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   32572篇
  免费   253篇
  国内免费   265篇
安全科学   756篇
废物处理   1386篇
环保管理   3485篇
综合类   8214篇
基础理论   7143篇
环境理论   12篇
污染及防治   8299篇
评价与监测   2178篇
社会与环境   1480篇
灾害及防治   137篇
  2022年   341篇
  2021年   295篇
  2019年   227篇
  2018年   501篇
  2017年   461篇
  2016年   732篇
  2015年   518篇
  2014年   894篇
  2013年   2387篇
  2012年   983篇
  2011年   1247篇
  2010年   1063篇
  2009年   1111篇
  2008年   1273篇
  2007年   1283篇
  2006年   1173篇
  2005年   1003篇
  2004年   974篇
  2003年   945篇
  2002年   864篇
  2001年   1132篇
  2000年   747篇
  1999年   530篇
  1998年   333篇
  1997年   344篇
  1996年   346篇
  1995年   409篇
  1994年   398篇
  1993年   329篇
  1992年   355篇
  1991年   381篇
  1990年   385篇
  1989年   345篇
  1988年   318篇
  1987年   259篇
  1986年   276篇
  1985年   283篇
  1984年   283篇
  1983年   281篇
  1982年   296篇
  1981年   290篇
  1980年   234篇
  1979年   258篇
  1978年   229篇
  1974年   224篇
  1967年   257篇
  1966年   216篇
  1965年   222篇
  1964年   243篇
  1963年   210篇
排序方式: 共有10000条查询结果,搜索用时 453 毫秒
71.
This study examines 45 cases of trisomy 13 and 59 cases of trisomy 18 and reports an algorithm to identify pregnancies with a fetus affected by trisomy 13 or 18 by a combination of maternal age fetal nuchal translucency (NT) thickness, and maternal serum free β-hCG and PAPP-A at 11–14 weeks of gestation. In this mixed trisomy group the median MoM NT was increased at 2.819, whilst the median MoMs for free β-hCG and PAPP-A were reduced at 0.375 and 0.201 respectively. We predict that with the use of the combined trisomy 13 and 18 algorithm and a risk cut-off of 1 in 150 will for a 0.3% false positive rate allow 95% of these chromosomal defects to be identified at 11–14 weeks. Such algorithms will enhance existing first trimester screening algorithms for trisomy 21. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
72.
73.
74.
Stormwater runoff is now a major contributor to the pollution of coastal waters in the United States. Public agencies are responding by requiring stormwater monitoring to satisfy the National Pollutant Discharge Elimination System stormwater permit. However, studies to understand the utility of the current programs or to improve their usefulness have not yet been performed. In this paper, we evaluate the land-use-based program, the industrial stormwater permit program, and beach water-quality monitoring in the County of Los Angeles, California, to determine if the results will be helpful to planners and regulators in abating stormwater pollution. The utility of the program has been assessed based on the programs' ability to accurately estimate the emissions for different classes of land use. The land-use program appears successful, while the industrial monitoring program does not. Beach water-quality monitoring suffers from a lack of real-time monitoring techniques. We also provide suggested improvements, such as sampling method and time, and parameter selection.  相似文献   
75.
76.
Two cases with a large cystic mass within the placenta are reported. By ultrasonography, it was found that both women had a subchorionic hypoechoic lesion (11.0 × 4.8 × 4.0 cm and 6.6 × 3.7 × 2.2 cm, respectively) at 24 and 35 weeks of gestation, respectively. In both cases, turbulent blood flow generated by a pulsatile jet flow (pulse rate; 40 to 60 beats per minute) into the cystic lesion seen on real-time imaging and lesions being low intensity on T1-weighted and isointensity on T2-weighted magnetic resonance image suggested that they contained fresh maternal blood. In both cases, the sonolucency of the lesions did not change until cesarean deliveries of females, both of whom were small-for-gestational-age infants (1940 g at 37 weeks and 2195 g at 37 weeks, respectively). Biochemical analysis of the fluid in the cystic lesion sampled during the cesarean section in the latter case confirmed that the fluid had originated from the maternal blood. These lesions histologically corresponded to large avillous areas surrounded by normal villi. Thus, a huge placental lake was diagnosed in both cases. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
77.
78.
79.
80.
A study was undertaken to establish the role of free amino acids (FAA) in aerobic energy dissipation in embryos of turbot (Scophthalmus maximus) which contain an oil globule in the egg. Laboratory-reared developing eggs and larvae (15°C, 34 salinity) were measured for oxygen uptake, ammonia excretion, contents of FAA, protein, and ammonium, and volumes of yolksac and oil globule. Newly spawned eggs from different batches contained 55 to 90 nmol egg–1 of FAA. Resorption of FAA occurred in parallel with the consumption of yolk. Resorption of the oil globule, however, occurred predominantly after hatching and mainly after yolk resorption. The combined data suggest that approximately 70% of the FAA are utilized as an energy substrate, while the rest are polymerized into body proteins. FAA become a significant energy substrate in the early egg stage and account for 100% of the aerobic energy dissipation 2 d after Fertilization then decrease to ca. 60% at the time of hatching. Lipids derived from the oil globule seem to be the main fuel after hatching and account for ca. 90% of the energy dissipation at the onset of first-feeding. Thus, the energetics of fish embryos which contain an oil globule seems to be different from those that depend exclusively on the nutritional reserves of the yolk.  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