全文获取类型
收费全文 | 299754篇 |
免费 | 7018篇 |
国内免费 | 1838篇 |
专业分类
电工技术 | 5738篇 |
技术理论 | 5篇 |
综合类 | 1177篇 |
化学工业 | 49566篇 |
金属工艺 | 11684篇 |
机械仪表 | 8595篇 |
建筑科学 | 7920篇 |
矿业工程 | 1732篇 |
能源动力 | 6563篇 |
轻工业 | 31187篇 |
水利工程 | 3270篇 |
石油天然气 | 6106篇 |
武器工业 | 249篇 |
无线电 | 30059篇 |
一般工业技术 | 56025篇 |
冶金工业 | 56093篇 |
原子能技术 | 5812篇 |
自动化技术 | 26829篇 |
出版年
2021年 | 3509篇 |
2020年 | 2462篇 |
2019年 | 2885篇 |
2018年 | 4891篇 |
2017年 | 4867篇 |
2016年 | 5279篇 |
2015年 | 3911篇 |
2014年 | 6034篇 |
2013年 | 14132篇 |
2012年 | 10085篇 |
2011年 | 12506篇 |
2010年 | 9881篇 |
2009年 | 10323篇 |
2008年 | 11298篇 |
2007年 | 11170篇 |
2006年 | 9635篇 |
2005年 | 8364篇 |
2004年 | 7575篇 |
2003年 | 7018篇 |
2002年 | 6919篇 |
2001年 | 6754篇 |
2000年 | 6337篇 |
1999年 | 6420篇 |
1998年 | 15688篇 |
1997年 | 11256篇 |
1996年 | 8386篇 |
1995年 | 6208篇 |
1994年 | 5644篇 |
1993年 | 5506篇 |
1992年 | 4141篇 |
1991年 | 3946篇 |
1990年 | 4025篇 |
1989年 | 3957篇 |
1988年 | 3700篇 |
1987年 | 3166篇 |
1986年 | 3212篇 |
1985年 | 3590篇 |
1984年 | 3486篇 |
1983年 | 3196篇 |
1982年 | 2837篇 |
1981年 | 3055篇 |
1980年 | 2783篇 |
1979年 | 2959篇 |
1978年 | 2840篇 |
1977年 | 3009篇 |
1976年 | 4060篇 |
1975年 | 2534篇 |
1974年 | 2356篇 |
1973年 | 2388篇 |
1972年 | 2023篇 |
排序方式: 共有10000条查询结果,搜索用时 625 毫秒
991.
This paper studies a particular single-stage power-factor-correction (PFC) switching regulator employing a discontinuous-conduction-mode (DCM) boost-input cell and a continuous-current-mode (CCM) forward output cell. Although this single-stage PFC regulator can provide a reasonably high power factor when its PFC stage is operating in discontinuous mode, substantial reduction in line-current harmonics is possible by applying a suitable frequency-modulation scheme. This paper derives a frequency-modulation scheme and proposes a practical implementation using a simple translinear analog circuit. A quantitative analysis on the total harmonic distortion (THD) of the line current when the circuit is subject to a limited range of frequency variations is presented along with some considerations for practical design. Experimental data obtained from a prototype confirms the effectiveness of the proposed frequency-modulation scheme. The proposed analog translinear circuit allows custom integrated circuit implementation, making it a viable low-cost solution to the elimination of line-current harmonics in switching regulators 相似文献
992.
We report a case of sperm cell seminoma caused by trauma. The data in the literature indicate the frequency is less than 5% of all seminomas. This case was exclusively located in the gonads and was a pure form. Orchidectomy with high ligature of the cord and adjuvant radiotherapy at the dose of 25 Gy centered on the para-aortic and subdiaphragmatic chains is adequate treatment. When inguinoscrotal surgery is performed, this zone must be irradiated with 25 Gy. Prognosis is satisfactory: 5-year survival is 100% with this protocol. 相似文献
993.
994.
995.
996.
W. Sjoerd Kijlstra Joop C.M.L. Daamen Jolinde M. van de Graaf Bart van der Linden Eduard K. Poels Alfred Bliek 《Applied catalysis. B, Environmental》1996,7(3-4):337-357
The effect of water on the selective catalytic reduction (SCR) of nitric oxide with ammonia over alumina supported with 2–15 wt.-% manganese oxide was investigated in the temperature range 385–600 K, with the emphasis on the low side of this temperature window. Studies on the effect of 1–5 vol.-% water vapour on the SCR reaction rate and selectivity were combined with TPD experiments to reveal the influence of water on the adsorption of the single SCR reactants. It turned out that the activity decrease due to water addition can be divided into a reversible inhibition and an irreversible deactivation. Inhibition is caused by molecular adsorption of water. TPD studies showed that water can adsorb competitively with both ammonia and nitric oxide. Additional kinetic experiments revealed that adsorbed ammonia is present in excess on the catalyst surface, even in the presence of water. Reduced nitric oxide adsorption is responsible for the observed reversible decrease in the reaction rate; the fractional reaction order changes from 0.79 in the absence of water to 1.07 in its presence. Deactivation is probably due to the dissociative adsorption of water, resulting in the formation of additional surface hydroxyls. As the amount of surface hydroxyls formed is limited to a saturation level, the deactivating effect on the catalyst is limited too. The additional hydroxyls condense and desorb in the temperature range 525–775 K, resulting in a lower degree of deactivation at higher temperature. A high temperature treatment at 775 K results in a complete regeneration. The amount of surface hydroxyls formed per unit surface area decreases at increasing MnOx-loading. The selectivity to the production of nitrogen is enhanced significantly by the presence of gas phase water. 相似文献
997.
A. Andreev L. Prahov M. Gabrovska A. Eliyas V. Ivanova 《Applied catalysis. B, Environmental》1996,8(4):365-373
The catalytic activity of a wide range of transition metal oxides in oxidation of sulphide ions by air in aqueous medium was studied. Some specific features of the reaction mechanism on some of the studied oxides were considered. The transition metal oxides are promising catalysts for practical application. Some of these oxides will allow the preparation of catalysts possessing activity comparable to that of the cobalt phthalocyanine based catalysts, popular in industrial practice. 相似文献
998.
999.
C Julian-Reynier G Macquart-Moulin JP Moatti A Loundou Y Aurran F Chabal S Aymé 《Canadian Metallurgical Quarterly》1993,13(7):613-627
The objective of this study was to explore women's attitudes towards prenatal diagnosis of trisomy 21 and to examine some of the factors possibly responsible for these attitudes before implementing in real practice serological screening of pregnant women at risk for trisomy 21. We carried out a telephone survey on a representative sample of women who had recently had a normal livebirth delivery in the Marseille district in 1990. The participation rate was 80 per cent and the average age of the mothers was 28.9 years. Among the 514 women interviewed, 78 per cent stated that they would ask for an amniocentesis for a 1 per cent risk of trisomy 21 at their next pregnancy. When adjusting for confounding factors, the decision to have or not to have an amniocentesis was found to depend not only on the women's attitude towards induced abortion, but also on their understanding of the risk involved and on the social context (knowing a handicapped child, discussion with the father). It also depended on the women's age and on what they knew about amniocentesis from the medical point of view. The risk of miscarriage can influence a woman's choice but this objection was not found to affect the women's decisions significantly in our survey. The data showed the existence of a high potential demand for fetal karyotyping. 相似文献
1000.