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   5914篇
  免费   299篇
  国内免费   8篇
  6221篇
  2024年   19篇
  2023年   92篇
  2022年   216篇
  2021年   312篇
  2020年   146篇
  2019年   166篇
  2018年   180篇
  2017年   153篇
  2016年   270篇
  2015年   294篇
  2014年   310篇
  2013年   467篇
  2012年   424篇
  2011年   460篇
  2010年   263篇
  2009年   236篇
  2008年   318篇
  2007年   287篇
  2006年   260篇
  2005年   216篇
  2004年   200篇
  2003年   169篇
  2002年   168篇
  2001年   47篇
  2000年   41篇
  1999年   39篇
  1998年   30篇
  1997年   32篇
  1996年   18篇
  1995年   27篇
  1994年   28篇
  1993年   22篇
  1992年   18篇
  1991年   16篇
  1990年   15篇
  1989年   17篇
  1988年   17篇
  1987年   18篇
  1986年   7篇
  1985年   23篇
  1984年   21篇
  1983年   15篇
  1982年   25篇
  1981年   13篇
  1980年   16篇
  1979年   9篇
  1978年   13篇
  1977年   10篇
  1974年   6篇
  1966年   7篇
排序方式: 共有6221条查询结果,搜索用时 15 毫秒
61.
This paper represents an attempt to investigate the mating behaviour of Symmorphus allobrogus, explaining the willingness of male to mount and copulate. The male displays including mode and frequency of antennation and position while copulating, the displays further comprises of intensity and frequency of rejecting behaviour. The presence of the male’s copulatory and postcopulatory courtship studies, understands the maintenance of monandry. The wasp has numerous secondary sexual characters, and the mating behaviour follows a phyletic and the specific sexual mating characters in context of sexual selection. The duration of mating phases and the number of male antennation series during precopulatory, copulatory and postcopulatory phases of mounting, differs significantly. Mating success depends mostly on the activities of male in the premounting phase and the behaviour of both sexes has a roughly equal importance for it in precopulatory phase. While during copulation, activity of male has little influence on its duration; however, behaviour of female has crucial effect, inducing its earlier termination.  相似文献   
62.

Over the last decades, web services are used for performing specific tasks demanded by users. The most important task of service’s classification system is to match an anonymous input service with the stored pre-classified web services. The most challenging issue is that web services are currently organized and classified according to syntax while the context of the requested service is ignored. Due to this motivation, Cloud-based Classification Methodology is proposed as it presents a new methodology based on semantic web service’s classification. Furthermore, cloud computing is used for not only storing but also allocating the high scale of web services with both high availability and accessibility. Fog technology is employed to reduce the latency and to speed up response time. The experimental results using the suggested methodology show a better performance of the proposed system regarding both precision and accuracy in comparison with most of the methods discussed in the literature of the current study.

  相似文献   
63.
Objectives:The study aimed to determine the effect of adding a school-based plyometric training program (PMT) to physical education (PE) sessions on the strength, balance, and flexibility in primary school girls.Methods:Students from grades 3-6 were randomized equally to a plyometric or control group. In the control group, students took their regular PE classes twice a week. In the plyometric group, students performed PMT twice a week during the initial 20 minutes of every PE session. The Lido Linea closed kinetic chain isokinetic dynamometer, Star excursion balance test (SEBT), and sit-and-reach test were used to assess muscle strength, balance, and flexibility, respectively, before and after nine weeks of training.Results:The improvement in extension peak force (p=0.04) and extension total work (p<0.001) was more prevalent in the PMT group than in the control group. SEBT scores had improved significantly (p<0.05) for all directions in the PMT group, except in the anterior direction, which was highly significant (p<0.001). Hamstring and lower back flexibility had improved more in the PMT group than in the control group (p<0.001).Conclusion:Adding PMT to regular PE classes has a positive and notable effect on muscle strength, balance, and flexibility in primary school students.  相似文献   
64.
Malaria is a lethal parasitic disease affecting over two hundred million people worldwide and kills almost half a million people per year. Until now, there is no curative treatment for this disease that has a substantial morbidity. The available chemotherapeutic agents are unable to completely control the infection with the continuous appearance of drug resistance. Consequently, the search for new therapeutic agents with high safety profiles and low side effects is of paramount importance. Several natural products have been investigated and proven to have antimalarial effects either in vivo or in vitro. A large number of plants have been studied globally for their antimalarial activities. However, studies that have been conducted in this field in Saudi Arabia are not enough. This article presents global and local research on the need for novel natural antimalarial agents with a particular emphasis on studies involving plants from Saudi Arabian flora.  相似文献   
65.
Loranthaceae family includes hemiparasitic members which are seen invading a wide range of commercial crops. Helicanthes elasticus (Desv.) Danser is very common on mango trees. Though parasitic in nature, this mistletoe is also medicinally important as fetoprotective, against vesicular calculi and kidney infections. This study is an attempt to document macro-microscopical features of parasitic root, fruit and host-mistletoe tissue interaction in the haustorium of H. elasticus growing on mango stems. Collection, preservation, sectioning, staining and photomicrography of the root, fruit and host-mistletoe union were done as per standard methodologies of anatomical studies. Though there is resemblance to the normal roots in morphology as well as anatomy, the microscopic finding of large number of branched stone cells in the roots is interesting. The morpho-anatomical features recorded would help in understanding the infection biology of this mistletoe. The eradication during the earlier stages of its establishment from seed or from the root creeping over the surface of the host can help in controlling this parasite infection on commercially important host plants.  相似文献   
66.
67.
Heavy metals in the soil and ground water have endangered our environment and human bodies by direct or indirect pathways. Currently, bioremediation is a developing process that offers the possibility to destroy various contaminants using natural biological activity. Biopolymers are industrially attractive because of their capability of lowering transition metal ion concentrations to parts per billion, they are widely available, and they are environmentally safe. This paper deals with the preparation of an ethylamine-modified biopolymer (chitosan) and carbon from biowaste (rice husk) composite beads (EAM-CCRCB) for metal ion removal. The prepared adsorbent was used for the adsorption of hexavalent chromium ions from aqueous solutions. The activation and surface properties of the adsorbent were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) analyses. The effect of process variables such as initial metal ion concentration, adsorbent dosage, and pH of the solution on the performance of percentage removal and adsorption capacity were studied. Various isotherm and kinetic models were fitted with experimental data to describe the solute interaction and nature of adsorption with the adsorbent through batch studies. Mass thermodynamic parameters were determined. Regeneration studies were attempted to check the stability and activity of the adsorbent.  相似文献   
68.
A validated simple and sensitive spectrofluorimetric method was developed for the determination of chlorpromazine hydrochloride, promethazine hydrochloride, trifluperazine hydrochloride, thioridazine hydrochloride, perazine maleate and oxomemazine. The method was based on condensation of malonic acid/acetic anhydride (MAA) under the catalytic effect of the tertiary amine moiety of the studied phenothiazines to provide a deep yellow to brown colour with green florescence. Relative fluorescence intensity of the products was measured at λexc 398 nm and λem 432 nm. Different variables affecting the reaction were studied and optimized. The method was successfully applied for the determination of the studied drugs in commercial dosage forms. The lower detection limits allowed the application of this method for the determination of the compounds in plasma as an example of a biological fluid. In addition, the method was considered specific for the determination of tertiary amines in the presence of primary and secondary amines; as a result, it was deemed suitable for the determination of the cited drugs in the presence of their degradation products resulting from N‐dealkylation or oxidation of the corresponding sulphoxides or sulphones. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
69.
Abstract

A series of some new acetylated S-glycosides of 2-thioxoquinazolin-4-ones, their thiono analogues and 3,1-benzothazin-2,4-dithione derivatives, including a D-glucose and a D-galactose derivatives and a D-xylose, and an L-arabinose derivatives have been synthesized. The conformation and configuration of these carbohydrate derivatives were determined by analysing their 1H and 13C NMR chemical shifts and coupling constants. The biological activity of these compounds has been studied.  相似文献   
70.
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