全文获取类型
收费全文 | 553538篇 |
免费 | 12229篇 |
国内免费 | 4078篇 |
专业分类
电工技术 | 13257篇 |
技术理论 | 6篇 |
综合类 | 4824篇 |
化学工业 | 83162篇 |
金属工艺 | 23934篇 |
机械仪表 | 22405篇 |
建筑科学 | 18054篇 |
矿业工程 | 4014篇 |
能源动力 | 13896篇 |
轻工业 | 49436篇 |
水利工程 | 5351篇 |
石油天然气 | 8573篇 |
武器工业 | 473篇 |
无线电 | 69263篇 |
一般工业技术 | 103921篇 |
冶金工业 | 83939篇 |
原子能技术 | 7392篇 |
自动化技术 | 57945篇 |
出版年
2022年 | 3483篇 |
2021年 | 5471篇 |
2020年 | 3908篇 |
2019年 | 4294篇 |
2018年 | 14401篇 |
2017年 | 15341篇 |
2016年 | 10685篇 |
2015年 | 6584篇 |
2014年 | 9160篇 |
2013年 | 23386篇 |
2012年 | 16973篇 |
2011年 | 27549篇 |
2010年 | 23123篇 |
2009年 | 23454篇 |
2008年 | 23940篇 |
2007年 | 26097篇 |
2006年 | 16539篇 |
2005年 | 18880篇 |
2004年 | 15594篇 |
2003年 | 14766篇 |
2002年 | 13299篇 |
2001年 | 12772篇 |
2000年 | 12153篇 |
1999年 | 12774篇 |
1998年 | 25683篇 |
1997年 | 18917篇 |
1996年 | 15249篇 |
1995年 | 12114篇 |
1994年 | 10703篇 |
1993年 | 10040篇 |
1992年 | 7644篇 |
1991年 | 7209篇 |
1990年 | 6700篇 |
1989年 | 6343篇 |
1988年 | 6077篇 |
1987年 | 5012篇 |
1986年 | 4930篇 |
1985年 | 6058篇 |
1984年 | 5654篇 |
1983年 | 4864篇 |
1982年 | 4537篇 |
1981年 | 4467篇 |
1980年 | 4228篇 |
1979年 | 4091篇 |
1978年 | 3774篇 |
1977年 | 4553篇 |
1976年 | 6104篇 |
1975年 | 3118篇 |
1974年 | 3012篇 |
1973年 | 2891篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
众所周如,西藏地区是研究地球深部结构和构造的窗口,是研究大陆板块学说的重要场所,也是寻找热源和其它矿产资源的有利地区.因此,世界各国地学科学家都希望在本地区进行科学研究工作,或与我国科学工作者协作共同探索这一重要地区.中国和法国的地质、地球物理科学家经商讨共同组成“中法喜马拉雅山地质研究队”,对西藏中部广大地区进行地质、地球物理的科学研究工作.深部地震探测是地球物理研究中的重要内容.中法双方共同决定,在中国西藏南部的日喀则地区至北边的那曲地区作地震探测工作.为了探测地壳上地幔的结构,1981年在西藏的南部佩枯错至普莫雍错500公里的喜马拉雅山北麓——雅鲁藏布江地区进行了人工爆炸地震探测工作.本文是中法地质研究队合作的深部地震探测资料的解释结果. 相似文献
992.
Schonfeld D. Goutsias J. 《IEEE transactions on pattern analysis and machine intelligence》1991,13(1):14-29
A theoretical analysis of morphological filters for the optimal restoration of noisy binary images is presented. The problem is formulated in a general form, and an optimal solution is obtained by using fundamental tools from mathematical morphology and decision theory. Consideration is given to the set-difference distance function as a measure of comparison between images. This function is used to introduce the mean-difference function as a quantitative measure of the degree of geometrical and topological distortion introduced by morphological filtering. It is proved that the class of alternating sequential filters is a set of parametric, smoothing morphological filters that best preserve the crucial structure of input images in the least-mean-difference sense 相似文献
993.
This paper discusses the testing accomplished to determine the radio frequency (RF) susceptibility of a typical electroencephalograph (EEG) and to determine the effectiveness of lead X-ray shielding as an RF shield. New data are presented on threshold field strengths and frequencies which just begin to cause interference on a typical EEG brain scan. Results are combined with data from other studies to show that a typical EEG machine would be immune to the RF environment expected in the vicinity of most hospitals. Therefore, expensive RF shielding in a hospital EEG suite is usually not required. Finally, an inexpensive shielding system utilizing X-ray shielding which was developed by the U.S. Veterans Administration is presented. 相似文献
994.
As a major pollutant source to urban receiving waters, the non-point source pollution from urban runoff needs to be well studied and effectively controlled. Based on monitoring data from urban runoff pollutant sources, this article describes a systematic estimation of total pollutant loads from the urban areas of Beijing. A numerical model was developed to quantify main pollutant loads of urban runoff in Beijing. A sub-procedure is involved in this method, in which the flush process influences both the quantity and quality of stormwater runoff. A statistics-based method was applied in computing the annual pollutant load as an output of the runoff. The proportions of pollutant from point-source and non-point sources were compared. This provides a scientific basis for proper environmental input assessment of urban stormwater pollution to receiving waters, improvement of infrastructure performance, implementation of urban stormwater management, and utilization of stormwater. 相似文献
995.
Analysis of multiple FSS screens of unequal periodicity using an efficient cascading technique 总被引:1,自引:0,他引:1
In this paper, we present an efficient cascading procedure for analyzing frequency selective surface (FSS) systems consisting of multiple FSS screens of unequal periodicity embedded in multiple dielectric layers. In this procedure, we first find a global period for the FSS system by studying the composite in its entirety. Next, we compute the scattering matrix [S] of each of the FSS subsystems for the global Floquet harmonics by applying a relationship we establish that maps the [S] matrix of the subsystem for the individual Floquet harmonics to that for the global harmonics. This mapping-cum-filling process substantially reduces the effort needed to compute the [S] matrix of a subsystem. Finally, we compute the [S] of the entire system by applying a modified cascading formulation, in which one matrix inversion step is eliminated, resulting in a reduction in the total computing resource requirement as well as time. Two numerical examples are given to illustrate the efficiency and effectiveness of the technique. 相似文献
996.
Luo J. Ulukus S. Ephremides A. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2005,51(8):2760-2769
The optimal signature sequences that maximize the sum capacity of a direct sequence code-division multiple-access (CDMA) system are characterized in the general case of symbol delay profile and user power constraints. It is shown that the optimal sum capacity of the symbol asynchronous system equals that of the symbol synchronous system with the same user power constraints. With the optimal signature sequence set, the maximum sum capacity is achieved with white Gaussian input signals. The existence of the optimal signature sequence set is proved by the proposal of an explicit construction method for arbitrary user delay profiles and power constraints. 相似文献
997.
V. Anjos M.J.V. Bell E.F. da Silva Jr. R.W.A. Franco I.A. Esquef 《Microelectronics Journal》2005,36(11):977-980
In this paper we report the use of photothermal techniques such as Thermal lens (TL) spectrometry, Photoacoustic and heat capacity, ρcp, to determine the thermo-optical parameters, such as thermal conductivity (K), thermal diffusivity (D), specific heat (cp) and the optical path dependence with temperature (ds/dT), of an undoped polycrystalline 3C-SiC. To our knowledge, this is the first time that Thermal lens technique is used for wide band-gap systems. Results obtained for the polycrystalline sample with TL technique indicates that ds/dT is negative at room temperature. Moreover, the obtained values of thermal diffusivity and thermal conductivity are in good agreement with that found in the literature, indicating that the phototermal techniques can be used to obtain the referred parameters in circumstances where other techniques cannot be used, for example, in harsh environments. 相似文献
998.
Cogburn D.L. Mueller M. McKnight L. Klein H. Mathiason J. 《Communications Magazine, IEEE》2005,43(12):12-14
Who should control the Internet? A dozen years after the Internet became a mass medium, this issue has continued to grow in urgency, becoming white hot in fall 2005. At the September 2005 preparatory meeting for the UN World Summit on the Information Society (WSIS), a coalition of countries criticized the United States' unilateral control of the Internet's domain name system (DNS) and proposed the establishment of a multinational Council to supervise it. This proposal emerged from the Final Report of the UN Working Group on Internet Governance. Researchers from the Internet Governance Project, a university-based consortium for policy analysis, have concluded that the United States should internationalize governance of the Internet, but in a way that avoids intrusive, centralized control. 相似文献
999.
Lasers containing a nanopatterned active layer demonstrating excellent threshold characteristics are presented. The nanopatterned active layer is fabricated using high-resolution electron beam lithography and selective-area metal organic chemical vapour deposition crystal growth. Results demonstrating an order of magnitude improvement over previous results are reported. 相似文献
1000.
本文评价了塑料闪烁片用作软β核素探测器时的计数效率、β谱、本底谱和康普顿电子谱的特征。文中强调:采用胶合光导剂构成的系统是一种新型探测器(软β核素内部样品塑料闪烁探测器)。本文提出内部样品固体探测器、胶合光导剂和胶合闪烁光导剂的新概念。 相似文献