全文获取类型
收费全文 | 436072篇 |
免费 | 4848篇 |
国内免费 | 1230篇 |
专业分类
化学 | 233784篇 |
晶体学 | 6489篇 |
力学 | 18764篇 |
综合类 | 13篇 |
数学 | 49929篇 |
物理学 | 133171篇 |
出版年
2021年 | 3771篇 |
2020年 | 4186篇 |
2019年 | 4519篇 |
2018年 | 5853篇 |
2017年 | 5744篇 |
2016年 | 8833篇 |
2015年 | 5563篇 |
2014年 | 8543篇 |
2013年 | 20184篇 |
2012年 | 15647篇 |
2011年 | 19268篇 |
2010年 | 13498篇 |
2009年 | 13350篇 |
2008年 | 17648篇 |
2007年 | 17542篇 |
2006年 | 16252篇 |
2005年 | 14552篇 |
2004年 | 13526篇 |
2003年 | 11892篇 |
2002年 | 11728篇 |
2001年 | 13429篇 |
2000年 | 10086篇 |
1999年 | 7916篇 |
1998年 | 6569篇 |
1997年 | 6314篇 |
1996年 | 6081篇 |
1995年 | 5470篇 |
1994年 | 5392篇 |
1993年 | 5197篇 |
1992年 | 5828篇 |
1991年 | 5924篇 |
1990年 | 5664篇 |
1989年 | 5473篇 |
1988年 | 5493篇 |
1987年 | 5374篇 |
1986年 | 5091篇 |
1985年 | 6560篇 |
1984年 | 6694篇 |
1983年 | 5301篇 |
1982年 | 5387篇 |
1981年 | 5367篇 |
1980年 | 4955篇 |
1979年 | 5352篇 |
1978年 | 5491篇 |
1977年 | 5385篇 |
1976年 | 5328篇 |
1975年 | 4914篇 |
1974年 | 4885篇 |
1973年 | 4880篇 |
1972年 | 3455篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
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
912.
A system for display of magnetic resonance (MR) spectroscopic imaging (SI) data is described which provides for efficient review and analysis of the multidimensional spectroscopic and spatial data format of this technique. Features include the rapid display of spectra from selected image voxels, formation of spectroscopic images, spectral and image data processing operations, methods for correlating spectroscopic image data with high resolution 1H MR images, and hardcopy facilities. Examples are shown for 31P and 1H spectroscopic imaging studies obtained in human and rat brain. 相似文献
913.
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
914.
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
915.
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
916.
Mutual precipitates of poly (N, N-dimethyl acrylamide) and poly (4-hydroxystyrene) were collected from dioxane, methanol, or acetone. The glass transition (Tg) temperatures of the precipitates are higher than the weight-average values. Clear films cast from dimethylformamide solutions have lower Tg values. Complexation also occurred between poly (ethyl oxazoline) and poly (4-hydroxystyrene) in dioxane and between poly (vinyl pyrrolidone) and poly (4-hydroxystyrene) in methanol. Again, the glass transition temperatures of the precipitates are higher than the values for the blend films. The ΔCp values associated with the glass transitions of the complexes are smaller than those of the blends having the same compositions. Negative excess heat capacities of mixing have been observed for several precipitates. 相似文献
917.
918.
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
919.
A simple High performance liquid chromatographic (HPLC) method for the specific determination of the molecular weight and concentration of hyaluronic acid (HA) in complex mixtures has been developed. Hyaluronate-binding proteins isolated from bovine cartilage labelled by 125I or fluoresceinisothiocyanate were used as specific markers. The specific binding affinities of the markers were compared and were found to have association constants of 1.6 x 10(7) M-1 and 1.2 x 10(7) M-1 respectively. The HA levels and molecular weight distributions can be easily determined in the range 10-500 ng/mL in complex mixtures by the use of markers, molecular sieving HPLC columns and appropriate detectors. It has been demonstrated clearly that the method is useful for the highly specific determination of the parameters in complex biological samples such as serum and synovial fluids and is recommended for clinical applications. 相似文献
920.
We consider applications of elliptic differential operators and their associated spectral functions in quantum field theory problems. The role of zeta functions and traces of heat kernels in the regularization of Casimir energies is emphasized, and the renormalization procedure is discussed with simple examples. 相似文献