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   87篇
  免费   14篇
  国内免费   1篇
  2023年   2篇
  2022年   3篇
  2020年   3篇
  2019年   1篇
  2018年   4篇
  2017年   1篇
  2016年   2篇
  2015年   5篇
  2014年   7篇
  2013年   15篇
  2012年   5篇
  2011年   7篇
  2010年   5篇
  2009年   7篇
  2008年   3篇
  2007年   5篇
  2006年   3篇
  2005年   5篇
  2004年   6篇
  2003年   1篇
  2002年   1篇
  2001年   1篇
  2000年   1篇
  1999年   2篇
  1998年   2篇
  1992年   1篇
  1989年   3篇
  1987年   1篇
排序方式: 共有102条查询结果,搜索用时 140 毫秒
21.
22.
Under hydrothermal condition, two novel organic-inorganic hybrid compounds, [Cu(bpp)][Cu2.5(bpp)3(Hbpp)]H0.5[BW12O40]·1.5H2O (1) and [Cu(en)2(H2O)]{[Cu(bpp)]3[AlW12O40]}·H2O (2) (bpp = 1,3-bis(4-pyridyl)propane; en = ethylenediamine), have been synthesized based on B/Al atom-centered Keggin-type polyoxometalates combined with Cu ions and bpp ligands. The two compounds are characterized through single-crystal X-ray diffraction analysis, elemental analyse, IR, UV and TG. For compound 1, as the nodes, the [BW12O40]5− polyanions link to the [Cu2.5(bpp)3(Hbpp)]3.5+ oligomers, leading to the formation of 1D helical chains which further attach to the macrocycles [Cu2(bpp)2]2+ via the Cu-O weak interaction to construct the 2D “wave-like” layers. For compound 2, the {[Cu(bpp)]4[AlW12O40]2} unit is obtained by the interaction between two Keggin-type [AlW12O40]5− polyanions and one tetranuclear macrocycle composed by four [Cu(bpp)]+ complex cations. Furthermore, the units are sandwiched by two 1D “wave-like” polymeric chains resulting in a new 1D structure. In addition, the electrochemical properties and electrocatalytic activities of these two compounds have been studied in this paper.  相似文献   
23.
The project brought together researchers from 9 EU-Countries and resulted in a number of actions, in particular the following: (a) There is an urgent need of defining the concept of flexible working hours, since it has been used in many different and even counterintuitive ways; the most obvious distinction is where the influence over the working hours lies, that is between the “company-based flexibility” and the “individual-oriented flexibility”; (b) The review of the Legislation in force in the 15 European countries shows that the regulation of working times is quite extensive and covers (Council Directive 93/104/EC) almost all the various arrangements of working hours (i.e., part-time, overtime, shift, and night work), but fails to provide for flexibility; (c) According to the data of the Third EU Survey on Working Conditions, longer and “irregular” working hours are in general linked to lower levels of health and well-being; moreover, low (individual) flexibility and high variability of working hours (i.e., company-based flexibility) were consistently associated with poor health and well-being, while low variability combined with high autonomy showed positive effects; (d) Six substudies from different countries demonstrated that flexible working hours vary according to country, economic sector, social status, and gender; overtime is the most frequent form of company-based flexibility but has negative effects on stress, sleep, and social and mental health; individual flexibility alleviates the negative effects of the company-based flexibility on subjective health, safety, and social well-being; (e) The literature review was able to list more than 1,000 references, but it was striking that most of these documents were mainly argumentative with very little empirical data. Thus, one may conclude that there is a large-scale intervention ongoing in our society with almost completely unknown and uncontrolled effects. Consequently, there is a strong need for systematic research and well-controlled actions in order to examine in detail what flexible working hours are considered, what and where are their positive effects, in particular, as concerns autonomy, and what regulation seem most reasonable.  相似文献   
24.
The three-substituted dipyridyl ligand bis(3-pyridylmethyl)sulfide (L1) was prepared by the reaction of 3-(chloromethyl)pyridine hydrochloride with thioacetamide under basic conditions. L1 was reacted with CuI to give complexes with 1:2 and 1:1 molar ratios. Crystal structures of [(CuI)2(L1)] (1) and [CuI(L1)] (2) were determined. In complex 1 the CuI species formed a one-dimensional staircase polymer to which L1 was bound in a side-by-side fashion with π-π interactions between the ligands on each side. Complex 2 consisted of a one-dimensional ribbon polymer of metallomacrocycles formed from two L1 ligands bridging Cu2I2 dimers which were fused within the macrocyclic ring. The analogous disulfide ligand bis(3-pyridylmethyl)disulfide (L2) was prepared by oxidation of the corresponding thiol 3-(sulfanylmethyl)pyridine. L2 was reacted with CuI in 1:2 and 1:1 molar ratios and products isolated but only the 1:1 product was able to be crystallised. The crystal structure of [CuI(L2)] (3) consisted of a one-dimensional ribbon polymer of metallomacrocycles formed from two L2 ligands linked through Cu2I2 dimers. The difference in the metallomacrocycle linking between the related structures 2 and 3 was attributed to the difference in ligand conformation.  相似文献   
25.
Flexible working hours can have several meanings and can be arranged in a number of ways to suit the worker and/or employer. Two aspects of “flexible” arrangement of working hours were considered: one more subjected to company control and decision (variability) and one more connected to individual discretion and autonomy (flexibility). The aim of the study was to analyze these two dimensions in relation to health and well-being, taking into consideration the interaction with some relevant background variables related to demographics plus working and social conditions. The dataset of the Third European Survey on working conditions, conducted in 2000 and involving 21,505 workers, was used. Nineteen health disorders and four psycho-social conditions were tested by means of multiple logistic regression analysis, in which mutually adjusted odds ratios were calculated for age, gender, marital status, number of children, occupation, mode of employment, shift work, night work, time pressure, mental and physical workload, job satisfaction, and participation in work organization. The flexibility and variability of working hours appeared inversely related to health and psycho-social well-being: the most favorable effects were associated with higher flexibility and lower variability. The analysis of the interactions with the twelve intervening variables showed that physical work, age, and flexibility are the three most important factors affecting health and well-being. Flexibility resulted as the most important factor to influence work satisfaction; the second to affect family and social commitment and the ability to do the same job when 60 years old, as well as trauma, overall fatigue, irritability, and headache; and the third to influence heart disease, stomachache, anxiety, injury, and the feeling that health being at risk because of work. Variability was the third most important factor influencing family and social commitments. Moreover, shift and night work confirmed to have a significant influence on sleep, digestive and cardiovascular troubles, as well and health and safety at work. Time pressure also showed a relevant influence, both on individual stress and social life. Therefore, suitable arrangements of flexible working time, aimed at supporting workers' coping strategies, appear to have a clear beneficial effect on worker health and well-being, with positive consequences also at the company and social level, as evidenced by the higher “feeling to be able to work until 60 years of age”.  相似文献   
26.
根据棉花的数量性状,利用聚类分析方法将17个亲本分为4类.根据棉花的综合性状和性状距离,给出了两套亲本选配方案.试验结果表明,这些方案能提高育种工作中亲本选配的效率.  相似文献   
27.
De'ath  Glenn 《Plant Ecology》1999,144(2):191-199
It is widely accepted that reliable ordination of ecological data requires a strong linear or ordinal relationship between the dissimilarity of sites, based on species composition, and the ecological distance between them. Certain dissimilarity measures, having the property that they take a fixed maximum value when sites have no species in common, have been shown to be strongly correlated with ecological distance. For ecological gradients of moderate length (moderate beta diversity), such measures, in conjunction with non-metric multidimensional scaling, will reliably yield successful ordinations. However, as beta diversity increases, more sites have no species in common, and such measures invariably under-estimate ecological distance for such sites. Thus ordinations of data with high species turnover (high beta diversity) may fail.Extended dissimilarities are defined using an iterative adaptation of flexible shortest path adjustment applied to the matrix of dissimilarities with fixed maximum values. By means of theoretical argument and simulations, this is shown to lead to far stronger correlations between the adjusted site dissimilarity and ecological distance for ecological gradients of greater length than previously considered. Hence ordinations of extended dissimilarities, by means of either metric or non-metric scaling techniques, are shown to outperform corresponding ordinations of unadjusted dissimilarities, with the difference increasing with increasing beta diversity.  相似文献   
28.

Background

Flexible video bronchoscopes, in particular the Olympus BF Type 3C160, are commonly used in pediatric respiratory medicine. There is no data on the magnification and distortion effects of these bronchoscopes yet important clinical decisions are made from the images. The aim of this study was to systematically describe the magnification and distortion of flexible bronchoscope images taken at various distances from the object.

Methods

Using images of known objects and processing these by digital video and computer programs both magnification and distortion scales were derived.

Results

Magnification changes as a linear function between 100 mm (×1) and 10 mm (×9.55) and then as an exponential function between 10 mm and 3 mm (×40) from the object. Magnification depends on the axis of orientation of the object to the optic axis or geometrical axis of the bronchoscope. Magnification also varies across the field of view with the central magnification being 39% greater than at the periphery of the field of view at 15 mm from the object. However, in the paediatric situation the diameter of the orifices is usually less than 10 mm and thus this limits the exposure to these peripheral limits of magnification reduction. Intraclass correlations for measurements and repeatability studies between instruments are very high, r = 0.96. Distortion occurs as both barrel and geometric types but both types are heterogeneous across the field of view. Distortion of geometric type ranges up to 30% at 3 mm from the object but may be as low as 5% depending on the position of the object in relation to the optic axis.

Conclusion

We conclude that the optimal working distance range is between 40 and 10 mm from the object. However the clinician should be cognisant of both variations in magnification and distortion in clinical judgements.  相似文献   
29.
30.
摘要 目的:探讨经尿道输尿管镜钬激光碎石术后尿路感染的影响因素及其对患者细胞免疫功能和生活质量的影响。方法:选择2019年6月-2021年1月期间来我院接受经尿道输尿管镜钬激光碎石术治疗的150例患者。采用流式细胞仪检测T淋巴细胞亚群指标:CD4+、CD8+,计算CD4+/CD8+。手术前后采用生活健康量表简表(SF-36)评价患者生活质量。分析经尿道输尿管镜钬激光碎石术后尿路感染的影响因素。结果:150例患者中,术后有18例发生尿路感染,发生率为12.00%(18/150)。根据是否发生尿路感染分为感染组(n=18)和未感染组(n=132)。单因素分析结果显示,经尿道输尿管镜钬激光碎石术后尿路感染与结石直径、年龄、合并糖尿病、住院时间、预防性应用抗菌药物、结石残留情况、手术时间、双J管留置时间有关(P<0.05)。多因素Logistic回归分析结果显示:年龄为61~岁、结石直径≧2 cm、手术时间≧2 h、合并糖尿病、双J管留置时间≧14 d、有结石残留是术后发生尿路感染的危险因素,而预防性应用抗菌药物是其保护因素(P<0.05)。两组术后CD4+、CD4+/CD8+下降,但未感染组高于感染组;CD8+升高,但未感染组低于感染组(P<0.05)。两组术后精神健康、总体健康、社会功能、情感职能、活力、躯体疼痛、生理职能、生理功能评分均升高,且未感染组高于感染组(P<0.05)。结论:经尿道输尿管镜钬激光碎石术后尿路感染患者细胞免疫功能下降,生活质量降低,且其尿路感染受年龄、结石直径、手术时间等因素影响,预防性应用抗菌药物可降低尿路感染的发生风险。  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