全文获取类型
收费全文 | 111221篇 |
免费 | 11055篇 |
国内免费 | 6110篇 |
专业分类
电工技术 | 8293篇 |
技术理论 | 8篇 |
综合类 | 7959篇 |
化学工业 | 17759篇 |
金属工艺 | 6272篇 |
机械仪表 | 7001篇 |
建筑科学 | 9290篇 |
矿业工程 | 3494篇 |
能源动力 | 3263篇 |
轻工业 | 7551篇 |
水利工程 | 2437篇 |
石油天然气 | 6740篇 |
武器工业 | 1079篇 |
无线电 | 13014篇 |
一般工业技术 | 13163篇 |
冶金工业 | 4852篇 |
原子能技术 | 1165篇 |
自动化技术 | 15046篇 |
出版年
2024年 | 599篇 |
2023年 | 2301篇 |
2022年 | 4211篇 |
2021年 | 5815篇 |
2020年 | 4232篇 |
2019年 | 3369篇 |
2018年 | 3610篇 |
2017年 | 4025篇 |
2016年 | 3628篇 |
2015年 | 4724篇 |
2014年 | 6008篇 |
2013年 | 6839篇 |
2012年 | 7746篇 |
2011年 | 8143篇 |
2010年 | 7104篇 |
2009年 | 6714篇 |
2008年 | 6547篇 |
2007年 | 6150篇 |
2006年 | 5899篇 |
2005年 | 5167篇 |
2004年 | 3390篇 |
2003年 | 2867篇 |
2002年 | 2541篇 |
2001年 | 2361篇 |
2000年 | 2333篇 |
1999年 | 2340篇 |
1998年 | 1756篇 |
1997年 | 1493篇 |
1996年 | 1343篇 |
1995年 | 1165篇 |
1994年 | 968篇 |
1993年 | 673篇 |
1992年 | 533篇 |
1991年 | 410篇 |
1990年 | 366篇 |
1989年 | 266篇 |
1988年 | 207篇 |
1987年 | 130篇 |
1986年 | 134篇 |
1985年 | 71篇 |
1984年 | 50篇 |
1983年 | 28篇 |
1982年 | 43篇 |
1981年 | 28篇 |
1980年 | 24篇 |
1979年 | 14篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1959年 | 8篇 |
1951年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Hao Li Pengcheng Zhang Shengquan Yu Hongyu Yang Bin Tang Fuhai Li Shuren Zhang 《Ceramics International》2019,45(9):11639-11647
Mg2(Ti1-xSnx)O4 (x?=?0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg2(Ti1-xSnx)O4 average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg2(Ti1-xSnx)O4 compounds were obtained (= 12.18, ?=?170,130?GHz, ?=??53.1?ppm/°C, x?=?0.2). 相似文献
4.
5.
选取具有代表性的碳钢、不锈钢、镀锌碳钢、铝、钛和铜挂片,在某企业不同部位投放后进行为期半年的大气腐蚀试验,得出了在半年期中的腐蚀规律。在此基础上,对每种金属材料的腐蚀特点进行总结,并对大气腐蚀的各种影响因素进行分析,提出了行之有效的防护措施。 相似文献
6.
在探讨了Web查询服务的典型模型后,论文阐述了服务器端会话追踪机制的实现方式,设计了基于服务器端会话追踪的Web查询服务的典型解决方案。该设计方案可以较为安全地保存和维护客户的上下文信息,可靠地跟踪用户查询所处的状态,有效地实现有状态的Web查询服务。 相似文献
7.
8.
宽带DDS跳频源设计 总被引:1,自引:0,他引:1
直接数字合成(DDS)简单可靠、控制方便,具有很高的频率分辨率,高速转换,非常适合快速跳频的要求。在对DDS基本原理进行了简要介绍和分析后,提出宽带跳频源设计方案。 相似文献
9.
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
10.
Feng-Wen Sun Yimin Jiang Baras J.S. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(1):180-190
Many issues in signal processing involve the inverses of Toeplitz matrices. One widely used technique is to replace Toeplitz matrices with their associated circulant matrices, based on the well-known fact that Toeplitz matrices asymptotically converge to their associated circulant matrices in the weak sense. This often leads to considerable simplification. However, it is well known that such a weak convergence cannot be strengthened into strong convergence. It is this fact that severely limits the usefulness of the close relation between Toeplitz matrices and circulant matrices. Observing that communication receiver design often needs to seek optimality in regard to a data sequence transmitted within finite duration, we define the finite-term strong convergence regarding two families of matrices. We present a condition under which the inverses of a Toeplitz matrix converges in the strong sense to a circulant matrix for finite-term quadratic forms. This builds a critical link in the application of the convergence theorems for the inverses of Toeplitz matrices since the weak convergence generally finds its usefulness in issues associated with minimum mean squared error and the finite-term strong convergence is useful in issues associated with the maximum-likelihood or maximum a posteriori principles. 相似文献