全文获取类型
收费全文 | 453篇 |
免费 | 20篇 |
国内免费 | 1篇 |
专业分类
化学 | 405篇 |
晶体学 | 2篇 |
力学 | 1篇 |
数学 | 27篇 |
物理学 | 39篇 |
出版年
2023年 | 2篇 |
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 12篇 |
2015年 | 11篇 |
2014年 | 16篇 |
2013年 | 21篇 |
2012年 | 20篇 |
2011年 | 24篇 |
2010年 | 19篇 |
2009年 | 7篇 |
2008年 | 33篇 |
2007年 | 22篇 |
2006年 | 29篇 |
2005年 | 31篇 |
2004年 | 28篇 |
2003年 | 20篇 |
2002年 | 22篇 |
2001年 | 14篇 |
2000年 | 13篇 |
1999年 | 12篇 |
1998年 | 7篇 |
1997年 | 10篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 9篇 |
1983年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有474条查询结果,搜索用时 15 毫秒
1.
2.
In Germany, very little empirical data about the mathematical competence of adults is available. The aim of this study is to test the level of mathematical competence in an adult sample. For this purpose, a mathematics test was constructed using the PISA mathematics framework as a guideline. The test consisted of fourteen public items from the mathematics test in PISA 2000. The study was implemented by carrying out house visits (Germany). The sample was comprised of 64 adults (90% women, age: 41 years). The test results were scaled in the metric of PISA 2000. Compared with the average German student competencies in PISA 2000, the average competency in the adult sample was on the same level as the fifteen-year-old comprehensive secondary school student (at a German Gymnasium). Further analysis shows that the mathematics competency level in the adult sample is positively connected to the individual vocational education degree. 相似文献
3.
Supercritical fluid extraction for liquid chromatographic determination of carotenoids in Spirulina Pacifica algae: a chemometric approach 总被引:1,自引:0,他引:1
Careri M Furlattini L Mangia A Musc M Anklam E Theobald A von Holst C 《Journal of chromatography. A》2001,912(1):61-71
An experimental design procedure was used to investigate the effects of some operating parameters on the supercritical fluid extraction of carotenoids beta-carotene, beta-cryptoxanthin and zeaxanthin from Spirulina Pacifica algae, a carotenoid-rich dietary product. Variables tested were temperature and pressure of the supercritical fluid, dynamic extraction time and percentage of ethanol added as the modifier. Each variable was tested at three levels; 31 experiments were performed in random order. Analyses of the extracts were performed by high-performance liquid chromatography with UV-Vis photodiode array detection. Analytical responses (chromatographic peak areas) were processed by using a stepwise multiple regression analysis, in order to find polynomial functions describing the relationships between variables and responses. For all the analytes the experimental conditions providing the highest extraction yield inside the experimental domain considered were found. Supercritical fluid extraction results obtained in these conditions were compared with those obtained by performing solvent extraction in order to evaluate the effectiveness of the supercritical fluid extraction procedure. 相似文献
4.
Effects of chemical, physical, and technological processes on the nature of food allergens 总被引:1,自引:0,他引:1
A review is presented of studies of different processing techniques and their effect on the allergenicity and antigenicity of certain allergenic foods. An overview of investigated technologies is given with regard to their impact on the protein structure and their potential application in the production of hypoallergenic foods. The use of physical processes (such as heating, high pressure, microparticulation, ultrafiltration, and irradiation), chemical processes (such as proteolysis, fermentation, and refining by extraction), and biotechnological approaches, as well as the effects of these processes on individual allergenic foods, are included. Additionally, the implications of food processing for food allergen analysis with respect to food safety assessment and industrial quality control are briefly discussed. 相似文献
5.
Elke Barsties Stefan Schaible Marc-Heinrich Prosenc Ursula Rief Werner Rll Oliver Weyand Birgit Dorer Hans-Herbert Brintzinger 《Journal of organometallic chemistry》1996,520(1-2):63-68
Bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (2-(CH3)2N-C9H6)2ZrCl2, and dimethylsilyl-bridged bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (CH3)2Si(2-(CH3)2N-C9H5)2ZrCl2, were prepared by reaction of the corresponding ligand lithium salts with ZrCl4 in toluene. Diffractometric structure determinations reveal C2-symmetric complex geometries for both complexes. An increased electron density at the Zr center of the dimethylamino-substituted complexes is indicated by reduction potentials which are 0.3–0.4 V more negative than those of their unsubstituted analogs. When activated with methyl aluminoxane in toluene solution, (CH3)2Si(2-(CH3)2N-C9H5)2ZrCl2 catalyzes the polymerization of propene to polymers with a microstructure comparable with that of polymers produced with other Me2Si-bridged bis(indenyl)ZrCl2 complexes, but with a substantially increased fraction of i-propyl end groups derived from alkyl exchange between Zr-polymer and Al---Me species. 相似文献
6.
Ternary Lithium Rare Earth Nitrates with Lonesome Nitrate Ions: Li3[M(NO3)5](NO3) (M = Gd? Lu, Y). The Crystal Structure of Li3Er(NO3)6 Single crystals of the ternary nitrate Li3Er(NO3)6 are obtained from a solution of “Er(NO3)3” in the melt of LiNO3. In Li3Er(NO3)6 (monoclinic, P21/n, Z = 4; a = 776.0(1); b = 748.86(8); c = 2 396(1) pm; β = 90.76(3)°; R1 = 0.0490; wR2 = 0.0792), Er3+ is surrounded by five bidentate nitrate ligands yielding the anionic units [Er(NO3)5]2?. These are arranged in the direction of the 21 screw axis. Two lonesome NO3? ions are in the middle of such a “helix” and are connected by Li+ with the anions [Er(NO3)5]2?. The helices are moved against each other by about half of the lattice constant a and are connected by further Li+ ions. 相似文献
7.
E. Anklam Maria Rosa Bassani Thomas Eiberger Stefan Kriebel Markus Lipp Reinhard Matissek 《Analytical and bioanalytical chemistry》1997,357(7):981-984
Pyrolysis Mass Spectrometry (Py-MS) was used for the discrimination of cocoa butters from other vegetable fats. Mass spectra
ranging from 50 amu to 250 amu were analyzed by principal component analysis (PCA) and with neural nets. The application of
neural nets leads to a good discrimination between the two classes. Detailed analysis of the nets revealed that only the first
60 masses were used within the net. The use of PCA requires a careful selection of the number of masses included in the calculation.
Canonical variance analysis was applied to determine the significant masses. Optimal performance of PCA was observed only
using the first 22 significant masses. Most of these masses were different from the ones used by the neural net. It seems
that the mass spectra obtained by Py-MS contain sufficient information for the discrimination of pure cocoa butter from other
vegetable fats, but none of the methods seems to be able to extract all information available. Neural net provides a very
robust method for this task and no prior data selection was necessary.
Received: 13 May 1996 / Revised: 7 August 1996 / Accepted: 7 August 1996 相似文献
8.
Batrice Alpha Elke Anklam Robert Deschenaux Jean-Marie Lehn Marek Pietraskiewiez 《Helvetica chimica acta》1988,71(5):1042-1052
Sythetic procedures have been deweloped for the preparation of sodium and lithium cryptates of the macrabicyclic ligands 1–11 containing pyridine, bipyridine, and biisoquinoline groups. They involve stepwise construction of the bicyclic system as will as direct macrobicyclisation procedures (Scheme 1) and give access to both symmetrical and dissymmetrical structures. Marked cation template effects have been found that facilitate the cyclisation processes. The ligands 1–11 were isolated as their cryptates with Na+ or Li+ cations. 相似文献
9.
Guldi DM Hirsch A Scheloske M Dietel E Troisi A Zerbetto F Prato M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(20):4968-4979
Control over the interchromophore separation, their angular relationship, and the spatial overlap of their electronic clouds in several ZnP-C(60) dyads (ZnP=zinc porphyrin) is used to modulate the rates of intramolecular electron transfer. For the first time, a detailed analysis of the charge transfer absorption and emission spectra, time-dependent spectroscopic measurements, and molecular dynamics simulations prove quantitatively that the same two moieties can produce widely different electron-transfer regimes. This investigation also shows that the combination of ZnP and C(60) consistently produces charge recombination in the inverted Marcus region, with reorganization energies that are remarkably low, regardless of the solvent polarity. The time constants of electron transfer range from the mus to the ps regime, the electronic couplings from a few tens to several hundreds of cm(-1), and the reorganization energies remain below 0.54 eV and can be as low as 0.16 eV. 相似文献
10.
Funk T Kennepohl P Di Bilio AJ Wehbi WA Young AT Friedrich S Arenholz E Gray HB Cramer SP 《Journal of the American Chemical Society》2004,126(18):5859-5866
We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d(x2-y2) having much greater ligand character. 相似文献