全文获取类型
收费全文 | 24432篇 |
免费 | 703篇 |
国内免费 | 22篇 |
专业分类
电工技术 | 217篇 |
综合类 | 18篇 |
化学工业 | 3791篇 |
金属工艺 | 322篇 |
机械仪表 | 403篇 |
建筑科学 | 1005篇 |
矿业工程 | 49篇 |
能源动力 | 502篇 |
轻工业 | 1679篇 |
水利工程 | 120篇 |
石油天然气 | 59篇 |
武器工业 | 1篇 |
无线电 | 1587篇 |
一般工业技术 | 3061篇 |
冶金工业 | 9306篇 |
原子能技术 | 188篇 |
自动化技术 | 2849篇 |
出版年
2023年 | 157篇 |
2022年 | 296篇 |
2021年 | 461篇 |
2020年 | 341篇 |
2019年 | 366篇 |
2018年 | 424篇 |
2017年 | 390篇 |
2016年 | 491篇 |
2015年 | 423篇 |
2014年 | 520篇 |
2013年 | 1072篇 |
2012年 | 856篇 |
2011年 | 1116篇 |
2010年 | 767篇 |
2009年 | 822篇 |
2008年 | 862篇 |
2007年 | 759篇 |
2006年 | 702篇 |
2005年 | 594篇 |
2004年 | 491篇 |
2003年 | 438篇 |
2002年 | 420篇 |
2001年 | 295篇 |
2000年 | 317篇 |
1999年 | 523篇 |
1998年 | 2739篇 |
1997年 | 1596篇 |
1996年 | 1111篇 |
1995年 | 652篇 |
1994年 | 593篇 |
1993年 | 649篇 |
1992年 | 223篇 |
1991年 | 247篇 |
1990年 | 203篇 |
1989年 | 212篇 |
1988年 | 199篇 |
1987年 | 199篇 |
1986年 | 185篇 |
1985年 | 207篇 |
1984年 | 135篇 |
1983年 | 126篇 |
1982年 | 123篇 |
1981年 | 144篇 |
1980年 | 123篇 |
1979年 | 98篇 |
1978年 | 103篇 |
1977年 | 273篇 |
1976年 | 450篇 |
1975年 | 91篇 |
1974年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
BACKGROUND: The efficacy and predictability of photorefractive keratectomy and radial keratotomy become increasingly relevant. This retrospective study compares one surgeon's experience with photorefractive keratectomy and radial keratotomy over a 3-year period from 1990 to 1993. METHODS: Photorefractive keratectomy was performed on 103 eyes of 76 patients that met the inclusion criteria for the phase IIb, phase III, and phototherapeutic keratectomy studies as delineated by the United States Food and Drug Administration. Radial keratotomy was performed on 117 eyes of 81 patients with up to 9.00 diopters (D) of myopia. RESULTS: In the photorefractive keratectomy group, 83% of the eyes achieved uncorrected visual acuity of at least 20/40; 37% saw 20/20; 88% had a refraction within 1.00 D of emmetropia, and 63% within 0.50 D of emmetropia. For the radial keratotomy group, 85% of the eyes achieved an uncorrected visual acuity of 20/40 or better; 27% saw 20/20; 88% had a refraction within 1.00 D of emmetropia; and 55% within 0.50 D of emmetropia. There were no serious complications, and only one single eye in each of the photorefractive keratectomy and radial keratotomy groups lost two lines or more of spectacle-corrected visual acuity. CONCLUSION: Photorefractive keratectomy and radial keratotomy are both effective procedures, and result in similar refractive outcomes for myopia of -1.00 to -9.00 D. 相似文献
123.
Grard J Martin Doubou Danho Claude Vallet 《Journal of the science of food and agriculture》1991,56(4):419-434
More than 500 carbohydrate samples have been characterised by hydrogen, carbon and oxygen isotopic parameters measured on ethanol and water resulting from controlled fermentation. Different chemical states of the carbohydrate pool have been considered: polysaccharides from cereals, tubers and leguminosae (maize, sorghum, rice, wheat, barley, potato, bean), glucose, fructose and sucrose from fruits (pineapple, citrus, apple, soft fruits), sucrose from sugar plants (beet, cane). The overall carbon-13 content of ethanol and the deuterium and oxygen-18 contents of water before fermentation were determined by mass spectrometry of isotope ratios whereas the investigation of site-specific natural isotope fractionation by deuterium NMR has provided access to the deuterium contents in the methyl (I) and methylene (II) sites of ethanol. The results have been analysed in the multidimensional space of these isotopic variables. Hierarchical clustering, principal component and discriminant analyses have been performed. Among fruits, for example, the pineapple group exhibits a 100% discrimination with respect to the apple and citrus groups which are themselves well distinguished. A still higher discrimination is reached between the three groups, citrus, cane and beet, and the addition of 10% exogenous cane or beet sucrose to citrus juice is unambiguously detected. 相似文献
124.
125.
An adaptive analog continuous-time biquadratic filter is realized in a 2-μm digital CMOS process for operation at 300 kHz. The biquad implements the notch, bandpass and low-pass transfer functions. The only parameter adapted is the resonant frequency of the biquad, which is identical to the notch frequency and the bandpass center frequency. The update method is based on a least-means-square algorithm which adapts the notch frequency to minimize the power at the notch filter output. The actual update is modified to reduce the circuit complexity to one biquad and one correlator. When the filter is tracking a sinusoid, this update generates a ripple-free gradient that decreases tracking error. Applications include phase-frequency detectors, FM demodulators (linear and frequency shift keying), clock extractors, and frequency acquisition aids for phase-locked loops and Costas loops. Measured results from experimental prototypes are presented. Nonidealities of an all-analog implementation are discussed, along with suggestions to improve performance 相似文献
126.
Chang C.-R. Steer M.B. Martin S. Reese E. Jr. 《Microwave Theory and Techniques》1991,39(10):1735-1745
The authors present a newly developed free-running steady-state oscillator analysis algorithm suited to large-signal oscillator analysis. Kurokawa's oscillation condition is coupled with the modified nodal admittance form of the circuit equations to avoid degenerate solutions. The algorithm was implemented by using both harmonic balance and frequency-domain spectral balance techniques. It was used in the simulation of monolithically integrated varactor-tuned MESFET oscillator. Good agreement between simulated power and oscillation frequency results was obtained 相似文献
127.
Auguste Commeyras Hlne Collet Laurent Boiteau Jacques Taillades Odile Vandenabeele‐Trambouze Herv Cottet Jean‐Philippe Biron Raphaël Plasson Louis Mion Olivier Lagrille Herv Martin Franck Selsis Michel Dobrijevic 《Polymer International》2002,51(7):661-665
Addressing the still open question of the prebiotic origin of sequential macromolecules (peptides, nucleic acids) on the primitive Earth, we describe a molecular engine (the primary pump), which works at ambient temperature and continuously generates, elongates and complexifies sequential peptides. This new scenario is based on a cyclic reaction sequence, whose keystep is the activation of amino acids into their N‐carboxyanhydrides (NCA) through nitrosation by NOx. This process could have taken place on tidal beaches; it requires a buffered ocean, emerged land and a nitrosating atmosphere. With the help of geochemical studies and computer simulations of atmosphere photochemistry, we show that the primitive Earth during the Hadean may have satisfied all these requirements. © 2001 Society of Chemical Industry. 相似文献
128.
Martin J. Beckmann 《The Annals of Regional Science》1992,26(3):199-207
The question explored in this paper is whether the center attracts more spending by all agents in a spatial economy than do peripheral locations, and why this might be so. This is the macroeconomics pendant to the microeconomic concern with accessibility.This paper is the result of discussions on the subject of time and space in economics with my good friend, Tonu Puu. I dedicate it to the memory of another dear friend, Claude Ponsard. 相似文献
129.
H.W. Choi C.W. Jeon M.D. Dawson P.R. Edwards R.W. Martin 《Photonics Technology Letters, IEEE》2003,15(4):510-512
High-performance, two-dimensional arrays of parallel-addressed InGaN blue micro-light-emitting diodes (LEDs) with individual element diameters of 8, 12, and 20 /spl mu/m, respectively, and overall dimensions 490 /spl times/490 /spl mu/m, have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallization, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have current-voltage (I-V) characteristics approaching those of broad-area reference LEDs fabricated from the same wafer, and give comparable (3-mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction. 相似文献
130.