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   2870篇
  免费   369篇
  国内免费   787篇
  2024年   5篇
  2023年   115篇
  2022年   143篇
  2021年   190篇
  2020年   174篇
  2019年   208篇
  2018年   170篇
  2017年   209篇
  2016年   185篇
  2015年   177篇
  2014年   218篇
  2013年   294篇
  2012年   162篇
  2011年   175篇
  2010年   106篇
  2009年   118篇
  2008年   133篇
  2007年   140篇
  2006年   121篇
  2005年   102篇
  2004年   81篇
  2003年   76篇
  2002年   74篇
  2001年   71篇
  2000年   64篇
  1999年   34篇
  1998年   35篇
  1997年   36篇
  1996年   40篇
  1995年   48篇
  1994年   44篇
  1993年   36篇
  1992年   29篇
  1991年   25篇
  1990年   24篇
  1989年   22篇
  1988年   17篇
  1987年   16篇
  1986年   15篇
  1985年   17篇
  1984年   13篇
  1983年   11篇
  1982年   10篇
  1981年   8篇
  1980年   5篇
  1979年   6篇
  1978年   5篇
  1977年   8篇
  1976年   2篇
  1973年   5篇
排序方式: 共有4026条查询结果,搜索用时 15 毫秒
1.
Global biodiversity is facing an extinction crisis. Australia has one of the highest terrestrial species extinction rates in the world. Scientists, policy advisors and governments have recommended that the issue be addressed at a landscape-scale, while noting that there are significant knowledge gaps that are hampering implementation of such an approach. From 2011–2015, the Australian Government funded a transdisciplinary research program, the Landscapes and Policy Hub, to meet this need. Transdisciplinary research is widely acknowledged as essential to address the complexity of contemporary environmental problems. Given that such research programs are in their infancy, it is important to evaluate their efficacy and provide an empirical basis for improving their design. This paper presents an evaluation of the strategies fostering transdisciplinarity adopted by the Landscapes and Policy Hub. A heavy emphasis on communication, with skilled knowledge brokering, regular face-to-face meetings using participatory activities and shared field engagements enhanced transdisciplinary interaction between researchers and research users. However, establishing a fully integrated interdisciplinary research program remained a challenge. Efforts to enable shared conceptual frameworks to emerge through adaptive application of theory in practice could have been balanced with increased effort at the outset for researchers and research users to collaboratively formulate shared research questions, leading to the establishment of teams that could address these questions through cross-mobilisation of interdisciplinary expertise.  相似文献   
2.
3.
PurposeNew promising detectors are available for measuring small field size output factors (OFs). This study focused on a multicenter evaluation of two new generation detectors for OF measurements on CyberKnife systems.MethodsPTW-60019 microDiamond and W1 plastic scintillation detector (PSD) were used to measure OFs on eight CyberKnife units of various generations for 5–60 mm fixed cones. MicroDiamond and PSD OF were compared to routinely used silicon diodes data corrected applying published Monte Carlo (MC) factors. PSD data were corrected for Čerenkov Light Ratio (CLR). The uncertainties related to CLR determination were estimated.ResultsConsidering OF values averaged over all centers, the differences between MC corrected diode and the other two detectors were within 1.5%. MicroDiamond exhibited an over-response of 1.3% at 7.5 mm and a trend inversion at 5 mm with a difference of 0.2%. This behavior was consistent among the different units. OFs measured by PSD slightly under-responded compared to MC corrected diode for the smaller cones and the differences were within 1%. The observed CLR variability was 2.5% and the related variation in OF values was 1.9%.ConclusionThis study indicates that CyberKnife microDiamond OF require corrections below 2%. The results are enhanced by the consistency observed among different units. Scintillator shows a good agreement to MC corrected diode but CLR determination remains critical requiring further investigations. The results emphasized the value of a multi-center validation over a single center approach.  相似文献   
4.
The development of habitat suitability models requires a large amount of data which are rarely available. In this case, researchers need to get information on the ecological features of the studied species, based on the opinion of experts or on the literature, to construct a qualitative model. However, such models cannot be rigorously evaluated, as in most cases absence points are not available. In this paper, we assess the habitat suitability for a vulnerable insectivorous plant, Pinguicula crystallina Sibth. et Smith subsp. hirtiflora (Ten.) Strid (Lentibulariaceae) in the Campania region. Our aim was to develop an expert-based, presence-only model in support of possible conservation actions. Topographic and geological features of this species suggested by the literature were used in our model. Both the Boyce index and field surveys were chosen to evaluate the model's reliability. During field surveys, 31 absence sites and 1 new presence site were identified, and differences between sites with regard to water chemistry and quality were investigated, water being an element in the species habitat. Factors that affect reliability of the model, such as the lack of a large amount of information on the species and the limited spatial resolution of geographical information system data, are discussed.  相似文献   
5.
6.
7.
Environmental management, being an important component in strategies for achieving sustainable development of processes and products, has emerged as a proactive approach in majority of the manufacturing organizations. From the strategic perspective environmentally conscious manufacturing (ECM) programs lead to better environmental management practice. The objective of the current paper is to present an integrated and holistic framework to evaluate ECM programs. This framework combines three multi-criteria decision analysis (MCDA) methods to consider eight major environmentally conscious manufacturing indicators (ECMI) in order to identify the efficiency of each ECM program. First the interdependence relationship among the ECMIs is established using decision-making trail and evaluation laboratory (DEMATEL). Then a range of weightage (i.e. upper and lower bounds) is created for each ECMI using analytic network process (ANP) to include managerial preferences. Finally, this range of weightage for each indicator is applied to perform restricted multiplier data envelopment analysis (RMDEA). Results show that the technical efficiency of the inefficient ECM programs for integrated RMDEA, on average, is calculated as 53.2% whereas traditional input oriented DEA provides the same score as 72.3%. This clearly indicates that integrated RMDEA is better than the input oriented DEA because same level of output could be produced with lesser resources if the ECM programs perform on the frontier. Hence, the advantage of this methodology is that the managerial preferences are successfully implemented through this newly developed hybrid methodology that will help to reduce less resource consumption and lead to better environmental policy.  相似文献   
8.
1IntreductionTheliteratUreonmulti-Criteriondecisionmaking(MCDM)problemshas~tremendouslyintherecentpast.TwomajorareashaveevolvedwhiChbothconcentrateondecisionmakingwithseveralcriteria:multiobjectivedecisionmaking(MODM)andmulti-attributedecisionmaking(MADM).TheformerconcentratesoncontinuousdecisionspaceandthelatterfocusesonproblemswithdiscreteSPace.FuzzysettheoryhascontributedtoMODMproblemsaswellastheMADMProblems.ThegeneralMODMproblemcanbedeft.edLllasfollows:Twostagescangenerallybe…  相似文献   
9.
We develop an improved approach to evaluate car sharing options under uncertain environments with the combination of Fuzzy Analytic Hierarchy Process (F-AHP) and Fuzzy Technique for Order Preference by Similarity to Ideal Solution (F-TOPSIS), which consists of three steps. In the first step, we propose a SCUMN (Specific, Comprehensive, Understandable, Measurable, and Neutral) methodology to identify appropriate indicators and obtain a final list of 24 indicators according to their relevance to car sharing options. In the second step, we determine the weight of each indicator with F-AHP and conduct consistency check of the comparison matrix of selected indicators. In the third step, comparison of different options is performed with selected indicators and F-TOPSIS. A case study is provided to validate the proposed approach. Twenty-four indicators are identified to evaluate five different car sharing options and rank them according to their closeness coefficients in decreasing order. And thirty-one sensitivity analysis experiments are conducted to figure out the influence of indicators on decision making. The experimental results show that the proposed approach is capable of evaluating car sharing options with uncertainty and vagueness. F-AHP is able to determine the weight for each selected indicator and F-TOPSIS demonstrates its advantage in comparing potential options.  相似文献   
10.
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