全文获取类型
收费全文 | 8645篇 |
免费 | 106篇 |
国内免费 | 9篇 |
学科分类
工业技术 | 8760篇 |
出版年
2021年 | 68篇 |
2020年 | 51篇 |
2019年 | 61篇 |
2018年 | 60篇 |
2017年 | 82篇 |
2016年 | 65篇 |
2015年 | 47篇 |
2014年 | 104篇 |
2013年 | 315篇 |
2012年 | 160篇 |
2011年 | 237篇 |
2010年 | 211篇 |
2009年 | 231篇 |
2008年 | 260篇 |
2007年 | 240篇 |
2006年 | 245篇 |
2005年 | 218篇 |
2004年 | 178篇 |
2003年 | 198篇 |
2002年 | 200篇 |
2001年 | 162篇 |
2000年 | 160篇 |
1999年 | 198篇 |
1998年 | 760篇 |
1997年 | 481篇 |
1996年 | 420篇 |
1995年 | 298篇 |
1994年 | 252篇 |
1993年 | 253篇 |
1992年 | 113篇 |
1991年 | 129篇 |
1990年 | 137篇 |
1989年 | 134篇 |
1988年 | 135篇 |
1987年 | 136篇 |
1986年 | 107篇 |
1985年 | 127篇 |
1984年 | 94篇 |
1983年 | 100篇 |
1982年 | 69篇 |
1981年 | 88篇 |
1980年 | 70篇 |
1979年 | 79篇 |
1978年 | 70篇 |
1977年 | 110篇 |
1976年 | 198篇 |
1975年 | 72篇 |
1974年 | 71篇 |
1973年 | 63篇 |
1972年 | 54篇 |
排序方式: 共有8760条查询结果,搜索用时 0 毫秒
131.
Martin R.K. Walsh J.M. Johnson C.R. Jr. 《Signal Processing, IEEE Transactions on》2005,53(4):1324-1334
Channel shortening is often employed as a means of mitigating intersymbol and intercarrier interference (ISI and ICI) in systems using multicarrier modulation. The Multicarrier Equalization by Restoration of RedundancY (MERRY) algorithm has previously been shown to blindly and adaptively shorten a channel to the length of the guard interval in a multicarrier system. This paper addresses synchronization and complexity reduction issues that were not dealt with in previous work and provides extensions to and generalizations of the MERRY algorithm. A modification is presented that removes the square root and division needed at each iteration without introducing additional complexity, with the added benefit of allowing the use of constraints other than a unit norm equalizer; an extension is proposed that allows for the use of more data in the MERRY update; the algorithm is generalized to the multiple-input multiple output (MIMO) and fractionally spaced cases; a low-complexity, blind symbol synchronization technique is proposed, and a method is proposed for blind initialization of the algorithm to avoid slow modes of convergence. Each extension to the basic MERRY algorithm is accompanied by an illustrative simulation example. 相似文献
132.
Li‐Ping Lu Dinesh Kabra Kerr Johnson Richard H. Friend 《Advanced functional materials》2012,22(1):144-150
A study of an efficient blue light‐emitting diode based on a fluorescent aryl polyfluorene (aryl‐F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO3 as electron‐ and hole‐injecting electrodes, respectively. Charge‐carrier balance and color purity in these structures are achieved by incorporating poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl)‐diphenylamine (TFB) into aryl‐F8. TFB is known to be a hole‐transporting material but it is found to act as a hole trap on mixing with aryl‐F8. Luminance efficiency of ≈6 cd A?1 and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl‐F8. Study of charge injection and transport in the single‐carrier devices shows that the addition of a small fraction of hole traps is necessary for charge‐carrier balance. Optical studies using UV–vis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl‐F8, which is the emitting material in these devices. Luminance efficiency of up to ≈11 cd A?1 and EQE values of 5.7% are achieved in these structures with the aid of improved out‐coupling using index‐matched hemispheres. 相似文献
133.
Evaluating copper lysine and copper sulfate sources for heifers 总被引:2,自引:0,他引:2
Rabiansky PA McDowell LR Velasquez-Pereira J Wilkinson NS Percival SS Martin FG Bates DB Johnson AB Batra TR Salgado-Madriz E 《Journal of dairy science》1999,82(12):2642-2650
The effects of feeding different sources and quantities of Cu to heifers were evaluated in a 211-d experiment. Forty crossbred predominantly Brahman x Hereford heifers averaging 13.5 mo of age and 301 kg were initially depleted of Cu. The depletion diet was fed for 70 d and consisted of low Cu and high antagonist minerals, Fe, S, and Mo at 1000 mg/kg, 0.5%, and 5 mg/kg (dry basis), respectively. On d 71, heifers continued to receive the antagonistic minerals and were allotted equally to five Cu treatments: 1) control, no additional Cu source; 2) 8 mg of Cu/kg from CuSO4; 3) 16 mg of Cu/kg from CuSO4; 4) 8 mg of Cu/kg from Cu lysine; and 5) 16 mg of Cu/kg from Cu lysine. When no notable change in concentration of Cu in the liver was observed, d 169, a second diet was formulated. The heifers were fed the same Cu treatments, but S and Mo were removed and Fe was lowered to 50 mg/kg. This diet was then fed for the final 42 d of the experiment. In addition to performance, concentrations of Cu, Fe, and Zn in the plasma and liver, plasma ceruloplasmin, hemoglobin, superoxide dismutase (SOD) activity of neutrophils and lymphocytes, and a cell mediated immune response (phytohemagglutinin-P, PHA) were measured. Heifers in this study had increased growth over time, but there were no treatment differences for growth and average daily gain. Liver and plasma Cu concentrations were not greatly influenced by different supplemental Cu sources. However, compared with other treatments, Cu lysine (16 mg/kg) increased liver Cu in cattle that were deficient and tended to increase plasma Cu in animals that were marginally deficient in Cu. Iron concentrations decreased over time in liver and plasma, but there was no difference in Fe and Zn concentrations in liver and plasma among treatments. Differences in ceruloplasmin and hemoglobin concentrations were significant over time but not among treatments. The SOD activity in neutrophils did not change over time, but SOD activity of lymphocytes increased over time. For the PHA immune response test, there was no effect of time or a time by treatment interaction. These data suggest that all Cu sources were available, but Cu at 16 mg/kg from Cu lysine was more beneficial than were other sources and particularly for heifers with low Cu status. 相似文献
134.
Nutritive value of corn silage as affected by maturity and mechanical processing: a contemporary review 总被引:2,自引:0,他引:2
Johnson L Harrison JH Hunt C Shinners K Doggett CG Sapienza D 《Journal of dairy science》1999,82(12):2813-2825
Stage of maturity at harvest and mechanical processing affect the nutritive value of corn silage. The change in nutritive value of corn silage as maturity advances can be measured by animal digestion and macro in situ degradation studies among other methods. Predictive equations using climatic data, vitreousness of corn grain in corn silage, starch reactivity, gelatinization enthalpy, dry matter (DM) of corn grain in corn silage, and DM of corn silage can be used to estimate starch digestibility of corn silage. Whole plant corn silage can be mechanically processed either pre- or postensiling with a kernel processor mounted on a forage harvester, a recutter screen on a forage harvester, or a stationary roller mill. Mechanical processing of corn silage can improve ensiling characteristics, reduce DM losses during ensiling, and improve starch and fiber digestion as a result of fracturing the corn kernels and crushing and shearing the stover and cobs. Improvements in milk production have ranged from 0.2 to 2.0 kg/d when cows were fed mechanically processed corn silage. A consistent improvement in milk protein yield has also been observed when mechanically processed corn silage has been fed. With the advent of mechanical processors, alternative strategies are evident for corn silage management, such as a longer harvest window. 相似文献
135.
The authors describe two chips which form the basis of a high-speed lossless image compression/decompression system. They present the transform and coding algorithms and the main architectural features of the chips and outline some performance specifications. Lossless compression can be achieved by a transformation process followed by entropy coding. The two application-specific integrated circuits (ASICs) perform S -transform image decomposition and the Lempel-Ziv (L-Z) type of entropy coding. The S -transform, besides decorrelating the image, provides a convenient method of hierarchical image decomposition. The data compressor/decompressor IC is a fast and efficient implementation of the L-Z algorithm. The chips can be used independently or together for image compression 相似文献
136.
K.V. Vassilevski N.G. Wright A.G. O’Neill C.M. Johnson 《Microelectronic Engineering》2006,83(1):150-154
High temperature silicon carbide diodes with nickel silicide Schottky contacts were fabricated by deposition of titanium-nickel metal film on 4H-SiC epitaxial wafer followed by annealing at 550 °C in vacuum. Room temperature boron implantation have been used to form single zone junction termination extension. 4H-SiC epitaxial structures designed to have theoretical parallel-plain breakdown voltages of 1900 and 3600 V have been used for this research. The diodes revealed soft recoverable avalanche breakdown at voltages of 1450 and 3400 V, respectively, which are about 80% and 95% of theoretical values. I-V characteristics of fabricated 4H-SiC Schottky diodes have been measured at temperatures from room temperature up to 400 °C. The diodes revealed unchangeable barrier heights and ideality factors as well as positive coefficients of breakdown voltage. 相似文献
137.
M. Reddy D. D. Lofgreen K. A. Jones J. M. Peterson W. A. Radford J. D. Benson S. M. Johnson 《Journal of Electronic Materials》2013,42(11):3114-3118
HgCdTe dual-band mid-wave infrared/long-wave infrared focal-plane arrays on CdZnTe are a key component in advanced electrooptic sensor applications. Molecular beam epitaxy (MBE) has been used successfully for growth of dual-band layers on larger CdZnTe substrates. However, the macrodefect density, which is known to reduce the pixel operability and its run-to-run variation, is larger when compared with layers grown on Si substrate. This paper reports the macrodefect density versus size signature of a well-optimized MBE dual-band growth and a cross-sectional study of a macrodefect that represents the most prevalent class using focused ion beam, scanning transmission electron microscopy, and energy-dispersive x-ray spectroscopy. The results show that the macrodefect originates from a void, which in turn is associated with a pit on the CdZnTe substrate. 相似文献
138.
Andrew Taylor Clay Mortensen Raimar Rostek Ngoc Nguyen David C. Johnson 《Journal of Electronic Materials》2010,39(9):1981-1986
This article demonstrates that carrier concentrations in bismuth telluride films can be controlled through annealing in controlled
vapor pressures of tellurium. For the bismuth telluride source with a small excess of tellurium, all the films reached a steady
state carrier concentration of 4 × 1019 carriers/cm3 with Seebeck coefficients of −170 μV K−1. For temperatures below 300°C and for film thicknesses of 0.4 μm or less, the rate-limiting step in reaching a steady state for the carrier concentration appeared to be the mass transport
of tellurium through the gas phase. At higher temperatures, with the resulting higher pressures of tellurium or for thicker
films, it was expected that mass transport through the solid would become rate limiting. The mobility also changed with annealing,
but at a rate different from that of the carrier concentration, perhaps as a consequence of the non-equilibrium concentration
of defects trapped in the films studied by the low temperature synthesis approach. 相似文献
139.
E. P. G. Smith E. A. Patten P. M. Goetz G. M. Venzor J. A. Roth B. Z. Nosho J. D. Benson A. J. Stoltz J. B. Varesi J. E. Jensen S. M. Johnson W. A. Radford 《Journal of Electronic Materials》2006,35(6):1145-1152
High-performance 20-μm unit-cell two-color detectors using an n-p+-n HgCdTe triple-layer heterojunction (TLHJ) device architecture grown by molecular beam epitaxy (MBE) on (211)-oriented CdZnTe
substrates with midwavelength (MW) infrared and long wavelength (LW) infrared spectral bands have been demonstrated. Detectors
with nominal MW and LW cut-off wavelengths of 5.5 μm and 10.5 μm, respectively, exhibit 78 K LW performance with >70 % quantum
efficiency, reverse bias dark currents below 300 pA, and RA products (zero field of view, 150-mV bias) in excess of 1×103 Ωcm2. Temperature-dependent current-voltage (I–V) detector measurements show diffusion-limited LW dark current performance extending
to temperatures below 70 K with good operating bias stability (150 mV ± 50 mV). These results reflect the successful implementation
of MBE-grown TLHJ detector designs and the introduction of advanced photolithography techniques with inductively coupled plasma
(ICP) etching to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance.
These detector improvements complement the development of high operability large format 640×480 and 1280×720 two-color HgCdTe
infrared focal plane arrays (FPAs) to support third generation forward looking infrared (FLIR) systems. 相似文献
140.
Yutaka Hasegawa Ryohei Harada Kazuki Tokuda Kimihiro Kimura Hideo Ogawa Toshikazu Onishi Atsushi Nishimura Johnson Han Makoto Inoue 《Journal of Infrared, Millimeter and Terahertz Waves》2017,38(5):638-652
A new stepped septum-type waveguide circular polarizer (SST-CP) was developed to operate in the 230 GHz band for radio astronomy, especially submillimeter-band VLBI observations. For previously reported SST-CP models, the 230 GHz band is too high to achieve the design characteristics in manufactured devices because of unexpected machining errors. To realize a functional SST-CP that can operate in the submillimeter band, a new method was developed, in which the division surface is shifted from the top step of the septum to the second step from the top, and we simulated the expected machining error. The SST-CP using this method can compensate for specified machining errors and suppress serious deterioration. To verify the proposed method, several test pieces were manufactured, and their characteristics were measured using a VNA. These results indicated that the insertion losses were approximately 0.75 dB, and the input return losses and the crosstalk of the left- and right-hand circular polarization were greater than 20 dB at 220–245 GHz on 300 K. Moreover, a 230 GHz SST-CP was developed by the proposed method and installed in a 1.85-m radio telescope receiver systems, and then had used for scientific observations during one observation season without any problems. These achievements demonstrate the successful development of a 230 GHz SST-CP for radio astronomical observations. Furthermore, the proposed method can be applicable for observations in higher frequency bands, such as 345 GHz. 相似文献