Freeze-cast Ceramics Research Articles

A Brief Review on Microstructure Control of Freeze-casted Porous Alumina during Sintering

Freeze-cast ceramics provide a wide range of applications, including filters, catalyst supports, heat-resistant materials, and biomaterials. The unique porous structure of freeze-cast ceramics offers new perspectives in various fields. However, for the functionality and durability of freeze-cast ceramics, a sufficient mechanical strength of the porous architecture is essential. In the freeze-casting process, the final microstructure is determined during sintering at high temperature through densification and grain growth, highlighting the importance of sintering techniques, which can be originated from sintering strategies in the bulk system. In other words, enhancing densification while suppressing grain growth should be realized by tailoring processing variables and/or materials variables. In this review, we suggest an effective way to control the microstructure by addition of dopants in porous alumina prepared by freeze-casting. The results provide a facile method for microstructure control and provide insight into the selection of promising dopants for better mechanical strength of porous ceramics.

Ambient Pressure Drying of Freeze-Cast Ceramics from Aqueous Suspension.

Freeze-casting has been wildly exploited to construct porous ceramics but usually requires costly and demanding freeze-drying (high vacuum, size limit, and supercooled chamber), which can be avoided by the ambient pressure drying (APD) technique. However, applying APD to freeze-cast ceramic based on an aqueous suspension is still challenging due to inert surface chemistry. Herein, a modified APD strategy is developed to improve the drying process of freeze-cast ceramics by exploiting the simultaneous ice etching, ionic cross-linking, and solvent exchange under mild conditions (-10-0 °C, ambient pressure). This versatile strategy is applicable to various ceramic species, metal ions, and freezing techniques. The incorporated metal ions not only enhance liquid-phase sintering, producing ceramics with higher density and mechanical properties than freeze-cast counterparts, but also render customizable coloration and antibacterial property. The cost-/time-efficient APD is promising for mass production and even successive production of large-size freeze-cast ceramics that exceed the size of commercial freeze-dryers.

Bi-directional freezing to fabricate freeze-cast ceramics with orientation gradient and uniaxial compressive deformation behavior of infiltrated composites

Structural orientation gradient in living organisms is attributed to their enhanced surface protection to penetration and deformation, providing bioinspiration to fabricate engineering materials with gradient in mechanical properties. Freeze-casting is a promising technique to fabricate bioinspired materials. However, in the conventional freeze-casting process, due to the applied unidirectional temperature gradient, all the material layers are also unidirectional, oriented in the direction of applied gradient. While bi-directional freezing conditions have been used but still preserving the characteristic unidirectional alignment. The main objective of this work was to demonstrate that bi-directional freezing conditions could be employed to create and control orientation gradient in freeze-cast materials and thereby develop multilayered composites with orientation gradient. Bi-directional freezing conditions were used to generate both horizontal freezing front (HFF) and vertical freezing front (VFF), where the ice crystals associated with VFF tilted the ice crystals associated with HFF, resulting in orientation gradient. 4 different bi-directional freezing conditions were achieved to change gradient microstructure. In situ studies were performed to investigate different growth front movements and calculate the velocities. Porous ceramics were infiltrated to develop bi-directionally freeze-cast composites. While the current literature sheds invaluable insights into the mechanical behavior of freeze-cast composites with unidirectional orientation, this investigation characterized uniaxial compressive mechanical response and damage characteristics of bi-directionally freeze-cast composites, which is another objective of this work. It was revealed that bi-directional freezing conditions can tune material layer orientation throughout freeze-cast microstructure, strongly influencing compressive mechanical response and damage characteristics in the resulting composite materials.

Image data analysis of high resolution μCT data for the characterization of pore orientation and pore space interconnectivity in freeze cast ceramics

Unidirectional and non-unidirectional freeze cast SiOC monoliths were characterized by means of digital image processing. For this purpose a novel method of pore segmentation in complex dendritic pore morphologies was developed. For the first time, the proposed method made it possible to perform an extensive geometrical and structural pore space characterization and analysis on single pore level. The orientation of the primary dendritic channels was obtained from high resolution μCT images. Based on the mean directional vector distribution, the pore space was segmented. As a result, 95% (unidirectional) and 88% (non-unidirectional) of the original binary pore volume could be allocated to individual pores. The pore interconnectivity could be derived from this segmented pore space. It has been shown that the degree of interconnectivity between pores of the same orientation is higher than between pores of different orientation. This could be the reason for peculiar wicking behavior in non-unidirectional samples.

Freeze-cast alumina pore networks: Effects of processing parameters in steady-state solidification regimes of aqueous slurries

Aqueous alumina slurries with varying solids loading and particle size were freeze cast under seven freezing conditions to investigate the influence of these on pore network characteristics including pore size and geometric specific surface area. Slurry temperatures were recorded in situ to determine freezing front position and velocity during solidification, which were then analyzed via regression and modeled using solidification theory. Classic mathematical models for the time dependence of freezing front position and velocity were found to hold for freeze-cast slurries. Building on these, a one-phase Stefan problem was used to describe freezing kinetics. Models for freezing front velocity were combined with solidification theory to obtain predictions of microstructural feature size from freezing kinetics. Results showed that, while there may be a dependence on solids loading, the combination of mathematical modeling of solidification and classic solidification theory is applicable to freeze-cast ceramics and accurately describes pore network characteristics from processing parameters.

Anisotropic sintering behavior of freeze-cast ceramics by optical dilatometry and discrete-element simulations

Directional freeze-casting of ceramic slurries followed by freeze drying and partial sintering results in materials with highly anisotropic properties parallel and transverse to the freezing direction. Physical measurements and optical dilatometry confirm that, during sintering, freeze-cast structures experience more strain along their freezing direction than transverse to it. Discrete Element (DEM) simulations of equivalent freeze-cast structures confirm this behavior. These simulations indicate that not only is sintering anisotropic on the macroscopic scale but within the walls and macropores themselves. It was determined that the anisotropic particle contact network that resulted from the aligned macropores led to anisotropic shrinkage during sintering.

Structure-processing correlations and mechanical properties in freeze-cast Ti-6Al-4V with highly aligned porosity and a lightweight Ti-6Al-4V-PMMA composite with excellent energy absorption capability

In contrast to freeze-cast ceramics and polymers, few freeze-cast metals have been described, to date. Our systematic study on structure-processing correlations in freeze-cast Ti-6Al-4V scaffolds reports, how processing parameters determine the architecture formed during the directional solidification of water-based metal slurries, and after sintering. In addition, sedimentation in the slurry during freezing and volume shrinkage during burnout and sintering were found to significantly affect both structure and properties of the scaffolds. Using two freezing rates, 1 and 10 °C min−1, two water-based polymer solutions as binders (chitosan and carboxymethyl cellulose) and two different metal volume fractions in the slurry, 20 and 30 vol%, Ti-6Al-4V scaffolds could be prepared with pore length, width, and porosity ranging from 41 to 523 μm, 14.5 to 76.5 μm, and 65 to 34%, respectively. Their compressive strength, stiffness, and toughness (work to 20% strain) fall in the range of 83–412 MPa, 7–29 GPa, and 14–122 MJ m−3, respectively. To further improve the properties, a select composition was infiltrated with poly(methyl methacrylate). This increased the average yield strength by a factor of 2.3 from 83 to 193 MPa and the average toughness (work to 50% strain) by a factor of 2.7 from 28.1 to 76.8 MJ m−3.

Effects of alcohol additives on pore structure and morphology of freeze-cast ceramics

The porous alumina ceramics with lamellar structure were fabricated successfully by freeze casting. The viscosities of alumina slurries, pore structures, porosities and mechanical properties of the sintered ceramics were investigated by introducing both types of alcohols as water solidification modifier into the initial slurries, such as ethanol and 1-propanol. With the addition of ethanol or 1-propanol, the viscosities of slurries increased and porosities of sintered ceramics decreased. The compressive strengths of the sintered porous alumina ceramics were improved due to a good connectivity between lamellae with the addition of both types of alcohols. The lowest porosities of 68.52% and 73.72% and highest compressive strengths of 18.2 MPa and 15.0 MPa were obtained by the addition of 30% ethanol in mass fraction and 1-propanol, respectively.

Control of pore structures and sizes in freeze cast ceramics

Freeze casting of aqueous ceramic suspension was investigated as a method for preparing porous ceramic with interconnected, aligned lamellar pores. The manipulation of the pore structures and sizes by adjusting the processing parameters was studied. Al2O3 suspensions with different viscosities were prepared by controlling the solid loadings and particle sizes. Larger lamellar pore width and thinner lamellar ceramic wall without any defect were achieved by the utilisation of microsized Al2O3 powder. Lamellar pore width, ceramic wall thickness and porosity can be tailored ranging from 12 to 49 μm, 4–26 μm and 37·4–56·2% respectively by changing the cooling rates. The incorporation of organic additives can modify not only the pore size but also the pore morphology. Double templates resulted in the coexistence of lamellar pores and large pores. The ability to produce an oriented long range ordered architecture and its manipulation flexibility indicated the potential of this method for different needs in application.