Porous ceramics and how to use them right

Besides the determination of strength, the determination of open porosity and bulk density is the most common analysis we perform in-house – and also one of the simplest. Nevertheless, it is one of the most important test methods for characterising a sample (sometimes even non-destructively). This is because conclusions about other physical properties can be drawn directly from the open porosity.

It is often generally said that the lower the porosity, the better the material. After all, it is known that dense ceramics sometimes have the highest strengths, little surface area for chemicals to attack and a high hardness. In one of our current projects, for example, the ceramic must also be gas-tight so that it can be used as a catalyst membrane.

But there are also cases where the porosity needs to be in a certain (higher) range, or generally as high as possible.

But why?

We would like to explain this in more detail in this blog post, starting by highlighting the use of ceramics as filters. Here, the properties of ceramics such as chemical inertness, temperature and wear resistance make them ideal candidates for many different applications in the chemical and pharmaceutical industries, as well as in gas, water and wastewater treatment.

In filtration, a gas or solution flows through the channels of a porous structure, retaining particles whose size exceeds the radius of the pores. The residue flows through the pores of the filter.

Ceramic filters with large pores in the micrometre range function with simple sieving action (macro- and microfiltration); if the pore size is reduced in microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), other forces such as pressure or chemical potential are required for effective filtration.

Regardless of the filtration principle, separation always occurs through open pores. Their presence is critical to the function of filtration (dense membranes, in which mass transport occurs by ion diffusion through the crystal lattice, are excluded). Closed pores, on the other hand, are dead ends and obviously unfavourable for mass transport.

The opposite is the case with ceramics, which should have the highest possible thermal shock resistance. Whereas with filters the aim is to keep the open pore space as high as possible, for these materials there is a pore optimum – preferably closed.

In the case of ceramics, temperature fluctuations lead to thermal stresses and trigger microcracks that can damage the entire component.

Experimental studies of various dense and porous ceramics confirm that introducing a certain number of pores into the ceramic structure can potentially improve the crack-pore interaction and increase the fracture energy required for crack propagation. This means that by introducing a certain amount of porosity, we can improve the σ/E ratio (strength to elastic modulus) and thus optimize thermal shock resistance.

Another topic that we do not want to ignore in our article is the application in medicine. Here, some ceramics have a very special property: bioactivity.

Bioactivity is the property of materials to develop a direct, adherent and strong bond with the surrounding tissue, which is the key parameter for the development of further connections between implant and bone. Bone implants, for example, are often used to support bone growth as they dissolve in the body. Therefore, for many biomaterials, good biocompatibility and good strength and degradation rates are desirable.

In a porous bioactive ceramic component (implant or scaffold), the size of the pores and the interconnectivity between the pores are the critical factors for bone or living cell ingrowth. The optimal pore size for bone ingrowth has been reported to be 100-400 µm; long or narrow interconnects beyond this point hinder tissue ingrowth and should be avoided.

In summary, porous ceramics are used in many, very different areas. However, the formation of the pores cannot be left to chance. Rather, an attempt is made to create a porous structure whose pores are precisely defined in terms of size, shape and distribution.

AM techniques offer promising possibilities for the production of porous ceramics and the precise control of pore size distribution. In binder jetting, this is provided by controlling the print layer thickness and the ink applied. By varying the print layer thickness or using multiple print heads, membrane supports and membrane layers with different porosities can also be produced in a single manufacturing step. In addition, the use of burnout materials in the powder mixture is an option when a specific pore size is required. The pore size of 3D-printed porous ceramics can vary from one to hundreds of micrometers for fine ceramics and up to several millimeters for coarser structures or, if required, a functionally graded porosity structure, resulting in a solid material with remarkably high thermal shock resistance.

Many advanced AM processes are already used to produce customized bioceramic parts with specific structure and desired porosity, which have both biocompatible and bioactive properties for various applications.

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