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   314922篇
  免费   3753篇
  国内免费   1091篇
化学   170921篇
晶体学   4471篇
力学   13066篇
综合类   7篇
数学   36311篇
物理学   94990篇
  2020年   1989篇
  2019年   1957篇
  2018年   1974篇
  2017年   1881篇
  2016年   3865篇
  2015年   3239篇
  2014年   4430篇
  2013年   13993篇
  2012年   10742篇
  2011年   13386篇
  2010年   8292篇
  2009年   8277篇
  2008年   12351篇
  2007年   12596篇
  2006年   12233篇
  2005年   11164篇
  2004年   10042篇
  2003年   8867篇
  2002年   8716篇
  2001年   10126篇
  2000年   7721篇
  1999年   6141篇
  1998年   4911篇
  1997年   4741篇
  1996年   4789篇
  1995年   4374篇
  1994年   4129篇
  1993年   3947篇
  1992年   4442篇
  1991年   4297篇
  1990年   4017篇
  1989年   3837篇
  1988年   4125篇
  1987年   3793篇
  1986年   3691篇
  1985年   5320篇
  1984年   5408篇
  1983年   4386篇
  1982年   4755篇
  1981年   4796篇
  1980年   4551篇
  1979年   4665篇
  1978年   4673篇
  1977年   4646篇
  1976年   4600篇
  1975年   4511篇
  1974年   4361篇
  1973年   4528篇
  1972年   2573篇
  1971年   1882篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Sr incorporation in the molecules of amorphous calcium phosphate, apatitic tricalcium phosphate, hydroxyapatite, octacalcium phosphate and dicalcium phosphate dihydrate was investigated. The concentration of Sr ranged from 225 to 1010 μ g / g, i.e. it overlapped with the physiological range of Sr concentrations in human bone. The leading experimental technique was extended X-ray absorption fine structure (EXAFS) at the Sr K edge. Results of these studies demonstrated the following: (1) Sr incorporation in the calcium phosphates is compound-dependent, (2) the coordination of incorporated Sr atoms in the Ca-P molecules is similar to that of Ca atoms, but interatomic distances are ≈0.015 nm larger, (3) in apatitic tricalcium phosphate, hydroxyapatite and octacalcium phosphate lattices Sr atoms may occupy selected Ca sites, which was not the case for dicalcium phosphate dihydrate, (4) in the apatite lattice Sr atoms are coordinated by 6 PO4 tetrahedrals and (5) EXAFS spectra at the K edge of the incorporated Sr may be used to distinguish the structures of amorphous calcium phosphate, dicalcium phosphate dihydrate as well as apatite and its derivatives (apatitic tricalcium phosphate, octacalcium phosphate).  相似文献   
92.
Summary Single layers and layer systems on diverse substrates were measured by Total Reflection X-Ray Fluorescence (TXRF) spectrometry. The angular dependence of the fluorescence intensities at grazing incidence allows the elemental composition, density and thickness of the layers to be evaluated using model calculations.  相似文献   
93.
We report results of ellipsometric measurements of the dielectric tensor of YBa2Cu4O8 as well as calculations employing density functional theory in the local density approximation (LDA). For the computation of the one-electron eigenvalues and states the linear-muffin-tin-orbital method (LMTO) was used. Measured and calculated results show good agreement. In particular, the calculation predicts different band structures for YBa2Cu3O7 and YBa2Cu4O8 in the vicinity of the Fermi energy which lead to significant differences in the optical spectra. One such difference is a region of small ε2 in the near infrared which is a fingerprint of the high (relative to the top of the highest fully occupied band) Fermi energy as compared to the situation in YBa2Cu3O7. Our experimental results confirm the theoretical findings.  相似文献   
94.
In this paper we consider the nonnegative matrix system ciA+uiB=ci+1 where the nonnegative matrix A is allowed to vary, within bounds. The cone control problem is to find a nonnegative matrix B such that if Ci is a nonnegative vector in a specified cone, then there is a nonnegative vector ui such that ci+1 is in that cone. We extend this problem to input control by finding a B such that the cone, generated by the rows of B, is as small as possible. Thus, the percent distribution of ∣uiB∣ through the states of the sustem by uiB is either constant or varies little.  相似文献   
95.
The authors study concentration profiles of solutes undergoingequilibrium absorption in the vicinity of a water well. Forthe case of a contamination event, the limit problem of vanishingwell radius, which is of self-similar nature, is analysed indetail. Existence, uniqueness, and qualitative properties ofsolutions of the corresponding ordinary differential equationsare shown. Some numerical examples are also presented.  相似文献   
96.
97.
98.
99.
100.
A number of biphenyl, terphenyl analogues and ethynes which contain a pyrazine ring have been made and their liquid crystal transition temperatures, together with examples of birefringence measurements, are reported. All the 2,5-disubstituted pyrazine systems are liquid crystalline showing high birefringence values for the biphenyl and terphenyl analogues, whereas the 1,5-disubstituted systems are not liquid crystalline. The pyrazine ethyne systems exhibit very high birefringence values. X-ray diffraction has been used to identify the liquid crystal phases of 2-n-nonyloxy-5-(4'-propylbiphenyl-4-yl)pyrazine.  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