P. Kavouras

Microstructural changes of processed vitrified solid waste products

Abstract Toxic lead-rich solid industrial wastes were stabilized by the vitrification method. Vitrification was attained by the addition of SiO 2 and Na 2 O as vitrifying and melting agent, respectively. The non-toxic, homogeneous, vitreous products studied in the present work, contain 60 wt.% of solid waste. Products with such a high content of solid waste comprise an economically realistic suggestion, but are easily devitrified in conditions of large-scale production due to the difficulty to achieve rapid cooling conditions in the whole volume of a large piece of stabilized product. Thus, it must be ascertained that the loss of homogeneity is not accompanied with the loss of chemical stability. Differential Thermal Analysis (DTA) was applied in order to inspect the prospect to crystal phase separation. The separated crystal phases were characterized by X-ray diffraction and transmission electron microscopy. Possible devitrification processes are investigated in order to interconnect the microstructure with the chemical stability of the devitrified products.

Vitrification of lead-rich solid ashes from incineration of hazardous industrial wastes

Lead-rich solid industrial wastes were vitrified by the addition of glass formers in various concentrations, to produce non-toxic vitreous stabilized products that can be freely disposed or used as construction materials. Toxicity of both the as-received industrial solid waste and the stabilized products was determined using standard leaching test procedures. The chemically stable vitreous products were subjected to thermal annealing in order to investigate the extent of crystal separation that could occur during cooling of large pieces of glass. Leaching tests were repeated to investigate the relation between annealing process and chemical stability. X-ray, scanning and transmission electron microscopy techniques were employed to identify the microstructure of stabilized products before and after thermal treatment. Relation between synthesis and processing, chemical stability and microstructure was investigated.

Incineration of tannery sludge under oxic and anoxic conditions: study of chromium speciation.

A tannery sludge, produced from physico-chemical treatment of tannery wastewaters, was incinerated without any pre-treatment process under oxic and anoxic conditions, by controlling the abundance of oxygen. Incineration in oxic conditions was performed at the temperature range from 300°C to 1200°C for duration of 2h, while in anoxic conditions at the temperature range from 400°C to 600°C and varying durations. Incineration under oxic conditions at 500°C resulted in almost total oxidation of Cr(III) to Cr(VI), with CaCrO4 to be the crystalline phase containing Cr(VI). At higher temperatures a part of Cr(VI) was reduced, mainly due to the formation of MgCr2O4. At 1200°C approximately 30% of Cr(VI) was reduced to Cr(III). Incineration under anoxic conditions substantially reduced the extent of oxidation of Cr(III) to Cr(VI). Increase of temperature and duration of incineration lead to increase of Cr(VI) content, while no chromium containing crystalline phase was detected.

Effect of composition and annealing temperature on the mechanical properties of a vitrified waste

The microhardness and indentation fracture toughness of a vitrified industrial waste, in the form of ash, were characterized by the method of static indentation test. These properties were investigated as a function of composition and thermal treatment conditions. To further understand the influence of the constitutive oxides, additional measurements were made in a series of synthetic products, where a mixture of iron and lead oxides was used as a substitute for the ash. The underlying deformation mechanisms controlling the mechanical properties of such complex systems as determined by static indentation tests are discussed. It was found that plasticity, in the amorphous products, is mainly governed from the silica content, even in the cases where silica is not found in sufficient quantity in order to build an extended three-dimensional vitreous network. On the other hand, the morphology of the separated crystalline phases is the dominant factor affecting plasticity in the glass-ceramic products.

Effect of heat treatment and in vitro aging on the microstructure and mechanical properties of cold isostatic-pressed zirconia ceramics for dental restorations

OBJECTIVES The temperature variations during the veneering firing cycles of a zirconia dental ceramic can negatively affect its mechanical properties. A possible synergistic effect of both heat-treatment and aging while exposed to the oral environment could result to catastrophic failure. The aim of the present study was to investigate the effect of heat treatment followed during veneering and in vitro aging on the mechanical and microstructural properties of zirconia dental ceramics. METHODS Three specimens from each of two zirconia blocks (Ivoclar IPS e.max ZirCAD (IV) and Wieland ZENO Zr (WI)) were cut by CAD/CAM technology, fully sintered and polished. Each one was cut in four equal parts. One part was used as control (C), one was heat-treated (H), one was aged (A) (134°C, 2bar, 10h) and one was heat-treated and subsequently aged (HA). The mechanical properties (nano-hardness (H) and elastic modulus (E*)) were investigated by nano-indentation tests while the surface characterization was carried out with XRD, FTIR and SEM. RESULTS Different treatments on IV and WI samples resulted in a reduction of both H and E* values, however the differences were not statistically significant (p>0.05). The combination of treatments imposes an overall effect (p<0.001), enhancing the influence on both H and E* values. This reduction in mechanical properties was followed by an increase of monoclinic content. Greater variations in both H and E* values were recorded for WI samples. SIGNIFICANCE The clinical performance of zirconia dental ceramics may be affected during firing and aging resulting in increased probability of failure.

A parametric study of implantation-induced variations on the mechanical properties of epitaxial GaN

Doping of GaN through ion implantation permits improved control of the dopant profile and dose, while it also modifies the films mechanical properties such as microhardness. We discuss the effect of Si + - and Mg + -ion implantation on the mechanical properties of GaN films grown by electron cyclotron resonance plasma-assisted molecular beam epitaxy on sapphire substrates. The changes of the mechanical properties are studied using the static indentation method and they are discussed in conjunction with results from near-edge x-ray absorption fine-structure spectroscopy. Transmission electron microscopy is used for the microstructural characterization of selected implanted specimens. The static indentation test method was applied utilizing Vickers and Knoop diamond indenters. The microhardness results are absolved from the influence of the substrate through the implementation of a well-established deconvolution method. The elastoplastic response of the films are compared and discussed on the basis of the proportional specimen resistance model and the form of the indentation size effect curves.

The effect of photo-activated glazes on the microhardness of acrylic baseplate resins

OBJECTIVE A comparative investigation of acrylic denture base surface microhardness, induced through glazing with different photo-activated liquids. MATERIALS AND METHODS Thermopolymerized acrylic resin Paladon 65 (Kulzer) was used for this study. The samples were mechanically thinned by silicon carbide grinding papers and finally, mechanically polished by alumina pastes. The samples were then glazed with Palaseal, Plaquit and Lightplast-Lack photo-activated liquids. Microhardness tests were carried out via a Zeiss optical microscope equipped with an Anton Paar microhardness tester fitted with a Knoop indenter. RESULTS Microhardness testing performed on surfaces glazed by Plaquit, Lightplast-Lack, and Palaseal photo-activated liquids showed enhanced microhardness values compared to the mechanically polished acrylic resin denture base material. CONCLUSIONS Comparative microhardness tests performed on acrylic base resin treated with photo-activated acrylic glazes showed that all increases the surface microhardness. The enhancement of surface microhardness of acrylic denture bases suggests that they are likely to resist wear during service.

Anisotropic microhardness and crack propagation in epitaxially grown GaN films

The anisotropic character of microhardness of cubic and hexagonal GaN films epitaxially grown on silicon and sapphire, respectively, is investigated. A series of Vickers indentations demonstrate anisotropic indentation induced crack propagation in cubic GaN, whereas in the hexagonal material no cracks are formed for indentations with the same load. Directions of easy crack propagation are observed. A series of Knoop indentations is conducted in order to inspect the probable microhardness orientation dependence relative to the main crystallographic directions of films. The measurements reveal an orientation dependence of microhardness at room temperature only in hexagonal GaN films. The indentations are made with an angular interval of five degrees and the microhardness-orientation curve is obtained. The curve has a periodicity close to sixty degrees.

Devitrification routes of a vitrified chromium-loaded ash

The devitrification routes of vitrified materials containing chromium-loaded ash were studied in this work. Chromium-loaded ash originates from the incineration of tannery sludge. Vitrification was applied using SiO2, Na2O and CaO. Three different batch compositions were studied with the relative proportions of SiO2 and Na2O kept constant, and varying proportions of chromium-loaded ash and CaO. All vitrified products were thermally treated in order to produce glass–ceramic materials, i.e. to induce devitrification. Thermal treatment temperatures were selected by application of differential thermal analysis. All products were characterized with X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. The resulting glass–ceramic products possessed different microstructures, i.e. composition, morphology and spatial distribution of separated ceramic phases, depending on the chromium-loaded ash content and thermal treatment temperature. The results show that a combination of differential thermal analysis with morphological, structural and elemental characterization methods renders microstructural tailoring and control of physical properties feasible.

Deformation and fracture in (0001) and (10-10) GaN single crystals

ABSTRACT Indentation techniques were utilised to induce deformation on polar (0001) c-plane and non-polar m-plane GaN single crystal. Cracking was more sensitively dependent on the orientation of the indenter tip, compared to hardness. The indentation-induced plastic deformation and fracture sequences were studied by cathodoluminescence imaging and optical microscopy, respectively. Polar GaN was harder than non-polar, while pop-in discontinuities occurred at lower loads in polar than non-polar GaN. Dislocation arrangements were more isotropic at the polar than the non-polar orientation. Polar GaN was more susceptible to cracking compared to non-polar. Indentation at the high load regime fostered radial and lateral crack formation at both indenter orientations in polar GaN. Post-indentation lateral crack propagation was observed in situ in polar GaN. This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.