全文获取类型
收费全文 | 563篇 |
免费 | 20篇 |
国内免费 | 2篇 |
专业分类
化学 | 393篇 |
晶体学 | 2篇 |
力学 | 19篇 |
数学 | 43篇 |
物理学 | 128篇 |
出版年
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 13篇 |
2020年 | 11篇 |
2019年 | 20篇 |
2018年 | 13篇 |
2017年 | 19篇 |
2016年 | 32篇 |
2015年 | 15篇 |
2014年 | 23篇 |
2013年 | 44篇 |
2012年 | 54篇 |
2011年 | 54篇 |
2010年 | 29篇 |
2009年 | 25篇 |
2008年 | 31篇 |
2007年 | 39篇 |
2006年 | 24篇 |
2005年 | 31篇 |
2004年 | 20篇 |
2003年 | 15篇 |
2002年 | 7篇 |
2001年 | 4篇 |
2000年 | 2篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
排序方式: 共有585条查询结果,搜索用时 15 毫秒
131.
A new approach to carbasugars in enantiomerically pure form is reported. The key step involves ring-closing metathesis of dienols 6 derived from a (R)-(+)-glyceraldehyde derivative 4 to form the substituted cyclopentenol 9 and cyclohexenol 34a. Stereocontrolled addition of hydroxyl groups followed by conversion of the ketal unit to hydroxymethyl group in these intermediates led to carbapentoses and -hexoses. Stereoselectivity during introduction of hydroxyl groups arises through the steric hindrance posed by the allylic substituents. A remarkable feature of the present approach is the accessibility of both d- and l-series of carbapentoses as illustrated by the synthesis of beta-D- and beta-L-carbaribofuranoses 17 and 20, respectively. Carba-alpha-D-ribofuranose 25, the biosynthetic intermediate to the antibiotic aristeromycin, has also been synthesized from the same cyclopentenol 9. Functional group manipulation in the cyclopentenol 9a also enabled access to carbaarabinofuranose 32. The present synthetic strategy can be extended for the synthesis of carbahexopyranose, as illustrated by the synthesis of carba-alpha-L-gulopyranose 40b. 相似文献
132.
We propose a new approach for estimating the phase in the presence of a nonlinear response of a phase-shifting device: a piezoelectric transducer (PZT). The method is complemented well by the high resolution and the maximum likelihood estimation techniques in the estimation of the phase step and the nonlinear coefficient. The advantage of the proposed method is that it can be extended to the extraction of multiple phases in configurations involving multiple PZTs in the presence of nonlinearity. Symmetricity in the phase steps is not required in this method. Hence hysteresis of the PZT does not have any influence on the accuracy of the phase estimation. The effectiveness of the method is shown by experimental results. 相似文献
133.
A phase-shifting piezo device commonly employed in phase-shifting interferometry exhibits a nonlinear response to applied voltage. Hence, a method for estimation of phase distribution in the presence of nonlinear phase steps is presented. The proposed method compensates for the harmonics present in the intensity fringe, allows the use of arbitrary phase-step values between 0 and tau rad, and does not impose constraints on the selection of particular phase-step values for minimizing nonlinearity and compensating for the harmonics. The comparison of the proposed method with other well-known benchmarking algorithms shows that our method is highly efficient and also works well in the presence of noise. 相似文献
134.
Wang Ke Hazra Raj Shankar Ma Qian Jiang Long Liu Zhaohui Zhang Yuanming Wang Shudong Han Guangting 《Cellulose (London, England)》2022,29(3):1647-1666
Cellulose - Silk fibroin (SF) has been intensively studied as a biobased renewable material for biomedical and other applications. However, its relatively low mechanical properties have limited its... 相似文献
135.
Meccanica - Structures such as turbomachinery blades, industrial fans, propellers, etc. can be modeled as twisted beams. The study of dynamics of these structures is vital as operational failure of... 相似文献
136.
We study a sourcing problem where a buyer reserves capacity from a set of suppliers. The suppliers have finite capacity and their unit production cost is a decreasing function of their capacity, implying scale economies. The capacity of each supplier and therefore the cost is his private information. The buyer and other suppliers only know the probability distribution of the supplier’s capacity. The buyer’s demand is random and she has to decide how much capacity to reserve in advance from a subset of suppliers and how much to source from marketplace. In this study we determine the buyer’s optimum reservation quantity and the size of the supply base. We find the presence of such capacity cost correlation leads to supply base reduction. 相似文献
137.
The spectroscopy and dissociation of I2–Rg (Rg = Kr and Xe) van der Waals complexes have been studied in detail using MP2 and CCSD(T) methods in conjunction with
the correlation-consistent triple-ζ and quadruple-ζ quality basis sets. The large-core Stuttgart–Dresden–Bonn (SDB) relativistic pseudopotential is used for all heavy elements.
The dissociation energy and depth of the potential well have been calculated using potential method and supermolecular approach
in order to remove the discrepancy among the existing theoretical and experimental values. Most of the spectroscopic properties
are first reported, and the rest agree very well with the theoretical and experimental values wherever available. 相似文献
138.
The synthesis, molecular and supramolecular structure, spectroscopy and electrochemistry of a dialkoxo-bridged diuranyl(VI) compound [(UO2)2(L)2(dimethylformamide)2] (1) derived from the Schiff base ligand H2L, obtained on condensation of 3-methoxysalicylaldehyde with 2-aminoethanol, have been described. The compound has been characterized by IR, UV–Vis, NMR and mass spectra, as well as by single crystal X-ray structure determination. The title compound crystallizes in the monoclinic P21/n space group with the following unit cell parameters a = 10.5713(2) Å, b = 11.9895(2) Å, c = 12.9372(2) Å, β = 102.773(3)° and Z = 2. The structure of 1 reveals that it is a dialkoxo-bridged dinuclear compound of uranium(VI) containing two deprotonated ligands, [L]2−, two dimethylformamide (dmf) molecules and two UO22+ centers. The coordination geometry around the uranium(VI) center is distorted pentagonal bipyramidal; two uranyl oxygens occupy the axial positions, while the basal pentagonal plane is defined by a phenoxo oxygen, two bridging alkoxo oxygens, one imine nitrogen, and one dmf oxygen. Three C–H?O type hydrogen bonds involving one uranyl oxygen, two dmf hydrogens and the imine hydrogen link the dinuclear units into a two-dimensional network. The ESI-MS spectrum of 1 in dimethylsulfoxide exhibits two peaks at m/z = 464.17 and 927.26, which are assignable to [(UO2)2L2H]+ (60%) and [(UO2)2LH]+ (100%) cations, respectively. Cyclic voltammetric measurements of 1 reveal that the uranium(VI) center is reduced quasireversibly at E1/2 = −1112 mV with ΔEP = 97 mV. 相似文献
139.
Precise conductance measurements have been performed for lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, sodium perchlorate, and sodium tetraphenylborate in 2-methoxyethanol–water mixtures at four different mole fractions, i.e., 0.056, 0.136, 0.262, and 0.486 of 2-methoxyethanol (69.73 D 26.55) at 25°C in the concentration range 0.0004–0.0642 mol-dm–3. The limiting molar conductivity °, the association constant K
A, and the association distance R for the solvent mixtures have been evaluated from the conductance concentration data using the 1978 Fuoss conductance equation. The single-ion conductances have been estimated using the reference electrolyte tetrabutylam-monium tetraphynylborate(Bu4NBPh4). The analysis of the data indicates that for most salts ion association is appreciable in the solvent mixtures with a mole fraction of the cosolvent of 0.262 or higher. The results have been interpreted in terms of ion-solvent interactions and structural changes in the mixed solvent media. 相似文献
140.