全文获取类型
收费全文 | 662篇 |
免费 | 20篇 |
国内免费 | 3篇 |
专业分类
化学 | 424篇 |
晶体学 | 9篇 |
力学 | 31篇 |
数学 | 134篇 |
物理学 | 87篇 |
出版年
2023年 | 7篇 |
2022年 | 25篇 |
2021年 | 30篇 |
2020年 | 26篇 |
2019年 | 24篇 |
2018年 | 23篇 |
2017年 | 10篇 |
2016年 | 27篇 |
2015年 | 14篇 |
2014年 | 30篇 |
2013年 | 79篇 |
2012年 | 45篇 |
2011年 | 39篇 |
2010年 | 34篇 |
2009年 | 27篇 |
2008年 | 32篇 |
2007年 | 36篇 |
2006年 | 23篇 |
2005年 | 13篇 |
2004年 | 14篇 |
2003年 | 10篇 |
2002年 | 20篇 |
2001年 | 4篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 7篇 |
1997年 | 2篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1979年 | 6篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1968年 | 1篇 |
1965年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有685条查询结果,搜索用时 31 毫秒
71.
Heydari-Gorji A Belmabkhout Y Sayari A 《Langmuir : the ACS journal of surfaces and colloids》2011,27(20):12411-12416
Poly(ethyleneimine) (PEI) supported on pore-expanded MCM-41 whose surface is covered with a layer of long-alkyl chains was found to be a more efficient CO(2) adsorbent than PEI supported on the corresponding calcined silica and all PEI-impregnated materials reported in the literature. The layer of surface alkyl chains plays an important role in enhancing the dispersion of PEI, thus decreasing the diffusion resistance. It was also found that at low temperature, adsorbents with relatively low PEI contents are more efficient than their highly loaded counterparts because of the increased adsorption rate. Extensive CO(2) adsorption-desorption cycling showed that the use of humidified feed and purge gases affords materials with enhanced stability, despite limited loss due to amine evaporation. 相似文献
72.
Youssef El KhouryRuth Hielscher Mariana VoicescuJulien Gross Petra Hellwig 《Vibrational Spectroscopy》2011,55(2):258-266
In this work we analyzed the specificity of the amide VI band for different types of secondary structure elements in protein structures. This band involves the bending motion of the CO group of the peptide chain that is typically observed in the spectral region from 590 to 490 cm−1. The infrared absorbance spectra of a set of polypeptide model compounds of well known secondary structure was obtained at defined pH, including poly (l-lysine), poly (l-tyrosine), poly (l-alanine) and poly (l-histidine). In addition spectra of membrane proteins from the respiratory chain, namely the NADH:ubiquinone oxidoreductase, the cytochrome c oxidase and its CuA fragment, the cytochrome bc1 complex, a Rieske-type protein and in addition myoglobin, have been comparatively investigated. The systematic analysis of the amide VI band of the polypeptides and the proteins allowed correlating the signal appearing at ∼525 cm−1 to α-helical structures and signals at ∼545 cm−1 to β-sheet contributions. Random coils have been found to contribute at ∼535 cm−1 while the β-turns were observed at ∼560 cm−1. 相似文献
73.
Enantiomeric Thin-Layer Chromatographic Assay of Escitalopram in Presence of “In-Process Impurities”
JPC – Journal of Planar Chromatography – Modern TLC - A simple, rapid and precise thin-layer chromatographic (TLC) method for analysis of escitalopram oxalate (ESC-OX) (S-enantiomer) in... 相似文献
74.
Adsorption of CO2, N2, CH4 and H2 on triamine-grafted pore-expanded MCM-41 mesoporous silica (TRI-PE-MCM-41) was investigated at room temperature in a wide
range of pressure (up to 25 bar) using gravimetric measurements. The material was found to exhibit high affinity toward CO2 in comparison to the other species over the whole range of pressure. Column-breakthrough dynamic measurements of CO2-containing mixtures showed very high selectivity toward CO2 over N2, CH4 and H2 at CO2 concentrations within the range of 5 to 50%. These conditions are suitable for effective removal of CO2 at room temperature from syngas, flue gas and biogas using temperature swing (TS) or temperature-pressure swing (TPS) regeneration
mode. Moreover, TRI-PE-MCM-41 was found to be highly stable over hundreds of adsorption-desorption cycles using TPS as regeneration
mode. 相似文献
75.
Eubank JF Mouttaki H Cairns AJ Belmabkhout Y Wojtas L Luebke R Alkordi M Eddaoudi M 《Journal of the American Chemical Society》2011,133(36):14204-14207
A new blueprint network for the design and synthesis of porous, functional 3D metal-organic frameworks (MOFs) has been identified, namely, the tbo net. Accordingly, tbo-MOFs based on this unique (3,4)-connected net can be exclusively constructed utilizing a combination of well-known and readily targeted [M(R-BDC)](n) MOF layers [i.e., supermolecular building layers (SBLs)] based on the edge-transitive 4,4 square lattice (sql) (i.e., 2D four-building units) and a novel pillaring strategy based on four proximal isophthalate ligands from neighboring SBL membered rings (i.e., two pairs from each layer) covalently cross-linked through an organic quadrangular core (e.g., tetrasubstituted benzene). Our strategy permits the rational design and synthesis of isoreticular structures, functionalized and/or expanded, that possess extra-large nanocapsule-like cages, high porosity, and potential for gas separation and storage, among others. Thus, tbo-MOF serves as an archetypal tunable, isoreticular MOF platform for targeting desired applications. 相似文献
76.
Ethyl 7‐amino‐3‐(3‐methyl‐5‐oxo‐1‐phenyl‐2‐pyrazolin‐4‐yl)‐5‐aryl‐5H‐thiazolo[3,2‐a]pyrimidine‐6‐carboxylate was hydrolyzed with an ethanolic sodium hydroxide and the sodium salt thus formed underwent cyclization with acetic anhydride to afford 2‐methyl‐7‐(3‐methyl‐5‐oxo‐1‐phenyl‐2‐pyrazolin‐4‐yl)‐5‐arylthiazolo[3,2‐a]pyrimido[4,5‐d]oxazin‐4(5H)‐one. This compound was transformed to related heterocyclic systems via its reaction with various reagents. The biological activity of the prepared compounds was tested against Gram positive and Gram negative bacteria as well as yeast‐like and filamentous fungi. They revealed in some cases excellent biocidal properties. 相似文献
77.
Pillay AE Bassioni G Stephen S Kühn FE 《Journal of the American Society for Mass Spectrometry》2011,22(8):1403-1408
Knowledge of trace metal ‘grains’ in asphaltenes could play a significant role in enhancing refining and processing of crudes
and also in providing useful information on mechanistic and migratory features linked to asphaltenes. These metals originate
directly from interaction of oils with source-rock, mineral matter, and formation water and their accumulation in asphaltene
matrices could vary from oil well to oil well. Suitable asphaltene samples were subjected to high-performance ICP-MS laser
depth profiling (213 nm) to depths of 50 μm at 5 μm intervals. The study was conducted in the absence of standardization and
characteristic intensities originating from the metals of interest were measured. Ten metal profiles were investigated (Na,
Mg, Al, Mn, Fe, Zn, Sr, Pb, V, and Ni). The experimental results showed non-uniform distribution of trace metals and identified
areas where such metals agglomerate. The data suggested that certain chemical and physical conditions within the structure
of asphaltenes are favorable for metal ‘grain’ formation at specific points. The exact mechanism for this behavior is not
clear at this stage, and has considerable scope for future studies, including mathematical modeling simulations of asphaltenes.
We also found that solid asphaltenes could be a useful forerunner of scale formation. 相似文献
78.
Reaction of 6‐amino‐2‐thiouracil 1 with ethyl bromoacetate yielded ethyl 2‐(7‐amino‐2,5‐dioxo‐3,5‐dihydro‐2H‐thiazolo[3,2‐a]pyrimidin‐6‐yl)acetate 2 . Reaction of 2 with sodium ethoxide afforded the pyrrolothiazolopyrimidine derivative 3 . Compound 2 reacted with hydrazine hydrate to give 7‐amino‐thiazolopyrimidine‐carbohydrazide 4 . The latter compound 4 reacted with carbon disulphide to form 7‐amino‐6‐(oxadiazolylmethyl) thiazolopyrimidine 5 . Compound 5 was heated in methanol to yield 9‐thioxotriazolopyrrolothiazolopyrimidine 6 . Also, the reaction of 3 with aromatic aldehydes afforded the diarylmethylenepyrrolothiazolopyrimidine derivatives 7a‐c . The latter compounds 7a‐c underwent cyclocondensation with hydroxylamine to give diaryldioxazolopyrrolothiazolopyrimidine derivatives 8a‐c . The new prepared compounds were subjected for antioxidant and antituomer studies, some of these compounds exhibited promising activity. 相似文献
79.
Chiraz Youssef Hamed Ben Ammar Mohamed Belhouchet Kassem Beydoun Ridha Ben Salem Henri Doucet Pierre H. Dixneuf 《Journal of heterocyclic chemistry》2011,48(5):1126-1131
2‐(Arylidene)cyanomethylbenzoxazoles have been prepared in water from benzoxazole‐2‐ylacetonitriles. Using multi‐component reactions, a variety of heterocycles containing benzoxazole and nitrile functionality has been prepared. J. Heterocyclic Chem., (2011). 相似文献
80.
3-Amino-3-thioxopropanamide (1) reacted with ethyl acetoacetate to form 6-hydroxy-4-methyl-2-thioxo-2,3-dihydropyridine-3-carboxamide (2), which reacted with α-haloketones 3 to produce 2,3-disubstituted-8-hydroxy-6-methyl-2H,5H-pyrido[3,2-f]-[1,4]thiazepin-5-ones 4a-c. Benzoylation of 4c led to the formation of the dibenzoate derivative 9. Compounds 4a-c could be prepared stepwise through the formation of S-alkylated derivatives 10a-c. Compounds 2, 4a-c, 9 and 10a-c were prepared using microwave as a source of heat, and gave better yields in shorter times than those achieved by traditional methods. Coupling of 4a-c with arenediazonium chlorides proceeded unusually to give the 6-hydroxy-4-methyl-2-(arylazo)thieno[2,3-b]pyridin-3(2H)-one ring contraction products 14. Structures of the newly synthesized compounds were proven by spectral and chemical methods. 相似文献