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Effect of Additives on the Densification Processes and Properties of Silica-Based CMCs

Ko S.-Y., Yong S.-M., Lee S. J., Cheong D.-I., Baek S.

Archives of Metallurgy and Materials

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2018

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Vol. 63, iss. 3

1485--1488

EN

The effect of additives on the densification behavior and mechanical properties of pure and additive (Zr, B and Mg)-added silica ceramics were investigated for their application to the matrix phase of a silica fiber reinforced silica (SiO2 /SiO2f ) composite. The additives affected the rate of densification and crystallization (or transformation) of the amorphous silica. Among the compositions, pure silica ceramics sintered at 900°C for 1h showed the maximum flexural strength. Based on the results, SiO2 /SiO2f was fabricated by a repeated vacuum-assisted infiltration method followed by the heat treatment at 900°C for 1h. The relative density of the composite was 78.2% with a flexural strength of 22.4 MPa. Fractography revealed that the composite was damaged by strong bonding at the fiber/matrix interface and the fracture of fiber.

2

Electrical and piezoresistive sensing capacities of cement paste with multi-walled carbon nanotubes

Yoo D. Y., You I., Lee S. J.

Archives of Civil and Mechanical Engineering

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2018

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Vol. 18, no. 2

371--384

EN

This study examined the feasibility of using multi-walled carbon nanotubes (MWCNTs) in cement paste for piezoresistive sensors. Two types of MWCNTs with different carbon content (>90% and >99%) were incorporated into cement paste at 1% by weight of binder (1 wt%). Plain cement paste and cement composites including 1 wt% graphite nanofiber (GNF) and graphene (G) were also considered for comparisons of the electrical conductivity. The test results indicate that the MWCNTs more effectively improved the conductivity of the cement paste than GNF and G. In addition, composites with MWCNTs with lower flowability had less resistivity than those with higher flowability. The size effect in electrical resistivity was observed in the cement pastes with and without the carbon-based nanomaterials, but it was mitigated by incorporating MWCNTs in the cement paste. The stresses and strains under cyclic compression and monotonic tension were well simulated by the measured fraction change in the resistivity of the composites with 1 wt% MWCNTs. The gauge factor (GF) for the composites with 1 wt% MWCNTs was higher than that of commercially available strain gauges, and it was affected by the loading condition: a higher GF obtained under compression than under tension.

3

Surface magnetic anisotropy of Co Hf thin films

Lim W. Y., Baek J. S., Kim Y. Y., Lee S. J., Yu S. C., Lee S. H.

Electron Technology

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1998

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Vol. 31, No. 1

60-64

EN

In order to investigate the effect of Hf content and the thickness of Co₁₀₀₋xHfx (x=16,24, 32 at.%) films we observed the absorption lines of spin wave resonance through the ferromagnetic resonance (FMR) measurement. One volume spin wave mode and surface spin wave models were observed for all the samples because the surface anisotropy constants of both sides of the films areless than zero. When the annealing temperature for Co84Hf16 film increased up to 225°C the surface magnetic anisotropy constant Ks2 of the film-substrate interface decreased from -0,07 erg/cm² to -0.32 erg/cm² and the Ks1 constant Ks2 of the film-air interface varied from 0.18 erg/cm² to -47 erg/cm². In Co76Hf24 film Ks2 decreased a little from -0.31 erg/cm² to -0.41 erg/cm² and Ks1 decreased rapidly from -0.19 erg/cm² to-0.60 erg/cm². In genearal it is shown that the surface anisotropy at the film-air interface is very sensitive to Hf content and annealing temperature. This result is due to the increase of Co content caused by oxidation of Hf atoms near the film-air interface during low temperature annealing (150~175°C) and the diffusion of Co atoms during high temperature annealing (200~220°C).