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   313871篇
  免费   4291篇
  国内免费   1081篇
化学   170594篇
晶体学   4460篇
力学   13038篇
综合类   7篇
数学   36280篇
物理学   94864篇
  2020年   1985篇
  2019年   1955篇
  2018年   1969篇
  2017年   1875篇
  2016年   3860篇
  2015年   3239篇
  2014年   4422篇
  2013年   13996篇
  2012年   10749篇
  2011年   13390篇
  2010年   8277篇
  2009年   8268篇
  2008年   12346篇
  2007年   12581篇
  2006年   12227篇
  2005年   11174篇
  2004年   10048篇
  2003年   8861篇
  2002年   8709篇
  2001年   10113篇
  2000年   7717篇
  1999年   6122篇
  1998年   4897篇
  1997年   4736篇
  1996年   4777篇
  1995年   4367篇
  1994年   4119篇
  1993年   3944篇
  1992年   4438篇
  1991年   4299篇
  1990年   4014篇
  1989年   3837篇
  1988年   4121篇
  1987年   3789篇
  1986年   3681篇
  1985年   5330篇
  1984年   5399篇
  1983年   4382篇
  1982年   4755篇
  1981年   4794篇
  1980年   4551篇
  1979年   4666篇
  1978年   4667篇
  1977年   4647篇
  1976年   4603篇
  1975年   4510篇
  1974年   4361篇
  1973年   4527篇
  1972年   2571篇
  1971年   1880篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
Oxidation of the title compounds yields, besides the reported isoimides 3 and/or the amides 4 , also the imides 5 . The observed product dichotomy is considered as the result of an intramolecular nucleophilic attack on the aroyl group, of the pressumed zwitterionic intermediate 2 , by O or N present in the ambident N-aroylimine site of 2 . The results of AM1 calculations agree with the product studies and both permit the formulation of a set of rules correlating structure and selectivity.  相似文献   
152.
153.
We describe a semi-analytical numerical method for coherent isotropic scattering time-dependent radiative transfer problems in slab geometry. This numerical method is based on a combination of two classes of numerical methods: the spectral methods and the Laplace transform (LTSN) methods applied to the radiative transfer equation in the discrete ordinates (SN) formulation. The basic idea is to use the essence of the spectral methods and expand the intensity of radiation in a truncated series of Laguerre polynomials in the time variable and then solve recursively the resulting set of “time-independent” SN problems by using the LTSN method. We show some numerical experiments for a typical model problem.  相似文献   
154.
Muon electron pairs were detected in an Al multiplate spark chamber, exposed to a neutrino beam from the CERN PS. The leptons were not accompanied by other particles, except occasionally by protons. The background came mainly from muon associated π0 production, with one decay gamma lost. It was determined empirically, together with the small contribution from υ e reactions. For electron energies above 2 GeV the background is 5.7±1.5 events, whereas 18 (μe)-candidates have been observed. Hence the effect is established, with a rate of about 10?4 as compared to the muonic reactions above 3 GeV. Charm creation as the origin of this (μe)-production process is excluded; heavy neutral lepton production does not fit the kinematics observed. Instead the events are compatible with the two-body decay of an object with variable invariant mass of order 1 GeV, possibly resulting from axion interactions.  相似文献   
155.
This paper presents an alternative to the beta continuous probability distribution for risk analysis. Particular attention has been given to two major applications of distributions, namely project management risk and critical path analysis (PERT). In conjunction with the beta, the triangular and normal distributions are frequently employed in order to give sufficient robustness to risk analysis. The beta distribution, as used in PERT, has a major theoretical implementation flaw. The new distribution was developed to give a possible alternative method of assessing risk. It is shown that the requirement to estimate the most pessimistic variate may be replaced by the probability to exceed the mode. Proposals for other simplifications in risk analysis are discussed. Practical means to validate the most appropriate distributions for risk analysis are outlined, and a cost-data case study is included.  相似文献   
156.
157.
158.
We consider the growth of a spherical crystal in a supersaturatedsolution. In the first part, existence and uniqueness resultsfor radially symmetric growth are obtained, provided that thesupersaturation is not too large; conversely, when the far-fieldsupersaturation exceeds a critical value, it is shown that theradially symmetric solution ceases to exist in finite time.In the second part, we examine the linear stability of a radiallysymmetric similarity solution (in which the radius grows ast?) to shape perturbations. The results are compared with previousquasi-static analyses, and, in particular, the critical radiusat which the crystal becomes unstable is found to be largerfor small supersaturations, but smaller for large supersaturations,than those predicted by the quasi-static analysis  相似文献   
159.
160.
We report morphologic changes of metallic surfaces at the onset of ablation, starting from gentle ablation to the emergence of ablation craters. The evolution of both observed melting zones and of ablation craters therein are investigated in dependence of the ablation laser fluence for nanosecond ultraviolet laser pulses. Further, consequences of crater formation for cluster synthesis within the released atomic vapor are pointed out. PACS 52.38.Mf; 79.20.Ds; 65.40.De  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