全文获取类型
收费全文 | 8066篇 |
免费 | 91篇 |
国内免费 | 242篇 |
专业分类
安全科学 | 155篇 |
废物处理 | 394篇 |
环保管理 | 919篇 |
综合类 | 1358篇 |
基础理论 | 1977篇 |
污染及防治 | 1991篇 |
评价与监测 | 607篇 |
社会与环境 | 950篇 |
灾害及防治 | 48篇 |
出版年
2023年 | 58篇 |
2022年 | 128篇 |
2021年 | 104篇 |
2020年 | 67篇 |
2019年 | 99篇 |
2018年 | 145篇 |
2017年 | 154篇 |
2016年 | 213篇 |
2015年 | 159篇 |
2014年 | 259篇 |
2013年 | 696篇 |
2012年 | 280篇 |
2011年 | 378篇 |
2010年 | 310篇 |
2009年 | 345篇 |
2008年 | 395篇 |
2007年 | 401篇 |
2006年 | 308篇 |
2005年 | 291篇 |
2004年 | 288篇 |
2003年 | 280篇 |
2002年 | 269篇 |
2001年 | 340篇 |
2000年 | 211篇 |
1999年 | 111篇 |
1998年 | 85篇 |
1997年 | 83篇 |
1996年 | 107篇 |
1995年 | 126篇 |
1994年 | 96篇 |
1993年 | 84篇 |
1992年 | 90篇 |
1991年 | 92篇 |
1990年 | 79篇 |
1989年 | 71篇 |
1988年 | 69篇 |
1987年 | 75篇 |
1986年 | 52篇 |
1985年 | 63篇 |
1984年 | 63篇 |
1983年 | 59篇 |
1982年 | 58篇 |
1981年 | 64篇 |
1980年 | 47篇 |
1979年 | 46篇 |
1977年 | 38篇 |
1976年 | 36篇 |
1974年 | 34篇 |
1973年 | 43篇 |
1972年 | 51篇 |
排序方式: 共有8399条查询结果,搜索用时 296 毫秒
991.
Thermal and mechanical properties of poly(lactic Acid) and poly(ethylene/butylene Succinate) blends 总被引:2,自引:0,他引:2
In this study, blends of poly (lactic acid) (PLA) with poly(ethylene/butylene succinate) (Bionolle) have been investigated
for their thermal and mechanical properties as a function of the concentration of Bionolle. Differential scanning calorimetry
(DSC), dynamic mechanical analysis (DMA), and tensile tests were used to characterize the blends. From the results of the
DMA and DSC, it was found that this blend system was not miscible within the compositions studied. DSC results showed that
adding Bionolle aids in crystallization of PLA. It was observed that increasing the Bionolle concentration led to a slight
increase in the strain-at-break of the blends but a decrease in the Young’s modulus and ultimate tensile strength. Biaxially
oriented films showed an increase in tensile strength, modulus, and strain-at-break. 相似文献
992.
The starch content and its composition have important consequences for the yield of the harvested crop and the materials extracted from it. The functional properties of the foods or other processed materials derived from these crops are also affected by the structure and composition of the starch. Recently, genetic engineering has been used to produce plants with an elevated starch content, achieved by transforming the plant with a mutated bacterial gene coding for an ADPglucose pyrophosphorylase that is active in the presence of metabolites which inhibit the plant enzyme. Besides the practical implications of these results, this experiment provided direct evidence for the regulatory role of the ADPglucose pyrophosphorylase in starch synthesis. Other bacterial enzymes, such as glycogen synthase and branching enzyme, could be introduced in order to modify starch structure. However, a more elegant (but longer-term) approach would be to learn enough about the structure-function relationships of the plant enzymes so that the product of their action could be changed. To achieve this objective, much more will have to be learned about the enzymes involved in the biosynthesis of starch than is presently known. Here, the basic properties of starch and the current research approaches to understanding its biosynthesis are described, together with a perspective of how genetic manipulation of starch structure may be achieved.Paper presented at the Bio/Environmentally Degradable Polymer Society—Third National Meeting, June 6–8, 1994, Boston, Massachusetts. 相似文献
993.
In many locations, regulatory agencies do not permit tree planting above landfills that are sealed with a capping clay, because
roots might penetrate the clay barrier and expose landfill contents to leaching. We find, however, no empirical or theoretical
basis for this restriction, and instead hypothesize that plant roots of any kind are incapable of penetrating the dense clays
used to seal landfills. As a test, we excavated 30 trees and shrubs, of 12 species, growing over a clay-lined municipal sanitary
landfill on Staten Island, New York. The landfill had been closed for seven years, and featured a very shallow (10 to 30-cm)
soil layer over a 45-cm layer of compacted grey marl (Woodbury series) clay. The test plants had invaded naturally from nearby
forests. All plants examined—including trees as tall as 6 m—had extremely shallow root plates, with deformed tap roots that
grew entirely above and parallel to the clay layer. Only occasional stubby feeder roots were found in the top 1 cm of clay,
and in clay cracks at depths to 6 cm, indicating that the primary impediment to root growth was physical, although both clay
and the overlying soil were highly acidic. These results, if confirmed by experimental research should lead to increased options
for the end use of many closed sanitary landfills. 相似文献
994.
995.
Disturbance of natural vegetation by camping: Experimental applications of low-level stress 总被引:3,自引:0,他引:3
David N. Cole 《Environmental management》1995,19(3):405-416
Previously undisturbed sites in four different vegetation types were camped on for one night and for four nights. Changes
in vegetation cover and vegetation height were measured after camping and one year later. Results are presented separately
for different campsite zones—parts of the site where campers slept, cooked meals, and stored their packs. Just one night of
camping was sufficient to cause evident impact in all four vegetation types, although the amount of impact varied significantly
between zones and between vegetation types. Vegetation impact on campsites used four nights was generally less than twice
as severe as impact on the sites used one night. The effects of camping on vegetation were also predicted for 12 other vegetation
types on the basis of vegetational responses to experimental trampling. These results suggest that impact can almost always
be minimized by confining camping to a small number of campsites instead of dispersing use across many campsites. 相似文献
996.
997.
998.
999.
S. K. Jain G. N. Yoganarasimhan S. M. Seth 《Journal of the American Water Resources Association》1992,28(6):1037-1043
ABSTRACT: Many approaches are available for operation of a multipurpose reservoir during flood season; one of them is allocation of storage space for flood control. A methodology to determine a reservoir operation policy based on explicit risk consideration is presented. The objective of the formulation is to maximize the reservoir storage at the end of a flood season while ensuring that the risk of an overflow is within acceptable limits. The Dynamic Programming technique has been used to solve the problem. This approach has been applied to develop operation policies for an existing reservoir. The performance of the policy was evaluated through simulation and was found to be satisfactory. 相似文献
1000.
Ronald A. Chadderton Robert G. Traver Jayaram N. Rao 《Journal of the American Water Resources Association》1992,28(6):1069-1076
ABSTRACT: This paper presents a study which used a standard, hydraulic computer model to generate detailed design information to support conflict analysis of a water resource use issue. As an extension of previous studies, the conflict analysis in this case included several scenarios for stability analysis - all of which reached the conclusion that compromising, shared access to the water resources available would result in the most benefits to society. This expected equilibrium outcome was found to maximize benefit-cost estimates. 相似文献