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991.
Complex systems of paramagnetic centres existing in demineralised flame coal (71.4 wt% C), medium-rank coal (85.6 wt% C) and
anthracite (94.9 wt% C) were analysed by electron paramagnetic resonance spectroscopy (EPR). Different groups of paramagnetic
centres were searched in macerals (exinite, vitrinite, inertinite) of coals with carbon contents of 73.8 wt% C and 85.6 wt%
C. Experimental EPR spectra were fitted by different superposition of theoretical lines. Total concentration of paramagnetic
centres both in coal and macerals increases with carbon content in the sample. Total concentration of paramagnetic centres
in macerals increases from exinite to inertinite. Four groups of paramagnetic centres were found in medium-rank coal, and
only two groups exist in flame coal and anthracite. The EPR spectrum of medium-rank coal is a superposition of two broad and
two narrow lines. Both broad and narrow, and two narrow lines were measured in EPR spectra of flame coal and anthracite, respectively.
Two types of paramagnetic centres with broad lines and one group with narrow lines exist in exinite and vitrinite from medium-rank
coal. Two narrow components were observed in EPR spectra of its inertinite. Two different broad electron paramagnetic resonance
signals were detected for macerals from low-coalificated coal. Paramagnetic centres systems of medium-rank coal samples reveal
the most complex character. Broad EPR lines were not observed for the higher coalificated samples, i.e., anthracite and inertinite
from medium-rank coal. Narrow lines do not occur in EPR spectra of low-coalificated macerals. Strong dipolar interactions
and fast spin-lattice relaxation processes are characteristic for paramagnetic centres with broad lines. Strong exchange interactions
and short spin-lattice relaxation time were measured for paramagnetic centres with the narrowest EPR signals. 相似文献
992.
The kinetics of the oxidation of five catecholamines viz., dopamine (A), L-dopa (B), methyldopa (C), epinephrine (D) and norepinephrine
(E) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in presence of HClO4 was studied at 30±0.1 °C. The five reactions followed identical kinetics with a first-order dependence on [CAT]
o
, fractional-order in [substrate]
o
, and inverse fractional-order in [H+]. Under comparable experimental conditions, the rate of oxidation of catecholamines increases in the order D>E>A>B>C. The
variation of ionic strength of the medium and the addition of p-toluenesulfonamide or halide ions had no significant effect
on the reaction rate. The rate increased with decreasing dielectric constant of the medium. The solvent isotope effect was
studied using D2O. A Michaelis-Menten type mechanism has been suggested to explain the results. Equilibrium and decomposition constants for
CAT-catecholamine complexes have been evaluated. CH3C6H4SO2NHCl of the oxidant has been postulated as the reactive oxidizing species and oxidation products were identified. An isokinetic
relationship is observed with β=361 K, indicating that enthalpy factors control the reaction rate. The mechanism proposed
and the derived rate law are consistent with the observed kinetics. 相似文献
993.
The title compounds 6a—6f were prepared with high yield via intramolecular Wittig reaction of methyl 3-perfluoroalkyl-6-peifluoroacyl-2-triphenylphosphoranylidenchex-3,5-dienoates (5a—5i) which were obtained from the reaction of 3-perfluoroacylprop-2-enylidenetriphenylphosphoranes (3a—3c) with methyl perfluoroalkynoates (4a—4c). 相似文献
994.
995.
Jinzhang Gao Yongjun Liu Wu Yang Lumei Pu Jie Yu Quanfang Lu 《Central European Journal of Chemistry》2005,3(3):377-386
A plasma induced degradation process has been studied to treat 4-nitrotoluene (4-NT) present as an aqueous pollutant. The
plasma was locally generated from a glow discharge around a tip of a platinum anode in an electrolytic solution. The influence
of initial pH and Fe2+ on the degradation was examined. Major intermediates resulting from the degradation process were identified. Amongst the
aromatic intermediates, p-hydroxybenzoic acid was the predominant degradation product. The formation of oxalic acid, malic
acid was also observed. The final products of degradation were NH
4
+
, NO
3
−
and CO2. Based on the analysis of intermediates and the kinetic considerations, the degradation was shown to follow a pseudo-first
order reaction hence, a possible reaction pathway was proposed. 相似文献
996.
Hammed H. A. M. Hassan 《Central European Journal of Chemistry》2005,3(4):803-829
A concise method for a stereocontrolled synthesis of a set of selectively protected disaccharides is reported. Coupling of
the donor 11 onto acceptors 23 and 24, promoted by trimethylsilyl triflate-N-iodosuccinimide (TMSOTf-NIS), generated the disaccharides 25 and 26. Under typical conditions, condensation of the fully protected donor 12 onto acceptors 23 and 24 produced the disaccharides 27 and 28. The building blocks 25–28 were prepared in moderate yields having exclusive β-stereoselectivity. A unique pattern of protecting groups distinguished
clearly between positions to be sulfated and functional groups remaining as free hydroxyl groups. Acetyl and/or levulinoyl
esters temporarily protected the positions to be sulfated, while benzyl ethers were used for permanent protection. The anomeric
positions were protected as allyl ethers, whereas the 4′-positions were masked as p-methoxybenzyl (PMB) ethers. The orthogonality of the PMB and allyl groups can then be used for further elongation of the
chain by recurrent deprotection and activation steps. The hydroxyl group, OH-6, of glucosamine moieties was protected as a
TBDPS ether to avoid oxidation. A five-step deprotection/sulfonation sequence was applied to the disaccharide 27 to generate the corresponding sulfated [β-D-GlcUA-2-OSO3Na-(1→4)-β-D-Glc pNAc]-(1→O-Pro) 34. 相似文献
997.
Shvartsburg AA Tang K Smith RD 《Journal of the American Society for Mass Spectrometry》2005,16(1):2-12
Field asymmetric waveform ion mobility spectrometry (FAIMS) holds significant promise for post-ionization separations in conjunction with mass-spectrometric analyses. However, a limited understanding of fundamentals of FAIMS analyzers has made their design and operation largely an empirical exercise. Recently, we developed an a priori simulation of FAIMS that accounts for both ion diffusion (including anisotropic components) and Coulomb repulsion, and validated it by extensive comparisons with FAIMS/MS data. Here it is corroborated further by FAIMS-only measurements, and applied to explore how key instrumental parameters (analytical gap width and length, waveform frequency and profile, the identity and flow speed of buffer gas) affect FAIMS response. We find that the trade-off between resolution and sensitivity can be managed by varying gap width, RF frequency, and (in certain cases) buffer gas, with equivalent outcome. In particular, the resolving power can be approximately doubled compared to "typical" conditions. Throughput may be increased by either accelerating the gas flow (preferable) or shortening the device, but below certain minimum residence times performance deteriorates. Bisinusoidal and clipped-sinusoidal waveforms have comparable merit, but switching to rectangular waveforms would improve resolution and/or sensitivity. For any waveform profile, the ratio of two between voltages in high and low portions of the cycle produces the best performance. 相似文献
998.
Characterization and electrochemical investigation of boron-doped mesocarbon microbead anode materials for lithium ion batteries 总被引:1,自引:0,他引:1
Mao-Hui Chen Guo-Tao Wu Guang-Ming Zhu Jin-Kua You Zu-Geng Lin 《Journal of Solid State Electrochemistry》2002,6(6):420-427
The structure and anodic performance of boron-doped and undoped mesocarbon microbeads (MCMBs) have been comparatively studied
and the results obtained by XPS, XRD, SEM, Raman spectroscopy and electrochemical measurements are discussed. It is found
that boron doping introduces a depressed d
002 spacing and the larger amount of "unorganized carbon", which induces vacancy formation in the graphite planes and leads to
a quite different morphology from that of the undoped material. Electrochemical charge/discharge cycle tests indicated that
after boron doping the lithium intercalation was carried through at a somewhat higher potential, being attended by greater
irreversible capacity loss.
Electronic Publication 相似文献
999.
SAPO—11分子筛用i—Pr2NH导向合成及其酸性研究 总被引:3,自引:0,他引:3
以AlPO4、H3PO4、SiO2等为原料采用单一模板剂iPr2NH合成了SAPO11分子筛,利用Avrami方程等对合成过程进行了定量描述。还利用红外(IR)、程序升温脱附(TPD)、线性丁烯骨架异构反应催化活性测试等手段表征了SAPO11的酸性。结果表明,合成过程随温度有明显变化,晶化阶段的活化能远高于核化阶段的值。可用Avrami方程的参数来表征合成过程的难易程度。分子筛中存在B酸和L酸位,主要在中等强度范围,从而对线性丁烯骨架异构反应有较好的催化活性。提高分子筛中硅含量可明显增大酸位浓度,酸强度也有轻度增强。 相似文献
1000.
Serganov A Yuan YR Pikovskaya O Polonskaia A Malinina L Phan AT Hobartner C Micura R Breaker RR Patel DJ 《Chemistry & biology》2004,11(12):1729-1741
Metabolite-sensing mRNAs, or "riboswitches," specifically interact with small ligands and direct expression of the genes involved in their metabolism. Riboswitches contain sensing "aptamer" modules, capable of ligand-induced structural changes, and downstream regions, harboring expression-controlling elements. We report the crystal structures of the add A-riboswitch and xpt G-riboswitch aptamer modules that distinguish between bound adenine and guanine with exquisite specificity and modulate expression of two different sets of genes. The riboswitches form tuning fork-like architectures, in which the prongs are held in parallel through hairpin loop interactions, and the internal bubble zippers up to form the purine binding pocket. The bound purines are held by hydrogen bonding interactions involving conserved nucleotides along their entire periphery. Recognition specificity is associated with Watson-Crick pairing of the encapsulated adenine and guanine ligands with uridine and cytosine, respectively. 相似文献