A metal–polymer composite with unusual properties

Abstract

Electrically conductive composites that contain conductive filler dispersed in an insulating polymer matrix are usually prepared by the vigorous mixing of the components. This affects the structure of the filler particles and thereby the properties of the composite. It is shown that by careful mixing nano-scale features on the surface of the filler particles can be retained. The fillers used possess sharp surface protrusions similar to the tips used in scanning tunnelling microscopy. The electric field strength at these tips is very large and results in field assisted (Fowler-Nordheim) tunnelling. In addition the polymer matrix intimately coats the filler particles and the particles do not come into direct physical contact. This prevents the formation of chains of filler particles in close contact as the filler content increases. In consequence the composite has an extremely high resistance even at filler loadings above the expected percolation threshold. The retention of filler particle morphology and the presence of an insulating polymer layer between them endow the composite with a number of unusual properties. These are presented here together with appropriate physical models.

Metal–polymer composite with nanostructured filler particles and amplified physical properties

“…12 These show that all the metal particles are coated in polymer, which adheres intimately to the nickel, and that the spiky surface morphology of the filler particles is retained in the composite. New scanning electron microscope ͑SEM͒ images of cut surfaces of stretched samples ͓Figs.…”

“…The projections on the surfaces of the filler particles have tip radii below 10 nm. 12 The local field at the these tips will be much larger than that at the surface of a spherical particle. 13 Localized discharge to air when 240 V ac is applied to compressed cylindrical QTC™ samples, i.e., there are internal fields Ͼ3 ϫ 10 6 V m −1 , supports this hypothesis.…”

“…As reported previously the resistance of a 1 ϫ 20ϫ 20 mm sheet, measured in the direction of elongation, fell from ϳ10 12 to 20 ⍀ at 36% elongation. 12 Ryvkina et al 8 note that in a heavily loaded, uniaxially compressed composite the random network of percolation paths will contain more lateral contacts than axial contacts between filler particles. Lateral expansion accompanying compression accounts for the small, unexpected decrease in conductivity they observed.…”

“…Further evidence for this is that removal of the sharp features from the filler particles, by mechanical working, oxidation, or etching, drastically reduces the sensitivity of QTC™ to deformation. 12 Although the uncoated filler particles can be damaged the composite is remarkably robust. Properties are recovered after Ͼ80% compression.…”

Temperature Dependence of Electrical Transport in a Pressure-Sensitive Nanocomposite

“…Tunneling processes may occur in composites comprising filler particles in close proximity, but not in direct contact, and with nanoscale features that act to enhance local electric fields 19 . Such systems may be modeled as a combination of ohmic connections between filler particles in physical contact and field-assisted tunneling between separated filler particles and/or clusters of filler particles 20 .…”