Yong S. Cho

Homogeneous grain growth and fast-firing of chemically modified nanocrystalline MnZn ferrites

Abstract A chemical method utilizing sol-gel reactions was investigated to uniformly incorporate small amounts of additives of Si and Ca into nanocrystalline Mn 0.6 Zn 0.4 Fe 2 O 4 powders (≈14 nm particle size). Sintering behavior of the chemically modified nanocrystalline MnZn ferrites was studied with regard to the effects of the chemical additives and sintering conditions on densification and grain growth. The nanopowder samples without the additives exhibited abnormal grain growth regardless of heating rate after pressing at a relatively low pressure and sintering at 1200°C, while the chemical addition of 2 wt% SiO 2 and 0.5 wt% CaO was found to improve microstructural characteristics, i.e., homogeneous grain growth and less porosity. Fast-firing did not seem to favorably affect densification of the chemically modified MnZn ferrite nanopowder, but was found to contribute to the suppression of grain growth in the nanocrystalline ferrite compacts.

Crystallization and microstructural evolution of cordierite-based thick film dielectrics

Abstract Microstructural evolution of glass–ceramic dielectric thick films based on a nonstoichiometric cordierite, 2.4MgO·2Al 2 O 3 ·5SiO 2 , containing B 2 O 3 , P 2 O 5 , and PbO were investigated in conjunction with nucleation and crystal growth. The cordierite thick films were deposited by screen printing on a 96% alumina substrate, and then fired in the temperature range of 850–950°C in a N 2 atmosphere. Surface microstructure characteristics of the thick films depended on PbO content. Heterogeneous nucleation originated predominantly from the interface between the densified thick film and the alumina substrate. The added PbO segregated to the remaining glass during crystallization, and the relative concentration of Pb in the glass was proven to increase with rising temperature. Most of the remaining glass protruded from the film surface at the final temperature of 950°C. Penetration of the alumina substrate by the remaining glass was also observed. The influences of temperature and PbO on the microstructural evolution including nucleation and growth will be discussed with possible explanations for the observed results.

Improved magnetic properties and growth anisotropy of chemically modified Sr ferrites

Magnetic properties and microstructural characteristics of SrO⋅5.9Fe2O3 chemically modified with Si and Ca were investigated by changing experimental parameters such as additive composition, the ratio of Ca/Si, and sintering condition. A novel particulate coating method utilizing sol–gel reactions was used to uniformly incorporate the additives of Si and Ca. This method was very successful in obtaining homogeneous grain growth and fine grains. A sample containing the gel additives of 0.6 wt % SiO2 and 0.7 wt % CaO and sintered at 1200 °C for 4 h was found to significantly suppress abnormal grain growth, resulting in submicron-sized grains and high density. A distinct grain boundary phase containing Si and Ca was observed by increasing the sintering temperature to 1250 °C. The resultant microstructural characteristics favorably affected magnetic properties. For example, the chemically modified sample exhibited a higher coercivity of 3530 Oe compared to a value of 2050 Oe obtained for the sample without the...

Grain boundaries and growth kinetics of polycrystalline ferrimagnetic oxides with chemical additives

Grain boundaries and grain growth kinetics of two ferrimagnetic oxides, Y2Gd1Fe5O12 and Li0.3Zn0.4Fe2.3O4, modified by a sol–gel particulate coating process were investigated on the basis of their microstructural characteristics. Interestingly, the addition of small amounts of MnO2 and SiO2 using the sol–gel coating process led to different results for the two magnetic materials. In the case of Y2Gd1Fe5O12, the additives had a role as a grain growth inhibitor because Si-rich precipitates were segregated along the grain boundary and exerted a drag force against grain boundary movement. On the other hand, the same additives acted as an accelerator for grain growth by forming a glassy phase at the grain boundaries for Li0.3Zn0.4Fe2.3O4. The results were analyzed quantitatively by calculating activation energies for grain growth and grain boundary mobilities. For example, the calculated grain boundary mobilities, i.e., 5.6×10−15 m3/N s at 1400 °C for Y2Gd1Fe5O12 and 22.3×10−15 m3/N s at 1150 °C for Li0.3Zn0.4...

Microstructural Aspects of Nanocrystalline LiZn Ferrites Densified with Chemically Derived Additives

Densification behavior and microstructural characteristics of nanocrystalline LiZn ferrites with chemically derived additives were investigated. Nanocrystalline Li{sub 0.3}Zn{sub 0.4}Fe{sub 2.3}O{sub 4} powders having a {approx} 15 nm size were prepared at a low temperature of 450 C by a chemical synthesis using a combustible polyacrylic acid (PAA). Small amounts of Si, Ca and Mn were incorporated into the nanocrystalline ferrites via sol-gel reactions utilizing tetraethyl orthosilicate, calcium isopropoxide and manganese acetate. This process was believed to give a homogeneous distribution of the additives over the nanocrystalline ferrites. A uniform microstructure was obtained without any evidence of exaggerated grain growth after sintering at 1,100 C. Saturation magnetization and coercive force were found to increase with the chemical additives. The results were compared with those of the same composition, but processed by the conventional batch-mixing of corresponding oxide additives.