全文获取类型
收费全文 | 411篇 |
免费 | 43篇 |
国内免费 | 37篇 |
专业分类
化学 | 129篇 |
晶体学 | 5篇 |
力学 | 46篇 |
综合类 | 7篇 |
数学 | 154篇 |
物理学 | 150篇 |
出版年
2023年 | 5篇 |
2022年 | 15篇 |
2021年 | 14篇 |
2020年 | 11篇 |
2019年 | 19篇 |
2018年 | 9篇 |
2017年 | 17篇 |
2016年 | 12篇 |
2015年 | 20篇 |
2014年 | 21篇 |
2013年 | 22篇 |
2012年 | 24篇 |
2011年 | 29篇 |
2010年 | 24篇 |
2009年 | 16篇 |
2008年 | 31篇 |
2007年 | 24篇 |
2006年 | 18篇 |
2005年 | 15篇 |
2004年 | 19篇 |
2003年 | 17篇 |
2002年 | 19篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1997年 | 10篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 7篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1981年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有491条查询结果,搜索用时 15 毫秒
1.
2.
Fukuji Higashi Kanako Sugishita 《Journal of polymer science. Part A, Polymer chemistry》2004,42(1):44-51
A two‐stage co‐oligomerization of the oligomers initially formed from an equimolar mixture of isophthalic acid (IPA) and terephthalic acid (TPA) and 2,2‐bis(4‐hydroxyphenyl)propane (BPA, 50 mol %) with bisphenols (BPs, 20 mol %) was carried out using a tosyl chloride/dimethylformamide/pyridine condensing agent. The distributions of the resulting oligomers (nx‐mers), which were quenched with methanol, were determined by a combination of gel permeation chromatography (GPC) and NMR. These distributions (presented by molar percentage) were conveniently calculated with the equation nx (mol %) = nx (% mol by GPC) × n0 (mol % by NMR)/n0 (% mol by GPC), where nx (% mol) = nx (wt % by GPC)/its molecular weight. The results showed the distributions of the preformed IPA/TPA‐BPA oligomers to be in fairly good accord with those obtained directly from GPC and to be supported by the NMR results. The calculation was applied to the co‐oligomers prepared up to a reaction of 0.7, at which there was an increase in the number of higher oligomers indivisible by GPC and the distributions could no longer be determined by molar percentage. The calculated distributions are discussed in relation to the results of copolycondensation. The sequence distributions in the resulting co‐oligomers, which were also examined by NMR, are compared with those in the copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 44–51, 2004 相似文献
3.
A direct central collision of two identical bodies of revolution is studied. A nonstationary mixed boundary-value problem with an unknown moving boundary is formulated. Its solution is represented by a series in term of Bessel functions. An infinite system of Volterra equations of the second kind for the unknown expansion coefficients is derived by satisfying the boundary conditions. The basic characteristics of the collision process are determined depending on the curvature of the frontal surface of the bodies 相似文献
4.
Frost R. L. Weier M. L. Erickson Kristy L. 《Journal of Thermal Analysis and Calorimetry》2004,76(3):1025-1033
Struvite (NH4MgPO4·6H2O) is a mineral often found in urinary tracts and kidneys. Thermal decomposition using slow low heating shows that the 'kidney'
stone can be decomposed at temperatures below 40°C. At this temperature both ammonia and water are evolved. If more rapid
heating is employed the decomposition occurs at around 80°C. The implication of this work rests with the use of low slow heat
for the decomposition of the kidney stones.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
One of the major applications for dielectrophoresis is selective trapping and fractionation of particles. If the surrounding medium is of low conductivity, the trapping force is high, but if the conductivity increases, the attraction decreases and may even become negative. However, high-conductivity media are essential when working with biological material such as living cells. In this paper, some basic calculations have been performed, and a model has been developed which employs both positive and negative dielectrophoresis in a channel with interdigitated electrodes. The finite element method was utilized to predict the trajectories of Escherichia coli bacteria in the superpositioned electrical fields. It is shown that a drastic improvement of trapping efficiency can be obtained in this way, when a high conductivity medium is employed. 相似文献
6.
7.
To make clear the reason of unsuitability of poly(vinyl alcohol) (PVA) protective colloid for the emulsion polymerization
of conjugated monomers, a model experiment of emulsion polymerization of methyl methacrylate (MMA) was carried out with ammonium
persulfate (APS) or azobis(isobutyronitrile) (AIBN) initiators, where a small amount of MMA (1/100th of the concentration
compared with ordinary emulsion polymerization) was employed. This corresponds to the initial stage of the emulsion polymerization.
Grafting of MMA onto PVA took place remarkably irrespective of the kind of the initiators. Formation of homo-poly(MMA) was
observed to a small extent. The formation of new emulsion particles smaller than 100 nm continued to increase to almost the
end of the polymerization. PVA molecules in the grafted polymer are supposed to act as stabilizers of newly formed particles.
From kinetic treatment using the experimental data, the important issues were derived as follows. Firstly, the sulfate anion
radical from APS is much more reactive than the isobutyronitrile radical from AIBN in terms of hydrogen abstraction from PVA.
Secondly, high grafting ability of the latter initiator system, notwithstanding the much lower reactivity in the hydrogen
abstraction compared with the APS system, is attributed to the relative reactivity of the primary radicals, i.e., hydrogen
abstraction reaction from PVA to initiation reaction with MMA. The much slower rate of addition of the isobutyronitrile radical
to the monomer compared with that of hydrogen abstraction from PVA facilitates the grafting, although the rate constant of
hydrogen abstraction is far smaller than that with the sulfate anion radical by 10−4 times.
Received: 26 April 2001 Accepted: 6 September 2001 相似文献
8.
9.
ELECTROCHEMICAL POLYMERIZATION OF ANILINE IN PHOSPHORIC ACID AND THE PROPERTIES OF POLYANILINE 总被引:1,自引:0,他引:1
The behavior of the electrochemical polymerization of aniline in a weak acid, phosphoric acid, is very similar to that in strong acids, i.e. its polymerization rate increases quickly with the electrolysis time. The FTIR spectra of polyaniline samples synthesized in phosphoric acid indicate that the counter ion H2PO4^- is present in both the oxidized form and the reduced form of polyaniline. The counter ion plays an important role in adjusting the pH value at the electrode surface of polyaniline during the oxidation and reduction processes. As a result, a pair of redox peaks still appear in cyclic voltammograms of polyaniline in a solution of sodium sulfate of pH 5.5 and in a solution of NaH2PO4 of pH 7.0,respectively, at low potential scan rate; and the color of polyaniline film also changes with applied potential at pH 7.0. Thus,the pH region for the electrochemical activity and the electrochromism of polyaniline is extended to pH 5.5 for a solution of sodium sulfate and to pH 7.0 for a solution of NaH2PO4. The conductivity of polyaniline is 3.3 S cm^-1, depending on the concentration of phosphoric acid used in the stage of polymerization of aniline. The result of elemental analysis of polyaniline is presented here. 相似文献
10.
Mechanisms for the initial stage of glyceraldehyde and glycine Maillard reaction under different pH conditions have been proposed, following usually the Hodge-scheme. Computations have been performed on the mechanisms at the standard state to test the possibility of the formation of different compounds, through evaluating the changes in Gibb's free energy during the reaction. Electronic energy changes during the reaction have also been evaluated. Glyceraldehyde+deprotonated glycine reaction has been found to be the most favorable for the formation of the Amadori rearrangement products in both gaseous and aqueous states. Due to the possibility of the production of both enol and keto forms of the Amadori rearrangement product, the rate of browning in glyceraldehyde+deprotonated glycine reaction is assumed to be faster than the others. Glyceraldehyde+unionized glycine reaction has been found to be more plausible for the formation of the keto form of the Amadori rearrangement products, particularly, in the gaseous phase. Glyceraldehyde+protonated glycine and glyceraldehyde+glycine zwitterion reactions are not favorable for the formation of the Amadori rearrangement products. Formation of hydroxyacetaldehyde from glyceralaldehyde, as one of the possible C2-fragmentation product, has been found to be favorable in the aqueous state. 相似文献