Preparation of molecular alloys by the ball-milling technique
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Preparation of molecular alloys by the ball-milling technique J. Font and J. Muntasell Departament de F´ısica i Eng. Nuclear, Univ. Polit`ecnica de Catalunya, Avda. Diagonal 647, 08028 Barcelona, Spain
E. Cesari and J. Pons Departament de F´ısica, Univ. Illes Balears, Crtra. de Valldemossa km 7.5, 07071 Palma de Mallorca, Spain (Received 10 April 1995; accepted 18 August 1995)
Ball milling is a technique used extensively in metallic powders to obtain adequated properties for their applications. In this study we show that this technique is also useful for preparing organic molecular alloys. Pentaglycerin/Pentaerythritol alloys have been obtained mechanically at room temperature. Calorimetric and crystallographic characterizations establish that the mechanically obtained alloys have the same solid crystalline structure and transform to the plastic phase as the conventionally prepared alloys from the melt of the pure compounds.
In the last few years, milling of materials has become of prime interest and economic importance to the powder/metallurgy industries. The objectives of the ballmilling technique1 are varied (particle size reduction, particle size growth, agglomeration, solid-state alloying, etc.), and it is applied to metallic powders, principally. The obtainment of amorphous2 – 4 or nanocrystalline5,6 alloys, having adequate properties for their applications, constitutes two of the main scopes of the solid-state alloying (mechanical alloying). Our interest is to obtain molecular binary alloys of polyols derived from Neopentane, pertaining to a series of plastic crystals we have extensively studied.7 – 12 These compounds show an orientationally disordered mesophase (plastic phase) before melting. The transition between solid crystalline and plastic phases can be used in thermal energy storage applications.13 – 15 By means of thermal and crystallographic characterizations, temperature-concentration binary phase diagrams of some polyols have been previously established.7 – 10 The phase diagrams were determined with alloys prepared from the melt of the two pure compounds, in the desired proportions, and slow cooling from the liquid phase to room temperature. This method, being satisfactory when low quantities of alloy are required, is not useful for large quantities because of the high vapor pressure and/or the possibility of thermal degradation. We will show in this paper that ball milling is a suitable method to obtain those crystalline molecular alloys. No information about mechanical alloying of organic compounds has been found in the literature. As a first step we apply this technique to obtain stable alloys of Pentaerythritol (PE) C(CH2 OH)4 and Pentaglycerin (PG) C(CH2 OH)3 CH3 at room temperature. These compounds J. Mater. Res., Vol. 11, No. 5, May 1996
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have the same crystalline structure in both phases: the solid crystalline phase is body-centered tetragonal while the plastic phase is face-centered cub
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