全文获取类型
收费全文 | 153857篇 |
免费 | 22661篇 |
国内免费 | 17937篇 |
专业分类
化学 | 104543篇 |
晶体学 | 1881篇 |
力学 | 10083篇 |
综合类 | 1386篇 |
数学 | 19496篇 |
物理学 | 57066篇 |
出版年
2024年 | 457篇 |
2023年 | 2685篇 |
2022年 | 4794篇 |
2021年 | 5120篇 |
2020年 | 5382篇 |
2019年 | 5265篇 |
2018年 | 4803篇 |
2017年 | 4633篇 |
2016年 | 6606篇 |
2015年 | 6832篇 |
2014年 | 8269篇 |
2013年 | 10929篇 |
2012年 | 12902篇 |
2011年 | 13490篇 |
2010年 | 9476篇 |
2009年 | 9465篇 |
2008年 | 10196篇 |
2007年 | 9064篇 |
2006年 | 8485篇 |
2005年 | 7107篇 |
2004年 | 5650篇 |
2003年 | 4528篇 |
2002年 | 4300篇 |
2001年 | 3679篇 |
2000年 | 3351篇 |
1999年 | 3233篇 |
1998年 | 2752篇 |
1997年 | 2468篇 |
1996年 | 2535篇 |
1995年 | 2193篇 |
1994年 | 2018篇 |
1993年 | 1746篇 |
1992年 | 1571篇 |
1991年 | 1314篇 |
1990年 | 1136篇 |
1989年 | 940篇 |
1988年 | 760篇 |
1987年 | 669篇 |
1986年 | 619篇 |
1985年 | 608篇 |
1984年 | 403篇 |
1983年 | 323篇 |
1982年 | 269篇 |
1981年 | 207篇 |
1980年 | 151篇 |
1979年 | 134篇 |
1978年 | 102篇 |
1977年 | 97篇 |
1974年 | 94篇 |
1973年 | 105篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Mexiletine— and lysine hydrochloride—o-phthalaldehyde and mexiletine hydrochloride—, cysteine—, cysteamine—, homocysteine— and lysine hydrochloride—fluorescamine derivatives were subjected to Triton and β-cyclodextrin enhancement treatments. Of several classical fluorescence-enhancing reagents tested (Triton, β-cyclodextrin, sodium dodecyl sulphate, Brij), Triton provided the best results, followed by β-cyclodextrin. Increases in fluorescence emission by a factor of up to about 10 (mexiletine—fluorescamine—Triton X-100) were observed, with a generally negligible influence of the enhancing reagents on the excitation and emission maxima. Fluorescence enhancement by the addition of a suitable reagent solution to the final analyte solution may, in specific instances, enhance the detectability of native or chemically induced fluorophores. 相似文献
992.
SYNTHESIS OF MESOPOROUS POLY(STYRENE-co-MALEIC ANHYDRIDE)/SILICA HYBRID MATERIALS VIA A NONSURFACTANT-TEMPLATED SOL-GEL PROCESS* 总被引:1,自引:0,他引:1
Jie-bin Pang Kun-yuan Qiu Yen Wei Department of Polymer Science Engineering College of Chemistry Molecular Engineering Peking University Beijing China. Department of Chemistry Drexel University Philadelphia Pennsylvania USA. 《高分子科学》2000,(5):469-472
Mesoporous poly (styrene-co-maleic anhydride)/silica hybrid materials have been prepared. The synthesis wasachieved by the HCl-catalyzed sol-gel reactions of tetraethyl orthosilicate (TEOS) and styrene-maleic anhydride copolymerin the presence of 3-aminopropyl triethoxysilane (APTES) as a coupling agent and citric acid as a nonsurfactant template orpore-forming agent, followed by ethanol extraction. Characterization results from nitrogen sorption isotherms and powder X-ray diffraction indicate that polymer-modified mesoporous materials with large specific surface areas (e.g. 900 m~2/g) andpore volumes (e.g. 0.6 cm~3/g) could be prepared. As the citric acid concentration is increased, the specific surface areas, porevolumes and pore diameters of the hybrid materials increase. 相似文献
993.
Poly(4-methyl-1-pentene) (PMP) hollow fiber membranes were prepared by the melt-spun and cold-stretch(MSCS) method. Scanning electronic microscopy (SEM) was used to characterize the section and surface structures of themembranes with special asymmetric structure. The preliminary results of gas permeation measurements indicated that the resultant hollow fiber membranes have the potential ability for oxygen/nitrogen separation. 相似文献
994.
Adsorption, desorption and activities of acid phosphatase on various colloidal particles from an Ultisol 总被引:1,自引:0,他引:1
Adsorption, desorption and activity of acid phosphatase on various soil colloidal particles and pure clay minerals were studied. Higher adsorption amounts and low percentage of desorption of acid phosphatase were found on fine soil clays (<0.2 μm). Electrostatic force and ligand exchange are the major driving forces that are involved in the adsorption of enzymes on soil clays. More enzyme molecules were adsorbed on soil clays in the presence of organic components. However, enzymes on organic clays were more easily released. One-third of the enzyme on goethite was adsorbed via ligand exchange process. Some other interactions, such as van der Waals force, hydrophobic force and hydrogen bonding may be more important in the adsorption of enzyme on kaolinite and the enzyme in this system cannot be easily removed. Coarse clays (0.2–2 μm) and inorganic soil clays had higher affinities for enzyme molecules than fine clays and organic clays, respectively. The activity of enzyme bound on soil clays was inhibited and the thermal stability was increased in the presence of organic matter. Data obtained in this study are helpful for a better understanding of the interactions of enzymes with inorganic and organic constituents in soil and associated environments. 相似文献
995.
The bifunctional chiral thiourea-tertiary amine organocatalysts have been applied to a direct asymmetric vinylogous Michael addition of α,α-dicyanoolefins to nitroolefins with 2-10 mol % catalyst loadings. The electronic properties of the thiourea-based catalysts have significant influences on this reaction. Moderate to excellent enantioselectivities (57-95% ee) have been achieved with low to good isolated yields through fine tuning the structures of the bifunctional organocatalysts. Much better ees were obtained for some α,α-dicyanoolefinic substrates compared with that catalyzed by modified cinchona alkaloids. 相似文献
996.
溶胶凝胶法制氧化铝负载铜基超细粒子催化剂的研究 总被引:9,自引:1,他引:9
采用溶胶凝胶法制氧化铝负载铜基超细粒子催化剂的制备了氧化铝负载的铜基超细粒子催化剂,用TG-DTA,XRD,BET以及TEM等技术手段,对催化剂的物相结构,表面孔结构,粒子形貌以及催化性能等进行了研究。结果表明,利用溶胶凝胶法,可以直接帛备出高比表面积,低堆积密度的纤维状纳米级负载型Cu/Al2O3超细粒子;活性组分以远低于纳米级的微晶粒子簇状态,均匀地分散在纳米级氧化铝载体表面上,而且在低于50 相似文献
997.
998.
水中酚的膜萃取分离流动注射技术测定的研究 总被引:6,自引:1,他引:6
本文提出了硅橡胶膜在线萃取分离流动注射技术测定水样中苯酚的方法。由于水样中的苯酚透过硅橡胶进入萃取液的萃取量随时间的增加和温度的升高而增大,故采用停流技术及升高温度以降低酚的检出限。 相似文献
999.
任意多阶梯度场强毛细管凝胶电泳中谱带的迁移和展宽 总被引:2,自引:0,他引:2
在自行制备的毛细管凝胶电泳柱上,通过实验考察毛细管凝胶电泳(CGE)中场强(E)和组分迁移率(μ)的关系,发现在一般CGE使用的场强范围内,μ随E增大而成近似线性的增加。并讨论了产生这种现象的原因。以此为基础提供了任意多阶梯度场强毛细管凝胶电泳中组分的迁移时间和距离的计算公式,用于编制计算机程序。 相似文献
1000.