全文获取类型
收费全文 | 9250篇 |
免费 | 184篇 |
国内免费 | 47篇 |
专业分类
化学 | 5957篇 |
晶体学 | 85篇 |
力学 | 292篇 |
数学 | 1713篇 |
物理学 | 1434篇 |
出版年
2021年 | 97篇 |
2020年 | 94篇 |
2019年 | 79篇 |
2018年 | 70篇 |
2017年 | 63篇 |
2016年 | 143篇 |
2015年 | 134篇 |
2014年 | 154篇 |
2013年 | 522篇 |
2012年 | 374篇 |
2011年 | 479篇 |
2010年 | 267篇 |
2009年 | 240篇 |
2008年 | 483篇 |
2007年 | 445篇 |
2006年 | 444篇 |
2005年 | 437篇 |
2004年 | 342篇 |
2003年 | 327篇 |
2002年 | 355篇 |
2001年 | 148篇 |
2000年 | 137篇 |
1999年 | 104篇 |
1998年 | 97篇 |
1997年 | 119篇 |
1996年 | 147篇 |
1995年 | 88篇 |
1994年 | 106篇 |
1993年 | 110篇 |
1992年 | 112篇 |
1991年 | 106篇 |
1990年 | 101篇 |
1989年 | 84篇 |
1988年 | 95篇 |
1987年 | 96篇 |
1986年 | 73篇 |
1985年 | 154篇 |
1984年 | 148篇 |
1983年 | 121篇 |
1982年 | 136篇 |
1981年 | 153篇 |
1980年 | 149篇 |
1979年 | 125篇 |
1978年 | 132篇 |
1977年 | 127篇 |
1976年 | 117篇 |
1975年 | 104篇 |
1974年 | 106篇 |
1973年 | 105篇 |
1972年 | 65篇 |
排序方式: 共有9481条查询结果,搜索用时 93 毫秒
231.
Paul J. O’Connor George E. Leroi John Allison 《Journal of the American Society for Mass Spectrometry》1991,2(4):322-335
The traditional approach for obtaining highly collimated low energy (<200 eV) beams from high-energy (1–10 keV) sector mass spectrometers involves the use of a complex exponential deceleration lens assembly. Through ion optical modeling of the exponential lens and other less-complex lens designs, the fundamental features that contribute to deceleration lens performance are investigated. From this evaluation, a simple high-performance lens was designed, constructed, and characterized. The novel lens described here decelerates a 3-keV ion beam down to the 3-200-eV range while providing low angular divergence, excellent focusing, and good transmission. 相似文献
232.
Theoretical studies on the optical properties of gold triangular prisms in solution are presented to determine how structural modifications affect the extinction spectrum. Well-defined trends in the particle extinction are found to depend on the triangular edge length and the prism thickness. Calculations performed on large, thin triangular prisms indicate multipolar excitation and display numerous peaks in the extinction spectrum. The dominant peaks are assigned to different in-plane modes corresponding to the lowest three orders of a multipole expansion. Vector polarization plots are presented to support the peak assignments. Altering the prisms by snipping off the points of the triangular cross section significantly blueshifts the dipole peak, but the higher-order modes are only slightly affected. Snipping off large volumes can lead to the suppression of high-order multipoles in the extinction spectrum. 相似文献
233.
An efficient route to various sugar lactones has been developed. Key to the overall transformation is the sequential osmium-catalyzed dihydroxylation of 2,4-dienoates. The simplest (one-step/racemic) example of this reaction occurs when the dihydroxylation is performed with aqueous NMO in MeOH. When the first dihydroxylation is performed using the AD-mix procedure, an enantioselective variant results. When a matched AD-mix procedure is used for the second dihydroxylation, an exceedingly diastereo- and enantioselective synthesis of galacto-1,4-lactone results. [Reaction: see text] 相似文献
234.
Let H={a
0, a
1, a
2, b
0, b
1, b
2} be the poset defined by a
0<a
2<a
1, b
0<b
2<b
1, a
0<b
1, and b
0<a
1. For an infinite regular cardinal
, we describe the free
-lattice on H. This continues the work of I. Rival and R. Wille who accomplished the same for
=. In subsequent papers, we show how to apply this result to describe the free
-lattice on a poset for a large class of posets, called slender posets. 相似文献
235.
Tyler GW 《The Journal of the Operational Research Society》1984,35(3):257-259
McClean's model is applied to an analysis of Consultant Staff in the Royal Infirmary of Edinburgh from 1801 to 1968. 相似文献
236.
Photochemical and thermal transformations of carboxylic dithiocarbamic anhydrides and acyl xanthates
The reaction of diphenylchloroacetyl chloride with different dithiocarbamates gave the corresponding dithiocarbamic anhydrides. Under analogous conditions, dithiocarbamyl phenylacetic acids and dithiocarbamyl acetic acids were formed from the reaction of dithiocarbamates with phenylchloroacetyl chloride and chloroacetyl chloride, respectively. O-Ethyl S-acyl xanthates were formed from the reaction of potassium O-ethyl xanthate with diphenylacetyl chloride, diphenylchloroacetyl chloride, cyclopentylphenylacetyl chloride and triphenylacetyl chloride. Photolysis of dithiocarbamic anhydrides gave a mixture of products consisting of 1,2-dichlorotetraphenylethane, carbon monoxide and the corresponding thiocarbamoyl sulfides. Acyl xanthates, on the other hand, gave carbon monoxide and the corresponding substituted ethane derivatives. When dithiocarbamic anhydrides were refluxed in hydroxylic solvents such as methanol, ethanol and n-propanol, the corresponding dithiocarbamyl acetates were formed. Thermal decomposition of acyl xanthates gave carbon disulfide and the corresponding esters. 相似文献
237.
238.
George Debney Edward E. Fairchild Jr. Stephen T. C. Siklos 《General Relativity and Gravitation》1978,9(10):879-887
In the Yang-Mills formulation of gravitational dynamics based uponSL(2,C) spin transformations acting on Dirac spinors, the vacuum field equations are R
+C
R
= 0 and and
. HereR
is the Ricci curvature andC
is the Weyl conformal curvature; is a coupling constant. We show the equivalence between solutions of these equations and the vacuum Einstein equationsR
= 0. The proof uses the Newman-Penrose formalism.Supported by a NATO fellowship.Supported by a SRC fellowship. 相似文献
239.
The luminescence properties of Eu2+, Sn2+, and Pb2+ in SrB6O10 have been studied both at room-temperature and liquid-helium temperature and the decay times of Sn2+ and Pb2+ in this matrix have been measured and analyzed. According to the emission spectrum of Eu2+ there seems to be three different cation sites in SrB6O10. Europium, tin, and lead were also used as sensitizers for Mn2+ and the energy transfer processes were characterized. Eu2+-Mn2+ energy transfer was inefficient due to the transfer within different Eu2+ centers. The sensitization action of Sn2+ and Pb2+ on Mn2+ was different because lead-lead energy transfer occurs (even at 4.2 K) but tin-tin transfer can be neglected. A fast diffusion model for the Pb2+ system is suggested. 相似文献
240.
Alcala R Shabaker JW Huber GW Sanchez-Castillo MA Dumesic JA 《The journal of physical chemistry. B》2005,109(6):2074-2085
Reaction kinetics studies were conducted for the conversions of ethanol and acetic acid over silica-supported Pt and Pt/Sn catalysts at temperatures from 500 to 600 K. Addition of Sn to Pt catalysts inhibits the decomposition of ethanol to CO, CH4, and C2H6, such that PtSn-based catalysts are active for dehydrogenation of ethanol to acetaldehyde. Furthermore, PtSn-based catalysts are selective for the conversion of acetic acid to ethanol, acetaldehyde, and ethyl acetate, whereas Pt catalysts lead mainly to decomposition products such as CH4 and CO. These results are interpreted using density functional theory (DFT) calculations for various adsorbed species and transition states on Pt(111) and Pt3Sn(111) surfaces. The Pt3Sn alloy slab was selected for DFT studies because results from in situ (119)Sn M?ssbauer spectroscopy and CO adsorption microcalorimetry of silica-supported Pt/Sn catalysts indicate that Pt-Sn alloy is the major phase present. Accordingly, results from DFT calculations show that transition-state energies for C-O and C-C bond cleavage in ethanol-derived species increase by 25-60 kJ/mol on Pt3Sn(111) compared to Pt(111), whereas energies of transition states for dehydrogenation reactions increase by only 5-10 kJ/mol. Results from DFT calculations show that transition-state energies for CH3CO-OH bond cleavage increase by only 12 kJ/mol on Pt3Sn(111) compared to Pt(111). The suppression of C-C bond cleavage in ethanol and acetic acid upon addition of Sn to Pt is also confirmed by microcalorimetric and infrared spectroscopic measurements at 300 K of the interactions of ethanol and acetic acid with Pt and PtSn on a silica support that had been silylated to remove silanol groups. 相似文献