Fernando A. Costa Oliveira

Development of Carbon Coatings for Cordierite Foams : An Alternative to Cordierite Honeycombs

Cordierite foams were prepared by replication of polyurethane foams and were coated with three types of carbon xerogels. The dip coating and synthesis conditions were optimized, and the coated foams were characterized exhaustively. The composition of the starting solution, coat loading, and carbonization temperature are the most important parameters determining both textural and mechanical properties. Carbon xerogel coatings obtained from aqueous solutions of resorcinol (R) and formaldehyde (F) are macro-, meso-, and microporous but present the greatest shrinkage, which causes a loss of adhesion between ceramic foams and carbon coatings. The coatings from polyfurfuryl alcohol (PFA) and RF-poly(vinyl butyral) (Butvar) resin are highly microporous and present very good adhesion even after carbonization. In all cases, coatings induce the improvement of the mechanical properties, which is related to the fact that the coating fills the defects present in the cordierite foams, thereby affecting both the rigidity and the way cracks propagate through the coated samples. These materials, due to the synergetic role of the highly porous coatings and the tortuous channels of the ceramic foams, are suitable materials for adsorption or catalytic treatments of fluids.

Hydroxyapatite Foams for Bone Replacement

Hydroxyapatite, often in the form of synthetic porous blocks, has been used in the repair of bone defects for over 20 years owing to its biocompatibility and osseoconductive behaviour. Bone ingrowth requires the existence of open and interconnected pores with diameters larger than 150 µm for proper circulation of nutrients. Hence, currently available materials are characterised by poor mechanical properties. Collapse of such products is therefore a major source of concern to surgeons using these weak materials in bone surgery. There is a need to develop stronger highly porous structures through adequate control over the size, shape and volume fraction of pores. In this work, highly porous open-cell hydroxyapatite foams were fabricated by the polymer foam replication process, where two types of polyurethane (PU) foams were infiltrated with optimised slurries containing appropriate binders and ceramic particles, followed by the removal of excess slurry, burning out of the polymer to leave a ceramic replica of the polyurethane and finally high temperature sintering. Open-cell HAP foams with porosities of about 80% were obtained, i.e. 30% higher than that determined for commercial ones (50%). Many of the commercial foam cells approach 500 µm in diameter whereas the developed foam cell size ranged from 300 up to 500 µm. The ultimate compressive strength of the developed foams (1-2 MPa) was found to be higher than that recorded for the commercial ones (0.7 MPa) indicating that these foams can more easily be modelled in theatre. Both the elastic moduli and the compressive strength of the developed foams were found to increase with increasing of the relative density, in accordance with the predictions of available micro-mechanical models.

Fabrication of Cellular Cordierite Foams

Cordierite foams were fabricated by the polymer foam replication process, where a polyurethane (PU) template is infiltrated with slurries containing appropriate binders and ceramic particles, followed by the removal of excess slurry, burning out of the polymer to leave a ceramic replica of the polyurethane and finally high temperature sintering. Two PU foam grades of the same density (0.021 g⋅cm -3 ) were used as templates. The structure of the sintered foams was evaluated through density measurements, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Rheological studies showed that optimum dispersion and stabilisation conditions were achieved for aqueous slurries containing 40-vol% of solids, 2-wt% of bentonite and 0.8-wt% of dispersant. Differential thermal analysis (DTA) was used to optimise the burnout of the polymer. Upon heating, decomposition of the polymer occurs, resulting in vaporisation as well as formation of a melt around 300oC. Cracks were observed by SEM on the struts’ surfaces of the fabricated foams. The struts had an angular cross-section and cracks were seen along the cell edges and the cell walls. The density of the sintered foams was typically 20% of the struts density. The volumetric shrinkage was approximately 30% and the linear shrinkage appeared fairly isotropic.

Microwave Sintering - a Novel Approach to Powder Technology

The present paper focus on preliminary work carried out at INETI concerning the use of microwave radiation applied to sintering of both ceramic and metal powders. Due to the characteristics of materials-radiation interaction, microwaves can become an interesting power source in powder technology and other processing routes, since it is possible to lower the sintering temperature and shorten the sintering cycles, leading to time and energy savings. Alumina, hydroxyapatite, titanium and stainless steel powder compacts were sintered in a modified commercial oven of 2.45GHz and 1000W nominal power. Microwave susceptors were used to enable temperature rise during the initial stage of the sintering cycles. Results on densification and microstructural evaluation of microwave sintered samples are reported and compared to conventionally sintered ones, when available. For similar porosity levels upon sintering, microwave radiation generally reduces sintering times from several hours to minutes. The results obtained so far are quite encouraging since in the case of alumina and stainless steel compacts, a decrease of about 200°C in the sintering temperature was achieved. It was also found that the green density plays a key role in the densification of both metallic and ceramic powders.

Zeolite-coated ceramic foams for VOCs removal

Abstract Copper (Cu) and Platinum (Pt) based MFI catalysts have been supported on cordierite foams by an improved washcoating method. Uniform catalyst coatings with a mean thickness as low as 27 μm presenting a good adherence onto the foam surface (weight losses of about 0.4±0.05 wt.%) were obtained. With catalyst contents around 8–10 wt.%, these coated-foam catalysts revealed better catalytic behaviours than their bulk-form counterparts for the deep oxidation of isopropanol. Conversions into CO 2 of about 100% are reached for the platinum-coated foam at 160°C.

Cordierite Foam Supports Washcoated with Zeolite-Based Catalysts for Volatile Organic Compounds (VOCs) Combustion

Reticulated cordierite foams produced by a direct foaming method were successfully washcoated with platinum-based zeolite catalysts. For comparison purposes, commercial cordierite monoliths were also washcoated. The effect of the structural properties on the fluid dynamics and catalytic behaviour for the toluene combustion were evaluated. Foam supports revealed highest performances, in terms of conversion into CO2, when compared to conventional honeycomb monoliths. The experimental results suggest that the catalytic behaviour is critically dependent on the fluid dynamics provided by structural characteristics of the supports, such as porosity, density and size of pores. The randomness and tortuosity of foams enhance reactant mixing, as it was evidenced by the higher axial and radial dispersions of the gas flow across the foam structure. This leads to better mass and heat transfers in the reaction system, thus improving the catalytic behaviour.

Surface Singularity Upon Solar Radiation Heating of Graphite/Tungsten Powder Mixture Compacts to Temperatures in Excess of 1600°C

Synthesis of single-phase tungsten sub-carbide W2C was attempted by heating pellets made out of a source of carbon (graphite-G) and W powders with G/W atom ratio between 0.35 and 0.50 to two target temperatures, namely 1600°C and 1900°C in an argon atmosphere using a solar furnace at PROMES-CNRS in Odeillo (France). The results showed that synthesis of single-phase W2C phase was difficult at either target temperature yielding the W2C co-existed with free metallic W. It was noted that the thin top surface layer of the solar-synthesised tungsten carbide pellets heated to 1900°C was distinguishable from the rest of the bulk specimen showing localised growth of nano-meter scale WC whiskers over W2C grains. Detailed XRD (X-ray diffraction) results on the effect of both G/W ratio and temperature on W2C lattice parameters are discussed.

Optimization of the Synthesis of Hydroxyapatite Powders for Biomedical Applications Using Taguchi's Method

A conventional method of chemical precipitation was used to produce hydroxyapatite (HAp) powders, with a molar ratio of Ca/P=1.67, for biomedical applications. The aim of this work was to study the effect of four control factors, namely pH, reaction temperature, reagent concentration and flow rate addition, at three levels, on the Ca/P ratio of the synthesized powders, using the Taguchi design of experiments method. Nine powders were synthesized using Ca(NO3)2ּ4H2O and (NH4)2HPO4 as starting reagents. The Ca/P molar ratio of each powder was determined by ICP-AES. Data obtained were used in the Taguchi’s design of experiments to optimize the Ca/P molar ratio. Results showed that the pH of the reaction is the main control parameter (74% contribution) affecting the Ca/P molar ratio of the powders. Therefore, attention should be paid to the control of the pH during the synthesis, in order to obtain HAp powders in a reproducible fashion. High pH values (9.5) and reaction temperature at 70 °C favoured the synthesis of HAp powders with a Ca/P ratio close to the target value of 1.67.

Contrasting Morphology of Calcium Phosphate Powders Prepared by the Wet Method Using Poly (Ethylene Glycol)

When preparing β-tricalcium phosphate (β-TCP), it is difficult to industrialise traditional methods because of the problem of particle aggregation. In this paper, β-TCP was prepared by calcining calcium-deficient hydroxyapatite (CDHA) powders obtained by the wet precipitation method at pH=6 and 40 °C, together with additions of poly (ethylene glycol) (PEG), in order to prevent powder agglomeration. The effect of the Ca/PEG mass ratio on the particle size distribution and the morphology of the resulting powders was evaluated. For comparison purposes, one powder without PEG addition was also synthesized. All synthesized powders were characterized by means of X-ray diffraction analysis, particle size analysis and scanning electron microscopy prior and after calcination in static air at 1000 °C for 15 h. Single-phase β-TCP powders have been obtained by calcining CDHA powders in the temperature range of 850°C-1150°C. It was also found that increasing the PEG content in solution decreased the particle size distribution of the agglomerated particles and the β→α-TCP transition temperature. A plausible explanation consistent with these experimental findings is proposed.

Strength Improvement of Cordierite Foams by a Dip Coating Method

The structure and morphological aspects of highly porous (higher than 90%) cordierite (Mg2Al4Si5O18) foams, prepared by a direct foaming method, have been evaluated by Scanning Electron Microscopy analysis. The resulting ceramic foams consisted of a three-dimensional array of struts forming a well-defined open-cell structure. This type of structure seems very attractive for catalyst support purposes. Attempts have been made in order to control the pore structure since it directly affects the physical properties, namely the mechanical strength. In this respect, the use of a dip coating method to improve the strength of the resulting foams was found to be effective in reducing defects (e.g. pores, flaws) in the struts. Based on image analysis, estimated mean cell sizes were about 550 µm whereas strut thicknesses varied in the range of 60-70 µm. The compressive strength of the developed foams increased by one order of magnitude (0.1 to 1 MPa) by increasing the relative density from 0.06 to 0.18.