[en] The development of high-quality ceramic materials depends on the reliable evaluation of their structural, mechanical, thermal and fluid dynamic properties. However, particularly in the case of dense materials, the experimental evaluation of air permeability parameters involves much work and reliable measurements of high pressure and extremely low flow rate values. The objective of this study was to optimize the analytical routine of a new and promising permeametry technique, whose main advantages are its easy operation and fully electronic data acquisition. Tests carried out on a high-alumina ultra-low cement refractory castable sample showed that permeability constants k₁ and k₂ fitted reliably to Forchheimer's equation only when the original data set was screened at pressure intervals of 0.2 bar. It was also observed that the broader the pressure-drop range used for fitting purposes, the more reliable the resulting k₁ and k₂ values, even though permeability constants obtained by different techniques were best compared in similar pressure-drop ranges

[en] Detection of transition phases in alumina, is very important in the receiving control and calcination of aluminium hydroxide. The non alfa or transition phases difficults the processability and causes localized shrinkage on sintering compromising the dimensional and mechanical aspects of the product. In this research using refraction index, absorption of dyes, specific density, X-ray diffraction and scanning electron microscopy, analyses, are done in calcined hydroxides submited to different thermal treatments. The limits and facilities of each technique are discussed and compared. (Author)

[en] The utilization of free calcium hydraulic binder in refractory castables of Al₂ O₃ - Si O₂ system affords a better thermochemical performance. Silica fume improves castable flowability and workability, promotes mullite formation, and the presence of free calcium hydraulic blinder it can act as a sintering additive promoting strength development at low firing temperatures. The main objective of this study was to evaluate the sintering steps and strength development of high alumina castables prepared with or without cement/silica fume. The dialometric and X-ray diffraction analysis were applied to evaluate the influence of phases formation on the mechanical characteristics. (author)

[en] The production of tetragonal zirconia polycrystalline (TZP) ceramics and identification of factors controlling the stability of the tetragonal phase in the ZrO₂.TiO₂ system gas been investigated. In this binary system, it was not possible to retain tetragonal zirconia polycrystals at room temperature for compositions sintered above 1200⁰C. A decrease in the martensitic transformation temperature of zirconia was observed with titania addition, but the effect was insufficient to retain the tetragonal phase at room temperature. In solid solution, TiO₂ additions act to suppress ZrO₂ densification, this leading to grain growth when attempts are made to attain higher densities. The use of fine powders, fast firing or sintering in reducing conditions altered densification but was not able to retain a final grain size sufficiently small to avoid spontaneous tetragonal up and down monoclinic transformation on cooling. (author)

[en] In this paper Ti O₂ has been added to a composition in the Zr O₂.Ce O₂ system which retains the eutectoid molar ratio for the Zr O₂ and Ce O₂ oxides (100 Ce O₂/(Ce O₂ + Zr O₂) 6.7). For a particular composition fully tetragonal structures with theoretical final density have been achieved after sintering at theoretical final density have been achieved after sintering at 1350⁰ C for 2 hours. It is also presented results for densification rate, toughness, fracture energy and mechanical strength for the composition with the highest sinterability. (author)

[en] Short communication

[en] Ceramic-metal composites have been obtained trough S.H.S.- reactions (Self-Propagating High Temperature Synthesis). The resulted material is normally porous and for their application as structural components it is necessary to produce them with high density. This can be carried out by the simultaneous application of mechanical pressure. For this aim, a pressing system and a reaction chamber were constructed. At high temperatures taking advantage of the material plasticity immediately after the occurrence of the reaction, a hydraulic system acts on the sample and compacts it. With this equipment it is possible to change pressure, atmosphere, pre-heating temperature of reactants, final sample dimensions and the delay time for the pressing action. A mold-punger set was specially developed to allow gas escape during pressing. Gases are generated due to S.H.S.-reaction and its removal is important for a successful densification. Alumina-zirconia-niobium composites were obtained from a niobia-aluminium S.H.S.- reaction with additions of zirconia as diluent. This work shows the evolution of the densification in the system mentioned above. (author)

[en] Calcium aluminate cement (CAC) can induce the development of high green mechanical strength to refractory castables in a short period of time (24 h). However, the production of this hydraulic binder is energy-intensive and releases a large content of carbon dioxide (CO₂ ), which adds to global warming. Thus, aiming to develop an alternative binding system, this study investigated the combined addition of aluminum lactate (0.25-1.0 wt%) and calcium carbonate (3.2 wt%) to alumina castables. The prepared samples were analyzed using different experimental techniques, such as cold and hot modulus of rupture, hot elastic modulus, X-ray diffraction, and apparent porosity. According to the results, the lactate addition to the castables improved their green mechanical strength after drying at 110 °C/24 h (4.89 MPa) when compared to the one containing plain calcium carbonate (1.72 MPa). Furthermore, the carbonate-containing refractories presented cold and hot modulus of rupture similar or even superior to the castable containing cement in a wide temperature range (600-1500 °C). (author)

[en] Short communication

[en] Short communication