全文获取类型
收费全文 | 722篇 |
免费 | 30篇 |
国内免费 | 104篇 |
专业分类
化学 | 637篇 |
晶体学 | 7篇 |
力学 | 4篇 |
综合类 | 1篇 |
数学 | 37篇 |
物理学 | 170篇 |
出版年
2024年 | 1篇 |
2023年 | 11篇 |
2022年 | 13篇 |
2021年 | 16篇 |
2020年 | 24篇 |
2019年 | 16篇 |
2018年 | 13篇 |
2017年 | 14篇 |
2016年 | 22篇 |
2015年 | 15篇 |
2014年 | 32篇 |
2013年 | 37篇 |
2012年 | 50篇 |
2011年 | 42篇 |
2010年 | 46篇 |
2009年 | 39篇 |
2008年 | 41篇 |
2007年 | 48篇 |
2006年 | 38篇 |
2005年 | 41篇 |
2004年 | 23篇 |
2003年 | 23篇 |
2002年 | 17篇 |
2001年 | 22篇 |
2000年 | 15篇 |
1999年 | 19篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 13篇 |
1995年 | 20篇 |
1994年 | 16篇 |
1993年 | 18篇 |
1992年 | 13篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1974年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有856条查询结果,搜索用时 15 毫秒
821.
Superconductivity is ascribed to a tendency of pairwise localization of conduction electrons. As a necessary condition in k space bands with very small as well as large dispersion need to be present at the Fermi level (“flat band/steep band”). The sufficient condition is a strong enough coupling of the flat band states to specific phonons in case of phonon-mediated superconductivity. The electron–phonon coupling parameter λ reveals a peak structure in the phonon q-space. These aspects are discussed for actual examples, e.g., rare earth carbides and carbide halides as well as MgB2. 相似文献
822.
AgCuSe nanorods were prepared at room temperature by a redox reaction. The as-prepared product was characterized by X-ray powder diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.X-ray powder diffraction shows that the as-prepared product is the tetragonal phase of AgCuSe. Transmission electron microscopy shows that the sample consists of nanorods with a diameter varying from 5 nm to 20 nm and a length varying from 200 nm to 600 nm. X-ray photoelectron spectroscopy shows that the purity of the sample is high. The formation mechanism of AgCuSe and the growth mechanism of AgCuSe nanorods were discussed.Thermodynamic calulations show that the final product in the synthetic system is pure AgCuSe. The solvent ethylenediamine did not only acts a didentate ligand to form relatively state Ag + and Cu + complexes, but also dissolved Se and enhanced the reactivity of Se. It played an important role in controlling the nucleation and growth of AgCuSe nanorods. 相似文献
823.
Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction 下载免费PDF全文
Cheng Wan Yagya N. Regmi Prof. Brian M. Leonard 《Angewandte Chemie (International ed. in English)》2014,53(25):6407-6410
Molybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β‐Mo2C with an Fe2N structure. Here, four phases of Mo‐C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine–metal oxide composite material including γ‐MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X‐ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ‐MoC. γ‐MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution. 相似文献
824.
Mark S. Bailey Helmut Claus John A. Schlueter 《Journal of solid state chemistry》2007,180(4):1333-1339
Li2Rh3B2 has been synthesized at 1000 °C from a stoichiometric mix of rhodium and boron and an excess of lithium. Li2Rh3B2 crystallizes in the orthorhombic space group Pbam (no. 55, Z=2) with room temperature lattice constants a=5.7712(1) Å, b=9.4377(2) Å, c=2.8301(1) Å and cell volume 154.149(6) Å3. The structure was solved from single crystal X-ray diffraction yielding the final R indices (all data) R1=2.8% and wR2=4.7%. The structure is a distortion of the CeCo3B2 structure type, containing a network of Rh6B trigonal prisms and short Li-Li contacts of 2.28(2) Å. Li2Rh3B2 is a diamagnetic metal with a room temperature resistivity of 19 μΩ cm, as determined by magnetic susceptibility and single crystal transport measurements. The measured diamagnetism and electronic structure calculations show that Li2Rh3B2 contains rhodium in a d10 configuration. 相似文献
825.
826.
A series of 13 silver(I) double and multiple salts containing 1,3‐butadiynediide, C42?, were synthesized by dissolving the silver carbide Ag2C4 in a concentrated aqueous solution of one or more of the silver salts AgNO3, AgCF3CO2, AgC2F5CO2, AgF, AgBF4, and AgPF6. The 1,3‐butadiynediide anion invariably adopts a μ4,μ4 coordination mode in these compounds, which indicates that the Ag4?C?C? C?C?Ag4 moiety can be used as a new type of metalloligand supramolecular synthon for the construction of coordination networks. Fine‐tuning with various ancillary anionic ligands caused the Ag4 aggregate at each ethynide terminus to adopt a butterfly‐shaped, planar, or barblike configuration, within which the silver–ethynide interactions can be classified into three types: σ, π, and mixed (σ,π). The effect of coexisting nitrile ligands and quaternary ammonium salts on supramolecular assembly with the above synthon was also explored. The hydrolysis of PF6? and BF4? led to the formation of the quadruple salt Ag2C4?4 AgNO3?AgPF2O2?Ag3PO4 and a novel (F)2(H2O)18 hydrogen‐bonded tape in the triple salt Ag2C4?2 AgF? 10 AgC2F5CO2?CH3CN?12 H2O, respectively. The largest silver–ethynide cluster aggregate described to date, (C4)3@Ag18, occurs in 3 Ag2C4? 12 AgC2F5CO2?5[(BnMe3N)C2F5CO2]? 4 H2O (Bn=benzyl). 相似文献
827.
A novel ternary borate, sodium strontium pentaborate, NaSrB5O9, has been prepared by solid-state reaction below 800 ℃. The single-crystal X-ray structural analysis showed that NaSrB5O9 crystallizes in monoclinic, space group P21/c with a = 6.499(2), b = 13.979(3), c = 8.045(2) , β = 106.92(2)°, V = 699.2(3) 3, Z = 4, Mr = 308.66, Dc = 2.932 g/cm3, μ = 7.804 mm-1, F(000) = 584, R = 0.0264 and wR = 0.0621 for 2426 observed reflections and 146 variables. NaSrB5O9 is a layered compound containing double ring B5O11 building units composed of two BO4 tetrahedra and three BO3 triangles. Each B5O11 unit is connected to four other equivalent units through exocyclic oxygen atoms to form a two-dimensional ∞2 [B5O9]3- layer. Symmetry-center related layers are stacked along the b axis and held together by Na+ and Sr2+ cations via electrostatic interactions. 相似文献
828.
Susan M. Kauzlarich Cathie L. Condron Teruyuki Ikeda 《Journal of solid state chemistry》2009,182(2):240-245
Single crystals of SrAl2Si2 were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl2Si2 is isostructural to CaAl2Si2 and crystallizes in the hexagonal space group P-3m1 (90 K, a=4.1834 (2), c=7.4104 (2) Å, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at ∼1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of ∼7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from ∼−78 μV K−1 at room temperature to −34 μV K−1 at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm−1 K, and contributes to the overall low zT for this compound. 相似文献
829.
Natalia L. Pacioni A. Guillermo Bracamonte Alicia V. Veglia 《Journal of photochemistry and photobiology. A, Chemistry》2008,198(2-3):179-185
The effect of the addition of different amounts of organic solvents (S) on the fluorescence of aromatic compounds (C) and their inclusion complexes with β-cyclodextrin (βCD) and hydroxypropyl-β-cyclodextrin (HPCD) has been examined using steady-state measurements. Carbamate pesticides with different aromatic moiety, such as carbofuran (CF), promecarb (PC), carbaryl (CY) and bendiocarb (BC) were used, as well as indole derivatives with different polarity in their lateral chains, such as melatonin (M, neutral), 5-methoxytryptamine (MT, cation) and auxin (IA, anion). Their complexes in water show a fluorescence signal higher than that obtained for the free substrates in solvent:water mixtures (30%, v/v n-propanol or acetonitrile, and 50%, v/v methanol). The isofluorescent point (IF), the %IF and the F85% are defined in order to evaluate the use of CD nanocavities as a non-polluting alternative for the analysis of the compounds analyzed.Apparent formation constants (KAP, M−1) for the complexes of C:HPCD at different solvent percentages were determined for CF and PC with methanol (MeOH), n-propanol (ProOH) and acetonitrile (ACN), and for indole compounds with ACN. A decrease in the KAP values for the CF:HPCD (120–30) and PC:HPCD (2000–400) complexes occurs in accordance with the solvent affinities for CDs (MeOH < ACN < ProOH). Nevertheless, in the indolic series, the polar characteristics of MT, IA and M determine their behaviour in the presence of ACN. For the neutral substrate M, KAP decreases with the increasing percentage of ACN (100–10). In contrast, for IA and MT (ionic substrates) KAP increases (10–100).These results may be accounted for by two different mechanisms: the competition between C and S for the cavity of the receptor or the formation of ternary complexes C:S:CD with additional stabilization. 相似文献
830.
Alexander Gribanov Andriy Grytsiv Peter Rogl Gerald Giester 《Journal of solid state chemistry》2008,181(11):2964-2975
Phase relations in the ternary system Ce-Pt-Si have been established for the isothermal section at 800 °C based on X-ray powder diffraction, metallography, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) techniques on about 120 alloys, which were prepared by various methods employing arc-melting under argon or powder reaction sintering. Nineteen ternary compounds were observed. Atom order in the crystal structures of τ18-Ce5(Pt,Si)4 (Pnma; a=0.77223(3) nm, b=1.53279(8) nm c=0.80054(5) nm), τ3-Ce2Pt7Si4 (Pnma; a=1.96335(8) nm, b=0.40361(4) nm, c=1.12240(6) nm) and τ10-CePtSi2 (Cmcm; a=0.42943(2) nm, b=1.67357(5) nm, c=0.42372(2) nm) was determined by direct methods from X-ray single-crystal CCD data and found to be isotypic with the Sm5Ge4-type, the Ce2Pt7Ge4-type and the CeNiSi2-type, respectively. Rietveld refinements established the atom arrangement in the structures of Pt3Si (Pt3Ge-type, C2/m, a=0.7724(2) nm, b=0.7767(2) nm, c=0.5390(2) nm, β=133.86(2)°), τ16-Ce3Pt5Si (Ce3Pd5Si-type, Imma, a=0.74025(8) nm, b=1.2951(2) nm, c=0.7508(1) nm) and τ17-Ce3PtSi3 (Ba3Al2Ge2-type, Immm, a=0.41065(5) nm, b=0.43221(5) nm, c=1.8375(3) nm). Phase equilibria in Ce-Pt-Si are characterised by the absence of cerium solubility in platinum silicides. Cerium silicides and cerium platinides, however, dissolve significant amounts of the third component, whereby random substitution of the almost equally sized atom species platinum and silicon is reflected in extended homogeneous regions at constant Ce content such as for τ13-Ce(PtxSi1−x)2, τ6-Ce2Pt3+xSi5−x or τ7-CePt2−xSi2+x. 相似文献