全文获取类型
收费全文 | 186篇 |
免费 | 2篇 |
国内免费 | 1篇 |
专业分类
化学 | 119篇 |
力学 | 15篇 |
数学 | 35篇 |
物理学 | 20篇 |
出版年
2023年 | 1篇 |
2022年 | 5篇 |
2021年 | 2篇 |
2020年 | 1篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 2篇 |
2014年 | 2篇 |
2013年 | 14篇 |
2012年 | 8篇 |
2011年 | 17篇 |
2010年 | 10篇 |
2009年 | 8篇 |
2008年 | 6篇 |
2007年 | 16篇 |
2006年 | 17篇 |
2005年 | 10篇 |
2004年 | 9篇 |
2003年 | 2篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1994年 | 1篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有189条查询结果,搜索用时 62 毫秒
1.
The major goal of this paper is to examine the hypothesis that stock returns and return volatility are asymmetric, threshold nonlinear, functions of change in trading volume. A minor goal is to examine whether return spillover effects also display such asymmetry. Employing a double-threshold GARCH model with trading volume as a threshold variable, we find strong evidence supporting this hypothesis in five international market return series. Asymmetric causality tests lend further support to our trading volume threshold model and conclusions. Specifically, an increase in volume is positively associated, while decreasing volume is negatively associated, with the major price index in four of the five markets. The volatility of each series also displays an asymmetric reaction, four of the markets display higher volatility following increases in trading volume. Using posterior odds ratio, the proposed threshold model is strongly favored in three of the five markets, compared to a US news double threshold GARCH model and a symmetric GARCH model. We also find significant nonlinear asymmetric return spillover effects from the US market. 相似文献
2.
3.
Characterization of archaeological frankincense by gas chromatography-mass spectrometry 总被引:1,自引:0,他引:1
A simple gas chromatography-mass spectrometry (GC-MS) method has been developed for the characterization of frankincense in archaeological samples. After trimethylsilylation of the methanolic extract, 15 triterpenoids have been found among the chemical constituents of commercial olibanum (alpha-boswellic acid, 3-O-acetyl-alpha-boswellic acid, beta-boswellic acid, 3-O-acetyl-beta-boswellic acid, alpha-amyrin, beta-amyrin, lupeol, 3-epi-beta-amyrin, 3-epi-beta-amyrin, 3-epi-lupeol, alpha-amyrenone, beta-amyrenone, lupenone, 3alpha-hydroxy-lup-20(29)-en-24-oic acid and 3-O-acetyl-hydroxy-lup-20(29)-en-24-oic acid). These compounds have been unequivocally identified by retention time and mass spectral comparison with pure standards previously isolated, for the most part, in our laboratory. Within these triterpenes, acid ones, the corresponding O-acetates, and their products of degradation were found to be characteristic of frankincense (Boswellia resin). The presence of these unusual triterpenic compounds in an archaeological resinous sample, recovered during excavations from Dahshour site (Egypt, XIIth Dynasty), enabled us to identify unambiguously frankincense resin among several other materials. Additional chromatographic peaks of this sample were assigned to broad chemical classes using retention time and mass spectra features. 相似文献
4.
2,2,6,6-Tetramethyl-1-piperidinyloxy catalyzes efficient oxidation of primary alcohols to aldehydes by N-chlorosuccinimide, in a biphasic dichloromethane-aqueous pH 8.6 buffer system in the presence of tetrabutylammonium chloride. Aliphatic, benzylic, and allylic alcohols are readily oxidized with no overoxidation to carboxylic acids. Secondary alcohols are oxidized to ketones with a much lower efficiency. Very high chemoselectivities are observed when primary alcohols are oxidized in the presence of secondary ones. Primary-secondary diols are selectively transformed into hydroxy aldehydes, with, in some cases, no detectable formation of the isomeric keto alcohols. 相似文献
5.
With the accelerated accumulation of genomic sequence data, there is a pressing need to develop computational methods and advanced bioinformatics infrastructure for reliable and large-scale protein annotation and biological knowledge discovery. The Protein Information Resource (PIR) provides an integrated public resource of protein informatics to support genomic and proteomic research. PIR produces the Protein Sequence Database of functionally annotated protein sequences. The annotation problems are addressed by a classification-driven and rule-based method with evidence attribution, coupled with an integrated knowledge base system being developed. The approach allows sensitive identification, consistent and rich annotation, and systematic detection of annotation errors, as well as distinction of experimentally verified and computationally predicted features. The knowledge base consists of two new databases, sequence analysis tools, and graphical interfaces. PIR-NREF, a non-redundant reference database, provides a timely and comprehensive collection of all protein sequences, totaling more than 1,000,000 entries. iProClass, an integrated database of protein family, function, and structure information, provides extensive value-added features for about 830,000 proteins with rich links to over 50 molecular databases. This paper describes our approach to protein functional annotation with case studies and examines common identification errors. It also illustrates that data integration in PIR supports exploration of protein relationships and may reveal protein functional associations beyond sequence homology. 相似文献
6.
7.
8.
Oncogenic MicroRNAs Biogenesis as a Drug Target: Structure–Activity Relationship Studies on New Aminoglycoside Conjugates
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
9.
The use of nano- or microfibrillar cellulose (NFC or MFC) in papermaking is generally hampered by high cost and potentially wasteful use in typical wet end applications. The solubility and fines nature of the material makes it inefficient to retain, and when retained it is generally inefficiently applied within the spatial distribution of the paper fibre matrix. The benefits of capturing the important NFC in a layer structure, to enhance surface and stiffness properties of paper, board and laminates whereby NFC is entrapped at the surface of a fibrous web by forming an in situ composite, were previously shown for the exemplified case of modified porous calcium carbonate, as might be used in an inkjet coating application (Ridgway and Gane in Cellulose 19(2):547–560, 2011). The NFC is seen to integrate itself within the larger interparticle porous structure providing excellent holdout and thin layer continuity, essential in developing an efficient concentration of the NFC at the surface of the substrate. The effect is likened to the well-known I-beam construction. The concept need not be confined to porous pigments, as any pigment coating structure that absorbs and holds the NFC, thus creating an in situ composite, could be used. The aim of this study is to look at a range of different pigments and investigate how these could be used as coating structures by measuring the effect on the pore structure before and after absorbing NFC. This is achieved by using model porous tablet blocks made from the respective anionic coating formulations. The penetration of cationic starch solution, as might be applied for surface sizing on paper, is studied for comparison. The use of cationic starch is considered in the industry to provide reasonably effective surface concentrations due to the electrostatically driven adsorption to the anionic pore surfaces. The effect of water alone on the coating structure has also been measured to allow for structural relaxation, considered to be mainly related to the swelling properties of the anionic polyacrylic coating pigment dispersant. The results illustrate the size-exclusion properties of the pore structure in relation to the material being absorbed and partial resistance to bulk penetration by pore wall adsorption in the case of oppositely charged species. The distribution of the absorbate throughout the pore network can be derived using mercury intrusion porosimetry and electron microscopy, and is deemed critical in respect to controlling the end performance properties, be they, for example, barrier, strength-enhancing applications, or both. 相似文献
10.
Cathy J. Ridgway Joachim Schoelkopf Patrick A. C. Gane 《Transport in Porous Media》2011,86(3):945-964
Fine calcium carbonate is frequently used as coating pigment, e.g. in paints, paper and board. Emulsion polymer latex provides
a typical binder incorporated in such coatings. Exposure of the resulting porous structure to liquid, depending on latex concentration
level, results in ab- and adsorption as the liquid is drawn into the structure by capillarity and the nature of the relative
surface chemistry of the pores. Previous work has shown that typical coating structures act to separate oil components according
to their surface energy characteristics and solubility index in respect to pigment surface and intra-latex diffusion. This
study considers water and alkane (hexadecane) as model liquids. Absorption rate into compressed tablets of ground calcium
carbonate is observed with and without the presence of latex. Polyacrylate dispersant used in manufacture renders the pigment
surface both hydrophilic and hygroscopic and, therefore, controls the differential dynamic wetting characteristics of the
porous structure for either polar or non-polar liquid. In competition with hexadecane saturating the pore structure of a latex-free
sample, polar water is seen to displace the non-polar liquid instantaneously, causing disintegration of the packed porous
structure. In the presence of latex binder, however, the structurally destructive pressure this creates is contained, and
retardation of the polar water is observed. The required pre-wetting action of water vapour diffusion on the polyacrylate
pigment surface is considered to be an integral part of the successful competition between oil and water in the offset printing
process, as non-polar oil absorbs steadily into a water pre-saturated structure. 相似文献