全文获取类型
收费全文 | 2402篇 |
免费 | 107篇 |
国内免费 | 4篇 |
专业分类
化学 | 2099篇 |
晶体学 | 5篇 |
力学 | 27篇 |
数学 | 212篇 |
物理学 | 170篇 |
出版年
2024年 | 3篇 |
2023年 | 26篇 |
2022年 | 116篇 |
2021年 | 128篇 |
2020年 | 61篇 |
2019年 | 66篇 |
2018年 | 54篇 |
2017年 | 40篇 |
2016年 | 113篇 |
2015年 | 82篇 |
2014年 | 109篇 |
2013年 | 140篇 |
2012年 | 153篇 |
2011年 | 212篇 |
2010年 | 111篇 |
2009年 | 115篇 |
2008年 | 174篇 |
2007年 | 157篇 |
2006年 | 134篇 |
2005年 | 125篇 |
2004年 | 94篇 |
2003年 | 78篇 |
2002年 | 59篇 |
2001年 | 20篇 |
2000年 | 10篇 |
1999年 | 16篇 |
1998年 | 15篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 10篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1972年 | 2篇 |
1938年 | 1篇 |
1933年 | 2篇 |
1932年 | 1篇 |
排序方式: 共有2513条查询结果,搜索用时 15 毫秒
101.
Ethanol production in a bioreactor with integrated membrane distillation (MD) module has been investigated. A hydrophobic
capillary polypropylene membrane (Accurel PP V8/2 HF), with an external/internal diameter ratio, d
out/d
in = 8.6 mm/5.5 mm and pore size 0.2 μm, was used in these studies. The products (mainly ethanol and acetic acid) formed during
the fermentation of sugar with Saccharomyces cerevisiae inhibited the process. These products were selectively removed from the fermentation broth by the MD process, which increased
the efficiency of the conversion of sugar to alcohol from 0.45 g to 0.5 g EtOH per g of fermented sucrose. The bioreactor
efficiency also increased by almost 30 %. Separation of alcohol by the MD generates a higher yield of ethanol in the permeate
than in the broth. The enrichment coefficient amounted to 4-8, and depended on the ethanol concentration in the broth. The
separated solutions did not wet the membrane in use for 2500 h of the MD experiments and the retention of inorganic solutes
was close to 100 %. 相似文献
102.
Miguel Gd Pérez-Morales M Martín-Romero MT Muñoz E Richardson TH Camacho L 《Langmuir : the ACS journal of surfaces and colloids》2007,23(7):3794-3801
The molecular organization of a mixed film, containing a water-soluble tetracationic porphyrin (TMPyP) and a p-tert-butyl calix[8]arene octacarboxylic acid derivative (C8A), at the air-water interface and on a solid support (LB film), has been investigated. Although the TMPyP aggregation was not detected at the air-water interface, TMPyP J-aggregates have been found in the LB films (Y-type). Unlike tetraanionic porphyrins, for example TSPP, the TMPyP J-aggregates are not induced by a zwitterion formation. The TMPyP J-aggregation is a result of a "double comb" configuration, where porphyrins from opposite layers are interwoven in a linear infinite J-aggregate. Our results confirm that TMPyP molecules tend to self-aggregate strongly, provided the electrostatic repulsions of their peripheral groups are cancelled by the anionic groups of the C8A matrix. 相似文献
103.
In order to study the influence of the side-chain orientation on the peptide backbone conformation we have synthesised the model dipeptides t-BuCO-l-Pro-(1S,2R)-c6Phe-NHMe and t-BuCO-l-Pro-(1R,2S)-c6Phe-NHMe, incorporating each enantiomer of the trans cyclohexane analogue of phenylalanine (trans-1-amino-2-phenylcyclohexanecarboxylic acid). The orientation of the aromatic side-chain determines the β-turn type accommodated by these peptides to the point that the (1S,2R)-c6Phe derivative retains the type I β-turn in the crystalline state, in contrast to the behaviour exhibited by the natural counterpart t-BuCO-l-Pro-l-Phe-NHMe. 相似文献
104.
Prof. Michael North Marta Omedes‐Pujol Courtney Williamson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(37):11367-11375
The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}2] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO3 or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}2O] and triphenylphosphine oxide, a non‐linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron‐deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case. 相似文献
105.
Cordierite porous ceramics Z, X, and K were prepared using three mixtures of clay minerals: Z from kaolinite, talc, and aluminum
hydroxide, X from kaolinite, talc, vermiculite, and aluminum hydroxide, and K from kaolinite, talc, and magnesium oxide. Ceramics
were different in porosity, specific surface area, cordierite polymorphs, and secondary crystalline phases. Vermiculite influenced
textural architecture of calcined cordierite ceramics X and predestinated crystallization of the high-temperature hexagonal
α-cordierite with secondary minerals enstatite, spinel and corundum. Ceramics Z contained low-temperature orthorhombic β-cordierite,
enstatite, and corundum, K was diphase of β-cordierite and forsterite. Total pore area (TPA) and specific surface area (SSA)
of X, in spite of the higher porosity and the pore size distribution in the range of 300–1000 nm, were smaller in comparison
with TPA and SSA of Z. Ceramics K retained high porosity, two maxima at 300–1000 nm and 50–200 nm in the pores size distribution,
and the highest TPA and SSA compared to those observed in ceramics Z and X.
Presented at the 8th Conference on Solid State Chemistry, 6-11 July 2008, Bratislava, Slovak Republic. 相似文献
106.
Marta Sevilla Teresa Valds-Solís Pedro Tartaj Antonio B. Fuertes 《Journal of colloid and interface science》2009,340(2):230-236
A synthetic method for the fabrication of silica-based mesoporous magnetic (Fe or iron oxide spinel) nanocomposites with enhanced adsorption and magnetic capabilities is presented. The successful in situ synthesis of magnetic nanoparticles is a consequence of the incorporation of a small amount of carbon into the pores of the silica, this step being essential for the generation of relatively large iron oxide magnetic nanocrystals (10 ± 3 nm) and for the formation of iron nanoparticles. These composites combine good magnetic properties (superparamagnetic behaviour in the case of SiO2–C–Fe3O4/γ–Fe2O3 samples) with a large and accessible porosity made up of wide mesopores (>9 nm). In the present work, we have demonstrated the usefulness of this kind of composite for the adsorption of a globular protein (hemoglobin). The results obtained show that a significant amount of hemoglobin can be immobilized within the pores of these materials (up to 180 mg g−1 for some of the samples). Moreover, we have proved that the composite loaded with hemoglobin can be easily manipulated by means of an external magnetic field. 相似文献
107.
Acetato-bis(pyrazole) complexes [Mo(η3-methallyl)(O2CMe)(CO)2(pz∗H)2], (methallyl = CH2C(CH3)CH2) and fac-[M(O2CMe)(CO)3(pz∗H)2], (pz∗H = pyrazole or 3,5-dimethylpyrazole, dmpzH; M = Mn, Re) are obtained from [Mo(η3-methallyl)Cl(CO)2(NCMe)2] or fac-[MBr(CO)3(NCMe)2] [M = Mn (synthesized in situ), Re], 2 equiv. of pyrazole, and 1 equiv. of sodium acetate for Mo complexes, or silver acetate for Mn or Re complexes. The chlorido-complexes [Mo(η3-methallyl)Cl(CO)2L2] (L = pzH, dmpzH), obtained from the same starting material by substitution of MeCN by pzH or dmpzH, are also described. The crystal structures of the fac-acetato-bis(dimethylpyrazole) complexes present the same pattern of intramolecular hydrogen bonds between the acetate and the dimetylpyrazole ligands, whereas the crystal structures of the fac-acetato-bis(pyrazole) complexes show different hydrogen bonds patterns, with intermolecular interactions. NMR data indicate that these interactions are not maintained in solution. 相似文献
108.
San Miguel V Alvarez M Filevich O Etchenique R del Campo A 《Langmuir : the ACS journal of surfaces and colloids》2012,28(2):1217-1221
Photoreactive surfaces derived from a new photocleavable surface modification agent and with photosensitivity in the Vis and IR region are described. A ruthenium(II) caged aminosilane, [Ru(bpy)(2)(PMe(3))(APTS)](PF(6))(2), was synthesized and attached to silica surfaces. Light irradiation removed the cage and generated surface patterns with reactive amine groups. The photosensitivity of this compound under single (460 nm) and two-photon (900) excitation is demonstrated. Functional patterns with site-selective attachment of other molecular species are described. 相似文献
109.
The inhibition of the model enzyme, haloalkane dehalogenase from Sphingomonas paucimobilis, was investigated by a combination of electrophoretically mediated microanalysis with a partial filling technique, followed by indirect or direct detection. In this setup, part of the capillary is filled with a buffer suitable for the enzymatic reaction (20 mM glycine buffer, pH 8.6) whereas the rest of the capillary is filled with the background electrolyte optimal for separation of substrates and products. Two different background electrolytes and corresponding detection approaches were used to show the versatility of the developed method. The inhibition effect of 1,2-dichloroethane on the dehalogenation of brominated substrate 1-bromobutane was studied by means of 10 mM chromate - 0.1 mM cetyltrimethylammonium bromide (pH 9.2) in combination with indirect detection or 20 mM beta-alanine - hydrochloric acid (pH 3.5) in combination with direct detection. The method was used to estimate the inhibition constant K(I) (0.44 mM by indirect detection and 0.63 mM by of direct detection) and to determine the inhibition type. Compared to spectrophotometric and other discontinuous assays, the method is rapid, can be automated, and requires only small amount of reagents that is especially important in the case of enzymes and inhibitors. 相似文献
110.
Díaz MC Illescas BM Martín N Viruela R Viruela PM Ortí E Brede O Zilbermann I Guldi DM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(8):2067-2077
A new class of pi-extended TTF-type electron donors (11 a-c) has been synthesized by Wittig-Horner olefination of bianthrone (9) with 1,3-dithiole phosphonate esters (10 a-c). In cyclic voltammetry experiments, donors 11 a-c reveal a single, electrochemically irreversible oxidation-yielding the corresponding dicationic products-at relatively low oxidation potentials (approximately 0.7-0.8 V). Theoretical calculations, performed at the DFT level (B3 P86/6-31 G*), predict a highly-folded C(2h) structure for 11 a. In the ground state, the molecule adopts a double saddle-like conformation to compensate the steric hindrance. The calculations suggest that the intramolecular charge transfer associated with the HOMO-->LUMO transition is responsible for an absorption band observed above 400 nm. While the radical cation 11 a*+ retains the folded C(2h) structure predicted for the neutral molecule as the most stable conformation, the dication 11 a(2+) has a fully aromatic D(2) structure, formed by an orthogonal 9,9'-bianthryl central unit to which two singly-charged dithiole rings are attached. The drastic conformational changes that compounds 11 undergo upon oxidation account for their electrochemical properties. By means of pulse radiolysis measurements, radical-induced one-electron oxidation of 11 a-c was shown to lead to the radical cation species (11 a-c*+), which were found to disproportionate with generation of the respective dication species (11 a-c(2+)) and the neutral molecules (11 a-c). 相似文献