全文获取类型
收费全文 | 113661篇 |
免费 | 2154篇 |
国内免费 | 500篇 |
专业分类
电工技术 | 1231篇 |
综合类 | 2356篇 |
化学工业 | 17195篇 |
金属工艺 | 5501篇 |
机械仪表 | 3749篇 |
建筑科学 | 3301篇 |
矿业工程 | 733篇 |
能源动力 | 2409篇 |
轻工业 | 6305篇 |
水利工程 | 1579篇 |
石油天然气 | 490篇 |
无线电 | 12099篇 |
一般工业技术 | 22036篇 |
冶金工业 | 7487篇 |
原子能技术 | 563篇 |
自动化技术 | 29281篇 |
出版年
2023年 | 293篇 |
2022年 | 711篇 |
2021年 | 828篇 |
2020年 | 645篇 |
2019年 | 688篇 |
2018年 | 15079篇 |
2017年 | 13963篇 |
2016年 | 10706篇 |
2015年 | 1209篇 |
2014年 | 1176篇 |
2013年 | 2279篇 |
2012年 | 4409篇 |
2011年 | 11078篇 |
2010年 | 9464篇 |
2009年 | 6803篇 |
2008年 | 8066篇 |
2007年 | 8904篇 |
2006年 | 1194篇 |
2005年 | 2175篇 |
2004年 | 1943篇 |
2003年 | 1904篇 |
2002年 | 1237篇 |
2001年 | 575篇 |
2000年 | 628篇 |
1999年 | 541篇 |
1998年 | 763篇 |
1997年 | 557篇 |
1996年 | 624篇 |
1995年 | 531篇 |
1994年 | 480篇 |
1993年 | 440篇 |
1992年 | 409篇 |
1991年 | 333篇 |
1990年 | 346篇 |
1989年 | 310篇 |
1988年 | 307篇 |
1987年 | 276篇 |
1986年 | 247篇 |
1985年 | 307篇 |
1984年 | 295篇 |
1983年 | 270篇 |
1982年 | 271篇 |
1981年 | 258篇 |
1980年 | 221篇 |
1979年 | 219篇 |
1978年 | 170篇 |
1977年 | 179篇 |
1976年 | 238篇 |
1975年 | 159篇 |
1973年 | 138篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
Ting Gang Zhu Uttiya Chowdhury Michael M. Wong Jonathan C. Denyszyn Russell D. Dupuis 《Journal of Electronic Materials》2002,31(5):406-410
In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky
rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers
grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5
V at 100 A/cm2 are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage
in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm2, and a forward-voltage drop of 3.5 V at a current density of 100 A/cm2. It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC
substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge
terminations in mesageometry GaN Schottky rectifiers has also been studied. 相似文献
93.
RB Singh R Beegom S Ghosh MA Niaz V Rastogi SS Rastogi NK Singh S Nangia 《Canadian Metallurgical Quarterly》1997,11(10):679-685
OBJECTIVES: To determine age-specific prevalence of hypertension and blood pressure (BP) levels in relation to diet and lifestyle factors in North Indians. DESIGN AND SETTING: Cross-sectional survey in 20 randomly selected streets in Moradabad, North India. SUBJECTS AND METHODS: A total of 1806 subjects from North India (904 males and 902 females) age range 25-64 years. The survey methods were as follows: dietary diaries for 7 days food intake record; BP measurements; physician administered questionnaire and anthropometric measurements. Diagnosis of hypertension was based on new World Health Organization/International Society of Hypertension (WHO/ISH) criteria. Risk factors were assessed based on WHO guidelines. RESULTS: The prevalence of hypertension according to WHO/ISH criteria was 23.7% and by old WHO criteria 13.3%. In the WHO/ISH hypertensive group, isolated diastolic hypertension was present in 47.3% males and 40.6% females. Males have a slightly higher prevalence than females in the young age group, however, the prevalence rates are comparable in the older age groups. In both sexes, the prevalence rates and BP level increased with older age. Multivariate analysis revealed that age, higher body mass index, central obesity and higher socioeconomic status were independently and strongly associated with hypertension in both sexes. Higher dietary fat and salt intake and lower physical activity were weakly but significantly associated with hypertension. CONCLUSION: Association of higher socioeconmic status, higher body mass index and central obesity in North Indian adults with higher fat intake, lower physical activity and higher prevalence and level of hypertension indicate that these populations may benefit by decreasing the dietary fat intake and increasing physical activity, with an aim to decrease central obesity for decreasing hypertension in North Indians. 相似文献
94.
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
95.
Jan van Eijck 《Formal Aspects of Computing》1994,6(1):766-787
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
96.
97.
98.
99.
100.
The electrochemical and corrosion behaviour of a nickel base super alloy (C-263) has been investigated in the deaerated binary and ternary solution mixture of concentrated phosphoric acid, acetic acid, sulphuric acid, nitric acid or water using potentiostatic technique at 35°C. The possibilities of electropolishing of this alloy in these solution mixtures have been also explored. The alloy showed distinct active, passive and transpassive behaviour in the experimental solutions. The alloy remained active and turned passive in the negative potential region. Transpassive dissolution of the alloy is observed and electropolishing is achieved in this region. The best electropolishing is obtained in 50% H3PO4 + 40% CH3COOH + 10% H2SO4. Higher content of water in the electrolytic solution is not useful for electropolishing of the alloy The experimental results also suggest that a current plateau in the transpassive potential region is not a sufficient condition to achieve electropolishing. 相似文献