全文获取类型
收费全文 | 1553828篇 |
免费 | 26729篇 |
国内免费 | 7923篇 |
专业分类
电工技术 | 35730篇 |
技术理论 | 5篇 |
综合类 | 7431篇 |
化学工业 | 275034篇 |
金属工艺 | 65587篇 |
机械仪表 | 44483篇 |
建筑科学 | 47807篇 |
矿业工程 | 11963篇 |
能源动力 | 50469篇 |
轻工业 | 119708篇 |
水利工程 | 15818篇 |
石油天然气 | 38542篇 |
武器工业 | 379篇 |
无线电 | 199036篇 |
一般工业技术 | 291629篇 |
冶金工业 | 178451篇 |
原子能技术 | 34150篇 |
自动化技术 | 172258篇 |
出版年
2021年 | 16122篇 |
2020年 | 12194篇 |
2019年 | 14924篇 |
2018年 | 16772篇 |
2017年 | 16223篇 |
2016年 | 22115篇 |
2015年 | 18023篇 |
2014年 | 29294篇 |
2013年 | 88805篇 |
2012年 | 37650篇 |
2011年 | 51020篇 |
2010年 | 44526篇 |
2009年 | 52610篇 |
2008年 | 47403篇 |
2007年 | 45110篇 |
2006年 | 46709篇 |
2005年 | 41368篇 |
2004年 | 42667篇 |
2003年 | 42375篇 |
2002年 | 41411篇 |
2001年 | 38277篇 |
2000年 | 36632篇 |
1999年 | 35701篇 |
1998年 | 49304篇 |
1997年 | 41935篇 |
1996年 | 37148篇 |
1995年 | 31848篇 |
1994年 | 29700篇 |
1993年 | 29510篇 |
1992年 | 26579篇 |
1991年 | 23680篇 |
1990年 | 23942篇 |
1989年 | 23093篇 |
1988年 | 21581篇 |
1987年 | 19772篇 |
1986年 | 19195篇 |
1985年 | 22527篇 |
1984年 | 22573篇 |
1983年 | 20502篇 |
1982年 | 19403篇 |
1981年 | 19541篇 |
1980年 | 18179篇 |
1979年 | 18700篇 |
1978年 | 17887篇 |
1977年 | 17874篇 |
1976年 | 19290篇 |
1975年 | 16117篇 |
1974年 | 15613篇 |
1973年 | 15696篇 |
1972年 | 13172篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Sadka A.H. Eryurthlu F. Kondoz A.M. 《Vision, Image and Signal Processing, IEE Proceedings -》1997,144(6):369-376
An algorithm is presented to enhance the resilience of block-based video-coding algorithms against channel errors. The error-resilience algorithm described suggests the use of fixed length coding to alleviate the effect of Huffman coding on the synchronisation of the decode under erroneous conditions. The proposed mechanism modifies the order of transmission of the fixed-length video parameters to increase the chance of their arrival. Synchronisation words are sent at fixed-length intervals within the bit stream to reduce the possibility of the decoder falling on similar bit patterns within the bit stream. To limit the effect of error accumulation, the motion prediction process is halted and the differential coding of motion vectors is not applied. FEC techniques are applied on some error-sensitive segments of the reordered video bitstream. The effectiveness of the proposed error-resilience algorithm is evidenced by both subjective and objective results 相似文献
992.
J.P.R. Lacey S.J. Madden M.A. Summerfield 《Photonics Technology Letters, IEEE》1997,9(10):1355-1357
Multichannel wavelength converters may be important components in the cross-connects in future wavelength-division multiplexed (WDM) transport networks. We demonstrate a multichannel, polarization-insensitive, optically transparent wavelength converter, based on four-wave mixing in two semiconductor optical amplifiers in a polarization-diversity arrangement. Bit-error-rate (BER) measurements with four input 2.5-Gb/s WDM channels, spaced by 2 nm, show penalties for wavelength conversion less than 2.6 dB at 10/sup -9/ BER. Changes in the state of polarization of the input signals cause the output power to change by less than 1.2 dB, and the corresponding power penalties change by less than 0.9 dB. 相似文献
993.
Utilizing multidimensional signaling techniques, a generalized multirate wavelet-based modulation format for orthogonally multiplexed communication systems is presented. Wavelet packet modulation (WPM) employs the basis functions from an arbitrary pruning of a dyadic tree structured filter bank as orthogonal pulse shapes for conventional quadrature amplitude modulation (QAM) symbols. This generalized framework affords an entire library of basis sets with increased flexibility in time-frequency (T-F) partitioning. The bandwidth efficiency and power spectral density figures of merit for the general signal are derived and shown to be that of standard QAM 相似文献
994.
Electronic equivalents of traditional cash payment systems are being launched worldwide. Electronic cash can combine the benefits of traditional cash with those of payment by debit and credit card, while circumventing both their shortcomings. As with traditional cash, electronic cash should have high acceptability and be suitable for low value payment from person to person. With the possible exception of online payment platforms such as the Internet, it is preferred that payments be verifiable offline, without the bank's involvement, for reasons of cost effectiveness and speed. To facilitate electronic cash payments over the phone and the Internet, physical proximity of payer and payee should not be necessary. Moreover, electronic cash should offer privacy of payments. In particular, payments by an honest payer should be untraceable, and information about transaction content should remain privy to payer and payee. Yet a payer ought to always be able to trace the payee; traceability suits electronic cash and is as open to extortion, money laundering, and bribery as a check or wire transfer. Lastly, as with payments by debit and credit card, electronic cash should be convenient to store and transport, while protecting users against loss, theft, and accidental destruction 相似文献
995.
Wavelength selection for low-saturation pulse oximetry 总被引:1,自引:0,他引:1
Mannheimer P.D. Cascini J.R. Fein M.E. Nierlich S.L. 《IEEE transactions on bio-medical engineering》1997,44(3):148-158
Conventional pulse oximeters are accurate at high oxygen saturation under a variety of physiological conditions but show worsening accuracy at lower saturation (below 70%). Numerical modeling suggests that sensors fabricated with 735 and 890 nm emitters should read more accurately at low saturation under a variety of conditions than sensors made with conventionally used 660 and 900 nm band emitters. Recent animal testing confirms this expectation. It is postulated that the most repeatable and stable accuracy of the pulse oximeter occurs when the fractional change in photon path lengths due to perturbations in the tissue (relative to the conditions present during system calibration) is equivalent at the two wavelengths. Additionally, the penetration depth (and/or breadth) of the probing light needs to be well matched at the two wavelengths in order to minimize the effects of tissue heterogeneity. At high saturation these conditions are optimally met with 660 and 900 nm band emitters, while at low saturation 735 and 890 nm provide better performance 相似文献
996.
Yu. N. Denisyuk N. M. Ganzherli I. M. Maurer S. A. Pisarevskaya 《Technical Physics Letters》1997,23(4):279-280
A thick-layer light-sensitive material consisting of glycerin-containing bichromated gelatin has been prepared and used to
record real-time volume holograms with 0.44 μm helium-cadmium laser radiation. The thickness of the layers is 400–600 μm.
The holographic sensitivity of the material is 1 J/cm2. The highest diffraction efficiency of holograms recorded using a symmetric system with parallel beams is 32%.
Pis’ma Zh. Tekh. Fiz. 23, 62–65 (April 12, 1997) 相似文献
997.
998.
The perfectly matched layer for use with the finite-difference time-domain method is adapted to our transmission-line matrix simulation as what we call a heaviside absorbing layer (HAL). It is shown that the reflection coefficient for the wave incident on a HAL is less than that of the wave incident on a matched-load termination at all angles of incidence. Furthermore, the dispersion relation of a transmission-line matrix mesh of a symmetrical condensed node with both electric and magnetic loss stubs is derived. It provides guidance on how to choose the losses of HAL and other simulation parameters properly 相似文献
999.
Space diversity reception and forward-error correction coding are powerful techniques to combat multipath fading encountered in mobile radio communications. In this paper, we analyze the performance of a discrete-time switched diversity system using trellis-coded modulation multiple phase-shift keying (TCM-MPSK) on slow, nonselective correlated Nakagami (1960) fading channels. Analytical upper bounds using the transfer function bounding technique are obtained and illustrated by several numerical examples. A simple integral expression for calculating the exact pairwise error probability is presented. The use of optimum adaptive and fixed switching thresholds is considered. Monte Carlo simulation results, which are more indicative of the exact system performance, are also given 相似文献
1000.
A new method based on a two-wire line has been developed for noncontact diagnostics of a low-temperature plasma and its fluxes
in channels with complex profiles. This method provides good spatial and time resolution.
Pis’ma Zh. Tekh. Fiz. 23, 1–7 (July 26, 1997) 相似文献