Freeze Casting Process Research Articles

Bioinspired intrinsic control of freeze cast composites: Harnessing hydrophobic hydration and clathrate hydrates

Bioinspired ZrO2-epoxy, two-phase composite materials were fabricated by the freeze casting fabrication technique followed by polymer infiltration. These materials were intrinsically controlled by adding varying concentrations of the monofunctional alcohols ethanol (EtOH), n-propanol (n-PrOH) and n-butanol (n-BuOH). The microstructures of freeze cast scaffolds created with these alcohol additives demonstrated maximum pore areas (peak Ap) at concentrations of 10, 5–7 and 3 vol% for EtOH, n-PrOH and n-BuOH respectively. Differential scanning calorimetry analyses of binary mixtures of these additives and water suggested only n-PrOH was capable of developing clathrate hydrates. Measurements of the adiabatic compressibility of complete freeze casting slurries showed that a similar room temperature phenomenon, hydrophobic hydration, was occurring in all cases with the maximum effect occurring at the same additive concentrations as the peak Ap values. This highlights that effects occurring within the slurry at room temperature and before freezing may have a significant effect on the freeze casting process. Analysis of the mechanical properties shows that infiltration of the scaffolds can provide resistance to Euler buckling, resulting in strengths of ∼3 orders of magnitude greater than uninfiltrated (and therefore unsupported) scaffolds. This suggests that layered structural design elements, found throughout nature, may be harnessing this Euler buckling resistance to increase strength.

Effect of two-step sintering on micro-honeycomb BaTiO3 ceramics prepared by freeze-casting process

Micro-honeycomb barium titanate (BaTiO3) ceramics, which have uniformly distributed one-dimensional pore channels, were prepared by freeze-casting suspensions of hydrothermally synthesized BaTiO3 nanopowder with a particle size of 100nm. The effects of sintering schedules such as sintering temperature and holding time on pore size distribution, porosity, pore wall density and grain size were studied. The grain size on the pore wall of BaTiO3 is around 1–2μm after two-step sintering. The crystallographic structure of all samples was tetragonal phase regardless of the sintering schedules. Ferroelectric domains were shaped in a stripe and the domain width obviously increased after polarization. The micro-honeycomb BaTiO3 sintered at lower temperature or shorter holding time showed larger piezoelectric coefficient d33 and smaller dielectric constant εr.

Requirements for Designing a Freeze Casting Process for Drugs – Rheological Properties of Suspensions

AbstractFreeze casting is a useful molding technique for the production of porous, fast dissolving tablets of foods and pharmaceuticals. In order to optimize the freeze casting process, the slurries, and also the characteristics of the freeze‐casted products, multifunctional additives are usually employed. For further applications of these procedures in industrial processes, the suspension characteristics should be known. This case study focuses on the rheological properties of paracetamol suspensions. The effects of modified starch as an additive, the solid loadings, and the temperature on the flow behavior and viscosities of the suspensions were investigated. It was found that most suspensions are thixotropic systems. The viscosity of the suspensions is highly sensitive with respect to the studied factors, such as shear rate, shearing time, temperature, solid loading, and additive content. Optimized conditions were found and are recommended for industrial use.

Freeze-casting: Fabrication of highly porous and hierarchical ceramic supports for energy applications

The manufacture of structured ceramic porous support knows an important boom since more than a decade with the development of new shaping techniques. Among the most promising ones, the freeze-casting also called Ice-Templating allows the fabrication of ceramic parts exhibiting high porosity (>50%) and vertically aligned and hierarchically organized pores. Such structures were firstly conceived for biomedical applications like bone substitute and tissue engineering, but the distinctive features of freeze-cast structures have attracted the attention of diverse scientific fields, especially in high temperature ceramic-based energy production systems. Indeed, technologies like (a) Solid Oxide Fuel Cell (SOFC) and Electrolyser Cell (SOEC), (b) gas separation (O2, H2) by asymmetric supported membranes based on mixed ionic and electronic conductors (MIEC) or hydrogen-permeable metals, and (c) Catalytic Membrane Reactor (CMR) systems present a porous component in their physical structure. This latest, presenting a tortuous pathway for gas access and as a consequence, a high transport limitation, is known to be a limiting component for the operation at high flow streams that would enable to reach industrial target.The aim of this paper is to give an overview of the freeze-casting ceramic shaping method and to show how its implementation could be useful for several energy applications where key components comprise a porous scaffold. A detailed presentation of the freeze-casting process and of the characteristics of the resulting porous parts is firstly given. The characteristic of freeze-cast parts and the drawbacks of conventional porous scaffolds existing in energy applications are drawn in order to highlight the expected beneficial effect of this new shaping technique as possible substitute to the conventional ones. Finally, a review of the state of the art freeze-cast based energy applications developed up to now and expected to be promising is given to illustrate the large perspectives opened by the implementation of the freeze-casting of ceramics for energy fields. Here we suggest discussing about the feasibility of incorporate freeze-cast porous support in high temperature ceramic-based energy applications.

Production of fire clay porous structure with no aligned macro porosity from water-based slurry by freeze casting process: A new approach

Despite the directional growing of ice crystals, fire clays structures with non-allied macroscopically porosity were prepared using a water-based freeze casting process. Pores were generated subsequently by sublimation of the columnar ice during freeze casting. By sintering this green body, a fire clay with the amount of porosity as high as 74% was obtained. Initially, the cell size and size distribution of the fire clay foams could be efficiently adjusted by the control of some parameters such as type of fire clay and gelatin content. The influence of each parameter on the properties of the obtained structures were investigated. The analysis suggests as useful this route as a new approach to produce spherical porous ceramics by freeze casting. The characteristics (morphology, strength, chemical and thermal resistance) of the foams suggest that they could be employed in applications as thermal insulation and soundproofing materials.

Graded ceramic polymer composites for orthodontic applications

Graded ceramic polymer composites for orthodontic applications

Properties of Calcium Phosphate Scaffolds Produced by Freeze-Casting

The pore structure of three-dimensional scaffolds applied in tissue engineering may influence the mechanical properties and cellular activity. As the optimal pore size is dependent on the specifics of the biomaterial or tissue engineering application, the ability to alter the pore size over a wide range is necessary for several scaffolds in order to meets the requirements of the applications. The aim of this study is to develop methodologies to produce calcium phosphate scaffolds with acceptable pore size and defined pore-channel interconnectivity. The pore size of calcium phosphate scaffolds is established during the freeze-drying fabrication process. In this process, material suspension is simply frozen and then dried by freeze-drier, which able to produce material with unique porous architectures, where the porosity is almost a direct replica of the frozen solvent crystals. There are two different method of freeze-casting carried out in order to study the effect of freezing temperature by which in the first method; sample being soaked with liquid nitrogen (-196°C) for about 10minutes before been place inside a freezer (-40°C). In the second method, the sample was directly placed inside a freezer for casting at temperature of -40 ̊C. The results show that the pore size of the scaffolds decreased as the freezing temperature was reduced. Taken together, these results demonstrate that the methodologies applied in this study can be used to produce a range of calcium phosphate scaffolds exhibiting better compressive strength, approximately 665-875 KPa for 54-64.3% of porosity with mean pore size from 102-113μm. The methods developed in this study provide a basis for the investigation on the effects of different freezing temperature in freeze-casting process on the porosity, morphology, and compressive properties of the calcium phosphate scaffolds.

Biomimetic multidirectional scaffolds for osteochondral repair by sequential freeze casting

Event Abstract Back to Event Biomimetic multidirectional scaffolds for osteochondral repair by sequential freeze casting Drew Clearfield1 and Mei Wei1 1 University of Connecticut, Department of Materials Science and Engineering, Institute of Materials Science, United States Introduction: By the year 2030, osteoarthritis is expected to affect 67 million Americans[1]. Osteoarthritis commonly leads to the creation of osteochondral lesions[2]. These lesions disrupt the unique zonal organization of osteochondral extracellular matrix, which proves vital to its function[3]. There exists a need for tissue engineering strategies to induce the regeneration of new tissue that recapitulates this zonal organization and corresponding functionality. In the current work, a sequential freeze casting process is detailed that allows for the fabrication of multi-zonal collagen-based scaffolds that mimic three zonal features of native osteochondral extracellular matrix: the superficial zone (SZ), the transition zone (TZ), and the deep zone (DZ) morphologies [1], zonal pore size gradient [2], and zone-specific compositions [3] of articular cartilage and subchondral bone in a single construct. To mimic the zonal structure, a multizonal scaffold has been designed through a novel sequential freezing process for osteochondral defect repair. Zonal anisotropy is created through two stages of unidirectional freezing. Zone-specific pore size is controlled through applied cooling rate and zone-specific composition is controlled by adjusting initial suspension formulations. Methods: Sequential freeze casting was performed to create multizonal collagen (Col) and collagen-hyaluronic acid (Col-HYA)/collagen-hydroxyapatite (Col-HA) composite scaffolds. For collagen-only scaffolds, 2 wt% type I collagen was homogenized in deionized water, degased, and unidirectionally frozen. The frozen structure was flipped 90º, partially melted, and a second collagen suspension was loaded and unidirectionally frozen atop the first. Sequentially frozen scaffolds were lyophilized at -40ºC for 3 days, crosslinked with 2% EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride):NHS (N-hydroxysuccinimide) (1:0.25 molar ratio), and lyophilized for another 3 days. The effect of cooling rate on zonal pore size was investigated. SEM analysis was performed to view zonal morphologies (longitudinal sections) and quantify pore sizes (transverse sections). To further mimic the zonal composition of native osteochondral extracellular matrix, HYA and HA was added to the collagen suspensions and the scaffolds were prepared using the approached described above. A Col-HYA (1.8 wt% Col, 0.2 wt% HYA) suspension was first unidirectionally frozen at 2ºC/min. A second Col-HYA slurry of increased HYA and decreased Col content was frozen 90º to the first at 0.5ºC/min, where a third Col-HA slurry was layered atop. Results and Discussion: SEM analysis reveals three distinct zones of collagen fiber orientation seamlessly integrated together (Figure 1). A 200-300 µm transition region of isotropic collagen integrates between two zones of unidirectionally aligned fibers, perpendicular to one another. Pore size quantification revealed a trend of increasing pore sizes through the depth of the scaffold. Zonal pore sizes were controlled by the applied cooling rate. Quicker cooling led to the formation of smaller pores, while slower cooling promoted larger pores. Fig. 1: Scanning electron micrograph of collagen-based multizonal scaffolds. SZ - superficial zone, TZ - transition zone, DZ - deep zone. Conclusions: Sequentially freeze cast multizonal collagen-based scaffolds were fabricated that mimic the zone-specific collagen fiber orientation, pore size, and composition present in native osteochondral tissue. The authors would like to thank NSF grants (CBET-1133883 and CBET-1347130) for their support

Toward highly compressible graphene aerogels of enhanced mechanical performance with polymer

The highly compressive durable graphene aerogels with enhanced strength was prepared by combining the freeze-casting process with the binding effect of polymer.

Reproducibility of ZrO2-based freeze casting for biomaterials

The processing technique of freeze casting has been intensely researched for its potential to create porous scaffold and infiltrated composite materials for biomedical implants and structural materials. However, in order for this technique to be employed medically or commercially, it must be able to reliably produce materials in great quantities with similar microstructures and properties. Here we investigate the reproducibility of the freeze casting process by independently fabricating three sets of eight ZrO2–epoxy composite scaffolds with the same processing conditions but varying solid loading (10, 15 and 20vol.%). Statistical analyses (One-way ANOVA and Tukey's HSD tests) run upon measurements of the microstructural dimensions of these composite scaffold sets show that, while the majority of microstructures are similar, in all cases the composite scaffolds display statistically significant variability. In addition, composite scaffolds where mechanically compressed and statistically analyzed. Similar to the microstructures, almost all of their resultant properties displayed significant variability though most composite scaffolds were similar. These results suggest that additional research to improve control of the freeze casting technique is required before scaffolds and composite scaffolds can reliably be reproduced for commercial or medical applications.

Porous SiC ceramics prepared via freeze-casting and solid state sintering

Porous SiC ceramics were prepared by freeze-casting process. In order to enhance the mechanical properties of the porous SiC, poly(vinyl alcohol) (PVA) was added as binder and pore morphology controller in this work. The results indicated that high porosity (>60%) SiC ceramics was obtained although the sintering temperature was over 2000°C. The pore structure could be divided into two kinds: macropores generated by sublimation of large ice crystals, and micropores in the ceramic matrix caused by sublimating of small ice crystals, stacking of SiC particles, and burning out of PVA. With the increase of the sintering temperature, the specimens exhibited higher density, thus resulted in higher strength. Porous SiC ceramics sintered at 2100°C showed a good flexural strength of 11.25MPa with an open porosity as high as 66.46%.

Control of pore size and wall thickness of 3-1 type porous PZT ceramics during freeze-casting process

3-1 type porous lead zirconate titanate (PZT) ceramics with unidirectionally aligned pore channels were prepared by freeze-casting process of which the freezing temperature was varied at −10~−100°C, and the solid content of slurries was changed at 15~40vol%. Characterization of pore size distributions and wall thickness distributions demonstrated the freezing temperature significantly influenced the pore size and wall thickness, while the solid content had strong influence on the porosity. When the solid content was stationary, the pore size decreased with decreasing freezing temperature and the wall thickness also tended to decrease. When the freezing temperature was constant, the porosity decreased as the initial solid content increased, meaning decreased ratio of pore size and wall thickness. The fabricated samples exhibited high piezoelectric coefficient and there was a trend that it increased with the simultaneous decrease of pore size and wall thickness.

Preparation, Microstructure and Properties of ZrO<sub>2</sub> Gradient Porous Ceramics by Freeze-Casting Process

The ZrO2gradient porous ceramics were prepared by a novel freeze-casting process. The porous structure of the ZrO2ceramics was investigated by scanning electron microscopy (SEM). And the porosity and compressive strength were also measured. Experimental results indicated that the porous structure of the specimens was remarkably affected by the sintering temperature. The dendritic pores were obtained after sintered at 1300 °C. The porosity and compressive strength of the specimen were 56.79% and 4.37 MPa, respectively. As the sintering temperature reached to 1500 °C, a lamellar structure was obtained on the cross-section of the specimen. Meanwhile, the porosity decreased to 26.77% and the compressive strength increased to 8.26 MPa. The ceramics can be divided subsequently into three distinctive zones along the solidification direction, i.e. lamellar zone, transition zone and cellular zone.

Directionally aligned macroporous SiOC via freeze casting of preceramic polymers

A commercially available polysiloxane was used as a preceramic polymer for solution freeze casting to obtain directionally aligned porous silicon oxycarbide. We show how choice of solvent, polymer concentration, and freezing rate can affect the final pore network of the freeze-cast ceramic. Solvents of cyclohexane and camphene resulted in dendritic pores, while tert-butyl alcohol (TBA) produced intersecting cellular pores in the freeze-cast ceramic. Characterization of pore size distribution by mercury intrusion porosimetry of ceramics produced from cyclohexane–polysiloxane solutions with varying polymer concentrations and freezing rates demonstrated trends consistent with solidification theory. Fourier transform infrared spectroscopy and X-ray diffraction were employed to confirm that the freeze-casting process resulted in silicon oxycarbide of comparable chemistry and crystallinity to that produced via traditional preceramic polymer processing techniques.

Bioinspired composites from freeze casting with clathrate hydrates

Freeze casting with isopropanol (IPA)-H2O as a freezing agent has shown the potential to create porous scaffolds with enlarged pores. Though not experimentally proven, this effect has been suggested to be the result of non-stoichiometric structures called clathrate hydrates forming during the freezing process. In this manuscript, we build upon these results to provide experimental evidence of the formation of clathrate hydrates during the freeze casting process when using IPA-H2O as a freezing agent and explain the observed maximum in pore area through observations of the enthalpy of the transitions. Additionally, these enlarged pores are harnessed in order to create two-phase bioinspired composites. These compos- ites exhibit an increase in mechanical strength over both constituents, a phenomena that is common to complex biological composites. Previous reports have shown that the characteristic method of failure for many freeze cast scaffolds is buckling of the lamellar walls within the porous microstructure. It is proposed that this increase in mechanical strength is due to the support of the second phase, which resists this characteristic buckling failure mode.

Sugar alcohols – Multifunctional agents in the freeze casting process of foods

The successful production of a new convenient dosage form, namely, a fast and easily dissolving/dispersal tablet was presented in a previous study (Nguyen and Ulrich, 2014). The highlight and interesting point of that work is the introduction of sugar as a binder which dramatically improved both the tensile strength and dispersal behavior of produced tablets. In this work, the investigation on the effect of sugar alcohols (i.e., isomalt and xylitol) was carried out in order to find the mechanism on how the binders (sugar alcohols) change the properties of the tablets. The effects of sugar, isomalt and xylitol on the key properties of the tablets (i.e., the tensile strength, the dispersal/dissolution behavior), the morphology of pores and tablets, especial the thermodynamic properties of aqueous suspensions were investigated. It was found that sugar alcohols depress the freezing points of aqueous suspensions, reduce the ice growth rate and induce to form a new pore morphology, a sphere-like porous solid body. Isomalt as binder improves the properties of the produced tablets to a higher degree than sugar and xylitol. A fast dispersal cocoa tablet (16mm ∅, 10mm height) with a fast dispersal time of lower than 1min and a better crushing force of 45N (i.e. 0.18N/mm2 diametral tensile strength) was obtained.

Fabrication, bioactivity, in vitro cytotoxicity and cell viability of cryo-treated nanohydroxyapatite–gelatin–polyvinyl alcohol macroporous scaffold

Freeze casting and cryogenic treatment both low temperature process have been employed to fabricate nanobiocomposite hydroxyapatite (HA)–gelatin–polyvinyl alcohol (PVA) macroporous scaffolds from synthesized three different spherical, rod and fibrous HA nanoparticles and composition optimized vis-á-vis porosity architecture, content and compressive strength. A critical HA morphology, solid loading and liquid nitrogen interaction time have a significant effect to enhance the mechanical response of developed scaffolds. Cryo-treated 40 wt.% nanorod HA–gelatin–PVA scaffold posses interconnected pore structure with 80 vol.% porosity, average pore diameter 50–200 μm and highest 5.8 MPa compressive strength. Different degree of the apatite deposition phenomenon in simulated body fluid solution at 37 °C and pH ∼ 7.4 varies with respect to time. In vitro cytotoxicity and L929 mouse fibroblast cell culture in the presence of Dulbecco's Modified Eagle Medium and 10% Fetal Bovine Serum at 37 °C and 5% CO2 atmosphere exhibit excellent cytocompatibility and cell viability at low extract concentration up to 25%.

High-strength porous Si3N4 ceramics prepared by freeze casting and silicon powder nitridation process

Porous Si3N4 ceramics with uniform morphology were fabricated via freeze casting and silicon powder nitridation process. Because of the interlocking grains, Si3N4 porous ceramics obtained high porosity of 70.51% and high flexural strength of 16.3MPa despite of its nitridation degree 88.8%. With SiC addition, a much higher nitridation degree 97.9% was achieved. But the relatively low porosity of 66.7% with low flexural strength of 9.6MPa was gained owing to the restrained growth and deflection of grains. After post-sintering, the increased aspect ratio of grains gave rise to the improved flexural strength for Si3N4 porous ceramics, while the contrary phenomenon was observed with SiC addition.

Novel oxide fiber composites by freeze casting

Abstract A freeze casting process with gelatine as gelling agent is presented to manufacture novel oxide fiber composites. The growth of ice crystals causes an in situ densification of the green matrix when the composite is frozen after the lamination of 2D-fabrics. This freeze casting process leads to a porous matrix with a cellular morphology, showing no shrinkage during sintering. The porous structure is suitable for a subsequent infiltration with a precursor to densify the matrix. As the matrix is crack-free, the sintered composites have high mechanical properties. The processing is demonstrated by manufacturing laminated Nextel™610/YAG-ZrO 2 oxide fiber composites.

Multi-scale 3D imaging of absorbing porous materials for solid oxide fuel cells

The performance of advanced functional materials for fuel cell applications are closely linked to the material composition and morphology at the micro and nano-scales. 3D characterization techniques that can provide bulk information at these fine scales are therefore essential for microstructure optimization of these materials. Here, the X-ray nano-holotomography technique is used to image various multi-phase and absorbing solid oxide fuel cell electrodes. Different porous structures for typical commercial cells and innovative electrode designs obtained using a freeze-casting process are studied. Taking advantage of the geometrical setup and the use of high energy X-rays, both large reconstructions (field of view: 150 µm) and local tomography at higher resolution (field of view: 50 µm) can be performed on the same sample to have a multi-scale approach. This produces highly representative sample volumes with a size/resolution ratio that allows the geometric and physical properties of the materials to be calculated, e.g., connectivity of each phase, mean particles diameters, specific surface area, particle size distributions, tortuosity factors, and densities of triple boundary lengths.