全文获取类型
收费全文 | 22428篇 |
免费 | 48篇 |
国内免费 | 53篇 |
专业分类
系统科学 | 225篇 |
丛书文集 | 493篇 |
教育与普及 | 45篇 |
理论与方法论 | 60篇 |
现状及发展 | 9480篇 |
研究方法 | 879篇 |
综合类 | 10976篇 |
自然研究 | 371篇 |
出版年
2013年 | 129篇 |
2012年 | 276篇 |
2011年 | 665篇 |
2010年 | 101篇 |
2008年 | 325篇 |
2007年 | 376篇 |
2006年 | 400篇 |
2005年 | 392篇 |
2004年 | 377篇 |
2003年 | 383篇 |
2002年 | 306篇 |
2001年 | 707篇 |
2000年 | 706篇 |
1999年 | 412篇 |
1992年 | 410篇 |
1991年 | 363篇 |
1990年 | 387篇 |
1989年 | 326篇 |
1988年 | 369篇 |
1987年 | 366篇 |
1986年 | 342篇 |
1985年 | 485篇 |
1984年 | 363篇 |
1983年 | 303篇 |
1982年 | 238篇 |
1981年 | 256篇 |
1980年 | 343篇 |
1979年 | 687篇 |
1978年 | 580篇 |
1977年 | 551篇 |
1976年 | 472篇 |
1975年 | 531篇 |
1974年 | 663篇 |
1973年 | 579篇 |
1972年 | 588篇 |
1971年 | 701篇 |
1970年 | 925篇 |
1969年 | 726篇 |
1968年 | 624篇 |
1967年 | 652篇 |
1966年 | 596篇 |
1965年 | 433篇 |
1964年 | 113篇 |
1959年 | 264篇 |
1958年 | 400篇 |
1957年 | 303篇 |
1956年 | 269篇 |
1955年 | 238篇 |
1954年 | 263篇 |
1948年 | 176篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Cobalt (Co)-modified brownmillerite KBiFe2O5 (KBFO; [KBiFe2(1?x)Co2xO5 (x = 0, 0.05)]) polycrystalline is synthesized following the solid-state reaction route. Rietveld refinement of X-ray diffraction data confirmed the phase purity of KBFO and KBiFe1.9Co0.1O5 (KBFCO). The optical bandgap energy (Eg) of KBFO decreased from 1.59 to 1.51 eV because of Co substitution. The decrease in bandgap can be attributed to the tilting of the Fe–O tetrahedral structure of KBFCO. The observed room-temperature Raman peaks of KBFCO shifted by 3 cm?1 toward a lower wavenumber than that of KBFO. The shift in Raman active modes can be attributed to the change in the bond angles and bond lengths of the Fe–O tetrahedral structure and modification in response to oxygen deficiency in KBFO because of Co doping. Compared with that of KBFO, the frequency-dependent dielectric constant and dielectric loss of KBFCO decrease at room temperature, which is a con-sequence of the reduction in oxygen migration and modification in response to vibrational modes present in the sample. 相似文献
2.
This is a contribution towards a history and philosophy of modeling in its early stages in electromagnetism. In 1873, James Clerk Maxwell (1831–1879) hinted at the methodology of modeling at the end of his Treatise on Electricity and Magnetism. We focus on Maxwell's impact on physicists who immediately followed him, specifically Oliver Lodge (1851–1940) and George Francis FitzGerald (1851–1901). We begin with the role that the scientific concept of model played in the late nineteenth century, as assessed by Ludwig Boltzmann (1844–1906). We then discuss the role of hypothesis as a methodology, the appeal to (dynamical) illustration, and the way Maxwell applied model and working model in his studies of electromagnetism. We show that for Maxwell these key terms were kept distinct, but Lodge did not maintain these distinctions and, in this regard, FitzGerald followed Lodge. Notwithstanding Lodge's influence, Fitzgerald modified Maxwell's theory based on the mechanical model he designed, thereby implicitly taking the first step towards modeling. This methodology consists in drawing consequences from the (mechanical) model to the (electrodynamic) theory and modifying the latter in light of the functioning of the former. At the core of our argument is the thesis that it was a methodological novelty to move from the concept of model to the methodology of modeling. The introduction of modeling as a new methodology into physics in the late nineteenth century was a major event which deserves proper recognition. 相似文献
3.
Methods for backcasting,nowcasting and forecasting using factor‐MIDAS: With an application to Korean GDP 下载免费PDF全文
We utilize mixed‐frequency factor‐MIDAS models for the purpose of carrying out backcasting, nowcasting, and forecasting experiments using real‐time data. We also introduce a new real‐time Korean GDP dataset, which is the focus of our experiments. The methodology that we utilize involves first estimating common latent factors (i.e., diffusion indices) from 190 monthly macroeconomic and financial series using various estimation strategies. These factors are then included, along with standard variables measured at multiple different frequencies, in various factor‐MIDAS prediction models. Our key empirical findings as follows. (i) When using real‐time data, factor‐MIDAS prediction models outperform various linear benchmark models. Interestingly, the “MSFE‐best” MIDAS models contain no autoregressive (AR) lag terms when backcasting and nowcasting. AR terms only begin to play a role in “true” forecasting contexts. (ii) Models that utilize only one or two factors are “MSFE‐best” at all forecasting horizons, but not at any backcasting and nowcasting horizons. In these latter contexts, much more heavily parametrized models with many factors are preferred. (iii) Real‐time data are crucial for forecasting Korean gross domestic product, and the use of “first available” versus “most recent” data “strongly” affects model selection and performance. (iv) Recursively estimated models are almost always “MSFE‐best,” and models estimated using autoregressive interpolation dominate those estimated using other interpolation methods. (v) Factors estimated using recursive principal component estimation methods have more predictive content than those estimated using a variety of other (more sophisticated) approaches. This result is particularly prevalent for our “MSFE‐best” factor‐MIDAS models, across virtually all forecast horizons, estimation schemes, and data vintages that are analyzed. 相似文献
4.
Tanja Wilke Wilko H. Ahlrichs Olaf R.P. Bininda-Emonds 《Journal of Natural History》2019,53(7-8):413-423
The morphology of soft-bodied rotifers, including those of Synchaeta spp, can be strongly affected by preparation artefacts including contraction and deformation. The long-standing, valid species Synchaeta monopus is known exclusively from ethanol- or formaldehyde-preserved material and no live specimens of it have ever been described. Although this alone is cause for concern, we could also reproduce unique characteristics diagnostic for this species (e.g. the swollen body and the rudimental foot) by subjecting specimens of Synchaeta pectinata to the preservation conditions under which it was first described. This proxy experiment and comparisons to other Synchaeta species indicate that literature occurrences of S. monopus likely represent preserved and deformed specimens of Synchaeta cecilia or other marine species of Synchaeta, thereby highlighting the importance of thorough morphological investigations of the habitus using live specimens and of features that are unaffected by preservation (e.g. the trophi). We therefore recommend that S. monopus be listed as a species inquirenda until topotypes are examined. Furthermore, in ecological studies including rotifers, where the examination of preserved material is often unavoidable, we stress that light-microscopical images of the habitus and trophi of the specimens minimally be included to facilitate independent verification of the species assignments. 相似文献
5.
Juliana L. Segadilha Priscila S. Nascimento Fábio M. Mauro Cristiana S. Serejo Taiara R. Ramos Irene A. Cardoso 《Journal of Natural History》2018,52(1-2):1-11
A total of 3109 crustaceans belonging to 50 taxa distributed in 42 families were found in 117 analysed stomachs of flying gurnard (Dactylopterus volitans). Samples were obtained in April 2008 by the R/V Gyre using a bottom trawl towed in 12 stations at 14–100 m depth on the continental shelf of the Campos Basin, Brazil. The carcinofauna was analysed and the order Calanoida (Copepoda) found to be the most important item in terms of relative abundance and frequency of occurrence, followed by the order Amphipoda (Peracarida), the infraorder Brachyura (Decapoda), the order Stomatopoda and the subclass Myodocopa (Ostracoda). In the order Calanoida, the species Pontellopsis cf. villosa (Pontellidae) represented 98.04% of total crustacean abundance. The diet of Dactylopterus volitans varied according to fish size, with higher diversity of Crustacea at smaller size classes, decreasing in larger fishes. A similar pattern regarding depth was obtained, with greater diversity of taxa in gurnard stomachs caught at shallower depths. Flying gurnard is considered a generalized carnivore of invertebrates, eating mobile macrobenthic organisms, such as crustaceans, and its diet varies with its life stage, without any specific group as its main food source. 相似文献
6.
Steven J. Collins Carolin Tumpach Bradley R. Groveman Simon C. Drew Cathryn L. Haigh 《Cellular and molecular life sciences : CMLS》2018,75(17):3231-3249
Neurogenesis continues in the post-developmental brain throughout life. The ability to stimulate the production of new neurones requires both quiescent and actively proliferating pools of neural stem cells (NSCs). Actively proliferating NSCs ensure that neurogenic demand can be met, whilst the quiescent pool makes certain NSC reserves do not become depleted. The processes preserving the NSC quiescent pool are only just beginning to be defined. Herein, we identify a switch between NSC proliferation and quiescence through changing intracellular redox signalling. We show that N-terminal post-translational cleavage products of the prion protein (PrP) induce a quiescent state, halting NSC cellular growth, migration, and neurite outgrowth. Quiescence is initiated by the PrP cleavage products through reducing intracellular levels of reactive oxygen species. First, inhibition of redox signalling results in increased mitochondrial fission, which rapidly signals quiescence. Thereafter, quiescence is maintained through downstream increases in the expression and activity of superoxide dismutase-2 that reduces mitochondrial superoxide. We further observe that PrP is predominantly cleaved in quiescent NSCs indicating a homeostatic role for this cascade. Our findings provide new insight into the regulation of NSC quiescence, which potentially could influence brain health throughout adult life. 相似文献
7.
Pedro R. Bartholomay Kevin A. Williams Roberto A. Cambra Marcio L. Oliveira 《Journal of Natural History》2019,53(43-44):2639-2683
ABSTRACTTraumatomutilla duplicata duplicata (Gerstaecker, 1874), T. duplicata feia Casal, 1969, T. bruchi André, 1908, T. cristata (Gerstaecker, 1874), T. aterrima (Gerstaecker, 1874) and T. lorena (Cresson, 1902) are proposed as junior synonyms of Traumatomutilla bivittata (Gerstaecker, 1874). Traumatomutilla immaculiceps André, 1901, T. bivittata rubroguttata André, 1901 and T. estrella (Cresson, 1902) are proposed as junior synonyms of T. juvenilis (Gerstaecker, 1874). Traumatomutilla bispiculata (André, 1907) is proposed as a junior synonym of T. miniata (Gerstaecker, 1874). The previously unknown males of T. guarata Casal, 1969 and T. juvenilis are described and illustrated. All species of the T. juvenilis species group are redescribed and illustrated. A new species, Traumatomutilla juvenindica Bartholomay & Williams sp. nov., is described based on males and females from northern South America. Additionally, identification keys to the species and known colour forms of the T. juvenilis species group are provided.http://www.zoobank.org/urn:lsid:zoobank.org:pub:3563B9EE-A45B-4939-8436-7A808D4F8996 相似文献
8.
Tatiane Lima da Silva Rísia Brígida Gonçalves Cabral Lucas de Assis Silva Andrade Noemia Falcão Nogueira Mariana de Souza Oliveira Ildemar Ferreira 《Journal of Natural History》2018,52(3-4):225-235
Fluvicola nengeta is an insectivorous bird that belongs to the Tyrannidae family, which lives close to bodies of water and can occur in urban areas as well. It makes use of natural and man-made substrates to build its nests. This study aims at describing the reproductive biology of F. nengeta by comparing nests built on vegetation and man-made structures as to their shapes, nest dimensions, building materials, distance from bodies of water and reproductive success. Forty-four nests were found, where 30 were located in vegetation and 14 on man-made structures. Nests did not exhibit any single pattern and were divided into three distinct shapes: closed/globular/base; closed/ovoid/base (45.5%; n = 5 for both) and low cup/base (9%; n = 1). Nests built on man-made structures exhibited a larger amount of anthropic material, such as plastic and paper, instead of plant-derived materials. Nests on man-made structures were found to be higher up from the ground (459.8 ± 46.9 m) and farther from water (232.9 ± 54.8 m) in comparison to those on vegetation (92.2 ± 28.6 m and 7.3 ± 4.7 m, respectively), aside from exhibiting a higher Mayfield reproductive success (37%) when compared to nests built on vegetation (34%). The estimated values of the daily survival rate (DSR) for vegetation nests were 0.997 for the incubation period and 0.928 for the nestling period. In man-made structure nests, DSRs were 0.968 and 0.964, respectively, during the incubation and nestling periods. The period survival rate (PSR) of nests in vegetation was significantly higher than the PSRs of nests in man-made structures during the incubation period. DSRs did not differ between incubation and nestling periods for either nesting sites. 相似文献
9.
R.S.K. Barnes 《Journal of Natural History》2018,52(1-2):87-113
Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobia’ knysnaensis (Krauss), ‘Assiminea’ capensis (Sowerby) and ‘Assiminea’ globulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m?2), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m2 samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere. 相似文献
10.