The morphology of porous materials can be of particular interest for a wide range of applications in different fields such as electronics, energy storage, biomedical implants, water cleaning, air filtration, waste disposal, composite materials, and many more.
There are a lot of different approaches to produce porous structures in a defined way. However, one that has attracted increasing scientific attention over the past two decades is a method called ice-templating, since it provides a simple and cost-efficient way to manufacture porous materials with aligned and defined pores. It is based on the directional solidification of aqueous suspensions or solutions, which are solidified by simply cooling the suspension/solution at one boundary. Due to the nature of ice-crystal growth, particles suspended or dissolved in water are rejected from the ice during solidification. This leads to an accumulation of particles in certain areas, which forces a lamellar growth of ice resulting in a structure of alternating phases of particles and ice-lamellae. The ice in a frozen sample can be extracted by simple freeze-drying methods leaving a porous structure made up of the materials that have been formerly suspended/dissolved in water. Although the experimental execution of the process is simple, a lot of physical details are not fully understood and, hence, by far not all theoretical possibilities of the process are realized experimentally.
A collaboration between two research groups of the Otto Schott Institute of Material Research (OSIM) and the Center for Energy and Environmental Chemistry (CEEC) at the Friedrich Schiller University Jena could recently take the next step towards a better control over the ice-templating process. It was already known that the pore sizes of materials produced by ice-templating are directly related to the velocity with which the solidification takes place in a lot of aqueous systems. Thus, varying ice front velocities during the ice-templating process lead to inhomogeneous pore sizes across the sample height.
By applying a specific exponential cooling function, i.e. the time-dependent temperature applied to one boundary of the aqueous sample in order to cool the sample and evoke the solidification, the scientists at the Friedrich Schiller University demonstrated a directional solidification with a preset and constant ice-front velocity all over the process for the first time. The exact velocity of the ice front in the experiments was successfully predicted by calculating the cooling function from the thermal properties of the sample materials.
While the researchers had already demonstrated that the usage of the exponential cooling function results in a homogenous pore size over the whole sample height in a previous work, the new publication provides a direct time-resolved measurement of the ice front evolution as well as a detailed theoretical description and discussion of the process. The latter includes a complete mathematical derivation of the exponential cooling function and a mathematical expression for calculating the naturally limited maximum sample height which can be achieved with a constant ice front velocity. This new approach helps to provide further means for the production of custom-tailored porous materials using the ice-templating process.