全文获取类型
收费全文 | 91072篇 |
免费 | 657篇 |
国内免费 | 433篇 |
专业分类
化学 | 31721篇 |
晶体学 | 918篇 |
力学 | 7192篇 |
数学 | 34234篇 |
物理学 | 18097篇 |
出版年
2022年 | 161篇 |
2021年 | 223篇 |
2020年 | 227篇 |
2019年 | 246篇 |
2018年 | 10572篇 |
2017年 | 10368篇 |
2016年 | 6339篇 |
2015年 | 1112篇 |
2014年 | 621篇 |
2013年 | 1161篇 |
2012年 | 4358篇 |
2011年 | 11129篇 |
2010年 | 6053篇 |
2009年 | 6431篇 |
2008年 | 7132篇 |
2007年 | 9217篇 |
2006年 | 690篇 |
2005年 | 1699篇 |
2004年 | 1946篇 |
2003年 | 2293篇 |
2002年 | 1350篇 |
2001年 | 503篇 |
2000年 | 519篇 |
1999年 | 325篇 |
1998年 | 327篇 |
1997年 | 307篇 |
1996年 | 397篇 |
1995年 | 271篇 |
1994年 | 223篇 |
1993年 | 283篇 |
1992年 | 239篇 |
1991年 | 251篇 |
1990年 | 191篇 |
1989年 | 196篇 |
1988年 | 211篇 |
1987年 | 270篇 |
1986年 | 239篇 |
1985年 | 274篇 |
1984年 | 249篇 |
1983年 | 201篇 |
1982年 | 236篇 |
1981年 | 239篇 |
1980年 | 238篇 |
1979年 | 246篇 |
1978年 | 225篇 |
1977年 | 198篇 |
1976年 | 175篇 |
1975年 | 192篇 |
1974年 | 150篇 |
1973年 | 157篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Oxidative addition of 1-chloro-1-nitroethane to trans-IrCl(CO)-[P(CH3)2C6H5]2 followed by treatment of the initial product with pyridine yields a new iridium(III) complex IrCl(py)[COC(NO2)CH3][P(CH3)2C6H5]2, whose structure has been confirmed by X-rays crystallography. Two intermediate products have been observed by NMR spectroscopy; their structures have been tentatively assigned. The reaction of the corresponding bromine derivatives yields two isomers of the composition IrBr2(CO)[CH(NO2)CH3][P(CH3)2C6H5]2, and these are not affected by pyridine. The reaction of 1-chloro-1-nitroethane with Pt[P(C6H5)3]4 takes a completely different course in that yields nitrorethane and cis-PtCl2[P(C6H5)3]2 as the main products, with no detectable formation of the products of oxidation addition. A brief mechanistic investigation points towards the participation of radicals and radical anions as transient intermediates and a mechanism is proposed which explains most of the experimental results. 相似文献
962.
Haoshuang Gu Yongming Hu Hao Wang Xiangrong Yang Zhenglong Hu Ying Yuan Jin You 《Journal of Sol-Gel Science and Technology》2007,42(3):293-297
Single crystalline nanowires of lead titanate (PbTiO3) were fabricated by hydrothermal method at 200°C using lead acetate and n-tetrabutyl titanate as starting materials, where sodium hydroxide was served as a mineralizer. Crystalline phases, microstructure
and optical properties of PbTiO3 nanowires were investigated. The PbTiO3 nanowires were uniform and continuous along the long axis, and were composed of single crystalline PbTiO3 with a tetragonal perovskite structure. The diameter of a single nanowire was around 12 nm and the length reached up to 3 μm.
The chemical composition of the samples and the valence states of elements were determined by X-ray photoelectron spectroscopy
(XPS). The ultraviolet/visible absorption spectroscopic investigation suggested that the absorption edge of optical transition
of the first excitonic state occurred at around 320 nm. A blue-green light emission peaking at about 471 nm (2.63 eV) is observed
at room temperature, and the intensity of this emission increased with increasing excitation wavelength. Oxygen vacancies
are responsible for the light emission of PbTiO3 nanowires. 相似文献
963.
Mátyás Milen László Hazai Pál Kolonits György Kalaus Lajos Szabó Ágnes Gömöry Csaba Szántay 《Central European Journal of Chemistry》2005,3(1):118-136
Transformation of β-carboline derivatives into optically active entities were studied and the de and ee values of the resulted compounds were detected.
Dedicated to Professor Károly Lempert on his 80th birthday. 相似文献
964.
Norman L. Allinger Lothar Schäfer K. Siam V. J. Klimkowski C. Van Alsenoy 《Journal of computational chemistry》1985,6(5):331-342
A series of ab initio calculations have been carried out, using the 4-21G basis set. Ethane and propane were first studied to obtain reference points. The effect of adding an electronegative atom (fluorine, or carbonyl oxygen) onto the framework was then studied as a function of the torsional angle about the single bond. Some pronounced trends in structural changes were observed, and these can in part be correlated with hyperconjugative effects. For example, fluoroethane has bond lengths which are shorter than those in ethane itself, by 0.024 Åin the C C bond, and 0.003 Åin the α C H bonds. These changes are essentially torsionally independent. On the other hand, in propionaldehyde, the C C bond length of the methyl group and the C H bond lengths of the hydrogens attached to the alpha carbon vary as a function of the torsion angle. If the methyl C C bond in the carbonyl plane is taken as a reference, the bond stretches .016 Åwhen the torsion angle is increased to 90°, an α C H bond similarly stretches up to .007 Å. Many of these geometric changes are large, well beyond the experimental errors in modern measurements. 相似文献
965.
Thermodynamics and crystallization kinetics of the hydrothermal synthesis of Na4Ti4Si3O10 (NaTS) were systemically studied by both experiments and model simulation. Experimental results showed that the curve of
crystallinity with time was a characteristic signmoid in the shape that indicated the crystallization of Na4Ti4Si3O10 was a typical spontaneous nucleation process on the laboratory scale. Crystallization of NaTS belongs to the liquid-liquid
transformation mechanism and the reaction is endothermic (ΔH = 15.3 kJ/mol). A mathematic model of crystallization kinetics was developed to simulate the synthesis of NaTS. Runge-Kutta
and simplex methods were adopted to solve the partial differential equations. Model results fitted well with the experimental
data and showed that the synthesis process belongs to spontaneous nucleation and crystal growth. Moreover, the very small
crystal growth constant (5.6·10−7) and gel dissolution constant (7.0·10−7) indicate they are the rate-limiting steps of the whole synthesis process. 相似文献
966.
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
967.
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
968.
We optimized a capillary zone electrophoretic method for separation of six angiotensin-II-receptor antagonists (ARA-IIs): candesartan, eprosartan, irbesartan, losartan potassium, telmisartan, and valsartan. A three-level, full-factorial design was applied to study the effect of the pH and molarity of the running buffer on separation. Combination of the studied parameters permitted the separation of the six ARA-IIs, which was best carried out using 60 mM sodium phosphate buffer (pH 2.5). The same system can also be applied for the quantitative determination of these compounds, but only for the more soluble ones. Some parameters (linearity, precision and accuracy) were validated. 相似文献
969.
The synthesis of a family of 1,1-diarylethylenes from an α-stannyl β-silylstyrene through a combination of a Stille coupling and a protodesilylation reaction is described. This approach avoids the problematic cine-substitution, which is a well documented side reaction during the palladium-assisted elaboration of α-substituted vinylstannanes to 1,1-disubstituted ethylenes. 相似文献
970.
The implementation of an expert system for the automated qualitative interpretation of energy-dispersive x-ray spectra is discussed. The first step in the interpretation process is the extraction of the relevant data from the spectrum, which is done by a preprocessor program, written in FORTRAN. The expert system itself consists of three parts. The knowledge base contains specific information on energy-dispersive x-ray fluorescence spectrometry presented in the form of IF/THEN rules. The data base contains the reduced spectral data and an array of certainty factors associated with each element; the certainty factor for an element represents the probability of its being present in the sample from which the spectrum was taken. Finally, the inference engine performs manipulation of the knowledge. For a particular state of the data base, the certainty factors for all the elements are iteratively modified until convergence is reached by using the rules from the knowledge base. During each cycle, the inference engine selects one rule from the knowledge base and executes it. Rules are selected on the basis of the chemical elements contained in their IF part and according to their previously assigned focus levels. Execution of the THEN part of the selected rule modifies the certainty factors of a number of elements. At the end of the interpretation session, the system lists the elements which have a high probability of being present in the sample. Optionally, the user can be provided with explanations of the reasoning steps taken during the interpretation. Application of the expert system to a particular spectrum shows that it is useful for the reliable interpretation of spectral data obtained from electron microprobe analysis of industrial aerosol particles. 相似文献