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   14178篇
  免费   1446篇
  国内免费   686篇
电工技术   989篇
综合类   916篇
化学工业   2502篇
金属工艺   829篇
机械仪表   861篇
建筑科学   1380篇
矿业工程   362篇
能源动力   399篇
轻工业   854篇
水利工程   284篇
石油天然气   752篇
武器工业   125篇
无线电   1744篇
一般工业技术   1640篇
冶金工业   602篇
原子能技术   233篇
自动化技术   1838篇
  2024年   83篇
  2023年   260篇
  2022年   493篇
  2021年   669篇
  2020年   499篇
  2019年   397篇
  2018年   457篇
  2017年   535篇
  2016年   442篇
  2015年   534篇
  2014年   701篇
  2013年   884篇
  2012年   869篇
  2011年   979篇
  2010年   847篇
  2009年   754篇
  2008年   706篇
  2007年   718篇
  2006年   725篇
  2005年   528篇
  2004年   473篇
  2003年   582篇
  2002年   631篇
  2001年   542篇
  2000年   384篇
  1999年   341篇
  1998年   246篇
  1997年   213篇
  1996年   184篇
  1995年   131篇
  1994年   117篇
  1993年   85篇
  1992年   66篇
  1991年   58篇
  1990年   46篇
  1989年   31篇
  1988年   19篇
  1987年   15篇
  1986年   18篇
  1985年   12篇
  1984年   7篇
  1983年   6篇
  1982年   4篇
  1981年   4篇
  1980年   8篇
  1979年   3篇
  1973年   1篇
  1951年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The application and effects for a series of rare earth (RE) drought resistant materials used in arid, salina,hungriness, wind defending and sand fixing matter, withdraw farming and return to grass and forest in western of China were reported.The important discussion was technological innovation within two years: such as seed clothing agent,RE liquid field film, RE grass and woods transplant living agent, and RE complex pesticide development and application.  相似文献   
42.
The distribution of neutrons produced by the reaction of 50MeV/u ^12C-ion on a thick Cu target are studied.The neutrons are measured with threshold activation detectors.Al.F,C,Al and In activation samples were used to measure neutrons with energy greater than 7,11,20,50MeV and thermal neutrons,respectively,The fluence rate,energy and angular distributions of neutrons,total neutron yield of 12C-ion and the emission rate in the forward direction of neutrons over 11 and 20MeV were obtained.  相似文献   
43.
44.
熊友泉 《材料保护》2004,37(3):12-13
为了降低电解制氢的电耗,提高经济效益.作者对枪黑色Sn-Ni合金在NaOH溶液中的析氢电流化性能进行了研究.试验测出的电化学参数表明,在NaOH溶液中电解制氢时,枪黑色Sn-Ni合金的析氢电催化性能比45钢和高硫镍的强很多,而且具有很高的电化学稳定性.用它取代目前工业上使用的45钢和高硫镍作为电解制氢的阴极,可显著降低制氢的生产成本.  相似文献   
45.
46.
Placenta accreta spectrum (PAS) accounts for 7% of maternal mortality and is associated with intraoperative and postoperative morbidity caused by massive blood loss, infection, and adjacent organ damage. The aims of this study were to identify the protein biomarkers of PAS and to further explore their pathogenetic roles in PAS. For this purpose, we collected five placentas from pregnant subjects with PAS complications and another five placentas from normal pregnancy (NP) cases. Then, we enriched protein samples by specifically isolating the trophoblast villous, deeply invading into the uterine muscle layer in the PAS patients. Next, fluorescence-based two-dimensional difference gel electrophoresis (2D-DIGE) and MALDI-TOF/MS were used to identify the proteins differentially abundant between PAS and NP placenta tissues. As a result, nineteen spots were determined as differentially abundant proteins, ten and nine of which were more abundant in PAS and NP placenta tissues, respectively. Then, specific validation with western blot assay and immunohisto/cytochemistry (IHC) assay confirmed that heat shock 70 kDa protein 4 (HSPA4) and chorionic somatomammotropin hormone (CSH) were PAS protein biomarkers. Further tube formation assays demonstrated that HSPA4 promoted the in vitro angiogenesis ability of vessel endothelial cells, which is consistent with the in vivo scenario of PAS complications. In this study, we not only identified PAS protein biomarkers but also connected the promoted angiogenesis with placenta invasion, investigating the pathogenetic mechanism of PAS.  相似文献   
47.
A superior drug controlled release system capable of achieving efficient osteogenesis is in imperative demand because of limited bone substitute tissue for the treatment of bone defect. In the present study, we investigated the potential of using poly(ε-caprolactone)–hydroxyapatite (PCL–HA) composite microspheres as an injectable bone repair vehicle by controlled release of alendronate (AL), a medicine that belongs to the bisphosphonates family. The PCL/HA–AL microspheres were prepared with solid/oil/water emulsion technique, which included two processes: (1) AL was loaded on the hydroxyapatite nanoparticles; (2) the HA–AL complex was built in the PCL matrix. The spherical PCL/HA–AL microspheres were characterized with its significantly improved encapsulation efficiency of hydrophilic AL and better sustained release. Human bone mesenchymal stem cells (hMSCs) were cultured on the surface of these microspheres and exhibited high proliferative profile. Specifically, in osteogenic medium, hMSCs on the surface of PCL/HA–AL microspheres displayed superior osteogenic differentiation which was verified by alkaline phosphatase activity assay. In conclusion, by presenting strong osteogenic commitment of hMSCs in vitro, the PCL/HA–AL microspheres have the potential to be used as an injectable vehicle for local therapy of bone defect.  相似文献   
48.
分析Peercast流媒体直播系统的结构。根据分析结果,指出其在实际网络应用中的弊端。改进下载流程,将单线程下载改为多线程下载,设计多线程F载任务分配模块。重新设计缓冲区,维护多对一情况F的缓冲区。介绍在多线程下载情况下直播延时方面的改进,对改进方案的性能进行分析。  相似文献   
49.
针对当前可靠性预测模型的预测精度问题,提出一种增强贝叶斯组合的短期软件可靠性预测模型。该模型以基于小波分解的单个可靠性预测模型作为基本预测模型, 根据当前相邻几个失效时间间隔的预测精度,更新组合模型中各个基本预测模型的权重,解决了贝叶斯组合模型权重计算采用全部历史数据而导致某个基本预测模型权值占主导地位的缺陷,提高了贝叶斯组合模型对软件可靠性的预测精度。实验结果表明,增强贝叶斯组合预测模型的预测精度不仅优于单一的预测方法,而且也优于传统的软件组合预测模型,能显著提高软件可靠性预测的精度和模型对数据的适应性。  相似文献   
50.
Many emerging online stream processing services require the consideration of quality of service (QoS), which is highly dependent on the placement of services at various hosts. This paper investigates the QoS-aware placement problems of stream processing services under different contexts. On condition that the client demands are stable, the QoS-aware placement problem aiming to minimize the cost when servers are CPU-uncapacitated, is equivalent to the set cover problem, and can be solved by a greedy algorithm with approximation factor O(log?n), where n is the number of clients. However, when CPU capacity constraints on servers are taken into account, the QoS-aware placement problem cannot be approximated unless P=NP. Therefore, we propose two heuristic algorithms: (1) ISCA (Iterated Set Cover-based Algorithm) and (2) KBA (Knapsack-Based Algorithm). We also consider the placement problem of client demands increasing over time. Two objectives, called extension factor and system lifetime, are proposed for demand increment-blind and increment-aware models respectively. Both of them can be solved by extending ISCA and KBA. The experimental results show that ISCA and KBA have distinct effects on different demand sizes. ISCA is more efficient when client demands are relatively small, while KBA performs better for larger demands.  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