全文获取类型
收费全文 | 52295篇 |
免费 | 232篇 |
国内免费 | 551篇 |
专业分类
系统科学 | 1227篇 |
丛书文集 | 280篇 |
教育与普及 | 290篇 |
理论与方法论 | 518篇 |
现状及发展 | 31248篇 |
研究方法 | 643篇 |
综合类 | 16674篇 |
自然研究 | 2198篇 |
出版年
2013年 | 805篇 |
2012年 | 607篇 |
2011年 | 2630篇 |
2009年 | 610篇 |
2008年 | 798篇 |
2007年 | 880篇 |
2006年 | 944篇 |
2005年 | 1139篇 |
2004年 | 2228篇 |
2003年 | 1860篇 |
2002年 | 1535篇 |
2001年 | 1152篇 |
2000年 | 690篇 |
1999年 | 813篇 |
1998年 | 640篇 |
1997年 | 758篇 |
1996年 | 510篇 |
1994年 | 681篇 |
1993年 | 687篇 |
1992年 | 790篇 |
1991年 | 695篇 |
1990年 | 759篇 |
1989年 | 592篇 |
1988年 | 571篇 |
1987年 | 563篇 |
1986年 | 651篇 |
1985年 | 793篇 |
1984年 | 701篇 |
1983年 | 624篇 |
1982年 | 751篇 |
1981年 | 803篇 |
1980年 | 882篇 |
1979年 | 1308篇 |
1978年 | 1183篇 |
1977年 | 1199篇 |
1976年 | 1036篇 |
1975年 | 1013篇 |
1974年 | 875篇 |
1973年 | 1120篇 |
1972年 | 1208篇 |
1971年 | 1174篇 |
1970年 | 1251篇 |
1969年 | 1170篇 |
1968年 | 1138篇 |
1967年 | 981篇 |
1966年 | 821篇 |
1965年 | 662篇 |
1964年 | 476篇 |
1958年 | 497篇 |
1957年 | 446篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Aluminum alloy matrix composites have found a predominant place in research, and their applications are explored in almost all industries. The aerospace industry has been using precipitation-hardenable alloys in structural applications. However, insufficient literature is available on the influence of multiwalled carbon nanotubes (MWCNTs) on precipitation-hardenable alloy composite materials; thus, this work was designed to elucidate the effect on MWCNT reinforcement on AA2219 with and without precipitation hardening. Reinforcement with MWCNTs has been reported to accelerate precipitation and to achieve greater hardness within a much shorter time. The addition of 0.75wt% MWCNTs resulted in maximal hardness at 90 min, which is approximately 27% of improvement over the maximum hardness achieved by the corresponding monolithic alloy after 10 h of aging. The sample reinforced with 0.75wt% MWCNTs showed an improvement of 82% in hardness by solutionizing and aging compared to that achieved by sintering. 相似文献
42.
Matthew L. Buffington Massimo Giorgini Chia-Hua Lue Giorgio Formisano Pasquale Cascone Mattias Forshage 《Journal of Natural History》2020,54(9-12):565-583
ABSTRACT In the search for native Asian parasitoids of Drosophila suzukii, the notorious spotted-wing Drosophila (SWD), an odd new species of Eucoilinae was discovered. Leptopilina lasallei sp. nov. is herein described and diagnosed relative to other eucoilines associated with drosophilid hosts. Morphologically, L. lasallei is somewhat aberrant within Leptopilina; phylogenetically, L. lasallei is sister group to the core Leptopilina. In the process of investigating L. lasallei, a de novo molecular phylogeny of Leptopilina was generated and is included here. The integrated approach used for the characterisation of L. lasallei, and the resulting phylogeny of Leptopilina, produced data useful to select parasitoid species for SWD biological control.http://www.zoobank.org/urn:lsid:zoobank.org:act:402D504A-4616-4524-85D7-1C13A6276F06 http://www.zoobank.org/urn:lsid:zoobank.org:act:402D504A-4616-4524-85D7-1C13A6276F06 相似文献
43.
Generation of CRISPR/Cas9-mediated gene-targeted pigs via somatic cell nuclear transfer 总被引:1,自引:1,他引:0
44.
Beryl?Royer-Bertrand Carlo?RivoltaEmail author 《Cellular and molecular life sciences : CMLS》2015,72(8):1463-1471
The past decade has seen the emergence of next-generation sequencing (NGS) technologies, which have revolutionized the field of human molecular genetics. With NGS, significant portions of the human genome can now be assessed by direct sequence analysis, highlighting normal and pathological variants of our DNA. Recent advances have also allowed the sequencing of complete genomes, by a method referred to as whole genome sequencing (WGS). In this work, we review the use of WGS in medical genetics, with specific emphasis on the benefits and the disadvantages of this technique for detecting genomic alterations leading to Mendelian human diseases and to cancer. 相似文献
45.
46.
47.
Chung D. Ngo Binh V. Ngo Thuong T. Hoang Thi T.T. Nguyen Hai P. Dang 《Journal of Natural History》2015,49(39-40):2417-2436
We studied the feeding ecology of Eutropis multifasciata in the tropical plains of central Vietnam to understand better the foraging mode, spatiotemporal and sexual variation in dietary composition, and rarefaction curves of prey-taxon richness for males and females. Stomach contents (n = 161) were collected from October 2013 to May 2014 using a nonlethal stomach-flushing technique. A total of 680 food items (624 animal items and 56 plant items) was found in 161 stomachs of skinks, representing 19 unique animal categories. We found that the diet of E. multifasciata is composed mainly of small, sedentary and clumped prey and that this skink specialises on spiders, insect larvae, snails, grasshoppers and crickets (with a combined importance index of 60%). Dietary composition, prey size and total prey volume in E. multifasciata changed between dry and rainy seasons and among regions. The total volume of food items consumed by males was larger than that of females, and the diversity and evenness index of prey categories were larger in males than in females. However, using rarefaction curves revealed that females have the higher prey-taxon richness after points between 130 and 140 prey items for frequency, and between 160 and 170 prey items for number of items, and the differences were not statistically significant. The foraging behaviour of E. multifasciata best fits a ‘widely foraging’ model. 相似文献
48.
A remarkable new eucoiline genus and species, Muhaka icipe, is described herein. The genus is clearly a Kleidotomini, but is distinguished from other genera in the tribe by a unique head and scutellar morphology. The genus belongs to the ‘wedge-head’-syndrome group of species that, to date, is unique to Afrotropical eucoilines. The new genus and species is reminiscent of Stentorceps Quinlan and Nanocthulhu Buffington, but is readily distinguished from these genera. Muhaka was collected from a threatened kaya (sacred forest) of coastal Kenya. The biological importance of this and other kaya forests, as well as their protection, is discussed.http://www.zoobank.org/urn:lsid:zoobank.org:pub:6918ED2C-69A4-48FC-A1E4-2B5DFF58E876 相似文献
49.
Two new species of tarantulas from Mexico are described and illustrated: Bonnetina tenuiverpis and Bonnetina juxtantricola, from the states of Mexico and Guerrero, respectively. Male palpal bulbs, tibial apophyses and spermatheca are among the most taxonomically informative characters. Male bulb microstructure is revealed from scanning electron microscopy; both new and homologous structures to other Theraphosinae genera are identified and described. Nomenclatural changes for male tibial apophyses are also proposed. The holotype male and allotype female of one of the species are molecularly characterized and matched from CO1 partial sequence.http://zoobank.org/urn:lsid:zoobank.org:pub:BBDBBEDE-6983-4F5C-B34C-4622DF494C84 相似文献
50.
Parasites of the grey mullet Mugil cephalus collected from Lower K?z?l?rmak Delta in Turkey were investigated. Trichodina puytoraci, Trichodina lepsii, Ligophorus mediterraneus, Ligophorus cephali, Gyrodactylus sp., Microcotyle mugilis, Ascocotyle (Phagicola) longa, Haplosplanchnus pachysomus, Diplostomum spathaceum, Tylodelphys clavata, Posthodiplostomum sp., Neoechinorhyncus agilis and Ergasilus lizae were identified. Overall infection prevalence (%), mean intensity and mean abundance values were 99.61%, 458.10 ± 75.72 and 456.30 ± 75.44, respectively. Infection indices of parasite species in relation to season, lakes/lagoons and some physicochemical parameters of water such as temperature, salinity, dissolved oxygen and nitrate are determined and discussed. Posthodiplostomum sp. is a new parasite record for mugilids and Diplostomum spathaceum is the first record in M. cephalus, in the present study. Moreover, Ligophorus mediterraneus, Ligophorus cephali and Ergasilus lizae are new records for Turkish parasite fauna of fish and Mugil cephalus is a new host record for Neoechinorhyncus agilis in Turkey. 相似文献