全文获取类型
收费全文 | 13418篇 |
免费 | 369篇 |
国内免费 | 46篇 |
专业分类
化学 | 10697篇 |
晶体学 | 74篇 |
力学 | 210篇 |
数学 | 1687篇 |
物理学 | 1165篇 |
出版年
2023年 | 107篇 |
2022年 | 449篇 |
2021年 | 503篇 |
2020年 | 283篇 |
2019年 | 333篇 |
2018年 | 253篇 |
2017年 | 213篇 |
2016年 | 436篇 |
2015年 | 442篇 |
2014年 | 416篇 |
2013年 | 842篇 |
2012年 | 859篇 |
2011年 | 972篇 |
2010年 | 588篇 |
2009年 | 576篇 |
2008年 | 814篇 |
2007年 | 796篇 |
2006年 | 680篇 |
2005年 | 620篇 |
2004年 | 525篇 |
2003年 | 441篇 |
2002年 | 431篇 |
2001年 | 169篇 |
2000年 | 135篇 |
1999年 | 115篇 |
1998年 | 114篇 |
1997年 | 99篇 |
1996年 | 127篇 |
1995年 | 88篇 |
1994年 | 84篇 |
1993年 | 77篇 |
1992年 | 60篇 |
1991年 | 75篇 |
1990年 | 54篇 |
1989年 | 73篇 |
1988年 | 62篇 |
1987年 | 51篇 |
1986年 | 52篇 |
1985年 | 63篇 |
1984年 | 82篇 |
1983年 | 43篇 |
1982年 | 66篇 |
1981年 | 58篇 |
1980年 | 57篇 |
1979年 | 46篇 |
1978年 | 54篇 |
1977年 | 39篇 |
1976年 | 33篇 |
1975年 | 25篇 |
1963年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
A multicomponent background electrolyte (BGE) was developed and its composition optimized using artificial neural networks (ANN). The optimal BGE composition was found to be 90 mM boric acid, 115 mM Tris, and 0.75 mM EDTA (pH 8.4). A separation voltage of 20 kV, 20 degrees C and detection at 210 nm were used. The method was applied to characterize several humic acids originating from various countries of the American continent: soil (Argentina), peat (Brazil), leonardite (Guatemala and Mexico) and coal (United States). Comparison with humic acids of International Humic Substances Society (IHSS) standard samples was also done. Well reproducible electropherograms showing a relatively high number of peaks were obtained. Characterization of the samples by elemental analysis and UV spectrophotometry was also done. In spite of the very different origins, the similarities between humic acids are high and by matrix assisted desorption/ionization-time of flight (MALDI-TOF)-mass spectrometry it was shown that most of the m/z patterns are the same in all humic acids. This means that humic acids of different origin have the same structural units or that they contain the same components. 相似文献
992.
Density functional theory calculations on [LnCp2APh] (Ln = La, Lu; Cp = eta 5-C5H5; A = O, S; Ph = C6H5) suggest that the linearity of the Ln-O-C vectors arises largely as a result of electrostatic repulsion between the alpha carbon and the trivalent metal centre. 相似文献
993.
994.
995.
Sastre G Vidal-Moya JA Blasco T Rius J Jordá JL Navarro MT Rey F Corma A 《Angewandte Chemie (International ed. in English)》2002,41(24):4722-4726
996.
Koziołkiewicz M Owczarek A Wójcik M Domański K Guga P Stec WJ 《Journal of the American Chemical Society》2002,124(17):4623-4627
A new method of analysis has allowed the exonucleolytic cleavage by human 3'-exonuclease to be determined. Hydrolysis by human plasma 3'-exonuclease proceeds with retention of configuration at phosphorus. The new method determines the sense of chirality at phosphorus in isotopomeric adenosine 5'-O-[(18)O]phosphorothioates. This is based on stereospecific two-step conversion of the mono-thionucleotide into the corresponding deoxyadenosine 5'-O-alpha-[(18)O]thiotriphosphate, followed by the use of terminal deoxyribonucleotidyl transferase and MALDI TOF mass spectrometry of the resulting elongated primer. Retention of configuration in the reaction of plasma 3'-exonuclease implies a two-step mechanism with two displacements on phosphorus. Inversion at each step leads to overall retention. 相似文献
997.
Ballardini R Balzani V Clemente-León M Credi A Gandolfi MT Ishow E Perkins J Stoddart JF Tseng HR Wenger S 《Journal of the American Chemical Society》2002,124(43):12786-12795
We have designed, synthesized, and investigated a self-assembling supramolecular system which mimics, at a molecular level, the function performed by a macroscopic electrical extension cable. The system is made up of three components, 12+, 2-H3+, and 3. Component 12+ consists of two moieties: a [Ru(bpy)3]2+ unit, which plays the role of an electron donor under light excitation, and a DB24C8 crown ether, which fulfills the function of a socket. The wire-type component 2-H3+ is also composed of two moieties, a secondary dialkylammonium-ion center and a bipyridinium unit, which thread into the DB24C8 crown-ether socket of 12+ and the 1/5DN38C10 crown-ether socket 3, respectively. The photochemical, photophysical, and electrochemical properties of the three separated components, of the 12+ superset 2-H3+ and 2-H3+ subset 3 dyads, and of the 12+ superset 2-H3+ subset 3 triad have been investigated in CH2Cl2 solution containing 2% MeCN. Reversible connection/disconnection of the two plug/socket systems can be controlled independently by acid/base and redox stimulation. The behavior of the various different dyads and triad has been monitored by light absorption and emission spectroscopies, as well as by electrochemical techniques. In the fully connected 12+ superset 2-H3+ subset 3 triad, light excitation of the [Ru(bpy)3]2+ unit of component 12+ is followed by electron transfer (k = 2.8 x 108 s-1) to the bipyridinium unit of component 2-H3+, which is plugged into component 3. Possible schemes to obtain improved molecular-level electrical extension cables are discussed. 相似文献
998.
In the paper calculation of the moments of inertia for nuclei from the region 87 ≤ Z ≤ 100 and 130 ≤ N ≤ 156 was made in dependence on the angular momentum of their rotational states. The experimental values of the moments of
inertia were calculated for rotational energy of the classic rotor in its quantum form, with the use of a simple formula.
The moment of inertia term appearing in the formula was treated as a variable. The calculations were carried out on the basis
of experimental data for the energies of the rotational levels for 51 bands built on ground states for even-even nuclei and
for nuclei with odd mass number A. In addition, 30 rotational bands built on excited states were also analysed in the investigated region in case of even-even
nuclei. For many bands and nuclei the considered dependence of the moment of inertia on angular momentum has been found in
the analytical form by fitting polynomials to the experimental data. It turned out that obtained results for the moments of
inertia made it possible to describe the energies of rotational levels with a relative deviation not greater or only slightly
greater than 1%. In general, in the case of 12 bands of ground level the maximum relative deviation of obtained level energies
is smaller than 1%.
相似文献
999.
Marina Quartu Maria Pina Serra Marianna Boi Viviana Ibba Tiziana Melis Marina Del Fiacco 《BMC neuroscience》2008,9(1):108
Background
The polysialylated neuronal cell adhesion molecule (PSA-NCAM) is considered a marker of developing and migrating neurons and of synaptogenesis in the immature vertebrate nervous system. However, it persists in the mature normal brain in some regions which retain a capability for morphofunctional reorganization throughout life. With the aim of providing information relevant to the potential for dynamic changes of specific neuronal populations in man, this study analyses the immunohistochemical occurrence of PSA-NCAM in the human trigeminal ganglion (TG) and brainstem neuronal populations at prenatal and adult age. 相似文献1000.
The Dirac-Fock equations are the relativistic analogue of the well-known Hartree-Fock equations. They are used in computational chemistry, and yield results on the inner-shell electrons of heavy atoms that are in very good agreement with experimental data. By a variational method, we prove the existence of infinitely many solutions of the Dirac-Fock equations "without projector", for Coulomb systems of electrons in atoms, ions or molecules, with Z h 124, N h 41, N h Z. Here, Z is the sum of the nuclear charges in the molecule, N is the number of electrons. 相似文献