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   252989篇
  免费   16185篇
  国内免费   36111篇
安全科学   23748篇
废物处理   11663篇
环保管理   29201篇
综合类   103472篇
基础理论   47703篇
环境理论   211篇
污染及防治   50487篇
评价与监测   16679篇
社会与环境   15071篇
灾害及防治   7050篇
  2024年   681篇
  2023年   3284篇
  2022年   9016篇
  2021年   9058篇
  2020年   8668篇
  2019年   6800篇
  2018年   7937篇
  2017年   9254篇
  2016年   9272篇
  2015年   9269篇
  2014年   11450篇
  2013年   20983篇
  2012年   16162篇
  2011年   18123篇
  2010年   15859篇
  2009年   15645篇
  2008年   16067篇
  2007年   15726篇
  2006年   14658篇
  2005年   11336篇
  2004年   8776篇
  2003年   7732篇
  2002年   6876篇
  2001年   6243篇
  2000年   5280篇
  1999年   3609篇
  1998年   2416篇
  1997年   2202篇
  1996年   2222篇
  1995年   2296篇
  1994年   2027篇
  1993年   1497篇
  1992年   1509篇
  1991年   1366篇
  1990年   1251篇
  1989年   1145篇
  1988年   1014篇
  1987年   907篇
  1986年   859篇
  1985年   870篇
  1984年   895篇
  1983年   824篇
  1982年   918篇
  1981年   817篇
  1980年   627篇
  1979年   686篇
  1978年   585篇
  1977年   513篇
  1975年   467篇
  1974年   461篇
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
251.
In the high technology industry, small and medium sized technology enterprises (SMSTEs) play a pivotal role in advancing the whole industry. To achieve sustainable development, they need to extend their scope of business activities beyond a national view and exploit international market actively to meet international competitions that increase quickly in the form of allocating resources within the scope of the world. However, the SMSTEs are also facing risks associated with themselves during the process of exploiting international market owing to their own restrictions, so what they should do is to consider risk evaluations in exploiting the international market.  相似文献   
252.
253.
254.
Abstract:  We surveyed shallow, rocky reefs in southwestern Apulia (Mediterranean Sea) to assess the effects on coastal fish assemblages of the date mussel (  Lithophaga lithophaga ) fishery, an illegal practice that strips the rocky reef bare. We visually sampled fish four times over 15 months at three locations, one affected by date-mussel fishery and two controls. The fish assemblage at the affected location differed significantly from those at the control locations over all sampling times. Herbivorous fishes, sparids, and labrids (genus Symphodus ) contributed most to the differences between the affected location and controls. Lower densities of Symphodus spp. were observed at the affected location, whereas detritivorous fishes were recorded exclusively at control sites. Small serranids and sparids showed temporal trends that differed between the affected location and the control locations. Our results suggest that the date-mussel fishery affects fish assemblages chiefly through reduction of arborescent macroalgae (contributing to habitat complexity and primary production) and emphasize the need for more effective policing against this destructive practice.  相似文献   
255.
256.
257.
258.
To comply with EU recycling targets for packaging waste, the UK relies on an industry run and market based eco-tax levied by reprocessors on obligated businesses. After examining the details of this approach, the use of Packaging Waste Recovery Notes is assessed in terms of its allocative impact and the contribution made to eco-efficiency. The evidence indicates the UK system has minimized the cost of delivering current EU targets, but has not been able to deliver allocative efficiency or to demonstrate dynamic improvements in resource use. Its effectiveness in meeting impending higher targets remains open to question.  相似文献   
259.
260.
The structure of the choroid plexus was studied in five normal human embryos, three normal fetuses and three fetuses with choroid plexus cysts. These were detected by ultrasound and the fetuses were karyotypically normal. The choroid plexus appears in the lateral cerebral ventricles at the seventh developmental week. The early structure is lobulated with vessels running in the mesenchymal stroma and forming capillary nets under the single-layered ependymal epithelium. This embryonal structure is converted into the fetal type during the ninth developmental week as the embryonal capillary net is replaced by elongated loops of wavy capillaries that lie under regular longitudinal epithelial folds. The choroid plexus cysts exhibited accumulation of fluid within distended mesenchymal stroma and did not show the wavy folds on this surface, which was smooth. Within this connective tissue of the cyst wall were distended angiomatous interconnecting thin-walled capillaries. Therefore, filled cavities were not lined by any epithelium. We suggest that fetal choroid plexuses cysts (at least in many cases) are in fact pseudocysts exhibiting angiomatous patterns of capillaries in their walls. Copyright © 2002 John Wiley & Sons, Ltd.  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