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   325篇
  免费   30篇
  国内免费   8篇
  2024年   3篇
  2023年   7篇
  2022年   5篇
  2021年   10篇
  2020年   15篇
  2019年   10篇
  2018年   9篇
  2017年   15篇
  2016年   12篇
  2015年   16篇
  2014年   14篇
  2013年   28篇
  2012年   16篇
  2011年   31篇
  2010年   26篇
  2009年   24篇
  2008年   12篇
  2007年   24篇
  2006年   23篇
  2005年   16篇
  2004年   7篇
  2003年   7篇
  2002年   5篇
  2001年   2篇
  2000年   6篇
  1999年   2篇
  1997年   1篇
  1995年   2篇
  1994年   2篇
  1993年   2篇
  1992年   1篇
  1991年   2篇
  1988年   2篇
  1985年   1篇
  1984年   1篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
  1973年   1篇
排序方式: 共有363条查询结果,搜索用时 15 毫秒
241.
Eu3+‐doped transparent glass ceramics containing MF2 (M = Ba, Ca, Sr) nanocrystals were fabricated using a melt–quenching method, and the resulting structures were studied using X‐ray diffraction. Levels 5D1 and 5D0 of Eu3+ ions were verified as thermally coupled levels using the fluorescence intensity ratio method. The fluorescence intensity ratios, optical temperature sensitivity and thermal quenching ratios of the transparent glass ceramics were studied as a function of temperature. With an increase in temperature, the relative sensitivity (S R) decreased sharply at first, then slowly increased, before finally decreasing. The minimum S R values of GCBaF2 (GCB), GCCaF2 (GCC) and GCSrF2 (GCS) were 2.8 × 10?4, 0.8 × 10?4 and 1.9 × 10?4 K?1 at 360, 269 and 319 K, respectively. Glass ceramics with an intense emission intensity can be used to convert the measured spectrum into temperature and may have an important role in temperature detectors.  相似文献   
242.
目的:研究锁定钢板固定术后内侧柱的支撑能力与肱骨近端骨折患者预后的相关关系。方法:选取107例肱骨近端骨折患者作为研究对象,根据不同内侧柱支撑重建方式将所有患者分为四组,其中A组患者48例,均接受肱骨近端内侧骨皮质解剖复位以重建内侧柱支持;B组患者20例,均使用1枚支撑螺钉置入肱骨头内下方的软骨下骨,C组患者14例,均使用2枚或2枚以上支撑螺钉置入肱骨头内下方的软骨下骨;D组患者25例,均未进行肱骨近端内侧骨皮质解剖复位亦未使用锁定螺钉固定。比较各组患者术后Constant评分、VAS(visual analogue scale)评分、骨折愈合时间、肱骨头高度丟失值、肱骨头内翻角、并发症发生情况及二次手术率。结果:与无支撑重建组相比,骨皮质解剖复位组、单枚螺钉支撑重建组以及多枚螺钉支撑重建组的VAS评分、骨折愈合时间、肱骨头高度丟失值以及肱骨头内翻角均明显降低,而Constant评分明显升高,其中骨皮质解剖复位组的变化幅度最大多枚螺钉支撑重建组次之,单枚螺钉支撑重建组变化幅度最小,差异具有统计学意义(t=23.100,22.130,7.267,68.440,47.900,均P0.001);与无支撑重建组相比,骨皮质解剖复位组、单枚螺钉支撑重建组以及多枚螺钉支撑重建组的术后总并发症发生率和二次手术率均明显降低,其中骨皮质解剖复位组的降低幅度最大,单枚螺钉支撑重建组次之,多枚螺钉支撑重建组降低幅度最小差异具有统计学意义(X~2=12.938,11.904,P=0.005,0.008)。结论:锁骨钢板固定术后内侧柱的支撑能力与肱骨近端异型解剖钢板患者预后相关,内侧柱支撑能力的越高患者术后骨折愈合、肩关节恢复越佳,而并发症发生率以及二次手术率越低。  相似文献   
243.
Classification of gene microarrays by penalized logistic regression   总被引:2,自引:0,他引:2  
Classification of patient samples is an important aspect of cancer diagnosis and treatment. The support vector machine (SVM) has been successfully applied to microarray cancer diagnosis problems. However, one weakness of the SVM is that given a tumor sample, it only predicts a cancer class label but does not provide any estimate of the underlying probability. We propose penalized logistic regression (PLR) as an alternative to the SVM for the microarray cancer diagnosis problem. We show that when using the same set of genes, PLR and the SVM perform similarly in cancer classification, but PLR has the advantage of additionally providing an estimate of the underlying probability. Often a primary goal in microarray cancer diagnosis is to identify the genes responsible for the classification, rather than class prediction. We consider two gene selection methods in this paper, univariate ranking (UR) and recursive feature elimination (RFE). Empirical results indicate that PLR combined with RFE tends to select fewer genes than other methods and also performs well in both cross-validation and test samples. A fast algorithm for solving PLR is also described.  相似文献   
244.
Research on the molecular systematics of higher taxa in the butterfly family Nymphalidae (Lepidoptera) is only just beginning. Outgroup selection is difficult at the moment due to the lack of consensus on the basal relationships of the major groups in Nymphalidae. We identify four major clades in the Nymphalidae based on a cladistic analysis of one mitochondrial gene sequence (COI, 1450 bp) and two nuclear gene sequences (EF-1alpha, 1064 bp, and wingless, 412-415 bp) from 54 exemplar species sampled from all currently recognized subfamilies. The COI data set was found to be highly incongruent with the nuclear data sets and a Partitioned Bremer Support analysis shows that the COI data set largely undermines support for most clades. Transitions at the third codon positions of the COI data set were highly saturated, but analyzing the combined data set with the COI third positions removed did not change the results. The major clades we found are termed the danaine clade (including Danainae), the satyrine clade (including Charaxinae, Satyrinae, Calinaginae, and Morphinae), the heliconiine clade (including Heliconiinae and Limenitidinae excluding Biblidini, Cyrestini, Pseudergolini, and Coeini) and the nymphaline clade (including Nymphalinae, Apaturinae, and Coeini, Cyrestini, Pseudergolini, and Biblidini from Limenitidinae). The heliconiine and nymphaline clades are sister groups, while the most parsimonious explanation for the combined data set places the danaine clade as the most basal large group of Nymphalidae. Our results give one of the strongest hypotheses for the subfamilial relationships within Nymphalidae. We were able to resolve the polyphyletic nature of Limenitidinae, which we recommend to be split into three subfamilies: Limenitidinae, Biblidinae, and Cyrestinae. The tribe Coeini belongs in Nymphalinae.  相似文献   
245.
As one important post-translational modification of prokaryotic proteins, pupylation plays a key role in regulating various biological processes. The accurate identification of pupylation sites is crucial for understanding the underlying mechanisms of pupylation. Although several computational methods have been developed for the identification of pupylation sites, the prediction accuracy of them is still unsatisfactory. Here, a novel bioinformatics tool named IMP–PUP is proposed to improve the prediction of pupylation sites. IMP–PUP is constructed on the composition of k-spaced amino acid pairs and trained with a modified semi-supervised self-training support vector machine (SVM) algorithm. The proposed algorithm iteratively trains a series of support vector machine classifiers on both annotated and non-annotated pupylated proteins. Computational results show that IMP–PUP achieves the area under receiver operating characteristic curves of 0.91, 0.73, and 0.75 on our training set, Tung's testing set, and our testing set, respectively, which are better than those of the different error costs SVM algorithm and the original self-training SVM algorithm. Independent tests also show that IMP–PUP significantly outperforms three other existing pupylation site predictors: GPS–PUP, iPUP, and pbPUP. Therefore, IMP–PUP can be a useful tool for accurate prediction of pupylation sites. A MATLAB software package for IMP–PUP is available at https://juzhe1120.github.io/.  相似文献   
246.
This paper reports results from a 3-year field experiment which examined Nitrogen (N) leaching loss from land under various set-aside managements. Four treatments were examined: three ploughed plots which were sown with wheat, ryegrass or maintained fallow; the fourth treatment was unploughed and natural weed growth (volunteers) permitted. The l-year set-aside was followed by two winter wheat test crops. Ceramic suction cups were installed at a depth of 90 cm and used to collect drainage water. N leaching loss was calculated by multiplying drainage volume, calculated from meteorological data, by its inorganic N concentration.Set-aside management significantly affected N leaching loss over the three years. During the set-aside year, the peak nitrate concentration from the unploughed treatment growing volunteer weeds was significantly lower than that from ploughed plots. Of the latter, by the spring, crop (i.e. wheat and ryegrass) assimilation of N significantly reduced N concentration compared to the fallow. The four set-aside treatments had a carry-over effect to the following year (first wheat test crop) resulting in significant differences in N losses. Leaching following the ryegrass treatment was very small and we believe that the grass residues minimised rates of net-N mineralization.The influence of set-aside management continued to the second wheat test crop where N loss was greater under the all wheat rotation because take-all had reduced yield and therefore crop N uptake.  相似文献   
247.
The mechanical structure of plant tissues has recently attracted a lot of attention. The present study aims to evaluate the importance of collenchymatous and sclerenchymatous tissues in supporting plant species in their harsh environments like road and street plant habitats. Dicots and monocots are classified into different models according to the types of supporting mechanisms. Mass cell percentage and soil analysis are used in this investigation. The tissues are distributed with different percentage masses and arrangements to overcome various severe conditions. Statistical analyses enhance the role of these tissues and clarify their significant values. The gear support mechanism is claimed to be the perfect mechanical method used.  相似文献   
248.
A packed bed bioreactor was investigated as means for the cultivation of mammalian cells. The packed bed is comprised of porous ceramic particles with pores sufficiently large for cell immobilization as well as for intraparticle convective flow. In this way, the transport of limiting nutrients such as oxygen can be significantly enhanced, allowing maintenance of cell viability and productivity in an environment protective of adverse shear effects. The extent of intraparticle convective medium flow was experimentally quantified relative to the reactor operating conditions, and was found to be the dominant mechanism of nutrient transport to cells immobilized in the particle interior. An approximate linear relationship was obtained between overall reactor productivity and the extent of intraparticle convection. As the latter can be controlled at the single-particle level through total flow rate control, this relationship is a useful scale-up tool for the design of bioreactors. The high cell densities and the high volumetric productivities achieved by using small lab-scale reactors underline the potential of this simple bioreactor configuration for large-scale cell culture applications. (c) 1993 John Wiley & Sons, Inc.  相似文献   
249.
Birth attendance has been proposed as a distinguishing feature of humans (Homo sapiens) and it has been linked to the difficulty of the delivery process in our species. Here, we provide the first quantitative study based on video-recordings of the social dynamics around three births in captive bonobos (Pan paniscus), human closest living relative along with the chimpanzee. We show that the general features defining traditional birth attendance in humans can also be identified in bonobos. As in humans, birth in bonobos was a social event, where female attendants provided protection and support to the parturient until the infant was born. Moreover, bystander females helped the parturient during the expulsive phase by performing manual gestures aimed at holding the infant. Our results on bonobos question the traditional view that the “obligatory” need for assistance was the main driving force leading to sociality around birth in our species. Indeed, birth in bonobos is not hindered by physical constraints and the mother is self-sufficient in accomplishing the delivery. Although further studies are needed both in captivity and in the wild, we suggest that the similarities observed between birth attendance in bonobos and humans might be related to the high level of female gregariousness in these species. In our view, the capacity of unrelated females to form strong social bonds and cooperate could have represented the evolutionary pre-requisite for the emergence of human midwifery.  相似文献   
250.
Membrane protein plays an important role in some biochemical process such as signal transduction, transmembrane transport, etc. Membrane proteins are usually classified into five types [Chou, K.C., Elrod, D.W., 1999. Prediction of membrane protein types and subcellular locations. Proteins: Struct. Funct. Genet. 34, 137-153] or six types [Chou, K.C., Cai, Y.D., 2005. J. Chem. Inf. Modelling 45, 407-413]. Designing in silico methods to identify and classify membrane protein can help us understand the structure and function of unknown proteins. This paper introduces an integrative approach, IAMPC, to classify membrane proteins based on protein sequences and protein profiles. These modules extract the amino acid composition of the whole profiles, the amino acid composition of N-terminal and C-terminal profiles, the amino acid composition of profile segments and the dipeptide composition of the whole profiles. In the computational experiment, the overall accuracy of the proposed approach is comparable with the functional-domain-based method. In addition, the performance of the proposed approach is complementary to the functional-domain-based method for different membrane protein types.  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