全文获取类型
收费全文 | 349584篇 |
免费 | 4690篇 |
国内免费 | 1254篇 |
专业分类
电工技术 | 5989篇 |
综合类 | 2647篇 |
化学工业 | 49900篇 |
金属工艺 | 15129篇 |
机械仪表 | 11948篇 |
建筑科学 | 8403篇 |
矿业工程 | 1630篇 |
能源动力 | 7864篇 |
轻工业 | 26978篇 |
水利工程 | 3809篇 |
石油天然气 | 4259篇 |
武器工业 | 68篇 |
无线电 | 45279篇 |
一般工业技术 | 67585篇 |
冶金工业 | 49574篇 |
原子能技术 | 5802篇 |
自动化技术 | 48664篇 |
出版年
2021年 | 2339篇 |
2020年 | 1767篇 |
2019年 | 2231篇 |
2018年 | 17773篇 |
2017年 | 16802篇 |
2016年 | 13780篇 |
2015年 | 3159篇 |
2014年 | 4626篇 |
2013年 | 12076篇 |
2012年 | 10139篇 |
2011年 | 18621篇 |
2010年 | 15448篇 |
2009年 | 13710篇 |
2008年 | 15690篇 |
2007年 | 16450篇 |
2006年 | 8249篇 |
2005年 | 8558篇 |
2004年 | 8216篇 |
2003年 | 8075篇 |
2002年 | 7128篇 |
2001年 | 6752篇 |
2000年 | 6522篇 |
1999年 | 6485篇 |
1998年 | 14579篇 |
1997年 | 10451篇 |
1996年 | 8279篇 |
1995年 | 6513篇 |
1994年 | 5813篇 |
1993年 | 5654篇 |
1992年 | 4595篇 |
1991年 | 4281篇 |
1990年 | 4083篇 |
1989年 | 3809篇 |
1988年 | 3655篇 |
1987年 | 3199篇 |
1986年 | 3082篇 |
1985年 | 3623篇 |
1984年 | 3396篇 |
1983年 | 3046篇 |
1982年 | 2861篇 |
1981年 | 2969篇 |
1980年 | 2770篇 |
1979年 | 2655篇 |
1978年 | 2502篇 |
1977年 | 2955篇 |
1976年 | 3565篇 |
1975年 | 2321篇 |
1974年 | 2316篇 |
1973年 | 2319篇 |
1972年 | 1849篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
High-performance surface-micromachined inchworm actuator 总被引:1,自引:0,他引:1
de Boer M.P. Luck D.L. Ashurst W.R. Maboudian R. Corwin A.D. Walraven J.A. Redmond J.M. 《Journal of microelectromechanical systems》2004,13(1):63-74
This work demonstrates a polycrystalline silicon surface-micromachined inchworm actuator that exhibits high-performance characteristics such as large force (/spl plusmn/0.5 millinewtons), large velocity range (0 to /spl plusmn/4.4 mm/sec), large displacement range (/spl plusmn/100 microns), small step size (/spl plusmn/10, /spl plusmn/40 or /spl plusmn/100 nanometers), low power consumption (nanojoules per cycle), continuous bidirectional operation and relatively small area (600 /spl times/ 200/spl mu/m/sup 2/). An in situ load spring calibrated on a logarithmic scale from micronewtons to millinewtons, optical microscopy and Michelson interferometry are used to characterize its performance. The actuator consists of a force-amplifying plate that spans two voltage-controlled clamps, and walking is achieved by appropriately sequencing signals to these three components. In the clamps, normal force is borne by equipotential rubbing counterfaces, enabling friction to be measured against load. Using different monolayer coatings, we show that the static coefficient of friction can be changed from 0.14 to 1.04, and that it is load-independent over a broad range. We further find that the static coefficient of friction does not accurately predict the force generated by the actuator and attribute this to nanometer-scale presliding tangential deflections. 相似文献
992.
The major steps of sewerage rehabilitation include inspection of sewerage, assessment of structural conditions, computation of structural condition grades, and determination of rehabilitation methods and materials. Conventionally, sewerage rehabilitation planning relies on experts with professional background that is tedious and time-consuming. This paper proposes an automation model of planning optimal sewerage rehabilitation strategies for the sewer system by integrating image process, clustering technology, optimization, and visualization display. Firstly, image processing techniques, such as wavelet transformation and co-occurrence features extraction, were employed to extract various characteristics of structural failures from CCTV inspection images. Secondly, a classification neural network was established to automatically interpret the structural conditions by comparing the extracted features with the typical failures in a databank. Then, to achieve optimal rehabilitation efficiency, a genetic algorithm was used to determine appropriate rehabilitation methods and substitution materials for the pipe sections with a risk of mal-function and even collapse. Finally, the result from the automation model can be visualized in a geographic information system in which essential information of the sewer system and sewerage rehabilitation plans are graphically displayed. For demonstration, the automation model of optimal sewerage rehabilitation planning was applied to a sewer system in east Taichung, Chinese Taiwan. 相似文献
993.
Frantz Rowe 《欧洲信息系统杂志》2006,15(3):244-248
994.
A new unsymmetrical chiral thioindigo dopant 6‐[(R,R)‐2,3‐difluorooct‐1‐yloxy]‐5′‐nitro‐6′‐[(R)‐2‐octyloxy]thioindigo ( 4 ) designed to photoinvert the sign of spontaneous polarization (PS) in a ferroelectric chiral smectic C (SmC*) liquid crystal was prepared using a synthetic approach previously developed in our laboratory. In this new “ambidextrous” design, the (R)‐2‐octyloxy side‐chain is sterically coupled to the thioindigo core and induces a positive PS, whereas the (R,R)‐2,3‐difluorooctyloxy side‐chain is decoupled from the core and induces a larger negative PS. In the trans form, this dopant induces a negative polarization in the SmC host (+)‐4‐(4‐methylhexyloxy)phenyl 4‐decyloxybenzoate ( PhB ). Irradiation of a 1 mol‐% mixture of 4 in PhB at λ = 510 nm caused a sign inversion of PS, from –0.88 to +0.42 nC cm–2 at T – TC = –5 °C, which is consistent with an increase in the polarization power of the coupled 2‐octyloxy/thioindigo unit over that of the 2,3‐difluorooctyloxy unit, due to the increase in transverse dipole moment of the thioindigo core upon trans–cis photoisomerization. The PS sign inversion was confirmed by a surface‐stabilized ferroelectric liquid crystal photoswitching experiment. Spectroscopic measurements on films of the doped liquid crystal mixtures showed that trans–cis photoisomerization is gradually suppressed with increasing dopant mole fraction, possibly as a result of increased dopant aggregation. 相似文献
995.
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
996.
Grosu D. Chronopoulos A.T. 《IEEE transactions on systems, man, and cybernetics. Part B, Cybernetics》2004,34(1):77-84
Computational grids are promising next-generation computing platforms for large-scale problems in science and engineering. Grids are large-scale computing systems composed of geographically distributed resources (computers, storage etc.) owned by self interested agents or organizations. These agents may manipulate the resource allocation algorithm in their own benefit, and their selfish behavior may lead to severe performance degradation and poor efficiency. In this paper, we investigate the problem of designing protocols for resource allocation involving selfish agents. Solving this kind of problems is the object of mechanism design theory. Using this theory, we design a truthful mechanism for solving the static load balancing problem in heterogeneous distributed systems. We prove that using the optimal allocation algorithm the output function admits a truthful payment scheme satisfying voluntary participation. We derive a protocol that implements our mechanism and present experiments to show its effectiveness. 相似文献
997.
This paper presents a fiber-optic vehicle axle detector for roadways. It is based on a fiber-optic Michelson interferometer that is mounted directly into the road surface. The fully dielectric design allows for remote operation of the sensor via a long section of optical fiber. A simple signal-processing scheme of the output signals has been adopted that allows for reliable detection of commercial and personal vehicles. 相似文献
998.
Miaris G. Kaitas T. Zaharis Z. Babas D. Vafladis E. Samaras T. Sahalos J.N. 《Antennas and Propagation Magazine, IEEE》2003,45(4):35-46
The current work presents the procedure of designing and carrying out measurements of radiation emission from an air-traffic surveillance radar. The system requirements are first given, and then the equipment used is described. The steps to determine the operating characteristics of the radar are specified in detail. The necessary methods for manual power measurements and calculations are next explained. Since the final measurement system was fully automated, its main features (topology, protocol of operation) are also given. Finally, some measurement results taken during the initial phase of system operation are presented and discussed. 相似文献
999.
A. V. Banshchikov L. A. Burlakova V. D. Irtegov M. A. Novikov 《Cybernetics and Systems Analysis》1992,28(1):119-127
Algorithms are proposed that construct first integrals and Lyapunov functions for stability analysis of stationary solutions of differential equations describing mechanical systems of linked bodies.Translated from Kibernetika i Sistemnyi Analiz, No. 1, pp. 138–148, January–February, 1992. 相似文献
1000.