全文获取类型
收费全文 | 293篇 |
免费 | 5篇 |
学科分类
数理化 | 298篇 |
出版年
2022年 | 2篇 |
2021年 | 4篇 |
2020年 | 6篇 |
2019年 | 6篇 |
2018年 | 8篇 |
2017年 | 6篇 |
2016年 | 8篇 |
2015年 | 4篇 |
2014年 | 18篇 |
2013年 | 17篇 |
2012年 | 16篇 |
2011年 | 18篇 |
2010年 | 33篇 |
2009年 | 29篇 |
2008年 | 38篇 |
2007年 | 15篇 |
2006年 | 13篇 |
2005年 | 5篇 |
2004年 | 2篇 |
2003年 | 8篇 |
2002年 | 13篇 |
2001年 | 9篇 |
2000年 | 8篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 2篇 |
排序方式: 共有298条查询结果,搜索用时 234 毫秒
81.
D. Martínez-Martínez C. López-Cartes A. Justo A. Fernández J.C. Sánchez-López 《Solid State Sciences》2009,11(3):660-670
This paper is devoted to the development of Ti(C,N)-based nanocomposite protective coatings consisting of nanocrystals of a hard phase (TiN or TiCxNy) embedded in an amorphous carbon-based matrix (a-C or a-CNx). The objective here is the achievement of a good compromise between the mechanical and tribological properties by the appropriate control of the hard/soft phase ratio and the microstructural characteristics of the film. To achieve this purpose, dual magnetron sputtering technique was employed following two different strategies. In the first one, we use Ti and graphite targets and Ar/N2 gas mixtures, while in the second case, TiN and graphite targets are sputtered in an Ar atmosphere. By changing the sputtering power applied to each magnetron, different sets of samples are prepared for each route. The effect of the bias voltage applied to the substrate is also studied in some selected cases. The mechanical and tribological properties of the films are characterized and correlated with the microstructure, crystallinity and phase composition. The establishment of correlations enables the development of advanced coatings with tailored mechanical and tribological properties for desired applications. 相似文献
82.
Debora Cobo Jairo Quiroga Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(9):o444-o446
The title compound, C14H11NS, crystallizes with Z′ = 0.75 in the space group C2/m. Two independent molecules are present, one of which lies with all the non‐H atoms on a mirror plane, while the other is fourfold disordered across a site of 2/m symmetry. The ordered molecules are stacked such that they enclose continuous channels running along twofold rotation axes, and the disordered molecules are positioned within these channels. 相似文献
83.
Juan C. Castillo Rodrigo Abonía Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(8):o423-o430
7‐Benzyl‐3‐tert‐butyl‐1‐phenyl‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C22H25N3O, (I), and 3‐tert‐butyl‐7‐(4‐methylbenzyl)‐1‐phenyl‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C23H27N3O, (II), are isomorphous in the space group P21, and molecules are linked into chains by C—H...O hydrogen bonds. In each of 3‐tert‐butyl‐7‐(4‐methoxybenzyl)‐1‐phenyl‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C23H27N3O2, (III), which has cell dimensions rather similar to those of (I) and (II), also in P21, and 3‐tert‐butyl‐1‐phenyl‐7‐[4‐(trifluoromethyl)benzyl]‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C23H24F3N3O, (IV), there are no direction‐specific interactions between the molecules. In 3‐tert‐butyl‐7‐(4‐nitrobenzyl)‐1‐phenyl‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C22H24N4O3, (V), a combination of C—H...O and C—H...N hydrogen bonds links the molecules into complex sheets. There are no direction‐specific interactions between the molecules of 3‐tert‐butyl‐7‐(2,3‐dimethoxybenzyl)‐1‐phenyl‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C24H29N3O3, (VI), but a three‐dimensional framework is formed in 3‐tert‐butyl‐7‐(3,4‐methylenedioxybenzyl)‐1‐phenyl‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C23H25N3O3, (VII), by a combination of C—H...O, C—H...N and C—H...π(arene) hydrogen bonds, while a combination of C—H...O and C—H...π(arene) hydrogen bonds links the molecules of 3‐tert‐butyl‐1‐phenyl‐7‐(3,4,5‐trimethoxybenzyl)‐6,7‐dihydro‐1H,4H‐pyrazolo[3,4‐d][1,3]oxazine, C25H31N3O4, (VIII), into complex sheets. In each compound, the oxazine ring adopts a half‐chair conformation, while the orientations of the pendent phenyl and tert‐butyl substituents relative to the pyrazolo[3,4‐d]oxazine unit are all very similar. 相似文献
84.
This paper deals with the problem of locating path-shaped facilities of unrestricted length on networks. We consider as objective functions measures conceptually related to the variability of the distribution of the distances from the demand points to a facility. We study the following problems: locating a path which minimizes the range, that is, the difference between the maximum and the minimum distance from the vertices of the network to a facility, and locating a path which minimizes a convex combination of the maximum and the minimum distance from the vertices of the network to a facility, also known in decision theory as the Hurwicz criterion. We show that these problems are NP-hard on general networks. For the discrete versions of these problems on trees, we provide a linear time algorithm for each objective function, and we show how our analysis can be extended also to the continuous case. 相似文献
85.
The spatial dependence of the electron quasi-Fermi level (QFL) in the interior of dye sensitized nanocrystalline solar cells (DSC) under short circuit conditions can be inferred from calculations based on a diffusive electron transport model. The calculations predict that the difference in the QFL between the electrolyte and contact sides of the TiO(2) layer under short circuit conditions at 1 sun could be as much as 0.5-0.7 eV. The predicted QFL profiles depend on assumptions made about energy positions, electron mobility, and the conduction band density of states. In this work, the position of the QFL at the electrolyte side of the dye sensitized TiO(2) film in a DSC has been measured using a thin passivated titanium contact deposited on top of the nanocrystalline TiO(2) by evaporation. The method allows changes in the electron QFL at all points on the IV characteristic of the cell to be monitored under dark and photostationary conditions. In addition, cells incorporating the titanium electrode can give information about the behavior of the QFL under dynamic conditions. 相似文献
86.
We present a methodology with which to evaluate translations and rotations of wavefront aberration measurements of systems in which the exit pupil suffers displacements and rotations with respect to the reference frame of the measuring device. We propose to use image registration techniques to account for rotations, translations, and scale changes of the pupil. We present a proof of principle, using an artificial eye in addition to computer simulations. The method is software based and requires no additional hardware. 相似文献
87.
In this paper we introduce the Single Period Coverage Facility Location Problem. It is a multi-period discrete location problem
in which each customer is serviced in exactly one period of the planning horizon. The locational decisions are made independently
for each period, so that the facilities that are open need not be the same in different time periods. It is also assumed that
at each period there is a minimum number of customers that can be assigned to the facilities that are open. The decisions
to be made include not only the facilities to open at each time period and the time period in which each customer will be
served, but also the allocation of customers to open facilities in their service period. 相似文献
88.
Location problems with extensive facilities represent a challenging field of research. According to the specialized literature, a facility is called extensive if, for purposes of location, it is too large in relation to its environment to be considered a point. There are many examples of this type of structures that appear in real-world applications both in the continuous space (straight lines, circles, strips) and in networks (paths, cycles, trees). There exists a recent literature review on the location of dimensional facilities on continuous space (Díaz-Báñez et al. in TOP 154:22–44, 2004; Schöbel in Location of dimensional facilities in a continuous space, 2015) that does not cover similar problems on networks. The goal of this paper is to review the location of dimensional facilities in networks. We mainly concentrate on the location of paths and trees considering the most common objective functions in the location literature, namely median and center. However, we also consider some other alternative criteria generalizing them, as the ordered median objective function, or related to equity, reliability, and robustness. We include the basic tools and techniques that are applicable to develop algorithms for this kind of problems. Moreover, we present the best known complexity results for each of the considered problems. Finally, some suggestions are also made for possible directions of future research. 相似文献
89.
Rodrigo Abonía Paola Cuervo Michael B. Hursthouse Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(1):o44-o46
The bond distances in the molecule of the title compound, C19H14N2O4, provide evidence for electronic polarization in the aminoarylpropenone fragment and for bond fixation in the quinolinone unit. Molecules are linked by N—H...O and C—H...O hydrogen bonds into chains in which centrosymmetric rings of R22(8) and R22(18) types alternate, and these chains are linked into sheets by a single aromatic π–π stacking interaction. 相似文献
90.
Fernando Cuenú Rodrigo Abonía Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(12):o589-o592
The intramolecular dimensions of the title compound, C14H12N2O, provide evidence for a polarized electronic structure. The molecule, which is almost completely planar, contains an intramolecular N—H...O hydrogen bond, and the molecules are linked by a combination of N—H...N, C—H...O and C—H...π(arene) hydrogen bonds to form a three‐dimensional framework structure. 相似文献