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   3778篇
  免费   285篇
  国内免费   27篇
  4090篇
  2024年   6篇
  2023年   30篇
  2022年   28篇
  2021年   89篇
  2020年   89篇
  2019年   94篇
  2018年   165篇
  2017年   154篇
  2016年   171篇
  2015年   254篇
  2014年   250篇
  2013年   299篇
  2012年   362篇
  2011年   330篇
  2010年   193篇
  2009年   155篇
  2008年   240篇
  2007年   197篇
  2006年   160篇
  2005年   165篇
  2004年   141篇
  2003年   127篇
  2002年   103篇
  2001年   40篇
  2000年   34篇
  1999年   33篇
  1998年   10篇
  1997年   15篇
  1996年   17篇
  1995年   10篇
  1994年   10篇
  1993年   10篇
  1992年   11篇
  1991年   14篇
  1990年   8篇
  1989年   9篇
  1988年   6篇
  1987年   7篇
  1986年   4篇
  1985年   8篇
  1984年   6篇
  1981年   2篇
  1977年   4篇
  1976年   3篇
  1975年   5篇
  1974年   4篇
  1973年   3篇
  1969年   2篇
  1967年   3篇
  1965年   2篇
排序方式: 共有4090条查询结果,搜索用时 0 毫秒
151.
In this work, we fabricated highly aligned electrospun poly(ε-caprolactone)(PCL)/collagen biocomposites, which were consisted of multi-layered structure. The aligned directions of the composites were controlled with two rotating collectors, and various weight fractions (1, 2, 3 wt%) of collagen were embedded between the mat of PCL microfibers to improve the mechanical property and biological activities of osteoblast-like cells (MG63). The PCL/collagen biocomposite showed nine times of increment in mechanical strength of random PCL/collagen composite. An increase in collagen content in the biocomposites displayed significant increase of mechanical properties, hydrophilic property, water-absorption ability, and even cell viability of osteoblast-like cells (MG63).  相似文献   
152.
本研究旨在克隆西方蜜蜂Apis mellifera磁受体基因AmMagR的序列,并分析该基因在工蜂不同日龄、不同组织中的表达特征,以期为该基因的功能研究提供理论基础。以西方蜜蜂工蜂为材料,提取其总RNA,通过RT-PCR技术克隆西方蜜蜂MagR基因的序列;利用多种生物信息学软件对其核酸及氨基酸序列分析;采用实时荧光定量PCR技术(qPCR)分析在西方蜜蜂工蜂不同日龄(1日龄、5日龄和21日龄)和不同组织(头部、胸部和腹部)中MagR基因的相对表达量。克隆到西方蜜蜂MagR基因,并命名为AmMagR(GenBank登录号:MH635411),其序列长度为748 bp,编码区长390 bp,编码130个氨基酸,其蛋白质分子量为14.142 kDa,理论等电点为9.06,无信号肽,无跨膜结构,且在第24~126位氨基酸之间得到一个结构域Fe-S_biosyn。系统发育树显示,西方蜜蜂AmMagR与小蜜蜂Apis florae AfIscA(IscA别称MagR)、中华蜜蜂Apis cerana cerana AcIscA基因聚成一支。荧光定量PCR结果表明,AmMagR基因在西方蜜蜂不同日龄的工蜂头部、胸部和腹部均有表达。5日龄工蜂的相对表达量最高,5日龄工蜂头部和胸部的AmMagR基因表达量显著高于1日龄(P0.05)和21日龄(P0.05),腹部AmMagR基因表达量显著高于21日龄(P0.05),但与1日龄的比较差异不显著(P0.05)。1日龄、5日龄和21日龄工蜂胸部的AmMagR基因表达量均显著高于头部和腹部(P0.05);1日龄工蜂头部AmMagR基因表达量均高于腹部(P0.05),但5日龄和21日龄工蜂AmMagR基因在头部与腹部的表达量无显著差异(P0.05)。明确了该基因在西方蜜蜂工蜂不同日龄和不同组织中的表达模式,并推测AmMagR可能参与了西方蜜蜂的定位、归巢过程。  相似文献   
153.
ABSTRACT

The recombinant human growth hormone (GH) has been used for the treatment of growth hormone deficiency (GHD) and diverse short stature state, and its physiological and therapeutic effects are well documented. However, since the effect of GH treatment on metabolic disorders has not been well characterized, we injected GH to Western diet-fed low-density lipoprotein receptor-deficient (Ldlr ?/?) mice to understand the exact effect of GH on metabolic diseases including atherosclerosis, hepatic steatosis, and obesity. Exogenous GH treatment increased plasma IGF-1 concentration and decreased body weight without affecting serum lipid profiles. GH treatment changed neither atherosclerotic lesion size nor collagen and smooth muscle cells accumulation in the lesion. GH treatment reduced macrophage accumulation in adipose tissue. Importantly, GH treatment attenuated hepatic steatosis and inflammation. The hepatic expression IL-1β mRNA were decreased by GH treatment. The mRNA and protein levels of CD36 were markedly decreased in GH treated mice without significant changes in other molecules related to lipid metabolism. Therefore, the treatment of GH treatment could attenuate hepatic steatosis and inflammation with downregulation of CD36 expression in hyperlipidemic condition.  相似文献   
154.
The record efficiency of the state‐of‐the‐art polymer solar cells (PSCs) is rapidly increasing, due to the discovery of high‐performance photoactive donor and acceptor materials. However, strong questions remain as to whether such high‐efficiency PSCs can be produced by scalable processes. This paper reports a high power conversion efficiency (PCE) of 13.5% achieved with single‐junction ternary PSCs based on PTB7‐Th, PC71BM, and COi8DFIC fabricated by slot‐die coating, which shows the highest PCE ever reported in PSCs fabricated by a scalable process. To understand the origin of the high performance of the slot‐die coated device, slot‐die coated photoactive films and devices are systematically investigated. These results indicate that the good performance of the slot‐die PSCs can be due to a favorable molecule‐structure and film‐morphology change by introducing 1,8‐diiodooctane and heat treatment, which can lead to improved charge transport with reduced carrier recombination. The optimized condition is then used for the fabrication of large‐area modules and also for roll‐to‐roll fabrication. The slot‐die coated module with 30 cm2 active‐area and roll‐to‐roll produced flexible PSC has shown 8.6% and 9.6%, respectively. These efficiencies are the highest in each category and demonstrate the strong potential of the slot‐die coated ternary system for commercial applications.  相似文献   
155.
Candeia (Eremanthus erythropappus (DC.) MacLeish), a native forest species from South America, has garnered commercial interest due to its production of essential oil that contains alpha-bisabolol. This compound is widely used in the pharmaceutical and cosmetics industry, with approximately 80% of Brazilian production being exported. Since candeia rust (Puccinia velata) has only been reported in Brazil, little is known about its epidemiology and control. There is no methodology to quantify rust severity in candeia, justifying the elaboration and validation of a diagrammatic scale containing eight levels of disease severity based on leaf area coverage (0.25%, 0.5%, 1%, 2%, 4%, 8%, 16% and 32%). In a natural sampling of disease in the field, 95% of the leaves showed severity below 16%, with the remaining 5% showing severities between 16% and 32% of leaf area. Validation of the proposed diagrammatic scale was performed by assessing the results from 10 inexperienced evaluators, performing evaluations of three leaves with different severity levels. The evaluations were performed at 7-day intervals; in the first instance, severity values were assigned without the diagrammatic scale, and for the second attempt, the scale proposed in this study was used. The accuracy and precision of the severity estimates produced by each evaluator compared to the real severity was analysed by linear regression and by Lin's statistics. The reproducibility of the estimates was evaluated by analysing the coefficient of determination of linear regressions by pairs of evaluators. The scale provided adequate levels of accuracy, precision, repeatability and reproducibility, indicating the proposed scale was a suitable method for quantifying the severity of candeia rust.  相似文献   
156.
Highly conductive and ultrathin 2D nanosheets are of importance for the development of portable electronics and electric vehicles. However, scalable production and rational design for highly electronic and ionic conductive 2D nanosheets still remain a challenge. Herein, an industrially adoptable fluid dynamic exfoliation process is reported to produce large quantities of ionic liquid (IL)‐functionalized metallic phase MoS2 (m‐MoS2) and defect‐free graphene (Gr) sheets. Hybrid 2D–2D layered films are also fabricated by incorporating Gr sheets into compact m‐MoS2 films. The incorporated IL functionalities and Gr sheets prevent aggregation and restacking of the m‐MoS2 sheets, thereby creating efficient and rapid ion and electron pathways in the hybrid films. The hybrid film with a high packing density of 2.02 g cm?3 has an outstanding volumetric capacitance of 1430.5 F cm?3 at 1 A g?1 and an extremely high rate capability of 80% retention at 1000 A g?1. The flexible supercapacitor assembled using a polymer‐gel electrolyte exhibits excellent resilience to harsh electrochemical and mechanical conditions while maintaining an impressive rate performance and long cycle life. Successful achievement of an ultrahigh volumetric energy density (1.14 W h cm?3) using an organic electrolyte with a wide cell voltage of ≈3.5 V is demonstrated.  相似文献   
157.
We analyzed a portion of mitochondrial COI gene sequences (658 bp) to investigate the genetic diversity and geographic variation of the swallowtail butterfly, Papilio xuthus L. (Lepidoptera: Papilionidae), and the cabbage butterfly, Pieris rapae L. (Lepidoptera: Pieridae). Papilio xuthus showed a moderate level of sequence divergence (0.91% at maximum) in 15 haplotypes, whereas Pi. rapae showed a moderate to high level of sequence divergence (1.67% at maximum) in 30 haplotypes, compared with other relevant studies. Analyses of population genetic structure showed that most populations are not genetically differentiated in both species. The distribution pattern of both species appears to be consistent with category IV of the phylogeographic pattern sensu Avise: a phylogenetic continuity, an absence of regional isolation of mtDNA clones, and extensive distribution of close clones. The observed pattern of genetic diversity and geographic variation of the two butterfly species seem to reflect the abundant habitats, abundant host plants, and flying abilities in connection with the lack of historical biogeographic barriers.  相似文献   
158.
159.

Objectives

In the present study, we aimed to determine the effect of both active and passive smoking on the prevalence of the hearing impairment and the hearing thresholds in different age groups through the analysis of data collected from the Korea National Health and Nutrition Examination Survey (KNHANES).

Study Design

Cross-sectional epidemiological study.

Methods

The KNHANES is an ongoing population study that started in 1998. We included a total of 12,935 participants aged ≥19 years in the KNHANES, from 2010 to 2012, in the present study. Pure-tone audiometric (PTA) testing was conducted and the frequencies tested were 0.5, 1, 2, 3, 4, and 6 kHz. Smoking status was categorized into three groups; current smoking group, passive smoking group and non-smoking group.

Results

In the current smoking group, the prevalence of speech-frequency bilateral hearing impairment was increased in ages of 40−69, and the rate of high frequency bilateral hearing impairment was elevated in ages of 30−79. When we investigated the impact of smoking on hearing thresholds, we found that the current smoking group had significantly increased hearing thresholds compared to the passive smoking group and non-smoking groups, across all ages in both speech-relevant and high frequencies. The passive smoking group did not have an elevated prevalence of either speech-frequency bilateral hearing impairment or high frequency bilateral hearing impairment, except in ages of 40s. However, the passive smoking group had higher hearing thresholds than the non-smoking group in the 30s and 40s age groups.

Conclusion

Current smoking was associated with hearing impairment in both speech-relevant frequency and high frequency across all ages. However, except in the ages of 40s, passive smoking was not related to hearing impairment in either speech-relevant or high frequencies.  相似文献   
160.
The participation of protein serine/threonine kinases in memory formation and retrieval is well established. In contrast, relatively little is known on the role of protein tyrosine kinases (PTKs). Previous work showed that intra-hippocampal infusion of the Src-PTK inhibitor radicicol inhibits memory acquisition, consolidation, and retrieval of one-trial step-down inhibitory avoidance task. In this study, we investigated the possible interaction between levels of Src-PTK activity in hippocampus and memory acquisition, formation, and retrieval of this task. Radicicol (0.5 μg/ml) was infused into the CA1 region of the hippocampus of rats trained in a one-trial step-down inhibitory avoidance task. Radicicol infused 15 min before training decreased Src-PTK activity, as measured 0, 1.5, and 24 h after training, and impaired memory acquisition of the task. When given immediately after training, there was a decrease in Src-PTK activity 1.5 h, but not 0 or 24 h after training. This treatment depressed memory consolidation. Radicicol infused into CA1 10 min prior to retrieval testing inhibited hippocampal Src-PTK activity, as measured immediately after the test session. The results suggest that Src-PTKs participate in memory acquisition, consolidation, and retrieval processes, but the timing of the role of the enzyme is different in each case.  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