Freeze Casting Process Research Articles

Dynamically Tunable All-Weather Daytime Cellulose Aerogel Radiative Supercooler for Energy-Saving Building.

A passive cooling strategy without any electricity input has shown a significant impact on overall energy consumption globally. However, designing tunable daytime radiative cooler to meet requirement of different weather conditions is still a big challenge, especially in hot, humid regions. Here, a novel type of tunable, thermally insulating and compressible cellulose nanocrystal (CNC) aerogel coolers is prepared via chemical cross-linking and unidirectional freeze casting process. Such aerogel coolers can achieve a subambient temperature drop of 9.2 °C under direct sunlight and promisingly reached the reduction of ∼7.4 °C even in hot, moist, and fickle extreme surroundings. The tunable cooling performance can be realized via controlling the compression ratio of shape-malleable aerogel coolers. Furthermore, energy consumption modeling of using such aerogel coolers in buildings in China shows 35.4% reduction of cooling energy. This work can pave the way toward designing high-performance, thermal-regulating materials for energy consumption savings.

Poly(d,l-lactide)-Grafted Bioactive Glass Nanoparticles: From Nanobricks to Freeze-Cast Scaffolds for Bone Substitution

This paper focuses on an integrative “bricks-and-mortar” approach involving bioactive glass nanoparticles (bricks) covalently functionalized with a customized polymer (mortar) combined with the freeze-casting process. With the aim of obtaining a macroporous composite for bone substitution, composed of a spatially homogeneous assembly of nanoscale objects, we establish a method for the systematic elaboration of nanocomposite scaffolds. It was implemented through several steps from the synthesis of functionalized poly(d,l-lactide) (PDLLA) and SiO2–CaO binary bioactive glass nanoparticles (diameter around 164 nm) to the unidirectional freeze-casting process. The different stages include the first description of controlled PDLLA (Mn 8400 g·mol–1) grafting onto bioactive glass nanoparticle surfaces, their fine characterization and grafting quantification, and their mixing with free PDLLA chains (83 400 g·mol–1) during suspension formulation. This paper emphasizes the effect of the working temperature during the freeze-casting process on the multiscale spatial organization of resulting scaffolds such as the porosity morphology (lamellar and tubular), size (from 30 to 380 μm), anisotropy, and orientation. In addition to porosity, our results demonstrate a rosary-like organization of PDLLA-grafted nanoparticles in pore walls. The higher homogeneity in the spatial distribution of grafted nanoparticles over the height of scaffolds and at a micron scale confirms the validity of the “bricks-and-mortar” concept to prevent or limit aggregation. In particular, this study highlights the correlation between nanoparticle functionalization and mechanical properties, especially the recovery rate after compression tests. These results lay the foundation for the development of tunable materials for bone substitution, via potential enhancement of bioactivity and cell colonization.

Bioinspired Techniques in Freeze Casting: A Survey of Processes, Current Advances, and Future Directions

Freeze casting, popularly known as ice templating or freeze gelation, is a mechanical method to fabricate scaffolds of desirable properties and materials. Aerospace engineering, the healthcare sector, manufacturing department, and automotive industries are the different fields where freeze casting has been used. Bioinspiration refers to the translation of biological systems into new and innovative creations. Bioinspired materials are extensively used in freeze casting methods such as ceramide, spines of porcupine fish, and collagen. Due to the tunable properties and production of complex structures with ease, biomaterials have found numerous applications in the ice templating method. This review rigorously explains the freeze casting process and the effect of thermal conductivity, stress, and electrostatic repulsion on the porous materials. Also, we have discussed the different biomaterial polymers used in freeze casting along with different methods involved.

Tailoring the Microstructure of Lamellar Ti3C2Tx MXene Aerogel by Compressive Straining.

Aerogels are attracting increasing interest due to their functional properties, such as lightweight and high porosity, which make them promising materials for energy storage and advanced composites. Compressive deformation allows the nano- and microstructure of lamellar freeze-cast aerogels to be tailored toward the aforementioned applications, where a 3D nanostructure of closely spaced, aligned sheets is desired. Quantitatively characterizing their microstructural evolution during compression is needed to allow optimization of manufacturing, understand in-service structural changes, and determine how aerogel structure relates to functional properties. Herein we have developed methods to quantitatively analyze lamellar aerogel domains, sheet spacing, and sheet orientation in 3D and to track their evolution as a function of increasing compression through synchrotron phase contrast X-ray microcomputed tomography (μCT). The as-cast domains are predominantly aligned with the freezing direction with random orientation in the orthogonal plane. Generally the sheets rotate toward flat and their spacing narrows progressively with increasing compression with negligible lateral strain (zero Poisson’s ratio). This is with the exception of sheets close to parallel with the loading direction (Z), which maintain their orientation and sheet spacing until ∼60% compression, beyond which they exhibit buckling. These data suggest that a single-domain, fully aligned as-cast aerogel is not necessary to produce a post-compression aligned lamellar structure and indicate how the spacing can be tailored as a function of compressive strain. The analysis methods presented herein are applicable to optimizing freeze-casting process and quantifying lamellar microdomain structures generally.

Co Nanoparticles Encapsulated in Carbon Nanotubes Decorated Carbon Aerogels Toward Excellent Microwave Absorption

Here, a porous hybrid carbon aerogel (CNS/Co@NCNTs) is fabricated by growing cobalt nanoparticles in N-doped carbon nanotubes on three-dimensional (3D) continuous carbon nanosheets through a directional freeze-casting process and a carbonization process. The resulting hybrid carbon aerogels exhibit excellent attenuation performance owing to a unique 3D porous structure with a large surface, multiple interfaces, abundant defects, and efficient electron conductivity. Consequently, the optimized CNS/Co@NCNT-900 aerogel demonstrates extraordinary microwave absorption (MA) performance in terms of minimum reflection loss (−27.5 dB) and effective absorption bandwidth (7.4 GHz). This work proposes a novel approach for designing and fabricating highly effective magnetic porous hybrid carbon aerogels to be applied in MA fields.

In situ imaging of three dimensional freeze printing process using rapid x-ray synchrotron radiography.

Three dimensional freeze printing (3DFP) combines the advantages of freeze casting and additive manufacturing to fabricate multifunctional aerogels. Freeze casting is a cost-effective, efficient, and versatile method capable of fabricating micro-scale porous structures inside the aerogels for many different applications. The 3DFP provided the capability of fabricating highly customized geometries with controlled microporous structures as well. However, there are still many unexplained phenomena and features because of the complexity of post-processes and indirect observation methods. This study demonstrates the design and construction of the in situ imaging systems, which use the x-ray synchrotron radiography to observe freeze casting and 3DFP processes. With the advantages provided by the in situ x-ray imaging techniques, the ice crystal growth with its unique lamellar structures can be observed during the freeze casting process. The movement of freeze front, material deposition, and growth of ice crystals can also be visualized during the inkjet-based 3DFP process.

A comparative study on dielectric and mechanical properties of porous β-Si3N4 ceramics by controlling porosity and microstructure

To ensure the porous Si3N4 ceramics with an excellent comprehensive performance for the application of radome, the commonly neglected effect of designed microstructures on dielectric and mechanical properties was investigated. Two typical porous Si3N4 ceramics with similar porosity but different microstructures (unidirectionally aligned and uniformly micropore structure with a different dimension of grains) were obtained by freeze casting and gel-casing process. The results indicate that the microstructure has a significant influence on the mechanical properties yet not an obvious effect on the dielectric properties, which means the dielectric constant can be reliably designed based on the porosity without consideration of different microstructures, meanwhile, the mechanical properties can be optimized by the microstructure. Besides, the results of the dielectric properties predicted by Finite Element Analysis show a high agreement with the experimental results. The study could be helpful to design a wave-transmitting component with integrated structure and function.

Fabrication and characterization of composite PMMA/HA scaffold using freeze casting method

ABSTRACT A bone implant is one of the important areas in biomedical applications where biomaterial like polymers, ceramics and metals plays a deciding role. Polymethyl(methacrylate) (PMMA) is one of the biocompatible materials that can be used for fabricating bone implants. In this work, the combinations of PMMA, PMMA/HA, PMMA/Mg and PMMA/HA/Mg scaffolds were fabricated by the freeze casting process. The variation of pore size, porosity and compressive strength was studied for PMMA and its combinations with HA and Mg. The pore size in all four combinations was greater than 200 µm. This indicates that all four types of scaffolds are suitable for bone implants in their biological functions. The compressive strength of the PMMA/HA (26.08 MPa) scaffold was higher than the cancellous bone. In this work, PMMA/HA scaffold with better pore size and compressive strength was fabricated which has given way to fabricate bone implants in the near future.

Nickel Wick by Continuous Freeze-Casting: Influences of the Particle Size on the Capillarity and Mechanical Properties.

The aim of this work was to study the effect of the particle size range, the freeze casting temperature and sintering temperature on the capillarity performance and mechanical properties of Ni wicks manufactured by freeze-casting. The use of Ni/camphene-polystyrene suspensions creates wicks with an open porosity above 80% and average pore sizes of 38 μm to 17 μm by tailoring the particle size ranges and freezing temperatures employed. The incorporation of PS and the use of a continuous freeze-casting process reduces the particle sedimentation and generates a highly interconnected pore structure with regular pore sizes across the sample. The capillarity performances exhibit a fast and complete water adsorption, especially in Ni wicks freeze-casted at 10 °C and sintered at 800 °C, but only when the smaller particle size range is used do Ni wicks achieve sufficient mechanical strength.

Morphological, mechanical, and in-vitro bioactivity of gelatine/collagen/hydroxyapatite based scaffolds prepared by unidirectional freeze-casting

The fabrication of biomaterials to be used in segmental bone defects, mimicking the bone's organic-inorganic architecture and mechanical properties to induce osteogenesis, persists as a key challenge. The purpose of this study was to elucidate the effect of a lightweight, morphologically graded, and multiphase self-standing scaffold structure prepared from a combination of gelatine (Gel), collagen type 1 (Col) and/or hydroxyapatite (HAP) nanoparticles by a unidirectional freeze-casting process at different temperatures (−20, −40, −60 °C), followed by carbodiimide induced cross-linking, on their in-vitro mechanical stability and bioactive properties. In addition, the rheological study of differently formulated Gel solutions has been performed to determine the effect of Col and HAP content on their microstructural arrangement, which, together with the freezing kinetic, affects Gel/Col orientation and cross-linking, and, thus, the scaffold's mechanical strength and stability.A bone-like anisotropic, interconnected, and graded porosity (from 120 to a few μm) scaffold structure with up to 30% total porosity and ~61 μm average pores' diameter is obtained by using a higher Col content (Col: Gel = 2:5) and freezing temperature (−20 °C) while forming a few μm thick close-to-parallel lamellae, separated with a 10–100 μm space when prepared at −60 °C. Such a structure influenced in-vitro stability strongly (lower swelling without weight loss), being accompanied with a ~76% increase of compression strength (to 37 kPa) and ~67% decrease of elastic modulus (to 17 kPa) when prepared with HAP and incubated in HBSS for 7 days. On the other hand, a significant reduction of both strength (~78%, to 15 kPa) and elasticity (~95%, to 5 kPa) was noted for a scaffold prepared with HAP at −60 °C, being related to faster degradation and the formation of a highly opened structure on the bottom, required to stimulate the bone ingrowth, while a more closed network structure on the top to adhere with the surrounding soft tissue. None of the scaffolds induced cytotoxicity to human bone-derived osteoblasts, even after 19 days of incubation, but rather improved their viability while promoting cells' adhesions, proliferation, and differentiation, being supported with an increased alkaline phosphatase activity and rod-like CaP formation.

Anisotropic cellulose nanofiber/chitosan aerogel with thermal management and oil absorption properties

Attributed to low cost, renewable, and high availability, cellulose-based aerogels are desirable materials for various applications. However, mechanical robustness and functionalization remain huge challenges. Herein, we synthesized a recoverable, anisotropic cellulose nanofiber (CNF) / chitosan (CS) aerogel via directional freeze casting and chemical cross-link process. The chitosan was performed as strength polymers to prohibits the shrinkage and retains the structural stability of 3D cellulose nanofiber skeleton, endowing the composite aerogel with satisfactory deformation recovery ability (without loss under 60 % stress cycled 100 times). The CNF/CS composite aerogel has ultralow density (∼8.4 mg/cm3), high temperature-invariant (above 300 °C) and high porosity (98 %). The CNF/CS aerogel demonstrates anisotropic thermal insulation properties with low thermal conductivity (28 mWm−1 K−1 in rational direction and 36 mW m−1 K−1 in the axial direction). Moreover, the composite aerogel (water contact angle ∼148°) exhibited outstanding oil/water selectivity and high absorption capacity (82–253 g/g) for various oils and organic solvents. Therefore, the multifunctional CNF/CS composite aerogels are potential materials for thermal management and oil absorption applications.

Modelling and simulation of the freeze casting process with the phase-field method

The freeze casting process is a novel manufacturing method for both near net-shape parts as well as directed porous structures as employed by filters and implants. Depending on the choice of liquid and processing conditions a very wide range of pore shapes and sizes can be achieved. In order to predict the resulting microstructure, a phase-field model is developed on the basis of the grand potential formalism. The model and its parametrization approximate the freeze-casting process of water by linking its thermodynamics with established theory. Directional solidification simulations with varying suspension concentrations, velocities and temperature gradients are carried out. From these, microstructural lengths are determined and linked with the processing parameters, so as to derive linkages between the microstructure and the processing conditions.

Fabrication of high permeability SiC ceramic membrane with gradient pore structure by one-step freeze-casting process

A highly permeable porous SiC ceramic membrane with multilayered unidirectional gradient pore microstructure was prepared by one-step freeze-casting process. The multilayer microstructures, mechanical properties and filtering performances of freeze-casting SiC ceramic membranes were controlled by optimizing the preparation process. It was found that with the increase of solid content from 30 vol% to 40 vol%, the directional pore channels gradually narrowed by about 50% and the mechanical properties increased from 0.47 MPa to 5.44 MPa. In addition, the increase of coarse particle content not only thickened the separation layer and the transition layer, but also widened the directional pore channel, which is beneficial to improve the permeability efficiency. When the coarse particle content of SiC ceramic membranes was 90%, the average pore size of separation layer, transition layer and lamellar structure was 5.52 μm, 7.63 μm and 36.09 μm, respectively. The maximum water permeability of S35C90 ceramic membrane reached up to 5.67 × 105 L/m2 h bar, and the rejection rate for ground calcium carbonate was 96.1%, confirming the superior perviousness and separation efficiency of freeze-casting ceramic membranes in the field of filtration.

Freeze-casting applied to ceramic materials: a short review of the influence of processing parameters

In recent years there is a high interest in the freeze-casting process because it is a simple, economical, and environmentally friendly method for obtaining highly porous materials. Most of the materials obtained by this technique have directional pore structure and anisotropic properties. However, a wide range of pore size and morphology can be obtained by the control of processing parameters, such as solvent type, solids concentration, particle size, freezing rate, and types of additives. The varied morphologies permit the application of freeze-casted materials in various technological applications. However, despite the high amount of studies about freeze-casting of ceramic materials in the last years, review articles addressing the influence of processing parameters on the pore characteristics of obtained ceramics are still scanty. Therefore, this review addresses the influence of freeze-casting process parameters on the pore characteristics of the ceramic materials.

In-situ X-Ray observations and thermal modeling of unidirectional and bidirectional freeze casting

Freeze casting is a cost effective, efficient, and versatile technique capable of producing 3D structures with controlled pore shapes, orientation of crystals, and components' geometries in many porous materials. Freeze casting of hydroxyapatite (HAP) has been widely applied to bone tissue engineering due to HAP's biodegradable, biocompatible, and osteoconductive properties. It provides interconnected porous structures with a relatively high mechanical strength. However, there are still many unexplained phenomena and features because of the complexity of the process. This study demonstrates the use of X-ray synchrotron micro-radiography for providing time-resolved, in-situ imaging of ice crystal growth in the HAP suspensions. The experimental results show the ice crystal growth behavior under unidirectional and bidirectional freeze casting conditions. The finite element modeling (FEM) of the freeze casting process has been used to predict the development of ice front position and temperature gradient in the suspensions during the freeze casting.

Bioinspired Manufacturing of Aerogels with Precisely Manipulated Surface Microstructure through Controlled Local Temperature Gradients.

The freeze casting process has been widely used for fabricating aerogels due to its versatile and environmentally friendly nature. This process offers a variety of tools to tailor the entire micropore morphology of the final product in a monolithic fashion through manipulation of the freezing kinetics and precursor suspension chemistry. However, aerogels with nonmonolithic micropore morphologies, having pores of various sizes located in certain regions of the aerogels, are highly desired by certain applications such as controlled drug-delivery, bone tissue engineering, extracellular simulation, selective liquid sorption, immobilized catalysts, and separators. Furthermore, aerogels composed of micropores with predesigned size, shape, and location can open up a new paradigm in aerogel design and lead to new applications. In this study, a general manufacturing approach is developed to control the size, shape, and location of the pores on the aerogel surface by applying a precise control on the local thermal conductivity of the substrate used in a unidirectional freeze casting process. With our method, we created patterned low and high thermal conductivity regions on the substrate by depositing patterned photoresist polymer features. The photoresist polymer has a much lower thermal conductivity, which resulted in lower cooling/freezing rates compared to the silicon substrate. Patterned thermal conductivity created a designed temperature profile yielding to local regions with faster and slower freezing rates. Essentially, we fabricated aerogels whose micropore morphology on their surface was a replica of the patterned substrates in terms of size and location of the micropores. Using the same substrates, we further showed the possibility of 3D printed aerogels with precisely controlled, surface micropore morphologies. To the best of our knowledge, this is the first study that reports aerogels having micropore morphologies (e.g., size, shape, and location) that are precisely controlled through locally controlled thermal conductivity of the substrates.

Controlling Porosity By Freeze Casting in Tubular Solid Oxide Fuel Cell Anode Supports

Solid oxide fuel cells (SOFCs) have the ability to demonstrate high energy conversion efficiency with commercially available fuels over a wide array of power applications. In order to achieve commercial viability, the cost and durability of the technology require significant improvements. Here we report high performance of a tubular solid oxide fuel cell utilizing a freeze-casting process to form the anode support. In our full-cell demonstration, we employed a standard nickel cermet anode with a standard lanthanum strontium manganite perovskite cathode to show the effect of a novel pore structure within the supporting anode. When evaluated with 30 vol% H2 as the fuel at 800 oC, a decrease of concentration polarization was achieved by increasing the porosity which enabled a 42% increase in the electrochemical performance from 0.35 to 0.50 W/cm2. Controlling porosity within the SOFC active layers is highly desirable for improving the electrochemical performance which can both affect the overall cost and durability of the system. Here we show how freeze casting allows for fine tuning of the aligned porosity within the tubular, anode support of the SOFC with simple adjustments in the solids/water ratio in the casting slurries. Permeation measurements of the anode structures support these findings and show how future improvements can be made by further optimization of the anode interface between the bulk anode and the electrolyte.

Recombinant collagen polypeptide as a versatile bone graft biomaterial

Autografts and allografts are currently considered the gold standard for grafting surgery; however, to meet the growing demand in fast-aging societies, synthetic biomaterials will play an increasingly important role. Here we report a biodegradable scaffold material composed of recombinant polypeptide based on the human type I collagen alpha 1 chain (RCPhC1) as a source of hydrogel-based graft materials. The flexibility to engineer ideal characteristics for bone grafts was demonstrated. The critical internal isotropic pore structure was generated through a designed thin-layer freeze casting process. The optimized biodegradation rate was controlled by dehydrothermal crosslinking by adjusting the amino acid composition of RCPhC1. As a result, RCPhC1 bone grafts manufactured by a highly scalable streamlined production protocol induced robust regeneration of mature bone tissue while being completely resorbed in pre-clinical animal models.

Preparation and properties of high-porosity ZrB2-SiC ceramics by water-based freeze casting

The development of novel cermet composites based on porous ceramics with high porosity, interconnected pore structure and good mechanical property has attracted considerable attention in engineering application. In this work, water-based freeze casting process was employed to fabricate ZrB2-SiC porous ceramic with aligned lamellar-channels structure using PAA-NH4 as the dispersant. The results revealed that the well-dispersed suspension with best rheological behavior was obtained using 1.0 wt% PAA-NH4 at pH 9. The crack-free porous ceramic exhibited small volume shrinkage ranging from 2.59 % to 1.87 %. By varying the solid loading, the fabricated samples displayed a tailored porosity ranging from 76.12% to 59.37% and an excellent compressive strength of 7 MPa to 78 MPa. After oxidation, the samples displayed a decreased porosity and an increased compressive strength. The ZrB2­SiC porous ceramic fabricated in this work will be a promising candidate for the framework of cermet composite.

Pd supported on 3D graphene aerogel as potential electrocatalyst for alkaline direct methanol fuel cells

Direct methanol fuel cells (DMFCs) have been shown potential interest in several portable electronics for “green” power generation, however their large-scale commercial production are mostly hampered due to the low activity and high cost of the anode catalysts. In the present study, we report a facile, environmentally friendly, and cost-effective Pd supported on 3D graphene aerogel (Pd/GA) by one step freeze-casting process. The as-synthesized Pd/GA was broadly characterized by XRD, Raman, SEM, and TEM. Morphology observations show that 3D-porous cellular structure of Pd/GA with uniform distribution of 10 nm Pd nanoparticles on the surface of graphene sheets. Based on structural merits, the performance of Pd/GA has been investigated as an anode catalyst for alkaline DMFCs. Pd/GA catalyst shows significantly superior electrocatalytic performance, abnormally high poison tolerance, and stability as compared to the conventional Pd/ carbon black (Vulcan XC-72R) with the same loading (∼20%) of Pd.The superior electrocatalytic performance of Pd/GA towards methanol oxidation can be used in practical application as a promising anode material for DMFCs.