全文获取类型
收费全文 | 50785篇 |
免费 | 589篇 |
国内免费 | 639篇 |
专业分类
安全科学 | 1735篇 |
废物处理 | 2359篇 |
环保管理 | 6691篇 |
综合类 | 8155篇 |
基础理论 | 13334篇 |
环境理论 | 25篇 |
污染及防治 | 12814篇 |
评价与监测 | 3460篇 |
社会与环境 | 3112篇 |
灾害及防治 | 328篇 |
出版年
2022年 | 478篇 |
2021年 | 531篇 |
2020年 | 361篇 |
2019年 | 446篇 |
2018年 | 778篇 |
2017年 | 798篇 |
2016年 | 1192篇 |
2015年 | 927篇 |
2014年 | 1403篇 |
2013年 | 4119篇 |
2012年 | 1732篇 |
2011年 | 2354篇 |
2010年 | 1911篇 |
2009年 | 1948篇 |
2008年 | 2336篇 |
2007年 | 2331篇 |
2006年 | 2073篇 |
2005年 | 1783篇 |
2004年 | 1681篇 |
2003年 | 1741篇 |
2002年 | 1580篇 |
2001年 | 1942篇 |
2000年 | 1374篇 |
1999年 | 833篇 |
1998年 | 609篇 |
1997年 | 619篇 |
1996年 | 633篇 |
1995年 | 750篇 |
1994年 | 706篇 |
1993年 | 594篇 |
1992年 | 633篇 |
1991年 | 619篇 |
1990年 | 610篇 |
1989年 | 570篇 |
1988年 | 506篇 |
1987年 | 449篇 |
1986年 | 416篇 |
1985年 | 425篇 |
1984年 | 469篇 |
1983年 | 442篇 |
1982年 | 465篇 |
1981年 | 386篇 |
1980年 | 311篇 |
1979年 | 340篇 |
1978年 | 300篇 |
1977年 | 228篇 |
1975年 | 245篇 |
1973年 | 232篇 |
1972年 | 243篇 |
1971年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 22 毫秒
661.
662.
A survey is given on fluid-dynamic effects caused by the structure and properties of biological surfaces. It is demonstrated
that the results of investigations aiming at technological applications can also provide insights into biophysical phenomena.
Techniques are described both for reducing wall shear stresses and for controlling boundary-layer separation. (a) Wall shear
stress reduction was investigated experimentally for various riblet surfaces including a shark skin replica. The latter consists
of 800 plastic model scales with compliant anchoring. Hairy surfaces are also considered, and surfaces in which the no-slip
condition is modified. Self-cleaning surfaces such as that of lotus leaves represent an interesting option to avoid fluid-dynamic
deterioration by the agglomeration of dirt. An example of technological implementation is discussed for riblets in long-range
commercial aircraft. (b) Separation control is also an important issue in biology. After a few brief comments on vortex generators,
the mechanism of separation control by bird feathers is described in detail. Self-activated movable flaps (=artificial bird
feathers) represent a high-lift system enhancing the maximum lift of airfoils by about 20%. This is achieved without perceivable
deleterious effects under cruise conditions. Finally, flight experiments on an aircraft with laminar wing and movable flaps
are presented. 相似文献
663.
The main objective of this project was to evaluate the possibility of reducing the quantity of fibre wasted at one of many hydrocyclone (centrifugal) cleaning processes in a paper mill. It was found that the application of elutriation water to both the tertiary and quaternary cleaners was essential to minimise the fibre discharged to the sewer, and the pressure of this elutriation water had a dramatic effect of reducing the fibre wastage. Accordingly, it has been shown that 150–160 kPa as the optimum pressure range to apply elutriation water to minimise the product grade fibre wasted whilst sending undesired shive fibre to the sewer. Also, monitoring of the press uhle box wastewater revealed that the paper mill has the potential to make substantial cost savings by reducing the waste stream. Further investigation is necessary to determine the types of fibre that are being wasted, and the viability of a screen to recycle the wasted fibre to the process. However, these fibres may be unsuitable to reuse in the process and alternative uses must be found. 相似文献
664.
Fifty-three patients who elected to reduce their pregnancies to a twin gestation in our centre are known to have subsequently undergone genetic amniocentesis. Five of these patients lost their entire pregnancy following the genetic amniocentesis procedure. This is equivalent to a 9·4 per cent pregnancy loss rate for reduced twin gestations in comparison with an expected loss rate of 2 per cent for non-reduced twin gestations. 相似文献
665.
Three cases of unusual chromosomal mosaicism are reported for which the cytogenetic data show inconsistent findings between CVS and AC or fetal tissue, and which cannot be explained simply by non-disjunction. For case 1, in CVS the karyotype was 46,XY, whereas lymphocytes and fibroblasts revealed 69,XXY. DNA fingerprinting indicated one paternal and two maternal chromosome sets, the latter most probably due to omission of maternal meiosis II. For case 2, in CVS mos 46,XX/47,XX,+mar de novo was observed. Amniotic fluid cells had the karyotype 46,XX. The origin of the marker chromosome might be explained by at least two events of unknown order (a somatic chromosome/chromatid deletion and non-disjunction of the homologous chromosome). In case 3 (CVS: mos 46,XY/46,XY,19q+ de novo; amniotic fluid cells, lymphocytes, and fibroblasts: 46,XY), the surplus of chromosome material in 19q+ might be explained on the basis of a somatic translocation. The idea of a chimera is less convincing, as the mosaic finding is restricted to one tissue. Furthermore, there was no hint of a vanishing twin. Hitherto, no case of structural chromosome mosaicism in CVS has been reconfirmed in fetal tissues. 相似文献
666.
A multiple pregnancy of high rank may occur in a couple at risk for a Mendelian disorder. Prenatal diagnosis is hampered by the difficulty of (1) obtaining chorionic villi from each zygote arid (2) unequivocally relating each sample to the corresponding embryo. The calculation of the genetic risk according to the number of zygotes led us to propose a diagnostic strategy based on embryo reduction, a technique initially designed to improve the perinatal outcome of multiple pregnancies with normal embryos. We report a case in which this approach allowed rational use of first-trimester chorionic villus sampling in a quintuplet pregnancy at risk for non-ketotic hyperglycinaemia, resulting in the selective birth of unaffected twins. 相似文献
667.
We describe a new method for the post-mortem examination of the fetal central nervous system. The brain is immobilized in gelatin prior to ultrasonic examination. Correlation with prenatal ultrasound is excellent and subsequent pathological examination is enhanced and facilitated. 相似文献
668.
A case is described of the prenatal diagnosis of choroid plexus cysts at 17 weeks' gestation which persisted beyond 36 weeks but could not be detected after delivery. At birth the child was found to have trisomy 18. 相似文献
669.
A. G. W. Hunter M.D. 《黑龙江环境通报》1987,7(6):433-441
Three hundred and fifty-four women who underwent midtrimester genetic amniocentesis were matched for age, parity, indication, and smoking history, with an equivalent number of women who had declined the procedure, for the purpose of comparing the neonatal respiratory status of their offspring. There was no evidence that the infants exposed to genetic amniocentesis were compromised. 相似文献
670.