全文获取类型
收费全文 | 373866篇 |
免费 | 28104篇 |
国内免费 | 15053篇 |
专业分类
电工技术 | 20617篇 |
技术理论 | 35篇 |
综合类 | 23199篇 |
化学工业 | 63564篇 |
金属工艺 | 21504篇 |
机械仪表 | 24139篇 |
建筑科学 | 29423篇 |
矿业工程 | 12290篇 |
能源动力 | 10434篇 |
轻工业 | 22788篇 |
水利工程 | 6225篇 |
石油天然气 | 25590篇 |
武器工业 | 2945篇 |
无线电 | 40512篇 |
一般工业技术 | 43762篇 |
冶金工业 | 19427篇 |
原子能技术 | 3776篇 |
自动化技术 | 46793篇 |
出版年
2024年 | 1543篇 |
2023年 | 5892篇 |
2022年 | 10309篇 |
2021年 | 14747篇 |
2020年 | 10998篇 |
2019年 | 9284篇 |
2018年 | 10493篇 |
2017年 | 11884篇 |
2016年 | 10294篇 |
2015年 | 14377篇 |
2014年 | 17866篇 |
2013年 | 21315篇 |
2012年 | 23150篇 |
2011年 | 25377篇 |
2010年 | 22071篇 |
2009年 | 20968篇 |
2008年 | 20277篇 |
2007年 | 19806篇 |
2006年 | 20615篇 |
2005年 | 18242篇 |
2004年 | 11829篇 |
2003年 | 10383篇 |
2002年 | 9550篇 |
2001年 | 8454篇 |
2000年 | 9161篇 |
1999年 | 10666篇 |
1998年 | 8643篇 |
1997年 | 7311篇 |
1996年 | 6852篇 |
1995年 | 5700篇 |
1994年 | 4752篇 |
1993年 | 3355篇 |
1992年 | 2701篇 |
1991年 | 2038篇 |
1990年 | 1509篇 |
1989年 | 1212篇 |
1988年 | 980篇 |
1987年 | 658篇 |
1986年 | 500篇 |
1985年 | 358篇 |
1984年 | 251篇 |
1983年 | 159篇 |
1982年 | 170篇 |
1981年 | 117篇 |
1980年 | 95篇 |
1979年 | 40篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1959年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Although cortico steroids are effective anti-inflammatory agents in ameliorating asthma symptoms and bronchial hperreactivity, their mechanism of action is unknown. Interleukin (IL)-5 is known to play a key role in regulating eosinophil proliferation and activation. Therefore, we examined the changes of IL-5 mRNA expressions in PBMC semi-quantitatively with RT-PCR as well as serum ECP levels and MCH-PC20 values in asthmatics before and after being treated with corticosteroids. The results revealed that there were significant decrease in the level of IL-5 mRNA and serum ECP concentration after therapy (P < 0.05) and there was remarkable improvement in the values of MCH-PC20 and FEV1% (P < 0.05). It was also found that the changes of serum ECP levels or MCH-PC20 values were accompanied by a reduction in IL-5 mRNA expression (r = 0.5426 or 0.4857, P < 0.05). These results suggested that the therapeutic effects of corticosteroids in asthma may result from modulation of IL-5 gene expression with consequent inhibition of eosinophil activation. 相似文献
82.
83.
本文对各种运输方式易腐货物的运价进行了比较分析,对铁路冷藏运输的现行运价和保本运价进行了讨论,提出了铁路冷藏运输运价改革的若干建议,可供铁路进行冷藏运输改革参考。 相似文献
84.
Jin-Wei Shi Chi-Kuang Sun 《Lightwave Technology, Journal of》2002,20(11):1942-1950
We present the theory and design of a tapered line distributed photodetector (TLDP). In the previously demonstrated velocity-matched distributed photodetector (VMDP), high electrical bandwidth is achieved by proper termination in the input end to absorb reverse traveling waves, sacrificing one-half of the quantum efficiency. By utilizing the tapered line structure and phase matching between optical waves and microwaves in our analyzed structure, a traveling-wave photodetector is more realizable and ultrahigh bandwidth can be attained due to removal of the extra input dummy load that sacrifices one-half of the total quantum efficiency. To investigate the advantages of TLDP over VMDP, we calculate their electrical bandwidth performances by using an analytic photodistributed current model. We adopted low-temperature-grown (LTG) GaAs-based metal-semiconductor-metal (MSM) traveling-wave photodetectors as example unit active devices in the analytic bandwidth calculation for their high-speed and high-power performances. Both VMDP and TLDP in our simulation are assumed to be transferred onto glass substrates, which would achieve high microwave velocity/impedance and make radiation loss negligible. The simulated bandwidth of a properly designed LTG GaAs MSM TLDP is /spl sim/325 GHz, which is higher than the simulated bandwidth of the LTG GaAs MSM VMDP with an open-circuit input end (/spl sim/240 GHz) and is almost comparable to the simulated bandwidth of an input-terminated LTG GaAs MSM VMDP (/spl sim/330 GHz). This proposed method can be applied to the design of high-bandwidth distributed photodetectors for radio-frequency photonic systems and optoelectronic generation of high-power microwaves and millimeter waves. 相似文献
85.
86.
Buttari D. Chini A. Meneghesso G. Zanoni E. Moran B. Heikman S. Zhang N.Q. Shen L. Coffie R. DenBaars S.P. Mishra U.K. 《Electron Device Letters, IEEE》2002,23(2):76-78
Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs 相似文献
87.
根据我院专用测试设备计量确认现状及需求,对开展试飞专用测试设备计量确认工作的要求,步骤、确认原则,以及编定计量确认方法进行了必要的讨论分析。 相似文献
88.
Development of the Chinese Scientometric Indicators (CSI) 总被引:1,自引:0,他引:1
We describe the Chinese Scientometric Indicators (CSI), an indicator database derived from the Chinese Science Citation Database (CSCD). Its design is supported by the Natural Sciences Foundation of China (NSFC). In this indicator database data of a statistical nature are organized and categorized leading to ranked lists and providing bases for comparisons among Chinese institutions and regions. 相似文献
89.
90.