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   123418篇
  免费   3148篇
  国内免费   446篇
电工技术   1065篇
综合类   2344篇
化学工业   18656篇
金属工艺   5528篇
机械仪表   3805篇
建筑科学   3455篇
矿业工程   633篇
能源动力   1913篇
轻工业   8880篇
水利工程   1602篇
石油天然气   514篇
武器工业   4篇
无线电   11438篇
一般工业技术   23138篇
冶金工业   13655篇
原子能技术   468篇
自动化技术   29914篇
  2023年   362篇
  2022年   360篇
  2021年   915篇
  2020年   764篇
  2019年   684篇
  2018年   15493篇
  2017年   14458篇
  2016年   11155篇
  2015年   1703篇
  2014年   1645篇
  2013年   2980篇
  2012年   5436篇
  2011年   11323篇
  2010年   9787篇
  2009年   6912篇
  2008年   8352篇
  2007年   9103篇
  2006年   1334篇
  2005年   2211篇
  2004年   2084篇
  2003年   2000篇
  2002年   1303篇
  2001年   612篇
  2000年   685篇
  1999年   717篇
  1998年   3611篇
  1997年   2166篇
  1996年   1421篇
  1995年   835篇
  1994年   711篇
  1993年   697篇
  1992年   249篇
  1991年   274篇
  1990年   246篇
  1989年   231篇
  1988年   224篇
  1987年   198篇
  1986年   204篇
  1985年   230篇
  1984年   207篇
  1983年   150篇
  1982年   180篇
  1981年   190篇
  1980年   174篇
  1979年   140篇
  1978年   126篇
  1977年   215篇
  1976年   476篇
  1975年   114篇
  1973年   91篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
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   
72.
The purpose of this study was to assess cortical and cancellous bone responses to unilateral limb immobilization and, subsequently, to remobilization with exercise, in a young adult canine model. Right forelimbs of 14 1-2-year old mongrel dogs were immobilized in a non-weight-bearing position by a bandage for 16 weeks. Six control dogs were untreated. At 16 weeks, seven immobilized and three control dogs were euthanized. The remaining seven immobilized dogs began a recovery protocol consisting of 16 weeks of kennel confinement (without the right forelimb bandaged) followed by 16 weeks of treadmill exercise conducted three times per week. These seven dogs and three control dogs were euthanized at 48 weeks. Bone mineral density of the proximal radii was determined with dual-energy X-ray absorptiometry and humeral middiaphyseal cross-sectional areas were determined with computed tomography. Humeri were tested in cranio-caudal three-point bending to failure. Cancellous bone cores from the lateral humeral condyles had wet apparent density determined and were tested to failure in compression. Mechanical properties, bone density, and cross-sectional areas were compared between immobilized (right forelimb), contralateral weight bearing (left forelimb), and control forelimbs with Kruskal-Wallis and post hoc tests. At 16 weeks, bone mineral density, cortical load, yield, and stiffness as well as cancellous bone failure stress, yield stress, and modulus were significantly lower (p < 0.02) for immobilized limbs than control limbs. Immobilized limb cancellous bone mechanical properties were 28%-74% of control values, and cortical bone mechanical properties were 71%-98% of control values. After 32 weeks of remobilization, cortical and cancellous bone mechanical properties were not different from control values except that cortical bone failure stress and modulus were significantly higher (p < 0.01) between remobilized and control limbs. In summary, 16 weeks of forelimb immobilization was associated with significantly lower mechanical properties, and with greater differences in cancellous than cortical bone properties. Mechanical properties were not different from control values after 32 weeks of recovery that included 16 weeks of treadmill exercise.  相似文献   
73.
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   
74.
75.
76.
77.
78.
This article examines the relationships between the employment suburbanisation from central cities towards their suburbs, and the process of intra-urban specialisation that occurred simultaneously in the fifty largest French metropolitan areas. A methodology is proposed to identify urban subcentres and to analyse the effects of the intra-urban specialisations on suburbanisation patterns. We conclude that the specialisation of both subcentres and central cities has a significant effect on suburbanisation rates. Lastly, an intra-metropolitan shift/share analysis provides additional insights into the employment dynamics of central cities and suburbs during the last twenty years. Received: 25 July 2000 / Accepted: 29 May 2001  相似文献   
79.
Three interesterified vegetable oils (IVOs) prepared from palm, cottonseed, and olive oil, respectively, have been studied for application to Frankfurters. Beef fat (10%) was replaced by one of the IVOs at 20, 60 or 100%. Fatty acid composition of the lipid fraction in the Frankfurters and saturated/unsaturated ratios were established for all batches. We found that total or partial replacement of beef fat by IVOs did not lead to significant changes in appearance, colour, texture, flavour or sensory score. We conclude that IVOs may be successfully applied as beef fat substitutes in Frankfurters.  相似文献   
80.
This study empirically examined the organisational learning effects of the nonfinancial performance information provided by management accounting information systems (MAISs) under advanced manufacturing technology (AMT). In this study, a target costing system and the frequent and quick reporting of information were considered the facilitators of learning. First, we examined the relationships between AMT level and the amount of nonfinancial performance information produced by MAISs. The empirical results showed that there are significant positive relationships between the level of AMT and nonfinancial performance information. With a systems approach, we also proved the impact of the relationships among AMT levels, nonfinancial performance information and learning facilitators on the organisational performance of a firm. The results of our research suggest that under a high level of AMT, for the provision of information to result in an increase of performance through organisational learning, a target costing system must be introduced and a large amount of information should be provided frequently and quickly. The results of this study also showed that although AMT level may be low, fairly well-arranged facilitators and a moderately large amount of information may be necessary for the improvement of performance. In conclusion, effective organisational learning depends on the provision of relevant information as well as efficient learning support mechanisms.  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