Freeze-casting Technique Research Articles

Effects of nanocellulose on the structure and properties of poly(vinyl alcohol)-borax hybrid foams

Nanocellulose-borax-polyvinyl alcohol (PVA) hybrid foams were prepared using a facile approach in an aqueous medium followed by a freeze-casting technique. Nanocellulose was well-dispersed in the PVA-borax (PB) matrix and acted as a cross-linking agent and nanofiller to bridge the 3D network, leading to enhanced mechanical and thermal performance. The effects of particle size, aspect ratio, surface charge and crystallinity on the microstructure and performance were investigated. With the increasing size and aspect ratio, cellulose nanofiber-PB foam with a density of ~0.110 g/cm3 exhibited the most pronounced honeycomb-like structure with a porosity of 92.2%, the smallest cell diameter (~0.93 μm) and the highest mechanical strength (bearing more than 7560 times its own weight). Chemical cross-linking of nanocellulose-PVA foams with borax led to uniform porous structure, small pores and high mechanical strength. Possible lyophilization-induced assembly mechanisms, relationships between microstructure and mechanical properties, and complexation reactions between building blocks are proposed.

Fabrication and Electrochemical Characterization of Freeze-Cast Tubular Solid Oxide Fuel Cells

NASA and others have shown improved power density of planar solid oxide fuel cells (P-SOFCs) fabricated via the freeze casting technique. As a manufacturing process, freeze casting is capable of producing ceramic, metal, and cermet components with unique and tailorable microstructural features that enhance SOFC performance. Low thermo-cyclic durability and slow startup times are common problems for P-SOFCs and have similarly limited the development of freeze cast P-SOFCs. Tubular SOFCs (T-SOFCs) have demonstrated superior thermal cycling capability and relatively fast startup times but have been reported to have lower volumetric power density compared to P-SOFCs. The utilization of freeze casting for fabrication of tubular geometries is suggested as a solution to significantly increase the volumetric power density of T-SOFCs by enhancing gas diffusivity and triple phase boundary reactions and to be at par or better than the power densities of state-of-art P-SOFCs. This paper reports the fabrication and characterization of freeze cast tubular anode supports for SOFCs. NiO-8YSZ (nickel oxide yttria-stabilized zirconia) anode supports were fabricated using three methods: gelation casting, center pin method, and a freeze and drain method. Slurry rheology was modified for vertical dip coating, and a dual purpose freezing and drying chamber was designed and manufactured. The effects of slurry properties and process parameters were studied with respect to the resulting microstructures. The freeze and drain method was determined to be optimum for producing highly porous tubular anode supports with hierarchical, acicular or dendritic micro-pore channels. With an optimized freeze-drying cycle, drying time was reduced significantly with minimal residual moisture in the green bodies. Additionally, the electrochemical performance of the T-SOFCs with freeze cast anode was evaluated. Figure 1

The fabrication and characterization of barium titanate/akermanite nano-bio-ceramic with a suitable piezoelectric coefficient for bone defect recovery

In recent years, due to the controllable mechanical properties and degradation rate, calcium silicates such as akermanite (Ca2MgSi2O7) with Ca-Mg and Si- containing bio-ceramics have received much more attention. In addition, the piezoelectric effect plays an important role in bone growth, remodeling and defect healing. To achieve our objective, the porous bioactive nano-composite with a suitable piezoelectric coefficient was fabricated by the freeze-casting technique from the barium titanate and nano-akermanite (BT/nAK) suspension. The highest d33of 4pC/N was obtained for BT90/nAK10. The compressive strength and porosity were for BT75/nAK25 and BT60/nAK40 at the highest level, respectively. The average pore channel diameter was 41 for BT75/nAK25. Interestingly enough, the inter-connected pore channel was observed in the SEM images. There was no detectable transformation phase in the XRD pattern for the BT/nAK composites. The manipulation flexibility of this method indicated the potential for the customized needs in the application of bone substitutes. An ((3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide)) MTT assay indicated that the obtained scaffolds have no cytotoxic effects on the human bone marrow mesenchymal stem cells.

YSZ monoliths promoted with Co as catalysts for the production of H2 by steam reforming of ethanol

Steam reforming of ethanol was investigated between 400°C and 600°C over 3mol% Y2O3 doped ZrO2 monolith infiltrated with Co, known as an effective catalyst for this reaction. Monoliths were fabricated by implementing the freeze-casting technique in order to obtain hierarchical, vertical and highly ordered porous microstructure resulting in very low pressure drop, high surface area and accessibility to active sites. Catalytic monoliths were firstly loaded with 3wt% of Co. Catalytic activity of Co-containing monolith was compared with the catalytic activity of a powdered material with the same chemical composition than the catalytic monolith. The results show a clear improvement all over the temperature range of the ethanol conversion and H2 yield and a decrease of the by-products contents when using a freeze-cast monolith support. The incorporation of La enabled a strong decrease in coke deposition over the activated monolith. The effect of the addition of 0.25wt.% of Pt was also studied. Pt makes possible to inhibit carbon deposition but has a detrimental effect on ethanol conversion at temperatures above 500°C and on H2 selectivity at low temperature. A higher Pt loading of 0.8wt% did not bring any beneficial effect and was even detrimental at low temperature. Finally, the beneficial effect of the implementation of freeze-cast monolith in comparison with the not structured fixed-bed configuration is explained by the difference of packing since and improved fluid dynamics at similar GHSV, the former one promotes in a higher proportion the Water Gas Shift Reaction (WGSR) further in the monolith length.

Construction of horizontal stratum landform-like composite foams and their methyl orange adsorption capacity

Chitosan (CS)/rectorite (REC)/carbon nanotubes (CNTs) composite foams with good mechanical properties were successfully fabricated by unidirectional freeze-casting technique. The morphology of the foam showed the well-ordered porous three-dimensional layers and horizontal stratum landform-like structure. The holes on the layers looked like the wings of butterfly. Additionally, the X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy results indicated the successful addition of CNTs and REC. The intercalated REC with CS chains was confirmed by small-angle X-ray diffraction. The surface structure of the foams was also analyzed by Raman spectroscopy. The adsorption experiments showed that when the mass ratio of CS to REC was 10:1 and CNTs content was 20%, the composite foam performed best in adsorbing low concentration methyl orange, and the largest adsorption capacity was 41.65mg/g.

Porous mullite/alumina-layered composites with a graded porosity fabricated by camphene-based freeze casting

A freeze casting technique was processed to fabricate porous mullite/alumina-layered composite with a gradient in porosity and pore size. In this work, a camphene/coal fly ash slurry system with an appropriate addition of Al2O3 was used. The pore channels with circular-shaped cross-sections were aligned along the solidification direction of molten camphene and connected with each other. The pore morphology was influenced by the starting solids loading and sintering temperature. The compressive strength of monolithic specimen greatly decreased when the porosity increased. However, the layered composites with a graded porosity exhibited improved compressive strength with minor decrease in porosity, probably due to the formation of residual compressive stress and relatively dense glass phase in the outer layer.

Preparation of nacre-like composites by reactive infiltration of a magnesium alloy into porous silicon carbide derived from ice template

Lightweight and high-strength AZ91/SiC composites with lamellar structures were successfully prepared by using freeze casting (ice-templating) and reactive infiltration techniques. The infiltration dynamics was measured and activation energy calculated to be 7.74kJ/mol. The compressive and flexural strengths of the composites with 30vol% initial solid load reached 743±20MPa and 599±44MPa, about 1.8 and 1.5 times of those of the AZ91 alloy, respectively.

Physicochemical and mechanical properties of freeze cast hydroxyapatite-gelatin scaffolds with dexamethasone loaded PLGA microspheres for hard tissue engineering applications

Hydroxyapatite (HA)-gelatin scaffolds incorporated with dexamethasone-loaded polylactic-co-glycolic acid (PLGA) microspheres were synthesized by freeze casting technique. Scanning electron microscopy (SEM) micrographs demonstrated a unidirectional microstructure and a decrease in the pore size as a function of temperature gradient. Higher amounts of HA resulted in a decrease in the pore size. According to the results, at lower cooling rates, the formation of a lamellar structure decreased the mechanical strength, but at the same time, enhanced the swelling ratio, biodegradation rate and drug release level. On the other hand, higher weight ratios of HA increased the compressive strength, and reduced the swelling ratio, biodegradation rate and drug release level. The results obtained by furrier transform infrared spectroscopy (FTIR) and bioactivity analysis illustrated that the interactions of the materials support the apatite formation in the simulated body fluid (SBF) solution. Based on the obtained results, the synthesized composite scaffolds have the necessary mechanical and physicochemical features to support the regeneration of defects and to maintain their stability during the neo-tissue formation.

Behaviour of freeze-casting iron oxide for purifying hydrogen streams by steam-iron process

Alternating reductions with hydrogen and oxidations with steam have been carried out on solids with improved redox capacity showing the benefits of different synthesis methods in the steam-iron process (SIP). Solids chosen were a commercial hematite, a lab-made iron oxide doped with ceria and alumina and hematite synthesized by freeze-casting technique. The first one shows a high loss of reactivity from the first cycle on while the others keep their reduction capacity along three consecutive cycles and beyond. Isothermal experiments were also performed with the best two solids. Although all of them had hematite as the starting material, they have shown significant differences in behaviour respecting the original commercial hematite depending on the chemical (inclusion of additives that account for only 2 wt% of the solid mass) or physical (macroporosity) properties. Also the effect of long lasting runs with up to 10 cycles (reduction + oxidation) have been performed at different reduction temperatures showing a slightly higher stability of the pure iron oxide pellets produced by freeze-casting versus an iron oxide with alumina and ceria selected as structural stabilizers and accelerators of the reduction/oxidation reaction.

Processing and mechanical properties of lamellar-structured Al–7Si–5Cu/TiC composites

Inspired by nacre, we designed biomimetic metal–ceramic composites in which hard and ductile phases were alternately arranged. We first prepared lamellar-structured TiC scaffolds with 25, 30 and 35vol% solid loadings using a unidirectional freeze casting technique and then measured wetting and infiltration dynamics for a commercial Al cast alloy, ZL107, with the main composition of Al–7Si–5Cu on the porous TiC scaffold using a modified sessile drop method at 850°C. The wetting was affected by liquid spreading at horizontal surface and infiltration in depth, while the latter played a dominant role in the decrease in contact angle. On the basis of the wetting results, the lamellar ZL107/TiC composites were successfully fabricated by pressure infiltration of the molten ZL107 alloy into porous TiC scaffolds and their mechanical properties were evaluated. The compressive strength and elastic modulus increased while bending strength decreased with increasing TiC content, giving maximum values of 1166±20MPa, 205±2GPa and 363±5MPa, respectively. Formation of (Alm,Si1−m)3Ti, Al3Ti and Al4C3 phases in the composites was presumed to deteriorate the toughness of the composites, leading to brittle fracture under loading.

Preparation of tubular hierarchically porous silicate cement compacts via a tert-butyl alcohol (TBA)-based freeze casting method

Tubular porous silicate cement compacts with hierarchical porosity were successfully fabricated via a tert-butyl alcohol (TBA)-based freeze-casting technique in one single step. These samples were characterized by unidirectional aligned pore channels and completely open porosity. Various experimental parameters affecting the macropore size, mesopore size, porosity, and compression strength were investigated, including the initial solids loading, freezing temperature and curing period. Both the research findings of microstructure observations and property tests show that the pore size decreased significantly with decreasing freezing temperature and increasing solids loading. The porosity increased significantly with decreasing solids loading, while the compressive strength decreased remarkably. In addition to their tunable physical characteristics, these brand-new inorganic porous materials with oriented aligned pore channels and dense layer surface showed high permeation performance and desirable separation ability. These results endow the silicate cement, a kind of low-cost and high-performance architecture material, with great potential as a novel ordered porous inorganic material for numerous applications, especially in separation process.

New multifunctional porous Yb2SiO5 ceramics prepared by freeze casting

New porous Yb2SiO5 ceramics were prepared by a water-based freeze casting technique using synthesized Yb2SiO5 powders. The prepared porous Yb2SiO5 ceramics exhibit multiple pore structures, including lamellar channel pores and small pores, in its skeleton. The effects of the solid content and sintering temperature on the pore structure, porosity, dielectric and mechanical properties of the porous Yb2SiO5 ceramics were investigated. The sample with 20vol% solids content prepared at 1550°C exhibited an ultra-low linear shrinkage (i.e. 4.5%), a high porosity (i.e. 79.1%), a high compressive strength (i.e. 4.9MPa), a low dielectric constant (i.e. 2.38) and low thermal conductivity (i.e. 0.168W/(mK)). These results indicate that porous Yb2SiO5 ceramics are good candidates for ultra-high temperature broadband radome structures and thermal insulator materials.

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.

Preparation and characterization of highly porous Yb2SiO5 ceramics using water-based freeze-casting

This study introduces a new, highly porous Yb2SiO5 ceramic prepared using a water-based freeze-casting technique. The prepared porous Yb2SiO5 ceramic had ultra-high porosity (82–86 %) and tailored pore structures. The effects of sintering temperature on the pore structure, thermal conductivity, and mechanical properties of these ceramics were investigated. The sample with 15 vol% solid content prepared at 1450 °C had ultra-low linear shrinkage (4.85 %), ultra-high porosity (83.95 %), high compressive strength (2.15 MPa), and low thermal conductivity (0.121 W/m K). In addition, this sample showed a relatively high compressive strength (1.15 MPa) at 1500 °C. These results highlight the potential application of highly porous Yb2SiO5 ceramics as thermal insulator materials in ultra-high temperature environments.

Optical properties of ZrB2 porous architectures

Porous ceramic materials are currently used as volumetric sunlight absorbers in concentrating solar power systems. As the efficiency of thermodynamic cycles rapidly increases with the operating temperature, the favorable characteristics of so-called ultra-high temperature ceramics (UHTCs) can be successfully exploited in novel solar absorbers. The present work reports, for the first time to the best of our knowledge, on optical properties and microstructural analysis of novel ice-templating porous ZrB2 UHTCs, to evaluate their potential as volumetric solar receivers. We demonstrate that the different complex structures that can be obtained with the freeze casting technique show promising optical properties. The idea of conjugating an highly tailorable morphology, useful for optimizing gas fluxes and heat exchanges between absorber and gas, to the spectral selectivity which is a characteristics of ZrB2 can be a promising route for increasing the efficiency of thermal solar systems.

The influence of Fe2O3 doping on the pore structure and mechanical strength of TiO2-containing alumina obtained by freeze-casting

This work investigated TiO2/Fe2O3 doped alumina prepared by the freeze-casting technique and using camphene as the solvent. Dendritic pores were formed in the TiO2 doped alumina, a structure conferred by the frozen camphene. Contrary to this trend, further Fe2O3 doping of TiO2-containing alumina resulted in the formation of non-dendritic structures. This behavior was attributed to the higher density of α-Fe2O3 (5.24gcm−3) when compared to α-Al2O3 (3.95gcm−3) and anatase TiO2 (3.89gcm−3), which reduced critical solidification front velocity, thus forming material with different pore shape. Fe2O3 doping also improved the densification of TiO2–alumina and inhibited the formation of cracks, reflected by superior mechanical strength with best results ~150% higher for 10% Fe2O3 loaded samples as compared to TiO2–alumina samples.

Porous Y2SiO5 ceramics with a centrosymmetric structure produced by freeze casting

A novel water-based freeze-casting technique was used to prepare porous yttrium silicate (Y2SiO5) ceramics. The formation mechanism of lamellar channels of porous ceramics that were arranged centrosymmetrically along the radial axis is discussed in this study. The effect of solid content on the pore structure, porosity, dielectric properties, and mechanical properties of porous Y2SiO5 ceramics were investigated. The increase in solid content reduced the porosity, decreased the pore size, and changed the pore structure. Upon increasing the solid content from 15vol% to 30vol%, the compressive strength and dielectric constant of the porous Y2SiO5 ceramics increased from 1.87 to 8.42MPa and from 1.86 to 2.85, respectively. Porous Y2SiO5 ceramics with these properties can be used in ultra-high temperature broadband radome structures.

Synthesis and magnetoelectric effect of composites with CoFe2O4-epoxy embedded in 3–1 type porous PZT ceramics

Three-phase magnetoelectric (ME) composites of lead zirconate titanate (PZT)/cobalt ferrite CoFe2O4 (CFO)/epoxy were synthesized in sequence by a tert-butyl alcohol (TBA)-based freeze-casting technique, a chemical solution deposition method, and an impregnation process. The novel pseudo 3–0–1 type composite structure with a high content of CFO was examined using SEM and XRD. The composites showed not only stable and excellent dielectric behavior in a wide frequency range, but also good ferroelectricity and ferromagneticity at the same time, as can be seen from the typical electrical and magnetic hysteresis loops. The ME effect of the resultant 3–0–1 type ferroelectric/ferromagnet/polymer composites was found to be significantly dependent on the content of PZT and CFO. The maximum ME coefficient of 2.2mV/cmOe was observed when the PZT volume fraction is 0.5 and CFO mass fraction is 0.2. This synthesis route we proposed provides a new connectivity type of ME composite and shows potential applications in sensors and transducers.

Effect of titania addition on the properties of freeze-cast alumina samples

This work investigated the behavior of TiO2-containing α-Al2O3 samples prepared by the freeze-casting technique. Camphene and liquid nitrogen were used as the solvent and cooling fluid, respectively. Camphene resulted in the formation of dendritic pores, in the direction of the freeze-casting cold front during sample preparation. The formation of β-Al2TiO5 phase occurred at 1300°C, and became more evident as the sintering temperatures reached 1500°C. The TiO2 loading did not affect the sample porosity at a given temperature, but it was detrimental in the case of mechanical properties under certain conditions. For instance, the flexural strength slightly improved with increasing the TiO2 loading and sintering temperature from 1100 to 1300°C. This effect was attributed to the occurrence of a more effective sintering of alumina. However, as the heat treatment temperature was raised from 1300 to 1500°C, the flexural strength did not increase as a function of the TiO2 loading, even though the densification occurred with loss of porosity. The loss of mechanical strength was found to be associated with the formation of microcracks which stemmed from the formation of β-Al2TiO5 phase for TiO2 loadings in excess of 4wt% at these high sintering temperatures.

Biomimetic gradient scaffold from ice-templating for self-seeding of cells with capillary effect

One of the most important issues in bone tissue engineering is the search for new materials and processing techniques to create novel scaffolds with 3-D porous structures. Although many properties such as biodegradability and porosity have been considered in designing bone scaffolds, very limited attention is paid to their capillary effect. In nature, capillary effect is ubiquitously used by plants and animals to constantly transport water and nutrients based on morphological and/or chemical gradient structures at multiple length-scales. In this work, we developed a modified freeze-casting technique to prepare ceramic scaffolds with gradient channel structures. The results show that our hydroxyapatite (HA) scaffolds have interconnected gradient channels that mimic the porous network of natural bone. More importantly, we demonstrate that such a scaffold has a very unique capillary behavior that promotes the self-seeding of cells when in contact with a cell solution due to spontaneous capillary flow generated from gradient channel structures. The strategy developed here provides a new avenue for designing “smart” scaffolds with complex porous structures and biological functions that mimic natural tissues.