首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   474499篇
  免费   4996篇
  国内免费   1792篇
化学   242813篇
晶体学   6065篇
力学   22174篇
综合类   72篇
数学   64259篇
物理学   145904篇
  2020年   3277篇
  2019年   3588篇
  2018年   11842篇
  2017年   12623篇
  2016年   10169篇
  2015年   5066篇
  2014年   6784篇
  2013年   17109篇
  2012年   16918篇
  2011年   26707篇
  2010年   17425篇
  2009年   17417篇
  2008年   23775篇
  2007年   26696篇
  2006年   14183篇
  2005年   18282篇
  2004年   14368篇
  2003年   13245篇
  2002年   11258篇
  2001年   11963篇
  2000年   9291篇
  1999年   6762篇
  1998年   5467篇
  1997年   5281篇
  1996年   5038篇
  1995年   4721篇
  1994年   4721篇
  1993年   4460篇
  1992年   5231篇
  1991年   5206篇
  1990年   4974篇
  1989年   4935篇
  1988年   4920篇
  1987年   4851篇
  1986年   4677篇
  1985年   6182篇
  1984年   6293篇
  1983年   5101篇
  1982年   5380篇
  1981年   5215篇
  1980年   4823篇
  1979年   5326篇
  1978年   5539篇
  1977年   5600篇
  1976年   5474篇
  1975年   5019篇
  1974年   5059篇
  1973年   5127篇
  1972年   3761篇
  1968年   3298篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
A model of the explosive fragmentation of a thin ring is developed which takes into account the statistical dispersion of the relative fracture deformation along the length of the ring. A formula is proposed for calculating the velocity of the boundary of the region near a plastic rupture in which the plastic flow of the ring material ceases. Methods for the numerical and analytical calculation of the average number of fragments of the ring are developed. The calculation results are compared with available experimental data.  相似文献   
92.
In this paper, we have significantly modified an existing model for calculating the zeta potential and streaming potential coefficient of porous media and tested it with a large, recently published, high-quality experimental dataset. The newly modified model does not require the imposition of a zeta potential offset but derives its high salinity zeta potential behaviour from Stern plane saturation considerations. The newly modified model has been implemented as a function of temperature, salinity, pH, and rock microstructure both for facies-specific aggregations of the new data and for individual samples. Since the experimental data include measurements on samples of both detrital and authigenic overgrowth sandstones, it was possible to model and test the effect of widely varying microstructural properties while keeping lithology constant. The results show that the theoretical model represents the experimental data very well when applied to model data for a particular lithofacies over the whole salinity, from 10?5 to 6.3 mol/dm3, and extremely well when modelling individual samples and taking individual sample microstructure into account. The new model reproduces and explains the extreme sensitivity of zeta and streaming potential coefficient to pore fluid pH. The low salinity control of streaming potential coefficient by rock microstructure is described well by the modified model. The model also behaves at high salinities, showing that the constant zeta potential observed at high salinities arises from the development of a maximum charge density in the diffuse layer as it is compressed to the thickness of one hydrated metal ion.  相似文献   
93.
In this work, the effect of various properties of materials on vacuum breakdown initiated by laser radiation is considered. Estimating calculations are performed which show that the material of the target electrode distinctly affects the minimum energy of laser radiation needed for igniting a vacuum spark. The experimental studies carried out confirm the estimating calculations, and a number of materials are revealed which can be arranged in order of increase in the energy needed for the formation of breakdown in vacuum by the impact of a laser pulse.  相似文献   
94.
In order to investigate the characteristics of force chains in a granular flow system, a parallel plate shear cell is constructed to simulate the shear movement of an infinite parallel plate and observe variations in relevant parameters. The shear dilatancy process is divided into three stages, namely, plastic strain, macroscopic failure, and granular recombination. The stickslip phenomenon is highly connected with the evolution of force chains during the shear dilatancy process. The load–distribution rate curves and patterns of the force chains are utilized to describe the load-carrying behaviors and morphologic changes of force chains separately. Force chains, namely, “diagonal gridding,” “tadpole-shaped,” and “pinnate” are defined according to the form of the force chains in the corresponding three stages.  相似文献   
95.
96.
97.
98.
Russian Journal of General Chemistry - A method was developed for the synthesis of an organofluorine derivative of β-D-ribofuranoside. Biological tests revealed high insecticidal activity and...  相似文献   
99.
Journal of Thermal Analysis and Calorimetry - Over the last few decades, tremendous consideration is drawn towards corrugation surfaces because of their advantages over the improvement in thermal...  相似文献   
100.
ABSTRACT

The authors present the results of an investigation in Fe–Ni-Cr austenitic alloys of the low-temperature deformation-induced segregations of nickel that form in the micro regions being (i) located close to grain- and subgrain boundaries and (ii) characteristic of the concentration and magnetic inhomogeneities indicated by the appearance of a dark diffraction contrast at the electron diffraction patterns taken from these regions typical (at the same time) of an enhanced value of Curie temperature. The observed effects were connected with the micro distortions caused by the local change of lattice parameter because of an increase in nickel concentration, as well as in the result of a magnetostriction dilatation. Using methods of the X-ray energy dispersive spectroscopy (XEDS) and atomic-probe body-section radiography (tomography – APT) has made it possible to determine the borders of those regions of austenite that were characteristic of an enhanced concentration of nickel in the fields of the localisation of a deformation-induced segregation of nickel in the vicinity of grain (subgrain) boundaries of austenitic alloys of the types Fe–13Cr–30Ni and Fe–37Ni–3Ti.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号