全文获取类型
收费全文 | 2932篇 |
免费 | 5篇 |
国内免费 | 231篇 |
专业分类
化学 | 2737篇 |
力学 | 7篇 |
数学 | 13篇 |
物理学 | 411篇 |
出版年
2022年 | 2篇 |
2021年 | 1篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 1篇 |
2014年 | 5篇 |
2013年 | 1篇 |
2012年 | 258篇 |
2011年 | 324篇 |
2010年 | 40篇 |
2009年 | 125篇 |
2008年 | 427篇 |
2007年 | 300篇 |
2006年 | 350篇 |
2005年 | 348篇 |
2004年 | 244篇 |
2003年 | 185篇 |
2002年 | 139篇 |
2001年 | 87篇 |
2000年 | 107篇 |
1999年 | 35篇 |
1998年 | 13篇 |
1997年 | 8篇 |
1996年 | 37篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 46篇 |
1992年 | 24篇 |
1991年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1986年 | 3篇 |
1979年 | 1篇 |
排序方式: 共有3168条查询结果,搜索用时 15 毫秒
1.
2.
The effect of oxidation pretreatment temperature(500 ~ 1 000 ℃) on the catalytic activity of Kovar applied on hydrocarbon CO2reforming was examined. Catalytic performance evaluation using tetradecane at 800 ℃ with 70 μmol/s CO2revealed 700 and 1 000 ℃ as the best pre-oxidation temperature in producing CO and H2,respectively. XRD and SEM-EDX analyses showed that a separate metal oxide layer composed of iron oxide(Fe2O3and F3O4),nickel,cobalt,and possibly their respective oxides started to form when oxidation was conducted at 700 ℃ or higher.The presence of iron enhanced the stability of nickel in the structure while the compact structure of Fe3O4resulted into the formation of a thick and rigid metal oxide layer on the surface of the Kovar tube. The strong physical bond between the metal oxide layer and Kovar tube provided the catalyst good mechanical strength and consequently good catalytic activity. 相似文献
3.
本文设计开发了一种以2,6-二甲酰基对甲苯酚为母体的新型荧光探针HMI,可用于高效识别EtOH-H2O (8/2, v/v, HEPES 10 mM, pH =7.4)体系中的CO32-。HMI在660 nm处显示发射带,加入CO32-后,在600 nm的等吸收点激发时,原来在660 nm处的荧光淬灭,而以540 nm为中心的新发射带荧光显着增加,为比率型荧光探针。HMI对CO32-表现出高选择性且具有较强的抗干扰能力。此外,荧光探针HMI对CO32-荧光响应的检测限较低,可达到3.938×10-6 M。更具有意义的是,HMI探针对CO32-的检测能够在实际水样中起到很好的应用,而且细胞成像研究表明,HMI可用于活体MCF-7细胞中CO32-的成像。 相似文献
4.
Gómez-Graña S Hubert F Testard F Guerrero-Martínez A Grillo I Liz-Marzán LM Spalla O 《Langmuir : the ACS journal of surfaces and colloids》2012,28(2):1453-1459
Gold nanorods in aqueous solution are generally surrounded by surfactants or capping agents. This is crucial for anisotropic growth during synthesis and for their final stability in solution. When CTAB is used, a bilayer has been evidenced from analytical methods even though no direct morphological characterization of the precise thickness and compactness has been reported. The type of surfactant layer is also relevant to understand the marked difference in further self-assembling properties of gold nanorods as experienced using 16-EO(1)-16 gemini surfactant instead of CTAB. To obtain a direct measure of the thickness of the surfactant layer on gold nanorods synthesized by the seeded growth method, we coupled TEM, SAXS, and SANS experiments for the two different cases, CTAB and gemini 16-EO(1)-16. Despite the strong residual signal from micelles in excess, it can be concluded that the thickness is imposed by the chain length of the surfactant and corresponds to a bilayer with partial interdigitation. 相似文献
5.
In this study the formation of the lactyl-thiamin diphosphate intermediate (L-ThDP) is addressed using density functional theory calculations at X3LYP/6-31++G(d,p) level of theory. The study includes potential energy surface scans, transition state search, and intrinsic reaction coordinate calculations. Reactivity is analyzed in terms of Fukui functions. The results allow to conclude that the reaction leading to the formation of L-ThDP occurs via a concerted mechanism, and during the nucleophilic attack on the pyruvate molecule, the ylide is in its AP form. The calculated activation barrier for the reaction is 19.2?kcal/mol, in agreement with the experimental reported value. 相似文献
6.
AM García-Deibe C Portela-García M Fondo AJ Mota J Sanmartín-Matalobos 《Chemical communications (Cambridge, England)》2012,48(79):9915-9917
A decanuclear Cd complex has been found as a carbonate-containing capsule. The structure strongly resembles a ten-blade waterwheel with a central carbonate ligand surrounded by two superimposed Cd(5)O(5) crowns with a pentagonal antiprism-like disposition. The capsule is doubly capped by two pentadentate perchlorate anions. 相似文献
7.
Rodriguez AM Cebrián C Prieto P García JI de la Hoz A Díaz-Ortiz Á 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(20):6217-6224
A DFT computational mechanistic study of the [2+2+2] cyclotrimerization of a diyne with benzonitrile, catalyzed by a cobalt complex, has been carried out. Three alternative catalytic cycles have been examined together with the precatalytic step (responsible for the induction period). The favored mechanism takes place by means of an intramolecular metal-assisted [4+2] cycloaddition. The beneficial role of microwave activation has been studied. It is concluded that microwave irradiation can decrease the catalytic induction period through thermal effects and can also increase the triplet lifetime and promote the reaction, thus improving the final yield. 相似文献
8.
Electronic devices endowed with camera platforms require new and powerful machine vision applications, which commonly include moving object detection strategies. To obtain high-quality results, the most recent strategies estimate nonparametrically background and foreground models and combine them by means of a Bayesian classifier. However, typical classifiers are limited by the use of constant prior values and they do not allow the inclusion of additional spatiodependent prior information. In this Letter, we propose an alternative Bayesian classifier that, unlike those reported before, allows the use of additional prior information obtained from any source and depending on the spatial position of each pixel. 相似文献
9.
Villaverde-de-Sáa E Quintana JB Rodil R Ferrero-Refojos R Rubí E Cela R 《Analytical and bioanalytical chemistry》2012,402(1):509-518
Perfluorinated compounds (PFCs) have been used for over 40 years in different commercial and industrial applications mainly
as surfactants and surface protectors and have become an important class of marine emerging pollutants. This study presents
the development and validation of a new analytical method to determine the simultaneous presence of eight PFCs in different
kinds of mollusks using matrix solid-phase dispersion (MSPD) followed by liquid chromatography–tandem mass spectrometry (LC-MS/MS).
Simplicity of the analytical procedure, low volume of solvent and quantity of sample required, low global price, and integration
of extraction and clean-up into a single step, are the most important advantages of the developed methodology. Solvent, solid
support (dispersing agent), clean-up sorbent, and their amounts were optimized by means of an experimental design. In the
final method, 0.5 g of sample are dispersed with 0.2 g of diatomaceous earth and transferred into a polypropylene syringe
containing 4 g of silica as clean-up sorbent. Then, analytes are eluted with 20 mL of acetonitrile. The extract is finally
concentrated to a final volume of 0.5 mL in methanol, avoiding extract dryness in order to prevent evaporation losses and
injected in the LC-MS/MS. The combination of this MSPD protocol with LC-MS/MS afforded detection limits from 0.05 to 0.3 ng g−1. Also, a good linearity was established for the eight PFCs in the range from limit of quantification (LOQ) to 500 ng mL−1 with R
2 > 0.9917. The recovery of the method was studied with three types of spiked mollusk and was in the 64–126% range. Moreover,
a mussel sample was spiked and aged for more than 1 month and analyzed by the developed method and a reference method, ion-pair
extraction, for comparison, producing both methods statistically equal concentration values. The method was finally applied
to the determination of PFCs in different kinds of mollusks revealing concentrations up to 8.3 ng g−1 for perfluoroundecanoic acid. 相似文献
10.
Reboredo S Reyes E Vicario JL Badía D Carrillo L de Cózar A Cossío FP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(23):7179-7188
The catalytic enantioselective [3+2] cycloaddition between azomethine ylides and α,β-unsaturated aldehydes catalyzed by α,α-diphenylprolinol has been studied in detail. In particular, the reaction has been extended to the use of 2-alkenylidene aminomalonates generated in situ as azomethine ylide precursors. These reactions lead to the formation of pyrrolidines containing a 5-alkenyl side chain with potential for chemical manipulation. Moreover, a detailed and concise computational study has been carried out to understand the exact nature of the mechanism of this reaction and especially the consequences derived from the incorporation of the chiral secondary amine catalyst on the reaction pathway. 相似文献