全文获取类型
收费全文 | 12124篇 |
免费 | 1548篇 |
国内免费 | 364篇 |
专业分类
电工技术 | 36篇 |
综合类 | 639篇 |
化学工业 | 4342篇 |
金属工艺 | 530篇 |
机械仪表 | 76篇 |
建筑科学 | 202篇 |
矿业工程 | 1936篇 |
能源动力 | 203篇 |
轻工业 | 1665篇 |
水利工程 | 58篇 |
石油天然气 | 461篇 |
武器工业 | 6篇 |
无线电 | 118篇 |
一般工业技术 | 653篇 |
冶金工业 | 2931篇 |
原子能技术 | 102篇 |
自动化技术 | 78篇 |
出版年
2024年 | 78篇 |
2023年 | 200篇 |
2022年 | 335篇 |
2021年 | 428篇 |
2020年 | 391篇 |
2019年 | 359篇 |
2018年 | 298篇 |
2017年 | 403篇 |
2016年 | 503篇 |
2015年 | 456篇 |
2014年 | 656篇 |
2013年 | 728篇 |
2012年 | 837篇 |
2011年 | 865篇 |
2010年 | 613篇 |
2009年 | 630篇 |
2008年 | 549篇 |
2007年 | 735篇 |
2006年 | 731篇 |
2005年 | 589篇 |
2004年 | 551篇 |
2003年 | 515篇 |
2002年 | 433篇 |
2001年 | 385篇 |
2000年 | 313篇 |
1999年 | 288篇 |
1998年 | 207篇 |
1997年 | 178篇 |
1996年 | 124篇 |
1995年 | 116篇 |
1994年 | 86篇 |
1993年 | 87篇 |
1992年 | 67篇 |
1991年 | 68篇 |
1990年 | 52篇 |
1989年 | 42篇 |
1988年 | 35篇 |
1987年 | 25篇 |
1986年 | 15篇 |
1985年 | 12篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1974年 | 4篇 |
1968年 | 2篇 |
1964年 | 2篇 |
1951年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
Nitrate leaching as affected by cropping system/crop rotation, history of farmyard manure application or fertilizer nitrogen application (0 N, 0.5 N and 1 N) was studied at nine sites on loamy soils during 1986/87, 1987/88 and 1988/89. Soil solution from 80 to 90 cm soil depth was sampled every second week in the period November to May by the use of porous ceramic cups and analysed for NO3-N and Cl. Climatical conditions were obtained from standard meteorological observations in the region. Drainage from soil profiles was calculated from measured and simulated values of precipitation and actual evapotranspiration, respectively.The results show that type of crop is of the utmost importance for the leaching magnitude of nitrate as 40% of the total variance in nitrate concentrations in the soil solution could be explained by the type of crop.The second factor of importance was the history of farmyard manure (FYM) application, which was able to explain 28% of the total variation in nitrate concentration in the soil solution. Nitrate concentration/leaching from arable land without FYM ever being applied was considerably lower than from arable land which received periodical FYM applications until the early 70's or from arable land which besides periodical FYM applications in the past presently still receives regular applications of FYM. Only about 1% of the total variation in nitrate concentration in the soil solution was explainable by the level of fertilizer nitrogen application.Differences between years explained 14% of the total variation in nitrate concentration in the soil solution due to differences between the years in soil temperatures and water run-off. The run-off during the autumn and winter of 1986/87, 1987/88 and 1988/89 was 95, 275 and 55 mm, respectively. As expected nitrate leaching increased with increasing run off. However, nitrate leaching at the 275 mm run off was considerably lower than expected, which seems explainable by a substantial denitrification. The soil at the sites in question seems thus partly to purify the soil solution for nitrate before it leaves the root zone at the observed high run off conditions. 相似文献
132.
In order to supplement the literature data on VLE, isobaric bubble temperature data were collected for the binary systems phenol-i-amyl alcohol and phenol-n-amyl alcohol over the entire composition range by indirect method using a standard Swietoslawski type ebulliometer. The measurements were made at three sub-atmospheric pressures. The experimental t-x data were correlated with both equation of state method and gamma-phi methods. In the former approach, the Soave equation of state is used, whereas in the latter case Wilson, NRTL and UNIQUAC equations are used. 相似文献
133.
Acrylonitrile butadiene rubber (NBR) was used as a polymeric plasticizer in poly(vinyl chloride) (PVC) for the partial replacement of di(2‐ethyl hexyl)phthalate (DEHP). The effect of this partial replacement on DEHP leaching from the PVC was evaluated at 10, 25, and 40°C. The study shows that the incorporation of NBR reduces the rate of DEHP leaching, the reduction being prominent at lower temperatures. Gas permeability of the NBR‐modified samples was also evaluated at the above temperatures using oxygen and carbon dioxide. A reduction in gas permeability is observed in NBR‐modified samples compared to the PVC plasticized with DEHP alone particularly in the case of carbon dioxide. Water vapor transmission rates of the NBR‐modified samples are higher than that of the control sample. The NBR‐modified PVC sample was found to be noncytotoxic in the in vitro cytotoxic evaluation both by direct contact and test on extract methods. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:4720–4727, 2006 相似文献
134.
135.
采用半连续乳液聚合的方法合成聚醋酸乙烯乳液。通过对乳液的粒子形态、压缩剪切强度、粘度、冻融稳定性的分析,讨论了不同性质的聚乙烯醇作为保护胶体,对聚醋酸乙烯乳液胶粘剂性能的影响。 相似文献
136.
维生素C催化合成乳酸异戊酯 总被引:2,自引:0,他引:2
首次利用维生素C催化乳酸和异戊醇合成了乳酸异戊酯。在0.1mol乳酸,0.2mol异戊醇和0.2g维生素C作用下,回流分水60min,酯收率达60.0%。结果表明:维生素C能够代替硫酸作为乳酸酯化的催化剂。 相似文献
137.
采用共沉淀法制得物质的量比为n(Cu2+)∶n(Mg2+)∶n(Al3+)=10∶65∶25的CuMgAl类水滑石前体(CMA-HT),经过不同温度焙烧制得CuMgAl水滑石催化剂。通过傅里叶变换红外光谱(FTIR)、热重分析(TG)、X射线粉末衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、N2-物理吸附、程序升温还原(H2-TPR)、H2-程序升温脱附(H2-TPD)、CO2-程序升温脱附(CO2-TPD)和NH3-程序升温脱附(NH3-TPD)等对催化剂的结构进行表征,在高压反应釜中考察了CuMgAl水滑石催化剂催化糠醇(FFA)加氢制1,2-戊二醇(1,2-PeD)和1,5-戊二醇(1,5-PeD)的催化性能。研究结果表明,焙烧温度对催化剂的结构及其催化性能具有显著的影响,金属活性中心和碱性位随焙烧温度的升高先增加后减少。经600℃焙烧的CMA催化剂表面存在适宜的金属中心和碱性位,在金属位和碱性中心的协同催化下,表现出了优异的催化性能。在140℃,H2压力为4 MPa的条件下,反应8 h,糠醇的转化率和戊二醇的收率分别达74.13%和58.36%。 相似文献
138.
139.
The Peel-Harvey estuary on the Swan Coastal Plain, Western Australia has become eutrophic partly because of the leaching of fertiliser phosphorus from sandy soils. The acid, coarse textured sandy soils are predominantly quartz, have a low iron and aluminium content and do not retain phosphorus. Red mud, derived from bauxite, is a by-product of the alumina industry and has the ability to retain phosphorus. Retention of phosphorus is enhanced when the red mud is neutralised with gypsum. Red mud has been suggested as a soil amendment to reduce phosphorus leaching.To investigate the reduction in the leaching of phosphorus from soils amended with red mud, weirs were constructed at the outlets from a pair of catchments to quantify the amount of phosphorus in the streamflow. Both catchments were deep grey Bassendean sand. One of the catchments was treated with 80 t/ha of red mud which had been neutralised with waste gypsum from the phosphate industry. The red mud was applied to the soil surface using conventional fertiliser spreading equipment. The other catchment was untreated.The red mud reduced phosphorus loss by 70% from 13.8 kg/ha on the untreated catchment, to 4.2 kg/ha on the treated catchment. Both catchments were treated with 20.4 kg/ha of phosphorus as superphosphate. The catchment treated with red mud also received a further 41.5 kg/ha of phosphorus from the phosphogypsum that was used to neutralise the red mud.Our results show that red mud reduces phosphorus leaching and is potentially a nutrient management option in sandy soils. Red mud has the potential to reduce the impact of agriculture on the estuarine environment and has implications for the continued expansion and intensification of agriculture in the Peel-Harvey catchment. 相似文献
140.