**4. Conclusions**

Biocompatible new materials will become increasingly important in the future. Ceramic structures based on Freeze Foaming allow ecological manufacturing without a need for organic scaffolds. Tailoring these ceramic foams to specific applications, a defined and reproducible adjustment of their structure and mechanical properties is necessary. However, the formation of the foam structure during Freeze Foaming is not ye<sup>t</sup> fully understood. Their manufacturing is influenced by a complex interaction of different steps and material's properties within the process.

Using a novel in situ μCT device, it was possible to examine the foaming process and the stages of the foaming process. Due to an integrated pressure control system, the foaming could be stopped at any applied pressure.

Radiographic imaging gathered information about the beginning, maximum, and end of foaming, depending on the temperature of the suspension. Independent of the temperature, foaming starts between 450 and 550 mbar. An earlier end of foaming was detected when increasing the suspension temperature to 40 ◦C, due to a higher water vapor partial pressure and a lower viscosity. Suspensions with an initial temperature of 5 ◦C did not exhibit a foaming maximum in our device, due to their high viscosity.

To observe the pore formation during Freeze Foaming, μCT scans were performed using the new μCT device. Virtual volumes of Freeze Foam scaffolds were created and analyzed. Foaming was executed with varying pressure reduction rates. While the porosity changed only slightly with varying pressure reduction rates, the pores were oriented differently. During foaming, 40 ◦C tempered suspensions collapsed before reaching the triple point, due to their low viscosity. On the other hand, the growth of 5 ◦C suspensions were inhibited by their high viscosity. As a result of radiographic examinations, five pressure values were identified as holding stages of interest. Those stages revealed a large influence of air bubbles introduced during mold filling on the final foam structure. Independent of the initial temperature of the suspension, there is a continuous rise in porosity during the foaming process, in general, while the variance of pore size increases. Furthermore, the results of compression testing of sintered samples show a distinct force maximum for 5 ◦C and 23 ◦C tempered and devolatilized Freeze Foams. On the other hand, samples with a less homogeneous structure (40 ◦C and 23 ◦C not devolatilized) exhibit a force plateau and a maximum force about half that of samples.

Approaches for the defined production of Freeze Foams have been achieved. However, the complexity of the Freeze Foaming process requires more experiments and evaluation, in order to truly control the pore structure and, thus, make them more suitable for larger industries and applications.

**Author Contributions:** T.B. and J.M. are the DFG-funded project managers from TU-DD ILK, writing the manuscript and working on the experiments presented in this paper; V.G. is working on the X-ray and is concerning on the experiments; M.A. and D.W. are the DFG project's manager and editors from the Fraunhofer IKTS, scientifically conceiving, guiding and supervising experiments and evaluations; T.M., A.M., and M.G. helped with higher-ranked issues and questions.

**Funding:** This research has been funded by DFG, gran<sup>t</sup> number 310892168.

**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
