A.L. Cavalieri

Electrical properties and thermal expansion of cordierite and cordierite-mullite materials

Abstract Commercially available cordierite and mullite powders were used to obtain cordierite and composite materials with mullite content up to 65 wt.% by attrition milling, uniaxial pressing and sintering. The employed cordierite powders were the coarse, medium and fine single granulometric fractions and the binary mixtures of them with 30, 50 and 70 wt.% of the smaller size component. Mullite powder employed in composites was a 7 h-attrition milled one. The dielectric constant (e), dielectric loss tangent (tan δ), resistivity (ρ) and thermal expansion coefficient (α) were measured. The influence of the porosity, mullite and glassy phase contents and grain size in the electrical parameters was analyzed. The thermal expansion coefficient as a function of the composition was studied.

Thermal Evolution of Alumina Prepared by the Sol-Gel Technique

The thermal evolution of an alumina gel synthesized by hydrolysis of aluminium alkoxide (sol-gel technique) was studied by thermal analysis (DTA and TGA), X-ray diffraction, FTIR and NMR spectroscopies, and specific surface area measurements. Between 400 and 900°C, γ- and δ-aluminas were formed showing aluminium vacancies preferentially located in tetrahedral sites. The atomic rearrangements produced during α-alumina formation are oriented to the progressive elimination of tetrahedral aluminium in the ultimate phase. The evolution of the specific surface area during heating is explained by changes in structure and microstructure.

Mechanical behavior of cordierite and cordierite–mullite materials evaluated by indentation techniques

Abstract Commercially available cordierite and mullite powders were used to obtain cordierite and composite materials with mullite content up to 65 wt.% by attrition milling, uniaxial pressing and sintering. Cordierite powders were the coarse, medium and fine single granulometric fractions and the binary mixtures of them with 30, 50 and 70 wt.% of the smaller component. Mullite powder employed in composites was the 7 h-attrition milled one. The Vickers hardness (HV) and fracture toughness (KIC) were measured by Vickers indentation techniques. The Knoop hardness (HK) and Youngs modulus (EK) were estimated by Knoop indentation method. Fracture surfaces of sintered materials were analyzed using a scanning electron microscope (SEM). The indentation load to determinate the fracture parameters was previously determined. The influence of the porosity, increasing mullite and glassy phase contents and grain size on the mechanical parameters were analyzed and a fracture mechanism was proposed.

Elongated mullite crystals obtained from high temperature transformation of sillimanite

The high temperature transformation of sillimanite to mullite has been studied by means of dilatometry, X-ray diffraction, FT-IR and scanning electron microscopy (SEM-EDS) techniques. The results obtained showed that total transformation of sillimanite to mullite takes place between 1550 and 1600°C. A rapid nucleation occurs above 1550°C due to the resemblance of sillimanite and mullite structures. As a consequence of the high temperature of mullitization of sillimanite, the grain growth of elongated mullite crystals takes place almost simultaneously with the nucleation stage. Finally, a step of acid leaching was employed to remove the glassy phase and high-grade purity mullite powders were obtained.

A study by density measurements and indentation tests of a calcium silicophosphate bioactive glass with different MgO or SrO contents

Abstract Vickers and Knoop indentation techniques and density measurements have been employed to study the structure of two series of glasses in the system CaOROSiO2P2O5CaF2 (R = Sr, Ca, Mg) with: (a) constant alkaline earth oxide content, but using different cations (Ca, Ca + Sr, Ca + Mg), and (b) increasing alkaline earth concentration (Mg and/or Ca). Samples have been prepared by melting the raw materials at 1400°C, and pouring the liquid on a steel plate. From the experimental data, microhardness, HV, fracture toughness, KIC, and values for mean atomic volume, VA, and ionic volume fraction, VF, were obtained. The last parameter is proposed as a suitable one for the analysis of variations in cationic coordination numbers, n. The results show that Mg, Ca and Sr have similar values of n. The value of n is proposed to be six.

Behaviour of cordierite materials under mechanical and thermal biaxial stress

Abstract A commercial cordierite powder (< 0.17 wt-% impurities) was selected for a study of material behaviour under mechanical and thermal stresses. Disks were slip cast, sintered for 2 h at 1450°C, and indented (Vickers, 44.1 N) at the centre of the surface to be subjected to mechanical and thermal shock tests. The sintered bodies (84 wt-% cordierite, 10 wt-% mullite, 6 wt-% glass) reached 95% of theoretical density. The microstructure consisted of homogeneous, mainly equiaxed grains (mean size ≈0.5 μm) and a few elongated grains (aspect ratio ≈1.9). A glass phase was identified at triple points, and intergranular pores (< 10 μm) and a few isolated larger pores (up to 40 μm) were observed. The fracture strength σF was measured by biaxial flexure, employing a ball on discontinuous ring configuration with displacement con1 trol (0.05 mm min -1). In each thermal shock test, the indented specimen was heated to a selected temperature and the disk centre was then suddenly cooled using a high velocity air jet at room temperature. The initial temperature was increased by increments of 10°C until crack propagation was detected and the value of the thermal shock resistance Δ TC was evaluated. The values obtained were compared with cordierite disks without indents and with alumina materials. The fracture features of the specimens broken in both mechanical and thermal shock tests (crack patterns and fracture surfaces) were characterised, taking into account the developed microstructures (grains, phases, pores) and the fracture origin at the controlled size defect introduced by indentation.

Modifications of unit-cell parameters in bentonite-ethylene glycol complexes

Abstract The unit-cell parameters of a natural bentonite and several of its monoionic forms (H,Na,K,Ca,Al) were studied by XRD, both in the 60° C-dry state and after complexing with ethylene glycol. The value of the b-dimension did not depend on the nature of the exchangeable cations in the 60°C-dry samples, but it increased to 9.028 and 9.024±0.006 A , respectively, in the original bentonite (mainly sodic) and their potassium and sodium forms when ethylene glycol was present, forming a complete double-layer in the interlamellar space. For the other forms, the b-values were about 9.011 A. This behavior was related to the positions of the exchangeable cations. The value of d(001) in the glycolated bentonites was in the range of 17.2–18.1 A, according to a two-layer ethylene glycol-bentonite complex. The slight differences measured depended on the nature of the exhangeable cations. In 60°C-dry samples, d(001) was smaller for the original, sodium and potassium samples (13.2, 13.3 and 12.2 A), and the higher value corresponded to the Ca-bentonite (16.9 A). The values were related to the hydration state of the exchangeable cations and to their positions in the interlayer space or into the hexagonal cavities of the silicate tetrahedral layers.

Experimental procedure for the mechanical evaluation of oxide-carbon refractories by strain measurement

Experimental issues regarding the implementation of a methodology for obtaining strain-stress curves at high temperatures and in a controlled atmosphere of oxide-C refractories are presented. These curves give a detailed description of the material’s mechanical behavior that is not attainable using conventional tests. The method to measure the specimen strain by contact extensometry and the system to control the atmosphere by a gas flow are described. As an example, the experimental study of commercial Al2O3-MgO-C refractory bricks used in steel ladles at high temperature (1260 °C) in both air and N2 atmospheres is presented showing the valuable information obtained applying strain-stress measurements.

Pressureless sintering of sol–gel alumina matrix composites

Abstract The effect of the temperature and the conditions of the reducing atmosphere on the sintering behaviour of pressureless-sintered sol–gel alumina matrix composites are studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and density measurements. The results are compared with those obtained with fine commercial alumina/SiC W composites, and analyzed by means of a thermodynamic study of the system. A notable improvement in sintering behaviour and a more extensive grain growth as the sintering temperature increases are determined for sol–gel alumina matrix samples. The amount of melt observed in the microstructural analysis of sol–gel alumina matrix materials is lower than that detected in fine alumina matrix samples. The mechanisms of formation of the glassy phase due to the reducing conditions of the atmosphere can operate in both series of samples, but the higher content of impurities in fine alumina matrix materials produces a more extensive liquid formation. The presence of glassy phase, higher starting densities, and lower weight losses during sintering, contribute to the higher final densities of fine alumina matrix composites.

Densification improvement of Al2O3SiCw composites by impregnation

Abstract The effect of the impregnation of Al 2 O 3 SiC w composites (0, 5, 10 and 15 vol% SiC) was studied by Hg porosimetry, density measurements and SEM analysis. Samples were prepared by slip casting, precalcined, impregnated with aluminium nitrate, treated with NH 3 and pressureless-sintered in reducing atmosphere. Higher fired densities were obtained because of the lower pore volume and higher green densities resulting from the impregnation. Also, the high reactivity of the small particles filling the pores in the impregnated samples contributed to the densification improvement.