全文获取类型
收费全文 | 46919篇 |
免费 | 10896篇 |
国内免费 | 2281篇 |
专业分类
化学 | 48901篇 |
晶体学 | 498篇 |
力学 | 1010篇 |
综合类 | 74篇 |
数学 | 3156篇 |
物理学 | 6457篇 |
出版年
2024年 | 26篇 |
2023年 | 219篇 |
2022年 | 264篇 |
2021年 | 574篇 |
2020年 | 1652篇 |
2019年 | 2907篇 |
2018年 | 1339篇 |
2017年 | 945篇 |
2016年 | 3830篇 |
2015年 | 3917篇 |
2014年 | 3966篇 |
2013年 | 4858篇 |
2012年 | 4011篇 |
2011年 | 3286篇 |
2010年 | 3450篇 |
2009年 | 3437篇 |
2008年 | 3158篇 |
2007年 | 2491篇 |
2006年 | 2083篇 |
2005年 | 2214篇 |
2004年 | 1826篇 |
2003年 | 1626篇 |
2002年 | 2319篇 |
2001年 | 1617篇 |
2000年 | 1523篇 |
1999年 | 586篇 |
1998年 | 244篇 |
1997年 | 181篇 |
1996年 | 176篇 |
1995年 | 192篇 |
1994年 | 175篇 |
1993年 | 139篇 |
1992年 | 114篇 |
1991年 | 97篇 |
1990年 | 100篇 |
1989年 | 56篇 |
1988年 | 51篇 |
1987年 | 52篇 |
1986年 | 54篇 |
1985年 | 49篇 |
1984年 | 35篇 |
1983年 | 25篇 |
1982年 | 37篇 |
1981年 | 21篇 |
1980年 | 18篇 |
1979年 | 14篇 |
1978年 | 21篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
This study examines the reagent gas pressure and ion source temperature dependence for dimethyl ether chemical ionization (DME CI) mass spectra recorded with an external source ion trap mass spectrometer (ITMS). Information for better controls of the reagent gas pressure in order to obtain fair CI spectra is provided. The origin of M+? ions observed in DME CIMS is discussed in detail. Furthermore, the ion source temperature effect on the DME CI is also investigated. 相似文献
992.
Manuel Snchez‐Chaves Gerardo Martínez Enrique Lpez Madruga Carmen Fernndez‐Monreal 《Journal of polymer science. Part A, Polymer chemistry》2002,40(8):1192-1199
Free‐radical copolymerization of glycidyl methacrylate (GMA) with N‐vinylpyrrolidone (VPD) was carried out at 50 °C using 3.0 mol · L?1 of N,N′‐dimethylformamide solution and 9.0 · 10?3 mol · L?1 of 2,2′‐azobisisobutyronitrile as an initiator. The modification reaction of GMA‐VPD copolymers with a model bioactive carboxylic acid, 6‐methoxy‐α‐methyl‐2‐naphthaleneacetic acid (naproxen), was studied in the homogeneous phase using basic catalysts. The influence of the type of catalyst and the GMA content was evaluated. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1192–1199, 2002 相似文献
993.
Abdullah A. Al‐Karim Al‐Shara'ey 《中国化学会会志》2004,51(3):547-552
6‐Aryl‐5‐cyano‐4‐pyrimidinone‐2‐thion derivatives 1a‐c reacted with methyl iodide (1:2) to give the corresponding 2‐S,N‐dimethyl pyrimidine‐4‐one derivatives 2a‐c . Compounds 2a‐c were in turn, reacted with hydrazine hydrate to give the sulfur free reaction products 3a‐c . These reaction products were taken as the starting materials for the synthesis of several new heterocyclic derivatives. Reaction of 3a‐c with acetic anhydride and formic acid gave pyrimido triazines 4a‐c and 7a‐c , respectively. Their reactions with active methylene containing reagents gave the corresponding 2‐(1‐pyrazonyl)‐N‐methyl pyrimidine derivatives 9a‐c and 10a‐c , respectively. Their reactions with aromatic aldehydes afforded the corresponding 2‐hydrazono pyrimidine derivatives 11a‐c . The structure of these reactions products were established based on both elemental analysis and spectral data studies. 相似文献
994.
A stable inorganic-organic hybrid multilayer film with homogeneous and dense inorganic nanoparticle deposition was constructed by coating ZrO2 nanoparticles with poly(4-sodium styrenesulfonate) (PSS) and irradiating multilayer film assembled from PSS-coated ZrO2 nanoparticles and a diazo-resin (DR). 相似文献
995.
Two new cyclopeptides, named arenariphilin A ( 1 ) and arenariphilin B ( 2 ), were isolated from the whole plants of Arenaria oreophila. Their structures were determined as cyclo‐(Thr‐Gly) ( 1 ) and cyclo‐(Ser1‐Gly ‐Ser2‐Ile ‐Phe1‐Phe2) ( 2 ) on the basis of spectral data, especially by 2D‐NMR. 相似文献
996.
Elisabete Curti Douglas de Britto Srgio Paulo Campana‐Filho 《Macromolecular bioscience》2003,3(10):571-576
N,N,N‐trimethylchitosan (TMC) was prepared by reacting purified chitosan with iodomethane, in the presence of sodium hydroxide, water and sodium iodide, at room temperature. The reaction medium was N‐methyl‐2‐pyrrolidone. Different samples of TMC were obtained by adding to the reaction medium a fixed volume (5.5 mL) of aqueous NaOH solutions at different concentrations (15, 20, 30 and 40 wt.‐%) and carrying out the reactions for 9 or 24 h. The features observed in the 1H and 13C NMR spectra of these chitosan derivatives, in respect of the chemical shift, number and relative intensity of the signals, depended strongly on the excess of NaOH and H3CI added to the reaction medium, but when the lowest excess was employed, the characteristics of the derivative were not affected by the reaction time to the same extent. The average degree of quaternization of these N‐methylated derivatives of chitosan ranged from 10.5% to 44.8%, according to the reaction conditions. Increasing the excess of NaOH, in reactions carried out for 9 h, resulted in TMC samples with progressively higher content of trimethylated sites however, the reaction yields were correspondingly lower and O‐alkylation was favored in these cases.
997.
998.
999.
Ben‐Yong Lou Yan‐Bin Huang 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(4):o246-o248
4,4′‐Bipyridyl N,N′‐dioxide crystallizes with 3‐hydroxy‐2‐naphthoic acid to give a centrosymmetric three‐component adduct, C10H8N2O2·2C11H8O3, which is engineered into a two‐dimensional layer structure by two kinds of π–π interactions. Weak C—H⋯O interactions further link the two‐dimensional structure into a three‐dimensional structure. 相似文献
1000.
Some azeto[2,1-a]isoquinolin-2-ones were synthesized from 2-(3,4-dimethoxyphenyl)ethylamine in three steps in good yield. 相似文献