M.A. Camerucci

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.

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.

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.

High-Temperature Elastic Properties of Ceramics in the System MgO-Al2O3-SiO2 Measured by Impulse Excitation

Youngs moduli of talc-based ceramics from the system MgO-Al2O3-SiO2 are measured for temperatures up to 1000 °C via impulse excitation. It is shown that, after pressing at 50 MPa and firing at 1280 °C, MgO-rich compositions exhibit higher porosity and lower Youngs moduli (approximately 2030 % lower than predicted via micromechanical relations). The Young moduli of materials with less MgO decrease with temperature, but those of MgO-rich ceramics increase with temperature and exhibit a large hysteresis between heating and cooling. Lower absolute values are mainly due to increased porosity, but the reason for the modulus increase with temperature and the hysteresis is the higher enstatite content in the MgO-rich compositions. For a special composition the Youngs moduli are more or less temperature-independent and without significant hysteresis effects, probably due to the low content of enstatite and the high content of sapphirine.