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   520779篇
  免费   5683篇
  国内免费   1650篇
化学   282202篇
晶体学   7442篇
力学   21454篇
综合类   20篇
数学   65752篇
物理学   151242篇
  2020年   4381篇
  2019年   4881篇
  2018年   6226篇
  2017年   6058篇
  2016年   9612篇
  2015年   6489篇
  2014年   9172篇
  2013年   23616篇
  2012年   18092篇
  2011年   21481篇
  2010年   14777篇
  2009年   14224篇
  2008年   19581篇
  2007年   19685篇
  2006年   18112篇
  2005年   16758篇
  2004年   15024篇
  2003年   13378篇
  2002年   13113篇
  2001年   13924篇
  2000年   10864篇
  1999年   8517篇
  1998年   7298篇
  1997年   7222篇
  1996年   7014篇
  1995年   6353篇
  1994年   6456篇
  1993年   6223篇
  1992年   6841篇
  1991年   6747篇
  1990年   6544篇
  1989年   6352篇
  1988年   6478篇
  1987年   6268篇
  1986年   5994篇
  1985年   8015篇
  1984年   8431篇
  1983年   6999篇
  1982年   7378篇
  1981年   7259篇
  1980年   7126篇
  1979年   7075篇
  1978年   7453篇
  1977年   7329篇
  1976年   7527篇
  1975年   6871篇
  1974年   7007篇
  1973年   7479篇
  1972年   4932篇
  1971年   3997篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
91.
This research extends previous work with dynamic models to manage groundwater quality by using the consumptive nitrate use rate instead of the nitrate application rate. The analysis indicates that misspecification results in overestimation of economic benefits, and supra-optimum nitrogen fertilizer application rates and groundwater nitrate stocks at a steady state.  相似文献   
92.
93.
94.
Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described.  相似文献   
95.
We describe a general method that enables us to obtain all the singular terms of helicity amplitudes of n-parton processes at one loop. The algorithm uses helicity amplitudes at tree level and simple color algebra. We illustrate the method by calculating the singular part of the one-loop helicity amplitudes of all 2 → 3 parton subprocesses. The results are used to derive the soft gluon limit of the cross sections of all 2 → 4 parton scattering subprocesses which provide a useful initial condition for the angular ordering approximation to coherent multiple soft gluon emission, incorporated in existing Monte Carlo simulation programs.  相似文献   
96.
In this paper we construct a primitive, non-symmetric 3-class association scheme with parameters v = 36, v1 = 7, p111 = 0 and p211 = 4 and show that such a scheme is determined by its parameters.  相似文献   
97.
Silver nitrate-acetonitrile and π iodine-benzene complexes in thermotropic liquid crystals have been studied by 1H, 2H, and 13C NMR spectroscopy and by optical microscopy. Evidence for at least two silver complexes in each liquid crystal is presented.  相似文献   
98.
 National measurement systems are infrastructures to ensure, for each nation, a consistent and internationally recognised basis for measurement. Such complex systems have historical, technical, legal, organisational and institutional aspects to connect scientific metrology with practical measurements. Underlying any valid measurement is a chain of comparisons linking the measurement to an accepted standard. The ways the links are forged and the etalons (measurement standards) to which they connect are defining characteristics of all measurement systems. This is often referred to as traceability which aims at basing measurements in common measurement units – a key issue for the integration of quantitative chemical analysis with the evolving physical and engineering measurement systems. Adequate traceability and metrological control make possible new technical capabilities and new levels of quality assurance and confidence by users in the accuracy and integrity of quantitative analytical results. Traceability for chemical measurements is difficult to achieve and harder to demonstrate. The supply of appropriate etalons is critical to the development of metrology systems for chemical analysis. An approach is suggested that involves the development of networks of specialised reference laboratories able to make matrix-independent reference measurements on submitted samples, which may then be used as reference materials by an originating laboratory using its practical measurement procedures. Received: 31 July 1995 Accepted: 19 August 1995  相似文献   
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