Solid-state mechanical alloying of plastic crystals
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Solid-state mechanical alloying of plastic crystals J. Font and J. Muntasell Departament de F´ısica i Eng. Nuclear, Univ. Polit`ecnica de Catalunya, Avda. Diagonal 647, E-08028 Barcelona, Spain
E. Cesari and J. Pons Departament de F´ısica, Univ. Illes Balears, Crtra. de Valldemossa km 7.5, E-07071 Palma de Mallorca, Spain (Received 8 August 1996; accepted 18 March 1997)
Ball milling has been used as a solid-state mechanical alloying technique in two binary systems of plastic crystals: neopentylglycol/pentaglycerin (NPG/PG), showing a partial solubility in the ordered phase, and 2-amino-2-methyl-1,3-propanediol/tris(hydroxymethyl) (AMP/TRIS) whose immiscibility in this ordered solid phase is almost total. For the AMP/TRIS system the stable state at room temperature was reached by milling. Contrarily, for NPG/PG, DSC measurements reveal that an annealing period is required after milling. These results have been compared with those of the pentaglycerin/ pentaerythritol (PG/PE) binary system, previously studied, whose miscibility is total at room temperature.
I. INTRODUCTION
Crystals with “globular” molecules generally show a transition from an ordered phase (form II) to an orientationally disordered solid state called plastic phase (form I). The great entropy change of this process, compared with that of melting, indicates the considerable disorder of this mesophase. For a few years we have been interested in polyalcohols derived from neopentane with transition to the plastic phase taking place at low temperatures (from room temperature to 470 K). These compounds, having tetrahedral molecules with a C atom bonded to the groups CH3 , CH2 OH, NH2 , and NO2 , were previously studied by means of thermal and crystallographic techniques. Phase diagrams for several binary mixtures were also obtained in order to contribute to the comprehension of the molecular interactions in organic crystals. The great solubility between the plastic phases of these compounds (fcc or bcc) is due in part to their molecular mobility. However, a large degree of immiscibility can be obtained between the solid ordered phases.1–4 Due to the high enthalpy variation of the solid-solid transition, these polyalcohols are of special interest for thermal energy storage systems. Moreover, the different transition temperatures obtained by alloying these polyalcohols make it possible to increase their potential applications. Although it is possible to prepare the alloys from the melt of the two pure compounds and slow cooling to room temperature, the melting method may not be useful when large quantities of compound are required because of their high vapor pressures and the possibility of thermal degradation.5 The aim of the present work is to analyze the ability of ball milling as a technique to prepare the molecular 3254
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 12, Dec 1997
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alloys of these plastic crystals. Sublimation and thermal degradation of the sample can b
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