Introduction
Motivation
Directional freeze casting creates directionally aligned pores using solidification crystals that push particles or separate preceramic polymer aside and act as sacrificial templates. A long history of theoretical and experimental research on alloy solidification provides the basis for using the freeze-casting method to achieve control over pore characteristics.
Objectives
Therefore, it is essential not only to know the desired pore characteristics and properties, but also to have a deep understanding of processing methods for pore manipulation. While this is an example of engineering porous microstructures to improve the functional properties of the material space, other examples are the use of pore space.
Thesis organization
Thus, most reports in the literature focus on suspension-based freeze-casting. This leads to the accumulation of particles on the surface of the membrane, as shown in the enlarged image (Figure 7.11b).
Background
Review of the porous ceramic processing method
This method replicates the porous structure of cellular materials by infiltrating a ceramic suspension or precursor solution. A critical step in this method is the stabilization of pores in the liquid with surfactant to avoid unwanted coalescence of the incorporated pores.
Freeze casting
In suspension-based freeze casting, it is desirable to repel most of the particles while ensuring that the critical freezing front velocity is not exceeded so that pores are formed by growing crystals. In suspension-based freeze casting, control of rheological properties requires additives such as glycerol [41], polyethylene glycol [42] or gelatin [43].
Solidification
A diagram of the constitutional gradient during solidification and the liquidus temperature gradient prior to the freezing front. The Jackson factor assesses the change in free energy of the adatoms to join the solid phase.
Polymer-derived ceramics (PDC)
The IND of the sample, coarsened for three hours at 4◦C (Figure 5.14c), contains many light blue bands. Some of the bridges produced during the second phase are indicated by arrows in Figure A.2b.
Gradient-Controlled Freeze Casting
Introduction
Although many freeze-casting studies have demonstrated the control of the freezing front velocity (V), there are few studies focusing on the effect of the temperature gradient (G) in freeze-casting structures. In this study, freeze-cast structures are created by controlling G and V based on the basic theory of constitutional subcooling, with the aim of manipulating pore size and pore morphology.
Experimental methods
As a result, the pore size distribution exhibits a unimodal distribution instead of the bimodal distribution characteristic of dendritic structures. Pore size distribution was measured by mercury intrusion porosimetry (MIP; Auto Pore IV, Micromeritics, Norcross, GA, USA).
Results
Figures 3.4 a-e show SEM images of samples cast with 20 wt. % of the polymer solution, in the transverse and longitudinal directions. SEM images show that 10 wt. % of the preceramic polymer solution gave dendritic structures with larger pore size, which is further confirmed by the pore size distribution data.
Discussions
For example, the concentration of the solute, C0, can be reduced, leading to a decrease in the slope of the boundary in the stability-microstructure map. Due to the short gelation time of the cyclohexane solution and the boiling point of cyclohexane (80.7◦C), the slowest V and the highest G investigated in this study were not sufficient to achieve long-range cellular growth (Figure 3.6b).
Conclusion
In the case of convection-enhanced freezing, the density gradient is reversed, leading to the convective flow ahead of the freezing front (Figure 4.4b). IND of the sample roughened at 2◦C for one hour (Figure 5.14b) contains a spot (purple arrow) at ~35◦ from the center.
Freeze-cast Honeycomb Structures via Gravity-Enhanced Con-
Introduction
Motivated by studies in directional solidification of metal alloys with convective flow, this chapter focuses on the effect of the convective flow induced by gravity during freeze casting. In alloy systems, depending on the density of the composition in alloys, convective flow may be present during the directional solidification [9, 10].
Experimental methods
In this study, solidification was carried out with solutions in both the conventional arrangement, where gravity is opposite to the freezing point direction, and a convection-enhanced arrangement, where gravity is opposed to the freezing point direction. . In particular, the freezing front velocity and temperature gradient are compared between two freezing conditions, and the resulting pore morphologies and pore sizes are investigated.
Results
The longitudinal images of the conventional freeze-cast sample and the convection-strengthened freeze-cast sample are shown in Figures 4.2c and Figure 4.2f. In stark contrast, the convection-enhanced freeze-cast sample shows cellular pores, leading to honeycomb-like structures, in the slow freezing region (FFP =~1.6 mm, Figure 4.2d) while the fast freezing region (FFP =~5) mm) exhibits dendritic structures (Figure 4.2 e).
Discussions
This discrepancy in the freezing front velocity between conventional freezing and convection-assisted freezing can be explained by constitutional supercooling of the solution. This leads to a greater driving force for dendritic growth, and the speed of the freezing front increases.
Conclusion
These large elliptical pores likely cause a slight shift of the secondary pore peak in the coarse sample at 2◦C (Figure 5.11a). The microstructure section in Figure 5.14e has purple spots corresponding to the point in IND.
Coarsening of Dendrites in Freeze-Cast Systems
Introduction
To gain further insight into the coarsening processes in freeze-cast systems in three dimensions, X-ray computed tomography has allowed us to quantitatively analyze morphologies and directionality using interfacial shape distributions (ISD) and interfacial normal distributions (IND). By linking images, pore size distributions with tomography-derived ISDs, and dendritic pore direction via their INDs, our studies provide new insight into the coarsening in freeze-casting systems, enable comparisons with the coarsening behavior of alloy systems, and provide an additional means for pore network customization.
Experimental methods
All samples were frozen at freezing front velocities of 15 µm/s for 20 and 30 wt. % solutions and temperature gradients of ~2.6 K/mm to maintain a homogeneous pore structure. All samples for MIP were machined with a core drill (∅= 15.9 mm) to remove the edges, and an approximately 1.8 mm disc was cut from the center of the sample.
Analysis of XCT images
The principal curvatures (𝜅1 and 𝜅2) and normal vectors were calculated at each of the triangular spots. If the porous structure has perfectly spherical shapes, the orientation of the normals is isotropic, resulting in a uniform probability distribution in the IND.
Results and discussion
Figure 5.13e and f show the same section of the thick sample at 2◦C for 1 h as in Figure 5.12e, but the structures are colored according to the interfacial shapes of interest. [001] stereographic projections are shown as IND in Figure 5.14 for (a) the control sample, (b) the sample thickened at 2◦C for one hour, and (c) the sample thickened at 4◦C for three hours.
Conclusions
A set of confocal microscopy images (left column: superimposed brightfield and fluorescent images, right column: fluorescent images) is shown in Figure 7.10. As shown by Pang et al., transformation-induced cracking can be mitigated by tuning the ZrO2-CeO2 composition by manipulating crystallographic phase compatibility [4].
Application of Freeze-Cast Structure: Microstructural Engineer-
Introduction
The thin cell walls would mimic the features of oligocrystalline pillars, providing a possible approach to exhibit the shape memory effect in a bulk structure. In this study, in addition to the shape-memory effect, the superelastic effect was also examined.
Experimental methods
A method for measuring freezing point velocity and temperature gradient is available in Chapter 3. In this work, four different conditions of freezing velocity at a constant temperature gradient were studied.
Results and discussion
As Figure 6.10b shows, the monoclinic composition gradually decreased after the second set of five charge-discharge cycles. To investigate the formation of microcracks during the test, the slope of the stress-strain curve during loading was plotted as a function of the applied stress (Figure 6.11a).
Conclusions
A schematic illustration of the side view of the acrylic holder holding the composite is shown in Figure 7.13c. In contrast, Figures A.2b and A.2c show SEM images of two-step freeze-cast SiOC produced with 5 vol.% and 10 vol.% polymer concentration in the second step, respectively.
Applications of Freeze-Cast Ceramics: Pore Space Design for
Size-based filtration by dendritic pores
As can be seen in an enlarged image in Figure 7.11d, the amount of particles that collected on the surface was significantly reduced. The pore size distribution further confirmed the presence of cellular pores in addition to dendritic pores (Figure 7.12d).
Ceramic/polymer composites for membrane chromatography
On the other hand, the compressive strength of two-stage freeze-cast specimens increases as the bridge density increases (and the porosity decreases). The permeability of single-stage freeze-cast and two-stage freeze-cast samples was compared.
Summary and Future Work
Summary and conclusions
Although the velocity of the freeze front had been an important solidification parameter to control the pores in freeze casting, the temperature gradient was often neglected. In contrast, the secondary pore sizes were found to depend on the cooling rate, the product of the freezing front velocity, and the temperature gradient.
Suggestions for Future work
In most freeze-casting studies, the preceramic polymer solution is directionally frozen immediately after the addition of the cross-linking agent. In most cases of freeze casting of preceramic polymers, a crosslinking agent is added prior to curing.
Introduction
This chapter builds on work from the journal article, "Hierarchical Porous Ceramics via Two-Phase Freeze Casting of Preceramic Polymers," by N. In this chapter, two-phase freeze-casting is explored to create a second set of lamellar bridges between (and perpendicular to) lamellae in a hierarchical fashion by solution-based freeze-thaw casting.
Experimental methods
In the first stage of two-stage frozen casting, a solution with 20 vol. % polymethylsiloxane was poured into a cylindrical glass mold (h = 20 mm;𝜙= 25 mm), placed on a PID-controlled thermoelectric plate, which was continuously cooled by silicone oil circulating in the refrigerator. Control samples were also prepared using the same first-stage procedure as above, but followed by pyrolysis instead of solidification; this process is referred to in this document as one-step freeze-casting.
Results and discussion
In addition, it is noteworthy that two-stage freeze-cast samples always have lower permeability than single-stage freeze-cast samples. -stage freeze-cast specimens have comparable compressive strength to dendritic pore structures with similar permeability constants.
