全文获取类型
收费全文 | 94558篇 |
免费 | 1331篇 |
国内免费 | 456篇 |
专业分类
电工技术 | 948篇 |
综合类 | 2342篇 |
化学工业 | 13038篇 |
金属工艺 | 4934篇 |
机械仪表 | 3308篇 |
建筑科学 | 2707篇 |
矿业工程 | 580篇 |
能源动力 | 1386篇 |
轻工业 | 4218篇 |
水利工程 | 1335篇 |
石油天然气 | 397篇 |
武器工业 | 4篇 |
无线电 | 10929篇 |
一般工业技术 | 17946篇 |
冶金工业 | 5093篇 |
原子能技术 | 338篇 |
自动化技术 | 26842篇 |
出版年
2022年 | 83篇 |
2021年 | 133篇 |
2020年 | 98篇 |
2019年 | 127篇 |
2018年 | 14585篇 |
2017年 | 13492篇 |
2016年 | 10110篇 |
2015年 | 765篇 |
2014年 | 426篇 |
2013年 | 771篇 |
2012年 | 3587篇 |
2011年 | 9918篇 |
2010年 | 8609篇 |
2009年 | 5992篇 |
2008年 | 7274篇 |
2007年 | 8255篇 |
2006年 | 621篇 |
2005年 | 1625篇 |
2004年 | 1465篇 |
2003年 | 1512篇 |
2002年 | 810篇 |
2001年 | 340篇 |
2000年 | 443篇 |
1999年 | 332篇 |
1998年 | 609篇 |
1997年 | 381篇 |
1996年 | 327篇 |
1995年 | 268篇 |
1994年 | 200篇 |
1993年 | 226篇 |
1992年 | 178篇 |
1991年 | 149篇 |
1990年 | 122篇 |
1989年 | 116篇 |
1988年 | 120篇 |
1987年 | 124篇 |
1986年 | 119篇 |
1985年 | 148篇 |
1984年 | 98篇 |
1983年 | 95篇 |
1982年 | 113篇 |
1981年 | 101篇 |
1980年 | 79篇 |
1979年 | 76篇 |
1978年 | 79篇 |
1977年 | 77篇 |
1976年 | 94篇 |
1975年 | 74篇 |
1955年 | 70篇 |
1954年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 191 毫秒
41.
42.
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
43.
N. Mitchell A. Alekseev R. Gallix D. Holland R. Meyder A. Panin M. Shimada F. Wong E. Zapretelina 《Journal of Fusion Energy》1997,16(1-2):25-35
The ITER magnet system consists of structurally linked sets of toroidal (TF) and poloidal (PF) field coils, central solenoid (CS), and various support structures. The coils are superconducting, force flow Helium cooled with a Kapton-Glass-Epoxy multilayer insulation system. The stored magnetic energy is about 100GJ in the TF system and 20GJ in the PF-CS. Coils and structure are maintained at 4 K by enclosing them in a vacuum cryostat. The cryostat, comprising an outer envelope to the magnets, forms most of the second radioactivity confinement barrier. The inner primary barrier is formed by the vacuum vessel, its ports and their extensions. To keep the machine size within acceptable bounds, it is essential that the magnets are in close proximity to both of the nuclear confinement barriers. The objective of the magnet design is that, although local damage to one of the barriers may occur in very exceptional circumstances, large scale magnet structural or thermal failure leading to simultaneous breaching of both barriers is not credible. Magnet accidents fall into three categories: thermal (which includes arcing arising from insulation failure and local overheating due to discharge failure in the event of a superconductor quench), structural (which includes component mechanical failure arising from material inadequacies, design errors and exceptional force patterns arising from coil shorts or control failures), and fluid (Helium release due to cooling line failure). After a preliminary survey to select initial faults conceivable within the present design, these faults are systematically analyzed to provide an assessment of the damage potential. The results of this damage assessment together with an assessment of the reliability of the monitoring and protective systems, shows that the magnets can operate with the required safety condition. 相似文献
44.
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
45.
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
46.
47.
A failed effort to implement new technology is always a failure to understand and adequately manage the change process. IS professionals are now in the change business and must become experts in managing organizational and human change. 相似文献
48.
Three sporulation-specific genes (orfA, sigE, sigG) from clostridium acetobutylicum ATCC 824 are arranged in a cluster, encoding the putative sigma E-processing enzyme, sigma E, and sigma sigma G respectively. When they were transformed into Clostridium acetobutylicum while on a plasmid functional in this organism, transformants did not survive. Three kinds of recombinations were then attempted with nonreplicative plasmids: duplication of orfA and sigE, replacement of all of the three genes, and inactivation of orfA. While the wild-type strain ceased to grow and produce solvents in batch cultures after approximately 24 h, mutant strains were isolated that showed sustained growth for a much longer time and produced a threefold increase in acetone and butanol in test tube cultures. In addition, one of the derived strains showed a significantly higher growth rate. Features of the restriction maps of the recombinants did not correlate with expected maps, indicating possible complications occurring during the recombination events. 相似文献
49.
The phenotypic characteristics of three Serpulina pilosicoli strains isolated from humans with diarrhoea (WesB, Kar, Hrm7) and two porcine S. pilosicoli strains isolated from pigs with intestinal spirochaetosis (1648, 3295), were compared with the type strain of the species P43/6/78T (T = type strain) and other intestinal spirochaetes within the genus Serpulina. All S. pilosicoli strains had a characteristic ultrastructural appearance, displayed similar growth rates, hydrolysed hippurate, lacked beta-glucosidase activity, utilised D-ribose as a growth substrate, and had similar sensitivities to rifampicin and spiramycin. The only consistent phenotypic characteristic that differentiated human strains from porcine strains of S. pilosicoli was that the human strains all utilised the pentose sugar D-xylose. These distinguishing phenotypic traits appear useful for identifying S. pilosicoli. 相似文献
50.
Certain new sulfide-smelting processes and coal-gasification processes generate high-strength sulfur dioxide streams, for
which a new process for converting sulfur dioxide to elemental sulfur needs to be developed because no process exists that
is generally and economically applicable to the treatment of such streams. A thermodynamic and experimental investigation
to develop a new process for converting sulfur dioxide to elemental sulfur by a cyclic process involving calcium sulfide and
calcium sulfate without generating secondary pollutants was carried out. In this process, the starting raw material, calcium
sulfate, is reduced by a suitable reducing agent, such as hydrogen, to produce calcium sulfide, which is used to reduce sulfur
dioxide to elemental sulfur vapor and calcium sulfate. The latter is, in turn, reduced to regenerate calcium sulfide. In this
Part I, detailed experimental results are presented on the kinetics of the reaction between sulfur dioxide and calcium-sulfide
pellets, which produces elemental sulfur and calcium sulfate. The experiments were carried out at temperatures between 1023
and 1088 K and sulfur-dioxide partial pressures between 9 and 60 kPa by the use of a thermogravimetric analysis (TGA) technique.
The rate of this reaction was demonstrated by the conversion of 40 pct calcium-sulfide pellets obtained from the hydrogen
reduction of fresh calcium sulfate in 10 minutes at 1073 K under a sulfur-dioxide partial pressure of 43 kPa. The reactivity
decreased somewhat during the first three cycles but remained largely unchanged thereafter up to the tenth cycle. This characteristic
of the pellets is important because the solids must be reusable for repeated cycles to avoid generating secondary pollutants.
A pore-blocking model was found to fit the reaction rate. The reaction is first order with respect to sulfur-dioxide partial
pressure and has an activation energy of 101 to 134 kJ/mol (24 to 32 kcal/mol) for calcium-sulfide pellets reacted and regenerated
several different times. Sulfur dioxide-containing streams from certain sources, such as the regenerator off-gas from an integrated-gasification,
combined-cycle, desulfurization unit and new sulfide-smelting plants, contain much higher partial pressures of SO2. In these cases, the rate of the first reaction is expected to be proportionally higher than in the test conditions reported
in this article. The reduction kinetics of calcium-sulfate pellets with hydrogen gas is reported in the accompanying Part
II. 相似文献