全文获取类型
收费全文 | 6668篇 |
免费 | 479篇 |
国内免费 | 18篇 |
专业分类
电工技术 | 128篇 |
综合类 | 9篇 |
化学工业 | 1469篇 |
金属工艺 | 245篇 |
机械仪表 | 409篇 |
建筑科学 | 126篇 |
矿业工程 | 2篇 |
能源动力 | 236篇 |
轻工业 | 627篇 |
水利工程 | 22篇 |
石油天然气 | 6篇 |
无线电 | 1159篇 |
一般工业技术 | 1416篇 |
冶金工业 | 429篇 |
原子能技术 | 75篇 |
自动化技术 | 807篇 |
出版年
2024年 | 8篇 |
2023年 | 86篇 |
2022年 | 124篇 |
2021年 | 219篇 |
2020年 | 153篇 |
2019年 | 178篇 |
2018年 | 219篇 |
2017年 | 220篇 |
2016年 | 248篇 |
2015年 | 226篇 |
2014年 | 321篇 |
2013年 | 439篇 |
2012年 | 449篇 |
2011年 | 529篇 |
2010年 | 348篇 |
2009年 | 395篇 |
2008年 | 350篇 |
2007年 | 264篇 |
2006年 | 279篇 |
2005年 | 226篇 |
2004年 | 211篇 |
2003年 | 183篇 |
2002年 | 178篇 |
2001年 | 146篇 |
2000年 | 123篇 |
1999年 | 132篇 |
1998年 | 210篇 |
1997年 | 139篇 |
1996年 | 83篇 |
1995年 | 84篇 |
1994年 | 67篇 |
1993年 | 60篇 |
1992年 | 45篇 |
1991年 | 38篇 |
1990年 | 31篇 |
1989年 | 23篇 |
1988年 | 11篇 |
1987年 | 17篇 |
1986年 | 16篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1975年 | 4篇 |
排序方式: 共有7165条查询结果,搜索用时 15 毫秒
51.
The research on optical packet switching (OPS) has witnessed considerable progress in the 1990s. We examine the future potential of OPS in the core network by discussing this switching approach and the current status of a number of its enabling technologies. Many of these technologies are still in the stage of research and experimentation. We see that optical packet switching may be deployed in the long-term future subject to satisfaction of three main conditions/developments. First, additional technological developments have to take place to overcome remaining implementation challenges while making OPS cost-effective to deploy. Second, a rational migration scenario of the network toward gradual deployment of packet-based optical switching approaches should exist. Finally, carriers have to become more interested in packet-based optical switching solutions 相似文献
52.
This paper presents a new push-push voltage-controlled oscillator (VCO) technique that extracts a second harmonic output signal from a capacitive common node in a negative-gm oscillator topology. The generation of the second harmonics is accounted for by the nonlinear current-voltage characteristic of the emitter-base junction diode causing: 1) significant voltage clipping and 2) different rise and fall times during the switching operation of the core transistors. Comparative investigations show the technique is more power efficient in the high-frequency region than a conventional push-push technique using an emitter common node. A prototype 17-GHz VCO realized in GaInP/GaAs HBT technology produces an output power of -6dBm and a phase noise of -110.4dBc/Hz at 1-MHz offset, which is equivalent to a VCO figure-of-merit of -184.3dBc/Hz, while drawing 4.38 mA from a 3.0-V supply 相似文献
53.
A noncontact NDE method using a laser generated focused-Lamb wave with enhanced defect-detection ability and spatial resolution 总被引:1,自引:0,他引:1
The laser generation method of a focused Lamb wave is expected to have high defect-detection abilities and spatial resolution, with the added advantage of noncontact testing. In this method, the laser beam is illuminated on the surface of an object through an arrayed arc slit. The energy of the generated Lamb wave is then concentrated at a focus of an arc. This focusing effect improves the NDE (nondestructive evaluation) performance, which is dependent on the geometries of an arrayed arc slit. In this paper, the relationships between the parameters determining the slit geometry and the focusing effect of the generated Lamb wave was investigated using a parametric study, and appropriate values of the parameters were obtained to maximize the focusing ability. In order to validate the performance of this method, an NDE system was constructed and experiments were performed to detect through-hole defects in a plate. The results showed that the proposed system could clearly detect defects as small as 0.3 mm in diameter, while the conventional line array method failed to detect defects smaller than 1 mm in diameter. Moreover, this method showed possible detection of defects much smaller than 0.3 mm, and great improvements in the spatial resolution as compared with the line array method. 相似文献
54.
The photografting of methacrylic acid (MAA) on a linear low‐density polyethylene film (thickness = 30 μm) under air and nitrogen atmospheres was investigated at 60°C in mixed solvents consisting of water and an organic solvent, with xanthone as a photoinitiator. The organic solvents used were acetone, methanol, tetrahydrofuran, and dioxane. A maximum percentage of grafting occurred at a certain concentration of the organic solvent in the mixed solvent. This was observed for the systems under both air and nitrogen. The grafting reaction under air exhibited an induction period, but the rate of grafting after the period was greater than that under nitrogen. The formation of poly(ethylene peroxide)s by photoirradiation seemed to be a factor for the accelerated photografting under air. On the basis of attenuated total reflection infrared spectroscopy and scanning electron microscopy of the grafted film, the MAA‐grafted chains of the sample prepared under air tended to penetrate more deeply inside the film than those of the sample prepared under nitrogen. The resulting grafted films exhibited a pH‐responsive character: the grafted films shrank in an acidic medium but swelled in alkaline medium. This was evaluated from measurements of dimensional changes in the grafted films. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 992–998, 2003 相似文献
55.
A wireless/mobile network supporting multilevel quality of service (QoS) is considered. In such a network, users or applications can tolerate a certain degree of QoS degradation. Bandwidth allocation to users can, therefore, be adjusted dynamically according to the underlying network condition so as to increase bandwidth utilization and service provider's revenue. However, arbitrary QoS degradation may be unsatisfactory or unacceptable to the users, hence resulting in their subsequent defection. Instead of only focusing on bandwidth utilization or blocking/dropping probability, two new user-perceived QoS metrics, degradation ratio and upgrade/degrade frequency, are proposed. A Markov model is then provided to derive these QoS metrics. Using this model, we evaluate the effects of adaptive bandwidth allocation on user-perceived QoS and show the existence of trade offs between system performance and user-perceived QoS. We also show how to exploit adaptive bandwidth allocation to increase system utilization (for the system administrator) with controlled QoS degradation (for the users). By considering various mobility patterns, the simulation results are shown to match our analytical results, demonstrating the applicability of our analytical model to more general cases. 相似文献
56.
We propose an asymmetric integral imaging method to adjust the resolution and depth of a three‐dimensional image. Our method is obtained by use of two lenticular sheets with different pitches fabricated under the same F/#. The asymmetric integral imaging is the generalized version of integral imaging, including both conventional integral imaging and one‐dimensional integral imaging. We present experimental results to test and verify the performance of our method computationally. 相似文献
57.
58.
The growth of carbide particles in TiC-XC-2 vol.% Ni and TiC-XC-30 vol.% Ni alloys, where X=Zr, Cr, W, Ta and Mo, was fitted
to an equation of the form d3-do
3=Kt. The grain growth behavior during liquid phase sintering at 1673K decreased markedly with the addition of Mo2C or WC, changed little for TaC, and increased with the addition of ZrC or Cr3C2. The grain contiguity decreased with increasing Ni content in the TiC−Mo2C−Ni alloy and was greater in the alloys with smaller growth rate constant. Consequently, the effect of carbide addition on
the grain growth of 2 vol.% Ni alloys was found to be similar to that of 30 vol.% Ni alloys. The grain growth mechanism could
be explained by the effect of contiguous carbide grain boundaries in restricting the overall grain growth, as well as the
area of the solid/liquid interfaces in the alloy by the usual solution/reprecipitation model. 相似文献
59.
Woo-Seog Song Seungbae Lee Dong-Shin Shin Yang Na 《Journal of Mechanical Science and Technology》2006,20(6):905-913
The instability curve of a Rijke tube system was obtained accurately by following different paths of heat power and flow-rate
for three regions and by defining its locus from the criterion based on the measured sound pressure levels. The unstable limits
in the region of flow-rate lower than that at the minimal power are compared with previous data. To observe the effect of
turbulence on the unstable limits, inflow turbulence was introduced by placing a bundle of circular cylinders upstream of
the heating part (50< Red< 700). The large-amplitude inflow fluctuation may delay the transition of the chamber acoustic mode to the unstable zone
even at a sufficient power. 相似文献
60.
Shin S.-C. Ming-Da Tsai Ren-Chieh Liu Lin K.-Y. Huei Wang 《Microwave and Wireless Components Letters, IEEE》2005,15(7):448-450
A 24-GHz low-noise amplifier (LNA) was designed and fabricated in a standard 0.18-/spl mu/m CMOS technology. The LNA chip achieves a peak gain of 13.1 dB at 24 GHz and a minimum noise figure of 3.9 dB at 24.3 GHz. The supply voltage and supply current are 1 V and 14 mA, respectively. To the author's knowledge, this LNA demonstrates the lowest noise figure among the reported LNAs in standard CMOS processes above 20 GHz. 相似文献