全文获取类型
收费全文 | 176881篇 |
免费 | 2133篇 |
国内免费 | 704篇 |
专业分类
电工技术 | 3290篇 |
综合类 | 101篇 |
化学工业 | 27875篇 |
金属工艺 | 7774篇 |
机械仪表 | 5119篇 |
建筑科学 | 4409篇 |
矿业工程 | 875篇 |
能源动力 | 4687篇 |
轻工业 | 16370篇 |
水利工程 | 1702篇 |
石油天然气 | 3126篇 |
武器工业 | 2篇 |
无线电 | 19845篇 |
一般工业技术 | 33313篇 |
冶金工业 | 33107篇 |
原子能技术 | 4291篇 |
自动化技术 | 13832篇 |
出版年
2021年 | 1314篇 |
2019年 | 1246篇 |
2018年 | 2096篇 |
2017年 | 2079篇 |
2016年 | 2186篇 |
2015年 | 1570篇 |
2014年 | 2716篇 |
2013年 | 7778篇 |
2012年 | 4551篇 |
2011年 | 6361篇 |
2010年 | 5033篇 |
2009年 | 5883篇 |
2008年 | 5860篇 |
2007年 | 5837篇 |
2006年 | 5010篇 |
2005年 | 4680篇 |
2004年 | 4472篇 |
2003年 | 4143篇 |
2002年 | 4101篇 |
2001年 | 4125篇 |
2000年 | 3922篇 |
1999年 | 4055篇 |
1998年 | 10381篇 |
1997年 | 7389篇 |
1996年 | 5653篇 |
1995年 | 4281篇 |
1994年 | 3617篇 |
1993年 | 3564篇 |
1992年 | 2604篇 |
1991年 | 2537篇 |
1990年 | 2416篇 |
1989年 | 2435篇 |
1988年 | 2376篇 |
1987年 | 2126篇 |
1986年 | 2066篇 |
1985年 | 2368篇 |
1984年 | 2185篇 |
1983年 | 2012篇 |
1982年 | 1880篇 |
1981年 | 1944篇 |
1980年 | 1796篇 |
1979年 | 1816篇 |
1978年 | 1776篇 |
1977年 | 2108篇 |
1976年 | 2687篇 |
1975年 | 1557篇 |
1974年 | 1549篇 |
1973年 | 1607篇 |
1972年 | 1350篇 |
1971年 | 1262篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Candelier P. Mondon F. Guillaumot B. Reimbold G. Martin F. 《Electron Device Letters, IEEE》1997,18(7):306-308
A simplified flash EEPROM process was developed using high-temperature LPCVD oxide both as flash cells interpoly dielectrics and as peripheral transistors gate oxide (decoding logic). An O2 anneal at 850°C lowers charge trapping and interface trap density induced by Fowler-Nordheim injection. However, electron trapping remains slightly higher than with dry thermal oxide. Similar memory charge loss and write-erase endurance are obtained as for ONO-insulated cells. HTO thus proves to have the required quality and reliability to be used in flash EEPROMs 相似文献
82.
Salama H.F. Reeves D.S. Viniotis Y. 《Selected Areas in Communications, IEEE Journal on》1997,15(3):332-345
Multicast (MC) routing algorithms capable of satisfying the quality of service (QoS) requirements of real-time applications will be essential for future high-speed networks. We compare the performance of all of the important MC routing algorithms when applied to networks with asymmetric link loads. Each algorithm is judged based on the quality of the MC trees it generates and its efficiency in managing the network resources. Simulation results over random networks show that unconstrained algorithms are not capable of fulfilling the QoS requirements of real-time applications in wide-area networks. Simulations also reveal that one of the unconstrained algorithms, reverse path multicasting (RPM), is quite inefficient when applied to asymmetric networks. We study how combining routing with resource reservation and admission control improves the RPM's efficiency in managing the network resources. The performance of one semiconstrained heuristic, MSC, three constrained Steiner tree (CST) heuristics, Kompella, Pasquale, and Polyzos (1992), constrained adaptive ordering (CAO), and bounded shortest multicast algorithm (BSMA), and one constrained shortest path tree (CSPT) heuristic, the constrained Dijkstra heuristic (CDKS) are also studied. Simulations show that the semiconstrained and constrained heuristics are capable of successfully constructing MC trees which satisfy the QoS requirements of real-time traffic. However, the cost performance of the heuristics varies. The BSMA's MC trees are lower in cost than all other constrained heuristics. Finally, we compare the execution times of all algorithms, unconstrained, semiconstrained, and constrained 相似文献
83.
84.
Okabe S. Yorozuya T. Endo F. Yamagiwa T. Iwaasa S. 《Power Delivery, IEEE Transactions on》1992,7(3):1232-1238
The voltage-time characteristics of spacer surfaces for steep-front impulse waves are investigated under a particle-contaminated condition in SF6 gas. The characteristics are measured as a function of particle length, particle position, and space shapes. Flashover voltages monotonically increase in the submicrosecond region as time to flashover is shorter, and are a minimum in the 1 μs region. Applicability of the equal voltage-time area criterion for estimating the voltage-time characteristics is discussed and the estimation is clarified. Moreover, it is demonstrated that an optimized spacer with ribs greatly improves flashover voltages in the submicrosecond region as well as in the 1 μs and power frequency region 相似文献
85.
A value-based test is presented for economic screening of electric utility demand-side management (DSM) programs. The widely used least cost test is valid if the programs do not alter the amount or value of energy services provided to customers. But, in general, DSM programs have such effects and, as a result, the value consumers receive is changed. A more general economic efficiency test, the most value test, provides a practical method for considering the effects of DSM on customer value. The version presented allows for multiple load periods and can account for rate impacts on several customer classes. Four typical DSM programs are evaluated as illustrations 相似文献
86.
N Ferrara G Longobardi N Guerra A Nicolino D Acanfora G Furgi L Odierna R Rosiello F Rengo 《Canadian Metallurgical Quarterly》1996,34(1-2):37-41
We prospectively studied 10 patients with stable exertional ischaemia, selected from a larger group of patients referred for suspected coronary artery disease or to detect residual ischaemia after myocardial infarction, to evaluate pharmacokinetic changes during chronic treatment with gallopamil and its correlation with clinical efficacy in patients with coronary artery disease. Our study consisted of a 1-week run-in single-blind placebo treatment and a 4-week single-blind gallopamil treatment. At the end of the run-in period patients underwent two different exercise tests, the first 2 hours and the second 7 hours after placebo administration. During active treatment all patients underwent two different exercise tests, the first 2 hours and the second 7 hours after gallopamil (50 mg) administration on the 1st and 28th days of gallopamil therapy. On the same days in eight of the patients we evaluated gallopamil pharmacokinetic changes. Our data revealed a rapid increase of unchanged gallopamil and its metabolite (norgallopamil) in the plasma, and a peak concentration of these substances about 2 hour after oral administration on both the 1st and 28th day of observation. Moreover, our results demonstrated an increase between the first and 28th day of treatment in peak concentration of unchanged gallopamil in the plasma, and of AUC 0-infinity and AUC o-c values during chronic treatment with gallopamil. Our clinical data showed an improvement in exercise results during gallopamil therapy related to increased concentration of the drug. 相似文献
87.
88.
ON THE GLOBAL AND LOCAL STRUCTURAL PROPERTIES OF PACKED BEDS OF NONEQUILATERAL CYLINDRICAL PARTICLES
F. Benyahia 《Particulate Science and Technology》1996,14(3):221-237
The structure of a series of packed beds of non-equilateral cylindrical particles has been investigated and compared with those of beds of spheres and equilateral cylinders of similar dimensions. It is found that the mean voidage of beds of equilateral cylinders is consistently lower than the corresponding values for beds of spheres and non-equilateral cylinders of aspect ratios 0.25, 0.5, 2 and 3. End effects do not appear to be present in beds of cylinders of aspect ratios of 0.25 and 0.5 and generally, both wall and end effects are significantly weaker than corresponding effects in beds of spheres and equilateral cylinders. This suggests that chanelling is reduced in beds of non-equilateral cylindrical pellets. Whilst the axial voidage distribution of beds of non-equilateral cylinders appears to be almost uniform, the radial voidage distribution bears the effect of confining walls and an oscillatory profile is observed in most cases, but is seen to be irregular for aspect ratios 0.25 and 0.5. The orientation effect of particles of aspect ratio 0.25 in adjacent layers causes an irregular pattern in the axially averaged radial voidage distribution. Observed axially averaged radial voidage profiles suggest that a uniform structure may be achieved by modifying the basic cylindrical shape. 相似文献
89.
90.
Vernotte F. Zalamansky G. McHugh M. Lantz E. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1996,43(3):403-409
The concept of structure functions, which is an extension of the variance approach, is useful to determine the variance (the structure function) which is optimized for a type of noise and for an order of drift. The multivariance method was developed to use different variances over the same signal. It is then possible to select a set of variances in which each variance is optimized to the determination of one parameter (of one noise level, drift, or cutoff frequency). Recently, we adapted this method to irregularly spaced timing data. In this connection, we replaced the structure functions by another method of spectral density estimation: the lowest-mode estimator, introduced by J.E. Deeter and P.E. Boynton (1982, 1984) for the analysis of pulsar timing data. Different lowest-mode estimators can be constructed according to two priorities: the order of drifts that must be removed and the type of noise for which the sensitivity must be maximum. Thus, a multivariance system is developed using a set of different estimators. The details of this method are described, and the results for different signals are discussed in this paper 相似文献