全文获取类型
收费全文 | 2713篇 |
免费 | 124篇 |
国内免费 | 135篇 |
专业分类
系统科学 | 172篇 |
丛书文集 | 62篇 |
教育与普及 | 40篇 |
理论与方法论 | 12篇 |
现状及发展 | 13篇 |
研究方法 | 9篇 |
综合类 | 2664篇 |
出版年
2024年 | 17篇 |
2023年 | 29篇 |
2022年 | 42篇 |
2021年 | 67篇 |
2020年 | 42篇 |
2019年 | 17篇 |
2018年 | 34篇 |
2017年 | 53篇 |
2016年 | 43篇 |
2015年 | 84篇 |
2014年 | 124篇 |
2013年 | 82篇 |
2012年 | 102篇 |
2011年 | 142篇 |
2010年 | 107篇 |
2009年 | 98篇 |
2008年 | 132篇 |
2007年 | 131篇 |
2006年 | 111篇 |
2005年 | 124篇 |
2004年 | 96篇 |
2003年 | 95篇 |
2002年 | 79篇 |
2001年 | 90篇 |
2000年 | 85篇 |
1999年 | 140篇 |
1998年 | 100篇 |
1997年 | 93篇 |
1996年 | 111篇 |
1995年 | 92篇 |
1994年 | 77篇 |
1993年 | 60篇 |
1992年 | 70篇 |
1991年 | 61篇 |
1990年 | 45篇 |
1989年 | 34篇 |
1988年 | 37篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 1篇 |
1981年 | 3篇 |
排序方式: 共有2972条查询结果,搜索用时 15 毫秒
181.
<正>To detect the deformation of the tunnel structure based on image sensor networks is the advanced study and application of spatial sensor technology.For the vertical settlement of metro tunnel caused by internal and external stress after its long period operation,the overall scheme and measuring principle of tunnel deformation detection system is introduced. The image data acquisition and processing of detection target are achieved by the cooperative work of image sensor,ARM embedded system.RS485 communication achieves the data transmission between ARM memory and host computer.The database system in station platform analyses the detection data and obtains the deformation state of tunnel inner wall,which makes it possible to early-warn the tunnel deformation and take preventive measures in time. 相似文献
182.
变压器的安装调试运行必须遵循施工顺序和操作方法,达到现行的国家标准和验收规范的规定,方可运行生产.本文主要探讨变压器安装和调试应注意的问题. 相似文献
183.
Performance of wireless mesh networks can be greatly improved by spatial reuse (SR), therefore, lots of algorithms have been
proposed to maximize the number of concurrent transmissions. However, most methods only considered the interference brought
by the sender nodes. In this paper, we study the interference model in WiMAX mesh networks and propose a minimal SR-pair interference
model to perform concurrent transmissions in both uplink and downlink directions. Simulation results show that our model is
more efficient and stable, i.e., it increases the number of concurrent transmissions and its performance is constant and independent
of transmission directions.
Biography: XIONG Qing(1978–), male, Ph. D., research direction: wireless mesh networks. 相似文献
184.
Yin-gang Zhang Zheng-ming Sun Jiang-tao Liu Shi-jie Wang Feng-ling Ren Xiong Guo 《浙江大学学报(自然科学英文版)》2009,10(7):522-527
Objective: The age-related change is important part of degenerative disc disease. However, no appropriate animal model or objective evaluation index is available. This study aimed to investigate the features of intervertebral disc degeneration in aging process of rats. Methods: 22-month-old Sprague-Dawley (SD) rats were used as spontaneously occurring intervertebral disc degeneration models and 6-month-old rats as young controls. Expression of collagen types II and X was measured by immunohistochemistry. Degenerations of intervertebral discs were scored according to Miyamoto’s method. Numbers and areas of afferent vascular buds were measured. The thicknesses of non-calcified and calcified layers were measured and statistically analyzed. Results: There were less collagen type II expression and more collagen type X expression in the calcified layer of the cartilage endplates and nucleus pulposus in the rats of the aged group than in the young control. There were fewer and smaller afferent vascular buds in the rats of the aged group than in the young control group. The ratio of the non-calcified to the calcified layers in the rats of the aged group significantly decreased, compared with that of the young control group (P<0.01). Conclusion: Rats can spontaneously establish intervertebral disc age-related degeneration. The expression of collagen types II and X, numbers and areas of afferent vascular buds, the ratio of the non-calcified to the calcified layers, and water and glycosaminoglycan contents in the nucleus pulposus are sensitive indexes of intervertebral disc degeneration. 相似文献
185.
186.
187.
Sikkema-Kantorovitch算子在B_α空间的逼近 总被引:1,自引:0,他引:1
研究了Sikkema-kantorovitch算子在Bα空间的逼近估计. 相似文献
188.
Yongnian Yan Shengjie Li Renji Zhang Feng Lin Rendong Wu Qingping Lu Zhuo Xiong Xiaohong Wang 《清华大学学报》2009
The rapid prototyping and manufacturing technology (RPM), is an integration of many different disciplines. It is based on an advanced dispersed-accumulated forming principle and originated from 1980s. It generates an entity by first forming a series of layers according to the dispersed section information of the digital model, and then piling the formed layers sequentially together. It is capable of forming parts with complicated structures and non-homogeneous materials. Traditional RPM techniques are mainly used as prototypes in product invention process, such as stereolithography, three-dimensional printing, laminated object manufacturing, and fused deposition modeling. Later, with the progress of material and enabling technology, many new RPM techniques emerged out and have been already applied in the fields such as rapid tooling/moulding, direct formed usable part, nano-/micro-RPM, and biomanufacturing. This high flexible digital manufacturing method has a likely ability to become an almighty forming technology. 相似文献
189.
利用地质、测井和地震等多种资料进行的综合研究表明,东濮凹陷桥口和新霍地区沙三段中发育轴向重力流水道沉积,它们主要由灰黑色块状泥岩与平行层理砂岩、块状层理砂岩、递变层理砂岩及合卵石砂岩构成,在垂向上组合成鲍玛序列的AE、BE、ACE、BCE和CE等。依据研究区轴向重力流水道沉积特征,将其划分成近源和远源水道、近源和远源水道侧源等4个微相,并描述了这4个微相的沉积特征,建立了沉积模式。油气勘探实践表明,重力流水道砂体是我国东部陆相断陷湖盆中重要的油气勘探新目标,它具有良好的储集物性,并含有丰富的油气。 相似文献
190.
应用高效液相色谱(HPLC)--荧光检测,在一根带有预柱的LKB Superpac ODSC_(18)钢柱上,使用离子对系统做为洗脱液,对一组正常活动规律的树鼩脑组织液中的去甲肾上腺素(NE)、多巴胺(DA)、5-羟色胺(5-HT)及色氨酸(TRP)进行测定。荧光检测的波长为激发波长280nm,发射波长360nm得到各物质最小检测限量为:40ng NE和80ngDA(信噪比3:1)以及10ng5-HT和15ng TRP(信噪比4:1)。此方法具有实验操作简单、方便、快速、灵敏度较高、样品易于处理等特点。 相似文献