全文获取类型
收费全文 | 96782篇 |
免费 | 7679篇 |
国内免费 | 32篇 |
出版年
2018年 | 1300篇 |
2017年 | 1212篇 |
2016年 | 1635篇 |
2015年 | 1727篇 |
2014年 | 2084篇 |
2013年 | 2964篇 |
2012年 | 3240篇 |
2011年 | 3343篇 |
2010年 | 2493篇 |
2009年 | 2099篇 |
2008年 | 3044篇 |
2007年 | 2998篇 |
2006年 | 2824篇 |
2005年 | 2660篇 |
2004年 | 2655篇 |
2003年 | 2544篇 |
2002年 | 2367篇 |
2001年 | 4395篇 |
2000年 | 4151篇 |
1999年 | 3215篇 |
1998年 | 1015篇 |
1997年 | 1001篇 |
1996年 | 894篇 |
1995年 | 830篇 |
1994年 | 824篇 |
1993年 | 830篇 |
1992年 | 2441篇 |
1991年 | 2453篇 |
1990年 | 2457篇 |
1989年 | 2203篇 |
1988年 | 2063篇 |
1987年 | 1977篇 |
1986年 | 1824篇 |
1985年 | 1816篇 |
1984年 | 1495篇 |
1983年 | 1294篇 |
1982年 | 929篇 |
1981年 | 871篇 |
1979年 | 1436篇 |
1978年 | 1125篇 |
1977年 | 1016篇 |
1976年 | 970篇 |
1975年 | 1156篇 |
1974年 | 1255篇 |
1973年 | 1337篇 |
1972年 | 1183篇 |
1971年 | 1042篇 |
1970年 | 922篇 |
1969年 | 954篇 |
1968年 | 834篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Uninfected adultAphis gossypii(Homoptera: Aphididae) apterae (sentinel aphids) on cotton leaves were exposed for 8 h to the air over a commercial cotton field in Louisiana during the night of 1–2 July 1995. At 0015 h there were 90,437 primary conidia/m3air of the fungusNeozygites fresenii(Entomophthorales: Neozygitaceae) at the midfield position as determined from Rotorod samples. Forty-eight percent (n = 106) of the sentinel aphids exposed for 8 h at midfield were infected by aerial conidia ofN. fresenii.Exposure of sentinel aphids outside the cotton field, at 10 and 100 m downwind and 10 m upwind, resulted in 34.8% (n = 131), 24.0% (n = 129), and 17.4% (n = 146) infected aphids, respectively. These data demonstrate that wind-dispersed aerial conidia ofN. freseniiare infective and rapidly and efficiently disperse the pathogen throughout aphid populations within and between fields. 相似文献
62.
63.
G. Petyt O. Cougnenc A.-S. Defachelles J.-L. Cazin P. Carpentier 《Médecine Nucléaire》2009,33(12):729-736
IntroductionThe MIITOP-0607 protocol, studying the efficiency of administration of topotecan and myelosupressive [131I]-mIBG therapy in children affected by neuroblastoma, needed to assess irradiation risks on staff and family of children to obtain the agreement of the Autorité de sûreté nucléaire (ASN). Our aim was to quantify irradiation of the staff during preparation of the mIBG and to assay the irradiation and contamination of the accompanying persons.Patient and methodsRadiation exposure of the staff was measured during the preparation, transport and administration of the first treatment. Salivary and urinary excretions were monitored well as the atmospheric radioactivity. Radiation exposure and contamination of the accompanying persons were also measured.ResultsFinger dose of 3 mSv and whole body dose of 50 μSv were estimated for preparation of an 11.1 GBq syringe. Irradiation from urinary activity can be as low as 100 μSv if a dedicated device is used. Salivary excretion decreased rapidly during the first 24 hours. Atmospheric contamination always remained below 25 Bq m?3. Total irradiation of the accompanying persons is about 2.35 mSv for the two consecutive injections (9,3 and 11,1 GBq). Internal contamination occurred only once and corresponded to a 27 μSv whole body irradiation and 670 μSv thyroid irradiation.ConclusionThis study shows the safety of [131I]-mIBG treatments using high activities. The involved dose is not negligible but seems to be acceptable in the specific paediatric oncology context if radioprotection instructions are met and if optimization of protocols is performed. 相似文献
64.
New material ofTrischizolagus dumitrescuae from Moldova and Ukraine is described. The variation of p3 inTrischizolagus shows the gradual shift of morphotype frequencies from the ‘Hypolagus’ pattern in Turolian through the mixture of three patterns (including ‘Nekrolagus’ morphotype) in Early Ruscinian to the dominant ‘Alilepus’ pattern in the Late Ruscinian samples. These transformations took place parallel to that of the North AmericanNekrolagus. Probably North AmericanSylvilagus, Brachylagus, andRomerolagus had an North American origin fromNekrolagus, whereas Eurasiatic and AfricanOryctolagus, Caprolagus, Nesolagus, andPoelagus could have originated in the Old World fromTrischizolagus. 相似文献
65.
The diversity of cultured aerobic organisms collected from water samples and bottom sediment from two areas of natural oil
seepage on Lake Baikal has been researched. Representatives of Alpha-, Beta-, and Gammaproteobacteria have been found in samples
collected near the Bol’shaya Zelenovskaya River mouth, while near Cape Gorevoi Utes Betaproteobacteria were absent. Most cultures
are characterized by a sufficiently high homology level (96–100%) with nucleotide sequences from the international database. 相似文献
66.
W. O. van der Knaap Jacqueline F. N. van Leeuwen Tomasz Goslar Robert Krisai Willy Tinner 《Vegetation History and Archaeobotany》2012,21(1):37-60
Three mires and a small lake in the Swiss and Austrian Alps were studied palynologically at high resolution, covering the
last 1,000, 400, 50 and 1,200 years, respectively. Methodological lessons include: (1) Sub-decadal resolution in upper, little-decomposed
peat layers reveals recurrent marked fluctuations in both percentages and influx of regional tree-pollen types, reflecting
variations in pollen production rather than in plant-population sizes. (2) Intermittent, single-spectrum pollen maxima in
samples of sub-decadal resolution indicate pollen transport in clumps. This type of pollen transport may remain unrecognized
in sections with lower sampling resolution, which may then lead to inappropriate interpretation in terms of plant-population
sizes. (3) The detection of short-lived phases of human impact in decomposed peat requires sampling intervals as close as
0.2 cm. (4) PAR (pollen influx) may reflect vegetation dynamics more faithfully than percentages. Reliable PAR, however, is
difficult to achieve in Alpine mires due to past human impact on peat growth, even when complex depth–age modelling techniques
are used. Critical comparison of PAR with percentages is therefore essential. (5) Careful consideration of spatial scales
in pollen signals (local–regional and subdivisions) is essential for a realistic palaeo-ecological interpretation. Results
in terms of past human impact on vegetation are summarized as follows: (1) Trends in pollen types reflecting regional human
action are in general agreement with earlier findings for the western Swiss Alps, allowing for regional differences. (2) All
mires in the Alps investigated here and in an earlier study experienced human impact during the last millennium. The studied
small lake, lying in sub-alpine pasture, records forest dynamics at a lower elevation since a.d. 800. 相似文献
67.
C. Ghetti O. Ortenzia G. Serreli 《Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB)》2012,28(2):161-165
Although iterative reconstruction is widely applied in SPECT/PET, its introduction in clinical CT is quite recent, in the past the demand for extensive computer power and long image reconstruction times have stopped the diffusion of this technique. Recently Iterative Reconstruction in Image Space (IRIS) has been introduced on Siemens top CT scanners. This recon method works on image data area, reducing the time-consuming loops on raw data and noise removal is obtained in subsequent iterative steps with a smoothing process. We evaluated image noise, low contrast resolution, CT number linearity and accuracy, transverse and z-axis spatial resolution using some dedicated phantoms in single, dual source and cardiac mode. We reconstructed images with a traditional filtered back-projection algorithm and with IRIS. The iterative procedure preserves spatial resolution, CT number accuracy and linearity moreover decreases image noise. These preliminary results support the idea that dose reduction with preserved image quality is possible with IRIS, even if studies on patients are necessary to confirm these data. 相似文献
68.
69.
70.
Rubén Torregrosa-Mu?umer Steffi Goffart Juha A. Haikonen Jaakko L. O. Pohjoism?ki 《Molecular biology of the cell》2015,26(23):4197-4208
Mitochondrial DNA is prone to damage by various intrinsic as well as environmental stressors. DNA damage can in turn cause problems for replication, resulting in replication stalling and double-strand breaks, which are suspected to be the leading cause of pathological mtDNA rearrangements. In this study, we exposed cells to subtle levels of oxidative stress or UV radiation and followed their effects on mtDNA maintenance. Although the damage did not influence mtDNA copy number, we detected a massive accumulation of RNA:DNA hybrid–containing replication intermediates, followed by an increase in cruciform DNA molecules, as well as in bidirectional replication initiation outside of the main replication origin, OH. Our results suggest that mitochondria maintain two different types of replication as an adaptation to different cellular environments; the RNA:DNA hybrid–involving replication mode maintains mtDNA integrity in tissues with low oxidative stress, and the potentially more error tolerant conventional strand-coupled replication operates when stress is high. 相似文献