全文获取类型
收费全文 | 3134篇 |
免费 | 187篇 |
国内免费 | 43篇 |
专业分类
化学 | 2236篇 |
晶体学 | 35篇 |
力学 | 124篇 |
综合类 | 2篇 |
数学 | 362篇 |
物理学 | 605篇 |
出版年
2024年 | 33篇 |
2023年 | 59篇 |
2022年 | 345篇 |
2021年 | 298篇 |
2020年 | 179篇 |
2019年 | 168篇 |
2018年 | 148篇 |
2017年 | 124篇 |
2016年 | 196篇 |
2015年 | 128篇 |
2014年 | 139篇 |
2013年 | 239篇 |
2012年 | 209篇 |
2011年 | 238篇 |
2010年 | 132篇 |
2009年 | 113篇 |
2008年 | 128篇 |
2007年 | 111篇 |
2006年 | 77篇 |
2005年 | 62篇 |
2004年 | 37篇 |
2003年 | 35篇 |
2002年 | 34篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 6篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 10篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1976年 | 3篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1964年 | 1篇 |
排序方式: 共有3364条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
995.
Muhammad Faisal Danish Shahzad Aamer Saeed Bhajan Lal Shomaila Saeed Fayaz Ali Larik Pervaiz Ali Channar Parvez Ali Mahesar Jamaluddin Mahar 《Tetrahedron: Asymmetry》2017,28(11):1445-1461
Isoquinuclidines constitute the central structural nucleus of numerous biologically active natural products, for example, iboga alkaloids such as ibogamine and catharanthine as well as non-indole-containing alkaloids such as the dioscorine and the cannivonines. Furthermore, in medicinal and pharmaceutical chemistry, the isoquinuclidine core is commonly employed as a rigid azabicyclic scaffold, thus providing significant precursors in the synthesis of numerous valuable alkaloids. Summarizing well-organized approaches to access the chiral isoquinuclidine structural centerpiece signifies a significant endeavor not only for developing biologically active natural products but also enhancing biological researches that can lead to possible drug discovery. Over time, the values and methodologies for the asymmetric synthesis of chiral isoquinuclidines are increasing; hence to advance asymmetric synthesis, this review combines and discusses the pros and cons of each synthesis techniques from 2008. This review should be helpful for promoting further developments of asymmetric synthetic methodologies and for medicinal chemistry. 相似文献
996.
Muhammad Danish Rabia Ayub Butt Saba Akbar Muhammad Ashfaq Graham Smith 《Journal of chemical crystallography》2016,46(10-12):421-428
997.
Ring-opening reactions of oxetanes yield important functionalized products depending upon the nature of nucleophiles as well as substitution pattern on the oxetane ring. Ring opening of oxetanes can be carried out under a variety of reaction conditions. In this review article, an up-to-date overview of major synthetic methodologies involved in the ring opening of oxetanes as well as their synthetic applications has been presented. 相似文献
998.
999.
Khushi Muhammad Humaira M. Siddiqi Juliana Eccher Ivan H. Bechtold 《Liquid crystals》2017,44(4):628-642
Hydroxyhexyl esters of alkoxy and halostilbene carboxylic acids were prepared and studied for thermal, liquid crystalline and fluorescent properties. The decomposition temperatures were determined thermogravimetrically and the compounds were found stable at least up to 200°C. Differential scanning calorimetry (DSC) indicated two mesophases in alkoxystilbene caboxylates. The smectic nature of the liquid crystal (LC) compounds was identified from the optical textures and confirmed through X-ray diffraction (XRD) measurements, where SmA, SmB and CrE mesophases were observed. The compounds 3a-g and 3h-k show single absorption maxima in UV-visible spectra at around 338 and 322 nm, respectively. All the alkoxy compounds emit blue light in solution and in solid state in the wavelength range of 422–425 nm. 相似文献
1000.
Traditional information retrieval systems return a ranked list of results to a user’s query. This list is often long, and the user cannot explore all the results retrieved. It is also ineffective for a highly ambiguous language such as Arabic. The modern writing style of Arabic excludes the diacritical marking, without which Arabic words become ambiguous. For a search query, the user has to skim over the document to infer if the word has the same meaning they are after, which is a time-consuming task. It is hoped that clustering the retrieved documents will collate documents into clear and meaningful groups. In this paper, we use an enhanced k-means clustering algorithm, which yields a faster clustering time than the regular k-means. The algorithm uses the distance calculated from previous iterations to minimize the number of distance calculations. We propose a system to cluster Arabic search results using the enhanced k-means algorithm, labeling each cluster with the most frequent word in the cluster. This system will help Arabic web users identify each cluster’s topic and go directly to the required cluster. Experimentally, the enhanced k-means algorithm reduced the execution time by 60% for the stemmed dataset and 47% for the non-stemmed dataset when compared to the regular k-means, while slightly improving the purity. 相似文献