Papers by Author: H.N. Yoshimura

Abstract: Potassium hexa and octatitanate fibers have been proposed as reinforcement for friction materials. The aim of this work was to establish a calcination route to produce these fibers, using commercial anatase and potassium carbonate powders. These powders were dry mixed with TiO2/K2O molar ratio, n, of 3.0, 3.5, and 4.0, and then calcined at 950, 1050, and 1150°C for 3 h. Calcined powders were milled, washed in warm water with different pHs, and heat treated to crystallize the fibers. The best conditions to growth long fibers were n=3.0 and 1050°C, in the twofase field (liquid + K2Ti4O9). Controlled ion-exchange with water removed K+ ions from K2Ti4O9 fibers resulting in potassium hexa or octatitanate fibers after the second heat-treatment. Fibers with sub-micrometer thickness (~0.6 μm) and average length of ~20 μm could be prepared.

Abstract: The aim of this work was to establish a route to produce pre-sintered blocks of Y-TZP (yttria stabilized tetragonal zirconia polycrystal) suitable to be machined in a commercial CADCAM system, used to manufacture crowns and bridge frameworks for dental applications. Two commercial Y-TZP powders were investigated. The powders were pressed with different compaction pressures (40 to 500 MPa) and pre-sintered at temperature ranging from 900 to 1100°C. Vickers hardness, biaxial flexural strength, and linear shrinkage could be correlated to the relative density of pre-sintered samples. Using an empirical equation, pre-sintered blocks with a pre-defined density could be prepared. The blocks withstood the machining in a CAD-CAM system, and the machined and sintered crowns presented good adaptation.

Abstract: The aim of this work was to study the effect of temperature and heating rate on the densification of two leucite-based dental porcelains: one low-fusion and one high-fusion commercial leucite porcelains (Dentsply-Ceramco). Porcelain powders were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), particle size distribution, and helium picnometry. Bar samples were sintered from 650 to 1050oC, using heating rate of 55oC and 10oC/min. Sintered samples were characterized in terms of bulk density, measured by the Archimedes method in water, and fractured surface microstructure by scanning electron microscopy (SEM). The results show that densification increases with increasing temperature and the increase in heating rate has no effect on the densification of the porcelains studied.

Abstract: ZnO varistors are nonlinear resistors used as surge arresters in power transmission and distribution for the protection of electronic devices. Electrical characteristics of these materials have been extensively studied, but their mechanical behavior is not completely understood. It has been suggested that the breakdown of ZnO varistors is related to microstructure heterogeneities and processing defects. These defects are the same that usually control the mechanical strength of ceramic materials. In this work, mechanical properties (flexural strength, fracture toughness, elastic constants, and hardness) of five commercial blocks of ZnO varistors (class I) from different producers were measured and correlated to their microstructure. Pore fraction and size significantly affected the flexural strength.