Directional Freeze Casting Research Articles

Enhanced compressive strengths and induced cell growth of 1-3-type BaTiO3/PMMA bio-piezoelectric composites

Barium titanate (BaTiO3) has been used as a bone implant material because of its piezoelectric properties and the ability to promote cell growth when combined with hydroxyapatite. However, the brittleness of BaTiO3 inhibits its use as a bone replacement material at load-bearing sites, and the reduction of BaTiO3 content in the composite reduces its piezoelectric effect on bone growth. In this study, we explored a preparation method, which included directional freeze casting and self-solidification of bone cement, to obtain 1-3-type BaTiO3/PMMA bio-piezoelectric composites with a lamellar structure. The lamellar BaTiO3 layer through the composite from the bottom to the top significantly improved the piezoelectric properties of the composite. In addition, the dendritic ceramic bridges on the BaTiO3 pore walls can improve the compressive strength and elastic modulus of BaTiO3/PMMA bio-piezoelectric composites with a lamellar structure. More importantly, it was found that polarized lamellar BaTiO3 could induce osteoblasts to grow in the direction of the BaTiO3 layers. When the width of the BaTiO3 layer was in the range of 8–21 μm, osteoblasts along the BaTiO3 layer showed well growth, which can be of great value for the production of biomimetic bone units.

Integrated porous cobalt oxide/cobalt anode with micro- and nano-pores for lithium ion battery

An integrated porous cobalt-oxide/cobalt (Co3O4/CoO/Co) anode was prepared by facile processes, including directional freeze casting of a Co foam and its partial thermal oxidation to Co3O4/CoO, for use as a high-capacity anode material for lithium-ion batteries (LIBs). The thermal oxidation created a nanostructured oxide layer, 0.4 μm in thickness, on the surface of aligned, interconnected Co lamellae comprising the foam. In this electrode design, the Co foam was used as the current collector, and the nanowall-like Co oxide (CoO and Co3O4) layers acted as the anode that reacted with lithium ions during discharging and charging. The integrated porous Co3O4/CoO/Co anode exhibits highly reversible capacity of 989 mAh g−1 after 50 cycles with an coulombic efficiency of 99.4%, which is superior to that of the conventional Co foil anode (245 mAh g−1). The integrated porous Co3O4/CoO/Co architecture demonstrated in this study has promising potential applications for self-supporting advanced anodes with tailored macro- and microstructures for high capacity LIBs.

Co/C@cellulose nanofiber aerogel derived from metal-organic frameworks for highly efficient electromagnetic interference shielding

• 3D Co/C@CNF aerogel is designed for EMI shielding. • The lightweight aerogel shows excellent EMI shielding effectiveness. • It features absorption-dominated EMI shielding mechanism. • The effects of dielectric loss and magnetic loss promote EM waves attenuation. A three-dimensional (3D) ultralight Co/C@CNF aerogel is fabricated by directional freeze-casting of cellulose nanofiber (CNF) anchored Zeolitic Imidazolate Framework-67 (ZIF-67) and subsequently annealing process of ZIF-67/CNF aerogel at different temperature from 700℃ to 900℃. The resultant Co/C@CNF aerogels are interconnected carbon sheet networks inlaid with Co/C nanoparticles, and the corresponding EMI shielding properties are proved to be dependent on the annealing temperature. Remarkably, the Co/C@CNF aerogel at 900℃ displays high EMI shielding effectiveness of 35.1 dB at a density of only 1.74 mg cm −3 , corresponding to the highest specific SE reaching 20172.4 dB cm 3 g −1 , and an extremely high absorption characteristic. This good performance is benefiting from magnetic nanoparticles of Co embedded in carbon sheet and 3D interconnected networks, enhancing magnetic loss and dielectric loss ability. This study offers a facile approach to design novel aerogels with ultra-high absorption for highly efficient EMI shielding application in aerospace, portable electronics, and so on.

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.

Fabrication of ultralight 3D porous composite for Ag nanowire/cellulose nanofiber with tunable mechanical and electrical properties via directional freeze casting

Use of nanoscale architecturing that exhibits superior strength-to-weight ratio has been of recent interest, and here we demonstrate a bulk fabrication of a fully recoverable, ultralight 3D porous composite with high strength composed of Ag nanowire/cellulose nanofiber using a freeze-casting method. A one-step process for highly efficient bulk formation of 3D porous structure via ice crystal formation followed by sublimation was demonstrated that can overcome the cost and scalability associated with lithography methods. 3D porous composite with controlled geometry was fabricated by controlling the nucleation and growth kinetics of ice formation that resulted in highly anisotropic compressive strength and resilience that depends on the wall orientations and composition of composite wall. Strength of the 3D porous composite increased with an increase in the Ag nanowire content as the deformation transitioned from bending dominant toward stretch dominant behavior for the case of vertically aligned walls, and an optimized concentration of Ag nanowires resulted in the compressive strength of 100 kPa at a relative density of 0.96%, which has 1.5 times higher strength when normalized by the material density (2.3 ± 0.2 MPa⋅cm3/g) in comparison to that of metal microlattice (1.7 MPa⋅cm3/g) fabricated by lithography methods. Horizontally oriented 3D porous structure interestingly showed a fully reversible deformation while also maintaining sufficient conductivity that makes this new material well-suited for a variety of flexible electronics applications.

Direct Cryo Writing of Aerogels via 3D Printing of Aligned Cellulose Nanocrystals Inspired by the Plant Cell Wall

Aerogel objects inspired by plant cell wall components and structures were fabricated using extrusion-based 3D printing at cryogenic temperatures. The printing process combines 3D printing with the alignment of rod-shaped nanoparticles through the freeze-casting of aqueous inks. We have named this method direct cryo writing (DCW) as it encompasses in a single processing step traditional directional freeze casting and the spatial fidelity of 3D printing. DCW is demonstrated with inks that are composed of an aqueous mixture of cellulose nanocrystals (CNCs) and xyloglucan (XG), which are the major building blocks of plant cell walls. Rapid fixation of the inks is achieved through tailored rheological properties and controlled directional freezing. Morphological evaluation revealed the role of ice crystal growth in the alignment of CNCs and XG. The structure of the aerogels changed from organized and tubular to disordered and flakey pores with an increase in XG content. The internal structure of the printed objects mimics the structure of various wood species and can therefore be used to create wood-like structures via additive manufacturing technologies using only renewable wood-based materials.

Structural evolution of directionally freeze-cast iron foams during oxidation/reduction cycles

Cyclical oxidation/reduction behavior of iron-based powders and porous pellets is of great interest for iron-air batteries, steam-iron, and chemical looping processes, but extended cycling is limited by degradation via sintering or pulverization. To address these problems, we use directional freeze casting to fabricate porous iron foams, consisting of colonies of parallel iron lamellae and open channels of sufficient width (20–40 and 20–50 μm, respectively) to accommodate iron/iron oxide volume changes during redox cycling. Iron foams of three different initial channel porosities (48, 61 and 65 vol.%) are fabricated via water-based freeze casting of Fe2O3 powders followed by reduction with H2 and sintering. The evolution of these iron foams is examined after 5 and 10 redox cycles between Fe3O4 and Fe at 800 °C (via steam and H2) using optical microscopy, scanning electron microscopy, and synchrotron X-ray tomography. Redox cycling causes a macroscopic foam shrinkage as the iron lamellae grow closer together, decreasing (and even sometimes eliminating) the channel width between lamellae. Smaller micropores within individual iron lamellae are partially preserved, consistent with new porosity formation via vacancy diffusion and clustering in the oxide phase. Additionally, a dense Fe shell forms on the exterior surface of most samples, caused by lamellae contacting and sintering during oxidation, followed by formation of an impermeable Fe layer during reduction. Strategies are proposed to reduce both channel constriction and shell formation, which are undesirable as they restrict gas phase transport.

Porous Titanium Scaffolds with Aligned Lamellar Pore Channels by Directional Freeze-Casting from Aqueous TiH2 Slurries

Porous titanium scaffolds with aligned lamellar pore channels and smooth inner walls were fabricated by directional freeze casting using aqueous TiH2 slurries. The formation of dendrites was inhibited by increasing the TiH2 content in slurry. The pore structures and compression strengths of porous titanium scaffolds that were made with different solid contents were characterized. The formation mechanism of aligned lamellar pore channels and the reason for the improvement of compression strength were analyzed. With increasing TiH2 content in slurry, the open porosity reduced from 38.57 to 29.03% while lamellar spacing decreased. Young’s modulus was decreased with the increasing open porosity. Directional lamellar pore channels were obtained by controlling the TiH2 content in slurry, which can reduce the number of branches and prevent stress concentrations during compression. Also, the porous titanium scaffolds prepared by this process had a good biocompatibility with osteoblasts.

Fabrication of porous alumina by directional freeze casting and annealing in TBA–water–sucrose system

The volume change difference between water and tert-butyl alcohol (TBA) hydrate was approximately 10.1% when ceramic slurry with water–TBA was frozen for fabrication of porous ceramics. Residual stress may be produced at the pore walls and led to low compressive strength of the ceramics. In this regard, sucrose was used in the ceramic slurry. Several water–TBA crystals were observed in the sucrose amorphous bodies after directional freezing. Water and TBA in the sucrose amorphous bodies were recrystallized during subsequent annealing. Sucrose was used as a protective agent and annealing additive in the TBA–water–sucrose system to eliminate residual stress and change the pore structure from snowflakes and dendrites into tiny cylinders. Adding sucrose and annealing process exerted minimal effect on the open porosity of porous ceramics but remarkably improved their compressive strength. When TBA and sucrose contents were 30 and 20 wt%, respectively, in the ceramic slurry with 20 vol% alumina content, the compressive strength of porous ceramics was 49.9 MPa after annealing at − 15 °C for 2 h. The value was 3.6-fold higher than that of the ceramic without annealing. Results provide a new method for improving the mechanical properties of porous ceramics prepared by freeze casting.

Anisotropic sintering behavior of freeze-cast ceramics by optical dilatometry and discrete-element simulations

Directional freeze-casting of ceramic slurries followed by freeze drying and partial sintering results in materials with highly anisotropic properties parallel and transverse to the freezing direction. Physical measurements and optical dilatometry confirm that, during sintering, freeze-cast structures experience more strain along their freezing direction than transverse to it. Discrete Element (DEM) simulations of equivalent freeze-cast structures confirm this behavior. These simulations indicate that not only is sintering anisotropic on the macroscopic scale but within the walls and macropores themselves. It was determined that the anisotropic particle contact network that resulted from the aligned macropores led to anisotropic shrinkage during sintering.

High-performance 3D directional porous LiFePO4/C materials synthesized by freeze casting

The directional freeze casting technology has received increasing attention as a processing method to fabricate the aligned porous materials due to the advantages of easy operation and free of byproduct. Here, we innovatively applied the directional freezing technology to synthesize the 3D directional porous LiFePO4 materials. The distinctive 3D directional porous structure of LiFePO4 is templated from the aligned ice crystals frozen under the temperature gradient in liquid nitrogen. When used as a positive electrode material in lithium-ion batteries, the directional porous LiFePO4/C composite exhibits high reversible capacities of 162 mA h g−1 at 0.1 C and 72 mA h g−1 at 10 C, and a high capacity retention of 93.3% after 800 cycles at 1 C. The 3D directional porous LiFePO4/C is promising for application in lithium-ion batteries and the synthesis strategy could be potentially extended to prepare other high performance electrode materials.

Annealing effects on the pore structures and mechanical properties of porous alumina via directional freeze-casting

The effects of the annealing methods and annealing temperatures on the pore structures and mechanical properties of porous alumina were investigated. The amorphisation behavior and solidification behavior of the sucrose solutions during annealing were discussed. The pore morphology of porous alumina changed noticeably after uniform annealing. As annealing temperature increased from −25 ℃ to −5 ℃, the pore morphology of porous alumina changed gradually from irregular lamellar channels to circular channels. After directional annealing, the pore morphology of porous alumina was similar to that after uniform annealing; however, the uniformity of pore channels and the density of pore walls were increased. During directional annealing at −15℃, the compressive strength of porous alumina reached 58.8 MPa, which was 35% higher than that of unannealed porous alumina.

Assembling 2D Mxene into 3D Ordered Structures for High Power Li-Ion Capacitors

MXenes are a family of two-dimensional (2D) materials with interesting electrical and electrochemical properties and their high potential as electrode materials for Li-ion capacitors has been recently demonstrated. However, the conventional methods for electrode assembly and device fabrication are limiting factors in exploiting the full potential of these electrode materials. The research on the development of electrochemical energy storage devices in the past few years has led to the realization that improved performance can be achieved by assembling the electrode materials into three-dimensional (3D) architectures instead of the conventional 2D “laminate” architecture. In this talk, we present a simple, efficient, and scalable method to assemble 2D Ti3C2 into ordered 3D and porous aerogel structures with high specific capacitances and rate capabilities. MXenes aerogels are fabricated by directional freeze casting and show large specific surface areas. The porous structure of MXene aerogels is dependent on the processing conditions and affect their mechanical and electrochemical properties. MXene aerogels show significantly high specific capacities (1200 mAh/g at 0.05 A/g), excellent rate capability (200 mAh/g at 10 A/g) and outstanding cycling performance for reversible lithium-ion energy storage. We believe that the fabricated 3D MXene structures are promising for a broad range of applications.

Directional freeze-cast hybrid-backbone meso-macroporous bodies as micromonolith catalysts for gas-to-liquid processes

Directional freeze casting enables the assembly of bimodally porous micronomolith Fischer–Tropsch catalyst bodies with enhanced effective thermal conductivity and pore molecular transport rates, leading to a high selectivity to liquid olefin products.

Lightweight spongy bone-like graphene@SiC aerogel composites for high-performance microwave absorption

Lightweight spongy bone-like graphene@silicon carbide (SiC) aerogels were fabricated by the directional freeze-casting of graphene oxide (GO) coated SiC whiskers slurry and thermal reduction of GO@SiC aerogels. The unique hierarchically ordered structure of graphene@SiC aerogels is formed by graphene wrapped SiC whiskers, such a structure possess great advantages such as low density (72 mg/cm3) and high microwave absorption performance for aerogels. The minimum reflection loss of typical sample achieves −47.3 dB at 10.52 GHz with a thickness of 3 mm, and the effective bandwidth of reflection loss ≤ −10 dB can reach 4.7 GHz. These results indicate that the novel graphene@SiC aerogels is an optimal material for high-performance microwave absorption application in aeronautics.

Fabrication of lamellar porous alumina with graded structures by combining centrifugal and directional freeze casting

Lamellar porous alumina with graded structures was fabricated through a novel method of simultaneous centrifugal casting and directional freeze casting. Alumina particles in ceramic slurry moved outward from the center by centrifugal force and formed stable gradient distribution because of viscous resistance and electrostatic repulsion among particles. The porous and pore-size gradient structure were obtained after freeze drying. With increased rotational speed, the pore size of the lamellar pore channels also gradually increased. However, the sizes of the ice crystals and lamellar pores shrunk at 200rpm due to the destabilization of laminar flow during ice-crystal growth. Compressive strength obviously improved with increased rotational speed, and that of the resulting porous ceramic was 2.3 times higher than that in the absence of centrifugation at 1000rpm rotational speed.

Iron foams created by directional freeze casting of iron oxide, reduction and sintering

Iron foams with aligned, lamellar channels were created by directionally freeze-casting slurries of water and Fe2O3 nanopowders, sublimating the ice, and reducing and sintering under hydrogen the powders accumulated in the interdendritic spaces. Foam porosity decreases from 85 to 50% with Fe2O3 slurry fractions increasing from 14 to 23vol.%. For foams created from 18.5vol.% Fe2O3 slurries, with increasing sintering temperature from 900 to 1100°C, a decrease in porosity from 71 to 61% and an increase in unidirectional compression strength from 8 to 20MPa are observed.

Self-supported ceramic substrates with directional porosity by mold freeze casting

Manufacture of thin-film ceramic substrates with high permeability and robustness is of high technological interest. In this work thin (green state thickness ∼500μm) porous yttria-stabilized zirconia self-supported substrates were fabricated by pouring stable colloidal aqueous suspensions in a mold and applying directional freeze casting. Use of optimized suspension, cryoprotector additive and mold proved to deliver defect free ceramic films with high dimensional control. Microstructure analysis demonstrated the formation of desirable aligned porosity at macro-structural scale and resulted to be highly dependent on colloidal behaviour and freeze casting conditions. Manufactured green films were joined by lamination at room temperature and sintered to obtain symmetrical cells consisting of two porous self-supported substrate electrodes (∼420μm) and dense yttria stabilized zirconia electrolyte (∼10μm).

Synthesis, structure and mechanical properties of ice-templated tungsten foams

Abstract

The role of CuO–TiO2 additives in the preparation of high-strength porous alumina scaffolds using directional freeze casting

Lamellar porous Al2O3 scaffolds with initial solid loading of 30 vol% were prepared by freeze casting using micron-sized Al2O3 powders as raw material and CuO–TiO2 additives as sintering aid. The effects of the composition of CuO–TiO2 on the microstructure, porosity and compressive property of the Al2O3 scaffolds were investigated and the mechanisms for sintering promotion addressed. The sintering aid effect was prominent when CuO:TiO2 was 1:2 in mass and their amount reached 3 wt% of the total ceramic powders. The corresponding compressive strength reached 176 ± 20 MPa with the porosity being 45 ± 1 % after sintering at 1450 °C for 2 h. Activation of the Al2O3 lattice due to partial substitution of Ti4+ for Al3+ and formation of a low-melting eutectic liquid were presumed to play a significant role in the sintering and strengthening of the Al2O3 scaffolds.