全文获取类型
收费全文 | 3079篇 |
免费 | 97篇 |
国内免费 | 10篇 |
专业分类
化学 | 2248篇 |
晶体学 | 30篇 |
力学 | 52篇 |
数学 | 405篇 |
物理学 | 451篇 |
出版年
2022年 | 18篇 |
2021年 | 31篇 |
2020年 | 47篇 |
2019年 | 48篇 |
2018年 | 24篇 |
2017年 | 32篇 |
2016年 | 66篇 |
2015年 | 59篇 |
2014年 | 67篇 |
2013年 | 116篇 |
2012年 | 166篇 |
2011年 | 161篇 |
2010年 | 100篇 |
2009年 | 81篇 |
2008年 | 115篇 |
2007年 | 144篇 |
2006年 | 164篇 |
2005年 | 158篇 |
2004年 | 131篇 |
2003年 | 109篇 |
2002年 | 112篇 |
2001年 | 43篇 |
2000年 | 19篇 |
1998年 | 31篇 |
1997年 | 38篇 |
1996年 | 39篇 |
1995年 | 26篇 |
1994年 | 30篇 |
1993年 | 42篇 |
1992年 | 43篇 |
1991年 | 32篇 |
1990年 | 34篇 |
1989年 | 42篇 |
1988年 | 37篇 |
1987年 | 23篇 |
1986年 | 30篇 |
1985年 | 57篇 |
1984年 | 39篇 |
1983年 | 38篇 |
1982年 | 50篇 |
1981年 | 54篇 |
1980年 | 52篇 |
1979年 | 57篇 |
1978年 | 65篇 |
1977年 | 34篇 |
1976年 | 56篇 |
1975年 | 33篇 |
1974年 | 28篇 |
1973年 | 35篇 |
1967年 | 17篇 |
排序方式: 共有3186条查询结果,搜索用时 79 毫秒
131.
Abstract— The kinetic factors which determine the rate at which Na+ channels in nerve membranes become photochemically modified were studied on giant axons from lobsters using the double sucrose gap voltage clamp technique. Axons were bathed in artificial sea water containing sensitizing dyes and illuminated from a Xe are source with light in the visible region while being repetitively step depolarized. Successive values of peak Na+ current and time-to-peak were monitored and rate constants for their change served as the assay for magnitude of modification. Action spectra for four sensitizers in the fluorescein series exhibited red shifts of roughly 17nm demonstrating that sensitizing species are not simply free in solution. Eosin Y diffuses to its sensitization sites with a half time of 70s indicating the existence of a major diffusion barrier which may mean that dye must penetrate to the interior of the membrane to be effective. Eosin Y is removed from sensitization sites by rinse with the same half time but shows two fractions: a faster fraction comprising 80% of sensitizing effectiveness and a slower fraction comprising 20%. The concentration dependence for Eosin Y is linear below 10 μ M and shows a progressive saturation at higher values, where the relationship is difficult to determine because of shielding. Different sensitizers vary in their ability to sensitize block of channels vs disruption of inactivation, demonstrating separate processes for the two modifications. It is suggested that both modifications proceed from single photon absorption events by individual sensitizer molecules bound or located close to the modification sites on the channels. 相似文献
132.
CELL MEMBRANE PHOTOMODIFICATION: RELATIVE EFFECTIVENESS OF HALOGENATED FLUORESCEINS FOR PHOTOHEMOLYSIS 总被引:1,自引:0,他引:1
Abstract— Relative potency of 10 fluorescein derivatives as sensitizers of delayed photohemolysis of human erythrocytes has been assessed. Dilute suspensions of washed cells were illuminated in the presence of sensitizer at different concentrations for 1 h and analyzed for percent hemolysis following 23 h of dark incubation. Plots of percent hemolysis versus concentration showed a steep dependence on concentration for all sensitizers. Additional measurements of octanol/water partition coefficients, photon absorption in octanol and in saline and photobleach rates were made. After correction for absorption cross section, the effectiveness values ranged over more than three orders of magnitude with fluorescein being the least potent and rose bengal the most. A reasonable prediction of potency is obtained by taking the product of partition coefficient, relative absorption in octanol versus water and molecular weight of the substituents added to the fluorescein skeleton. The results suggest that the influence of halogen substitution on sensitizing potency is exerted by four factors; (1) distribution of sensitizer into a low polarity region of the cell membrane, (2) absorption efficiency in a low polarity region, (3) triplet quantum yield, and (4) photobleach rate. 相似文献
133.
Fiedler D Leung DH Bergman RG Raymond KN 《Journal of the American Chemical Society》2004,126(12):3674-3675
A supramolecular metal-ligand assembly encapsulates a variety of cationic half-sandwich ruthenium complexes. Due to the chirality of both host and guest, chiral recognition is observed with diastereomeric excesses up to 70%. The chiral cavity can be used to carry out a dynamic resolution of the rapidly equilibrating enantiomers of the chiral organometallic guest. 相似文献
134.
Rachael Padman Dennis Bly 《International Journal of Infrared and Millimeter Waves》1992,13(10):1471-1485
High resolution millimetre and submillimetre wave astronomical spectrometers using hot-electron bolometer mixers as detectors often show marked standing wave patterns in the spectral baseline. LO phase noise contributes through two mechanisms: the phase noise side-bands may be converted to amplitude noise in the source because of the power-frequency characteristic of the source, or they can be converted to in-band amplitude noise through the action of the quasi-optical discriminator formed by the mixer, beam-splitter and telescope structure. The baseline ripple components due to each of these mechanisms have different characteristic periods, and under some circumstances can dominate the spectrometer baseline. The ripple levels estimated using the theory agree well with those observed in practice. It is shown that with careful design systematic effects due to this cause can be reduced to a negligible level. 相似文献
135.
We have experimentally investigated a basic ring laser gyroscope (RLG) with carbon dioxide gain, and studied the prospects of developing a practical CO2 RLG. Rotation sensing was demonstrated on a number of transitions in the 9.4 m and 10.4 m vibration-rotation bands. Gyro response is discussed with regard to lock-in, bias, homogeneous broadening effects, and high power operation. We show that such a system may offer important advantages over standard helium-neon RLGs, including reduced quantum limit and backscattering. The prospects and possible approaches for developing a practical high power CO2 RLG are discussed, and a method of eliminating cross-saturation at high pressure is proposed and analyzed. 相似文献
136.
137.
Chen JP Meziani ZE Beck D Boyd G Chinitz LM Day DB Dennis LC Dodge G Filippone BW Giovanetti KL Jourdan J Kemper KW Koh T Lorenzon W McCarthy JS McKeown RD Milner RG Minehart RC Morgenstern J Mougey J Potterveld DH Rondon-Aramayo OA Sealock RM Smith LC Thornton ST Walker RC Woodward C 《Physical review letters》1991,66(10):1283-1286
138.
The solvolysis rates and products of several 7-anti-substituted 2-endo-norbornyl p-toluenesulfonates 11 have been determined and compared with those of the previously reported 6-exo-substituted 2-exo-norbornyl p-toluenesulfonates 1. Although the number of bonds between the substituent and the reaction site is the same in the two series, the inductive effect of the substitutents is transmitted far more strongly in the 6-exo-2-exo-series 1 than in the 7-anti-2-endo-series 11 ; i.e. their inductivities differ widely. It is concluded that through space induction involves graded bridging of the substituent-bearing C-atom to the incipient cationic center at C(2) and that this involves differential bridging strain. The different reactivities of unsubstituted 2-exo- and 2-endo-norbornyl derivatives can then be ascribed to a stereoelectronic effect. 相似文献
139.
F. Munari A. Trisciani G. Mapelli S. Trestianu K. Grob J. M. Colin 《Journal of separation science》1985,8(9):601-606
Two-dimensional chromatography of gasoline by on-line coupled HPLC-HRGC, as described in this paper, allows separate GC analysis of paraffins and aromatics. The GC system contains a retention gap of only 10 m length for introducing HPLC fractions of 100 μl volume. This becomes possible through evaporation of part of the solvent during introduction of the HPLC eluent. This “partially concurrent solvent evaporation” technique allows transfer of large volumes of HPLC eluent into relatively short retention gaps, maintaining the full efficiency of the solvent effects in reconcentrating the bands of the early eluted solutes. 相似文献
140.
A nano-electrospray ionisation source has been designed and constructed for a high temperature ion mobility spectrometer. The drift cell was modified by replacement of the 63Ni atmospheric pressure chemical ionisation source with a tube lens/desolvation region and operated using commercial nano-electrospray capillaries. Ions were introduced into the drift region via a Bradbury-Nielson gate (pulse width 50 micros, repetition period 20 ms). A unidirectional flow of nitrogen was used as the drift gas at temperatures in the range 100-150 degrees C to aid desolvation. The performance of the nano-electrospray ion source has been demonstrated for analytes including crown ethers, amino acids and peptides. Reduced mobilities determined by nano-ESI were consistent with those reported using a 63Ni ion source. 相似文献