Y. K. Zheng

Effect of oxygen vacancies on transformation of zirconia at low temperatures

The effect of oxygen vacancy on the stability of phase structures and phase transformation of zirconia at low temperatures was studied using an electrochemical technique. It is suggested that the decrease of oxygen vacancies decreases the stability of metastable tetragonal zirconia and promotes its transformation to the monoclinic phase.

The effect of an electric field on the phase separation of glasses

The electromagnetic theory is used in order to characterize the polarization processes in glasses. The change in free energy owing to phase transformation of glasses in the presence of an electric field is . The analysis shows that the free energy may be reduced or enhanced by an electric field depending on the ratio of the dielectric constants of the old and new phases. This means that the electric field can either stimulate or inhibit the nucleation process. The theory is qualitatively consistent with the experimental results. Furthermore, we may extend it to the phase transformation of all dielectric materials upon application of an electric field. These results suggest a possible method for the preparation of nanophase glass ceramics.

Study of the diffusion of Al–Li alloys subjected to an electric field

An experimental investigation of the homogenization treatment of 2091 Al–Li alloy in the presence of an electric field, has revealed the phenomena of reduced volume fraction, small size, spherical shape and random distribution of second-phase particles, which bring about an increase in ductility. The results show that the dissolution of second-phase particles is promoted by means of the vacancy mechanism, because of the larger diffusion coefficient of solute atoms than the vacancy–solute complexes at the beginning of the homogenization treatment. By increasing the homogenization time and applying an electric field, the diffusion coefficient of vacancy-solute complexes is raised, whereas that of solute atoms is reduced, because of the decreased potential energy of the second phase at grain boundaries. Therefore, the non-equilibrium segregation of magnesium and copper elements is generated near the surface of the ingot by a complex mechanism. An experimental study of the solution treatment under an electric field, revealed that the lithium non-equilibrium segregation is induced at grain boundaries responsible for the δ precipitates.

Phase separation of CaO·Al2O3 ·SiO2 glasses induced by electric field

By means of S-4200 SEM, phase separation of CaO-Al2O3-SiO2 glasses upon application of an electric field is investigated. The experimental results show that externally applied electric field promotes the phase separation of the glass, and it leads to a different size of the droplet phase. In the vicinity of the anode of the glass, the smaller droplet phase is induced compared with the cathode of the glass. The accelerating phase separation of the glass in the presence of an electric field is due to the decrease in the free energy forming a critical nucleus size. The different size of the droplet phase of the cathode is due to the increase in the driving force of nucleation and decrease in the viscosity of the glass compared with that of the anode of the specimen.

Phase separation of CaO·B2O3·P2O5 glasses induced by polarization in an electric field

The electromagnetic theory is used to characterize the polarization processes in glasses. At low B2O3 content, the electric field treatment inhibits the phase separation of the glass. But, at low P2O5 content, electric field treatment promotes the process. The analysis shows that the free energy may be reduced or enhanced by the electric field depending on the ratio of the dielectric constants of the old and new phases. This means that the electric field can either stimulate or inhibit the nucleation process. The theory is in excellent agreement with the experimental results. And, we may extend it to the phase transformation of all dielectric materials applying an electric field. These results suggest a possible method for the preparation of nanophase glass-ceramics.

Controlled phase separation by an electric field in glasses

Abstract Externally applied electric field has a significant effect on phase separation of glasses. At low BaO contents, spherical barium-rich phase separation is promoted due to the greater ( e 2 / e 1 >1) ratio of the dielectric constants of the new phases than the old. Increasing BaO contents to 0.1 mol%, owing to the interconnected phase separation produced, the electric field has no effect on the phase separation. At higher BaO contents, spherical silica-rich phase separation is retarded. This means that the electric field can either stimulate or inhibit the phase separation process. Furthermore, we may control phase separation of glasses. These results suggest a new method for the preparation of nanocrystalline glass ceramics.

The phase separation mechanism of CaO–B2O3–P2O5 in an electric field

The effects of an electric field on phase separation in CBP (CaOB 2 O 3 P 2 O 5 ) have been studied. It was discovered that an electric field can inhibit the process of phase separation in CBP glasses. If the electric field strength is increased, the inhibiting effect will increase, too. The mechanism of phase separation in an electric field has been discussed in light of the ir spectrum of CBP glasses.