Note to reader
Milling is an important technology which has a wide range of potential applications in many different fields. It can be applied in different ways to any type of compound. Its effects range from obtaining nano-particles to chemical synthesis, from the increase in reactivity to the forced formation of alloys.
In all cases, changes in the physical state, in the form of polymorphic modification between crystalline varieties, amorphization etc. may occur during milling. The accidental formation of these new states can dramatically affect the stability and expected performance of a product. On the contrary, milling can be used to induce voluntarily changes of state, which makes it possible to produce new solid forms, with interesting new capacities (for example, in pharmacy, to obtain an amorphization, which allows to improve dissolution properties of poorly soluble compounds). Milling is then a "green" route of synthesis that avoids the use of a solvent or a chemically destructive heating. The amorphization induced during co-milling can be a desirable intermediate of a chemical synthesis. These physical transformations occur in conditions that are far from equilibrium. Their fundamental understanding is a challenge for materials physics. At the present time, there is no universal theoretical framework for describing and predicting transformations induced by milling. Even a simple definition of the relevant control parameters is an open problem.
In this paper, I will consider essentially the issue of changes in physical states and will only briefly mention the other aspects: chemical synthesis, forced formation of alloys or co-crystals. Similarly I will not go into the details of the multiple technical aspects associated with the performance of the mills.
After some general considerations, I will give a rapid description of the classical thermodynamic conditions for obtaining changes in physical states (polymorphisms, phase transitions, amorphization and glass transitions). This is useful for making a comparison between the thermodynamically induced and the mechanically forced changes. I will then briefly introduce the main experimental techniques that are useful to identify and quantify the changes. The numerous examples described later will give an idea of the relevance of these different techniques, according to the types of compounds studied. I will then present typical examples of transformations for different classes of compounds (elemental compounds, minerals and oxides, metal compounds, molecular and macromolecular compounds). I will dwell more on the behavior of these organic compounds, which have an important practical interest in the fields of food, pharmacy, pigments, energetic materials and so on. My area of expertise essentially concerns these compounds. They have a high sensitivity to milling and temperature changes. They present a very rich polymorphism and are good glass formers with a glass transition that is well marked and close to room temperature. It is a favorable situation to investigate in detail the effect of changing milling conditions on the nature of the end products.
I will conclude with a presentation and discussion of the many theories which compete to describe the physical transformations under milling. They are usually based on thermal equilibrium considerations. However there are many shortcomings in these approaches. I will show how more recent non equilibrium approaches could provide a more universal framework of description.