全文获取类型
收费全文 | 1146篇 |
免费 | 51篇 |
国内免费 | 6篇 |
专业分类
化学 | 924篇 |
晶体学 | 3篇 |
力学 | 10篇 |
数学 | 119篇 |
物理学 | 81篇 |
无线电 | 66篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 12篇 |
2021年 | 19篇 |
2020年 | 15篇 |
2019年 | 18篇 |
2018年 | 5篇 |
2017年 | 17篇 |
2016年 | 36篇 |
2015年 | 34篇 |
2014年 | 40篇 |
2013年 | 40篇 |
2012年 | 69篇 |
2011年 | 85篇 |
2010年 | 43篇 |
2009年 | 39篇 |
2008年 | 78篇 |
2007年 | 68篇 |
2006年 | 79篇 |
2005年 | 66篇 |
2004年 | 63篇 |
2003年 | 40篇 |
2002年 | 47篇 |
2001年 | 16篇 |
2000年 | 15篇 |
1999年 | 14篇 |
1998年 | 18篇 |
1997年 | 17篇 |
1996年 | 12篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 8篇 |
1990年 | 13篇 |
1989年 | 11篇 |
1988年 | 11篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有1203条查询结果,搜索用时 0 毫秒
21.
Potyrailo RA Chisholm BJ Morris WG Cawse JN Flanagan WP Hassib L Molaison CA Ezbiansky K Medford G Reitz H 《Journal of combinatorial chemistry》2003,5(4):472-478
Coupling of combinatorial chemistry methods with high-throughput (HT) performance testing and measurements of resulting properties has provided a powerful set of tools for the 10-fold accelerated discovery of new high-performance coating materials for automotive applications. Our approach replaces labor-intensive steps with automated systems for evaluation of adhesion of 8 x 6 arrays of coating elements that are discretely deposited on a single 9 x 12 cm plastic substrate. Performance of coatings is evaluated with respect to their resistance to adhesion loss, because this parameter is one of the primary considerations in end-use automotive applications. Our HT adhesion evaluation provides previously unavailable capabilities of high speed and reproducibility of testing by using a robotic automation, an expanded range of types of tested coatings by using the coating tagging strategy, and an improved quantitation by using high signal-to-noise automatic imaging. Upon testing, the coatings undergo changes that are impossible to quantitatively predict using existing knowledge. Using our HT methodology, we have developed several coatings leads. These HT screening results for the best coating compositions have been validated on the traditional scales of coating formulation and adhesion loss testing. These validation results have confirmed the superb performance of combinatorially developed coatings over conventional coatings on the traditional scale. 相似文献
22.
Karin Krüger Natalia V. Shakhlevich Yuri N. Sotskov Frank Werner 《The Journal of the Operational Research Society》1995,46(12):1481-1497
We consider a scheduling problem where a set of n jobs has to be processed on a set of m machines and arbitrary precedence constraints between operations are given. Moreover, for any two operations i and j values a ij >0 and a ji >0 may be given where a ij is the minimal difference between the starting times of operations i and j when operation i is processed first. Often, the objective is to minimize the makespan but we consider also arbitrary regular criteria. Even the special cases of the classical job shop problem J//Cmax belong to the set of NP-hard problems. Therefore, approximation or heuristic algorithms are necessary to handle large-dimension problems. Based on the mixed graph model we give a heuristic decomposition algorithm for such a problem, i.e. the initial problem is partitioned into subproblems that can be solved exactly or approximately with a small error bound. These subproblems are obtained by a relaxation of a subset of the set of undirected edges of the mixed graph. The subproblems are successively solved and a proportion of the results obtained for one subproblem is kept for further subproblem definitions. Numerical results of the algorithm presented here are given. 相似文献
23.
Frank Seela Karin Mersmann Jane A. Grasby Michael J. Gait 《Helvetica chimica acta》1993,76(5):1809-1820
A 7-deazaadenosine ( = tubercidin; c7A; 1 ) building block for solid-phase oligoribonucleotide synthesis was prepared. The amino group of 1 was protected with the (dimethylamino)methylidene residue (→ 3 ), and the monomethoxytrityl group was introduced at OH? C(5′) (→ 4 ). Protection of OH? C(2′) was carried out by silylation, showing that use of the (i-Pr)3Si group resulted in high 2′-O-selectivity (→ 5b , 80%). Reaction of 5b with PCl3 afforded the phosphonate 7 which was used in solid-phase oligoribonucleotide synthesis. The autocatalytic hydrolysis of hammerhead ribozymes using pG-G-G-A-G-U-C-A-G-U-C-C-C-U-U-C-G-G-G-G-A-C-U-C-U-G-A-A-G-A-G-G-C-G-C as substrate strand (S) and modified G-C-G-C-C-G-A-A-A-C-U-C-C-C as enzyme strand (E) was studied. When c7A replaced A13 or A14, a small decrease of catalytic activity was observed, while modification in position A15 enhanced the autocatalytic hydrolysis. The results demonstrate, that the atom N(7) of adenosine in any of these positions is not crucial for ribozyme action. 相似文献
24.
Sousa Pedrares A Teng W Ruhlandt-Senge K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):2019-2024
Novel magnesium pyridine-2-thiolates were prepared by using alkane elimination chemistry. The resulting complexes display a metal coordination environment composed of sulfur/nitrogen bonding from the intramolecularly stabilized mercaptopyridine ligand, in addition to coordination by the oxygen centers from two THF donors. The compounds are well-suited model compounds for the magnesium centers in Photosystem I, in which magnesium, situated in the central chlorophyll ligand, is bound to sulfur from a nearby methionine residue. All compounds were characterized by (1)H, (13)C NMR, and IR spectroscopy, in addition to Xray crystallography. 相似文献
25.
The formation of tin compounds on the surface of hydroxyapatite, the main crystalline component of teeth, is discussed controversially. SnF2 is used as an anticaries agent in toothpaste preparations. Pure hydroxyapatite (HAP) is treated with extracts of commercially available toothpastes containing tin fluoride and analyzed by electron spectroscopy (ESCA). Survey spectra and depth profiles are recorded. The chemical shifts measured for the HAP samples are compared with those of standard tin compounds. It is concluded that on the surface of hydroxyapatite a mixture of two-valent fluoro-phosphato compounds is formed. SnO is not found in the mixture. 相似文献
26.
What is “Molybdic Acid” or “Polymolybdic Acid”? According to a comparative study of the literature, supplemented by well-aimed experimental investigations and equilibrium calculations, the terms “molybdic acid” or “polymolybdic acid”, used for many substances, species, or solutions in the literature, are applicable to a species, a solution, and two solids:
- a) The monomeric molybdic acid, most probably having the formula MoO2(OH)2(H2O)2(? H2MoO4, aq), exists in (aqueous) solution only and never exceeds a concentration of ≈ 10?3 M since at higher concentrations it reacts with other monomemeric molybdenum (VI) species to give anionic or cationic polymers.
- b) A concentrated (>0.1 M MoVI) aqueous molybdate solution of degree of acidification P = 2 (realized, e. g., by a solution of one of the MoVI oxides; by any molybdate solutions whose cations have been exchanged by H3O+ on a cation exchanger; by suitable acidification of a molybdate solution) contains 8 H3O+ and the well-known polyanion Mo36O112(H2O)168? exactly in the stoichiometric proportions.
- c) A glassy substance, obtained from an alkali metal salt-free solution prepared according to (b), refers to the compound (H3O)8[Mo36O112(H2O)16]·xH2O, x = 25—29.
- d) A solid having the ideal composition [(H3O)Mo5O15(OH)H2O·H2O]∞ consists of a polymolybdate skeleton (the well-known ?decamolybdate”? structure), in the tunnels of which H3O+ and H2O are intercalate. The structure is very unstable if only H3O+ cations are present, but it is enormously stabilized by a partial exchange of H3O+ by certain alkali or alkaline earth metal cations.
27.
Structural modifications of multifidene ( 1 ), viridiene ( 2 ) and ectocarpene ( 5 ) led to the synthesis of eleven new pheromone components or imitatin substances mostly by Grignard alkylation of cyclopentene and cycloheptadiene synthons. A new lactone α,ω dienol conversion is reported. 相似文献
28.
Wouter Post Arijana Susa Rolf Blaauw Karin Molenveld Rutger J. I. Knoop 《高分子科学杂志,C辑:聚合物评论》2020,60(2):359-388
AbstractThe outstanding performance of conventional thermosets arising from their covalently cross-linked networks directly results in a limited recyclability. The available commercial or close-to-commercial techniques facing this challenge can be divided into mechanical, thermal, and chemical processing. However, these methods typically require a high energy input and do not take the recycling of the thermoset matrix itself into account. Rather, they focus on retrieving the more valuable fibers, fillers, or substrates. To increase the circularity of thermoset products, many academic studies report potential solutions which require a reduced energy input by using degradable linkages or dynamic covalent bonds. However, the majority of these studies have limited potential for industrial implementation. This review aims to bridge the gap between the industrial and academic developments by focusing on those which are most relevant from a technological, sustainable and economic point of view. An overview is given of currently used approaches for the recycling of thermoset materials, the development of novel inherently recyclable thermosets and examples of possible applications that could reach the market in the near future. 相似文献
29.
30.
Synthesis and Crystal Structure of the Nitrido Complex [Na-15-crown-5]2[MoNF4]2 · 2 CH3CN The title compound is synthesized by the reaction of [MoCl4(NSCl)]2 with excess NaF in boiling acetonitrile in the presence of the crown ether 15-crown-5. [Na-15-crown-5]2[MoNF4]2 · 2 CH3CN forms yellow crystals, which were characterized by an X-ray structure determination. Space group P1 , Z = 1. Lattice dimensions at ?90°C: a = 855.5, b = 1 069.9, C = 1 143.5 pm, α = 105.71°, β = 95.29°, γ = 102.25° (4 096 independent observed reflexions, R = 0.039). Short Na…?F contacts of 234 pm with the four axial fluoro ligands of the dimeric anion [MoNF4]22? allow formulation of a triple ion. The centrosymmetric anion is dimerized by bent fluoro bridges with Mo? F distances of 198 and 245 pm. The long Mo? F distances of the MoF2Mo ring are in transposition to the nitrido ligands, the bond lengths of which (165 pm) correspond to triple bonds. 相似文献