全文获取类型
收费全文 | 9156篇 |
免费 | 635篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 137篇 |
综合类 | 24篇 |
化学工业 | 1885篇 |
金属工艺 | 245篇 |
机械仪表 | 215篇 |
建筑科学 | 527篇 |
矿业工程 | 39篇 |
能源动力 | 168篇 |
轻工业 | 1421篇 |
水利工程 | 42篇 |
石油天然气 | 16篇 |
无线电 | 694篇 |
一般工业技术 | 2651篇 |
冶金工业 | 226篇 |
原子能技术 | 89篇 |
自动化技术 | 1415篇 |
出版年
2023年 | 119篇 |
2022年 | 60篇 |
2021年 | 190篇 |
2020年 | 145篇 |
2019年 | 140篇 |
2018年 | 315篇 |
2017年 | 330篇 |
2016年 | 367篇 |
2015年 | 271篇 |
2014年 | 358篇 |
2013年 | 683篇 |
2012年 | 456篇 |
2011年 | 571篇 |
2010年 | 448篇 |
2009年 | 469篇 |
2008年 | 546篇 |
2007年 | 499篇 |
2006年 | 352篇 |
2005年 | 301篇 |
2004年 | 258篇 |
2003年 | 252篇 |
2002年 | 228篇 |
2001年 | 195篇 |
2000年 | 157篇 |
1999年 | 125篇 |
1998年 | 131篇 |
1997年 | 124篇 |
1996年 | 106篇 |
1995年 | 67篇 |
1994年 | 75篇 |
1993年 | 62篇 |
1992年 | 47篇 |
1991年 | 61篇 |
1990年 | 43篇 |
1989年 | 47篇 |
1988年 | 43篇 |
1987年 | 42篇 |
1986年 | 57篇 |
1985年 | 109篇 |
1984年 | 74篇 |
1983年 | 60篇 |
1982年 | 53篇 |
1981年 | 44篇 |
1980年 | 53篇 |
1979年 | 41篇 |
1978年 | 45篇 |
1976年 | 28篇 |
1974年 | 30篇 |
1973年 | 31篇 |
1943年 | 26篇 |
排序方式: 共有9794条查询结果,搜索用时 31 毫秒
41.
Content This paper is dealing with the power density versus electric field characteristic of microwave-excited discharges on the example of typical CO2 laser gas discharges.The fundamentals of the method are a simple electric equivalent circuit for the excitation structure and the assumption of a fixed reduced electric field intensity within the plasma. This operating field is enforced by the plasma under stationary conditions and can be calculated using an electron balance under consideration of the non-Maxwellian electron energy distribution function. The calculations turn out that c.w. excitation with microwaves is involved with considerable plasma power densities. In the case of the CO2 laser this requires special discharge channel designs for effective gas cooling. Besides a discussion of the plasma characteristic, measurements taken from a microwave applicator are presented and compared to the calculations.
Modellierung mikrowellenangeregter Gasentladungen
Übersicht Der Artikel behandelt die Berechnung der Feldstärke-Leistungsdichte-Kennlinie mikrowellenangeregter Gasentladungen am Beispiel von Entladungen in typischen CO2-Lasergasgemischen. Das Berechnungsverfahren beruht auf einem einfachen, auf die meisten Anregungsstrukturen anwendbaren elektrischen Ersatzschaltbild und der Tatsache einer vom Plasma erzwungenen stationären Brennfeldstärke. Letztere wird über eine Elektronenbilanz unter Berücksichtigung der nicht Maxwell'schen Elektronen-Energieverteilung ermittelt. Es zeigt sich, daß bei einer CW-Anregung mit Mikrowellen hohe Mindestleistungsdichten im Plasma auftreten, wodurch beispielsweise im Fall des CO2-Lasers kein Laserbetrieb ohne sehr effektive Maßnahmen zur Gaskühlung möglich ist.Neben einer Diskussion der Plasmakennlinie werden Vergleichsmessungen an einer konkreten Anregungsstruktur vorgestellt.相似文献
42.
Droplet formation on Si surfaces bombarded by 10 keV beams from a liquid metal indium ion source has been observed. It has been verified that excess indium is transported in the ion beam, probably in the form of charged droplets. SIMS, SEM and RBS techniques have been used to study the distribution of droplets at the surface. At the specified ion source operating conditions, equilibrium droplet coverage is of the order of 25%. Individual droplets have an average diameter of 0.35 ωm and an average height of 2.2 nm. At a bombardment current density of 0.1 mA cm?2, equilibrium coverage is reached in about 1 s. 相似文献
43.
Per Gustafson 《Metallurgical and Materials Transactions A》1988,19(10):2547-2554
A thermodynamic evaluation of the C-Cr-Fe-W system has been made using a multi-sublattice model. A set of parameter values
describing the Gibbs energy of each individual phase is given and a number of calculated sections of the C-Cr-Fe-W phase diagram
are presented and compared with experimental data.
Formerly with the Division of Physical Metallurgy, Royal Institute of Technology, Stockholm, Sweden 相似文献
44.
The mechanism of the loss of stereospecificity in palladium-catalyzed nucleophilic substitution of allylic substrates has been investigated. Eight substrates (cis and trans isomers of 1a-d) and two nucleophiles (Et2NH and NaCH(SO2Ph)2) were studied. In the animation reactions two pathways are responsible for the formation of anomalous inversion product, viz., isomerization of the starting material (path B, Scheme 2) and isomerization of the π-allyl intermediate via displacement of palladium by Pd(0) (path C, Scheme 2), the latter of which predominates. In the alkylation the results indicate that loss of stereospecificity is caused only by path C. The use of a more reactive substrate increased the stereospecificity of the reaction and suppressed the isomerization pathway. An analysis of the kinetics is consistent with the hypothesis that path C is the major pathway for the stereochemical loss. 相似文献
45.
Kurzfassung Die Saltpool-Experimente von OSWALD (1998) und OSWALD & KINZELBACH (2004), im Jahre 1998 an der ETH Zürich ausgeführt, sind exzellente Benchmark-Tests für numerische Simulationsprogramme der dichtevariablen Strömung. Zahlreiche Autoren haben Ergebnisse dazu veröffentlicht. Die kritischen Probleme hierbei sind sowohl die zu dicke Übergangszone von hoher Konzentration an der Sohle der Versuchsbox zum Süßwasser am Top der Box in der Injektionsphase und in der Ruhephase als auch die extrem langen Rechenzeiten.Die Untersuchungen von HÄFNER & BOY (2003a) wurden fortgeführt mit dem erweiterten Code MODCALIF-MST (MODular Flow and CALIbration with Front Limitation algorithm—Multi Species and Temperature). Insbesondere werden Methoden zur Rechenzeitbeschleunigung diskutiert und verifiziert. Durch Nutzung eines algebraischen Multigrid- Gleichungslösers konnte die Rechenzeit um den Faktor 4 verringert werden. Zur Prüfung der Leistungsfähigkeit von MODCALIF-MST wurden die Saltpool Experimente, case 1 und case 2, nachsimuliert und Gitterkonvergenzstudien sowie eine Modellkalibrierung durchgeführt.
Simulation of density-dependent solute transport in groundwater and verification with saltpool experiments
Abstract The Saltpool experiments by OSWALD (1998) and OSWALD & KINZELBACH (2004), carried out in 1998 at the ETH Zurich, are an excellent benchmark test for numerical simulation programmes of the variable-density flow. Numerous authors published results on this subject. Here, critical problems are the thick transition zones from high concentration at the bottom to fresh water at the top of the box in the injection phase and the rest phase (phases 1–2) as well as the extremely long computing time.We have continued the investigations by HÄFNER & BOY (2003a) with the extended code MODCALIF-MST (MODular Flow and CALIbration with Front Limitation algorithm—Multi Species and Temperature) in double porosity porous media. Especially, the methods for code acceleration are discussed and verified. By incorporating the algebraic multigrid package as linear equation solver, computing times could be reduced by a factor of about 4. Simulations, a grid convergence study and a model calibration were carried out for the experiments case 1 and case 2 with MODCALIF.相似文献
46.
47.
48.
Wan-Yu Wu Jyh-Ming Ting Hayo Zutz Dominika Lyzwa Inga Gerhards Carsten Ronning Hans Hofsäss 《Diamond and Related Materials》2008,17(7-10):1494-1497
While experimenting with the growth of metal-containing amorphous carbon (a-c:Me) thin films using two different growth processes, self-assembled multilayered structures were observed. One of the processes is a reactive magnetron sputter deposition process. The other process is a mass selective ion beam deposition process. Despite of the differences in the growth method and the growth condition, self-assembled multilayered thin films, consisting of alternating dark layer and bright layer, were obtained in both processes. Based on the consideration of energy for atomic diffusion in the thin films, the growth mechanism is discussed. 相似文献
49.
Per Munk Nielsen Jesper Brask Lene Fjerbaek 《European Journal of Lipid Science and Technology》2008,110(8):692-700
It is well documented in the literature that enzymatic processing of oils and fats for biodiesel is technically feasible. However, with very few exceptions, enzyme technology is not currently used in commercial‐scale biodiesel production. This is mainly due to non‐optimized process design and a lack of available cost‐effective enzymes. The technology to re‐use enzymes has typically proven insufficient for the processes to be competitive. However, literature data documenting the productivity of enzymatic biodiesel together with the development of new immobilization technology indicates that enzyme catalysts can become cost effective compared to chemical processing. This work reviews the enzymatic processing of oils and fats into biodiesel with focus on process design and economy. 相似文献
50.