In this paper we report on the preparation and characterization of porous ceramic substrates in the group of NaZr
2(PO
4)
3, CaZr
4(PO
4)
6, SrZr
4(PO
4)
6 and BaZr
4(PO
4)
6 with low thermal ...expansion coefficients. The starting powders were obtained by a sol–gel process and attrition milling both before and after calcination. The resulting sintered bodies exhibited high porosity also after further thermal treatments at high temperature. On the other hand their thermal expansion coefficient (CTE) is low over the range 25–1000
°C. In particular the blend between NaZr
2(PO
4)
3 and SrZr
4(PO
4)
6 has a CTE close to zero in the whole temperature range. According to these properties the above mentioned materials seem to be potential candidates as ceramic substrates for catalytic applications in combustion reactions since they maintain a high level of micrometric porosity when operating at temperatures exceeding 1000
°C.
This study investigates the high temperature chemical resistance of zircon, ZrSiO
4, against Na
2O, CaCO
3, PbO, ZnO, Fe
2O
3, V
2O
5 and CeO
2. It was observed that lead and sodium oxides react with ...zircon only at 700 and 850 °C respectively, while the others do not react over the temperature range examined. Surprisingly, if V
2O
5 and PbO are present at the same time, the reaction of PbO is inhibited and zircon does not suffer degradation. On the other hand, the ternary mixture V
2O
5–PbO–Na
2O reacts slightly with zircon, the reaction was confirmed by XRD after thermal treatment of the mixtures at 650 °C and by FTIR after thermal treatment at 700 °C revealing the formation of lead silicates and lead zirconates. Not all the reactions between zircon and the other oxides are destructive and suggest that zircon is a suitable material for the production of diesel ceramic filters.
It is well known that the durability of the honeycomb structures made of cordierite, used as filters for diesel exhaust emissions, is affected by some components of the soot which can react with ...cordierite during regeneration cycles at high temperature (up to 1000 °C). Amongst all the components of the soot, sodium is particularly dangerous because of its destructive effect on the cordierite structure.
The interaction between sodium carbonate and cordierite has been studied by means of FTIR spectroscopy and the following statements can be made: even if very slowly, the attack on cordierite has already begun at 500 °C and proceeds very quickly onwards from 600 °C; an amorphous phase is formed at 700 °C; at higher temperature, the amorphous phase crystallizes releasing sodium which, during cooling, reacts with carbon dioxide, yielding again sodium carbonate; this carbonate will act as sodium source in the following thermal regeneration cycles. During this study it has also been shown that the first attack is not determined by the sodium ion diffusion into the pseudo-zeolitic channels of the cordierite lattice.
Dredging spoils, due to their composition could be considered a new potential source for the production of monolithic ceramics. Nevertheless, abundance of coloured oxides in these materials preclude ...the possibility of obtaining white products, but not that of producing ceramics with a good mechanical behaviour. As goal of the present research we have produced and studied samples using not only dredging spoils alone, but also mixtures with other waste materials such as bottom ashes from an incinerator of municipal solid waste, incinerated seawage sludge from a municipal seawage treatment plant and steelworks slag. Blending of different components was done by attrition milling. Powders were pressed into specimens which were air sintered in a muffle furnace and their shrinkage on firing was determined. Water absorption, density, strength, hardness, fracture toughness, thermal expansion coefficient of the fired bodies were measured; XRD and SEM images were also examined. The fired samples were finally tested in acidic environment in order to evaluate their elution behaviour and consequently their environmental compatibility. It is observed that, although the shrinkage on firing is too high for the production of tiles, in all the compositions studied the sintering procedure leads to fine microstructures, good mechanical properties and to a limitation of the release of many of the most hazardous metals contained in the starting powders.
Municipal sewage sludge from municipal wastewater treatment plants were thermally transformed into powders which were blended with two types of steelworks slags in different proportions. Mixtures of ...powders, performed by attrition milling, were pressed into specimens which were submitted to thermodilatometric tests in order to evaluate their shrinkage and then their softening temperatures on heating up to 1100
°C. Samples were then sintered in air using a muffle furnace and characterized by density, strength, hardness, fracture toughness measurements, X-Ray diffraction and SEM investigations.
The mechanical properties of the sintered specimens are satisfactory and this may be due to the formation of small grains that, in some cases, are embedded into a vitreous matrix.
Samples obtained by the various blends were aged in an acidic (HCl) water solution to evaluate the elution of the components and it was observed that, through the sintering procedure, it is possible to limit the release of many of the most hazardous metals contained in the starting powders.
Powders were obtained by coprecipitation of solutions of zirconyl chloride hydrate and barium nitrate. Several zirconia-baria solid solutions were obtained by sintering the powders. The fired ...materials had monoclinic, cubic or tetragonal structure depending on composition (4 to 16 mol% BaO). The pure cubic phase could be retained at room temperature for compositions from 10 to 12 mol% whereas materials containing 16 mol% BaO were tetragonal; for lower baria content a mixture of monoclinic and cubic phase was detected. The double phased materials can be fully transformed into the monoclinic form by grinding while for pure cubic or tetragonal samples the transformation is complete. Samples fired at 1450 C had coarse microstructures and the grains dimension is of several micron just after 1 h of sintering, but materials containing 10, 12 and 16-mol% BaO remain cubic or tetragonal, their transformability being the same as that of materials sintered at lower temperature. 24 refs.
We examined the production of macroporous monolithic ceramic from mortars prepared using aluminate cements, one or two different types of alumina powders, superfluidificant, water and polyurethane ...fibres. Some relevant parameters for the preparation process of the mortar are: (i) the weight ratio between cement powder and water, (ii) the weight ratio between superfluidificant and cement powder and (iii) that between superfluidificant and water. Thirty-six percent is the upper limit for the volumetric content of the fibres that can be introduced into the mortar without leading to a material that breaks up by handling after the successive thermal cycle. We also optimized the thermal cycle that may be used in order to avoid the spontaneous break-up of the material during the sintering treatment. Materials containing fine and coarse particles of alumina powders exhibit extended ruptures probably dependent on the thermal stresses that grow on cooling after the sintering process, whereas in materials containing only fine alumina powders the presence of ruptures is more limited so that the fired materials have a sufficient strength and can be handled without a particular care. The ageing time at the maximum temperature plays an important role on the microstructure of the sintered material since it has been seen that after 2
h at 1250
°C the residual porosity seems to be completely closed and the large grain size shows the presence of structures which can be due to the presence of aluminates.
The production of polycrystalline ceramics from YIG powders, obtained by reacting Y
2O
3 and Fe
2O
3 oxides, is a well understood and controlled industrial process. However, methods of production via ...chemical pathways, such as coprecipitation, possess the advantage that the garnet powder is more reactive and no longer requires prolonged grinding which can affect its purity. In this paper, we will show that the mastering of these two processes enables the production of ceramics possessing good magnetic characteristics for microwave applications.
Mullite is one of the most promising substitutes for cordierite as a material for the manufacture of diesel vehicle filters. This study investigated the sensitivity of mullite to chemical attack ...provoked by some of the oxides present in the particulate of diesel-engine exhaust gases. Filter thermal regeneration conditions were simulated by means of heat treatments between 400 and 1000°C. The following pollutants were used: V
2O
5, PbO, ZnO, Fe
2O
3, CeO
2, CaCO
3 and Na
2CO
3 (the last-mentioned as oxide generators). The results obtained were compared with those for cordierite. Mullite was less sensitive to attack than cordierite but was unable to effectively resist attack by PbO and Na
2CO
3.
