全文获取类型
收费全文 | 21908篇 |
免费 | 1619篇 |
国内免费 | 71篇 |
专业分类
电工技术 | 335篇 |
综合类 | 76篇 |
化学工业 | 5329篇 |
金属工艺 | 496篇 |
机械仪表 | 719篇 |
建筑科学 | 760篇 |
矿业工程 | 61篇 |
能源动力 | 912篇 |
轻工业 | 4102篇 |
水利工程 | 252篇 |
石油天然气 | 114篇 |
武器工业 | 2篇 |
无线电 | 1443篇 |
一般工业技术 | 3565篇 |
冶金工业 | 1855篇 |
原子能技术 | 162篇 |
自动化技术 | 3415篇 |
出版年
2024年 | 51篇 |
2023年 | 227篇 |
2022年 | 371篇 |
2021年 | 1041篇 |
2020年 | 687篇 |
2019年 | 801篇 |
2018年 | 930篇 |
2017年 | 945篇 |
2016年 | 955篇 |
2015年 | 693篇 |
2014年 | 977篇 |
2013年 | 1719篇 |
2012年 | 1450篇 |
2011年 | 1723篇 |
2010年 | 1177篇 |
2009年 | 1213篇 |
2008年 | 1092篇 |
2007年 | 938篇 |
2006年 | 740篇 |
2005年 | 561篇 |
2004年 | 517篇 |
2003年 | 526篇 |
2002年 | 504篇 |
2001年 | 370篇 |
2000年 | 310篇 |
1999年 | 277篇 |
1998年 | 606篇 |
1997年 | 453篇 |
1996年 | 324篇 |
1995年 | 217篇 |
1994年 | 162篇 |
1993年 | 146篇 |
1992年 | 92篇 |
1991年 | 86篇 |
1990年 | 76篇 |
1989年 | 65篇 |
1988年 | 65篇 |
1987年 | 40篇 |
1986年 | 39篇 |
1985年 | 50篇 |
1984年 | 42篇 |
1983年 | 40篇 |
1982年 | 28篇 |
1981年 | 29篇 |
1980年 | 29篇 |
1979年 | 24篇 |
1978年 | 19篇 |
1977年 | 39篇 |
1976年 | 55篇 |
1975年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
81.
82.
Artem Sokolov Darrell Whitley Andre’ da Motta Salles Barreto 《Genetic Programming and Evolvable Machines》2007,8(3):221-237
This paper evaluates different forms of rank-based selection that are used with genetic algorithms and genetic programming.
Many types of rank based selection have exactly the same expected value in terms of the sampling rate allocated to each member
of the population. However, the variance associated with that sampling rate can vary depending on how selection is implemented.
We examine two forms of tournament selection and compare these to linear rank-based selection using an explicit formula. Because
selective pressure has a direct impact on population diversity, we also examine the interaction between selective pressure
and different mutation strategies. 相似文献
83.
84.
The solid phase crystallization kinetics of chemically vapour-deposited amorphous silicon films were studied by in situ X-ray diffraction. We determined the crystalline volume directly from the Bragg peak intensities at various times during isothermal annealing in the temperature range 578 °C < T < 658 °C. From these experiments we deduced that the crystallization was due to nucleation predominantly at the substrate-film interface followed by crystal growth perpendicular to this interface. The crystal growth rate was thermally activated with an activation energy Ev of 3.1 eV. A strong 〈111〉 preferred orientation of the growing polycrystal was observed and the grain size remained constant at about 60 nm. Evidence of stresses at the amorphous-crystalline interface during the early stages of crystallization was observed. A comparison with previous conductivity measurements is also carried out. 相似文献
85.
A method to produce monodisperse magnetic composite spheres with diameters from less than 100 nm to more than 1 μm in water solution is reported. The spheres consist of a dielectric silica core and a cobalt/cobalt oxide shell which can be protected from further oxidation with an outer shell of silica or, alternatively, they can be covered with the polymer polyvinylpyrrolidone as a stabilizer. The formation of a uniform magnetic shell proceeds with the adsorption of metallic cobalt seeds, produced by the reduction of cobalt chloride with sodium borohydride, on a self‐assembled layer of polyelectrolytes on the silica core. In the second step, an outer silica shell can be formed by the hydrolysis and condensation of (3‐aminopropyl)trimethoxysilane and tetraethoxysilane. The double‐shell composite spheres show excellent sphericity, monodispersity, and a magnetic hysteresis loop at room temperature. 相似文献
86.
87.
J Lamoril C Andant C Bogard H Puy L Gouya JM Pawlotsky V Da Silva JC Soulé JC Deybach Y Nordmann 《Canadian Metallurgical Quarterly》1998,27(3):848-852
From 1995 to 1997, we prospectively evaluated the prevalence of hepatitis C virus (HCV) RNA in 124 patients with porphyria cutanea tarda (PCT) from Northern France (83 sporadic and 41 familial PCT). Serum samples were analyzed for ferritin, transaminases, HCV antibodies, and HCV RNA. In addition, genotyping of HCV and searches for HCV infection risk factors (blood transfusion, iv drug abuse, and surgical intervention) were performed. Twenty-six of 124 patients (21%; 95% CI: 13.9-28) were positive for serum HCV antibodies. All of them were also positive for HCV RNA. The prevalence of HCV infection was higher in the sporadic PCT group (26.5%, 22 out of 83) than in the familial PCT group (9.7%, 4 out of 41). Risk factors for hepatitis C infection were found to be significantly increased in the HCV-positive group when compared with the HCV-negative PCT group. In all HCV-positive patients with a risk factor, the suspected date of exposure to the virus always preceded the clinical onset of PCT. The HCV genotype pattern in PCT patients was similar to that observed in nonporphyric HCV patients in western European countries. Serum ferritin level was increased in both HCV-positive and HCV-negative porphyric patients. Transaminase levels were significantly higher in HCV-infected PCT patients. Sixty-seven out of 124 patients were retrospectively studied for hepatitis G virus (HGV) infection. Six of these 67 patients (8.9%; 95% CI: 2.1-15.8) were positive for HGV RNA. None of the six HGV-infected patients were positive for HCV RNA. The HGV-infected patients did not differ statistically from those without HGV infection with regard to age, ferritin, transaminase levels, and PCT treatment. These results support the view that sporadic cases of HGV infection may occur frequently. This study of a large cohort of HCV and PCT patients further documents an increasing gradient in HCV prevalence from northern to southern Europe, and shows that HCV infection acts as a triggering factor of PCT. Finally, the HGV prevalence found in the PCT patients was comparable with that found in French blood donors, suggesting that HGV is not a PCT triggering factor. 相似文献
88.
A Nu?ez C De la Roza ML Rodrigo-Angulo W Bu?o F Reinoso-Suárez 《Canadian Metallurgical Quarterly》1997,754(1-2):1-11
This study analyses the relationship between the use of rotating instruments, the production of a smear layer and the presence of alterations to enamel microstructures. The rotating instruments used were carbide (8-12 blade) and diamond tipped (30-15 m) cutters. Cavities were made in extracted teeth. Subsequently, half the sample was analysed using the rugosimeter before and after the application of ortophosphoric acid at 35% for 15 and the other half suing a Scansion Electronic microscope (SEM). The results obtained showed on the one hand that carbide cutters leave a smoother surfacer than diamond tipped cutters, and on the other that the smear layer is eliminated better by carbide cutters compared to diamond tipped cutters. Moreover, there are no major traumatic-type alterations at the level of the enamel affecting the microstructure after the use of carbide cutters. 相似文献
89.
90.