Cellular Ceramics Research Articles

Functional ceramic and nanocomposite fibers, cellular articles and microspheres via radiation curable colloidal dispersions

We present here new cellular ceramics, fibers and microspheres produced by novel and general microshaping techniques employing colloidal dispersions in solvent free radiation curable monomer mixtures. High loading, low viscosity dispersions of functional nanoparticles such as TiO 2 and α-Fe 2O 3 and colloidal Fe(C 2O 4)·2H 2O metal salt are achieved with comb-polyelectrolyte surfactants and/or mildly polar resins. TiO 2 dispersions are spun and solidified “on the fly” by UV radiation into continuous ceramic/polymer nanocomposite fibers with dimensions below 10 μm. Fe(C 2O 4)·2H 2O dispersions provide a UV curable alternative to α-Fe 2O 3 dispersions which can only be thermally cured. The Fe(C 2O 4)·2H 2O nanocomposites transform to α-Fe 2O 3 below 550 °C. Novel cellular Al 2O 3, TiO 2 and α-Fe 2O 3 articles with porosity >80% and precise replication of the pore-forming agents have also been produced from such dispersions. Al 2O 3 nanocomposite microspheres are produced by emulsification of the dispersions in an appropriate medium and UV curing.

Processing of Ceramic Foams with Hierarchical Cell Structure

AbstractSilicon carbide‐based cellular ceramics characterized by a hierarchical pore structure were processed. An open‐cellular PU foam template with a mean cell density of 10 pores per inch (ppi), equivalent to a cell diameter of 4 mm and a strut thickness of 250 µm, was coated with a primary SiC slurry. Cross‐linking of a polysiloxane binder at 190 h resulted in a SiC filled reticulated thermoset foam (first generation). Subsequently, this matrix foam was infiltrated with a second slurry of slightly different polysiloxane composition which upon heating to 290 °C caused bubble nucleation and formation of a second generation foam filling the cell space in the matrix foam skeleton. After pyrolysis at 1000 °C a SiC/SiOC reaction bonded composite foam with a hierarchical cell structure and a fractional density of 0.31 was obtained. SEM and X‐ray µ‐CT analyses confirm both generations of matrix as well as infiltrated foam to be open cellular but with a pronounced difference in cell size (matrix foam cell size 2.5 mm versus infiltrated foam cell size 0.29 mm). While the matrix foam skeleton provides control of macroscopic shape as well as density distribution in the component, single‐ or multistep foam infiltration may offer a high potential for improving the mechanical properties of hierarchical cellular materials. Thus, hierarchical structure formation offers a high potential to fabricate low density cellular ceramics which might be of interest for lightweight design of novel engineering materials.

Selection and experimental evaluation of low-cost porous materials for regenerator applications in thermoacoustic engines

This paper aims at evaluating three selected low-cost porous materials from the point of view of their suitability as regenerator materials in the design of thermoacoustic travelling-wave engines. The materials tested include: a cellular ceramic substrate with regular square channels; steel “scourers”; and stainless steel “wool”. Comparisons are made against a widely used regenerator material: stainless steel woven wire mesh screen. For meaningful comparisons, the materials are selected to have similar hydraulic radii. One set of regenerators was designed around the hydraulic radius of 200 μm. This included the ceramic substrate, steel “scourers”, stainless steel “wool” and stacked wire screens (as a reference). This set was complemented by steel “scourers” and stacked wire screens (as a reference) with hydraulic radii of 120 μm. Therefore six regenerators were produced to carry out the testing. Initial tests were made in a steady air flow to estimate their relative pressure drop due to viscous dissipation. Subsequently, they were installed in a looped-tube travelling-wave thermoacoustic engine to test their relative performance. Testing included the onset temperature difference, the maximum pressure amplitude generated and the acoustic power output as a function of mean pressure between 0 and 10 bar above atmospheric. It appears that the performance of regenerators made out of “scourers” and steel “wool” is much worse than their mesh-screen counterparts of the same hydraulic radius. However cellular ceramics may offer an alternative to traditional regenerator materials to reduce the overall system costs. Detailed discussions are provided.

Processing and Properties of Graded Ceramic Filters

A new processing route for high‐performance graded ceramic filters is presented. The direct foaming process is based on the transition of a surfactant‐stabilized alkane dispersion in a stabilized aqueous ceramic powder suspension into mechanically stable ceramic foams with porosities up to 90%. The cell size distribution and, consequently, the permeability of the filters could be efficiently adjusted by the control of the high alkane phase‐emulsified suspension (HAPES) microstructure during emulsification. Furthermore, open porous graded structures are produced by combining HAPES layers with different droplet sizes. The tailored microstructural features, providing controlled permeabilities as well as high compressive strength of the cellular ceramics are of special interest for applications where fluid transport through ceramic microstructures is required. These include filters, in particular for aerosol filtration, temperature control membranes in autothermal reforming, catalytic supports including supports for batteries as well as matrices for immobilized microorganisms.

Preparation of γ-alumina ceramic foams employing hydrophilated polyester polyurethane sponges

Single phase c-Al2O3 ceramic foams with high surface area (179 m/g) and porosity (67%) were prepared by the polyurethane sponge replica method from slurries containing c-Al2O3. Ceramic foams are highly porous brittle materials with large voids (cells) [1]. These cellular ceramics are special classes of porous materials comprised of cells with size ranging from a few microns to a few millimetres, where the cells can be surrounded by ceramic walls or contain solid material at only cell edges (struts), thus creating an interconnected structure (open cell foam) [1–3]. Ceramics foams are finding increasing applications as catalytic supports, ceramic membranes, sensors, filters, thermal and acoustic insulators, due to their properties such as low density, low thermal conductivity, high temperature stability, and high resistance to chemical attack [4]. Due to the high calcination temperatures employed in the conventional preparation of alumina foams, a-Al2O3 is always obtained [5]. For catalytic applications, post-treatments such as washcoating of the a-Al2O3 monoliths with suspensions of c-Al2O3 or sol–gel deposition procedures can be employed in order to obtain materials with higher surface areas [6]. However, when the foam is coated with c-alumina or any hydrated form of alumina, the maximum useful temperature is limited to approximately 700 C to avoid loss of surface area as the result of the transformation to more crystalline forms of alumina [7]. To our best knowledge, there has been only one, very recent report on the preparation of single phase c-Al2O3 ceramic foam [8]. In the present work, the preparation of single phase c-alumina foams of high surface area employing the polyurethane sponge replica method is reported, which consists of dipping the polymeric foam into a slurry containing the ceramic precursors, followed by a thermal treatment that leads to the burning out of the sponge and to the pressureless sintering at elevated temperatures of the ceramic skeleton [9]. It is expected that the meso/ macroporous characteristics of the ceramic foams will parallel those of the templating organic material. A commercial polymeric sponge (ACE Hardware Corp.), corresponding to hydrophilated polyester polyurethane with pore density of 122 ppi, was tested as template for the preparation of ceramic foams. c-Alumina which has 283 m/g BET area, 0.53 cm/g pore volume, and mean pore radius of 7.6 nm, was prepared in this laboratory following the method reported by Xu et al. [10], and was used in the present study for preparing the slurries employed in the impregnation process of polyurethane sponge. Slurries were prepared using the following procedure: 21.8 g of the synthesized alumina was mixed with 24 mL of deionized water and 5 mL of concentrated HCl. The viscosity of the slurry prepared with c-alumina at pH 3.5 was 103 9 10 Pa s. The alumina coating was applied by repeated immersiondrying cycles of the substrate (PU sponge) in the alumina slurries. A minimum of four immersion-drying cycles were Electronic supplementary material The online version of this article (doi:10.1007/s10853-009-3693-3) contains supplementary material, which is available to authorized users.

Mechanical properties of cellular ceramics obtained by gel casting: Characterization and modeling

Dense and cellular ceramics were produced from yttria partially stabilized zirconia powders by gel-casting, using agar as a gelling agent and polyethylene spheres (125–300 μm diameter) as volatile pore forming agent to create 50–65 vol.% spherical macropores, uniformly distributed in a microporous matrix. The mechanical properties of both dense and porous samples were investigated at the microscale by nanoindentation testing. The influence of micro-porosity on the mechanical properties of samples was evaluated by the analysis of hardness and modulus depth profiles, coupled with FIB-SEM section observations of selected indentation marks. The intrinsic elastic modulus of the zirconia phase resulted to be of the order of 220 GPa. Mechanical characterization at the macroscale consisted of uniaxial compression tests and four point bending tests. Elastic moduli of about 170 GPa were measured for about 93% dense ceramics, lowering down to 44 and 13 GPa with the addition 50 and 65 vol.% macropores, respectively. Digital image based finite element analysis (DIB-FEA) procedures were implemented in order to verify their applicability for the prediction of mechanical behavior of this type of cellular materials: results confirmed that a very good match between measured and calculated values of elastic modulus can be achieved, provided that the effects of micro-porosity are considered by the proper choice of the elastic properties to be assigned to each individual phase identified by Image Analysis.

Natural additives in protein coagulation casting process for improved microstructural controllability of cellular ceramics

Aiming to improve the microstructural controllability of porous ceramic, starch was introduced as a natural additive into an environmental friendly process-protein coagulation casting. Results indicated that the viscosity of alumina/starch slurries was affected by the combined effect of the total solid loading and the water uptake of starch. Furthermore, as pore former and wet foam stabilizer, starch helped to increase the open porosity and obtain a more uniform distribution of microstructures in the fabricated alumina foams. A better microstructural controllability of the alumina foam prepared by the modified protein coagulation casting process was realised which is believed to be beneficial to the multifunctional application of these ceramic foams.

New cellular ceramics from high alkane phase emulsified suspensions (HAPES)

Open porous mechanically stable ceramic foams are developed by a simple direct foaming process. The new processing route is based on the transition of a surfactant stabilized highly concentrated alkane phase homogeneously distributed in a stabilized aqueous ceramic powder suspension into high performance ceramic foams with porosities up to 90% and cell sizes ranging from 3 to 200 μm. The droplet size distribution of the high alkane phase emulsified suspensions (HAPES) is efficiently controlled by the stirring velocity during emulsification experimentally investigated for varying powder particle contents. Stable foams with tailored structural features can be prepared by adjusting the rheological characteristics of HAPES being dependent on the system and process parameters. The influence of the emulsification stirring velocity on the resulting HAPES droplet size is analysed on the basis of the Taylor model of mechanical shearing describing the stresses responsible for the fragmentation of the droplets.

Mechanical characterization of cellular ceramics

Polymeric sponge replication technique has been widely used to obtain ceramic foams with a cellular structure for filtration applications. This process is based on an impregnation of a polymeric sponge with ceramic slurry. Ceramic filters must present simultaneously high permeability and strength. However, these parameters are influenced in different ways by the processing method and these properties are inversely proportional. In this work the mechanical properties by compression and flexural has been evaluated for 10 and 40 ppi (pores per linear inch) Al 2O 3–ZrO 2 filter materials. Characterization included mechanical properties, crystalline identification as well as the microstructural analyses of the fracture surface.

Preparation and characterization of porous cordierite for potential use in cellular ceramics

AbstractCordierite porous ceramics Z, X, and K were prepared using three mixtures of clay minerals: Z from kaolinite, talc, and aluminum hydroxide, X from kaolinite, talc, vermiculite, and aluminum hydroxide, and K from kaolinite, talc, and magnesium oxide. Ceramics were different in porosity, specific surface area, cordierite polymorphs, and secondary crystalline phases. Vermiculite influenced textural architecture of calcined cordierite ceramics X and predestinated crystallization of the high-temperature hexagonal α-cordierite with secondary minerals enstatite, spinel and corundum. Ceramics Z contained low-temperature orthorhombic β-cordierite, enstatite, and corundum, K was diphase of β-cordierite and forsterite. Total pore area (TPA) and specific surface area (SSA) of X, in spite of the higher porosity and the pore size distribution in the range of 300–1000 nm, were smaller in comparison with TPA and SSA of Z. Ceramics K retained high porosity, two maxima at 300–1000 nm and 50–200 nm in the pores size distribution, and the highest TPA and SSA compared to those observed in ceramics Z and X.

Thermal conductivity of sintered lithium orthosilicate compacts

The design of solid breeder blankets is strongly affected by the low values of thermal conductivity and density of ceramic breeder pebble beds. A significant rise of both quantities would enhance the thermal performance and lead to an increased tritium breeding ratio. In order to improve these quantities pretreated lithium orthosilicate pebble material was dry pressed and subsequently sintered. The thermal conductivity of cylindrical pellets was determined by the heat pulse method using a laser flash device. A pebble bed characteristic sample was also investigated in order to check the measurement accuracy in comparison with previous results. Furthermore, two samples of low density cellular ceramics were also prepared by infiltration of polymer foams with a ceramic slurry. The thermal conductivity results show that the values are affected both by the particle size and the sample density. Thermal conductivity values of higher than 2 W/m K were obtained using large particles and sintering at 1000 °C.

Characterization of cellular ceramics for high-temperature applications

Polymeric sponge replication technique is the most used process to obtain ceramic foams with a cellular structure for filtration applications. This technique is based on an impregnation of a polymeric sponge with ceramic slurry, removal by squeezing, followed by burning out polymer and high temperature sintering. Ceramic filters must present high permeability and strength. However, these parameters are influenced in different ways by the processing method and the consequent cellular structure. In this work the relationship between permeability and strength has been investigated for 10- and 40-ppi (pores per linear inch) Al2O3–ZrO2 filter materials. Characterization included the evaluation of the permeability and strength as well as the microstructural analyses of the fracture surface.

Cellular Ceramics by Direct Foaming of Emulsified Ceramic Powder Suspensions

A new direct foaming method to produce macroporous cellular ceramics using surfactants as foam stabilizers is presented. The technology relies on the transition of a stabilized aqueous ceramic powder suspension containing a homogeneously dispersed alkane or air–alkane phase into cellular ceramics. The stabilization of the powder suspension and the emulsion is realized with particular emphasis on the interaction of both mechanisms providing enduring stability of the system up to high foaming degrees. Anionic, cationic, and nonionic surfactants were studied with their stabilization and foaming effects. The presence and influence of air bubbles was proved to be of negligible importance. Foaming is then provided by the evaporation of the emulsified alkane droplets, leading to the expansion of the emerging foam and giving rise to solids foams with cell sizes from 0.5 to 3 mm and porosities up to 97.5% after sintering. The microstructures of these filigree ceramics are stable and rigid with dense struts and uniform distributions of the solid phase and the porosity.

Production of Cellular Ceramics by Gel Casting Ceramic Emulsions

Cellular ceramics have been produced by several methods, giving a wide range of macrostructures and properties. This work describes a novel route to produce cellular ceramics based on the gel casting of emulsions consisting in an aqueous ceramic suspension containing water-soluble organic monomers and an emulsified insoluble liquid phase. The effects of solids loading and kerosene fraction on the rheological properties of emulsions were characterized. Samples with different kerosene additions (20, 30, 40 and 50 %vol.) were produced and their green and sintered densities were measured. The results have shown a good correlation between the volumetric fraction of kerosene into the emulsion and the porosity of samples. Diametrical compression was used to evaluate the strength of sintered samples, which varied with the porosity between 18 and 37 MPa. The cellular structure was analyzed by SEM and revealed isolated pores for samples with low porosity, which changed to an interconnected network of pores as the porosity increased.

Improvement of Microstructural Controllability of Cellular Ceramic for Multifunctional Composites

By modifying the sequences in preparation of aqueous ceramic slurries, starch was successfully employed to improve the microstructural controllability of alumina foams fabricated through protein coagulation casting technique. Open porosity was increased and the uniform distribution of microstructures was improved by the dual functions of starch as pore former and wet foam stabilizer. The viscosity of slurry was affected by the increasing amount of starch additive due to the total solid loading increase and the water uptake of starch. A well controlled pore structure will benefit multifunctional applications of these porous foams.

Freeze‐Casting of Porous Ceramics: A Review of Current Achievements and Issues

AbstractFreeze‐casting, the templating of porous structures by the solidification of a solvent, have seen a great deal of efforts during the last few years. Of particular interest are the unique structure and properties exhibited by porous freeze‐casted ceramics, which opened new opportunities in the field of cellular ceramics. The objective of this review is to provide a first understanding of the process as of today, with particular attention being paid on the underlying principles of the structure formation mechanisms and the influence of processing parameters on the structure. This analysis highlights the current limits of both the understanding and the control of the process. A few perspectives are given, with regards of the current achievements, interests and identified issues.

Engineering porosity in polymer-derived ceramics

By employing carefully controlled processing methods, a large amount of porosity (>70 vol%) was introduced in ceramic materials derived from preceramic polymers (silicone resins) after pyrolysis at 1000–1200 °C in inert atmosphere. The resulting components have a bulk density ranging from ∼250 to 950 kg/m 3. Three main fabrication methods have here been employed: (1) direct foaming of a solution of a thermosetting silicone resin in a suitable solvent (with or without the addition of polyurethane precursors), acting also as a blowing agent; (2) the use of sacrificial fillers that decompose during pyrolysis, consisting in polymeric microbeads; (3) the mixing of preceramic polymers possessing different characteristics, in particular ceramic yield, depending on their molecular structure. In addition to that, several methods for developing micro- or meso-pores within the resulting SiOC macro-porous ceramics were explored, with the aim of fabricating components with hierarchical porosity. These include a controlled thermal treatment, the addition of fillers with a high specific surface area (SSA), the deposition of zeolites or meso-porous silica coatings, the infiltration with aerogels, selective etching of the SiOC material and the in situ formation of C-based nanostructures. Depending on the fabrication procedure adopted, cells with an average size ranging from the micrometer to the millimeter were obtained. All these processes are simple, economical and versatile, and large bodies with various shapes (tubes, plates, blocks) can be produced, possessing a wide range of morphologies and properties. Compression strength, flexural strength and Young's modulus vary with the morphology and density of the porous components. It is also possible to add to the preceramic polymers some filler powders, for instance possessing electrical conductivity or magnetic properties, leading to the production of functional cellular ceramics.

Fabrication of Porous Cellular SiC Ceramics from Wood by Embedding in Si/SiO<sub>2</sub> Powder Mixture

Wood has strongly anisotropic cellular structure with 50-80 vol% porosity. It can be converted into porous ceramics (e.g., SiC, SiC/Si, TiC, C/C, and TiO2) replicating the wood structure by various processes. Previously porous cellular SiC ceramic was prepared by reaction of wood charcoal with gaseous SiO generated remotely from an equimolar mixture of Si and SiO2. In the present work, poplar charcoal was simply embedded in the powder mixture of Si and SiO2 and heated at 1600°C for 1 h in Ar to produce porous SiC. Samples were also prepared by infiltration of Si melt (1500-1600°C, 2 h) and vapor (1700°C, 2 h) into the charcoal for comparison. Samples prepared by Si melt infiltration showed 15-52% conversion to SiC. Samples prepared by Si vapor infiltration showed severe damage such that the conversion degree could not be measured. In contrast, samples prepared by the embedding process showed full conversion to SiC (mostly β form) with good retention of the cellular structure of the original wood. The embedding process is a simple and efficient way to produce porous cellular SiC from wood.

Gelcasting of alumina foams using agarose solutions

Aqueous gelcasting of dense or cellular ceramics by using biopolymers as gel-formers, instead of monomers, is a promising technology mainly in terms of environmental aspects. The main difficulty of using biopolymer solutions in processing of cellular ceramics by foaming method is their high viscosity, which prevents the foaming capacity of the ceramic suspension. In this work, the procedure for preparing concentrated agarose solutions (4 wt.%) by dissolving under overpressure conditions was evaluated for the gelcasting of alumina foams, and the rheological behaviour of alumina suspensions containing agarose was studied. The viscosity of the gelling solution obtained under overpressure conditions was lower than that prepared by simply heating at 90 °C, thus providing high foaming capacity of the alumina suspensions and consequently manufacturing of highly porous ceramics (86–90%). The microstructure of alumina foams was typically composed of approximately spherical cells interconnected by circular windows. The use of different agarose concentrations in alumina suspensions effected the rheological conditions, which resulted in changes in the pore and window sizes of the resulting ceramics. Depending on agarose concentration (0.50–1.0 wt.% on a dry solids basis) in the starting (35 vol.%) alumina slurry, the mean pore size ranged from 529 to 375 μm, while the mean window size varied from 113 to 77 μm.

Properties of a hydrophone produced with porous PZT ceramic

Interconnected piezoelectric porous ceramics PZT have been tested for possible use in hydrophone applications. These materials consisted of: (1) ceramics with fine porosity having pores from 50 to 100 μm and (2) cellular ceramics having cavities of the order of the millimeters. Porous ceramics are coated with flexible polymer standard polyurethane for the hydrostatics tests. The effect of the size of the pores and porous volume on the sensitivity of the sensor is studied and the performances of the hydrophones devices are presented.