全文获取类型
收费全文 | 751篇 |
免费 | 37篇 |
国内免费 | 2篇 |
专业分类
化学 | 115篇 |
晶体学 | 1篇 |
力学 | 127篇 |
数学 | 258篇 |
物理学 | 289篇 |
出版年
2023年 | 23篇 |
2022年 | 15篇 |
2021年 | 18篇 |
2020年 | 30篇 |
2019年 | 14篇 |
2018年 | 18篇 |
2017年 | 39篇 |
2016年 | 64篇 |
2015年 | 73篇 |
2014年 | 74篇 |
2013年 | 28篇 |
2012年 | 42篇 |
2011年 | 52篇 |
2010年 | 43篇 |
2009年 | 38篇 |
2008年 | 27篇 |
2007年 | 34篇 |
2006年 | 23篇 |
2005年 | 25篇 |
2004年 | 9篇 |
2003年 | 10篇 |
2002年 | 11篇 |
2001年 | 25篇 |
2000年 | 13篇 |
1999年 | 18篇 |
1998年 | 12篇 |
1997年 | 6篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1986年 | 1篇 |
排序方式: 共有790条查询结果,搜索用时 15 毫秒
41.
L.X. Zhang B. Chen Z.F. Yang G.Q. Chen M.M. Jiang G.Y. Liu 《Communications in Nonlinear Science & Numerical Simulation》2009,14(6):2827-2836
An emergy-based comparison analysis is conducted for three typical mega cities in China, i.e., Beijing, Shanghai and Guangzhou, from 1990 to 2005 in four perspectives including emergy intensity, resource structure, environmental pressure and resource use efficiency. A new index of non-renewable emergy/money ratio is established to indicate the utilization efficiency of the non-renewable resources. The results show that for the three mega urban systems, Beijing, Shanghai and Guangzhou, the total emergy inputs were 3.76E+23, 3.54E+23, 2.52E+23 sej in 2005, of which 64.88%, 91.45% and 72.28% were imported from the outsides, respectively. As to the indicators of emergy intensity involving the total emergy use, emergy density and emergy use per cap, three cities exhibited similar overall increase trends with annual fluctuations from 1990 to 2005. Shanghai achieved the highest level of economic development and non-renewable resource use efficiency, and meanwhile, lower proportion of renewable resource use and higher environmental pressure compared to those of Beijing and Guangzhou. Guangzhou has long term sustainability considering an amount of local renewable resources used, per capita emergy used, energy consumption per unit GDP and the ratio of waste to renewable emergy. It can be concluded that different emergy-based evaluation results arise from different geographical locations, resources endowments, industrial structures and urban orientations of the concerned mega cities. 相似文献
42.
Y. Zhang Y.W. Zhao Z.F. Yang B. Chen G.Q. Chen 《Communications in Nonlinear Science & Numerical Simulation》2009,14(4):1758-1765
Cities as superorganisms confront disturbances from their metabolic processes, including large metabolic fluxes, low stocks of resources and products, and a low efficiency of the urban material metabolism. Based on ecological thermodynamics, an indicator system is established in this paper to evaluate the fluxes, stocks, and effiency of the urban material metabolism using emergy analysis. Also, a new model for the urban material metabolism is proposed to define the production possibility curve using a wealth index (WI) and an ecological effiency index (EEI). Then, six large Chinese cities including Beijing, Shanghai, Tianjin, Chongqing, Guangzhou, and Shenzhen are selected as typical cases to validate the proposed model. The results show that Shenzhen has the highest metabolic capacity, followed by Beijing, Shanghai, Guangzhou, Tianjin, and Chongqing. It is also reflected that different urban material metabolisms of the six cities are resulted from the varied regional metabolic capacities, thus providing insights into how cities improve their metabolic capacities. Meanwhile, Shenzhen has the highest WI and EEI, Chongqing has the lowest WI and EEI, and Beijing, Shanghai, Tianjin, and Guangzhou has low WI and high EEI. It is also shown that the sustainable operation of the urban material metabolism reflects the mutualism and symbiosis between socioeconomic development and ecological environment protection. 相似文献
43.
Peng-Xiang Li Meng-Wu Zhang You-Min Xi Wen-Tian Cui 《Computational & Mathematical Organization Theory》2009,15(3):169-190
This paper discusses chain of command networks that are most likely to exhibit the scale-free (SF) property in organizational
networks, explaining why organizational networks do not show SF distributions. We propose an evolving hierarchical tree network
model without explicit preferential attachment. The model simulates several kinds of chain of command networks with the span
of control ranging from extreme homogeneity to extreme heterogeneity. In addition to traditional degree distribution, a new
kind of cumulative-outdegree distribution p(K
cum
=k
cum
) is introduced and discussed that gives a probability that a randomly selected node has exactly k
cum
children nodes. Theoretical analysis and simulation results show that even if the network size is large enough to meet the
demand of large-scale networks, the SF property can emerge only when a hierarchical tree lies in two extreme situations: (1)
the exact same span of control exists at all levels of an organization; (2) the node outdegree (i.e. span of control) distribution
obeys a power-law distribution. The empirical investigations show that real organization networks are between the two extreme
situations. This is why organizational networks in reality do not show an SF degree distribution or SF cumulative-outdegree
distribution. This finding shows that the SF property is the consequence of extreme situations, even though it is very common
in nature and in society. In fact, the SF property is of no value in the study of problems in organizations. 相似文献
44.
《Communications in Nonlinear Science & Numerical Simulation》2010,15(9):2603-2616
Systems constituted by moving components that make intermittent contacts with each other can be modelled by a system of ordinary differential equations containing piecewise linear terms. We consider a soft impact bilinear oscillator for which we obtain bifurcation diagrams, Lyapunov coefficients, return maps and phase portraits of the response. Besides Lyapunov coefficients diagrams, bifurcation diagrams are represented in terms of both non-dimensional time instants of contact (when the mass impacts the obstacle) and of portions of contact duration (the percentage-time interval when the material point is inside the obstacle) vs. non-dimensional external force frequency (or amplitude). The second kind of diagrams is needed because the contact duration (or the complementary flight time duration) are quantities that can easily be measured in an experiment aiming at confirming the validity of the present model. Lyapunov coefficients are evaluated converting the piecewise linear system of ordinary differential equations into a map, the so-called impact map, where time and velocity corresponding to a given impact are evaluated as functions of time and velocity corresponding to the previous impact. Thus, the usual methods related to this last map are used. The trajectories are represented in terms of return maps (all points in the time-velocity plane involved in the impact events) and phase portraits (the trajectory-itself in the displacement-velocity plane). In the bifurcation diagrams, transition between different responses is evidenced and a perfect correlation between chaotic (periodic) attractors and positive (negative) values of the maximum Lyapunov coefficient is found. 相似文献
45.
The effect of a market factor on information flow between stocks using the minimal spanning tree 总被引:1,自引:0,他引:1
We empirically investigated the effects of market factors on the information flow created from N(N−1)/2 linkage relationships among stocks. We also examined the possibility of employing the minimal spanning tree (MST) method, which is capable of reducing the number of links to N−1. We determined that market factors carry important information value regarding information flow among stocks. Moreover, the information flow among stocks showed time-varying properties according to the changes in market status. In particular, we noted that the information flow increased dramatically during periods of market crises. Finally, we confirmed, via the MST method, that the information flow among stocks could be assessed effectively with the reduced linkage relationships among all links among stocks from the perspective of the overall market. 相似文献
46.
47.
48.
49.
《Colloids and surfaces. B, Biointerfaces》2010,75(2):492-497
Fats are widely present in a large variety of food and represent the main source of energy for the body. In the current study we investigate the behaviour of fatty acids at liquid–liquid interfaces, mimicking some steps of the very complex digestion process. Octanoic acid is used as an example of middle chain fatty acids. For the oil phase we choose sunflower oil as an industrial product and hexane as pure oil.The influence of the fatty acid concentration and the pH of the aqueous phase on the interfacial tension is determined by profile analyse tensiometry (PAT), which allows to examine the way of adsorption and transition of the fatty acids from one phase to the other. Predominantly, the pH affects the dissociation and thereby the strength of the hydrophilic character of the fatty acid. The adsorption behaviour indicates the different interfacial activity of the studied octanoic acid. 相似文献
50.
F. Maucher D. Buccoliero S. Skupin M. Grech A. S. Desyatnikov W. Krolikowski 《Optical and Quantum Electronics》2009,41(4):337-348
We study the formation of azimuthons, i.e., rotating spatial solitons, in media with nonlocal focusing nonlinearity. We show
that whole families of these solutions can be found by considering internal modes of classical non-rotating stationary solutions,
namely vortex solitons. This offers an exhaustive method to identify azimuthons in a given nonlocal medium. We demonstrate
formation of azimuthons of different vorticities and explain their properties by considering the strongly nonlocal limit of
accessible solitons. 相似文献