首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   157536篇
  免费   1341篇
  国内免费   649篇
电工技术   3176篇
综合类   194篇
化学工业   25078篇
金属工艺   5867篇
机械仪表   5007篇
建筑科学   4716篇
矿业工程   381篇
能源动力   4162篇
轻工业   17789篇
水利工程   1209篇
石油天然气   640篇
武器工业   5篇
无线电   21000篇
一般工业技术   29797篇
冶金工业   24135篇
原子能技术   2375篇
自动化技术   13995篇
  2021年   953篇
  2019年   1073篇
  2018年   1192篇
  2017年   1232篇
  2016年   1455篇
  2015年   1183篇
  2014年   1952篇
  2013年   6846篇
  2012年   3459篇
  2011年   4933篇
  2010年   3876篇
  2009年   4405篇
  2008年   4912篇
  2007年   5138篇
  2006年   4548篇
  2005年   4280篇
  2004年   4154篇
  2003年   4026篇
  2002年   4040篇
  2001年   4036篇
  2000年   3812篇
  1999年   3741篇
  1998年   6695篇
  1997年   5279篇
  1996年   4515篇
  1995年   3748篇
  1994年   3409篇
  1993年   3218篇
  1992年   2808篇
  1991年   2716篇
  1990年   2651篇
  1989年   2642篇
  1988年   2476篇
  1987年   2191篇
  1986年   2135篇
  1985年   2573篇
  1984年   2336篇
  1983年   2213篇
  1982年   2088篇
  1981年   2004篇
  1980年   1879篇
  1979年   1890篇
  1978年   1778篇
  1977年   2099篇
  1976年   2572篇
  1975年   1586篇
  1974年   1437篇
  1973年   1459篇
  1972年   1199篇
  1971年   1121篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
Abstract There are still many institutions of higher education in the UK, both universities and polytechnics, that have not taken on board the application of information technology to foreign language learning. Apart from considerations of cost, there is also the problem of time—deciding how best to use the limited time at one's disposal for exploring computer applications to foreign language learning. Both problems may be compounded by the mistaken belief that effective use of this 'new' technology necessarily involves learning one or more programming languages. This paper endeavours to show that recent advances in both software and hardware have produced computing tools of particular interest to translation and the translator, and that, since translation is a reality for large numbers of undergraduates in the UK, such tools represent an obvious starting point for the introduction of IT into the undergraduate curriculum.  相似文献   
132.
The main objective of this paper is to simulate the effects of soil erosion on river water quality and on agricultural production as a result of the transformation of forestlands in the catchment of the upstream Phong River. Suspended solids carry down attached nutrients and agricultural chemicals causing water pollution in the downstream. There are four different types of land use in this simulation, namely forestlands, flatland and highland sugarcane plantation areas, and paddy fields. The highest mean annual amount of soil erosion is from paddy fields (585,700 tons/year), followed by highland (73,800 tons/year) and flatland (63,950 tons/year) sugarcane plantation areas and forestlands (41,800 tons/year), respectively. However, as most of paddy fields are located in a low land and are wet type cultivations, the soil erosion occurred has less impact on river water quality and its production compared to the soil erosion from the steeper slopes of highland plantation areas. Under the resource-based agriculture, the sugarcane production is mainly increased by expanding the plantation areas leading to a significant loss of topsoil and a considerable reduction of agricultural production. Soil erosion contributes to an increase in the average annual suspended solids concentration by 72 mg/l.  相似文献   
133.
A jet-printed digital-lithographic method, in place of conventional photolithography, was used to fabricate 64 /spl times/ 64 pixel (300 /spl mu/m pitch) matrix addressing thin-film transistor (TFT) arrays. The average hydrogenated amorphous silicon TFT device within an array had a threshold voltage of /spl sim/3.5 V, carrier mobility of 0.7 cm/sup 2//V/spl middot/s, subthreshold slope of 0.76 V/decade, and an on/off ratio of 10/sup 8/.  相似文献   
134.
This paper examines the current status and methodologies of study of material and system reliability in Microelectromechanical Systems (MEMS). This includes: a review of the current literature in the area of MEMS regarding failure analysis experimental investigations; testing methods and philosophies for material characterization and possible mechanistic analytical solutions for estimating material properties. The paper proposes a reliability framework that encompasses all the available information. This statistical platform will enable the MEMS design engineer to distill all the available information in the literature into a stand-alone semi-empirical material reliability model, and a holistic system-level model for a complete system.  相似文献   
135.
We introduce optical local-area network (LAN) architectures based on multimode optical fiber and components, short wavelength lasers and detectors, and the widely used fast Ethernet protocol. These architectures are designed to lower the costs associated with passive optical LAN implementation. Further reduction in overall cost is achieved through decreased network downtime, lower maintenance cost, extended geometrical spans, and larger headroom for future capacity increase. These optically transparent networks represent a novel approach for implementing fiber-to-the-desk.  相似文献   
136.
137.
This paper evaluates experimentally the performance of a novel axial velocity estimator, the 2D autocorrelator, and its Doppler power estimation counterpart, the 2D zero-lag autocorrelator, in the context of ultrasound color flow mapping. The evaluation also encompasses the well-established 1D autocorrelation technique for velocity estimation and its corresponding power estimator (1D zero-lag autocorrelator), to allow performance comparisons under identical conditions. Clutter-suppressed in vitro data sets from a steady-flow system are used to document the effect of the range gate and ensemble length, noise level and angle of insonation on the precision of the velocity estimates. The same data sets are used to examine issues related to the estimation of the Doppler signal's power. The first-order statistics of power estimates from regions corresponding to flow and noise are determined experimentally and the ability of power-based thresholding to separate flow signals from noise is characterized by means of ROC analysis. In summary, the results of the in vitro evaluation show that the proposed 2D-autocorrelation form of processing is consistently better than the corresponding 1D-autocorrelation techniques, in terms of both velocity and power estimation. Therefore, given their relatively modest implementation requirements, the 2D-autocorrelation algorithms for velocity and power estimation appear to represent a superior, yet realistic, alternative to conventional Doppler processing for color flow mapping  相似文献   
138.
The crystal structure of CaMgGeO4 is described. CaMgGeO4, Mr = 200.9, orthorhombic, Pnam, A = 11.285(5) Å, B = 5.016(2) Å, C = 6.435(2) Å, V = 364.36 Å3, Dx = 3.664 Mg/m3.λ(MoKa = 0.71069 Å, F(000) = 384, room temperature, final R = 0.045 for 1752 observed reflections. The structure is isomorphous with CaMgSiO4 (monticellite).  相似文献   
139.
The authors have fabricated n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBTs) with base doping graded exponentially from 5×1019 cm-3 at the emitter edge to 5×1018 cm-3 at the collector edge. The built-in field due to the exponentially graded doping profile significantly reduces base transit time, despite bandgap narrowing associated with high base doping. Compared to devices with the same base thickness and uniform base doping of 1×1019 cm-3 , the cutoff frequency is increased from 22 to 31 GHz and maximum frequency of oscillation is increased from 40 to 58 GHz. Exponentially graded base doping also results ill consistently higher common-emitter current gain than uniform base doping, even though the Gummel number is twice as high and the base resistance is reduced by 40%  相似文献   
140.
The effect of noncatastrophic positive human body model (HBM) electrostatic discharge (ESD) stress on n-channel power MOSFETs is radically different from that on p-channel MOSFETs. In n-channel transistors, the stress causes negative shifts of the current-voltage characteristics indicative of positive charge trapping in the gate oxide. In p-channel transistors, the stress increases the drain-to-source leakage current, probably due to localized avalanche electron injection from the p-doped drain  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号