全文获取类型
收费全文 | 112497篇 |
免费 | 11705篇 |
国内免费 | 5810篇 |
专业分类
电工技术 | 13566篇 |
技术理论 | 5篇 |
综合类 | 7950篇 |
化学工业 | 18290篇 |
金属工艺 | 3241篇 |
机械仪表 | 4253篇 |
建筑科学 | 12245篇 |
矿业工程 | 7229篇 |
能源动力 | 7554篇 |
轻工业 | 6074篇 |
水利工程 | 2072篇 |
石油天然气 | 20203篇 |
武器工业 | 743篇 |
无线电 | 5228篇 |
一般工业技术 | 7105篇 |
冶金工业 | 5167篇 |
原子能技术 | 1535篇 |
自动化技术 | 7552篇 |
出版年
2024年 | 455篇 |
2023年 | 1558篇 |
2022年 | 3059篇 |
2021年 | 3937篇 |
2020年 | 4279篇 |
2019年 | 3584篇 |
2018年 | 3301篇 |
2017年 | 3689篇 |
2016年 | 4326篇 |
2015年 | 4355篇 |
2014年 | 7146篇 |
2013年 | 7221篇 |
2012年 | 8448篇 |
2011年 | 8751篇 |
2010年 | 6126篇 |
2009年 | 6332篇 |
2008年 | 5606篇 |
2007年 | 6705篇 |
2006年 | 6714篇 |
2005年 | 5503篇 |
2004年 | 4767篇 |
2003年 | 4125篇 |
2002年 | 3489篇 |
2001年 | 3014篇 |
2000年 | 2628篇 |
1999年 | 2110篇 |
1998年 | 1665篇 |
1997年 | 1360篇 |
1996年 | 1082篇 |
1995年 | 940篇 |
1994年 | 837篇 |
1993年 | 578篇 |
1992年 | 467篇 |
1991年 | 389篇 |
1990年 | 331篇 |
1989年 | 294篇 |
1988年 | 160篇 |
1987年 | 118篇 |
1986年 | 100篇 |
1985年 | 88篇 |
1984年 | 70篇 |
1983年 | 47篇 |
1982年 | 47篇 |
1981年 | 68篇 |
1980年 | 50篇 |
1979年 | 14篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1959年 | 10篇 |
1951年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
91.
Klaus G. Johannsen 《International Journal of Satellite Communications and Networking》1987,5(3):233-247
Several domestic satellite systems have been more than 10 years in operation. During this time, the world has experienced a dramatic growth in communication. While the demand for more telephony and data circuits and television channels is still on the rise, new methods of signal transmission and circuit compression have been developed which may balance or reduce the need for more satellite trunk capacity. And while whole national networks are converting to digital transmission, new transmission media like optical fibre, are finding application, and may take over some of the heavy traffic routes previously carried by satellite. On the other side, business data communication between roof-top terminals, thin route traffic to remote users, television distribution by direct satellite broadcasting are becoming more popular and satellite land mobile communication may soon be a reality. Thus the domestic satellite's role appears to be slowly changing towards thin route and mobile traffic and DBS-type distribution and perhaps even radio position determination. System planners are adapting to the new trends in satellite communication and are trying to accommodate the various service payloads on a single satellite. This article discusses some aspects of the emerging multi-purpose domestic satellite, which carries fixed and mobile services, TV direct broadcasting and radio position determination, in view of network digitalization, network expansion, circuit compression, synchronization. The discussion centres on transponder supply and demand, satellite replenishment, Ku-band vs. C-band for TV direct satellite broadcasting, choice of modulation/multiple access for mobile communication, and expected accuracy in radio position determination. 相似文献
92.
Gamma distributions are some of the most popular models for hydrological processes. In this paper, a very flexible family which contains the gamma distribution as a particular case is introduced. Evidence of flexibility is shown by examining the shape of its probability density function (pdf). A treatment of the mathematical properties is provided by deriving expressions for the n th moment. Estimation and simulation issues are also considered. Finally, a detailed application to drought data from the State of Nebraska is illustrated. 相似文献
93.
根据旧有锅炉存在的问题提出了改造方案,并对其进行可行性分析,选择了符合企业实际情况的改造方案,从而取得了良好的经济效益。 相似文献
94.
95.
96.
C. N. Cacaval E. C. BuruianÎ D. Rou E. Rusu G. E. Grigoriu V. Brboiu 《Polymer International》1985,17(4):343-346
Thermogravimetry and pyrolysis in combination with gas chromatography and infrared spectroscopy were the experimental techniques applied to the thermal degradation of cinnamoylated poly(vinyl alcohol) samples, constituted from vinyl alcohol-vinyl cinnamate photocrosslinkable copolymers. The thermal decomposition products include gases, liquids and solids. The gases are formed from saturated and unsaturated volatile hydrocarbons C1? C4, carbon monoxide and carbon dioxide. The liquid fraction includes aromatic hydrocarbons and some oxygenated organic compounds. The solid product identified in the greatest amount was cinnamic acid. The content in the thermal decomposition products varies considerably both with copolymer composition and temperature. 相似文献
97.
为使北京谱仪端盖族射计数器更好地工作在自猝灭流光SQS模式下,提高其能量分辨以及线性等指标,用Ar,CO2,Lsobutane三元工作气体对探测器的工作性能进行了仔细研究,找出了一适合于主雇地数器的工作气体,此外,还研究了异丁的纯度对探测器性能的影响。 相似文献
98.
本文结合大庆油田天然气矿场加工实际生产装置,讨论了应用HYSIM烃类工艺模拟软件对含有不确定组分的体系进行流程模拟计算的特点、塔设备严格模拟计算过程中输入规定的选择、以及多股流换热器和吸附脱水工艺的模拟计算方法。 相似文献
99.
Although the gas metal arc welding (GMAW) process is widely used for joining metals, its planning is still a cumbersome task. The planning procedure involves many complex interrelationships among the process variables and depends heavily on empirical knowledge. Therefore, a solution method that allows a convenient way to represent the domain knowledge and evaluate the process variable relationships is needed. Further, the lack of a strong domain theory to quantify these interrelationships is a deficiency in the planning of the GMAW process. This paper presents a solution method that overcomes this deficiency and introduces a convenient knowledge representation scheme. The solution method considers the planning procedure to be a constraint satisfaction problem (CSP). The AC-4 algorithm is used to obtain arc consistency among nodes in the constraint network and the PC-4 algorithm is applied to obtain path consistency in the network. A modified version of the AC-4 algorithm is developed to reduce the search space and correspondingly reduce the computation time. An example is presented to demonstrate the capability of the method and its potential to reduce lead time. 相似文献
100.
G. Bretschko 《河流研究与利用》1992,7(1):17-22
The ‘hyporheic zone’ is described. According to the classical definition, water below the sediment surface is groundwater and the hyporheic interstitial zone is part of the phreatic system. This ignores the vertical distribution of the epigeic benthic fauna. The uppermost layer of the sediment (bed sediments) of the hyporheic zone is dominated by epigeic faunal elements and is therefore part of the surface system. In contrast to hypogeic species, the depth penetration of epigeic species is limited. Possible explanations are discussed. 相似文献