National Radio Astronomy Observatory, Charlottesville
ROHACELL Foam Compression Strength and Heat ConductivityJ. R. Fisher - November 18, 2013ROHACELL HF is a low-density foam with excellent RF properties as described at 1 shows the mechanical properties of three density grades of this product reproduced from the web page, and Table 2 shows the dielectric properties.Table 1. Mechanical Properties of ROHACELL HFPropertyUnitROHACELL31 HFROHACELL51 HFROHACELL71 HFDensitykg/m3325275lbs./cu,ft.2.003.254.68Compressive StrengthMPa0.40.91.5psi58130217Tensile StrengthMPa1.01.92.8psi145275406Shear StrengthMPa0.40.81.3psi58116188Elastic ModulusMPa367092psi52201015013340Shear ModulusMPa131929psi188527554205Strain at Break%3.54.04.5Table 2. Electrical Properties of ROHACELL HFPropertyFrequency (GHz)ROHACELL31 HFROHACELL51 HFROHACELL71 HFDielectric Constant2.51.0501.0571.0755.01.0431.0651.10610.01.0461.0671.09326.51.0411.0481.093Loss Tangent2.5<0.0002<0.0002<0.00025.00.00160.00080.001610.00.00170.00410.003826.50.01060.01350.0155Since one standard atmosphere is 0.1013 MPa the compressive strength of a nearly solid block of all three grades of ROHACELL is adequate to support a flexible vacuum window without yielding. From the elastic moduli we can see that a solid block of each grade of foam would compress by 0.28%, 0.145%, and 0.11% of its thickness under one atmosphere pressure.From the web page and links therein the thermal conductivities of grades 31 HF, 51 HF and 71 HF are 0.031, 0.029, and 0.030 W/m-K, respectively. If we use an example of a cylindrical piece of solid foam 20 cm thick and 70 cm in diameter that would cover a 19-element PAF, the heat flux through the top surface with a temperature differential of 270 K would be 0.03 * 270 * π * 0.352 / 0.2 = 15.6 W. This doesn’t count heat flux through the sides of the cylinder nor through the dipoles that stick up into the foam, but it shows the order of magnitude of the insulating properties of this foam. ................
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