Sun-Ho Kang

The effect of nonstoichiometry (δ) on the magnetic properties of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite

The effect of nonstoichiometry (δ) on the magnetic properties of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite was investigated. The nonstoichiometry was controlled via a solid state electrochemical method to encompass from δ=+0.005 to −0.006. Initial permeability was found to be affected significantly by nonstoichiometry. From the natural resonance frequency in the magnetic loss spectra and initial permeability at low frequencies, magnetocrystalline anisotropy constant (K1) of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite was calculated as a function of nonstoichiometry, which increased from −1.03×104 to −8.60×103 ergs/cm3 with decreasing nonstoichiometry from +0.005 to −0.006. The effect of nonstoichiometry on the initial permeability of (Mg0.22Mn0.07Fe0.71)3−δO4 ferrite is discussed in terms of saturation magnetization and magnetic anisotropy.

Non-stoichiometry, electrical conductivity and defect structure of hyper-stoichiometric UO2+x at 1000°C

Abstract The oxygen non-stoichiometry (x) and electrical conductivity (σ) of hyper-stoichiometric UO2+x have been measured as a function of partial pressure (PO2) at 1000°C by a solid-state coulometric titration technique and a dc 4-probe method, respectively. Both of the properties were found to be proportional to PO21/2 at the high oxygen partial pressure regime, and PO21/5 at the low oxygen partial pressure regime. These PO2-dependencies of the non-stoichiometry and the electrical conductivity are well explained with the (2:2:2) cluster model: (2Oia2Oib2VO)′ and (2Oia2Oib2VO)″″ are predominant at high and low PO2, respectively. The electron–hole mobility of UO2+x at 1000°C has been determined by the combination of the non-stoichiometry and electrical conductivity combined on the basis of the (2:2:2) cluster model.

Nonstoichiometry (δ) of (MgxFe1 − x)3 − δO4 ferrite spinel

Abstract The nonstoichiometry (δ) of (Mg x Fe 1 − x ) 3 − δ O 4 ferrite spinel has been measured by a coulometric titration method as a function of the thermodynamic variables-temperature ( T ), oxygen partial pressure ( P o 2 ) and Mg content ( x ). The nonstoichiometry amounts to ± 1% order in its extent and varies sine-hyperbolically against P o 2 . The latter is explained in terms of Frenkel disorder as the majority defect type. The effect of temperature and Mg content on nonstoichiometry is discussed.

Fracture behavior and reliability of brazed alumina joints via Mo–Mn process and active metal brazing

Alumina/alumina and alumina/kovar/alumina joints were produced via two joining techniques: (i) a moly-manganese process and (ii) active metal brazing. These joints were heat treated at 400 °C for 100 h to evaluate their fracture strength, fracture behavior, and high-temperature reliability. Depending on the joining methods employed, the optimum microstructure of ceramics for joining is different. It was found in most cases, that the active metal brazing resulted in superior joint strength and reliability over the moly-manganese process. The fracture strength and reliability of alumina/kovar/alumina joints were lower than those of alumina/alumina joints and showed different fracture behaviors. After heat treatment, fracture strength and reliability decreased due to the formation of brittle phases at the joint interface. The fraction of brittle debonding at the interface increased with heat treatment, while that of fracture within the alumina decreased.

Dielectric properties of Pb(Zn1/3, Nb2/3)O3 ceramics modified by Ba(Zn1/3, Nb2/3)O3 and BaTiO3

Dielectric properties of lead zinc niobate (PZN) ceramics modified by barium zinc niobate (BZN) and BaTiO3 (BT) were investigated. By adding the modifier of BT and BZN, the stabilization of perovskite phase of PZN increased, but its Curie temperature decreased linearly with the amount of added modifier. Room temperature dielectric constant of PZN increased by addition of stabilizers up to 12 and 15 mol% of BZN and BT, respectively. The maximum room temperature dielectric constant was observed to be 7800 at 12 mol% of BZN, and 9800 at 15 mol% of BT, respectively.

Redox reaction kinetics of UO2+x

Abstract Oxidation and reduction kinetics of UO 2+ x was investigated in CO 2 /CO gas mixtures at elevated temperatures of 1000°C to 1300°C by monitoring the electrical conductivity relaxation induced by an abrupt change of ambient oxygen partial pressure. By fitting the experimental relaxation data to theoretical equations, the oxidation and reduction kinetics was found to be controlled mostly by the surface reaction step in the ranges of oxygen partial pressure and temperature examined and the surface reaction rate constants were derived therefrom.

High Temperature Transport Properties and Reaction Kinetics of (CeyUt-y)O2+x

The electrical conductivity (σ) of (CeyU1-y)O2+x (y=0.05,0.15,0.25) has been measured as a function of oxygen partial pressure (P02) at elevated temperatures by a d.c. 4-probe method. From the PO2-dependence of O, Ce ions were found to exist as Ce3+ on U sites and, consequently, to act as electron acceptors. From the electrical conductivity and the charge carrier concentration, hole mobility was calculated; activation energy for hopping was obtained from the temperature dependence of the hole mobility. Furthermore, redox reaction kinetics of (CeyU1-y)O2+x was investigated via conductivity relaxation experiment; surface reaction was found to be predominant over chemical diffusion and the surface reaction rate constants increased with decreasing Po2.