Electrical behavior of oxidized metal powders during and after compaction
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The electrical behavior during compaction of tantalum and aluminum powders was characterized before and after thermal oxidation. The resistivity of unoxidized powders decreased by >106 over a narrow range of stress between 1 and 10 MPa. Thermal oxidation of the powders to produce submicrometer thick oxide layers on the particles increased the precompaction electric breakdown strength from 5 kV/cm but did not have a significant effect on the low field resistivity of the powders during compaction. At higher fields, the decrease in resistivity during compaction occurred at lower stresses and over a much narrower stress range since catastrophic electrical breakdown occurred once a certain level of stress was reached. The breakdown field at constant stress also decreased as the stress was increased for the oxidized powders. These effects are caused by the cracking of the brittle oxide coatings at the contact points between the particles during compaction.
I. INTRODUCTION
The electrical behavior of various metal powders during compaction has been studied previously.1–5 Although copper has been used in most studies,1–3 nickel,4 copper– nickel mixtures,5 and iron3 have also been investigated. Copper has been studied the most because of its remarkable behavior during compaction. When certain copper powders are compacted, their electrical conductivity increases by many orders of magnitude over a relatively narrow range of applied pressure. At low pressure, the powders are insulating due to the presence of a thin native oxide film. Once these films are broken down at higher pressure, the conductivity of the compact approaches that of pure copper. For example, Euler1 and co-workers reported that the conductivity of a copper powder with a 40-Å-thick oxide layer increased by over seven orders of magnitude as the compaction pressure was increased from 2 to 5 MPa. Therefore, we have studied this phenomenon both to gain a better scientific understanding and to extend the range of materials and properties so that this effect can be utilized in new applications.6 In this work, the electrical behavior of two metal powders, tantalum and aluminum, is reported. The change in the resistivity of both powders during compaction is similar to that of copper. However, their oxides are very insulating, unlike copper oxide which is semiconducting. The effect on electrical behavior of increasing the thickness of the oxide layer on the powder particles through thermal oxidation was investigated. The resistivity of oxidized powders was measured at several levels of electric
field. The dielectric breakdown strength of the oxidized powders was determined as a function of pressure both during compaction and after the application and removal of a compacting load. Finally, nanoindentation was used to study the cracking of the thin oxide layer on tantalum due to contact stresses to better understand the mechanism responsible for these effects.
II. EXPERIMENTAL
Two metals were evaluated because preliminary experiments indicated that they exhibited th
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