全文获取类型
收费全文 | 1278篇 |
免费 | 18篇 |
国内免费 | 3篇 |
专业分类
化学 | 645篇 |
晶体学 | 33篇 |
力学 | 30篇 |
数学 | 159篇 |
物理学 | 432篇 |
出版年
2021年 | 9篇 |
2020年 | 15篇 |
2019年 | 10篇 |
2018年 | 8篇 |
2016年 | 14篇 |
2015年 | 14篇 |
2014年 | 17篇 |
2013年 | 62篇 |
2012年 | 36篇 |
2011年 | 53篇 |
2010年 | 31篇 |
2009年 | 18篇 |
2008年 | 39篇 |
2007年 | 58篇 |
2006年 | 42篇 |
2005年 | 44篇 |
2004年 | 40篇 |
2003年 | 42篇 |
2002年 | 34篇 |
2001年 | 38篇 |
2000年 | 33篇 |
1999年 | 22篇 |
1998年 | 17篇 |
1997年 | 32篇 |
1996年 | 33篇 |
1995年 | 19篇 |
1994年 | 42篇 |
1993年 | 40篇 |
1992年 | 27篇 |
1991年 | 42篇 |
1990年 | 23篇 |
1989年 | 15篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 20篇 |
1985年 | 20篇 |
1984年 | 32篇 |
1983年 | 13篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1979年 | 16篇 |
1978年 | 9篇 |
1977年 | 15篇 |
1976年 | 9篇 |
1975年 | 14篇 |
1974年 | 8篇 |
1973年 | 18篇 |
1972年 | 7篇 |
1937年 | 6篇 |
排序方式: 共有1299条查询结果,搜索用时 15 毫秒
101.
The difluoromethylene analogue of aspartyl phosphate 6 has been prepared by the fluoride catalysed coupling of diethyl trimethylsilyldifluoromethyl phosphonate with an appropriate aldehyde followed by Dess-Martin oxidation and deprotection; the deprotected compound inhibited (KI 95 microM) aspartate semi-aldehyde dehydrogenase, a key enzyme involved in bacterial amino acid and peptidoglycan biosynthesis. 相似文献
102.
103.
We study the syzygies of a codimension two ideal . Our main result is that the module of syzygies vanishing (scheme-theoretically) at the zero locus is generated by the Koszul syzygies iff is a local complete intersection. The proof uses a characterization of complete intersections due to Herzog. When is saturated, we relate our theorem to results of Weyman and Simis and Vasconcelos. We conclude with an example of how our theorem fails for four generated local complete intersections in and we discuss generalizations to higher dimensions.
104.
In this work we investigated the relative merits of conventional single-photon confocal laser scanning fluorescence microscopy (CLSM) and two-photon laser scanning fluorescence microscopy (2p-LSM) for the study of mitochondria in living neurons. Dorsal root ganglion neurons were loaded with the mitochondrion-specific fluorescent dye JC-1, the ratio between red (J-aggregates) and green (monomer) fluorescence of which reflects mitochondrial membrane potential. Cells were illuminated at 488 nm for single-photon excitation or at 870 nm for two-photon excitation. In both modalities we found that mitochondria showed: (i) similar appearance; (ii) similar fluorescence ratio values over both whole cell bodies and individual mitochondria; and (iii) similar responses to mitochondrial uncoupler, which dropped the ratio values by 50%. However, 2p-LSM exhibited several advantages over CLSM: (i) better signal/noise ratio in the green emission channel; (ii) less phototoxicity upon repetitive scanning in the focal plane; and (iii) no significant loss of image quality upon repetitive scans in the z direction. We conclude that, while both techniques enable visualisation of individual mitochondria in living cells, 2p-LSM has significant advantages for physiological work requiring time-lapse experiments or four-dimensional reconstructions of mitochondria. 相似文献
105.
This paper describes a technique to develop high-resolution three-dimensional (3D) images of microvasculature structures in curettage, hysterectomy or endometrial resection biopsies using parallel histological serial sections. Employing a labelled streptavidin-biotin-alkaline phosphatase (LSAB(+)) method and visualising by using DAB(+) with the primary antibody, mouse anti human Q-Bend-10, the images were directly digitised from a light microscope into the KS400 Universal Image Processing and Analysis software via a CCD colour camera; binary images of the structures were created and the binary images were exported into VoxBlast 3D rendering software to view still and rotating 3D images on a computer monitor. This in turn enabled hard copies of the full sequence to be printed. 相似文献
106.
Supershell structure in alkali metal nanowires 总被引:1,自引:0,他引:1
Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure. 相似文献
107.
High resolution x-ray powder diffraction measurements on poled PbZr1-xTixO3 (PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the so-called morphotropic phase boundary) have shown that for both rhombohedral and tetragonal compositions the piezoelectric elongation of the unit cell does not occur along the polar directions but along those directions associated with the monoclinic distortion. This work provides the first direct evidence for the origin of the very high piezoelectricity in PZT. 相似文献
108.
Karyne M. Rogers Kerry Somerton Pamela Rogers Julie Cox 《Rapid communications in mass spectrometry : RCM》2010,24(16):2370-2374
Carbon isotope analyses (δ13C) of some New Zealand Manuka honeys show that they often fail the internationally recognised Association of Official Analytical Chemists sugar test (AOAC method 998.12) which detects added C4 sugar, although these honeys are from unadulterated sources. Failure of these high value products is detrimental to the New Zealand honey industry, not only in lost export revenue, but also in brand and market reputation damage. The standard AOAC test compares the carbon isotope value of the whole honey and corresponding protein isolated from the same honey. Differences between whole honey and protein δ13C values should not be greater than +1.0‰, as it indicates the possibility of adulteration with syrups or sugars from C4 plants such as high fructose corn syrup or cane sugar. We have determined that during the standard AOAC method, pollen and other insoluble components are isolated with the flocculated protein. These non‐protein components have isotope values which are considerably different from those of the pure protein, and can shift the apparent δ13C value of protein further away from the δ13C value of the whole honey, giving a false positive result for added C4 sugar. To eliminate a false positive C4 sugar test for Manuka honey, prior removal of pollen and other insoluble material from the honey is necessary to ensure that only the pure protein is isolated. This will enable a true comparison between whole honey and protein δ13C isotopes. Furthermore, we strongly suggest this modification to the AOAC method be universally adopted for all honey C4 sugar tests. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
109.
Finite monodisperse two-dimensional clusters of bubbles are shown to behave like a crystal rather than a liquid. They attain their lowest-energy configuration when they find an arrangement close to a hexagonal lattice, as predicted by the Wulff construction. The deviation from hexagonal shape of the bubbles in a cluster and the variation in bubble pressure throughout the structure are calculated. It is shown that the effect of reducing the length of the boundary is negligible, explaining why configurations retaining the structure of the lattice will give the lowest energy. 相似文献
110.
The energy in a string subject to positive viscous damping is known to decay exponentially in time. Under the assumption that the damping is of bounded variation, we identify the best rate of decay with the supremum of the real part of the spectrum of the infinitesimal generator of the underlying semigroup. We analyze the spectrum of this nonselfadjoint operator in some detail. Our bounds on its real eigenvalues and asymptotic form of its large eigenvalues translate into criteria for over/underdamping and a proof that the decay rate achieves its (negative) minimum over those dampings whose total variation does not exceed a prescribed value. 相似文献