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   61954篇
  免费   7629篇
  国内免费   6280篇
化学   12835篇
晶体学   969篇
力学   10962篇
综合类   1067篇
数学   31306篇
物理学   18724篇
  2024年   70篇
  2023年   614篇
  2022年   926篇
  2021年   1241篇
  2020年   1614篇
  2019年   1495篇
  2018年   1520篇
  2017年   2028篇
  2016年   2320篇
  2015年   1795篇
  2014年   3179篇
  2013年   4532篇
  2012年   3653篇
  2011年   4228篇
  2010年   3618篇
  2009年   4234篇
  2008年   4150篇
  2007年   4194篇
  2006年   3783篇
  2005年   3516篇
  2004年   2968篇
  2003年   2729篇
  2002年   2365篇
  2001年   1985篇
  2000年   1789篇
  1999年   1574篇
  1998年   1448篇
  1997年   1300篇
  1996年   1041篇
  1995年   901篇
  1994年   805篇
  1993年   631篇
  1992年   609篇
  1991年   478篇
  1990年   410篇
  1989年   299篇
  1988年   264篇
  1987年   202篇
  1986年   155篇
  1985年   209篇
  1984年   177篇
  1983年   101篇
  1982年   137篇
  1981年   116篇
  1980年   79篇
  1979年   79篇
  1978年   66篇
  1977年   65篇
  1976年   44篇
  1973年   28篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
《Discrete Mathematics》2022,345(10):112984
Let G be a generalized dicyclic group with identity 1. An inverse closed subset S of G?{1} is called minimal if S=G and there exists some sS such that S?{s,s?1}G. In this paper, we characterize distance-regular Cayley graphs Cay(G,S) of G under the condition that S is minimal.  相似文献   
32.
本研究采用水热法,以柠檬酸为螯合剂,通过控制n(Sn4+)/n(Sn2+)的数值,合成了由具有丰富氧空位的SnO2纳米晶体组装成的微球。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、傅里叶红外光谱(FT-IR)、X射线光电子能谱(XPS)及UV-Vis漫反射光谱对SnO2纳米微球进行表征分析,结果表明:在酸性水热条件和柠檬酸的螯合作用下,二氧化锡纳米晶体聚集形成微球;在Sn4+/Sn2+摩尔比例为3:7时,其微球尺寸最小,整体分散性较好;同时适量二价锡离子的掺杂使得该样品氧空位浓度达到最佳,氧空位的存在将使得样品光吸收范围拓展至可见光,因而该样品显示出较强的可见光催化效率,在8 min内完全降解甲基橙。  相似文献   
33.
34.
35.
36.
37.
38.
39.
Chengwei Deng 《中国物理 B》2022,31(11):118702-118702
RNAs play crucial and versatile roles in cellular biochemical reactions. Since experimental approaches of determining their three-dimensional (3D) structures are costly and less efficient, it is greatly advantageous to develop computational methods to predict RNA 3D structures. For these methods, designing a model or scoring function for structure quality assessment is an essential step but this step poses challenges. In this study, we designed and trained a deep learning model to tackle this problem. The model was based on a graph convolutional network (GCN) and named RNAGCN. The model provided a natural way of representing RNA structures, avoided complex algorithms to preserve atomic rotational equivalence, and was capable of extracting features automatically out of structural patterns. Testing results on two datasets convincingly demonstrated that RNAGCN performs similarly to or better than four leading scoring functions. Our approach provides an alternative way of RNA tertiary structure assessment and may facilitate RNA structure predictions. RNAGCN can be downloaded from https://gitee.com/dcw-RNAGCN/rnagcn.  相似文献   
40.
There is a growing attention to the bio and renewable energies due to fast depletion of fossil fuels as well as the global warming problem. Here, we developed a modeling and simulation method by means of artificial intelligence (AI) for prediction of the bioenergy production from vegetable bean oil. AI methods are well known for prediction of complex and nonlinear process. Three distinct Adaptive Boosted models including Huber regression, LASSO, and Support Vector Regression (SVR) as well as artificial neural network (ANN) were applied in this study to predict actual yield of Fatty acid methyl esters (FAME) production. All boosted utilizing the Adaptive boosting algorithm. The important influencing parameters on the biodiesel production such as the catalyst loading (CAO/Ag, wt%) and methanol to oil (Soybean oil) molar ratio were selected as the input variables of models while the yield of FAME production was selected as output. Model hyper-parameters were tuned to maintain generality while improving prediction accuracy. The models were evaluated using three distinct metrics Mean Absolute Error (MAE), Root Mean Square Error (RMSE), and R2. Error rates of 8.16780E-01, 4.43895E-01, 2.06692E + 00, and 3.92713 E-01 were obtained with the MAE metric for boosted Huber, SVR, LASSO and ANN models. On the other hand, the RMSE error of these models were about 1.092E-02, 1.015E-02, 2.669E-02, and 1.01174E-02, respectively. Finally, the R-square score were calculated for boosted Huber, boosted SVR, and boosted LASSO as 0.976, 0.990, 0.872, and 0.99702, respectively. Therefore, it can be concluded that although the boosted SVR and ANN models were better models for prediction of process efficiency in terms of error, but all algorithms had high accuracy. The optimum yield of 83.77% and 81.60% for biodiesel production were observed at optimum operating values from boosted SVR and ANN models, respectively.  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