全文获取类型
收费全文 | 1178篇 |
免费 | 49篇 |
国内免费 | 6篇 |
专业分类
化学 | 1008篇 |
晶体学 | 5篇 |
力学 | 11篇 |
数学 | 125篇 |
物理学 | 84篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 19篇 |
2020年 | 17篇 |
2019年 | 15篇 |
2018年 | 5篇 |
2017年 | 14篇 |
2016年 | 37篇 |
2015年 | 32篇 |
2014年 | 38篇 |
2013年 | 39篇 |
2012年 | 76篇 |
2011年 | 87篇 |
2010年 | 44篇 |
2009年 | 38篇 |
2008年 | 85篇 |
2007年 | 71篇 |
2006年 | 84篇 |
2005年 | 65篇 |
2004年 | 60篇 |
2003年 | 41篇 |
2002年 | 48篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 13篇 |
1998年 | 16篇 |
1997年 | 17篇 |
1996年 | 12篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 16篇 |
1992年 | 11篇 |
1991年 | 6篇 |
1990年 | 14篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1972年 | 3篇 |
1970年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有1233条查询结果,搜索用时 15 毫秒
31.
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
32.
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
33.
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.
34.
Schwarzer R 《Analytical and bioanalytical chemistry》2002,374(4):699-702
Automated backscatter Kikuchi diffraction in the SEM enables rapid discrimination of phases and the automated acquisition of phase-distribution maps at sub-micron resolution, if the phases belong to different Laue groups and/or if their lattice constants are significantly different. The acquired database can be further used for quantitative determination of phase contents, for construction of crystal orientation maps, for characterization of grain and phase boundaries, for crystal texture analysis on a grain-specific level by calculating pole figures and orientation density functions separately for each phase, and for orientation stereology in general. Pattern-quality maps provide a clear reproduction of the microstructure on a meso-scale level. 相似文献
35.
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
36.
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
37.
Bobby L. Wilson Rudolph R. Schwarzer Callista O. Chukwuenye 《Microchemical Journal》1982,27(4):558-563
This paper reports the development of a field electrode method for the determination of total sulfide in water. The method involves the use of preweighed sodium sulfide crystals in the standardization process. Sodium sulfide crystals were weighed and sealed in air-tight plastic volumetric flasks. Standards were prepared in the field by adding a sulfide antioxidant buffer to the flasks containing the sulfide crystals and diluting it to mark with deionized deaerated water. Standards of lower concentration were prepared by serial dilutions of first standards.The results of the reproducibility determination revealed that sulfide concentrations as low as 6 ppb could be measured with a reproducibility of better than ±10%.Water samples were collected from a series of lakes in Fort Bend County, Texas (near Houston) and analyzed for sulfide content. The sulfide ion concentrations of these samples were determined directly in the field by use of an Orion Model 407A/F specific meter equipped with a silver/sulfide ion selective electrode in conjunction with a double junction reference electrode. 相似文献
38.
The determination of residual amounts of bromide in a local anaesthetic hydrochloride by capillary electrophoresis was developed. Direct UV detection at 200 nm was used for the determination of the bromide content. The separation capacity of the system must be sufficient when bromide is determined in the presence of a large excess of chloride since electromigration dispersion of the highly concentrated chloride peak may impair the resolution. The background electrolyte (BGE) contained both acetonitrile and methanesulphonic acid in order to improve the selectivity and minimise the electromigration dispersion. The system was optimised with respect to resolution of the chloride and the bromide peaks by statistical experimental design using a multivariate optimisation program. The developed method was validated in accordance with the ICH guidelines and proved to be suitable for its intended use. 相似文献
39.
40.
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