首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1149篇
  免费   122篇
  2023年   6篇
  2021年   31篇
  2020年   15篇
  2019年   23篇
  2018年   24篇
  2017年   17篇
  2016年   32篇
  2015年   41篇
  2014年   61篇
  2013年   86篇
  2012年   82篇
  2011年   99篇
  2010年   43篇
  2009年   60篇
  2008年   63篇
  2007年   60篇
  2006年   44篇
  2005年   33篇
  2004年   44篇
  2003年   39篇
  2002年   44篇
  2001年   17篇
  2000年   17篇
  1999年   8篇
  1998年   11篇
  1997年   9篇
  1996年   9篇
  1995年   4篇
  1994年   12篇
  1993年   11篇
  1992年   13篇
  1991年   15篇
  1990年   8篇
  1989年   15篇
  1988年   20篇
  1987年   6篇
  1986年   10篇
  1985年   12篇
  1984年   12篇
  1983年   9篇
  1982年   7篇
  1981年   7篇
  1980年   10篇
  1979年   9篇
  1978年   6篇
  1975年   6篇
  1973年   8篇
  1972年   6篇
  1970年   4篇
  1965年   3篇
排序方式: 共有1271条查询结果,搜索用时 25 毫秒
51.
Weather can shape movements of animals and alter their exposure to anthropogenic threats. Bald eagles (Haliaeetus leucocephalus) are increasingly at risk from collision with turbines used in onshore wind energy generation. In the midwestern United States, development of this energy source typically occurs in upland areas that bald eagles use only intermittently. Our objective was to determine the factors that cause wintering bald eagles to occupy riparian areas and riskier, upland areas. We tracked 20 bald eagles using telemetry in the Upper Midwest (MN, IA, MO, WI, IL, USA) during winter 2014–2015 and 2015–2016 and evaluated habitat use by eagles in response to variation in weather and time of year. Eagles used riparian areas more when wind speed and atmospheric pressure were low. Exclusive use of uplands was more frequent during weather systems with low pressure and high humidity and after long periods of cold weather. There was a non-linear response to time of year (measured by days before migration) in the frequency of exclusive use of uplands or riparian areas. Probability of exclusive use of either landscape was generally constant within 95 days prior to migration. The probability of use of riparian areas, however, was markedly less during dates >100 days before migration. Our results suggest that eagles are most likely to be exposed to wind energy developments located in upland areas during low pressure systems, after long periods of cold weather, and several months before the onset of spring migration. This information helps to better understand the factors influencing bald eagle habitat use in winter and will be useful to managers and developers wishing to establish effective strategies to avoid, minimize, and mitigate take, and to survey for mortalities at wind energy developments. © 2020 The Wildlife Society.  相似文献   
52.
The field of neuroimaging dedicated to mapping connections in the brain is increasingly being recognized as key for understanding neurodevelopment and pathology. Networks of these connections are quantitatively represented using complex structures, including matrices, functions, and graphs, which require specialized statistical techniques for estimation and inference about developmental and disorder-related changes. Unfortunately, classical statistical testing procedures are not well suited to high-dimensional testing problems. In the context of global or regional tests for differences in neuroimaging data, traditional analysis of variance (ANOVA) is not directly applicable without first summarizing the data into univariate or low-dimensional features, a process that might mask the salient features of high-dimensional distributions. In this work, we consider a general framework for two-sample testing of complex structures by studying generalized within-group and between-group variances based on distances between complex and potentially high-dimensional observations. We derive an asymptotic approximation to the null distribution of the ANOVA test statistic, and conduct simulation studies with scalar and graph outcomes to study finite sample properties of the test. Finally, we apply our test to our motivating study of structural connectivity in autism spectrum disorder.  相似文献   
53.
Abstract

A method for the parameterisation of molybdenum disulphide is presented which reproduces the crystal structure accurately. The method involves calculating parameters such that there is no net force contribution from any individual term of the potential on any atom. Ideal bond lengths and bond angles are taken from the X-ray crystal structure; stretching and bending force constants are calculated from a combination of spectroscopic data and quantum mechanics calculations, whereby the energy function with bond length or bond angle is calculated and fitted with an harmonic potential. For the non-bonded Lennard-Jones parameters, the dispersion coefficient C was calculated by an interpolation of existing published parameters using a multiple regression and then the crystal energy was minimised with respect to the van der Waals radius r0 using a fixed crystal fragment.

These parameters were tested for various models of the hexagonal and rhombohedral forms of MoS2. RMS fits between structures minimised with molecular mechanics and experimental models ranged from 0.006 Å to 0.012 Å.  相似文献   
54.
A vitrification-based cryopreservation protocol was applied to in vitro sourced shoot-tips of four genotypes of Carica papaya; two female (70 and Z6) and two male (B2 and B4). Regeneration of ~58?% (70) and ~59?% (Z6) was recorded for the female genotypes confirming previously published results. Regeneration was at ~77 and ~53?% for the two male genotypes B2 and B4 respectively. Cryo-tube storage and regeneration was tested after 2?C18?months storage in one male (B2) and one female (70) genotype. Regeneration post cryo-storage was similar to 1?h exposure to liquid nitrogen. Individual shoot-tips from the two female and two male genotypes were grown into complete in vitro plants, potted and acclimatised without micropropagation to provide material for randomly amplified DNA fingerprinting (RAF) and amplified DNA methylation polymorphism (AMP) analysis of multiple individuals from in vitro control, plant vitrification solution 2 (PVS2) cryoprotectant control and short (1?h) and long-term cryopreservation treatment plants. No variations were detected for genotype Z6 control and treatment individuals and no RAF variations were detected in any individuals of genotype B2. Small numbers of RAF and AMP variations were detected in some individuals from genotypes B2 (AMP variation only), B4 and 70, but these were also found in controls. Genotype 70 showed the greatest level of variation; genomic DNA variation (RAF) was detected in control and cryopreservation treatment individuals, and the PVS2 control group was the only treatment group without variations for the respective AMP analysis. The variations observed could not be correlated with any phenotypic characteristics 2?months after acclimatisation.  相似文献   
55.
Herein, we report the synthesis and structure–activity relationship studies of new analogs of boceprevir 1 and telaprevir 2. Introduction of azetidine and spiroazetidines as a P2 substituent that replaced the pyrrolidine moiety of 1 and 2 led to the discovery of a potent hepatitis C protease inhibitor 37c (EC50 = 0.8 μM).  相似文献   
56.
Fish oil-enriched long chain n-3 polyunsaturated fatty acids disrupt the molecular organization of T-cell proteins in the immunological synapse. The impact of fish oil derived n-3 fatty acids on antigen-presenting cells, particularly at the animal level, is unknown. We previously demonstrated B-cells isolated from mice fed with fish oil-suppressed naïve CD4+ T-cell activation. Therefore, here we determined the mechanistic effects of fish oil on murine B-cell major histocompatibility complex (MHC) class II molecular distribution using a combination of total internal reflection fluorescence, Förster resonance energy transfer and confocal imaging. Fish oil had no impact on presynaptic B-cell MHC II clustering. Upon conjugation with transgenic T-cells, fish-oil suppressed MHC II accumulation at the immunological synapse. As a consequence, T-cell protein kinase C theta (PKCθ) recruitment to the synapse was also diminished. The effects were independent of changes in B-T cell adhesion, as measured with microscopy, flow cytometry and static cell adhesion assays with select immune ligands. Given that fish oil can reorganize the membrane by lowering membrane cholesterol levels, we then compared the results with fish oil to cholesterol depletion using methyl-B-cyclodextrin (MβCD). MβCD treatment of B-cells suppressed MHC II and T-cell PKCθ recruitment to the immunological synapse, similar to fish oil. Overall, the results reveal commonality in the mechanism by which fish oil manipulates protein lateral organization of B-cells compared to T-cells. Furthermore, the data establish MHC class II lateral organization on the B-cell side of the immunological synapse as a novel molecular target of fish oil.  相似文献   
57.
Through a bioreplication approach, we have fabricated artificial visual decoys for the invasive species Agrilus planipennis—commonly known as the Emerald Ash Borer (EAB). The mating behavior of this species involves an overflying EAB male pouncing on an EAB female at rest on an ash leaflet before copulating. The male spots the female on the leaflet by visually detecting the iridescent green color of the female's elytra. As rearing EAB and then deploying dead females as decoys for trapping is both arduous and inconvenient, we decided to fabricate artificial decoys. We used a dead female to make a negative die of nickel and a positive die of epoxy. Decoys were then made by first depositing a quarter-wave-stack Bragg reflector on a polymer sheet and then stamping it with a pair of matched negative and positive dies to take the shape of the upper surface of an EAB female. As nearly 100 artificial decoys were fabricated from just one EAB female, this bioreplication process is industrially scalable. Preliminary results from a field trapping test are indicative of success.  相似文献   
58.
59.

Background

Tuberculosis is a major occupational hazard in low and middle-income countries. Limited data exist on serial testing of healthcare workers (HCWs) with interferon-γ release assays (IGRAs) for latent tuberculosis infection (LTBI), especially in low and middle-income countries. We sought to evaluate the rates of and risk factors for LTBI prevalence and LTBI test conversion among HCWs using the tuberculin skin test (TST) and QuantiFERON-TB Gold In-tube assay (QFT-GIT).

Methods

A prospective longitudinal study was conducted among HCWs in the country of Georgia. Subjects completed a questionnaire, and TST and QFT-GIT tests were performed. LTBI testing was repeated 6-26 months after baseline testing.

Results

Among 319 HCWs enrolled, 89% reported prior BCG vaccination, and 60% worked in TB healthcare facilities (HCFs). HCWs from TB HCFs had higher prevalence of positive QFT-GIT and TST than those from non-TB HCFs: 107/194 (55%) vs. 30/125 (31%) QFT-GIT positive (p<0.0001) and 128/189 (69%) vs. 64/119 (54%) TST positive (p = 0.01). There was fair agreement between TST and QFT-GIT (kappa = 0.42, 95% CI 0.31–0.52). In multivariate analysis, frequent contact with TB patients was associated with increased risk of positive QFT-GIT (aOR 3.04, 95% CI 1.79–5.14) but not positive TST. Increasing age was associated with increased risk of positive QFT-GIT (aOR 1.05, 95% CI 1.01–1.09) and TST (aOR 1.05, 95% CI 1.01–1.10). High rates of HCW conversion were seen: the QFT-GIT conversion rate was 22.8/100 person-years, and TST conversion rate was 17.1/100 person-years. In multivariate analysis, female HCWs had decreased risk of TST conversion (aOR 0.05, 95% CI 0.01–0.43), and older HCWs had increased risk of QFT-GIT conversion (aOR 1.07 per year, 95% CI 1.01–1.13).

Conclusion

LTBI prevalence and LTBI test conversion rates were high among Georgian HCWs, especially among those working at TB HCFs. These data highlight the need for increased implementation of TB infection control measures.  相似文献   
60.
The Respiratory Syncytial Virus (RSV) and Influenza A Virus (IAV) are both two major causative agents of severe respiratory tract infections in humans leading to hospitalization and thousands of deaths each year. In this study, we evaluated the immunogenicity and efficacy of a combinatory DNA vaccine in comparison to the single component vaccines against both diseases in a mouse model. Intramuscular electroporation with plasmids expressing the hemagglutinin (HA) of IAV and the F protein of RSV induced strong humoral immune responses regardless if they were delivered in combination or alone. In consequence, high neutralizing antibody titers were detected, which conferred protection against a lethal challenge with IAV. Furthermore, the viral load in the lungs after a RSV infection could be dramatically reduced in vaccinated mice. Concurrently, substantial amounts of antigen-specific, polyfunctional CD8+ T-cells were measured after vaccination. Interestingly, the cellular response to the hemagglutinin was significantly reduced in the presence of the RSV-F encoding plasmid, but not vice versa. Although these results indicate a suppressive effect of the RSV-F protein, the protective efficacy of the combinatory vaccine was comparable to the efficacy of both single-component vaccines. In conclusion, the novel combinatory vaccine against RSV and IAV may have great potential to reduce the rate of severe respiratory tract infections in humans without increasing the number of necessary vaccinations.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号