Unidirectional Freeze Casting Research Articles

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.

Anisotropic Electromagnetic Absorption of Aligned Ti3C2Tx MXene/Gelatin Nanocomposite Aerogels.

Assembling Ti3C2Tx MXene nanosheets into three-dimensional (3D) architecture with controllable alignment is of great importance for electromagnetic wave absorption (EMA) application. However, it is a great challenge to realize it due to the weak van der Waals interconnection between MXene nanosheets. Herein, we propose to introduce gelatin molecules as a "chemical glue" to fabricate the 3D Mxene@gelatin (M@G) nanocomposite aerogel using a unidirectional freeze casting method. The Ti3C2Tx MXene nanosheets are well aligned in the M@G nanocomposite aerogel, yielding much enhanced yet anisotropic mechanical properties. Due to the unidirectional aligned microstructure, the M@G nanocomposite aerogel shows significantly anisotropic EMA properties. M@G-45 shows a -59.5 dB minimum reflection loss (RLmin) at 14.04 GHz together with a 6.24 GHz effective absorption bandwidth in the parallel direction (relative to the direction of unidirectional freeze casting). However, in the vertical direction of the same M@G aerogel, RLmin is shifted to a much lower frequency (4.08 GHz) and the effective absorption bandwidth decreases to 0.86 GHz. The anisotropic electromagnetic energy dissipation mechanism was deeply investigated, and the impendence match plays a critical role for electromagnetic wave penetration. Our lightweight M@G nanocomposite aerogel with controllable MXene alignment is very promising in EMA application.

Effect of unidirectional freezing using a thermal camera on polyvinyl (alcohol) for aligned porous cryogels

ABSTRACT Unidirectional freeze casting for polymers and ceramics has been performed extensively; however, the mechanisms involved in unidirectional freezing are not thoroughly understood and therefore, in this study, polyvinyl (alcohol) hydrogels were freeze-thawed by unidirectional freezing and the resultant freezing profile was investigated using thermal imagery to comprehend the physical mechanism involved in the production of these aligned cryogels. The approach used follows a slight variation from what is currently used in literature, by enclosing the mould with insulating materials and leaving one side with a metal encasing for a fast freezing flow from the direction of the desired aligned of the structure. This method was cost-effective and simple; however, it was able to induce unidirectional freezing resulting in two distinct structures. These structures exhibited a cellular region interspersed with a fibrous and porous morphology. The method developed without the need of a freeze-dryer was confirmed to produce a unidirectional freezing which can be used for future studies.

A novel approach to the fabrication of lamellar Al2O3/6061Al composites with high-volume fractions of hard phases

Freeze casting has been widely used in the preparation of nacre-inspired composites because of its prominent advantages in controlling the fine structure of ceramic scaffolds. However, the ceramic-phase content in composites prepared by this method is much lower than that in nacre due to the limitations of the dispersion and viscosity of the ceramic slurry. In this work, we combined the ideas of freeze casting and in-situ reaction to prepare nacre-inspired lamellar composites with high hard-phase contents. We first prepared the TiO2-−SiO2 porous scaffold with an initial solid loading of 20 vol% via unidirectional freeze casting and then infiltrated it with a 6061Al alloy. During the reaction process, the TiO2-SiO2 layers were progressively transformed into Al2O3 layers, and the Al layers were transformed into Al3Ti. Composites with different hard-phase volume fractions (approx. 29%, 38%, 52% and 65%) and different interfacial bonding strengths could be obtained by altering the infiltration temperature. Increases in the hard-phase content and the interfacial bonding strength substantially enhanced the bending and compressive strengths of the composites, with maximum values of 350 MPa and 854 MPa, respectively, achieved in the composites with approx. 65 vol% hard phases. However, the highest fracture toughness of 42.9 MPa m1/2 and maximum fracture work of 3.6 kJ/m2 appeared when the hard-phase content in the composite was approx. 38 vol%, sharing an appropriate interfacial bonding strength. A robust interface favors stress transfer but impedes crack deflection, while a weak interface fails to bear loads, even though it is conducive to crack deflection. A moderately-strong interface together with a considerable amount of layered soft phase contributes to the generation of multiple-cracking, and thereby greatly toughens the metal−ceramic composite.

Pore Architectures and Mechanical Properties of Porous α-SiAlON Ceramics Fabricated via Unidirectional Freeze Casting Based on Camphene-Templating.

Porous α-SiAlON ceramics were fabricated using the camphene-based unidirectional freeze casting method, in which a gradient porous structure was formed as a result of the decreased solidification velocity in the freezing direction. Microstructure, porosity and pore size distribution of different parts of as-prepared samples were examined and compared, and correlated with their mechanical properties. For a given solid loading, the overall pore size and porosity of the top part were greater than those of the bottom part. Interestingly, despite its higher porosity, the flexural strength and fracture toughness of the top part were both higher than those of the bottom part, suggesting that apart from porosity, pore morphology and size affected mechanical properties of as-prepared porous α-SiAlON ceramics.

Achieving vertically aligned SiC microwires networks in a uniform cold environment for polymer composites with high through-plane thermal conductivity enhancement

Pre-constructing vertically aligned 3D thermally conductive fillers networks via unidirectional freeze casting method to prepare polymer composites with high through-plane thermal conductivity (TC) has recently attracted much attention. Here we proposed a strategy to fabricate anisotropic silicon carbide microwires (SiCMWs) networks in a uniform cold environment. The key point of our strategy was to design and make the anisotropic molds composed of materials with very different TC. We used a cuboid mold made of alumina sheets as top and bottom and PVC foam plates as side surfaces to fabricate vertically aligned SiCMWs networks in a cryogenic refrigerator. After impregnating of the networks into epoxy matrix, the obtained composites exhibited significant TC enhancement at an ultralow loading. The composite containing only 1.32 vol % of SiCMWs had a TC value of 0.62 W/(m K), which was 3.9 and 2.3 times that of pure epoxy and epoxy composite with random SiCMWs, respectively. Moreover, utilizing our method, the 3D networks with different orientation direction, dimensions or shapes could be also easily prepared only by designing and utilizing different molds.

Effect of freeze speed on the microstructure and damage-tolerance behavior of bio-inspired ZL205A/silicon carbide composites

In the present research, we produced porous silicon carbide (SiC) scaffolds with lamellar and uniformly distributed structures by unidirectional and quick freeze casting, respectively. Subsequently, we prepared the ZL205A/SiC composites with lamellar and uniformly distributed structures by infiltrating a molten ZL205A alloy into the porous SiC scaffolds. The influence of freezing speed on the damage-tolerance behavior and the toughening mechanism of the resultant composites were investigated. We found that because of multiple toughening mechanisms, the layered composites exhibited much higher strength and toughness as compared with the uniformly distributed composites. The lamellar composites manifested superior specific modulus and a unique combination of specific strength and toughness, which was comparable to those of titanium and magnesium alloys. We also observed that the thicknesses of alloy layers and SiC lamellae of the composites reduced with decreasing cooling temperature in freeze casting. Furthermore, the refinement of lamellae size at higher freezing speeds caused a decrease in the damage-tolerance ability of the composites because of weak toughening effect.

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.

In situ growth of one-dimensional nanowires on porous PDC-SiC/Si3N4 ceramics with excellent microwave absorption properties

Abstract Enhanced microwave absorption properties of porous PDC-SiC/Si 3 N 4 ceramics were successfully prepared through in situ growth of Si 3 N 4 nanowires (NWs) by the unidirectional freeze casting of Si 3 N 4 /camphene slurries with various polycarbosilane (PCS) contents at 1400 °C in N 2 . Si 3 N 4 NWs were in situ formed in the pore channels via a vapor-solid (VS) mechanism, the content of Si 3 N 4 NWs increased with increasing of PCS content, which tailored the microstructure and mechanical property of porous PDC-SiC/Si 3 N 4 ceramics. With the increase of PCS content, the minimum reflection coefficient (RC) of porous PDC-SiC/Si 3 N 4 ceramic decreased from −20.2 dB to −56.2 dB, and the effective absorption bandwidth (EAB) increased from 5.3 to 7 GHz, demonstrating that the porous PDC-SiC/Si 3 N 4 ceramics decorated with Si 3 N 4 NWs had a superior microwave-absorbing ability, which could be ascribed to the electronic dipole polarization and interfacial scattering enhanced by the various interfaces among Si 3 N 4 , in-situ formed PDC-SiC nanograins, nanosized carbon and Si 3 N 4 NWs phases. These findings provide significance guidance in the development of new broadband EM absorbing materials.

Ultrahigh dielectric constant and low loss of highly-aligned graphene aerogel/poly(vinyl alcohol) composites with insulating barriers

A novel two-step route to prepare graphene aerogel (GA)/poly(vinyl alcohol) (PVA) composites with exceptional dielectric performance is reported. The composites obtained from the initial unidirectional freeze casting possess a highly-aligned, conducting graphene skeleton with an ultralow density, a high porosity and remarkable thermal stability, showing ultrahigh dielectric constants combined with moderate losses. The losses are significantly reduced by means of PVA barriers introduced between the neighboring conductive main skeletons of the aligned GA/PVA networks in the 2nd freeze casting process. The insulating barriers effectively block the current leakage by removing the transversely interconnected, conductive ligaments. The compaction of barrier-shielded, porous GA/PVA composites yield fully consolidated, solid composites which deliver both exceptional dielectric constants and very stable, low losses. The approach developed here paves the way for rational design and assembly of GA/PVA composites as a lightweight, tunable and high-performance dielectric material, satisfying various requirements for emerging applications.

Ultralow Electrical Percolation in Graphene Aerogel/Epoxy Composites

Graphene aerogels (GAs) with a highly aligned, porous structure are prepared using a novel unidirectional freeze casting method, followed by thermal reduction. The unique graphene orientation in a preferred direction is achieved due to the large temperature gradient generated during freeze casting, in which graphene oxide (GO) sheets are expelled by the rapidly advancing ice front to assemble between the aligned ice crystals. The resulting unidirectional GAs (UGAs) possess ultralow densities, high porosities, and large surface areas, as well as excellent electrical conductivities. The solid UGA/epoxy composites fabricated by vacuum-assisted infiltration of liquid epoxy present an extremely low percolation threshold of 0.007 vol %, which is the lowest value for all graphene/polymer composites reported in the literature. Besides, the anisotropic structure of UGAs gives rise to significant anisotropic electrical conductivities of UGA/epoxy composites, a potentially useful attribute for many important applica...

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.

溶液含浸法によるスピネル多孔体の作製におけるアルミナ骨格の微細構造の影響

Porous MgAl2O4 ceramics were fabricated by impregnating porous Al2O3 bodies calcined at 1250℃ with Mg(NO3)2 saturated aqueous solution under reduced pressure, followed by firing at 1500℃. Porous Al2O3 bodies with continuously long-ranged pore structure and cross-linked one were prepared by a unidirectional freeze casting using aqueous Al2O3 slurries containing urethane latex binder (ULB) and water-soluble binder (PVA), respectively. The effect of the micro structure of Al2O3 bodies on MgAl2O4 formation was investigated. In the case of the porous Al2O3 body with continuously long-ranged pore structure, the amount of MgAl2O4 formed on firing at 1500℃ was larger than that for the porous Al2O3 body with cross-linked pore structure and a single-phase MgAl2O4 porous ceramic was obtained by four times impregnation. However, in this case, the surface of the porous Al2O3 body peeled off gradually with repeating impregnation. On the other hand, in the case of the porous Al2O3 body with cross-linked pore structure, about 10% Al2O3 remained after four times impregnation, but the structural disintegration caused by repeating impregnation was not observed. The cross-linked pore wall was effective to increase the strength of the calcined porous Al2O3 bodies but it was difficult to make the solution of Mg(NO3)2・6H2O into the small pore, resulting in a decrease in the rate of formation of MgAl2O4.

Investigation of structural collapse in unidirectionally freeze cast collagen scaffolds.

Though unidirectional freeze casting is a facile method for the production of structurally anisotropic biomedical scaffolds, challenges exist in optimizing the drying process that are often overlooked. In particular, structural collapse may occur if the material's frozen-state glass transition temperature (Tg') is exceeded. It was discovered that unidirectionally freeze cast collagen matrices were highly deformed following lyophilization, rendering them incapable of further use. In this study, modulated differential scanning calorimetry was performed to identify Tg's of unidirectionally freeze cast collagen scaffolds, and product temperatures during sublimation were recorded. It was observed that cast matrices from 0.5 to 0.05 M acetic acid (HAc) sublimed at a lyophilizer shelf temperature of -25 °C underwent structural collapse and exceeded their Tg's for the majority of the drying cycle. The use of a low pH suspension (0.5 M HAc) promoted the formation of a non-porous surface, which in turn contributed to the increase of the product temperature above its Tg' during drying. This study has revealed that use of a low shelf temperature (-40 °C) and a low HAc concentration (0.05 M) is effective in maintaining product temperatures under Tg' thereby preventing collapse in unidirectionally freeze cast collagen scaffolds.

Improving bone repair of femoral and radial defects in rabbit by incorporating PRP into PLGA/CPC composite scaffold with unidirectional pore structure.

In this study, a platelet-rich plasma poly(lactic-co-glycolic acid) (PRP-PLGA)/calcium phosphate cement (CPC) composite scaffold was prepared by incorporating PRP into PLGA/CPC scaffold with unidirectional pore structure, which was fabricated by the unidirectional freeze casting of CPC slurry and the following infiltration of PLGA. The results from in vitro cell experiments and in vivo implantation in femoral defects manifested that incorporation of PRP into PLGA/CPC scaffold improved in vitro cell response (cell attachment, proliferation, and differentiation), and markedly boosted bone formation, angiogenesis and material degradation. The incorporation of PRP into scaffold showed more outstanding improvement in osteogenesis as the scaffolds were used to repair the segmental radial defects, especially at the early stage. The new bone tissues grew along the unidirectional lamellar pores of scaffold. At 12 weeks postimplantation, the segmental radial defects treated with PRP-PLGA/CPC scaffold had almost recuperated, whereas treated with the scaffold without PRP was far from healed. Taken together, the PRP-PLGA/CPC scaffold with unidirectional pore structure is a promising candidate to repair bone defects at various sites.

凍結成形法で作製した多孔性ジルコニアセラミックスの微細構造に及ぼすPVA添加の影響

Porous ZrO2 ceramic was prepared by a unidirectional freeze casting using aqueous slurries containing PVA. The effects of PVA content on the microstructure of sintered bodies were investigated by a scanning electron microscopy and a mercury intrusion porosimetry. Porous ZrO2 ceramic with continuously long-ranged pore structure was obtained by the unidirectional freeze casting of the aqueous ZrO2 slurry, followed by sublimation of ice and firing at 1400℃ for 1h in air. The PVA loading of the slurry scarcely affected the pore volume of sintered bodies. However, the pore size decreased and the compressive strength increased with increasing PVA loading in the slurry. When PVA was added, branch and ladder structures occurred in pore walls. Therefore, three dimensional pore structures developed due to the increased in the connectivities of the pore walls, resulting in the decrease in pore size and the increase in compressive strength.

Effect of cooling rate and gelatin concentration on the microstructural and mechanical properties of ice template gelatin scaffolds

In the current study, a controlled unidirectional freeze casting method was employed to fabricate highly porous gelatin scaffolds. Different gelatin concentrations of 1, 3, and 5 wt% were dissolved in distilled water, and the constant value of glutaraldehyde cross-linking agent (0.5 wt%) was added to the solution. Then, the solutions freeze casted at different cooling rates of 1, 3, and 6°C/Min and freeze dried. Finally, pore morphology, mechanical properties, and water adsorption characteristics of scaffolds were assessed. Results showed that the increase in gelatin concentration caused the pore shapes to change from oblate and polygon to almost round but the cooling rate had no obvious effect on pore morphology. Compressive strength of the scaffolds improved as a function of increase in cooling rate and gelatin concentration from 20 to 1,150 kPa. The value of water adsorption was decreased with augmentation in gelatin concentration and cooling rate in the range of 2,000%-500%. Therefore, this study suggests that the use of a controllable freeze casting method and cooling rate can tailor the pore morphology, mechanical properties, and water adsorption of gelatin scaffolds and could be a novel approach to avoid the use of a toxic dose of a cross-linking agent like glutaraldehyde.

Bi-layered calcium phosphate cement-based composite scaffold mimicking natural bone structure

In this study, a core/shell bi-layered calcium phosphate cement (CPC)-based composite scaffold with adjustable compressive strength, which mimicked the structure of natural cortical/cancellous bone, was fabricated. The dense tubular CPC shell was prepared by isostatic pressing CPC powder with a specially designed mould. A porous CPC core with unidirectional lamellar pore structure was fabricated inside the cavity of dense tubular CPC shell by unidirectional freeze casting, followed by infiltration of poly(lactic-co-glycolic acid) and immobilization of collagen. The compressive strength of bi-layered CPC-based composite scaffold can be controlled by varying thickness ratio of dense layer to porous layer. Compared to the scaffold without dense shell, the pore interconnection of bi-layered scaffold was not obviously compromised because of its high unidirectional interconnectivity but poor three dimensional interconnectivity. The in vitro results showed that the rat bone marrow stromal cells attached and proliferated well on the bi-layered CPC-based composite scaffold. This novel bi-layered CPC-based composite scaffold is promising for bone repair.

Improvement of cell response of the poly(lactic-co-glycolic acid)/calcium phosphate cement composite scaffold with unidirectional pore structure by the surface immobilization of collagen via plasma treatment

In this study, calcium phosphate cement (CPC)-based scaffold with unidirectional lamellar pore structure was fabricated by unidirectional freeze casting. Poly(lactic-co-glycolic acid) (PLGA) was infiltrated into the CPC scaffold to improve its strength and toughness, which compromised the bioactivity and osteoconductivity of CPC. Collagen (Col) was immobilized on the pore surface of the PLGA/CPC scaffold to enhance the bioactivity of the scaffold using plasma treatment under the ammonia (NH3) atmosphere. The immobilization of collagen was characterized by infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Compared to the PLGA/CPC composite scaffold, the Col/PLGA/CPC composite scaffold had higher contact angle, porosity and water absorption, while the compressive strength of both scaffolds was comparable. Rat bone marrow mesenchymal stem cells (rMSCs) seeded on the Col/PLGA/CPC scaffold showed markedly improved cell seeding, attachment, proliferation and differentiation than those on the PLGA/CPC scaffold. These results suggest that the surface immobilization of collagen by plasma treatment can improve the bioactivity of the PLGA/CPC scaffold and the Col/PLGA/CPC composite scaffold is a promising candidate for bone tissue engineering.