Amorphous states of alkali phosphates obtained through high-energy ball milling: how different are they from the glassy state?
SCHNEIDER J. 1,2
1 Universidade de São Paulo, São Carlos, Brazil; 2 Center for Research, Technology and Education in Vitreous Materials, São Carlos, Brazil
High-energy ball milling (HEBM) has been proposed as a low-temperature processing method to generate fine powders, with particles having structural characteristics different from the starting materials. The high pressure associated to the collision forces is known to generate surface amorphization or phase transformations at nanometric scale. The process of amorphization well below melting temperatures avoids potential volatilization of some chemical species and allows better energy efficiency. A lingering question about HEBM amorphization is whether the resulting material can be considered a glass or some form of highly distorted nanocrystals, or a mixture of both. In this work, the structural transformations induced by HEBM on Li and Na phosphate compounds were systematically analyzed, with focus on LiPO3 and NaPO3 crystalline phases. As phosphate compositions with good ionic conductivity are important materials to develop solid state batteries, and fabrication methods for electrodes involving HEBM have been proposed, it is important to understand the response of these materials to the milling conditions and the nature of the resulting structures and phases. We studied the effect of the synthesis conditions of the phosphates (reaction temperatures and times) and the milling parameters (kinetic energy, milling time and mixing fluid) on the structure of the milling products, using 7Li, 31P and 23Na nuclear magnetic resonance, differential scanning calorimetry and x-ray diffraction. The results show that dry HEBM generate fully amorphous materials at moderate kinetic energies (400 rpm rotation with 10mm balls) after 60 hours milling. The nature of the amorphous state differs from the one obtained by melt-quench vitrification, as evidenced by the absence of a glass transition event and a broader 31P-NMR resonance line, pointing to a higher degree of disorder of the phosphate chains. The activation energy for Li diffusive jumps in this structure, determined from 7Li-NMR techniques, is lower than in the glass state. The obtained material can be better described as a strongly disordered state derived from the parent crystal form. On the other hand wet HEBM with isopropyl alcohol leads to the formation of new crystalline structures and much less degree of amorphization than the dry process, under comparable milling parameters. This behaviour can be attributed to fast structural relaxation of the amorphous state into crystalline phases induced by the enhanced kinetic energy transfer attained with the use of fluid.