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   24628篇
  免费   316篇
  国内免费   185篇
安全科学   698篇
废物处理   1094篇
环保管理   3859篇
综合类   3728篇
基础理论   6715篇
环境理论   13篇
污染及防治   6273篇
评价与监测   1558篇
社会与环境   1013篇
灾害及防治   178篇
  2021年   176篇
  2019年   180篇
  2018年   296篇
  2017年   329篇
  2016年   496篇
  2015年   399篇
  2014年   558篇
  2013年   2070篇
  2012年   725篇
  2011年   1032篇
  2010年   819篇
  2009年   824篇
  2008年   1035篇
  2007年   1072篇
  2006年   959篇
  2005年   795篇
  2004年   805篇
  2003年   798篇
  2002年   761篇
  2001年   828篇
  2000年   663篇
  1999年   414篇
  1998年   324篇
  1997年   335篇
  1996年   342篇
  1995年   391篇
  1994年   356篇
  1993年   339篇
  1992年   338篇
  1991年   317篇
  1990年   338篇
  1989年   316篇
  1988年   276篇
  1987年   270篇
  1986年   257篇
  1985年   234篇
  1984年   295篇
  1983年   252篇
  1982年   305篇
  1981年   267篇
  1980年   224篇
  1979年   219篇
  1978年   204篇
  1977年   167篇
  1976年   148篇
  1975年   156篇
  1974年   164篇
  1973年   167篇
  1972年   154篇
  1971年   158篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
691.
692.
693.
We report two cases of apparently balanced complex de novo chromosomal rearrangements (BCCR) detected prenatally at 17 weeks and 10 weeks of gestation, respectively. Chromosomes were studied using GTG-banding and fluorescent in situ hybridization (FISH). In one case four chromosomes and in the other case three chromosomes were involved in the rearrangements. One of the pregnancies was terminated and no external or internal abnormalities were detected at autopsy. The other pregnancy continued to term. Level III ultrasound examination showed no abnormalities. The child is now 3 years old and has neither congenital anomalies nor evidence of delayed psychomotor development.  相似文献   
694.
Prenatal diagnosis in a pregnancy at risk for a juvenile B1 variant of GM2-gangliosidosis was carried out. The biochemical study of the cultured amniocytes and the affected fetal brain is reported. The results obtained show that the sulphated artificial substrate can be used in the diagnosis of B1 variant, but not the neutral one. The accumulation of GM2-ganglioside in the fetal brain of the B1 juvenile form and an infantile form of GM2-gangliosidosis (0 variant) was compared.  相似文献   
695.
We have applied our multimarker approach of maternal serum alpha-fetoprotein (AFP) and free-beta human chorionic gonadotropin (hCG) for Down syndrome screening to multiple gestations to assess its efficacy for improved detection of twin and triplet pregnancies. This study matched 225 cases of twin pregnancy and 39 cases of triplet pregnancy each with ten singleton pregnancies based on gestational week, race, time to receive sample, time of year of sample, and geographical area. The ratios of the MOM for each group at the tenth, 50th, and 90th percentiles were compared by the Wilcoxon test. Risks for twins were calculated using Bayes' rule, the age-related incidence of twins, and the levels of AFP and free-beta hCG. The tenth, 50th, and 90th percentiles of free-beta hCG MOMs in twin and triplet cases were 0.85, 1.99, and 4.51, and 1.38, 2.78, and 4.07, respectively. For AFP, the MOMs at these percentiles were 1.26, 1.91, and 2.99, and 2.02, 2.68, and 5.30, respectively. The twin and triplet distributions for each marker were statistically significantly different from the singleton distributions (P<0.0001) and from each other (P=0.0012). At a twin risk cut-off of 1 in 50, 77.4 per cent of all twin gestations can be detected in a second-trimester AFP and free-beta hCG screening protocol with 5.1 per cent of singleton pregnancies falsely identified as at risk for twins. Our dual marker protocol for mid-trimester pregnancy screening combining AFP and free-beta hCG can identify over 77 per cent of twin pregnancies in women less than 35 years of age. This benefit may contribute to an improved outcome of pregnancy by early detection of multiple gestation.  相似文献   
696.
Mitigation and Adaptation Strategies for Global Change -  相似文献   
697.
Industrial ecology: a new field or only a metaphor?   总被引:1,自引:0,他引:1  
In the 10 years since industrial ecology first became a topic of academic interest, it has grown as a field of inquiry and has produced a community of practice in several sectors including academia, business, and government. Even as the shape of industrial ecology becomes clearer, questions remain as to its lasting power beyond the metaphor that gave it its distinctiveness. This paper examines the development of industrial ecology and assesses its progress towards becoming a field of academic inquiry. And, in a related analysis, I look at the progress industrial ecology has made in establishing itself as an institutional (cultural) basis for action in the above sectors. Ideas like industrial ecology must become institutionalized if they are to have much effect on the reality of everyday activities.  相似文献   
698.
A modified method of severe plastic deformation (SPD) entitled constrained groove pressing-cross route (CGP-CR) was introduced for imposing a high magnitude of equivalent strain of about 2.32 per pass on the sheet form samples. The major benefit of this improved route compared to previous common route was the more homogeneity of strain in the rolling (RD) and transverse (TD) directions of sheets. In this study, low carbon steel samples were used for examination of evolutions in microstructure and mechanical properties during SPD via CGP-CR process. Mechanical properties improvement were measured by tensile and macro hardness tests. The results indicate that CGP-CR process can effectively improve tensile strength; and also, yield stress and hardness of as-received low carbon steel samples were improved up to about 100% after two deformation passes. Also, high magnitude of inhomogeneity can be observed in hardness distribution through first pass of the process which diminishes in the subsequent passes. Microstructural evolutions during process were monitored by optical microscopy observations and X-ray diffraction analysis. The results demonstrate that initial ferritic microstructure with grain size of about 30 μm was refined to a 225 nm cell structure after two passes of CGP-CR process.  相似文献   
699.
700.
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