全文获取类型
收费全文 | 599篇 |
免费 | 16篇 |
专业分类
化学 | 400篇 |
晶体学 | 19篇 |
力学 | 5篇 |
数学 | 32篇 |
物理学 | 159篇 |
出版年
2023年 | 7篇 |
2022年 | 10篇 |
2021年 | 17篇 |
2020年 | 20篇 |
2019年 | 21篇 |
2018年 | 16篇 |
2017年 | 10篇 |
2016年 | 20篇 |
2015年 | 14篇 |
2014年 | 26篇 |
2013年 | 31篇 |
2012年 | 37篇 |
2011年 | 40篇 |
2010年 | 29篇 |
2009年 | 46篇 |
2008年 | 44篇 |
2007年 | 30篇 |
2006年 | 23篇 |
2005年 | 31篇 |
2004年 | 15篇 |
2003年 | 14篇 |
2002年 | 26篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 10篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有615条查询结果,搜索用时 218 毫秒
61.
Westrup KC Gregório T Stinghen D Reis DM Hitchcock PB Ribeiro RR Barison A Back DF de Sá EL Nunes GG Soares JF 《Dalton transactions (Cambridge, England : 2003)》2011,40(13):3198-3210
The reversible thermochromic behaviour of homoleptic [{V(OR)(4)}(n)] complexes in solution [R = Pr(i) (product I), Bu(s) (B(s)), Nep (N) and Cy (C)] is accounted for the existence of an aggregation equilibrium involving dimeric and monomeric species in which vanadium(iv) is respectively five- and four-coordinate. Bulky R groups such as Bu(t) and Pe(t) (tert-pentoxide) prevent aggregation and therefore give rise to exclusively mononuclear compounds (B(t) and P(t), respectively) that are not thermochromic. The complexes and their temperature-dependent interconversion were characterised by single crystal X-ray diffractometry, magnetic susceptibility measurements and electronic, FTIR and EPR spectroscopies in a wide temperature range. Equilibrium constants and enthalpy and entropy changes for the dimerization reactions have been determined and compared with literature data. 相似文献
62.
Nunes AP Peres AE de Araujo AC Valadão GE 《Journal of colloid and interface science》2011,361(2):632-638
The reverse apatite flotation with fatty acids has been widely used for the reduction of phosphorus content of magmatic origin iron ores. However, the occurrence of phosphorus intensely disseminated as secondary minerals such as wavellite renders the anionic reverse flotation a challenge. Zeta potential measurements and microflotation tests of wavellite with the use of anionic and cationic collectors were carried out in this work. The wavellite's IEP value was achieved at pH 4.5. Below the IEP value, the surface positively charged sites are made up of aluminum ions. The species H(+), Al(OH)(2)(+), Al(OH)(2+), Al(3+), OH(-), H(2)PO(4)(-), HPO(4)(2-), and PO(4)(3-) play a role in the protonation and deprotonation reactions that will determine the wavellite-solution interface properties. The highest values of wavellite's floatability under basic pH conditions were achieved in the presence of cationic collectors (1 × 10(-4) mol L(-1)). The formation of surface complexes and the precipitation of insoluble salt of aluminum onto wavellite surface seems to be the most likely hypothesis for the chemical nature interactions between amines and wavellite. The surface formation of aluminum oleate on the wavellite's surface seems to be the most probable hypothesis for the adsorption mechanism and the resultant high floatability of wavellite between pH 7.5 and pH 10.0 in the presence of sodium oleate (1 × 10(-4) mol L(-1)). The results showed that the cationic reverse flotation of secondary phosphates is a promising route to reduce the phosphorus content of iron ores from deposits that underwent a supergene enrichment process, since wavellite floatability in the alkaline pH range, using amine as collector, was not significantly affected by the presence of corn starch. 相似文献
63.
Lopes S Nunes CM Gómez-Zavaglia A Pinho e Melo TM Fausto R 《The journal of physical chemistry. A》2011,115(7):1199-1209
Methyl 4-chloro-5-phenylisoxazole-3-carboxylate (MCPIC) has been synthesized, isolated in low temperature argon and xenon matrices, and studied by FTIR spectroscopy. The characterization of the low energy conformers of MCPIC was made by undertaking a systematic investigation of the DFT(B3LYP)/6-311++G(d,p) potential energy surface of the molecule. The theoretical calculations predicted the existence of three low energy conformers. Two of them (I and II) were observed experimentally in the cryogenic matrices. The third one (III) was found to be converted into conformer II during deposition of the matrices, a result that is in agreement with the predicted low III → II energy barrier (<0.3 kJ mol(-1)). In situ UV irradiation (λ > 235 nm) of matrix-isolated MCPIC yielded as final photoproduct the corresponding oxazole (methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate). Identification of the azirine and nitrile-ylide intermediates in the spectra of the irradiated matrices confirmed their mechanistic relevance in the isoxazole → oxazole photoisomerization. 相似文献
64.
Alécio R. Nunes Aline O. Moura Alexandre G. S. Prado 《Journal of Thermal Analysis and Calorimetry》2011,106(2):445-452
H-magadiite was applied to remove the pesticides 2,4-d, diuron, and atrazine from water. The H-magadiite containing herbicides adsorbed were investigated by FTIR, DRX, N2 adsorption and desorption isotherms, and surface area. Calorimetric studies were carried out to determine the heat of interaction
between pesticides and magadiite. It was possible to ride the cycle of interaction of magadiite–pesticide for a better understanding
of the process involved. From the results of the thermal effect of the interaction of magadiite–pesticide, the thermochemical
parameters can be determined by using the relationship between the data obtained from adsorption and calorimetry results.
The interaction of pesticide–magadiite follows the sequence of adsorption: diuron > atrazine > 2,4-d. The ΔH values for the interactions were determined to be −20.62 ± 1.08, −24.04 ± 0.86–26.34 ± 0.93 kJ mol−1 for 2,4-d, diuron and atrazine, respectively. All the interactions were spontaneous, enthalpically and entropically favored demonstrating
the effectiveness and efficiency of the method. 相似文献
65.
Ronaldo S. Nunes Gilbert Bannach José M. Luiz Flávio J. Caires Claudio T. Carvalho Massao Ionashiro 《Journal of Thermal Analysis and Calorimetry》2011,106(2):525-529
Solid-state compounds of general formula Ln2L3·nH2O, where L represents 1,4-bis(3-carboxy-3-oxo-prop-1-enyl)benzene and Ln = La, Ce, Pr, Nd, Sm, were synthesized. Complexometric
titrations with EDTA, thermogravimetry (TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC),
X-ray powder diffractometry, elemental analysis and infrared spectroscopy have been employed to characterize and to study
the thermal behavior of these compounds in dynamic air atmosphere. The results led to information about the composition, dehydration,
crystallinity, and thermal decomposition of the synthesized compounds. 相似文献
66.
Ferreira da Silva F Almeida D Antunes R Martins G Nunes Y Eden S Garcia G Limão-Vieira P 《Physical chemistry chemical physics : PCCP》2011,13(48):21621-21629
Electron transfer to uracil (U), 5-chlorouracil (5-ClU) and 5-fluorouracil (5-FU) yielding anion formation has been investigated in 30-100 eV potassium-molecule collisions. The rich fragmentation patterns of all three molecules suggest that electron transfer in collisions with electronegative neutrals may cause efficient damage to RNA. The main ring fragment anion in all the mass spectra was NCO(-) while the production of X(-) (X = F, Cl) was a strong decomposition of the halouracil temporary negative ions. Cl(-) was the most intense fragment anion in the 5-chlorouracil measurements, whereas NCO(-) production dominated in the U and 5-FU data. Arguments based on energetics and vibrational dynamics have been proposed to explain these differences. Electronic coupling between dipole- and valence-bound states may play a particularly important role in the fragmentation pathways of the 5-ClU parent anion. The stabilizing influence of the potassium cation following electron transfer (ionic scattering) on the observed fragmentation patterns is discussed, notably in the context of comparisons with free electron attachment processes. 相似文献
67.
Y. Kim M. Ree T. Chang C. S. Ha T. L. Nunes J. S. Lin 《Journal of Polymer Science.Polymer Physics》1995,33(14):2075-2082
Homogeneous precursor/precursor solutions with various compositions were obtained with appreciably high solid contents in N-methyl-2-pyrrolidone from soluble poly(amic diethyl ester) precursors of rodlike poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) and flexible poly(4,4′-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA), which are hydrolytically more stable as well as more soluble than the corresponding poly(amic acid)s being equilibrated with the constituent monomers. Both optical microscopic and light scattering measurements showed that the dried precursor blend films and resultant polyimide composite films were optically transparent, regardless of compositions and process conditions. The composite films showed a single Tg behavior. However, for the composite of 30 wt % BPDA-PDA dispersed in the matrix of 70 wt % BPDA-ODA, a smectic crystalline-like aggregation of the BPDA-PDA component was detected on wide-angle x-ray diffraction patterns, indicative of microscopic phase separation between the two components. This phase separation was not detected on the optical microscopy, light scattering, and dynamic mechanical thermal analysis because of their resolution limits: Optical microscopy has a resolution of submicrometers, whereas dynamic mechanical thermal analysis and light scattering have a resolution of ca. 50 Å. Therefore, it is speculated that in the composite films BPDA-PDA and BPDA-ODA polyimide molecules have demixed on the scale of a few nanometers. The mean long periodicity, which was estimated from the small-angle x-ray scattering pattern, varied from 134 to 170 Å as the content of BPDA-ODA component increased. In addition, mechanical properties of the composite films were characterized. ©1995 John Wiley & Sons, Inc. 相似文献
68.
D. Fagundes-Peters A.S.S. de Camargo L.A.O. Nunes 《Applied physics. B, Lasers and optics》2006,85(1):101-104
Thulium doped fluoroindogallate glass was characterized by means of excited state absorption experiment in the 0.95 to 1.55 μm spectral range. The three bands corresponding to the electronic transitions 3
F
4→3
F
2 (at 1.05 μm), 3
F
4→3
F
3 (at 1.125 μm), and 3
F
4→3
H
4 (at 1.45 μm) were observed. The energy transfer microscopic parameters for the reverse cross relaxation process 3
F
4, 3
F
4→3
H
6, 3
H
4 were calculated for different multipolar interaction mechanisms using the Kushida model, and it was verified that the probability of this process is 100 times lower than that of the direct 3
H
4, 3
H
6→3
F
4, 3
F
4 cross relaxation, responsible for the 1.8 μm emission pumping. PACS 78.20.-e; 78.55.Qr; 42.70.Hj; 42.55.Wd 相似文献
69.
We show that the Sal'nikov thermokinetic oscillator system has at least two limit cycles, an unstable one lying inside a stable one. The proof uses the elementary technique of Liénard's equation. 相似文献
70.
O.A.C. Nunes 《Solid State Communications》1983,47(11):873-875
The magnon scattering by free carriers in the presence of an intense laser field is discussed. A kinetic equation for electrons is derived from which the multiphonon absorption coefficient is calculated. It is found that for very large fields the absorption coefficient is vanishing small. A physical explanation of this result is also available. 相似文献