全文获取类型
收费全文 | 110439篇 |
免费 | 2482篇 |
国内免费 | 857篇 |
专业分类
电工技术 | 1558篇 |
综合类 | 2895篇 |
化学工业 | 15623篇 |
金属工艺 | 5706篇 |
机械仪表 | 4089篇 |
建筑科学 | 3454篇 |
矿业工程 | 806篇 |
能源动力 | 1937篇 |
轻工业 | 6031篇 |
水利工程 | 1608篇 |
石油天然气 | 843篇 |
武器工业 | 34篇 |
无线电 | 12561篇 |
一般工业技术 | 20272篇 |
冶金工业 | 6930篇 |
原子能技术 | 529篇 |
自动化技术 | 28902篇 |
出版年
2024年 | 72篇 |
2023年 | 262篇 |
2022年 | 519篇 |
2021年 | 731篇 |
2020年 | 549篇 |
2019年 | 610篇 |
2018年 | 14994篇 |
2017年 | 13890篇 |
2016年 | 10574篇 |
2015年 | 1282篇 |
2014年 | 1114篇 |
2013年 | 1780篇 |
2012年 | 4405篇 |
2011年 | 10871篇 |
2010年 | 9456篇 |
2009年 | 6753篇 |
2008年 | 7925篇 |
2007年 | 8864篇 |
2006年 | 1154篇 |
2005年 | 2174篇 |
2004年 | 1819篇 |
2003年 | 1880篇 |
2002年 | 1267篇 |
2001年 | 776篇 |
2000年 | 769篇 |
1999年 | 693篇 |
1998年 | 1409篇 |
1997年 | 931篇 |
1996年 | 746篇 |
1995年 | 575篇 |
1994年 | 476篇 |
1993年 | 441篇 |
1992年 | 285篇 |
1991年 | 251篇 |
1990年 | 223篇 |
1989年 | 222篇 |
1988年 | 216篇 |
1987年 | 167篇 |
1986年 | 166篇 |
1985年 | 207篇 |
1984年 | 161篇 |
1983年 | 127篇 |
1982年 | 121篇 |
1981年 | 134篇 |
1980年 | 115篇 |
1979年 | 114篇 |
1977年 | 141篇 |
1976年 | 191篇 |
1973年 | 78篇 |
1954年 | 69篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
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
62.
Dyspnea may be easily appreciated during exercise with dyspneic scales, but methodological standardisation still needs to be specified. Authors review the basic physiological mechanism relating dyspnea to indices obtained during a stress test. They propose to use the dyspnea/VE relationship. With the concept of dyspneic threshold (close to the ventilatory threshold) and the ramp that both could be modified (for instance by rehabilitation programmes including exercise training). Interpretation of dyspnea during an exercise test obviously needs to be integrated with other parameters studied during exercise. 相似文献
63.
C.H. Chu C.I. Hung Y.H. Wang M.P. Houng 《Photonics Technology Letters, IEEE》1997,9(9):1262-1264
We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques. 相似文献
64.
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
65.
CL Mesh BL Cmolik DW Van Heekeren JH Lee D Whittlesey LM Graham AS Geha SJ Bowlin 《Canadian Metallurgical Quarterly》1997,11(6):612-619
A premise of cardiac risk stratification is that the added risk of coronary artery bypass grafting (CABG) is offset by the improved safety of subsequent vascular reconstruction (VR). We questioned if elective CABG is patients with severe peripheral vascular disease (PVD) is a relatively high-risk procedure. A cohort study of 680 elective CABG patients from January 1993 to December 1994 was performed using three mutually exclusive outcomes of complication-free survival, morbidity, and mortality. Patient characteristic, operative, and outcome data were prospectively collected. Retrospective review determined that 58 patients had either a standard indication for or a history of VR. Overall CABG mortality was 2.5%, with statistically similar but relatively higher rates for PVD as compared to non-PVD patients. In contrast, major morbidity occurred at rates 3.6-fold higher in PVD patients (39.7%) than in disease-free patients (16.7%) after adjustment for the effects of patient and operative variables (odds ratio [OR] 3.67, 95% confidence interval [CI] 1.93-6.99). CABG morbidity in the PVD patient was most likely in those patients with aortoiliac (OR 9.51, CI 3.20-28.27) and aortic aneurysmal (OR 5.24, CI 1.28-21.41) disease types. CABG in PVD patients is associated with significant major morbidity. Such morbidity may preclude or alter the timing of subsequent VR. 相似文献
66.
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
67.
68.
Bert Van Keulen 《国际强度与非线性控制杂志
》1994,4(4):521-552
》1994,4(4):521-552
In this paper we extend the finite-dimensional results for the H∞-control problem with measurement-feedback to a large class of infinite-dimensional systems, allowing for a certain type of unboundedness in the input and output operators (the Pritchard-Salamon class). The main result of the paper relates the solvability to the suboptimal H∞-control problem to the existence of stabilizing solutions to certain operator Riccati equations. Furthermore, a characterization of all suboptimal controllers is given. 相似文献
69.
First break picking is a pattern recognition problem in seismic signal processing, one that requires much human effort and is difficult to automate. The authors' goal is to reduce the manual effort in the picking process and accurately perform the picking. Feedforward neural network first break pickers have been developed using backpropagation training algorithms applied either to an encoded version of the raw data or to derived seismic attributes which are extracted from the raw data. The authors summarize a study in which they applied a backpropagation fuzzy logic system (BPFLS) to first break picking. The authors use derived seismic attributes as features, and take lateral variations into account by using the distance to a piecewise linear guiding function as a new feature. Experimental results indicate that the BPFLS achieves about the same picking accuracy as a feedforward neural network that is also trained using a backpropagation algorithm; however, the BPFLS is trained in a much shorter time, because there is a systematic way in which the initial parameters of the BPFLS can be chosen, versus the random way in which the weights of the neural network are chosen 相似文献
70.
Wesley W. Chu Ph.D. Ion T. leong Ph.D. Ricky K. Taira Ph.D. 《The VLDB Journal The International Journal on Very Large Data Bases》1994,3(4):445-477
We introduce a semantic data model to capture the hierarchical, spatial, temporal, and evolutionary semantics of images in pictorial databases. This model mimics the user's conceptual view of the image content, providing the framework and guidelines for preprocessing to extract image features. Based on the model constructs, a spatial evolutionary query language (SEQL), which provides direct image object manipulation capabilities, is presented. With semantic information captured in the model, spatial evolutionary queries are answered efficiently. Using an object-oriented platform, a prototype medical-image management system was implemented at UCLA to demonstrate the feasibility of the proposed approach. 相似文献