全文获取类型
收费全文 | 46661篇 |
免费 | 5442篇 |
国内免费 | 3322篇 |
专业分类
电工技术 | 14315篇 |
技术理论 | 1篇 |
综合类 | 3738篇 |
化学工业 | 5748篇 |
金属工艺 | 2850篇 |
机械仪表 | 2056篇 |
建筑科学 | 2164篇 |
矿业工程 | 1216篇 |
能源动力 | 1688篇 |
轻工业 | 1391篇 |
水利工程 | 1156篇 |
石油天然气 | 1723篇 |
武器工业 | 484篇 |
无线电 | 6047篇 |
一般工业技术 | 4628篇 |
冶金工业 | 1577篇 |
原子能技术 | 1044篇 |
自动化技术 | 3599篇 |
出版年
2024年 | 215篇 |
2023年 | 838篇 |
2022年 | 1134篇 |
2021年 | 1426篇 |
2020年 | 1582篇 |
2019年 | 1315篇 |
2018年 | 1328篇 |
2017年 | 1784篇 |
2016年 | 1924篇 |
2015年 | 2011篇 |
2014年 | 2863篇 |
2013年 | 2888篇 |
2012年 | 3326篇 |
2011年 | 3772篇 |
2010年 | 2803篇 |
2009年 | 2929篇 |
2008年 | 2829篇 |
2007年 | 3383篇 |
2006年 | 2945篇 |
2005年 | 2403篇 |
2004年 | 2001篇 |
2003年 | 1623篇 |
2002年 | 1265篇 |
2001年 | 1191篇 |
2000年 | 1015篇 |
1999年 | 855篇 |
1998年 | 645篇 |
1997年 | 561篇 |
1996年 | 500篇 |
1995年 | 396篇 |
1994年 | 334篇 |
1993年 | 243篇 |
1992年 | 215篇 |
1991年 | 175篇 |
1990年 | 146篇 |
1989年 | 140篇 |
1988年 | 91篇 |
1987年 | 67篇 |
1986年 | 54篇 |
1985年 | 48篇 |
1984年 | 49篇 |
1983年 | 30篇 |
1982年 | 23篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1959年 | 3篇 |
1951年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
近年来,大电流的监测成了人们十分关心的问题,这是由于大电流有强磁、高压、电流大等一系列的直接监测所不能克服的问题。这就需要把高压大电流的强电信号,经过提取后变成能在实验室中直接观测的弱电信号。为此,如何从高压大电流的信号中提取出随实时信号变化而实时变化的小信号就变成了一个关键问题。详细介绍了监测信号提取的光收发电路原理和后续信号处理电路,分析了各部分的作用,给出了必要的分析结果。 相似文献
92.
Fon-Chieh Chang R. R. Fessler B. D. Merkle J. M. Borton W. M. Goldberger 《Particulate Science and Technology》2004,22(1):35-50
Electroconsolidation® is a process for densifying complex-shaped parts by using electrically conductive particulate solids as a pressure-transmitting medium. The part is immersed in a bed of the particulate medium contained in a die chamber. Sintering temperature is achieved by resistive heating of the medium while applying compaction pressure. The process is capable of ultrahigh temperatures and short cycle times and offers the potential for low processing costs.
Control of the process and selection of process conditions require knowledge of the temperatures within the die. Temperature gradients exist because of the high heating rate and because of variations of density and electrical resistivity of the medium due to the presence of the part. Direct measurement of temperature with thermocouples or other conventional means is impractical because of the high temperatures, high currents, and high pressures that are involved. Therefore, a computer model was developed to predict temperature as a function of time and applied voltage for any location in the die. The computer model is composed of three parts: a geometrical model to approximate the density and resistivity variations in the medium, a finite-element model to calculate the rate of resistive heating within each element, and a finite-difference model to calculate the temperature distribution based on solution of the heat-transfer equations. Predicted temperatures have been shown to be in excellent agreement with measurements, and numerical simulation provided encouraging consistency and reasonably accurate predictions of temperature profiles within the die. The model demonstrated the feasibility of a new process to achieve simultaneous application of pressure and heat to powder densification in Electroconsolidation. 相似文献
Control of the process and selection of process conditions require knowledge of the temperatures within the die. Temperature gradients exist because of the high heating rate and because of variations of density and electrical resistivity of the medium due to the presence of the part. Direct measurement of temperature with thermocouples or other conventional means is impractical because of the high temperatures, high currents, and high pressures that are involved. Therefore, a computer model was developed to predict temperature as a function of time and applied voltage for any location in the die. The computer model is composed of three parts: a geometrical model to approximate the density and resistivity variations in the medium, a finite-element model to calculate the rate of resistive heating within each element, and a finite-difference model to calculate the temperature distribution based on solution of the heat-transfer equations. Predicted temperatures have been shown to be in excellent agreement with measurements, and numerical simulation provided encouraging consistency and reasonably accurate predictions of temperature profiles within the die. The model demonstrated the feasibility of a new process to achieve simultaneous application of pressure and heat to powder densification in Electroconsolidation. 相似文献
93.
随着苏丹1/2/4区块的深入开发,为了加快开发主力油层,提高原油产量,降低钻井成本,GNPOC在此区块布置水平井开发任务。所开发的区域主要分布于Heglig,Munga,Simbir,Bamboo区块,油层所在是Zaqar和Bentiue地层。此层位具有高渗透,地层温度高,易塌、气窜等复杂问题,尤其是Heglig和Munga地区井壁坍塌严重。为此,根据地质资料、井身结构和对水平井注水泥的技术要求,科学合理设计施工工艺,优选出适合此地区固井施工的冲洗液和隔离液,特别是经过努力实践,开发出了新型膨胀水泥浆体系,此体系具有高强度、低失水、无析水、防气窜之功效。结合使用哈里伯顿CPT—Y4大功率固井车,精确计量和自动密度控制,该体系水泥浆目前已在7口水平井施工中得以应用,均为优质井。现正在被大量使用,此体系的开发,解决了该固井技术难题,创造了中国固井在海外首次水平井作业一次成功的历史记录,具有较高的经济和社会效益。 相似文献
94.
95.
真空行业现状及未来国内市场 总被引:1,自引:1,他引:0
简单扼要的概述了真空行业的发展历史,以较详细的统计数据阐述了2002年行业经济运行情况,并通过对相关行业“十五”发展规划的研究,对今后5~10年真空设备的市场进行了分析预测。 相似文献
96.
P. K. Khare J. M. Keller M. S. Gaur Ranjeet Singh S. C. Datt 《Polymer International》1994,35(4):337-343
The electrical conductivity of solution-grown ethyl cellulose (EC) films, 5–30 μm thick, has been studied in the sandwich configuration (metal–EC–metal) as a function of iodine concentration from 0.5 to 5.0 wt% ratio. The studies were conducted in the temperature range 333–383 K, while the field was varied over the range (3.0–5.5) × 104V/cm. Aluminium was used as the lower electrode, while the upper electrode was of Al, Ag, Cu, Au or Sn. Certain transient effects such as a large burst of current immediately after the application of field were observed. An attempt was made to identify the nature of the current by comparing the observed dependence on electric field, electrode material and temperature with the respective characteristic features of the existing theories of electrical conduction. The results show that the electrical conduction follows Ohm's law at lower fields, while at higher fields, space-charge limited current (SCLC) was observed. It was also found that Richardson–Schottky emission was responsible, to some extent, for the transport of charge carriers in the polymer. The conductivity of the films increased on doping with iodine. The dopant molecules are considered to act as additional trapping centes and provide links between the polymer molecules in the amorphous region, thus resulting in the formation of charge transfer complexes. 相似文献
97.
98.
99.
100.
In this work we conceived a model of a multilayer solar cell composed by four layers of opposite conductivities: an n-type 6H-SiC used as a frontal layer to absorb high energy photons (energy gap equals 2.9 eV), a p-type Si layer, an n-type Si layer and a p-type SiGe back layer to absorb low energy photons (Si0.8Ge0.2 with an energy gap equal to 0.8 eV). The impurity concentration in every layer of the model is taken equal to 1017 cm−3 to ensure abrupt junctions inside the cell. The optical properties of the separate layers have been fitted and tabulated to be used for thin films devices numerical simulation. We developed the equations giving the minority carrier concentration and the photocurrent density in each abscissa of the model. We used Matlab software to simulate and optimize the layers thicknesses to achieve the maximum photocurrent generated under AM0 solar spectrum. The results of simulation showed that the optimized structure could deliver, assuming 105 cm/s surface recombination velocity, a photocurrent density of more than 53 mA/cm2, which represents 88.3% of the ideal photocurrent (59.99 mA/cm2) that can be generated under AM0 solar spectrum. 相似文献