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   509019篇
  免费   6794篇
  国内免费   1629篇
电工技术   9454篇
综合类   739篇
化学工业   75948篇
金属工艺   16872篇
机械仪表   14490篇
建筑科学   13450篇
矿业工程   1315篇
能源动力   14443篇
轻工业   52367篇
水利工程   4183篇
石油天然气   5307篇
武器工业   13篇
无线电   65220篇
一般工业技术   94205篇
冶金工业   95478篇
原子能技术   7845篇
自动化技术   46113篇
  2021年   3687篇
  2020年   2765篇
  2019年   3451篇
  2018年   5616篇
  2017年   5493篇
  2016年   5899篇
  2015年   4311篇
  2014年   7362篇
  2013年   23985篇
  2012年   12679篇
  2011年   17897篇
  2010年   13920篇
  2009年   15901篇
  2008年   16745篇
  2007年   16742篇
  2006年   15104篇
  2005年   13817篇
  2004年   13449篇
  2003年   13141篇
  2002年   12549篇
  2001年   13019篇
  2000年   12114篇
  1999年   13005篇
  1998年   31022篇
  1997年   21980篇
  1996年   16961篇
  1995年   13075篇
  1994年   11649篇
  1993年   11347篇
  1992年   8303篇
  1991年   7914篇
  1990年   7618篇
  1989年   7369篇
  1988年   7128篇
  1987年   5905篇
  1986年   5918篇
  1985年   7112篇
  1984年   6409篇
  1983年   5957篇
  1982年   5409篇
  1981年   5589篇
  1980年   5169篇
  1979年   4967篇
  1978年   4618篇
  1977年   5508篇
  1976年   7035篇
  1975年   3961篇
  1974年   3748篇
  1973年   3814篇
  1972年   3044篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
141.
142.
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   
143.
144.
Reviews the book, Constructing the subject: Historical origins of psychological research by Kurt Danziger (1990). Kurt Danziger's Constructing the Subject: Historical Origins of Psychological Research is a book of singular importance because it provides such a penetrating analysis, and does so in a manner that is cause for considerable reflection. In brief, Danziger provides a history lesson that not only situates the names and the projects of experimental psychology in the first part of this century, but also aims to clarify the project of knowledge generation both past and present. Indeed, shades of Quine, Kuhn, and Hesse permeate this book in a way that demands psychologists examine their own investigative practices and logics of justification. Through Wundt, through Galton, through Ebbinghaus and others, Danziger illuminates the development of experimental psychology along with the historical and philosophical vicissitudes that have given rise to numerous psychological knowledge claims. If it is true that we must understand our history in order to understand our present, then Danziger's book should be required reading in all research laboratories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
145.
Particulate TiB2 reinforced aluminum-based metal matrix composites (MMCs) were successfully fabricated by means of the reaction processing method. TiB2 particulates were formed in situ through the reaction of Ti and B in Ti-Al-B, TiO2 and B in TiO2-Al-B, and TiO2 and B2O3 in TiO2-Al-B2O3 systems. The results showed that in situ TiB2 particulates formed in the Ti-Al-B system had a size of 5 μm and they exhibited block and rodlike structures. Moreover, coarse Al3Ti blocks several tens of micrometers in size were also formed simultaneously. On the other hand, equiaxed Al2O3 and TiB2 particulates with a size of less than 2 μm were formed in situ in the TiO2-Al-B and TiO2-Al-B2O3 systems. The Al3Ti phase was completely eliminated in the TiO2-Al-B system with increasing B content. Tensile tests revealed that the Al2O3 · TiB2/Al composite fabricated from the TiO2-Al-B system exhibits excellent mechanical properties. The yield strength of the Al2O3 · TiB2/Al composite appeared to increase with increasing TiB2 content. The yield strength of the Al2O3 · TiB2/Al composite could be further increased by introducing CuO into the TiO2-Al-B system. Such an increment in mechanical strength arose from the strengthening effect caused by the Al2Cu precipitates. The incorporation of CuO had no effect on the in situ reaction process of the TiO2-Al-B system. Finally, the effect of SiC addition on the microstructure and mechanical properties of the composites fabricated from the TiO2-Al-B and TiO2-Al-B-CuO systems was also investigated.  相似文献   
146.
The human 4 hour patch test provides an opportunity to identify substances with significant skin irritation potential without recourse to the use of animals. The protocol is designed to avoid the production of more than mild irritant reactions and meets the highest ethical standards. This paper provides the background to the development of the method and comments on its performance in the light of recent intra- and inter-laboratory investigations. In particular, the value of the method in providing 'gold standard' data for the identification of those substances (or preparations) which should, or should not, be classified as irritant to skin in European legislation is discussed. On the basis of the published data and supplementary investigations, recommendations are made on both the conduct and interpretation of the human 4 hour patch test. Finally, the lack of any necessity for formal validation of this assay is addressed.  相似文献   
147.
Morgagni hernias are the least common form of diaphragmatic hernias. Although they are congenital, most of them are not diagnosed until later in life. The indication for surgery is based on the patient's symptoms or on the radiological evidence of incarcerated tissue, and until quite recently involved a laparotomy or thoracotomy. Laparoscopy not only permits the suspected diagnosis to be confirmed--which is otherwise often difficult--but also makes it possible to close the hernia site by suturing. For improved security, the hernia site is augmented by fixing in place a non-absorbable mesh. The operative technique employed is described.  相似文献   
148.
149.
cDNAs for green fluorescent protein (GFP) and for a GFP fusion protein containing the presequence of human ornithine transcarbamylase (pOTC-GFP) were transfected into cultured human fibroblasts. GFP cDNA gave diffuse fluorescence throughout the cytoplasm and the nucleus, whereas pOTC-GFP cDNA gave mitochondria-associated fluorescence. Fluorescent mitochondrial structures could be classified into five patterns: thread-like mitochondria, fine thread-like ones, rod-like ones, granular ones, and granular ones with weak cytosolic fluorescence. pOTC-GFP mutants resulted in a loss of mitochondrial fluorescence and an appearance of weak fluorescence throughout the cytoplasm. pOTC-GFP cDNA was transfected into fibroblasts from patients with various mitochondrial diseases. Higher ratios of fibroblasts with granular mitochondria and those with fine thread-like ones were observed in a patient with Reye's syndrome and a patient with Kearns-Sayre syndrome. Weak cytosolic fluorescence was sometimes observed in fibroblasts from these patients. This method will be useful to analyze mitochondrial structural alterations and disorders of mitochondrial protein import.  相似文献   
150.
The physical widths of reference features incorporated into electrical linewidth test structures patterned in films of monocrystalline silicon have been determined from Kelvin voltage measurements. The films in which the test structures are patterned are electrically insulated from the bulk-silicon substrate by a layer of silicon dioxide provided by SIMOX (Separation by the IMplantation of OXygen) processing. The motivation is to facilitate the development of linewidth reference materials for critical-dimension (CD) metrology-instrument calibration. The selection of the (110) orientation of the starting silicon and the orientation of the structures' features relative to the crystal lattice enable a lattice-plane-selective etch to generate reference-feature properties of rectangular cross section and atomically planar sidewalls. These properties are highly desirable for CD applications in which feature widths are certified with nanometer-level uncertainty for use by a diverse range of CD instruments. End applications include the development and calibration of new generations of CD instruments directed at controlling processes for manufacturing devices having sub-quarter-micrometer features  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