全文获取类型
收费全文 | 419428篇 |
免费 | 18217篇 |
国内免费 | 8842篇 |
专业分类
电工技术 | 15656篇 |
技术理论 | 13篇 |
综合类 | 11896篇 |
化学工业 | 68538篇 |
金属工艺 | 19241篇 |
机械仪表 | 18616篇 |
建筑科学 | 19136篇 |
矿业工程 | 6822篇 |
能源动力 | 10419篇 |
轻工业 | 37162篇 |
水利工程 | 5740篇 |
石油天然气 | 15810篇 |
武器工业 | 1435篇 |
无线电 | 46345篇 |
一般工业技术 | 65969篇 |
冶金工业 | 53984篇 |
原子能技术 | 7854篇 |
自动化技术 | 41851篇 |
出版年
2023年 | 3727篇 |
2022年 | 7640篇 |
2021年 | 10582篇 |
2020年 | 7850篇 |
2019年 | 7057篇 |
2018年 | 9188篇 |
2017年 | 9800篇 |
2016年 | 9368篇 |
2015年 | 9796篇 |
2014年 | 13260篇 |
2013年 | 21395篇 |
2012年 | 18329篇 |
2011年 | 21106篇 |
2010年 | 17490篇 |
2009年 | 17551篇 |
2008年 | 17425篇 |
2007年 | 16773篇 |
2006年 | 16138篇 |
2005年 | 13958篇 |
2004年 | 11100篇 |
2003年 | 10026篇 |
2002年 | 9641篇 |
2001年 | 9135篇 |
2000年 | 8890篇 |
1999年 | 9428篇 |
1998年 | 15988篇 |
1997年 | 11790篇 |
1996年 | 9569篇 |
1995年 | 7514篇 |
1994年 | 6518篇 |
1993年 | 5908篇 |
1992年 | 4698篇 |
1991年 | 4161篇 |
1990年 | 4149篇 |
1989年 | 3933篇 |
1988年 | 3665篇 |
1987年 | 3142篇 |
1986年 | 3137篇 |
1985年 | 3496篇 |
1984年 | 3330篇 |
1983年 | 2977篇 |
1982年 | 2822篇 |
1981年 | 2888篇 |
1980年 | 2781篇 |
1979年 | 2652篇 |
1978年 | 2709篇 |
1977年 | 2970篇 |
1976年 | 3827篇 |
1975年 | 2377篇 |
1973年 | 2381篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Phase equilibria involving spinel solid solutions, delafossite, and hematite in the Fe–Cu–O system are studied by emf measurements in solid-electrolyte galvanic cells. The results demonstrate that, above 1250 K, Fe3O4 and CuFe2O4 form a continuous series of solid solutions. At lower temperatures, the solid solution disproportionates with the formation of delafossite and Fe2O3, and two spinel solid solutions appear: one based on Fe3O4 and the other based on Cu2FeO4. The compositions of the spinel phases in equilibrium with delafossite and Fe2O3 are determined in the range 1100–1250 K. 相似文献
52.
Thomas S. Lowry John C. Bright Murray E. Close Christina A. Robb Paul A. White Stewart G. Cameron 《国际水资源开发杂志》2003,19(4):579-592
The primary objective of this project is to identify gaps, whether real or perceived, that hinder effective groundwater management in New Zealand. These gaps show as gaps in information, gaps in implementation, gaps in technological and management tools, and gaps in understanding of fundamental processes. The secondary objective is to propose a management strategy to close the identified gaps. Several methods are used to meet these objectives: surveys distributed to selected staff in each regional council; the review of various written reports; the analysis of land-use databases; and private consultation within each regional council. Results show that groundwater management in New Zealand is generally reactionary with the main gaps being in strategic planning and national guidelines. Most gaps appear to be predominantly information and implementation issues. In some cases there are gaps in the understanding of fundamental processes within an aquifer system, including the long-term effects of land-use on groundwater quality. An adaptive management approach is suggested as a means of closing these gaps. 相似文献
53.
54.
Sheldon H. Jacobson Julie L. Virta Jon M. Bowman John E. Kobza John J. Nestor 《IIE Transactions》2003,35(3):259-269
Aviation security protects vital national interests, as well as passengers and aircraft. Key components of an aviation security system include baggage and passenger screening devices and operations. Determining how and where to assign (deploy) such devices can be quite challenging. Moreover, even after such systems are in place, it can be difficult to measure their effectiveness. This paper describes how discrete optimization models can be used to address these questions, based on three performance measures that quantify the effectiveness of airport baggage screening security device systems. These models are used to solve for optimal airport baggage screening security device deployments considering the number of passengers on a set of flights who have not been cleared using a security risk assessment system in use by the Federal Aviation Administration (i.e., passengers whose baggage is subjected to screening), the number of flights in this set, and the size of the aircraft for such flights. Several examples are provided to illustrate these results, including an example that uses data available from the Official Airline Guide. 相似文献
55.
Introduces the current issue of the American Psychologist, which examines the ramifications of the Brown vs. Board of Education decision and its sequelae over the last 50 years, with a special focus on the mixed role of psychological research and practice. Despite the great strides toward a more just society since May 17, 1954, inequities remain in schooling, in social relations, and in economic opportunity. How will history judge American psychology 50 years from now vis-a-vis the possibilities it helped create via Brown? The articles in this special issue suggest several important directions of research and action our field will need to take if the verdict of history is to be a positive one. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
56.
Object segmentation and labeling by learning from examples 总被引:1,自引:0,他引:1
We propose a system that employs low-level image segmentation followed by color and two-dimensional (2-D) shape matching to automatically group those low-level segments into objects based on their similarity to a set of example object templates presented by the user. A hierarchical content tree data structure is used for each database image to store matching combinations of low-level regions as objects. The system automatically initializes the content tree with only "elementary nodes" representing homogeneous low-level regions. The "learning" phase refers to labeling of combinations of low-level regions that have resulted in successful color and/or 2-D shape matches with the example template(s). These combinations are labeled as "object nodes" in the hierarchical content tree. Once learning is performed, the speed of second-time retrieval of learned objects in the database increases significantly. The learning step can be performed off-line provided that example objects are given in the form of user interest profiles. Experimental results are presented to demonstrate the effectiveness of the proposed system with hierarchical content tree representation and learning by color and 2-D shape matching on collections of car and face images. 相似文献
57.
A.L. Pan H.G. ZhengZ.P. Yang F.X. Liu Z.J. Ding Y.T. Qian 《Materials Research Bulletin》2003,38(5):789-796
Small Ag particles or clusters dispersed mesoporous SiO2 composite films were prepared by a new method: First the matrix SiO2 films were prepared by sol-gel process combined with the dip-coating technique, then they were soaked in AgNO3 solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO2 films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements. 相似文献
58.
59.
A new topology for a photonic signal processor, which overcomes the basic recursive frequency response problem that limits the passband range, is presented. The structure is based on a new multiple-wavelength offset-cavity structure that is cascaded with a series of unbalanced delay line structures. This not only can synthesize a very narrow notch response with good shape factor but also permits a multifold extension of the free spectral range (FSR) and passband width. Results on the interference mitigation filter demonstrate a stopband of 1% of center frequency and a fourfold increase in the FSR and passband width, while also having a very small shape factor, in excellent agreement with predictions. 相似文献
60.