全文获取类型
收费全文 | 171164篇 |
免费 | 2318篇 |
国内免费 | 713篇 |
专业分类
化学 | 92168篇 |
晶体学 | 2466篇 |
力学 | 7606篇 |
综合类 | 7篇 |
数学 | 17795篇 |
物理学 | 54153篇 |
出版年
2021年 | 989篇 |
2020年 | 1125篇 |
2019年 | 1133篇 |
2018年 | 1155篇 |
2017年 | 1170篇 |
2016年 | 2340篇 |
2015年 | 1924篇 |
2014年 | 2499篇 |
2013年 | 7714篇 |
2012年 | 6285篇 |
2011年 | 7964篇 |
2010年 | 4898篇 |
2009年 | 5052篇 |
2008年 | 7319篇 |
2007年 | 7303篇 |
2006年 | 7025篇 |
2005年 | 6554篇 |
2004年 | 5868篇 |
2003年 | 5054篇 |
2002年 | 4967篇 |
2001年 | 6032篇 |
2000年 | 4565篇 |
1999年 | 3482篇 |
1998年 | 2633篇 |
1997年 | 2627篇 |
1996年 | 2584篇 |
1995年 | 2288篇 |
1994年 | 2179篇 |
1993年 | 2035篇 |
1992年 | 2349篇 |
1991年 | 2316篇 |
1990年 | 2031篇 |
1989年 | 2010篇 |
1988年 | 2033篇 |
1987年 | 1966篇 |
1986年 | 1880篇 |
1985年 | 2784篇 |
1984年 | 2757篇 |
1983年 | 2214篇 |
1982年 | 2395篇 |
1981年 | 2275篇 |
1980年 | 2231篇 |
1979年 | 2178篇 |
1978年 | 2253篇 |
1977年 | 2209篇 |
1976年 | 2133篇 |
1975年 | 2124篇 |
1974年 | 2030篇 |
1973年 | 2114篇 |
1972年 | 1203篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
891.
A. Ross M. Czisch G.C. King 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,124(2):355-365
A theoretical approach to calculate the time evolution of magnetization during a CPMG pulse sequence of arbitrary parameter settings is developed and verified by experiment. The analysis reveals that off-resonance effects can cause systematic reductions in measured peak amplitudes that commonly lie in the range 5–25%, reaching 50% in unfavorable circumstances. These errors, which are finely dependent upon frequency offset and CPMG parameter settings, are subsequently transferred into erroneousT2values obtained by curve fitting, where they are reduced or amplified depending upon the magnitude of the relaxation time. Subsequent transfer to Lipari–Szabo model analysis can produce significant errors in derived motional parameters, with τeinternal correlation times being affected somewhat more thanS2order parameters. A hazard of this off-resonance phenomenon is its oscillatory nature, so that strongly affected and unaffected signals can be found at various frequencies within a CPMG spectrum. Methods for the reduction of the systematic error are discussed. Relaxation studies on biomolecules, especially at high field strengths, should take account of potential off-resonance contributions. 相似文献
892.
893.
I. Ortalli G. Pedrazzi K. Jiang X. Zhang C. Carlo-Stella V. Rizzoli 《Il Nuovo Cimento D》1992,14(4):351-358
Summary A new approach to cancer treatment using low-energy gamma-rays is discussed. Cultures of bone marrow mononuclear cells collected
from normal donors and patients with chronic myelogenous leukaemia have been studied after the addition of hematin, at different
concentrations, and irradiation by a M?ssbauer gamma-ray source. Growth stimulation has been observed when hematin is added
to normal bone marrow cultures, while growth inhibition is observed when hematin is added to leukaemic cultures. The effect
becomes then more pronounced when hematin is used in combination with M?ssbauer gamma-rays. 相似文献
894.
We show that it is a NP-complete problem to decide whether a finite poset arises as the (Birkhoff) dual of the Frattini sublattice of some finite distributive lattice.This work was supported in part by Swiss NSF grant 20-32644.91. 相似文献
895.
C. A. Beattie M. B. Boisen Jr. L. W. Johnson 《Journal of Optimization Theory and Applications》1991,68(1):203-203
Corrections to the last two equations of Ref. 1 are given. 相似文献
896.
Spectroscopic determinations of the concentrations associated with the onset of intermolecular interaction in solutions of poly(ethylene terephthalate) (PET) are described. The intersegment forces perturb the electronic spectra and induce ground-state aggregation. These phenomena become observable at different chain densities, reflecting the influence of concentration on the coil dimensions, as well as the dynamical nature of the interactions. An estimate of the equilibrium constant for PET dimer formation in hexafluoroisopropanol is reported. 相似文献
897.
J. S. Vrentas D. C. Venerus C. M. Vrentas 《Journal of Polymer Science.Polymer Physics》1991,29(5):537-545
An integral constitutive equation which includes strain-coupling effects is proposed. The constitutive equation is more general than the K-BKZ equation, and it is shown that this equation can explain various important aspects of double-step stress-relaxation data. 相似文献
898.
Five different analytical schemes for examining isothermal and nonisothermal degradation of polymers were reviewed and found inadequate for describing multistage decomposition. The different schemes were experimentally tested using thermogravimetric analysis data for an ethylene-vinyl acetate (EVA) copolymer, which exhibited a well-behaved two-step decomposition process in a nitrogen environment. Based on these experimental and analytical findings, a generalized methodology was developed capable of describing the exhibited complex decomposition behavior of polymers by combining two or more kinetic mechanisms in a series or parallel arrangement. This composite of combinational methodology thus extends established analytical schemes for describing complex decomposition of polymers in a rational manner, incorporating both experimental and theoretical considerations. 相似文献
899.
A E Abdel-Megied P Hansen E B Pedersen C M Nielsen 《Acta chemica Scandinavica (Copenhagen, Denmark : 1989)》1991,45(10):1060-1063
The 3'-iodonucleoside 4 and the 3'-O-methylsulfonylthymidine 9 have been synthesized by condensation of silylated uracils 2 with methyl 5-O-tert-butyldiphensilyl-2,3-dideoxy-3-iodo-D-threo-pentofuran oside (3) and methyl 5-O-tert-butyldiphenylsilyl-2-deoxy-3-methylsulfonyl-D-erythro- pentofuranoside (8), respectively. The nucleoside 4 and 9 produced the corresponding 2',3'-didehydro-2',3'-dideoxynucleosides 5 in an elimination reaction on treatment with sodium methoxide. The compounds 5b showed no antiviral activity against HIV-1. 相似文献
900.
K. Ishikawa H. Nakano H. Masuda C. Kusano Y. Takahashi T. Hamagishi N. Endo 《Fiber and Integrated Optics》1992,11(4):385-394
A 10-Gbit/s optical receiver is developed using low-capacitance HBT IC technology. The HBT allows a cutoff frequency of 45 GHz and a maximum oscillation frequency of 70 GHz. The receiver contains an automatic gain-control amplifier IC with a gain of 16 dB, a bandwidth of 10.7 GHz, and a decision-circuit IC with an ambiguity of 66 mVp-p at a data rate of 10 Gbit/s. The sensitivity of this receiver is -26.5 dBm at a bit error rate of 10-9. Transmission over a 140-km fiber was achieved successfully. 相似文献