全文获取类型
收费全文 | 206篇 |
免费 | 11篇 |
专业分类
化学 | 139篇 |
力学 | 11篇 |
数学 | 35篇 |
物理学 | 32篇 |
出版年
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 3篇 |
2019年 | 8篇 |
2018年 | 5篇 |
2017年 | 2篇 |
2016年 | 11篇 |
2015年 | 5篇 |
2014年 | 7篇 |
2013年 | 11篇 |
2012年 | 5篇 |
2011年 | 8篇 |
2010年 | 7篇 |
2009年 | 13篇 |
2008年 | 11篇 |
2007年 | 10篇 |
2006年 | 5篇 |
2005年 | 7篇 |
2004年 | 6篇 |
2003年 | 5篇 |
2002年 | 8篇 |
2001年 | 2篇 |
1998年 | 2篇 |
1996年 | 2篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1991年 | 2篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1980年 | 3篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1970年 | 2篇 |
1969年 | 4篇 |
1966年 | 1篇 |
1957年 | 1篇 |
1944年 | 1篇 |
1939年 | 1篇 |
1936年 | 1篇 |
1913年 | 1篇 |
1908年 | 1篇 |
1905年 | 1篇 |
1904年 | 1篇 |
1903年 | 1篇 |
1900年 | 1篇 |
1893年 | 1篇 |
1891年 | 3篇 |
1877年 | 1篇 |
1875年 | 1篇 |
排序方式: 共有217条查询结果,搜索用时 15 毫秒
1.
The steel ASTM A213 P22 is used for superheater outlet header in power plants. During duty cycles lasting over tens of years the temperature is of the order of 545°C and the pressure 125 atm. The microscopic changes in these steels are hard to analyze under working conditions, but they are believed to be responsible for the appearance of creeps in such devices. Investigation of the microscopic modifications will help to predict future failures due to creeps, increasing the reliability and saving a lot of money.We report the investigation of such changes in ASTM A213 P22 steel in use for 30 years as a Superheater header at Haifa power station. The analysis has been carried out using Mössbauer Spectroscopy, X-ray diffraction and optical microscopy. As a result it seems that the morphology and phase change of the carbide phases M3C/M7C3 and M23C6 are the most prominent changes leading to the material failures. 相似文献
2.
Giovanni Greco Ettore Novellino Maurizio Pellecchia Carlo Silipo Antonio Vittoria 《Journal of computer-aided molecular design》1994,8(2):97-112
Summary An example of a CoMFA study is described with the aim to discuss one of the major problems of this 3D QSAR method: lack of variable selection. It is shown that the use of nonrelevant energy parameters might produce CoMFA contour maps which poorly reflect the actual nature of the binding site and are in part statistical artefacts. The data set employed in our analysis comparises triazine inhibitors of dihydrofolate reductase (DHFR), isolated from chicken liver, which have already been the object of a QSAR study by other authors. Since three-dimensional structures of triazine-DHFR complexes are known, it was possible not only to reduce ambiguities in the superimposition of the ligands, but also to compare the resulting CoMFA contour maps with the enzyme active site.Supplementary material available: The Cartesian coordinates and the atomic charges of the PM3-optimized structures used in the CoMFA study are available as MOL2 files upon request.To whose memory this paper is dedicated. 相似文献
3.
Ettore Carletti 《Annali dell'Universita di Ferrara》1983,29(1):37-44
Riassunto Scopo di questo lavoro è dare una formula asintotica per il numero degli zeri di ReF
K(λ+it) e di ImF
K(λ+it), dove
eζ
K(8) è la funzione zeta di Dedekind associata al campo numericoK, con 0<t<T e λ numero reale fissato tale che 1−1/n<λ<1 doven è il grado diK.
Summary The aim of this paper is to give an asymptotic formula for the number of zeros of ReF K(λ+it) and ImF K(λ+it), where andζ K(8) is the Dedekind zeta function for a number fieldK, with 0<t<T and λ fixed real number such that 1−1/n<λ<1, wheren is the degree ofK.相似文献
4.
Davide Barreca Ettore Fois Alberto Gasparotto Chiara Maccato Mario Oriani Gloria Tabacchi 《Molecules (Basel, Switzerland)》2021,26(7)
Transition metal complexes with β-diketonate and diamine ligands are valuable precursors for chemical vapor deposition (CVD) of metal oxide nanomaterials, but the metal-ligand bond dissociation mechanism on the growth surface is not yet clarified in detail. We address this question by density functional theory (DFT) and ab initio molecular dynamics (AIMD) in combination with the Blue Moon (BM) statistical sampling approach. AIMD simulations of the Zn β-diketonate-diamine complex Zn(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine), an amenable precursor for the CVD of ZnO nanosystems, show that rolling diffusion of this precursor at 500 K on a hydroxylated silica slab leads to an octahedral-to-square pyramidal rearrangement of its molecular geometry. The free energy profile of the octahedral-to-square pyramidal conversion indicates that the process barrier (5.8 kcal/mol) is of the order of magnitude of the thermal energy at the operating temperature. The formation of hydrogen bonds with surface hydroxyl groups plays a key role in aiding the dissociation of a Zn-O bond. In the square-pyramidal complex, the Zn center has a free coordination position, which might promote the interaction with incoming reagents on the deposition surface. These results provide a valuable atomistic insight on the molecule-to-material conversion process which, in perspective, might help to tailor by design the first nucleation stages of the target ZnO-based nanostructures. 相似文献
5.
Valeria Di Bussolo Giorgio Catelani Ettore Mastrorilli Cristina Di Bugno Raffaello Giorgi 《Tetrahedron: Asymmetry》1996,7(12):3585-3592
(±)-1-Cyclohexyl-c-2-hydroxymethyl-r-1-cyclohexanol 3, a precursor of the antimuscarinic drug Rociverine 1, was obtained diastereospecifically in very high yield, from the Grignard reaction between C6H11MgCl and an appropriately protected 2-(hydroxymethyl)cyclohexanone. The preparation of enantiomerically enriched cis diol (+)-(1R,2S)-3 and the corresponding 2-acetoxymethyl derivative (+)-(1S,2R)-12 was achieved by lipase PPL-catalyzed transesterification of racemic diol (±)-3. 相似文献
6.
Abstract –The individual tryptophanyl contributions to the near-ultraviolet dichroic activity of apomyoglobin in its native conformation have been resolved. This was accomplished by comparing the spectra of two classes of apomyoglobin with different aromatic residue contents and observing the effect of a specific modification of indole residues. The circular dichroism (CD) spectra of apomyoglobins containing two tryptophanyl residues, i.e. Trp A-5 and A-12, show the presence of a positive peak centered at 292 nm, attributable to indolic chromophore, which is missing in the CD spectrum of tuna apomyoglobin possessing only Trp A-12. Moreover, the specific modification of Trp A-5 by 2-hydroxy-5-nitrobenzyl bromide is shown by the lack of the 292 nm peak and the appearance of a positive band at longer wavelength. The pH dependence of the position of this band suggests that it arises from the 2-hydroxy-5-nitrobenzyl moiety. The results suggest that Trp A-12 does not substantially contribute to the optical activity in the near ultraviolet. 相似文献
7.
Benzyl 5,6-dihydroxyindole-2-carboxylate 3 and the free acid 4 were synthesized by oxidative cyclization of dopa benzyl ester with ceric ammonium nitrate and subsequent removal of the benzyl group by hydrogenation. 相似文献
8.
Inherently Chiral Macrocyclic Oligothiophenes: Easily Accessible Electrosensitive Cavities with Outstanding Enantioselection Performances 下载免费PDF全文
Prof. Francesco Sannicolò Prof. Patrizia R. Mussini Prof. Tiziana Benincori Dr. Roberto Cirilli Prof. Sergio Abbate Serena Arnaboldi Simone Casolo Dr. Ettore Castiglioni Prof. Giovanna Longhi Dr. Rocco Martinazzo Dr. Monica Panigati Dr. Marco Pappini Dr. Elsa Quartapelle Procopio Dr. Simona Rizzo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(47):15298-15302
Linear conjugated oligothiophenes of variable length and different substitution pattern are ubiquitous in technologically advanced optoelectronic devices, though limitations in application derive from insolubility, scarce processability and chain‐end effects. This study describes an easy access to chiral cyclic oligothiophenes constituted by 12 and 18 fully conjugated thiophene units. Chemical oxidation of an “inherently chiral” sexithiophene monomer, synthesized in two steps from commercially available materials, induces the formation of an elliptical dimer and a triangular trimer endowed with electrosensitive cavities of different tunable sizes. Combination of chirality with electroactivity makes these molecules unique in the current oligothiophenes literature. These macrocycles, which are stable and soluble in most organic solvents, show outstanding chiroptical properties, high circularly polarized luminescence effects and an exceptional enantiorecognition ability. 相似文献
9.
In this paper we consider discrete-time positive switched systems, switching among autonomous subsystems, characterized either by monomial matrices or by circulant matrices. Necessary and sufficient conditions are provided guaranteeing either (global uniform) asymptotic stability or stabilizability (i.e. the possibility of driving to zero the state trajectory corresponding to any initial state by resorting to some switching sequence). Such conditions lead to simple algorithms that allow to easily detect, under suitable conditions, whether a given positive switched system is not stabilizable. 相似文献
10.