Vernon L. Burdick

The effect of liquid‐phase sintering on the properties of Pr6O11‐based ZnO varistors

Conventionally batched varistors in the ZnO–Co3O4–Pr6O11 system that were sintered at or above 1280 °C exhibited a sharp increase in the average breakdown voltage per grain boundary. In these samples, barrier heights and donor concentration values, derived from capacitance‐voltage analysis, exhibited sharp changes at the same soaking temperature. At or above 1280 °C, grain growth in this system proceeds with the assistance of an eutectic liquid. The improvement of the distribution of Pr6O11, by precipitation of Pr(OH)3 during batching, resulted in varistors markedly more insensitive to firing temperature than conventionally batched ones. The presence of a liquid phase, at least during grain growth, results in an efficient distribution of grain boundary materials and dopants in general. Suitable chemisorbed gases along the grain boundaries are believed to be responsible for the formation of interface states. Segregated materials appeared to provide for enhanced transport of gaseous species along the grain ...

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.

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...

The Corrosive Nature of Molten Glass

One of the outstanding characteristics of molten glass is its ability to chemically and physically attack the refractories which serve to contain the glass. The physical attack usually consists of penetration of voids, cracks and joints by the molten glass or in some cases refractories may be abraded. Chemical corrosion is much more deleterious to the life of the refractories since it involves the dissolution of some or all of their components. Additionally, this chemical interaction is potentially a source of defective glass product.