全文获取类型
收费全文 | 728篇 |
免费 | 18篇 |
专业分类
化学 | 474篇 |
晶体学 | 3篇 |
力学 | 18篇 |
数学 | 55篇 |
物理学 | 196篇 |
出版年
2023年 | 2篇 |
2022年 | 19篇 |
2021年 | 12篇 |
2020年 | 16篇 |
2019年 | 10篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 14篇 |
2015年 | 13篇 |
2014年 | 23篇 |
2013年 | 44篇 |
2012年 | 49篇 |
2011年 | 61篇 |
2010年 | 44篇 |
2009年 | 23篇 |
2008年 | 38篇 |
2007年 | 32篇 |
2006年 | 31篇 |
2005年 | 33篇 |
2004年 | 36篇 |
2003年 | 30篇 |
2002年 | 19篇 |
2001年 | 9篇 |
2000年 | 16篇 |
1999年 | 7篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 13篇 |
1995年 | 3篇 |
1994年 | 11篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 4篇 |
排序方式: 共有746条查询结果,搜索用时 15 毫秒
721.
2,3,4,5-Tetrasubstituted pyrroles have been prepared with high regioselectivity by a formal cycloaddition of alpha-(alkylideneamino)nitriles and nitroolefins followed by elimination of HCN and HNO2. The reaction allows the convergent construction of the pyrrole ring in four steps from a nitroalkane and three aldehydes. 相似文献
722.
Vartanyants IA Singer A Mancuso AP Yefanov OM Sakdinawat A Liu Y Bang E Williams GJ Cadenazzi G Abbey B Sinn H Attwood D Nugent KA Weckert E Wang T Zhu D Wu B Graves C Scherz A Turner JJ Schlotter WF Messerschmidt M Lüning J Acremann Y Heimann P Mancini DC Joshi V Krzywinski J Soufli R Fernandez-Perea M Hau-Riege S Peele AG Feng Y Krupin O Moeller S Wurth W 《Physical review letters》2011,107(14):144801
Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser, the Linac Coherent Light Source, are presented. Single-shot measurements were performed at 780?eV x-ray photon energy using apertures containing double pinholes in "diffract-and-destroy" mode. We determined a coherence length of 17 μm in the vertical direction, which is approximately the size of the focused Linac Coherent Light Source beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.55?fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode. 相似文献
723.
Nikola Burić Ines Grozdanović Kristina Todorović Nebojša Vasović 《Journal of statistical physics》2011,145(1):175-186
Influence of small time-delays in coupling between noisy excitable systems on the coherence resonance and self-induced stochastic
resonance is studied. Parameters of delayed coupled deterministic excitable units are chosen such that the system has only
one attractor, namely the stationary state, for any value of the coupling and the time-lag. Addition of white noise induces
qualitatively different types of coherent oscillations, and we analyzed the influence of coupling time-delay on the properties
of these coherent oscillations. The main conclusion is that time-lag τ≥1, but still smaller than the refractory period, and sufficiently strong coupling drastically change signal to noise ratio
in the quantitative and qualitative way. An interval of noise values implies quite large signal to noise ratio and different
types of noise induced coherence are greatly enhanced. We also observed coincident spiking for small noise intensity and time-lag
proportional to the inter-spike interval of the coherent spike trains. On the other hand, time-lags τ<1 and/or weak coupling induce negligible changes in the properties of the stochastic coherence. 相似文献
724.
Mancini E De Martino L Marandino A Scognamiglio MR De Feo V 《Molecules (Basel, Switzerland)》2011,16(9):7725-7735
The chemical composition of the essential oil of Helichrysum italicum (Roth) Don ssp. italicum, collected in the National Park of Cilento and Diano Valley, Southern Italy, was studied by means of GC and GC/MS. Forty four compounds of 45 constituents were identified in the oil, mainly oxygenated sesquiterpenes. The essential oil was evaluated for its potential in vitro phytotoxic activity against germination and early radicle elongation of radish and garden cress. The radicle elongation of radish was significantly inhibited at the highest doses tested, while germination of both seeds was not affected. 相似文献
725.
Nada Horvatinčić Andreja Sironić Jadranka Barešić Ines Krajcar Bronić Jovana Nikolov Nataša Todorović Jan Hansman Miodrag Krmar 《Central European Journal of Physics》2014,12(10):707-713
The analyses of radioactive isotopes 14C, 137Cs and 210Pb, and stable isotope 13C were performed in the sediment cores, top 40 cm, taken in 2011 from karst lakes Pro??e and Kozjak in the Plitvice Lakes National Park, central Croatia. Frozen sediment cores were cut into 1 cm thick layers and dried. 14C activity in both carbonate and organic fractions was measured using accelerator mass spectrometry technique with graphite synthesis. 137Cs, 210Pb, 214Pb and 214Bi were measured by low level gamma spectrometry method on ORTECHPGe detector with the efficiency of 32%. Distribution of 14C activity from both lakes showed increase of the 14C activity in the top 10–12 cm in both carbonate and organic fractions as a response to thermonuclear bomb-produced 14C in the atmosphere in the sixties of the 20th century. Anthropogenically produced 137Cs was also observed in sediment profiles. Sedimentation rates for both lake sediments were estimated based on the unsupported 210Pb activity. Different 14C activity of the carbonate fraction (63–80 pMC, percent of modern carbon) and organic fraction (82–93 pMC) is the result of geochemical and biological processes of the sediment precipitation in the lake waters. This is also confirmed by the δ 13 C values of both fractions. Carbon isotope composition, a 14 C and δ 13 C, was compared with the lake sediments from the same lakes collected in 1989 and 2003. 相似文献
726.
Margaux Renaudat St‐Jean Maria Ines Amanti Alice Bernard Ariane Calvar Alfredo Bismuto Emilio Gini Mattias Beck Jerome Faist H. C. Liu Carlo Sirtori 《Laser \u0026amp; Photonics Reviews》2014,8(3):443-449
Compact laser sources operating in mid infrared spectral region with stable emission are important for applications in spectroscopy and wireless communication. Quantum cascade lasers (QCL) are unique semiconductor sources covering mid infrared frequency range. Based on intersubband transitions, the carrier lifetime of these sources is in the ps range. For this reason their frequency response to direct modulation is expected to overcome the limits of standard semiconductor lasers. In this work injection locking of the roundtrip frequency of a QCL emitting at 9 μm is reported. Inter modes laser frequency separation is stabilized and controlled by an external microwave source. Designing an optical waveguide embedded in a microstrip line a flat frequency response to direct modulation up to 14 GHz is presented. Injection locking over MHz frequency range at 13.7 GHz is demonstrated. Numerical solutions of injection locking theory are discussed and presented as tool to describe experimental results. 相似文献
727.
Carmen S. Renamayor Alejandra Pastoriza Cesar L. Usma Ines F. Pierola 《Colloid and polymer science》2013,291(8):2017-2021
This work attempts to study the interaction of two ionic liquids (1-ethyl-3-methylimidazolium tosylate and 1-hexyl-3-methylimidazolium chloride) with chemically crosslinked poly(N-vinylimidazole) in aqueous media, and to compare it with that of NaCl, a typical salting-out electrolyte. The three salts show a salting-in effect whose intensity was measured on the basis of the decrease of the polymer–solvent interaction parameter and the increase of swelling with increasing ionic strength. It was thus found that the salting-in effect is the same for the two salts with the same anion (chloride), while the intensity of the salting-in effect exerted by the tosylate ionic liquid is larger. The coefficient of selective sorption, which expresses the salt excess inside the swollen gel with respect to the external solution was determined by comparing the initial and equilibrium compositions of the immersion bath. These results are discussed in terms of the hydrophobic character of the ions involved. 相似文献
728.
Michael Raessler Jan Rothe Ines Hilke 《International journal of environmental analytical chemistry》2013,93(9):707-715
AI, Fe, Mn and Zn were determined in two ditTerent species of woodlouse: Porcellio scaber and Porcellio dilalalus. Both species were cultivated under standardized conditions in a climatic chamber. Moreover, skins of the cultivated animals were collected and analyzed separately to examine a possible way of decontamination by moulting. To obtain enough sample material for each species, IS animals of the same age and size were pooled. For skin analysis, 10–12 skins were collected and pooled. The animals and their skins were dried, ground and digested in pure concentrated nitric acid using multiwave-assisted high-pressure digestion and. finally, analyzed by ICP-OES. Special emphasis was given to quality control: the reference materials Dorm-2, Dogfish Muscle (Squalus acanthias) and SRM 1577b Bovine Liver, were used to evaluate the whole analytical process including sample digestion. These reference materials of animal origin were selected to match the matrix of the samples as closely as possible. While concentrations of Fe. Mn and Zn were traced to both reference materials, the concentration of AI was checked by Dorm-2 only. Analyses of the elements in the reference materials were carried out using four ditTerent wavelengths for each element simultaneously. aiming at the determination of the best suited wavelength for each element. Analyses of woodlouse samples and their skins were finally carried out using the wavelengths with the highest sensitivities after absence of spectral interferences had been demonstrated. 相似文献
729.
Ines Ghorbel Aljia Afli Souhir Abid Martine Tessier Rachid El Gharbi Alain Fradet 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(6):433-440
A solution polyaddition procedure was applied to prepare furanic polyacylsemicarbazides based on 2,2′-isopropylidene-bis(5-(2-furoyl) hydrazide) and aliphatic or aromatic diisocyanates. Model compounds were prepared to facilitate the synthesis and the characterization of the polymers. A systematic study of reaction parameters (the nature of the organic phase, the temperature, the reaction time, and the concentration of monomers) was carried out. High MW polymers were obtained after 12 h reaction at 20°C with 0.2 M monomer solutions in dimethylacetamide. In fact, the polyacylsemicarbazides obtained had inherent viscosities ranging from 0.63 to 0.85 dL/g. There showed good solubility in aprotic polar solvents. The 1H and 13C-NMR spectra were consistent with the expected structures. These furan-based polyacylsemicarbazides are thermally stable up to 290°C, but exhibit a complex thermal behavior. This was explained by the cyclohydration of acylsemicarbazide groups into NH-substituted oxadiazoles, resulting in rigid and high melting polymers, less stable, however, than conventional aromatic polyoxadiazoles. 相似文献
730.
A new tecton-like tetracarboxylic compound 1 featuring two isophthalic acid groups attached to both ends of a rigid 1,3-butadiyne spacer unit is synthesised using acetylene blocking/deblocking and coupling techniques. Crystallisation of 1 from DMSO-chloroform solution gives rise to the formation of an unusual mixed solvent crystalline framework structure 1a containing DMSO and chloroform as secondary and tertiary components in 1:2:1 stoichiometric ratio. The X-ray crystal structure of 1a shows interesting stacking mode and hydrogen-bonded, 3D network architecture with the two solvent species being involved in different behaviour pattern. 相似文献