全文获取类型
收费全文 | 38828篇 |
免费 | 3500篇 |
国内免费 | 21篇 |
专业分类
42349篇 |
出版年
2023年 | 235篇 |
2022年 | 494篇 |
2021年 | 1061篇 |
2020年 | 610篇 |
2019年 | 772篇 |
2018年 | 917篇 |
2017年 | 770篇 |
2016年 | 1194篇 |
2015年 | 1985篇 |
2014年 | 2210篇 |
2013年 | 2345篇 |
2012年 | 3315篇 |
2011年 | 3022篇 |
2010年 | 1940篇 |
2009年 | 1666篇 |
2008年 | 2397篇 |
2007年 | 2349篇 |
2006年 | 2124篇 |
2005年 | 2006篇 |
2004年 | 1843篇 |
2003年 | 1736篇 |
2002年 | 1614篇 |
2001年 | 339篇 |
2000年 | 238篇 |
1999年 | 342篇 |
1998年 | 413篇 |
1997年 | 273篇 |
1996年 | 247篇 |
1995年 | 211篇 |
1994年 | 214篇 |
1993年 | 220篇 |
1992年 | 217篇 |
1991年 | 203篇 |
1990年 | 191篇 |
1989年 | 167篇 |
1988年 | 174篇 |
1987年 | 162篇 |
1986年 | 129篇 |
1985年 | 129篇 |
1984年 | 165篇 |
1983年 | 124篇 |
1982年 | 153篇 |
1981年 | 145篇 |
1980年 | 133篇 |
1979年 | 103篇 |
1978年 | 107篇 |
1977年 | 99篇 |
1976年 | 82篇 |
1974年 | 71篇 |
1973年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
Mohamed Boukhary Omar Cherif Ahmed N. El-Barkooky Samy A. Mohamed Yasmine Hussein-Kamel Daniel Girgis Hanna 《Historical Biology》2016,28(5):613-655
The stratigraphic framework of the Neogene sequence drilled by two offshore wells located in the north-eastern shore of the Nile Delta (the wells Sekhmet-1 and Sekhmet-2) has been established. The lithostratigraphic units with their sequences, from older to younger, are as follows: the Sidi Salim Formation (including Sr1 SB, Sr2 SB, Sr2 MFS, Sr3 SB and Sr3 MFS), a sequence representing the uppermost part of the Sidi Salim and most of the lower part of the Qawasim Formations (including Tor 1.1 SB, Tor 1.2 SB, Tor 1.3 SB, Tor 1.4 SB and ?Tor 2 SB), a sequence representing the uppermost part of Qawasim and the lower part of the Abu Madi Formations (including ?Me1 SB, Me2 SB and Me2 MFS), the Kafr El Sheikh Formation (including alternatively Za 1 and 2 SB and MFS and Ge 1 SB and MFS), the El Wastani Formation (including Ge 2 SB and MFS) and a Quaternary sequence represented by the topmost part of El Wastani and Mit Ghamr/Bilqas Formation (including alternatively ?Cala 1 and 2 SB and MFS and ?Io 2 SB). The lower part of the Qawasim in well Sekhmet-2 includes two LST: Tor 2 LST and Me 1 LST. 相似文献
962.
963.
High‐Performance Liquid Chromatographic Resolution of Neutral and Cationic Hetero[6]Helicenes 下载免费PDF全文
Geraldine M. Labrador Johann Bosson Zachary S. Breitbach Yeeun Lim Eric R. Francotte Rocchina Sabia Claudio Villani Daniel W. Armstrong Jérôme Lacour 《Chirality》2016,28(4):282-289
Cationic hetero[6]helicenes 1+ , 2+ and 3+ have been recently disclosed. Herein we report on their enantiomeric separation using high‐performance liquid chromatography. Separation of the antipodes can be achieved in preparative scale on neutral adducts with Chiralcel OD‐I or Chiralpak ID CSP. Selectivity factors of 1.90, 1.67, and 1.96 were obtained for 1-H , 2-H , and 3-H , respectively. Separation can also be performed on the carbenium ions on regular Chiralpak IA CSP using water‐containing eluents, thus allowing for enantiomeric purity determinations in aqueous environments. Resolution of neutral and cationic helicenes is also achieved on more recently developed LARIHC columns. The versatility of the cyclofructan phases allows for baseline separations for both cases and their loading capabilities are demonstrated. Finally, the configurational stability of 1+ , 2+ , and 3+ was measured. For each replacement of an oxygen atom by an amino group, the racemization barrier increases significantly (ΔG? = 29.8, 36.3 and >37 kcal mol‐1 for 1+ , 2+ , and 3+ respectively). Chirality 28:282–289, 2016. © 2016 Wiley Periodicals, Inc. 相似文献
964.
Timothée Poisot Benjamin Baiser Jennifer A. Dunne Sonia Kéfi François Massol Nicolas Mouquet Tamara N. Romanuk Daniel B. Stouffer Spencer A. Wood Dominique Gravel 《Ecography》2016,39(4):384-390
The study of ecological networks is severely limited by 1) the difficulty to access data, 2) the lack of a standardized way to link meta‐data with interactions, and 3) the disparity of formats in which ecological networks themselves are stored and represented. To overcome these limitations, we have designed a data specification for ecological networks. We implemented a database respecting this standard, and released an R package (rmangal) allowing users to programmatically access, curate, and deposit data on ecological interactions. In this article, we show how these tools, in conjunction with other frameworks for the programmatic manipulation of open ecological data, streamlines the analysis process and improves replicability and reproducibility of ecological network studies. 相似文献
965.
Reconstructing changes in the genotype,phenotype, and climatic niche of an introduced species 下载免费PDF全文
Daniel Z. Atwater U. Uzay Sezen Valorie Goff Wenqian Kong Andrew H. Paterson Jacob N. Barney 《Ecography》2016,39(9):894-903
An introduced species must contend with enormous environmental variation in its introduced range. In this study, we use niche models and ordination analyses to reconstruct changes in genotype, phenotype, and climatic niche of Johnsongrass Sorghum halepense, which is regarded as one of the world's most threatening invasive plants. In the United States, Johnsongrass has rapidly evolved within‐ and among‐population genetic diversity; our results show that genetic differentiation in expanding Johnsongrass populations has resulted in phenotypic variation that is consistent with habitat and climatic variation encountered during its expansion. Moreover, Johnsongrass expanded from agricultural to non‐agricultural habitat, and now, despite occupying overlapping ranges, extant agricultural and non‐agricultural populations are genetically and phenotypically distinct and manifest different plastic responses when encountering environmental variation. Non‐agricultural accessions are broadly distributed in climatic and geographic space and their fitness traits demonstrate plastic responses to common garden conditions that are consistent with local specialization. In contrast, agricultural accessions demonstrate ‘general purpose’ plastic responses and have more restricted climatic niches and geographic distributions. They also grow much larger than non‐agricultural accessions. If these differences are adaptive, our results suggest that adaptation to local habitat variation plays a crucial role in the ecology of this invader. Further, its success relates to its ability to succeed on dual fronts, by responding simultaneously to habitat and climate variability and by capitalizing on differential responses to these factors during its range expansion. 相似文献
966.
Estefania Ugarte‐Berzal Erik Martens Lise Boon Jennifer Vandooren Daniel Blockmans Paul Proost Ghislain Opdenakker 《Journal of cellular and molecular medicine》2019,23(1):576-585
Gelatin zymography analysis is a sensitive method and commonly used to characterize and quantify the presence of the gelatinases (MMP‐2 and MMP‐9) in biological samples. In human plasma samples from healthy controls and systemic lupus erythematosus (SLE) patients, we observed a gelatinolytic molecule at 80 kDa, suggestive for activated human MMP‐9. However, by developing and using the EDTA/gelatin zymography method and after purification of the 80 kDa entity, we proved that this molecule was the C1s subunit of the complement system. The zymolytic capacity of C1s was validated and found to be enhanced, in the absence of calcium and in the presence of EDTA. Our findings indicate that for correct identification of gelatinolytic proteins in complex biological samples the use of EDTA/gelatin zymography for enzyme development is advised. In addition, by quantification of EDTA/gelatin zymography analysis and ELISA, we observed that the levels of C1s were higher in plasma and immune complexes of SLE patients than of healthy individuals. Therefore, our data imply that C1s may become a marker for the diagnosis of SLE. 相似文献
967.
Christopher B. Cooper Edward J. Beard lvaro Vzquez‐Mayagoitia Liliana Stan Gavin B. G. Stenning Daniel W. Nye Julian A. Vigil Tina Tomar Jingwen Jia Govardhana B. Bodedla Song Chen Lucía Gallego Santiago Franco Antonio Carella K. R. Justin Thomas Song Xue Xunjin Zhu Jacqueline M. Cole 《Liver Transplantation》2019,9(5)
Data‐driven materials discovery has become increasingly important in identifying materials that exhibit specific, desirable properties from a vast chemical search space. Synergic prediction and experimental validation are needed to accelerate scientific advances related to critical societal applications. A design‐to‐device study that uses high‐throughput screens with algorithmic encodings of structure–property relationships is reported to identify new materials with panchromatic optical absorption, whose photovoltaic device applications are then experimentally verified. The data‐mining methods source 9431 dye candidates, which are auto‐generated from the literature using a custom text‐mining tool. These candidates are sifted via a data‐mining workflow that is tailored to identify optimal combinations of organic dyes that have complementary optical absorption properties such that they can harvest all available sunlight when acting as co‐sensitizers for dye‐sensitized solar cells (DSSCs). Six promising dye combinations are shortlisted for device testing, whereupon one dye combination yields co‐sensitized DSSCs with power conversion efficiencies comparable to those of the high‐performance, organometallic dye, N719. These results demonstrate how data‐driven molecular engineering can accelerate materials discovery for panchromatic photovoltaic or other applications. 相似文献
968.
Boyi Chen Jun Peng Heping Shen The Duong Daniel Walter Steve Johnston Mowafak M. Al‐Jassim Klaus J. Weber Thomas P. White Kylie R. Catchpole Daniel Macdonald Hieu T. Nguyen 《Liver Transplantation》2019,9(7)
A fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells (PSCs) with micrometer‐scale spatial resolution is developed. This imaging technique utilizes well‐defined and relatively symmetrical band‐to‐band luminescence spectra emitted from perovskite materials, whose spectral peak locations coincide with absorption thresholds and thus represent their optical bandgaps. The technique is employed to capture relative variations in optical bandgaps across various PSCs, and to resolve optical bandgap inhomogeneity within the same device due to material degradation and impurities. Degradation and impurities are found to both cause optical bandgap shifts inside the materials. The results are confirmed with micro‐photoluminescence spectroscopy scans. The excellent agreement between the two techniques opens opportunities for this imaging concept to become a quantified, high spatial resolution, large‐area characterization tool of PSCs. This development continues to strengthen the high value of luminescence imaging for the research and development of this photovoltaic technology. 相似文献
969.
Lee Ho-Joon Kremer Daniel M. Sajjakulnukit Peter Zhang Li Lyssiotis Costas A. 《Metabolomics : Official journal of the Metabolomic Society》2019,15(7):1-15
Metabolomics - Exposure to ricin can be lethal and treatments that are under development have short windows of opportunity for administration after exposure. It is therefore essential to achieve... 相似文献
970.
Salinas Daniel Mumey Brendan M. June Ronald K. 《Biomechanics and modeling in mechanobiology》2019,18(1):69-77
Biomechanics and Modeling in Mechanobiology - Chondrocytes use the pathways of central metabolism to synthesize molecular building blocks and energy for cartilage homeostasis. An interesting... 相似文献