Freeze Casting of Aqueous Alumina Slurries with Glycerol

Freeze-drying concepts were utilized in the shape forming of alumina parts by pressureless slip molding using aqueous slurries. A water solidification modifier, glycerol, was utilized to eliminate the defects associated with the expansion and ceramic particle rejection of water during freezing. Castable alumina slurries with solids loading up to 60 vol% were prepared and characterized using viscosity and zeta-potential measurements with and without glycerol additions. Frozen parts were dried under vacuum by sublimation of ice to obtain net-shape green bodies. The combined effects of high-solids-loading slurries, >57.5 vol%, and glycerol additions were essential for freeze casting to achieve highly dense alumina bodies with a uniform microstructure.     

Ceramic Bodies with Complex Geometries and Ceramic Shells by Freeze Casting Using Ice as Mold Material

Ceramic bodies with a complex shape and closed ceramic shells encapsulating other components like steel parts were fabricated by the freeze-casting technique using ice as a mold and as core material. The ice molds and cores were simultaneously removed with the frozen suspension liquid of the ceramic slip by subsequent freeze drying. In this way, cores can be eliminated from a closed shell by sublimation through the porosity of the shell. Moreover, the ice cores allowed to transfect other components into porous ceramic bodies. Complex dental parts such as steel rods encapsulated in porous zirconia shells achieved by this ice mold freeze casting are represented in this article.     

The controllable microstructure of porous Al2O3 ceramics prepared via a novel freeze casting route

In traditional aqueous slurry freezing casting processing, the growth method of ice crystals is hard to control, resulting in the uncontrollable pore's morphologies of the porous ceramics. In the experimental, the pure Al₂O₃ sol was used to substitute water as a medium for preparing ceramic slurry. With Al₂O₃ sol addition, it becomes easy to control the microstructure and pore's morphologies of the porous Al₂O₃ ceramics via adjusting of the solid loading, composition of the ceramic slurries, as well as the cooling methods. The SEM micrographs showed that the sol-contained ceramic slurry combined with freeze casting processing can easily prepare the porous Al₂O₃ ceramics with different pore sizes and different morphologies. The porous Al₂O₃ ceramics prepared from 70 wt.% to 90 wt.% solid loading sol-contained Al₂O₃ slurries and sintered at 1500 °C for 2 h have open porosities from 81.7% to 64.6%.     

Porous hydroxyapatite scaffolds produced by the combination of the gel-casting and freeze drying techniques

A technique combining gel-casting and freeze drying methods is introduced to prepare porous hydroxyapatite scaffolds which allow for better control of the scaffold microstructure and have improved mechanical properties. A monomeric system which is known to be a suitable gelling agent for setting ceramic suspensions into dense forms was selected to produce ceramic foams. Different concentrations of sodium lauryl sulphate solution were added into the hydroxyapatite gel suspension as a pore former. The effect of the solid content on the mechanical properties of the scaffold was also investigated. Rapid freezing with liquid nitrogen was performed according to the freeze drying technique and the porous structure and morphology of the scaffolds were analyzed by scanning electron microscopy. The mechanical properties of the hydroxyapatite scaffolds were determined by testing compressive strength using a universal testing machine. The prepared scaffolds were characterized by well-defined pore connectivity along with directional, uniform and completely open porosity. The maximum compressive strength of about 17?MPa obtained from the suspension consisted of 50% solid content with 20% concentration of sodium lauryl sulphate solution. The results show that sodium lauryl sulphate solution plays a significant role in changing the pore structure of hydroxyapaite scaffolds in systems having high solid content.     

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.     

冷冻干燥法制备层状多孔羟基磷灰石支架过程中的溶剂升华行为

采用水基羟基磷灰石(hydroxyapatite,HA)浆料,经冷冻干燥和烧结工艺(1 250℃烧结3 h)制备了层状多孔HA支架.研究了冷冻温度、干燥压力和干燥温度对水基HA浆料中溶剂升华行为的影响.结果表明:随着冷冻温度的降低,多孔HA支架的层间距逐渐减小,支架的升华时间增加;由于样品的干燥过程同时受到传热和传质的...     

Effect of particle size and freezing conditions on freeze-casted scaffold

Freeze casting is one of the emerging and novel manufacturing routes to fabricate porous scaffolds for various applications including orthopedic implants, drug delivery, energy storing devices etc. Thus, it becomes important to understand this process in a deeper sense. Present work was focused to study the effect/influence of basic parameters, particle sizes, and freezing conditions on the mechanical properties and microstructures of porous scaffold fabricated by freeze casting. β-tricalcium phosphate (β-TCP) and hydroxyapatite (HAp) powder with particle sizes of 10?μm and 20?nm were used. Prepared slurries were freeze casted at constant freezing temperature (5?°C) and constant freezing rate (1.86?°C/min) to study the effect of freezing conditions on mechanical and microstructural properties of the porous scaffold. It was observed that porous scaffold fabricated by nanoparticles has given better porosity (63.22–76.16%), than scaffold fabricated by microparticles (13–43.05%) at given solid loading of both freezing conditions. Although, the range of pore size of the scaffold fabricated by nanoparticles (CFR: 2.60–0.84?μm; CFT: 1.66–0.46?μm) was lower than that of scaffold fabricated by microparticles (CFR: 9.45–4.83?μm; CFT: 4.72–2.84?μm). The compressive strength of scaffolds prepared by nanoparticles was in the range of trabecular bone. Moreover, the results of present work will pave the way for the fabrication of porous scaffold with desired pore size and porosity for various implants, energy, and drug delivery applications.     

Tailored graded pore structure in zirconia toughened alumina ceramics using double-side cooling freeze casting

Ceramics with graded and continuously aligned open pores were investigated using a double-side cooling freeze casting setup. The ceramic preforms with tailored lamellae spacing (wavelength), wall thickness and graded pore structure were used to infiltrate with a second phase for the fabrication of graded interpenetrating phase composites. The effects of solid content, temperature setting and gradient, cooling rate and the introduction of electrophoretic deposition (EPD) on the freezing velocity of the ceramic suspension were analysed. On the bottom of the ceramic specimen, a dense layer was formed and tailored with the use of EPD. The ceramic was characterised by a graded open pore structure with wavelengths up to 115 μm and interconnected microstructure. The effect of solid content on the degree of supercooling and the effect of temperature gradient on the average freezing velocity were investigated. The addition of EPD before freeze casting affected significantly the microstructure, the wavelength decreased and the wavelength gradient became smaller compared to simple freeze casting.     

Generation of Large Pore Channels for Bone Tissue Engineering Using Camphene-Based Freeze Casting

The present study reports an innovative way to produce large pore channels with a size >100 μm for applications in bone tissue engineering using the camphene-based freeze casting method, and using an unusually high freezing temperature, which is close to the solidification temperature of the slurry, in order to allow the formation of excessively overgrown camphene dendrites due to the extremely low solidification velocity. To accomplish this, hydroxyapatite (HA)/slurries with various solid loadings (10, 15, and 20 vol%) were frozen at 35°C for 20 h. The frozen samples were freeze dried and sintered at 1250°C for 3 h. All of the fabricated samples showed highly porous structures with large pore channels >100 μm in size and dense HA walls without any noticeable defects, such as cracks or pores. As the initial solid loading was increased from 10 to 20 vol%, the porosity of the sample decreased linearly from 76% to 55%, while the pore channels became narrower. However, the compressive strength was remarkably improved from 2.5 to 16.7 MPa.     

TBA-based freeze/gel casting of porous hydroxyapatite scaffolds

Porous ceramic scaffolds with a controlled “designer” pore structure have been prepared by the freeze/gel casting route using a TBA-based hydroxyapatite slurry system with 20–40 wt.% solid content. The products were characterized in terms of sintered microstructure, together with physical and mechanical properties. After sintering at 1050–1250 °C, the advantages of freeze casting and gel casting appeared in the pore structure and compressive strength of the ceramics, i.e., unidirectional aligned macro-pore channels developed by controlling the solidification direction of the TBA solvent used in the freeze casting together with small diameter (micron sized) isolated pores formed in the dense outer walls of the pore channels when processed by gel casting. The sintered porosity and pore size generally resulted in a high solid loading giving low porosity and small pore size, this leading to higher compressive strengths. The scaffolds obtained exhibited an average porosity and compressive strength in the range 41.9–79.3% and 35.1–2.7 MPa, respectively, depending on the processing conditions used.     

Microstructure and mechanical properties of ZrB2–SiC porous ceramic by camphene-based freeze casting

Camphene-based freeze casting technique was adopted to fabricate ZrB₂–SiC porous ceramic with 3-dimensional (3D) pore network. ZrB₂–SiC/camphene slurries (initial solid loading: 20 vol%, 25 vol% and 30 vol%) were prepared for freeze casting. Regardless of initial solid loading, the fabricated sample had dense/porous dual microstructure. The thickness of dense layer was about 200–300 μm. The microstructures of ZrB₂–SiC porous ceramics were significantly influenced by the initial solid loading, which determines the pore size, porosity and mechanical properties of the final products.     

Ice templating of ZrB2 porous architectures

For the first time, freeze casting of aqueous ZrB₂ based suspension was performed in order to produce porous architectures with main unidirectional anisotropic pores interconnected by diffuse globular-isotropic pores. The porosity characterizing the green bodies after the sublimation process is the replica of the ice crystals. The effects of solid content, suspension stabilization and cold transmission depending on mold type were studied since micro and macrostructure and, hence, the properties of the sintered body are determined by controlling the growth direction of the ice crystals during freezing. Improved mechanical performances are obtained by changing the above mentioned parameters, in virtue of the formation of thinner and more homogenously distributed ceramic lamellae. Possible applications of these materials include high temperature porous volumetric absorbers for concentrating solar power systems.     

Preparation of porous TiO2 by a novel freeze casting

Highly porous and open interconnected pore structural TiO₂ were prepared by a novel freeze casting method. In the experiment, the well-dispersed aqueous slurries were first frozen, and then dried at a reduced vacuum. Since the sublimation of ice crystals developed in the freezing process, the green bodies with highly porous were obtained. The phase composition and the microstructure of the sintered samples were characterized by XRD, SEM, porosity and the pore size distribution was measured by mercury porosimetry. The results demonstrated that the PVA concentration in the slurries remarkably affect the microstructure of TiO₂ ceramics. The pore morphology of TiO₂ ceramics with 3 wt.% polyvinyl alcohol (PVA) addition was dendritic, and however, the pore morphology of TiO₂ ceramics with 6 wt.% PVA addition changed into columnar. The reason for the variation of the pore morphology was ascribed to the effect of the PVA gelation on the growth behavior of the ice crystals.     

Freezing Dilute Ceramic/Camphene Slurry for Ultra-High Porosity Ceramics with Completely Interconnected Pore Networks

Highly porous alumina ceramics with completely interconnected pore channels were fabricated by freezing dilute alumina/camphene slurries with solid loadings ranging from 5 to 20 vol%. This method fundamentally made full use of the three-dimensional camphene dendritic network for producing interconnected pore channels and the concentrated alumina powder network for achieving dense alumina walls. Firstly, alumina/camphene slurries were prepared at 60°C using ball milling and then cast into molds at 20°C. After subliming the frozen camphene, the samples were sintered at 1400°C for 5 h. This method enabled us to freeze very dilute ceramic slurries with a low solid loading of ≤20 vol% without the collapse of the sample after sintering. As the initial solid loading decreased from 20 to 5 vol%, the porosity linearly increased from 66% to 90% with an increase in the pore size, while completely interconnected pore networks were obtained in all cases. In addition, the free surfaces of the alumina walls showed full densification after sintering even at a low temperature of 1400°C, while some pores were present in the inner regions of the alumina walls.     

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

Freeze 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.     

Additive manufacturing of green ceramic by selective laser gasifying of frozen slurry

The slurry-based additive manufacturing (AM) of ceramics involves a drying process to form solid support; however, the drying process is time-consuming, and the support is not easily removed. We propose a new AM process for green ceramic that includes freezing a layer of aqueous ceramic slurry, laser gasifying of the frozen-layer ice to process 2D green ware, and removing the support in water to release the 3D ceramic part. With a suitable laser power and scanning speed, this approach can yield a layer that has a thickness of 90 μm, a cantilever structure with a wall thickness of 115 μm and a span of 30 mm without deflection. The casting layer cannot be damaged by using a cryopanel to rapidly freeze the slurry, and redundant frozen materials can be melted in water without swelling. Therefore, this new process can rapidly form a solid support and has a high removal efficiency.     

Fabrication of Gradient Pore TiO2 Sheets by a Novel Freeze–Tape-Casting Process

Gradient pore structure TiO₂ sheets were fabricated by a novel freeze–tape-casting process. Aqueous TiO₂ ceramic slurries were prepared by the traditional tape-casting processing and were then cast onto an aluminum foil carrier. The slurries were immediately frozen on the substrate, whose temperature was about −18°C. After freezing completely, the green sheets were then dried in a lyophilizer. Freeze–tape casting led to formation of a gradient pore microstructure of the TiO₂ sheet. The results showed that the solid loading of slurry considerably affected the pore microstructure, pore morphology, and the porosity. Solid loadings of 10, 20, 30, 40, and 50 wt% slurries were used, respectively, and the gradient pore structure TiO₂ sheets with different porosities of 75%–88% were obtained.     

Porous Ceramic Bodies with Interconnected Pore Channels by a Novel Freeze Casting Technique

Porous ceramic bodies with interconnected pore channels were fabricated by a novel freeze casting technique using camphene-based slurries. The pore channels are surrounded by almost fully dense walls and have nearly circular cross-sections. The pore volume fraction and the channel size were controllable by the solid content in the slurry. The channels are replicas of entangled dendrites of frozen camphene, which sublimed during the freeze-drying process. This porous structure with entangled pore channels is considered potentially useful in many applications such as implantable bone scaffolds.     

Slip casting using wet-jet milled slurry

Green bodies fabricated by slip casting using wet-jet milled slurries at different solid contents (10, 30, and 50 vol.%), which had low viscosity, had very high relative density and showed very small shrinkage during sintering, as comparing to the ball-milled slurries. The square of the thickness of green bodies was proportional to casting time indicating slip casting was implemented well. The relative density of the green body prepared by the wet-jet milled slurries was 67%, indicating much higher than that of the ball-milled slurries. After sintering, the linear shrinkage of the sintered bodies prepared by wet-jet slurries at different solid contents was constant at 11%. On the other hand, the linear shrinkage of the sintered bodies prepared by the ball-milled slurries was dependent on the solid contents in slurries. Thus, it was possible to produce not only green bodies with high density but also sintered bodies with low shrinkage by using the wet-jet milled slurries.     

Control of pore structure during freeze casting of porous SiC ceramics by different freezing modes

The frozen moulds, including homogeneous, unidirectional and bidirectional freezing, were designed using materials with different thermal conductivities, and the temperature variations of the moulds and samples during the freezing process were simulated by finite element analysis. Highly porous SiC ceramics with significant differences in pore structure were fabricated by using the SiC/water slurries prepared via uniform or oriented freeze casting with various freezing modes, and porosity and compressive strength of the as-fabricated ceramics were investigated. The results showed that the pore structure of ceramics prepared by homogeneous freezing was relatively intricate and inconsistent, and had a higher compressive strength. In contrast, the pore structure of ceramics fabricated using bidirectional freezing mode was more ordered and higher porosity was observed. Moreover, porous ceramics prepared by unidirectional freezing mode exhibited a typical gradient structure with increased pore size from tens of micrometers in the bottom to hundreds of micrometers in the top.