Roger Honorato Piva

Synthesis by coprecipitation of india-stabilized zirconia and codoping with MoO3, WO3, TaO2.5, or NbO2.5 for application as thermal barrier coatings

Coprecipitation synthesis of nanocrystalline india-stabilized zirconia with high surface area and codoping with MoO3, WO3, TaO2.5, or NbO2.5 is reported. The concentration of codopants was defined by the charge-compensating mechanism. Ethanol washing followed by azeotropic distillation and freeze drying were compared as dehydration techniques for the gels. As determined by XRD and Raman scattering, 9 mol% of InO1.5 plus charge-compensating dopants is sufficient to completely stabilize the high-temperature tetragonal phase of zirconia. The effect of alloying hexavalent and pentavalent oxides was secondary compared to the InO1.5 concentration in the retention of the tetragonal structure. Improved specific surface area of 106.1 m2 g‒1 and crystallite size between 8 and 9 nm were achieved through ethanol washing and subsequent azeotropic distillation even after calcination at 600 oC. This result is attributed to the effect of the incorporation of ethoxy and butoxy groups after the treatment of the gels in organic medium, as detected by FT-IR spectroscopy and DSC/TG.

Compatibility Analyses of BICUVOX.10 as a Cathode in Yttria-stabilized Zirconia Electrolytes for Usage in Solid Oxide Fuel Cells

Copper-substituted bismuth vanadate has been considered a promising material for composite cathodes in SOFC. However, high reactivity of BICUVOX.10 towards the electrolytes still has been its greatest shortcoming. This paper describes reactions between BICUVOX.10 and yttria-stabilized zirconia (YSZ) electrolytes. Secondary phases formed were evaluated by X-ray diffraction, scanning electron microscopy and a.c. impedance spectroscopy. A deleterious interaction between BICUVOX.10 and YSZ was observed, mainly regarding the yttrium depletion from ZrO2 lattice through reaction with VO2.5, resulting in YVO4 phase nucleation and destabilization of the tetragonal and cubic ZrO2 polymorphs to monoclinic. The ZrO2 destabilization and YVO4 nucleation are related phenomena and were interpreted through a theoretical mechanism using charge-compensating dopants description. Thus, these reactions were seen as detrimental to the cathode/electrolyte contact, especially regarding the highly resistive layer formed in the BICUVOX.10/YSZ junction, discouraging further usage of BICUVOX.10 as a cathode in yttria-stabilized zirconia electrolytes.

Influence of oxidant and fuel on the powder characteristics of LiNbO3 synthesized by combustion method

Lithium niobate (LiNbO3) is widely recognized as a promising material for replacing lead-based piezoelectric ceramics. Although the LiNbO3 synthesis by combustion method has been investigated with particular attention recently, the influence of oxidants and different fuels’ sources on the synthesized powders has not yet been thoroughly studied. In this work we investigate the influence of urea and maleic hydrazide as fuels and ammonium nitrate as an oxidant on the powder characteristics of LiNbO3 synthesized by combustion method. In addition, powder characteristics and sinterability of powders prepared by combustion method are compared with those of powders prepared by solid-state reaction. The results show that the second phase LiNb3O8 was detected only when an oxidant agent was used in the synthesis process. Among all combustion reactions, the powders prepared with excess of urea presented better final characteristics. As a result, the sintering temperature for LiNbO3 powders prepared by combustion method was appreciably lowered when compared with those prepared by solid-state reaction.

Iron-Rich Glass-Ceramics Obtained from Mill Scale

Mill scale contains high amounts of iron and it is classified as residue Class 2 – Not Inert (ABNT NBR 10.004:2004). Considering its availability, this study aimed to investigate glass-ceramics from this residue focused on its valorization. The influence of lithium oxide and zirconia was also evaluated. Six formulations were melted at 1350 °C and obtained frits were wet ground, dried and characterized: X-ray fluorescence, X-ray diffraction, thermal differential analysis and dilatometry. Pressed bodies were dried and heat treated at 950 °C. After, crystallized samples were characterized by different techniques: X-ray diffraction, apparent and theoretical density, coefficient of thermal expansion, hardness and bending strength. Results showed that lithium oxide and zirconia significantly influenced the thermal and structural behavior of obtained glass-ceramics.