Reply To: turn heat into electricity

[Henning]

I’ve got a different proposal for insulation material: Kapton.

The discussion is on the forum with the title Corona Resistant Kapton Insulation.

Henning wrote: I just stumbled over a heat resistant insulator, which maybe can be used in the DPF or switches.

It’s called Kapton. Maybe it’s already used by LPP anyway. But in case the insulation is an issue, I’m posting some information here.

Different versions of Kapton:
http://www2.dupont.com/Kapton/en_US/products/index.html

General overview:
http://www2.dupont.com/Kapton/en_US/assets/downloads/pdf/summaryofprop.pdf

The corona resistant version of Kapton:
http://www2.dupont.com/Kapton/en_US/assets/downloads/pdf/CR_H-54506-1.pdf

Insulation capability:
Electrical Strength: 291 kV/mm

One supplier is here (and there are others):
http://www.kaptontape.com/default.php

Henning wrote: Got some more from Wikipedia:

According to a NASA internal report, space shuttle “wires were coated with an insulator known as Kapton that tended to break down over time, causing short circuits and, potentially, fires.” The NASA Jet Propulsion Laboratory has considered Kapton as a good plastic support for solar sails because of its long duration in the space environment.

Kapton is also commonly used as a material for windows of all kinds at X-ray sources (synchrotron beam-lines and X-ray tubes) and X-ray detectors. Its high mechanical and thermal stability as well as its high transmittance to X-rays make it the preferred material. It is also relatively insensitive to radiation damage. Another prominent material for these purposes is beryllium.

The thermal conductivity of Kapton in temperatures from 0.5 to 5 kelvin is rather high κ = 4.638×10−3 T0.5678 W·m−1·K−1. This, together with its good dielectric qualities and its availability as thin sheets have made it a favorite material in cryogenics. Kapton is regularly used as an insulator in ultra-high vacuum environments due to its low outgassing rate.

So on the one hand Wikipedia states it has great mechanical properties, on the other hand it breaks down over time.

With a reply from pulser:

pulser wrote: Kapton is good for vacuum systems, but as mentioned will break down over time. Use 4x the thickness associated with dielectric strength for long term reliability.
Dealing with high voltages can be challenging. Use of semi-insulators will help with limiting electrostatic charge build-up and localized breakdown. I’ve found most people are not familiar with semi-insulators and the concept of what they do and how to use them is challenging even for electrical engineers working with high voltage devices for a living. Semi-insulators include materials like SIPOS and silicon rich silicon nitride (more like amorphous silicon than Si3N4). Carbon could also be used but I have no experience on the best form or deposition methods. Sheet resistances of 1e12 to 1e15ohms/sq are typical. Using alternating layers of semi-insulators and high dielectric strength insulators is a very good way to stand off very high fields even with ionization taking place creating localized high field spots. For pulsed applications such as this, much higher conductivity films could be utilized as long as the power dissipation does not cause significant self heating.

But I think that break-down is the result of real mechanical stress, and the wear over time is measured in years — much more than it’s needed for the experiments. The only thing is that it’s not as mechanically flexible as Mylar (biaxially oriented PET film). But for protecting the insulator within the switch or even DPF, this might be a good candidate (if it doesn’t introduce more non-symmetries).