全文获取类型
收费全文 | 29207篇 |
免费 | 3647篇 |
国内免费 | 3169篇 |
专业分类
化学 | 21101篇 |
晶体学 | 363篇 |
力学 | 1432篇 |
综合类 | 234篇 |
数学 | 3545篇 |
物理学 | 9348篇 |
出版年
2024年 | 77篇 |
2023年 | 454篇 |
2022年 | 795篇 |
2021年 | 886篇 |
2020年 | 928篇 |
2019年 | 940篇 |
2018年 | 807篇 |
2017年 | 772篇 |
2016年 | 1152篇 |
2015年 | 1181篇 |
2014年 | 1401篇 |
2013年 | 1956篇 |
2012年 | 2308篇 |
2011年 | 2509篇 |
2010年 | 1763篇 |
2009年 | 1736篇 |
2008年 | 1932篇 |
2007年 | 1760篇 |
2006年 | 1579篇 |
2005年 | 1392篇 |
2004年 | 1109篇 |
2003年 | 904篇 |
2002年 | 882篇 |
2001年 | 740篇 |
2000年 | 623篇 |
1999年 | 580篇 |
1998年 | 504篇 |
1997年 | 443篇 |
1996年 | 474篇 |
1995年 | 386篇 |
1994年 | 370篇 |
1993年 | 342篇 |
1992年 | 323篇 |
1991年 | 278篇 |
1990年 | 232篇 |
1989年 | 191篇 |
1988年 | 167篇 |
1987年 | 129篇 |
1986年 | 142篇 |
1985年 | 155篇 |
1984年 | 101篇 |
1983年 | 64篇 |
1982年 | 71篇 |
1981年 | 53篇 |
1980年 | 52篇 |
1979年 | 58篇 |
1978年 | 40篇 |
1976年 | 37篇 |
1974年 | 40篇 |
1973年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
251.
O. de la Iglesia M. Pedernera R. Mallada Z. Lin J. Rocha J. Coronas J. Santamaría 《Journal of membrane science》2006,280(1-2):867-875
MCM-48 membranes have been prepared on alumina supports of different pore sizes. A battery of characterization techniques has been used to study the physical properties and the quality of the membranes prepared. The highest quality membranes were prepared on supports with pore size of up to 60 nm. The MCM-48 membranes were tested in the separation of gas phase mixtures and a cyclohexane/O2 selectivity higher than 270 was obtained. The selective separation of organic compounds from inert components is a result of the cooperative effects of capillary condensation in MCM-48 pores and of the specific interactions of the permeating compounds and the membrane material. 相似文献
252.
The synthesis of three difuryl analogs of the oxophlorins is described. In common with the tetrapyrrolic oxophlorins, these compounds favor the keto tautomers and little, if any, of the enol tautomers are present in neutral solutions. Unlike the tetrapyrrolic systems, the difuryl oxophlorins give well resolved nmr spectra. Attempts to extend these studies to the synthesis of dithienyl oxophlorin analogs were unsuccessful. 相似文献
253.
We have shown that affinity NMR can be used to edit a NMR spectrum so that ligands that have affinity to DNA can be observed in the presence of other nonbinding molecules. Diffusion encoded spectroscopy (DECODES) can be used to identify the binding ligands. We were able to identify Hoechst 33342 as binding to the Drew-Dickerson dodecamer d(CGCGAATTCGCG)2 in the presence of the nonbinding molecules adenine, adenosine, and thiamine. Affinity NMR appears to be readily applicable to DNA systems for the following reasons. (1) The relaxation rate of the DNA oligonucleotides is favorable, thus the signal intensity loss due to relaxation is not severe. (2) A comparison of the patterns of the DNA cross-peaks upon binding in the two-dimensional total correlation spectroscopy and correlation spectroscopy spectrum are easily performed, and the ligand signals in the two-dimensional DECODES spectrum can be readily identified. (3) The aromatic part of the DNA spectrum is devoid of 2D cross-peaks in these correlation spectra, greatly facilitating the interpretation of the bound ligand in the DECODES spectrum. 相似文献
254.
Rate coefficients of the reaction O((3)P) + CH(3)OH in the temperature range of 835-1777 K were determined using a diaphragmless shock tube. O atoms were generated by photolysis of SO(2) with a KrF excimer laser at 248 nm or an ArF excimer laser at 193 nm; their concentrations were monitored via atomic resonance absorption excited by emission from a microwave-discharged mixture of O(2) and He. The rate coefficients determined for the temperature range can be represented by the Arrhenius equation, k(T) = (2.29 +/- 0.18) x 10(-10) exp[-(4210 +/- 100)T] cm(3) molecule(-1) s(-1); unless otherwise noted, all the listed errors represent one standard deviation in fitting. Combination of these and previous data at lower temperature shows a non-Arrhenius behavior described as the three-parameter equation, k(T) = (2.74 +/- 0.07) x 10(-18)T(2.25 +/- 0.13) exp[-(1500 +/- 90)T] cm(3)molecule(-1) s(-1). Theoretical calculations at the Becke-3-Lee-Yang-Parr (B3LYP)6-311 + G(3df,2p) level locate three transition states. Based on the energies computed with coupled clusters singles, doubles (triples) [CCSD(T)]/6-311 + G(3df,2p)B3LYP6-311 + G(3df,2p), the rate coefficients predicted with canonical variational transition state theory with small curvature tunneling corrections agree satisfactorily with the experimental observations. The branching ratios of two accessible reaction channels forming OH + CH(2)OH (1a) and OH + CH(3)O (1b) are predicted to vary strongly with temperature. At 300 K, reaction (1a) dominates, whereas reaction (1b) becomes more important than reaction (1a) above 1700 K. 相似文献
255.
256.
Anodic deposition of iodide ion on silver at 25° in aqueous (0.5 M KNO3) and in 90% (w/w) ethanol-water (0.05 M KClO4) solutions was studied galvanostatically. The exchange current density, transfer coefficient and the rate constants for the electrode reaction were evaluated. The experimental results revealed that the overall electrode reaction and the charge-transfer step was the same one, i.e., , which might be assumed highly reversible as reflected by the exchange current density (i) and transfer coefficient (α). The numerical values of the rate constants, and at 25° were, in aqueous solution, 1.02×10?5 and 2.88×10?6, and in ethanol solution, 2.88×10?5 and 6.3×10?6 cm sec?1, respectively. 相似文献
257.
To explore the properties of cyclodextrins (CDs) as an optical sensing phase, the behavior of immobilized CD in interaction with analytes was studied in this work. CDs having different cavity sizes were immobilized onto the surface of infrared (IR) internal reflection-sensing element (IRE) to kinetically monitor the behavior of CD in interaction with analytes. Several aromatic compounds having various molecular sizes and functional groups were used to characterize the interaction mechanism. A two-layer modification method was proposed in this work, which utilized a thin hydrophobic film (polyvinyl benzyl chloride) to stick on the IRE and to covalently bond to the CDs through an ethylene diamine linker. The synthesized CD phases exhibited high stability in aqueous solution. To analyze the behavior during the formation of complexes between the guest molecules and the CD phases, we modeled the interaction behavior and treated the kinetic data with the theoretical equations developed in this work. The results indicate that the behavior of the interaction between guest molecules and CDs was explained by considering the formation of two types of complexes: adsorbed complexes and inclusion complexes. The formation of the inclusion complexes was relatively fast, the time required to reach equilibrium could be shorter than a few minutes. The adsorbed complexes were also observed, but their rate of formation was relatively slow; equilibrium could be reached at times greater than 60 min. Based on the signals observed under equilibrium conditions, the concentration of inclusion complexes was approximately three times than that of the adsorbed complexes. 相似文献
258.
LiLi Liu WanHong Ma WenJing Song ChunCheng Chen Jun Lin JinCai Zhao XinHua Qian ShiBo Zhang 《中国科学:化学(英文版)》2007,50(6):770-775
This study investigated the visible-light catalysis mediated by zeolite NaY on the oxidation of dyes with H2O2. The results demonstrated that zeolite NaY acts as a sink for the electron from the photo-excited dye in the heterogeneous catalysis. Furthermore, the electron can effectively activate H2O2 to produce ·OH radical that is a powerful oxidant for the oxidation of dye at room temperature. The effects of the framework topology, Si/Al ratio, and exchangeable cation of the zeolite on the oxidation of various dyes were also shown. 相似文献
259.
Hulin Li Boyan Zhang Lin Ma Liliu Wu James Q. Chambers 《Transition Metal Chemistry》1995,20(6):552-556
Summary The spectrochemical, electrochemical and electrocatalytic properties of Co[15]aneN4 ([15]aneN4 = 1,4,8,12-tetraazacyclopentadecane) have been investigated. The results show that, in aqueous solution, this compound mainly exists as three species whose axial coordination positions are occupied by water and/or hydroxy ligands; it is marginal whether other substrates such as Cl– and NO
inf3
sup–
interact with the central ion in acid-base solutions. The approximate Pourbaix diagram of CoIII/II[15]ane N4 was determined. There is an electrochemically-induced isomerization between two trans conformational isomers of the Co[15]aneN4 complexes in acid and netural solutions. The Co[15]aneN4 complex has electrocatalytic properties for reduction of nitrate and nitrite only in strong alkaline solution. 相似文献
260.
Some novel beta-biarylacryl ferrocene derivatives were synthesized via Pd-catalytic Suzuki cross-coupling reactions of beta-(2-bromophenyl)acrylferrocene and arylboronic acids. The structures were determined with elemental analyses, IR spectra, and (1)H-NMR spectra. 相似文献