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   25999篇
  免费   286篇
  国内免费   216篇
安全科学   751篇
废物处理   969篇
环保管理   3609篇
综合类   4832篇
基础理论   6673篇
环境理论   17篇
污染及防治   6404篇
评价与监测   1598篇
社会与环境   1453篇
灾害及防治   195篇
  2022年   215篇
  2021年   214篇
  2020年   170篇
  2019年   216篇
  2018年   341篇
  2017年   366篇
  2016年   517篇
  2015年   443篇
  2014年   601篇
  2013年   2099篇
  2012年   782篇
  2011年   1056篇
  2010年   860篇
  2009年   990篇
  2008年   1086篇
  2007年   1093篇
  2006年   1000篇
  2005年   853篇
  2004年   780篇
  2003年   809篇
  2002年   743篇
  2001年   951篇
  2000年   709篇
  1999年   426篇
  1998年   303篇
  1997年   320篇
  1996年   348篇
  1995年   376篇
  1994年   371篇
  1993年   347篇
  1992年   326篇
  1991年   349篇
  1990年   335篇
  1989年   308篇
  1988年   256篇
  1987年   225篇
  1986年   254篇
  1985年   237篇
  1984年   249篇
  1983年   259篇
  1982年   258篇
  1981年   250篇
  1980年   230篇
  1979年   241篇
  1978年   167篇
  1977年   182篇
  1975年   136篇
  1974年   160篇
  1973年   152篇
  1972年   159篇
排序方式: 共有10000条查询结果,搜索用时 672 毫秒
461.
462.
The nature of the cosmic redshift is one of the most fundamental questions in modern science. Hubble's discovery of the apparent Expansion of the Universe is derived from observations on a small number of galaxies at very low redshifts. Today, quasar redshifts have a range more than 1000 times greater than those in Hubble's sample, and represent more than 100 times as many objects. A recent comprehensive compilation of published measurements provides the basis for a study indicating that quasar observations are not in good agreement with the original predictions of the Expanding Universe theory, but are well fit by the predictions of an alternative theory having fewer adjustable parameters.  相似文献   
463.
464.
465.
466.
Uterine lavage affords the potential for non-invasive human blastocyst recovery, with obvious potential for preimplantation genetic diagnosis. In an effort to duplicate in women the multiple blastocyst recovery per cycle that can be achieved in several other species, we initiated a programme in which fertile women underwent superovulation, followed by lavage and embryo collection. We superovulated 15 fertile women, aged 21–40, in 29 cycles using one of four regimens. Insemination was by either intercourse or artificial intracervical donor insemination with cryopreserved sperm from men of proven fertility. In 28 of 29 cycles, the uterus was lavaged daily for 1, 2, or 3 days between 5 and 10 days after human chorionic gonadotropin (hCG) administration or luteinizing hormone (LH) surge. Almost total fluid volume was recovered in every lavage. There were no retained pregnancies and no complications. Surprisingly, only two morulae, one blastocyst, and four unfertilized ova were recovered. Thus, alterations in ovulation induction, insemination timing, or lavage techniques must be contemplated in order to increase the blastocyst yield and thus fulfil the potential of uterine lavage for preimplantation diagnosis.  相似文献   
467.
A population of 1639 patients were seen for chorionic villus sampling (CVS). Embryonic death was identified at ultrasound in 5.3 per cent of patients. The number of patients undergoing CVS was 1551, with 1416 transcervical procedures and 135 transabdominal procedures. The most common indication for CVS was advanced maternal age. Spontaneous pregnancy losses identified by increased risk of pregnancy loss with increasing aspiration attempts. The total fetal loss for this population was 5.4 per cent with the pregnancy loss estimated due to procedure being 1.2 per cent. Analysis of placentae from patients having CVS and amniocen-tesis showed no differences. Microbiological assessment prior to CVS was similar to previous publications.  相似文献   
468.
The natural desire for couples to be parents and the medical practitioner's inability to treat most genetic diseases have been responsible for some of the most exciting research into infertility and genetic disorders. This has led in the United Kingdom to the establishment of the Warnock Committee of Inquiry into Human Fertilization and its report in 1984, and to a Review of the guidance on Research Use of Fetuses and Fetal Material published in 1989 and known as the Polkinghorne Report. The Warnock Report, among other ethical issues, considers the most fundamental question which has been debated for thousands of years, namely, What is life and when does it begin? More recently, the report has been responsible for new legislation which imposes ethical and legal restrictions on the scientific and medical community. The Polkinghorne Report recommends a voluntary code of practice which is morally and ethically acceptable within our society. We are also fortunate in the U.K. to have a parliamentary structure which allows debate on such important human issues and is prepared to impose ethical restrictions.  相似文献   
469.
470.
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