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931.
Andrew K. Yaluma Peter J. T. Tait John C. Chadwick 《Journal of polymer science. Part A, Polymer chemistry》2006,44(5):1635-1647
Active center determinations on different Ziegler–Natta polypropylene catalysts, comprising MgCl2, TiCl4, and either a diether or a phthalate ester as internal donor, have been carried out by quenching propylene polymerization with tritiated ethanol, followed by radiochemical analysis of the resulting polymers. The purpose of this study was to determine the factors contributing to the high activities of the catalyst system MgCl2/TiCl4/diether—AlEt3. Active center contents (C*) in the range 2–8% (of total Ti present) were measured and a strong correlation between catalyst activity and active center content was found, indicating that the high activity of the diether‐containing catalysts is due to an increased proportion of active centers rather than to a difference in propagation rate coefficients. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1635–1647, 2006 相似文献
932.
933.
We describe two data structures that preprocess a set S of n points in
(d constant) so that the sum of Euclidean distances of points in S to a query point q can be quickly approximated to within a factor of . This preprocessing technique has several applications in clustering and facility location. Using it, we derive an O(nlogn) time deterministic and O(n) time randomized -approximation algorithm for the so called Fermat–Weber problem in any fixed dimension. 相似文献
934.
935.
Khang Hoang 《固体物理学:研究快报》2015,9(12):722-725
Understanding the luminescence of GaN doped with erbium (Er) requires a detailed knowledge of the interaction between the rare‐earth dopant and the nitride host, including intrinsic defects and other impurities that may be present in the host material. We address this problem through a first‐principles hybrid density functional study of the structure, energetics, and transition levels of the Er impurity and its complexes with N and Ga vacancies, substitutional C and O impurities, and H interstitials in wurtzite GaN. We find that, in the interior of the material, ErGa is the dominant Er3+ center with a formation energy of 1.55 eV, ErGa–VN possesses a deep donor level at 0.61 eV which can assist in the transfer of energy to the 4f ‐electron core. Multiple optically active Er3+ centers are possible in Er‐doped GaN. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献
936.
937.
在水热条件下合成了四个氨三乙酸配合物[Ln(NTA)(H2O)]n(Ln=Sm(III)、Gd(III)、Dy(III)和Er(III);NTA=氨三乙酸),分别标记为1、2、3和4;利用元素分析、红外光谱和X射线单晶衍射等对其进行了结构表征.结果表明,配合物1-4同晶同构,中心原子采取N1O7的配位模式形成扭曲的十二面体几何构型.配合物以菱形四面体Ln8C14O28为基本构筑块,通过O—C—O和氢键形成有序的三维结构.此外,四个配合物均存在镧系收缩效应;配合物1显示出反铁磁性,配合物2对Hg2+有良好的荧光选择性. 相似文献
938.
Investigation of the distinction between van der Waals interaction and chemical bonding based on the PAEM‐MO diagram
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In recent years, the basic problem of understanding chemical bonding, nonbonded, and/or van der Waals interactions has been intensively debated in terms of various theoretical methods. We propose and construct the potential acting on one electron in a molecule‐molecular orbital (PAEM‐MO) diagram, which draws the PAEM inserted the MO energy levels with their major atomic orbital components. PAEM‐MO diagram is able to show clear distinction of chemical bonding from nonbonded and/or vdW interactions. The rule for this is as follows. Along the line connecting two atoms in a molecule or a complex, the existence of chemical bonding between these two atoms needs to satisfy two conditions: (a) a critical point of PAEM exists and (b) PAEM barrier between the two atoms is lower in energy than the occupied major valence‐shell bonding MO which contains in‐phase atomic components (positive overlap) of the two considered atoms. In contrast to the chemical bonding, for a nonbonded interaction or van der Waals interaction between two atoms, both conditions (a) and (b) do not be satisfied at the same time. This is demonstrated and discussed by various typical cases, particularly those related to helium atom and H? H bonding in phenanthrene. There are helium bonds in HHeF and HeBeO molecules, whereas no H? H bonding in phenanthrene. The validity and limitation for this rule is demonstrated through the investigations of the curves of the PAEM barrier top and MO energies versus the internuclear distances for He2, H2, and He2+ systems. © 2014 Wiley Periodicals, Inc. 相似文献
939.
Two coordination polymers, namely [Pb(Hbtc)(phen)]n(1) and {[Cd3(btc)2(H2O)4]·2H2O}n(2, H3btc = biphenyl-2,5,3'-tricarboxylic acid, phen = 1,10-phenanthroline), were assembled. Single-crystal X-ray diffraction studies show that compound 1 possesses a zigzag chain 1D coordination network, which is further extended into a 3D supramolecular architecture via O–H···O hydrogen bonds and π-π packing interactions. Compound 2 features a 3D open framework based on a tricadmium(II) subunit, generating a trinodal 4,4,6-connected net with the 4,4,6T24 topology defined by the point symbol of(44·62)3(46·69)2. Thermal stability and luminescent properties of both compounds were studied and discussed. 相似文献
940.
Xiqi Zhang Zhiyong Ma Yang Yang Xiaoyong Zhang Zhenguo Chi Siwei Liu Jiarui Xu Xinru Jia Yen Wei 《Tetrahedron》2014
Three 9,10-bis[(N-alkylphenothiazin-3-yl)vinyl]anthracene derivatives with different propyl, hexyl, and dodecyl side chains (AnPh3, AnPh6, and AnPh12) were synthesized and confirmed by standard spectroscopic methods. All of the compounds exhibited obvious aggregation induced emission (AIE) and piezofluorochromic (PFC) properties. The PFC behaviors were investigated and showed that proportional alkyl-length dependent relationship existed not only in the ground states of the compounds, but also in the melted states compared with the fumed states of the compounds. The PFC mechanism was explored and ascribed to the crystalline–amorphous phase transformation. More importantly, these derivatives showed reversible significant PFC properties and reproducibility in various states including fumed, ground, annealed, and melted states, making them promising stimuli-responsive and smart luminescent materials for pressure-sensors, information-recording, and light-emitting device applications. 相似文献