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11.
A Power Control MAC Protocol for Ad Hoc Networks 总被引:6,自引:0,他引:6
This paper presents a power control MAC protocol that allows nodes to vary transmit power level on a per-packet basis. Several researchers have proposed simple modifications of IEEE 802.11 to incorporate power control. The main idea of these power control schemes is to use different power levels for RTS–CTS and DATA–ACK. Specifically, maximum transmit power is used for RTS–CTS, and the minimum required transmit power is used for DATA–ACK transmissions in order to save energy. However, we show that these schemes can degrade network throughput and can result in higher energy consumption than when using IEEE 802.11 without power control. We propose a power control protocol which does not degrade throughput and yields energy saving. 相似文献
12.
A method of optimal beamforming for flat Rayleigh faded channels using the Fractional Fourier Transform (FRFT) is considered
in this paper. It has been demonstrated through simulations that optimal beamforming with FRFT allows smaller mean-square
errors in restoring signals degraded with linear time-or frequency variant distortions and Additive White Gaussian Noise.
This is made possible by the additional flexibility that comes with free parameter ‘a’ of the fractional Fourier transform as oppose to the classical Fourier transform (FT). The method is especially useful in
moving source problems, where Doppler Effect produces frequency shift when the source is moving, as in mobile and wireless
communication where user produces the frequency shift while moving. In this paper it is shown through simulations that beamforming
in fractional domain reduces BER as compared to time or frequency domain. 相似文献
13.
“Recently, various paradigms, for instance, device-to-device communications, LTE-unlicensed and cognitive radio based on an opportunistic spectrum access (OSA) are being envisioned to improve the average spectrum utilization. In OSA, secondary (unlicensed) users (SUs) need decision making policies (DMPs) to identify and transmit over optimum frequency bands without any interference to the primary (licensed) users as well as minimize the number of collisions among SUs. In this paper, we have proposed a two-stage DMP consisting of Bayesian Multi-armed Bandit algorithm to accurately characterize the frequency band statistics independently at each SU and frequency band selection scheme for orthogonalization of SUs. The analytical and simulation results show that the proposed DMP leads to 45% improvement in the average spectrum utilization compared to 36–39% in the existing DMPs. Furthermore, the number of collisions are 58.5% lower in the proposed DMP making SU terminals energy-efficient. The performance of the proposed DMP has been verified on the proposed USRP testbed in real radio environment and the experimental results closely match the simulated results .” 相似文献
14.
Rajiv K. Singh 《Journal of Electronic Materials》1996,25(1):125-129
The absorption of laser energy by the plasma during pulsed laser deposition of thin films has been analyzed theoretically.
The amount of laser energy absorbed in the plasma termed as the “plasma shielding factor” is a function of the incident laser
wavelength, and time dependent plasma dimensions and electron density. Due to time varying parameters, a quantitative analysis
of the plasma absorption is difficult. A model which takes into account the absorption of laser energy by the plasma has been
developed. In this model, the time-dependent plasma dimension is replaced by the time dependent ablation depth. Using simulated
absorption coefficient values, the ablation characteristics of silicon and high Tc superconductors are computed and compared with experimental results. The plasma shielding factor was found to vary approximately
linearly with absorbed laser energy. The calculations also showed that the plasma shielding was strongly dependent on the
laser fluence but varies very weakly with the simulated plasma absorption coefficient values. Experimental results on plume
shielding showed good agreement with the calculations. 相似文献
15.
Media as Educator,Media as Disruptor: Conceptualizing the Role of Social Context in Media Effects 下载免费PDF全文
We document how social contexts serve to refract media effects. We theorized the relationship between media use and individual‐level knowledge (and attitude) would be stronger when community‐level knowledge (and attitude) was low than when it was high. Data come from a national survey (N = 12,608 women and 1,237 men) conducted in Nepal. Knowledge and stigma toward people living with HIV were the 2 dependent variables. Hypotheses were tested 12 times: across the use of 3 media (newspaper, radio, television) × 2 study outcomes (knowledge and attitudes) × 2 genders. Predicted interactions were supported in 9 of the 12 tests. Findings point to the need to take into account the role of community factors in theorizing about media effects. 相似文献
16.
The high inherent surface roughness of as-deposited polycrystalline diamond films has made effective planarization processing
of these films essential for most industrial applications. We have investigated the efficacy of ion beam sources for planarization
in an electron cyclotron resonance plasma system using both direct substrate biasing and an accelerating grid system. Rough
polycrystalline diamond films were synthesized using hot filament chemical vapor deposition. Both the etching rates and the
resultant surface roughnesses were found to decrease as the angle of incidence (relative to the substrate surface normal)
of the ion beam was increased. In the case of direct biasing of the sample, acicular features were observed following processing
at higher incident angles. The use of double ion-extraction grids in conjunction with concomitant sample rotation was found
to produce more uniform planarization of the diamond films. The rate of surface roughness reduction was found to be nonlinear
and decreased with time. For both ion extraction methods investigated, the average film roughness (Ra) was significantly reduced from 0.2 to 0.05–0.06 μm. 相似文献
17.
Rajiv Kumar Marcin Nyk Tymish Y. Ohulchanskyy Christopher A. Flask Paras N. Prasad 《Advanced functional materials》2009,19(6):853-859
Here, novel nanoprobes for combined optical and magnetic resonance (MR) bioimaging are reported. Fluoride (NaYF4) nanocrystals (20–30 nm size) co‐doped with the rare earth ions Gd3+ and Er3+/Yb3+/Eu3+ are synthesized and dispersed in water. An efficient up‐ and downconverted photoluminescence from the rare‐earth ions (Er3+ and Yb3+ or Eu3+) doped into fluoride nanomatrix allows optical imaging modality for the nanoprobes. Upconversion nanophosphors (UCNPs) show nearly quadratic dependence of the photoluminescence intensity on the excitation light power, confirming a two‐photon induced process and allowing two‐photon imaging with UCNPs with low power continuous wave laser diodes due to the sequential nature of the two‐photon process. Furthermore, both UCNPs and downconversion nanophosphors (DCNPs) are modified with biorecognition biomolecules such as anti‐claudin‐4 and anti‐mesothelin, and show in vitro targeted delivery to cancer cells using confocal microscopy. The possibility of using nanoprobes for optical imaging in vivo is also demonstrated. It is also shown that Gd3+ co‐doped within the nanophosphors imparts strong T1 (Spin‐lattice relaxation time) and T2 (spin‐spin relaxation time) for high contrast MR imaging. Thus, nanoprobes based on fluoride nanophosphors doped with rare earth ions are shown to provide the dual modality of optical and magnetic resonance imaging. 相似文献
18.
The rising global demand for energy has triggered emphasis on conservation of energy. Buildings are one of the important energy consuming sectors. Passive solar architecture encompasses a wide range of strategies and options resulting in energy efficient building design and increased occupant's comfort. Passive solar design, aiming at increasing direct solar gains during winter period and to avoid overheating during summer period should make use of specific shading devices over energy efficient window. The static sunshades are most effective for solar control inside the buildings.Countries like India have composite climate, which can be classified under summer, winter and rainy season. Depending on the seasonal requirements, this paper introduces a new geometry of a static sunshade, designed by calculating the sun angles for the two dates. The static sun shading design methodology is validated with the help of small scale modeling experimentation technique, carried out in Pilani, Rajasthan (India). Although insulating materials can be used as a part of a building structure, its feasibility should be checked before particular application. In the present paper, the two small-scale experimental models of actual construction material with varying static sunshades, i.e. horizontal and the proposed one have been constructed and analyzed with the models of insulating material (Polyurethane Foam [PUF]). Depending upon the solar intersection over south facade wall, sunlit area and shaded area have been correlated with temperature inside the models to decide the effectiveness of the proposed sunshade. 相似文献
19.
The bactericidal characteristics of nano-copper oxide or functionalized zeolite coated concrete pipes against Acidithiobacillus thiooxidans were studied by measuring the temporal variation of bacterial dry cell weight measurement, cellular Adenosine Triphosphate production, as well as oxygen uptake rate of the aforementioned bacterium. Uncorroded (UC), severely corroded (SC), and moderately corroded (MC) concrete pipes were electrochemically coated with a nano-copper oxide, while another uncorroded concrete pipe was used to apply functionalized zeolite coating (Z2). Specimens were characterized by field emission-scanning electron microscopy, and optical microscopy. Oxygen uptake rate of the bacterium was the highest in UC followed by the MC. Oxygen uptake rate and cellular Adenosine Triphosphate decreased progressively in Z2 and SC throughout the duration of the experiment due to decline in live bacterial cell. The maximum bacterial specific growth rate was 1.1 × 10−2 day−1 for both UC and MC, with a decay rates varying from 1.4 × 10−2 to 2.6 × 10−2 day−1. The minimum concentration limits for the inhibition of the bacterium in the nano-copper oxide coated concrete pipes ranged from 2.3 mg to 2.6 mg Cu per mg dry cell weight. 相似文献
20.
S. Prasanth Vaidya P. V. S. S. R. Chandra Mouli 《Multimedia Tools and Applications》2018,77(5):5609-5635
Multimedia Tools and Applications - Ownership identification and copyright protection are two major concerns for digital data. Digital watermarking provides the best solution for these issues. In... 相似文献