• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1998.11a
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• pp.311-314
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• 1998

The effects of the additions of transition metal oxides on ZrO$_2$ - Y$_2$O$_3$ (Y$_2$O$_3$ - containing tetragonal zirconia polycrystals : Y-TZP) system has been studied by investigating fracture toughness and phase stability of the sintered specimens. In the specimens sintered at 1450$^{\circ}C$ for 2hrs in air the phase transformation from tetragonal to monoclinic was observed. The ratios of monoclinic phase to tetragonal phase were changed with the additions of CoO, Fe$_2$O$_3$ and MnO$_2$, respectively, from 0.00 to 8.00wt%. The fracture toughness was increased with increasing the monoclinic to tetragonal phase ratio and was maximum at the ratio of about 18%. However, the hardness was decreased with increasing the ratio. The additions of CoO, Fe$_2$O$_3$ and MnO$_2$ together into Y-TZP resulted in more complex behaviors of fracture toughness and hardness. The specimen with the additions of 1.5wt% Fe$_2$O$_3$, 3.0wt% Al$_2$O$_3$ and 1.5wt% CoO showed the monoclinic to tetragonal phase ratio of 18% and the highest toughness of 10.8 MPa.m$\^$$\frac{1}{2}$/ and Vickers hardness of 1201kgf/mm$^2$.

• Journal of the Korean Ceramic Society
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• v.32 no.12
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• pp.1337-1348
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• 1995

TZP/SUS- and ZT/SUS-functionally gradient materials (FGM) were fabricated by pressureless sintering in Ar-atmosphere. The sintering defects such as warping, frustrum formation, splitting and cracking which originated from shrinkage and sintering behaviors of metal and ceramics different from each other could be controlled by the adjustment with respect to the particle size and phase type of zirconia. The residual stresses generated on the metal and ceramic regions in FGM were characterized with X-ray diffraction method, and relaxed as the thickness and number of compositional gradient layer were increased. The residual stress states in TZP/SUS-FGM have irregular patterns by means of the different sintering behavior and cracking at ceramic-monolith. While in ZT/SUS-FGM, compressive stress is induced on ceramic-monolith by the volume expansion of monoclinic ZrO2 at phase transformation. Also, compressive stress is induced on metal-monolith by the constraint of warping which may be created to the metal direction by the difference of coefficient of thermal expansions. As a consequence, it has been verified that the residual stress generated on FGM is dominantly influenced by the thickness and number of compositional gradient layer, and the sintering defects and residual stress can be controlled by the constraint of the difference of shrinkage and sintering behaviors of each component.

• Journal of the Korean Ceramic Society
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• v.37 no.4
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• pp.359-367
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• 2000

Particulate composites of Al2O3/SiC, Al2O3/ZrO2 and Al2O3/ZrO2/SiC have been fabricated to investigate their R-curve behaviors and toughening mechanisms. Al2O3 containing 30 vol% SiC particles of 3${\mu}{\textrm}{m}$ showed rising R-curve behavior owing to the strong crack bridging by SiC particles. The fracture toughness reached 9.1 MPa {{{{ SQRT {m} }} at the crack length of 1000${\mu}{\textrm}{m}$. On the other hand, ZrO2-toughened Al2O3 had a high flat R-curve since it rose steeply in the short crack region due to the well known transformation toughening. For Al2O3/ZrO2/SiC composites, the R-curve behavior was similar to that of Al2O3/SiC but with slightly higher toughness. The SiC particles in this composite decreased the amount of transformable tetragonal phase to reduce the effect of transformation toughening by 50%. It was also found that the fracture toughness of this composite with two different toughening mechanisms was markedly lower than that estimated by the simple addition of two contributions.

• Journal of the Korean Ceramic Society
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• v.48 no.5
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• pp.354-359
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• 2011

Silicon nitride ceramics were prepared by microwave gas phase reaction sintering. By this method higher density specimens were obtained for short time and at low temperature, compared than ones by conventional pressureless sintering, even though sintering behaviors showed same trend, the relative density of sintered body inverse-exponentially increases with sintering temperature and/or holding time. And grain size of ${\beta}$-phase of the microwave sintered body is bigger than one of the conventional pressureless sintered one. Also they showed good bending strengths and thermal shock resistances.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1896-1901
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• 2008

Single crystalline copper nanowires are subjected to bending tests using molecular dynamics simulations and the embedded atom method. To observe behaviors of nanowire, bending tests are performed for various rates of deflection and different boundary conditions: fixed-free and fixed-fixed. When the deflection of nanowire becomes large, twinnings and dislocations appear, and <100> crystal structure transforms to <110>. At high rates, phase transformation occurs in whole nanowire. But, at low rates, atomic structure changes to <110> phase partially. The final deformed structures are affected by the rate of deflection and boundary conditions. These effects can be important design parameters at nanoscale.

• Journal of Powder Materials
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• v.6 no.2
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• pp.171-177
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• 1999

Ti-45.2at.%Ni-5at.%Cu and Ti-40.2at.%Ni-10atat.%Cu alloy powders were fabricated by gas atomization process. The microstructures, Shape, hardness and phase transformation behaviors of the powders were investigated by means of optical microscopy, scanning electron microscopy, micro-hardness measurement, x-ray diffraction analyses and differential scanning calorimetry. The hardness of the Ti-Ni-XCu alloy powders decreased as Cu-content increased. The x-ray diffraction analyses were carried out for powders without heat treatment, and those that treated at 85$0^{\circ}C$ for an hour in a vaccum state($10^5$ torr) and then quenched into ice water. The intensity of B$19^t$ phase increased with heat treating. The monoclinic B$19^t$ martensite was formed in the Ti-Ni-XCu alloy powders during cooling.

• Journal of Magnetics
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• v.6 no.3
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• pp.94-100
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• 2001

Fe-Co-Ni particles with an average size of 45 and 135 nm are characterized in terms of magnetic phase transformation and magnetic properties at room temperature. BCC structure of Fe-Co-Ni spherical particles can be synthesized from Fe-Co-Ni-Al-Cu precursor films by heating at 600-80$0^{\circ}C$ for the phase separation of Fe-Co rich Fe-Co-Ni particles, followed by a post heating at $600^{\circ}C$ for 5 hours. The average size of nanoparticles was directly determined by the thickness of precursor films. Exchange interactive hysteresis was observed for the nano-composite (Fe-Co-Ni)+(Fe-Ni-Al) films resulting from the short exchange interface between ferromagnetic Fe-Co-Ni particles surrounded by almost papramagnetic Ni-Al-Fe matrix. Arraying the isolated Fe-Co-Ni nano-particles in a random arrangement on $Al_2O_3$substrate the particle assembly showed a behavior of dipole interactive ferromagnetic clusters depending on their volume and inter-particle distance.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.3
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• pp.200-206
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• 2011

The mechanical properties of the most widely used cryogenic materials, i.e. austenitic stainless steel (ASS), aluminum alloy and invar steel, strongly depend on temperatures and strain rates. These phenomena show very complicated non-linear behaviors and cannot be expressed by general constitutive equation. In this study, an unified constitutive equation was proposed to represent the effect of temperature and strain rate on the materials. The proposed constitutive equation has been based on Tomita/Iwamoto and Bodner/Partom model for the expression of 2nd hardening due to martensite phase transformation of ASS. To simulate ductile fracture, modified Bodner/Chan damage model was additionally applied to the model and the model validity was verified by comparison of experimental and simulation results.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.160.2-160.2
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• 2014

Recently silicon has attracted intense interest as a promising anode material of lithium-ion batteries due to its extremely high capacity of 4200 mA/g (for Li4.2Si) that is much higher than 372 mAh/g (for LiC6) of graphite. However, it seriously suffers from large volume change (even up to 300%) of the electrode upon lithiation, leading to its pulverization or mechanical failure during lithiation/delithiation processes and the rapid capacity fading. To overcome this problem, Si nanowires have been considered. Use of such Si nanowires provides their facile relaxation during lithiation/delithiation without mechanical breaking. To design better Si electrodes, a study to unveil atomic-scale mechanisms involving the volume expansion and the phase transformation upon lithiation is critical. In order to investigate the lithiation mechanism in Si nanowires, we have developed a reactive force field (ReaxFF) for Si-Li systems based on density functional theory calculations. The ReaxFF method provides a highly transferable simulation method for atomistic scale simulation on chemical reactions at the nanosecond and nanometer scale. Molecular dynamics (MD) simulations with the ReaxFF reproduces well experimental anisotropic volume expansion of Si nanowires during lithiation and diffusion behaviors of lithium atoms, indicating that it would be definitely helpful to investigate lithiation mechanism of Si electrodes and then design new Si electrodes.

• Journal of the Korean Physical Society
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• v.73 no.10
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• pp.1529-1534
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• 2018

We used in-situ synchrotron X-ray scattering to investigate phase transformations of octahedral tilted monoclinic $SrRuO_3$ (MSRO) and tetragonal SRO (TSRO) thin films on $SrTiO_3$ (STO) substrates. The octahedral tilted MSRO thin films were highly crystalline and the monoclinic distortion angle was $0.45^{\circ}$. The phase transition temperature from the MSRO to TSRO phase occurred at approximately $200^{\circ}C$ as a second order transition. Conversely, no phase transformations of the TSRO thin film occurred within the range from RT to $250^{\circ}C$. The octahedral $RuO_6$ rotation was strongly affected by the phase transformation in the SRO thin films.