全文获取类型
收费全文 | 3270篇 |
免费 | 560篇 |
国内免费 | 396篇 |
专业分类
化学 | 2418篇 |
晶体学 | 37篇 |
力学 | 242篇 |
综合类 | 17篇 |
数学 | 325篇 |
物理学 | 1187篇 |
出版年
2024年 | 7篇 |
2023年 | 77篇 |
2022年 | 128篇 |
2021年 | 140篇 |
2020年 | 155篇 |
2019年 | 136篇 |
2018年 | 143篇 |
2017年 | 113篇 |
2016年 | 179篇 |
2015年 | 166篇 |
2014年 | 167篇 |
2013年 | 230篇 |
2012年 | 339篇 |
2011年 | 366篇 |
2010年 | 231篇 |
2009年 | 177篇 |
2008年 | 188篇 |
2007年 | 190篇 |
2006年 | 171篇 |
2005年 | 119篇 |
2004年 | 89篇 |
2003年 | 96篇 |
2002年 | 57篇 |
2001年 | 49篇 |
2000年 | 58篇 |
1999年 | 54篇 |
1998年 | 55篇 |
1997年 | 60篇 |
1996年 | 44篇 |
1995年 | 30篇 |
1994年 | 28篇 |
1993年 | 34篇 |
1992年 | 33篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 5篇 |
1986年 | 16篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 7篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1973年 | 1篇 |
1970年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有4226条查询结果,搜索用时 31 毫秒
981.
采用共沉淀法制备了一系列 CexZr0.9-xLa0.1O1.95-Al2O3 储氧材料 (其中 CexZr0.9-xLa0.1O1.95 与 Al2O3 的质量比为 1; x = 0, 0.3, 0.5, 0.7 和 0.9), 以其为载体, 制备了一系列负载铁基整体式催化剂. 考察了该系列催化剂催化甲烷燃烧反应性能, 并用低温 N2 吸附-脱附、储氧能力测试、X 射线衍射和 H2 程序升温还原等手段对载体和催化剂进行了表征. 活性测试结果表明, 当 x = 0.7 时, 催化剂活性最高, 可在 V(CH4) = 1% 和 50 000 h1 条件下使甲烷 465 oC 起燃, 615 oC 完全转化. CexZr0.9-xLa0.1O1.95-Al2O3 样品同时具有较高的储氧能力、较高的比表面积和抗热老化能力. 当 x = 0.5 和 0.7 时, 催化剂载体以 CeZrLaAlO 固溶体存在, 抗老化能力最好, 且 x = 0.7 时, 催化剂最易被还原. 相似文献
982.
A new chitosan imprinting adsorbent using diatomite as core material was prepared by using the surface molecular imprinting
technology with the Pb2+ as imprinted ion. The preparation process conditions of the surface molecular imprinting adsorbent were studied. The adsorbent
was characterized by using Fourier transform infrared (FTIR) spectrum. FTIR spectrum indicated that it was cross-linked by
epichlorohydrin. The new imprinting adsorbent could provide a higher adsorption capacity for Pb2+, which reached 139.6 mg/g increasing 32.3% compared with cross-linking chitosan adsorbent (the initial Pb2+ concentration of 600 mg/L). The adsorption velocity was quick and the equilibration time of the imprinting adsorbent for
Pb2+ was 3 h that shortened about 40% compared with cross-linking chitosan adsorbent. It had a more wide pH range of 5–7 than
that of cross-linking chitosan adsorbent. The new imprinting adsorbent can be reused for up to ten cycles without loss of
adsorption capacity. In the kinetics and isotherm study, the pseudosecond order model and Langmuir model could represent the
adsorption process. 相似文献
983.
Ke-Jing Huang Cong-Hui Han Jun-Yong Sun De-Jun Niu Zhi-Wei Wu Ya-Ping Xue Li-Juan Zhang Xiao-Qin Xiong 《Chromatographia》2010,71(11-12):1049-1054
The purpose of the present work is to develop a simple, rapid, sensitive and accurate method for the derivatization and subsequently preconcentration of Hg(II) and the determination of its derivative, diphenylmercury, in natural water samples using gas chromatography-flame ionization detection. The method is based on the diphenylation using phenyl boronic acid, subsequent extraction of phenylmercury into a single drop of an organic solvent (toluene), followed by gas chromatography-flame ionization detection GC-FID analysis of the extract. The pH of the feed solution was kept in pH 5 with acetate buffer solution. Thus, the optimized conditions are: organic solvent, toluene; derivatization time, 10 min; extraction time, 15 min; microdrop volume, 1.6 μL; stirring rate, 600 rpm; sample volume, 5 mL. The limit of detection (LOD), calculated on the basis of five replicates was 0.02 μg mL?1. The relative standard deviation of the method (RSD%, n = 5) was 3.0. Linear range was between 0.05 and 5 μg mL?1 and preconcentration factor obtained for phenyl-mercury was 105. 相似文献
984.
对于底物不溶于水的纤维素降解反应而言,为了增强纤维素酶的活性,在丁二酸二异辛酯磺酸钠(AOT)/异辛烷反胶束体系中加入非离子表面活性剂TritonX-100进行纤维素降解实验.结果表明,在AOT中加入非离子表面活性剂TritonX-100可以使纤维素酶的活性提高,非离子表面活性剂TritonX-100与AOT的最佳物质的量之比是0.20.考察了水与表面活性剂的物质的量之比(Wo)、不同酸度(pH)和不同温度(T)等其他反应条件对纤维素降解反应的影响.研究结果表明,反应的最佳条件是:Wo为3.3,T为315.11K,pH为5.10. 相似文献
985.
A facile strong cation-exchange stationary phase based on propylsulfonic acid functionalized silica was prepared through the oxidation in situ of 3-mercaptopropyl silica and characterized by elemental analysis and X-ray photoelectron spectroscopy. The phase was used to separate monovalent and divalent inorganic cations with strong acid as mobile phase. Simultaneous separation of alkali metals and alkaline earth metals was examined and the cations sodium, magnesium and calcium in tap water were determined. 相似文献
986.
Shengxiao Zhang Hongyun Niu Zhengjun Hu Yaqi Cai Yali Shi 《Journal of chromatography. A》2010,1217(29):4757-4764
The carbon coated Fe3O4 nanoparticles (Fe3O4/C) were synthesized by a simple hydrothermal reaction and applied as solid-phase extraction (SPE) sorbents to extract trace polycyclic aromatic hydrocarbons (PAHs) from environmental water samples. The Fe3O4/C sorbents possess high adsorption capacity and extraction efficiency due to strong adsorption ability of carbon materials and large surface area of nanoparticles, and only 50 mg of sorbents are required to extract PAHs from 1000 mL water samples. The adsorption attains equilibrium rapidly and analytes are eluted with acetonitrile readily. Salinity and solution pH have no obvious effect on the recoveries of PAHs, which avoids fussy adjustment to water sample before extraction. Under optimized conditions, the detection limits of PAHs are in the range of 0.2–0.6 ng L−1. The accuracy of the method was evaluated by the recoveries of spiked samples. Good recoveries (76–110%) with low relative standard deviations from 0.8% to 9.7% are achieved. This new SPE method provides several advantages, such as high extraction efficiency, high breakthrough volumes, convenient extraction procedure, and short analysis times. To our knowledge, this is the first time that Fe3O4/C nanoparticles are used for the pretreatment of environmental water samples. 相似文献
987.
Huang YL Wang R Niu TC Kera S Ueno N Pflaum J Wee AT Chen W 《Chemical communications (Cambridge, England)》2010,46(47):9040-9042
Molecular dipole chain arrays of chloroaluminium phthalocyanine (ClAlPc) on the graphite surface have been investigated by scanning tunneling microscopy. The inter-chain spacing can be tuned by the co-adsorption of di-indenoperylene (DIP) via nanoscale phase separation. 相似文献
988.
Jixia Song Junhua Yuan Fei Li Dongxue Han Jiangfeng Song Li Niu 《Journal of Solid State Electrochemistry》2010,14(10):1915-1922
Oxygen electrochemical reduction on gold–polyaniline (Au–PANI) porous nanocomposite-modified glassy carbon electrode in basic
media was described. The as-prepared Au–PANI porous nanocomposite showed superior tunable activity for electrochemical reduction
of oxygen. The specific surface area of Au–PANI porous nanocomposites was evaluated to be about 11.3 m2 g−1 through a convenient voltammetric approach. Rotating ring-disk electrode experiments further demonstrated the number of electrons
exchanged in oxygen reduction increased from 2e to 4e with increasing the trigger potential from 300, to 500, 700 mV. The
tunable activity in electrochemical reduction of oxygen was achieved as a result of positive potential-induced formation and
reduction of Au surface oxide. However, the tunable oxygen reduction reaction is fit for applying potential in a linear positive-going
potential sweep. Irreversible ORR tunability was found after a more active surface formed at 700 mV. To optimize the applied
potential window on these Au-based porous materials has potential applications such as in electrochemical sensing, fuel cells,
or getting rid of the interference from the coexisted substances. 相似文献
989.
Niu Z He J Russell TP Wang Q 《Angewandte Chemie (International ed. in English)》2010,49(52):10052-10066
The generation of novel multifunctional materials with hierarchical ordering is a major focus of current materials science and engineering. For such endeavors, fluid interfaces, such as air-liquid and liquid-liquid interfaces, offer ideal platforms where nanoparticles or colloidal particles can accumulate and self-assemble. Different assembly processes and reactions have been performed at fluid interfaces to generate hierarchical structures, including two-dimensional crystalline films, colloidosomes, raspberry-like core-shell structures, and Janus particles, which lead to broad applications in drug delivery and controlled release, nanoelectronics, sensors, food supplements, and cosmetics. 相似文献
990.