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   145466篇
  免费   5896篇
  国内免费   3248篇
电工技术   4175篇
技术理论   6篇
综合类   6405篇
化学工业   21898篇
金属工艺   8723篇
机械仪表   6822篇
建筑科学   6935篇
矿业工程   2814篇
能源动力   3069篇
轻工业   7129篇
水利工程   2354篇
石油天然气   4975篇
武器工业   471篇
无线电   15642篇
一般工业技术   23383篇
冶金工业   6020篇
原子能技术   992篇
自动化技术   32797篇
  2024年   261篇
  2023年   1029篇
  2022年   1777篇
  2021年   2544篇
  2020年   1891篇
  2019年   1693篇
  2018年   16033篇
  2017年   15226篇
  2016年   11632篇
  2015年   3000篇
  2014年   3308篇
  2013年   3732篇
  2012年   6842篇
  2011年   13380篇
  2010年   11913篇
  2009年   8796篇
  2008年   10038篇
  2007年   10977篇
  2006年   3383篇
  2005年   4010篇
  2004年   3179篇
  2003年   3344篇
  2002年   3055篇
  2001年   2202篇
  2000年   1821篇
  1999年   1798篇
  1998年   1319篇
  1997年   1111篇
  1996年   1164篇
  1995年   866篇
  1994年   654篇
  1993年   480篇
  1992年   383篇
  1991年   297篇
  1990年   209篇
  1989年   169篇
  1988年   164篇
  1987年   90篇
  1986年   69篇
  1985年   39篇
  1984年   38篇
  1968年   43篇
  1966年   42篇
  1965年   44篇
  1959年   35篇
  1958年   37篇
  1957年   36篇
  1956年   34篇
  1955年   63篇
  1954年   68篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
The purpose of this paper is to evaluate two methods of assessing the productivity and quality impact of Computer Aided Software Engineering (CASE) and Fourth Generation Language (4GL) technologies: (1) by the retrospective method; and (2) the cross-sectional method. Both methods involve the use of questionnaire surveys. Developers' perceptions depend on the context in which they are expressed and this includes expectations about the effectiveness of a given software product. Consequently, it is generally not reliable to base inferences about the relative merits of CASE and 4GLs on a cross-sectional comparison of two separate samples of users. The retrospective method that requires each respondent to directly compare different products is shown to be more reliable. However, there may be scope to employ cross-sectional comparisons of the findings from different samples where both sets of respondents use the same reference point for their judgements, and where numerical rather than verbal rating scales are used to measure perceptions.  相似文献   
22.
Presuppositions of utterances are the pieces of information you convey with an utterance no matter whether your utterance is true or not. We first study presupposition in a very simple framework of updating propositional information, with examples of how presuppositions of complex propositional updates can be calculated. Next we move on to presuppositions and quantification, in the context of a dynamic version of predicate logic, suitably modified to allow for presupposition failure. In both the propositional and the quantificational case, presupposition failure can be viewed as error abortion of procedures. Thus, a dynamic assertion logic which describes the preconditions for error abortion is the suitable tool for analysing presupposition.  相似文献   
23.
24.
Different hierarchical models in pattern analysis and recognition are proposed, based on occurrence probability of patterns. As an important application of recognizing handprinted characters, three typical kinds of hierarchical models such asM 89-89,M 89-36 andM 36-36 have been presented, accompanied by the computer algorithms for computing recognition rates of pattern parts. Moreover, a comparative study of their recognition rates has been conducted theoretically; and numerical experiments have been carried out to verify the analytical conclusions made. Various hierarchical models deliberated in this paper can provide users more or better choices of pattern models in practical application, and lead to a uniform computational scheme (or code). The recognition rates of parts can be improved remarkably by a suitable hierarchical model. For the modelM 89-36 in which case some of the Canadian standard handprinted characters have multiple occurrence probabilities, the total mean recognition rates of the given sample may reach 120% of that by the model proposed by Li et al., and 156% of that obtained from the subjective experiments reported by Suen.  相似文献   
25.
EEPROM的参数设置控制着系统的基本功能,设置的改变对系统的起动和运行起着重要作用。本文详细地列出了EEPROM的地址分配表,并通过上机实验对那些重要的地址了详细的说明。  相似文献   
26.
实验研究了以工业硫酸亚铁及废硫酸为原料,分别采用NaClo_3、KClO_3、H_2O_2及(o_2+NaNO_2)为氧化剂成聚硫酸铁的条件。在以(O_2+NaNO_2)为氧化剂的合成过程中,发现添加微量助催化剂(型号为HG-1、HG-2和HG-3),对提高合成速度及反应过程的稳定性具有重大作用。  相似文献   
27.
本文报导了最近研制成功的用于提高国产氩离子激光器相干长度的相干扩展器。利用这种基于F-P理论的相干扩展器可以使国产氩离子激光器的相干长度由不足5cm一下提高到m的量级,功率损耗在25%~40%之间,有着很好的应用前景。  相似文献   
28.
A human fibroblastic cell line transformed by the SV40-T antigen sequence and continuously cultured for 7 months displayed large periodic variations in cell proliferation. This contrasted with other characteristics of this cell line that remained constant: mosaic cell shape, absence of cell contact inhibition, and predominance of a hypodiploid population. Similar fluctuations in proliferative capacity were also found during the long-term growth of a transformed but nonimmortalized human fibroblastic line prior to senescence, and in the established hamster fibroblastic Nil cell line. This growth pattern suggests a recurrent stimulation of growth in these three transformed cell lines. The proliferation pattern from cultured transformed cells may thus be complex and requires further investigation. These variations presumably influence major cell functions. This observation has important implications for the analysis of data from such cell lines.  相似文献   
29.
30.
从光率体出发,利用坐标轴旋转,导出了光在晶体中传播时折射率及振动方向的公式。  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