全文获取类型
收费全文 | 496153篇 |
免费 | 5617篇 |
国内免费 | 2037篇 |
专业分类
电工技术 | 8933篇 |
综合类 | 695篇 |
化学工业 | 71353篇 |
金属工艺 | 18946篇 |
机械仪表 | 14656篇 |
建筑科学 | 11739篇 |
矿业工程 | 2298篇 |
能源动力 | 13315篇 |
轻工业 | 40758篇 |
水利工程 | 4901篇 |
石油天然气 | 8397篇 |
武器工业 | 58篇 |
无线电 | 62433篇 |
一般工业技术 | 98551篇 |
冶金工业 | 94844篇 |
原子能技术 | 11420篇 |
自动化技术 | 40510篇 |
出版年
2021年 | 4172篇 |
2020年 | 3138篇 |
2019年 | 3836篇 |
2018年 | 6476篇 |
2017年 | 6539篇 |
2016年 | 6858篇 |
2015年 | 4455篇 |
2014年 | 7757篇 |
2013年 | 21785篇 |
2012年 | 12300篇 |
2011年 | 16978篇 |
2010年 | 13356篇 |
2009年 | 15378篇 |
2008年 | 16074篇 |
2007年 | 16060篇 |
2006年 | 14017篇 |
2005年 | 12763篇 |
2004年 | 12524篇 |
2003年 | 12440篇 |
2002年 | 11853篇 |
2001年 | 12326篇 |
2000年 | 11434篇 |
1999年 | 12236篇 |
1998年 | 31183篇 |
1997年 | 21756篇 |
1996年 | 16654篇 |
1995年 | 12598篇 |
1994年 | 11147篇 |
1993年 | 11193篇 |
1992年 | 7934篇 |
1991年 | 7609篇 |
1990年 | 7373篇 |
1989年 | 7190篇 |
1988年 | 6805篇 |
1987年 | 5967篇 |
1986年 | 5878篇 |
1985年 | 6481篇 |
1984年 | 5957篇 |
1983年 | 5628篇 |
1982年 | 5303篇 |
1981年 | 5192篇 |
1980年 | 5129篇 |
1979年 | 4795篇 |
1978年 | 4657篇 |
1977年 | 5422篇 |
1976年 | 7325篇 |
1975年 | 3929篇 |
1974年 | 3860篇 |
1973年 | 3833篇 |
1972年 | 3289篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
911.
In addition to satisfying several competing objectives, many real-world applications are also characterized by a certain degree of noise, manifesting itself in the form of signal distortion or uncertain information. In this paper, extensive studies are carried out to examine the impact of noisy environments in evolutionary multiobjective optimization. Three noise-handling features are then proposed based upon the analysis of empirical results, including an experiential learning directed perturbation operator that adapts the magnitude and direction of variation according to past experiences for fast convergence, a gene adaptation selection strategy that helps the evolutionary search in escaping from local optima or premature convergence, and a possibilistic archiving model based on the concept of possibility and necessity measures to deal with problem of uncertainties. In addition, the performances of various multiobjective evolutionary algorithms in noisy environments, as well as the robustness and effectiveness of the proposed features are examined based upon five benchmark problems characterized by different difficulties in local optimality, nonuniformity, discontinuity, and nonconvexity 相似文献
912.
K.F. Chan C.K. Wan K.N. Yu 《Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms》2007,262(1):128-134
In the present work, we have studied the feasibility of a method based on polyallyldiglycol-carbonate (PADC) films to investigate the effects of alpha particles on HeLa cervix cancer cells. Thin PADC films with thickness of about 20 μm were prepared from commercially available CR-39 films by chemical etching to fabricate custom-made petri dishes for cell culture, which could accurately record alpha particle hit positions. A special method involving “base tracks” for aligning the images of cell nuclei and alpha particle hits has been proposed, so that alpha particle transversals of cell nuclei can be visually counted. Radiobiological experiments were carried out to induce DNA damages, with the TdT-mediated dUTP Nick-End Labeling (TUNEL) fluorescence method employed to detect DNA strand breaks. The staining results were investigated by flow cytometer. The preliminary results showed that more strand breaks occurred in cells hit by alpha particles with lower energies. Moreover, large TUNEL positive signals were obtained even with small percentages of cells irradiated and TUNEL signals were also obtained from non-targeted cells. These provided evidence for the bystander effect. 相似文献
913.
914.
Ohne Zusammenfassung 相似文献
915.
Tiwari S.P. Namdas E.B. Ramgopal Rao V. Fichou D. Mhaisalkar S.G. 《Electron Device Letters, IEEE》2007,28(10):880-883
Solution-processed n-type organic field-effect transistors (OFETs) based on the fullerene derivative {6}-1-(3-(2- thienylethoxycarbonyl)-propyl)-{5}-l-phenyl-[5,6]-C61 (TEPP) and phenyl-C61-butyric acid methyl ester (PCBM) in a multiring source/drain structure are reported. Devices with TEPP show high electron mobility up to 7.8 x 10-2 cm2/Vs in the saturation regime for bottom-contact OFETs with Au S/D electrodes with a solution-processed fullerene derivative. The ON/OFF ratios reported in this letter, which are in the range of 105 -106, are among the highest values reported for such devices. This mobility is always higher compared to PCBM devices prepared in identical conditions. The mobility of TEPP and PCBM increased with increasing temperatures in the range of 100-300 K with activation energy of 78 and 113 meV, respectively, which suggests that the thermally activated hopping of electrons is dominant in TEPP. 相似文献
916.
El-Husseini M.H. Venet P. Rojat G. Joubert C. 《Industry Applications, IEEE Transactions on》2002,38(3):713-718
In this paper, the authors use an analytical model to calculate the losses in the metallized polypropylene film capacitors. The model is validated experimentally for capacitors having the same capacitance but different geometry. For each group of capacitors, a temperature distribution in the roll is assumed with the aim of optimizing its thermal performance. It appears that the heating of a long capacitor is higher than that of an equivalent flat capacitor subjected to the same electric stresses 相似文献
917.
Metal Science and Heat Treatment - 相似文献
918.
Mesh network resiliency using GMPLS 总被引:4,自引:0,他引:4
Lang J.P. Drake J. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2002,90(9):1559-1564
The generalized multiprotocol label switching (GMPLS) is being developed as the control plane for the evolving photonic network. We describe how GMPLS can be used with mesh networks to provide efficient network resiliency. In particular we examine the key aspects of GMPLS that are used to support protection and restoration. We also study the various protection and restoration techniques, and we highlight the tradeoffs between recovery time and resource redundancy 相似文献
919.
S. Abiraman H. K. Varma T. V. Kumari P. R. Umashankar Annie John 《Bulletin of Materials Science》2002,25(5):419-429
This study investigates quantitatively and qualitatively the sol-gel derived bioactive glass-ceramic system (BGS)—apatite-wollastonite
(AW) type granules in the size range of 0.5–1 mm, as an effective graft material for bone augmentation and restoration. Scanning
electron micrographs (SEM) of the sintered granules revealed the rough material surface with micropores in the range 10–30
μm. X-ray diffraction (XRD) pattern of the granules revealed the presence of crystalline phases of the hydroxyapatite and
wollastonite, and the functional groups of the silicate and phosphates were identified by Fourier transform infrared spectroscopy
(FT-IR). Thein vitro cell culture studies with L929 mouse fibroblast cell line showed very few cells adhered on the BGS disc after 24 h. This
could be due to the highly reactive surface of the disc concomitant with the crystallization but not due to the cytotoxicity
of the material, since the cellular viability (MTT assay) with the material was 80‰ Cytotoxicity and cytocompatibility studies
proved that the material was non-toxic and biocompatible. After 12 weeks of implantation of the BGS granules in the tibia
bone of New Zealand white rabbits, the granules were found to be well osteointegrated, as observed in the radiographs. Angiogram
with barium sulphate and Indian ink after 12 weeks showed the presence of microcapillaries in the vicinity of the implant
site implicating high vascularity. Gross observation of the implant site did not show any inflammation or necrosis. SEM of
the implanted site after 24 weeks revealed good osteointegration of the material with the newly formed bone and host bone.
New bone was also observed within the material, which was degrading. Histological evaluation of the bone healing with the
BGS granules in the tibial defect at all time intervals was without inflammation or fibrous tissue encapsulation. After 2
weeks the new bone was observed as a trabeculae network around the granules, and by 6 weeks the defect was completely closed
with immature woven bone. By 12 weeks mature woven bone was observed, and new immature woven bone was seen within the cracks
of the granules. After 24 weeks the defect was completely healed with lamellar bone and the size of the granules decreased.
Histomorphometrically the area percentage of new bone formed was 67.77% after 12 weeks and 63.37% after 24 weeks. Less bone
formation after 24 weeks was due to an increased implant surface area contributed by the material degradation and active bone
remodeling. The osteostimulative and osteoconductive potential of the BGS granules was established by tetracycline labelling
of the mineralizing areas by 2 and 6 weeks. This sol-gel derived BGS granules proved to be bioactive and resorbable which
in turn encouraged active bone formation. 相似文献
920.
Robert H. Doremus Denis Murphy Narottam P. Bansal William A. Lanford Chandra Burman 《Journal of Materials Science》1985,20(12):4445-4453
When liquid water contacts a zirconium-barium-lanthanum fluoride glass, at least three different processes occur. Barium and zirconium fluoride dissolve into the water, water penetrates into the glass, and zirconium fluoride crystals grow on the glass surface, in static solution. The rate of dissolution, as measured by solution analysis, is possibly controlled by diffusion in the solid surface; surface blockage and surface reactions are other possible kinetic steps involved. Diffusion in solution is not the controlling mechanism. Hydrogen profiles in the glass surface suggest that the penetration rate of water into the glass is controlled by diffusion and a surface reaction. 相似文献