全文获取类型
收费全文 | 88904篇 |
免费 | 9368篇 |
国内免费 | 5764篇 |
学科分类
工业技术 | 104036篇 |
出版年
2024年 | 512篇 |
2023年 | 1579篇 |
2022年 | 3027篇 |
2021年 | 4115篇 |
2020年 | 3113篇 |
2019年 | 2453篇 |
2018年 | 2762篇 |
2017年 | 3069篇 |
2016年 | 2741篇 |
2015年 | 3793篇 |
2014年 | 4793篇 |
2013年 | 5706篇 |
2012年 | 6760篇 |
2011年 | 7113篇 |
2010年 | 6449篇 |
2009年 | 6141篇 |
2008年 | 5998篇 |
2007年 | 5829篇 |
2006年 | 5338篇 |
2005年 | 4275篇 |
2004年 | 3157篇 |
2003年 | 2619篇 |
2002年 | 2616篇 |
2001年 | 2364篇 |
2000年 | 1782篇 |
1999年 | 1358篇 |
1998年 | 841篇 |
1997年 | 682篇 |
1996年 | 622篇 |
1995年 | 537篇 |
1994年 | 394篇 |
1993年 | 341篇 |
1992年 | 267篇 |
1991年 | 187篇 |
1990年 | 156篇 |
1989年 | 105篇 |
1988年 | 108篇 |
1987年 | 59篇 |
1986年 | 73篇 |
1985年 | 39篇 |
1984年 | 27篇 |
1983年 | 18篇 |
1982年 | 21篇 |
1981年 | 11篇 |
1980年 | 18篇 |
1979年 | 17篇 |
1977年 | 7篇 |
1970年 | 6篇 |
1959年 | 7篇 |
1951年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
L-带掺铒光纤放大器的研究与进展 总被引:6,自引:1,他引:5
通过理论计算,比较了C-带掺铒光纤放大器和L-带掺铒光纤放大器的增益特性,并较为全面地阐述了近几年来国内外对L-带掺铒光纤放大器的研究情况。通过对L-带掺铒光纤放大器工作原理的分析,总结出新的设计思路与方案,提出了一些新的研究点。 相似文献
62.
Jia-sheng Tian Tian-lin Dong 《Journal of Infrared, Millimeter and Terahertz Waves》2004,25(6):981-988
Based on the established rigorous theory for two-dimensionally periodic (2DP) medium, Longitudinal propagation in Dielectric Waveguide Array (DWA) was analyzed in detail. Firstly, given longitudinal wave vector (k z), Brillouin dispersion relations between transversal wave vectors were analyzed. Interactions between space harmonics were also studied. Secondly, on condition that transversal wave vector k x equaled k y, dispersion relations between longitudinal and transversal wave vectors were analyzed in detail. Because of interactions between space harmonics increasingly getting stronger with k z or modulation coefficients increasing, stop-bands could be distinctly displayed and thus longitudinal propagation in DWA could be comprehensively analyzed. The applications about longitudinal propagation in DWA were also discussed. 相似文献
63.
Dong Sik Kim 《Signal Processing, IEEE Transactions on》2003,51(6):1632-1641
The vector quantizer (VQ) codebook is usually designed by clustering a training sequence (TS) drawn from the underlying distribution function. In order to cluster a TS, we may use the K-means algorithm (generalized Lloyd (1982) algorithm) or the self-organizing map algorithm. In this paper, a survey of trained VQ performance is conducted to study the effect of the training ratio on training quantizers. The training ratio, which is defined by the ratio of the TS size to the codebook size, is dependent on the VQ structure. Hence, different VQs may show different training properties, even though the VQs are designed for the same TS. A numerical comparison of trained VQs is then conducted in conjunction with deriving their training ratios. Through the comparison, it is shown that structured VQs can achieve better performance than the full-search scheme if the codebooks are trained by a finite TS. Further, we can derive a design or comparison guideline that maintains equal training ratios in training different VQs. 相似文献
64.
65.
66.
Disagreement exists in the literature concerting the activity of NiO in (Ni,Mg)O solid solutions. Ideal behavior, as well as strong positive deviations and strong negative deviations from ideality, has been reported. In the present study, the activity of NiO was determined by electromotive force measurements using the solid-state cell Pt/Ni-NiO/0.92ZrO2 -0.08Y2 O3 /Ni-(Ni,Mg)O/Pt. The compositions studied had NiO mole fractions of 0.1,0.3,0.5,0.7, and 0.9. In agreement with theoretical expectations, the solid solutions exhibited ideal behavior at 882° and 982°C. Possible reasons for the conflicting results in the literature arc presented. 相似文献
67.
The energy release rate, which is the total derivative of the energy with respect to crack length, is recognized as corresponding to the shape sensitivity analysis with the crack length change represented by the tangential component of design velocity. In this paper the sensitivity formula recently developed for a changing boundary condition has been further extended to cover crack kinking under mixed mode loading. Due to difficulty in defining the velocity field at the corner, the energy release rate at the onset of crack kinking is obtained by extrapolating the energy release rates for finite length kinked cracks. A rectangular plate with a single edge slant crack under uniform tension is taken as a numerical example. The multi-region technique in the numerical implementation of the boundary integral equation is adopted to consider the asymmetry of the problem. Excellent accuracy is observed as compared with reference analytic solutions. 相似文献
68.
本文主要介绍了硅中硼离子注入校准样品的制备与研究。分别用三台SIMS仪器对样品进行了深度剖析与比对,并对用作校准目的的样品主要参数进行了定值。 相似文献
69.
70.
n-Al2O3/Ni复合镀层的组织与滑动磨损性能研究 总被引:24,自引:4,他引:20
用电刷镀技术制得了镍基n-Al2O3复合镀层,并对镀层的滑动磨损性能进行了试验研究。纳米复合镀层的表面形貌比较细腻,镀层中纳米粒子分布均匀,与基质金属结合紧密。镀层显微硬度达到HV700,比快速镍镀层提高约40%。滑动磨损试验结果表明,随着纳米粒子含量的增大,镀层的耐磨性提高,摩擦系数也呈增大趋势;但当镀层中n-Al2O3粒子的超过2.56%(质量分数)时,镀层的耐磨性显著下降。纳米复合镀层的磨损机制以疲劳磨损为主,而快速镍底层以粘着磨损为主。 相似文献