全文获取类型
收费全文 | 471710篇 |
免费 | 7415篇 |
国内免费 | 2614篇 |
专业分类
电工技术 | 9022篇 |
技术理论 | 5篇 |
综合类 | 1346篇 |
化学工业 | 72876篇 |
金属工艺 | 17708篇 |
机械仪表 | 13779篇 |
建筑科学 | 12357篇 |
矿业工程 | 2152篇 |
能源动力 | 11830篇 |
轻工业 | 48460篇 |
水利工程 | 4634篇 |
石油天然气 | 7660篇 |
武器工业 | 248篇 |
无线电 | 55540篇 |
一般工业技术 | 87080篇 |
冶金工业 | 87520篇 |
原子能技术 | 8275篇 |
自动化技术 | 41247篇 |
出版年
2021年 | 3908篇 |
2020年 | 2805篇 |
2019年 | 3458篇 |
2018年 | 5458篇 |
2017年 | 5439篇 |
2016年 | 6015篇 |
2015年 | 4601篇 |
2014年 | 7500篇 |
2013年 | 21463篇 |
2012年 | 13086篇 |
2011年 | 18067篇 |
2010年 | 14198篇 |
2009年 | 15626篇 |
2008年 | 16743篇 |
2007年 | 16880篇 |
2006年 | 15215篇 |
2005年 | 13518篇 |
2004年 | 12549篇 |
2003年 | 12116篇 |
2002年 | 11674篇 |
2001年 | 12071篇 |
2000年 | 11095篇 |
1999年 | 11546篇 |
1998年 | 26746篇 |
1997年 | 19186篇 |
1996年 | 14625篇 |
1995年 | 11106篇 |
1994年 | 9969篇 |
1993年 | 9709篇 |
1992年 | 7245篇 |
1991年 | 6880篇 |
1990年 | 6815篇 |
1989年 | 6559篇 |
1988年 | 6291篇 |
1987年 | 5226篇 |
1986年 | 5150篇 |
1985年 | 6099篇 |
1984年 | 5703篇 |
1983年 | 5251篇 |
1982年 | 4704篇 |
1981年 | 4962篇 |
1980年 | 4530篇 |
1979年 | 4610篇 |
1978年 | 4366篇 |
1977年 | 4758篇 |
1976年 | 6113篇 |
1975年 | 3804篇 |
1974年 | 3592篇 |
1973年 | 3673篇 |
1972年 | 3006篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Varani L. Reggiani L. Kuhn T. Gonzalez T. Pardo D. 《Electron Devices, IEEE Transactions on》1994,41(11):1916-1925
We present a microscopic interpretation of electronic noise in semiconductor materials and two-terminal devices. The theory is based on Monte Carlo simulations of the carrier motion self-consistently coupled with a Poisson solver. Current and voltage noise operations are applied and their respective representations discussed. As application we consider the cases of homogeneous materials, resistors, n+nn + structures, and Schottky-barrier diodes. Phenomena associated with coupling between fluctuations in carrier velocity and self-consistent electric field are quantitatively investigated for the first time. At increasing applied fields hot-carrier effects are found to be of relevant importance in all the cases considered here. As a general result, noise spectroscopy is found to be a source of valuable information to investigate and characterize transport properties of semiconductor materials and devices 相似文献
122.
123.
124.
McDonald K.C. Zimmermann R. Kimball J.S. 《Geoscience and Remote Sensing, IEEE Transactions on》2002,40(9):2063-2082
Spatial and temporal variations in vegetation dielectric properties strongly influence the microwave backscatter characteristics of forested landscapes. This paper examines the relationship between xylem tissue dielectric constant, xylem sap flux density, and xylem sap chemical composition as measured in the stems of two Norway Spruce (Picea abies [L.] Karst.) trees in the Fichtelgebirge region of Northern Bavaria, Germany. Dielectric constant and xylem sap flux were monitored continuously from June through October 1995, at several heights along the tree trunks. At the end of the measurement series, each tree was harvested, and its xylem sap extracted and analyzed to determine the concentrations of amino acids and cations. Results show that the sap flux density was correlated with vapor pressure deficit (VPD) at all heights in the stem. In contrast, the xylem tissue dielectric constant is influenced by VPD but can exhibit a significant temporal lag relative to changes in VPD. This lag varies with position along the tree trunk. The temporal variability of the dielectric constant is compared with both trees at several positions along the tree trunks. Results of xylem sap chemical analysis are presented. We show that spatial and temporal variability in the xylem tissue dielectric constant is influenced not only by water content, but by variations in xylem sap chemistry as well. This has important implications for microwave remote sensing of forested landscapes, as useful information may be acquired regarding stand physiology and water relations and where variations in dielectric properties within individual trees and across geographic areas can be significant error sources for forest inventory mapping. 相似文献
125.
MB Heyman JM LaBerge KA Somberg P Rosenthal C Mudge EJ Ring JD Snyder 《Canadian Metallurgical Quarterly》1997,131(6):914-919
The transjugular intrahepatic portosystemic shunt procedure is an accepted treatment for adults with complications of portal hypertension. We performed a retrospective review of all pediatric TIPS placements performed at the University of California, San Francisco between 1990 and 1996. Twelve procedures were attempted in nine children, with a mean age (+/- SD) of 9.4 +/- 3.9 years (range, 5 to 15 years) and a mean weight of 31 +/- 18 kg (range, 16 to 70 kg). The indications for TIPS placement were portal hypertension complicated by chronic variceal hemorrhage not controlled with sclerotherapy (n = 7) and hypersplenism with thrombocytopenia (n = 2). TIPS placement was successfully completed initially in seven of nine (78%) patients. Unfavorable vascular anatomy was the cause of failure in two cases. The seven patients who underwent successful TIPS placement were followed up for an average of 136 days (range, 1 to 800 days); two still have patent shunts, three underwent liver transplantation, one had a splenorenal shunt after stenosis, and one died of underlying liver disease. Variceal bleeding was controlled in four of five patients who successfully underwent TIPS placement. Shunt occlusion occurred in four patients; patency was restored by transjugular shunt revision in three, and a splenorenal shunt was performed in one. 相似文献
126.
A modification to the `shape-invariant' sinusoidal speech model is proposed, whereby the phases of the component sinewaves used for the excitation are made to add coherently at each glottal closure. Applied to pitch and time-scale modification, higher quality synthetic speech is produced when large changes are required 相似文献
127.
The authors report the design of a new current-mode A/D converter, based on a modified successive-approximations model, in 1.2 μm CMOS technology. The proposed circuit is characterised by good accuracy and fast dynamic performance, low power consumption and small occupation area. SPICE simulations allow the design approach to be validated and the electrical performance of the ADC to be predicted 相似文献
128.
Weian Huang Fuhrmann D.R. Politte D.G. Thomas L.J. Jr. States D.J. 《IEEE transactions on bio-medical engineering》1998,45(4):422-428
In four-color fluorescence-based automated DNA sequencing, a 4×4 filter matrix parameterizes the relationship between the dye-intensity signals of interest and the data collected by an optical imaging system. The filter matrix is important because the estimated DNA sequence is based on the dye intensities that can only be recovered via inversion of the matrix. Here, the authors present a calibration method for the estimation of the columns of this matrix, using data generated through a special experiment in which DNA samples are labeled with only one fluorescent dye at a time. Simulations and applications of the method to real data are provided, with promising results 相似文献
129.
Dissolved organic carbon (DOC) dynamics were investigated over a two year period in the Upper Rhǒne River to examine the role of side-arms in providing DOC to the main channel in relation to discharge fluctuations, especially floods. Concentrations of DOC are shown to remain low in space and in time (average 1.5 mg ?1) and to be more related to global hydrological events (precipitation) than to any local flushing for backwaters during floods. The results do not support the general assumption that side-arms are the providers of DOC to the main channel. 相似文献
130.
A case of traumatic extracranial vertebral arterial dissection leading to vertebrobasilar thrombosis and respiratory compromise requiring mechanical ventilation was managed with intraarterial thrombolysis and stenting of the vertebral intimal dissection. In contrast to similar, previously reported cases, this critically ill patient made a full recovery, returning to his job as a secondary school teacher. 相似文献