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   222412篇
  免费   2728篇
  国内免费   1330篇
化学   123547篇
晶体学   3236篇
力学   8930篇
综合类   61篇
数学   24550篇
物理学   66146篇
  2020年   1775篇
  2019年   1912篇
  2018年   2494篇
  2017年   2544篇
  2016年   3675篇
  2015年   2441篇
  2014年   3564篇
  2013年   8955篇
  2012年   7745篇
  2011年   9466篇
  2010年   6703篇
  2009年   6472篇
  2008年   8836篇
  2007年   8962篇
  2006年   8438篇
  2005年   7749篇
  2004年   6921篇
  2003年   6080篇
  2002年   6030篇
  2001年   6740篇
  2000年   5100篇
  1999年   3682篇
  1998年   3055篇
  1997年   3024篇
  1996年   3026篇
  1995年   2646篇
  1994年   2750篇
  1993年   2618篇
  1992年   2859篇
  1991年   2919篇
  1990年   2720篇
  1989年   2631篇
  1988年   2580篇
  1987年   2508篇
  1986年   2537篇
  1985年   3328篇
  1984年   3373篇
  1983年   2820篇
  1982年   3066篇
  1981年   2829篇
  1980年   2630篇
  1979年   2809篇
  1978年   3021篇
  1977年   3052篇
  1976年   3083篇
  1975年   2820篇
  1974年   2920篇
  1973年   2985篇
  1972年   2330篇
  1971年   1859篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
982.
Angra dos Reis/Itaguai region of the state of Rio de Janeiro, Brazil, is a very problematic area due to the instability of slope and landslides, due to geological and geomorphological conditions and to the significant and continuous human occupation over favorable areas is prone to the triggering of landslides. The samples were analyzed by thermogravimetry (TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA). This paper analyzes and characterizes the clay minerals and presence of water, gravitational mass movements’ sites and compares it with sites where gravitational mass movements do not occur. Indeed, this analysis also attempts to verify the presence of minerals.  相似文献   
983.
984.
A deflecting system formed by two parallel-plate capacitors on opposite sides of an optoelectronic element that alters the energy of a beam of charged particles, is calculated numerically. The conditions for correcting the chromatic beam spreading in the plane of the target are found. Good agreement with the results of an approximate analytical calculation is demonstrated. Zh. Tekh. Fiz. 67, 86–88 (December 1997)  相似文献   
985.
The well-known equations for the optical functions of reflectance, transmittance and phase changes on reflection and transmission for a solid material have been rearranged into the form of simple geometric figures in the u, v (n, k) plane. The curves for Fp show a singularity on the n-axis.  相似文献   
986.
Levels up to 9.3 MeV excitation in22Ne were studied with the19 F(α, pγ) 22Ne reaction atE α = 12 MeV. Level energies, branching ratios, mean lifetimes, spins and mixing ratios were obtained for a number of levels. Relevant results are compared with many-particle shell-model calculations.  相似文献   
987.
The propagation of Alfven and magnetoacoustic waves in a cold magnetized plasma to great distances from the expanding plasma cloud that is the source of these waves is investigated. Allowance is made for back-ground-plasma permittivity anisotropy, which leads to Alfven-and magnetoacoustic-wave interaction and channeling along the external magnetic field.Moscow Engineering-Physics Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 9, pp. 882–891, September, 1993.  相似文献   
988.
A Kind of direct methods is presented for the solution of optimal control problems with state constraints.These methods are sequential quadratic programming methods.At every iteration a quadratic programming which is obtained by quadratic approximation to Lagrangian function and Linear approximations to constraints is solved to get a search direction for a merit function.The merit function is formulated by augmenting the Lagrangian funetion with a penalty term.A line search is carried out along the search direction to determine a step length such that the merit function is decreased.The methods presented in this paper include continuous sequential quadratic programming methods and discreate sequential quadrade programming methods.  相似文献   
989.
990.
This study was performed under the joint TRMC/INER program for the determination of low level85Kr and133Xe concentrations in the environmental air samples. Based on cryogenic adsorption of krypton and xenon on charcoal followed by chromatographic separation from other gases, the85Kr and133Xe recovered from 200 liters of atmospheric air can be determined by either on-line gas flow proportional counter or liquid scintillation counting. The recovery yields of krypton and xenon examined by using85Kr and133Xe tracers were nearly 100%. The minimum detectable activity of85Kr and133Xe by gas flow proportional counting is about 7.40 Bq. The method is satisfactory for environmental monitoring applications under abnormal conditions of nuclear facilities. However, for lower level environmental85Kr and133Xe measurements, the liquid scintillation counting method can be applied due to their extremely low detection limits (i.e. 0.107 Bq and 0.093 Bq for85Kr and133Xe, respectively). Using this method, the measurable limits of concentrations are 0.535 Bq/m3 and 0.466 Bq/m3 for85Kr and133Xe, respectively.  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