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181.
Riccardo Po Giorgio Bornengo Letterio DiStefano Carla Cavagna Ernesto Occhiello Fabio Garbassi 《先进技术聚合物》1995,6(2):63-68
Poly(ethylene terephthalate) and a series of other amorphous copolyesters based on ethylene glycol and terephthalic/isophthalic or 2,6-naphthalenedicarboxylic/isophthalic acid mixtures were prepared by melt polycondensation. Comonomers consisting of azoic dyes having two polymerizable functions on the aromatic rings (either one alcoholic and one carbomethoxyl, two alcoholic or two carbomethoxyl groups) were also added, in amounts lower than 1% mol with respect to the sum of diesters used as starting monomers. In these conditions (P<0.1 torr and T=280°C), only few azoic dyes were found to be stable. Among the prepared polymers, those based on dyes having sufficient thermal stability were characterized by differential scanning calorimetry and solution properties. Inherent viscosities suggest that molecular weights are high. Glass transition and melting temperatures as well as mechanical properties of colored polyesters are not affected by the presence of the azoic comonomers. UV–visible spectra of copolyester solutions were registered, and absorption coefficients were correlated with the amount of dye. 相似文献
182.
Virgil Percec Anatoliy V. Popov Ernesto Ramirez‐Castillo Oliver Weichold 《Journal of polymer science. Part A, Polymer chemistry》2004,42(24):6364-6374
The accelerated single electron transfer–degenerative chain transfer mediated living radical polymerization (SET–DTLRP) of vinyl chloride (VC) in H2O/tetrahydrofuran (THF) at 25 °C is reported. This process is catalyzed by sodium dithionite (Na2S2O4)‐sodium bicarbonate (NaHCO3). Electron transfer cocatalysts (ETC) 1,1′‐dialkyl‐4,4′‐bipyridinum dihalides or alkyl viologens were also employed in this polymerization. The resulting poly(vinyl chloride) (PVC) has a number‐average molecular weight (Mn) = 2,000–12,000, no detectable amounts of structural defects, and both active chloroiodomethyl and inactive chloromethyl chain ends. The molecular weight distribution of PVC obtained is Mw/Mn = 1.5. The surface active agents afford the final polymers as a powder and provide an acceleration of the rate of polymerization. The role of ETC is to accelerate the single electron transfer (SET) step, whereas THF enhances the degenerative chain transfer (DT) step. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6364–6374, 2004 相似文献
183.
María L. Cerrada Rosario Benavente Ernesto Prez Jorge Moniz‐Santos M. Rosrio Ribeiro 《Journal of Polymer Science.Polymer Physics》2004,42(20):3797-3808
The relationship between the structure and mechanical properties has been established for several copolymers of ethylene and 5,7‐dimethylocta‐1,6‐diene synthesized with a metallocene catalyst. A dependence on the composition and polymerization temperature has been found. The branches cannot be incorporated into the orthorhombic crystal lattice, and consequently, structural parameters such as the crystallinity and crystal size are considerably affected as the 5,7‐dimethylocta‐1,6‐diene content increases in the copolymers. The viscoelastic relaxations have been analyzed and compared with those exhibited by high‐density polyethylene (HDPE). The β relaxation does not appear in HDPE and is exclusively seen in the copolymers. As the 5,7‐dimethylocta‐1,6‐diene content rises, the intensity of this process is increased, and its location is shifted to a lower temperature up to comonomer contents of approximately 6–8 mol % in the copolymers. On the other hand, the α mechanism associated with motion within the crystalline regions is also moved to a lower temperature and its intensity is diminished as the 5,7‐dimethylocta‐1,6‐diene molar fraction increases in the copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3797–3808, 2004 相似文献
184.
The liquid crystal behaviour of a family of non-symmetric liquid crystalline dimers is reported. These systems contain two bibenzoate rigid units that are linked to distinct terminal groups at one end, and to a flexible interconnecting spacer at the other. Several systems having different terminal and central chains are studied using calorimetric, microscopic and diffraction techniques. All the samples form phases with variable degrees of order (from low ordered smectic to crystalline phases) depending on the chemical constitution of the different segments. The influence of the length, parity and lateral substitution of the spacers on the transitional properties and the symmetry of the mesophases that are formed is analysed. It is found that a decrease in the transition temperatures and enthalpies occurs when the length of the flexible spacers increases, when lateral methyl substituents are introduced, or when the parity of the central spacer changes from an even to odd number of carbon atoms or ether groups. The arrangement of the mesogens and dissimilar flexible groups within the ordered structure is discussed with respect to the observed L/d ratios. Different values were obtained depending on the parity of the central spacer and on the degree of order. Interpenetrated structures, in which the flexible groups of different lengths are mixed, seem to be compatible with low ordered smectic phases, but sterically disfavoured when constructing crystalline phases. 相似文献
185.
The problem of classification of images that have a perfectly random nature is considered. We propose a new approach to solve this problem that is based on the use of the synthetic discriminant functions being synthesized to separate linearly the power spectra of random image fields to be classified. In the stage of both discriminant function synthesis and classification, the statistical technique of power spectrum estimation is employed. The realization of the proposed approach by means of a hybrid computer-optical technique is discussed, and its efficiency is illustrated by two examples of real-world texture classification. 相似文献
186.
Products of the reaction of OH radicals with 1-butene have been investigated in the presence of NO in one atmosphere of air at room temperature using gas chromatography and in situ long pathlength Fourier transform infrared absorption spectroscopy. The major product observed was propionaldehyde, with a formation yield (after allowing for its subsequent loss processes) of 0.94 ± 0.12. Minor yields of organic nitrates (RONO2) and of peroxypropionyl nitrate, a secondary product arising from propionaldehyde, were also observed. However, none of the products expected from the reactions subsequent to H-atom abstraction from 1-butene by OH radicals were observed, allowing an upper limit of 10% for this process to be derived. These data are compared with the available literature results and the implications are discussed. 相似文献
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