[Zalo]

I'm having little luck trying to pin down a material that has low (far infrared) reflectivity AND emissitivity.  It's usually one or the other.  Does such a material exist, and if so, how expensive is it (no germanium (too transparent anyway)).

[Enthalpy]

The second law of thermodynamics tells that, at one wavelength, emissivity equals absorptivity.

Then, incoming infrared light can be either transmitted, absorbed or reflected.

You see? These three properties are totally linked.

If you accept a transparent material, which is the only possibility to minimize both reflectivity and emissivity, there are cheap ones. Polyethylene for instance is transparent near 10µm, the wavelength of thermal radiation at 300K; more plastics and ceramics are. Used as a window in movements detectors.

There are turnarounds, if you can distinguish among angles, precise wavelengths... Thermodynamics is tricky and leads often to wrong conclusions.

If you hope to go undetected from thermal imaging cameras, that'll be difficult.