全文获取类型
收费全文 | 1683篇 |
免费 | 113篇 |
国内免费 | 50篇 |
专业分类
化学 | 1284篇 |
晶体学 | 12篇 |
力学 | 77篇 |
综合类 | 2篇 |
数学 | 185篇 |
物理学 | 286篇 |
出版年
2024年 | 3篇 |
2023年 | 9篇 |
2022年 | 19篇 |
2021年 | 49篇 |
2020年 | 62篇 |
2019年 | 90篇 |
2018年 | 90篇 |
2017年 | 72篇 |
2016年 | 128篇 |
2015年 | 86篇 |
2014年 | 122篇 |
2013年 | 208篇 |
2012年 | 143篇 |
2011年 | 143篇 |
2010年 | 114篇 |
2009年 | 90篇 |
2008年 | 120篇 |
2007年 | 58篇 |
2006年 | 60篇 |
2005年 | 27篇 |
2004年 | 47篇 |
2003年 | 16篇 |
2002年 | 18篇 |
2001年 | 6篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1970年 | 1篇 |
1964年 | 1篇 |
1941年 | 1篇 |
1937年 | 2篇 |
1927年 | 3篇 |
排序方式: 共有1846条查询结果,搜索用时 15 毫秒
131.
Hamid Reza Memarian Asadollah Farhadi 《Monatshefte für Chemie / Chemical Monthly》2009,12(5):1217-1220
Abstract
UV light irradiation of Biginelli 3,4-dihydropyrimidin-2(1H)-ones in chloroform in an argon atmosphere leads to dehydrogenation of these compounds to their corresponding pyrimidin-2(1H)-ones in excellent yields. Irradiation in the same solvent under an oxygen atmosphere generates, in addition, various hitherto unidentified products. A light-induced electron transfer from the substrate to the solvent is proposed as the initial event, supported by the detection of dichloromethane and hydrogen chloride in the photolysate. 相似文献132.
Ajayan Vinu Pranjal Kalita Hamid Oveisi Ajayan Mano B.V. Subba Reddy 《Tetrahedron letters》2009,50(51):7132-1489
Here we demonstrate for the first time the synthesis of α-aminophosphonates through the three-component coupling reaction of aldehydes, amines, and diethyl phosphite by using highly acidic 3D mesoporous aluminosilicate nanocage catalyst, which gave excellent yield with a high selectivity in a short reaction time due to its high acidity, 3D pores, and a huge space in the nanocages. 相似文献
133.
Experimental Techniques - 相似文献
134.
Dinuclear copper(II) complex with the general formula [(diamine)Cu(OH)]2(ClO4)2, where diamine stands for N,N-dialkyl,N′-benzyl-ethylenediamine, were prepared. The complexes were characterized by elemental analysis, spectroscopic, conductance measurements, and X-ray structural analysis. The complexes are soluble in various organic solvents and show positive solvatochromism. Thermochromism was also observed particularly in strong donor solvents. 相似文献
135.
Mohammad Ali Karimi Mohammad Mazloum Ardakani Hamid Abdollahi Fatemeh Banifatemeh 《Analytical sciences》2008,24(2):261-266
Simultaneous determination of hydrazine (HZ) and phenylhydrazine (PHZ) by H-point standard addition method (HPSAM) and partial least squares (PLS) regression was carried out based on kinetic data from novel potentiometry methods. The rate of chloride ion production in the reaction of HZ and PHZ with N-chlorosuccinimide (NCS) was monitored by a chloride ion-selective electrode. The experimental data show the good ability of ion-selective electrodes (ISEs) as detectors not only for the direct determination of chloride ion but also for simultaneous kinetic-potentiometric analysis using HPSAM and PLS methods. The methods are based on the differences observed in the production rate of chloride ions. The results show that simultaneous determination of HZ and PHZ can be performed in concentration ranges of 0.5 - 20.0 and 0.8 - 25.0 microg mL(-1), respectively. The total relative standard error for applying the PLS method to 8 synthetic samples in the concentration ranges of 1.0 - 16.0 microg mL(-1) for HZ and 2.0 - 16.0 microg mL(-1) for PHZ was 3.96. In order for the selectivity of the method to be assessed, we evaluated the effects of certain foreign ions upon the reaction rate and assessed the selectivity of the method. Both methods (PLS and HPSAM) were evaluated using a set of synthetic sample mixtures and then applied for simultaneous determination of HZ and PHZ in water samples. 相似文献
136.
137.
138.
An unexpected three-component condensation reaction between an isocyanide, isoquinoline and a strong CH-acid efficiently provides 1,2-dihydroisoquinoline derivatives in a one-pot reaction in water at 70 °C without using any catalyst. 相似文献
139.
A tri-enzyme electrode has been developed for determining starch in a flowing stream based on amperometric monitoring of hydrogen peroxide at a potential of +600 mV versus a silver--silver chloride reference electrode. The nylon-based starch-sensing membranes (over a platinum electrode) were prepared from an enzyme cocktail containing various ratios of amyloglucosidase (AMG), mutarotase (MUT) and glucose oxidase (GO). The best starch-sensing membrane (to give the type A electrode) was made from an enzyme cocktail of AMG--MUT--GO (2000 + 100 + 100 U; where 1 U = 16.67 nanokatals), i.e., containing a high level of AMG. In this system, starch samples were first incubated with soluble alpha-amylase (alpha-AMY) (1000 U) for 1 h at room temperature prior to analysis with the tri-enzyme electrode. Attempts were also made to immobilize alpha-AMY on to nylon net, either alone or as a component of a four-membrane starch-enzyme electrode but starch signals were weak compared with those generated by starch pre-treated with soluble alpha-AMY. This system, associated with the type A starch electrode, not only exhibited a calibration of wide linear range (1 x 10(-4)-0.1% m/v starch) but also showed promising operational properties. It has excellent thermal stability over the range 30-70 degrees C. 相似文献
140.
Synthesis, characterization and properties of novel thermally stable poly(urethane-oxazolidone) elastomers 总被引:2,自引:0,他引:2
Oxazolidone-incorporated polyurethane elastomers based on hydroxyl terminated polycaprolactone, were synthesized and characterized. Reaction of epoxy-terminated polyurethane with isocyanate librated from a blocked polyisocyanate was the strategy followed. The reaction condition was optimized through preparation of model oxazolidones. Epoxy-terminated polyurethanes were prepared from reaction of glycidol with NCO-terminated polyurethane prepolymers and curing agent was synthesized from the reaction of trimethylol propane, toluene diisocyanate and N-methyl-aniline. Incorporation of heterocyclic oxazolidone groups into the polyurethane backbone caused improvements in thermal and mechanical properties. Investigation of structure-property relationship for prepared elastomers showed that the main determining factors for observed properties were crosslink density, crystallinity and content of oxazolidone rings. 相似文献