全文获取类型
收费全文 | 173708篇 |
免费 | 2652篇 |
国内免费 | 741篇 |
专业分类
电工技术 | 3430篇 |
综合类 | 232篇 |
化学工业 | 28912篇 |
金属工艺 | 6233篇 |
机械仪表 | 5371篇 |
建筑科学 | 6095篇 |
矿业工程 | 535篇 |
能源动力 | 4598篇 |
轻工业 | 19290篇 |
水利工程 | 1312篇 |
石油天然气 | 673篇 |
武器工业 | 6篇 |
无线电 | 22031篇 |
一般工业技术 | 32737篇 |
冶金工业 | 26347篇 |
原子能技术 | 2467篇 |
自动化技术 | 16832篇 |
出版年
2021年 | 1285篇 |
2019年 | 1148篇 |
2018年 | 1552篇 |
2017年 | 1542篇 |
2016年 | 1789篇 |
2015年 | 1517篇 |
2014年 | 2457篇 |
2013年 | 8028篇 |
2012年 | 4217篇 |
2011年 | 6052篇 |
2010年 | 4647篇 |
2009年 | 5155篇 |
2008年 | 5852篇 |
2007年 | 5976篇 |
2006年 | 5263篇 |
2005年 | 4940篇 |
2004年 | 4738篇 |
2003年 | 4559篇 |
2002年 | 4561篇 |
2001年 | 4367篇 |
2000年 | 4114篇 |
1999年 | 4093篇 |
1998年 | 7055篇 |
1997年 | 5585篇 |
1996年 | 4794篇 |
1995年 | 3987篇 |
1994年 | 3633篇 |
1993年 | 3463篇 |
1992年 | 3009篇 |
1991年 | 2887篇 |
1990年 | 2815篇 |
1989年 | 2830篇 |
1988年 | 2626篇 |
1987年 | 2334篇 |
1986年 | 2277篇 |
1985年 | 2776篇 |
1984年 | 2510篇 |
1983年 | 2365篇 |
1982年 | 2223篇 |
1981年 | 2163篇 |
1980年 | 1976篇 |
1979年 | 2008篇 |
1978年 | 1901篇 |
1977年 | 2195篇 |
1976年 | 2693篇 |
1975年 | 1695篇 |
1974年 | 1524篇 |
1973年 | 1530篇 |
1972年 | 1255篇 |
1971年 | 1169篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
First-principles calculations using quantum-mechanical density functional theory (DFT) are carried out to study the geometric structure and electronic properties of dehydrogenated nanodiamonds with diameters varying from 0.8 nm to 1.6 nm. The results show that the electronic properties of dehydrogenated nanodiamond are quite different from those of bulk diamond or hydrogenated nanodiamond. Surface atoms play an important role in the electronic structure, especially the states near the Fermi level, for dehydrogenated nanodiamond. In addition, it has been revealed that the size-dependent feature in the electronic properties for dehydrogenated diamonds is also contributed by the surface effect, in addition to the quantum confinement effect. 相似文献
982.
Ralph W. Pridmore 《Color research and application》2008,33(1):77-81
In present convention, chromatic induction (simultaneous and successive contrast) is usually held to be an opponent color process. Fifty years ago, it was an accepted complementary color process. The latter was never disputed yet apparently overlooked, and is here shown to be the more accurate account by inspecting afterimages and published data on simultaneous and successive hue induction. © 2007 Wiley Periodicals, Inc. Col Res Appl, 33, 77–81, 2008 相似文献
983.
This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency. 相似文献
984.
W. Kulisch C. Popov H. Rauscher L. Sirghi T. Sasaki S. Bliznakov F. Rossi 《Diamond and Related Materials》2008,17(7-10):1116-1121
Nucleation and growth, but especially the development of the morphology of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been investigated by systematic variation of three important parameters, namely the deposition time, the growth rate, and the substrate pre-treatment used to enhance the nucleation density. The films have been characterized, among others, by scanning electron microscopy, atomic force microscopy, and Fourier transform infrared spectroscopy. It is shown that, by successive addition of ultradispersive diamond powder to the suspension of nanocrystalline diamond powder in n-pentane used for the ultrasonic pre-treatment, the nucleation density can be enhanced by two orders of magnitude from 1 · 108 cm− 2 to > 1 · 1010 cm− 2. This reduces the thickness required to achieve closed films from 1 µm to 100 nm. However, once coalescence of the individual nodules emerging from the nucleation sites has taken place the films loose “memory” of the nucleation step and start to develop the typical NCD morphology consisting of larger features with diameters of some hundreds of nm which are in turn composed of much smaller features. Irrespective of the feature size and of the parameters used, the films of this investigation possess AFM rms roughnesses of 9–13 nm, indicating that rms values are not sufficient to characterize NCD surfaces. 相似文献
985.
Lucila R. Geymonat de Destefani T. W. Allan Whitfield 《Color research and application》2008,33(1):55-60
A feature of the empirical literature in colour psychology is that little attention has been given to how people in real settings engage in colour selection. Surprisingly, with rare exceptions, we know almost nothing about this. Besides its theoretical significance within psychology, this question has practical implications for industries where colour choice is important. A study is reported into that most ubiquitous of activities, selecting a paint colour for the home. This used a retrospective method supported by qualitative data analysis (NVivo) in which participants described their process of colour selection. The results indicate a process that is far from uniform, but one that consists of two essential stages. The first involves arriving at what we refer to as an affective specification of the qualities sought. Once constructed, this leads to the second stage of matching colour attributes to that specification. Selecting a wall colour emerges as a surprisingly complex process, and one that corresponds more closely to a search than to a simple affective choice. A parallel is drawn with decision research, particularly the field of naturalistic decision‐making. From participants' reports, colour selection also emerges as a process that women appear to understand better than men. © 2007 Wiley Periodicals, Inc. Col Res Appl, 33, 55–60, 2008 相似文献
986.
987.
W. S. Singleton 《Journal of the American Oil Chemists' Society》1948,25(1):15-20
- The ternary systems oleic acid-stearic acid-commercial hexane and oleic acid-stearic acid-acetone containing varying amounts of the three components have been equilibrated at 0°C., −10°C., −20°C., −30°C., and −40°C.
- From compositional data of the liquid and solid phases in equilibrium at each isotherm, ternary phase diagrams have been constructed. From these diagrams it is possible to predict the degree of separation which can be obtained with any given mixture of oleic and stearic acids, using either acetone or commercial hexane as solvent.
- With practical solvent ratios the phase diagrams at −20°C., −30°C., and −40°C., exhibit closed areas representing liquid phase composition. The liquid phase boundaries have been established for each isotherm investigated.
- The intersolubilizing effect of oleic acid on stearic acid, greater in commercial hexane than in acetone, and the possible formation of mixed crystals of oleic and stearic acid have been noted.
- Oleic acid of high purity can be obtained as one of the practical applications of these data.
988.
If an IT system is so so hopelessly and fundamentally flawed that there's no practical way to address its deficiencies one by one, a clean slate initiative might be the only way out of the swamp. The key is to make sure the drivers are sound, the process is retrospective, the solution is robust, and expectations are realistic. 相似文献
989.
This approach imitates human reasoning to enable flexible context recognition. Its usefulness is demonstrated by employing audio-signal processing to recognize several everyday situations. 相似文献
990.
Self-stabilizing somersaults 总被引:1,自引:0,他引:1
Mombaur K.D. Bock H.G. Schloder J.P. Longman R.W. 《Robotics, IEEE Transactions on》2005,21(6):1148-1157
We investigate the open-loop stability of a planar biped robot performing a periodic motion of forward somersaults with alternating single-leg contacts. The robot has a trunk and two actuated telescopic legs with point feet which are coupled to the trunk by actuated hinges. There is compliance and damping in the hip and in the legs. The concept of open-loop control implies that all actuators of the system receive predetermined inputs that are never altered by any feedback interference. Only with the right choice of model parameters and actuator inputs is it possible to create such self-stabilizing motions exploiting the natural stability properties of the system. These unknowns have been determined using special-purpose stability-optimization methods. The resulting motion is not only stable, but also a more efficient form of forward motion than running for the investigated robot. 相似文献