Vincenzo M. Sglavo

Bauxite 'red mud' in the ceramic industry. Part 1: thermal behaviour

Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon heating was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H2O release from aluminium hydroxides, followed by carbonate decomposition with CO2 evolution below 900°C. Alkaline oxides, mainly CaO and Na2O, lead to the formation of Ca3Al2O6 and NaAlSiO4 between 900 and 1100°C. At the highest temperatures, reduction of Fe3+ to Fe2+, involving O2 release, promotes the formation of Fe2TiO4, with the disappearance of the rutile-TiO2 phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components.

Production of sharp cracks in ceramic materials by three-point bending of sandwiched specimens

Abstract An easy procedure for precracking ceramic specimens is proposed in this paper. The method is based on the three-point bending of a sandwich formed by a ceramic beam inserted between two steel beams. Theoretical features of this procedure are analyzed on the basis of beam theory and fracture mechanics arguments. Crack length can be controlled by the initial saw cut length and by the relative stiffnesses of the ceramic and metal beams. This precracking procedure was applied to two different alumina ceramics. Straight-through sharp cracks with controlled length were proved to be easily introduced and the obtained crack lengths were in good agreement with theoretical predictions.

Micro- and nano-hydroxyapatite as active reinforcement for soft biocomposites.

Pectin-based biocomposite hydrogels were produced by internal gelation, using different hydroxyapatite (HA) powders from commercial source or synthesized by the wet chemical method. HA possesses the double functionality of cross-linking agent and inorganic reinforcement. The mineralogical composition, grain size, specific surface area and microstructure of the hydroxyapatite powders are shown to strongly influence the properties of the biocomposites. Specifically, the grain size and specific surface area of the HA powders are strictly correlated to the gelling time and rheological properties of the hydrogels at room temperature. Pectin pH is also significant for the formation of ionic cross-links and therefore for the hydrogels stability at higher temperatures. The obtained results point out that micrometric-size hydroxyapatite can be proposed for applications which require rapid gelling kinetics and improved mechanical properties; conversely the nanometric hydroxyapatite synthesized in the present work seems the best choice to obtain homogeneous hydrogels with more easily controlled gelling kinetics.

Theoretical and phenomenological analogies between flash sintering and dielectric breakdown in α-alumina

α-alumina pre-sintered samples were subjected to flash sintering at 1200 °C under differing electric field strengths. The analysis of the relation between the incubation time and pre-sintering temperature clearly shows that the presence of pores and surfaces within the sample plays a central role in field-assisted sintering behavior of the material. The observed behavior is accounted for by the strong non-linear electrical conductivity shown by porous alumina at high field strength. The observed non-ohmic conductivity can also be related to the “pre-breakdown” behavior previously described by Frenkel. Literature results suggest that the field involved in flash sintering of alumina is comparable with the dielectric strength at high temperature. We can also state that dielectric breakdown and flash sintering in alumina are associated with similar physical phenomena.

Crack decorating technique for fracture-toughness measurement in alumina

Abstract A technique for fracture toughness measurement in alumina is presented. Alumina with bimodal grain-size distribution and containing about 10% inter-granular glassy phase was used as test material. Controlled defects were introduced by indentation using loads from 1.5 N to 39.2 N. Some specimens were annealed at 1000 °C in order to remove the indentation residual stress field. Crack shape and dimension were analyzed by a decorating technique. Red colorant was poured on indented specimens and decorated fracture surfaces were then observed by optical microscopy on later drying. This technique allowed the complete analysis of indentation crack evolution upon bending. Both indented and annealed specimens were considered. Shape-factor evolution during crack growth was investigated and constant fracture toughness values (3–3.5 MPa √ m independent from crack growth and indentation load were calculated.

The sub-critical indentation fracture process in soda-lime-silica glass

The sub-critical propagation of indentation cracks in soda-lime silicate glass was investigated. Vickers indentations were produced using a force of 9.8 N, and both as-indented and annealed samples were studied. Indented bars were subjected to various flexural loading and unloading cycles in a water environment. Fractographical analysis revealed that the radial crack propagation process could be divided into two successive stages. Initially, the radial crack growth is strongly influenced by interaction with the lateral crack, which obstructs crack growth. This “pinning” effect of the lateral crack leads to bowing of the radial crack front into a complex shape. In the second stage, the radial cracks escape from the lateral cracks interaction and assume a semi-elliptical shape. Measurement of radial crack speed from the markings left on the fracture surface allowed the crack shape factor, ψ and the indentation-residual stress factor, χ, to be calibrated as a function of the crack size, c. The shape factor was found to be a decreasing function of c during the first stage of propagation, then it abruptly increases when radial cracks escape from lateral crack interaction. For further crack extension, ψ decreases again, assuming absolute values larger than those measured during the first stage. Conversely, the residual stress factor is constant during the first phase of radial crack growth while it suddenly decreases and becomes zero for larger c. The implications of the trend in ψ(c) and ψ(c) on the stable growth and on the time-to-failure predictions in static fatigue tests are discussed.

Vertical sintering to measure the uniaxial viscosity of thin ceramic layers

Abstract A novel method is proposed for the determination of the uniaxial viscosity of porous ceramic layers upon sintering. This approach is based on the application of a continuous but very low tensile stress to the densifying powder compact whose deformation is continuously monitored by an optical system. The viscosity of the system can be determined as a function of temperature and density from the sintering rate differences measured between loaded and unloaded samples. The uniaxial viscosity of porous Y 2 O 3 doped ZrO 2 (YSZ) and NiO–YSZ composites was measured using the proposed approach. The results were used to predict the curvature evolution of bilayers used in solid oxide fuel cell applications, obtaining a fairly good agreement between the model and the data recorded experimentally.

Synthesis and characterization of strontium-substituted hydroxyapatite nanoparticles for bone regeneration.

The production of stable suspensions of strontium-substituted hydroxyapatite (Sr-HA) nanopowders, as Sr ions vector for bone tissue regeneration, was carried out in the present work. Sr-HA nanopowders were synthesized via aqueous precipitation methods using Sr2+ amount from 0 to 100mol% and were characterized by several complementary techniques such as solid-state Nuclear Magnetic Resonance spectroscopy, X-ray diffraction, Infrared spectroscopy, N2 physisorption and Transmission Electron Microscopy. The substitution of Ca2+ with Sr2+ in HA is always isomorphic with gradual evolution between the two limit compositions (containing 100% Ca and 100% Sr), this pointing out the homogeneity of the synthesized nanopowders and the complete solubility of strontium in HA lattice. Strontium addition is responsible for an increasing c/a ratio in the triclinic unit cell. A significant variation of the nanopowders shape and dimension is also observed, a preferential growth along the c-axis direction being evident at higher strontium loads. Modifications in the local chemical environment of phosphate and hydroxyl groups in the apatite lattice are also observed. Stable suspensions were produced by dispersing the synthesized nanopowders in bovine serum albumin. Characterization by Dynamic Light Scattering and ζ-potential determination allowed to show that Ca2+→Sr2+ substitution influences the hydrodynamic diameter, which is always twice the particles size determined by TEM, the nanoparticles being always negatively charged as a result from the albumin rearrangement upon the interaction with nanoparticles surface. The biocompatibility of the suspensions was studied in terms of cell viability, apoptosis, proliferation and morphology, using osteosarcoma cell line SAOS-2. The data pointed out an increased cell proliferation for HA nanoparticles containing larger Sr2+ load, the cells morphology remaining essentially unaffected.

A new HA/TTCP material for bone augmentation: an in vivo histological pilot study in primates sinus grafting.

Objective:Synthetic calcium phosphate bone substitutes are widely used in sinus graft procedures due to their osteoconductive and biocompatible properties. Hydroxyapatite (HA), beta-tricalcium phosphate (&bgr;-TCP), and HA/&bgr;-TCP composite are the most applied materials. The aim of this study was to propose a new mineralogical formulation, HA/tetracalcium phosphate (TTCP), as biomaterial for bone regeneration in the maxillary sinus. Methods:Sinus grafts were performed by using granules of a HA/TTCP blend and a collagen membrane. Bone response at time points of 14 and 17 weeks was histologically evaluated. Results:After 14 weeks of healing, histomorphometric analysis showed the formation of new bone trabeculae among HA/TTCP granules. After 17 weeks, the bone trabeculae were thicker and HA/TTCP granules were still present. Histomorphometric analysis revealed a bone graft contact (BGC) of 64%. Conclusions:After 17 weeks from implantation, HA/TTCP synthetic bone graft performed very well as osteoconductive material: BGC was found very high, and bone volume and vital bone showed an ideal bone density for implant placement. HA/TTCP granules are accounted for to accelerate new bone formation and to reduce the time needed for the graft healing, thus achieving high quantity of the new bone formed.

Protective Coatings of Metallic Interconnects for IT-SOFC Application

The development of high-performing planar solid oxide fuel cell (SOFC) stacks operating at intermediate temperature (700-850°C) is based on thin-electrolyte anode supported cells (ASCs) and interconnects made by ferritic stainless steels. These metallic materials match very well the thermal expansion behavior of the ASCs and can be manufactured and formed using cheaper and easier processes than ceramics or chromium alloys. Nevertheless, some problems remain to be solved with these components as the performance degradation due to the oxide scale growth at the cathodic contact surface and the evaporation of volatile Cr-containing species, which poisons the cathodic materials. Both effects strongly limit the stack performance compared to single cells and increase the degradation rate with time. Providing the steel composition is carefully controlled, the above problems can be limited and some special ferritic stainless steels have been developed in the past years for SOFC application. Unfortunately, no commercial alloy is still able to satisfy the limit in degradation rate required for stationary applications (SECA target is <0.25% upon 1000 h on a minimum service life of 40,000 h). To achieve these goals a further improvement of composition should be required but this cannot be easily obtained in a cost-effective large-scale metallurgical production. An alternative and probably simpler way is to coat the surface of the steel with a protective layer with the twofold aim to limit Cr evaporation and to develop a conductive scale. In the present work, the effect of different oxide coatings on the chromium evaporation rate and on the contact resistance of ferritic stainless steel has been investigated. To obtain a conductive layer, spinel compositions containing Co, Mn, and Cu have been considered. Steels surfaces have been spray-coated using alcoholic suspensions, and the microstructural evolution of the interface between the metallic substrate and oxide layers has been investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy linescan analysis for exposure at high temperature. The variation with time of the areaspecific resistance at 800°C has been recorded up to 1000 h. The evaporation rate of Cr-containing species has been also evaluated by a qualitative method.