全文获取类型
收费全文 | 1515篇 |
免费 | 251篇 |
国内免费 | 336篇 |
专业分类
2102篇 |
出版年
2024年 | 8篇 |
2023年 | 48篇 |
2022年 | 35篇 |
2021年 | 56篇 |
2020年 | 79篇 |
2019年 | 88篇 |
2018年 | 73篇 |
2017年 | 99篇 |
2016年 | 89篇 |
2015年 | 102篇 |
2014年 | 89篇 |
2013年 | 114篇 |
2012年 | 70篇 |
2011年 | 79篇 |
2010年 | 56篇 |
2009年 | 89篇 |
2008年 | 97篇 |
2007年 | 79篇 |
2006年 | 78篇 |
2005年 | 65篇 |
2004年 | 57篇 |
2003年 | 52篇 |
2002年 | 48篇 |
2001年 | 34篇 |
2000年 | 43篇 |
1999年 | 26篇 |
1998年 | 49篇 |
1997年 | 24篇 |
1996年 | 27篇 |
1995年 | 28篇 |
1994年 | 17篇 |
1993年 | 13篇 |
1992年 | 18篇 |
1991年 | 22篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 18篇 |
1987年 | 17篇 |
1986年 | 14篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 9篇 |
1982年 | 17篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1973年 | 1篇 |
1958年 | 3篇 |
排序方式: 共有2102条查询结果,搜索用时 15 毫秒
1.
An Extreme Value Analysis of Pollutant Concentrations in Surface Soils Due to Atmospheric Deposition
Risk assessments often rely on deterministic models using long-term averages or “steady-state” values of input variables. Such models do not provide the information needed to estimate acute exposures. This study uses extreme value theory to examine the frequency and magnitude of daily pollutant concentrations in surface soils predicted at six U.S. locations. Concentrations are predicted using a deposition-leaching model and 50 years of historical precipitation data. A stochastic model also is used to generate 1000 years of precipitation data as modeling inputs for each location. The annual maximum concentrations at each site are fitted to a Gumbel type I distribution to estimate occurrence probability. For soluble pollutants, the predicted concentration varied substantially with precipitation, and the maximum daily concentrations exceeded annual averages by 4 to 8 times. Observed and synthetic precipitation data produced similar results at most study locations, though the synthetic data provided a slightly better fit to the Gumbel type I distribution. The precipitation model allows the generation of representative precipitation data that extend limited historical records. The extreme value analysis facilitates the evaluation of maximum pollutant concentrations, return periods, and other statistics that are important in evaluating acute exposures. 相似文献
2.
S. Ofer G.C. Papaefthymiou R.B. Frankel H.A. Lowenstam 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,676(2):199-204
Dermal granules containing hydrous ferric oxide cores from Molpadia intermedia were studied by Mössbauer spectroscopy from 1.5 t0 300 K and in magnetic fields up to 80 kOersted at 4.2 K. A magnetic phase transition to an antiferromagnetically ordered state is observed at 10 K. The results are compared with the magnetic behavior of micellar cores of ferritin from eukaryotes and iron-storage materials from prokaryotes. 相似文献
3.
Qualitative analyses were made from whole-body cuticular extracts of Scapteriscus abbreviatus, Scapteriscus acletus, Scapteriscus vicinus, and Neocurtilla hexadactyla by isothermal and temperature-programmed gas chromatography. Adults of both sexes and nymphs of each species were collected in Florida. The chromatographic profiles of peaks were distinct and easily recognizable for each species, regardless of sex or developmental stage. Distinct sexual differences were found in S. acletus and S. abbreviatus. Specimens of S. abbreviatus from Puerto Rico and S. vicinus from Bolivia produced gas chromatography (GC) traces very similar to those of conspecifics collected in Florida. Evidence is presented to illustrate the potential importance of volatile cuticular lipid analysis as a tool for mole cricket identification. Cuticular extracts of an undescribed short-winged species of Scapteriscus from Bolivia were also examined and produced GC traces unlike those of any other species analyzed to date. 相似文献
4.
Cuticles of several plant species are covered by tubular wax aggregates that are known to consist mainly of (S)-nonacosan-10-ol. The present work addresses the question whether minor wax components may additionally contribute to these tubules. Thin layer chromatography was used to prepare secondary alkanediol fractions from leaf cuticular waxes of Nelumbo nucifera and Thalictrum flavum, containing nonacosane-3,10-diol, nonacosane-4,10-diol, nonacosane-5,10-diol, nonacosane-7,10-diol, nonacosane-9,10-diol and nonacosane-10,13-diol. From organic solutions all these compounds crystallized in tubular shapes. Possible crystal structures of relevant alkanediol isomers are proposed, in analogy to the lattice geometries of comparable aliphatic compounds. The resulting structural model shows that nonacosan-10-ol and various secondary alkanediols may join in metastable mixed crystals. According to the structural model proposed the admixture of alkanediols to nonacosan-10-ol aggregates should enhance the stability of their tubular habit. 相似文献
5.
Fibroblast heterogeneity in glucocorticoid regulation of collagen metabolism: Genetic or epigenetic?
James D. Russell Shirley B. Russell Kathryn M. Trupin 《In vitro cellular & developmental biology. Plant》1982,18(6):557-564
Summary Cultured fibroblasts derived from normal human dermis show a consistent 62% inhibition of collagen synthesis by hydrocortisone,
whereas cultures derived from keloids average only 30% inhibition and show a much larger strain to strain variation ranging
from 75% inhibition to 49% stimulation. Examination of fibroblast clones indicates that this high variation among keloid strains
is not due to differences in the proportion of normal and keloid cells in the mass culture populations. Small but significant
differences in the effect of hydrocortisone on collagen deposition are also seen among these clonal populations, but are not
related to the type of tissue from which cultures were derived. Two to three-fold differences among clones derived from a
single individual were observed, possibly suggesting functional heterogeneity of dermal fibroblasts with regard to collagen
metabolism under control conditions and in response to hydrocortisone. However, this variation among clones may simply reflect
differences in clonal growth, inasmuch as both collagen synthesis and deposition, and the effect of hydrocortisone on these
processes, are strongly affected by population density.
This work was supported in part by PHS grants, CA-17229 from the National Cancer Institute and AG-02046 from the National
Institute on Aging, DHHS; and by Grant RIM 78-17313 from the National Science Foundation. 相似文献
6.
Estimating the uptake of traffic-derived NO2 from 15N abundance in Norway spruce needles 总被引:1,自引:0,他引:1
The 15N ratio of nitrogen oxides (NOx) emitted from vehicles, measured in the air adjacent to a highway in the Swiss Middle Land, was very high [δ15N(NO2) = +5.7‰]. This high 15N abundance was used to estimate long-term NO2 dry deposition into a forest ecosystem by measuring δ15N in the needles and the soil of potted and autochthonous spruce trees [Picea abies (L.) Karst] exposed to NO2 in a transect orthogonal to the highway. δ15N in the current-year needles of potted trees was 2.0‰ higher than that of the control after 4 months of exposure close to
the highway, suggesting a 25% contribution to the N-nutrition of these needles. Needle fall into the pots was prevented by
grids placed above the soil, while the continuous decomposition of needle litter below the autochthonous trees over previous
years has increased δ15N values in the soil, resulting in parallel gradients of δ15N in soil and needles with distance from the highway. Estimates of NO2 uptake into needles obtained from the δ15N data were significantly correlated with the inputs calculated with a shoot gas exchange model based on a parameterisation
widely used in deposition modelling. Therefore, we provide an indication of estimated N inputs to forest ecosystems via dry
deposition of NO2 at the receptor level under field conditions.
Received: 7 November 1997 / Accepted: 16 September 1998 相似文献
7.
8.
MARIANNE PHILIPP LISE BACH HANSEN HENNING ADSERSEN HANS R. SIEGISMUND 《Botanical journal of the Linnean Society. Linnean Society of London》2004,146(2):171-180
Lecocarpus pinnatifidus is an endemic member of the Asteraceae occurring on only one island in the Galápagos archipelago. The capitula are large with female ray florets and male disc florets. They are self-compatible but this study suggests fruit set is pollen limited. Visits from Xylocopa darwini and other larger insect pollinators are rare, and small insects seem to be the main pollinators. Small insects carry few pollen grains and most likely mediate self-pollinations. Self-compatibility and seed set after selfing are the most common reproductive strategy in the Galápagos Islands and L. pinnatifidus seemingly fits well into this group. © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 171–180. 相似文献
9.
Thomas P. Brennan Pendar Ardalan Han‐Bo‐Ram Lee Jonathan R. Bakke I‐Kang Ding Michael D. McGehee Stacey F. Bent 《Liver Transplantation》2011,1(6):1169-1175
Functioning quantum dot (QD) sensitized solar cells have been fabricated using the vacuum deposition technique atomic layer deposition (ALD). Utilizing the incubation period of CdS growth by ALD on TiO2, we are able to grow QDs of adjustable size which act as sensitizers for solid‐state QD‐sensitized solar cells (ssQDSSC). The size of QDs, studied with transmission electron microscopy (TEM), varied with the number of ALD cycles from 1‐10 nm. Photovoltaic devices with the QDs were fabricated and characterized using a ssQDSSC device architecture with 2,2',7,7'‐tetrakis‐(N,N‐di‐p methoxyphenylamine) 9,9'‐spirobifluorene (spiro‐OMeTAD) as the solid‐state hole conductor. The ALD approach described here can be applied to fabrication of quantum‐confined structures for a variety of applications, including solar electricity and solar fuels. Because ALD provides the ability to deposit many materials in very high aspect ratio substrates, this work introduces a strategy by which material and optical properties of QD sensitizers may be adjusted not only by the size of the particles but also in the future by the composition. 相似文献
10.
Effects of grass species and grass growth on atmospheric nitrogen deposition to a bog ecosystem surrounded by intensive agricultural land use 下载免费PDF全文
Miriam Hurkuck Christian Brümmer Karsten Mohr Oliver Spott Reinhard Well Heinz Flessa Werner L. Kutsch 《Ecology and evolution》2015,5(13):2556-2571
We applied a 15N dilution technique called “Integrated Total Nitrogen Input” (ITNI) to quantify annual atmospheric N input into a peatland surrounded by intensive agricultural practices over a 2-year period. Grass species and grass growth effects on atmospheric N deposition were investigated using Lolium multiflorum and Eriophorum vaginatum and different levels of added N resulting in increased biomass production. Plant biomass production was positively correlated with atmospheric N uptake (up to 102.7 mg N pot−1) when using Lolium multiflorum. In contrast, atmospheric N deposition to Eriophorum vaginatum did not show a clear dependency to produced biomass and ranged from 81.9 to 138.2 mg N pot−1. Both species revealed a relationship between atmospheric N input and total biomass N contents. Airborne N deposition varied from about 24 to 55 kg N ha−1 yr−1. Partitioning of airborne N within the monitor system differed such that most of the deposited N was found in roots of Eriophorum vaginatum while the highest share was allocated in aboveground biomass of Lolium multiflorum. Compared to other approaches determining atmospheric N deposition, ITNI showed highest airborne N input and an up to fivefold exceedance of the ecosystem-specific critical load of 5–10 kg N ha−1 yr−1. 相似文献