全文获取类型
收费全文 | 27609篇 |
免费 | 2499篇 |
国内免费 | 927篇 |
专业分类
电工技术 | 1129篇 |
技术理论 | 1篇 |
综合类 | 2177篇 |
化学工业 | 7159篇 |
金属工艺 | 1173篇 |
机械仪表 | 461篇 |
建筑科学 | 3251篇 |
矿业工程 | 1657篇 |
能源动力 | 959篇 |
轻工业 | 4108篇 |
水利工程 | 757篇 |
石油天然气 | 1403篇 |
武器工业 | 51篇 |
无线电 | 797篇 |
一般工业技术 | 1787篇 |
冶金工业 | 1925篇 |
原子能技术 | 143篇 |
自动化技术 | 2097篇 |
出版年
2024年 | 114篇 |
2023年 | 335篇 |
2022年 | 742篇 |
2021年 | 863篇 |
2020年 | 890篇 |
2019年 | 798篇 |
2018年 | 777篇 |
2017年 | 933篇 |
2016年 | 1008篇 |
2015年 | 1052篇 |
2014年 | 1752篇 |
2013年 | 1716篇 |
2012年 | 2026篇 |
2011年 | 2108篇 |
2010年 | 1539篇 |
2009年 | 1586篇 |
2008年 | 1298篇 |
2007年 | 1739篇 |
2006年 | 1541篇 |
2005年 | 1310篇 |
2004年 | 1220篇 |
2003年 | 1051篇 |
2002年 | 855篇 |
2001年 | 646篇 |
2000年 | 588篇 |
1999年 | 470篇 |
1998年 | 357篇 |
1997年 | 297篇 |
1996年 | 241篇 |
1995年 | 220篇 |
1994年 | 175篇 |
1993年 | 135篇 |
1992年 | 113篇 |
1991年 | 96篇 |
1990年 | 70篇 |
1989年 | 63篇 |
1988年 | 52篇 |
1987年 | 45篇 |
1986年 | 30篇 |
1985年 | 31篇 |
1984年 | 28篇 |
1983年 | 13篇 |
1982年 | 21篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1964年 | 4篇 |
1951年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
221.
Transport of phosphorus forms and of nitrate through a clay soil under grass and cereal production 总被引:1,自引:0,他引:1
Many nutrients are lost from soil to water viatile drains. However, there are very few reliable studies of such phosphorus(P)losses under Swedish agriculture practices, especially in connection tofertiliser and slurry applications and related to nutrient balances. Tile drainlosses were measured from nine experimental plots in south-west Sweden; fourplots were measured for 7 years and five plots for 2–3 years. Cereals,mainly spring barley (Hordeum vulgare, L.) and oats(Avena sativa, L.), were grown in six plots, while oneplotwas cultivated with grass, timothy (Phleum pratense L.)andmeadow fescue (Festuca pratensis, L.), one with lucerne(Medicago sativa, L.) and meadow fescue (F.pratensis, L.), and the last one was a set-aside withgrass (Lolium perenne, L. and Trifoliumrepens, L.) that was neither fertilised nor used for crop removal.Attention was paid to the forms in which P was transported to water since thishas important ecological implications. Average losses of particulate P (PP)fromthe set-aside land was significantly lower than from the cerealproduction, but the average losses of soluble reactive P (SRP) and dissolvednonreactive P (DUP) were the same. Average loss of PP from the grass ley during 3years (0.09 kg per ha–1 y–1) wassignificantly less than the losses from cereal production. Total P loss of 0.3kg per ha–1 y–1 was equal to the averagelossfrom the Swedish monitoring network of observation fields. Compared to theaverage nitrate-nitrogen (NO3-N) leaching, which was 7 kg perha–1 y–1 from cereals, the NO3-Nleaching was very low from the plots with lucerne-grass and set-aside.After applying and directly ploughing in 80 kg ha–1 ofsuperphosphate for cereal production in November 1998, 0.6–1.8 kgSRP ha–1 y–1 was lost through the drain tiles.Surface application of cattle slurry in spring 1999 to the grass ley alsoresulted in a very pronounced increased base level of SRP. This paper documentsthe importance of applying slurry during dry conditions and of placingfertiliser into the soil according to crop requirements in the time perspectiveof one year. 相似文献
222.
223.
224.
Effects of N application and water supply on yield, oil content and N accumulation by canola, cultivar Marnoo, grown on a heavy clay soil in the Goulburn Murray Irrigation Region were investigated. Treatments were rainfed (Rf) or watered at a deficit of 50 mm (40–60 mm, I50) beginning in the spring. N treatments were 0, 50, 100 or 200 kg N ha–1 at sowing or as split applications of 20/80, and 50/50 kg N ha–1 at sowing and rosette, respectively.Yield (Yg) ranged from 170 to 520 g m–2. Irrigation and N increased yield in both years. Grain yields were increased by N application on the irrigated treatments when 100 or 200 kg N ha–1 was applied. Oil concentrations ranged from a maximum of 46.4% in treatment N0 to a minimum of 40.6% in treatment N200 and was inversely related to seed N concentration. Although fertilizer N decreased oil concentration, it increased the yield of oil.Nitrogen accumulation (Nb) limited yield of all treatments and was described by the equation, Yg = 806[1-EXP(–0.039*Nb)]. This implied a decrease in yield per unit of Nb at the higher rates of fertilizer addition with consequent increases in grain N concentration.The efficiency of water use in the production of grain (WUEg) and biomass (WUEb) were 7.5 and 23 kg ha–1 mm–1 respectively. Nitrogen additions increased WUEg and WUEb in both seasons. Maximum values of 8.9 (WUEg 1986) and 26.8 (WUEb 1987) were measured from treatment N200. These data suggest that the crops made efficient use of the applied water. 相似文献
225.
The white pine cone beetle, Conophthorus coniperda, exhibited dose and enantiospecific responses to -pinene in stands of mature eastern white pine, Pinus strobus, in a seed orchard near Murphy, North Carolina, USA. (–)--Pinene significantly increased catches of cone beetles to traps baited with (± )-trans-pityol. (+)--Pinene did not increase catches of beetles to pityol-baited traps and interrupted the response of beetles to traps baited with (±)-trans-pityol and (–)--pinene. Maximal attraction of cone beetles to pityol-baited traps was obtained with lures releasing (–)--pinene at a rate of 103 mg/day at 23°C. Lures releasing (–)--pinene at rates lower or higher than 103 mg/day resulted in reduced catches to traps baited with (±)-trans-pityol. The sex ratio in all catches was heavily male biased. Attraction of the clerid predator, Thanasimus dubius, to traps baited with (±)-trans-pityol increased significantly with the presence of -pinene, irrespective of enantiomeric composition. Maximal attraction of T. dubius to pityol-baited traps occurred with devices releasing (–)--pinene at the highest rate tested, 579 mg/d at 23°C, a sub optimal rate for cone beetles. 相似文献
226.
The insecticidal nonprotein amino acid S-(-carboxyethyl)-cysteine (S-CEC) is the major free amino acid in the seeds of severalCalliandra spp. where it accounts for up to 2.9 % of dry weight. Lesser amounts of other related S-containing amino acids and an array of nonprotein imino acids derived from pipecolic acid are other constituents. High concentrations of imino acids, which also show insecticidal activity, are maintained in the mature leaves, but sulfur compounds are lacking. In this study the disappearance of S-CEC from the germinating seeds and young seedlings ofC. rubescens was monitored over time. After 10 weeks, S-CEC continues to be found in high concentrations in the stems and new leaves. As young leaves mature, sulfur compounds quickly decrease in concentration. Traces of S-CEC are found in new leaves of plants up to nine months after germination. Whether high concentration of S-CEC in young leaves is due to transportation from the seed or de novo synthesis is unclear. The ecological implications are discussed. 相似文献
227.
Analysis of fly ash heavy metal content and disposal in three thermal power plants in India 总被引:2,自引:0,他引:2
The study investigates the heavy metal content of fly ash and bottom ash from three major power plants in North India, using flame atomic absorption spectrometry. It also studies the prevalent disposal methods used at these sites. The ashes were analysed for the presence of Cr, Mn, Pb, Zn, Cu, Ni and Co and detectable levels of all were found in both fly ash and bottom ash. The concentrations of Cr and Zn were highest while Co concentration was less. The wet disposal method is used in two of the power plants (site 1 and site 3). Neither of the sites uses ash pond lining in the construction of the ash ponds, hence leaching of the heavy metals is possible. Site 2 has adopted 100% dry disposal system which allows better utilization but incurs additional costs. Better management practices, increased utilization and proper disposal practices need to be undertaken to minimize the adverse environmental impact. 相似文献
228.
One major constraint of the agricultural uses of fly ash (FA) is the low availability of different plant nutrients despite their high occurrence in the total amount. However, degrading FA through increased microbial activity can improve the availability of these nutrients substantially. It has been found that intestines of epigeic earthworms contain a high concentration of different microorganisms. Therefore, in the present study we addressed the effects of vermicomposting technology on the solubility of some micronutrient cations (Fe, Mn, Cu, and Zn) and some heavy metals (Pb, Cd, and Cr) in different combinations of fly ash and organic matter, applied in the form of cow dung (CD). Various combinations of FA and CD were treated with and without an epigeic earthworm (Eisenia fetida) and the solubility of different trace elements in the treatments were estimated periodically. The results revealed that the inclusion of epigeic earthworm Eisenia fetida in different combinations of fly ash and cow dung converted a considerable amount of the micronutrients into bio-available forms. On the other hand, the solubility of heavy metals tended to be reduced by the microorganisms, presumably by formation of some organo-metallic complex. Application of these vermicomposted FA and CD combinations to a red lateritic soil was found to improve the soluble Fe, Mn, Cu, and Zn status of the soil. Furthermore, the use of vermicomposted FA and CD (1:1) in potato cultivation demonstrated that use of this mixture at 10 ton per hectare (t ha−1; fresh weight) was able to compensate 80% of the recommended NPK fertiliser, along with farm yard manure application, without compromising the crop yield. 相似文献
229.
The effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample has been investigated. The samples were tested for mercury adsorption using a fixed‐bed with a simulated flue gas. The activated fly ash carbon sample has lower mercury capacity than its precursor fly ash carbon (0.23 vs. 1.85 mg/g), although its surface area is around 15 times larger, 863 vs. 53 m2/g. It was found that oxygen functionality and the presence of halogen species on the surface of fly ash carbons may promote mercury adsorption, while the surface area does not seem to have a significant effect on their mercury capacity. 相似文献
230.
Two kinds of fly ash, discharged in the combustion of either refused derived fuel (RDF) or car shredder dust (SD), were examined for the emission of heavy metals in melting process under oxidizing and reducing conditions. The residual fractions of heavy metal in slag were experimentally estimated. As a result, it was confirmed that several volatile heavy metals were readily emitted during melting process. The type of atmosphere provided for the melting process was found to affect the emission of some volatile metals in RDF ash, but not in SD ash. The emission of volatile heavy metals in RDF ash under oxidizing conditions was lower than under any other conditions in this study. The emission behavior of iron and heavy metals in RDF ash under reducing conditions was similar to that in SD ash. These facts indicated that phosphorous in RDF ash had the property of fixing the volatile metals in the slag only under oxidizing conditions. Then the mixture of SD ash with phosphorous oxide powder was also tested in a melting process, and the result was consistent with the above inference of the effect of phosphorous. 相似文献