Freeze-casting Technique Research Articles

Freeze Casting: From Low-Dimensional Building Blocks to Aligned Porous Structures-A Review of Novel Materials, Methods, and Applications.

Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of well-controlled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freeze-casting processes are described and the relationships between processing and structure are examined. Recent progress in freeze-casting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored micro- and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freeze-cast geometries-beads, fibers, films, complex macrostructures, and nacre-mimetic composites-are presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freeze-casting techniques and low-dimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.

Fabricating metal wicks by LMC-like continuous directional freeze casting

A novel continuous directional freeze casting technique derived from LMC (liquid metal cooling) is developed in the present study, which is named as “LMC-like freeze casting”. In this novel freeze casting method, a quartz mold containing slurry is dipped into a low-temperature bath with a constant rate to achieve directional freezing of the slurry. Under quasi-steady state conditions, the solidification front is kept above the liquid level of the low-temperature bath at a constant distance. The heat released during freezing is conducted downward axially within the solidified slurry, and then is conducted axially and radially to the low-temperature bath through the mold wall. As a result, the thermal resistance between the solidification front and the low-temperature bath is kept constant, and the solidification velocity is exactly the same as the dipping rate of the casting mold. With casting molds made from quartz, a good compromise between high casting rate and unidirectional freezing is achieved without introducing complex temperature control systems. To demonstrate the feasibility of the LMC-like freeze casting, long wicks (about 10 cm) with uniform pore size along the freezing direction are freeze-casted from aqueous nickel slurry. Permeabilities and capillary performance parameter of the fabricated wicks are measured, proving that LMC-like freeze casting technique is feasible to fabricate biporous metal wicks with aligned porosity.

Porous Titanium Cylinders Obtained by the Freeze-Casting Technique: Influence of Process Parameters on Porosity and Mechanical Behavior

The discrepancy between the stiffness of commercially pure titanium and cortical bone tissue compromises its success as a biomaterial. The use of porous titanium has been widely studied, however, it is still challenging to obtain materials able to replicate the porous structure of the bones (content, size, morphology and distribution). In this work, the freeze-casting technique is used to manufacture cylinders with elongated porosity, using a home-made and economical device. The relationship between the processing parameters (diameter and material of the mold, temperature gradient), microstructural features and mechanical properties is established and discussed, in terms of ensuring biomechanical and biofunctional balance. The cylinders have a gradient porosity suitable for use in dentistry, presenting higher Young’s modulus at the bottom, near the cold spot and, therefore better mechanical resistance (it would be in contact with a prosthetic crown), while the opposite side, the hot spot, has bigger, elongated pores and walls.

Inverse Nacre-like Epoxy-Graphene Layered Nanocomposites with Integration of High Toughness and Self-Monitoring

Summary Epoxy nanocomposites have many promising applications in the fields of aerospace and aeronautics, as well as many others. Achieving tough epoxy nanocomposites remains a great challenge, however. One inspiration for improving the mechanical properties of epoxy nanocomposites is nacre, which has remarkable fracture toughness for its layered “brick-and-mortar” architecture. Inspired by this, we fabricated a lamellar graphene scaffold by the freeze-casting technique. An alternating-layered epoxy-graphene nanocomposite was made by infiltrating epoxy into this graphene scaffold. As our epoxy-graphene nanocomposite consists of ∼99 wt % organic epoxy, in contrast to nacre containing ∼96 wt % inorganic aragonite, we call it an “inverse nacre-like” epoxy-graphene layered nanocomposite. It exhibits exceptional fracture toughness, 3.61 times that of pure epoxy, and demonstrates anisotropic conductivity due to the anisotropic graphene scaffold, which can be used to detect cracks. Our bioinspired strategy provides a promising approach to combine excellent mechanical properties with functional properties to fabricate high-performance nanocomposites.

Fabrication of polymer-derived ceramics with hierarchical porosities by freeze casting assisted by thiol-ene click chemistry and HF etching

The freeze casting technique assisted with cryo thiol-ene photopolymerization is successfully employed for the fabrication of macroporous polymer-derived silicon oxycarbide with highly aligned porosity. It is demonstrated that the free radical initiated thiol-ene click reaction effectively cross-linked the vinyl-containing liquid polysiloxanes into infusible thermosets even at low temperatures. Furthermore, mixed solution- and suspension-based freeze casting is employed by adding silica nanopowders. SiOC/SiO2 foams with almost perfect cylindrical shapes are obtained, demonstrating that the presence of nano-SiO2 does not restrict the complete photoinduced cross-linking. The post-pyrolysis HF acid treatments of produced SiOC monoliths yields hierarchical porosities, with SiOC/SiO2 nanocomposites after etching demonstrating the highest specific surface area of 494 m2/g and pore sizes across the macro-, meso- and micropores ranges. The newly developed approach gives a versatile solution for the fabrication of bulk polymer-derived ceramics with controlled porosity.

Isotropic freeze casting of through-porous hydroxyapatite ceramics

It has well known that hydroxyapatite (HA) is a kind of excellent materials for biomolecular absorption and separation, and the absorption and separation performances of HA would be improved if HA had been processed into desirable porous structures. In this paper, we reported on the combination of gel casting and freeze casting to develop the through-porous hydroxyapatite ceramic monoliths. Experiments demonstrated that the gel-containing freeze casting technique was an isotropic pore-forming technique and could prepare the near-net-shape forming green bodies with good mechanical strength no matter what the HA content in green bodies was. Further green body sintering formed the through-porous ceramics whose grain size, pore size, and porosity depended on and could be controlled by the content of HA in green bodies. The formation of through-pores in ceramics resulted from the gels and water in green bodies, which acted as the templates of the pores with size < 1 μm and the pores with size > μ μm, respectively. The gel-freeze casting technique is simple, repeatable, and cost-effective, therefore being hopeful for industrial applications.

Novel high-capacity and reusable carbonaceous sponges for efficient absorption and recovery of oil from water

Ever increasing demand for crude oil has resulted in frequent accidents of oil spill. To effectively separate and recover spilled oil from water, there has been a dire need for effective absorbent materials. In this work, we synthesized an innovative hydrophobic and recyclable sponges using low-cost raw materials and by the unidirectional freeze-casting technique. The composite material showed unusually high capacities for various oil (up to 32.26 g g−1) and organic solvents (up to 52.51 g g−1). FT-IR and XPS analyses revealed a highly oriented inter-linked network of the polymers, which accounts for the high absorption capacities and strong mechanical stability. Upon regeneration, the sponges can be reused >5 times without a significant drop of the absorption capacity. Moreover, the sponges can collect spilled oil continuously from water surface with the aid of an oil-cleanup pumping device. Due to the facile and environment-friendly synthesis, high absorption capacity and thermal stability, the new materials hold potential for more cost-effective separation and recovery of crude oil or other hydrophobic organic pollutants from water.

Fabrication and characterization of nacre-inspired alumina-epoxy composites

Abstract Nacre consisting of aragonite layers and organic layers exhibits extraordinary strong and tough properties due to its hierarchical structure. Generally, nacre-like structure can be produced by freeze casting technique. However the waviness of aragonite layers of nacre is still not replicated in the existing freeze-cast materials. Here, a novel method is proposed to create the waviness in freeze-cast lamellar alumina scaffolds through a heat-treatment process of their green-bodies whose side-faces are glued to Teflon plates. The heat-treatment temperatures of 60 °C, 120 °C and 180 °C are used in the heat-treatment process, respectively. Lamellar composites with nacre-like waviness are obtained through infiltrating the alumina scaffolds with epoxy. The waviness heights and mechanical properties of the alumina-epoxy composites show a strong dependence on the heat-treatment temperature. When the heat-treatment temperature is 120 °C, the alumina-epoxy composites exhibit the optimal waviness heights and better comprehensive mechanical properties: relatively high flexural strength, flexural modulus, toughness and ultimate strain.

Ice-Templating for the Elaboration of Oxygen Permeation Asymmetric Tubular Membrane with Radial Oriented Porosity

An original asymmetric tubular membrane for oxygen production applications was manufactured in a two-step process. A 3 mol% Y2O3 stabilized ZrO2 (3YSZ) porous tubular support was manufactured by the freeze-casting technique, offering a hierarchical and radial-oriented porosity of about 15 µm in width, separated by fully densified walls of about 2 µm thick, suggesting low pressure drop and boosted gas transport. The external surface of the support was successively dip-coated to get a Ce0.8Gd0.2O2−δ – 5mol%Co (CGO-Co) interlayer of 80 µm in thickness and an outer dense layer of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) with a thickness of 30 µm. The whole tubular membrane presents both uniform geometric characteristics and microstructure all along its length. Chemical reactivity between each layer was studied by coupling X-Ray Diffraction (XRD) analysis and Energy Dispersive X-Ray spectroscopy (EDX) mapping at each step of the manufacturing process. Cation interdiffusion between different phases was discarded, confirming the compatibility of this tri-layer asymmetric ceramic membrane for oxygen production purposes. For the first time, a freeze-cast tubular membrane has been evaluated for oxygen permeation, exhibiting a value of 0.31 mL·min−1·cm−2 at 1000 °C under air and argon as feed and sweep gases, respectively. Finally, under the same conditions and increasing the oxygen partial pressure to get pure oxygen as feed, the oxygen permeation reached 1.07 mL·min−1·cm−2.

Enhanced arsenite removal from water by radially porous Fe-chitosan beads: Adsorption and H2O2 catalytic oxidation.

Although Fe-chitosan adsorbents are attractive for removing arsenite from water, the practical applications of these granular adsorbents are mainly limited by slow adsorption kinetics. In this study, radially porous Fe-chitosan beads (P/Fe-CB) were prepared using freeze-casting technique. The P/Fe-CB particles possess radially aligned micron-sized tunnels from the surface to the inside as well as excellent acid resistance. Kinetic studies show that the adsorption equilibrium time of P/Fe-CB to 0.975 mg/L As(III) (within 240 min) is considerably shorter than that of compact Fe-chitosan beads (over 600 min). The maximal adsorption capacity of P/Fe-CB for As(III) is 52.7 mg/g. It can work effectively in a wide pH range from 3 to 9, and the coexisting sulfate, carbonate, silicate and humic acid have no significant effect on As(III) removal. The addition of H2O2 can further accelerate and promote the As(III) removal except at high pH (11) and phosphate concentration (50 mg/L). The fixed-bed experiments demonstrate that the P/Fe-CB column can effectively treat about 3000 bed volume (BV) of simulated As(III)-containing groundwater to meet the drinking water standard (<10 μg As/L). This study would extend the potential applicability of porous Fe based chitosan adsorbent and millimeter-sized adsorbent combined with H2O2 to a great extent.

Oriented porous LLZO 3D structures obtained by freeze casting for battery applications

A novel freeze casting technique was employed to obtain 3D porous LLZO solid-electrolyte scaffolds that were infiltrated with NMC-622 cathode material to form thick composite electrodes for all-solid-state batteries.

Bi-directional freeze casting of porous alumina ceramics: A study of the effects of different processing parameters on microstructure

Highly aligned lamellar ceramic scaffolds were produced using a bi-directional freeze casting technique. A specially designed, sloped copper mould was covered with a polymer to modulate the temperature field. Effects of different processing parameters (cooling rate, mould slope angle, ceramic solid loading and binder concentration) on lamellar orientation were systematically studied. The results showed that freezing under a dual temperature gradient produced highly aligned ceramic scaffolds. Increasing both the cooling rate and the mould slope angle increased the size of the ordered ceramic region. Using different alumina solid loadings in the initial suspension had little effect on the aligned lamellar structure. Increasing the binder concentration affected ice crystal growth in a highly aligned direction. Therefore, freeze casting using a dual temperature gradient can be used to fabricate highly aligned porous materials.

Effects of solid loadings and silica addition on microstructure and compressive strength of hydroxyapatite specimens fabricated by freeze casting technique

Attributed to its bioactivity, osteoconductivity, and chemical compositions similarly to those of human bones, hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is a well-known material suitable for bone graft applications. To achieve physical and mechanical properties desired for the practical applications, microstructure of the HAp specimen needs to be controlled. This study therefore aimed at fabricating freeze-cast hydroxyapatite specimens with the porosity, pore sizes and compressive strength with acceptable values for the applications. Effects of solids loadings and silica addition on compressive strength were also examined in this study. Experimental results indicated that enhanced strength could be obtained in the specimens prepared from slips with higher solids loadings. The results also revealed that the HAp specimens achieved average porosity, pore sizes, and compressive strength in the ranges of 43.3–72.8%, 94.2–128.4 µm, and 1.29–29.35 MPa, respectively. The highest compressive strength was evident in the specimens prepared from silica-added slip with 35 vol% solids loading. The pH value of simulated body fluid solution was relatively stable upon sample submersion. Slight increase of specimen weight and formation of apatite layer were observed. The findings suggested that the specimens could be potential candidates for biomedical applications.

Dense freeze‐cast Li7La3Zr2O12 solid electrolytes with oriented open porosity and contiguous ceramic scaffold

AbstractFreeze casting is used for the first time to prepare solid electrolyte scaffolds with oriented porosity and dense ceramic walls made of Li7La3Zr2O12 (LLZO), one of the most promising candidates for solid‐state battery electrolytes. Processing parameters—such as solvent solidification rate, solvent type, and ceramic particle size—are investigated, focusing on their influence on porosity and ceramic wall density. Dendrite‐like porosity is obtained when using cyclohexane and dioxane as solvents. Lamellar porosity is observed in aqueous slurries resulting in a structure with the highest apparent porosity and densest ceramic scaffold but weakest mechanical properties due to the lack of interlamellar support. The use of smaller LLZO particle size in the slurries resulted in lower porosity and denser ceramic walls. The intrinsic ionic conductivity of the oriented LLZ ceramic scaffold is unaffected by the freeze casting technique, providing a promising ceramic scaffold for polymer infill in view of designing new types of ceramic‐polymer composites.

Magnetic filter produced by ZnFe2O4 nanoparticles using freeze casting

Zinc ferrite magnetic nanoparticles were synthesized by combustion method and the obtained powders were processed by the freeze casting technique, to produce magnetic filters. Green bodies were obtained by freezing a water-based slurry of nanoparticles and polyethylene glycol (PEG) of molecular mass 8000. The suspension was frozen to −140°C under the cooling rate of 10°C/min. The obtained bodies were sintered at 1300°C during 1h and the X-ray diffraction (XRD) analyses showed ferrite as the only crystalline phase. Scanning electron microscopy (SEM) analyses revealed formation of porous channels in freezing direction and Archimedes’ density measurements showed porosity values ranging from 48.0% to 63.0%, depending on PEG content.

A Simple and Economical Device to Process Ti Cylinders with Elongated Porosity by Freeze-Casting Techniques: Design and Manufacturing

Nowadays, the development of materials with gradient porosity is an important aim for many applications, especially in the field of bone tissue replacement. This research work shows the design and manufacture of a simple and economical device to process Ti cylinders with elongated and high interconnected porosity by freeze-casting techniques. The influence of the vessel material on the internal lamellar structure of the porous Ti samples is also studied. The device has been validated with: (i) a thermal gradient from-10 °C at the cold surface to 20 °C at the hot surface; and (ii) a cylindrical vessel with 12 mm diameter of alumina or Teflon. These working conditions have allowed maintaining the freezing conditions during the full process (alumina vessel: 30 minutes; Teflon vessel: 3 hours). After the solidification of the Ti aqueous suspension, the ice is sublimated (24 hours at-50 °C and 0.070 mbar). Then, the resulting powder is sintered (1150 °C for 5 h at high vacuum ~ 10-5 mbar), obtaining the porous Ti sample with the final structural strength. Finally, a detailed study of the most relevant porosity features is performed: porosity ratio and interconnectivity degree by Archimedes’ method, and size and shape of the pores by image analysis at three different zones along the longitudinal cross section. The results indicate the viability of the device to enhance the directional freezing and thus, the elongated porosity. Teflon vessel present the best results, with an average porosity ratio and porosity size of 39.5 % and 123.3 μm, respectively. This suggests an optimal biomechanical and bifunctional balance of this porous material.

Mechanics of bioinspired lamellar structured ceramic/polymer composites: Experiments and models

Creation of super-tough ceramics is one of the main goals of materials science. Bioinspired design is shown to be the most effective method to achieve this goal. Previous studies on the mechanical performance of biological multilayered materials such as nacre have shown that their outstanding mechanical properties are direct results of the small-scale features and optimized arrangement of the elements in their microstructure. Hence, the freeze casting technique has been recently introduced as a novel method to create a new class of bioinspired polymer/ceramic composites. However, the method is cumbersome and the mechanics that controls the overall performance of these composites is not well-known. In this study, the mechanical performance of bioinspired alumina/polydimethylsiloxane (Al2O3/PDMS) and alumina/polyurethane (Al2O3/PU) composite samples with lamellar structure are experimentally and analytically investigated. Bioinspired multilayered samples with micron-size layers are fabricated using the challenging freeze casting technique. Different parameters such as solution concentration, freezing rate, and sintering temperature affect the structure, and subsequently, the mechanical performance of these multilayered materials. Moreover, in order to fully understand the underlying toughening and deformation mechanisms, a micromechanics model of the mechanical response of lamellar composites is presented. The closed-form solutions for the displacements of the layers as a function of constituent properties are derived to calculate the mechanical response of lamellar structured composites such as elastic modulus, strength, and tensile toughness. The experimental results agree well with the proposed analytical models. Fracture mechanics tests are also used to study the Resistance-Curve (R- Curve) behavior of the samples. Furthermore, important toughening mechanisms in these samples are discussed and governing equations for fiber bridging and fiber pull-out in lamellar ceramic/polymer composites are presented. Finally, detailed material design relationships are derived to identify future directions in the design of next generation structural composites.

Effects of ceramic lamellae compactness and interfacial reaction on the mechanical properties of nacre-inspired Al/Al2O3–ZrO2 composites

Freeze casting has been proven to be a versatile processing approach for the design of biomimetic composites that exhibit unique combinations of strength and toughness. However, to date, most studies have focused on polymer–ceramic composites, and very limited work has examined metal–ceramic composites, even though the latter possess much superior mechanical properties. In this work, we prepared nacre-like Al/Al2O3–ZrO2 composites using freeze casting and pressure infiltration techniques and then investigated the influences of the ceramic lamellae compactness and interface reaction on the mechanical properties and fracture behavior of the composites. The results show that the compactness of the ceramic lamellae greatly influences the Al penetration and subsequent interfacial reaction with ZrO2. The presence of Al3Zr reaction product layers facilitates crack initiation and interface debonding, weakening the fracture toughness of the composites. The ceramic lamellae can be densified by using fine ceramic particles and increasing the ceramic loading and sintering temperature, thus greatly reducing the extent of the interfacial reaction and improving the strength and toughness of the materials.

Formation mechanism and control of a large-scale lamellar structure in freeze-cast Al2O3 ceramics under dual temperature gradients

We prepared a large-scale aligned structure in Al2O3 porous ceramics by a simple and novel bidirectional freeze-casting technique. This technique uses a side metal plate to achieve cold-surface cooling from the bottom to generate dual temperature gradients to control the nucleation and growth of ice crystals. By recording the slurry temperature in situ, we obtained various temperature gradients for the samples prepared by this technique using 3 mm thick copper, aluminium and stainless-steel plates as the cold surfaces. The architectures of the scaffolds produced under these freezing conditions were investigated to determine the effect of the temperature gradients on the morphology of the microstructure. Finally, a model was proposed to account for the temperature gradient regulatory mechanism of the transition from a dendritic structure to a parallel structure.

3D long-range ordered porous ceramics prepared by a novel bidirectional freeze-casting technique

We developed a novel and simple bidirectional freeze-casting technique to prepare porous ceramics with long-range ordered lamellar structures in three dimensions (3D). A side copper or stainless-steel plate was used as the cold source to generate dual temperature gradients for the control of the nucleation and growth of ice crystals. The Al2O3 architectures prepared with the side copper plate exhibited well-assembled lamellar structures with abundant cross connections of the ceramic lamellae. The compressive strength of the resultant Al2O3 architectures with ∼ 50% porosity reached 158 ± 3MPa in the primary freezing direction and 42 ± 6MPa in the secondary freezing direction, ∼ 32% and ∼ 130% higher than that of the counterparts prepared by unidirectional freeze casting. In contrast, the Al2O3 architectures prepared with a side stainless-steel plate exhibited thicker ceramic lamellae and wider pore channels as well as decreased cross connections, leading to much weaker compressive strength.