全文获取类型
收费全文 | 103496篇 |
免费 | 9675篇 |
国内免费 | 7549篇 |
专业分类
电工技术 | 3943篇 |
技术理论 | 5篇 |
综合类 | 11767篇 |
化学工业 | 13804篇 |
金属工艺 | 6604篇 |
机械仪表 | 5445篇 |
建筑科学 | 27335篇 |
矿业工程 | 4398篇 |
能源动力 | 2549篇 |
轻工业 | 5401篇 |
水利工程 | 5344篇 |
石油天然气 | 4437篇 |
武器工业 | 895篇 |
无线电 | 5413篇 |
一般工业技术 | 10056篇 |
冶金工业 | 3795篇 |
原子能技术 | 661篇 |
自动化技术 | 8868篇 |
出版年
2024年 | 354篇 |
2023年 | 1374篇 |
2022年 | 2712篇 |
2021年 | 3185篇 |
2020年 | 3061篇 |
2019年 | 2568篇 |
2018年 | 2414篇 |
2017年 | 3028篇 |
2016年 | 3234篇 |
2015年 | 3495篇 |
2014年 | 6472篇 |
2013年 | 5360篇 |
2012年 | 7225篇 |
2011年 | 8006篇 |
2010年 | 6266篇 |
2009年 | 6718篇 |
2008年 | 6083篇 |
2007年 | 7313篇 |
2006年 | 6676篇 |
2005年 | 5949篇 |
2004年 | 4895篇 |
2003年 | 4284篇 |
2002年 | 3572篇 |
2001年 | 2948篇 |
2000年 | 2552篇 |
1999年 | 2113篇 |
1998年 | 1674篇 |
1997年 | 1392篇 |
1996年 | 1060篇 |
1995年 | 925篇 |
1994年 | 847篇 |
1993年 | 538篇 |
1992年 | 505篇 |
1991年 | 402篇 |
1990年 | 331篇 |
1989年 | 232篇 |
1988年 | 182篇 |
1987年 | 109篇 |
1986年 | 87篇 |
1985年 | 85篇 |
1984年 | 74篇 |
1983年 | 62篇 |
1982年 | 76篇 |
1981年 | 31篇 |
1980年 | 71篇 |
1979年 | 48篇 |
1978年 | 12篇 |
1975年 | 12篇 |
1959年 | 12篇 |
1951年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
951.
In this paper efficiency of applied fertilizers under tropical conditions is examined. Understanding of the fertilizer efficiency is particularly important for the developing countries mainly because there is need for enhancement of crop yields at a reduced cost. Many of the soil, plant and climatic factors of the tropical regions that contribute to the efficiency of the applied fertilizers have been discussed. Many of the tropical soils are acidic in nature and in these soils efficiency of applied fertilizer is relatively poor, mainly because plant roots are unable to grow and function to their fullest extent in utilizing the soils available nutrients. To enhance yield potentials there is need for understanding of interaction between crop species and soil and climatic variables. Incorporation and adoptation of a suitable application time can greatly enhance efficiency of urea form of nitrogen. Research findings in tropical soils have shown that an initial broadcast application of P and subsequent band treatment is more effective than either method of application alone. Current crop yields in tropical countries are far below the known yield potentials. Such low production potentials are attributed to the lack of suitable crop germplasms and understanding of improved agronomic practices. Intensification of research activities in fertilizer use efficiency in tropical countries is suggested.Senior author formerly was a Research Advisor to EMBRAPA/IICA/World Bank Program in Brazil. 相似文献
952.
薛泷辉 《水动力学研究与进展(A辑)》2017,(2):228-236
该文针对感潮河段魁岐排涝二站?型复杂流道的不利冲刷问题,通过建立整体水工模型,研究城市异形排涝通道消能防冲新技术,提出导流坎、消力坎和消力墩联合消能防冲技术,探索其在宽幅水位变化、多工程交替运行及通航情况下的适应能力,并给出水力设计方法。研究表明,联合消能工对复杂工况适应良好,能有效解决异形排涝通道的消能防冲问题。 相似文献
953.
An easy pot incubation method for measuring nitrogen mineralization from easily decomposable organic material under well defined conditions 总被引:1,自引:0,他引:1
K. Thorup-Kristensen 《Nutrient Cycling in Agroecosystems》1994,38(3):239-247
A pot incubation method for measuring mineralization dynamics from fresh plant material was tested. The aim was to develop a method which under well-defined conditions could produce mineralization data suited for estimating model input parameters for nitrogen prediction models. The results showed that the water tension of the soil could be controlled easily and precisely by diffusion through porous ceramic cups, and that nitrogen mineralization or immobilization could be measured already after 15 days at 15°C. The results showed that for the incubated catch crop residues carbon, nitrogen and nitrate-N contents were the most important factors determining mineralization. No significant effects ould be ascribed to other parameters measured. 相似文献
954.
针对向家坝地面电站,采用三维有限元法分析比较蜗壳不同埋设方式下的流道及其外围混凝土结构的静力特性,为蜗壳埋设方式的选择提供依据。研究表明:直埋方案所需配筋最多,不经济,且可能产生贯穿性裂缝;垫层方案应力普遍较小,垫层沿水流向敷设至蜗壳45°断面的组合方案仅局部区域有较大拉应力。推荐采用垫层埋设方式或垫层敷设至蜗壳45°断面的组合埋设方式,但同时应采取有效工程措施避免局部产生贯穿性裂缝。 相似文献
955.
956.
Groundnut Yield Response to Single Superphosphate, Calcitic Lime and Gypsum on Acid Granitic Sandy Soil 总被引:1,自引:0,他引:1
Phosphorus and calcium are the major nutrients limiting groundnut production. The objectives were to determine (a) optimum
application levels of P and Ca, and (b) compare the effectiveness of calcitic lime (40% Ca, 4.5% Mg) and gypsum (22% Ca, 17%
S) as sources of Ca for groundnut grown on sandy soils. Field experiments were established in smallholder farming areas using
four levels of P (0, 8.5, 17 and 34 kg ha−1) combined factorially with calcitic lime (0, 200, 400 and 800 kg ha−1) to give 16 treatments. Similar levels of P were combined factorially with gypsum (0, 100, 200 and 400 kg ha−1) to give sixteen treatments. Experiments were laid in a randomized complete block design with three replications. Phosphorus
had a significant effect on groundnut yield at the majority of the experimental sites. Application of P at 8.5 kg ha−1 gave the optimum groundnut yield response. The optimum application rates for calcitic lime and gypsum were 200 and 100 kg ha−1. Gypsum and calcitic lime were not significantly different as sources of Ca for groundnut. Soil chemical properties were
significantly improved following application of P and Ca sources. 相似文献
957.
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
958.
Permeability characteristics of carbonated concrete considering capillary pore structure 总被引:3,自引:0,他引:3
During carbonation process, the calcium phases present in cement are attacked by CO2 and converted into CaCO3 and the permeability of concrete is changing due to the change in porosity. The rate of carbonation depends upon porosity and moisture content of the concrete. Especially in underground reinforced concrete structures, the interior portion of concrete surface may be exposed to carbonation and the exterior portion of concrete surface exposed to wet soil or underground water. As carbonation proceeds from outer surface into internal portion of concrete, microstructure is also changed continuously from outer surface into internal portion of concrete. Even the deteriorations in the structures due to the carbonation have been reported more, research on permeability characteristics of concrete considering carbonation and micro-structural information is very scarce.In this study, the permeability coefficient in carbonated concrete is derived by applying a capillary pore structure formation model in carbonated cement mortar and assuming that aggregates do not affect carbonation process in early-aged concrete as a function of porosity. The permeability obtained from the micro-level modeling for carbonated concrete is verified with the results of accelerated carbonation test and water penetration test in cement mortar. 相似文献
959.
960.
Peter?S.?Piispanen Marcus?Persson Per?Claesson Torbj?rn?NorinEmail author 《Journal of surfactants and detergents》2004,7(2):147-159
Several novel and some previously known, mostly sugar-based, surfactants have been synthesized and some of their surface properties
have been characterized and compared with those of commercial nonylphenol ethoxylates. The surfactant solubility in water,
ethanol, and dodecane was studied. The properties of these compounds as emulsification agents in systems composed of the surfactant
with water/isopropyl myristate, water/rapeseed oil, and water/dodecane are presented. The aqueous solubility of the surfactants
follows the general trend expected from their hydrophilic-lipophilic balance according to Griffin (HLBG), but it is also clear
that the nature of the headgroup and the structure of the nonpolar part affect the solubility in a manner not aptured in the
standard HLBG concept. An ester or amine group as the connecting unit between the hydrophile and the hydrophobe produces a
more water-soluble surfactant than the corresponding amide derivative. Some effective emulsifiers were found. For instance,
the surfactants with a dehydroabietic nonpolar group appear to be promosing emulsifiers. Most sugar-based surfactants were
able to form macroemulsions of up to around 2 wt/vol% of oil. The stability of many of these amulsions was very high, extending
for months. 相似文献