全文获取类型
收费全文 | 95725篇 |
免费 | 1853篇 |
国内免费 | 713篇 |
专业分类
电工技术 | 1168篇 |
综合类 | 2683篇 |
化学工业 | 13612篇 |
金属工艺 | 5213篇 |
机械仪表 | 3457篇 |
建筑科学 | 2899篇 |
矿业工程 | 717篇 |
能源动力 | 1453篇 |
轻工业 | 4539篇 |
水利工程 | 1423篇 |
石油天然气 | 635篇 |
武器工业 | 34篇 |
无线电 | 10826篇 |
一般工业技术 | 18073篇 |
冶金工业 | 4483篇 |
原子能技术 | 333篇 |
自动化技术 | 26743篇 |
出版年
2023年 | 134篇 |
2022年 | 238篇 |
2021年 | 411篇 |
2020年 | 254篇 |
2019年 | 253篇 |
2018年 | 14732篇 |
2017年 | 13659篇 |
2016年 | 10254篇 |
2015年 | 943篇 |
2014年 | 718篇 |
2013年 | 935篇 |
2012年 | 3790篇 |
2011年 | 10295篇 |
2010年 | 9017篇 |
2009年 | 6232篇 |
2008年 | 7564篇 |
2007年 | 8434篇 |
2006年 | 777篇 |
2005年 | 1708篇 |
2004年 | 1552篇 |
2003年 | 1586篇 |
2002年 | 919篇 |
2001年 | 401篇 |
2000年 | 458篇 |
1999年 | 340篇 |
1998年 | 375篇 |
1997年 | 299篇 |
1996年 | 293篇 |
1995年 | 203篇 |
1994年 | 169篇 |
1993年 | 131篇 |
1992年 | 95篇 |
1991年 | 79篇 |
1990年 | 61篇 |
1989年 | 58篇 |
1988年 | 52篇 |
1987年 | 37篇 |
1986年 | 34篇 |
1968年 | 43篇 |
1967年 | 34篇 |
1966年 | 42篇 |
1965年 | 44篇 |
1963年 | 28篇 |
1960年 | 30篇 |
1959年 | 35篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
51.
ZhaoYahong ZhangZhongpei WuWeiling 《电子科学学刊(英文版)》2003,20(3):177-182
Adaptive modulation and power allocation is introduced into the multicarrier DSCDMA system to improve the system performance and bandwidth efficiency.First,the system design appropriate for adaptive modulation and power allocation is given,then the algorithm of adaptive modulation and power allocation is applied.Simulation results demonstrate great performance improvement compared with the fixed modulated one. 相似文献
52.
As an aid towards improving the treatment of exchange and correlation effects in electronic structure calculations, it is
desirable to have a clear picture of the errors introduced by currently popular approximate exchange-correlation functionals.
We have performed ab initio density functional theory and density functional perturbation theory calculations to investigate
the thermal properties of bulk Cu, using both the local density approximation (LDA) and the generalized gradient approximation
(GGA). Thermal effects are treated within the quasiharmonic approximation. We find that the LDA and GGA errors for anharmonic
quantities are an order of magnitude smaller than for harmonic quantities; we argue that this might be a general feature.
We also obtain much closer agreement with experiment than earlier, more approximate calculations. 相似文献
53.
Effect of doping of carbon nanotubes by magnetic transition metal atoms has been considered in this paper. In the case of
semiconducting tubes, it was found that the system has zero magnetization, whereas in metallic tubes the valence electrons
of the tube screen the magnetization of the dopants: the coupling to the tube is usually antiferromagnetic (except for Cr). 相似文献
54.
Jui-Ting Weng Jiunn Ru Lai Wanjiun Liao 《Wireless Communications, IEEE Transactions on》2006,5(7):1676-1684
In this paper, we analyze node mobility for reliable packet delivery in mobile IP networks. In mobile IP, packets destined to roaming nodes are intercepted by their home agents and delivered via tunneling to their care of addresses (CoA). A mobile node may roam across multiple subnets. At each boundary crossing, a handoff is initiated such that the CoA is updated and a new tunnel is established. We consider both basic mobile IP handoff and smooth handoff. We find that reliable packet delivery in mobile IP networks can be modeled as a renewal process, because the retransmission over a new tunnel after each boundary crossing is independent of the previous history. We then derive the probability distribution of boundary crossings for each successful packet, based on which the packet reliable delivery time can be obtained. Our analytical model is derived based on a general distribution of residence time in a subnet and a general distribution of successful retransmission attempts in each subnet. The results can be readily applied to any distributions for both items. We also provide numerical examples to calculate the probability distribution of boundary crossings, and conduct simulations to validate our analytical results 相似文献
55.
DeLongis Anita; Capreol Martha; Holtzman Susan; O'Brien Tess; Campbell Jennifer 《Canadian Metallurgical Quarterly》2004,18(3):470
In response to recent calls in the literature for within-person examinations of social support processes over time, this study explores the relationships of spousal support, spousal strain, and well-being among husbands and wives, both within the same day and across days. Eighty-three couples were interviewed and completed a structured diary twice daily for 1 week. The results of multilevel hierarchical modeling suggest that both spousal support and spousal strain made significant, independent contributions to concurrent negative affect, although only spousal support was a significant predictor of next-day negative affect. Spousal strain interacted with spousal support to predict next-day negative affect. Direct and moderating effects of perceived marital adjustment on negative affect were discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
56.
57.
Chou Y.C. Leung D. Lai R. Grundbacher R. Barsky M. Kan Q. Tsai R. Wojtowicz M. Eng D. Tran L. Block T. Liu P.H. Nishimoto M. Oki A. 《Electron Device Letters, IEEE》2003,24(6):378-380
The authors have investigated the reliability performance of G-band (183 GHz) monolithic microwave integrated circuit (MMIC) amplifiers fabricated using 0.07-/spl mu/m T-gate InGaAs-InAlAs-InP HEMTs with pseudomorphic In/sub 0.75/Ga/sub 0.25/As channel on 3-in wafers. Life test was performed at two temperatures (T/sub 1/ = 200 /spl deg/C and T/sub 2/ = 215 /spl deg/C), and the amplifiers were stressed at V/sub ds/ of 1 V and I/sub ds/ of 250 mA/mm in a N/sub 2/ ambient. The activation energy is as high as 1.7 eV, achieving a projected median-time-to-failure (MTTF) /spl ap/ 2 /spl times/ 10/sup 6/ h at a junction temperature of 125 /spl deg/C. MTTF was determined by 2-temperature constant current stress using /spl Delta/G/sub mp/ = -20% as the failure criteria. The difference of reliability performance between 0.07-/spl mu/m InGaAs-InAlAs-InP HEMT MMICs with pseudomorphic In/sub 0.75/Ga/sub 0.25/As channel and 0.1-/spl mu/m InGaAs-InAlAs-InP HEMT MMICs with In/sub 0.6/Ga/sub 0.4/As channel is also discussed. The achieved high-reliability result demonstrates a robust 0.07-/spl mu/m pseudomorphic InGaAs-InAlAs-InP HEMT MMICs production technology for G-band applications. 相似文献
58.
Dong-Soo Yoon Jae Sung Roh Sung-Man Lee Hong Koo Baik 《Journal of Electronic Materials》2003,32(8):890-898
The effect of a thin RuOx layer formed on the Ru/TiN/doped poly-Si/Si stack structure was compared with that on the RuOx/TiN/doped poly-Si/Si stack structure over the post-deposition annealing temperature ranges of 450–600°C. The Ru/TiN/poly-Si/Si
contact system exhibited linear behavior at forward bias with a small increase in the total resistance up to 600°C. The RuOx/TiN/poly-Si/Si contact system exhibited nonlinear characteristics under forward bias at 450°C, which is attributed to no
formation of a thin RuOx layer at the RuOx surface and porous-amorphous microstructure. In the former case, the addition of oxygen at the surface layer of the Ru film
by pre-annealing leads to the formation of a thin RuOx layer and chemically strong Ru-O bonds. This results from the retardation of oxygen diffusion caused by the discontinuity
of diffusion paths. In particular, the RuOx layer in a nonstoichiometric state is changed to the RuO2-crystalline phase in a stoichiometric state after post-deposition annealing; this phase can act as an oxygen-capture layer.
Therefore, it appears that the electrical properties of the Ru/TiN/poly-Si/Si contact system are better than those of the
RuOx/TiN/poly-Si/Si contact system. 相似文献
59.
The objective is to have uniformly distributed tangential stresses on the transition profile of a stepped bar subjected to tensile and torsional loading using an axisymmetric boundary element formulation. The transition curve is represented by the Langragian interpolation polynomial with progressive degrees to avoid shape distortion during the optimization procedure. The calculated result is compared with that obtained by fluid dynamics given in the literature. It is seen that the transition curve obtained in this paper is reasonable. 相似文献
60.