Eugenio H. Otal

The influence of defects formed by Ca excess and thermal post-treatments on the persistent luminescence of CaTiO_3:Pr

Red emitting CaTiO3:Pr phosphors with a nominal composition of Ca0.998+xPr0.002TiO3+δ (0.02≤x≤0.04) were prepared by solid state reactions with different thermal post treatments and characterized by X-ray diffraction, transmission electron microscopy and photoluminescence. The Ca excess exhibited complete solubility up to 4% in the samples treated at 1400 °C but segregation in the form of Ruddlesden-Popper phases (Ca3Ti2O7 - Ca4Ti3O10) was observed in samples prepared at 1500 °C. The increase in temperature for stoichiometric samples showed a monotonic increase of decay time due to the reduction of non-radiative recombination defects. It was found that the Ca excess favored the formation of oxygen vacancies which are known to act as trap. In the samples treated at 1400 °C, 3% of Ca excess showed to be the best concentration to increase the decay time of persistent luminescence. For the samples treated at 1500 °C, the segregation of Ruddlesden-Popper phases left a constant amount of Ca soluble in all the CaTiO3 samples. This constant concentration of Ca caused the same density of defects and, consequently, the same decay time in all samples.

Improved photoluminescence and afterglow of CaTiO 3 :Pr 3+ by ammonia treatment

The phosphor CaTiO3:Pr3+ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH3. Comparatively large off-center displacements of Ti in the TiO6 octahedra were confirmed for as-synthesized CaTiO3:Pr3 by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH3 treatment are explained as a result of the reduced concentration of oxygen excesses with simultaneous relaxation of the lattice strain.

Transport and magnetic properties of PrCo1−xNixO3 (x = 0.0–0.7)

The influence of Ni substitution on the transport and magnetic properties of the perovskite PrCo1−xNixO3 (x = 0.0–0.7) has been investigated by synthesizing and characterizing different samples. The unit cell volume of orthorhombic crystal structure expands with increasing Ni content, affecting the distortion of M–O (I, II)-M bond angles (M = Co, Ni). The expansion of the unit cell volume can be attributed to the larger ionic radii of Ni2+ and to the stabilization of the intermediate-spin state in the low-spin cobalt matrix. The electrical conduction mechanism varies with the Ni content with noticeable change from variable range hopping to nearest neighbour hopping in compositions 0.07 ≤ x ≤ 0.29. The Seebeck coefficient is positive for samples with 0.0 < x < 0.49. Only PrCo0.3Ni0.7O3 has a negative Seebeck coefficient. The ZFC/FC curves and M (H) hysteresis loops reveal both a spin-glass behaviour and ferromagnetic ordering below the Curie temperature (TC) for Ni-substituted samples. By the combination of Seebeck coefficient data and the measured effective magnetic moment μeff, the mixed valences of Co3+/Co4+ and Ni2+/Ni3+ in PrCo1−xNixO3 compounds were determined and the values are in agreement with x-ray photoemission spectroscopy measurements.

Debinding Mechanisms in Thermoplastic processing of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ‐ Stearic Acid–Polystyrene Mixture

In the present study, we address the interaction between a thermoplastic binder system and Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) (BSCF) during thermal treatment of the thermoplastic feedstock. BSCF powder was coated with different amounts of stearic acid (SA) acting as a surfactant. Oxygen release from the uncoated BSCF surface changes the decomposition of polystyrene (PS) in inert atmospheres from a pyrolytic to a thermoxidative mechanism, thereby decreasing the break-down temperature and the activation energy. In mixtures with coated BSCF powder, the decomposition products of SA carbonatize the BSCF surface, which inhibits oxygen release. Mass spectrometry of the breakdown products indicates that the decomposition of SA in the presence of BSCF also modifies the decomposition pathway of PS. The influence of BSCF on the polymer decomposition reaction in air is not as strong. Oxygen diffusion seems to be responsible for the differences to pure PS in reaction rates and the activation energies.

Phase transitions of BaTi{sub 0.9}Rh{sub 0.1}O{sub 3±δ} perovskite-type oxides under reducing environments

Perovskite-type oxides of composition BaTi0.9Rh0.1O3±δ were prepared following a new chemical route that avoids the formation of hydroxyl species and precipitation, and allows the homogeneous distribution of Rh in the final mixed metal oxide. The high dispersion of Rh and the formation of the solid solution between Rh and the BaTiO3 perovskite is confirmed by means of X-ray diffraction (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS). The presence of Rh stabilized the hexagonal BaTi0.9Rh0.1O3±δ phase, which decomposes into barium orthotitanate (BaTi2O4) and metallic Rh° in reducing environment. This phase transition starts already at 700 °C and is only partially completed at 900 °C suggesting that part of the Rh present in the perovskite lattice might not be easily reduced by hydrogen. These aspects and further open questions are discussed.