全文获取类型
收费全文 | 538982篇 |
免费 | 5461篇 |
国内免费 | 1230篇 |
专业分类
电工技术 | 9714篇 |
综合类 | 486篇 |
化学工业 | 83567篇 |
金属工艺 | 24326篇 |
机械仪表 | 18503篇 |
建筑科学 | 12579篇 |
矿业工程 | 3482篇 |
能源动力 | 11540篇 |
轻工业 | 46848篇 |
水利工程 | 6040篇 |
石油天然气 | 11532篇 |
武器工业 | 44篇 |
无线电 | 56575篇 |
一般工业技术 | 107366篇 |
冶金工业 | 93536篇 |
原子能技术 | 12483篇 |
自动化技术 | 47052篇 |
出版年
2021年 | 3835篇 |
2019年 | 3592篇 |
2018年 | 12794篇 |
2017年 | 13137篇 |
2016年 | 10484篇 |
2015年 | 4691篇 |
2014年 | 7443篇 |
2013年 | 20577篇 |
2012年 | 13767篇 |
2011年 | 22342篇 |
2010年 | 18959篇 |
2009年 | 19571篇 |
2008年 | 20146篇 |
2007年 | 21688篇 |
2006年 | 13404篇 |
2005年 | 14779篇 |
2004年 | 12998篇 |
2003年 | 12389篇 |
2002年 | 11698篇 |
2001年 | 10955篇 |
2000年 | 10654篇 |
1999年 | 10522篇 |
1998年 | 25116篇 |
1997年 | 18342篇 |
1996年 | 14258篇 |
1995年 | 10867篇 |
1994年 | 9872篇 |
1993年 | 9605篇 |
1992年 | 7493篇 |
1991年 | 7182篇 |
1990年 | 7186篇 |
1989年 | 7103篇 |
1988年 | 6930篇 |
1987年 | 5943篇 |
1986年 | 5998篇 |
1985年 | 6922篇 |
1984年 | 6606篇 |
1983年 | 6012篇 |
1982年 | 5636篇 |
1981年 | 5836篇 |
1980年 | 5644篇 |
1979年 | 5483篇 |
1978年 | 5554篇 |
1977年 | 6308篇 |
1976年 | 8125篇 |
1975年 | 5022篇 |
1974年 | 4778篇 |
1973年 | 4912篇 |
1972年 | 4191篇 |
1971年 | 3875篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
The problem of synthesis of an isotropic reradiator providing for the transparency of a circular perfectly conducting cylinder illuminated by a TE-polarized plane wave is investigated. A general solution to the problem of diffraction from the cylinder with a reradiator is obtained, and a system of nonlinear equations for the synthesis problem is derived. Numerical solution of the synthesis problem is illustrated by a number of examples. 相似文献
32.
We report for the first time optical signal-to-noise penalties which lead to performance degradations in single-fiber long-reach optical access networks when compared to identical dual-fiber systems. A simplified architecture, with reduced optical amplifier count compared to previous work, for single-fiber operation of a symmetrical 10-Gb/s, 1024-way split, 110-km long-reach optical access network is presented and demonstrated. In addition, a possible solution to remove the optical signal-to-noise penalty is suggested 相似文献
33.
The objective of this study was to compare methane production and characteristics of digested material in anaerobic digestion concepts according to the Animal By-Products Regulation (ABP-Regulation) of the EC (hygienisation of biowaste for 1 hour at 70 degrees C, particle size < 12 mm) and Finnish national regulations (treatment temperature 55 degrees C, feeding interval 24 h, hydraulic retention time (HRT) 20 d, particle size < 40 mm) and with small variations in treatment methods for treating manure and biowaste. Moreover, the survival of three different salmonella bacteria in these processes was studied. Hygienisation of biowaste prior to digestion at 35 degrees C enhanced methane production by 14-18% compared to similar treatment without hygienisation. The differences in treatment temperature, HRT and hygienisation of biowaste prior to digestion did not significantly affect the characteristics of digested material. The concepts according to the ABP-Regulation and Finnish national regulations were effective in destroying salmonella bacteria to an undetectable level. 相似文献
34.
The strong tendency of organic nanoparticles to rapidly self‐assemble into highly aligned superlattices at room temperature when solution‐cast from dispersions or spray‐coated directly onto various substrates is described. The nanoparticle dispersions are stable for years. The novel precipitation process used is believed to result in molecular distances and alignments in the nanoparticles that are not normally possible. Functional organic light‐emitting diodes (OLEDs)—which have the same host–dopant emissive‐material composition—with process‐tunable electroluminescence have been built with these nanoparticles, indicating the presence of novel nanostructures. For example, only changing the conditions of the precipitation process changes the OLED emission from green light to yellow. 相似文献
35.
An increase in hydrogen evolution from the hydrogen-evolving enzyme in the actinomycete Frankia was recorded in the presence of nickel. Immunogold localisation analysis of the intracellular distribution of hydrogenase proteins indicated that they were evenly distributed in the membranes and cytosol of both hyphae and vesicles. In addition, molecular characterisation of the hydrogen-evolving enzyme at the proteomic level, using two-dimensional gel electrophoresis combined with mass spectrometry, confirmed that the Frankia hydrogen-evolving enzyme is similar to the cyanobacterial bidirectional hydrogenase of Anabena siamensis. 相似文献
36.
The effects of the cuticle and epicuticular waxes of grapefruit, strawberry and apple on the photodegradation and penetration of chlorpyrifos-methyl were studied. Photodegradation experiments were conducted by exposing the insecticide to the light of a xenon lamp in the presence of a film of wax extracted from the fruit surface. The half-life of chlorpyrifos-methyl irradiated in absence of waxes was 9.6 min. The half-lives of pesticide irradiated in the presence of wax extracts of apple, grapefruit and strawberry were 83, 34 and 26 min, respectively. In penetration studies, fruit with and without wax layers were treated with an aqueous suspension of pesticide. The penetration of the pesticide from the cuticle to the pulp was measured after 24 h. Samples without wax contained a higher total amount of insecticide than those with wax. No pesticide was detected in samples of apple and grapefruit pulp. Residues were detected in all fractions of strawberry. The waxes and cuticle appear to have some effect on the photodegradation and penetration of chlorpyrifos-methyl in fruit samples. 相似文献
37.
卫星电视上行站信号自动监测系统 总被引:1,自引:0,他引:1
鄂志东 《信息安全与通信保密》2006,(3):84-85
每个卫星电视上行站都必须对卫星转发器上的电视信号进行监测,才能及时发现卫星电视上行信号的异常变化,准确处理异常突发干扰。为实现自动、连续、实时监测,就需要建立卫星电视上行站射频信号自动监测系统。文章详细介绍了一个自动监测系统的设计及实现。 相似文献
38.
39.
40.
The catalytic activities of alumina prepared from an Al alkoxide-amine adduct monomer for the reaction of cyclopentene oxide with piperidine was determined after various pretreatments, including calcination and exposure to moisture. They were compared with the activity of alumina prepared by the conventional hydrolysis method. It was found that the as-prepared sample from the alkoxide-amine monomer preparation was five times more active than a conventional preparation, suggesting that it has a higher density of surface Lewis acid sites. However, its activity was much more severely suppressed by exposure to moisture. 相似文献