全文获取类型
收费全文 | 6506篇 |
免费 | 306篇 |
国内免费 | 67篇 |
专业分类
化学 | 4555篇 |
晶体学 | 76篇 |
力学 | 290篇 |
综合类 | 1篇 |
数学 | 726篇 |
物理学 | 1231篇 |
出版年
2024年 | 21篇 |
2023年 | 70篇 |
2022年 | 530篇 |
2021年 | 410篇 |
2020年 | 277篇 |
2019年 | 289篇 |
2018年 | 257篇 |
2017年 | 200篇 |
2016年 | 350篇 |
2015年 | 210篇 |
2014年 | 281篇 |
2013年 | 525篇 |
2012年 | 400篇 |
2011年 | 379篇 |
2010年 | 237篇 |
2009年 | 200篇 |
2008年 | 251篇 |
2007年 | 252篇 |
2006年 | 207篇 |
2005年 | 156篇 |
2004年 | 155篇 |
2003年 | 131篇 |
2002年 | 151篇 |
2001年 | 69篇 |
2000年 | 47篇 |
1999年 | 41篇 |
1998年 | 37篇 |
1997年 | 27篇 |
1996年 | 37篇 |
1995年 | 39篇 |
1994年 | 41篇 |
1993年 | 34篇 |
1992年 | 37篇 |
1991年 | 39篇 |
1990年 | 34篇 |
1989年 | 49篇 |
1988年 | 44篇 |
1987年 | 29篇 |
1986年 | 32篇 |
1985年 | 31篇 |
1984年 | 39篇 |
1983年 | 28篇 |
1982年 | 19篇 |
1981年 | 25篇 |
1980年 | 24篇 |
1979年 | 18篇 |
1978年 | 23篇 |
1977年 | 18篇 |
1976年 | 16篇 |
1973年 | 11篇 |
排序方式: 共有6879条查询结果,搜索用时 16 毫秒
991.
Pesticides are necessary for increasing agricultural productivity; however, their enormous use contaminates air, water and food. Among various organic pesticides, organochlorine pesticides (OCPs) are most persistent; and though their use is banned, they are still used illegally. In contrast to OCPs, organophosphorous pesticides are less persistent and used most extensively, while synthetic pyrethroid pesticides are the least toxic and used as insecticides. Extensive use of these pesticides is vulnerable to the ecosystem. Various extraction methods are used worldwide both by the regulatory bodies and private laboratories for the determination of multi-residue pesticides in leafy vegetables. This mini review presents an update on extraction procedure in gas chromatographic methods of pesticides analysis in various samples with special emphasis on leafy vegetables. We have covered six years of work from 2008–2013, discussing various extraction methods and their applications. 相似文献
992.
Phenol is very useful intermediate in the manufacture of petrochemicals, drugs, agrochemicals, and plastics. Commercially, phenol is produced by a three-step, high-energy consumption process known as the cumene process. The conversion of a chemical to a value-added product is always economically desirable. More than 90% of phenol consumption in the world is manufactured by the multistep cumene process, in which acetone is coproduced in 1: 1 molar ratio with respect to phenol. However, the drawbacks of the three-step cumene process have spurred the development of more economical routes to decrease energy consumption, avoid the formation of explosive cumene hydroperoxide, and increase the yield. The objective of this article is to highlight benzene-to-phenol conversion technologies with emphasis on direct conversion methods. Gas phase and liquid phase reactions are the two main routes for direct oxidation of benzene to phenol. Indirect methods, such as the cumene process, and direct methods of benzene-to-phenol conversion are discussed in detail. Also discussed is the single-step reaction of benzene to phenol using oxidants such as O2, N2O, and H2O2. Catalytic conversion of benzene to value-added phenol using a chemically converted graphene-based catalyst, a cost-effective carbon material, is discussed. 相似文献
993.
Ikram Ahmad Tahseen Kamal Sher Bahadar Khan Abdullah M. Asiri 《Cellulose (London, England)》2016,23(6):3577-3588
Re-use of a catalyst is an important task, which is usually achieved by loading it on easily separable supports such as magnetic substrates. However, we demonstrate here the process of easy and fast catalyst separation from a reaction medium by loading it onto an economically feasible and microscopically high surface substrate of filter paper (FP) made up of cellulose microfibers as catalyst support. To achieve the goal, we coated chitosan (CH) on filter paper (CH-FP) to impart a high affinity of the substrate for metal ion absorption. AgNO3 dissolved in water with a 0.1 M concentration was used as the Ag ion carrier solution, and CH-FP strips with known rectangular dimensions were submerged into it for the metal ion absorption. The metal ion-laden CH-FP strips were dip treated with sodium borohydride (NaBH4) aqueous solution to prepare Ag-nanoparticle loaded CH-FP (Ag/CH-FP). X-ray diffraction and energy dispersive X-ray spectroscopy confirmed the formation of the Ag/CH-FP hybrid. Ag/CH-FP morphology was examined through scanning electron microscopy analysis, which showed the presence of Ag nanoparticles attached to the cellulose microfibers. The prepared Ag/CH-FP was employed as a dip catalyst for the degradation of nitroarene compounds of 2-nitophenol (2-NP) and 4-nitrophenol (4-NP) by NaBH4. Remarkably, the rate constants for 4-NP and 2-NP were 3.9 × 10?3 and 1.7 × 10?3 s?1, respectively. In addition, we discussed the ease of the catalyst retrievability from the reaction mixture and its re-usability. 相似文献
994.
Sanaullah Khan Shahnaz Khan Sajida Batool Mushtaq Ahmed 《Natural product research》2016,30(5):570-573
Acid phosphatase-I (Apase-I) from seeds of Nelumbo nucifera was purified to electrophoretic homogeneity by combination of ammonium sulfate precipitation, size-exclusion and ion exchange chromatography. SDS-PAGE of purified Apase-I gave a single band with molecular mass of 80 kDa under reducing and non-reducing conditions, indicating that the enzyme was a monomer. The purified enzyme showed maximum activity at 50°C and at pH 5. The Km, Vmax and Kcat for p-nitrophenyl phosphate were 132 μM, 10 μmol/min/mg and 6.7/sec respectively. Apase-I activity was strongly inhibited by Zn2+, W2+; weakly inhibited by Cu2+, Mo2+ and Cr6+ and moderately activated by Mg2+. The enzyme was shown to be thermolabile as it lost 50% of its activity at 50°C after incubation for 1 hour. The amino acid analysis of enzyme revealed high proportion of acidic amino acids, which is very similar to that of tomato Apase-I and lower than potato Apase. 相似文献
995.
Tijani A. Apalara Salim A. Messaoudi Ahmed A. Keddi 《Mathematical Methods in the Applied Sciences》2016,39(10):2671-2684
In this work, we study the well‐posedness and the asymptotic stability of a one‐dimensional linear thermoelastic Timoshenko system, where the heat conduction is given by Cattaneo's law and the coupling is via the displacement equation. We prove that the system is exponentially stable provided that the stability number χτ=0. Otherwise, we show that the system lacks exponential stability. Furthermore, in the latter case, we show that the solution decays polynomially. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
996.
Synthesis of Hollow Platinum–Palladium Nanospheres with a Dendritic Shell as Efficient Electrocatalysts for Methanol Oxidation
下载免费PDF全文
![点击此处可从《化学:亚洲杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Qingqing Lu Prof. Hongjing Wang Kamel Eid Zeid Abdullah Alothman Dr. Victor Malgras Prof. Yusuke Yamauchi Prof. Liang Wang 《化学:亚洲杂志》2016,11(13):1939-1944
Engineering the size, composition, and morphology of platinum‐based nanomaterials can provide a great opportunity to improve the utilization efficiency of electrocatalysts and reinforce their electrochemical performances. Herein, three‐dimensional platinum–palladium hollow nanospheres with a dendritic shell (PtPd‐HNSs) are successfully fabricated through a facile and economic route, during which SiO2 microspheres act as the hard template for the globular cavity, whereas the triblock copolymer F127 contributes to the formation of the dendritic shell. In contrast with platinum hollow nanospheres (Pt‐HNSs) and commercial platinum on carbon (Pt/C) catalyst, the novel architecture shows a remarkable activity and durability toward the methanol oxidation reaction (MOR) owing to the coupled merits of bimetallic nanodendrites and a hollow interior. As a proof of concept, this strategy is also extended to trimetallic gold–palladium–platinum hollow nanospheres (AuPdPt‐HNSs), which paves the way towards the controlled synthesis of other bi‐ or multimetallic platinum‐based hollow electrocatalysts. 相似文献
997.
In this paper we present new results on two‐weight Hardy, Hardy–Poincaré and Rellich type inequalities with remainder terms on a complete noncompact Riemannian Manifold M. The method we use is flexible enough to obtain more weighted Hardy type inequalities. Our results improve and include many previously known results as special cases. 相似文献
998.
This paper explores enterprise development and commercialization in the field of graphene. Firm characteristics and relationships, value chain positioning, and factors associated with product entry are examined for a set of 65 graphene-oriented small and medium-sized enterprises located in 16 different countries. As well as secondary sources and bibliometric methods to profile developments in graphene, we use computerized data mining and analytical techniques, including cluster and regression modeling, to identify patterns from publicly available online information on enterprise web sites. We identify groups of graphene small and medium-sized enterprises differentiated by how they are involved with graphene, the materials they target, whether they make equipment, and their orientation toward science and intellectual property. In general, access to finance and the firms’ location are significant factors that are associated with graphene product introductions. We also find that patents and scientific publications are not statistically significant predictors of product development in our sample of graphene enterprises. We further identify a cohort of graphene-oriented firms that are signaling plans to develop intermediate graphene products that should have higher value in the marketplace. Our findings suggest that policy needs to ensure attention to the introduction and scale-up of downstream intermediate and final graphene products and associated financial, intermediary, and market identification support. The paper demonstrates novel data methods that can be combined with existing information for real-time intelligence to understand and map enterprise development and commercialization in a rapidly emerging and growing new technology. 相似文献
999.
1000.