Making a Conductive Elastomer*
by sjacoby
*maybe
Inspired by a paper we read last week, I attempted to produce a conductive silicone elastomer. While the paper used a variety of conductive fillers—silver, aluminum, nickel—I was limited by the materials on-hand, and decided to focus on nickel, which was available in the lab, and gives fairly good conductive results.
I mixed a number of silicone samples using two different conductive fillers at a range of volumetric percentages. After letting the silicon, none of the samples were shown to be conductive. This indicates procedural or technical problems in my implementation, which diverged from that in the paper’s.
Substrate
SkinRITE10 Base & Catalyst
To cast the silicone, I used SkinRITE10, a two-component silicone frequently used for casting. It consists of a silicone base and a catalyst to begin the curing process. 10 parts base are mixed with 1 part catalyst, and the silicone cures over the course of several hours. It is widely-available.
Conductive Filler
The datasheet indicated that these nickel flakes were manufactured specifically for conductive applications, and had been specially-treated to prevent oxidization. Each flake is milled to be 1 micron thick, but has a 14-16 micron diameter. The manufacturer suggests that this allows for a great deal of surface contacts to be made. It is typically used in conductive paints.
Novamet HCA-1 Nickel Flakes
Novamet 60% Nickel Coated Graphite
To get a better sense of relative conductivities, I also used nickel-coated graphite. Having had some experience with carbon conductors, I was curious how this material would behave. The primary application of this substance, though, is in shielding and I expected to see less conductivity than with the pure nickel flakes.
Novamet Nickel-coated Graphite
Procedure
Using the calculated density of the conductive fillers—1.45 g/ml for both—specific volumes of filler were weighed out. Volumetric ratios of 20%, 30%, 40%, and 50% filler were calculated and weighed.
Weighing Filler Samples
Each was combined with 5ml of catalyzed silicone and mixed together as vigorously as possible with a toothpick. A mold with cavities of 5ml was cut using a laser cutter. The resulting mixture was spread across a mold and allowed to cure. The resistivity of the cured silicone panel was the tested with a digital multimeter.
Cast & Curing Silicone Panels
Challenges
Mixing up silicone is a messy process, as is measuring and preparing the fine filler samples. The quantities involved are small, and it is difficult to maintain accuracy. The SkinRITE silicone takes about a day to cure,.
Outcome
No samples were tested as conductive with a digital multimeter. A number of conditions were attempted—stretching, compressing, piercing the silicon panels—all unsuccessful. There are a number of possible factors, many of which are discussed in the relevant paper:
- Insufficient filler concentrations to achieve conduction
- Inadequate agitation of filler-silicone mixture
- Overly-elastic substrate which does not compress filler into a conductive lattice
- Oxidization or corrosion of powdered filler
- Procedural issues in the preparation of the SkinRITE silicon
- Inconsistent casting and conductor distribution
Testing the conductive silicone mixture
Going Forward
The first step will be to attempt to repeat the experiment with higher concentrations of conductive filler. I would also like to improve the consistency of the casting, to acquire thin, uniform sheets. A useful, controllable, highly-elastic conductive substrate could be poured and cast in many different shapes and forms. It could be used to coat other materials, or form conductive contacts on unusual or ‘squishy’ forms.