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   1877篇
  免费   212篇
  国内免费   10篇
  2024年   14篇
  2023年   53篇
  2022年   75篇
  2021年   88篇
  2020年   61篇
  2019年   76篇
  2018年   62篇
  2017年   74篇
  2016年   83篇
  2015年   101篇
  2014年   111篇
  2013年   127篇
  2012年   105篇
  2011年   113篇
  2010年   73篇
  2009年   107篇
  2008年   115篇
  2007年   92篇
  2006年   69篇
  2005年   72篇
  2004年   54篇
  2003年   37篇
  2002年   45篇
  2001年   33篇
  2000年   25篇
  1999年   30篇
  1998年   20篇
  1997年   24篇
  1996年   24篇
  1995年   19篇
  1994年   21篇
  1993年   11篇
  1992年   10篇
  1991年   4篇
  1990年   11篇
  1989年   12篇
  1988年   5篇
  1987年   12篇
  1986年   6篇
  1985年   3篇
  1984年   5篇
  1983年   2篇
  1982年   5篇
  1981年   3篇
  1980年   1篇
  1978年   2篇
  1977年   1篇
  1974年   1篇
  1973年   2篇
排序方式: 共有2099条查询结果,搜索用时 14 毫秒
51.
The present work aims to synthesize zinc oxide (ZnO) nanoparticles via green approaches using leaf extract of Parthenium hysterophorus. UV–vis and FT-IR tests confirmed the existence of biomolecules, active materials, and metal oxides. The X-ray diffraction structural study exposes the ZnO nanoparticles formation with hexagonal phase structures. SEM and TEM analysis reveal surface morphologies of ZnO nanoparticles and most of them are spherical with a size range of 10 nm. ZnO nanoparticles were revealed strong antimicrobial activity against both bacterial and fungal strains. The germination of seeds and vegetative growth of Sesamum indicum has been greatly improved.  相似文献   
52.
Inbreeding depression is the reduction in fitness caused by mating between related individuals. Inbreeding is expected to cause a reduction in offspring fitness when the offspring themselves are inbred, but outbred individuals may also suffer a reduction in fitness when they depend on care from inbred parents. At present, little is known about the significance of such intergenerational effects of inbreeding. Here, we report two experiments on the burying beetle Nicrophorus vespilloides, an insect with elaborate parental care, in which we investigated inbreeding depression in offspring when either the offspring themselves or their parents were inbred. We found substantial inbreeding depression when offspring were inbred, including reductions in hatching success of inbred eggs and survival of inbred offspring. We also found substantial inbreeding depression when parents were inbred, including reductions in hatching success of eggs produced by inbred parents and survival of outbred offspring that received care from inbred parents. Our results suggest that intergenerational effects of inbreeding can have substantial fitness costs to offspring, and that future studies need to incorporate such costs to obtain accurate estimates of inbreeding depression.  相似文献   
53.
The evolution of social traits may not only depend on but also change the social structure of the population. In particular, the evolution of pairwise cooperation, such as biparental care, depends on the pair‐matching distribution of the population, and the latter often emerges as a collective outcome of individual pair‐bonding traits, which are also under selection. Here, we develop an analytical model and individual‐based simulations to study the coevolution of long‐term pair bonds and cooperation in parental care, where partners play a Snowdrift game in each breeding season. We illustrate that long‐term pair bonds may coevolve with cooperation when bonding cost is below a threshold. As long‐term pair bonds lead to assortative interactions through pair‐matching dynamics, they may promote the prevalence of cooperation. In addition to the pay‐off matrix of a single game, the evolutionarily stable equilibrium also depends on bonding cost and accidental divorce rate, and it is determined by a form of balancing selection because the benefit from pair‐bond maintenance diminishes as the frequency of cooperators increases. Our findings highlight the importance of ecological factors affecting social bonding cost and stability in understanding the coevolution of social behaviour and social structures, which may lead to the diversity of biological social systems.  相似文献   
54.
A challenging activity in the promotion of sustainable development is to synthesise indicators that can support decision-making. In developing countries such as Brazil, the use of information in support of the health and health-related environment of humans needs improvement. This paper presents a proposal for a system that can evaluate human health and urban environment sanitation in an integrated manner. The indicators selected for human health were child mortality rate, mortality rate due to acute diarrhoea for all ages and 1–4-year-old mortality rate due to acute respiratory tract infections. Environmental sanitation has been represented by indicators of sanitation coverage, such as sewerage. The paper describes the design of a fuzzy linguistic model that synthesises these indicators. The designed fuzzy rule-based system has proved to be a useful tool to integrate information and assist in the public planning of interrelated areas.  相似文献   
55.

Aim

This survey is performed to learn about the structure of radiotherapy in México.

Background

Radiation oncology practice is increasing because of the higher incidence of cancer. There is no published data about radiotherapy in México.

Materials and methods

A questionnaire was sent to the 83 registered centers in the database of the Mexican regulatory agency. One out of the 32 states has no radiotherapy. 27 centers from 14 states provided their answers.

Results

829 patients are treated annually with any radiotherapy modality in each center. Two centers have one cobalt machine, 7 have a cobalt and a linac and 10 have more than one linac. Five centers use 2D planning systems, 22 use 3D; 9, conventional simulators; 22, CT based simulation, and 1 center has no simulation. Most of the centers verify beams with films, electronic portal image devices and cone beam CTs are also used. Intensity modulated and image guided radiotherapy are performed in 5 states. Breast, prostate, cervix, lung, rectum and head and neck cancer are the six most common locations. There are 45 public and 38 private centers, 2 dedicated to children. Two gamma knife units, 5 Novalis systems, 1 tomotherapy and 2 cyberknife machines are working. All centers have at least one radiation oncologist, one physicist and one radiotherapist.

Conclusions

Definitive conclusions cannot be drawn from this limited feedback due to a low participation of centers. This survey about radiotherapy in Mexico shows the heterogeneity of equipment as well as medical and technical staff in the whole country.  相似文献   
56.
A highly sensitive chemiluminescence (CL) immunoassay was incorporated into a low‐cost microfluidic paper‐based analytical device (μ‐PAD) to fabricate a facile paper‐based CL immunodevice (denoted as μ‐PCLI). This μ‐PCLI was constructed by covalently immobilizing capture antibody on a chitosan membrane modified μ‐PADs, which was developed by simple wax printing methodology. TiO2 nanoparticles coated multiwalled carbon nanotubes (TiO2/MWCNTs) were synthesized as an amplification catalyst tag to label signal antibody (Ab2). After sandwich‐type immunoreactions, the TiO2/MWCNTs were captured on the surface of μ‐PADs to catalyze the luminol‐p‐iodophenol‐H2O2 CL system, which produced an enhanced CL emission. Using prostate‐specific antigen as a model analyte, the approach provided a good linear response range from 0.001 to 20 ng/mL with a low detection limit of 0.8 pg/mL under optimal conditions. This μ‐PCLI showed good reproducibility, selectivity and stability. The assay results of prostate‐specific antigen in clinical serum samples were in good agreement with that obtained by commercially used electrochemiluminescence methods at the Cancer Research Center of Shandong Tumor Hospital (Jinan, Shandong Province, China). This μ‐PCLI could be very useful to realize highly sensitive, qualitative point‐of‐care testing in developing or developed countries. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
57.
58.
ObjectiveTo evaluate the influence of a change in the management of admissions on the activity and care outcomes of a Geriatric Functional Recovery Unit (GFRU).Material and methodsA retrospective observational study was conducted. Since 2000, the Hospital Central Cruz Roja GFRU has been collecting data grouped into periods of 4 years, except for the centralised admissions (September 2016-December 2018). The data collected on admission included the Red Cross Functional and Mental scales, the Barthel index, the main diagnosis of the functional decline (grouped into stroke, orthopaedic problem, and multifactorial immobility episodes), and comorbidity evaluated by the Charlson index. The following outcome variables were analysed: the overall and relative functional gain at discharge; length of hospital stay; the functional efficiency, discharges to nursing homes, and transfers to acute care units. An analysis was made of the relationship between the admissions from the centralised unit and the previous period (directly admission managed by GFRU), using multivariate analysis (linear regression for continuous outcome variables and logistic regression for the dichotomous ones), adjusted for admission variables.ResultsPatients admitted from the centralised unit showed a greater overall and relative functional gain (difference between both means: 3.49 points, 95% CI; 1.65-5.33, and 12.41%, 95% CI; 0.74-24.08, respectively), longer stay (12.92 days, 95% CI; 11.54-14.30) and lower efficiency (−0.36, 95% CI; −0.16 to −0.57), higher risk of institutionalisation (OR 1.61, 95% CI; 1.19-2.16), and transfers to acute care units (OR 3.16, 95% CI; 2.24-4.47).ConclusionsA centralised admissions system had an influence on the improvement of functional parameters in the patients, but with a longer length of hospital stay, and lower efficiency. Increases in institutionalisation at discharge and transfers to acute care units were also observed.  相似文献   
59.
Prolactin-releasing peptide2 (PrRP2) was administered intraperitoneally to male intertidal blenny Rhabdoblennius nitidus, a species with male uniparental care of eggs, to investigate the effect on their feeding activity. A significant inhibitory effect on appetite was observed in the breeding season, but not in the nonbreeding season. These results suggest that PrRP2 and PrRP2 receptors are more active during the breeding season. The presence of a mechanism to inhibit feeding activity while parents take care of their offspring may be important for the success of parental care.  相似文献   
60.
This study describes a recreational fishing method focused on marine catfish, Genidens barbus (Endangered-EN), through the adoption of its eggs as a natural bait during its reproductive period in southern Brazilian ecosystems. Male G. barbus perform parental care, collecting eggs released by females after the fertilization process. Owing to this male behavioural pattern during the reproductive period, these individuals are easily caught in recreational fisheries that use eggs of the same species as a natural bait. The current adoption of G. barbus eggs as bait may intensify the fishing pressure on its populations.  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