全文获取类型
收费全文 | 45022篇 |
免费 | 17472篇 |
国内免费 | 13篇 |
专业分类
62507篇 |
出版年
2023年 | 34篇 |
2022年 | 54篇 |
2021年 | 484篇 |
2020年 | 2833篇 |
2019年 | 4377篇 |
2018年 | 4659篇 |
2017年 | 4629篇 |
2016年 | 4339篇 |
2015年 | 4202篇 |
2014年 | 4097篇 |
2013年 | 4585篇 |
2012年 | 3878篇 |
2011年 | 4064篇 |
2010年 | 3536篇 |
2009年 | 2397篇 |
2008年 | 2558篇 |
2007年 | 1974篇 |
2006年 | 1964篇 |
2005年 | 1655篇 |
2004年 | 1312篇 |
2003年 | 1404篇 |
2002年 | 1220篇 |
2001年 | 924篇 |
2000年 | 458篇 |
1999年 | 296篇 |
1998年 | 34篇 |
1997年 | 51篇 |
1996年 | 46篇 |
1995年 | 51篇 |
1994年 | 45篇 |
1993年 | 44篇 |
1992年 | 43篇 |
1991年 | 24篇 |
1990年 | 32篇 |
1989年 | 23篇 |
1988年 | 16篇 |
1987年 | 25篇 |
1986年 | 18篇 |
1985年 | 17篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 19篇 |
1981年 | 8篇 |
1980年 | 19篇 |
1979年 | 7篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 4篇 |
1972年 | 3篇 |
1970年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Twelve groups of fossils, including graptolites, brachiopods, nautiloids, trilobites, corals, crinoids, bryozoans, conodonts, ostracods, gastropods, chitinozoans, and acritarchs expired in different but substantial magnitude and global extent during the late Caradoc to latest Ashgill. It indicates a multiple‐episodic mass extinction containing the possible Prologue (late Caradoc), Climax episode (Rawtheyan) and Epilogue (late Hirnantian). The main causes of this mass extinction are recognized as a global sea‐level lowering in the climax and remarkable rapid rise at the final, and global cooling. The Chinese data, especially from the South China Paleoplate, are evaluated first. They are significant for explaining this global bioevent. 相似文献
182.
Godfrey C. Akani Luca Luiselli Edem A. Eniang Charity C. Amuzie Nwabueze Ebere 《African Journal of Ecology》2008,46(4):533-539
The ecology of the blindsnake, Typhlops punctatus punctatus in five selected study plots in the River Niger Delta (Port Harcourt, Nigeria) was investigated. A total of 207 specimens were collected, 43% occurring under stones, 30% under leaf litter and 15% under logs. The number of snakes found under cover objects was significantly influenced by the number of cover objects lifted up. GLM analysis revealed that there was no difference in terms of number of snakes observed, once the relative effect of the different number of cover objects lifted up was taken into account, either among study plots or among types of cover objects. These snakes appeared to have a strong affinity for loamy soils, with moisture content of 8–18%, high organic matter (1–6%) and slightly acid pH of 5.4–6.9. Stomach content analysis revealed that these snakes fed mainly upon ants and termites. A case of predation by the toad Bufo maculatus on a juvenile blindsnake was also reported. 相似文献
183.
Ahmad Golaraei Kamdin Mirsanaye Yeji Ro Serguei Krouglov Margarete K. Akens Brian C. Wilson Virginijus Barzda 《Journal of biophotonics》2019,12(1)
Polarization‐dependent second‐harmonic generation (P‐SHG) microscopy is used to characterize molecular nonlinear optical properties of collagen and determine a three‐dimensional (3D) orientation map of collagen fibers within a pig tendon. C6 symmetry is used to determine the nonlinear susceptibility tensor components ratios in the molecular frame of reference and , where the latter is a newly extracted parameter from the P‐SHG images and is related to the chiral structure of collagen. The is observed for collagen fibers tilted out of the image plane, and can have positive or negative values, revealing the relative polarity of collagen fibers within the tissue. The P‐SHG imaging was performed using a linear polarization‐in polarization‐out (PIPO) method on thin sections of pig tendon cut at different angles. The nonlinear chiral properties of collagen can be used to construct the 3D organization of collagen in the tissue and determine the orientation‐independent molecular susceptibility ratios of collagen fibers in the molecular frame of reference. 相似文献
184.
Functional genomic studies and drug candidate testing both require high throughput, parallel experimentation strategies to screen for variable cellular behaviors. In this article we describe the use of an impedance sensing electrode array that is capable of sensing cell "presence" as well as the extent of cell (focal) attachment to the substrate. The signals provided by mouse fibroblasts on a sensing structure containing four different sized electrodes are reported. In the absence of cells, each electrode's impedance was found to depend as expected on electrode size and frequency. The impedance increased by several-fold when fibroblasts attached and spread out over time. More notably, the sensors also detected the cellular response to the protein kinase C inhibitor, H-7. H-7 inhibits actomyosin contractility; thereafter, the loss of focal adhesion complexes occurs. The sensors, in turn, detected an impedance decrease after H-7 addition and an increase in impedance after H-7 removal. 相似文献
185.
The roles of allopatric fragmentation and niche divergence in intraspecific lineage diversification in the common midwife toad (Alytes obstetricans) 下载免费PDF全文
186.
187.
Mechthilde?MennickenEmail author Wolfgang?Maier Pedro?W.?Crous Franz?Oberwinkler 《Mycological Progress》2005,4(3):215-224
Rust fungi on Aizoaceae in southern Africa have been examined and reported based on 27 specimens collected during a biodiversity
study and previously collected herbarium specimens. Eight species including five new species have been recognized, and they
are described in detail and illustrated. Together with two additional species in literature, ten species of rust fungi are
now recognized on Aizoaceae in southern Africa.
Part 223 in the series “Studies in Heterobasidiomycetes” from the Botanical Institute, University of Tübingen. 相似文献
188.
189.
Lee JY Lim JM Kim DK Zheng YH Moon S Han BK Song KD Kim H Han JY 《Molecular reproduction and development》2008,75(1):184-190
Members of the Pumilio (Pum) family of RNA-binding proteins act as translational repressors and are required for germ cell development and asymmetric division. We identified the chicken Pum1 and Pum2 genes and analyzed their expression patterns in various tissues. Comparative sequence analysis of the Pum1 and Pum2 proteins from the drosophila, chicken, mouse, and human revealed a high degree of evolutionary conservation in terms of the levels of homology of the peptide sequences and the structure of Pumilio homology domain (PUM-HD), C-terminal RNA-binding domain, with similar spacing between the adjacent Pum eight tandem repeats. In addition, phylogenetic patterns of pumilio family showed that Pum 1 and 2 of chicken are more closely related to those of mouse and human than other species and Pum1 is more conserved than Pum2. Using real-time RT-PCR, the expression levels of the Pum1 and Pum2 genes were found to be highest in hatched female gonads, and high-level expression of Pum2 was detected in 12-day and hatched gonads among the various chicken embryonic tissues tested. In adult tissues, the expression levels of Pum1 and Pum2 were expressed at higher levels in the testis and muscle than in any other tissue. The characteristics of the tissue-specific expression of Pum genes suggest that Pum1 and Pum2 have effects crucially in particular stage during development of chicken gonads depending on sexual maturation. 相似文献
190.
Shuming Dou Jie Xu Xiaoya Cui Weidi Liu Zhicheng Zhang Yida Deng Wenbin Hu Yanan Chen 《Liver Transplantation》2020,10(33)
Functional nanomaterials are playing a crucial role in the emerging field of energy‐related devices. Recently, as a novel synthesis method, high‐temperature shock (HTS), which is rapid, low cost, eco‐friendly, universal, scalable, and controllable, has provided a promising option for the rational design and synthesis of various high‐quality nanomaterials. In this report, the HTS technique, including the equipment setup and operating principle, is systematically introduced, and recent progress in the synthesis of nanomaterials for energy storage and conversion applications using this HTS method is summarized. The growth mechanisms of nanoparticles and carbonaceous nanomaterials are thoroughly discussed, followed by the summary of the characteristic advantages of the HTS strategy. A series of nanomaterials prepared by the HTS method, including carbon‐based films, metal nanoparticles and compound nanoparticles, show high performance in the diverse applications of storage energy batteries, highly active catalysts, and smart energy devices. Finally, the future perspectives and directions of HTS in nanomanufacturing for broader applications are presented. 相似文献