全文获取类型
收费全文 | 307885篇 |
免费 | 2977篇 |
国内免费 | 480篇 |
专业分类
电工技术 | 5774篇 |
综合类 | 341篇 |
化学工业 | 47453篇 |
金属工艺 | 15285篇 |
机械仪表 | 12355篇 |
建筑科学 | 6702篇 |
矿业工程 | 2041篇 |
能源动力 | 5772篇 |
轻工业 | 25915篇 |
水利工程 | 3477篇 |
石油天然气 | 6152篇 |
武器工业 | 32篇 |
无线电 | 31308篇 |
一般工业技术 | 62553篇 |
冶金工业 | 50438篇 |
原子能技术 | 6787篇 |
自动化技术 | 28957篇 |
出版年
2021年 | 2108篇 |
2019年 | 2017篇 |
2018年 | 11055篇 |
2017年 | 11526篇 |
2016年 | 8023篇 |
2015年 | 2688篇 |
2014年 | 3982篇 |
2013年 | 10235篇 |
2012年 | 8525篇 |
2011年 | 15856篇 |
2010年 | 13872篇 |
2009年 | 13369篇 |
2008年 | 13617篇 |
2007年 | 15661篇 |
2006年 | 6953篇 |
2005年 | 9307篇 |
2004年 | 7673篇 |
2003年 | 7174篇 |
2002年 | 6294篇 |
2001年 | 5624篇 |
2000年 | 5515篇 |
1999年 | 5323篇 |
1998年 | 12575篇 |
1997年 | 8897篇 |
1996年 | 6869篇 |
1995年 | 5340篇 |
1994年 | 4764篇 |
1993年 | 4648篇 |
1992年 | 3681篇 |
1991年 | 3410篇 |
1990年 | 3593篇 |
1989年 | 3472篇 |
1988年 | 3310篇 |
1987年 | 2895篇 |
1986年 | 2932篇 |
1985年 | 3373篇 |
1984年 | 3219篇 |
1983年 | 2892篇 |
1982年 | 2738篇 |
1981年 | 2821篇 |
1980年 | 2719篇 |
1979年 | 2624篇 |
1978年 | 2684篇 |
1977年 | 2948篇 |
1976年 | 3805篇 |
1975年 | 2368篇 |
1974年 | 2242篇 |
1973年 | 2376篇 |
1972年 | 1970篇 |
1971年 | 1858篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Type testing for fault currents for medium-voltage circuit breakers rated at 15 kV to 72.5 kV is addressed. The ANSI designation of a new class of definite-purpose circuit breakers capable of switching transformer secondary faults and the introduction of the requirement for short-line-fault tests for breakers rated 72.5 kV and below have increased the difficulty of performing type tests in a testing laboratory. It is concluded that synthetic testing using the parallel current injection circuit is described in ANSI C37.081 provides the best available method for proving these complex requirements for medium-voltage circuit breakers 相似文献
902.
M. E. O'Neill 《Chemical Engineering Communications》1996,148(1):161-182
Few exact solutions of the Stokes equations are known, even for steady or quasi-steady flows, involving finite sized bodies, and numerical techniques generally have to be resorted to for finding solutions. However, quite effective modelling of flows involving complicated boundary geometries is possible using the three-dimensional Stokeslet and rotlet point singularities. Two problems are studied in detail. In the first example, exact solutions for the three-dimensional Stokeslet and rotlet placed axisymmetrically along the axis of a circular disc are found and combined with Brenner's first order interaction formulae to determine the effect of the presence of the disc on the force and torque acting on a particle whose dimensions are small compared with its distance from the disc. The results are compared with those of a full numerical integration of the Stokes equations for a sphere translating towards a disc. In the second example, Brenner's first order wall correction theory is applied to the motion of a particle in a circular cylinder using the exact solutions for a torus translating or rotating in isolation. The theoretical predictions for the drag on a torus settling symmetrically in a circular cylinder are compared with those determined experimentally. 相似文献
903.
904.
The ElectroScan environmental scanning electron microscope (ESEM) enables wet samples to be observed by eliminating air but allowing water vapour into the sample chamber. However, evaporation from, and condensation on, the sample may occur during the pumpdown sequence used to reach this state, which means that the sample may not be in its natural state when viewed if due care is not taken. In this paper, the pumping system of the ESEM is described mathematically and expressions are derived for the evaporation and condensation. This treatment is then used to calculate the optimum pumpdown sequence. The importance of using the optimized procedure is illustrated by micrographs of fat emulsions. 相似文献
905.
Abstract: Maintainability problems associated with traditional software systems are exacerbated in rule-based systems. The very nature of that approach — separation of control knowledge and data-driven execution — hampers maintenance. While there are widely accepted techniques for maintaining conventional software, the same is not true for rule-based systems. In most situations, both a knowledge engineer and a domain expert are necessary to update the rules of a rule-based system. This paper presents, first, an overview of the software engineering techniques and object-oriented methods used in maintaining rule-based systems. It then discusses alternate paradigms for expert system development. The benefits of using case-based reasoning (from the maintenance point of view) are illustrated through the implementation of a case-based scheduler. The main value of the scheduler is that its knowledge base can be modified by the expert without the assistance of a knowledge engineer. Since changes in application requirements can be given directly to the system by the expert, the effort of maintaining the knowledge base is greatly reduced. 相似文献
906.
We propose a new principle for a low temperature semiconductor detector of charged particles with possibly submicron lateral resolution based on point contacts formed in the 2-dimensional electron gas (2DEG) of a GaAs-AlxGa1-xAs heterostructure using split-gates. The detector operates up to liquid nitrogen temperatures. Impinging particles excite locally extra donors in the doping layer of the heterostructure. By measuring simultaneous increase in the conductances of 3 point contacts due to the impacts of charged particles the position of penetration is calculated using the Thomas-Fermi approximation for screening in 2DEG. 相似文献
907.
R Gouider E LeGuern J Emile S Tardieu F Cabon M Samid J Weissenbach Y Agid P Bouche A Brice 《Canadian Metallurgical Quarterly》1994,44(12):2250-2252
Fistulas secondary to Crohn's disease occur in about 33% of patients. A colobronchial fistula complicating Crohn's disease is very rare, having been reported only twice previously. We present an unusual fistula secondary to Crohn's colitis that originated from the splenic flexure and crossed the diaphragm to involve the bronchial tree. 相似文献
908.
E. C. Kirby 《Fullerenes, Nanotubes and Carbon Nanostructures》1994,2(4):395-404
The possibility of toroidal cages of carbon with repeating five and seven membered ring pairs (azulene units) upon their surfaces is briefly discussed. Some of the difficulties in recognising possible fullerene cage shapes are pointed out. 相似文献
909.
Investigates adaptive digital notch filters for the elimination of powerline noise from biomedical signals. Since the distribution of the frequency variation of the powerline noise may or may not be centered at 60 Hz. Three different adaptive digital notch filters are considered. For the first case, an adaptive FIR second-order digital notch filter is designed to track the center frequency variation. For the second case, the zeroes of an adaptive IIR second-order digital notch filter are fixed on the unit circle and the poles are adapted to find an optimum bandwidth to eliminate the noise to a pre-defined attenuation level. In the third case, both the poles and zeroes of the adaptive IIR second-order filter are adapted to track the center frequency variation within an optimum bandwidth. The adaptive process is considerably simplified by designing the notch filters by pole-zero placement on the unit circle using some suggested rules. A constrained least mean-squared algorithm is used for the adaptive process. To evaluate their performance, the three adaptive notch filters are applied to a powerline noise sample and to a noisy EEG as an illustration of a biomedical signal 相似文献
910.
Cadiou J.F. Guena J. Penard E. Legaud P. Minot C. Palmier J.F. Le Person H. Harmand J.E. 《Electronics letters》1994,30(20):1690-1691
For the first time, a direct optically injection-locked oscillator based on the Esaki-Tsu effect in superlattices is reported. The microwave output power is -7 dBm at 20 GHz with an optical input power of -10 dBm at a wavelength of 1.3 μm 相似文献