全文获取类型
收费全文 | 62166篇 |
免费 | 7070篇 |
国内免费 | 4144篇 |
专业分类
电工技术 | 17712篇 |
综合类 | 5085篇 |
化学工业 | 3280篇 |
金属工艺 | 3094篇 |
机械仪表 | 6683篇 |
建筑科学 | 2989篇 |
矿业工程 | 1665篇 |
能源动力 | 1700篇 |
轻工业 | 1234篇 |
水利工程 | 1381篇 |
石油天然气 | 1174篇 |
武器工业 | 820篇 |
无线电 | 9772篇 |
一般工业技术 | 6097篇 |
冶金工业 | 2252篇 |
原子能技术 | 1293篇 |
自动化技术 | 7149篇 |
出版年
2024年 | 253篇 |
2023年 | 790篇 |
2022年 | 1290篇 |
2021年 | 1588篇 |
2020年 | 1778篇 |
2019年 | 1373篇 |
2018年 | 1395篇 |
2017年 | 2173篇 |
2016年 | 2400篇 |
2015年 | 2696篇 |
2014年 | 3957篇 |
2013年 | 3462篇 |
2012年 | 4630篇 |
2011年 | 4947篇 |
2010年 | 3662篇 |
2009年 | 3845篇 |
2008年 | 3874篇 |
2007年 | 4695篇 |
2006年 | 4145篇 |
2005年 | 3318篇 |
2004年 | 2864篇 |
2003年 | 2394篇 |
2002年 | 1903篇 |
2001年 | 1728篇 |
2000年 | 1505篇 |
1999年 | 1223篇 |
1998年 | 935篇 |
1997年 | 957篇 |
1996年 | 719篇 |
1995年 | 561篇 |
1994年 | 511篇 |
1993年 | 390篇 |
1992年 | 304篇 |
1991年 | 218篇 |
1990年 | 186篇 |
1989年 | 137篇 |
1988年 | 110篇 |
1987年 | 90篇 |
1986年 | 51篇 |
1985年 | 46篇 |
1984年 | 63篇 |
1983年 | 33篇 |
1982年 | 35篇 |
1981年 | 20篇 |
1980年 | 11篇 |
1977年 | 10篇 |
1964年 | 7篇 |
1959年 | 13篇 |
1956年 | 7篇 |
1955年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
一种测量建筑材料和土壤氡面析出率的新方案 总被引:2,自引:1,他引:1
在分析了直接测量法测量氡面析出率的原理和影响测量效果各因素的基础上,对建筑材料、土壤氡面析出率的测量方案进行了探讨,并提出了一种新的测量方案。该方案能够较好地克服氡面析出率测量中各种因素的影响,并且具有装置简单、可大批量重复测量、成本低等优点。 相似文献
152.
Bayazit Mahmut; Hammer Tove Helland; Wazeter David L. 《Canadian Metallurgical Quarterly》2004,89(4):738
Methodological problems in studies of union commitment were identified and illustrated with data from 4,641 members and 479 stewards in 297 local teachers' unions. Using a 20-item union commitment scale, results confirmed the existence of 3 substantive factors and 1 method factor at the individual level of analysis: loyalty to the union, responsibility to the union, willingness to work for the union, and a factor of negatively worded items. Tests of measurement invariance showed that the scale captured commitment for rank-and-file members but not for union stewards. The authors also found partial measurement invariance between long-time and newer members and full measurement invariance between men and women. Finally, the authors found that violation of the statistical assumption of independence reduced model fit when individual commitment scores were analyzed without attention to the hierarchical nature of the data. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
153.
Predictions of the discharge and the associated sediment concentration are very useful ingredients in any water resources reservoir design, planning, maintenance, and operation. Although there are many empirical relationships between the discharge and sediment concentration amounts, they need estimation of model parameters. Generally, parameter estimations are achieved through the regression method (RM), which has several restrictive assumptions. Such models are locally valid and their structures and parameter values are questionable from region to others. This paper proposes a new approach for sediment concentration prediction provided that there are measurements of discharge and sediment concentration. The basis of the methodology is a dynamic transitional model between successive time instances based on two variables, namely, discharge and sediment concentration measurements. The transition matrix elements are estimated from the measurements through a special form of the artificial neural networks as perceptrons. The sediment concentration predictions from discharge measurements are achieved through a perceptron Kalman filtering (PKF) technique. In the meantime, this technique also provides temporal predictions. A certain portion of the measurement sequence is employed for the model parameter estimations through training and the remaining part is used for the model verification. Detailed comparisons between RM and PKF approaches are presented and, finally, it is shown that the latter model works dynamically by simulating the observation scatter diagram in the best possible manner with smaller prediction errors. The application of the methodology is performed for the discharge and sediment concentration measurements obtained from the Mississippi River basin at St. Louis, Missouri. It is found that the PKF methodology has smaller average relative, root-mean-square, and absolute errors than RM. Furthermore, graphical representation, such as the scatter and frequency diagrams, indicated that the PKF approach has superiority over the RM. 相似文献
154.
万用表电压测量法查找直流接地故障 总被引:1,自引:0,他引:1
直流电源是发电站的主要设备,一旦直流系统发生接地故障,将会严重地危及到发电站的安全和经济运行。因此,本文介绍一种简单易行,能快速找出直流接地点的方法一万用表电压测量法。 相似文献
155.
无磷洗涤剂的现状及存在问题 总被引:1,自引:0,他引:1
含磷洗涤剂中含的“磷”主要是STPP,作用很大但对环境的危害也很大,它是造成地表水体出现富营养化的重要原因之一,所以许多地方政府都在实行“禁磷”——禁止销售、使用含磷洗涤剂。现在我国使用的无磷洗涤剂中代磷助剂主要有3种,其中4A沸石用量最大,但它会对环境带来新的问题。主要介绍了含磷洗涤剂的危害及无磷洗涤剂的应用现状,指出了我国禁磷后将面临的问题。 相似文献
156.
157.
158.
J. Gegner Priv.‐Doz. Dr. W. Nierlich M. Brückner 《Materialwissenschaft und Werkstofftechnik》2007,38(8):613-623
Due to clearly distinguishable damage symptoms, it is differentiated between the surface and sub‐surface failure mode of rolling bearings. Material states red out by X‐ray diffraction (XRD) residual stress measurements point to a variety of loading conditions especially at raceway surfaces that are associated with several competing failure mechanisms. The corresponding lifetime reduction can range from the lower fatigue strength region to material ratcheting in extreme cases. Relevant position of the microstructural changes and nature of the failure mechanisms are characterized. The time alteration of the XRD material parameters measured at or near the surface and at the depth of the maximum equivalent stress correlates, in a different manner, with the statistical parameter of the 10 % bearing life. Both failure modes are illustrated by concrete examples. Contaminated lubricant and boundary lubrication, which represent practically important surface‐induced failures, are discussed in more detail. Gray staining, i.e. shallow pitting, often occurs without distinct indication of global material aging by means of XRD characteristics. Here, scanning electron microscopy observations and electron microprobe analyses point to corrosion fatigue as acting surface failure mechanism. The interaction between material and lubricant under complex loading regimes particularly of mixed friction and corrosion opens further failure research areas in the field of tribology. 相似文献
159.
160.
电阻式电压传感器结构设计及误差特性研究 总被引:2,自引:0,他引:2
从路的角度分析了电阻式电压传感器的误差特性 ,从场的角度比较了几种结构方案的幅值误差、相角误差及最大场强 ,优化出最佳方案 ,并分析了该方案幅值误差的频率特性 相似文献