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   266793篇
  免费   20830篇
  国内免费   13463篇
电工技术   17502篇
技术理论   52篇
综合类   36933篇
化学工业   28647篇
金属工艺   10986篇
机械仪表   15545篇
建筑科学   37571篇
矿业工程   15014篇
能源动力   8476篇
轻工业   14123篇
水利工程   14128篇
石油天然气   11085篇
武器工业   2626篇
无线电   17427篇
一般工业技术   18646篇
冶金工业   15388篇
原子能技术   2665篇
自动化技术   34272篇
  2024年   591篇
  2023年   2366篇
  2022年   4593篇
  2021年   5776篇
  2020年   6062篇
  2019年   5070篇
  2018年   4731篇
  2017年   5792篇
  2016年   7057篇
  2015年   8005篇
  2014年   14857篇
  2013年   13445篇
  2012年   17787篇
  2011年   18927篇
  2010年   15042篇
  2009年   15692篇
  2008年   14658篇
  2007年   19331篇
  2006年   18333篇
  2005年   15982篇
  2004年   13681篇
  2003年   12302篇
  2002年   10135篇
  2001年   8542篇
  2000年   7190篇
  1999年   5877篇
  1998年   4452篇
  1997年   3908篇
  1996年   3610篇
  1995年   3071篇
  1994年   2723篇
  1993年   2059篇
  1992年   1810篇
  1991年   1331篇
  1990年   1176篇
  1989年   1029篇
  1988年   821篇
  1987年   559篇
  1986年   416篇
  1985年   353篇
  1984年   331篇
  1983年   243篇
  1982年   217篇
  1981年   166篇
  1980年   131篇
  1979年   111篇
  1978年   62篇
  1977年   72篇
  1976年   55篇
  1975年   55篇
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
991.
�����׿�����������ǰ��   总被引:2,自引:0,他引:2  
本文分析了四川盆地川西地区中三叠统雷口坡组油气成藏的地质条件,进而评价和预测了雷三段的有利勘探地区和构造。  相似文献   
992.
针对UO_3的N_2还原反应建立了多孔挡板流化床反应器模型。将多孔挡板床的每一级视为具有相似的流动状态,每一级的上、下段称为稀、密相段,分别用鼓泡床和活塞流模型描述。该模型的计算值与¢63mm的多孔挡板床H_2还原UO_3的热态实验值能较好的吻合。还预计了还原温度、操作气速和挡板结构参数对固相转化率的影响,为该类反应器的工业设计、放大和优化提供了依据。  相似文献   
993.
本文介绍了可在微机上实现大规模电信网践模拟的实用化软件SIMUNET,及基所采用的无时钟事件追踪法。该软件可用规划中的电信网络和实际网路的模拟及性能评价,也可用于不同流量控制算法的评价。文中给出一个评价实例。  相似文献   
994.
The primary purpose of this work is to review the literature about what is and is not known about using ethylene vinyl acetate (EVA0 copolymer as the encapsulant (or pottant) material in photovoltaic (PV) modules. Secondary purposes include elucidating the complexity of the encapsulation problem, providing an overview about encapsulation of PV cells and modules, providing a historical overview of the relevant research and development on EVA, summarizing performance losses reported for PV systems deployed since ca. 1981, and summarizing the general problems of polymer stability in a solar environment. We also provide a critical review of aspects of reported work for cases that we believe are important.Failure modes resolved in the early work to establish reliability of deployed modules and the purposes and properties of pottants, are summarized. Typical performance losses in large field-deployed, large-scale systems ranging from 1% to 10% per year are given quantitatively, and qualitative reports of EVA discoloration are summarized with respect to ultraviolet (UV), world-wide location and site dependence.The general stability of polymers and their desirable bulk properties for solar utilization are given. The stabilization formulation for EVA, its effectiveness, and changes in it during degradation are discussed. The degradation mechanisms for the base resin, e.g., unstabilized Elvax 150TM, and stabilized EVA are indicated for literature dating to the early 1950s, and the role played by unsaturated chromophores is indicated. The limited number of studies relating discoloration and PV cell efficiency are summarized.Observed degradation of EVA or the unstabilized base resin in the laboratory and examples used to measure the degradation are summarized in sections entitled: (1) thermally-induced degradation; (2) photodegradation and photothermal degradation of EVA in different temperature regimes; (3) photobleaching and photodegradation of the UV absorber and cross-linking agent; (4) acetic acid and metal and metal-oxide catalyzed oxidative degradation; and (5) discolaration and PV cell efficiency losses.Processing effects/influences on EVA stability are discussed in sections entitled: (1) EVA raw materials and extruded, uncured films; (2) thermal encapsulation processes; (3) effects of lamination, curing, and curing peroxide on gel content and chromophores formed; and (4) incomplete shielding of curing-generated chromophores. A summary is given for the limited number of accelerated lifetime testing efforts and examples of erroneous service lifetime predictions for EVA are discussed. The known factors that effect the discoloration rate of several EVA formulations are discussed in which the reduction in rate by using UV-absorbing superstrates is a prime example. A summary is given of what is and is not known about EVA degradation mechanisms, degradation from exposures in field-deployed modeules and/or laboratory testing, and factors that contribute to EVA stability or degradation. Finally, conclusions about using Elvax 150 in EVA formulations are summarized, and future prospects for developing the next-generation pottant for encapsulating PV modules are discussed.  相似文献   
995.
王皓月  汤俊雄 《计量学报》1996,17(4):310-313
本文报导了用铷原子7800A光电流谱来稳定半导体激光器的工作,在光电流谱线的中部发现了有利于稳频的凹陷,将半导体激光器频率锁定在凹陷的中央,估算的频率稳定度为1MHz。  相似文献   
996.
本文根据压力传感器(静态)国家计量检定规程,对压力传感器静态检定装置进行误差分析,得出该装置的测量不确定度。最后,通过实例说明实际测量不确定度与理论分析相符合。  相似文献   
997.
管内含蜡原油降温过程中的放热问题   总被引:1,自引:0,他引:1  
利用两种管段和两种原油,进行了降温过程自然对流放热参数的测试和研究。采用滞流点确定降温第二阶段中凝油边界的移动,区分管内的对流区与导热区。由实验数据回归得到适用于含蜡原油降温过程的自然对流放热准则方程式。用该方程式计算的自然对流放热系数值比按无限空间自然对流准则方程式计算值小20%~50%。  相似文献   
998.
Recent research suggests that implementing information systems presents considerable difficulties and that many implementations are total or partial failures. This paper argues that what both practitioners and students require are richer and more acceptable models of information systems implementation. Accordingly, case study data concerning the introduction of manufacturing resource planning (MRP II) are used to illustrate five patterns of behaviour (rites) which capture important social actions; and four components of changing psychological orientation. It is argued that appreciation of the rites and psychological developments identified here will support effective change programmes. Guidelines based on our model are provided for practitioners.  相似文献   
999.
以热轧宽卷板作为焊管坯料,在宽度方向上纵向剪切成规格的带钢,各种焊管规格坯料搭配剪裁,提高管坯利用率  相似文献   
1000.
The engineering and management of human safety is an important societal objective that includes extensive efforts by governments, both legislative and administrative, to enhance the health and safety of the public. Although the achievement of safety goals depend primarily on individuals and organizations responsible for safety, much support is drawn from expertise in diverse scientific and engineering disciplines. The activities range from structural safety (dams, tunnels, bridges to tall buildings) to safe operation of hazardous industrial installations (energy generation facilities, LNG terminals, petrochemical plants) to transportation systems (airline, rail, car safety) to technologies designed to minimize adverse impacts on the environment. All these activities are crucially concerned with risk: with the likelihood and the probable effects of various measures on life and health. We have developed a unified rationale and a clear basis for effective strategic management of risk across diverse sectors. Safety is an important objective in society but it is not the only one. The allocation of society's resources devoted to safety must be continually appraised in light of competing needs, because there is a limit on the resources that can be expended to extend life. The paper presents the Life Quality Index (LQI) as a tool for the assessment of risk reduction initiatives that would support the public interest and enhance safety and quality of life. The paper provides an intuitive reformulation of the LQI as equivalent to a valid utility function that is consistent with the principles of rational decision analysis. The LQI is further refined to consider the issues of discounting of life years, competing background risks, and population age and mortality distribution. The LQI is applied to quantify the societal willingness-to-pay, which is an acceptable level of public expenditure in exchange for a reduction in the risk of death that results in improved life-quality.  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