全文获取类型
收费全文 | 80049篇 |
免费 | 401篇 |
国内免费 | 383篇 |
专业分类
化学 | 25474篇 |
晶体学 | 792篇 |
力学 | 6759篇 |
数学 | 32285篇 |
物理学 | 15523篇 |
出版年
2021年 | 70篇 |
2020年 | 62篇 |
2019年 | 74篇 |
2018年 | 10479篇 |
2017年 | 10280篇 |
2016年 | 6129篇 |
2015年 | 885篇 |
2014年 | 371篇 |
2013年 | 415篇 |
2012年 | 3898篇 |
2011年 | 10606篇 |
2010年 | 5701篇 |
2009年 | 6117篇 |
2008年 | 6664篇 |
2007年 | 8821篇 |
2006年 | 292篇 |
2005年 | 1374篇 |
2004年 | 1578篇 |
2003年 | 2012篇 |
2002年 | 1058篇 |
2001年 | 263篇 |
2000年 | 309篇 |
1999年 | 185篇 |
1998年 | 206篇 |
1997年 | 161篇 |
1996年 | 214篇 |
1995年 | 140篇 |
1994年 | 91篇 |
1993年 | 118篇 |
1992年 | 71篇 |
1991年 | 82篇 |
1990年 | 60篇 |
1989年 | 77篇 |
1988年 | 67篇 |
1987年 | 64篇 |
1986年 | 65篇 |
1985年 | 55篇 |
1984年 | 58篇 |
1983年 | 43篇 |
1982年 | 54篇 |
1981年 | 48篇 |
1980年 | 57篇 |
1979年 | 65篇 |
1978年 | 49篇 |
1914年 | 48篇 |
1913年 | 41篇 |
1912年 | 45篇 |
1909年 | 44篇 |
1908年 | 41篇 |
1907年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
741.
Stefan Mecking 《Colloid and polymer science》2007,285(6):605-619
By contrast to traditional free radical emulsion polymerization, catalytic polymerization allows for polymer microstructure
control. In terms of polymerizable monomers, both techniques are largely complementary. Since the beginning of this decade,
an increasing number of reports on polyolefin, polybutadiene, polyalkenamer, polynorbornene, polyketone, and polyacetylene
dispersions prepared by catalytic polymerization in disperse aqueous systems has appeared. This contribution reviews the preparation
of these dispersions, their colloidal properties, particle formation mechanisms, particle morphologies, and polymer microstructures. 相似文献
742.
In the present work, corrosion resistance of surface-coated galvanized steel was quantitatively determined by an analysis
of the alternating current (AC) impedance spectra measured on the salt-spray-tested specimen. To evaluate the corrosion resistance
of the surface-coated galvanized steel, AC impedance spectroscopy was performed on the salt-spray-tested specimen previously
exposed to salt-sprayed corrosive environment. From the analysis of the impedance spectra, the area fraction transient of
white rust θ
2(t) was theoretically derived from the equivalent circuit equation by using two fitting parameters. The values of the two fitting
parameters were determined by fitting the empirical transient equations to the area fraction of the resin coating layer and
to the total resistance obtained from the impedance spectra measured, respectively. From the analyses of θ
2(t) for four kinds of surface-coated galvanized steels with various resin coating layers, it is indicated that as the values
of the two fitting parameters decrease in the order of CP, GI, OD and OM (commercial trade names) specimens, the corrosion
resistance increases in that order as well. Furthermore, from the quantitative comparison of the two fitting parameters with
the polarization resistance of the upper resin coating layer R
p determined from the potentiodynamic polarization curve, it is suggested that the two fitting parameters decrease in value
as well with increasing R
p. 相似文献
743.
Rollie J. Myers 《Journal of solution chemistry》2007,36(3):395-403
We have measured the second acid dissociation constant, K
2a
, at several ionic strengths for hydrogen telluride (H2Te) using the Charge Transfer to Solvent (CTTS) uv spectra of its anions HTe− and Te2−. Since it is produced in our solutions, we have also determined the spectra of Te2
2− both in the uv and in the visible regions. At 25 ∘C, K
2a
= (1.28 ± 0.02) × 10−12 by extrapolation to zero ionic strength. Its value at an ionic strength equal to 0.5 mol.dm-3 was estimated to be (8.7 ± 0.2) × 10−12. The solution thermodynamics of these species are also discussed and comparisons are made to related acids. 相似文献
744.
Naomi Masuda Yatsuhisa Nagano T. Kimura 《Journal of Thermal Analysis and Calorimetry》2005,81(3):533-535
Summary The standard molar enthalpy of formation of methyl methylthiomethyl sulfoxide, CH3(CH3SCH2)SO, at T=298.15 K in the liquid state was determined to be -199.4±1.5 kJ mol-1 by means of oxygen rotating-bomb combustion calorimetry. 相似文献
745.
The enthalpy change of formation of the reaction of hydrous dysprosium chloride with ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen?H2O) in absolute ethanol at 298.15 K has been determined as (-16.12 ± 0.05) kJ?mol-1 by a microcalormeter. Thermodynamic parameters (the activation enthalpy, the activation entropy and the activation free energy), rate constant and kinetics parameters (the apparent activation energy, the pre-exponential constant and the reaction order) of the reaction have also been calculated. The enthalpy change of the solid-phase reaction at 298.15 K has been obtained as (53.59 ± 0.29) kJ?molt-1 by a thermochemistry cycle. The values of the enthalpy change of formation both in liquid-phase and solid-phase reaction indicated that the complex could only be synthesized in liquid-phase reaction. 相似文献
746.
Mesoporous amorphous MnO<Subscript>2</Subscript> as electrode material for supercapacitor 总被引:1,自引:0,他引:1
Mao-Wen Xu Dan-Dan Zhao Shu-Juan Bao Hu-Lin Li 《Journal of Solid State Electrochemistry》2007,11(8):1101-1107
A kind of novel mesoporous, electrochemical active material, amorphous MnO2 has been synthesized by an improved reduction reaction and using supramolecular as template. The synthesized sample was characterized
physically by thermogravimetric analysis, X-ray diffraction, transmission electron microscope (TEM), and Brunauer–Emmett–Teller
(BET) surface area measurement, respectively. Electrochemical characterization was performed using cyclic voltammetry and
chronopotentiometry in 2 mol/l KOH aqueous solution electrolyte. The results of BET and TEM analysis indicated that supramolecular
template plays an important role in the process of big specific surface area mesoporous material forming. After sintering
at 200 °C, the sample still remained an amorphous structure, and its specific capacitance reached 298.7 F/g and presented
a very stable capacitance after 500 cycles. In addition, the electrochemical process, such as ion transfer and electrical
condition, was also investigated with electrochemical impedance spectroscopy. 相似文献
747.
The characters, dynamics, and relaxation pathways of low-lying excited states of the complexes [W(CO)(5)L] [L = 4-cyanopyridine (pyCN) and piperidine (pip)] were investigated using theoretical and spectroscopic methods. DFT calculations revealed the delocalized character of chemically and spectroscopicaly relevant molecular orbitals and the presence of a low-lying manifold of CO pi-based unoccupied molecular orbitals. Traditional ligand-field arguments are not applicable. The lowest excited states of [W(CO)(5)(pyCN)] are W --> pyCN MLCT in character. They are closely followed in energy by W --> CO MLCT states. Excitation at 400 or 500 nm populates the (3)MLCT(pyCN) excited state, which was characterized by picosecond time-resolved IR and resonance Raman spectroscopy. Excited-state vibrations were assigned using DFT calculations. The (3)MLCT(pyCN) excited state is initially formed highly excited in low-frequency vibrations which cool with time constants between 1 and 20 ps, depending on the excitation wavelength, solvent, and particular high-frequency nu(CO) or nu(CN) mode. The lowest excited states of [W(CO)(5)(pip)] are W --> CO MLCT, as revealed by TD-DFT interpretation of a nanosecond time-resolved IR spectrum that was measured earlier in a low-temperature glass (Johnson, F. P. A.; George, M. W.; Morrison, S. L.; Turner, J. J. J. Chem. Soc., Chem. Commun. 1995, 391-393). MLCT(CO) excitation involves transfer of electron density from the W atom and, to a lesser extent, the trans CO to the pi orbitals of the four cis CO ligands. Optical excitation into MLCT(CO) transition of either complex in fluid solution triggers femtosecond dissociation of a W-N bond, producing [W(CO)(5)(solvent)]. It is initially vibrationally excited both in nu(CO) and anharmonicaly coupled low-frequency modes. Vibrational cooling occurs with time constants of 16-22 ps while the intramolecular vibrational energy redistribution from the v = 1 nu(CO) modes is much slower, 160-220 ps. No LF excited states have been found for the complexes studied in a spectroscopically relevant range up to 6-7 eV. It follows that spectroscopy, photophysics, and photochemistry of [W(CO)(5)L] and related complexes are well described by an interplay of close-lying MLCT(L) and MLCT(CO) excited states. The high-lying LF states play only an indirect photochemical role by modifying potential energy curves of MLCT(CO) states, making them dissociative. 相似文献
748.
Ab initio molecular orbital theory and density functional theory have been used to study nine isomers of N7 ionic clusters with low spin at the HF/6-31G*, MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311(+)G* levels of theory. All stationary
points are examined with harmonic vibrational frequency analyses. Four N7
+ isomers and five N7
− isomers are determined to be local minima or very close to the minima on their potential-energy hypersurfaces, respectively.
For N7
+ and N7
−, the energetically low lying isomers are open-chain structures (C
2
v
and C
2
v
or C2). The results are very similar to those of other known odd-number nitrogen ions, such as N5
+, N9
+, and N9
−, for which the open-chain structures are also the global minima. This research suggests that the N7 ionic clusters are likely to be stable and to be potential high-energy-density materials if they could be synthesized.
Received: 16 July 2001 / Accepted: 8 October 2001 / Published online: 21 January 2002 相似文献
749.
The ionic liquid 1-N-butyl-3-methylimidazolium chloride ([C4mim]+Cl−) was investigated as reaction media for the homogeneous acylation of cellulose with 2-furoyl chloride in the presence of
pyridine. The preparation of cellulose furoate depending on the reaction conditions, the cellulose type and the pyridine content
was studied. Cellulose furoates with a degree of substitution in the range from 0.46 to 3.0 were accessible, i.e., under mild
conditions, with a low excess of reagent and in a short reaction time. The products were characterized by elemental analysis,
perpropionylation, 1H- and 13C NMR spectroscopy and FTIR spectroscopy.
Thomas Heinze is the member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu 相似文献
750.
In this study, poly(N-methylolacrylamide)/polymethylacrylamide (PNMA/PMAA) hybrids were produced successfully by frontal free-radical polymerization
at ambient pressure. In a typical run, the appropriate amounts of reactants (N-methylolacrylamide, NMA; methylacrylamide, MAA) and initiator (ammonium persulfate) were dissolved in dimethyl sulfoxide
at ambient temperature. Frontal polymerization (FP) was initiated by heating the wall of the tube with a soldering iron, and
the resultant hot fronts were allowed to self-propagate throughout the reaction vessel. Once initiated, no further energy
was required for polymerization to occur. The dependences of the front velocity and front temperature on the initiator concentration,
reactant dilution, and NMA/MAA components were thoroughly investigated. The front temperatures were between 69 and 116 °C,
depending on the persulfate concentration. We have also investigated the FP of PNMA/PMAA hybrids with N-methyl-2-pyrrolidone as solvent. Results show that FP can be exploited as a means for the preparation of PNMA/PMAA hybrids
with the potential advantage of higher throughput compared to the traditional mode. 相似文献