全文获取类型
收费全文 | 43761篇 |
免费 | 4787篇 |
国内免费 | 3949篇 |
专业分类
电工技术 | 1651篇 |
技术理论 | 4篇 |
综合类 | 5680篇 |
化学工业 | 3576篇 |
金属工艺 | 1390篇 |
机械仪表 | 1805篇 |
建筑科学 | 15323篇 |
矿业工程 | 2367篇 |
能源动力 | 1172篇 |
轻工业 | 1714篇 |
水利工程 | 4725篇 |
石油天然气 | 1239篇 |
武器工业 | 331篇 |
无线电 | 2323篇 |
一般工业技术 | 2617篇 |
冶金工业 | 1597篇 |
原子能技术 | 335篇 |
自动化技术 | 4648篇 |
出版年
2024年 | 189篇 |
2023年 | 601篇 |
2022年 | 1147篇 |
2021年 | 1439篇 |
2020年 | 1405篇 |
2019年 | 1234篇 |
2018年 | 1126篇 |
2017年 | 1503篇 |
2016年 | 1594篇 |
2015年 | 1691篇 |
2014年 | 2895篇 |
2013年 | 2381篇 |
2012年 | 2893篇 |
2011年 | 3328篇 |
2010年 | 2705篇 |
2009年 | 2837篇 |
2008年 | 2668篇 |
2007年 | 3352篇 |
2006年 | 3031篇 |
2005年 | 2696篇 |
2004年 | 2269篇 |
2003年 | 1929篇 |
2002年 | 1592篇 |
2001年 | 1274篇 |
2000年 | 1038篇 |
1999年 | 818篇 |
1998年 | 619篇 |
1997年 | 462篇 |
1996年 | 375篇 |
1995年 | 290篇 |
1994年 | 239篇 |
1993年 | 153篇 |
1992年 | 138篇 |
1991年 | 97篇 |
1990年 | 86篇 |
1989年 | 62篇 |
1988年 | 59篇 |
1987年 | 46篇 |
1986年 | 38篇 |
1985年 | 28篇 |
1984年 | 30篇 |
1983年 | 18篇 |
1982年 | 15篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 42篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1959年 | 6篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Modeling and assimilation of root zone soil moisture using remote sensing observations in Walnut Gulch Watershed during SMEX04 总被引:1,自引:0,他引:1
Soil moisture status in the root zone is an important component of the water cycle at all spatial scales (e.g., point, field, catchment, watershed, and region). In this study, the spatio-temporal evolution of root zone soil moisture of the Walnut Gulch Experimental Watershed (WGEW) in Arizona was investigated during the Soil Moisture Experiment 2004 (SMEX04). Root zone soil moisture was estimated via assimilation of aircraft-based remotely sensed surface soil moisture into a distributed Soil-Water-Atmosphere-Plant (SWAP) model. An ensemble square root filter (EnSRF) based on a Kalman filtering scheme was used for assimilating the aircraft-based soil moisture observations at a spatial resolution of 800 m × 800 m. The SWAP model inputs were derived from the SSURGO soil database, LAI (Leaf Area Index) data from SMEX04 database, and data from meteorological stations/rain gauges at the WGEW. Model predictions are presented in terms of temporal evolution of soil moisture probability density function at various depths across the WGEW. The assimilation of the remotely sensed surface soil moisture observations had limited influence on the profile soil moisture. More specifically, root zone soil moisture depended mostly on the soil type. Modeled soil moisture profile estimates were compared to field measurements made periodically during the experiment at the ground based soil moisture stations in the watershed. Comparisons showed that the ground-based soil moisture observations at various depths were within ± 1 standard deviation of the modeled profile soil moisture. Density plots of root zone soil moisture at various depths in the WGEW exhibited multi-modal variations due to the uneven distribution of precipitation and the heterogeneity of soil types and soil layers across the watershed. 相似文献
2.
卜寺沟水电站预可阶段正常蓄水位是考虑规划成果、地形、地质务件、梯级衔接、水库淹没、筑坝技术、水能资源利用等因素综合初选的.从理论角度分析,水库淹没时水库内寺庙的影响,按全部淹没进行经济补偿处理代价较小;其社会环境敏感因素对正常蓄水位的选择不应起决定性作用.但是,考虑到少数民族群众、僧侣和地方政府的要求,适当降低正常蓄水位,以减少对扎西日岗寺庙的影响,将带来较积极的社会影响. 相似文献
3.
用连续分级提取和反复多次酸提取方法研究了长期土壤酸化对红壤固相铝库中铝含量及亏损程度的影响。研究结果表明:嵊县红壤的活性铝库大于永春红壤和屯溪红壤;因永春红壤和屯溪红壤的酸化程度大于嵊县红壤,前两者的有机铝库和无机铝库都比嵊县红壤的亏损,因此反复酸提取过程中铝的释放量也比嵊县红壤少;当高强度酸输入土壤后,弱键合的有机络合态铝可快速活化并亏损,剩余铝库因活性小而释放速率减小,但长期酸化过程中,动力学控制的低活性铝库的活化可能对铝的溶解量仍有重要贡献。 相似文献
4.
5.
Huai Wen-xin Wuhan University of Hydraulic & Electric Engineering Wuhan Hubei P.R.China Prof.Li Wei 《水动力学研究与进展(B辑)》1993,(3)
Turbulence represents an essential aspect in atmospheric and oceanic circulations.In particu-lar,it is a preponderant factor in the dispersion of pollutants of artificial or natural origin.Thevertical turbulent jets,plumes and buoyant jets discharging into static environment is one of themost important and basic flow patterns related with the environment pollution.These flows havenumerous important applications,for example,thrust augmentors,waste disposal plumes fromstacks and combustion systems,and significant problems of turbulent diffusion. 相似文献
6.
The authors highlight several possible ways in which the construction industry could lower present emissions of the greenhouse gas, carbon dioxide. It is an excellent discussion document and is recommended reading. 相似文献
7.
C-VISION中文窗口系统综合采用了面向对象与窗口环境设计的程序方法,彻底更新了在微机上进行中文窗口设计的编程思想,同时作为一个基础系统,为软件设计节约了大量的人力和时间,实现了微机程序的共享。 相似文献
8.
涂珍梅 《数字社区&智能家居》2006,(11)
中国的网络远程教育目前也正处于蓬勃发展的阶段,各种远教机构如雨后春笋般涌现。但在远教课程向网络转型的过渡时期,难免会有各种不尽如人意的地方。设计不够贴近学生、缺乏师生互动和技术运用的片面与僵化等是很多网络课程常有的通病。如何在课程设计上进行改进,使网络课程更具人性化,真正实现以学生为中心,是中国网络教育更上一层楼的关键问题之一。 相似文献
9.
Aboubakar Sako Kenneth G. MacLeod Catherine M. O’Reilly 《Journal of Great Lakes research》2007,33(4):806-815
In this study, we measured growth trends in oxygen and carbon isotopic ratios in whole sagittal otoliths from three adult centropomid fish (Lates stappersii) from each of three sub-basins of Lake Tanganyika, East Africa. Sampling density was 20 to 50 samples per otolith. Both δ18O and δ13C values increase with age. The δ18O data suggest that otoliths were precipitated near the expected equilibrium with the ambient environment (ca. +3.5‰) and support a migration pattern from surface waters during larval stages to deeper waters (40 to 80 m) for mature fish. Relatively high δ18O values in the southern sub-basin are consistent with cooler temperatures in the region during seasonal upwelling. The δ13C increase from otolith core to edge is large (up to 4‰) and is interpreted as due to ontogenetic changes in diet and contributions from a decrease in the proportion of respired CO2 incorporated into otolith carbonate as metabolic rates of the fish dropped with maturity. The data seem to successfully reveal life strategy and migration patterns of L. stappersii, document regional differences in lake conditions, and provide a record of temperature within the water column during which the fish lived. Higher resolution studies and analyses of historical samples could be used to constrain modern and past growth patterns, and to reconstruct past temperature gradients and productivity patterns in the lake. 相似文献
10.
袁永强 《有色冶金设计与研究》2003,24(3):39-42
结合工程实例,对采用传统的短木桩加新兴的土工合成材料——土工网及碎石垫层构成复合基础,处理河岸挡墙软基进行探讨,该方法在软土层厚度不超过4.0m的条件下,技术可靠,施工简单,工期短,造价低。 相似文献