全文获取类型
收费全文 | 128984篇 |
免费 | 19137篇 |
国内免费 | 3694篇 |
专业分类
电工技术 | 5733篇 |
技术理论 | 10篇 |
综合类 | 6523篇 |
化学工业 | 31629篇 |
金属工艺 | 5476篇 |
机械仪表 | 6606篇 |
建筑科学 | 8320篇 |
矿业工程 | 3036篇 |
能源动力 | 3865篇 |
轻工业 | 12560篇 |
水利工程 | 1702篇 |
石油天然气 | 6164篇 |
武器工业 | 571篇 |
无线电 | 16155篇 |
一般工业技术 | 21934篇 |
冶金工业 | 5402篇 |
原子能技术 | 970篇 |
自动化技术 | 15159篇 |
出版年
2024年 | 356篇 |
2023年 | 1449篇 |
2022年 | 2440篇 |
2021年 | 3554篇 |
2020年 | 3886篇 |
2019年 | 5280篇 |
2018年 | 5562篇 |
2017年 | 6071篇 |
2016年 | 6174篇 |
2015年 | 7021篇 |
2014年 | 7755篇 |
2013年 | 9711篇 |
2012年 | 7663篇 |
2011年 | 7841篇 |
2010年 | 7404篇 |
2009年 | 7114篇 |
2008年 | 6563篇 |
2007年 | 6250篇 |
2006年 | 6166篇 |
2005年 | 5258篇 |
2004年 | 4494篇 |
2003年 | 5014篇 |
2002年 | 5635篇 |
2001年 | 4856篇 |
2000年 | 3775篇 |
1999年 | 3065篇 |
1998年 | 2043篇 |
1997年 | 1685篇 |
1996年 | 1617篇 |
1995年 | 1336篇 |
1994年 | 1119篇 |
1993年 | 856篇 |
1992年 | 629篇 |
1991年 | 489篇 |
1990年 | 395篇 |
1989年 | 290篇 |
1988年 | 262篇 |
1987年 | 176篇 |
1986年 | 137篇 |
1985年 | 111篇 |
1984年 | 74篇 |
1983年 | 50篇 |
1982年 | 48篇 |
1981年 | 36篇 |
1980年 | 28篇 |
1979年 | 17篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
21.
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
22.
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
23.
24.
25.
Flame structure and NO emission characteristics in counterflow diffusion flame of blended fuel of H2/CO2/Ar have been numerically simulated with detailed chemistry. The combination of H2, CO2 and Ar as fuel is selected to clearly display the contribution of hydrocarbon products to flame structure and NO emission characteristics due to the breakdown of CO2. A radiative heat loss term is involved to correctly describe the flame dynamics especially at low strain rates. The detailed chemistry adopts the reaction mechanism of GRI 2.11, which consists of 49 species and 279 elementary reactions. All mechanisms including thermal, NO2, N2O and Fenimore are taken into account to separately evaluate the effects of CO2 addition on NO emission characteristics. The increase of added CO2 quantity causes flame temperature to fall since at high strain rates a diluent effect is prevailing and at low strain rates the breakdown of CO2 produces relatively populous hydrocarbon products and thus the existence of hydrocarbon products inhibits chain branching. It is also found that the contribution of NO production by N2O and NO2 mechanisms are negligible and that thermal mechanism is concentrated on only the reaction zone. As strain rate and CO2 quantity increase, NO production is remarkably augmented. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
26.
STEP-NC是由欧美提出的新的数控接口标准,目的是取代旧的数控标准ISO6983。本文首先分析了STEP-NC的两种应用标准:ISO14649(ARM)和STEPAP238(AIM),并且分析了Part21和Part28的应用领域及Part21文件格式和其中的信息产生方法。然後介绍了ST-Developer环境下ROSEC++颠库编程接口及其应用程序的早、晚编联方式,并且以此为基础设计了整个程序的编程流程,重点介绍了工艺规划设计。最后给出了在C++编程环境下利用ROSE库函数写入形成STEP AP238文件的方法。 相似文献
27.
Hong Jeong Jeong‐Ho Park 《International Journal of Adaptive Control and Signal Processing》2002,16(1):1-23
Tracking targets of interest is one of the major research areas in radar surveillance systems. We formulate the problem as incomplete data estimation and apply EM to the MAP estimate. The resulting filter has a recursive structure analogous to the Kalman filter. The advantage is that the measurement‐update deals with multiple measurements in parallel and the parameter‐update estimates the system parameters on the fly. Experiments tracking separate targets in parallel show that tracking maintenance ratio of the proposed system is better than that of NNF and RMS position error is smaller than that of PDAF. Also, the system parameters are correctly obtained even from incorrect initial values. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
28.
Interface characteristics of carbon fibre reinforced copper matrix composites materials with various interface states and their effect on the flexural strength of composites have been studied. Interfacial states are mechanical bonding, dissolution bonding and reaction bonding. To a certain extent, raising the interfacial strength enables an increase in the flexural strength due to prevention of carbon fibre being pulled out under low stress during fracture process of composites. Raising the interfacial bondage strength, causes the brittleness of composites to increase; the fracture surface of composites is converted from a fibre pull-out model to a fibre even model. While strengthening the interface bondage, the extent of chemical reaction and dissolution at the interface must be controlled to avoid degrading the carbon fibre. 相似文献
30.
Yu‐Qing Cao Zhan Zhang Yan‐Xin Guo 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2008,83(10):1441-1444
A new efficient method for synthesising nitriles, important organic reagents, is reported in this paper. In an environmentally benign solvent‐free system, aryl carboxylic acids were converted into the corresponding nitriles via one‐pot reactions, by amidation with ethyl carbamate followed by dehydration with thionyl chloride, in excellent yields. The results showed that the method has the advantages of lower cost, higher yield, less pollution and greater ease of work‐up. Copyright © 2008 Society of Chemical Industry 相似文献