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   3169篇
  免费   230篇
  2023年   25篇
  2021年   50篇
  2020年   33篇
  2019年   42篇
  2018年   94篇
  2017年   83篇
  2016年   154篇
  2015年   102篇
  2014年   145篇
  2013年   253篇
  2012年   142篇
  2011年   141篇
  2010年   132篇
  2009年   125篇
  2008年   114篇
  2007年   82篇
  2006年   124篇
  2005年   87篇
  2004年   72篇
  2003年   77篇
  2002年   69篇
  2001年   57篇
  2000年   55篇
  1999年   55篇
  1998年   68篇
  1997年   41篇
  1996年   46篇
  1995年   44篇
  1994年   52篇
  1993年   52篇
  1992年   29篇
  1991年   29篇
  1989年   23篇
  1988年   31篇
  1987年   24篇
  1985年   27篇
  1984年   21篇
  1983年   23篇
  1981年   27篇
  1980年   35篇
  1979年   21篇
  1978年   24篇
  1975年   21篇
  1974年   34篇
  1973年   26篇
  1972年   22篇
  1971年   23篇
  1969年   22篇
  1967年   20篇
  1966年   23篇
排序方式: 共有3399条查询结果,搜索用时 187 毫秒
181.
182.
183.
184.
The Antarctic minke whale (Balaenoptera bonaerensis) is a difficult species to study because of its low visual detectability and preference for living within the sea ice habitat, accessible only by ice‐strengthened vessels. Recent identification of the Antarctic minke whale as the source of the seasonally ubiquitous bio‐duck call has allowed the use of this sound, as well as downsweeps, to investigate seasonality trends and diel patterns in Antarctic minke whale call production, and their relationship to sea ice cover. Passive acoustic data were collected using an autonomous Acoustic Recording Package (ARP) off the western Antarctic Peninsula. Bio‐duck calls were classified into four distinct call variants, with one variant having two subtypes. Bio‐duck calls were detected between April and November, with increasing call duration during the austral winter, indicating a strong seasonality in call production. Downsweeps, which were also attributed to Antarctic minke whales, were present throughout most months during the recording period, with a peak in July, and an absence in March and April. Both bio‐duck and downsweeps were significantly correlated with sea ice cover. No diel patterns were observed in bio‐duck calls or in downsweep call production at this site.  相似文献   
185.
186.
187.
188.
This study examined the effects of school climate (open classroom and positive student relationships) and personality dispositions (shyness and need for cognition) on adolescents’ political self-efficacy. Data were collected in 2014 from 1,954 Czech ninth- and tenth-graders (mean age?=?15.60). A multilevel analysis showed that school-level political self-efficacy predicted self-efficacy for local politics. However, schools and classrooms were rather homogeneous in terms of students’ mean political self-efficacy and students’ self-reported acquisition of civic skills at school. Hence, school characteristics had only limited associations with adolescents’ political self-efficacy. At the same time, students’ political efficacy had a considerable association with lower shyness and higher need for cognition. These results suggest that the development of political efficacy at school goes beyond simple general influences of school environment and individual differences between students must be considered.  相似文献   
189.
To characterize the impact of the potentially functional mutation--the G to A transition at the position 3400 of the leukemia inhibitory factor (LIF; a pluripotent cytokine that plays a central role in the control of the embryo implantation) gene that leads to the exchange of valine with methionine at codon 64 we evaluated the association of the LIF gene mutation and the levels of antiphospolipid antibodies (aPLs) in the peripheral blood of infertile women (the aPLs examination was part of our routine immunological test during the infertility check-up). Eight infertile mutation-positive women were diagnosed with idiopathic infertility (n=5) and endometriosis (n=3) and their levels of aPLs in serum were compared with 115 infertile women without any LIF gene mutation. Enzyme-linked immunosorbent assay was used for the detection of seven antiphospholipid antibodies; the results were statistically assessed by the Fisher's 2 by 2 exact test to evaluate the association of the LIF gene mutations and aPLs in serum of infertile patients. The presence of aPLs was significantly higher in our study group (100%) than in 30% of aPLs-positives in control infertile patients (p = 0.0035) which indicates that the aPLs are elevated in women with LIF gene mutations.  相似文献   
190.
LC-MS/MS analysis on a linear ion trap LTQ mass spectrometer, combined with data processing, stringent, and sequence-similarity database searching tools, was employed in a layered manner to identify proteins in organisms with unsequenced genomes. Highly specific stringent searches (MASCOT) were applied as a first layer screen to identify either known (i.e. present in a database) proteins, or unknown proteins sharing identical peptides with related database sequences. Once the confidently matched spectra were removed, the remainder was filtered against a nonannotated library of background spectra that cleaned up the dataset from spectra of common protein and chemical contaminants. The rectified spectral dataset was further subjected to rapid batch de novo interpretation by PepNovo software, followed by the MS BLAST sequence-similarity search that used multiple redundant and partially accurate candidate peptide sequences. Importantly, a single dataset was acquired at the uncompromised sensitivity with no need of manual selection of MS/MS spectra for subsequent de novo interpretation. This approach enabled a completely automated identification of novel proteins that were, otherwise, missed by conventional database searches.  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