Shinhoo Kang

Effect of various carbides on the dissolution behavior of Ti(C0.7N0.3) in a Ti(C0.7N0.3)-30Ni system

Abstract HfC, TaC, or WC were individually added to a Ti(C0.7N0.3)–30 wt% Ni system, in order to investigate microstructural changes and the dissolution behaviors of Ti(C0.7N0.3) and the carbides. Of these systems, the Ti(C0.7N0.3)–WC–Ni system proved to be the most favorable for the refinement of microstructure. The fraction for the cross-sectional area of the Ti(C0.7N0.3) cores, which is related to the amount of Ti(C0.7N0.3) dissolved, increases and the thickness of solid solution rims decreases in the order of HfC, TaC, and WC added. This finding indicates that the dissolution rate of Ti(C0.7N0.3) in a Ni melt is the lowest when WC is added. In addition, it was found that the average dissolution rate of Ti(C0.7N0.3) in the HfC-containing system is ∼1.6 and 1.9 times higher than those for Ti(C0.7N0.3) in the TaC- and WC-containing systems at the same sintering conditions. Further, the dissolution rates of TaC and WC were ∼80% of the rate found for HfC during the formation of the outer rim.

Microstructure and tribo-mechanical properties of ultrafine Ti(CN) cermets

Abstract Role of Ti(CN) particle size on the microstructure of the Ti(CN)–WC–Ni cermets has been evaluated. The systems containing ultrafine grade Ti(CN) shows better structural homogeneity and a large volume fraction of rim phase in the core-rim structure when compared to the coarse Ti(CN) cermets. Measurable shift in the lattice parameter of the Ti(CN) core occurs in the ultrafine grade cermet possibly due to the diffusion of W and/or Ni. The improved structural features in the ultrafine grade led to considerably enhanced tribo-mechanical properties of the cermets.

Effect of HfC addition on microstructure of Ti(CN)–Ni cermet system

Abstract Ti(CN) based cermets have a core/shell microstructure, which gives rise to outstanding mechanical properties and chemical stability for cutting tool applications. The HfC content and C/N ratio in Ti(C1-x Nx ) were varied in order to investigate the effect of HfC addition on the microstructure of Ti(C1-x Nx )–Ni systems. The presence of HfC in the solid solution, (Ti,Hf)C, was found to cause the surface energy to be more anisotropic in the presence of liquid Ni. A phase separation of (Ti,Hf)C and (Hf,Ti)C was noted when >20 wt-%HfC was added to TiC–HfC–Ni. In addition, the Ti(C0·3 N0·7 ) based system showed the lowest coarsening for all sintering times of the various Ti(C1-x Nx ) systems examined. This study focuses on the role of various Ti(CN) and HfC cermets and how they affect the coarsening of microstructures.

Synthesis of Zn 2− x Mn x SiO 4 phosphors using a hydrothermal technique

Zn 2− x Mn x SiO 4 phosphors were synthesized by a hydrothermal process using ZnO and SiO 2 . Prismatic Zn 4 Si 2 O 7 (OH) 2 · H 2 O particles were formed through heterogeneous nucleation on the surface of the ZnO particles. The Zn 2 SiO 4 phase was formed through an isometric phase transition of Zn 4 Si 2 O 7 (OH) 2 · H 2 O. Water pressure, as well as the particle size of the raw materials, had a profound effect on the temperature of phase formation. The effect of solution pH on the morphology and growth of the particles was investigated. The photoluminescence of the synthesized Zn 2− x Mn x SiO 4 phosphor was also examined. The color coordinate of the Zn 2− x Mn x SiO 4 phosphor obtained in this study showed a green color, which was shifted toward short wavelength.

Some issues in Ti(CN)-WC-TaC cermets

Some issues in the dissociation behaviour of a carbonitride solid solution, the transport and precipitation phenomena of second phases in the binder phase were investigated with a Ti(CN)-WC-TaC-Ni-Co system. It was shown experimentally that the dissociation of the cubic phase occurred at a fixed ratio of carbon to nitrogen. This indicated that the nitrogen in the cubic phase has a stronger affinity with metallic species than expected, making the system relatively stable at high temperatures. For the reactions occurring in the binder phase, the effects of sintering conditions on the binder structure were measured with coercive force (Hc) and magnetic saturation (σ). This study showed some evidence that the precipitation in the heating stage influence the binder microstructure extensively and nitrogen can transport to the surface in the form of voids during sintering. The issues were discussed with the results of an analytical electron microscopy and thermodynamic principles.

First-principles and experimental investigation of the morphology of layer-structured LiNiO2 and LiCoO2

LiNiO2 (LNO)-based layered materials for use as cathodes in lithium ion batteries generally take the form of agglomerates composed of small particles. Such morphologies produce low electrode densities compared with LiCoO2 (LCO). In this study, the surface energies of various LNO and LCO crystal facets were calculated, and the morphological characteristics of the materials were interpreted based on these results. Crystal models were constructed and the energy of each facet was calculated using density functional theory methods. The chemical mechanisms underlying the atomic structure at each type of facet surface were analyzed using molecular orbital methods. All facet planes yielded surface energies for LNO that were lower than those for LCO. Atoms in the LNO crystal were found to be less ionized than in LCO, which provided weaker ionic bonding and a lower surface energy. The surface energy was proposed to be the main driving force underlying the morphological differences between LNO and LCO. LNO was expected to favor a high fraction of Li atoms at the facet planes. The interpretations based on the theoretical calculations were supported by the experimental results and showed excellent agreement.

Stable phase domains of the TiO2–Ti3O5–Ti2O3–TiO–Ti(CxOy)–TiC system examined experimentally and via first principles calculations

The stable phase domains of the TiO2–Ti3O5–Ti2O3–TiO–Ti(CxOy)–TiC system were examined experimentally and using first principles calculations. The stable phase domains before, after, and during (i.e., meta-stable phases) the carbothermal reduction of TiO2 are reported here in detail because this increasingly important reaction has not been systematically studied. Stable phase domains were experimentally assessed from the isothermal study of the reduction of TiO2; the thermodynamic activities and the Gibbs free energies of the formation of TiC and TiO in Ti(CxOy) solid solution phases were calculated using the Gibbs–Duhem equation. The first principles calculations of stable-phase-domain diagrams at different temperatures were used to assess the effects of temperature. Formation energies of the solid solutions were calculated using special quasi-random structures. Zero-point energies and phonon vibration effects were calculated for the solid solutions by density perturbation theory implemented in the Vienna ab initio simulations package. A thorough understanding of the stabilities of the initial, final, and intermediate phases will allow the optimization of synthesis conditions and aid the development of new materials. Therefore, this study is expected to aid the synthesis of titanium oxides, titanium carbides, and their solid solutions for energy storage materials and other useful applications.

A study of the characteristics of Ti(CN) solid solutions

The activities of TiC and TiN in Ti(CN) solid solutions can be calculated by means of an integration method. In order to obtain accurate activities for a given set of data, two approaches were evaluated. A linear regression with one curve was found to be more proper than a two-curve approach. The activity exhibited a strong negative deviation from ideal solution behavior. The free energy of formation was obtained at given temperatures using the activity values. Isothermal curves of the free energy of formation with respect to composition had a minimum value near Ti(C0.3N0.7) and Ti(C0.6N0.4) at 1700 and 2100 K, respectively. The investigation shows that the Ti(CN) solid solution cannot be treated as a regular solution. Thus, a subregular solution model was introduced to describe the temperature dependence of free energy.

First-principles thermodynamic calculations and experimental investigation of Sr–Si–N–O system—synthesis of Sr2Si5N8:Eu phosphor

Thermodynamic stabilities of the phases of Sr–Si–N–O system were evaluated by simulating phase diagrams at various conditions based on first-principles density functional theory calculations. Synthesis conditions and stability of the compounds belonging to the system, in which oxidation and nitridation reactions are involved complicatedly, could be interpreted through this first systematic investigation on the two-gas system. Practical synthetic methods of nitrides, such as hydrogen-reduction and nitridation or carbothermal reduction and nitridation reactions, were studied with special attention. This study enabled us to calculate proper conditions for synthesis of the Sr2Si5N8 phase, which is drawing attention as a new phosphor material for light emitting diodes. The types of impurities appearing with deviation from the proper synthetic conditions were also investigated, which may provide information about optimizing synthesis conditions. Synthesis of Sr2Si5N8:Eu phosphor using SiO2, instead of conventionally used Si3N4, was predicted by first-principles calculations, and we succeeded in synthesizing Sr2Si5N8:Eu phosphor for the first time using all oxide raw materials under normal pressure on this basis. The results of this study are expected to provide useful guidelines for synthesis of nitrides and the established simulation method may effectively be applied to other multi-gas systems.

Calculation of Formation Energy of Oxygen Vacancy in ZnO Based on Photoluminescence Measurements

The formation energy of an oxygen vacancy in ZnO was calculated. The photoluminescence intensity of green emission was used as a measure of vacancy concentration, and its variation as a function of reduction temperature was monitored. This enabled a thermodynamic approach based on experimental data, which is in contrast with most previous studies, which focused on a theoretical treatment based on first principles methods. Two reduction conditions were used: hydrogen gas flow and a CO/CO(2) atmosphere generated using activated carbon. The two cases were compared, and mechanisms for the formation of oxygen vacancies during thermal treatment were investigated. The similar results obtained for the two cases indicate that the proposed models of V(O) formation are reasonable.