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   9868篇
  免费   113篇
  国内免费   93篇
安全科学   283篇
废物处理   460篇
环保管理   1244篇
综合类   1194篇
基础理论   2677篇
环境理论   2篇
污染及防治   2785篇
评价与监测   713篇
社会与环境   668篇
灾害及防治   48篇
  2022年   105篇
  2021年   80篇
  2020年   65篇
  2019年   72篇
  2018年   145篇
  2017年   141篇
  2016年   231篇
  2015年   164篇
  2014年   237篇
  2013年   743篇
  2012年   297篇
  2011年   442篇
  2010年   364篇
  2009年   397篇
  2008年   472篇
  2007年   478篇
  2006年   425篇
  2005年   368篇
  2004年   324篇
  2003年   376篇
  2002年   324篇
  2001年   503篇
  2000年   331篇
  1999年   192篇
  1998年   137篇
  1997年   133篇
  1996年   138篇
  1995年   162篇
  1994年   125篇
  1993年   102篇
  1992年   123篇
  1991年   118篇
  1990年   130篇
  1989年   130篇
  1988年   94篇
  1987年   87篇
  1986年   65篇
  1985年   90篇
  1984年   89篇
  1983年   86篇
  1982年   82篇
  1981年   73篇
  1980年   61篇
  1979年   68篇
  1977年   53篇
  1976年   48篇
  1975年   53篇
  1974年   51篇
  1973年   53篇
  1970年   45篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Defense Mechanisms of Arthropods, no. 96; no. 95: Eisner et al.: Chemoecology (in press)  相似文献   
142.
Placenta growth factor (PIGF), an angiogenic factor belonging to the vascular endothelial growth factor family, pregnancy-associated plasma protein A (PAPP-A) and free β-human chorionic gonadotrophin (β-hCG) were measured in maternal serum from 45 pregnancies with trisomy 21, 45 with trisomy 18 and 493 normal controls at 10–13 completed weeks of gestation. In the normal pregnancies maternal serum PIGF levels increased exponentially with gestation. The median multiple of the median (MoM) PIGF concentration in the trisomy 21 group (1.26 MoM) was significantly higher (p<0.0001) than in the control group (1.00 MoM). In the trisomy 18 group the median PIGF was lower (0.889 MoM) but this did not quite reach significance (p=0.064). The corresponding median MoM values for PAPP-A were 1.00 MoM for the controls, 0.49 MoM for trisomy 21 and 0.16 MoM for trisomy 18. The median MoM values for free β-hCG were 1.00 MoM for the controls, 2.05 MoM for trisomy 21 and 0.38 MoM for trisomy 18. In the control group there was a small but significant correlation of PIGF with free β-hCG (r=+0.1024) and PAPP-A (r=+0.2288). In the trisomy 18 group there was a significant association between PIGF and free β-hCG (r=+0.2629) but not with PAPP-A (r=+0.0038). In the trisomy 21 group there was a small but significant association with PAPP-A (r=+0.1028) but not with free β-hCG (r=+0.0339). The separation of affected and unaffected pregnancies in maternal serum PIGF is small, and therefore it is unlikely that measurement of PIGF would improve screening for these abnormalities provided by the combination of fetal nuchal translucency and maternal serum PAPP-A and free β-hCG. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
143.
某些剧毒有机废料在高温至超临界水中的处理实验   总被引:8,自引:0,他引:8  
李统锦  Brill T. B. 《环境科学》1998,19(4):43-46,51
为了探索毒害有机废料的处理条件和反应机制,用二氨基乙二肟,氨基氰和蜜胺作初始试料,在温度150-400℃,压力100-700×10^5Pa的条件下进行了水热实验研究,部分密胺实验还添加了H2O2,实验结构表明,在水热条件下,尤其在超临界水中,二氨基乙二肟和氨基氰既可以通过热解作用而聚合成较高分子量的氮杂环混合物,进而水解转化成CO2和NH3,也可以直接发生水解作用,生成CO2和NH3,添加氧化剂可  相似文献   
144.
Congenital limb deficiency (CLD) occurs in 0.54 to 0.59/1000 live-born infants and varies according to its anatomic location, type and cause. Our aim was to present a fetus with a prenatal ultrasonographic diagnosis of CLD (transverse reduction deficiency of the left upper limb), and to determine the prevalence and clinical characteristics of CLD at a tertiary medical centre in Israel. Among 78 500 live-born infants, there were 24 cases of CLD (0.31/1000): 45.8% of the CLD cases with affected upper limbs, 45.8% with affected lower limbs, and 8.4% with both limbs affected; 88.4% of the limb deficiencies were longitudinal and 11.6% were transverse; and, 33.3% (8/24) of the affected newborn infants had additional congenital anomalies. We conclude that CLD is not an infrequent finding in live-born infants. Comprehensive ultrasonography of the fetus allows early prenatal diagnosis of CLD and provides the parents with important information and helps them in their decision regarding the fate of the pregnancy. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
145.
Adoption of glyphosate-resistant canola (Brassica napus L.) has increased glyphosate applications to this crop, and concerns have been raised about unintended consequences of these multiple applications. A field trial was conducted to evaluate the effects of pre-seed and in-crop glyphosate and alternative herbicides on soil microbial community functional structure, diversity and biomass. Pre-seed treatments were 2,4-D, glyphosate and 2,4-D + glyphosate, and in-crop treatments were glyphosate applied once, glyphosate applied twice, ethalfluralin, ethalfluralin + sethoxydim + ethametsulfuron + clopyralid, and sethoxydim + ethametsulfuron. Rhizosphere and bulk soil was collected at flowering stage of canola and analyzed for bacterial community-level substrate utilization patterns and microbial biomass C (MBC). Where differences were significant, pre-seed application of both 2,4-D and glyphosate altered the functional structure and reduced the functional diversity of soil bacteria, but increased MBC. These effects were not necessarily concurrent. The reduction in functional diversity was due to reduction in evenness, which means that the soil where both pre-seed herbicides had been applied was dominated by only few functional groups. In 1 year, two in-crop applications of glyphosate also reduced the functional diversity of soil bacteria when applied after pre-seed 2,4-D, as did in-crop sethoxydim + ethametsulfuron following pre-seed glyphosate. Even though significant differences between herbicides were fewer than non-significant differences, i.e., there were no changes in soil microbial community structure, diversity or biomass in response to glyphosate or alternative herbicides applied to glyphosate-resistant canola in most cases, the observed changes in soil microbial communities could affect soil food webs and biological processes.  相似文献   
146.
Understanding the product's ‘end-of-life’ is important to reduce the environmental impact of the products' final disposal. When the initial stages of product development consider end-of-life aspects, which can be established by ecodesign (a proactive approach of environmental management that aims to reduce the total environmental impact of products), it becomes easier to close the loop of materials. The ‘end-of-life’ ecodesign methods generally include more than one ‘end-of-life’ strategy. Since product complexity varies substantially, some components, systems or sub-systems are easier to be recycled, reused or remanufactured than others. Remanufacture is an effective way to maintain products in a closed-loop, reducing both environmental impacts and costs of the manufacturing processes. This paper presents some ecodesign methods focused on the integration of different ‘end-of-life’ strategies, with special attention to remanufacturing, given its increasing importance in the international scenario to reduce the life cycle impacts of products.  相似文献   
147.
148.
149.
For this study, field measurements of current profiles, buoy trajectories and the lag of two tidal stations were performed to explore the flow characteristics of Taichung Harbour. in order to distinguish the effects of wind drift current on circulation inside Taichung Harbour, field surveys during both summer and monsoon (winter) season were completed separately. the speed of the drift current was about 3.5% that of the, wind speed, based on the field data. the horizontal dispersion coefficients were close to Bowden's (1965) formula for the tidal current. the water temperature showed a two-layer profile, with the mixing thickness about 4 to 6 metres for the upper layer. the corresponding Richardson number was about 70. the tidal lag between the two tidal stations was 9.68 minutes. All the results agree well with the theoretical results.  相似文献   
150.
Paternity and paternal care in the polygynandrous Smith's longspur   总被引:4,自引:0,他引:4  
In species where females copulate with more than one male during a single breeding attempt, males risk investing in offspring that are not their own. In the polygynandrous Smith's longspur (Calcarius pictus), females copulate sequentially with one to three males for each clutch of eggs and most of these males later assist in feeding the young. Using multilocus DNA profiling, we determined that there was mixed paternity in >75% of broods (n=31) but that few offspring (<1% of 114 nestlings) were sired by males outside the polygynandrous group. Male feeding rate increased significantly with the number of young sired, with males siring four nestlings feeding the brood at double the frequency of males siring only a single nestling. However, male Smith's longspurs appear to show a graded adjustment of paternal care in response to paternity only when other males are available to compensate for reduced care: feeding rate did not vary in relation to paternity when only one male provisioned young at the nest. There was no evidence that males could recognise their own offspring within a brood and feed them preferentially. The number of offspring sired by each male was significantly correlated with the number of days spent copulating with the attending female: on average, a male sired one offspring for every 2 days of copulatory access. If males use their access to females to estimate paternity (and thereby decide on their subsequent level of parental investment), a positive relationship is expected between the amount of female access and the subsequent feeding rate to the nestlings. Nonetheless, male feeding effort was only weakly correlated with female access and more study is needed to determine how males estimate their paternity in a brood. Received: 1 June 1997 / Accepted after revision: 1 April 1998  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