Colloidal processing of chemically prepared zinc oxide varistors. Part II: Near-net-shape forming and fired electrical p
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Chemically prepared zinc oxide powders were processed for the production of high aspect ratio varistor components (length/diameter >5). Near-net-shape casting methods including slip casting and agarose gelcasting were evaluated for effectiveness in achieving a uniform green microstructure that densifies to near theoretical values during sintering. The structure of the green parts was examined by mercury porisimetry. Agarose gelcasting produced green parts having low solids loading values and did not achieve high fired density. Isopressing the agarose cast parts after drying raised the fired density to greater than 95%, but the parts exhibited catastrophic shorting during electrical testing. Slip casting produced high green density parts, which exhibit high fired density values. The electrical characteristics of slip-cast parts are comparable with dry-pressed powder compacts.
I. INTRODUCTION
Zinc oxide ceramics are used as varistors due to their non-ohmic electrical properties and their ability to withstand power surges. These properties find application in surge protection and voltage stabilization.1 Tailoring of these properties relates to control over the ceramic microstructure, addition of dopants, and overall porosity. Microstructurally, varistors are commonly modeled as conducting grains in a matrix of resistive intergranular compound, which commonly is bismuth oxide.2 The behavior of the interfaces between these components is expected to dominate many of the bulk electrical properties of the ceramic.3 A number of studies have examined the influence of dopants on the sintering behavior, microstructure, and electrical properties.4–7 Nanomaterials used to form varistors have shown increased values for breakdown voltage, attributed to the increased number of grain boundaries per unit area.8 Study of the interfaces in these multiphase ceramics has found, for example, that direct current (dc) degradation effects and low-frequency losses can result from bismuth-rich phases segregated at the corners of grain junctions.9 However, a definitive understanding of electrical “flaws” related to microstructure is not currently available.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0180
The fabrication of varistor components that meet target electrical breakdown characteristics requires processing materials into geometries that minimize flaws, inhomogeneities, and compositional gradients. Powder processing of mixed oxides to form varistors has traditionally mixed the desired component oxides in the target ratio and sintered the material at temperatures that promote the diffusion of all components to form a homogeneous microstructure. The high temperatures needed can promote the formation of large grain sizes (1–5 m) and thereby impact the electrical characteristics. A doped zinc oxide material has been developed at Sandia National Laboratories in which the dopants (Bi, Mn, Co, Al, and Na) are incorporated in the material during fabrication and in which the average primar
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