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   1335537篇
  免费   26488篇
  国内免费   8481篇
化学   666429篇
晶体学   20259篇
力学   74576篇
综合类   161篇
数学   241931篇
物理学   367150篇
  2021年   13585篇
  2020年   16016篇
  2019年   16106篇
  2018年   12216篇
  2016年   27760篇
  2015年   20889篇
  2014年   30545篇
  2013年   74616篇
  2012年   37874篇
  2011年   34339篇
  2010年   37061篇
  2009年   39492篇
  2008年   33872篇
  2007年   29409篇
  2006年   36015篇
  2005年   28436篇
  2004年   29614篇
  2003年   28046篇
  2002年   29141篇
  2001年   27389篇
  2000年   24215篇
  1999年   22228篇
  1998年   20942篇
  1997年   20952篇
  1996年   21160篇
  1995年   19219篇
  1994年   18668篇
  1993年   18222篇
  1992年   17992篇
  1991年   18258篇
  1990年   17449篇
  1989年   17487篇
  1988年   17050篇
  1987年   17057篇
  1986年   16022篇
  1985年   22497篇
  1984年   23766篇
  1983年   19980篇
  1982年   21699篇
  1981年   20885篇
  1980年   20275篇
  1979年   20351篇
  1978年   21714篇
  1977年   21295篇
  1976年   21019篇
  1975年   19662篇
  1974年   19318篇
  1973年   19788篇
  1972年   14293篇
  1967年   12476篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The problem of exciting bulk elastic waves at the surface of a piezoelectric with symmetries 422,622 has been solved by a successive approximation method. In the approximation of a fixed electric field, created at the surface of the piezoelectric crystal by a two-electrode transducer, the distributions are found for the shear wave stress and the energy flux density in the far zone. The equivalent circuit parameters for a two-electrode radiator are determined taking account of the dynamic piezoelectric correction obtained in the second approximation. The equivalent circuit parameters and the transducer loss are treated for TeO2 crystals. A realistic possibility of using surface transducers in the development of acousto-optic modulators in the s.h.f. region is shown. Tomsk State Academy of Control Systems and Radio Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 8–15, January, 1997.  相似文献   
82.
83.
An analysis is made of the thermodynamic stability of the dislocation structure in polycrystalline samples of Fe-C, deformed under conditions of high rates and hydrostatic pressures, based on experimental and theoretical data concerning the internal energy and on diffractometer measurements of the broadening of x-ray lines. The method of deformation calorimetry was used to determine the internal energy in a wide range of deformations. A theoretical model is proposed for estimating the change in internal energy in deformed alloys. An investigation is made of the dependence of the interdislocation interaction parameter on the deformation rate for different stressed state-schemes and large plastic deformations. It is shown that the relative quantity δU/A is correlated in a wide range of deformations with the relative root-mean-square distortions of the crystal lattice of the deformed solid solution. The mechanisms for the accumulation of energy in deformed solid solutions having a body-centered cubic lattice are considered. Don State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 77–83.  相似文献   
84.
In this paper we study constant mean curvature compact surfaces with two Jordan curves in parallel planes as boundary and we investigate the point at which the surface inherits the symmetries of its boundary.  相似文献   
85.
Thirteen fifth graders were given an assignment to invent their own numeration systems, following a unit on bases and a look at early events in the history of numbers. The task presented options that required the students to make decisions (such as whether to use a base, which base to use, design of symbols, etc.), and build a rationale for the elements of their system. Analyses of patterns embedded in their invented systems provided an assessment of student understanding of numeration. The progression of more and less complex thinking related to the student's choice of a base other than 10, consistency of logic throughout the system in words and symbols, rationale for change, and perception of real life examples that would change if the system was adopted. The invention task is presented as another way to make connections.  相似文献   
86.
87.
88.
 The effect of iodine sorption on the free volume of polycarbonate is investigated by Positron Annihilation Lifetime method. The observed results are interpreted in terms of the Charge Transfer Complex formation and precipitation of iodine at the initial and final stages of sorption, respectively. At higher levels of sorption, changes in the second lifetime and its intensity seem to suggest a conformational transformation, probably due to the net change in the amorphous-crystalline boundary regions of the polymer matrix. The free volume cavities have been predominantly occupied by I- 3 species and the diffusion process obeys Fick’s law. An exponential type correlation has been observed between fractional free volume and the diffusion coefficient. Received: 13 June 1996/Accepted: 20 September 1996  相似文献   
89.
We provide a characterization of the limit periodic sets for analytic families of vector fields under the hypothesis that the first jet is non-vanishing at any singular point. Also, applying the family desingularization method, we reduce the complexity of some of these sets.  相似文献   
90.
We report a strong dependence of the thermal stability of Nafion® perfluorosulfonate ionomer on the nature of the counterion associated with the fixed sulfonate site. These results were obtained using thermal gravimetric analysis on a series of alkali metal and alkyl ammonium cation-exchanged Nafion films. We have found that the temperature of decomposition of Nafion is inversely dependent on the size of the exchanged cation; i.e., Nafion films show improved thermal stability as the size of the counter cation decreases. We attribute this inverse relationship of thermal stability with counterion size to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-coun-terion interaction. © 1993 John Wiley & Sons, Inc.  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