全文获取类型
收费全文 | 514860篇 |
免费 | 7953篇 |
国内免费 | 1793篇 |
专业分类
电工技术 | 9852篇 |
综合类 | 526篇 |
化学工业 | 78694篇 |
金属工艺 | 17981篇 |
机械仪表 | 15067篇 |
建筑科学 | 14792篇 |
矿业工程 | 1174篇 |
能源动力 | 13706篇 |
轻工业 | 54749篇 |
水利工程 | 4059篇 |
石油天然气 | 2270篇 |
武器工业 | 17篇 |
无线电 | 68004篇 |
一般工业技术 | 95061篇 |
冶金工业 | 93329篇 |
原子能技术 | 7028篇 |
自动化技术 | 48297篇 |
出版年
2021年 | 3211篇 |
2019年 | 3035篇 |
2018年 | 4560篇 |
2017年 | 4650篇 |
2016年 | 4995篇 |
2015年 | 4133篇 |
2014年 | 6911篇 |
2013年 | 23850篇 |
2012年 | 12212篇 |
2011年 | 17672篇 |
2010年 | 13640篇 |
2009年 | 15636篇 |
2008年 | 16901篇 |
2007年 | 17351篇 |
2006年 | 15566篇 |
2005年 | 14554篇 |
2004年 | 14139篇 |
2003年 | 13807篇 |
2002年 | 13427篇 |
2001年 | 13631篇 |
2000年 | 12612篇 |
1999年 | 13150篇 |
1998年 | 28772篇 |
1997年 | 21298篇 |
1996年 | 16975篇 |
1995年 | 13447篇 |
1994年 | 12039篇 |
1993年 | 11488篇 |
1992年 | 8958篇 |
1991年 | 8502篇 |
1990年 | 8113篇 |
1989年 | 7860篇 |
1988年 | 7501篇 |
1987年 | 6414篇 |
1986年 | 6320篇 |
1985年 | 7712篇 |
1984年 | 7143篇 |
1983年 | 6371篇 |
1982年 | 5995篇 |
1981年 | 5877篇 |
1980年 | 5512篇 |
1979年 | 5394篇 |
1978年 | 5024篇 |
1977年 | 6117篇 |
1976年 | 8043篇 |
1975年 | 4259篇 |
1974年 | 4033篇 |
1973年 | 3941篇 |
1972年 | 3176篇 |
1971年 | 2792篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Lasers containing a nanopatterned active layer demonstrating excellent threshold characteristics are presented. The nanopatterned active layer is fabricated using high-resolution electron beam lithography and selective-area metal organic chemical vapour deposition crystal growth. Results demonstrating an order of magnitude improvement over previous results are reported. 相似文献
992.
End-to-End QoS for Video Delivery Over Wireless Internet 总被引:6,自引:0,他引:6
Zhang Q. Zhu W. Zhang Y. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2005,93(1):123-134
Providing end-to-end quality of service (QoS) support is essential for video delivery over the next-generation wireless Internet. We address several key elements in the end-to-end QoS support, including scalable video representation, network-aware end system, and network QoS provisioning. There are generally two approaches in QoS support: the network-centric and the end-system centric solutions. The fundamental problem in a network-centric solution is how to map QoS criterion at different layers respectively, and optimize total quality across these layers. We first present the general framework of a cross-layer network-centric solution, and then describe the recent advances in network modeling, QoS mapping, and QoS adaptation. The key targets in end-system centric approach are network adaptation and media adaptation. We present a general framework of the end-system centric solution and investigate the recent developments. Specifically, for network adaptation, we review the available bandwidth estimation and efficient video transport protocol; for media adaptation , we describe the advances in error control, power control, and corresponding bit allocation. Finally, we highlight several advanced research directions. 相似文献
993.
A mass spectrometric technique combined with a double Knudsen cell was used to determine the antimony and copper activities
in the Cu-Sb binary system at 1373 K and in the two-melt composition range of the Cu−S−Sb ternary system at 1423 K. The antimony
and copper activities were calculated based on the intensity ration of the gaseous Sb and Cu species, over the unknown and
known activity samples, respectively. γ
Sb
o
were found to be 1.1×10−2 in molten copper at 1373 K, and 1.8×10−2 and 0.44 in a copper-rich phase and in a matter phase, of the Cu−S−Sb ternary system at 1423 K, respectively. These values
indicate, that antimony can be removed during the matte smelting and slagging stage of the copper smelting process. Interaction
parameters of antimony in molten copper slagging stage of the copper smelting process. Interaction parameters of antimony
in molten copper at 1423 K were calculated and found to be 10.7, −5.4, and 6.3 for ε
Sb
Sb
· ρSb
Sb, and ε
Sb
S
, respectively.
M. HINO, formerly Visiting Scientist at the University of Toronto 相似文献
994.
A new time domain reflectometry algorithm is presented. It is intended for identification of the impedance profile of the non-uniform lossless transmission line. Experimental results are presented to illustrate the validity of this new technique. 相似文献
995.
Self-Adapting Linear Algebra Algorithms and Software 总被引:2,自引:0,他引:2
Demmel J. Dongarra J. Eijkhout V. Fuentes E. Petitet A. Vuduc R. Whaley R.C. Yelick K. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2005,93(2):293-312
One of the main obstacles to the efficient solution of scientific problems is the problem of tuning software, both to the available architecture and to the user problem at hand. We describe approaches for obtaining tuned high-performance kernels and for automatically choosing suitable algorithms. Specifically, we describe the generation of dense and sparse Basic Linear Algebra Subprograms (BLAS) kernels, and the selection of linear solver algorithms. However, the ideas presented here extend beyond these areas, which can be considered proof of concept. 相似文献
996.
T. S. Srivatsan I. A. Ibrahim F. A. Mohamed E. J. Lavernia 《Journal of Materials Science》1991,26(22):5965-5978
The critical need for high strength, lightweight and high stiffness materials has, in recent years, resurrected much interest in discontinuously reinforced powder metallurgy metal matrix composites. These hybrid materials have combined both standard wrought alloys of aluminium and a wide variety of discontinuous reinforcements such as particulates and whiskers of ceramic materials. Renewed interest in these materials as attractive candidates for use in the aerospace and transportation industry has resulted from an attractive and unique combination of physical and mechanical properties, and an ability to offer near isotropic properties coupled with the low cost of these materials when compared with existing monolithic materials. In this paper, the primary processing categories for discontinuously-reinforced metal-matrix composites are highlighted and the salient features of the various techniques in each category are discussed. The variables involved in each processing technique are examined, and the influence of alloy chemistry highlighted. Novel processing techniques for these materials such as the variable co-deposition method is presented as a means to process these novel engineering materials in order to improve their overall mechanical performance. 相似文献
997.
P.O. Fanger B.M. Ipsen G. Langkilde B.W. Olessen N.K. Christensen S. Tanabe 《Energy and Buildings》1985,8(3):225-236
Groups of 32 and 16 subjects of both sexes were exposed in an environmental chamber to radiant asymmetry caused by a cool wall, a warm wall, and a cool ceiling. Each subject was tested individually while seated and clothed at 0.6 clo. During each 3.5-hour experiment the subject was exposed to six radiant temperature asymmetries. He was asked whether and where he experienced any local cool or warm sensation, and whether it was felt to be uncomfortable. During the entire experiment he was kept thermally neutral by changing the air temperature according to his wishes.For cool walls, warm walls, and cool ceilings curves have been established showing the percentage of dissatisfied subjects as a function of the radiant asymmetry. Radiant asymmetry at a warm wall caused less discomfort than a cool wall. A cool ceiling caused less discomfort than a warm ceiling. Accepting that 5% of the subjects may feel uncomfortable. a radiant temperature asymmetry of 10°C is allowable at a cool wall, 23°C at a warm wall, and 14°C under a cool ceiling. A previous study showed that 4°C is allowable under a warm ceiling. Radiant asymmetry had no significant impact on the operative temperatures preferred by the subjects. No significant differences were observed between the responses of men and women exposed to radiant asymmetry. 相似文献
998.
C. W. Draper J. M. Gibson D. C. Jacobson J. M. Poate S. M. Shin J. M. Rigsbee 《Journal of Materials Science》1985,20(7):2303-2312
Evaporated thin tantalum films on single and polycrystalline nickel have been laser surface alloyed using either continuous-wave CO2 or Q-switched Nd-YAG radiation. In the case of the continuous-wave laser, surface alloys contain amorphous tantalum-rich regions, intermediate polycrystalline bands of TaNi, and an underlying Ni(Ta) solid solution. In the Q-switched laser case, a much more laterally uniform amorphous phase with approximately equal atomic fractions of nickel and tantalum is found, with little evidence of polycrystalline intermetallics.In situ annealing with the electron beam of the microscope results in formation of microcrystallites, predominantly nickel. 相似文献
999.
Ruthenium colloidal particles prepared by the decomposition of a ruthenium acid sulfite complex increased the electrochemical reduction of benzene from 45% to 60% at room temperature and 10 psig of hydrogen. The reduction was carried out in a 21 mole % ethanol - 79 mole % hexamethylphosporamide. The significant increase in the percent of benzene reduced was accompanied by an apparent current efficiency of 98%. Comparison with analogous experiments employing colloidal platinum particles indicates that the above increase in benzene reduction can be attributed to direct catalytic hydrogenation by the ruthenium. 相似文献
1000.
Cationic thermal motions in the MPS3 have been related to substitution abilities in this layer family. They have also been correlated to the crystal field stabilization energy. These considerations explain why some MPS3 materials may or may not undergo a low temperature substitution intercalation reaction. From interatomic distances comparisons, the bond between (PS3) groups in the anionic (P2S6)4? octahedra, is found to be easily stretched upon cation substitution. Cationic radii follows the expected variation according to their high spin configuration in octahedral sulfur environment. 相似文献