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   2034篇
  免费   178篇
  国内免费   355篇
  2567篇
  2024年   14篇
  2023年   69篇
  2022年   84篇
  2021年   82篇
  2020年   101篇
  2019年   113篇
  2018年   106篇
  2017年   89篇
  2016年   103篇
  2015年   121篇
  2014年   130篇
  2013年   183篇
  2012年   71篇
  2011年   85篇
  2010年   72篇
  2009年   100篇
  2008年   102篇
  2007年   109篇
  2006年   93篇
  2005年   75篇
  2004年   73篇
  2003年   66篇
  2002年   58篇
  2001年   59篇
  2000年   46篇
  1999年   47篇
  1998年   37篇
  1997年   20篇
  1996年   27篇
  1995年   34篇
  1994年   24篇
  1993年   35篇
  1992年   25篇
  1991年   26篇
  1990年   19篇
  1989年   13篇
  1988年   10篇
  1987年   11篇
  1986年   6篇
  1985年   5篇
  1984年   7篇
  1983年   4篇
  1982年   6篇
  1981年   1篇
  1979年   1篇
  1978年   2篇
  1976年   1篇
  1975年   1篇
  1974年   1篇
排序方式: 共有2567条查询结果,搜索用时 13 毫秒
91.
Abstract

The deployment of high-altitude vehicles in near space with the purpose of providing Internet, communication, and other services represents the new frontier of aerospace activities. Near-space operations are attracting growing interest due to their mult-purpose nature and their anticipated high profitability. Despite such positive perceptions, near-space plans are, however, hampered by the uncertain international legal status of near space. Using the precedent of the exclusive economic zone (EEZ), this article suggests a new categorization of the near space as the exclusive utilization space (EUS) and a set of rules to manage its utilization.  相似文献   
92.
93.
94.
Among other concerns, safeguarding the supply chains of raw materials is an important task for industrial companies. Therefore, not surprisingly, the number of scientific publications concerning the evaluation of resource criticality has increased in recent years. However, it was noticed that currently published methodologies are too complex to be applied by industrial companies on a daily basis. For this reason, the need to develop a methodology that would allow not only assessing resource criticality, but could also be integrated into widely applied methodological frameworks as an additional driver to improve resource efficiency was identified. Geostrategic supply risk and economic importance were chosen as key indicators to analyze and assess relative resource criticality. The developed methodology was field tested by applying it to a resource‐intensive nitrogen fertilizer production company. Five scenarios for resource efficiency improvements, consisting of cleaner production and industrial symbiosis measures, were investigated. If all the proposed measures were implemented, consumption of natural gas would decrease by 3.552 million cubic meters per year (0.3% of the total consumption). However, not all identified measures contribute to a reduction of the overall criticality of resources for the production company. Nevertheless, the integration of criticality assessments into the widely applied methodologies for development and implementation of resource efficiency innovations is a valuable addition and should be included in the analysis for sustainable innovations and development.  相似文献   
95.
Phenotypic integration and plasticity are central to our understanding of how complex phenotypic traits evolve. Evolutionary change in complex quantitative traits can be predicted using the multivariate breeders’ equation, but such predictions are only accurate if the matrices involved are stable over evolutionary time. Recent study, however, suggests that these matrices are temporally plastic, spatially variable and themselves evolvable. The data available on phenotypic variance‐covariance matrix ( P ) stability are sparse, and largely focused on morphological traits. Here, we compared P for the structure of the complex sexual advertisement call of six divergent allopatric populations of the Australian black field cricket, Teleogryllus commodus. We measured a subset of calls from wild‐caught crickets from each of the populations and then a second subset after rearing crickets under common‐garden conditions for three generations. In a second experiment, crickets from each population were reared in the laboratory on high‐ and low‐nutrient diets and their calls recorded. In both experiments, we estimated P for call traits and used multiple methods to compare them statistically (Flury hierarchy, geometric subspace comparisons and random skewers). Despite considerable variation in means and variances of individual call traits, the structure of P was largely conserved among populations, across generations and between our rearing diets. Our finding that P remains largely stable, among populations and between environmental conditions, suggests that selection has preserved the structure of call traits in order that they can function as an integrated unit.  相似文献   
96.
This paper expands the existing literature on ethnicity and economic activity in Britain by studying the impact of religion and class. It argues that while the class location of the different South-Asian groups is important in determining their labour market outcomes, it does not operate independently from ethnicity; rather it is highly influenced by ethnicity in the process of determining the labour market participation of these groups. We use data obtained from the 2001 UK Census on Indian, Pakistani, and Bangladeshi men and women aged between twenty and twenty nine. Our findings confirm that class structure of the South-Asian groups is highly ethnicized, in that the ethno-religious background and class are interwoven to the extent that the separation between them is not easy, if not impossible.  相似文献   
97.
稻瘟病菌致病突变体的REMI诱变与鉴定   总被引:2,自引:0,他引:2  
以水稻“爱知旭”(Aichi—aShahi)为寄主,将带选择标记的质粒pCSN43和pBF101为外源DNA,利用限制酶诱导整合(REMI)这种新方法转化稻瘟病菌原生质体,从筛选到的数百个转化体中分离出3个与致病能力密切相关的突变体R2H65、R2H69和R281565。其中,R2H65和R2H69只产生畸形分生孢子,分生孢子的发育和附着胞的形成以及黑色素的合成均受到极大影响,致病性测试证明完全丧失致病能力;R281565的许多表型与野生型的相似,但致病能力却大大降低。  相似文献   
98.
With school days seemingly getting shorter and packed with more and more obligations, teachers integrate core subjects like science and reading in the hopes of delivering a curriculum that is meaningful and content rich. Using trade books, primary teachers can develop and deliver science content designed to introduce students to proenvironmental behaviors built around the Next Generation Science Earth and Human Activity Standard, Human Impacts on Earth Systems, ESS3-C. The Next Generation Science standards call for integrating literacy skills and science to develop knowledge of scientific topics. This article shares ideas on how primary teachers can use a variety of children’s literature as a foundation to inspire young students to take small steps toward developing proenvironmental behaviors. Under an umbrella of three environmental themes, proenvironmental behavior, trash, and water pollution, the article describes key concepts, vocabulary, and activities designed to develop a mindfulness of proenvironmental behaviors for each selected text. While texts and learning experiences are intentionally focused on young learners (Kindergarten/first grade), they can be adapted and used in all primary grades.  相似文献   
99.
T. Ren  Z. Zhang  R. Fu  Y. Yang  W. Li  J. Liang  G. Mo  W. Luo  X. Zhang 《Animal genetics》2020,51(4):568-578
Parathyroid hormone (PTH) is a crucial regulator of calcium homeostasis and bone remodeling, and the parathyroid hormone 1 receptor (PTH1R) belongs to a class II G-protein-coupled receptor. PTH activates PTH1R, which mediates catabolic and anabolic processes in the skeleton. However, the functional mechanism of PTH1R has not been thoroughly elucidated in organisms. This study identified a 51 bp indel mutation in the first intron of the PTH1R gene and elucidated the effect of this gene mutation on the growth and carcass traits in chickens. The results indicated that the 51 bp indel was significantly associated with subcutaneous fat thickness, abdominal fat weight, body weight and daily gain over 4–8 weeks. Furthermore, we found that PTH1R gene expression was highest in the kidney and liver tissues, and it showed a trend of decreasing in leg and breast muscle tissues at different embryonic stages. In addition, we examined the expression of the three genotypes of the PTH1R gene in the liver, breast muscle and abdominal fat and found that the II genotype was significantly higher than the DD and ID genotypes. In summary, these findings suggest that the PTH1R gene can serve as a potential molecular marker for chicken breeding.  相似文献   
100.
Decoding health and disease phenotypes is one of the fundamental objectives in biomedicine. Whereas high-throughput omics approaches are available, it is evident that any single omics approach might not be adequate to capture the complexity of phenotypes. Therefore, integrated multi-omics approaches have been used to unravel genotype–phenotype relationships such as global regulatory mechanisms and complex metabolic networks in different eukaryotic organisms. Some of the progress and challenges associated with integrated omics studies have been reviewed previously in comprehensive studies. In this work, we highlight and review the progress, challenges and advantages associated with emerging approaches, integrating gene expression and protein-protein interaction networks to unravel network-based functional features. This includes identifying disease related genes, gene prioritization, clustering protein interactions, developing the modules, extract active subnetworks and static protein complexes or dynamic/temporal protein complexes. We also discuss how these approaches contribute to our understanding of the biology of complex traits and diseases. This article is part of a Special Issue entitled: Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.  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