Simeon Agathopoulos

The fluorapatite–anorthite system in biomedicine

Glasses and glass ceramics of fluorapatite-anorthite (eutectic composition) were produced and characterized in order to evaluate their potential application in biomedicine. Bio-reactivity was determined by in vitro tests by immersion of powders in simulated plasma liquids as well as by in vivo experiments by implantation in rabbits. According to the results, the investigated materials are bio-acceptable since no toxic or other harmful evidence was detected. Glass-ceramics showed remarkable inertness, whereas glasses spontaneously dissolved in SBF and after 1 week moderate formation of apatite was observed, that however ceased within a month.

Hydroxyapatite scaffolds hydrothermally grown from aragonitic cuttlefish bones

Scaffolds of pure AB-type carbonated hydroxyapatite (HA) were successfully produced via hydrothermal transformation (HT) of aragonitic cuttlefish bones at 200 °C. Beyond low production cost, worldwide availability and natural biological origin of raw materials, the produced scaffolds preserved the initial structure of the cuttlefish bone, featuring pore size of ∼80 µm in width and ∼100 µm in height. The transformation was complete after 9 h and no intermediate products were registered. The kinetics were fast, since, HA was the dominant crystalline phase after only 1 h of HT. The HA crystallites formed had a size of nanoscale (∼20–50 nm) and were randomly oriented.

Photoluminescence properties of Eu2+-activated CaSi2O2N2: Redshift and concentration quenching

Highly efficient CaSi2O2N2:Eu2+ green phosphors were synthesized via solid-state reaction method. The produced phosphors are effectively excited with UV-vis light of wavelength between 300 and 460 nm and emit a single, intense, and broad emission band centered at 538 nm. The experimental results and their analysis suggest that the energy transfer mechanism should occur due to dipole-dipole interactions among Eu2+ ions, resulting in a shift in emission spectrum toward longer wavelengths with increasing Eu2+ concentration. The quenching concentration of Eu2+ (i.e., where the emission intensity maximizes) is approximately 2 at. %. Accordingly, the produced CaSi2O2N2:Eu2+ green phosphors are qualified for further consideration and experimentation for potential use in white light emitting diodes.

Luminescence and Energy-Transfer Mechanism in SrSi2O2N2 : Ce3 + , Eu2 + Phosphors for White LEDs

A series of Ce 3+ and Eu 2+ co-doped SrSi 2 O 2 N 2 phosphors, whose features qualify them for consideration in white-light UV or blue light-emitting diodes (LEDs), was synthesized via a high temperature solid-state reaction under a reductive atmosphere. The dependence of luminescence properties of the produced powders on the concentration of an activator (Eu 2+ ) and a coactivator (Ce 3+ ) was investigated. The experimentally recorded luminescence spectra and the calculations of the efficiency of energy transfer from Ce 3+ to Eu 2+ and the critical distance between Ce 3+ and Eu 2+ suggest a resonance-type energy-transfer mechanism from Ce 3+ to Eu 2+ due to dipole-dipole interactions.

Synthesis, bioactivity and preliminary biocompatibility studies of glasses in the system CaO–MgO–SiO2–Na2O–P2O5–CaF2

New compositions of bioactive glasses are proposed in the CaO–MgO–SiO2–Na2O–P2O5–CaF2 system. Mineralization tests with immersion of the investigated glasses in simulated body fluid (SBF) at 37°C showed that the glasses favour the surface formation of hydroxyapatite (HA) from the early stages of the experiments. In the case of daily renewable SBF, monetite (CaHPO4) formation competed with the formation of HA. The influence of structural features of the glasses on their mineralization (bioactivity) performance is discussed. Preliminary in vitro experiments with osteoblasts’ cell-cultures showed that the glasses are biocompatible and there is no evidence of toxicity. Sintering and devitrification studies of glass powder compacts were also performed. Glass-ceramics with attractive properties were obtained after heat treatment of the glasses at relatively low temperatures (up to 850°C).

Role of oxygen vacancy on the photoluminescence of BaMgSiO4:Eu phosphors: experimental and theoretical analysis.

Pure BaMgSiO4:Eu(2+) phosphor, prepared by a solid state reaction method under N2 atmosphere, exhibited a strong green emission at 500 nm and a weak emission at 405 nm. Heat treatment under NH3 atmosphere causes changes in the PL intensity: the green emission at 500 nm gradually decreases and completely disappears after heat treatment for 3 h, whereas a new blue emission peak, centered at 445 nm, appears and becomes very strong. The results of the analyses with electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) spectroscopy suggest that the heat treatment causes the generation of a large amount of oxygen vacancies. This resulted in the aforementioned color changes of the BaMgSiO4:Eu phosphor, which are confirmed by the results of DFT+U calculations. In particular, these calculations showed that Eu prefers to occupy Ba(3) sites, which are six coordinated to oxygen atoms. The emission at 500 nm was attributed to the 4f-5d transition energy of Eu in Ba(3) site, calculated as 2.54 eV. It was also shown that Eu 4f energy level decreases when oxygen is removed from the oxygen position adjacent to Eu, which results in a larger Eu 4f-5d transition energy and shorter wavelengths of emission peaks.

Sintering behavior and properties of reinforced hydroxyapatite/TCP biphasic bioceramics with ZnO-whiskers

Composite materials of hydroxyapatite/β-tricalcium phosphate (HA/β-TCP) biphasic bioceramics and ZnO whiskers, added in amounts up to 3 wt% to an HA/β-TCP matrix, were prepared by sintering. The sintering behavior, the mechanical reinforcement (due to ZnO), and the mineralization ability of the resulting materials in simulated body fluid (SBF) were experimentally investigated. The experimental results showed that the densification and mechanical properties were improved with increasing amounts of ZnO. The presence of ZnO appeared to affect the proportion of HA/β-TCP after sintering. Doping with a high amount of ZnO favored the dissolution of the materials in SBF.

The influence of BaO additives on the reaction of Al2O3–SiO2 ceramics with molten Al and Al–Si alloys

BaO considerably affects the composition and the microstructure of the reaction zone formed between BaO-doped aluminosilicate ceramics and molten aluminium alloys under vacuum. The reduced Ba and Si form AlBaSi precipitates, found adhered to the interface and dispersed in the metal Al-matrix, whose formation apparently controls the reaction kinetics.

Reactions at the interface between Al 2 O 3 -SiO 2 ceramics with additives of alkaline-earth oxides and liquid Al-Si alloy

The interfacial reactions between aluminosilicate ceramics doped with MgO, CaO, or BaO and A1-7 wt% Si alloy were investigated at 1023, 1173, and 1323 K under vacuum for 4 h. Alkaline-earth oxide additives defined phase formation and microstructure of the sintered ceramics and subsequently controlled the ceramic/metal interfacial reactions, which were always intensive. In general, reaction zones consisted of Al 2 O 3 , infiltrated with Al. In the case of CaO- and BaO-doped ceramics, precipitates formed into the metal phase and concentrated the reduced Ca and Ba, respectively. A reaction mechanism is proposed, which anticipates an active role of SiO 2 .

Numerical Simulation of Thermal Conductivity of Particle Filled Epoxy Composites

A finite element method was developed to predict the effective thermal conductivity of particle filled epoxy composites. Three-dimensional models, which considered the effect of filler geometry, filler aspect ratio, conductivity ratio of filler to matrix, and interfacial layer were used to simulate the microstructure of epoxy composites for various filler volume fractions up to 30%. The calculated thermal conductivities were compared with results from existing theoretical models and experiments. Numerical estimation of ellipsoids-in-cube model accurately predicted thermal conductivity of epoxy composites with alumina filler particles. The number of length division during mesh process and particle numbers used in the finite element analysis affect the accuracy of calculated results. At a given value of filler content, the numerical results indicated a ratio of conductivity of filler to matrix for achieving the maximum thermal conductivity.