Konstantinos M. Paraskevopoulos

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.

Hydroxy Carbonate Apatite Formation on Particulate Bioglass In Vitro as a Function of Time

The mechanism of bonding of bioactive glasses with living tissues has been reported to be associated with the development of a layer consisting of carbonate-containing hydroxyapatite similar to that of bone on the surface of the materials. This layer is also formed in vitro, in solutions with ion concentrations similar to those of human blood plasma, like SBF (Simulated Body Fluid). The development of HCAp (Hydroxy carbonate apatite) layer on the surface of a commercially available Bioglass® Synthetic Bone Graft Particulate (Perioglas®) after immersion in SBF solution using Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) with associated Energy Dispersive Spectroscopy (EDS). PerioGlas® powder with particle size 20-63μm, pressed in a vacuum press in order to produce pellets. The pellets were soaked in SBF for 12, 18, 24 and 48 hours at 37°C. Results revealed the formation of an amorphous CaO-P 2 O 5 - rich layer on the surface of the specimens after 12 hours in the solution and a well crystalline HCAp layer after 24 hours immersion time.

Mechanical properties, electrochemical corrosion and in-vitro bioactivity of yttria stabilized zirconia reinforced hydroxyapatite coatings prepared by gas tunnel type plasma spraying

Yttria stabilized zirconia reinforced hydroxyapatite coatings were deposited by a gas tunnel type plasma spray torch under optimum spraying conditions. For this purpose, 10, 20 and 30 wt% of yttria stabilized zirconia (YSZ) powders were premixed individually with hydroxyapatite (HA) powder and were used as the feedstocks for the coatings. The effect of YSZ reinforcement on the phase formation and mechanical properties of the coatings such as hardness, adhesive strength and sliding wear rates was examined. The results showed that the reinforcement of YSZ in HA could significantly enhance the hardness and adhesive strength of the coatings. The potentiodynamic polarization and impedance measurements showed that the reinforced coatings exhibited superior corrosion resistance compared to the HA coating in SBF solution. Further the results of the bioactivity test conducted by immersion of coatings in SBF showed that after 10 days of immersion of the obtained coatings with all the above compositions commonly exhibited the onset of bioactive apatite formation except for HA+10%YSZ coating. The cytocompatibility was investigated by culturing the green fluorescent protein (GFP)-labeled marrow stromal cells (MSCs) on the coating surface. The cell culture results revealed that the reinforced coatings have superior cell growth than the pure HA coatings.

Highly anisotropic crystal growth and thermoelectric properties of K2Bi8−xSbxSe13 solid solutions: Band gap anomaly at low x

The thermoelectric properties of solid solutions of the type β-K2Bi8−xSbxSe13 (0<x<8) were studied with respect to thermal behavior, band gap variation, and charge transport properties as a function of x. At x values between 0 and 1.5, the energy band gap is observed to decrease (anomalous) before it widens with increasing x values as would be expected. For selected members of the solid solutions, the Bridgman technique was applied to obtain well-grown oriented ingots that were used to measure the thermal conductivity and charge transport properties in different growth directions. The measurements showed a strong anisotropy in thermoelectric properties with the largest anisotropy observed in the electrical conductivity. Lattice thermal conductivities of the selected solid solutions were observed to decrease when the x value increases. Preliminary doping studies on the x=1.6 member were carried out and it was shown that it is possible to significantly increase the power factor.

High temperature charge and thermal transport properties of the n-type thermoelectric material PbSe.

We present a detailed study of the charge transport, infrared optical reflectivity, and thermal transport properties of n-type PbSe crystals. A strong scattering, mobility-limiting mechanism was revealed to be at play at temperatures above 500 K. The mechanism is indicative of complex electron-phonon interactions that cannot be explained by conventional acoustic phonon scattering alone. We applied the first-order nonparabolicity approximation to extract the density-of-states effective mass as a function of doping both at room temperature and at 700 K. The results are compared to those of a parabolic band model and in light of doping-dependent studies of the infrared optical reflectivity. The thermal conductivity behavior as a function of temperature shows a strong deviation from the expected Debye-Peierls high-temperature behavior (umklapp dominated) indicating an additional heat-carrying channel, which we associate with optical phonon excitations. The correlation of the thermal conductivity observations to the high-temperature carrier mobility behavior is discussed. The thermoelectric figure of merit exhibits a promising value of {approx} 0.8 at 700 K at {approx} 1.5 x 10{sup 19} cm{sup -3}.

Effect of in vitro aging on the flexural strength and probability to fracture of Y-TZP zirconia ceramics for all-ceramic restorations.

OBJECTIVES Dental zirconia restorations should present long-term clinical survival and be in service within the oral environment for many years. However, low temperature degradation could affect their mechanical properties and survival. The aim of this study was to investigate the effect of in vitro aging on the flexural strength of yttrium-stabilized (Y-TZP) zirconia ceramics for ceramic restorations. METHODS One hundred twenty bar-shaped specimens were prepared from two ceramics (ZENO Zr (WI) and IPS e.max(®) ZirCAD (IV)), and loaded until fracture according to ISO 6872. The specimens from each ceramic (nx=60) were divided in three groups (control, aged for 5h, aged for 10h). One-way ANOVA was used to assess statistically significant differences among flexural strength values (P<0.05). The variability of the flexural strength values was analyzed using the two-parameter Weibull distribution function, which was applied for the estimation of Weibull modulus (m) and characteristic strength (σ0). The crystalline phase polymorphs of the materials (tetragonal, t, and monoclinic, m, zirconia) were investigated by X-ray diffraction (XRD) analysis, Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. RESULTS A slight increase of the flexural strength after 5h, and a decrease after 10h of aging, was recorded for both ceramics, however statistically significant was for the WI group (P<0.05). Both ceramics presented a t→m phase transformation, with the m-phase increasing from 4 to 5% at 5h to around 15% after 10h. SIGNIFICANCE The significant reduction of the flexural strength after 10h of in vitro aging, suggests high fracture probability for one of the zirconia ceramics tested.

Synthesis and characterization of LiMn2O4 for use in Li-ion batteries

Lithiated spinel manganese dioxide was synthesised from electrochemical MnO 2 and Li2CO 3 with deficiency or excess lithium (Li x Mn 2 O 4 , 0.8 <x < 1.3) for use in Li/Li x Mn 2 O 4 and Li-ion cells. Micron-sized Li 1.05 Mn 2 O 4 prepared at 730°C showed high Li utilization, excellent cyclability and good rate capability with an initial discharge capacity of 123 mA h/g and 10% discharge capacity reduction after 20 cycles. Different types of commercial carbonaceous materials were also investigated with respect to their electrochemical performance vs. Li. Unoptimised Li-ion cells, using Li 1.05 Mn 2 O 4 prepared at 730°C as the cathode material, EC-DMC-LiPF 6 electrolyte and carbon fibres, showed promising performance characteristics.

Structure inhomogeneities, shallow defects, and charge transport in the series of thermoelectric materials K2Bi8−xSbxSe13

The charge transport properties of the low-dimensional thermoelectric materials K2Bi8−xSbxSe13 (0<x⩽8) were studied as a function of temperature and composition. The Seebeck coefficient shows an evolution from n- to p-type character with increasing incorporation of Sb, and at the same time the temperature dependence of the electrical conductivity changes from that of a degenerate semiconductor to that of an intrinsic or compensated semiconductor. These changes, however, are not monotonic with composition due to the nonuniform substitution of Sb atoms at the Bi sites of the structure. Three separate composition regions can be assigned depending on x each with different charge transport characters. Electronic transport in K2Bi8−xSbxSe13 was analyzed on the basis of the classical semiconductor theory and discussed in the context of recent band calculations. The results suggest that the K2Bi8−xSbxSe13 materials possess coexisting domains with semimetallic and semiconducting characters whose ratio is influence...

Electrical and optical properties of lithium intercalated III–VI compounds

Abstract III-VI layered compounds such as InSe and GaSe are intercalated with lithium by spontaneous reaction in n-butyllithium and electrochemical process. These compounds show to be good hosts for the insertion of Li at room temperature. Electrical and optical properties of intercalated materials are presented. The insertion reaction is characterized by the time resistivity variation of the host material. Electrical transport measurements on Li intercalated InSe and GaSe show a change by 3-orders of magnitude of the conductivity with respect to non intercalated samples. The effect of Li intercalation on the emission spectrum of host material is presented and interpretation of observed deviation is given. Li intercalation in InSe induces in the luminescence spectra characteristics which are the manifestation of new luminescence centers.

Thermal characterization of carbonate rocks, Kozani area, North-western Macedonia, Greece

Representative carbonate rock samples collected from the broader area of Kozani (NW Macedonia, Greece) are examined. The participation of constituents in the formations was determined combining three different methods of analysis, atomic absorption spectroscopy (AAS), X-ray diffraction analysis (XRD) and thermogravimetry (TG). The three methods are used in a complimentary way in order to specify the exact composition of the samples. Although the question about the composition can be answered by AAS, the percentage of the included calcite and dolomite can be determined with accuracy, only through TG analysis, under the appropriate conditions, regarding the gas carrier. The results of the analysis lead to a more complete view of the geological conditions that predominated in the studied area.