全文获取类型
收费全文 | 2162篇 |
免费 | 39篇 |
国内免费 | 9篇 |
专业分类
化学 | 1471篇 |
晶体学 | 13篇 |
力学 | 65篇 |
数学 | 435篇 |
物理学 | 226篇 |
出版年
2020年 | 17篇 |
2019年 | 26篇 |
2016年 | 47篇 |
2015年 | 24篇 |
2014年 | 42篇 |
2013年 | 112篇 |
2012年 | 83篇 |
2011年 | 83篇 |
2010年 | 66篇 |
2009年 | 52篇 |
2008年 | 101篇 |
2007年 | 90篇 |
2006年 | 84篇 |
2005年 | 98篇 |
2004年 | 72篇 |
2003年 | 56篇 |
2002年 | 53篇 |
2001年 | 42篇 |
2000年 | 39篇 |
1999年 | 23篇 |
1998年 | 24篇 |
1997年 | 20篇 |
1996年 | 30篇 |
1995年 | 23篇 |
1994年 | 22篇 |
1993年 | 26篇 |
1992年 | 24篇 |
1991年 | 19篇 |
1990年 | 24篇 |
1989年 | 31篇 |
1988年 | 36篇 |
1987年 | 35篇 |
1986年 | 23篇 |
1985年 | 36篇 |
1984年 | 29篇 |
1983年 | 31篇 |
1982年 | 38篇 |
1981年 | 37篇 |
1980年 | 29篇 |
1979年 | 34篇 |
1978年 | 24篇 |
1977年 | 31篇 |
1975年 | 21篇 |
1974年 | 29篇 |
1973年 | 24篇 |
1972年 | 14篇 |
1971年 | 14篇 |
1966年 | 25篇 |
1965年 | 52篇 |
1964年 | 27篇 |
排序方式: 共有2210条查询结果,搜索用时 671 毫秒
91.
Electrochemical behavior of the alpha-[SiMo(3)(-)(x)()V(x)()W(9)O(40)]((4+)(x)()())(-) and alpha-[PMo(3)(-)(x)()V(x)()W(9)O(40)]((3+)(x)()())(-) anions with x = 1, 2, or 3 were studied. Electrochemical reduction of each compounds was consistent with its Mo/V ratio, reduction of vanadium and molybdenum atoms occurring in the +0.6 to -0.6 V potential range. The one-electron-reduced species were prepared by electrolysis and then characterized by ESR spectroscopy. The g and A values for V(4+)ions appeared to depend on the nature of the surrounding atoms (Mo(VI), W(VI), and V(V)). In solution at 330 K, the ESR spectrum of the protonated alpha-H[SiMoV(IV)VW(9)O(40)](6)(-) anion displayed 29 superhyperfine lines which were related to the partial localization of the electron on one vanadium nucleus. The ESR spectra at room temperature for the divanadium-substituted anions showed a strong anisotropy of the A tensor which would be related to the electron transfer along a preferential axis. An isolated V(4+) signal was not observed, even at 12 K, indicating that the electron is never firmly trapped on one single vanadium atom. 相似文献
92.
Xu X Zhao Z Song P Zhou G Xu J Deng P Bourdet G Chanteloup JC Zou JP Fulop A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(11-12):2444-2447
Infrared (1.2-1.6 microm) luminescence in a ytterbium aluminium garnet (YbAG) crystal, doped with Cr (0.05at.%) ions, was investigated under CW laser diode pumping (lambda=940 nm). The Cr4+ emission band was observed with its peak at 1.34 microm and measured to be about 1.3 times with respect to Yb3+ IR luminescence (lambda=1.03 microm). We demonstrate that for the excitation wavelength of 940 nm Yb3+ ions act as sensitizers of the 3B2(3T2)-3B1(3A2) emission of Cr4+ ions. This crystal is promising as a high-efficient system for tunable laser (1.2-1.6 microm) output. 相似文献
93.
The enantiomeric separation of various kinds of basic pharmaceuticals has been investigated in nonaqueous capillary electrophoresis (NACE) systems using an ion-pairing reagent in combination with cyclodextrins (CDs). The simultaneous addition to the methanolic background electrolyte (BGE) of (+)-S-camphorsulfonate or alkanesulfonates and an anionic beta-cyclodextrin derivative, heptakis(2,3-dimethyl-6-sulfato)-beta-cyclodextrin (HDMS-beta-CD), led to partial or complete enantioresolution in most cases. In the absence of ion-pairing reagent, the enantiomeric resolution obtained with this CD derivative was most often completely lost or strongly reduced, indicating the important role of ion-pairing in the chiral recognition mechanism in these NACE systems. The influence of the nature and concentration of the counterion and the anionic CD derivative on the enantioseparation of basic compounds was studied. Synergistic effects between these two kinds of charged additives were clearly observed. 相似文献
94.
[reaction: see text] An expedient synthesis of diverse 2-amino-4-heteroarylpyrimidines via a 2-chloropyrimidine intermediate is described. A series of potentially biologically active analogues have been synthesized in two parallel steps to afford focused arrays. 相似文献
95.
Jäger S Rasched G Kornreich-Leshem H Engeser M Thum O Famulok M 《Journal of the American Chemical Society》2005,127(43):15071-15082
To broaden the applicability of chemically modified DNAs in nano- and biotechnology, material science, sensor development, and molecular recognition, strategies are required for introducing a large variety of different modifications into the same nucleic acid sequence at once. Here, we investigate the scope and limits for obtaining functionalized dsDNA by primer extension and PCR, using a broad variety of chemically modified deoxynucleotide triphosphates (dNTPs), DNA polymerases, and templates. All natural nucleobases in each strand were substituted with up to four different base-modified analogues. We studied the sequence dependence of enzymatic amplification to yield high-density functionalized DNA (fDNA) from modified dNTPs, and of fDNA templates, and found that GC-rich sequences are amplified with decreased efficiency as compared to AT-rich ones. There is also a strong dependence on the polymerase used. While family A polymerases generally performed poorly on "demanding" templates containing consecutive stretches of a particular base, family B polymerases were better suited for this purpose, in particular Pwo and Vent (exo-) DNA polymerase. A systematic analysis of fDNAs modified at increasing densities by CD spectroscopy revealed that single modified bases do not alter the overall B-type DNA structure, regardless of their chemical nature. A density of three modified bases induces conformational changes in the double helix, reflected by an inversion of the CD spectra. Our study provides a basis for establishing a generally applicable toolbox of enzymes, templates, and monomers for generating high-density functionalized DNAs for a broad range of applications. 相似文献
96.
If chemistry is to be taught successfully, teachers must have a good subject matter knowledge (SK) of the ideas with which they are dealing, the nature of this falling within the orbit of philosophy of chemistry. They
must also have a good pedagogic content knowledge (PCK), the ability to communicate SK to students, the nature of this falling within the philosophy and psychology of chemical
education. Taking the case of models and modelling, important themes in the philosophy of chemistry, an interview-based study
was conducted into the SK and PCK of a sample of teachers in Brazil. This paper focuses on the results of the university chemistry
teacher sub-sample in that enquiry, analyses their SK and PCK, and speculates on the implications of this for the education
of school teachers. Finally, it suggests approaches to the professional development of university chemistry teachers that
place an emphasis on the philosophy of chemistry.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
97.
In Computerized Tomography (CT), an image must be recovered from its sampled projections in the form of values of the Radon transform. In this work a method of recovering the image is based on the properties of the raised-cosine wavelet. This wavelet has a closed form which allows for certain precomputations and avoids convolution. The rate of convergence of the resulting algorithm to the image density function is found under suitable hypotheses. This algorithm is then tested on the standard Shepp–Logan 相似文献
98.
99.
100.