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   190623篇
  免费   19997篇
  国内免费   10602篇
电工技术   9255篇
技术理论   5篇
综合类   11547篇
化学工业   44678篇
金属工艺   34120篇
机械仪表   6838篇
建筑科学   14071篇
矿业工程   4756篇
能源动力   4456篇
轻工业   15015篇
水利工程   3626篇
石油天然气   8590篇
武器工业   1236篇
无线电   14033篇
一般工业技术   23392篇
冶金工业   14305篇
原子能技术   1885篇
自动化技术   9414篇
  2024年   922篇
  2023年   3315篇
  2022年   7373篇
  2021年   8295篇
  2020年   6529篇
  2019年   5900篇
  2018年   5807篇
  2017年   7357篇
  2016年   8381篇
  2015年   8980篇
  2014年   11592篇
  2013年   12044篇
  2012年   12311篇
  2011年   12458篇
  2010年   9768篇
  2009年   10421篇
  2008年   8615篇
  2007年   11802篇
  2006年   11638篇
  2005年   9656篇
  2004年   7940篇
  2003年   7349篇
  2002年   5988篇
  2001年   4837篇
  2000年   4101篇
  1999年   3100篇
  1998年   2412篇
  1997年   2139篇
  1996年   1730篇
  1995年   1515篇
  1994年   1297篇
  1993年   934篇
  1992年   876篇
  1991年   670篇
  1990年   617篇
  1989年   571篇
  1988年   289篇
  1987年   233篇
  1986年   191篇
  1985年   174篇
  1984年   169篇
  1983年   117篇
  1982年   110篇
  1981年   81篇
  1980年   101篇
  1979年   32篇
  1966年   34篇
  1964年   43篇
  1962年   66篇
  1959年   34篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
In this paper, an off‐grid direction of arrival (DoA) estimation method is proposed for wideband signals. This method is based on the sparse representation (SR) of the array covariance matrix. Similar to the time domain DoA estimation methods, the correlation function of the sources was assumed to be the same and known. A new measurement vector is obtained using the lower‐left triangular elements of the covariance matrix. The DoAs are estimated by quantizing the entire range of continuous angle space into discrete grid points. However, the exact DoAs may be located between two grid points; therefore, this estimation has errors. The accuracy of DoA estimation is improved by the minimization of the difference between the new measurement vector and its estimated values. Simulation results revealed that the proposed method can enhance the DoA estimation accuracy of wideband signals.  相似文献   
4.
A first‐principles‐based effective Hamiltonian is developed and employed to investigate finite‐temperature structural properties of a prototype of perovskite halides, that is CsPbI3. Such simulations, when using first‐principles‐extracted coefficients, successfully reproduce the existence of an orthorhombic Pnma state and its iodine octahedral tilting angles around room temperature. However, they also yield a direct transformation from Pnma to cubic P m 3 ¯ m upon heating, unlike measurements that reported the occurrence of an intermediate long‐range‐tilted tetragonal P4/mbm phase in‐between the orthorhombic and cubic phases. Such disagreement, which may cast some doubts about the extent to which first‐principle methods can be trusted to mimic hybrid perovskites, can be resolved by “only” changing one short‐range tilting parameter in the whole set of effective Hamiltonian coefficients. In such a case, some reasonable values of this specific parameter result in the predictions that i) the intermediate P4/mbm state originates from fluctuations over many different tilted states; and ii) the cubic P m 3 ¯ m phase is highly locally distorted and develops strong transverse antiphase correlation between first‐nearest neighbor iodine octahedral tiltings, before undergoing a phase transition to P4/mbm under cooling.  相似文献   
5.
Pinhão seed is an unconventional source of starch and the pines grow up in native forests of southern Latin America. In this study, pinhão starch was adjusted at 15, 20 and 25% moisture content and heated to 100, 110 and 120 °C for 1 h. A decrease in λ max (starch/iodine complex) was observed as a result of increase in temperature and moisture content of HMT. The ratio of crystalline to amorphous phase in pinhão starch was determined via Fourier transform infra red by taking 1045/1022 band ratio. A decrease in crystallinity occurred as a result of HMT. Polarised light microscopy indicated a loss of birefringence of starch granules under 120 °C at 25% moisture content. Granule size distribution was further confirmed via scanning electron microscopy which showed the HMT effects. These results increased the understanding on molecular and structural properties of HMT pinhão starch and broadened its food and nonfood industrial applications.  相似文献   
6.
7.
The effect of SO2 gas was investigated on the activity of the photo-assisted selective catalytic reduction of nitrogen monoxide (NO) with ammonia (NH3) over a TiO2 photocatalyst in the presence of excess oxygen (photo-SCR). The introduction of SO2 (300 ppm) greatly decreased the activity of the photo-SCR at 373 K. The increment of the reaction temperature enhanced the resistance to SO2 gas, and at 553 K the conversion of NO was stable for at least 300 min of the reaction. X-ray diffraction, FTIR spectroscopy, thermogravimetry and differential thermal analysis, x-ray photoelectron spectroscopy (XPS), elemental analysis and N2 adsorption measurement revealed that the ammonium sulfate species were generated after the reaction. There was a strong negative correlation between the deposition amount of the ammonium sulfate species and the specific surface area. Based on the above relationship, we concluded that the deposition of the ammonium sulfate species decreased the specific surface area by plugging the pore structure of the catalyst, and the decrease of the specific surface area resulted in the deactivation of the catalyst.  相似文献   
8.
通过制备不同晶相结构〔单斜相(m-ZrO_2)、四方相(t-ZrO_2)和无定型(a-ZrO_2)〕ZrO_2载体,再通过沉积沉淀法制得Cu/m-ZrO_2、Cu/t-ZrO_2和Cu/a-ZrO_2催化剂,分别用于催化二乙醇胺脱氢合成亚氨基二乙酸反应。采用XRD、氮气物理吸附脱附、XPS、H_2-TPR、CO_2-TPD对催化剂的结构进行了表征。结果表明,Cu/m-ZrO_2催化剂界面更加有利于Cu~+/Cu~0稳定存在,具有更多的碱性位点,且抗氧化性较好。在二乙醇胺脱氢反应中,Cu/m-ZrO_2催化剂性能最好,反应时间为2.5 h,亚氨基二乙酸收率为97.64%。  相似文献   
9.
10.
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