Structural study of a K-substituted synthetic cordierite

The crystal structure of a K-substituted hexagonal cordierite with composition K_0.17Mg₂Al_4.17Si_4.83O₁₈ has been solved by single crystal X-ray techniques. The K atoms have been clearly located inside the channels of a typical cordierite framework. They are statistically distributed among the four positions of a 4e ($\text{0, 0,}\text{z}\text{;}\text{z}\text{? 0.15}$) site. This original location was confirmed by X-ray analysis of a powder sample with composition K_0.5Mg₂Al_4.5Si_4.5O₁₈. A very convenient semi-quantitative test to determine the presence and content of K in cordierite is proposed.     

Impedance spectroscopy of CuO-doped Y-TZP ceramics

The effects of CuO additions on the grain interior and grain boundary resistivity in coated 2.5 mol% Y-TZP ceramics have been studied by using impedance spectroscopy. In addition, the effects of hydrothermal ageing on the conductivities of the ceramics were evaluated. The results showed that the grain interior and grain boundary resistivities increased with increasing CuO content. In particular, the grain boundary resistivity was observed to increase significantly in the doped ceramics, which could have been due to the formation of compounds, and resulted in lower conductivity. Impedance spectroscopy performed on the hydrothermally aged Y-TZPs revealed that the grain boundary regions were significantly affected, i.e. the grain boundary resistivities of the undoped and CuO-doped samples were increased by 65% and 45% respectively after ageing for 200 h.Thus, the indication is that the ageing-induced tetragonal to monoclinic phase transformation is closely associated with changes in the grain boundary regions.     

ESR study of degraded bovine bone

A comparison is made between the electron spin resonance (ESR) spectra obtained from bovine bone samples after (i) mechanical degradation, (ii) -irradiation, and (iii) mechanical degradation followed by -irradiation. Mechanical degradation is achieved by filing in either liquid nitrogen or air at room temperature. -irradiation is performed at 77 K. The stability of the radical species produced at 77 K is studied as a function of temperature. It is concluded that the spectra obtained in all instances arise from radicals formed both in the collagen and mineral phase of the bone, those from the collagen predominating. The relative radical concentrations from the collagen and mineral phase vary with the degradation method employed. It is observed that additional hydrogen atom radicals are produced only in the case of the -irradiated samples.On leave of absence from C.N.R. Centre for Macromolecular Physics, University of Bologna, Italy.     

Zirconia toughened cordierite

Zirconia-cordierite ceramic composites were fabricated by the hot pressing of cordierite and zirconia powders at 1400 ° C/20 MPa for 10 min. Both the fracture toughness and fracture strength have been enhanced by the addition of zirconia. Such improvements are due to stress-induced transformation toughening and to the generation of residual stresses. Micro-crack toughening has been found not to occur in the composite as a consequence of the large thermal expansion mismatch. The transformability of zirconia inclusions in the cordierite matrix is higher than in the case of an alumina matrix, although the inclusions have a rounded morphology, resulting from the presence of a liquid phase at the hot-pressing temperature.     

Positron lifetime study of the devitrification process in a cordierite glass

The devitrification of the 13.3% MgO-32.0% Al₂O₃-52.7% SiO₂-2.0% BaO ceramic-glass via isochronal heating up to a stable crystalline structure has been studied by the positron annihilation technique. The lifetime spectra showed two components. This devitrification results as a two-stage process. The long-lifetime ₂, attributed to the pick-off annihilation ofortho-Ps states, decreased on crystallization, but it was insensitive to the hexagonal-orthorhombic transformation of the crystalline cordierite. On the contrary, the effective lifetime of positrons annihilating in states that are not Ps, increased. This increase has been attributed to the Al/Si order process inherent to the hexagonal-orthorhombic transformation. Moreover, the Ps fraction in the crystalline phases was higher than in the glassy phase. However the free volume model might qualitatively explain the relationship observed betweenI ₂ and ₂.     

Powder neutron diffraction study of the thermal expansion of a K-substituted cordierite

A structural study of a potassium-substituted cordierite with K_0.5Mg₂Al_4.5Si_4.5O₁₈ formula was performed between 20 and 825° C using a time-of-flight powder neutron diffraction technique, on sintered samples obtained by glass crystallization. The thermal evolution of this compound is essentially of the same nature as for pure hexagonal cordierite, but is very dependent on the presence of potassium atoms within the channels. The very low value of the thermal expansion coefficient ( between 20 and 825° C) results from an opposite evolution of thea andc lattice parameters which may be explained in terms of a reorganization and a regularization of (Al, Si)O₄ and MgO₆ polyhedra. It strongly depends on the Al/Si ordering degree.     

The crystallization of cordierite glass

The results of a study of the crystallization characteristics of a glass near the cordierite composition are discussed. The devitrification of the base glass was studied by the method of DTA and the development of this technique is described. The effects of additions of vanadium pentoxide and tungstic oxide on the behaviour of the base glass were also determined and are discussed.     

The crystallisation of cordierite glass

A study has been made of the crystallisation of a glass near the cordierite composition (2MgO.2Al₂O₃.5SiO₂) using the techniques of differential thermal analysis and multiple attenuated total reflectance infra-red spectrophotometry. An extensive review of the theoretical aspects of glass crystallisation, and in particular the crystallisation of cordierite-type glasses, was undertaken and is presented as part 1 of this work.     

Microscopic characterization of sol-gel processed cordierite

Different analytical techniques have been used to characterize green and sintered sol-gel processed cordierite on a microscopic scale. The sintered material, although identified macroscopically as -cordierite, contains many small precipitates of different chemical composition. Larger precipitates, characterized by an increased Mg concentration, result from inhomogeneities in the precursor gel. Smaller precipitates, identified as mullite and spinel, could not be related to gel heterogeneities. The amorphous phase surrounding the cordierite grains and the precipitates is Mg depleted and Si enriched. Neither -cordierite nor -cordierite were observed; slight variations of the -cordierite lattice parameters are attributed to some local perturbations due to a slight departure from stoichiometry or Si, Al ordering on tiny domains.     

Mechanical properties of alkoxy-derived cordierite ceramics

Mechanical properties of cordierite ceramics prepared by the controlled hydrolysis and following polycondensation of aluminium, magnesium and silicon alkoxides were investigated in great detail. Flexural strengths of- and-cordierite ceramics are about 60 and 100 MPa, respectively. The flexural strengths of these ceramics are mainly influenced by cracks arising from thermal mismatch between - and -cordierite precipitated during sintering. High fracture toughness of-cordierite ceramics prepared by this method is ascribed to the fine microstructure of the ceramics. The high-temperature flexural strength of-cordierite ceramics is little reduced below 1000° C because of high purity of the ceramics.[/p]     

Crystallization of nickel-bearing stoichiometric cordierite glasses

A complete crystalline solid solution exists between magnesium cordierite (Mg₂Al₄Si₅O₁₈) and nickel cordierite (Ni₂Al₄Si₅O₁₈). Isothermal crystallization experiments were conducted at 1 atm on stoichiometric powdered glasses in this compositional range in which high quartz crystallized and then transformed to hexagonal cordierite. When nickel is present, these three phases can be simply identified by colour changes: the glass is brown, the high quartz is red and the cordierite is yellow. The sequence of appearance of the phases is independent of the nickel content of the glass. At any given temperature, the higher the nickel content, the shorter are both the time required for crystallization of high quartz from the glass and that required for the transformation to hexagonal cordierite. This implies that the processes involved in both steps might be similar. Measurements of lattice constants of cordierite crystallization products show a linear decrease in a and a linear increase in c as a function of increasing nickel content between the magnesium and nickel end members. There is no change of the cell volume with the substitution of nickel into the structure. The increase in reaction rates caused by increasing the Ni/Mg ratio of the glass starting material may be due to the stronger network modifying effect of nickel in comparison to magnesium.     

Copper diffusion during copper cordierite cosintering

The migration of copper into cordierite during a cosintering thermochemical treatment was studied mainly by energy-dispersive X-ray spectroscopy. Various samples were studied, using pressed pellets or green tapes on which copper was applied by either screen printing or sputtering. The influence of various parameters (mainly the atmosphere and the structural state of the ceramic) is presented. It is concluded that the presence of copper in the ceramic near the interface results from two processes: a mechanical transport of copper oxide particles at low temperature and a diffusion in the amorphous phase of the ceramic at high temperature.     

Plasma-sprayed cordierite: structure and transformations

Powdered cordierite (fused and mechanically reduced) feedstock was plasma-sprayed onto a steel substrate and subsequently removed. The substrate-free deposit was thermally cycled and investigated using differential thermal analysis, X-ray diffraction and transmission electron microscopy (TEM). The deposit as sprayed is amorphous, with crystallization to -cordierite occurring at ca. 830° C, followed by a transition to high-cordierite at ca. 1000° C. The transition temperature is influenced by the heating rate and the geometry of the sample. Isothermal kinetic studies of crystallization were carried out, with the growth of the crystalline phase being described by a Johnson-Mehl-Avrami equation. The activation energies were obtained from the differential thermal analysis.     

Thermal expansion behavior of plasma sprayed cordierite

Bulk cordierite has unusually low thermal expansivity. We here report on a study of the thermal expansion behavior of cordierite formed by plasma spraying. It is found that the thermal expansivity is strongly influenced by crystal structure and that the thermal expansion coefficient of the high-cordierite structure is very low (1.84 × 10⁻⁶/°C). Furthermore, the as-sprayed cordierite expands 27% in volume when it is placed next to zirconia in a high-temperature furnace. This expanded structure results in even lower thermal expansivity in the later thermal cycling.     

Anisotropic mullitization in CuO-doped oxide mixture activated by high-energy ball milling

Mullite phase formation and grain growth, in a CuO-doped Al₂O₃ and silica mixture, has been investigated. The oxide mixture was activated with both stainless steel and tungsten carbide milling media. The milled powders demonstrated a much lower mullite formation temperature when compared to the conventional solid-state reaction process. Anisotropic grain growth was observed in the powders milled with stainless steel media, while well-shaped mullite whiskers were produced in the WC milled samples. The lowered mullitization temperature together with the anisotropic grain growth behavior was attributed to the refined microstructure of the powders as a result of the high-energy ball milling process.     

ESR study of the equimolar PbO-V2O5 system

The semiconducting system 50 mol% PbO-V₂O₅ has been studied by electron spin resonance (ESR) in the temperature range 150 to 675 K. Depending on the cooling rate, this system can be obtained from the melt as an amorphous phase, as polycrystalline leadmetavanadate (PbV₂O₆), or as two different mixtures containing polycrystalline and amorphous phases. In all these materials the ratio V⁴⁺/V_tot has been measured by wet analysis showing that the V⁴⁺ concentration is nearly constant. It has been possible to show that the appearance of very different ESR line-shapes for the various materials is due to the difference in the hopping rate of the charge carriers. In particular the presence of a well-defined hyperfine structure is characteristic of the amorphous phase, in which the mobility of the charge carriers is lowered by the presence of a termW _D in the expression of the hopping rate. The lead-metavanadate ESR spectrum shows the presence of two lines of different widths; this fact has been explained assuming the presence of two distinct jumping rates. No temperature variation of the spectra has been observed at low temperatures. The high temperature spectra have permitted us to follow the thermal evolution of the system.     

An ESR study on the photoreactivity of tio2 pigments

The photoreactivity of commercial TiO₂ pigments has been studied by monitoring the generation of radicals at the TiO₂- water interface upon illumination with ultraviolet light. The ESR spin-trapping technique has been employed, using 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) as spin trap. In illuminated TiO₂ dispersions the only radical species identified has been the OH. The amount of radicals generated was found to depend upon the surface properties of the pigment, such as impurities and surface treatments, and upon the oxygen concentration in the slurry. A feasible mechanism for radical generation, involving O₂ adsorption is proposed. The initial rate of radical generation has been found to be in good agreement with other methods commonly used to characterize the photostability of paints.