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   215656篇
  免费   3064篇
  国内免费   651篇
电工技术   4181篇
综合类   138篇
化学工业   30105篇
金属工艺   7813篇
机械仪表   5790篇
建筑科学   4560篇
矿业工程   792篇
能源动力   4936篇
轻工业   17647篇
水利工程   1913篇
石油天然气   2916篇
武器工业   14篇
无线电   28122篇
一般工业技术   40235篇
冶金工业   49535篇
原子能技术   3633篇
自动化技术   17041篇
  2021年   1671篇
  2020年   1314篇
  2019年   1702篇
  2018年   2882篇
  2017年   2825篇
  2016年   2897篇
  2015年   2052篇
  2014年   3311篇
  2013年   9313篇
  2012年   5571篇
  2011年   7023篇
  2010年   5699篇
  2009年   6496篇
  2008年   6893篇
  2007年   6889篇
  2006年   6074篇
  2005年   5678篇
  2004年   5635篇
  2003年   5440篇
  2002年   5215篇
  2001年   5510篇
  2000年   5110篇
  1999年   5699篇
  1998年   16978篇
  1997年   11179篇
  1996年   8492篇
  1995年   6061篇
  1994年   5244篇
  1993年   5311篇
  1992年   3482篇
  1991年   3356篇
  1990年   3291篇
  1989年   3122篇
  1988年   2858篇
  1987年   2294篇
  1986年   2364篇
  1985年   2687篇
  1984年   2358篇
  1983年   2093篇
  1982年   1955篇
  1981年   2111篇
  1980年   1842篇
  1979年   1688篇
  1978年   1678篇
  1977年   2112篇
  1976年   2890篇
  1975年   1437篇
  1974年   1385篇
  1973年   1329篇
  1972年   1131篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Lenin Dnepropetrovsk Pipe Plant. Translated from Metallurg, No. 8, p. 39, August, 1989.  相似文献   
43.
A continuous phase quadrature phase shift keyed (CPQPSK) modulation technique is presented. This method utilizes a conventional QPSK modulator and a phase trajectory converter to approximate M=4, h=1/4 continuous phase signal and allows low cost, low complexity, and high rate (>1 Gbit/s) CPM modem implementation for bandwidth efficient transmission through nonlinear satellite channels. Using a communications analysis computer program it has been found that CPQPSK has 99 percent out-of-band power of 0.8R (MSK has 99 percent out-of-band power of 1.2 R where R is defined as bit rate), continuous phase trajectories, and nearly constant envelope amplitude. Simulation of realistic hardware designs indicate that the CPQPSK will require an Eb/No of 14 dB to achieve a bit error rate (BER) of 10-6. Forward error correcting techniques using block codes with an overhead of 10 percent indicate that the Eb/No requirements can be reduced to 11.2 dB for 10-6 BER  相似文献   
44.
This paper reports on the first demonstration of a half-bridge power inverter constructed from silicon carbide gate turn-off thyristors (GTOs) operated in the conventional GTO mode. This circuit was characterized with input bus voltages of up to 600 VDC and 2 A (peak current density of 540 A/cm2) with resistive loads using a pulse-width modulated switching frequency of 2 kHz. We discuss the implications of the thyristor's electrical characteristics and the circuit topology on the overall operation of the half-bridge circuit. This work has determined the conservative critical rate of rise value of the off-state voltage to be 200 V/μs in these devices  相似文献   
45.
46.
The strong interaction of electrons with the flat surfaces of small crystals has been investigated by high resolution CTEM and STEM instruments. When cubic crystals of MgO smoke with edges 20–300 nm are oriented so that the ?001? or ?011? zone axis is parallel to the optical axis, then two kinds of external fringes are observed at (100) surfaces. One kind is parallel to the surface, having spacings up to 0.4 nm. These are caused by interference among the electron channelled along the surface. Fresnel-diffracted ones and the remnant of the incident beam. Fringes of the other kind, which appear as fine structure in the first kind of fringes, are perpendicular to the crystal edge. When an electron beam is parallel to the ?011? axis, the second kind of fringe, whose spacing is 0.3 nm corresponding to d011, shows the difference of the surface potential between magnesium atoms and oxygen atoms. Selected area diffraction patterns and microdiffraction patterns also show the same periodicities as in the two kinds of fringes. Simulated images, using the scattering amplitudes for ions, are compared with observations.  相似文献   
47.
Ohtoshi  T. 《Electronics letters》1987,23(11):570-571
A theoretical expression is derived for the far fields of semiconductor lasers with coated facets. It is shown that the far fields are different for uncoated, antireflection-coated and high-reflection-coated lasers. The correction factor for the far fields is shown to depend on the transmission coefficient of the facets.  相似文献   
48.
We propose an asynchronous structure for implementation on a SoC. An intersegment topological arrangement preserves parallelization and, through a so-called central arbiter, efficiently organizes communication with high signaling speed in the proposed structure. Researchers proposed the concept of segmenting buses primarily for multicomputer architectures. More recent approaches address on-chip implementation of segmented buses. We present an asynchronous segmented-bus architecture targeted for the modular design of high-performance SoC applications. The structure not only enables faster operation than a conventional bus system but also offers lower power consumption per transferred data item. This is possible because segmentation is realized in such a way that the majority of data transfers in the system are intrasegment transactions on relatively short wires with low or moderate capacitive loads.  相似文献   
49.
With the creation of balancing authorities by the North American Reliability Council that span large portions of the North American interconnection, and stringent requirements for real time monitoring of power system evolution, faster and more accurate state estimation algorithms that can efficiently handle systems of very large sizes are needed in the present environment. This paper presents a distributed state estimation algorithm suitable for large-scale power systems. Synchronized phasor measurements are applied to aggregate the voltage phase angles of each decomposed subsystem in the distributed state estimation solution. The aggregated state estimation solution is obtained from the distributed solution using a sensitivity analysis based update at chosen boundary buses. Placement of synchronized phasor measurements in the decomposed subsystems is also investigated in this paper. Test results on the IEEE 118-bus test bed are provided  相似文献   
50.
The cooling and solidification of melted drops during their movement in an immiscible cooling medium is widely employed for granulation in the chemical industry, and a study of these processes to provides a basis for the design of the granulation tower height and the temperature of the cooling medium is reported. A physical model of the cooling and solidification of the drop is established and the numerical calculation is performed. The influences of the key factors in the solidification, i.e., Bi number, drop diameter, temperature of the cooling medium, etc. are presented. The cooling and solidification during wax granulation in a water‐cooling tower and during urea granulation in an air‐cooling tower (spraying tower) are described in detail. Characteristics of the solidification and temperature distribution within the particle at different times are shown. The model and calculations can be used for structure design of the granulation tower and optimization of the operation parameters.  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