全文获取类型
收费全文 | 2569篇 |
免费 | 49篇 |
国内免费 | 6篇 |
专业分类
化学 | 1769篇 |
晶体学 | 12篇 |
力学 | 31篇 |
数学 | 390篇 |
物理学 | 422篇 |
出版年
2020年 | 21篇 |
2019年 | 27篇 |
2018年 | 20篇 |
2017年 | 21篇 |
2016年 | 41篇 |
2015年 | 51篇 |
2014年 | 46篇 |
2013年 | 112篇 |
2012年 | 101篇 |
2011年 | 97篇 |
2010年 | 76篇 |
2009年 | 90篇 |
2008年 | 101篇 |
2007年 | 116篇 |
2006年 | 83篇 |
2005年 | 98篇 |
2004年 | 96篇 |
2003年 | 61篇 |
2002年 | 72篇 |
2001年 | 39篇 |
2000年 | 62篇 |
1999年 | 63篇 |
1998年 | 56篇 |
1997年 | 56篇 |
1996年 | 45篇 |
1995年 | 34篇 |
1994年 | 49篇 |
1993年 | 54篇 |
1992年 | 52篇 |
1991年 | 39篇 |
1990年 | 26篇 |
1989年 | 29篇 |
1988年 | 32篇 |
1987年 | 42篇 |
1986年 | 47篇 |
1985年 | 46篇 |
1984年 | 44篇 |
1983年 | 36篇 |
1982年 | 51篇 |
1981年 | 34篇 |
1980年 | 54篇 |
1979年 | 31篇 |
1978年 | 35篇 |
1977年 | 24篇 |
1976年 | 34篇 |
1975年 | 29篇 |
1974年 | 20篇 |
1973年 | 16篇 |
1972年 | 17篇 |
1971年 | 14篇 |
排序方式: 共有2624条查询结果,搜索用时 656 毫秒
991.
Schuster ME Hävecker M Arrigo R Blume R Knauer M Ivleva NP Su DS Niessner R Schlögl R 《The journal of physical chemistry. A》2011,115(12):2568-2580
Diesel soot (Euro IV and Euro VI) was investigated with spectroscopic methods such as near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoemission spectroscopy (XPS). C and O K-edge NEXAFS show that structural disorder on the surface is accompanied by a higher amount of oxygen functional groups. O K-edge NEXAFS and O1s XPS results are discussed with the aim to elucidate the nature of the oxygen surface species. The analysis of the data presented here allows the postulation of a hypothetical structure for soot samples emitted by diesel engines. 相似文献
992.
Detlef Schiel Olaf Rienitz Reinhard J?hrling Bernd G??ttler Ralf Matschat Holger Scharf J??rgen Birkhahn Guillaume Labarraque Paola Fisicaro Ulrich Borchers David Schwesig 《Accreditation and quality assurance》2011,16(10):489-498
Within the scope of a project of the “European Association of National Metrology Institutes??(EURAMET), a European metrological dissemination system (network) providing traceable reference values assigned to matrix materials for validation purposes is described and put to the test. It enables testing laboratories (TL) to obtain comparable results for measurements under the “EU Water Framework Directive 2000/60/EC??(WFD) and thus, to comply with a core requirement of this very directive. The dissemination system is characterized by the fact that it is available to all laboratories throughout Europe which intend to perform measurements in the context of the WFD and that it can ensure sustainable metrological traceability to the International System of Units (SI) as a reference point for the measurement results. This dissemination system is set up in a hierarchical manner and links up the level of the national metrology institutes (NMI) with that of the TLs via an intermediate level of calibration laboratories (CL) by comparison measurements. The CLs are expert laboratories with respect to the measurement of the analytes considered here (within the project, the CLs are called potential calibration laboratories (PCL)) and are additionally involved in the organization of comparison measurements within the scope of regional quality assurance (QA) systems. Three comparison measurements have been performed to support the approach. A total of about 130 laboratories participated in this exercise with the focus on the measurement of the priority substances Pb, Cd, Hg, and Ni defined in the WFD. The elemental concentrations in the water samples roughly corresponded to one of the established environmental quality standards (EQS), the annual average concentration (AA-EQS), which is defined in the daughter Directive 2008/105/EC of the WFD. It turned out that a significant number of TLs still need to improve their measurement methods in order to be able to fulfill the minimum requirements of the WFD, in particular, with regard to the elements Cd and Hg probably due to their low EQS values. Furthermore, it became obvious that the hierarchical dissemination system suggested here actually corresponds to the measuring capabilities of the three participating groups (NMIs, PCLs, and TLs). 相似文献
993.
994.
Makarević J Štefanić Z Horvat L Žinić M 《Chemical communications (Cambridge, England)》2012,48(59):7407-7409
Chiral amino acid and biphenyl incorporating oxalamide gelators 4-7 with large, 9 bond distance between chiral centres and biphenyl units have been studied. CD investigation of 4-octanol gel and the crystal structure of rac-4 reveal that efficient central to axial chirality transfer occurs by intermolecular interactions in gel and solid state assemblies. 相似文献
995.
996.
Manka A Pathak H Tanimura S Wölk J Strey R Wyslouzil BE 《Physical chemistry chemical physics : PCCP》2012,14(13):4505-4516
We report homogeneous ice nucleation rates between 202 K and 215 K, thereby reducing the measurement gap that previously existed between 203 K and 228 K. These temperatures are significantly below the homogenous freezing limit, T(H)≈ 235 K for bulk water, and well within no-man's land. The ice nucleation rates are determined by characterizing nanodroplets with radii between 3.2 and 5.8 nm produced in a supersonic nozzle using three techniques: (1) pressure trace measurements to determine the properties of the flow as well as the temperature and velocity of the droplets, (2) small angle X-ray scattering (SAXS) to measure the size and number density of the droplets, and (3) Fourier Transform Infrared (FTIR) spectroscopy to follow the liquid to solid phase transition. Assuming that nucleation occurs throughout the droplet volume, the measured ice nucleation rates J(ice,V) are on the order of 10(23) cm(-3) s(-1), and agree well with published values near 203 K. 相似文献
997.
Adam Slabon Eduardo Cuervo‐Reyes Christof Kubata Michael Wörle Christian Mensing Prof. Dr. Reinhard Nesper 《无机化学与普通化学杂志》2012,638(10):1417-1423
The new ternary phase Eu2–xMg2–yGe3 (x = 0.1, y = 0.5) was obtained by solid‐state synthesis and the structure determined by means of Single Crystal X‐ray Diffraction. The compound crystallizes with the orthorhombic space group Cmcm (no. 63) having structural features of the low‐temperature modification of LaSi. The crystal structure contains two different types of germanium anions: isolated Ge4– and $\rm^{2}_{\infty}$ [Ge2–x–y] chains. The cation substructure is partially disordered and is best represented assuming a split position. The chemical bonding is well represented by the Zintl‐Klemm concept. Resistivity measurements reveal that the compound is metallic. DFT band structure calculations were carried out on the ideal stoichiometric compound Eu2Mg2Ge3, showing that this model (x = 0; y = 0) would be also metallic as a consequence of the ecliptic stacking of anions. Susceptibility and specific heat measurements evidence the presence of weak, and probably frustrated, antiferromagnetic interactions between disordered europium atoms. 相似文献
998.
A sensitive electrochemical approach for the detection of thrombin was designed by using densely packed hierarchical dendritic gold microstructures (HDGMs) with secondary and tertiary branches as matrices, and thionine-functionalized mesoporous silica nanospheres as signal tags. To prepare the signal tags, the positively charged thionine (as an indicator) was initially adsorbed onto the mesoporous silica nanoparticles (MSNs). Then [AuCl4]− ions were in situ reduced on the thionine-modified MSNs by ascorbic acid to construct nanogold-decorated MSNs (GMSNs). The formed GMSNs were employed as label of the aminated aptamers. The assay was carried out in PBS, pH 7.4 with a sandwich-type assay mode by using the assembled thionine in the GMSNs as indicators. Compared with the pure silica nanoparticles, mesoporous silica could provide a larger surface for the immobilization of biomolecules and improve the sensitivity of the aptasensor. Under optimal conditions, the electrochemical aptasensors exhibited a wide linear range from 0.001 to 600 ng mL−1 (i.e. 0.03 pM to 0.018 μM thrombin) with a low detection limit (LOD) of 0.5 pg mL−1 (≈15 fM) thrombin at 3σ. No obvious non-specific adsorption was observed during a series of analyses to detect target analyte. The precision, selectivity and stability of the aptasensors were acceptable. Importantly, the methodology was evaluated with thrombin spiked samples in blank fetal calf serum, and the recoveries were 94.2–112%, indicating an exciting potential for thrombin detection. 相似文献
999.
Reinhard Kaindl Daniel M. Tbbens Volker Kahlenberg 《Journal of Raman spectroscopy : JRS》2011,42(1):78-85
Y2Si2O7 is an intriguing material combining a complex structural polymorphism with several important technological applications. Raman spectra were experimentally determined for most of the seven known modifications of Y2Si2O7 except the form ε, and in the case of β, γ, δ and ζ for the first time. The error‐prone procedure of mode assignment to the measured Raman bands, usually done by comparison with similar or related structures, has been replaced by quantum chemical calculations of the spectra of the polymorphs. Various functionals were evaluated considering the agreement of the calculated modes with the experimental data. The average and maximum deviations between calculated and experimental spectra are ± 8 cm−1 and 20 cm−1, respectively. Assignments of most of the observed bands to vibrational modes are given. The relationship between selected Raman bands, Si O and Y O polyhedra stretching and bending modes, and the crystal structures are discussed. Y2Si2O7 offers the possibility to study the relationship between structural and spectral changes in a chemically fixed system. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
1000.
Aleksandra Newirkowez Reinhard Neumann Christina Trautmann Kay-Obbe Voss 《Phase Transitions》2013,86(4):285-292
The Facility for Antiproton and Ion Research (FAIR) to be built at GSI in Darmstadt will be equipped with superconducting magnets. Due to the high ion beam intensities and related beam losses, radiation damage of the Cu/NbTi superconducting wires used in the magnet coils has to be considered. Here we report first experimental results on NbTi multifilament wires exposed to 2.6 GeV U ions at room temperature. Radiation-induced effects were examined using X-ray diffraction and transmission electron microscopy. With increasing ion fluence the amount of the α-Ti phase decreases. This is crucial because α-Ti precipitates are the main flux-pinning centres in the Nb–Ti superconducting alloy. The equilibrium hcp α-Ti transforms into the hexagonal ω-Ti phase. 相似文献