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   68513篇
  免费   5137篇
  国内免费   6400篇
化学   25450篇
晶体学   277篇
力学   6255篇
综合类   1243篇
数学   26037篇
物理学   20788篇
  2023年   529篇
  2022年   800篇
  2021年   1127篇
  2020年   1275篇
  2019年   1531篇
  2018年   1271篇
  2017年   1523篇
  2016年   1738篇
  2015年   1574篇
  2014年   2267篇
  2013年   4394篇
  2012年   2661篇
  2011年   3079篇
  2010年   2664篇
  2009年   3771篇
  2008年   4223篇
  2007年   4415篇
  2006年   4128篇
  2005年   3502篇
  2004年   3232篇
  2003年   3295篇
  2002年   3023篇
  2001年   2472篇
  2000年   2384篇
  1999年   2134篇
  1998年   1996篇
  1997年   1652篇
  1996年   1463篇
  1995年   1406篇
  1994年   1306篇
  1993年   1102篇
  1992年   1071篇
  1991年   843篇
  1990年   693篇
  1989年   605篇
  1988年   548篇
  1987年   435篇
  1986年   374篇
  1985年   453篇
  1984年   441篇
  1983年   215篇
  1982年   352篇
  1981年   405篇
  1980年   303篇
  1979年   297篇
  1978年   233篇
  1977年   215篇
  1976年   161篇
  1974年   114篇
  1973年   106篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Total glucosides of paeony are the active constituents of Paeoniae Radix Alba. In this study, a novel strategy was proposed to find more metabolites and the differences between paeoniflorin, albiflorin and total glucosides of paeony (TGP). This strategy was characterized as follows: firstly, the animals were divided into three groups (paeoniflorin, albiflorin and TGP) to identify the source of TGP metabolites from paeoniflorin or albiflorin; secondly, a generic information‐dependent acquisition scan for the low‐level metabolites was triggered by the multiple mass defect filter and dynamic background subtraction; thirdly, the metabolites were identified with a combination of data‐processing methods including mass defect filtering, neutral loss filtering and product ion filtering; finally, a comparative study was used in the metabolism of paeoniflorin, albiflorin and TGP. Based on the strategy, 18 metabolites of TGP, 10 metabolites of paeoniflorin and 13 metabolites of albiflorin were identified respectively. The results indicated that the hydrolysis, conjugation reaction and oxidization were the major metabolic pathways, and the metabolic sites were the glycosidic linkage, the ester bond and the benzene ring. This study is first to explore the metabolism of TGP, and these findings enhance our understanding of the metabolism and the interactions of paeoniflrin and albiflorin in TGP. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
92.
We give a sheaf theoretic interpretation of Potts models with external magnetic field, in terms of constructible sheaves and their Euler characteristics. We show that the polynomial countability question for the hypersurfaces defined by the vanishing of the partition function is affected by changes in the magnetic field: elementary examples suffice to see non-polynomially countable cases that become polynomially countable after a perturbation of the magnetic field. The same recursive formula for the Grothendieck classes, under edge-doubling operations, holds as in the case without magnetic field, but the closed formulae for specific examples like banana graphs differ in the presence of magnetic field. We give examples of computation of the Euler characteristic with compact support, for the set of real zeros, and find a similar exponential growth with the size of the graph. This can be viewed as a measure of topological and algorithmic complexity. We also consider the computational complexity question for evaluations of the polynomial, and show both tractable and NP-hard examples, using dynamic programming.  相似文献   
93.
《中国物理 B》2021,30(5):57303-057303
A novel super-junction LDMOS with low resistance channel(LRC), named LRC-LDMOS based on the silicon-oninsulator(SOI) technology is proposed. The LRC is highly doped on the surface of the drift region, which can significantly reduce the specific on resistance(Ron,sp) in forward conduction. The charge compensation between the LRC, N-pillar,and P-pillar of the super-junction are adjusted to satisfy the charge balance, which can completely deplete the whole drift,thus the breakdown voltage(BV) is enhanced in reverse blocking. The three-dimensional(3 D) simulation results show that the BV and R_(on,sp) of the device can reach 253 V and 15.5 mΩ·cm~2, respectively, and the Baliga's figure of merit(FOM = BV~2/R_(on,sp)) of 4.1 MW/cm~2 is achieved, breaking through the silicon limit.  相似文献   
94.
We report a general method for selective cross‐coupling of α,β‐unsaturated carboxylic acids with aryl tosylates enabled by versatile Pd(II) complexes. This method features the general cross‐coupling of ubiquitous α,β‐unsaturated carboxylic acids by decarboxylation. The transformation is characterized by its operational simplicity, the use of inexpensive, air‐stable Pd(II) catalysts, scalability and wide substrate scope. The reaction proceeds with high trans selectivity to furnish valuable (E)‐1,2‐diarylethenes.  相似文献   
95.
Distance between two vertices is the number of edges in a shortest path connecting them in a connected graph G. The transmission of a vertex v is the sum of distances from v to all the other vertices of G. If transmissions of all vertices are mutually distinct, then G is a transmission irregular graph. It is known that almost no graphs are transmission irregular. Infinite families of transmission irregular trees of odd order were presented in Alizadeh and Klav?ar (2018). The following problem was posed in Alizadeh and Klav?ar (2018): do there exist infinite families of transmission irregular trees of even order? In this article, such a family is constructed.  相似文献   
96.
97.
Elastic scattering angular distributions and total reaction cross-sections of ~(7,10,11,12)Be projectiles are predicted by the systematic ~9 Be global phenomenological optical model potential for target mass numbers ranging from24 to 209. These predictions provide a detailed analysis by their comparison with the available experimental data.Furthermore, these elastic scattering observables are also predicted for some targets out of the mass number range.The results are in reasonable agreement with the existing experimental data, and they are presented in this study.  相似文献   
98.
99.
100.
This paper reports the results of a research exploring the mathematical connections of pre-university students while they solving tasks which involving rates of change. We assume mathematical connections as a cognitive process through which a person finds real relationships between two or more ideas, concepts, definitions, theorems, procedures, representations or meanings or with other disciplines or the real-world. Four tasks were proposed to the 33 pre-university students that participated in this research; the central concept of the first task is the slope, the last three tasks contain concepts like velocity, speed and acceleration. Task-based interviews were conducted to collect data and later analysed with thematic analysis. Results showed most of the students made mathematical connections of the procedural type, the mathematical connections of the common features type are made in smaller quantities and the mathematical connection of the generalization type is scarcely made. Furthermore, students considered slope as a concept disconnected from velocity, speed and acceleration.  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