全文获取类型
收费全文 | 3612篇 |
免费 | 474篇 |
国内免费 | 650篇 |
出版年
2024年 | 5篇 |
2023年 | 89篇 |
2022年 | 85篇 |
2021年 | 160篇 |
2020年 | 180篇 |
2019年 | 197篇 |
2018年 | 173篇 |
2017年 | 179篇 |
2016年 | 199篇 |
2015年 | 168篇 |
2014年 | 210篇 |
2013年 | 285篇 |
2012年 | 190篇 |
2011年 | 182篇 |
2010年 | 165篇 |
2009年 | 185篇 |
2008年 | 183篇 |
2007年 | 207篇 |
2006年 | 187篇 |
2005年 | 174篇 |
2004年 | 118篇 |
2003年 | 124篇 |
2002年 | 109篇 |
2001年 | 86篇 |
2000年 | 84篇 |
1999年 | 98篇 |
1998年 | 77篇 |
1997年 | 65篇 |
1996年 | 56篇 |
1995年 | 53篇 |
1994年 | 56篇 |
1993年 | 56篇 |
1992年 | 50篇 |
1991年 | 53篇 |
1990年 | 31篇 |
1989年 | 35篇 |
1988年 | 23篇 |
1987年 | 18篇 |
1986年 | 22篇 |
1985年 | 21篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 20篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1958年 | 1篇 |
排序方式: 共有4736条查询结果,搜索用时 578 毫秒
101.
Recent studies point to the need for improved understanding of environmental management frameworks designed to combine qualitative public and quantitative technical inputs in decision-making processes. Flux in public perception and concern about risks imply frameworks must be iterative in nature and incorporate a variety of assessment triggers in the form of decision points. A conceptual model is proposed here to explain the de facto operation of standard risk analytic frameworks within the broader sociopolitical milieu of public policy. The model is presented as a decision flow diagram that emphasizes setting environmental management goals based on societal input and the formulation of decision criteria for selecting management actions to achieve those goals. Prospective and retrospective decision control points operate to select management options that, respectively, avoid or reduce actual or predicted effects. Feedback loops that modify risk management outcomes are identified. Technical and scientific inputs (i.e., risk analysis) are assigned an essential information role within the framework and are responsible for informing the management process with the results of appropriately conducted and reviewed investigations. The proposed model is intended primarily to indicate how environmental risk management decision-making and associated technical assessments may be influenced by social pressures. It is hoped this understanding will lead to analytical transparency and better public communication of the environmental implications of policy options. 相似文献
102.
103.
Thomas G. Tornabene 《Journal of molecular evolution》1978,11(3):253-257
Summary
Halobacterium cutirubrum was successfully cultivated under aerobic and microaerobic conditions. The early stationary phase of growth was obtained at 2.2 days and 45–55 days for aerated and non-aerated cultures, respectively. The dry cell yields were 0.7–1.2 gm/l in all preparations grown to early stationary growth phase. The cellular ratio of squalene to dihydro- and tetra-hydrosqualene decreased proportionately with decreased aeration rates. 相似文献
104.
A checklist of identified oribatid mite taxa from riverine freshwater environments from six islands in Polynesia (New Caledonia, Tahiti, Moorea, Rurutu, Tubuai, Raiatea) is presented; 18 species, 16 genera and eight families were recorded. Trhypochthoniellus longisetus (Berlese, 1904) and Trimalaconothrus albulus Hammer, 1972 prevailed on distribution. Fortuynia smiti
sp. n. (Fortuyniidae) is described from New Caledonia. The new speciesis morphologically most similar to Fortuynia marina Hammen, 1960 from New Guinea, but it differs from the latter by the longer notogastral setae dm, lm, c2, p1, epimeral setae 3b and adanal setae ad1 and the presence of prodorsal lateral ridges. 相似文献
105.
R Seaton 《New Zealand journal of zoology.》2013,40(1):16-27
Abstract Internationally, birds of prey are often reported as being relatively prone to collision with wind turbines in comparison to other groups of birds. However, as yet it is unclear to what extent New Zealand's only endemic bird of prey, the New Zealand falcon (Falco novaeseelandiae), is at risk. In this paper we summarise the potential for wind farms to impact New Zealand falcon, evaluate the efficacy of a range of risk assessment and post-consent monitoring practices, and present options for mitigating and/or offsetting any residual effects. We conclude that the lack of knowledge on the effects of wind farms on New Zealand falcon is the result of inconsistency in the assessment methods thus far employed and the absence of a coordinated approach to monitoring methods and the dissemination of results. To remedy this we present a risk assessment framework that, if adopted, will provide the information necessary to ensure alternative energy targets can be met without compromising the conservation of this threatened species. 相似文献
106.
Life history responses of fishes to culture 总被引:2,自引:0,他引:2
J. E. Thorpe† 《Journal of fish biology》2004,65(S1):263-285
The protected environment in culture permits fishes to reduce the proportion of energy normally channelled into the costs associated with competition for food, shelter and mates, avoidance of predators and counteracting parasites and diseases. The surplus energy so released is allocated to growth and reproduction, accelerating development through increased growth rate, earlier maturation and increased relative fecundity. Cultivators manipulate the rearing environment to remove seasonal variation in availability of resources, so that the fishes grow and develop through otherwise unproductive seasons. Such environmental manipulations exaggerate the basic accelerative effects. Since maturation deflects energy from growth, farmers also manipulate the fishes nutritionally, physiologically, hormonally and genetically to postpone maturation. As environmental regulators determine sex in many fish species, environmental manipulation in culture may have unintended effects on sex ratios. Mortality in culture should be very low, but survival of fishes released from culture is rarely as high as that of wild conspecifics. Finally, while short life‐cycles and simplified population age‐structures permit high rates of production in farms, they lead to ecological instability when the fishes are cultured for support of wild populations. 相似文献
107.
The Escherichia coli d-xylose isomerase (d-xylose ketol-isomerase, EC 5.3.1.5) gene, xylA, has been cloned on various E. coli plasmids. However, it has been found that high levels of overproduction of the d-xylose isomerase, the protein product of the xylA gene, cannot be accomplished by cloning the intact gene on high copy-number plasmids alone. This is believed to be due to the fact that the expression of the gene through its natural promoter is highly regulated in E. coli. In order to overcome this, the xylA structural gene has been fused with other strong promoters such as tac and lac, resulting in the construction of a number of fused genes. Analysis of the E. coli transformants containing the fused genes, cloned on high copy-number plasmids, indicated that a 20-fold overproduction of the enzyme can now be obtained. It is expected that overproduction of the enzyme in E. coli can still be substantially improved through additional manipulation with recombinant DNA techniques. 相似文献
108.
109.
Yuanzhi Zhu Joshua Sokolowski Xiancheng Song Yanghua He Yi Mei Gang Wu 《Liver Transplantation》2020,10(11)
Carbon‐based heteroatom‐coordinated single‐atom catalysts (SACs) are promising candidates for energy‐related electrocatalysts because of their low‐cost, tunable catalytic activity/selectivity, and relatively homogeneous morphologies. Unique interactions between single metal sites and their surrounding coordination environments play a significant role in modulating the electronic structure of the metal centers, leading to unusual scaling relationships, new reaction mechanisms, and improved catalytic performance. This review summarizes recent advancements in engineering of the local coordination environment of SACs for improved electrocatalytic performance for several crucial energy‐convention electrochemical reactions: oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, CO2 reduction reaction, and nitrogen reduction reaction. Various engineering strategies including heteroatom‐doping, changing the location of SACs on their support, introducing external ligands, and constructing dual metal sites are comprehensively discussed. The controllable synthetic methods and the activity enhancement mechanism of state‐of‐the‐art SACs are also highlighted. Recent achievements in the electronic modification of SACs will provide an understanding of the structure–activity relationship for the rational design of advanced electrocatalysts. 相似文献
110.