-8) g/mL, which is similar to the sensitivity of the fixed-wavelength UV detector to benzene (approximately 5 x 10(-8) g/mL). It would appear that the new design has potentially a sensitivity at least an order of magnitude lower than that reported here. 相似文献
33.
Optical directional couplers with longitudinal periodic perturbations or gratings are analyzed by a multiple-scale solution to the coupled-mode equations. The use of two length scales in the analysis becomes the key to obtaining globally valid analytic solutions, which are shown to be in excellent agreement with the exact numerical solutions. Because of the nonorthogonality of the coupled modes in the structure, two different coupling lengths, a maximum power transfer length and a zero crosstalk length, are predicted for the power exchange. The spectral properties of the coupler are also studied through a generalized multiple scale method 相似文献
34.
EFFECTS OF FLUENCE FRACTIONATION ON UV-INDUCED MALIGNANT TRANSFORMATION AND CELL KILLING IN NON-CYCLING PLATEAU PHASE MOUSE CELLS 总被引:1,自引:0,他引:1
Plateau phase C3H 10T 1/2 mouse cells were used to measure the response to split fluence UV light irradiation in the absence of any cell cycle effects. It was found that fluence fractionation with up to 24 h between the fluences resulted in no survival enhancement. The frequency of malignant transformation was potentiated 2.5-fold when the time interval between the fluences was greater than 4h. This potentiation of transformation was attributed to plateau phase holding rather than to fluence fractionation per se. 相似文献
35.
Adjustable point-contact Josephson junctions have been used as self-oscillating mixers at 34 GHz, with an intermediate frequency of 300 MHz. The average conversion loss was about 18 dB, but mismatch losses and waveguide losses may amount to 10 dB at present. 相似文献
36.
R. Daniel Little George W. Muller Manuel G. Venegas Gary L. Carroll Ahmed Bukhari Larry Patton Keith Stone 《Tetrahedron》1981,37(25):4371-4383
A comparison of the inter- and intramolecular diyl trapping routes to linearly fused tricyclopentanoids is presented. In addition, several of the factors which are responsible for the stereoselectivity which is associated with the intramolecular process are examined and it is concluded that conformational rather than electronic (secondary orbital) factors play the dominant role. It is shown that gem methyl groups located on the acyclic chain which joins the diyl and diylophile (in reference to 32 and 35, but not to 47) have no practical effect upon the outcome of the trapping reaction. The intramolecular process is stereospecific with respect to diylophile geometry, and highly stereoselective with respect to the ring junction stereochemistry. Finally, an abortive attempt to synthesize the marine natural product Δ9(12)-capnellene (19) as well as a successful synthesis of the mold metabolite d,l- hirsutene (18) is presented. 相似文献
37.
Lagoja IM Pochet S Boudou V Little R Lescrinier E Rozenski J Herdewijn P 《The Journal of organic chemistry》2003,68(5):1867-1871
The synthesis of fully [13C/15N] labeled pyrimidine nucleosides has been achieved from 13C-glucose and labeled nucleobases. The reaction scheme leads directly to the protected nucleosides without the need for the inversion of configuration of C-3 of 13C-glucose. This was achieved by an oxitative ring-opening reaction removing the carbon with the wrong configuration. 相似文献
38.
Wavelength conversion in GaAs micro-ring resonators 总被引:2,自引:0,他引:2
Absil PP Hryniewicz JV Little BE Cho PS Wilson RA Joneckis LG Ho PT 《Optics letters》2000,25(8):554-556
Tightly confined, low-loss waveguides in highly nonlinear materials permit nonlinear optical interactions to occur over much shorter distances than do fibers. The nonlinear interactions are further enhanced in resonators. Both theory and experiment of enhanced four-wave mixing in micro-ring resonators are presented that can be used for many applications. A conversion efficiency of 14% achievable with only 10-mW peak pump power is predicted under realizable conditions. The experiment, the first one to the authors' knowledge in nonlinear optics performed in micro-rings, shows, even in a lossy GaAs/AlGaAs ring, a 26-dB improvement in the conversion efficiency compared with that of an equivalent straight waveguide, in agreement with theory. 相似文献
39.
Filter synthesis for periodically coupled microring resonators 总被引:3,自引:0,他引:3
The theory of filter synthesis using periodically coupled microring resonators is developed as a means to overcome the fabrication sensitivities inherent in conventional higher-order filters, while still achieving desirable spectral characteristics. Each resonator in the array can compensate for deficiencies in any of the others. These filters exhibit a boxlike shape and very high extinction ratios. 相似文献
40.
C.J. Tun C.H. Kuo Y.K. Fu C.W. Kuo Mitch M.C. Chou G.C. Chi 《Journal of Crystal Growth》2009,311(14):3726-3730
This study demonstrates a pure c-plane AlGaN epilayer grown on a γ-LiAlO2 (1 0 0) (LAO) substrate with an AlN nucleation layer grown at a relatively low temperature (LT-AlN) by metal-organic chemical vapor deposition (MOCVD). The AlGaN film forms polycrystalline film with m- and c-plane when the nucleation layer grows at a temperature ranging from 660 to 680 °C. However, a pure c-plane AlGaN film with an Al content of approximately 20% can be obtained by increasing the LT-AlN nucleation layer growth temperature to 700 °C. This is because the nuclei density of AlN increases as the growth temperature increases, and a higher nuclei density of AlN deposited on LAO substrate helps prevent the deposition of m-plane AlGaN. Therefore, high-quality and crack-free AlGaN films can be obtained with a (0 0 0 2) ω-rocking curve FWHM of 547 arcsec and surface roughness of 0.79 nm (root-mean-square) using a 700-°C-grown LT-AlN nucleation layer. 相似文献