全文获取类型
收费全文 | 8036篇 |
免费 | 573篇 |
国内免费 | 574篇 |
专业分类
电工技术 | 385篇 |
技术理论 | 11篇 |
综合类 | 952篇 |
化学工业 | 674篇 |
金属工艺 | 155篇 |
机械仪表 | 182篇 |
建筑科学 | 1575篇 |
矿业工程 | 397篇 |
能源动力 | 550篇 |
轻工业 | 619篇 |
水利工程 | 511篇 |
石油天然气 | 1452篇 |
武器工业 | 24篇 |
无线电 | 312篇 |
一般工业技术 | 392篇 |
冶金工业 | 432篇 |
原子能技术 | 213篇 |
自动化技术 | 347篇 |
出版年
2024年 | 21篇 |
2023年 | 78篇 |
2022年 | 149篇 |
2021年 | 182篇 |
2020年 | 233篇 |
2019年 | 315篇 |
2018年 | 244篇 |
2017年 | 220篇 |
2016年 | 249篇 |
2015年 | 276篇 |
2014年 | 406篇 |
2013年 | 430篇 |
2012年 | 492篇 |
2011年 | 661篇 |
2010年 | 551篇 |
2009年 | 488篇 |
2008年 | 431篇 |
2007年 | 480篇 |
2006年 | 515篇 |
2005年 | 501篇 |
2004年 | 395篇 |
2003年 | 351篇 |
2002年 | 293篇 |
2001年 | 269篇 |
2000年 | 269篇 |
1999年 | 163篇 |
1998年 | 99篇 |
1997年 | 89篇 |
1996年 | 75篇 |
1995年 | 59篇 |
1994年 | 64篇 |
1993年 | 32篇 |
1992年 | 24篇 |
1991年 | 24篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1980年 | 1篇 |
1959年 | 3篇 |
排序方式: 共有9183条查询结果,搜索用时 15 毫秒
51.
In recent years, the socio-economic impacts of winter extreme climate events have underscored the importance of winter climate anomalies in Southwest China (SWC). The spatio-temporal variability of surface air temperature (SAT) and precipitation in SWC and their possible causes have been investigated in this paper based on observational data from 1961 to 2010. The results indicate that SAT anomalies in SWC have two dominate modes, one is homogenous, and the other a zonal dipole. The former is caused by the anomalies of East Asian winter monsoon; the latter arises from the anomalies of both subtropical west Pacific high and regional cold air in lower troposphere. The most dominant mode of precipitation anomalies in SWC is homogenous and it has a high correlation with northern hemisphere annular mode (NAM, AO). Neither NAM nor ENSO has significant impacts on SAT in SWC.The anomalies of NAM are associated with the anomalies of tropical circulations, and there-fore precipitation over the SWC. When NAM is in positive (negative) phase; the winter pre-cipitation is more (less) than normal in SWC. Winter precipitation increase over the whole SWC is associated with the El Ni o. However, during La Nifia winter, the pattem is not uni-form. There is an increase in precipitation over the central parts and a decrease in western and eastern parts of SWC. The severe drought in SWC in winter 2010 is more, likely caused by anomalies of NAM, not EI Ni(n)o. 相似文献
52.
53.
现存的标准管理系统中可维护的只是标准的结构化信息,并不能反映标准在修订过程中的阶段信息,无法跟踪标准修订的历史。针对这些不足之处,提出一种符合CNONIX(China Online Information Exchange)标准维护需求、基于标准内容动态维护的解决方案。该方案首先将不同版本标准的内容分别以二叉树的结构组织起来。然后对两棵二叉树中对应节点的内容进行比对,针对节点中变化的属性进行兼容性测试。最后,将标准的修订历史和兼容性测试结果清晰地呈现给用户,从而更好地满足用户的需求。 相似文献
54.
以中铝集团有限责任公司PS版基在国内的营销为研究对象和研究内容,运用营销管理的理论和方法,在系统分析中铝公司PS版基产品国内营销环境的基础上,从理论和实际两个方面来探讨中铝公司在国内市场营销中面临的竞争和存在的问题,提出了中铝公司国内市场PS版基产品的营销策略。 相似文献
55.
针对刚变态的中国林蛙幼蛙雌雄不易鉴定的问题,本研究找出了幼蛙的固定、生殖腺的剖取、性腺组织切片的制作过程及性别鉴定方法。 相似文献
56.
Evaluating MODIS data for mapping wildlife habitat distribution 总被引:2,自引:0,他引:2
Andrs Via Scott Bearer Hemin Zhang Zhiyun Ouyang Jianguo Liu 《Remote sensing of environment》2008,112(5):2160-2169
Habitat distribution models have a long history in ecological research. With the development of geospatial information technology, including remote sensing, these models are now applied to an ever-increasing number of species, particularly those located in areas in which it is logistically difficult to collect habitat data in the field. Many habitat studies have used data acquired by multi-spectral sensor systems such as the Landsat Thematic Mapper (TM), due mostly to their availability and relatively high spatial resolution (30 m/pixel). The use of data collected by other sensor systems with lower spatial resolutions but high frequency of acquisitions has largely been neglected, due to the perception that such low spatial resolution data are too coarse for habitat mapping. In this study we compare two models using data from different satellite sensor systems for mapping the spatial distribution of giant panda habitat in Wolong Nature Reserve, China. The first one is a four-category scheme model based on combining forest cover (derived from a digital land cover classification of Landsat TM imagery acquired in June, 2001) with information on elevation and slope (derived from a digital elevation model obtained from topographic maps of the study area). The second model is based on the Ecological Niche Factor Analysis (ENFA) of a time series of weekly composites of WDRVI (Wide Dynamic Range Vegetation Index) images derived from MODIS (Moderate Resolution Imaging Spectroradiometer – 250 m/pixel) for 2001. A series of field plots was established in the reserve during the summer–autumn months of 2001–2003. The locations of the plots with panda feces were used to calibrate the ENFA model and to validate the results of both models. Results showed that the model using the seasonal variability of MODIS-WDRVI had a similar prediction success to that using Landsat TM and digital elevation model data, albeit having a coarser spatial resolution. This suggests that the phenological characterization of the land surface provides an appropriate environmental predictor for giant panda habitat mapping. Therefore, the information contained in remotely sensed data acquired with low spatial resolution but high frequency of acquisitions has considerable potential for mapping the habitat distribution of wildlife species. 相似文献
57.
58.
59.
利用华北地区90个气象站点1951~2013年逐日降水量资料,计算了8个极端降水指数,采用滑动平均、线性回归、Mann\|Kendall非参数检验和Morlet小波分析等方法,分析了华北地区极端降水事件的变化规律。结果表明:华北地区近63 a连续干旱日数(CDD)呈明显的增加趋势,连续5 d降雨量、连续湿日、年总降雨量趋于减少并通过0.05显著性检验,其他4个极端降水指数都表现为减少趋势且不显著。各极端降水指数与年总降水量(PRCPTOT)有很好的相关性,除连续干旱日数与之呈负的弱相关性以外,其他指数均呈较强的正相关性。在近63 a的时间尺度上各项极端降水指数都存在2~4 a显著周期,多数指数也存在8 a、16 a和32 a的周期。在时间转折上,连续湿日(CWD)的突变点时间较早,发生于1964年。从空间分布来看,京津冀地区及山西大部是极端降水指数变化最为显著的区域,连续干旱日数增加,其他指数均呈现减小趋势。
相似文献
60.
在中国二元经济结构的背景之下,从理论上探讨了纺织业优化要素配置的技术选择路径,通过对1985—2005年中国纺织业要素产出效率的实证分析,考察了.人均资本产出的演变轨迹,并从优化要素配置的角度提出了中国纺织业未来产业结构调整和技术进步的政策建议。 相似文献