全文获取类型
收费全文 | 59156篇 |
免费 | 7805篇 |
国内免费 | 5084篇 |
专业分类
化学 | 31016篇 |
晶体学 | 1026篇 |
力学 | 9896篇 |
综合类 | 598篇 |
数学 | 14834篇 |
物理学 | 14675篇 |
出版年
2024年 | 135篇 |
2023年 | 718篇 |
2022年 | 1479篇 |
2021年 | 1630篇 |
2020年 | 2468篇 |
2019年 | 1986篇 |
2018年 | 1829篇 |
2017年 | 2107篇 |
2016年 | 2916篇 |
2015年 | 2390篇 |
2014年 | 3209篇 |
2013年 | 4263篇 |
2012年 | 3518篇 |
2011年 | 3953篇 |
2010年 | 3353篇 |
2009年 | 3641篇 |
2008年 | 3503篇 |
2007年 | 3417篇 |
2006年 | 3147篇 |
2005年 | 2912篇 |
2004年 | 2601篇 |
2003年 | 2288篇 |
2002年 | 2750篇 |
2001年 | 1550篇 |
2000年 | 1386篇 |
1999年 | 1314篇 |
1998年 | 1246篇 |
1997年 | 1030篇 |
1996年 | 815篇 |
1995年 | 714篇 |
1994年 | 612篇 |
1993年 | 487篇 |
1992年 | 493篇 |
1991年 | 387篇 |
1990年 | 313篇 |
1989年 | 237篇 |
1988年 | 205篇 |
1987年 | 149篇 |
1986年 | 154篇 |
1985年 | 132篇 |
1984年 | 116篇 |
1983年 | 63篇 |
1982年 | 72篇 |
1981年 | 57篇 |
1980年 | 37篇 |
1979年 | 55篇 |
1978年 | 43篇 |
1977年 | 43篇 |
1976年 | 20篇 |
1974年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
91.
A.A. FilaretovM.G. Zhizhin L.N. KomissarovaV.P. Danilov V.V. ChernyshevB.I. Lazoryak 《Journal of solid state chemistry》2002,166(2):362-368
A new ammonium indium phosphate (NH4)In(OH)PO4 was prepared by hydrothermal reaction in the In2O3-NH4H2PO4-NH3/OH system (T=200°C, autogenous pressure, 7 days). The formula (NH4)In(OH)PO4 was determined on the basis of chemical and thermal analysis (TG/DSC), X-ray powder diffraction and IR-spectroscopy. (NH4)In(OH)PO4 crystallizes in the tetragonal system with space group P43212 (No. 96); a=9.4232(1) Å, c=11.1766(1) Å, V=992.45(2) Å3; Z=8. The crystal structure was refined by the Rietveld method (Rw=6.35%, Rp=5.10%). The second-harmonic generation study confirmed that structure of (NH4)In(OH)PO4 does not have a center of symmetry. The cis-InO4(OH)2 octahedra form helical chains, parallel to the c-axis. The In-O-In bonds are nearly equidistant. The chains are interconnected by phosphate tetrahedra and create tunnels containing the NH4+ ions along the c-axis. (NH4)In(OH)PO4 is isostructural with RbIn(OH)PO4. 相似文献
92.
It has been shown that the “internal” isotope distribution within a given molecular species at the natural abundance level is accessible by a new method, SNIF-NMR, which is based on deuterium NMR. Relative internal factors, Ri/j,have been defined which enable the isotope content of a given site, i, to be compared to that of another molecular site, j, taken as the reference. Several referencing methods intended to provide direct access to relative , Ti , and , (D/H)i , site-specific parameters, are now discussed from both the theoretical and the experimental points of view. In the , which involves a time-consuming chemical transformation of the sample, the risk exists of more or leas systematic errors resulting from discriminating fractionation effects. However this technique offers, conversely, an interesting way of investigating kinetic isotope effects without the need for specific labelling. In spite of its lower spectral precision the has the advantage of being fast and less sensitive to systematic errois and may be used for direct rough routine determinations of the site-specific isotope contents. More precise results can be obtained, at the price of contaminating the sample, when an is added and signal heights are used, remembering however that the intensity parameters then have no strict physical meaning in terms of absolute isotope contents. The site-specific parameters, Ti and (D/H)i thus accessible, provide new information on the mechanisms of the fractionation effects occurring in natural conditions and examples are considered. 相似文献
93.
A novel Ugi-five-centre-four-component reaction (U-5C-4CR) of aldehydes, primary amines, trimethylsilylazide and 2-isocyanoethyltosylate yielding tetrazolopiperazine building blocks is described. 相似文献
94.
95.
严继民 《中国科学B辑(英文版)》1990,(7)
An ab initio calculation of the hydrogen bond complex (HF)_2 is given with the 6-311 G~(**) basis set, according to which the potential surface around the balance point of the distancès and the orientations between two HF molecules is obtained. The atomic charges in the system are calculated with the PD/LSF method (potential-derived/least-square-fitting method) and then an analysis of the hydrogen bond interaction between two HF molecules is given with the (exp-6-1) potential function, by means of which it is shown that the main interaction between them is not an electro-static but a charge transfer one. The potential curve between two HF molecules is like a Morse function. 相似文献
96.
根据在自由基引发剂存在下,8,9-苯并-2-亚甲基-1,4,6-三氧螺(4,4)壬烷(Ⅰ)可能存在的竞争反应,推导了动力学方程式。用红外光谱法测定了单体的消失速率和苯酞的生成速率。实验结果与推导动力学方程式相符。进一步证明了单体(Ⅰ)的增长反应由加成聚合和开环—异构化—加成这两个反应组成。这两个反应对总速率的相对贡献与单体浓度有关。得到了共聚物的结构。 相似文献
97.
Numerical results for the ground state of the HN
2
+
and HCO+ molecular ions at their near equilibrium geometry, obtained by the complex molecular orbitals (CMO) method in the extended basis set, are reported. The CMO wavefunction of the HN
2
+
ion is compared with the CI wavefunction obtained in the same basis set. This reveals the nature of approximations inherent in the CMO method. A peculiar feature of the occupation numbers of the CMO natural orbitals is also explained.Alexander von Humboldt Fellow. On leave from the Institute Rudjer Bokovi, Zagreb, Croatia, Yugoslavia. 相似文献
98.
Combinatorial library of isoxazoles was prepared by 1,3-dipolar cycloaddition on solid support. p-Cyano derivatives showed nematic and/or smectic A phases. A bilayer smectic phase for 2h is proposed by the combination of molecular mechanics calculation and X-ray diffraction experiment. 相似文献
99.
This paper presents in detail a robust, efficient and accurate methodology for the computation of equilibrium composition in gaseous mixtures. The methodology is founded on the concept of the chemical basis, which is defined and formalized using a powerful matricial approach. The method is specially designed to be general, thus providing basic thermodynamic data in several areas, such as combustion, plasma chemistry and, more generally speaking, computational fluid dynamics. The performance of the method is given in terms of CPU usage and the computed results are compared with those in the published literature. The method is shown to yield results of very high quality in terms of accuracy and smoothness. 相似文献
100.
Robert O. Jones 《Angewandte Chemie (International ed. in English)》1991,30(6):630-640
The geometrical structure of any aggregate of atoms is one of its basic properties and, in principle, straightforward to predict. One chooses a structure, determines the total energy E of the system of electrons and ions, and repeats the calculation for all possible geometries. The ground state structure is that with the lowest energy. A quantum mechanical calculation of the exact wave function Ψ would lead to the total energy, but this is practicable only in very small molecules. Furthermore, the number of local minima in the energy surface increases dramatically with increasing molecular size. While traditional ab initio methods have had many impressive successes, these difficulties have meant that they have focused on systems with relatively few local minima, or have used experiment or experience to limit the range of geometries studied. On the other hand, calculations for much larger molecules and extended systems are often forced to use simplifying assumptions about the interatomic forces that limit their predictive capability. The approach described here avoids both of these extremes: Total energies of predictive value are calculated without using semi-empirical force laws, and the problem of multiple minima in the energy surface is addressed. The density functional formalism, with a local density approximation for the exchange-correlation energy, allows one to calculate the total energy for a given geometry in an efficient, if approximate, manner. Calculations for heavier elements are not significantly more difficult than for those in the first row and provide an ideal way to study bonding trends. When coupled with finite-temperature molecular dynamics, this formalism can avoid many of the energetically unfavorable minima in the energy surface. We show here that the method leads to surprising and exciting results. 相似文献