Microstructure and permeability of porous zirconia ceramic foams prepared via direct foaming with mixed surfactants

The permeability performance of porous ceramics from particle-stabilized foams is closely associated with the connectivity between bubble-evolved pores. In order to regulate the connectivity of pore structure, the zirconia ceramic foams were fabricated by direct foaming with mixed surfactants of cetyltrimethyl ammonium bromide (CTAB) and sodium N-lauroyl sarcosinate (SLS). Different solid loadings and CTAB:SLS mixing ratios were used in this study. The results indicate that the pore structures of zirconia ceramic foams were interconnected by open windows on the cell walls, and the porosity and average size of cell and cell windows could be tailored by adjusting the solid loading and CTAB:SLS mixing ratio. The decrease in solid loading and CTAB:SLS mixing ratio caused larger porosity and size of cells and cell windows, and thus resulted in the obvious augment of Darcian (k₁) and non-Darcian (k₂) permeability constants. The ranges of k₁ and k₂ of the as-fabricated zirconia ceramic foams are 6.92 × 10^?13-4.05 × 10^?10 m² and 2.09 × 10^?5-3.19 × 10^?9 m respectively.     

The microstructural characterization and mechanical response of YSZ ceramic foams fabricated via volume-controlled foaming

Yttria-stabilized zirconia (YSZ) ceramic foams are a promising class of materials for lightweight, high specific strength catalyst supports or insulation. Foam morphology is one of the most significant factors that dominate the mechanical properties of the YSZ ceramic foams. However, the foam morphology as a function of gravity and foam film strength for YSZ ceramic foams has been seldom reported up to now. Our work focuses on YSZ ceramic foams fabricated via a novel foam-gelcasting method using Isobam as gelling agent. The relative magnitudes of the foam film strength and the gravitational force can be changed by controlling the foaming yield of slurries. Both the remaining high-temperature strength and the critical difference temperature (△T_c) of YSZ (3.0) ceramic foams were higher than those of YSZ (5.0) ceramic foams, mainly owing to high closed-cells and relatively uniform distributed pore structure. In addition, the YSZ ceramic foams could not break suddenly like dense ceramics. This work demonstrates that tuning the foaming yield of slurries is a viable route to improved thermomechanical property in ceramic foams for use as insulation or catalyst supports in extreme environments.     

Preparation of zirconium carbide foam by direct foaming method

Ultra light, highly porous, closed-cell structured ZrC foam can be produced in two steps. First, pre-ceramic foam is prepared by direct foaming of zirconia sol and phenolic resin. In the next step, the foamed green body is converted into ZrC foam after carbothermal reduction at 1600 °C under argon atmosphere. The obtained ZrC foam has porosity of 85% and possesses uniform cells with an average size of about 40 μm. The foam also displays excellent thermal stability up to 2400 °C. Its compressive strength and thermal conductivity at room temperature are 0.4 MPa and 0.94 W/(m K), respectively.     

Preparation and microstructure evolution of novel ultra-low thermal conductivity calcium silicate-based ceramic foams

In this study, municipal solid waste incineration fly ash (MSWI FA) was used as a new raw material for the ceramics industry and a novel ultra-low thermal conductivity calcium silicate-based foams (CSFs) was prepared by the direct foaming method. The effects of the addition of foam and borax on the sintering behavior and microstructural evolution of the CSFs were investigated. With the optimal amount of foam, the CSFs had an apparent porosity of 63.43%–67.49%, bulk density of 0.75–0.84 g/cm³, compressive strength of 1.83–3.21 MPa, and room-temperature thermal conductivity of 0.213–0.235 W/(m·K). Notably, the whisker morphology, pore structure, and sintering behavior of the samples can be controlled by changing the amount of borax. The prepared ceramic foams can be applied in the fields of thermal insulation, filtration, and catalyst carriers.     

Preparation of high open porosity ceramic foams via direct foaming molded and dried at room temperature

Herein an alternative approach was considered for addressing one difficulty of ceramic foams that the foam slurry with a high content of bubbles which were obtained via direct foaming, cannot maintain well for a long time at room temperature. It is fascinating that the foam slurry mentioned above could stably mold and dry at room temperature, based on an animal protein as foaming agent, kaolin, talc powder and alumina as raw materials, alpha-tricalcium phosphate prepared via co-precipitation as curing agent, and hydrophobic activated carbon powders as stabilizing agent. Effects of the calcination temperatures, the contents of alpha-tricalcium phosphate and activated carbon powder on microstructures, crystal phases, compressive strength and open porosities of ceramic foams were studied systematically. The results indicated that ceramic foams with a high open porosity and uniform pore distribution and sizes sought for application in catalysts supports, could be produced by adjusting these parameters.     

Thermal and mechanical behavior of ZrTiO4-TiO2 porous ceramics by direct foaming

This paper reports the production of micro porous ceramics consisting of TiO₂ and ZrO₂ by direct foaming. ZrO₂ particles in a colloidal suspension were partially hydrophobized using propyl gallate as an amphiphile at a suitable pH range of around 3.5–4.5. A TiO₂ suspension with different mole ratios was added to the surface modified ZrO₂ suspension to obtain ZrTiO₄-TiO₂ porous ceramics in the sintered sample. The influence of the TiO₂ content and calcination temperature on the phase transformation, microstructure, and thermal properties of the materials was determined by thermal analysis, X-ray diffraction, field emission scanning electron microscopy, and dilatometry. The crystallization of ZrTiO₄ (orthorhombic) was observed at 1100 °C on the thermal hysteresis curve due to anisotropic thermal expansion. The compressive load and displacement of the sintered porous ceramics samples were calculated using the Hertzian indentation method.     

Performance study of foam ceramics prepared by direct foaming method using red mud and K-feldspar washed waste

In this study, foam ceramics were prepared via a direct foaming method at high temperatures (1080–1120 °C), using red mud (RM) and K-feldspar washed waste (KFW) as the raw materials and SiC as the foaming agent, respectively. The chemical compositions and crystalline phases of the raw materials as well as the structural and mechanical properties of the foam ceramics were investigated. By adjusting the formulation and sintering process parameters, the porous structure of the foam ceramics could be effectively modulated. In addition to some residual crystalline phases in the raw materials, new phases, including rutile (TiO₂) and anorthite (CaAl₂Si₂O₈), were generated in foam ceramics. The compressive strength of the foam ceramics decreased with an increase in the KFW/RM ratio and sintering temperature, which was mainly related to the low density of the foam ceramics and the poor support of the pore walls to the structure. Among all the foam ceramics investigated, the foam ceramic with the KFW/RM ratio of 1:1, SiC content of 1 wt%, sintering temperature of 1100 °C and sintering time of 60 min showed the best overall performance with a bulk density, an apparent porosity, an average pore size and a compressive strength of 0.77 g/cm³, 61.89%, 0.52 mm, and 3.64 MPa, respectively. Its excellent porous structure and mechanical properties rendered it suitable for application as insulation materials or decorative materials for building partition walls.     

Microstructure and damage evolution of SiCf/PyC/SiC and SiCf/BN/SiC mini-composites: A synchrotron X-ray computed microtomography study

SiC_f/PyC/SiC and SiC_f/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapor deposited PyC or BN interface layers are fabricated. The microstructure evolutions of the mini-composite samples as the oxidation temperature increases (oxidation at 1000, 1200, 1400, and 1600?°C in air for 2?h) are observed by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction characterization methods. The damage evolution for each component of the as-fabricated SiC_f/SiC composites (SiC fibre, PyC/BN interface, SiC matrix, and mesophase) is mapped as a three-dimensional (3D) image and quantified with X-ray computed tomography. The mechanical performance of the composites is investigated via tensile tests.The results reveal that tensile failure occurs after the delamination and fibre pull-out in the SiC_f/PyC/SiC composites due to the volatilization of the PyC interface at high temperatures in the air environment. Meanwhile, the gaps between the fibres and matrix lead to rapid oxidation and crack propagation from the SiC matrix to SiC fibre, resulting in the failure of the SiC_f/PyC/SiC composites as the oxidation temperature increases to 1600?°C. On the other hand, the oxidation products of B₂O₃ molten compounds (reacted from the BN interface) fill up the fracture, cracks, and voids in the SiC matrix, providing excellent strength retention at elevated oxidation temperatures. Moreover, under the protection of B₂O₃, the SiC_f/BN/SiC mini-composites show a nearly intact microstructure of the SiC fibre, a low void growth rate from the matrix to fibre, and inhibition of new void formation and the SiO₂ grain growth from room to high temperatures. This work provides guidance for predicting the service life of SiC_f/PyC/SiC and SiC_f/BN/SiC composite materials, and is fundamental for establishing multiscale damage models on a local scale.