Young-Hwan Han

Spark Plasma Sintering

Spark Plasma Sintering (SPS) involves pressure driven powder consolidation under pulsed direct electric current, usually using graphite dies.. In general, the SPS process results in a finer grain size when high pressures are used for densification. These findings can be rationalised, at least initially, by recognising that while the pressure greatly adds to the driving force for sintering, it does not affect the driving force for grain growth. Therefore, since sintering is achieved more quickly, there is little time for grain growth, which preserves, very nearly, the original grain size in the powder.

Characterisation of transparent hydroxyapatite nanoceramics prepared by spark plasma sintering

Abstract Hydroxyapatites (HA) have good biocompatibility and are used as bioceramics for artificial bones. The application areas can be extended further if transparent and dense HA ceramics can be prepared. The preparation of dense and transparent HA ceramics were attempted using a spark plasma sintering technique at relatively low temperatures (900–1000°C) under a pressure of 80 MPa for a short time of 10 min. The sintered body was almost fully dense (>99%) and transparent with a transmittance >70%. The microstructure was examined by SEM, TEM, STEM and EDX. The HA ceramics exhibited a microstructure with grains, approximately 100 nm size. A number of intragranular voids, 5–10 nm in size, with flat boundaries were also observed. The voids were believed to have been generated by evaporation during spark plasma sintering and were stabilised during cooling. The grain boundaries were clean without a glassy phase.

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.

Characterization of multiwalled carbon nanotube-reinforced hydroxyapatite composites consolidated by spark plasma sintering.

Pure HA and 1, 3, 5, and 10 vol% multiwalled carbon nanotube- (MWNT-) reinforced hydroxyapatite (HA) were consolidated using a spark plasma sintering (SPS) technique. The relative density of pure HA increased with increasing sintering temperature, but that of the MWNT/HA composite reached almost full density at 900°C, and then decreased with further increases in sintering temperature. The relative density of the MWNT/HA composites increased with increasing MWNT content due to the excellent thermal conductivity of MWNTs. The grain size of MWNT/HA composites decreased with increasing MWNT content and increased with increasing sintering temperature. Pull-out toughening of the MWNTs of the MWNT/HA composites was observed in the fractured surface, which can be used to predict the improvement of the mechanical properties. On the other hand, the existence of undispersed or agglomerate MWNTs in the MWNT/HA composites accompanied large pores. The formation of large pores increased with increasing sintering temperature and MWNT content. The addition of MWNT in HA increased the hardness and fracture toughness by approximately 3~4 times, despite the presence of large pores produced by un-dispersed MWNTs. This provides strong evidence as to why the MWNTs are good candidates as reinforcements for strengthening the ceramic matrix. The MWNT/HA composites did not decompose during SPS sintering. The MWNT-reinforced HA composites were non-toxic and showed a good cell affinity and morphology in vitro for 1 day.

Microscale tribological behavior and in vitro biocompatibility of graphene nanoplatelet reinforced alumina.

Graphene nanoplatelets were added as reinforcement to alumina ceramics in order to enhance microscale tribological behavior, which would be beneficial for ceramic-on-ceramic hip implant applications. The reduction in microscale wear is critical to hip implant applications where small amounts of wear debris can be detrimental to patients and to implant performance. The addition of the GNPs lead to improvements in fracture toughness and wear (scratch) resistance of 21% and 39%, respectively. The improved wear resistance was attributed to GNP-induced toughening, which generates fine (~100nm) microcracks on the scratch surface. In addition, active participation of GNPs was observed in the scratch subsurface of GNP-reinforced samples through focused ion beam sectioning. Friction coefficients are not significantly influenced by the addition of GNPs, and hence GNPs do not act as solid state lubricants. In vitro biocompatibility with human osteoblasts was assessed to evaluate any possible cytotoxic effects induced by GNPs. Osteoblast cells were observed to survive and proliferate robustly in the GNP-reinforced samples, particularly those with high (10-15vol%) GNP content.

Prevention of Grain Growth during the Liquid-Phase Assisted Sintering of β-SiC

In our previous studies, continuous SiC fiber-reinforced SiC-matrix composites (SiCf/SiC) had been fabricated by two different slurry infiltration methods: vacuum infiltration and electrophoretic deposition (EPD). 12 wt% of Al₂O₃-Y₂O₃-MgO with respect to SiC powder was used as additives for liquid-phase assisted sintering. After hot pressing at 1750°C under 20MPa for 2 h in Ar atmosphere, a high composite density could be achieved for both cases, whereas the problems such as large grain size and non-uniform distribution of liquid phase were observed, which was resulted in the relatively poor mechanical properties of composites. Therefore, efforts have been made to reduce the grain growth during the sintering, including the optimization for hot pressing condition and utilization of spark plasma sintering using a SiC monolith. Based on the results, spark plasma sintering was found to be effective method in decreasing the amount of sintering additive, time and grain growth, which will be explained in comparison to the results of hot pressing in this paper.

Facile preparation of in situ coated Ti3C2Tx/Ni0.5Zn0.5Fe2O4 composites and their electromagnetic performance

A novel family of Ti3C2Tx/ferrite composites with high reflection loss was developed using a facile in situ co-precipitation method. The as-synthesized Ti3C2Tx/ferrite composite with a 5 wt% Ti3C2Tx MXenes loading exhibited high reflection loss (−42.5 dB) at 13.5 GHz. The effective absorption bandwidth of the 5 wt% Ti3C2Tx/Ni0.5Zn0.5Fe2O4 composite reached ∼3 GHz (12–15 GHz) in the K-band. The incorporation of Ti3C2Tx MXenes improved the electromagnetic impedance of the Ti3C2Tx/Ni0.5Zn0.5Fe2O4 composite resulting from the enhanced electrical conductivity. The potential electromagnetic wave absorption mechanisms were revealed, which may contain magnetic loss, dielectric loss, conductivity loss, multiple reflections, and scattering. The technique is facile, fast, scalable, and favorable for the commercialization of this composite. This study provides a potential way to develop EM wave absorbing materials for a large family of MXenes/ferrite composites.

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.

Characteristics of SiCf/SiC hybrid composites fabricated by hot pressing and spark plasma sintering

Abstract Abstract SiC fibre reinforced SiC–matrix ceramic composites were fabricated by electrophoretic deposition (EPD) combined with ultrasonication. Fine β-SiC powder and Tyranno-SA fabrics were used as the matrix and fibre for reinforcement, respectively. Different amounts of fine Al2O3–Y2O3 were added for liquid phase assisted sintering. For EPD, highly dispersed slurry was prepared by adjusting the zeta potentials of the constituent particles to ⩾+40 mV for homogeneous deposition. The composite properties were compared after using two different consolidation methods: hot pressing for 2 h at 20 MPa and spark plasma sintering (SPS) for 3 min at 45 MPa at 1750°C to minimise the damage to the SiC fibre. The maximum flexural strength and density for the 45 vol.-% fibre content composites were 482 MPa and 98% after hot pressing, respectively, whereas those for SPS were 561 MPa and 99·5%, indicating the effectiveness of SPS.