Michiharu Okano

Densification, Microstructure, and Behavior of Hydroxyapatite Ceramics Sintered by Using Spark Plasma Sintering

This paper discusses the dependence of the mechanical properties and microstructure of sintered hydroxyapatite (HA) on the sintering temperature and pressure. A set of specimens was prepared from as-received HA powder and sintered by using a spark plasma sintering (SPS) process. The sintering pressures were set at 22.3 MPa, 44.6 MPa, and 66.9 MPa, and sintering was performed in the temperature range from 800°C to 1000°C at each pressure. Mechanisms underlying the interrelated temperature-mechanical and pressure-mechanical properties of dense HA were investigated. The effects of temperature and pressure on the flexural strength, Youngs modulus, fracture toughness, relative density, activation energy, phase stability, and microstructure were assessed. The relative density and grain size increased with an increase in the temperature. The flexural strength and Youngs modulus increased with an increase in the temperature, giving maximum values of 131.5 MPa and 75.6 GPa, respectively, at a critical temperature of 950°C and 44.6 MPa, and the fracture toughness was 1.4 MPa m 1/2 at 1000°C at 44.6 MPa. Increasing the sintering pressure led to acceleration of the densification of HA.

Fabrication of Hydroxyapatite-incorporated ZrO2-20 wt% Al2O 3 by Spark Plasma Sintering and Characterization

Yttria-stabilized tetragonal zirconia/20 wt% alumina (TZP-3Y20A) composites were fabricated by spark plasma sintering at various sintering temperatures over the range of 1000—1400°C. Hydroxyapatite (HA) additive was added in volume fractions of 10—50% in order to increase the biocompatibility of the composite. The densification behavior of the TZP-3Y20A composite and that of the composites with HA were investigated. In the case of TZP-3Y20A composites, the density increases steadily with temperature and reaches a maximum value of 97.8% of the theoretical density at 1400°C. The solution interface formed between zirconia (ZrO2) and alumina (Al 2O3) strengthens the bond between ZrO2 and Al 2O3 grains and facilitates densification. In the case of TZP-3Y20A/HA composites, sintering at 1400°C led to the formation of tricalcium phosphate in the samples, which resulted from the decomposition of HA due to its limited thermal stability at high temperature; no reaction was observed between ZrO2 and HA. The addition of HA imposes a barrier effect on the diffusion between ZrO2 and Al2O3 grains, thus limiting the grain growth of ZrO2 and Al2O 3.

Driver's Mental Stress Analysis Using Discrete Wavelets Transform

Discrete wavelets have been applied to analyzing a R-R Interval tachogram. Consequently, a specific wavelet level indicates the dominant frequency that indicates drivers mental stress with the time process. This method is able to replace former Fourier base analysis in the drivers circumstances. The experiments are carried out with a driver carrying an electrocardiograph in a passenger car circulating a test course with 140km/h. The R-R Interval tachogram was obtained from the driver before and while driving. The motivation behind this work is to establish a drivers mental model while the driver operates a vehicle. The model contributes to safe and comfortable drivers circumstances.