全文获取类型
收费全文 | 228114篇 |
免费 | 2781篇 |
国内免费 | 691篇 |
专业分类
电工技术 | 3988篇 |
综合类 | 203篇 |
化学工业 | 34924篇 |
金属工艺 | 11130篇 |
机械仪表 | 7674篇 |
建筑科学 | 4547篇 |
矿业工程 | 2217篇 |
能源动力 | 5184篇 |
轻工业 | 14582篇 |
水利工程 | 3112篇 |
石油天然气 | 7867篇 |
武器工业 | 23篇 |
无线电 | 23933篇 |
一般工业技术 | 49066篇 |
冶金工业 | 40318篇 |
原子能技术 | 7026篇 |
自动化技术 | 15792篇 |
出版年
2021年 | 2213篇 |
2019年 | 2157篇 |
2018年 | 3911篇 |
2017年 | 3940篇 |
2016年 | 4252篇 |
2015年 | 2411篇 |
2014年 | 4087篇 |
2013年 | 9523篇 |
2012年 | 6171篇 |
2011年 | 7948篇 |
2010年 | 6405篇 |
2009年 | 7098篇 |
2008年 | 7279篇 |
2007年 | 7200篇 |
2006年 | 6168篇 |
2005年 | 5786篇 |
2004年 | 5182篇 |
2003年 | 5167篇 |
2002年 | 5016篇 |
2001年 | 5097篇 |
2000年 | 4752篇 |
1999年 | 4767篇 |
1998年 | 12220篇 |
1997年 | 8516篇 |
1996年 | 6482篇 |
1995年 | 4822篇 |
1994年 | 4188篇 |
1993年 | 4448篇 |
1992年 | 3321篇 |
1991年 | 3319篇 |
1990年 | 3225篇 |
1989年 | 3183篇 |
1988年 | 3150篇 |
1987年 | 2772篇 |
1986年 | 2797篇 |
1985年 | 3124篇 |
1984年 | 2896篇 |
1983年 | 2750篇 |
1982年 | 2473篇 |
1981年 | 2482篇 |
1980年 | 2539篇 |
1979年 | 2523篇 |
1978年 | 2524篇 |
1977年 | 2764篇 |
1976年 | 3265篇 |
1975年 | 2304篇 |
1974年 | 2173篇 |
1973年 | 2281篇 |
1972年 | 2028篇 |
1971年 | 1825篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
In this communication, the structural, micro-structural, dielectric, electrical, magnetic, and leakage-current characteristics of a double perovskite (Y2CoMnO6) ceramic material have been reported. The material was synthesized via a high-temperature mixed-oxide route. The compound crystallizes in a monoclinic structure which is confirmed from preliminary X-ray structural study. The morphological study by using scanning electron micrograph reveals the almost homogeneous distribution of grains throughout the surface of the sample. The nature of frequency-dependence of dielectric constant has been described by the Maxwell-Wagner model. The occurrence of a dielectric anomaly in the temperature dependence of dielectric permittivity study demonstrates the ferroelectric-paraelectric phase transition in the material. From the Nyquist plots, we found the existence of both grain and grain boundary effects. The frequency dependence of conductivity was studied by the Jonscher’s Power law, and the conduction phenomenon obeys the large overlapping polaron tunneling model. By using the Arrhenius equation, the activation energy has been calculated which is nearly equal to the energy required for the hoping of the electron. Both impedance and conductivity analysis demonstrate that the sample exhibits negative temperature coefficient of resistance (NTCR) properties indicating the semiconducting type of material at high temperatures. The anti-ferromagnetic character of the material is observed from the nature of magnetic hysteresis loop. The leakage current analysis suggests that the conduction process in the material follows the space charge limited conduction phenomenon. Such material will be helpful for modern electronic devices and spintronic applications. 相似文献
32.
Popov N. N. Presnyakov D. V. Lar’kin V. F. 《Journal of Machinery Manufacture and Reliability》2020,49(3):232-236
Journal of Machinery Manufacture and Reliability - A device based on smart alloys has been designed for use in machine building items, in particular, in nuclear engineering, for preventing... 相似文献
33.
Kobrakov K. I. Kurnetsov D. N. Ruchkina A. G. Sharpar N. M. 《Theoretical Foundations of Chemical Engineering》2020,54(5):1052-1056
Theoretical Foundations of Chemical Engineering - New research results, as well as those published earlier by the authors, on the synthesis of azo compounds based on nitroanilines obtained by the... 相似文献
34.
Bhattacharya Subhayan Sinha Sankhamita Roy Sarbani Gupta Amarnath 《The Journal of supercomputing》2020,76(12):9882-9900
The Journal of Supercomputing - Currently, all online social networks (OSNs) are considered to follow a power-law distribution. In this paper, the degree distribution for multiple OSNs has been... 相似文献
35.
Dorofeyev V. Yu. Sviridova A. N. Svistun L. I. 《Russian Journal of Non-Ferrous Metals》2020,61(3):354-361
Russian Journal of Non-Ferrous Metals - Rolling bearing rings comprise a large reserve for expanding powder metallurgy production. This reserve is being incompletely implemented. The hot forging of... 相似文献
36.
37.
38.
39.
40.
C. Doga Demirhan William W. Tso Joseph B. Powell Efstratios N. Pistikopoulos 《American Institute of Chemical Engineers》2019,65(7):e16498
Current ammonia production technologies have a significant carbon footprint. In this study, we present a process synthesis and global optimization framework to discover the efficient utilization of renewable resources in ammonia production. Competing technologies are incorporated in a process superstructure where biomass, wind, and solar routes are compared with the natural gas-based reference case. A deterministic global optimization-based branch-and-bound algorithm is used to solve the resulting large-scale nonconvex mixed-integer nonlinear programming problem (MINLP). Case studies for Texas, California, and Iowa are conducted to examine the effects of different feedstock prices and availabilities. Results indicate that profitability of ammonia production is highly sensitive to feedstock and electricity prices, as well as greenhouse gas (GHG) restrictions. Under strict 75% GHG reductions, biomass to ammonia route is found to be competitive with natural gas route, whereas wind and solar to ammonia routes require further improvement to compete with those two routes. © 2018 American Institute of Chemical Engineers AIChE J, 65: e16498 2019 相似文献