全文获取类型
收费全文 | 826099篇 |
免费 | 10787篇 |
国内免费 | 2059篇 |
学科分类
工业技术 | 838945篇 |
出版年
2021年 | 7603篇 |
2020年 | 5776篇 |
2019年 | 7367篇 |
2018年 | 12563篇 |
2017年 | 12682篇 |
2016年 | 13369篇 |
2015年 | 8570篇 |
2014年 | 14316篇 |
2013年 | 39190篇 |
2012年 | 22537篇 |
2011年 | 30722篇 |
2010年 | 24535篇 |
2009年 | 27552篇 |
2008年 | 28057篇 |
2007年 | 27665篇 |
2006年 | 24247篇 |
2005年 | 21920篇 |
2004年 | 20921篇 |
2003年 | 20688篇 |
2002年 | 19671篇 |
2001年 | 19282篇 |
2000年 | 18301篇 |
1999年 | 18576篇 |
1998年 | 45422篇 |
1997年 | 32210篇 |
1996年 | 25002篇 |
1995年 | 18902篇 |
1994年 | 16911篇 |
1993年 | 16702篇 |
1992年 | 12361篇 |
1991年 | 11775篇 |
1990年 | 11666篇 |
1989年 | 11206篇 |
1988年 | 10639篇 |
1987年 | 9604篇 |
1986年 | 9254篇 |
1985年 | 10600篇 |
1984年 | 9633篇 |
1983年 | 8990篇 |
1982年 | 8185篇 |
1981年 | 8350篇 |
1980年 | 7817篇 |
1979年 | 7891篇 |
1978年 | 7734篇 |
1977年 | 8740篇 |
1976年 | 11122篇 |
1975年 | 6849篇 |
1974年 | 6563篇 |
1973年 | 6672篇 |
1972年 | 5647篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
A.S. Fomichev I. David S.M. Lukyanov Yu.E. Penionzhkevich N.K. Skobelev O.B. Tarasov 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》1994,350(3):605-607
The mass and charge identification of secondary particles with Z < 4 by a large CsI(T1) scintillation detector is performed using pulse shape analysis and time-of-flight methods. The dependence of the light output on E, A and Z is studied in the energy range of 1–20 MeV/A and special attention is paid to the integration time of the photomultiplier anode signal. It is found that the behaviour of the calibration curves strongly depends on the choice of the integration time interval. 相似文献
992.
Carroll R.D. Merritt S.W. Branciforte E.J. Tanski W.J. Cullen D.E. Sacks R.N. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1994,41(3):416-418
Heterostructure Acoustic Charge Transport (HACT) devices have been fabricated with a new nondestructive sense (NDS) electrode structure that provides for the recovery of base-band signals without the use of an integrating capacitor. This electrode structure provides an output signal comprising an RF carrier at the SAW frequency, amplitude modulated by the sampled input signal which has been delayed by a period proportional to the output electrodes distance from the input diode. The output of the NDS electrode structure is subsequently demodulated to provide the base-band signal 相似文献
993.
994.
995.
ABSTRACT: The objective of this work was to observe the microstructure of feta cheese using confocal scanning laser microscopy. Low fat cheese and cheese containing capsule-producing cultures were made. The protein network was observed using the reflectance mode of the confocal microscope. Fat was stained by Nile red dye diluted with whey. More even distribution of fat with a larger number of smaller globules was observed in cheese made with noncapsule-forming culture compared to that made with the capsule-forming culture. Nonfat cheese had compact structure when made with noncapsule-forming culture which contrasted with the open structure observed in cheese made with the capsule-forming culture 相似文献
996.
McDonald K.C. Zimmermann R. Kimball J.S. 《Geoscience and Remote Sensing, IEEE Transactions on》2002,40(9):2063-2082
Spatial and temporal variations in vegetation dielectric properties strongly influence the microwave backscatter characteristics of forested landscapes. This paper examines the relationship between xylem tissue dielectric constant, xylem sap flux density, and xylem sap chemical composition as measured in the stems of two Norway Spruce (Picea abies [L.] Karst.) trees in the Fichtelgebirge region of Northern Bavaria, Germany. Dielectric constant and xylem sap flux were monitored continuously from June through October 1995, at several heights along the tree trunks. At the end of the measurement series, each tree was harvested, and its xylem sap extracted and analyzed to determine the concentrations of amino acids and cations. Results show that the sap flux density was correlated with vapor pressure deficit (VPD) at all heights in the stem. In contrast, the xylem tissue dielectric constant is influenced by VPD but can exhibit a significant temporal lag relative to changes in VPD. This lag varies with position along the tree trunk. The temporal variability of the dielectric constant is compared with both trees at several positions along the tree trunks. Results of xylem sap chemical analysis are presented. We show that spatial and temporal variability in the xylem tissue dielectric constant is influenced not only by water content, but by variations in xylem sap chemistry as well. This has important implications for microwave remote sensing of forested landscapes, as useful information may be acquired regarding stand physiology and water relations and where variations in dielectric properties within individual trees and across geographic areas can be significant error sources for forest inventory mapping. 相似文献
997.
An adaptive antenna array is incorporated into a decorrelatingmulti-user detector to effectively increase the DS-CDMA system capacity.Capacity improvement is due to beamforming gain, spatial diversity gain(assuming large angle spread), and the decorrelating effect. Thereceiver has been analyzed for the cases of sufficiently andinsufficiently spaced receiving antennas. The receiver consists of afront-end beamformer for every user in the cell and has knowledge of allusers' signature sequences. The beamformer estimates the desired userchannel vector, enhancing its signal and reducing the co-channelinterference from other directions. The multi-user detection, exploitingknowledge of other users, rejects those interferers whose arrival anglesare close to that of the desired user. The average uncoded Bit ErrorRate (BER) as a function of the number of in-beam active users, theaverage antenna Signal-to-Noise Ratio (SNR), and the number of receivingantennas is examined in both single-path and multi-path Rayleigh fadingchannels. Analysis shows an increase in system capacity proportional tothe number of receiving antennas. 相似文献
998.
999.
1000.
Z Abbas AH Khan SM Jafri SS Hamid SH Shah S Abid JA Qureshi 《Canadian Metallurgical Quarterly》1997,12(11):703-706
The authors compared in a controlled clinical study two groups of patients after a first renal transplantation treated by triple drug immunosuppressive therapy. In a group of 31 patients the triple combination comprised Sandimmune Neoral. In the control group there were 30 patients who received Sandimmune. No differences were found between the two groups as regards the effectiveness of this treatment and the authors did not confirm a lower incidence of rejections described in patients treated with Sandimmune Neoral. They confirmed, however, a lower interindividual variability of Cy-A levels assessed specifically in patients treated with Sandimmune Neoral. 相似文献