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961.
962.
Random walks on square lattice percolating clusters were followed for up to 2×105 steps. The mean number of distinct sites visited (S
N
>
gives a spectral dimension ofd
s = 1.30±0.03 consistent with superuniversality (d
s
=4J3) but closer to the alternatived
s
= 182/139, based on the low dimensionality correction. Simulations are also given for walkers on anenergetically disordered lattice, with a jump probability that depends on the local energy mismatch and the temperature. An apparent fractal behavior is observed for a low enough reduced temperature. Above this temperature, the walker exhibits a crossover from fractal-to-Euclidean behavior. Walks on two- and three-dimensional lattices are similar, except that those in three dimensions are more efficient.Supported by NSF Grant No. DMR 8303919 and Nato Grant No. SA 5205 RG 295J82. 相似文献
963.
Q. Ching Wang Frantiek vec Jean M. J. Frchet 《Journal of polymer science. Part A, Polymer chemistry》1994,32(13):2577-2588
The porous structure of monodisperse macroporous beads can be controlled by using soluble polymers with well-defined structural characteristics as part of the porogenic mixture. In general, the use of linear polystyrene as a porogen in the preparation of poly (styrene-co-divinylbenzene) beads shifts the pore size distribution towards larger pores. While a direct correlation between pore size and molecular weight of the porogen has been established, the chemical composition of the polymer porogen has no effect on the porous and chromatographic properties of the beads. These findings suggest that the average molar volume of the porogenic system is important while the miscibility of the polymer porogen with the crosslinked polymer that is formed is of little relevance. © 1994 John Wiley & Sons, Inc. 相似文献
964.
In the present study, three different methods for packing of 3 microm PBD-ZrO2 particles in 0.5 mm i.d. glass-lined stainless steel columns have been examined. The two first methods were based on a traditional downstream high-pressure technique using tetrachloromethane (Method I) or aqueous Triton X-100 (Method II) as slurry solvents, while Method III was an upstream high-pressure flocculating method with stirring, using isopropanol both as the slurry and packing solvent. Method I was found to be superior in terms of efficiency, producing 0.5 mm i.d. x 10 cm columns with almost 90,000 plates m(-1) for toluene (R.S.D. = 8.7%, n = 3), using a slurry concentration of 600 mg ml(-1), ACN-water (50:50 (v/v)) as the packing solvent and a packing pressure of 650 bars. For Method I, the slurry concentration, column i.d., column length and initial packing pressure were found to have a significant effect on column efficiency. Finally, the long-term temperature stability of the prepared columns was investigated. In isothermal mode, using ACN-20 mM phosphate buffer, pH 7 (50:50 (v/v)) as the mobile phase, the columns were found to be stable for at least 3,000 void volumes at 100 degrees C. At this temperature, the solute efficiencies changed about 5-18% and the retention factors changed about 6-8%. In temperature programming mode (not exceeding 100 degrees C), on the other hand, a rapid decrease in both column efficiency and retention factors was observed. However, when the columns were packed as initially described, ramped up and down from 50 to 100 degrees C for 48 h and refilled, fairly stable columns with acceptable efficiencies were obtained. Although not fully regaining their initial efficiency after refilling, the solute efficiencies changed about 19-28% (32-37%) and the retention factors changed about 4-5% (13-17%) after running 3,000 (25,000) void volumes or 500 (3,900) temperature programs. 相似文献
965.
Pfeffer-Hennig S. Piechon P. Bellus M. Goldbronn C. Tedesco E. 《Journal of Thermal Analysis and Calorimetry》2004,77(2):663-679
The physico-chemical properties and polymorphism of a new active pharmaceutical ingredient entity has been analyzed and the
gain of knowledge during the chemical development of the substance is described. Initial crystallization revealed an anhydrous
crystal form with good crystallinity and a single, sharp DSC melting peak at 171°C and a straightforward development of this
crystal form seemed possible. However, during polymorphism screening, new crystalline forms were detected that were often
analyzed as mixtures of crystal forms. The process of characterization and identification of the different crystalline forms
and its thermodynamical relationship has been supported by a combination of experimental and computational work including
determination of the three-dimensional structures of the crystal forms. The crystal structure of one polymorphic form was
solved by single crystal X-ray structure analysis. Unfortunately, Mod B resisted in formation of suitable single crystals,
but its structure could be solved by high resolution powder diffraction data analysis using synchrotron radiation. Calculation
of the theoretical X-ray powder diffraction pattern from three dimensional crystal coordinates allowed an unambiguous identification
of the different crystalline forms. Two polymorphic crystal forms of the API-CG3, named Mod A and Mod B, are enantiotropic
whereas Mod B is the most stable polymorph at room temperature up to about 50°C and Mod A at temperatures above 50°C. The
mechanism of the solid-solid transition can be explained by analyzing the molecular packing information gained from the single
crystal structures. A third crystalline form with the highest melting peak turned out to be not a polymorphic or pseudopolymorphic
crystal modification of our API-CG3 but a chemically different substance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
966.
DTA study of an Indian Kaolinite has been performed by varying packing density and rate of heating. Both these two parameters
influence the intensities of both endothermic and exothermic peaks related to the dehydration, dehydroxylation and then crystallizations
of Al-Si spinel, mullite and cristobalite phases. Significantly, the study reveals that mullitization takes place by two separate
reactions as indicated by two exotherms in the 1200-1400°C range of DTA trace.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
967.
Pentazole Derivates and Azides Formed from them: Potassium‐Crown‐Ether Salts of [O3S—p‐C6H4—N5]— and [O3S—p‐C6H4—N3]— —O3S—p‐C6H4—N2+ was reacted with sodium azide at —50 °C in methanol, yielding a mixture of 4‐pentazolylbenzenesulfonate and 4‐azidobenzenesulfonate (amount‐of‐substance ratio 27:73 according to NMR). By addition of KOH in methanol at —50 °C a mixture of the potassium salts K[O3S—p‐C6H4—N5] and K[O3S—p‐C6H4—N3] was precipitated (ratio 60:40). A solution of this mixture along with 18‐crown‐6 in tetrahydrofurane yielded the crystalline pentazole derivate [THF‐K‐18‐crown‐6][O3S—p‐C6H4—N5]·THF by addition of petrol ether at —70 °C. From the same solution upon evaporation and redissolution in THF/petrol ether the crystalline azide [THF‐K‐18‐crown‐6][O3S—p‐C6H4—N3]·THF was obtained. A solution of the latter in chloroform/toluene under air yielded [K‐18‐crown‐6][O3S—p‐C6H4—N3]·1/3H2O. According to their X‐ray crystal structure determinations [THF‐K‐18‐crown‐6][O3S—p‐C6H4—N5]·THF and [THF‐K‐18‐crown‐6][O3S—p‐C6H4—N3]·THF have the same kind of crystal packing. Differences worth mentioning exist only for the atomic positions of the pentazole ring as compared to the azido group and for one THF molecule which is coordinated to the potassium ion; different orientations of the THF molecule take account for the different space requirements of the N5 and the N3 group. In [K‐18‐crown‐6][O3S—p‐C6H4—N3]·1/3H2O there exists one unit consisting of one [K‐18‐crown‐6]+ and one [O3S‐C6H4—N3]— ion and another unit consisting of two [O3S‐C6H4—N3]— ions joined via two [K‐18‐crown‐6]+ ions and one water molecule. The rate constants for the decomposition [O3S‐C6H4—N5]— → [O3S‐C6H4—N3]— + N2 in methanol were determined at 0 °C and —20 °C. 相似文献
968.
化学短程序和中程序的Voronoi多面体分析 总被引:1,自引:0,他引:1
液体和玻璃中的化学短程序是其物理模型的一个基本问题.Voronoi 多面体的计算和统计,提供了一种研究原子间短程化学有序的方法.然而,Voronoi 多面体只能描述原子最近邻的局域堆垛结构,为考察化学有序与距离的关系,特别是短程序、中程序及其间的过渡,我们提出Voronoi 多面体套的概念,并建立了它的计算方法.Voronoi 多面体套(详称胀套Voronoi 多面体)是Voronoi 多面体的直接推广.随着胀套级的递增,它由近到远地描述了原子的各层次的近邻之堆垛结构,细致地刻划出化学有序 相似文献
969.
970.
E. M. Aver’yanov 《Journal of Structural Chemistry》2006,47(3):465-475
Mass dimension D has been studied analytically and by computer simulation within the framework of the atomic cluster model for large numbers of lacunary molecules (lacunars) from the class of polycyclic hydrocarbons and polyphenyls. These molecules have a central framework and radial peripheral fragments. Due to the presence of lacunas (voids) between the peripheral fragments and because of growth of lacunas with increasing length of these fragments, these lacunary molecules, which are not self-similar fractal objects, have fractional dimension 1 < D c < 2 in the region of cores. It is established that D c(q) depends on the number q and length of peripheral fragments, on the core shape and the local symmetry and local anisotropy of the core, and on the character of space occupation by the molecule. For both classes of molecules, analytical dependences D c(q) have been obtained that explain the results of computer simulation. Similarity and differences between lacunars and fractals are discussed. 相似文献