全文获取类型
收费全文 | 163527篇 |
免费 | 19936篇 |
国内免费 | 12815篇 |
专业分类
电工技术 | 16364篇 |
技术理论 | 1篇 |
综合类 | 15442篇 |
化学工业 | 20219篇 |
金属工艺 | 10766篇 |
机械仪表 | 11574篇 |
建筑科学 | 12749篇 |
矿业工程 | 5649篇 |
能源动力 | 4609篇 |
轻工业 | 17702篇 |
水利工程 | 6266篇 |
石油天然气 | 5467篇 |
武器工业 | 2319篇 |
无线电 | 18396篇 |
一般工业技术 | 15273篇 |
冶金工业 | 6051篇 |
原子能技术 | 2558篇 |
自动化技术 | 24873篇 |
出版年
2024年 | 1143篇 |
2023年 | 3259篇 |
2022年 | 7028篇 |
2021年 | 9207篇 |
2020年 | 6586篇 |
2019年 | 4630篇 |
2018年 | 4828篇 |
2017年 | 5486篇 |
2016年 | 4970篇 |
2015年 | 7457篇 |
2014年 | 9187篇 |
2013年 | 10966篇 |
2012年 | 13155篇 |
2011年 | 13658篇 |
2010年 | 12862篇 |
2009年 | 12412篇 |
2008年 | 12613篇 |
2007年 | 12200篇 |
2006年 | 10485篇 |
2005年 | 8658篇 |
2004年 | 6267篇 |
2003年 | 4181篇 |
2002年 | 3903篇 |
2001年 | 3600篇 |
2000年 | 2851篇 |
1999年 | 1372篇 |
1998年 | 588篇 |
1997年 | 499篇 |
1996年 | 435篇 |
1995年 | 353篇 |
1994年 | 297篇 |
1993年 | 201篇 |
1992年 | 176篇 |
1991年 | 120篇 |
1990年 | 107篇 |
1989年 | 85篇 |
1988年 | 81篇 |
1987年 | 61篇 |
1986年 | 57篇 |
1985年 | 29篇 |
1984年 | 21篇 |
1983年 | 16篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 34篇 |
1979年 | 32篇 |
1977年 | 6篇 |
1973年 | 3篇 |
1959年 | 27篇 |
1951年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
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
2.
3.
通过龙潭水电站对励磁装置可控硅元件实施过电压保护改进前后实际效果的分析,介绍了一种有效的过压保护方式. 相似文献
4.
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
5.
Data mining extracts implicit, previously unknown, and potentially useful information from databases. Many approaches have been proposed to extract information, and one of the most important ones is finding association rules. Although a large amount of research has been devoted to this subject, none of it finds association rules from directed acyclic graph (DAG) data. Without such a mining method, the hidden knowledge, if any, cannot be discovered from the databases storing DAG data such as family genealogy profiles, product structures, XML documents, task precedence relations, and course structures. In this article, we define a new kind of association rule in DAG databases called the predecessor–successor rule, where a node x is a predecessor of another node y if we can find a path in DAG where x appears before y. The predecessor–successor rules enable us to observe how the characteristics of the predecessors influence the successors. An approach containing four stages is proposed to discover the predecessor–successor rules. © 2006 Wiley Periodicals, Inc. Int J Int Syst 21: 621–637, 2006. 相似文献
6.
小波变换在光谱特征提取方面的应用 总被引:4,自引:1,他引:3
人们在处理高光谱图像时一般要对一些典型地物进行光谱分析、特征波段的提取,以便提取出最大量的有效信息,剔除无用或冗余的信息,然后再进行分类识别.采用小波变换的分析方法,选用合适的小波进行分解,根据分解后的高频分量中包含的重要信息,利用局部相邻的正负极值点找出对应于原始光谱曲线上每个吸收带的左右边界;利用局部过零点,即可比较精确的提取出各个吸收带的中心波长.该方法比传统的光谱特征提取方法更简洁、有效,实验证明为一种比较理想的光谱特征提取方法. 相似文献
7.
大型往复式压缩机功率大、占地面积大,管道振动问题也更为突出。笔者结合实践经验,对大型往复式压缩机的配管设计进行探讨,说明了平面布置与管道防振设计的密切关系,并介绍了管道防振设计的方法和步骤,以及相对简单的复杂管系气柱固有频率的转移矩阵计算方法。 相似文献
8.
高功率脉冲水中放电的应用及其发展 总被引:7,自引:0,他引:7
首先介绍了高功率脉冲水下放电的机理,然后综合论述了其应用和研究的状况,从较为成熟的脉冲水处理技术到医疗领域的ESWL应用技术,以及在脉冲电场食品杀菌和水下目标探测方面的最新发展和研究,最后指出了高功率脉冲电技术应用前景以及进行更系统和深入研究的必要性。 相似文献
9.
浅谈如何有效学习计算机软件知识 总被引:1,自引:0,他引:1
黄霞 《湖南工业职业技术学院学报》2003,3(2):79-80
计算机软件知识的学习并不是完全线性的,而是不断循环往复、迭代的过程。本对如何深入、广泛、有效地学习计算机软件知识做出了一些探讨。 相似文献
10.
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