全文获取类型
收费全文 | 312篇 |
免费 | 14篇 |
专业分类
化学 | 150篇 |
晶体学 | 1篇 |
力学 | 9篇 |
数学 | 18篇 |
物理学 | 148篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 5篇 |
2016年 | 3篇 |
2015年 | 9篇 |
2014年 | 10篇 |
2013年 | 5篇 |
2012年 | 13篇 |
2011年 | 13篇 |
2010年 | 7篇 |
2009年 | 8篇 |
2008年 | 16篇 |
2007年 | 17篇 |
2006年 | 19篇 |
2005年 | 19篇 |
2004年 | 11篇 |
2003年 | 18篇 |
2002年 | 11篇 |
2001年 | 9篇 |
2000年 | 8篇 |
1996年 | 6篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 20篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1983年 | 3篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1966年 | 1篇 |
1964年 | 1篇 |
1963年 | 1篇 |
1939年 | 1篇 |
1932年 | 1篇 |
1897年 | 2篇 |
1893年 | 1篇 |
排序方式: 共有326条查询结果,搜索用时 15 毫秒
51.
Dr. Soukaina Bennaamane Barbara Rialland Lhoussain Khrouz Dr. Marie Fustier-Boutignon Dr. Christophe Bucher Dr. Eric Clot Dr. Nicolas Mézailles 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2023,135(3):e202209102
Ammonia, NH3, is an essential molecule, being part of fertilizers. It is currently synthesized via the Haber–Bosch process, from the very stable dinitrogen molecule, N2 and dihydrogen, H2. This process requires high temperatures and pressures, thereby generating ca 1.6 % of the global CO2 emissions. Alternative strategies are needed to realize the functionalization of N2 to NH3 under mild conditions. Here, we show that boron-centered radicals provide a means of activating N2 at room temperature and atmospheric pressure whilst allowing a radical process to occur, leading to the production of borylamines. Subsequent hydrolysis released NH4+, the acidic form of NH3. EPR spectroscopy supported the intermediacy of radicals in the process, corroborated by DFT calculations, which rationalized the mechanism of the N2 functionalization by R2B radicals. 相似文献
52.
We prove that there is a gap between \(\sqrt 2 and\left( {1 + \sqrt 5 } \right)/2\) for the exponential growth rate of nontrivial free products. For amalgamated products G = A*CB with ([A: C] ? 1)([B: C] ? 1) ≥ 2, we show that an exponential growth rate lower than \(\sqrt 2 \) can be achieved. Indeed, there are infinitely many amalgamated products for which the exponential growth rate is equal to ψ ≈ 1.325, where ψ is the unique positive root of the polynomial z3?z?1. One of these groups is \(PGL\left( {2,\mathbb{Z}} \right) \cong \left( {{C_2} \times {C_2}} \right){*_{{C_2}}}{D_6}\). However, under some natural conditions the lower bound can be put up to \(\sqrt 2 \). This answers two questions by Avinoam Mann [The growth of free products, Journal of Algebra 326, no. 1 (2011), 208–217]. We also prove that ψ is a lower bound for the minimal growth rates of a large class of Coxeter groups, including cofinite non-cocompact planar hyperbolic groups, which strengthens a result obtained earlier by William Floyd, who considered only standard Coxeter generators. 相似文献
53.
Du Y Chen L McGruer NE Adams GG Etsion I 《Journal of colloid and interface science》2007,312(2):522-528
An elastic-plastic microcontact with adhesion was studied using a finite element model. This model differs from the existing models, in that it includes the effect of adhesion on the deformation and stresses field, making it applicable to a wide range of material properties. It shows two distinct separation modes, brittle separation and ductile separation. To the best of our knowledge this is the first time that a finite element model has predicted ductile separation in an adhesive contact. Three key parameters affecting the contact and separation modes are also discussed. Further work is expected to fully reveal the effect of these parameters on the separation modes. 相似文献
54.
Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene terminus have been synthesized and subjected to thermal C2-C6 cyclization. The ratio of ene versus formal Diels-Alder products could be rationalized on the basis of steric effects. Only the thermolysis of 4c, equipped with the fast diphenylcyclopropylcarbinyl radical clock, afforded a 1,3-butadienyl benzofulvene clearly formed via cyclopropyl ring opening. This finding provides unambiguous evidence for a stepwise mechanism of the C2-C6 cyclization making it possible to suggest a lifetime for the intermediate diradical of >1x10(-10) s (at 170 degrees C). An interesting corollary was the isolation of an unexpected silyl shift product in the thermolysis of all three enyne-allenes that allows explanation of the loss of the TIPS group in some of the Diels-Alder products. For a full understanding of the mechanism, silyl and hydrogen shift processes were interrogated using DFT. 相似文献
55.
56.
George O. Buica Christophe Bucher Jean‐Claude Moutet Guy Royal Eric Saint‐Aman Eleonora M. Ungureanu 《Electroanalysis》2009,21(1):77-86
Complexing polymer‐coated electrodes have been synthesized by oxidative electropolymerization of ethylenediamine tetra‐N‐(3‐pyrrole‐1‐yl)propylacetamide (monomer L ). The presence of four polymerizable pyrrole fragments on the same EDTA skeleton was thought to confer enhanced rigidity and controlled dimensionality to the resulting complexing materials, which were used for the electrochemical detection of Hg(II), Cu(II), Pb(II) and Cd(II) ions by means of the chemical preconcentration‐anodic stripping technique. The polyamide electrode material showed particularly a significant selectivity towards mercury ions, even in the presence of a large excess of other metal cations. Moreover, the use of imprinted polymer‐coated electrodes prepared by electropolymerization of L in the presence of metal cations turned out to significantly improve the detection limits, down to 5×10?10 mol L?1 for Hg(II) and Cu(II) species. 相似文献
57.
Eduardo Pereira Bernabé L. Rivas Marie Heitzman Jean-Claude Moutet Christophe Bucher Guy Royal Eric Saint Aman 《Macromolecular Symposia》2011,304(1):115-125
Summary: The complexing properties of poly (3-(pyrrol-1-yl)propylmalonic acid) (poly1) and poly(N,N′-ethylenebis[N-[(3-(pyrrole-1-yl)propyl) carbamoyl) methyl]-glycine (poly2) coated electrodes (C|poly1 and C|poly2) towards Cu(II), Pb(II), Hg(II) and Cd(II) cations using the open circuit chemical preconcentration-anodic stripping technique were studied. Sorption process of metal cations onto complexing surfaces was readily investigated through the combination of a chemical pre-concentration-anodic stripping technique with a Langmuir isotherm model. The modified electrodes were used for the voltammetric determination of Cu(II), Pb(II), Hg(II) and Cd(II) ions, giving low detection limits for Cu(II) (5 × 10−9 mol L−1) and Pb(II) (5 × 10−10 mol L−1). The ability of the modified electrodes to analyze Cu(II) ions in natural sample has been demonstrated by the analysis of a tap water sample. The results of the preconcentration process under competitive conditions clearly shows that the selectivity of complexing molecular electrode materials can be subtly tuned upon playing on the accumulation time, polymer thickness and/or memory effect of the binding polymers, opening up new avenues towards evolutive and efficient smart sensing materials. 相似文献
58.
Bui TT Iordache A Chen Z Roznyatovskiy VV Saint-Aman E Lim JM Lee BS Ghosh S Moutet JC Sessler JL Kim D Bucher C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(19):5853-5859
A new member of the cyclo[n]pyrrole class of expanded porphyrins could be prepared from the corresponding thiophene-containing terpyrrole precursor through use of a mild electrochemical oxidative procedure. The isolated macrocycle, featuring nine heterocyclic subunits directly connected through their α,α'-positions, is the largest cyclo[n]pyrrole derivative reported to date (see figure). 相似文献
59.
60.
K. Friemelt S. Akari M.-Ch. Lux-Steiner T. Schill E. Bucher K. Dransfeld 《Annalen der Physik》1992,504(4):248-253
Atomic resolution images of layered transition metal-dichalcogenide ReS2 single-crystals (n-type semiconductor) were obtained using a scanning tunneling microscope with a positive tip. In most cases only unresolved clusters of four rhenium atoms could be seen. Occasional images with higher resolution showed that these bright structures consist of four separated atoms. The symmetry of the imaged atoms is identical to that of the rhenium sublattice but not to that of the sulfur atoms. We conclude therefore that the main contribution to the tunneling current is due to the rhenium atoms, although the sulfur atoms are placed by about 0.15 nm closer to the tip. Thus for our positive bias of the tip the tunneling electrons originate from occupied rhenium states in the valence band of the semiconductor. 相似文献