全文获取类型
收费全文 | 80112篇 |
免费 | 403篇 |
国内免费 | 384篇 |
专业分类
化学 | 25612篇 |
晶体学 | 798篇 |
力学 | 6781篇 |
数学 | 32235篇 |
物理学 | 15473篇 |
出版年
2019年 | 34篇 |
2018年 | 10448篇 |
2017年 | 10286篇 |
2016年 | 6113篇 |
2015年 | 898篇 |
2014年 | 348篇 |
2013年 | 449篇 |
2012年 | 3870篇 |
2011年 | 10596篇 |
2010年 | 5707篇 |
2009年 | 6101篇 |
2008年 | 6712篇 |
2007年 | 8871篇 |
2006年 | 346篇 |
2005年 | 1442篇 |
2004年 | 1629篇 |
2003年 | 2042篇 |
2002年 | 1072篇 |
2001年 | 285篇 |
2000年 | 322篇 |
1999年 | 190篇 |
1998年 | 214篇 |
1997年 | 172篇 |
1996年 | 228篇 |
1995年 | 138篇 |
1994年 | 99篇 |
1993年 | 121篇 |
1992年 | 80篇 |
1991年 | 82篇 |
1990年 | 69篇 |
1989年 | 78篇 |
1988年 | 74篇 |
1987年 | 73篇 |
1986年 | 82篇 |
1985年 | 72篇 |
1984年 | 79篇 |
1983年 | 68篇 |
1982年 | 68篇 |
1981年 | 58篇 |
1980年 | 66篇 |
1979年 | 60篇 |
1978年 | 50篇 |
1977年 | 43篇 |
1976年 | 33篇 |
1973年 | 44篇 |
1914年 | 46篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
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
992.
Endre Nagy 《Central European Journal of Chemistry》2003,1(2):160-177
A heterogeneous, multi-layer mass transfer model is proposed for prediction of the effect of multi-layer packing of catalyst
particles adhered to the gas-liquid interface. The behavior of the mass transfer rate with respect to the multi-layer packing,
to the particle size and mass transfer coefficient without particles is discussed. It is shown that enhancement can be considerably
increased by multi-layer packing compared to that of mono-layer packing, depending on the values of particle size and mass
transfer coefficient. The predicted mass transfer rates using the proposed model was verified with experimental data taken
from the literature. The model presented should be superior to that of published in the literature. 相似文献
993.
Ihalainen JA Croce R Morosinotto T van Stokkum IH Bassi R Dekker JP van Grondelle R 《The journal of physical chemistry. B》2005,109(44):21150-21158
Light-harvesting complex I (LHCI), which serves as a peripheral antenna for photosystem I (PSI) in green plants, consists mainly of four polypeptides, Lhca1-4. We report room temperature emission properties of individual reconstituted monomeric Lhca proteins (Lhca1, -2, -3, and -4) and dimeric Lhca1/4, performed by steady-state and time-resolved fluorescence techniques. The emission quantum yields of the samples are approximately 0.12, 0.085, 0.081, 0.041, and 0.063 for Lhca1, -2, -3, -4, and the -1/4 dimer, respectively, which is considerably lower than the value of 0.22 found for light-harvesting complex II (LHCII), the main peripheral antenna complex of photosystem II in green plants. The decay components of LHCI proteins can be divided in two categories: Lhca1 and Lhca3 have decay times of 1.1-1.6 ns and 3.3-3.6 ns, and Lhca2 and Lhca4 have decay times of 0.7-0.9 ns and 3.1-3.2 ns. These categories seem to correlate with the pigment composition of the samples. All decay times are faster than that observed previously for LHCII. When the absolute emission yields and the lifetimes of the Lhca samples are combined, the overall emission properties of the individual Lhca proteins are expressed in terms of their emitting dipole moment strength. In the samples without extreme red states, that is, Lhca1 and Lhca2, the emitting dipole moment has a value close to unity (relative to monomeric chlorophyll in acetone), which is similar to that for LHCII, whereas, in the samples with the red-most state (F-730), that is, Lhca3, -4, and the -1/4 dimer, the emitting dipole moment has a value less than unity (0.6-0.8), which can be explained by mixing the red-most (exciton) state with a dark charge-transfer state, as suggested in previous PSI red pigment studies. In addition, we find a lifetime component of approximately 50-150 ps in all red-pigment-containing samples, which cannot be due to "slow" energy transfer, but is instead assigned to an unrelaxed state of the pigment-protein, which, on this time-scale, is converted into the final emitting state. 相似文献
994.
Petrignani A van der Zande WJ Cosby PC Hellberg F Thomas RD Larsson M 《The Journal of chemical physics》2005,122(1):14302
We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed. 相似文献
995.
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
996.
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
997.
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
998.
A new compound (5R, 10R)-3,8-dihydroxy-5,10-diethoxy-5,10-dihydrochromeno[5,4,3-cde]chromene monohydrate was obtained from 3,4-dihydroxybenzaldehyde in aerobic basic aqueous ethanol solution in the presence
of manganese chloride and triethylamine and crystallized in orthorhombic P212121 space group (denoted as 1). When 1 was recrystallized from aqueous methanol, it was transformed to another crystal (2) with the same composition but in P21/n space group. The drastic difference in the extensive hydrogen bond network makes 1 a 3D and 2 a 2D infinite supramolecular structure, respectively. 相似文献
999.
M. Jayalakshmi P. Radhika K. Phani Raja M. Mohan Rao 《Journal of Solid State Electrochemistry》2007,11(2):165-172
Nanoparticles of α-phase nickel hydroxide were synthesized by a single-step hydrothermal method using urea as the hydrolytic agent. Precipitated powders were of pure turbostratic α-phase as confirmed by x-ray diffraction profile. The ageing of α-Ni(OH)2 in 1.0 M alkali solutions is investigated for pure non-intercalated α-Ni(OH)2 and thiourea intercalated/absorbed α-phase nanomaterials. The α-Ni(OH)2 powder immobilized on the surface of graphite electrodes shows a gradual α→β phase transformation with continuous voltammetric cycling, and the concentration gradient of water that exists in the layered-double-hydroxide-like interlayers of α-phase and the solution was shown to play a crucial role on the high electrochemical activity of this phase nickel hydroxide. To understand the role of water in the ageing process, concomitant entries of non-aqueous solvents like ethanol and acetonitrile along with thiourea were effected. Cyclic voltammetric measurements of thiourea-treated α-Ni(OH)2 samples revealed that hydroxyl ion influx during the anodic oxidation depends on the counter flux of solvent molecules, and if the intercalated the solvent is acetonitrile, then the electrochemical activity of α-Ni(OH)2 reduced drastically; Q
a/Q
c>1 for water as solvent in the interlayers α-Ni(OH)2 and Q
a/Q
c<1 for ethanol and acetonitrile as solvents. The α-phase gets stabilized in the presence of thiourea with water and ethanol as co-intercalates. Transmission electron microscope images of α-Ni(OH)2 and thiourea-treated samples show a change in particle size and morphology. Elemental CHNS analysis confirms the presence of sulphur in the thiourea intercalated samples. 相似文献
1000.
Two novel RuII complexes [Ru(phen)2(PNOPH)]2+ and [Ru(dmp)2 (PNOPH)]2+ (phen = 1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, PNOPH = 2-(4-nitrophenyl)imidazo-[4,5-f][1,10]phenanthroline) and their deprotoned complexes were synthesized and characterized by ES–MS, 1H - n.m.r, u.v.–vis. and electrochemistry. The crystal structure of the deprotonated complex [Ru(dmp)2 (PNOP)][ClO4] · CH3CN was determined by means of X-ray single crystal diffraction. Nonlinear optical properties of the RuII complexes were investigated by Z-scan techniques in DMF solution, and all of them exhibited both NLO absorption and self-defocusing effect. The corresponding effective NLO susceptibilities |3 | of the complexes are 2.39 × 10-12–5.80 × 10-12 esu. 相似文献