全文获取类型
收费全文 | 1077篇 |
免费 | 39篇 |
国内免费 | 9篇 |
专业分类
化学 | 804篇 |
晶体学 | 4篇 |
力学 | 26篇 |
数学 | 135篇 |
物理学 | 156篇 |
出版年
2023年 | 14篇 |
2022年 | 12篇 |
2021年 | 22篇 |
2020年 | 27篇 |
2019年 | 18篇 |
2018年 | 16篇 |
2017年 | 5篇 |
2016年 | 26篇 |
2015年 | 17篇 |
2014年 | 17篇 |
2013年 | 49篇 |
2012年 | 70篇 |
2011年 | 79篇 |
2010年 | 41篇 |
2009年 | 42篇 |
2008年 | 63篇 |
2007年 | 68篇 |
2006年 | 69篇 |
2005年 | 58篇 |
2004年 | 38篇 |
2003年 | 37篇 |
2002年 | 31篇 |
2001年 | 15篇 |
2000年 | 14篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 16篇 |
1996年 | 11篇 |
1995年 | 13篇 |
1994年 | 13篇 |
1993年 | 17篇 |
1992年 | 14篇 |
1991年 | 15篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1987年 | 7篇 |
1985年 | 12篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 13篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 9篇 |
1975年 | 4篇 |
1974年 | 7篇 |
1971年 | 4篇 |
1933年 | 5篇 |
排序方式: 共有1125条查询结果,搜索用时 15 毫秒
141.
In this study, we report the protonation reactions for astaxanthin and canthaxanthin radical anions in methanol, alkaline methanol, and aqueous 2% Triton X-100 at different pH values. The pKa values for the corresponding alpha-hydroxy radical derivatives of astaxanthin, canthaxanthin, and beta-apo-8'-carotenal were estimated in 2% Triton X-100. Also, the effects of the microenvironment and the structure of the carotenoids on the protonation rate constant are discussed. 相似文献
142.
Wallace KJ Fagbemi RI Folmer-Andersen FJ Morey J Lynth VM Anslyn EV 《Chemical communications (Cambridge, England)》2006,(37):3886-3888
The detection of chemical warfare simulants is attained by the PET mechanism that gives an "off-on" fluorescent response with a half-life of approximately 50 ms upon phosphorylation of a reactive oximate functionality; the X-ray crystal structure of the oximate was also obtained and is discussed. 相似文献
143.
144.
Zhanar Berikkyzy Steve Butler Jay Cummings Kristin Heysse Paul Horn Ruth Luo Brent Moran 《Discrete Mathematics》2018,341(2):497-507
Consider the following process on a simple graph without isolated vertices: order the edges randomly and keep an edge if and only if it contains a vertex which is not contained in some preceding edge. The resulting set of edges forms a spanning forest of the graph.The probability of obtaining components in this process for complete bipartite graphs is determined as well as a formula for the expected number of components in any graph. A generic recurrence and some additional basic properties are discussed. 相似文献
145.
John Asplund Kossi Edoh Ruth Haas Yulia Hristova Beth Novick Brett Werner 《Discrete Mathematics》2018,341(10):2938-2948
For a graph and, the shortest path reconfiguration graph of with respect to and is denoted by . The vertex set of is the set of all shortest paths between and in . Two vertices in are adjacent, if their corresponding paths in differ by exactly one vertex. This paper examines the properties of shortest path graphs. Results include establishing classes of graphs that appear as shortest path graphs, decompositions and sums involving shortest path graphs, and the complete classification of shortest path graphs with girth 5 or greater. We include an infinite family of well structured examples, showing that the shortest path graph of a grid graph is an induced subgraph of a lattice. 相似文献
146.
Zoltán Füredi Alexandr Kostochka Ruth Luo Jacques Verstraëte 《Discrete Mathematics》2018,341(5):1253-1263
The Erd?s–Gallai Theorem states that for , any -vertex graph with no cycle of length at least has at most edges. A stronger version of the Erd?s–Gallai Theorem was given by Kopylov: If is a 2-connected -vertex graph with no cycle of length at least , then , where . Furthermore, Kopylov presented the two possible extremal graphs, one with edges and one with edges.In this paper, we complete a stability theorem which strengthens Kopylov’s result. In particular, we show that for odd and all , every -vertex 2-connected graph with no cycle of length at least is a subgraph of one of the two extremal graphs or . The upper bound for here is tight. 相似文献
147.
PLD-assisted VLS growth of aligned ferrite nanorods, nanowires, and nanobelts-synthesis, and properties 总被引:4,自引:0,他引:4
Morber JR Ding Y Haluska MS Li Y Liu JP Wang ZL Snyder RL 《The journal of physical chemistry. B》2006,110(43):21672-21679
We report here a systematic synthesis and characterization of aligned alpha-Fe2O3 (hematite), epsilon-Fe2O3, and Fe3O4 (magnetite) nanorods, nanobelts, and nanowires on alumina substrates using a pulsed laser deposition (PLD) method. The presence of spherical gold catalyst particles at the tips of the nanostructures indicates selective growth via the vapor-liquid-solid (VLS) mechanism. Through a series of experiments, we have produced a primitive "phase diagram" for growing these structures based on several designed pressure and temperature parameters. Transmission electron microscopy (TEM) analysis has shown that the rods, wires, and belts are single-crystalline and grow along <111>m or <110>h directions. X-ray diffraction (XRD) measurements confirm phase and structural analysis. Superconducting quantum interference device (SQUID) measurements show that the iron oxide structures exhibit interesting magnetic behavior, particularly at room temperature. This work is the first known report of magnetite 1D nanostructure growth via the vapor-liquid-solid (VLS) mechanism without using a template, as well as the first known synthesis of long epsilon-Fe2O3 nanobelts and nanowires. 相似文献
148.
Diana B. Muñiz-Márquez Guillermo C. Martínez-Ávila Jorge E. Wong-Paz Ruth Belmares-Cerda Raúl Rodríguez-Herrera Cristóbal N. Aguilar 《Ultrasonics sonochemistry》2013,20(5):1149-1154
Bay leaves (BL) (Laurus nobilis L., Family: Laureceae) are traditionally used to treat some symptoms of gastrointestinal problems, such as epigastric bloating, impaired digestion, eructing and flatulence. These biological properties are mainly attributed to its phenolic compounds. In this paper, ultrasound-assisted extraction of phenolic compounds from Laurus nobilis L. (Laureceae) was studied.Effects of several experimental factors, such as sonication time, solid/liquid ratio and concentration of solvent on extraction of phenolic compounds were evaluated through a randomized complete block design with factorial treatment arrangement (33). The best extraction conditions were: 1 g plant sample with 12 mL of 35% ethanol, for 40 min, obtaining a yield of phenolic compounds of 17.32 ± 1.52 mg g?1 of plant. In addition, free radical-scavenging potential of DPPH and lipid oxidation inhibition, by linoleic acid peroxidation of the selected extract was measured in order to evidence their antioxidant properties. Results indicated that high amounts of phenolic compounds can be extracted from L. nobilis by ultrasound-assisted extraction technology. 相似文献
149.
Sanchez-Prado L Barro R Garcia-Jares C Llompart M Lores M Petrakis C Kalogerakis N Mantzavinos D Psillakis E 《Ultrasonics sonochemistry》2008,15(5):689-694
The sonochemical degradation of 5 μg l−1 triclosan, a priority micro-pollutant, in various environmental samples (seawater, urban runoff and influent domestic wastewater) as well as in model solutions (pure and saline water) was investigated. Experiments were conducted with a horn-type sonicator operating at 80 kHz frequency and a nominal applied power of 135 W, while solid-phase microextraction coupled with gas chromatography–electron capture detector (SPME/GC–ECD) was employed to monitor triclosan degradation. The latter followed pseudo-first order kinetics with the rate constant being (min−1): 0.2284 for seawater > 0.1051 for 3.5% NaCl in deionised water > 0.0597 for centrifuged urban runoff 0.0523 for untreated urban runoff > 0.0272 for deionised water > 0.0063 for wastewater influent. SPME/GC–ECD and SPME coupled with gas chromatography–mass spectrometry (SPME/GC–MS) were also used to check for the formation of chlorinated and other toxic by-products; at the conditions in question, the presence of such compounds was not confirmed. 相似文献
150.
Steffen Reich Patrick Kaiser Mahsa Mafi Holger Schmalz Daniel Rhinow Ruth Freitag Andreas Greiner 《Macromolecular bioscience》2019,19(5)
Living Micrococcus luteus (M. luteus) and Escherichia coli (E. coli) are encapsulated in poly(vinyl alcohol), poly(vinylpyrrolidone), hydroxypropyl cellulose, and gelatin by high‐temperature spray drying. The challenge is the survival of the bacteria during the standard spray‐drying process at temperatures of 150 °C (M. luteus) and 120 °C (E. coli). Raman imaging and transmission electron microscopy indicate encapsulated bacteria in hollow composite microparticles. The versatility of the spray‐dried polymer bacteria microparticles is successfully proved by standard polymer solution–processing techniques such as electrospinning, even with harmful solvents, to water‐insoluble polyacrylonitrile, polystyrene, poly(methyl methacrylate), and poly(vinyl butyrate) nanofiber nonwovens, which opens numerous new opportunities for novel applications. 相似文献