全文获取类型
收费全文 | 1833篇 |
免费 | 93篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 23篇 |
综合类 | 1篇 |
化学工业 | 386篇 |
金属工艺 | 45篇 |
机械仪表 | 30篇 |
建筑科学 | 52篇 |
矿业工程 | 2篇 |
能源动力 | 76篇 |
轻工业 | 159篇 |
水利工程 | 10篇 |
石油天然气 | 2篇 |
无线电 | 179篇 |
一般工业技术 | 363篇 |
冶金工业 | 272篇 |
原子能技术 | 18篇 |
自动化技术 | 310篇 |
出版年
2023年 | 15篇 |
2022年 | 13篇 |
2021年 | 54篇 |
2020年 | 35篇 |
2019年 | 57篇 |
2018年 | 52篇 |
2017年 | 43篇 |
2016年 | 57篇 |
2015年 | 53篇 |
2014年 | 75篇 |
2013年 | 117篇 |
2012年 | 108篇 |
2011年 | 163篇 |
2010年 | 116篇 |
2009年 | 103篇 |
2008年 | 104篇 |
2007年 | 85篇 |
2006年 | 81篇 |
2005年 | 58篇 |
2004年 | 34篇 |
2003年 | 39篇 |
2002年 | 32篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 23篇 |
1998年 | 59篇 |
1997年 | 50篇 |
1996年 | 38篇 |
1995年 | 27篇 |
1994年 | 25篇 |
1993年 | 23篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 11篇 |
1976年 | 14篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 4篇 |
排序方式: 共有1928条查询结果,搜索用时 15 毫秒
991.
Strachan JP Strukov DB Borghetti J Yang JJ Medeiros-Ribeiro G Williams RS 《Nanotechnology》2011,22(25):254015
Memristors are memory resistors promising a rapid integration into future memory technologies. However, progress is still critically limited by a lack of understanding of the physical processes occurring at the nanoscale. Here we correlate device electrical characteristics with local atomic structure, chemistry and temperature. We resolved a single conducting channel that is made up of a reduced phase of the as-deposited titanium oxide. Moreover, we observed sufficient Joule heating to induce a crystallization of the oxide surrounding the channel, with a peculiar pattern that finite element simulations correlated with the existence of a hot spot close to the bottom electrode, thus identifying the switching location. This work reports direct observations in all three dimensions of the internal structure of titanium oxide memristors. 相似文献
992.
Carrier depletion and transport in a single ZnO nanowire Schottky device have been investigated at 5?K, using cathodoluminescence measurements. An exciton diffusion length of 200?nm has been determined along the nanowire axis. The depletion width is found to increase linearly with the reverse bias. The origin of this unusual dependence in semiconductor material is discussed in terms of charge location and dimensional effects on the screening of the junction electric field. 相似文献
993.
Adrian J Postal V Moessinger M Rascle N Charles A 《Accident; analysis and prevention》2011,(5):1652-1659
The present study investigated how executive functions and personality traits are related with driving performance among older drivers. Forty-two participants aged 60 and over were recruited to complete a battery of cognitive tests, measures of personality traits and an on-road driving test. Significant correlations were found between poor driving performances and low scores on tests assessing shifting and updating functions. In addition, extraversion had a negative relation with driving performance and made the only contribution, among the psychological factors, to the prediction of driving performance. Finally, gender and age emerged as the best predictors of on-road driving performance. Gender, personality traits and executive functions should be taken into account when studying safety among older drivers. 相似文献
994.
In this work, the analytical potential of cyclical electrical field flow fractionation (CyElFFF) for nanomaterial and colloidal particle characterization has been experimentally demonstrated. Different operating parameters were investigated in order to evaluate their effect on the mechanisms of retention and fractionation power of CyElFFF. The voltage and frequency of the oscillating electrical field appeared to be the most influential parameters controlling the separation mode. Mobile phase flow rate was also found to be a key parameter controlling the fractionation efficiency. This work allowed the definition of operating conditions such that a reliable CyElFFF analysis could be performed on different nanoparticles on the basis of the direct comparison of their theoretical and experimental behavior. The results show that this technique in optimized conditions is a powerful tool for electrophoretic mobility based separation and characterization of various nanoparticles. 相似文献
995.
Size exclusion chromatography (SEC) on a semipreparative scale (10 mg and more) was used to size-select ultrasmall gold nanoclusters (<2 nm) from polydisperse mixtures. In particular, the ubiquitous byproducts of the etching process toward Au(38)(SR)(24) (SR, thiolate) clusters were separated and gained in high monodispersity (based on mass spectrometry). The isolated fractions were characterized by UV-vis spectroscopy, MALDI mass spectrometry, HPLC, and electron microscopy. Most notably, the separation of Au(38)(SR)(24) and Au(40)(SR)(24) clusters is demonstrated. 相似文献
996.
Robert Zierold Zhenyu Wu Johannes Biskupek Ute Kaiser Julien Bachmann Carl E. Krill III Kornelius Nielsch 《Advanced functional materials》2011,21(2):226-232
This work presents a synthesis route for low‐aspect‐ratio nanotubes consisting of a layer of magnetite (Fe3O4) sandwiched between SiO2 layers. In this template‐based strategy, self‐ordered porous alumina membranes are combined with the atomic layer deposition of SiO2 and Fe2O3. An optimized electrochemical setup yields nanoporous Al2O3 membranes on 4‐inch Al substrates, which serve as templates for the large‐scale fabrication of nanotubes. A selective chemical etching step releases the magnetic tubes for suspension in a carrier fluid and permits recycling of the underlying aluminum foils for the fabrication of subsequent nanotube batches. The nanotubes consisting of an iron oxide layer protected by a silica shell are magnetically characterized in suspensions as well as in dried form on a substrate. High‐resolution transmission electron imaging reveals a polycrystalline, magnetite spinel structure of iron oxide, with the proper stoichiometry proven by the presence of the Verwey transition. Furthermore, field‐dependent viscosity measurements show an enhancement of the magnetoviscosity, thus demonstrating the technological potential of nanotube suspensions as a new class of ferrofluidic solutions. Owing to the tubular shape being closed at one end, these nanoparticles might additionally function as magnetic containers for targeted drug‐delivery or as chemical nanoreactors. 相似文献
997.
The objective of this study is to estimate the technical and economic feasibilities of hydrogen applied to automotive traction. The problems of mass storage and transportation of hydrogen, capillary distribution, storage aboard vehicles and those concerning hydrogen thermal engines and hydrogen fuel cells are investigated. The different ways of using hydrogen, either compressed or liquefied or combined in hydrides, are taken into account.Energy and economic balance sheets lead to the conclusion that hydrogen internal combustion engines cannot compete with gasoline engines with regard to primary energy consumption and fuel cost. To the contrary, a hydrogen fuel cell, thanks to its high efficiency, provides for appreciable energy saving and leads to a fuel expense of the same order of magnitude as premium gasoline in an urban vehicle. 相似文献
998.
Petit HV 《Journal of dairy science》2003,86(8):2637-2646
Forty midlactation Holstein cows averaging 635 kg of body weight (SE = 8) were allotted at wk 25 of lactation to ten groups of four cows blocked for similar calving dates to determine the effects of formaldehyde treatment of flaxseed and sunflower seed on fatty acid composition of blood and milk, milk yield, feed intake, and apparent digestibility. Cows were fed a total mixed diet based on grass silage and supplements for ad libitum intake over a 10-wk period. Cows within each block were assigned to one of the four isonitrogenous supplements based on either untreated whole flaxseed, formaldehyde-treated whole flaxseed, untreated whole sunflower seed, or formaldehyde-treated whole sunflower seed. Cows fed whole flaxseed compared with sunflower seed maintained greater dry matter (DM) intake (20.3 vs. 18.9 kg/d). Intake of DM, expressed as a percentage of body weight, was increased by adding formaldehyde to oilseeds (3.24 vs. 2.98%). Milk production was similar for cows fed flaxseed and those fed sunflower. Formaldehyde treatment of flaxseed and sunflower seed increased milk production by an average of 2.65 kg/d. Efficiency of fat-corrected milk yield per kilogram of DM intake was increased by formaldehyde treatment (1.31 vs. 1.21), and it was greater with sunflower seed than with flaxseed (1.33 vs.1.21). Protein concentration in milk was greater for cows fed flaxseed (3.38%) compared with those fed sunflower seed (3.21%) and formaldehyde had no effect. Apparent digestibility of DM was not affected by type of seed but it was greater for cows fed formaldehyde-treated seeds. Cows fed formaldehyde-treated flaxseed had the greatest apparent digestibilities of acid detergent and neutral detergent fiber compared with those fed the other diets. Apparent digestibilities of fatty acids were greater for sunflower seed than for flaxseed-based diets. In general, formaldehyde treatment had limited effect on milk fatty acid composition, suggesting that formaldehyde was not very effective in protecting polyunsaturated fatty acids against ruminal biohydrogenation. Feeding flaxseed resulted in the lowest omega 6 to omega 3 fatty acid ratio. The data suggest that both flaxseed and sunflower seed are acceptable fat sources for midlactating cows and that flaxseed increases milk protein percentage compared to sunflower seed. 相似文献
999.
This experiment was conducted to investigate the effect of feeding level and oilseed supplementation during the close-up dry period on energy balance (EB), hepatic lipidosis, metabolic status, and productivity in early lactation. Seventy-seven Holstein cows were blocked according to parity and expected calving date and then assigned randomly to 1 of 6 treatments in a 2 × 3 factorial allocation with 2 feeding levels: ad libitum (AL) or 30% feed restriction (FR), and 3 dietary fatty acid sources: canola seed, linola seed, or flaxseed at 8% of dietary dry matter (DM), to enrich the rations with oleic, linoleic, or linolenic acids, respectively during the last 4 wk of gestation. After parturition, all cows were fed a common lactation diet. Cows fed AL lost less body weight (−2.9 vs. −6.0%) and body condition score (+0.67 vs. −2.30%), and consequently were in more positive EB (+4.6 vs. −0.3 Mcal) during the prepartum period than cows subjected to FR. Postpartum, FR cows lost less body weight (−9.7 vs. −12.4%) and experienced less severe negative EB (−4.5 vs. −7.0 Mcal) than AL cows. Cows fed AL had higher plasma insulin (6.8 vs. 4.4 μIU/mL) and lower nonesterified fatty acid concentrations (436 vs. 570 mEq/mL) during the close-up period than cows subjected to FR. Cows fed AL tended to have lower liver glycogen content in early lactation than cows subjected to FR (4.4 vs. 2.9 μg/g of DM), but had similar triglyceride content (13.1 ± 1.2 μg/g of DM). Fatty acid source did not influence response variables. In conclusion, eliminating intake depression by FR during the close-up period had positive carryover effects on EB and metabolic status during early lactation, but feeding linoleic and linolenic acids via unprotected oilseeds only had negligible effects on peripartum responses. 相似文献
1000.
Xu T Sulerzycki J Nys JP Patriarche G Grandidier B Stiévenard D 《Nanoscale research letters》2011,6(1):113
We report the growth of Si and Ge nanowires (NWs) on a Si(111) surface by molecular beam epitaxy. While Si NWs grow perpendicular
to the surface, two types of growth axes are found for the Ge NWs. Structural studies of both types of NWs performed with
electron microscopies reveal a marked difference between the roughnesses of their respective sidewalls. As the investigation
of their length dependence on their diameter indicates that the growth of the NWs predominantly proceeds through the diffusion
of adatoms from the substrate up along the sidewalls, difference in the sidewall roughness qualitatively explains the length
variation measured between both types of NWs. The formation of atomically flat {111} sidewalls on the <110>-oriented Ge NWs
accounts for a larger diffusion length. 相似文献