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   21370篇
  免费   2150篇
  国内免费   16篇
  2022年   181篇
  2021年   434篇
  2020年   240篇
  2019年   302篇
  2018年   396篇
  2017年   322篇
  2016年   461篇
  2015年   890篇
  2014年   875篇
  2013年   1209篇
  2012年   1514篇
  2011年   1465篇
  2010年   914篇
  2009年   834篇
  2008年   1178篇
  2007年   1205篇
  2006年   1109篇
  2005年   1153篇
  2004年   1094篇
  2003年   1011篇
  2002年   1025篇
  2001年   234篇
  2000年   178篇
  1999年   268篇
  1998年   278篇
  1997年   182篇
  1996年   170篇
  1995年   160篇
  1994年   149篇
  1993年   174篇
  1992年   170篇
  1991年   160篇
  1990年   142篇
  1989年   171篇
  1988年   164篇
  1987年   132篇
  1986年   130篇
  1985年   150篇
  1984年   145篇
  1983年   147篇
  1982年   191篇
  1981年   196篇
  1980年   148篇
  1979年   121篇
  1978年   138篇
  1977年   114篇
  1976年   107篇
  1975年   95篇
  1974年   99篇
  1973年   85篇
排序方式: 共有10000条查询结果,搜索用时 991 毫秒
991.
A novel type of solar cell has been developed based on charge separation at the heterojunction formed by a transparent conducting MXene electrode and an n‐type silicon (n‐Si) wafer. A thin layer of the native silicon dioxide plays an important role in suppressing the recombination of charge carriers. A two‐step chemical treatment can increase the device efficiency by about 40%. Promisingly, an average power conversion efficiency of over 10% under simulated full sunlight is achieved for this novel class of solar cell with the application of an antireflection layer. The efficiencies of these novel solar cells based on a MXene‐Si heterojunction achieved in this work point to great promise in emerging photovoltaic technology. In addition to their high efficiency, the excellent reproducibility of such devices establishes a solid base for possible future commercialization.  相似文献   
992.
Twenty‐three resin samples have been obtained by tapping from individual Pinus pinaster adult trees grown in Corsica and submitted to acido‐basic partition. Identification and quantitative determination of resin acids has been carried out using 13C‐NMR spectroscopy following a method developed by our group. The main components were dehydroabietic acid (up to 37.6 %), levopimaric acid (up to 35.5 %) and abietic acid (up to 24.7 %). A lignan, pinoresinol, has been identified in some samples. Within the 23 compositions, submitted to k‐means analysis and Principal Component Analysis, two clusters have been perfectly differentiated, whose compositions were dominated by dehydroabietic acid (Group I, M=23.5 %, SD=6.3) and levopimaric acid (Group II, M=21.2 %; SD=6.2), respectively. Both compositions have been observed in the three locations of harvest.  相似文献   
993.
Biomechanics and Modeling in Mechanobiology - The process of vision begins in the retina, yet the role of biomechanical forces in the retina is relatively unknown and only recently being explored....  相似文献   
994.
Protein posttranslational modifications critically regulate a range of physiological and disease processes. In addition to tyrosine, serine, and threonine phosphorylation, reversible N‐ε acylation and alkylation of protein lysine residues also modulate diverse aspects of cellular function. Studies of lysine acyl and alkyl modifications have focused on nuclear proteins in epigenetic regulation; however, lysine modifications are also prevalent on cytosolic proteins to serve increasingly apparent, although less understood roles in cell regulation. Here, the methyl‐lysine (meK) proteome of anucleate blood platelets is characterized. With high‐resolution, multiplex MS methods, 190 mono‐, di‐, and tri‐meK modifications are identified on 150 different platelet proteins—including 28 meK modifications quantified by tandem mass tag (TMT) labeling. In addition to identifying meK modifications on calmodulin (CaM), GRP78 (HSPA5, BiP), and EF1A1 that have been previously characterized in other cell types, more novel modifications are also uncovered on cofilin, drebin‐like protein (DBNL, Hip‐55), DOCK8, TRIM25, and numerous other cytoplasmic proteins. Together, the results and analyses support roles for lysine methylation in mediating cytoskeletal, translational, secretory, and other cellular processes. MS data for this study have been deposited into the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD012217.  相似文献   
995.
996.
997.
998.
Calorie restriction (CR), which lengthens lifespan in many species, is associated with moderate hyperadrenocorticism and attenuated inflammation. Given the anti‐inflammatory action of glucocorticoids, we tested the hypothesis that the hyperadrenocorticism of CR contributes to its attenuated inflammatory response. We used a corticotropin‐releasing‐hormone knockout (CRHKO) mouse, which is glucocorticoid insufficient. There were four controls groups: CRHKO mice and wild‐type (WT) littermates fed either ad libitum (AL) or CR (60% of AL food intake), and three experimental groups: (a) AL‐fed CRHKO mice given corticosterone (CORT) in their drinking water titrated to match the integrated 24‐hr plasma CORT levels of AL‐fed WT mice, (b) CR‐fed CRHKO mice given CORT to match the 24‐hr CORT levels of AL‐fed WT mice, and (c) CR‐fed CHRKO mice given CORT to match the 24‐hr CORT levels of CR‐fed WT mice. Inflammation was measured volumetrically as footpad edema induced by carrageenan injection. As previously observed, CR attenuated footpad edema in WT mice. This attenuation was significantly blocked in CORT‐deficient CR‐fed CRHKO mice. Replacement of CORT in CR‐fed CRHKO mice to the elevated levels observed in CR‐fed WT mice, but not to the levels observed in AL‐fed WT mice, restored the anti‐inflammatory effect of CR. These results indicate that the hyperadrenocorticism of CR contributes to the anti‐inflammatory action of CR, which may in turn contribute to its life‐extending actions.  相似文献   
999.
Natural history museums harbour a plethora of biological specimens which are of potential use in population and conservation genetic studies. Although technical advancements in museum genomics have enabled genome‐wide markers to be generated from aged museum specimens, the suitability of these data for robust biological inference is not well characterized. The aim of this study was to test the utility of museum specimens in population and conservation genomics by assessing the biological and technical validity of single nucleotide polymorphism (SNP) data derived from such samples. To achieve this, we generated thousands of SNPs from 47 red‐tailed black cockatoo (Calyptorhychus banksii) traditional museum samples (i.e. samples that were not collected with the primary intent of DNA analysis) and 113 fresh tissue samples (cryopreserved liver/muscle) using a restriction site‐associated DNA marker approach (DArTseq?). Thousands of SNPs were successfully generated from most of the traditional museum samples (with a mean age of 44 years, ranging from 5 to 123 years), although 38% did not provide useful data. These SNPs exhibited higher error rates and contained significantly more missing data compared with SNPs from fresh tissue samples, likely due to considerable DNA fragmentation. However, based on simulation results, the level of genotyping error had a negligible effect on inference of population structure in this species. We did identify a bias towards low diversity SNPs in older samples that appears to compromise temporal inferences of genetic diversity. This study demonstrates the utility of a RADseq‐based method to produce reliable genome‐wide SNP data from traditional museum specimens.  相似文献   
1000.
The role of nutrition in mental health is becoming increasingly acknowledged. Along with dietary intake, nutrition can also be obtained from “nutrient supplements”, such as polyunsaturated fatty acids (PUFAs), vitamins, minerals, antioxidants, amino acids and pre/probiotic supplements. Recently, a large number of meta‐analyses have emerged examining nutrient supplements in the treatment of mental disorders. To produce a meta‐review of this top‐tier evidence, we identified, synthesized and appraised all meta‐analyses of randomized controlled trials (RCTs) reporting on the efficacy and safety of nutrient supplements in common and severe mental disorders. Our systematic search identified 33 meta‐analyses of placebo‐controlled RCTs, with primary analyses including outcome data from 10,951 individuals. The strongest evidence was found for PUFAs (particularly as eicosapentaenoic acid) as an adjunctive treatment for depression. More nascent evidence suggested that PUFAs may also be beneficial for attention‐deficit/hyperactivity disorder, whereas there was no evidence for schizophrenia. Folate‐based supplements were widely researched as adjunctive treatments for depression and schizophrenia, with positive effects from RCTs of high‐dose methylfolate in major depressive disorder. There was emergent evidence for N‐acetylcysteine as a useful adjunctive treatment in mood disorders and schizophrenia. All nutrient supplements had good safety profiles, with no evidence of serious adverse effects or contraindications with psychiatric medications. In conclusion, clinicians should be informed of the nutrient supplements with established efficacy for certain conditions (such as eicosapentaenoic acid in depression), but also made aware of those currently lacking evidentiary support. Future research should aim to determine which individuals may benefit most from evidence‐based supplements, to further elucidate the underlying mechanisms.  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