Effect of Polystyrene Addition on Freeze Casting of Ceramic/Camphene Slurry for Ultra-High Porosity Ceramics with Aligned Pore Channels

We investigated the effect of polystyrene (PS) addition on the freezing behavior of a very dilute alumina/camphene slurry with an initial solid loading of 5 vol% for the fabrication of ultra-high porosity ceramics with aligned pore channels. To accomplish this, slurries with various PS contents (10, 20, and 30 vol% in relation to the alumina powders) were prepared by ball milling at 60°C and then cast into molds at a constant temperature of 20°C. After removing the frozen camphene, the samples were heat treated to burn out the organic phases and sinter the alumina walls. The addition of the PS binder remarkably enhanced the green strength of the sample, making it possible to handle it without difficulty. All of the sintered samples showed ultra-high porosities of >88% without the collapse of the porous structure, wherein the pore channels were completely interconnected. Three distinctive zones having different pore structures were observed, viz. the outer shell with elongated pores, the inner zone with long aligned pore channels, and the center with equiaxed pores. The content of the PS polymer significantly affected the pore morphologies in the three distinctive zones. In addition, it was found that the addition of the PS polymer was highly beneficial to the alignment of the pore channels.     

Fabrication of Porous PZT–PZN Piezoelectric Ceramics With High Hydrostatic Figure of Merits Using Camphene-Based Freeze Casting

Porous lead zirconate titanate–lead zinc niobate (PZT–PZN) piezoelectric ceramics with interconnected pore channels were fabricated using the camphene-based freeze-casting method. In this method, warm PZT–PZN/camphene slurries with various solid loadings (10, 15, 20, and 25 vol%) were prepared by ball milling at 60°C and then cast into molds at 20°C, resulting in the formation of solidified green bodies comprised of three-dimensionally interconnected camphene dendrite networks and concentrated ceramic particle walls. After the removal of the frozen camphene via sublimation, the samples were sintered at 1200°C for 2 h. All of the fabricated samples showed highly porous structures, consisting of fully dense PZT–PZN walls without defects, such as cracks or pores. As the initial solid loading was decreased from 25 to 10 vol%, the porosity was linearly increased from 50% to 82%. This increase in the porosity led to a reduction in the permittivity, a moderate decline in the d ₃₃ value, and a rapid decline in the d ₃₁ value, which endowed the porous samples with a high hydrostatic figure of merit (HFOM). The highest HFOM value of 35650 × 10⁻¹⁵ Pa⁻¹ was achieved for the sample with a porosity of 82%, as well as ɛ₃₃=284, d _h =298 pC/N, and g _h =118 × 10⁻³ V·(m·Pa)⁻¹.     

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.     

Highly Aligned Porous Silicon Carbide Ceramics by Freezing Polycarbosilane/Camphene Solution

We fabricated highly aligned porous silicon carbide (SiC) ceramics with well-defined pore structures by freezing a polycarbosilane (PCS)/camphene solution. In this method, the solution prepared at 60°C was cast into a mold at temperatures ranging from 20° to −196°C, which resulted in a bicontinuous structure, in which each phase (camphene or PCS) was interconnected in a regular pattern. After the removal of the frozen camphene network, the samples showed highly porous structures, in which long straight and short elongated pore channels were formed parallel and normal to the direction of freezing, respectively. Thereafter, porous SiC ceramics were produced by the pyrolysis of the porous PCS objects at 1400°C for 1 h in a flowing Ar atmosphere, while preserving their mother pore structures having aligned pore channels.     

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.     

Fabrication of a Porous Bioactive Glass–Ceramic Using Room-Temperature Freeze Casting

The room-temperature freeze-casting method was used to fabricate porous bioactive glass–ceramics. In this method, a glass/camphene slurry prepared at 60°C was cast into a mold at 20°C, resulting in the production of a rigid green body that was comprised of three-dimensional dendritic camphene networks surrounded by highly concentrated glass powder walls. After the sublimation of camphene, the samples were sintered for 3 h at elevated temperatures ranging from 700° to 1100°C. As the sintering temperature was increased to 1000°C, the densification of the glass–ceramic wall was remarkably enhanced, while its highly porous structure was preserved. The sample sintered at 1000°C showed a high porosity of 53% and pore channels with a size of several tens of micrometers, as well as dense glass–ceramic walls. In addition, the fabricated samples effectively induced the deposition of apatite on their surfaces when immersed in simulated body fluid, implying that they are very bioactive.     

Liquid Phase Sintering of Alumina, III. Effect of Trapped Gases in Pores on Densification

The microstructure evolution and densification kinetics of alumina containing 10 and 20 vol% calcium aluminosilicate glass were studied, for sintering under vacuum and air at 1600°C. Residual porosity was always present in the air-fired samples. The kinetic analysis lent strong support to the notion that trapped gases inhibited the densification and limited the attainment of full density. The samples containing 20 vol% glass were able to reach full density during vacuum sintering. However, the samples containing 10 vol% glass contained some residual porosity even after vacuum sintering, which was attributed to the preferential volatalization of liquid phase.     

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.     

Highly porous ZrO2 ceramics fabricated by a camphene-based freeze-casting route: Microstructure and properties

A camphene-based freeze-casting method was adopted to create ceramics with aligned, equiaxed pores applied so far exclusively for ceramics—is demonstrated for ZrO₂ porous ceramics. The pore volume fraction, channel size and pore shape were controlled by varying the freezing temperature, solid content and sintering condition. After sublimation of camphene, the samples were sintered for 2 h at elevated temperatures ranging from 1400 to 1550 °C. The initial level of solid loading played a primary role in the resulting porosity of the product. The porosity decreased from 82.5 to 65.5 vol.% when the solid loading was increased from 10 to 20 vol.%. The relationship of the compressive strength versus initial solid loading and sintering temperature was discussed. This technique is considered potentially useful in fabricating novel porous ceramics with special structure, and introduces a new application field of freeze-casting.     

Preparation of porous alumina ceramic with ultra-high porosity and long straight pores by freeze casting

Porous alumina ceramic was prepared by freeze casting method using tert-butyl alcohol as the solvent. The as?Cprepared porous alumina ceramic possessed long straight porous structure. The non-dendrite pore feature was quite distinguished from that prepared based on common solvents such as water and camphene. The porosity of the ceramic could be regulated through the solid loading. When the solid loading in the slurry was 20?vol%, the porosity of the alumina ceramic was 65%. With decreasing the solid loading, the porosity of the alumina ceramic increased linearly. The relationship between the total porosity (P) and initial solid loading (X) can be expressed as P?=?98.8?1.7X. The ultra-high porosity of 82% could be achieved when the solid loading was 10?vol%. Moreover, the density of the porous alumina ceramic with the porosity of 82% was even lower than water??s. The compressive strength of the porous alumina ceramic with the porosity of 63 and 82% was determined to be 37.0 and 2.6?MPa, respectively.     

Novel camphene/photopolymer solution as pore-forming agent for photocuring-assisted additive manufacturing of porous ceramics

This study proposes camphene/photopolymer solutions as a novel pore-forming agent for the photocuring-assisted additive manufacturing of porous ceramics. Unlike conventional techniques using molten camphene, solid camphene can be directly dissolved in the photocurable monomer hexanediol diacrylate (HDDA) at room temperature, which can then crystallize with a dendrite-like morphology based on phase separation at lower temperatures. This unique approach allows alumina suspensions to solidify at ―2 °C and then effectively be photopolymerized using a digital light processing engine, resulting in camphene-rich crystals surrounded by photopolymerized alumina/HDDA walls. Sintered samples exhibited a highly porous structure, with the pores created after the removal of the camphene-rich crystals. Two different pore sizes were obtained in the lower and upper regions of a single layer, due to a decrease in the solidification rate along the building direction, although their porosities were similar (～ 52 vol%). The porous samples exhibited a compressive strength of ～ 265 MPa.     

Synthesis of Porous Si3N4 Ceramics with Rod-Shaped Pore Structure

Porous Si₃N₄ ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si₃N₄, 5 wt% Y₂O₃+2 wt% Al₂O₃, and 0–60% organic whiskers composed of phenol–formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800°C in air and sintered at 1850°C in a N₂ atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0–60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si₃N₄ matrix.     

Photocurable ceramic slurry using solid camphor as novel diluent for conventional digital light processing (DLP) process

This study presents the utility of solid camphor as a novel type of diluent for the preparation of photocurable ceramic slurries with sufficiently low viscosity at high solid loading (48 vol%), which can be applicable for the conventional digital light processing (DLP) process. The camphor addition remarkably decreased the viscosity of calcium phosphate (CaP) ceramic slurries without affecting their photopolymerization behavior. This approach could effectively mitigate the clogging of pores with residual slurries, and thus the porous structure of porous CaP scaffolds with 3D channels could be tightly controlled. Furthermore, the high densification of CaP frameworks after sintering at 1250 °C for 3 h could be achieved owing to the use of the high solid loading in the CaP slurry. The porous CaP scaffolds produced displayed high compressive strength (˜ 23.8 MPa) and modulus (˜ 276 MPa) at a high porosity of ˜ 50.6 vol%.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Freeze Casting of Porous Bioactive Glass and Bioceramics

Highly porous network structures of hydroxyapatite, tricalcium phosphates, a bioactive glass as well as their composites have been fabricated using variations of camphene-, glycerol-, and ice-based freeze-casting techniques. The ball-milled slurries containing 10%–60% solid loading were cast at ambient temperature, followed by sublimation at temperatures between −70° and 60°C. The green body was sintered in air to a maximum temperature of 1100°C for 4 h, which produced excellent three-dimensionally (3-D) interconnected structures with open pores. The nature of the pore channels varied from dendritic, columnar, and cellular to mixed geometry, with dense outer shells in some cases, depending on the particular method used. A monotonic increase in porosity with loading was observed with a decrease in the loading volume. Microstructural analysis was used for porous structures to extract information on geometry, porosity, and pore size distribution. This proved particularly useful to assess whether some of the 3-D structures produced by these methods are suitable for tissue engineering applications. Differential thermal analysis–thermogravimetry, scanning electron microscopy, X-ray diffraction, and density were used to characterize precursor powders, slurry, and sintered products.     

Fabrication and Characterization of Dual-Channeled Zirconia Ceramic Scaffold

A novel porous ceramic with a structure containing two three-dimensional (3D) pore channels in a tetragonal zirconia polycrystals (TZP) ceramic was fabricated using a combination of a CNC-machining method and slurry coating process. A graphite scaffold with a single interconnected 3D channel as a template was prepared using CNC machining and lamination. The surfaces of the graphite scaffold were then coated uniformly with the TZP slurry, followed by heat treatment at 900°C for 3 h in air to remove the graphite material completely via thermal oxidation and at 1400°C for 3 h in air to sinter the TZP walls. This process produced a dual-channeled TZP scaffold with an additional 3D channel, which replicated the 3D graphite structure with the pre-existing channel. The fabricated scaffold showed ultra-high porosity (91%), high surface area, and high compressive strength (2.04 MPa), as well as a tightly controlled pore structure.     

Near-net shape manufacture of macroporous nanosized alumina monolith by aqueous gel casting method

A potentially near-net-shape manufacturing procedure for macroporous alumina monoliths with 20%-75% porosity was presented by gel casting of nano γ-Al₂O₃. Although monolith obtained by nano alumina had a high surface area and low sintering temperature, an optimum fraction of micron alumina needed to be added to achieve the proper rheological and mechanical properties of slurries. The preparation parameters including alumina loading, sintering temperature, monomer concentration, and the fraction of nano alumina were investigated. The green densities ranging from 0.66 to 0.86 g cm⁻³ were obtained by raising alumina loading from 10 to 20 vol%. Depending on the monomer concentration and sintering temperature, the mean pore size ranging from 45 to 412 nm, total porosity (20%–75%), and open porosity varying from 12% to 89% were obtained. The sintered density (from 0.95 to 3.15 g cm⁻³) and compressive strength (CS) (from 4.2 to 31.46 MPa) were suitable for use in different fields.     

Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

The fabrication of composites formed by alumina grains (95 vol%) in the micrometer size range and aluminium titanate nanoparticles (5 vol%) by reaction sintering of alumina (Al₂O₃) and titania (TiO₂) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400 °C were established from dynamic sintering experiments. Specimens cooled at 5 °C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400 °C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5 °C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.     

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.     

Egg White as an Environmentally Friendly Low-Cost Binder for Gelcasting of Ceramics

Slurries with different solids loading (34–50 vol%) of alumina were made with egg white (ovalbumin)–water premix. The ovalbumin content in the premix was decreased with increase in solids loading to lower slurry viscosity. The slurries were cast into bulk shapes and heated to 80°C, resulting in denaturation of ovalbumin forming a gel. The gelled samples were dried under controlled humidity and sintered in air to densities 94%–97% of theoretical. Alumina slurries formed with egg white (no water) were used to form tapes as thin as 60 μm in the sintered state.