全文获取类型
收费全文 | 65444篇 |
免费 | 7932篇 |
国内免费 | 5459篇 |
学科分类
工业技术 | 78835篇 |
出版年
2024年 | 423篇 |
2023年 | 1138篇 |
2022年 | 2739篇 |
2021年 | 3381篇 |
2020年 | 2416篇 |
2019年 | 1693篇 |
2018年 | 1800篇 |
2017年 | 2081篇 |
2016年 | 1859篇 |
2015年 | 2928篇 |
2014年 | 3649篇 |
2013年 | 4267篇 |
2012年 | 5518篇 |
2011年 | 5522篇 |
2010年 | 5333篇 |
2009年 | 5144篇 |
2008年 | 5337篇 |
2007年 | 5006篇 |
2006年 | 4426篇 |
2005年 | 3732篇 |
2004年 | 2691篇 |
2003年 | 1746篇 |
2002年 | 1682篇 |
2001年 | 1569篇 |
2000年 | 1299篇 |
1999年 | 480篇 |
1998年 | 170篇 |
1997年 | 138篇 |
1996年 | 106篇 |
1995年 | 74篇 |
1994年 | 62篇 |
1993年 | 62篇 |
1992年 | 51篇 |
1991年 | 53篇 |
1990年 | 51篇 |
1989年 | 40篇 |
1988年 | 25篇 |
1987年 | 19篇 |
1986年 | 21篇 |
1985年 | 5篇 |
1984年 | 11篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 24篇 |
1979年 | 17篇 |
1966年 | 1篇 |
1964年 | 1篇 |
1959年 | 10篇 |
1951年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
In this work,the ionospheric variability is analyzed by applying the wavelet decomposition technique to the noontime fo F2,F10.7,interplanetary magnetic field(IMF)Bz,Ap,and lower thermospheric temperature at pressure of 10?4 h Pa in 2002.Results show that the variance of periodic oscillations in the ionosphere is largest in the 2–4-day period and declines with the increase of the period.The maximum variance of the periodic oscillations in solar irradiation is in the 16–32-day period.For geomagnetic activities,most of the variance is about equally distributed on intervals of periods shorter than 32 days.Variance distributions of IMF Bz and lower thermospheric temperature are similar to those of the ionosphere.They show the maximum in the 2–4-day period and decline with the increase of the period.By analyzing the distributions of the variances,the potential connections between the ionosphere and the external sources are discussed. 相似文献
3.
Biao Chen Hao Wang 《Signal Processing, IEEE Transactions on》2004,52(7):2047-2057
Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed. 相似文献
4.
5.
由于干态聚合物电解质目前还不能满足聚合物锂离子电池的应用要求,人们致力于开发含液体增塑剂的聚合物电解质,包括凝胶型和微孔型两类体系。本文综述了含液聚合物电解质的最新进展,重点论述了各种新体系和新方法。 相似文献
6.
Hao Wang Ying Lin Biao Chen 《Signal Processing, IEEE Transactions on》2003,51(10):2613-2623
We investigate non data-aided channel estimation for cyclically prefixed orthogonal frequency division multiplexing (OFDM) systems. By exploiting channel diversity using only two receive antennas, a blind deterministic algorithm is proposed. Identifiability conditions are derived that guarantee the perfect channel retrieval in the absence of noise. In the presence of noise, the proposed method has the desired property of being data efficient-only a single OFDM block is needed to achieve good estimation performance for a wide range of SNR values. The algorithm is also robust to input symbols as it does not have any restriction on the input symbols with regard to their constellation or their statistical properties. In addition, this diversity-based algorithm is computationally efficient, and its performance compares favorably to most existing blind algorithms. 相似文献
7.
制作压力传感器时,在二氧化硅层上淀积多晶硅膜,既可利用优良的机械特性,又可保证压敏电阻与衬底间具有良好的绝缘性,由此可大大提高器件的温度特性。介绍了一种多晶硅压力传感器的原理和设计。实验结果表明,这类传感器具有灵敏度好,精度高等特点,电路工作范围为0-250℃,且具有良好的温度稳定性。 相似文献
8.
石油测井仪器可靠性指标探讨 总被引:5,自引:1,他引:4
在分析常用可靠性指标及其应用现状的基础上,根据可靠性理论及石油测井仪器的特点。提出了石油测井仪器的可靠性模型。在探讨常用可靠性指标对石油测井仪器适用性的同时,提出将平衡稳定工作时间NWTURE和免维修测井井次NOLWNM作为石油测井仪器可靠性指标的思想;探讨了获取可靠性指标的途径。给出了推荐使用的石油测井仪器可靠性指标。 相似文献
9.
10.