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   35422篇
  免费   1072篇
  国内免费   228篇
化学   22956篇
晶体学   172篇
力学   781篇
综合类   1篇
数学   6974篇
物理学   5838篇
  2022年   184篇
  2021年   369篇
  2020年   455篇
  2019年   404篇
  2018年   436篇
  2017年   462篇
  2016年   989篇
  2015年   874篇
  2014年   939篇
  2013年   2063篇
  2012年   1753篇
  2011年   2155篇
  2010年   1217篇
  2009年   1031篇
  2008年   1686篇
  2007年   1630篇
  2006年   1676篇
  2005年   1469篇
  2004年   1410篇
  2003年   1191篇
  2002年   1154篇
  2001年   551篇
  2000年   484篇
  1999年   483篇
  1998年   472篇
  1997年   521篇
  1996年   478篇
  1995年   438篇
  1994年   463篇
  1993年   410篇
  1992年   389篇
  1991年   381篇
  1990年   332篇
  1989年   339篇
  1988年   328篇
  1987年   286篇
  1986年   311篇
  1985年   451篇
  1984年   453篇
  1983年   388篇
  1982年   416篇
  1981年   386篇
  1980年   419篇
  1979年   345篇
  1978年   376篇
  1977年   362篇
  1976年   287篇
  1975年   241篇
  1974年   262篇
  1973年   238篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
91.
Received December 31, 2001 / Published online May 21, 2002  相似文献   
92.
Ohne ZusammenfassungDiese Arbeit wurde während eines Aufenthaltes von Schmidt in der Bundesrepublik Deutschland als Preisträger der Alexander von Humboldt-Stiftung geschrieben.  相似文献   
93.
Butyltins were analyzed in waters from California river and lake marinas and were detected at the part per trillion (ppt) level in most locations. The tributyltin:dibutyltin (TBT/DBT) ratios in the fresh waters were similar to those in their saline counterparts indicating like rates of degradation in the two salinity regimes.  相似文献   
94.
We give a new and comparably short proof of Gittins’ index theorem for dynamic allocation problems of the multi-armed bandit type in continuous time under minimal assumptions. This proof gives a complete characterization of optimal allocation strategies as those policies which follow the current leader among the Gittins indices while ensuring that a Gittins index is at an all-time low whenever the associated project is not worked on exclusively. The main tool is a representation property of Gittins index processes which allows us to show that these processes can be chosen to be pathwise lower semi-continuous from the right and quasi-lower semi-continuous from the left. Both regularity properties turn out to be crucial for our characterization and the construction of optimal allocation policies.  相似文献   
95.
The complete set of self-consistent parameters of nominally undoped LiNbO3 crystals of congruent composition that describe the electro-optic, piezoelectric, elasto-optic, elastic, and dielectric response has been determined by numerically evaluating available measurements. The parameters were determined at room temperature and consist of the low-frequency clamped dielectric constants εS ij, elastic stiffness constants at constant electric field CE ijkl, piezoelectric stress coefficients eijk, elasto-optic constants at constant electric field pE ijkl, and clamped electro-optic coefficients rS ijk. It is shown that the complete set is required for calculating the effective electro-optic coefficients and dielectric constants in photorefractive applications of LiNbO3. Received: 4 January 2002 / Revised version: 1 February 2002 / Published online: 14 March 2002  相似文献   
96.
Summary The paper addresses the problem of the implementation of nonhomogeneous essential Dirichlet type boundary conditions in thep-version of the finite element method.Partially supported by the Office of Naval Research under Grant N-00014-85-K-0169Research partially supported by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant Number AFOSR 85-0322  相似文献   
97.
This study analyzes the economics of transshipping biomass from truck to train in a North American setting. Transshipment will only be economic when the cost per unit distance of a second transportation mode is less than the original mode. There is an optimum number of transshipment terminals which is related to biomass yield. Transshipment incurs incremental fixed costs, and hence there is a minimum shipping distance for rail transport above which lower costs/km offset the incremental fixed costs. For transport by dedicated unit train with an optimum number of terminals, the minimum economic rail shipping distance for straw is 170 km, and for boreal forest harvest residue wood chips is 145 km. The minimum economic shipping distance for straw exceeds the biomass draw distance for economically sized centrally located power plants, and hence the prospects for rail transport are limited to cases in which traffic congestion from truck transport would otherwise preclude project development. Ideally, wood chip transport costs would be lowered by rail transshipment for an economically sized centrally located power plant, but in a specific case in Alberta, Canada, the layout of existing rail lines precludes a centrally located plant supplied by rail, whereas a more versatile road system enables it by truck. Hence for wood chips as well as straw the economic incentive for rail transport to centrally located processing plants is limited. Rail transshipment may still be preferred in cases in which road congestion precludes truck delivery, for example as result of community objections.  相似文献   
98.
99.
100.
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