Electromagnetic and Thermal Properties of a Conductively Loaded Epoxy

Abstract

We discuss the mechanical, thermal, and electromagnetic properties of a castable microwave absorber consisting of a mixture of stainless steel powder suspended in a commercially available epoxy. The resulting mixture is well suited for cryogenic applications. Its coefficient of thermal expansion closely matches most metals to reduce mechanical strain during cool down. The absorption can be tuned by varying the volume filling fraction of the stainless steel powder in the mixture and exhibits little change from room temperature to 4 K. We provide simple expressions for the real and imaginary parts of the dielectric permittivity as a function of frequency and the stainless steel filling fraction.

Appendices

Appendix

Recommended steelcast sample preparation procedure

The following components are used in the preparation of the Stycast – stainless steel powder mixtures we refer to as Steelcast:

Emerson & Cuming Stycast 2850 FT, Catalyst 23 LV [22]Carpenter 16 micron Stainless Steel Powder Micro-Melt 316L [23]

The vendor specifies a density for Stycast 2850 FT in the range of ∼2.35 to 2.45 g/cm³ [22]. This variability in density can have a significant influence on the complex dielectric constant and was traced to settling of the Al₂O₃ in the 2850 FT over the timescale of weeks. We alleviate this issue by mixing the 2850 FT in the storage container for about 2–to–3 minutes prior to transferring the required amount of material to a separate mixing container. Next, the 2850 FT is heated in a mixing container to 40 to 50°C for about 30 minutes. Catalyst 23 LV (7.5% by weight) is added and mixed vigorously for ∼3 minutes. Stainless steel powder (30% by volume, 59% by weight) is then added and mixed vigorously for ∼3 minutes or until the mixture is smooth and free of clumps.

Acetone (up to 12% by volume) can be added to make the mixture easier to pour and cast, however, this increases the curing time by up to 300% if used in a mold with limited openings for venting. The introduction of acetone reduces the elasticity and mechanical strength of the resulting material as indicated in Table 1. Other material properties of the final product are largely unaffected by the addition of acetone during the casting process.

The mixture is then degassed in order to remove the air incorporated during mixing by placing in a vacuum-oven at ∼40–to–50°C and evacuating to <6.5 kPa. The mixture will rapidly rise when the pressure drops to ∼15 kPa and then collapse. After the mixture collapses, it is degassed for about 30 seconds and then returned to ambient pressure. At this point the material is removed from vacuum and gently folded without re-incorporation of air. Since the material quickly thickens upon cooling, for best results, it is poured into the intended mold as soon as practical. The addition of carbon black to these mixtures is recommended if a higher DC conductivity is required.

The properties of the Steelcast formulations described can be well controlled: Over the course of a year, 36 individual waveguide shim samples (WR28.0) were filled with the 30% Steelcast formulation and a complex dielectric constant of $\varepsilon _r^* = 10.8 + 2.4i\left( {\mu _r^* \approx 1} \right)$ with a 4% scatter in the derived parameters. The highest dielectric volume loading fraction explored in practice was realized by an alternative method–a mold was filled with stainless steel powder and cyanoacrylate (i.e., “superglue”) was employed as the host dielectric media. To eliminate the formation of voids in the sample; after packing, the binder was pulled through the steel powder with a vacuum pump. The observed electrical properties at 30 GHz for this composite material were $\varepsilon _r^* \sim 20 + 20i$ and $\mu _r^* \approx 1$.