全文获取类型
收费全文 | 27890篇 |
免费 | 274篇 |
国内免费 | 272篇 |
专业分类
安全科学 | 765篇 |
废物处理 | 1329篇 |
环保管理 | 3422篇 |
综合类 | 3601篇 |
基础理论 | 7371篇 |
环境理论 | 6篇 |
污染及防治 | 7697篇 |
评价与监测 | 2163篇 |
社会与环境 | 1921篇 |
灾害及防治 | 161篇 |
出版年
2022年 | 280篇 |
2021年 | 274篇 |
2020年 | 191篇 |
2019年 | 232篇 |
2018年 | 440篇 |
2017年 | 433篇 |
2016年 | 697篇 |
2015年 | 482篇 |
2014年 | 748篇 |
2013年 | 2330篇 |
2012年 | 900篇 |
2011年 | 1201篇 |
2010年 | 1062篇 |
2009年 | 1080篇 |
2008年 | 1252篇 |
2007年 | 1337篇 |
2006年 | 1188篇 |
2005年 | 997篇 |
2004年 | 961篇 |
2003年 | 982篇 |
2002年 | 908篇 |
2001年 | 1262篇 |
2000年 | 879篇 |
1999年 | 539篇 |
1998年 | 376篇 |
1997年 | 350篇 |
1996年 | 391篇 |
1995年 | 406篇 |
1994年 | 350篇 |
1993年 | 312篇 |
1992年 | 330篇 |
1991年 | 302篇 |
1990年 | 319篇 |
1989年 | 318篇 |
1988年 | 273篇 |
1987年 | 231篇 |
1986年 | 179篇 |
1985年 | 212篇 |
1984年 | 240篇 |
1983年 | 231篇 |
1982年 | 259篇 |
1981年 | 198篇 |
1980年 | 173篇 |
1979年 | 206篇 |
1978年 | 151篇 |
1977年 | 154篇 |
1976年 | 139篇 |
1975年 | 126篇 |
1974年 | 128篇 |
1973年 | 125篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
371.
372.
Prenatal cytogenetic diagnosis after transabdominal chorionic villus sampling in the first trimester
First trimester prenatal cytogenetic diagnosis was attempted in 350 pregnancies after trans-abdominal chorionic villus sampling. The cytogenetic investigation was performed using both a short-term method (24 h incubation) and cell culture. Adequate samples were obtained in 99·1 per cent and in all these cases the fetal karyotype was established. A chromosome abnormality was found in 2·0 per cent of cases. A discrepancy between the karyotype obtained after 24 h incubation and the karyotype in cell culture was observed in 2·3 per cent. Maternal cell contamination in the cultures was confirmed in 13 of 181 cases where the 24 h incubation revealed a male karyotype. Studies of culture morphology showed that colonies of convoluted cells may serve as a marker for contamination with maternal cells in culture. For the present, we recommend using a short-term method as well as cell culture for cytogenetic investigation until the problems with karyotype discrepancy and maternal cell contamination have been further clarified. 相似文献
373.
374.
We present a remarkable chain of events in which percutaneous umbilical cord sampling was performed in an attempt to clarify a situation of possible fetal sex chromosome mosaicism in an amniotic fluid culture and led to the discovery that the mother herself had a 45,X/46,XX/ 47.XXX chromosome constitution. This may have simply represented the chance concurrence of pseudo-mosaicism in the amniotic fluid culture of a woman with an abnormal sex chromosome constitution, but it is also possible that the 45,X colony was maternal in origin. Although clearly a most unusual circumstance, the possibility should be kept in mind when termination of a pregnancy is being considered because of apparent mosaicism in a prenatal diagnostic study. 相似文献
375.
Self-diffusion coefficients were studied for the highly polar liquid N-methylformamide at pressures up to 200 MPa between the melting pressure curves and 420 K by the spin-echo method. N-Methylformamide exists as a mixture of two conformers in the neat liquid. These conformers have large differences at lower
temperatures in their dynamic and structural properties. The self-diffusion coefficient of the cis-conformer being 17% lower than that of the trans-conformer at the same T and p. This is the first observation of such an effect. The experimental study is supported by Monte Carlo (MC) calculations which
show that the first neighbors around a cis conformer are arranged differently than in an all trans liquid. The difference leads in the simulations to a much lower dielectric constant for the trans-cis mixture and might also explain the retardation of diffusion for the cis conformer.
Received: 15 November 1999 / Accepted in revised form: 28 February 2000 相似文献
376.
Robert T. LackeyAuthor vitae 《Environmental Science & Policy》1998,1(4):329-335
Fisheries management is the practice of analyzing and selecting options to maintain or alter the structure, dynamics, and interaction of habitat, aquatic biota, and man to achieve human goals and objectives. The theory of fisheries management is: managers or decision makers attempt to maximize renewable `output' from an aquatic resource by choosing from among a set of decision options and applying a set of actions that generate an array of outputs. Outputs may be defined as a tangible catch, a fishing experience, an existence value, or anything else produced or supported by renewable aquatic resources. Overall output is always a mix of tangible and intangible elements. However defined, management goals and objectives are essential components of fisheries management or any other field of renewable natural resource management. Reaching consensus on management goals and objectives has never been a simple task. Beyond the broad and often conflicting goals of an agency, managers must decide who should set specific management objectives — agency personnel, the public, or a combination of the two. Historically, rhetoric aside, fisheries managers in North America nearly always have consulted with professionals in governmental roles to set management objectives. In a strongly pluralistic society, this often resulted in protracted political and legal conflict. Increasingly, there are calls for use of risk assessment to help solve such ecological policy and management problems commonly encountered in fisheries management. The basic concepts of ecological risk assessment may be simple, but the jargon and details are not. Risk assessment (and similar analytical tools) is a concept that has evoked strong reactions whenever it has been used. In spite of the difficulties of defining problems and setting management objectives for complex ecological policy questions, use of risk assessment to help solve ecological problems is widely supported. Ecological risk assessment will be most useful (and objective) in political deliberations when the policy debate revolves around largely technical concerns. To the extent that risk assessment forces policy debate and disagreement toward fundamental differences rather than superficial ones, it will be useful in decision making. 相似文献
377.
Maternal serum CA 125 levels were determined at 9–11 menstrual weeks for 26 cases of trisomy 13 (n = 4), trisomy 18 (n = 7), trisomy 21 (n = 15), and appropriate controls. There were no statistically significant differences between groups. 相似文献
378.
H. Brandenburg MD W. De Koning M. G. J. Jahoda Th. Stijnen M. A. J. De Ridder E. S. Sachs J. W. Wladimiroff 《黑龙江环境通报》1992,12(12):1031-1035
One hundred and fifty-one women of advanced maternal age who underwent genetic termination of pregnancy (TOP) were studied for their reproductive behaviour and the type of procedure for prenatal diagnosis in a subsequent pregnancy. A total of 59 women (39 per cent) had a further pregnancy. In all continuing pregnancies prenatal diagnosis was performed, of which 75 per cent consisted of chorionic villus sampling (CVS). Reproductive behaviour following a genetic termination was negatively correlated with maternal age and parity. Both reproductive behaviour and the choice to undergo a diagnostic procedure in the next pregnancy were independent of the type of diagnostic procedure in the previous affected pregnancy. 相似文献
379.
380.