首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   572篇
  免费   5篇
  国内免费   8篇
系统科学   9篇
丛书文集   1篇
教育与普及   1篇
理论与方法论   9篇
现状及发展   113篇
研究方法   71篇
综合类   351篇
自然研究   30篇
  2021年   3篇
  2020年   2篇
  2019年   5篇
  2018年   3篇
  2017年   5篇
  2016年   4篇
  2015年   6篇
  2014年   6篇
  2013年   12篇
  2012年   47篇
  2011年   76篇
  2010年   13篇
  2009年   6篇
  2008年   36篇
  2007年   38篇
  2006年   46篇
  2005年   43篇
  2004年   34篇
  2003年   24篇
  2002年   27篇
  2001年   8篇
  2000年   12篇
  1999年   3篇
  1998年   2篇
  1996年   3篇
  1995年   6篇
  1994年   5篇
  1991年   2篇
  1990年   5篇
  1989年   7篇
  1987年   9篇
  1986年   4篇
  1985年   6篇
  1984年   6篇
  1983年   2篇
  1982年   2篇
  1979年   5篇
  1978年   2篇
  1977年   3篇
  1975年   4篇
  1974年   5篇
  1973年   7篇
  1972年   7篇
  1971年   4篇
  1970年   3篇
  1968年   2篇
  1966年   9篇
  1965年   5篇
  1956年   1篇
  1945年   1篇
排序方式: 共有585条查询结果,搜索用时 15 毫秒
481.
Obtaining the optimal values of the parameters for th e design of a required mould and the operation of the moulding process are diffi cult, this is due to the complexity of product geometry and the variation of pla stic material properties. The typical parameters for the mould design and mouldi ng process are melt flow length, injection pressure, holding pressure, back pres sure, injection speed, melt temperature, mould temperature, clamping force, inje ction time, holding time and cooling time. This pape...  相似文献   
482.
This paper presents a new methodological approach for the synthesis of a neuro-fuzzy controller, using an on-line learning procedure. A simple algebraic formulation of a Sugeno fuzzy inference system that ensures a coherent universe of discourse, making easy its interpretation by a human being, is proposed and implemented in the case of the control of a bioreactor, which is considered as a complex non linear process.  相似文献   
483.
Work-team has emerged as an important unit composi ng the organization today. The concepts relating to the properties of team-role t hat fit with today‘s business environment have attracted much attention from the theorists and researchers. A team having a balanced composition of team-ro les is called a balanced team. Theoretically, a balanced team usually renders a better team performance. Therefore, to improve the performance of the team, mana gement needs to ensure that their work teams are composed...  相似文献   
484.
Bv8 regulates myeloid-cell-dependent tumour angiogenesis   总被引:1,自引:0,他引:1  
Shojaei F  Wu X  Zhong C  Yu L  Liang XH  Yao J  Blanchard D  Bais C  Peale FV  van Bruggen N  Ho C  Ross J  Tan M  Carano RA  Meng YG  Ferrara N 《Nature》2007,450(7171):825-831
Bone-marrow-derived cells facilitate tumour angiogenesis, but the molecular mechanisms of this facilitation are incompletely understood. We have previously shown that the related EG-VEGF and Bv8 proteins, also known as prokineticin 1 (Prok1) and prokineticin 2 (Prok2), promote both tissue-specific angiogenesis and haematopoietic cell mobilization. Unlike EG-VEGF, Bv8 is expressed in the bone marrow. Here we show that implantation of tumour cells in mice resulted in upregulation of Bv8 in CD11b+Gr1+ myeloid cells. We identified granulocyte colony-stimulating factor as a major positive regulator of Bv8 expression. Anti-Bv8 antibodies reduced CD11b+Gr1+ cell mobilization elicited by granulocyte colony-stimulating factor. Adenoviral delivery of Bv8 into tumours was shown to promote angiogenesis. Anti-Bv8 antibodies inhibited growth of several tumours in mice and suppressed angiogenesis. Anti-Bv8 treatment also reduced CD11b+Gr1+ cells, both in peripheral blood and in tumours. The effects of anti-Bv8 antibodies were additive to those of anti-Vegf antibodies or cytotoxic chemotherapy. Thus, Bv8 modulates mobilization of CD11b+Gr1+ cells from the bone marrow during tumour development and also promotes angiogenesis locally.  相似文献   
485.
Yang J  Goldstein JI  Scott ER 《Nature》2007,446(7138):888-891
In our Solar System, the planets formed by collisional growth from smaller bodies. Planetesimals collided to form Moon-to-Mars-sized protoplanets in the inner Solar System in 0.1-1 Myr, and these collided more energetically to form planets. Insights into the timing and nature of collisions during planetary accretion can be gained from meteorite studies. In particular, iron meteorites offer the best constraints on early stages of planetary accretion because most are remnants of the oldest bodies, which accreted and melted in <1.5 Myr, forming silicate mantles and iron-nickel metallic cores. Cooling rates for various groups of iron meteorites suggest that if the irons cooled isothermally in the cores of differentiated bodies, as conventionally assumed, these bodies were 5-200 km in diameter. This picture is incompatible, however, with the diverse cooling rates observed within certain groups, most notably the IVA group, but the large uncertainties associated with the measurements do not preclude it. Here we report cooling rates for group IVA iron meteorites that range from 100 to 6,000 K Myr(-1), increasing with decreasing bulk Ni. Improvements in the cooling rate model, smaller error bars, and new data from an independent cooling rate indicator show that the conventional interpretation is no longer viable. Our results require that the IVA meteorites cooled in a 300-km-diameter metallic body that lacked an insulating mantle. This body probably formed approximately 4,500 Myr ago in a 'hit-and-run' collision between Moon-to-Mars-sized protoplanets. This demonstrates that protoplanets of approximately 10(3) km size accreted within the first 1.5 Myr, as proposed by theory, and that fragments of these bodies survived as asteroids.  相似文献   
486.
487.
488.
The mountains of north central Washington contain a variety of habitat types, from shrubsteppe to high alpine meadows. While native bee surveys of some surrounding areas like the Columbia Basin are fairly complete, little work has been done in the mountains of north central Washington to document the diversity of bees found therein. We conducted a survey of native bees in the Tonasket Ranger District of the Okanogan-Wenatchee National Forest during summer 2004. Collections yielded a diverse bee fauna (140 species in 24 genera) visiting diverse floral elements (57 plant species in 18 families). These preliminary data suggest that a rich bee fauna exists in the Okanogan Basin and surrounding mountains.  相似文献   
489.
Circadian clocks orchestrate 24-h oscillations of essential physiological and behavioral processes in response to daily environmental changes. These clocks are remarkably precise under constant conditions yet highly responsive to resetting signals. With the molecular composition of the core oscillator largely established, recent research has increasingly focused on clock-modifying mechanisms/molecules. In particular, small molecule modifiers, intrinsic or extrinsic, are emerging as powerful tools for understanding basic clock biology as well as developing putative therapeutic agents for clock-associated diseases. In this review, we will focus on synthetic compounds capable of modifying the period, phase, or amplitude of circadian clocks, with particular emphasis on the mammalian clock. We will discuss the potential of exploiting these small molecule modifiers in both basic and translational research.  相似文献   
490.
A robust digital receiver based on a matched filter (MF) is proposed for the radio frequency identification (RFID) reader system to enhance the reliability of signal processing in the electronic product code (EPC) sensor network (ESN). The performance of the proposed receiver is investigated by examining the anti-collision algorithm in the EPC global Class1 Generation2 protocol. The validity and usefulness are demonstrated by both computer simulations and experiments. Based on the verification  results, comparing with the conventional zero crossing detector (ZCD) based receiver, the proposed receiver is very robust against strong amplitude distortions and considerable frequency deviations happening on the backscattered signal from a passive tag.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号