Preparation of high strength lamellar porous calcium silicate ceramics by directional freeze casting

In this study, porous calcium silicate (CS) ceramics with oriented arrangement of lamellar macropore structure were prepared by directional freeze casting method. The lamellar macropores were connected by the micropores on the pore wall, which had good pore interconnectivity. The effects of solid loading of the slurry, freezing temperature, sintering additive content, and sintering temperature on the microstructures and compressive strength of the synthesized porous materials were investigated systematically. The results showed that with the increase of solid loading (≤20 vol%) and sintering additive content, the sizes of lamellar pores and pore walls increased gradually, the open porosity decreased and the compressive strength increased. The sintering temperature had little effect on the pore size of the ceramics, but increasing the sintering temperature (≤1050 °C) promoted the densification of the pore wall, reduced the porosity, and improved the strength. The decrease of freezing temperature had little effect on porosity, but it reduced the size of lamellar pore and pore wall, so as to improve the strength. Finally, porous CS ceramics with lamellar macropores of about 300–600 μm and 2–10 μm micropores on the pore wall were obtained. The porous CS ceramics had high pore interconnectivity, an open porosity of 66.25% and a compressive strength of 5.47 MPa, which was expected to be used in bone tissue engineering.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

The sound absorption performance of the highly porous silica ceramics prepared using freeze casting method

Highly porous silica ceramics with unidirectional pores were prepared using the freeze casting method. By adjusting the solid content and freezing temperature, the porosity of the ceramics was tailored in the range of 78.20%-84.59% and pore size in the range of 8.4-71.4 μm, respectively. Sound absorption properties of porous silica ceramics was studied and the effect of structural factors was systematically investigated. The results showed that higher porosity and smaller pores of the porous ceramics favored the sound absorption in the entire sound wave frequency. By backing the sample with small pore size porous ceramics, the sound absorption property was enhanced, particularly in the low and medium frequency range, thus the sound absorption peak shifted towards lower frequency. The presence of air gap in the back would also favor sound absorption in low and medium frequency range. The as-fabricated porous silica ceramics owed excellent sound absorption properties due to their unidirectional pores and low flow resistances.     

Porous asymmetric microfiltration membranes shaped by combined alumina freeze and tape casting

An assembled asymmetric alumina microfiltration membrane with high performance was prepared by combining freeze and tape casting techniques followed by two sintering steps. Freeze casting was used for manufacturing of the porous support layer with a highly interconnected pore network. Tape casting was applied on the top layer to form a pre-membrane with smaller pore size and controlled thickness, which was set on the sintered support. Morphology influences were investigated for different solid loadings, additives content and the assembled layer membrane structures. No delamination among the layers was observed. The assembled ceramic membrane had an average pore size between 30 and 50 μm together with a top surface layer around 0.35 μm, which is suitable to the microfiltration separation process. Porosity in the range of 26–50 % and water flux of 11–32 m³ m^?2 h^?1 bar^?1 were reached for samples prepared with two sintering steps at 1600 and 1300 °C for 2 h.     

Intrinsic microstructural anisotropy in tape cast ceramics by ice templating

Ice templating concepts based on tape cast processing technology are capable of creating continuous columnar-graded pore structures using a wide variety of ceramic oxides with thicknesses from 100 µm up to 1 cm for applications including fuel cells, compositional grading templates, filtration/separation membranes, and catalyst supports. The merging of the tape casting process allows for the preparation of large area flexible green tapes as well as the long-range alignment of the acicular ice crystals transverse to the cross-section, yielding an additional degree of microstructure control beyond traditional freeze cast processing. Moreover, optimization of the freeze tape casting processing parameters has proven effective in tailoring porosity in aqueous cast ceramics. Magnetic resonance microscopy (MRM) combined with scanning electron microscopy (SEM) has revealed inherent microstructural anisotropy in the morphology of pores related to the direction of the cast tape. Due to the ability of MRM to image noninvasively with volume averaging over a range of slice thicknesses, the MRM images provide complementary information to the higher resolution single plane SEM images. Morphological variance as observed at different orientations through the cross-sections of cast yttria stabilized zirconia is reported.     

Optimization of non-aqueous tape casting of high solid loading slurry for aluminum nitride ceramic substrates

Aluminum nitride (AlN) ceramic substrates have been fabricated using non-aqueous tape casting and pressureless densification under flowing N₂ atmosphere. Considering the economic and environmental impact, a new strategy of solvent and dispersant system was adopted to prepare AlN slurries with high solid loading. According to the viscosity characteristics of AlN slurries, dispersant content was adjusted to be 0.5 wt% of AlN powder in order to optimize the rheological behavior of AlN slurries. The addition contents of binder and plasticizer were both optimized as 5 wt% of AlN powders by combining the viscosity of slurries and tensile strength of green tapes. Green AlN tapes were fabricated with an optimized tape casting process such as dry temperature. The exclusion process of organic additives was investigated by employing thermogravimetric analysis. Flat and dense AlN ceramic substrates with a relative bulk density over 99.75 % were achieved after being sintered under 1800°C for 6 hours, which had a maximum size of 110 × 110 mm. The thermal conductivity of the AlN substrate could reach 145 Wm⁻¹K⁻¹.     

Microstructure of TiO2 porous ceramics by freeze casting of nanoparticle suspensions

During freeze casting of TiO₂ porous ceramics, the porous architecture is strongly influenced by TiO₂ particle size, solids loading, and cooling temperature. This work investigates the influences of particle size, freezing substrate, and cooling temperature on the TiO₂ green bodies prepared by freeze casting. The results show that the lamellar channel width with 100 nm particles is larger than that of 25 nm particles, yet the ceramic wall thickness is noticeably decreased. The lamellar structure is more ordered when using a copper sheet than glass as its freezing substrate. A finer microstructure results when frozen at − 50 ℃ than − 30 ℃. Such porous materials have application potentials in a wide range of areas such as photocatalysis, solar cells, and pollutant removal and should be further studied.     

Microwave hybrid and conventional sintering of Al2O3 and Al2O3/ZrO2 multilayers fabricated by aqueous tape casting

In this study, microwave hybrid sintering and conventional sintering of Al₂O₃- and Al₂O₃/ZrO₂-laminated structures fabricated via aqueous tape casting were investigated. A combination of process temperature control rings and thermocouples was used to measure the sample surface temperatures more accurately. Microwave hybrid sintering caused higher densification and resulted in higher hardness in Al₂O₃ and Al₂O₃/ZrO₂ than in their conventionally sintered counterparts. The flexural strength of microwave-hybrid-sintered Al₂O₃/ZrO₂ was 70.9% higher than that of the conventionally sintered composite, despite a lower sintering temperature. The fracture toughness of the microwave-hybrid-sintered Al₂O₃ increased remarkably by 107.8% despite a decrease in the relative density when only 3 wt.% t-ZrO₂ was added. The fracture toughness of the microwave-hybrid-sintered Al₂O₃/ZrO₂ was significantly higher (247.7%) than that of the conventionally sintered composite. A higher particle coordination and voids elimination due to the tape casting and the lamination processes, the microwave effect, the stress-induced martensitic phase transformation, and the grain refinement phenomenon are regarded as the main reasons for the mentioned outcomes.     

Optimization of the tape casting process for the development of high performance silicon nitride substrate

In this paper, tape casting of Si₃N₄ substrate were investigated and optimized. The effects of dispersant content, binder, plasticizer/binder ratio, and solid loading on the green sheet properties were studied. An optimal formulation for the tape casting slurries was proposed, green tape with homogeneous microstructure and higher relative density of 56.08% was developed. After gas‐pressure sintering and annealing, Si₃N₄ substrate with a relative density of above 99% and thermal conductivity as 58 W/m/K was obtained. Results showed that the combination of tape casting and gas‐pressure sintering is feasible for the development of Si₃N₄ circuit substrates for power electronic devices.     

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.     

Fabrication of porous silica with controllable and tunable porosity via freeze casting

Porous ceramics offer unique properties that can bring advances to many application areas. The freeze-casting process has a strong potential for fabricating porous ceramics; however, the effects of process parameters on part porosity must be well understood for scalable manufacturing via freeze casting. This paper presents an experimental analysis of the freeze-casting process that correlates the freeze-casting parameters with pore characteristics. A full-factorial design of experiments is conducted on a unidirectional freeze-casting testbed using silica as the ceramic material and camphene as the solvent. The effects of solid loading, particle size, cooling temperature, and the distance from the cooling surface on porosity characteristics are evaluated. The fabricated samples are cross-sectioned vertically and horizontally and imaged using scanning electron microscopy. Image processing is used to obtain the porosity characteristics of areal porosity, pore size, pore shape, and pore orientation. The capability to steer the pore orientation is also demonstrated through bidirectional freezing experiments supported by a finite-element model. As a result, a quantitative understanding of the effects of freeze-casting process parameters on porosity characteristics is gained for the silica–camphene system. These results and the presented approach can be used for reproducible manufacture of porous ceramics with controlled porosity.     

Aqueous tape casting of B4C ceramics

B₄C green tapes are prepared by aqueous tape casting and a spark plasma sintering (SPS) process using polyethylenimine (PEI) as dispersant, hydroxypropyl methyl cellulose (HPMC) as binder and polyethylene glycol (PEG) as plasticiser. The influences of solid content, dispersant content, mass ratio of plasticiser to binder (R value) and milling time on the slurry viscosity are studied. The samples are characterised by means of hardness tester, universal testing machine and scanning electron microscopy. The results indicate that the solid content of B₄C slurry achieves 47.5?wt-% with milling time of 12?h when the content of PEI, HPMC and PEG is 1.5, 5 and 5?wt-%, respectively. The relative density of B₄C ceramics subject to SPS at 1600°C and 50?MPa for 8?min is up to 97.2%. The Vickers hardness, flexural strength and fracture toughness of B₄C ceramics reach 36.5?±?0.7?GPa, 510.3?±?19.4?MPa and 5.04?±?0.29?MPa?m^?1/2, respectively.     

Microstructure and permeability of porous YSZ ceramics fabricated by freeze casting of oil-in-water suspension

Porous yttria-stabilized zirconia (YSZ) ceramics are fabricated through freeze casting of oil-in-water suspension followed by sintering at 1250−1550 °C. The pore structure, compressive strength and permeability of porous YSZ ceramics are tailored via altering the emulsion content and sintering temperature. The samples obtained using higher emulsion content or at lower sintering temperature show larger Darcian and non-Darcian constants due to their higher open porosity and larger pore size. Furthermore, the investigation on individual contributions of viscous and inertial resistances on the total pressure drop during permeation process indicates that the viscous resistance increases but the inertial resistance decreases with increasing the emulsion content or decreasing the sintering temperature for samples. Porous YSZ ceramics obtained in this work with a k₁ range of 3.14 × 10⁻¹³–1.12 × 10⁻¹² m² are appropriate for applications in filters and membrane supports.     

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.     

New multifunctional porous Yb2SiO5 ceramics prepared by freeze casting

New porous Yb₂SiO₅ ceramics were prepared by a water-based freeze casting technique using synthesized Yb₂SiO₅ powders. The prepared porous Yb₂SiO₅ ceramics exhibit multiple pore structures, including lamellar channel pores and small pores, in its skeleton. The effects of the solid content and sintering temperature on the pore structure, porosity, dielectric and mechanical properties of the porous Yb₂SiO₅ ceramics were investigated. The sample with 20 vol% solids content prepared at 1550 °C exhibited an ultra-low linear shrinkage (i.e. 4.5%), a high porosity (i.e. 79.1%), a high compressive strength (i.e. 4.9 MPa), a low dielectric constant (i.e. 2.38) and low thermal conductivity (i.e. 0.168 W/(m K)). These results indicate that porous Yb₂SiO₅ ceramics are good candidates for ultra-high temperature broadband radome structures and thermal insulator materials.     

Application of polypropylene carbonate for the tape casting of silicon nitride ceramics

QPAC40 (polypropylene carbonate), with a little decomposition residue, is commonly used as a binder in aluminum nitride (AlN) tape casting. In this paper, we tried to explore its application in silicon nitride (Si₃N₄) tape casting. By studying the influence of dispersant, binder, plasticizer/binder ratio, and solid loading on slurry and green tape properties, the optimum formulation of the tape casting of Si₃N₄ slurry was determined, and the green tape with a uniform structure and relative density up to 63.16% was prepared. Si₃N₄ ceramics were obtained by debinding at 600°C for 1 h in vacuum and gas-pressure sintering at 1830°C for 2 h in N₂. The thermal conductivity and flexural strength of Si₃N₄ ceramics were 56.28 ± 1.21 W/(m·K) and 1130.67 ± 23.58 MPa, respectively. These results indicated that QPAC40 can be used to prepare Si₃N₄ sheets through tape casting.     

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.     

Characterization of functionalized zirconia membranes manufactured by aqueous tape casting

In this study, porous zirconia membranes were developed by aqueous tape casting. The influence of poly (methyl methacrylate) (PMMA) as a pore former, and sintering temperatures (1300, 1400, and 1500 °C) on open porosity and pore size was investigated. The rheological behaviour of the suspensions was measured. The slurries showed pseudoplastic behavior, which is desirable for tape casting. Functionalization with an amino silane precursor (3-aminopropyl triethoxysilane, APTES) was carried out to increase the hydrophilic properties of the membranes. The functionalized samples were characterized by SEM-EDX to identify the moieties attached to the surface. Membranes with open porosity ranging from 27% to 51% and average pore sizes from 0.2 to 1.4 μm were obtained. Samples sintered at 1400 °C with added pore former yielded the highest water flux, 257 Lm⁻²h⁻¹, which increased to 642 Lm⁻²h⁻¹ after functionalization. Membranes with tailored porosity and pore size obtained in this study are indicated for applications involving separation processes, especially for microfiltration systems.     

Dendritic porous alumina with high porosity by directional freeze casting using a binary solution for bacterial removal

High-porosity dendritic porous alumina was fabricated by using tertiary butanol (TBA) hydrate crystals combined with directional freeze casting. The porosity of this porous alumina approximated 80 %, and its high porosity resulted in high water flux. Dendritic pores improved the physical interception capability of porous ceramics due to the intrinsic moving paths and intercepts from the pore structure. Changes in the TBA content (from 70 vol.% to 85 vol.%) caused a change in pore size from 36.58 μm to 11.54 μm and pore structure (change order: snowflake, dendritic, rod-like, and needle-like), which are important factors affecting water flux and interception capability. The interception and removal of Escherichia coli by 7 mm-height porous ceramics with dendritic structure and an average pore size of 27.90 μm reached 100 % at pH 7.2. This study provides a simple and low-cost method for the effective removal of bacteria.     

Effect of additives on porosity of alumina materials obtained by freeze casting

The influence of two different additives, glycerol and polyvinyl alcohol (PVA), on the rheological behaviour and freeze casting performance of 35?vol.-% aqueous alumina suspensions is studied. Suspensions with PVA were prepared by either mixing all the components together or adding the PVA in a second step on the dispersed slurry. Although the slight increase in viscosity suggests that competitive adsorption occurs, the microstructure seems not to change depending on the order of addition of additives. Considering its lower molecular weight, glycerol provides lower viscosity, and as a consequence of its cryoprotective character, sintering leads to smaller pore size, being the porosity 35% for an added content of 10?wt-% with respect to solids. In the case of PVA, contents of 2?wt-% are enough to obtain solid firm bodies with a porosity of 48%. The porosity and the size of the pores decrease with increasing concentration of PVA.