Authors: Teresa Marcelo, João M.G. Mascarenhas, Fernando A. Costa Oliveira
Abstract: 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.
946
Authors: Fernando A. Costa Oliveira, Hiroaki Nitani, T. Nakagawa, T.A. Yamamoto, Nobumitsu Shohoji
Abstract: The use of ultra-bright, highly directional, high and variable energy associated to synchrotron radiation beams at SPring-8 when compared with conventional X-Ray diffraction analysis applied to slate powders heat treated at temperatures in the range of 600°C up to 1000°C is reported. Fe K-edge XAFS measurements of the products formed upon heating slate showed that in the temperature range 600-800°C, conversion of FeO present in the as-received slate powder into a mixed -Fe2O3/-Fe2O3 has occurred providing useful information on the short-range order atomic structure of the slate. Above 800°C, mainly -Fe2O3 was identified to be present in the slate. The change of the Fe valences within the slate network is most likely responsible for the detected colour change with the increasing temperature.
922
Authors: Elisabete R. Silva, J.M. Silva, Fernando A. Costa Oliveira, M. Fatima Vaz, M. Filipa Ribeiro
Abstract: 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.
104
Authors: Fernando A. Costa Oliveira, Bernard Granier, J.M. Badie, Jorge Cruz Fernandes, Luis Guerra Rosa, Nobumitsu Shohoji
Abstract: 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.
993
Authors: Susana Dias, Fernando A. Costa Oliveira, C. Henriques, F.R. Ribeiro, Carmen M. Rangel, M. Filipa Ribeiro
Abstract: The reactors used for Selective Catalytic Reduction (SCR) of NOx require low pressure
drop structured catalyst packing. Structured packings, such as ceramic foams, are gaining increasing
interest for application in low pressure drop reactors, membrane reactors and catalytic distillation
units. In this work, cobalt ion exchanged mordenite (Co-HMOR)-coated cordierite-based foams
produced by the replication method were evaluated for catalytic reduction of NOx with methane.
The addition of 0.3 wt.% Pd to 2 wt.% Co-HMOR leads to a material that can convert 50 % NOx to
N2 at 450 °C in a reaction mixture containing 2000 ppm CH4, 1000 ppm NOx, 5 % O2 and balance
helium, at GHSV=17000 h-1. Although in an early stage of development, an efficient coating
procedure was explored and different ways of exchange of Co and Pd cations into mordenite
(Si/Al=10) were studied. Additions of 2 wt.% fumed silica enhanced adhesion of the zeolite onto
the ceramic foam. Pd-exchanged Co-HMOR showed to be very sensitive to steam. A 50 % decrease
in NOx conversion to N2 was observed after Pd/Co-HMOR samples were exposed at 450 °C to a
reaction mixture containing 2 vol% H2O. Although further research is needed to ascertain the
mechanism of this deactivation behaviour, agglomeration of Pd forming PdO particles is envisaged.
810
Authors: Elisabete R. Silva, J.M. Silva, Fernando A. Costa Oliveira, F.R. Ribeiro, João C. Bordado, M. Fatima Vaz, M. Filipa Ribeiro
Abstract: 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.
123
Authors: Fernando A. Costa Oliveira
Abstract: The damage imposed on open-cell mullite ceramic foams was evaluated in pre-mixed
radiant gas burners. After exposure to the prevailing combustion environment, foams suffered
moderate strength degradation as a result of thermal stresses being imposed on the material during
service. There was evidence of chemical attack during combustion although thermal shock
measurements suggest that damage sustained by the foams results mainly from thermal shock rather
than chemical degradation. Indeed, samples from burners subjected to ageing tests did not show
additional damage compared to those subjected to short ageing tests indicating that most of damage
occurred during start-up. For comparison purposes, a set of ceramic foam samples were subjected to
a water quench test so that the extent to which the foams were damage by exposure to the
combustion environment, under well controlled conditions, could be established. The strength
retained after thermal shock by open-cell mullite foams decreased gradually with increasing quench
temperatures. This suggests a cumulative damage mechanism reflecting an increase in damage
throughout the material rather than sudden failure owing to propagation of pre-existing cracks along
a plane. Damage in mullite foams was mainly localised at the top layer of the burners where higher
temperatures and steeper thermal gradients were imposed on the material. Surprisingly, needle-like
mullite crystals with a large aspect ratio were also found to have grown at the surface of the burners
via a vapour feed gas-liquid catalyst-solid needle-like growth (VLS) mechanism.
99
Authors: Vera Lourenço, Susana Dias, Carlos A. Nogueira, Fernando A. Costa Oliveira
Abstract: This study focus on the synthesis of Ca-deficient apatites (CDA) by using the wet
chemical precipitation method in demineralised water solutions at pH=7 and synthesis temperature
of 90 °C. Upon calcination at 1000°C, the resulting HAP powders decomposed leading to the
formation of small amounts of β-TCP. Surprisingly, under some conditions, minor amounts of α-
TCP were also formed at 1000°C together with β-TCP. The appearance of α-TCP is favoured by the
absence of mineral ions in the precipitation medium, which in turn depends on the purity of the
water used. Factors affecting the formation of both β-TCP and α-TCP are discussed.
37
Authors: Susana Dias, Vera Lourenço, Carlos A. Nogueira, Fernando A. Costa Oliveira
Abstract: 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.
32
Authors: Fernando A. Costa Oliveira, Patricia Pascaud, Joaquim Domingues, Teresa Marcelo, Jorge Cruz Fernandes, Luis Guerra Rosa
Abstract: Hydroxyapatite (HA) is widely used as a bone repair material. The use of microwave
radiation as energy source for powders densification opens new opportunities for sintering HA at
lower temperatures than by conventional sintering (electric furnace). The aim of this work was to
compare the densification behaviour of a commercial hydroxyapatite powder under microwave and
conventional heating conditions through evaluation of the elastic properties of the resulting sintered
materials. The effect of green density, sintering temperature and dwell time on the Young’s
modulus E (measured by the impulse excitation of vibration method) of microwave sintered HA
was evaluated by using the Taguchi method. Under the set of controlled parameters investigated,
green density was the most significant factor affecting E. It was verified that the highest E values
obtained with each sintering process are similar (104 and 105 GPa for microwave and conventional
sintering, respectively). These values were obtained at 1250°C for 2 h in conventional sintering and
at 1200°C for 10 min with microwave sintering. This illustrates the advantage of microwave
processing on energy and time savings over conventional sintering.
17