Colloidal processing of chemically prepared zinc oxide varistors. Part I: Milling and dispersion of powder
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Chemically prepared zinc oxide powders are fabricated for the production of high aspect ratio varistor components. Colloidal processing was performed to reduce agglomerates to primary particles, form a high solids loading slurry, and prevent dopant migration. The milled and dispersed powder exhibited a viscoelastic to elastic behavioral transition at a volume loading of 43–46%. The origin of this transition was studied using acoustic spectroscopy, zeta potential measurements, and oscillatory rheology. The phenomenon occurs due to a volume fraction solids dependent reduction in the zeta potential of the solid phase. It is postulated to result from divalent ion binding within the polyelectrolyte dispersant chain and was mitigated using a polyethylene glycol plasticizing additive. This allowed for increased solids loading in the slurry and a green body fabrication study to be presented in our companion paper.
I. INTRODUCTION
Colloidal processing of ceramic materials has led to the development of several novel forming methods for casting bulk ceramic pre-forms.1 These processes can be superior to conventional forming methods when structural uniformity is a key issue and in situations where loss of material from postsintering machining and finishing processes has a significant cost impact. Often of equal importance to product quality is the negative effect that machining-induced flaws can have on performance. For example, in varistor elements, material defects or flaws (such as machining cracks or pores) severely impact electrical performance.2 Inhomogeneities can concentrate the pulse current and nucleate high-voltage breakdown of the varistor. The purpose of the work presented here was to investigate the application of colloidal processing–based forming methods to prepare high field ZnO varistor components. The varistor powder used in this study was prepared using an aqueous precipitation process developed at Sandia National Laboratories.3–6 One of the challenges of developing colloidal forming processes using these powders is that their primary particles are below 200 nm in size. This small size makes them highly sinterable but complicates slurry processing. Bergstrom et al. have shown for very fine particle size powders that there is a significant effect on densification dependent on the particle interactions and formation of a homogeneous green a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0179
microstructure.7 This implies that achieving a maximum solids loading in the slurry and optimizing the formation method effects on green structure will dictate the success of part production. Rheological characterization is commonly used to determine the maximum solids loading using the KriegerDoughtery expression1
冉
= s 1 −
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冊
−关兴max
(1)
.
In this expression, is the true volume fraction with a maximum at max, s is the solvent viscosity and [] is the intrinsic viscosity, which relates to the effect on the relative viscosity of nonint
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