Elastic properties of anisotropic monolithic samples of compressed expanded graphite studied with ultrasounds
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A. Celzard,a) J.F. Mareˆche´, and S. Puricelli Laboratoire de Chimie du Solide Mine´ral, UMR-CNRS 7555, BP 239, 54506 Vandoeuvre-le`s-Nancy, France (Received 11 May 2000; accepted 21 November 2000)
The elastic properties of cubic samples of compressed expanded graphite determined by means of ultrasonic velocity measurements are presented. These materials are highly porous and exhibit porosity-dependent anisotropic moduli. The results are analyzed according to two approaches. The first involves semi-empirical equations fitted to the experimental data, resulting in information about the shape and the connectivity of pores. It is found that pores are oblate ellipsoids, connected parallel to their direction of flatness. The second approach is based on application of the percolation theory near the rigidity threshold. The value of the critical exponent indicates that compressed expanded graphites behave like elastic networks in which central forces are predominant. Results of this study give evidence that ultrasound is a convenient and accurate method for investigation of the critical behavior of the elastic properties in highly tenuous structures.
I. INTRODUCTION
The exfoliation phenomenon of intercalated graphite has been used and developed industrially over the past ten years. The resultant expanded graphite (EG) is obtained in the form of very light wormlike particles. Compressing such a material leads to solid blocks or sheets characterized by interesting mechanical, electrical, and chemical properties.1 Therefore, compressed expanded graphite (CEG) is used in chemical, nuclear, and mechanical industries. CEG is also used as a substratum for adsorption of gases because of its great specific surface area and its homogeneity.2 As will be discussed further in this article, moderate compaction of EG gives highly porous solid blocks. It is possible to improve considerably their adsorption properties using them as support for polymeric species. Indeed, if polymers are subsequently pyrolyzed and activated, they behave like a thin active carbon coating supported on CEG.3 Such materials could have industrial applications, provided that their mechanical properties are satisfactory. As a matter of fact, the CEG blocks having various porosities presented in this paper exhibited very good elastic properties. Dynamic mechanical properties (dynamic elastic moduli) are studied with the molecular acoustics meth-
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J. Mater. Res., Vol. 16, No. 2, Feb 2001 Downloaded: 04 Apr 2015
ods. Molecular acoustics is the study of interactions of sonic waves with matter. The aim of molecular acoustics4,5 is to obtain, on the basis of measurements of some acoustics parameters, such as velocity of acoustic wave propagation and absorption coefficients, information about molecular structure, physical properties of materials, phenomena of phase transitions, and also kinetics of physical processes occurring in matter during ext
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