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   4539篇
  免费   36篇
  国内免费   2篇
电工技术   17篇
综合类   2篇
化学工业   253篇
金属工艺   52篇
机械仪表   39篇
建筑科学   124篇
矿业工程   7篇
能源动力   67篇
轻工业   217篇
水利工程   14篇
石油天然气   11篇
无线电   178篇
一般工业技术   377篇
冶金工业   2976篇
原子能技术   13篇
自动化技术   230篇
  2022年   14篇
  2019年   18篇
  2018年   14篇
  2017年   21篇
  2016年   13篇
  2015年   20篇
  2014年   29篇
  2013年   125篇
  2012年   60篇
  2011年   87篇
  2010年   71篇
  2009年   62篇
  2008年   72篇
  2007年   79篇
  2006年   68篇
  2005年   71篇
  2004年   65篇
  2003年   61篇
  2002年   44篇
  2001年   51篇
  2000年   47篇
  1999年   124篇
  1998年   957篇
  1997年   538篇
  1996年   320篇
  1995年   208篇
  1994年   189篇
  1993年   201篇
  1992年   47篇
  1991年   43篇
  1990年   47篇
  1989年   44篇
  1988年   54篇
  1987年   53篇
  1986年   37篇
  1985年   46篇
  1984年   21篇
  1983年   17篇
  1982年   34篇
  1981年   31篇
  1980年   34篇
  1979年   24篇
  1978年   27篇
  1977年   80篇
  1976年   150篇
  1975年   18篇
  1974年   13篇
  1973年   19篇
  1972年   14篇
  1968年   13篇
排序方式: 共有4577条查询结果,搜索用时 15 毫秒
1.
Lévy flights have gained prominence for analysis of animal movement. In a Lévy flight, step-lengths are drawn from a heavy-tailed distribution such as a power law (PL), and a large number of empirical demonstrations have been published. Others, however, have suggested that animal movement is ill fit by PL distributions or contend a state-switching process better explains apparent Lévy flight movement patterns. We used a mix of direct behavioural observations and GPS tracking to understand step-length patterns in females of two related butterflies. We initially found movement in one species (Euphydryas editha taylori) was best fit by a bounded PL, evidence of a Lévy flight, while the other (Euphydryas phaeton) was best fit by an exponential distribution. Subsequent analyses introduced additional candidate models and used behavioural observations to sort steps based on intraspecific interactions (interactions were rare in E. phaeton but common in E. e. taylori). These analyses showed a mixed-exponential is favoured over the bounded PL for E. e. taylori and that when step-lengths were sorted into states based on the influence of harassing conspecific males, both states were best fit by simple exponential distributions. The direct behavioural observations allowed us to infer the underlying behavioural mechanism is a state-switching process driven by intraspecific interactions rather than a Lévy flight.  相似文献   
2.
3.
The transjugular intrahepatic portosystemic shunt procedure is an accepted treatment for adults with complications of portal hypertension. We performed a retrospective review of all pediatric TIPS placements performed at the University of California, San Francisco between 1990 and 1996. Twelve procedures were attempted in nine children, with a mean age (+/- SD) of 9.4 +/- 3.9 years (range, 5 to 15 years) and a mean weight of 31 +/- 18 kg (range, 16 to 70 kg). The indications for TIPS placement were portal hypertension complicated by chronic variceal hemorrhage not controlled with sclerotherapy (n = 7) and hypersplenism with thrombocytopenia (n = 2). TIPS placement was successfully completed initially in seven of nine (78%) patients. Unfavorable vascular anatomy was the cause of failure in two cases. The seven patients who underwent successful TIPS placement were followed up for an average of 136 days (range, 1 to 800 days); two still have patent shunts, three underwent liver transplantation, one had a splenorenal shunt after stenosis, and one died of underlying liver disease. Variceal bleeding was controlled in four of five patients who successfully underwent TIPS placement. Shunt occlusion occurred in four patients; patency was restored by transjugular shunt revision in three, and a splenorenal shunt was performed in one.  相似文献   
4.
Gastric bacteria from cheetahs with gastritis were used to inoculate specific-pathogen free kittens and conventional mice. Helicobacter sp. and Gastrospirillum sp. colonized kittens, while only Gastrospirillum sp. colonized mice. In kittens, both bacterial species induced mild lymphofolliclar gastritis which did not change over the course of the 11 months observation period. In mice, Gastrospirillum sp. induced lymphoplasmacytic and follicular gastritis which increased in severity over 6 months and persisted for the 12 month observation period. Gastric ulcers and gastric mucosal hypertrophy were present in chronically infected mice. These results indicate that host but not bacterial factors influence the severity of gastritis, and that in mice, bacterial gastritis increases in severity with time and may lead to gastric ulceration in some individuals.  相似文献   
5.
6.
Many current implementations of protocols such as the Transmission Control Protocol/Internet Protocol (TCP/IP) are inefficient because data are often accessed more frequently than necessary. Three techniques that reduce the need for memory bandwidth are proposed. The techniques are copy-on-write, page remapping, and single-copy. Afterburner, a network-independent card that provides the services that are necessary for a single-copy protocol stack, is described. The card has 1 MByte of local buffers and provides a simple interface to a variety of network link adapters, including HIPPI and asynchronous transfer mode (ATM). Afterburner can support transfers to and from the link adapter card at rates up to 1 Gbit/s. An implementation of TCP/IP that uses the features provided by Afterburner to reduce the movement of data to a single copy is discussed. Measurements of the end-to-end performance of Afterburner and the single-copy implementation of TCP/IP are presented  相似文献   
7.
Elasticity is discussed as an aspect of viscoelasticity, which is described by the tube model. The effects of both crosslinks and entanglements contribute to this model and a discussion of how these effects can be quantified is given. At high enough concentration, entanglements ensure the existence of elastic effects even without crosslinks, and a theory is presented on how this dynamical phase change comes about.  相似文献   
8.
9.
Image processing allows the automation of the business transaction, turning “paper factories” into “image factories.” This burgeoning new area of computerization needs new paradigms, theories, and methods to maximize its effectiveness at cutting time and costs. This paper investigates what this new area of computerization can gain from CIM technology. It examines the similarities between the processing of computer images in a paperless business environment and the processing of materials into products in the manufacturing plant. These similarities will allow us to apply mature, well-tested CIM techniques to emerging image factories.  相似文献   
10.
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