全文获取类型
收费全文 | 2178篇 |
免费 | 203篇 |
国内免费 | 338篇 |
专业分类
化学 | 195篇 |
晶体学 | 9篇 |
力学 | 213篇 |
综合类 | 51篇 |
数学 | 1706篇 |
物理学 | 545篇 |
出版年
2024年 | 6篇 |
2023年 | 16篇 |
2022年 | 39篇 |
2021年 | 43篇 |
2020年 | 48篇 |
2019年 | 51篇 |
2018年 | 49篇 |
2017年 | 55篇 |
2016年 | 52篇 |
2015年 | 45篇 |
2014年 | 78篇 |
2013年 | 264篇 |
2012年 | 77篇 |
2011年 | 108篇 |
2010年 | 90篇 |
2009年 | 103篇 |
2008年 | 156篇 |
2007年 | 151篇 |
2006年 | 143篇 |
2005年 | 103篇 |
2004年 | 104篇 |
2003年 | 119篇 |
2002年 | 107篇 |
2001年 | 99篇 |
2000年 | 90篇 |
1999年 | 84篇 |
1998年 | 88篇 |
1997年 | 62篇 |
1996年 | 55篇 |
1995年 | 46篇 |
1994年 | 42篇 |
1993年 | 22篇 |
1992年 | 16篇 |
1991年 | 16篇 |
1990年 | 20篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 3篇 |
1984年 | 12篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1936年 | 2篇 |
排序方式: 共有2719条查询结果,搜索用时 15 毫秒
101.
通过分离式霍普金森杆对层状千枚岩施加动态载荷,得到不同层理倾角下层状千枚岩的动态抗压强度与宏观破坏模式。采用三维激光仪获得断裂面细观形貌,引入分形几何定量计算断口面粗糙度;结合SEM观察到的微观尺度下不同层理倾角断口破坏机理,分析了不同层理倾角下层状岩石的动态破坏机制。研究结果表明:动态压缩下层理弱面对岩石的抗压强度影响较大;不同层理倾角千枚岩的断口形貌分形维数随层理倾角增大呈U型变化;从强度与裂纹扩展两方面考虑层理弱面对层状岩石破坏特征的影响,对于层理倾角为0°的试样,强度由岩石基质控制,但层理弱面仍对岩石破坏的裂纹分布与走向产生较大影响;对于层理倾角为22.5°的试样,强度与裂纹走向受岩石基质与层理弱面共同控制;对于层理倾角为45°~67.5°的试样,强度与裂纹走向受层理弱面控制;而对于层理倾角为90°的试样,动态抗压强度受岩石基质的影响较大,在层理弱面较早形成纵向宏观裂纹,导致该层理弱面角度下裂纹受层理弱面的影响较大。 相似文献
102.
D. Sternik P. Staszczuk J. Pękalska G. Grodzicka B. Gawdzik J. Osypiuk-Tomasik P. Witer 《Journal of Thermal Analysis and Calorimetry》2006,86(1):85-91
The adsorption of a polar (water)
and a non-polar (n-octane) liquid on silica
gels, modified by adsorption of proteins, has been studied by thermal analysis.
Silica gels with physically adsorbed BSA and ovalbumin layers were used. Thermodesorption
energies were determined from Q-TG and Q-DTG curves recorded under quasi-equilibrium
conditions. Significant differences in liquid desorption were observed from
the surfaces due to heterogeneous changes (energetic and geometrical) as a
result of modification. These results are compared with those obtained for
the samples heated at 160°C for 1 h. 相似文献
103.
P. Staszczuk D. Sternik G. W. Chądzyński E. Robens M. Błachnio 《Journal of Thermal Analysis and Calorimetry》2006,86(1):133-136
Nitrogen
adsorption measured at 77 K was used to characterize the surface heterogeneity
of high-temperature superconductor surfaces. Properties relating to adsorption
and porosity of the solids (adsorption capacity, specific surface area, radii
and volume of the pores, pore-size distribution function) were determined
from nitrogen adsorption–desorption isotherms and atomic force microscopy
(AFM) for a series of oxide superconductors. It is shown that the adsorption
isotherms of all samples are S-shaped and belong to type II according to the
IUPAC classification. On the basis of the nitrogen adsorption isotherms and
AFM data, fractal dimensions were determined and correlations found with adsorption
and porosity parameters. 相似文献
104.
Jing Liu Ji-jun Wang Ting Li Yong Zhou De-yan Shen Yun Huang Shou-ke Yan State Key Laboratory of Polymer Physics Chemistry Institute of Chemistry The Chinese Academy of Sciences Beijing China Physics Department Peking University Beijing China 《高分子科学》2007,(2):207-215
Aggregation process of isotactic poly(methyl methacrylate) (i-PMMA) has been studied extensively for many years, and considerable progress has been made in both experimental and theoretical studies. They are, however, seldom sustained by real-space observations of the underlying morphology. In this paper, the aggregation process of i-PMMA in concentrated acetone solutions and the fractal structure of the resulting three-dimensional clusters were characterized on the basis of real-space AFM observations of their two-dimensional projection. It was found that spherical multiple-chain particles formed upon collapse and aggregation of the involving chains as a whole during quenching the solution to room temperature. By keeping the solution at room temperature, the initially formed particles stick together upon contact to form larger particles through reassembling very slowly. The succeeding collision of the enlarged spherical particles leads to the formation of small clusters. These newly formed small clusters grow when they meet with other clusters or single Brownian particles. This leads to the formation of large clusters with fractal dimension of 1.95$±0.05, which suggest a reaction-limited cluster aggregation of i-PMMA in a concentrated acetone solution. This is in accordance with the conclusion obtained by light scattering measurements. 相似文献
105.
106.
粗糙表面分维计算的立方体覆盖法 总被引:16,自引:0,他引:16
针对三角形棱柱表面积法和投影覆盖法在计算粗糙表面分形维数中存在的问题,提出了计算粗糙表面分维的立方体覆盖法,对计算结果进行了对比分析,并进一步对表面分维计算中的有关理论问题进行了分析,发现立方体覆盖法作为一种几何意义上的覆盖法,并计算结果比三角形棱柱表面积法和投影覆盖法更接近实际。 相似文献
107.
Soil aggregate stability as a key indicator of soil structure, is a product of interactions between soil environment, management practices, and land use patterns. The objective of this study was to analyze the impact of various land use patterns on soil aggregate stability in Sichuan Basin of southwestern China. The dry- and water-stable aggregate size distributions were determined by manual dry sieving procedure and Yoder's wet sieving procedure, respectively, while microaggregates and its mechanical and chemical stabilities by Kachisky's method, oscillator method, and citrate-dithionate (C-D) reagent method, separately. The results indicated that fractal dimension and surface fractal dimension were useful indicators to reflect soil aggregate distribution. Land use patterns have an obvious influence on soil aggregate stability. In the study area, water stability, mechanical stability, and chemical stability followed the sequence, Barren landforestlandorchardcropland, and the original stability and collapse velocity were sensitive to soil properties and soil structure. The difference of aggregate stability under different land use patterns is mainly due to the intensity of human disturbance and cultivation. Improper land use patterns will lead to breakdown of unstable aggregates, producing finer and more-easily transportable particles and microaggregates. In the future, inappropriate cultivation and land use patterns should be changed to protect soil structure, to improve soil aggregate stability and soil fertility in Sichuan Basin. 相似文献
108.
109.
Sibe Marde
i 《Topology and its Applications》1988,30(3):291-306
The well-known factorization theorems for covering dimension dim and compact Hausdorff spaces are here established for the cohomological dimension dim
using a new characterization of dim
In particular, it is proved that every mapping f: X → Y from a compact Hausdorff space X with
to a compact metric space Y admits a factorization f = hg, where g: X → Z, h: Z → Y and Z is a metric compactum with
. These results are applied to the well-known open problem whether
. It is shown that the problem has a positive answer for compact Hausdorff spaces X if and only if it has a positive answer for metric compacta X. 相似文献
110.
Hitting probabilities and the Hausdorff dimension of the inverse images of a class of anisotropic random fields
下载免费PDF全文
![点击此处可从《数学学报(英文版)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Let X = {X(t):t ∈ R~N} be an anisotropic random field with values in R~d.Under certain conditions on X,we establish upper and lower bounds on the hitting probabilities of X in terms of respectively Hausdorff measure and Bessel-Riesz capacity.We also obtain the Hausdorff dimension of its inverse image,and the Hausdorff and packing dimensions of its level sets.These results are applicable to non-linear solutions of stochastic heat equations driven by a white in time and spatially homogeneous Gaussian noise and anisotropic Guassian random fields. 相似文献