全文获取类型
收费全文 | 2627篇 |
免费 | 236篇 |
国内免费 | 6篇 |
专业分类
电工技术 | 38篇 |
综合类 | 11篇 |
化学工业 | 746篇 |
金属工艺 | 42篇 |
机械仪表 | 47篇 |
建筑科学 | 138篇 |
矿业工程 | 37篇 |
能源动力 | 60篇 |
轻工业 | 275篇 |
水利工程 | 29篇 |
石油天然气 | 3篇 |
无线电 | 182篇 |
一般工业技术 | 533篇 |
冶金工业 | 290篇 |
原子能技术 | 10篇 |
自动化技术 | 428篇 |
出版年
2023年 | 48篇 |
2022年 | 37篇 |
2021年 | 115篇 |
2020年 | 82篇 |
2019年 | 70篇 |
2018年 | 105篇 |
2017年 | 101篇 |
2016年 | 120篇 |
2015年 | 119篇 |
2014年 | 153篇 |
2013年 | 167篇 |
2012年 | 161篇 |
2011年 | 195篇 |
2010年 | 112篇 |
2009年 | 130篇 |
2008年 | 131篇 |
2007年 | 102篇 |
2006年 | 83篇 |
2005年 | 80篇 |
2004年 | 63篇 |
2003年 | 57篇 |
2002年 | 42篇 |
2001年 | 25篇 |
2000年 | 29篇 |
1999年 | 30篇 |
1998年 | 71篇 |
1997年 | 40篇 |
1996年 | 49篇 |
1995年 | 37篇 |
1994年 | 20篇 |
1993年 | 27篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 16篇 |
1974年 | 8篇 |
1973年 | 5篇 |
排序方式: 共有2869条查询结果,搜索用时 546 毫秒
21.
22.
In this article we describe our recent efforts in the area of palladium- and nickel-catalysed aromatic substitution reactions. Main focus is on low cost and low waste production methods. The use of aromatic carboxylic anhydrides in the Heck reaction leads to a waste-free protocol. In addition these reactions are easy to work up as no ligands or bases are used. For Heck reactions where substrates or products do not tolerate high temperatures we found that use of a bulky phosphoramidite (13b) as ligand for palladium leads to a very fast reaction at low temperatures. Recycle of palladium in ligand-free Heck and Suzuki reactions is easily accomplished by treating the palladium black that precipitates at the end of the reaction on a carrier material with a small excess of I2 prior to its re-use in the next run. Use of aryl chlorides in the palladium- and nickel-catalysed formation of biaryls can be accomplished by using the nickel-catalysed coupling with arylzinc chlorides. Better still, it was possible to make use of the arylgrignard and use a catalytic amount of ZnCl2. Whereas the strength of these aromatic substitution reactions lies in their broad tolerance of functional groups, one exception was the Sonogashira reaction on 3-bromoaniline. The problem was solved by making use of in situ catalytic protection of the NH2 group with benzaldehyde. 相似文献
23.
In this article, we formulate and study quantum analogues of randomized search heuristics, which make use of Grover search (in Proceedings of the 28th Annual ACM Symposium on Theory of Computing, pp. 212–219. ACM, New York, 1996) to accelerate the search for improved offsprings. We then specialize the above formulation to two specific search heuristics: Random Local Search and the (1+1) Evolutionary Algorithm. We call the resulting quantum versions of these search heuristics Quantum Local Search and the (1+1) Quantum Evolutionary Algorithm. We conduct a rigorous runtime analysis of these quantum search heuristics in the computation model of quantum algorithms, which, besides classical computation steps, also permits those unique to quantum computing devices. To this end, we study the six elementary pseudo-Boolean optimization problems OneMax, LeadingOnes, Discrepancy, Needle, Jump, and TinyTrap. It turns out that the advantage of the respective quantum search heuristic over its classical counterpart varies with the problem structure and ranges from no speedup at all for the problem Discrepancy to exponential speedup for the problem TinyTrap. We show that these runtime behaviors are closely linked to the probabilities of performing successful mutations in the classical algorithms. 相似文献
24.
In many studies on lipoxygenase catalysis, nonionic detergents are used to obtain an optically transparent solution of the
fatty acid substrate. In order to resolve some controversies that exist with regard to the interpretation of kinetic data
obtained with solutions containing nonionic detergents, a systematic investigation was undertaken into the effects of Lubrol,
Tween-20 and Triton X-100 (0–0.8 g/L) on the kinetics of linoleate (2.5–110 μM) dioxygenation, catalyzed by lipoxygenase-1
or lipoxygenase-2 from soybean, at pH 9 or 10, at 25°C. Under most conditions, it was found that the detergents slowed down
the reaction. However, at high linoleate concentrations, where substrate inhibition of lipoxygenase is significant, small
amounts of detergent increased the dioxygenation rate. In a quantitative analysis of the results, a kinetic model in which
the incorporation of linoleate in the detergent micelles is formulated as a simple reversible equilibrium, and in which both
lipoxygenase-1 and-2 interact with free linoleate, but not with linoleate incorporated in the micelles, appeared to be sufficient
to predict experimental results over a wide range of experimental conditions. According to this model, the changes in the
dioxygenation kinetics caused by the presence of nonionic detergents are similar (but not equal) to those caused by competitive
inhibitors. The conclusions that monomeric, nonmicellar linoleate is the preferred substrate for lipoxygenase and that the
observed inhibition and stimulation are solely due to changes in the effective linoleate concentration strongly corroborate
the earlier observations by Galpin and Allen [Biochim. Biophys. Acta 488 (1977), 392–401]. 相似文献
25.
Johannes G. A. Terlingen Gijsbert A. J. Takens Frederik J. Van Der Gaag Allan S. Hoffman Jan Feijen 《应用聚合物科学杂志》1994,52(1):39-53
Poly(acrylic acid) (PAAc) films were treated with either an argon or a tetrafluoromethane (CF4) plasma and subsequently analyzed with X-ray photoelectron spectroscopy (XPS). PAAc films were decarboxylated during both types of plasma treatments. In addition, during the CF4 plasma treatment, the PAAc films became fluorinated. The plasma phase during the argon plasma treatment of PAAc films was investigated with optical emission spectroscopy. It was shown that during this plasma treatment carbon dioxide, water, and possibly hydrogen were liberated from the PAAc surface. By covering the surface of PAAc films with different materials (lithium fluoride, UV fused silica, and glass) during the plasma treatment, it was possible to differentiate between photochemically induced and particle-induced changes of the surface. This method was used to show that decarboxylation during the argon plasma treatment was caused by vacuum UV radiation (wavelength < 150 nm) and the decarboxylation/fluorination during the CF4 plasma treatment was induced by reactive fluorine-containing species from the plasma phase. Furthermore, during both processes, etching of the PAAc surface occurred. Based on these mechanisms, kinetic models were derived that could be used to describe the measured kinetic data adequately. © 1994 John Wiley & Sons, Inc. 相似文献
26.
In this paper we present a novel approach to simulate image formation for a wide range of real world lenses in the Monte Carlo ray tracing framework. Our approach sidesteps the overhead of tracing rays through a system of lenses and requires no tabulation. To this end we first improve the precision of polynomial optics to closely match ground‐truth ray tracing. Second, we show how the Jacobian of the optical system enables efficient importance sampling, which is crucial for difficult paths such as sampling the aperture which is hidden behind lenses on both sides. Our results show that this yields converged images significantly faster than previous methods and accurately renders complex lens systems with negligible overhead compared to simple models, e.g. the thin lens model. We demonstrate the practicality of our method by incorporating it into a bidirectional path tracing framework and show how it can provide information needed for sophisticated light transport algorithms. 相似文献
27.
Attila Bereck Silke Dillbohner Burkhard Weber Dieter Riegel Jrg Mosel Johannes Michael Reper Axel Brakelmann 《Coloration Technology》1997,113(11):322-326
A simple method for the characterisation of fabric softness using a conventional tensile tester and a special measurement device has been developed. The method is demonstrated to be especially useful in detecting changes in softness of a given substrate due to different treatments. The results obtained, particularly hysteresis at 75% of the maximum extension, provide a good correlation with subjective ranking for samples that are subjectively distinguishable; they also differentiate well between samples which seem subjectively indistinguishable. Screening tests were carried out, mainly on wool fabrics, to find the effect of different treatments such as oxidative (alkaline and acidic) and reductive (Blankit IN) bleaching, dyeing with acid and a 1:2 metal complex, chrome and (mono and bi)reactive dyes, the effect of the antisetting agent Basolan AS (BASF) and of crosslinking with Irgasol HTW (CGY) on fabric softness. 相似文献
28.
Francisco Salmerón-Miranda Birgitta Båth Henrik Eckersten Johannes Forkman Maria Wivstad 《Nutrient Cycling in Agroecosystems》2007,79(2):125-139
The main objective of this field study was to estimate the total plant uptake of soil mineral N in maize (Zea mays L.) and common bean (Phaseolus vulgaris L.) grown in crop rotations under different N content in Nicaragua. Secondary objectives were to estimate the fraction of
the measured soil mineral N content taken up in this way, and to determine how the measured N in plant aboveground parts was
related to the total mineral N uptake. A large variation in N content was obtained by using data from fertilisation experiments.
Plant total N uptake was estimated as the residual N in a mass balance calculation of soil mineral N. Mineral N content in
the top 0–0.3 m soil layer in the field cultivations and in tubes isolated from root uptake, and N content in aboveground
plant parts were measured every 30 days. Estimated plant total uptake of soil mineral N varied considerably (2.5–14 g N m−2 30 day−1) over periods and N treatments. The range of variation was similar for maize and bean. The fraction of the soil mineral N
that was taken up by the plant daily varied more in maize (about 0.03–0.12 day−1) than in bean (about 0.05–0.08 day−1). Our results suggest that monthly changes in N in aboveground plant parts were linearly related to plant total N uptake
during the same period. Aboveground plant N constituted between about 55% and 80% of total uptake of soil mineral N in maize
depending on period within season, whereas for bean it was more constant and smaller (about 40%). 相似文献
29.
New microporous hybrid organic/inorganic materials were obtained by modification of zeolites using chlorination of their surface,
followed by treatment with a Grignard reagent. Loading of butyl groups and retained crystallinity of ZSM-5 type zeolites was
higher than for zeolites Y. In contrast to zeolites Y, the lattice parameters of zeolites ZSM-5 increased after surface modification.
The obtained hybrid materials on the basis of high-silica zeolites possess a high degree of crystallinity at increased hydrophobicity. 相似文献
30.
Philipp Hauck Andreas Jentys Johannes A. Lercher 《Applied catalysis. B, Environmental》2007,70(1-4):91-99
In order to meet the stricter NOx and particulate emission limits for commercial vehicles, the selective catalytic reduction (SCR) with urea is currently seen having the highest potential. The conversion of urea into ammonia and carbon dioxide consists of two consecutive reactions, in which isocyanic acid is an intermediate that is hydrolyzed over TiO2. The intrinsic kinetics and the surface chemistry for this reaction are explored. Up to a temperature of 132 °C the reaction was in the intrinsic kinetic regime (EA = 73 kJ/mol), while at higher temperatures the reaction was limited by pore and external diffusion constraints, respectively. In the presence of NO, NH3 and NO2, the catalytic activity was negatively influenced, increasing in severity in the sequence mentioned indicating that nitrates formed from NO2 were most effective in blocking cations and anions of TiO2. IR spectroscopy indicates that dissociative adsorption of HNCO on TiO2 forms Ti–NCO and hydrogen bonded OH species. In the presence of water, isocyanic acid was so rapidly hydrolyzed that only adsorbed ammonia was observed on the catalyst surface. The presence of NO, NH3 and NO2 retards hydrolysis leading to the appearance of isocyanate species on the surface. 相似文献