摘要:

_We have conducted an investigation of the interaction of a series of n-alkanes with the surface of highly ordered pyrolytic graphite (HOPG) using scanning tunneling microscopy (STM) and molecular dynamics (MD) computer simulation. The alkanes studied in this work (CnH2n+2with n=16, 17, 20, 28, and 32) yield STM images which are highly suggestive of an ordered quasi-two-dimensional phase on the graphite surface. Individual molecules on the graphite surface adopt a rod-like conformation and are arranged in a 'rank-and-file' structure. Analysis of the STM height profiles indicates the presence of two dominant spatial frequency components. The high-frequency component has a period of about 0.30 nm and is independent of the carbon number of the alkane. The low frequency components have periods ranging from 3 nm to 5.5 nm. These values correlate closely with those obtained from estimates of the length of fully extended alkanes and from molecular dynamics computer simulations of surface-adsorbed alkanes. These direct experimental observations of surface ordering and the associated molecular dynamics simulations provide key new insights in elucidating the mechanisms of molecular adsorption on surfaces and thin films. In addition, the data helped resolve two conflicting mechanisms of alkane adsorption which have appeared in the recent literature.

DOI:10.1163/156855494X00021

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The interface microstructure between α-silicon nitride whiskers and the 6061 aluminum matrix in a squeeze cast composite was observed by transmission electron microscopy. No reaction product was found at the interface. Only a strain field in the matrix near the interface was observed. The strain field induced a thin layer along the interface in which the aluminum lattice was distorted. This layer occasionally looked like an amorphous layer when observed under the exact Bragg diffraction condition of the matrix, but it seemed not to be really amorphous. After prolonged reaction at 800°C, the whiskers reacted with the matrix to produce aluminum nitride and silicon.

DOI:10.1163/156855494X00030

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Non-reactive pure metals do not wet ionocovalent oxides like alumina and silica. It is believed generally that a strong improvement of wetting can be produced by alloying elements presenting a high reactivity with the oxide substrate. In this study, existing data on contact angles are reviewed and analysed in order to answer the following three questions related with the modelling of reactive wetting: (1) Does a relation between wettability, which is a 2D phenomenon, and bulk reactivity (3D) really exist? (2) If so, what are the mechanisms by which an interfacial reaction can modify wettability? (3) What are the criteria for selection of alloying elements able to act as wetting agents? From the answers to these questions, a new description of the relationship between reactivity, wettability and adhesion in metal/oxide systems is given.

DOI:10.1163/156855494X00049

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The residual stresses in a unidirectionally reinforced P100 Gr/6061 Al metal matrix composite with different thermal histories have been measured using X-ray diffraction (XRD) methods. The study reveals that the maximum residual stresses of the Gr/A1 composite measured by the XRD method are strongly dependent on the yield strength of the aluminium matrix which depends on the thermal history of the sample. The yield strength deduced from the XRD residual stress measurement was used as input data in a finite element analysis program described elsewhere for predicting the residual stresses in the composite. Agreement was found between the measured residual stresses and the finite element analysis over a range of thermal histories. To better understand the thermal-mechanical behavior of the composites, the microstructural features of the composite were characterized by means of electron microscopy and differential scanning calorimetry (DSC).

DOI:10.1163/156855494X00058

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The formation of macromolecular aggregates in dilute and semidilute solutions of the blend of two thermodynamically miscible polymers, polycarbonate and poly- Σ -caprolactone was studied. It was found that the critical concentration of crossover in solutions of blends depends on the ratio of components. Adsorption on aerosil from solutions of blends in dilute and semidilute regimens has been investigated and is characterized by the appearance of the maxima on isotherms at concentrations near the critical concentration of crossover. Increasing concentration of the solution leads to a reduction of the fraction of bound segments. The thickness of the adsorbed layer was estimated at various concentrations, assuming that all the surface of the sorbent is saturated at low solution concentration and that the increase of adsorption associated with the increase in concentration is associated with the adsorption of aggregates and the rearrangement of the structure of adsorption layer.

DOI:10.1163/156855494X00067

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor