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   1573篇
  免费   300篇
  国内免费   174篇
电工技术   528篇
综合类   182篇
化学工业   43篇
金属工艺   21篇
机械仪表   63篇
建筑科学   71篇
矿业工程   47篇
能源动力   72篇
轻工业   25篇
水利工程   33篇
石油天然气   309篇
武器工业   24篇
无线电   169篇
一般工业技术   42篇
冶金工业   28篇
原子能技术   46篇
自动化技术   344篇
  2024年   3篇
  2023年   28篇
  2022年   66篇
  2021年   62篇
  2020年   72篇
  2019年   62篇
  2018年   69篇
  2017年   81篇
  2016年   69篇
  2015年   90篇
  2014年   126篇
  2013年   115篇
  2012年   158篇
  2011年   147篇
  2010年   107篇
  2009年   95篇
  2008年   97篇
  2007年   117篇
  2006年   112篇
  2005年   83篇
  2004年   61篇
  2003年   34篇
  2002年   35篇
  2001年   24篇
  2000年   28篇
  1999年   23篇
  1998年   14篇
  1997年   10篇
  1996年   10篇
  1995年   10篇
  1994年   9篇
  1993年   8篇
  1992年   7篇
  1991年   2篇
  1990年   2篇
  1989年   2篇
  1988年   1篇
  1986年   3篇
  1985年   1篇
  1981年   1篇
  1959年   2篇
  1951年   1篇
排序方式: 共有2047条查询结果,搜索用时 31 毫秒
1.
新疆阿尔泰南缘北塔山组火山岩较为发育。本文对新疆阿尔泰南缘默色克奥依一带北塔山组火山岩进行地球化学分析和年代学研究。研究表明:U-Pb锆石年龄为(384.3±4.0Ma)Ma,为中泥盆世;该组火山岩属于钾质钙碱性系列,全碱(Na2O+K2O=2.07%-6.49%)和YiO2(0.24%-0.64%)含量较低,K2O含量(1.11%-4.04%)变化较大,稀土元素含量(∑REE)较低。稀土元素显示其轻稀土富集且分馏不明显;δEu值均大于1,为Eu富集型;δCe值均小于1,为Ce亏损型。该组火山岩微量元素P明显富集,K、Rb、Ba等大离子亲石元素(LILE)略富集,亏损Ta、Zr、Hf、Ti等高场强元素(HISF)。结合前人研究,本文认为默色克奥依一带北塔山组火山岩形成于岛弧及与俯冲过程有关的活动大陆边缘造山带环境,为泥盆纪古大洋俯冲消减过程中,俯冲板片脱水产生的流体交代上覆地幔楔部分熔融的产物。  相似文献   
2.
四川盆地震旦系灯影组丘滩体发育分布及对储层的控制   总被引:2,自引:0,他引:2  
四川盆地震旦系灯影组储层主要发育于微生物丘滩体中,但不同成因类型丘滩体对储层的影响尚不明确。通过钻井岩心和野外露头观测、岩石薄片分析和物性测试等手段,将丘滩体划分为丘基、丘核、丘翼和丘坪4种微相,并根据微相的组合划分出台缘并进型丘滩体、台缘追补型丘滩体和台内饥饿型丘滩体3种丘滩体类型。台缘并进型丘滩体的生长速率快,与可容空间的增加速率相同步,其微相组合以"丘基+丘核"垂向加积为主,在海平面升降过程中易暴露于大气淡水环境,造成海水胶结作用弱、淡水溶蚀作用强,形成以格架溶蚀孔洞为主的储集空间,储层物性好,储集性能最优。台缘追补型丘滩体的生长速率次之,其早期的生长速率滞后于可容空间增长速率但后期逐渐同步,呈现出"丘基+丘核+丘翼/丘坪"侧向加积的微相组合;由于暴露于大气淡水环境的几率减小,其海水胶结作用强,淡水溶蚀作用减弱,储集性能稍差。台内饥饿型丘滩体的生长速率最慢,严重滞后于可容空间的增长速率,呈"云坪(丘基)+丘核+丘坪"微相组合,难以暴露于大气淡水环境,淡水溶蚀作用最弱,储集性能也最差。台缘带古地貌对丘滩体的生长速率影响较大,可控制丘滩体的沉积样式。陡坡台缘带有利于台缘并进型丘滩体发育,缓坡台缘带主要发育台缘追补型丘滩体,台内高地中发育台内饥饿型丘滩体。胡家坝-阆中-安岳地区一带的震旦系灯影组四段发育于典型的陡坡台缘带,是储层发育的最有利相带。  相似文献   
3.
张宗堂  陈喆  戴卫国 《计算机应用》2019,39(5):1364-1367
针对传统集成算法不适用于不平衡数据分类的问题,提出基于间隔理论的AdaBoost算法(MOSBoost)。首先通过预训练得到原始样本的间隔;然后依据间隔排序对少类样本进行启发式复制,从而形成新的平衡样本集;最后将平衡样本集输入AdaBoost算法进行训练以得到最终集成分类器。在UCI数据集上进行测试实验,利用F-measure和G-mean两个准则对MOSBoost、AdaBoost、随机过采样AdaBoost(ROSBoost)和随机降采样AdaBoost(RDSBoost)四种算法进行评价。实验结果表明,MOSBoost算法分类性能优于其他三种算法,其中,相对于AdaBoost算法,MOSBoost算法在F-measureG-mean准则下分别提升了8.4%和6.2%。  相似文献   
4.
A directional control method (DCM) for power flows on a set of interface lines between two regions of power system considering static voltage stability margin is developed in this paper. A surface approximation approach is firstly used to obtain the relationship between the interface flow solution and the generation direction of generator (the portion of generation variation in each participating generator to satisfy the desired power increase on the interface and the system loss). Then, an optimization model is built to determine the optimum dispatching scheme of generators. This method not only can control the total power on the interface to satisfy the power demand but also can realize the directional control of power on each interface line based on the needs of operation. The proposed DCM is further extended to determine the optimum dispatching scheme of generators for maximizing the interface flow margin (IFM), which is the active power margin of the key transmission lines between two regions of power system constrained by static voltage stability. A modified continuation power flow (MCPF) is used to show and evaluate the impacts of the DCM on the IFM. The New England 39-bus system and the IEEE 300-bus system have been employed to verify the effectiveness of the DCM.  相似文献   
5.
Operator reliability in complex systems is influenced by various performance shaping factors (PSFs). Time is a particularly important PSF; however, empirical studies of human reliability analysis (HRA) are rarely focused on modeling the effect of time PSF on human error probability (HEP). This study contributes to HRA literature by investigating the empirical relationship between time margin and HEP. Time margin is defined as the difference between the time available to complete a task and the time required to successfully complete the task, divided by the required time. We investigate and compare two models (logistic and linear) to explain HEP based on time margin. The empirical HEP data for model testing were extracted from a microworld simulator (Study 1) and a full-scope simulator (Study 2) in two existing studies relevant to procedural tasks in nuclear power plants. For Study 1, both models exhibited an acceptable, equivalent explanatory power; for Study 2, although both models exhibited an acceptable explanatory ability, the logistic model explained more variance in HEP. Our findings indicate the potential of the logistic model in explaining and predicting HEP based on time margin in time-critical tasks.  相似文献   
6.
南阳盆地是白垩纪晚期以来持续发育的断陷盆地,通过对南阳盆地钻孔及河流阶地的研究,并结合ESR测年方法,对南阳盆地北缘更新世以来的构造演化特征进行了分析。结果表明:研究区冲洪积层主要由全新统、上更新统、中更新统组成,分别构成了湍河的Ⅰ级、Ⅱ级、Ⅲ级阶地;中更新统以来,研究区垂直升降运动较为明显,湍河下蚀作用十分强烈,多级阶地的结构体现了更新世中晚期以来的总体持续下蚀过程,且时间和空间上表现出不均衡性,其下蚀速率由慢至快又逐渐减缓。  相似文献   
7.
This work proposes a novel design method for generalized order lead/lag compensators. With respect to the traditional lead/lag compensator, it introduces a new parameter, β, which is a non-integer number. A new design method of the compensator is introduced in order to quantify its design parameters. Compared to its integer order counterpart, the generalized order lead compensator facilitates with the unique solution of desired design specifications with maximum phase at the desired gain cross-over frequency to achieve reshaping of the loop frequency domain characteristics. On the other hand, generalized order lag compensator is designed so as to allow minimum phase lag at the new gain crossover frequency. Examples with simulation and real-time results are presented to validate the efficacy of the proposed approach.  相似文献   
8.
The technology of explosion venting with a duct can effectively reduce the destructive effect generated from gas accidental explosion in the place of intensive industrial production, while it is not applicable to the production site designed for the technology without ducts. Since it is not clear how to quantitatively evaluate the regularity of energy release in explosion venting especially under high cracking pressure, it is still challenging on the safe application of the technology. This paper aimed to explore the basic characteristics of explosion pressure in a 20 L sphere vessel and venting duct during hydrogen explosion venting for safety design. The effects of the cracking pressure and duct length on explosion pressure at different positions were systematically analyzed according to the test of the hydrogen with different concentrations in an explosion venting device with a duct. Comparing the results of explosion pressure characteristics with experiments conducted in a device without a duct, and quantitatively analyzing the pressure release rule, manifested that the presence of the duct reduced the explosion venting efficiency to a certain extent. In the meantime, by comparing the standard NFPA 68 (Pcra ≤ 0.075 MPa, 0.1 m3 ≤ V ≤ 10,000 m3) for safety design of explosion venting devices, the extended implementation scheme with a venting duct was assessed, implying that the calculation results from NFPA 68 were relatively stable in over-scope (Pcra > 0.075 MPa, V < 0.1 m3) measurement. In future industrial productions, the design of high-pressure hydrogen venting is suggested, leaving sufficient safety margin when referring to the result.  相似文献   
9.
武双杰  马保振 《煤矿机械》2007,28(8):110-113
因小孔类零件检测比较困难,使用快速检测装置有助于小孔类零件同轴度误差的检测速度和精度的提高。列举实例采用2种方法进行对比分析,解决了在批量生产条件下小型孔类零件的同轴度误差测量。  相似文献   
10.
总结高层建筑工程施工过程中对垂直度的各种控制方法,及其精度与误差分析,结合工程实例介绍各种控制方法的选择.  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