The Thermal Shock Resistance Analysis of Ceramic Foams

This article studies the thermal shock resistance behavior of ceramic foams under sudden thermal load induced by a sudden temperature variation. Two types of thermal shock loading conditions are considered: cold shock and hot shock. Variations of the stress and stress intensity factor with thermal shock time, location, crack size, medium thickness, and relative density of the ceramic foam are given. Crack growth behavior is studied and crack growth velocity is explained from energy equilibrium consideration. The thermal shock resistances of ceramic foams are established from the view points of energy criterion and fracture mechanics concept.     

Hierarchically porous lanthanum zirconate foams with low thermal conductivity from particle-stabilized foams

Lanthanum zirconate (LZO) ceramic foams with hierarchical pore structure were fabricated by particle-stabilized foaming method for the first time, and the as-prepared ceramics have high porosity of 90.7%-94.9%, low thermal conductivity, and relatively high compressive strength. The LZO powder was synthesized by solid-state method. The porosity of the ceramic foams was tailored by suspensions with different solid loadings (20-40 wt%). The sample with porosity of 94.9% has thermal conductivity of 0.073 W/(m·K) and compressive strength of 1.19 MPa, which exhibits outstanding property of thermal insulation and mechanical performance, indicating that LZO ceramic foam is a promising thermal insulation material in high temperature applications.     

Thermal Fracture Resistance of Ceramic Coatings Applied to Metal: I, Elastic Deformation

A study was made of the resistance to thermal fracture of four ceramic coatings of the cobalt-bearing ground-coat type applied to enameling-grade iron specimens. The study was made of coated-metal systems in the unsteady state, symmetrically cooled, and in the absence of viscous or plastic flow. Determinations were made of the elastic characteristics of the coating-metal composites, the effective coefficient of linear expansion, the temperature at which the coating and base metal were at dimensional equilibrium, and the temperature differential sufficient to induce coating fracture when water quenched. Coating-metal thickness ratios were correlated with the maximum specimen temperature withstood in water quenching without coating fracture. Studies indicated that ceramic coatings, after receiving a given thermal treatment, fracture when subjected to a thermal shock by a critical temperature differential. When no residual coating stress is present, thermal shock resistance is inversely related to the thermal expansion characteristics of the coating. The critical stress at which coating fracture occurs may be expressed as the sum of thermal and residual stresses developed in annealed systems in which viscous or plastic flow does not occur. Residual compressive stress in a coating is a major factor in improved thermal shock resistance. Increased thermal shock resistance is gained by decreased coating thickness.     

A Simple Direct Casting Route to Ceramic Foams

A simple direct foaming and casting process using ovalbumin-based aqueous slurries for fabricating ceramic and metal foams is demonstrated. Foaming of aqueous ceramic slurries and the foam microstructure were seen to be a strong function of slurry rheology. Setting of foams with ceramic solids loading above 20 vol% was achieved by addition of acid, which also prevented binder migration. Acid addition resulted in excessive shrinkage, causing cracking of foams with ceramic loading below 20 vol%. Addition of sucrose to the slurries suppressed shrinkage leading to defect-free foams with porosity exceeding 95%. Overall porosity and foam microstructure could be controlled through ceramic solids loading, ovalbumin–water ratio, foaming time and sucrose amount, and sintering temperature. The ceramic foams fabricated by the process were strong enough to be green machined to different shapes.     

Preparation and properties of enstatite ceramic foam from talc

The effective method of preparation, stabilization and high temperature treatment of enstatite ceramic foam is described in this work. The technique is based on foaming of suspension of talc, on the stabilization of foam structure and on final high temperature treatment after drying. The spontaneous delamination of aggregates and the redistribution of talc particles in foam are driven by decreasing surface energy. The changes of phase composition as well as the mechanism and the kinetics of processes which take place during the thermal treatment were described. The treatment within the temperature range from 1150 to 1250 °C provides the ceramic foam via sintering without melted phase, whereas a liquid phase sintering occurs at higher temperatures. The final temperature of sintering is 1300 °C. Increasing amount of melted phase supports the formation of enclosed porosity and formed glass stabilizes the high temperature protoenstatite polymorph in the foam.     

Thermal properties of copper-coated carbon foams

Carbon foams, with 97% porosity, were electroplated with copper for different periods of time to achieve desired copper thicknesses and foam porosity. A light flash diffusivity instrument was used to measure the thermal conductivity of the coated samples. An analytical model was developed to calculate the effective thermal conductivity of the coated foams. It was observed that the copper-coated carbon foam with 50% porosity can attain a thermal conductivity of 180 W/m K. The results from the analytical model were compared to the experimental results and they were in a very good agreement. The above analyses demonstrated the significance of copper coating in tailoring carbon foam thermal properties. The developed analytical model was adopted to predict the thermal conductivity of the copper-coated carbon foams.     

Strength Evaluation of Piezoelectric Ceramics under Transient Thermal Environments

The strength of piezoelectric ceramics is analyzed for a plate suddenly exposed to an environmental medium of different temperatures. The admissible temperature jump the material can sustain is studied using the stress- and fracture-toughness-based failure criteria. The critical parameters governing the level of the transient thermal stress in piezoelectric ceramics are identified. Solutions are obtained for the maximum thermal shock that the plate can sustain without failure, under the conditions that (i) maximum local tensile stress equals the tensile strength of the ceramic, and (ii) maximum stress intensity factor for representative pre-existing cracks equals the fracture toughness of the ceramic.     

Evolution of internal cracks and residual stress during deposition of TBC

Thermal barrier coating (TBCs) are ceramic coatings that are deposited on metallic substrates to provide high thermal resistance. Residual stress is among the critical factors that affect the performance of TBCs. It evolves during the process of coating deposition and in-service loading. High residual stresses result in significant cracking and premature delamination of the TBC layer. In the present study, a hybrid computational approach is used to predict the evolution of internal cracks and residual stress in TBC. Smooth particle hydrodynamics (SPH) is first used to model the deposition of yttria-stabilized zirconia (YSZ) layer that contains various interfaces and micropores on a steel substrate. Then, three-dimensional (3D) finite element analysis is utilized to predict the evolution of internal cracks and residual stress in the ceramic coating layer. It is found that multiple cracks emerge during the solidification of the coating layer due to the development of high tensile (quenching) stresses. The cracking density is higher at regions near the coating interface. It is also found that compressive (residual) stresses are developed when the deposited coating is cooled to room temperature. The residual stress state is equibiaxial and nonlinear across the thickness/width of the TBC layer. The residual stress profile predicted compares well with that of hole drilling experiments.     

Al2O3基泡沫陶瓷的研究

以Al2O3和锆英石为主要原料制备了具较高强度及抗热震性的泡沫陶瓷并进行了性能研究与应用试验。结果表明：当锆英石含量为30%时,材料的抗弯强度最大,达3.84MPa;随锆英石含量的提高,抗热震性能增强;泡沫陶瓷的孔隙率与锆英石含量无关,为80-81%,泡沫陶瓷过滤器能有效滤除铸液中夹杂物。     

Novel ZrP2O7 ceramic foams with controllable structures and ultra-low thermal conductivity

This study demonstrated the synthesis of novel zirconium pyrophosphate (ZrP₂O₇) ceramic foams via a two-step method using a foam casting technique. The synthesised foams functioned as thermal insulators with a highly controllable performance. We investigated the effects of the addition of foaming and thickening agents as well as the solid content of the slurries on the slurry, mechanical properties, thermal conductivities, and microstructure of ZrP₂O₇ ceramic foams. The ZrP₂O₇ ceramic foams synthesised at 1473 K exhibited a porosity, compressive strength, and thermal conductivity of 75.2–89.1 %, 1.95–0.02 MPa, and 0.144–0.057 W/(m K) (298–573 K), respectively. The increase in the porosity to >60 % will facilitate applications based on the low thermal conductivities of the foams.     

Highly Porous Zirconia Ceramic Foams with Low Thermal Conductivity from Particle‐Stabilized Foams

Zirconia ceramic foams with ultra‐high porosity of 96%–98% have been fabricated using sodium dodecyl sulfate (SDS) as the particle stabilizer of zirconia particles for the first time. The wet foams stabilized by zirconia particles are ultra‐stable due to partially hydrophobic zirconia particles modified by SDS. Zirconia foams exhibit close cells with thin cell wall and small grain size. Increasing SDS concentration favors the foamability of the suspension, and further increases the porosity of ceramic foams. Zirconia ceramic foams with porosity of 98.1% have compressive strength of 0.26 ± 0.05 MPa. Decreasing solid loading leads to the porosity of ceramic foams. The compressive strength could be improved significantly by increasing the sintering temperature. Zirconia ceramic foams with porosity of 97.9% has low thermal conductivity of 0.027 ± 0.004 W·(m·K)^?1, which could be used as thermal insulation and refractory material.     

Ultra low-density mullite foams by reaction sintering of thermo-foamed alumina-silica powder dispersions in molten sucrose

Ultra low-density mullite foams are prepared by thermo-foaming followed by reaction sintering of alumina-silica powder dispersions in molten sucrose. The foaming & setting time, foam rise, sintering shrinkage, porosity, cell size and compressive strength are studied as a function of ceramic powder loading, foaming temperature and magnesium nitrate (blowing agent and setting agent) concentration. Phase pure mullite is produced by reaction sintering at 1600 °C. The mullite foams produced without magnesium nitrate have porous struts and cell walls due to improper densification. The magnesium nitrate drastically decreases the foaming & setting time and increases the foam rise and cell interconnectivity. The MgO produced from the magnesium nitrate assists the densification of the mullite as evidenced from the non-porous struts and cell walls at higher magnesium nitrate concentrations. The maximum porosity of 94.92 and 96.28 vol.% achieved without and with magnesium nitrate, respectively, is the highest reported for mullite foams.     

泡沫碳化硅陶瓷材料的研究进展

泡沫碳化硅陶瓷材料除了孔隙率高、比表面积大,还具有相对密度小、优良的热学、力学、电学、声学性能等特性,已经广泛应用于化工、机械、生物、环保等领域。本文总结了泡沫碳化硅陶瓷材料的主要制备技术,包括粉末烧结法、固相反应烧结法、含硅树脂热解法以及气相沉积法等。阐述了泡沫碳化硅陶瓷材料的几种优良特性,包括结构特征、流体阻力、抗氧化性、吸波性等。最后举例介绍了该陶瓷在催化、过滤、生物学等领域的应用现状,重点介绍了其作为塔内件在化工领域中的应用,指出为满足对泡沫碳化硅陶瓷更高性能的需求,不仅要对现有技术进行集成创新,更要挖掘和开发泡沫碳化硅的潜在优势。     

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.     

High-porosity mullite ceramic foams prepared by selective laser sintering using fly ash hollow spheres as raw materials

A novel method to prepare high-porosity mullite ceramic foams by selective laser sintering (SLS) using fly ash hollow spheres (FAHSs) as raw materials was reported. The complex-shaped FAHS green bodies and ceramic foams without delamination or cracks were prepared by SLS. The influence of sintering temperatures on linear shrinkage, phase composition, porosity and mechanical properties was investigated. With the increase of sintering temperature from 1250?°C to 1400?°C, the compressive strength of ceramic foams increased from 0.2?MPa to 6.7?MPa causing the fracture mechanism change from fracturing along FAHSs to across FAHSs, while the porosity of ceramic foams decreased from 88.7% to 79.9% which was higher than those of ceramic foams prepared by the conventional methods. The relatively high porosity of ceramic foams was resulted from the inner hollow structure of FAHSs, the interspaces between stacking FAHSs, and the gaps between FAHSs directly related to SLS. The results above indicated that the fabrication of high-porosity FAHS ceramic foams by SLS could achieve the advanced utilization of FAHS solid waste.     

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.     

Aerogel-like ceramic foams with super-high porosity and nanoscale cell wall from sol nanoparticles stabilized foams

Ultralight ceramic foam materials with high porosity play an important role in increasingly hi-tech areas due to the combinative merit of ceramic material and highly porous structure. So far, it remains challenging to fabricate alumina ceramic foams with extremely high porosity and high specific surface area that are comparable to aerogel materials by employing a low cost, eco-friendly and convenient approach. For the first time, we propose the preparation of aerogel-like ceramic foams with nanoscale cell wall and unprecedentedly high porosity using boehmite sol as both ceramic source and bubble interface stabilizer, based on sol nanoparticles stabilized foams using sodium lauryl sulfate (SDS) as modifier. The obtained ultra-stable sol foams allow for the achievement of bulk foams with ultrathin cell wall with thickness in the range of 30-90 nm, super-high porosity up to 99%, and large specific surface area of 280 m²/g, which is attributed to the well-organized assembly of nanoparticles at the liquid/air interfaces. This novel foam material demonstrates excellent adsorption ability for polar volatile organic gases (VOCs) due to its extremely high porosity and large specific surface area.     

Experimental and numerical analysis of the complex permittivity of open-cell ceramic foams

Open-cell ceramic foams are promising materials in the field of microwave heating. They can be manufactured from susceptor materials and can, therefore, be used as selective heating elements. In this study, the complex permittivities of ceramic foam materials, including silicon-infiltrated silicon carbide (SiSiC), pressureless sintered silicon carbide (SSiC), silicate-bonded silicon carbide (SBSiC), and cordierite were determined. The dielectric properties of the foams were determined by the cavity perturbation technique using a TE₁₀₄ WR340 waveguide resonator at 2.45 GHz. Samples were preheated in a tubular furnace, enabling temperature-dependent permittivity measurements up to 200 °C. The effective dielectric constant and effective loss factor were found to depend on the porosity and material composition of the foam. The SiSiC material had a higher effective dielectric constant than the SSiC and SBSiC ceramics. The effective dielectric constant of the foams showed a trend of gradual increase with increasing temperature. Some selected dielectric mixing relations were then applied to describe the effective permittivity of the foams and compare them with predictions from finite element simulations performed using the CST Studio Suite. The foam morphologies and simple block inclusions were used in the simulations.     

Degradation and recovery of adhesion properties of deformed metal–polymer interfaces studied by laser induced delamination

Adhesion properties of polymer coatings on metals are of great interest in various industrial applications, including packaging of food and drinks. Particular interest is focused on polymer–metal interfaces that are subjected to significant deformations during manufacturing process. In this work steel samples laminated with polyethylene terephthalate (PET) were subjected to uniaxial tensile deformations followed by annealing treatments. The measurements have demonstrated degradation of adhesion of the metal–polymer interface as the strain introduced by the deformation increased. Moreover, it was observed that within the geometry used in the experiments tensile deformations of the metal substrate introduced in-plane compressive stresses in the bulk of the coating. After applying a thermal treatment restoration of the adhesion has been achieved.Laser induced delamination technique was used to monitor the adhesion properties. In this technique a coating is subjected to a series of infrared laser pulses with a stepwise increase of intensity. Upon increasing the laser pulse intensity, the pressure which is formed inside the blisters reaches a critical value, resulting in further delamination of the coating. To process the experimental data an elastic model was developed. From the analysis of the experimental data the critical stresses required for the delamination and the practical work of adhesion are derived. The model accounts for the compressive in-plane stress present in the coating of the deformed samples.     

Processing,Characterization, and Modeling of Room‐Temperature‐Vulcanized Silicone‐Derived Ceramic Foams

A novel method was developed to produce ceramic foams from a silicone precursor which was foamed and vulcanized at room temperature. Silicone foams were prepared by platinum‐catalyzed cross‐linking and dehydrogenation of reactive polysiloxanes. Silicone foams were converted to ceramic foams after being pyrolyzed at 1200°C in argon. Near‐net‐shape polymer‐to‐ceramic conversion was achieved when SiC particles were added to the polymer as a solid filler. A simple physical model was created to describe the rising and pyrolysis of the silicone foam, and was validated by experimental data. Foam density was largely dependent on the content of ethanol, which was used as a chemical blowing agent. Up to 1.8 wt% ethanol was effective in driving foam rising without leading to foam collapse. SiC filler helped reduce weight loss and volumetric shrinkage during pyrolysis, and slightly increased foam density. Scanning electron microscopy indicated that although incorporating a solid filler helps to reduce the bulk shrinkage, it cannot prevent local microcracking and residual porosity.