全文获取类型
收费全文 | 663篇 |
免费 | 78篇 |
国内免费 | 182篇 |
专业分类
安全科学 | 101篇 |
废物处理 | 17篇 |
环保管理 | 83篇 |
综合类 | 370篇 |
基础理论 | 164篇 |
污染及防治 | 62篇 |
评价与监测 | 31篇 |
社会与环境 | 62篇 |
灾害及防治 | 33篇 |
出版年
2024年 | 4篇 |
2023年 | 7篇 |
2022年 | 28篇 |
2021年 | 20篇 |
2020年 | 25篇 |
2019年 | 25篇 |
2018年 | 30篇 |
2017年 | 34篇 |
2016年 | 35篇 |
2015年 | 43篇 |
2014年 | 36篇 |
2013年 | 63篇 |
2012年 | 67篇 |
2011年 | 70篇 |
2010年 | 46篇 |
2009年 | 66篇 |
2008年 | 41篇 |
2007年 | 61篇 |
2006年 | 40篇 |
2005年 | 31篇 |
2004年 | 20篇 |
2003年 | 10篇 |
2002年 | 23篇 |
2001年 | 13篇 |
2000年 | 14篇 |
1999年 | 15篇 |
1998年 | 11篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1987年 | 3篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1975年 | 2篇 |
排序方式: 共有923条查询结果,搜索用时 250 毫秒
731.
目的 探究不同电流密度下,直流和脉冲2种电流模式对钢筋混凝土电化学除氯的影响,降低钢筋渗氢量,减小通电后阴极产生的氢气使钢筋发生氢脆的风险,提高除氯效率,改善电化学除氯效率的均匀性。方法 通过在电流密度为2、3、4 A/m2的条件下,对混凝土进行直流和脉冲电流电化学修复,测定处理后的钢筋的释氢电流,计算释氢总电量,来表示钢筋渗氢量,对比处理后强电流区和弱电流区混凝土的氯离子含量,分析2种方式除氯效率的均匀性,同时用扫描电子显微镜观察电化学除氯前后混凝土的微观结构。结果 计算得到不同通电电流密度下钢筋的释氢总电量和除氯效率,脉冲电流的释氢总电量比直流电流低约20%,而除氯效率提高了约10%。结论 增加电流密度可以提高除氯效率,但是会增加钢筋发生氢脆的风险。相较于直流电流,脉冲电流电化学修复可以降低钢筋渗氢量,显著提高弱电流区的除氯效率,从而改善除氯效率的均匀性。 相似文献
732.
733.
734.
Extrapolating cetacean densities to quantitatively assess human impacts on populations in the high seas
下载免费PDF全文
![点击此处可从《Conservation biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Laura Mannocci Jason J. Roberts David L. Miller Patrick N. Halpin 《Conservation biology》2017,31(3):601-614
As human activities expand beyond national jurisdictions to the high seas, there is an increasing need to consider anthropogenic impacts to species inhabiting these waters. The current scarcity of scientific observations of cetaceans in the high seas impedes the assessment of population‐level impacts of these activities. We developed plausible density estimates to facilitate a quantitative assessment of anthropogenic impacts on cetacean populations in these waters. Our study region extended from a well‐surveyed region within the U.S. Exclusive Economic Zone into a large region of the western North Atlantic sparsely surveyed for cetaceans. We modeled densities of 15 cetacean taxa with available line transect survey data and habitat covariates and extrapolated predictions to sparsely surveyed regions. We formulated models to reduce the extent of extrapolation beyond covariate ranges, and constrained them to model simple and generalizable relationships. To evaluate confidence in the predictions, we mapped where predictions were made outside sampled covariate ranges, examined alternate models, and compared predicted densities with maps of sightings from sources that could not be integrated into our models. Confidence levels in model results depended on the taxon and geographic area and highlighted the need for additional surveying in environmentally distinct areas. With application of necessary caution, our density estimates can inform management needs in the high seas, such as the quantification of potential cetacean interactions with military training exercises, shipping, fisheries, and deep‐sea mining and be used to delineate areas of special biological significance in international waters. Our approach is generally applicable to other marine taxa and geographic regions for which management will be implemented but data are sparse. 相似文献
735.
密度场干涉测量及其条纹处理计算 总被引:2,自引:0,他引:2
本文将普通纹影仪改造成激光差分干涉仪,并用于火焰等密度场的显示与测量。编制了条纹预处理和细化软件系统,并对干涉条纹进行了处理和细化计算。另外,针对细化后的条纹,编制了密度场的计算软件,并对蜡烛火焰和超声速轴对称绕流流场进行了计算,得到了密度场定量的结果。 相似文献
736.
选取了轨道交通车辆用量较多的橡胶典型功能部件折棚式内风挡、外风挡和空气弹簧胶囊用材料,依据最新的业内通用标准EN 45545-2∶2020,同时进行烟毒性和热释放速率试验。试验结果表明,现用橡胶典型功能性部件除烟毒性指数(CITG)外,烟密度(Ds)和最大平均热释放速率(MARHE)均无法满足EN 45545-2∶2020防火标准的HL2级要求。面对这一问题,生产厂家应在满足部件功能性的前提下,尽量提高产品的防火性能;同时,为确保部件自身的功能性,标准制定方在制定指标要求时也应适当放宽这些材料的防火阻燃要求。 相似文献
737.
Zhang Z Zhan X Yan L Li M Hu J Wei F 《Environmental science and pollution research international》2009,16(1):79-84
Background, aim, and scope Foraging patches can be described as a nested hierarchy of aggregated resources, implying that study of foraging by wild animals
should be directed across different spatial scales. However, almost all previous research on habitat selection by the giant
panda has concentrated upon one scale. In this research, we carried out a field study to understand foraging patch selection
by giant pandas in winter at both microhabitat and feeding site scales and, for the first time, attempted to understand how
long it would stay at the feeding sites before moving on.
Materials and methods The field survey was conducted from November 2002 to March 2003 at Fengtongzhai Nature Reserve (102°48′–103°00′ E, 30°19′–30°47′
N), Baoxing County of Sichuan Province, China, to collect data in both microhabitat and control plots. The microhabitat plots
were located by fresh feces or foraging traces left by giant pandas, and the control plots were established to reflect the
environment. Within each microhabitat plot, one 1 × 1 m2 plot was centralized at the center of each feeding site, in which numbers of old bamboos and old shoots, including eaten
and uneaten, were counted, respectively.
Results The results showed that winter microhabitats selected by this species were characteristic of gentle slopes and high old-shoot
proportions and that the latter was even higher at feeding sites. Two selection processes, namely, from the environment to
microhabitats and from the latter to feeding sites, were found during this species’ foraging patch utilization. Giant pandas
preferred to eat old shoots to old bamboo at feeding sites in winter and did not leave unless old-shoot density fell to lower
than the average in the environment.
Discussion Both microhabitats and feeding sites selected by giant pandas were characteristic of high old-shoot density, indicating that
the preferred food item had a significant influence upon its foraging patch selection. The preference for gentle slopes by
giant pandas was presumed to save energy in movement or reflect the need to sit and free its fore-limbs to grasp bamboo culms
when feeding but also seemed to be correlated with an easier access to old shoots. The utilization of old shoots at feeding
sites was assumed to help maximize energy or nutrient intake during their foraging.
Conclusions The difference between microhabitat plots and control plots and between microhabitats and feeding sites uncovered a continuous
selection process from the environment via microhabitats to feeding sites. The utilization of old shoots at feeding sites
was parallel to the marginal value theorem. The selection and abandonment of foraging patches by giant pandas was an optimal
behavioral strategy adapted to their peculiar food with high cellulose and low protein.
Recommendations and perspectives Our results uncovered the importance of multiple scales in habitat selection research. To further understand the process of
habitat selection, future research should pay more attention to resolve the question of how to locate foraging patches under
dense bamboo forests by the giant panda, which was traditionally considered to have poor eyesight, although our research has
answered what type of habitats the giant panda prefers and when to leave. 相似文献
738.
In this paper new analytical inspection strategies, based on hyperspectral imaging (HSI) in the VIS–NIR and NIR wavelength ranges (400–1000 and 1000–1700 nm, respectively), have been investigated and set up in order to define quality control logics that could be applied at industrial plant level for polyolefins recycling. The research was developed inside the European FP7 Project W2Plastics “Magnetic Sorting and Ultrasound Sensor Technologies for Production of High Purity Secondary Polyolefins from Waste”. The main aim of the project is the separation of pure polyethylene and polypropylene adopting an innovative process, the magnetic density separation (MDS). Spectra of plastic particles and contaminants resulting from post-consumer complex wastes and of virgin polyolefins have been acquired by HSI and by Raman spectroscopy. The classification results obtained applying principal component analysis (PCA) on HSI data have been compared with those obtained by Raman spectroscopy, in order to validate the proposed innovative methodology. Results showed that HSI sensing techniques allow to identify both polyolefins and contaminants. Results also demonstrated that HSI has a great potentiality as a tool for quality control of feed (identification of contaminants in the plastic waste) and of the two different pure polypropylene and polyethylene flow streams resulting from the MDS-based recycling process. 相似文献
739.
Persistent organic pollutants (POPs), xenobiotics that accumulate in fat tissue, may impair bone metabolism. We studied (1) the association of bone mineral density (BMD) with POPs and (2) whether associations of fat mass (FM) or lean mass (LM), two components of body composition, with BMD differed depending on levels of POPs. Participants aged ?20 in the National Health and Nutrition Examination Survey 1999-2004 were included (n = 2769). Eight POPs with detection rate ?80% and three skeletal subregions (left arm, pelvis, and right leg) were selected. All analyses were stratified by gender and age (cutpoint 50 years or more). POPs at background concentrations were mostly unassociated with BMD. However, the associations of FM and LM with BMD depended on POPs concentrations, in particular with BMD of the left arm (usually not weight-bearing) in postmenopausal women. When POPs concentrations were low, FM showed inverse associations with BMD while LM showed positive associations. However, when POPs levels were high, FM showed positive associations with BMD while the positive associations between LM and BMD weakened. POPs may biologically modify the associations of FM and LM with BMD, especially among postmenopausal women, possibly explaining inconsistent associations between FM and BMD in previous epidemiological studies. 相似文献
740.
Abstract: Roots of riparian vegetation increase streambank erosion resistance and structural stability; therefore, knowledge of root density and distribution in streambanks is useful for stream management and restoration. The objective of this study was to compare streambank root distributions for herbaceous and woody vegetation and to develop empirical models to predict root density. Root length density, root volume ratio, soil physical and chemical properties, and above‐ground vegetation densities were measured at 25 sites on six streams in southwestern Virginia. The Mann‐Whitney test was used to determine differences in root density along stream segments dominated by either woody or herbaceous vegetation. Multiple linear regression was used to develop relationships between root density and site characteristics. Study results showed that roots were evenly distributed across the bank face with the majority of roots having diameters less than 2 mm. Soil bulk density and above‐ground vegetation were key factors influencing root density. While significant relationships were developed to predict root density, the predictive capabilities of the equations was low. Because of the highly variable nature of soil and vegetation properties, it is recommended at this time that soil erodibility and root density be measured in the field for design and modeling purposes, rather than estimated based on empirical relationships. 相似文献