DC Electrical, Thermal, and Spectroscopic Properties of Various Condensation Polyimides Containing Surface Cobalt Oxide

Abstract

Doping polyimides with cobalt ion causes the room temperature direct current electrical resistivity to decrease relative to the polymer alone, the reduction being most pronounced for the air-side of the cobalt modified polyimides. At a constant electrical field, resistivity for the volume, air-side and glass-side modes decreases yet further with an increase in temperature as expected for semiconductors and insulators. X-ray photoelectron spectroscopy indicates the air-side of the cobalt modified polyimides is predominately Co304. The bulk resistivity of the air-side and activation energy of conduction for this surface are comparable to high purity sintered Co304. Charging characteristics at room temperature indicate a substantial polymer matrix contribution to both the glass-side and volume mode measurements but a negligible contribution to the air-side electrical properties. Volume electrical resistivity for similar additive levels is reduced by increasing the molecular flexibility of the host polymer.Polyimides, having useful mechanical properties over a wide temperature range (1), have extensive applications as specialty engineering materials because of their favorable properties such as radiation resistance, hydrolytic and thermo-oxidative stability, low crystallinity, and extremely high electrical resistivity (2). The properties of polyimides have been modified by many techniques. Modification of conventional polyimides with siloxane (3) to produce materials having superior adhesion to microelectronic devices and resistance to water and modification with dispersed metals (4), metal salts, and metal complexes (5-9) to produce materials having unique combinations of properties are a few examples.One of the systems evaluated by our research group is cobalt modified condensation polyimides, chosen because the cobalt ion can be probed by a variety of spectroscopic techniques (10). Polyamic acid solutions were obtained by the condensation reaction between various dianhydrides and diamines in N,N-dimethylacetamide at room temperature. Cobalt(II) chloride was next dissolved in the polyamic acid solution. A film was then cast which, after thermal imidization, resulted in a cobalt modified polyimide film. Highly flexible films were obtained wherein one film surface exhibited a tightly adhering cobalt oxide surface. Because a principal goal of the laboratory work was to produce materials having substantially modified electrical properties, direct current electrical resistivity (bulk and each surface) of the polyimide films has now been measured at both room temperature in air and elevated temperature in vacuum. The field dependence and time dependence of the electrical current as well as the time dependence of charge accumulation have been evaluated. Data pertaining to the lowered resistivity of a variety of polyimides, each of which has been comparably doped with COC12, have been obtained. A discussion of these results constitutes the focus of this report. Experimental Polymer film preparation.--Monomers and cobalt(II...

Preparation and properties of surface-conductive polyimide films via in situ codeposition of metal salts

“…Information from surface-sensitive spectroscopic techniques and a variety of electrical measurements has been used to propose a structural model that represents the metal ion modified polyimide films in terms of both their spectroscopic properties and dc electrical resistivity. 6 In addition, a deeper understanding of the surface characteristics of these novel materials has guided additional research. In this regard, reportedly a small amount of lithium oxide stoichiometrically substituted for cobalt(II) in cobalt oxide (Co304) greatly enhances the electrical conductivity of the resulting mixed oxide relative to the pure cobalt oxide.…”