Chapter VIII: Summary and Future Work
8.2 Suggestions for Future work
Figure 8.1: SEM images of freeze-cast structures using cyclooctane as a solvent in longitudinal direction. As the higher temperature gradient is applied, the di- rectionality of pores improved. Left image is taken from a study by Naviroj et al.
[1]
In this work, effects of temperature gradient and coarsening process in solution- based freeze casting were studied in detail. However, only cyclohexane was studied as a solvent, and there are other solvents which produce seaweed structures, lamellar structures, and highly anisotropic, two-dimensional dendritic structures [1] which deserve attention. Figure 8.1 shows freeze-cast structures templated by cyclooctane crystals. Cyclooctane-based microstructure is seaweed-like [2] and the resulting ceramic microstructure is isotropic and non-directional, as shown in the left im- age. However, as the higher temperature gradient is applied, pore directionality is improved (the middle and right images were freeze-cast with a slightly lower preceramic polymer concentration than the one from the left image).
8.2.2 Rheology
Rheology is another important parameter in solution-based freeze casting. In suspension-based freeze casting, changing viscosity requires additives or higher solid loading. However, in solution-based freeze casting, rheology can be con- trolled by changing the molecular weight of preceramic polymers through chemical cross-linking or thermal cross-linking. This gives further control in pore structure and material property, and should be studied further.
Figure 8.2: SEM images showing freeze-cast lamellar structures with (a) 5 minutes and (b) 6 hours of stirring after adding the cross-linking agent.
Enhanced mechanical properties
In most freeze-casting studies, the preceramic polymer solution is directionally frozen right after a cross-linking agent is added. Figure 8.2 shows SEM images of freeze-cast lamellar structures from dimethyl carbonate. Although they are freeze-cast from the same solvent and preceramic polymer, they exhibit significantly different morphologies due to the different cross-linking time, the time the solution had been stirring after addition of the cross-linking agent. Figure 8.2a shows a typical lamellar structure which was fabricated after five minutes of cross-linking time. In contrast, Figure 8.2b shows a porous structure after six hours of cross- linking time. This porous structure has bridges between lamellar walls. Although bridges are effective at enhancing the mechanical properties of lamellar structures, the mechanism by which bridge formation occurs is unknown. Further investigations are required to elucidate this morphology change. In addition, it would be interesting to explore how viscosity affects different porous structures, such as dendritic and isotropic-like structures, and the resultant mechanical and transport properties.
Precise control of pore structure
As shown in Chapter 3, the stability criterion for stable planar front can be expressed as follows:
𝐺 𝑣
= 𝑚 𝐷
𝑘0−1 𝑘0
𝐶0 .
In most examples of freeze-casting of preceramic polymers, a cross-linking agent is added prior to solidification. Once the cross-linking agent is added, the cross-linking
Figure 8.3: Compressive strength and permeability constants of different structures.
Data for "Lamellar 15 µm/s" and "Dendritic 15µm/s" are taken from the work by Naviroj [2].
process starts and increases the molecular weight of preceramic polymer, resulting in a change in the diffusion coefficient, D, as a function of time. This is one of the reasons why it is challenging to achieve cellular pores in solution-based freeze casting because the stability criterion becomes more stringent as the time proceeds (Figure 8.4).
However, recent developments of photopolymerization-assisted freeze casting [3]
enables the precise control of the diffusion coefficient. With this photopolymeriza- tion route, one can start with desired molecular weight of the polymer or viscosity of solution, freeze-cast the solution with fixed rheological properties of the solu- tion, and then cross-link after solidification to ensure the integrity of the sample for pyrolysis. With photopolymerization and gradient-controlled freeze casting, freezing front velocity, temperature gradient and diffusion coefficient can be inde- pendently controlled, which allows to fine-tuning of the porous structure. Hence, it would be interesting to investigate how primary pore spacing, primary pore size and
Figure 8.4: A stability-microstructure map with an arrow indicating an increase of diffusion coefficient results in change in stability criterion.
secondary pore size change with the diffusion coefficient of preceramic polymers.
Additionally, one can explore how conditions for cellular growth can be altered with a change of diffusion coefficient.
8.2.3 Porous shape-memory zirconia with precise dopant control
In Chapter 6, it was demonstrated that improved shape-memory properties were possible by making honeycomb structures. However, the suspension contains both zirconium oxide (zirconia) and cerium oxide (ceria) were mixed by ball-milling.
Because there are two different materials in suspension, one of them might sediment faster during the directional solidification, leading to a variation in composition. As demonstrated by Pang et al., the transformation-induced cracking can be mitigated by tuning the composition of ZrO2-CeO2by manipulating the crystallographic phase compatibility [4]. In their study, it was shown that a variation in composition as small as 0.5 mol.% could have significant impact on crack-resistance properties.
Hence, preparing pre-doped powders [5] and freeze-casting them would be ideal to avoid composition variation within samples, and warrants exploration.
References
[1] Maninpat Naviroj, Peter W. Voorhees, and Katherine T. Faber. “Suspension- and solution-based freeze casting for porous ceramics”. In:Journal of Mate- rials Research32.17 (2017), pp. 3372–3382.
[2] Maninpat Naviroj. “Silicon-based porous ceramics via freeze casting of pre- ceramic polymers”. PhD thesis. Northwestern University, 2017.
[3] Richard Obmann et al. “Porous polysilazane-derived ceramic structures gen- erated through photopolymerization-assisted solidification templating”. In:
Journal of the European Ceramic Society39.4 (2019), pp. 838–845.
[4] Edward L. Pang, Caitlin A. McCandler, and Christopher A. Schuh. “Reduced cracking in polycrystalline ZrO2-CeO2 shape-memory ceramics by meeting the cofactor conditions”. In:Acta Materialia177 (2019), pp. 230–239.
[5] A.L. Quinelato et al. “Synthesis and sintering of ZrO2-CeO2 powder by use of polymeric precursor based on Pechini process”. In: Journal of Materials Science36.15 (2001), pp. 3825–3830.
A p p e n d i x A
HIERARCHICAL PORE STRUCTURE
This chapter is based on the work from the journal article, "Hierarchical porous ceramics via two-stage freeze casting of preceramic polymers," by N. Arai, and K.T.
Faber. This article has been published in Scripta Materialia.
Arai N., and Faber K. T. Hierarchical porous ceramics via two-stage freeze casting of preceramic polymers. Scripta Materialia. 2019;162:72–76. https://doi.org/
10.1016/j.scriptamat.2018.10.037