全文获取类型
收费全文 | 14926篇 |
免费 | 1287篇 |
国内免费 | 14篇 |
专业分类
电工技术 | 141篇 |
综合类 | 12篇 |
化学工业 | 3774篇 |
金属工艺 | 183篇 |
机械仪表 | 402篇 |
建筑科学 | 657篇 |
矿业工程 | 39篇 |
能源动力 | 403篇 |
轻工业 | 2962篇 |
水利工程 | 126篇 |
石油天然气 | 35篇 |
无线电 | 1015篇 |
一般工业技术 | 2730篇 |
冶金工业 | 776篇 |
原子能技术 | 46篇 |
自动化技术 | 2926篇 |
出版年
2024年 | 41篇 |
2023年 | 195篇 |
2022年 | 295篇 |
2021年 | 556篇 |
2020年 | 416篇 |
2019年 | 438篇 |
2018年 | 639篇 |
2017年 | 641篇 |
2016年 | 709篇 |
2015年 | 582篇 |
2014年 | 741篇 |
2013年 | 1361篇 |
2012年 | 1299篇 |
2011年 | 1226篇 |
2010年 | 812篇 |
2009年 | 773篇 |
2008年 | 837篇 |
2007年 | 739篇 |
2006年 | 591篇 |
2005年 | 446篇 |
2004年 | 399篇 |
2003年 | 330篇 |
2002年 | 326篇 |
2001年 | 180篇 |
2000年 | 181篇 |
1999年 | 127篇 |
1998年 | 163篇 |
1997年 | 137篇 |
1996年 | 112篇 |
1995年 | 116篇 |
1994年 | 97篇 |
1993年 | 77篇 |
1992年 | 54篇 |
1991年 | 48篇 |
1990年 | 33篇 |
1989年 | 38篇 |
1988年 | 34篇 |
1987年 | 19篇 |
1986年 | 25篇 |
1985年 | 35篇 |
1984年 | 39篇 |
1983年 | 27篇 |
1982年 | 25篇 |
1981年 | 29篇 |
1980年 | 19篇 |
1979年 | 33篇 |
1978年 | 17篇 |
1977年 | 16篇 |
1976年 | 14篇 |
1975年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Several 1 wt% Pt/KL catalysts doped with different concentrations of either BaO or La2O3 were prepared by successive impregnation of a zeolite KL and then characterized by H2-O2 titration, TPR, CO–FTIR, TEM-XEDS and XPS. Catalytic activity measurements in the hydroconversion of n-heptane showed that barium highly enhances the aromatization activity of Pt/KL, the more so the higher the barium concentration. The yield to aromatics increases to a lesser degree by adding 1 wt% La, but it is little modified at higher La concentrations. The CO–FTIR results suggest that the promoter effect of barium is related to an electron enrichment of Pt produced by the BaO Pt0 interaction which, according to the TEM–XEDS and XPS results, is more favored than the La2O3 Pt interaction. 相似文献
992.
Study of the aromatic by-products formed from ozonation of anilines in aqueous solution 总被引:6,自引:0,他引:6
Aqueous solutions of aniline and p-chloroaniline were treated with ozone in order to study the reaction and oxidation by-products. Aniline solutions were ozonated at low and high pH, so as to compare both molecular and hydroxyl free radical mechanisms, respectively. The main identified aromatic by-products were nitrobenzene and azobenzene when the experiment was carried out at acid pH. Formation of nitrobenzene, azobenzene, azoxybenzene and 2-pyridine-carboxylic was observed when the ozonation was carried out at basic pH. p-Chloroaniline was treated with ozone only at high pH and the identified by-products were in accordance with those obtained in the ozonation of aniline: p-chloronitrobenzene, 4,4'-dichloroazobenzene and 4-chloro-2-pyridine-carboxylic acid. All the aromatic by-products found were less toxic than the raw materials. The pseudo-first-order constants in aniline concentration were calculated, whilst kinetic in p-chloroaniline concentration could not be adjusted to a first-order reaction. 相似文献
993.
Pichia sorbitophila grows rapidly in the presence of very high NaCl concentrations. Under these conditions, even when the K(+) concentration is low, P. sorbitophila cells can maintain low Na(+) and high K(+) contents. This remarkable capacity of P. sorbitophila fails when the external pH is not acidic. This indicates that Na(+) efflux is mediated by an electroneutral Na(+)/H(+) antiporter. We have cloned and sequenced two genes designated as PsNHA1 and PsNHA2, which probably encode two antiporters of this type. The genes present high similarity with the corresponding genes from other yeasts. The heterologous expression of PsNHA1 or PsNHA2 in a Saccharomyces cerevisiae mutant lacking the Na(+) efflux systems and sensitive to high concentrations of Na(+) and K(+) rescued the tolerance and the ability to extrude both cations. The Accession Nos of the sequenced DNA fragments are: PsNHA1, AJ496431; PsNHA2, AJ496432. (TC 2.A.36) Copyright 2002 John Wiley & Sons, Ltd. 相似文献
994.
Deletion of Saccharomyces cerevisiae BIG1 causes an approximately 95% reduction in cell wall beta-1,6-glucan, an essential polymer involved in the cell wall attachment of many surface mannoproteins. The big1 deletion mutant grows very slowly, but growth can be enhanced if cells are given osmotic support. We have begun a cell biological and genetic analysis of its product. We demonstrate, using a Big1p-GFP fusion construct, that Big1p is an N-glycosylated integral membrane protein with a Type I topology that is located in the endoplasmic reticulum (ER). Some phenotypes of a big1Delta mutant resemble those of strains disrupted for KRE5, which encodes another ER protein affecting beta-l,6-glucan levels to a similar extent. In a big1Deltakre5Delta double mutant, both the growth and alkali-soluble beta-l,6-glucan levels were reduced as compared to either single mutant. Thus, while Big1p and Kre5p may have similar effects on beta-l,6-glucan synthesis, these effects are at least partially distinct. Residual beta-l,6-glucan levels in the big1Deltakre5Delta double mutant indicate that these gene products are unlikely to be beta-l,6-glucan synthase subunits, but rather may play some ancillary roles in beta-l,6-glucan synthase assembly or function, or in modifying proteins for attachment of beta-l,6-glucan. 相似文献
995.
Quiroga JA Servin M Cuevas F 《Journal of the Optical Society of America. A, Optics, image science, and vision》2002,19(8):1524-1531
The fringe orientation angle provides useful information for many fringe-pattern-processing techniques. From a single normalized fringe pattern (background suppressed and modulation normalized), the fringe orientation angle can be obtained by computing the irradiance gradient and performing a further arctangent computation. Because of the 180 degrees ambiguity of the fringe direction, the orientation angle computed from the gradient of a single fringe pattern can be determined only modulo pi. Recently, several studies have shown that a reliable determination of the fringe orientation angle modulo 2pi is a key point for a robust demodulation of the phase from a single fringe pattern. We present an algorithm for the computation of the modulo 2pi fringe orientation angle by unwrapping the orientation angle obtained from the gradient computation with a regularized phase tracking method. Simulated as well as experimental results are presented. 相似文献
996.
Simão AG Guimarães LG de Mendonça JP 《Journal of the Optical Society of America. A, Optics, image science, and vision》2002,19(10):2053-2063
A theoretical and numerical investigation is devised for resonant light scattering of an off-axis normally incident Gaussian beam by two parallel nonabsorbing cylinders based on the related beam theory developed in J. Opt. Soc. Am. A 14, 640 (1997). By varying the half-beam width, we show that the multireflection process between the two scatterers can be minimized. Moreover, the study is an attempt to understand the underlying physics present in the process of resonance excitation by evanescent wave coupling. 相似文献
997.
Güdel M 《Philosophical transactions. Series A, Mathematical, physical, and engineering sciences》2002,360(1798):1935-1949
More than two years of observation with Chandra and XMM-Newton has provided a rich harvest of new results on the physics of stellar coronae and winds. High-resolution X-ray spectroscopy in particular has opened new windows to the structure, the dynamics and the composition of stellar atmospheres. The present paper presents selected results from the areas of hot and cool stars and star formation, summarizing new views of the thermal structure and energy release in stellar coronae, observations of magnetically active brown dwarfs, the structure of winds in hot stars, the physics in colliding-wind binary systems, and X-rays from protostars and stellar jets. 相似文献
998.
del Mar Cordero-Rando M Hidalgo-Hidalgo de Cisneros JL Blanco E Naranjo-Rodríguez I 《Analytical chemistry》2002,74(10):2423-2427
In this work, a novel sol-gel-based procedure to obtain solid carbon composite electrodes is proposed. The procedure is based on the use of sonocatalysis to apply high-energy ultrasound directly to the precursors; this way, ultrasonic cavitation is achieved so that hydrolysis with acidic water is promoted in the absence of any additional solvent and the time needed to get a unique phase is reduced drastically. The mix of sonogel with spectroscopic grade graphite leads to a new type of sol-gel electrode: the Sonogel-Carbon electrode. The amount of water, necessary for hydrolysis to occur, has been studied, as well as the sol pH value and the electrochemical behavior of the Sonogel-Carbon electrode in Britton-Robinson buffer. Mechanical renewal of the electrodes surfaces and background signal stability for at least 42 days have been also tested. We have carried out some tests to check the ability of the synthesized material to give electrochemical response when redox species are present in the background electrolyte. Mechanical and electrochemical studies indicate that the Sonogel-Carbon electrode shows good properties for use as an electrochemical sensor. 相似文献
999.
To better detect and identify beeswax in ancient organic residues from archaeological remains, we developed a new analytical methodology consisting of the analysis of (i) the trimethylsilylated organic extract by GC/MS and (ii) the crude extract by ESI-MS. Selective scanning modes, such as SIM or MRM, permit separate quantification of each chemical family (fatty acids, monoesters, monohydroxyesters, and diesters) and allow an improvement in sensitivity and selectivity, allowing the crude extract to be treated without further purification. GC/MS (SIM) was revealed to be a powerful method for the detection of components, with a detection limit down to a total lipid extract in the range of approximately 50 ng in a complex matix, such as archaeological degraded material, whereas ESI-MS/MS is instead used for the detection of nonvolatile biomarkers. Identification by GC/MS (SIM) and ESI-MS/ MS (MRM) of more than 50 biomarkers of beeswax in an Etruscan cup at the parts-per-million level provides the first evidence for the use of this material by the Etruscans as fuel or as a waterproof coating for ceramics. 相似文献
1000.
A liquid-cell shearing interferometer was developed to measure refractive-index variations (delta n) in transparent materials. The cell was filled with a liquid having a matched refractive index. The achieved resolution was better than 1/1000 of a fringe shift and resulted in a delta n measurement sensitivity down to 10(-7) for 1-mm-thick samples. A refractive-index increment in photothermorefractive glass of up to 5 x 10(-6) was observed after UV exposure at 325 nm. A refractive-index decrement of up to 1 x 10(-3) was observed after thermal development of the exposed sample. It was proved that photothermorefractive glass obeys the reciprocity law; i.e., delta n depends on the UV dosage but does not depend on the irradiance. 相似文献