全文获取类型
收费全文 | 131026篇 |
免费 | 11478篇 |
国内免费 | 5944篇 |
专业分类
电工技术 | 7860篇 |
技术理论 | 8篇 |
综合类 | 8781篇 |
化学工业 | 21693篇 |
金属工艺 | 7465篇 |
机械仪表 | 8936篇 |
建筑科学 | 10097篇 |
矿业工程 | 4174篇 |
能源动力 | 3799篇 |
轻工业 | 8766篇 |
水利工程 | 2350篇 |
石油天然气 | 8163篇 |
武器工业 | 1061篇 |
无线电 | 14828篇 |
一般工业技术 | 15642篇 |
冶金工业 | 6361篇 |
原子能技术 | 1447篇 |
自动化技术 | 17017篇 |
出版年
2024年 | 453篇 |
2023年 | 2198篇 |
2022年 | 4118篇 |
2021年 | 5615篇 |
2020年 | 4211篇 |
2019年 | 3524篇 |
2018年 | 3940篇 |
2017年 | 4524篇 |
2016年 | 3862篇 |
2015年 | 5380篇 |
2014年 | 6749篇 |
2013年 | 8165篇 |
2012年 | 8598篇 |
2011年 | 9612篇 |
2010年 | 8362篇 |
2009年 | 8010篇 |
2008年 | 7816篇 |
2007年 | 7225篇 |
2006年 | 7010篇 |
2005年 | 6006篇 |
2004年 | 3951篇 |
2003年 | 3403篇 |
2002年 | 3117篇 |
2001年 | 2768篇 |
2000年 | 2750篇 |
1999年 | 2978篇 |
1998年 | 2530篇 |
1997年 | 2052篇 |
1996年 | 1996篇 |
1995年 | 1716篇 |
1994年 | 1369篇 |
1993年 | 1051篇 |
1992年 | 769篇 |
1991年 | 625篇 |
1990年 | 466篇 |
1989年 | 362篇 |
1988年 | 311篇 |
1987年 | 193篇 |
1986年 | 148篇 |
1985年 | 115篇 |
1984年 | 76篇 |
1983年 | 60篇 |
1982年 | 62篇 |
1981年 | 43篇 |
1980年 | 43篇 |
1979年 | 25篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 7篇 |
1951年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
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
182.
威海市污水处理深海排放工程是为改善其水域环境而建设的一英包括市区排水系统,污水预处理厂,输水管道和海洋放流管系统在内的完整而典型的工程,因此其工程设计扣的经验将会对同类项目的设计与工程建设有所借鉴。 相似文献
183.
在活齿传动理论及齿轮泵工作原理相结合的基础上,介绍了双相活齿泵的结构及其工作原理,研究结果表明,双相活齿泵具有流量大、流量均匀性好、噪声低以及可变量等优点。 相似文献
184.
185.
186.
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
187.
188.
Liang Y.C. Wenjiang Zeng Pick Hong Ong Zhaoxia Gao Jun Cai Balasubramanian N. 《Electron Device Letters, IEEE》2002,23(12):700-703
In this letter, a concise process technology is proposed for the first time to enable the fabrication of good quality three-dimensional (3-D) suspended radio frequency (RF) micro-inductors on bulk silicon, without utilizing the lithography process on sidewall and trench-bottom patterning. Samples were fabricated to demonstrate the applicability of the proposed process technology. 相似文献
189.
190.
Interfacial segregation of Ti in the brazing of diamond grits onto a steel substrate using a Cu-Sn-Ti brazing alloy 总被引:1,自引:0,他引:1
Wen-Chung Li Shun-Tian Lin Cheng Liang 《Metallurgical and Materials Transactions A》2002,33(7):2163-2172
Diamond grits were brazed onto a steel substrate using a prealloyed Cu-10Sn-15Ti (wt pct) brazing alloy at 925 °C and 1050
°C. Due to the relatively high concentration of Ti in the brazing alloy, the braze matrix exhibited a composite structure,
composed of β-(Cu,Sn), a Cu-based solid solution, and various intermetallic compounds with different morphologies. The reaction of Ti with
diamond yielded a continuous TiC layer on the surfaces of the diamond grits. On top of the TiC growth front, an intermetallic
compound, composed of Sn and Ti, nucleated and grew into a randomly interwoven fine lacey structure. An interfacial structure
developed as the interwoven fine lacey phase was semicoherently bonded to the TiC layer, with the Cu-based braze matrix filling
its interstices. The thickness of such a composite layer was increased linearly with the square root of isothermal holding
time at 925 °C, complying with the law of a diffusion-controlled process. However, at 1050 °C, the segregation behavior of
Ti and Sn to the interfaces between the TiC layer and the braze matrix diminished, due to the increased solubility of Ti in
the Cu-based liquid phase. The enhanced dissolution of Ti in the Cu-based liquid phase at 1050 °C also caused the precipitation
of rod-like CuTi with an average diameter of about 0.2 μm during cooling. SnTi3 was the predominant intermetallic compound and existed in three different forms in the braze matrix. It existed as interconnected
grains of large size which either floated to the surface of the braze matrix or grew into faceted grains. It also exhibited
a nail-like structure with a mean diameter of about 1 μm for the rod section and a lamellar structure arising from a eutectic reaction during cooling. 相似文献