全文获取类型
收费全文 | 8795篇 |
免费 | 304篇 |
国内免费 | 1篇 |
专业分类
化学 | 4598篇 |
晶体学 | 91篇 |
力学 | 175篇 |
数学 | 1858篇 |
物理学 | 2378篇 |
出版年
2022年 | 70篇 |
2021年 | 126篇 |
2020年 | 139篇 |
2019年 | 119篇 |
2018年 | 192篇 |
2017年 | 203篇 |
2016年 | 432篇 |
2015年 | 297篇 |
2014年 | 338篇 |
2013年 | 735篇 |
2012年 | 429篇 |
2011年 | 533篇 |
2010年 | 354篇 |
2009年 | 281篇 |
2008年 | 300篇 |
2007年 | 222篇 |
2006年 | 159篇 |
2005年 | 102篇 |
2004年 | 112篇 |
2003年 | 125篇 |
2002年 | 139篇 |
2001年 | 115篇 |
2000年 | 101篇 |
1999年 | 102篇 |
1998年 | 97篇 |
1997年 | 100篇 |
1996年 | 99篇 |
1995年 | 89篇 |
1994年 | 81篇 |
1993年 | 80篇 |
1992年 | 88篇 |
1991年 | 74篇 |
1988年 | 78篇 |
1987年 | 85篇 |
1986年 | 81篇 |
1985年 | 106篇 |
1984年 | 99篇 |
1983年 | 86篇 |
1982年 | 84篇 |
1981年 | 83篇 |
1980年 | 95篇 |
1979年 | 87篇 |
1978年 | 95篇 |
1977年 | 96篇 |
1976年 | 79篇 |
1975年 | 76篇 |
1974年 | 88篇 |
1973年 | 93篇 |
1966年 | 75篇 |
1965年 | 70篇 |
排序方式: 共有9100条查询结果,搜索用时 62 毫秒
991.
Barbieriková Z Bella M Kučerák J Milata V Jantová S Dvoranová D Veselá M Staško A Brezová V 《Photochemistry and photobiology》2011,87(1):32-44
Novel 7‐substituted 6‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐h]quinoline ( SeQ(1–6) ) and 8‐substituted 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f ]quinoline derivatives ( SeQN(1–5) ) with R7, R8 = H, COOC2H5, COOCH3, COOH, COCH3 or CN were synthesized and their spectral characteristics were obtained by UV/Vis spectroscopy. Ultraviolet A photoexcitation of the selenadiazoloquinolones in dimethylsulfoxide or acetonitrile resulted in the formation of paramagnetic species coupled with molecular oxygen activation generating the superoxide radical anion or singlet oxygen, evidenced by electron paramagnetic resonance spectroscopy. The cytotoxic/photocytotoxic impact of selenadiazoloquinolones on murine and human cancer cell lines was demonstrated using the derivative SeQ5 (with R7 = COCH3). 相似文献
992.
This study deals with O2− generation in corona discharge (CD) in point to plane geometry for single flow ion mobility spectrometry (IMS) with gas outlet located behind the ionization source. We have designed CD of special geometry in order to achieve the high O2− yield. Using this ion source we have achieved in zero air conditions that up to 74% all negative ions were O2− or O2−(H2O). It has been demonstrated that the non-electronegative nitrogen positively influences the efficiency of O2− generation in O2/N2 mixtures. The reduced ion mobility of 2.27 cm2 V−1 s−1 has been measured for O2−/O2−(H2O) ions in zero air. Additional ions detected in zero air (less than 200 ppb CO2) using the mass spectrometric and IMS technique were, NO2−, N2O2− (2.37 cm2 V−1 s−1), NO3−, N2O3− and N2O3−(H2O). The CO3− and CO4− ions have been detected after the introduction of 5 ppm CO2 into zero air. 相似文献
993.
Bojan Šarac Janez Cerkovnik Bernard Ancian Guillaume Mériguet Gaëlle M. Roger Serge Durand-Vidal Marija Bešter-Rogač 《Colloid and polymer science》2011,289(14):1597-1607
The complex aggregation processes of dodecyltrimethylammonium chloride (DTAC) have been studied in dilute solutions of sodium
salicylate (NaSal) by isothermal titration calorimetry and electrical conductivity at temperatures between 278.15 K and 318.15 K.
A structural transformation that was dependent on the concentrations of DTAC and NaSal was observed. The micellization process
in dilute solutions of DTAC has been subjected to a detailed thermodynamic analysis and shown to occur at considerably lower
critical micelle concentrations than reported for DTAC in water and NaCl solutions. Gibbs free energy, Δ
mic
G
o, and entropy, Δ
mic
S
o, were deduced by taking into account the degree of micelle ionization, β, estimated from conductivity measurements. From the temperature dependence of the enthalpy of micellization, Δ
mic
H
o, the heat capacities of micellization, Dmic cpo {\Delta_{{{\rm mic} }}}c_p^o were determined and discussed in terms of the removal of large areas of non-polar surface from contact with water upon micellization.
The process is exothermic at all temperatures, indicating, in addition to the hydrophobic effect, the presence of strong interactions
between surfactant and salicylate ions. These were confirmed by 1H NMR spectroscopy and diffusion NMR experiments. Salicylate ions not only interact with the headgroups but also insert further
into the micelle core. At c
NaSal/c
DTAC > 2.5, the structural rearrangements occur even at relatively low concentrations of NaSal. 相似文献
994.
Jiménez MV Lahoz FJ Lukešová L Miranda JR Modrego FJ Nguyen DH Oro LA Pérez-Torrente JJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(29):8115-8128
The treatment of [{Rh(μ-SH){P(OPh)(3)}(2)}(2)] with [{M(μ-Cl)(diolef)}(2)] (diolef=diolefin) in the presence of NEt(3) affords the hydrido-sulfido clusters [Rh(3)(μ-H)(μ(3)-S)(2)(diolef){P(OPh)(3)}(4)] (diolef=1,5-cyclooctadiene (cod) for 1, 2,5-norbornadiene (nbd) for 2, and tetrafluorobenzo[5,6]bicyclo[2.2.2]octa-2,5,7-triene (tfb) for 3) and [Rh(2)Ir(μ-H)(μ(3)-S)(2)(cod){P(OPh)(3)}(4)] (4). Cluster 1 can be also obtained by treating [{Rh(μ-SH){P(OPh)(3)}(2)}(2)] with [{Rh(μ-OMe)(cod)}(2)], although the main product of the reaction with [{Ir(μ-OMe)(cod)}(2)] was [RhIr(2)(μ-H)(μ(3)-S)(2)(cod)(2){P(OPh)(3)}(2)] (5). The molecular structures of clusters 1 and 4 have been determined by X-ray diffraction methods. The deprotonation of a hydrosulfido ligand in [{Rh(μ-SH)(CO)(PPh(3))}(2)] by [M(acac)(diolef)] (acac=acetylacetonate) results in the formation of hydrido-sulfido clusters [Rh(3)(μ-H)(μ(3)-S)(2)(CO)(2) (diolef)(PPh(3))(2)] (diolef=cod for 6, nbd for 7) and [Rh(2)Ir(μ-H)(μ(3)-S)(2)(CO)(2)(cod)(PPh(3))(2)] (8). Clusters 1-3 and 5 exist in solution as two interconverting isomers with the bridging hydride ligand at different edges. Cluster 8 exists as three isomers that arise from the disposition of the PPh(3) ligands in the cluster (cis and trans) and the location of the hydride ligand. The dynamic behaviour of clusters with bulky triphenylphosphite ligands, which involves hydrogen migration from rhodium to sulfur with a switch from hydride to proton character, is significant to understand hydrogen diffusion on the surface of metal sulfide hydrotreating catalysts. 相似文献
995.
Glavnik Vesna Simonovska Breda Vovk Irena Pavlović Dragana Mutavdžić Ašperger Danijela Babić Sandra 《平面色谱法杂志一现代薄层色谱法》2011,24(6):482-486
JPC – Journal of Planar Chromatography – Modern TLC - Validation of a method for the quantification of (−)-epicatechin and procyanidin B2 in chocolates is reported. After a simple... 相似文献
996.
Voignier J Frey J Kraus T Buděšínský M Cvačka J Heitz V Sauvage JP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(19):5404-5414
By using the "gathering-and-threading" effect of copper(I) with rigid ring-and-string conjugates, daisy-chain-type [3]- and [4]pseudorotaxanes could be prepared in high yields. The organic fragment used consisted of a 2,9-diphenyl-1,10-phenanthroline (dpp)-containing ring attached to a coordinating filament capable of threading through the ring of another molecule by coordination to copper(I). The bidentate chelate introduced in the axis was also a 1,10-phenanthroline (phen) derivative with two methyl groups ortho to the nitrogen atoms of the phen unit. The organic component was prepared following a multistep strategy, one of the key steps being the attachment of the ring to the lateral axis. This connection was done by a condensation reaction between an ortho dione located at the back of a ring-incorporated phen and an aromatic aldehyde, which was the end-function of the thread. An oxazole nucleus was obtained after the condensation, which provided a rigid connection between the ring and the axis. In this way, the coordination axes of the ring-incorporated bidentate chelate and of the ligand belonging to the lateral filament were approximately orthogonal to one another. The design was such that the tetrameric complex, a [4]pseudorotaxane, seemed to be the most stable species, owing to the mutual geometrical arrangement of the filament and the ring. Various spectroscopic techniques, such as (1)H NMR spectroscopy, including DOSY, and electrospray ionization mass spectroscopy (ESI-MS), clearly demonstrated that a mixture of cyclic [3]- and [4]pseudorotaxane was obtained; the proportions of both components depended on concentration and temperature. Copper(I) was not the only metal center leading to the formation of cyclic pseudorotaxanes. A similar effect was observed with silver(I) as the templating metal: quantitative formation of threaded species was observed, with a higher proportion of trimer over tetramer than in the copper(I) case. Concentration and temperature effects were investigated for both series of Cu(I) - and Ag(I) -complexed threaded species showing that formation of the trimer was favored upon dilution or heating of the solution. 相似文献
997.
Zelenka K Trnka T Tišlerová I Monti D Cinti S Naitana ML Schiaffino L Venanzi M Laguzzi G Luvidi L Mancini G Nováková Z Šimák O Wimmer Z Drašar P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(49):13743-13753
Solvent-driven aggregation of a series of porphyrin derivatives was studied by UV/Vis and circular dichroism spectroscopy. The porphyrins are characterised by the presence in the meso positions of steroidal moieties further conjugated with glucosyl groups. The presence of these groups makes the investigated macrocycles amphiphilic and soluble in aqueous solvent, namely, dimethyl acetamide/water. Aggregation of the macrocycles is triggered by a change in bulk solvent composition leading to formation of large architectures that express supramolecular chirality, steered by the presence of the stereogenic centres on the periphery of the macrocycles. The aggregation behaviour and chiroptical features of the aggregates are strongly dependent on the number of moieties decorating the periphery of the porphyrin framework. In particular, experimental evidence indicates that the structure of the steroid linker dictates the overall chirality of the supramolecular architectures. Moreover, the porphyrin concentration strongly affects the aggregation mechanism and the CD intensities of the spectra. Notably, AFM investigations reveal strong differences in aggregate morphology that are dependent on the nature of the appended functional groups, and closely in line with the changes in aggregation mechanism. The suprastructures formed at lower concentration show a network of long fibrous structures spanning over tens of micrometres, whereas the aggregates formed at higher concentration have smaller rod-shaped structures that can be recognised as the result of coalescence of smaller globular structures. The fully steroid substituted derivative forms globular structures over the whole concentration range explored. Finally, a rationale for the aggregation phenomena was given by semiempirical calculations at the PM6 level. 相似文献
998.
We present results of ab-initio scattering calculations for electron collisions with pyrazine using the R-matrix method, carried out at various levels of approximation. We confirm the existing experimental and theoretical understanding of the three well-known π? shape resonances. In addition, we find numerous core-excited resonances (above 4.8 eV) and identify their most likely parent states. We also present differential cross sections, showing high sensitivity to the scattering model chosen at low energies. We make recommendations regarding the selection of models for scattering calculations with this type of targets. 相似文献
999.
Lísa M Netušilová K Franěk L Dvořáková H Vrkoslav V Holčapek M 《Journal of chromatography. A》2011,1218(42):7499-7510
Fatty acid (FA) and triacylglycerol (TG) composition of natural oils and fats intake in the diet has a strong influence on the human health and chronic diseases. In this work, non-aqueous reversed-phase (NARP) and silver-ion high-performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometry detection and gas chromatography with flame-ionization detection (GC/FID) and mass spectrometry detection are used for the characterization of FA and TG composition in complex samples of animal fats from fallow deer, red deer, sheep, moufflon, wild boar, cock, duck and rabbit. The FA composition of samples is determined based on the GC/FID analysis of FA methyl esters. In total, 81 FAs of different acyl chain length, double bond (DB) number, branched/linear, cis-/trans- and DB positional isomers are identified. TGs in animal fats contain mainly monounsaturated and saturated FAs. High amounts of branched and trans-FAs are observed in the samples of ruminants. In NARP mode, individual TG species are separated including the separation of trans- and branched TGs. Silver-ion mode provides the separation of TG regioisomers, which enables the determination of their ratios. Great differences in the preference of unsaturated and saturated FAs in the sn-2 position on the glycerol skeleton are observed among individual animal fats. Unsaturated FAs are preferentially occupied in the sn-2 position in all animal samples except for wild boar with the strong preference of saturated FAs in the sn-2 position. 相似文献
1000.
M. Hartmanová V. Navrátil V. Buršíková F. Kundracik C. Mansilla 《Russian Journal of Electrochemistry》2011,47(5):505-516
Electrical conductivity, dielectric permittivity and mechanical hardness of the polycrystalline CeO2 + xSm2O3 (x = 0, 10.9–15.9 mol %) films prepared by Electron Beam Physical Vapour Deposition (EB-PVD) and Ionic Beam Assisted Deposition,
(IBAD), techniques were investigated in dependence on their structure and microstructure influenced by the deposition conditions,
namely composition, deposition temperature and Ar+ ion bombardment. The electrical conductivity of doped ceria prepared without Ar+ ion bombardment and investigated by the impedance spectroscopy, IS, was found to be predominantly ionic one under the oxidizing
atmosphere/low-temperature conditions and the higher amounts of Sm2O3 (>10 mol %) used. The bulk conductivity as a part of total measured conductivity was a subject of interest because the grain
boundary conductivity was found to be ∼3 orders of magnitude lower than the corresponding bulk conductivity. Ar+ ion bombardment acted as a reducer (Ce4+ → Ce3+) resulting in the development of electronic conductivity. Dielectric permittivity determined from the bulk parallel capacitance
measured at room temperature and the frequency of 1 MHz, similarly as the mechanical hardness measured by indentation (classical
Vickers and Depth Sensing Indentation-DSI) techniques were also found to be dependent on the deposition conditions. The approximative
value of hardness for the investigated films deposited on the substrate was estimated using a simple phenomenological model
described by the power function HV = HV
0 + aP
b
and compared with the so-called apparent hardness (substrate + investigated film) determined by the classical Vickers formula.
Results obtained are analyzed and discussed. 相似文献