Jondo Yun

Preliminary investigation of hydroxyapatite microstructures prepared by flash sintering

Flash sintering is a novel and emerging route for sintering ceramics within a few seconds, even under pressure-less conditions. In the current study, hydroxyapatite (HA) was fully densified by flash sintering at a furnace temperature of 1020°C. Flash sintering with constant electric fields of 750 and 1000 V cm−1 reduced the grain growth rate significantly compared to that sintered in the absence of an electric field at 1400°C. The microstructure of HA consolidated by flash sintering was compared with that of the without electric field sintered samples. The flash-sintered samples showed smaller grains (160 ∼ 320 nm) than the without electric field sintered samples (∼15 µm). The samples with a higher applied electric field showed slightly better densification than those with the lower field by flash sintering. Overall, the electric flash reduces the sintering temperature effectively and decreases the holding time to densify highly insulating ceramics, such as HA.

Spark plasma sintered superplastic deformed transparent ultrafine hydroxyapatite nanoceramics

Hydroxyapatites (HAs) are used as bioceramics for artificial bone substitutes because of their good biocompatibility. In this study, highly transparent ultrafine HA ceramics with a mean grain size of 170 nm were synthesised by spark plasma sintering at 900–1000°C and 80 MPa. Phase analysis revealed the presence of a pure HA phase even after sintering at 1000°C. The sintered body was almost fully dense (>99%). The total forward transmission was >70% at 400 nm, and it approached the theoretical value of 89% in the infrared wavelength range. The HA ceramics contained several intragranular voids of 5–10 nm. An evaluation of the superplastic flow behaviour of this sintered HA sample at 950–1050°C revealed superplasticity with a maximum elongation and initial strain rate of 486% and 1.0x10− 4 s− 1 respectively at 1000°C. The deformed microstructure of HA indicated activated dislocation motion assisted grain boundary sliding to be the major mechanism of superplastic flow with stress exponent values ranging from 4 to 5 based on interface reaction controlled creep. Surprisingly, no dislocations were observed by transmission electron microscopy.

Mechanism of superplastic deformed transparent hydroxyapatite

Abstract Superplastic deformed hydroxyapatite (HA) samples were examined by TEM to elucidate the deformation mechanism in the HA. The present paper was to study the main mechanism of the superplastic behaviour of nanocrystalline transparent HA with a mean grain size of 170 nm fabricated by spark plasma sintering, which has exhibited superplasticity at temperatures ranging from 950 to 1050°C with a maximum elongation of 486% at 1000°C at an initial strain rate of 1·0×10−4 s−1. The post-deformation microstructure of HA indicated that activated dislocation motion assisted grain boundary sliding was the predominant mechanism of superplastic flow with the values of the stress exponent, 4 to 5, according to a theoretical model of interface reaction controlled creep. Unexpectedly, no dislocation was observed by TEM.

Quantitative analysis of carbon in silicon carbide coated with carbon.

Nonconductive specimens for scanning electron microscopy or X-ray microanalysis are coated with conductive carbon in order to reduce charging. But carbon film absorbs X-ray fluxes causing errors in measuring chemical composition. Especially when the carbon content is measured, carbon coating not only blocks X-rays but also becomes a source of carbon X-rays. It is thus necessary to determine how much errors are induced by carbon coating, and how thick coating is allowed for the accurate measurement. In this study, quantitative analysis of carbon on silicon carbide with carbon coating films was attempted by electron probe microanalyzer. It was found that measured carbon content increased in a nonlinear manner up to 40% with a film thickness, whereas silicon content decreased slightly. Carbon X-ray intensity was determined by computer simulation, which increased in a linear manner with the thickness. The discrepancy was due to a nucleation and growth of islands and thus a change of density with a thickening of coating film.

Eutectic structure from amorphous Al2O3-ZrO2-Y2O3 system by rapid quenching technique for potential hybrid solar cell application

Abstract Ternary Al2O3-ZrO2-Y2O3 samples with a eutectic composition were prepared using the rapid quenching method, with some samples further annealed at 1300°C for 30 min and then slow cooled. The microstructural evolution was observed with XRD, SEM, and TEM. The SEM and TEM observations of the ternary samples agreed with the XRD. The rapid quenched sample was an amorphous phase with a small amount of crystalline phase mixed in. Observations showed that the rapid quenched and annealed sample was completely crystalline with a granular structure and well defined crystals of 40-60 nm in size.

Eutectic structure evolution of Al2O3―ZrO2-Y2O3 system for potential hybrid solar cell application

Abstract Ternary Al2O3-ZrO2-Y2O3 samples with a eutectic composition were prepared by slow cooling. The microstructural evolution was observed with X-ray diffraction (XRD), scanning electron microscopy (SEM). The SEM observation of the ternary samples agreed with the XRD with a completion of crystallisation by slow cooling. The target materials commonly have ‘cantaloupe skin’ microstructures as shown in the previous studies by Han et al. The nanocomposite may have experienced different cooling rates with two different microstructures, near the surface having experienced optimal conditions for the eutectic reaction during their cooling and thus formed the eutectic microstructure, near the centre having experienced a slower cooling rate. The crystallised eutectic ternary Al2O3-ZrO2-Y2O3 system had three different phases with a 3Y2O3-5Al2O3 (yttrium-aluminium garnet phase), an alumina phase formed by the eutectic reaction, and a solid solution of ZrO2 and Y2O3.

Effect of silver on superplastic deformation in YBa2Cu3O7-x/Ag composites

Superplastic deformation was studied in fine-grained (0.7-1. 1 μm) YBa 2 Cu 3 O 7 - x /Ag composites containing 2.5-25 vol% Ag. The compression tests were conducted in the temperature range of 750-875 °C and at strain rates of 10 - 5 to 10 - 3 /s. For the YBa 2 Cu 3 O 7 - x /25%Ag composites with grain size of 0.7-1.1 μm, deformed at 800-850 °C and 10 - 5 to 10 - 3 /s, the stress exponent, grain size exponent, and the activation energy of deformation were 2.0 ′ 0.1, 2.5 ′ 0.7, and 760 ′ 100 kJ/mol, respectively. These values were the same as those of the pure YBa 2 Cu 3 O 7 - x , indicating that the deformation of the composite was controlled by that of the rigid YBa 2 Cu 3 O 7 - x phase. However, the strain rate was increased by the addition of silver as explained by the soft inclusion model of Chen. The dependence of the flow stress on the silver content was in close agreement with the prediction of the model.