全文获取类型
收费全文 | 176903篇 |
免费 | 2137篇 |
国内免费 | 704篇 |
专业分类
电工技术 | 3290篇 |
综合类 | 101篇 |
化学工业 | 27874篇 |
金属工艺 | 7774篇 |
机械仪表 | 5118篇 |
建筑科学 | 4409篇 |
矿业工程 | 875篇 |
能源动力 | 4677篇 |
轻工业 | 16409篇 |
水利工程 | 1702篇 |
石油天然气 | 3126篇 |
武器工业 | 2篇 |
无线电 | 19845篇 |
一般工业技术 | 33314篇 |
冶金工业 | 33105篇 |
原子能技术 | 4291篇 |
自动化技术 | 13832篇 |
出版年
2021年 | 1312篇 |
2019年 | 1246篇 |
2018年 | 2096篇 |
2017年 | 2077篇 |
2016年 | 2189篇 |
2015年 | 1570篇 |
2014年 | 2718篇 |
2013年 | 7781篇 |
2012年 | 4552篇 |
2011年 | 6362篇 |
2010年 | 5034篇 |
2009年 | 5888篇 |
2008年 | 5865篇 |
2007年 | 5842篇 |
2006年 | 5013篇 |
2005年 | 4680篇 |
2004年 | 4472篇 |
2003年 | 4144篇 |
2002年 | 4102篇 |
2001年 | 4125篇 |
2000年 | 3922篇 |
1999年 | 4054篇 |
1998年 | 10380篇 |
1997年 | 7389篇 |
1996年 | 5653篇 |
1995年 | 4281篇 |
1994年 | 3617篇 |
1993年 | 3564篇 |
1992年 | 2604篇 |
1991年 | 2537篇 |
1990年 | 2416篇 |
1989年 | 2435篇 |
1988年 | 2376篇 |
1987年 | 2126篇 |
1986年 | 2066篇 |
1985年 | 2368篇 |
1984年 | 2185篇 |
1983年 | 2012篇 |
1982年 | 1880篇 |
1981年 | 1944篇 |
1980年 | 1796篇 |
1979年 | 1816篇 |
1978年 | 1776篇 |
1977年 | 2108篇 |
1976年 | 2687篇 |
1975年 | 1557篇 |
1974年 | 1549篇 |
1973年 | 1607篇 |
1972年 | 1350篇 |
1971年 | 1262篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
991.
Ma Shizhou Beazley Karen F. Nussey Patrick Greene Christopher S. 《Water Resources Management》2021,35(14):4825-4840
Water Resources Management - The Active River Area (ARA) is a spatial approach for identifying the extent of functional riparian area. Given known limitations in terms of input elevation data... 相似文献
992.
W. F. M. Groot Zevert A. J. A. Winnubst G. S. A. M. Theunissen A. J. Burggraaf 《Journal of Materials Science》1990,25(8):3449-3455
Two wet chemical preparation methods are described for yttria-doped tetragonal zirconia powders. Both methods yield powders with an extremely small crystallite size (8 nm) and a narrow size distribution. The agglomerate and aggregate structure of these powders have been investigated by several techniques. Gel precipitation from an alkoxide solution in water (alkoxide synthesis) results in a ceramic powder with irregular-shaped weak and porous agglomerates, which are built up from dense aggregates with a size of 18 nm. Gel precipitates formed from a metal-chloride solution in ammonia (chloride synthesis) do not contain aggregates. Both types of agglomerate are fractured during isostatic compaction. Hydrolysis and washing under (strong) basic conditions probably decrease the degree of aggregation. The aggregate morphology and structure are key parameters in the microstructure development during sintering of a ceramic. Several characteristics of these powders are compared with those of a commerical one (Toyo Soda TZ3Y). 相似文献
993.
By using optical displacement-measuring techniques we obtain the crack tip fields in terms of crack tip strain, crack opening angle (COA) and crack tip opening angle (CTOA). Thin plates with different crack configurations made of six elastic-plastic materials with strain hardening exponent ranging from 3 to are studied. We find that the COA resistance curve assumes certain characteristics common to all the materials. The experimental results indicate that the critical value of COA may be used as a fracture toughness parameter for these materials.
Résumé En utilisant une technique de mesure optique des déplacements, on obtient les champs régnant à l'estrémité d'une entaille, qui s'experiment par la déformation d'extrémité d'entaille, l'angle d'ouverture de la crique (COA et l'angle d'ouverture de la pointe de la crique (CTOA). On étudie des feuillards présentant différentes configurations de fissures et réalisés en six matériaux élasto-plastiques dont les modules d'écrouissage s'échelonnent de 3 à l'infini.On trouve que la courbe de résistance COA suppose certaines caractéristiques communes à tous les matériaux. Les résultats expérimentaux indiquent que la valeur critique du COA peut être utilisée comme paramètre de ténacité à la rupture pour les matériaux étudiés.相似文献
994.
M. H. El-Fouly A. F. Maged H. H. Amer M. A. Morsy 《Journal of Materials Science》1990,25(4):2264-2266
Temperature-induced transformations are considered to be interesting characteristic properties of amorphous materials including the Si
x
Te60–x
As30Ge10 system, withx=5, 10, 12 and 20. Density (), X-ray diffraction and differential thermal analysis (DTA) were used to characterize the compositions. DTA traces of each glass composition at different heating rates from 5 to 30° C min–1 were obtained and interpreted. Fast and slow cooling cycles were used to determine the rate of structure formation. Cycling studies of materials show no memory effect but only ovonic switching action. The compositional dependence of the crystallization activation energy (E) and the coefficient of glass-forming tendency (K
gl) have been calculated. The thermal transition temperatures and associated changes in specific heat have been examined as a function of the Te/Si ratio by differential scanning calorimetry. It was found that andE increase linearly with increasing tellurium content, while the heat capacity (c
p) andK
gl, decrease with increasing tellurium content.E=1.54 eV andc
p=0.246 J g–1 K–1 forx=20 whileE=2.74eV andc
p=0.22 J g–1 K–1 forx=5. 相似文献
995.
The effect of thallium on the phenomena accompanying the thermally induced structural changes in different compositions of the chalcogenide system Tl
x
Se1–x, withx = 0.0 to 0.9, has been investigated using differential thermal analysis measurements. It is possible to prepare massive homogeneous glassy samples with thallium contents up to 10 at %. Glasses with some embedded crystallites are obtained up to 40 at % Tl. The coefficient of the glass-forming tendency (K
gl) increases with increasing selenium content. In the binary system Tl-Se, one (or more) of the phases are likely to be more unstable with respect to crystallization than was the initially homogeneous system. The activation energy of crystallization for the glasses fromx = 0.0 to 0.3, varies from 1.0 to 4.5 eV for the low-temperature phase and from 1.7 to 2.5 eV for the high-temperature phase. 相似文献
996.
The influence of several sinter-HIP variables on the densification behaviour of silicon nitride-based ceramics has been investigated. The processing conditions were studied for Si3N4 powder mixtures containing controlled amounts of Y2O3+Al2O3 or Y2O3+MgO. The specimens were subjected to sinter-HIP cycles under argon or nitrogen atmospheres at various temperatures and pressures. The final density of the powder compacts exhibited a strong dependence not only on the applied pressure, the composition and the processing temperature, but also on the pressurization rate and the initial pressurization time. The microstructural changes induced by the application of high pressure were followed by transmission electron microscopy. On examination by TEM, large concentrations of dislocations, generated inside some-Si3N4 grains, were observed. Characterization of these dislocations showed thatb=0001 is their most frequently found Burgers vector. Also, two relaxation mechanisms, tending to release the stored energy of deformation in the-Si3N4 grains, namely grain penetration (a form of strain-induced boundary migration) and grain fragmentation (the formation of subgrains due to rearrangement of dislocations into low energy configurations), have been identified. The intergranular phases formed were characterized by energy dispersive X-ray spectroscopic analysis, X-ray diffraction and electron diffraction. The influence of different sinter-HIP cycles on the transformation of silicon nitride without additives was also investigated by X-ray diffractometry. 相似文献
997.
Microstructural characteristics of (Pb, Ca)TiO3 piezoelectric ceramic particles have been deduced from computer analysis of their images and the corresponding statistical treatment of the data obtained. It is intended that this study should be used as a nexus, or link, between the geometrical parameters of these particles and some of their physical properties. 相似文献
998.
The microstructural and mechanical properties of mullite-zirconia composites with TiO2 (0.25 and 1.0 mol) additions have been studied, after ageing the samples over a wide temperature range (1000 to 1500° C) for long periods of time (100 to 200 h). In the sample with 0.25 mol TiO2 addition, changes in mullite composition and in the solid state compatibility at temperatures below 1450° C were detected. In the sample containing 1 mol TiO2, decomposition of Al2TiO5 occurs atT1200° C. Both compositions exhibit no increment in zirconia average grain size during ageing and, concomitantly, there is no strength degradation until higher temperatures (>1400° C) are reached, which become more drastic when Al2Ti5 is present. 相似文献
999.
XPS investigation on AISI 420 stainless steel corrosion in oil and gas well environments 总被引:3,自引:0,他引:3
G. Fierro G. M. Ingo F. Mancia L. Scoppio N. Zacchetti 《Journal of Materials Science》1990,25(2):1407-1415
The corrosion behaviour of 13Cr-martensitic stainless steel (AISI 420) was investigated in CO2-H2S-Cl– environments typical of oil and gas wells under different CO2 and H2S partial pressures. The corrosion tests indicated that the AISI 420 steel was highly corrosion resistant to CO2-induced phenomena (general corrosion and carbonate S.C.C.), while in the H2S environment a high S.S.C.C. (Sulphide Stress Corrosion Cracking) susceptibility and high corrosion rates were found. Moreover, CO2 in CO2-H2S-Cl– systems inhibited general corrosion and S.S.C.C. phenomena by favouring the formation of a protective film. By means of X-ray photoelectron spectroscopy (XPS) the chemical nature of the films grown on AISI 420 in different environmental conditions was investigated and the following statements were drawn out:
Furthermore from XPS results an index of protectivenessI
p = Cr+3/ (Cr+3 + Fe
OX
was defined and related to the environmental parameter
and to the corrosion rates. 相似文献
| CO2 favours the growth of a hydrated Cr-oxide rich protective film with a low Fe-oxide and sulphide content; |
| the presence of H2S favours the formation of less protective Fe-sulphide and Fe-oxide rich layers. |
1000.
The teaching of literature through CAI raises problems of both a linguistic and instructional nature; student involvement and creativity in studying literature, and especially poetry, is difficult to build into a computer-based lesson. We have confronted these difficulties in the lessonPoetry I, which introduces undergraduates to basic concepts of poetic verse in a design using screen display, speech synthesis, and verse processing to maximize interactivity and student involvement.The lesson contains instructional modules which include the student's composition of a limerick. Computational processing of the limerick's text enables the program to offer guidance as the student composes and revises the verse and gains first-hand experience with metrical language. The significant problems inherent in processing verse are addressed by adapting Digital Equipment Corporation's DECTalk speech synthesizer as an engine converting natural language verse text into accessible strings of phonemic symbols. Although the verse processor cannot scan verse, it can interpret DECTalk's symbols reliably enough to successfully stimulate students' thinking about their verse compositions.Preliminary responses to the program have been favorable and demonstrate its effectiveness in involving students more deeply in learning about poetic verse. This suggests that the techniques of verse processing prototyped inPoetry I might be usefully extended to other types of verse and levels of study.W. Webster Newbold is an Assistant Professor of English at Ball State University, Muncie, Indiana, USA, who has recently served as English Department Computer Coordinator. His research interests include computer-assisted instruction and computer-based composition.Herbert F. W Stahlke is a Professor of English, specializing in linguistics, and currently serves as Associate Director for Academic Computing, Ball State University Computing Services. His research interests include computer-assisted instruction, natural language processing, and lexical databases. 相似文献