全文获取类型
收费全文 | 734636篇 |
免费 | 11251篇 |
国内免费 | 2663篇 |
学科分类
工业技术 | 748550篇 |
出版年
2021年 | 6869篇 |
2020年 | 4997篇 |
2019年 | 6358篇 |
2018年 | 10595篇 |
2017年 | 10522篇 |
2016年 | 11030篇 |
2015年 | 7595篇 |
2014年 | 12628篇 |
2013年 | 33537篇 |
2012年 | 19922篇 |
2011年 | 27530篇 |
2010年 | 21468篇 |
2009年 | 24323篇 |
2008年 | 24721篇 |
2007年 | 24366篇 |
2006年 | 21152篇 |
2005年 | 19407篇 |
2004年 | 18998篇 |
2003年 | 18733篇 |
2002年 | 17824篇 |
2001年 | 17912篇 |
2000年 | 16480篇 |
1999年 | 17442篇 |
1998年 | 45212篇 |
1997年 | 31701篇 |
1996年 | 24229篇 |
1995年 | 18193篇 |
1994年 | 15915篇 |
1993年 | 15709篇 |
1992年 | 11193篇 |
1991年 | 10628篇 |
1990年 | 10353篇 |
1989年 | 10077篇 |
1988年 | 9371篇 |
1987年 | 8404篇 |
1986年 | 8285篇 |
1985年 | 9160篇 |
1984年 | 8546篇 |
1983年 | 7642篇 |
1982年 | 7233篇 |
1981年 | 7324篇 |
1980年 | 7076篇 |
1979年 | 6769篇 |
1978年 | 6670篇 |
1977年 | 7744篇 |
1976年 | 10320篇 |
1975年 | 5643篇 |
1974年 | 5353篇 |
1973年 | 5495篇 |
1972年 | 4523篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
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
982.
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
983.
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
984.
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
985.
986.
987.
Summary This paper proposes the use of special sensitivities, called nodal sensitivities, as error indicators and estimators for numerical analysis in mechanics. Nodal sensitivities are defined as rates of change of response quantities with respect to nodal positions. Direct analytical differentiation is used to obtain the sensitivities, and the infinitesimal perturbations of the nodes are forced to lie along the elements. The idea proposed here can be used in conjunction with general purpose computational methods such as the Finite Element Method (FEM), the Boundary Element Method (BEM) or the Finite Difference Method (FDM); however, the BEM is the method of choice in this paper. The performance of the error indicators is evaluated through two numerical examples in linear elasticity. 相似文献
988.
M Susser 《Canadian Metallurgical Quarterly》1997,105(6):566-570
The development of multimedia image review stations offers the cardiologist the ability to review multiple image sets in a single setting rather than moving from one review station to another. To accomplish this goal there are three essential barriers: 1) memory requirements for digital images; 2) standardization of data format; and 3) practical user interfaces. Image compression algorithms can be used to overcome the digital barrier; however, these must be used cautiously so as to not adversely affect image quality. The lack of standardization is being addressed by the implementation of the Digital Communication in Medicine (DICOM) standard, and practical user interfaces are being developed every day with the widespread implementation of World Wide Web technology. These solutions will allow the clinician to review all of a patient's image data in one location, such as in the office, on the nursing unit, or at home. 相似文献
989.
T. I. Voronina T. S. Lagunova M. P. Mikhailova K. D. Moiseev M. A. Sipovskaya Yu. P. Yakovlev 《Semiconductors》1997,31(8):763-767
Galvanomagnetic phenomena and photoconductivity in broken-gap type-II GaInAsSb/p-InAs heterojunctions with different levels of doping of the solid solution with donor (Te) or acceptor (Zn) impurities have
been investigated. It has been determined that in such structures an electronic channel, which determines the galvanomagnetic
effects in a wide range of doping levels, is present at the heterojunction. A sharp decrease of the Hall mobility was observed
in the experimental heterostructures with a high level of doping of the epitaxial layer with an acceptor impurity. The observed
effect is due to exhaustion of the electronic channel as a result of carrier localization in potential wells at the heterojunction.
Fiz. Tekh. Poluprovodn. 31, 897–901 (August 1997) 相似文献
990.
M Suzui T Ushijima N Yoshimi H Nakagama A Hara T Sugimura M Nagao H Mori 《Canadian Metallurgical Quarterly》1997,20(4):389-393
In 1992, 1531 women aging 25-45 years from 5 urban and rural regions were interviewed about infertility and subfecundity within the German part of a study by the European community. 1248 of them had a positive reproductive history with 3018 pregnancies. 400 (= 13.25%) occurred during contraception. This affected 296 women (= 19.6% of all women). Smoking seems to have a promotion effect. The risk of ectopic pregnancies seems to be higher in contraceptive users. The highest fetal loss was found in women using an IUD. There were no differences in the course and outcome of pregnancy between users and nonusers of contraceptives. About 40% of the women in the corresponding group reported an irregular application of the contraceptive technique. 相似文献